Optical waveguides – With optical coupler – Particular coupling structure
Reexamination Certificate
1999-03-31
2002-12-24
Font, Frank G. (Department: 2877)
Optical waveguides
With optical coupler
Particular coupling structure
C385S094000, C385S039000
Reexamination Certificate
active
06498879
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to fused-fiber wavelength division multiplexers (WDM) and, in particular, to a multi-window dense WDM structure.
BACKGROUND OF THE INVENTION
With existing fiber optic networks, there is often the need to increase information transmission capacity. However, both physical and economic constraints can limit the feasibility of increasing transmission capacity. For example, installing additional fiber optic cable to support additional signal channels can be cost prohibitive, and electronic system components may impose physical limitations on the speed of information that can be transmitted. The use of wavelength division multiplexers (WDMs) provides a simple and economical way to increase the transmission capacity of fiber optic communication systems by allowing multiple wavelengths to be transmitted and received over a single optical fiber through signal wavelength multiplexing and demultiplexing. In addition, WDMs can be used in fiber optic communication systems for other purposes, such as dispersion compensation, noise reduction, and gain flatting, i.e., maintaining a uniform gain within the usable bandwidth for erbium doped amplifiers.
WDMs can be manufactured using, for example, biconical tapered fusion (BTF) technology. Typically, two optical fibers are fused together along an interior portion to form a fused-fiber coupler, so that light of two wavelengths (i.e., 1310 nm and 1550 nm) entering the input terminals of the first and second fibers, respectively, are multiplexed onto a single fiber. The coupling ratios for the two channels (the signals at 1310 nm and 1550 nm) exhibit complementary sinusoidal behavior for amplitude as a function of frequency within the passband of the WDM, with each channel having one or more peaks (or windows) within the passband. Information carried by the two signals along the single fiber is then demultiplexed at the WDM outputs. Light at 1550 nm is particularly desirable because minimal absorption is exhibited by optical fibers around this wavelength. Commercially available fused-fiber WDMs typically also couple and decouple light at 1550 nm and 980 nm and at 1550 nm and 1480 nm.
The principles of WDM can be extended to further increase data transmission capability by coupling additional discrete wavelengths or channels onto a single fiber using devices known as dense WDMs (DWDMs). DWDM is a one-to-N device, as shown in FIG.
1
. Fused-fiber DWDM
10
may couple N discrete communication channels &lgr;
1
through &lgr;
N
onto a single optic fiber &lgr;. For example, 8, 16, or even 32 discrete communication channels may be coupled onto a single optic fiber. However, because the usable bandwidth of the light is limited, increasing the number of wavelengths necessarily results in smaller channel separation between the discrete wavelengths. In general, smaller channel spacing can be achieved by increasing the length of the fused portion of a fused-fiber DWDM. However, decreasing channel spacing presents different types of problems, such as increased sensitivity to temperature fluctuations.
A DWDM may comprise several or a plurality of multi-window WDMs (MWDMs). An MWDM is a one-to-two device, as shown in FIG.
2
. Light with wavelength &lgr; enters MWDM
20
which decouples wavelength &lgr; into two groups, one consisting wavelengths &lgr;
1
to &lgr;
N−1
and one group consisting wavelengths &lgr;
2
to &lgr;
N
, where N is an even number.
MWDMs have two or more peaks of amplitude as a function of frequency (or operational windows) for each channel within a passband. MWDMs can also be made using BTF technology by putting two optical fibers in parallel, fusing the center portion together, and pulling the fibers until a desired multi-window transmission spectrum appears at a monitored fiber output terminal.
Fused-fiber couplers generally exhibit polarization-dependent loss (PDL). This PDL is induced by the difference of two polarization-dependent coupling coefficients in the tapered regions of the coupler, where two optical fibers are fused together and elongated for optical power coupling. The cross-sectional shape of the tapered section is elliptical or dumbbell-shaped which produces birefringence along the tapered section.
FIG. 3
illustrates spectral transmitted ratios for two polarizations, e.g., x-polarization and y-polarization. Curve
30
, representing, e.g., x-polarization, is shifted from curve
31
, representing, e.g., y-polarization, in terms of wavelength. The difference of transmitted power for the two modes at the same wavelength &lgr;
1
is defined as the PDL between x-polarization curve
30
and y-polarization curve
31
. Curve
30
is shifted from curve
31
because the two principle polarizations x and y have different polarization coefficients. Hence, a light propagating along the x-polarization sees a different geometrical structure than a light propagating along the y-polarization, thus, a difference in the coupling strength. PDL is undesirable because it limits the system performance, e.g., less passbands and larger signal fluctuation.
One way to reduce the polarization sensitivity of BTF MWDM is to use longer pull length to reach optimum phase match condition between the propagation mode of light. Such a long-tapered-fusing technology is discussed in commonly-owned U.S. Pat. No. 5,809,190, entitled “Apparatus and Method of Making a Fused Dense Wavelength-Division Multiplexer”, which is incorporated by reference herein in its entirety. However, methods of quenching the polarization sensitivity generally lead to an increased temperature sensitivity. In addition, this method requires longer pull length, which reduces the diameter of the tapered section and increases the chance of breakage.
Another method, a paper by I. J. Wilkinson and C. J. Rowe entitled “Close-Spaced Fused Fibre Wavelength Division Multiplexers with Very Low Polarisation Sensitivity” (the Wilkinson paper), Electronics Letters, vol. 26, No. 6, pp. 382-384, Mar. 15, 1990, describes how the polarization sensitivity (birefringence) of a wavelength multiplexing fused fiber 2×2 coupler can be substantially nulled-out by elasticity twisting the coupler after its fabrication. However, this paper fails to provide precise wavelength spacing control during the device fabrication, which is critical to a MWDM. The precise control of channel spacing is critical because of the cumulative effect of channel spacing offset across the operating window. For example, if the first wavelength in an operating window, having n wavelengths, is set at a desired value precisely, an inaccuracy of d&lgr; in channel spacing will accumulate across the entire operating window, resulting in a significant offset of (n−1)×d&lgr; for the last wavelength in the operating window. In addition, this paper also fails to provide peak position control during the device fabrication.
U.S. Pat. No. 5,408,555 (the '555 patent) entitled “Polarization Insensitive Wavelength Multiplexing 2×2 Fibre Couplers,” by Fielding et al., added an additional step of requiring continuous monitoring of the twisting process to enable termination of the manufacturing process at a particular moment, i.e., providing a relatively high level of precision in the spectral positioning of the minimum and maximum power transfer wavelengths for one of the principal planes of polarization of the coupler. However, the method described in the '555 patent is complicated and time consuming because a polarized light source and a polarization controller are required. In addition, this method requires numerous torching steps during the polarization adjustment process, thereby increasing the insertion loss of the device. Furthermore, due to the use of a single-wavelength light source, the actual channel spacing and the polarization states across the entire desired wavelength range of the MWDM are not measurable. Therefore, this method is only suitable for fabricating WDMs, not MWDMs.
A problem associated with the long-tapered couplers is that when the
Chon Joseph C.
Huang Chi-hung
Huang Robert I.
Font Frank G.
Hsue James S.
Lauchman Layla
Skjerven Morrill LLP
Wavesplitter Technologies, Inc.
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