Broadband Noise Limit in the Photodetection of Ultralow Jitter Optical Pulses

Abstract

Applications with optical atomic clocks and precision timing often require the transfer of optical frequency references to the electrical domain with extremely high fidelity. Here we examine the impact of photocarrier scattering and distributed absorption on the photocurrent noise of high-speed photodiodes when detecting ultralow jitter optical pulses. Despite its small contribution to the total photocurrent, this excess noise can determine the phase noise and timing jitter of microwave signals generated by detecting ultrashort optical pulses. A Monte Carlo simulation of the photodetection process is used to quantitatively estimate the excess noise. Simulated phase noise on the 10 GHz harmonic of a photodetected pulse train shows good agreement with previous experimental data, leading to the conclusion that the lowest phase noise photonically generated microwave signals are limited by photocarrier scattering well above the quantum limit of the optical pulse train.

Photonic microwave signals with zeptosecond-level absolute timing noise

“…Although we do observe and confirm such a dependency, the measured white phase noise floor of the microwave signal obtained illuminating the photodiode with 800 fs pulses deviates from the theoretical prediction by more than 20 dB (-196 dBc.Hz -1 ). This significant discrepancy has been thought to originate from photocarrier scattering and absorption in the photodetector resulting in an increased timing jitter of the electrical pulse train33 .Phase noise characterization. The 12 GHz harmonics from the FOFC under test is selected by a narrow filter (10 MHz bandwidth) and split in two parts.…”

Excess Loss in Homodyne Detection Originating from Distributed Photocarrier Generation in Photodiodes

“…2 underestimates the excess loss, especially for InGaAs. A complete prediction of the excess loss can be given by the Monte-Carlo simulation of photocarrier generation and transportation [11]. The 1 mW input power is small enough to avoid the effect of saturation of the photodiodes.…”

“…Even in this limit, non-uniform impulse response H(ω; x) can result in residual of the vacuum term. Since the excess loss is caused by the variation of the transfer functions respective to the absorption positions, it is non-zero only above the frequency where the transit-time roll-off takes place [11]. Note that photodiodes with high quantum efficiency must have enough thickness, and the excess loss term inevitably appears because of the distributed absorption within the absorption distribution e −αx , which is characteristic to the photodiode material.…”

Optically referenced broadband electronic synthesizer with 15 digits of resolution

“…The optical photodetector in our experiments are based on a modified uni‐traveling carrier (MUTC) photodetector design that is optimized for high power handling, high speed, and high linearity . Upon photodetection the optical pulse train is converted to a train of stable electronic pulses that carry the timing and stability of the optical pulse train, but with some loss in dynamic range at higher offset frequencies . Pulse interleaving of the optical pulse train effectively multiplies the native pulse repetition rate from 1 GHz to 2 GHz, which alleviates nonlinearities in photodetection by reducing the energy per pulse .…”