Enhancing the capabilities of binary phase only filter

In this paper, we demonstrate that the capabilities of a binary phase-only filter (BPOF) can be enhanced to identify targets irrespective of rotation, scale or the imaging spectral band by utilizing the concept of log-polar transform and image fusion. Till date, BPOFs have been considered to be the simplest of all filters and incapable of identifying distorted images or images of different spectral bands like the visible or infrared (IR) bands. The novelty of this work lies in the approach adopted to demonstrate that a BPOF is equally capable of distortion-invariance like any other distortion-invariant complex matched filter. This is done by suitably fusing the images of visible and IR bands and then taking the log-polar transformation of the fused image to synthesize the BPOF. A single BPOF is thus sufficient to identify (0-360)° in-plane rotated images, (50-190)% scaled images, combination of rotation and scale changes of the target, noisy image of both the visible and IR spectral bands. A further enhancement of the correlation peak intensity (CPI) is achieved by modifying this BPOF with Mexican-hat wavelet. The designed filter was implemented in the hybrid digital-optical correlation scheme. Correlation peak intensity and peak correlation energy (PCE) have been calculated as metrics of goodness of the proposed approach. Experimental results have been presented.     

Analysis of polarization-independent tunable optical comb filter by cascading MZI and phase modulating Sagnac loop

A novel scheme of tunable optical comb filter constructed by cascading a polarization-swapping MZI and a phase modulating Sagnac loop is proposed. The key features of this comb filter design are its polarization-independence and high speed in wavelength tuning. The wavelength tunability can be readily achieved by incorporating an electro-optic phase modulator into the Sagnac loop. In addition, the filter can be made amplitude adjustable as well. The filter will be desirable for multi-wavelength fiber lasers, multi-channel optical signal processing and optical WDM systems.     

Ultra-fast tunable optical filters for WDM networks

One missing component in the next generation optical communications link is an ultra-fast tunable filter. This is a crucial element in dense wavelength division multiplexing systems. Here, a novel fiber loop configuration is proposed, which operates as such a filter. The filter implementation is based upon currently available technologies. It is shown that with the suggested technique, optical selective elements can be generated with a tuning speed of a few nanoseconds. This is three orders of magnitude faster than other currently available filters. The filter can be designed as compact and is immunized to external instabilities. Using compound configurations, the filter can be designed with a very wide range of filter finesse and free spectral range.     

Performance analysis of dispersion compensating optical FIR filters designed using a quasi-analytic synthesis approach

In this paper we investigate the performance of a quasi-analytic synthesis approach for dispersion compensating optical finite impulse response filters. It is shown by eye opening penalty and optical signal to noise ratio penalty calculations that with this method the filters dispersion can be adjusted reliably and with low effort to compensate for a specific amount of residual chromatic dispersion in optical transmission systems. The maximum attainable dispersion is then only limited by the filter order and the free spectral range rather then by optimization problems within the filter design.     

Spectral characteristics of non-collinear acousto-optic tunable filters

The bandpass width and the diffraction efficiency of a non-collinear acousto-optic tunable filter are considered as functions of the optical wavelength for fixed values of the direction of propagation of the optic wave, the length of the acousto-optic interaction and the power density of the controlling RF-signal. A comparison between the calculated and the experimentally measured spectral dependencies is discussed and illustrated from the viewpoint of practical application for a TeO₂ filter. It is shown that in the approximation used, satisfactory agreement can be achieved between the experimental and calculated dependencies.     

Angular and power characteristics of noncollinear acousto-optic tunable filters

The angular and the power characteristics of noncollinear acousto-optic tunable filters are investigated both experimentally and theoretically. The dependencies of the acoustic wave frequency, the bandpass width and the diffraction efficiency of the filter are studied as functions of the angle of incidence of the optical wave and of the acoustic signal power density. These dependencies are illustrated and tested on the basis of a realized such a filter based on tellurium dioxide (TeO₂). A good agreement of theory and experiment is demonstrated, which indicates that the approach used can be effective in designing such devices.     

1 × N add-drop filter structures using one dense wavelength division multiplexing thin-film filter

This paper proposes 1 × N add-drop filter structures in which only one thin-film filter (TF) is used. Our key idea is based on a combination of an angle-multiplexing concept and the flexibility of the optical fiber to allow a multiwavelength optical beam hit the TF several times, each time at a different angle but same position. Due to the TF angle sensitivity, the desired wavelength optical beam corresponding to the incident angle is therefore spatially isolated from the main optical beam. Our first TF-based 1 × N add-drop filter structure is arranged in a reflective design in which N wavelength optical beams can be dropped out from the main channel. For our transmissive architecture, N − 2 channels are directed to their associated output terminals while the remaining λ_N−1 and λ_N wavelength optical beams are sent out at the same port. Experimental proof of concept for our reflective TF-based 1 × 3 add-drop filter using one off-the-shelf TF, a triple fiber-optic collimator, and an optical circulator separates two wavelength optical beams with their channel spacing of 0.8 nm from the main channel. In this case, measured optical losses of 0.67 dB, 1.66 dB, and 2.59 dB are obtained for the first, the second, and the remaining dropped wavelength optical beams, respectively. Optical crosstalk and polarization dependent loss of <−18 dB and <0.08 dB are also investigated, respectively.     

含有光纤光栅的萨尼亚克环的透射特性分析

详细分析了由光纤光栅和萨尼亚克（Sagnac)环构成的透射滤波器的特性,给出了在均匀光纤光栅的情况下,该滤波器透过率的解析公式,结果表明该滤波器的透射光谱是余弦函数调制后的光纤光栅反射光谱,当光纤光栅是“强”耦合时,该滤波器可以构成多通带滤波器,而当纤光栅为“弱”耦合时,该滤波器可以构成单通带滤波器,这些滤波器可应用于现代光纤通信系统中。     

一种通道数可变的光纤光栅梳状滤波器

利用级联长周期光纤光栅与具有宽带反射功能的啁啾光纤光栅,构成了一种反射型的光纤光栅梳状滤波器.由于光在输出前经过两次干涉滤波作用,所以该梳状滤波器的消光比与透射输出型(即反射前)相比,增加了一倍.而且,通过调节啁啾光纤光栅的反射带宽,可以改变滤波器输出信道的数目.实验中利用弹性梁弯曲的方法,获得了两个可调的光纤光栅梳状滤波器,其信道数可分别在1～5和3～9之间的奇数间调谐(如果改变啁啾光纤光栅的初始中心波长,也可以获得偶数信道间的调谐).     

光子晶体超窄带滤波器

本文利用光学传输矩阵法研究了一种类似于谐振腔结构的光子晶体.研究发现在这种光子晶体的光子带隙的中央存在一个极窄的透光窗口,窗口的宽度在红外波段1500nm附近可以做到0.0001nm以下,窗口内的透光率峰值可以接近100%,窗口以外的区域的透光率可以做到0.01%以下.通过改变所研究结构中的周期厚度、中间夹层厚度、两边光子晶体的周期重复数和波的入射角可以改变窄带透过窗口的位置和窄带的宽度.这种光子晶体可以用来制作超窄带滤波器,有望在光通信超密度波分复用技术和光学信息精密测量技术当中获得应用.     

复振幅光瞳滤波器的三维超分辨性能研究

超分辨技术中现有的纯振幅型或纯相位型光瞳滤波器,大部分只能实现轴向或横向超分辨而不能实现三维超分辨,三维超分辨在三维成像系统中有着重要的作用。因此为提高成像系统中的三维分辨能力,设计了一种复振幅光瞳滤波器,并对其三维超分辨性能进行了研究。详细分析了该光瞳滤波器的第一区半径和透射率对施特雷尔比、轴向和横向超分辨因子以及旁瓣能量的影响。通过一系列的模拟证明,借助于复振幅光瞳滤波器可以实现三维超分辨。该滤波器的优点是容易实现三维超分辨,且有比较高的施特雷尔比,缺点是三维超分辨的实现总是伴随着旁瓣能量的增加,但可以采用共焦扫描成像系统来加以抑制。     

Tunable all-optical microwave notch filter with a negative tap based on a semiconductor optical amplifier and a dispersive medium

A tunable all-optical microwave notch filter with a negative tap based on a semiconductor optical amplifier and a dispersive medium is demonstrated. A negative tap is realized through the cross gain modulation in the semiconductor optical amplifier. The free spectrum range of the notch filter can be tuned by tuning the wavelength of one of the two laser sources. The influence of the amplitude mismatch of the two lasers on the performance of the filter is analyzed. The present notch filter is easy to implement and cost-effective.     

Demonstration of high extinction ratio modal interference in a two-mode fiber and its applications for all-fiber comb filter and high-temperature sensor

We show that high extinction ratio (>20 dB) modal interference in a two-mode dispersion compensating fiber can be utilized to build a compact, easy-to-fabricate tunable all-fiber optical comb filter. Wavelength tunability over the full free spectral range of the comb filter is demonstrated with an electrical power of 115mW using an on-fiber thin film micro-heater deposited directly on the fiber. In another configuration, the comb filter is used as a temperature sensor with dynamic range of >300 °C and sensitivity of <0.1 °C. The temperature sensor is capable of measuring a temperature as high as 500 °C.     

Broad omnidirectional high-precision filters design using genetic algorithm

An application of the genetic algorithm in designing omnidirectional optical filters is reported in this paper. Concerning different periodic numbers and thickness ratios in the heterostructure, we gave some optimization examples and finally achieved a photonic heterostructure with very broad omnidirectional filter bandwidth as well as a very narrow transmission window. And it is found that when the normal incident beam is tilted at a negligibly small angle, the perfect transmittance peak will vanish. Hence, this heterostructure can be regarded as omnidirectional high-precision filters with potential application in optical filters, optical switches, and many other optical telecommunication areas.     

Continuously tunable photonic microwave filter based on a spatial light modulator

An optical microwave filter architecture which can be continuously tuned is proposed and demonstrated. The architecture is based on a nematic liquid crystal spatial light modulator in parallel configuration and has the potential to control a large number of taps. Proof-of concept experimental results are provided.     

基于啁啾相移光纤光栅的滤波器设计及应用

相移光纤光栅作为透射型带通滤波器件降低了使用成本和插入损耗.在光通信和传感领域有着较高的应用价值.啁啾相移光纤光栅的透射峰波长和数量由相移点的位置和数量来确定,透射峰特性受相移量的影响.采用分段切趾的方法获得满足光通信要求的滤波特性,利用传输矩阵法分析了切趾前后透射峰特性的变化,结果表明:切趾改善了滤波器的幅度和相位响应,依据滤波器的设计要求选取不同的切趾函数以及切趾比例;切趾还大大减弱了透射峰波长和深度对相移量的敏感度,适当切趾后相移量的影响可以忽略.并提出一种应用双通道啁啾相移光纤光栅滤波器产生脉冲毫米波副载波信号的方案.     

Synthesis algorithm of a multi-channel lattice-form optical delay-line circuit

A novel synthesis algorithm for multi-channel (M?2) lattice form optical delay-line circuit is presented in this paper. This circuit offers multi-port FIR optical filter with delay time of N Δτ (Δτ: unit delay time). Synthesis algorithm is based on division of total transfer matrix into unit blocks. Developed method confirms that 1×M optical delay-line circuit offers same transmission characteristics as 1×M FIR digital filter. Band-pass flat group delay type filter is considered as an example in this paper. It is also confirmed that proposed delay-line circuit can realize 100% power transmittance.     

薄膜截止滤光片的消偏振设计

薄膜截止滤光片在倾斜入射时不可避免地会产生s和p二个偏振分量的分离,因而在许多应用,特别是光通讯的应用中成为一个棘手的难题。提出了一种新的设计方法,对最常用的45°入射角,实现了长波通和短波通两种截止滤光片的完全消偏振, 在透射率为50%处,其偏振分离分别为0.3 nm和0.1 nm。基本的设计方法是采用宽带法布里珀罗薄膜干涉滤光片中心波长两侧的干涉带作为长波通或短波通截止滤光片的初始膜系,然后经过适当的优化以提高透射带的透射率。宽带干涉滤光片的间隔层常由半波长厚度的高、低折射率混合膜层组成,如2H2L2H或2L2H2L。由于这种设计的截止区和透射带带宽常嫌不足,故提出了展宽截止区和透射带的方法。对一个典型的短波通截止滤光片,在波长1550 nm,截止区和透射带宽均达到了200 nm。这种设计方法不仅简单、性能优良,而且膜厚控制容差较大,故易于制造。     

四元量化的综合鉴别函数滤波器实现比例不变相关识别

设计一种基于综合鉴别函数(SDF)的比例不变相关识别滤波器，将频域中被SDF调制的相关输出函数进行复数形式的四元量化（QFCQ），并采用叠代算法调整SDF的系数，使输出的相关峰尖锐，区分能力强，同时满足不同比例图像等相关峰输出的准则，从而达到了比例不变识别的要求.计算机仿真实验对属于训练图像系列的目标图像进行了识别，并与基本的SDF及二元化纯相位SDF做了比较，结果表明该方法具有较好的识别效果.     

Design of an optical filter using photonic band gap material

The article presents a design of an optical filter using Photonic Band Gap (PBG) materials in the visible and infrared region of the electromagnetic spectrum. This idea is based on the famous Kronig-Penny model in the band theory of solids. The suggested filter may work at any range of the wavelength that can be achieved by choosing the appropriate values of the controlling parameters. This structure is also able to pass the light emitted by the Ruby laser and this acts as a monochromator. Also, we have studied the anomalous behavior of refractive index for the structure having alternate layers of air and GaAs.