Abstract:
WDM demultiplexers are provided utilizing, for example, cascaded asymmetric MZI filters, wherein the (asymmetric) differential delay is such that at the output of the first stage MZI demultiplexes the WDM channels in groups of two at its output. The result is a relaxed specification for the frequency response of the filters.

Description:
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 (λ 1 , . . . , λ 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 (Δl) needed to separate channels with channel spacing can be calculated through Δl≅λ 0   2 /(2ηΔλ), where λ 0  is the central wavelength and η 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. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Exemplary embodiments of the invention will now be further described with references to the drawings in which same reference numerals designate similar parts throughout the figures thereof, and wherein: 
     FIG. 1 shows an MZI having additional delay in one arm to provide wavelength filtering; 
     FIG. 2 shows MZI utilizing Y-junction couplers; 
     FIG. 3 shows cascaded MZIs to provide a wavelength demultiplexer according to the prior art; 
     FIGS. 4 a  and  4   b  show the results of filtering after the first stage using the prior art demultiplexer of FIG. 3; 
     FIG. 5 shows a demultiplexer according to the present invention; 
     FIGS. 6 a  and  6   b  show the results of filtering after the first stage of the demultiplexer of FIG. 5; 
     FIGS. 7 a  and  7   b  show the results of filtering after the second stage of the demultiplexer of FIG. 5; 
     FIGS. 8 a  and  8   b  show the results of filtering after the third stage of the demultiplexer of FIG. 5; and 
     FIGS. 9 a  and  9   b  show filtering results after the first stage of an alternative demultiplexer to that shown in FIG. 5, wherein the first stage divides the wavelength channels in groups of four, instead of two, at its output. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1 of the drawings, it shows an MZI having a four-port input coupler  10  and an output four-port coupler  11 , and two branches (or arms)  12  and  13  interconnecting the couplers  10  and  11 . The branch  13  provides slightly more delay Δl than the branch  12 . In FIG. 2, a similar MZI 14  is shown, but using Y-junctions  15  and  16  as the input and output couplers. An asymmetric MZI is the building block of the cascaded demultiplexers both of the prior art and the present invention; the difference being in the asymmetric delay Δl, which causes the wavelengths in the WDM or DWDM input signal to be split in pairs or singly at the output of the first MZI in the cascade. This is shown in FIGS. 3 and 5, where in FIG. 3 the component lambdas at one output of MZI 11  and even numbered lambdas at the other output of the MZI 11 . While in FIG. 5, according to the present invention, the two outputs of the MZI 11  split the lambdas into pairs (λ 1 , λ 2 ) to (λ 13 , λ14 2 ) and (λ 3 , λ 4 ) to (λ 15 , λ 16 ) between them. This is shown in the transfer function is of the MZI 11  of FIG. 3 in FIGS. 4 a  and  4   b ; and of the MZI 11  of FIG. 5 in FIGS. 6 a  and  6   b . This is achieved by the differential delay Δl if the MZI 11  in FIG. 5 being approximately one-half of the Δl of the MZI 11  in FIG.  3 . With the further important characteristic that the delay in the next stages of the MZIs (that is MZI  21 , 22 ; MZI  31 ,  32 ,  33 ,  34 ; and MZI  41 , 42 , 43 , 44 , 45 , 46 , 47 , 48 ) are also reduced by one-half from their values in FIG.  3 . 
     The transfer functions and channels at the outputs of the second and third stages of the demultiplexer of FIG. 5 are shown in FIGS. 7 a, b  and  8   a  and  b , respectively. 
     FIGS. 4,  6 ,  7  and  8  show generally wavelength channels specified by ITU, with channel spacing of 100 GHz (0.8 nanometers). In FIG. 4, the sharp filtering required means a differential delay Δl in MZI 11  of approximately 1001 micrometers; this Δl is reduced in the MZI 11  of FIG. 5 to approximately 500 micrometers. This means that the passband of the first stage filter is as large as that of second stage filters in the prior art. The same filter is used for the second stage (MZI 21  and MZI 22 ) with a small shift in the frequency response. This frequency shift can easily be performed by a small change in the optical length difference of the MZI. In this case this value is in the range of 0.5 micrometers. The output for one of the second stage filters is shown in FIG. 7 b . The attenuation caused by this method is also shown in output signals of the first and second stages. We should also note that a small cross-talk of adjacent channels exists. This cross-talk gradually decreases in each stage of the filtering. 
     As shown in FIG. 3, the third stage, and any stages after in accordance with the invention, are the same as those used in the prior art tree structure demultiplexing (shown in FIG. 3) with a small configuration of the connections as shown in FIG.  5 . There, the two outputs from MZI 21  and MZI 22 , going to the inputs of MZI 32  and MZI 33  in FIG. 3, are now interchanged. This is necessary due to the pairing of odd and even channels at the outputs of MZI 11 , which feed MZI 21  and MZI 22 . 
     In the above description in was shown how pair filtering could help to relax the filtering requirements at the fist stage. However, it is reasonable to ask what would be the result if the channels were divided in groups of four, eight or more. Although, the present scheme is still applicable, the resulting cross talk and filter distortion may not be acceptable. An example is shown in FIGS. 9 a  and  9   b  for a 4-by-4 case. As shown, the middle pairs in each block ( 17  and  18  in FIG. 9 b ) of 4 channels have higher energy than the other two. The cross talk effect at the other outputs ( 19  in FIG. 9 b ) is also noticeable. We should also note that it is not easy to separate the channels in each block in the second stage as was done in the paired case.