Micro-optical 1 � 4 fiber switch for multimodefibers with 600-�m core diameters

Jacques Duparre, Bernt Gotz, and Rolf Goring

The design, manufacture, and test of a 1 � 4 micro-optical fiber switch for multimode fibers with 600-�mcore diameters are described. Microlens array telescopes allow for variable and fast beam deflectionwhen the positions of the cylindrical microlens arrays relative to each another are altered by speciallydesigned piezomechanical actuators. Standard achromats are used for collimation of light emitted bythe input multimode fiber and for focusing of the deflected light onto a linear array of output multimodefibers. Design and assembly of micro-optical as well as of optomechanical components are discussed.Insertion loss and cross talk are measured, and the results are compared with those of numerical opticalsimulations. Measurements of switching time and long-term stability, as well as of thermal behavior,are also presented. © 2003 Optical Society of America

OCIS codes: 350.3950, 060.1810, 230.6120, 060.2350, 220.4880.

1. Introduction

There are various concepts of micromechanicalswitches for multimode fibers. In a simple ap-proach, one moves the fibers directly to achieve opti-mum coupling of the input fiber end face to one of thevarious output fiber end faces.1,2 This concept iswell suited for multimode fiber transmission systemswith small fiber core diameters. For the large corediameter fibers commonly used in fiber-coupled spec-trometric systems, problems appear because of thestiffness of thick fibers. The fibers cannot be movedaccurately because mechanisms for fast movementwith large strokes do not generate enough force.This is why a system has to be developed in which thefibers themselves do not need to be moved for switch-ing among channels. Instead, micro-optical compo-nents that facilitate fast beam steering can be used todeflect the light into the selected channel. Some sys-tems have already been proposed for this purpose andexamined in the literature.3–6

Light emitted by multimode fibers with large core

J. Duparre �jacques.duparre@iof.fraunhofer.de� is with theFraunhofer-Institut fur Angewandte Optik und Feinmechanik,Winzerlaer Strasse 10, D-07745 Jena, Germany. B. Gotz is withPiezosystem Jena GmbH, Prussingstrasse 27, D-07745 Jena, Ger-many. R. Goring is with Pyramid Optics GmbH, Lindenstrasse,D-07589 Lederhose, Germany.

Received 22 April 2003; revised manuscript received 5 August2003.

0003-6935�03�346889-08$15.00�0© 2003 Optical Society of America

diameters and even moderate numerical apertureshas a large space–bandwidth product.3 If highly ef-ficient coupling in the same type of fiber as the inputfiber is desired, the transferring micro-optics may notincrease the space–bandwidth product. Taking dif-fraction effects into account means that the overallsystem aperture must be large. However, to achievefast beam deflection requires that the beam steeringcomponents have short focal lengths. The numeri-cal apertures of the light-deflecting components, how-ever, must be small for aberrations to remain small.Thus the light bundle in the beam-steering device hasto be sectioned.7 This can be done with systems likethose shown in Figs. 1 and 2 that use decenterablecylindrical microlens array �MLA� telescopes forbeam deflection.3,8–11 A multimode fiber with coreradius Rin and numerical aperture NA1 emits a lightbundle. Bulk collimating and focusing optics withfocal lengths FCOL and FFOC image the input fiberend face onto the output fiber end faces. After col-limation of the light by the first collimating optics thebundle has a diameter of approximately A � 2NA1�FCOL and a residual divergence �half-angle� of �max �arctan�Rin�FCOL�.3 Beam deflection is achievedwith two MLAs with pitches p1 and p2 �equal to eachother for this type of application� and focal lengths f1and f2. The first MLA focuses the collimated beaminto a large number of subfoci, and the second MLArecollimates each beam. These micro-optical ele-ments permit fast beam deflection because evensmall displacements result in large deflection anglesas a result of the short focal lengths of the micro-

1 December 2003 � Vol. 42, No. 34 � APPLIED OPTICS 6889

lenses. Deflection angle � as a function of the focallengths of the lenslets in the second MLA, f2, and ofthe relative positions of the two MLAs, r0, is given by� � arctan�r0�f2�.8 The maximum angle of deflec-tion is determined by �max � arctan�p2�2f2� becauser0max

� p2�2. The displacement of the spot in theimage plane deflected from the optical axis, v � FFOCtan ���, can then be given by v � FFOCr0�f2. Theoverall magnification of the imaged fiber end face isMnet � f1�f2 �FFOC�FCOL�. We present here, for thefirst time to our knowledge, a prototype of a fiberswitch for multimode fibers with large core diametersbased on the principles discussed above. This studyis a continuation of previous, more theoreticalones.3,7,11,12

2. Layout and Assembly

A. One-Dimensional Optical Layout

In a previous paper Duparre and Goring12 showedthat for sources and receivers with a large lateralextension, such as multimode fibers with core diam-eters of 600 �m, the grating effect of the MLAtelescopes13–15 plays a minor role for the transfer ef-ficiencies. Thus, large transfer efficiencies of thebeam-deflection system can be achieved with cylin-drical MLAs with 200-�m pitch. The system’swavelength dependence is negligible.

Cylindrical MLAs fabricated by the reflow processhave been used because two-dimensional MLAs withlarge fill factors can be fabricated only with greattechnological effort �by gray-tone lithography or di-rect laser writing�. At present, no high-quality con-cave cylindrical MLAs are available on the market.To minimize aberrations, we decided to use convexlenslets in a reverse Keplerian orientation �lenses areoriented in opposite directions; substrate back sur-faces face one another�. The focal length of the mi-

crolenses is 874 �m, so small aberrations can beexpected because the numerical aperture is only 0.11.The homogeneity of the lenslets in the MLAs ishigh, which allows for high transfer efficiency.16

The parameters of the MLAs are listed in Table 1.The multimode fibers have core diameters of 600 �mand numerical apertures of 0.22. We used anti-reflection- �AR-� coated standard achromats with fo-cal lengths of 20 mm and free aperture diameters of12 mm as the collimating and focusing optics.

In determining the maximum number and separa-tion of output channels, one has to take into accountthat a larger separation of channels leads to a largerinsertion loss for the outer channels owing to thegeneration of spurious light. Less cross talk be-tween neighboring channels results if the separationis increased, however. Additionally, spurious light9

may not be deflected in one of the channels, whichwould otherwise increase cross talk, as illustrated inFig. 3. For the given set of MLAs and collimatingand focusing optics, the distance of spurious lightfrom the deflected beam in the image plane was 4.4mm. Thus we decided on an arrangement of fouroutput fiber channels with a separation of 1 mm.

B. Two-Dimensional Optical Arrangement for ReducingCross Talk

Previous tests with a one-dimensional beam-deflection system that used only one cylindrical MLAtelescope showed that besides the deflected beamthere is a strong corona in the direction of transfor-mation of the MLA telescopes. The corona results inincreased cross talk because it overlaps the array of

Fig. 1. Schematic of a two-dimensional micro-optical scannerbased on MLA telescopes.

Fig. 2. One-dimensional micro-optical fiber switch that uses aMLA telescope for beam deflection.

Table 1. Parameters of Cylindrical Microlens Arrays

MLA Parameter Convex Lenslets

Radius of curvature 450 �mPitch 200 �mFill factor 0.995Surface deviation of lenslets

from perfect circle0.1 �m peak to valley

Free aperture 19 � 14 mmLens array material OrmocerSubstrate material Borofloat 33Substrate thickness 1.13 mm

Fig. 3. Schematic drawing showing the spatial separation be-tween the deflected beam and spurious light �NA2 is defined asp2�2f2�.

6890 APPLIED OPTICS � Vol. 42, No. 34 � 1 December 2003

output fiber end faces, as shown in Fig. 4. This co-rona is caused by imperfections in the MLA, such asdeviation of the surface profile from a circle and a fillfactor smaller than 1, but also by the grating effect ofthe MLAs. By using this one-dimensional beam-deflection system we could achieve cross-talk valuesof only approximately �25 dB, which were muchpoorer than values of �50 dB achieved in existingoptical fiber switches. Improving the microlensquality does not improve these values. Thus we de-cided to use the two-dimensional setup shown in Fig.1 with two crossed cylindrical MLA telescopes andthe array of output fiber endfaces oriented at 45° withrespect to the direction of transformation of the MLAtelescopes. This configuration results in a distribu-tion of light in the image plane as shown in Fig. 5.The corona does not overlap the array of output fiberend faces, and thus an improvement in the cross talkcan be achieved.

C. Piezomechanical Actuation

Piezoelectric elements exhibit ideal parameters forhigh resolution and highly dynamic motion.17,18

They proved to be valuable translation mechanismswhen they were combined with microoptical elementsin micro optoelectromechanical systems.4,5,19

For application of MLA telescopes in commercialdevices such as multimode fiber switches, stabilityand repeatability are of major importance. Toler-ance analysis showed that the decentration for beamdeflection needs to be accurate on a submicrometerscale. Additionally, rotation of two MLAs that buildup a telescope about the optical axis led to a strongdecrease in transfer efficiency. This is why we de-veloped a new piezobased actuator for highly accu-rate and highly parallel decentration of two MLAs forbeam deflection, as shown in Fig. 6.

Here each beam-deflection system consists of twoactuators operating in opposite directions; each ac-tuator contains one cylindrical MLA. In each of theactuators a parallelogram principle based on solid-state hinges19 is used, which allows for movement inthe desired direction only. By orienting the piezo-stacks in the actuators in a reverse V arrangementwe can achieve that a movement in the direction inwhich the cylindrical MLAs shall be moved onlyresults during application of the correct set of volt-ages that cause one actuator to contract and the otherto expand. �The axes of the cylindrical MLAs areoriented vertically in Fig. 6�. If both piezostackscontract or expand for any reason �for instance, be-cause of a change in temperature�, movement of thecylindrical MLAs only in the direction of the cylinderaxis will result; thus there will be no deflection oflight.

Focusing and recollimating MLAs are accom-plished in two completely identical actuators, asshown in Fig. 6. The actuators are moving in oppo-site directions, with a maximum decentration of the

Fig. 4. Influence of the corona caused by MLA imperfections andgrating effect on cross talk among channels with a cylindrical MLAtelescope used for beam deflection.

Fig. 5. Influence of the corona caused by MLA imperfections andgrating effects on cross talk among channels with two crossedcylindrical MLA telescopes used for beam deflection and orienta-tion of the output fiber array at 45° with respect to the direction oftransformation of the MLA telescopes.

Fig. 6. Mechanical setup for highly parallel decentration of cy-lindrical MLA telescopes.

1 December 2003 � Vol. 42, No. 34 � APPLIED OPTICS 6891

MLAs of 110 �m �the maximum necessary decen-tration is half of the lens pitch,9 which in our casemeans 100 �m�. In this way the desired beamdeflection can be achieved and, because of symmetry,additional stabilization of the deflection will result.The system is centered when both actuators are inthe middle of their translation ranges.

Because of this highly symmetrical setup of theone-dimensional beam-deflection systems it can beexpected that the system will operate stably and in-dependently of temperature. Strain-gauge mea-surement systems are applied to permit adetermination of relative positions of the MLAs foractive control. The driving and amplification elec-tronics are:20 2� ENV 150 PSH �amplifier�, 1 �ENT 400 �power supply�, 2 � ER1 �control circuit�,and 1� EDA2 �computer interface�.

To address output channels oriented in imagespace at a 45° angle with the cylinder axes of theMLAs requires beam deflection into two perpendicu-lar directions. Thus we placed two one-dimensionalbeam-deflection systems perpendicular to each otherinto the optical path, as shown in Fig. 7. The align-ment of the two beam-deflection systems relative toeach other and to the collimated beam is less criticalthan the alignment of two MLAs into one actuator,which we describe next.

D. Assembly

Tolerance analysis has shown that two MLAs for one-dimensional beam deflection need to be alignedwithin submicrometer accuracy to achieve minimuminsertion loss. Thus we decided for an active align-ment controlled by the optical transfer efficiency byusing six-axis translation stages. In the opticalalignment path we used the same components as

were used in the final fiber switch. Figure 8 showsthe system for active alignment of the MLAs. TheMLAs were glued into specially designed frames topermit active alignment in the optical path byclamps, as shown in Fig. 9. Both MLAs �via framesand clamps� and the actuator were attached to six-axis manipulators to achieve the correct alignmentwith the optical path. After curing by UV light theframes could be released and the clamps removed.To fix the MLAs into frames and the frames intoactuators we used the UV-light curing glue Vitralit1507 because of its low shrinkage during hardening.With this procedure two identical beam-deflectionsystems were actively assembled.

The comb structure for building up an array of fourfibers with 600-�m core diameters and 1-mm separa-tion is shown in Fig. 10. Because of the large outerdiameter of the multimode fibers with 2.45-mm buff-ers, compromises between length and stress in fiber

Fig. 7. Two-dimensional beam deflection system that uses twoperpendicularly oriented devices from Fig. 6 with holders for col-limating and focusing optics.

Fig. 8. MLAs in frames need to be actively aligned in the piezo-mechanical stage.

Fig. 9. Gripping tool for mounting the frames that hold the MLAsinto the actuators.

6892 APPLIED OPTICS � Vol. 42, No. 34 � 1 December 2003

bending had to be made when the distance of the fibercores was changed from the 1-mm separation in theimage plane to the minimum possible fiber distanceincluding all buffers, 2.45 mm. The structure shownin Fig. 10 allows strain relief and also incorporation ofthe bare fibers in a very compact arrangement.

We completed the micro-optical fiber switch based onbeam deflection by cylindrical MLA telescopes by assem-bling the two beam-deflection systems, the two achro-mats, the input fiber, and the array of output fibers intoa commercially available optomechanical stage.

Figure 11 shows the completely assembled proto-type micro-optical 1 � 4 fiber switch for multimodefibers with 600-�m core diameters without controlelectronics. The overall system dimensions are 15cm � 10 cm � 13 cm with an overall weight of theoptomechanics of 3 kg. We achieved switchingamong channels by the variable beam deflector by dig-itally memorizing the values of voltage for the largestpossible transfer efficiency for each channel and usinga PC to assign a channel number to this voltage value.

3. Parameter Measurements and Their Comparisonwith Simulations

A. Insertion Loss and Cross Talk

For measurements of insertion loss we used a fiber-coupled LED at 635-nm wavelength as the source andan Advantest TQ 8210 optical powermeter with aQ82104A fiber-coupled Si photodiode for signal detec-tion.

We measured an insertion loss of 0.4 dB for thefiber imaging system using only two achromats with-out beam-deflecting components in the light path �re-flection losses of two additional fiber end faces of 0.36dB were not taken into account�. These losses couldbe caused by cladding modes in the fiber. The in-sertion losses of only the assembled beam-deflectionmodules for undeflected throughput were measuredin the alignment setup to be 1.3 and 1.2 dB. Theycan be reduced to 0.95 and 0.85 dB, respectively, byuse of AR coatings for the air–glass interfaces. �Be-tween the substrates of the MLAs that build up onetelescope we used immersion oil of a suitable refrac-tive index.�. Without beam deflection for the overallsystem we can expect an insertion loss of 3.25 dB,including all Fresnel losses and losses that are due tothe fiber imaging system.

Table 2 lists the measured results for insertionlosses and cross talk for the four output fibers. Theaddressed channels are listed in the rows, and themeasured channels are listed in the columns, of thetable. Thus on the diagonal the values of insertionloss can be seen and in all other sectors the values ofcross talk are given. Channels 1 and 4 are outerchannels in the linear fiber array of four, and chan-nels 2 and 3 are center channels �Fig. 2�.

Because there are four additional uncoated sur-faces of the MLAs in the experimental light path thevalues of insertion loss can be approximately 0.72 dBlower when AR coatings are used on the MLAs �if oneassumes approximately 0.18-dB Fresnel loss perglass–air interface�. AR coating the two additionalfiber end faces of input and output fibers and neglect-ing the losses of the bulk fiber-imaging optics of 0.4dB would lead to theoretically achievable values ofthe overall insertion loss 1.48 dB lower than wereexperimentally achieved. Thus insertion losses of2.4 dB for the inner channels and 2.9 dB for the outerchannels can be obtained.

Fig. 10. Holder for multimode fibers in a linear array of four, withseparation of the bare fibers and a strain-relief clamp for thebuffers. The lateral distance between fiber end faces is 1 mm.

Fig. 11. Overall view of the prototype �without the driving elec-tronics for the actuators�.

Table 2. Measured Values of Insertion Loss and Cross Talk �dB� ofPrototype 1 � 4 Fiber Switch for Multimode Fibers with 600-�m Core

Diametersa

MeasuredChannel

Activated Channel

1 2 3 4

1 4.5 �37.5 �44.1 �43.52 �38.2 4.0 �34.3 �44.73 �43.8 �34.2 3.7 �38.84 �39.8 �43.4 �37.4 4.4

aIncluding all reflection losses and losses of the bulk fiber imag-ing optics.

1 December 2003 � Vol. 42, No. 34 � APPLIED OPTICS 6893

The parameter variations that can be observed inthe insertion-loss and cross-talk values are due toinhomogeneities of the cut and polished fibers in thelinear fiber array. Comparing the experimentallyobtained insertion losses for beam deflection as listedin Table 2 with the value of 3.25 dB when there is nodeflection, we can see that deflection-dependentlosses caused by spurious light9,13 have a major effecton the outer channels. The losses could be dramat-ically reduced if field lens arrays were placed uponone substrate with the recollimating MLAs.9

In Table 3 the values of insertion loss and cross talkcalculated by numerical wave optical simulations12 forthe parameters of the system are shown �without anyreflection losses�. It can be seen from the results ofthe simulation that the values of insertion losses andcross talk were expected to be much better than exper-imentally achieved. The insertion loss of the innerchannels should theoretically be approximately 1 dBbetter than was experimentally achieved. The theo-retical values for the outer channels match the exper-imental values well if the reflection losses and thelosses that are due to the collimating–focusing optics,which add to 1.5 dB, are taken into account whenexperimental and simulated data are compared. Theobserved differences in measured and simulated val-ues of insertion loss for the inner channels were due tomisalignment of the beam-steering devices.

For commercial fiber switch applications a cross talkof �50 dB is considered a standard value. This valuetheoretically can be achieved for all channels that arenot direct neighbors of the addressed channel. As canbe seen from Table 3, for direct neighbors of the ad-dressed channel, values of �42 to �43 dB can be ob-tained. From the simulations it is not obvious whatcauses the relatively large cross talk in the experi-ments. The experimental values of cross talk are�34.2 to �44.7 dB. Additional MLA imperfectionsand the presence of scattered light are possible reasonsfor the large amount of cross talk, which we can de-crease further only by increasing the distance betweenthe output fiber end faces in the image plane.

B. Switching Time

For measurements of switching time we applied arectangular signal to the analog input of the driving

electronics to obtain switching between two previ-ously defined coordinates for the X and Y beam-deflecting modules. The amount of time for datatransfer from PC to PC interface in the driving elec-tronics and for digital-to-analog conversion should beadditionally considered in measurements of switch-ing time when one is switching via a PC.

Figure 12 shows the measured switching behaviorof the 1 � 4 fiber switch for switching among chan-nels with different amounts of separation. The sig-nal curves for arriving at the switching stateschannel on and channel off are shown. The maxi-mum optical switching times correspond quite well tothe values that can be expected from the dynamicbehavior of the mechanics and the driving and controlelectronics.17,18 The ramps for switching betweenfarther-separated channels Figs. 12�a� and 12�b�� aresteeper than for channels that are closer to each otherFigs. 12�c� and 12�d�� because of the behavior of thecontrol electronics. So switching off to farther-separated channels is faster �3 ms� than switching tocloser channels �9 ms�. Switching on from farther-separated channels takes place in 7 ms and fromcloser channels takes 9 ms. These switching timesfor thick fibers are attractive for practical use.

C. Long-Term Stability and Repeatability and ClimaticExposure Test

Long-term stability and repeatability are also essen-tial parameters of optical fiber switches. We used atemperature-stabilized LED at a wavelength of 850nm as a fiber-coupled source and an Exfo IQ 203optical test system with an IQ 1200 four-channel pow-ermeter with data acquisition every 10 s to test thelong-term stability and repeatability of the prototypedescribed here. Each channel was switched on for30 s during one experiment cycle. The absolute du-ration of one measurement was 12 h; thus 360 cycleswere driven. Two channels were tested at one time sowe could monitor the intensity of the LED light sourceas a reference. The experiments took place underthermally unstabilized laboratory conditions.

Figure 13 shows a curve of long-term stability andrepeatability of channel 4. It can be seen that thechange in insertion loss over 12 h was only 0.011 dB.The drift of intensity of the LED light source was0.004 dB at maximum and was subtracted to producethe curve presented here. The maximum change ininsertion loss for all four channels was 0.017 dB.

This excellent behavior results from the doublycompensated actuator mechanisms and the controlcircuit with active position control that uses straingauge measurement systems. The stability of theactuator and also its large displacement of 220 �mcombined with the short switching time shows theutility of the piezomechanical actuator concept andsuggests many new applications. At present it is notclear whether the small residual instability is causedby the optomechanics or by the driving electronics.

Measurements of stability in the climatic exposuretest cabinet were made with the same source andinsertion-loss measurement setup as for long-term

Table 3. Numerical Wave Optical Simulated Values �dB� of InsertionLoss and Cross Talk of Prototype for 1 � 4 Fiber Switch for Multimode

Fibers with 600-�m Core Diametersa

MeasuredChannel

Activated Channel

1 2 3 4

1 3.1 �42.4 �50.2 �51.82 �43.5 1.3 �42.0 �51.53 �51.5 �42.0 1.3 �43.54 �51.8 �50.2 �42.4 3.1

aNo reflection losses or losses due to imperfections of collimatingand focusing optics, which sum to 1.5 dB, were taken into account.For direct comparison with Table 2 they would need to be added tothe values of insertion loss presented here.

6894 APPLIED OPTICS � Vol. 42, No. 34 � 1 December 2003

stability and repeatability. Only the fiber switchwas placed inside the climatic exposure test cabinet;the necessary electronics were placed outside. Amaximum change of insertion loss of less than 0.5 dBwas observed for a temperature change from 25 °C to60 °C within 90 min and for a 2-h constant temper-ature at 60 °C. After cooling, the system easilycould be brought back to its original values after anew calibration of the deflection coordinates.

4. Conclusions and Outlook

We have developed a micro-optical fiber switch formultimode fibers with 600-�m core diameters. Twocrossed cylindrical microlens array telescopes wereused for two-dimensional beam deflection, and the ar-ray of output fiber end faces was oriented at 45° withrespect to the cylinder axes of the MLA telescopes.This special design proved to be necessary to minimizecross talk that was due to MLA imperfections and tothe grating effect. With this arrangement we wereable to obtain values for the cross talk of �34.2 to�44.7 dB, which was still not so low as we expectedfrom numerical simulations. Because of the applied

cascading of beam-deflecting telescopes to improve thecross talk, doubling of the insertion loss had to beaccepted. Thus we reached an average of 3.85 dB forthe two center channels and 4.4 dB for the two outerchannels �including reflection losses on six glass–airinterfaces�. Switching times of 3–9 ms were mea-sured, depending on whether switching was takingplace between two neighboring channels or betweentwo more widely separated channels and on whetherthe on or the off state was measured. Testing long-term stability and repeatability, we determined a max-imum change in insertion loss over all four channels of0.017 dB measured over 12 h and 360 cycles underthermally unstable conditions. This result gives riseto the assumption that the applied piezomechanicalconcept potentially will find wide applications. Pre-liminary measurements in a climatic exposure testcabinet showed only a small change in insertion losswith change in temperature. Major advantages ofthe new design are large translation ranges and fastswitching times simultaneously with good stabilityand accuracy.

One can easily increase the number of output chan-

Fig. 12. Activation and deactivation behavior of the four channels �approximate switching times are given here in parentheses�: �a�Activation of channel 4 when one is switching from channel 1 �7 ms�. �b� Deactivation of channel 1 when one is switching to channel 4�3 ms�. �c� Activation of channel 2 when one is switching from channel 3 �9 ms�. �d� Deactivation of channel 2 when one is switching tochannel 3 �9 ms�.

1 December 2003 � Vol. 42, No. 34 � APPLIED OPTICS 6895

nels from four to eight by using the same beam-deflection setup but doubling the focal length of thecollimating and focusing optics. In this case the lat-eral translation of the deflected spot in the plane ofthe output fiber end faces will double by the sameamount that the MLA is off center. However, onehas to take into account that the bundle diameteralso doubles, and so consequently does the necessarysize of the MLAs. The alignment of the MLA tele-scopes gets more critical with regard to twisting alongthe optical axis.

Introducing an array of field lenses as proposed byWatson,9 which can be on the same substrate withthe recollimating MLA, will lead to a constant trans-fer efficiency independently of the value of deflection.This property can be used to prevent larger insertionlosses for the outer channels. Future examinationswill therefore concentrate on implementing field lensarrays with high precision upon one substrate withrecollimating MLAs.

This research was performed while the authorswere with Piezosystem Jena GmbH. We are grate-ful to Peter Dannberg �Fraunhofer-Institut fur Ange-wandte Optik und Feinmechanik, Jena� forfabricating and characterizing the cylindrical lens ar-rays. Frank Wyrowski �Lighttrans GmbH� for tem-porary free use of the wave optical simulationsoftware DIFFRACTICA, and Andreas Tunnermannfor many valuable discussions.

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Fig. 13. Repeatability and long-term stability for the example ofchannel 4. The change of insertion loss is so small that the curvesare of the magnitude of the resolution of the insertion-loss mea-surement system, 0.001 dB. The drift of the intensity of the LEDlight source was also monitored �maximum 0.004 dB� and sub-tracted to achieve the curve shown.

6896 APPLIED OPTICS � Vol. 42, No. 34 � 1 December 2003