1. Introduction

The advantages of optically pumped semiconductor disk lasers (SDLs) include their capability of producing multi-Watt output powers with excellent beam quality [1]. The external cavity of SDLs allows various optical components to be conveniently integrated into the cavity, and for power scaling by increasing the mode area on the gain element or by using multiple gain elements. However, the most prominent feature of SDLs is their spectral versatility as the range of accessible wavelengths for semiconductor technology spans from the ultraviolet to 5 μm [2-3].

Significant progress has been achieved in recent years in SDL technology based on InGaAs quantum wells (QWs) emitting at wavelengths 920-1180 nm. High infrared (IR) power obtained in continuous wave regime [4–6] has been efficiently converted into blue, green and yellow wavelengths by intracavity frequency doubling [4, 7–10]. Further extension of the SDL spectral coverage to the 600-900 nm and the 1200-1600 nm spectral ranges requires special efforts and different approaches. One way to extend the operation wavelength of SDLs is to use in-well pumping instead of the traditional barrier pumping. For emission wavelengths in the range 830-870 nm, this scheme can utilize the inexpensive and powerful pump sources emitting around 800 nm and has enabled up to 1 W of output power from a GaAs/AlGaAs SDL [11,12]. In-well pumping has also been proposed as a solution to cover the 700-820 nm spectral band [13]. This approach, however, suffers from the low power and the high cost of the available pump diodes emitting around 670 nm. Using frequency-doubled solid-state systems, e.g. Nd:YVO₄ based lasers, for pumping SDLs emitting below 800 nm is another option. Although this approach is unlikely to be a practical solution, output powers of 1 W at 675 nm from a GaInP QW SDL [14] and 52 mW at 716-755 nm from an InP quantum-dot (QD) system have been reported [15].

Contrary to the SDLs emitting at 600-830 nm, various sources are readily available for pumping SDLs operating at 920-1600 nm. Though the state-of-the-art pumping technology has contributed significantly to the rapid progress of high power InGaAs QW SDLs, extending the emission wavelength of InGaAs structures beyond 1180 nm is limited by the increased lattice mismatch between GaAs and InGaAs. The operation wavelength can be shifted beyond 1180 nm by adding nitrogen into the InGaAs QWs as demonstrated in [16], but this approach requires higher accuracy in the growth conditions and suffers from enhanced non-radiative processes with increasing nitrogen content needed to tailor the operating wavelength to longer wavelengths [17, 18]. Although the dilute nitride GaInNAs structures have produced 11 W at 1180 nm [19], the reported output power drops down to 600 mW at 1320 nm [20]. Alternatively, quantum dot (QD) nitrogen-free structures have been employed to produce radiation at wavelengths above 1180 nm [21] and orange emission through frequency doubling [22]. The QD structures, however, suffer from lower gain as compared to the QW structures and the prospect for their exploitation to achieve lasing beyond 1300 nm is still unclear.

InP-based QWs are ideal for semiconductor lasers operating at the 1200-1600 nm spectral band [23]. Monolithic InP-based lasers with vertical cavity, however, suffer from low refractive index contrast and poor thermal conductivity of the compounds lattice-matched to InP [24], requiring thick distributed Bragg reflectors (DBRs) with reduced performance [25, 26]. This problem can be overcome by wafer fusion [27], which allows the integration of InP-based QWs and high quality GaAs/AlGaAs DBRs. Particularly, the high power wafer-fused SDLs operating at 1200-1600 nm [28–31] have enabled 3 W of output power at 650 nm by frequency doubling [32].

We have previously reported 2.6 W of output power from a wafer-fused SDL operating at 1580 nm [31]. In this Letter, we scale this power up to 4.6 W which allows 1 W of frequency-doubled output power at 785 nm to be obtained. SDLs operating at the 700-800 nm wavelength range could provide an alternative to the expensive and bulky Ti:sapphire laser when broad wavelength tunability is not required [33]. The SDL concept also allows for enhanced power scaling with diffraction limited beam quality as compared to laser diodes [34]. In all, intracavity frequency doubling of SDLs emitting at 1400-1600 nm allows to cover the 700-800 nm spectral range and represents a practical scheme since it uses powerful and inexpensive 980 nm pump diodes.

2. Wafer-fused SDL emitting at 1580 nm

The active medium of the SDL discussed here was grown on an InP substrate by low pressure metalorganic vapor phase epitaxy (LP MOVPE). The 5 pairs of compressively strained (1%) AlGaInAs QWs were positioned at the antinodes of the optical field. The measured photoluminescence peak is centered near 1520 nm at room temperature. The DBR grown by solid-source molecular beam epitaxy (SS MBE) on a GaAs substrate consists of 35 pairs of quarter wave thick Al_0.9Ga_0.1As and GaAs layers. The wafers were then processed using the 2-inch wafer fusion technique described in [35]. After the fusion, the InP substrate was removed by wet-etching using HCl. The GaInAsP etch-stop was removed using H₃PO₄:H₂O₂:H₂O (3:1:3). The structure then was cut into pieces of 2.5 x 2.5 mm². The thermal management of the structure was achieved by capillary bonding a 300 μm thick transparent diamond on top of the gain section using de-ionized water [36]. The assembly was then placed between water-cooled copper blocks using thin indium foil to ensure good mechanical and thermal contact. Finally, an antireflection (AR) coating at the signal wavelength was deposited on the diamond surface. The role of the antireflection coating is to preserve good beam quality while the spectral fringes cannot be avoided completely.

The optical pumping was performed with a 980 nm fiber-coupled diode laser that was focused onto a spot with a diameter of 300 μm at the gain element. The cavity geometry ensured that the size of the laser mode matched closely to the pumped spot. A schematic of the cavity is given in Fig. 1 . The output power characteristics with the corresponding spectra are shown in Fig. 2 . The maximum output powers of 4.2 W and 4.6 W were obtained at gain element temperatures of 15 °C and 8°C, respectively. The output power was limited by thermal roll-over. The beam quality parameter M² was measured to be below 1.25 in both transverse directions at all pump powers. The multiline spectra shown in the inset of Fig. 2 originate from the Fabry-Pérot etalon effect induced by the intracavity diamond heat spreader.

 

Fig. 1 Cavity schematic of the wafer-fused SDL producing 4.6 W of output power at 1580 nm. The output coupling ratio is close to 1.5%.

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Fig. 2 Output power as function of pump power for two gain element temperatures. The laser spectra and the output beam taken at the highest output power are shown in the insets.

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Compared to our previous report [31], two key cavity parameters responsible for power scaling were thoroughly optimized to achieve the observed increase in the output power. First, the pump spot and the cavity mode diameter on the gain element were increased from 180 μm to 300 μm, which resulted in reduced temperature rise in the gain element at a given pump power. Second, since the laser efficiency critically depends on the output coupling ratio, a precise setting of the output coupling ratio is crucial for the laser optimization. A tunable output coupler was employed for accurate adjustment of the output coupling ratio. The transmission of the output mirror could be changed by moving the mode location across the mirror surface. With an optimal value of the output coupling ratio, a notable increase in output power was achieved.

3. Generation of 785 nm radiation by frequency conversion

The schematic of the cavity comprising the nonlinear crystal for frequency doubling is shown in Fig. 3 . The second harmonic output at 785 nm is obtained through mirror 2 (M₂), which is transparent for the frequency-doubled radiation. Mirror 3 (M₃) is highly reflective for both fundamental and second harmonic wavelengths. The nonlinear crystal is a critically phase-matched, Type I, 6 mm long lithium triborate (LBO) crystal that was AR coated for the signal and the second harmonic wavelengths. The temperature of the LBO crystal was fixed to 25 °C by mounting it into a Peltier cooler in order to preserve the phase matching condition in the LBO crystal [37, 38]. The 785 nm output power as a function of 980 nm pump power and the corresponding spectra are shown in Fig. 4 . The output power reaches 1 W at 42 W of pump power and is limited by thermal roll-over. The beam quality parameter M² was measured to be below 1.45 in both transversal directions at all pump powers, allowing a 70% coupling efficiency into a single mode fiber.

 

Fig. 3 Cavity schematic of the frequency-doubled SDL producing 1 W of output power at 785 nm. The mode diameter in the 6 mm long LBO crystal was estimated to be 140 μm.

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Fig. 4 Frequency-doubled output power as a function of pump power. The temperature of the gain element was kept at 15°C. The laser spectra and the output beam taken at the highest output power are shown in the insets.

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The output power characteristic for the frequency-doubled radiation, shown Fig. 4, is typical for lasers for which the conversion efficiency is relatively low and the fundamental intracavity power increases approximately linearly with pump power [39–41]. One factor responsible for the relatively low conversion efficiency could be a limited phase matching bandwidth of the LBO crystal. It should be noted, however, that the acceptance bandwidth of the LBO crystal is ≈5.3 nm, which approximately corresponds to the −3 dB spectral width of the laser. Nevertheless, improvements in the conversion efficiency could be obtained by using crystals with wider acceptance bandwidths or/and by reducing the bandwidth of optical spectrum by using appropriate filters or etalons [42]. Further improvement in the conversion efficiency could be achieved by applying polarization control of the fundamental wave to prevent the LBO crystal from affecting the laser polarization.

4. Conclusion

We demonstrate a wafer-fused optically pumped semiconductor disk laser generating 4.6 W of output power at 1580 nm. Through intracavity frequency doubling the disk laser produces up to 1 W of output power at 785 nm. This disk laser concept allows for substantial tailoring of the operation wavelength and offers the potential for power scaling with good beam quality. Future work includes further power scaling of long-wavelength semiconductor disk lasers and improvement of the frequency conversion efficiency. This progress is expected through thermal management enhancement of the gain element and optimization of the laser cavity.

New applications for long-wavelength semiconductor disk lasers are expected to arise due to the unique combination of characteristics they offer – high power and low-noise operation. In particular, optical pumping of Raman fiber lasers and amplifiers with 1200-1600 nm semiconductor disk lasers provides significant advantages over conventional pumping techniques based on diode lasers and/or fiber converters and could radically change Raman fiber technology. Finally, high power SDLs operating at 700-800 nm could significantly increases the number of potential SDL applications, including spectroscopy, atomic cooling and biophotonics.