High-precision, wavelength flexible frequency division for metrology

We realize and investigate a wavelength-flexible phase-coherent all-optical frequency-by-2 divider. Frequency division is obtained via self-phase locking in a degenerate continuous-wave (cw) optical parametric oscillator (OPO). The wavelength flexibility of the divider is based on the use of quasiphase matching (QPM) with perpendicular polarizations of the OPO output waves (type II). Mutual injection of the subharmonic waves is achieved by using an intracavity quarter-wave plate. A locking range of up to 160 MHz is observed experimentally, and a stable, self-phase-locked operation of the OPO is achieved over typically 15 minutes. For the first time, we measure the frequency stability of the divider by recording the relative phases of the subharmonic waves as a function of time. For a measurement time interval of 40 s, we measure a residual frequency instability of the divider of 8 · 10−18. We demonstrate a full control of the OPO’s output waves’ phase difference and observe the related change in power ratio of the subharmonic waves in agreement with the theoretically expected behavior. We propose that this possibility to monitor the divider’s drift within the locking range via the power ratio can be used for stabilizing that drift in order to achieve a significant improvement of the long-term stability of the frequency-by-2 divider.