J. Eur. Opt. Society-Rapid Publ. 22, 7( 2026) 67
Using the commonly accepted values r gas =( 4.3 ± 0.5) 10 �19 cm 2 and s a = 3.8 ± 0.2ls, equation( 2) yields I sat 6.1 MW / cm 2.( This value lies comfortably within the commonly accepted range of 4 – 7 MW / cm 2 for Cr 4 +: YAG at 1064 nm). The physical consequence of this relatively low saturation intensity is crucial: once the intracavity intensity reaches only a few times I sat( typically 20 – 30 MW / cm 2, easily achieved during the rapid growth phase of a Q-switched pulse), the absorber is bleached in just a few cavity round trips, causing an abrupt drop in cavity loss and the release of a giant pulse. When the intracavity intensity significantly exceeds I sat, the absorber becomes fully bleached, allowing the release of a giant pulse. Following pulse emission, the intracavity intensity collapses rapidly. Because the excited-state recovery time of Cr 4 + is on the order of microseconds – much longer than the cavity roundtrip time( nanoseconds) in compact resonators – the absorber quickly regains its high absorption. This cycle of bleaching and recovery can repeat multiple times during the gain lifetime, producing the characteristic burst-mode operation with a train of nanosecond pulses per pump cycle [ 1, 2, 7 ].
This strongly nonlinear, intensity-dependent loss – high absorption at low intensities, rapid bleaching near
~ 6MW / cm2, and recovery after intensity collapse – is the
fundamental mechanism enabling passive Q-switching and the generation of multiple giant pulses in the present laser system.
3 Experimental setup
The experimental setup is illustrated in Figure 7. The laser cavity consists of a concave high-reflectivity rear mirror( radius of curvature 500 mm) and a flat output coupler with 60 % reflectivity( 40 % transmission) at 1064 nm. The Nd: YAG rod( 100 mm length, 5 mm diameter) and the xenon flash lamp( 115 mm length, 6 mm diameter) are placed parallel inside the pumping chamber with a center-to-center distance of 17 mm, providing side-pumping configuration. The distance between the cooling-water inlet and outlet ports of the chamber is 97 mm. The internal surface of the chamber has high reflectivity. The xenon flash lamp is made of a quartz tube filled with xenon gas and equipped with tungsten-alloy electrodes at both ends. The length of the flashlamp is chosen approximately equal to the Nd: YAG rod length to ensure full coverage and uniform excitation of the active medium. The flashlamp is powered by a pulsed power supply and delivers high-energy pulses with a peak electrical power of 5.17 mV over a temporal width of approximately 3.03 ms. The input pump energy density( fluence) on the Nd: YAG rod was varied from 10 to 50 J / cm 2 by changing only the electrical energy supplied to the flashlamp, while all other flashlamp parameters were kept strictly constant.
The optical resonator adopts a hemispherical linear configuration, comprising a plano-concave high-reflectivity mirror( HR) with a radius of curvature of 500 mm( reflectivity: 99.9 %) and a flat output coupler( OC) with a transmission coefficient of 40 %( reflectivity: 60 %). The output
Fig. 7. The top-view of the Nd: YAG laser cavity:( A) concave HR mirror( R = 500 mm),( B) laser head housing the Nd: YAG crystal and flashlamp, and( C) flat output coupler( T = 40 %).
coupler was a flat mirror with a reflectivity of 60 % at 1064 nm( R = 60 %, T = 40 %) to provide moderate intracavity round-trip losses, which is essential to achieve long burst mode operation with multiple nanosecond pulses per pump cycle in Q-switched lasers. Output couplers with higher reflectivity typically limit the output to only a few intense pulses, while too low reflectivity will over-extend the Q-switch threshold. The internal geometry of the cavity was varied throughout the experiments to study the effect of cavity length on pulse characteristics. The total cavity lengths used for different configurations ranged from 275 mm to 465 mm.
To maintain thermal stability and prevent thermal lensing effects in the gain medium, the laser head was actively cooled using a water-cooling system set at 18.0 ° C. This was critical in ensuring the consistency and repeatability of the pulse generation under high-repetition-rate and burst-mode operation.
For temporal characterization of the laser pulses, we employed a DSO5200 200 MHz digital oscilloscope connected to an InGaAs photodetector( model M / DET08C). This detector offers high sensitivity in the near-infrared range, particularly around 1064 nm, and operates linearly within its dynamic range. The peak voltage signals detected by the InGaAs photodetector were used as a relative measure of the laser output power, as a calibrated joulemeter was not available during the experiments. Given the linear relationship between the incident optical power and the detector output voltage, this method provided a consistent and reproducible way to monitor pulse intensity trends across different cavity configurations and operating modes.
The experimental procedure involved adjusting cavity parameters, including mirror separation distances, and measuring the resulting output waveforms under both free-running and passively Q-switched regimes. The measurements included pulse duration, peak voltage, pulse repetition rate, and burst characteristics such as the number of pulses per burst and inter-pulse spacing. In the case of second-harmonic generation, a KTP crystal was placed external to the laser cavity to convert the fundamental 1064 nm radiation into 532 nm green light. A short-pass optical filter was employed to isolate the second-harmonic signal, and a polarizer oriented at 54.7 ° was placed near the concave mirror to enhance SHG efficiency through proper polarization alignment.