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Fig. 8. Variations of the pulse length in relation to the length of the cavity.
Fig. 9. Variations in pulse duration with respect to input energy density.
4 Results and discussion
A
) Nd: YAG laser cavity configuration in free-running mode
In the initial phase of this study, the Nd: YAG laser system was configured to operate in free-running mode. In this regime, laser emission persists as long as the pumping energy exceeds the lasing threshold, allowing population inversion to be sustained without active or passive modulation of cavity losses. As a result, the output pulse durations extend into the microsecond or even millisecond range, governed primarily by the duration of the pump pulse and the relaxation dynamics of the gain medium.
The primary objective of investigating the laser in freerunning mode is to characterize the natural temporal response of the laser and establish a baseline for subsequent optimization in Q-switched operation. Figure 7 shows the cavity experimental setup and Figure 2 shows the schematic diagram of the cavity design in free-running mode. In this design, L 1 and L 2 represent the free-space distances between the optical components and d represents the fixed length of the Nd: YAG rod, which is 100 mm. The total cavity length, L total, is the sum of these components and defines the longitudinal configuration of the resonator. Five different cavity configurations with lengths ranging from 312 mm to 352 mm were examined( Table 1). In each case, we evaluated the pulse duration and peak output voltage at a constant input energy density of 40 J / cm 2.
As shown in Figure 8, the output pulse duration remained nearly constant( ~ 618 ls) across all five configurations, regardless of cavity length. This observation confirms the well-established notion that in free-running lasers, the cavity length has minimal influence on pulse duration. Instead, pulse duration is predominantly determined by the lifetime of the upper laser level and the characteristics of the pumping pulse. The cavity length primarily affects the longitudinal mode structure and mode spacing, which have negligible impact on temporal pulse width under free-running conditions [ 18 ].
To explore the role of input energy density, the pulse duration was measured at varying fluence levels( from 10 to 40 J / cm 2) for a fixed cavity length of 322 mm( Fig. 9). A gradual increase in pulse duration from 601 ls to 618 ls was observed with increasing pump fluenec. This trend suggests that higher input energy leads to extended emission duration due to prolonged stimulated emission and slower energy depletion in the gain medium [ 19, 20 ].
In a separate analysis, we investigated the correlation between the input flux and the peak output voltage( Fig. 10). This experiment shows that laser emission only begins when the input flux exceeds 10 J / cm 2. No laser output is detected for input flux levels below the threshold. Over the entire range of free-running cavity lengths investigated, 312 – 352 mm( Table 1), the laser oscillation threshold remains essentially constant at 10 J / cm 2. The peak pulse voltage – used here as a relative measure of output power – was observed to slightly decrease from 12.88 mV at 10 J / cm 2 to 12.47 mV at 40 J / cm 2.
Figure 11 shows a typical oscilloscope trace of the freerunning pulse at the highest input energy. The pulse exhibits a smooth envelope with a duration of approximately 618.42 ls and a peak voltage of 12.8 mV, further validating the consistency of the free-running regime under our experimental conditions.
B
) Passive Q-switching mode of the Nd: YAG laser using a Cr: YAG saturable absorber
Figure 12 shows the experimental setup of a passively Q-switched Nd: YAG laser cavity. Starting from the left, the cavity begins with a highly reflective concave mirror( radius of curvature = 500 mm, R 99.9 % at 1064 nm), followed by the laser head, which contains a 100 mm long Nd: YAG rod with a diameter of Ø5 mm pumped by a flashlamp inside an optical reflector. Next, a 5 mm thick