High-performance tunable filter

The tuning performance of a liquid-crystal, electro-optically tunable, Lyot filter is described. This filter is widely tunable across the visible region up to a wavelength of approximately 630 nm and has very high performance characteristics that are preserved over the entire tuning range. These include a narrow passband of 0.1 nm, a large-aperture performance of at least 30 mm, a wide field of view of ?8.5 degrees , and the ability to be tuned to an accuracy that is one tenth of the linewidth of the passband. Possible applications for such a filter include remote sensing, multispectral sensing, and laser environmental and underwater monitoring.     

Analysis and demonstration of single-passband response and tuning characteristics in a chirped ladder interferometric filter for a widely tunable laser diode

We have designed and demonstrated a chirped ladder-type tunable filter and discussed its potential application for a tunable diode laser. A ladder interferometric filter normally has a periodic passband, which makes it impossible to stabilize laser oscillation frequency. To overcome this drawback, we have designed, fabricated, and characterized a novel chirped tunable ladder filter. We have successfully demonstrated a single-passband response in the fabricated device. Furthermore, a tuning operation of more than 30 nm was achieved by introducing a current injection structure and optimizing electrode lengths at each single-stage ladder interferometer.     

Spectral resolution analysis of resonant grating waveguides

A high spectral resolution analysis of narrowband reflection filters based on resonant grating waveguide structures is presented. A tunable high‐performance dye laser with ～0.15 cm^‐1 line width and a beam analyzing system consisting of three simultaneously controlled CCD cameras were used to investigate grating waveguide resonances at wavelengths ～694 nm and ～633 nm. A reflectivity of 91 % and a line width of ～0.3 nm were measured and theoretically modeled for a resonant reflection filter specifically designed for the ruby laser wavelength 694.2 nm. A resonance shift of several nanometers was observed for a second grating waveguide structure in the region of the helium‐neon laser emission wavelength 632.8 nm by changing the sample temperature. We discuss the potential of grating waveguide devices combining the narrow line width and the tunability of the resonant response as promising candidates for implementation in innovative concepts for reflection filter and sensor applications.     

Design of a lithium niobate Fabry-Perot étalon-based spectrometer

The design of a tunable Fabry-Perot étalon-based filter that has a passband of 0.01 nm and a free spectral band larger than 50 nm when operated in the center of the visible spectrum is described. The filter consists of two Y-cut lithium niobate étalons having thicknesses in a vernier ratio. The polarization state of light passing through the tandem étalons is rotated 90 degrees before again being transmitted through the étalon pair. If the components are arranged in a symmetrical manner, the filter will operate with unpolarized incident light. Each étalon in double pass will have a greater transmittance than two individual étalons of the same average optical thicknesses, since variations in the physical thickness due to fabrication errors are correlated.     

一种基于SIFW的小型化宽带带通滤波器

为实现射频系统的小型化,本文设计出一种基于基片集成折叠波导( Substrate integrated folded waveguide,简称SIFW)的小型化宽带带通滤波器,并给出了仿真结果.为了改善带外抑制,滤波器通过交叉耦合,在通带低端引入两个传输零点.为改善通带高端滤波特性,在顶层和底层金属微带上刻蚀互补型开口谐振环(Complementary Split Ring Resonators,简称CSRR).仿真结果表明,所实现的滤波器中心频率在7.1 GHz,相对带宽约为47％,通带内回波损耗优于-15dB,插入损耗小于0.7dB.     

Narrow bandpass tunable terahertz filter based on photonic crystal cavity

We have fabricated a very narrow bandpass tunable terahertz (THz) filter based on a one-dimensional photonic crystal cavity. Since the filter consists of silicon wafers and air spacers, it has a very high quality factor of about 1500. The full width at half maximum (FWHM) of the passband is only about 200 MHz, and the peak transmission is higher than -4 dB. Besides, the central frequency can be tuned rapidly over the entire bandgap with the length of cavity adjusted by a motorized linear stage. Further analytical calculations indicate that a high-Q tunable filter with both high peak transmission and wide tunable range is possible if thinner silicon layers are used.     

Terahertz narrow-band filter based on rectangle photonic crystal

A terahertz filter with a channel drop cavity and a resonant reflection cavity in a two-dimensional photonic crystal is theoretically proposed. The channel drop cavity is used to trap photons at resonant frequency from the bus waveguide through coupling and emit them to a drop waveguide, while the resonant reflection cavity is used to realize wavelength selective reflection feedback in the bus waveguide. The transmission properties of the terahertz filter are simulated using the finite element method. It is found that a peak with the central frequency of 1.12 THz is existed in the transmission spectrum. The full width at half maximum of the passband is only 5 GHz, and the peak drop efficiency is up to 94.8%.     

基于EBG结构的基片集成波导超宽带带通滤波器

为拓展带通滤波器的通带带宽,设计了一种基于EBG结构的基片集成波导（SIW）超宽带带通滤波器．该滤波器利用箭头形电磁带隙结构的阻波特性,将不同大小箭头形结构单元蚀刻在SIW上金属面,以获得超宽带通带．所设计的滤波器中心频率在985 GHz,相对带宽为3959%,通带内的最大插入损耗约为154 dB,相比于类似结构的带通滤波器,其带内回波损耗较大,且整体电路面积较小．测量结果与仿真结果基本吻合,有效地验证了该设计方法的有效性．     

Infrared-ultraviolet sum-frequency generation spectrometer with a wide tunability of the ultraviolet probe

We have constructed a multiplex infrared-ultraviolet (IR-UV) sum-frequency generation (SFG) spectrometer that has a wide tunable range (235-390 nm) of the UV probe wavelength. The tunable UV probe was obtained by doubling the signal output of an optical parametric amplifier pumped by a 400 nm picosecond pulse. A prism monochromator was used as a tunable sharp-cut bandpass filter to reduce stray light due to the scattering of the UV probe so that any wavelength within the tunable range can be chosen as that of the UV probe. The SFG spectra of p-mercaptobenzoic acid on a gold substrate was measured with 289 and 334 nm UV probes. The SFG vibrational band intensities due to the carbonyl stretch mode and a phenyl ring stretch mode measured with the 289 nm probe were approximately three times larger than those measured with the 334 nm probe. The enhancement was ascribed to an electronic resonant effect.     

Flattopped tunable wavelength-division-multiplexer filter design

We present what is to our knowledge the first design of a tunable optical filter with a flattopped passband. The filter consists of a Fabry-Perot etalon with multiple reflection gratings as dielectric mirrors. Wavelength tunability is achieved by modulation of the refractive indices of the cavity and the dielectric mirrors. Specifically, a filter with a 1-nm linewidth and a 40-nm wavelength-tuning range is designed for applications in wavelength-division-multiplexing (WDM) optical fiber communication systems. We also discuss several factors, including absorption and variations of other design parameters, that may affect the performance of the filter.     

Switchable dual-wavelength single-longitudinal-mode erbium-doped fiber laser using an inverse-Gaussian apodized fiber Bragg grating filter and a low-gain semiconductor optical amplifier

We present a stable and switchable dual-wavelength erbium-doped fiber laser. In the ring cavity, an inverse-Gaussian apodized fiber Bragg grating serves as an ultranarrow dual-wavelength passband filter, a semiconductor optical amplifier biased in the low-gain regime reduces the gain competition of the two wavelengths, and a feedback fiber loop acts as a mode filter to guarantee a stable single-longitudinal-mode operation. Two lasing lines with a wavelength separation of approximately 0.1 nm are obtained experimentally. A microwave signal at 12.51 GHz is demonstrated by beating the dual wavelengths at a photodetector.     

Wide wavelength range tunable guided-mode resonance filter based on incident angle rotation

A wide wavelength range tunable guided-mode resonance filter (GMRF) with high peak efficiencies, narrow linewidths, and low sidebands is experimentally demonstrated. The resonance wavelength can be tuned under TM polarized light illumination by rotating the angle of incidence. The GMRF composed of a one-dimensional grating layer and two waveguide layers on a glass substrate is designed with rigorous coupled-wave analysis. The grating structure of the GMRF is patterned by interference lithography with two ultraviolet laser beams. The reflection colours of fabricated GMRF can be shifted from blue to red by rotating the angle of incidence. The measured full width at half-maximums of the reflection peaks located at 450.6, 500.9, 551.0, 601.1, and 651.3?nm are 3.1, 3.9, 4.3, 3.8, and 4.1?nm, respectively. The corresponding sidebands of measured spectra are below 0.2. Compared with previous experimental studies on tunable structures, the narrow linewidths and low sidebands of the proposed GMRF are remarkably improved.     

Ultrahigh-efficiency, very narrowpassband, tunable optical filter

Recently developed technologies have been uniquely integrated to produce optical filters with unprecedented performance. The precise design and fabrication of narrow-passband and high-efficiency optical filters with a central frequency of 532 nm are reported. Measurements with these filters demonstrate a performance that accurately mirrors the design specification, with one filter having a passband of 70 pm and an efficiency greater than 90% and the second filter having a passband of 7 pm and an efficiency greater than 80%.     

Chalcogenide coatings of Ge15Sb20S65 and Te20As30Se50

Chalcogenide coatings are investigated to obtain either optical components for spectral applications or optochemical sensors in the mid-infrared. The deposition of Ge(15)Sb(20)S(65) and Te(20)As(30)Se(50) chalcogenide glasses is performed by two physical techniques: electron-beam and pulsed-laser deposition. The quality of the film is analyzed by scanning electron microscopy, atomic force microscopy, and energy dispersive spectroscopy to characterize the morphology, topography, and chemical composition. The optical properties and optical constants are also determined. A CF(4) dry etching is performed on these films to obtain a channeled optical waveguide. For a passband filter made by electron-beam deposition, cryolite as a low-refractive-index material and chalcogenide glasses as high-refractive-index materials are used to favor a large refractive-index contrast. A shift of a centered wavelength of a photosensitive passband filter is controlled by illumination time.     

Development of an eye-safe solid-state tunable laser transmitter in the 1.4-1.5 microm wavelength region based on Cr4+:YAG crystal for lidar applications

An experimental optimization of the efficiency of a gain switched tunable Cr4+:YAG laser at 10 Hz is described. The thermal lensing during pulsed operation was measured. Optimal performance occurred at a crystal temperature of 34 degrees C and resulted in an output energy of approximately 7 mJ and a pulse duration of approximately 35 ns. Tunability in the range of 1350-1500 nm, spectral linewidth of approximately 200 GHz, and M2<4 are demonstrated. The main laser material parameters are estimated. Such a laser could be employed in a laboratory-based nonscanning lidar system if a narrowband birefringent filter is installed. The tunability will permit the improvement of the Cr4+:YAG transmitter for water-vapor differential absorption lidar if injection seeding is applied.     

Widely tunable multiwavelength filter based on cascaded long period fiber gratings

A temperature-controlled, continuously and linearly tunable multiwavelength filter based on cascaded long period fiber gratings has been realized. For the first time, a tunable wavelength range of 2.1 nm has been achieved with a temperature variation of 62°C and a regression coefficient of 0.997. The experimental results are in good agreement with the theory.     

Tunable optical filter for colorimetric applications

A temperature-controlled tunable optical filter for use in the visible range and based on the Christiansen effect, in which the solid particles have been substituted with glass fibers, has been fabricated and tested. The construction of the filter and transmission properties are reported. For an operating temperature of 95°C, the peak wavelength was 509.5 nm, with a peak transmission of 50.8% and a FWHM of 71 nm.     

基于光折变光栅的可调谐滤波方案及特性分析

在LiNbO3单模波导上利用双光束干涉可形成光折变波导光栅,针对粗波分复用系统提出了通过实时改变双光束之间的夹角进而改变光栅间距,实现可调谐滤波的方案.利用光折变动力学理论,得到光栅调谐时间在毫秒数量级.研究发现,适当提高光强和增加杂质掺杂浓度可提高衍射效率、减少光栅初次建立时间.基于耦合波理论的数值模拟结果表明,增大光栅长度能使衍射效率提高且带宽变小,而增加折射率调制度使衍射效率提高但同时带宽变宽.在折射率调制度为8×10-5,光栅长度大于12mm时,可获得91％以上的衍射效率和小于0.08 nm的带宽.     

Wavelength division multiplexing based on apodized planar holographic Bragg reflectors

We report wavelength division multiplexing based on lithographically fabricated slab-waveguide-contained planar holographic Bragg reflectors (HBRs). Partial HBR diffractive contour writing and contour displacement are successfully demonstrated to enable precise bandpass engineering of multiplexer transfer functions and make possible compact-footprint devices based on hologram overlay. Four- and eight-channel multiplexers with channel spacings of approximately 50 and approximately 100 GHz, improved sidelobe suppression, and flattop passbands are demonstrated. When a second-order apodization effect, comprising effective waveguide refractive-index variation with written contour fraction, and the effect of hologram overlap on the hologram reflective amplitude are included in the simulation, excellent agreement between predicted and observed spectral passband profiles is obtained. With demonstrated simulation capability, the ability to fabricate general desired passband profiles becomes tractable.     

A novel broadband terahertz filter for photonic crystal

Abstract

The influence of the change of the radius of point defect cylinders, scattering dielectric cylinders, dielectric cylinders on both sides of line waveguide on S parameter is studied. According to the resonant coupling principle between micro-cavity and waveguide, a novel broadband terahertz filter is designed. The novel filter is formed by introducing scattering dielectric cylinders into the resonant microcavity, and the point defect cylinders are composed of HgTe material and adjusting the radius of dielectric cylinders on both sides of line waveguide. Results show that the 3 dB bandwidth reaches 74.2 GHz, the return loss is less than ?12.02 dB, the maximum insertion loss in-band reaches 0.35 dB and its drop efficiency is up to 96.79%. The novel terahertz filter has flat passband, sharp rejections at out-bands and its central frequency is 0.338THz. The good performances show that it can meet the requirements of high speed and broadband in terahertz atmosphere communication I window.