IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 19, NO. 23, DECEMBER 1, 2007
1925
Enhancement of Pumping Efficiency in a Vertical-External-Cavity Surface-Emitting Laser Ki-Sung Kim, Jaeryung Yoo, Gibum Kim, Sangmoon Lee, Soohaeng Cho, Junyoun Kim, Taek Kim, and Yongjo Park
Abstract—We report an improved optical pumping efficiency in a vertical-external-cavity surface-emitting laser (VECSEL) by employing the double-band mirror (DBM), which acts as a pump beam reflector as well as a cavity mirror. The optical pumping efficiency of a VECSEL with the DBM was increased by 23%, as compared to one with the conventional distributed Bragg reflector. It was found that the use of the DBM was more effective in the VECSEL with the shorter absorption layer thickness. Index Terms—Double-band mirror (DBM), high-power lasers, InGaAs quantum well (QW), pumping efficiency, resonant periodic gain (RPG), semiconductor lasers, vertical-external-cavity surface-emitting laser (VECSEL).
I. INTRODUCTION
H
IGH-POWER blue/green lasers are of commercial interest in laser projection display. Among various types of lasers, an optically pumped vertical-external-cavity surface-emitting laser (VECSEL) combined with intracavity frequency doubling has been suggested as a promising solution to provide visible light sources for displays [1], [2], because an optical pumping makes it possible to increase the gain volume in surface-emitting devices to emit high output power with good beam quality. When a conventional VECSEL chip is optically pumped by over tens of watts, a great deal of pump beam is generally not absorbed at the absorption layer and wasted through the substrate. This deteriorates the thermal properties and the output performance of the optically pumped VECSEL. On the other hand, the increase of the number of resonant periodic gain (RPG) for the complete consumption of pump beam can induce the undesirable effect on the laser performance such as the increase of the threshold and the deterioration of the thermal characteristics. In order to solve the inherent drawbacks in an optically pumped VECSEL, the concepts for the recycling of the pump beam were suggested by some research groups [3], [4]. However, the practical demonstrations have not been reported and analyzed in terms of the improvement of pump beam absorption efficiency. Here, we report a promising method for enhancing the laser performance of a 920-nm VECSEL through recycling the wasted pumping energy.
Manuscript received June 15, 2007; revised August 13, 2007. The authors are with Photonics STU, Samsung Advanced Institute of Technology, Suwon 440-600, Korea (e-mail:
[email protected]). Color versions of one or more of the figures in this letter are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/LPT.2007.908771
Fig. 1. (a) DBM stack, and (b) reflectivity spectra of DBM only and DBM RPG.
+15
II. EXPERIMENT The VECSEL chip structure was grown on a (001) GaAs substrate by a metal–organic vapor phase epitaxy reactor. We initially grew a double-band mirror (DBM) on GaAs substrate. The DBM was designed to have two stopbands centered at 920 and 808 nm, and reflectivities of both wavelengths were intended to be over 99.9%. To achieve this reflector, a multilayer stack of the : where H Al GaAs and L [AlAs] are form [4] ( LH quarterwave layers of high and low refractive index at the center wavelength of 860 nm) was used, as shown in Fig. 1(a). The numbers and were carefully chosen to have the designed wavelength separation between two band and high reflectivities, respectively. It can be seen in Fig. 1(b) that there are two distinct stopbands centered at 920 and 808 nm on a sample having only DBMs from the reflectance measurement. This clearly shows
1041-1135/$25.00 © 2007 IEEE
1926
IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 19, NO. 23, DECEMBER 1, 2007
Fig. 2. Light output characteristics of VECSEL chips with a DBM (circle) and a DBR (triangle). L–L curves were measured at room temperature and an output mirror with reflectivity of 94%.
that the designed DBM is well optimized for reflecting the fundamental laser light and pump light. Then InGaAs quantumwell (QW) RPGs were grown on the DBM. Multi-QWs were placed at the every antinode of the standing wave in the RPG. The barriers between QWs were composed of the strain compensating GaAs P and pump laser absorbing Al GaAs or GaAs layers. An Al GaAs barrier prevents the excited carriers from recombining at the wafer surface. A 10-nm-thick GaAs layer finishes the structure. The reflectance trace of a VECSEL chip (15 RPG DBM) is also shown in Fig. 1(b). Besides the observed ripples in the only DBM reflectance trace, several additional dips appear periodically, which come from the absorption by resonance between the DBM and GaAs–air surface. The offset between gain and resonance wavelength is 10 nm. After the structure growth, an optically transparent single crystal diamond was capillary bonded on the VECSEL chip surface [5]. The laser setup used in this study consisted of a pumping laser diode, an external mirror, and a VECSEL chip, where the cavity of the laser was formed between a DBR and an external spherical mirror (radius of curvature 50 mm). An 808-nm pump beam was focused on to a VECSEL chip at an angle of 30 with respect to the surface normal. The pump beam diameter illuminated on the chip surface was found to be 250 m. The length of cavity was varied from 45 to 50 mm to match a mode size with a pump size. III. RESULTS AND DISCUSSION Fig. 2 shows the typical output power curves of VECSEL chips with a DBM (circle) and a DBR (triangle). The reflectivity of an external mirror and a heat sink temperature were 94% and 20 C, respectively. The RPG structures were identical for both samples, that is, 15 RPG and double 4-nm-thick QWs at every antinode. Assuming an absorption coefficient of 8000 cm at the absorption layer of RPG and an input power of 24 W, 80% of pump beam power is absorbed when they propagate through the absorption layer with the thickness of about 2 m and the rest of pump beam power (20%) is expected to be reflected from the DBM. With increasing incident input power, the output powers of both samples are linearly increased without suffering thermal
Fig. 3. Maximum output powers and thresholds as a function of heat sink temperature. The maximum output power was measured at an input power of 24 W and an output mirror with reflectivity of 94%.
rollover for all operating range. The maximum output power is limited by the available pump input power. However, it is clearly shown that the maximum output power and the output efficiency for the DBM sample are increased by 8.5% and 12%, respectively, compared to those of the DBR one. As a result, the DBM samples reache a maximum output power of more than 11 W at an incident power of 24 W, corresponding to a slope conversion efficiency of up to 56%. The pump absorption efficiencies were measured to be 80% and 65% for the DBM and the DBR sample, respectively. The beam quality was not affected by the employment of the of both samples were measured DBM. The beam qualities to be 1.3 at the threshold pump power and increased to 2 at the maximum output. In order to investigate the thermal characteristics of DBM samples, we measured the output power and the threshold input powers at various heat sink temperatures, as shown in Fig. 3. Compared to the DBR one, the DBM sample exhibits higher W and lower maximum output powers input power thresholds for all temperatures. Based on the maximum output power at 20 C, the power reductions at 60 C are measured to be 22% and 19% for the DBM and the DBR sample, respectively. To further assess the DBM effect on the VECSEL performance, another RPG structure was also investigated. The details of the RPG structure are shown in the inset of Fig. 4. As an attempt to make better lasing performances than those of Fig. 2, several concepts for a RPG structure were introduced [6]. First, we inserted an Al Ga As nonabsorbing layer (carrier confinement layer) at node positions of standing wave not to generate excessive carriers which tended to recombine in the GaAs barriers. The Al Ga As layer was also expected to act as a diffusion barrier which prevented carriers from diffusing to the longer path to the QW. Second, we made the structure stepwise by locating the GaAs P layer far from the QWs and used the GaAs layer as barriers. This RPG scheme is very similar to a separate confinement heterostucture lasers. Therefore, effective carrier confinement within the RPG unit was expected by the employed structure. Third, since an actual thickness of the absorption layers (GaAs and GaAsP layer) was reduced to 1 m, more than 45% of pump beam power can be wasted through a
KIM et al.: ENHANCEMENT OF PUMPING EFFICIENCY IN A VECSEL
1927
power at the same input power is increased by 32% (7.35 W) when the pump beam reflector is introduced on the DBR layers. In addition to this, the threshold pump power is decreased from 2 to 1.3 W in case of the presence of the pump beam reflector. Therefore, it can be concluded that the pump beam reflection scheme is more effective in the thinner RPG structures. IV. CONCLUSION
Fig. 4. DBM effect of a VECSEL with nonabsorbing layers at node position and ten RPG on the VECSEL performance. L–L curves were measured at room temperature and an output mirror with reflectivity of 94%. Inset shows conduction band energy profile of the RPG structure.
DBR. Of course, pump beam can be more utilized by increasing the number of RPGs. However, this causes the undesirable effects on lasing performances such as a higher threshold and a temperature rise of chip due to a longer distance between a heatspreader and RPGs. Accordingly, a pump beam recycling scheme was also employed to solve the problem. Fig. 4 shows the power transfer characterization of the 920-nm VECSEL with and without pump beam reflector on DBR layers. Unlike our expectation, both cases show significantly lower lasing performances than those of RPG structure of Fig. 2. It was experimentally proven that the deterioration of lasing performances in the RPG structure of Fig. 4 is strongly related to the nonabsorbing layers and barrier layer composition. A detailed study of the barrier structures of a 920-nm RPG structure will be published elsewhere. As far as the pump beam recycling is concerned, the recycling effects on lasing performance of the RPG structure in Fig. 4 are more prominent, compared to those of Fig. 2. In the case of conventional VECSEL (without pump beam reflector), the W with maximum output power is 5.58 W input power weak thermal roll-over. On the other hand, the maximum output
The lasing performance of an optically pumped InGaAs-based VECSEL with a DBM, which reflects a pump beam as well as a fundamental one, was investigated. The higher pump absorption efficiency was achieved by the pump beam recycling. As a result, a CW output power of 11.4 W with a high efficiency of 56% and good beam quality was demonstrated at room temperature. The use of the DBM did not deteriorate the thermal characteristics of the VECSEL chip, resulting in the only 22% reduction of maximum output power as the heat sink temperature was increased up to 60 C. The DBM effect was proven to be more dominant for a thinner RPG condition. REFERENCES [1] M. Kuznetsov, F. Hakimi, R. Sprague, and A. Mooradian, “Design and characteristics of high power (>0.5-W CW) diode-pumped vertical-external-cavity surface-emitting semiconductor lasers with circular TEM beams,” IEEE J. Sel. Topics Quantum Electron., vol. 5, no. 3, pp. 561–573, May/Jun. 1999. [2] J. L. Chillar, H. Zhou, E. Weiss, A. L. Caprara, Q. Shou, S. V. Govorkov, M. K. Reed, and L. Spinelli, “Blue and green optically pumped semiconductor lasers for display,” Proc. SPIE, vol. 5740, pp. 41–47, 2005. [3] R. Haring, R. Paschotta, E. Gini, F. Morier-genoud, D. Martin, H. Melchior, and U. Keller, “Picosecond surface-emitting semiconductor laser with >200 mW average power,” Electron. Lett., vol. 37, pp. 766–767, 2001. [4] S. Calvez, D. Burns, and M. D. Daeson, “Optimization of an optically pumped 1.3-m GaInNAs vertical-cavity surface-emitting laser,” IEEE Photon. Technol. Lett., vol. 14, no. 2, pp. 131–133, Feb. 2002. [5] J. M. Hopkins, S. A. Smith, C. W. Jeon, H. D. Sun, D. Burns, S. Calvez, M. D. Dawson, T. Jouhti, and M. Pessa, “0.6 W CW gainnas vertical external-cavity surface emitting laser operating at 1.32 m,” Electron. Lett., vol. 40, no. 1, pp. 30–31, Jan. 2004. [6] J. Y. Yoo, K. S. Kim, S. M. Lee, S. J. Lim, G. B. Kim, J. Y. Kim, S. H. Cho, J. H. Lee, T. Kim, and Y. J. Park, “Gain structure optimization of vertical external cavity surface emitting laser at 920 nm,” Appl. Phys. Lett., vol. 89, pp. 131125–131127, 2006.
