Wavelength Calibration Method Research Articles

Comparison of On-Sky Wavelength Calibration Methods for Integral Field Spectrograph

With advancements in technology, scientists are delving deeper in their explorations of the universe. Integral field spectrograph (IFS) play a significant role in investigating the physical properties of supermassive black holes at the centers of galaxies, the nuclei of galaxies, and the star formation processes within galaxies, including under extreme conditions such as those present in galaxy mergers, ultra-low-metallicity galaxies, and star-forming galaxies with strong feedback. IFS transform the spatial field into a linear field using an image slicer and obtain the spectra of targets in each spatial resolution element through a grating. Through scientific processing, two-dimensional images for each target band can be obtained. IFS use concave gratings as dispersion systems to decompose the polychromatic light emitted by celestial bodies into monochromatic light, arranged linearly according to wavelength. In this experiment, the working environment of a star was simulated in the laboratory to facilitate the wavelength calibration of the space integral field spectrometer. Tools necessary for the calibration process were also explored. A mercury–argon lamp was employed as the light source to extract characteristic information from each pixel in the detector, facilitating the wavelength calibration of the spatial IFS. The optimal peak-finding method was selected by contrasting the center of weight, polynomial fitting, and Gaussian fitting methods. Ultimately, employing the 4FFT-LMG algorithm to fit Gaussian curves enabled the determination of the spectral peak positions, yielding wavelength calibration coefficients for a spatial IFS within the range of 360 nm to 600 nm. The correlation of the fitting results between the detector pixel positions and corresponding wavelengths was >99.99%. The calibration accuracy during wavelength calibration was 0.0067 nm, reaching a very high level.

Fraunhofer line-based wavelength-calibration method without calibration targets for planetary lander instruments

High-accuracy wavelength calibration is critical for qualitative and quantitative spectroscopic measurements. Many spectrometers employed in planetary-exploration missions have onboard calibration sources, including standard lamps and calibration targets. However, such calibration sources are not always available because planetary missions, particularly landing missions, usually have limitations in size and mass. Thus, a wavelength calibration method without requiring hardware addition can be highly beneficial. In this study, we demonstrate a method for wavelength calibration using solar Fraunhofer lines observed in the reflectance spectra of planetary surfaces. Using a Raman spectrometer prototype developed for a Phobos rover, we measured the spectrum of the sunlight reflected from a spectral standard, manufactured to provide similar reflectance spectra to the surface of Phobos. We identified 35 Fraunhofer absorption lines in the wavelength range between 530 and 700 nm and utilized these features for the wavelength calibration of the spectrometer. This approach using Fraunhofer lines achieved good results (better than +0.04/−0.06 nm), comparable to the results achieved using a conventional Ne lamp. The wavelength accuracy corresponds to a wavenumber accuracy better than ±1.5 cm−1 in the 0–4000 cm−1 Raman shift (Stokes shift) range with a 532 nm excitation laser. This result enabled the estimation of the magnesium number (Mg#) of olivine, achieving a value more precise than 1.5% based on the Raman peak positions. In addition, we examined the number of solar Fraunhofer lines detectable at different wavelength resolutions by binning the solar spectrum acquired in this study. We found that more than 10 Fraunhofer lines could be detected as prominent absorption lines when the wavelength resolution is higher than 1 nm/pix (30 cm−1/pix at 1000 cm−1). This result suggests that the target-free wavelength-calibration method using solar Fraunhofer lines can be applied to other spectrometers simply by observing sunlit planetary surfaces.

Wavelength Calibration for the LIBS Spectra of the Zhurong Mars Rover

China’s first Mars rover, Zhurong, landed on the southern region of Utopia Planitia, Mars, on 14 May 2021 (UTC). Zhurong is equipped with the Mars Surface Composition Detection Package (MarSCoDe), which analyzes the Martian surface’s material composition. Composed of laser-induced breakdown spectroscopy (LIBS), short-wave infrared spectroscopy (SWIR), and a microimaging camera, MarsCoDe can work at a distance of 1.6–7 m to analyze element abundance and the mineralogy of targets on the Martian surface. Analysis shows that the wavelengths of MarSCoDe onboard LIBS spectra acquired within the same probe period will have different degrees of drift, leading to deviation in qualitative and quantitative elemental analysis. This paper finds that the spectrum drift follows a quadratic function relationship with the CCD temperature of the MarSCoDe spectrometer, based on which a wavelength calibration method is established. According to the function, the drift of a certain channel is calculated by the corresponding CCD temperature, and then the wavelength of the spectrum is calibrated by the drift. The accuracy of this calibration method for the position of peak wavelength in the LIBS spectrum can reach about 1/5 of the apparatus spectral width, and the cross-validation analysis using a norite standard sample shows that it is comparable to the wavelength calibration accuracy of the ChemCam onboard data product.

Robust Full-Screen Wavelength Calibration Algorithm

In spectrometer measurement, it is very important to accurately calibrate the wavelength of all target characteristic spectra. Although wavelength calibration methods have long been investigated, no techniques have been designed for the scanning, double-layer, secondary diffraction, linear-array CCD spectrometer, to the best of our knowledge. Based on the grating diffraction equation and experimental results, a mathematical model of wavelength calibration was established for the scanning, double-layer, secondary diffraction, linear-array CCD spectrometer. In this study, a robust, full-screen, wavelength calibration algorithm is proposed, based on the related working principle and the requirements of both accuracy and robustness. The detailed steps are as follows. First, we established a wavelength calibration model at central pixels, following the grating diffraction equation. Then, according to the relationship between the difference in pixels and the feedback values of the grating ruler, a model was established to show the association between these factors. Finally, we combined the two models and built a full-screen wavelength calibration model. We theoretically and experimentally demonstrate that the proposed calibration algorithm is an excellent calibration tool, which can conveniently and accurately calibrate the wavelengths of central and non-central pixels at the same time.

Dynamic wavelength calibration based on synchrosqueezed wavelet transform.

With the developments of the tunable laser source (TLS), there are increasing demands for high-resolution dynamic wavelength calibration in recent years. Considering mutual constraints between wide measurement range and high calibration resolution, we propose a dynamic wavelength calibration method based on an auxiliary Mach-Zehnder interferometer (MZI) and the synchrosqueezed wavelet transform (SSWT). Our proposed method can achieve a calibration resolution of 5 fm and a tuning range of 10 nm. Moreover, the measurement range and spatial resolution of the optical frequency domain reflectometer (OFDR) system are improved to ∼80 m and ∼mm, respectively. Our proposed approach can substantially reduce the subtle spectrum distortion (tens of fm) in coherent optical spectrum analyzer (COSA) systems.

A wavelength calibration method for photoelastic-modulated Fourier transform spectrometers

PurposeAn interferogram is produced by modulating the difference between the extraordinary refractive index and the ordinary refractive index for photoelastic crystals in photoelastic-modulated Fourier transform spectrometers (PEM-FTs). Due to the influence of the refractive index dispersion characteristics on the maximum optical path difference of the interferogram, it is necessary to study wavelength calibration methods.Design/methodology/approachA wavelength calibration method for PEM-FTs was proposed based on the modulation principle of the photoelastic-modulated interferometer and the relationship between the maximum optical path difference and the refractive index difference. A 632.8 nm narrow-pulse laser was used as a reference source to measure the maximum optical path difference () of the interferogram, and the parameter was used to calculate the discrete frequency points in the frequency domain. To account for the influence of refractive index dispersion on the maximum optical path difference, the refractive index curve for the photoelastic crystal was used to adjust the discrete frequency coordinates.FindingsThe error in the reconstructed spectral frequency coordinates clearly decreased. The maximum relative error was 2.5%. A good solar absorption spectrum was obtained with a PEM-FT experimental platform and the wavelength calibration method.Originality/valueThe interferogram is produced by adjusting the difference between extraordinary refractive index and ordinary refractive index for the photoelastic crystal in the PEM-FTs. Given the wavelength dependence on the refractive indices, in view of the modulation principle of the photoelastic modulated interferometer, the relationship between the maximum optical path difference and the refractive index difference, the variation law of the refractive index of the photoelastic crystal and the process of spectral reconstruction is presented in this paper.

Application of high precision wavelength calibration method for plasmas rotation measurement based on Fabry-Pérot etalon on experimental advanced superconducting tokamak.

Analyzing the radiation spectra of impurity ions is a widely applied diagnostic scheme for plasma ion temperature and rotation measurements on tokamaks. High precision wavelength calibration is a prerequisite for the accurate measurement of plasma parameters, especially for plasma rotation. Furthermore, the sparseness or absence of the standard spectral lines brings calibration challenges due to the narrow wavelength range. A precise wavelength calibration method is demonstrated in which the comb-like spectra generated by the Fabry-Pérot etalon can lock a series of fixed peaks as reference points in a wide wavelength range. The equal frequency intervals of the comb-like spectra are further corrected using several characteristic neon lines of known wavelengths. The experimental results indicate that the wavelength accuracy obtained by this calibration method is less than 0.005 nm, which corresponds to a rotation speed of 2.3 km/s in the toroidal direction for the beam emission spectroscopy spectrometer installed on the experimental advanced superconducting tokamak. Taking the O V(650.024 nm, n = 4 → 3) line as an example, the maximum difference in the oxygen ion rotation velocity is 3.8 km/s for the absolute rotation of ∼25 km/s, when compared with the calibration results of a standard lamp.

A novel multi-band plenoptic pyrometer for high-temperature applications

A snapshot, multi-band imaging pyrometer was developed for two-dimensional, high-temperature measurements. The multi-band pyrometer is composed of a plenoptic camera and a continuous wavelength filter (450 nm–850 nm), located at the aperture plane of the primary imaging lens. A microlens array forms micro-images of the color filter with a resulting spectral resolution of approximately 25 nm per pixel. A wavelength calibration method was developed to use eight narrow-band light-emitting diodes at discrete wavelengths with sub-pixel wavelength location determined using a weighted intensity centroid algorithm. Temperature calibration was conducted using a graphite block as a gray-body radiator placed in a box furnace with controlled temperatures of 600 ∘C–1100 ∘C. Temperature is estimated at each microlens location using the spectral pyrometry method, which reduces the dependency of the measurement on a priori knowledge of material emissivity. Simulated data with Gaussian noise demonstrates reduced uncertainty at higher temperatures and larger bit-depths. Experimental measurements with a heated graphite block demonstrated accuracy within the uncertainty bounds of a reference thermocouple (±0.75%). In addition, validation experiments were conducted in the more dynamic environments of a strand burner and a solidifying copper pool, which contain high-temperature, moving particles and phase dependent emissivity, respectively. The addition of alumina particles to solid fuel produced an expected increase of over 400 ∘C that was measured by both a multi-band imaging pyrometer and a thermocouple. The pyrometer demonstrated the ability to measure temperatures for liquid and solid phases of copper simultaneously with temperature accuracy better than 5%, despite larger differences in material emissivity.

Wavelength calibration methods in laser wavelength measurement.

At present, accurate wavelength calibration plays an important role in laser spectrum measurements. Although the wavelength calibration methods have been investigated for a long time, there are no techniques that are particularly designed for laser spectral calibration to the best of our knowledge. A mathematical model for calibrating a pulse laser wavelength is first established, to the best of our knowledge. According to the analysis formula of dispersion aberration, a flat-field concave grating in the near-infrared band is designed. Then, a wavelength calibration model based on concave grating spectroscopy is proposed. Through adjusting the spectra of each pixel, we design a calibration algorithm based on the cubic spline interpolation and kernel regression methods. By compensating and interpolating spectral data, accurate wavelengths are obtained. Finally, some experiments verify the calibration performance of the proposed method. Meanwhile, the uncertainty of measurement is also analyzed.

A wavelength calibration method for variable-included-angle plane grating monochromators with sine drives

The wavelength calibration is a key step for a variable-included-angle plane grating monochromator (VIA-PGM). At present, the wavelength calibration of the VIA-PGM is done separately for sine drives of the optical components. But the positions of the sine drives are linked by the grating equation and the focusing condition. A new wavelength calibration method for VIA-PGM is presented in which the relationship between the wavelength and both sine drive positions are included in a mathematical model. By this method the structural parameters for both sine drives are calibrated at the same time. It has been successfully applied to the wavelength calibration of a VIA-PGM in the Hefei Light Source, China. The results show that this method is effective and can perform accurate wavelength calibration.

Spectrometer calibration with reduced dispersion for optical coherence tomography

A wavelength calibration method is proposed for Fourier domain optical coherence tomography. In the present study, the wavelength remapping procedure is based on the spectral phase function determined by the calibration signal. To accomplish high accuracy feature for wavelength calibration, a common-path interferometer is employed. Two autocorrelation interferograms generated from the common-path interferometer are utilized as the calibration signals. The advantage of the interferometer proposed here is that the accurate optical path difference of the calibration signals could be acquired easily. The wavelength distribution in the spectrometer was deduced with the phase signal. The approach was compared to a wavelength-determined approach using a standard light source with a characterized spectrum. With the result that the mean spectrometer calibration error is 0.1 nm, it demonstrates that the proposed method is more superior in spectrometer calibration. Furthermore, the proposed method allows for higher axial imaging resolution and signal-to-noise ratio (SNR) in the FD-OCT system.

Recovered HCN Absorption Spectrum-Based FBG Demodulation Method Covering the Whole C-Band for Temperature Changing Environment

Calibration by hydrogen cyanide (HCN) absorption spectrum can make the fiber Bragg grating (FBG) demodulation system immune to environment temperature changes. The transmittance function of Fabry-Perot (F-P) filter will result in the distortion of the HCN absorption peaks. A demodulation scheme based on deconvolution is proposed to eliminate the impact of the F-P filter. A recovered HCN absorption spectrum with deeper, narrower and more symmetrical absorption peaks is obtained for the Bragg wavelength calibration. It is proved experimentally that this method can reduce the standard deviation of demodulated FBG wavelengths by at least 45.9% at a constant temperature, and 42.9% in the environment of varying temperature from 10 °C to 60 °C. Compared with the method of Bragg wavelength calibration based on the original HCN absorption spectrum, the spectral bandwidth that can be accurately demodulated is also expanded to the whole C-band.

宽谱段高光谱成像仪星上波长定标方法

宽谱段高光谱成像仪星上波长定标方法

Comparison and analysis of wavelength calibration methods for prism – Grating imaging spectrometer

Abstract To identify spectral information obtained from the prism - grating imaging spectrometer, it is essential to know the center wavelength for each spectral channel. In this letter, the wavelength calibration principle of push-broom imaging spectrometers is described, and a wavelength calibration device is built. The wavelength calibration of the self-developed prism-grating imaging spectrometer using the peak method and the gravity method is completed, and the center wavelength of each spectral channel is obtained by the two methods respectively. Compared with the peak method, the gravity method can effectively avoid the errors caused by the uneven light spot energy in the process of wavelength calibration, and can more effectively complete wavelength calibration. The two methods can meet the practical application and can be selected according to actual demand.

Intelligent and automatic wavelength calibration method

Wavelength calibration is carried out before a spectrometer is used normally. The usual calibration process measures a standard light source of known wavelength, such as that of a mercury argon lamp. The spectral lines formed by the standard light source are divided to find the peak value; then, the peak value of the light source is matched with the wavelength of the standard light source. At present, the calibration of the spectrometer needs to be carried out with human intervention, which requires a lot of work, and the accuracy is not high. In our previous work, we proposed an efficient and accurate peak-finding method and a matching method. This paper will expand on this basis to illustrate a new intelligent and automated wavelength calibration method. In terms of peak searching, we discussed the implementation and defects of the maximum discrete cosine transform, Gaussian mixed model, and polynomial fitting methods; we also compared the maximum linear matching method with the maximum matching method and proposed the advantages of our method. Our experiments show that spectrum segmentation and peak-seeking methods based on spectral energy can effectively solve the problem of segmentation and peak seeking in wavelength calibration and is superior to other fitting methods. The matching method, combining the Hough transform and random sample consensus, can effectively solve the matching problem in wavelength calibration without manual intervention. In addition, the average accuracy and average recall rate exceed the traditional manual matching method and maximum linear matching method.

Error analysis of mechanical system and wavelength calibration of monochromator.

This study focuses on improving the accuracy of a grating monochromator on the basis of the grating diffraction equation in combination with an analysis of the mechanical transmission relationship between the grating, the sine bar, and the screw of the scanning mechanism. First, the relationship between the mechanical error in the monochromator with the sine drive and the wavelength error is analyzed. Second, a mathematical model of the wavelength error and mechanical error is developed, and an accurate wavelength calibration method based on the sine bar's length adjustment and error compensation is proposed. Based on the mathematical model and calibration method, experiments using a standard light source with known spectral lines and a pre-adjusted sine bar length are conducted. The model parameter equations are solved, and subsequent parameter optimization simulations are performed to determine the optimal length ratio. Lastly, the length of the sine bar is adjusted. The experimental results indicate that the wavelength accuracy is ±0.3 nm, which is better than the original accuracy of ±2.6 nm. The results confirm the validity of the error analysis of the mechanical system of the monochromator as well as the validity of the calibration method.

Maximum linear matching: Intelligent and automatic wavelength calibration method

SummaryWavelength calibration is a necessary means to ensure the normal operation of the spectrometer. In general, calibration light sources that can emit fixed wavelength (such as mercury‐argon lamp) are utilized to generate spectral line on the linear array charge‐coupled device detector. Thus, it is a prerequisite for wavelength calibration to match the pixel position of the well‐divided spectral line with light of different wavelength emitted by calibration light source correctly. In this paper, we aim to present a method to make calibration procedure intelligent and automatic by machine. We establish a pixel‐wavelength model based on bipartite graph and propose the maximum linear matching (MLM) algorithm to find the correct set of pixel‐wavelength automatically. Meanwhile, we calculate precision and recall to measure the effectiveness of MLM and analyze the practical application of MLM by comparing it with conventional artificial methods. Experiments show that the autocalibration method based on MLM can calibrate many types of grating spectrometer more accurately and more reliably. With MLM, we report 98.33% precision and 94.59% recall on 8 groups of experiments.

Accurate wavelength calibration method for compact CCD spectrometer.

Wavelength calibration is an important step in charge-coupled device (CCD) spectrometers. In this paper, an accurate calibration method is proposed. A model of a line profile spectrum is built at the beginning, followed by noise reduction, bandwidth correction, and automatic peak-seeking treatment. Experimental tests are conducted on the USB4000 spectrometer with a mercury-argon calibration light source. Compared with the traditional method, the results show that this wavelength calibration procedure obtains higher accuracy and the deviations are within 0.1 nm.

Wavelength calibration method of temperature FBG based on FPGA and DS18B20

Wavelength calibration method of temperature FBG based on FPGA and DS18B20

Optical Wavelength Meter Calibration Using Iodine Stabilized He-Ne Laser by Direct Measurement Method

Non-stabilized He-Ne lasers are widely used in calibration laboratories and industries. Wavelength calibration is a requirement to know the real wavelength of a non-stabilized He-Ne laser and its deviation. More importantly, calibration process creates a traceability link from the calibrated device to the primary standard of length measurement. One major wavelength calibration method for non-stabilized He-Ne lasers is using direct measurement method which requires a wavelength meter. The wavelength meter has to be calibrated first using a stabilizedHe-Ne laser prior to its use. The recommended radiation power of the stabilized laser in RCM-LIPI, IodineStabilized He-Ne Laser (KIM-1), is (50 ± 25) μW. Here, we examine the calibration process of the high precision wavelength meter with a measurement range of -25 dBm to +10 dBmor equals to 3.16 μW to 10 mW for a wavelength measurement range of 600 nm to 1650 nm. Several uncertainty evaluations for measurement were applied to assess the wavelength meter's performance. The results show that the wavelength meter produces a wavelength reading that is still within the desired band of measurement, thus making it a valid measurement instrument to calibrate the wavelength of non-stabilized He-Ne lasers with 10-3 nm of resolution. The wavelength meter's measurement capability can also be traced back to the length main standard, Iodine Stabilized He-Ne Laser (KIM-1).