Abstract:
A device is provided for demultiplexing a DWDM composite light signal into distinct signal channels or frequencies. The device includes a plurality of resonators, which each acts to slice an incoming signal into two equal parts. The free spectral range characteristics (FSR) of the resonators are successively increased by an even multiple to achieve the slicing effect. As a result, the resonators of the subject invention can be formed with relatively low finesse values because of the slicing effect.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to Provisional Patent Application No. 60/155,307, filed on Sep. 21, 1999. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to nanophotonic devices, and, more particularly, to optical resonator devices used in demultiplexing devices. 
     BACKGROUND OF INVENTION 
     Wave-division multiplexing (WDM), and similarly, dense WDM (DWDM) and ultra-dense WDM (UDWDM), provide the ability to simultaneously transmit multiple signals through a single optical fiber or waveguide, with each signal being transmitted on a separate wavelength or channel and each typically carrying either 2.5 or 10-gigabit-per-second signals. 
     The International Telecommunications Union (ITU) has set standards for the basic wavelength and channel spacing used in WDM. Light, like radio waves has a wavelength. For light this is measured in nanometers (millionths of a millimeter). The ITU standards set a “window” from 1500 nm to 1535 nm for WDM, subdivided into 43 “channels”, sometimes referred to as “colors”, whose centers are separated by 0.8 nm. This represents a channel bandwidth of about 100 GHz regarded as the current practical limit for manufacturing precision tunable optical transceivers. In future, however, the channel spacing will be halved to provide up to 80 channels per fiber. 
     In practice, each channel can be treated as an independent optical transmission path and therefore can be modulated at whatever speed is appropriate for an application. A hierarchy of optical fiber transmission speeds has been standardized for the two major optical network systems—Synchronous Optical NETwork (SONET) in the US, and the ITU&#39;s standard Synchronous Digital Hierarchy (SDH) in the rest of the world. There are differences between the terminology and the details of hierarchy of speeds but the standards are not completely incompatible. 
     DWDM combines multiple optical signals so that they can be amplified as a group and transported over a single fiber or waveguide to increase capacity. Each signal carried can be at a different rate (OC-3/12/24, etc.) and in a different format (SONET, ATM, data, etc.) For example, a DWDM network with a mix of SONET signals operating at OC-48 (2.5 Gbps) and OC-192 (10 Gbps) over a DWDM infrastructure can achieve capacities of over 40 Gbps. A system with DWDM can achieve all this gracefully while maintaining the same degree of system performance, reliability, and robustness as current transport systems—or even surpassing it. Future DWDM terminals will carry up to 80 wavelengths of OC-48, a total of 200 Gbps, or up to 40 wavelengths of OC-192, a total of 400 Gbps—which is enough capacity to transmit 90,000 volumes of an encyclopedia in one second. 
     Micro-ring resonators are known in the prior art, such as that disclosed in U.S. Pat. No. 5,926,496. In addition, it is known in the prior art to use micro-ring resonators as filters, wherein the resonators act to separate desired wavelengths (i.e., channels) of a light signal from a DWDM input light signal. For example, FIG. 1 depicts a prior art filter arrangement  1  having an input waveguide  2 , with an input port  3  and an output port  4 , and an output waveguide  5 , with an output port  6 . A micro-ring resonator  7  is interposed between the input waveguide  2  and the output waveguide  5  and is tuned to a predetermined wavelength. To understand the operation of the filter  1 , with a DWDM light signal propagating through the input waveguide  2  (in a direction from the input port  3  and towards the output port  4 ), part of the light signal (i.e., the wavelength of the input signal that is on-resonance with the resonator  7 ) will couple from the input waveguide to the resonator  7 . That wavelength is thus demultiplexed or dropped from the input signal. The resonator  7 , in turn, couples that wavelength to the output waveguide  5  and a light signal having that particular wavelength propagate through the output waveguide  5  towards the output port  6 . The remaining wavelengths of the input signal, i.e., those which are not on-resonance with the resonator  7 , by-pass the resonator  7  and continue propagating through the input waveguide  2  and towards the output port  4 . 
     Using this basic methodology, full-scale demultiplexing systems have been built for lightwave systems. With reference to FIG. 2, a demultiplexing device  10  is shown having a single input waveguide  11 , with an input port  12  and an output port  13 . A series of micro-ring resonators  14 A-D are arranged along the length of the input waveguide  11 . Although not shown in FIG. 2, the resonators  14 A-D would generally be each formed with a different radius; with the radius of the resonator determining, at least in part, the resonant wavelength of the resonator. Additionally, an output waveguide  15 A-D is provided for each resonator  14 A-D, with each output waveguide  15 A-D having an output port  16 A-D. The demultiplexing device  10  is referred to as a 1×5 device: the first number (1) signifying a single input, while the second number signifies the number of outputs (5). Other combinations are possible, including 1×8 and 1×16. With the structural arrangement of the device  10 , a DWDM light signal propagating through the input waveguide  11 , in a direction from the input port  12  and towards the output port  13 , will be sequentially demultiplexed (also known as “demuxed”) by the resonators  14 A-D into four different wavelengths, with a remainder signal portion (i.e., those wavelengths that are not demuxed) propagating through the input waveguide  11 . The various wavelengths will respectively propagate towards the output ports  13  and  16 A-D. 
     With reference to FIG. 3, a chart is provided to symbolically represent the coupling of wavelengths of a light signal by a resonator. The arrows along line A′ represent different light signal wavelengths or channels LS. Trapezoidal blocks T on line B′ represents the transfer characteristic of a resonator, such as resonator  7  (FIG.  3 ). With the DWM light signal having a plurality of wavelengths or channels propagating through input waveguide  2 , the wavelengths or channels LS that coincide with the trapezoidal blocks T are coupled to the resonator  7 , as represented by coupled wavelengths or channels CLS shown on line C′ in FIG.  3 . Wavelengths that are not coincident with the trapezoidal blocks T by-pass the resonator  7  and continue to propagate through (or are guided by) the input waveguide  2 , as depicted on line D′ and identified as SLS. 
     The spacing S between the trapezoidal blocks T is a free spectral range (FSR) characteristic of the resonator  7 , whereas, the full-width half-maximum (FWHM) width W of the trapezoidal blocks T is indicative of the linewidth of the resonator  7 . In addition, the finesse F of a resonator is equal to the FSR/linewidth. As can be appreciated, a narrow linewidth will result in a large finesse F, while a large linewidth will result in a small finesse F. 
     Although effective, the system of FIG. 2 has limitations. Each of the resonators  14 A-D requires a narrow linewidth to only select a specific wavelength of the input signal. Where a large number of wavelengths are required to be demultiplexed, the finesse of the resonators  14 A-D will be relatively high, thereby requiring relatively stringent tolerances, finer tunability, etc., and high manufacturing standards. 
     Thus, there exists a need in the art for an optical device that overcomes the above-described shortcomings of the prior art. 
     SUMMARY OF THE INVENTION 
     The subject invention overcomes the deficiencies of the prior art, wherein a demultiplexing device is provided for selectively demultiplexing wavelengths or channels of a DWDM light signal. The device includes a plurality of resonators, preferably micro-ring, which are arranged to “slice” a signal into wavelengths or channels (those terms being used interchangeably herein), rather than couple desired wavelengths. By “slicing” the signal in sequential steps, the resonators can each be formed with a lower finesse than resonators arranged in a prior art device. Prior art demultiplexing devices using resonators typically include a plurality of resonators arranged in a generally linear and cascaded array. All the resonators are required to have a finesse that is proportional to the number of wavelengths in the DWDM signal. For very broadband DWDM signals (high channel count), the required FSR is also proportionally larger, which means that the resonators will be very small. 
     With the subject invention, a plurality of resonators having different physical and optical characteristics are optically coupled to a plurality of waveguides, thus defining a plurality of stages. In each stage, the number of channels in the DWDM signal is reduced by two (or by 2N, an even number). Thus, the architecture of the present invention may also be referred to as a divide-by-2N architecture. After the first resonator stage, the DWDM signals will have been separated into even-number and odd number channels, with half of the channels being dropped by the first resonator. The other half will continue through the input waveguide, having by-passed the resonator. 
     Thereafter (i.e., in subsequent, downstream stages), a plurality of resonators are utilized to continue slicing, until single wavelength or channel light signals remain. It should be noted that the same number of resonators will generally be required for the subject invention as in a prior art demultiplexer. However, the resonators that are required can be of a much lower finesse than that of the resonators used in the prior art. In addition, the resonators will generally be arranged in parallel, thereby, cutting down the distance signals must propagate. 
     The invention accordingly comprises the features of construction, combination of elements, and arrangement of parts which will be exemplified in the disclosure herein, and the scope of the invention will be indicated in the claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawing figures, which are not to scale, and which are merely illustrative, and wherein like reference numerals depict like elements throughout the several views: 
     FIG. 1 is a top plan view of a prior art optical filter having input and output waveguides and a resonator optically coupled therebetween; 
     FIG. 2 is a top plan view of a prior art optical demultiplexer device; 
     FIG. 3 is a schematic representing transfer characteristics of a typical optical resonator; 
     FIG. 4 is a top plan view of a demultiplexer formed in accordance with the subject invention that spatially separates eight wavelength channels that constitute the DWDM input signal; 
     FIGS. 5A-G are schematics representing various transfer characteristics of the resonators depicted in FIG. 4; and 
     FIG. 6 are plots showing relative FSR and linewidth characteristics of the resonators depicted in FIG.  4 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 4, a device  1000  for demultiplexing a DWDM input light signal is shown. The device  1000  generally includes an input waveguide  1002 , a first resonator  1004 , a connecting waveguide  1006 , two secondary resonators  1008 ,  1010 , two secondary waveguides  1012 ,  1014 , four tertiary resonators  1016 ,  1018 ,  1020 ,  1022 , and four tertiary waveguides  1024 ,  1026 ,  1028 ,  1030 . Preferably, all resonators are micro-ring resonators. 
     The device  1000  is shown for use in a 1×8 demultiplexing application. It is to be understood that this description of the device  1000  is provided to illustrate the structure and functioning of the subject invention, and the subject invention is not limited to 1×8 applications. Other applications are possible, such as 1×16 demultiplexing applications, consistent with the teachings herein. 
     In a preferred embodiment, the FSR of the first resonator  1004  is equal to two times the channel spacing of the DWDM input signal. In this manner, the first resonator  1004  couples half the wavelengths propagating through the input waveguide  1002 . To demonstrate the subject invention, FIGS.  4  and  5 A-G depict arrows, with each arrow representing a particular wavelength or channel of the DWDM signal. As indicated above, the device  1000  is shown to be a 1×8 demultiplexer; therefore, as represented by line A in FIGS. 4 and 5A, an original input signal having eight channels is directed into the input waveguide  1002 . The first resonator  1004  has transfer characteristics as represented by line B in FIG. 5A, with the four signal channels represented in line C being coincident with the transfer characteristics and coupled to the connecting waveguide  1006  by the first resonator  1004 . The four signal channels represented by line D by-pass the first resonator  1004  and continue through the input waveguide  1002 . As can be appreciated, the first resonator  1004  acts to slice the original input signal into two halves, with each half having the same number of generally evenly-spaced wavelengths or channels. 
     In turn, the coupled channels represented by line C propagate through the connecting waveguide  1006  and into proximity with the secondary resonator  1010 . As represented in FIG. 5B, the channels of the light signal coinciding with the transfer characteristics of the secondary resonator  1010 , represented by line E, are coupled to the secondary waveguide  1014 . The coupled channels are represented by line F, whereas the channels by-passing the secondary resonator  1010  are represented by the line G. 
     In similar fashion, the secondary resonator  1008 , having transfer characteristics represented by line H, causes the channels represented by line  1  to be coupled to the secondary waveguide  1012 . The channels represented by line J by-pass the secondary resonator  1008 . 
     In a preferred embodiment, the secondary resonators  1008 ,  1010  have FSR characteristics which are an even multiple (i.e., n times, with n being an integer) of the FSR of the first resonator  1004 . More preferably, the FSR characteristics of the secondary resonators  1008 ,  1010  are two times the FSR of the first resonator  1004 . This relationship between FSR (in wavelength units) and resonator radius is given by:              FSR   =       λ   m   2       2                 π                   Rn   eff                 (   1   )                                
     where n eff  is the effective refractive index of the waveguide (i.e., the resonator waveguide) and is approximately equal to 3, and λ m  is the center wavelength (e.g., 1.55 microns or 1550 nm). FSR may be expressed in frequency or in wavelength units according to the following relationship: 100 GHz=0.8 nm, 200 GHz=1.6 nm, etc. Thus, for FSR of 100 GHz, the resonator radius R will be equal to 160 microns. Likewise, for FSR of 200 and 400 GHz, the resonator radius will be equal to 80 and 40 microns, respectively. As the FSR increases from an earlier stage to a later stage, so does the linewidth. As adjacent channels are filtered out, the linewidth required can be relaxed. In fact, the ratio of FSR over linewidth, i.e., the finesse, will remain constant at a relatively low value. 
     In a preferred embodiment, the radius R 1  will be two times the radius R 2  of the secondary resonators  1008 ,  1010 , since it is desired that the FSR characteristics of the secondary resonators  1008 ,  1010  be two times the FSR of the first resonator  1004 . As known by those skilled in the art, the size of the FSR characteristics of a micro-ring resonator is inversely proportional to the radius of the resonator. 
     The tertiary resonators  1016 ,  1018 ,  1020 ,  1022  work in similar fashion to the secondary resonators  1008 ,  1010 , in slicing wavelengths or channels. The tertiary resonators  1016 ,  1018 ,  1020 ,  1022  have transfer characteristics represented respectively by lines K, L, M, N. As a result, the channels represented by line V are coupled to the tertiary waveguide  1024 ; the channels represented by line Y are coupled to the tertiary waveguide  1026 ; the channels represented by line P are coupled to the tertiary waveguide  1028 ; and the channels represented by line R are coupled to the tertiary waveguide  1030 . Additionally, the channels represented by line X by-pass the tertiary resonator  1016  to continue propagating through the secondary waveguide  1012 ; the channels represented by line Z by-pass the tertiary resonator  1018  to continue propagating through the input waveguide  1002 ; the channels represented by line Q by-pass the tertiary resonator  1020  to continue propagating through the secondary waveguide  1014 ; and, the channels represented by line U by-pass the tertiary resonator  1022  to continue propagating through the connecting waveguide  1006 . As a net result, eight distinct channels are separated from the DWDM input signal represented by the line A. 
     Preferably, the tertiary resonators  1016 ,  1018 ,  1020 ,  1022  are each, respectively, formed to have a FSR which is two times the FSR of the secondary resonators  1008 ,  1010 . As such the radius R 2  of the secondary resonators  1008 ,  1010  is two times the radius R 3  of the tertiary resonators  1016 ,  1018 ,  1020 ,  1022 . Alternatively, the FSR characteristics of the tertiary resonators can be other even multiples of the FSR characteristics of the secondary resonators. 
     To further describe the preferred embodiment of the invention, the following is an exemplary embodiment. With an input signal having a 50 GHz channel spacing, the FSR of the first resonator  1004  will be 100 GHz. Accordingly, the FSR of the secondary resonators  1008 ,  1010  will be 200 GHz, while that of the tertiary resonators  1016 ,  1018 ,  1020 ,  1022  will be 400 GHz. Meanwhile, the radius R 1  of the first resonator  1004  may be 160 microns, the radius R 2  of the secondary resonators  1008 ,  1010  may be 80 microns, and the radius R 3  of the tertiary resonators  1016 ,  1018 ,  1020 ,  1022  will be 40 microns. 
     Referring to FIG. 6, stage  1  represents the relative FSR and linewidth characteristics of the first resonator  1004 ; stage  2  represents the relative FSR and linewidths of the secondary resonators  1008 ,  1010 ; and stage  3  represents the relative FSR and linewidth characteristics of the tertiary resonators  1016 ,  1018 ,  1020 ,  1022 . As a result, in contrast to the prior art, the finesses of all of the resonators does not have to change throughout the device  1000  and can be maintained at relatively low levels. As the FSR increases from an earlier stage to a later stage, so does the linewidth. As adjacent channels are filtered out, the linewidth required can be relaxed. In fact, the ratio of FSR over linewidth, i.e., the finesse, will remain constant at a relatively low value. This is depicted in Table 1 in which possible FSR and linewidth relationships for the aforementioned three stages, with the finesse being constantly held at the value 4 are depicted: 
     
       
         
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Stage 
                 FSR 
                 Linewidth 
                 Finesse 
               
               
                   
                   
               
             
             
               
                   
                 1 
                 2Δλ 
                 0.5Δλ 
                 4 
               
               
                   
                 2 
                 4Δλ 
                 Δλ 
                 4 
               
               
                   
                 3 
                 8Δλ 
                 2Δλ 
                 4 
               
               
                   
                   
               
             
          
         
       
     
     Advantageously, the subject invention does not require high finesse values, thereby avoiding high manufacturing requirements. 
     It will be understood by those skilled in the art that the FSR of the first resonator  1004  may be other even multiples of the channel spacing of the input waveguide  1002 . Also, the FSR characteristics of the first, secondary, and tertiary resonators may be of other even multiples. 
     Thus, while there have been shown and described and pointed out fundamental novel features of the invention as applied to preferred embodiments thereof, it will be understood that various omissions and substitutions and changes in the form and details of the disclosed invention may be made by those skilled in the art without departing from the spirit of the invention. It is the intention, therefore, to be limited only as indicated by the scope of the claims appended hereto.