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We present a passively mode-locked semiconductor disk laser (SDL) emitting at 650nm with intra-cavity second harmonic generation to the ultraviolet (UV) spectral range. Both the gain and the absorber structure contain InP quantum dots (QDs) as active material. In a v-shaped cavity using the semiconductor samples as end mirrors, a beta barium borate (BBO) crystal is placed in front of the semiconductor saturable absorber mirror (SESAM) for pulsed UV laser emission in one of the two outcoupled beams. Autocorrelation (AC) measurements at the fundamental wavelength reveal a FWHM pulse duration of 1.22ps. With a repetition frequency of 836MHz, the average output power is 10mW per beam for the red emission and 0.5mW at 325nm.
© 2015 Optical Society of America
1. Introduction
Semiconductor disk lasers, also called vertical external-cavity surface-emitting lasers (VECSELs), have many advantageous properties such as a near diffraction limited beam quality [1], high output power [2] and the possibility of bandgap engineering. By using different semiconductor compositions as gain material, VECSELs have been realized at many different wavelengths in the visible [3–6] and in the infrared spectral range [6, 7]. In continuous-wave (CW) operation, the extremely high power density inside the cavity has been exploited to achieve efficient intra-cavity second harmonic generation to the green [8] and yellow [9] spectral range with 20W of frequency-doubled output power.
Since the first demonstration [10] of an AlGaInP SDL in 2002, the performance of these compact laser systems directly emitting in the visible spectrum has been steadily improved reaching CW output powers of 1.6W at 666nm with quantum wells [11] and 1.39W at 654nm with quantum dots [12]. Furthermore, their external cavities allow the implementation of intra-cavity elements such as birefringent filters for wavelength tuning, even during laser operation. Large tuning ranges of more than 20nm [13] make red-emitting VECSELs perfect laser sources for spectroscopic applications.
On the one hand, by including a saturable absorber mirror [14] into the cavity, mode-locked operation of AlGaInP VECSELs has been demonstrated with pulse durations in the picosecond [15,16] and femtosecond [17] regime, expanding the field of possible applications. On the other hand, second harmonic generation to the ultraviolet spectral range [18] by inserting frequency doubling crystals such as beta barium borate has been reported with maximum output powers of up to 260 mW at 329nm [19].
By intra-cavity frequency doubling of a passively mode-locked InGaAs/GaAs semiconductor disk laser, 3.9ps pulses have been produced at a wavelength of 489nm [20]. In this article, we present the combination of SESAM mode locking and second harmonic generation in order to realize an optically pumped VECSEL emitting picosecond pulses in the ultraviolet spectral range. The application of such a compact pulsed UV laser source can be anticipated in many different areas including biophotonics and spectroscopy, e.g. as excitation laser for time-resolved investigations of GaN and ZnO based materials.
2. Design and epitaxial growth
Our semiconductor structures are fabricated by metal-organic vapor-phase epitaxy (MOVPE) in an AIX-200 horizontal reactor and include an AlAs/Al0.45GaAs distributed Bragg reflector (DBR) with 55 pairs of λ/4 layers on a (100) n+-GaAs substrate misoriented 6° toward the [111]A direction. The active region of the VECSEL contains seven layers of InP QDs, which are embedded in strain compensating Al0.1GaInP barriers and Al0.55GaInP cladding layers. Each QD layer is placed in an antinode of the standing wave of the electric field (resonant periodic gain [21,22]) and the semiconductor/air interface is positioned close to an antinode of the electric field (near-anti-resonant design). The gain structure and its CW laser characteristics in a linear setup have been described in detail elsewhere [12].
The refractive index and the simulated electric field intensity of the absorber structure is shown in Fig 1. The active region of the SESAM includes one of the same QD layers as used in the gain structure, also embedded in Al0.1GaInP barriers and Al0.55GaInP cladding layers. It is fabricated in a near-anti-resonant design with a 10nm GaInP capping layer to prevent oxidation. An additional fused silica layer with a thickness of 97nm is deposited on top of the semiconductor absorber sample to increase the field enhancement inside the structure. This near-resonant design of the overall structure results in a higher modulation depth and a lower saturation fluence compared to the sample without the SiO2 layer, leading to a more stable mode locking behavior.
Fig. 1 Index of refraction and electric field intensity in the SESAM simulated with the transfer matrix method. A 97nm SiO2 coating is used to increase the field enhancement at the QD layer position by a factor of ∼ 1.8 compared to the uncoated semiconductor structure.
3. Experimental setup
The cavity setup used in this experiment (see Fig. 2) is similar to the setups described previously [16, 17], with a total cavity size of 179.4mm resulting in a repetition frequency of the emitted pulse train of 836MHz. The outcoupling mirror with a reflectivity of ∼ 99.7% for the fundamental wavelength and a radius of curvature of 50mm also serves as folding mirror. With a 30mm length of the resonator arm between SESAM and outcoupling mirror, the cavity is aligned close to its stability limit. This leads to tight focussing of the laser mode onto the absorber region with diameters of ∼ 20μm on the SESAM and ∼ 100μm on the gain structure. The semiconductor structures used as end mirrors are placed on copper heatsinks, which can be cooled by Peltier elements. Due to the poor thermal conductivity of the DBR layers, the large amount of heat introduced by the 532nm pump laser is removed by a diamond heat spreader without anti-reflection coating bonded onto the gain chip via liquid capillary bonding [23]. During the measurements, a pump power of 5.4W was used and the VECSEL heatsink temperature was held at around −23°C while the SESAM was operated at room temperature. For second harmonic generation, a BBO crystal with a length of 3mm and anti-reflection coated facets optimized for the wavelengths of 660nm and 330nm is placed close to the absorber structure. Therefore, only one of the two outcoupled beams contains the frequency-doubled laser emission.
Fig. 2 Experimental setup (left) and photograph (right) of the mode-locked VECSEL. The v-shaped cavity is used for tight focussing of the laser mode onto the absorber. For second harmonic generation, a BBO crystal is placed in front of the SESAM. The emitted UV light becomes visible by the fluorescence on a sheet of paper.
4. Results
The laser beam which only contains emission at the fundamental wavelength was characterized by using an autocorrelator in a collinear configuration with a scan range of 15ps. A typical intensity autocorrelation trace is plotted in Fig. 3 and fitted by a squared hyperbolic secant (sech2) function showing a FWHM pulse duration of 1.22ps. As observed in previous experiments [16, 17], side pulses with a time delay of about 9ps appeared due to the intra-cavity use of a plane diamond heat spreader without anti-reflection coating.
Fig. 3 Autocorrelation trace at the fundamental wavelength. The data are fitted by a sech2 showing a FWHM pulse duration of 1.22ps. Parts of the side pulses occuring due to the intra-cavity heat spreader can be observed.
At the same time, the other beam was guided through a mirror to partly suppress the fundamental wavelength and the laser emission was measured by a spectrometer with a resolution limit of ∼ 0.8nm. The resulting optical spectrum is plotted in Fig. 4 with sech2 fits for both the fundamental and the second harmonic peak, showing emission wavelengths of 649.8nm and 324.9nm with a FWHM of 1.02nm and 0.84nm, respectively. The typical response function of the spectrometer was recorded with a helium-neon laser at 632.8nm and is plotted in the inset of Fig. 4 with a Gaussian fit showing a FWHM of 0.78nm.
Fig. 4 Optical spectrum of the laser emission measured simultaneously with the AC trace plotted in Fig. 3 and sech2 fits showing FWHM values near the resolution limit. The intensities are not to scale, since an additional mirror was used to partly suppress the fundamental wavelength. Inset: Typical response function of the spectrometer measured with a helium-neon laser.
Measuring the pulse duration of ultra-short UV pulses is very challenging, requiring a rather complex setup and an additional pulse with comparable duration for difference-frequency mixing [24] or high UV pulse energies for two-photon absorption in thin crystals [25]. Therefore, an estimation of the pulse broadening effect due to second harmonic generation is given. The FWHM phase-matching bandwidth of the nonlinear crystal was calculated [26] to be 0.63nm for the fundamental wavelength. Taking into account the resolution limit of the spectrometer, the deconvolution of the measured optical spectrum at the fundamental wavelength results in a FWHM of 0.62nm. Furthermore, the group velocity mismatch (GVM) was calculated from the refractive index of BBO [27] to estimate the length Lg characterizing the temporal walk-off between the fundamental and the second harmonic pulse [28]:
For the measured pulse duration of τ = 1.22ps, the characteristic length is 3.35mm, slightly higher than the actual BBO crystal length of 3mm. Therefore, we believe that there is no considerable increase in the pulse duration of the second harmonic.The pulse train emitted by the mode-locked laser (fundamental wavelength) was further analyzed with a 12.5GHz photodiode and a 16GHz oscilloscope. A time window of 15ns is plotted in Fig. 5 demonstrating a clear zero level and a repetition frequency of 836MHz.
Fig. 5 Extract from the laser emission measured with a 16GHz oscilloscope. The pulse train of the mode-locked laser is stable over the whole measurement range of 50μs with a clear zero level.
At a pump power of 5.4W, the average output power of the mode-locked laser was measured to be 10mW at the fundamental wavelength. For the power measurement of the second harmonic, a lens transmitting ∼ 63% and a bandpass colored glass filter transmitting ∼ 91% of the UV emission were used, resulting in a measured value of > 0.3mW.
In order to increase the intra-cavity power and therefore the power of the second harmonic, the outcoupling mirror was replaced by a mirror optimized for high UV transmission and high reflectivity in the red spectral range. While the autocorrelation signal became very unstable with a large background, the increased frequency conversion had no noticeable effect on the signal measured with the oscilloscope. Considering the higher intra-cavity power, we assume that these instabilites are caused by the effect of kinetic hole filling [29, 30] together with the nonlinear second harmonic generation. With refilling processes on the timescale of a few picoseconds, the stimulated charge carrier recombination in the gain medium leads to a decrease in gain for the following pulse. In addition, the increased frequency conversion is counteracting the pulsed operation of the laser. The combination of these effects possibly prevents the generation of short pulses which can be observed in the autocorrelation trace. However, mode-locked operation remains with a pulse duration in the order of tens of picoseconds, as indicated by the stable oscilloscope signal. The measured average output power of the second harmonic emission from this setup was more than 1mW, corresponding to a total emitted UV power of > 1.7mW.
Despite the successful demonstration of intra-cavity second harmonic generation of a mode-locked AlGaInP VECSEL, the setup is not perfectly optimized for frequency doubling and can be improved in many ways. With the BBO crystal placed in front of the absorber structure, the frequency-doubled emission in one direction hits the SESAM, where most of it is absorbed, leading to a reduced UV output power of the laser and probably damaging the absorber structure. In a z-shaped cavity, an additional folding mirror with high transmission in the UV can be used to avoid the irradiation of the sample and double the total second harmonic output power due to a second outcoupled UV beam. A further advantage of this cavity geometry is the possibility of positioning the frequency doubling crystal in the beam waist which can be adjusted for optimum conversion values. Thus the thickness of the nonlinear crystal can be reduced, minimizing the temporal walk-off between the fundamental and the second harmonic pulse. Moreover, the BBO crystal used in our experiment is optimized for a fundamental laser wavelength of 660nm, with a plane cut at an angle of 36.1° for type I critical phase matching and the respective anti-reflection coatings on the facets. At a laser wavelength different from this optimum, this may lead to losses and mode locking instabilities.
5. Conclusion
We have demonstrated a passively mode-locked semiconductor disk laser emitting at a wavelength of 650nm with intra-cavity second harmonic generation to the ultraviolet spectral range. Autocorrelation measurements at the fundamental wavelength reveal a FWHM pulse duration of 1.22ps and pulse train measurements with a fast oscilloscope show a repetition frequency of 836MHz. For a pump power of 5.4W, the average output power of the fundamental and the frequency doubled laser emission is 10mW and 0.5mW, respectively. Our results show that despite the counteracting effect of intra-cavity second harmonic generation on the pulse formation, it can be used in mode-locked red-emitting VECSELs to realize a compact pulsed UV laser source, which is a promising candidate for many applications.
Acknowledgments
The authors would like to thank U. Keller and her team at the ETH Zürich for the fused silica deposition, E. Kohler for technical assistance with the MOVPE system, J. Elling for SEM analysis and the German research foundation (DFG) for funding ( JE 422/9-1).
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