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We report power scaling experiments of a GaInNAs/GaAs-based semiconductor disk laser operating at ~1180 nm. Using a single gain chip cooled to mount temperature of ~10 °C we obtained 11 W of output power. For efficient thermal management we used a water-cooled microchannel mount and an intracavity diamond heat spreader. Laser performance was studied using different spot sizes of the pump beam on the gain chip and different output couplers. Intracavity frequency-doubling experiments led to generation of ~6.2 W of laser radiation at ~590 nm, a wavelength relevant for the development of sodium laser guide stars.
©2010 Optical Society of America
1. Introduction
The advantages offered by the optically-pumped semiconductor disk lasers (OP-SDLs), also referred to as vertical external cavity surface-emitting lasers [1], have been proved using a large variety of semiconductor gain media for generating high-brightness radiation at visible, infrared, and mid-IR wavelengths [2,3]. Demonstrated first for the generation of radiation at 1.32 µm [4], dilute nitride (GaInNAs/GaAs) OP-SDLs have recently emerged as a viable solution for developing high-power laser sources emitting in the wavelength range between 1.17 µm [5] and 1.24 µm [6]. The increased interest in dilute nitride SDLs has been largely motivated by their ability to produce orange-red laser light via intracavity frequency doubling [3]. GaInNAs/GaAs quantum wells (QWs) can be monolithically integrated with AlAs/GaAs distributed Bragg reflectors (DBRs), which exhibit superior optical and thermal properties compared to InP-based DBRs. By alloying only a few percent of nitrogen into GaInAs, both the band-gap and lattice constant of GaIn(N)As/GaAs QWs are simultaneously decreased. Thus the use of dilute nitrides offers a higher flexibility in designing the gain mirrors and alleviates the problems related to the high compressive lattice strain present in GaInAs/GaAs QWs emitting at wavelengths above 1100 nm [7]. Another important aspect for SDL operation is that GaInNAs/GaAs QWs exhibit a large conduction-band offset and electron effective mass providing efficient carrier confinement and excellent temperature behaviour [8].
On the other hand, the incorporation of nitrogen is usually accompanied by the introduction of lattice defects which ultimately decrease the performance of dilute nitride lasers. However, dilute nitride OP-SDLs with emission wavelengths in the range of 1180–1240 nm require a nitrogen content of less than 1% and can exhibit multi-watt output powers in both fundamental [6] and frequency-doubled operation [9,10]. The highest reported output power (~7 W) and slope efficiency (~30%) exhibited by a dilute nitride OP-SDL incorporating one gain element have been obtained for operation at ~1180 nm [11]. This wavelength range is of particular interest for important applications in medicine [12], optical frequency metrology [13], and astrophysics [14]. For example, high power emission at the frequency doubled wavelength of 589 nm is required to create sodium laser guide stars employed in earth-based telescopes with adaptive optics systems. Frequency doubled SDLs emitting at the 589 nm D2-sodium absorption line are anticipated to fulfill the specifications of future generation sodium laser guide stars [15], providing that significant advances are made in increasing the output power and achieving stable operation with narrow linewidth.
Assuming that sufficient heat dissipation is ensured, the output power of an OP-SDL can be scaled up by increasing the mode size on the laser gain mirror or by using multiple gain chips in a single SDL cavity or both [16]. The first approach is attractive due to its simplicity; in particular for a laser guide star application, the narrow linewidth and the spectral stability requirements might be hard to achieve by using multiple gain elements. Increasing the mode size allows for keeping the power density, and therefore the heat generated per unit area constant while increasing the pump and output power. In this paper, we report power scaling of an OP-SDL based on a dilute nitride gain mirror emitting at ~1180 nm. We used a water-cooled microchannel mount for thermal management and studied the influence of pump spot diameter and output coupler transmission on the laser performance. As a result, a record-high output power of 11 W is demonstrated.
2. Gain mirror fabrication
The dilute nitride gain mirror was grown on n-GaAs(100) substrate using solid source molecular beam epitaxy (SS-MBE) equipped with a radio frequency (RF) plasma source for incorporating nitrogen into the crystal. The design of the resonant periodic gain (RPG) structure is shown in Fig. 1 .
Fig. 1 Schematic representation of the gain mirror design revealing the positioning of the QWs at the antinodes of the optical field distribution at 1180 nm.
The main elements of the gain mirror are a 25.5-pair AlAs/GaAs DBR and a gain region with 10 GaInNAs QWs. The QWs are distributed in five identical pairs at the standing wave anti-nodes formed within the Fabry–Pérot (FP) cavity defined by the DBR and the semiconductor-air interface. The quantum well pairs are embedded within GaAs, which absorbs the pump light and supplies carriers to QWs for providing optical gain. The GaAs layers surrounding the QW group located deepest in the structure are thicker than those surrounding the QW groups placed closer to the surface; this configuration reflects the pump intensity depletion due to absorption and should ensure a more uniform carrier distribution between the QW groups. The compressive lattice strain of the QWs was partially compensated by 4-nm thick tensile-strained GaAsN layers grown on both sides of each QW. The GaAsN layers have a beneficial effect also in terms of red-shifting the emission wavelength [17]. A 0.75λ thick Al0.31Ga0.69As window layer was grown on top of the structure to confine the photo-generated carriers within the active region and to avoid non-radiative surface recombination. Finally, a 5 nm thick GaAs cap layer was grown to prevent oxidation of the window layer. The QWs were grown at a low temperature of about 450 °C with a growth rate of 0.95 μm/h and a high As/III beam equivalent pressure ratio of 25, which was found to lead to preferential formation of N-In bonds and better stability and optical quality of the as-grown material [18]. The N2 flow rate to the plasma source was 0.18 sccm and the RF power was 175 W. These settings correspond to a relative small amount, estimated to be ~0.6%, of nitrogen incorporated into the QWs. After the growth, the structure was in situ annealed in the MBE chamber for 7 minutes at a temperature of 680 °C.
Figure 2 reveals the evolution of the reflectivity spectra and photoluminescence (PL) signal with increasing gain chip temperature. The reflectivity spectra were measured from an as-grown gain mirror whereas the PL was measured in the laser setup from a gain mirror bonded to a diamond heat spreader with an anti-reflection coating. The reason for making the PL measurement in the laser setup was to take advantage of the temperature control. The power of the PL excitation laser was kept low (a few tens of mW) in order to prevent undesired heating of the sample.
Fig. 2 Reflectivity and photoluminescence spectra of the gain mirror measured at different temperatures.
From these measurements we estimated that the DBR stop-band red-shifts at a rate of only 0.06 nm/K while the PL peak wavelength red-shifts at a rate of 0.3 nm/K. Hence, the detuning between QW absorption edge and the resonant wavelength of the gain mirror decreases with increasing temperature, as revealed by the resonance-enhanced absorption dip in the stop-band. However, owing to a higher rate of non-radiative recombination, the PL intensity decreases with increasing temperature regardless of the better match between luminescence spectrum and the resonant wavelength of the gain mirror. This behavior emphasizes the importance of wavelength detuning between the PL and resonant wavelength and, on the other hand, reveals the detrimental effect of temperature rise on light emission. For the gain mirror studied here, the room temperature PL peak wavelength (as measured from a quantum well sample grown prior to the gain mirror) was detuned to about 30 nm below the desired laser operation wavelength; this is the same detuning as for the chip used in Ref [11].
3. Laser characterization
3.1 Laser cavity and thermal management
The V-shaped laser cavity (Fig. 3 ) comprised the gain mirror, a high-reflectivity curved folding mirror and a planar output coupler. A fibre-coupled 808 nm diode laser was used for optical pumping of the gain mirror. The pump beam was incident onto the gain mirror at a 27° angle relative to the surface normal. The diameter of the pump spot on the gain chip was varied between ~320 μm and ~460 μm. The cavity was adjusted to achieve high power operation with a good beam shape. The as-grown gain chip was 2.5 × 2.5 mm2 in size and was capillary-bonded with water to a ~3 × 3 × 0.3 mm3 synthetic single-crystal diamond heat spreader to ensure efficient heat removal from the gain region [19]. The diamond had a 2° wedge angle with respect to semiconductor surface and was antireflection coated to avoid etalon effects and to minimize the pump reflection. The bonded chip was mechanically clamped to a water-cooled microchannel mount.
Fig. 3 The cavity design of the SDL (left) and a drawing of the water-cooled microchannel mount (right).
One of the factors limiting the output power of the 1180 nm SDLs we reported earlier [11] was linked to the use of a cooling mount that did not provide sufficient heat extraction from the gain chip at high pump powers. To ensure efficient heat extraction for the experiments reported here, we developed a metallic water-cooled microchannel mount (see Fig. 3). The microchannels were machined as close as possible to the diamond heat spreader, i.e. at a distance of about 1 mm from the closest edge of the diamond. The channel cross section was about 2 mm2 at the narrowest point. The thermal dissipation ability was simulated using COMSOL finite element tools. The pressure difference between the water at the input and output of the microchannels was estimated to be about 100 kPa. With this pressure difference, the water flow and the geometry of the mount was not sufficient to maintain constant temperature when the pump power was increased; the variation of the mount temperature with the pump power for constant pressure of the circulating water is shown in Fig. 4 . Nevertheless, the mount enabled more efficient heat dissipation compared to a standard mount without microchannels placed in the proximity of the diamond heat-spreader.
Fig. 4 The dependence of the mount temperature (TMount) on the pump power absorbed by the gain mirror during lasing at a constant water flow through the microchannel mount(measurements for pump spot diameter of 320 µm and output coupler transmission of 1.5%).
3.2 Influence of the output coupler on the output characteristics
The power conversion characteristics corresponding to different output couplers are shown in Fig. 5 . For these measurements, the pump spot diameter on the gain mirror was ~320 μm and the temperature for the cooling water was ~16 °C. As revealed in Fig. 6 , the slope efficiency and the threshold pump power increased with increasing output coupling. The maximum slope efficiency of 27%, estimated between the threshold and a pump power of 20 W, was reached with a 3% output coupling. However, it is noticed that regardless of the higher value at low pump powers, the slope efficiency with the 2.5% and 3% output couplers decreased more rapidly at high pump powers than it did with smaller output coupling. This is most likely due to higher gain required to match the increased output coupling, which means higher carrier concentration and, thus, higher rate of non-radiative recombination. Ultimately, the 1.5% output coupler resulted in the highest output power of ~7.5 W at a pump power of ~41.1 W and at the measured mount temperature of TMount = 21 °C. The maximum power conversion efficiency of 21% (pump light to signal light) was achieved with this output coupler at a pump power of 30.3 W. The pump power reflected from the gain mirror surface was measured to be 7% of the incident power.
Fig. 5 Output characteristics of the 1.18 μm SDL corresponding to output couplers with different transmission. The temperature of the cooling water was set to 16 °C and the diameter of the pump spot was ~320 μm.
Fig. 6 Variation of the slope efficiency, maximum output power, and the threshold pump power with the output coupling ratio, for a cooling water temperature of 16 °C and a pump spot diameter of ~320 μm. The slope efficiency was determined from linear fit in the range between threshold and a pump power of 20 W.
The deterioration of the slope efficiency at high pump powers due to non-radiative recombination can be alleviated by reducing the gain mirror temperature. In order to further increase the output power, the temperature of the cooling water was set to 1°C. Accordingly, the slope efficiency was increased, especially for higher output coupling, and the thermal roll-over point was shifted to higher pump powers. The maximum output power of ~9 W was achieved with TMount = 8 °C, again for the output coupler with a transmission of 1.5% as revealed in Fig. 7 .
Fig. 7 Output characteristics of the 1.18 μm SDL with cooling water temperature set to 1 °C. Pump spot diameter was ~320 μm.
3.3 Power scaling by increasing the pump spot size
Next, different pump spot sizes were tested with the temperature of the cooling water set to 1°C. For each of the pump spot sizes, we also tested the operation with different output couplers. In all cases, the 1.5% output coupler was found to be optimal with respect to maximum achievable output power. Thus, only the results with this particular output coupler are presented below. The pump spot size was changed by modifying the focusing lens between the pump laser and the gain mirror, followed by cavity readjustment. Figure 8 reveals that over 11 W of output power was generated by the SDL with increased pump spot diameter of 390 μm and with TMount = 10 °C. As the figure suggests, this was achieved by shifting the thermal roll-over to higher pump powers by decreasing the pump intensity. The slope efficiency, however, decreased with increasing spot size and, thus, thermal roll-over prevented the maximum output power from being increased further when pump spot diameter was increased to 460 µm. This can be explained by the non-ideal heat extraction from the gain mirror, as the heat extraction efficiency increases at a rate smaller than area [20]. Another, potentially important, limiting factor for the power scaling with the pump spot size, is the increase of the number of nonradiative defects within the optically pumped area. The larger the pump spot, the greater the risk to reach areas with higher density of defects or dislocation lines. We believe that further power scaling is possible by improving the heat dissipation and selecting gain chips with a lower density of defects.
Fig. 8 Left: Output characteristics of the 1.18 μm SDL for three pump spot sizes. Cooling water temperature was set to 1 °C and the transmission of the output coupler was 1.5%. Right: Typical output spectrum at an output power of 5 W.
Figure 8 reveals also a typical output spectrum corresponding to an output power of 5 W. Multiple peaks with spacing of about 0.156 nm were observed in the spectra, which were otherwise relatively broad and flat. The peaks originate from the etalon effect taking place in the 3 mm thick output coupler, despite the use of an antireflective coating on its back surface. To provide a smooth spectral shape, also the output coupler should be wedged. The origin of the dip in the spectrum around 1177.5 nm is not obvious, but we suspect it to be caused by the competition between etalon effects associated with the output coupler, diamond heat-spreader, and the resonant wavelength of the RPG mirror.
3.4 Frequency doubling to yellow
The same V-cavity SDL (Fig. 3) was used for frequency doubling experiments. For these experiments the output coupler M2 was replaced by a flat mirror with high reflectivity for both the fundamental and frequency doubled radiation. The folding mirror M1 had high transmission (of more than 90%) for the frequency doubled light and high reflectivity for the fundamental laser radiation. A 4 mm long type-I critically phase-matched BBO crystal, which was antireflection coated for 1220 nm radiation, was inserted into the cavity about 5 mm away from the flat mirror. The estimated mode diameter within the BBO crystal was 180 μm. With TMount = 9 °C (cooling water was set to 1 °C), we achieved about 6.2 W of frequency doubled output power. The power conversion characteristics are shown in Fig. 9 together with a typical output spectrum. With cooling water temperature set to 16 °C (TMount = 22 °C), the maximum output power of the frequency converted beam decreased to 5.2 W. A more thorough study of the frequency doubling behavior of the laser is currently underway.
Fig. 9 Frequency doubled output power as a function of absorbed pump power (left), and output spectrum and output beam shape corresponding to 4.4 W of output power (right). Cooling water temperature was set to 1 °C (TMount = 9 °C) during the output power measurement. The output spectrum and beam shape were measured close to room temperature.
4. Conclusions
We demonstrated a GaInNAs/GaAs-based semiconductor disk laser emitting a record-high output power of 11 W at ~1180 nm. This performance was achieved by using a single gain chip attached to a water-cooled microchannel mount with the temperature of ~10 °C. We studied the laser operation with various cavity arrangements corresponding to different sizes of the pump spot on the gain chip and different output couplers. The results are encouraging for future developments, which will aim at making dilute nitride SDLs reliable and practical sources of high-brightness yellow-orange laser radiation. In particular we will focus on further improvements of the gain structure and heat management and on the demonstration of narrow linewidth operation for meeting the laser guide star specifications.
Acknowledgments
We acknowledge the help received from Jari Nikkinen for the antireflection coating deposition. This work was financially supported by the Academy of Finland within the Mignon project (number 128364), Pirkanmaa TE-center and Areté Associates.
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