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Measurement of Si—OH content in fused silica with extended dynamic range by Fourier transform infrared spectroscopy
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Abstract
Fourier transform infrared (FTIR) spectroscopy Si—OH group is widely used in Si—OH content measurement of fused silica optics. However, the measurement accuracy is influenced by water molecular absorption bands in low Si—OH content samples and absorption saturation in high Si—OH content samples, leading to limited measurement range. FTIR spectroscopy is employed to measure 2500~5000 cm-1 transmittance spectra of fused silica samples with different OH contents and thicknesses. The interference of water molecular absorption band to 3673 cm-1 is eliminated. Then Si—OH contents, corresponding measurement errors and limits of detection at 3673 cm-1 and 4522 cm-1 bands are calculated. Based on the experimental results and Beer's law, a model to correlate Si—OH content, sample thickness, measurement error of transmittance, and measurement error of Si—OH content is established. From this model, by using 3673 cm-1 band for fused silica samples with Si-OH content less than 8.17×10-4 and 4522 cm-1 band for samples with Si—OH content more than 8.17×10-4 to measure the Si—OH content, a dynamic range from 0.4×10-6 to10-2 Si—OH can be achieved with optimized accuracy for fused silica samples with 2 mm thickness. -
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References
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Overview
Si-OH group is the most common impurity in fused silica, which has a significant impact on the optical performance of fused silica optics from infrared to ultraviolet spectral regions. The Si-OH content can be directly measured from Si-OH stretching band at 3673 cm-1 with molar absorptivity of (77.5±1.5) L/(mol·cm). However, measurement range is limited by interference of absorption bands from water molecule in low Si-OH content samples and absorption saturation in high Si-OH content samples. Fourier transform infrared (FTIR) spectroscopy is employed to measure 2500 cm-1~5000 cm-1 transmittance spectra of fused silica samples with different Si-OH contents ranging from 0.0 to 10-3 and thicknesses ranging from 2.0 mm to 8.0 mm. Since 3000 cm-1~3100 cm-1 absorption bands from H2O molecule is not covered by other infrared (IR) absorption bands, absorption at 3673 cm-1 from H2O molecule can be eliminated using transmittance from 3000 cm-1~3100 cm-1 region and line shape of H2O band obtained by Gaussian fit from low Si-OH content samples. Compared with 4522 cm-1 absorption peak which is not interfered by any absorption bands, the Si-OH content calculated from 3673 cm-1 band without influence of water absorption band has a relative error about 2.5% lower than that directly calculated by 3673 cm-1 band in Suprasil 501 sample with about 5.0×10-5 Si-OH content and 2.0 mm thickness. After eliminating the influence of water absorption band, corresponding measurement errors and limit of detection for Si-OH content at 3673 cm-1 and 4522 cm-1 bands are calculated. Based on experimental data and Beer’s law, a model to correlate Si-OH content, sample thickness, measurement error of transmittance, and measurement error of Si-OH content is established. From this model, Si-OH content measurement error is mainly influenced by logarithm of transmittance and influence of water absorption band in low Si-OH content samples, and absorption saturation in high Si-OH content samples. Since the molar absorptivity at 4522 cm-1 is nearly 50 times weaker than that at 3673 cm-1, the corresponding transmittance at 4522 cm-1 is much lower than that at 3673 cm-1, leading to a lower relative root mean square error for Si-OH content measurement at 4522 cm-1 than that at 3673 cm-1 in high OH content samples. With 2.0 mm thickness, a relative error less than 0.13% can be achieved in sample with 10-2 Si-OH content at 4522 cm-1 absorption band. As a result, by eliminating water absorption band at 3673 cm-1 for low Si-OH content samples and employing 4522 cm-1 band for high Si-OH content samples to measure the Si-OH content, the measurement range is increased from (6.0~1810.0)×10-6 to (0.4~10000.0)×10-6 at 2.0 mm thickness with improved measurement accuracy.
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Figure 1.
Transmittance spectra of 9 fused silica samples with 2.0 mm thickness at 2500 cm-1~5000 cm-1 wavenumber measured 30 times. The results show Si—OH absorption bands with the absorption intensity becoming smaller: Infrasil 302, HOQ 310, Suprasil 711, Suprasil 501, Herasil 3, Suprasil 401, Herasil 102, Corning 7980, Spectrosil 2000.
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Figure 2.
Transmittance spectra of Corning7980 samples with different thicknesses at 2500 cm-1~5000 cm-1 wavenumber measured 30 times.
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Figure 3.
Transmittance spectra (full line) and resolved water molecule band (dotted line) of 9 fused silica samples with 2.0 mm thickness at 2750 cm-1~4000 cm-1 wavenumber.
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Figure 4.
Measured absorbance values at 3673 cm-1 and 4522 cm-1 of 6 fused silica samples.
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Figure 5.
Absorbance spectra for Infrasil 302 fused silica sample with 2.0 mm thickness (black line) at 2800 cm-1~5000 cm-1 wavenumber measured 30 times and H2O molecule (red dot line) resolved by absorbance at 3050 cm-1.
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Figure 6.
Relative root mean square error |dC/C| of Si—OH content measured at 4522 cm-1 band (black line), 3673 cm-1 bands with water band (blue line) and 3673 cm-1 bands without water band (red line) versus Si—OH content of fused silica samples with 2.0 mm thickness, and relative errors calculated with 30 times measurements for Infrasil 302, Herasil 102 and Spectrosil 2000 samples with 2.0 mm thickness at 3673 cm-1 (red points) and 4522 cm-1 (black points).
