Corner cube reflector

In a transmission insertion probe (see Figure 6.1) light makes only a single pass through the sample gap. In these probes either the fiber is forced into a U bend at the end of the probe, or corner-cube reflectors must be used to turn the light path 180°. Again, the smallest practical gap is 0.5 mm (0.5-mm optical path). 

The beam enters the 1.75 m Teflon-lined White cell containing a corner cube reflector (3) and undergoes 102 passes before exiting to the detector. Absorptions at least as low as 10 s can be measured which, for a total path length of 150 m corresponds to detection limits in the range 25 to 100 parts per trillion by volume for most atmospheric gases. 

A very convenient way to extract the laser power is the Littow-mounted grating combined with a mirror at right angle to form a "corner cube reflector" (Fig.3.8). The zeroth order reflection of the grating is used to couple the laser power out of the cavity. This loss is present anyway, and thus we can avoid a partly transmitting end mirror and use a gold coated mirror instead. 

Figure 4.69 illustrates the principle of a traveling-wave Michelson-type interferometer as used in our laboratory. Such a wavemeter was first demonstrated in a slightly different version by Hall and Lee [184] and by Kowalski et al. [190]. The beams 5r of a reference laser and of a laser with unknown wavelength Xx traverse the interferometer on identical paths, but in opposite directions. Both incoming beams are split into two partial beams by the beam splitters BSl and BS2, respectively. One of the partial beams travels the constant path BS1-P-T3-P-BS2 for the reference beam, and in the opposite direction for the beam Bx. The second partial beam travels the variable path BS1-T1-M3-M4-T2-BS2 for 5r, and in the opposite direction for Bx. The moving corner-cube reflectors T1 and T2 are mounted on a carriage, which either travels with wheels on rods or slides on an airtrack. 

The corner-cube reflectors guarantee that the incoming light beam is always reflected exactly parallel to its indicent direction, irrespective of slight... 

These corner-cube reflectors guarantee that the incoming light beam is always reflected exactly parallel to its indicent direction, irrespective of slight misalignments or movements of the travelling reflector. The two partial beams (BS1-T1-M3-M4-T2-BS2 and BS1-P-T3-P-BS2) for the reference laser interfere at the detector PDl, and the two beams BS2-T2-M4-M3-T1-BS1 and BS2-P-T3-P-BS1 from the unknown laser interfere at the... 

Fig.5.25a,b> Two examples of possible ring laser configurations, (a) Using total reflection in corner-cube reflectors and frustrated total reflection for output coupling, (b) Three mirror arrangements with beam combining prism... 

The principle of long-path absorption techniques is illustrated in Fig. 10.17. A laser beam is transmitted continuously into the atmosphere against a corner-cube retro-reflector (Fig. 6.21) that is placed at a distance of up to 10 km. The reflected beam is received by an optical telescope that is placed at the site of the laser and is directed towards the retro-reflector. The received light intensity is measured photo-electrically as a function of the laser wavelength. The absorption spectrum of the atmosphere between the laser and the retro-reflector is then recorded and the mean concentrations Nj of pollutant molecules can be determined using the Beer-Lambert relation... 

If a spectral measurement is disturbed, it is important to understand the origin of the disturbance first and take proper action to eliminate it. For example, if the reflectors on the two arms of the interferometer are plane mirrors (not cube-corner retroreflectors), the above case of the infrared radiation emitted by a high-temperature sample may be dealt with in the following way. It is effective to block half of the incident beam from the light source at a point close to the sample position where the incident beam is focused. Then, the infrared radiation from the sample will advance to the interferometer through the remaining half and will be reflected by the interferometer. When it comes back toward the sample. 

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