Spontaneous Raman Measurements in an Optical Fiber Perform Flame.

Spontaneous Raman Spectroscopy (SRS) has been Applied to methane/oxygen diffusion flames used to deposit silica particles on preforms for the synthesis of optical fiber. A Vapor Axial Deposition (VAD) torch was used in the experiments. Axial and radial temperature profiles of the flame were determined by measuring the vibrational Raman spectrum of flame oxygen and performing least-squares fits of theoretically-determined spectra to the data. Steep temperature gradients were found in both radial and axial directions, with peak temperatures in the absence of silica particles on the centerline of the burner exceeding 2500K. SRS measurements were feasible in the presence of silica particles (formed by adding silicon tetrachloride vapor to the flame), but were limited by optical breakdown in the flame (the threshold of which decreases when large numbers of particles are present). The addition of silicon tetrachloride significantly alters the axial temperature profile of the flame. From this we infer that the introduction of silicon tetrachloride to the flame dramatically alters flame structure.