Multichannel Fourier-transform infraredspectrometer

Mamoru Hashimoto and Satoshi Kawata

A compact Fourier-transform IR spectrometer without a moving mechanism was developed. The

spectrometer consists of a shearing interferometer for forming a spatially distributed interferogram and

an IR array detector for observing the interferogram. The shearing interferometer of the developed

system is a birefringent interferometer with a Savert plate; the IR array detector is a PtSi Schottky-

barrier detector with 4096 elements. The optics and the system configuration are described in detail,

and the experimental results of the IR absorption spectra of polystyrene and polyethylene terephthalate

film are shown. The developed optics is as small as 20 x 6 cm k in size. The spectral resolution of the

prototype system is 27.6 cm-1 between 5000 and 2000 cm-'. The methods and their possibilities of

resolution improvement are also described.Key words: Interferometers, infrared spectroscopy, Fourier-transform spectroscopy, multichannel

detectors, infrared detectors.

Introduction

Thanks to the recent availability of various types ofhigh-performance multichannel solid-state detectors,grating dispersive spectrometers as polychrometersequipped with multichannel detectors are becomingpopular.'3 Such a type of spectrometer, which iscalled a multichannel dispersive spectrometer(MCDS), features no grating scanning, and hence, itis capable of the instantaneous measurement ofspectra or time-resolved spectroscopy. Because thereare no moving parts, MCDS can be compactly builtand is rugged against vibration. It is hence applica-ble to field use, on-line measurement, and in-processmonitoring as well as time-resolved spectroscopy.

The essential disadvantages of the MCDS tech-nique compared with Fourier transform spectros-copy, which currently dominates the IR spectroscopyresearch field and market, are its small opticalthroughput as a result of the necessity of an entranceslit for resolving the spectrum and the overlapping ofhigher-order diffraction onto the spectrum.

When this study was done the authors were with the Depart-ment of Applied Physics, Osaka University, Suita, Osaka 565,Japan. M. Hashimoto is now with the Kanagawa Academy of

Science and Technology, KSP East 301, 3-2-1 Sakato, Takatu-ku,

Kawasaki 213, Japan.Received 10 December 1991.

0003-6935/92/286096-06$05.00/0.o 1992 Optical Society of America.

Multichannel Fourier-transform spectroscopy(MCFT),G'3 which is a combination of a multichanneldetection (MC) technique and Fourier-transform in-terferometric spectroscopy (FT), eliminates such prob-lems with MCDS, although it uses multichanneldetectors. This idea was originally proposed by theauthors,5 and then various types of interferometerwere used for this type of spectrometry.6 8 Besidesresearch on resolution enhancement,'12- 5 backgroundsuppression9" 0 and other revisions" of this spectrom-etry have also been reported.

However, most published demonstrations are forvisible and near-IR spectroscopy, while this method isneeded in the IR field because the MCFT spectrome-ter is more advantageous in the IR region than in theshort-wavelength region in terms of the signal-to-noise ratio. There are no applications of MCFT to theIR region because IR multichannel detectors of highsensitivity and high resolution are still only at theresearch and development stage, and few practicalapplications to spectroscopy have been reported.Although compound semiconductors, such as HgCdTeand InSb, provide high quantum efficiency for photo-electric conversion, the integration of a large numberof photodetector elements made of compound semicon-ductors with CCD or a metal oxide semiconductorfield-effect transister (MOSFET) switches based onsilicon is still difficult with current technology. 6"17

A realistic IR multichannel detector is a Schottky-barrier detector.1821 This type of detector is con-structed as a monolithic silicon structure and utilizes

6096 APPLIED OPTICS / Vol. 31, No. 28 / 1 October 1992

the internal-photoemission effect of the Schottkybarrier between metal-silicide and p-type silicon.A silicon-base Schottky-barrier multichannel detec-tor with up to 4096 elements has been developed.2 'Because this detector is based on silicon, CCD's forsignal readout can also be integrated into a singlechip.

We describe an IR spectrometer that we havedeveloped with a platinum silicide Schottky-barrierdetector. We believe that this is the first paperreporting the development of IR MCFT as well as thefirst report of the application of a Schottky-barrier IRCCD to IR spectrometry.

OpticsFigure 1 shows an optical diagram of the developedMCFT spectrometer. The IR radiation to be mea-sured is polarized by a linear polarizer and is split intotwo polarized components by a Savert plate.82223

This birefringent prism shears the two componentslaterally, and the lens reunites these two componentsin its focal plane. The analyzer is used to make twocomponents interfere.

Compared with a Michelson interferometer used ina conventional Fourier transform spectrometer,24 theMCFT spectrometer shown in Fig. 1 produces aninterferogram (or the Fourier transform of the radia-tion spectrum) to be measured in space on thedetector plane rather than in time according to thereference-mirror scanning.

Figure 2 shows optics that are equivalent to thosein Fig. 1. The extended radiation source is split intotwo; they are laterally sheared by the Savert plate.Since the detection plane is the focal plane of the lens,this optics is a source-doubling setup; the interfer-ence fringe produced on the detector plane is equiva-lent to the one of equal inclination. The visibility ofthe interference fringe is therefore independent ofthe light source size.25 This means that theoreti-cally this optical setup permits an optical throughputthat is much larger than the conventional Fourier-transform spectrometer. In the conventional Fou-rier-transform spectrometer the allowable source sizeis limited by the necessary spectral resolution. Theequivalent optics shown in Fig. 2 are also equivalentto the triangle common-path interferometer, whichwas originally used for the MCFT research.5

In Fig. 2 the path difference A between two compo-nents at the x position in the detector plane is given

Virtualsources

Lens Interferogram

Multichanneldetector

Fig. 2. Schematic optical diagram equivalent to Fig. 1.

by

dx

f (1)

in paraxial approximation (the focal length f is as-sumed to be long enough compared with x); d in Eq.(1) is the distance between the corresponding twopoints of the two split virtual sources. In a conven-tional Fourier-transform spectrometer the path differ-ence A between two arms of the Michelson interferom-eter is a function of time t, while in the MCFTspectrometer shown in Fig. 2 it is a function of thespatial location x in the detector plane. An arraydetector placed along the x axis can read out thisinterferogram as a sequential signal without mechan-ical scanning. This is the principle of the MCFT.

The interferometer may form both the in-phaseand the antiphase interferograms in space by rotatingeither the polarizer or the analyzer by 90 deg. Bysubtraction of these two interferograms the back-ground distribution that is caused by noninterferencelight can be removed.

Since the two divided beams travel along almost thesame path, this interferometer is inherently ruggedagainst vibrations and can be made compact by use ofonly a polarizer, a Savert plate, an analyzer, and alens.

Savert Plate

The key component in the interferometer is a Savertplate. Figure 3 shows the coordinate system of thisprism. The Savert plate consists of two identicaluniaxial crystals that are cut so that their optic axesare aligned at 45 deg to the optical axis of the system.(The direction of the optic axes is shown in Fig. 3.)

In the first crystal the incident beams are dividedinto two components, the ordinary ray and the extraor-

Polarizer Analyzer Interfere rn

SourcSavert plate f

Lens Multichanneldetector

Fig. 1. Optical diagram of the MCFT spectrometer based on aSavert plate birefringent interferometer.

Fig. 3. Savert plate and the paths of ordinary-extraordinary (OE)and extraordinary-ordinary (EO) rays.

1 October 1992 / Vol. 31, No. 28 / APPLIED OPTICS 6097

dinary ray. The ordinary ray satisfies Snell's lawwhile the extraordinary one does not. As a result theextraordinary ray deviates from the ordinary one.The optic axis of the second crystal is perpendicular tothe first one. The ordinary ray in the first crystalchanges to the extraordinary one in the second crys-tal, while the extraordinary ray changes to the ordi-nary one. The output rays are hence parallel later-ally sheared but not longitudinally sheared. As aresult the Savert plate splits the source into twovirtual sources.

The distance d between the two virtual sources isgiven by22

n, 2- n,2d= 2 t, (2)

d=J(fe2 +l o2)t

where n0 and ne are the ordinary and extraordinaryrefractive indices and t is the thickness of the Savertplate.

System Configuration

Figure 4 shows the total system of the developedMCFT IR spectrometer. A Nichrome wire is usedfor an IR source. The Savert plate is made of twoTiO2 crystals. Its size is 15 x 15 x 15 mm3 . Thedistance between the two virtual sources split by theSavert plate is 1 mm. The polarizer and theanalyzer are aluminum wire grids on a CaF2 base(IGP227, Cambridge Physical Science). The lensesare made of CaF2. The focal length and the diameterof the imaging lens L1 are f = 40 mm and 4 = 30 mm,respectively, and for the Fourier-transform lens L2f = 60mmand4 = 50mm.

We used a Schottky-barrier detector of platinumsilicide with 4096 elements as an IR multichanneldetector.9 This detector was designed and producedby Kimata et al. for an earth resources satellite. Thespectral range of this detector covers - 1.3-5 pm.The size of each element is 10 x 10 pum2, and the totalwidth of the detection area is 40.96 mm. The maxi-mum path difference between two beams at the end ofthe detector corresponds to 0.033 cm. Hence theresolution of this spectrometer is 30 cm-'.

The detector is mounted in a cryostat cooled byliquid nitrogen at 77 K. The window of the cryostatis a CaF2 plate. The diameter of the window is 70

Source

Fig. 4. Total system of te developed MCFT IR spectrometer.

mm, and the thickness is 5 mm. The inside of thecryostat is vacuumed by a rotary pump to avoid dewcondensation.

The output signal from the detector is amplifiedand converted to a 12-bit digital signal. The elec-tronic circuitry is attached to the cryostat. Thedigital signal is sent to a personal computer (PC9801,NEC Corporation) with direct memory access datatransfer. The maximum scan rate for 4096 ele-ments is 300 Hz, i.e., 3.5 ms/1 scan.

Experiment

We carried out the following IR spectroscopy experi-ments by using the MCFT system. Figures 5(a) and5(b) show the interferograms of radiation from theNichrome wire source without sample absorption.Figure 5(a) is the interferogram measured with thepolarization axis of the analyzer in parallel to that ofthe polarizer, while Fig. 5(b) is the interferogramwhen the axis of the analyzer is perpendicular to thatof the polarizer. The parallel arrangement exhibitsa positive peak at the zero-path difference betweentwo beams as seen in Fig. 5(a), while the perpendicu-lar arrangement shows a negative peak. Here is anoninterference background distribution in bothcases. This background pattern can be removed bysubtracting the antiphase interferogram from thein-phase one. The interference component becomestwice as large by this subtraction, while the noninter-ference component disappears. Figure 5(c) showsthe result of subtraction. Figure 5(d) shows thecentral half of the interferogram of Fig. 5(c) expandedhorizontally by a factor of 2.

Figure 6 shows the Fourier transform of the inter-ferogram shown in Fig. 5(c), i.e., the reconstructedspectrum of the Nichrome wire source. The calcula-tion of the Fourier transform is done by fast Fouriertransform with the apodization by a triangle function.This spectrum includes an absorption of the TiO2crystal of the Savert plate at 3300 cm-'.

Figure 7(a) shows a transmittance spectrum of apolystyrene film measured by the system. We calcu-lated this spectrum by dividing the reconstructedspectrum by the reference spectrum of the source.The reconstructed spectrum, Fig. 7(a), includes theoverlapping of absorption peaks of C-H bonds withstretching vibrations between 3.0 and 3.7 pm.

As a comparison the spectrum of the same sampleis shown in Fig. 7(b) measured by a high-resolutiondispersive spectrometer (Perkin-Elmer 983). Thespectral resolution of the developed system [Fig. 7(a)]is less than the resolution of a grating spectrometer.The resolution of the present system is 27.6 cm-'.

The scanning rate for obtaining Fig. 7(a) was 7ms/scan (1.66 jis/element) in the experiment, and100 frames (700 ms) were averaged. The total expo-sure time was 2.8 s for collecting both in-phase andantiphase interferograms with and without the sam-ple. As a comparison it took 1200 s to collect thespectrum of Fig. 7(b) with a grating spectrophotome-ter.

6098 APPLIED OPTICS / Vol. 31, No. 28 / 1 October 1992

0 1000 2000PIXEL NUMBER

(c)

1500 2000PIXEL NUMBER

3000 4000

3000

(d)

Fig. 5. Interferograms of the Nichrome wire source: (a) in-phase interferogram; (b) antiphase interferogram; (c) subtractionfrom (a) by (b); (d) central half of (c).

30x103

25

COzz

20

15

10

5L ....,I.-.. ..... I.... .

5000 4500 4000 3500 3000 2500 2000

WAVE NUMBER (cm-,)

Fig. 6. Reconstructed spectrum of a Nichrome wire source.

We performed experiments with a number of othersamples. Figure 8(a) shows a transmittance spec-trum of a polyethylene terephthalate film measuredby the system that we have developed. This spec-

WAVELENGTH(gm)

100

80

w0z

zc:I.-

60

40

20

O L i i i tI i 5000 4500

1002!

80

_

:0z

U,zIn

H

60

40

20

L5000 4500

4000 3500 3000 2500 2000

WAVE NUMBER (cm 1)(a)

WAVELENGTH(gm)3

lI

4000 3500 3000

4 5

2500 2000

WAVE NUMBER (cm 1)

(b)Fig. 7. (a) Reconstructed transmittance spectrum of the polysty-rene film by the developed MCFT IR spectrometer. (b) Thespectrum of the same sample but by a dispersive grating spectrom-eter.

1 October 1992 / Vol. 31, No. 28 / APPLIED OPTICS 6099

4000

3000

a,Z 2000

z

1000

0

4000

3000

coI2000

1000

0

PIXEL NUMBER(a)

PIXEL NUMBER

(b)

2000

CDzz

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-20002500

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I , I . I . . . . I .. . . I ....I I I 1 1 I

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WAVELENGTH(gm) mum choice of d and f, which is 10 times better than3 4 5 the current value of 27.6 cm-' (2.5 cm-' of 5000 cm-'

divided by 4096/2 elements) and is comparable withcommercialized Fourier transform IR spectrometers.

To increase the distance d between the virtualsources, a thicker and more birefringent material

.... . .... . ..--*,,,+ - - - - .... .. must be chosen as a Savert plate. It is known that'lK i ||I the Savert plate distorts the interference fringe in the

direction along the fringes.22 However, since the....4-*4- .. -.....------ t--t! + E ~array detector used is one dimensional and each

element is effectively only 30 ALm long along the

..... .... .... .._. .... fringe, the distortion is not particularly significant.The effective spectral range of 2000-5000 cm is

... , ii| i due to the spectral response of the PtSi Schottky: 00 4500 .L~ 4000 3500 300..L±~ 2500barrier. It has been reported that an IrSi Schottky-

JO 4500 4000 3500 3000 2500 2000 barrier detector covering 1200-5000 cm-' was devel-WAVE NUMBER (cm ') oped.2 6 We hope that a multichannel detector cover-

(a) ing the mid-IR completely will be commerciallyWAVELENGTH(gm) available in the near future and will be used in the

2 _ __ 3 4 5 MCFT's.We did not discuss the signal-to-noise ratio of the

developed spectrometer. The signal-to-noise ratio of-1 - -- - _ X - this type of spectrometer is compared by the authors

theoretically27 with a conventional Fourier-transform_. _ -_ _ _ _ _ _ _ _ _ _ _ _4 _ _ _ _ _ spectrometer and dispersive grating spectrometers

with a single-channel detector and a multichanneldetector. The fundamental results derived through

, C - t the theoretical analysis in the paper is that MCFT isadvantageous in terms of the signal-to-noise ratio

-_. _ _ _. _- _ _ _ __ __ __ _ __ _____ when the optical throughput advantage of sourcedoubling optics is valid.

)--- L_________ __ ___The PtSi IR CCD used in the experiment was4500 4000 3500 3000

WAVE NUMBER (cm 1)

2500

(b)

Fig. 8. (a) Reconstructed transmittance spectrum of the polyeth-ylene terephthalate film by a developed MCFT IR spectrometer.(b) The spectrum of the same sample but by a dispersive grating

spectrometer.

trum includes an interference fringe caused by theinternal reflection of the film. Figure 8(b) shows thespectrum of the same sample measured by a Perkin-Elmer 983 for comparison.

Discussion

We built an MCFT IR spectrometer using a PtSi IRCCD. The spectra obtained were in the range of2000 and 5000 cm-' or 2 and 5 pm. The experimen-tal results indicate that this type of spectrometer maybe useful for IR spectral analysis.

The resolution of the present system is limited to27.6 cm-'. The reason for this limitation is notbecause of the number of CCD elements but becauseof the lack of a maximum path difference at the end ofthe multichannel detector Xma. The path differencecan be enlarged either by widening the separation d oftwo virtual sources or by using a lens with a shorterfocal length. The highest resolution Av attainablewith a 4096-element detector is 2.5 cm-' in the

2000-5000-cm- 1 range with the use of the opti-

developed for the earth observation system of EarthResource Satellite-1 of the Japan Resources Observa-tion System Organization (JAROS). The authorsthank Masafumi Kimata of Mitsubishi Electric for hishelp and discussion in designing and assembling thePtSi CCD system. The authors thank Shigeo Mi-nami of Osaka University and Hiro-o Hamaguchi,Laboratory Head of Kanagawa Academy of Scienceand Technology, for encouraging and supporting thisresearch. The authors note that this research wasinitiated in 1986 by Yasushi Inoue, who was at OsakaUniversity.

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