System Measures Raman Gain in Bulk Glasses

Breck Hitz

Scientists from the University of Central Florida in Orlando have developed an optical system to measure stimulated Raman gain coefficients in bulk glasses. They have used it to investigate two tellurite glasses developed at the Centre National de la Recherche Scientifique's Institute for Condensed Matter in Pessac, France, that have peak Raman gain coefficients 30 times better than fused silica.

Raman amplification is growing in importance not only for long-haul fiber links, but also for local-area and metro networks. Unlike amplification in erbium-doped fiber amplifiers, it may be used across the spectrum of transmission fibers and thus may enable expansion from the C-band into the L- and S-bands, and perhaps into other regions of the spectrum.

A new optical system measures the stimulated Raman gain coefficient of millimeter-thick samples of bulk glass.

The Raman gain in conventional fused-silica fiber is relatively narrow spectrally, and multiple pumps therefore are required to provide flat gain across the C-band. Moreover, other glasses are known to have more intense spontaneous Raman spectra than fused silica. These considerations have led to an active investigation of Raman gain in other glasses, but it had been necessary to draw the glasses into fiber prior to testing -- a laborious and expensive procedure.

A mode-locked, Q-switched Nd:YAG laser provides both the 1.06-µm radiation to excite Raman gain in the sample and the 532-nm light to pump an optical parametric generator-amplifier (see figure). The output of the optical parametric generator-amplifier can be tuned from 780 to 2300 nm and serves as the probe signal to be amplified by Raman gain in the glass sample. The probe beam is polarized at 45° to the pump beam, which passes through an adjustable delay line to ensure temporal overlap with the probe. The two wavelengths combine at a beamsplitter.

After passing through the 2-mm-thick sample, the pump wavelength is filtered from the probe with a monochromator. Because Raman gain is polarization-sensitive and because half the probe energy is polarized parallel to the pump and half polarized orthogonal to it, only the half that is parallel to the pump is amplified. Thus, the ratio of energy in the amplified probe's two polarization components -- which is determined by separating them with a polarizing beamsplitter and measuring each with a calibrated germanium detector -- quantifies the Raman gain.

The researchers found that the stimulated gain data were in good spectral agreement with spontaneous Raman curves for the glasses tested. The peak Raman gain of one sample was 30 times better than that of fused silica, and gain of the other was 20 times better. The Raman gain bandwidth of both was roughly twice that of fused silica because of the greater variety of structural bonds in the tellurite glasses. They associated many spectral features of the Raman gain with specific vibrational modes of the glass constituents and concluded that a Raman profile can be tailored by carefully adjusting the composition of the glass.