Three-nanosecond-equal interval sub-pulse Nd:YAG laser with multi-step active Q-switching

The pulse laser which can be widely used in different laser fields. The multiple pulses string laser can be obtained by selecting from high frequency pulses. High-power pulses are usually obtained by Q-switched technology. The characteristics of the Q-switched method is that single energy storage firstly and then normal lasing actively. The energy stored in the active medium before is suddenly released in the form of very short pulse of light. The electro-optic Q-switched, acousto-optic Q-switched and passive Q-switched technology are difficult to obtain the frequency above MHz. Mode-locking technology is difficult to obtain the frequency below GHz. Therefore, it is hardly impossible to achieve the pulse group output with nanosecond pulse interval. It is very difficult to accumulate the population inversion well above the threshold under nanosecond intervals pumping as the microsecond orders of the fluorescence lifetime. This is also the reason for obtaining nanosecond intervals pulse output by the traditional Q-switched technology difficultly. In recent years, the application in laser drilling, laser medical and other fields is to obtain multiple pulses with nanosecond interval by a new method of controlling the laser output delay, but a multi-channel signal generator is needed to control the Q-switched delay of multiple lasers. The number of pulses depends on the number of the multiple lasers, and so the total size is large and the price is expensive.

 

Dr. Wang Chao from school of science of Changchun University of science and technology proposed a method of obtaining multi pulse by multi-step Q-switching in Chinese Optics Letters, Vol. 19, No. 7, 2021(Wang Chao, et al., Three nanosecond-equal interval sub-pulses Nd:YAG laser with multi-step active Q-switching). This method has the characteristics of energy storage first and then multiple normal laser effects. Taking three pulse output as an example, the process of multi pulse output is illustrated. The figure shows a typical time sequence for generating a three-step Q-switched pulse train. The resonant cavity loss presents a three-step distribution. The cavity loss shows the first step decline at time after the population inversion reached peak value by the action of pumping. The population inversion is rapidly transformed into photons and the first sub-pulse laser output is emitted. Then, after an appreciable delay, the resonator loss shows the second step decline, and the population inversion is rapidly converted into the photons and the second sub-pulse laser output is emitted. Finally, after an appreciable delay, the resonator loss drops in the third step, and the population inversion drops again and turns into photons, the third sub-pulse laser output is emitted. In order for the peak power of the three pulses to be equal, each step loss must to be strictly controlled. For more pulses output, more step distribution is needed for the cavity loss, and each step loss must be strictly controlled. Multiple nanosecond pulse intervals can be obtained without reaccumulating the population inversion consequently.

 

Diagram of three nanosecond-interval sub-pulses laser

 

A typical time sequence of the generation of three-step Q-switched pulse

 

The method was applied to the traditional Q-switched Nd:YAG laser. The three pulses group with 360kW of peak power was obtained stably at the time interval between 100ns and 1000ns, which is hardly impossible to achieve by adopting traditional Q-switching methods. Three pulses with the same nanosecond interval and the same peak power were obtained at pulse widths of 24ns, 28ns, and 36.6ns, respectively. The total energy of the group is about 32.5mJ, and the corresponding optical efficiency is 10.8%. The number of pulses can be increased by increasing the initial gain and decreasing the peak power. The method shows that multi-step Q-switching method proposed by the research group is practicable to achieve nanosecond interval sub-pulses. Therefore, the researchers believe that compared with other methods of achieving nanosecond interval pulses, this method is simple without inserting other optical elements on conventional Q-switched laser, which provides a new way for pulse group outputs with several nanosecond pulse intervals.