Influence of Temperature on the Optical Properties of Narrow-band Interference Filters

For narrow-band interference filters, we generally observe a small variation of peak wavelength with time; this variation of filter characteristics depends to a large degree on the materials of which these filters are made, and on the manufacturing process. Several explanations have been given to take account of this effect, which is particularly dramatic in the case of very narrow-band filters, so it is of the utmost importance to study in detail the influence of parameters such as the shift of temperature within the plant during deposition. The aim of this work is to know how to produce filters whose optical characteristics are steady with time. A significant advance in the study of this problem has been obtained by simulating the monitoring process on a computer. However, it is necessary, for each material used, to determine the dilatation coefficients of both refractive index and thickness. In this work we show how these coefficients may be deduced from measurements of shifts with temperature of the peak wavelength of complete narrow-band filters of two different designs. A theoretical study of the effect of temperature on the position of the passband of a dielectric narrow-band filter is performed, and from experimental measurements on complete filters we estimate the dilatation coefficients of the optical thicknesses of the component materials. The results obtained are slightly different from those given by other authors; this can be easily explained because the approximations made in their calculations of the dilatation coefficients are not accurate enough. Good agreement is obtained if an accurate check is performed from their experimental results.     

Error Compensation Mechanisms in Some Thin-film Monitoring Systems

Theoretical techniques which were developed for the study of errors and error compensation in the turning value monitoring of thin-film narrow-band filters are applied to the monitoring of broad-band components, especially anti-reflection coatings and edge filters. The technique of pre-coating the test plate in semi-direct monitoring is examined. It is shown that in the turning value monitoring of broad-band components the most important factor is the small consistent overshoot made by experienced plant operators and that, to be effective, a pre-coating must minimize the interactions between errors caused by the successive overshoots. Techniques for predicting the magnitudes of such interactions are given. The stability of an alternative technique for edge filters, that of semi-direct level monitoring, is considered and the way in which pre-coatings can improve the accuracy of edge position demonstrated.     

FRM滤波器中应用积分梳状滤波器的研究

传统的频率响应屏蔽(Frequency Response Masking,FRM)滤波器由多个含有乘法器的滤波器构成,计算复杂度较高。提出了一种改进窄带低通FRM数字滤波器设计复杂度的方法,使用积分梳状滤波器作为屏蔽滤波器,以达到设计窄过渡带的滤波器的要求。仿真结果表明,在降低窄带低通FRM滤波器计算复杂度方面效果明显,有效地减少了乘法器的使用,降低了目标滤波器设计的阶数。该方法可以应用于窄过渡FRM数字滤波器的设计中。     

Postfabrication optimization of an autoregressive planar waveguide lattice filter

Narrow-band planar waveguide filters are sensitive to fabrication errors that make it difficult to realize multistage filters successfully and maintain tight center wavelength tolerances unless a tuning technique is available. An analysis algorithm combined with waveguide heaters has been successfully used to demonstrate that fabrication errors can be compensated in an autoregressive (AR) lattice architecture that makes it possible to concatenate conveniently multiple stages and to achieve higher-order filter functions. Compensation of fabrication errors on filter functions for a second- and third-order AR lattice filter is presented, and issues related to the application of the analysis algorithm are discussed. In combination with postfabrication tuning of the coupling ratios, the analysis algorithm will allow complete control of the filter function after fabrication.     

Application of in situ ellipsometry in the fabrication of thin-film optical coatings on semiconductors

Thin-film interference filters, suitable for use on GaAs- and InP-based lasers, have been fabricated by use of the electron-cyclotron resonance plasma-enhanced chemical vapor deposition technique. Multilayer film structures composed of silicon oxynitride material have been deposited at low temperatures with an in situ rotating compensator ellipsometer for monitoring the index of refraction and thickness of the deposited layers. Individual layers with an index of refraction from 3.3 to 1.46 at 633 nm have been produced with a run-to-run reproducibility of 0.005 and a thickness control of 10 A. Several filter designs have been implemented, including high-reflection filters, one- and two-layer anitreflection filters, and narrow-band high-reflection filters. It is shown that an accurate measurement of the filter optical properties during deposition is possible and that controlled reflectance spectra can be obtained.     

Direct optical monitoring on the rotating substrate holder

Single wavelength optical monitoring is frequently applied in large box coaters. In state of the art optical coating systems a testglass changer with stationary test glasses is used for thickness monitoring while the substrates are located on a rotating substrate holder. The accuracy of this so called indirect monitoring technique is mainly limited by variations of the tooling factor. Direct monitoring on a substrate which is located on the rotating substrate holder avoids these errors and takes advantage of automatic thickness error compensation effects. The paper reports substantial progress which has been achieved for coating systems with large area substrate holders (up to Ø 1500 mm). Intermittent monitoring on a substrate or a witness was applied. This technique enables rapid prototyping with tight specifications and high yields in large area batch coaters. Application results of challenging optical multilayer systems are demonstrating clearly the potential of this powerful monitoring technique. The monitoring capability was investigated for a lot of different layer systems such as dielectric mirrors, anti‐reflection coatings, sophisticated edge filters, polarizer coatings, beam splitters multiple cavity band‐pass filters and notch filter coatings. Strong coincidence of theory with experiment was achieved with plasma ion assisted deposition (PIAD) and magnetron sputtering. Reproducibility experiments have clearly shown the benefits of this monitoring technique.     

Global sensitivity analysis of bandpass and antireflection coating manufacturing by numerical space filling designs

We present the effectiveness of global sensitivity analyses of optical coatings manufacturing to assess the robustness of filters by computer experiments. The most critical interactions of layers are determined for a 29 quarter-wave layer bandpass filter and for an antireflection coating with eight non-quarter-wave layers. Two monitoring techniques with the associated production performances are considered, and their influence on the interactions classification is discussed. Global sensitivity analyses by numerical space filling designs give clues to improve filter manufacturing against error effects and to assess the potential robustness of the coatings.     

Effect of stress on performance of dense wavelength division multiplexing filters: thermal properties

Thin-film filters used for dense wavelength division multiplexing (DWDM) applications are processed by a variety of deposition techniques, including ion-beam sputtering. Ion-beam sputtering produces high-quality coatings and provides flexibility of coating materials. However, DWDM filters consisting of oxide films that are reactively deposited by ion-beam sputtering, as in most sputter techniques, typically exhibit high levels of compressive stress. This affects the thermal characteristics of the filters. We have identified three thermal effects: center wavelength drift with temperature, center wavelength creep, and permanent center wavelength shift. The latter two are strongly dependent on the stress state of the filter. Models are presented that support the data that were taken.     

连续激光辐照窄带滤光片软损伤研究

利用YAG激光对ZnSe/MgF2窄带滤光片进行连续辐照研究,发现滤光片的通带发生了漂移, 严重时通带完全飘离使用波长,使其光学性能完全失效,即滤光片出现软损伤。根据实验结果和温度场分布,模拟了滤光片在连续激光辐照下的温度响应和光学性能失效过程,结果表明ZnSe材料的折射率温度梯度是引起软损伤的主要因素。     

Dynamic Filters for the Spectral Analysis of Random Signals

It is proposed that the narrow-band filter in filtering spectrum analyzers be made dynamic, i.e., operate in a transient mode, with the characteristics adjusting according to given laws. The initial ratios are obtained for the optimal synthesis of those characteristics of narrow-band filters used in the direct filtering and the correlation-filtering methods. The problem of optimizing the laws governing the tuning of filter characteristics is formulated.     

Absorption in Turning Value Monitoring of Narrow Band Thin-film Optical Filters

The effects of absorption on the theoretical accuracy of the turning value method of film thickness monitoring in the production of thin-film all-dielectric narrow band optical filters are investigated. It is shown that the performance of the method is at least as good as it is in the absence of absorption, although care must be taken in the interpretation of the monitoring signals for certain critical layers.     

Interest of broadband optical monitoring for thin-film filter manufacturing

Broadband optical monitoring for thin-film filter manufacturing is more and more developed thanks to better performances of spectrometers with array detectors. We compare this optical monitoring with turning point monitoring and quartz monitoring of different designs. The sensitivity to thickness errors and to refractive index errors is evaluated. We show that real time determination of deposited thickness is a valuable criterion. We also present our experimental setup of transmittance and reflectance broadband optical monitoring. The use of a 400-1000 nm range combined with a signal-to-noise ratio of ~2500 in transmittance and 1000 in reflectance permits us to expect the manufacturing of high-performance non-quarter-wave designs. A first manufacturing of an 18-layer non-quarter-wave high-pass filter is provided.     

Compact bandpass filters in coplanar strip lines

The authors describe a compact bandpass filter design implemented in asymmetric coplanar strip lines. For initial demonstration, it was built on print circuit boards using modified two-pole Butterworth bandpass filter topology. The novel filter design does not use bonding wires, vias or any interconnecting structure. It is not based on quarter wavelength sections either. It uses the creation of an electrode pattern on the coplanar strip electrodes to emulate an equivalent circuit. The pattern is fine-tuned using full-wave simulations. Bandpass filters for 2.4 GHz ISM (Industrial, Scientific and Medical) and 5.15-5.85 GHz UNII (Unlicensed National Information Infrastructure) bands were designed, fabricated and measured. Measurement responses agree well with full-wave simulation results. The electrode pattern can be scaled for different centre frequency and improved with fine-tuning procedure. For chip-level implementation, the filter size can be greatly reduced because of much smaller minimum electrode width     

Performance of minimum-mean-square-error filters for spatially nonoverlapping target and input-scene noise

Using computer simulations, we investigate the performance of a minimum-mean-square-error filter for input-scene noise that is spatially nonoverlapping (disjoint) with a target for a limited set of images. Different input-scene-noise statistics are used to test the filter performance. We show that in the presence of spatially nonoverlapping target and input-scene noise, the output of the minimummean- square-error filter has a well-defined correlation peak, small sidelobes, and a high peak-to-correlationenergy ratio compared with other widely used filters such as the classical matched filter, the phase-only filter, and the inverse filter. We also test the robustness of the minimum-mean-square-error filter to errors in noise statistics used in the filter design. We show that, for the images tested here, the performance of the minimum-mean-square-error filter is not sensitive to errors in noise statistics and the filter can detect the target even if a considerable error exists. The discrimination capability and the illumination sensitivity of the minimum-mean-square-error filter are also tested.     

Narrow-band interference filters with unconventional spacer layers

Classical narrow-band interference dielectric filters with all-dielectric reflectors have quarter-wave stacks separated by a half-wave thickness (or a multiple-) spacer layer. These filters are essentially Fabry-Perot filters; hence the theory developed for those filters applies in full. The theory of narrow-band interference dielectric filters with unconventional spacer layers is presented. This spacer layer consists of two different materials. The optical features of these filters are compared with the features of Fabry-Perot filters. The influence of the errors of the layers on spectral characteristics is analyzed. The theory presented can be applied to any spectral range as well as to any thin-film material, including absorbing and nonlinear materials.     

Automatic defect inspection system of colour filters using Taguchi-based neural network

A coloured filter is a critical part of an LCD panel, especially to present a high quality colour display. At present, the defect detection of colour filters is conducted by manual inspection in the final product stage. However, poor detection efficiency and subjective judgment of manual inspection undermine accuracy. Therefore, this study applied image processing technology and the neural network to detect surface defects of colour filters in order to prevent losses arising from incorrect detection, lower production costs, and effectively improve yield. A back-propagation neural network (BPNN) classifier was selected to train the features. The results showed that the proposed method can be successfully applied in defect detection of colour filters to reduce artificial detection errors. In addition, the Taguchi method was used with BPNN to save time searching optimal learning parameters by the trial and error method, which achieves faster convergence, smaller convergent errors and better recognition rate. The results proved that the root-mean-square error (RMSE) of the Taguchi-based BPNN at final convergence is 0.000254, and recognition rate reaches 94%. Therefore, the proposed method has good effects in detecting the micro defects of a colour filter panel.     

Tolerances For Layer Thicknesses in Dielectric Multilayer Coatings and Interference Filters

A theory is developed for dielectric multilayer coatings in which the layers depart from calculated thickness. The theory is applied to alternating systems of quarter wave layers of ZnS and MgF₂. The effects of thickness errors are: (1) A shift of the wavelength at which maximum reflectance occurs; and (2) a change in phase shift upon reflection. The magnitude of these effects, and also their dependence on various parameters, are determined. Statistical tolerances for layer thicknesses are computed for given tolerances on the multilayer performance. The accuracy required for producing dielectric interference filters is up to about 40 times higher than the accuracy sufficient for the production of dielectric mirrors and beam splitters. Various techniques of experimentally controlling film thicknesses, and their accuracies, are discussed. The production of mirrors and beam splitters deviating from theoretical maximum reflectance by only 1 percent seems to be possible with Dufour’s simple single photocell method of monitoring film thicknesses. With more precise methods, such as those developed by Giacomo and Jacquinot, or Traub, the production of interference filters appears to be possible to within plus or minus one half their half widths.     

Acousto-optic superlattice modulation in fiber Bragg gratings

The superposition of a long-period grating and a fiber Bragg grating, which we call an optical superlattice, causes high-efficiency narrow-band reflections to be induced on either side of the Bragg wavelength. This effect was recently observed experimentally in a fiber-based acousto-optic superlattice modulator. We develop in detail the theory of optical superlattices in fiber Bragg gratings, treating both the acousto-optic and the fixed-grating cases. Applications include reconfigurable wavelength division multiplexers, fiber lasers and sensors, tunable filters, modulators, and frequency shifters.     

Ultraviolet Rayleigh-Mie lidar with Mie-scattering correction by Fabry-Perot etalons for temperature profiling of the troposphere

A Rayleigh-Mie-scattering lidar system at an eye-safe 355-nm ultraviolet wavelength that is based on a high-spectral-resolution lidar technique is demonstrated for measuring the vertical temperature profile of the troposphere. Two Rayleigh signals, which determine the atmospheric temperature, are filtered with two Fabry-Perot etalon filters. The filters are located on the same side of the wings of the Rayleigh-scattering spectrum and are optically constructed with a dual-pass optical layout. This configuration achieves a high rejection rate for Mie scattering and reasonable transmission for Rayleigh scattering. The Mie signal is detected with a third Fabry-Perot etalon filter, which is centered at the laser frequency. The filter parameters were optimized by numerical calculation; the results showed a Mie rejection of approximately -45 dB, and Rayleigh transmittance greater than 1% could be achieved for the two Rayleigh channels. A Mie correction method is demonstrated that uses an independent measure of the aerosol scattering to correct the temperature measurements that have been influenced by the aerosols and clouds. Simulations and preliminary experiments have demonstrated that the performance of the dual-pass etalon and Mie correction method is highly effective in practical applications. Simulation results have shown that the temperature errors that are due to noise are less than 1 K up to a height of 4 km for daytime measurement for 300 W m(-2) sr(-1) microm(-1) sky brightness with a lidar system that uses 200 mJ of laser energy, a 3.5-min integration time, and a 25-cm telescope.     

Comparison of antireflection surfaces based on two-dimensional binary gratings and thin-film coatings

A comparison of antireflection surfaces based on the two-dimensional binary gratings and thin-film coatings is presented. First, a two-dimensional hybrid binary grating is proposed and analyzed by use of a vector-based implementation of the rigorous coupled-wave analysis method. The optimum parameters of the structure are determined and the effects that changing them have on spectral characteristics of the structure are studied. Then this structure is compared with multilayer thin-film antireflection filters. These filters are designed by genetic algorithm and needle methods, which are powerful methods for multilayer filter design. The comparison results show that the sensitivity of the grating to changes in the incident wavelength is high. However, a reflectance of the order of 10(-3)% at the design wavelength can be achieved. The sensitivity of designed antireflection thin-film filters to wavelength changes is lower, however, and the minimum achievable reflectance is higher.