4.3        New OLCR set-ups

4.3.1        Time multiplexing OLCR design

Fig. 4-6 presents the OLCR scheme with the time multiplexing of the broad-band source and the reference laser. The signals under investigation are the OLCR amplitude and the phase difference between the OLCR signal and the reference laser phase.

Fig. 4-6 Time multiplexing OLCR set-up : low coherence light source (SLD), tunable laser (TL), optical switch (OpS), circulator (C), coupler (CPL), piezoelectric plate (PZT), mirror (MIR), translation stage (TS), polarization controller (POLA), test FBG (FBG), fiber end in index matching fluid (IMF), attenuator (A), detectors (D), voltage difference module (VD) and lock-in amplifier (L-I)

The major feature is a time multiplexing by the optical switch of the low coherent light source and of the laser source operating at the same wavelength. The SLD is centered at 1318 nm and has a bandwidth of 40 nm FWHM corresponding to a coherence length L_C = 12.8 mm in the fiber (single mode telecom fiber). The laser source is tunable and its wavelength is set to the Bragg wavelength l_B = 2n_effL, where n_eff is the effective reflective index and L the FBG period. The 3 dB coupler splits the light to equally illuminate the reference and test arms. The reference arm includes a mirror (MIR) placed on a 25 cm translation stage used to scan the OPLD. The phase of the reference signal is ramp modulated by a piezoelectric plate over the OPLD of two fringes at a frequency of f = 178 Hz. The lens couples the light beam from fiber to free space and back to the fiber. The test arm contains the FBG under test and a polarization controller that optimizes the interference pattern. The fiber end of the test arm is placed in an index matching fluid to avoid unwanted light reflections. A balanced detection scheme is used, including a coupler, a circulator, an attenuator, two detectors and a voltage difference module. For a given mirror position, the interfering part of the total intensity (OLCR signal) is a 2f-sinus and the constant part of the total intensity signal is cancelled by the balanced detection. The dual-phase lock-in amplifier extracts the amplitude and the phase information directly for the OLCR and the laser signals. The time interval between the laser and the OLCR phase measurements is 54 ms thus limiting phase drifts to below p/100. The measurement chronology for each OPLD is presented in Fig. 4-7. OPLD discretization from 1 to 200 mm have been used depending on the required resolution.

Fig. 4-7 Measurement chronology for the time multiplexing OLCR set-up