Microwave noise downconversion in interband cascade laser frequency combs

TL;DR

This paper introduces a simplified method to assess the coherence of chip-scale semiconductor laser frequency combs by analyzing MHz-range noise, bypassing the need for high-speed photodetectors.

Contribution

The work demonstrates a novel approach linking low-frequency noise to microwave beat notes, enabling easier coherence measurement of interband cascade lasers.

Findings

Microwave noise is downconverted to near-DC frequencies intrinsically within the laser.

Correlation analysis in frequency and time domains reveals the noise origin.

The method offers a new way to characterize frequency combs in wavelength regions with limited photodetectors.

Abstract

Chip-scale semiconductor laser frequency combs offer remarkable prospects for compact and power-efficient optical sensors. For the laser to be suitable for typical comb applications, its degree of coherence must first be assessed from a microwave self-mixing signal. Unfortunately, such measurements require scarcely available high-speed photodetectors with multi-GHz bandwidths and radio-frequency electronics. However, in this work, we demonstrate a simplified approach to comb coherence assessment for interband cascade lasers based on a relationship between easily-accessible MHz-frequency (baseband) noise and the multi-GHz-frequency intermode beat note. The downconversion of microwave noise to near-DC frequencies is found to originate intrinsically from the laser, which simultaneously acts as a frequency mixer due to electrical nonlinearities and a phase-to-amplitude noise converter due to the linewidth enhancement factor. Correlation between the electrical signals is explored in both frequency and time domains. Since this phenomenon is potentially universal in semiconductor lasers, it creates a new opportunity for frequency comb characterization, which may be particularly valuable in wavelength regions where fast photodetectors have limited availability.