Wavelength Error Research Articles

Performance evaluation of an all-fiber ratiometric wavelength monitor system using edge filters based on sms fiber structures

This article presents performance evaluation of the use of one or two edge filter(s) based on singlemode-multimode-singlemode (SMS) fiber structure on an all-fiber ratiometric wavelength monitor system.Several aspects of noise, polarization-dependent loss (PDL), and temperature-dependent loss (TDL) that can influence the system's performance were investigated. It was demonstrated that the use of two SMS edge filters can improve the system's resolution and wavelength error when compared with the use of one edge filter. © 2013 Wiley Periodicals, Inc. Microwave Opt Technol Lett 55:1645–1649, 2013; View this article online at wileyonlinelibrary.com. DOI 10.1002/mop.27619

Etch depth mapping of phase binary computer-generated holograms by means of specular spectroscopic scatterometry

Detailed analysis of etch depth map for phase binary computer-generated holograms intended for testing aspheric optics is a very important task. In particular, diffractive Fizeau null lenses need to be carefully tested for uniformity of etch depth. We offer a simplified version of the specular spectroscopic scatterometry method. It is based on the spectral properties of binary phase multi-order gratings. An intensity of zero order is a periodical function of illumination light wave number. The grating grooves depth can be calculated as it is inversely proportional to the period. Measurement in reflection allows one to increase the phase depth of the grooves by a factor of 2 and measure more precisely shallow phase gratings. Measurement uncertainty is mainly defined by the following parameters: shifts of the spectrum maximums that occur due to the tilted grooves sidewalls, uncertainty of light incidence angle measurement, and spectrophotometer wavelength error. It is theoretically and experimentally shown that the method we describe can ensure 1% error. However, fiber spectrometers are more convenient for scanning measurements of large area computer-generated holograms. Our experimental system for characterization of binary computer-generated holograms was developed using a fiber spectrometer.

Study on Error Factors of Magneto–Optical Current Transformer

The working principles of MOCT(Magneto–Optical Current Transformer) based on improved dual optical path detection method is studied. The MOCT’s output could be influenced by both the fluctuation of operating wavelength and the error of intersecting angle of optical axis between polarizer and analyzer . Based on IEC 60044-8 standard, the scope of the photosource wavelength and angle error of photic axis are obtained. The results would be useful for researchers and designers working in the MOCT area.

Ultracompact, narrowband three-dimensional plasmonic waveguide Bragg grating in metal/multi-insulator/metal configuration

Ultracompact three-dimensional (3D) waveguide plasmonic Bragg gratings in a metal/multi-insulator/metal (MMIM) configuration with sinusoidal width modulations are presented. A semi-analytical approach from the eigenvalue problem and finite transmission-line network perspectives is described to facilitate the 3D designs with Bragg wavelength errors being within the range of 0.12%-3.99%. A narrowband design operating in the 1550 nm band with a FWHM bandwidth of 10.8 nm and an extinction ratio of approximately 12 dB is numerically demonstrated within a footprint of <17 μm(2) (10 periods). Unlike other types of plasmonic Bragg gratings, the bandwidth is increased as the MMIM grating length increases. The number of distinct plasmonic z-directed Poynting vector patterns within one period is found to be identical to the corresponding Bragg order. Narrowband characteristics are attributed to delicate, concurrent contra-flow interactions in and between photonic and plasmonic modes occurring simultaneously in multiple places within one period.

Quantifying Resolving Power in Astronomical Spectra

AbstractThe spectral resolving power R = λ/δλ is a key property of any spectrograph, but its definition is vague because the ‘smallest resolvable wavelength difference’ δλ does not have a consistent definition. Often, the FWHM is used, but this is not consistent when comparing the resolution of instruments with different forms of spectral line-spread function. Here, two methods for calculating resolving power on a consistent scale are given. The first method is based on the principle that two spectral lines are just resolved when the mutual disturbance in fitting the fluxes of the lines reaches a threshold (here equal to that of sinc2 profiles at the Rayleigh criterion). The second criterion assumes that two spectrographs have equal resolving powers if the wavelength error in fitting a narrow spectral line is the same in each case (given equal signal flux and noise power). The two criteria give similar results and give rise to scaling factors that can be applied to bring resolving power calculated using the FWHM on to a consistent scale. The differences among commonly encountered line-spread functions are substantial, with a Lorentzian profile (as produced by an imaging Fabry–Perot interferometer) being a factor of two worse than the boxy profile from a projected circle (as produced by integration across the spatial dimension of a multi-mode fibre) when both have the same FWHM. The projected circle has a larger FWHM than its true resolution, so using FWHM to characterise the resolution of a spectrograph which is fed by multi-mode fibres significantly underestimates its true resolving power if it has small aberrations and a well-sampled profile.

Wavelength Anomalies in Ultraviolet-Visible Spectrophotometry

Spectral scans of atomic line emission sources with a Shimadzu UV2101PC spectrophotometer show that the nominal wavelength depends upon the instrumental slit width, the wavelength sampling interval, and for some slit widths, also on the specified spectral range. The range dependence is manifested as a smoothing that occurs when the range includes >65 sampled wavelengths, and it affects both the wavelength and the line shape. For spectra not subject to this smoothing, the wavelength error looks like a one-sample misassociation of the wavelength and photometric readings. However, the instrument does reliably move to a specified wavelength, independent of the scan parameter settings. These behaviors do not seem to be documented anywhere but have been present in the software for operating this instrument for about two decades.

Study of a Novel Demodulation Method for FBG Sensor

A novel demodulation method for Fiber Bragg Grating (FBG) sensor based on matching grating demodulation is proposed. It is means using several fiber gratings to cascade, and programming to make the computer automatically select different wavelengths. So that it can expand demodulation range based on high sensitivity. The photoelectric detector (PD) makes the reflected light intensity which form FBG change into voltage. And the voltage is amplified to 0~5v by amplifying circuit. Then through the data collection box, voltage sent to computer for data acquiring. Real-time analysis data and display the variable of center wavelength for sensing FBG. Because the same type of sensing FBG and demodulation FBG, they have the same temperature drift, so it can control the influence of environmental temperature. Therefore, this system can successfully solve the temperature/strain cross-sensitivity issue. The experimental results show that it is feasible to demodulate wavelength using the cascaded FBG. And the experimental measuring wavelength error is less than 0.009 nm, the measuring micro-strain error is less than 7.5με.

Regional surface mass anomalies from GRACE KBR measurements: Application of L‐curve regularization anda priori hydrological knowledge

We present a method of directly estimating surface mass anomalies at regional scales using satellite‐to‐satellite K‐band Ranging (KBR) data from the Gravity Recovery and Climate Experiment (GRACE) twin‐satellite mission. Geopotential differences based primarily on KBR measurement are derived using a modified energy integral method with an improved method to calibrate accelerometer measurements. Surface mass anomalies are computed based on a downward continuation process, with optimal regularization parameters estimated using the L‐curve criterion method. We derive the covariance functions in both space‐ and space‐time domains and use them as light constraints in the regional gravity estimation process in the Amazon basin study region. The space‐time covariance function has a time‐correlation distance of 1.27 months, which is evident that observations between neighboring months are correlated and the correlation should be taken into account. However, most of the current GRACE solutions did not consider such temporal correlations. In our study, the artifact in the regional gravity solution is mitigated by using the covariance functions. The averaged commission errors are estimated to be only 6.86% and 5.85% for the solutions based on the space‐covariance function (SCF) and the space‐time covariance function (STCF), respectively. Our regional gravity solution in the Amazon basin study region, which requires no further post‐processing, shows enhanced regional gravity signatures, reduced gravity artifacts, and the gravity solution agrees with NASA/GSFC's GRACE MASCON solution to about 1 cm RMS in terms of water thickness change over the Amazon basin study region. The regional gravity solution also retains the maximum signal energy while suppressing the short wavelength errors.

Airborne Gravity Surveys Over Taiwan Island and Strait, Kuroshio Current and South China Sea: Comparison of GPS and Gravity Accuracies at Different Flight Altitudes

We compare the accuracies of GPS and gravity observations obtained from three airborne gravity surveys over Taiwan Island at altitude 5000 m and over Kuroshio Current, Taiwan Strait, and Dongsha Atoll at altitude 1500 m. A kinematic network adjustment was used to determine the positions of the aircrafts. GPS positioning errors are at the decimeter-level, which are not entirely propagated to velocity and acceleration errors due to cancellation of long wavelength errors. Outliers are downweighted in the Gaussian filtering to improve the gravity accuracy, especially at altitude 1500 m. Compared with the upward-continued gravity, the gravity anomalies from the 1500-m surveys show a consistent accuracy of about 3 mgal; the accuracy from the 5000-m survey is degraded, especially over high mountains. The RMS crossover differences at 1500 m and 5000 m are all below 3 mgal, suggesting flight altitudes do not affect the crossover difference. Coherence analysis suggests that the spatial resolvable wavelengths of the airborne gravity range from 4 km (altitude 1500 m) to 6 km (altitude 5000 m).

High Flattened Comb Filter with Precise Channel Spacing Based on Linearly Chirped Fiber Bragg Grating and Phase Compensation

A novel approach to implement high flattened comb filters with precise channel spacing based on DC phase compensation is proposed. Channel wavelength error and peak reflectivity variation are discussed and calculated. A phase compensation method by simply changing the DC refractive index is proposed. Fiber Bragg gratings with different DC phase shifts are fabricated, which indicates that an arbitrary DC phase shift can be introduced precisely. A high flattened comb filter with precise 50-GHz channel spacing is realized with the help of a micrometer-precision translation stage. Then the peak reflectivity variation with and without phase compensation for different chirp coefficients is researched.

Method for improving the accuracy of step height measurement based on optical frequency comb

An improved method of step height measurement by optical frequency comb and tunable diode laser is presented. The tunable diode laser is phase locked to the optical frequency comb, so that the output wavelength of the laser can be locked and measured in a high accuracy. Based on the principle of variable synthetic wavelength chain, the step height can be measured by tuning the wavelength of the locked tunable diode laser. This method can be used to eliminate the effect of the synthetic wavelength error on the measurement uncertainty of the step height. The stability of the phase locked laser is tested by locking a tunable diode laser to an optical frequency comb for about 5000 s. It is shown that the frequency stability of the phase locked laser is 1.810-12. The uncertainty of the step height measurement by this method is estimated to be 7.9 nm accordingly. Moreover, the measurement result of the step height can be traced to the time and frequency standard.

Identification and elimination of half-synthetic wavelength error for multi-wavelength long absolute distance measurement

A multi-wavelength absolute distance measurement method based on synchronous occurrence and phase measurement of coarse and refined synthetic wavelength is proposed to remove the effect of vibration on the measurement distance. The phase value of refined synthetic wavelength is monitored to see if half-synthetic wavelength error occurs by judging whether it is near to 2π or 0. If yes, a new suitable refined synthetic wavelength is generated to re-measure the distance with phase near π. A measuring system has been built with a He–Ne laser source and three acousto-optic frequency shifters for implementation of this method. A comparative measurement has been performed using a counting laser interferometer at a distance of 20 m. Actual measurements indicate that half-synthetic wavelength error has been identified and eliminated with uncertainty of smaller than 60 µm under laboratory conditions.

Si micro-ring MUX/DeMUX WDM filters

We demonstrate 3rd order micro-ring filters, 100 GHz-spaced 16 channels and 50 GHz-spaced 32 channels. Fabrication-induced resonant wavelength errors, σ = 0.237 nm, and temperature-dependent wavelength shift, 0.043 nm/°C tolerable to ΔT>10 °C, has been measured on filters based on the fundamental TM mode. The problem of CMOS-compatible photolithography is solved, while maintaining a small radius, R = 9 μm. As some dummy channels are arranged, it is shown that an on-chip optical network for many cores CPU can be constructed by 16 channel ring filters with the currently available technology.

The Measurement of LED Domain Wavelength Based on LabVIEW

The domain wavelength is the most important colorimetric parameter of tested LED, it decides the LED color quality. The data acquisition system is build of linear CCD, monochromator，and PCI data acquisition card, which can obtain the chart of distribution value of 1urninous intensity and color coordinate of LED with LabVIEW. This paper present measurement method based on the table method, the measurement method is looking-up two domain wavelengths which distance is 1nm through slope which get from testing LED color coordinate and white light LED coordinate, and calculate the domain wavelength of testing LEDs using LabVIEW and mathematic interpolation, the measurement error of domain wavelength is 1 nm.

Application of Fiber Bragg grating for determining positions of gas absorption lines

Fiber Bragg grating (FBG) is used as a wavelength reference device to calibrate the position of gas absorption peak in the intracavity absorption gas sensor (ICAGS) based on erbium-doped fiberring laser. This system can detect both the reflectance spectrum of FBGs and absorption spectrum of measured gas during a single wavelength sweeping process by linearly varying the driving voltage of optic filter. The voltages corresponding to center wavelength positions of four FBGs in the spectrum are determined through Gaussian peak fitting. Then, the wavelengths of gas absorption lines are deduced from the correspondence between driving voltage and wavelength obtained by quadratic curve fitting. The maximum error of wavelength of acetylene absorption lines between experimental results and those from HITRAN database is 0.106 nm and the resolving accuracy of two adjacent absorption lines is about 93.593%. By using this method, ICAGS can theoretically recognize the measured gas type and monitor multi-gas components.

High-throughput accurate-wavelength lens-based visible spectrometer

A scanning visible spectrometer has been prototyped to complement fixed-wavelength transmission grating spectrometers for charge exchange recombination spectroscopy. Fast f/1.8 200 mm commercial lenses are used with a large 2160 mm(-1) grating for high throughput. A stepping-motor controlled sine drive positions the grating, which is mounted on a precision rotary table. A high-resolution optical encoder on the grating stage allows the grating angle to be measured with an absolute accuracy of 0.075 arc sec, corresponding to a wavelength error ≤0.005 Å. At this precision, changes in grating groove density due to thermal expansion and variations in the refractive index of air are important. An automated calibration procedure determines all the relevant spectrometer parameters to high accuracy. Changes in bulk grating temperature, atmospheric temperature, and pressure are monitored between the time of calibration and the time of measurement to ensure a persistent wavelength calibration.

High‐fidelity spectroscopy at the highest resolutions

AbstractHigh‐fidelity spectroscopy presents challenges for both observations and in designing instruments. High‐resolution and high‐accuracy spectra are required for verifying hydrodynamic stellar atmospheres and for resolving intergalactic absorption‐line structures in quasars. Even with great photon fluxes from large telescopes with matching spectrometers, precise measurements of line profiles and wavelength positions encounter various physical, observational, and instrumental limits. The analysis may be limited by astrophysical and telluric blends, lack of suitable lines, imprecise laboratory wavelengths, or instrumental imperfections. To some extent, such limits can be pushed by forming averages over many similar spectral lines, thus averaging away small random blends and wavelength errors. In situations where theoretical predictions of lineshapes and shifts can be accurately made (e.g., hydrodynamic models of solar‐type stars), the consistency between noisy observations and theoretical predictions may be verified; however this is not feasible for, e.g., the complex of intergalactic metal lines in spectra of distant quasars, where the primary data must come from observations. To more fully resolve lineshapes and interpret wavelength shifts in stars and quasars alike, spectral resolutions on order R = 300 000 or more are required; a level that is becoming (but is not yet) available. A grand challenge remains to design efficient spectrometers with resolutions approaching R = 1 000 000 for the forthcoming generation of extremely large telescopes (© 2010 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Optimal combination of InSAR and GPS for measuring interseismic crustal deformation

High spatial resolution measurements of interseismic deformation along major faults are critical for understanding the earthquake cycle and for assessing earthquake hazard. We propose a new remove/filter/restore technique to optimally combine GPS and InSAR data to measure interseismic crustal deformation, considering the spacing of GPS stations in California and the characteristics of interseismic signal and noise using InSAR. To constrain the longer wavelengths (>40 km) we use GPS measurements, combined with a dislocation model, and for the shorter wavelength information we rely on InSAR measurements. Expanding the standard techniques, which use a planar ramp to remove long wavelength error, we use a Gaussian filter technique. Our method has the advantage of increasing the signal-to-noise ratio, controlling the variance of atmosphere error, and being isotropic. Our theoretical analysis indicates this technique can improve the signal-to-noise ratio by up to 20%. We test this method along three segments of the San Andreas Fault (Southern section near Salton Sea, Creeping section near Parkfield and Mojave/Big Bend section near Los Angeles), and find improvements of 26%, 11% and 8% in these areas, respectively. Our data shows a zone of uplift to the west of the Creeping section of the San Andreas Fault and an area of subsidence near the city of Lancaster. This work suggests that after only 5 years of data collection, ALOS interferograms will provide a major improvement in measuring details of interseismic deformation.

Focus on test and measurement

Focus on test and measurement

Breadboard Model of On-orbit Calibration Equipment for Small Hyperspectral Sensor

This paper examined the effect of atmospheric correction on that hyperspectral sensors have wavelength error. Also we examined the effects of classification result on erroneous atmospheric correction. Additionally, this paper proposes new spectral calibration device named OCE (On-orbit Calibration Equipment) which consists of the visible and near infrared LEDs. The calibration accuracy by OCE compared with spectral lines of mercury and xenon has been evaluated. The OCE-BBM to central pixel of spatial direction can be calibrated with sufficient accuracy.