Hyperspectral Imager, from Ultraviolet to Visible, with a KDP Acousto-Optic Tunable Filter

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yperspectral imager, from ultraviolet to visible,ith a KDP acousto-optic tunable filter

eelam Gupta and Vitaly Voloshinov

Hyperspectral imaging in the ultraviolet to visible spectral region has applications in astronomy, biology,chemistry, medical sciences, etc. A novel electronically tunable, random-wavelength access, compact,no-moving-parts, vibration-insensitive, computer-controlled hyperspectral imager operating from 220 to480 nm with a spectral resolution of 160 cm�1, e.g., 2 nm at 350 nm, has been developed by use of a KDPacousto-optic tunable filter �AOTF� with an enhanced CCD camera and a pair of crossed calcite Glan–Taylor polarizing prisms. The linear and angular apertures of the AOTF are 1.5 � 1.5 cm2 and 1.2°,respectively. Imager setup and spectral imaging results as well as analyses and discussion of variousfactors affecting image quality are presented. © 2004 Optical Society of America

OCIS codes: 110.0110, 230.1040, 260.7190, 110.3000, 110.2970.

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. Introduction

ltraviolet- �UV� to-visible range hyperspectral im-gers are useful in a wide variety of applications instronomy, atmospheric sciences, medicine, spectros-opy, etc. Astronomers use these spectral images totudy galaxies, stars, and planets; study of ozone inhe Earth’s atmosphere is done in this spectral regions well.1 In medicine, fluorescence microscopy usesuch images to study in vitro tissue samples to dif-erentiate the cancerous cells from normal ones,2 andpectroscopy in this region is used to get chemicalomposition information. Such an imager can be de-igned with an acousto-optic �AO� tunable filterAOTF� with a camera sensitive in the UV-to-visibleange with a suitable optical train. Previously weeveloped hyperspectral imagers operating in the vis-ble and infrared wavelengths by using AOTFs.3–10

Until recently, efficient UV AOTFs were not avail-ble. Most of the AOTFs operating in the UV regionave been designed mainly with single crystals ofuartz ��-SiO2� in a collinear configuration. Otherrystals that can be used in this spectral region areapphire ��-Al2O3�, ammonium dihydrophosphate

N. Gupta �[email protected]� is with the U.S. Army Researchaboratory, 2800 Powder Mill Road, Adelphi, Maryland 20783-197. V. Voloshinov is with the Department of Physics, M. V.omonosov Moscow State University, 119992 Moscow, Russia.Received 21 August 2003; revised manuscript received 22 Jan-

ary 2004; accepted 18 February 2004.0003-6935�04�132752-08$15.00�0© 2004 Optical Society of America

752 APPLIED OPTICS � Vol. 43, No. 13 � 1 May 2004

ADP�, potassium dihydrophosphate �KDP�, andagnesium fluoride �MgF2�. Limited work has been

one in designing AOTFs operating in the UVegion.11–15 We have developed new AOTFs bysing KDP and MgF2 crystals operating in the UV-o-visible region. Based on our results in charac-erizing these filters for imaging, KDP-basedOTFs have the best performance in terms of

ransmission efficiency and optical throughputeeded to yield high-quality spectral images. Theeason for better performance of KDP is that it hashe highest value of the AO figure of merit, M2,mong all the available crystals. The acoustic andptical properties of KDP and its application in theesign of an AOTF were described in detail in an-ther paper.16

An AOTF is an agile filter that filters incidenthite light into narrowband light at a wavelengthetermined by the applied radio frequency �rf � signal.y changing the frequency of the applied signal,avelength of the filtered light can be changed with-ut any physical movement of the filter. The timeequired for making a wavelength change is quitehort, i.e., tens of microseconds. The wavelength ac-ess can be random or sequential, and multiple wave-ength operation can be carried out. Usually thepectral range of operation is one octave in wave-ength, but it can be expanded over more than twoctaves by use of multiple transducers.An AOTF is fabricated by bonding a piezoelectric

ransducer to a specially cut birefringent crystal.17–19

hen an rf signal is applied to the transducer, itroduces an ultrasonic wave that travels through the

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rystal with an acoustic frequency that is the same ashe applied rf. This sets up a moving diffractionrating in the crystal with the acoustic velocity spe-ific to the material and the grating period equal tohe acoustic wavelength in the material. An acous-ic absorber absorbs the sound wave after it traverseshe crystal. When light is incident upon such a crys-al, it is diffracted by the traveling acoustic wave androduces a diffracted beam with a Doppler shift for aarticular wavelength based on the phase-matchingondition. The diffracted wavelength is inverselyroportional to the acoustic frequency. The pass-and of the diffracted light depends on the incidentavelength, the birefringence of the material, the

ength of the transducer, and the geometry of theide-angle AO diffraction. There are two main

ypes of AOTF: collinear and noncollinear. Most ofhe filters used in imaging applications are noncol-inear and filter unpolarized broadband light intohree spatially separated beams: one frequency up-hifted, one frequency downshifted, and one of undif-racted light �this consists of both an ordinary and anxtraordinary wave� that contains incident light atll wavelengths except the diffracted wavelength.he two frequency-shifted beams have orthogonal po-

arizations, and they are spatially separated.20 Ineneral, to carry out spectral imaging using anOTF, one uses one of the diffracted beams with a

wo-dimensional focal-plane array while the otherwo output beams are blocked.

A hyperspectral imager was set up with a KDPOTF that operates from 220 to 480 nm with a pass-and of 160 cm�1, e.g., 2 nm at 350 nm, by tuning thef from 164 to 60 MHz in front of an uncooled UV-nhanced CCD camera with a suitable optical trainonsisting of quartz lenses, irises, and a pair of calcitelan–Taylor polarizer–analyzer prisms. The tun-

ng of the applied frequency and the image acquisi-ion and storage were carried out under full computerontrol.

In this paper we summarize the performance pa-ameters of the AOTF and the laboratory setup forhe imager and present the hyperspectral imagingesults obtained. To understand the spectral imag-ng results from an AOTF hyperspectral imager, wearried out a detailed quantitative analysis of variousactors that influence the image quality and spatialesolution of the filtered images, and we discuss theesults for our filter.

. Acousto-optic Tunable Filter Development

ne noncollinear AOTF cell was developed by using aarge single crystal of KDP. The AO figure of merit,

2, for KDP is 4.6 � 10�18 s3�g for the shear acousticave propagation, which is three times larger than

he corresponding value for quartz. Also, KDP isasily available in large sizes because it is a widelysed electro-optic material in Q switches and opticalarametric oscillators. A drawing of the cell ishown in Fig. 1. The AO interaction plane in therystal is �010�, and a 1.5 � 2.8 cm2 thin plate of x-cutithium niobate is bonded onto the crystal as a piezo-

lectric transducer such that it forms an angle, �, of° with respect to the optical axis of the crystal. Thenput optical facet of the crystal forms an 84° angleith the transducer facet of the prism. For ordinaryolarized light incident normal to the input opticalacet, the direction of optical beam propagation in therystal is determined by the polar angle �i, which isqual to 12° in this case, in the �010� plane relative tohe optical axis. The Bragg incidence angle for thisrdinary polarized beam in the cell is given by �o �

i � � � 6°. The input optical aperture of the crystals 1.5 � 1.5 cm2. When an rf signal is applied to theransducer, an acoustic wave with velocity 1.66 � 105

m�s is launched in the crystal. The moving gratingraverses the crystal in �10 s; this means that morehan 1.1 � 105 spectral images per second can bebtained. Basic parameters of KDP crystal are sum-arized in Table 1. A photograph of the filter is

hown in Fig. 2.The lithium niobate transducer is made of eight

ections connected in series. Transducer bondingith the crystal was carried out to minimize thecoustic power loss, and a 50- impedance-matchinglectrical network was used to minimize the electricalower loss. Measurements on a network analyzerhowed that the coupling of rf power into the trans-ucer is close to 100%. As a rule, the internal loss oflectric energy in well-designed and -fabricated pi-zoelectric transducers is low. Also, matching net-orks for the transducers consist of reactivelements, i.e., inductors and capacitors that do notissipate energy. Since less than 10% of the input rfower was reflected from the electric terminals of thelter, it is reasonable to conclude that most of theriving rf electric power applied to the AOTF cell wasffectively transformed into acoustic power. More-ver, measurement of the diffracted light intensity athe output of the KDP cell was close to the theoreti-ally predicted value. The observed high efficiencyf diffracted light at the filter output was anotherndicator of the highly efficient coupling between thelectric and acoustic powers in the piezoelectricransducer.

The transducer VSWR �voltage standing-wave ra-io� response is obtained by measuring both the re-ected and the transmitted RF signals in the circuitonsisting of the RF generator and the transducer.SWR is given by the ratio of the absorbed power and

Fig. 1. KDP filter prism.

1 May 2004 � Vol. 43, No. 13 � APPLIED OPTICS 2753

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he incident power, and it determines the electricalerformance of the device. The transducer operatesver a frequency range from 60 to 164 MHz measuredt VSWR � 6.0; the corresponding wavelength tun-ng range is from 480 to 220 nm. It was found thathe spectral resolution of the filter at FWHM was 160m�1, e.g., 2 nm at 350 nm, allowing us to obtainmages in 154 independent spectral bands. Figure 3hows the tuning curve of the AOTF, and Fig. 4 showshe frequency dependence of the acoustic power cou-ling into the transducer, which is directly related toSWR. The passband of the filter at 633 nm ishown in Fig. 5. The measured transmission coeffi-ients, T, of the filter �the ratio of the diffracted andncident light intensities� for 2 W of rf power appliedo the filter terminals is 70% at 250 nm, 60% at 350m, and 20% at 480 nm. The difference in the trans-ission coefficient values are due to the 1��2 depen-

ence of the diffracted light intensity as well as theower efficiency for ultrasound generation for acousticrequencies �70 MHz as shown in Fig. 4. Combina-ion of these effects resulted in drastic decrease in thealue of T for � � 400 nm. Details of the AOTFesign along with the characterization results wereiscussed in another publication.16

The tuning relationship for a noncollinear filtersing wide-angle diffraction geometry for n�no �� 1

n the negative uniaxial crystal is given by the fol-owing equation:

�0 � � nV�f �sin2��d � ��

sin �, (1)

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here �0 is the diffracted wavelength, n is the bi-efringence �difference of two refractive indices�, V ishe acoustic velocity in the material, f is the appliedadio frequency, �d is the diffraction angle for thextraordinary polarized beam, and � is the tilt angle.he angle �d is related to the Bragg angle of incidenceo, and the two indices of refraction, no �for the ordi-ary wave� and nd �for the extraordinary wave�, as

ollows:

�d � arccos��no�nd�cos �o�, (2)

here the refractive index nd can be expressed as

nd � no�1 � nno

sin2��d � �� . (3)

ince the angle �o is known, the value of �d can bebtained by solution of Eqs. �2� and �3�.The angular field of view �the angular separation

etween the incident and the diffracted beams�, � �

o � �d, in the crystal for small values of � is giveny the following expression:

� � � n�n �sin2�� cot � . (4)

Fig. 2. Photograph of KDP filter with side plate removed.
Fig. 3. Tuning relationship for the KDP filter.
Table 1. Parameters of KDP Crystal

Parameter

At Wavelength �nm�

633 nm 480 nm 350 nm 220 nm 200 nm

Index of Refractionno 1.507 1.515 1.532 1.596 1.622ne 1.467 1.470 1.487 1.543 1.562

Density �, 2.34 g�cm3

Effective photoelastic coefficient peff at 12° relative to Z axis in XZ plane, 0.067Acoustic phase velocity, V at 6° relative to X axis in XZ plane, 1.66 � 105 cm�sAO figure of merit M2, 4.6 � 10�18 s3�g

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The optical passband, �, and filter resolvingower, R, for small values of acoustic walk-off anglere given by

� �0.8�0

2 cos �o

nL sin2��d � ��, (5)

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�� 1.25

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�0 cos �o, (6)

here L is the length of the transducer.For the KDP AOTF no � 1.51 and ne � 1.47, giving

n � 0.04 at 633 nm; with �i � 12°, �o � 6°, �d �.36°, � � 6°, L � 2.8 cm, and V � 1.66 � 105 cm�s,e get � � 0.64° in the crystal and 1.0° in the air.he acoustic frequency f corresponding to 633 nm isqual to 43.6 MHz. Also, at 350 nm, n � 0.045 and� 88.7 MHz; at 250 nm, n � 0.05 and f � 138 MHz.rom Eq. �4� it is clear that the angle of spatial sep-ration of the incident and the diffracted beams out-ide the KDP crystal is larger at UV wavelengths250 nm. For example, � � 1.1° at 250 nm, and� � 1.2° at 220 nm, because data in Table 1 indicate

ig. 4. Frequency dependence of acoustic power coupling intoransducer of the KDP filter.

Fig. 5. Measured passband of the KDP filter at 633 nm.

hat n has larger values in the UV �0.05 at 250 nm�han in the visible spectral region.16

The output optical facet of the cell is cut at anngle, �, close to 3.5° relative to the input optical faceto form an optical wedge to eliminate the chromaticlurring that is due to spectral scene shift in air as aesult of the refractive-index dispersion. The wedges designed for operation with the ordinary �vertically�olarized incident light that gives a Doppler upshiftedxtraordinary �horizontally� polarized diffracted beam.ecause of the highly hygroscopic nature of KDP, aermetic seal was applied, all facets of the prism wereoated by a special chemical compound to protect therism from moisture after its fabrication. Both in-ut and output optical facets of the prism were anti-eflection coated to minimize the Fresnel reflectionosses.

AO properties—spectral range, passband, andransmission—of this AOTF were evaluated experi-entally. Two separate measurements of the spec-

ral bandpass of the filter were carried out with twoifferent lasers at 633 nm and 532 nm. The rf re-ponse and transmission of the filter were measuredith a network analyzer. The measured perfor-ance parameters of this AOTF are listed here:

Spectral range, 220–480 nmSpectral passband A at 350 nm, 2 nm; at 633 nm, 67

nmRf range, 60–164 MHzLinear aperture, 1.5 � 1.5 cm2

Angular aperture, 1.2°Applied power, 2.0 WTransmission coefficient, 60%

. Experimental Setup

he UV-to-visible imager is set up on an optical tabley placing the KDP AOTF in between a pair of calcitelan–Taylor polarizer–analyzer prisms in front of anncooled UV-enhanced CCD camera. A couple ofuartz lenses were used for beam forming, and aamera lens was used for imaging on the camera. Aenon lamp was used as a source of light. The sche-atic drawing of the imager is shown in Fig. 6.ince the diffracted and undiffracted beams have aather small angular separation, irises and spatiallters were used to block unwanted light beams. Aesolution chart and microscope slides with plantpecimens were placed in the path of the light fromhe source before the AOTF to form the spectral im-ges with the diffracted light. The rf power neces-

ig. 6. Experimental arrangement for UV-to-visible hyperspec-ral imager.


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ary to yield high-quality spectral images was only00 mW. The operation of the rf synthesizer forhanging the frequency and intensity and the imagecquisition by the CCD and data storage on the hardrive was carried out under full computer control.he CCD output is captured and digitized with a

rame grabber and stored on the hard drive. Weeveloped a graphical user interface for seamless op-ration of the imager. The block diagram of theomputer-controlled experimental setup for the hy-erspectral imager is shown in Fig. 7, and a photo-raph of the imager setup is shown in Fig. 8.

. Results

pectral images were obtained for a wide range ofavelengths. The highest-quality spectral imagesere obtained from 340 to 500 nm; some of these are

hown in Figs. 9 and 10. The shortest wavelengthor which the filtered signal was obtained for thisOTF was 230 nm. The images of the resolutionhart were obtained only at wavelengths longer than70 nm because it was made of glass and could notransmit efficiently in the UV region. The resolu-ion chart image in Fig. 9 shows that both the hori-ontal and the vertical lines are not blurred. Theicroscope slides were made of quartz and could

ransmit light at shorter wavelengths. A large num-er of spectral images were obtained. The imagentensity is higher for longer-wavelength images.lso, a spectral scene shift �even with a wedge cor-

Fig. 7. Block diagram of imager experimental setup.

Fig. 8. Photograph of imager experimental setup.


ection�, when the image shifts in space, was ob-erved at the shorter wavelengths when filter wasuned down from 480 nm to below 250 nm.

. Spatial Resolution During Acousto-optic Processingf Images

o better understand our results, it is important toook at various factors that influence the optical qual-ty and spatial resolution of an image filtered by anOTF.21,22 In particular, the quality of an imageeteriorates owing to nonuniform distribution of colorver a frame and longitudinal and transverse shifts offiltered scene, as well as optical distortion and blur-

ing in an image introduced by various optical ele-ents. Here we consider mainly the optical blur

nd scene shifts produced in the image because of theOTF.The two most important parameters that can quan-

itatively determine the quality of a filtered imagere the spatial resolution, r, and the number of re-olvable spots, N, in the image. In general, the well-nown Raleigh criterion is used to evaluate thepatial resolution in an image, and the total numberf spots resolved in a two-dimensional picture isqual to a product of the corresponding numbers of

Fig. 9. Spectral image of a resolution chart at 470 nm.

ig. 10. Spectral images of a lilac flower specimen at four differ-nt wavelengths: 380, 420, 460, and 480 nm.

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ixels along two orthogonal directions, namely, the xnd y axes, i.e., N � Nx � Ny. If an image is formedy a collimated monochromatic light beam of wave-ength � by an optical system with the linear aperture, and the angular aperture �, then r � �� andx � ��, where the angular spreading �� 0.9

�a is due to the diffraction limit of this optical sys-em.

For the KDP filter under investigation, the angularperture, �, is 1.1° and the linear aperture is 1.5 cm.onsequently, the number of resolvable spots alongne of the directions evaluated at 633 nm gives Nx �00. Since the KDP filter was designed with ap-roximately equal angular and linear apertureslong the x and y axes, Nx � Ny. In other words, theotal number of resolvable spots at the KDP filterutput is given by N � Nx

2 � 2.5 � 105. This valueets the upper limit of the spatial resolution becausehe performance of a real optical device is alwaysorse than what is predicted by the diffraction limit.The above discussion of image quality parametersas for monochromatic illumination and is a much

impler case than for a device that operates over aide spectral range, i.e., from the UV to the visibleavelengths, as would be the case for an AOTF thatlters light over a wide spectral range. When thelter operates with a nonmonochromatic beam, thenhe dominant factors are the spectral resolution, R �� , and the spectral passband, �, of the AOTFnd not the diffraction limit. In general, both thesearameters can dramatically influence the quality offiltered image.21,22 It is well known that poor spec-

ral resolution always causes undesirable blurring infiltered image and decreases the total number of

esolvable spots in it.In the case of noncoherent illumination, the num-

er of resolvable spots in a line, Nx, can still be eval-ated by use of the general equation Nx � ��d,here ��d is the angular spread of the diffracted op-

ical beam at the filter output corresponding to aingle pixel in the image. For the case of the wide-ngle geometry for the anisotropic AO interaction,�d in KDP can be expressed as

��d �0.8�0

no L sin �0. (7)

sing Eq. �4� and relation �7� and assuming that �s small, we obtain the number of resolvable spotshat can be observed in a filtered image as follows:

Nx �1.25 nL

�0sin2��d � ��cos �d. (8)

Examination of inequality �8� shows that the num-er of spots along one line is inversely proportional tohe optical wavelength; i.e., the number is greater forhorter wavelengths. This result agrees with theeneral assumption that better spatial resolution ofn optical system can be obtained with shorter wave-engths of light. In other words, the size of a re-

olved pixel in a picture decreases with decreasingptical wavelength.It is also clear from inequality �8� that the number

f spots in a filtered image increases linearly with theransducer length as well as the birefringence of theaterial, n. This number also increases with in-

reasing incidence angle �0. To obtain less blurring,e need to use large transducers. In other words, areater AO interaction length gives higher spatialesolution as well as narrower spectral bandwidth�. The reason for this is that for greater AO inter-ction lengths the incident optical beam traverseshrough more periods of the ultrasonically inducediffraction grating. Similarly, a large value of bire-ringence and the far-off-axis propagation of light in airefringent material result in a large number ofrating periods for the light beam to pass throughnd interact with.For the KDP AOTF, we get Nx � 95 by using n �

.04, L � 2.8 cm, � � 633 nm, �d � 5.36°, and � � 6°n inequality �8�. The corresponding value of spatialesolution in the filter is r � Nx�a � 63 lines�cm with� 1.5 cm. On the other hand, if the filter is tuned

n the UV spectral region, then at 350 and 250 nm thealues of number of resolvable spots and spatial res-lution are 195 and 130 lines�cm and 300 and 200ines�cm, respectively. These numbers of resolvablepots in a line may be considered adequate for mostmaging applications because the upper limit of theumber of resolvable spots in a line for the filter is00 as discussed above.This analysis clearly shows that the real number of

esolvable spots in a filtered image depends on thepectral resolution of the filter. The actual spatialesolution in the case of nonmonochromatic illumina-ion is a few times worse than the diffraction-limitedesolution computed at � � 633 nm. The lower spa-ial resolution inevitably results in blurring of theltered image. Consequently, in practice good spa-ial resolution and good optical quality for a filteredmage can be expected only if the AOTF is fabricatedith a large angular and a large linear aperture asell as a narrow bandpass. AOTFs with low spec-

ral resolution will produce filtered images that areot sharp.As mentioned earlier, good images were obtainedith the KDP filter at driving power levels as low as00 mW. However, the transmission coefficient ofhe device at these driving signals was low, e.g., a fewercent. To operate with high transmission coeffi-ient values, it was necessary to increase the drivingf power to 2.0 W, especially at longer optical wave-engths. Crystal heating caused by the absorbedcoustic power had no registered influence on theltered image quality during the experiments.ven at driving signals as high as 3.0 W, it wasbserved that heating did not influence the filter op-ration. The best explanation for the observed filterehavior is that both the area of the transducer �2.8 �.5 cm2� and the size of the KDP crystal �3.6 � 2.4 �.0 cm3� were relatively large. The heat sink alsolayed a key role in the dissipation of acoustic energy


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nd in maintaining a low temperature gradient in theell, which resulted in the absence of noticeable im-ge distortion.Since TeO2 imaging AOTFs are most commonly

sed, it is interesting to compare the image quality ofhe KDP filter with TeO2 filters. The KDP filternder investigation has a relatively narrow angularperture of 1.2° in comparison to 4°–9° angular ap-rtures typically obtained with TeO2.3–10,17–23 Thiseans that the filtered image quality for this KDP

lter is expected to be worse than that for the TeO2lters mainly because of the lower birefringence ofDP. Since at present KDP is the best material for

abricating AOTFs operating at shorter UV wave-engths �below the absorption edge of TeO2�, our bestractical choice is limited to small-angular-apertureDP filters.

. Influence of Dispersion on Image Shifts

ne major factor that gives a low-quality processedmage from an AOTF is the transverse shift of theltered scene as a function of wavelength. This shift

s due to the effect of optical dispersion, i.e., the spec-ral dependence of the refractive indices of the crys-al.9,10 In the KDP filter this dispersion is verytrong at wavelengths below 250 nm. This gives riseo a significant variation in the relative lengths of thencident and the diffracted wave vectors as a functionf wavelength. Consequently the angle � betweenhe wave vectors of the incident and the diffractedight changes with wavelength, instead of having axed value in the crystal. As a result the position ofhe diffracted optical beam is not fixed in space butoves relative to the undiffracted beam, causing a

hift in the filtered image each time the filter is tunedo a different optical wavelength.

To compensate for the spatial shift, the KDP filteras fabricated with an optical wedge at the output, as

hown in Fig. 1. The wedge was formed by tiltinghe output optical facet of the crystal by an angle �elative to the input facet. The value of � can bealculated by use of the following expression:

� � ��2�

no��2��1 �

� � 1� � 1� , (9)

here � � ��2�� 1� is the ratio of separationngles between the incident and the diffracted beamsnd � � no��2��no��1� is the ratio of the indices ofefraction at two limit wavelengths �1 and �2 of theuning range.

The measured value of the tuning range of thecoustic frequencies from Fig. 4 is from 60 to 164Hz. According to data in Fig. 3, the corresponding

uning range for the optical wavelengths is from 220o 480 nm. From Table 1, the values of the ordinaryndex of refraction in KDP at these limit wavelengthsre no��1� � 1.596 and no��2� � 1.515. Conse-uently the computed values of � at �1 and �2 were.2° and 1.0°, respectively. Using these values inelation �9�, the computed value of angle � is equal to


.0° which is close to the value 3.5° obtained with aore rigorous expression for �.During our experiment, observation of the position

f the filtered image showed that inclusion of theedge had practically compensated for the undesired

cene shifts over the entire tuning range of the filter.he shifts were limited to �0.04°. But when thelter was tuned from relatively long optical wave-

engths to the shorter UV wavelengths �near the ab-orption edge of the material�, it was noticed that theltered scene was not fixed in space and was movingway from the transmitted light. We should pointut that if the filter did not contain the wedge, thepatial shifts at the filter output would be ��1� ��2� � 0.2°, i.e., �20% of the image frame � � �.0°�. In many practical cases such a shift is notcceptable and may require further compensation bydditional electronic components and during postrocessing of images.

. Summary and Conclusion

novel noncollinear AOTF cell was designed andabricated with a single KDP crystal; at the presentime KDP is the best crystal available for designing aide-angle diffraction AO interaction geometry de-ice because its AO figure of merit for is three timesigher than that for quartz. This cell was used withUV-enhanced uncooled CCD camera and a pair of

alcite Glan–Taylor prisms in setting up a fullyomputer-controlled hyperspectral imager operatingrom 220 to 480 nm. The linear aperture of the cells 1.5 � 1.5 cm2, but the angular aperture at 1.2° isather small and required the use of irises and spatiallters to improve the signal-to-noise ratio. Thepectral passband of the filter was designed to beelatively wide to increase the light throughput andmprove the signal-to-noise performance of the im-ger. High-quality hyperspectral images were ob-ained with this imager. The filter operatedfficiently at low applied power, showing that such aell has relatively higher transmission than quartznd that its imaging performance is better. Theavelength tuning time for this filter is less than 10s, and the imager can be used to obtain an imageube with 154 independent wavelengths. Such UV-o-visible range imagers with random wavelength ac-ess can be used in a wide variety of applications instronomy, environmental monitoring, medical im-ging, and chemical analysis.The effects of spectral resolution and transversal

pectral scene shift on the image quality were dis-ussed in detail and analyzed quantitatively for theDP AOTF. The results show that filtered imagesith a good spatial resolution and high optical qual-

ty can be obtained from an AOTF with large angularnd linear apertures as well as a narrow spectralandpass. The main limiting factor for the imageuality of the KDP filter is its low birefringence.lso, the wedge in the AOTF compensated for thecene shifts over most of the spectral interval, buthis compensation is not effective when the filter isuned down from the longest to the shortest wave-

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engths ��250 nm� owing to effect of the large rise inhe values of the two refractive indices near the ma-erial absorption edge. At present, because of theack of a better material, KDP offers the best optionor designing AOTFs operating at shorter wave-engths than is possible with other available AO ma-erials. Filter design will be further improved to getigher transmission and improved performance.

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3. N. Gupta, R. Dahmani, and S. Choy, “Acousto-optic tunablefilter based visible-to-near-infrared spectropolarimetric im-ager,” Opt. Eng. 41, 1033–1038 �2002�.

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5. N. Gupta and R. Dahmani, “Tunable infrared hyperspectralimagers,” Proceedings of the International Symposium onSpectral Sensing Research �ISSSR� 2001 �Science and Tech-nology Corporation, Hampton, Va., 2001�, pp. 233–236.

6. N. Gupta, R. Dahmani, K. Bennett, S. Simizu, D. R. Suhre, andN. B. Singh, “Progress in AOTF Hyperspectral Imagers,” inAutomated Geo-Spatial Image and Data Exploitation, W. E.Roper and M. K. Hamilton, eds., Proc. SPIE 4054, 30–38�2000�.

7. D. A. Glenar, J. J. Hillman, B. Saif, and J. Bergstralh,“Acousto-optic imaging spectropolarimetry for remote sens-ing,” Appl. Opt. 33, 7412–7424 �1994�.

8. L. J. Cheng, J. C. Mahoney, G. F. Reyes, and H. R. Suiter,“Target detection using an AOTF hyperspectral imager,” inOptical Pattern Recognition V, D. P. Casasent and T. Chao,eds., Proc. SPIE 2237, 251–259 �1994�.

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2. P. Katzka and I. C. Chang, “Non-collinear acousto-optic filterfor the ultraviolet in active optical devices,” in Infrared Tech-nology for Target Detection and Classification, P. M. Narendra,ed., Proc. SPIE 202, 26–32 �1979�.

3. I. C. Chang and J. Xu, “High performance AOTFs for theultraviolet,” Proceedings of the IEEE Ultrasonics Symposium�Institute of Electrical and Electronics Engineering, New York,1998�, pp. 1289–1292.

4. I. B. Belikov, V. B. Voloshinov, A. B. Kasyanov, and V. N.Parygin, “Acousto-optic spectral filtration of radiation in theultraviolet region,” Sov. Tech. Phys. Lett. 16, 645–650 �1989�.

5. V. B. Voloshinov, “Acousto-optic filtration of electromagneticradiation in the ultraviolet region,” in Physical Acoustics, Fun-damental and Applications, O. Leroy and M. Breazeale, eds.�Plenum Press, New York, 1991�, pp. 665–670.

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2. L. J. Denes, B. Kaminsky, M. Gottlieb, and P. Metes, “Factorsaffecting AOTF image quality,” in Proceedings of the FirstArmy Research Laboratory AOTF Workshop, N. Gupta, ed.,Army Research Laboratory Report ARL-SR-54, �Army Re-search Laboratory, Adelphi, Md., 1997�, pp. 179–188.

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Hyperspectral Imager, from Ultraviolet to Visible, with a KDP Acousto-Optic Tunable Filter