Multimode resonant cavities

A. Dodabalapur, L.J. Rothberg, and T.M. Miller, Color variation with electroluminescent organic semiconductors in multimode resonant cavities, Appl. Phys. Lett., 65 2308-2310 (1994). 

Chiba, A. Fujiwara, H. Hotta, J. Takeuchi, S. Sasaki, K., Fano resonance in a multimode tapered fiber coupled with a microspherical cavity, Appl. Phys. Lett. 2005, 86, 261106... 

Figure 2.8 A schematic diagram of the gain spectral profile, G(v), of a laser transition (solid line), together with the axial resonator modes (dotted line) of a cavity in which the frequency separation between adjacent modes is A v. (a) Multimode and (b) single-mode operation. The frequencies of those modes for which the gain exceeds the losses have been marked.

With an iodine cell inside the resonator of a cw multimode dye laser, an enhancement factor of g = 10 could be achieved, allowing the detection of I2 molecules at concentrations down to n < 10 /cm [20]. This corresponds to a sensitivity limit of aL < 10 . Instead of the laser output power, the laser-induced fluorescence from a second iodine cell outside the laser resonator was monitored as a function of wavelength. This experimental arrangement (Fig. 1.16) allows demonstration of the isotope-specific absorption. When the laser beam passes through two external iodine cells filled with the isotopes l2 and l2, tiny traces of l2 inside the laser cavity are sufficient to completely quench the laser-induced fluorescence from the external l2 cell, while the l2 fluorescence is not affected [21]. This demonstrates that those modes of the broadband dye laser that are absorbed by the internal l2 are completely suppressed. 

If the modulator is placed inside the laser resonator with the mirror separation d and the mode frequencies = vo m cjld (m = 0,1,2,...), the sidebands coincide with resonator mode frequencies if the modulation frequency / equals the mode separation Av = cjld. The sidebands can then reach the oscillation threshold and participate in the laser oscillation. Since they pass the intracavity modulator they are also modulated and new sidebands y = vq i 2/ are generated. This continues until all modes inside the gain profile participate in the laser oscillation. There is, however, an important difference from normal multimode operation the modes do not oscillate independently, but are phase-coupled by the modulator. At a certain time the amplitudes of all modes have their maximum at the location of the modulator and this situation is repeated after each cavity round-trip time T = 2d c (Fig. 6.8c). We will discuss this in more detail The modulator has the time-dependent transmission... 

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