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Spaceborne infra-red interferometer of the IASI instrument François Hénaulta, Christian Builb, Benoît Chidainea, Denis Scheidela a

Aérospatiale, 100, Boulevard du Midi, BP 99, 06132 Cannes La Bocca, France b CNES, 18, Avenue Edouard Belin, 31401 Toulouse, France ABSTRACT

Within the IASI (Infrared Atmospheric Sounding Interferometer) instrument, the principal function of the Interferometer and Hot Optics Subassembly (IHOS) is to create modulated interferograms which will be recorded by the Cold Box Subsystem. The IHOS is a spaceborne Fourier Transform Spectrometer (FTS) working in the 3.6-15.5 µm wavelength range. It is basically a Michelson interferometer where the flat mirrors are replaced with hollow corner-cubes retro-reflectors, one of which is moving along the optical axis in order to create a variable Optical Path Difference (OPD). The IHOS is mainly composed of the following components mounted on the same optical bench : • The Michelson interferometer, including the beamsplitter and the fixed and moving hollow cube-corners. • The cube-corner driving mechanism. • The entrance and exit optics. In addition, a reference interferogram is created by means of a reference laser source in order to generate a pulsed electrical signal defining the interferogram sampling times. The laser module consists in a highly stable laser source (10-7) and its control loop electronics, and is deported from the optical bench by means of an optical fiber. The most critical requirements of the IASI interferometer are the lateral shifts of the moving cube-corner (seen through the beamsplitter), the mis-alignment of its scanning axis, and the reference laser alignment error. These contributions must not exceed 15 µm, 200 µrad and 300 µrad respectively during the five years mission in orbit. Keywords: Michelson interferometer, Fourier transform spectrometer, cube-corner

1. INTRODUCTION The IASI instrument (Infrared Atmospheric Sounding Interferometer) is a Fourier-transform spectrometer which will be launched on the European MetOp meteorological platform, in order to provide spectrums of the Earth's atmosphere observed from space1,2. The data acquired by the instrument will be ground-processed in order to derive accurate measurements of the temperature, humidity, and composition of the atmosphere. AEROSPATIALE has been selected by the French National Agency (CNES) as the prime contractor for the realization of both the instrument and its interferometer (IHOS). The two main functions of the IHOS are : - To create modulated interferograms which will be recorded by the Cold Box Subsystem (CBS) of the IASI instrument. The IHOS basically is a Michelson interferometer where the flat mirrors are replaced with hollow corner-cubes retro-reflectors (CC), one of which is displaced along the optical axis in order to create a variable OPD between both interferometer arms. - To generate a Reference Path Difference (RPD) electrical signal defining the interferogram sampling times and corresponding to equal OPD increments. This is realized by means of a reference laser source (LAS) injected into both interferometer arms through an optical fiber and the LCO collimating optics. The reference interferogram is then collected by the Receiver Assembly Unit (RAU) electronics in charge of generating the RPD signal.

The IHOS is divided into three modules as shown in Figure 2.1-1 : 1) The optical module, or “equipped optical bench”, which is constituted of the beamsplitter (BS) and two hollow cube-corners, one of which is axially displaced by means of a driving mechanism (CCA), the entrance and exit optics (M1, M2, M3, and M4 mirrors), the emission and detection heads of the reference laser beam (LCO and RAU), and the optical bench itself. 2) The laser module, which mainly consists in the laser source and its control loop electronics, and is deported from the optical module by means of an optical fiber. 3) The electronics module (CCE), which main function is to drive the cube-corner guiding mechanism. The CCE is also in charge of most of the IHOS electrical interfaces, including power distribution, telecommand and telemetry of the laser and RAU, and the thermal control of the optical module.

EQUIPPED OPTICAL BENCH CC2

M1 CC1 CCA

M2 Beamsplitter

M4

M3 Optics

Cold Box Subsystem (CBS)

RAE

LCO

RAU

RPD (Reference Path difference) ASE (Acquisition Start/End)

RPD

Optical fiber

CCE LASER

LAU

CD (cube direction) NS (sample number) ASE half-period Power, TM/TC

IHOS

Instrument Management Subsystem (IMS)

Power, TM/TC

Figure 2.1-1 : Block-diagram of the IASI interferometer

2. ALIGNMENT AND PERFORMANCE REQUIREMENTS The basic requirements of the IASI interferometer are summarized in the Table 2.1-1. Spectral coverage OPD increment OPD range Dwell time Cube-corner half period OPD scanning speed Field of View

645 -2760 cm-1 768.85 ± 5 nm -20.7 mm < OPDmin < -20.4 mm +20.4 mm < OPDmax < +20.7 mm 151 ms 216 ms 27.36 ± 0.2 cm/s 47 x 47 mrad square

Table 2.1-1 : IHOS primary requirements

2.1.

Spectral performance

The spectral performance of a FTS usually is characterized by means of its Line Shape Function (LSF). The main performance criteria are the spectral resolution (full width at half-maximum), the spectral calibration (spectral shift of the LSF centroid), and LSF deformations with respect to its ideal shape. In the case of the IASI interferometer, the LSF characteristics mainly depend on two types of geometrical defects, which are the cube-corner lateral shifts and the misalignment of the reference laser3.

2.1.1.

Cube-corner lateral shifts

Due to the choice of cube-corner retro-reflectors, the IASI interferometer is affected by a typical misalignment that is known as the CC lateral shift4 (or apparent trajectory), as represented in the Figure 2.1-2. Moving cornercube CC 1 dy( ξ)

y

(O'',x,y,z) Stroke reference frame Beginning of the acquisition

O"

dz( ξ)

α

x IHOS optical axis

Average scanning axis

O'

z

End of the acquisition

Y Fixed cornercube CC 2 O X (O,X,Y,Z) : IHOS reference frame Z

Figure 2.1-2 : Cube-corner apparent trajectory Let us denote ξ the OPD created between both arms of the interferometer, while the moving cube-corner CC1 is mechanically displaced along the X scanning axis, and dy(ξ) and dz(ξ) its apparent lateral shifts, seen through the beamsplitter along the Y and Z axes respectively. For an ideal interferometer dy(ξ) = dz(ξ) = 0, and the image of the CC1 apex is perfectly merged with the CC2 fixed cube-corner apex. In addition the scanning axis must be perfectly aligned with the IHOS optical axis, so that CC1 and CC2 apex are staying merged along the full measurement stroke. For the real interferometer dy(ξ) and dz(ξ) can be decomposed into four different types of cube-corner lateral shifts :

dy(ξ ) = y 0 + y 1 dz(ξ ) = z 0 + z 1

ξ ξMax ξ ξMax

2

 ξ  + y2   + y Osc (ξ )  ξMax  2

 ξ  + z2   + z Osc (ξ )  ξMax 

(1)

where y0 and z0 are the constant lateral shifts, resulting from cube-corners and beamsplitter alignment errors and thermoelastic effects, y1 and z1 are the linear shifts (with respect to ξ) having the same origins, y2 and z2 are the parabolic shifts essentially due to thermo-elastic effects and gravity influence on the CCA, and yOsc and zOsc are the periodic or oscillatory

lateral shifts resulting from micro-vibrations generated by the spacecraft, the IASI Scanning Mirror, or the CCA mechanism itself. The required performance for the IHOS lateral shifts is summarized in the Table 2.1-2. The typical 80 seconds duration corresponds to periodical spectral and radiometric calibrations of the instrument performed in-flight and on-ground. LATERAL SHIFT

ABSOLUTE VALUE < 15 µm

SHORT TERM STABILITY (80 seconds) < 1 µm

MEASUREMENT ACCURACY < 1 µm

y 20 + z 20 y 12 + z 12

< 2 µm1

< 0.2 µm

< 1 µm

y 22 + z 22

< 2 µm

< 0.5 µm

< 1 µm

y 2Osc. + z 2Osc.

< 1 µm

< 1 µm

Table 2.1.2 : IHOS lateral shifts requirements

2.1.2.

Misalignment of the reference laser

FTS usually are affected by axial errors known as “sampling jitters”5 modifying the assumed positions of the moving cube-corner along the scanning axis. When the interferograms are sampled at constant laps of time, these errors may originate from cube-corner scanning speed variations, stability of the optical components, electronic time delays, etc... Within the IASI interferometer, these effects are compensated by means of a highly stable laser source at a reference wavelength λ0 generating an interference pattern which zero-crossings are used to trigger the acquisition chain : thus the OPD created by the moving cube-corner are multiples of λ0/2. However this is not true if the laser beam is not correctly aligned with respect to the interferometer scanning axis. In that case the actual OPD increment will be λS/2, where λS is the synthetic wavelength of the laser sampled by the RAU at the exit of the interferometer. It can be shown that λS depends on the spectral stability δλ of the reference laser, on one hand, and on its alignment accuracy δθ, on the other hand :



λS ≈ λ0  1 + 

δλ   cos δθ λ0 

(2)

As a significant difference between the real and synthetic wavelengths of the LAS would result in a spectral calibration error of the optical spectra generated by the interferometer, the λS/λ0 ratio (or “OPD reference error”) is submitted to the following requirements : ◊ Absolute knowledge better than 5 10-7 on-ground. ◊ Relative stability better than 2 10-7 during the whole lifetime in orbit. Such specifications have two main consequences on the design of the IASI interferometer : firstly it imposes the integration of a high quality laser source having a spectral stability better than 10-7 (see § 4.4), and secondly the optical axis of the reference laser, which is defined by the LCO, must be set parallel to the cube-corner scanning axis (seen through the beamsplitter) by means of mechanical adjustments around the Y and Z axes, with an accuracy better than 300 µrad. The alignment has to be maintained during the five years mission in-orbit.

1

equivalent to a 200 µrad angular misalignment with respect to the optical axis.

2.2.

Radiometric performance

The most stringent of the IHOS radiometric requirements probably is its spectral transmission which must be higher than 0.31 over the whole IASI spectral range. Practically, it dictates the choice of the Potassium Bromide (KBr) material for the beamsplitter plates substrate (see § 4.2).

3. INTERFEROMETER DESCRIPTION

3.1.

Optical architecture

The optical layout of the “equipped optical bench” is illustrated on the Figure 3.1-1. It is constituted of the following components, mounted on the Optical Bench Unit (OBU) and working at a temperature of 300 ± 2 K. The heart of the IHOS is a Michelson interferometer constituted by a beamsplitter and two hollow cube-corners (CC1 and CC2) each defining one arm of the interferometer. In operation CC1 is continuously moved along the IHOS optical axis in order to create a variable OPD between both interferometer arms, while CC2 stays fixed. After reflection on the cubecorners, the incident rays are recombined on the beamsplitter and interfere. The variations of the created fringe pattern represent the Fourier Transform of the incident light. The Michelson interferometer is surrounded by an input and an output telescope called the Hot Optics. The entrance optics is an afocal telescope made of two confocal off-axis parabolic mirrors (M1 and M2), and the exit optics are constituted of a folding mirror M3 and an off-axis parabolic mirror M4. The rays collected by the IHOS are focused at the M4 focal plane, where a field stop is defining the instrument field of view at the CBS entrance. The IHOS internal pupil is located on the fixed cube-corner and imaged (through the afocal telescope M1/M2) at the entrance pupil of the instrument, which corresponds to the location of the IASI Scanning Mirror. The cube-corners are also imaged with a cold aperture stop located inside the Cold Box Subsystem. Scanning mirror M0 Fixed cornercube CC2

M1 mirror Z IHOS

X IHOS

M2 mirror

Moving cornercube CC1

Beamsplitter

Cold Box Subsystem

M4 mirror

M3 folding mirror

Y IHOS

Figure 3.1-1 : IHOS optical configuration

3.2.

Mechanical and thermal architecture

The design of the equipped optical bench is strongly driven by the need to preserve the alignments of the optical components from the last tests performed on-ground until the orbital end of life. One of the main contributors to the instrument performance degradation is the instability of the CC lateral shift (including both cube-corner and beamsplitter relative positions) during a 80 seconds duration span, which concerns all the micro-vibration effects during the IASI operational life. These perturbations are restrained by means of passive isolators located at the instrument interface with MetOp, and by the seek of optimized stiffness decoupling between the primary structure, the optical bench, and the involved optical components. Alignment bias and lower frequency drifts (with respect to the 80 seconds period) remain major contributors to the interferometer performance. They are mainly due to : • Residual misalignments during the integration of the optical components. • Optical bench distortion due to IHOS integration inside the instrument. • Residual launch distortions including micro sliding effects of the optical components. These errors roughly are within the same magnitude range. The alignment bias due to integration are kept benign (lower than 5 µm) through accurate shimming of the critical components (fixed and moving cube-corners). All distortions originating from the instrument are filtered by the implementation of an isostatic mount between the optical bench and the primary structure. This interface is made of three flexible blades having their flexible axis concurrent. In order to minimize launch distortions the mechanical interfaces of the optical components are sized with respect to a 1 µm maximal sliding, on one hand, and the OBU is dimensioned so that its residual distortions are lower than 1 µm/m, on the other hand. Other sources of instability like thermal distortion, moisture expansion, or gravity effects are reduced to negligible values thanks to the choice of the OBU material (carbon fiber sandwich with cyanate resin) combining low thermal and moisture expansion coefficients and high specific stiffness, and to the use of an accurate active PI thermal control within the optical bench enclosure.

Thermal enclosure (MLI) Instrument primary structure Radiative surface Cube Corner Mechanism Beam Splitter Optical Bench

Laser Collimating Optics Receiver Assembly Unit Laser Source

Figure 3.2-1 General overview of the equipped optical bench

Thermally, the interferometer enclosure is coupled to a radiator having a direct view to space, and isolated as much as possible from conductive and radiative heat transfers with the instrument. A first line of operational heaters is accommodated on the rear side of the OBU and another line is laid on the external radiator. Each regulation line is in cold redundancy. In operational mode the thermal sensors are interrogated every 8 seconds and corrective power pulses are then sent to the heaters. In safe mode, autonomous heaters will prevent the interferometer components from extreme temperatures injuries.

3.3.

Electrical architecture

The IASI architecture is formed around the Instrument Management Subsystem (IMS) which is the main interface with the MetOp platform. Other subsystems are star connected to the IMS, except for specific signals driven by the spacecraft as well as timing critical lines. IASI is high level controlled by MetOp according to the OBDH bus whereas science data are transmitted by a specific channel. All the IASI subsystems are configured and monitored through the dedicated serial digital channels. Specific monitoring lines are also used between subsystems and IMS, in order to comply with the observability rules even if they are switched off. Subsystems are primary supplied and monitored under IMS control. Power distribution is realized from the IMS power distribution unit switching the main unregulated instrument power bus. The Cube Corner Electronics (CCE) is the main electronic equipment of the interferometer and is therefore under direct IMU computer control. Its primary function consists in controlling the Cube Corner Mechanism, while the secondary function is to realize the powering and communications transfer for the lasers, the RAU and the IASI calibration black body. The overall architecture is shown in the Figure 3.3-1 with redundancy indications. The nominal half period of the CCM is about 216 ms. It can be altered of ±1 ms according to the phase error between the Acquisition Start End (ASE) transition and the 8 seconds absolute clock generated by the platform and received by the IMS on its OBDH bus. An order is then transmitted to the CCE to adapt the cube-corner half period by choosing between nominal, lower or higher speed. Laser 1

RAU_N

RAU_R

IF_N IF_R

Laser 2 IF_N IF_R

Black Body IF_N IF_R

Supply (IMS) Cube Corner Electronics

Cmd / Ctrl (IMS) IMS_N

IMS_R

Mechanism release (Metop)

Cube Corner Mechanism

Figure 3.3-1 : IHOS simplified architecture

4. INTERFEROMETER EQUIPMENTS

4.1.

Cube-Corner Mechanism (CCM)

The fundamental requirements of the CCM are its guiding performance (see § 2.1.1), a long operational lifetime corresponding to 7.3 108 scan cycles in-orbit, and the limited forces and torques exported to the instrument and spacecraft, which require the integration of a compensation mechanism. The architecture of the cube-corner driving unit is based on the main following elements (see Figure 4.1-1) : ◊ A main structure made in Aluminum alloy, fixed to the OBU by three flexible feet. ◊ Two flexible double compensated parallelogram constituted by two machined membranes in high-strength copper-beryllium alloy (thickness 0.33 mm), ensuring the guiding accuracy. ◊ An intermediate stage linking the upper parts of both parallelograms. ◊ A mobile stage supporting the hollow cube-corner and the motor coil. ◊ A balancing counter-mass mounted on the mobile stage at the opposite side of the cube-corner. ◊ A driving lever constraining the parallelogram motion in order to compensate residual guiding errors. ◊ A voice coil motor which yoke is mounted on the fixed structure. ◊ An optical linear encoder with a resolution better than 0.125 µm in order to control the cube-corner velocity. ◊ A locking device based on Shape Memory Alloy technology in order to hold the mechanism during the launch phase.

Figure 4.1-1 : Cube-corner Mechanism overview

4.2.

Beamsplitter

Ideally the spectral transmission of the IASI beamsplitter should be higher than 0.35 on the whole interferometer spectral range (3.6-15.5 µm). However such radiometric performance is difficult to attain in the 15 µm region. The candidate materials for the beamsplitter plates are Zinc Selenide (ZnSe), Potassium Bromide (KBr) and Potassium Chloride (KCl). Although ZnSe presented by far the best mechanical properties, it was eliminated because its spectral transmission is rapidly decreasing above 14 µm. In spite of its very hygroscopic behavior and high coefficient of thermal expansion, KBr has finally be chosen as the baseline material. The IASI beamsplitter is composed of a 20 mm-thick KBr separating plate, coated with semi-reflective layers having an excellent reflection-transmission factor deposited on its internal face, and of a compensating plate having the same geometrical characteristics in order to compensate chromatic effects in the interferometer field of view. Protective layers are deposited on the three other faces of the BS in order to prevent moisture effects of the KBr material. The use of KBr leads to additional constraints : firstly the moisture environment of the beamsplitter should not exceed 50 % of humidity in order to preserve its optical transmission during all its on-ground life, and secondly the mechanical design of the mount must preserve the material from the high launch loads and thermo-elastic stresses. Axial restraining of the plates is ensured by a spring-loaded cover inducing a preload within the plates, while radial restraining is realized by means of an annular flexible joint located between the substrate and the structural mount. An intermediate ring (with thermal expansion coefficient close to the KBr material) is set between the separating and compensating plates and defines the mount reference surface.

4.3.

Hot optics (mirrors and cube-corners)

The most critical components of the Hot Optics probably are the hollow cube-corners, since the image quality of their retro-reflected wavefronts directly affects the radiometric contrast of the interferometer3. On the IASI instrument, the required contrast factor is 80 %, which corresponds to an image quality better than 0.6 µm Peak-to-Valley, or to angular deflections lower than 0.5 arcsec between the three reflecting faces. The hollow cube-corners are constituted of three SiC plates bonded together with epoxy adhesive. The final shape is obtained by bonding the three faces of the plates while they are maintained by molecular adhesion on a master cubecorner. They are coated with protected silver ensuring a global reflection factor higher than 0.94. The reflective surfaces of the Hot Optics mirrors are made in Zerodur glass bonded on invar mounts in order to limit thermo-elastic deformations. The mirrors are coated with hard gold having a 97.5 % reflectance from 3.6 µm to 5 µm and 98.5 % above 5 µm.

4.4.

Laser Source Subsystem (LAS)

The Laser Source Subsystem is composed of two laser units (LAU) pigtailed to an optical fiber and located on the upper panel of the IASI instrument. The laser beams are mixed in cold redundancy within the Laser Collimating Optics (LCO) unit, by means of a non-polarizing beamsplitter and an optical fiber connection system. The LCO is placed on the optical bench behind a small opening in the M3 mirror. Its design allows an easy disconnection and reconnection of the optical fibers without modifying the alignment of the output RPD beam.

The IASI laser source must present a high spectral stability (better than 10-7) during the whole instrument lifetime. Due to their intrinsic low-frequency and drift instabilities, no laser diodes can fulfill these long-term requirements, thus it is necessary to implement an active control of the laser frequency. The basic principle (see Figure 4.4-1) consists in enslaving the frequency of the laser diode on a physical reference defined by a well characterized molecular absorption line in the 1.5 µm wavelength region, so that it is possible to use commercial telecommunication diodes. The frequency position of the spectral absorption line is very stable and only depends on the nature of the chosen molecule. The most important design features are the following6. Optical output Temperature controller Isolator Laser module DFB

Optical coupler

Current source Attenuator

Oscillator 350 MHz

Absorption cell

Phase shifter

Integrator

Mixer

Detector

Figure 4.4-1 : Principle of LAS frequency stabilization 1) The chosen frequency reference is an Acetylene 13(C2H2) molecular absorption line. This gas presents numerous absorption peaks in the spectral region of interest, from which the 1537.7 nm line was selected. The acetylene is enclosed in a titanium gas cell of 80 mm length, under a pressure of 20 Torr. 2) The enslaved laser diode is a tunable Distributed FeedBack (DFB) having a continuous frequency adjustment capability with respect to its temperature and current. Both parameters are used to maintain the emission frequency of the laser at the center of the absorption peak : the temperature control is used to approximate the reference wavelength while the current control allows accurate frequency adjustment. The DFB module includes an InGaAsP laser diode, a Peltier unit for temperature regulation (both cooling or warming) and a -45 dB optical isolator. 3) A frequency modulation (350 MHz with a weak modulation ratio) and an homodyne phase demodulation of the emitted laser power propagating into the acetylene cell is used to generate an error signal, which zero-crossing represents the detection of the reference absorption line.

4.5.

Receiver Assembly Unit (RAU)

Once returned from the interferometer output port, the modulated laser beam enters in the Receiver Assembly Unit (RAU), also located behind the M3 mirror, where it is divided into two beams by means of a non-polarizing beamsplitter. Each beam is focused on an InGaAs detector. Two identical Receiver Assembly Electronics (nominal and redundant) are in charge of the zero-crossing detection and the generation of the RPD digital pulses.

5. CONCLUSION Within the IASI instrument, the most critical performance of the interferometer undoubtedly is the cube-corner apparent trajectory and the alignment of the reference laser beam. These requirements can only be attained through the realization of customized equipments such as the Cube-corner Mechanism, the beamsplitter, and the reference laser source, on one hand, and accurate alignment of the fixed and moving cube-corners and the Laser Collimating Optics, directly performed on the optical bench, on the other hand. Several functional breadboards of critical interferometer components such as the CCM, LAS, or beamsplitter have already been realized in the frame of the IASI development program : they all led to promising results with respect to the attained performance as well as their ability to withstand the specified launch and flight environments.

6. REFERENCES 1 2 3

4 5 6

P. Javelle, “IASI instrument overview”, Proceedings of the 5th Workshop on ASSFTS, November 30th - December 2nd, 1994, Tokyo K. Dohlen, F. Hénault, D. Scheidel, D. Siméoni, F. Cayla, G. Chalon, P. Javelle, “Infrared Atmospheric Sounding Interferometer”, Proceedings of the 5th Workshop on ASSFTS, November 30th - December 2nd, 1994, Tokyo F. Hénault, D. Miras, D. Scheidel, F. Boubault, “Infrared Atmospheric Sounding Interferometer (IASI) performance evaluation”, Proceedings of the 6th Workshop on ASSFTS, October 3rd - 5th, 1995, San Juan Capistrano J. Kauppinen, P. Saarinen, “Line-shape distortions in misaligned cube corner interferometers”, Applied optics, Vol. 31, n° 1, January 1992 A. Zachor, “Drive non-linearities : their effects in Fourier spectroscopy”, Applied optics, Vol. 16, n° 5, May 1977 L. Pujol, “Spatialized laser source frequency locked onto a molecular line”, ICSO’97, December 2nd - 4th, 1997, Toulouse