Indiana University-Purdue University Indianapolis (IUPUI) / This research investigates characteristics of PT (parity-time) symmetry breaking in a system of two optically-coupled, time-delayed semiconductor lasers. A theoretical rate equation model for the lasers' electric fields is presented and then reduced to a 2x2 Hamiltonian model, which, in the absence of time-delay, is PT-symmetric. The important parameters we control are the temporal separation of the lasers, the frequency detuning, and the coupling strength. The detuning is experimentally controlled by varying the lasers' temperatures, and intensity vs. detuning behavior are examined, specifically how the PT-transition and the period and amplitude of sideband intensity oscillations change with coupling and delay. Experiments are compared to analytic predictions and numerical results, and all are found to be in good agreement. Eigenvalues, eigenvectors, and exceptional points of the reduced Hamiltonian model are numerically and analytically investigated, specifically how nonzero delay affects existing exceptional points.

A new fully integrated wavelength stabilization scheme based on grating-coupled surface-emitting lasers is explored. This wavelength stabilization scheme relies on two gratings. The first grating is fabricated on the p-side of the semiconductor laser in close proximity to the laser waveguide such that it couples light out of the guided mode of the waveguide into a propagating mode in the substrate; this grating is known as the grating coupler. The second grating is fabricated on the n-side of the substrate such that for the stabilization wavelength, this second grating operates in the Littrow condition and is known as the feedback grating. Furthermore with the proper design of the two gratings, the feedback grating will operate under total internal reflection conditions allowing a near unity retro-reflection of the light of the stabilization wavelength. The grating coupler and feedback grating together comprise a dual grating reflector (DGR). The DGR wavelength stabilization scheme is investigated both theoretically by means of numerical modeling and experimentally by integration of a DGR as a wavelength selective reflector into a single quantum well semiconductor laser with a gain peak centered at 975nm. Numerical modeling predicts a peak reflection of approximately 70% including losses and a spectral width of 0.3nm. The integration of a DGR into a semiconductor laser proved both the efficacy of the scheme and also allowed us to experimentally determine the effective reflectivity to be on the order of 62%; the spectral width of light output from these devices is typically on the order of 0.2nm. Furthermore, these devices had light-current characteristic slopes greater than 0.84W/A operating under continuous wave conditions. The DGR was then modified to provide a reflection with two spectral peaks. A semiconductor device incorporating this dual wavelength DGR was fabricated and tested. These devices showed a peak optical power of in excess of 5.5W and a light-current characteristic slope of 0.86W/A in quasi continuous wave operation; these devices also exhibit a large operating current range in which both wavelengths have comparable output powers. Another modified DGR design was investigated for the purpose of providing an even narrower spectral reflection. Devices incorporating this modified design provided an output with a spectral width as narrow as 0.06nm. DGRs were also integrated into an extremely broad area device of an unorthodox geometry; square devices that lase in two orthogonal directions were fabricated and tested. The last idea investigated was combining a DGR wavelength stabilized laser with a tapered semiconductor optical amplifier into a master oscillator power amplifier device, with the optical coupling between the two components provided by identical grating couplers disposed on the p-side surfaces of each of the devices. These master oscillator power amplifiers provide a peak power of 32W when operating under quasi continuous wave operation.

External cavity semiconductor mode-locked lasers can produce pulses of a few picoseconds. The pulses from these lasers are inherently chirped with a predominant linear chirp component that can be compensated resulting in sub-picosecond pulses. External cavity semiconductor mode-locked lasers can be configured as multiwavelength pulse sources and are good candidates for time and wavelength division multiplexing applications. The gain medium in external cavity semiconductor mode-locked lasers is a semiconductor optical amplifier (SOA), and passive and hybrid mode-locked operation are achieved by the introduction of a saturable absorber (SA) in the laser cavity. Pump-probe techniques were used to measure the intracavity absorption dynamics of a SA in an external cavity semiconductor mode-locked laser and the gain dynamics of a SOA for the amplification of diverse pulses. The SOA gain dynamics measurements include the amplification of 750 fs pulses, 6.5 ps pulses, multiwavelength pulses and the intracavity gain dynamics of an external cavity multiwavelength semiconductor mode-locked laser. The experimental results show how the inherent chirp on pulses from external cavity semiconductor mode-locked lasers results in a slow gain depletion without significant fast gain dynamics. In the multiwavelength operation regime of these lasers, the chirp broadens the temporal pulse profile and decreases the temporal beating resulting from the phase correlation among wavelength channels. This results in a slow gain depletion mitigating nonlinearities and gain competition among wavelength channels in the SOA supporting the multiwavelength operation of the laser. Numerical simulations support the experimental results.

This dissertation details work on high repetition rate semiconductor mode-locked lasers. The qualities of stable pulse trains and stable optical frequency content are the focus of the work performed. First, applications of such lasers are reviewed with particular attention to applications only realizable with laser performance such as presented in this dissertation. Sources of timing jitter are also reviewed, as are techniques by which the timing jitter of a 10 GHz optical pulse train may be measured. Experimental results begin with an exploration of the consequences on the timing and amplitude jitter of the phase noise of an RF source used for mode-locking. These results lead to an ultralow timing jitter source, with 30 fs of timing jitter (1 Hz to 5 GHz, extrapolated). The focus of the work then shifts to generating a stabilized optical frequency comb. The first technique to generating the frequency comb is through optical injection. It is shown that not only can injection locking stabilize a mode-locked laser to the injection seed, but linewidth narrowing, timing jitter reduction and suppression of superfluous optical supermodes of a harmonically mode-locked laser also result. A scheme by which optical injection locking can be maintained long term is also proposed. Results on using an intracavity etalon for supermode suppression and optical frequency stabilization then follow. An etalon-based actively mode-locked laser is shown to have a timing jitter of only 20 fs (1Hz-5 GHz, extrapolated), optical linewidths below 10 kHz and optical frequency instabilities less than 400 kHz. By adding dispersion compensating fiber, the optical spectrum was broadened to 2 THz and 800 fs duration pulses were obtained. By using the etalon-based actively mode-locked laser as a basis, a completely self-contained frequency stabilized coupled optoelectronic oscillator was built and characterized. By simultaneously stabilizing the optical frequencies and the pulse repetition rate to the etalon, a 10 GHz comb source centered at 1550 nm was realized. This system maintains the high quality performance of the actively mode-locked laser while significantly reducing the size weight and power consumption of the system. This system also has the potential for outperforming the actively mode-locked laser by increasing the finesse and stability of the intracavity etalon. The final chapter of this dissertation outlines the future work on the etalon-based coupled optoelectronic oscillator, including the incorporation of a higher finesse, more stable etalon and active phase noise suppression of the RF signal. Two appendices give details on phase noise measurements that incorporate carrier suppression and the noise model for the coupled optoelectronic oscillator.

Highly coherent light, although beneficial in specific applications, suffers from the formation of speckles, resulting in poor imaging, lighting, and projection/display quality. Moreover, the long coherence length limits the resolution in interference based sensing. This has led to the emergence of edge-emitting semiconductor low coherence light sources (e.g., broadband lasers, superluminescent diodes, etc.), which have been used in display applications, optical coherence tomography, and random bit generation. However, edge emission prevents the ease of fabricating two-dimensional arrays. Conversely, vertical-cavity surface-emitting lasers (VCSELs) have recently been widely used in consumer electronics due to the unique advantages of surface emission. Nevertheless, they still suffer from issues caused by high coherence. The aim of this dissertation is to design low-coherence surface-emitting lasers to push simultaneous illumination and optical wireless communication (OWC) toward reliable implementation with higher speeds. To that end, we demonstrate, for the first time, the use of chaotic cavities to lower the coherence of VCSELs without increasing their emission area, which would lower their speed. Not only did the chaotic cavity result in doubling the number of modes (lowering the coherence) compared to conventional VCSELs, but it also resulted in an increase in the optical power of up to 60%. We also show that chaotic-cavity broad-area VCSELs can achieve significantly broader modulation bandwidths (up to 5 GHz) and higher data rates (up to 12.6 Gb/s) compared to other low-coherence light sources, while achieving a lower speckle contrast. We further report a novel technique of lowering the speckle contrast 2 by carefully designing the AC signal used for communication. We show that the apparent spatial coherence is dramatically decreased by inserting a short chirp signal between symbols. Using this method with a chaotic-cavity VCSEL, the number of apparent modes can be up to 450 modes, compared to 88 modes measured from a conventional broad-area VCSEL. The simplicity of implementing the reported design, which requires no additional fabrication steps, makes it a promising solution for applications that would benefit from the lower speckle density of the emitted light as well as those that rely on lower temporal coherence.

Quaternary GalnAsN containing a small amount of nitrogen (<2%) is a potentially promising material for realizing long-wavelength emission lasers for applications in optical communication systems. Such devices should have better high-temperature characteristics than conventional InGaAsP lasers due to an increase of the conduction band offset. In this thesis, the GS-MBE growth of quaternary GalnAsN and ternary GaAsN was carried out. Active N was produced by passing high purity nitrogen gas into either an RF or an ECR plasma source. The RF plasma source was found to produce better quality nitrides. Characterization techniques such as photoluminescence, X-ray diffraction, TEM, SIMS, and Hall effect measurements were used to characterize thick layers (e.g. 1 pm) and quantum wells of these nitride materials. The concentration of N incorporated into GalnAs and GaAs is very dependent on growth conditions and plasma conditions. The incorporation of a small amount of N into compressively strained InGaAs reduces the strain and produces a red-shift of photoluminescence peak. However, compared to N-free InGaAs materials, the optical quality is dramatically degraded yielding reduced photoluminescence intensity and a broadened FWHM of the PL peak. Hall effect measurements on un-doped, Si-doped, Bedoped thick GalnAsN layers indicate the presence of a high concentration of electron and hole traps. The results of SIMS suggest that impurity H might be responsible for the deep level defects formed. However, the nature of the defects is currently unknown. From TEM observations and comparison to samples grown with a He-plasma instead of a Nplasma, spinodal decomposition and ion-induced damage in GalnAsN may produce the reduced quality of materials, but these are not the major reasons responsible for the dramatic degradation of optical quality. Thermal annealing was found to be an effective method for significantly improving the optical quality of GalnAsN with a low N concentration. Optimum annealing conditions were obtained. Hall effect measurements on annealed samples indicate that electron and hole traps are reduced but still present after anneal. / Thesis / Master of Engineering (ME)

Semiconductor lasers are considered essential for the advancement in the field of photonics where compact and energy-efficient lasers are necessary. Advancements in integrated photonic technologies will help push the performance of semiconductor lasers in the coming years and expand the technology to several other applications. Semiconductor lasers offer several key features such as high energy efficiency, mass production, availability at a myriad of wavelengths, and high integration capabilities. However, limitations in noise performance, pulse energy, and duration hold back semiconductor lasers from being utilized to their full potential. This dissertation reviews the utilization and development of external techniques that enable semiconductor mode-locked lasers to be used in multi-photon imaging and microwave photonic applications. We first review a two-color external cavity mode-locked laser system operating at wavelengths 834 nm and 974 nm that can generate synchronized picosecond pulses with peak powers exceeding 80 W and 100 W respectively. We verify the feasibility of this system to induce non-linear processes by demonstrating two-photon excitation in commercially available dyes. Next, we introduce the concepts of optical injection locking and discuss the development of a multi-tone optical self-injection locking technique to improve the noise performance and optical linewidth of a chip-scale InP based mode-locked laser. We utilize a Fabry-Perot etalon as the optical comb filter, which also serves to suppress the super-mode noise that arises from external cavity feedback. In addition to this, we also implement a coupled opto-electronic loop and reference it to an external RF source demonstrating exceptional timing stability. This approach along with the usage of fully integrated and ultra-compact components in subsequent versions has the potential to realize compact frequency comb lasers for microwave photonic and other practical applications.

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We live in an era where the impact of climate change is felt in everyday life, with warming temperatures leading to environmental destruction and unprecedented shifts in natural and anthropic activities. Before these adverse effects can be addressed, it is crucial to explore, monitor, and collect data on the Earth's environment on both the micro- and macro-scale. Additionally, once environmental solutions have been implemented, it is imperative to continue data collection to gauge success. Standing in the way of such exploration and monitoring efforts are the shortcomings and limitations of pervasive communication technologies, namely radio frequency technologies (e.g., WiFi and Bluetooth). In this thesis, we peer beyond the confines of the radio frequency spectrum and explore the use of light — namely visible light, along with its infrared neighbor — to create next-generation communication and sensing systems. To generate highly tunable light, we explicitly turn to lasers. Unlike traditional luminaries, e.g., light-emitting diodes, laser diodes provide superior communication and sensing performance thanks to their GHz modulation speeds, narrow spectral wavelengths, intrinsic polarization, high-power densities, and high electro-optical conversion ratios. In this thesis, we exploit the versatility of laser light to enable micro- and macro-scale exploration and monitoring systems in aquatic, terrestrial, and aerial environments. We begin by exploring the powerful potential of laser light in communication and sensing contexts, then dive into the overarching challenges and techniques needed to realize practical and robust solutions. Armed with our research methodology, we then break through the air-water boundary with laser light and demonstrate two systems supporting bidirectional communication and 3D localization between aerial and underwater robots. Continuing on land and in the air, we next present two systems enabling laser tethering, communication, and wireless power delivery for high-mobility targets. We then complement these micro-scale systems with a macro-scale approach for laser polarization sensing over terrestrial fiber-optic networks. Finally, we conclude by expounding on the remaining challenges associated with laser-based communication and sensing systems, as well as the bright future of laser light in these novel contexts.