Variable energy, high peak power, passive Q-switching diode end-pumped Yb:LuAG laser

One of the oldest, well-established, but still vital research fields in laser physics is passive Q-switching [1–23]. Although this regime has evident advantages, a main deficiency, which limits its possible application, is the near constant energy of output pulse for a given laser configuration (i.e. gain medium, passive Q-switch and out-coupling mirror). In a typical passively Q-switched laser, a change in the pump rate results mainly in a change in the repetition period accompanied by a small change in the pulse energy and duration due to second-order effects. Thus, to date, these types of lasers suffer in comparison with other types of pulsed, high-repetition-rate lasers (e.g. actively Q-switched lasers or (fibre) master oscillator power amplifiers).

With passive Q-switching, the second threshold condition defines the necessary requirements to obtain an effective short pulse Q-switching regime (see e.g. [1, 3]). From this point of view, it is required that the pair of the gain medium/saturable absorber should have as high a ratio of absorption cross section σ_abs to gain cross section σ_em as possible, and that the recovery time of the absorber must be much longer than the round-trip time of the cavity. The most frequently used, well-established Nd:YAG and Cr:YAG pair provides a ratio of σ_abs/ σ_em~15, which although, sufficient, in some cases does not lead to effective pulse generation due to the non-negligible part of stored energy dissipated in the process of bleaching of the saturable absorber.

It appears that much better performance could be obtained with the same absorber in the pair with Yb-doped gain medium, e.g. σ_abs/ σ_em > 100 for a Yb:YAG and Cr:YAG pair. Moreover, because of the much longer lifetimes of Yb media compared to Nd media, higher pulse energies and peak powers can be achieved for the same pump powers (see e.g. [13]). One of the most important properties of Yb media is the occurrence of reabsorption on the laser wavelength as a result of the partial occupation of a lower laser level (about 3–5% of relative occupation at room temperature). The examination of several Yb-doped media [4, 6, 13, 23–33] in crystal growing [23–27, 31, 33] or ceramic technology [13, 28–30, 32] showed its feasibility for the continuous-wave, mode locking and passive Q-switching regimes. Besides the low quantum defect of Yb active media (3–5%), the design of an efficient continuous-wave pumped Yb laser is a challenging task because of the requirement to overcome reabsorption losses by bleaching with high pump densities and the resulting high densities of heat sources. To mitigate thermal problems the effective cooling, even to cryo temperatures has been used (see e.g. [25]). Alternatively, special setups based on multi-pass pumping of thin disk geometry [26, 27] or edge-pumping [28] have been proposed for room temperature operation.

With the advent of high-brightness fibre-coupled laser diode sources offering dozens of watts of power in 100 μm fibre core, the high excitation levels, much above that required to minimize reabsorption effects, can be achieved in the gain medium for single pass pumping even at room temperatures. To start the passive Q-switching process, the intracavity laser field intensity should overcome the saturation intensity of the saturable absorber. Further, in such a channel of bleached transparent media (e.g. Yb:YAG and Cr:YAG), the laser giant pulse action develops inside the cavity and enables the efficient extraction of a significant part of the energy by means of the out-coupling mirror. The method of optimizing Q-switched lasers [14–16] can also be widened in the case of quasi-III-level gain media, especially for the case of short (compared to gain medium lifetime), repetition periods [3, 19, 22]. The measure of quality in the Q-switching regime is the extraction efficiency, i.e. output energy/power in the Q-switching regime compared to the free-running output obtained under the same circumstances. Agnesi et al [13] recently showed above 86% extraction efficiency for a passively Q-switched regime in a Yb:YAG/Cr:YAG diode-pumped laser.

Our idea of variable energy passive Q-switching consists of controlling the starting point of saturable absorber bleaching via a change in the effective pump area (see figure 1).

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Modelling results of the dependence of an effective pump area on the control defocusing parameter are shown in figure 2. If we increase the effective pump area (e.g. simply by moving the gain medium along the caustics), the volume of inversion increases simultaneously with a decrease in the effective and maximum pump density.

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As a result, the starting point of the passive Q-switching process moves to longer pump durations and higher stored energies because of an increase in the perpendicular size of the excited area. Therefore, we have a method to control the stored and extracted energy for the same laser configuration and pump power. If the fundamental laser mode area is larger than the averaged pump area, which is feasible for high-brightness fibre-coupled laser diodes, the laser operation is enforced in the fundamental mode. In summary, giant pulse generation starts for the same pump power with variable energy controlled by the change in pumping area width.

Such a simple theoretical model is completed by accompanying processes, which can play a detrimental role as follows: a decrease in the effective absorption efficiency as a result of absorption saturation in the gain medium, an increase in the effective temperature of the excited volume due to a change in heat power density, and an increase in gain and thermally induced guiding effects affecting the fundamental laser mode size.

The experimental set-up is shown in figure 3.

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For the pump source we used the 20 W, 100 μm core fibre-coupled diode HLU20F100 made by LIMO. The pump beam after relay optics creates the caustics, with a diameter of 0.135 mm and the parameter M² = 21. The pump source wavelength was tuned to 968 nm—the absorption peak of Yb : LuAG.

We examined two cavities in a semi-symmetrical configuration having the same confocal parameter and laser mode radius w₀ = 0.16 mm:

• (a)  
  longer cavity with a flat rear mirror and output coupler with curvature radius r_curv = 150 mm and transmission T_OC = 30% at λ = 1030 nm, cavity length L_rez=75 mm;
• (b)  
  short cavity r_curv = 300 mm, T_OC = 30% at λ = 1030 nm, L_rez = 25 mm.

In both cases the gain medium was AR coated, 15 at% Yb:LuAG crystal (grown by Crytur Inc.) with 1 mm thickness. During the experiment, the Yb:LuAG crystal was wrapped by indium foil and fixed in a water-cooled cupreous heat sink mounted on the XYZ-translation stage. The temperature of the cooling water was 20 °C. An effective absorption coefficient of our Yb:LuAG sample was estimated to be ~12 cm⁻¹. With an increase in pump power absorption, the efficiency value slightly decreased (from 72% to 65%) due to saturation. For maximum incident pump power of 15 W we have not observed significant change in absorption efficiency with defocusing (see figure 4).

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To neglect thermo-optically induced effects we conducted all experiments in the quasi-continuous wave pumping regime with a 2 ms pump duration and repetition of 10 Hz, corresponding to duty cycle 0.02 of the pumping regime. In such a case, the averaged increase in effective temperature inside the gain medium was less than 12 K, thus, it did not affect threshold conditions and fundamental mode.

For passive Q-switching, the Cr : YAG saturable absorber with initial transmission of 83% at 1030 nm was applied in both cases. The measurement results of the pulse energy and repetition rates in dependence on the defocusing parameter for both cases are shown in figures 5 and 6, respectively.

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For the long cavity case, the energy changed from 0.145 up to 0.51 mJ for 1.4 mm defocusing. For the latter case (see figure 6) the energy increased from 0.26 up to 0.43 mJ for the 1.5 mm defocusing.

The pulse duration was about 12 ns for the longer cavity and was decreased to about 4 ns for the shorter 25 mm cavity. The obtained peak powers for both cases (see figure 7) showed the same dependence on defocusing as the pulse energies. The maximal peak power of 113 kW for a pulse duration of 3.9 ns was demonstrated in the 25 mm, short cavity laser, whereas maximal 37 kW peak power in the 14 ns pulse duration was achieved in the 75 mm long cavity laser. In both cases we have a linearly polarized output beam with polarization azimuth determined by the orientation of Cr : YAG crystal. The estimated temporal jitter was less than 2%.

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In the last part of the experiments, we compared the averaged output powers demonstrated in free-running and passive Q-switching regimes for both cavities (see figure 8). The observed differences in output powers and slope efficiencies can be explained by the different averaged pump areas in both lasers and additional, higher order effects.

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The extraction efficiency defined as the ratio of maximal average powers obtained in both regimes was 78.7% for the long cavity and 88.5% for the shorter cavity, which evidences the high quality of the passively Q-switched lasers presented here.

In both cases with longer and shorter cavities, the divergence of the output beam was not dependent on the defocusing parameter and showed evidence of near fundamental mode operation. The divergence was about 5.4 mrad for the longer cavity and the resulting estimated parameter was M² < 1.1, whereas for the shorter cavity the parameter was M² < 1.3 and a divergence angle of 4.3 mrad.

We proposed and experimentally verified a new method of pulse energy control in passively Q-switched lasers. This letter presents experiments for a pair of Yb:LuAG as a gain medium and a Cr:YAG as a passive saturable absorber. The concept was proven; the range of demonstrated energy was 145–510 μJ for the longer 75 mm cavity (pulse duration 12 ns) and 258–430 μJ for the shorter 25 mm cavity (pulse durations 4 ns) for the absorbed pump power of 10 W and duty cycle of 0.02.

Notice that we achieved near 90% average power in the passive Q-switching regime compared to the free-running regime. We suppose that the same concept can be exploited for other types of passively Q-switched lasers (including microchips lasers), such as Nd:YAG–Cr:YAG at 1064 nm and Nd:YAG–V:YAG at 1340 nm. The method proposed here can be an alternative to changing the repetition rate, as exploited in the case of actively Q-switched lasers to control the pulse energy.

We have only shown preliminary results here, which have demonstrated the principle of the energy control method in experiments which were far from optimal. The gain medium length and dopant level, initial transmission of the passive Q-switch, and out-coupling losses for given pump unit parameters should be the control parameters in the process of optimizing this type of laser. To understand the necessary requirements and limitations of the proposed method, a 2D numerical model of such a laser will be developed and described in the near future.

This research has been supported by the Czech Ministry of Education, Youth and Sport project RVO 6840770 and by the Polish National Centre of Science under the project 2012/06/M/ST7/00425. We acknowledge Crytur Inc. for delivering of Yb : LuAG and Cr:YAG samples.