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We report for the first time on the performance of 1300 nm waveband semiconductor disc lasers (SDLs) with wafer fused gain mirrors that implement intracavity diamond and flip-chip heat dissipation schemes based on the same gain material. With a new type of gain mirror structure, maximum output power values reach 7.1 W with intracavity diamond gain mirrors and 5.6 W with flip-chip gain mirrors, using a pump spot diameter of 300 µm, exhibiting a beam quality factor M2< 1.25 in the full operation range. These results confirm previously published theoretical modeling of these types of SDLs.
© 2014 Optical Society of America
1. Introduction
1300 nm waveband semiconductor disc lasers (SDLs) emitting in the range of 1250 nm- 1350 nm, made possible the demonstration of novel picosecond pulse generators employing passive mode-locking of 1300 nm SDLs [1], 1380 nm Raman fiber lasers pumped with 1300 nm SDLs [2] and 1330 nm mode-locked seed SDLs in combination with Bi-doped fiber amplifiers [3]. Besides obvious fiber-optics applications in optical communications and optical clocking in supercomputers, this wavelength range presents interest also in spectroscopy [4], free-space optical communications [5], etc. In addition, 1300 nm waveband SDLs offer an excellent solution for producing high power lasers emitting in the red spectral region at around 650 nm by intra-cavity frequency doubling of 1300 nm SDLs [6].
Watt-level SDLs emitting in the 1300 nm band were first demonstrated using the wafer fusion fabrication technique [7]. In this approach, gain mirrors include AlGaInAs/InP quantum well (QW) active regions that are wafer-bonded to AlGaAs/GaAs distributed Bragg reflectors (DBRs). Typically, 980 nm diode lasers are used for optical pumping of these gain mirrors for generating electron-hole pairs that recombine in the QWs of the active region to produce laser emission in the 1300 nm band. The difference in energy of pump photons and emitted photons, non-radiative recombination and absorption of pump light outside the active region produce heating of the gain mirrors that needs to be extracted by appropriate heat-spreading schemes.
As in other types of gain mirrors, the heat generated in 1300 nm wafer-fused mirrors may be quite efficiently dissipated either via intra-cavity diamond heat-spreaders [6,7], or in the flip-chip (thin disc) heat dissipation scheme [8,9]. So far, the maximum output power levels of 1300 nm SDLs reached 6.6 W using the intra-cavity diamond heat-dissipation configuration with a 300 µm pump spot [6] and, very recently, 6.1W with the flip-chip heat dissipation configuration with a 400 µm pump spot [9]. The fabrication of gain mirrors in the flip-chip heat dissipation configuration is more complex, but this approach allows circumventing optical distortions and losses that are typically introduced when using intra-cavity diamond heat spreaders. On the other hand, the modeling results of Ref. 10 show that gain mirrors with intracavity diamond heat spreaders should have the advantage of better heat dissipation for pump-spot sizes below ̴500 µm. In 1300 nm SDLs reported so far, the pump spot size values vary between 180 µm and 400 µm. These values of pump spots are quite practical because for smaller pump spot sizes the quality uniformity of both diamond and fused mirror structure are quite sufficient for reaching a good beam quality factor M2 close to 1.2. However, in the previous publications there were no reports on 1300 nm gain mirrors implementing both heat dissipation schemes on the same active sub-cavity material, which would confirm the advantage of the intra-cavity diamond heat dissipation scheme for the pump spot sizes in the mentioned range.
In this letter, we report for the first time on 1300 nm waveband wafer fused gain mirrors that implement either intra-cavity diamond or flip-chip heat dissipation schemes based on the same active material. We demonstrate maximum output power values of 7.1 W with intracavity diamond gain mirrors and 5.6 W with flip-chip gain mirrors using a pump spot size of 300 µm, with a beam quality factor M2< 1.25 in the full operation range.
2. Gain mirror design, fabrication and characteristics
In the previously reported 1300 nm SDL that produced maximum output power of 6.6W [6], the gain mirror included an active region that comprised 5 groups of 2 (2-2-2-2-2) AlGaInAs QWs placed in the anti-nodes of the electric field distribution. On the other hand, our work on 1490 nm SDLs [11] showed that replacing five AlGaInAs QWs groups (the 2-2-2-2-2 structure) with three groups (4-2-2 structure), every group being placed in anti-nodes of the electric field distribution, results in improved performance due to a better matching of the QWs to the absorption profile of the pump laser and improved thermal properties. Following this optimization concept, in the current work we have implemented a 3-3-2-2 QWs structure taking into consideration the lower optical confinement factor and barrier height in the 1300 nm QWs. In addition, in this work the active region was fused to a binary 24 pairs AlAs-GaAsDBR to allow the implementation of the flip-chip heat dissipation scheme. The refractive index profile and the calculated standing wave pattern of the electric field in the gain mirror are shown in Fig. 1.
Fig. 1 Refractive index profile (green) and the calculated standing wave pattern of the electric field (blue) across the gain mirror structure.
Compressively strained (1%) 6.7 nm thick AlGaInAs QWs, with room temperature photoluminescence (PL) peak at 1255 nm, were embedded between 10 nm thick AlGaInAs strain compensation layers and positioned by un-strained AlGaInAs/InP spacers to form a resonant periodic gain structure. The topmost InP layer of the gain section provides additional carrier confinement, preventing carrier diffusion to the surface and subsequent non-radiative recombination. The total sub-cavity length was designed for emission at 1280 nm. The InP-based 3-3-2-2 QW active gain structure was grown by metalorganic vapor phase epitaxy (MOVPE) on (100) InP substrates.
Figure 2 depicts the measured photoluminescence (PL) features at different temperatures of the 3-3-2-2 QWs gain structure before bonding it to the AlAs-GaAs DBR. The PL intensity decreases with temperature as expected but the AlGaInAs QWs still show considerable luminescence intensity at temperatures as high as 100°C (Fig. 2(a)). Retaining more than 50% of the room temperature PL intensity at about 70°C, which corresponds to the gain mirror temperature at maximum SDL output power, attests to the high optical quality of the active material for applications in SDLs (Fig. 2(b)). The peak of the PL spectra red-shifts with temperature by 0.5 nm/°C (Fig. 2(c)).
Fig. 2 Photoluminescence spectra of the 3-3-2-2 QWs gain structure at different temperatures, from 10 to 100°C (a), PL peak intensity variation with temperature (b), and PL peak wavelength vs. temperature (c).
As depicted on Fig. 3, starting from the same gain mirror fused stack (Fig. 3(a)) one can fabricate either gain mirrors with intracavity diamond configuration after etching the InP substrate and performing molecular bonding to diamond (Fig. 3(b)), or gain mirrors with flip-chip heat dissipation scheme after selectively etching the GaAs substrate, bonding to diamond using Au-Au bonding layers and finally etching the remaining InP substrate (Fig. 3(c)).
Fig. 3 Schematics of the initial fused gain mirror stack (a), intra-cavity diamond (b) and flip-chip (c) configurations.
After bonding the InP-based active region to the AlAs-GaAs DBR, we first etch on a part of the fused wafer the InP substrate for gain mirror sub-cavity tests and lasing tests in the intra-cavity diamond heat-dissipation configuration. Figure 4 depicts the PL spectra of the fused gain mirror sub-cavity in the temperature range of 20 - 80 °C. The PL peak intensity versus temperature has a maximum at 40 °C, indicating that at this temperature the maximum of the QW PL intensity spectrum is aligned with the sub-cavity mode. The PL peak of the sub-cavitymode red-shifts with temperature by 0.22 nm/°C, reflecting the temperature dependence of the refractive index of the cavity material.
Fig. 4 Photoluminescence spectra of the fused sub-cavity at 20 to 80°C (left) and peak PL intensity variation with temperature (right).
In the intracavity diamond heat spreader configuration, the gain chip is capillary-bonded onto a 3x3x0.350 mm3 CVD diamond heat spreader using deionized water. The top surface of the CVD diamond is then pressed against a water-cooled copper block with a 2 mm diameter circular aperture for passing the signal and the pump beams. Thin indium foil is placed between the diamond and the copper block to ensure good mechanical and thermal contact. Finally, the top surface of the diamond is antireflection (AR) coated with TiO2/SiO2 layers designed for the signal wavelength. This is done to minimize the losses and beam distortion that can be introduced by the intracavity diamond.
To fabricate gain mirrors in the flip-chip heat-dissipation scheme, we selectively etch the GaAs substrate on another part of the fused gain mirror stack. This is followed by e-beam evaporation of Ti-Au on the DBR side and cleaving the gain mirror into 3x3 mm2 chips. These chips undergo bonding to diamond heat-spreaders. To this end, we perform a low-temperature Au-Au thermo-compression wafer bonding using a bonder machine similar to the one employed for the wafer fusion of the gain mirrors. With this technique, gain mirrors are Au-Au bonded at 150°C on 5x5x0.3 mm3 highest thermal grade polycrystalline CVD diamond heat-spreaders, with thermal conductivity > 1800 W/mK. The bonding process is followed by InP substrate selective etching and lasing characterization tests. This process shows a high degree of repetabily from batch to batch and a very good uniformity of the performance across the gain mirror. More fabrication details can be found in Ref. 8.
3. Laser characteristics
The output power of the SDL as a function of pump power (Fig. 5) was measured in a V-type laser configuration (see [6]) with a pump-spot size diameter of 300 µm and an output mirror of 2.5% coupling. The cavity mode was closely matched to the pump spot. The temperature of the circulating water used to stabilize the chip temperature was kept at 7°C. The absorbed pump power in the flip chip configuration comprises the total pump diode power from which the 38% of reflected pump power is subtracted. In the intracavity diamond configuration, the reflected pump power was 7% of the total incident pump power. We reached maximum output power of 7.1 W with the gain mirror in intra-cavity diamond heatdissipation configuration. This value is by 0.5 W higher compared to the record value of 6.6 W that was previously obtained using an intracavity diamond spreader with the same pump spot diameter and the same heat sink temperature [6]. The SDL with the flip-chip gain mirror configuration reached saturation at the maximum output power of 5.6 W. For both SDL configurations, the slope efficiency is close to 21%. The beam quality parameter M2 with the flip-chip SDL was independent of pump power and was measured to be 1.19 in the vertical direction and 1.24 in the horizontal direction. With the intracavity diamond SDL, the M2 increased from 1.21 at threshold to 1.24 at the highest output power in both directions. The higher output power of the SDL with intra-cavity diamond heat-spreader compared with the flip-chip configuration can be explained by better thermal properties for the 300 µm pump spot size, in accordance with the results of Ref.11. In addition, previous numerical studies [12] have shown that antireflection coating of the flip-chip gain mirror would further decrease the maximum output power. On the other hand, the higher threshold pump power in the intracavity diamond configuration can be explained by reduced sub-cavity gain enhancement due to the AR-coated intracavity diamond [13]. Similar slope efficiency values for both configurations suggests that optical losses induced by intracavity diamond could be very low using a pump spot size of 300 µm.
Fig. 5 Output power versus pump power for the flip-chip and intracavity diamond heat-spreading SDL configurations. Inset shows the schematics of the V-type laser configuration.
Figures 6 and 7 show SDL lasing spectra acquired with the circulating cooling water temperature set to 15 °C at different pump power levels for both types of gain mirrors considered in this work. The emission spectra wavelengths, close to 1280 nm, correspond well to the emission wavelength of the designed structure. As one can observe from Fig. 6, anothereffect of the intra-cavity diamond consists in the fragmentation of the lasing spectra into multiple lines that are spaced according to the free spectral range of the intracavity diamond Fabry-Perot resonator. On the other hand, the lasing spectra of the SDL employing a gain mirror with flip-chip heat dissipation scheme (see Fig. 7) are much less perturbed. Nevertheless, spectral distortions induced by the intracavity diamonds can be considerably reduced by employing wedged diamonds and applying antireflective coatings as shown, for example, in Ref.6. In addition, the gain mirrors employing intracavity diamond are easier to fabricate and can be used in applications where reaching maximum power levels in the 1300 nm waveband are required
4. Conclusions
1300 nm waveband SDL fabrication approach based on the wafer fusion technique allows producing gain mirrors with intracavity diamond or flip-chip heat dissipation schemes from the same gain material. Based on a new gain mirror structure, we demonstrate maximum output power values of 7.1 W with intracavity diamond gain mirrors and 5.6 W with flip-chip gain mirrors employing a pump spot diameter of 300 µm, with a beam quality factor M2< 1.25 in the full operation range. These results are in line with previous modeling work of these types of SDLs. The output power should be extendible to the 10W regime by further optimizing the gain enhancement and optical losses and improving the lateral uniformity of the gain mirrors and diamond heat-spreaders that will allow using larger spots of the pumping laser.
Acknowledgments
This work was partially supported by a grant from the Swiss National Foundation, the Swiss Nanotera Valorization fund and by the Academy of Finland via project T-REPTILE (#269121).
References and links
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