Optical waveguides – With optical coupler – Particular coupling structure
Reexamination Certificate
2002-01-03
2003-12-02
Lee, John D. (Department: 2874)
Optical waveguides
With optical coupler
Particular coupling structure
C385S014000, C385S028000, C385S051000, C385S096000, C385S099000
Reexamination Certificate
active
06658182
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to tapered optical fiber components, such as fused and tapered fiber couplers and tapered fiber filters, which are mounted on a rigid substrate which forms part of a packaging design that provides a temperature compensating effect and leads to athermal behaviour of the optical properties of the component. The invention also includes a method of securing the component to the rigid substrate and adjusting or controlling it so as to achieve the desired athermal behaviour.
2. Description of the Prior Art
It is known to provide fiber optic components, such as fiber couplers, with packaging such that the thermal expansion of the substrate used in the packaging matches that of the coupler. For example, in U.S. Pat. No. 5,243,680 a stiffener is provided in the package which has an equivalent thermal expansivity to that of the coupler.
U.S. Pat. No. 5,367,589 provides an optical fiber package where the sleeve is made of a material with a coefficient of thermal expansion that is about the same as that of the fiber material. In this patent it is explained that if the materials selected for the package and the fiber have identical coefficients of thermal expansion, then the materials will contract or expand to the same degree in response to a change in temperature. Thus, if the package and fiber exhibit the same such response to a change in temperature, the fiber will not be subjected to strains it would otherwise experience if the package and fiber expanded or contracted to different degrees in response to a change in temperature.
Also, in U.S. Pat. No. 5,430,821 there is provided a protective case for a fused optical fiber coupler where the coefficient of thermal expansion of the fused region of the optical fiber coupler is equivalent to that of the case, thereby reducing the stress occurrence due to the change in temperature.
It is also known to produce a fiber grating package to stabilize Bragg filters by introducing in an optical fiber containing embedded gratings a strain that compensates for temperature-induced changes in the wavelength reflected by the grating elements. One such device is disclosed in U.S. Pat. No. 5,841,920 where a tension adjusting member and a compensating member are provided and are formed of materials each selected so that as the temperature of the device decreases, the tension adjusting member contracts more than the compensating member and thus imposes an axial strain on the grating.
SUMMARY OF THE INVENTION
It has been surprisingly found that the basic principle of compensating the temperature dependent optical effect as used for Bragg gratings is also applicable to tapered optic devices, such as fused and tapered fiber couplers or tapered fiber filters, despite the fact that the problem in such devices is quite different from Bragg where the compensation is fiber dependent and requires compression rather than extension of the substrate as the temperature increases. Thus, the compensation of tapered devices, which mostly depends on their taper profile, represents a different problem to which this invention provides a solution.
As is known, fused and tapered fiber couplers are made by fusing two or more single mode optical fibers and fiber filters are made by heating and tapering single mode optical fibers until desired filtering properties are obtained. The tapering creates a region of smaller cross-section where the fiber cores cease to guide light, thereby creating cladding modes. Such cladding modes propagate in the tapered region, each mode having its own propagation constant; thus, they accumulate phase differences. The relative phase difference between the modes will determine if the light will interfere constructively or destructively in the output cores. This interference at the end of the tapered region will create either a phase dependent power transfer between optical fibers in couplers or filtering effects in single-fiber tapered filters. The principle behind couplers and filters is thus interferometric in nature and the phase difference and interference depends on the optical waveguide created in the tapered region. The phase difference generally increases approximately linearly as a function of wavelength, thus creating a sine-like spectral response of fiber couplers and filters. Because these components are fragile, the tapered fibers are usually bonded to a rigid substrate and enclosed in a tube or otherwise packaged for protection. It is the package that permits the tapered component to hold its properties over time. Though the spectral response of the tapered component can be ideally formed and packaged, it is not inherently stable with temperature, because the refractive index of silica depends on the temperature, and the interferometer phase will change as a function of temperature thus creating a shift in the wavelength response of the component The change in refractive index is small and may be of no consequence for components with very small phase difference, but can be very significant for components that have a sharper wavelength response because their modal phase difference is large.
According to the present invention, such optical change in a tapered component is compensated by a mechanical effect using a rigid substrate within a specially designed package that changes the length of the tapered component. This change of length creates an additional phase accumulation that opposes the phase change due to the change of index of refraction. This method can be applied not just to compensate the temperature change, but also to control it. It can thus enable the realization of components of predetermined positive or negative dependence in temperature shifts.
In order to achieve the control of the temperature dependence of the tapered components, this invention uses several particular properties of tapered structures and combines them to realize the desired effects.
Thus, in a longitudinally invariant waveguide, the phase difference between two modes is given by the equation
&phgr;=&Dgr;&bgr;L
where &phgr; is the phase difference, &Dgr;&bgr;=&bgr;
1
-&bgr;
2
represents the difference between the propagation constants of the modes &bgr;
1
and &bgr;
2
and L is the length of the tapered section of the waveguide.
Fiber couplers and fiber tapers are waveguides formed by an optical fiber core, an optical fiber cladding and a surrounding optical medium, which is often air.
In these waveguides, &Dgr;&bgr; depends on the waveguide cross-section, on the indices of refraction and on wavelength. For a given cross-section, &Dgr;&bgr; increases with wavelength. At a given wavelength, &Dgr;&bgr; increases exponentially with the relative dimension of the cross-section. The smaller the taper wavelength, the larger the &Dgr;&bgr;.
A tapered profile is generally not perfectly uniform since the cross-section changes along the length of the tapered section. Thus, the total accumulated phase difference between the modes is the integral of all the local differences in propagation constants of the modes along the length of the taper sections as shown in the following equation:
&phgr;=∫
L
O
&Dgr;&bgr;dz
If the length L is changed, the phase will change. This is easily verified experimentally by pulling on the tapered component, thus changing its length. The cycles that are observed during fabrication are due to the increase in phase because the component is pulled. After fabrication, the tapered component is flexible and can be mechanically elongated like a spring. One may not change the phase a lot before it brakes, but one will always observe a shift of the oscillation towards the lower wavelengths, corresponding to an increase of the phase.
On the other hand, if the coupler is only heated, a shift toward the higher wavelength is observed, showing that the decrease in the phase due to the decrease in the effective indices of the modes has a larger effect on the phase than the small increase in length due to the thermal expansion of silica.
If the tapere
Doan Jennifer
ITF Technologies Optiques Inc.
Lee John D.
Primak George J.
LandOfFree
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