Efficient Third-harmonic Generation Of Broadband Nd: Glass By Frequency Mxing Of Broadband And Narrowband Nd: Glass Lasers

A broadband bandwidth is key factor for the performance of the laser-aperture (～300 mm) and high energy (-kJ) Nd:glass laser facility, such as avoiding damage to optical components, improving laser-beam uniformity, improving extracted efficiency from energy-storage of laser amplifer. A broadband laser is also helpful to the experiment in high energy density science (HEDS), such as improving absorption of laser by a target and obtaining a higher target compression, suppressing the production of hot electron in the laser-plasma interaction. An important trend of the high-power laser delivers for inertial confinement fusion (ICF) is to apply broadband laser light.A frequency doulber and tripler has become a standard auxiliary device in high-power laser facilities because a the third-harmonic component of Nd:glass laser is thought to be and ideal wavelength for ICF. Efficient third-harmonic generation (THG) is limited to narrow bandwidth cases because of the dispersion of KDP crystal, so the need of efficient broadband frequency-tripling conversion become more urgent for the development of broadband laser delivers. Several approaches have benn proposed to increase the acceptance bandwidth, however, they are too complicated to be adopted in fusion lasers or still insufficient. Therefore, the efficient broadband frequency conversion technique is an urgent and difficult research subject.We report an efficient frequency tripling scheme for 351 nm broadband pulses by use of broadband and narrowband Nd:glass lasers. Avoiding any additional large dispersive optical element (i.e. diffraction grating) caused energy loss or beam distortion, with the assist of a narrowband laser in the sum-frequency generation (SFG) process and the addition of a second tripler, results in a large bandwidth acceptance (over 5 nm) as well as high tripling conversion efficiency (>80%). It is expected to overcome the obstacle of broadband Nd:glass laser delivers.The major research work can be listed as follows:1. We propose and numerically study a simple and efficient broadband THG scheme for Nd:glass laser system based on mixing narrowband and broadband laser pulses in the SFG process. The group-velocity mismatching (GVM) effects of the SFG process is substantially alleviated with the assist of a narrowband laser, which is the dominate obstacle to efficient broadband THG. Under the small signal the phase-matching bandwidth of the proposed frequency conversion scheme are studied analytically and numerically, respectively. The results show that the narrowband pulse-related GVMs have little effect on phase-matching bandwidth.2. A dual-tripler scheme has been successfully demonstrated and seems to be more promising, and the acceptance bandwidth of 1.2 nm almost can satisfy the need of practical application. But it does not allivated the GVM effects substantially, result in a conversion from the frequency modulation (FM) of fundamental pulse to amplitude modulation (AM) of output third-harmonic pulse. Two GVM values with considerable difference exit in the SFG process, the bigger one has little effect on frequency convrsion if the long wavelength laser is chosen as narrowband laser in the proposed scheme. As a result, the FM-to-AM conversion is substantial suppressed and a pulse with a well-temporal shape is obtained.3. Since a typical Nd:glass petawatt laser delivers has a bandwidth of 4～5 nm, the ultimate solution for frequency tripling needs to exploit the whole bandwidth of 5 nm efficiently. Based on the frequency-mixing of broadband chirped-pulse with a narrowband laser and a dual-tripler scheme, a broadband Nd:galss chirped-pulse with a bandwidth of 5 nm can be efficiently converted to ultraviolet pulse (efficiency >80%). The generated ultraviolet pulse can be compressed to a duration shorter than that of the initial Nd:glass petawatt laser pulse and ultraviolet petawatt peak power can be increased by 2.5 times as high as that of the Nd:glass petawatt laser. The demonstrated THG scheme may provide a promising route to efficient generation of high-power lasers at short wavelengths.4. The sensitivities of conversion efficiency on both the intensity variation and crystal-orientation offset in the proposed scheme are also discussed for practical applications. Designs that involve two doublers offer a high dynamic range of conversion efficiency versus intensity variation, at the same time, a dual-doubler is quite insentive to input angle offset.