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A single frequency wafer-fused semiconductor disk laser at 1.56 µm with 1 watt of output power and a coherence length over 5 km in fiber is demonstrated. The result represents the highest output power reported for a narrow-line semiconductor disk laser operating at this spectral range. The study shows the promising potential of the wafer fusion technique for power scaling of single frequency vertical-cavity lasers emitting in the 1.3-1.6 µm range.
©2013 Optical Society of America
1. Introduction
Semiconductor disk lasers (SDLs) combine the power scaling properties of thin disk lasers with the wavelength scalability inherent to semiconductor technology. The performance of these devices critically depends on efficient heat removal from the active medium, which is conventionally based on the flip-chip approach or the intracavity heat spreader approach [1, 2]. While in the flip-chip design heat is extracted through the distributed Bragg reflector (DBR), in the intracavity heat spreader design heat is extracted directly from the light emitting top surface of the gain chip using a transparent heat dissipater with high thermal conductivity. The optimal strategy for thermal management depends largely on the targeted power level, the wavelength range that determines the composition of the SDL structure and the application.
The flip-chip approach is more appropriate for devices with high thermal conductivity DBRs and large pump spot diameters, whereas the intracavity heat removal is preferred with low thermal conductivity DBRs and small pump spots [1, 2]. Generally speaking, the flip-chip design is superior for the spectral range around 1 μm where GaAs/AlAs DBRs offer high thermal conductivity, while the intracavity heatspreader technique is preferred with low performance DBRs at other wavelengths [3, 4]. In particular, monolithically grown InP-based SDLs operating at wavelengths 1.3-1.6 μm suffer from increased DBR layer thicknesses, a greater number of constituent DBR layer pairs needed to cope with the low refractive index contrast between the layers, and significantly lowered thermal conductivity of the available compounds. Accordingly, SDLs intended for operation at 1.3-1.6 μm mainly utilize intracavity diamond heatspreaders that avoid the poor heat transfer through the DBR by creating the short heat removal path between the gain medium and heat spreader [2, 5, 6]. The intracavity heatspreader also provides an efficient means for suppressing thermal lensing in the gain element [5, 7].
In addition to their wavelength and power scalability, the high-Q cavity inherent to SDLs makes them ideal for single frequency operation [8–10] with several applications including laser communications, light detection and ranging (LIDAR), spectroscopy, sensing and seed laser operation [11, 12]. The single longitudinal mode operation of SDLs is typically obtained by utilizing short-length cavities of 5-30 mm [10, 13, 14] or by introducing wavelength selective elements into the cavity [15, 16]. However, it should be noted that the noise of the pumping source could limit the achievable linewidth of an optically-pumped SDL [10, 13, 15].
Though numerous narrow-line SDLs operating at broad spectral range have been demonstrated [13, 17–19], single frequency SDLs emitting in the telecom range of 1.55 μm represent a big challenge because monolithically grown structures suffer from low-quality DBRs that limit the obtainable output power [20–23]. To date, the best performance in single frequency operation around 1.55 μm has been obtained with a monolithic InP-based laser utilizing a thin intracavity diamond heatspreader that also acted as an intracavity filter [24]. The obtained output power at room temperature was 170 mW.
Recently, we have demonstrated a wafer-fusion technique that allows the integration of high-quality GaAs/AlGaAs DBRs with InP-based active regions and, consequently, the power scaling of 1.3-1.6 μm SDLs to multi-watt levels [25–29]. In this letter, we report a wafer-fused single frequency SDL with 1 watt of output power and a coherence length longer than 5 km in single mode fiber. The result represents the highest output power obtained from a single frequency SDL at this wavelength range and is the first demonstration of a narrow-line wafer-fused SDL.
2. Experimental
The active region of the resonant periodic gain structure was grown by low pressure metalorganic vapor phase epitaxy (LP MOVPE) on an InP substrate. The gain region comprises 5 pairs of compressively strained AlGaInAs quantum wells (QWs) that were placed at the antinodes of the optical field. The measured photoluminescence peak is centered near 1520 nm at room temperature. The DBR was grown by solid source molecular beam epitaxy (SS MBE) on a GaAs substrate and comprises 35 pairs of quarter-wave thick GaAs/Al0.9Ga0.1As pairs. The active region and the DBR were fused together using a process described in [30]. The InP substrate was then removed by wet-etching using HCl and the GaInAsP etch-stop was removed using H3PO4:H2O2:H2O. The assembly was cut into pieces of 2.5 × 2.5 mm2 and the gain chip was capillary bonded to a 3 × 3 × 0.3 mm3 intracavity diamond using de-ionized water [31]. Finally, the top surface of the diamond was pressed against a copper plate using a piece of Teflon [32]. The copper plate had a 2 mm diameter circular aperture for the signal and pump beams. Thin indium foil was placed between the diamond and the copper plate to ensure good mechanical and thermal contact. The temperature of the gain element was controlled by placing it on a Peltier cooler.
A schematic of the laser V-cavity is shown in Fig. 1 . The optical pumping was performed with a 980 nm fiber-coupled diode laser that was focused onto an approximately Gaussian spot with a diameter of 300 µm at the gain element. The total cavity length was 32 cm with cavity arms of 12 cm and 20 cm. The cavity design, low thermal lensing in the gain element and accurate cavity alignment ensured the overlap between the pump spot and the fundamental cavity mode [5, 7, 33]. Two fused silica etalons with thicknesses of 500 µm and 750 µm were inserted into the cavity to facilitate single frequency operation. Single frequency operation was also obtainable using just one etalon, but only at small intervals of pump power with a given etalon angle. The second etalon allowed fixing the laser wavelength to the middle of the gain bandwidth and, consequently, enabled single frequency operation at all pump powers without any further alignment. The optimal alignment of the etalons corresponded to nearly normal incidence of the intracavity radiation. The actual position of the etalons within the cavity was not critical to the single frequency operation.
The output power in single frequency operation as a function of pump power is shown in Fig. 2 , with the output spectrum shown in the inset. Variation up to 25% was observed in the output power by changing the location of spot on the gain material. The single frequency operation was confirmed with a scanning Fabry-Perot interferometer (FPI) with a free spectral range of 1.5 GHz. The FPI spectrum is shown in Fig. 3 . The inset of Fig. 3 displays a close-up of the instrument resolution limited 18 MHz (FWHM) line.
Fig. 2 Output power in single frequency regime as a function of pump power at two gain element temperatures. The output spectrum and the output beam at the highest output power are shown in the inset.
Fig. 3 Scanning Fabry-Perot spectrum taken at output power of 950 mW. The free-spectral range of the FPI is 1.5 GHz. A close-up of the 18 MHz (FWHM) line is shown in the inset.
The linewidth of the laser was then characterized using a delayed self-heterodyne interferometer (DSHI) with a 25 µs delay and 100 MHz acousto-optic modulator [34]. The DSHI output is shown in Fig. 4 with 40 kHz oscillations in the wings of the signal. These oscillations correspond to the inverse of the DSHI delay time and indicate that the coherence length of our laser is longer than the 5 km DSHI fiber delay [35–37]. Figure 4 also shows a theoretical fitting of a purely Lorentzian-shaped signal that is provided as a reference for the overall signal shape, excluding the peak in the middle [35]. It should be noted that an accurate value for the laser linewidth would not be reliably obtainable using a more advanced fitting procedure accounting for both Loretzian and Gaussian spectral distributions [38, 39] due to the large number of free parameters [40, 41]. Nevertheless, the DSHI measurement allows concluding that the coherence length of our laser is longer than the 5 km fiber delay. The corresponding linewidths for Lorentzian and Gaussian spectral distributions are 13 kHz and 18 kHz, respectively, with the Gaussian spectral distribution providing a realistic approximation for the upper limit of the laser linewidth [13, 18]. The positioning of the operation spot over the gain material did not have measurable effect on the laser linewidth.
Fig. 4 Delayed self-heterodyne interferometer spectrum taken at output power of 600 mW. In red: theoretical fitting.
4. Conclusion
We demonstrated a single frequency semiconductor disk laser operating at wavelength 1.56 μm with 1 watt of output power and a coherence length longer than 5 km in fiber. The result represents the highest output power reported from a single frequency SDL operating at this wavelength range and is the first demonstration of a wafer-fused single frequency SDL. The study shows the promising potential of wafer-fusion for obtaining high-power narrow-linewidth operation of SDLs in the wavelength range of 1.3-1.6 μm.
Acknowledgments
The authors acknowledge the technical help of Vladimir Iakovlev from EPFL Lausanne, Switzerland, and Sanna Ranta, Miki Tavast and Jari Lyytikäinen from the Optoelectronics Research Centre, Tampere University of Technology.
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