Fiber grating laser: a performance study on coupling efficiency of fiber microlens and the Bragg reflectivity

Abstract The λ=1.55 μ m fiber grating lasers (FGLs), comprised of an uncoated tapered hemispherical-end fiber (THEF) microlens for strong coupling the fiber grating external cavity and the HR/AR-coated diode laser, have been investigated in four different combinations of coupling efficiency ( η ) and Bragg reflectivity ( R g ). The FGL of higher η =72% and R g =0.52 has a stronger resonant feedback as the spectral output shows a single longitudinal mode (SMSR>45 dB), high output power (>5 mW), and lower threshold current. However, for the case of R g =0.35 and η =68%, the FGL exhibits more stable SMSR against pumping current and temperature. We have also performed numerical simulation on the SMSR at different η and R g for the FGLs. The calculated SMSR shows an excellent agreement with the measured data for the FGLs of higher η .