High spatial resolution fiber-optic distributed lateral-stress sensing by stepwise frequency modulation of a super structure grating distributed Bragg reflector laser diode

We have reported recently a distributed lateral-stress sensor based on the polarization mode coupling in polarization-maintaining fiber (PMF), in which the frequency of a super structure grating distributed Bragg reflector laser diode (SSG-DBR-LD) source was modulated stepwise to synthesize the optical coherence function into a narrow peak for spatially resolved detection. Its spatial resolution, however, was limited to 2.5 m due to the instability of the lasing frequency (J. Lightw. Technol., vol. 20, p. 1715, 2002). In this paper, the effect of the frequency instability on the synthesized coherence function is studied per simulation. By improving the driving electronics and the frequency calibration of the SSG-DBR-LD, a stable stepwise frequency modulation over a range of 5 THz is obtained, which enables the synthesis of a coherence peak as narrow as 24 μm in the experiment. The spatial resolution of the distributed stress sensor is thus improved to 20 cm. This technique can be used for smart materials and structures.