To compensate for the gain saturation effect in the high-energy laser amplifier, a modified polarization beam combination (PBC) method is introduced to reshape temporal waveform of the injected laser pulse to obtain a controlled high-energy laser pulse shape after amplification. One linearly polarized beam is divided into two orthogonal polarized beams, which spatially recombine together collinearly after propagating different optical paths with relative time delay in PBC structure. The obtained beam with polarization direction being rotated by the following half wave plate is divided and combined again to reform a new beam in another modified polarization beam structure. The reformed beam is injected into three cascaded laser amplifiers. The amplified pulse shape can be controlled by the incident pulse shape and amplifier gain, which is agreeable to the simulation by the Frank–Nodvik equations. Based on the simple method, the various temporal waveform of output pulse with tunable 7 to 20 ns pulse duration can be obtained without interferometric fringes.
An accurate evaluation method with an amplified spontaneous emission (ASE) as the irradiation source has been developed for testing thin-film damage threshold. The partial coherence of the ASE source results in a very smooth beam profile in the near-field and a uniform intensity distribution of the focal spot in the far-field. ASE is generated by an Nd: glass rod amplifier in SG-II high power laser facility, with pulse duration of 9 ns and spectral width (FWHM) of 1 nm. The damage threshold of the TiO2 high reflection film is 14.4J/cm2 using ASE as the irradiation source, about twice of 7.4 J/cm2 that tested by a laser source with the same pulse duration and central wavelength. The damage area induced by ASE is small with small-scale desquamation and a few pits, corresponding to the defect distribution of samples. Large area desquamation is observed in the area damaged by laser, as the main reason that the non-uniformity of the laser light. The ASE damage threshold leads to more accurate evaluations of the samples damage probability by reducing the influence of hot spots in the irradiation beam. Furthermore, the ASE source has a great potential in the detection of the defect distribution of the optical elements.
Pre-amplifier between the frontend and main power amplifier is the key unit of high power laser divers. The recent
progresses on the off-axis quadruple pass amplifier are presented, which include the beam path design, parasitic
oscillation research and experimental results. A single longitudinal mode, temporally shaped laser pulse with 5ns pulse
duration at 1053nm is injected into a Nd: Glass regenerative amplifier, which can provide a 12mJ energy output with
0.5% long term energy stability. The quadruple pass amplifier is designed as an off-axis pattern. With 1.3mJ energy
injection, amplified pulse with 16.5J can be achieved, and the measured output energy stability of the amplifier is 7.3%
(PV) at this output energy level, corresponding to a 21 shot result. The total gain of the amplifier is more than 10,000.
The parasitic oscillation was analyzed and discussed, and the parasitic mode and pencil beam are neither observed in the