2.3 Method of electro-optical cavity-dumping

According to the E-O Q-switching theory, the laser pulse width is limited by its repetition frequency, the former widens significantly with the increase of the latter. E-O cavity-dumping is a special Q-switching technique used to generate high repetition frequency and narrow pulse width. Generally, the energy storage unit of the E-O Q-switched laser is the laser gain medium, but the energy storage unit of the E-O cavity-dumping system is the resonant cavity, so that the laser pulse width obtained by cavity-dumping has nothing to do with the repetition frequency, but only depends on the length of the resonant cavity and the switching speed of the Q-switch. Theoretically, as long as the switching speed of the Q-switch is high enough, the pulse width obtained by cavity-dumping is equal to the round-trip time of the oscillating beam in the cavity.

In 2006, people conducted a comparative experiment between the E-O cavity-dumping technique and the traditional E-O Q-switching technique. This experiment used 888 nm LD end-pumped Nd:YVO₄ as the laser source and BBO crystal (www.wisoptic.com) as the Q-switching material. When applying the traditional E-O Q-switching technique, the pulse width changes from 15 ns at repetition frequency of 20 kHz to 45 ns at repetition frequency of 100 kHz. However, the pulse width stays at 6±0.2 ns with E-O cavity-emptying technique at different repetition frequencies. That’s to say, laser output with repetition frequency of 100 kHz and pulse width of 6 ns can be realized by the E-O cavity-dumping technique.

In 2014, researchers directly pumped Nd:YVO₄ by LD at 880 nm to effectively reduce the thermal effect of the laser crystal, and used BBO crystal as the Q-switching element in the E-O cavity-dumping system. By optimizing the resonator, the laser was kept stable in a wide pumping range and the mode matching between the pump light and the oscillating light is improved. Stable laser output with single pulse energy of 20 μJ and pulse width of 6 ns was obtained at repetition rate 500 kHz. The structure diagram of this E-O cavity-dumping design is shown in figure below.

In summary, E-O Q-switched laser can produce high repetition rate beam with narrow pulse width. Benefit from the fast switching ability of E-O Q-switch, narrow pulse width laser output can be obtained by compressing the cavity length. The length of the cavity is usually compressed by coating the end face of the gain medium instead of using a mirror, and the other end face of the gain medium is cut by Brewster angle instead of using a polarizer. The quarter-wave plate can be discarded by applying the voltage-decreased E-O Q-switching technique to further compress the cavity length. The use of E-O deflector makes it possible to realize Q-switching operation in the resonator without adding any other cavity elements except the deflector, which effectively compress the cavity length, reduce the insertion loss, and more easily obtain the narrow pulse width laser output. However, with the increase of the repetition frequency, the pulse width will be broadened accordingly. Laser output with higher repetition frequency and narrow pulse width can be obtained by using E-O cavity-dumping technique which characterize that the pulse width of the output beam is independent of the repetition frequency.