S

N

1

OmibcsdatumistofststaMas

tuCR

J

pace-multiplexed optical scanner

abeel A. Riza and Zahid Yaqoob

A low-loss two-dimensional optical beam scanner that is capable of delivering large �e.g., �10°� angularscans along the elevation as well as the azimuthal direction is presented. The proposed scanner is basedon a space-switched parallel-serial architecture that employs a coarse-scanner module and a fine-scannermodule that produce an ultrahigh scan space-fill factor, e.g., 900 � 900 distinguishable beams in a 10°�elevation� � 10° �azimuth� scan space. The experimentally demonstrated one-dimensional version ofthe proposed scanner has a supercontinuous scan, 100 distinguishable beam spots in a 2.29° total scanrange, and 1.5-dB optical insertion loss. © 2004 Optical Society of America

OCIS codes: 120.5800, 230.5440, 110.0110.

pef

nsooessosos

2S

FSuTfimrpadFwfitac

. Introduction

ptical scanners are critical building blocks in nu-erous optical applications such as optical switch-

ng, laser ultrasonics, laser imaging for nonsurgicaliopsies, free-space laser communications, infraredountermeasures, and laser radar.1,2 Therefore atate-of-the-art scanner with features such as two-imensional �2-D� scan capability, large �e.g., �1-cm�perture, wide �e.g., �10°� angular scan range alonghe directions of elevation as well as azimuth, anltrahigh scan space-fill factor, and microsecond do-ain scan setting speeds is desired to meet needs in

ndustry, research, and defense. Other technologiesuch as acousto-optics, electro-optics, and microelec-romechanical systems have been used in the designf scanners, but the scanners had application-specificeatures and trade-offs. Recently a new scanner de-ign methodology called multiplexed optical scannerechnology �MOST� was introduced3 for laser beamteering that promises low power consumption andrue rapid three-dimensional �3-D� beam forming toccurately control beam position, power, and shape.OST exploits the various parameters of light such

s time, spatial code, frequency, polarization, andpace to deliver scanner designs that incorporate im-

The authors are with the Photonic Information Processing Sys-ems Laboratory, School of Optics—Center for Research and Ed-cation in Optics and Lasers, University of Central Florida, 4000entral Florida Boulevard, Orlando, Florida 32816-2700. N. A.iza’s e-mail address is nriza@mail.ucf.edu.Received 23 September 2003; revised manuscript received 23

anuary 2004; accepted 13 February 2004.0003-6935�04�132703-06$15.00�0© 2004 Optical Society of America

roved specific scanner performance parameters,.g., microsecond-speed random-access scans withocus–defocus controls.4–8

Earlier, we showed how wavelength and space ma-ipulations can be used to achieve agile 2-D beamcanners.9 In this paper we describe, to the best ofur knowledge for the first time, a space-multiplexedptical scanner �S-MOS� with a unified scanner ap-rture and 2-D coarse–fine-scan capability. We de-cribe the design and analysis of the proposedcanner and a proof-of-concept experiment that dem-nstrates the scanner’s performance parametersuch as angular scan range, scan resolution, numberf distinguishable beam spots, insertion loss, andwitching speed.

. Two-Dimensional Space-Multiplexed Opticalcanner

igure 1 shows the proposed high-resolution 2-D-MOS based on parallel-serial architecture thatses a cascade of coarse- and fine-scanner modules.he coarse scanner is based on a 1 � N broadbandber-coupled optical switch in which N output single-ode fibers �SMFs� couple with a spherical lens ar-

anged in a Fourier geometry. Specifically, theolished ends of the SMFs in the 2-D M � M � N fiberrray are parallel to one another and are located aistance of one focal length from the achromatic lens.or the setup shown in Fig. 1 a coordinate systemas chosen with its origin at the tip of one of thebers �e.g., the central fiber in the 2-D array� suchhat the z axis is aligned with the principal axis of thechromatic lens. Because the 2-D fiber array coin-ides with the xy plane, elevation angle � of the col-

1 May 2004 � Vol. 43, No. 13 � APPLIED OPTICS 2703

lg

wpd

wifiabTvsFwSwtceatbs

t

ficv

w

aaa

IobcT2lts1lsseso

Fn

2

imated beams after the collimating spherical lens isiven by

� � �tan�1�r�f �, (1)

here r � �x2 y2 such that �x, y� represent theositions of fibers in the xy plane. The 1�e2 beamiameter de of the collimated beams can be written as

de �2f cos �

�winc, (2)

here f is the focal length of the achromatic collimat-ng lens and winc is the beam waist �half of the modeeld diameter� of the Gaussian beam emerging fromSMF. Note that the collimated beams cross in theack focal plane of the collimating lens �Fig. 1�.hus, by electronically controlled switching amongarious output fibers of the optical switch, the coarse-canner module of the proposed S-MOS is achieved.urthermore, unlike the S-MOS described in Ref. 9,hich exhibited a distributed scanner aperture, the-MOS described here has a coarse-scanner moduleith a unified scanner aperture defined by the size of

he collimated beams in the back focal plane of theollimating lens. Note that the unified scanner ap-rture is paramount in the proposed S-MOS design,s it permits the placement of a single fine scanner inhe coarse-scanner module’s aperture plane �i.e., theack focal plane of the collimating lens� to attain aupercontinuous beam scan in 2-D space.Assuming that the number of fibers M along direc-

ion x or y is odd and that the central fiber in the 2-D

ig. 1. Schematic of the proposed high-resolution two-dimensioumber of fibers along the x or y direction in the 2-D fiber array.

Table 1. Parameters

Number of OutputPorts of Fiber-Optic

Switch �N�

Number of Fibersalong the x or y

Axis �M�Fiber Pitch

xp �mm�

100 10 1100 10 2400 20 1

aFor a coarse scanner.

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

ber array is aligned with the principal axis of theollimating lens, the output angular scan along ele-ation or azimuth is given by

� � �tan�1�nxp�f �, (3)

here n is an integer given by

��M � 12 � � n � �M � 1

2 � (4)

nd xp is the pitch of the optical fibers in the 2-D fiberrray. Thus the total switch-based angular scanlong the direction of elevation or azimuth is given by

�� � 2 tan�1��M � 1� xp

2f � . (5)

f �p is the elevation angle that corresponds to pitch xpf the optical fibers in the 2-D fiber array, the switch-ased coarse angular scan along elevation or azimuthan also be given approximately by �M � 1� � �p.hus, by suitably choosing the pitch of the fibers in a-D fiber array and the focal length of the collimatingens, one can obtain the desired coarse-scanner aper-ure as well as the angular scan range of the coarse-canner module. For instance, SMFs for the550-nm band along with a 50-mm focal-length col-imating lens will deliver a switch-based coarse-canner module with 1-cm aperture. Table 1hows the angular scan range �along directions oflevation and azimuth� and the scan step of thewitch-based coarse scanner versus the number ofutput ports of the fiber-optic switch and the pitch of

-MOS: N, number of output ports of the fiber-optic switch; M,

rious S-MOS Design

Angular Scan Rangealong Elevation or

Azimuth �deg�a

Switch-BasedScan Step

�deg�a

Fine-ScannerDesired Range

�deg�

10.28 1.146 1.14620.41 2.29 2.2921.51 1.145 1.145

nal S

of Va

tToc�brss

ul�ulNqldttdgcxa�

amwaNTmNssasT�asSusgpsgd

s�sTatsfcTfeawgthidHnap

3

Fo1cwctfit8it

Ffidfl

he fibers in the 2-D fiber array. As indicated inable 1, a coarse scanner made with a 1 � 100 fiber-ptic switch and 50-mm focal-length collimating lensan deliver 10.28° and 20.41° angular scan rangesalong the directions of elevation as well as azimuth�ased on fiber pitches xp � 1 mm and xp � 2 mm,espectively. Table 1 also shows the desired fine-canner scan range that matches the scan step of thewitch-based coarse scanner.There are various options for the fine scanner to be

sed in the proposed S-MOS. For instance, a spatialight modulator such as a nematic liquid crystalNLC� single-pixel programmable deflector can besed as a fine scanner to improve the scanner reso-

ution along the elevation or azimuthal direction.ote that polarization-maintaining optics will be re-uired for the state of polarization �SOP� of the col-imated light �reaching the NLC deflector� along theirector of the programmable NLC cell to be main-ained. However, the need to maintain the SOP ofhe incoming light can be eliminated by use of twoifferent NLC programmable deflectors stacked to-ether such that the directors of the individual NLCells are orthogonal to each other, i.e., one along theaxis and the other along the y axis, yet both gener-ting programmable wedges along the same axise.g., the x axis; see Fig. 2�a��. The goal of achieving

ig. 2. One-dimensional x-direction scan polarization-insensitivene-scanner design options for the S-MOS: �a� design with cross-irector NLC deflectors, �b� design with aligned-director NLC de-ectors with a half-wave plate �HWP� at 45° to the director axis.

polarization-insensitive fine scanner can also beet by use of two similar NLC deflectors with a half-ave plate with its axis at 45° to the two deflectors’xes �Fig. 2�b��. Note that the directors of the twoLC deflectors are parallel, i.e., along the x axis.hus if the s polarization sees the first NLC program-able wedge, the p polarization will see the secondLC wedge for deflection to the desired angular po-

ition in space. Because the paired NLC deflectorshown in Fig. 2 deliver fine angular scans along onexis, similar arrangements will be required for finecans along the other orthogonal scan direction.herefore a combination of two paired NLC deflectors

90° rotated with respect to each other� will producefine-scanner unit that is capable of delivering fine

cans in two dimensions regardless of input beam’sOP. Furthermore, multiple ultrathin fine-scannernits can be cascaded to match the coarse-scannertep size. If the fine scanner can generate P distin-uishable beam spots in two dimensions, the S-MOSroduced by a combination of a switch-based coarsecanner and a NLC-deflector-based fine scanner willenerate N � P distinguishable beam spots in twoimensions.Because of its large aperture and thin-stacked de-

ign, the polarization-multiplexed optical scanner8

P-MOS� is another attractive candidate for the finecanner in the proposed high-resolution S-MOS.he P-MOS is a peer member of the MOST familynd has a binary-switched serial beam control archi-ecture for 3-D beam forming. The P-MOS usesingle-pixel 90° linear polarization rotators such aserroelectric liquid-crystal cells to act as electricallyontrolled polarization-multiplexing components.he 3-D beam forming information, such as tilt and

ocus–defocus refractive-index gradients, is stored inlectrically controlled birefringent-mode devices suchs NLC single-pixel programmable cells. Note that,ith P ferroelectric liquid-crystal cells and P pro-rammable phase plates, 2P different 3-D beam pat-erns can be generated. Thus the maximumardware compression as well as the 3-D beam form-

ng capabilities of the P-MOS make it a suitable can-idate for the fine scanner in the proposed S-MOS.ence a combination of a switch-based coarse scan-er and a P-MOS can deliver a high-resolution widengular scan range S-MOS with depth scanning ca-abilities as well.

. Experiment

igure 3 shows a proof-of-concept high-resolutionne-dimensional �1-D� scanner. The choice for the-D scanner demonstration is based on the in-timeommercial availability of a sample 1-D fiber arrayithin a specific cost constraint. Light from a fiber-

oupled 1550-nm laser source is input via a polariza-ion controller �PC� to a 1 � 32 optical switch. Therst 8 output ports of the optical switch are connectedo the 8-channel SMF array. Each SMF in the-channel fiber array is cut at 8°, polished, and placedn a V-groove assembly to hold it aligned parallel tohe other SMFs. The fiber pitch �core-to-core dis-

1 May 2004 � Vol. 43, No. 13 � APPLIED OPTICS 2705

tV�tapctstpmcu0cfaaapnnrcas

ppcb55TtiuctNbdr

fitoacdcdurokoaoseafs

F

F�t

2

ance between two adjacent fibers� is 250 �m. The-groove assembly is mounted on a tip-tilt stage

such that the fiber array is parallel to the opticalable� and placed at one focal length’s distance fromn f1 � 50 mm lens. Fiber 4 is aligned with therincipal axis of the collimating lens. By electronicontrol of the optical switch, the input light is routedo output ports 1–8 of the optical switch. Thiswitching operation causes the collimated beams �af-er the collimating lens� to scan in a plane that isarallel to the optical table. Note that the colli-ated beams cross in the back focal plane of the

ollimating lens. Because the pitch xp � 250 �m,sing an f1 � 50 mm focal-length lens yields �p �.286°. Thus choosing N � 8 produces an 8-beamoarse scanner with a total angular scan of 2.00°. Aocusing lens of focal length f2 � 100 mm is placed at

distance f1 f2 from the collimating lens to focusnd map the angularly scanned beams along the xxis. A CCD camera is placed at the back focallane of the focusing lens to image the discrete scan-ing beam spots. Figure 4 shows images of the scan-ing beam spots from left to right as the input light isouted to output ports 1–8 of the optical switch byomputerized control. Figure 4 also illustrates thengular scans �in degrees� that correspond to opticalwitch output port settings.To implement the fine scan stage of the S-MOS we

lace a NLC programmable wedge10 in the back focallane of the collimating lens. The liquid crystal �LC�ell is filled with NLC Merck BL006, which has airefringence �n � �ne � no� of 0.286 at 20 °C and �89.3 nm. The LC layer has a uniform thickness of0 �m. The clear aperture of the devices is 5 mm.wo substrates are deposited, with two different elec-rodes. One substrate is coated with a low-mpedance layer of a material as indium tin oxide forse as the ground electrode and the other substrate isoated with a uniform layer of a high-impedance ma-erial for use as the control electrode. To use theseLC cells as angular deflectors, one applies voltageetween two parallel linear metallic contacts that areeposited at the edges of the control electrode, whichesults in a linearly varying electric field between the

ig. 3. Proof-of-concept experimental setup for a 1-D high-resolut

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

ront �control� and the back �ground� electrodes, caus-ng the index to vary linearly across the clear aper-ure of the device. By controlling either the voltager the frequency of the ac voltage signal appliedcross the voltage terminals of the LC cell, one canontrol the wedge angle of the programmable cell asesired. As the index modulation is seen by theomponent of the input polarization that is along theirector of the liquid crystal, the PC in the setup issed to optimize the SOP of the collimated light thateaches the NLC deflector. Changing the frequencyf the NLC deflector’s drive signal from 200 Hz to 5Hz causes the incident beam to undergo a deflectionf 0.286°, which is equal to the discrete deflectionchieved by changing the optical switch from oneutput port setting to the next output port. Figure 5hows images of six sample scanning beam spots gen-rated between optical switch settings S2 �at the left�nd S3 �at the right� for several NLC drive signalrequency settings. The NLC deflector used a driveignal of 3.8 V and frequency settings of 400, 800,

-MOS with an 8-channel SMF array: PC, polarization controller.

ig. 4. Images of 1-D scan S-MOS scanning beam spots generatedfrom left to right� as the input light is routed to output ports 1hrough 8 of the optical switch.

ion S

1fimobfi

odiiwoottlooowbfis dlor

bmb1alvac

tgdb1agwtt

4

IaS�StbnaifisSlofalfaoatm

R

R

FsSwg

400, 2200, 3500, and 4200 Hz to generate the sixne-scan spots �shown from left to right�. In sum-ary, the proposed S-MOS that uses the combination

f a switch-based coarse scanner and a NLC-deflector-ased fine scanner indeed delivers a high scan spacell factor for our demonstrated 1-D scanner.The loss of this scanner depends on the loss of the

ptical switch and on the NLC programmableeflector�s� used in the setup. The maximum opticalnsertion loss of the optical switch used in the setups �0.7 dB. The programmable NLC deflector thate used is a single-pixel cell that has an optical loss

f 0.75 dB. Thus, for any scanned beam, the dem-nstrated S-MOS loss is 1.5 dB. The switchingime of the S-MOS is defined by the switching time ofhe optical switch or the NLC deflector, whichever isonger. The 1 � 32 channel optomechanics-basedptical switch used in the setup has a switching timef �4 ms. The response time of the NLC cell fromne frequency setting to another is 1 s. However,hen these NLC cells are used to realize a P-MOS �toe used as a fine scanner� in the proposed S-MOS, thene scanner’s response time is limited by the re-ponse time of the ferroelectric liquid cells, which is35 �s �Ref. 11� at 1310 nm. On the switch side,epending on the fabrication technology used, e.g.,ithium niobate integrated optics, bulk acousto-ptics, or thermo-optics, the switching times canange from milliseconds to nanoseconds.12

Because the mode field diameter of the opticaleam exiting the SMFs is 10 �m,13 Eq. �2� deter-ines that the 1�e2 beam size of the collimated

eams after the collimating lens will be 9.86 mm at550 nm. However, the sizes of collimated beamsfter they pass through the NLC deflector will beimited to the size of the clear aperture of the de-ice, i.e., 5 mm. Thus the optical beam divergencet 1550 nm will be 0.3947 mrad, or 0.0226°. Be-ause the programmable NLC cell can deliver a

ig. 5. Images of six sample 1-D scan S-MOS scanning beampots generated between optical switch settings S2 �at the left� and3 �at the right�. The NLC deflector used a 3.8-V drive signalith the frequency settings shown �Hz� to generate the six NLC-enerated spots �from left to right�.

otal scan of 0.286°, the total number of LC-enerated distinguishable scan spots is 12. As theemonstrated scanner setup uses an 8-channel fi-er array, the total number of scan spots becomes2 � 8 � 96. Note that, by using a 1-cm clear-perture NLC deflector cell, one gets a beam diver-ence �at 1550 nm� equal to 0.0113°. This in turnill double the LC-generated distinguishable spots

o 25 and the S-MOS total number of spots to 200 inhe 2.29° angular scan range.

. Conclusions

n conclusion, we have proposed and demonstratednew member of the MOST family called the

-MOS. The S-MOS can provide 2-D large �e.g.,10°� angular scans with high resolution. The-MOS is based on a parallel-serial architecturehat includes a fiber-optic switch-based coarse-eam scanner and a large-aperture fine-beam scan-er �e.g., a P-MOS�, leading to a large unifiedperture with supercontinuous beam scan capabil-ty in 2-D space. When it is equipped with a fastber-optic switch �e.g., a 1 � N acousto-opticwitch� and a fast fine scanner �e.g., a P-MOS�, the-MOS is capable of microsecond-speed wide angu-

ar scans in 2-D space. The experimentally dem-nstrated 1-D version of the proposed S-MOSeatures a supercontinuous scan, 100 distinguish-ble beam spots in 2.29° of total scan range, and aow, 1.5-dB, optical insertion loss. The S-MOS alsoeatures an optically reversible design, permittingpplications for which transmit and receive beamperations are required. The fiber-based designlso makes the S-MOS useful for remote applica-ions, perhaps when space is at a premium or wheninimum front-end complexity is required.

This research is supported by Defense Advancedesearch Projects Agency grant N66001-98-D-6003.

eferences1. L. Beiser and R. B. Johnson, “Scanners,” in Handbook of Op-

tics, M. Bass, E. W. Van Stryland, D. R. Williams, and W. L.Wolfe, eds. �McGraw-Hill, New York, 1995�, Vol. II, pp. 19.1–19.57.

2. L. Beiser, S. F. Sagan, and G. F. Marshall, eds., Optical Scan-ning: Design and Application, Proc. SPIE 3787, �1999�.

3. N. A. Riza, “Multiplexed optical scanner technology,” U.S.patent 6,687,036 �3 February 2004�.

4. Z. Yaqoob, A. A. Rizvi, and N. A. Riza, “Free-space wavelengthmultiplexed optical scanner,” Appl. Opt. 40, 6425–6438 �2001�.

5. Z. Yaqoob and N. A. Riza, “Free-space wavelength-multiplexedoptical scanner demonstration,” Appl. Opt. 41, 5568–5573�2002�.

6. Z. Yaqoob, M. A. Arain, and N. A. Riza, “High-speed two-dimensional laser scanner based on Bragg gratings stored inphotothermo-refractive glass,” Appl. Opt. 42, 5251–5262�2003�.

7. N. A. Riza and M. A. Arain, “Code-multiplexed optical scan-ner,” Appl. Opt. 42, 1493–1502 �2003�.

8. N. A. Riza and S. A. Khan, “Programmable high-speed polar-ization multiplexed optical scanner,” Opt. Lett. 28, 561–563�2003�.

9. Z. Yaqoob and N. A. Riza, “Agile optical beam scanner using

1 May 2004 � Vol. 43, No. 13 � APPLIED OPTICS 2707

1

1

1

1

2

wavelength and space manipulations,” in Algorithms andSystems for Optical Information Processing V, B. Javidi andD. Psaltis, eds., Proc. SPIE 4471, 262–271 �2001�.

0. M. Yu. Loktev, V. N. Belopukhov, F. L. Vladimirov, G. V.Vdovin, G. D. Love, and A. F. Naumov, “Wave front controlsystems based on modal liquid crystal lenses,” Rev. Sci. In-strum. 71, 3290–3297 �2000�.

1. S. A. Khan and N. A. Riza, “High speed polarization-

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

multiplexed optical scanner for three-dimensional scanningapplications,” in Free-Space Laser Communication and ActiveLaser Illumination III, D. G. Voelz and J. C. Ricklin, eds., Proc.SPIE 5160, 3–8 �2003�.

2. A. Marrakchi, Photonic Switching and Interconnects �MarcelDekker, New York, 1993�.

3. Product specification sheet, Corning SMF-28 optical fiber�Corning, Inc., Corning, N.Y., 1998�.