Multimodal laser sources

The Goeppert-Mayer two- (or multi-) photon absorption, mechanism (ii), may look similar, but it involves intennediate levels far from resonance with one-photon absorption. A third, quasi-resonant stepwise mechanism (iii), proceeds via smgle- photon excitation steps involvmg near-resonant intennediate levels. Finally, in mechanism (iv), there is the stepwise multiphoton absorption of incoherent radiation from themial light sources or broad-band statistical multimode lasers. In principle, all of these processes and their combinations play a role in the multiphoton excitation of atoms and molecules, but one can broadly... 

At the same time very often the real optical field interacting with atoms ha.s rather broad spectral profile, width of which is broader or comparable with the inhomogeneous width of the atomic transition. In this case, a broad spectral line approximation for quantum density matrix approach has proved to be verj- rewai d-ing. This approximation was introduced in the 1960s by C. Cohcn-Taimoudji for excitation of atoms with ordinai-y light sources [10]. This was an era before lasers. Later on it was adjusted for application for exedtation of atoms wdth multimode lasers [11] and for excitation of molecules in the case of large angular momentum states [3, 12]. 

If only the axial modes TEMqo participate in the laser oscillation, the laser beam transmitted through the output mirrors has a Gaussian intensity profile (5.32), (5.42). It may still consist of many frequencies = qcl 2nd) within the spectral gain profile. The spectral bandwidth of a multimode laser oscillating on an atomic or molecular transition is comparable to that of an incoherent source emitting on this transition ... 

For spectroscopic applications of multimode lasers one has to keep in mind that the spectral interval Ay within the bandwidth of the laser is, in general, not uniformly filled. This means that, contrary to an incoherent source, the intensity 7(y) is not a smooth function within the laser bandwidth but exhibits holes. This is particularly true for multimode dye lasers with Fabry-Perot-type resonators where standing waves are present and hole burning occurs (Sect. 5.3.4). 

The intensity I(t) of a cw laser is not completely constant, but shows periodic and random fluctuations and also, in general, long-term drifts. The reasons for these fluctuations are manifold and may, for example, be due to an insufficiently filtered power supply, which results in a ripple on the discharge current of the gas laser and a corresponding intensity modulation. Other noise sources are instabilities of the gas discharge, dust particles diffusing through the laser beam inside the resonator, and vibrations of the resonator mirrors. In multimode lasers, internal effects, such as mode competition, also contribute to noise. In cw dye lasers, density fluctuations in the dye jet stream and air bubbles are the main cause of intensity fluctuations. 

The 5P4E samples where purchased from Monsanto Co and directly used in the pressure cell. The exciting laser source was a COHERENT INOVA 90-3 either in a single mode (500 mW) or multimode (1 W) output at 5145 A depending on the analysis technique. 

The ultrashort laser sources used to generate the CARS are also efficient in exciting other multiphoton processes, such as TPF and SHG. As the signals are spectrally separated, it is possible to integrate a multimodal nonlinear microscope, able to acquire simultaneously CARS, TPF, and SHG. Figure 14.4 shows an example of the signals that can be acquired in a multimodal CARS system, where the SHG is produced by the pump beam of an erbium fiber laser emitting at 780 nm. 

Lasers can be pulsed (where laser output occurs suddenly in a short pulse, followed by a long interval to restore the population inversion by pumping) or continuous-wave (CW where the output is almost continuous, because the next population inversion occurs very soon). Secondary lasers, such as dye lasers, must use a primary (usually pulsed) laser as the excitation source. When all modes are allowed in the optical cavity, the laser is said to be in multimode operation. 

The most common light source for all applications has been the blue LED (470 nm) and red LED (635 nm), generally used with multimode fibers. Lasers are used with single-mode fibers because the small core requires more intense power. Eor spectrophotometry, devices utilize full-spectrum light sources such as mercury-vapor, halogen, or tungsten lamps. This allows... 

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