1. Introduction

High-energy and high repetition rate nanosecond pulse lasers play significant roles in the applications such as biomedical imaging [1], industrial materials processing [2], microwave generation [3], laser ignition [4], and so on. Q-switching is the main technique for generating pulsed lasers with the pulse duration in the microsecond and nanosecond even sub-nanosecond region [5]. Passive Q-switching is capable of generating nanosecond or sub-nanosecond pulse laser output with compact structure. However, it has some disadvantages of uncontrolled pulse repetition rate and poor pulse instabilities [6–8]. Active Q-switching typically using Acousto-optical (AO) or Electro-optical (EO) modulator can generate nanosecond pulse laser with certain pulse repetition rate and better pulse stability. Compared to AO Q-switching, EO Q-switching has advantages of better hold-off ability, short switching time, which makes it more suitable for generating the narrower pulse duration, higher pulse energy and peak powers [9]. What’s more, with the innovation and development of laser-diode (LD) technology, Quasi-continuous wave laser diodes (QCW-LDs) play more and more important roles in generating high energy lasers. Combined with EO Q-switching, it is considered as the most effective method to realize high energy nanosecond lasers with the repetition rates in 50-1000 Hz region.

In 2003, Wu et al. reported a QCW-LD side-pumped Q-switched Yb:YAG slab laser with an output energy of 3.07 mJ and a pulse duration of 56.27 ns at a repetition rate of 25 Hz [10]. Later, they further realized a pulse energy of 10.7 mJ with a pulse duration of 18.4 ns at repetition rate of 1–10 Hz by side-pumping configuration [11]. To achieve higher efficiency and better beam quality, QCW-LD end pumped EO Q-switched Yb:YAG lasers have also been studied. In 2014, Liu et al. presented an output energy of 32.6 mJ and pulse duration of 13.4 ns at a repetition rate of 5 Hz, but the beam quality degraded to 1.55 and 1.40 [12]. In 2019, Gao et al. studied an EO Q-switched Yb:YAG slab laser with a pulse duration of 30 ns at a repetition rate of 2 kHz [13], the corresponding output energy of 14.6 mJ with beam quality factor of ∼1.32 was achieved by continuous wave (CW) pumping. In 2015, Jambunathan et al. achieved an output energy of 1 mJ with a pulse duration of 38 ns for a repetition rate of 250 Hz at the temperature of 100 K by CW pumping [14]. However, many application fields need the nanosecond laser sources with high energy, high repetition rate (50-1000 Hz) and good beam quality.

In this paper, a QCW-LD end-pumped EO Q-switched Yb:YAG laser with high-efficiency, high repetition rate and good-beam-quality was realized. When the repetition rate was 100 Hz, the shortest pulse duration of 17.4 ns was obtained with an optical–optical efficiency of 23%. As the repetition rate increase up to 500 Hz, a maximum output power of 2.69 W was obtained with the pulse width of 18.7 ns, corresponding to the slope efficiency of 21.6% and the beam quality factor of $M_x^2 = 1.04$, and $M_y^2 = 1.01$. The long-term output power instability was measured to be ± 0.75% (RMS) in two hours. Moreover, the theoretical analysis by numerical simulation of the rate equations was agreed well with the experimental results.

2. Experiment setup

The schematic diagram of the QCW-LD end-pumped EO Q-switched Yb:YAG laser is shown in Fig. 1. The pump source is a fiber-coupled LD at 940 nm with a core diameter of 105 µm and N.A. of 0.22, which matches the strong absorption peak of Yb:YAG [12]. The LD works in the QCW mode with the pumping pulse width tuning range from 100 to 2000 µs and the repetition rate tuning range from 1 to 1000 Hz, respectively. The pump beam is focused into an uncoated Yb:YAG crystal with a radius of 300 µm by two coupling lens. A commercial 20 mm thick Yb (2 at.%):YAG crystal is applied as the gain medium, which is tightly wrapped in indium foil and mounted onto a heat sink that is maintained at 17 °C by circulating cooled-water. The optical components labeled as M1(R=∞), M2(R = −300 mm) in Fig. 1 are input mirror and fold mirror, which are anti-reflection coated at 940 nm on the outside surface, high-reflection (HR) coated at 1030 nm and high-transmission (HT) coated at 940 nm on the inside surface. M3 was flat mirror with the transmittance of 50% at 1000-1100 nm served as the output coupler. The arms of the three-mirror resonator are designed with the length of 225 mm and 150 mm, allowing the oscillating beam radius on crystal to be about 240 µm, matching well with the pump light. A 3 × 3 × 25 mm³ BBO crystal with a corresponding quarter wave voltage of 3200 V is employed as the Pockels cell for the advantages of high optical damage threshold (5000 MW∕cm²), high extinction ratio (>2000:1) and not easily deliquescent.

 

Fig. 1. Experimental setup of the QCW-LD end-pumped EO Q-switched Yb:YAG laser.

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3. Experimental results and discussions

Compared to CW pumping, QCW pumping has the advantages of relatively lower thermal loading both of the LD and laser crystal, and thus is beneficial to high energy output with good beam quality and stability. By optimizing the pumping pulse width of 1.6 ms and setting the repetition rate at 100, 300 and 500 Hz, the high repetition rate, high energy nanosecond pulsed laser was realized. As shown in Fig. 2(a), the output energies at the repetition rate of 500 Hz under free running operation which means the operation without active Q-switch in QCW pumping mode increased almost linearly with the augment of incident pump energies. Under an incident pump energy of 68 mJ, the maximum output energy was 7.62 mJ, corresponding to a slope efficiency of 30.52%.

 

Fig. 2. (a) Average output energies versus incident energies for free running and Q-switching operations. (b) Pulse durations versus incident pump energies at different repetition rates. (c) Pulse peak powers versus incident pump energies at different repetition rates. (d) Emission spectra of Q-switched lasers.

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Stable EO Q-switched laser operation was achieved at different repetition rates by synchronizing the time delay of the pumping light and the modulation signal of BBO Pockels cell. As shown in Fig. 2(a), the obtained maximum output energies were 5.38, 5.5, and 5.1 mJ for the repetition rates of 500, 300, and 100 Hz, respectively, corresponding to the maximum output powers of 2.69, 1.65 and 0.51 W and slope efficiencies of 21.63%, 21.09%, and 23.02%. Figure 2(b) shows the relationships between the pulse width and the incident pump energies. It can be found that the pulse durations decrease with the increase of incident pump energies. As the EO Q-modulation frequency decreases, the output pulse duration also gradually decreases. The shortest pulse durations were 18.7, 17.8, and 17.4 ns, respectively, corresponding to the calculated peak powers of 282.56, 315.73, and 286.19 kW, as shown in Fig. 2(c). Figure 2(d) displays the emitted central wavelength of 1030.28 nm at the repetition rate of 500 Hz and the incident energy of 68 mJ detected by a spectrometer with a resolution of 0.05 nm (YOKOGAWA AQ6370C, Japan).

A digital oscilloscope was used to record laser pulses (1 GHz bandwidth, Tektronix DPO 7102, USA). Figure 3 shows the single pulse profile and oscilloscope pulse train of the Q-switched laser at a pulse repetition rate of 100 Hz. The shortest pulse duration was 17.4 ns at the repetition rate of 100 Hz. The output beam quality at the highest output energy under a repetition rate of 500 Hz was shown in Fig. 4(a). The M² factors were measured to be 1.04 and 1.01 in the tangential and sagittal directions, respectively. The output laser stability was measured and shown in Fig. 4(b), with a root mean square (RMS) of ±0.75%, indicating the good output stability.

 

Fig. 3. (a) Single pulse profile and (b) Oscilloscope pulse train recorded at the repetition rate of 100 Hz.

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Fig. 4. (a) Beam quality factor M² of the EO Q-switched Yb: YAG laser beam at the maximum output power. The inserts show the 2D or 3D laser profile. (b) Output power stability for 2 hours operation at the maximum output power.

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The pulsed laser performance with the CW pumping mode at the repetition rate of 500 Hz was also studied, where the output energy, the pulse width, the beam quality, and the stability were 5.36 mJ, 20 ns, M²=∼1.1, and RMS=±1.44%, respectively. Thus, the pulsed laser output characteristics of the QCW pumping were better than those of the CW pumping. Table 1 lists the main results achieved for QCW-LD pumped EO Q-switched Yb: YAG lasers. Among them, the results obtained in this work provided the highest slope efficiency and the best beam quality. The high efficiency and the better beam quality mainly attributed to the QCW-LD end-pumping structure, the proper cavity design and insertion loss controlling of the EO modulator.

Table 1. Development of EO Q-switched Yb:YAG Lasers^a

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4. Theoretical analysis

For simulating the dynamical process of EO Q-switched Yb:YAG laser pumped by CW and QCW, a numerical analysis was applied based on the laser rate equations. The energy level system of Yb:YAG consists of the Stark splitting ₂F^7/2 ground state and the ₂F^5/2 excited state [15,16]. Because of small energy splitting of each manifold, the relaxation times for energy levels within a manifold are assumed to be very small, hence it justifies the use of a two-energy-level system for simulating the Yb:YAG laser mechanism [17–20]. The following rate equations can be applied

Under QCW pumping, ${R_{pump}}$ is the QCW pumping rate and can be defined as

Using the parameters shown in Table 2, a theoretical investigation of the experimental results can be provided by Eqs. (1) and (2).

Table 2. Parameters of Theoretical Calculation

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Figure 5 shows the numerical calculation of the rate equations for shortest pulse durations at different repetition rates under CW and QCW pumping, which was agreed well with the experimental results. Under CW pumping EO Q-switched lasers, pump light is continuously injected into the gain crystal, which can cause the accumulation of heat, ultimately leading to a more severe thermal effect. However, the QCW pump light is injected into the crystal at a certain frequency, reducing the thermal loading of the gain medium and improving laser output performance. Compared with CW pumping EO Q-switched lasers, QCW pumping can achieve pulsed lasers with better beam quality, higher output pulse energy and shorter pulse width. It can be seen from Fig. 5 that the experimental results agree well with the theoretical ones.

 

Fig. 5. Simulation of pulse durations at repetition rate of 100, 300, 500 Hz under CW and QCW pumping. Symbol, experimental data; solid curve, theoretical result.

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5. Conclusion

In conclusion, a QCW-LD end-pumped EO Q-switched Yb:YAG laser was successfully explored. When the repetition rate was 100 Hz, the shortest pulse width of 17.4 ns was obtained with a maximum single pulse energy of 5.1 mJ. The maximum output energy of 5.38 mJ was obtained with a high stability of RMS=±0.75% in two hours at repetition rate of 500 Hz, the corresponding beam quality was determined to be $M_x^2$=1.04, and $M_y^2$=1.01. A theoretical study by numerical simulation of the rate equations was carried with the results agreed well with the experimental ones. Our results indicate that QCW-LD pumping with Yb-doped crystals should be a promising way for high repetition rate high-energy nanosecond pulsed laser generation.