Mode Selection in Lasers

In Chap.5 we saw that in an active laser resonator, all those modes 

Let us first consider the selection of transverse modes. In Sect.5.4 it was shown that the higher transverse TEM modes have radial field dis-tributions which are less and less concentrated along the resonator axis with increasing transverse order n or m. This means that their diffraction losses are much higher than that of the fundamental modes TEI when an aperture is inserted inside the resonator. The field distribution of the modes and therefore also their diffraction losses depend on the resonator parameters such as the radii R. of curvature of the mirrors, the mirror separation d, and of course the Fresnel number N (see Sect.5.3). Only those resonators which fulfil the stability condition [6.10] 

A stability diagram in the 9 92 plane, as shown in Fig.6.9, allows the stable and unstable regions to be identified. In Fig.6.10, the ratio y q/yqq of the diffraction losses for the TEM q and the TEMqq modes is plotted for different values of g as a function of the Fresnel number N. From this diagram one can obtain for any given resonator the diameter 2a of an aperture which suppresses the TEM q mode but still has sufficiently small losses for the fundamental TEMqq mode. In gas lasers, the diameter 2a of the discharge tube generally forms the limiting aperture. One has to choose the resonator parameters in such a way that a 3w/2 (see Sect.5.11) because this assures that the fundamental mode nearly fills the whole active medium but still suffers less than 1% diffraction losses. 

From the discussion in Sect.5.7 it should have become clear that simultaneous oscillation on several longitudinal resonator modes is possible when the inhomogeneous width Av of the gain profile exceeds the mode spacing c/2d (Fig.6.12). A simple way to achieve single-mode operation is 

In single-mode operation we can expect roughly the fraction j om g the multimode power, where is the homogeneous width within the in- 

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This illustrates that the MI can be regarded either as a wavelength-dependent filter for the transmitted light, or as a wavelength-selective reflector. In the latter function it is often used for mode selection in lasers (Fox-Smith selector. Sect. 5.4.3). 

P.W. Smith Mode selection in lasers. Proc. IEEE 60, 422 (1972)... 

We shall describe the experimental techniques that are necessary for achieving optimal results in spectroscopic applications. These techniques comprise mode selection in lasers, wavelength and intensity stabilization of single-mode lasers, and experimental realizations of controlled wavele7 gth tuning. Furthermore we briefly discuss the interesting question of why a lower limit exists for the laser linewidth. At the end of this chapter some methods of relative and absolute frequency measurements in the optical region will be presented. 

Figure B2.5.18 compares this inter molecular selectivity with intra molecular or mode selectivity. In an IR plus UV, two-photon process, it is possible to break either of the two bonds selectively in the same ITOD molecule. Depending on whether the OFI or the OD stretching vibration is excited, the products are either IT -t OD or FIO + D [24]- hr large molecules, mirmnolecular selectivity competes with fast miramolecular (i.e. unimolecular) vibrational energy redistribution (IVR) processes, which destroy the selectivity. In laser experiments with D-difluorobutane [82], it was estimated that, in spite of frequency selective excitation of the...

My answer would be, using an analogy between molecules and human beings, that it is neither nice nor possibly easy to use brute force on the molecules. However, often the molecules may be in a state where they do not really know what they want to do. Then we might use some very mild means to seduce them to do what we would wish them to do. As an early example for such mild seduction I might quote the theoretical scheme for potentially mode selective infrared laser chemistry of ozone [1, 2], which predates some of the more widely publicized subsequent schemes using excited electronic states. 

Polyatomic molecules are characterized by complex internal motions leading to several possible rearrangements. With the advent of ultrafast lasers, much hope arose for achieving mode selectivity in chemical reactions based on the interaction between the laser pulses and the dynamics of the molecule. In the quest to steer complex systems, an especially attractive control scheme is the adaptive optimal... 

There are several ways to narrow down the bandwidth, the most popular method being the insertion of an etalon into an SRO resonator, in the same manner as for mode selection in standard tuneable laser resonators (see Section 4.3). Injection seeding is another possibility to achieve reliable single-mode operation, since only very small seed powers are required (often less than 1 mW), and CW semiconductor lasers have become a popular choice for this implementation. Using this type of CW seeding, tuneable pulsed OPO operation with line width below 0.5 GHz has been demonstrated. 

Fig. S.40a,b. Mode selection in the case of broad gain profiles. The prisms narrow the net gain profile and two etalons enforce single-mode operation (a) experimental realization for a jet stream cw dye laser (b) schematic diagram of gain profile and transmission curves of the two etalons...

Fig. 5.41. Mode selection in the cw dye laser with a folded cavity using a birefringent filter, a tilted etalon, and a prism FPI (Coherent model 599). The folding angle is chosen for optimum compensation of astigmatism introduced by the dye jet...

Mode selection in an argon laser is often accomplished with an intracavity etalon. What is the frequency drift of the transmission maximum... 

D. Kuhlke, W. Diehl Mode selection in cw-laser with homogeneously broadened gain. Opt. Quant. Electron. 9, 305 (1977)... 

The specific characteristics of a cw ring dye laser regarding output power and linewidth have been studied in [489]. A theoretical treatment of mode selection in Fabry-Perot-type and in ring resonators can be found in [490]. Because of the many optical elements in the ring resonator, the losses are generally slightly higher than... 

An etalon is used for single-mode selection in a 1-m rhodamine 6G laser. If the dye gain bandwidth is commensurate with the width of the rhodamine 6G fluorescence spectrum shown in Problem 8.2, what etalon separations and etalon surface reflectivities would ensure that only one axial mode is selected at any time ... 

Now we will study a nmnber of laser types more closely. We distinguish between fixed-frequency lasers and tunable lasers. In a limited sense, the former type is also tunable since different modes under the gain curve can be selected. In lasers for which the active medium is a solid, the gain curve can... 

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