13.5 μJ, 20 kHz Repetition Rate, Single Frequency Pr³⁺:YLF Master Oscillator Power Amplifier System

Abstract

This article describes a master oscillator and power amplifier (MOPA) system with a single longitudinal mode (SLM) and high-repetition-frequency Pr³⁺:YLF active medium that was end-pumped by two 444 nm laser diodes. The Pr³⁺:YLF MOPA laser system produced a maximum pulse energy of 13.5 μJ with a pulse width of 130.2 ns at a pulse repetition frequency of 20 kHz, translating to a peak power of around 103.7 W. The Pr³⁺:YLF MOPA laser system’s output wavelength was 639.7 nm, and the line-width of its laser spectra was roughly 168 MHz. Additionally, at the highest output level, the laser beam quality did not decrease much due to amplification.

1. Introduction

Recently, there have been many reports about the application of Pr³⁺:YLF visible-band laser to laser light sources [1]. Especially, a 639 nm laser can pump holmium (Ho)-doped upconversion fiber to generate 544 nm green laser radiation [2]. Numerous other applications, including lidar, strontium-based optical clocks [3], and spectroscopic analysis, benefit from the employment of high-repetition-rate, directly single-frequency visible emitting lasers. A crucial aspect of visible lasers is their simple second-harmonic generation (SHG) conversion to the UV range. Due to their ease of use and compactness, UV sources based on visible lasers are more favorable than traditional frequency-tripled or -quadrupled neodymium (Nd)-doped solid-state laser systems [4]. Applications for such UV coherent sources include polymer processing, maskless lithography, and, most significantly, semiconductor wafer inspection [5].

The most promising materials for definitively emitting lasers are praseodymium (Pr)-doped fluorides, particularly LiYF₄ (YLF) crystals. Numerous emission lines from the Pr³⁺ ion may be seen in this material. Moreover, Pr³⁺’s stimulated emission cross-sections are orders of magnitude greater than those of the other rare-earth ions with visible emission lines. Due to its low quantum defect (~7% at 639 nm), high conversion (70% at 639 nm), and broad emission cross-section (22 × 10⁻²⁰ cm²), Pr-doped laser fluorides are suited for producing high-power visible laser light. Due to its excellent optical quality and strong thermal conductivity, YLF hosts are favored in the high-energy MOPA 639 nm laser. In addition, because Pr:YLF crystals have longer energy level lifetimes (35.7 μs) than Pr:YAP (14 μs) and Pr:CaF₂ (25 μs), they are particularly well suited for Q-switching and power amplification. Large-energy and high-repetition-frequency Pr-doped visible lasers are being extensively researched due to these benefits [6].

In this work, we report a SLM pulsed Pr³⁺:YLF MOPA system that operates at a wavelength of 639.7 nm. An isolator, a half-wave plate, a first 444 nm pump source, and an A-O Q-switch were used to construct the typical straight cavity SLM pulsed Pr³⁺:YLF oscillator. By using the second 444 nm pump power as the pump of the pulsed Pr³⁺:YLF amplifier, the 639 nm laser power was further boosted. We are aware of no previous reports of SLM pulsed MOPA system at 639 nm with an isolator. The 10 μJ-level SLM Pr-doped pulsed laser has also never before been described.

2. Materials and Methods

The principle of pre-lase Q-switching technology in the oscillation stage is described below. The acousto-optic Q-modulation operation mechanism is shown in the schematic diagram in Figure 1. The upper-level particles of Pr³⁺ ions are quickly released when the intra-cavity loss is reduced. The first pulse gap is created as a result of the Q-switch’s increased intra-cavity loss induced by the initial drop in photon density in the crystal. The second pulse gap is produced after a short period of time (less than the life of the upper energy level), along with the second reduction in the total photon density in the A-O Q-switch crystal. When the first pulse is close to the threshold, the second pulse is amplified depending on it; therefore, the second pulse will be output in an SLM. Based on the previous research on pre-lase Q-modulation, the signal generator for acousto-optic Q-modulation was parameterized as follows: at 20 kHz re-frequency (50 μs per pulse period), the seed signal generation time of the oscillating stage was set to 15.7 μs, and the pulse output time was set to 35 μs (which is less than the lifetime of the upper energy stage) [7].

A simple energy level diagram of Pr:YLF is shown in Figure 2. The ground state ions in the ³F₂ Pr manifold can be stimulated to the higher lasing level of the ³P₀ Pr manifold when the YLF amplifier crystal is pumped with a 444 nm laser.

We take into consideration an amplifier crystal that is cylindrical and is end-pumped by a pump laser beam from one side. The rate-equations for the local population densities of the Pr manifolds are given by [8], and they are as follows:

$$\begin{array}{ll}\frac{d{N}_2\left(r,z,t\right)}{dt}=& \frac{c_0}{n}{\sigma }_{\mathrm{abs}}\left({\lambda }_p\right){\eta }_{p}P\left(r,z,t\right)\\ & \times \left[N_1\left(r,z,t\right)-\frac{N_2\left(r,z,t\right)}{F\left({\lambda }_p,T\right)}\right]\\ & -k_{\mathsf{\Sigma }}{N}_2^2\left(r,z,t\right)-\frac{N_2\left(r,z,t\right)}{{\tau }_2}\end{array},$$

(1)

$$N_1=N_{\mathrm{tot} }-N_2,$$

(2)

where $c_0$ is the speed of light, ${\sigma }_{abs}$(${\lambda }_p$) is the absorption cross-sections at the pump wavelength, ${\eta }_p$ is the effective pump efficiency of the upper lasing level, $P$ is the local photon densities of the pump laser fields, $N_i$ are the population densities of the Pr-ions of manifold i, $N_{\mathrm{tot}}$ is the praseodymium dopant concentration, ${\tau }_2$ is the upper level lifetime, $k_{\mathsf{\Sigma }}$ is the total upconversion loss constant [9], and $F\left({\lambda }_p,T\right)$ is the ratio of Boltzmann population distributions of Pr [10].

Along the length of the amplifier crystal, the propagating pump fields $P$ are iterated in accordance with

$$\begin{array}{ll}\frac{dP\left(r,z,t\right)}{dz}=& P\left(r,k,t\right){\sigma }_{\mathrm{abs}}\left({\lambda }_p\right)\\ & \times \left[N_1\left(r,z,t\right)-\frac{N_2\left(r,z,t\right)}{F\left({\lambda }_p,T\right)}\right]\end{array}.$$

(3)

The propagating amplified laser fields $S$ (photon densities) inside the crystal are iterated throughout its length in accordance with

$$\begin{array}{ll}\frac{dS\left(r,z,t\right)}{dz}=& S\left(r,z,t\right){\sigma }_{\mathrm{eff}}\left({\lambda }_l\right)\\ & \times \left[N_2\left(r,z,t\right)-F\left({\lambda }_l,T\right)N_1\left(r,z,t\right)\right]\\ & +\frac{\Delta \Omega }{4\pi }\frac{N_2\left(r,z,t\right)}{{\tau }_2{c}_0}\end{array}$$

(4)

where ${\sigma }_{\mathrm{eff}}\left({\lambda }_l\right)$ is the effectively stimulated emission cross-section at the Pr wavelength ${\lambda }_l$. The final element in (4) is amplified spontaneous emission, where $\Delta \Omega$ is the solid angle determined by the pump spot radius ${\omega }_p$ and the amplifier crystal’s length $L$.

We determine the extracted photon densities of the laser field after amplification by computing the local population in (1) and repeating steps (3) and (4) along the length of the amplifier crystal.

3. Experimental Setup

Figure 3 depicts the configuration of the SLM actively Q-switched Pr³⁺:YLF MOPA system. A 444 nm laser diode (CNI MDL-X, Changchun, China) with a 7.8 W maximum output power served as the initial pump source. Through the employment of a fiber coupling lens, the 444 nm pump laser was focused with a 1/e² spot radius of 100 μm into the first Pr³⁺:YLF crystal in the oscillator. The oscillator had a cavity that was straight. It had a 95 mm-long cavity. A highly reflective coating at 639 nm and a highly transmissive coating at 444 nm were present on the flat-flat mirror IM. With 100 mm of curvature, the flat-concave mirror OC was an output coupler with optimized 1.5% transmittance at 639 nm. An a-cut Pr³⁺:YLF crystal with a concentration of Pr-doping of 0.5 at.% was utilized in the oscillator, which had a cross-section of 3 mm × 3 mm and a length of 5 mm, to lessen the impact of thermal effects. In a copper heat sink that was water-cooled at 15 °C, the laser crystal was securely placed. Due to the higher polarization emission cross-section of the a-cut Pr³⁺:YLF crystal (~14.4 × 10⁻²⁰ cm² at 639 nm for σ-polarization vs. ~0.8 × 10⁻²⁰ cm² at 639 nm for π-polarization), the Pr³⁺:YLF oscillator could only create a single polarization laser. As the pre-lase technology controller, a TeO₂ A-O Q-switching crystal (Gooch & Housego 3080-125, Ilminster, UK) anti-reflective (AR) coated at 639 nm was put into the cavity. TeO₂ crystal with electric modulator modulation has an acousto-optic absorption property, which indicates that TeO₂ crystal’s intra-cavity loss is lessened by the greater 639 nm photon density. Although the electro-optic Q-switching device turns off faster, it is not used due to its larger insertion loss and volume.

The oscillator and amplifier were separated by a half-wave plate and an isolator (Thorlabs IO-3 series 613-653 nm, Newton, NJ, USA). The isolator’s isolation was ~40 dB. The pump source for the amplifier was a second 444 nm laser diode (CNI MDL-X, Changchun, China) with a maximum output power of 10.5 W. Through a fiber coupling lens, it was focused into a second Pr³⁺:YLF crystal with a 200 μm 1/e² spot radius. To increase the conversion efficiency of the amplifier, a laser diode pump source with a substantially wider spot radius than the oscillator was utilized in the amplifier. For additional power scaling in the amplifier, a single-pass pulsed Pr³⁺:YLF amplifier was created. The amplifier a-cut Pr³⁺:YLF crystal had a cross-section of 3 mm × 3 mm and a length of 5 mm with a concentration of Pr³⁺-doping of 0.5 at.%. The temperature of the copper heat sinks, which were installed with the second Pr³⁺:YLF crystal amplifier, was kept at 15 °C by a water cooler and coated in indium foils. Two plane convex lenses (L1, L2) with a 100 mm focal length were used to modify the seed beam.

4. Experimental Results and Discussion

4.1. Oscillator

Figure 4 displays the output parameters of the SLM Pr³⁺:YLF oscillator in relation to 639 nm pump power. Because the pump source is a multimode fiber coupled semiconductor laser, its output laser polarization is poor. For YLF crystals, their absorption efficiency is much lower than that of linearly polarized pump sources (>70%). At the maximum pump power, it was calculated that the Pr³⁺:YLF crystal absorbed around 35% of the pump power. At an incident pump power of 7.8 W and absorbed power of 2.73 W, the Pr³⁺:YLF oscillator achieved a maximum power of 0.81 W, translating to an optical-to-optical efficiency of 29.6%. The Pr³⁺:YLF oscillator worked in pre-lase Q-switched mode when the A-O crystal was placed into the oscillator. The optical-to-optical efficiency of the SLM Q-switched Pr³⁺:YLF oscillator was 5.9%, with a maximum average output power of 0.16 W under the absorbed power of 2.73 W. Due to the large pulse duty ratio and long pre-lase modulation time of the Q-switched laser, the slope efficiency of the Q-switched laser is significantly lower than that of the continuous-wave (CW) laser. The energy of a single pulse may be computed to be 8 μJ at 20 kHz repetition frequency.

As illustrated in Figure 5, an oscilloscope (Tektronix MSO series, Beaverton, OR, USA) was used to display the output longitudinal mode of the SLM Pr³⁺:YLF oscillator, which was being monitored by a Fabry–Perot scanning interferometer with a free spectral range (FSR) of 1.5 GHz. The Fabry–Perot scanning interferometer’s purple line represents the driving voltage of the PZT. The Fabry–Perot scanning interferometer’s corresponding cavity length lowers first, then grows over a time of voltage change when the voltage climbs first and then declines. As a result, one voltage change period corresponded to one set of peaks (yellow line) being detected. No other untidy peaks were recorded, and the 1.5 GHz free spectral range (FSR) separated the two peaks of each set of peaks, indicating that the Pr³⁺:YLF oscillator only possessed one longitudinal mode. The line width may be approximated to be around 140 MHz.

Figure 6 depicts the pulsed Pr³⁺:YLF oscillator’s pulse repetition frequency and pulse width. The oscillator’s pulse repetition frequency was 20 kHz, and its pulse width was 154.8 ns at 7.8 W pump power. The pulse width versus pump power is shown in Figure 4. Additionally, at higher average laser power densities, there was significant variability in the pulse energy, which may indicate that the TeO₂ A–O crystal had not fully recovered from saturation. The maximum single pulse energy was 8.3 μJ.

4.2. Amplifier

To increase the output power, a single-pass pulsed Pr³⁺:YLF amplifier was employed. The SLM Pr³⁺:YLF amplifier’s output characteristics are depicted in Figure 5 in relation to the initial 444 nm pump power. At the maximum pump power of the amplification stage (10.5 W), the Pr³⁺:YLF crystal absorbed about 36% of the pump power (3.77 W). The solid lines in Figure 7 indicate the theoretical output calculations of the amplifier performed using the amplifier theoretical model. The pulsed Pr³⁺:YLF amplifier’s highest average output power of 0.27 W was 1.69 times higher than the oscillator’s average output power of 0.16 W. The optical-to-optical efficiency was achieved by using the pulsed Pr³⁺:YLF MOPA system, and it was 2.9%. The results of the experiment concur with the theoretical calculations. When the pump power is decreased and the pump laser spot and seed laser spot get larger, the amplifier’s output power decreases. Due to space constraints between mirror mounts, the performance of the pulsed Pr³⁺:YLF amplifier with a spot size of less than 200 μm was not tested, but the theoretical maximum average output power was estimated to be 0.32 W when the amplifier spot radius was about 120 μm, which was consistent with the experimental results reported in this paper.

The changed linewidth was observed from the Fabry–Perot scanning interferometer when the amplifier pump power was changed from 0 W to 10.5 W, as shown in Figure 8. The yellow curve represents the SLM intensity peak. A mode spread was observed when the amplifier pump power was changed. At the maximum output power, the output line width of the Pr³⁺:YLF SLM MOPA system is 168 MHz. It can be found by the amplifier theoretical model that this is caused by amplified spontaneous emission.

At the maximum output power of the 20 kHz SLM laser amplifier, the pulse width was measured to be 130.2 ns, as shown in Figure 9. Compared with the seed laser, the pulse width is slightly shortened at this time. This is because the seed light is a Gaussian waveform, and during the amplification process, it will experience an exponential change from a low to high extraction rate of photons at the upper energy level.

As shown in Figure 8, the output spectra time averages of the Pr³⁺:YLF oscillator and amplifier were measured. The spectrum analyzer (Yokogawa AQ6373, Musashino, Japan) captured the spectra. The center wavelength of the output laser of the Pr³⁺:YLF oscillator was determined to be 639.75 nm, as shown in Figure 10. The Pr³⁺:YLF amplifier’s laser emitted light at a wavelength of 639.77 nm. The losses of the isolator and a half-wave plate were the major causes of the modest discrepancy in the center wavelengths between oscillator and amplifier. To avoid the precision of the spectrometer analyzer (0.02 nm), the change of center wavelength was also verified in the wavelength meter (HighFinesse WS-U, Tübingen, Germany). The output laser frequency jittered over time due to small variations in the water-cooling temperature of the oscillator stage and amplifier and device jitter.

Using the beam profilers (Ophir SP620, Israel and Spiricon M2-200, Logan, UT, USA), the beam quality of the SLM Pr³⁺:YLF amplifier at the greatest output power was also examined. By using a normal Gaussian function fitting on the experimental data, the M² factors of the SLM Pr³⁺:YLF oscillator and amplifier were calculated to be about 1.47 and 1.6, respectively. The comparison of Figure 11a,b reveals that the beam profile has some distortion following the laser amplifier. This phenomenon is generally due to the reflection of the M1 mirror and the uneven power density of the second LD pump source.

5. Conclusions

This work demonstrated an SLM Pr³⁺:YLF laser with a high repetition frequency around the 444 nm absorption line by means of electrically modulated pre-laser Q-modulation and MOPA amplification technology. The SLM Pr³⁺:YLF laser’s output wavelength was 639.7 nm. The Pr³⁺:YLF laser resonant cavity had a virtually one longitudinal mode spacing when employing pre-lase technology as the SLM seed technique. A scanning Fabry–Perot interferometer determined that the SLM Pr³⁺:YLF laser’s instrument-limited linewidth was about 168 MHz. The SLM Pr³⁺:YLF laser was given a boost in output power using an amplifier setup with just one crystal. Up to 13.5 μJ of output single-pulse energy with a gain of 2.27 dB was achieved from the amplifier at the absorbed power of 3.77 W and 8 μJ of master oscillator energy injection. According to estimates, the SLM Pr³⁺:YLF amplifier’s M² factor was about 1.6.