Kiyokazu Hagiwara

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.

Holographic Fourier diffraction gratings with a high diffraction efficiency optimized for optical communication systems

Novel diffraction gratings in a near-infrared region that is specially optimized for optical communication systems are studied and demonstrated by using a simple holographic exposure process. These gratings have a unique groove profile. The gratings of 770 lines/mm have a very high efficiency of 95% at a wavelength of 1.31 microm and more than 85% at a wavelength of 1.52 microm for nonpolarized light in the Littrow mounting. They also show a very low dependence (less than 2%) of the efficiency on the polarization.

Narrow-band ten-channel optical multiplexer and demultiplexer using a Fourier diffraction grating

A narrow-band ten-channel optical multiplexer and demultiplexer for densely spaced wavelength division multiplexing in multimode fiber communication systems are discussed. These devices consist of a Fourier diffraction grating having very high diffraction efficiency (85%), a telecentric lens system with low aberration, and a unique fiber array. The devices had a very narrow channel spacing of 1.9 nm and low dependence on polarizations in the range from 831.6 to 848.5 nm. The multiplexer had a bandwidth of 0.7 nm and the demultiplexer had a bandwidth of 1.1 nm.