Abstract:
An optical wavelength multiplexing device for use in an optical networking environment is presented. The device features optically dispersive elements combined with an array of reflectors that permit both realization of flat-top filter characteristics together with noise reduction, and in some embodiments wavelength routing, blocking or add-drop functionality. Additionally, the design of the device is easily modified to dissipate light having a wavelength that does not correspond to any predetermined wavelength channel.

Description:
FIELD OF THE INVENTION 
   The invention relates to the design of integrated wavelength (de)multiplexers and more specifically to integrated wavelength management devices having flat-top passband behavior employing selective retro-reflection in combination with optically dispersive elements. Such management devices include, but are not limited to, programmable demultiplexers, reconfigurable add-drop multiplexers, and s optical interleavers. 
   BACKGROUND OF THE INVENTION 
   Planar waveguide grating devices have been widely proposed and implemented within wavelength multiplexing/demultiplexing, routing, and optical add-drop applications for dense wavelength-division multiplexing (DWDM) transmission in advanced optical networks. Commonly, this is accomplished with either an arrayed waveguide grating (AWG), or an etched reflecting or transmissive diffraction grating, such as an echelle grating. Such planar devices have gained widespread market acceptance by leveraging semiconductor manufacturing&#39;s cost reductions to optical components that traditionally were assembled from multiple discrete elements with very high labor content. When the device performs a demultiplexing function, multiple signal channels of different wavelengths which are transmitted in an optical fiber are launched into an input waveguide of the device, are exposed to a dispersive element which separates them according to their wavelength, and then each signal channel is directed to a predetermined output waveguide of a plurality of output waveguides. A typical spectral response of such a device is shown in FIG.  1 . One of the most desired features of such devices is a spectral response having a wide and flat response within a passband of each signal channel. This feature allows system designers several benefits, including higher modulation frequency or data rate of the transmitted network signals whilst minimizing distortions onto the signals from the elements themselves. Further, a spectral response graph having a flat wide portion throughout the passband is indicative of a device with a large tolerance to wavelength drift of an input signal received at the input waveguide and one that is tolerant to passband wavelength drift of the device resulting from, for example, temperature variations, aging or manufacturing offsets. Also, a spectral response as indicated above reduces the effect of polarization dispersion resulted from the planar waveguide geometry. Moreover, a device having such a flat and wide passband is particularly important in DWDM networks where multiple filters are cascaded and the cumulative passband is much narrower than that of a single stage filter. 
   It is also highly desirable that the transmission coefficient drops sharply at the edges of the passband within the spectral response so that adjacent channels can be closely spaced without causing unacceptable crosstalk. Evidently, a sharper change in transmission coefficient also results in signals within the passband being passed with approximately equivalent attenuation, thereby, rendering the entire passband similar in response. 
   In a planar waveguide demultiplexing device, the shape of the spectral response is determined by a convolution of the amplitude distribution at the output focal plane, which is an image of the input waveguide mode profile formed by the dispersive grating, with the mode profile of the output waveguide itself. The channel spectral response is approximately Gaussian shaped when single-mode waveguides are used for both input waveguide and output waveguide in a conventional device. The passband is narrow, the passband top is not flat and the transition is slow resulting in relatively high crosstalk on adjacent channels unless very wide spacings are used, thereby limiting the number of channels within the system. 
   Many improved designs have been proposed to flatten and widen the passband spectral response. However, they all have limitations and drawbacks. 
   In U.S. Pat. No. 6,381,052, Delisle et al, propose a variety of different devices for providing flat top spectral response. The design of these devices includes a first wavelength dispersive element, an inversion means and second wavelength dispersive element. Unfortunately, using present manufacturing technology, this type of device is not easily produced in a maimer that is commercially viable. 
   SUMMARY OF INVENTION 
   The invention describes a flat-top optical multiplexing device comprising: an input port for receiving an optical signal; 
   a first wavelength dispersive element for separating a multiplexed optical signal into a plurality of optical signals, each of said optical signals corresponding to a predetermined wavelength channel, said first wavelength dispersive element in optical communication with the input port; and, 
   an at least an optical element for reflecting said at least one of said optical signals; and, 
   a second wavelength dispersive element for receiving the reflected optical signals from the at least an optical element, wavelength multiplexing the reflected optical signals to provide a multiplexed reflected optical signal and providing the multiplexed reflected optical signal, and 
   at least an output port optically coupled to said second wavelength dispersive element, the output port for propagating the multiplexed reflected optical signal. 
   Further, the invention teaches a method of manipulating a wavelength multiplexed optical signal to produce a flat-top response, comprising: 
   providing a first wavelength multiplexed optical signal at an input port; 
   dispersing said first wavelength multiplexed optical signal in dependence upon wavelength; 
   providing a plurality of optical signals incident an array of mirrors, said plurality of optical signals having a first characteristic spatial separation, said plurality of optical signals corresponding to predetermined wavelength channels; 
   reflecting said plurality of optical signals using said array of mirrors; 
   providing a second plurality of optical signals from the array of mirrors, said second plurality of optical signals having a second characteristic spatial separation, said second characteristic spatial separation being less than said first characteristic spatial separation. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will now be described with reference to the figures in which: 
       FIG. 1  is a wavelength profile featuring a first output signal from a flat-top demultiplexer and a second optical signal having a same center wavelength provided by a conventional Gaussian demultiplexer; 
       FIG. 2  is a top view of a prior art, Delisle et al, flat top wavelength dispersive element having a flat top output characteristic; 
       FIG. 3  is a top view diagram representing an optical ray diagram schematic that acts as a simple reflector; 
       FIG. 4  is a top view diagram representing an optical ray diagram schematic of a trivial embodiment of the invention that acts as a continuous waveguide; 
       FIG. 5  is a top view diagram representing an optical ray diagram schematic of an embodiment of the invention, similar to  FIG. 4 , with a multi-facetted mirror disposed within the optical path; 
       FIG. 6  is a top view diagram representing an optical ray diagram schematic demonstrating a set of optical paths provided by an embodiment of the invention in the form of a saw-like mirror which redirects the optical rays, changes the physical separation between the rays incident on the adjacent teeth of the mirror and filters unsupported wavelengths; 
       FIG. 7  is a top view of a wavelength demultiplexer according to the invention, with cascaded grating based DWDM elements and multi-facetted reflecting surface; 
       FIG. 8  is a top view of a wavelength switch according to the invention; 
       FIG. 9  is a top view of a multi-wavelength filter according to the invention; 
       FIG. 10  is a switch device according to the invention for use with an add-drop node in which individual wavelength channels are accessible from output ports of a common substrate; 
       FIG. 11  is a switch device according to the invention for use with an add-drop node in which the optical signals are wavelength multiplexed proximate a common output port; 
       FIG. 12  is a diagram of an optical interleaver incorporating a multi-facetted mirror according to the invention; and, 
       FIG. 13  is a ray diagram associated with an optical interleaver designed according to the invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring to  FIG. 1 , a pair of wavelength profiles are shown. A vertical axis  110  denotes optical intensity while the horizontal axis  111  corresponds to the wavelength of an optical signal. A first profile  101  is consistent with a prior art wavelength demultiplexer, such as a conventional arrayed waveguide grating (AWG). As can be seen, the attenuation of an optical signal according to this profile is very sensitive to variations in the wavelength of the optical signal. A second profile  102  is shown representing the system designers requirements for a flat-top profile. In this case variations in wavelength of the transmitted signal result in significantly less variation in attenuation within a predetermined wavelength range, the clear passband as this is referred to by designers, where said wavelength range is defined between λ 1  and λ 2 . Also, the steeper slopes on either side of the flat-top profile  102  inhibit cross talk with adjacent wavelength channels, as is well known to a person of skill in the art of multi-wavelength telecommunications. 
   Since a flat top device provides a more consistent insertion loss within the channel a system built around these types of devices is able to tolerate variations in wavelengths of different sources more easily than a system using conventional components. Indeed, as more wavelength channels are multiplexed on a same optical fibre it is necessary to ensure that lasers providing the optical signals have very accurate wavelengths characteristics. Consequently, these lasers are quite costly and harder to produce. Using a flat top device permits the use of less expensive more readily available lasers, and other source devices, such as wavelength converters. It also allows the same optical system to transmit signals at higher datarates, as the equivalent spectrum of these signals is broader and covers a wider portion of the optical spectrum. 
   Referring to  FIG. 2 , a top view diagram of a prior art flat top demultiplexer is shown. This device is consistent with the invention of U.S. Pat. No. 6,381,052 by Delisle et al. issued 30 Apr. 2002. This device, based upon arrayed waveguide grating concepts (AWG), features an input waveguide  200 , a first waveguide array  201 , an inversion means  202 , a second waveguide array  203  and output waveguides  204 . In this prior art device an optical signal propagates within the input waveguide  200 , is expanded and addresses the first waveguide array  201 . The wavelengths are spatially dispersed when recombined after propagating within this first waveguide array  201 . The optical signal is then inverted by the inversion means  202 . The signal then propagates within the second waveguide array  203  and is provided at the output waveguides  204 . In this prior art the inversion means  202  applies the inversion to only a narrow central portion of each channel. In this case, inverting a portion of the optical signal and subtracting this fraction of the intensity of the optical signal from the original optical signal reduces the intensity of the optical signal at the center of the supported wavelength channels. While this technique does provide flat top behaviour it is unclear how it should be implemented. Indeed, while an inversion means is specified within the prior art it is not specifically described. Conventional inversion means are fairly sophisticated and therefore, producing this type of device would likely be fairly difficult and quite expensive. Obviously, additional loss must be introduced since the light should pass several extra interfaces in the optical elements implementing spectral inversion. 
   Referring to  FIG. 3 , the equivalent of a simple curved mirror device according to a prior art is shown in order to explain the basic principles of the invention using simple ray diagrams. Light incident to a dispersive element  301  such as an AWG, echelle grating or other dispersive element is directed towards different regions of a spherical mirror  302 . As is well understood by a person of skill in the art, the wavelength dispersion element  301  separates light in dependence upon wavelength. Thus, light having a first wavelength is directed to a first position on the spherical mirror  302  and light having a second other wavelength is directed to a second position on the spherical mirror  302 . The spherical mirror  302  reflects all wavelengths equally back towards the wavelength dispersion element  301 . After a second pass through the dispersion element  301  the light is returned along a path from which it propagated. The performance in this case is trivial, and such a system behaves as a simple mirror. 
   Referring to  FIG. 4  a schematic according to a prior art is shown having a more complex system compared to  FIG. 3 , but again for explanation of the inventions operating principle. Light propagating along a first waveguide  400  is dispersed in dependence upon wavelength by the wavelength dispersion element  401 . Optical signals propagating from the wavelength dispersion element  401  propagate to the spherical mirror  402 . The spherical mirror  402 , now orientated with it radius of axis with the dispersion element  401 , directs the optical signals onto a second dispersion element  403 ,.The second dispersion element  403  receives the various optical signals. The second dispersion element  403  shown is functionally similar to the first dispersion element. The overall performance of this device is equivalent to a straight-through connection. 
   In order to provide non-trivial optical performance the mirror in  FIG. 4  should be changed as it is demonstrated in FIG.  5 . 
   Referring to  FIG. 5  an embodiment of the invention is shown featuring a segmented, or saw-like, mirror  502 . In this embodiment of the invention, light propagates to a first wavelength dispersion element  501  and is dispersed in dependence upon wavelength. The dispersed light propagates to the segmented mirror  502 . The segmented mirror reflects light incident thereon. Due to the segmented mirrors surfaces being reflective in different directions as we move spatially across it the segmented mirror causes some of the optical signals to propagate towards the second wavelength dispersive element  503 , and others to be directed away from the second dispersive element. Due to the geometry of the reflecting surfaces of the mirror, the reflected light incident the second wavelength dispersive element  503  is not combined into a single output optical beam. Instead the various optical beams propagating from the second wavelength dispersive element  503  are provided at slightly different angles or exit points in dependence upon their wavelength. 
   Referring to  FIG. 6  an operation of the system of  FIG. 5  is illustrated in more detail. The system includes a first wavelength dispersion element  601 , a segmented, or saw-like, mirror  602 , and a second wavelength dispersion element  603 . In use, an optical signal is provided to the first wavelength dispersion element  601 . The optical signal is dispersed in dependence upon wavelength and propagates to the segmented mirror  602 . As is well known to those of skill in the art, optical communications components supporting multiple wavelengths typically support wavelengths corresponding to an industry standard known as the ITU grid. The ITU grid provides a wavelength range for each supported channel. In order to reduce the effects of wavelength dispersion the ITU grid includes unsupported wavelength ranges between the channels. Optical signals if present within these unsupported wavelength ranges are likely to introduce undesired noise into the system. Referring again to  FIG. 6 , the optical signals propagating from the first wavelength dispersion element  601  are shown by parallel rays  606   a  to  606   f . The rays  606   a  to  606   f  are shown as being parallel. This has been done for illustrative purposes only. The rays  606   a  to  606   f  are incident on the segmented mirror  602 . For illustrative purposes, the rays  606   a  to  606   f  are projected through the segmented mirror  602  and shown on a panel  604 . A first region  605   a  corresponds to a first supported channel, a second region  605   b  corresponds to a second supported channel. The distance  609   a  corresponds to an unused region of between the channels corresponding to regions  605   a  and  605   b . The segmented mirror is disposed to ensure that the optical signals corresponding to the first region  605   a  and the second region  605   b  are reflected to the second wavelength dispersion element  603 . The distance  609   b  corresponds to the distance  609   a  however after reflection the optical signals have been shifted closer together and therefore  609   b  is substantially less than  609   a . This abrupt reduction in spacing between channels corresponds to a discontinuous change in the direction of the multiplexed optical signal and leads to an improved passband flattened performance of the device. In this way, a device according to this embodiment of the invention achieves the optical performance described by Delisle et al. without an inversion means. Indeed, the fabrication of optical gratings and reflective facets are very well known in the art and relatively simple to manufacture. Further, the segmented mirror  602  acts to filter out light incident on the segmented mirror in a region that does not correspond to a supported wavelength channel. Thus, an optical signal having a wavelength corresponding to position  608  on the panel  604  would not be reflected to the second wavelength dispersion element  603 . Optical signals present between adjacent supported wavelength channels are optionally diverted from optical signals corresponding to the supported wavelength channels. Thus, the device as described with reference to  FIG. 6  acts as a wavelength filter for a wavelength multiplexed optical signal. In fact the transition can be extremely abrupt according to the implementation of the segmented mirror. 
   Referring to  FIG. 7 , a top view of a first embodiment of the invention is shown. This embodiment features an input waveguide  700 , an input slab  701 , a first array of waveguides  702  and an intermediate slab region  703 , a mirror array  704 , a second array of waveguides  705 , an output slab  706  and an output waveguide  707 . This embodiment is useable as a filter for reducing the optical intensity of any optical noise present between predetermined wavelength channels. In operation, an optical signal propagates within the input waveguide  700  and through the input slab  701  into the first array of waveguides  702 . The optical signal is then provided to the intermediate slab region  703  where it interacts with the mirror array  704 . The mirror array  704  reflects only those portions of the optical signal that correspond to a predetermined set of wavelength channels. Thus, optical signals that do not correspond to the predetermined wavelength channels are attenuated. The remaining optical signals are reflected from the mirror array  704  and propagate within the intermediate slab region  703 . These optical signals are then coupled to the second array of waveguides  705  and then to an output slab  706 . The optical signals then propagate within the output waveguide  707 . In this way, unwanted optical noise is filtered from the multiplexed optical signal without a costly optical to electrical and back to optical (OEO) conversion. In order to provide a mirror array  704  having a high reflectivity it is suggested that the mirrors support total internal reflection of the optical signals incident thereon. Additionally, it is suggested that the mirror array  704  be produced using a chemical etching process incorporating a photolithographical mask. 
   Referring to  FIG. 8 , a second embodiment of the invention is shown featuring, an input waveguide  800 , an input slab  801 , a first array of waveguides  802 , an intermediate slab  803 , an array of adjustable mirrors  804 , a second array of waveguides  805 , a first output slab  806 , a first set of output ports  807 , a third array of waveguides  808 , a second output slab  809  and a second set of output ports  810 . In this embodiment, a wavelength multiplexed optical signal is provided to the input waveguide  800  and propagates to the input slab  801  where it is coupled to the first array of waveguides  802 . The wavelength multiplexed optical signal propagates within the intermediate slab region  803  and focuses on the array of adjustable mirrors  804 . The device is designed such that the optical signals corresponding to predetermined wavelength channels are provided to specific mirrors of the array of adjustable mirrors  804 . Optical signals corresponding to gaps between the predetermined wavelength channels are attenuated as they propagate past the mirror elements  804  and are dissipated. The array of adjustable mirrors is controlled to redirect the optical signals incident on the mirrors to either of the second and third arrays of waveguides  805  and  808 . Optical signals provided to the second array of waveguides  805  propagate within first output slab  806  to the first output port  807 . Alternatively, the optical signals with “switched” mirrors are provided to the third array of waveguides  808 . Optical signals provided to the third array of waveguides  808  propagate within the second output slab  809  to the second port  810 . Thus, this embodiment of the invention permits the switching of individual channels while simultaneously reducing unwanted noise between the predetermined channels. Although the direction of the optical signal is clearly specified as propagating from the input waveguide  800  to the array of adjustable mirrors  804  this need not be the case as this device is optionally using in a fully bi-directional manner. 
   Referring to  FIG. 9 , a third embodiment of the invention is shown. The function and description of elements shown in  FIG. 9  are equivalent to elements of other figures having the same reference numerals. An intermediate waveguide  907  receives a multiplexed optical signal from the output slab  706 . The multiplexed optical signal propagates to the demultiplexer input slab  908 . Portions of the multiplexed optical signal then propagate along the demultiplexer array of waveguides  909  and are received by the demultiplexer output slab  910 . A set of optical signals is provided to the array of output waveguides  911 . The geometry and physical properties of the structure are chose such that each of the optical signals present in array of output waveguides  911  corresponds to a predetermined wavelength channel. Thus, this embodiment of the invention receives an multiplexed optical signal, filters the signal and then separates the filtered multiplexed optical signal into a set of optical signals each optical signal of the set of optical signals corresponding to a specific predetermined wavelength channel. 
   Referring to  FIG. 10 , a top view of a fourth embodiment of the invention is shown. This embodiment includes a first waveguide  1001 , a second waveguide  1002  and a third waveguide  1003 , a first circulator  1004 , a waveguide substrate  1005  with an input port  1006 , a first slab region  1007 , a first waveguide array  1008 , an intermediate slab region  1009 , an array of adjustable mirrors  1010 , a second waveguide array  1011 , a second slab region  1012 , a plurality of output ports  1013 , a fourth waveguide  1014 , a fifth waveguide  1015 , a sixth waveguide  1016  and a second circulator  1017 . In operation, an optical signal propagating within the first waveguide  1001  is coupled to the second waveguide  1002  through the first circulator  1004 . The optical signal propagates along the second waveguide  1002  and enters the waveguide substrate  1005  at the input port  1006 . From the input port  1006  the optical signal propagates through the first slab region  1007  and into the first array of waveguides  1008 . The optical signal then propagates though an intermediate slab region  1009  and is demultiplexed into a set of channel signals. Each of the channel signals is reflected by the array of adjustable mirrors  1010 . In this embodiment of the invention, the optical signal is a wavelength multiplexed optical signal supporting a plurality of channel signals, each having a characteristic wavelength range. Each characteristic wavelength range corresponds to a predetermined wavelength channel. Optical noise present within the optical signal between the predetermined wavelength channels is incident on a region of array of adjustable mirrors  1010  that causes the noise to dissipate. Thus, the optical noise is substantially prevented from coupling into either the first array of waveguides  1008  or the second array of waveguide  1011 . 
   The mirrors of the array of adjustable mirrors  1010  are actuated to propagate individual channel signals corresponding to specific wavelength channels to either the first array of waveguides  1008  or the second array of waveguides  1011 . A channel signal directed from the array of adjustable mirrors  1010  to the first array of waveguides  1008  will propagate within the first array of waveguides  1008  and continue along an optical path through the input port  1006  along the second waveguide  1002  to the first circulator  1004  The first circulator  1004  then couples this channel signal to the third waveguide  1003 . Alternatively, when a channel signal is directed from the array of adjustable mirrors  1010  to the second array of waveguides  1011 , the channel signal then propagates through this second array of waveguides  1011  and into the second slab region  1012 . The optical signals provided to the second array of waveguides  1011  are multiplexed to create a multiplexed output signal. This multiplexed output signal is provided to and propagates along waveguide  1018 . In order to provide optical signal corresponding to predetermined individual channels, the multiplexed output signal is coupled to an arrayed waveguide grating having an AWG input slab  1019 , an AWG array of waveguides  1020  and an AWG output slab  1021 . An output waveguide  1014  couples one such optical signal corresponding to a predetermined individual channel, henceforth referred to as a channel signal to a second circulator  1017 . When the channel signal is received by the second circulator  1017 , it is coupled to the fifth waveguide  1015 . This channel signal is described as having been “dropped”. A second channel signal corresponding to a same wavelength channel as the aforementioned channel signal propagating towards the second circulator  1017  along the sixth waveguide  1016  will be coupled into the fourth waveguide  1014  by the second circulator  1017 . The second channel signal propagates along a path similar to the optical path that the first channel signal propagated along, however as the direction of propagation of the second channel signal is opposite to that the first channel signal, the second channel signal with interact with the first circulator in a different manner. Specifically, the second channel signal propagates along the second waveguide  1002  and is received by the first circulator  1004 . The second channel signal is then provided to, and propagates along, the third waveguide  1003 . This second channel signal described as having been “added”. The terminology of “adding” and “dropping” channel signals will be well known to one of skill in the art of add/drop demultiplexing modules. 
   Advantageously, the device according to this embodiment also serves to reduce optical noise between the supported wavelength channels. Although only one output port is shown as being coupled to other optical equipment for adding and dropping channel signals this need not be the case. Clearly, each of the output ports  1013  is optionally coupled to a set of optical components analogous to the fourth waveguide  1014 , the fifth waveguide  1015 , the sixth waveguide  1016  and the second circulator  1017 . Since each of the output ports  1013  is optionally coupled to an external optical circuit it is a simple matter to add additional waveguides and circulators as desired to support any available channels. A variety of different options are available to actuate the adjustable mirrors  1010 . For example, the mirrors optionally incorporate a micro-electro-mechanical system (MEMS) actuator. 
   Clearly, a person of skill in the art will easily envision a wide variety of variations to the embodiment of FIG.  10 . For example, rather than providing a plurality of channel signals at the various output ports  1013 , the plurality of channel signals is optionally wavelength multiplexed and provided at a single common output port  1100  as described in FIG.  11 . In this configuration, other equipment  1101  in the form of conventional arrayed waveguide gratings is used to separate the individual channel signals and optionally provide other channel signals. Thus, in this configuration the number of circulators is reduced in comparison to the embodiment of  FIG. 6  in which many optical wavelength channels are optionally switched. 
   Referring to  FIG. 12  a ray diagram representative of the optical performance of an embodiment of the invention is shown. The diagram shows a set of rays  1  to  6 , and a segmented mirror  7 . For simplicity, the rays  1  to  6  are shown as being parallel. The facets of the segmented mirror  7  are disposed to reflect the even numbered rays  2 ,  4  and  6  is a first direction and to reflect the odd numbered rays  1 ,  3  and  5  in a second, other direction. A person of skill in the art will be aware that the optical behavior of this device is beneficial for providing an optical interleaver. 
   Alternatively, as shown in  FIG. 13 , a ray diagram corresponding to a design of an optical interleaver according to the invention is shown. The interleaver directs optical signals incident a segmented mirror  1309  to one of four output slabs. In this embodiment of the invention there are four sets of facets in the segmented mirror  1309 . A set of facets is for directing an optical signal provided from a predetermined input port to a specific output slab. Thus, rays  1301  and  1305  are directed to a first port, rays  1302  and  1306  propagate to a second port, rays  1303  and  1307  propagate to a third port and rays  1304  and  1308  propagate to a fourth port. Hence 25 GHz channels are generated and separated from 4 streams of 100 GHz channels. Clearly, other such variations will be apparent to one of skill in the art. 
   It will be apparent to one of skill in the art that the segmented, saw-like mirror structure is provides flat channel shapes. These channel shapes are highly beneficial. Numerous other embodiments of the invention may be envisioned by one of skill in the art of optical communications and arrayed waveguide devices.