GaSb-based VECSEL exhibiting multiple-Watt output

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GaSb-based VECSEL exhibiting multiple-Watt output power and high beam quality at a lasing wavelength of 2.25 µm. B.Rösener, N. Schulz, M. Rattunde, ...
Invited Paper

GaSb-based VECSEL exhibiting multiple-Watt output power and high beam quality at a lasing wavelength of 2.25 µm B.Rösener, N. Schulz, M. Rattunde, C. Manz, K. Köhler, and J. Wagner Fraunhofer-Institut für Angewandte Festkörperphysik, Tullastrasse 72, 79108 Freiburg, Germany ABSTRACT We report the realization of GaSb-based optically pumped vertical-external-cavity surface-emitting lasers (VECSELs) emitting at 2.25 µm which are capable of multiple-Watt output power. VECSEL structures were grown on GaSbsubstrates by molecular beam epitaxy. SiC heat spreaders were capillary bonded onto the surface of the VECSEL chip in order to facilitate efficient heat removal. A continuous-wave output power of more than 3.4 W was recorded at a heat sink temperature of -10 °C. At room temperature (20 °C) we still obtained more than 1.6 W output power. A beam propagation factor in the range of M2≤5 was measured at maximum output power. In adjusting the fundamental mode diameter on the VECSEL chip to the pump spot diameter the beam quality could be further improved resulting in a beam propagation factor of M2~1.5. Furthermore, initial results on a GaSb-based dual-chip VECSEL are reported, capable of delivering a maximum output power of 3.3 W for a heat sink temperature of 20 °C and an emission wavelength of 2.25 µm. Keywords: Vertical-external-cavity surface-emitting laser (VECSEL), optically pumped semiconductor disk laser (OPSDL), infrared, (AlGaIn)(AsSb), beam quality

1. INTRODUCTION During the last years, considerable efforts have targeted on advanced device concepts for semiconductor lasers. Although conventional edge-emitting laser diodes have established themselves as powerful and compact coherent light sources in many applications their poor beam quality still represents a serious drawback for numerous applications. The concept of surface-emitting semiconductor lasers is a well-known approach which allows for the realization of devices exhibiting an almost circular output beam with a Gaussian transversal intensity profile. The vast majority of research activities in the field of surface-emitting semiconductor lasers has so far addressed the development of monolithically integrated devices such as VCSELs (Vertical-Cavity Surface-emitting lasers). However, while showing good performance in the mW regime the VCSEL concept is not suited if high output power is desired. Therefore, a new category of surface-emitting lasers has recently attracted considerable interest in current research. These devices are referred to as optically-pumped semiconductor disk lasers (OPSDL) or vertical-external-cavity surface-emitting lasers (VECSEL) [1], [2] and possibly show more similarities to doped-dielectric thin disk lasers [3] than to conventional semiconductor laser diodes. The principles of a VECSEL can be briefly described as follows (see Fig. 1): The active (or gain) medium is optically pumped either by high-power diode lasers or any other laser source with an appropriate emission wavelength. For simplicity reasons, cost-effective high-power laser diodes emitting at wavelengths below 1 µm are commonly used for this purpose. Radiation emitted from the pump source is focused on a semiconductor gain medium by appropriate focusing optics resulting in a pump spot on the chip surface of typically some hundreds of microns in diameter. The active medium is an epitaxially grown semiconductor structure that comprises multiple quantum wells (MQW), the latter providing sufficient gain for laser operation when optically pumped. In its simplest form, the laser cavity is constituted by a distributed Bragg mirror (DBR) embedded into the semiconductor structure underneath the MQW active region on the one hand and an external output coupling mirror on the other. In case of a VECSEL, the transversal mode pattern is mainly determined by the respective external cavity configuration and is therefore independent of the actual semiconductor structure. If the pump spot size is properly matched to the cavity mode diameter on the chip surface a circular, nearly diffraction-limited output beam can be achieved. For high power operation, an appropriate thermal management is crucial since not all of the pump radiation is converted into laser output. Due to the fact that VECSEL structures have to be pumped at a wavelength shorter than the actual emission wavelength, a considerable heat load can arise depending on the size of this quantum deficit. Therefore, Semiconductor Lasers and Laser Dynamics III, edited by Krassimir P. Panayotov, Marc Sciamanna, Angel A. Valle, Rainer Michalzik, Proc. of SPIE Vol. 6997, 699702, (2008) · 0277-786X/08/$18 · doi: 10.1117/12.781143

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different approaches have been developed in order to provide an efficient heat removal [4]. VECSELs with an improved thermal management have shown to be capable of several Watts of output power and therefore impressively demonstrated the potential of these devices for a variety of new applications.

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Transparent Heatspreader Fig. 1: Schematic VECSEL set-up: The laser cavity is formed by a distributed Bragg reflector (DBR) integrated into the semiconductor VECSEL 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 50-500 µm in diameter.

Research activities have up to now been predominantly focused on AlGaAs/GaInAs/GaAs-based devices emitting in the 0.8 - 1 µm wavelength range where high output powers exceeding 30 Watts have been realized [5], [6]. Due to the external cavity configuration, VECSELs lend themselves for non-linear optical frequency conversion using appropriate crystals as intra-cavity elements. Based on VECSEL structures emitting around and below 1 µm wavelength highbrightness blue and green lasers could hence be realized in the past using intra-cavity frequency doubling [7], [8]. Addressing applications like free space optical communications, remote sensing or infrared countermeasures coherent light sources emitting in the mid-infrared regime are required. The realization of VECSELs emitting at wavelengths beyond 2 µm is, however, a challenging task for several reasons. On the one hand, materials based on quaternary group-III-antimonides have to be employed which up to the present have been generally considered to be less powerful in terms of their thermal and electro-optical properties if compared to well-established ternary compounds like e.g. GaInAs, which are commonly used in VECSELs emitting in the 0.8 - 1 µm wavelength range. Secondly, the operation of a long-wavelength VECSEL is typically associated with an increased heat load due to the large quantum-deficit arising from the use of diode laser pump modules emitting at 1 µm and below. Taking a 2.25 µm emitting VECSEL pumped at 980 nm as an example, the quantum deficit can be as high as 57%. Therefore, special care has to be taken to optimize such VECSEL structures with respect to optimized heat removal and thermal robustness when high output powers are required. In this contribution, we prove the capability of VECSELs emitting at wavelength > 2.25µm for high-brightness operation and output powers of several Watts. Besides detailed operation characteristics, results of an in-depth investigation of the beam quality under different operating conditions will be presented. Furthermore, first results on a VECSEL consisting of two coherently coupled gain chips in a common cavity will be reported. These results demonstrate the potential of the VECSEL concept for a further scaling of the output power.

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2. VECSEL DESIGN AND FABRICATION The semiconductor chip is certainly the most essential element of every VECSEL. Basically, this chip is formed by a complex sequence of thin epitaxial layers of semiconductor alloys with different compositions that are deposited on a single-crystalline substrate. The layer structure of such an epitaxially grown wafer can be divided into three main sections according to the respective function of the layers. As already mentioned above, a DBR is commonly embedded into the semiconductor structure which consists of many layer pairs of two alternating materials, each layer having an optical thickness of a quarter wavelength. Typically, some tens of layer pairs are being grown on the substrate in order to constitute a reflector providing a reflectivity at the design wavelength of more than 99%. The actual gain region of a VECSEL is formed by several quantum wells (QWs) which are enclosed within barrier and spacing layers of a higher band gap material. VECSEL structures in which pump radiation is predominantly absorbed in the barrier layers are also referred to as “barrier-pumped”. Besides, VECSEL structures can be designed for absorption of the pump light in the QWs – these structures are therefore referred to as “in-well-pumped” VECSELs [9], [10], [11]. Free carriers generated in the barrier and/or QW layers can be efficiently confined to the gain region if the whole structure is terminated by a high band gap window layer. Barrier, QW and window layers form a microcavity bounded by the DBR and the interface semiconductor-air (which acts as a second mirror of that microcavity). If the thickness of the microcavity is chosen such that an anti-node of the internal standing-wave pattern is located at the interface between the window layer and the gain section, the field intensity in the gain region can be resonantly enhanced (see Fig. 2) . If the QWs are furthermore located at the anti-nodes of the internal standing-wave pattern the modal gain can be still more increased. This approach which has already been used in VCSEL design is also referred to as “resonant periodic gain” (RPG) structure [12].

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Within the scope of this work, VECSEL structures were grown on 2-inch GaSb substrates by solid-source molecularbeam epitaxy (MBE) employing valved-cracker effusion cells for both group V constituents. The DBR region consists of alternating quarter-wavelength thick layer pairs out of GaSb as the high-index material (nGaSb = 3.89) and lattice-matched AlAs0.08Sb0.92 (nAlAsSb = 3.16) for the low-index layers. Using 21.5 GaSb/AlAs0.08Sb0.92 layer pairs (total thickness = 7.1µm) results in a calculated reflectivity (neglecting absorption and scattering effects) of the DBR of 99.8% at a wavelength of 2.3µm. The active region consists of compressively strained 10 nm thick quaternary GaxIn1-xAsySb1-y QWs with varying composition depending on the desired emission wavelength. The QWs are embedded between latticematched Al0.30Ga0.70As0.02Sb0.98 barrier and absorbing layers and the whole structure is terminated by a Al0.85Ga0.15As0.068Sb0.932 window layer, also lattice matched to the GaSb-substrate. Since aluminum-rich semiconductor alloys are vulnerable to oxidation, a thin (a few nm thick) GaSb capping layer is grown as the topmost layer to protect the structure from the ambient air.

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In order to obtain epitaxial VECSEL structures exhibiting high crystalline quality and consisting of a large number of different layers with well defined composition, thickness and strain, a thorough post-growth analysis of the structures is inevitable. Characterization tools applied to the present (AlGaIn)(AsSb)-based VECSEL structures were high-resolution X-ray diffraction (HRXRD), secondary ion mass spectrometry (SIMS), as well as photoluminescence and reflectivity measurements as optical characterization techniques. With the help of HRXRD measurements we were able to asses the crystalline structure of the MBE grown samples. Fig. 3 displays the HRXRD profile of a 2.25 µm emitting VECSEL structure. Apart from the 004 reflection of the GaSb substrate and the almost perfectly lattice-matched spacer and window layers superlattice (SL) diffraction peaks arising from the compressively strained GaInAsSb MQW active region can also be identified. The quaternary Ga0.67In0.33As0.11Sb0.89 QWs exhibited a lattice mismatch (∆a/a0)⊥ of 2.7 %, corresponding to a compressive strain of εzz of 1.3 %. 5

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Subsequently, the thicknesses of all semiconductor layers forming the VECSEL structure were verified by means of secondary ion mass spectroscopy (SIMS). In particular, the exact thickness of the microcavity could be verified. Furthermore, SIMS measurements proved that the quantum wells were located precisely at the antinodes of the standing wave pattern as intended for the present RPG design. In order to check for the alignment of the central wavelengths of the DBR, the micro-cavity resonance and the QW gain spectrum temperature-dependent reflectivity and photoluminescence (PL) spectra were measured. Such data are shown in Fig. 4 for a GaSb-based VECSEL structure designed for emission at 2.25µm. In the reflectivity spectrum, the rectangular DBR stop band (R ≥ 99.5%) is interrupted by a dip arising from the QW absorption which is resonantly enhanced by the microcavity. It has to be pointed out that the peak wavelength of the material gain shifts at a higher rate with increasing temperature than the wavelength of the microcavity resonance. In case of our VECSEL structures, we observed a temperature-tuning rate of 0.3 nm/K for the microcavity resonance which is mainly determined by the temperature dependence of the refractive index of the semiconductor materials employed in the active region. The PL peak position, however, shifts with increasing temperatures at a tuning rate of 1.5 nm/K. This shift is essentially given by

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the temperature-dependence of the semiconductor bandgap energy and the mentioned tuning-rate also applies to the temperature-dependent tuning of the QW absorption edge and the QW gain curve maximum. Due to the different temperature-tuning we observe that the absorption dip within the stop band of the DBR changes in spectral position and depth with increasing temperature. At some temperature Tres, we observe a minimum value for the reflectivity at the spectral position of the absorption dip. At this temperature, the onset of QW absorption (which approximately corresponds to the position of the peak value of the QW gain curve) coincides with the wavelength of the microcavity resonance. Taking into account the different temperature-tuning rates of the gain spectrum and the microcavity resonance, the gain peak offset with respect to the microcavity resonance could be calculated at room temperature. It has been shown that the gain peak offset at room temperature is a key parameter for the design of high performance VECSELs [13].

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For the present VECSEL structures the MQW active region was grown such as to provide a gain peak which is blueshifted with respect to the spectral position of the cavity resonance (see Fig. 4) so that the peak wavelength of the material gain and the cavity resonance coincide at an elevated temperature. This approach enables an improved laser operation at increased pump powers since the latter induce a pronounced self-heating of the VECSEL chip. For the barrier-pumped VECSEL emitting at 2.25µm, we estimated the spectral offset between gain peak and microcavity resonance to a value of 60 nm at room temperature. Apart from a precise control of the modal gain, high power operation has been facilitated by the use of an intra-cavity heat spreader [14]. Barrier-pumped VECSELs emitting at wavelengths > 2µm generally suffer from an increased heat load which is due to the large quantum deficit. Furthermore, the DBR and substrate of GaSb-based VECSELs consist of materials exhibiting a low thermal conductivity. Therefore, heat cannot be efficiently removed from the active region through the DBR mirror and the substrate for structures without any additional provision for an improved thermal management. In fact, we observed a thermal impedance about two orders of magnitude higher in case of samples without a heat spreader. When employing an intra-cavity heat spreader heat can be extracted directly from the VECSEL chip surface where it is generated, i.e. it is possible to bypass the limited thermal conductivity of the epitaxial DBR layer stack and semiconductor substrate. Due to its high thermal conductivity and transparency at the laser and the pump wavelength, we

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use SiC as heat spreader material [15], which is bonded to the VECSELs chip surface using the liquid capillary bonding technique [16].

3. LASING CHARACTERISTICS 3.1 Barrier-pumped VECSEL emitting at 2.25 µm For the basic characterization of the lasing properties we used a linear cavity consisting of a curved output coupling mirror (radius of curvature: 100 mm, R=95%) and the VECSEL chip with the DBR as a plane end mirror. A fibercoupled diode laser module emitting at 980 nm served as pump source. The pump light leaving the fiber (core diameter: 100 µm) was imaged on the VECSEL chip using a combination of a collimating and a focusing lens. The VECSEL chip was irradiated by the focused pump beam from the heat spreader side at an approximate angle of incidence of 30° which resulted in a slightly elliptical pump spot with a diameter of 375 µm × 425 µm on the chip surface.

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Fig.5: Absorbed threshold pump power density vs. heat sink temperature for a barrier-pumped VECSEL, emitting at 2.25 µm. A linear cavity and an output coupling mirror with a reflectivity of 95% was used for this experiment.

In a first step, we investigated the behavior of the threshold pump power when the heat sink temperature was varied. The resulting curve is shown in Fig. 5 for a VECSEL structure with a room-temperature gain peak offset of 60 nm (see Fig. 4). The temperature characteristic of the threshold pump power is mainly determined by the temperature dependence of the modal gain (i.e. the product of QW material gain and cavity enhancement factor) and by the temperature dependence of various loss mechanisms. On the one hand, the different temperature tuning of the cavity resonance wavelength and the wavelength of maximum QW gain tend to maximize the modal gain at Tres (see section 2). On the other hand, the decrease of the peak value of the QW gain spectrum and the increase of the various loss mechanisms like e.g. Auger recombination tend to increase the threshold power monotonically with rising temperature. The combination of both effects leads to a minimum of the threshold power at a temperature Tmin below Tres [17]. For the present VECSEL structure and an output coupler reflectivity of 95% we observed a minimum threshold pump power density of 1.02 kW/cm2 at a heat sink temperature Tmin=20°C (see Fig.5). Power transfer curves for different heat sink temperatures are shown in Fig. 6. A maximum output power of 3.4 W was measured at -10°C and an absorbed pump power of 21 W [18]. Apart from a slight decrease of the slope efficiency at elevated pump power no pronounced thermally induced roll-over was observed in this case. Instead, the maximum output power was limited by the maximum pump power available. From the linear part of the power transfer curve measured at -10°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 e.g. 17.6% at 20°C and the thermally

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induced rollover already occurred within the available range of pump powers. Nevertheless, we still observed a maximum output power of >1.6 W at 20°C.

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The beam quality was investigated by determining the beam propagation factor M2 according to the International Organization for Standardization ISO 11146 Standard which is based on the measurement of beam diameters defined by second order moments. The beam propagation factor was determined both in the direction parallel to the plane of incidence of the pump beam (horizontally) and in the direction perpendicular to the latter (vertically). Within our experiments we varied the fundamental mode diameter on the VECSEL chip by changing the cavity length while the pump spot diameter was kept constant. In Fig. 7b, we display the measured beam propagation factor as a function of the fundamental mode diameter, the latter being calculated for the given cavity lengths by means of the ray transfer matrix formalism. The smallest mode diameter shown in Fig. 7b corresponds to the resonator alignment used for the measurement of the power transfer curves shown in Fig. 6. All M2 data in Fig. 7b were measured at maximum output power for the respective resonator alignment. Furthermore, we plotted the maximum output power and brightness, the latter calculated from the respective maximum output power and the corresponding M2-values (Fig. 7a).

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Fig.6: Continuous-wave (CW) output power vs. absorbed pump power recorded at different heat sink temperatures for a barrier-pumped VECSEL, emitting at 2.25 µm. A SiC intra-cavity heat spreader was used and an out-coupling rate of 5 %. The inset shows a spectrum recorded at an output power of 2.7 W and a heat sink temperature of 0°C.

From Fig. 7b it becomes evident that the elliptical shape of the pump spot introduces a distinct difference between the beam quality in the vertical and horizontal direction. Up to a fundamental mode diameter of ~350µm, an inferior beam quality was observed in the horizontal direction which was attributed to the extended pump spot diameter in this direction. Since the pump spot acts as a gain aperture, only transversal modes with a mode diameter comparable to or smaller than the pump spot diameter can oscillate whereas other modes are subjected to increased absorption losses and will therefore be suppressed. This explains the larger beam propagation factors measured in the horizontal direction where the pump spot diameter is generally larger compared to the fundamental mode diameter. In the same way, we can explain the stepwise decrease of the beam propagation factor in the horizontal direction at certain mode diameters. We attribute the drop from M2~5 down to values in the range between 3.5 and 2.5 to a suppression of the TEM2X-modes at fundamental mode diameters ≥ 230 µm. When the mode diameter is further increased and finally exceeds ~340 µm we obtain a drop from M2~3 to M2~1.5 which was attributed to the suppression of the TEM10-Mode. In the vertical direction, a corresponding decrease is observed when increasing the mode spot diameter to values >220 µm. With the resonator aligned for highest beam quality (M2~1.5 in both horizontal and vertical direction) no output power dependence of the beam propagation factor was found. We still observed a slope efficiency of 19% at 0°C while the

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maximum output power was slightly decreased to still more than 2.2 W at that temperature. A maximum brightness of ~21 MW/cm2 was calculated for this configuration.

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Fig.7: Output power, brightness and beam propagation factor M2 as a function of the fundamental mode diameter on the VECSEL chip. Different propagation factors were obtained depending on the scanning direction (horizontal or vertical). The pump spot was kept at a constant size of 375µm × 425µm diameter throughout the experiments which were carried out at a heat sink temperature of 0°C.

3.2 Dual-Chip Configuration The basic power-scaling scheme of a thin disk laser is to increase the pumped area while holding the pump power intensity at a constant level. This approach of course also applies to semiconductor disk lasers. However, depending on the heat extraction scheme employed for thermal management, there is an upper limit for the maximum pump spot size: 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 capability decreases with increasing pump spot size and eventually a transition to one-dimensional heat flow takes place [14]. As a result, heating of the active region increases and leads to a decreasing efficiency and thermally induced roll-over. In addition to these thermal aspects, it has already been pointed out that power-scaling of (semiconductor) disk lasers by increasing the pumped area can also be limited by the onset of amplified spontaneous lateral emission (ASE) [19]. Another power-scaling scheme which has been applied to solid state thin disk lasers as well as to VECSELs is the use of several separately pumped gain media in a common laser cavity. This approach has been successfully demonstrated for VECSEL structures emitting at wavelengths below 1 µm. Here, impressive results such as an output power of more than 19 W at a lasing wavelength of 970 nm have been reported recently [20]. In this paper, we describe the realization of a laser cavity consisting of two VECSEL chips emitting at 2.25 µm wavelength and yielding several Watts of output power. For the present experiments, we employed two VECSEL chips from the same wafer which was used for the barrierpumped single-chip VECSEL described in section 3.1. VECSEL chips were liquid-capillary bonded to SiC heat spreaders, again. In order to coherently couple the two VECSEL chips we used a cavity in which one VECSEL chip (chip 1) served as an end mirror while the other chip (chip 2) was used as a folding mirror (see Fig. 8). We used an output coupler with a radius of curvature (ROC) of 150 mm placed at a distance of ~140 mm with respect to chip 2. An HR coated folding mirror (ROC=50 mm) was placed at a distance of 100 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 in changing the length of the resonator arm between chip 2 and the output coupler. Due to a small folding angle of 6°

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negligible astigmatism was introduced to the cavity mode by the employment of the curved folding mirror. Chip 1 and chip 2 were pumped by fiber-coupled diode laser modules emitting at 980 nm.

Output Coupler (Radius of Curvature: 150 mm)

Folding Mirror (Radius of Curvature: _,'.J 1111111

//\\H Fig. 8: Sketch of the dual-chip VECSEL configuration used for the experiments.

VECEL CW Output Power (W)

Power transfer curves for the dual-chip configuration recorded at 20°C heat sink temperature are shown in Fig. 9. Using an 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. We thus obtained a slightly reduced efficiency when comparing with the results obtained for a single-chip VECSEL (see results in subsection 3.1). Furthermore, output coupler with lower reflectivity had to be used in order to obtain similar efficiencies as in case of the single-chip VECSEL. We attributed these findings to slightly higher cavity losses caused by the increased number of optical elements forming the cavity.

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Fig. 9: Power transfer characteristic of the present dual-chip VECSEL recorded for an output coupler reflectivity of 92 %. Both chips were run at the same heat sink temperature of 20°C.

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z (nm) z (nm) Fig. 10: Band-edge profile (solid lines) and energy levels (dotted lines) of a Ga0.65In0.35As0.10Sb0.90 QW (emission wavelength: 2.35 µm) embedded between Al0.30Ga0.70As0.02Sb0.98 barrier layers. (a) Electronic transitions for barrier pumping at a pump wavelength of 1.064 µm. (b) Electronic transitions for in-well pumping at 1.94 µm.

3.3 In-Well pumped VECSEL emitting at 2.35 µm The VECSELs discussed so far were barrier-pumped devices, i.e. the absorption of the short-wavelength (980 nm) pump radiation mainly takes place in the barrier layers surrounding the quantum wells (see Fig. 10a). In the thick barrier layers, the incident pump radiation is almost completely absorbed in a single pass. On the other hand, the quantum defect can be quite large. For a 2.3µm VECSEL pumped at 980 nm, the quantum defect amounts to ~57 %, which imposes a severe limit to high efficiency VECSEL operation – and, indeed, is the origin for the strong device heating during CW operation in spite of an already optimized heat removal using intra-cavity heat spreaders. These limitations can be overcome by employing the alternative in-well optical pumping concept which was initially developed for GaAs-based VECSELs emitting at wavelengths around 1 µm [9], [10]. In case of in-well pumped VECSELs, the pump photons are directly absorbed in the QWs instead of the barrier layers (Fig. 10 b). Using a pump wavelength of 1.94µm and an VECSEL wavelength of 2.3 µm reduces the quantum defect to 16 % - representing a maximum of a 3.5-fold reduction in the heat loading and opening up the feasibility for significantly improved VECSEL performance at wavelengths above 2 µm.

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Absorbed Pump Power (W) Fig. 12: CW output power vs. absorbed pump power recorded at different heat sink temperatures for an in-well VECSEL , emitting at 2.35 µm (solid lines). A SiC intra-cavity heat spreader was used for thermal management and an outcoupling rate of 3.6 %. The inset shows a lasing spectrum recorded at a heat sink temperature of –15°C and an absorbed pump power of 10W. For comparison, the CW power transfer characteristic of a comparable barrier pumped 2.3 µm-VECSEL is also displayed (thin, solid line).

Since the active QWs are only several nanometers thick, the pump absorption per QW is rather low, typically ≤ 1%. To increase the total pump absorption in the VECSEL structure for a given number of QWs, the structure can be designed to be simultaneously resonant at the laser wavelength and the pump wavelength [11]. The back reflection of the pump radiation into the active region is achieved using a two-band DBR with two high-reflective bands centered at the pump wavelength and the lasing wavelength. As the pump radiation is also partially back reflected at the semiconductor-air interface at the VECSEL chips top surface, a standing-wave-type optical intensity distribution at the pump and the emission wavelength builds up inside the VECSEL structure. In this way, a certain degree of internal pump recycling is realised leading to resonantly enhanced pump absorption. By positioning the QWs in regions of maximal overlap between the pump and laser standing-wave optical fields within the VECSEL micro-cavity, the pump absorption and the overlap of each QW with the laser mode can simultaneously be maximized, as shown in Fig. 11. An (AlGaIn)(AsSb)-based VECSEL epitaxial layer structure according to this doubly-resonant design has been fabricated and experimentally characterized [11]. The structure was designed for a pump wavelength of 1.92–1.98 µm and an emission wavelength of 2.35 µm. For initial experiments, a thulium-doped fiber laser emitting at 1.96 µm was used as a pump source. The pump radiation was focused to a spot diameter of 300 µm on the VECSEL chip under an angle of 35°. The pump absorption within the VECSEL structure was measured to be 60% on the bare VECSEL chip. However, the attachment of a SiC heat spreader caused a decrease in surface reflectivity and a reduced pump absorption of 25% was thus observed. By re-focusing the pump light reflected from the surface of the VECSEL chip (pump recycling), a total pump absorption of 40% was achieved. The VECSEL chip was placed in a linear cavity configuration, using a -50 mm ROC output coupler with a reflectivity of 96.4 %. In Fig. 12, the CW power transfer characteristics of the in-well-pumped VECSEL are shown together with a lasing spectrum recorded at a heat sink temperature of –15°C and an absorbed pump power of 10 W. The in-well pumped VECSEL shows a remarkable low threshold power density of 300 W/cm2 at –15°C heat sink temperature, increasing gradually to 400 W/cm2 at +15°C. An output power of 3.2 W was achieved at a heat sink temperature of -15°C without any indication of thermal rollover. The power efficiency, calculated with respect to the absorbed pump power, amounts to 29%. At +15°C, the maximum output power still exceeds 2 W with a power efficiency of 20 %. Also shown in Fig. 14 is the CW output power characteristic at a heat sink temperature of –15°C of a similar barrier-pumped VECSEL device which was fabricated during the same MBE growth campaign. It can be seen that the in-well pumped VECSEL gives significant performance benefits over the barrier pumped device.

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For practical VECSEL devices, diode laser pumping is of course preferable instead of using a Tm-based fibre laser as pump source. Initial experiments have shown that the in-well pumped VECSEL operation can also be achieved using a GaSb-based single emitter diode laser emitting at 1.96 µm [21]. The next step will be to use GaSb-based high-power diode laser bars, that are able to emit up to 20W in CW operation at 1.98 µm [22] also developed at Fraunhofer IAF as well as fibre-coupled diode laser modules, based on these 2 µm laser bars [23].

4. SUMMARY In conclusion, recent progress in GaSb-based VECSELs emitting at 2.25 µm – 2.35 µm wavelength has been presented. VECSEL structures were grown by solid-source MBE and bonded to intra-cavity SiC heat spreaders for efficient heat extraction from the VECSEL active region. Barrier-pumped VECSELs (using 980 nm diode lasers as pump source) were capable of emitting up to 3.4 W output power in CW operation at 2.25 µm lasing wavelength an a heat sink temperature of –10°C. Close to room temperature, they still exhibited watt-level output power (e.g. 1.6 W at 20 °C heat sink temperature). Despite the high quantum deficit of >50%, these lasers reached an optical power conversion efficiency of almost 18 % at room temperature. By optimizing the resonator configuration for highest beam quality, a nearly diffraction limited TEM00 emission was achieved with M2 values in the range of 1.1 – 1.5. Even with the resonator aligned for maximum output power rather than highest beam quality, the beam quality parameter remains in the range of M2 ≤5. We also presented first results on a VECSEL comprising two separately pumped gain media in a common laser cavity. Barrier-pumped VECSEL structures emitting at 2.25 µm wavelength were used for these experiments, too. With this power scaling approach, CW output powers up to 3.3 W were recorded at a heat sink temperature of 20°C. In order to increase the maximum power efficiency, in-well pumped 2.35 µm VECSELs optimized for 1.96 µm pump radiation have been realized. Using a Tm-doped fiber laser, a pump-power limited output power of 3.2W (2W) was achieved for a heat sink temperature of –15°C (+15°C). GaSb-based fiber coupled diode laser modules, emitting at 1.96 µm, have been shown to be also capable to serve as a pump source and will be used for further exploitation of the in-well pumping concept.

ACKNOWLEDGEMENTS The authors would like to thank L. Kirste and T. Fuchs for expert SIMS and X-ray diffraction analyses, and W. Fehrenbach for VECSEL chip processing. Financial support by the European Community through the project VERTIGO (EU contract number 034692) is gratefully acknowledged.

REFERENCES [1]

[2] [3] [4]

[5]

M. Kuznetsov, F. Hakimi, R. Spraque, and A. Mooradian, “High-Power (>0.5-W CW) Diode-Pumped VerticalExternal-Cavity Surface-Emitting Semiconductor Lasers with Circular TEM00 Beams”, IEEE Photonics Technol. Lett. 9, 1063 (1997). J. Rivers, 2. A. Tropper, H. Foreman, A. Garnache, K. Wilcox, and S. Hoogland, “Vertical-external-cavity semiconductor lasers,“ J. Phys. D: Appl. Phys. 37, R75-R85 (2004). A. Giesen, H. Hügel, A. Voss, K. Wittig, U. Brauch, and H. Opower, "Scalable Concept For Diode-Pumped HighPower Solid-State Lasers," Applied Physics B - Lasers and Optics 58, 365 (1994). A. J. Kemp, G. J. Valentine, J.-M. Hopkins, J. E. Hastie, S. A. Smith, S. Calvez, M. D. Dawson, and David Burns, “Thermal Management in Vertical-External-Cavity Surface-Emitting Lasers: Finite-Element Analysis of a Heatspreader Approach”, IEEE J. Quantum Electronics 41(2), 148 (2007). S. Lutgen, T. Albrecht, P. Brick, W. Reill, J. Luft, and W. Späth, "8-W high-efficiency continuous-wave semiconductor disk laser at 1000 nm," Appl. Phys. Lett. 82, 3620 (2003).

Proc. of SPIE Vol. 6997 699702-12

[6] [7]

[8]

[9] [10]

[11]

[12] [13]

[14]

[15]

[16] [17] [18]

[19] [20]

[21] [22] [23]

J. Chilla, S. Butterworth, A. Zeitschel, J. Charles, A. L. Caprara, M. K. Reed, and L. Spinelli, "High Power Optically Pumped Semiconductor Lasers," Proc SPIE 5332, 143 (2004). J. Lee, S.-M. Lee, T. Kim, and Y. Park, “7 W high-efficiency continuous-wave green light generation by intracavity frequency doubling of an end-pumped vertical external-cavity surfaceemitting semiconductor laser”, Appl. Phys. Lett. 89, 241107 (2006). J.-Y. Kim, S. Cho, S.-J. Lim, J. Yoo, G. B. Kim, K.-S. Kim, J. Lee, S.-M. Lee, T. Kim, and Y. Park, “Efficient blue lasers based on gain structure optimizing of vertical-externalcavity surface-emitting laser with second harmonic generation”, Journal of Applied Physics 101, 033103 (2007). M. Schmid, S. Benchabane, F. Torabi-Goudarzi, R. Abram, A. Ferguson, and E. Riis, „Optical in-well pumping of a vertical-external-cavity surface-emitting laser,“ Appl. Phys. Lett. 84, 4860, 2004. S.-S. Beyertt, M. Zorn, T. Kübler, H. Wenzel, M. Weyers, A. Giesen, G. Tränkle, and U. Brauch, „Optical In-Well Pumping of a Semiconductor Disk Laser With High Optical Efficiency,“ IEEE J. Quantum Electronics 41, 1439, 2005. N. Schulz, M. Rattunde, C. Ritzenthaler, B. Rösener, C. Manz, K. Köhler, and J. Wagner, „Resonant optical in-well pumping of an „(AlGaIn)(AsSb)-based vertical-external-cavity surface-emitting laser emitting at 2.35 µm,“ Appl. Phys. Lett. 91, 091113, 2007. S. W. Corzine, R. S. Geels, J. W. Scott, R.-H. Yan, and L..A. Coldren, “Design of Fabry-Perot surface-emitting lasers with a periodic gain structure,” IEEE J. Quantum Electronics 25, 1513 (1989). N. Schulz, M. Rattunde, C. Ritzenthaler, B. Rösener, C. Manz, K. Köhler, and J. Wagner, “Effect of the Cavity Resonance-Gain Offset on the Output Power Characteristics of GaSb-Based VECSELs,,“ IEEE Photonics Technolgy Letters 19, 1741 (2007). A. J. Kemp, J.-M. Hopkins, A. J. Maclean, N. Schulz, M. Rattunde, J. Wagner, and D. Burns, “Thermal Management in 2.3-µm Semiconductor Disk Lasers: A Finite Element Analysis,“ IEEE J. Quantum Electronics 44, 125 (2008). J. E. Hastie, J.-M. Hopkins, S. Calvez, C. W. Jeon, D. Burns, R. Abram, E. Riis, A. I. Ferguson, and M. D. Dawson, “0.5-W Single Transverse-Mode Operation of an 850-nm Diode-Pumped Surface-Emitting Semiconductor Laser,“ IEEE Photonics Technology Letters 15, 894 (2003). Z. L. Liau, “Semiconductor wafer bonding via liquid capillarity,” Appl. Phys. Lett. 77, 651 (2000). J. Piprek, Y.A. Akulova, D.I. Babic, L.A. Coldren, J.E. Bowers, “Minimum temperature sensitivity of 1.55 µm vertical-cavity lasers at 30 nm gain offset,“ Appl. Phys. Lett. 72, 1814 (1998). B. Rösener, N. Schulz, M. Rattunde, C. Manz, K. Köhler, and J. Wagner, „High-Power, High-Brightness Operation of a 2.25µm (AlGaIn)(AsSb)-based Barrier-pumped Vertical-External-Cavity Surface-Emitting Laser”, accepted for publication in IEEE Photon. Technol. Lett. R. G. Bedford, M. Kolesik, J. L. A. Chilla, M. K. Reed, T. R. Nelson, J. V. Moloney: „Power-limiting mechanisms in VECSELs,“ Proceedings of SPIE 5814, 199 (2005). L. Fan, M. Fallahi, J. Hader, A. R. Zakharian, J. V. Moloney, J. T. Murray, R. Bedford, W. Stolz, S. W. Koch, “Multichip vertical-external-cavity surface-emitting lasers: a coherent power scaling scheme,“ Optics Letters 31, 3612 (2006). M. Rattunde, J. Schmitz, G. Kaufel, M. Kelemen, J. Weber, and J. Wagner, “GaSb-based 2.X µm quantum-well diode lasers with low beam divergence and high output power”, Appl. Phys. Lett. 88, 081115 (2006). M.T. Kelemen, J. Weber, M. Rattunde, G. Kaufel, J. Schmitz, R. Moritz, M. Mikulla, J. Wagner, „High-power 1.9µm diode laser arrays with reduced far-field angle,“ IEEE Photon. Technol. Lett. 18, 628 (2006). M. Haverkamp, K. Wieching, M. Traub, K. Bouke, “Fiber-coupled laser modules with wavelength around 2 µm,“ Proc. SPIE 6456, 64560U-1 (2007).

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