Transmission Characteristics Of Filters Research Articles

Design and analysis of polarization-beam-splitter-based fiber optical comb filter

In this paper, we proposed and demonstrated a novel switchable fiber comb filter based on polarization beam splitter (PBS). The proposed filter transmission characteristics is theoretically analyzed in detail by using the transmission matrix theory. The simulated results show that the filter can generate two different output channels selected by adjusting the polarization controllers (PC) in front of the PBS, and the two different channels’ wavelength intervals can be changed with polarization-maintaining fiber’s (PMF’s) characteristics. Different PMFs give a new switchable combination of channel intervals. A multi-wavelength fiber laser (MWFL) based on the proposed filter is experimentally demonstrated. Its wavelength interval can be switched among 0.85 nm, 0.40 nm, which is consistent with the simulated results of the filter. The power fluctuation of the two channel interval outputs in one hour is less than 0.88 dB and 1.37 dB, and the wavelength fluctuation is less than 0.06 nm and 0.18 nm, respectively.

Multiband notch filter based guided-mode resonance for mid-infrared spectroscopy

A dielectric guided-mode resonance (GMR) notch filter with multiple channels was designed to operate in the mid-wave infrared (MWIR, 3∼4μm). The transmission characteristics of filters were analyzed using a finite difference time domain (FDTD) method. In the structure of notch filter, two different periodic (p1=1.6μm and p2=2.0μm) silicon (Si) grating waveguide layers (GWLs) are constructed on the CaF2 substrate. Si waveguide layer thickness and CaF2 cladding layer thickness were designed based on optical interference principle to give maximum transmission across the MWIR. The transmission spectrum presents two narrow band located at 3.410μm and 3.833μm with not only a high bandstop coefficient (>99%), but also a broadband high transmission (>90%) off-resonance. The resonant wavelengths can be flexible modulated in wide range by changing grating periods or filling factors, and it is proved that filter is stable and useful in actual atmospheric conditions. Because of the good performance in the bandstop features, the multiband GMR notch filter designed in this method may have a potential for a variety of imaging or detection applications.

Low-pass filter without the end effect for estimating transmission characteristics—Simultaneous attaining of the end effect problem and guarantee of the transmission characteristics

A Gaussian filter (GF) is the most commonly used low-pass filter of measuring surface roughness. However, undesirable distortions, called “end effects," occur near the end points of the data ends in GF. The transmission characteristics are one of the most important indicators that show the quality of a filter. Previously, it was only possible to obtain theoretical values for the transmission characteristics of filters whose weights were given by an explicit function. In recent years, it has also become possible to obtain the transmission characteristics of a filter whose weights are given by an implicit function. However, this method has a problem in that the values near the end points of the measurement data become significantly different from each other. The consequence is that end effects may occur in the filter outputs due to side effects of the periodic extension. In the case of a spline filter (SF) applied to open profiles, the transmission characteristics of a periodic SF with an end effect can be obtained uniquely. However, the transmission characteristics of a nonperiodic SF, which has no end effect, cannot be uniquely obtained. This results in a trade-off between the two states: end effects exist in a filter whose transmission characteristics can be obtained, and the transmission characteristics of a filter without end effects cannot be uniquely obtained. To address this problem, we propose a method for the GF processing that uses shearing, point symmetric extension, and periodic extension, and produces no end effect but allows the transmission characteristics to be obtained. Previously, there was a problem with the proposed method in that the rationale was unclear regarding how to determine the reference points for point symmetric extension. We resolved this and optimized the reference points. As a result, the proposed method was shown to be successful in not only resolving the end effects in the GF, but also obtaining the transmission characteristics.

High Resolution High Power Low Frequency Digital-to-Analog Converter

The architectural scheme of the designed Sigma-Delta DAC on the FPGA is considered. The place of the interpolator in Sigma-Delta DACs is briefly discussed. The summarized structure of the most common interpolators is presented. More applicable structures of interpolators were suggested and analyzed, providing the comparison with [1]. Having changed the structure of the incomplete interpolator and having optimized the stages, it was possible to improve the characteristic of amplitude frequency response with a smaller number of non-zero coefficients and much lower FPGA resources. The paper provides simulated results of the interpolator filter transmission characteristics as well as Sigma-Delta modulator quantization noise parameters. It is demonstrated that simulation of the complete converter system (interpolator + modulator + output filter) allows to identify places of the interpolator, where hardware resources could be saved, thereby reducing the chip area occupied by the converter, which is not always obvious when analyzing nodes separately. Therefore another version of the interpolator was proposed for the system ensuring larger suppression of the additional frequency band in the whole system compared with the previous interpolator. Simulated results related to occupied chip resources are also confirmed by the experiment, which was implemented in Xilinx Spartan FPGA.

A Refined Calibration Procedure of Two-Channel Sun Photometers to Measure Atmospheric Precipitable Water at Various Antarctic Sites

Abstract Two-channel sun photometers can be easily employed at Antarctic sites, where harsh environmental conditions prevail, to carry out measurements of precipitable water W. In the very dry air conditions observed in the Antarctic atmosphere, water vapor does not produce strong absorption features along the sun path. Therefore, these instruments need to be calibrated using analytical forms different from the square root regime, which can be determined by simulating the output voltages measured at Antarctic sites, for the spectral near-IR curves of extraterrestrial solar irradiance, instrumental responsivity parameters, and atmospheric transmittance, relative to various measurement periods. For this purpose, average models of the Antarctic atmosphere from the ground level up to the 30-km altitude were considered for different solar zenith angles and relative humidity conditions. The ratios between the output voltages simulated in the band and window channels were plotted as a function of total water vapor content Cw, for each site and each period, to define the best-fit calibration curves, which were subsequently normalized to the field measurements to take into account the aging effects on the filter transmission characteristics. Each of the five calibration curves was found to present a slope coefficient decreasing gradually with Cw from values higher than 0.8 to about 0.6. Using these curves, measurements of W were obtained, which differ appreciably at both sea level and high-altitude sites from those given by the square root calibration curves, avoiding large overestimation errors of 10%–40% at the high-altitude sites and underestimation errors of 5%–15% at the sea level site.

Design of multipulse Thomson scattering diagnostic for SST-1 tokamak

A multipulse Nd:YAG (Yttrium aluminum garnet) Thomson scattering (TS) system is designed and developed for measuring electron temperature (T(e)) and density (n(e)) profiles of SST-1 tokamak. The system operates at vertical, divertor, and horizontal (midplane) regions of plasma and measures the electron temperature of 20 eV to 1.5 keV and density of 10(18)-10(19) m(-3). Six Nd:YAG lasers synchronized with external control is used to get three different temporal resolutions (30 Hz, 180 Hz, and 1 kHz). The entire system is laboratory tested for the stability of alignment and performance over a distance of 30 m. Different imaging lens assemblies are designed to image the scattered photons from each of the scattering region to an array of optical fibers. A low cost and compact five-channel interference filter polychromator is designed, fabricated, and tested for its image quality and the filter transmission characteristics. Detection system with an avalanche photodiode and required signal conditioning electronics is developed for detecting the scattered photons. A data acquisition and control module operating on PXI bus is developed for the real time data acquisition and system control. A detailed description of design and testing of TS subsystems is presented in this article.

Performance of lightwave systems incorporating multilongitudinal mode laser and optically preamplified receiver combinations

The performance of optical communication systems employing multilongitudinal mode laser and semiconductor laser preamplified receiver combinations is investigated theoretically, emphasis being placed on the influence of mode partition noise (MPN). Two types of MPN are considered in the analysis: one owing to wavelength-dependent fibre dispersion and the other owing to the wavelength-dependent attenuation mechanism associated with both the amplifier gain and optical filter transmission characteristics. To reduce the influence of amplifier gain ripple and to ensure high receiver performance the mode spacing and the centre mode wavelength of the laser source may need to match closely those of the amplifier, especially for high internal amplifier gains and substantial facet reflectivity. For example, for 1% amplifier facet reflectivity, 30 dB internal optical gain, and 3 nm half-spectral-width laser the receiver sensitivity degrades by 5.1 dB when the lengths of the laser and the amplifier differ by 5%. In contrast with 0.1 and 0.01% reflectivity this reduces to 1.95 dB and 0.4 dB, respectively. We also show that for multimode laser operation there is an optimum bandwidth for the bandpass optical filter at which the required detectable optical power for a given bit error rate (BER) is minimum. Operating with filter bandwidth less than this optimum value will degrade dramatically the sensitivity and may, even in the absence of fibre dispersion, lead to a BER floor in the presence of MPN. The analysis presented here may be used to provide guidelines for the design of optical systems based on laser amplifiers and multimode or nearly single mode laser sources.

Transmission Characteristics of Filters

A study is made of the light transmission of several optical filters, used individually and in combination. Measurements are made over a wavelength range of 0.36–0.75 μ. A strip-chart recorder is used to plot the output of a Barnes ES-100 educational spectrometer.

Properties of All-Dielectric Interference Filters II Filters in Parallel Beams of Light Incident Obliquely and in Convergent Beams

The basic formulas derived in Part I are used to determine the wavelength of the transmission band when all-dielectric interference filters are used in parallel beams incident obliquely. Experimental results agree well with the theoretical predictions provided that the dispersion of the layers is taken into account.The transmission characteristics of filters in convergent beams are evaluated on the basis of the expressions for oblique incidence.