Narrow-linewidth laser diode with compact optical-feedback system

A compact narrow-linewidth semiconductor laser source is demonstrated. Spectral linewidth of a single-mode laser diode (LD) utilized as this source can be reduced significantly by using a compact optical negative feedback system, which requires only two optical components, namely a lens and an optical filter. Our numerical analysis predicts that the spectral linewidth of LDs can be reduced by 40 dB by the proposed compact and simple optical negative feedback system. A proof-of-concept experiment confirms that the proposed system reduces the linewidth of a distributed-feedback laser from 6.4 MHz to 6.5 kHz (i.e., a reduction ratio of 30 dB). The proposed light source can be formed as a compact integrated light source by integrating a waveguide-type optical filter with a LD.