Nonlinear amplification process

Figure 22.31 shows the directional ASE spectra obtained by increasing either the excitation intensity I (a) or the excitation stripe length L (b) [96]. The results are virtually identical in both cases SN of the polymer emission was observed above certain threshold values for both I and L. This directional SE can be successfully modeled using the ASE approximation and Equation 22.11. Since y has a maximum at A 630 nm, then 7ase (630 nm) experiences the maximum gain whereas amplification at other As is relatively smaller. Consequently, this nonlinear amplification process leads to SN when either I or L increases. 

A third way of generating continuously tunable coherent radiation uses more complicated systems based on the principle of optical parametric oscillation (and amplification). Since the gain in these systems is not originated by stimulated emission, but by means of a nonlinear frequency conversion process, we will treat them in a separate section. 

Photoacoustic Raman spectroscopy (PARS) Photoacoustic Raman spectroscopy (PARS) is again a nonlinear spectroscopic technique. In this technique, selective population of a given energy state of a system (transitions must involve change in polarizability) is amplified using coherent Raman amplification (also known as stimulated Raman scattering). In this process, it is also important that the frequency difference of the two incident laser beams must be adjusted to equal the frequency of Raman-active transition. 

Self-amplification, suppressed in the latter conditions, is thus the process that is responsible for the existence of an abrupt threshold when the level of extracellular cAMP is allowed to vary freely. The fact that the response in fig. 5.39 does not display any threshold confirms that the latter owes its existence, in fig. 5.32, to the dynamic property of excitability linked to self-amplification, and not to the nonlinear coupling between the receptor and the cyclase, since the latter process is also at work in system (5.16). 

We may conclude that many important biological rhythms originate from positive feedback mechanisms whose nonlinearity is further strengthened by the cooperative nature of the regulatory process. Although the detailed molecular implementation of the feedback process differs in each case, it is the self-amplification with which it is associated that gives rise to instabilities followed by sustained oscillations in biochemical systems as well as in cardiac or neural cells (Goldbeter, 1992). 

Thus it appears that amplification in the cochlea resides in the gating of the outer hair cell cflia, while the motility is due to passive piezoelectric properties of the cell wall. The flow through the gate has significant nonlinearity at small amplitudes of displacement of the cilia (10 nm). Sufficiently high amplitudes of displacement of the cilia will cause the tip links to buckle. We estimate that this will occur at around 70 dB sound pressure level, thereby turning off the active process for higher sound intensity. 

E. C. Khoo, ""Optically induced molecular reorientation and third order nonlinear process in nematic liquid crystals. Phys. Rev., vol. A23, pp. 2077-2081, 1981 see also 1. C. Khoo and S. L. Zhuang, " Nonlinear light amplification in a nematic liquid crystal above the Freedericks transition, Appl. Phys. Lett., vol. 37, pp. 3-5, 1980 I. C. Khoo, ""Theory of optically induced molecular reorientation and quantitative experiments on wave-mixings and self-focusing of light, Phys. Rev., vol. A25, pp. 1636-1644, 1982 1. C. Khoo, ""Reexamination of the theory and experimental results of optically induced molecular reorientation and nonlinear diffraction in nematic liquid crystals spatial frequency and temperature dependence, Phvs. Rev., vol. A27, p. 2747, 1983. 

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