Employing an acousto-optic modulator (AOM), the Q-switched laser characteristics of a Tm,Y:CaF2 crystal have been investigated. With T = 1%, 2% and 5% output couplers (OCs), output laser performance in both continuous-wave (CW) and Q-switching regimes have been presented and are discussed in details. The AOM Q-switched Tm,Y:CaF2 laser can run at high repetition rates ranging from 1 kHz to 10 kHz. Under the modulation frequency of 1 kHz, pulses with the shortest duration of 280 ns and the maximum pulse energy of 0.335 mJ have been delivered, corresponding to a maximum peak power of 1.19 kW. The results indicate that Tm,Y:CaF2 crystals can act as a promising candidate of gain medium for pulsed 2 μm laser.
© 2017 Optical Society of America
Eye-safe 2 μm laser has gained a lot of interests due to its wide applications in a number of fields such as gas detection, coherent lidar, tissue welding and medical application [1–4]. Additionally, 2 μm laser is an important pumping source for optical parametric oscillators (OPOs) to generate mid-infrared laser with longer wavelength [5, 6]. In general, 2 μm laser sources can be realized by either OPOs  or rear-earth ions doped laser crystals. Compared with the complicated setup for OPOs, the latter method is more advantageous in realizing a compact and efficient 2 μm laser source. Tm3+ ions doped crystals are essential gain media for such laser sources which rely on the transition from 3F4 to 3H6. However, typical quasi-three-level system for Tm3+ ions doped crystal has intrinsic disadvantage of serious reabsorption loss that strongly degrades the laser performance. One method to solve this problem is employing high-brightness laser diode (LD) as pump source, which can greatly increase the degree of population inversion, subsequently improving the gain capability of the laser crystal . However, LD pumping would introduce excess thermal load, which conversely enhances the temperature-dependent reabsorption loss [9, 10]. To overcome the reabsorption loss, large Stark splitting in the ground-state of active ions would be helpful. In addition, originating from the up-conversion process, the subsequent phonon-assisted non-radiation process is also adverse to the laser emission in Tm3+ ions doped crystal, which depopulates the upper laser level and leads to heat generation . Employing low-phonon-energy crystal as host material is an efficient way to conquer this problem. Furthermore, low phonon energy also endows a long lifetime to upper laser level, which obviously enhances the energy storage ability of laser crystal and makes it much more suitable for Q-switching operation. Thus efforts on exploring novel host crystals with low phonon energy and providing large Stark splitting for Tm3+ ions has been paid in recent years.
CaF2 crystal, the host material in this experiment, is one kind of low-phonon-energy crystals with the maximum phonon energy of 495 cm−1 , while this value for popular host crystal such as YAG is 800 cm−1 . CaF2 crystal also has an attractive property of broad-band transparency from UV to infrared spectral domain. Thus it is a very advantageous candidate for laser host crystal. Employing the Tm3+ doped CaF2 single crystal and ceramic as gain media, laser radiation at ~1.9 μm have been experimentally demonstrated since 2004 [14–16]. Taking advantage of the mass difference between Tm3+ ion and Y3+ ion, the lattice field of Tm,Y:CaF2 crystal can be efficiently distorted, which is beneficial for enhancing the Stark splitting . In our previous work, the Tm,Y:CaF2 laser crystal has been successfully grown and employed to generate lasing emission in both CW and wavelength tunable operation regimes , and a broadly wavelength tunable range of ~190 nm was achieved, which was much broader than that of Tm:CaF2 crystal (~135 nm) . Additionally, Tm,Y:CaF2 crystal has an emission cross-section as high as 4.6 ⨯ 10−21 cm−2 , which is more than twice of that for Tm:YAG (2 ⨯ 10−21 cm−2) . As it is well known, the pulse duration of a Q-switched laser is inversely proportional to the emission cross-section , thus an efficient Q-switched Tm,Y:CaF2 laser with short pulse duration is highly expected.
In this paper, an actively Q-switched Tm,Y:CaF2 laser has been successfully realized with an AOM, and the pulsed laser characteristics in terms of modulation frequencies and pump powers have been investigated and discussed in details. The shortest pulse duration of 280 ns was achieved at the repetition rate of 1 kHz, corresponding to a maximum pulse energy of 0.335 mJ and a maximum peak power of 1.19 kW.
2. Experimental setup
Figure 1 shows the experimental setup of the actively Q-switched Tm,Y:CaF2 laser. Restricted to the size of AOM, a 13.5 cm-long V-type cavity was employed to investigate the output laser performance in both CW and Q-switching regimes. The pump source (Class IV laser product, produced by Lissotschenko Mikrooptik corporation) was a fiber-coupled diode laser (LD) with a emission wavelength of 785 nm and a maximum output power of 50 W. The coupling fiber had a core diameter of 200 μm and NA of 0.22. Using a 1:1 imaging module, the pump light was focused into the 3 at.% Tm, 3 at.% Y:CaF2 laser crystal, which was grown by Temperature Gradient Technique method. To remove the excess heat, the laser crystal was wrapped by indium foil and held in a brass heat-sink cooled at 14 °C by a water cooler. Both surfaces of the laser crystal were antireflection (AR) coated from 750 nm to 850 nm and 1800 nm to 2150 nm. The employed AOM (The 26th Electronics Institute, Chinese Ministry of Information Industry) was fabricated by fused quartz and had a physical length of 45 mm with an acoustic aperture of 2 mm. Both surfaces of fused quartz were AR coated at ~2 μm to minimize the insertion loss. The AOM had a work pattern of two-dimension modulation which effectively enhances the diffraction efficiency compared to one-dimension modulation pattern. Driven with radio frequency of 41 MHz and power of 50 W, the modulation frequency of AOM could be varied from 1 kHz to 50 kHz and the delivered diffraction efficiency was 90%. To efficiently remove the heat generated by electroacoustic transducer and prevent the AOM from being damaged, the AOM was also cooled at 14 °C by the water cooler. In the employed folded cavity, both mirrors M1 and M2 had curvature radii of 75 mm and were AR coated from 750 nm to 850 nm (reflectivity<2%), high-reflectivity (HR) coated (reflectivity >99.9%) from 1850 nm to 2100 nm. Several OC mirrors M3 with different transmittances of 1%, 2% and 5% at ~2 μm were employed for comparisons.
3. Results and discussions
Firstly, the Tm,Y:CaF2 laser was investigated in CW regime. The average output powers were recorded by a laser power-meter (MAX 500AD, Coherent, USA). As shown in Fig. 2, output powers increased almost linearly with absorbed pump powers. For T = 1%, 2% and 5% OCs, the oscillation thresholds were 337 mW, 461 mW and 752 mW, respectively. Under a maximum absorbed pump power of 2.6 W, the maximum output powers of 473 mW, 500 mW and 420 mW were obtained for T = 1%, 2% and 5% OCs, respectively. Due to the relatively low damage threshold of the Tm,Y:CaF2 crystal (7.96 kW/cm2) , the incident pump power has not been further increased. A maximum slope efficiency of 24.1% was obtained with the T = 2% OC employed, corresponding to a maximum optical to optical conversion efficiency of 19.3%. Thus in the following Q-switching experiment, the 2% OC was chosen for output power performance investigation. The relatively low slope efficiency is mainly attributed to the low absorbance of Tm,Y:CaF2 crystal at 785 nm since a large amount of unabsorbed pump power will easily introduce extra thermal loss inside the laser crystal. By using a pump source emitting at the absorption peak wavelength of 767 nm, where the absorbance of Tm,Y:CaF2 crystal is more than three times of that at 785 nm , a much improved slope efficiency is highly expected.
To evaluate the insertion loss of AOM, with the AOM inserted in the cavity and turned off, the average output powers were also recorded as shown in Fig. 2(b), from which one can see that the average output powers only dropped a little due to the insertion loss of the AOM, which was attributed to the AR coating for both surfaces of the fused quartz.
With the T = 2% OC employed, the dependences of average output powers on absorbed pump powers under different modulation frequencies were investigated, which are shown in Fig. 3. When the pulse repetition frequency (PRF) varied from 1 kHz to 10 kHz, the average output powers and slope efficiencies all increased, which was due to the decreasing loss induced by increasing modulation frequency. Maximum average output powers of 335 mW, 365 mW and 396 mW, under the PRF of 1 kHz, 5 kHz and 10 kHz, were obtained, respectively, corresponding to slope efficiencies of 19.5%, 18.5% and 18.1%.
The dependences of pulse durations, pulse energies and peak powers on different PRFs under the maximum pump power were recorded and shown in Fig. 4. At the modulation frequency of 1 kHz, the shortest pulses with durations of 290 ns, 280 ns and 330 ns were obtained for T = 1%, 2% and 5% OCs, respectively. The corresponding maximum single pulse energies for T = 1%, 2% and 5% OCs were 0.320 mJ, 0.335 mJ, and 0.332 mJ, respectively, corresponding to the maximum calculated peak powers of 1.0 kW, 1.16 kW, and 1.19 kW.
The temporal pulse profiles were detected by a fast InGaAs photodetector with a rise time of 35 ps (ET-5000, EOT, USA) and monitored by a digital oscilloscope with a bandwidth of 1 GHz (Tektronix DPO 7102, USA). Under the PRF of 1 kHz, the temporal pulse trains at the maximum output power are shown in Fig. 5, from which it can be seen that stable Q-switching operation was successfully achieved. The bottom of Fig. 5 shows the temporal profile of the shortest pulse with duration of 280 ns. The sharp decline in the falling edge of the pulse was caused by the very short turn-off time of the AOM.
A laser spectrometer (APE WaveScan, APE Inc.) with a resolution of 0.4 nm was employed to record the output spectra. Figure 6 shows the output spectra for different operation regimes of the Tm,Y:CaF2 laser. The spectral bandwidth for CW regime was 38 nm when T = 2% OC was employed, while in Q-switching regime, the output spectral bandwidths for T = 1%, 2% and 5% OCs were 14 nm, 17 nm and 19 nm, respectively. The bandwidths in Q-switching regime were much narrower than that in CW regime, which was attributed to the insertion loss induced wavelength selection role of AOM. Additionally, the central wavelengths were found to depend on the operation regimes. As shown in Fig. 6, central wavelengths for the CW and Q-switching regimes were 1944 nm, 1912 nm, 1899 nm and 1884 nm, respectively. From the results we can see that blue shift happened to the emission wavelength with the increase of intracavity loss, which was caused by the required increased inversion rate in such a typical three-level laser system. A 90.0/10.0 scanning-knife-edge method was employed to evaluate the laser beam quality at the maximum output power under a PRF of 1 kHz. As shown in Fig. 7, the M2 factors in the tangential and sagittal planes were measured to be 1.53 and 1.46, respectively.
In this paper, a diode-pumped acoustic-optically Q-switched Tm,Y:CaF2 laser has been successfully realized. The actively Q-switched laser could run at high repetition rates ranging from 1 kHz to 10 kHz. At the modulation frequency of 1 kHz, the shortest pulses with duration of 280 ns have been achieved, corresponding to maximum pulse energy of 0.335 mJ and maximum pulse peak power of 1.19 kW. The results indicate the promising prospect of Tm,Y:CaF2 crystal in generating efficient 2-μm pulsed lasers with high repetition rates and short pulse durations.
National Natural Science Foundation of China (NSFC) (61475088, 61775119, 61378022, 61422511); Young Scholars Program of Shandong University (2015WLJH38); Open Research Fund of the State Key Laboratory of Pulsed Power Laser Technology, Electronic Engineering Institute, Hefei, China (SLK2016KF01).
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