We report an optically-pumped semiconductor disk laser passively mode-locked with a semiconductor saturable-absorber mirror. Both the absorber and the gain media were made of dilute nitride compound semiconductor, GaInNAs, which enables operation around 1.2 µm wavelengths. The laser generated 5 ps optical pulses with an average output power up to 275 mW. Our demonstration provides an attractive approach for efficiently generating red-wavelengths through external cavity frequency doubling.
©2008 Optical Society of America
Semiconductor disk lasers (SDLs) operating in continuous wave (CW) mode  have been used successfully to produce multi-watt visible radiation by means of intracavity frequency conversion. Recently, we have demonstrated a 2.7 W continuous wave red SDL based on GaInNAs/GaAs quantum-wells (QWs) and intracavity doubling [2–4]. The dilute nitride material provides gain at 1.2 µm–1.3 µm while taking advantage of GaAs/AlAs technology for the fabrication of semiconductor mirrors . The SDLs can be passively mode-locked with semiconductor saturable absorber mirrors (SESAMs) to generate ultrashort optical pulses [6,7], which can be then efficiently frequency doubled using external cavity nonlinear conversion .
Practical optical sources emitting short pulses in the yellow-orange-red spectral range are on the demand list of many important applications in spectroscopy and advanced imaging techniques; this is because many chemical markers have a spectral signature in this wavelength domain. However, visible light generation via frequency doubling from SDL is especially challenging at red wavelength because it requires fundamental oscillation at 1.2 µm wavelength range which indeed limits the choice of semiconductor materials able to provide efficient gain . In this study we focus on demonstrating a mode-locked disk oscillator using gain structure and SESAM both based on dilute nitride technology. The main results of our demonstration is the generation of a 5 ps optical pulse train with an average output power of 275 mW at around 1220 nm.
2. GaInNAs-based SDL gain mirror and SESAM
The monolithic gain mirror was grown on an n-type GaAs (100) substrate by solid source molecular beam epitaxy equipped with a radio frequency plasma source for incorporating nitrogen into the crystal. Fig. 1(a) shows the schematic cross section of the gain mirror. The structure consists of a 30-pair GaAs/AlAs distributed Bragg reflector (DBR) and a gain region comprising 10 GaInNAs quantum wells (QWs) with relatively low nitrogen content, ~0.6–0.7 %. The QWs have a thickness of 7 nm and are arranged in five identical pairs placed at the antinodes of the standing wave formed within the Fabry-Pérot cavity defined by the DBR and the semiconductor-air interface. The compressive lattice strain of the QWs was compensated by 4-nm thick tensile-strained GaAsN layers grown on both sides of each QW. A 0.75-λ thick Al0.37Ga0.63As window layer was grown on the top of the active region to confine the photo-generated carriers within the active region and to avoid non-radiative surface recombination. Finally, a thin GaAs capping layer was grown to prevent oxidation of the window layer.
The SESAM structure, shown in Fig. 1(b), comprises two 6.5-nm thick compressively strained GaInNAs quantum wells with a nitrogen content higher than what has been used in the gain structure. The QWs are separated by a 15-nm thick GaAs barrier. The DBR of the SESAM comprises 24.5 pairs of quarter-wave thick GaAs/AlAs layers. The QWs are placed within an antiresonant Fabry-Pérot GaAs microcavity. The SESAM structure was not strain compensated. The red solid plot in Fig. 2 shows the measured reflectivity spectrum of the gain component. The other plot represents the photoluminescence spectrum of the gain structure.
Since the gain mirror operates under intense pumping conditions, thermal management is a crucial issue for the disk laser. In our experiments we used a transparent diamond as an intracavity heat spreader . A 2.5×2.5 mm2 gain chip was scribed from the as-grown wafer and capillary bonded with de-ionized water to a 3×3×0.05 mm3 natural type IIa diamond heat spreader. In this bonding technique, two flat and smooth surfaces are pulled into close contact by the surface tension of water, methanol or another suitable liquid, and bonded together by intermolecular surface forces . The bonded component was assembled between two metallic plates with indium foil in between to ensure good thermal and mechanical contact. The topmost metal plate had a circular aperture for signal and pump beams. A 2-layer SiO2-TiO2 antireflection coating was deposited onto the diamond surface. At the 1220 nm laser emission wavelength the diamond surface has a reflectivity of about 0.5 %. The reflectivity at the pump wavelength for the beam launched at a 35 degrees angle was about 5 %.
3. Experimental and Results
The gain mirror was pumped with a 808-nm beam delivered by a fiber-coupled diode. The pump beam was focused onto the gain mirror in a spot of about 180-µm in diameter. The barrier and spacer layers absorbed the pump radiation creating carriers that were trapped in the QWs to generate gain at 1.2 µm. The temperature of the gain mirror was kept at 15°C by using circulating water to transfer the heat from the gain mount. The temperature of the SESAM was set to 15°C by using a Peltier element. The maximum nonlinear reflectivity change of the SESAM was ~1 % ensuring a reliable start-up of the passive mode-locking when the mode diameter of the laser beam on the absorber was 30 µm.
The asymmetric Z-shaped laser cavity, shown in Fig. 3, comprised two curved mirrors, the gain structure, and the SESAM. This configuration provides a convenient option for setting the suitable mode size on the gain mirror and on the absorber. The cavity was first adjusted for maximum emission in TEM00 mode. Self-starting mode-locked operation could then be initiated by optimizing the SESAM axial position near the focal point of the focusing mirror.
The laser generated pulses with duration of ~5 ps which was found to be fairly independent on the output power. The output power characteristic is shown in Fig. 4. The maximum output power was about 275 mW corresponding to 8 W of pump power. The repetition rate of the mode-locked laser was measured to be 840 MHz.
As it can be seen in Fig. 5, the autocorrelation traces of the pulses measured near the laser threshold, i.e. corresponding to an output power of 25 mW, and to an output of 213 mW show no notable variation. The pulse energy at 213 mW average output power was about 250 pJ.
The corresponding spectra for the autocorrelation traces are shown in Fig. 6. The pulse duration is independent of the output power, however, the time-bandwidth product increases linearly with the pump power from 0.47 to 1.63. The gradual increase of the time-bandwidth product is due to nonlinear effects in the gain and saturable absorber regions [11, 12].
The central wavelength of the pulse is defined by the interplay between the resonance of the intracavity diamond heatspreader, acting also as a Fabry-Pérot etalon, and the spectral profile of the intrinsic gain provided by the semiconductor mirror. The spectrum shift to the longer wavelength, seen in Fig. 6, is due to temperature increase in the gain medium at high pump power. The step red shift in wavelength of about 6.2 nm corresponds to the free spectral range of the diamond heatspreader etalon.
Figure 7 shows the microwave spectrum of the mode-locked pulse train measured from a 9 GHz photodiode. The extinction ratio of the RF harmonics is ≥60 dB. As it can be inferred from this picture, the fundamental frequency of the cavity, and hence the pulse repetition rate, was 840 MHz.
We have demonstrated a passively mode-locked semiconductor disk laser operating at 1.22 µm. The laser produces pulses with duration of 5 ps independent of the average output power. The laser uses dilute nitride hetero-structures for both gain and absorber media and has promising potential for red light generation through external cavity frequency doubling.
The main factor limiting the maximum output power in the experimental setup we used was the optical damage of the SESAM. Higher output powers can be obtained by optimizing the SESAM towards reducing the optical losses and reducing the saturation energy, which would enable to increase the output coupling and reduce the fluence on the SESAM. At the same time the SESAM can be coated with dielectric thin films to increase the resilience to optical damage.
Future work will be devoted to increasing the output power and the development of frequency-doubled system producing short optical pulses at red wavelengths.
This research was supported in part by the European Commission through the FP6 research project NATAL, Ulla Tuominen Foundation, and Emil Aaltonen Foundation.
References and links
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