Narrow Bandpass Optical Filters Research Articles

Design and fabrication of a low-cost, multispectral imaging system

This paper reports the design and construction of a low-cost, multispectral imaging system using a single, large format CCD and an array of 18 individual lenses coupled to individual spectral filters. The system allows the simultaneous acquisition of 18 subimages, each with potentially different optical information. The subimages are combined to create a composite image, highlighting the desired spectral information. Because all the subimages are acquired simultaneously, the composite image shows no motion artifact. Although the present configuration uses 17 narrow bandpass optical filters to obtain multispectral information from a scene, the system is designed to be a general purpose, multiaperture platform, easily reconfigured for other multiaperture imaging modes.

Narrow bandpass optical filters fabricated with one-dimensionally periodic inhomogeneous thin films

Thin films with sinusoidally varying refractive index profiles display photonic band gap effects. Intentional deviations from the periodic index profile can be used to tailor the optical properties of the resulting thin film. We present experimental characterization of TiO2 films with periodic index profiles fabricated using a deposition technique known as glancing angle deposition (GLAD). The resulting porous thin films have a microstructure consisting of vertically aligned columns. Sinusoidal porosity gradients, and therefore sinusoidal index profiles, can be introduced in the direction of the substrate normal by fabricating columns with a periodically varying diameter. Local modifications of the index profile are achieved by inserting thin layers of constant porosity into the center of the film, or by discontinuously changing the phase of the sinusoidal gradient. The introduction of these structural defects creates a narrow optical passband inside the larger band gap, and we demonstrate how the properties of this passband can be controlled through modification of the defect parameters. The magnitude of the phase shift constituting the defect is shown to control the location of the resulting passband. By inserting a layer with in-plane birefringence, we show that it is possible to create two separate passbands, one for each polarization, within a single stop band. The results illustrate how nanoscale porosity engineering using GLAD is a precise technique for fabricating one-dimensionally periodic films with a variety of optical characteristics.

Nanostructured gradient index optical filter for high-speed humidity sensing

A high-speed optical humidity sensor was fabricated from a nanostructured titanium dioxide thin film. A refractive index depth profile designed to yield a narrow-bandpass optical interference filter was obtained through nanoscale porosity variations produced by a physical vapor deposition technique known as glancing angle deposition (GLAD). The GLAD process provides film morphology control on the 10 nm scale through a combination of deposition at highly oblique vapor incidence angles and controlled substrate motion. Under varying humidity conditions the transmittance spectrum of the filter shifts due to effective index changes of the porous structure resulting from water vapor adsorption/desorption. Performance analysis has shown that this device is highly sensitive, exhibits minimal hysteresis, and is extremely fast. The adsorption and desorption response times were measured to be 270 ms and 160 ms, respectively.

Sub-Second Humidity Sensing based on Nanostructured Narrow-Bandpass Optical Filters

AbstractAn optical-based humidity sensor with a sub-second response time was fabricated from a nanostructured titanium dioxide thin film. A refractive index profile designed to yield a narrow-bandpass optical interference filter was obtained through nanoscale porosity variations produced by glancing angle deposition (GLAD). Under varying humidity conditions the transmittance spectrum of the filter shifts due to effective index changes of the porous structure resulting from adsorption/desorption of water vapor. In the following we will show that this device is highly sensitive, exhibits minimal hysteresis, and is extremely fast. The adsorption and desorption response times were measured to be 270 ms and 160 ms, respectively.

Secondary ion mass spectrometry backside analysis of barrier layers for copper diffusion

Secondary ion mass spectrometry (SIMS) backside analyses have been performed on a Cu/TaN/Ta/SiO2/Si structure to determine barrier effectiveness for Cu diffusion. Sample backside access to the barrier layers was obtained by removal of the Si substrate using a polishing method that maintains parallelism between the sample surface and the polished back side by monitoring changes in facets at the four corners of the specimen. Determination of the Si thickness remaining during the polishing process was improved through the use of optical interference measurements using a narrow band pass optical filter. Samples having slopes with respect to the original surface less than 6 nm over 60 μm have been obtained. A difference in polishing rate between SiO2 and Si was exploited to obtain this parallelism. For SIMS analyses, the presence of a SiO2 layer required electron gun charge neutralization for the O2+ 0.5 keV impact energy analysis. SIMS analyses show the ability to distinguish all layers and to monitor copper through the barrier material. With the high depth resolution conditions used, SIMS analyses were able to provide detailed elemental distribution information such as the presence of nitrogen at specific interfaces.

Thickness monitoring of optical filters for DWDM applications

A method for thickness monitoring and turning-point prediction during deposition of narrow band pass optical filters (NBPF) for dense-wavelength-division-multiplexing (DWDM) applications is proposed. The method is based on a recurrent approach, with relative transmittance .tting, and includes partial coherence and monochromator bandpass e.ects. We show that the partial coherence e.ects in thin .lm structures are signi.cant and can not be neglected. The proposed method is applicable for precise thickness monitoring and deposition control of any complex multilayer coating.

253.7nm放射照度測定値の国内比較

The 253.7nm irradiance standards were compared among five Japanese organizations-Sankyo Denki Co., Ltd, Toshiba Lighting & Technology Corp., Iwasaki Electric Co., Ltd., Topcon Corp., and Matsushita Electric Industrial Co., Ltd. Three of the participants calibrated their standard by optical filters and thermal detector having its absolute responsivity. Those of the other two participants were calibrated using a Si-PD, silicon photodiode, having absolute responsivity to 253.7nm line spectral irradiance.All the standards were measured with a 253.7nm irradiance transferring detector consisting of a GaAsP-photodiode and a narrow bandpass optical interference filter around 253.7nm.As a result, the 253.7nm irradiance scales of the two of the participants using thermal detector calibration and the one of them using Si-PD calibration agreed well within±1.5% deviation. The other two's scales were deviated by 5% to 10%from the average of the first three's.

Optical all-pass filters for phase response design with applications for dispersion compensation

Lossless all-pass optical filters are introduced, which can approximate any desired phase response while maintaining a constant, unity amplitude response. Architectures using cascade and lattice structures based on ring resonators and cavities defined by reflectors are discussed. Two applications are presented: 1) for fiber dispersion compensation and 2) for compensation of the nonlinear phase response of narrow bandpass optical filters such as thin-film or Bragg grating filters. All orders of dispersion can be compensated in principle, and the filters are periodic so multiple channels can be compensated with a single device. The architectures are very compact compared to alternatives such as chirped Bragg gratings.

Spectrally selective gas cell for electrooptical infrared compact multigas sensor

Abstract Progress in a new electrooptical compact gas sensor system is reported. The sensor measures pollutant concentrations in the atmosphere by comparing the absorption of infrared light in a gas cell at specific wavelengths. This task is performed by a non-dispersive spectral analyzer that operates at discrete wavelengths and has no moving parts (spectral retina). Several gases can be detected with the same detection principle, by simply selecting different signal channels. The spectral analyzer is made by coupling an array of integrated narrow band-pass optical filters to an array of PbSe detectors. Simultaneous detection of CO and SO 2 with no cross-talk is demonstrated. A specific spectrally selective gas cell has been designed and fabricated. The new design greatly simplifies the fabrication process of the spectral analyzer and expands the range of operation towards longer wavelengths, useful for more sensitive detection of SO 2 and NO 2 .

A computer controlled narrow bandpass optical tunable filter using a Fourier diffraction grating in the range of 1.3-1.55 μm

A novel tunable filter using a Fourier grating is studied and discussed. It consists of a Fourier grating, a collimator lens, single-mode fibers, and a computer controlled rotary actuator. It has very low polarization dependence of less than 0.3 dB, an insertion loss of less than 3 dB, and a narrow bandwidth of 1.5 nm. Wavelength selection was realized by rotating the grating, which was controlled by a computer. The device is able to select the light at the desired wavelength from more than 100 WDM optical data streams with 2 nm spacing in the range of 1.3-1.55 /spl mu/m.

Cotton Moisture Measurement with a Black-and-white Video Camera

Moisture meters employed in a prototypical cotton gin control system use lead-sulfide sensors which measure near-infrared energy at wavelengths beyond the range of standard silicon-video sensors. These meters are expensive and must be constructed and installed separate from color and trash sensing equipment. Because silicon-video sensors have some capacity to measure energy in the near-infrared range closer to visible light wavelengths, a standard, silicon-chip, charge-coupled device, infrared-sensitive, black-and-white video camera was examined for its cotton moisture measurement ability. Three narrow band-pass optical filters centered at 925, 940, and 950 nm were placed individually in front of the camera lens, and digital images of cotton samples were recorded for each filter at six different moisture levels. Pixel-intensity averages were calculated for sample images. This data was compared to oven moisture tests of the samples. Multiple polynomial regression with oven moisture as the dependent variable and with various combinations of filtered intensities as independent variables indicated that the technique was significantly correlated (at the 0.05 probability level) with cotton moisture content of both lint and seed cotton. Regression on four-replication averages increased the correlation between predicted and actual moisture content in seed cotton. However, all correlations were weak. While the results were not entirely promising, they did demonstrate the ability of silicon sensors to detect moisture content in cotton, and the usefulness of further research to refine the technique.

Analysis of optical gain enhanced erbium-doped fiber amplifiers using optical filters

Erbium-doped fiber amplifiers (EDFAs) with enhanced optical gain obtained by incorporating narrow-bandpass optical filters into the amplifier length are studied. It is shown in theory that it is possible to increase optical gain by more than 10 dB for optical signals around the wavelength of 1.55 mu m, compared with conventional EDFAs without filters. It is also shown that the gain improvement at longer wavelengths away from the amplifier gain peak is much higher than that of the EDFA with an optical isolator within the amplifier length. The optimum filter position is found to be around 42% of the total amplifier length from the input end. The effects of filter insertion loss and pump loss are discussed. This amplifier can be used as an optical preamplifier in a receiver for a wide range of wavelengths. >

Use of narrow bandpass optical filters in Zeeman polarimetry for poloidal field measurements on the Texas Experimental Tokamak (abstract)

Very narrow bandpass optical fibers have been used in Zeeman polarimetry of Li I 6708 Å line emission from a monoenergetic lithium beam as a spectral source makes the measurement well localized. Initial experimental results for measurements of q0 will be presented.

Use of optical filters for Tokamak plasma spectroscopy

A description is given of a method for using narrow-bandpass optical interference filters to make a number of simultaneous chord observations of plasma radiation from a Tokamak. Employed with the Canberra Tokamak LT-3 the method has been applied to provide measurements of O III, O IV and O V impurity radiation from a hydrogen plasma.