Measurement of Si―OH content in fused silica with extended dynamic range by Fourier transform infrared spectroscopy

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 cm-1 ~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.