Rf Performance Of Optical Injection Locking

Coherent photonic systems promise novel functionality and/or improved performance compared to direct detection photonic systems, but have the disadvantage of being sensitive to optical phase noise. The most common approach to this problem is to force one laser to track the phase of the other with a phase locked loop (PU), so that the phase noise of the lasers cancels out of the RF heterodyne beat note. Although the PLL approach has been implemented for semiconductor lasers, the large linewidth of these lasers and the resulting large PLL loop bandwidth severely constrain the design and limit performance. This disadvantage of the PLL approach is particularly relevant for many applications, since semiconductor lasers are preferred for system insertion.An alternative approach for establishing coherence between two lasers is optical injection locking. Standard theory indicates that injection locking can act in the same way as a first order PLL with a bandwidth as high as several GHz, which is large enough to achieve state of the art noise levels (e.g. -130 dBc/Hz @ 1 MHz offset) with semiconductor lasers. We present phase noise measurements on the beat note of two injection locked semiconductor lasers. Our results (phase noise @ 1 MHz offset as low as -125 dBc/Hz) indicate that state of the art phase noise performance from injection locked lasers should be obtainable in practice. We found that it is necessary to length match the two paths in the experiment (master -> detector and master -> slave -> detector) to avoid excess noise due to delay decorrelation of the master, and also that environmental noise seems dominant at offsets < 20 kHz.