I don’t think the polarization combiner is the right tool for the job here. I am not familiar with OOK coherent detection techniques, although it looks like the MZM you are using is also acting as a phase modulator, so it looks like some kind of PSK. Anyways try the 90 degree optical hybrid found in the Receivers Library/Optical Receivers.
I think this structure is generally used for coherent detection, if you are not familiar with the component I would direct you to one of the sample files in the Advanced Modulation folder to see how you could connect it up to photodetectors. For example the “50 Gbps Coherent QPSK_Trans_detail with DSP” in the Advanced modulation systems\PSK systems\QPSK folder. In this example they don’t actually use the 90 degree optical hybrid component, but they have explicitly made it out of cross couplers and phase shifts so you could just copy that as well.
Hope this helps!