With the sub-carrier (SC) laser source under repair, we are using an auxiliary beam to align the SC path on OPO; this is obtained by bypassing the PLL fiber setup, see first attached picture: the pick-off from main SQZ laser is injected in fiber coupler at port A and the fiber is connected to the SC collimator at port B.

The SC beam is injecte in the external Faraday Isolator and overlapped to the bright alignment beam (BAB). The overlap was optimized by slightly mistuning the Faraday Isolator with the internal motorized waveplate, and injecting about 10% of the BAB power into the optical fiber at port B, then maximizing the coupled power at port A. So far we reached a coupling efficiency of about 60% from port B to port A by simply tuning the steering mirrors on the SC path. Better coupling might be achieved by fine  axial tuning of the SC telescope lenses.

As the auxiliary beam is a pick-off of the main SQZ laser, it resonates in the OPO together with the BAB; thus the SC beam can not be separated from the BAB either in transmission and in reflection from the OPA. The second picture shows the intensity at the Faraday isolator output port C with the scanning OPA. On resonance the interference of BAB and SC produces a dispersion-like shape due to the nonlinear phase shift. With the OPA locked, the interference of BAB and reflected SC as measured at FI output port C provides a reliable measurement of the relative mode matching. With about 60% of coupling efficiency in the optical fiber, we observed a fringe contrast around 83%. The contrast can be further improved by better balancing the optical powers and then by tuning the refractive telescope to optimize the mode matching of SC into OPA.