Electro-optic (EO) Q-switching technology has been extensively used to fabricate pulsed lasers. Its advantages of a faster switching rate, better hold-off ability, and controllable repetition rates enable the generation of energetic short laser pulses. To date, practical EO crystals include LiNbO₃ (LN), LiTaO₃ (LT), KD₂PO₄ (DKDP), and RbTiOPO₄ (RTP). To achieve a low driving voltage, the laser must propagate along the non-optical-axis direction of these crystals, which introduces additional phase retardation induced by natural birefringence. Using a second crystal that is rotated 90° with respect to the first crystal is necessary to compensate for the natural birefringence and its strong thermal fluctuations. However, achieving double-crystal EO Q-switches with a high extinction ratio is difficult because both crystals should have high transverse optical homogeneity and should be carefully matched. The matching quality may be affected by numerous factors, including the optical inhomogeneity of the crystals, optical processing accuracy, and temperature changes. To date, factors affecting the extinction ratio of double-crystal EO Q-switches have not been systematically studied, limiting the development and application of double-crystal EO Q-switches. In this study, we comprehensively analyze the factors affecting the extinction ratio and fabricated double-crystal LT EO Q-switches.

First, using double-crystal LT EO Q-switches as examples, we analyze the factors that affect the extinction ratio of double-crystal EO Q-switches. To maximize the EO effect, an LT Q-switch is fabricated from two x-cut LT crystals with light propagating along the x axis and voltage applied along the z axis. A set of analytical phase-shift formulas that consider the optical inhomogeneity, crystallographic orientation deviation, length deviation, and temperature change of the two matching crystals are derived. Combined with the transmittance formula for the parallel-polarization system, the tolerances of these factors are calculated using an extinction ratio of 100∶1. Accordingly, we fabricate two LT EO Q-switches with aperture of 9 mm×9 mm and lengths of 10 mm and 5 mm, respectively. The matching crystals are polished using the same polishing lap to ensure that the deviations in length and orientation satisfy the requirements. Each face of the crystals is finely ground, and the x surface is precisely polished and coated with anti-reflection films at 1064 nm. The z surface is then plated with gold and chromium. The two matching crystals are packaged in an elastic holder. The matching quality and extinction ratios are measured and characterized.

According to univariate analysis, the extinction ratio of double-crystal EO Q-switches is strongly related to the optical inhomogeneity, crystallographic orientation deviation, and the length and temperature differences of the two matching crystals. The extinction ratio is inversely proportional to the square of the optical inhomogeneity [Fig. 2(b)] and those of the length and temperature differences when the other parameters are kept constant. In addition, the crystal length significantly affects the extinction ratio. When the optical inhomogeneity, crystallographic orientation deviation, and temperature difference are set, the extinction ratio is inversely proportional to the square of the crystal length [Fig. 2(a)]. In addition, even the same change in temperature in the two matching crystals may affect the extinction ratio when a difference in crystal length is observed. To achieve an extinction ratio of 100∶1 under a crystal length of 10 mm, laser spot radium of 2.5 mm, and wavelength of 1064 nm, the optical homogeneity must be better than 6.8×10^-6/cm, the x and z orientation deviations should be less than 1.3° and 3.1°, respectively, the length difference should be less than 4.6 μm, and the temperature difference must be less than 0.16 ℃. For the two prepared LT EO Q-switches (Fig. 4), the shorter switch exhibits a better matching quality and higher extinction ratio (Fig. 6). The extinction ratio of the shorter LT Q-switch is approximately three times that of the longer switch (Table 1). However, the extinction ratios of both Q-switches are low due to poor optical homogeneity (Fig. 5).

Using double-crystal LT EO Q-switches as an example, we systematically analyze the factors affecting the extinction ratio and calculate their tolerances. Based on the theoretical results and practical difficulties of crystal growth and optical processing, the optical inhomogeneity and length and temperature differences are verified as critical factors that must be strictly controlled. In addition, the extinction ratio is found to be inversely proportional to the square of the crystal length when the optical inhomogeneity, crystallographic orientation deviation, and temperature difference are constant. Accordingly, we prepare two double-crystal LT EO Q-switches with different lengths. A shorter LT Q-switch is verified to have a better matching quality and higher extinction ratio. However, the extinction ratios of both Q-switches are low due to poor optical homogeneity. Double-crystal LT-EO Q-switches with high extinction ratios can be achieved using crystals with high optical quality. This work can be of significance in guiding the development of double-crystal EO Q-switches with high extinction ratios.