Takemi Hasegawa

Measurement of Brillouin gain spectrum distribution along an optical fiber by direct frequency modulation of a laser diode

A novel scheme for measuring Brillouin gain spectrum distribution along an optical fiber is developed. This scheme utilizes direct frequency modulation of a laser diode for two purposes: generation of the pump and probe lightwaves, and position-selective excitation of stimulated Brillouin scattering. A laser diode is modulated with a rectangular-wave signal, so that the pump and probe lightwaves for Brillouin gain spectrum measurement are generated in time-division manner. A sinusoidal-wave signal is mixed with the modulation signal for spreading the spectra of the pump and probe lightwaves. Because of the spread spectra, stimulated Brillouin scattering occurs in a small section of the fiber where the two lightwaves are highly correlated. The Brillouin gain spectrum at the section is obtained selectively. The section to be measured is chosen by varying the period of the sinusoidal modulation. Experimental result of measurement of the Brillouin spectrum distribution with a spatial resolution of 70 cm is presented. As a comparison, another system is demonstrated, where the probe lightwave is generated by intensity modulation with LN modulator. Spatial resolution of about 6.5 cm is achieved.

Measurement of Brillouin gain spectrum of an optical fiber using direct frequency modulation of a laser diode

Summary form only given. Brillouin gain spectrum measurement for optical fibers offers an efficient means of distributed strain or temperature sensing. Although there are several sensing schemes of good performance, they appear expensive for commercial application. What makes them expensive is the requirement for narrow-linewidth pump and probe lightwaves with a precisely controlled frequency difference. Typically, a frequency difference of 10 GHz with a precision of 1 MHz is required. We present a novel simple scheme for measuring the Brillouin gain spectrum of optical fibers. In the scheme, we generate the pump and probe lightwaves using direct modulation of a laser diode. This simple scheme would enable a cost-effective sensing system.