External cavity semiconductor laser with a Fourier grating and an aspheric lens

To obtain a stable single longitudinal mode oscillation of an external cavity laser diode, we must achieve strong coupling between the laser diode and the external cavity. An external cavity laser with an AlGaAs/GaAs laser diode, a Fourier grating, and an aspheric glass lens has been designed and fabricated. The Fourier grating has high diffraction efficiency independent of polarization. The antireflection-coated aspheric lens decreased optical coupling losses because of its low aberration and high transmittance. This laser oscillated stably in a single longitudinal mode at ~ 0.8 microm, and the oscillation wavelength was tunable in the region of 45 nm with a side-mode suppression ratio of less than -35 dB. The coupling efficiency presented by the effective amplitude reflectivity of the external cavity was estimated to be 0.63. A linewidth of less than 100 kHz was obtained.