Optical isolator and circulator

Details Optical isolator and circulator Optical isolator and circulator

1999-04-16

2001-11-13

Spyrou, Cassandra (Department: 2872)

Optical waveguides

Polarization without modulation

C385S024000, C385S036000, C359S199200, C359S199200, C359S199200

Reexamination Certificate

active

06317527

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of Invention
The present invention relates generally to bidirectional fiber optic communication systems and, more particularly, to an optical device which may be configured as an optical isolator and circulator for use in both high and low bit rate applications in such fiber optic systems.
2. Description of the Prior Art
Communication service providers are experiencing significant consumer demands to accommodate additional bandwidth in optically-based communications systems and the demand is ever-increasing. Today's optical communication systems and networks field rising consumer demands for e-mail, video, multimedia, data and voice-data transmission requirements across a variety of communication protocols. In the future, all indications are that the use of fiberoptic networks will become even more prevalent as a preferred medium for transferring information as the marketplace for wide-band services matures. It is anticipated that additional services such as enhanced pay-per-view, video-on-demand, interactive television and gaming, image networking, video telephony, CATV and ISDN switching services will be depend on and be substantial users of such systems. Because capacity is a critical parameter for system viability, bidirectional systems are desirable when the increased capacity or other attributes afforded by the bidirectional fiber is required. Enabling bidirectional use of installed and developing fiber in fiberoptic systems will permit communication service provider to gain additional utility from limited system resources.
Lasers are employed in numerous applications, particularly within fiberoptic communications networks, in which the laser emits an information-carrying light signal to an optical fiber which transmits the light signal to a photodetector for further processing. Typically, the optical signal propagates in one direction over a signal optical fiber. In a bidirectional fiber optic configuration, an optical signal propagates in both directions over an optical fiber. However, due to the sensitivities of these systems even a small amount of reflected light will cause instability to the laser source in terms of its power and frequency characteristics.
To reduce some of the problems of reflected light, optical circulators and isolators, non-reciprocal devices, may be installed at each end of a fiber link in a system thereby enabling the bit carrying capacity of an existing unidirectional fiber optic link. The use of Faraday isolators in optic systems is well known as an integral component for removing reciprocal light based on the use of polarizers which are rotated by 45 degrees relative to each other on either side of a magnetic medium, as optical isolators passes a signal in the forward direction from a first optical port to a second optical port. Optical circulators are employed in bidirectional systems for multiplexing the forward and reverse paths of an optical light source, such as a laser. Optical circulators provide a non-reciprocal coupling of light between two fiber paths, based on the Faraday rotation of light, as light is treated differently depending on its entry port into an optical port.
As is generally known, an optical circulator is a non-reciprocal optical device which allows the passage of light from a first port to a second port while a reverse optical signal into the second port is transmitted in totality to a third port; similar transmissions continue for remaining ports thusly creating a circulating operation. Effectively, any two consecutive ports of an optical circulator are an optical isolator as signals are transmitted only one way.
FIG. 1
is illustrative of the operation employing optical circulators to provide simultaneous, bidirectional communication in a single fiber optic link. In
FIG. 1
, optical circulators
100
and
200
, each comprised of ports
10
,
20
, and
30
, are installed at opposite ends of a fiber optic link
150
. Communication transmitters
110
and
210
are connected to each port
10
, communication receivers
120
and
220
are connected to each port
30
, and the fiber optic link
150
is connected between ports
20
at each optical circulator. Light entering port
10
exits the optical circulator at port
20
as directionally indicated
160
. Light that enters the optical circulator at port
20
exits at port
30
as directionally indicated
170
. Light travels bidirectionally across a single fiber
150
as directionally indicated
180
.
Using traditional beam splitting plates in bidirectional systems may result in substantial reductions in light intensity each time a light beam passes through a beam splitting plate (e.g. on the order of 50% loss). Typically these “ping-pong” type of fiber communications occur at low speeds. Additionally, there is an increase in noise to the system due to insertion loss, cross-talk and coupling loss. With the advent of long-haul applications, these reflective problems may cause the communication to fail; similarly, low speed rates are not well-suited for long-haul application. Wavelength dependent beam splitter cubes and dichroic mirrors are known in the field. U.S. Pat. No. 5,210,643 (Fuji et al.) discloses a wave combining apparatus having dichroic mirrors for combining laser beams having the same direction of oscillation by waveform division, respectively, and a polarizing beam splitting prism for combining the first and second resultant beams into a single combined waveform. However, the separation and recombination of optical beams as described in this reference is suited only for low-speed applications.
Therefore, the need exists for an apparatus which improves over the light intensity losses, virtually eliminates ghost images resulting from reflections, and is able to communicate in both high and low bit rate applications at a reasonably low-cost.
SUMMARY OF THE INVENTION
In accordance with one aspect of the present invention, the optical device of the present invention is an optical isolator and circulator comprising a Faraday rotator, a polarizer and a polarizing sensitive beam splitting cube to provide optical isolation for the transmitting laser source and at the same time reflect the optical beam to the receiver.
The Faraday rotator rotates linearly polarized light by forty-five degrees such that the rotation may be clockwise or counterclockwise; the Faraday rotator is a traditional Faraday rotator requiring a magnetic field or preferably, is a latched-Faraday rotator which does not utilize a magnetic field. For the traditional Faraday rotator, the presence of an additional magnet ring having a different direction of magnetic field may be required depending on the configuration employed. In the present invention, a first Faraday rotator rotates linearly polarized light counterclockwise forty-five degrees, while a second Faraday rotator rotates linearly polarized light clockwise forty-five degrees in the same reference frame as to the first source.
The Faraday rotator and polarizer may preferably be integrated into a generic bidirectional module capable of transmitting, receiving or both. In this preferred arrangement, the module requires isolation from both specular reflections from fiber, connectors, and transmitted source signals from other emitters or transmitter devices.
In a further preferred arrangement, the bidirectional module comprises an isolator, a circulator, or both.
The polarizing beam splitting cube preferably is comprised of two triangular prisms connected (i.e. affixed) together by a dielectric film or coating. Undesirable light reflections from the cube's faces are eliminated by orienting the cube about the dielectric film's normal axis so that light is incident with nonzero incidence angle at the cube's faces. Preferably, a broadband coating is applied to the cube faces to virtually eliminate reflected light. In an additional preferred embodiment, an absorbing filter is optically cemented on the back face of the cube to absorb stray light.
It is an object of the present invention to pr

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