The present invention relates to a distributed feedback type or distibuted Bragg reflection type semiconductor laser device which is equipped with a diffraction grating. The device performs laser oscillations at a wavelength of 660 to 890 nm in a single longitudinal mode.
When a semiconductor laser device is used as a light source in an optical information transmission system or an optical measurement system utilizing optical fibers, the laser device is required to have such operation characteristics which oscillate in a single longitudinal mode. Relative to the laser device structure suited for achieving 1 desired laser oscillation in a single longitudinal mode, a distributed feedback type device is known or distributed Bragg reflection type having a periodically corrugated diffraction grating in the active region or in the vicinity thereof.
FIG. 3 shows the structure of a conventional distributed feedback type semiconductor laser device, which comprises an n-InP substrate 10, an n-InP cladding layer (buffer layer) 20, a non-doped InGaPAs active layer 30, a p-InGaPAs optical guiding layer 40, a p-InP cladding layer 50 and a p-InGaPAs cap layer 60 formed in sequence. A p-side ohmic electrode 70 and an n-side ohmic electrode 80 are disposed on the cap layer 60 and the substrate 10 respectively, and a diffraction grating 401 for controlling the laser oscillation wavelength is formed on the upper surface of the optical guiding layer 40. This laser device employs an InGaPAs-InP semiconductor configuration with oscillation characteristics that effect laser oscillations at relatively long wavelengths of 1,300 nm. Meanwhile, a similar structure can also be applied to a semiconductor laser device for general use at shorter oscillaton wavelengths of 890 nm or less. Such a laser device comprises, as shown in FIG. 4, an n-GaAs substrate 100, an n-GaAlAs cladding layer 200, a non-doped GaAs or GaAlAs active layer 300, a p-GaAlAs optical guiding layer 400, a p-GAlAs cladding layer 500 and a p-GaAs cap layer 600 formed in sequence. In this structure, the GaAlAs cladding layer 500 is grown on a diffraction grating 401 which is formed on the GaAlAs optical guiding layer 400. However, since a crystal layer of GaAlAs or the like containing Al as a component is readily oxidized in the atmosphere to form an oxide film momentarily, regrowth of the crystal on GaAlAs is difficult to render. Consequently, with regard to laser devices operating at any oscillation wavelength shorter than 890 nm, complete requisite technology for the semiconductor lasers with a diffraction grating has not yet been practically accomplished in the present stage.
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