Ultraviolet-Visible Imaging Acousto-Optic Tunable Filters in KDP

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ltraviolet–visible imaging acousto-optic tunableilters in KDP

italy Voloshinov and Neelam Gupta

There is a need to develop large-aperture acousto-optic tunable filters �AOTFs� in the UV region forapplications in astronomy, environmental sciences, biology, etc. We have developed a high-qualitynoncollinear AOTF cell that uses a single crystal of KDP that has nearly a four times larger acousto-opticfigure of merit, M2, than quartz. The linear and angular apertures of this cell are 1.5 cm � 1.5 cm and1.2°, respectively. The spectral range is 220–480 nm, with 160-cm�1 spectral resolution and hightransmission in the UV. We present an analysis of the design and describe the characterization results.

OCIS codes: 230.1040, 120.2440, 160.1050, 260.7190, 220.4610.

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. Introduction

cousto-optic tunable filters �AOTFs� are useful inhe design of high-resolution spectrometers whenhey are used with a single detector and a hyperspec-ral imager with a two-dimensional detector array.uch filters are also used to tune laser wavelengths inither an external cavity or in an intracavity config-ration. An AOTF can also be used with a broad-and source to provide a tunable light source.1–5

ost of these filters have been designed to operate inhe visible and the infrared regions.6,7 The opera-ion of the filter is based on the well-known phenom-non of the diffraction of light by acoustic-waveropagation in a birefringent crystal.1–4 So far, ul-raviolet �UV� filters have been designed mainly in aollinear configuration with high spectral resolutionnd have used single crystals of quartz, a few non-ollinear filters with wide-angle fields of view havelso been designed.8–13 Such noncollinear devicesequire rather large single crystals. A UV AOTFan be used for a multitude of applications in variouselds, from atmospheric monitoring to medicaliagnosis.10–14 Such applications include monitor-ng of both terrestrial and extraterrestrial atmo-pheric ozone concentrations and the diurnal andeasonal changes in them, in fluorescence spectro-

V. Voloshinov is with the Department of Physics, M. V.omonosov Moscow State University, 119992 Moscow, Russia.. Gupta �[email protected]� is with the U.S. Army Researchaboratory, 2800 Powder Mill Road, Adelphi, Maryland 20783-197.Received 18 June 2003; revised manuscript received 21 January

004; accepted 30 March 2004.

copic imaging of biological samples to differentiateormal from cancerous tissues, in the design of tun-ble UV sources to carry out both fluorescence andesonance Raman spectroscopy, and in the detectionf bacterial and viral samples.In this paper we examine the prospect of utilizing

ingle crystals of KH2PO4 �KDP� to design and fab-icate imaging AOTFs. First we discuss design con-iderations; then we review the optical, photoelastic,nd acoustic properties of the KDP material and ap-ly acousto-optic �AO� theory to determine the pa-ameters that will ensure a wide-angle AOnteraction. Next, we calculate the Bragg incidencengle as a function of the acoustic frequency, theuning relationship, and the transmission coefficientor the filter. Finally, we discuss the filter design,abrication, and the laboratory characterization ex-eriments, and we include a detailed analysis of theesults of our investigation.

. Design Considerations

oncollinear UV AOTFs can be designed with singlerystals of NH4H2PO4 and MgF2 as well as KDP.13–15

ased on our experience, the best AOTF performancet UV wavelengths longer than 200 nm is obtainedor KDP rather than for the other two crystals. Theain reason for this result is the relatively higher

alue of the AO figure of merit for KDP than for thether materials,1–4 KDP is a well-known ubiquitousonlinear optical material that is used extensively inhe design of electro-optic Q switches and opticalarametric oscillators, specifically for second-armonic generation and for frequency doubling.articular attention had not been paid to this mate-ial for designing AO devices because of its lower AO

1 July 2004 � Vol. 43, No. 19 � APPLIED OPTICS 3901

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gure of merit13,14 than in the crystals used in theisible region.Designing an AOTF for application in UV spectral

maging, especially for outdoor monitoring and re-ote sensing, requires a special effort to get sufficient

ntensity and optical throughput to form a high-uality image on a camera. A single crystal of KDPan be used as a spectral filter.13–15 It is importanto design a filter that can both transmit higher in-ensities of light and possess higher throughput thann the previous designs.10–15 Because the photons inhe UV region of the spectrum at and below the vac-um edge �250 nm� are absorbed by air to a signifi-ant degree, we must raise the throughput of thelter by increasing its diffraction efficiency and trans-ission coefficient, its passband, and both its linear

nd angular apertures. Consequently, the filter willossess relatively lower spectral resolution, R �� 100, in comparison with the filters intendedor traditional spectroscopic applications for whichigh resolution, R � 1000, is required.

. Physical Properties of KDP Crystals

t is well-known that a single crystal of KDP is anfficient crystal in electro-optics and nonlinear op-ics.1,16 It is widely used as an electro-optic Q switchnd as an optical parametric oscillator. However, arawback of this material is that it is rather hygro-copic; special care needs to be taken to prevent itrom deteriorating over time. Because not many bi-efringent crystals are transparent in the UV region,DP is one of only two or three available low-costirefringent crystals in which the so-called wide-ngle AO interaction geometry is possible.

. Optical and Photoelastic Properties of KDP

he KDP crystal belongs to the tetragonal class ofrystalline materials of point group 4�2m.1,13–15 Theensity of the crystal, � 2.34 g�cm3, is low com-ared with that of the commonly used crystal TeO2.DP is an optically negative uniaxial crystal; i.e., the

ndex of refraction of an extraordinary polarizedeam, ne, is smaller than that for the ordinary polar-zed beam, no. Its birefringence, �n � 0.04, is closeo one-fourth that of TeO2.1 The material is trans-arent to � � 200 nm,16 and �n is fairly constant for� 300 nm but increases sharply near the KDP

bsorption edge �� 200 nm�. Because the acousticriving frequency of a filter, f, is proportional to theirefringence, �n��, the wavelength dependence ofhe acoustic driving frequency is stronger near theDP absorption edge. This means that the spectral

uning range of the filter, which is in excess of anctave in wavelength, can be obtained only with aransducer generating ultrasound in an interval ofcoustic frequencies that exceeds an octave.Photoelastic coefficients, pij, of KDP are known

rom the literature.2,14,17 The values of p11, p12, p13,31, and p33 are approximately equal to 0.23. It cane shown that, for longitudinal acoustic-wave propa-ation in the material, the effective photoelastic con-tant p for the anisotropic interaction of light and
eff
902 APPLIED OPTICS � Vol. 43, No. 19 � 1 July 2004

ound in KDP is proportional to the p11–p12, p11–p13,r p11–p31. As the coefficients have the same signsnd they are nearly equal to one another, their dif-erences are small for KDP and the value of peff islose to zero. Small values of peff are also typical forar-axis interactions when shear acoustic waves areaunched away from the crystalline axes X, Y, and Zf the material. Due to the small values of peff, theirection of sound propagation 110� and the plane1�10� of the AO interaction are not recommended foresigning an AO device in KDP. In this respect,DP is different from the well-known AO materialeO2, in which slow shear acoustic waves propagatelose to the direction 110�.18

Based on the considerations described above, onlyhear acoustic-wave propagation close to the KDPrystal axes X and Y in the �010� and �100� planes isf interest for a practical AOTF device. For thisind of AO interaction, only two coefficients, p44 and66, are involved in the diffraction process. Becausehe value p44 � �0.034 is only half that of p66 �0.068, the only coefficient of interest in the design offilter is p66. Unfortunately, the magnitude of this

oefficient is �0.1. Thus it is reasonable to predicthat the AO figure of merit for this crystal will haverelatively low value.

. Acoustic Properties of KDP

he results of calculations to compute the acoustichase velocities for the shear waves in KDP showhat phase velocity Vt for the transverse acousticave in the �001� plane of the material is Vt � c44�2.34 � 105 cm�s, where c44 is the coefficient of the

lastic tensor of KDP that determines the acousticroperties of the crystal. The slow shear acousticode propagating along the X and Y axes has a phase

elocity V � c66� � 1.65 � 105 cm�s that is ap-roximately 1.5 times smaller than for this moderopagating along the 100� axis. It can be shown,owever, that if the slow, shear wave propagates inlane �010� or �100� at an angle � to the X or Y axis,he phase velocity will continuously change from.34 � 105 to 1.65 � 105 cm�s, depending on the valuef �. To obtain a high value of peff with a reasonablealue of M2 �needed for the design of a practical wide-ngle device� requires that the slow shear wave prop-gate in either the �010� or the �100� plane of the KDPingle crystal.We use the following equation to calculate the

hase velocity of the shear waves propagating at anngle � with respect to the 100� axis in the �010�lane of the crystal:

V�� V1002 cos2 � � V001

2 sin2 ��1�2, (1)

here V100 and V001 are the velocities along the X andaxes, respectively. The angular dependence of the

hase velocity in the �010� plane is shown in Fig. 1.coustic walk-off angle �, i.e., the angle between the

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he relationship of acoustic walk-off angle � to tiltngle � is also shown in Fig. 1. Comparison of theata in Fig. 1 with the corresponding data for TeO2Ref. 18� shows that, in KDP, the dependence of thehase velocity on angle � is weaker and the absolutealues of � are much smaller. These characteristicsre advantageous because they result in use ofhorter lengths of the crystal than what is requiredhen there is a large acoustic walk-off. If the aniso-

ropic AO interaction plane includes the 001� and100� axes and if the slow shear acoustic mode is used,he highest value of M2 will be obtained at � � 0, with

small value of incident angle �. The maximumalue of the figure of merit is limited to M2 � 6.2 �0�18 s3�g because peff � p66 � �0.068, with n � 1.5nd V � 1.65 � 105 cm�s. This figure of merit isore than 200 times smaller than in TeO2 but �4

imes larger than in fused silica.18 At present, KDPs the best material for operation in the 200–300-nmV spectral region, even though the AO figure oferit for this crystal is relatively low, because of a

ack of better materials.

. Diffraction of Light by Ultrasound in KDP

. Wide-Angle Acousto-Optic Interaction Geometry

any characteristics of light diffraction by acousticaves can be obtained from the analysis of waveector diagrams that illustrate the law of momentumonservation during the AO interaction.1–4,13 Theector diagram that corresponds to plane of interac-ion �010� in KDP is shown in Fig. 2. Here ki, kd,nd K are the wave vectors that correspond to thencident and the diffracted light and the acousticave, respectively. The incident light is an ordinaryave, whereas the diffracted light is extraordinary.he angle of incidence of the light in the figure, �, iseasured with respect to the direction of the acoustic

ig. 1. Acoustic phase velocity V and walk-off angle � in a �010�lane as a function of tilt angle.

ave front. The lengths of these wave vectors are ki2� no��, kd � 2� nd��, and K � 2� f�V, where nd

orresponds to the refractive index of the diffractedave and ne � nd � no. These refractive indices are

elated to one another as follows:

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no2 sin2��d � �� ne

2 cos2��d � ��1�2 , (3)

here �d is the angle of diffraction. We can make aurther simplification in Eq. �3� based on the fact thatn�no �� 1 to get the following simpler expression ford:

nd � no1 � ��n�no�sin2��d � �� . (3a)

aking the cosine projections of the wave vectors ofight on the acoustic-wave vector, we can write

no cos � � nd cos �d . (4)

r diffraction angle �d can be computed by

�d � arccos�no�ne�cos �� . (4a)

hen the separation between the incident and theiffracted waves is small, deflection angle �� dan be expressed as

�� n�no�sin2�� cot �d . (4b)

sing the sine projections of these vectors in theirection of the acoustic-wave vector, we can writehe following expression for the acoustic frequency:

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1�2 � nd sin �d� . (5)

ig. 2. Wave vector diagram for wide-angle AO interaction in aingle crystal of KDP.


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n approximate expression for Eq. �5� can be writtenor �n�no �� 1 as

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e can use Eqs. �4� and �5� to find the relationship ofngle of incidence �, angle of diffraction �d, and fre-uency f. This relationship is shown in Fig. 3. Ithould be noted that, if the polarization of the inci-ent light is changed from ordinary to extraordinary,urve o for ��f� in Fig. 3 will correspond to the fre-uency dependence of diffraction angle �d�f�, whereasurve e is for the incident light. It is important toote that all the optical waves propagating in KDPrystal stay relatively close to the optical axis formall values of �. Because the KDP crystal hasractically no optical activity,1 a set of cross polariz-rs can be used to block off the zero-order diffractedeam, unlike for TeO2.4–9,19

. Calculation of Bragg Incidence Angles

he frequency dependence of the incidence angles forhe ordinary �o� and the extraordinary �e� polarizedaves in KDP is presented in Fig. 3. These valuesere calculated from the optical and the acousticroperties of KDP.13–17 The calculation was carriedut with Eqs. �3�–�5� at � � 350 nm with � � 6°.his tilt angle was chosen to keep the upper limit of

he acoustic frequency range below 200 MHz to pre-ent strong attenuation of ultrasound in the crystal.ompared with those for TeO2, the Bragg angles forDP for the ordinary wave are larger than those for

he extraordinary wave. The difference in these an-les is due to the fact that KDP is optically negativehereas TeO2 is optically positive.It is clear from Fig. 3 that a 6° Bragg incidence

ngle corresponds to the wide-angle regime of thenisotropic AO interaction in KDP, because here df�� 0, and the corresponding angle of diffraction is.3°. This means that the diffracted and the undif-

ig. 3. Frequency dependence of angles of incidence for both or-inary �o� and extraordinary �e� incident waves.


racted beams are separated by a deflection anglequal to 0.7° in the crystal and as much as 1.2° in air.n an optimized filter, deflection angle �� is equal tocceptance angle �� determined by a range of inci-ence angles over which the filter has efficient oper-tion. In the optimal case, the filtered and theransmitted beams do not overlap. In general, spa-ial overlap results in a poor signal-to-noise ratio athe filter output, even if a polarizer is used to sup-ress the undiffracted light and to select the filteredadiation.19 It is clear that the deflection angle de-ends on the birefringence and increases with �n,hereas the acceptance angle also depends on the

ransducer length, i.e., by the divergence of ultra-ound. Unfortunately, deflection angles �� for KDPre a few times smaller than for TeO2 because ofower birefringence of KDP.19,20 However, it is pos-ible to increase angular aperture in KDP by chang-ng the value of �, i.e., by selecting other cuts of therystal. For example, an angular aperture in air of2° may be obtained for � � 12°. In this case thepper limit of acoustic frequency increases to 400Hz, which is problematic from a practical consider-

tion because the attenuation of ultrasound at highrequencies, especially when the filter has a largeinear aperture, becomes a dominant factor. Also,esign of electrical matching networks for the trans-ucers at 300–400 MHz is quite difficult.

. Tuning Curve and Spectral Transmission of the Filter

he theoretical tuning curves for a filter that uses theut of the crystal at � � 6° with �solid curve� andithout �dashed curve� birefringence dispersionn�� are shown in Fig. 4, with �n � 0.04. It is clear

rom these curves that the dispersion broadens theange of driving frequency. For example, the acous-ic frequency range is 60–110 MHz without includingispersion in the calculation, and it is 65–140 MHzith dispersion when the filter is tuned over wave-

engths from 250 to 450 nm. This difference is par-icularly noticeable at the UV wavelengths �300 nm.

ig. 4. Tuning curves for the filter with dispersion �solid curve�nd without dispersion �dashed curve� for a 6° tilt angle.

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We can calculate the spectral transmission of thelter for low diffraction efficiency, if the acoustic fre-uency bandwidth of diffraction is known, by usinghe following relationship:7,8,13,18

�f � �0.8V��

l cos � �no V��cos�� 0� � �f0 sin �

no V��sin �0 � �f0, (6)

here angle of incidence �0 and acoustic frequency f0orrespond to the wide-angle diffraction and l is theength of the transducer. The data in Fig. 1 showhat the acoustic walk-off angle at � � 6° is small ands limited to � � 5°. In this case Eq. �6� may beimplified to

�f � 0.8V��1 � tan �0 tan ��l tan �0� . (7)

he spectral passband of the filter and the acousticrequency interval are related by the well-known ex-ression

�� f�f0 . (8)

quations �5�, �7�, and �8� can be combined to yieldhe following expression for �� for small values of �:

�� 0.8�2 cos �0

�nl sin2��d � ��. (8a)

sing � � 633 nm, l � 2.8 cm, f0 � 43.4 MHz, V �.66 � 105 cm�s, �0 � 6°, �d � 5.3°, and no � 1.51, weet �f � 0.45 MHz and �� 64 Å; and for � � 350 nm,� � 20 Å; whereas in the UV region at � � 250 nm,� � 10 Å; i.e., the passband is roughly 3 and 6 timesarrower. The spectral resolution of the filter thatorresponds to these passbands is 160 cm�1.

The calculated value of the acceptance angle, ��, inhe filter for a transducer length equal to 2.8 cm wasimited to �0.6° in the plane of the AO interaction010�, and it was limited to �0.8° in the orthogonallane. A shorter transducer will result in wider ac-eptance angles, giving rise to overlapping of the fil-ered and the unfiltered radiation at the filter output.

longer transducer �l � 2.8 cm� will ensure spatialeparation of the two beams but will decrease thengular aperture and the throughput of the device.

. Transmission Coefficient of the Accousto-Opticevice

he transmission coefficient of a filter, T, is deter-ined by the ratio of diffracted and the incident light

ntensities Id and I0, respectively; i.e., T � 100% �Id�0�.1–5 It can be expressed as

T � �100%�sin2��

� �M2Pl2d

cos �

cos�� 0�� , (9)

here P is the acoustic �practically equal to electricower pel� driving power applied to the filter and d ishe height of the transducer.7,8,13 Figure of merit M2n Eq. �9� is given by1–5

M2 �peff

2 nd3no

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here peff is the effective photoelastic constant ofDP in plane �010�, as follows:

peff � p66 cos � cos��0 � �� p44 sin � sin��0 � �� .(11)

sing these expressions at � � 633 nm, we get M2 �.6 � 10�18 s3�g and peff � �0.067. It is likely thathe figure of merit may be 10–20% higher at shorterV wavelengths because of the dispersion of the re-

ractive indices and the photoelastic coefficients.alculating the transmission coefficient by use of ex-ressions �8�–�11� with the linear optical aperture ofhe cell a � 1.5 cm �equal to d� with a transducerength of l � 2.8 cm at the moderate driving acousticower of 2 W, we get for � � 350 nm, T � 72%; for � �32 nm, T � 30%; and for � � 633 nm, T � 22%.

. Fabrication of an Acousto-Optic Tunable Filter

e fabricated an AOTF cell, using a single crystal ofDP in the shape of a prism. The configuration of

he cell is shown in Fig. 5. Because the crystal isighly hygroscopic, it must be prevented from absorb-

ng water. Therefore, just after the fabrication ofhe prism, we applied a hermetic seal by coating allacets of the specimen with a special compound torotect the specimen from moisture. The trans-ucer facet of the prism formed a tilt angle equal to 6°ith respect to the optical axis in the �010� interac-

ion plane of the crystal. The transducer was fabri-ated from a thin plate of X-cut LiNbO3 crystal withimensions l � 2.8 cm along the direction of lightropagation and d � 1.5 cm in the orthogonal direc-ion along 010�. Cold-indium vacuum-weldingechnology was used in bonding the transducer to therystal. After the bonding, the transducer was pol-shed to a thickness of �20 �m and then cut intoeven sections connected in series for easier broad-and matching with the 50-� driving rf generator.

Fig. 5. Layout of the UV AOTF cell.


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special electrical matching network circuit wassed for the 50-� impedance matching. In additiono the electrical matching, acoustic impedance match-ng was also carried out, by evaporation of thin inter-

ediate layers of indium and tin in the acoustic bondetween the crystal of KDP and the LiNbO3 plate.his resulted in better acoustic coupling and lower rfower loss.The input optical facet of the crystal makes an

ngle of �84° with the transducer facet of the prism,s shown in Fig. 5. The output optical facet of theell was rotated by �4° relative to the input opticalacet to form an optical wedge to eliminate the spec-ral scene shift in the filtered light caused by theefractive-index dispersion.19,20 If the incidenteam has ordinary polarization, the resultant up-hifted diffracted beam has extraordinary polariza-ion; when the incident beam has oppositeolarization, the diffracted beam is downshifted withrthogonal polarization. Both the input and the out-ut optical facets of the cell were antireflectionoated. The antireflection coating reduced the re-ection loss to 1.0–1.5% from 4% over the entire op-ical tuning range. The housing of the cell was maderom aluminum for better heat dissipation.

. Laboratory Testing of the Filter

e first characterized the frequency response of theiezoelectric transducer bonded onto the crystal.he frequency dependence of the driving acousticower applied to the filter was measured with a net-ork analyzer. We obtained the transducer’soltage–standing-wave ratio, i.e., the frequency de-endence of the standing-wave ratio that determineshe electrical performance of the device, by evaluat-ng both the reflected and the transmitted rf signalsn the circuit, which consisted of the rf generator andtransducer. The result of measurement of the ra-

io of absorbed power P and incident power P0 ishown in Fig. 6.Next we obtained the tuning curve of the filter,

sing 633-nm He–Ne �f � 43.2 MHz�, 532-nm Nd:AG �f � 52.7 MHz�, and 337-nm nitrogen �f � 92.5Hz� lasers and broadband light from tungsten,

uartz, and mercury lamps. Even though two of therequencies used with the coherent light sources areuch smaller than the FWHM response points of the

ransducer at 60 and 164 MHz, the response of theransducer was sufficient to filter the light even athese relatively long optical wavelengths. Also, theame measurements in UV region with the nitrogenaser, the quartz, and the mercury lamps confirmedhe theoretical predictions.

The frequency dependence of the Bragg incidencengle for the ordinary and the extraordinary polar-zed radiation at � � 633 nm was both calculated and

easured, as shown in Fig. 7. In this figure theluses correspond to the measured values of the in-idence angles for the ordinary polarized light of thee–Ne laser.The spectral bandpass of the filter was measuredith 532- and 633-nm lasers, one at a time, by the


raditional rf sweeping method. In each case thepplied frequency was varied and the intensity of theiffracted light was measured for a fixed incidentptical wavelength. The results are shown in Fig. 8or the 633-nm laser at a driving rf power of 2 W.

We measured the transmission coefficient of thelter by varying the applied rf power from a givenource, and the results are shown in Fig. 9. In thisgure the 532-nm curve is labeled 1, and we obtainedurve 2 by extrapolation of the experimental data tohe shorter wavelength at 250 nm. This extrapola-ion is based on the fact that the efficiency of trans-ission of electric power into acoustic power, i.e., the

oltage–standing-wave ratio coefficient of the trans-ucer at f � 52.7 MHz and � � 532 nm, was �20%Fig. 6�, and at 250 nm and f � 140 MHz it was higherhan 80%. The diffracted light’s intensity at 250 nmas 4.5 times higher than at 532 nm, however, be-

ause of the �2 dependence in Eq. �9�. Consequently,

ig. 6. Frequency dependence of the relative acoustic power inhe cell.

ig. 7. Calculated and measured frequency dependence of thencidence angles. Pluses, measured values of the incidence anglesor an ordinary polarized 633-nm He–Ne beam.

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t is reasonable to predict that operation of the devicet 250 nm will be �20 times better than at 532 nm.his means that, at � � 250 nm, close to 100% trans-ission may be obtained with a driving power of �1.5, as indicated in Fig. 9.For the UV measurements, light from a nitrogen

aser as well as from quartz and mercury lamps wassed, and a photomultiplier tube that is sensitivenly in the UV range and not in the visible range wassed for detection of light. During the experimenthe filter was continuously tuned over the entire tun-ng range. Extremely high intensities of filteredight were observed at the laser wavelength of 337 nmnd at some other acoustic frequencies within theuning range corresponding to the spectral lines inhe lamps.

. Analysis of Results and Discussion

t is clear from Fig. 6 that the driving rf power isfficiently absorbed by the transducer and subse-uently converted into acoustic energy in the FWHM

ig. 8. Frequency dependence of the filter transmission coeffi-ient. Spectral bandpass �f is marked at FWHM points.

ig. 9. Dependence of the transmission coefficient on rf power.urve 1, measurements at 532 nm; curve 2 was calculated for 250m.

nterval from 60 to 164 MHz. The frequencies athese two limits determine the transducer’s tuningange �based only on the transducer’s characteris-ics�, and their ratio is 2.7, implying that the tuningange is significantly greater than 1 octave. Thiseans that the acoustic and electric matching for the

ransducer is working well. The acoustic and elec-rical matchings compensate well for the mismatch inhe acoustic impedances of LiNbO3 and KDP mediand the large static capacitance of the transducerlate.One can use the solid curve in Fig. 4 to predict the

imits of the calculated optical wavelength range thatorrespond to the acoustic frequency limits discussedbove. The values of these limiting optical wave-ength are 220 and 480 nm. The ratio of these twoimiting wavelengths is 2.2, which is less than 2.7, theatio of the limiting frequencies, because of the effectf optical dispersion. We experimentally verifiedhe wide tuning range of the filter by using 633-, 532-,nd 377-nm lasers as well as tungsten, quartz, andercury lamps. The experimental results agreedell with the predicted values.One can obtain the measured value of the optimal

ncidence angle, �0 that corresponds to the wide-angleiffraction regime in KDP by examining the curve ofhe measured Bragg angle of incidence for the ordi-ary polarized light as a function of acoustic fre-uency in Fig. 7 �shown by pluses�. The figurehows that the measured value of �0 is 6°, as pre-icted by the theory. This figure also shows thathere is some discrepancy between the theoreticalnd the measurement data. For example, the mea-ured value of the acoustic frequency that corre-ponds to the condition df�d� � 0 is 43.2 MHz,hereas the theory predicts 43.4 MHz; the difference

s well within the experimental error.The measured angle of spatial separation of beams

t the filter’s output is 1.2° � 0.1°, in agreement withhe theoretical prediction. We use this angle to cal-ulate the angular aperture of the filter. The mea-ured aperture in KDP is a few times narrower thanhe angular aperture for a typical TeO2 filter.7,8,13

he reason for this is the smaller birefringence forDP than for TeO2, i.e., �n � 0.04 compared with.15. Higher values of the acoustic phase velocity inDP, however, are responsible for its higher acoustic

requencies than those of TeO2. As mentionedbove, the angular aperture can be increased if theilt angle is �6°, but unfortunately this greater tiltngle corresponds to higher driving frequencies andbsorption of ultrasound.There is another way to improve the throughput of

he filter, and that is by inserting the filter between arossed polarizer and analyzer to block the zero-orderiffracted beam, as discussed above. As a result,ven for spatial overlap of the incident and the dif-racted beams at the filter output, the signal-to-noiseatio during light filtering in some specific cases mayemain tolerable, at least in the plane of the AOnteraction.19 Use of a crossed polarizer/analyzerair doubles the angular aperture; i.e., �� 2.4°.


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The measured spectral bandpass of the filter at � �33 nm is shown in Fig. 8. The maximum transmis-ion coefficient T � 2.0% is observed for an acousticrequency of 43.2 MHz for 2-W driving power. Theow value of the transmission coefficient at 633 nm isetermined by poor efficiency of the transducer oper-tion out of the FWHM range.The data in Fig. 9 also show that �f � 0.46 MHz, in

ood agreement with the theoretical prediction off � 0.45 MHz. The measured frequency intervalorresponds to the spectral resolution of the filter,

� ��2 � 165 cm�1. This resolution value de-ermines the transmission passband �� 67 Å at � �33 nm. Using a 532-nm laser to measure the spec-ral response gave �f � 0.45 MHz, which is close tohe value at 633 nm within the experimental errorargin. As discussed above, the experimentallyeasured resolution R � 165 cm�1 is only a few

ercent higher than the calculated value of R � 160m�1. Finally, the spectral bandwidth of the filterransmission at the midpoint of the tuning range at50 nm is �20 Å, and it is 10 Å at 250 nm.Look at the sidelobes in Fig. 8; it is clear that the

elative intensity of the lobes in the cell was limitedo a few percent, which is close to the theoreticallyredicted value of �5% for low drive power and lowiffraction efficiency. It is likely that the smallsymmetry in the lobes is caused by a nonuniformistribution of the acoustic power over the length ofhe transducer and by phase delays among separateections of the transducer. Theory predicts that, forigh diffraction efficiency, the total intensity of the1 and the �1 sidelobes will be �11%. At the UVavelengths the measured transmission coefficient of

he lobes does not exceed 15%, in agreement with theheory. Also, all experimental data show that thentensity in the left lobe was less than in the rightobe.

Calculation of transmission coefficient T of the fil-er confirms that the poor diffraction efficiency inDP for � � 550 nm is due to KDP’s low AO figure oferit. A theoretical prediction �without including

he frequency response of the transducer� is that, at ariving power of 1 W, T should be 11%, 15%, and 40%,orresponding to wavelengths of 633, 532, and 350m, respectively. However, Fig. 6 shows that theatio P�P0, or voltage–standing-wave ratio response,.e., the efficiency of ultrasound generation in the cell,s close to 12% at 633 nm �f � 43.2 MHz�, 20% at 532m �f � 52.7 MHz�, and 90% at 350 nm �f � 89 MHz�.esults of some of the transmission measurementsre plotted in Fig. 9, where curve 1 shows results for32 nm and curve 2 those for 250 nm �curve 2 ob-ained by extrapolation of the experimental data tohorter wavelengths�. At 532 nm, T � 8% for P � 2

from measurements, which is a little less than theredicted value of 9%. During testing of the cell inhe UV, extremely high intensities of the filtered lightere observed only at some radio frequencies within

he tuning interval of the transducer. For example,adiation of the nitrogen laser at � � 337 nm wasltered with the efficiency T � 50% at f � 92.5 MHz


or 1.5-W rf power. Comparison of the measuredesults with known spectral data for the quartz andercury lamps confirmed that the frequencies corre-

pond to the line spectra of these noncoherent sourcesf radiation.Imaging experiments with this filter were carried

ut by use of a xenon arc lamp source, a pair oflan–Taylor prisms, quartz lenses, and a UV-

ensitive camera.21

. Conclusions and Summary

he acousto-optic properties of a single crystal ofDP were studied and we carried out calculations toesign an imaging acousto-optic tunable filter with ainear aperture of 1.5 cm � 1.5 cm operating from 220o 480 nm. Such a filter was fabricated and tested.t has a spectral resolution of 160 cm�1 and a pass-and of 20 Å at 350 nm. The wide tuning range isossible because of careful matching of the electricalnd acoustic impedances of the transducer. A trans-ission coefficient of better than 50% for the filteras obtained for applied rf power of 2 W in a wide

ange of optical wavelengths from the UV to the vis-ble. The angular aperture of the filter is �1.2° inir, and it can be increased by use of a pair of crossolarizers. Imaging experiments with this filterere carried out with a xenon-arc lamp source, a pairf Glan–Taylor prisms, quartz lenses, and an UV-ensitive camera.21

In general, the AO figure of merit, M2, in KDP forhe shear acoustic wave regime is 6.2 � 10�18 s3�g,nd it is 4.6 � 10�18 s3�g for the regime of the wide-ngle diffraction used in the filter. Consequently,he figure of merit in KDP is 3–4 times greater thann fused silica. Because of the unavailability of bet-er birefringent materials that are transparent in theV, single crystals of KDP are the best material forse in fabricating an AOTF in the this region of spec-rum.

The authors are grateful to V. Ya. Molchanov forabrication of the acousto-optic cell.

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4. A. Ponomarev, I. Rodionov, and G. Teterin, “Wide apertureacousto-optic tunable filters for visible and UV lights,” in Fiber

Optic Network Components, S. Najafi and H. Porte, eds., Proc.SPIE 2449, 200–207 �1995�.

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Ultraviolet-Visible Imaging Acousto-Optic Tunable Filters in KDP