Optical waveguides – With optical coupler – Plural
Reexamination Certificate
1999-08-26
2001-04-24
Lee, John D. (Department: 2874)
Optical waveguides
With optical coupler
Plural
C385S027000, C385S042000, C359S199200, C356S329000, C356S329000
Reexamination Certificate
active
06222958
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to generally to a multiplexing/demultiplexing circuit, and more particularly, to a circuit that is suitable for interleaving/de-interleaving channels in an optical system.
BACKGROUND OF THE INVENTION
Using optical signals as a means of carrying channeled information at high speeds through an optical path such as an optical waveguide i.e. optical fibres, is preferable over other schemes such as those using microwave links, coaxial cables, and twisted copper wires, since in the former, propagation loss is lower, and optical systems are immune to Electro-Magnetic Interference (EMI), and have higher channel capacities. High-speed optical systems have signaling rates of several mega-bits per second to several tens of giga-bits per second.
Optical communication systems are nearly ubiquitous in communication networks. The expression herein “Optical communication system” relates to any system that uses optical signals at any wavelength to convey information between two points through any optical path. Optical communication systems are described for example, in Gower, Ed. Optical communication Systems, (Prentice Hall, New York) 1993, and by P. E. Green, Jr in “Fiber optic networks” (Prentice Hall New Jersey) 1993, which are incorporated herein by reference.
As communication capacity is further increased to transmit an ever-increasing amount of information on optical fibres, data transmission rates increase and available bandwidth becomes a scarce resource.
High speed data signals are plural signals that are formed by the aggregation (or multiplexing) of several data streams to share a transmission medium for transmitting data to a distant location. Wavelength Division Multiplexing (WDM) is commonly used in optical communications systems as means to more efficiently use available resources. In WDM each high-speed data channel transmits its information at a pre-allocated wavelength on a single optical waveguide. At a receiver end, channels of different wavelengths are generally separated by narrow band filters and then detected or used for further processing. In practice, the number of channels that can be carried by a single optical waveguide in a WDM system is limited by crosstalk, narrow operating bandwidth of optical amplifiers and/or optical fiber non-linearities. Moreover such systems require an accurate band selection, stable tunable lasers or filters, and spectral purity that increase the cost of WDM systems and add to their complexity. This invention relates to a method and system for filtering or separating closely spaced channels in a manner that would otherwise not be suitably filtered by conventional optical filters.
Currently, internationally agreed upon channel spacing for high-speed optical transmission systems, is 100 Ghz, equivalent to 0.8 nm, surpassing, for example 200 Ghz channel spacing equivalent to 1.6 nanometers between adjacent channels. Of course, as the separation in wavelength between adjacent channels decreases, the requirement for more precise demultiplexing circuitry capable of ultra-narrow-band filtering, absent crosstalk, increases. The use of conventional dichroic filters to separate channels spaced by 0.4 nm or less without crosstalk, is not practicable; such filters being difficult if not impossible to manufacture.
In a paper entitled Multifunction optical filter with a Michelson-Gires-Tumois interferometer for wavelength-division-multiplexed network system applications, by Benjamin B. Dingle and Masayuki Izutsu published 1998, by the Optical Society of America, a device hereafter termed the MGTI (Michelson-Gires-Tournois Interferometer) device provides some of the functionality provided by the instant invention. For example, the MGTI device as exemplified in
FIG. 1
a
serves as a narrow band wavelength demultiplexor; this device relies on interfering a reflected E-field with an E-field reflected by a plane mirror
16
. The etalon
10
used has a 99.9% reflective back reflector
12
r
and a front reflector
12
f
having a reflectivity of about 10%; hence an output signal from only the front reflector
12
f
is utilized. A beam splitting prism (BSP)
18
is disposed to receive an incident beam and to direct the incident beam to the etalon
10
. The BSP
18
further receives light returning from the etalon and provides a portion of that light to the plane mirror
16
and a remaining portion to an output port. Although the MGTI device appears to perform its intended function, it appears to have certain limitations. For example, the MGTI device requires an optical circulator to extract the output signal adding to signals loss and increased cost of the device; and the requirement of a BSP which is known to have a significant polarization dependent loss.
FIG. 10
shows a graph with a linear plot of the phase difference between the two reflected E-fields from the GT and from a mirror with an optical path difference.
A wavelength interferometer can be made using a Mach-Zehnder interferometer(MZI). Notwithstanding, the spectral response of the MZI is sinusoidal and consequently does not have a desired flat-top characteristic passband; hence, its spectral window for low crosstalk, is small.
A paper by K. Oda et al., entitled “A wide-band guided-wave periodic multi/demultiplexer with a ring resonator for optical FDM transmission systems, JLT, vol. 6, no. 6, pp 1016-1022, June 1988, discloses improving the spectral response of the MZI and a suitable step-like response can be obtained by adding an all-wave filter such as a ring resonator to one arm of the MZ as is shown in FIG.
1
. However, it is generally difficult to implement a low-loss ring resonator in a system having a free-spectral range (FSR) of 100 GHz or 50 Ghz.
The instant invention obviates the problems associated with the bulk optics GT device described heretofore, and obviates a device requiring a ring resonator.
It is an object of this invention to provide embodiments of the invention, some of which are planar waveguide implementations for a wavelength interleaver based on an MZ interferometer.
It is a further object of the invention to provide an interleaver that uses an asymmetric MZ combined with a suitable resonator disposed on the shorter arm of the MZ. Advantageously, the use of a planar waveguide MZ interferometer allows the setting of a required length difference between two arms of the MZ, very accurately.
SUMMARY OF THE INVENTION
In accordance with the invention, there is provided, a first coupler having an input port and at least two output ports;
a second coupler having two input ports and two output ports, the at least two output ports of the first coupler being optically coupled with the two output ports of the second coupler;
an optical cavity having a partially reflective surface E at one end and a reflective surface at another end with a higher reflectivity E′, the optical cavity being optically coupled to one of the at least two output ports of the first coupler and at one of the two output ports of the second coupler.
In accordance with the invention there is further provided, an interleaver/de-interleaver circuit comprising:
a Mach-Zehnder interferometer having two optical branch paths between two 50/50 couplers, one of the two branch paths including an etalon within the path, such that light traversing said one branch path is coupled from one of the couplers to the other after resonating within the etalon, light traversing another of the two branch paths interfering with the light traversing the other branch path within one of the two couplers.
In accordance with the invention, there is still further provided, an interleaver/de-interleaver comprising:
a MZ-interferometer having an input/output port and two output/input ports;
a GT resonator optically coupled with the MZ-interferometer, the GT resonator for providing additional filtering and for lessening the sinusoidal shape of de-interleaved channels.
REFERENCES:
patent: 5499308 (1996-03-01), Arai et al.
patent: 6055345 (2000-04-01), Ahn et al.
patent: 6125220 (2000-09-0
Connelly-Cushwa Michelle R.
JDS Fitel Inc.
Lee John D.
Teitelbaum Neil
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