Short Pulse Width and High Peak Power 457 nm Deep Blue Laser with V-Type Cavity

An intracavity frequency doubling acousto-optically Q-switched Neody-mium-doped Yttrium Orthvanadate (Nd:YVO 4 ) 457 nm blue laser by employing a three-mirror folded cavity was demonstrated. With the incident pump power of 40.4 W, the maximum average output power of 439 mW 457 nm laser, and the minimum pulse duration of 86.14 ns and the maximum peak power of 510 W were achieved at 10 kHz. The M 2 factors are 1.23 and 1.61 in X and Y directions, respectively. The power stability in two hours is better than 2%.

Journal of Applied Mathematics and Physics researchers focused on developing Nd:YVO 4 /LBO continuous-wave blue lasers [6] [9] [10], which are applicable for color displays, flow cytometry, and highdensity optical data storage. But for deep-seawater research, the high peak power and the short pulse width and the high repetition rate of the pulsed deep blue laser will help to improve its transmission distance and speed. So far, there are only a few reports on pulsed 457 nm lasers. Gao et al. [11] used a four-mirror-folded Z-shaped cavity; the pulse duration of 217 ns and the peak power of 258 W of 457 nm laser were achieved at 10 kHz. It can be seen that the laser cavity length is too long using Z-shaped cavity, laser pulses show wide pulse duration and therefore low peak power could be obtained at high-repetition rate operation. Some of the above problems can be avoided by using a three-mirror-folded V-shaped cavity.
In this paper, a three-mirror-folded V-shaped cavity was adopted in the pulsed laser system at 457 nm, using an 808 nm laser diode (LD) to pump the Nd:YVO 4 laser crystal, through acousto-optically Q-switched and intracavity frequency doubling, with the incident pump power of 40.4 W, the maximum average power of 439 mW 457 nm laser was achieved at 10 kHz, with the pulse duration of 86.14 ns and the peak power of 510 W. To the best of our knowledge, this is the shortest pulse width and the highest peak power of the 457 nm deep blue laser so far.

Cavity Parameter Optimization
In this experiment, a three-mirror folded cavity was introduced to reduce the whole length of the cavity and to produce two separate beam waists. One is aimed at improving the output power of the fundamental frequency laser, the other is for high efficiency frequency doubling in LBO. According to the ABCD matrix theory and the stability conditions of the thermally insensitive cavity, the parameters of the resonator were optimized by MATLAB simulation calculation.
Experimental setup is shown in Figure 1. Figure 2 and Figure 3 show the influence of the lengths of arm L1 and L2 on the stability of the cavity. It is obvious that different thermal lens focal lengths do not affect the stable region of the cavity. When the length of L1 changes by 20 mm, the stability parameter only changes less than 0.1, so the stability of the cavity is insensitive to the length of arm L1. However, we should notice that a 10 mm change of the second arm L2   will change the cavity stability parameter by 0.3. Therefore the length of L2 must be adjusted carefully in the experiment. In addition, it can also be seen that the stability parameter of the cavity is closest to 0.5 with L1 = 140 mm and L2 = 70 mm.

Experiment Setup
Experimental setup is shown in Figure 1.

Results and Discussions
At 10 kHz, the average output power and pulse width of the 457 nm deep laser as a function of the incident pump power are presented in Figure 4. It can be seen that the average output power is increased and the pulse width is reduced exponentially with the increase of the incident pump power. However, the relationship between average output power and pumping power is not linear. When the pump power is less than a certain level, the slope efficiency of the 457 nm average output power is very low, whereas, when the pump power is higher than that level, the slope efficiency increases dramatically. The reason for this phenomenon is caused by the saturation of the re-absorption loss of the quasi-three-level laser system. At the lower pump level, the population inversion and fundamental gain are very small, and the re-absorption loss will lead to relatively low slope efficiency, as the pump power increased, the fundamental gain is large enough to offset the impact of re-absorption. When the incident pump power is 40.4 W, the maximum average power of 439 mW, and the minimum pulse duration of 86.14 ns and the peak power of 510 W for 457 nm laser were achieved. Figure 5 and Figure 6 show the pulse trains and the typical pulse profile of the 457 nm deep blue laser at the maximum average output power (439 mW). Here the lost pulse and the double pulses phenomenon are not observed.
Laser beam intensity profile of the pulsed 457 nm deep blue laser at the maximum average output power of 439 mW is shown in Figure 7. It can be seen that the laser intensity distribution is very symmetrical and near Gaussian-distribution.

Conclusion
We have demonstrated an intracavity frequency doubling acousto-optically Q-switched Nd:YVO 4 457 nm blue laser by employing a three-mirror folded cavity for the first time. A three-mirror folded cavity is employed to enhance the conversion efficiency. With an incident pump power of 40.4 W, the maximum average power of 439 mW 457 nm laser was achieved at 10 kHz, with the minimum pulse duration of 86.14 ns and the maximum peak power of 510 W. The M 2 factors are 1.23 and 1.61 in X and Y directions, respectively. The power stability in two hours is better than 2%.