Optimization of fiber-laser coupling with respect to both the coupling efficiency and carrier-to-noise ratio: a novel design

Fiber-coupled laser diodes are indispensable for optical fiber communications. The design of laser-fiber coupling is often guided by maximizing the optical power coupled into fiber, and by relaxing the fiber-laser alignment tolerances. Most of them are susceptible to optical back-reflections from the fiber resulting in not only a reduced coupling efficiency but also an increased optical noise of the signal. The high bit rate transmission through optical fiber is degraded by reflected intensity noise (RIN) due to multiple reflections from refractive index discontinuities. An offset misaligned coupling scheme involving lensed fiber is proposed to reduce reflected intensity noise. The effect of such misalignment on back reflection is studied theoretically. The results show a significant improvement in the reflected intensity noise and carrier to noise ratio (CNR). Such an improvement of RIN and CNR is important for maintaining quality of laser and high-speed data transmission. The results enable one to optimize the design of any laser-fiber coupling with respect to CNR.