Broadband Noise Limit in the Photodetection of Ultralow Jitter Optical Pulses

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.

Figure 1

Schematic diagram of photonic microwave generation via the detection of ultralow jitter optical pulses. In the lower right, the black noise sidebands represent the noise from the optical pulse train, and the blue line is the total noise after photodetection.

Figure 2

Simplified schematic illustration of photocarrier transport and resulting photocurrent from a reverse-biased photodiode. Both the randomness in the timing of the photoelectron events and the shape of the elementary impulses give rise to noise in the photocurrent.

Figure 3

(a) $|H(f){|}^2$ for a few representative current impulses for 16 V bias, $I_{\text{avg}}$ of 15 mA, and pulse width of 15 ps. (b) Calculated $F_H$ at average current 15 mA and pulse width of 15 ps.

Figure 4

Average photocurrent and optical pulse width dependence of $F_H$ at 10 GHz.

Figure 5

Simulated single sideband phase noise level at three selected pulse widths. Dashed lines are the corresponding experimental results from Ref. [10].

Figure 6

Phase noise deviation from the long pulse limit. Red dash curve is the analytical calculation from Ref. [11]. Closed circle symbols are the experimental measurement from Ref. [10] with error bars. Open triangle symbols are Monte Carlo simulation of shot noise only. Closed triangle symbols are Monte Carlo simulation of shot noise and distributed absorption only. Open circle symbols are Monte Carlo simulation of shot noise, photon absorption, and carrier scattering.