External modulator for optical communication

Optical: systems and elements – Optical modulator – Light wave temporal modulation

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385 2, 385 13, G02F 111

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active

055067212

DESCRIPTION:

BRIEF SUMMARY
TECHNICAL FIELD

This invention relates to an external modulator for optical communication for applying a modulation signal to an optical fiber from the exterior and thereby indirectly modifying a light propagating in the optical fiber, and more particularly to an external modulator for optical communication which can be easily manufactured, which can suppress the resonance of elastic waves in the optical fiber, which has a small insertion loss so that a large number of external modulators can be connected to a single transmission line, which has a flat frequency characteristic, and which can be used for high frequency modulation.


BACKGROUND ART

Conventionally, most of the modulators incorporated in an optical communication system generally utilize a direct modulation system for changing an application current to a semiconductor laser or light emitting diode which is a light source and directly modulating an output light therefrom.
However, in the case of this system, there is a problem that the light property is made unstable according to the modulation of the intensity in an LD (laser diode) generally used as a light source and the transmission distance of an optical signal is considerably limited.
Further, in this system, one signal source (signal transmission source) is used as to correspond to one transmission line, it becomes necessary to insert an optical coupler between the individual signal source and the transmission line in order to couple optical signals composed of a plurality of signal sources into one transmission line.
However, in the above direct modulation system, an insertion loss caused by the insertion of the optical coupler is large. For example, even in a case of a waveguide type optical coupler whose insertion loss is said to be relatively small, it is approx. 0.5 dB. Therefore, the number of signals which can be coupled to one transmission line is limited.
In order to overcome the above limitation in the direct modulation system, recently, a modulator utilizing an external modulation system is developed.
The external modulation system is a system having a D.C. light source whose intensity is constant with time, and modulating a propagation light by use of a modulator arranged on a half-way of the optical transmission line and can exclude obstacles based on instability of the light source. Further, it has an advantage that the insertion loss caused when the modulator is inserted to the optical transmission line is small, and therefore, a large number of modulators can be incorporated into the optical transmission line.
As a modulator incorporated into an optical communication system of the above external modulation system, the following examples are known as representative examples.
One of them is obtained by forming a waveguide pattern having a large refractive index in a substrate made of LiNbO.sub.3 by ion exchanging operation, for example, and disposing an electrode for voltage application near the waveguide.
The external modulator is operated on the basis of the electrooptic effect, and the modulation characteristic in the high frequency region (several GHz) is excellent, but the dependency thereof on variations in the temperature and moisture is large, the dependency of the light output intensity on the bias voltage fluctuates according to deterioration with time and expansion/contraction of the waveguide base plate caused by the above variations, thus providing a problem that it is difficult to attain the stable operation. Further, since the single crystal of LiNbO.sub.3 is extremely expensive and has a difficulty in the practical application in the industry. In the case of this modulator, the insertion loss is as large as 2 to 4 dB, and therefore, it is difficult to dispose a large number of modulators in one optical transmission line.
Further, there is provided such a type as shown in FIG. 1 and disclosed by D. S. Czaplak and F. S. Hickernell in Ultrasonics Symposium, 1987, pp 491 to 493. The external modulator has such a structure that the outer periphery of a clad laye

REFERENCES:
patent: 3771856 (1973-11-01), Eschler
patent: 4703287 (1973-11-01), Fournier, Jr. et al.
Conference on Lasers and Electro-Optics, Jun. 1986, San Francisco, California, pp. 292-293, S. S. Tarng et al., "Acoustooptic Modulators for Single-Mode Fibers".
Journal of the Institution of Electronic and Radio Engineers, vol. 58, No. 5, Jul., 1988, London, England, pp. 566-578, M. J. F. Digonnet, et al., "Single-Mode Fibre-Optic Components".
Paper entitled "All-Fiber Acoutso-Optic Phase Modulators Using Zinc Oxide Films on Glass Fiber", A. A. Godil, et al., presented at the Optical Fiber Sensors Topical Meeting, Jan. 27-29, 1988, New Orleans, Louisana, and printed in `Optical Fiber Sensors`, 1988, Technical Digest Series, vol. Part 1, Optical Society of America, Wash. D.C. pp. 159-162.
Article Entitled "Acousto-Optic Phase Modulator for Single Mode Optical Fiber", H. F. Taylor, Navy Technical Disclosure Bulletin, vol. II, No. 2, Dec. 1985, Navy Tech. Cat. No. 4900, Case No. 68094, pp. 65-70.
Article entitled "Phase Shift Nonlinearity at Resonance in a Piezofilm-Based Fiber-Optic Phase Modulator", V. S. Sudarshanam, et al., Journal of Applied Physics 68 (1990) Sep., No. 5, New York,, N.Y. pp. 1975-1980.
Article entitled "AO Phase Modulator for Single-Mode Fibers Using Cylindrical ZnO Transducers" by D. S. Czaplak, et al., Naval Research Laboratory, 1987 Ultrasonics Symposium, IEEE Research Laboratory, Wash. D.C., pp. 491-493.
Article entitled "All-Fiber Acoustooptic Phase Modulators Using Zinc Oxide Films on Glass Fiber", A. A. Godil, et al, Journal of Lightwave Technology, vol. 6, No. 10, Oct., 1988, IEEE, pp. 1586-1590.
Article entitled "Detachable 400-MHZ Acousto-Optic Phase Modulator for A Single-Mode Optical Fiber", D. B. Patterson, Received Sep. 8, 1988 and accepted Nov. 30, 1988, Published in Optics Letters, vol. 14, No. 4, Feb. 15, 1989 Optical Society of America, Washington, D.C., pp. 248-250.
D. S. Czaplak; AO Phase Modulator for Single-Mode Fibers Using Cylindrical ZnO Transducers; 1987; pp. 491-493; IEEE.
A. A. Godil; All-Fiber Acoustooptic Phase Modulators Using Zinc Oxide Films on Glass Fiber; 1988; pp. 1586-1590; IEEE.
D. B. Patterson; Detachable 400-MHz Acousto-optic Phase Modulator For A Single-mode Optical Fiber; 1989; pp. 248-250; Optics Letters.

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