Optical waveguides – With optical coupler – Particular coupling structure
Reexamination Certificate
2003-03-17
2004-12-14
Healy, Brian (Department: 2883)
Optical waveguides
With optical coupler
Particular coupling structure
C385S028000, C065S385000
Reexamination Certificate
active
06832030
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to tapered optical fiber filters with a non-sinusoidal wavelength response.
2. Description of the Prior Art
PCT international publication No. WO 00/41016 describes an optical fiber filter which produces a predetermined sinusoidal response in amplitude and wavelength period of the filter. Such filter comprises an essentially adiabatic taper in a single mode fiber, which taper has an elongated central zone with a sloped portion at each end, and on each slope of the adiabatic taper, there is provided a non-adiabatic taper. This leads to the formation of two coupling regions at the extremities of the central zone, which produce a predetermined sinusoidal response in amplitude and wavelength period of the filter.
The above filter works very well for a sinusoidal Mach-Zehnder response. However, more complex, non-sinusoidal responses may also be desirable, because with a purely sinusoidal response, the bandwidth decreases as the period of the filter decreases, leading to a situation where the bandwidth is too narrow and the laser must be extremely well controlled in wavelength and is limited in its modulation frequency.
It is therefore advantageous in some cases to flatten the apex of the bands produced by the optical fiber filter in order to increase the bandwidth of the transmission band and thereby allow satisfactory laser operation and improved isolation.
In U.S. Pat. No. 5,930,441 such more complex non-sinusoidal filtering characteristics are achieved in a split-beam Fourier filter by a special plate distribution positioned between two lenses glued to the ends of respective optical fibers. This is a fairly complex arrangement specifically adapted to a Fourier filter. Such filtering characteristics may also be achieved using all-fiber devices by concatenating several sine filters as disclosed in PCT application PCT/CA00/00250. In accordance with the present invention, ways are provided of designing a complex filter that has the function of flattening the passband of the sinusoidal wavelength response, using a single component with an integrated multi-taper approach.
OBJECTS AND SUMMARY OF THE INVENTION
It is an object of the present invention to provide a non-sinusoidal optical fiber filter.
Another object of the invention is to obtain such filter by a simple and efficient method of tapering an optical fiber.
Yet another object of this invention is to provide an optical filter that has a periodic response that matches a periodic sine response of an all-fiber Mach-Zehnder interferometer that will flatten the optical passband of the Mach-Zehnder response and add isolation.
Other objects and advantages of the invention will become apparent from the following description thereof.
In essence, the optical fiber filter of the present invention comprises an adiabatic taper in a single mode fiber, having an elongated central zone with a sloped portion at each end thereof, thereby forming a slope at each extremity of the central zone, a non adiabatic taper on each slope adapted to produce a predetermined wavelength response in the filter, and a non-adiabatic taper in the middle of the elongated central zone adopted to produce a predetermined mode coupling and phase shift in said response resulting in desired non-sinusoidal filtering characteristics. The non-adiabatic taper in the middle of the central zone is preferably so located that the phase shift on each side thereof is essentially identical. The non-sinusoidal filtering characteristics are controlled by adjusting the widths of the non-adiabatic tapers relative to each other.
The optical fiber filter of the present invention is therefore substantially the same as disclosed in PCT international publication No. WO 00/41016, but with the additional provision of a non-adiabatic taper in the middle of the central zone, which provides the desired phase shift and mode coupling effect, resulting in a predetermined non-sinusoidal wavelength response.
The preferred method of manufacturing such non-sinusoidal filter comprises:
(a) producing an essentially adiabatic taper in a single-mode fiber, said taper having an elongated central zone with a sloped portion at each end thereof;
(b) forming a non-adiabatic taper on one sloped portion to produce a predetermined coupling region in said sloped portion;
(c) forming a non-adiabatic taper in the middle of the central zone to produce a coupling region in said central zone with a desired phase shift relative to the coupling region in the sloped portion produced in (b) above; and (d) forming a non-adiabatic taper on the sloped portion at the other end of the central zone to produce a coupling region which is essentially the same as the coupling region produced in (b) above and with a phase shift from the coupling region in the middle of the central zone being essentially the same as produced in (c) above.
Additional steps may be performed to fine-tune or adjust the widths of the non-adiabatic tapers relative to each other to achieve the final desired non-sinusoidal wavelength response.
As already described in the above mentioned PCT international publication No. WO 00/41016, the adiabatic taper is normally produced by approaching a torch with a flame to a section of the single-node fiber stripped of its protective jacket, and having the flame brush this section over a predetermined length while pulling both ends of the fiber until a desired reduction in diameter (preferably about 50%) is obtained in the central zone.
The non-adiabatic tapers on each slope and in the middle of the central zone are formed by approaching a torch with a small flame to a predetermined spot on each slope and in the middle of the elongated zone, and slowly pulling both ends of the fiber until power has decreased to a desired value, and repeating this procedure for each non-adiabatic taper.
The non-adiabatic tapers are produced so as to achieve a desired power splitting at both ends and in the middle to form a desired non-sinusoidal profile.
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T.J. Brophy et al.; In-Line Singlemode Fibre Interferometer . . . ; Jul. 8, 1993; pp. 1276-1277; Electronic Letters.
D.A. Nolan et al.; Tapered Lattice Filters; Feb. 21, 1999; pp. 85-87; Optical Fiber Communication Conference; San Diego, CA.
Healy Brian
Hughes James P.
ITF Optical Technologies Inc.
Primak George J.
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