An effective nonreciprocal circuit for semiconductor laser-to-optical-fiber coupling using a YIG sphere

A practical nonreciprocal optical circuit for laser-diode (LD)-to-optical-fiber coupling in the 1.3-μm wavelength region is described. It consists of a yttrium iron garnet- (YIG) sphere graded-index (GRIN) rod lens and a polarizer. The YIG sphere can function not only as a Faraday rotator but also as an effective coupling lens. High coupling efficiency of more than -5 dB for a single-mode fiber, and more than -2 dB for a multimode fiber, is easily achievable. Alignment sensitivities and coupling characteristics of the proposed circuit are also discussed theoretically and experimentally. The increase in LD relative intensity noise (RIN), caused by light injected into the LD, is estimated using the reciprocal characteristics of LD-to-single-mode-fiber coupling. By comparing the LD-RIN increase in the proposed non-reciprocal circuit with that of the reciprocal, high isolation of about 32 dB is confirmed. The fact that reflected light from the proposed circuit has little influence on LD characteristics is also clarified.