Calibration wavelength

Most tunable lasers show an optical frequency u(V) which deviates more-or-less strongly from the linear relation u = oV+b between laser frequency u and input voltage V to the scan electronics. For a visible dye laser the deviations may reach 100 MHz over a 20 GHz scan. These deviations can be monitored and corrected for by comparing the measured frequency markers with the linear expression u = z o+mc/(2nd) (m = 0,1,2...). 

For absolute wavelength measurements of spectral lines the laser is stabilized onto the center of the line and its wavelengths A is measured with one of the wavemeters described in Sect.4.4. For Doppler-free lines (Chaps. 7-11) one may reach absolute wavelength determinations with an uncertainty of smaller than lO cm (=210 /im at A = 500/xm). 

Often calibration spectra are used which are taken simultaneously with the unknown spectra. Examples are the I2 spectrum, which has been published in the iodine atlas by Gerstenkorn and Luc [5.90) in the range of 14800 to 20000 cm . For wavelengths below 500 nm thorium lines can be utilized [5.91] measured in a hollow cathode by optogalvanic spectroscopy (Sect.6.5) or uranium lines [5.92] 

If no wavemeter is available, two FPFs with slightly different mirror separations d and d can be used for wavelength determination (Fig.5.59). Assume 1/ 2 = p/q equals the ratio of two rather large integers p and q with no common divisor and both interferometers have a transmission peak at A  

Let us assume that Aj is known from calibration with a spectral line. When the laser wavelength is tuned, the next coincidence appears at A2 = A +AA where 

Between these two wavelengths A-i and A.2 the maximum of a spectral line with the unknown wavelength kx may appear in a linear wavelength scan at the distance 8x from the position of A-i. Then we obtain frxim Fig. 5.63 

This controlled frequency shift of the laser frequency against a reference frequency can be also realized by electronic elements in the servo loop. This dispenses with the modulation by the Pockels cell of the previous method. Such a frequency-offset locking technique has been demonstrated by HALL [6.33], who locked a tunable single-mode laser with a variable frequency offset to an ultrastable He-Ne laser, stabilized onto the center of a CH line. 

A very precise method of measuring the frequency separation of two closely spaced spectral lines is based on a heterodyne technique. In this method, two single-mode lasers are used. Each laser is stabilized onto the centers Vp V2 of two spectral lines. The output of both lasers is superimposed onto a detector with nonlinear response generating the difference frequency - V2 which is electronically filtered from the frequencies Vp V2 and + V2 and which directly yields the desired line separation. This heterodyne method is illustrated by several examples in Sect.10.6. 

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Calibration. Wavelength calibration to measure the dispersion relation between wavelength and position on the detector requires illumination... 

Calibration. In general, standards used for instrument calibration are physical devices (standard lamps, flow meters, etc.) or pure chemical compounds in solution (solid or liquid), although some combined forms could be used (e.g., Tb + Eu in glass for wavelength calibration). Calibrated lnstr iment parameters include wavelength accuracy, detection-system spectral responsivity (to determine corrected excitation and emission spectra), and stability, among others. Fluorescence data such as corrected excitation and emission spectra, quantum yields, decay times, and polarization that are to be compared among laboratories are dependent on these calibrations. The Instrument and fluorescence parameters and various standards, reviewed recently (1,2,11), are discussed briefly below. 

Table I. Spectral Lines from External Sources Used for Monochromator Wavelength Calibration...

Figure 25-1 Symbolic graphical depiction of a two-wavelength calibration.

Standard data reduction, i.e. bias and flat field correction, has been performed with Iraf. The Iraf task APEXTRACT/APALL was used to extract the spectra, with interactively selected background sampling, in order to avoid contamination for the star spectrum. The wavelength calibration has been done using daily He, Ne, HgCd arcs, and, in order to improve the calibration, wavelengths values for the transitions used were taken from http //physics.nist.gov/. 

The wavelength calibration can be checked by comparing the maximum absorbance wavelength for a known substance as measured by the instrument to what it is reported to be otherwise. 

Quality assurance measures such as pre-analytical checks on instrumental stability, wavelength calibration, balance calibration, tests on resolution of chromatography columns, and problem diagnostics are not included. For present purposes they are regarded as part of the analytical protocol, and IQC tests their effectiveness together with the other aspects of the methodology. 

S. J. Choqnette, J.C. Travis, L.E. O Neal, C. Zhn, and D.L. Dnewer, SRM 2035 a rare earth oxide glass for the wavelength calibration of near infrared dispersive and Fonrier transform spectrometers. Spectroscopy, 16(4), 14—19... 

DLS- 700S light scattering photometer at 633nm, calibrated with benzene. The optical clarification was performed with teflon filters. The specific refractive index increment (dn/dc) was obtained with Chromatix KMX-16 reffactometer at the same wavelength, calibrated with NaCl solution. 

There are many atomic emission lamps which give very precise line spectra. These are little used in photochemical applications, but are useful as wavelength calibration standards. A small selection of available wavelengths is listed in Table 7.1. 

Fig. 2.9. A Average Raman spectra of hyperplastic (n = 20 solid line) and adenomatous (n = 34 broken line) colon polyps collected ex vivo (power = 200 mW 30-s collection time). B Average Raman spectra of hyperplastic (n = 9 solid line) and adenomatous (n = 10 broken line) colon polyps collected in vivo (power = 100mW 5-s collection time). The spectra have been intensity corrected, wavelength calibrated, and fluorescence background subtracted (modified from [25], with permission)...

Finally, the impact of possible calibration instability on the prediction quality must be understood. In this case, spectrograph and laser wavelength calibrations were very stable and easy to update, but of minor concern because of the broad bands being used. However, since the intensity calibration function is non-linear, any changes in it could unequally affect bands used in the calibration ratio and introduce error in the prediction. Newer equipment offers easy intensity calibration routines but this can be difficult to use automatically with immersion probes since it requires that they be removed from the process. 

Wavelength accuracy. In order to evaluate the ability of each system to locate spectral lines, a preliminary wavelength calibration was carred out with the emission spectrum of a mercury pen lamp and then the peak maxima of several atomic lines from an iron hollow cathode lamp were located. The root mean square (RMS) prediction error, which is the difference between the predicted and the observed location of a line, for the vidicon detector system was 1.4 DAC steps. Because it is known from system calibration data that one DAC increment corresponds to 0.0125 mm, the absolute error in position prediction is 0.018 mm. For the image dissector, the RMS prediction error was 7.6 DAC steps, and because one DAC step for this system corresponds to 0.0055 mm, the absolute error in the predicted coordinate is 0.042 mm. The data in Table II represent a comparison of the wavelength position prediction errors for the two detectors. 

Calibration methods. Wavelength calibration is achieved with spectral lamps, e.g. mercury (Fig. 2) or helium. The separation of mercury lines 2 nm apart is readily achievable. Calibration in intensity (relative) is done using an incandescent source of known temperature. 

Figure 2. Wavelength calibration curve of the microspectrofluorometer using mercury and helium...

These simple one-wavelength calibration models with no intercept term are severely limited. Spectral data is used from only one wavelength, which means a lot of useful data points recorded by the instrument are thrown away. Nonzero baseline offsets cannot be accommodated. Worst of all, because spectral data from only one wavelength is used, absorbance signals from other constituents in the mixtures can interfere with analysis. Some of the problems revealed for models without an intercept term can be reduced when an intercept term is incorporated. 

During the course of laser resonance experiments it was noticed that the central wavelengths shift depending on the helium density. Thus, the resonance line shapes at various target gas conditions were measured precisely with a reduced laser bandwidth and an improved wavelength calibration [18]. Figure 5 shows resonance profiles taken for the 597.26 nm line at different pressures ranging from 530 mb to 8.0 bar at temperatures of 5.8-6.3 K. The results are summarized in Table 2. 

Two measurements [31,10] were conducted at a low-inductance vacuum spark plasma and a tokamak plasma respectively. In both cases only the w line was reported. The first study used a double Johann spectrograph and characteristic K lines were used for calibration [31]. The energy of the w was 5.20558(55) keV or a 105 ppm result The second study [10] used a tokamak plasma and claimed an uncertainty of 40 ppm. Close lying Lyman series lines were used for calibration so this was a relative measurement chain assuming one-electron QED. Shorter wavelength calibration lines and helium-like resonances were observed in second order diffraction suggesting the significant systematic shifts discussed above. The third study [11] was a relative measurement to the w line and, as such, can not be compared to absolute measurements. 

Care should be taken to make sure that the wavelength calibration of the monochromator is correct. For elements with complex emission spectra, such as iron, a small wavelength calibration error may result in the wrong wavelength being employed accidentally. This may seriously adversely effect sensitivity and precision. 

If in doubt, check that the wavelength thought to be the one giving optimal sensitivity does indeed do so. Alternatively check the wavelength calibration in that vicinity using a hollow cathode lamp of an element with a much simpler, and thus unambiguous, spectrum in the region of interest. 

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