Multiphonon edge

Figure 11.02 Absorption and scattering losses for a range of optical materials plotted on a log-log scale which gives straight tines for Rayleigh scattering but a slight curvature to the multiphonon edge (After France ct al., 1986)...

The multiphonon edge, although somewhat difficult to measure accurately, is not always strictly exponential, probably due to intrinsic compositional characteristics. In addition, non-Rayleigh scattering has often been measured, e.g. exhibiting 1/A or 1/A° dependence, perhaps due to microcrystals, bubbles, etc., and residual transition metal ion, lanthanide ion and OH group absorptions will always be present. In conclusion, the estimates for intrinsic minimum loss in HMFG systems appear optimistic but, ultimately, the actual values at ultralow loss levels will be determined mostly by extrinsic factors. 

These absorptions can be divided into three categories impurity absorptions due to gases or bound hydrogen isotopes, the infrared cutoff or multiphonon edge, and the fundamental structural vibrations. 

Figure 10.4 Infrared transmission spectra showing the shift in the multiphonon edge for glasses of different compositions...

G. Spitzer, Multiphonon Lattice Absorption D. L. Slierwall and R F. Potter, Emittance Studies H- R- Philipp and H. Ehrenveich, Ultraviolet Optical Properties M. Cardona, Optical Absorption above the Fundamental Edge . J. Johnson, Absorption near the Fundamental Edge... 

For ZBLAN, the IR edge is in the 5-8 m range with a 50% transmission located at 7.1 /on (1400 cm-1) for a 2 mm-thick sample. This is to be compared with silica glass whose transparency starts decreasing at 3 pm. The IR edge is due to multiphonon absorption processes related to the fundamental vibration frequencies... 

One of the main spectroscopic properties that differentiate fluoride glasses from silica-based glasses is the low multiphonon emission rate. These non-radiative relaxations that may strongly compete with radiative processes in rare-earth ions are nearly three orders of magnitude lower in ZBLAN glass than in silicate, as shown in Fig. 2. This property is directly related to the fundamental vibration modes of the host and, therefore, varies basically in the same manner as the infrared absorption edge. 

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