Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
1998-08-14
2001-10-09
Negash, Kinfe-Michael (Department: 2633)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C359S494010
Reexamination Certificate
active
06301030
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to the field of optical devices and systems. More particularly, the invention relates to a system for multiplexing polarized signals for simultaneous transmission through a communication fiber. The invention also relates to a system for demultiplexing a beam into polarized components.
2. Description of the Related Art
In the prior art, optical fiber capacity has been increased by multiplexing signals of different wavelengths. The technique is known as wavelength division multiplexing (WDM). A problem with wavelength division multiplexing is that the propagation rate of light is wavelength dependent. Consequently, the different propagation rates of different signals must be taken into account to accurately extract all transmitted information. Wavelength division multiplexing is also limited by the number of different wavelength signals that can be accommodated in a single mode fiber under commercial conditions.
There is a need in the art for a convenient system for multiplexing signals of the same or similar wavelength.
SUMMARY OF THE INVENTION
The disadvantages of the prior art are overcome to a great extent by the present invention, in which signals having substantially the same wavelength but different polarizations are multiplexed into a single beam. The multiplexed beam may be transmitted through a single mode fiber without interference between the two signals. The signals may be polarization demultiplexed at a downstream location.
In one aspect of the invention, a compact multiplexing system is provided with first and second sources (such as pump lasers) for producing first and second linearly polarized signal beams, and a birefringent device for combining the two beams into a single multiplexed beam.
In a preferred embodiment of the invention, the birefringent device is in the form of a wedge, with first and second planar surfaces arranged at an angle to each other. The first and second surfaces are not parallel to each other. The polarized beams are incident on the first surface. The multiplexed beam exits through the second surface. Thus, the multiplexing system is arranged to align and linearly walk the polarized beams together such that they are parallel and coincident with each other as they exit the birefringent wedge.
The birefringent wedge may be formed of a suitable uniaxial crystal material, such as rutile. The birefringent wedge may be a modified walkoff plate or prism.
In another aspect of the invention, a single graded index lens is used to collimate the signal beams and launch them into the birefringent wedge. In a preferred embodiment, the lens causes the signal beams to slightly diverge from each other, such that they are differentially refracted into a parallel condition by the birefringent wedge.
In a preferred embodiment of the invention, the multiplexed beam is focused into a communication fiber by a second graded index lens. The birefringent lens may be sandwiched between the two graded index lenses to form a compact, uncomplicated apparatus. An advantage of the invention is that it can be constructed as a compact package, without using any beam splitters or right angle prisms.
The present invention also relates to a communication system with a multiplexer for launching a polarization multiplexed signal into a single mode fiber, and a downstream demultiplexer for separating polarization dependent signals from the multiplexed signal. In a preferred embodiment of the invention, the demultiplexer may be formed of essentially the same components as those of the multiplexer, but arranged in reverse.
A retarder mechanism may be provided to align the polarized components of the multiplexed signal. The retarder mechanism may be controlled to maximize the strength (intensity) of the demultiplexed signals. The retarder mechanism may be, for example, a set of paddles that are actively tuned to provide a desired amount of retardance.
The present invention also relates to a method of multiplexing optical signals. Thus, in one aspect of the invention, linearly polarized signal beams are independently generated by separately modulating respective lasers. A birefringent wedge combines the beams into a single multiplexed beam. The multiplexed beam is propagated through a single mode fiber, and polarization demultiplexed at a downstream location.
The present invention permits a single optical fiber, or optical capillary, to transmit, without interference or interaction, two beams of light that are of the same or similar wavelength. Thus, the present invention may be used to increase fiber data capacity by at least two fold, over known wavelength division multiplexing/demultiplexing techniques. The invention may also be used to provide pump laser diversity in optical amplifiers.
These and other features and advantages of the invention will become apparent from the following detailed description of preferred embodiments of the invention.
REFERENCES:
patent: 3671747 (1972-06-01), Duguay
patent: 5111322 (1992-05-01), Bergano et al.
patent: 5113458 (1992-05-01), Taylor
patent: 5223975 (1993-06-01), Naganuma et al.
patent: 5982539 (1999-11-01), Shirasaki
Lucent Technologies - Inc.
Negash Kinfe-Michael
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