External-cavity semiconductor-laser array insensitive to paraxial misalignment

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OPTICS LETTERS / Vol. 20, No. 20 / October 15, 1995

External-cavity semiconductor-laser array insensitive to paraxial misalignment Ramadas Pillai* and Elsa Garmire† Center for Laser Studies, University of Southern California, University Park, DRB-17, Los Angeles, California 90089-1112 Received June 15, 1995 We report a tilted-mirror external-cavity gain-guided laser array whose output power is insensitive to small misalignments of the cavity elements. This novel configuration delivers a near-diffraction-limited (1.5 times diffraction-limited) single-lobed beam in the entire current range without the need for realignment of the cavity. The light-current characteristic is linear and kink free, with a higher efficiency compared with the free-running array. A compact graded-index lens design that can emit a collimated beam with a circular cross section is also described.  1995 Optical Society of America

Spatially coherent high-power semiconductor lasers1 – 8 have demanding applications in the pumping of fiber amplifiers, in high-speed printing and recording, and in various biomedical /industrial systems. Extracting coherent high power from commercial laser arrays and broad-area lasers based on an external-cavity scheme by Chang-Hasnain et al.1 (C-H) is very attractive owing to its simplicity and scalability to multiwatt output power. Their scheme uses a collimated geometry in which the array facet and the external mirror are placed at the focal planes of a lens. We propose and demonstrate here an alternative focused geometry in which the array near field is imaged on the external mirror. We show that this geometry has 3-orders-ofmagnitude higher cavity-misalignment tolerance and superior mode discrimination, and we demonstrate the extraction of near-diffraction-limited single-lobed output from commercial gain-guided arrays. Compact multiwatt versions of this hybrid geometry are expected to be low-cost competitors to the f laredamplifier single-lobe emitters.8 In the C-H geometry a 0.25-pitch graded-index (GRIN) lens collimates the array output, and a thinstripe mirror at the other end of the GRIN lens (Fourier plane) acts as the external mirror (EM). The thinstripe mirror selects and feeds back a diffractionlimited lobe into the array, which amplifies it, forming a near-diffraction-limited single-lobed output beam. The present geometry performs equally well in emitting a near-diffraction-limited beam of width 0.8± (1.5 times diffraction limit) at the rated output of the array of 100 mW. At the same time, it does not have the submicrometer misalignment sensitivity of the C-H geometry, making it a practical scheme. The experimental setup is shown in Fig. 1. The EM is placed at the image plane of the lens [Fig. 1(a)], and the output is taken from an output coupling mirror (OC) inserted at the focal plane of the lens [Fig. 1(b)]. We used a commercial ten-stripe gainguided GaAs –AlGaAs array (SDL2410-C, 100 mW) with individual stripe widths of 5 mm, center-to-center spacing of 10 mm, and back and front ref lectivities of ,95% and 5%, respectively. The array was mounted on an open heat sink without any temperature control. A 0.15-pitch GRIN lens (N.A. 0.6, f ­ 2.1 mm, antiref lection coated) was used as a focusing element 0146-9592/95/202108-03$6.00/0

whose back focal plane was ,2 mm outside its back aperture. The distance between the EM and the focal plane of the GRIN lens was equal to the EM’s curvature, ,150 mm. A glass slide of 1-mm thickness, with one end polished f lat at a 45± angle, was used as the OC and was placed at the second focal plane of the lens, where the laser lobe had its waist.7 The EM formed a real inverted image of the OC, as shown in Fig. 1(b). The OC and its image together form a narrow aperture that acts as a bandpass spatial filter. The full width w of this aperture is given by w ­ 2Cu 2 2a ,

(1)

where C is the curvature of the EM, a is the distance from the edge of the OC to the array axis, and u is the tilt angle of the EM, as def ined in Fig. 1(b). This aperture is similar to the aperture formed by the stripe mirror in the C-H cavity. We could adjust the EM tilt and the OC position separately to select an optimum width and position of the aperture in the present geometry. In our experiment w , 40 mm.

Fig. 1. Experimental setup of paraxial-misalignmentinsensitive external-cavity laser array, showing (a) out-of-plane and ( b) in-plane views. The long solid lines represent the beam profile of the lasing beam. P, pitch.  1995 Optical Society of America

October 15, 1995 / Vol. 20, No. 20 / OPTICS LETTERS

The back surface of the OC was painted black to make it opaque for the lasing lobe. Since the ref lectivity of the OC did not inf luence the performance parameters of the external-cavity laser, we did not take time to coat the polished surface for high ref lection for the present measurements. A relay lens was used to magnify and reimage the far-field pattern formed at the focal plane of the GRIN lens to a convenient location at which a detector array (1024 pixels) could be placed. The linear dimension of the far-f ield pattern was calibrated in terms of degrees by use of the known positions of the far-f ield lobes of the free-running array. Far-f ield intensity patterns of the free-running array [Fig. 2(a)] show a lobe separation of ,6.3± at 1.4I th. The drive currents are normalized to the free-running threshold of 284 mA. In Fig. 2(b) far fields of the external-cavity laser are shown, with drive currents normalized to 274 mA, which is the threshold of the external-cavity laser. The near-f ield patterns for the same drive currents are shown in Fig. 3. The far field was optimized for maximum peak power at the highest current, and the same alignment was used to generate the curves at lower drive currents. The external-cavity laser delivered sharp single-lobed far-f ield patterns throughout the current range without any need for realignment. The FWHM of the lobe increased from 0.7± near threshold to 0.84± at the highest drive current of 428 mA. A majority of the power was contained in the main lobe, and the light-current characteristic was linear and kink free. The slope eff iciency of the external-cavity laser was 0.88 mWymA, higher than that of the free-running array (0.84 mWymA) and consistent with the experimental results of C-H. At the highest current the external-cavity laser delivered 135 mW in a single-lobed far field, compared with 120 mW for the free-running array in a double-lobed far field. The output powers of the external-cavity laser are determined from the far-f ield patterns by comparison with the free-running far field, measured under identical conditions. Even though the external-cavity alignment remained fixed, as the drive current was varied the peak of the lobe shifted to lower angles at the lowest drive currents. Since there was negligible shift in the medium- and high-current ranges, lobe shifts should not cause any serious problem for high-power operation and modulation. This beam steering is not surprising since the free-running lobes themselves were steered with drive current. This in turn steered the lasing lobe, which was driven by one of the free-running lobes. The width of the spatial-filtering aperture is ,40 mm, ,2.5 times wider than that of the C-H geometry. A wider width is tolerable in the present geometry, perhaps because of enhanced discrimination from mode mixing owing to the tilted external mirror.4 The C-H geometry could not tolerate beam steering of this order because of the smaller width of the stripe mirror. The tolerance to out-of-plane displacement misalignment of the GRIN lens in the present geometry is given by the GRIN lens aperture size of ,1 mm, whereas in the C-H cavity it is limited by the beam size of ,1 mm at the facet. This shows an improvement in the misalignment tolerance of approximately 3 orders of mag-

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nitude. Though the output power is stable against small misalignments, the width and the position of the far-f ield lobe change with the in-plane misalignments because of the spatial-f iltering action of the OC–EM combination. The present geometry can be easily set up with bulk optics to deliver stable output without any vibration isolation, as a result of its paraxial-misalignment insensitivity. Long-term stability was tested for a period of two days; the laser remained aligned, delivering single-lobed output with only ,5% reduction in output power. A comparable collimated geometry (with the array moved closer to the lens in Fig. 1) delivered an output power that f luctuated as much as 30% (less than 10 Hz), and the cavity remained aligned only for ,10 min. Thus the present setup is well suited to typical laboratory environments. In the present setup, the out-of-plane beam angle after passing the GRIN lens is ,0.4±, with its waist located at the image plane. The in-plane beam waist is located at the focal plane, with a beam angle of ,2.7±. An external optic can be used to collimate this beam or focus it

Fig. 2. Far-f ield intensity patterns of (a) the free-running array and ( b) the external-cavity laser. The far-field angles are defined with respect to the array axis. The far field is formed at the focal plane of the GRIN lens.

Fig. 3. Near-field intensity distribution of the array in the external cavity for the drive currents shown in Fig. 2( b).

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Fig. 4. GRIN-lens compact design of the misalignmentinsensitive external-cavity laser array. All transmitting surfaces are antiref lection coated. The lines with arrows represent symmetry axes of the lasing beam. HR, highref lection; TIR, total internal ref lection.

down to a near-diffraction-limited spot size. The present single-lobed beam power (135 mW) is limited by the rated power of the array used. However, the output power may be scaled to multiwatts by use of a multiwatt array. A rugged compact design of the present geometry can be easily fabricated and put into a small package (Fig. 4). It consists of two 0.25-pitch GRIN lenses and an OC sandwiched between the lenses. The OC is made from two high-index prisms spaced by an air gap or glued together with a thin film of lower-index epoxy such that the light incident at 45± undergoes total internal ref lection at the film. Moving lens 2 in the in-plane direction is equivalent to tilting the EM in Fig. 1. The separation between the lower edge of the air gap and the array axis determines the dominant mode component of the lasing lobe, whereas the separation between the lower edge and the axis of lens 2 controls the width of the lasing lobe. Computation of the paraxial round-trip matrix for the compact external cavity shows that it remains aligned regardless of either the OC thickness or the misalignment of the individual components. However, to minimize diffraction effects, the OC thickness should be less than the Rayleigh range (assuming a Gaussian lobe shape), i.e., ,2.5 mm for a beam of 0.7± FWHM.7 Since the OC is placed at the waist of the lasing lobe it introduces minimal or no aberrations. This design delivers a collimated output with an elliptical beam cross section. However, one could minimize the ellipticity by permitting the beam to be incident upon the output face of the OC at an appropriate angle in a plane of incidence parallel to the larger dimension of the beam. The output face should be antiref lection coated for this angle of incidence, since incidence will be very close to the critical angle for a large ellipticity of ,40:1, as in the present case. Once the output face of the OC is made at this angle, the angle of incidence may be fine tuned to the desired value by small adjustments of the position of the array in the out-of-plane direction relative to the external cavity. Such small adjustments do not misalign the cavity. Hence the ellipticity can be corrected exactly without the use of any astigmatism-correcting optics. In conclusion, we have demonstrated a paraxialmisalignment-insensitive external-cavity laser array that emits a near-diffraction-limited beam through-

out the driving current range without any need for cavity realignment. The present geometry is orders-of-magnitude less sensitive to small cavity misalignments compared with that reported previously. Owing to the tilted external mirror in the image plane (or, equivalently, to an axially displaced 0.25-pitch lens/mirror in the compact design), the oscillating array modes are mixed together,4 locking to a single composite cavity spatial mode more easily with improved discrimination. The width of the spatial-filtering aperture can be optimized by small adjustments of the EM and OC. Note that, for mode-locking applications, we can make the present cavity as long as necessary while maintaining good output stability. Finally, it is applicable to arrays of arbitrary power and arbitrary spatial distribution of emitters, since the ref lected light is imaged back to fall exactly on the individual lasing elements, even in the presence of any residual misalignments of the external cavity. Therefore it can be used with a stack of arrays9 that may not have good spacing uniformity. This study was funded in part by the National Science Foundation’s Lightwave Technology program and by the Advanced Research Projects Agency through the National Center for Integrated Photonic Technology. Useful discussions with Prem Kumar are gratefully acknowledged. *Present address, Department of Electrical Engineering and Computer Science, Northwestern University, 2145 North Sheridan Road, Evanston, Illinois 60208-3118. †Present address, Thayer School of Engineering, Dartmouth College, Hanover, New Hampshire 03759-8000. References 1. C. J. Chang-Hasnain, J. Berger, D. R. Scifres, W. Streifer, J. R. Whinnery, and A. Dienes, Appl. Phys. Lett. 50, 21 (1987). 2. M. L. Tilton, G. C. Dente, A. H. Paxton, J. Cser, R. K. DeFreez, C. E. Moeller, and D. Depatie, IEEE J. Quantum Electron. 27, 2098 (1991). 3. S. T. Srinivasan, C. F. Schaus, S. Z. Sun, S. D. Hersee, J. G. McInerney, A. H. Paxton, and D. J. Gallant, Appl. Phys. Lett. 60, 1272 (1992). 4. J. Salzman, R. Lang, S. Margalit, and A. Yariv, Appl. Phys. Lett. 47, 1 (1985). 5. J. A. Ruff, A. E. Siegman, and S. C. Wang, in Conference on Lasers and Electro-Optics, Vol. 11 of 1989 OSA Technical Digest Series (Optical Society of America, Washington, D.C., 1989), paper THK18. 6. L. Goldberg and M. K. Chun, Appl. Phys. Lett. 53, 20 (1988). 7. R. Pillai, ‘‘Experiment and analysis of alignmentstabilized external-cavity laser arrays emitting near-diffraction-limited single-lobed beam,’’ Ph.D. dissertation (University of Southern California, Los Angeles, Calif., 1995). 8. J. S. Osinski, D. Mehuys, D. F. Welch, R. G. Waarts, J. S. Major, Jr., K. M. Dzurko, and R. J. Lang, Appl. Phys. Lett. 66, 556 (1995). 9. M. Sakamoto, D. F. Welch, G. L. Harnagel, W. Streifer, H. Kung, and D. R. Scifres, Appl. Phys. Lett. 52, 26 (1988).