Optical waveguides – With optical coupler – Input/output coupler
Reexamination Certificate
2001-12-06
2003-10-21
Healy, Brian (Department: 2874)
Optical waveguides
With optical coupler
Input/output coupler
C385S002000, C385S013000, C501S004000, C501S007000
Reexamination Certificate
active
06636667
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to fiber optic communication devices, and in particular to a tunable optical fiber grating package with low temperature dependency.
BACKGROUND OF THE INVENTION
Optical gratings are becoming more and more important for selectively controlling specific wavelengths of light transmitted within optical communication systems. Various fiber Bragg gratings (FBG) are a particularly advantageous group of gratings for manipulating optical signals based on their wavelengths. A fiber Bragg grating is generally fabricated by exposing a photosensitive fiber to UV light thereby creating a permanent refractive-index grating along the core of a fiber.
It is familiar to those skilled in the art that the Bragg wavelength &lgr; of a fiber Bragg grating is related to the period (&Lgr;) of the fiber Bragg grating and the effective index of refraction (n
eff
) as shown in equation (1):
&lgr;=2
n
eff
&Lgr;(1) (1)
The effective index of refraction n
eff
of the fiber varies significantly over an expected working temperature range of, e.g. −20° C. to 80° C., mainly due to the thermal expansion of the fiber material-itself and the temperature dependency of the fiber's effective index of refraction n
eff
. Over this expected working temperature range, the grating wavelength shifting of an uncompensated 1550 nm grating can exceed 1 nm which is not acceptable to an optical communication system.
In equation (1), both the period &Lgr; of the fiber Bragg grating and the effective index of refraction n
eff
vary with temperature. The wavelength &lgr; can be changed by changing the effective index of refraction n
eff
or changing the period &Lgr; of the fiber Bragg grating. This principle has been used to develop various devices to compensate wavelength shifting of fiber grating mainly through adjusting the strain applied in the optical fiber.
From equation (1), following equation (2) can be qualitatively derived:
d&lgr;/&lgr;=dn
eff
eff
+d&Lgr;/&Lgr;
(2)
According to equation (2), it is apparent to those skilled in the art that the wavelength &lgr; of a fiber Bragg grating can be changed by changing the effective index of refraction n
eff
and/or by changing the period &Lgr;. Up to now, the method by changing the effective index of refraction n
eff
is relatively expensive and difficult to achieve. Thus a preferred method is to vary the period &Lgr; so as to tune the wavelength of a fiber grating. The period of a fiber Bragg grating is generally varied by stretching, compressing or deforming the fiber. If the effective index of refraction n
eff
is kept constant, the changing of the period of the fiber Bragg grating is proportional to the changing of length of the fiber portion in which the fiber Bragg Grating is written. Unfortunately, the effective index of refraction n
eff
in equation (2) is not constant over an expected temperature range. Therefore after tuning a fiber Bragg grating to an expected wavelength by changing the period, this wavelength is subject to shifting and needs to be compensated against temperature variation.
U.S. Pat. No. 5,042,898 discloses a temperature compensated fiber Bragg device having a fiber portion with two ends. Each end of the fiber portion is attached to a different one of two compensating members made of materials with different positive Coefficients of Thermal Expansion (CTE) relative to one another such that the longitudinal strain applied to the fiber varies with temperature in such a manner that the changes in the central wavelength that are attributable to the changes in the longitudinal strain substantially compensate for those attributable to the changes in the temperature of the grating. This device is relatively complicated and is not suitable for temperature compensation of a tunable device with more than one central wavelength.
U.S. Pat. No. 5,841,920 discloses a similar temperature compensating optical wave-guide device having two supporting components with two different positive Coefficients of Thermal Expansion (CTE). This prior art reference teaches that a single compensating member is not practical since a material possessing the required expansion coefficient properties is not commonly known or readily available (column 1, lines 62-65). This teaching is generally not consistent with the present technical possibilities in the art.
The devices disclosed in U.S. Pat. Nos. 6,144,789 and 6,181,851 also use two supporting components with two different positive Coefficients of Thermal Expansion (CTE). These devices are also not suitable for temperature compensation of a tunable package with more than one central wavelength.
U.S. Pat. No. 6,233,382 teaches a thermal compensating package for an optical fiber Bragg grating having a supporting member which is formed of a composite structure having a first material with a first negative CTE in one direction and a second material with a second CTE in another directions which is different from that of the first CTE. The composite structure is formed of two types (e.g. polymer fibers and carbon fibers) of fibers embedded in an epoxy resin. This device is generally not suitable for temperature compensation of a tunable fiber package. This prior art reference also teaches that it is particularly difficult to provide one single negative CTE material that precisely compensates for temperature variations of a fiber package (column 2, lines 6-9). This teaching is also generally not consistent with the present technical possibilities in the art.
U.S. Pat. No. 6,240,220 teaches a tunable optical fiber grating package which can cause a change in the center wavelength of a fiber Bragg grating. The package provides a controlled and predetermined change in wavelength response by subjecting a fiber portion having a fiber Bragg grating written therein to a controlled strain. The strain in the fiber is induced by varying a longitudinal displacement of a support member which supports the fiber. A PZT actuator is used to linearly displace the fiber support member. Unfortunately, this tunable optical fiber grating package itself has a relatively high temperature dependency due to following reasons:
The support member of the package is made of brass, stainless steel, aluminum, Cu/Be alloy or the like. These materials have relatively high positive CTE and cause the longitudinal displacement of the support member to vary with temperature; and
The supporting member of the package includes a magnification structure with a mechanical advantage between 5 and 15. The variation of the longitudinal displacement of the support member caused by the CTE of the PZT actuator itself is also magnified to an unacceptable amount, e.g. 5 times to 15 times larger than the thermal dimensional variation of the PZT actuator itself.
In view of the above, it would be an advance in the art to provide a tunable optical fiber grating package with low temperature dependency.
OBJECTS AND ADVANTAGES
It is a primary object of the present invention to provide a tunable optical fiber grating package with low temperature dependency having a first supporting member being formed of a material having ultra low CTE and a second supporting member being formed of a material having a negative CTE. The optical characteristic that varies with strain of the fiber portion, e.g. the wavelength of a Bragg grating, is tunable by changing the dimension of the first supporting member along the longitudinal direction of the fiber by a tuning means. The effect of temperature fluctuation on the fiber grating can be substantially compensated by the second supporting member.
It is another object of the present invention to provide a tunable optical fiber grating package with low temperature dependency having an optical fiber portion having at least one grating between a first end and a second end of the fiber portion. The first end and the second end of the fiber portion are fixed on the first supporting member and the second supporting member respectively.
It is yet another o
Wang Qinglin
Wu Weiti
Healy Brian
Lin Tina M
Lumen Intellectual Property Services
Oplink Communications Inc.
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