Abstract: In measurement method of dispersion characteristics of an optical fiber, a laser for measurement connected to one end of the fiber and variable in output light wavelength and an oscillation for modulating a measurement light signal sent out from the laser are provided. The modulation frequency of the output from the oscillator is used as an reference electric signal. A measuring light signal obtained through the modulator 8 and transmitted through the fiber is converted into an measuring electric signal. And the characteristics is measured from the phase difference between the reference electric signal and the measuring electric signal, wherein an external modulator, an optical divider and a monitoring section are provided. The modulator modulates the measuring light signal sent out from the laser and having a narrow spectral linewidth. The divider divides the output light signal from the modulator, at a prescribed ratio.

Abstract: In a controller for a semiconductor laser light source, a portion of light emitted from the source is fed back to regulate the spectral line width of the source and to regulate the oscillation frequency of the source in terms of the phase of the portion or the driving current for the source, the very small portion of the light is allowed to pass through an external resonator so as to be subjected to optical frequency discrimination by an optical frequency discriminator, the portion does not need to be divided from a communication signal light. Therefore, the communication signal light can be undergo a loss.