Browse Prior Art Database

Tunable Laser and Microwave Source

IP.com Disclosure Number: IPCOM000086810D
Original Publication Date: 1976-Oct-01
Included in the Prior Art Database: 2005-Mar-03
Document File: 3 page(s) / 36K

Publishing Venue

IBM

Related People

Garwin, RL: AUTHOR

Abstract

It is known that microwave and submillimeter radiation can be produced by beating two selected laser frequencies together. For example, two laser beams having wavelengths of 600 nanometers will produce a beat frequency at DC, and the beat frequency will shift to 1 millimeter wavelength if one of the laser frequencies is shifted by 600 parts per million. Thus, the entire range of frequencies from 0 to 3x10/11/ sec/-1/ is covered by a fractional frequency deviation of a single visible laser beam by 600 parts per million (ppm).

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Tunable Laser and Microwave Source

It is known that microwave and submillimeter radiation can be produced by beating two selected laser frequencies together. For example, two laser beams having wavelengths of 600 nanometers will produce a beat frequency at DC, and the beat frequency will shift to 1 millimeter wavelength if one of the laser frequencies is shifted by 600 parts per million. Thus, the entire range of frequencies from 0 to 3x10/11/ sec/-1/ is covered by a fractional frequency deviation of a single visible laser beam by 600 parts per million (ppm).

For matching a laser to a line of an atom or molecule, many more lasers can be realized if a given laser can be shifted by a few hundred parts per million. If a required frequency in the infrared is obtained by mixing two higher-frequency laser beams, a lesser fractional deviation on either of the input beams would provide the required tuning of the infrared source.

Thus, a simple stable means is described for shifting a laser beam by a few hundred parts per million, whether this means is employed relative to a laser beam later to be used without further change of wavelengths or to one of the input beams which are used to make a final and different frequency.

The present device makes use of the doppler effect on the frequency of light reflected from a moving mirror. If light is normally reflected from a mirror, the change of frequency (for low-mirror velocities) is given by expression [1], where c is the velocity of light, f the frequency of the incident light, v the velocity of the mirror, and (f+delta f) the frequency of the resulting light. For future reference, a mirror velocity v=3x10/4/ cm/sec (that is, equal to the velocity of sound in air), corresponds to a frequency shift by 2x10/-6/ or 2 parts per million. Delta f = (2v/c)f [1].

The velocity of a mirror carried on a rotating disk or arm is limited by the "centrifugal force" involved with the circular motion. For instance, a thin ring rotating about an axis through its center puts the material of the ring into uniaxial stress of magnitude S = rho v/2/ [2]. A thin steel ring at sonic velocity, thus, has a circumferential tensile stress of S = (8)(3x10/4/)/2/ = 7x10/9/ = 7000 bar = 100,000 psi [3].

On the other hand, a carbon fiber rod of density 1.5 would have a largely radial uniaxial stress which drops to zero at the end. Tapered carbon fiber or beryllium rods can effectively run at 6x10/4/cm/sec peripheral velocity, corresponding to a doppler shift on reflection of ppm.

If the laser beam is reflected from the moving mirror, thence to a stationary mirror, thence returned to the moving mirror, etc., the resultant frequency shift will be simply n delta f, where n is the number of reflections from the moving mirror. Thus, 50 reflections from the moving mirror (100 reflections total) would correspond in the example given of a mirror running at 3x10/4/cm/sec to a total f...