4.3        New OLCR set-ups

4.3.6       Discussion of different OLCR designs

The first aspects concern both designs. The laser wavelength is used as phase reference and thus an acurate knowledge of the wavelength and its stability is essential. The tunable laser used in the time multiplexing set-up guaranties a wavelength stability better than 1 pm and the DFB laser used in the wavelength multiplexing set-up is temperature stabilized to ensure the same stability. A wavemeter has been used in parallel to track the exact wavelength position and eventual drifts (laser wavelength drifts smaller than 0.1 pm were observed). The thermal stability of the test FBG is fundamental as the measurement time goes from several minutes to several hours. Temperature changes in the test FBG modify its spectral properties (in first approximation a frequency shift) that are seen in the OLCR measurement by a phase difference slope modifications due to a different local Bragg condition.

The time multiplexing design shows several advantages over the wavelength multiplexing scheme :

-         The laser reflective reference is the FBG itself and thus cancels completely the laser phase noise problem encountered with the cleaved fiber end as the reference

-         The phase difference is ranged in tens of radians compared to the millions of radians range of the phase itself for the same scan distance (e.g. a few centimeters)

-         The sampling is not limited by the Nyquist condition and this reduces considerably the measurement time (several minutes instead of several hours) and the amount of data (factor 10 to 100)

-         The single wavelength operation divides by two the number of optical and electronic components (adding on the other hand an optical switch) and guaranties the perfect 2p modulation for both OLCR and laser phase signals

The wavelength multiplexing exhibits nevertheless interesting features :

-         The wavelength symmetry enables measurements of FBG in the 1310 and 1550 nm range by simple source exchange

-         For a dynamic measurement configuration (§4.3.2), the laser signal can be used directly as reference signal for the lock-in amplifier and then the phase difference is directly extracted without any other operation (accurate constant mirror velocity is required)

-         The time delay between the laser and OLCR phase measurement is very small and can either be totally suppressed if the voltage difference of both lock-in is measured, reducing the marginal phase drifts well bellow the p/100 encountered for the time multiplexing design

We want now to discuss the minimal sampling distance required for FBGs OLCR measurements when the OLCR phase is obtained from the difference with the laser reference phase. The phase difference change Df for two different wavelength l₁ and l₂ over the same distance d is given by

where Dl  = l₂-l₁. The following table gives the minimal distance d_2p for which a complete 2p phase change is obtained for l₁ = 1310 nm.

When measuring the phase difference between the laser and OLCR signals, the sampling has to be half the minimal distance d_2p to fulfill the Nyquist criteria. We observe that the wavelength multiplexing design could also be used in the difference phase mode but with smaller sampling intervals and an independent distance measurement. In this discussion, we have neglected the phase changes introduced by the test FBG itself. For homogeneous FBGs, additional p shifts in the impulse response phase are observed due to global reflections at the grating interfaces and they are spread in the OLCR measurement over the broadband light source coherence length by convolution. For chirped grating, the Bragg condition change is usually smaller than 10 to 20 nm and then the corresponding d_2p remains under 100 mm. The typical OPLD sampling distance (that is twice the incremental change of the mirror position) used in time multiplexing design was 20 mm, corresponding to 10 mm mirror step. Several experiments have also been conducted with 1 mm mirror steps for precise OLCR measurements and 100 mm for fast measurements.