Toward tunable thin-film filters for wavelength division multiplexing applications

We provide a detailed analysis of the various problems connected with the development of tunable thin-film filters for wavelength-division multiplexing applications. We examine the relation between the change in layer thickness and the central wavelength shift for various configurations and point out the significance of the structure of the reflectors, the spacer thickness, and the location of the active layers. We describe and compare practical arrangements using either temperature or an electric field as the driving parameter.     

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.     

Bandwidth reduction technique for multilayer wavelength division multiplexing (WDM) bandpass filters

The push to develop 100 GHz and smaller bandwidth WDM filters is at demanding levels. Currently, 200 GHz is the standard bandwidth for multilayer interference coatings with high efficiencies, and enormous processing effort is going into the development of standard 100 GHz filters. This paper outlines a simple design that will reduce bandwidth up to 40% when applied to 200 GHz bandpass filters. This design method can also be used in existing 100 GHz designs to achieve even smaller bandwidths.     

Holographic edge-illuminated polymer Bragg gratings for dense wavelength division optical filters at 1550 nm

We discuss the use of holographic photopolymer materials for use as dense wavelength division multiplexing filters in the C-band of the optical communication spectrum. An edge-illuminated hologram configuration is described that effectively extends the grating length to achieve narrow band filters operating near 1550 nm in photopolymers that are 100-200-microm thick. This configuration enables the formation of apodized and cascaded filter systems. Rouard's method is used to examine the properties of both apodization and cascaded gratings and indicates the potential for narrow spectral bandwidths (< 0.2 nm) and high side-lobe suppression (<-- 30 dB). Initial experimental results with a commercially available photopolymer are provided that verify narrowband spectral-transmittance properties (< 0.6 nm) and the ability to apodize the index profile. The primary limitation of the design is the absorption of existing photopolymer materials. Optimizing the polymer chemistry for filter design at 1550 nm may solve this problem.     

光谱光电高温计的波长和带宽参数对温度测量准确度的影响

本文依据普朗克黑体辐射定律,研究了光谱光电高温计的光谱波长和光谱带宽对温度测量准确度的影响,导出了温度测量误差关系式,并在钢铁热处理实验中进行了验证,介绍了减少外界环境对温度测量准确度干扰的几种实用方法,讨论了温度测量的方式所带来的不同测量误差,为钢铁工业测温正确使用光谱光电高温计提供了有用的参考。     

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.     

Effect of stress on performance of dense wavelength division multiplexing filters: optical 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 optical characteristics of the filters. Details of the filter passband characteristics and wave-front distortion illustrate the influence of the stress. Spatial variation of the stress on the filter surface causes the filter center wavelength to have spatial variation, and it causes the filter to have an asymmetric passband characteristic.     

Design of DOEs for wavelength division and focusing

We propose a technique for calculating the color separation gratings aimed at separating plane light beams of different wavelengths into different diffraction orders. The technique is based on a special-type optimization criterion. With this criterion, the problem of calculating the piecewise-constant grating profile is reduced to sequentially solving independent problems of optimization of the step heights. We derived an analytical expression for the profile of a color separation grating that generalizes the familiar analytical solutions. The criterion introduced is used to design a diffractive optical element (DOE) that generates required light beams when it is illuminated by different wavelengths. Design of color separation gratings able to separate three and five different wavelengths and DOEs to demultiplex and focus the two- and three-wavelength beams is presented.     

Demodulation scheme for fiber bragg grating sensors based on active control of the spectral response of a wavelength division multiplexer

We present a closed-loop technique for measuring wavelength shifts associated with fiber Bragg gratings by using a fused biconical wavelength division multiplexer (WDM). The spectral response of the WDM is actively tuned by stretching of the coupling region to maintain a fixed coupling ratio at the reflected Bragg wavelength. The closed-loop operation allows sensitivities usually associated with a highly selective WDM to be obtained without compromising the measurement range. A simple theoretical model is presented together with experimental results for temperature and strain measurements.     

Cross-shaped bandpass filters for the near- and mid-infrared wavelength regions

A photolithographic process has been used to form cross-shaped patterns in 3-μm-thick nickel foils. Patterns with cross arm dimensions in the 10-20-μm range, and with periodicities in the 16-26-μm range, yield self-resonant bandpass filters for wavelengths in the 20-25-μm region. Transmittances as high as 80% were achieved with center wavelength-to-bandwidth ratios (λ(R)/Δλ) of ~5. We present a simple empirical formula that relates the wavelength of peak transmittance, or resonant frequency, with cross dimensions and periodicity.     

Effective frequency technique for finite spectral bandwidth effects

A technique that uses a single effective frequency to represent the effects of finite spectral bandwidth for active and passive measurements centered on an absorption line, a trough region, or a slowly varying spectral feature is described. For Gaussian and rectangular instrumental line shapes, the effective frequency is shown to have a simple form that depends only on the instrumental line shape and bandwidth and not on the absorption line profile. The technique is applicable to a large class of active and passive measurements and simulations in both the laboratory and the atmosphere. Simulations show that the technique yields accuracies better than 0.1% for bandwidths less than 0.2 times the atmospheric linewidth for a rectangular line shape or better than 0.2% for a Gaussian.     

Accurate and precise wavelength calibration for wide bandwidth array spectrometers

Calibrating the wavelength scale of an array spectrometer typically involves measurements of lines at well-known wavelengths from a calibration lamp such as a mercury-argon source. This process is relatively straightforward when the lines are well separated, relative to the bandwidth of the spectrometer. When the spectrometer's bandwidth is large, compared with the distance between calibration wavelengths, it becomes increasingly difficult to accurately locate lines in the calibration spectrum. Even calibrations for instruments with a modest bandwidth of 12 nm can be difficult. Here we present results from a simple approach to improve the accuracy of wavelength calibration for an instrument with a large bandwidth (12 nm, center-to-center pixel spacing 3.3 nm). A monochromator has been used to filter the source so that each calibration line can be measured separately. For ten spectrometers, we were able to achieve accuracy better than 0.12 nm, or 0.09 nm on average; this is less than 3% of the pixel spacing. We anticipate this approach will be useful for improving the accuracy of measurements on array spectrometers and particularly in transferring multivariate calibrations between instruments.     

Design of blocking filters of any narrow bandwidth

A design approach is described to achieve spectral blocking filters of any spectral width and optical density for narrow blocking bands. The ratio of the thickness of the high-index material to the low-index material in the layer pairs is adjusted to obtain the desired bandwidth in the first-harmonic reflection band. The number of layer pairs is adjusted to provide the required optical density. Equations are provided for estimating the ratios and number of layer pairs needed to achieve a given bandwidth and optical density. This approach can be useful for laser line blocking filters, night vision filters, and other general applications.     

Gain-clamped erbium-doped fibre amplifier for wavelength division multiplexed systems

Abstract

Amplification of multiwavelength signals by an erbium-doped fibre amplifier (EDFA) is becoming critical, due to the proliferation of wavelength division multiplexed systems. However, when a standard EDFA is employed, dissimilarities between signal gains may prove unacceptable. Thus, a novel gain-clamped EDFA is proposed here to tackle the issue of gain tilt, a measure of maximum gain difference. The suitability of the configuration is then investigated through a numerical model, which is developed based on the standard EDFA and fibre laser models.     

High-resolution two-grating spectrometer for dual wavelength spectral imaging

A two-grating high-resolution spectrometer for dual wavelength imaging is demonstrated based on the standard Czerny-Turner mounting with an auxiliary grating and a mirror. A two-dimensional charge-coupled device (CCD) detector in the spectrometer focal plane allows simultaneous detection of two spectral intervals. Each spectrometer grating is driven by a high-precision stepper motor interfaced to a computer via home-made software. The software allows fast tuning of the gratings to a desirable spectral interval anywhere between 200 nm and 800 nm. The spectral interval widths are 2-3 nm for a 'high-resolution' (2400 grooves/mm) grating and 4-5 nm for a 'low-resolution' (1200 grooves/mm) grating. The resolution varies between 0.01 nm and 0.02 nm depending on the grating used. The performance of the spectrometer is demonstrated by detecting spectrally resolved images from a back-illuminated template and from a laser-induced plasma. The spectrometer can be useful for two-line spectroscopic diagnostics or can be expanded for multi-element spectral analysis.     

Bandpass filters for 8 GHz using solidly mounted bulk acoustic wave resonators

Frequency shift, design, and fabrication issues have been investigated for the realization of 8 GHz bandpass filters based on AlN thin film bulk acoustic wave resonators. Fabrication includes well-textured AlN thin films on Pt (111) electrodes and SiO/sub 2//AlN Bragg gratings for the solidly mounted resonators. The chosen ladder filter design requires the tuning of the shunt resonators with respect to the series one. For this purpose, mass loading of the shunt resonators with aluminum (Al) and SiO/sub 2/ were studied. Design simulations showed that the channel bandwidth can be doubled by shifting more than the difference of resonance and antiresonance frequency. Bandpass filters at 8 GHz were successfully fabricated with -5.5 dB insertion loss, -26 dB out-of-band rejection, 99 MHz (1.2%) /spl plusmn/0.2 dB channel bandwidth, and 224 MHz (2.8%) 3 dB bandwidth. The group delay variations within any 30 MHz channel inside the channel bandwidth amounts to <0.2 ns. Comparisons with simulation calculations and single resonator characteristics show that each /spl pi/-section includes a parasitic series resistance and inductance.     

Multichannel wavelength division multiplexing with photonic crystals

A multichannel wavelength-division-multiplexing system consisting of a two-dimensional photonic crystal is proposed. The system consists of two parts, a waveguiding element, realized by defects in a photonic crystal, and frequency-selective elements, realized by photonic crystal microcavities. Simulations, performed with a two-dimensional finite-difference time-domain technique with a perfectly matched layer absorbing boundary condition, showed the ability to filter an incident pulse into six spectral channels with a FWHM of 2 nm.     

Study of the effect of source signal bandwidth on ratiometric wavelength measurement

We derive an analytic equation for a ratiometric wavelength measurement system and analyze the influence of the optical source signal bandwidth. Our investigation shows that in a particular optical sensing system, the higher the bandwidth of the optical signal, the better resolution the system will achieve. Experiments based on two types of optical signals (output signal of a tunable laser and a fiber Bragg grating) were carried out, and experimental results verified both the simulation results and the theoretical analysis.     

Dispersion-optimized optical fiber for high-speed long-haul dense wavelength division multiplexing transmission

Four non-zero-dispersion-shifted fibers with almost the same large effective area (A(eff)) and optimized dispersion properties are realized by novel index profile designing and modified vapor axial deposition and modified chemical vapor deposition processes. An A(eff) of greater than 71 μm(2) is obtained for the designed fibers. Three of the developed fibers with positive dispersion are improved by reducing the 1550 nm dispersion slope from 0.072 ps/nm(2)/km to 0.063 ps/nm(2)/km or 0.05 ps/nm(2)/km, increasing the 1550 nm dispersion from 4.972 ps/nm/km to 5.679 ps/nm/km or 7.776 ps/nm/km, and shifting the zero-dispersion wavelength from 1500 nm to 1450 nm. One of these fibers is in good agreement with G655D and G.656 fibers simultaneously, and another one with G655E and G.656 fibers; both fibers are beneficial to high-bit long-haul dense wavelength division multiplexing systems over S-, C-, and L-bands. The fourth developed fiber with negative dispersion is also improved by reducing the 1550 nm dispersion slope from 0.12 ps/nm(2)/km to 0.085 ps/nm(2)/km, increasing the 1550 nm dispersion from -4 ps/nm/km to -6.016 ps/nm/km, providing facilities for a submarine transmission system. Experimental measurements indicate that the developed fibers all have excellent optical transmission and good macrobending and splice performances.