Research on the Extinction Ratio of Dual-Crystal Electro-Optic Q-Switches (01)

1. Introduction

Lasers with high peak power and narrow pulse width are widely used in fields such as laser processing, optoelectronic detection, nonlinear optics, and medicine. Q-switching is a primary method for generating pulsed lasers; among the various techniques, electro-optic Q-switching is extensively employed in short-pulse laser systems due to its high switching speed, robust shut-off capability, active controllability, and the ease with which it can be synchronized with high precision to other instruments.

Electro-optic crystals (www.wisoptic.com) are the critical components that determine the performance of electro-optic Q-switches. Currently, the electro-optic crystals that have reached practical application primarily include lithium niobate (LiNbO₃, LN), potassium dideuterium phosphate (KD₂PO₄, DKDP) (www.wisoptic.com), and rubidium titanyl phosphate (RbTiOPO₄, RTP). As uniaxial crystals, LN and DKDP are typically used in electro-optic Q-switches with light propagating along the optical axis; while this configuration allows for Q-switching using a single crystal, it fails to utilize the crystal's maximum effective electro-optic coefficient, resulting in a high half-wave voltage. If light propagates along a non-optical axis direction, natural birefringence—which is also highly temperature-dependent—comes into play; consequently, a pair of crystals must be used to compensate for both the natural birefringence and the temperature-induced effects. RTP, an electro-optic crystal introduced more recently, is a biaxial crystal that also requires a paired configuration to compensate for natural birefringence. Furthermore, lithium tantalate (LiTaO₃, LT) shares a similar structure with LN and possesses excellent electro-optic properties. Its growth technology is well-established, enabling the production of large-sized, high-optical-quality crystals, and it exhibits a damage threshold significantly higher than that of LN. However, because the electro-optic coefficient (γ₂₂) of LT is very small (approximately 0.1 pm·V⁻¹), it cannot be configured like an LN Q-switch—where light propagates along the optical axis while an electric field is applied along the x-axis. When light propagates along other directions, a pair of LT crystals must be used to compensate for natural birefringence. At present, there are relatively few reports on the use of LT crystals in electro-optic Q-switching devices.

In summary, due to limitations imposed by crystal growth techniques and the intrinsic properties of the crystals themselves, there are relatively few electro-optic crystals suitable for practical application. While employing a dual-crystal configuration is an effective method for developing electro-optic Q-switches with low half-wave voltage, this approach places more stringent requirements on the optical quality of the crystals, as well as on the precision of their fabrication and temperature control.

Taking a dual-crystal LT (lithium tantalate) electro-optic Q-switch as an example, this paper systematically investigates the factors affecting the extinction ratio of such devices. It establishes the necessary specifications for parameters including crystal optical quality, fabrication parameters, and temperature control precision. Based on these findings, two dual-crystal LT electro-optic Q-switches of different dimensions were fabricated using optical cold-working techniques—specifically, a simultaneous (co-planar) polishing process. Finally, the dual-crystal matching quality and the extinction ratio of the Q-switches were evaluated.