Open Access
Add to BibSonomy
Abstract
A vertical external cavity surface emitting laser (VECSEL) has been developed for a sodium guide star application. Stable single frequency operation with 21 W of output power near 1178 nm with multiple gain elements while lasing in the TEM00 mode has been achieved. Higher output power results in multimode lasing. For the sodium guide star application, the 1178 nm can be frequency doubled to 589 nm. The power scaling approach used involves using multiple gain mirrors in a folded standing wave cavity. This is the first demonstration of a high power single frequency VECSEL using a twisted-mode configuration and multiple gain mirrors located at the cavity folds.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
VECSELs typically have wavelengths in the near infrared but can range from 665 nm to 5 µm [1–5]. Using VECSELs in combination with techniques such as second harmonic generation allows for a large wavelength range into the UV providing many potential applications [6–8]. The 1178 nm laser developed here is intended to be frequency doubled to 589 nm and used as the laser source for a sodium guide star. A sodium guide star application requires that the laser source be high power, stable, and have a narrow spectral linewidth [9].
When power scaling a VECSEL one of the main challenges is thermal management. As the pump power increases the semiconductor gain chip temperature increases and the output power eventually starts to roll over. Two ways to deal with the thermal load involve adding more gain chips (longitudinal scaling) or using a larger beam diameter (lateral scaling). Lateral scaling uses a larger diameter beam to use more of the gain mirror and distribute the thermal load. Increasing the beam diameter has a fundamental limitation since the rollover pump power scales linearly with the beam radius while the lasing threshold scales with the square of the beam radius. This eventually creates a situation where the threshold pump power is higher than the roll over pump power [10]. Longitudinal scaling involves adding more gain chips without changing the beam diameter. Adding more gain chips distributes the pump power and heat load to avoid roll over. This technique has been demonstrated by Hunziker et al. to achieve single frequency output of 55 W at 532 nm [10]. Longitudinal scaling is our chosen approach for high power single frequency operation and this paper is focused on reporting our initial design and results and its narrow-linewidth operation which is not discussed in [10].
2. VECSEL design
A folded standing wave cavity with eight legs was used with four gain mirrors. A schematic of the cavity is shown in Fig. 1. In this configuration there is a gain mirror (GM) placed at each beam waist. The remaining mirrors are four high reflectivity (HR) spherical mirrors, with a 200 mm radius of curvature (ROC), and one 93% reflective output coupler with a 250 mm ROC. The seven short cavity legs are each 141 mm and the long leg is 210 mm for a total cavity length of 1197 mm. Each leg has an angle of incidence (AOI) of 6° on each gain mirror to accommodate the 30 mm spacing between the gain mirrors. This combination of mirrors and cavity lengths has a cavity mode spot radius of 185 µm on each gain mirror.
The gain mirrors are commercial components developed by Vexlum. Each gain mirror has 10 quantum wells made of GaInNAs providing gain at 1178 nm. The quantum wells are designed to be used in a standing wave cavity so each well aligns with an antinode of the 1178 nm standing wave. The structure employs a “flip-chip” design with a monolithic GaAs based distributed Bragg mirror grown on top of the epi-layers [1]. The front side of the gain mirror structure is antireflection coated for the lasing wavelength of 1178 nm and the pump wavelength of 808 nm. Pump absorption takes place within GaAs regions separating the quantum wells. To cool the gain mirror it is soldered onto a diamond heat spreader and the heat spreader is soldered on a gold-plated copper heat sink. Two gain mirrors are placed on each copper heatsink with an on center spacing of 30 mm. The gain mirrors are placed with an accuracy < 0.1 mm and the copper heat sink is flat enough to keep the angle between the gain mirrors < 3 mrad. The arrangement is shown in Fig. 2. The copper heat sink is attached to a water cooled cold plate and a thermoelectric chiller is used to keep the water temperature at 20° C.
Each gain mirror is pumped with a fiber coupled 808 nm diode laser module (DILAS M1F1S22-808.3-35C-SS2.18). Four 35 W fiber modules, one for each gain mirror, were used in total. The output fiber has a core diameter of 105 µm and a NA of 0.22. The pump light is collimated and then imaged on to the gain mirror with a magnification of 3.5 producing a pump spot radius of 182 µm. The collimating and imaging lenses are standard off the shelf spherical lenses. Higher order transverse cavity modes have a larger spot size on the gain mirror than the fundamental mode so the lasing of higher order modes can be controlled by selecting a pump spot that is similar to the fundamental spot size. Due to the compact nature of the setup out of plane pumping was used. The pump optics are arranged to shoot down from above the resonator plane and a mirror is placed just under the resonator plane to reflect the light onto the gain mirror with an AOI of about 15°. The pump optics arrangement is depicted in Fig. 1.
At each gain mirror there is a forward and backward traveling wave that create an interference pattern. First consider the case where the cavity only contains a birefringent filter; which is discussed more later. The birefringent filter is inserted into the cavity at Brewster’s angle which creates two linear eigenpolarizations that have different amounts of loss. The eigenpolarization with the lowest loss becomes the dominant polarization. The interference of four linearly polarized waves, incident and reflected for each forward and backward wave, is shown in Fig. 3, where ϕ is the relative phase difference between the forward and backward going waves. The intensity distribution seen by a quantum well is not uniform creating a similar effect to spatial hole burning as seen in solid-state lasers with standing wave cavities. The nodes in the intensity distribution in the quantum well plane make it impossible for a single longitudinal mode to saturate the gain of the quantum well resulting in unstable operation.
Utilizing a “twisted-mode” configuration changes the interference pattern allowing for a single mode to saturate the gain [13]. In the twisted-mode configuration two quarter wave plates are used in the cavity to change the eigenmode polarization from linear to circular at the gain mirrors while maintaining linear polarization throughout the rest of the cavity [11–13]. Now there are four circularly polarized waves interfering at each gain mirror but the forward and backward traveling waves are counter-rotating so they are orthogonal. The resulting interference pattern has uniform antinodes at each quantum well as illustrated in Fig. 4.
Fig. 4. Interference pattern created by counter-rotating circularly polarized waves at each gain mirror [11,12].
Coarse wavelength selection is provided by an uncoated crystalline quartz birefringent filter. The quartz plate is a-cut and placed in the cavity at Brewster’s angle. After the plate is tilted it is rotated around the normal to the plate’s surface to adjust the lasing wavelength. Fine adjustment of the filter is achieved through temperature control of the quartz plate. For single longitudinal mode operation two a-cut 2.6 mm thick crystalline quartz etalons were used. Both etalons are placed in the cavity near normal incidence. The first etalon has its fast axis in either the horizontal or vertical position and remains static. The second etalon is then rotated around the optical axis until single mode operation is seen near 1178 nm. Rotating the second etalon allows for adjustment of the lasing wavelength while maintaining single mode operation. The gain mirrors have gain properties that vary with pump power therefore single mode operation cannot be maintained at all power levels with the same birefringent filter and etalon settings. The birefringent filter and etalon settings were optimized for maximum output power. Using two birefringent etalons complicates the behavior of the cavity filter function and a more rigorous analysis is planned for future work.
3. Results
The VECSEL was able to achieve 39 W of output power with 116 W of pump power without the twisted-mode and wavelength control, representing 34% o-o efficiency with respect to the incident power. A plot of the pump power versus output power is shown in Fig. 5. Figure 5 also shows the output power curves for empty one and two gain mirror cavities. The one gain mirror cavity is V shaped with 4% output coupling and the two gain mirror cavity is W shaped with 7% output coupling. Longitudinal scaling increases the output power of the lasers but some diminishing returns can be seen with each device added. After inserting the quarter wave plates, birefringent filter, and both etalons the output power was reduced to 21 W near 1178 nm with a total pump power of 100 W. As expected, the output power drops due to the increase in cavity loss created by the additional elements. At 21 W the laser was frequency stable and operated in the fundamental TEM00 mode. Single mode operation was verified with a scanning Fabry Perrot interferometer that has a resolution of 7.5 MHz. The laser would maintain single mode operating for at least 5 minutes before jumping to multimode operation but would return to single frequency operation within a few seconds with an open-air setup. This was considered stable enough for proof of concept considering the cavity is open making it susceptible to air currents and thermal effects. While the laser was frequency stable the output power did fluctuate by as much as half a watt. Adjusting the rotation angle of either etalon 1 or etalon 2 allowed for some frequency tunability while remaining in single frequency operation.
Fig. 5. Output power versus pump power for one, two, and 4 gain mirror cavities without the twisted-mode and wavelength control components.
At 24 W of output the power the laser is qualitatively judged to be semi-stable. Shifting between single and multimode operation is much more common and output power fluctuations have larger swings. An etalon or combination of etalons with a narrower transmission peak would be required for higher power operation.
4. Future work and conclusions
Future plans to improve this design involve a rigorous analysis of the filter function of the cavity and etalon requirements for higher power operation. Improvements to stability can be made by enclosing the system to isolate it from air currents and thermal effects. The etalons can also be temperature controlled for better stability.
This is the first demonstration of a single frequency VECSEL that uses a twisted-mode configuration and multiple gain mirrors in a folded cavity. We have achieved an output power of 21 W at 1178 nm while maintaining stable single longitudinal mode operation. The configuration shown has high potential to meet the needs of a sodium guide star laser source.
Funding
U.S. Air Force (FA9451-19-C-0580).
Disclosures
The authors declare no potential conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time. Requests for data are subject to approval by the Unites States Air Force.
References
1. M. Guina, A. Rantamaki, and A. Harkonen, “Optically Pumped VECSELs: Review of Technology and Progress,” J. Phys. D: Appl. Phys. 50(38), 383001 (2017). [CrossRef]
2. J. E. Hastie, S. Calvez, and M. D. Dawson, “High power CW red VECSEL with linearly polarized TEM00 output beam,” Opt. Express 13(1), 77–81 (2005). [CrossRef]
3. M. Rahim, A. Khiar, F. Felder, M. Fill, H. Zogg, and M. W. Sigrist, “5-µm vertical external-cavity surface-emitting laser (VECSEL) for spectroscopic application,” Appl. Phys. B 100(2), 261–264 (2010). [CrossRef]
4. M. Gaulke, J. Heidrich, B. O. Alaydin, M. Golling, A. Barh, and U. Keller, “High average output power from a backside-cooled 2-µm InGaSb VECSEL with full gain characterization,” Opt. Express 29(24), 40360–40373 (2021). [CrossRef]
5. J. Heidrich, M. Gaulke, B. O. Alaydin, M. Golling, A. Barh, and U. Keller, “324-fs Pulses from a SESAM modelocked backside-cooled 2-µm VECSEL,” IEEE Photonics Technol. Lett. 34(6), 337–340 (2022). [CrossRef]
6. Y. Kaneds, J. M. Yarborough, Y. Merzlyak, A. Yamaguchi, K. Hayashida, N. Ohmae, and H. Katori, “Continuous-wave, single-frequency 229 nm laser source for laser cooling of cadmium atoms,” Opt. Lett. 41(4), 705–708 (2016). [CrossRef]
7. P. H. Moriya, Y. Singh, K. Bongs, and J. E. Hastie, “Sub-kHz-linewidth VECSELs for cold atom experiments,” Opt. Express 28(11), 15943–15953 (2020). [CrossRef]
8. J. Paul, Y. Kaneda, T.-L. Wang, C. Lytle, J. V. Moloney, and R. J. Jones, “Doppler-free spectroscopy of mercury at 253.7 nm using a high-power, frequency-quadrupled, optically pumped external-cavity semiconductor laser,” Opt. Lett. 36(1), 61–63 (2011). [CrossRef]
9. X. Huo, Y. Qi, Y. Zhang, B. Chen, Z. Bai, J. Ding, Y. Wang, and Z. Lu, “Research development of 589 nm laser for sodium laser guide stars,” Opt. Lasers Eng. 134, 106207 (2020). [CrossRef]
10. L. Hunziker, Q.-Z. Shu, D. Bauer, C. Ihli, G. Mahnke, M. Rebut, J. Chilla, A. Caprara, H. Zhou, E. Weiss, and M. Reed, “Power-scaling of optically pumped semiconductor lasers,” Proc. SPIE 6451, 64510A (2007). [CrossRef]
11. Y. Kaneda, D. Mitten, M. Hart, S. H. Warner, J.-P. Penttinen, and M. Guina, “Longitudinal scaling of VECSEL output power maintaining narrow linewidth,” Proc. SPIE 11263, 9 (2020). [CrossRef]
12. Y. Kaneda, M. Hart, S. H. Warner, J.-P. Penttinen, and M. Guina, “Narrow-linewidth operation of folded 1178 nm VECSEL with twisted-mode cavity,” Opt. Express 27(19), 27267–27272 (2019). [CrossRef]
13. V. Evtuhov and A. E. Siegman, “A “Twisted-Mode” Technique for Obtaining Axially Uniform Energy Density in a Laser Cavity,” Appl. Opt. 4(1), 142–143 (1965). [CrossRef]
