A passively Q-switched Tm,Ho:GdVO4 laser operating at cryogenic temperature with a Cr2+:ZnS saturable absorber pumped with continuous wave LDs was demonstrated. The performance of the laser was investigated through changing the distance between Cr2+:ZnS and output coupler. The maximum pulse energy of 70.5 μJ was obtained at 10 W input power. The maximum average output power of PQS laser was up to 3.2 W at the pump power of 22.8 W, corresponding to CW output power of 7.4 W, pulse repetition frequency of 52 kHz, and a pulse width of 389 ns. The M2 factor measured by the traveling knife-edge method was ~1.1 in x and y directions with near-diffraction limited beam quality.
© 2013 Optical Society of America
Diode-pumped Q-switched solid state lasers operating in the 2 μm eye-safe spectral range are applied to environmental atmosphere monitoring, wind lidar [1–3], medicine , and so on. Especially, 2 μm Q-switched lasers are attractive pumping sources for OPOs and solid state lasers which can efficiently convert radiation to the mid-infrared spectral range .
Passively Q-switched (PQS) lasers with saturable absorbers (SAs) were usually accompanied with significant advantages such as inherent compactness, simplicity, and low cost of cavity design. Up to now, several PQS Tm-doped [6–11], Ho-doped , and Tm,Ho-codoped [13, 14] lasers emitting in ~2 μm have been reported with different SAs based on semiconductor and nanometer materials. Tm-doped lasers’ Q-switching performance is inferior to Ho-doped ones because of smaller stimulated emission cross section and shorter lifetime of excited state energy level comparing with Ho3+ ions . Tm,Ho-codoped laser crystals are favorable owing to strong absorption of Tm3+ ions as sensitization ions at 800nm. Compared with other hosts, the GdVO4 host is superior with a stronger and broader absorption band. This favorable spectroscopic property allows us to efficiently pump these lasers by commercially available high-power LDs which is beneficial for realizing all-solid-state laser system .
Besides, Tm,Ho-codoped lasers have to operate with quasi-three level at room temperature which results in very high excited state densities to achieve population inversion. While the upconversion processes (5I7, 3F4→5I5, 3H6) of the Tm:Ho-codoped laser are sensitive to excited state (5I7) densities. High 5I7 upper level density not only leads up to upconversion losses, but also decreases leaving holmium ions for the Tm 3F4→Ho 5I7 energy transfer. At cryogenic temperature, the quasi-four level nature of Tm,Ho-codoped laser not only provides low excited state densities which are benefit for decreasing upconversion losses but also obtains the higher output power and better beam quality by weakening thermal effect of the gain medium originating the large thermal conductivity of GdVO4 (0.117 Wcm−1K−1) .
In addition, the suitable SAs are considerably important for the effectively operating of PQS lasers. Compared with other SAs, the Cr2+:ZnS material naturally possesses higher optical damage threshold (1.5 J/cm2)  and larger thermal conductivity (0.27 Wcm−1K−1) , thus leading to weaker thermal lens effect. Particularly, Cr2+:ZnS SAs are very promising for passively Q-switching of the rare-earth lasers owing to about two orders of magnitude greater absorption and emission cross-sections than that of the rare-earth ions .
Based on the predominance mentioned, we reported the output characteristics of a PQS Tm,Ho:GdVO4 laser using Cr2+:ZnS as SA at liquid nitrogen cooling. The influence of the Q-switch position was particularly investigated.
2. Experimental setup
The PQS Tm,Ho:GdVO4 laser setup with dual crystals was schematically shown in Fig. 1(a). The laser resonator was composed of the cavity mirror M1-M5. The M1-M4 plane mirrors were anti-reflection (AR) coated around ~798 nm and high-reflection coated at ~2 μm. The output coupler M5 had 500 mm radius of curvature and transmittance of 40% at 2 μm. The distances among M1-M5 were orderly 50 mm, 130 mm, 50 mm and 45 mm, with a total cavity length of 275 mm. The emission wavelength of the LD1 and LD2 were in the range of 798-802 nm depending on the heat sink temperature and the pump current, with fiber diameter of 400 μm and numerical aperture of 0.22. Each collimated pump laser was averaged into two beams by a spectroscope and were coupled into Tm,Ho:GdVO4 crystals from two sides by lenses. The focused pump beam in the laser media had a diameter of ~800 μm. Both a-cut Tm,Ho:GdVO4 crystals with the same Tm3+ (4 at.%) and Ho3+ (0.4 at.%) doping concentration had a cross section of 4 × 4 mm2 and length 8 mm (crystal 1) and 10 mm (crystal 2) respectively. To effectively remove the heat generated in the crystals for high power operation, they were respectively wrapped in indium foil and held in copper heat-sinks connected with two small dewars with each filled with liquid-N2. In addition, the dual crystals configuration with dual-end pumped for each gain crystal cannot only further facilitate the distribution of the thermal load but also be benefit for adjusting the mode-coupling efficiency for each crystal. The configuration can be simplified as a plano-concave resonator with two thin thermal lenses. Our group had analyzed the thermal effect of Tm,Ho-codoped vanadates laser crystal [17, 21, 22]. By the equation in  and the parameters in , the estimated thermal focal length was ~60 cm for each Tm,Ho:GdVO4 at the pump power of 35 W, considering the well-known ABCD matrix, corresponding to |A + D|/2 value of the resonator lower than 0.82, which indicated the resonator always keep stable. The radii of TEM00 mode on the laser crystal 1 and 2 calculated were respectively ~370 μm and ~480 μm at the pump power of ~10 W. It should be pointed out that the actual thermal focal length may be much shorter than the estimated value at the liquid-N2 cooling from the thermal parameters’ change , however no power degradation or resonator instability was observed in the experiment.
A thin Cr2+:ZnS SA produced by a diffusion doped method was cut into 9 × 9 mm2 cross section and 2 mm thickness with small-signal transmission of ~82%. The absorption coefficient of Cr2+:ZnS is shown in Fig. 1(b). The ground-state absorption cross section is approximately 8.9 × 10−20 cm2 at 2.05 μm. The saturation fluence was 1.1 J/cm2. The SA was placed in the resonator with a variable distance L from the output coupler to change the mode radius on it. And, it was mounted in a copper heat sink which was cooled by water. The radii of the TEM00 mode on SA with different L calculated were respectively ~520 μm (L = 25 mm), ~530 μm (L = 15 mm), ~540 μm (L = 7 mm) at the pump power of 10 W.
3. Experimental results and discussion
The change of CW output power with the pump power is shown in Fig. 2(a). The CW output power of the Tm,Ho:GdVO4 laser at 77 K increases linearly with the pump power with a slope efficiency of 35.5%. The maximum output power is up to 10.5 W at the pump power of 32.2 W and no power saturation is observed. The higher output power and slope efficiency, compared with ones at room temperature  is attributed to cryogenic cooling to reduce greatly thermo-optic effects  and decrease effectively upconversion losses .
The output performance of the Tm, Ho:GdVO4 laser with Cr2+:ZnS SA inserted at different L are investigated. The average output power of the PQS laser as a function of the incident pump power was depicted in Fig. 2(b). At the lower pump power (<16W), the average output power increases almost linearly with the pump power. And the larger the distance L, the higher slope efficiency and output power could be obtained [see Fig. 2(b)]. It results from the lower saturation absorption loss of Cr2+:ZnS SA at the larger L due to the distribution of intracavity laser field . With the increase of the pump power, the slope efficiency begins to decrease. Especially, when the SA is near the gain medium (L = 25 mm), the Q-switched laser trends towards saturation. The reason is related to the power density on the Cr2+:ZnS SA. With the increase of L, the laser mode radii on SA became smaller, which led to the increase of the power density on SA. For the lower pump power, it brought a better performance of Q-switching. While, when the pump power was larger, the larger power density on SA led to the bleaching of the SA that deteriorates the performance of Q-switching . The Q-switch efficiency  of PQS laser operating stably were found to be 39%, 43.5%, and 48.8% for 24.3 W, 22.8 W, and 16.1 W pump power at L of 7 mm, 15 mm, and 25 mm, respectively. The maximum average output power is 3.2 W for the pump power of 22.8 W at L of 15 mm.
The variation of the single pulse energy and pulse width versus the pump power at the different L is shown in Fig. 3. It is different from other references reported with almost constant pulse energy to pump power [11, 14]. From Fig. 3, the pulse energy of ~10 μJ and the pulse width of microsecond magnitude are obtained at the pump power of ~3.5 W. These results are close to ones of PQS Tm,Ho-codoped laser at room temperature [13, 14]. Figure 3(a) shows also that the pulse energy at L of 25 mm increases abruptly with the pump power, and reaches the maximum value of 70.5 μJ at the pump power of ~10 W, then decreases to 53 μJ. For L of 15 mm, the maximum pulse energy of 51.9 μJ occurs also at the pump power of 10 W, and the pulse energy changes slightly between 51.9 μJ and 43.3 μJ at the pump power of >10 W. As comparison, the maximum pulse energy for L of 7 mm appears at pump power of 18.3 W. From Fig. 3(b), for the pump power of <10 W, the pulse width decreases fast monotonically from a few microseconds to hundreds of nanoseconds. As shown in Fig. 3(b), the pulse width exhibits no obvious dependence on the larger pump power than 10 W. And at L of 25 mm, the minimum pulse width of ~350 ns is obtained. Figure 4 shows the pulse repetition frequency (PRF) with the pump power at the different L. The PRF increases monotonically from 8 kHz to 57 kHz with the pump power though the change among different L is unobvious. Base on the above analysis, it is found that the maximum single pulse energy of 70.5 μJ was obtained, corresponding to the pulse width of 354 ns and PRF of 16.7 kHz at the pump power of 9.9 W at L of 25 mm. When the L equaled 15 mm, the pulse energy keeps almost constant when the input power changes from 10 W to 22.8 W.
The pulse temporal trace was recorded by a Lecroy digital oscilloscope (600 MHz bandwidth) with a fast PIN photodiode. Figure 5(a) shows the oscilloscope trace of a single expanded shape pulse at the pump power of 9.9 W (L = 25 mm). The inset in Fig. 5(a) shows the stable pulses for this pump power and distance. When the pump power was more than 16.1 W (L = 25 mm), 22.8 W (L = 15 mm), and 24.3 W (L = 7 mm), we can’t observed the stable pulses train. Figure 5(b) shows the unstable train of output pulses at the pump power of 22.8 W for the distance L of 25 mm. It is meaningless to measure the PRF and pulse width in this unstable condition. So, the parameters for the pump power weren’t given in Fig. 3 and Fig. 4. The instabilities in the pulse spacing (“jitter”) observed in Fig. 5(b) had previously been attributed to technical factors such as thermal and mechanical instability of the SAs. The recent researches showed that the poor pulse-to-pulse stability of the laser could be induced by the intrinsic nonlinear dynamics of the system, and was ruled by low-dimensional deterministic chaos [25, 26]. Besides, local heating of the active channel from the uniformity of the dopant concentration could also induce visible instability of the Q-switching pulses .
The laser beam quality was measured by the traveling knife-edge method along x- and y-axis. By fitting Gaussian beam to these data, the M2 factors were determined to be 1.10 and 1.14 in x and y directions respectively [see Fig. 6]. The inset shows the transverse output beam profile measured by a beam analyzer with a close to fundamental transverse electromagnetic mode.
The emission spectrum of CW and corresponding Q-switched laser was also recorded with a WDG-30 monochrometer with entry slice of less than 0.04mm. The emission spectrum of CW laser kept good consistency when the CW laser output power was increased from 2W to 10.5W. At the beginning of Q-switching, for the pump power of 7.6W, the spectral width of the Q-switched laser was broadened [see Fig. 7(a)]. With the increase of pump power, the PQS laser brings the narrowing spectral width and lengthening wavelength. At the fixed pump power such as 10W, the emission central wavelength of PQS laser, comparison with CW operation, generates also slightly red shift from 2.051um to 2.053um for different L [see Fig. 7(b)]. The slightly red shift of the central wavelength results possibly from the combined effect of both the emission cross section relation to emitting wavelength and the change of intracavity losses for inserted the SA .
In conclusion, we realized, first, to our knowledge, diode-pumped PQS Tm,Ho:GdVO4 laser cooled by liquid-N2 with a Cr2+:ZnS SA. The influence of the different distance of Cr2+:ZnS from the output coupler was investigated. The highest pulse energy of 70.5 μJ was obtained at input power of 10 W at the appropriate position L of 25 mm, corresponding to pulse repetition frequency of 16.8 kHz and a pulse width of 354 ns. The maximum Q-switch efficiency is up to 48.8% for ~16.1 W pump power at L of 25 mm. Furthermore, the M2 factor of ~1.1 in x and y directions shows near-diffraction limited beam quality. The stable PQS Tm,Ho:GdVO4 laser is useful for generating different wavelength laser inside nonlinear optical media.
This work is supported by National Natural Science Foundation of China (60878011, 61078008, 61308009), Fundamental Research funds for the Central Universities (Hit.NSRIF.2014044), and Program for New Century Excellent Talents in University (NCET-10-0067).
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