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Abstract
We demonstrated a compact cavity-dumped Q-switched Yb:YAG laser based on a 72-pass pump module. A highly stable nanosecond pulse laser with a duration of 18.38 ns was achieved at a repetition rate of 100 kHz, which is pumped by multimode laser diode stacks at 969 nm. The average output power of more than 150 W is delivered in a good output beam characterized by M2 = 1.53. The result shows that the pulse duration derived from simulation agrees well with the one retrieved from the experimental measurements. To the best of our knowledge, the 43.5% optical-optical efficiency is the highest reported to date from a cavity-dumped Q-switched Yb:YAG TDL (thin-disk laser).
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
High-energy, narrow-pulse-duration, high-peak-power Q-switched lasers are of great importance in a variety of applications such as radar, precision micromachining, optical communication, laser rangefinders, frequency conversion, three-dimensional laser writing of waveguides [1–5]. In addition to pulse energy and duration, the repetition rate is a critically important parameter for lasers used in material processing as it determines the material removal rate due to heat accumulation [6]. Therefore, narrow pulse width, high average power and high repetition rate are the development trend in this field. With the increase of average output power, traditional lasers such as fiber lasers have strong nonlinear effects (i.e. stimulated Brillouin scattering), resulting in wide pulse duration and limited peak power [7,8]. Slab lasers have yielded nanosecond pulses with pulse energies of 341 mJ at 20 Hz [9], but a strong asymmetric thermal lens tends to compromise the output beam quality at high power levels.
Fortunately, the emergence of laser systems based on the thin-disk gain geometry has effectively improved this situation [10]. The multi-pass pumping technology makes up for the drawbacks of low single-pass absorption of the gain medium and improves the pump absorption efficiency, while the backward cooling of single-side pump further reduces the thermal lens effect of the thin-disk. The small quantum defect of Yb:YAG crystal minimizes the heat load while the availability of high-brightness diodes allows for high-intensity pumping for power scaling, the thin-disk geometry is a proven technique for enhanced cooling, as well as mitigation of thermo-optical effects [11–13]. Therefore, the laser systems based on the thin-disk gain geometry have been widely used in continous and pulsed fields [14–16]. Combined with electro-optical cavity-dumped Q-switched technology, several hundred watts output powers have been demonstrated with thin-disk gain geometry, and have been applied in material processing [17,18].
In this paper, we present a millijoule-level nanosecond TDL based on cavity-dumped Q-switched technology, which is developed as seed laser for kW-class nanosecond multipass amplifier. Based on the 72-pass pump module, at the repetition rate of 100kHz, the maximum average output power reaches 150.2 W, the output pulse sequence is highly stable, and the pulse duration is as narrow as 18.38 ns. Because of excellent thermal management and cavity design, we have achieved highly stable nanosecond pulse output with near-diffraction-limited beam quality and an optical-optical efficiency of more than 43.5%. The theoretical predictions are verified by experiments, and the experimental results are significantly higher than those reported in the previous literature, which have important reference significance for related research work.
2. Experimental setup
As shown in Fig. 1, the cavity-dumped Q-switched laser consists of one 72-pass disk module equipped with a thin-disk Yb:YAG crystal which has a thickness of $\sim$200 $\mu$m, a diameter of 15 mm, a doping concentration of 7 at.% and a radius of curvature of 3.6 m and pumped by 400 W diode laser stacks with thin disk laser(TDL) adjusting technology [19].
They are integrated into a Z-type resonator of 2 m optical path length, which accommodates a 1/4 waveplate and a single-crystal BBO Pockels Cell(crystal size is $4\times 4 \times 25 mm$), introducing $\lambda /4$ phase shift required for Q-switching.
The pumping source is a fiber-coupled diode laser with a core diameter of 400 $\mu$m and a numerical aperture of 0.22 operating at wavelength of 969 nm and is stabilized by narrow-band volume Bragg grating (VBG). Pumping at 969 nm, the so called zero-phonon line(ZPL) excites electrons directly to upper laser level, which reduces quantum defect from 8.7% to 5.9%, and, as a consequence, amount of waste heat generated in the laser crystal is 32% decreased [20], as compared to the usual 940 nm pump wavelength. The pump spot on disk has a size of 4.0 mm, implying a pump intensity of 2745 $W/cm^2$ when pumped at 345 W.
The electro-optically cavity-dumped Q-switched laser based on thin-disk geometry is schematically depicted in Fig. 2. The Z-shaped cavity has been chosen because it satisfies the laser mode matching of the resonator, resulting in excellent beam quality.
Fig. 2. Schematic diagram of the electro-optically cavity-dumped Q-switched Yb:YAG TDL. QWP, quarter-wave plate; PC, Pockels cell; TFP, thin-film polarizer.M3 denotes a 1.7 m RoC convex mirror, and M2 is a plane HR with an incident angle of 50$^{\circ }$. All other mirrors are plane.
The Pockels cell is made of BBO crystal and driven by voltage impluses set at the quarter-wave voltage of approximate 3.5 kV and the rise time is about 10 ns. The TFP is a thin film polarizer with high reflectance for s-polarized light at 1030 nm and high transmittance for p-polarized light at the Brewster angle of 45$^\circ$.
The cavity-dumping cycle has the following three steps. Firstly, no voltage is applied to the Pockels cell. Therefore, s-polarized light from the TFP passes through the QWP twice in the round-trip and turns into p-polarized light before being transmitted out of the resonator. Secondly, the quarter-wave voltage is applied to the Pockels cell. The Pockels cell works as a QWP and the light can pass through the QWP and PC twice, this is equivalent to passing QWP four times. Therefore, the output coupling rate of the resonator is zero, and s-polarized light can be amplified between M1 and M3, and store energy in the form of photons. At the end of the cycle, the voltage is removed. The laser in the cavity can be transmitted out by the TFP during a round-trip and a new cycle starts. Thus, the round-trip time, which is the theoretical pulse duration regardless of the switching time, is approximate 17.8 ns.
The resonator mode(see Fig. 3) was calculated by ABCD matrix formalism and is designed to fulfill two requirements: (1)to reduce the peak power on the optical components: it keeps a mode size of 2.0 mm on PC and (2) to ensure a mode size of 3.6 mm on the disk.
3. Model and theory
3.1 Aborption efficiency of the 72-pass pump module
A multipass pumped TDL is generally composed of a parabolic mirror and several folding prisms, as shown in Fig. 4. When the pump power is injected into the Yb:YAG thin disk, since the thin-disk gain medium has a certain thickness, the thin-disk gain medium will absorb part of the pump power and the remaining reflected light will be refocused on the thin-disk crystal after being reflected by the folding prisms. After multiple times of reflection, the pump light will pass through the thin-disk crystal many times. It makes up for low single-pass absorption efficiency due to the limited thickness of the disk. Assuming that the incident pump power is $P$, the reflectivity of the folding prism is $R_1$, the reflectivity of the parabolic mirror is $R_2$, the reflectivity of the back surface of the thin-disk crystal is $R_Y$, the absorption coefficient of the Yb:YAG crystal is $\alpha$, and the thickness of thin disk is $L$, the angle of incidence of the crystal is $\theta$. When the pump laser passes through the thin-disk crystal once, the thin-disk gain medium absorbs the pump laser twice.
The power absorbed for the first time passing through the thin disk [21]:
The remaining power after one pass: The power absorbed for the second time passing through the thin disk:In order to simplify the formula, let $A=e^{-{\alpha L} /{\cos \theta }}$. Therefore, the absorbed power of $N-1$ and $N$ times can be written as:
Thus, according to the above Eq. (1)–(4), absorption efficiency after $N$ passes can be obtained.
The absorption coefficient of Yb:YAG thin-disk crystal $\alpha$ is $10.9 cm^{-1}$, and the incident angle is 30${}^\circ$. Substitute the constant into Eq. (5) in the cases when the thickness of the thin-disk crystal is 100$\mu m$, 200$\mu m$, 300$\mu m$, respectively. Thus, the relationship between the number of pump passes and absorption efficiency is obtained, as shown in Fig. 5.
It can be seen from Fig. 5 that the absorption efficiency of the same thin-disk thickness increases with the increase of the number of pump passes. At the same number of pump passes, the thicker the thin disk is, the higher the absorption efficiency is. When the number of pump passes is equal to 72, the absorption efficiency of thin disk with thickness of 200 $\mu$m and 300 $\mu$m is higher than that of thin disk with thickness of 100 $\mu$m, while the absorption efficiency of 200 $\mu$m is almost the same as that of 300 $\mu$m. Since when the number of pump passes reaches a certain number, the absorption efficiency will reach saturation. But the thermal lens effect is enhanced significantly as the thickness of the thin disk inceases. In order to achieve high absorption efficiency and low thermal lens effect, it is an effective method to reduce the thickness of the crystal and increase the pump passes [22]. However, since processing of crystals thinner than 100 $\mu$m is extremely difficult, the thickness of the thin-disk crystal of the multi-pass pump module is chosen to be 200 $\mu$m and the number of pump passes $N$ is 72, in order to balance the absorption efficiency, the thermal lens effect and the processing difficulty. Under these conditions, the calculated absorption efficiency is 99.10%, which basically reaches saturated absorption.
3.2 Simulation of cavity-dumped Q-switched laser
When an external electric field is applied to the BBO crystal, its refractive index will change, so that a phase difference between light passing through the different polarization directions of the crystal, thereby changing the polarizaion state of the light. Therefore, by controlling the high voltage power supply, the state of the cavity-dumped laser can be changed. Thus, the variations of laser field energy in the cavity and population inversion energy in the active medium show the dynamic process of cavity dumping.
For a simple cavity-dumped Q-switched laser, it is assumed that the Q-switch can be turned on instantly, the rate equations of the laser resonator after the Q-switch is turned on are as follows:
As can be seen from Fig. 6, during the period of $0\sim 2\times 10^{-4}$ s, the Q switch is turned off and the particle number accumulation is reversed. At the time of $2\times 10^{-4}$ s, the Q switch is turned on. The number of accumulated inversion particles is released in an instant, and photons in the cavity is emptied to form nanosecond pulses. The calculated pulse duration is 17.89 ns. For a cavity-dumped Q-switched laser, the pulse duration is only related to the length of the resonator and the speed of the Q switch, but has nothing to do with the gain characteristics and repetition rate. Theoretically, as long as the speed of the Q switch is fast enough, the pulse duration of output laser is equal to the round-trip time in the cavity. Therefore, in order to obtain a narrower pulse duration, it is necessary to shorten the cavity length and choose a faster Q switch.
4. Results and discussion
In order to verify the output performance of the 72-pass pump module, first of all, the V-shape cavity laser experiment based on the module was carried out. As shown in Fig. 7, the length of the two arms of the V-shape cavity is 980 mm and 1700 mm, respectively. The end mirror is a convex mirror and its radius of curvature is 1.7 m. The output couping mirror is plane and its output coupling rate is 5.36%.
As can be seen from Fig. 8, when the pump power is 345 W, the average output power of 187.2 W is obtained, and the corresponding optical-optical efficiency is 54.26%. At the maximum pump power of 345 W , the M2 factor of the laser beam is about 1.7, and the beam shape at the waist is shown in the illustration. This optical-optical conversion efficiency far exceeds the previous 24-pass pump module [21], which shows the excellent output potential of the module.
Fig. 8. (a) Average output power and optical-optical conversion efficiency of the V-shape cavity. (b) Beam quality of the output beam measured at 345 W pump power. The inset shows the beam shape at the waist. The measured valuse are $M_x^2=1.720$ and $M_y^2=1.681$.
In the experiment, the quarter-wave voltage applied to the Pockels cell is 3.5 kV. At different repetition rates, the variation of average output power with incident pump power is shown in Fig. 9(a). As can be seen from Fig. 9(a), at the same pump power, with the increase of the repetition rate, the average power will increase as the single-pass gain increases. It can be seen from Fig. 9(b) that at a repetition rate of 100 kHz, a maximum average output power of 150 W was achieved at the incident pump power of 345 W, corresponding to the slope efficiency of 54.7% and the optical-optical conversion efficiency of 43.5%.
Fig. 9. (a) Average output power versus incident pump power for cavity-dumped Q-switched laser under different repetition rates. (b) Average output power and optical-optical efficiency at repetition rate of 100 kHz.
Figure 10(a) shows the sequence of highly stable laser pulses at a repetition rate of 100 kHz under the maximum pump power. At the repetition rate of 100 kHz, the maximum pulse energy is 1.5 mJ, and the peak-to-peak instability of pulse duration and energy are $\pm$2.75% and $\pm$3.52%. The corresponding single pulse profile is shown in Fig. 10(b). The pulse duration was kept at apprroximately 18.38 ns. Taking into account the changes of the parameters of the high voltage driving power supply and the measurement errors, it can be considered that the pulse duration is basically consistent with the theoretical calculation of the single round-trip time of the laser in the resonator.
Fig. 10. Temporal pulse train trace of cavity-dumped Yb:YAG thin-disk laser. (a) Pulse train at repetition rate of 100 kHz. (b) Pulse profile with pulse duration of 18.38 ns.
Fig. 11. Beam quality of the output beam measured at 345 W pump power and a repetition rate of 100 kHz. The inset shows the beam shape at the waist. The measured valuse are $M_x^2=1.529$ and $M_y^2=1.538$.
The high stability of pulse-pulse energy output can be explained by the fact that in the range of 50-100 kHz, the time interval of adjacent laser pulses is significantly larger than the upper level lifetime of Yb:YAG 980 $\mu$s, which ensures that there is sufficient time for continous pump light to pump Yb:YAG thin-disk crystal and enabled the gain of each pulse to reach the maximum value. In other words, the balance between the number of inversion paticles comsumed in the amplification process of stimulated radiation and the number of inversion particles increased in the process of stimulated absorption is always maintained, and the adjacent laser pulses are not affected by each other in the process of oscillation amplification.
In the experiment, the beam quality of a laser beam with a repetition rate of 100 kHz is analyzed by the Beamage beam quality analyzer (Gentec-EO, Canada). At the maximum pump power of 345 W and the repetition rate of 100 kHz, the M2 factor of the laser beam is about 1.53, and the beam shape at the waist is shown in Fig. 11.
5. Summary and outlook
In conclusion, we demonstrated a cavity-dumped Q-switched thin-disk nanosecond laser with more than 150 W of average output power at 100 kHz repetition rate. Based on the 72-pass pump module independently designed by the laboratory and optimized cavity design, the maximum output power can reach 150.2 W and the optical-optical conversion efficiency is 43.5% when the pump power is 345 W. The laser is able to deliver a very stable output, while still maintaining a good beam shape and $M^2$ factor of 1.53. To achieve this good beam quality and highly stable pulse sequence, a Z-type resonator is used for the cavity-dumped Q-switched TDL. Moreover, the duration of output pulse of the cavity-dumped Q-switched laser is theoretically calculated and it is in good agreement with the measured results. At present, the output potential of the 72-pass pump module has not been fully explored. By reducing the thickness of the disk crystal, enlarging the pump spot and placing the resonator in vacuum, the output power of the nanosecond laser oscillator can be scaled to kilowatt level while maintaining high stability and near-diffraction-limited beam quality. The laser has proven to be an excellent and highly stable seed laser suitable for 10-kW-class nanosecond amplifier and will lay the foundation for further energy scaling of nanosecond thin-disk lasers.
Funding
Dalian Science and Technology Innovation Fund (2018J11CY013).
Acknowledgments
We gratefully acknowledge technical support from CASTECH, lnc. At the same time, we are very grateful for the 72-pass pump module provided by Key Laboratory of Chemical Lasers, Dalian Institute of Chemical Physics.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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