Laser counterpropagating

The system used to measure the optical fiber signals employs two separate frequency tunable laser light sources operating at about 1320 nm wavelength. One laser acts as a pump laser, whereas the other serves as the probe laser that sends optical pulses down the fiber to interact with the counterpropagating laser light wave pumped into the fiber from its opposite end. 

Two different experimental geometries are used for DFWM one with counterpropagating (Fig. 6a) and one with copropagating (Fig. 6b) laser pulses. The setup with counter propagating beams will be discussed first. 

The IR pulse is split into a weak probe beam, which passes down a computer-controlled variable delay line with up to 12 ns of delay and a strong pump beam. The pump and probe pulses are counterpropagating and focused into the center of the SCF cell. Typical spot sizes (1/e radius of E-field) were oj0 120 pm for the pump beam and oj0 60 pm for the probe beam. A few percent of the transmitted probe beam is split off and directed into an InSb detector. A reference beam is sent through a different portion of the sample. The reference beam is used to perform shot-to-shot normalization. The pump beam is chopped at half the laser repetition rate (900 Hz). The shot-to-shot normalized signal is measured with a lock-in amplifier and recorded by computer. 

Since —ih2d/dQa is decoupled from the rest of the Floquet Hamiltonian, it acts trivially and can be omitted. This effective model is valid only if two different frequencies are assumed. The derivative term represents the relative number of photon pairs, one otj photon minus one 002 photon. Thus the absorption of one effective photon of frequency 8 in the effective model (312) corresponds in the complete model (304) to the absorption of one photon of frequency G>i and the emission of one photon of frequency 002- If the two laser fields are counterpropagating, perpendicularly to the atomic beam, this double photon exchange results in a net transfer of momentum to the atom of + 0)2)/ c which manifests as a deflection of the beam. 

It predicts the adiabatic plateaus of Fig. 17, which can be interpreted as a topological quantization of the number of exchanged photons. The dips are due to nonadiabatic Landau-Zener transitions when the pulse overlap is in the neighborhood of the intersections. With a configuration of counterpropagating laser fields, perpendicular to an atomic beam, this translates into the possibility of deflection of the beam by the quantized transfer of a momentum kh(ffli +... 

Fig. 4. Schematic representation of the Bragg scattering experiment, Including the laser configuration. The angles 0l and 9p are about 45°. The probe and Bragg reflected beams are counterpropagating to lattice laser beams.

Figure 5.19 Two color apparatus for the study of an overtone-induced reaction followed by time-delayed LIF or FE analysis of products. The two laser beams counterpropagate in the pressurized sample cell. Taken with permission from Rizzo et al. (1984). 

Note In the derivation of (2.32) we have only regarded population changes due to saturation effects. We have neglected coherence phenomena that may, for instance, result from interference between the two waves. These effects, which differ for copropagating waves from those of counterpropagating waves, have been treated in detail in [203-205]. For sufficiently small laser intensities (5-Cl) they do not strongly affect the results derived above, but add finer details to the spectral structures obtained. 

For the experimental realization of optical cooling, which uses a collimated beam of atoms and a counterpropagating cw laser (dye laser or diode laser. Fig. 9.6) the following difficulties have to be overcome during the deceleration time the Doppler-shifted absorption frequency o) t) = k v(t) changes with the decreasing velocity V, and the atoms would come out of resonance with the monochromatic laser. 

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]...

Fig. 9.14 Optical molasses with six pairwise counterpropagating laser beams...

Depending on the polarization of the two counterpropagating laser beams the lin J. lin configuration can be used with two orthogonal linear polarizations or the a -a configuration with a circularly polarized a+ wave, superimposed by the reflected cr wave. With Sisyphus cooling temperatures as low as 5-10 pK can be achieved. 

Fig. 9.91 Exciting the hydrogen 15-25 transition with two counterpropagating photons in a standing-wave field at 243 nm. This radiation is obtained from a dye laser frequency doubled in a BBO crystal and stabilized to a reference cavity. While scanning the hydrogen resonance the frequency of this laser is measured with a frequency comb to be 2 466 061413 187 074 (34) Hz for the hy-perfine centroid [1327]...

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