Design of binary long-period fiber grating filters by the inverse-scattering method with genetic algorithm optimization

An approach is presented to the design of binary long-period fiber grating (LPFG) filters based on the Gel'fand-Levitan-Marchenko (GLM) inverse-scattering method and genetic algorithm optimization. The nonuniform coupling strength of the binary grating can be realized by varying the local duty ratio. A coupled-mode theory combined with the Poisson sum formula for treating the binary index perturbation is developed for the application of the GLM synthesis method. Since the coupled-mode theory, which smears out the discrete coupling nature, can be regarded only as an approximation to the modeling of a binary LPFG, we use instead the transfer-matrix model to analyze the coupling behavior of a nonuniform binary LPFG. Based on the synthesized grating patterns from the GLM method, a real-coded genetic algorithm with the transfer-matrix model is used to compensate for the discrepancies resulting from use of the coupled-mode theory and to optimize the design. We exemplify the above procedure by designing a flatband LPFG filter and a high-visibility all-fiber Mach-Zehnder filter.     

Making optical low-pass spatial filters by thin-film phase grating

A modified rule was applied to design a grating optical low-pass spatial filter (GOLF) with a thin-film phase grating. This filter differed from the crystal type of optical low-pass spatial filter (OLPF). The fabrication method of the thin-film phase GOLF was also different from the conventional lithographic process. A new fabrication method utilizing a metal mesh mask and a thin-film coating process was applied to make the GOLF. The process had the advantages of being simple and cheap, and would thus be good for mass-production. The modulation transfer function (MTF) of the device was measured and the results agree well with the theoretical calculations. Finally, the GOLF was installed on a digital camera which was used to capture images of the circular zone plane (CZP), to show the Moiré pattern suppression ability.     

Application of thin-film optical filters to the temperaturecompensation of optical fiber grating-based devices

A major problem currently affecting the implementation of in-fiber refractive-index grating-based optical fiber devices is the drift in wavelength modulation due to the change in the ambient temperature. For accurate and reliable long-term operation of these devices, suitable temperature compensation techniques are a necessity. This paper presents a novel temperature compensation technique for in-fiber refractive index grating-based devices and components. The proposed technique is based on the temperature-dependent spectral characteristics of a dielectric multilayer thin-film interference filter fabricated on the endface of refractive-index grating impressed optical fiber. Temperature compensation is achieved by comparing the reflected intensities at the grating-reflected and the interference filter-reflected wavelengths. The proposed scheme also compensates for light source fluctuations and lead-in fiber bending losses     

Beam displacement and distortion effects in narrowband optical thin-film filters

A three-dimensional model for beam propagation through optical interference filters is presented. The model predicts a wavelength-dependent lateral beam displacement of tens or hundreds of micrometers in narrowband filters at an angle of incidence of only 3 degrees to 5 degrees . The effects of filter bandwidth, wavelength offset, angle of incidence, and beam size are investigated. The effect is experimentally confirmed for a 100 GHz filter at a 3.5 degrees angle of incidence.     

Efficient evolutionary algorithm for the thin-film synthesis of inhomogeneous optical coatings

We propose an efficient evolutionary approach for the thin-film synthesis of inhomogeneous optical coatings. The proposed approach consists of global and local strategies by integration of decreasing-based mutations and self-adaptive mutations by means of family competition and adaptive rules. Numerical results indicate that the proposed approach performs robustly and is competitive with other approaches. Our approach, although somewhat slower, is flexible and can easily be adopted to other application domains. Our approach is also able to generate homogeneous solutions with two materials available.     

Tunable phase-only optical filters with a uniaxial crystal

A novel method of fabricating phase-only optical filters that is based on the properties of a uniaxial crystal is proposed. With these optical filters, the phase differences are tunable among the different filter zones. Many focal patterns can be obtained if these optical filters are placed in front of a lens; furthermore, these optical filters can also be used to make up for the distortions in fabrications in which they were used only as untunable optical filters.     

Mechanically tunable optical filters with a microring resonator

A new type of optical filter based on mechanical tuning and a microring resonator is proposed. The proposed filter is expected to consume much less standing power compared to the conventional thermo-optic and carrier-injection tunable filters. In this work, two methods are used to prove the concept of the proposed device: (1) the analytical method and (2) the finite-difference time-domain method. The dependence of the filter characteristics on some of the device parameters is studied as well.     

Local characterization of fiber-Bragg gratings through combined use of low-coherence interferometry and a layer-peeling algorithm

The technique presented here allows us to obtain an accurate determination of the refractive index modulation amplitude, the mean effective index, and the chirp of fiber-Bragg gratings. A layer-peeling algorithm is used to extract this information from low-coherence interferometry measurements. Finally, we present a systematic study over 10 uniform and chirped gratings to proof the reliability and accuracy of this technique.     

Fabrication methods of optical resonant filters with a close-to-rectangle filtering profile

The potential applications of optical resonant filters for optical communication systems can be significant. Three approaches to achieve a close-to-rectangle filtering profile for optical resonant filter were investigated and proven experimentally. The combination of the filtering sharpness, which comes from the resonance nature, and a flattened filtering window realized the close-to-rectangle filtering profile that can enhance the bandwidth efficiency.     

Three-dimensional optical correlator with general complex filters

A new type of electro-optical three-dimensional (3-D) correlator is proposed and demonstrated. A 3-D object scene, observed by multiple cameras from several points of view, is correlated with a 3-D complex computer-generated function. This correlator is a hybridization of the joint transform and the VanderLugt correlators, and, as such, it allows correlations to be made between 3-D real-world objects and 3-D general complex functions. Experimental results are presented.     

Center wavelength shift dependence on substrate coefficient of thermal expansion for optical thin-film interference filters deposited by ion-beam sputtering

Single-layer films of Ta2O5 and multilayer thin-film filters of Ta2O5 and SiO2 were deposited by ion-beam-sputter deposition. Postdeposition annealing of the structures resulted in increased optical thickness of the films, resulting in an upward shift in the wavelength of the transmission-reflection spectra. Modeling of the single-layer films by means of the effective media approximation indicates an increase in the void fraction of the film after annealing. This increase is consistent with an observed decrease in refractive index and an increase in physical film thickness. The multilayer structures, deposited on substrates of varying coefficient of thermal expansion (CTE), were annealed at various temperatures, and the change in the center wavelength was measured. The measured change is dependent on the annealing temperature and the substrate CTE, indicating that the increase in the void fraction is caused in part by thermally induced stress during the annealing process. A simple model is proposed that relates the void fraction present in the films after annealing with the substrate CTE and the annealing temperature.     

Temperature stability of thin-film narrow-bandpass filters produced by ion-assisted deposition

Four types of single-cavity, thin-film, narrow-bandpass filter whose full width at half-maximum ranges from 0.5 to 1.1 nm are produced by ion-assisted deposition of alternating TiO(2)/SiO(2) or Ta(2)O(5)/SiO(2) layers upon eight substrates having differing coefficients of linear expansion, and the temperature stability of their center wavelengths is examined in the 1540-nm wavelength region. The temperature stability is shown to be greatly dependent on the coefficient of linear expansion of the substrate upon which the filter is deposited. For the eight substrates whose coefficients of linear expansion range from 0 to 142 × 10(-7)/°C, the temperature stability of the filters ranges from +0.018 to -0.005 nm/°C. Calculations based on a newly developed elastic strain model reveal that the main reason temperature stability of the center wavelengths exhibits substrate dependency is due to a reduction in film packing density brought about by volumetric distortion of the film, which is caused by stress induced from the substrate.     

Dominant wavelength sensitivity of thin-film color filters to spectral centering

The dominant wavelength shift of 16 color filters is presented for a ±1% theoretical change in spectral centering and compared with the threshold-perceived color difference (dominant wavelength). The resulting change in dominant wavelength exceeds a threshold limit of a just-noticed color difference for some filters. Spectral-centering tolerance limits are given for the dominant wavelength to be at the threshold limit. The change in dominant wavelength is also presented from normal incidence to 60° angle of incidence, with incidence-angle limits and corresponding f-numbers for optical systems. For comparison, the color difference Δ E*(uv), from the L*u*v* color space, is calculated for each filter for ±1% spectral centering.     

光学镀膜仪的监控稳定性研究

光学镀膜仪的膜厚监控稳定性影响着镀膜的合格率，并且对镀膜经验控制，膜系修正和自动化系统开发有重要的意义。对真空镀膜仪进行4种薄膜系列的膜厚监控实验，建立一套稳定性评价体系，获取镀膜仪的稳定性指标。将修正的方差分析方法、不确定度分析方法、标准偏差与相对偏差方法和线性回归方法等综合起来，用6项指标从各个方面对光学薄膜厚度监控的稳定性做出评判，甚至对膜厚控制精度和特征波长做出分析。实验结果表明：上述的综合评价体系是合理的；多层膜比单层膜的监控稳定性稍差一些，膜层材料对稳定性的影响比较大，膜厚控制精度的标准偏差≤0．45％；薄膜层数对膜厚控制精度没有明显的影响，甚至多层膜比少层膜的控制精度要高，出现了“中心层”效应；用线性回归方法可以估计实际镀膜的特征波长长移量。     

Fourier transform technique with frequency filtering for optical thin-film design

A Fourier analysis of existing and potentially complex refractive index profiles is often useful in the resolution of thin-film design problems. Applications of a frequency-filtering technique, which consists of the suppression of undesired spatial frequencies from the refractive index profiles, are described. A new, extremely simple antireflection coating synthesis method based on this concept is proposed.     

Wavelength-division multiplexed thin-film filters used in tilted incident angles of light

The properties of wavelength-division multiplexed (WDM) narrowband filters used in tilted collimated light are described. The wavelength shift for s polarization in a low-index-spacer filter is larger than that for p polarization when the filter is tilted, but it is smaller in a high-index-spacer filter. Therefore the passbands of the tilted filter can be centered at the same wavelength for two polarization modes by use of high- and low-index materials, as appropriate, as spacers or by selection of a moderate-index material instead of a spacer. With such spacers, WDM filters used for incident angles of 20 degrees or even greater are constructed. Experimental results agree with the computed ones.     

Optical thin-film reflection filters based on the theory of photonic crystals

Based on the theory of photonic crystals and the framework of a single-channel reflection filter that we presented before, structures of reflection filters with multiple channels are proposed. These structures can overcome some drawbacks of conventional multichannel transmission filters and are much easier to fabricate. We have practically fabricated the reflection filters with two and three channels, and the tested results show approximate agreement with theoretical simulation. Moreover, the superprism effect is also simulated in the single-channel reflection filter, the superiorities to transmission filters are discussed, and these analyses may shed some light on new applications of reflection filters in optical communication and other systems.     

Optical filters with prescribed optical thickness and refined refractive indices

We propose to refine the refractive index of the layers composing optical filters while keeping their optical thicknesses constant. Using this technique, one can optimize filters made of quarter-wave layers using conventional optimization techniques, while preserving the possibility to use turning-point monitoring during their fabrication. Application of this method to the design of a dual narrowband filter and a tilted edge filter demonstrates its effectiveness.     

InP薄膜内包层光纤放大性的理论计算

将纳米技术和光纤技术相结合，对研制出的纳米级InP薄膜内包层光纤进行光放大性能的测试，测试结果表明该光纤在很短的长度上具有比较好的放大性能，单位长度放大系数在906-1044nm的波段上为1．40-5．12dB／m，在1080-1491nm的波段上为1．40-15．35dB／m，在1524-1596nm的波段上为1．86-7．44dB／m。根据氢原子模型计算了InP微粒产生量子尺寸效应的相对粒径αB=8．313nm，以及纳米级InP微粒的粒径与能级改变的关系得到，当厚度d=16nm，△E=0.071eV,满足当d≤2aB时的强限域，量子尺寸效应明显，光纤具有光放大性,计算结果与实验测试结果相符。     

Convergence properties of a quadratic approach to the inverse-scattering problem

The local-minima question that arises in the framework of a quadratic approach to inverse-scattering problems is investigated. In particular, a sufficient condition for the absence of local minima is given, and some guidelines to ensure the reliability of the algorithm are outlined for the case of data not belonging to the range of the relevant quadratic operator. This is relevant also when an iterated solution procedure based on a quadratic approximation of the electromagnetic scattering at each step is considered.