MODELING PHASE NOISE AND NONLINEARITY IN PHOTODETECTORS
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Date
2020-01-20
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Department
Computer Science and Electrical Engineering
Program
Engineering, Electrical
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Distribution Rights granted to UMBC by the author.
This item may be protected under Title 17 of the U.S. Copyright Law. It is made available by UMBC for non-commercial research and education. For permission to publish or reproduce, please see http://aok.lib.umbc.edu/specoll/repro.php or contact Special Collections at speccoll(at)umbc.edu
This item may be protected under Title 17 of the U.S. Copyright Law. It is made available by UMBC for non-commercial research and education. For permission to publish or reproduce, please see http://aok.lib.umbc.edu/specoll/repro.php or contact Special Collections at speccoll(at)umbc.edu
Abstract
High-power photodetectors are important devices in the analog optical links that are present in many RF-photonic systems. The wide bandwidth of RF-photonic links, along with their immunity to electromagnetic interference, and their decreased size, weight, and power requirements compared to other microwave systems make them an appropriate choice in a variety of applications. Applications include antenna remoting and radio-over-fiber, beamforming in phased-array radars, and optical signal processing of microwave signals. Phase noise in photodetectors is a critical limiting factor in many RF-photonic applications, particularly metrology applications in which phase noise limits the extent to which the inherently low noise of an optical comb can be transferred to microwaves.
Bleaching or absorption saturation can occur in a high-current photodetector when the high density of photogenerated carriers either saturate the number of available final energy states or depopulate the initial states.
Additionally, the high density of photogenerated electrons can increase the possibility that they are recaptured.
Bleaching leads to a reduction in the photodetector's responsivity as the peak intensity and hence the average power increases.
Bleaching can, in turn, lead to nonlinear distortion of an incoming RF-photonic signal and limit the performance of photonic analog-to-digital converters (PADCs).
In this dissertations, we first describe prior work on a one-dimensional (1-D) drift-diffusion model that we used to study phase noise in a modified uni-traveling carrier (MUTC) photodetector and the bleaching effect in p-i-n and MUTC photodetectors. We then 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 an 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. Finally, we calculate the phase noise of an MUTC photodetector.
Applying our approach, we found good agreement between our results, the Monte Carlo simulation results, and experimental results. We showed that phase noise is minimized by having a fast photocurrent response with a tail that is as small as possible. Our approach is much faster computationally than Monte Carlo simulations, making it possible to carry out a broad parameter study to optimize the device performance. We propose a new optimized structure with lower phase noise and reduced nonlinearity.
We next study the impact of photodetector nonlinearity on RF-modulated frequency combs. Frequency combs can be used in RF-photonic systems to disambiguate radar signals and to increase the threshold for Brillouin scattering in optical fiber links. In addition to the sources of nonlinearity that are present when detecting continuous wave (CW) signals, the high peak power of optical frequency combs can bleach the photodetectors and contribute to the nonlinear distortion of the RF signal.
We developed an empirical model of bleaching, which we added to the 1-D drift-diffusion model that we previously developed. We determined the parameters of this model by comparison with experimental results in both pulsed and CW modes in a p-i-n photodetector and in the pulsed mode in an MUTC photodetector.
We calculated the impact of the bleaching on device nonlinearity as a function of average optical power. We used the three-tone modulation technique to calculate the second- and third-order intermodulation distortions (IMD2 and IMD3) in the pulsed mode. We calculated the second- and third-order output intercept points (OIP2 and OIP3) to characterize IMD2 and IMD3. The output of modulated optical pulse trains in the photodetector corresponds to a set of frequency comb lines in the frequency domain. With a CW input, there is a single IMD2 and IMD3 and a single OIP2 and OIP3.
By contrast, with an optical frequency comb input, there is a different IMD2n, IMD3n, OIP2n, and OIP3n associated with each comb line n.
We determined the behavior of IMD2n, IMD3n, OIP2n, and OIP3n as a function of comb line frequency (f=nf_r, where n is the comb line number and f_r is the repetition frequency) both with and without bleaching to determine the impact of bleaching on nonlinear distortion products in the \pin~and MUTC photodetectors. We found that when bleaching is included, OIP2n and OIP3n are higher in the p-i-n photodetector than the MUTC photodetector, and the difference between them increases as the comb line frequency increases. We calculated the distortion-to-signal strength ratios rho2n=IMD2n/Sin and rho3n=IMD3n/Sin, where Sin is the fundamental power as a function of comb line frequency with and without bleaching. We found that these ratios increase as the comb line number increases, which means that the impact of nonlinearity becomes larger as the comb line number increases. We showed that the impact of bleaching on the ratios rho2n and rho3n is complex and not always detrimental.
In the MUTC photodetector, when $n\lesssim100$ ($\lesssim5$ GHz), we found that the ratio is higher with bleaching.
On the other hand, when $n\gtrsim100$ ($\gtrsim5$ GHz), the ratio is lower with bleaching, so that bleaching actually improves this ratio.
We found nonlinear distortion is greater for the p-i-n photodetector than it is for the MUTC photodetector at low comb line frequencies and the opposite is true at high comb line frequencies.