- Email: [email protected]
The role of monolithic integration in advanced laser products
ARTICLE IN PRESS
Journal of Crystal Growth 288 (2006) 2–6 www.elsevier.com/locate/jcrysgro
The role of monolithic integration in advanced laser products John H. Marsh Intense Ltd, 4 Stanley Blvd, Hamilton International Technology Park, Blantyre, Glasgow G72 0BN, UK Available online 20 January 2006
Abstract The design and performance of single-mode high-power (4100 mW) semiconductor lasers suitable for integration into large arrays are reported. In 830 nm lasers, quantum well intermixing (QWI) has been used to increase the bandgap of the waveguide in the facet region by 120 meV, and the catastrophic optical damage threshold of uncoated devices increased by a factor of 43 as a result. The passive waveguides are relatively cool, bringing high reliability, improving the single-mode waveguide stability and enabling high-temperature operation. Furthermore, the passive waveguides relax the cleaving and packaging alignment tolerances, giving a high yield process suitable for manufacture. A far-field reduction layer is included in the lasers giving a fast axis divergence of o201 FWHM. Arrays in which each emitter operates at several 100 mW, have excellent uniformity of laser parameters such as kink power, operating power and optical beam profile. r 2005 Elsevier B.V. All rights reserved. PACS: 42.55.Px Keywords: A1. Nanostructures; B2. Semiconducting gallium arsenide; B3. Laser diodes
1. Introduction High-power single-mode semiconductor lasers have long had many applications including material processing, printing and optical pumping. Recently device reliability and processing yield has increased to the point where it is feasible to manufacture large arrays of single-mode devices, each operating at a power of several 100 mW. Individually addressable arrays of such lasers are currently driving new developments in digital image transfer including industrial printing and high-quality thermal printing. This paper addresses the technology developments that have made it possible to realize monolithic arrays containing up to 100 emitters. The primary challenge in creating semiconductor laser arrays is to develop a laser design that can deliver a power of several 100 mW, reliably, in a single-transverse mode, and that is robust against manufacturing tolerances. The work described here uses quantum well intermixing (QWI) to create relatively long (4100 mm) passive waveguides Fax: +44 1698 827262.
E-mail address: [email protected]. 0022-0248/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2005.12.021
immediately adjacent to each facet of a ridge waveguide laser. As a result of the QWI processing, the passive waveguides have a larger bandgap than the gain section so raising the threshold power at which catastrophic optical damage (COD) of the facet mirror occurs. The length of the passive waveguides enables large monolithic arrays to be cleaved from a wafer with high yield and also relaxes packaging tolerances as the passive waveguide sections can overhang the laser carrier. A novel ‘V-profile’ layer has also been included in the structure to reduce the fast axis farfield divergence and to make the laser performance less vulnerable to processing variations across the wafer and from wafer to wafer. 2. Catastrophic optical damage and quantum well intermixing COD is the main failure mode of high-power semiconductor lasers operating in the 650–1000 nm wavelength range, and is known to arise from a thermal run-away process at the laser facet. The semiconductor crystal terminates at the facet, and the resulting dangling bonds give rise to defect states. Electrons and holes recombine via
ARTICLE IN PRESS J.H. Marsh / Journal of Crystal Growth 288 (2006) 2–6
these states, emitting heat instead of light. As the facet heats up relative to the bulk of the device, the local bandgap decreases leading to two effects: firstly, a potential gradient which causes carriers to drift towards the facets, and secondly, increased optical absorption at the facet. Both these factors lead to increased non-radiative recombination, and hence to a further rise in the local temperature. Ultimately, this thermal run-away effect leads to the irreversible optical damage known as COD. The optical intensity at which this occurs is approximately 2 106 W cm 2; for a mode size of 1 3 mm2 this implies an output power of only 60 mW. By integrating passive waveguides in the facet region, the facet becomes a nonabsorbing mirror (NAM). NAMs prevent COD in two ways: by restricting the number of carriers that drift from the bulk of the laser to the facet region and by reducing band-to-band absorption facet region. QWI is by far the most common technique for fabricating the passive waveguide NAMs. In QWI, point defects are generated at or close to the semiconductor surface; diffusion of these point defects at elevated temperatures results in the intermixing of the wells with the barriers, and a consequent increase in the bandgap energy [1], so creating a passive waveguide. QWI is a regrowth-free process, and the resulting active/passive interfaces are defect-free and in perfect alignment with the gain section. Impurity-induced disordering using Zn or Si is a widely used QWI process, and the first reported NAM laser [2], and more recent reports [3,4] used these impurities. However, Zn and Si are electrically active, so their use leads to free-carrier absorption losses and current leakage, both of which are undesirable in high-performance NAMs. An alternative approach relies on the fact that vacancies are created during sputter deposition of SiO2 [5]. The vacancies diffuse through the semiconductor and, as a result, individual atoms hop from one lattice site to another so intermixing the QW with the adjacent barrier material. The resulting passive waveguides have low optical propagation losses (few dB cm 1) and uncoated NAM lasers have been demonstrated to have a greater COD level than standard lasers by a factor of 2.6 [6]. More detailed theoretical and experimental studies have been carried out [7] to quantify the benefits of NAMs by separating the role of excluding the carriers from the facet region (non-injection mirror (NIM)) and the increase in the local band-gap at the facet (NAM). The results were compared with conventional lasers, which had no special facet region processing. The laser consisted of a QW epistructure emitting at 830 nm, which was fabricated into ridge-waveguide (RWG) lasers. The lasers had a total cavity length of 1700 mm and the waveguides were 2 mm wide. A schematic of the device is shown in Fig. 1. Lasers were fabricated with (i) conventional facets, (ii) NIMs where the injecting contact was not present close to the facet, and (iii) NAMs, which were bandgap widened and the injecting contact was not present close to the facet.
3
QWI was carried out prior to the ridge-etch stage to increase the bandgap in the NAM regions, whilst maintaining the original bandgap in the active/gain regions. The differential bandgap shift between the active section and NAM varied from 45 to 65 nm (85–120 meV). Devices were fabricated using a mask-set with designs for conventional lasers, NIM only devices, and NAM/NIM lasers. Devices were cleaved into bars and tested. Fig. 2 shows a selection of CW light–current (L–I) curves from bars of uncoated devices with (a) conventional facets, (b) NIMs only, (c) NAM/NIMs with 45 nm bandgap shift, and (d) NAM/ NIMs with 65 nm shift. The L–I curves show that the conventional laser has a COD power of 80 mW, the NIM laser of 125 mW, and the NAM/NIM lasers of 200 mW (for 45 nm NAM shift) and 4300 mW (for 65 nm shift). In the latter case the L–I curve rolls over before COD occurs. The experimental results are in good qualitative agreement with the theoretical calculations [7].
Fig. 1. Schematic of the ridge waveguide lasers showing the nonabsorbing mirror/non-injection mirror (NAM/NIM) regions.
Fig. 2. CW light/current characteristics of 830 nm un-coated ridge waveguide lasers with (a) no-NAM/NIM (i.e. a conventional laser), (b) NIM only, (c) a NAM/NIM with a 45 nm bandgap shift, and (d) a NAM/NIM with 65 nm shift.
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J.H. Marsh / Journal of Crystal Growth 288 (2006) 2–6
Fig. 4. Laser array mounted on subcarrier. Because no heat is generated in passive waveguide sections, the chip can overhang the subcarrier.
Fig. 3. Laser waveguides are not exactly orthogonal to the cleavage planes. For an alignment tolerance of 70.051, the run-out across a 1 cm facet length is 79 mm.
3. Passive waveguides and yield The facets of the vast majority of semiconductor lasers are formed by breaking the semiconductor crystal along a cleavage plane. This gives an extremely flat mirror surface, but cleaving is a mechanical step whose precision is difficult to control. The typical precision to which cleaving can be carried out is around 75 mm. For conventional lasers, the cleave has to be located o10 mm from the end of the gain contact, but in a laser with passive waveguides the cleaving position only needs to fall somewhere within the passive region. The inclusion of passive waveguides therefore relaxes the cleaving tolerance. Angular variations in the cleaving direction have a large impact on yield, especially for arrays. During photolithography, laser waveguides are aligned to the crystallographic orientation of the wafer by reference to the major flat. On wafers with high precision flats, the tolerance with which the major flat is aligned to the crystal plane is 70.011. The lithography system also introduces error, giving a total alignment variation of 70.051. However, the facets are formed by cleaving along the crystal planes, so they are precisely aligned with the crystallographic orientation and are not exactly perpendicular to the laser waveguides. The misorientation is too small to affect the lasing characteristics, but introduces a significant uncertainty in the position of the facet relative to the end of the gain section. Fig. 3 shows that, for a misalignment of 70.051, the run-out distance across a 1 cm facet length is as large as 79 mm. Run-out severely impacts the manufacturing yield of laser arrays without passive waveguides; on the other hand, by incorporating suitable lengths of passive waveguides, the yield of arrays can be increased dramatically. 4. Packaging alignment Incorporating passive waveguides brings further yield benefits at the packaging stage. Semiconductor lasers are
among the most efficient at converting energy to light, with wall-plug efficiencies as high as 50%. However, the remaining energy is dissipated as heat into a very small volume. A low thermal resistance is therefore required between the active region and a heatsink, so the entire active section must be in close proximity to a subcarrier. The beam divergence of the laser beam imposes further constraints on the alignment. A FWHM of 301 in the vertical plane is common for a laser operating around 800–1000 nm. Taken together, these constraints mean that the laser must be aligned to the subcarrier with great precision. This constraint is particularly serious for an array, when a facet with a length of 410 mm has to be aligned with an angular precision of o0.11 to the subcarrier. Again the incorporation of passive waveguides allows these constraints to be relaxed; little heat is generated in the passive sections, so the passive waveguides can safely overhang the subcarrier (Fig. 4).
5. V-profile In addition to a high kink-free output power, desirable features of the output beam include small beam divergence and a reasonably symmetric beam profile. However, conventional epi-structure designs as in Fig. 5(a) result in large beam divergence and asymmetry of the far-field in the two orthogonal planes, typically about 301 in the vertical direction and 5–101 in the horizontal direction [8]. Another problem with RWG lasers using a conventional epitaxial structure is low kink-free power. The power kinks are caused by higher order mode lasing at high injection current levels. In order to reduce beam divergence without much impact on optical overlap, unconventional cladding structures in which higher refractive index layers are inserted in both the lower and upper cladding layers have been proposed [9,10]. The reported far-field can be as low as 211, however our simulation shows that this approach puts a very tight requirement on growth tolerance, as even a slight deviation from the design specification results in a very different laser performance. Indeed, our modeling shows the mole fraction of aluminum must be controlled to
ARTICLE IN PRESS J.H. Marsh / Journal of Crystal Growth 288 (2006) 2–6
within 1.3% in the case of AlGaAs-based laser structures, if the laser far-field is to be within 5% of that specified. We have proposed a new epitaxial structure [11,12] in which a graded far-field reduction layer (FRL) is inserted in the lower cladding (see Fig. 5(b)) to reduce the far-field and suppress higher mode lasing. Instead of using a layer with a constant material composition, hence constant refractive index, a graded V-profile is used. Simulations show that this structure leads to considerable improvement in growth tolerance. As a consequence, the FRL can be designed to reduce the beam divergence and to suppress higher mode lasing simultaneously. The vertical far-field can be reduced to about 171, and higher mode lasing can be greatly suppressed or even completely eliminated. For 830 nm lasers, whose epi-structure included a graded FRL
5
similar to that in Fig. 5(b), the measured kink-free power was typically 4400 mW (Fig. 6). 6. Array performance Full 3 in wafer processing using state-of-the-art photolithography and dry-etching techniques, in combination with NAMs and the V-profile laser design, allows very large element arrays of high-performance single-mode lasers to be fabricated with high yield. The combination of QWI and the V-profile design has been optimized to give extremely good uniformity across arrays containing up to 100 individually addressable lasers. Laser arrays have been die-bonded, using a hard solder process, to a metallic substrate. Laser kink power and output power at a
Fig. 5. Diagram of (a) conventional laser structure and (b) novel structure containing V-profile far-field reduction layer.
Fig. 6. Measured characteristics of 830 nm laser containing FRL: (a) light-current (L–I) curve and (b) horizontal and vertical far-field patterns.
Fig. 7. (a) Laser kink power and constant-current power measured under continuous-wave operating conditions for each element in a 60-element laserdiode array. (b) Threshold power and horizontal and vertical far-field FWHM show excellent performance for each laser in the multi-element array.
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J.H. Marsh / Journal of Crystal Growth 288 (2006) 2–6
constant current of 300 mA have been measured under continuous-wave operating conditions for each element in a 60-element laser-diode array. Fig. 7(a) shows the kink power lies just below 300 mW, with excellent uniformity, and the constant-current power is 225 mW. The threshold current and vertical and horizontal far-field divergences shown in Fig. 7(b) also exhibit excellent performance and uniformity for each laser in the multi-element array. 7. Conclusions Recent developments in integrated optics and semiconductor processing have enabled large monolithic arrays of individually addressable semiconductor lasers to be realized for the first time. Such arrays are likely to find increasing application in areas such as printing, material processing and life sciences. The output power of individual elements can be 4400 mW, with many tens of elements on a single bar. Acknowledgments I am particularly grateful to all my colleagues at Intense who have contributed to this work, in particular B.C. Qiu, M. Silver, S. Najda, O.P. Kowalski, S. D. McDougall, X.F. Liu, G. Bacchin and C.J. Hamilton.
The author is on secondment to Intense Ltd. from the University of Glasgow.
References [1] J.H. Marsh, Semicond. Sci. Technol. 8 (1993) 1136. [2] Y. Suzuki, Y. Horikoshi, M. Kobayashi, H. Okamoto, Electron. Lett. 20 (1984) 383. [3] J.K. Lee, K.H. Park, D.H. Jang, H.S. Cho, C.S. Park, K.E. Pyun, J. Jeong, IEEE Photon. Technol. Lett. 10 (1998) 1226. [4] K. Hiramoto, M. Gasawa, T. Kikawa, S. Tsuji, IEEE J. Select. Top. Quant. Electron. 5 (1999) 817. [5] O.P. Kowalski, C.J. Hamilton, S.D. McDougall, J.H. Marsh, A.C. Bryce, R.M. De La Rue, B. Vo¨gele, C.R. Stanley, C.C. Button, J.S. Aitchison, Appl. Phys. Lett. 72 (1998) 581. [6] C.L. Walker, A.C. Bryce, J.H. Marsh, IEEE Photon. Technol. Lett. 14 (2002) 1394. [7] M. Silver, O.P. Kowalski, I. Hutchinson, X. Liu, S.D. McDougall, J.H. Marsh, CLEO 2005, Baltimore, 22–28 May 2005, Paper CMX2. [8] H. Horie, H. Ohta, T. Fujimori, IEEE J. Select. Top. Quant. Electron. 5 (1999) 832. [9] S.T. Yen, C.P. Lee, Opt. Quant. Electron. 28 (1996) 1229. [10] G.W. Yang, J.Y. Xu, Z.T. Xu, L.H. Chen, Q.M. Wang, Proc. SPIE 2886 (1996) 258. [11] S.P. Najda, G. Bacchin, B. Qiu, X. Liu, O.P. Kowalski, M. Silver, S.D. McDougall, C.J. Hamilton, J.H. Marsh, Proc. SPIE 5365 (2004) 1. [12] B.C. Qiu, S.D. McDougall, X.F. Liu, G. Bacchin, J.H. Marsh, IEEE J. Quant. Electron. 41 (2005) 1124.
Journal of Crystal Growth 288 (2006) 2–6 www.elsevier.com/locate/jcrysgro
The role of monolithic integration in advanced laser products John H. Marsh Intense Ltd, 4 Stanley Blvd, Hamilton International Technology Park, Blantyre, Glasgow G72 0BN, UK Available online 20 January 2006
Abstract The design and performance of single-mode high-power (4100 mW) semiconductor lasers suitable for integration into large arrays are reported. In 830 nm lasers, quantum well intermixing (QWI) has been used to increase the bandgap of the waveguide in the facet region by 120 meV, and the catastrophic optical damage threshold of uncoated devices increased by a factor of 43 as a result. The passive waveguides are relatively cool, bringing high reliability, improving the single-mode waveguide stability and enabling high-temperature operation. Furthermore, the passive waveguides relax the cleaving and packaging alignment tolerances, giving a high yield process suitable for manufacture. A far-field reduction layer is included in the lasers giving a fast axis divergence of o201 FWHM. Arrays in which each emitter operates at several 100 mW, have excellent uniformity of laser parameters such as kink power, operating power and optical beam profile. r 2005 Elsevier B.V. All rights reserved. PACS: 42.55.Px Keywords: A1. Nanostructures; B2. Semiconducting gallium arsenide; B3. Laser diodes
1. Introduction High-power single-mode semiconductor lasers have long had many applications including material processing, printing and optical pumping. Recently device reliability and processing yield has increased to the point where it is feasible to manufacture large arrays of single-mode devices, each operating at a power of several 100 mW. Individually addressable arrays of such lasers are currently driving new developments in digital image transfer including industrial printing and high-quality thermal printing. This paper addresses the technology developments that have made it possible to realize monolithic arrays containing up to 100 emitters. The primary challenge in creating semiconductor laser arrays is to develop a laser design that can deliver a power of several 100 mW, reliably, in a single-transverse mode, and that is robust against manufacturing tolerances. The work described here uses quantum well intermixing (QWI) to create relatively long (4100 mm) passive waveguides Fax: +44 1698 827262.
E-mail address: [email protected]. 0022-0248/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2005.12.021
immediately adjacent to each facet of a ridge waveguide laser. As a result of the QWI processing, the passive waveguides have a larger bandgap than the gain section so raising the threshold power at which catastrophic optical damage (COD) of the facet mirror occurs. The length of the passive waveguides enables large monolithic arrays to be cleaved from a wafer with high yield and also relaxes packaging tolerances as the passive waveguide sections can overhang the laser carrier. A novel ‘V-profile’ layer has also been included in the structure to reduce the fast axis farfield divergence and to make the laser performance less vulnerable to processing variations across the wafer and from wafer to wafer. 2. Catastrophic optical damage and quantum well intermixing COD is the main failure mode of high-power semiconductor lasers operating in the 650–1000 nm wavelength range, and is known to arise from a thermal run-away process at the laser facet. The semiconductor crystal terminates at the facet, and the resulting dangling bonds give rise to defect states. Electrons and holes recombine via
ARTICLE IN PRESS J.H. Marsh / Journal of Crystal Growth 288 (2006) 2–6
these states, emitting heat instead of light. As the facet heats up relative to the bulk of the device, the local bandgap decreases leading to two effects: firstly, a potential gradient which causes carriers to drift towards the facets, and secondly, increased optical absorption at the facet. Both these factors lead to increased non-radiative recombination, and hence to a further rise in the local temperature. Ultimately, this thermal run-away effect leads to the irreversible optical damage known as COD. The optical intensity at which this occurs is approximately 2 106 W cm 2; for a mode size of 1 3 mm2 this implies an output power of only 60 mW. By integrating passive waveguides in the facet region, the facet becomes a nonabsorbing mirror (NAM). NAMs prevent COD in two ways: by restricting the number of carriers that drift from the bulk of the laser to the facet region and by reducing band-to-band absorption facet region. QWI is by far the most common technique for fabricating the passive waveguide NAMs. In QWI, point defects are generated at or close to the semiconductor surface; diffusion of these point defects at elevated temperatures results in the intermixing of the wells with the barriers, and a consequent increase in the bandgap energy [1], so creating a passive waveguide. QWI is a regrowth-free process, and the resulting active/passive interfaces are defect-free and in perfect alignment with the gain section. Impurity-induced disordering using Zn or Si is a widely used QWI process, and the first reported NAM laser [2], and more recent reports [3,4] used these impurities. However, Zn and Si are electrically active, so their use leads to free-carrier absorption losses and current leakage, both of which are undesirable in high-performance NAMs. An alternative approach relies on the fact that vacancies are created during sputter deposition of SiO2 [5]. The vacancies diffuse through the semiconductor and, as a result, individual atoms hop from one lattice site to another so intermixing the QW with the adjacent barrier material. The resulting passive waveguides have low optical propagation losses (few dB cm 1) and uncoated NAM lasers have been demonstrated to have a greater COD level than standard lasers by a factor of 2.6 [6]. More detailed theoretical and experimental studies have been carried out [7] to quantify the benefits of NAMs by separating the role of excluding the carriers from the facet region (non-injection mirror (NIM)) and the increase in the local band-gap at the facet (NAM). The results were compared with conventional lasers, which had no special facet region processing. The laser consisted of a QW epistructure emitting at 830 nm, which was fabricated into ridge-waveguide (RWG) lasers. The lasers had a total cavity length of 1700 mm and the waveguides were 2 mm wide. A schematic of the device is shown in Fig. 1. Lasers were fabricated with (i) conventional facets, (ii) NIMs where the injecting contact was not present close to the facet, and (iii) NAMs, which were bandgap widened and the injecting contact was not present close to the facet.
3
QWI was carried out prior to the ridge-etch stage to increase the bandgap in the NAM regions, whilst maintaining the original bandgap in the active/gain regions. The differential bandgap shift between the active section and NAM varied from 45 to 65 nm (85–120 meV). Devices were fabricated using a mask-set with designs for conventional lasers, NIM only devices, and NAM/NIM lasers. Devices were cleaved into bars and tested. Fig. 2 shows a selection of CW light–current (L–I) curves from bars of uncoated devices with (a) conventional facets, (b) NIMs only, (c) NAM/NIMs with 45 nm bandgap shift, and (d) NAM/ NIMs with 65 nm shift. The L–I curves show that the conventional laser has a COD power of 80 mW, the NIM laser of 125 mW, and the NAM/NIM lasers of 200 mW (for 45 nm NAM shift) and 4300 mW (for 65 nm shift). In the latter case the L–I curve rolls over before COD occurs. The experimental results are in good qualitative agreement with the theoretical calculations [7].
Fig. 1. Schematic of the ridge waveguide lasers showing the nonabsorbing mirror/non-injection mirror (NAM/NIM) regions.
Fig. 2. CW light/current characteristics of 830 nm un-coated ridge waveguide lasers with (a) no-NAM/NIM (i.e. a conventional laser), (b) NIM only, (c) a NAM/NIM with a 45 nm bandgap shift, and (d) a NAM/NIM with 65 nm shift.
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J.H. Marsh / Journal of Crystal Growth 288 (2006) 2–6
Fig. 4. Laser array mounted on subcarrier. Because no heat is generated in passive waveguide sections, the chip can overhang the subcarrier.
Fig. 3. Laser waveguides are not exactly orthogonal to the cleavage planes. For an alignment tolerance of 70.051, the run-out across a 1 cm facet length is 79 mm.
3. Passive waveguides and yield The facets of the vast majority of semiconductor lasers are formed by breaking the semiconductor crystal along a cleavage plane. This gives an extremely flat mirror surface, but cleaving is a mechanical step whose precision is difficult to control. The typical precision to which cleaving can be carried out is around 75 mm. For conventional lasers, the cleave has to be located o10 mm from the end of the gain contact, but in a laser with passive waveguides the cleaving position only needs to fall somewhere within the passive region. The inclusion of passive waveguides therefore relaxes the cleaving tolerance. Angular variations in the cleaving direction have a large impact on yield, especially for arrays. During photolithography, laser waveguides are aligned to the crystallographic orientation of the wafer by reference to the major flat. On wafers with high precision flats, the tolerance with which the major flat is aligned to the crystal plane is 70.011. The lithography system also introduces error, giving a total alignment variation of 70.051. However, the facets are formed by cleaving along the crystal planes, so they are precisely aligned with the crystallographic orientation and are not exactly perpendicular to the laser waveguides. The misorientation is too small to affect the lasing characteristics, but introduces a significant uncertainty in the position of the facet relative to the end of the gain section. Fig. 3 shows that, for a misalignment of 70.051, the run-out distance across a 1 cm facet length is as large as 79 mm. Run-out severely impacts the manufacturing yield of laser arrays without passive waveguides; on the other hand, by incorporating suitable lengths of passive waveguides, the yield of arrays can be increased dramatically. 4. Packaging alignment Incorporating passive waveguides brings further yield benefits at the packaging stage. Semiconductor lasers are
among the most efficient at converting energy to light, with wall-plug efficiencies as high as 50%. However, the remaining energy is dissipated as heat into a very small volume. A low thermal resistance is therefore required between the active region and a heatsink, so the entire active section must be in close proximity to a subcarrier. The beam divergence of the laser beam imposes further constraints on the alignment. A FWHM of 301 in the vertical plane is common for a laser operating around 800–1000 nm. Taken together, these constraints mean that the laser must be aligned to the subcarrier with great precision. This constraint is particularly serious for an array, when a facet with a length of 410 mm has to be aligned with an angular precision of o0.11 to the subcarrier. Again the incorporation of passive waveguides allows these constraints to be relaxed; little heat is generated in the passive sections, so the passive waveguides can safely overhang the subcarrier (Fig. 4).
5. V-profile In addition to a high kink-free output power, desirable features of the output beam include small beam divergence and a reasonably symmetric beam profile. However, conventional epi-structure designs as in Fig. 5(a) result in large beam divergence and asymmetry of the far-field in the two orthogonal planes, typically about 301 in the vertical direction and 5–101 in the horizontal direction [8]. Another problem with RWG lasers using a conventional epitaxial structure is low kink-free power. The power kinks are caused by higher order mode lasing at high injection current levels. In order to reduce beam divergence without much impact on optical overlap, unconventional cladding structures in which higher refractive index layers are inserted in both the lower and upper cladding layers have been proposed [9,10]. The reported far-field can be as low as 211, however our simulation shows that this approach puts a very tight requirement on growth tolerance, as even a slight deviation from the design specification results in a very different laser performance. Indeed, our modeling shows the mole fraction of aluminum must be controlled to
ARTICLE IN PRESS J.H. Marsh / Journal of Crystal Growth 288 (2006) 2–6
within 1.3% in the case of AlGaAs-based laser structures, if the laser far-field is to be within 5% of that specified. We have proposed a new epitaxial structure [11,12] in which a graded far-field reduction layer (FRL) is inserted in the lower cladding (see Fig. 5(b)) to reduce the far-field and suppress higher mode lasing. Instead of using a layer with a constant material composition, hence constant refractive index, a graded V-profile is used. Simulations show that this structure leads to considerable improvement in growth tolerance. As a consequence, the FRL can be designed to reduce the beam divergence and to suppress higher mode lasing simultaneously. The vertical far-field can be reduced to about 171, and higher mode lasing can be greatly suppressed or even completely eliminated. For 830 nm lasers, whose epi-structure included a graded FRL
5
similar to that in Fig. 5(b), the measured kink-free power was typically 4400 mW (Fig. 6). 6. Array performance Full 3 in wafer processing using state-of-the-art photolithography and dry-etching techniques, in combination with NAMs and the V-profile laser design, allows very large element arrays of high-performance single-mode lasers to be fabricated with high yield. The combination of QWI and the V-profile design has been optimized to give extremely good uniformity across arrays containing up to 100 individually addressable lasers. Laser arrays have been die-bonded, using a hard solder process, to a metallic substrate. Laser kink power and output power at a
Fig. 5. Diagram of (a) conventional laser structure and (b) novel structure containing V-profile far-field reduction layer.
Fig. 6. Measured characteristics of 830 nm laser containing FRL: (a) light-current (L–I) curve and (b) horizontal and vertical far-field patterns.
Fig. 7. (a) Laser kink power and constant-current power measured under continuous-wave operating conditions for each element in a 60-element laserdiode array. (b) Threshold power and horizontal and vertical far-field FWHM show excellent performance for each laser in the multi-element array.
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J.H. Marsh / Journal of Crystal Growth 288 (2006) 2–6
constant current of 300 mA have been measured under continuous-wave operating conditions for each element in a 60-element laser-diode array. Fig. 7(a) shows the kink power lies just below 300 mW, with excellent uniformity, and the constant-current power is 225 mW. The threshold current and vertical and horizontal far-field divergences shown in Fig. 7(b) also exhibit excellent performance and uniformity for each laser in the multi-element array. 7. Conclusions Recent developments in integrated optics and semiconductor processing have enabled large monolithic arrays of individually addressable semiconductor lasers to be realized for the first time. Such arrays are likely to find increasing application in areas such as printing, material processing and life sciences. The output power of individual elements can be 4400 mW, with many tens of elements on a single bar. Acknowledgments I am particularly grateful to all my colleagues at Intense who have contributed to this work, in particular B.C. Qiu, M. Silver, S. Najda, O.P. Kowalski, S. D. McDougall, X.F. Liu, G. Bacchin and C.J. Hamilton.
The author is on secondment to Intense Ltd. from the University of Glasgow.
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