To stabilize the luminous intensity of a diode laser and the fluctuation of frequency by using a Bragg type fiber diffraction grating.

The diode laser 26 is connected to a fiber 32 having a diffraction grating on the inside by employing an optical instrument device. The diffraction grating 34 is formed in the induction mode region of the optical fiber. The wavelength of the maximum reflectivity of the diffraction grating is selected so as to approximate the maximum value of the gain band of the diode laser 26. The magnitude and band width of the reflectivity of the diffraction grating 34 does not so much lower optical output power from the front end of the fiber, and stabilizes an output from the diode laser 26. The selection of the band width of the optical spectrum of the diode laser 26 is determined by a distance from the diode laser 26 of the diffraction grating 34.