Double-cutting beam shaping techniquefor high-power diode laser area lightsource

Zhihua HuangLingling XiongHui LiuZhenfu WangPu ZhangZhiqiang NieDihai WuXingsheng Liu

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Double-cutting beam shaping technique for high-powerdiode laser area light source

Zhihua HuangChinese Academy of SciencesXi’an Institute of Optics and Precision MechanicsState Key Laboratory of Transient Optics and

PhotonicsXi’an 710119, Chinaand

University of Chinese Academy of SciencesBeijing 100049, China

Lingling XiongHui LiuZhenfu WangPu ZhangZhiqiang NieDihai WuChinese Academy of SciencesXi’an Institute of Optics and Precision MechanicsState Key Laboratory of Transient Optics and

PhotonicsXi’an 710119, China

Xingsheng LiuChinese Academy of SciencesXi’an Institute of Optics and Precision MechanicsState Key Laboratory of Transient Optics and

PhotonicsXi’an 710119, Chinaand

Focuslight TechnologiesXi’an 710119, ChinaE-mail: liuxs@opt.ac.cn

Abstract. A new beam-shaping technique is proposed to improve thebeam quality of a high-power diode laser area light source. It consistsof two staggered prism arrays and a reflector array, which can cut theslow axis beam twice and rearrange the divided beams in fast axis tomake the beam quality of both axes approximately equal. Furthermore,the beam transformation and compression can be carried out simultane-ously, and the assembly error of this technique induced by the machiningaccuracy of prism’s dimensions also can be greatly decreased. By thistechnique, a fiber-coupled system for one three-bar laser diode stack isdesigned and characterized. The experimental results demonstrate thatthe laser beams could be transformed into the required distribution with∼93.4% reshaped efficiency and coupled into a 400 μm∕0.22 NA fiber,which are consistent with the theory. © 2013 Society of Photo-OpticalInstrumentation Engineers (SPIE) [DOI: 10.1117/1.OE.52.10.106108]

Subject terms: beam shaping; beam parameter product; laser diode; fiber coupling.

Paper 131340 received Aug. 30, 2013; revised manuscript received Sep. 27, 2013;accepted for publication Oct. 1, 2013; published online Oct. 21, 2013.

1 IntroductionThe high-power laser diode (HPLD), having benefits of highreliability, compactness, affordable cost, and high electro-optical conversion efficiency, has been widely applied inmaterials processing, the medical field, solid-state lasers,and fiber laser pumping.1 In recent years, although the opti-cal power of single bar can achieve 30 to 150 W with acontinuous wave (CW) output in commercial form, manyapplications still require higher brightness by the fiber cou-pling of multiple bars or stack beams.1,2 For typical laserdiode bars, the emitter’s width is 50 to 200 μm in slow axis,while it is only 1 μm in fast axis with typical divergenceangles of 5 and 35 deg (1∕e2, half-angle), respectively. Dueto the poor beam quality of HPLD, a significant optical beamtransformation technique is needed to reshape the beambefore fiber coupling.2

During the past few years, different approaches have beenproposed to perform the beam shaping, including the diffrac-tive and geometrical optical elements. By using the diffrac-tive methods, the large range of wavelength3,4 and low

reshaped efficiency5,6 reduce the transmission of beam trans-formation and limit the performance in applications (e.g.,laser pumping source). Generally, the geometrical opticalelements, such as the micro-optics array devices7–9 andsub-beams rearrange devices10–12 are used to reduce thebeam parameter product (BPP).

The beam along the slow axis is usually cut into Nsections by microprisms and increased by the same ratioin the fast axis.13 In order to obtain good beam quality, thesize of the microprisms should be very small, even <1 mm.However, the actual beam quality cannot always meet that ofthe theoretical design due to the process error of shapingprisms, which is caused by the low machining accuracy.8,10

Also, most of such methods are only effective for the singlebar, and the high fabricating and mounting efforts usuallyresult in relatively high costs per bar or low reshaped effi-ciency for the systems.14 Furthermore, to achieve more com-pact systems in multiple bars or stacks coupling, moreoptical elements should be employed even though that willincrease the complexity of system.10,15–17

In this work, a new beam shaping method with two stag-gered prism arrays and a reflector array is proposed toimprove the beam quality. Compared with the existing0091-3286/2013/$25.00 © 2013 SPIE

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microprism elements for dividing into the same amount offolds, this solution has an advantage in fabricating andassembling since it carries out the beam division twiceand makes the thickness of each prism doubled. At thesame time, the beam width in the fast axis will also becompressed during the reshaping process without usingother optical components. Based on this technique, a trans-formation device for a three-bar LD stack was designed.After reshaping, it makes the beam quality of both axesapproximately equal, and the rearranged beam also couldbe coupled into a silica fiber with typical parameters of400 μm∕0.22 NA, which was demonstrated both by simula-tion and experiment. The results verified that the new tech-nique is effective for performing the area light sourcestransformation such as stacks or multiple bars.

2 Principle of TechniqueIn order to evaluate the beam quality of a laser diode array,the most convenient way is to characterize the BPP, definedas θ ×W, where W is the half beam width and θ is the beamdivergence at half angle in the far field. Due to the highlyasymmetric output beam profile from the diode laser bar,efficient fiber coupling is only possible if the beam qualityis adapted by cutting the beam into Nfold parts in the slowaxis and deflecting them to proper places in the fast axis,and then meet the requirement given by Eq. (1):10,18

BPPtotal ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiBPP2fast þ BPP2slow

q≤ BPPfiber ¼

dfiber2

× NA;

(1)

where BPPtotal is the BPP of the laser beam in a diagonaldirection, and dfiber and NA denote the diameter and numeri-cal aperture of the optical fiber, respectively. The number ofsubdivisions Nfold can be found from the following formula:

Nfold ≅�WslowθslowWfastθfast

�1∕2

; (2)

where Nfold is an even number. An example construction ofthe new beam shaper with a schematic diagram of the trans-formation process is illustrated in Fig. 1. The device is organ-ized with three main components.

The first one is array1 of Nfold∕2 pieces of prism, with anidentical increment in the length of adjacent prisms. Whilethe incident beam passes through prism array1 vertically, it

carries out the first cutting, tailoring the beam into Nfold∕2segments along the slow axis and compressing the beamwidth in the fast axis by refracting on different inclinedplanes.

The second one is array2 consisting of Nfold∕2þ 1 piecesof prism that has two kinds of length placed alternately and isset at an appropriate angle to the X-Z plane. With a staggeredposition in the slow axis, the borderline of two adjacentprisms in array2 overlaps the bisectors of prism in array1 asshown in Fig. 2(a). The thickness of each prism in botharray1 and array2 is 2L∕Nfold, where L is the beam widthin the slow axis at the incident face of prim array1. Whenthe beam further propagates to the prism array2 in a similarway, each section after array1 will be tailored into two partsand compressed again since one half-section passes throughthe long prism and the other passes through the short one.After the second cutting,N sections of sub-beams in the slowaxis are achieved and the propagation direction is revertedparallel to X-Z plane.

The last component is the reflector array3 that has Nfold

pieces of prism placed at a 45-deg angle toward the beampropagation direction. After reflecting on array3, Nfold seg-ments will be rearranged linearly in the fast axis and thetransformation process is accomplished.

As it can be seen from Fig. 2(b), the displacement of theadjacent segmented beam after the second cutting (h2) isdetermined by the length difference of the contiguous prismin array1 (Δl1) and array2 (Δl2). In order to avoid the beamsoverlapping after rearrangement, the following conditionscalculated by the theory of analytic geometry should besatisfied.

Fig. 1 Example of double-cutting beam shaping device.

Fig. 2 Schematic of double-cutting beam shaping method by takingsix folds; the numbers represent the order of output beam parts in slowaxis. (a) Top view. (b) Side view.

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δ ¼ arcsinðn · cos θÞ − ð90 deg −θÞ; (3)

M ¼ sin2 θ

cos2½arcsinðn · cos θÞ� ; (4)

h2 ≥ w0∕M; (5)

Δl1 ¼−2h2 · sin θ

cos½arcsinðn · cos θÞþ θ� · cos½arcsinðn · cos θÞ� ; (6)

Δl2 ¼h2

sin½arcsinðn · cos θÞ þ θ − 90 deg � ; (7)

where w0 andM represent the incident beam width and com-pression ratio in the fast axis. The symbols (n; θ; δ) are therefractive index, slant angle of prism, and the angle of array2set to X-Z plane, respectively.

While the divided amount Nfold is an odd number, it isalso suitable for employing the new technique. In case thefolds number Nfold ¼ 5þ 4k (k ¼ 0; 1; 2: : : : : : ), the devicehas a similar structure as shown in Fig. 3(a). The differencefrom the one described previously is that array1 and array2both have ðNfold þ 1Þ∕2 pieces of prisms, and the originalbeam just passes through one half of the last prism inarray1. Figure 3(b) represents the structure of Nfold ¼3þ 4k; however, the difference of length (Δl1) betweenthe longest and the secondary prisms in array1 is halved,which assures the profiles are close to each other alongthe fast axis after rearranging. The expressions of all param-eters are the same as the situations of the other folds.

3 Case StudyBased on this new type technique, a fiber-coupled module isdesigned by using ZEMAX ray tracing software, and the

experiment has been done in accordance with the simulationresults.

3.1 Optical System Design

The simulated schematic of the beam combination system isshown in Fig. 4. The laser source is a three-bar LD stack witha maximum 60 W (CW) per bar at a central wavelength of976 nm, and its structural parameters are given in Table 1.

To suppress the large divergences and improve beamquality, the emission beams from the LD bars are first colli-mated in the fast axis by plane-convex aspheric columnlenses (f ¼ 0.91 mm, NA ¼ 0.8) and then in the slow axisby microcylindrical lens arrays (f ¼ 3.2 mm, NA ¼ 0.1).In theory, the full divergences (95%E) should be reducedto 2.6 mrad in the fast axis and 70 in slow axis.Actually, they are increased to ∼5 and 80 mrad,respectively, owing to the smile effect. Thus, the BPPs(95%E) of the incident beam are calculated to be∼5 ðfastÞ and 240 ðslowÞmm ⋅mrad with a beam size of4 mm ðfastÞ × 12 mm ðslowÞ after collimation. In order tofurther improve beam quality in the slow axis, a polarizationmultiplexing technique is used before reconfiguring the

Fig. 3 Examples of beam transforming system by N folds, where N isan odd number: (a) N ¼ 5þ 4k . (b) N ¼ 3þ 4k (k ¼ 0;1;2: : : : : : ).

Fig. 4 Layout of beam combination in fiber-coupled module.

Table 1 Typical values of high-power laser diode stack.

Parameter Values

Chip width 10 mm

Cavity length 2 mm

Number of emitters 19

Emission width 150 μm

Emitter pitch 500 μm

Fill factor 30%

Pitch of bars 1.5 mm

θslow (FWHM) 8 deg

θfast (FWHM) 35 deg

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beam, which can reduce the BPPslow to ∼132 mm ⋅mrad(6.6 mm in the slow axis).

Based on calculations from Eq. (2), the beams should bedivided into five parts in the slow axis direction. However,considering the large remaining divergence leading to a seri-ous deterioration of BPPslow during the propagation process,it is proper to be tailored into six sections.

Based on the character of the incident beam, the relation-ships among each parameter are shown in Fig. 5 according toEqs. (3) to (7). It indicates that the compression ratiodecreases while the length difference in array1 and array2increases quickly as the prism’s slant angle increases. Mean-while, it can get a more compact system with the same slantangle by the shorter Δl1 and Δl2 when the index of the prismis larger. When the slant angle is less than a certain value, thecompression ratio will increase sharply. However, the diver-gence in the fast axis will also be increased by the same ratioafter compressing according to the Lagrange invariant. Thus,it is important to choose the suitable compressibility basedon the whole couple system. Here, defining the material ofprism as N-LAF21 (n ¼ 1.771), the slant angle θ ¼ 60 deg,and h2 ¼ 1.4 mm, we can get the pivotal variable Δl1 ¼9.76mm, Δl2 ¼ 2.62 mm, M ¼ 3.47, and δ ¼ 32.3 degfrom Eqs. (3) to (7), and the thickness of the prism inarray 1 and 2 is 2.2 mm.

Last, the output beams after transformation goes througha set of lenses to be focused into an optical fiber of standard

400-μm core diameter and NA ¼ 0.22. The Galilean beamexpander is used to reduce the residual divergence inthe slow axis, including a concave-plane cylindrical lens(effective focal length ¼ −9.7 mm) and a convex-planecylindrical lens (EFL ¼ 38.2 mm). After that, an asphericlens with an EFL ¼ 26 mm is chosen to focus the beam.

3.2 Experimental Results

According to the design parameters, the beam shaping ele-ments were fabricated as shown in Fig. 6.

The simulation and corresponding experiment results ofbeam transformation process are given in Figs. 7 and 8,respectively. It was found that the beam in the experimenthad been transformed into a required distribution, whichis basically consistent with the simulation. The profiles, asshown in Figs. 7(a) and 8(a), represent the output beam afterpolarization combination. Figures 7(b) and 8(b) are the irra-diance distributions after passing through prism array1. Thefacula was divided into three parts in the slow axis and sep-arated in the fast axis. Figures 7(c) and 8(c) show the sim-ulation and measured intensity distribution after prismarray2. As can be seen, the beams were divided into six sec-tions, and the beam width in the fast axis of each section wasreduced to ∼1.2 mm, 3.33 times less than its initial value.Correspondingly, the half divergence angle in the fast axiswas increased to ∼8.1 mrad according to Lagrange invariantprinciple. The layouts of the beams emerging from thereflector unit are given in Figs. 7(d) and 8(d). After rear-rangement, the measured beam size (95%E) was changed to∼8.4 mm ðfastÞ × 1.5 mm ðslowÞ (it was shown ∼8.6 mm×2.3 mm detecting in the place 10 mm after the output face).Therefore, the beam quality (95%E) was turned to 34 mm ⋅mrad in the fast axis and 30 mm ⋅mrad in the slow axis.

Fig. 5 The relationships among the index (n), slant angle of prism (θ),compression ratio (M), length difference of array1 (Δl1) and array2(Δl2).

Fig. 6 Fabricated double-cutting beam transforming elements.

Fig. 7 Simulation of beam transformation process: (a) after polarizationmultiplexing, (b) after first cutting, (c) after second cutting, and (d) afterrearranging. All detectors were set as 18 × 18 mm2 and placed 10 mmafter output face. Vertical: fast axis, horizontal: slow axis.

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Consequently, a BPPtotal of ∼45.3 mm ⋅mrad was obtained,which means that most of the beam still can be coupled intothe optical fiber, although the measured result was slightlylarger than the calculated one (BPPfiber ¼ 44 mm ⋅mrad).

At the coolant temperature of 22°C, flow of 10 L∕min,and CWoperating mode, the results in terms of the reshapedpower and reshaped efficiency are shown in Fig. 9. Thereshaped efficiency is defined as the ratio of measured powerbefore to after passing through the beam shaping device.Although the reshaped efficiency slightly decreased withincreasing injection current, which could be due to theincreasing of the slow-axis divergence,19,20 the reshaped effi-ciency achieved was 93.4% at an operating current of 65 A,corresponding to an output power of 145.1 W.

In the same operating situation described above, the mea-sured maximum power after collimating was 166.8 W whilethe fiber output was 129.8 W, the results of which are givenin Fig. 10. The electro-optics (E-O) conversion efficiency of

40.5% (whole system) and a slope efficiency of 2.24 W∕A(fiber output) testing from 7 to 65 A were obtained.Considering all factors, the low polarization degree (only∼93%) of the incident beam is one reason for the loss ofthe optical power in this system. Selecting an LD with ahigher polarization degree can further enhance the E-O effi-ciency. Besides, the directional error of the LD stack alsoreduces the beam quality and leads to the energy loss infocusing the beam into the fiber.

4 ConclusionsIn summary, we have demonstrated a double-cutting beamshaping method to reconfigure the beams of HPLD stackor multiple bars by employing two staggered prism arraysand a reflector array. The proposed technique is easy to pro-duce due to the larger size and tolerance, and has a highreshaped efficiency. Meanwhile, it also makes the systemmore compact by compressing the beam width in the fastaxis simultaneously. Based on this beam shaping technique,an efficient system for a three-bar LD stack was designed tocouple the beams into a 400 μm∕NA ¼ 0.22 fiber. About93.4% reshaped efficiency of the shaping elements and40.5% electro-optics efficiency of overall system wereobtained in the experiment. Hence, this novel technique issuitable for the HPLD area light source.

AcknowledgmentsThis work was supported by the Western Light InnovationFoundations of Chinese Academy of Sciences(Y229B21213, Y329431213) and the Important ResearchEquipment Foundation of Chinese Academy of Sciences(Y329001232). The authors thank the colleagues inFocuslight Technologies for their help in device fabricationand optical path adjusting.

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Fig. 9 Output power of the reshaped beam and reshaped efficiencyfor the beam transforming system versus different current situations.

Fig. 10 Measured power and electro-optical (E-O) conversion effi-ciency of fiber output in different current situations.

Fig. 8 Experimental results of beam transformation process, corre-sponding to Fig. 7.

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Zhihua Huang received his BS degree fromShenzhen University, Shenzhen, China, in2011. He is currently working toward hisMS degree majoring in optics from theState Key Laboratory of Transient Opticsand Photonics, Xi’an Institute of Optics andPrecision Mechanics, Chinese Academy ofSciences (CAS), Xi’an, Shaanxi, China. Hisfocus is on the beam shaping of high-power diode laser arrays. He has held aninternship position for Xi’an Focuslight Tech-

nologies, Xi’an, Shaanxi, China.

Lingling Xiong received her BS and MSdegrees from Chongqing Normal University,Chongqing, China, in 2003 and 2006,respectively. She received her PhD degreefrom Sichuan University, Sichuan, China, in2009. Her major is optics. She has joinedXian Institute of Optics and PrecisionMechanics of CAS and is engaging in thebeam shaping of high-power diode lasers.

Hui Liu received his PhD degree from theKey Laboratory for Physical Electronicsand Devices, Ministry of Education, and theShaanxi Key Laboratory of Information Pho-tonic Technique, School of Electronics andInformation Engineering, Xi’an Jiaotong Uni-versity, Xi’an, China, in 2011. He has beenwith the Xi’an Institute of Optics and Preci-sion Mechanics, CAS, Xi’an, since 2011.His current research interests include high-power semiconductor lasers.

Zhenfu Wang received his PhD degree inphysics from the Changchun Institute ofOptics, Fine Mechanics and Physics, CAS,Jilin, China, in 2011. He is currently withthe Key Laboratory of Excited State Proc-esses, CAS, and is involved in research onthe fabrication and characterization of semi-conductor lasers, including edge-emittinglasers, vertical cavity surface-emitting lasers,vertical external cavity surface-emittinglasers (VECSEL), and optically pumped

VECSEL. His current research interests include the testing methods,degradation, and reliability of high-power diodes.

Pu Zhang received his BS degree from theDepartment of Physics, University of Scienceand Technology of China, Hefei, China, in2003 and his PhD degree from the Depart-ment of Chemical Physics, University of Sci-ence and Technology of China, Hefei, China,in 2010. He joined Xi’an Institute of Opticsand Precision Mechanics of CAS in 2010,and his research field focuses on high-power semiconductor lasers.

Zhiqiang Nie received his PhD degree fromthe Department of Electronic Science andTechnology, Xi’an Jiaotong University, Xi’an,China, in 2011. He has been with the Xi’anInstitute of Optics and Precision Mechanics,CAS, Xi’an, since 2011. His current researchinterests include the testing of high-powersemiconductor lasers, atto-second andfemto-second polarization beats of four-wave mixing (FWM) processes and hetero-dyne detection of FWM and six-wave mixing

(SWM) processes, Raman-, Rayleigh- and Brillouin-enhanced polari-zation beat (PB), multidressing FWM processes, and spatial modula-tion of FWM.

Dihai Wu received his BS degree from XidianUniversity, Xi’an, China, in 2013. He is work-ing toward his PhD degree in microelectron-ics and solid-state electronics at Xi’anInstitute of Optics and Precision Mechanics,CAS, Xi’an, Shaanxi, China.

Xingsheng Liu received his PhD degree inmaterials science and engineering fromVirginia Polytechnic Institute and StateUniversity, Blacksburg, in 2001. He waswith Corning Inc., Corning, NY; CoherentInc., Coherent, CA; and nLight PhotonicsCorporation, Hillsboro, OR. He then joinedthe Xi’an Institute of Optics and PrecisionMechanics, CAS, Xi’an, China, as a profes-sor. He is currently the president of Xi’anFocuslight Technologies Co. Ltd., Xi’an,

where he is involved in the research, development, manufacturing,and sale of high-power semiconductor lasers. His current researchinterests include developing packaging methodologies and productsof high-power semiconductor lasers.

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