|
Thus, for the needs of PEGASE mission – a possible DARWIN in flight demonstration- SAGEIS-CSO has been asked by CNES to design a fine longitudinal sensor able to work at 120 K while performing displacement measurements at a working distance range of 25 to 250 m. Its required performances are a resolution and a precision of 25 nm. This activity succeeds to the MOUSE II system development, which has demonstrated the ability to obtain the required laser metrology using a frequency stabilised laser, a compact and totally passive Michelson type sensor head plus a detection unit for data processing. Optical signals are routed using fibres, allowing the sensor head to be alone in a cryogenic environment. Now, the goal is to obtain a validated prototype at a MQ level by the end of 2007. For that, the laser source will be an update of the flight models made for IASI, using a more powerful DFB diode, pin-to-pin compatible with the previous design, and then giving minor changes. The current regulation was optimized in order not to degrade the narrow diode spectral width. The opto-thermo-mechanical design of the sensor head, in collaboration with AAS, is also under progress, and constitutes the major evolution of the MOUSE II. |
1PRESENTATION OF THE SYSTEMThe principle of the longitudinal measurement is based on the use of an interferometric system. A Michelson interferometer splits an incoming coherent light into two arms: a measuring arm, going output to the target and reflected back, and a reference arm, staying inside the sensor head. So, the overall system is composed of three major units: a laser source, a sensor head including the Michelson, and a detection unit for data processing. That is the minimal configuration, able to do relative optical path difference (OPD) measurements. By adding a second laser source, associated to a wavelength-demultiplexing element or by using a second optical head, it is then possible to do absolute distance metrology [2], [4]. Such a system, called “MOUSE II” [3], but not designed for 120 K, is now operating for displacement measurements at a working distance in the 25 – 250 m range (see Fig. 1). The head contains optical parts only, meaning that only optical signals go to and come from it, routed by fibers. In this way, the sensor unit can be located in cryogenic environment whilst electronic units are in a thermally controlled area. And the use of a totally passive sensor allows not having an effect on the metrologic measurement. 2DESIGN OF THE NEW SENSOR HEAD2.1Optical designIn “MOUSE II”, the overall interferometric function is realized using a Fresnel prism for beams separation and recombination, π/2 phase shift between s and p polarizations of reference beam [3], [4]. A Foster prism is then used to separate polarizations of the return measuring beam and the reference one, in order to obtain two optical signals called SIN and COS, necessary to know the direction of the displacement and the value of the displacement by an arctangent calculation, more accurate method than calculating an arcsinus. The great advantage of using a Fresnel prism is to have all the major optical functions realized by a single element, which has also the advantage to equalize the BK7 path crossed by beams, making easier the sensor’s assembly and allowing having a compact and robust head. But this main component is composed of three rhombohedra, bonded together by optical glue. If these elements separate from each other, changing the refractive index at optical interfaces, their geometry is such that measuring beam can’t go output. Unfortunately, no optical glue remaining clear for 1.5 μm wavelength at 120 K, approved for space flights programs in cryogenic environment, is actually known. As thermal expansion can also be a source of problem, it has been decided not to use glue at optical interfaces. Molecular adherence was not a good alternative, due to the coating for beam separation. The Foster prism is also composed of two parts in calcite, and the glue has this time also an optical function, due to its specific refractive index allowing transmitting one polarization then reflecting the second. In conclusion, the cryogenic environments lead us to define a new Michelson scheme. This has been done in close collaboration with Alcatel Alenia Space (AAS), in charge of the thermo-mechanical design. By iterations, optics and mechanics have progressed together in order to gain volume and mass, while keeping the system performant at ambient, during launch (vibrations, cool down to 120 K) and operational phase (vacuum, 120 K ± 1 K, ageing), and for two distance measurement modes: relative (displacement) and absolute one (OPD). The optical scheme obtained is the following:
2.2Thermo - mechanical designThe main function of this architecture is to locate and to maintain within the specified positioning and stability accuracies every optical component along the operational environments (mechanical and cryogenic). It is composed of the main following parts:
The estimated preliminary volume is 170 mm x 150 mm x 60 mm (see Fig. 2), without collimators, for a mass of 2 kg. The choice of material and of optical component fixation technologies is backed on the expertise got by AAS from the qualified cryogenic instruments ISOCAM (operational temperature = 4 K) and IASI Cold Box Structure (operational temperature = 90 K). 3COHERENT LASER SOURCEThe laser source associated to “MOUSE II” is an upgrade of a prototype of flight models manufactured for IASI project [5]. Its performances are key points for the longitudinal sensor:
That’s why the first major change was to replace the initial 1.55 μm DFB diode laser by a pin to pin compatible new one, provided by JDS Uniphase. It emits 60 mW instead of 2 mW and line width value on the order of 430 kHz is expected, the goal being to have a visibility factor at least equal to 0.1 for a 500 m long OPD. The evolution of this visibility factor versus OPD is shown in Fig. 3, taking as hypothesis a lorentzian lineshape. 3.1Performances of up-graded laser sourceThe laser source is the entire system that provides a frequency locked monochromatic radiation. It includes the fibered diode laser, its current and temperature regulation, optics and electronics necessary for the servo-loop [5]. The frequency stabilization is made using an absorption peak of acetylene as frequency reference and by modulating the diode’s emitted frequency via its driving current, at 350 MHz. Performances of the new laser are summarized in: updated MOUSE II laser source performances. Comparing to IASI source performances, the increase of total power budget (6.5 W instead of 5 W) comes from the fact that the use of a powerful diode results inevitably in higher driving and thermoelectric cooler currents. As shown in Fig. 5, this value depends on the environment temperature but can be reduced by optimizing electronic components, not chosen initially for this kind of diode. The linewidth value was measured during a test campaign at ONERA/CERT, using their delayed self-heterodyne setup. This method avoids the requirement of a separate local oscillator laser by taking advantage of the large optical delays obtainable with optical fibers. Incident light is split into two paths by an interferometer. The optical frequency of one arm is shift by a value of ω using an acousto-optic modulator, and the optical length of the second arm is such that the delay exceeds the coherence time of the source. Then the two combining beams interfere as if they originated from two independent lasers offset in frequency by ω. As shown in Table 1, and according to Fig. 3, the spectral characteristics are not compliant with the need. Table 1:updated MOUSE II laser source performances
Going on further by doing tests with other diodes lasers and low noise laboratory current driver, it appeared:
Thus, investigations were performed to understand the origin of the deterioration of spectral performances. 3.2Improvement of the coherence length3.2.1Preliminary tests based on noise measurementsThe first phase was to compare electronics schemes by performing spectral analysis comparison tests. The circuit shown in Fig. 6 depicts the existing circuit. For these tests the laser was bypassed with a short circuit, so as to prevent any damage to the Laser diode. The current was sampled through a 1 ohm resistor via a low noise screened coaxial cable, and fed into the input of a wide band electrical spectrum analyzer. Fig. 7 compared to Fig. 8 shows a great deal of noise modulating the actual laser diode current. This electrical noise modulates the laser diode, and is believed to have the effect of widening the laser optical bandwidth. To solve this problem, an improved Howland-based circuit with protection was designed and tested (see Fig. 9). Electronic noise measurements shown improvements (see Fig. 10) but a series of comprehensive optical tests could then be undertaken to further qualify the two types of current sources. 3.2.2Optical measurementsTo be sure that the laser source is useable to do interferometric measurements at long distance range, the best way was to try to do it. This was done easily using a fibered Mach-Zehnder interferometer developed by IRCOM and lent by CNES. The setup is given in Fig. 11. One of its arms has a fixed length while the second one length can be variable, allowing optical path difference until 500 m. All the fibers are polarization maintaining ones and a piezo-actuator acts on the reference arm to create small variations of the OPD. The two beams interfere on the photodiode P3 and fringes contrast can then be measured. The contribution coming from photometric budget is estimated by simultaneous measurements of optical power on each arm, deduced from P1 and P2 outputs. So, after properly calibration, it is possible to measure the variation of the contrast according to the optical path difference. Firstly, measurements performed using the up-dated laser source for MOUSE II, and shown in Fig. 12, have confirmed the result obtained with the self-heterodyne method (~ 2 MHz). Preliminary measurements have also been done using a second DFB JDS Uniphase diode, reference 500183, driven by a laboratory current regulation. Self-heterodyne method gave a linewidth of 321 kHz, value confirmed by interferometric measurements (see Fig. 13). Secondly, the influence of the “Howland 2” circuit was tested with several diodes, resulting in improvements, as shown in Fig. 14 and Fig. 15, this last one taking also into account all the optical setup of the laser source. Finally, the impact of the frequency modulation was also measured. Fig. 16 proves, as expected, that modulation depth damaged spectral purity of the output optical radiation due to the increased amplitude of sidebands 350 MHz apart the carrier. But the usually modulation used for IASI sources remains acceptable and we obtain a correct visibility factor of about 0.27 for an optical path difference of 500 m. 4EXPECTED PERFORMANCESFor the opto-mechanical design of the sensor head, tolerance calculations were made by taking into account influence of any optical misalignment on the amplitude of detected signals, especially of the measuring beam. One interesting feature of this measuring system is that a calibration is made just before beginning measurements. Gain, offset and phase shift of SIN and COS can consequently be corrected. Budget for integration and launch was given on the basis of the results obtained with the detection electronics developed for MOUSE II, allowing obtaining a resolution of 25 nm. To know the maximum allowable loss to withstand with an accuracy of 25 nm during measurement, a specific simulation tool was developed. This loss is represented by the term η, defined by Eq.1, with P0 the initial optical power of the measuring beam and Ploss its new value. The example given in Fig. 17 shows that, according to the hypothesis and for the given value of η, we do an error of 23 nm, due to the fact that the real Lissajous figure (SIN versus COS) is not the one used to perform displacement calculations. Error due to phase shift variations are also considered in the tolerance budget. The spectral characteristics of the laser source are also of a major importance. The coherence, analysed in the previous paragraph, is taken into account in the simulation tool through the value of the visibility factor. Finally, an accuracy of 25 nm at 250 m required a relative frequency stability of 10-10, performance measured with the upgraded laser source, as shown in 5CONCLUSIONIn conclusion for this first phase of definition of a fine longitudinal sensor for the needs of PEGASE mission, we can say that:
66REFERENCESLe Duigou J.M., Ollivier M. et al.,
“PEGASE: a free flying interferometer for the spectroscopy of giant exo-planets,”
in Proceedings ICSO 2006,
Google Scholar
R. Thibout, L. Pujol, P. Juncar, J. Berthon,
“Absolute distance measurements by laser interferometry,”
in Proceedings ICSO 97,
Google Scholar
Le Duigou J.M., Poupinet A.,
“The MOUSE II sensors for longitudinal measurements,”
in Proceedings OPTRO,
(2005). Google Scholar
Poupinet A., Pujol L., Berthon J., Le Duigou J.M.,
“Laser metrology: new generation of MOUSE sensors extends distance and displacement measurement performances,”
in Proceedings ICSO 2004,
509
–513
(2004). Google Scholar
L. Pujol,
“Spatialized laser source frequency locked onto a molecular line,”
in Proceedings ICSO 97,
Google Scholar
|
