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masers and lasers spontaneous and stimulatd emmisions he ne laser
Dear student,
A semiconductor laser is a special kind of light-emitting diode. It produces a beam of light in the near infrared with a divergence of about 15o, but the cone of emitted light is elliptical rather than circular, so that the beam appears to have originated from a line rather than a point. If the astigmatism of this beam is corrected by means of aspherical optics, a spatially-coherent beam can be obtained, and this has been used experimentally for making holograms. The main attraction of semiconductor lasers is that they are cheap and very small. They also operate at comparatively low voltages and art similar power range to that of He-Ne lasers.
There are a number of things to be considered in the choosing of a laser. A laser used to produce holograms needs good stability, and must be free from vibrations. The laser beam must be as plane as possible. A laser beam with multi modes is useless for making holograms. We want that the laser should have a circular beam diameter without any noise. The beam diameter is the important parameter in the calculation of the pinhole of the spatial filter.
The coherence length of the laser should be as large as possible. If the coherence length is small, the requirements of the path difference between the object and reference beam become harder to meet. This means that the path difference between these beams must be nearly zero. The number of modes in the laser is also an important parameter. In holography we prefer a laser with as few as possible modes. If we use a multi mode laser, we have problems with low visibility and the contrast in the hologram will be low.
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Sagar Singh
B.Tech, IIT Delhi
sagarsingh24.iitd@gmail.com
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