In this letter, we report the fabrication of an antiguided laser array using zinc diffusion ... facets) and 1.6-W quasi-continuous-wave (CW) (100- s pulses; total, both ...
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IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 10, NO. 3, MARCH 1998
High-Power Antiguided Laser Array Fabricated Without the Need for Overgrowth J. M. Gray, J. H. Marsh, Senior Member, IEEE, and J. S. Roberts
Abstract— The fabrication of conventional semiconductor antiguided laser array structures involves etching of the array profile followed by an overgrowth step. In this letter, we report the fabrication of an antiguided laser array using zinc diffusion induced intermixing of a superlattice to create the necessary index step. The technique was used to fabricate a five-element, 10-m center, antiguided laser array operating at 0.860 m. The device operated at 1.22 diffraction limit to 3-W pulsed (total, both facets) and 1.6-W quasi-continuous-wave (CW) (100-s pulses; total, both facets). Index Terms— Gallium arsenide, intermixing, semiconductor laser arrays, superlattice. Fig. 1. Structure of antiguided array laser.
I. INTRODUCTION
M
ONOLITHIC phase-locked semiconductor array lasers appear to be the best candidates for fulfilling the quest to achieve high coherent output powers (≥1 W) delivered in single-moded diffraction-limited beams. There are four main types [1], [2] of phase-locked arrays, these are: evanescentwave coupled, diffraction-coupled, Y-junction coupled and leaky-wave coupled. The devices make use of a periodic variation of the real part of the effective index, and operate by the coupling of adjacent lateral modes. Two classes of modes characterize the operation of the devices: evanescent-type array modes, for which the peak of the optical field is confined to the high-index regions of the array, and leaky-type array modes, for which the peak of the optical field is confined to the low-index regions of the array. Positive-index guided arrays (evanescent-type coupling) have been shown to exhibit weak overall coupling, giving rise to poor intermodal discrimination [3]–[5]. The resulting positive-index guided devices are sensitive to spatial hole burning [6] and thermal effects. Negative-index guided (antiguided) arrays in turn show strong overall coupling (“parallel-coupling”), exhibiting good intermodal discrimination [7]. Unlike positive-index guided devices, in antiguided arrays there is no limitation on the effective-index step, , (for evanescently-coupled arrays has to be below the cutoff for high-order modes [8], typically ≤5 10 ) which allows one to fabricate devices of high-index steps (0.02–0.05) Manuscript received August 25, 1997; revised December 1, 1997. J. M. Gray and J. H. Marsh are with the Department of Electronics and Electrical Engineering, University of Glasgow, Glasgow G12 8QQ, Scotland, U.K. J. S. Roberts is with Department of Electronic and Electrical Engineering, University of Sheffield, Sheffield S1 3JD, U.K. Publisher Item Identifier S 1041-1135(98)01840-0.
that are stable against thermal and/or carrier-induced index variations. Both types of devices depend on the creation of a lateral effective-index step. Conventionally, this index step has been fabricated in GaAs–AlGaAs systems by etching between the array elements to within 0.1–0.2 m of the active layer [9]. Overgrowth is then used to create a passive waveguide structure in these etched regions. In this letter, we report using the optical properties of a GaAs–AlAs superlattice to create the necessary index step in the fabrication of antiguided array lasers in the GaAs–AlGaAs system at 0.860 m. GaAs–AlAs superlattices have the desirable property that their refractive index can differ by as much as 6% [10] from that of bulk material of the same average aluminum composition. More specifically, in separate experiments we measured a refractive index change in the range 4.8%–5.2% in GaAs–AlAs superlattices after intermixing, this being sufficient to create an effective index step, 0.015 between the antiguide and interelement regions of the array. Significant progress has been made since our earlier work [11] with our latest devices recording output powers of 1.5 W per facet (pulsed) and quasi-continuous-wave (CW) far-field profiles exhibiting near-diffraction limited 1.2 (FWHM) beams from five element, 10- m center arrays operating at 0.860 m. II. DESIGN
AND
FABRICATION
A schematic diagram of the array structure is shown in Fig. 1. The active/waveguide region consisted of a double ˚ GaAs quantum quantum well (DQW) made up of 2 100 A ˚ As barrier, centrally placed wells with a 100-A Al Ga
1041–1135/98$10.00 1998 IEEE
GRAY et al.: HIGH-POWER ANTIGUIDED LASER ARRAY FABRICATED WITHOUT THE NEED FOR OVERGROWTH
Fig. 2. Light output against current for typical laser array at 0.86 m. Single facet, 400-ns pulses at 1-kHz repetition rate.
in a 0.30- m Al Ga As waveguide. The upper cladding layer was an undoped superlattice consisting of 223 periods of ˚ AlAs ˚ GaAs ( 5.65 5 monolayers) and 16.98 A 28.25-A ( 5.66 3 monolayers) to make up a 1.0- m superlattice layer of average Al composition 37.5%. The entire structure was grown at Sheffield University using the metal–organic vapor pressure epitaxy (MOVPE) method. The arrays were fabricated by first coating the devices with a masking layer of SiO and opening windows for 5 elements with 10- m antiguide/interelement centers. After photolithography the superlattice areas beneath the windows were diffused using a solid source of zinc arsenide at a temperature of 650 C for 15 min in an annealing furnace, with the experimental configuration designed to create an arsenic overpressure. This diffusion served two purposes, firstly, to intermix the superlattice layers beneath the windows and generate an effective index step between these regions and the adjacent interelement regions, and secondly, to confine current to the low-index antiguide regions by doping these regions p-type ( 10 cm ). The one-dimensional (1-D) approximation to the resonance in-phase coupling condition for fundamental modes of the antiguided array is given by (1)
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Fig. 3. Far-field emission pattern during quasi-CW operation (100-s pulses, 10% duty cycle at 800-mW output, single facet). Theoretical diffraction limit is 1.0 .
array with a relatively high value ≥ 5.7, giving effective device dimensions of 8.5 m and 1.5 m. III. RESULTS 800- m-long devices, with no antireflection coatings were tested using current pulses of 400 ns at a 1-kHz repetition rate. Typical devices had threshold currents of 0.75 A, corresponding to threshold current densities of 2 10 A cm , and external differential quantum efficiencies 60%. The devices were tested pulsed to a limit of 4.5 A (the maximum output of our equipment) and Fig. 2 shows the light output against current for a typical device. The device shown had a threshold current of 0.65 A, threshold current density of 1625 A cm , external differential quantum efficiency of 62.7% and, at a drive current of 4.0 A (6.2 ), the peak power was 1.5 W per facet. Fig. 3 shows the experimental far-field pattern of this device under quasi-CW operation (10% duty cycle). Near-diffraction limited operation (the theoretical diffraction limit for this type of near-resonant array is 1.0 ) is achieved in a 1.2 (FWHM) wide beam (at 4 ) to powers ≥800 mW per facet. At drive currents higher than this there is some broadening of the beam and at 7 the beam had broadened to 3.6 , but remained single-lobed throughout. IV. CONCLUSION
where is the number of interelement standing-wave peaks (odd for the in-phase condition), is the laser emission wavelength, is the effective refractive index of the antiguide is the effective region, is the width of the antiguide region, refractive index of the interelement region, and is the width of the interelement region. A series of devices with varying and dimensions was made and the results for the best of these are displayed in Figs. 2 and 3, which correspond to a device with SiO mask dimensions 6 m and 4 m. The far-field pattern (Fig. 3) for this device shows a central lobe containing 80%–85% of the power output. At first glance this would appear to be an inconsistency as a ratio of 1.5 should give strong side lobes at ±4.9 [2], however zinc diffusion produces an intermixed-material front which proceeds laterally as well as vertically and the true and parameters cannot be assumed from the dimensions of the SiO mask. The results shown are consistent with a 1 near-resonant antiguided
We have demonstrated a novel method for the fabrication of high-power antiguided array lasers in the GaAs–AlGaAs system without the need for overgrowth. The uncertainty in the profile of the intermixed-material front makes it very difficult to produce devices with definitive and parameters and the approach taken was to produce a series of devices in which these parameters were varied in a methodical way. Our results are consistent with a near-resonant antiguided array where 1 [12]. The relatively high threshold-current densities ( 1.6–2 kA cm ) are probably due to the small number of array elements, (5) and the relatively small effectiveindex step, (0.015). Such low values for and will provide high-edge radiation losses [2], our next generation of devices is planned to include a higher (by increasing the thickness of the superlattice layer and by altering the composition of the lower cladding layer) and an increase in (10 or 20). As the edge radiation losses decrease as and 1/ this should significantly lower .
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ACKNOWLEDGMENT The authors would like to thank M. R. S. Taylor for the use of his vector finite difference computer program, used in the ) step for the structure. calculation of the effective-index ( REFERENCES [1] N. W. Carlson, Monolithic Diode-Laser Arrays. Berlin, Germany: Springer-Verlag, 1994. [2] D. Botez and D. Scifres, Eds., Diode-Laser Arrays. Cambridge, U.K.: Cambridge Univ. Press, 1994. [3] J. K. Butler, D. E. Ackley, and D. Botez, “Coupled-mode analysis of phase-locked injection laser arrays,” Appl. Phys. Lett., vol. 44, pp. 293–295, 1984. [4] E. Kapon, J. Katz, and A. Yariv, “Supermode analysis of phase-locked arrays of semiconductor lasers,” Opt. Lett., vol. 10, pp. 125–127, 1984. [5] J. K. Butler, D. E. Ackley, and M. Ettenberg, “Coupled-mode analysis of gain and wavelength oscillation characteristics of diode laser phased arrays,” IEEE J. Quantum Electron., vol. 21, pp. 458–464, 1985.
[6] K. L. Chen and S. Wang, “Spatial hole burning in evanescently coupled semiconductor laser arrays,” Appl. Phys. Lett., vol. 47, pp. 555–557, 1985. [7] G. R. Hadley, D. Botez, and L. I. Mawst, “Modal discrimination in leaky-mode (antiguided) arrays,” IEEE J. Quantum Electron., vol. 27, pp. 921–930, 1991. [8] W. Streifer, A. Hardy, R. D. Burnham, and D. R. Scifres, “Single-lobe phased-array diode-lasers,” Electron. Lett., vol. 21, no. 3, pp. 118–120, 1985. [9] D. Botez, “High-power monolithic phase-locked arrays of antiguided semiconductor diode lasers,” Proc. Inst. Elect. Eng., pt. J, vol. 139, pp. 14–23, 1992. [10] J. P. Leburton, K. Hess, N. Holonyak, J. J. Coleman, and M. Camras, “Index of refraction of AlAs-GaAs superlattices,” J. Appl. Phys., vol. 54, no. 7, pp. 4230–4231, 1983. [11] J. M. Gray, J. H. Marsh, and J. S. Roberts, “High power antiguided laser array fabricated using a superlattice structure,” Electron. Lett., vol. 30, no. 24, pp. 2040–2041, 1994. [12] L. J. Mawst, D. Botez, M. Jansen, T. J. Roth, and G. Peterson, “Highpower, narrow single-lobe operation from 20-element phase-locked arrays of antiguides,” Appl. Phys Lett., vol. 55, no. 20, pp. 2060–2062, 1989.
