Optical waveguides – With optical coupler – Plural
Reexamination Certificate
2001-03-16
2004-06-22
Glick, Edward J. (Department: 2882)
Optical waveguides
With optical coupler
Plural
C385S050000, C398S082000
Reexamination Certificate
active
06754411
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to wave division multiplexing (WDM) and dense WDM (DWDM), and particularly to demultiplexers for WDM and DWDM multiplexed optical signals. More particularly still, it relates to demultiplexers utilizing cascaded asymmetric Mach-Zehnder Interferometers (MZI).
BACKGROUND OF THE INVENTION
WDM is the current favorite in optical communications. In this technology, data channels are multiplexed on different wavelengths. The number of available wavelength channels is growing rapidly as the technology improves. Currently, the International Telecommunication Union (ITU) specification is considering channel spacing as low as 50 GHz. This new generation of WDM is usually referred to as DWDM. On the other hand, data communication speeds have also increased significantly, so that OC-192 with around 10 Giga-bits per second is used in current optical network designs.
In WDM, devices are needed to multiplex and demultiplex different data channels. These devices are, however, very expensive and hard to manufacture. MZIs may be used as building blocks to build the optical multiplexer and demultiplexer. It should be noted that each of the input ports of the MZIs might be used. In the following descriptions, only one of the input ports, say input port 1, of the MZIs is used. It is also possible to use three-port combiner/splitter Y-junctions instead of four-port couplers in an MZI.
In a typical configuration, a number of MZIs are cascaded in a tree to separate all the WDM channels. For example, assume that there are 16 WDM channels (&lgr;
1
, . . . , &lgr;
16
) to be demultiplexed. The first level MZI
11
separates the odd and even channels into different output ports sent to MZI
21
and MZI
22
, respectively. In the second stage, MZI
21
and MZI
22
redirect every other input to one of the outputs. For example, in the upper path, MZI
21
redirects channels 1, 5, 9, 13 to MZI
31
, and channels 3, 7, 11, 15 to MZI
32
, and so on. In the final stage all the channels are demultiplexed to different output ports. In this scheme, the additional (or differential) delay required in one of the asymmetric MZI arms is half of the delay in the previous demultiplexing stage.
Methods are effective when the numbers of channels and the data rates are low. This is mainly because of the fact that MZI filtering characteristics change based on its differential delay or equivalently length difference of the two limbs of the MZI. The length difference (&Dgr;l) needed to separate channels with channel spacing can be calculated through &Dgr;l≅&lgr;
0
2
/(2&eegr;&Dgr;&lgr;), where &lgr;
0
is the central wavelength and &eegr; is the refractive index of the waveguide (e.g. optical fiber). As an example, for 100 GHz spacing (wavelength spacing of 0.8 nm) and refractive index of 1.5, the length difference needed for the first stage filter (MZI
11
) is around 1001 micrometers. The length difference for the second stage filters (MZI
21
and MZI
22
) is around 500.5 micrometers, 250.25 micrometers for the third stage MZI
31
, MZI
32
, MZI
33
, MZ
134
) and finally 125.12 micrometers for the last stage.
As technology moves towards higher data rates, such as the one for the OC-192 standard, the long delay caused by one of the limbs of the MZI might be comparable to the optical pulse width in the time domain. As a result, the MZI can cause dispersion or pulse widening of the data signals. Consequently, the MZI tree structure cannot be used.
It should be noted that this problem might also exist in other filtering techniques. For example, design of sharp filters is usually very difficult no matter what technique is used. The novel technique may be successfully applied to filters/demultiplexers utilizing other than MZIs.
SUMMARY OF THE INVENTION
In this invention, a technique is introduced which relaxes the requirement of having very sharp frequency response in the first stage of the tree structure as used at present. In the novel approach, the channels are separated in pairs rather than singly. As a result, the filter frequency response for the first stage not need to be as sharp as in the prior art. This means that the delay can be kept in the range that does not cause any deterioration of the high-speed signal in the time domain. The channels are separated in pairs in the first stage, i.e. channels (1,2), (5,6), (9,10), (13,14) in one port and (3,4), (7,8), (11,12), (15,16) in the adjacent port. The same filtering is used with a small shift in the frequency response to separate the channels in the pairs. This technique, however, causes some attenuation to the signal that may be compensated by amplification.
REFERENCES:
patent: 5590226 (1996-12-01), Wolf et al.
patent: 5611007 (1997-03-01), Wolf et al.
patent: 5852505 (1998-12-01), Li
patent: 6388783 (2002-05-01), Weller-Brophy
patent: 6487335 (2002-11-01), Yang et al.
patent: 6546166 (2003-04-01), Liu et al.
Ahmadvand Nima
Hatami-Hanza Hamid
Glick Edward J.
Peleton Photonic Systems Inc.
Suchecki Krystyna
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