Passive Laser Beam Combining With Intracavity Interferometric Combiners

IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 15, NO. 2, MARCH/APRIL 2009 301

Passive Laser Beam Combining With IntracavityInterferometric Combiners

Amiel A. Ishaaya, Nir Davidson, and Asher A. Friesem, Life Fellow, IEEE

(Invited Paper)

Abstract—In this paper, we present an approach for passivelaser beam combining using plane-parallel intracavity interfero-metric combiners. With this approach, efficient coherent combin-ing of Gaussian laser beams, transverse single high-order-modelaser beams, and even transverse multimode laser beams is possi-ble. We experimentally obtained a combining efficiency of about90% when coherently combining 16 solid-state laser channels, aswell as when coherently combining four fiber laser channels. Wealso present an arrangement for simultaneous coherent combin-ing and spectral combining, which could possibly overcome cur-rent upscaling limits. We present the basic coherent combiningapproach, review our past and recent investigations and resultswith both solid-state and fiber laser configurations, and discuss thepossible upscaling and future prospects of this approach.

Index Terms—Coherent combining, laser arrays, laser res-onators, phase locking.

I. INTRODUCTION

THE POSSIBILITY of combining several lasers that typi-cally have excellent beam quality into one high-brightness

powerful laser beam is appealing to many practical applica-tions. Over the years, various beam combining approaches havebeen investigated [1], [2]. These can be divided into three maincategories: incoherent, active coherent, and passive coherentcombining approaches. With the incoherent approaches, fieldswith either random relative phase or different wavelengths arecombined. With the active coherent approaches, the fields areactively phase-locked by means of feedback loops and coher-ently combined. With the passive coherent approaches, the fieldsare self-phase-locked by means of a passive coupling mecha-nism and coherently combined. Significant progress has beenachieved in recent years with incoherent spectral beam combin-ing [3]–[5], with active phase locking of fiber amplifiers [6]–[8],and with passive coherent beam combining where the emphasishas been on interferometric combining [9]–[16]. Yet, scalabilityof the various approaches, both in terms of the number of com-bined lasers and the absolute total output power (brightness),

Manuscript received October 7, 2008; revised November 6, 2008 andNovember 20, 2008. First published January 27, 2009; current version pub-lished April 8, 2009.

A. A. Ishaaya is with the Department of Electrical and Computer Engineer-ing, Ben Gurion University of the Negev, Beer-Sheva 84105, Israel (e-mail:[email protected]).

N. Davidson and A. A. Friesem are with the Department of Physics of Com-plex Systems, Weizmann Institute of Science, Rehovot 76100, Israel (e-mail:[email protected]; [email protected]).

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/JSTQE.2008.2010409

has proved to be very challenging and so far showed limitedsuccess.

The passive interferometric coherent combining approachesare based on intracavity interference between laser channels,which promotes mutual coherence and phase locking amongthem [9]–[26]. An early configuration introduced in the 1960sinvolved the Vernier–Michelson resonator [17]. In this configu-ration, two intracavity Gaussian channels are phase-locked andcoherently combined by using discrete beam splitters. Duringthe 1980s, a similar configuration involving Y -branch arrayswas investigated with semiconductor diode laser arrays [22].Here, the beam splitter was replaced by single-mode Y junctionsthat depend on the phase of the incoming beams [18]–[21]. Withthe advent of double-clad fiber lasers, interferometric phaselocking and coherent addition of several fiber laser channelswere investigated using standard fiber couplers [9], [10], [24],[26], [27]. Here, efficient addition of up to eight intracavity laserchannels was demonstrated [27].

In this paper, we focus on passive interferometric laser beamcombining using compact, plane parallel, and intracavity com-biners. We have investigated this approach over the past decade,demonstrating efficient coherent combining of Gaussian laserbeams, transverse single high-order-mode laser beams, andeven transverse multimode laser beams. Furthermore, we suc-ceeded to upscale this approach experimentally achieving phase-locking and coherent combining of up to 25 laser channels. In thefollowing, we present the basic configurations, review our pastand recent results with both solid-state and fiber laser configura-tions, and discuss various aspects and the possible upscalabilityof this approach.

II. PLANE-PARALLEL INTERFEROMETRIC COMBINERS

A basic configuration for intracavity phase-locking and coher-ent addition of two Gaussian laser field distributions is schemati-cally presented in Fig. 1 [12]. The resonator is composed of a flatrear mirror, an output coupler that could be either flat or concavefor stable laser operation, two gain mediums, two apertures withdiameters suitable for independent fundamental TEM00 opera-tion in each of the two channels, and a planar interferometriccombiner. The use of interferometric combiners and commonend mirrors is of great advantage. Together, they alleviate thecomplexity of alignment and significantly improve the stability,thereby allowing for practical implementation.

The interferometric combiner is composed of a high-precisionplane-parallel plate, with specially designed coatings. For chan-nels with equal gain, half of the front surface is coated with an

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302 IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 15, NO. 2, MARCH/APRIL 2009

Fig. 1. Intracavity coherent addition of two laser channels using a singleplane-parallel interferometric combiner.

antireflection (AR) layer and the other half with a 50% beamsplitter layer, while half of the rear surface is coated with ahighly reflecting layer and the other half is coated with an ARlayer. In case of different gain in each channel, an appropriatebeam splitter transmission should be chosen. The beam at thelower channel is directly incident on the beam-splitter-coated re-gion, while the beam at the upper channel is transmitted throughthe AR-coated region, reflected back from the rear surface, andthen is incident on the beam-splitter-coated region. The thick-ness d of the interferometric combiner and its angle relative tothe beams are designed to match the distance between the twobeams, so they overlap optimally and propagate collinearly afterexiting the combiner.

In a simplified manner, the operation of the combined res-onator can be explained as follows. If the two Gaussian beamdistributions are incoherent (random relative phase between thebeams or different frequencies), then each beam will suffer a50% loss passing through the interferometric combiner; thus,typically, no lasing will occur. On the other hand, if the beamsadd coherently, then the losses introduced by the interferomet-ric combiner may be completely suppressed. Specifically, tothe right of the combiner, constructive interference occurs withlittle, if any, losses, while destructive interference occurs inthe lower path from the combiner, as shown in Fig. 1. Indeed,the combined laser will tend to operate so that the losses areminimum, whereby the phases of the individual beams will beautomatically matched (self-phase-locking) such that coherentaddition takes place.

In addition to the loss mechanism introduced by the combinerthat promotes phase-locking of the channels, the combiner alsostrongly couples light from one channel to the other. This isevident by examining the light path from the output couplertoward the rear mirror, where half of the field is diverted towardeach channel. The combination of these two mechanisms, lossand coupling, provide the basis for the robust phase locking thatis achieved in this configuration.

The aforementioned operation can be achieved only for thoselongitudinal modes (frequencies) that are common in the twolaser channels. The optical length of each resonator channeldetermines its frequency comb of longitudinal modes. Sincethe combiner introduces an optical length difference betweenthe two channels, there will be several common longitudi-nal modes (or bands) depending on the optical length differ-ence. The combiner and the optical length difference ∆L mustbe designed so as to obtain one or more common frequen-cies (longitudinal modes) within the gain bandwidth ∆ν. Note

Fig. 2. Configurations for intracavity coherent combining of 4 and 16 laserchannels using several interferometric combiners. (a) Four laser channels.(b) Sixteen laser channels [12], [14].

that a slight change in the combiner’s angle will result in achange of the common frequencies, and thus affect the out-put spectrum, but will not have a drastic effect on the beamcombining.

By using more such intracavity interferometric beam com-biners, it is possible to coherently combine more laser chan-nels [12], [14]. Fig. 2(a) and (b) shows possible configurationsfor combining 4 and 16 laser channels, respectively. To en-sure that common frequencies exist, the difference in the op-tical length between the individual resonator channels shouldbe accurately controlled. When coherently adding two chan-nels, the optical length difference ∆L need only be greaterthan ∼c/2∆ν to obtain stable operation [25]. This lower limitensures that the corresponding frequency spacing of the com-mon longitudinal modes is smaller than the gain bandwidth,and thus, at least one common mode is amplified. In general,with more resonator channels, each with a different opticallength L, L + ∆L1, L + ∆L2, etc., the probability for havingcommon frequencies within the gain bandwidth is drasticallydecreased.

In the case where the resonator channels end mirrors arecommon, the interferometric combiners introduce exact opti-cal length differences, integer number of ∆L (i.e., ∆L, 2∆L,3∆L, etc.) between the channels. For example, in the config-uration presented in Fig. 2(a) for coherently adding four laserchannels, the resonator length of one channel is L, two channelsL + ∆L, and the remaining channel L + 2∆L. Thus, resortingto length differences of n∆L ensures that common frequencybands equally exist in this configuration. In Fig. 2(b), two pairsof interferometric combiners are exploited for intracavity coher-ent addition of 16 Gaussian channels. The configuration consistsof a laser resonator with a large-diameter gain medium to enable

ISHAAYA et al.: PASSIVE LASER BEAM COMBINING WITH INTRACAVITY INTERFEROMETRIC COMBINERS 303

Fig. 3. Coherent combining with interferometric sequential combiners.(a) Sequential combiner for intracavity coherent addition of five laser chan-nels. (b) Resonator configuration for coherently combining 25 laser channelswith two orthogonally oriented sequential combiners [29].

16 separate channels, two common end mirrors, and an aperturethat selects only the Gaussian (TEM00 mode) distribution. Twopairs of interferometric combiners, similar to those previouslydescribed, are inserted into the resonator. To allow for properbeam displacement, the thickness of the combiners in the firstpair is twice that of the combiners in the second pair. The 16laser channels right after the gain medium are folded verticallyafter the first interferometric combiner to get 8 channels, whichare then folded horizontally to form 4 channels by the secondinterferometric combiner, and these 4 distributions are foldedby the next pair to form a single output beam of Gaussian dis-tribution. The special design of the interferometric combinersand the self-organization property of the laser ensure that 16separate Gaussian distributions are all phase-locked, and coher-ently add into one high-power Gaussian beam distribution atthe output. Each additional pair of interferometric combinerswould increase the number of beams in the array by a factorof 4. The optical length differences between all channels couldstill be n∆L, thereby ensuring the existence of common longi-tudinal modes. In general, the number of channels in the arraywill scale as 2N , where N is the number of interferometriccombiners (N = 2, 4, 6, . . .).

Somewhat more complex interferometric combiners can beused to combine more than two laser channels with a singleplanar element, as shown in Fig. 3 [28], [29]. The interfero-

metric combiner for coherently combining five laser channels isschematically shown in Fig. 3(a). It is formed of a high-precisionplane-parallel plate, with specially designed coated regions. Thefirst region in the front surface is coated with an AR layer, thesecond with a layer of 50% reflectivity (and 50% transmittiv-ity), the third with a layer of 66.6% reflectivity, the fourth witha layer of 75% reflectivity, and the fifth region with a layer of80% reflectivity. The rear surface is mainly coated with a highlyreflecting layer, except for a small region that is coated with anAR layer. When placed in a laser resonator, this interferometriccombiner will lead to sequential coherent addition of five laserchannels. Moreover, with such a single interferometric com-biner, the path length differences between the channels will bea multiple of ∆L, where ∆L is the path length difference be-tween two adjacent channels. This will ensure the existence ofcommon longitudinal modes even when combining more chan-nels sequentially. Note that practical issues such as deviationsfrom exact parallelism and variations of the optical lengths dueto the coatings in the interferometric combiners should be takeninto account in order to optimize combining efficiency. We per-formed some cold cavity calculations and found that variationsin the optical length differences of about 1 µm (easily obtainedwith standard dielectric coatings) will not significantly affectthe combining efficiency with a modest number of lasers.

With the use of sequential interferometric combiners, phase-locking and coherent addition of large 2-D arrays of laser chan-nels can be achieved in a compact manner with only two or-thogonally oriented combiners. For example, Fig. 3(b) shows aschematic configuration in which 25 laser channels are com-bined with two orthogonally oriented sequential combiners.These combiners are more complex and harder to fabricate thanthe two-beam combiners; however, the resonator configurationis more compact, alignment is easier, and internal losses arereduced.

III. EXPERIMENTAL RESULTS WITH SOLID-STATE LASERS

Experiments performed with pulsed Nd:YAG lasers con-firmed the ability to efficiently phase-lock and coherently com-bine a multiplicity of intracavity laser channels with plane-parallel interferometric combiners [12]–[14], [28]–[32]. In theseexperiments, 2, 4, 9, 16, and even 25 Gaussian distributions werephase-locked and combined, resulting in one high-power outputbeam with good beam quality (equal or better than that of theequivalent single-channel resonator).

Experimental results of coherently combining two Nd:YAGlaser channels in a configuration similar to that of Fig. 1 areshown in Fig. 4 [12]. The resonator was 70 cm long and includeda common Nd:YAG gain medium with two 1.6-mm-diameterapertures for the two intracavity apertures. Experiments wereperformed both in free running and Q-switched operation andresulted in 18.0 mJ of combined energy per pulse comparedto 9.8 mJ for single-channel operation, representing 92% com-bining efficiency. Fig. 4(a)–(d) shows the near- and far-fieldintensity distributions for each of the two channels when op-erated independently. The calculated M 2 for these individualbeams was M 2

x = 1.15 and M 2y = 1.20 for the first channel,


Fig. 4. Experimental intensity distributions of the separate independentGaussian beams, and the combined output beam obtained using a single two-beam combiner configuration. (a) and (b) Near- and far-field intensity distribu-tions of the first channel output beam. (c) and (d) Near- and far-field intensitydistributions of the second channel output beam. (e) and (f) Near- and far-fieldintensity distributions of the combined channel output beam [12].

and M 2x = 1.14 and M 2

y = 1:21 for the second channel, indi-cating nearly pure Gaussian TEM00 distributions. The near- andfar-field intensity distributions of the combined beam are shownin Fig. 4(e) and (f). The calculated M 2values for the combinedoutput beam was M 2

x = 1.12 and M 2y = 1.18, indicating that

the original, nearly Gaussian distribution was preserved.Experimental results with the configuration shown in

Fig. 2(b), where 16 channels are combined with four two-beamcombiners, are shown in Fig. 5 [14]. Fig. 5(a) shows the 16 sep-arate laser distributions escaping from the rear mirror. Fig. 5(b)and (c) shows the near- and far-field intensity distributions ofthe output beam, respectively. As evident, the combined inten-sity distribution has the modal structure of the single TEM00laser distribution. The output energy of a single channel oper-ated independently was 2.6 mJ per pulse, while that of the 16coherently added laser channels was 37 mJ per pulse, corre-sponding to 88% combining efficiency. The beam quality of thecombined beam was calculated from the measured output to beM 2 = 1.3, practically the same as the single-channel beam qual-ity. Finally, experimental results with the configuration shown inFig. 3(b), where a 5 × 5 solid-state laser channel array was com-bined with two orthogonal sequential combiners, are shown inFig. 6 [29]. Fig. 6(a) shows the far-field intensity distribution ofthe combined output, and Fig. 6(b) shows the near-field intensitydistribution of residual light leaking through the rear mirror.

In all the aforementioned experiments, spectral frequencybands within the gain bandwidth were observed in the com-

Fig. 5. Experimental results for coherent addition of 16 laser distributions.(a) Detected intensity distribution of light escaping from the rear mirror.(b) Detected near-field intensity distribution of output. (c) Detected far-fieldintensity distribution of output [14].

Fig. 6. Experimental coherent addition of 25 laser distributions. (a) Far-fieldintensity distribution of the combined output. (b) Near-field intensity distribu-tions of the residual light leaking through the rear mirror [29].

bined output beam. As expected, the difference between theband peaks corresponded to the difference in optical lengthof the channels. With a slight tilt of the combiner, the outputspectrum could be controlled without significantly affecting thecombining efficiency.

The effect of thermal lensing in the laser rods was also in-vestigated experimentally [33]. Results with four channels in acommon laser rod revealed that as the pulse repetition increases,i.e., increased thermal lensing, there is a degradation of the com-bining efficiency. However, by adding a proper compensatinglens inside the laser cavity close to the laser rod, it was possibleto almost completely suppress the deleterious thermal lensingeffects. Thus, in these solid-state configurations, efficient coher-ent combining can be achieved for a specific work point (pumplevel). Changing the work point would require change in theresonator optics.

Coherent combining of laser beam distributions with intra-cavity interferometric combiners need not be limited only toGaussian beam distributions. Efficient intracavity coherent


Fig. 7. Experimental intensity distributions of two separate transverse multi-mode channels and the combined laser output beam distribution [13].

addition of two high-order (TEM01 or TEM02) degenerateLaguerre–Gaussian modes was demonstrated with a pulsedNd:YAG setup using a similar configuration to that shown inFig. 1 [32]. Single high-order-mode operation in each channelwas obtained by inserting a suitable binary phase element eitheralong the combined channel, or along one of the separate chan-nels where the desired high-order-mode operation is imposedon the other channel [30]. Combining efficiency was more than95%, with considerably more output energy than for an outputwith Gaussian distribution.

Since multimode laser light has typically several transverseand longitudinal modes with random relative phase, the con-cept of efficient coherent combining of transverse multimodelaser beam distributions does not seem plausible at first glance.Indeed, it is not feasible with independent multimode beamsoriginating from independent lasers. However, with intracavityinterferometric combiners, coherent combining of multimodefield distributions becomes possible. The interferometric lossmechanism can cause simultaneous self-phase-locking of allthe corresponding transverse modes in the channels, enablingthe coherent combining of the incoherent beam distributions.

A basic configuration for intracavity coherent combining oftwo transverse multimode field distributions is identical to thatof combining two Gaussian beams (see Fig. 1), except that theapertures are chosen such as to obtain a multimode distribu-tion in each of the two channels [13]. If the two multimodebeam distributions have similar mode composition, and if eachof the transverse modes in one distribution adds coherently withits counterpart in the other beam, then destructive interferenceoccurs, and the losses introduced by the combiner may be com-pletely suppressed. The combined laser configuration tends tooperate so that the losses are minimal, whereby the phases ofthe corresponding individual transverse modes automaticallymatch and coherent combining takes place. The combined mul-timode beam is thus composed of many pairs of phase-lockedmodes, where the phase difference between the pairs is stillcompletely undefined. Fig. 7 shows representative experimen-tal near- and far-field intensity distributions of each of the twochannels (when operated independently) and the combined laser

output. The M 2 of each of the two laser channels, when oper-ated separately, was 4.5, corresponding to roughly 13 transversemodes. The combining efficiency was 91%, and the combinedoutput beam quality was not only preserved, but also slightlyimproved.

If the aperture diameter in one channel is reduced, then thischannel with the lower transverse mode content will imprint itsmodal content on the other channel, obtaining phase-lockingand coherent combining of the corresponding modes in the twodistributions [30]. This self-imprinting of the modal content oc-curs because the higher transverse modes in the channel with thelarger aperture diameter do not have corresponding counterpartsin the other channel, so they suffer considerable losses by theinterferometric combiner. The ability to combine many multi-mode beam distributions can serve as another degree of freedomwhen designing lasers for applications where the required beamquality need not be strictly M 2 = 1. In these cases, combiningmultimode channels instead of Gaussian channels can reduceconsiderably the number of channels to be added, greatly sim-plifying the coherent addition configuration.

The passive interferometric beam combining approach canalso be exploited to increase the optical brightness of multimodelaser resonators [31]. Here, the intracavity multimode beam issplit into a tightly packed array of high-beam-quality laser chan-nels that are coherently combined within the resonator to forma single high-power, high-quality, output beam. The phase lock-ing and coherent combining are achieved with intracavity planarinterferometric beam combiners. As the individual channels aremore tightly “packed,” i.e., with high fill factor, the brightnessincreases considerably.

Experiments were performed with a configuration similar tothat shown in Fig. 2(b). In order to achieve a high fill factor fora 4 × 4 array of Gaussian distributions, the configuration wasmodified by replacing the single small round aperture with alarge square aperture, by using a concave output coupler withappropriate radius, and by adjusting the angular orientationsof the interferometric combiners. Representative experimentalresults are shown in Fig. 8. Fig. 8(a) schematically shows thesingle large square aperture that can support the tightly packedarray. Operating the laser with the single square aperture butno interferometric combiners resulted in an output beam with asquare multimode distribution shown in Fig. 8(b). Inserting thefour interferometric beam combiners generated a 4 × 4 arrayof tightly packed Gaussian distributions that were phase-lockedand coherently combined to obtain a single Gaussian outputbeam, as shown in Fig. 8(c). The multimode output energy,measured with the large square aperture and no interferometriccombiners, was 147 mJ with a beam quality factor of M 2 = 15.The output energy of the combined beam after insertion of theinterferometric combiners was 33 mJ with a beam quality factorof 1.3 (the reduced output energy is mainly due to the fill factor).Accordingly, the measured increase in brightness was about 30.

IV. EXPERIMENTAL RESULTS WITH FIBER LASERS

The intracavity interferometric combining approach de-scribed in the preceding sections was also successfully applied


Fig. 8. Increasing the brightness of multimode laser resonators. (a) Squareintracavity aperture for generating a tightly packed 4 × 4 array. (b) Experimen-tal far-field intensity distribution without the intracavity interferometric beamcombiners. (c) Experimental far-field intensity distribution when coherentlycombining a tightly packed 4 × 4 array of Gaussian distributions with fourintracavity interferometric combiners.

with fiber lasers. Compared to other leading fiber combiningapproaches, this approach has several attractive features. Thefirst feature is coherence, i.e., the fields are added coherently re-sulting in high output power without an increase in the spectralbandwidth. The second feature is the self-phase-locking mecha-nism, which eliminates the need for active control and complexfeedback loops. The third feature is that there is one combinedoutput beam in free space rather than in the output fiber core,thereby leading to higher thresholds for damage and deleteriousnonlinear effects that limit the output powers of fiber lasers.

A basic configuration for adding two fiber lasers is similarto that shown in Fig. 1, whereby the solid-state laser rods arereplaced by active fibers. There are several key differences be-tween fiber and solid-state laser combining configurations. Thegain bandwidth of a fiber laser is typically much broader thanthat of a solid-state laser, and the typical resonator length issignificantly longer (several meters compared to several tens ofcentimeters). This translates into a significantly larger number ofclosely spaced longitudinal modes. In addition, the single-passgain in a fiber laser is considerably higher than that of a typi-cal solid-state laser, leading to relatively high output couplingand low cavity finesse. These differences increase the proba-bility for having common longitudinal modes in the fiber laserconfiguration, which is a strict requirement for interferometricbeam combining. On the other hand, unlike in the solid-stateconfiguration, it is very hard, if not impossible, to control theoptical length of the intracavity channels in the fiber configu-ration. This means that when combining several channels, evenif plane-parallel combiners are used, the optical length differ-ences between the channels will not be an integer number of∆L. Thus, in the case of fiber lasers, the prospects for scalingthe interferometric beam combining approach to a large numberof laser channels are uncertain (see Section V).

Fig. 9. Coherent combining of four fiber lasers. (a) Experimental setup.(b) Experimental intensity distributions without intracavity combiners, withone combiner, and with two combiners [34].

Efficient coherent combining of two and four fiber laser chan-nels was recently demonstrated with continuous-wave (CW)Er-doped fiber lasers [34]. The experimental arrangement forcombining four fiber laser channels and results are shown inFig. 9. The experimental arrangement shown schematically inFig. 9(a) includes four fiber lasers and two, orthogonally posi-tioned, intracavity interferometric combiners. Each fiber laserchannel consists of 7 m polarization maintaining Er-doped fiber,where one end is attached to a high-reflection fiber Bragg grating(FBG) (5 nm spectral bandwidth) and the other end is splicedto a collimating-graded index lens with an AR layer to suppressany reflections back into the fiber, and a flat output couplerof 20% reflection is common for all laser channels. Pumpingis achieved with 5-W, 911-nm diode lasers that are spliced tothe fiber Bragg gratings. Fig. 9(b) shows the detected outputintensity distributions without the intracavity combiners, withone combiner, and two combiners. An output power of 35 mWwas obtained for single-channel operation, and 121 mW forfour combined channels operation, indicating 86% combiningefficiency, while maintaining the single-channel beam quality.

Coherent combining of two fiber lasers, each operating witha single high-order transverse mode, was also investigated [35].The experimental configuration for adding two such fiber lasersis schematically shown in Fig. 10. The two lasers were com-posed of a 12-m- and 15-m-long Yb-doped, double-clad, non-polarization maintaining fibers (22 µm core diameter, numericalaperture of 0.07), gradient index (GRIN) lenses, high-reflectingrear mirrors of R > 99.8% at 1064 ± 1 nm, and an output


Fig. 10. Configuration for coherent addition of two fiber lasers each operatingwith a high-order mode [35].

Fig. 11. Coherent addition of two fiber lasers each operating with LP11 mode.(a) Intensity distribution of one laser. (b) Intensity distribution from the otherlaser. (c) Intensity distribution of the combined output [35].

coupler with 10% reflectivity. The fibers ends were cleaved atan 8◦ angle to avoid reflections, and pumped with 915 nm diodelasers. Intracavity polarizers were used to control and fix the po-larization and orientation of the transverse mode shape emergingfrom each fiber laser. Fig. 11 shows the experimental intensitydistributions when combining two fiber lasers operating withLP11 mode. Fig. 11(a) shows the intensity distribution of lightemerging from one laser, and Fig. 11(b) shows the distributionemerging from the other laser. Fig. 11(c) shows the intensitydistribution of the combined output. As expected, the combinedoutput distribution closely resembles that obtained for each fiberlaser independently. The combining efficiency was measured tobe over 90%.

Intracavity coherent combining of fiber lasers was furtherextended by simultaneously performing coherent and spectralbeam combining of a 2-D fiber laser array [15]. In one dimen-sion, coherent combining was achieved by means of an interfer-ometric combiner in free space, while in the other dimension,spectral combining was achieved by means of a diffraction grat-ing [5]. This approach of simultaneous coherent and spectralcombining opens an alternative route to upscaling combiningto large arrays of lasers, possibly overcoming the limitation ofeach individual approach. The experimental configuration forthe simultaneous coherent and spectral combining is presentedschematically in Fig. 12. It contains four end-pumped single-mode Er-doped CW fibers arranged in a 2 × 2 square arraywith a distance of 3.5 mm between adjacent fibers. Each fiber is

Fig. 12. Basic configurations for simultaneously performing coherent andspectral combining of a 2 × 2 fiber laser array. Coherent combining is performedin the horizontal direction by means of an interferometric combiner, and spectralcombining is performed in the vertical direction by means of a linear diffractiongrating. Insets show the four beams before the combiner, the two beams afterthe combiner, and the single combined output beam after the grating [15].

about 7 m long, and has a 10-µm core diameter with a numericalaperture of 0.19. One end of each fiber is attached to an FBGthat serves as the rear mirror. The other fiber end is connectedto a collimator coated with AR layers to suppress any reflec-tions back into the fiber. The four parallel beams (parallelismbetter than 0.1 mrad) emerging from the fiber collimators arefirst coherently added in the horizontal direction by means ofan intracavity interferometric combiner to obtain two parallelbeams. These beams are then deflected in the vertical directionby means of a 500-mm-focal-length lens, so they exactly over-lap on the surface of a 1200-lines/mm blazed diffraction gratingand diffracted toward a collimating lens and a common flat out-put coupler of 20% reflectivity. The wavelength of each beam isself-selected to ensure normal incidence on the output coupler,and consequently, exact self-reflection back into the fibers toobtain lasing [4]. Thus, the two beams are spectrally added intoa single output beam, maintaining the high beam quality of eachof the individual lasers.

The M 2 of the combined output beam was measured to be1.15, essentially identical to that of each individual fiber laser.The overall combining efficiency, after both coherent and spec-tral combining, was measured to be 82%. The reduction inefficiency for this experiment is attributed mainly to losses toundesired diffraction orders from the blazed grating and smallmisalignment between the fiber lasers channels. The spectra ofthe fiber lasers and the combined output were detected and mea-sured by means of a grating spectrometer. The results are pre-sented in Fig. 13. Fig. 13(a) shows the expected single spectrumwhen only the lasers in the upper pair are coherently added.Fig. 13(b) shows the corresponding spectrum when only thelasers in the lower pair are coherently combined. As evident,each pair of lasers operates at a different wavelength, with aspectral separation of 1.3 nm. Fig. 13(c) shows the spectrum ofthe combined output beam. As evident, there is a simple addi-tion of the individual spectrum of each pair of lasers, indicatingthat there is no interaction between them. Accordingly, bothcoherent combining and spectral combining simultaneously oc-cur. Measurement of the longitudinal mode separation withinthe spectrum of each coherently combined pair confirmed that


Fig. 13. Spectra of the fiber lasers and combined output when simultaneouslyperforming coherent and spectral combining of four fiber lasers with a 1200-lines/mm intracavity grating. (a) Spectrum when only the lasers in the upper pairare coherently combined. (b) Spectrum when only the lasers in the lower pairare coherently combined. (c) Spectrum of the spectrally combined output [15].

Fig. 14. Configurations for measuring the level of phase-locking (fringe visi-bility) of two fiber lasers as a function of coupling strength. In one configuration,the fiber ends are cleaved at 8◦ to yield a configuration without individual feed-back, and in the other, the fiber ends are cleaved at 0◦ to yield a configurationwith individual feedback. BS denotes 50% beam splitter [16].

there were enough common longitudinal modes to achieve thecoherent combining.

An interesting characteristic of the intracavity interferometricbeam combining approach was recently demonstrated with fiberlasers. It was shown that with this approach, efficient phaselocking and coherent combining can be achieved even at verylow coupling levels between intracavity laser channels, muchlower than expected from other systems of coupled oscillators[16]. The phase locking dynamics and its dependence on thecoupling level were investigated for two different fiber laserconfigurations.

The two different fiber laser configurations are shownschematically in Fig. 14. They include two fiber laser chan-nels, a 50% beam splitter, and an output coupler. In the firstconfiguration, the two fiber ends are cleaved at 0◦ yielding twoindependent lasers with individual feedback, whereby exter-nal coupling is achieved with the beam splitter and the outputcoupler. Here, even if the output coupler is removed (i.e., nocoupling), the two lasers will still lase (incoherently) and lose50% on the beam splitter. In the second configuration, without

Fig. 15. Experimental and calculated fringe visibility as a function of cou-pling strength. Crosses denote experimental results for the configuration with-out individual feedback. Dots denote experimental results for the configurationwith individual feedback. Solid curves denote corresponding numerically cal-culated results, and dashed curves denote corresponding analytically calculatedresults [16].

individual feedback, the fiber ends are cleaved at 8◦. Here, ex-ternal feedback is provided only by the common output coupler.The beam splitter that introduces a phase-dependent loss is nowinside the common laser cavity. This is similar to the configura-tions discussed previously [see Figs. 9(a) and 10]. The couplingstrength, defined as the fraction of output intensity of each laserthat returns into the other laser cavity, was controlled by varyingthe amount of light reflected from the output coupler back intothe fibers. This was achieved by inserting a focusing lens infront of the output coupler whose location could be varied.

In the experiments, two polarization maintaining Yb-dopedfibers, with a core diameter of 15 and 125 µm inner clad, wereused. Each fiber was about 10 m long, attached at one end to anFBG (R > 99.8% at 1064 ± 1 nm, high transmission at 915 nm),and pumped with 915 nm diode lasers. The reflectivity of theoutput coupler was 4%. To determine the degree of phase lock-ing between the lasers, parts of the light from each laser weredeflected and interfered in order to determine the fringe visi-bility. The results of fringe visibility as a function of couplingstrength for the configurations with and without individual feed-back are presented in Fig. 15. As clearly evident, a high fringevisibility level occurs at a significantly lower coupling strengthfor the configuration without individual feedback. Specifically,the transition from no phase locking to almost complete phaselocking for the configuration without individual feedback oc-curs over a range of 0.001 coupling strengths, whereas for theconfiguration with individual feedback, the range is 0.01. Thisdramatic difference is also seen in the inset photographs of fringepattern taken at the same coupling strength of 0.001 for the twoconfigurations. Numerical and analytical modeling of these con-figurations was in good agreement with the experimental results(see solid and dashed curves in Fig. 15) [16]. It should be notedthat the combined output power at very low coupling levelsis higher for the configuration with individual feedback [36].The ability to exploit the intracavity interferometric combiningapproach with very low output coupler reflectivity can prevent


deleterious feedback damage, and thus be advantageous in high-power fiber laser systems.

Finally, phase locking dynamics of coupled fiber lasers nearthreshold was investigated [37]. It was shown both experimen-tally and with a relatively simple theoretical model that the de-gree of phase locking between two fiber lasers, when operatednear threshold, depends not only on the coupling strength butalso on the total output power. The results indicate by how muchthe coupling strength must be increased when quantum noise isdominant. These results are of basic aspect and significant onlywhen the lasers operate near threshold.

V. SCALABILITY AND FUTURE PROSPECTS

Scalability, in the context of laser beam combining, can berelated to two main parameters: the number of efficiently com-bined laser beams and the absolute value of output power. Thedesired values for these parameters strongly depend on the ap-plication. For example, typical high-power laser systems forcutting and welding applications have CW output powers of5–10 kW. If a single fiber laser can deliver 500 W CW,then combining 10–20 such fiber lasers may be adequate. Onthe other hand, with laser diode bars and stacks, the poweris relatively low but the number of sources to combine ismuch larger. Thus, from a practical point of view, scalabil-ity should be discussed along with the relevant laser type andapplication.

We have shown experimentally that the number of efficientlycombined laser beams with the intracavity interferometric beamcombining approach, when applied to pulsed solid-state lasersin the tens of millijoules regime, can reach 16 or more beams.Further extending this approach to larger arrays of solid-statelaser channels is yet to be investigated. With very large laser ar-rays, the mode discrimination between the desired phase-lockedglobal eigenstate and other possible global eigenstates is ex-pected to be lower so that small perturbations could lead toinstabilities and poor operation.

With fiber lasers, the situation is somewhat different. Asmentioned earlier, the optical length of fiber lasers is harderto control, so the probability of finding a common longitudi-nal mode (frequency) decreases drastically with the number ofcombined lasers. Indeed, it was recently claimed that if one as-sumes fibers with random optical lengths, the increase in bright-ness, with these and similar interferometric fiber combiningconfigurations, is limited to about a factor of 10 (i.e., a limit ofabout ten efficiently combined lasers) [38]–[40]. Nevertheless,even though we expect that the passive interferometric com-bining approach would be limited in terms of the number ofcombined fiber lasers, we believe that the limit may be higherthan ten combined lasers, for example, by resorting to simul-taneous coherent and spectral combining, or coherent and po-larization combining, or even to new configurations in whichglobal coupling is not used.

Regarding absolute output power, the interferometric beamcombining approach has not yet been demonstrated with highpowers. With pulsed solid-state Nd:YAG configurations inwhich intracavity plane-parallel beam combiners were incorpo-

rated, the maximum output energy when combining 16 channelswas tens of millijoules [14]. With CW fiber lasers, powers lessthan 500 W were experimentally achieved [5]. Furthermore, sev-eral investigations of coherent beam combining of high-powerfiber lasers indicated that nonlinear and thermal effects mayhave a detrimental influence on phase locking [41]. Currently,it seems that additional investigations and development are re-quired in order to scale coherent combining of fiber lasers whenconsidering several kilowatts for the combined output.

The use of newly developed multicore fibers (standard stepindex or photonic crystal fibers) might enable further upscalingcoherent combining of fiber lasers. The more controlled opticallengths with multicore fibers may allow for common longitu-dinal modes with a larger number of laser channels. Here, anadditional difficulty is the coherent combining of the beamsemerging from the closely spaced fiber cores.

The future practical implementation of the intracavity inter-ferometric beam combining approach will depend not only onfuture progress in scalability of this approach (number of com-bined lasers or absolute power), but also on the progress madewith other competing approaches such as spectral or active beamcombining. In applications where narrow spectral bandwidthof the combined beam is crucial, the passive interferometricapproach might have an advantage over spectral combining.In pulsed applications, where the time for active phase stabi-lization is limited, the passive interferometric approach mighthave an advantage over the active beam combining approach. Insome solid-state applications where the required output power ismoderate, but system compactness is of crucial importance, theself-phase-locking and compact configurations with the planarinterferometric combiners could be highly beneficial. Finally, inaddition to their practical aspects, the intracavity interferomet-ric beam combining configurations may serve as experimentalplatforms for investigating more fundamental topics such as co-herence formation and collective phenomena in large ensemblesof coupled oscillators [42]–[47].

ACKNOWLEDGMENT

The authors would like to thank L. Shimshi, V. Eckhouse, andM. Fridman for their contributions.

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Amiel A. Ishaaya received the B.Sc. and M.Sc. de-grees in physics from Tel Aviv University, Tel Aviv,Israel, in 1987 and 1995, respectively, and the Ph.D.degree in physics from Weizmann Institute of Sci-ence, Rehovot, Israel, in 2005.

From 1994 to 2001, he was with ELOP Ltd., Re-hovot, where he was engaged in various laser devel-opment projects. From 2001 to 2005, he was engagedin research on high-order transverse mode selectionand coherent beam combining in various laser config-urations. From 2005 to 2007, he was a Postdoctoral

Research Associate at Cornell University, Ithaca, NY, where he was engaged instudying collapse dynamics of high-intensity optical beams and nonlinear inter-actions in photonic crystal fibers. Since 2007, he has been with the Departmentof Electrical and Computer Engineering, Ben Gurion University of the Negev,Beer-Sheva, Israel. His current research interests include lasers and nonlinearoptical devices based on photonic crystal fibers and silicon waveguides. He hasauthored or coauthored more than 50 journal and conference publications, 20technical reports, and holds a U.S. patent application, all in the field of opticsand lasers.

Dr. Ishaaya is a member of the Optical Society of America (OSA) and theInternational Society for Optical Engineers (SPIE).


Nir Davidson received the B.Sc. degree in physicsand mathematics from Hebrew University, Jerusalem,Israel, in 1983, the M.Sc. degree in physics from theTechnion, Haifa, Israel, in 1988, and the Ph.D. de-gree in physics from Weizmann Institute of Science,Rehovot, Israel, in 1993.

He was a Postdoctoral Fellow at Stanford Univer-sity. He is currently the Peter and Carola KleemanProfessor of Optical Sciences in the Department ofPhysics of Complex Systems, Weizmann Institute ofScience. His current research interests include the

areas of laser cooling and trapping of atoms, quantum chaos, quantum optics,Bose–Einstein condensation, laser physics, and physical optics. He has authoredor coauthored more than 130 journal publications.

Prof. Davidson was the recipient of the Allon Award, the Yosefa and LeonidAlshwang Prize for Physics, and the Bessel Award from the Alexander vonHumboldt Foundation. He is the President of the Israeli Laser and Electro-Optics Society.

Asher A. Friesem (S’57–M’62–SM’79–F’95–LF’05) received the B.Sc. and Ph.D. degrees fromthe University of Michigan, Ann Arbor, in 1958 and1968, respectively.

From 1958 to 1963, he was with Bell AeroSystems Company and Bendix Research Labora-tories. From 1963 to 1969, he was with the In-stitute of Science and Technology, University ofMichigan, where he was engaged in investigationsof coherent optics, mainly in the areas of opti-cal data processing and holography. From 1969 to

1973, he was a Principal Research Scientist in the Electro-Optics Center,Harris, Inc., where he was involved in the areas of optical memories and dis-plays. In 1973, he joined Weizmann Institute of Science, Rehovot, Israel, wherehe has been a Professor of optical sciences since 1977, and was subsequentlythe Department Head, the Chairman of the Scientific Council, and the Chairmanof the Professorial Council. He has also been a Visiting Professor in Germany,Switzerland, France, and USA. His current research interests include new holo-graphic concepts and applications, optical image processing, electrooptic de-vices, and new laser resonator configurations. He has authored or coauthoredmore than 250 scientific papers. He is a Co-Editor of four scientific volumes,and a holder of 30 international patents.

Prof. Friesem was the Vice President of the International Commission ofOptics (ICO) and the Chairman of the Israel Laser and Electro-Optics Society.He is a Fellow of the Optical Society of America (OSA), and a member of theInternational Society for Optical Engineers (SPIE) and the Sigma Xi. He hasbeen a member of numerous program and advisory committees of national andinternational conferences.

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Passive Laser Beam Combining With Intracavity Interferometric Combiners