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Fig. 27. The pulse produced in the nanosecond time scale from the second harmonic with a wavelength of 532 nm.
walk-off between the fundamental and second-harmonic waves in the 5-mm-long KTP crystal limit the effective interaction length for low-intensity portions of the pulse, thereby reinforcing the observed pulse narrowing. Such nonlinear pulse shortening during nanosecond SHG is a well-known and commonly reported phenomenon in highpeak-power Q-switched laser systems.
A visual confirmation of the green light emission is provided in Figure 28, which captures the green laser spot at 532 nm. The high brightness and spatial uniformity of the green spot further confirm the success of the SHG process. The implementation of a carefully aligned optical system – with optimized polarizer angle, phase-matching alignment, and thermal stabilization – was crucial in achieving this result.
Such coherent and intense green output beams are of significant interest for a wide range of photonic applications, including nonlinear optics, fluorescence microscopy, laser spectroscopy, biomedical imaging, and optical pumping in laser systems [ 40, 41 ]. The outcomes of this section demonstrate that, despite the inherent challenges posed by the green problem, the passive Q-switched Nd: YAG laser – when paired with a properly engineered SHG stage – can reliably produce nanosecond green pulses with high peak power and excellent beam quality.
5 Conclusion
Fig. 28. Image of green light spots of the second harmonic with a wavelength of 532 nm.
rate of the 532 nm pulses( f rep = 3.846kHz), andapulse spacing of 26 ns. These metrics highlight the effectiveness of the external-cavity SHG configuration in delivering high-quality, short-duration green pulses. The substantial reduction in pulse width from the 45 ns fundamental( 1064 nm) pulses to 6 ns green pulses can be attributed to nonlinear compression and pulse reshaping effects within the KTP crystal under optimized conditions.
The pronounced shortening of the pulse duration from
~ 45 ns at 1064 nm to ~ 6 ns at 532 nm originates primarily
from the inherently nonlinear nature of the secondharmonic generation( SHG) process in the KTP crystal. In SHG, the conversion efficiency scales quadratically with the instantaneous intensity of the fundamental wave( g / I 2). Consequently, only the high-intensity portion of the fundamental pulse, near its temporal peak, contributes efficiently to the generation of the second harmonic, while the low-intensity leading and trailing edges of the pulse are strongly suppressed. This nonlinear intensity discrimination results in effective temporal compression and pulse reshaping of the SHG output.
This effect is further enhanced in our external frequency-doubling configuration due to the high peak power of the passively Q-switched pulses and optimized phase-matching conditions in the KTP crystal. In addition, group-velocity mismatch( GVM) and spatial / temporal
In this study, the performance of an Nd: YAG laser was systematically investigated in both free-running and passive Q-switched regimes utilizing a Cr: YAG saturable absorber. In the free-running mode, the influence of cavity length on the output pulse duration was examined, revealing that the pulse duration remained essentially invariant across different cavity lengths, maintaining a consistent value of approximately 618 ls. Additionally, the effect of pump energy density on the pulse characteristics was analyzed, showing that an increase in pump energy density from 10 to 40 J / cm 2 resulted in a slight elongation of the pulse duration from 601 ls to618ls, accompanied by a concomitant decrease in peak output voltage from 12.88 mV to 12.47 mV.
Subsequently, in the passive Q-switching regime with the Cr: YAG saturable absorber, key parameters including the input energy density threshold, repetition rate, and number of pulses per burst were investigated as functions of cavity length and pump conditions. It was demonstrated that reducing the cavity length lowers the input energy density threshold required to initiate laser oscillation, while simultaneously increasing both the repetition rate and the number of pulses within each burst. This behavior is attributed to the shorter photon round-trip time in compact cavities, which enhances the rate of gain build-up and facilitates multiple pulse generation per pump cycle. Conversely, longer cavities demand higher pump fluence to saturate the absorber, leading to reduced repetition rates and fewer pulses per burst.
Experimental results identified a threshold energy density of 33 J / cm 2 for the shortest cavity length of 275 mm,