A compact diode-pumped continuous wave and passively Q-swithced Tm,Ho:LuLiF4 laser is demonstrated. The maximal output power of 381 mW at 2069 nm in continuous wave regime is obtained at an absorbed pump power of 1.5 W. By using a Cr2+:ZnS saturable absorber, the maximum Q-switched average output power of 74 mW is obtained at 2055 nm. The pulse width and pulse energy are almost independent of the absorbed pump power, with the maximal values of 1.2 μs and of 13 μJ respectively. The pulse repetition frequency can be tuned almost linearly from 1 to 5.8 kHz by changing the absorbed pump power. Furthermore, a comparative analysis of laser performances with different output couplers is first carried out.
© 2013 OSA
Q-switched solid state lasers emitting in 2 µm eye-safe spectral region have attracted much attention for various applications including coherent Doppler wind lidars, differential absorption lidars, photo-medicine, and nonlinear frequency conversion [1–3]. Tm-doped and Tm-Ho-codoped laser materials can produce 2 µm laser. However, the Tm-Ho-codoped materials are usually preferred for high output energy application because of long upper level lifetime and large emission cross section [4–10]. Among those known Tm-Ho-codped materials, Tm,Ho:LuLiF4 (LLF) is a kind of excellent laser crystal for producing 2 µm laser because of high natural birefringence, low upconversion effect, and large energy spread of the manifolds [11, 12]. For Tm,Ho:LLF crystal, the emission cross sections of the Ho 5I7 manifold are about 1.5 × 10−20 cm2 at 2.05 μm and 1.1 × 10−20 cm2 at 2.06 μm respectively, and the upper level lifetime of 5I7 manifold is as long as 14.8 ms . Therefore, Tm,Ho:LLF crystal offers good characteristics for Q-switched operations. At present, continuous wave (CW) and actively Q-switched Tm,Ho:LLF lasers have been researched . However, the output performances of passively Q-switched (PQS) Tm,Ho:LLF laser is seldom reported.
Passive Q-switching of solid state lasers with saturable absorbers (SAs) is a simpler and lower cost method to obtain pulse laser. In the past years, diode pumped PQS solid state laser at 2 µm waveband mainly focused on Tm-doped lasers. The PQS Tm:KY(WO4)2 laser using Cr2+:ZnSe SA was first demonstrated with a pulse duration of 57 ns and a single pulse energy of 4 µJ . The PQS Tm:KY(WO4)2 laser using a PbS-quantum-dot-based SA also was reported, which emitted pulse with duration of 8 ns and pulse energy of 30 µJ . Using Tm:KLu(WO4)2 and Cr2+:ZnS SAs, the laser pulses were produced with the single pulse energy of 145 µJ and the pulse durations in the 24-30 ns range . Almost simultaneously, the stable diode-pumped PQS Tm:YLF and Tm:LLF lasers using Cr2+:ZnS SAs were demonstrated which emitted pulses with durations of 14 ns and 7.6 ns, and the corresponding pulse energies of 0.9 mJ and 1.26 mJ respectively [17, 18]. It is worthwhile to note that the above Tm-doped lasers have very short pulse width. Tm-Ho-codopded solid state lasers are very ideal optical sources for 2 μm waveband coherent Doppler laser lidars. To improve the precision of wind velocity measurement, Q-switched mode coherent Doppler laser lidars require microsecond long pulse width . Injection seeding by a long pulse and single frequency seed laser can directly produce the pulses of proper duration, without requiring excessively long resonators or elaborate cavity loss control schemes . As a local oscillator, the long pulse PQS laser is required to be single frequency output. Operation with long pulses makes the generation of single frequency difficult to achieve. However, the single frequency oscillation of long pulse PQS laser can still be realized by the twisted-mode technique  and using volume Bragg grating to form the laser resonator . Recently, the long pulse PQS Tm,Ho:YLF laser has been reported, however it shows evident thermal effect at only 0.9 W absorbed pump power . LLF crystal is one of fluoride isomorphs, and compared with YLF, it has better thermo-optical and thermomechanical properties . Its thermal conductivity (a-axis), linear expansion coefficient (a-axis), and thermal coefficient of refractive index dn/dT (π poarization) are 5 Wm−1K−1, 13.6 × 10−6 K−1, and −6 × 10−6 K−1 respectively . In this paper, we first report a comprehensive investigation of the CW and PQS laser actions in a diode pumped Tm,Ho:LLF laser. In CW operation, the maximal output power of 381 mW at 2069 nm with slope efficiency of 29.2% is obtained. In PQS operation, a maximal average output power of 74 mW at 2055 nm is obtained, and the pulse width and energy are near constants of 1.2 μs and 13 μJ at the repetition frequency between 1 kHz and 5.2 kHz, respectively.
2. Experimental setup
A simple plane–concave cavity configuration is shown in Fig. 1. The pump laser is a fiber-coupled laser diode temperature-tuned to 792 nm emission wavelength, with a maximum output power of 3 W. The diameter and numerical aperture of the fiber core are 100 μm and 0.22 respectively. A coupling optics system is used to focus the pump beam. The pump spot diameter in the crystal is about 100 µm. The total transmission efficiency of the beam-reshaping system is over 90% at 792nm. The Tm,Ho:LLF laser crystal has dopant concentrations of 5% Tm, 0.5% Ho with dimension of 5 mm × 5 mm × 2.5 mm. A dichromatic coating on the front face of the crystal is high transmitting at 792 nm, but is totally reflecting at 2 µm. The other face is antireflection coated at 792 nm and 2 µm. The near hemispherical resonator is formed between the planar crystal front face and the output coupler. The curvature radius of the output coupler is 103 mm. To efficiently remove the heat generated with pump power from the crystal, the crystal is wrapped with indium foils and held in a brass heat sink. Temperature of the heat sink is held at 283K with a thermoelectric cooler. For PQS operation, a Cr2+:ZnS SA which is 2.1 mm thick, with aperture of 10 mm × 5.4 mm, is inserted inside the laser cavity. The faces of Cr2+:ZnS crystal are antireflection coated near 2 µm leading to an initial transmission of T0 = 92%. The laser pulse is recorded by a Tektronix TDS3032B digital oscilloscope (300MHz bandwidth, 5G samples/s) and a fast InGaAs PIN photodiode. The average output power is measured by a power meter (MolectronPM10). The spectra are measured with a monochrometer (WDG-30) and a InGaAs PIN photodiode. The transverse output beam profile is measured with a beam analyzer (Electrophysics, MicronViewer 7290A).
3. Results and discussion
The CW operation of Tm,Ho:LLF laser is firstly investigated at an optimal cavity length of 100 mm. The mode radius of 2 µm laser in the crystal is calculated to be about 100 µm, which is well matched with the pump spot. The change of the output power with the absorbed pump power for three different output couplers of 2%, 5%, and 10% is shown in Fig. 2(a). Straight lines are results of a linear fit, and the calculated slope efficiencies are also given in Fig. 2(a). As can be noted from Fig. 2(a), the best result is obtained with the 5% output coupler. Under an absorbed pump power of 1.5 W, a highest output power of 381 mW is obtained with a slope efficiency of 29.2%. Reducing the output coupling transmittance to 2%, the maximum output power and slope efficiency decrease to 300 mW and 24.2%, respectively.
The output performances of the PQS Tm,Ho:LLF laser are investigated by inserting the Cr2+:ZnS SA into the cavity. Figure 2(b) shows the average output power as a function of absorbed pumped power with the output couplers of 2%, 5%, and 10%, respectively. As can be noted from Fig. 2(b), the average output power linearly depends on the absorbed pump power, and the best result is from the 10% output coupler. At 1.5 W absorbed pump power, the highest average output powers of 73 mW with a slope efficiency of 9.3% is obtained. When the transmittance of output coupler is changed to 5% and 2%, the maximum average output powers of 56 mW and 28.3 mW are obtained with the corresponding slope efficiency of 5.6% and 2.8%, respectively.
The output of Tm,Ho:LLF laser is π-polarized in all cases. The laser spectra are recorded at the absorbed pump power of 1.5 W and shown in Fig. 3. As can be seen from Fig. 3(a), the CW laser operates in three different centric wavelengths of 2070 nm, 2069 nm and 2055 nm for three different output transmittances of 2%, 5%, and 10%. The laser centric wavelength shortens with increasing the transmittance of output coupler, due to the quasi-three-level characteristic of laser crystal near room temperature . In the CW operation, for output couplers with transmittances of 2% and 5%, the losses in the resonator are relatively low. The effective gain at the wavelength of 2069-2070 nm exceeds that at 2055 nm, therefore the laser at the wavelength of 2069-2070 nm oscillates, and the 2055 nm laser is suppressed. With the 10% output coupler, the losses in the resonator are increased, and a higher inversion level is required to reach lasing threshold. The effective gain at 2055 nm exceeds that at 2069-2070 nm, so the output wavelength of the CW laser shifts to shorter 2055 nm .
When the laser operates in the PQS mode, the output wavelengths all shifts to the shorter wavelength of 2055 nm for the above three coupling transmittances as shown in Fig. 3(b). In the PQS mode, the intracavity losses are greatly increased because of inserting the Cr2+:ZnS SA, which will lead to a very high inversion population density when the laser pulse begins to build. In this case, the emission cross section becomes the key factor, then lasing will occur on the transition with the largest emission cross section at 2055 nm [12,13].
The dependence of the pulse energy on the absorbed pump power is shown in Fig. 4. As been seen, the pulse energy nearly keeps constant with the augment of the absorbed pump power, and the pulse energies are 5, 9, and 13 μJ for the output couplers of 2%, 5%, and 10% respectively. The best result is achieved with the 10% output coupler.
Figure 5(a) shows the variation of the pulse width versus the absorbed pump power for the three coupling transmittances. From Fig. 5(a), it can be noted that the pulse width almost keeps a constant of 1.2 μs with the increase of absorbed pump power for the output couplers of 5% and 10%. However, the pulse width obtained with the output coupler of 2% slightly decreases from 1.2 to 0.9 μs with increasing the absorbed pump power from 0.7 to 1.5 W. Figure 5(b) shows the pulse repetition frequency versus the absorbed pump power. As be noted from Fig. 5(b) that the pulse repetition frequency almost linearly increases with increasing the absorbed pump power [16–18]. The pulse repetition frequency increases from 1 to 5.8, 5.2, 4.8 kHz for 5%, 10% and 2% output couplers respectively. Through the above analysis, it is found that the pulse repetition frequency of the Tm,Ho:LLF laser can be effectively controlled by changing the absorbed pump power, moreover, in the progress, the pulse energy and pulse width can keep near constants. The characteristic of the PQS Tm,Ho:LLF laser is very meaningful as a seed laser for accurate wind velocity measuring.
The output pulse train with the repetition frequency of 4 kHz and sing pulse profile with duration of 1.2 μs are shown in Fig. 6(a). It can be found from the inset of Fig. 6(a) that the pulse amplitude variation of pulse-to-pulse is less than 5%. Moreover, the output pulses always show good stability when the absorbed pump power is changed. With the transmittance of 10%, the transverse output beam profile is measured under the absorbed pump power of 1.5 W, and shown in Fig. 6(b). It can be seen from Fig. 6(b) that the output beam is close to fundamental transverse electromagnetic mode (TEM00). The beam radius at different positions along the beam propagation direction measured by the traveling knife-edge method is also shown in Fig. 6(b), and the M2 factor is calculated to be 1.17.
Finally, it is worthwhile to note that the results of the PQS Tm,Ho:LLF laser reported here are much better, compared with the previously reported PQS Tm,Ho:YLF laser under similar conditions . For the PQS Tm,Ho:YLF laser at 2053 nm, the maximal pulse energy is 4 µJ, the highest repetition frequency is 2.6 KHz, and the maximal average output power is 16 mW. Furthermore, the average output power shows obvious saturation at only 0.9 W absorbed pump power due to the thermal effect.
We have reported on a compact diode-pumped CW and PQS Tm,Ho:LLF laser. The influence of output coupling transmittance on laser performances is comparatively analyzed. A maximal CW output power of 381 mW at 2069 nm is obtained for the output coupling transmittance of 5%, corresponding to a slope efficiency of 29.2%. Furthermore, the centric wavelength shifts to the shorter value with the increase of output coupling transmittance. With a Cr2+:ZnS SA, the centric wavelength of the PQS laser is always at 2055 nm for the three output coupling transmittances of 2%, 5%, and 10%. The maximum average output power of 74 mW is obtained for the output coupling of 10%. The pulse width and pulse energy are near constants of 1.2 μs and 13 μJ respectively, at the pulse repetition frequency from 1 to 5.2 kHz which can be nearly linearly tuned by changing the absorbed pump power. The stable long pulse PQS Tm,Ho:LLF laser is very suitable to 2 μm coherent laser lidar system for accurate wind velocity measurement.
This work is supported by the Specialized Research Fund for the Programe for New Century Excellent Talents in University (Grant No. NCET-11-269), the National Natural Science Foundation of China (Grant Nos. 11204048, 61275138, 10804022, 60878016), the Fundamental Research Funds for the Central Universities (Grant Nos. HEUCFZ1221, HEUCFZ11217), and the 111 project to the Harbin Engineering University (Grant No. B13015).
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