Infrared semiconductor lasers for DIRCM applications

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1 Infrared semiconductor lasers for DIRCM applications J. Wagner a, N. Schulz a, B. Rösener a, M. Rattunde a, Q. Yang a, F. Fuchs a, C. Manz a, W. Bronner, a C. Mann a, K. Köhler a, M. Raab b, E. Romasev b, H. D. Tholl b a Fraunhofer-Institut für Angewandte Festkörperphysik, Tullastrasse 72, 7918 Freiburg, Germany; b Diehl BGT Defence, Alte Nussdorfer Strasse 13, Überlingen, Germany ABSTRACT We report on the development and characteristics of infrared semiconductor lasers as compact and robust light sources for Directed Infrared Countermeasures (DIRCM). The short-wavelength side of the 2-5 µm wavelength band of interest can be covered by GaSb-based optically pumped semiconductor disk lasers (OPSDLs), delivering a continuous-wave (cw) or temporally modulated multiple-watt output with a high beam quality (M 2 <3). For the µm wavelength range InP-based quantum cascade (QC) lasers are the best suited semiconductor laser source, delivering several hundreds of mw of average output power in a nearly diffraction limited output beam (M 2 <2). Further up-scaling of the output power can be achieved for OPSDLs by intra-cavity coupling of several semiconductor chips as gain elements in a multiple-disk cavity arrangement. For a 2.3 µm emitting dual-disk OPSDL, a doubling of the maximum roomtemperature output power compared to that of a comparable single-chip OPSDL has been demonstrated. For QC lasers power scaling by beam-quality-preserving beam combining has been demonstrated via polarization coupling of the output beams of two individual QC lasers, yielding a coupling efficiency of 82%. Keywords: Infrared semiconductor lasers, optically pumped semiconductor disk laser (OPSDL), vertical-external-cavity surface-emitting laser (VECSEL), power-scaling, laser beam coupling, directed infrared countermeasures (DIRCM) 1. INTRODUCTION Deception-class directed infrared countermeasure (DIRCM) systems call for compact and robust mid-infrared (2-5 µm wavelength range) laser sources which are capable of injecting false information into the tracking sensor of a heatseeking missile [1]. Performance requirements for such a laser source are average output powers in excess of 1 W up to the multiple Watt level combined with a high beam quality, characterized by a beam propagation factor M 2 <3. While currently solid-state laser based optical parametric oscillator (OPO) systems are most widely used and thus have to be considered as state-of-the-art, the output power levels reached by infrared semiconductor lasers is progressing rapidly, thus making this class of lasers a serious contender for future DIRCM systems. More specifically, as will be shown in detail below, the performance of state-of-the-art 2-5 µm emitting infrared semiconductor lasers is such that these lasers are now well suited to be implemented as jammers as well as for tracking in compact and lightweight DIRCM systems for use on e.g. small signature platforms [1]. The 2-3 µm wavelength range can be covered by conventional interband diode lasers based on the (AlGaIn)(AsSb) materials combination [2,3,4], delivering output powers well in excess of 1 W for broad-area single emitters [5] and >1 W for fiber-coupled linear arrays of broad area lasers [6] emitting at around 2 µm. In spite of this impressive output power performance, this type of semiconductor lasers suffers from an inherently low beam quality, which is not compatible with the above mentioned requirements for laser sources to be used in DIRCM systems. Narrow ridgewaveguide diode lasers are capable of emitting an almost diffraction limited output beam (M 2 1.5), but suffer from a significantly reduced output power of typically only 1 mw at 2 µm emission wavelength [5]. Tapered diode lasers to some extend combine the advantages of ridge-waveguide and broad-area diode lasers, delivering at an emission wavelength of 2 µm output powers up to 1.5 W with a simultaneously high beam quality (M 2 1.5) [7]. These lasers, however, feature a very asymmetric beam aperture, requiring rather complex beam shaping optics to achieve a symmetric collimated beam. They further exhibit a pronounced astigmatism, i.e. different location of the virtual source point for the fast-axis (perpendicular to the epitaxial layer plane) and the slow-axis (parallel to the epitaxial layer plane) orientation with respect to the output facet, which is also strongly current dependent [7]. Technologies for Optical Countermeasures V, edited by David H. Titterton, Mark A. Richardson Proc. of SPIE Vol. 7115, 7115A 28 SPIE CCC code: X/8/$18 doi: / Proc. of SPIE Vol A-1

2 Recently, the concept of the optically pumped semiconductor disk laser (OPSDL) [8], also known as optically pumped vertical external cavity surface emitting laser (VECSEL), has been extended also to wavelengths 2 µm [9]. This class of semiconductor laser combines attractive features of classical solid-state lasers, which are a high output power of up to several Watts with a high quality (1.1 M 2 5) circular output beam, with the wavelength versatility and tunability of a semiconductor laser. More details on 2 µm emitting OPSDLs will be given below in Section 2. For the wavelength range >4 µm InP-based quantum cascade (QC) lasers are the best suited semiconductor laser variant at present, delivering now at room-temperature average output powers up to >1 W range with a high-quality (M 2 <2), even though strongly divergent, output beam [1,11]. Data on QC lasers covering the µm and the µm wavelength bands will be given in Section 3. Furthermore, different concepts for power-scaling by beam-qualityconserving coupling of the output of several individual lasers or gain elements will be discussed and experimental data presented. 2. OPSDL COVERING THE µm SPECTRAL RANGE The basic layout of an OPSDL is shown in Fig. 1. The actual OPSDL chip is an epitaxially grown semiconductor heterostructure consisting of a semiconductor-based distributed Bragg reflector (DBR), which acts as a planar highly reflective end mirror of the OPSDL cavity, and a semiconductor active region grown on top of the DBR. The active region consists of thin layers of semiconductor material with a smaller energy gap, so called quantum wells (QWs), embedded between layers of a wider bandgap semiconductor, so called barriers. The QWs are appropriately placed at the antinodes of the optical standing wave formed in the semiconductor heterostructure. The barrier layers separating the QWs act simultaneously as pump absorbing layers. For µm emitting OPSDLs of interest here, the DBR is composed of alternating quarter-wave GaSb and AlAsSb layers while the QW layers consist of GaInAsSb and the barrier and pump absorbing layers of AlGaAsSb. The OPSDL cavity in its simplest form is terminated by an external concave output coupling mirror. The active region is optically pumped by high-power diode lasers emitting at shorter wavelength than the OPSDL. The pump light emitted from the diode laser pump module is focused onto the OPSDL chip by appropriate focusing optics, resulting in a pump spot on the chip surface with a diameter of a few hundreds of microns. The transversal mode pattern of an OPSDL is essentially determined by the respective external cavity configuration and is therefore independent of the actual semiconductor heterostructure. If the pump spot size is properly matched to the cavity mode diameter on the OPSDL chip surface a circular, nearly diffraction-limited output beam can be achieved. For high power operation, an optimized thermal management is essential to remove from the active region the excess heat generated by the impinging pump beam (a) due to the less than unity internal quantum efficiency of the gain medium and (b) due to the, for infrared ( 2 µm) OPSDLs particularly large, quantum deficit (i.e. the difference between the pump photon energy and that of the emitted laser photon). For a 2.25 µm emitting OPSDL pumped at 98 nm the quantum deficit is as high as 57%. This implies that, even if the internal quantum efficiency was unity, at an absorbed pump power of 1 W a heat-load of 5.7 W could be imposed on the OPSDL chip. Therefore, different approaches have been developed in order to provide an efficient heat removal [12,13]. OPSDLs with an optimized thermal management by using highly thermally conductive and infrared transparent intra-cavity heat-spreaders are capable of delivering several Watts of output power at wavelengths of µm [14,15]. Typical output power vs. absorbed pump power characteristics are shown in Fig. 2 for a 2.26 µm emitting OPSDL (see the lasing spectrum displayed in the inset of Fig. 2). A linear cavity was employed consisting of a curved output coupling mirror (radius of curvature: 1 mm, reflectivity R=95%) and the OPSDL chip with the DBR as a plane end mirror. A fiber-coupled diode laser module emitting at 98 nm served as pump source. The pump light delivered through a 1 µm core diameter optical fiber was focused onto the OPSDL chip through a transparent SiC intra-cavity heat-spreader using a combination of a collimating and a focusing lens. The approximate angle of incidence was 3 which resulted in a slightly elliptical pump spot with dimensions of 375 µm 425 µm on the OPSDL chip surface. A maximum output power of 3.4 W was recorded at a heat-sink temperature -1 C and an absorbed pump power of 21 W [15]. Apart from a slight decrease of the slope efficiency at elevated pump powers no pronounced thermally induced roll-over was observed; instead, the maximum output power was limited by the maximum pump power available. From the linear part of the power transfer curve measured at -1 C, a slope efficiency of 23.8% was calculated resulting in a differential quantum efficiency of 54.5%. For higher heat-sink temperatures, the slope efficiency slightly decreased to for instance Proc. of SPIE Vol A-2

3 17.6% at 2 C and the thermally induced rollover already occurred within the available range of pump powers. Nevertheless, we still observed a maximum output power of >1.6 W at 2 C. The beam quality was assessed by determining the beam propagation factor M 2 according to the International Organization for Standardization ISO procedure, which is based on the measurement of beam diameters defined by second order moments. Resulting values for beam propagation factors were M 2 ~5 in case the cavity was optimized for maximum output power and as low as M 2 ~1.5 if aligned for highest beam quality. In the latter case the output power dropped to 7% of its maximum value, resulting in a maximum brightness of ~21 MW/cm 2. The associated beam divergence angle is on the order of 1 full width at half maximum (FWHM). OPSDL Laser Output Heat Sink Transparent Heatspreader External Mirror Fig. 1: Schematic OPSDL set-up: The laser cavity is formed by a distributed Bragg reflector (DBR) integrated into the semiconductor OPSDL chip, which also contains the gain region, and an external out-coupling mirror. A pump laser is focused on the chip surface yielding a pump spot typically 5-5 µm in diameter. OPSDL CW Output Power (W) Intensity (a.u.) 2,24 2,26 2,28 Wavelength (µm) 2 C 1 C -1 C C Absorbed Pump Power (W) Fig. 2: Continuous-wave (cw) output power vs. absorbed pump power recorded at different heat-sink temperatures of an OPSDL, emitting at 2.25 µm. A SiC intra-cavity heat-spreader was used; output coupler reflectivity was 95 %. The inset shows a spectrum recorded at an output power of 2.7 W and a heat-sink temperature of C. A straightforward scheme for further up-scaling the output power of a thin disk laser is to increase the pumped area while holding the pump power intensity at a constant level. This approach can also be applied to semiconductor disk lasers. However, depending on the heat extraction scheme employed, there is an upper limit for the maximum pump spot Proc. of SPIE Vol A-3

4 size; i.e. above a certain pump spot diameter no further increase of the output power is observed. This behavior has been attributed to the fact that the relative contribution of the radial heat flux to the total heat removal capacity decreases with increasing pump spot size and eventually a transition from a two-dimensional to a one-dimensional heat flow pattern occurs [13]. As a result, heating of the active region increases and leads to a decrease in efficiency and thermally induced roll-over. In addition to these thermal aspects, it has been pointed out that power-scaling of (semiconductor) disk lasers by increasing the pumped area can also be limited by the onset of lateral amplified spontaneous emission (ASE) [16]. Another power-scaling scheme which has already been applied to solid state thin disk lasers as well as to near-infrared (~1 µm) OPSDLs is the use of several separately pumped gain chips in a common laser cavity. This approach has been successfully demonstrated for OPSDL structures emitting at a wavelength of 97 nm, yielding impressive results such as an output power of more than 19 W [17]. Here we report on the realization of a dual-chip OPSDL emitting at 2.25 µm with an output power of several Watts at room-temperature. The layout of the present dual-chip OPSDL is shown in Fig. 3. Two OPSDL chips were employed bonded to SiC intracavity heat-spreaders. The chips were cleaved from the same wafer as used for the above described single-chip OPSDL (data shown in Fig. 2). In order to coherently couple the two OPSDL chips a cavity in which one OPSDL chip (chip 1) served as an end mirror and the other one (chip 2) as a folding mirror (see Fig. 1) was used. An output coupler with a radius of curvature (ROC) of 15 mm was placed at a distance of ~14 mm with respect to chip 2. An HR coated folding mirror (ROC=1 mm) was positioned at a distance of 1 mm with respect to both, chip 1 and chip 2. We thus obtained equal fundamental cavity mode diameters on chip 1 and on chip 2, which could be easily controlled by changing the length of the resonator arm connecting chip 2 and the output coupler. Due to a small folding angle of 6 a negligible astigmatism was introduced by the use of a curved folding mirror. Chip 1 and chip 2 were pumped by two separate fibercoupled diode laser modules emitting at 98 nm. Fig. 3: Schematic drawing of the dual-chip OPSDL configuration used for the present experiments. The power transfer characteristic of the dual-chip OPSDL operated at a heat-sink temperature of 2 C is shown in Fig. 4. Using an optimized output coupler reflectivity of 92 %, we obtained a maximum output power of more than 3.3 W. From the linear part of the power transfer curve a maximum optical-to-optical conversion efficiency of ~15% was calculated. For comparison the power transfer characteristics of the two OPSDL chips operated in a linear single-chip cavity (see Fig. 1) are also shown. The difference in the power characteristics of chip 1 and chip 2 are due to the fact that the SiC heat-spreader bonded to chip 2 was antireflection coated to avoid multiple internal reflections while that attached to chip 1 was left uncoated. As can be seen, the output power of the dual-chip OPSDL, which amounts to 3.3 W at an absorbed pump power of 3 W, almost equals the sum of the output powers of the two chips when operated individually in single-chip configuration at an absorbed pump power of 15 W each, which is 3.4 W. Thus the coupling efficiency of the dual-chip OPSDL setup reaches a high value of 97%. Proc. of SPIE Vol A-4

5 OPSDL CW Output Power (W) T=2 C OPSDL Chip 1 OPSDL Chip 2 Dual-Chip OPSDL Absorbed Pump Power (W) Fig. 4: Power transfer characteristic of a 2.3 µm emitting dual-chip OPSDL recorded for an output coupler reflectivity of 92 %; both chips were operated at the same heat- sink temperature of 2 C. For comparison, the power transfer characteristics of the two OPSDL chips operated individually in a single-chip cavity are also shown. Pulsed mode operation of the above dual-chip OPSDL is readily achieved by pulsed pumping. Injecting 98 nm pump pulses by synchronized operation of both pump modules a maximum output power of 5.5 W has been achieved, limited by the maximum available pump power and not by thermal roll-over. Maximum repetition rates in this mode of operation are in the 2-5 khz range, limited by the pump laser diodes and associated driving electronics. Using a 95 nm emitting nanostack laser diode in combination with driving electronics especially designed for short pulse operation, pulse length as short as 156 ns have been reported for a 2.3 µm emitting single-chip OPSDL with a linear cavity configuration [18]. Fig. 5: Photograph of a compact 2 µm emitting OPSDL module. The approx. dimensions are 8 mm x 3 mm x 5 mm (L x W x H). Optical pumping is achieved by a fiber coupled 98 nm diode laser module (not shown). Proc. of SPIE Vol A-5

6 VECSEL CW Output Power (W) 3, 2,5 2, 1,5 1,,5, Laboratory OPSDL setup Compact OPSDL module λ = 2. µm R = 95.2% T = 2 C Absorbed Pump Power (W) Fig. 6: Power transfer characteristic of the 2. µm emitting single-chip OPSDL module displayed in Fig. 4 and, for comparison, of the same OPSDL chip operated in a lab bench laboratory setup. Whereas the above results have all been achieved using lab bench laboratory setups, OPSDLs can be fabricated also as compact small footprint modules as illustrated in Fig. 5. There the photograph of a 2 µm emitting single-chip OPSDL module with a linear cavity configuration is shown. Optical pumping is achieved by a commercial fiber-coupled 98 nm diode laser module of similar size. A typical room-temperature (2 C) power transfer characteristic recorded from this compact OPSDL module is displayed in Fig. 6, showing a maximum output power of 2.8 W at an absorbed pump power of 18 W, limited by thermal roll-over. For comparison the corresponding characteristic recorded using the same OPSDL chip but placed in a lab bench cavity, featuring more mechanical degrees of freedom for optimizing the alignment, is also plotted in Fig. 6. As can be seen from this direct comparison, the resulting power transfer characteristic is within the experimental accuracy identical to that of the compact OPSDL module, which demonstrates the high level of performance achieved by the present OPSDL module. 3. QC LASER COVERING THE µm SPECTRAL RANGE QC lasers are based on optical transitions between electron subbands in a sequence of multiple QW active regions, which are connected in a cascading scheme by so-called injector regions [19]. This way, one electron injected into this stack of alternating active and injector regions has multiple chances to generate a lasing photon. Due to the polarization selection rules inherent to inter-subband transitions, the electric field vector of the laser radiation emitted is perpendicular to the epitaxial layer plane. Therefore, direct surface emission as employed in the OPSDL concept described in the preceding section is not possible for QC lasers [19] and hence QC lasers are inherently edge-emitting devices. Surface emission from QC lasers can be achieved when diffractive grating or photonic crystal structures are used for re-directing the propagation vector of the lasing radiation [2]. However, the maximum peak output power of surface emitting QC lasers is still inferior to that of corresponding edge-emitting devices. Fig. 7 shows the secondary electron micrograph (SEM) of the cross-section of a double-trench waveguide QC laser with a GaInAs/AlInAs active region, GaInAs separate confinement layers, and InP cladding and contact layers. As the maximum room-temperature wall-plug efficiency of QC lasers covering the present wavelength range is typically around 1 % [1,11], appropriate heat-removal from the active region is also an important issue for this kind of devices. Therefore a several µm thick layer of electroplated gold is deposited on top of the double-trench waveguide structure for Proc. of SPIE Vol A-6

7 both vertical and lateral heat extraction. In addition, to further facilitate efficient heat removal high-power QC lasers feature narrow waveguides with typical widths in the 1-2 µm range and large resonator lengths of 2-4 mm. As a direct consequence of the narrow vertical and lateral waveguide width, a nearly diffraction limited output beam with M 2 <2 is readily achieved at the expense, however, of large slow-axis and in particular fast-axis beam divergence angles of typically 3-4 and 5 (FWHM), respectively. Applying the large optical waveguide concept to QC lasers, the fast axis beam divergence could be reduced to around 3 for 4.3 µm emitting QC lasers [21]. A drawback of the large optical waveguide design is, however, an increased thermal resistance of the active waveguide core severely limiting the average pulsed mode or even cw mode output power [21]. Temperature-dependent pulsed output power-vs.-current characteristics of a 4.9 µm emitting QC laser are shown in Fig. 8. At a heat-sink temperature of 27 K (-3 C) the maximum single-ended peak output power amounts to 3.2 W, dropping to 2.6 W if the temperature is raised to 3 K (27 C). The maximum power-conversion efficiency in pulsed mode operation is close to 9 % at 27 K (-3 C) (Fig. 9). Fig. 7: SEM micrograph of the cleaved facet of an InP substrate based QC laser in double-trench waveguide configuration. Laser emission is perpendicular to the drawing plane. Voltage (V) K 27 K 3 K 36 K Peak power / facet (W) Total power efficiency (%) K 3 K 36 K Current (A) Current (A) Fig. 8: Temperature-dependent output power-vs.-current characteristics of a 4.9 µm emitting QC laser in pulsed mode operation. The 3 K voltage-currentcharacteristic is also shown. Ridge width and cavity length were 16 µm and 3 mm, respectively. Fig. 9: Temperature-dependent total power efficiencyvs.-current characteristics of a 4.9 µm emitting QC laser in pulsed mode operation. Ridge width and cavity length were 16 µm and 3 mm, respectively. Proc. of SPIE Vol A-7

8 Figs. 1 and 11 show temperature-dependent output power-vs.-current characteristics and lasing spectra of a QC laser designed for µm laser emission, featuring a GaInAs/AlAsSb active region with a larger conduction band offset as the above GaInAs/AlInAs active region design [22,23]. At 77 K a maximum peak output power in excess of 1 W has been achieved, dropping to 2.1 W at 25 K (-23 C), a heat-sink temperature readily achievable by thermoelectric cooling (Fig. 1) [24]. Corresponding power conversion efficiencies are 21 % and 3 %. The center wavelength of the multiple longitudinal mode lasing spectrum shifts from 3.66 µm at 77 K to 3.79 µm at 3 K (27 C) (Fig. 11). Peak Power (W) 1 77 K Current (A) intensity (normalized) 77 K 19 K 3 K wavelength (µm) Fig. 1: Temperature-dependent output power-vs.-current characteristics of a µm emitting GaInAs/AlAsSb QC laser in pulsed mode operation. The ridge width was 18 µm and the cavity length 2 mm with a high-reflectivity coated rear facet. Fig. 11: Temperature-dependent lasing spectra of a GaInAs/AlAsSb QC laser in pulsed mode operation. To increase the output power beyond the level achievable with a single QC laser, polarization coupling of two µm emitting QC lasers has been demonstrated as a beam-quality-preserving means of extra-cavity beam coupling. A schematic of the polarization coupling concept using a silicon Brewster plate as the beam combining element is shown in Fig. 12, exploiting the intrinsic linear polarization of the emission of a QC laser. Fig. 13 shows the normalized average output power in short-pulse operation plotted versus the pulse repetition rate for (a) QC laser 1 and QC laser 2 individually as measured directly behind the beam collimation optics of each laser and (b) for the combined output beam of QC laser 1 and QC laser 2 i.e. behind the Brewster plate. Also shown is the sum of the average output powers recorded individually for QC laser 1 and QC laser 2. The average output powers increase almost linearly with repetition rate, which indicates that the QC laser performance is not severely limited by thermal cross-talk between individual pulses. A detailed analysis showed that the beam quality of the combined output beam was the same as that of the individual QC lasers with collimating optics attached. The coupling efficiency of the beam combining system can be defined as the ratio between the combined output power of the polarization coupled QC lasers and the sum of the individual output powers of both QC lasers. For operation at maximum average output power the coupling efficiency amounts to 82 %. Even though this value still falls short of the 97 % coupling efficiency reported in the preceding section for intra-cavity coupling of two OPSDL chips, it is quite acceptable for an extra-cavity beam combining scheme. The major part of the 18 % coupling losses arises from reflection losses for the output beam of QC laser 1 when bouncing off the Brewster plate. Proc. of SPIE Vol A-8

9 QCL 2 QCL 1 Brewster plate Output beam Average power (%) QCL 1 QCL 2 QCL 1 + QCL 2 combined beam Repetition rate (khz) Fig. 12: Beam combining of two QC lasers via polarization coupling. Fig. 13: Average output power vs. repetition rate for two polarization coupled QC lasers as well as for each QC laser measured individually. 4. SUMMARY In conclusion, we have reported on different semiconductor laser variants capable of delivering >1 W of output power in the 2-5 µm wavelength band with a beam quality suitable for its use in future DIRCM systems. For the short wavelength portion of that band, i.e. the 2-3 µm wavelength range, the GaSb-based optically pumped semiconductor disc laser (OPSDL) is the most promising candidate, with demonstrated cw output powers at µm of 2-3 W at a heat-sink temperature of 2 C and a simultaneously high beam quality (M 2 <3). For the 4-5 µm spectral range quantum cascade (QC) lasers are the most promising semiconductor laser variant, delivering average output powers of several hundreds of mw in short-pulse high duty cycle or even continuous wave (cw) operation. The output beam of a QC laser is typically nearly diffraction limited (M 2 <2) at the expense of a much larger beam divergence angle than that observed for an OPSDL or for classical solid state lasers. For both semiconductor lasers, up-scaling of the output power has been demonstrated by either intra-cavity coupling of two OPSDL chips, yielding an extraordinarily high coupling efficiency of 97 %, or by extra-cavity polarization coupling of two QC lasers, resulting in a coupling efficiency of 82 %. Future R&D will focus on further increasing the output power at high beam quality and improving the power efficiency of the above two semiconductor laser variants, as well as on increasing the wavelength coverage to fill the 3-4 µm wavelength gap. Furthermore, high-frequency modulation of OPSDL will be a topic of particular interest for future DIRCM applications. ACKNOWLEDGEMENTS The authors would like to thank W. Fehrenbach, M. Moritz, M. Lesic, K. Schäuble, K. Schwarz, and U. Weinberg for expert technical assistance. Financial support by the German Ministry of Defence and by the European Community through project VERTIGO (EU contract number 34692) is gratefully acknowledged. Proc. of SPIE Vol A-9

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