Beam Quality Improvement in Broad-Area Semiconductor Lasers Via Evanescent Spatial Filtering Jordan P

IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. 48, NO. 10, OCTOBER 2012 1269 Beam Quality Improvement in Broad-Area Semiconductor Lasers via Evanescent Spatial Filtering Jordan P. Leidner and John R. Marciante, Member, IEEE

Abstract— In semiconductor lasers, the unpumped cladding to the angled grating as power increased, which limited region outside the optical waveguide is nominally lossy. Since performance to ∼1.5 W [6]. In the end, the observed power the evanescent tail of higher-order waveguide modes extends limitations of these devices limits them to a small niche further into the waveguide cladding, the modal loss increases for high spatial frequency perturbations, creating a weak spatial of applications that need only slightly more power than a filter. Increasing the effective laser cavity length in a broad-area single-mode diode laser can provide. laser enhances the effect of this spatial filter, resulting in The most widespread use of BALs is in pumping improved beam quality. Simulations predict that by optimizing high-power dual-clad fiber lasers, which do not require the lateral index step and output coupler reflectivity, a factor of diffraction-limited beam quality from the BALs. A dual-clad two increases the beam brightness with only a 10% penalty in electrical-to-optical efficiency. fiber (DCF) has three optical layers in the radial direction. The core is nominally single-mode for the desired lasing Index Terms— Brightness, broad-area pump lasers, diode wavelength and doped as a gain medium for use as a fiber lasers, optical filters, semiconductor lasers. laser or amplifier, while the pump light is confined to the inner cladding region of the fiber by the outer-cladding [7]. I. INTRODUCTION The light of the pump laser, now in the multi-mode region INGLE-SPATIAL-MODE diode lasers are limited in of the DCF, is required to be absorbed by the gain medium Smaximum power to about 1 W due to catastrophic optical to be useful to the intended fiber laser or amplifier. However, damage at the output facet [1]. For applications requiring since the gain cross-sectional area is much smaller than the higher power, the damage problem can be avoided by using pumped area, the absorption of the pump light occurs over a diode laser that is broadened in the lateral dimension longer distances compared to the lower-power core-pumped (perpendicular to the growth direction of the semiconductor), configuration. Higher brightness BALs would allow smaller thus reducing the intensity at the output. However, such a inner claddings for shorter absorption lengths or higher powers broad-area laser (BAL) is no longer a single-mode waveguide via higher pumping density. The motivation for this work is and is highly susceptible to free-carrier-induced self-focusing, therefore to improve but not perfect the beam quality of BALs which causes chaotic beam filamentation [2]. This filamen- using a simple technique that does not require new equipment tation results in a massive degradation in beam quality that or fabrication processes to implement. causes difficulties when trying to couple the beam into optical In this work, we numerically explore the enhancement fiber or in any other applications that require tight focusing. of evanescent spatial filtering via waveguide and cavity The last three decades have been witness to a significant design. Specifically, BAL brightness enhancement is studied amount of research dedicated to improving the beam quality of by exploiting the loss resulting from the modal evanescent tails BALs. Tapered gain regions, unstable resonator configurations, propagating in the lossy cladding as a function of the index and external cavity diode lasers have achieved near diffraction step that defines the optical waveguide and the reflectivity limited powers up to ∼2 W with varying degrees of com- of the output facet. In Section II, losses in the evanescent mercial viability [3]Ð[5]. All of these designs are based on a field are compared at totally internally reflecting (TIR) angles. single lumped spatial filter and therefore eventually fail when In Section III, numerical simulations are used to study the filamentation occurs within one round trip of the laser cavity. effects of these losses. In Section IV, practical issues involved An alternative approach called the angled distributed feedback with the beam quality improvement technique are presented. laser implemented continuous filtering which enabled scaling Section V contains a discussion of the results and concluding to higher powers, but the design began lasing perpendicular remarks.

Manuscript received April 3, 2012; revised May 31, 2012; accepted July 1, 2012. Date of publication July 10, 2012; date of current version July 31, 2012. II. SPATIAL FILTERING VIA EVANESCENT FIELD LOSS The authors are with the Institute of Optics, University of Rochester, Rochester, NY 14627 USA (e-mail: [email protected]; marciante@ Filamentation in BALs creates an optical field profile within optics.rochester.edu). the laser cavity that contains high spatial frequencies. The Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. desirable fundamental mode of the BAL waveguide, which Digital Object Identifier 10.1109/JQE.2012.2207881 allows tight focusing and coupling into single-mode-fiber, 0018Ð9197/$31.00 © 2012 IEEE 1270 IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. 48, NO. 10, OCTOBER 2012 consists of low spatial frequencies. Therefore, by designing Fundamental mode Typical filamentation range a laser cavity with increased loss at high spatial frequencies, it 1 is reasonable to suggest that the beam quality can be improved. In a laser cavity, fine lateral spatial structure is indicative l of coherent light traveling at large angles with respect to the a Incre nominal propagation axis, i.e. high spatial frequencies. It is ment 10 well known that light impinging on a dielectric interface at da as

an angle exceeding the critical angle “samples” the medium in eff, fun eff, g α / on the other side of the waveguide even though no net Lo 100 eff ss power propagates beyond the interface due to total internal α reflection [8]. In waveguide terminology, this is the evanescent tail of the mode that exists outside the boundary of the waveguide but propagates parallel to its surface. In the context 1000 of filamentation, high spatial frequencies propagating in the 90.0 89.7 89.4 89.1 88.8 88.5 laser cavity will “sample” more of the material outside the Angle of incidence (degrees) waveguide than lower spatial frequencies. Since this region is un-pumped and therefore lossy in the semiconductor materials Fig. 1. Relative effective absorption as a function of sidewall incident angle with respect to the interface. The cavity is assumed to be 100-μm wide with of interest, this evanescent loss acts as a weak spatial filter a refractive index step of 0.001 and a corresponding TIR angle of 88.6¡. The against high spatial frequencies. incident angles for the fundamental mode and typical filamentation patterns From the ray perspective, the Goos–Hänchen shift describes are also shown in the plot. the apparent axial dislocation of a beam upon total internally reflection (TIR) from the interface as [8]: we refer to this difference in effective modal loss as evanescent θ = 2 tan c spatial filtering. zGH / (1) 2 2 1 2 k1 sin θ − sin θc Since this filtering mechanism provides loss while the beam is propagating, extending propagation in the cavity naturally θ where zGH is the shift down the wall of the interface, c leads to a higher level of spatial filtering. The effective exten- is the TIR critical angle as measured from the perpendicular sion of propagation in the cavity can easily be accomplished θ with the interface, is the angle of incidence of the beam of by increasing the output coupling reflectivity. Increasing the = π λ λ interest, and k1 2 n1/ where is the optical wavelength cavity lifetime and thus increasing the filtering distance allows and n1 is the refractive index of the waveguide core. We use for additional attenuation of high spatial frequencies and thus this distance zGH to describe the interaction length of the better beam quality of the laser output. Moreover, the index optical field with the absorbing cladding material upon each step at the waveguide interface provides an additional degree reflection. The ratio of 2 zGH to the total axial distance of freedom through the TIR angle in Eqn. (1). traveled in a single lateral round trip reflection creates an effective absorption for spatial frequencies corresponding to III. SIMULATIONS OF EVANESCENT SPATIAL FILTERING a range of angles of reflection. The axial distance traveled per sidewall reflection depends geometrically on the angle of This concept was explored numerically using a Fox-Li incidence and the width of the waveguide w. This relation Beam Propagation Method (BPM) simulation of a BAL can be used with zGH and the absorption of the un-pumped cavity [2]. The parameters used in the BPM, shown in material beyond the interface to create an effective absorption Table 1, were chosen to mimic typical high-efficiency due to lossy material outside the waveguide: commercial BALs [9], with resulting simulated performance (power, efficiency, threshold, beam quality) that closely match z α = α GH . their commercial products [10]. Once these parameters were ef f w θ + (2) tan zGH matched, simulations were performed on a typical 2 mm-long Fig. 1 shows effective absorption for TIR light in a laser with a 100-μm lateral waveguide stripe. A high- 100-μm-wide cavity as a function of sidewall incident angle. reflectivity coating (95%) was used at the rear facet, while the Since the modes of the cavity have different amounts of front (output) facet reflectivity was varied to study the effect power beyond the waveguide, this picture is not complete. of effective cavity length on the output beam characteristics. However, this figure does illustrate the vast relative difference The first evidence of spatial filtering comes from examining in absorption between the fundamental mode containing the the field inside the laser cavity. Once transients were damped waveguide’s low spatial frequencies and that of higher spatial out in the simulation, the forward propagating half of the frequencies characteristic of filamentation; a filamenting beam intra-cavity field was captured and Fourier transformed to will experience 10Ð30x more loss than the fundamental mode. reveal the spatial frequency content of the light re-circulating Including a weighting factor for the modal power in the in the cavity. Fig. 2 displays the Fourier amplitude of these cladding will only exacerbate this picture, as the higher- fields for various output facet reflectivities. As the output order modes have increasingly more power propagating in the facet reflectivity increases, the width of the spatial frequency cladding. Since the properties of this loss are dependent on spectrum decreases, implying lower beam divergence and the refractive properties of the incident light with the sidewall, therefore better beam quality. The reduced Fourier spectral LEIDNER AND MARCIANTE: BEAM QUALITY IMPROVEMENT IN BROAD-AREA SEMICONDUCTOR LASERS 1271

TABLE I PARAMETERS USED IN NUMERICAL SIMULATION 95/01 Physical quantity Value Cavity length 2mm Contact stripe width 100.5 μm/variable 95/05 Active-layer thickness 8.5 nm Transverse conﬁnement factor 0.0135 Facet power reﬂectivities variable Laser wavelength 980 nm 95/10 .u.)

Effective index 3.35 a e (

Broad-area waveguide step 0.001/variable d

Linewidth-enhancement factor, α 2.3 litu

p 95/20 −1 Internal loss 1cm m a

Pump Current 16 A/variable d − Gain cross-section 1.5 × 10 15 cm2 r-fiel

Diffusion constant 33 cm2/s a 95/30

− Ne Transparency carrier density 1.5 × 1018 cm 3 Non-radiative lifetime 4ns − Spontaneous emission coefﬁcient 1.4 × 10 10 cm3/s 95/40

Propagation 95/50 Lateral

-50 50 Lateral cavity position (µm)

Fig. 3. Simulated snapshot of the lateral electric ﬁeld amplitude of a laser beam exiting the cavity for various facet reﬂectivities. Dashed (red) lines indicate the waveguide boundary.

for low output facet reflectivity, a significant amount of light resides outside the pumped/wave-guided region of the laser that is bounded by dashed (red) lines. This is related to the fine structure in Fig. 2 and is also indicative of light being scattered beyond the critical angle of the waveguide. Figures 2 and 3 clearly demonstrate that increasing the number of effective round trips in the cavity via increased cavity reflectivity results in increased evanescent spatial filtering. Nominally, spatial characteristics of laser fields can be quantified by analyzing the amplitude of their spatial Fourier transforms. However, the chaotic nature of the filamentation in BALs makes it necessary to average these quantities over time, represented by cavity round trips in the BPM simulation, in order to obtain “typical” (as opposed to instantaneous) characteristics. The results of this analysis under varying Fig. 2. Simulated normalized Fourier transform amplitude of a laser beam condition of output coupler reflectivity (1% to 50%) are shown as it passes through a laser cavity for various cavity reflectivities. The in Fig. 4. These plots show a clear narrowing of the diver- propagation direction goes from the high-reflectivity surface at the bottom of each subfigure towards the low-reflectivity surface at the top of each gence of the laser with increasing output coupler reflectivities. subfigure. Qualitatively, this far-field narrowing should result in a direct improvement in beam quality since the emission aperture is identical for all cases. width indicates that there is less fine structure on the beam, The 2σ divergence angle of the far-field distribution, shown which is verified with the snapshots of the electric field in Fig. 5, clearly depicts this expected far-field narrowing. amplitude outputs shown in Fig. 3. Fig. 3 also reveals that By replacing conventional low-reflectivity coatings (∼2Ð4%) 1272 IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. 48, NO. 10, OCTOBER 2012

1.0 95/01

y 0.9

95/05 0.8 e efficienc p

Slo 0.7 95/10

0.6 .u.) a 0 0.1 0.2 0.3 0.4 0.5 e (

d 95/20 Reflectivity litu p

m Fig. 6. Simulated slope efﬁciency as a function of output facet reﬂectivity. a

d 95/30

r-fiel 100 Fa 80 95/40

2 60 M

95/50 40 20 -8 8 Far-field angle (degrees) 0 0 0.1 0.2 0.3 0.4 0.5 Fig. 4. Simulated average far-field amplitudes of the output laser field for various facet reflectivities. Reflectivity

2 8.0 Fig. 7. M as a function of output facet reﬂectivity for various waveguide index steps: n = 0.01 (triangles), n = 0.001 (squares), and n = 0 (diamonds). ) s 7.0 ree g e

d of reﬂectivity. The beam quality in Fig. 7 is represented by 6.0 2

th ( the commonly accepted measure given by the parameter M , d which is deﬁned as the ratio of the divergence of a given beam wi

d 5.0 compared to the divergence of a Gaussian beam of the same 2σ beam width [11]. Figure 6 shows that the efficiency decreases r-fiel 4.0 linearly with increasing reflectivity, as would be expected by Fa letting less light escape from the laser cavity. However, Fig. 7 2 3.0 shows that M drops much more rapidly resulting in a net increase in beam brightness, coarsely defined as the output 0.0 0.1 0.2 0.3 0.4 0.5 power divided by the product of the emission aperture area Output facet reflectivity and the emitted beam divergence. In fact, by comparing typical Fig. 5. Angular width of the time-averaged far-field amplitude as a function low-reflectivity coatings (2Ð4%) to a simple uncoated output of output facet reflectivity. facet (∼30%), the beam quality can be improved by a factor of two while only sacrificing 10% in device efficiency. It was previously noted that the magnitude of the index with higher reflectivity coatings, the far-field distribution can step can play a role since it determines the TIR critical angle be narrowed by a factor of two. Of course, with higher and thus the spatial extent of the modes in the cladding. reflectivity coatings, it is reasonable to expect a reduction Fig. 8 shows M2 as a function of index step for output in laser output since more light is re-circulated within in facet reflectivities of 0.3 and 0.05. The index step in fact the cavity instead of emitting from the cavity. Fig. 6 shows plays a critical role in determining the beam quality via the device slope efficiency as a function of output facet evanescent spatial filtering. For large index steps, the modes reflectivity, while Fig. 7 shows the beam quality as a function are all highly confined, resulting in very weak evanescent LEIDNER AND MARCIANTE: BEAM QUALITY IMPROVEMENT IN BROAD-AREA SEMICONDUCTOR LASERS 1273

70 1.0 60 50 0.9 /A)

40 W ( 2 y M 30 0.8

20 fficienc

E 0.7 10 0 0.0001 0.001 0.01 0.6 020406080 Index step Beam waist (μm)

Fig. 8. Plot of simulated M2 versus broad-width index step at 30% (triangle) Fig. 9. Simulated slope efficiency as a function of 2σ beam waist for lasers and 5% (square) output facet reflectivity. with output facet reflectivities of 0.05 (square) and 0.27 (triangle). This plot contains slope efficiencies at a range of pumping levels. spatial filtering. For small index steps, the modes are weakly confined, resulting in high loss for all modes and therefore laser brightness. More importantly, the commercially practical reduced relative modal filtering. Fig. 8 also shows that there change, from a low reflectivity coating to an uncoated facet, is an optimum index step for providing evanescent filtering. reduces the efficiency by only 10% while offering a beam For traditional low reflectivity output couplers, optimizing the quality improvement of a factor of 2. index step can result in more than a factor of 2 improvement This point is further underscored in Fig. 9, which shows the in beam quality without sacrificing laser efficiency. A further slope efficiency as a function of the 2σ beam width for 5% 60% beam quality improvement can be obtained by simul- and 27% output facet reflectivities. For waveguides that can taneously optimizing the facet reflectivity. Regardless of the support more than a few spatial modes, the efficiency is nearly facet reflectivity chosen, the optimal index step is near 0.001. constant regardless of the waveguide width. This can largely This value will be used for the remainder of the simulations. be explained in terms of the filamentation dynamics. Since the Fig. 7 shows M2 as a function of the output facet reflectivity nominal filament spacing is a result of the interplay between for three cases of index step: strongly guided (n = 0.01), carrier-induced self-focusing and diffraction, the filamentation moderately guided (n = 0.001), and gain-guided (n = 0). spacing should be constant at a given pump level provided As discussed previously, the strongly guided case leaves the waveguide is sufficiently large to support many waveguide little mode power in the cladding, resulting in virtually no modes [12]. evanescent spatial filtering, as evidenced by the nearly con- 2 stant M . In contrast, the gain-guided case allows the modes V. C ONCLUSION to spread well beyond the waveguide boundaries, resulting in Free-carrier induced filamentation is a continuously seeded relatively high loss for all modes. Nevertheless, evanescent process, making lumped spatial filtering ineffective when high spatial filtering can still be effective in reducing the M2 down operating power causes the filamentation length to be shorter to the level of the strongly guided case. The moderately guided than the round trip of a cavity. A continuous filtering method case allows for full exploitation of refractive spatial filtering, is therefore required to combat filamentation in a power- with a ∼2x reduction in M2 compared to the conventional low scalable manner. This paper has shown that the sidewalls of reflectivity case (a few %). Moreover, the beam quality for the a BAL provide a weak spatial filter that can be enhanced by uncoated facet is 3x better than the strongly guided case, which optimizing the lateral waveguide index step and increasing the is a typical configuration for high efficiency diode lasers. cavity lifetime via the output facet reflectivity. Instantaneous analysis of the electric field within the cavity demonstrates that IV. PRACTICAL CONSIDERATIONS increased filtering occurs despite the increased power stored As was indicated previously, high power laser configurations in the cavity with high output facet reflectivities. A broad- nominally have low reflectivity coatings on the output facet in range of optimum index step exists to provide high loss order to allow the highest efficiency laser emission. By increas- only to the higher order modes and thus provide low-loss ing the facet reflectivity to enhance the effects of refractive spatial filtering. Higher confinement reduces the desirable spatial filtering, more of the light is retained in the laser cavity, effects by reducing the amount of light outside the waveguide, resulting in reduced laser emission. Fig. 6 shows the simulated while lower confinement allows even relatively low spatial electrical-to-optical slope efficiency as a function of output frequencies to see similar losses as high spatial frequencies. facet reflectivity. Although the efficiency linearly decreases Potentially, evanescent spatial filtering can be improved by with increasing facet reflectivity, the beam quality (Fig. 7) using a more strongly absorbing media in the un-pumped improves more rapidly, resulting in a net improvement in cladding regions of the lateral laser waveguide. 1274 IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. 48, NO. 10, OCTOBER 2012

In conclusion, spatial filtration of the intra-cavity [11] A. E. Siegman, “How to (maybe) measure laser beam quality?” in Proc. semiconductor laser beam has been numerically demonstrated Diode Pumped Solid State Lasers Appl. Issues, 1998, pp. 1Ð3. [12] J. R. Marciante and G. P. Agrawal, “Spatio-temporal characteristics of by optimizing the output facet reflectivity and the lateral filamentation in broad-area semiconductor lasers,” IEEE J. Quantum waveguide index step. The evanescent tail of waveguide modes Electron., vol. 33, no. 1, pp. 1174Ð1179, Jan. 1997. experience absorptive loss, which becomes increasingly higher for higher-order modes, equivalently creating a weak spatial filter. Increasing the effective laser cavity length in a broad- Jordan P. Leidner received the B.S. degree in area laser enhances the effect of the filter by extending electrical engineering from the University of Texas propagation in the cavity, leading to improved output beam at Austin, Austin, in 2007, with a focus in elec- quality. Simulations predicted a factor of two increase in tromagnetic and solid-state engineering. He is currently pursuing the Ph.D. degree with the Institute beam brightness with only a 10% efficiency penalty. of Optics, University of Rochester, Rochester, NY, researching the power limitations and filamentation suppression of high-brightness semiconductor lasers. REFERENCES [1] D. G. Mehuys, “Power limitations,” in Semiconductor Lasers: Mate- rials and Structures, E. Kapon, Ed. New York: Academic, 1999, pp. 260Ð263. [2] J. R. Marciante and G. P. Agrawal, “Nonlinear mechanisms of filamentation in broad-area semiconductor lasers,” IEEE J. Quantum Electron., vol. 32, no. 4, pp. 590Ð596, Apr. 1996. John R. Marciante (M’00) received the B.S. degree [3] R. Parke, D. F. Welch, A. Hardy, R. Lang, D. Mehuys, S. O’Brien, in engineering physics from the University of Illinois K. Dzurko, and D. Scifres, “2.0 CW, diffraction-limited operation of at Urbana-Champaign, Urbana, in 1991, and the a monolithically integrated master oscillator power amplifier,” IEEE M.S. and Ph.D. degrees in optics from the University Photon. Technol. Lett., vol. 5, no. 3, pp. 297Ð300, Mar. 1993. of Rochester, Rochester, NY, in 1992 and 1997, [4] M. L. Tilton, G. C. Dente, A. H. Paxton, J. Cser, R. K. DeFreeze, respectively. C. E. Moeller, and D. Depatie, “High power, nearly diffraction-limited He was with the Air Force Research Laboratory, output from a semiconductor laser with an unstable resonator,” IEEE J. Kirtland AFB, NM, in 1991, where he was involved Quantum Electron., vol. 27, no. 9, pp. 2098Ð2108, Sep. 1991. in research on high-brightness semiconductor lasers [5] R. M. R. Pillai and E. M. Garmire, “Paraxial-misalignment insensitive and fiber amplifiers and coherent beam combination. external-cavity semiconductor-laser array emitting near-diffraction lim- In 2001, he was with Corning Rochester Photonics ited single-lobed beam,” IEEE J. Quantum Electron., vol. 32, no. 6, pp. Corporation, Rochester, where he was engaged in research on high-index- 996Ð1008, Jun. 1996. contrast waveguides, metal-free diffraction gratings, and liquid crystal cells. [6] R. J. Lang, K. Dzurko, A. A. Hardy, S. Demars, A. Schoenfelder, and Since 2003, he has been with the University of Rochester, where he joined D. F. Welch, “Theory of grating-confined broad-area lasers,” IEEE J. the Laboratory for Laser Energetics in 2003 and researched fiber-based laser Quantum Electron., vol. 34, no. 11, pp. 2196Ð2210, Nov. 1998. sources, diagnostics, and timing systems, and since 2006, he has been an [7] H. Po, E. Snitzer, R. Tumminelli, L. Zenteno, F. Hakimi, N. Cho, and T. Associate Professor of optics with the Institute of Optics, engaged in research Haw, “Double clad high brightness Nd fiber laser pumped by GaAlAs on large-mode area fibers, high-power fiber lasers, single-frequency fiber phased array,” in Proc. Opt. Fiber Commun. Conf., 1989, pp. 1Ð4. lasers, all-fiber optical components, and high-brightness semiconductor lasers. [8] A. W. Snyder and J. D. Love, “Goos-Hänchen shift,” Appl. Opt., vol. 15, Prof. Marciante is a member of the Optical Society of America and the IEEE no. 1, pp. 236Ð238, Jan. 1976. Lasers and Electro-Optics Society. For two terms, he was a Topical Editor for [9] M. Kanskar, T. Earles, T. J. Goodnough, E. Stiers, D. Botez, and L. J. the Journal of the Optical Society of America B. He was an Adjunct Professor Mawst, “73% CW power conversion efficiency at 50 W from 970 nm with the Electrical and Computer Engineering Department, University of New diode laser bars,” Electron. Lett., vol. 41, no. 5, pp. 245Ð247, Mar. 2005. Mexico, Albuquerque, and the Chairman of the IEEE/LEOS Albuquerque [10] 6-Pin Uncooled Industrial Pump. (2012, Apr. 3) [Online]. Available: Chapter. He was involved in numerous conference committees and is currently http://www.alfalight.com/pdf/QS4225%206-pin%20Industrial%20v7- the Chairman of the Fiber Modeling and Fabrication Technical Group, Optical 4.pdf Society of America.