Comer-cube reflectors

In a transmission insertion probe (see Figure 4.2) 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 comer-cube reflectors must be used to turn the light path 180°. Again, the smallest practical gap is 0.5mm (0.5-nun optical path). 

The open-path experiments were carried out using a transmitter-retro-reflector-receiver configuration, where the laser radiation was transmitted into the atmosphere and returned back to the detector along a coaxial delivery/retum path. A comer-cube retro-reflector was placed at the required distances between 220 m and 2900 m from the transmitter/receiver site... 

Another way to free the mirror carriage from the high precision requirement is to use retroreflectors instead of flat mirrors. Then a small tilt of the reflector does not matter. The original instrument of Fellgett used comer cubes. High precision requirements are shifted from the motor design to that of the reflector, where they are easier to handle, but are still not trivial. The Cassini spacecraft (to arrive at Saturn in July 2004) and the Mars Express spacecraft (to arrive at Mars in December... 

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