Calibration technique for suppressing residual etalon artifacts in slit-averaged Raman spectroscopy

TL;DR

This paper compares two methods for correcting etaloning artifacts in slit-averaged Raman spectroscopy using BI-CCD arrays, demonstrating that row-by-row calibration can significantly improve spectral quality when etaloning dominates noise.

Contribution

It introduces a calibration technique for suppressing residual etalon artifacts at the row level in BI-CCD arrays used in Raman spectroscopy, enhancing spectral accuracy.

Findings

Row-by-row calibration improves spectral quality when etaloning is dominant.

Calibration has no effect when shot-noise limits the performance.

Correcting fixed-pattern effects is crucial for weak Raman signals with fluorescence background.

Abstract

Back-illuminated charged-coupled device (BI-CCD) arrays increase quantum efficiency but also amplify etaloning, a multiplicative, wavelength-dependent fixed-pattern effect. When spectral data from hundreds of BI-CCD rows are combined, the averaged spectrum will generally appear etalon-free. This can mask substantial etaloning at the row level, even if the BI-CCD has been treated to suppress the effect. This Note compares two methods of etalon correction, one with simple averaging and one with row-by-row calibration using a fluorescence standard. Two BI-CCD arrays, both roughened by the supplier to reduce etaloning, were used to acquire Raman spectra of murine bone specimens. For one array, etaloning was the dominant source of noise under the exposure conditions chosen, even for the averaged spectrum across all rows; near-infrared-excited Raman peaks were noticeably affected. In this case, row-by-row calibration improved the spectral quality of the average spectrum. The other CCD's performance was shot-noise limited and therefore received no benefit from the extra calibration. The different results highlight the importance of checking for and correcting row-level fixed pattern when measuring weak Raman signals in the presence of a large fluorescence background.