Optical Engineering 50(2), 025005 (February 2011)

Design of a fiber-collimated array for beam combining

Cindy BellangerArnaud BrignonThales Research and TechnologyCampus de Polytechnique1 Avenue Augustin Fresnel91767 Palaiseau Cedex, FranceE-mail: arnaud.brignon@thalesgroup.com

Bruno ToulonJerome PrimotOffice National de d’Etudes et de Recherches

AerospatialesChemin de la Huniere91761 Palaiseau Cedex, France

Faycal BouamraneThomas BouvetStephan MegtertUnite Mixte de PhysiqueUMR 137-CNRS/Thales et Universite Paris-sud XICampus de Polytechnique1 Avenue Augustin Fresnel91767 Palaiseau Cedex, France

Lionel QuetelTristan AllainIDIL Fibres Optiques42 rue Claude Chappe22300 Lannion, France

Abstract. In this paper, we present the design of a very precise collimatedfiber array that meets requirements for beam combining. Calculations per-mit to determine the tolerances toward key parameters and specify thecomponents to manufacture. Thus, the collimated fiber array is composedof a high quality commercial microlens array and an especially dedicatedfiber holder that we design and realize experimentally. Manufacture tech-niques for both the microlens and the holder are chosen to be collectiveand then compatible with a high number of fibers. With the collimatedfiber array hence obtained, the individual beam quality was measuredto be λ/10 and the pointing accuracy is under 0.6 mrad. C© 2011 Society ofPhoto-Optical Instrumentation Engineers (SPIE). [DOI: 10.1117/1.3537968]

Subject terms: collimated fiber array; beam combining; fiber lasers.

Paper 100739R received Sep. 14, 2010; revised manuscript received Dec. 9, 2010;accepted for publication Dec. 20, 2010; published online Feb. 25, 2011.

1 IntroductionFiber lasers provide an attractive means of reaching high-output laser power because of their advantages in terms ofcompactness, reliability, efficiency, and beam quality. Evengiven these advantages, it is desirable to increase the sys-tem power or energy levels beyond what is possible with asingle-mode fiber laser. This is of particular interest for appli-cations that require a narrow-linewidth source with polarizedemission.

A promising technology is coherent beam combining,where all the fiber lasers operate at the same wavelengthand are phase-locked so that their fields add coherently inthe far field. Active phase locking involves phase detectionand active compensation of phase errors.1–7 This techniquebrings additional functionalities such as beam deflection andbeam shaping. This can be useful to correct a wavefront dis-torted by a passage through atmosphere, or to point a receiverin free-space communication.8–10

The investigated architecture to combine a high numberof amplifier fibers by an active control of the phase is shownin Fig. 1. A master oscillator provides a signal which isdistributed to N single-mode polarization maintaining (PM)fiber amplifiers. The N end facets of the fibers are disposedin a matrix arrangement and collimated by a lens array. A

0091-3286/2011/$25.00 C© 2011 SPIE

small part of the resulting beam is sent to a phase sensor.This device measures the phase of each beam and drivesa phase modulator array through an electronic feedbackloop.

In this setup, one of the key components is the array ofcollimated fibers. To the best of our knowledge, commer-cial collimation systems available are not accurate enough toachieve beam combining with a high Strehl ratio. The aimof this paper is to present the design and the realization of afiber collimated system fully compatible with a high numberof fibers. The device we realized is made of 64 collimatedfiber lasers and works at the wavelength of 1.5 μm.

2 Requirements on Collimation to Realize BeamCombining

In order to evaluate the tolerances toward collimation ac-curacy on the quality of beam combining, we modelled thepropagation through far field of beams from a bundle ofsingle-mode fibers collimated by a lens array.6

We start with the propagation of a Gaussian distributionof the optical field u at the end of a fiber in a cylindricalcoordinate system (r,z), given by:

u(r, z) = E0w0

w(z)exp

[ −r2

w2(z)

]exp

[ik

r2

2R(z)

]× exp(i(kz−ϕ(z)), (1)

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Bellanger et al.: Design of a fiber-collimated array for beam combining

Fig. 1 General scheme of the beam combining setup with an activecontrol feedback loop.

where E0 is the maximum amplitude of the optical field, w0is the waist of the Gaussian beam, and⎧⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎨⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎪⎩

w(z) = w0

√1 + z

z2R

R(z) = z

(1 + z2

R

z

)

zR = πw20

λ

ϕ(z) = arctan( zzR

)

k = 2π

λ

, (2)

where λ is the wavelength and zR is the Rayleigh range. Eachbeam is then collimated by a lens of diameter 2R. The endfacet of the fiber is placed at z = 0 and at a distance f fromthe lens, where f is the focal length. For one fiber, the fieldbecomes

ul (r, f ) = u(r, f ) exp

[iπ

r2

λ f

]disc(|r|) (3)

where disc(x) = 1 if x ≤ R, and disc(x) = 0 for x > R.The fibers and their associated collimating lens are dis-

posed side by side in a matrix arrangement. Then the resultingtotal optical field U in the plane of the lens array is given by

U (x, y) = ul (x, y) ⊗ �(x, y) (4)

where ⊗ is the convolution operator,{x = |r| cos[arg(r)]

y = |r| sin[arg(r)], (5)

and

�(x, y) =N x∑j=1

N x∑k=1

δ(x − x j , y − yk). (6)

The Fourier transform of the near field U distribution givesthe far field U . The final intensity in the far field is given by

I (x ′, y′) = U (x ′, y′) · U (x ′, y′)∗. (7)

To evaluate the quality of the coherent combining, wecalculate the Strehl ratio S by comparing peak intensity onthe calculated far field I with that we would obtain with aGaussian beam (Igauss) with a waist at 1/e2 equal to the radiusof the total composite pupil:

S = I (0, 0)

Igauss(0, 0). (8)

The Strehl ratio is in direct proportion with the fillingfactor of the global pupil composed of individuals’ beams tocombine. As a consequence, the beam arrangement in nearfield should be as compact as possible.

The influence of the size of each beam compared withthe aperture size of its associated lens has been studied [seeFig. 2(a)]. For small beam diameters, the beams do not coverthe whole lens, the total fill factor is reduced, and the Strehlratio is low. Numerical calculation shows that a ratio of aboutρ = 84% between lens diameter and beam diameter at 1/e2

maximizes the Strehl ratio. Above this value, a fraction ofthe power is clipped by the lens aperture and the Strehl ratiodecreases. A value of ρ = 84% with a fiber numerical aper-ture ON of 0.1 gives collimating lens f-number N given byN = ρ/2O N ≈ 4.

The degradation of the Strehl ratio with usual collimationerrors has also been studied, when the phase of the fibersis perfectly controlled. It can be seen in Fig. 2(b) that animportant degradation of the Strehl ratio can be induced byan offset between the fiber axis and the axis of his associatedlens. Indeed, this offset leads to a tilt a of the beam after thelens. This tilt angle must be kept lower than about 0.6 mrad.The Strehl ratio decreases by 50% for a tilt angle of about1.3 mrad.

The precision of the focal lengths f of the collimatinglenses is an important parameter as shown in Fig. 2(c). It canbe seen that an accuracy of at least 1.8% on the focal length isrequired to keep a Strehl ratio above 85% of its higher value.The Strehl ratio decreases by 50% for a focal length variationof 3.9% peak-to-valley. Finally, a fiber tilt translates into anoffset of the collimated beam after the lens. This error, to firstorder, does not contribute to the degradation of far field onaxis intensity.

Table 1 summarizes the requirement for the collimatedfiber array for an optimization of the Strehl ratio in the farfield. All the described requirements lead to a collimationquality of λ/10 at least.

3 Microlens ArrayFibers used in the device are standard Corning PM fiberson which we measured a total divergence of 0.1 rad. Re-quirement toward individual lens fill factor ρ is establishedaround 84% and leads to chose an individual microlens withan f-number of 4 in order to match with the fiber divergence.1500 μm-pitch silicon microlens array, with an individualfocal length of 5.77 mm@1550 μm leading to an f-numberof 3.8, are chosen to match as close as possible this criteria.This component, described in Fig. 3, was realized by SussMicroOptics with a photolithographic manufacturing pro-cess. The 64 plano-spherical circular lenslets are placed ina 8×8 Cartesian arrangement. The quality of the microlensarray was checked up by measuring statistically the wave-front from one individual lens of the matrix enlightened bya large plane wave with a Quadri-Wave Lateral Shearing

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Bellanger et al.: Design of a fiber-collimated array for beam combining

Fig. 2 (a) Strehl degradation toward filling factor of individual lens, (b) pointing error and (c) focal length error.

Interferometer (QWLSI).11, 12 The experimental setup andthe results are shown in Fig. 4. A typical phase map showsa spherical aberration estimated to 0.7 rad (λ/9). This valueis in agreement with what we could expect from a plano-spherical lens, and matches the quality criterion for beamcombining. Reduction of this aberration would require theuse of aspherical lens array.

A regularity check was also performed on the mi-crolens array by systematic pitch and profile measurements(see Fig. 5). A typical profile shows a perfect match withthe corresponding spherical profile obtained by calculationon 90% of the lens aperture and was obtained with a per-fect regularity on a different lens of the array. This ensuresfocal length uniformity. Pitch regularity is respected with aprecision of 1 μm or better.

The whole set of measurements performed on our lensletarray gave values suitable for beam combining requirements.

4 Design of a Dedicated Positioning PlateThe next step is the positioning of the fibers. A previ-ous model showed that a great degradation of Strehl ratiocomes from an offset a between fiber and his associated lens

Table 1 Summary of the requirements on the collimated fiber arrayto realize beam combining with a Strelh ratio above 80% of his highervalue.

Parameter Required value

Phase error (peak-to-valley) < λ/10

Collimating lens f-numberρ

2ON≈ 4 with standard

single-mode fibers

Pointing error (peak-to-valley) < 0.6 mrad

Focal length error (peak-to-valley) < 1%

axis. The resulting tilt angle must be kept lower than about0.6 mrad. As our lens array has a focal length of 5.77 mm,this criterion translates into a fiber positioning accuracy of3.4 μm.

Fig. 3 Datasheet of our microlens array.

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Bellanger et al.: Design of a fiber-collimated array for beam combining

Fig. 4 (a) Scheme of the experimental setup used to measuremicrolens quality by QWLSI. (b) Typical phase profile obtained withour microlens array.

The fabrication of a high precision optical fiber holderwas achieved by means of deep x-ray lithography (DXRL)technology which can handle the requested high precision,i.e., 1 μm precision for the fiber hole diameter and for thepositioning between two fibers. The theoretical precision ofDXRL is mainly related to the vertical and horizontal di-vergences of the synchrotron source which are less than0.1 mrad in our operating conditions, well beyond the re-quired tolerance. The material chosen for the holder is poly-methylmethacrylate (PMMA), which is a standard DXRLresist that offers appropriate mechanical qualities to tightlymaintain a matrix of optical fibers.

To manufacture the DXRL mask, duplication is doneusing UV optical lithography. A standard chromium maskwas used, which guarantees an accuracy of better than1 μm for the dimensions and relative positions of the holes.The UV lithography was performed on a 300 μm thickgraphite substrate (SFG-2.3 from Poco Graphite) on topof which a 25 μm thick SU-8 layer was spun up. After

Fig. 5 Profile and pitch measurement on the microlens array.Photographs were taken through a binocular lens.

Fig. 6 Binocular photo of the PMMA plate, with detailed view of ahole.

development, gold was electroplated up to a thickness of20 μm. The duplication of the chromium mask to manufac-ture the DXRL mask maintains the precision of the hole diam-eter and of the positioning between holes within the toleranceof 1 μm.

To manufacture the high precision optical fiber holder, a1 mm thick PMMA sheet was baked 1 h at 110◦C in order toremove accumulated stress due to former treatments such ascutting and machining.

The PMMA resist was exposed to synchrotron light atANKA, Forschungszentrum Karlsruhe (FZK), on litho IILIGA station. This LIGA beam line is equipped with asilicon mirror covered with 200 nm thick nickel film, inorder to remove high energy photons (higher than 12 keV),a cutoff angle of 4.85 mrad was chosen. The used radiationdose at the bottom of the resist was 3 kJ/cm3. The PMMAsheet and the mask were kept at 20◦C during exposure inorder to avoid heating due to the absorbed high photon flux.After exposure, the self-standing PMMA sheet was thendeveloped in a standard GG bath13 for 24 h, rinsed in waterduring 20 mn, and then dried at room temperature. Figure 6presents a photograph taken through a binocular lens of thePMMA holder we manufactured.

We can note that our manufacturing process, based onmasking techniques, allows the realization of a holder com-patible with a much higher number of fibers with the sameaccuracy, for high power beam combining application asmentioned in Ref. 14. Also, the use of other materials thanPMMA, such as metallic plates, can be envisaged for hold-ing a higher power fiber amplifier and for better thermalmanagement.

5 Insertion and Positioning of the FibersThe 64 PM fibers were mounted on the matrix according tothe following steps. First, each PM fiber has been cleaved tobe able to see the stress applying parts of the PANDA PMfiber with a camera. In a second step, the fibers were insertedinto the 64 narrow holes of the matrix using a camera anda X, Y, and Z translation stage (see Fig. 7). When a fiber isinserted into their hole, a rotation stage and the same camerahave been used to align the stress applying parts in front of avertical line on the camera screen.

After this, the fibers were definitely fixed with epoxy glueon the matrix. When all the fibers were glued, the matrix wasfixed on a holder. This holder is a black anodized aluminumtube. The back side of the matrix with the 64 output fibersis perfectly protected by this tube as shown in Fig. 8(a).Thepolishing process is exactly identical to optical connectorpolishing one. We use three different polishing films with

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Bellanger et al.: Design of a fiber-collimated array for beam combining

Fig. 7 Photo of the fiber insertion setup in the lab: the fibers areinserted into the 64 narrow holes of the matrix using a camera and aX, Y, and Z translation stage.

different grades. A camera was also used to check the qualityof the work.

At the end, 64 PM FC/APC connectors have been mountedon the opposite side of the matrix. The final step of the processwas to measure the optical loss of each fiber by injectinglight by the connector. The fiber array realized is shown inhis definitive holder in Fig. 8(b).

Fig. 8 Photos of the fiber array.

6 Measurement of the Quality of the CollimationIn order to test the collimation device we have placed thefiber array in front of the microlens array creating 64 laserbeams for beam combining.

We now describe the procedure followed to align the 8×8array of fibers with the 8×8 array of microlenses. In thefollowing, the 8×8 fiber array is considered as the referencefor the alignment.

First of all, the array of microlens is adjusted so that thebeam issued by each fiber is centered on the associated lens.A way to do this preliminary alignment is to use a camera, andto slightly translate the microlens array: a good alignment isreached when the translation induces no rotation of the gridof beams on the camera.

From now on, the fiber array and microlens array havethe same orientation, i.e., the basis vectors are the same, butare not necessarily parallel. The parallelism between the twoarrays can be reached in two steps. First, the major paral-lelism error is evaluated and then compensated by imagingthe microlenses on the camera. The observed array has to beCartesian, and a slight translation of the microlenses shall in-duce uniform defocusing and thus broadening of the beams.The second step is to quantify the residual error of paral-lelism thanks to the use of a wave-front sensor, for example,the quadri-wave lateral shearing interferometer we had pre-viously used. The wavefront sensor is then used to measurethe phase of each beam issued by the microlenses and toevaluate the defocusing. Then we obtain a 8×8 defocusingaberrations array, and the correction of orientation of the

Fig. 9 (a) Scheme of the experimental setup for collimation align-ment tests. (b) Plot of the centroıd of each collimated beam on thefocal plane of the lens. The dotted circle represents the toleranceestablished by calculations.

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Bellanger et al.: Design of a fiber-collimated array for beam combining

microlens array can be estimated by projecting the array ofdefocusing values on two unit tilts.

To measure fiber position accuracy, the position of thecentroid of each beam was plotted in the far field plane.This has been done through a 200 mm focal length lens anda camera placed exactly at his focal point [see Fig. 9(a)].These data allowed us to retrieve the offsets between eachfiber and its associated lenslet axis. Our results are shown inFig. 9(b). A dashed circle is plotted in Fig. 9(b). This circlerepresents the offset limit to achieve beam combining withhigh Strehl ratio (greater than 85% of his maximal value).The maximum pointing error we measured on our collimationsystem was 0.52 mrad, and the average error was 0.2 mrad.According to this criteria our system meets requirements forbeam combining.

7 ConclusionTo conclude, we have designed, manufactured, and specifiedan array of 64 collimated fibers suitable for beam combin-ing at the wavelength of 1.5 μm. This bundle is made of amicrolens array and a dedicated holder that supports the 64single-mode fibers. Measured individual beam quality is λ/10and pointing accuracy is under 0.6 mrad, allowing beam com-bining operation with high Strehl ratio. The involved manu-facturing techniques are all collectives and scalable to a highnumber of fibers, and allows all kinds of spatial arrangementfor the fibers.

AcknowledgmentsThe authors would like to thank Dr. Lothar Hahn et al. fromthe IMT (Institut fur Mikrostrukturtechnik, Karlsruhe) fordedicated beam time allocated to this project and for theirhelp during processing x ray lithography. The authors aregrateful to Gerard Mourou for fruitful advices and discus-sions. This work was partially supported by the French Re-search National Agency (ANR).

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4. B. Bennai, L. Lombard, V. Jolivet, C. Delezoide, E. Pourtal, P. Bourdon,G. Canat, O. Vasseur, and Y. Jaouen, “Brightness scaling based on 1.55μm fiber amplifiers coherent combining,” Fiber Intergr. Opt. 27(5),355–369 (2008).

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Cindy Bellanger graduated from Ecole Cen-trale de Marseille (France) in 2006 and re-ceived her PhD from Universite Paris XIOrsay in 2010. Her thesis work was con-ducted in the Physics Group of Thales Re-search & Technology. She studied collectivetechniques to realize coherent beam com-bining with a high number of fibers lasers.She is currently working in the Theoreticaland Applied Optics Department at ONERA(French Aerospace Lab).

Arnaud Brignon received his EngineeringDegree in 1991 from the Institut d’OptiqueGraduate School, and his PhD in 1996 fromthe Paris University. His thesis work was con-ducted in the Corporate Research Labora-tory of Thales in Orsay, France. He studiednonlinear wave-mixing in laser media for theapplication of high beam quality solid-statelasers. From 1996 to 2007 he was workingon laser beam control including phase conju-gation, beam cleanup, beam shaping, beam

steering and beam combining, and high-power diode-pumped solid-state lasers. From 2007 to 2009 he was head of the Advanced Pho-tonics Laboratory and since 2009, he is head of the Micro and Nano-Physics Laboratory in Thales Research & Technology, Palaiseau,France. He has authored and co-authored more than 150 papers(including some 30 invited and tutorials) on laser beam control inscientific journals and conference proceedings, two books, and 30patents. In 1996 he received the Fabry-de-Gramont prize from theFrench Optical Society, in 2000 the Fresnel prize from the EuropeanPhysical Society, and in 2001 the Technology Review’s Young In-novators Award from the Massachusets Institute of Technology. Heis a member of the French Optical Society, the European PhysicalSociety, and the Optical Society of America.

Bruno Toulon graduated from the EcoleSuperieure d’Optique (Orsay, France) in2006 and received his PhD from UniversiteParis XI Orsay, where he worked on interfer-ometry techniques and their applications. Heis currently working as an optronics engineerat Sagem, Safran Group.

Jerome Primot is a graduate of the Instituteof Optics, France. He defended a PhD thesisin 1989, which demonstrated for the first timethe interest of deconvolution from wavefrontsensing (DWFS) in high angular resolutionastronomical imagery. Since 1991, he hasheld ten patents in the field of interferome-try. In particular, he discovered multi-lateralshearing interferometry, a bi-dimensional ex-tension of traditional lateral shearing interfer-ometry. Nowadays, these interferometers are

commonly used in major public research centers all around the worldand in industrial research centers, acknowledged by the “Prix Yves

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Bellanger et al.: Design of a fiber-collimated array for beam combining

Rocard de la Societe Francaise de Physique. He is a shareholder inPhasics, a start-up created in 2003 to commercialize these interfer-ometers. He has also made a significant contribution in the field ofTalbot imagery and of NonDiffracting Arrays. He is also involved in thefield of optical system design. For example, he has developed a newconcept in Fourier transform interferometry and designed advancedradiometric cameras, for airborne applications. He has written morethan 40 papers published in international journals.

Faycal Bouamrane is a research engineerat CNRS. In 1991 he graduated from the“ENSI de Caen,” in materials science and in1997 he received a PhD degree from Univer-sity Paris XI Orsay. He joined CNRS in 1998,first at the laboratory LURE (Laboratoirepour l’Utilisation du Rayonnement Electro-magnetique) in Orsay. Since 2005 he joinedthe Unite Mixte de Physique CNRS THALES.He is doing a research and development ac-tivity in the field of micro-fabrication with the

aim of optimizing the LIGA technology (Lithography, electrogrowth,molding) in order to produce microstructures. He has been respon-sible for microfabrication projects in different fields: sonotrodes toolsfor processing ceramics, Hartmann grids for x-ray optics, microelec-trodes for chemistry spectroscopy, microholders for fiber optics mul-tiplexer, and slow wave structures for THz amplifiers.

Thomas Bouvet is an assistant engineer atCNRS. He joined CNRS in 2001 at the lab-oratory LURE (Laboratoire pour l’Utilisationdu Rayonnement Electromagnetique) in Or-say. Since 2005 he joined the Unite Mixtede Physique CNRS THALES. He graduatedfrom Paris XI University in 2007. Parallel tohis activity, he received a Master 2 of Sciencein micro and nanotechnology. He is respon-sible for the development, the improvement,and valorization of all the studies around

the microfabrication by the LIGA process (lithography X, UV, elec-trogrowth, technology of x-ray mask), and the realization of all themanipulations in order to fabricate microstructures relative to theLIGA projects: sonotrodes tools for processing ceramics, Hartmanngrids for x-ray optics, microelectrodes for chemistry spectroscopy, mi-croholders for fiber optics multiplexer, and slow wave structures forTHz amplifiers.

Stephan Megtert received his State Doctor-ate in 1984 at Laboratoire de Physique desSolides Orsay in the field of structural prop-erties of low dimensional conductors usingx-ray and neutron diffuse scattering experi-ments. He further went on to study High TcSuperconductors in 1987 using the same x-ray technique and is still active in this areaalthough he started in 1992 to develop theDeep x-ray Lithography and related LIGA ac-tivity at the synchrotron centre LURE Orsay.

Since then, he is the head of the microfabrication team. He joined theUMR 137 CNRS-THALES in 2005 with the aim to transfer his teamexpertise as a tool for microsystem process engineers. He partici-pated in more than 20 publications on the development of the LIGAprocess.

Lionel Quetel received a Master in optical-electronical engineering in 1994 fromRennes I University and PhD degrees in op-tical engineering in 1997 from Lilles Univer-sity. From 1998 to 2003 he was Fiber BraggGrating project manager in Highwave OpticalTechnologies (France). Since 2004, he is theOptical Component manager in IDIL FibresOptiques (France).

Tristan Allain received his Bachelor degreein optoelectronics and imaging at the Univer-sity of Angers in 2000. As soon as he gradu-ated he started at IDIL fibres Optiques. He isnow the production manager of IDIL FibresOptiques.

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