Optical waveguides – Optical fiber waveguide with cladding
Reexamination Certificate
1999-03-11
2001-11-13
Bovernick, Rodney (Department: 2874)
Optical waveguides
Optical fiber waveguide with cladding
C385S124000, C385S014000, C385S042000, C359S341430
Reexamination Certificate
active
06317548
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to optical fibers and optical waveguides, and, more particularly, to precision control of the temperature and/or the optical length of an optical fiber or optical waveguide.
An optical fiber typically includes a core of a first glass, a casing of a second glass overlying the core, and a protective layer overlying the casing. Light introduced into the core is propagated by an internal reflection mechanism along the length of the optical fiber, following the path of the optical fiber with essentially no loss of energy. The light may be propagated over great distances and through complex paths. These same properties may also be obtained using optical waveguides, which are typically etched integrated optics devices formed on a substrate. As used herein, the term “optical fiber” will be understood to encompass both discrete optical fibers and integrated optical waveguides, unless the context indicates the contrary.
In some applications of optical fibers, it is important to determine and control the temperature and/or the optical length of the optical fiber very precisely. (The “optical length” is the product of the physical length of the optical fiber and its index of refraction.) The temperature and optical length of the optical fiber are linked through the effect of temperature on the refractive index of the glass and through the temperature coefficient of expansion of the glass of the optical fiber.
As an example of an application, an interferometer may be made with one or both of the two interfering light paths being optical fibers. The relative optical lengths of the two light paths of the interferometer must be controlled very precisely for many applications. Because the optical length of the optical fiber is a function of the temperature, one approach for controlling the optical length is to heat the optical fiber to increase its optical length and to cool the optical fiber to decrease its optical length. It may also be necessary to dynamically heat or cool the optical fiber to stabilize its optical length against changes in the environment.
Various types of heaters may be used to heat the optical fiber by applying heat, and to cool the optical fiber by reducing the heat input. In one type of heater, a wire is wrapped around the optical fiber. Thin film heaters contacting the optical fiber are also used. A flat-sided fiber may be heated by an applied electric field. These different approaches work to varying degrees, but have disadvantages in respect to response times for temperature changes, difficulty and cost of fabrication and maintenance, insertion loss, and physical length limitations on the optical length of the optical fiber.
There is a need for an improved approach to controlling the temperature and the optical length of an optical fiber. The present invention fulfills this need, and further provides related advantages.
SUMMARY OF THE INVENTION
The present invention provides an optical fiber device and method for its use, wherein the optical fiber device has an optical fiber with a precisely controlled temperature and/or optical length. The optical length of the optical fiber is related to its temperature through the effect of temperature on the refractive index of the glass and/or through the temperature coefficient of expansion of the glass of the optical fiber. The optical length of the optical fiber may thereby be controlled to a fraction of a wavelength of the propagated light energy signal. The response times for heating and cooling temperature changes are sufficiently fast for many applications. The thermal approach for control of the optical length of the optical fiber can achieve changes over time scales on the order of 0.1 to 1 milliseconds, or longer; this response time is established by physical limitations on how fast the fiber temperature may change.
The present invention also provides for faster response times (1 millisecond or less) by exploiting the fact that the index of refraction of the optical fiber and, hence, its optical length depend upon the polarizability of the fiber medium. Specifically, the polarizability depends on the properties of any dopants that might be present in the fiber as well as the properties of the glass that is used to make the fiber. Consequently, the polarizability may be rapidly controlled by changing the population distribution among the energy levels of the dopant ions. It may be necessary to dynamically adjust this population distribution to stabilize the fiber's optical length against changes in the environment.
The physical length of the optical fiber over which the temperature and optical length may be precisely controlled according to this invention is greater than for some other approaches in the art. That is, it is possible to fabricate longer lengths of optical fibers having controllable optical lengths than with prior techniques. Neither the heating and cooling, nor variations in the polarizability, adversely affects the signal transmission performance of the optical fiber in any significant way.
In accordance with the invention, an optical fiber device comprises a first optical fiber having a first-fiber core region transmissive to optical energy of a signal wavelength and absorptive to optical energy of an excitation wavelength different from the signal wavelength. A concentration of one or more dopant ions resides within the core region. The dopant ions are operable to absorb optical energy at the excitation wavelength. The absorbed optical energy is converted to heat, thereby raising the temperature, or modifies the population distribution among the energy levels of the dopant ions, thereby changing the polarizability and index of refraction of the fiber medium. A first source of optical energy of the excitation wavelength is coupled into the first optical fiber in order to excite the dopant ions and thence alter the optical length of the first optical fiber. A second source of optical energy at the signal wavelength is coupled into the first optical fiber as the source of the signal to be transmitted through the optical fiber.
The first source is operated to control either the temperature of the optical fiber, or the optical length of the optical fiber, via the dependence of the index of refraction on temperature or polarizability, or the temperature-dependence of the physical length of the optical fiber. The second source is operated to transmit a signal through the optical fiber. Because the two sources operate at different optical wavelengths and because the dopant ions are responsive to only the excitation wavelength of the first source, the heating and cooling performance is independent of the signal transmission performance.
In an application of interest to the inventors, the first optical fiber is one arm of an optical-fiber interferometer. Preferably, there is a second optical fiber serving as the second arm of the interferometer. The second arm may be doped or undoped. Even if the temperature of the second optical fiber is not to be controlled, it is preferred that the second optical fiber also be doped with the same concentration of the dopant ions so that the two optical fibers have the same optical and thermal expansion properties.
The temperature or the optical length of the optical fiber is controlled by operating the first source of optical energy to excite the dopant ions. (In each case, the optical length of the optical fiber is established, although for some applications the established temperature of the optical fiber is of paramount interest, and the established optical length is incidental.) Heat energy generated as a consequence of this excitation is conducted into the surrounding core material, thereby heating it and changing the index of refraction or the physical length of the optical fiber, while the change in polarizability also changes the index of refraction of the optical fiber. The optical fiber, once excited, may be maintained at the excited state by continuing to operate the first source. The optical fiber may be co
Minden Monica L.
Rockwell David A.
Bovernick Rodney
Gudmestad T.
Hughes Electronics Corporation
Pak Sung
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