Modeless laser

Unlike the typical laser source, the zero-point blackbody field is spectrally white , providing all colours, CO2, that seek out all co - CO2 = coj resonances available in a given sample. Thus all possible Raman lines can be seen with a single incident source at tOp Such multiplex capability is now found in the Class II spectroscopies where broadband excitation is obtained either by using modeless lasers, or a femtosecond pulse, which on first principles must be spectrally broad [32]. Another distinction between a coherent laser source and the blackbody radiation is that the zero-point field is spatially isotropic. By perfonuing the simple wavevector algebra for SR, we find that the scattered radiation is isotropic as well. This concept of spatial incoherence will be used to explain a certain stimulated Raman scattering event in a subsequent section. 

Which lasers . The above mentioned accuracy of the tilt measurements can be achieve if there are enough return photons. The average laser power required to get them is 2 x 20 W. Up to now, there is no cw laser available that powerful (see Ch. 14). In addition it raised the problem of saturation of the absorption by Na atoms in the D2 transition. These two problems have justified the development of the modeless laser (LSM) at LSP (Pique and Farinotti, 2003). 

Figure 28. Return flux from the modeless laser in the D2 line versus the input power. Top to bottom spectral FWHM = 3 GHz, 4.5 GHz and I MHz single mode (Pique and Faiinotti, 2003).

Moreover, in recent years broad band lasers have appeared which lack any frequency modal structure, at the same time retaining such common properties of lasers as directivity and spatial coherence of the light beam at sufficiently high spectral power density. The advantages of such a laser consist of fairly well defined statistical properties and a low noise level. In particular, the authors of [245] report on a tunable modeless direct current laser with a generation contour width of 12 GHz, and with a spectral power density of 50 /xW/MHz. The constructive interference which produces mode structure in a Fabry-Perot-type resonator is eliminated by phase shift, introduced by an acoustic modulator inserted into the resonator. 

Noise contribution from the many modes in the Stokes laser can be reduced by the use of a modeless dye laser invented by Ewart [67]. Fortunately nitrogen CARS spectra are less affected by this due to the large number of spectral lines that can be probed together [68]. 

Ewart, P. "A Modeless Variable Bandwidth Tunable Laser." Optics Communications 55 (1985) 124. 

Kaminski, C. R, and Ewart, P. "Multiples H2 Coherent Anti-Stokes Raman Scattering Thermometry with a Modeless Dye Laser." Applied Optics 38 (1997) 731. 

A very interesting alternative laser for optical cooling of atoms in a collimated beam is the modeless laser [1131], which has a broad spectral emission (without mode stmcture, when averaged over a time of T > 10 ns, with an adjustable bandwidth and a tunable center frequency). Such a laser can cool all atoms regardless of their velocity if its spectral width Ao>l is larger than the Doppler shift Acod = vok [1132]. 

With the following experimental trick it is possible to compress the velocity distribution N(Vz) of atoms in a beam into a small interval Av around a wanted final velocity Uf. The beam from the modeless laser propagates anticollinearly to the atomic beam and cools the atoms (Fig. 9.10). A second single-mode laser intersects the atomic beam under a small angle a against the beam axis. If it is tuned to the frequency... 

Fig. 9.10 Cooling of all atoms with a counterpropagating modeless laser. A cooling stop at a selectable velocity Vf can be realized with a second copropagating single-mode laser [1133]...

I.C.M. Littler, St. Balle, K. Bergmann The CW modeless laser spectral control, performance data and build-up dynamics. Opt. Commun. 88, 514 (1992)... 

I.C.M. Littler, H.M. Keller, U. Gaubatz, K. Bergmann Velocity control and cooling of an atomic beam using a modeless laser. Z. Physik D 18, 307 (1991)... 

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