### Browsing by Author "Wang, Shaokang"

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Item Analysis of Stability and Noise in Passively Modelocked Comb Lasers(2018-01-01) Wang, Shaokang; Menyuk, Curtis R; Computer Science and Electrical Engineering; Engineering, ElectricalThe search for robust, low-noise modelocked comb sources has attracted significant attention during the last two decades. Passively modelocked fiber lasers are among the most attractive comb sources. The most important design problems for a passively modelocked laser include: (1) finding a region in the laser's adjustable parameter space where it operates stably, (2) optimizing the pulse profile within that region, and (3) lowering the noise level. Adjustable parameters will typically include the cavity length, the pump power, and the amplifier gain, which may be a function of the pump power, the pump wavelength, and both the material and geometry of the gain medium. There are two basic computational approaches for modeling passively modelocked laser systems: the evolutionary approach and the dynamical approach. In the evolutionary approach, which replicates the physical behavior of the laser, one launches light into the simulated laser and follows it for many round trips in the laser. If one obtains a stationary or periodically-stationary modelocked pulse, the laser is deemed stable and, if no such pulse is found, the laser is deemed unstable. The effect of noise can be studied by using a random number generator to add computational noise. In the dynamical approach, one first obtains a single modelocked pulse solution either analytically or by using the evolutionary approach. Next, one finds the pulse parameters as the laser parameters vary by solving a root-finding algorithm. One then linearizes the evolution equations about the steady-state solution and determines the eigenvalues of the linearized equation, which we refer to as the equation's dynamical spectrum. If any eigenvalue has a positive real part, then the modelocked pulse is unstable. The effect of noise can be determined by calculating the noise that enters each of the modes in the dynamical spectrum, whose amplitudes are described by either a Langevin process or a random walk process. The evolutionary approach is intuitive and straightforward to program, and it is widely used. However, it is computationally time-consuming to determine the stable operating regions and can give ambiguous results near a stability boundary. When evaluating the noise levels, Monte Carlo simulations, which are based upon the evolutionary approach, are often prohibitively expensive computationally. By comparison, the dynamical approach is more difficult to program, but it is computationally rapid, yields unambiguous results for the stability, and avoids computationally expensive Monte Carlo simulations. The two approaches are complementary to each other. However, the dynamical approach can be a powerful tool for system design and optimization and has historically been undertilized. In this dissertations, we discuss the dynamical approach that we have developed for design and optimization of passively modelocked laser systems. This approach provides deep insights into the instability mechanisms of the laser that impact or limit modelocking, and makes it possible to rapidly and unambiguously map out the regions of stable operation in a large parameter space. For a given system setup, we can calculate the noise level in the laser cavity within minutes on a desktop computer. Compared to Monte Carlo simulations, we will show that the dynamical approach improves the computational efficiency by more than three orders of magnitude. We will apply the dynamical approach to a laser with a fast saturable absorber and to a laser with a slow saturable absorber. We apply our model of a laser with a slow saturable absorber to a fiber comb laser with a semiconductor absorbing mirror (SESAM) that was developed at National Institute of Standards and Technology (NIST), Boulder, CO. We optimize its parameters and show that it is possible to increase its output power and bandwidth while lowering the pump power that is needed.Item Calculation of the impulse response and phase noise of a high-current photodetector using the drift-diffusion equations(OSA, 2019) Mahabadi, Seyed Ehsan Jamali; Wang, Shaokang; Carruthers, Thomas F.; Menyuk, Curtis ; Quinlan, Franklyn J.; Hutchinson, Meredith N.; McKinney, Jason D.; Williams, Keith J.We describe a procedure to calculate the impulse response and phase noise of high-current photodetectors using the drift-diffusion equations while avoiding computationally expensive Monte Carlo simulations. We apply this procedure to a modified uni-traveling-carrier (MUTC) photodetector. In our approach, we first use the full drift-diffusion equations to calculate the steady-state photodetector parameters. We then perturb the generation rate as a function of time to calculate the impulse response. We next calculate the fundamental shot noise limit and cut-off frequency of the device. We find the contributions of the electron, hole, and displacement currents. We calculate the phase noise of an MUTC photodetector. We find good agreement with experimental and Monte Carlo simulation results. We show that phase noise is minimized by having an impulse response with a tail that is as small as possible. Since, our approach is much faster computationally than Monte Carlo simulations, we are able to carry out a broad parameter study to optimize the device performance. We propose a new optimized structure with less phase noise and reduced nonlinearity.Item Dissipative cnoidal waves (Turing rolls) and the soliton limit in microring resonators(Optical Society of America, 2019-09-30) Qi, Zhen; Wang, Shaokang; Jaramillo-Villegas, José; Qi, Minghao; Weiner, Andrew M.; D'Aguanno, Giuseppe; Carruthers, Thomas F.; Menyuk, CurtisSingle solitons are a special limit of more general waveforms commonly referred to as cnoidal waves or Turing rolls. We theoretically and computationally investigate the stability and accessibility of cnoidal waves in microresonators. We show that they are robust and, in contrast to single solitons, can be easily and deterministically accessed in most cases. Their bandwidth can be comparable to single solitons, in which limit they are effectively a periodic train of solitons and correspond to a frequency comb with increased power. We comprehensively explore the three-dimensional parameter space that consists of detuning, pump amplitude, and mode circumference in order to determine where stable solutions exist. To carry out this task, we use a unique set of computational tools based on dynamical system theory that allow us to rapidly and accurately determine the stable region for each cnoidal wave periodicity and to find the instability mechanisms and their time scales. Finally, we focus on the soliton limit, and we show that the stable region for single solitons almost completely overlaps the stable region for both continuous waves and several higher-periodicity cnoidal waves that are effectively multiple soliton trains. This result explains in part why it is difficult to access single solitons deterministically.Item Dynamical Methods for Studying Noise in Frequency Comb Sources(2022-03-01) Menyuk, Curtis; Wang, ShaokangItem Efficiently modeling the noise performance of short-pulse lasers with a computational implementation of dynamical methods(OSA Publishing, 2018-05-27) Wang, Shaokang; Carruthers, Thomas F.; Menyuk, CurtisLowering the output noise of short-pulse lasers has been a long-standing effort for decades. Modeling the noise performance plays a crucial role in isolating the noise sources and reducing them. Modeling to date has either used analytical or semianalytical implementation of dynamical methods or Monte Carlo simulations. The former approach is too simplified to accurately assess the noise performance in real laser systems, while the latter approach is too computationally slow to optimize the performance as parameters vary over a wide range. Here, we describe a computational implementation of dynamical methods that allows us to determine the noise performance of a passively mode-locked laser within minutes on a desktop computer and is faster than Monte Carlo methods by a factor on the order of 10³. We apply this method to characterize a laser that is locked using a fast saturable absorber—for example, a fiber-based nonlinear polarization rotation device—and a laser that is locked using a slow saturable absorber—for example, a semiconductor saturable absorbing mirror.Item Obtaining more energetic modelocked pulses from a SESAM-based fiber laser(OSA Publishing, 2020) Wang, Shaokang; Tu, Chaoran; Mahabadi, Seyed Ehsan Jamali; Droste, Stefan; Sinclair, Laura C.; Coddington, Ian; Newbury, Nathan R.; Carruthers, Thomas F.; Menyuk, CurtisA major design goal for femtosecond fiber lasers is to increase the output power but not at the cost of increasing the noise level or narrowing the bandwidth. Here, we perform a computational study to optimize the cavity design of a femtosecond fiber laser that is passively modelocked with a semiconductor saturable absorbing mirror (SESAM). We use dynamical methods that are more than a thousand times faster than standard evolutionary methods. We show that we can obtain higher pulse energies and hence higher output powers by simultaneously increasing the output coupling ratio, the gain, and the anomalous group delay dispersion. We can obtain output pulses that are from 5 to 15 times the energy of the pulse in the current experimental design with no penalty in the noise level or bandwidth.Item Slow saturable absorption for optimal operation in a soliton comb laser(Optica, 2022-04-05) Kang, Zhe; Wang, Yinghe; Wang, Shaokang; Zheng, Zi-Wei; Zhang, Xianting; Menyuk, CurtisIn this article, we study how the choice of parameters of a slow saturable absorber (SSA) affects the stable operation of a soliton fiber comb laser. We show that a shorter recovery time for the SSA does not always lead to shorter modelocked pulses. Instead, increasing the cavity gain plays a critical role in generating stable modelocked pulses with higher energy and shorter durations. We find that more stable, shorter, and more energetic output pulses can be achieved with lower saturation energies of the SSA and/or higher anomalous dispersion within the cavity.Item Stability and noise in frequency combs: efficient and accurate computation using dynamical methods(SPIE, 2022-11-02) Menyuk, Curtis; Wang, ShaokangKey issues in the design of any passively modelocked laser system are determining the parameter ranges within which it can operate stably, determining its noise perfomance, and then optimizing the design to achieve the best possible output pulse parameters. Here, we review work within our research group to use computational methods based on dynamical systems theory to accurately and efficiently address these issues. These methods are typically many orders of magnitude faster than widely used evolutionary methods. We then review our application of these methods to the analysis and design of passively modelocked fiber lasers that use a semiconductor saturable absorbing mirror (SESAM). These lasers are subject to a wake instability in which modes can grow in the wake of the modelocked pulse and destroy it. Even when stable, the wake modes can lead to undesirable radiofrequency sidebands. We demonstrate that the dynamical methods have an advantage of more than three orders of magnitude over standard evolutionary methods for this laser system. After identifying the stable operating range, we take advantage of the computational speed of these methods to optimize the laser performance over a three-dimensional parameter space.Item Stability of cnoidal wave frequency combs in microresonators(OSA Publishing, 2018) Qi, Zhen; Wang, Shaokang; Jaramillo-Villegas, Jose A.; Qi, Minghao; Weiner, Andrew M.; D’Aguanno, Giuseppe; Menyuk, CurtisWe determine the regions in the parameter space of microresonators where cnoidal waves (Turing rolls) are stable. Solitons are included as a special limit. We identify conditions to efficiently obtain broadband frequency combs.Item UMBC Computational Photonics LaboratoryWeiblen, R. Joseph; Wang, Shaokang; Hu, Yue