The basic physical principle of a laser (light amplification by stimulated emission of radiation) is the induced emission of photons. With induced emission the emitted photons have identical properties and thus produce coherent light of the same wavelength.
A laser consists of an optical cavity which contains the lasing material with a mirror placed at each end. The light, which is repeatedly reflected between the two mirrors, is amplified. As one of the mirrors is only partially reflecting, a small laser beam emanates from the cavity. To keep the process going, energy is supplied in order to excite the atoms in the lasing material.
The wave along the laser cavity is a standing wave and the cavity of length L only resonates when there is an integral number n of half wavelengths l between the mirrors:
n = 2L / l
The frequency W is given by W = nc/ 2L where c is the velocity of the light. The separation of the multiple wavelengths (longitudinal modes) is DW = c / 2L.
However, not all possible modes in a laser cavity will be excited. Only those that are within the gain profile of the lasing medium will generate an emission.
The maximum distance over which the phase relationship will exist is called coherence length. The relation between the coherence length and the bandwidth is c/Dl
The laser used for laser Doppler vibrometers is a helium neon (He-Ne) laser. This laser produces a visible red beam (l = 0.6328 µm). This gas laser is an extremely low-noise light source and therefore ideally suited for this application.
Such a laser can be stabilized so that only a single mode is excited. The line width is then a couple of MHz which yields coherence lengths of about 200 to 300m.
Laser vibrometers are usually operated with multimode lasers where the lasers oscillate at 2-3 modes at the same time. The coherence length is only 10-20 cm because of the interference of the beat frequencies of these different modes. However, this beat frequency generates a cos2 dependence of the visibility as the distance to the object is varied. The signal has a maxima at distances of 2mL and minima at (2m-1)L where m is an integer. It is therefore possible to make measurements at very long distances with such a laser. The measuring distance should be adjusted to a visibility peak for maximum signal strength.
|
