Laser transitions

Four-level lasers offer a distinct advantage over tlieir tliree-level counterjiarts, (figure C2.15.5). The Nd YAG system is an excellent example of a four-level laser. Here tlie tenninal level for tlie laser transition, 2), is unoccupied tlius resulting in an inverted state as soon as any atom is pumped to state 3. Solid-state systems based on tliis pumping geometry dominate tlie marketplace for high-power laser devices. 

The ground configuration of Ar is KL3s 3p, giving an inverted P /2 multiplet. The excited states involved in laser action involve promotion of an electron from the 3p orbital into excited As,5s,Ap,5p,3d,Ad,... orbitals. Similarly, excited states of Kr involved arise from promotion of an electron from the Ap orbital. In Ar the KL3s 3p configuration gives rise to 5, V, terms (see Section 7.1.2.3). Most laser transitions involve the core in one of the states and the promoted electron in the Ap orbital. 

Decay of the 1 and 2 lower levels of the laser transitions are rapid down to the 2 level this is depopulated mostly by collisions with helium atoms in the CO2 N2 Fie gas mixture which is used. 

Figure lb shows a four-level system. The terminal level, level 2, is ordinarily empty. Atoms are optically pumped to level 4. From level 4, the atoms make a rapid radiationless transition to level 3. The first few atoms to arrive begin to contribute to the population inversion. Therefore, laser operation can begin with much less intense pumping light. After the laser transition, the atoms return to the ground state (level 1) by a radiationless transition. 

Figure 9.22 Energy levels of most importance in the neodymium laser. The pump transition is from the ground state to the broad 5d 6s band. The main laser transition is between the 4F and 4Ih/2 levels. Internal transitions are marked with dotted lines.

Figure 2.6 An energy-level scheme for (a) four- and (b) three-level lasers, transition =, laser transitions , fast nonradiative transitions.

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.

The determining feature by which laser action can be efficiently obtained from this type of active medium is the fact that the atoms that form the dimmer are only bound in the excited state. Figure 2.9 shows a schematic diagram of the laser energy levels in a molecule of excimer. The laser transition is produced between two molecular electronic levels in which the potential energy curve for the fundamental state is repulsive. This ensures the population inversion. 

On the other hand, the energy released by the laser transition is associated with a dissociation process of the dimmer, which consequently takes place in the UV part of the spectrum. 

Usually, mainly Doppler broadening determines the gain profile of a particular laser transition. Indeed, due to the different configurations achievable with gas lasers (namely, a large cavity length), the laser line can be narrower than the Doppler linewidth. Different experimental realizations of single-mode lasers are detailed elsewhere (Demtroder, 2(X)3). 

With respect to intensity, most of the laser transitions therefore are superior to conventional lamps, especially for experiments which demand a narrow spectral range or a small solid angle of the incoming light. 

Light pulses with halfwidths of 10" sec have been generated mainly by Q-switched solid-state lasers but can be obtained principally with all high-gain laser transitions, as for instance CO2 lasers " ), nitrogen lasers ) (X = 3300 A), or dye lasers ). 

Excitatio.1 of COj by a Q-switched COj laser at >. = 10.6 ju may even lead to a population inversion in the pumped gas, resulting in a new laser transition at 4.3 p. Hocker et al. studied vibrational relaxation in CO2 by this technique. 

This situation corresponds to the well-known saturation effect in the emission of most gas laser transitions, where, for the same reason, fewer upper-state molecules can contribute to the gain of the laser transition at the center of the doppler-broadened fluorescence line than nearby. When tuning the laser frequency across the doppler-line profile, the laser intensity therefore shows a dip at the centerfrequen-cy, called the Bennet hole or Lamb dip after W.R. Bennet who discovered and explained this phenomen, and W.E. Lamb 2) who predicted it in his general theory of a laser. 

Fig. 16. Spectroscopic investigation of inelastic collisions in a gas discharge scheme of appropriate levels and corresponding transitions. The level b corresponds in case of Ne to the 3 2 level (Paschen Notation) which is partly depopulated by the laser transitions at X = 3.39 ixm and X = 0.6328 jum...

The emission intensity of the 4F3/2- 4/n/2 transition (laser transition) does not change appreciably for a range of x from 0.02 to 0.5. This is probably due to a balancing of the total capture cross section for the pump light with the reducing quantum efficiency of AFV1 state. Figure 32 shows the variation of threshold with concentration. Data were collected at room temperature. Curve A is for an FT-524 flash lamp with a half-width of approximately 100 /xsec, whereas curve B is for an FT-91 flash lamp with a half-width of approximately 10 psec in a small ellipse. 

A useful way of changing the wavelength of some lasers, for example the C02 infrared laser, is to use isotopically substituted material in which the wavelengths of laser transitions are appreciably altered. 

Fig. 11. (Upper) Splitting of pHe+ states due to magnetic interactions, and observable laser transitions between the F+ and F states according to Bakalov and Korobov [33]. (Lower) Observed hyperfine splitting of the unfavoured laser transition (n, L) = (38,34) —> (37, 35) [16]. The laser bandwidth is 1.2 GHz. The solid line is the result of a fit of two Voigt functions (a Gaussian fixed to the laser bandwidth convoluted with a Lorentzian to describe the intrinsic line width) to the spectrum. The intrinsic width of each lines was found to 0.4 0.1 GHz. From Widmann et al. [16]...

We now have the opportunity to perform a laser resonance experiment to measure the Lamb shift in muonic hydrogen. Taking into account the muon stop rate, entrance detector efficiency, long-lived metastable 2S population, laser transition probability, solid angles and detection efficiency of the X-ray detector we estimate our event rate on resonance to be 9 events per hour. 

Fig. 2. Laser (/ ), microwave ( hf)> and RF ( shf) transitions in pHe+. Vhf is a transition between the SHF states of same total angular momentum J = L that is suppressed by a factor 1 /L compared to the allowed AL = 1 transitions i f- The right-hand side shows the splitting of the parent state of a laser transition into a quadruplet, while on the left-hand side only the dominant doublet splitting for the daughter state is shown...

A CERN-Columbia Collaboration [6] [assisted by staff of the Alternating Gradient Synchrotron (AGS) Brookhaven National Laboratory] have looked into the possibility of performing an experiment of the same type (i.e. via laser transition in muonic ions), but with transitions between levels not belonging to S states in order to avoid the consequences of the electromagnetic form-factor uncertainties. ... 

Our last runs were with the (expensive) isotopic gas 13C1802 in our TEA laser system. We ran for some days with the diffracting grid set at 1 = 9.8595 urn (presumed resonance value), firing the laser with every other beam burst. During this period, the repetition rate of the AGS was 1.4 s. It is easy to see that a good quantity to look at to observe stimulated laser transitions is the ratio K /K. ... 

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