1. Repetition rate of electro-optic Q-switched laser

The repetition rate of E-O Q-switched laser is closely related to the characteristics of E-O Q-switching element and the optical characteristics of the gain medium. The E-O coefficient, piezoelectric ring effect and optical rotation effect of E-O crystal affect the repetition rate of laser. Compared with the E-O crystal as the Q-switching element, the E-O deflector made according to the E-O characteristics of the E-O crystal reduces the driving voltage and increases the repetition rate . In addition, the upper level lifetime of the gain medium will limit the repetition rate of the laser. In the following, the development of high repetition rate E-O Q-switched lasers is reviewed from two aspects: the selection of E-O Q-switching elements and gain media.

1.1 The electro-optic crystal

The refractive index of E-O crystal changes under the effect of external electric field, which leads to the change of laser propagation characteristics. The E-O effect is the physical basis of E-O Q switching. KD^*P(KD₂PO₄, www.wisoptic.com) and LN (LiNbO₃, www.wisoptic.com) are two traditional E-O crystals with mature technology and have been widely used in E-O Q-switched lasers. However, due to its high driving voltage and obvious piezoelectric ringing effect, high repetition rate laser output cannot be obtained. To increase the repetition rate of the laser, new E-O crystals such as RTP (RbTiOPO₄, www.wisoptic.com), BBO (β-BaB₂O₄, www.wisoptic.com), LGS (La₃Ga₅SiO₄, www.wisoptic.com) and PLZT [(Pb,La) (Zr,Ti) O₃] have gradually replaced traditional E-O crystals in high repetition rate lasers. The experimental data show that the new E-O crystal has excellent performance in increasing the repetition rate.

1.1.1 KD^*P E-O Q-swtiched laser

KD^*P has a high E-O coefficient. As an E-O Q-switching element, its technology is mature and has been widely used. However, the high driving voltage of KD^*P limits the increase of laser repetition rate. KD^*P E-O Q-switch is often used in lasers with repetition rate of 1kHz or lower. In 2012, some researcher used LD end-pumped Nd:YAG (as gain medium) and KD^*P Pockels cell to obtain laser output with single pulse energy of 2.7mJ and pulse width of 5ns under repetition rate of 1kHz. In 2018, people adopted a new LD end-pumped structure, using a three-column lens coupling system to effectively focus the pump light on the Nd:YAG laser crystal, to improve the absorption efficiency of the pump light, and obtain a high power laser output. Under the condition of using KD^*P Pockels cell with repetition rate of 1kHz, the laser output with single pulse energy of 11mJ is finally obtained.

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