Narrow Pass Filter Research Articles

Experimental Insights on Factors Influencing Sensitivity of Thin Film Narrow Band-Pass Filters

First time study for development of relationship between design and fabrication of thin film narrow-pass filters simulated using the Essential Macleod optical coating design program and verified through experimental data. In thin film narrow band-pass filter's design, can design better filter by changing some parameters, but the theoretical designs are often difficult to achieve. The sensitivity of thin film narrow band-pass filters are mainly influenced by the refractive index of cavity layers, number of mirror layers, interference order and number of cavities. Based on analysis of all these aspects, laws of influence are obtained. A narrow band-pass filter with super wide rejection band was designed and fabricated to verify the simulation results, showing a good agreement between the experimental and theoretical observations.

Mid-wavelength infrared p-on-n Hg1−xCdxTe heterostructure detectors: 30–120 kelvin state-of-the-Art performance

We report on Hg1−xCdxTe mid-wavelength infrared (MWIR) detectors grown by molecular-beam epitaxy (MBE) on CdZnTe substrates. Current-voltage (I-V) characteristics of HgCdTe-MWIR devices and temperature dependence of focal-plane array (FPA) dark current have been investigated and compared with the most recent InSb published data. These MWIR p-on-n Hg1−xCdxTe/CdZnTe heterostructure detectors give outstanding performance, and at 68 K, they are limited by diffusion currents. For temperatures lower than 68 K, in the near small-bias region, another current is dominant. This current has lower sensitivity to temperature and most likely is of tunneling origin. High-performance MWIR devices and arrays were fabricated with median RoA values of 3.96 × 1010 Ω-cm2 at 78 K and 1.27 × 1012 Ω-cm2 at 60 K; the quantum efficiency (QE) without an antireflection (AR) coating was 73% for a cutoff wavelength of 5.3 µm at 78 K. The QE measurement was performed with a narrow pass filter centered at 3.5 µm. Many large-format MWIR 1024 × 1024 FPAs were fabricated and tested as a function of temperature to confirm the ultra-low dark currents observed in individual devices. For these MWIR FPAs, dark current as low as 0.01 e−/pixel/sec at 58 K for 18 × 18 µm pixels was measured. The 1024 × 1024 array operability and AR-coated QE at 78 K were 99.48% and 88.3%, respectively. A comparison of these results with the state-of-the-art InSb-detector data suggests MWIR-HgCdTe devices have significantly higher performance in the 30–120 K temperature range. The InSb detectors are dominated by generation-recombination (G-R) currents in the 60–120 K temperature range because of a defect center in the energy gap, whereas MWIR-HgCdTe detectors do not exhibit G-R-type currents in this temperature range and are limited by diffusion currents.

Partial rotation-invariant pattern matching and face recognition with a joint transform correlator

We describe the circular-harmonic (CH) image CH(mr), which is based on CH components for rotationally invariant pattern recognition. CH components of the order m, derived from an image in polar coordinates, are used to form a two-dimensional space together with the radial variable r. Filtering the CH(mr) image leads to a reference image with some rotational invariance. For a narrow-pass filter we have a single CH component with full rotation invariance; for an all-pass filter we have the original image with no rotational invariance; for a low-pass filter we form a reference image containing multiple circular harmonics with partial rotation invariance. Results of numerical simulations and optical experiments with a joint transform correlator are given that illustrate partial-rotation-invariant recognition for human face images.

Self-aligning comparison beam methods for one-, two- and three-dimensional optical velocity measurements

The automatic beam superposition properties of the Wollaston prism are used to develop comparison beam systems for the simultaneous optical measurement of one, two and three components of the velocity vector in fluids. The systems do not require beam alignment for heterodyning, thus removing a traditional disadvantage of comparison beam systems compared with cross-beam systems and permitting full use of the fact that comparison beam systems are inherently better suited for multidimensional velocity measurement than cross-beam systems. The systems discussed use photodiodes (having one or more photosensitive areas) without preamplification. No electronic or optical high, low or narrow pass filters are used.

Lidar Observation of Cloud

Lidar (from "light detection and ranging") is the optical counterpart of meteorological radar. At optical wavelengths, very much smaller atmospheric particles can be detected than at microwave wavelengths. With a laser power source, a transmitter uses a lens system to beam very intense pulses of monochromatic light of extremely short duration. Light backscattered by the atmosphere is collected in a receiver system that is essentially a telescope coaligned with the transmitter, and a narrow-pass filter allows only light of the transmitted frequency to be detected by a photomultiplier. Data are presented on an oscilloscope as a trace of signal intensity versus range (the A-scope of radar practice) and photographed.

Photographic Photometry of a Flash Tube Using a High-Speed Streak Camera*

The General Radio Strobolux unit triggered by a Strobotac has been used extensively in this laboratory for visual threshold determinations. This photometric study was undertaken to provide information on the duration of the flash and the constancy of the total energy for successive flashes. By using a standardized ribbon filament lamp for a comparison source and four narrow pass filters, the total energy per flash in absolute units was found for four spectral regions. A high-speed camera developed by O’Brien was used for the photographic record. A narrow slit illuminated by the Strobolux was imaged on a strip of 16-mm Super-XX film carried on the inner circumference of a drum mounted on the shaft of a vertical motor. When the drum was driven at 10 800 rpm, the exposure time on the film was 0.236 µsec. The film was calibrated by exposing a standard lamp through a stepped slit on the same record. The peak luminance through a 550-mµ interference filter is 7660 c/cm2 of tube area. The duration of the flash is about 100 µsec with 90% of the energy delivered in 41 µsec.

Zeeman-Split Absorption Lines as Very Narrow Pass Filters

Zeeman-Split Absorption Lines as Very Narrow Pass Filters