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Modulating a Laser Beam

IP.com Disclosure Number: IPCOM000095118D
Original Publication Date: 1965-Sep-01
Included in the Prior Art Database: 2005-Mar-07
Document File: 2 page(s) / 35K

IBM

Related People

Magill, PJ: AUTHOR [+1]

Abstract

These methods are for generating a CW frequency on a laser beam which can be used as a carrier in a single or multichannel laser communication of one portion of the beam relative to another portion. When the beams are recombined and incidenced on a square law detector, a signal at the phase-modulated frequency is observed. One way of obtaining the phase shift is by varying the optical path length. In drawing 1, the difference in path length between beam 1 and beam 11 can be shown to be equal to x, the distance between point 2 and 3. To obtain a uniform change in phase as a function of time, either the prism or the light beam can be moved at a constant velocity. The change in phase/ unit time or frequency shift can be shown to be equal to theta/t = v = Kv/lambda.

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Modulating a Laser Beam

These methods are for generating a CW frequency on a laser beam which can be used as a carrier in a single or multichannel laser communication of one portion of the beam relative to another portion.

When the beams are recombined and incidenced on a square law detector, a signal at the phase-modulated frequency is observed. One way of obtaining the phase shift is by varying the optical path length. In drawing 1, the difference in path length between beam 1 and beam 11 can be shown to be equal to x, the distance between point 2 and 3. To obtain a uniform change in phase as a function of time, either the prism or the light beam can be moved at a constant velocity. The change in phase/ unit time or frequency shift can be shown to be equal to theta/t = v = Kv/lambda. In the equation, K is a constant dependent on the apex angle and index of refraction of the prism, v is the velocity in centimeters per second, and lambda is the wavelength of the radiation in centimeters. K has a value of approximately 0.2 for an angle of 35 degrees and an index of refraction of 1.4. The frequency increase or decrease for the He-Ne laser wavelength is therefore v = 3 x 10/3/v.

Another method of obtaining the relative velocity between the light beam and the prism involves moving the light beam. This can be accomplished in several ways. Drawing 2 illustrates one of these. A moving mirror can be oscillated back and forth, causing the beam to sweep from position 1 to...