Broadband Optoelectronic Mixer for Terahertz Frequency-Comb Measurements

TL;DR

This paper introduces a broadband optoelectronic mixer using a high-speed photodiode for phase-coherent detection of terahertz frequency combs, enabling precise measurements across microwave to THz frequencies.

Contribution

The work demonstrates harmonic generation and down-mixing of frequency combs with different repetition rates using nonlinear effects in a modified uni-traveling carrier photodiode, achieving phase-coherent detection up to 500 GHz.

Findings

Achieved phase-coherent detection of THz frequency combs.

Optimized photodiode bias for minimal conversion loss.

Demonstrated stable measurements with low Allan deviation.

Abstract

We demonstrate ultra-broadband optoelectronic mixing of frequency combs that provides phase-coherent detection of a repetition frequency up to 500 GHz, using a high-speed modified uni-traveling carrier (MUTC) photodiode. Nonlinear photo-electron effects in the photodiode itself enable harmonic generation and down-mixing process of combs with widely different repetition frequency. Specifically, we generate two, 25 GHz frequency combs and use an optical filter to explore coherent down-mixing to baseband of comb spectral components across microwave, millimeter wave, and terahertz (THz) frequencies. The exceptional noise performance of the optoelectronic mixer enables the phase-coherent measurement of millimeter-wave and THz frequency combs with an Allan deviation of 10^-13/t for a measurement time of t. We further investigate the dependence of conversion loss on the reverse bias voltage and photocurrent. The experimental results indicate that we can minimize the conversion loss by operating the photodiode at an optimal voltage and maximum available photocurrent. Our work provides a solution for millimeter-wave and THz frequency comb measurements and facilitates fully stabilized frequency combs with microresonators.