J. Eur. Opt. Society-Rapid Publ. 22, 7( 2026) 73
Fig
. 19. Intra-burst repetition rate( f rep) of pulses within the burst for the four cavity configurations.
Fig
. 20. Pulse separation time( t sep = 1 / f rep) within the burst.
Figure 26 presents two oscilloscope traces of the green output signal measured over different time windows – 2 ms and 400 ls – to examine both the macroscopic and fine temporal structure of the output. The pulse trains exhibit noticeable irregularities and amplitude fluctuations, particularly within the narrower observation window. These fluctuations are characteristic manifestations of what is widely referred to as the“ green problem” in nonlinear frequency conversion systems using KTP crystals [ 32, 33 ].
The green problem encompasses a collection of instabilities – temporal, spatial, and spectral – that degrade the performance and beam quality of green laser sources operating in the nanosecond pulsed regime. These instabilities become particularly pronounced in high-repetition-rate, high-peak-power Q-switched systems, such as those employed in this experiment. Several underlying mechanisms contribute to the green problem:
1. The observed temporal and spatial instabilities in the green output( pulse-to-pulse amplitude jitter, temporal drift and beam filamentation) are features of the well-known“ green problem” in KTP. Although the average pump energy is moderate, the burst nanosecond single pulses exhibit high peak power and are repeated at kHz rates per burst, creating much higher instantaneous intensities inside the crystal. At these intensities, nonlinear absorption( mainly two-photon absorption at 1064 nm), green-induced infrared absorption, optical refraction and gray tracking become significant despite negligible linear absorption, thereby inducing local refractive index variations and disrupting phase matching. The non-ideal antireflection coating at 532 nm further enhances these effects by partially reabsorbing the generated green light. Hence, instabilities arise from high peak intensity and intra-burst repetition rather than volumetric thermal loading of moderate power.
2. Thermal effects: Both 1064 nm and 532 nm light are partially absorbed within the KTP crystal, leading to localized heating and consequent changes in the refractive index( thermal lensing). These variations disturb the phase-matching condition required for SHG and can lead to spatial beam degradation [ 34 ].
3. Photorefractive effects: High-intensity nanosecond pulses can induce charge trapping in KTP, generating space-charge fields and permanent refractive index modulations, thereby creating phase mismatch and spectral broadening.
4. Gray tracking: Prolonged exposure to intense green light can result in the formation of color centers( gray tracks) that act as absorptive or scattering defects, progressively degrading beam quality over time.
5. Nonuniform phase matching: Inhomogeneities in the KTP crystal, whether intrinsic or thermally induced, can lead to localized deviations from optimal phasematching conditions, producing multiple beamlets or spatiotemporal modulation of the output.
These factors collectively contribute to pulse-to-pulse variability, timing jitter, and spatial beam distortion, as observed in Figure 26. Such effects limit the practical application of high-repetition green lasers in precision tasks unless compensated through rigorous thermal control, high-quality crystal selection, and cavity design optimization.
Although the characteristic instabilities of the“ green problem” were observed in the present burst-mode frequency-doubled output, several well-established and practically feasible mitigation strategies can be readily implemented to significantly suppress these effects and achieve highly stable 532 nm pulse trains. These include:
1. Extracavity placement of the KTP crystal, as implemented here, constitutes the most critical measure, eliminating exposure to high intracavity average power and drastically reducing cumulative green-light-induced photorefractive damage and gray-track formation compared to intracavity doubling schemes [ 35 ].