*mgeorges@ulg.ac.be ; Phone : 0032 4 3676668 ; Fax : 0032 4 3675613

Phase control strategies for stabilization of photorefractive holographic interferometer

Cédric Thizya, Marc Georges*

a, Philippe Lemaire

a, Yvan Stockman

a, Dominic Doyle

b

aCentre Spatial de Liège – Université de Liège, Liège Science Park, B-4031 Angleur, Belgium

bESTEC-European Space Agency, Keperlaan 1, NL-2200 AG Noordwijk ZH, The Netherlands

ABSTRACT

A current problem encountered in holographic interferometry systems is the phase variations that arise from vibrations, from the use of optical fiber in one arm, by air convection or by unwanted movement between the object and the measurement device. The holographic device developed by CSL based on photorefractive BSO crystals has the advantage of high resolution fringes with the capability of self-processing and indefinite reusability. The weakness of the technique is a response time that is often slow compared to external perturbations. For applications outside the laboratory, we decided to find solutions adapted to the holographic camera and which tend to limit or to eliminate the effect of such phase variations. An error signal is measured from one or another technique which measure the phase variations and which counteracts on an active element in the interferometer to stabilize the phase during recording. Different solutions have been analysed and are presented here. Interferograms were obtained with and without implementation of the phase control system that show the interest of such approach. Some of the system presented are only adapted to photorefractive holographic interferometry, making use of some crystal diffraction properties. Other systems can be used with other kind of interferometers. Keywords: Holographic interferometry, phase control, vibration compensation, photorefractive crystal

1. INTRODUCTION Optical interferometry techniques are commonly used in surface metrology, but their sensitivity to environmental perturbation confines them to optical laboratories. The particular case of holographic interferometry (HI) is even more critical since the set-up must be stable during the hologram recording in the appropriate media. To improve the range of applications of HI, it must be made compatible with an “industrial” environment. CSL has developed a holographic camera (HC) based on a BSO photorefractive crystal (PRC). This instrument has been widely described in the past1,-3 and is now commercially available4. Briefly its principle of working is the following. The laser light comes from the laser through a fiber and is further split into object and reference beams in the separation unit. The object beam illuminates the object which reflects (generally in a scattering manner) the light towards the frontal lens of the HC. The object beam interacts at the PRC with the reference beam, recording a hologram within the PRC response time which can be set at a chosen value (depending on the reference beam intensity). Once recorded, the hologram serves as reference state and any variation of the object is directly observed as interference pattern. The classical phase quantification techniques can be applied, mainly the phase-shifting by means of a piezotranslator mounted on a mirror of the reference beam. Once the readout is initiated, the hologram is erasing due to the dynamic behaviour of the PRC. If the object is sufficiently stable, a new hologram is recording during the readout and the interferogram finally disappears. Then a new measurement can be performed on the basis of the new hologram. The response time of the crystal for hologram recording is generally ranged between 1 to 10 seconds when dealing with continuous illumination laser, which limits its use in moderately stable environments. These values allow a proper

application of phase-shifting during the readout. Nevertheless this response time is somewhat long compared to external perturbations and is then a drawback. An increasing number of demands deals with the utilization of the device on-field or in less stable conditions. In particular, a recent project aimed at adapting the instrument to work within a space environment stimulator for the monitoring of space antennas deformations under thermal cycling. A vacuum compatible version of the camera has been manufactured and has been implemented on a holographic bench to perform tests on reflective surfaces using a diffuser5. During the test, some variations in the measurement set-up (long optical arms experiencing thermal and vibration perturbations) led us to envisage the stabilization of the hologram recording for obtaining better results. The technique that has been envisaged to adapt the HC to perturbed environment conditions is the Active Phase Control (APC) of the optical path. It consists in locking the optical path difference in the holographic interferometer with a closed loop control during the reference acquisition and readout steps. Two different types of solution will be presented. The first one makes use of an additional Mach-Zehnder (MZ) interferometer. The second one is implemented at the HC level : either the CCD detector is replaced with a CMOS detector, enabling fast phase variations measurement on a few pixels, or a photomultiplier tube (PMT) is used together with the CCD detector. These techniques will be implemented and tested on the holographic bench.

2. EXTERNAL MACH-ZEHNDER INTERFEROMETER SOLUTION

2.1. Principle The first solution of APC, depicted in figure 1, consists in inserting on the present holographic bench a secondary interferometer (dashed beams) taking its reference and object beam on the out/incoming object beam of the holographic interferometer, with help of beam samplers (BS). The holographic bench being highly stable itself, it allows controlling the remaining part of the object optical path with a piezo-electric (PZT) translator located on a folding mirror in the illumination arm. With this optical scheme, two phase-sensor techniques can be used: fringe locking with one detector (D1) or quadrature phase detection with two detectors (D1, D2).

Fig. 1. Sketch of principle for optical path control with external interferometer

2.2. Test with a parabolic mirror The tests were performed on a 200 mm diameter parabolic mirror with a radius of curvature of about 1 m and a 60 mm diameter central obstruction. This mirror is hanging on a pillar put on the laboratory floor. The mirror is about 1.7 meters from the floor. The holographic bench is on a separate table. The Fourier analysis of the interference signal shows that the frequencies are typically under 20 Hz. To obtain fringes, the imaging lens depicted in figure 1 is translated in-plane with a PZT. Figure 2a shows an interferogram and a phase map obtained without APC. The fringe contrast in the interferogram is about 82 grey levels. The phase shifting sequence to obtain the phase map did not performed efficiently because of mirror vibrations. Figure 2b shows results obtained with APC. The compensation frequency is set at 800 Hz. The fringe contrast is now about 170 grey levels and the phase shifting works properly.

Fig.2. Interferograms and phase maps obtained during the tests on the parabolic mirror Another way to see the effect of APC on the MZ interferometer fringe stability is to perform the APC with one detector and to use the second detector to acquire the fringe signal (Figure 3).

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2.3. Limitations The main limitation of the technique in terms of performance, is the PZT used to perform the APC. It is subject to hysteresis, the performance of the control loop can therefore not be optimum. Also, the response of the PZT has a cut off frequency of about 250 Hz (gain at -3 dB). If it must be driven above this frequency, the performance of the control loop is reduced. The system works well with specular surfaces, but it must be adapted to test a scattering surface. One solution is to attach a mirror or a retroreflector on the tested object. The other solution is to perform the APC by isolating and monitoring one speckle grain. This latter method has been experimented in the past but not sufficiently in deep to be presented here and shall be more investigated. At the present stage of development, the main drawback of the MZ technique is its bulkiness. It necessitates additional optics and detectors and it requires an alignment. An integrated solution could be envisaged.

3. ACTIVE PHASE CONTROL SOLUTION AT THE HOLOGRAPHIC CAMERA LEVEL

3.1. Principle This second solution consists in detecting the phase variation at the holographic camera level. Indeed, after the photorefractive crystal a continuous interference occurs between the holographically diffracted and the directly transmitted object wavefronts. On the basis of fringe locking technique, this signal (even weak in presence of object vibrations) can be used in order to stabilise the phase (with the same phase actuator of the first solution, figure 1) and progressively enhance the quality of holographic recording in a self consistent way. This means that compared to the MZ system, the setpoint in the fringe locking algorithm must evolve with the recording process until its optimum value, due to the dynamic behaviour of the PRC. While the MZ solution can be used with any holographic interferometer, this smart solution can be used only with PRC since it takes a maximum of advantages from the dynamical behaviour of the recording medium. Two techniques will be described, the first one involving a CMOS camera and the second one a PhotoMultiplier Tub (PMT).

3.2. CMOS technique The main achievement is to implement a CMOS matrix detector instead of the present CCD. With the ability to select the area of readout in CMOS sensor, some pixels are used at high frequency for the APC and the full matrix is used when image acquisition is requested. The CMOS camera that is available at CSL is a CCi4 camera from Vector International. The detector matrix is 1268x1024 pixels. The window of interest (WOI) can be as small as 4x1 pixels. The maximum frame rate at full resolution is 7.5 frames/seconds. Of course, the smaller the WOI, the faster the frame rate. The latter then mainly depends on the integration time of the camera which is 135 µs at the smallest. The main problem with this technique is the low sensitivity of the CMOS detector with respect to the CCD one. The consequence is that one must either increase the integration time, which means a lower compensation frequency, or increase the laser power. A WOI of 4x4 pixels is used to perform APC, the mean value is calculated and is used in the compensation algorithm. Due to the above limitations, APC with the CMOS camera did not allow to perform a test with the parabolic mirror, in the same configuration as for the MZ technique. Indeed, the frequencies of the mirror vibration are too high to be compensated. The set-up has been set in closed loop on the holographic bench. Instead of illuminating the parabolic mirror, the microscope objective illuminates directly the diffuser. This means that stable environmental conditions are achieved. It is therefore easier to evaluate the CMOS technique.

The signal from the CMOS is monitored with and without APC. A 20 ms integration time is set which means that the compensation frequency could reach 45 Hz. Despite the fact that the environmental conditions are quite stable, one can see the effect of the APC system (figure 4).

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Fig.4. APC with the CMOS system, the holographic set-up is in closed loop

3.3. PMT technique In order to keep the userfriendly character of the HC (which is not possible with the external interferometer) and to have an efficient APC solution (which is not possible with the CMOS detector due to its low sensitivity), we envisage to use a PMT located after the PRC, combined with a CCD detector (figure 5). The principle is equivalent to the CMOS technique, but instead of using a single detector that performs both image acquisition and APC, separate detectors are used for each task.

Fig.5. Implementation of the PMT technique in the holographic camera

This solution could be the good compromise. It has been tested in the past at a breadboard level, but it still has to be implemented in the HC. Additional tests will be performed to characterize the performance of the technique.

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3.4. Limitations The same limitations concerning the PZT discussed for the MZ technique, holds of course for these two APC systems. The main drawback of these techniques is that the phase shifting and APC systems are not independent, as it is the case in the MZ technique. The phase shifting must therefore be performed by changing the setpoint in the APC algorithm. This procedure will be tested with the PMT technique.

4. CONCLUSION Three techniques of APC have been presented. Among them, two have been implemented and tested. The first one, the MZ technique works well. It has been successfully tested in harsh environmental conditions with the tested parabolic mirror hanging on a column that is put on the laboratory floor. This system can be adapted to any holographic interferometer. The inconvenient of this technique is its bulkiness. Also, in its actual implementation, it cannot work with diffusive surfaces, but a solution exists to get rid of this problem and could be implemented in the future. The second one, the CMOS technique also works but not as well as the MZ technique. It is limited by the low sensitivity of the detector. CSL still has to implement the third one, the PMT technique. It is easy to implement and works for both diffusive and reflective surfaces, such as the CMOS technique. The performance should be the same as for the MZ technique.

5. ACKNOWLEDGEMENT These works were funded under the ESA contract number 16070/02/NL/CP and under the MULTIENCODE project, European Commission contract number SSP 006427.

6. REFERENCES 1. M. Georges, Ph. Lemaire, , "Real-time holographic interferometry using sillenite photorefractive crystals. Study

and optimization of a transportable set-up for quantified phase measurements on large objects", Appl. Phys. B 68, 1073-1083 (1999)

2. M. Georges, V. Scauflaire, Ph. Lemaire, "Compact and portable holographic camera using photorefractive crystals. Applications in various metrological problems", Appl. Phys. B 72, 761-765 (2001)

3. M. Georges, C. Thizy, V. Scauflaire, S. Ryhon, G. Pauliat, Ph. Lemaire, G. Roosen, "Holographic interferometry with photorefractive crystals : review of applications and advances techniques", Proceedings SPIE Vol. 4933 on Speckle Metrology 2003, Trondheim, Norway, June 2003, pp.250-255.

4. http://www.optrion-tech.com 5. Y. Stockman, C. Thizy, D. Doyle, P. Lemaire, Y. Houbrechts, A. Mazzoli, M. Georges, E. Mazy, I. Tychon, G.

Ulbrich, "Dynamic holography for the space qualification of large reflectors", Proceedings of SPIE, Vol 5965 on Optical Systems Design, Jena, Germany, September 2005.