Continuously variable fiber optic delay line

Details Continuously variable fiber optic delay line Continuously variable fiber optic delay line

1984-07-09

1987-06-30

Sikes, William L.

350 9629, G02B 626

Patent

active

046765859

DESCRIPTION:

BRIEF SUMMARY
BACKGROUND OF THE INVENTION

This invention relates to the processing of signals transmitted through optical fibers. More particularly the invention is directed to a continuously variable delay line.
The advantages of fiber optic delay lines are well-known in the art. Thus, for example, transversal filters capable of selectively filtering modulated light signals has been taught. Furthermore, the construction of transversal filters by helically wrapping a single fiber optic element around a series of v-grooves in a silicon chip, with taps at each groove, is known in the art. However, because no adjustment of the delay line lengths has been possible in prior art delay lines or transversal filters, the frequency vs. attenuation characteristics of prior transversal filters was of necessity, determined at the time of construction of the filter. There is therefore a need for an adjustable fiber optic delay line so that, for example, the frequency response of a transversal filter utilizing the delay line may be adjusted through a continuous range.


SUMMARY OF THE INVENTION

This invention provides a continuously variable delay line for use with single mode optical fibers. Such a device is useful in a variety of applications. For example, a continuously variable delay line may be used to change the frequency response of a transversal filter.
The invention comprises a single optical fiber, which is wrapped around a plate or chip having parallel v-grooves, so that successive portions of the fiber are mounted in adjacent v-grooves. A portion of the cladding on each optical fiber mounted on the plate or chip is removed along a lateral line normal to the length of the fiber in the v-grooves, thereby simultaneously creating a tap in each fiber portion. Light is selectively coupled from one of the taps by superimposing on the plate or chip, a second v-grooved plate or chip which supports a single optical fiber, the cladding of which has been similarly removed. When the fibers are superimposed, evanescent field coupling occurs between the fibers at a selected one of the taps on the first plate or chip. This selection depends on the relative position of the two plates.
The length of the tap or coupling region of the fiber portions in the first plate or chip is longer than the length of the tap or coupling region of the fiber in the second plate. By adjusting the position of the second plate's coupling region or tap along the length of the first plate's coupling region or tap, the amount of delay can be varied through a continuous range. Thus, the present invention provides a practical continuously variable delay line not heretofore possible.


DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

These and other advantages of the present invention are best understood through reference to the drawings, in which:
FIG. 1 is a cross-sectional view of the fiber optic coupler of the present invention, showing a pair of fiber optic strands mounted in respective arcuate grooves of respective bases;
FIGS. 2 and 3 are cross-sectional views of the coupler of FIG. 1, taken along the lines 2--2 and 3--3, respectively;
FIG. 4 is a perspective view of the lower base of the coupler of FIG. 1, separated from the other base, to show its associated fiber mounted thereon, and the oval-shaped, facing surface of the fiber;
FIG. 5 is a schematic diagram showing the evanescent fields of the pair of fibers overlapping at the interaction region;
FIG. 6 is a schematic drawing of the coupler of FIG. 1, illustrating the radius of curvature, core spacing, and interaction length, as being parameters of the coupler;
FIG. 7 is a schematic drawing of an "equivalent" coupler;
FIG. 8 is a graph of normalized coupled power as a function of interaction length for a given fiber core spacing;
FIG. 9 is a graph of normalized coupled power as a function of interaction length for another fiber core spacing;
FIG. 10 is a graph of normalized coupled power as a junction of minimum fiber core spacing (spacing surfaces superimposed);
FIG. 11 is a schematic representation o

REFERENCES:
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patent: 4302071 (1981-11-01), Winzer
patent: 4307933 (1981-12-01), Palmer
patent: 4342499 (1982-08-01), Hicks, Jr.
Applied Optics, vol. 20, No. 15, Aug. 1, 1981, F. J. Liao, et al., "Single Mode Fiber Coupler", pp. 2731-2734.
Applied Optics, vol. 20, No. 14, Jul. 15, 1981, O. Parriaux, et al., "Distributed Coupling on Polished Single-Mode Optical Fibers", pp. 2420-2423.
Electronics Letters, vol. 16, No. 7, Mar. 27, 1980, Bergh, et al., "Single-Mode Fibre Optic Directional Coupler", pp. 260-261.
SPIE, vol. 232, 1980 International Optical Computing Conference (1980), Palmer, et al., "Analog Matrix Multiplication by Directional Coupling between Optical Fibers", pp. 157-159.

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