Laser Phase Noise Characterization Using Parallel Linear Optical Sampling

We developed a laser phase noise characterization technique by using linear optical sampling. Thanks to the broad bandwidth nature of the optical sampling technology, the proposed technique provides the FM noise spectrum and phase error variance of a laser by characterizing the beat note between the laser under test and a reference laser, even when the wavelengths of the signal and reference beams differ considerably. Requirement of wavelengths of the two lasers is to overlap a spectrum of sampling pulse. This broad bandwidth nature eliminates the need to choose lasers with similar wavelengths to observe their beat note. We characterized the phase noise characteristics of different lasers and confirmed the validity of this technique by comparing reference measurement results. We also found that the measurement error with the technique is mainly due to the shot noise. We show that appropriate spectral filtering of the sampling pulse is effective in reducing this measurement error.