Precise Laser Frequency Measurement of Weak Power With Submegahertz Linewidth

We present an optical phase-locked loop (OPLL)-enabled frequency measurement for a stabilized laser with weak power and submegahertz linewidth. For frequency converting laser, the method uses an optical frequency comb (OFC) measuring the seed beam instability to bypass the mismatch problem between the primary laser and the OFC output wavelength range. The method utilizes a phase-locked secondary laser transferring the stability of the primary laser to the OFC, which solves the shortage power problem. The phase noise power spectra of the OPLL is characterized, and the residual phase noise variance 0.07 rad2 between two submegahertz linewidth lasers is achieved. Validation of the method is performed by comparing the instability of the OPLL and the OFC measurement.