Xinxiang Miao

Surface Contaminant Control Technologies to Improve Laser Damage Resistance of Optics

The large high-power solid lasers, such as the National Ignition Facility (NIF) of America and the Shenguang-III (SG-III) laser facility of China, can output over 2.1 MJ laser pulse for the inertial confinement fusion (ICF) experiments. Because of the enhancement of operating flux and the expansion of laser driver scale, the problem of contamination seriously influences their construction period and operation life. During irradiation by intense laser beams, the contaminants on the metallic surface of beam tubes can be transmitted to the optical surfaces and lead to damage of optical components. For the high-power solid-state laser facilities, contamination control focuses on the slab amplifiers, spatial filters, and final-optical assemblies. In this paper, an effective solution to control contaminations including the whole process of the laser driver is put forward to provide the safe operation of laser facilities, and the detailed technical methods of contamination control such as washing, cleanliness metrology, and cleanliness protecting are also introduced to reduce the probability of laser-induced damage of optics. The experimental results show that the cleanliness level of SG-III laser facility is much better to ensure that the laser facility can safely operate at high energy flux.

Bulk damage and stress behavior of fused silica irradiated by nanosecond laser

Abstract. The laser-induced bulk damage and stress behaviors of fused silica are studied by using a neodymium-doped yttrium aluminum garnet laser operated at 1064 nm with pulse width of 11.7 ns. Three zones of bulk damage are defined: columned cavity zone, compacted zone, and crack zone. The damage morphology and stress distribution are characterized by a three-dimensional digital microscope and a polarizer stress analyzer. The results show that the stress in the columned cavity zone and compacted zone is approximately zero. From the laser beam center to fringe, both tensile and compressive stresses in the crack zone increase abruptly and linearly and then decrease exponentially. Thermal annealing is used to prove the phase retardation caused by the residual stress. The formation mechanism of bulk damage is also discussed.

Influence of outgassing organic contamination on the transmittance and laser‐induced damage of SiO2 sol‐gel antireflection film

Abstract. The influence of organic contamination (rubber outgassing) on the transmittance of the SiO2 sol‐gel antireflection (AR) film and laser‐induced damage threshold (LIDT) at 355 nm for 3ω AR film and at 1064 nm for 1ω AR film is studied. The correlation between the contamination time and the transmittance loss/LIDT of 1ω/3ω AR film is also investigated both in atmospheric and vacuum environments. The results show that the transmittance loss increases with increasing contamination time, and the LIDT decreases with increasing contamination time for both in atmospheric and vacuum environments. In addition, the resistance against contamination of the 1ω film is stronger than 3ω film, and the contamination is more serious in vacuum than in an atmosphere environment for the same contamination time. Meanwhile, the damage mechanism is also discussed. It indicated that both the porous structure and photo‐thermal absorption contribute to the decreasing LIDT of the sol‐gel AR film.

Numerical simulation of modulation to incident laser by submicron to micron surface contaminants on fused silica

Modulation caused by surface/subsurface contaminants is one of the important factors for laser-induced damage of fused silica. In this work, a three-dimensional finite-difference time-domain (3D-FDTD) method is employed to simulate the electric field intensity distribution in the vicinity of particulate contaminants on fused silica surface. The simulated results reveal that the contaminant on both the input and output surfaces plays an important role in the electric field modulation of the incident laser. The influences of the shape, size, embedded depth, dielectric constant (er), and the number of contaminant particles on the electric field distribution are discussed in detail. Meanwhile, the corresponding physical mechanism is analyzed theoretically.

Quantitative measurement of CO2 laser-induced residual stress in fused silica optics

Abstract. The residual stress field of fused silica induced by continuous wave CO2 laser irradiation is investigated with specific photoelastic methods. Both hoop stress and axial stress in the irradiated zone are measured quantitatively. For the hoop stress along the radial direction, the maximum phase retardance of 30 nm appears at the boundary of the laser distorted zone (680-μm distance to center), and the phase retardance decreases rapidly and linearly inward, and decreases slowly and exponentially outward. For the axial stress, tensile stress lies in a thin surface layer (<280  μm) and compressive stress lies just below the tensile region. Both tensile and compressive stresses increase first and then decrease along the depth direction. The maximum phase retardance induced by axial tensile stress is 150 nm, and the maximum phase retardance caused by axial compression stress is about 75 nm. In addition, the relationship between the maximum axial stress and the deformation height of the laser irradiated zone is also discussed.

A Contamination Sensor Based on an Array of Microfibers with Nanoscale-Structured Film

A contamination sensor based on an array of microfibers with nanoscale-structured film using evanescent field is proposed and demonstrated theoretically and experimentally. When the molecular contaminants deposit on the nanoscale-structured film, the refractive index of the film will change and the additional loss will be produced due to the disturbance of evanescent field. The possibility of the sensor is demonstrated theoretically by using three-dimensional finite-difference time domain (3D-FDTD). The corresponding experiments have also been carried out in order to demonstrate the theoretical results. Microfibers are fabricated by using hydrogen-oxygen flame-heated scanning fiber drawing method and the nanoscale-structured film coated on the surface of microfibers is deposited by using dip coating process. Then an array of microfibers is assembled to demonstrate the feasibility of the device. The experimental results show that contaminants detection with the device can agree well with the results measured by the laser-scattering particle counter, which demonstrates the feasibility of the new type of contaminant sensor. The device can be used to monitor contaminants on-line in the high-power laser system.

A simple all-fiber comb filter based on the combined effect of multimode interference and Mach-Zehnder interferometer

A polarization-dependent all-fiber comb filter based on a combination effect of multimode interference and Mach-Zehnder interferometer was proposed and demonstrated. The comb filter was composed with a short section of multimode fiber (MMF) fusion spliced with a conventional single mode fiber on the one side and a short section of a different type of optical fiber on the other side. The second type of optical fiber is spliced to the MMF with a properly designed misalignment. Different types and lengths of fibers were used to investigate the influence of fiber types and lengths on the performance of the comb filter. Experimentally, several comb filters with free spectral range (FSR) values ranging from 0.236 to 1.524 nm were achieved. The extinction ratio of the comb filter can be adjusted from 6 to 11.1 dB by varying polarization states of the input light, while maintaining the FSR unchanged. The proposed comb filter has the potential to be used in optical dense wavelength division multiplexing communication systems.

Micro/Nanofiber with Hollow Silica Nanoparticles Thin-Film for Airborne Molecular Contaminants Real-Time Sensing

A novel chemical sensing approach detecting airborne molecular contaminants (AMCs) or compounds is demonstrated by using single-mode optical microfibre (OMF) coated with hollow silica nanoparticles (HSNs). The concentration of AMCs, which were volatilized on the surface of the tapered microfibre coated with HSNs, influences the transmission loss of the microfibre. Tapered OMF was fabricated using a high-precision electrically controlled setup, and coatings of HSNs were prepared by meniscus coating method. The transmission loss of three OMFs with different diameters and the same thick coating were tested to determine the relationship between AMC concentrations and transmission loss of coated OMFs. Experimental results showed that the transmission loss increases with increasing concentration of AMCs. The sensitivity for volatile simethicone was 0.0263 dB/mg/m3 obtained by the coated OMF with diameter of 2.5 μm, and the sensitivity values of coated OMF with diameters of 5 μm and 6 μm were 0.0024 and 0.0018 dB/mg/m3, respectively. Thus the proposed coated OMF can be used in enclosed space for AMCs sensing.

Cleaning mechanism of particle contaminants on large aperture optical components by using air knife sweeping technology

The cleaning mechanism of optical surface particle contaminants in the light pneumatic tube was simulated based on the static equations and JKR model. Cleaning verification experiment based on air knife sweeping system and on-line monitoring system in high power laser facility was set up in order to verify the simulated results. Results showed that the removal ratio is significantly influenced by sweeping velocity and angle. The removal ratio can reach to 94.3% by using higher input pressure of the air knife, demonstrating that the air knife sweeping technology is useful for maintaining the surface cleanliness of optical elements, and thus guaranteeing the long-term stable running of the high power laser facility.

Influence of secondary treatment with CO2 laser irradiation for mitigation site on fused silica surface

The ablation debris and raised rim, as well as residual stress and deep crater will be formed during the mitigation of damage site with a CO2 laser irradiation on fused silica surface, which greatly affects the laser damage resistance of optics. In this study, the experimental study combined with numerical simulation is utilized to investigate the effect of the secondary treatment on a mitigated site by CO2 laser irradiation. The results indicate that the ablation debris and the raised rim can be completely eliminated and the depth of crater can be reduced. Notable results show that the residual stress of the mitigation site after treatment will reduce two-thirds of the original stress. Finally, the elimination and the controlling mechanism of secondary treatment on the debris and raised rim, as well as the reasons for changing the profile and stress are analyzed. The results can provide a reference for the optimization treatment of mitigation sites by CO2 laser secondary treatment.