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United States Patent   Patent Number: 4,973,120
Jopson et al.  Date of Patent: Nov. 27,1990
 OPTICAL ISOLATOR WITH RESONANT
CAVITY HAVING GYROTROPIC MATERIAL
 Inventors: Robert M. Jopson; Julian Stone, both of Rumson, N.J.
 Assignee: AT&T Bell Laboratories, Murray Hill, N.J.
 Appl. No.: 345,861
 Filed: May 1,1989
 Int. CI.* G02F 1/09; G02F 1/21
 U.S. Q 350/96.13; 350/376;
 Field of Search 350/96.13, 375, 376,
350/377, 378, 384, 385; 356/351, 352
 References Cited
U.S. PATENT DOCUMENTS
3,523,718 8/1970 Crow 350/375
3,644,016 2/1972 Macken 356/352
4,033,670 7/1977 Tanton et al 350/375
4,516,073 5/1985 Doriath et al 350/377
4,563,092 1/1986 Kaiser 350/377
4,756,607 7/1988 Watanbe et al 350/375
4,830,451 5/1989 Stone 356/352
FOREIGN PATENT DOCUMENTS
62-257032 11/1987 Japan 356/352
"Bulk Optical Isolator Tunable from 1.2 jxm to 1.7 /xm",
by R. M. Jopson et al., Electronics Letters, Aug. 29, 1985, vol. 21, No. 18.
Primary Examiner—Bruce Y. Arnold
Assistant Examiner—Martin Lerner
Attorney, Agent, or Firm—Eli Weiss
The invention is an optical isolator. In one embodiment the optical isolator comprises two linear polarizers, one at the input of the isolator and the other at the output. Positioned between the input and the output linear polarizer is a gyrotropic medium located within a resonant cavity such as a Fabry-Perot cavity. Interposed between the linear polarizer at the input of the isolator and the resonant cavity is a first polarization conversion means for converting received plane polarized optical energy from said linear polarizer to circularly polarized optical energy and interposed between the resonant cavity and the linear polarizer at the output of the isolator is a second polarization conversion means for converting received circular polarized optical energy from said resonant cavity to plane polarized optical energy. In an embodiment, the resonant cavity comprising the gyrotropic medium becomes the filtering medium to block reflected optical radiation.
7 Claims, 2 Drawing Sheets
U.S. Patent Nov. 27,1990 Sheet 2 of 2
OPTICAL ISOLATOR WITH RESONANT CAVITY
HAVING GYROTROPIC MATERIAL
This invention relates generally to optical isolators and, more particularly, to optical isolators which utilize a resonant cavity to obtain a substantial reduction in size.
BACKGROUND OF THE INVENTION
Optical isolators are necessary to prevent reflected optical radiation from re-entering an optical device such as, for example, a laser. In optical communication systems, reflections of optical energy into a laser degrades the operation of the laser by causing amplitude fluctuations, mode partitioning, frequency shifts, and linewidth narrowing. Present optical communication systems use optical isolators at the output of a laser to 2Q prevent light from being reflected back into the laser. Such isolators are generally referred to as Faraday isolators and utilize the principles of linearly polarized (also referred to as plane polarized) electromagnetic energy in combination with Faraday rotation. Fre- ^ quently, two isolators are used in tandem to provide as much as 60 db isolation. Typically, when designed to operate with optical energy at a wavelength of 1.5 u-m, the optical isolator will have a length of several centimeters and require 8 optical surfaces. Present day opti- 30 cal isolators are large and bulky relative to optical fibers and, therefore, cannot readily be incorporated into a chip or become an integral part of an optical fiber.
The present invention relates to an optical isolator which is adapted to pass optical energy in one direction and block or substantially block optical energy in the opposite direction. In a typical installation an optical isolator is located at the output of a laser and passes 49 optical energy generated by the laser to a receiving device i.e., an optical fiber. In one embodiment, the optical isolator can comprise two linear polarizers, one at the input of the isolator and the other at the output. Positioned between the input and output linear polariz- 45 ers is a Faraday active gyrotropic medium located within a resonant cavity such as a Fabry-Perot cavity. Interposed between the linear polarizer at the input of the isolator and the resonant cavity is a first polarization conversion means for converting plane polarized opti- 50 cal energy received from said linear polarizer to circularly polarized optical energy and, interposed between the resonant cavity and the linear polarizer at the output of the isolator is a second polarization conversion means for converting circular polarized optical energy re- 55 ceived from said resonant cavity to plane polarized optical energy. In one embodiment, the resonant cavity comprising the gyrotropic medium becomes the filtering medium to block reflected optical radiation.
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 is a schematic of a conventional Faraday effect optical isolator;
FIG. 2 is a schematic of one embodiment of structure in accordance with the principles of the invention; 65
FIG. 3 is a view of a resonant cavity;
FIG. 4 is a schematic of a device incorporating the inventive optical isolator; and
FIG. 5 is a schematic of another device incorporating the inventive optical isolator.
Referring to FIG. 1, there is illustrated an optical isolator which uses the Faraday rotational effect with received plane polarized optical energy to pass optical energy in one direction and block optical energy in the other direction. The device comprises a gyrotropic medium 12 interposed between an input linear or plane polarizer 10 and an output linear or plane polarizer 14. The gyrotropic medium 12 is located within an applied longitudinal magnetic field represented by the arrow 16. The gyrotropic medium is characterized by a Verdet constant V, which is defined as the rotation of the plane of polarization of the optical energy per unit length, per unit of applied field. The rotation of the plane polarized optical energy, as it passes through the gyrotropic medium, is given by the expression:
V is the Verdet constant; and
H is the magnetic field. In some materials, such as yttrium iron garnet (YIG) the Faraday rotation saturates at some applied magnetic field, in which case, the rotation is given by the expression:
apis the saturated specific rotation and L is the length of the gyrotropic medium. Typically, at a wavelength of 1.5 |xm and a magnetic field of about 1 kgauss (which is readily obtainable from a samarium-cobalt magnet) a length of 2.6 \xm give a rotation of 45 degrees in YIG, the most commonly used isolator material in the 1.3 pim-1.5 u-m region of the spectrum.
Light 19 traveling through the optical isolator in the backward direction passes through linear polarizer 14 which is oriented at an angle; and the backward linearly polarized optical energy which emerges from polarizer 14 is oriented at this angle. The linearly polarized optical energy then enters Faraday rotator 12 which then rotates the plane of the linear polarized optical energy by the angle <?f. The linearly polarized optical energy from the Faraday rotator then enters polarizer 10 which is oriented to block the light from the Faraday rotator. In most isolators the Faraday rotators are designed to rotate the plane of the polarized optical energy by 45 degrees.
In the forward direction, light 18 passes through polarizer 10 and is linearly polarized at the angle of the polarizer 10. This light which is traveling to the right, then passes through the Faraday rotator where it is rotated further in the same sense as the rotation of the plane of polarization of optical energy traveling to the left. If the Faraday rotator is designed to rotate the plane of polarization of the linear polarized optical energy 45 degrees, then the optical energy which emerges from the Faraday rotator 12 is polarized parallel to the polarizer 14 and is fully transmitted.
Optical isolators which contain YIG or similar garnets have demonstrated low loss (~1 dB) and reasonably good isolation (—30 dB). However, they are rela