4.3        New OLCR set-ups

4.3.5       Wavelength multiplexing OLCR design

The wavelength multiplexing OLCR design is presented in Fig. 4-11.

Fig. 4-11 Wavelength multiplexing OLCR set-up : wavelength division multiplexer (WDM), cleaved fiber end (PC)

The low coherence light source around 1318 nm and the reference laser around 1550 nm are launched together in the reference and test arms. The FBG (in the 1300 nm range) has very low reflectivity at the laser wavelength, and then another laser reference point in the test arm is required and a cleaved fiber end behind the grating is used for this purpose. The distance between the FBG and the PC has to be as small as possible to limit the laser phase noise (i.e. the interference of lights emitted at different time). The laser wavelength is calibrated with a wavemeter to improve the distance accuracy (relative distance uncertainty less than 10^-6).

Fig. 4-12 Measurement chronology for the wavelength multiplexing OLCR set-up

The cross talk between the low coherence and reference laser signals is reduced by using 1310/1550 nm wavelength division multiplexers with isolation greater than 45 dB. The remaining cross talk disappears with the different levels used in the balanced detection of both signals. Spectral variation of the laser and broadband sources are kept below 2 pm with temperature control. Two dual-phase lock-in amplifiers extract amplitude (I_oclr, I_laser) and phase (j_olcr, j_laser) of the OLCR and laser signals. I_x and j_x are measured at each mirror (MIR) position. The perfect symmetry of the set-up allows measuring FBG's at 1550 nm with a matching broadband source and a laser source at 1310 nm. The OPLD is sampled in order to fulfill the Nyquist criteria for the reference laser phase. The absolute distance without phase drifts for the OLCR phase is then calculated (i.e. OPLD = l_laser×j_laser/2p). From the absolute distance, a linear resampling is applied to the OLCR amplitude and phase.

The measurement chronology is presented in Fig. 4-12. A small time delay of 4 ms is observed between both phase measurements.