Optical: systems and elements – Polarization without modulation – By relatively adjustable superimposed or in series polarizers
Reexamination Certificate
2001-11-29
2003-09-23
Robinson, Mark A. (Department: 2872)
Optical: systems and elements
Polarization without modulation
By relatively adjustable superimposed or in series polarizers
C359S490020, C359S199200, C359S199200, C359S577000
Reexamination Certificate
active
06624939
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to fiber optic communication system, and in particular to an optical de-interleaver for de-multiplexing optical signals.
BACKGROUND OF THE INVENTION
Fiber optic communications are becoming increasingly popular for data transmission due to their high speed and high data capacity capabilities. In order to reduce cost and the amount of time required to provide the increased capacity, wavelength division multiplexing (WDM) and dense wavelength division multiplexing (DWDM) have been developed, which can provide increased capacity without requiring new fiber optic cables.
Another approach to increasing fiber optic capacity is to use more closely spaced channels. An interleaver is essentially an optical router that allows existing DWDM filters designed for operation at wide channel spacing to be extended to system designs with narrow channel spacing, in the range of 50 GHz or even less. An interleaver combines two sets of channels into one densely packed set with half the channel spacing. Inversely an optical de-interleaver routes the single input set of channels into two output streams with double channel spacing. The general principal behind an interleaver/de-interleaver is an interferometric overlap of two light beams. The interference creates a periodic, repeating output as different integral multiples of wavelengths pass through the device. The desired channel spacings of a device are set by controlling the fringe pattern. Methods using fused-fiber Mach-Zehnder interferometer, Michelson interferometer, liquid crystals, birefringent crystals, Gires-Tournois interferometer (GTI) and other approaches are developed to build interleavers/de-interleavers.
U.S. Pat. No. 6,169,626 discloses an (100/200 GHz or 50/100 GHz) interleaver/de-interleaver that includes an unequal path Michelson interferometer to provide a linear phase response and a second non-linear interferometer (a Fabry-Perot Phase Shifter) to provide a non-linear phase response with slight attenuation. U.S. Pat. No. 6,169,604 discloses an optical de-interleaver that includes two non-linear interferometers (NLI). Each of the non-linear interferometers is a Gires-Tournois Interferometer (GTI) with an internal &lgr;/4 wave-plate and an external &lgr;/8 wave-plate.
For DWDM applications, there is a need to cascade two or more de-interleavers with different channel spacings. For example, cascading one 50/100 GHz de-interleaver and two 100/200 GHz de-interleavers to create a 50/200 GHz de-interleaver. The market of DWDM demands that a de-interleaver generally has output channel spacings that are 4 times, even 8 times of the input channel spacing.
U.S. Pat. No. 6,169,626 also discloses a 50/200 GHz de-interleaver which is cascaded by one 50/100 GHz de-interleaver and two 100/200 GHz de-interleavers. Optical fibers are used in this system to connect between the 50/100 GHz de-interleaver and two 100/200 GHz de-interleavers. However, simply cascading several de-interleavers together leads to substantially larger system size, high insertion loss and more optical components. Therefore there is a need to create an integral device as a de-interleaver having output channel spacings that are 4 times, even 8 times of the input channel spacing.
A Gires-Tournois Interferometer (GTI) based interleaver/de-interleaver has following advantages:
very low insertion loss;
uniform response over a wide wavelength range (flat-top spectrum); and
minimal polarization dependence effect.
However, relatively larger chromatic dispersion becomes the major disadvantage of a Gires-Tournois Interferometer (GTI) based interleaver/de-interleaver.
For a 100/200 GHz optical de-interleaver, the input channel spacing is 100 GHz. The typical insertion loss is about 1 dB and the maximum insertion loss is about 1.5 dB. The 0.5 dB passband width and −25 dB (isolation) stopband width are not less than 27.5 GHz. The absolute value of the chromatic dispersion is not larger than 30 ps
m. The specifications of a 200/100 GHz interleaver are the same as those of a de-interlesver, except for the isolation. For an interleaver, −15 dB isolation is sufficient.
When the fiber optic communications require higher data capacity, an optic fiber needs to transmit more channels in the same bandwidth, this means that the channel spacing of an interleaver/de-interleaver is getting narrower, i.e. 50/100 GHz, even 25/50 GHz. In order to retain high speed data transmission of 10 Gbit/sec, the required specifications of an interleaver/de-interleavers with the narrower channel spacing are almost the same as that of a an interleaver/de-interleavers with wider channel spacing. The fact is that when the channel spacing of an interleaver/de-interleavers is reduced to half, the passband and stopband widths are also reduced to half, and the chromatic dispersion values increase to 4 times. Therefore, there exists a need for an approach that allows a de-interleaver with narrower channel spacing to have a wider stopband width and a smaller dispersion value.
In co-pending U.S. patent application Ser. No. 09/929,875, entitled “Gires-Tournois Interferometer with Faraday Rotator for Optical Signal Interleaver”, the inventor of the present application discloses a Gires-Tournois Interferometer (GTI) with Faraday rotator (GTIFR) for use in an interleaver or a de-interleaver. A dispersion compensated GTIFR interleaver/de-interleaver is also disclosed in this co-pending patent application, which includes a second GTI for providing chromatic dispersion compensation. The absolute value of dispersion of the compensated device can reach as lower as only about 12% of that of an un-compensated device.
In another co-pending U.S. Patent Application entitled “De-Interleaver with high Isolation and Dispersion Compensation, and 50/200 GHz Interleaver and De-Interleaver”, the inventor of the present application discloses a polarization interferometer based 50/100 GHz de-interleaver with high isolation (the −25 dB stopband width is about 30 GHz) and dispersion compensation (the absolute value of dispersion is not larger than 24 ps
m) Three polarization interferometers are used in this 50/100 GHz de-interleaver with high isolation and dispersion compensation. The inventor of the present application also discloses an integral 50/200 GHz de-interleaver with dispersion compensation in this co-pending patent application. In this 50/200 GHz de-interleaver, three polarization interferometers are used and the (−25 dB isolation) stopband widths for the 50 GHz adjacent channels are 18.8 GHz.
In view of the above, it would be an advance in the art to provide a S/2S GHz de-interleaver with high isolation and dispersion compensation, which includes only one polarization interferometer. It would be an especially welcome advance to provide an integral de-interleaver, e.g. a S/4S GHz de-interleaver, with less optical components, lower cost, higher isolation (wider stopband) and lower dispersion for DWDM applications. Here S represents the channel spacing of an input beam, e.g. S=50 GHz.
OBJECTS AND ADVANTAGES
It is a primary object of the present invention to provide a S/2S optical de-interleaver, e.g. an 50/100 GHz optical de-interleaver, with high isolation and dispersion compensation, which has only one Polarization Interferometer.
It is a further object of the present invention to provide a S/4S optical de-interleaver, e.g. a 50/200 GHz de-interleaver, with less optical components, lower cost, higher isolation (wider stopband) and lower dispersion.
These and numerous other objects and advantages of the present invention will become apparent upon reading the detailed description.
SUMMARY
In accordance with the present invention, an optical de-interleaver for de-interleaving an input beam of odd and even channel signals having channel spacing S into a first output beam of odd channel signals having channel spacing
2
S and a second output beam of even channel signals having channel spacing
2
S is provided which has only one polarization interf
Fineman Lee
Lumen Intellectual Property Services
Oplink Communications Inc.
Robinson Mark A.
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