Optical waveguides – With optical coupler
Reexamination Certificate
2001-10-09
2003-07-01
Healy, Brian (Department: 2874)
Optical waveguides
With optical coupler
C385S027000, C385S039000, C385S122000, C385S123000, C359S341100, C359S341300, C359S348000, C359S199200, C359S337500
Reexamination Certificate
active
06587606
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to a signal regenerator for use in an optical communication system, and particularly to such a regenerator that combines dispersion compensation with the saturable absorption properties of a nonlinear optical loop mirror or a nonlinear amplifying loop mirror.
2. Technical Background
The concept of dispersion management emerged early in the development stages of optical waveguide systems. Dispersion management is enabled by the property of waveguides fibers that allows adjustment of the waveguide dispersion by alteration of the optical waveguide fiber refractive index profile. In particular, the waveguide dispersion can be made to substantially cancel, i.e., subtract from, the material dispersion to provide a waveguide fiber having a total dispersion near zero over an extended wavelength range. The total dispersion of a waveguide fiber (also called the chromatic dispersion or the group velocity dispersion) is the algebraic sum of the waveguide dispersion and the material dispersion of the optical waveguide fiber. The convention in the art is to assign a positive value to total dispersion if the total dispersion causes light of shorter wavelength to travel at a higher speed in the fiber in comparison to the speed of longer wavelength light. Conversely; negative total dispersion causes light of longer wavelength to travel at higher speed in the fiber.
The waveguide dispersion can be altered to provide waveguide fibers that have a zero dispersion wavelength at any point in a wide wavelength range. For example zero dispersion wavelength of a waveguide fiber can be placed anywhere in the range from 200 nm to 2000 nm. In addition, the slope of the total dispersion can be made positive or negative essentially independently of the placement of the zero dispersion wavelength. These capabilities allow dispersion compensation to be achieved by altering the sign of the total dispersion along the length of an individual waveguide fiber. Further, dispersion compensation can be achieved on an overall system basis by forming the system of positive and negative waveguide fibers. The accumulated dispersion of a system is determined by adding the dispersion products of the waveguides that make up the system length and dividing by the total system length. The dispersion product of a waveguide fiber is defined as the total dispersion of the waveguide fiber in ps
m-km multiplied by the length of the fiber.
As a valuable adjunct to dispersion management is the optical amplifier, which is used to manage attenuation. Dispersion management combined with optical amplification raises the possibility of a dispersion and attenuation free system, having repeater spacing limited only by spontaneous noise from the amplifiers, frequency chirping of the signal source, and non-linear optical effects.
A signal regenerator module that, in addition to compensating dispersion and attenuation, also removed spontaneous noise, pulse timing jitter, and reduced or eliminated non-linear effects would serve to greatly decrease system cost by preserving signal pulse integrity in systems having larger regenerator spacing than is possible with present systems using standard regenerators.
Recent theoretical and experimental (PCT WO 98/36512; Golovchenko et al, Electronics Letters, v. 33, n. 9, p. 73, (1997); Nakawaw, et al, IEEE Photonics Technology Letters, v. 8, n. 8, p. 1088, 1996; F. Favre. Et al, Electronic Letters, v. 33, n. 25, p. 2135, (1997); T. Yu, Optics Letters, v. 22, n. 11, p. 793, (1997)) work in dispersion managed systems employing a return-to-zero (RZ) format for soliton signals has shown that allowing the RZ soliton signals to alternately broaden in highly dispersive optical waveguide fiber of one sign and then contract in highly dispersive optical waveguide fiber of the other sign has a beneficial impact on overall signal integrity. For example, the RZ soliton signals could broaden in positive dispersion optical fiber and then contract in negative dispersion optical fiber. In particular in varying-soliton signals, timing jitter due to amplified spontaneous emission (ASE) is reduced, signal-to-noise-ratio in the receiver (SNR) is improved, the impact of discrete amplifier power perturbations is reduced, and the deleterious effects of signal collisions or multi-wavelength signal interactions is reduced.
An opportunity therefore exists to exploit this advantageous combination in systems using an RZ format. Furthermore, the combination can be configured to further enhance the beneficial effect on the varying-soliton signals.
SUMMARY OF THE INVENTION
Throughout this application the term varying-soliton(s) is used to describe RZ (return to zero) signal pulse(s), i.e., soliton pulses whose amplitude, width, or shape are caused to vary along at least a portion of the waveguide fiber, in particular along the waveguide fibers of the dispersion compensating optical regenerator in accord with the invention. This designation of varying-soliton distinguishes the signal pulses of the present application from those that propagate in systems designed to maintain ideal, i.e., invariant, soliton signals. It also distinguishes the signal pulses of the present application from non-return-to-zero pulses used in commercial systems today.
One aspect of the present invention is a dispersion compensating optical regenerator for use in a waveguide fiber telecommunications system. This passive optical component combines the functions of dispersion compensation with signal regeneration, where signal regeneration includes recovering signal amplitude and shape. The dispersion compensating optical regenerator system comprises a positive total dispersion waveguide fiber for the transmission fiber and a negative total dispersion waveguide fiber for dispersion compensation, where the negative dispersion waveguide fiber is a part of a non-linear optical loop mirror (NOLM) or a non-linear amplifying loop mirror (NALM). The respective positive and negative total dispersion waveguide fibers have respective lengths and total dispersion magnitudes selected to provide a pre-selected amount of dispersion compensation. In particular, the waveguide fiber dispersion products, i.e., the product obtained by multiplying fiber total dispersion by fiber length, of the respective waveguide fibers are added algebraically. The respective fiber lengths and dispersions are chosen such that the magnitude of the algebraic sum is made to fall within a desired range.
An advantageous range is one that does not include zero, thereby limiting resonant non-linear phenomenon of four wave mixing. Another advantageous range choice is one in which the algebraic sum is positive, thereby allowing for formation and propagation of soliton pulses. The invention is particularly suited for use in varying-soliton transmission as will be pointed out in the detailed description below. The upper limit of the algebraic sum should be small to limit dispersion power penalty. Thus a preferred range of total average dispersion of the system is 0.01 to 5.0 ps
m-km and a more preferred range is 0.1 to 1 ps
m-km.
In an embodiment of this first aspect of the invention, the phase shifting means is an asymmetrical coupler that divides the signal pulses into counter-propagating pulses having different amplitude. The higher amplitude pulses corresponding to one of the propagation directions will undergo a larger phase shift due to self phase modulation.
In another embodiment, the respective amplitudes of the counter-propagating pulses are made asymmetric by an optical amplifier asymmetrically placed along the length of the optical waveguide fiber comprising the loop mirror. An alternative statement of the asymmetric placement of the optical amplifier is that the length of the fiber of the loop mirror coupled to one port of the amplifier is different from the length coupled to the other port of the amplifier. In this case the optical amplifier has two ports, one for signal input and one for signal output
Corning Incorporated
Healy Brian
Lin Tina M
Redman Mary Y.
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