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United States Patent m
Vogel et al.
 LIGHT MODULATOR
 Inventors: Paul Vogel, Steffisburg; Rainer Battig, Beme, both of Switzerland
 Assignee: Ascom Tech Ltd., Berne, Switzerland
 Appl. No.: 153,984
 Filed: Nov. 18,1993
 Foreign Application Priority Data
Nov. 20, 1992 [CH] Switzerland 03566/92
 Int. CI.6 G02B 26/00
 U.S. CI 359/291; 359/263; 356/358
 Field of Search 359/249, 263,
359/260, 291, 318; 285/9, 18; 356/357,
 References Cited
U.S. PATENT DOCUMENTS 4,558,950 12/1985 Ulrich et al 356/345
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US005508840A [ii] Patent Number: 5,508,840  Date of Patent: Apr. 16, 1996
FOREIGN PATENT DOCUMENTS
0420468A2 4/1991 European Pat. Off. . 4031970A1 4/1992 Germany.
Primary Examiner—Georgia Y. Epps
Assistant Examiner—Thomas Robbins
Attorney, Agent, or Firm—Burns, Doane, Swecker & Mathis
A light modulator useful for modulating a diverging light beam exiting from an optical fiber. In one embodiment, the modulator has a focusing lens and two flat partial mirrors. The lens collimates the light, one half of which respectively strikes each of the partial mirrors orthogonally. These mirrors reflect the light back onto itself causing, depending on the relationship between the wavelength of the incident light and the differences in path lengths, varying degrees of constructive or destructive interference. The difference in path length can be varied electrically if the partial mirrors are embodied as etched semiconductor foils which are controllably displaced in an electrostatic field set up between the foils and a substrate electrode.
17 Claims, 2 Drawing Sheets
U.S. Patent Apr. 16, 1996 Sheet l of 2 5,508,840
FIELD OF THE INVENTION
The present invention relates to a light modulator for 5 modulating the intensity of a light beam exiting an optical fiber, and, more particularly, to a light modulator having a displaceable mirror for reflecting such a light beam.
BACKGROUND OF THE INVENTION
One type of optical reflection modulator is known from German Patent Publication DE 40 31 970. In the arrangement disclosed in the German Patent Publication, light exiting the blunt end of an optical fiber strikes a mirror disposed orthogonally to the fiber. This mirror reflects the 15 light back onto itself, thus creating a standing wave in the manner of a Fabry-Perot resonator between the reflecting fiber end and the mirror, provided their distance from each other corresponds to a multiple of half a wavelength of the light used. By changing the distance between the reflecting 20 fiber end and the mirror, particularly by displacing the mirror, the Fabry-Perot resonator can be detuned and the intensity of the light can be changed.
The described modulator is of a relatively simple design and has good modulating properties. However, it is depen- 25 dent on the absolute value of the distance between the fiber end and the mirror, which changes because of many effects, for example, as a function of the temperature. It is therefore necessary to provide a regulator which maintains the "optical distance" between the reflecting fiber end and the mirror 30 constant.
There is thus a need for a light modulator having modulating properties comparable to those of prior devices, but which does not require maintaining a length constant. There 35 is also a need for a highly sensitive light modulator which is nevertheless of a simple design. There is also a need to achieve these ends in a modulator of the species of optical reflection modulators with movable mirror surfaces.
SUMMARY OF THE INVENTION 40
The present invention meets these needs by providing a modulator for modulating the intensity of a light beam which exits and reenters an optical fiber, the light modulator including a first mirror arranged to reflect a first substantial 45 portion of the light beam emerging from said optical fiber back into said optical fiber and a second mirror arranged next to the first mirror to reflect a second substantial portion of the light beam emerging from the optical fiber back into said optical fiber. A length of a path traversed by light exiting 50 the optical fiber, being reflected by the second mirror, and returning to the optical fiber is controllably variable with respect to a length of a path traversed by light exiting the optical fiber, being reflected by the first mirror, and returning to the optical fiber so that light reflected by the second mirror 55 interferes a controllable amount with light reflected by the first mirror.
BRIEF DESCRIPTION OF THE DRAWINGS
These and other features of the invention will be described in detail below by way of example by means of four drawing figures, in which:
FIG. 1 is a perspective principal view of a first embodiment of a modulator according to the present invention; 55
FIG. 2 is an enlarged view of detail of the modulator shown in FIG. 1;
FIG. 3 is a sectional through a second embodiment of a modulator according to the present invention; and
FIG. 4 is a view of a third embodiment of a modulator according to the present invention.
DETAILED DESCRIPTION OF PREFERRED
FIG. 1 is a principal, perspective view of a first embodiment of a modulator 11. The light reaches the modulator 11 via an optical fiber 13, for example a monomode glass fiber. The light exits through the end of the fiber 13, widens conically in a known manner and is collimated by a focusing lens 16, one focus of which is located at the light exit point 14. The relatively wide, collimated light beam strikes a two-part mirror 19 disposed orthogonally with respect to the light beam in such a way that each partial mirror 19.1 and 19.2 receives approximately the same amount of light. The incoming light is reflected back onto itself because of the orthogonality, is focused by the focusing lens 16 and is finally fed back into the optical fiber 13. This is indicated by the two-headed arrow 17.
The two partial mirrors 19.1, 19.2 are flat and in a rest position are located at the same level or in the same plane. Because of this there is no phase difference between the light which was reflected by the two partial mirrors, and the returning light beam remains undisturbed.
The situation is different if the two partial mirrors are located on different levels, that is, at different distances from the end of the optical fiber 13. In this case a phase difference, which is a function of the difference in the levels, occurs between the partial light beams reflected by the two partial mirrors 19.1, 19.2, which weakens the reflected total light beam to a lesser or greater degree because of interference and thus modulates it. An arbitrary, absolute length is thus not important. Instead, the relative difference in distance and the resultant phase difference are important. In this way all external influences are largely removed. For example, temperature fluctuations have no effect on the modulation depth.
It is generally true that the phase difference is caused by a relative movement of the two partial mirrors 19.1, 19.2. The relative movement can be achieved in that one partial mirror, for example 19.1, is fastened stationary and the other partial mirror 19.2 is orthogonally displaced in relation to its mirror surface. The direction, whether to the front or the back, does not matter in this case. However, it is also possible to displace both partial mirrors simultaneously, for example at different distances in the same direction or preferably at the same distance in opposite directions. In the latter case a displacement of each partial mirror 19.1,19.2 by one-eighth of a wavelength of the incident light is sufficient to modulate or digitally switch the light from maximum brightness to minimum brightness or maximum extinction. With a typical wavelength of, for example, 3100 nm, this means a deflection of the partial mirrors 19.1,19.2 by respectively only 400 nm.
As already mentioned, each partial mirror should reflect one half of the incoming or outgoing light. If this is not exactly the case, a reduced modulation depth results. However, this does not depend on the shape of the mirror. Instead of the flat partial mirrors 19.1, 19.2 shown in FIG. 1, it is therefore possible to form the surface of the one partial mirror circularly and to dispose this partial mirror in the sector of the circle of the second partial mirror.
FIG. 2 shows a greatly enlarged detail view of the partial mirrors 19.1,19.2. In this preferred embodiment the partial