Symmetric interferometers unbalanced by using light exposure...

Details Symmetric interferometers unbalanced by using light exposure... Symmetric interferometers unbalanced by using light exposure...

2000-01-10

2002-06-04

Ngo, Hung N. (Department: 2874)

Optical waveguides

With optical coupler

Particular coupling structure

C385S048000, C385S050000, C385S043000, C385S003000

Reexamination Certificate

active

06400870

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to an optical filter and method of making an interferometric optical filter by irradiating one or more substantially similar waveguide arms with light of an intensity, wavelength and duration sufficient to vary the optical path length difference between the two arms to obtain a desired output response.
BACKGROUND OF THE INVENTION
Mach-Zehnder Interferometers (MZIs) are well-known devices in planar waveguide technology and in optical fibers. An MZI is comprised of two couplers having a phase shifting region therebetween which consists of two arms,
10
a
and
10
b
shown in
FIG. 1
, having differential propagation constants. Unbalancing the MZI, and more specifically the arms of the MZI produces a sinusoidally varying spectral output behavior as is shown in
FIG. 5
a
useful in a number of filtering or gain shaping applications. In planar waveguides, such as those made in lithium niobate or poled polymer materials, the application of a voltage and use of the electro-optic effect is used to controllably unbalance the MZI. Fiber-optic based MZI's are often unbalanced by using different fiber lengths in each arm, using dissimilar fibers in the two arms such as fibres having different core radii, or by varying the core dimension by using a bi-conical taper in one of the arms. Notwithstanding, these solutions have their limitations; for example, different fiber lengths are undesirable because of packaging difficulties and a large temperature sensitivity. The use of dissimilar fibers have an unwanted polarization and wavelength sensitivity. Furthermore, the use of dissimilar fibres lessens the amount of control afforded in the manufacture process. The use of a fused biconical taper in one of the interferometer arms introduces an unwanted wavelength sensitivity.
In 1989 researchers at the Communications Research Center (CRC) in Ottawa, Ontario, Canada have shown that an unbalanced MZI can be made using dissimilar fiber fused taper couplers. In particular, the optical path lengths of the two arms are different because the two fibers fused together to make the MZI are dissimilar and, therefore, have different propagation constants (B. Malo, F. Bilodeau, K. O. Hill, D. C. Johnson and J. Albert,
Electronics Letters,
Oct. 12, 1989, Vol. 25, pp. 1416-1417). Then, in 1990 the same researchers at CRC used an unbalanced MZI with dissimilar fibers as a measurement tool for the purpose of measuring the ultraviolet light photosensitivity in germanium-doped silica fiber. This was described by B. Malo, K. A. Vineberg, F. Bilodeau, D. C. Johnson and K. O. Hill, in
Optics Letters,
Sep. 1, 1990, Vol. 15, pp. 953-955.
There are a number of limitations of using dissimilar fibers in an MZI. First, a residual polarization dependence and polarization dependent loss can result from the use of dissimilar fibers. Another problem is that the couplers can vary with frequency, limiting the wavelength range over which the device operates accurately. Also, the only tunable parameter during the MZI manufacturing process given two dissimilar fibers is the position of the tapers.
In contrast, using the same fiber in both arms of the MZI reduces the polarization dependence and increases the wavelength range of operation.
As an improvement to the measurement system, the CRC researchers have also used an MZI made from the same fiber. However, an MZI made from identical fibers of equal arm length is not useful for the measurement of small optical phase shifts because it is balanced. In order to create an optical path-length difference or imbalance between the two interferometer arms, they used a novel design in which one of the fibers is pre-tapered in the middle over a 13-mm length, reducing the core-mode effective index in the tapered region (F. Bilodeau, D. C. Johnson, B. Malo, K. A. Vineberg, K. O. Hill, T. F. Morse, A. Kilian and L. Reinhart,
Optics Letters,
Oct. 15, 1990, Vol. 15, pp. 1138-1140). Thus, a symmetric MZI I purposefully tapered in one arm of the MZI to unbalance it. This unbalanced MZI was then used to measure refractive index changes caused by light exposure by irradiating the untapered arm.
In summary, unbalanced MZIs have been made and used for the purpose of measuring the photosensitivity in one of the waveguides. Unbalancing was achieved by using different optical fibres and an another instance by introducing a taper in one of the arms thereby reducing the core-mode effective index in the tapered region.
Hermetically sealed MZI couplers are known to have good environmental stability by inserting two dissimilar fibers in a glass tube. In particular, these have been fabricated by inserting fibers with differing core diameters and index deltas into a glass tube doped with barium oxide. The tube is then heated and collapsed on the fibers. The collapsed tube is then drawn to provide for the two couplers. The magnitude of the coupling can be easily adjusted by changing the taper length as descried by D. A. Nolan, W. J. Miller, R. Irion, Optical Fiber Conference, February 1998, San Jose, Calif.,
OFC'
98
Technical Digest,
pp. 339-340. Fiber-based lattice devices have also been made by cascading different MZI's. These lattice filters are made by using three unequal couplers surrounding two different phase shifting regions (D. A. Nolan, IEEE International Passive Components Workshop, September 1998). In addition, using a series of MZI's can lead to filter synthesis using Fourier expansion for applications in band splitters and combiners and gain equalization (Y. P. Li and C. H. Henry,
Optical Fiber Telecommunications IIIB,
eds. I. P. Kaminow and T. L. Koch. San Diego: Academic Press, 1997, Ch. 8, pp. 345-351).
The great amount of interest in wavelength division multiplexing (WDM) system applications has caused a surge in devices based on fused biconically tapered (FBT) fiber couplers. In general, the technologies used to produce passive components divide them into three main categories: integrated optics, micro-optics and all-fiber devices. FBT couplers are suitable for low-cost, low port count applications. Planar integrated-optic components are ideal for network branching applications requiring 16, 32 or higher port count. However, micro-optic couplers and wavelength splitters are more suitable for high-performance systems (R. Chua,
FiberOptic Product News,
November 1998, pp. 31-34).
In its simplest form, a FBT fiber coupler consists of two optical fibers whose optical cladding has been fused together. The structure is tapered by elongation while it is hot until appropriate coupling properties are achieved. It is then bonded to a substrate and encapsulated into a compact and rugged package. This is shown in FIG.
2
. Unfortunately, FBT's are polarization dependence and consequently exhibit polarization dependent loss (PDL) in a system. As in any two-path interferometer, the optical path difference must be large to yield high spectral selectivity. However, since the fiber coupler is a birefringent structure, longer length leads to greater polarization dependence. The FBT coupler of
FIG. 2
behaves much like the MZI of
FIG. 1
, where the optical path difference between the two arms results in a sinusoidal wavelength response. However, unlike the FBT the MZI has a very small polarization dependence and PDL because the couplers themselves in
FIG. 1
are very short (F. Gonthier,
FiberOptic Product News,
September 1998, pp. 54-56).
Recently UV-induced fiber Bragg gratings have been making a tremendous impact on fiber-optic communications. The gratings an be created directly in the germanium-doped core of optical fibers by holographic interference techniques, using phase or amplitude masks, or by point-by-point writing techniques (c.f. .A. E. White an S. G. Grubb,
Optical Fiber Telecommunications IIIB,
eds. I. P. Kaminow and T. L. Koch. San Diego: Academic Press, 1997, Ch. 7, pp. 273-276). At first, the photosensitivity of ordinary transmission fiber was too weak to write the strong gratings of interest for applications.

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