Resonant cavity optical modulator
AssigneeElectrical Power Research Institute, Inc.
InventorsThomas F. Gallagher, John P. Watjen
Life of this patent
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Abstract
A phase-matched resonant waveguide modulator suitable for very high frequency modulation and applications such as FM laser spectroscopy is described. The width of a crystal modulator section of the waveguide is chosen so that the operating frequency is just above cut-off to achieve optical and RF phase velocity matching at high frequencies such as 10 GHz by virtue of the dispersion of the waveguide near cut-off. The waveguide contains air in opposite end sections so that those sections are below cut-off and the microwave is evanescent, with the result that the end sections preferably are open for ease of optical coupling to the waveguide.
Claims18
A laser modulator comprising a waveguide for applying a selected high operating frequency modulating signal such as a radio frequency signal to a laser beam for frequency modulating the laser beam, and wherein the waveguide includes a crystal section having its width selected so that the high operating frequency is just above the cut-off frequency, whereby the associated phase velocity of the modulating signal in the crystal approximates the laser phase velocity in the crystal.
An RF laser modulator comprising a waveguide having a central crystal section along its length and wherein the width of the crystal is selected as c/(2n.sub.m f.sub.c) to approximate the condition (f.sub.c /f)=(1-(n.sub.o /n.sub.m).sup.2).sup.1/2 such that the microwave operating frequency is just above the cut-off frequency, to thereby establish high-frequency optical and microwave velocity phase matching, where c is the speed of light, f is the microwave operating frequency, f.sub.c is the microwave cut-off frequency, n.sub.o is the optical index of refraction of the laser beam in the crystal and n.sub.m is the microwave index of refraction in the crystal.
A high frequency RF laser modulator comprising a metal waveguide having open opposite ends for efficiently transmitting a pulsed monochromatic optical beam such as a laser beam therethrough;
- a modulator material in the form of an LiTaO.sub.3 crystal occupying a center section of the waveguide of length, .lambda., for transmitting the optical beam therethrough at a phase velocity c/n.sub.o, where c is the speed of light and n.sub.o is the optical index of refraction of the beam in the transmitting medium;
- the opposite end sections adjacent the open waveguide ends comprising a gaseous ambient such as air;
- the width, a, of the crystal modulator material being selected so that the dispersion of the waveguide near the cut-off frequency, f.sub.c, provides a microwave phase velocity, (c/n.sub.m)/(1-(f.sub.c /f).sup.2).sub.1/2, which approximately equals the optical phase velocity c/n.sub.o, where n.sub.m is the microwave index of refraction in the crystal modulator material, f is the selected operating frequency, f.sub.c is the cut-off frequency, and a is given by c/(2n.sub.m f.sub.c).
Dependencies are inferred from the claim text.