Optical waveguides – With optical coupler – Particular coupling structure
Reexamination Certificate
2002-09-03
2004-12-28
Lee, Diane I. (Department: 2876)
Optical waveguides
With optical coupler
Particular coupling structure
C385S042000, C385S014000
Reexamination Certificate
active
06836599
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to all-fiber Mach-Zehnder interferometers and a method of making the same. More particularly, the invention relates to single-mode, fused optical fiber couplers that are concatenated to produce interferometric paths between the couplers, thereby resulting in a Mach-Zehnder structure. The invention also includes mechanical stabilization of such structure and its packaging.
BACKGROUND OF THE INVENTION
All-fiber Mach-Zehnder interferometers generally comprise two fiber optic couplers fused in series on two parallel single-mode optical fibers. This basic structure is well described in the literature and in patents, such as U.S. Pat. Nos. 5,044,715 and 5,119,453. The all-fiber Mach-Zehnder appellation derives from the analogy between the classical two-path Mach-Zehnder interferometer and the fiber structure. A classical Mach-Zehnder interferometer is composed of two beam splitters and mirrors. A first beam splitter is used to split an input collimated light beam into two beams of equal intensity. These two beams are then redirected with mirrors at 45° incidence on opposite sides of the second beam splitter, so that the reflexion of any one of the two beams will exactly coincide with the transmitted portion of the other beam. Because of this coincidence, the two beams interfere constructively or destructively at the output beam splitter, depending on the phase difference between the amplitude of the light in the two beams. This phase difference depends on the optical path length difference in the two beams, and the Mach-Zehnder output power in one beam is a sine square function of the phase difference and a complementary cosine square for the second output beam. If the beam splitters are exactly 50%, then the power transfer from one beam to the other is 100%. This is called the Mach-Zehnder effect.
In the all-fiber version, fused couplers are equivalent to beam splitters. The two paths correspond to the optical fibers between the couplers and the two output ports of the second coupler correspond to the two output beams. The fused couplers are made by laterally fusing two single-mode optical fibers together by applying a heat source on two optical fibers which are longitudinally in contact. The heated structure is then pulled, creating a bi-tapered structure. In this tapering, the light escapes the single-mode core, which becomes too small to guide the light and excites a superposition of two cladding modes, one symmetric and the other asymmetric. These modes have different propagation constants and will accumulate a phase difference. Because of the transverse modal intensity profile, the total transverse optical field, resulting from the interference between the two modes, will show a concentration of power varying with the phase difference between the two modes that will shift from one side of the fused fiber structure to the other. At the output taper of the coupler, this power distribution will result in the coupling of the output power in one fiber or the other, thus creating an exchange of power between the two output fibers. The ratio of power transferred is called the coupling ratio and because of the symmetry of the coupler, this ratio can vary from 0 to 100%, 50% or 3 dB being the case where the power split between the fibers is equal. In a Mach-Zehnder structure this is used to split light between two fibers, and then recombine and interfere the light at the output. This interference will depend on the optical path difference in the lengths of the two fibers between the two couplers.
U.S. Pat. No. 5,119,453 describes a Mach-Zehnder interferometer with small path differences. However, Mach-Zehnder interferometers with large path differences can also be made and have applications such as multiplexing and demultiplexing wavelengths onto and from a single-mode fiber. In these applications, the Mach-Zehnder structure transmission varies sinusoidally with wavelength, with a maximum amplitude. Depending on the wavelength, the power can output completely in one or the other of the output fibers.
The principle of an all-fiber Mach-Zehnder interferometer is fairly straightforward for one versed in the art, but as an interferometer, the Mach-Zehnder is very sensitive to any perturbation in its structure, such as mechanical changes or temperature changes. Furthermore, the greater the path difference, the smaller the wavelength period, thus making the selectivity of the wavelength response sharper, but at the same time the structure becomes more sensitive to outside environmental influences. The latter will cause the Mach-Zehnder sinusoidal wavelength response to shift, rendering the structure very good for sensing applications, but unusable in reliable wavelength multiplexing and demultiplexing applications. The difficulty in creating a satisfactory Mach-Zehnder structure thus resides both in the fabrication and in the packaging of the structure.
In dense wavelength multiplexing or demultiplexing applications, great care must be taken to achieve wavelength accuracy in the spectral response of the Mach-Zehnder with the maximum contrast. The couplers must be accurately fabricated to obtain a 50% coupling ratio at the middle wavelength in the wavelength range of operation, to insure the largest contrast possible and the fiber length difference between the couplers must be accurately controlled to insure the correct spectral response. Both these characteristics are difficult to achieve simultaneously.
As an example, one can easily fabricate two 3 dB fused couplers with exactly the correct spectral properties, and subsequently by fusing splice the output fibers of the first coupler to the input fibers of the second coupler. In such a case, however, it is extremely difficult to obtain the correct length difference which must be controlled within a micron to achieve the appropriate spectral response. Furthermore, splicing requires fiber length and the longer the fibers between the couplers, the more sensitive the structure is to external environmental factors such as temperature and vibrations. There is thus a need to produce all-fiber Mach-Zehnder interferometers that would obviate these disadvantages.
OBJECTS AND SUMMARY OF THE INVENTION
It is an object of the present invention to fabricate improved all-fiber Mach-Zehnder interferometers that obviate the disadvantages mentioned above.
Another object of the invention is to provide smaller and mechanically more stable Mach-Zehnder structures.
A still further object of the invention is to achieve better control in the fabrication and packaging of all-fiber Mach-Zehnder devices.
Other objects and advantages of the invention will become apparent from the following descriptions thereof.
In essence, the present invention provides an all-fiber Mach-Zehnder interferometer that comprises two fiber optic couplers made in series by fusing and tapering two parallel single-mode fibers and a central fiber structure between the two fiber optic couplers, said central fiber structure having two fibers of a shape that produces a predetermined path difference required for achieving a desired Mach-Zehnder effect, and is characterized in that the fibers of said central fiber structure are bonded in predetermined spots so as to stabilize the central fiber structure within the interferometer.
Thus, the invention involves creating a fiber structure by bonding two single-mode optical fibers in a given shape adapted to produce a predetermined optical path difference, and thereafter making two fused fiber optic couplers on the input and output fibers of such central fiber structure, so as to produce an all-fiber Mach-Zehnder interferometer. The two single-mode optical fibers may be bonded to each other or to other supporting fibers or to a substrate in order to retain the given shape of the central fiber structure unmodified, while the couplers are made and thereafter while the interferometer is packaged. Also, the central fiber structure is preferably so created that the input and output fibers coming out of the structu
ITF Technologies Optiques Inc.
Lee Diane I.
Primak George J.
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