In a refractive index coupling distributed semiconductor laser having a .LAMBDA./2-phase-shift distributed feedback structure with a diffraction grating having a refractive index coupling property on an active layer, when viewed in a light distributed feedback direction, a value of (duty of a high refractive index portion)/(duty of a low refractive index portion) of a diffraction grating in a rear end face region is larger than that of a diffraction grating in a front end face region. In this manner, a coupling coefficient .kappa.2 in a front end face region of a conventional semiconductor laser is smaller than a coupling coefficient .kappa.1 in a rear end face region and is larger than 100 cm.sup.-1.

 
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> High power gas discharge laser with helium purged line narrowing unit

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