Optical isolator having surface mountable open core

Details Optical isolator having surface mountable open core Optical isolator having surface mountable open core

1998-09-24

2000-04-25

Spyrou, Cassandra

Optical: systems and elements

Polarization without modulation

Polarization using a time invariant electric, magnetic, or...

359495, 359500, 372703, 385 11, 335219, 335297, G02B 530, H01F 100, H01F 300

Patent

active

06055102&

DESCRIPTION:

BRIEF SUMMARY
The present invention relates to an optical isolator, and to optical devices employing optical isolators, in particular semiconductor laser modules.
An optical isolator permits the passage of light in one direction through the isolator, but prevents (or at least substantially attenuates) the passage of light in the opposite direction through the isolator. To achieve such non-reciprocal operation an optical isolator must comprise a component which behaves differently, dependent on the direction in which the light passes through it. This component is generally a Faraday rotator. A Faraday rotator comprises a material, typically a crystalline material, which is capable of rotating the plane of polarization of light passing through it in response to the application of an external magnetic field. The direction of rotation of the plane of polarization is dependent on the direction of passage of the light through the Faraday rotator, relative to the direction of the applied external magnetic field. The Faraday rotator is combined with one, or more, usually two, polarizers or birefringent walk off crystals, in order to form an isolator. The rotation of the plane of polarization provided by the Faraday rotator in one direction allows light to pass through both polarizers, whereas in the opposite direction the plane of polarization is rotated so that the passage of the light through the isolator is blocked by the polarizer.
Optical isolators find particular application in optical communications systems, especially for use with semiconductor lasers. As the transmission rates used in optical communications systems have increased, for example to several Gbits per second, the performance required of lasers used in such systems has also increased. It is well known that light reflected back from some part of an optical communications system will adversely affect the operation of such a high performance laser, for example leading to fluctuations in the spectrum, line width, or intrinsic noise of the laser. Optical isolators are utilised to protect such high performance semiconductor lasers from these reflections, and are increasingly being used not just in systems for research purposes, but in systems which are deployed for commercial use. To prevent isolators from adding an excessive additional cost to laser transmitters, isolators must be miniaturized, easily aligned to a laser, and of low cost.
A number of miniaturized optical isolators are known. WO 93/20465 discloses a laminated isolator element which is disposed within an optical fiber feedthrough tube. The fabrication and assembly technique for this isolator is complex, and thus expensive. Furthermore, although the isolator element itself is small, the cylindrical magnet employed renders this isolator arrangement bulk and cumbersome.
The optical isolator arrangement of U.S. Pat. No. 4,966,444 attempts to reduce the number of components employed by utilizing a Faraday rotator as a lens. In addition to compromising the performance of the isolator, this arrangement remains bulky due to the large radius of curvature required by the Faraday rotator lens, and the large cylindrical magnet employed.
U.S. Pat. No. 5,452,122 discloses a further isolator arrangement which utilizes laminated isolator elements, and a cylindrical magnet. Due to the use of a cylindrical magnet the isolator must be rotationally aligned to the output mode of the semiconductor laser.
According to a first embodiment of the present invention, there is provided an optical isolator mountable on a planar surface, the optical isolator comprising an isolator element, having an optical axis and comprising a Faraday rotator and at least one polarizer, which may be a grafting polarizer or birefringent walk off crystal and a magnet, wherein the magnet is an open-core magnet shaped so that when placed on a substantially planar surface an orifice is defined by the magnet and the substantially planar surface, and wherein the isolator element is located within said orifice so that the said optical axis passes throu

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