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Optics Express

Optics Express

  • Editor: Michael Duncan
  • Vol. 14, Iss. 26 — Dec. 25, 2006
  • pp: 12868–12871
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Power scalable semiconductor disk laser using multiple gain cavity

Esa. J. Saarinen, Antti Härkönen, Soile Suomalainen, and Oleg G. Okhotnikov  »View Author Affiliations


Optics Express, Vol. 14, Issue 26, pp. 12868-12871 (2006)
http://dx.doi.org/10.1364/OE.14.012868


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Abstract

We report on power scaling of optically-pumped semiconductor disk lasers using multiple gain scheme. The method allows for significant power improvement while preserving good beam quality. Total power of over 8 W was achieved in dual-gain configuration, while one-gain lasers could produce separately about 4 W, limited by the thermal rollover of the output characteristics. The results show that reduced thermal load to a gain element in a dual-gain cavity allows extending the range of usable pump powers boosting the laser output.

© 2006 Optical Society of America

1. Introduction

Optically pumped semiconductor disk lasers (SEDLs), also known as vertical-external-cavity surface-emitting lasers (VECSELs), can produce high power and good quality of the beam [1

1 . 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, 3620–3622 (2003). [CrossRef]

], however, both parameters are critically dependent on the efficiency of the heat removal from the gain structure operating under strong pumping condition. Heat spreaders with high thermal conductance, e.g. diamond, SiC or copper, providing efficient heat dissipation, are used in high power SEDLs to reduce rollover and thermal lensing [2–3

2 . M. Kuznetsov, F. Hakimi, R. Sprague, and A. Mooradian, “High-power (>0.5-W CW) diode-pumped vertical-external-cavity surface-emitting semiconductor lasers with circular TEM00 beams,” IEEE Photon. Technol. Lett. 9, 1063–1065 (1997). [CrossRef]

]. Although, technology of the wafer bonding and heat spreaders constantly improves, the beam quality and output powers achievable from SEDL would eventually be limited by the state-of-the-art of the thermal management. In order to increase the SEDL power, a number of approaches have been considered. The power scaling could obviously be achieved by increasing the mode size on the gain medium, however, some penalty to the beam quality can generally be expected [1

1 . 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, 3620–3622 (2003). [CrossRef]

]. It is also possible to distribute the pump power into a number of spots over the gain structure forming arrayed or “parallel” geometry [4

4 . J. E. Hastie, L. G. Morton, S. Calvez, M. D. Dawson, T. Leinonen, M. Pessa, G. Gibson, and M. J. Padgett “Red microchip VECSEL array,” Opt. Express 13, 7209–7214 (2005). [CrossRef] [PubMed]

]. The resulted increase in the output power would, however, correspond to the multiple beams with complicated spatial distribution. With gain segments formed on a large semiconductor chip and connected in series by the multibounce cavity, the fundamental-transverse mode with increased output power could be expected [5

5 . 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 TEM00 beams,” IEEE J. Sel. Top. Quantum Electron. 5, 561–573 (1999). [CrossRef]

]. This geometry, however, uses single heatsink, which may limit efficiency of heat extraction and is difficult to implement practically.

In this study we use two entirely separated gain elements with individual heat-spreaders placed in the same cavity for power scaling, while preserving high quality of the output beam. In principle, the concept can be applied to a number of gain elements. The multiple gain cavity could take on higher pump powers by sharing thermal load among different gain elements thus avoiding excessive heating and the rollover. It should be mentioned that similar approach was demonstrated to be attractive for solid-state disk lasers [6

6 . C. Stewen, K. Contag, M. Larionov, A. Giesen, and H. Hügel, “A 1-kW CW thin disc laser,” IEEE J. Sel. Top. Quantum Electron. 6, 650–657 (2000). [CrossRef]

]. The results show impressive power scalability up to 1 kW, however, with compromised beam quality.

2. Gain material and laser setup

The laser structure was designed for operation around 1050 nm and it comprises a 30.5-pair GaAs/AlGaAs distributed Bragg reflector and a gain section with 13 non-strain-compensated Ga0.74In0.26As quantum-wells grown monolithically on a GaAs substrate by molecular beam epitaxy. In order to improve the heat transfer from the gain section, transparent diamond heat spreaders of 300-µm thickness were capillary bonded on the top of the 2.5×2.5 mm gain samples. The gain samples were mounted in water-cooled copper heat sinks. The diamond-air interface was coated with a two-layer TiO2-SiO2 film to reduce pump and signal reflection. The Z-shaped laser cavity was defined by the two semiconductor gain mirrors, a curved 5%- output coupler mirror and a high reflective spherical folding mirror. The laser setup is shown in Fig 1. The gain structures were optically pumped at ~800 nm with multimode fiber-coupled diode systems at an angle of about 35°. The pump spot diameter on both gain media was about 180 µm, which matches the fundamental mode size of the cavity expected from numerical simulation. During all measurements, the temperature of the copper mounts was kept at 15°C.

Fig. 1. Semiconductor disk laser with the cavity including two separate gain reflectors and two curved mirrors. RoC=Radius of Curvature, R=Reflectivity

3. Results

The output characteristics, spectrum and beam quality factor M2 were measured from the dual-gain laser and two lasers each using single-gain element. The performance of each gain mirror was tested in a single-gain cavity with output coupling of 3%. The lasers were operated with the center wavelength near 1050 nm. Light output characteristics, shown in Fig. 2, demonstrate that power achieved from the dual-gain laser is increased by a factor of two compared with single-gain setup. This result indicates that the thermal load on the gain structures in dual-gain configuration is reduced compared with one-gain laser pumped with the same power. Consequently, the thermal rollover in a single-gain scheme limits usable pump power and prevents power scaling. In contrary, the dual-gain laser has an increased threshold of thermal rollover and allows for significant power scaling.

Fig. 2. Light output characteristics of dual-gain laser, single-gain lasers, and sum of the single-gain laser outputs versus total pump power.

The beam quality factor M2 was measured for orthogonal directions using an automated scanning slit device. For single-gain lasers M2 parameter was ~1.2 in both directions at the maximum power. The dual-gain disk laser has good beam stability with the output power indicating only minor increase of the M2- factor by 15–20% up to 1.28 / 1.45 at the output power of 8 W. The spatial beam distribution fitted with Gaussian shape is shown in Fig. 3.

Fig. 3. Intensity profile and the Gaussian fit of the output beam of the two-gain laser at output power of 8 W.

4. Conclusion

In conclusion, the motivation of this study is given by the ever growing pump power that cannot be entirely absorbed by a single gain element of the disk laser without an increase in the pump area. The solution based on the increase of the mode size is limited, since it is ultimately accompanied by certain degradation of the beam quality. We have demonstrated the method of power scaling in semiconductor disk lasers using dual-gain cavity. Dual-gain concept reduces the thermal load of the gain material, increases the threshold of rollover and extends capability for boosting the output power. Reduced thermal lensing also prevents the degradation in the beam quality. We have recognized that disk lasers could naturally adopt multi-gain cavity. With this geometry, the strong pump shared among gain media could be efficiently absorbed without thermal overload. The output from a high-power pump source can be split into a few beams to pump each gain structure separately. This geometry seems to be a promising solution for power scaling preserving the near diffraction-limited beam.

Acknowledgments

The authors acknowledge the support from the Academy of Finland, The Employment and Economic Development Centre for Pirkanmaa, EU FP6 NATAL (Contract No. 016769), Nokia foundation, Jenny and Antti Wihuri foundation and Emil Aaltonen’s foundation.

References and links

1 .

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, 3620–3622 (2003). [CrossRef]

2 .

M. Kuznetsov, F. Hakimi, R. Sprague, and A. Mooradian, “High-power (>0.5-W CW) diode-pumped vertical-external-cavity surface-emitting semiconductor lasers with circular TEM00 beams,” IEEE Photon. Technol. Lett. 9, 1063–1065 (1997). [CrossRef]

3 .

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 Photon. Technol. Lett. 15, 894–896 (2003). [CrossRef]

4 .

J. E. Hastie, L. G. Morton, S. Calvez, M. D. Dawson, T. Leinonen, M. Pessa, G. Gibson, and M. J. Padgett “Red microchip VECSEL array,” Opt. Express 13, 7209–7214 (2005). [CrossRef] [PubMed]

5 .

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 TEM00 beams,” IEEE J. Sel. Top. Quantum Electron. 5, 561–573 (1999). [CrossRef]

6 .

C. Stewen, K. Contag, M. Larionov, A. Giesen, and H. Hügel, “A 1-kW CW thin disc laser,” IEEE J. Sel. Top. Quantum Electron. 6, 650–657 (2000). [CrossRef]

OCIS Codes
(140.0140) Lasers and laser optics : Lasers and laser optics
(140.3480) Lasers and laser optics : Lasers, diode-pumped

ToC Category:
Lasers and Laser Optics

History
Original Manuscript: October 20, 2006
Revised Manuscript: November 30, 2006
Manuscript Accepted: December 15, 2006
Published: December 22, 2006

Citation
Esa J. Saarinen, Antti Härkönen, Soile Suomalainen, and Oleg G. Okhotnikov, "Power scalable semiconductor disk laser using multiple gain cavity," Opt. Express 14, 12868-12871 (2006)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-14-26-12868


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References

  1. 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, 3620-3622 (2003). [CrossRef]
  2. M. Kuznetsov, F. Hakimi, R. Sprague, and A. Mooradian, "High-power (>0.5-W CW) diode-pumped vertical-external-cavity surface-emitting semiconductor lasers with circular TEM00 beams," IEEE Photon. Technol. Lett. 9, 1063-1065 (1997). [CrossRef]
  3. 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 Photon. Technol. Lett. 15,894-896 (2003). [CrossRef]
  4. J. E. Hastie, L. G. Morton, S. Calvez, M. D. Dawson, T. Leinonen, M. Pessa, G. Gibson and M. J. Padgett "Red microchip VECSEL array," Opt. Express 13, 7209-7214 (2005). [CrossRef] [PubMed]
  5. 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 TEM00 beams," IEEE J. Sel. Top. Quantum Electron. 5, 561-573 (1999). [CrossRef]
  6. C. Stewen, K. Contag, M. Larionov, A. Giesen, and H. Hügel, "A 1-kW CW thin disc laser," IEEE J. Sel. Top. Quantum Electron. 6, 650-657 (2000). [CrossRef]

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