Polarisation independent tunable liquid-crystal Fabry-Perotinterferometer filter module with double-pass configuration

A novel tunable liquid-crystal Fabry-Perot interferometer (LC-FPI) filter module with a Faraday rotator and double-pass configuration has been fabricated. The filter module has a 3 dB bandwidth of ~0.4 nm, a 20 dB bandwidth of 1.5 nm, and a polarisation dependent loss (PDL) <0.8 dB over a tuning range of 56 nm. More than 49 dB isolation from the output port to the input port has been achieved     

Tunable optical filters for dense WDM networks

WDM is currently taking over as the leading technology in point-to-point transmission links. For optical implementation of WDM networks, logical functionalities such as wavelength (channel) selection should be carried out in the wavelength domain; thus, the development of dynamic optical devices is required. One key device is a tunable optical filter. Important features of such a filter include low insertion loss, narrow bandwidth, high sidelobe suppression, large dynamic range, fast tuning speed, a simple control mechanism, small size, and cost effectiveness. Here, an extensive overview of the different technologies used to produce tunable optical filters is presented. Among them, fiber filters such as fiber Bragg gratings and fiber Fabry Perot are the most commercialized, yet inherently limited in their dynamic speeds. For high demanding dynamics, micro-machined and acousto-optic filters can offer a good solution for microsecond tuning speeds. Faster tunable devices, in nanosecond tuning speeds, might emerge out of microresonators, electrooptic filters, and active DBR filters     

Superconducting Ultrahigh Q Tunable RE Preselector

The application of superconductivity to the problem of reducing radio frequency interference (RFI) is described. A lowloss, extremely narrowband superconducting filter was developed that is tunable from 6.3 to 21 MHz. This filter has an unloaded Q of 350 000 to 600 000 over the tuning band, bandwidth adjustable from 60 Hz to 50 kHz, calibrated external controls for frequency tuning and bandwidth adjustment, and good resettability and stability. An optimum circuit design provides minimum bandwidth variation with frequency tuning and minimum resonant frequency variation with changes in bandwidth. Performance tests on high-frequency receivers showed reductions in intermodulation interference susceptibility of 40 dB and improvements in FSK Teletype signals of 23 dB.     

A design of a tunable hybrid Solc filter for 40-Gbps DWDM optical systems

In this paper, a contribution for enhancing the performance of a hybrid tunable Solc filter for 40-Gbps DWDM systems is presented. By using liquid crystal cells with variable birefringence, we have achieved a wavelength tuning of 33 ITU channels, for a 100-GHz frequency spacing, from 1564.69 to 1530.32 nm via a 10-stage hybrid Solc filter. The designed device presents a tuning step of 0.1 nm by applying a voltage difference of 0.2 V. The optical cancelling technique is used to make a 3-dB bandwidth tuning between 0.31 and 1.4 nm. The channel’s bandwidth is adjusted for different kinds of 40-Gbps modulated channels spectra. The 10-stage hybrid Solc filter provides theoretically an insertion loss of 4.6 dB, by using polarizers and birefringent plates characterized by high anti-reflection coatings, while its crosstalk approximates 14 dB. The proposed filter provides very low polarization-dependent loss through the incorporation of a polarization beam splitter and a polarization beam combiner.     

采用ASK调制的波分复用系统中滤波器带宽的优化设计

提出了采用幅移键控（ＡＳＫ）调制的波分复用（ＷＤＭ）系统中光滤波器带宽的优化设计问题。并根据对长距离波分复用系统中影响信噪比的主要因素的分析,计算了优化设计问题,数值计算给出了一些有意义的结果     

Varactor-tuned combline bandpass filter using step-impedance microstrip lines

In this paper, a varactor-tuned combline bandpass filter using step-impedance microstrip lines is considered so that the absolute passband bandwidth can be maintained nearly constant within the tuning range. The difference between the odd- and even-mode characteristics of the coupled microstrip line makes it difficult to design a tunable bandpass filter with minimum degradation in passband performance. By using step-impedance microstrip lines, couplings between resonators can be controlled so that the constant bandwidth requirement could be satisfied with reasonable design parameter values. Lumped inductors are used for input and output coupling networks. Design equations are derived, and experimental results are compared with theoretical ones based on these equations.     

Calculation of penalties due to polarization effects in a long-haulWDM system using a Stokes parameter model

We derive a Stokes parameter model to calculate the penalties due to the combination of polarization mode dispersion (PMD), polarization dependent loss (PDL), and polarization dependent gain (PDG) in long-haul, dense wavelength division multiplexed (WDM) systems. In this model, we follow the Stokes parameters for the signal and the noise in each channel instead of following the full time domain behavior of each channel. This approach allows us to determine the statistical distribution of penalties with up to 10⁵ fiber realizations and 40 channels. We validate this model to the extent possible by comparison to full numerical simulations. Using this model, we find that the interaction of PMD and PDL is the major source of penalties and that the effect of PDG is negligible in WDM systems with more than ten channels     

基于波纹耦合微带线的可调谐带通滤波器

设计了一种紧凑5阶双零点可调谐带通滤波器。通过在传统梳状线滤波器中引入波纹耦合微带线结构,该滤波器可以在中心频率调谐过程中保持绝对带宽恒定,同时尺寸相较于传统方法可减小50%。谐振器中引入分布式补偿电容利于滤波器应用于高介电常数衬底的微带线结构。基于源与负载间的耦合,引入两个传输零点来提高滤波器的衰减特性。HFSS仿真结果显示,当加载的电容值为2.43 pF时,滤波器的中心频率为1.48 GHz,带宽为194 MHz,插入损耗为1.36 dB。随着加载电容值从2.15 pF增加到2.93 pF,滤波器的中心频率从1.55 GHz减小至1.37 GHz。保持绝对带宽恒定的情况下,通带内的插入损耗小于1.5 dB。     

Widely tunable chaotic fiber laser for WDM-PON detection

A widely tunable high precision chaotic fiber laser is proposed and experimentally demonstrated. A tunable fiber Bragg grating （TFBG） filter is used as a tuning element to determine the turning range from 1533 nm to 1558 nm with a linewidth of 0.5 nm at any wavelength. The wide tuning range is capable of supporting 32 wavelength-division mul- tiplexing （WDM） channels with 100 GHz channel spacing. All single wavelengths are found to be chaotic with 10 GHz bandwidth. The full width at half maximum （FWHM） of the chaotic correlation curve of the different wave- lengths is on a picosecond time scale, thereby offering millimeter spatial resolution in WDM detection.     

Tunable lumped-distributed capacitively-coupled transmission-line filter

A wide tuning range filter is proposed based on lumped-distributed capacitively-coupled transmission lines. The proposed structure maintains a constant inter-resonator coupling factor over a wide centre-frequency tuning range leading to a constant fractional bandwidth design. A bandpass filter with a two-octave tuning range has been designed, fabricated and measured.     

偏振模色散及偏振相关损耗对光传输链路相关特性的影响

在同时考虑偏振模色散(PMD)及偏振相关损耗(PDL)情况下，分别通过理论和蒙特一卡罗方法研究了光传输链路的输出信号偏振态相关函数。首先，从理论上得到了光传输链路的输出信号偏振态相关函数的解析表达式，而后，利用蒙特一卡罗方法进行了模拟研究，理论和模拟的结果能很好地吻合。研究结果表明，当光传输链路的偏振模色散值不变时，随着传输链路偏振相关损耗的增大，输出偏振态归一化自相关函数的线宽变宽；而在光传输链路的偏振相关损耗一定的情况下，随着光传输链路的偏振模色散的增大输出偏振态归一化自相关函数的线宽变窄。     

A two-pole tunable filter with constant fractional-bandwidth characteristics

In this paper, a tunable filter with constant fractional bandwidth is designed and fabricated on ε_r = 2.2, h = 0.508 mm duroid substrates. A new mixed electrical and magnetic coupling scheme is introduced by this structure to achieve an overall constant coupling coefficient during the tuning range, which can meet the requirement of constant fractional bandwidth. A detailed analysis on the external quality (Q_ext) and coupling coefficient is presented. Two-chip capacitors (C_M = 1 pF), using radio frequency (RF) and microwave capacitors, were used to enhance Q_ext. The Q_ext (22 ± 3) shows a relatively small variation over the entire tuning range. The tunable constant fractional-bandwidth filter shows a tuning range of 1.28–1.86 GHz, a 3-dB bandwidth of 5.3 ± 0.6% and an insertion loss of 5.11–2.69 dB. The measured and simulation results are in a good agreement.     

A tunable polarization-independent liquid-crystal Fabry-Perotinterferometer filter

The authors propose a simple structure for a tunable polarization-independent wavelength-selective filter. This successfully fabricated filter consists of a liquid crystal Fabry-Perot interferometer, a γ/4 plate, and a mirror. The input light is divided into a transmitted light and a reflected light. The polarization of this reflected light is rotated 90° by the γ/4 plate and the mirror and then this light is input again into the filter. The bandwidth is 0.48 nm and the tuning range is 50 nm. Fiber-to-fiber loss is approximately 3.9 dB and polarization dependence of the loss is approximately 0.3 dB     

Design Theory of Up-Converters for Use as Electronically-Tunable Filters (Sep. 1961 [T-MTT])

The up-converters discussed use a single diode, a wide-band impedance matching filter at their signal input, a moderately wide-band impedance matching filter at their pump input, and a narrow-band filter at their sideband output. With a narrow-band filter at the sideband output, the frequency which will be accepted by the amplifier can be controlled by varying the pump frequency. Analysis of the impedance matching problem involved shows that tuning ranges of the order of a half-octave to an octave are possible. Theory is presented for both the lower-sideband and upper-sideband types of tunable up-converters and for the design of the required impedance matching networks. It is shown that, because of the pump input bandwidth required, it will generally be necessary to accept some mismatch at the pump input. But, by use of a properly designed impedance matching filter, the reflection loss can be kept nearly constant across the pump band, and the incident pump power required is not unreasonable. It is seen that properly designed devices of this type using voltage-tunable pump oscillators should have wide tuning range, fast tuning capability a useful amount of gain, no image response, and a low noise figure.     

An adaptive wavelength tunable optical filter employing anangle-tuned interference filter and an intelligent digital controller

This paper describes an adaptive wavelength tunable optical filter, which is composed of an angle-tuned interference optical filter and an intelligent digital controller. The new angle-tuned interference filter consists of a dielectric interference optical filter and a piezoelectric angle-tuning mechanism. It achieves quick wavelength switching within 2.5 ms in a 30 nm tuning range and a sufficiently low crosstalk less than -30 dB. The intelligent digital controller has two functions: wavelength tracking and wavelength channel selection. Combining these technologies, we have developed a practical low-cost tunable filter suitable for a post-optical-amplifier filter in a high-sensitivity detection system and a channel selector in a WDM system that requires 10-100 ms channel selection time. With a wavelength tracking operation, we have confirmed -35 dBm high-sensitivity detection in 20 nm wavelength range in a 10 Gb/s system. We have also confirmed a wavelength channel selection operation within 18 ms in a three-channel wavelength division multiplexed (WDM) system whose channel spacing is 4.4 nm     

双信道解复用长周期光纤光栅的实验研究

长周期光纤光栅是近几年出现的一种波长选择滤波器件。利用其波长选择损耗特性,提出将之用于1310nm/1550nm波分复用系统的解复用。采用电弧熔融刻槽的方法,制备了长周期光纤光栅。并通过控制电弧放电过程和光栅的周期长短及周期数,得到了较为理想的谱特性。结果显示,其通道插入损耗能够小于0.5dB,20dB损耗阻带带宽可达15nm,信道隔离度估计约25dB。     

角度调谐滤光片光斑整形设计与实验

在测试制备的100 GHz密集波分复用(DWDM)低偏振相关损耗的角度调谐滤光片时,其透射光谱的插入损耗和带宽的展宽现象会随着入射角度的加大而迅速增加,与理论设计不符.通过多光束干涉原理建立模型模拟,发现在倾斜入射时滤光片的透射光束的光斑会出现展宽和形变,使得位于接收端处的光准直器接收不到足够的透射光束,导致了光谱变形.根据分析结果设计了一种利用直角棱镜的光斑整形方案,能将展宽的椭圆光斑压缩回近似的圆形光斑.制备了一组直角棱镜整形装置,实验证明该设计能够在保持接收端光准直器位置固定的情况下有效地减小透射波形的损耗和半宽展宽现象,使角度调谐滤光片能达到理论设计中20 D_m的可调谐范围.     

Rapid continuous tuning of a single-polarization fiber ring laser

We have developed and studied what we believe is the first fiber laser operating in a single polarization at 1550 nm with PM fiber output and capable of wavelength tuning over a 20-nm bandwidth in less than 50 μs. The fast wavelength tuning speed in a fiber laser is achieved by incorporating an acousto-optic tunable filter into the laser. The same filter is used in an optoelectronic feedback loop to suppress the relaxation oscillations induced by wavelength transitions     

Wavelength-division multi demultiplexers for two-channel single-mode transmission systems

Low-loss lensless 2-channel wavelength-division multiplexing (WDM) couplers may be produced on the beam-splitter principle with interference filters and three butt-coupled fiber ends not only for multi-mode Operation but also for single-mode transmission links. These devices are constructed with single-mode fibers-for bidirectional transmission only in the transmission channel, but for unidirectional operation in all channels. The interference filter, used in the experiment to separate the emissions of two LD's with wavelengths of 834 and 868 nm, may be implemented both as a longwave-pass (LWP) and a shortwave-pass (SWP) edge filter with high edge steepness and low polarization effects. WDM single-mode couplers exhibit very low insertion losses lying in the O.5-dB region. Their fat-end crosstalk attenuations attain values of between 15 and 30 dB and their near-end crosstalk attenuations are better than 65 dB.     

Tunable microstrip bandpass filter with constant fractional bandwidth based on cascade triplet topology

This paper presents an approach for designing 3-pole tunable microstrip bandpass filter with constant fractional bandwidth (CFBW). To achieve sharp roll-off responses, a cross-coupling scheme is adopted in the filter design. Cascade triplet (CT) topology and source-load coupling scheme are used to introduce three transmission zeros (TZs) in the design of 3-pole tunable filters. Thanks to tunable coupling coefficient K_c, the TZs allocated at the low-end of passbands can be tuned, which providing a reconfigurable selectivity. For the purpose of demonstration, the 3-pole tunable bandpass filters with CFBW are designed, fabricated, and measured. The proposed filter can be tuned over a wide fractional range up to 50%. The measured 3-pole filter shows a tuning range of 950–1460 MHz, with a fractional bandwidth (FBW) of 13.1% ± 2.5% and an insertion loss of 1.8–3.6 dB. Measured results of the filter present sharp roll-offs, which agree well with the simulated counterparts.