**5.1 Tunable laser**

66 Modern Metrology Concerns

In order to demonstrate the capabilities of FSI based sensors, two implementations are described in this section, one for FSI and another for Dual FSI, both tailored for space applications (i.e. long distances, no atmosphere refraction errors are considered, specific

The first absolute distance sensor, based in FSI, was designed for a space mission comprising a multiple aperture optical telescope (coherent array), to measure the OPD between two sub-telescopes and the combining-telescope with an uncertainty at the 10 µm

The second sensor, based in Dual FSI, was defined in view of a coronagraph mission composed of two spacecraft in a free-flying formation. The requirements on the absolute distance metrology sensor prototype were a measurement uncertainty below the 100 µm

In both sensors, the Laser and the FP are the key items. Complex tasks normally implemented in real time electronics were assigned to data processing, thus moving system

A drift compensation model using consecutive measurements was implemented and validated experimentally. Fig. 5 shows the principle behind the compensation technique using two consecutive measurements with different signs on the sweep range (Δν) and equal durations, corresponding to a symmetrical triangular shaped frequency sweep

Fig. 5. Principle of the Drift compensation model using two consecutive measurements with different signs on the sweep range and equal durations corresponding to a symmetrical

**5. Experimental implementation of FSI based sensors** 

level for a distance around 50 m at a 10 Hz measurement rate.

complexity to the software area.

triangular shaped frequency sweep.

(Δ*t2* = Δ*t1* = Δ*t* and Δν*2* = -Δν*1* = Δν *r2* = *r1* = *r*).

operational requirements associated to measurement rate and accuracy).

level at a 10 Hz measurement rate with the lowest possible technical complexity.

FSI needs a tunable laser, the frequency of which changes continuously without mode hop. The tunable range should be able to cover several tens of GHz, and this range must be completely swept in a time frame not exceeding a tenth of second. Laser linewidth must be small in order to generate a coherence length of several tens of metre. It is also important to have a compact, robust and with low complexity laser.

Although several options exist to provide wavelength/frequency tuning, the best solution to cope with the requirements is an External Cavity Diode Laser (ECDL) (Amann & Buus, 1998). Traditional diode-laser technology provides high reliability, high electrical efficiency, and a wide range of available wavelengths. By using an external cavity built around a diode laser, it is possible to achieve single-mode operation with narrow linewidth and precise frequency tuning. The extended cavity leads to a spectral narrowing of the laser output to below 1 MHz. The coherence length can thus be in the order of a few hundred metres.
