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J. Eur. Opt. Society-Rapid Publ. 22, 7( 2026)
Fig. 14. Number of train pulses formed in four cavity arrangements.
Table 5. Number of train pulses produced in four cavity arrangements.
Number of pulse |
Input energy |
Setup A |
Setup B |
Setup C |
Setup C |
|
density( J / cm 2)
( 275 mm)
( 345 mm)
( 405 mm)
( 405 mm)
|
30 |
2 |
0 |
0 |
0 |
35 |
8 |
6 |
3 |
3 |
40 |
12 |
10 |
9 |
9 |
47 |
18 |
17 |
16 |
16 |
50 |
22 |
21 |
20 |
20 |
The oscilloscope traces for all configurations( Figs. 15 – 18) confirm the trend in pulse train formation across varying pump energies. For each configuration, increasing input fluence not only raises the number of pulses but also alters the temporal profile of the pulse burst. Notably, setup A exhibits the most densely packed and temporally confined pulse bursts, a direct result of the optimized cavity geometry for fast saturation and efficient energy extraction.
In this work, the temporal characteristics of the burstmode output are described using the following clearly defined parameters: f rep: burst repetition rate, the rate at which complete pulse bursts are emitted per second. t sep: pulse-to-pulse separation time, the temporal interval between two consecutive individual nanosecond pulses within the same burst. f sep = 1 / t sep: intra-burst pulse repetition frequency, the instantaneous repetition frequency of the individual pulses inside a single burst.
Further analysis of intra-burst repetition rate as a function of pump fluence is shown in Figure 19. In configuration A, the repetition rate increases sharply from 1.612 kHz at 33 J / cm 2 to 6.12 kHz at 50 J / cm 2. Similarly, setup D sees a rise from 1.922 kHz to 6.25 kHz over the fluence range of 40 to 50 J / cm 2. These results underline a strong positive correlation between input fluence and repetition rate, with shorter cavities consistently exhibiting higher rates. The repetition rate is effectively governed by the interplay between gain buildup and absorber recovery, both of which are accelerated in geometrically compact resonators.
Figure 20 presents the inverse relationship between pulse period( i. e., the time interval between successive pulses) and input energy density. As fluence increases, the time between pulses decreases resulting in tighter pulse packing within each burst. For instance, in cavity A, raising the input fluence from 30 to 50 J / cm 2 compresses the pulse period from 62 ns to 14 ns. A similar trend is observed in cavity D, where the period contracts from 52 ns to 16 ns across the same energy range.
To further clarify this behavior, Figure 21 illustrates the direct correlation between the number of pulses per burst and both repetition rate and pulse period. As the number of pulses increases, the repetition rate rises, while the interval between pulses decreases. This indicates that the total duration of the pulse train remains nearly constant, and the system dynamically adjusts pulse timing to accommodate additional pulses – a hallmark of efficient burst-mode Q-switching.
A noteworthy observation during this study was the occasional formation of two distinct pulse bursts within a single excitation cycle. As shown in Figure 22, thetemporal gap between consecutive pulse trains was measured to be inter-burst interval of 13.12 ms. This double-burst behavior is attributed to incomplete energy extraction during the first burst, leaving residual energy in the gain medium that triggers a secondary burst once the absorber recovers. Such effects are more prominent under specific saturation dynamics and are closely linked to the recovery time of the Cr: YAG absorber [ 30, 31 ].
Figure 23 shows that each pulse burst lasts approximately 250 ls, further emphasizing the highly transient nature of burst-mode Q-switching. Lastly, high-resolution measurements( Fig. 24) reveal that individual pulses within the burst have a duration of 45 ns, peak voltage of 56 mV, and a repetition rate of 22.2 MHz – demonstrating the effectiveness of the optimized configuration in producing high-speed nanosecond pulses.
C) Second harmonic generation using a passive Q-switched Nd: YAG laser
In the final phase of this study, second-harmonic generation( SHG) was achieved using the optimized passively Q-switched Nd: YAG laser described previously, incorporating a Cr: YAG saturable absorber. The laser system was configured to generate high-intensity green emission at a wavelength of 532 nm by converting the fundamental 1064 nm output through an external KTP( potassium titanyl phosphate) nonlinear crystal. The KTP crystal( Laser Components( UK) Ltd.) had dimensions of 5 5 5 mm ³, with a 5 mm optical path length along the propagation direction, and was cut for type-II phase-matching( h = 90 °, u 23.5 °) to enable efficient second-harmonic generation from 1064 nm to 532 nm. The linear absorption