Astigmatism and beam quality of high-brightness tapered diode lasers

Transcription

1 Astigmatism and beam quality of high-brightness tapered diode lasers M. T. Kelemen *, J. Weber, S. Kallenbach, C. Pfahler, M. Mikulla, and G. Weimann Fraunhofer Institute for Applied Solid State Physics, Tullastr. 72, D Freiburg, Germany ABSTRACT Semiconductor lasers with high beam quality and high optical output power are very attractive for a variety of applications such as molecular spectroscopy, fiber optic communication and frequency conversion. In the used power regime, devices based on tapered gain sections are the most promising candidates to reach these demands. However, two disadvantages of the tapered laser concept are the reduced output power provoked by their additional resonator losses and the astigmatism of these diode lasers. In case of high brightness diode lasers it is important to discuss the methods needed for an advanced output power also from the point of view of beam quality. The knowledge about astigmatism is essential for designing micro-optics. For the experimental results low modal gain, single quantum well InGaAs/AlGaAs devices emitting at 980 nm were grown by molecular beam epitaxy. The influence of the thermal resistance and of the tapered section length on the output power as well as on the beam quality has been investigated. In addition the impact of these parameters on the astigmatism of tapered diode lasers has been analysed. The experimental results have been correlated with simulations of the current-power curves and BPM simulations of the nearfield behaviour. Keywords: High-Brightness Diode-Laser, High-Power Diode-Laser, Tapered Laser, Astigmatism, Beam Quality 1. INTRODUCTION High-power diode lasers are finding more and more applications such as molecular spectroscopy, fiber optic communication systems and nonlinear frequency conversion e.g. for laser TV. Most of these applications require high output power in combination with diffraction limited beam quality. Today broad-area diode lasers are used to achieve high outputs. But standard broad-area waveguide designs are susceptible to modal instabilities, filamentation and catastrophical optical mirror damage (COMD) failure. This results in low beam qualities and values for the brightness limited around 10 MW/(cm 2 sr). On the other hand high beam qualities are realized with ridge-laser emitting in diffraction limited optical beam. The reliable output power of these lasers is mainly limited by the onset of facet degradation that depends on the power density on the facet [1]. Due to the small stripe width of a few microns, the output power is limited to about 350 mw [2] resulting in a brightness of less than 30 MW/(cm 2 sr). Especially, for applications requiring fibre coupling, high brightness sources, this means sources with high output power in combination with high beam quality, are under development. We have shown previously that the tapered diode laser concept leads to a brightness of more than 270 MW/(cm 2 sr) for wavelengths around 1 µm [3]. However, one disadvantage in comparison to state-of-the-art broad-area diode lasers is the reduced output power provoked by additional optical loss mechanism. So commercially only tapered diode lasers up to 2 W together with a nearly diffraction limited behaviour are available. Within this power regime as a second disadvantage a highly astigmatism behaviour complicates the design of micro-optics and constricts the application of tapered diode laser systems in many cases. Here we want to discuss concepts for a remarkable improvement of the output power of tapered diode lasers. We have reached an enhancement of more than 50% of the output power up to output powers over 4 W at 6 A. Because tapered diode lasers are high brightness diode lasers it is important to discuss the methods needed for an advanced output power also from the point of view of beam quality. In addition the influences on the astigmatism as the third important parameter of tapered diode lasers has been analysed. Semiconductor Lasers and Laser Dynamics, edited by Daan Lenstra, Geert Morthier, Thomas Erneux, Markus Pessa, Proceedings of SPIE Vol (SPIE, Bellingham, WA, 2004) X/04/$15 doi: /

2 For the experimental results low modal gain, single quantum well InGaAs/AlGaAs devices emitting at 980 nm were grown by molecular beam epitaxy. The lateral design consists of a tapered and a ridge-waveguide section. The experimental results concerning the output power and the nearfield profiles have been correlated with BPM simulations. 2. LASER STRUCTURE The fabrication of high power lasers with high conversion efficiency requires an epitaxial layer sequence with both, low internal losses (< 2 cm -1 ) and high internal conversion efficiency (> 0.9). The reduction of the internal losses can be achieved by broadening the waveguide layers [4]. This reduces the overlap of the optical mode with the highly doped cladding layers. For this purpose we have grown a laser structure with a large optical cavity by molecular beam epitaxy (MBE). The epitaxial layer sequence is similar to those reported in [5]. The active region consists of a single InGaAsquantum well embedded in an 880 nm thick AlGaAs core region with 20 % Al content. The quantum well is 7 nm thick with a nominal In content of 20 %. The optical waveguide is surrounded by 1 µm thick AlGaAs claddings with 40 % Al. Si and Be have been used for n- and p-type doping, respectively. The doping concentrations start at a level of 5x10 17 cm -3 near the core and increase to a level of 2x10 18 cm -3 in the outer cladding regions. The GaAs cap layer is heavily p-doped (6x10 19 cm -3 ) in order to reduce the contact resistance. The layer design exhibits an overlap of the fundamental optical mode with the quantum well of 1.3 %. We have shown previously that this low modal gain epitaxial layer structure suppresses beam filamentation in tapered laser oscillators and tapered laser amplifiers [6]. As a further advantage of this layer sequence, 95 % of the optical power is concentrated in the undoped core layers and the overlap of the fundamental mode with the doped cladding layers is only 5 %. As a result, low internal losses of 1.5 cm -1 are obtained from Fabry-Perot laser diodes of different lengths. The high material quality of the MBE-grown laser structures yields a high internal efficiency of more than 90 %. The use of high-band gap (E g = 1.68 ev) AlGaAs core layers with 20 % of Al content leads to a strong carrier confinement. This results in laser diodes with relatively temperature insensitive characteristics. Experimentally, a characteristic temperature of T 0 = 160 K is observed for broad area diode lasers in the temperature range between 15 C and 80 C. The lateral design consists of a tapered and a ridge-waveguide section and is schematically shown in fig. 1. Both have been fabricated in a standard process [6]. In addition to a low modal gain, high beam quality requires a ridge-waveguide structure with a width W 1 of about 3 µm and a ridge length L 1 of 500 µm [7]. In contrast, high output powers need a broad pumped area, which is provided by a tapered section [6]. The tapered section length has been varied in the following between 1.5 mm and 2.5 mm. The tapered angle is 6. W 2 L 1 y z d W 1 x active region Fig. 1. Schematic of a tapered diode laser with a ridge waveguide for mode filtering. The length of the ridge section L 1 is 500 µm, whereas the length of the tapered section can be varied between 1.5 mm and 2.5 mm. The tapered angle is Proc. of SPIE Vol. 5452

3 The rear facets are coated with a highly reflective double-stack of Si and SiO 2 films (> 97 % reflectivity) and the front facets are anti-reflection coated by a single layer of SiN (0.6 % reflectivity). The devices were finally mounted junction side down on different copper heat sinks characterized by their thermal resistances. The reasons for different thermal resistances are different designs and volumes of the copper mounts. Uniform pumping of the laser medium is achieved by current injection using homogenously spread bond wires. 3. SIMULATION In broad-area devices, beam filamentation is the main effect that limits the device performance when high beam quality is required [8]. With increasing power level, spatial hole burning occurs due to the interaction between the amplified optical field and the carrier density in the active region. Therefore, the complex index of refraction becomes inhomogeneous, leads to self-focusing of the optical wave and results in filament formation that severely deteriorates the beam quality. The dependence of the complex index of refraction neff ( n C ) on the carrier density n c of the active layer is given by the following equation [6]: 1 i n ( n ) = n g ( n ) α + g ( n ) 2k 2k [ α ] eff C eff,0 m C H m C int In this equation, g m (n c ) = Γg(n c ) is the modal optical gain given by the product of the material gain g(n c ) and the optical confinement factor Γ, which is determined by the overlap between the vertical mode profile and the active material. k is the vacuum wavenumber, n eff,0 is the carrier independent part of the optical index and α int are the optical losses, respectively. In order to achieve lateral coherence and suppress filament formation in semiconductor lasers to reach high brightness, three principles are crucial: The differential optical index neff ( nc ) is proportional to the linewidth enhancement factor α H. So by finding ways to lower α H, it is possible to reduce the sensitivity against self-focusing and filamentation. E.g. diode lasers based on quantum dot structures have shown to posses an inherently low linewidth enhancement factor. Minimizing the optical losses α opt, especially the resonator and geometrical losses. As has been theoretically and experimentally analyzed in [5,9], the differential optical index neff ( nc ) is proportional to the confinement factor squared. So a reduced confinement factor also reduces the variation of the optical index due to spatial-hole burning. Within this work the last point has been actually realized through epitaxial design. The dependence of the beam quality and of the linewidth enhancement factor on the tapered design has been analysed elsewhere [10]. In this work we have varied the length of the tapered section and therefore we have changed the optical losses systematically. In the case of a tapered diode laser the optical losses consist of the internal optical losses α i, the resonator losses α R and additionally a geometrical part (as additional resonator losses) due to the tapered resonator design and are well described by [5]: 1 W α opt = α i + αr - ln 2(L + L ) 4λL 2 1 n eff Fig. 2 gives an impression of the magnitude of the different parts of the optical losses of a tapered diode laser in dependence on the tapered section length. In general (α i = const) the different parts of the optical losses become smaller with longer tapered section lengths. Proc. of SPIE Vol

4 losses (1/cm) internal losses geometrical losses resonator losses taper section length (µm) Fig. 2. Calculated losses of tapered diode lasers in dependence of the tapered section length. The optical losses consist of internal losses, geometrical losses and resonator losses. 4.0 broad-area diode laser tapered diode laser T = 20 C, cw 60 output power (W) conversion efficiency (%) Fig. 3. Comparison of the electro-optical characteristics of a tapered diode laser and a broad-area diode laser. All measurements have been done at a heatsink temperature of 20 C in cw operation. Both diodes have been mounted on standard c-mounts. The design parameters are given in section Proc. of SPIE Vol. 5452

5 The simulation program used is based on a Fox and Li type resonator calculation taking into account the nonlinear semiconductor medium between rear and front facet. To reduce the 3-dimensional problem to two dimensions the effective index approximation was used. Because of the large angles in the tapered diode laser design the widely used paraxial approximation is not sufficient any longer and the ì wide-angleî (1,1)-PadÈ approximation for the Helmholtz equation is used. Further details about the program ì Diosimî and the used parameters can be found elsewhere [12]. 4. ELECTRO-OPTICAL CHARACTERISATION Brightness is proportional to the output power divided by the beam propagation factors M 2 in vertical and parallel direction. So one way to reach a higher brightness is to increase the output power. To illustrate the main problems of tapered diode lasers concerning the output power, a comparison between a tapered laser (TL) and a broad-area diode laser (BA) has been shown in fig. 3. The TL consists of a ridge section length of 500 µm and a tapered section length of 2 mm. The broad-area diode laser has a resonator length of 2 mm. Both have the same vertical structure given in [13] emitting at 975 nm and both have the same width of the emitting facet. Table 1 summarize the main characterization data for both types of diode lasers. tapered laser broad-area laser I th (A) s.e. (W/A) η 4Α (%) P 4A (W) P loss (W) α opt (cm -1 ) T Table 1: Comparison between tapered diode laser and broad-area diode laser according to fig. 3. The optical losses α opt have been calculated from the design parameters. All values except T 0 have been measured at a heatsink temperature of 20 C in cw operation. Both diodes have been mounted on standard c-mounts. In order to increase the brightness it is necessary to reach output powers which are comparable to broad-area diode lasers. From table 1 two main problems are visible: (1) Because of the additional loss mechanism of tapered diode laser mentioned in the last section the TL laser has a threshold current which is nearly two times the threshold current of the BA laser. (2) The thermal dissipation energy P loss of the TL laser is also nearly twice as much as for the BA laser due to a smaller pumped area. Together with a dramatically reduced value for T 0 for the TL laser this leads to a higher internal laser temperature and so to a higher threshold current and lower slope efficiency (s.e.). From this point of view two concepts should be crucial to improve the output power of tapered diode lasers significantly: (1) A longer resonator length will blow up the area for cooling and the dissipation energy density will drop down. In addition for a constant tapered angle of 6 a longer resonator length leads to a broader facet width allowing for higher output powers. (2) The thermal management plays an important role for TL laser. So the packaging, especially the heatsinks, should be improved to reduce their thermal resistance. In fig. 4 (part a) some simulated power-current curves for different thermal resistances are shown. As initial parameters for the simulation the values of table 1 have been used. At the second part of fig.4 (part b) some experimental currentpower curves of TL laser with improved heatsink designs are shown. The thermal resistances used for the simulations have been measured from the diodes used in part b and can be calculated by measuring the wavelength shift with temperature and the wavelength shift with the current in pulse mode. Proc. of SPIE Vol

6 A thermal resistance of 7.1 K/W corresponds to the standard c-mount concept, mostly used today for tapered lasers. As one can see, a reduction of the thermal resistance down to 1.9 K/W leads experimentally to output powers up to 4.2 W (at 6 A) instead of 2.8 W before. This means an improvement of 50% in output power only by changing the heatsink design and volume. output power (W) simulation = 0 K/W = 1.9 K/W = 4.4 K/W = 7.1 K/W experiment T = 20 C, cw 1 (a) current(a) (b) Fig. 4. (a) Simulation of current-power curves for tapered diode laser for different values of thermal resistances. (b) Measured current-power curves for different heatsink designs. All measurements have been done at a heatsink temperature of 20 C in cw operation. output power (W) simulation T 0 = 70 K 10 T 1 = 500 K 3.5 mm = 2 K/W 3.0 mm 2.5 mm 2.0 mm mm (a) experiment 2.5 mm 2.0 mm 1.5 mm (b) Fig. 5. (a) Simulation of current-power curves for tapered diode laser for different lengths of the tapered section. (b) Measured current-power curves for different lengths of the tapered section. All measurements have been done at a heatsink temperature of 20 C in cw operation. 238 Proc. of SPIE Vol. 5452

7 The effect of using longer tapered sections has been demonstrated in fig. 5. Again on the left side some simulated power-current curves have been shown in comparison to some experimentally realized tapered designs on the right side. Up to now tapered section lengths up to 2.5 mm have been processed. Longer tapered section lengths are in progress. The length of the ridge section was 500 µm. From the experiment a criteria for a ì quasi COMDî in cw mode could be defined. There is a typical current of sudden mirror death corresponding linearly to the different facet widths of the TL lasers with the different tapered section lengths. From that argument one can define a maximum output power of W per µm facet widths. This number could be used as a ì break off argumentî for the simulation. The simulated power-current curves for the tapered section lengths between 1.5 mm and 2.5 mm are in very good agreement with the experimental data. Finally for a tapered section length of 3.5 mm a maximum output power of over 9 W could be predicted from these calculations. 5. BEAM QUALITY In the last section we have demonstrated that longer tapered sections lead to higher output powers. But the brightness will be only increased, if the beam quality remains constant at these higher output powers, too. So the influence of longer tapered section lengths on the beam quality should be analysed. By maintaining uniform thermal conductivity, filamentation may be avoided [14]. We have shown previously that a longer tapered section length up to 2.5 mm leads to a decrease of the beam quality parameter M 2 [7,10] for constant output power. In addition now we want to study filamentation effects for longer tapered section lengths. In fig. 6 there a comparison is shown between simulated and experimental nearfield profiles for two tapered section lengths of 1.5 mm and 2.5 mm. The curves have been simulated and measured for an output power of 2 W. For the simulation as well as for the experiment there is a remarkable shrinking of the filament growth visible for longer tapered section lengths. This behaviour is in good accordance with the predictions of section 3. So we can go a step forward to look for even longer tapered sections at higher output powers. (a) simulation (b) experiment 1.0 = 1.5 mm = 1.5 mm nearfield intensity (arbitrary units) = 2.5 mm = 2.5 mm lateral dimension (µm) Fig. 6. (a) BPM simulations of nearfield profiles for tapered diode lasers for tapered section lengths of 1.5 mm and 2.5 mm at an output power of 2 W. For the calculations an alpha-factor of 3 has been used. (b) Measured nearfield profiles for tapered diode lasers for tapered section lengths of 1.5 mm and 2.5 mm at an output power of 2 W. The measurements have been done at a heatsink temperature of 20 C in cw operation. All diodes have been mounted on standard c-mounts. Proc. of SPIE Vol

8 nearfield intensity (normalized units) simulation = 1.5 mm = 2.5 mm simulation = 2 mm = 3 mm lateral dimension (µm) Fig. 7. BPM simulations of nearfield profiles for tapered diode laser for different tapered section lengths between 1.5 mm and 3 mm at an output power of 4 W. For the calculations an alpha-factor of 3 has been used. For this purpose fig. 7 shows a simulation for nearfields in dependence on the tapered section length. The tapered section length has been varied between 1.5 mm and 3 mm. The ridge section length has been kept constant at 500 µm. The simulations have been done now for output powers of 4 W. In good accordance to the predictions of section 3 the period of the filaments grows with longer tapered section lengths. In addition the filament gain shrinks. So with increasing tapered section length the nearfield profile shows a more and more constant intensity profile. For tapered section lengths up to 3 mm a nearly rectangular behaviour has been observed in the simulation. 6. ASTIGMATISM At the output facet the curved wavefronts of tapered diode lasers diffract according to Snellís law. The beam is astigmatic, since in the direction perpendicular to the quantum well it diverges from the output facet, but in the plane of the quantum well it diverges from a virtual source that is approximately /n eff behind the output facet. Because n eff depends on the carrier density and the temperature, distortions of the carrier distribution or different thermal management concepts should have a remarkable influence on n eff and on the position of the virtual source. For the measurement, a fast lens is used to collimate the beam in the fast direction (the transverse direction perpendicular to the plane of the quantum well). Because of the astigmatism, the beam in this case is focused in the lateral plane, called corrected farfield. The distance between lens and corrected farfield provides a direct measure of the astigmatism using the simple lens equation and the known focal length f of the measurement lens. Similarly, the width of the waist is a measure of the width of the virtual image. In fig. 8 the astigmatism (distance between virtual source and facet) curves in dependence on the current for different design parameters are shown. On the left side (a) the tapered section length (L 1 = 500 µm), on the right side the ridge section length ( = 2 mm) have been varied. Whereas in the low current regime the ridge and the tapered section length 240 Proc. of SPIE Vol. 5452

9 have a remarkable influence on the position of the virtual source, visible in a change of the astigmatism, the influence of the design on the astigmatism has been decreased in the higher current regime. 0.8 = 2000 µm L 1 = 500 µm astigmatism (mm) = 1500 µm L 1 = 100 µm 0.3 (a) (b) Fig. 8. Astigmatism of tapered diode lasers in dependence on the current for different ridge- and tapered section lengths. All measurements have been done at a heatsink temperature of 20 C in cw mode. All diodes have been mounted on standard c-mounts = 4.4 K/W astigmatism = 7.1 K/W L 1 = 500 µm = 2 mm Fig. 9. Astigmatism of tapered diode lasers in dependence on the current for different thermal resistances. All measurements have been done at a heatsink temperature of 20 C in cw mode. The thermal resistance has been changed by using different heatsinks. Proc. of SPIE Vol

10 The astigmatism of tapered diode lasers is mainly temperature driven, as shown in fig. 9. A lower thermal resistance, leading to a better heat flow, results in a fast crossover into a flat curve. In contrast to this, higher values of the thermal resistance lead to a gradual changeover to a flat behaviour. As a results, the astigmatism remains nearly constant in the higher current regime respectively higher output regime. The same effect takes place by making the tapered section length longer. Here for longer tapered section lengths the power losses per area decrease, there is a better heat flow, and the astigmatism becomes more constant in a faster way. 7. CONCLUSION In conclusion we have shown that with the help of heatsinks with lower thermal resistances the output power of tapered diode lasers can be improved by 50% resulting in output powers of more than 4 W at 6 A for a tapered diode laser with a tapered section length of 2 mm and a ridge section length of 500 µm. Due to thermal simulations we can predict output powers of more than 9 W for a tapered section length of 3.5 mm. In good accordance to these simulations we have achieved output powers of around 7 W with a tapered section length of 2.5 mm. It has been demonstrated that the nearfield profile of a tapered diode laser shows less filamentation effects by making the tapered section length longer. Previously we have shown that in addition M 2 becomes smaller if the tapered section length grows. In addition it has been shown that for longer tapered section lengths the period of the filaments grows and the filament gain shrinks. The third important characteristic of tapered diode laser, the astigmatism, is mainly temperature driven. So by making the tapered section length longer as well as for decreasing the thermal resistance the dependence of the astigmatism on the current becomes more constant. This will be helpful for designing micro lenses for tapered diode laser systems. ACKNOWLEDGEMENT The authors gratefully acknowledge M. Walther, G. Bihlmann, R. Kiefer, J. Schleife, R. Moritz, P. Friedmann, N. Lehmann, B. Campillo-Lundbeck and W. Fehrenbach for perfect technical assistance. This work was supported by the German Federal Ministry of Education and Research. REFERENCES 1. A. Moser, E.E. Latta, Arrhenius parameters for the rate process leading to catastrophic damage of AlGaAs-GaAs laser facets, J. Appl. Phys., vol. 71, pp , G. Beister, F. Bugge. G. Erbert, J. Maege, P. Ressel, J. Sebastian, A. Thies, H. Wenzel, Monomode emission at 350 mw and high reliability with InGaAs / AlGaAs ridge waveguide laser diodes, Electron. Lett., vol. 34, pp. 778, M.T. Kelemen, J. Weber, F. Rinner, J. Rogg, M. Mikulla, and G. Weimann, High-brightness 1040-nm tapered diode lasers, SPIE Proc., vol. 4947, p , A. Al-Muhanna, L. J. Mawst, D. Botez, D. Z. Garbuzov, R. U. Martinalli, and J. C. Connolly, 14.3 W quasicontinuous wave front-facet power from broad-waveguide Al-free 970 nm diode lasers, Appl. Phys. Lett. 71, pp , M. Mikulla, P. Chazan, A. Schmitt, S. Morgott, A. Wetzel, M. Walther, R. Kiefer, W. Pletschen, J. Braunstein, and G. Weimann, High-Brightness Tapered Semiconductor Laser Oscillators and Amplifiers with Low-Modal Gain Epilayer-Structures, IEEE Photon. Techn. Lett., vol. 10, No. 5, pp , M. Mikulla, Tapered High-Power, High-Brightness Diode Lasers: Design and Performance, High-Power Diode Lasers, Topics Appl. Phys., vol. 78, pp , Proc. of SPIE Vol. 5452

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