Light sources that emit light in the mid-infrared wavelength region, which includes the absorption lines of numerous molecules, have great potential in various applications [1–4]. Because a 3-µm wavelength within mid-infrared light is well absorbed by hydroxyl (OH groups) or water-containing substances [5], there are worldwide attempts to develop an Er:YAG laser that radiates this wavelength. For example, Er:YAG lasers are potentially suitable for oral hard and soft tissue ablation and treatment in clinical dentistry [6, 7]. In addition, Er:YAG lasers have attracted attention in mid-IR spectroscopy by utilizing them as pumping light sources for other mid-IR light sources, such as Fe:ZnSe lasers [8]. This study focuses on the potential of using Er:YAG lasers for air-underwater communication and underwater SONAR in military applications [9, 10]. However, Er:YAG lasers in the 3 μm mid-infrared range are difficult to realize because of the laser’s short lifetime of the upper energy level [11–13]. Therefore, the short lifetime must be compensated for by high-concentration erbium ion doping and increasing the stimulated emission cross-section based on a loss-minimized resonator design to enable the Er:YAG lasers to emit light with a wavelength of 2,940 nm.

Furthermore, Er:YAG lasers can be combined with Q-switching technology to improve their effectiveness in these applications by emitting intense and short pulses [14–16]. Research on Q-switched Er:YAG lasers, such as frustrated total internal reflection (FTIR) [17], electro-optically [18–21], and spinning mirror [8, 22–24] Q-switched lasers, has been conducted in the past. Eichler et al. [17] developed FTIR Q-switched Er:YAG lasers with an energy of 50 mJ. Next, Skorczakowski et al. [18] obtained pulses with an energy of 15 mJ using electro-optic Q-switching technology based on an RTP Pockels cell. In addition, Zajac et al. [19] generated pulses with an energy of 137 mJ, and Yang et al. [21] produced pulses with an energy of 226 mJ from electro-optically Q-switched lasers with a LiNbO3 Pockels cell. Karki et al. [24] developed an Er:YAG pulsed laser using spinning mirror Q-switching and achieved energies up to 800 mJ. The electro-optic Q-switching technique has unique advantages, such as fast response time and accurate pulse timing, which are essential in some applications. However, several studies have reported that the damage threshold of the Pockels cells limits the maximum energy of lasers that can be implemented with electro-optic Q-switching [22].

In this study, we developed a high-energy (285 mJ) robust electro-optically Q-switched Er:YAG master oscillator power amplifier (MOPA) system to overcome the damage threshold of LiNbO3 Pockels cells. To the best of our knowledge, this is among the highest-energy Q-switched Er:YAG lasers reported for electro-optic Q-switching. We introduced a MOPA structure that boosts the output power independently of the Pockels cell by amplifying it behind a master oscillator. Moreover, we successfully increased the output by adjusting the pumping start timing of the flashlamp of the power amplifier relative to that of the flashlamp of the master amplifier. In addition, a barium pumping reflector design and cooling of up to 287 K were used to effectively remove the heat caused by high-power pumping. This paper explores the potential for high-energy generation of electro-optically Q-switched lasers, which are particularly advantageous for applications requiring short pulses and precise timing characteristics, such as SONAR and communications.

Figure 1 shows the generation of mid-infrared pulses using an electro-optically Q-switched MOPA system. The master oscillator of the MOPA system, constructed as shown in Fig. 1(a), generates pulses with the desired wavelength and pulse characteristics. The power amplifier operates behind the master oscillator to amplify the energy of the pulses without damaging the Q-switching material. The master oscillator consists of a highly reflective (HR) flat mirror, Pockels cell, 5 mm diameter Er:YAG rod heavily doped with 50% erbium ions, xenon flashlamp, quarter-wave plate (QWP), and 85% partial reflective (PR) flat mirror (Table 1). A key element for the Q-switching of the master oscillator was a Pockels cell made of a lithium niobate crystal, and we machined the Brewster’s angles on both sides of the crystal so that it could act as a polarizer. The use of the machined crystal can enhance the polarization efficiency inside the cavity, which has a positive effect on the laser output efficiency, leading to high-power laser oscillation. When a voltage was applied to the lithium niobate Pockels cell, polarization was rotated to prevent the transmission of light, whereas a large amount of light was transmitted when no voltage was applied, leading to the generation of short intense pulses. The driving voltage applied to the lithium niobate crystal can be expressed as follows:

Table 1 . Components of Q-switched Er:YAG MOPA system.

Components	Specification	Details
Er:YAG rod (oscillator)	5ϕ × 125 (mm)	50% Er3+, 5 m concave
Er:YAG rod (amplifier)	5ϕ × 125 (mm)	50% Er3+, AR, Flat
Pockels Cell	10 × 23 × 30 (mm)	LiNbO3
HR	15ϕ × 3 (mm)	100%, Flat
PR	15ϕ × 3 (mm)	85%, Flat
QWP	12.7ϕ × 0.5 (mm)	MgF2
FM	15ϕ × 4 (mm)	Flat
Flashlamp	4,343 W, 1,131 A	Xenon
Pump Cavity	Fused Silica Flow Tube	BaSO4

Figure 1. Mid-IR electro-optically Q-switched Er:YAG MOPA system with an adjustable pumping delay between Xenon flashlamps; (a) Schematic of the system, (b) photograph of the constructed system.

(1)$${\text{V}}_{\text{λ}\text{/4}}\text{=}\frac{{\text{λ}}_{\text{0}}\text{d}}{\text{4}{\text{r}}_{\text{22}}{\text{n}}^{\text{3}}\text{L}}$$

where λ₀ is the wavelength of the laser, n is the refractive index, r₂₂ is the electro-optic coefficient, L is the crystal length, and d is the distance between the electrodes. The refractive index at the central wavelength of 2.94 μm was 2.16. The electro-optic coefficient was 5.6 × 10₋₁₂ m/V [21]. The length of the lithium niobate crystal was 30 mm and the distance between the electrodes was 10 mm. The quarter-wave voltage was calculated as approximately 4.3 kV. In practice, a 4–4.5 kV voltage led to highly efficient electro-optic Q-switching. Driving a suitable high voltage was critical for a single-pulse generation; Otherwise, multi-pulsing occurred [19]. The magnesium fluoride quarter-wave plate was inserted before the PR mirror to compensate for the change in polarization state due to the birefringence that occurs when the rod is heated.

The output beam of the master oscillator is reflected by two 45-degree folding mirrors (FM) and delivered to the next power amplifier. The power amplifier contains a 5 mm diameter rod pumped by a Xenon flashlamp that outputs light with a wavelength range of 600–800 nm. A closed-coupled pumping arrangement using a barium sulfate pump cavity enables efficient light coupling to the Er:YAG rod [22]. The delay between the oscillator and amplifier flashlamp triggering was designed to be adjustable in order to optimize the amplification factor. Additionally, an industrial chiller (S&A CW-5200AI; Megatech, Cannock, UK) was used to cool both the master oscillator and power amplifier sections at a flow rate of 16 L/min to reduce the heat generated by the xenon flashlamps. During cooling, the laboratory temperature was maintained at approximately 294 K using a thermo-hygrostat.

3.1. Mid-IR Electro-optically Q-switched Er:YAG Master Oscillator

The configuration of the mid-IR electro-optically Q-switched Er:YAG master oscillator, shown in Fig. 2(a), enabled the generation of a giant short pulse. The experimental data in Fig. 2(b) and 2(c) are the measurements of the output energy per pulse of the master oscillator for different values of the key design parameters. The important design parameters for electro-optically Q-switched lasers include the coolant temperature, resonator length, pulse repetition rate, voltage, and switching time of the Pockels cell. First, it is essential to lower the temperature of the coolant to below room temperature to compensate for the thermal lensing effect, which causes optical damage to solid-state lasers and limits the beam quality and output energy [24]. Figure 2(b) shows the effect of the coolant temperature on the output energy of the Q-switched Er:YAG master oscillator. For instance, at 295 K, the energy was 117 mJ, and this increased by 40% to 165 mJ at 287 K at a flashlamp voltage of 950 V. Thus, the output energy increased as the coolant temperature decreased and tended to saturate when the temperature reached 287 K. Consequently, we adjusted the coolant temperature to 287 K, just above the dew point temperature in the experiment. Figure 2(c) shows the variation in the output energy with the resonator length when the temperature is cooled to 287 K. For a resonator length of 491 mm, the output energy was 165 mJ and increased to 180 mJ when the resonator length was shortened to 480 mm under a flashlamp voltage of 950 V. Shortening the resonator length to the same extent as above at a flashlamp voltage of 970 V enabled the laser energy to be increased from 185 to 206 mJ. A shorter resonator has the advantages of a shorter pulse duration and larger output energy owing to reduced diffraction losses [25]. Conversely, reducing the resonator length significantly results in high pulse energies being produced inside the resonator, increasing the risk of optical damage. Thus, the length of the resonator was optimized such that the intensity was only slightly below the damage threshold to prevent permanent damage to the resonator. Adjusting the resonator of the master oscillator to the optimal length of 480 mm and lowering the temperature to 287 K enabled the master oscillator to generate energy of 206 mJ and irradiance of 7.5 MW/cm². This was set below the damage threshold of 10 MW/cm² for LiNbO3 to ensure that the optical components were not damaged [26].

Figure 2. Mid-IR electro-optically Q-switched Er:YAG master oscillator. (a) Schematic of the master oscillator. Output pulse energy of the Er:YAG master oscillator as a function of the flashlamp voltage (b) at different temperatures and (c) at different resonator lengths.

3.2. Mid-IR Electro-optically Q-switched Er:YAG Master Oscillator Power Amplifier System

The mid-IR electro-optically Q-switched Er:YAG MOPA system displayed in Fig. 1 allowed a significant output power to be generated by decoupling the master oscillator and power amplifier to prevent the output power from being adversely affected by intracavity irradiance. The experimental data shown in Figs. 3–6 are the measurements of the output characteristics of the MOPA system. Figure 3 shows how the output energy of the Q-switched Er:YAG MOPA system changes at different repetition rates. As shown in Fig. 3, the energy was measured by adjusting the repetition rate from 2 to 5 Hz, and the highest output energy was obtained at the lowest repetition rate of 2 Hz. In addition to resonator cooling, lower repetition rates can compensate for the thermal lensing effect [24]. Based on these results, subsequent experiments using the MOPA system were performed by cooling the resonator to 287 K and using the optimal resonator length of 480 mm and a repetition rate of 2 Hz.

Figure 3. The output pulse energy of the Er:YAG MOPA system at a different repetition rate.

Figure 4. Dependence of the pulse energy of the Er:YAG master oscillator power amplifier (MOPA) system on the delay time at a flashlamp voltage of 800 V; (a) The output pulse energy of the Er:YAG MOPA system as the delay time increases; (b) The increase in the ratio of the output energy of the Er:YAG MOPA system with delay to that without delay.

Figure 5. Dependence of the pulse energy of the Er:YAG master oscillator power amplifier (MOPA) system on the flashlamp voltage; (a) The output pulse energy of the Er:YAG MOPA system as the flashlamp voltage increases; (b) The amplification factor of the Er:YAG MOPA system as the flashlamp voltage increases.

Figure 6. Temporal, spectral, and spatial characteristics of the mid-IR electro-optically Q-switched Er:YAG MOPA system; (a) The pulse width; (b) The frequency spectrum; (c) The 2D spatial intensity distribution of the system.

Figure 4 shows the dependence of the output energy of the Er:YAG MOPA system on the delay time. As shown in Fig. 4(a), the experiments were conducted by initially setting the output energy of the MOPA system to 100 mJ, and by adjusting the delay time between the timings at which the flashlamps of the oscillator and amplifier were triggered. As the delay time increased, the output energy gradually increased, saturated, and then decreased. Figure 4(b) shows the increase in the ratio of the output energy of the MOPA system with the delay to that without the delay. Ultimately, the 100 mJ energy from the MOPA system with no delay between the oscillator and amplifier flashlamp triggering increased up to approximately 130 mJ with a delay of approximately 120–200 μs, which is calculated as an increasing ratio of approximately 1.3. These results indicate that the laser pulse is effectively amplified by delaying the pumping of the flashlamp of the power amplifier compared to that of the master oscillator until the population inversion and stored energy are maximized [27].

Figure 5 shows the changes in the output energy and amplification factor as a function of the driving voltage of the flashlamp. The amplification factor was calculated as the ratio of the output energy of the power amplifier to that of the master oscillator. These calculations showed that the pulse energy of the master oscillator was increased by a factor of approximately 1.2 times by installing the power amplifier after the master oscillator to amplify the energy output by the oscillator. Furthermore, the pulse energy could be further increased by setting the delay time to an optimum value of 120 μs. The use of the power amplifier with a flashlamp that has a pumping time delay of 120 μs from that of the master oscillator amplified the output energy of the master oscillator by approximately 1.4 to 1.9 times. The energy of the master oscillator (approximately 200 mJ) was amplified by approximately 1.4 times to 285 mJ using the power amplifier with its flashlamp pumping time delay of 120 μs relative to that of the master oscillator. Consequently, the electro-optically Q-switched MOPA system could generate output energy as high as 285 mJ at a pulse repetition rate of 2 Hz. In theory, the overall output energy could be further increased by adding different amplifier stages in sequence; However, in practice, unwanted amplifier noise could be added during the operation of this amplifier chain.

Finally, the characteristics of the output beam were monitored using an energy meter, optical spectrum analyzer, photodetector, and beam profiler. Figure 6 shows the characteristics of the output pulse of the electro-optically Q-switched Er:YAG MOPA system. Figure 6(a) and 6(b) show the characteristics of the temporal and spectral domains, respectively, at the maximum energy of 285 mJ. The Er:YAG MOPA system emitted optical pulses with a full-width at half maximum (FWHM) pulse width of approximately 140 ns. The pulses were centered at a wavelength of 2,936.7 nm with a 3 dB spectral bandwidth of 1.24 nm. Fig. 6(c) shows the 2D spatial intensity distribution of the output beam profile when the maximum energy of 285 mJ was reached. Based on these results, the beam divergence was calculated to be less than 4 mrad.

We developed a mid-IR Q-switched Er:YAG laser that, to the best of our knowledge, has the highest energy among existing pulsed lasers equipped with electro-optic Q-switching technology at 2,940 nm. Pulses with a width of approximately 140 ns and energy of 285 mJ were generated at a repetition rate of 2 Hz. In the frequency domain, these pulses had a spectral width of 1.24 nm and were centered at 2,940 nm. The MOPA structure was used to attain high energies without damaging the Pockels cell, and the output energy of the master oscillator could be amplified by a factor of up to approximately 1.9 by the power amplifier. The output energy was significantly increased by cooling the resonator, optimizing the resonator length and flashlamp pumping time, and reducing the repetition rate. Because water-containing substances are excellent absorbers of the light emitted by this high-power Er:YAG MOPA system, the Er:YAG MOPA system can be used as a multipurpose light source for dental treatment, surgery, spectroscopy, underwater communication, and underwater object detection.

The authors declare no conflicts of interest.

The data underlying the results presented in this paper are not publicly available at the time of publication and can be obtained from the authors upon reasonable request.

This work was supported by the Agency for Defense Development by the Republic of Korea Government.

Agency for Defense Development.