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A high-power optically-pumped vertical-external-cavity surface-emitting laser (VECSEL) generating 10.5 W of cw output power at 615 nm is reported. The gain mirror incorporated 10 GaInNAs quantum wells and was designed to have an emission peak in the 1230 nm range. The fundamental emission was frequency doubled to the red spectral range by using an intra-cavity nonlinear LBO crystal. The maximum optical-to-optical conversion efficiency was 17.5%. The VECSEL was also operated in pulsed mode by directly modulating the pump laser to produce light pulses with duration of ~1.5 µs. The maximum peak power for pulsed operation (pump limited) was 13.8 W. This corresponded to an optical-to-optical conversion efficiency of 20.4%.
© 2015 Optical Society of America
1. Introduction
High-power red lasers are useful for many applications, including laser projection, spectroscopy and medicine. For example, there is an increasing need for single-frequency high-brightness lasers emitting in the long visible wavelength range for quantum optics applications [1,2]. In addition, laser projection relies on lasers emitting >10 W of red, green and blue light. Currently, laser diodes emitting around 640 nm are used as the red light sources in display technology [3]. However, the human eye is more sensitive to shorter red wavelengths, therefore a laser emitting closer to 620 nm would allow higher lumen efficacy. Decreasing the wavelength of laser diodes below 640 nm has proved difficult and would result in severe power and lifetime limitations [4], hence other light sources should be considered.
Frequency doubled vertical-external-cavity surface-emitting lasers (VECSELs) have emerged as compact and wavelength versatile laser platforms generating high-brightness radiation in the visible wavelength range [5,6]. Their external cavity enables efficient frequency conversion by placing a nonlinear crystal inside the laser cavity and thus allows the exploitation of the high intra-cavity field. Furthermore, wavelength selective components (e.g. birefringent filters and etalons) can also be added inside the cavity for narrowing the spectral linewidth and achieving single-frequency operation [7]. Frequency doubled VECSELs have already been extensively reported in the green spectral range [8,9] and high powers have also been demonstrated in the yellow-orange range [10]. However, there are only a few reports on the red spectral range, 610–630 nm, due to the difficulty associated with semiconductor material fabrication for direct emission, as well as for frequency doubled emission. The highest power reported for an AlGaInP quantum well gain mirror is 1.2 W with direct emission at a longer wavelength of 665 nm [11]. GaInAs material system is a good candidate for frequency doubled VECSELs at the shorter visible wavelength range, but it suffers from high lattice strain at wavelengths longer than 1100 nm. This makes it difficult to grow quantum wells emitting in the 1220–1260 nm range needed for frequency doubling to 610–630 nm.
Using semiconductor gain mirrors based on GaInNAs, a wavelength emission range covering 1.1 µm to 1.5 µm can be achieved [12,13]. Compared to GaInAs material systems, the addition of nitrogen decreases the lattice strain, as well as the band gap, enabling the fabrication of gain mirrors at longer wavelengths. So far, most of the efforts on developing GaInNAs-based VECSELS have been focused on demonstrating fundamental emission at 1180 nm and subsequent doubling to 590 nm with power levels above 10 W [14]. In contrast, the developments of red VECSELs based on GaInNAs gain mirrors have been scarce with the best reported power being 4.6 W at 610 nm [15].
In this article, we report on achieving output power of more than 10 W for a red frequency doubled continuous wave (cw) VECSEL utilizing GaInNAs quantum wells. We also report pulsed operation of such VECSEL by directly modulating the pump laser. High peak power operation with lower average power would especially benefit medical applications, because damage to healthy tissue can be decreased by decreasing the average power and thus the thermal load on the tissue.
2. Gain mirror design and fabrication
The gain mirror wafer was grown by molecular beam epitaxy on a GaAs substrate. The gain mirror structure was designed to be resonant at 1230 nm and comprised 10 GaInNAs quantum wells (QWs) surrounded by GaAs barrier layers. The QWs were located at the anti-nodes of the standing optical wave formed inside the active region (see Fig. 1 for an illustration of the structure). The thickness of a QW was 7 nm and the corresponding In and N compositions were 31% and 0.5%, respectively. A 50-nm-thick GaInP window layer terminated the active region and served as a carrier barrier layer preventing surface recombination. Clustering within the QW layers was limited by using a growth temperature of 375 °C. After the growth, the wafer was annealed for 4 minutes at ~700 °C to improve the material quality and diced into 3 x 3 mm2 chips. For efficient heat extraction, the gain chip was capillary bonded to a 300-µm-thick CVD diamond heat spreader. The diamond was nominally flat, but it became apparent during measurements that the surfaces were not parallel causing beam distortion. Furthermore, the diamond was attached to a liquid-cooled copper mount via indium foil. The outer surface of the diamond was coated with an anti-reflective coating for the pump radiation (808 nm) and the fundamental lasing wavelength (1230 nm).
Fig. 1 Schematic illustration of the active region of the gain chip. The In and N compositions were x ~35% and y ~0.5%, respectively.
3. Laser characterization
3.1 Experimental setup
The VECSEL was built into a V-shaped configuration formed by the gain mirror and two curved dielectric mirrors (external mirrors), as shown in Fig. 2. The folding mirror had a radius of curvature (RoC) of 50 mm and the cavity end mirror had a RoC of 25 mm. The gain mirror was optically pumped using an 808 nm diode laser delivering a maximum cw power of ~120 W. The pump radiation was focused to a ~400 µm diameter spot (1/e2) on the gain mirror using a 200-µm fiber and converging lenses. The total length of the cavity was ~147 mm with the first arm having a length of 89 mm. The mode diameter on the gain mirror was 240 µm tangential and 300 µm sagittal. The elliptical shape of the mode was a straight consequence of the ~30° folding angle of the V-shaped cavity.
Fig. 2 Illustration of the VECSEL configuration used for measuring the transmittance of the HR cavity end-mirror and long-wave-pass filter (λ > 900 nm). The HR cavity end-mirror is represented by the mirror inside the dashed box and is not part of the VECSEL cavity.
For efficient intra-cavity frequency doubling, both external cavity mirrors need to be highly reflective (HR) for the fundamental lasing wavelength of 1230 nm. We measured the infrared transmittance of the HR cavity end-mirror (RoC = 25 mm) in order to determine the intra-cavity power. For this measurement, the VECSEL cavity incorporated a HR folding mirror and a partially reflective mirror (output coupler) as the cavity end-mirror. The HR cavity end-mirror (also HR for the red spectral range) was then placed in front of the VECSEL’s output beam. A power head meter was used to measure the output power through the HR mirror as well as when the HR mirror was removed from the beam path. The percentage of infrared radiation transmitted through the HR mirror was calculated by dividing the measured power readings. The transmittance of the mirror is wavelength sensitive; hence, this process was repeated for several wavelengths around 1230 nm. The VECSEL was wavelength tuned by placing a 1.0-mm-thick birefringent filter inside the cavity (see Fig. 2 for a detailed illustration of the cavity setup). The transmittance of the long-wave-pass filter (used to filter red radiation leaking through the HR cavity end-mirror) was measured in a similar way. The transmittances of these optical components as a function of wavelength are shown in Fig. 3.
Fig. 3 Transmittance of the HR cavity end-mirror, Tmirror, and the long-wave-pass filter, Tfilter, (λ > 900 nm) as a function of wavelength.
For frequency doubling measurements, the output coupler in Fig. 2 was replaced by the HR mirror which was characterized as illustrated in Fig. 3. In addition, a 100-µm-thick etalon was added to the cavity for linewidth narrowing and wavelength control, and the 1.0-mm-thick BRF was kept in place for the same reason. The red spectral range was achieved by inserting a nonlinear lithium triborate (LBO) crystal inside the cavity, near the mode waist (~70–80 µm in diameter). The crystal was 10 mm long and specified for Type I second harmonic generation (SHG) cut for non-critical phase matching (NCPM). Phase matching is vital for efficient frequency conversion and it was achieved by temperature stabilizing the crystal with a copper oven to a temperature optimized for frequency doubling at 1230 nm. This was achieved by observing the frequency doubled output power while changing the temperature of the crystal’s oven. The temperature that resulted in the highest output power was 20.1 °C. The frequency doubled radiation was extracted from the cavity through the folding mirror, which was highly reflecting for the fundamental wavelength but had low reflectivity (< 5%) for the red spectral range. The cavity configuration is shown in Fig. 4. In addition, a short-wave pass filter was used between the folding mirror and the power head to filter out infrared radiation leaking through the folding mirror. Whereas, a long-wave pass filter was used between the HR cavity end-mirror and a power head to filter out the red light leaking through the cavity end-mirror.
3.2 Continuous wave operation
At first, the VECSEL was operated in continuous wave mode. The cooling liquid was set to a temperature of 1 °C, which corresponded to mount temperatures of 5–8 °C during operation. The maximum frequency doubled output power of 10.5 W was achieved for an absorbed pump power of 59.7 W. This corresponded to an optical-to-optical (absorbed pump power to red output power) conversion efficiency of 17.5%. Absorbed pump power was measured by subtracting the amount of 808 nm pump power reflected from the surface of the gain mirror. The amount of pump radiation reflected from the gain mirror surface was 4.2%. The emission wavelength of the VECSEL was centered at 615 nm (inset of Fig. 5) and exhibited a FWHM linewidth of 0.4 nm. The beam profile (inset of Fig. 5) was recorded with a CCD camera showing an elliptical shape horizontally, which might be a consequence of multi-transversal mode operation potentially caused by the larger pump spot diameter in comparison to the mode diameter. The output spectrum and the beam profile were recorded at 7 W of output power. The output power curve is shown in Fig. 5.
Fig. 5 Output power curve for red emission (output power vs. absorbed pump power). Insets: Output spectrum for red emission and lateral beam profile measured at 7 W.
The intra-cavity power of the VECSEL was determined by measuring the infrared radiation leaking through the HR cavity end-mirror, whose transmission was determined earlier. The intra-cavity power was then estimated by dividing the measured infrared power with the transmission of the mirror. Furthermore, the single-pass conversion efficiency of the LBO crystal was calculated using the measured red output power and the intra-cavity power results. The calculation assumed that there are no losses for the red light inside the cavity, and all of the red light escapes the cavity. At the highest output power, the intra-cavity power was estimated to be 640 W. The maximum single-pass conversion efficiency of 0.8% was obtained for a red output power of 9.6 W. An almost equal conversion efficiency value was obtained for the highest red output power. Results of the intra-cavity power and single-pass conversion efficiency calculations are shown in Fig. 6.
Fig. 6 Intra-cavity power (blue circles) and single-pass conversion efficiency (red squares) of the nonlinear crystal (LBO) vs. absorbed pump power.
3.3 Pulsed operation
The VECSEL was also pumped in pulsed mode. This reduces the thermal load on the chip and leads to higher conversion efficiency. The same cavity configuration was used for the pulsed measurements as for cw measurements except the cw 808 nm pump laser was switched to a pulsed 808 nm pump laser. The pump laser was pulsed using a driver capable of producing current pulses up to 1.5 µs. When the driver was set to produce 1.5 µs pulse widths, the pump laser produced peak powers up to 70.6 W. Si-based detectors were used to record the light pulses generated by the pump laser and the VECSEL. The pulse waveforms measured at the maximum output power are shown in Fig. 7 for the pump laser and the VECSEL. The cooling liquid of the gain mirror’s mount was set to near room temperature at 20 °C. Owing to the decreased thermal load on the gain mirror, the mount temperature remained at 20 °C throughout the pulsed measurements. For comparison, the VECSEL’s output power was also measured in cw mode while the mount temperature was near 20 °C. In this case, the cooling liquid was set to 16 °C. Figure 8 shows the output power curves for both, the pulsed and cw operation, as well as the VECSEL’s output spectrum. The maximum output power achieved in cw mode was 8.0 W, whereas in pulsed mode the maximum peak output power was 13.8 W for a pulse width of 1.16 µs. Furthermore, thermal rollover was reached for the cw operation, whereas in pulsed mode the maximum peak output power was pump power limited. The maximum average output power for the pulsed operation was 161 mW and the maximum optical-to-optical conversion efficiency (absorbed peak pump power to peak output power) was 20.4%.
Fig. 7 Pulse waveforms for the pulsed pump laser (black trace) and the red VECSEL output (red) recorded at maximum frequency doubled output for 1.5 µs pulse width setting.
Fig. 8 (a) Output power curves for continuous wave (red circles) and pulsed mode (blue triangles) when the mount temperature was ~20 °C. (b) Output spectrum for the red VECSEL measured at the maximum pulsed pump power.
4. Conclusions
A high-power GaInNAs VECSEL emitting at 615 nm, operating in continuous wave and pulsed modes, has been demonstrated. The VECSEL emitted fundamental radiation in the 1230 nm range and was frequency doubled to ~615 nm via an intra-cavity nonlinear LBO crystal. A maximum of 10.5 W was reached in cw mode with an optical-to-optical conversion efficiency of 17.5% for a mount temperature of 8 °C. The single-pass conversion efficiency of the LBO crystal was estimated to be 0.8%. In pulsed mode, the VECSEL generated a maximum of 13.8 W of peak output power for pulse duration of 1.16 µs. In this case the optical-to-optical conversion efficiency was 20.4% and the mount temperature stayed at 20 °C throughout the measurements. The maximum peak output power in pulsed mode was pump power limited; hence, even higher peak output powers are expected with a better pulsed pump laser. The cw power measurements were limited by thermal rollover.
Acknowledgments
This work was financially supported by the Academy of Finland project Qubit (decision #278388) and TEKES project ReLase (40016/14). The authors would like to acknowledge Jari Nikkinen for the deposition of the AR-coating on the diamond heat spreader. The main author would also like to acknowledge Walter Ahlström and Ulla Tuominen Foundations for financial support.
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