Abstract
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.
Highlights
In this paper, we study the impact of nonlinearity in a modified uni-traveling carrier (MUTC) photodetector (PD) on radio-frequency (RF) modulated frequency combs
The gap is larger for low comb line numbers
We have developed an empirical model of bleaching based on experimental data that were collected at the Naval Research Laboratory and we have incorporated this model into the 1-D drift-diffusion equations
Summary
We study the impact of nonlinearity in a modified uni-traveling carrier (MUTC) photodetector (PD) on radio-frequency (RF) modulated frequency combs. Frequency combs can be generated in the optical domain by creating a stream of short pulses, using for example a short pulse laser [9] or a continuous wave laser followed by an electro-optic modulator [10]. Regardless of its origin, bleaching leads to a reduction in the PD’s responsivity as the peak intensity and the average power increases This reduction in responsivity can lead to nonlinear distortion of an incoming RF-photonic signal. With a modulated CW input, the device nonlinearity can be characterized using the secondand third-order intermodulation distortion powers (IMD2 and IMD3 powers) and by the secondand third-order output intercept points (OIP2 and OIP3) This simple characterization is no longer possible when working with frequency combs since every electrical comb line is impacted by the nonlinearity in a different way.
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