Abstract:
An optical device for rearranging wavelength channels in an optical network is disclosed. The optical device has a wavelength selective coupler having one input port and a plurality of output ports coupled to a plurality of input ports of an optical grating demultiplexor such as an arrayed waveguide grating. The wavelength channels in each of the input ports are dispersed by the demultiplexor and are directed to a plurality of output ports of the optical grating demultiplexor. As a result, at least one wavelength channel at each of the input ports of the optical grating demultiplexor is coupled into a common output port. The optical device is useful in passive optical networks wherein a same demultiplexor is used for simultaneous multiplexing and demultiplexing of wavelength channels.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present invention claims priority from U.S. Provisional Patent Application No. 61/229,928 filed Jul. 30, 2009 which is incorporated herein by reference for all purposes. 
    
    
     TECHNICAL FIELD 
     The present invention relates to optical devices for routing and directing optical signals, and in particular to optical devices for rearranging wavelength-multiplexed optical signals in an optical communications network. 
     BACKGROUND OF THE INVENTION 
     The Internet services are currently provided using interconnected long-haul and metro optical networks. In modern long-haul and metro optical networks, optical signals are modulated with digital information and transmitted from one location to another, typically through a length of an optical fiber. To increase the information carrying capacity of the networks, modulated optical signals at different wavelengths, called “wavelength channels”, are grouped together (multiplexed) at one location of the network, transmitted through a common fiber to the other location of the network, and ungrouped (demultiplexed) at the other location. 
     As the Internet, Voice over Internet Protocol (VoIP) and streamed Internet Protocol (IP) television gain popularity, more and more subscribers desire to access these services from their premises. At present, these services are delivered to individual premises using either a twisted-pair Digital Subscriber Line (DSL) or a coaxial television cable. Due to the increased demand, the DSL and coaxial cable technologies are reaching their information carrying capacity limits, and optical technologies (so-called “Fiber To The Premises”, or FTTP) are increasingly used for delivering Internet services to individual premises. 
     Most FTTP technologies presently use a passive optical network (PON) architecture to provide fiberoptic access to the premises, because a PON architecture does not require expensive amplification and wavelength selective switching equipment commonly used in long-haul and metro optical networks. To deliver communication services from a central office to multiple individual subscribers, most PON systems use a passive star-type optical splitter and a form of time-division multiplexing (TDM) for delivering downstream and upstream information. 
     Disadvantageously, TDM-PON systems are quite complex and do not always provide a required degree of security of communications. A wavelength-division multiplexing (WDM) architecture can be attractive for a PON application, because in a WDM-PON, different wavelengths can be assigned to different subscribers or groups of subscribers, thus providing a higher degree of security of communications than a TDM-PON can provide. Furthermore, a WDM-PON architecture can potentially provide a broader bandwidth than a TDM-PON architecture. Nonetheless, WDM-PON systems so far have been relatively costly. For this reason, WDM-PON systems have not yet found a widespread utilization in cost-sensitive FTTH applications. 
     WDM-PON systems utilize wavelength-selective combiners and splitters of optical signals called “WDM multiplexors” and “WDM demultiplexors”, respectively. To save costs, a WDM multiplexor and a WDM demultiplexor of a WDM-PON system can be combined into a single unit, which is referred to as a “de/multiplexor”. Referring to  FIG. 1A , a prior-art arrayed waveguide (AWG) WDM de/multiplexor  100  is shown having a single input port  102  and four output ports  111  to  114 . Four wavelength channels λ 1   C , λ 2   C , λ 3   C , λ 4   C  of central (“C”) band of optical communications and four wavelength channels λ 1   S , λ 2   S , λ 3   S , λ 4   S  of short (“S”) band optical communications are present at the input port  102 . The WDM de/multiplexor  100  directs wavelengths λ 1   C , λ 1   S  to the output port  111 ; wavelengths λ 2   C , λ 2   S  to the output port  112 ; wavelengths λ 3   C , λ 3   S  to the output port  113 ; and wavelengths λ 4   C , λ 4   S  to the output port  114 . To direct different wavelengths to a same output port, the WDM de/multiplexor  100  uses a diffractive optical device having multiple orders of diffraction. The WDM de/multiplexor  100  is bidirectional, that is, the wavelength channels arriving at the output ports  111 - 114  can be combined into a single multi-channel signal at the input port  102 . Referring now to  FIG. 1B , a WDM-PON  120  has two nodes  121  and  122  coupled through a length of an optical fiber  123 . Each node  121  and  122  has one WDM de/multiplexor  100 . The input ports  102  of the WDM de/multiplexors  100  of the nodes  121  and  122  are connected together by the optical fiber  123 . The output ports  111  to  114  of the WDM de/multiplexors  100  are coupled to duplex optical filters  124  coupled to corresponding transmitters  126  and receivers  128 . The node  121  uses the wavelength channels λ 1   C , λ 2   C , λ 3   C , λ 4   C  for transmission and the wavelength channels λ 1   S , λ 2   S , λ 3   S , λ 4   S  for reception. The node  122  uses the wavelength channels λ 1   S , λ 2   S , λ 3   S , λ 4   S  for transmission and the wavelength channels λ 1   C , λ 2   C , λ 3   C , λ 4   C  for reception. The direction of flow of the signals is shown with arrows  127 . Thus, each WDM de/multiplexor  100  is used for both multiplexing and demultiplexing wavelength channels, whereby significant cost savings can be achieved. 
     Disadvantageously, in the AWG WDM de/multiplexor  100 , and in any diffraction grating based demultiplexor for that matter, the wavelengths of the channels λ i   S  and λ i   C  directed to a same i th  output port in different orders of diffraction m and m+1 are tied together by the grating equation: λ i   S ≈λ i   C m/(m+1) and therefore cannot be selected independently from each other. As a result, the WDM-PON  120  does not allow a system designer to select the wavelength channels λ 1   C , λ 2   C , λ 3   C , λ 4   C  independently from the wavelength channels λ 1   S , λ 2   S , λ 3   S , λ 4   S . This represents a considerable limitation, especially for a FTTP application where the available bandwidth needs to be utilized to a full extent to provide as broad coverage as possible at a given cost. 
     It is therefore an object of the invention to provide an optical device for directing and regrouping wavelength channels, wherein the wavelengths of the channels directed to the same output port are independently selectable. The independent wavelength selection improves bandwidth utilization and network efficiency. As a result, a deployment cost to provide a FTTH-based broadband Internet service to individual subscribers is reduced. 
     SUMMARY OF THE INVENTION 
     An optical device of the invention achieves independent routing of two or more wavelength channels to a same output port of an optical grating multiplexor by providing two or more separate input ports for the optical grating demultiplexor. The input ports are offset from each other so as to provide a required wavelength separation between the two or more wavelength channels intended for coupling to a same output port. The wavelength channels are initially separated into two or more groups of channels, one group per one input port of the optical grating multiplexor. The groups of wavelength channels are then separately coupled to the input ports of the optical grating multiplexor. 
     In accordance with the invention there is provided an optical device for rearranging wavelength channels, comprising: 
     a wavelength selective coupler having an input port and first and second output ports, for separating wavelength channels received at the input port into first and second groups of wavelength channels for output at the first and the second output ports, respectively;
 
an optical grating demultiplexor having first and second input ports optically coupled to the first and the second output ports of the wavelength selective coupler, respectively, and a plurality of output ports, for demultiplexing the first and the second groups of wavelength channels;
 
wherein the first and the second input ports of the optical grating demultiplexor are offset from each other so as to couple a wavelength channel of the first group from the first input port, together with a wavelength channel of the second group from the second input port, into a same output port of the optical grating demultiplexor.
 
     In one embodiment, the wavelength selective coupler includes an optical interleaver having one input and two outputs coupled to the first and the second input ports of the optical grating demultiplexor. Advantageously, this allows one to use the optical grating demultiplexor having channel spacing twice as big as the channel spacing of the wavelength channels. By way of example, this embodiment of the invention allows a 100 GHz demultiplexor to be used in an optical network having 50 GHz spaced channels. 
     In one embodiment, the optical device of the invention further includes a plurality of wavelength selective splitters. Each wavelength selective splitter is optically coupled to one of the plurality of the output ports of the optical grating demultiplexor, functioning as a separator of wavelength channels of the first group from wavelength channels of the second group. The wavelength selective splitters are preferably duplex filters for bidirectional communication, wherein the first group of channels carries information in one direction, and the second group of channel carries information in the other, opposite direction. 
     In accordance with another aspect of the invention there is further provided an optical network node comprising: 
     the optical device for rearranging the wavelength channels; 
     a plurality of receivers each coupled to a particular one of the duplex filters for receiving a transmission channel; and 
     a plurality of transmitters each coupled to a particular one of the duplex filters for transmitting a transmission channel. 
     In accordance with yet another aspect of the invention there is further provided an optical network comprising two optical network nodes and an optical transmission line that couples together the input ports of the wavelength selective couplers of the two optical network nodes, 
     wherein the transmission channels of the first optical network node are the reception channels of the second optical network node, and vice versa. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Exemplary embodiments will now be described in conjunction with the drawings in which: 
         FIG. 1A  is a block diagram of a prior-art arrayed waveguide demultiplexor; 
         FIG. 1B  is a block diagram of a WDM passive optical network having two demultiplexors of  FIG. 1A ; 
         FIG. 2A  is a block diagram of an optical device of the invention having a wavelength division multiplexor coupled to an optical grating demultiplexor; 
         FIG. 2B  is a block diagram of an optical device of the invention having an optical interleaver coupled to an optical grating demultiplexor; 
         FIG. 2C  is a block diagram of a variant of the optical device of  FIG. 2B  having a different offset between the input ports of the optical grating demultiplexor; 
         FIG. 3  is a spectrum of wavelength channels coupled to the input ports of the optical devices of  FIGS. 2B and 2C ; 
         FIG. 4  is a block diagram of an optical device of the invention having 1:N wavelength selective coupler and N:M optical grating demultiplexor; 
         FIG. 5  is a plan view of an optical device of  FIGS. 2A to 2C , having an arrayed waveguide grating; and 
         FIG. 6  is a block diagram of an optical network of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     While the present teachings are described in conjunction with various embodiments and examples, it is not intended that the present teachings be limited to such embodiments. On the contrary, the present teachings encompass various alternatives, modifications and equivalents, as will be appreciated by those of skill in the art. 
     Referring to  FIG. 2A , an optical device  200 A of the invention includes a wavelength division multiplexor  202 A coupled to an optical grating demultiplexor  210 . The wavelength division multiplexor  202 A has an input port  204  and first and second output ports  206  and  208 , respectively. The function of the wavelength division multiplexor  202 A is to separate wavelength channels λ 1  to λ 8  received at the input port  204  into first and second groups of wavelength channels λ 1  to λ 4  and λ 5  to λ 8 , respectively, and direct them to the first and the second output ports  206  and  208 , respectively. 
     The first and the second output ports  206  and  208  are coupled to first and second input ports  212  and  214 , respectively, of the optical grating demultiplexor  210 . The function of the optical grating demultiplexor  210  is to demultiplex the first and the second groups of wavelength channels λ 1  to λ 8  and to direct the demultiplexed channels towards a plurality of output ports  216  to  219  of the optical grating demultiplexor  210 . The first and the second input ports  212  and  214  of the optical grating demultiplexor  210  are offset from each other so as to couple a wavelength channel of the first group λ 1  to λ 4  from the first input port  212 , together with a wavelength channel of the second group λ 5  to λ 8  from the second input port  214 , into a same output port  216 ,  217 ,  218 , or  219 , of the optical grating demultiplexor  210 . Thus, the output port  216  has the wavelength channels λ 1  and λ 5 ; the output port  217  has the wavelength channels λ 2  and λ 6 ; the output port  218  has the wavelength channels λ 3  and λ 7 ; and the output port  219  has the wavelength channels λ 4  and λ 8 . Advantageously, the presence of two offset input ports  212  and  214  allows the wavelengths λ 1  and λ 5  to be individually selectable by adjusting the magnitude of the offset between the input ports  212  and  214 . The wavelength adjustability will be illustrated further below. 
     Turning now to  FIG. 2B , an optical device  200 B is an alternative embodiment of the optical device  200 A. One difference between the optical devices  200 A and  200 B is that the optical device  200 B includes an optical interleaver  202 B instead of the WDM filter  202 A. The function of the optical interleaver  202 B is to separate wavelength channels λ 1  to λ 8  received at the input port  204  into first and second groups of wavelength channels λ 1 , λ 3 , λ 5 , λ 7  and λ 2 , λ 4 , λ 6 , λ 8 , respectively, and direct them to the first and the second output ports  206  and  208 , respectively. The optical interleaver preferably has an input channel spacing twice as small as a channel spacing of the optical grating demultiplexor  310 . Advantageously, the optical grating demultiplexor  210  can have a larger channel spacing than the channel spacing of an optical network wherein the optical device  200 B is used. For example, the optical grating demultiplexor  210  can have a 100 GHz channel spacing, while the optical network it is used in can have a 50 GHz channel spacing. 
     As noted above, one important advantage of the invention is the adjustability of wavelengths of the channels that are coupled together in the same output port  216 ,  217 ,  218 , or  219  of the optical grating demultiplexor. Turning to  FIG. 2C , an optical device  200 C is shown. The optical device  200 C is a variant of the optical device  200 B. One difference between the optical devices  200 B and  200 C is that an optical grating demultiplexor  211  of the optical device  200 C has an input  220  that is offset by an additional amount of             as compared to a position of the corresponding input  214  of the optical grating demultiplexor  210  of the optical device  200 B of  FIG. 2B . The additional offset           is illustrated at  225  in  FIG. 2C . The additional offset           determines which ones of the wavelength channels λ 2 , λ 4 , λ 6 , λ 8  are coupled to which ones of the output ports  216  to  219  of the optical grating demultiplexor  211 .
     Referring now to  FIG. 3 , a spectrum  311  shows the wavelength channels λ 1  to λ 8  at the input port  204  of the optical devices  200 B and  200 C of  FIGS. 2B and 2C . In  FIG. 3 , a spectrum  312  shows the wavelength channels λ 1 , λ 3 , λ 5 , λ 7  at the upper input port  212  of the optical grating demultiplexors  210  and  211 . 
     A spectrum  313  shows even wavelength channels λ 2 , λ 4 , λ 6 , λ 8  at the lower input port  214  of the optical grating demultiplexor  210  of  FIG. 2B . In  FIG. 3 , the spectrum  313  is shifted so that the even wavelength channels λ 2 , λ 4 , λ 6 , λ 8  line up with the odd wavelength channels λ 1 , λ 3 , λ 5 , λ 7 , due to the offset between the input ports  212  and  214  of the optical grating demultiplexor  210  of  FIG. 2B . As a result of the offset, the pairs of wavelength channels λ 1  and λ 2 ; λ 3  and λ 4 ; λ 5  and λ 6 ; λ 7  and λ 8  are coupled into the output ports  216  to  219 , respectively. The output ports  216  to  219  are shown in  FIG. 3  lined up with the corresponding wavelength channel pairs λ 1  and λ 2 ; λ 3  and λ 4 ; λ 5  and λ 6 ; λ 7  and λ 8 . 
     A spectrum  314  shows the even wavelength channels λ 2 , λ 4 , λ 6 , λ 8  at the lower input port  220  of the optical grating demultiplexor  211  of  FIG. 2C . In  FIG. 3 , the spectrum  314  is shifted as shown at  325  so that the wavelength channels λ 4 , λ 6 , λ 8  line up with the wavelength channels λ 1 , λ 3 , λ 5  due to the additional offset             shown at  225 . As a result of the additional offset          , the pairs of wavelength channels λ 1  and λ 4 ; λ 3  and λ 6 ; λ 5  and λ 8  are coupled into the output ports  216  to  218 , respectively. The output ports  216  to  218  are shown in  FIG. 3  lined up with the corresponding wavelength channel pairs λ 1  and λ 4 ; λ 3  and λ 6 ; λ 5  and λ 8 . The remaining wavelength channels λ 2  and λ 7  are coupled into an additional output port  315  and the output port  219 , respectively. The additional output port  315  is not shown in  FIG. 2C .
     By properly selecting the additional offset            , one can increase the wavelength separation of the wavelength channels coupled together into a same output port of the optical grating demultiplexor  211 . In  FIG. 3 , for example, wavelength channel pairs λ 1  and λ 4  at the output port  216  are separated three times more than the input channels λ 1  and λ 2 . Advantageously, selecting wavelength channels that are separated by at least three times more than the input channel spacing to be directed to a same output port, simplifies subsequent demultiplexing of these channels, because of the increased wavelength separation of these wavelength channels. At the same time, the advantage brought in by the interleaver  202 B, specifically a wider channel spacing of the optical grating demultiplexor  211 , is kept. In other words, the optical grating demultiplexor  211  can have a channel spacing that is twice bigger than the channel spacing at the input of the optical device  200 C.
     Referring now to  FIG. 4 , a more general form of an optical device of the invention is presented. An optical device  400  of the invention has a 1:M wavelength selective coupler  402  having one input port  404  and M output ports  406 - 1  . . .  406 -M, wherein M≧3. The 1:M wavelength selective coupler  402  is coupled to an M:N optical grating demultiplexor  410  having M input ports  412 - 1  . . .  412 -M and N output ports  416 - 1  . . .  416 -N, wherein N≧3. The M output ports  406 - 1  . . .  406 -M of the 1:M wavelength selective coupler  402  are coupled to the M input ports  412 - 1  . . .  412 -M of the M:N optical grating demultiplexor  410 , respectively. The function of the 1:M wavelength selective coupler  402  is to separate wavelength channels λ 1   1  . . . λ N   1 , λ 1   2  . . . λ N   2 , . . . , and λ 1   M  . . . λ N   M  into M groups of wavelength channels λ 1   1  . . . λ N   1 ; λ 1   2  . . . λ N   2 ; . . . ; and λ 1   M  . . . λ N   M , each group being directed to a corresponding output port  406 - 1 ;  406 - 2 ; . . . ;  406 -M. The function of the optical grating demultiplexor  410  is to demultiplex wavelength channels of each of the M groups received at M input ports  412 - 1  . . .  412 -M and to direct the demultiplexed channels λ 1   1  . . . λ 1   M ; λ 2   1  . . . λ 2   M ; . . . ; and λ N   1  . . . λ N   M  towards the output ports  416 - 1  . . .  416 -N, respectively. By properly selecting the positions of the input ports  412 - 1  . . .  412 -M of the M:N optical grating demultiplexor  410 , one can select which wavelength channels are directed to which one of the output ports  416 - 1  . . .  416 -N. The positions of the input ports are selected based on a grating equation of an optical grating used in the M:N optical grating demultiplexor  410 . The grating equations of some commonly used optical gratings are given further below. 
     The WDM coupler  202 A or  402  can use any type of a wavelength selective filter such a dichroic (thin film) optical filter, for example. The WDM couplers  202 A and  402  and the interleaver  202 B can be replaced with any other type of a wavelength selective coupler for separating wavelength channels received at the input port  204  into at least two groups of (not necessarily adjacent) wavelength channels. The optical interleaver  202 B preferably includes at least one Mach-Zehnder (MZ) interferometer. Two serially coupled MZ interferometers forming a lattice filter are further preferable. The optical grating demultiplexors  210 ,  211 , and  410  can include an arrayed waveguide grating (AWG), a bulk Echelle grating, a slab Echelle grating, or a bulk diffraction grating. 
     Referring to  FIG. 5 , an optical device  500  of the invention includes serially coupled a 1×2 wavelength selective coupler  502  and an AWG demultiplexor  510  having an input slab section  521 , a waveguide section  522  coupled to the input slab section  521 , an output slab section  523  coupled to the waveguide section  522 , two input waveguides  512  and  514  coupled to the input slab section  521 , and a plurality of output waveguides  516  to  519  coupled to the output slab sections  523 . The AWG demultiplexor  510  is preferably based on an athermal AWG using any athermal AWG types known to a person skilled in the art. The wavelength selective coupler  502  is preferably waveguide based, so it can be integrated on the same waveguide substrate as the AWG demultiplexor  510 . 
     The principle of adjustability of which wavelength channel is directed to which output port (depending on the input port position) will now be explained. The relative position of the input ports  212  and  214  of the optical grating demultiplexor  210 ; the relative position of the input ports  212  and  220  of the optical grating demultiplexor  211 ; the relative position of the input ports  412 - 1  . . .  412 -M of the M:N optical grating demultiplexor  410 ; and the relative position of the input ports  512  and  514  of the arrayed waveguide grating demultiplexor  510  is defined by a grating equation of a particular optical grating used in these devices. The grating equations of various optical gratings are known to one of ordinary skill in the art. The grating equation of an arrayed waveguide grating, for example, is
 
 n   s (λ) p  sin(θ in )+ n   s (λ) p  sin(θout)+ n   w (λ)Δ L=mλ   (1),
 
     wherein n s (λ) is a refractive index of the slab sections  521  and  523 , n w (λ) is a refractive index of the waveguide section  522 , θ in  is an input beam angle of an optical beam emitted by the input waveguide  512  or the input waveguide  514 , θ out  is an output beam angle of an optical beam coupled into the output waveguides  516  to  519 , ΔL is an optical path difference between neighboring waveguides of the waveguide section  522 , p is a waveguide spacing of the waveguide section  522 , and m is an order of diffraction. According to the grating equation (1), by selecting proper angles θ in , which depends on a position of an input waveguide, different wavelength channels can be coupled into a same output waveguide in a different orders of diffraction m or even in a same order of diffraction m. 
     The grating equation of a free-space diffraction grating is similar to Equation (1) above:
 
 nd (sin θ in +sin θ out )= mλ   (2),
 
     wherein n is refractive index of a medium the diffraction grating is in, and d is a groove spacing of the diffraction grating. By properly selecting the input beam angles θ in , one can couple different wavelength channels into a same output port. The input beam angles θ in  and the output beam angles θ out  depend on position of the input and output ports of the free-space diffraction grating and on a focal length of a lens or lenses used to collimate the input and the output beams. These free space lenses correspond to the input and the output slabs  521  and  523  of the arrayed waveguide grating demultiplexor  510  of  FIG. 5 . 
     In the optical grating demultiplexors  210 ,  211 , and  410 , the input ports  212 ,  214 ,  220 , and  412 - 1  to  412 -M can be disposed so that different wavelength channels can be directed to a same output port by diffracting into different orders of diffraction. This provides for a freedom to space the input ports apart by enough of a distance to prevent crosstalk, for example. Furthermore, according to the present invention and the Equations (1) and (2) above, the input ports  212 ,  214 ,  220 , and  412 - 1  to  412 -M can also be disposed so that different wavelength channels are directed to a same output port by diffracting into a same order of diffraction m. This provides an important design benefit because the optical grating demultiplexors  210 ,  211 , and  410  do not need to be optimized for operation in different orders of diffraction, which allows one to achieve a better optical performance in a single order of diffraction m. 
     Turning now to  FIG. 6 , an optical network  600  of the invention includes nodes  602  and  604  coupled by a length of an optical fiber  606 . Each of the nodes includes the optical device  200 A of the invention, a plurality of duplex filters  612  coupled to the output ports  216  to  219  of the optical grating demultiplexors  210 , for separating wavelength channels present at the output ports  216  to  219 , a plurality of receivers  620  each coupled to a particular one of the duplex filters  612 , and a plurality of transmitters  630  each coupled to a particular one of the duplex filters  612 . As seen in  FIG. 6 , the wavelength channels λ 5  to λ 8  are transmission wavelength channels for the node  602  and are accordingly reception wavelength channels for the node  604 . The wavelength channels λ 1  to λ 4  are reception wavelength channels for the node  602  and are transmission wavelength channels for the node  604 . Of course, the wavelength selective coupler  502 , the interleaver  202 B, or the 1×M wavelength selective splitter  402  can be used in place of the wavelength division multiplexor  202 A, and the AWG demultiplexor  510 , the optical grating demultiplexor  211 , or the M×N optical grating demultiplexor  410  can be used in place of the optical grating demultiplexor  210 . The transmitters  630  are preferably laser diodes, although light emitting diodes (LEDs) can also be used. The receivers  620  are preferably PIN or avalanche photodiodes.