Phase Locking of a Tunable OEO to an Optical Frequency Comb for Microwave Synthesis from an Optical Reference

Microwave synthesis, from DC to 100 GHz, from an optical reference will greatly aid a wide range of applications. Higher spectral purity signals can allow longer coherence times in very long baseline interferometry (VLBI) for radioastronomy and the realization of an optical timescale. Here, we present the synthesis of ultra-high stability tunable microwave signals via phase locking of an OEO to a fiber comb using the intermodal RF comb generated by the fiber comb as a stepped precision microwave reference. This scheme allows the tunable OEO to be referenced to an optical signal, such as an optical clock. The experimental setup is shown in Figure 1(a). The tunable OEO, extensively described in ref [3], is mixed with the microwave harmonics of a photodetected 80 MHz optical pulse train generated with a femtosecond oscillator. The combined RF spectrum is shown in Figure 1(b) where the OEO scans along the harmonics of the femtosecond oscillator. An intermediate frequency, carrying the differential phase noise between the OEO and the microwave harmonic is detected (see Figure 1(c)). This intermediate frequency can be phase locked to any frequency between DC and 40 MHz (frep/2) by driving DDS1 with a user-defined divisor. We keep the repetition rate under 100 MHz to not use DDS1 at high frequency to not limit the synthesis (as opposed to ref [4]).