Abstract. A time-division Brillouin optical correlation domain analysis system was successfully achieved using simplified laser diode (LD) modulation and pump lightwave optimization. A complicated transfer function for a precise output waveform of a LD was required for the conventional system. However, a very simple modulation function gave a power output very close to a required ideal rectangle waveform without sacrificing optical output spectrum. An electrical input waveform applied into a gate in the pump lightwave path was also optimized for eliminating a probe lightwave included in a pump lightwave and for passing consecutive pump pulses alternatively. So the stimulated Brillouin scattering gain was attained without seriously distorting FM modulation, and the targeted spatial resolution was clearly accomplished. Additionally, using high speed response of a semiconductor optical amplifier (SOA), unlike an erbium-doped fiber amplifier (EDFA), the possibility was investigated that an SOA was going to replace an EDFA and a modulator used as a gate in the same time.
IntroductionOptical fiber sensors using Brillouin scattering have become effective tools for distributed measurement sensing strain or temperature in construction materials and structures. Brillouin scattering is a three-wave interaction between an incident photon, a backscattered photon, and an acoustic phonon. Pump and probe lightwaves propagating reversely to each other in an optical fiber interfere and excite the acoustic wave, and the refractive index grating caused by the acoustic wave couples the two lightwaves. This coupling gives rise to optical power transfer from the pump to the probe. The probe lightwave experiences gain through the stimulated Brillouin scattering (SBS) process when the optical frequency difference Δν between the probe and the pump is tuned to around the Brillouin frequency shift (ν B ) of the fiber.1,2 The Brillouin gain spectrum (BGS) is narrow (30 MHz) and can be recovered by sweeping the frequency difference between the two lightwaves.3 The center frequency of BGS is shifted in proportion to longitudinal strain or temperature applied to it.A pulse lightwave was first used for the wave interaction and the BGS was measured as a function of time. It is called Brillouin optical fiber time domain analysis (BOTDA). This technology made it possible to measure longer than a 10-km optical fiber. 4,5 Due to the finite time required for the acoustic wave to be excited by the interaction of pulse pump and continuous probe, however, the spatial resolution of the conventional BOTDA systems is known to be limited to ∼1 m. Small amounts of backscattering induced by a pulsed lightwave need a large number of light pulses to obtain a sufficient signal-to-noise ratio. So a long time is required to obtain the distribution information, and it has difficulty