Discussion on RIN improvement using a standard coupler

It is possible to increase the signal-to-noise ratio (SNR) of radio-frequency optical links by a factor of N by combining the light of N individual lasers. The individual relative intensity noises (RINs) are then averaged, and the equivalent noise is lowered. However in some cases, while using a standard 3-dB coupler for combining two lasers, some additional noise appears which limits the RIN (and, thus, the SNR) improvement. This noise is proved, in this letter, to originate from heterodyne beating between the peak of one laser and the sidemodes of the other laser. It is possible to suppress this additional noise by injecting the light beams in crossed polarization in the coupler. The experimental data found by doing so matches perfectly the expected behavior. A 3-dB RIN improvement can be achieved if the two lasers have the same output optical power and similar RIN.