An optically pumped semiconductor disk laser was mode-locked for the first time by employing a single-walled carbon nanotube saturable absorber. Stable passive fundamental mode-locking was obtained at a repetition rate of 613 MHz with a pulse length of 1.23 ps. The mode-locked semiconductor disk laser in a compact geometry delivered a maximum average output power of 136 mW at 1074 nm.
©2013 Optical Society of America
Ultrafast laser sources with high repetition rates in the range of hundreds of megahertz offer a wide range of applications in communications [1,2], clock generation [3–5], metrology  and life sciences .
Optically-pumped semiconductor disk lasers (OPSDLs)  provide a very promising solution to cover a wide spectral range in the visible and near-infrared (NIR)  as they can capitalize on the well-established techniques for bandgap and quantum structure engineering as well as reliable high precision growth methods in semiconductors. High homogeneity of the grown semiconductor gain structures enables straightforward scaling of the average output power in OPSDLs by increasing active aperture size , if thermal management is taken care of. The small length of the gain structure and quasi-one-dimensional heat flow in the disk geometry facilitate low thermal lensing and fundamental transversal mode operation even at high pump fluences. This is of crucial importance for stable mode-locked laser operation. On the other hand, the short length of the amplification structure typically consisting of only a few quantum wells contributes to the roundtrip gain of only about 0.1 dB which makes OPSDL rather sensitive to intracavity losses such as those due to mode-locking elements. Semiconductor quantum-well (QW) [11,12] and quantum-dot (QD) saturable absorber mirrors (SESAMs) have been successfully used for mode-locking of OPSDLs [13,14]. Where QDs feature a very low saturation fluence for high-repetition rate mode-locking and an absorption which depends on the QD size and not necessarily the material. High-quality semiconductor quantum-well absorber structures with low non-saturable loss enabled demonstration of passively mode-locked OPSDLs with average powers of up to 5.1 W with a pulse length of 683 fs using a QW-SESAM  and pulses as short as 60 fs  in fundamental mode-locking schemes as well as pulse repetition rate of 92 GHz with pulses shorter than 200 fs in a harmonic mode-locking scheme .
In view of broad potential spectral range of OPSDLs it would be of interest to utilize more universal saturable absorber technology which could operate over the whole near-infrared spectral range employing the same basic absorber material. As recently reported, semiconducting single-walled carbon nanotube saturable absorbers (SWCNT-SAs) offer a potential alternative for high-gain solid-state laser mode-locking over a wide NIR spectral range . They do not yet have the optimized ratio of saturable loss to non-saturable loss that SESAM possess, but output powers of 3.6 W with a pulse duration of 8.4 ps at a repetition rate of 122 MHz in a Nd:GdVO4 laser  and SWCNT-SA assisted mode-locking with pulses as short as 62 fs in a Ti:Al2O3 laser with an output power of 600 mW at a repetition rate of 99.4 MHz  have been achieved in fundamentally mode-locked solid-state lasers recently. A more comprehensive overview of mode-locked lasers using SWCNT-SA can be found in , showing the potential of SWCNT-SAs for mode-locking of solid-state lasers from 800 nm to 1940 nm. For OPSDLs, this would correspond to several different QW-materials: From GaAs QW for emission around 850 nm, over InGaAs QW in the range of 1 µm as used in this work, to GaInSb QW in the 2 µm region. SWCNT-SAs, on the other hand, can be manufactured on a variety of substrates in a cost-effective way by employing relatively simple techniques such as spray and spin-coating . Moreover, they possess broadband ultrafast saturable absorption behavior, which either can be used in reflection or transmission. Note that the latter is not a trivial task to achieve with semiconductor saturable absorber structures .
However, it is not obvious that SWCNT-SAs can be employed for mode-locking OPSDLs due to low roundtrip gain and large non-saturable absorption loss as reported previously in a typical SWCNT-SA. In the present work, we demonstrate for the first time to our knowledge that saturable absorbers based on SWCNTs with low loss operating in transmission can be used for mode-locking an OPSDL. This first application of a SWCNT-SA for mode-locking of an OPSDL represents an important step for a further enhancement of the versatility of both devices and can be seen as an alternative, easy to implement and cost effective design. Stable fundamental mode-locking was obtained for such a system generating 1.23-ps pulses at a repetition rate of 613 MHz, delivering output powers of up to 136 mW near 1074 nm.
2. Experimental setup
In the present experiments, we employed a V-type cavity with a total length of about 244.5 mm (see Fig. 1). The cavity consisted of the gain structure in the first beam waist (1/e2) of 60 µm and the SWCNT-SA for mode-locking placed in the second beam waist of 75 µm. A curved output coupler with a transmission of 0.6% and a radius of curvature (ROC) of 50 mm was used as the cavity end mirror. In addition, one highly reflective dielectric mirror with a ROC of 100 mm was utilized as the folding mirror to control the cavity mode waist. The semiconductor gain structure was grown by molecular beam epitaxy for the emission wavelength of 1060 nm. The structure includes a highly reflective Bragg mirror formed by 27 pairs of AlAs/GaAs layers and the gain region composed of 3 GaInAs quantum wells engineered for the emission wavelength of 1060 nm. The structure was grown in reverse order on an n-type GaAs substrate, which was etched away after the growth  and does not have a dielectric anti-reflection coating.
In the laser setup, the DBR of the semiconductor chip was capillary-force bonded on a 300-µm-thick CVD diamond heat spreader, which in turn was mounted on a water-cooled copper heat sink. During the experiment the heat-sink temperature was maintained at 15°C, in order to avoid condensation. The gain structure was optically pumped by a fiber-coupled 808 nm laser diode at an incident angle of 45°, focused to a spot size of 70 µm (1/e2 radius).
For the OPSDL mode-locking experiment, a transmission-type SWCNT-SA was inserted at Brewster’s angle into the beam waist of the cavity. The SWCNT-SA was fabricated in the following steps: The purified arc-discharge SWCNTs (Hanwha Chemical Co. Ltd., ~1.4 nm in diameter) were first dissolved in dichlorobenzene in a concentration of 0.1 mg/ml. The surfactant PmPV was added during an ultrasonic agitation to improve the solubility of the SWCNTs. After ultrasonication process, PMMA dissolved in dichlorobenzene was added to the SWCNT dispersion. The SWCNT/PMMA mixture was then stirred and centrifuged for about 12 hours. The well-dispersed mixture was finally deposited onto a 1-mm-thick quartz window by spin coating to form a homogeneous film. The thickness of the coated absorber layer was measured to be about 300 nm.
The fabricated SWCNT-SAs were characterized by linear and nonlinear transmission measurements. Single-pass transmission was measured using a spectrophotometer (Agilent Tech., Cary 5000) and a typical spectrum corrected for the Fresnel reflections is shown in Fig. 2. The broad absorption in the 1-µm and 1.8-µm regions is caused by the ensemble of the interband transitions in semiconducting SWCNTs of different diameters and chiralities . The linear transmission at the laser operation wavelength of 1074 nm was about 99%. The nonlinear transmission measurement was performed by using a synchronously-pumped NIR optical parametric oscillator (OPO), delivering tunable sub-200 fs pulses at wavelengths between 1088 and 1590 nm. The modulation depth of the SWCNT-SA used was measured to be 0.25% and the saturation fluence (Fsat) 11.36 µJ/cm2 at the shortest possible wavelength of OPO at 1088 nm (Fig. 3), whereas the linear and nonlinear characteristics of the SWCNT-SA in this region should not differ from those at the laser wavelength. The carrier recovery is also investigated by time-resolved pump-probe spectroscopy using the same OPO. The measured pump-probe trace was well-fitted with a biexponential function and delivered after deconvolution the fast and slow decay time constant of about 150 fs (limited by the excitation pulse duration) and 1.1 ps, respectively, corresponding to the intraband and interband transitions of SWCNTs (Fig. 3 (inset)) . A more in depth description of SWCNT-SA fabrication and characterization can be found in .
3. Results and discussion
We first investigated continuous wave (CW) operation of the OPSDL, before the SWCNT-SA was inserted into the cavity for mode-locking. A maximum average output power of 1.01 W at a pump power of 8.5 W was achieved, shown in Fig. 4(a). The slope efficiency in CW operation is limited by the undercoupling with a 0.6% transmission OC, which was specifically chosen to maximize the output power in the mode-locked regime. In addition, the slight decrease in the slope efficiency is related to the thermal rollover. The spectrum was measured with an optical spectrum analyzer with a resolution of 0.05 nm and found to be approximately 9 nm broad with a center wavelength of around 1074 nm (Fig. 4(b)). The modulation of the spectrum probably originates from an etalon effect of the gain chip.
For mode-locked operation, the transmitting SWCNT-SA was placed at Brewster’s angle between HR mirror and output coupler (OC) inside the cavity (Fig. 1). As shown in Fig. 3(a), the mode-locked maximum output power achieved was 136 mW with at a pump power of 6 W. The self-starting mode-locking could be achieved above the output power of 50 mW, where the calculated fluence on the absorber inside the cavity was 61 µJ/cm2. This value is about five times higher than the saturation fluence (Fsat = 11.36 µJ/cm2) of the SWCNT-SA used. At the maximum output power, the fluence was estimated to be 209 µJ/cm2. At higher pump powers, the mode-locked operation was sustained, but the output power was slightly decreasing. Since no damage of the SWCNT-SA was observed, the mode-locked operation was probably limited by the thermal loading in the gain.
When the OPSDL is mode-locked, the spectrum becomes narrower down to a bandwidth of FWHM = 1.6 nm centered around 1074 nm. Compared to the modulated CW spectrum, the mode-locked case shows well-defined shape of spectrum without any modulation. The wavelength at maximum power is shifted by 13 nm from the nominal design wavelength because of an increase of the temperature in the gain structure under operating conditions. An intensity-autocorrelation measurement was performed to determine the pulse duration of the mode-locked pulses. The measured autocorrelation trace corresponded to a pulse duration of 1.23 ps (Fig. 5), with a small deviation of the fitted sech2-pulse shape, which is usually used for passive mode-locking. The time-bandwidth product was about 1.6 times over the Fourier limit and no dispersion compensation was used inside the laser cavity. The total dispersion is given by the dispersion of the gain structure, which is around 50 fs2 and by the dispersion of the quartz window with <100 fs2. The laser was operating in TEM00 mode at all powers with a beam quality of M2 = 1.2. The SWCNT-SA had to be aligned precisely at Brewster’s angle. Due to the spin-coating process, the homogeneity of the sample was much better in the center of the sample, compared to the edges of the substrate.
To verify the stable operation of the mode-locked OPSDL, radio frequency (RF) spectra were recorded with a biased InGaAs-photodiode with a bandwidth of 3 GHz. Top trace in Fig. 6 shows an average scan RF spectrum (over 100 scans) taken with the resolution of 3 MHz. The lower trace shows a single, high resolution scan with a resolution bandwidth of 3 kHz of the fundamental frequency, normalized to the carrier maximum. Both measurements confirm the stability of the pulse train and absence of multipulsing.
In order to unequivocally verify the mode-locked operation in this OPSDL, we performed external second harmonic generation (SHG) measurement. A 5-mm-long lithium triborate (LBO) crystal for type-I phase matching was used for frequency doubling of the mode-locked OPSDL. The linearly-polarized output was focused onto LBO using a broadband AR coated lens with a focal length of 25 mm, resulting in a focus of 50 µm (1/e2 radius). At the maximum incident average power of 136 mW, an average SHG power of 10.56 mW was measured after a Schott BG 18 filter. The SNLO software (Sandia Nat. Labs) was used to estimate the expected SHG using plane-wave short-pulse mixing in LBO.
With an effective nonlinearity of 0.84 pm/V and the focus size of 50 µm (1/e2 radius), the calculated theoretical efficiency is 10.8%. The experimental result was with 8.6%, taking into account the losses by the filter, close to the theoretical value. For comparison, we replaced the SWCNT-SA in the cavity by a plane quartz plate mounted at Brewster’s angle to enforce single polarization in CW laser operation. Using 250-mW CW output from this laser in the same frequency doubling setup, a second harmonic power as low as 123 µW is generated as expected. This is a clear indication that the OPSDL operates mode-locked by the SWCNT-SA.
A passively mode-locked semiconductor disk laser was demonstrated where a transmitting SWCNT-SA mounted at Brewster’s angle in the cavity was used as a mode-locker. This is to our knowledge the first result of mode-locking with carbon nanotubes in a semiconductor disk laser. The laser setup represents a highly practical and simple solution for a high repetition rate solid-state laser operating in the near-infrared spectral region near 1074 nm and delivered stable cw single pulse mode-locking with a maximum average output power of 136 mW at a repetition rate of 613 MHz. The corresponding pulse energy is 0.22 nJ. Without dispersion compensation, a pulse duration of 1.23 ps was achieved. A next step would be to lower the non-saturable losses in an optimized SWCNT-SA sample production process and show the versatility of this approach by using different QW materials like antimonite based gain structures in the 2 µm spectral region.
This work was supported by the Swedish Research Council (VR) through its Linnaeus Center of Excellence ADOPT and the European FAST-DOT project, and by the NRF funded by the Korean Government (MEST) (2011-0017494, 2010-220-C00013).
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