Browsing by Author "Williams, Keith J."
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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 Efficient and Accurate Calculation of Photodetector RF Output Power(IEEE, 2022-11-13) Simsek, Ergun; Anjum, Ishraq Md; Carruthers, Thomas F.; Menyuk, Curtis; Tulchinsky, David A.; Williams, Keith J.; Campbell, Joe C.We present two different ways to calculate RF output power of today’s complex photodetectors accurately and efficiently by solving drift-diffusion equations. Numerical results show a very good agreement with measurements.Item Fast Evaluation of RF Power Spectrum of Photodetectors With Windowing Functions(IEEE, 2023-05-22) Simsek, Ergun; Anjum, Ishraq Md; Carruthers, Thomas F.; Menyuk, Curtis; Campbell, Joe C.; Tulchinsky, David A.; Williams, Keith J.Time window functions are used as broadband excitations to obtain the output power spectrum of photodetectors. Numerical results show a good agreement with experimental results. The proposed method yields a two-orders-of-magnitude reduction in computing time and memory compared to traditional monochromatic calculations. This large reduction makes it possible to design high-performance photodetectors with numerical optimization on desktop computers. We use this approach to design a device whose 3-dB bandwidth is four times larger than the initial design.Item Impact of Nonlinearity in an MUTC Photodetector on an RF-Modulated Frequency Comb(IEEE, 2019-11-21) Mahabadi, Seyed Ehsan Jamali; Carruthers, Thomas F.; Menyuk, Curtis; Hutchinson, Meredith N.; McKinney, Jason D.; Williams, Keith J.Abstract—We calculate the impact of nonlinearity in an MUTC photodetector on an RF-modulated frequency comb, taking into account bleaching (gain saturation). An RF-modulated frequency comb is characterized by a distinct IMD2ₙ and IMD3ₙ and hence a distinct OIP2ₙ and OIP3ₙ for each comb line nItem Impact of nonlinearity including bleaching in MUTC photodetectors on RF-modulated electro-optic frequency combs(OSA Publishing) Mahabadi, Seyed Ehsan Jamali; Carruthers, Thomas; Menyuk, Curtis; McKinney, Jason D.; Williams, Keith J.We use the drift-diffusion equations to calculate the responsivity of a modified uni-traveling carrier (MUTC) photodetector (PD) with a frequency comb input that is generated by a series of short optical pulses. We first use experimental results for the responsivity of the MUTC PD to obtain an empirical model of bleaching in pulsed mode. We incorporate our empirical bleaching model into a drift-diffusion model to calculate the impact of nonlinearity in an MUTC PD on RF-modulated electro-optic frequency combs. We quantify the nonlinearity using the second- and third-order intermodulation distortion powers (IMD2 and IMD3), from which we calculate the second- and third-order output intercept points (OIP2 and OIP3). In contrast to a continuous wave (CW) input for which there is a single IMD2 and IMD3 and hence a single OIP2 and OIP3, each comb line n has its own IMD2n, IMD3n, OIP2n, and OIP3n associated with it. We determine the IMD2n, IMD3n, OIP2n, and OIP3n, and we compare the results with and without bleaching. We find that the impact of bleaching is complex and, somewhat surprisingly, not always detrimental. The principal effect of bleaching is to lower the responsivity, which decreases the nonlinearity due to space charge. While bleaching always reduces the OIP2n and OIP3n, we find that bleaching leads to a decreased distortion-to-signal ratio for large n.Item Impact of Nonlinearity on RF-Modulated Frequency Combs with Different Modulation Depths in an MUTC Photodetector(IEEE, 2019-11-07) Mahabadi, Seyed Ehsan Jamali; Carruthers, Thomas F.; Menyuk, Curtis; Hutchinson, Meredith N.; McKinney, Jason D.; Williams, Keith J.We calculate the impact of nonlinearity in a modified uni-traveling carrier (MUTC) photodetector on an RF-modulated frequency comb that is generated using short optical pulses. We take into account bleaching (nonlinear saturation), which plays an important role due to the large peak-to-average power ratio. Nonlinear impairment of an RF-modulated continuous wave is characterized by the second- and third-order intermodulation distortion (IMD2 and IMD3). By contrast, an RF-modulated frequency comb is characterized by a distinct IMD2ₙ and IMD3ₙ for each comb line n. We study the effect of modulation depth on nonlinearity in frequency combs, comparing modulation depths of 4% and 8%. When we include bleaching, we find that the OIP3ₙ, third-order intercept point for the n-th comb line, increases when the modulation depth increases for comb lines at frequencies above 2 GHz (n >= 40), so that distortion decreases. We show that this effect occurs because bleaching attenuates lower frequencies more than higher frequencies and this preferential attenuation increases as the modulation depth increases.Item Impact on frequency combs of nonlinearity including bleaching in p-i-n photodetectors(IEEE, 2020-08-06) Mahabadi, Seyed Ehsan Jamali; Carruthers, Thomas F.; Menyuk, Curtis; McKinney, Jason D.; Williams, Keith J.We calculate the impact of nonlinearity in a p-i-n photodetector on an RF-modulated frequency comb taking into account bleaching due to the large peak-to-average power ratio in frequency combs. We characterized RF-modulated frequency combs by IMD2 n , IMD3 n , OIP2 n , and OIP3 n for each comb line n.Item Modeling Sources of Nonlinearity in a Simple p-i-n Photodetector(IEEE, 2015-10-15) Hu, Yue; Marks, Brian S.; Menyuk, Curtis; Urick, Vincent J.; Williams, Keith J.Nonlinearity in p-i-n photodetectors leads to power generation at harmonics of the input frequency, limiting the performance of RF- photonic systems. We use one-dimensional and two-dimensional simulations of the drift-diffusion equations to determine the physical origin of the saturation in a simple heterojunction p-i-n photodetector at room temperature. Incomplete ionization, external loading, impact ionization, and the Franz–Keldysh effect are all included in the model. Impact ionization is the main source of nonlinearity at large reverse bias (>10 V in the device that we simulated). The electron and hole current contributions to the second harmonic power were calculated. We find that impact ionization has a greater effect on the electrons than it does on the holes. We also find that the hole velocity saturates slowly with increasing reverse bias, and the hole current makes a large contribution to the harmonic power at 10 V. This result implies that decreasing the hole injection will decrease the harmonic power.Item Simulation of a partially depleted absorber (PDA) photodetector(2015) Hu, Yue; Carruthers, Thomas F.; Menyuk, Curtis; Hutchinson, Meredith N.; Urick, Vincent J.; Williams, Keith J.We use a 2D drift-diffusion model to study the nonlinear response of a partially depleted absorber (PDA) phododetector. The model includes external loading, incomplete ionization, the Franz-Keldysh effect, and history-dependent impact ionization. It also takes into account heat flow in the device. With all these effects included, we obtain excellent agreement with experiments for the responsivity and for the harmonic power at different modulation frequencies. The role of these different physical effects is elucidated, and we find that both the Franz-Keldysh effect and the load resistance play a key role in generating higher harmonic power at larger reverse biases. Increasing the size of the p-region absorption layers reduces the impact of the Franz-Keldysh effect. Decreasing the effective load resistance also decreases the higher harmonic powers. We also show that the model can suggest design changes that will improve device performance.Item Use of Evolutionary Optimization Algorithms for the Design and Analysis of Low Bias, Low Phase Noise Photodetectors(IEEE, 2023-11-03) Anjum, Ishraq Md; Simsek, Ergun; Mahabadi, Seyed Ehsan Jamali; Carruthers, Thomas F.; Menyuk, Curtis; Campbell, Joe C.; Tulchinsky, David A.; Williams, Keith J.With the rapid advance of machine learning techniques and the increased availability of high-speed computing resources, it has become possible to exploit machine-learning technologies to aid in the design of photonic devices. In this work we use evolutionary optimization algorithms, machine learning techniques, and the drift-diffusion equations to optimize a modified uni- traveling-carrier (MUTC) photodetector for low phase noise at a relatively low bias of 5 V. We compare the particle swarm optimization (PSO), genetic, and surrogate optimization algorithms. We find that PSO yields the solution with the lowest phase noise, with an improvement over a current design of 4.4 dBc/Hz. We then analyze the machine-optimized design to understand the physics behind the phase noise reduction and show that the optimized design removes electrical bottlenecks in the current design.