JEOS RP ISSN03 | Page 68

J. Eur. Opt. Society-Rapid Publ. 22, 7( 2026) 61

Fig. 1. Laser pulse train in the burst mode [ 11 ].

increased average power, higher temporal resolution, and enhanced interaction with nonlinear or fast-evolving targets. Burst pulses are especially beneficial in applications like material ablation, nonlinear frequency conversion, and advanced diagnostic systems where multiple pulses in a narrow time window can improve efficiency and control.

In 2010, Harris et al. developed a high-energy burstmode MOPA laser system for Thomson scattering diagnostics, combining a Q-switched Nd: YVO 4 oscillator with multiple Nd: YAG amplifiers. Their system produced burst trains of up to 30 pulses at 250 kHz, with pulse energies up to 0.53 J, 45 ns pulse widths, and a total burst duration of 2ms [ 15 ]. In 2014, Ma et al. demonstrated a diode-pumped Cr: YAG Q-switched Nd: YAG burst laser that generated up to eight pulses per burst at 10 Hz and achieved single-pulse energies up to 15.5 mJ with pulse widths of ~ 13 ns [ 11 ]. Similarly, in 2016, Li et al. introduced a compact laser based on a Cr: YAG / Nd: YAG / YAG composite crystal, achieving pulse widths as short as 1.7 ns and pulse energies of 210 lJ within 11-pulse bursts [ 10 ].

The high peak power and short pulse duration of Q-switched Nd: YAG lasers make them highly suitable for second-harmonic generation( SHG) via nonlinear optical conversion. SHG enables frequency doubling – typically converting the 1064 nm fundamental wavelength into visible or ultraviolet radiation – through the use of nonlinear crystals placed outside the laser cavity [ 4, 16, 17 ]. This technique offers a practical pathway to generate stable, high-quality laser output at shorter wavelengths.

In this work, we report the development of a flashlamppumped 1064 nm Nd: YAG laser operating in burst-mode with passive Q-switching using a Cr: YAG saturable absorber. The study focuses on a flat – concave cavity configuration optimized to enhance peak power and reduce the duration of the burst pulses. Three different operational modes are explored:( 1) free-running,( 2) passively Q-switched with Cr: YAG, and( 3) second-harmonic generation. We experimentally investigate the effect of cavity geometry – including variations in cavity length and mirror configurations – on pulse duration, repetition rate, and energy output. Finally, by incorporating a nonlinear crystal, we demonstrate successful second-harmonic generation using the optimized Q-switched cavity design.

This

work presents a significant advance in burst-mode passively Q-switched Nd: YAG lasers by demonstrating that a simple flashlamp-pumped system with an optimized short cavity( 275 mm) can generate the highest number of pulses per burst( 22 pulses) and the highest intra-burst repetition rate( 7.142 kHz) yet reported for conventional( non-composite, non-diode-pumped) configurations, while simultaneously reducing the Q-switching threshold by more than 17 % compared to longer cavities. Furthermore, successful extracavity second-harmonic generation in a single KTP crystal produced stable 6 ns green pulses at 532 nm with an intra-burst repetition rate of 3.846 kHz and high beam quality, despite the intense“ green problem” typically encountered under such high peak-power burst conditions. The present study thus simultaneously improves pulse duration control, burst pulse count, repetition rate, and frequency-conversion efficiency. It overcomes previous limitations on pulse density in burst-mode flashlamp-pumped lasers and provides a practical, reliable, and low-cost route for generating high-repetition-rate nanosecond visible pulse trains suitable for nonlinear optics, highprecision material processing, and fast diagnostic systems. These quantitative results serve as a valuable design guideline for future compact and inexpensive burst-mode lasers.

2 Theoretical analysis

In the analysis of the behavior of Gaussian beams in laser cavities, the beam transfer method( ABCD matrix) is widely used. This approach allows for the analytical modeling of beam propagation through all optical elements of the cavity – including free space gaps, mirrors, crystals, and passive components. Using this formalism, it is possible to determine the stability conditions of the cavity and obtain the Gaussian beam parameters, such as the beam waist, the spot size on the mirrors, and the waist location. In this framework, every optical element – such as free space, lenses, mirrors, and dielectric interfaces – is described by a2 2 matrix, and the propagation of a ray through an optical system is given by the product of the matrices of the corresponding elements. If the ray coordinates at the input are expressed as( y 1, h 1) and at the output as( y 2, h 2), the following linear relation holds [ 7 ]:

y 1 h 1

¼ A B

C D

y2 h 2

ð1Þ

where the coefficients A, B, C, D determine the overall optical characteristics of the system. The determinant of all linear optical elements is equal to unity, and consequently for the entire system we also have det( M)= 1.

For a cavity consisting of two mirrors with curvature radii R 1 and R 2 separated by a distance L, the round-trip ray-transfer matrix is calculated as follows:

M ¼ A B

¼ C D