Chemical sensor based on long-period fiber grating response to index of refraction

Abstract

high spatial resolution and wide dynamic range. However, in the FMCW system, highspeed optical frequency sweep is required for avoiding optical phase fluctuations in a long optical fiber. With 1 THz/sec frequency-sweep rate, 5 m spatial resolution have been achieved for measuring a reflection from 30-km-long optical fiber end.2 However, higher frequencysweep rate is necessary for the cm resolution. On the other hand, we have studied aphasemodulating optical coherence domain reflectometry by synthesis of coherence function (pOCDR).3'4 In the p-OCDR, the whole optical phase in the system should be stable only in a shorter time for obtaining the reflectivity at one position. Thus this system has less influences of optical phase fluctuations in the optical fiber. The system proposed for measuring reflectivity distribution at a long distance is shown in Fig. 1. The oscillation frequency ofthe 1.55 p m distributed-feedback laser diode (DFB-LD) is modulatedvia injection current in a wave form shown in Fig. l a for synthesis ofperiodic deltafunction-like coherence function. The frequency spacing is 11.8 MHz, the number of step-pairs 80, and the period of the step 0.05 p s . In this case, the coherence peaks with 5.3 cm width are periodically synthesized with the period of 8.5 m. While, the phase of the reference lightwave is synchronously modulated to be proportional to the shifted frequency, for example as shown in Fig. 1b. Such a phase modulation has the same role as changing the differential delay in this interferometer by the proportional coefficient in the phase modulat i ~ n . ~ , ~ Therefore the coherence peak can be scanned electrically by the phase modulation. Additionally, the acousto-optical (AO) switch after the isolator generates optical pulses so that only one coherence peak is located in the region under test by the pulse window, as shown in Fig. 1. The pulse window-width is set to be 8 m. We also select the flat region in each step of the frequency-modulation wave form of DFB-LD by this switching. Consequently the reflectivity distribution inside the pulse window can be measured with 5 km 20 m