The Gridless Plasma Ion Source (GIS)for Plasma Ion Assisted Optical Coating

High-quality optical coating is a key technology for modern optics. lon-assisted deposition technology was used to improve the vaporized coating in 1980's. The GIS (gridless ion source), which is an advanced plasma source for producing a high-quality optical coating in large area, can produce a large area uniformity>1000 mm(diameter), a high ion current density - 0.5mA/cm2, 20 eV - 200 eV energetic plasma ions and can activate reactive gas and film atoms. Now we have developed a GIS system. The GIS and the plasma ion-assisted deposition technology are investigated to achieve a high-quality optical coating. The GIS is a high power and high current source with a power of l kW - 7.5 kW, a current of 10 A - 70 A and an ion density of 200 μA/cm2 - 500 μA/cm2. Because of the special magnetic structure, the plasma-ion extraction efficiency has been improved to obtain a maximum ion density of 500 μA/cm2 in the medium power (- 4kW) level. The GIS applied is of a special cathode structure, so that the GIS operation can be maintained under a rather low power and the lifetime of cathode will be extended. The GIS has been installed in the LPSX-1200 type box coating system. The coated TiO2, SiO2 films such as antireflective films with the system have the same performance reported by Leybold Co, 1992, along with a controllable refractive index and film structure.

Experimental study of sweep control in e-beam evaporated optical coatings

High performance optical coating requires excellent uniformity of thin-film.Keeping the surface of evaporation material flat is propitious for the stability of vapor plume,and can improve the uniformity of thin-film.Based on the principle of electron beam spot sweep,a pattern controller in domestic coater is designed.For the purpose of even evaporation during auto-sweep,the influence of the depth of material surface in the crucible on the evaporation characteristic is considered.Pre-melting and evaporation experiments are performed on melting material（Ti3O5）,subliming material（SiO2）,and semi-melting, semi-subliming material（HfO2）.The sweeping experimental results show that using the designed sweep controller can make good performance on evaporation and pre-melting for the above materials.

Imaging photothermal microscopy for absorption measurements of optical coatings

For absorption measurement of large-aperture optical coatings, a novel method of imaging photothermal microscopy based on image lock-in technique is presented. Detailed theoretical analysis and numerical calculation are made based on the image photothermal technique. The feasibility of this imaging method is proved through the coincidence between the theoretical results of single spot method and multi-channel method. The measuring speed of this imaging method can be increased hundreds of times compared with that of the raster scanning. This technique can expand the applications of photothermal technique.

Accuracy design of ultra-low residual reflection coatings for laser optics

Refractive index inhomogeneity is one of the important characteristics of optical coating material, which is one of the key factors to produce loss to the ultra-low residual reflection coatings except using the refractive index inhomogeneity to obtain gradient-index coating. In the normal structure of antireflection coatings for center wavelength at 532 nm, the physical thicknesses of layer H and layer L are 22.18 nm and 118.86 nm, respectively. The residual reflectance caused by refractive index inhomogeneity（the degree of inhomogeneous is between -0.2 and 0.2） is about 200 ppm, and the minimum reflectivity wavelength is between 528.2 nm and 535.2 nm. A new numerical method adding the refractive index inhomogeneity to the spectra calculation was proposed to design the laser antireflection coatings, which can achieve the design of antireflection coatings with ppm residual reflection by adjusting physical thickness of the couple layers. When the degree of refractive index inhomogeneity of the layer H and layer L is-0.08 and 0.05 respectively, the residual reflectance increase from zero to 0.0769% at 532 nm. According to the above accuracy numerical method, if layer H physical thickness increases by 1.30 nm and layer L decrease by 4.50 nm, residual reflectance of thin film will achieve to 2.06 ppm. When the degree of refractive index inhomogeneity of the layer H and layer L is 0.08 and -0.05 respectively, the residual reflectance increase from zero to 0.0784% at 532 nm. The residual reflectance of designed thin film can be reduced to 0.8 ppm by decreasing the layer H of 1.55 nm while increasing the layer L of 4.94 nm.

High-dispersive mirror for pulse stretcher in femtosecond fiber laser amplification system

We present a high-dispersive multilayer mirror for pulse stretching in a femtosecond fiber laser amplification system.The designed mirror contains 54 layers with a total physical thickness of 7.3 μm, which can provide a positive group delay dispersion(GDD) of 600 fs2and a high reflectance over 99.9% from 1010 to 1070 nm. The samples were prepared by dual ion beam sputtering. The measured transmittance matches well with the theoretical result. The GDD characteristics of samples were tested by home-made white light interferometer. The measured GDD is higher than the design results,an average GDD of +722 fs2from 1010 nm to 1070 nm. The mirrors were employed in a Yb-doped large-mode-area photonic crystal fiber amplification system. An input pulse compressed by the gratings with autocorrelation function of83 fs is obtained with a stretched FWHM of 1.29 ps after 28 bounces between the dispersive mirrors. The results show that the multilayer dispersive mirror could be an effective and promising technique for pulse stretching in femtosecond amplification systems.

Study on Preparation and Ns-Laser Damage of HfO₂Single Layers

The effects of the main parameters of argon flux, oxygen flux and beam voltage on the surface morphology, transmittance spectrum and laser damage of the HfO2 single layers prepared by ion beam sputtering are studied. The HfO2 amorphous single layers have porous surface morphologies. Different processes will cause differences in coatings absorption and surface morphology, which in turn will cause changes in the spectral transmittance curve. The ion beam sputtering HfO2 single layers have high content of argon (4.5% - 8%). The laser damage of HfO2 single layers is related to argon inclusions and non-stoichiometric defects. The changes of argon flux and beam voltage have a greater impact on argon content and O/Hf ratio. When the argon content in the coatings is lower and the O/Hf ratio is higher, the laser damage thresholds of the HfO2 single layers are higher.

Comparative Study on Optical Properties and Scratch Resistance of Nanocomposite Coatings Incorporated with Flame Spray Pyrolyzed Silica Modified via in-situ Route and ex-situ Route

A new type of transparent scratch resistant coatings including in-situ modified SiO2 （g-SiO2） in flame spray pyrolysis （FSP） process was prepared. The maximum content of g-SiO2 in the coating was 15 wt%, which is higher than that of SiO2 modified by traditional wet chemical route （I-SiO2, only 10 wt%）. The results of transmission electron microscopy have demonstrated that in-situ surface modified g-SiO2 particles dispersed well with smaller agglomerates in the final coating, which was much better than the particles modified via wet chemical route. Visible light transmittance and haze tests were introduced to characterize the optical quality of the films. All coatings were highly transparent with the visible light transmittance of above 80%, especially for coatings containing g-SiO2, which exhibited slightly higher visible light transmittance than l-SiO2 embedded one. The haze value of coatings incorporated with 15 wt% g-SiO2 was 1.85%, even lower than the coating with 5 wt% I-SiO2 （haze value of 2.09%）, indicating much better clarity of g-SiO2. The excellent optical property of g-SiO2 filled coatings was attributed to the good dispersion and distribution of particles. Nano-indention and nano-scratch tests were con- ducted to investigate the scratch resistance of coatings on nano-scale. The surface hardness of the coatings rose by 18% and 14%, and the average friction coefficient decreased by 15% and 11%, respectively, compared to the neat coat due to the addition of 10 wt% g-SiO2 and I-SiO2. The pencil hardness of the coating with 15 wt% g-SiO2 increased from 2B for the neat coating to 2H. However, the pencil hardness of coating with 10 wt% I-SiO2 was only H. The results showed that the g-SiO2 embedded coatings exhibited higher scratch resistance and better optical properties.

Postprocessing treatments to improve the laser damage resistance of fused silica optical surfaces and SiO_2 coatings

The combination of deep wet etching and a magneto-rheological finishing （MRF） process is investigated to simultaneously improve laser damage resistance of a fused-silica surface at 355 nm. The subsequently deposited SiO2 coatings are researched to clarify the impact of substrate finishing technology on the coatings. It is revealed that a deep removal proceeding from the single side or double side had a significant impact on the laser-induced damage threshold （LIDT） of the fused silica, especially for the rear surface. After the deep etching, the MRF process that followed does not actually increase the LIDT, but it does ameliorate the surface qualities without additional LIDT degradation. The combination guarantee both the integrity of the surface＇s finish and the laser damage resistance of the fused silica and subsequent SiO2 coatings.

Optical property of an antireflection coating fabricated by an optimal spin-coating method with a pH-modified SiO2 nanoparticle solution

An antireflection （AR） coating is fabricated by applying an optimal spin-coating method and a pH-modified SiO2 nanoparticle solution on a cover glass. Because the pH value of the solution will affect the aggregation and dispersion of the SiO2 particles, the transmittance of the AR-treated cover glass will be enhanced under optimal fabricated conditions. The experimental results show that an AR coating fabricated by an SiO2 nano- particle solution of pH 11 enhances the transmittance approximately by 3% and 5% under normal and oblique incident conditions, respectively. Furthermore, the AR-treated cover glass exhibits hydrophobicity and shows a 65% enhancement at a contact angle to bare glass.

Sol-gel preparation of a silica antireflective coating with enhanced hydrophobicity and optical stability in vacuum

A new silica antireflective coating with improved hydrophobicity and optical stability in a vacuum is obtained by a two-step route. Firstly, silica sols are prepared with a sol-gel process, in which tetraethyl orthosilicate is utilized as a precursor. And by introduction of fluorine containing glycol into the sols, the porosity of silica particles and surface polarity of the coatings are decreased. Afterward, coatings are constructed with low surface roughness by modification of PMBA-PMMA. The coatings retain transmission of up to 99.6%, and laser damage threshold of about 50 J/cm^2 at a wavelenth of 532 nm （1-on-1. 10 ns）

A novel optical multilayer hydrophone with a triangular pyramid substrate

A novel concept for an optical multilayer ultrasonic hydrophone with the sensing film deposited on a triangular pyramid glass substrate is proposed. Using the calculation model for the spectral coefficients＇ derivatives of a dielectric multilayer optical coating, the acousto-optic sensitivity characteristic of the hy- drophone is analyzed with different measurement laser polarizations and incident angles. We present a reasonable method and adjusting strategy for the optimum working point selection of the ultrasound mea- surement. Analytic results show that the novel hydrophone possesses all the other merits of a plate glass substrate optical multilayer hydrophone but with improved detection sensitivity. A longer measurement time without distortion decreases the difficulty of high frequency signal circuits. Spatial split of the ultra- sound signal caused by the substrate＇s triangular pyramid roof simplifies the spatial spot area correction, which contributes to the accurate calibration of the hydrophone＇s wideband frequency response.

Variation of coating transmission under intense laser irradiation

A model that considers both thermal expansion and thermo-optical effects is developed to investigate the trans- mission variation of optical coatings when they are exposed to an intense laser beam. Our results indicate that a higher gradient of the transmission spectrum curve at a certain wavelength leads to a more evident variation of the coating transmission. Three customized HfO2-SiO2 multilayer coatings with different transmission spectra are used to measure the transmitted power under the irradiation of a 1080 nm continuous-fiber laser. Excellent agreement is found between the experimental result and the theoretical prediction. Our result is helpful for the improvement of such devices in the application of high-power laser systems.

Intensity modulated silver coated glass optical fiber refractive index sensor

Miniature optical fiber sensors with thin films as sensitive elements could open new fields for optical fiber sensor applications. Thin films work as sensitive elements and a transducer to get response and feedback from environments, in which optical fibers act as a signal carrier. A novel Ag coated intensity modulated optical fiber sensor based on refractive index changes using IR and UV-Vis （UV-visible） light sources is proposed. The sensor with an IR light source has higher sensitivity compared to a UV-Vis source. When the refractive index is en- hanced to 1.38, the normalized intensity of IR and UV-Vis light diminishes to 0.2 and 0.8. respectively.

Evolution of stress in evaporated silicon dioxide thin films

The evolution of stress in evaporated SiO2, used as optical coatings, is investigated experimentally through in situ stress measurement. A typical evolution pattern consisting of five subprocedures （thin film deposition, stopping deposition, cooling, venting the vacuum chamber, and exposing coated optics to the atmosphere） is put forward. Further investigations into the subprocedures reveal their features. During the deposition stage, the stresses are usually compressive and reach a stable state when the deposited film is thicker than 100 nm. An increment of compressive stress value is observed with the decrease of residual gas pressure or deposition rate. A very low stress of-20 MPa is formed in SiO2 films deposited at 3×10^-2 Pa. After deposition, the stress increases slightly in the compressive direction and is subject to the stabilization in subsequent tens of minutes. In the process of venting and exposure, the compressive component increases rapidly with the admission of room air and then reaches saturation, followed by a logarithmic decrement of the compressive state in the succeeding hours. An initial discussion of these behaviors is given.

Single-frequency operation of a diode-pumped green laser using multi-Brewster plates

In a diode-pumped, KTP intracavity frequency-doubled green laser, multi-Brewster plates are used to achieve high power single-frequency operation. With a simple experimental setup, single-frequency operation of the diode-pumped green laser with output of 46 and 218 mW is obtained by one and three Brewster plate, respectively.