The orthogonal Porro prism resonant cavity has strong anti-detuning characteristics under shock vibration conditions, so it has a wide range of applications in the military and aerospace fields. The ridge line is typically set at 45° when using Porro prisms. A wave plate must be added to compensate for the phase delay introduced by the Porro prism. Quartz Porro prisms are generally compensated by 0.57λ wave-plates (where λ is the wavelength of light). In practice, the optimal compensation azimuth angle range of 0.57λ wave-plates is relatively narrow, and there are numerous azimuths near the wave-plate’s best azimuth angle. Therefore, it is equivalent to weakening the anti-detuning ability of the Porro prism. This study reported that a 0.93λ wave-plate could compensate for the phase retardation of Porro prisms. The 0.93λ wave-plate has a wide optimal compensation range angle, and no angle is close to the optimal compensation range angle. It provides new ideas for the phase delay compensation of Porro prisms.
Objective In the optical system shown in Fig. 2, the effective reflectivity of the polarizer is calculated by using the Jones matrix and the relationship between the phase delay of the Porro prisms for linearly polarized light and the Porro ridge angle are analyzed. The resonant cavity shown in Fig. 4, the Porro prism ridge line, is placed at 45°/135°, and the resonant cavity is at a low-Q state when the Q-switch is without high voltage. It is calculated by Jones matrix that two groups of 0.57λ/0.43λ and 0.93λ/0.07λ can make the resonant cavity in a low-Q state. Since the two wave plates in each group of wave-plates are complementary, the 0.57λ and 0.93λ wave-plates are selected for theoretical analysis and experimental comparison. The Jones matrix is used to perform the theoretical simulation of the compensation of the two wave-plates, and the experimental verification is performed in the resonant cavity shown in Fig. 4 based on the hypothetical simulation results. The wave-plate is installed in an adjustable frame with an adjustable angle in the experiment. Under free-running conditions, the wave-plate is rotated while increasing the pumping current to determine the best azimuth angle range for the resonant to be in a low-Q state at the maximum pumping current.
Objective and Discussions In the optical system shown in Fig. 2, the phase delay produced by Porro prism is demonstrated in Fig. 3 when the ridge angle is 45°/135°, the effective reflectivity of the polarizer is the largest indicating that the Porro prism produces the largest phase delay at these angles. The Porro prism ridge line angle in the resonant cavity shown in Fig. 4 is 45°/135°. According to the Jones matrix simulation results, the 0.57λ and 0.93λ wave-plates can make the cavity in a low-Q state; at this state, the azimuth angle range of the 0.93λ wave-plate is 3.6°, which is about 3.6 times that of the 0.57λ, as shown in Fig. 5. It can also be seen in Fig. 5 that the 0.93λ wave-plate does not have any other angles that are close to the optimal compensation angle within the adjustment range of 0°--180°. The 0.57λ wave-plate, however, has multiple angles that are close to the optimal compensation angle. The experimental verification, according to the theoretical analysis, shows that the adjustment range of the 0.93λ wave-plate in the low-Q state of the resonator is about 6.8°, and the adjustment range of the 0.57λ wave-plate is about 2.2°, as shown in Table 1. The optimal adjustment range of the 0.93λ wave-plate is about 3.1 times that of the 0.57λ waveplate, which is consistent with the theoretical results. In addition, the output laser parameters of the two kinds of wave-plates are compared, and the results of the energy and pulse width of the two are the same, shown in Fig. 6 and Fig. 7.
Conclusions This paper analyzes the phase delay introduced by the Porro prism made of quartz, the number of wave-plates that can compensate the phase delay is calculated using Jones matrix: 0.57λ/0.43λ and 0.93λ/0.07λ. Since the two sets of wave-plates are complementary wave plates, the 0.93λ wave-plate and commonly used 0.57λ wave-plate have been theoretically analyzed and verified by experiments. The results show that the adjustment range of the 0.93λ wave-plate is about 6.8° to make the resonant cavity in a low-Q state, which is 3.1 times that of the 0.57λ wave-plate. The result is consistent with that of the theoretical analysis. In the theoretical analysis, the 0.57λ wave-plate has multiple azimuth angles that make the resonant cavity close to the low-Q state, whereas the 0.93λ wave-plate has only one azimuth angle. This result has been experimentally verified. In the actual adjustment process, determining the best azimuth angle of the 0.57λ wave-plate one by one is necessary, and the 0.93λ wave plate is more convenient. Compared with the 0.57λ wave-plate, the 0.93λ wave-plate has a wider range of optimal angles when compensating the phase delay of the Porro prism, and the adjustment is more convenient. This provides a new choice for the Porro prism compensation wave-plate and ideas for further improving the stability of the laser cavity.
