We demonstrate the first use of InGaAs/GaAs as a saturable absorber in the Q-switching of a diode pumped Tm3+ doped laser operating at the wavelengths of 1940 nm and 1986 nm. The influence of the semiconductor saturable absorber’s (SESA) position and thermal lens effect on the Q-switch characteristics was investigated. With a pump power of 35 W, the maximum pulse energy of 28.1 μJ with a pulse width of 447 ns at the pulse repetition frequency (PRF) of 43.7 kHz was obtained by selecting the appropriate position of the SESA.
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Solid-state lasers in the eye safe range of 2 μm , are important owing to the potential applications in atmospheric sounding , wind lidar , medicine , and so on. Q-switched 2 μm solid-state lasers are of particular interest since they provide short duration optical pulses required for ranging and nonlinear optical frequency conversion . Q-switching at 2 μm is obtained using electro-optic or acousto-optic devices to provide the required optical shutters . The passive saturable absorbers as alternatives of acousto-optic devices offer numerous advantages in cost, simplicity, and reliability. Ho:YVO4, Ho:YLiF4 and Ho:GaF2 crystals have been shown to be effective solid-state saturable absorbers Q-switch for the flash-lamp-pumped 2 μm Tm,Cr:Y3Al5O12 lasers [7–9]. With the laser diode (LD) pump technology growing up, the suitable saturable absorbers are needed to meet the requirement of continuous wave (CW) pump scheme. Semiconductor materials like GaAs as saturable absorber was used in Nd3+ doped crystal passively Q-switched lasers [10,11], and InGaAsP quantum-wells saturable absorber has been used for diode-pumped passively Q-switched 1.3 μm lasers . Recently, absorption saturation (bleaching effect) in 2 μm spectral range was demonstrated by PbS quantum-dot-doped glass and semiconductor saturable absorber mirror (SESAM) [13,14].
In this paper, for the first time, we present an InGaAs with GaAs barrier structure grown on an semi-insulating GaAs substrate to be a SESA for a Tm:YAP laser. Maximum pulse energy of 28.1 μJ with the repetition rate of 43.7 kHz and minimum pulse width of 447 ns (FWHM) was obtained at the pump power of 35 W. There were two central emission wavelengths of 1940 nm and 1986 nm from the Q-switched laser, simultaneously. Compared with SESAM, the double-pass configuration with an external output coupler is more beneficial to the flexibility of the cavity design and the optimization of the output coupler. The influence of the SESA position in the cavity on the Q-switching stability was also investigated.
2. Experimental setup
The laser setup used in our experiment is schematically shown in Fig. 1 . The pump light from a fiber coupled LD bar (DILAS GmbH) was reimaged into the laser crystal by the coupling lenses with 51 mm and 100 mm focal length, respectively. The core diameter of 200 μm pigtail fiber (NA = 0.22) is made in Germany by LIMO GmbH. The wavelength of the LD was calibrated before the experiment and met as the following equation, Wavelength = 0.1161I + 0.2571T + 783.8 (nm). I is the operating current and T is the operating temperature. The central emission wavelength of the LD is 794 nm. The focused pump beam in the laser medium has a diameter of about 392 μm. The a-cut Tm:YAP sample has a Tm3+ doping concentration of 4 at.% and a dimension of 3 × 3 mm2 in cross section and 10 mm in length. Both sides of the sample were anti-reflection (AR) coated with reflectance of less than 0.2% near 2 μm. To efficiently remove the generated heat during the experiment, the sample was wrapped with indium foil and tightly mounted in a TEC-cooled copper holder. The temperature of the sample was set at as low as 15 °C. A simple L shape cavity configuration was employed in this laser. The input mirror (M1) is HR coated in a broad band at ~2 μm and AR coated at ~794 nm. M2 is a 45° reflector (HR@~2 μm). The output coupler (OC) has a 100 mm radius of curvature (T = 12.8%). The SESA’s (provided by Batop GmbH) substrate is semi-insulating GaAs. The absorber layer consists of In0.769Ga0.231As with GaAs Barriers, whose band gap energy is 0.529 eV. The SESA has a cross section of 5 × 5 mm2. The thickness of the chip is 620 μm. The two sides of the chip were polished and AR coated for ~2 μm (1900-2100 nm). The modulation depth of the SESA device is 0.6% and the absorptance is 1% which means that the nonsaturable loss of the SESA is approximately 0.4%. Relaxation time constant of the chip is ~500 fs. Typically, there exist a short part of the relaxation < 1ps and a more slowly relaxation in the ps regime. The Saturation fluence is 300 μJ/cm2. The SESA was glued on a glided Cu-cylinder sink, which was mounted on a three dimensional translation. In our experiment, the cavity length was fixed as 123 mm, the distance L between the SESA and the OC was variable in order to change the mode radius on the SESA.
3. Experimental results and analysis
For the experimental configuration which is shown in Fig. 1, using the well-known ABCD matrix method and considering the thermal lens effect of the laser medium, the radii of the TEM00 mode on the laser crystal and on the SESA with different L were calculated as shown in Fig. 2 . With the increase of pump power, the variations of TEM00 mode radii are about 170-300 μm in the laser crystal. It can be also seen that TEM00 mode radii on the SESA increase with the pump power, preventing the SESA from damage under high pump power. The mode radii on the SESA decrease when L is increased. In this laser, thermal lens effect of the laser medium is very serious. The threshold is as high as 15 W (Fig. 3 ). One possible explanation is that the thermal focal length measured by the changing cavity length method was less than 100 mm in which range the cavity can keep stable according to the ABCD matrix theory (Fig. 2), when the pump power increased more than 15 W.
The CW output power increases almost linearly with the pump power and no power saturation is observed. The CW laser had a slope efficiency of 28.1% and a maximum output power of 5.44 W at the pump power of 35 W. The average output power of the lasers with the SESA inserted at different L was also investigated (see in Fig. 3). Larger the distance L, higher slope efficiency and maximum output power could be obtained. The Q-switching efficiencies (ratio of the Q-switched output power to the CW power at the maximum pump power) were found to be 13.1%, 22.6%, 33.2%, 40.2% at the L of 2, 15, 31, 47 mm respectively . The Q-switched laser operated stably for 4 hours, and no damage phenomenon was observed, when the L is in the range of 2-15 mm. However, when the L increased to 58 mm, the output power decreased abruptly which indicated that part of the SESA had been destroyed. The PRF and pulse widths at different L and pump power were also recorded, as shown in Fig. 4 and Fig. 5 . The pulse repetition frequencies increase monotonically with the pump power. On the other hand, the pulse widths decrease with the pump power. However, unlike typical passively Q-switched lasers, whose pulse energy is insensitive to the pump power , the pulse energy tends to increase from 3.1 to 16.5 μJ at L = 2 mm (red dashed line of Fig. 3) and from 13.7 to 28.1μJ at L = 15 mm (green solid line of Fig. 3), when the pump power increases from 18 W to 35 W. One possible explanation is that the variation of the pump wavelength causes the change of the absorption of the gain medium. The pulse energy reaches the maximum value of 28.1 μJ at the maximum pump power. In other words, the best position of the SESA is 15 mm from the OC.
The pulse temporal behavior was recorded by a Lecroy digital oscilloscope (WaveJet 332, 2G-samples/sec, 350 MHz bandwidth) with a fast PIN photodiode. The (a) and (b) of Fig. 6 show the typical oscilloscope trace of expanded shape of a single pulse and a train of output pulses at the pump power of 35 W (L = 15 mm), respectively. When the pump power was more than 25 W at L = 31 mm and 20 W at L = 47 mm, the unstable pulses emerged as shown in the (c) (d) of Fig. 6. So, it is meaningless to measure the PRF and pulse width in this immeasurable condition. When the L increased to the 58 mm, the mode radius on the SESA was so small that the chip bleached deeply and some spike shape pulse appeared. Finally, some bad points were observed on the surface of the SESA, which meant that the SESA was destroyed. Jitters in the peak power and repetition rate are observable in the (b) of Fig. 6. The instability of the Q-switched pulses at a high pump power level could be induced by the intrinsic nonlinear dynamics of the system such as the deterministic chaos . Another possible reason is related to the structure of the gain media. Local heating of the active channel could also induce visible instability of the Q-switching pulses .
The emission wavelength of Tm:YAP laser was measured with a WDG-30 monochrometer (300 mm focal length, 300 lines/mm grating blazed at 2 μm). The chopped light from exit slice was detected by an InGaAs photodetector connected with a Stanford SR850 lock in amplifier (Fig. 7 ). The free running central wavelength was 1940 nm when the pump power was slightly larger than the threshold (i.e. 17.45 W). When the pump power further increased, the central wavelength tended to be shifted to 1986 nm (i.e. 21.85 W). However, no matter how high the pump power was injected, dual-wavelength lasing was observed in the process of the passively Q-switched operation. The two spectra peaks are located at 1940 and 1986 nm, respectively.
The output laser mode was also measured by the traveling knife-edge method . The Integrated Gaussian profile indicates the TEM00 mode laser output (Fig. 8 ). The radiuses of the laser beam were also calculated by the knife-edge method. By fitting Gaussian beam standard expression to these data, we estimated the beam quality to be M 2 = 1.13 ± 0.02.
In conclusion, we demonstrated a dual-wavelength passively Q-switched operation of a diode pumped Tm:YAP laser by using a SESA based on InGaAs /GaAs at the wavelengths 1940 nm and 1986 nm for the first time. By selecting the appropriate position of the SESA (L = 15 mm), the maximum pulse energy of 28.1 μJ with the PRF of 43.7 kHz and pulse width of 447 ns was obtained at the pump power of 35 W. The InGaAs /GaAs looks as a perspective candidate for saturable absorbers in pulse solid-state lasers emitting in the range of 2 μm. The dual-wavelength Tm:YAP laser with 3.6 THz frequency separation is useful for terahertz difference frequency generation inside a nonlinear optical crystals.
References and links
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