Limiting linewidth

Figure 2 Schematic of the electronic absorption spectrum of a single chromophoric site in a condensed phase host environment at low temperatures. An extremely sharp electronic origin, exhibiting a radiatively limited linewidth is accompanied by a phonon sideband with vibrational sidelines. A second electronic excited state lies at higher energies. Vibrational sidelines and the second electronic excited state are lifetime broadened by rapid radiationless deactivation processes...

The instrumental resolution of the spectrometers is limited by the combined frequency fluctuation from each CO2 laser (about 15 kilohertz). This, of course, is less than any Doppler limited linewidth and, therefore, does not limit our resolution except for possible sub-Doppler work. This high resolution provides an excellent way of studying pressure shifts and line shape studies of spectral lines. The measurement of OH concentrations in our atmosphere as a function of altitude using absorption and emission measurements requires an accurate knowledge of its linewidth in the atmosphere. 

Figure 13. Stochastic simulation of the fluorescence intensity (top) and frequency (middle) correlation functions for a model of spectral diffusion in a glass. The molecule is coupled to a three dimensional distribution of tunneling systems with distributed microscopic parameters. The bottom panel shows the timescale (horizontal) and amplitude of the frequency jumps (relative to the lifetime limited linewidth, vertical scale). The inset in the top panel shows the line-shape and the lifetime limited linewidth as a small bar (from Ref. 77).

The sensitivity of systems which detect changes in phase or wavelength rrltimately depends on the linewidth of the sottrce, i.e., the narrower source linewidth the better. Similarly, the sensitivity of sensing systems which detect changes in intensity may depend on the source intensity noise. Fiber lasers can have close to quantum-limited linewidth and intensity noise, making them excellent sottrces for marty sensing applications. 

The ultimate linewidth of a laser oscillator is limited by the "contamination" of the coherent stimulated emission by Incoherent spontaneous emission. The resulting limiting linewidth was given by Schawlow and Townes as ... 

FIGURE 14.19 (a) Experimental setup used to demonstrate reduction of Iransit-tiine broadening. A beam of ND3 molecules is decelerated from 280m/sec to either 100 or 50m/sec, sent through a microwave zone, and subsequently detected, (b) Upon reducing the beam velocity, the transit-time-limited linewidth of the microwave transitions decreases to about 1 kHz at SOm/sec. (From van Veldhoven, J., Eur. Phys. J. D, 31, 337-349, 2004. Copyright EDP Sciences. With permission.)... 

Figure 5, Schmatic illustration of motional narrowing. A system has a low temperature limiting linewidth represented by When a motion, modulating the...

Going into the fundamental resolution limits, the transit times, the second-order Doppler effect, and the second-order laser wave-front curvatures, limit the obtainable resolution, as was also experimentally investigated in the 8 3 2 " Ps/zTPA in Bil, where a mainly transit time limited linewidth of -400 kHz was obtained. 

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