External chopper

Any interferometer in which the mirror moves at a constant velocity and the beam is not modulated by an external chopper or by phase modulation (see Section 20.3)... 

In the stepping mode, the signal is integrated at each rest position for a certain time, the dwell time. After that, the Michelson mirror advances to the following position and the next point is recorded. As in the case of an a.c. radiometer an external chopper may be used to modulate the incoming radiation. The stepping technique has been perfected by J. and P. Coimes. 

Frequently a modulation of light is introduced to the system in order to increase the signal to noise ratio. Flash lamps by their construction give pulses of light with repetition, which can be controlled by the user. Other lamps cannot be modulated through their driving current because the emitted radiation would be unstable over time. In this case, the application of an external modulator, e.g. a mechanical chopper, is the only solution. In both cases, the frequency of modulation is rather low - up to kilohertz. 

Monochromatic light can also be obtained from other types of lasers solid state, gas, ion, dye. Among them argon ion laser with its many lines is an especially valuable light source used in many sensors. However, these types of lasers are expensive, the modulation of the light cannot be done internally and external modulators (e.g. choppers) should be used. Wavelengths emitted by some exemplary lasers are presented in Table 1. 

The probe beam, multipass system and detector are also mounted in a stable frame. The intracavity frame and the deflection frame are rigidly connected to minimize mechanical vibrations between the two laser beams. The entire setup is sustained by soft cushions to decouple external low frequency vibrations, especially important because a low chopper frequency of 20 Hz was used. Air turbulences are minimized by shielding the whole frame. 

The measured time distribution of the prebunched positrons at the entrance slit of the chopper is about 2 ns (FWHM). This width is mainly caused by the energy spread(A sa 0.4eVFWHM) of the moderated positrons [133,136]. The positrons, which travel in a longitudinal magnetic guiding field of about 7 mT, are deflected by 90° and enter the new chopper device with an energy of 200 eV (see Figure 4.39). This chopper is a double-plate system with an external resonator, positioned outside the vacuum system. 

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