Study On Non-focusing Low-pass Spatial Filtering Technology For High-power Laser Beam

High-power laser with fine beam quality is significant for its applications in inertial confinement fusion system, laser manufacturing, laser weapon, and so on. Low-pass spatial filtering is one of the key techniques to control and refine the beam quality.Based on whether the beam must be focused or not, spatial filtering can be classified into two categories. One is the focusing method, and the other is the non-focusing method. The former method is very mature, while the latter one is in the preliminary situation.According to the special requirements of spatial filtering for high-power laser beam, three innovative spatial filtering techniques without focusing——volume grating, multilayer dielectric film, and Rugate coating are analyzed.Based on Kogelnik's coupled wave theory, rigorous coupled wave theory, and matrix transfer method, the principles of periodic and quasi-periodic optical structures are investigated. The feasibility of the substitution of the non-focusing spatial filter for pin-hole filter is validated experimentally. The main contents and conclusions are as follows.1. non-focusing spatial filtering technique based on volume gratingThe diffraction and propagation characteristics of quasi-monochromatic plane wave by volume grating are analyzed theoretically. And the diffraction and propagation characteristics of ultra-short Gaussian laser pulse by transmitting and reflecting volume grating are investigated intensively. The condition of the diffraction with high efficiency and fidelity is achieved. The diffraction efficiency equation for the ultra-short laser pulse by transmitting volume grating is rectified.The configuration of spatial filtering without focusing based on volume grating is designed. The angular spectrum selectivity and filtering performances between transmitting and reflecting volume grating are compared with each other. The results show that reflecting volume grating has the advantages of high stability and wavelength-tunability over transmitting one.The transmitting volume grating is fabricated. And it has the relative diffraction efficiency up to 90% at 632.8 nm. The bandwidths of its diffractive angular spectrum with the change of the grating's period and thickness are measured. The results show that the measured data are in good accordance with those by refined Kogelnik's coupled wave theory. Using the transmitting volume grating recorded in Photo-Thermal Refractive glass (PTR glass), the low-pass spatial filtering of the spatially-deformed laser is realized. Compared with the results by pin-hole filter, the reliability of the non-focusing spatial filter is validated experimentally. In the experiment, the spatially-deformed laser beams are produced by He-Ne laser modulated by the amplitude plates, which has duty cycle of 1: 1 and the frequency of 7.2 mm^-1 and 3.0 mm^-1, respectively. The spatial profile of the deformed beam with the high frequency modulation (7.2 mm^-1) diffracted by the volume grating is similar to that of the undeformed one, while that with the low frequency modulation (3.0 mm^-1) is not.The coincidence between the theoretical analysis and the experimental results show that the diffractive side-lobes of the volume grating may greatly influence its filtering performance. The relative permittivity of the common volume grating is sinusoidally-modulated. Though its angular spectrum bandwidth may be up to 100μrad, its spatial filtering performance can not achieve the fine performance of the 100μrad due to the side-lobes. Aiming at the limitations, the phase-shifted configuration of the combination of two pieces of non-slanted volume grating is conceived to improve the angular selectivity. The apodized and phase-shifted non-slanted reflecting volume grating is put forward to suppress the side-lobes in order to enhance the filtering performance.2. non-focusing spatial filtering technique based on multilayer dielectric filmBy the matrix transfer method, the diffraction efficiencies, angular spectrum bandwidths, and the light spectrum bandwidths of two basic categories of multilayer dielectric film——high reflectance film and bandpass filter are analyzed. The results show that high reflectance film has lots of side-lobes and broad bandwidth, while bandpass filter does not. So bandpass filter has better spatial filtering performance than high reflectance film. In this thesis, the configuration of two pieces of four-etalon bandpass filter cascaded is designed. And its two-dimensional spatial filtering performance is studied.3. non-focusing spatial filtering technique based on Rugate coatingBy the matrix transfer method, the diffraction characteristics of chirped, apodized and phase-shifted Rugate coatings are investigated. Their diffraction efficiencies, angular spectrum bandwidths and light spectrum bandwidths with the change of the parameters are analyzed. The results show that apodized and phase-shifted Rugate coatings can suppress side-lobes efficiently. And the angular spectrum bandwidth of the phase-shifted coating is narrower than that of the apodized one. Using apodized and phase-shifted Rugate coatings, the configurations of low-pass spatial filter are designed, respectively. And their performances are analyzed. Two kinds of the fabrication of Rugate coating——codoposition and glancing-angle decomposition method are reviewed.According to the special requirements of the high-power laser system, three aforementioned techniques are compared with each other. The results are as follows.Currently, the holography techniques can fabricate sinusodially-modulated refractive index easily. Though the angular spectrum bandwidth of the transmitting volume holographic gratings may be up to 100μrad, they can not meet the requirements of the high-power system due to their diffractive side-lobes and dispersion. Apodized and phase-shifted reflecting volume grating can meet the requirements (≤20～40 diffraction limit). Improving the refractive index modulation amplitude of the PTR glass, and fabricating apodized and phase-shifted volume grating with large cross section, are the requisite conditions to make the PTR volume grating facilitate its application for high-power laser system.The angular spectrum bandwidth of bandpass filter is up to 100μrad. It is very useful for the spatial filtering of the laser with narrow light spectrum. Rugate coating has lots of diversified kinds of periodic and quasi-periodic structures. The angular spectrum selectivity can be improved by the apodized and phase-shifted coating. Phase-shifted Rugate coating has the higher laser-damage threshold, lower stress, and more efficient side-lobe suppression than bandpass filter. So it facilitates the low-pass spatial filtering of high-power laser beam with large cross section.In the high-power laser system, the intensity at the focal plane of the pin-hole filter is more than 10¹² W/cm². So the plasma closure may be probably caused. And the material of the pin-hole plate is easily burned. As a hard-edge diaphragm, the low-frequency diffractive rings are likely to appear. The combination of lens and pin-hole has the performance to limit the enhancement of the output power of the laser. Low-pass spatial filtering without focusing can surmount the drawbacks of the pin-hole filter. The low-pass spatial filters based on Rugate coating, multilayer dielectric film, and volume grating are analyzed. And their filtering performances are validated. The non-focusing low-pass spatial filter is successfully explored to substitute for focusing one.There are several disadvantages of the non-focusing filter, such as the limitation of its usage for the ultra-short laser pulse and limited thermal stability. And it is difficult to fabricate the periodic optical structure with complex function. The techniques to overcome the drawbacks and solve the problem are discussed preliminarily.