Abstract

Conventional laser spectral analysis relies on direct measurement of power spectral density (PSD), which is not able to distinguish certain types of phase and frequency modulations without ambiguity. In this paper, we propose and demonstrate a modified scheme aimed at improving the specificity in laser frequency analysis. Our method is based on PSD measurement of the downshifted laser spectrum in the radio-frequency range but introduces an electronic frequency divider (EFD) prior to the spectrum analyzer. By monitoring the spectral changes caused by the EFD, we show, both theoretically and experimentally, that common scenarios of phase and frequency modulations, such as wideband frequency modulation (FM) and broadband phase modulation, can be differentiated without ambiguity. Moreover, the new method allows for quantitative assessment of wideband FM parameters such as modulation frequency, modulation index, and frequency deviation, which is not possible with any conventional spectral analysis methods. This technique may find potential applications in laser frequency metrology, microwave photonics, optical sensing, and radar/lidar technologies.

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