Transcript
OJECT A P SI-I/R/27/00 PR ASI-I/R/27/00 ROJECT R&D bread-board for a Fourier Transform Spectrometer to remotely sense the rotational water vapour band (10-1000 cm-1).
RMINE S RIO PP.I.: .I.: C SEERIO CAARMINE ANCESCO E POSITO,, L CA P LCHETTI,, R LANDO R ZZI I.: FFRRANCESCO C ESSPOSITO PAALCHETTI RIIZZI O..I.: OLANDO CO LUUCA RO IVE S ECUT MMARY E UT TIVE SU UMMARY EXXECU
BSU PDU
SMU RLS Laser
POTENZA, GIUGNO 2002
0.1 Summary The Radiation Explorer in the Far Infrared (REFIR) Science Team blends expertise in broadband radiometry, Fourier spectroscopy, Physics of detectors and optical devices, cloud and radiation remote sensing and climate modelling. The project team has identified a mature design of the REFIR spectrometer and has built-up a breadboard version of the instrument. The main characteristics of the breadboard interferometer are here summarised. -
Fourier Transform Spectrometer Polarising interferometer Double input/double output ports Spectral range: 10 to 100 micron Sampling rate 0.5 cm-1 (unapodized) Integration Time 7.0 s SNR better than 100 Un-cooled pyroelectric detector
One of the main difficulties in developing Fourier Transform Spectrometer in the far infrared is the lack of efficient polarising beam splitter and high performance detectors even a 77 K. Possible solutions, which overcome such difficulties, have been studied. For the polarising beam splitter polyprpropylene and diamond substrates have been investigated as possible alternative to Mylar membranes. For the detectors, the use of uncooled pyroelectric detectors seem to be feasible and extremely promising even in terms of performance and radiometric accuracy. However, these solutions have not yet been tested in terms of reliability and end-uses such as commercial/industrial applications. The REFIR bread-board has been developed just to address these hot issues in order to trade-off between all the instrument parameters: throughput and transmittance; detector detectivity, area and operating conditions; optical system, f-number; spectral resolution; spectral filtering; interferometer beam splitter; mirror motion mechanism; interferometer electronics; detector electronics; calibration. The experience we have gained with the breadboard will allow us to design REFIR balloon-borne and an aircraft prototypes.
0.2 Sommario Lo studio da satellite dell’energetica del pianeta ha ricevuto negli ultimi anni un notevole impulso. A seguito del progetto americano ERBE, altri due progetti concorreranno al miglioramento della conoscenza dei processi che influenzano il bilancio energetico della Terra. Essi sono l’Europeo GERB e l’americano CERES. Questi due progetti misureranno i flussi uscenti di onda lunga e corta ma in modo spettralmente indifferenziato. In particolare, per problemi di natura tecnologica, GERB non misurerà al di sopra dei 30 micron, lasciando cosi inesplorata una porzione dello spettro di emissione terrestre di notevole importanza per il budget energetico, in quanto sopra i 30 micron insiste la banda di assorbimento del vapore d’acqua. Per tale motivo, soprattutto su impulso dello science Team di GERB, si è organizzato un gruppo Europeo che ha sviluppato l’idea di REFIR (Radiation Explorer in the Far Infrared), che ha lo scopo principale di affiancare GERB per lo studio più approfondito dei processi che determinano il bilancio radiativo terrestre. E’ bene rilevare, che CERES e GERB non sono missioni operative. Si tratta di due esperimenti di ricerca che tendono a migliorare la nostra conoscenza dei processi fisico/chimici in atmosfera che determinano il bilancio radiativo. In questo contesto, REFIR è nato per supplire a mancanze conoscitive di missione di GERB/CERES ed è pertanto a maggior ragione un esperimento di ricerca nel senso più pieno. REFIR è stato finanziato dall’EU sino alla fine del 1999. Nell’ambito del progetto si è effettuata un’analisi di fase B0 per lo strumento. In virtù del notevole interesse scientifico dello strumento, si è proposto all’ASI di costruire un prototipo di REFIR su banco ottico (bread-board) al fine di meglio caratterizzare e risolvere tutti i possibili constraint tecnologici. L’obiettivo finale dello studio è di qualificare per applicazione spaziale un interferometro di moderata risoluzione (0.5 cm-1) nel range spettrale 10-100 micron. La parte alta di questo range spettrale non è attualmente coperta da nessun esperimento futuro da satellite. Da un punto di vista tecnologico, REFIR è uno spettrometro a trasformata di Fourier. Molte applicazioni nell’ambito dell’osservazione della Terra dallo Spazio trarranno vantaggio dallo sviluppo di spettrometri a larga banda di media risoluzione a trasformata di Fourier. Gli spettrometri a trasformata di Fourier, grazie al vantaggio multiplex, consentono di ottenere spettri di elevata qualità di sorgenti poco intense, che si presentano ad esempio nel caso dell'atmosfera e più in generale nel caso della spettroscopia di emissione. Oltre ai problemi ottici, ed alla scelta di appropriati coatings e windows, la difficoltà maggiore che si incontra nella progettazione di spettrometri/radiometri a larga banda, in grado di coprire intervalli spettrali al di sotto di 600 cm-1, è la mancanza di adeguati detectors. Gli usuali MCT o dispositivi InSb degradano immediatamente in efficienza non appena ci si approssima alla soglia dei 600-650 cm-1. Al fine di avere rapporti segnale/rumore compatibili con l’accuratezza delle misure richieste da future missioni spaziali, è giocoforza il ricorso a rivelatori che necessitano di sistemi di raffreddamento a 4 K che limitano immediatamente le doti di compattezza degli strumenti, oltre che la loro vita media. La possibilità di utilizzare rivelatori a temperatura ambiente è stata limitata fino ad ora dal fatto che questo tipo di rivelatori risultano notevolmente più lenti. Il conseguente aumento del tempo di integrazione della misura comporta così la perdita del principale vantaggio della spettroscopia di Fourier, cioè il vantaggio multiplex, per effetto della variabilità della scena durante l'acquisizione dell'interferogramma, con una conseguente degradazione del rapporto segnale/rumore. Di recente grazie allo sviluppo di un nuova tecnica di acquisizione, introdotta dal Dr. J.W. Brault del NOAA, Boulder, Co (USA), è stato possibile realizzare un campionamento molto più veloce, anche nel caso di rivelatori lenti, senza incorrere in problemi legati all'errore di campionamento. Il nuovo approccio ha così aperto la strada allo sviluppo di una nuova generazione di spettrometri a trasformata di Fourier di media risoluzione operanti su larga banda spettrale e a temperatura ambiente. Per realizzare uno strumento dalle dimensioni contenute ed altamente affidabile ed efficiente, come richiesto per una corretta qualificazione spaziale, il gruppo proponente ha sviluppato una soluzione originale per il design ottico dell’interferometro ed il detector in modo che le specifiche di progetto possano essere ottenute con uno strumento che lavori a temperatura ambiente. Proprio in virtù della originalità delle soluzioni adottate, il sistema va caratterizzato su banco ottico (bread-board). Lo scopo primario del progetto è consistito nella realizzazione su banco ottico di uno spettrometro a trasformata di Fourier con le seguenti caratteristiche generali: Tipo di spettrometro: Fourier Transform Spectrometer (FTS) a polarizzazione Doppia porta di ingresso/Doppia porta di uscita Larghezza di banda: 10 - 1000 cm-1 Intervallo di campionamento 0.5 cm-1 (non-apodizzato) Tempo d’integrazione 7.0 s Rapporto Segnale/Rumore migliore di 100 La realizzazione su banco ottico ha permesso di stabilire la funzionalità dei vari componenti e di verificare sperimentalmente il funzionamento della configurazione ottica innovativa adottata per l'interferometro. In particolare, si sono studiate le tolleranze dell'allineamento ottico e verificate le specifiche di progetto allo scopo di ottimizzare i diversi parametri costruttivi necessari per la realizzazione di un prototipo compatto per future missioni da aereo e satellitari.
1. Activity Report
This report summarises the basic conceptual design of the instrument which has led us to the final breadboard version of the instrument. A detailed account of the breadboard instrument and its realisation is given in the annex document. Four different teams whose affiliation and project leaders are listed in the table shown below have carried out the research activity.
Partners Affiliation
Responsible person for the research activity di Prof. Rolando Rizzi
ADGB, Università Bologna DIFA, Università della Dr Francesco Esposito Basilicata IMAA/CNR, Potenza Prof. Carmine Serio (P.I.) IFAC/CNR, Firenze Dr Luca Palchetti
The project team has realized the breadboard by carrying out a series of work packages which are here listed, and that will be described in the following pages. 1. 2. 3. 4. 5. 6.
Definition of instrument requirements (All Partners) Design and development of the opto-mechanical configuration of the interferometer (ADGB/IFAC/IMAA) Design and development of the data acquisition and pre-processing systems (DIFA) Mirror mounting and alignment (IMAA/IFAC) Design and development of the interferometer mirror mechanism (DIFA/IFAC) Development of the instrument model for calibration (IFAC/ADGB)
1. Definition of instrument requirements. The instrument requirements are a heritage of the REFIR basic concept and are summarized in the following table. We will refer to these requirements as the baseline option of the instrument. Table I. Instrument parameters. Spectral band Spectral resolution (non-apodised, ∆σr) Interferogram type Measurement time (tm) Instrument optical throughput Field of view inside interferometer Pupil diameter inside interferometer Instrument temperature Polariser Efficiency Light Collector Efficiency Magnetic Permeability of Mirror Coating (gold) Electrical Conductivity of Mirror Coating (gold) Mirror Roughness Tilt between Interfering Beams Lateral Shift between Interfering Beams Detector type
10 – 1000 cm-1 0.5 cm-1 Double side 7 sec AΩinstr=0.0178 cm2 sr ± 30 mrad 28.34 mm 300 K ηpol=0.90 ηcol=0.90 p=1 (Gaussian unit) s=38.3 sec-1 (Gaussian unit) 1 µm 10 arcsec 0.2 mm DLATGS 300 K pyroelectric 1.3 ⋅ 1.3 mm2 detector (GECMarconi Infrared Division) with 22 µm thick polypropylene, NEP ≈ 2.5 10-10 W/√Hz
2. Design and development of the opto-mechanical configuration of the interferometer The conceptual optical baseline configuration of the interferometer is shown in Figure 1. It was studied in order to define the opto-mechanical layout that minimises the collimated optical path. Several boundaries, as the circular shape of beam-splitters and mechanical interference between components and supports had to be taken into account in arranging optics. Folding angles and interdistances were minimised. This topic has a great impact on the instrument overall size.
Fig.1 Optical baseline configuration of the interferometer 2.1 Mechanical design The mechanical design of the baseline interferometer was studied in order to develop a flexible configuration able to be implemented both on optical bench and on a dedicated support. The mechanical layout was first developed in order to be used with a commercial linear translation stage by Physik Instrumente of the interferometer mirror movement. Figure 2 shows the top-view of the mechanical layout of the interferometer (a) with mechanical supports and the Physik Instrumente linear translation stage (PI) and the enlarged front-view of the back-to-back rooftop mirror unit (b). All the mirror supports are quite similar and allow to use both a single planar mirror for the two planes of the interferometer or two separated mirrors. The alignment can be adjusted along two-axes and with a sensitivity of about 0.12 mrad for the mirrors and 0.024 mrad for the beam-splitters. PI
(a)
(b)
Figure 2. Top-view of the mechanical layout of the interferometer (a), front-view of the detail of the rooftop mirror unit (b)
Particular attention was devoted to the design of the rooftop mirror unit and the two beam-splitter assembly (for the input and output polarisers and the interferometer beam-splitters). The rooftop mirror unit is a compact structure, with planar mirrors placed on the prisms in the bottom and in the top part of the support. After the manufacturing of the mechanical support, the mirrors can be aligned once for all by way of an auto-collimator in order to reflect the incoming beam exactly in the same direction (angle between the bottom and the top mirrors equal to 90°). As far as the beam-splitter support is concerned, it must have higher precision than the other supports. A special care was taken for the control of the beam-splitter planarity and the alignment precision. Actually the required planarity is obtained directly from the supporting ring, therefore the holder has to distort the component as little as possible. For this purpose, a clamping by using three spheres for the support of the beam-splitter ring is used. The complete overall mechanical layout is quite compact measuring (without input and output optics) only about 40x40 cm2. 2,2 Polariser alignment tolerances Calculations have been made in order to analyse the dependence of output signal from the misalignment of the polarising axis both in the input/output polarisers and in the beam splitters. Ideal polarisers and mirrors have been considered. The result not only demonstrates the feasibility of the optical layout selected for the interferometer but also shows that the misalignment of polarising axes is a second order effect, which does not pose any implementation problem.
3. Design and development of the data acquisition and pre-processing systems The data acquisition and pre-processing depend on the sampling method adopted for the acquisition of the interferogram. The sampling approach requires to implement a compensation for the slow response of the roomtemperature pyroelectric detector and the non-uniform speed of the scanning-mirror drive. The compensation is very difficult to obtain for large spectral bandwidths. Different sampling techniques were studied for the selection of the most suitable choice for this instrument. Equal-time-sampling with filter compensation and numerical re-sampling is the preferred choice, because it allows high performances without increasing the mechanical requirements for the mirror drive. It moves the complexity from hardware to software, with advantage in terms of costs and reliability. This method makes use of a digital filter that allows an amplitude and phase correction of the detector response, which can be more precise than that provided by either a constant time delay or a simple analogue electrical filter. With this operation, high quality spectra can be obtained without the need for a fast detector system or a high precision mechanical drive. The method follows the sampling approach introduced by J.W.Brault (Appl. Opt. 35, 2891-2896, 1996). An acquisition card has been chosen which is able to acquire multiple channels at the same time, with a full compatibility with LabView. The acquisition card is a National Instrument (NI) one, model PCI-6032E, with the following features: • PCI bus • 16-bit resolution • 16 analog inputs • Maximum acquisition rate 100 kS/s • 2 24-bit, 20 MHz counters • 8 digital input/output lines All input/output lines can be programmed by means of LabView. We choose to use NI LabView as software to use for all acquisition/file management/on-line analysis operations. This choice is due to the fact that by using LabView we can acquire analog channels, drive position stage, read counters, make analysis and save data on disk. The analysis has been done by using FORTRAN codes linked to LabView acquisition program. We have developed the relevant subroutines to acquire data simultaneously from different channels. This is necessary because an interferometer using the Brault technique has at minimum 3 channels, two of them coming from pyroelectric sensors, and the remaining one coming from the HeNe reference laser signal. The subroutines also provide to read counters, in order to have a time measurement, necessary for an optimal application of the Brault technique.
4. Mirror mounting A mirror support was manufactured, capable to mount one or two mirrors. The support is mounted on a square base with four holes for the mounting, and can rotate around a vertical axis. On the support one or two mirror holders can be mounted; in both cases equipped with two screws for angles adjustment. The supports can be mounted on an optical bench or on a specific support for the interferometer.
5. Design and development of the interferometer mirror mechanism The movement of the double-roof mirror is carried out by using the linear positioning stage Physik Instrumente M126.DG which is already available to the project team. The M-126.DG is a motorized, lead screw driven translation stage with a travel range of 25 mm. It utilizes a closed loop DC motor with shaft mounted position encoder and a backlash-free gear head providing 0.1 µm minimum incremental motion. Non-contacting switches are integrated to eliminate the possibility of over travel. Unidirectional repeatability is 0.2 µm, and bi-directional repeatability is about 2 µm, the normal load capacity is 20 Kg. Motion control parameters such as velocity and acceleration can be varied freely within the given limits. The maximum velocity is 1.5 mm/s. Motion control parameters can be changed via software operations. The linear position stage is completely programmable by means of drivers and VI for LabView. By using this kind of position stage with a well-designed software, we have been be able to completely control the movement, in order to measure both using a standard technique or a Brault technique, by simulating a known mirror velocity law. We have developed the relevant software routines that will be used for the acquisition program to vary the cinematic parameters (velocity, acceleration, etc.), and to test the movement stage.
6. Development of the instrument model for calibration and error budget A mathematical model for the instrument has been developed to help the calibration phase of the interferometer and to assess the expected error budget. The model takes into account the many parameters listed below: -
Instrument configuration Optical characteristics of mirrors (coating, reflectivity and emissivity) Instrument temperature and its expected time variability Optical characteristics of the beam splitter Angular responsivity to the input radiation
An example of the overall error budget obtained through the REFIR instrument model is shown in Fig. 3.
overall budgets 1,2E-03
radiance [W/m2/sr/cm-1]
1,0E-03
8,0E-04 NEdL BIAS
6,0E-04
DRIFT NOISE
4,0E-04
2,0E-04
0,0E+00 0
200
400
600
800
wavenumber [cm-1]
Figure 3
1000
1200
6. The breadboard The breadboard is shown in the cover picture of this report. Preliminary measurements have been performed in a vacuum chamber (picture below, see the annex report for further details) that confirms the expected performance of the instrument. Figure 5 shows the acquisition unit while recording the laser signal and the source signal (consisting here of the thermal emission of a hot emitter at a temperature of about 700 K). The instrument has been used within the ESA project to assess the performance of polypropylene polarising beamsplitter (ESA project “REFIR Beam Splitter Pre-development”, ESTEC No. 15612/01/NL/SF, 2001-2002)
Fig. 4. Experimental set up of the preliminary tests in a vacuum chamber.
Figure 5. REFIR data acqusisition system while recording from one REFIR channel the the reference laser (channel 1) and a the signal from a hot source (channel 2)
List of publication supported also by this project 1.
L.Palchetti and D.Lastrucci, “Spectral noise due to sampling error in Fourier transform spectroscopy”, Applied Optics Vol 40, No. 19, pp. 3235-3243, 2001.
2.
Maestri Tiziano e Rolando Rizzi, “A study of infrared diabatic forcing of ice clouds in the tropical atmosphere”. Sottomesso a J. Geophysical Research (2202JD002146)
3.
Mlynczak MG, JE Harries, R Rizzi, PW Stackhouse, DP Kratz, DG Johnson, CJ Mertens, RR Garcia, BJ Soden, “The Far-Infrared: A frontier in remote sensing of Earth’s climate and energy balance”, in Optical Spectroscopic Techniques, Remote Sensing and Instrumentation for Atmospheric and Space Research IV, Allen M. Larar and Martin G. Mlynczak Editors, Proceedings of SPIE vol.4485, 150-158 (2002).
4.
R. Rizzi, L. Palchetti, B. Carli, R. Bonsignori, J.E. Harries, J. Leotin, S. Peskett, C. Serio, A. Sutera, “Feasibility study of the space-borne Radiation Explorer in the Far InfraRed (REFIR)”, in Optical Spectroscopic Techniques, Remote Sensing and Instrumentation for Atmospheric and Space Research IV, Allen M. Larar and Martin G. Mlynczak Editors, Proceedings of SPIE vol.4485, 150-158 (2002).
5.
Rizzi R., Maestri T. and Amorati R., “Quantitative role of far-infrared emission on diabatic forcing of the middle and upper troposphere in clear and cloudy conditions”, in Optical Spectroscopic Techniques, Remote Sensing and Instrumentation for Atmospheric and Space Research IV, Allen M. Larar and Martin G. Mlynczak Editors, Proceedings of SPIE vol.4485, 159-170 (2002).
6.
Rizzi R., Serio C., Amorati R., “Sensitivity of broad-band and spectral measurements of outgoing radiance to changes in water vapour content”, in Optical Spectroscopic Techniques, Remote Sensing and Instrumentation for Atmospheric and Space Research IV, Allen M. Larar and Martin G. Mlynczak Editors, Proceedings of SPIE vol.4485, 181-190 (2002).
7.
R. Rizzi, B. Carli, J.E. Harries, J. Leotin, S. Peskett, C. Serio, A. Sutera, B. Bizzarri, R. Bonsignori, and S. Peskett, Mission objectives and instrument requirements for the REFIR – Radiation Explorer in the Far InfraRed – Mission: An outline after the end of Phase B0, in IRS 2000: Current Problems in Atmospheric Radiation, W. L. Smith and Yu. M. Timofeyev (Eds.). A. Deepak Publishing, Hampton, Virginia. pp. 105108.(2002).
8.
R. Rizzi, L. Palchetti and C. Serio, “The REFIR radiation explorer in the far infrared at the end of Phase B0”, in 9th International Workshop on Atmospheric Science from Space using Fourier Transform Spectrometry, Kyoto, 22-24 May, 2000 (CD-ROM version 2001):
