Optical resonators are capable of storing electromagnetic energies in the visible and infrared band. The light intensity is greatly enhanced within the resonator, which makes them suitable as a platform for nonlinear optics studies. Here, using silica microresonators as platforms, we explore the fundamental nonlinear dynamics of light induced by Kerr nonlinearity and Brillouin scattering. The first half of the thesis analyzes optical solitons as a result of Kerr nonlinearity, including its universal scaling, its dynamics in the presence of laser feedback, the analytical properties of its relativistic counterpart, as well as its applications as a wavelength reference. The second half of the thesis focuses on stimulated Brillouin lasers and their linewidth performance, demonstrating new performance levels of the Brillouin laser and two correction factors to its linewidth that have been established for semiconductor lasers.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Vahala, Kerry J.}, } @phdthesis{10.7907/10XE-G776, author = {Lai, Yu-Hung}, title = {Microresonator Brillouin Laser Gyroscope}, school = {California Institute of Technology}, year = {2019}, doi = {10.7907/10XE-G776}, url = {https://resolver.caltech.edu/CaltechTHESIS:05282019-120721714}, abstract = {Optical Gyroscopes are among the most accurate rotation-measuring devices and are widely used for navigation and accurate compasses. With the advent of integrated photonics for complex telecommunication chips, there has been interest in the possibility of chip-scale optical gyroscopes. Besides the potential benefits of miniaturization, such solid-state systems would be robust and resistant to shock. In this thesis, we investigate a chip-based optical gyroscope using counter-propagating Brillouin lasers on a monolithic silicon chip. The near-degenerate lasers mimic a commercial ring laser gyroscope including the existence of a locking band. By using physical properties associated with the Brillouin process, a solid-state unlocking method is demonstrated. We focus on three topics to explore the potential of the counter-propagating Brillouin-laser gyroscope. First, we explore the physics of the counter-propagating Brillouin lasers by deriving the theory to link the passive cavity mode with the lasing gain medium. We explicitly show how the dispersion, Kerr nonlinearity, dissipative coupling, and Sagnac sensing affect the beating frequency of the Brillouin lasers. Second, we experimentally demonstrate the performance of the gyroscope. Most notably, the gyroscope is used to measure the rotation of the Earth, representing an important milestone for chip-scale optical gyroscopes. Third, we investigate the non-Hermitian interaction between the counter-propagating Brillouin lasers. We test the recent prediction of the EP-enhanced Sagnac effect, and observe a Sagnac scale factor boost by over 4X by measurement of rotations applied to the resonator. Our research shows the feasibility of the chip-based Brillouin laser gyroscope. This gyroscope paves the way towards an all-optical inertial guidance system.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Vahala, Kerry J.}, } @phdthesis{10.7907/DWMX-S056, author = {Yang, Qifan}, title = {Nonlinear Physics in Soliton Microcombs}, school = {California Institute of Technology}, year = {2019}, doi = {10.7907/DWMX-S056}, url = {https://resolver.caltech.edu/CaltechTHESIS:05192019-220451795}, abstract = {Like rulers of light, optical frequency combs consist of hundreds to millions of coherent laser lines, which are capable of measuring time and frequency with the highest degree of accuracy. Used to rely on table-top mode-locked lasers, optical frequency combs have been recently realized in a miniaturized form, namely the microcomb, using monolithic microresonators. Besides a reduction of footprint, microcombs could also achieve parity with traditional frequency combs in performance by mode-locking through the formation of “light bullets” called dissipative Kerr solitons. These soliton microcombs not only serve as a unique platform to study nonlinear physics, but also offer scalable and cost-effective solutions to many groundbreaking applications, spanning spectroscopy to time standards. In this thesis I will trace the physical origin of soliton microcombs, followed by their experimental realization in high-Q silica microresonators. The impact of several nonlinear process on solitons will be discussed, which leads to novel soliton systems, e.g., Stokes solitons and counter-propagating solitons. Utilizing these nonlinear properties, we show that soliton microcombs can be adapted for high-precision spectroscopic applications. In the end, a real-time method for monitoring transient behavior of solitons will be presented.}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Vahala, Kerry J.}, } @phdthesis{10.7907/Z95H7DGT, author = {Oh, Dong Yoon}, title = {Integrated Nonlinear Photonic Devices}, school = {California Institute of Technology}, year = {2018}, doi = {10.7907/Z95H7DGT}, url = {https://resolver.caltech.edu/CaltechTHESIS:11122017-215023206}, abstract = {Chip-scale nonlinear optics can provide important new functions in communications, frequency metrology and spectroscopy. Optical microcavities enhance nonlinear optical effects through resonant recirculation. This recirculation dramatically reduces the required power in applications and also lowers signal noise. A key figure-of-merit is the optical Q factor, which provides a dimensionless scale of optical storage time within the microcavity. In this thesis, a novel integrated ultra-high-Q microcavity with Q as high as 230 million is presented. The device is applied to demonstrate multiple functions including electronic-rate soliton microcomb generation and stimulated Brillouin laser operation. For soliton generation, the resonator must be engineered to produce optical mode families that feature anomalous dispersion. This engineering is applied to generate solitons at wavelengths of 1064 nm and 778 nm. Systems-on-a-chip applications of these devices are discussed including compact optical synthesizers, optical clocks and rotation sensors. Finally, a compact array of silica ridge waveguides is described and applied for efficient and coherent ultraviolet-to-visible comb generation by dispersive-wave generation. Unlike other devices used to broaden spectra such as micro-structured fibers, these arrays provide a wide range of emission wavelength choices on a single chip. The arrays can also enable mode-locked lasers to attain greatly extended spectral reach for spectroscopy, bioimaging, tomography and metrology.}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Vahala, Kerry J.}, } @phdthesis{10.7907/Z96T0JTQ, author = {Yang, Kiyoul}, title = {Integrated Ultra-High-Q Nonlinear Photonic Platform for On-Chip Optoelectronic Systems}, school = {California Institute of Technology}, year = {2018}, doi = {10.7907/Z96T0JTQ}, url = {https://resolver.caltech.edu/CaltechThesis:10042017-102201104}, abstract = {Silicon technology provided a concrete basis of the integrated microelectronics revolution, and it might usher disruptive advances in photonics again. An integrated photonic system can potentially revolutionize instrumentation, time standards, spectroscopy, and navigation. Driven by these applications, various high-Q platforms have emerged over the last decade. However, applications require to satisfy challenging combinations of ultra-high-Q (UHQ) cavity performance, monolithic integration, and nonlinear cavity designs: the monolithic integration of UHQ devices still remains elusive. In this thesis, an integrated UHQ microcavity is demonstrated for the first time. A silicon nitride waveguide is monolithically integrated with a silicon oxide cavity, and the integrated waveguide can provide nearly universal interface to other photonic devices. Significantly, this thesis discusses far beyond setting a new record for integrated Q factor: the integrated UHQ cavity provides functionality as soliton source with electronic-repetition-rates. Demonstration of low-pump-power soliton generation at 15 GHz was previously possible in only discrete devices but essentially required for integrated self-referenced comb, which can unlock new level of performance and scale in an optoelectronic system. In addition, nonlinear cavity design is another outstanding challenge towards a further development on the optoelectronic system, and will be discussed in this thesis. The dispersion-engineered platform can potentially tailor the spectral bandwidth of frequency comb, and extend the frequency comb to visible and ultraviolet band. Importantly, the design methods are directly transferable to the integrated platform.}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Vahala, Kerry J.}, } @phdthesis{10.7907/Z92F7KG8, author = {Suh, Myoung-Gyun}, title = {Nonlinear Optics in Chip-based Microresonators and their Applications}, school = {California Institute of Technology}, year = {2017}, doi = {10.7907/Z92F7KG8}, url = {https://resolver.caltech.edu/CaltechTHESIS:06012017-163114372}, abstract = {Optical micro-resonators have been studied for decades as a platform to investigate optical physics, and to miniaturize bulky optical systems. In the last decade, optical frequency combs, which have revolutionized the precision measurement of time and frequency, have been demonstrated in optical micro-resonators via the combined effect of parametric oscillation and cascaded four-wave mixing. More recently, soliton mode-locking has made possible low-noise/reproducible generation of these miniature combs (microcombs). In this thesis, we demonstrated the generation of soliton microcombs from silica wedge disk micro-resonators and the characteristics of the soliton microcombs are described. We also applied soliton microcombs to dual-comb spectroscopy and distance measurement (LIDAR) for the first time. Also, ways to improve spectral resolution, signal-to-noise ratio, and spectral coverage are discussed. In addition to soliton microcombs, a novel spiral resonator is studied as a stable optical frequency reference. Combined with a frequency comb, this new type of chip-based reference cavity is also applied to generate stable microwaves via optical frequency division. Lastly, we generated a stimulated Brillouin laser (SBL) from the optical micro-resonator and its phonon-limited linewidth is studied. Application of the SBL for rotation measurement is also demonstrated. This thesis is organized into six chapters. Throughout the thesis, the implication and potential of my PhD work toward chip-based advanced optics system are discussed.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Vahala, Kerry J.}, } @phdthesis{10.7907/Z9FT8J22, author = {Yi, Xu}, title = {Physics and Applications of Microresonator Solitons and Electro-optic Frequency Combs}, school = {California Institute of Technology}, year = {2017}, doi = {10.7907/Z9FT8J22}, url = {https://resolver.caltech.edu/CaltechTHESIS:05232017-161027761}, abstract = {Frequency combs are having a broad impact on science and technology because they provide a way to coherently link radio/microwave-rate electrical signals with optical-rate signals derived from lasers and atomic transitions. A new, miniature realization, the microcomb, that uses chip-based microresonators can potentially revolutionize instrumentation, time keeping, spectroscopy, and navigation. Microcombs were first demonstrated using a form of cascaded four-wave mixing. However, the recent discovery of dissipative soliton microcombs enables phase-locked spectra with reproducible envelopes, as required in many frequency comb applications. In addition, these solitons are confined in a high-Q microresonator, thereby creating a rich landscape for research in nonlinear optical phenomena. In this thesis, these solitons are demonstrated for the first time in a silica microcavity. Significantly, the device provides a microwave-detectable soliton repetition rate, which is essential to many comb applications. The unusual properties of the solitons are studied from a theoretical viewpoint using a Lagrangian formalism and predictions of the theory are confirmed experimentally. In the course of this work, a new optical soliton, the Stokes soliton, was also discovered. In addition to soliton mode locking, another novel and compact platform, the electro-optical modulation frequency comb, was studied. This type of frequency comb was used to demonstrate a novel electro-optic form of frequency division for stable microwave synthesis. It was also modified to perform astronomical calibration for exoplanet detection at the Keck Observatory in Hawaii.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Vahala, Kerry J.}, } @phdthesis{10.7907/20G7-EM85, author = {Li, Jiang}, title = {Nonlinear Optics in Planar Silica-on-Silicon Disk Resonators}, school = {California Institute of Technology}, year = {2013}, doi = {10.7907/20G7-EM85}, url = {https://resolver.caltech.edu/CaltechTHESIS:05312013-151745220}, abstract = {Optical frequency combs (OFCs) provide direct phase-coherent link between optical and RF frequencies, and enable precision measurement of optical frequencies. In recent years, a new class of frequency combs (microcombs) have emerged based on parametric frequency conversions in dielectric microresonators. Micocombs have large line spacing from 10’s to 100’s GHz, allowing easy access to individual comb lines for arbitrary waveform synthesis. They also provide broadband parametric gain bandwidth, not limited by specific atomic or molecular transitions in conventional OFCs. The emerging applications of microcombs include low noise microwave generation, astronomical spectrograph calibration, direct comb spectroscopy, and high capacity telecommunications.
In this thesis, research is presented starting with the introduction of a new type of chemically etched, planar silica-on-silicon disk resonator. A record Q factor of 875 million is achieved for on-chip devices. A simple and accurate approach to characterize the FSR and dispersion of microcavities is demonstrated. Microresonator-based frequency combs (microcombs) are demonstrated with microwave repetition rate less than 80 GHz on a chip for the first time. Overall low threshold power (as low as 1 mW) of microcombs across a wide range of resonator FSRs from 2.6 to 220 GHz in surface-loss-limited disk resonators is demonstrated. The rich and complex dynamics of microcomb RF noise are studied. High-coherence, RF phase-locking of microcombs is demonstrated where injection locking of the subcomb offset frequencies are observed by pump-detuning-alignment. Moreover, temporal mode locking, featuring subpicosecond pulses from a parametric 22 GHz microcomb, is observed. We further demonstrated a shot-noise-limited white phase noise of microcomb for the first time. Finally, stabilization of the microcomb repetition rate is realized by phase lock loop control.
For another major nonlinear optical application of disk resonators, highly coherent, simulated Brillouin lasers (SBL) on silicon are also demonstrated, with record low Schawlow-Townes noise less than 0.1 Hz^2/Hz for any chip-based lasers and low technical noise comparable to commercial narrow-linewidth fiber lasers. The SBL devices are efficient, featuring more than 90% quantum efficiency and threshold as low as 60 microwatts. Moreover, novel properties of the SBL are studied, including cascaded operation, threshold tuning, and mode-pulling phenomena. Furthermore, high performance microwave generation using on-chip cascaded Brillouin oscillation is demonstrated. It is also robust enough to enable incorporation as the optical voltage-controlled-oscillator in the first demonstration of a photonic-based, microwave frequency synthesizer. Finally, applications of microresonators as frequency reference cavities and low-phase-noise optomechanical oscillators are presented.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Vahala, Kerry J.}, } @phdthesis{10.7907/QBR4-3980, author = {Chen, Tong}, title = {Ultralow-Loss Silica Resonators and Waveguides on a Silicon Chip}, school = {California Institute of Technology}, year = {2013}, doi = {10.7907/QBR4-3980}, url = {https://resolver.caltech.edu/CaltechTHESIS:02072013-163825736}, abstract = {Compared to fiber optic systems, on-chip optical devices provide reasonable optical performance and mechanical stability in a smaller footprint and at a lower cost. Such devices, including resonators and waveguides, have been applied in diverse areas of scientific research, including quantum information, nonlinear optics, cavity optomechanics, telecommunications, biodetection, rotation sensing, high stability microwave oscillators, and all-optical signal processing. As performance demands on these applications increase, resonators and waveguides with ultralow propagation loss become critical.
In this thesis, we first demonstrate a new resonator with a record Q factor of 875 million for on-chip devices. The fabrication of our device avoids the requirement for a specialized processing step, which in microtoroid resonators has made it difficult to control their size and achieve millimeter- and centimeter-scale diameters. Attaining these sizes is important in applications such as microcombs. The resonators not only set a new benchmark for the Q factor on a chip, but also provide, for the first time, full compatibility of this important device class with conventional semiconductor processing.
Meanwhile, we demonstrate a monolithic waveguide as long as 27 m (39 m optical path length), and featuring broadband loss rate values of (0.08 ± 0.01) dB/m measured over 7 m by optical backscattering. Resonator measurements show a further reduction of loss to 0.037 dB/m, close to that of optical fibers when first considered a viable technology. Scaling this waveguide to integrated spans exceeding 250 m and attenuation rates below 0.01 dB/m is discussed. This chip-based waveguide and resonator improve shock resistance, and afford the possibility of integration for system-on-a chip functionality.
We finally demonstrate a highly sensitive nanoparticle and virus detection method by using a thermal-stabilized reference interferometer in conjunction with an ultrahigh-Q microcavity. Sensitivity is sufficient to resolve shifts caused by binding of individual nanobeads in solution down to a record radius of 12.5 nm, a size approaching that of single protein molecules. A histogram of wavelength shift versus nanoparticle radius shows that particle size can be inferred from shift maxima.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Vahala, Kerry J.}, } @phdthesis{10.7907/ER2J-WT93, author = {Ostby, Eric Paul}, title = {Photonic Whispering-Gallery Resonators in New Environments}, school = {California Institute of Technology}, year = {2009}, doi = {10.7907/ER2J-WT93}, url = {https://resolver.caltech.edu/CaltechETD:etd-05292009-172708}, abstract = {Optical whispering-gallery devices, like the microtoroid or microdisk, confine light at resonant frequencies and in ultra-small volumes for long periods of time. Such ultra-low loss resonators have been applied in diverse areas of scientific research, including low-threshold lasers on-chip, biological sensing, and quantum computing. In this thesis, novel ultra-low loss microstructures are studied for their unique characteristics and utility. The author investigates the interaction between microcavities and various environments in order to quantify the results and lay the foundation for future applications.
The first optical cavity studied is the microtoroid, which possesses ultra-high quality factor (Q) on account of its nearly atomic smooth surface, produced by surface-tension induced laser reflow. Ytterbium-doped silica microtoroids are fabricated by a sol-gel technique. The ytterbium microtoroid laser achieves record-low laser threshold (2 µW) in air, and produces the first laser output for a solid-state laser in water. This laser in water can be developed as an ultra-sensitive biological sensor, with potentially record sensitivity enabled by gain-narrowed linewidth. Also, a novel CO2 laser reflow and microtoroid testing vacuum system is demonstrated. Fabrication and testing of microtoroids is performed in a vacuum chamber to study the effect of atmospheric water and upper limit of Q in microtoroids.
The selective reflow of microtoroids presents difficulties for integration of on-chip optical waveguides. As an alternative, dimension-preserving low-loss optical structures are researched for their unique applications. A gold-coated silica microdisk is fabricated, and demonstrates record and nearly-ideal quality factor (1,376) as a surface-plasmon polariton resonator. The hybrid optical-plasmonic mode structure is studied in simulation and experiment. The plasmonic resonator has ultra-low mode volume and high field confinement, making it suitable for short-range optical communication or sensing. Finally, a novel whispering-gallery optical delay line in a spiral geometry is designed and experimentally demonstrated. The center transition region of the spiral is optimized for low transmission loss by beam propagation simulation. A 1.4 m long spiral waveguide within a 1 cm^2 area is presented. The spiral waveguide structure is being developed as a real-time optical delay line with fiber-like loss, important for optical communication and signal processing.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Vahala, Kerry J.}, } @phdthesis{10.7907/CVT6-8J70, author = {Rokhsari Azar, Hossein}, title = {High-Q Microcavities: Optomechanical Nonlinearities, Measurement Techniques and Applications}, school = {California Institute of Technology}, year = {2006}, doi = {10.7907/CVT6-8J70}, url = {https://resolver.caltech.edu/CaltechETD:etd-11082005-204747}, abstract = {Optical microresonators have historically been perceived as structures that could efficiently confine optical energies. This is due to their exceedingly low losses at optical frequencies. This thesis has, for the first time, explored these structures in a starkly different frequency range. Optical microcavities like any other structure have mechanical eigenmodes or resonant modes of vibration with quality-factors representing the efficiency of energy storage at mechanical frequencies. It is shown here that micron size of these structures results in vibrations at radio frequencies (~1-100 MH), about seven orders of magnitude apart from the optical frequencies (~100 THz). Mechanical quality factors in excess of 5,000 are measured for toroidal microcavities revealing a heretofore unknown potential of these structures in storing energy at frequencies remarkably distant from their optical resonant modes. This thesis describes how radiation-pressure or the force due to impact of photons could result in exceptionally strong couplings between the mechanical and optical resonators collocated within the same device. The discovered optomechanical coupling present in toroid microcavities is shown to reach such a high level that regenerative mechanical oscillations of the cavity structure are initiated with only micro-Watts of optical power. This is the first demonstration of radiation-pressure-induced mechanical oscillations in any type of optomechanical system. Embodied within a microscale, chip-based device, this mechanism can benefit both research into macroscale quantum mechanical phenomena and improve the understanding of the mechanism within the context of Laser interferometer gravitational-wave observatory (LIGO). It also suggests that new technologies are possible that will leverage the phenomenon within photonics. Different physical functionalities are also realized in this thesis by a combination of ultra-high-Q microtoroids and extremely low-loss tapered optical fibers for efficient delivery of optical power to these structures. Using these tools an almost ideal optical band-pass filter is designed with efficiencies solely limited by intrinsic losses of the optical resonator. These intrinsic loss mechanisms are experimentally studied and differentiated by a powerful technique based on thermal nonlinearities of the microcavity material. By taking advantage of slow response times of thermal effects, an innovative pump and probe technique is also developed to unveil and measure the Kerr nonlinearity in microcavities, for the first time, at room temperature.}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Vahala, Kerry J.}, } @phdthesis{10.7907/KGE9-J792, author = {Min, Bumki}, title = {Ultrahigh–Q Microtoroid On-Chip Resonators for Low Threshold Microlasers}, school = {California Institute of Technology}, year = {2006}, doi = {10.7907/KGE9-J792}, url = {https://resolver.caltech.edu/CaltechETD:etd-07062006-135109}, abstract = {Recently demonstrated silica toroidal microcavities, as on-chip resonant cavities, become one of the most promising laser resonators due to their exceptional ability to confine optical energy temporarily and spatially (high Q-factor and small mode volume) while being integrated on a silicon substrate.
In the first part of this thesis, semianalytic theory is presented for an in-depth understanding of the high-Q toroidal microcavity coupled to a tapered fiber waveguide. Basic properties of toroidal microcavities such as cavity mode field, resonance wavelength, cavity mode volume, radiative Q-factor, and phase-matching condition are described within the limit of an iterative perturbation expansion method. With this theoretical background, various laser systems with different gain media, utilizing the high-Q toroidal microcavity as a laser resonator, are demonstrated in the latter parts.
As a first example, II-VI semiconductor nanocrystal, CdSe/ZnS (core/shell), quantum dots are coated on the surface of ultrahigh-Q toroidal microcavities. By pulsed excitation of quantum dots on the surface, either through tapered fiber waveguides or free-space, lasing is observed at both room and liquid nitrogen temperature. Use of a tapered fiber coupling substantially lowered the threshold energy when compared to the case of free-space excitation. Further threshold reduction down to 9.9 fJ was made possible by quantum dot density control.
Lasing from an erbium-implanted high-Q silica toroidal microcavity is demonstrated and analyzed in the next chapter. A minimum threshold power as low as 4.5 uW and a maximum output lasing power as high as 39.4 uW are obtained. Control of lasing wavelength is demonstrated by changing the cavity loading. Analytic formulas predicting threshold power, differential slope efficiency are derived and their dependence on cavity loading, erbium ion concentration and Q-factor is found and compared with the experimental results.
The nonlinear oscillation in an ultrahigh-Q silica toroidal microcavity is investigated in the last chapter. A controllable and reversible transition between parametric and Raman oscillation is experimentally demonstrated and theoretically analyzed. By direct change of cavity loading and indirect adjustment of frequency detuning, parametric and/or Raman oscillation can be accessed selectively without modification of cavity geometry in a toroidal microcavity with large enough aspect ratio. Based on an effective cavity gain theory, this transition is analyzed in terms of cavity loading and frequency detuning leading to a better understanding of the combined effects of parametric and Raman processes in silica microcavities.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Vahala, Kerry J.}, } @phdthesis{10.7907/HHQ8-VC25, author = {Yang, Lan}, title = {Fabrication and Characterization of Microlasers by the Sol-Gel Method}, school = {California Institute of Technology}, year = {2005}, doi = {10.7907/HHQ8-VC25}, url = {https://resolver.caltech.edu/CaltechETD:etd-06032005-115306}, abstract = {The present study explores the application of new materials systems for low threshold microlasers, and characterization of the microcavities. The sol-gel method is used for gain functionalization of high-Q microcavities. A detailed procedure for preparation of the sol-gel films by the spin-on or dip-coating method is presented. The effect of different process conditions on the properties and microstructure of the thin films is investigated through Fourier Transform Infrared (FTIR) Spectroscopy, Scanning Electron Microscopy (SEM), and etching rate test.
Surface gain functionalization of microsphere cavities is fabricated by coating the microsphere with a thin layer of Er³⁺-doped sol-gel films. The optical gain is due to the population inversion of rare earth ions in the sol-gel films. A fiber taper is used to both couple the pump power into and extract the laser power out of the microsphere laser. The laser dynamics change between continuous-wave and pulsating operation by varying the doping concentration and the thickness of the sol-gel films outside the microsphere.
Surface functionalization is also achieved on the microtoroid on a single silicon chip, which can be fabricated in parallel using wafer-scale processing and has characteristics that are more easily controlled than microsphere. The microtoroid can be selectively coated only at the periphery by making use of the variation of etching rate (in buffered HF) of sol-gel films with different degrees of densification. The laser performance of the gain functionalized microtoroids is investigated. Highly confined whispering gallery modes make possible single-mode microlasers. This work also shows that the high Q microtoroid laser has a linewidth much lower than 300 kHz.
The thesis explores fabrication of high Q microcavities directly from the sol-gel silica films deposited on a single silicon wafer. Quality factor as high as 2.5 x 10⁷ at 1561 nm is obtained in toroidal microcavities formed of silica sol-gel, which allows Raman lasing at absorbed pump power below 1 mW. Additionally, Er³⁺-doped microlasers are fabricated from Er³⁺-doped sol-gel layers with control of the laser dynamics possible by varying the erbium concentration of the starting sol-gel material. Continuous lasing with a record threshold of 660 nW for erbium-doped microlaser on a silicon wafer is also obtained.
Analytic formulas are derived to predict the laser performance, such as the laser output power, the threshold power, and the differential quantum efficiency, under different loading condition, i.e. the air gap between the fiber-taper coupler and the cavities. The effect of Er3+ concentration on the minimum threshold is also investigated. In addition, we present a theoretical model in which we include paired ions as the saturable absorber. It shows that self-pulsing operation can be expected with paired-ions-induced quenching in the system. The pulsation frequency increases linearly with the square root of the pumping level, which is consistent with the experimental observation.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Vahala, Kerry J.}, } @phdthesis{10.7907/EZHA-VY23, author = {Armani, Deniz Karapetian}, title = {Ultra-High-Q Planar Microcavities and Applications}, school = {California Institute of Technology}, year = {2005}, doi = {10.7907/EZHA-VY23}, url = {https://resolver.caltech.edu/CaltechETD:etd-05272005-113247}, abstract = {Ultra-high-Q (UHQ) silica microspheres have found research applications in diverse fields ranging from telecommunications to nonlinear optics to biological and chemical sensing. However, despite having quality factors greater than 108, the silica microsphere has not moved to an industrial setting because of several major drawbacks. The most hindering is the manual fabrication technique used that makes tight process control difficult and integration with other optical or electrical components impossible. Despite the strong desire to fabricate an integrated UHQ microresonator on a planar substrate, the highest quality factor achieved for any micro-fabricated planar micro-cavity (at the time of my first publication) was over 4 orders of magnitude lower than for silica microspheres. In this thesis, a process for creating planar micro-cavities with Q factors in excess of 400 million on silicon wafers is demonstrated. The advantage of these planar ultra-high-Q (UHQ) microtoroid resonators is that they successfully overcome the previously mentioned drawbacks of silica microsphere resonators while maintaining nearly identical, if not better, performance characteristics. Additionally, due to the planar nature of these new devices, functionality has been integrated in-situ while maintaining UHQ for the first time, such as active resonant frequency tuning, coupling control, and low-threshold lasing.}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Vahala, Kerry J.}, } @phdthesis{10.7907/MKYH-WV47, author = {Spillane, Sean Michael}, title = {Fiber-Coupled Ultra-High-Q Microresonators for Nonlinear and Quantum Optics}, school = {California Institute of Technology}, year = {2004}, doi = {10.7907/MKYH-WV47}, url = {https://resolver.caltech.edu/CaltechETD:etd-06012004-104250}, abstract = {The ability to confine optical energy in small volumes for long periods of time is desirable for a number of applications, ranging from photonics and nonlinear optics, to fundamental studies in quantum electrodynamics. Whispering-gallery-mode microresonators are a promising cavity to study, due to the ability to obtain quality factors exceeding 100 million in micron-scale volumes. This thesis investigates the suitability of ultra-high-quality factor silica microresonators (both silica microspheres and silica toroidal microresonators) for nonlinear and quantum optics. Crucial to the actual use of these structures is the ability to efficiently excite and extract optical energy. The first part of this thesis investigates the ability to achieve near lossless coupling between a fiber-taper waveguide and a silica microresonator. It is shown that a coupling ideality (which is the fraction of energy coupled into the desired fiber mode) in excess of 99.97% is possible, meaning that optical energy can be coupled both to and from the optical resonator with near perfect efficiency.
Using tapered fibers, low threshold stimulated Raman scattering is observed in both silica microspheres and silica microtoroids at record low incident pump powers below 100 microwatts, much lower than previous devices. High conversion efficiencies (>35%) are also realized. Furthermore, the conditions for optimized performance of both stimulated Raman scattering and parametric oscillation in a microcavity are described.
Lastly, the suitability of toroidal microcavities for strong coupling cavity quantum electrodynamics is investigated. Numerical modeling of the optical modes demonstrates a significant reduction of modal volume with respect to spherical cavities, while retaining high quality factors. The extra degree of freedom of toroid microcavities can be used to achieve improved strong-coupling characteristics, and numerical results for atom-cavity coupling strength, critical atom number and critical photon numbers for cesium are calculated and shown to exceed values currently possible using Fabry-Perot cavities. Modeling predicts atom-cavity coupling rates exceeding 700 MHz and critical atom numbers approaching 10⁻⁷.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Vahala, Kerry J.}, } @phdthesis{10.7907/T5B6-9R14, author = {Kippenberg, Tobias Jan August}, title = {Nonlinear Optics in Ultra-High-Q Whispering-Gallery Optical Microcavities}, school = {California Institute of Technology}, year = {2004}, doi = {10.7907/T5B6-9R14}, url = {https://resolver.caltech.edu/CaltechETD:etd-06072004-085555}, abstract = {Optical microcavities confine light at resonant frequencies for extended periods of time and fundamentally alter the interaction of light with matter. They are the basis of numerous applied and fundamental studies, such as cavity QED, photonics and sensing. Of all resonant geometries, surface tension-induced microcavities, such as silica micro-spheres, exhibit the highest Q-factor to date of nearly 9 billion. Despite these high Q-factor and the intense interest in these structures, the nonlinear optical properties of silica micro-spheres have remained nearly entirely unexplored. In this thesis the nonlinear optical phenomena which can occur in ultra-high-Q microcavities are investigated. To efficiently excite the whispering-gallery modes, tapered optical fibers are used and the coupling to ultra-high-Q modes studied. It is found, that microcavities with ultra-high enter a regime where scattering of light into the degenerate pair of clockwise and counter-clockwise mode is the dominant scattering process. In this regime the coupling properties are significantly altered, but the cavities still retain their ability to achieve significant cavity build-up fields. This allowed exceeding the threshold for all common nonlinearities encountered in silica. In particular, stimulated Raman scattering is observed in taper fiber coupled silica micro-spheres at threshold levels typically in the micro-Watt range, which usually is considered the regime of linear optics. Cascaded Raman lasing is also observed in these structures. The tapered optical fiber in these experiments functions to both pump WGMs as well as to extract the nonlinear Raman fields. In addition, the tapered-fiber coupling junction is highly ideal, making it possible to strongly over-couple ultra-high-Q cavities with negligible junction loss. This feature allows for the observation of very high internal differential photon conversion efficiencies approaching unity. Whereas micro-spheres are both compact and efficient nonlinear oscillators, their fabrication properties lack the control and parallelism typical of micro-fabrication techniques. A synergistic approach of micro-fabrication and a laser assisted reflow process, allows to create toroidally silica microcavities on a chip. In this thesis it is demonstrated, that these cavities can exhibit ultra-high-Q whispering-gallery modes, allowing to achieve ultra-high-Q modes on a chip. This results is a nearly four-order of magnitude improvement with respect to other wafer-scale microcavities. In addition their azimuthal mode-spectrum is strongly reduced. Nonlinear oscillation in these cavities has also been studied, and stimulated Raman scattering observed, allowing to achieve the first Raman laser on a chip. The devices show improved performance compared to micro-spheres due to a strongly reduced azimuthal mode spectrum, which allowed to observe single mode emission. The enhanced geometric control of these cavities is also studied and found to profoundly alter the nonlinear optical processes the toroid microcavities. Reduction of toroidal cross section is observed to cause a transition from stimulated Raman to parametric oscillation regime. This allowed to observe Kerr nonlinearity induced parametric oscillation in a microcavity for the first time. The parametrically generated “twin beams” exhibit high conversion efficiency and show near unity signal-to-idler ratio.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Vahala, Kerry J.}, } @phdthesis{10.7907/CMAB-WF06, author = {Bhardwaj, Ashish Ishwar Singh}, title = {All-Optical Logic Circuits Based on the Polarization Properties of Non-Degenerate Four-Wave Mixing}, school = {California Institute of Technology}, year = {2001}, doi = {10.7907/CMAB-WF06}, url = {https://resolver.caltech.edu/CaltechETD:etd-07132001-112041}, abstract = {
This thesis investigates a new class of all-optical logic circuits that are based on the polarization properties of non-degenerate Four-Wave Mixing. Such circuits would be used in conjunction with a data modulation format where the information is coded on the states of polarization of the electric field. Schemes to perform multiple triple-product logic functions are discussed and it is shown that higher-level Boolean operations involving several bits can be implemented without resorting to the standard 2-input gates that are based on some form of switching. Instead, an entire hierarchy of more complex Boolean functions can be derived based on the selection rules of multi-photon scattering processes that can form a new class of primitive building blocks for digital circuits.
Possible applications of these circuits could involve some front-end signal processing to be performed all-optically in shared computer back-planes. As a simple illustration of this idea, a circuit performing error correction on a (3,1) Hamming Code is demonstrated. Error-free performance (Bit Error Rate of < 10⁻⁹) at 2.5 Gbit/s is achieved after single-error correction on the Hamming word with 50 percent errors. The bit-rate is only limited by the bandwidth of available resources. Since Four-Wave Mixing is an ultrafast nonlinearity, these circuits offer the potential of computing at several terabits per second. Furthermore, it is shown that several Boolean functions can be performed in parallel in the same set of devices using different multi-photon scattering processes. The main objective of this thesis is to motivate a new paradigm of thought in digital circuit design. Challenges pertaining to the feasibility of these ideas are discussed.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Vahala, Kerry J.}, } @phdthesis{10.7907/JB4C-VR78, author = {Cai, Ming}, title = {Optical Fiber Taper Coupled Glass Microsphere Resonators}, school = {California Institute of Technology}, year = {2001}, doi = {10.7907/JB4C-VR78}, url = {https://resolver.caltech.edu/CaltechTHESIS:10212010-100027618}, abstract = {This thesis studies optical fiber taper coupled dielectric microsphere resonators and their applications. Fundamental properties including ideal coupling and critical coupling in an optical fiber taper to fused silica glass microsphere coupling system is investigated both theoretically and experimentally. A symmetrical dual-taper coupling configuration is proposed to obtain highly efficient power transfer from the taper coupler to the microsphere resonator. Applications as channel add/drop filters and microsphere lasers are also demonstrated.
The physical essence of the fiber taper to silica microsphere is analyzed using a two-dimensional model. The relationship between the coupling strength and the cavity loss is unveiled. Adiabatic tapers and high-quality microspheres are fabricated and used to demonstrate actual coupling systems. Perfect agreement between the experimental results and the theoretical prediction is presented.
Power transfer from the taper to a microsphere resonator has been significantly improved by employing a dual-taper symmetrically coupling configuration. Up to -28 dB extinction at the central resonant wavelength has been measured.
We then propose a device application of the taper-sphere-taper structure as a channel add/drop filter in the wavelength division multiplexing systems. For a filter with a bandwidth of 3.8 GHz and a dropping channel isolation of 26 dB, the bit-error-rate measurement shows no power penalty at 2.5 Gbit/s.
A 1.5 µm wavelength single-frequency fiber laser is also demonstrated using a single tapered fiber coupling to a highly doped erbium:ytterbium phosphate glass microsphere. The fiber taper serves the dual purpose of transporting optical pump power into the sphere and extracting the resulting laser emission. As low as 60 µW pump threshold and fiber-coupled output power as high as 3 µW with single mode operation are obtained. Imaging of photoluminescence from the sphere at visible wavelengths reveals the pump power is resonantly coupled into semiclassical orbits due to the strong absorption damping in the phosphate glass. A bi-sphere laser system consisting of two microspheres attached to a single fiber taper is also demonstrated.
Finally, a novel hybrid fiber taper, made from a combination of a 980 nm single mode fiber and a 1550 nm single mode fiber, is proposed and demonstrated as the microsphere laser coupler. Both the pump wave and laser emission are found to be more efficiently coupled to and from, respectively, the sphere modes. As high as 112 µW single-frequency laser output power is measured with a differential quantum efficiency of 12%.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Vahala, Kerry J.}, } @phdthesis{10.7907/r9qa-0n27, author = {Paiella, Roberto}, title = {Physics and application of four-wave mixing in semiconductor optical amplifiers}, school = {California Institute of Technology}, year = {1999}, doi = {10.7907/r9qa-0n27}, url = {https://resolver.caltech.edu/CaltechETD:etd-02242008-091907}, abstract = {This thesis investigates the physical mechanisms responsible for four-wave mixing (FWM) in semiconductor optical amplifiers (SOAs), and their application to quantum-well spectroscopy and all-optical signal processing. A microscopic theory of polarization-resolved FWM is developed, and the corresponding polarization selection rules are derived. It is then shown how these results can be used to study basic carrier dynamics in semiconductor active layers. Finally, a wavelength conversion device and a new class of all-optical logic gates, based on FWM in SOAs, are presented and characterized.
The first part of the thesis is devoted to several experimental and theoretical investigations of carrier transport dynamics in multiquantum-well SOAs, and of their relation to the FWM nonlinearity of these devices. A polarization-resolved FWM configuration is used to study interwell carrier transport in a SOA consisting of alternating pairs of tensile and compressively strained quantum wells. A similar structure with interwell coupling provided by resonant tunneling is then investigated theoretically; it is shown how FWM can be used to excite coherent electric-dipole oscillations in this device, leading to efficient generation of TeraHertz radiation. Finally, a novel wavelength-resolved FWM technique is demonstrated to directly study the capture of carriers in quantum wells.
The second part of the thesis focuses on the application of FWM to all-optical signal processing for WDM communication systems. A wavelength conversion device based on FWM in a long (1.5 mm) SOA is developed, and used to demonstrate error-free conversion of 10 Gbit/sec data over a record 30 nm wavelength span. Other configurations for wavelength conversion by FWM are then proposed and demonstrated, including: a dual-pump configuration for polarization insensitive operation; a self-pumped FWM converter, based on a fiber-Bragg-grating coupled diode laser; and a device based on injection-locked FWM in this same laser, characterized by a large resonance peak in its conversion efficiency. Finally, the last chapter is devoted to a novel class of all-optical logic gates, based on FWM, designed to operate on bytes of information encoded in wavelength (“byte-wide WDM”).
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Vahala, Kerry J.}, } @phdthesis{10.7907/n9df-zc75, author = {Camata, Renato Penha}, title = {Aerosol synthesis and characterization of silicon nanocrystals}, school = {California Institute of Technology}, year = {1998}, doi = {10.7907/n9df-zc75}, url = {https://resolver.caltech.edu/CaltechETD:etd-01182008-131457}, abstract = {NOTE: Text or symbols not renderable in plain ASCII are indicated by […]. Abstract is included in .pdf document. Synthesis and processing of optically active silicon nanocrystals are explored from an aerosol science perspective. Spark ablation, laser ablation and thermal evaporation in inert atmospheres are employed alternatively as vapor phase sources of nanocrystals. Nanocrystals generated employing these techniques comprise a highly polydisperse and morphologically diverse aerosol. After collection on a solid substrate, samples of these nanocrystals exhibit wide-band visible photoluminescence. A system for size classification of the initial polydisperse nanocrystal aerosol is demonstrated employing differential mobility analysis. Working at low nanocrystal concentrations (around […]) size control within 15% to 20% is achieved in the 2 to 10 nm size regime with a radial differential mobility analyzer at the expense, however, of low throughputs which make optical studies challenging. Seeking higher throughputs, the physics of aerosol size classification by this technique is investigated in detail by self-consistent numerical simulations of the particle transport inside the differential mobility analyzer. Our results lead to the identification of critical design characteristics required to maximize the analyzer performance from the viewpoint of semiconductor nanocrystal synthesis. With the guidance of these theoretical predictions, an optimized differential mobility analyzer design is suggested. This instrument has its parameters chosen to perform high resolution, high throughput size classification of nanocrystals in the 0.5 to 10 nm range. Optical characterization studies on polydisperse and size-classified silicon nanocrystal samples are performed. Results suggest that at least two mechanisms for light emission are at work in aerosol synthesized silicon nanocrystals. X-ray photoelectron measurements on size-classified silicon nanocrystals reveal that an oxide layer with thickness in excess of several nanometers forms on the silicon nanocrystals within a few minutes of air exposure. In order to preserve and control the surface chemistry of the nanocrystals, a system for anaerobic transfer of the size- classified silicon nanocrystals is designed and built. The system couples the nanocrystal synthesis experiment with the ultra high vacuum chamber of a surface analysis system via a load lock high vacuum chamber. Optical characterization capabilities are also installed. Preliminary results on nanocrystal synthesis and characterization using this in situ setup are presented and discussed.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Atwater, Harry Albert and Vahala, Kerry J. and Flagan, Richard C.}, } @phdthesis{10.7907/fcfk-7n73, author = {Hunziker, Guido H.}, title = {Spectroscopy and wavelength conversion by four-wave mixing in semiconductor optical amplifers}, school = {California Institute of Technology}, year = {1998}, doi = {10.7907/fcfk-7n73}, url = {https://resolver.caltech.edu/CaltechETD:etd-01232008-143421}, abstract = {The first part of this thesis is dedicated to the study of the physics of the four-wave mixing (FWM) optical non-linearity in semiconductor optical amplifiers (SOAs). We focus our attention on the polarization properties of FWM and spectroscopic measurements of ultrafast carrier dynamics in these amplifiers. The second part presents investigations of FWM applications in the context of high-speed optical communication systems.
The detuning and polarization dependence of the third-order non-linear susceptibility is presented with a model based on the density matrix formalism. Experimental verifications of the model for the polarization properties of the four-wave mixing are presented using an alternating compressive and tensile strained multiquantum-well semiconductor optical amplifier. The polarization selection rules are then used for spectroscopic measurements of the carrier dynamics in quantum well SOAs. In particular, we present new techniques to measure the stimulated carrier lifetime, the inter quantum-well transport lifetime as well as the intrinsic escape and capture time constants for quantum wells. The capture lifetime is further studied in a separate experiment involving wavelength resolved spectroscopy.
We then demonstrate that strongly saturated and long SOAs (1.5 mm) are very effective wide span wavelength converters. We present bit error rate measurements for 30 nm wavelength down-conversion and 15 nm wavelength up-conversion at 10 Gb/s. We also present an application of the polarization selection rules to generate a polarization independent conversion at 2.5 Gb/s. Then, we introduce two different configurations where we use a lasing optical amplifier with a fiber Bragg grating to enhance the conversion efficiency and simplify the converter design. In the first case, we used the laser mode as pump wave and in the second case the lasing mode is injection locked to the FWM signal generated within the cavity. Finally, we present a new paradigm to perform wavelength encoded logic operations on a byte-wide WDM bus. Again, we use the polarization properties of the FWM to perform the logic operations.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Vahala, Kerry J.}, } @phdthesis{10.7907/4yye-5f58, author = {Lee, Robert Bumju}, title = {All-Optical Wavelength Conversion by Four-Wave Mixing in a Semiconductor Optical Amplifier}, school = {California Institute of Technology}, year = {1997}, doi = {10.7907/4yye-5f58}, url = {https://resolver.caltech.edu/CaltechETD:etd-01102008-142830}, abstract = {Wavelength division multiplexed optical communication systems will soon become an integral part of commercial optical networks. A crucial new function required in WDM networks is wavelength conversion, the spectral translation of information-laden optical carriers, which enhances wavelength routing options and greatly improves network reconfigurability. One of several techniques for implementing this function is four-wave mixing utilizing ultra-fast intraband nonlinearities in semicondutor optical amplifiers.
The effects of input power, noise prefiltering and semiconductor optical amplifier length on the conversion efficiency and optical signal-to-noise ratio were examined. Systems experiments have been conducted in which several important performance characteristics of the wavelength converter were studied. A bit-error-rate performance of < 10-9 at 10 Gb/s was achieved for a record shift of 18 nm down in wavelength and 10 nm up in wavelength. Two cascaded conversions spanning a 40 km fiber link at 10 Gb/s are also demonstrated for conversions of up to 9 nm down and up in wavelength. The dynamic range of input signal power and its impact on the BER performance were studied at 2.5 Gb/s for both a single-channel conversion and a simultaneous 2-channel conversion. The crosstalk penalty induced by parasitic cross-gain modulation in 2-channel conversion is quantified. The spectral inversion which results from the conversion process is studied by time-resolved spectral analysis, and its application as a technique for dispersion compensation is demonstrated.
Finally, the application of selective organometallic vapor-phase epitaxy for the formation of highly-uniform and densely-packed arrays of GaAs quantum dots is demonstrated. GaAs dots of 15-20 nm in base diameter and 8-10 nm in height terminated by slow-growth crystallographic planes were grown within dielectric-mask openings and characterized by atomic force microscopy.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Vahala, Kerry J.}, } @phdthesis{10.7907/rqrk-zs39, author = {Geraghty, David Francis}, title = {Investigation of wavelength conversion by four-wave mixing in semiconductor optical amplifiers}, school = {California Institute of Technology}, year = {1997}, doi = {10.7907/rqrk-zs39}, url = {https://resolver.caltech.edu/CaltechETD:etd-01092008-081143}, abstract = {
NOTE: Text or symbols not renderable in plain ASCII are indicated by […]. Abstract is included in .pdf document.
Four-wave mixing (FWM) in semiconductor optical amplifiers (SOAs) is investigated for wavelength conversion in high-speed, all-optical networks. The design of the wavelength converter is optimized and the system performance limitations imposed by the fundamental physical principles involved in the SOA FWM process are characterized.
Single channel conversion performance is evaluated. The FWM efficiency ultimately determines many systems-level characteristics of the wavelength converter. The spectral range of our wavelength converter is characterized. Wide wavelength conversions of up to 18 nm and complete coverage of a 10 nm spectral range are demonstrated, while maintaining a BER performance of better than […] at 10 Gb/s. The converter also demonstrates a large dynamic input range of over 10 dB at 2.5 Gb/s. And the first characterization of cascaded FWM SOA wavelength converters, cascading conversion of up to 10 nm at 10 Gb/s, is performed.
With a simple modification of the converter design to a dual-pump configuration, the wavelength converter is able to provide nearly polarization insensitive performance. The converted signal’s magnitude varies by less than 1.5 dB and its sensitivity varies by less than 2 dB for 2.5 Gb/s signals over the entire range of input polarizations.
Time resolved spectral analysis is performed to evaluate the spectral properties of the wavelength converter. A pattern-dependent additional chirp is measured on the signal, primarily resulting from fluctuations in the gain saturation of the SOA. This degradation to the optical phase conjugation, intrinsic to the SOA FWM process, is minimal enough to allow dispersion compensation by mid-span spectral inversion. Error-free detection of a directly modulated 10 Gb/s signal is achieved over 120 km.
Additional demonstrations are also presented. Multi-channel wavelength conversion and dynamic routing are successfully performed. Finally, some work on a microcavity erbium-doped fiber laser, initially designed and developed for use as a tunable source for wavelength-division multiplexed networks, is presented.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Vahala, Kerry J.}, } @phdthesis{10.7907/vrda-c377, author = {Tsai, Charles Su-Chang}, title = {Optoelectronic structure fabrication by organometallic vapor-phase epitaxy and selective epitaxy}, school = {California Institute of Technology}, year = {1996}, doi = {10.7907/vrda-c377}, url = {https://resolver.caltech.edu/CaltechETD:etd-12222007-114128}, abstract = {NOTE: Text or symbols not renderable in plain ASCII are indicated by […]. Abstract is included in .pdf document.
The internal configuration and external supports of OMVPE reactors are examined. The quality of epitaxial layers deposited by an OMVPE reactor is strongly influenced by its internal configuration. The quality of the external supports determines the safety, the environmental impact, and the operating efficiency of the OMVPE reactor.
Optoelectronic structures are fabricated by selective epitaxy. The morphology and growth behavior of GaAs, AlGaAs, and InGaAs using selective epitaxy are presented. Highly selective growth can be achieved through the use of organometallic compounds which contain halogens. The selective growth of nanometer-scale GaAs wire and dot structures is demonstrated. Spectrally-resolved cathodoluminescence images as well as pectra from single dots and wires, passivated by an additional AIGaAs layer, are presented. A blue shifting of the GaAs luminescence peak is observed as the size scale of the wires and dots decreases. Formation of highly-uniform and densely-packed arrays of GaAs dots by selective epitaxy is described. The smallest GaAs dots formed are 15-20 nm in base diameter and 8-10 nm in height with slow-growth crystallographic planes limiting growths of individual dots. Completely selective GaAs growth within dielectric-mask openings at these small size-scales is also demonstrated. The technique of facet-modulation selective epitaxy and its application to quantum-well wire doublet fabrication are described. The smallest wire fabricated has a crescent cross-section less than 140 […] thick and less than 1400 […] wide.
The development of OMVPE epitaxial layers for a visible-wavelength vertical-cavity surface-emitting laser (VCSEL) is presented. The defect density of the mirror layers was reduced to a negligible level by optimizing gas switching. Electroluminescence spectrum of an InGaP heterostructure p-n diode is presented. The defect density of the active region was also reduced to a negligible level by optimizing the gas-switching sequences.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Vahala, Kerry J.}, } @phdthesis{10.7907/vmke-sz45, author = {Zhou, Jianhui}, title = {Four-wave mixing in semiconductor optical amplifiers for terahertz spectroscopy and wavelength conversion}, school = {California Institute of Technology}, year = {1995}, doi = {10.7907/vmke-sz45}, url = {https://resolver.caltech.edu/CaltechETD:etd-09082005-105846}, abstract = {Four-wave mixing in semiconductor gain media from GHz to THz detuning rates was used as a frequency-domain technique for analysis of carrier relaxation mechanisms having relaxation times extending from nanosecond to femtosecond time scales. Measurements of four-wave mixing in various semiconductor traveling-wave amplifiers were performed for detuning frequencies as large as 1.7 THz. Ultrafast intraband mechanisms having relaxation time constants of 650 fs, in agreement with dynamic carrier heating, and of less than 100 fs, in agreement with intraband carrier-carrier scattering, were determined in the measurements. A novel cross-polarized four-wave mixing technique was also developed to study the inter quantum well carrier transport process in quantum well amplifiers. A semiconductor optical amplifier having a structure of alternating tensile and compressively strained quantum wells was used. Polarization selection rule of the strained quantum wells enables selective excitation and probing of adjacent quantum wells according to polarization, thereby enabling study of inter-well carrier transport. A one-dimensional diffusion model was developed to illustrate the different transport efficiencies for carrier number and temperature modulations, thereby qualitatively explaining the experimental data. The inter-well carrier number transport rate in the device measured was determined to be greater than 100 GHz. Four-wave mixing in semiconductor optical amplifiers was also studied as a wavelength conversion technique. Conversion efficiency over spans up to 65 nm was measured, and wavelength conversion with gain was also demonstrated. It was found theoretically and confirmed experimentally that the conversion efficiency varies with the cube of the saturated amplifier gain. Noise characteristics of four-wave mixing wavelength converters and their dependence on various device and operational parameters were also studied. Noise reduction by introducing a filter between the preamplifier and the mixer was demonstrated and significant noise reduction was achieved. Finally, wavelength conversion of modulated signals at data rates of 2.5 Gb/s and 10 Gb/s was demonstrated.}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Vahala, Kerry J.}, } @phdthesis{10.7907/ZK03-KN91, author = {Park, Namkyoo}, title = {Application of fiber amplifiers to fiber lasers and terahertz spectroscopy}, school = {California Institute of Technology}, year = {1994}, doi = {10.7907/ZK03-KN91}, url = {https://resolver.caltech.edu/CaltechETD:etd-12212006-102347}, abstract = {Starting with a review of the Erbium doped fiber amplifier, this thesis will describe the construction, intensity noise, linewidth, stabilization techniques, and spectroscopic applications of the Erbium doped fiber ring laser developed as a part of the thesis research activity. This laser, which uses the Erbium doped fiber amplifier as its gain module within fiber based ring resonator, exhibits excellent sidemode suppression (>70dB) and intensity noise properties (shot noise limited beyond GHz regime) with ultranarrow linewidth (<4kHz).
To measure and improve these performance, several new techniques were developed. A new interferometer based on a loss-compensated recirculating delayed self heterodyne technique, for the measurement of ultranarrow linewidth. A novel intracavity filtering technique to make the laser operate at the shot noise floor of intensity noise. Extension of Pound-Drever locking technique into the laser cavity, to enable the laser be stabilized and locked to an external reference at the same time.
The laser was also applied as a spectroscopic tool to study the four wave mixing process in semiconductor optical amplifiers. Because of the ultranarrow linewidth and intensity noise characteristics of fiber laser, it was possible to resolve THz intraband dynamics in a quantum well amplifier.
This thesis will also cover mode locked operation of the fiber laser and related issues briefly in the appendix.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Vahala, Kerry J.}, } @phdthesis{10.7907/65s0-wf19, author = {Dawson, Jay W.}, title = {Single and multiple frequency fiber lasers}, school = {California Institute of Technology}, year = {1993}, doi = {10.7907/65s0-wf19}, url = {https://resolver.caltech.edu/CaltechETD:etd-08272007-084804}, abstract = {NOTE: Text or symbols not renderable in plain ASCII are indicated by […]. Abstract is included in .pdf document.
Single frequency, low intensity noise, widely tunable lasers operating in the 1.5 […]m region have potential applications in future wavelength division multiplexed optical communications systems, fiber sensor arrays and high resolution spectroscopic measurements. A single frequency fiber laser having these characteristics will be described in detail. The laser cavity contains an erbium doped fiber gain module, fiber isolators to ensure unidirectional travelling wave operation and two fiber Fabry-Perot filters acting in tandem, which provide broadband tunability (1530 nm -1560 nm) combined with stable single frequency operation. Shot noise limited operation of this laser has been observed at frequencies greater than 300 MHz. At lower frequencies (1-300 MHz) the intensity noise has been characterized in terms of sidemode suppression (> 60 dB of minimum sidemode suppression has been realized). Lower still (10 kHz - 1 MHz) the intensity noise is dominated by the laser’s relaxation resonance (30 kHz @ 1 mW output, -105 dBc/Hz). The linewidth of this laser has been measured to be less than 4 kHz using a loss compensated recirculating delayed self-heterodyne interferometer (RDSHI). The RDSHI is an improvement over the standard delayed self-heterodyne interferometer in that the effective delay line can be increased by a factor of 30 over the standard method, increasing the resolution by a corresponding amount. The RDSHI also allows measurement of the short term frequency jitter of a laser. In order to reduce laser frequency jitter, the Pound-Drever technique was employed to lock the laser frequency to an external fiber Fabry-Perot. The same technique also permitted the internal mode selection filter to track the laser frequency, completely eliminating residual mode hopping due to thermal length changes of the laser cavity. Finally, fiber laser configurations that allow multiple frequencies to be simultaneously produced in one laser cavity will be described.}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Vahala, Kerry J.}, } @phdthesis{10.7907/cmqv-p871, author = {Sercel, Peter C.}, title = {Semiconductor structures in the quantum size regime}, school = {California Institute of Technology}, year = {1992}, doi = {10.7907/cmqv-p871}, url = {https://resolver.caltech.edu/CaltechETD:etd-08202007-132916}, abstract = {NOTE: Text or symbols not renderable in plain ASCII are indicated by […]. Abstract is included in .pdf document. The physics of quantum wires and quantum dots is investigated theoretically. We develop an analytical formalism for determining the energy eigenstates and bandstructure of spherical quantum dots and cylindrical quantum wires. The technique is based upon a reformulation of second order […] theory in a basis of eigenstates of total angular momentum. We are led by analysis of quantum wires and dots based upon the InAs-GaSb material system to propose a novel class of self-doping nanostructures for carrier transport experiments and possible future application. The polarization dependence of linear optical absorption and gain spectra in cylindrical quantum wires is calculated. Applicability of the results derived for cylindrical quantum wires to the case of wires with lower symmetry is addressed using symmetry group theory. Fabrication of quantum wires and dots is attempted by several techniques. A method for fabricating nanometer-scale GaAs wire structures from quantum well material by selective impurity induced disordering is demonstrated. The technique produces lateral bandgap modifications on a 100 nm scale, as verified by cathodoluminescence imaging and spectroscopy. We demonstrate vapor phase synthesis of nanometer-scale III-V semiconductor clusters in the 5 to 20 nm diameter regime. Clusters form by homogeneous nucleation from a non-equilibrium vapor created by the explosive vaporization of a bulk semiconductor filament in an inert atmosphere. The clusters produced have zincblende crystal structure and are faceted. The optical absorption spectra of the clusters are suggestive of quantum confinement effects. A second method of cluster formation utilizes homogeneous nucleation from volatile metal-organic and hydride precursors to produce nanometer-scale, zincblende GaAs clusters.}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Vahala, Kerry J.}, } @phdthesis{10.7907/98ma-fx75, author = {Newkirk, Michael Avery}, title = {Investigations of semiconductor laser modulation dynamics and field fluctuations}, school = {California Institute of Technology}, year = {1991}, doi = {10.7907/98ma-fx75}, url = {https://resolver.caltech.edu/CaltechETD:etd-07102007-112900}, abstract = {Active-layer photomixing is an optical modulation technique to probe the fundamental modulation response of a semiconductor laser. By heterodyning two laser sources with a tunable frequency difference in the device’s active region, the gain, and hence the optical output, is modulated at the beat frequency of the sources. Using an equivalent circuit model for the laser diode, the optical modulation is shown to be decoupled from the electrical parasitics of the laser structure. The fundamental modulation response of the laser can thereby be studied independently of the parasitic response, which would otherwise mask the fundamental response. The photomixing technique is used on GaAs/GaAlAs lasers at room temperature, liquid nitrogen and liquid helium temperature, and it is verified that the modulation response appears ideal to millimeter-wave frequencies.
Application of the active-layer photomixing technique led to the discovery and explanation of a new effect called the “gain lever.” It enhances the modulation efficiency of a semiconductor laser with a quantum well active layer. By inhomogeneously pumping the device, regions with unequal differential gain are created. If the laser is above threshold, then the overall modal gain is clamped, and by modulating the section with larger differential gain, the output power can be modulated with greater than unity quantum efficiency.
The fundamental coupling between intensity noise and phase noise in semiconductor laser light is investigated. This coupling, described by the [alpha] parameter, causes the well-known linewidth enhancement, but also implies the fluctuations are correlated. By the technique of “amplitude-phase decorrelation,” the intensity noise can be passively reduced by the ratio 1/(1 + alpha). Using a Michelson interferometer as a frequency discriminator, intensity noise from a DFB laser is reduced below its intrinsic level up to a factor of 28.
A balanced homodyne detection scheme is used to study the noise reduction in relation to the photon shot noise floor. The decorrelated intensity noise can be reduced to within a dB of the shot noise level. Reduction below shot noise may be inhibited by uncorrelated phase noise in the lasing mode.}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Vahala, Kerry J.}, } @phdthesis{10.7907/dkyt-nc40, author = {Paslaski, Joel Stephen}, title = {High Speed Optoelectronics: Photodiodes, Q-Switched Laser Diode and Photoconductive Sampling}, school = {California Institute of Technology}, year = {1990}, doi = {10.7907/dkyt-nc40}, url = {https://resolver.caltech.edu/CaltechETD:etd-05092007-084117}, abstract = {In this thesis, a variety of topics related to high speed optoelectronic devices and measurement techniques using ultrafast optical pulses are presented.
Following a brief introduction, the second chapter describes a Q-switched semi-conductor laser using a multi-quantum well active layer both for gain and as an intracavity loss modulator. While Q-Switching does not produce as short a pulse as modelocking, it does offer the advantage of adjustability of the repetition rate making it attractive as a source for digital communication links. It is also found to be preferred to the similar approach of gain switching due to less demanding requirements on the rf modulation power level and waveform. Results include a pulse width of ~ 20 ps which is fairly independent of the repetition rate, and a limiting repetition rate of 3.2 GHz. The onset of an irregular pulse train which limits the maximum modulation frequency, is analyzed by a graphical approach.
The potential for optical interconnects has motivated a marriage between the two technologies of Si VLSI and GaAs optoelectronics. Direct integration by the growth of GaAs on Si had been impossible, but the MBE and MOCVD techniques now enable the growth of such layers and of a quality suitable for devices. The third chapter describes the operating characteristics of GaAs-on-Si lasers and photodiodes with particular attention to their high speed performance. Both the lasers and photodiodes show comparable high speed performance to similar structures fabricated on GaAs, with most of the shortcomings being in their dc characteristics.
In the fourth chapter, a novel approach to improving the resolution of photoconductive sampling is presented, called differential sampling. This technique obviates the need for carrier lifetime reduction usually used to improve temporal resolution, and is in principal only limited by a small (few ps) RC circuit time. An analysis of the minimum detectable signal voltage shows the technique does quite well compared with lifetime reduction techniques which also tend to reduce mobility and dark resistance. An experimental demonstration of this technique is presented in chapter five. Using a two gap sampler, accurate measurement (10 ps resolution) of a 60 ps pulse response from a photodiode is achieved using photoconductors with a recovery time of only 150 ps. Performance near the fundamental Johnson noise limit is also attained, though the minimum detectable signal is higher than predicted due to low response of the photoconductors (probably due to poor contacts).
Finally, in chapter six, the possibility of retrieving an impulse response from its autocorrelation is explored. The use of the logarithmic Hilbert transform for phase retrieval has been discounted in the literature since most such work is concerned with imaging problems for which it is not appropriate due to their symmetric nature. However, causality and the decay nature of transient phenomena make this technique very suitable for use with the impulse response of passive devices. Conditions for the validity of this technique for temporal problems are presented. Simulated retrieval of two functions with similar autocorrelations is demonstrated with sufficient clarity to distinguish them, as well as showing good agreement with the original. Practical limitations and aspects – such as noise, finite time domain, etc. – are also simulated and discussed.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Yariv, Amnon}, } @phdthesis{10.7907/ajzc-wx87, author = {Zarem, Hal}, title = {Investigations of quantum wires, carrier diffusion lengths, and carrier lifetimes in GaAs/AlGaAs heterostructures}, school = {California Institute of Technology}, year = {1990}, doi = {10.7907/ajzc-wx87}, url = {https://resolver.caltech.edu/CaltechETD:etd-11092007-090251}, abstract = {NOTE: Text or symbols not renderable in plain ASCII are indicated by […]. Abstract is included in .pdf document.
Nanometer scale wire structures are fabricated by selective disorder of a GaAs/AlGaAs quantum well. These structures are investigated by cathodoluminescence (CL). Spectrally resolved CL images of the structures as well as local CL spectra of the structures are resented. The effects of fabricational variations on quantum wire laser gain spectra and performance are discussed. A new technique for determining carrier diffusion lengths by cathodoluminescence measurements is presented. The technique is extremely accurate and can be applied to a variety of structures. The ambipolar diffusion length and carrier lifetime are measured in […] for several mole fractions in the interval 0 < […] < 0.38. These parameters are found to have significantly higher values in the higher mole fraction samples. These increases are attributed to occupation of states in the indirect valleys, and supporting calculations are presented.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Yariv, Amnon}, } @phdthesis{10.7907/ppm7-sq27, author = {Hoenk, Michael Eugene}, title = {Orientation selective effects in III-V heterostructure systems with application to nanostructure fabrication}, school = {California Institute of Technology}, year = {1990}, doi = {10.7907/ppm7-sq27}, url = {https://resolver.caltech.edu/CaltechETD:etd-06122007-091106}, abstract = {Orientation selective effects in the molecular beam epitaxial (MBE) growth of Al1-xGaxAs on a patterned (100) GaAs substrate are investigated. Under particular growth conditions, the spontaneous, selective formation of a superlattice or quantum wire array on the sides of grooves is observed. Experiments and modelling indicate that this growth technique, orientation selective epitaxy (OSE), has application to the in situ growth of manometer scale structures.
An optical fiber cathodoluminescence system is described. By using an optical fiber to collect light directly from the surface of the sample, spectrally resolved cathodoluminescence measurements can be done without precluding simultaneous measurement of other signals. The system attains comparable performance to alternative systems which utilize concave mirrors for light collection. A cryogenic cold stage, using a continuous flow of liquid helium or nitrogen, is described.
Experimental results on the MBE growth of AlxGa1-xAs on [011] grooves in a (100) GaAs substrate exhibit orientation selective effects. Spontaneous growth of a superlattice is observed on the sides of grooves. The superlattice, with a period of approximately 70A, exhibits strong luminescence, red-shifted by 127 meV from the emission of uniform Al0.25Ga0.75As grown on adjacent facets. This is the first observation of compositional modulation in the growth of AlGaAs on a {111} surface. The abrupt termination of the superlattice at the interface with adjacent facets constitutes a heterojunction oriented lateral to the growth direction, the first such structure to be formed spontaneously during growth.
Orientation selective epitaxy is modelled, using calculations and Monte Carlo simulations of growth on surfaces inclined with respect to the substrate rotation axis. The proposed mechanism predicts an in-plane variation of the composition, resulting from growth by step flow in the presence of time dependent fluxes. The time dependence is caused by the inclination of the surface with respect to the substrate rotation axis.
The dependence of OSE growth on surface orientation is investigated by varying groove orientation in a patterned (100) GaAs substrate. A strong dependence of cathodoluminescence emission on groove orientation is observed. In some ranges of groove angles, monotonic dependence of the peak emission on groove orientation is observed, suggesting the possibility of bandgap tuning by variation of surface orientation. In growth on [010] grooves, quantum wire arrays were observed, with a wire dimension of approximately 60A.
The growth of AlxGa1-xAs in oval defects is investigated. A strong dependence of the composition on the orientation of growth in oval defects is observed. Comparison with growth in grooves indicates that orientation selective epitaxy plays a role in determining AlxGa1-xAs composition in oval defects.
}, address = {1200 East California Boulevard, Pasadena, California 91125}, advisor = {Vahala, Kerry J.}, }