Optical waveguides – With optical coupler – Input/output coupler
Utility Patent
1999-01-26
2001-01-02
Lee, John D. (Department: 2874)
Optical waveguides
With optical coupler
Input/output coupler
Utility Patent
active
06169831
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to an optical fiber grating device and more particularly, a chirped grating.
BACKGROUND OF THE INVENTION
Optical fibers and fiber gratings are useful for telecommunication transmission and networking. Basically, optical fibers are thin strands of glass capable of transmitting information-containing optical signals over long distances with low loss. In essence, an optical fiber is a small diameter waveguide comprising a core having a first index of refraction surrounded by a cladding having a second (lower) index of refraction. As long as the refractive index of the core exceeds that of the cladding, a light beam propagated along the core exhibits total internal reflection, and it is guided along the length of the core. Typical optical fibers are made of high purity silica, and various concentrations of dopants may be added to control the index of refraction.
Optical gratings are important elements for selectively controlling the paths or properties of traveling light and specific wavelengths of light transmitted within optical communication systems. Gratings based on optical fibers are of particular interest as components in modem telecommunication systems. For example, in long-distance transmission of optical signals, the accumulation of signal dispersion may be a serious problem. This problem intensifies with an increase in the distance the signals travel and the number of channels in a wavelength-division-multiplexed (WDM) optical communication system. Optical fiber grating devices may be used to compensate for chromatic dispersion.
Optical gratings may include Bragg gratings, long-period gratings, and diffraction gratings. These gratings typically comprise a body of material with a plurality of spaced-apart optical grating elements disposed in the material. Often, the grating elements are non-chirped gratings in that they comprise substantially equally-spaced index perturbations, slits, or grooves. However, chirped gratings, comprising unequally-spaced perturbations, are used as well. Chirped gratings may be linearly-chirped (having perturbations that vary in a linear fashion), or non-linearly or randomly chirped.
An extrinsic chirp refers to a chirp in the grating that is obtained by applying an external perturbation-generating field (an “external gradient”) to the fiber. In the past non-chirped fibers have been subjected to external gradients to produce chirped fibers. To create a chirp, an external gradient has been applied non-uniformly along the length of a non-chirped fiber, resulting in non-uniform changes in properties of the fiber grating, thus creating a chirp. An extrinsic chirp is valuable in that it may be applied to adjust the parameters of the grating, and it may be used to control the dispersion of a fiber Bragg grating. External gradients are well-known and typically comprise strain gradients or temperature gradients. Use of a temperature gradient to impose a chirp on a fiber grating is described, for example, in U.S. Pat. No. 5,671,307 to Lauzon, issued Sep. 23, 1997, which is incorporated herein by reference. Similarly, it was proposed that a chirp could be induced in a grating using a strain gradient in P. C. Hill & B. J. Eggleton, ELECT. LETT. Vol. 30, 1172-74 (1994), incorporated herein by reference.
There are disadvantages, however, in forming chirped gratings with an external gradient. The maximum chirp rate (or range of chirping) that can be achieved is limited in that relatively large gradients (or forces) are required to obtain a range of chirping, but such forces may have a negative impact on the reliability of the fiber. For example, a temperature gradient may be applied along the length of the fiber to create a chirp in the fiber grating, and the chirp rate can be controlled by adjusting the temperature difference between the ends of the grating. The maximum chirp rate that can be imposed on the grating is limited, however, by the material properties of the fiber. The maximum temperature at which the gratings in typical fibers are thermally stable is on the order of about 200° to 500° C. At these temperatures, the grating strength can decay. See T.
Erdogan, V. Mizrahi, P. J. Lemaire, and D. Monroe, J. APPL. PHYS. Vol. 76, No. 1 (1994), at pp. 73-80. Also, moderate temperatures should be used to ensure that the grating devices remain thermally stable over long periods of time. These limitations restrict the range of chirp that may be obtained with a temperature gradient.
In the past, extrinsic chirp nevertheless has been used to obtain chirped gratings, using unchirped gratings. This is because unchirped gratings have been much easier to fabricate than intrinsically-chirped gratings. An intrinsically-chirped grating (or “intrinsic chirp”) refers to a grating in which the chirp has been incorporated into the fiber during the fabrication process. For example, an intrinsic chirp may be achieved by using a prescribed phase mask in which the period of the phase mask varies in some manner. When radiation is applied to the fiber through the phase mask, the resulting fiber will be inherently chirped. Using this technique, one may obtain broadband gratings that can compensate for dispersion slope in a multiple channel system. However, the intrinsic chirp prescribes a fixed amount of dispersion and a specified reflection spectrum. While such gratings may be valuable in communication systems where a specific amount of dispersion compensation is required, the dispersion and amplitude response of the grating is essentially fixed, and thus, the intrinsically-chirped gratings are not well suited to situations in which dynamically adjustable devices are required.
Those concerned with technologies involving optical communications systems continue to search for new grating designs such as broadband gratings that are well suited for dynamically adjustable devices. This invention discloses a method for making such a grating device and optical communication systems comprising such a device.
SUMMARY OF THE INVENTION
Summarily described, the invention embraces a method for making a chirped fiber grating device comprising providing a length of waveguide having an intrinsically-chirped grating region and applying an external gradient or force to the waveguide to alter the range of the chirping. The intrinsically chirped grating region may be linearly or non-linearly chirped. The external gradient is not confined to a particular type of external perturbation and may comprise a temperature gradient, a strain gradient, or other external perturbation, including, for example, a gradient induced by magnetic or mechanical forces. The grating device optionally may comprise a tunable dispersion compensator device comprising a length of waveguide having a chirped grating region, a body attached to the waveguide proximal the grating region, and a component for inducing elastic strain in the body. In one embodiment, the component for inducing elastic strain may comprise magnets disposed alongside the fiber. Many embodiments and applications for the device are contemplated, including dispersion compensator modules, amplifiers, and WDM systems.
REFERENCES:
patent: 5982963 (1999-11-01), Feng et al.
patent: 5999546 (1999-12-01), Espindola et al.
patent: 5999671 (1999-12-01), Jin et al.
patent: 6031950 (2000-02-01), Fujita
patent: 6055348 (2000-04-01), Jin et al.
Adams Laura Ellen
Eggleton Benjamin John
Espindola Rolando Patricio
Jin Sung-ho
Mavoori Hareesh
Connelly-Cushua Michelle R.
Lee John D.
Lucent Technologies - Inc.
Mathews, Collins, Sheperd & Gould, P.A.
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