Impact of near-end residual reflectivity on the spectral performance of high-power pump lasers

Abnormal spectral characteristics are observed in high-power semiconductor pump laser modules with ultralow chip front-facet reflectivity levels. We have identified the root cause as destructive interference in an external-cavity reflection from an antireflection (AR) coated fiber tip. This suppressive external cavity imposes a practical limit on the lower desirable bound for front-facet AR coatings for high-power lasers and uncooled fiber Bragg grating stabilized semiconductor devices.