Abstract:
Apparatus for filtering an optical input signal includes a coupler and an arrayed waveguide grating connected to the coupler having a phase shifting region. The phase shifting region is a back wall of a free space region of the arrayed waveguide grating with concentric image and reflector surfaces. Radial waveguides between the surfaces are divided into alternating regions of wider and thinner sets of waveguides. A corresponding change in optical length alters the phase of the input signal which forces an output signal of a desired wavelength to appear at a desired port of the filter.

Description:
FIELD OF INVENTION 
   The invention relates generally to the field of optical communications and, more specifically, to an apparatus for slicing or interleaving individual wavelength signals with improved passband transfer function characteristics. 
   BACKGROUND OF INVENTION 
   Channel adding/dropping filters in high quality optical networks form a key functional element in dense wavelength-division multiplexed (DWDM) optical fiber networks. They are used to either separate (slice) or multiplex (interleave) signals that are on equally spaced apart channels (wavelengths). For example, a typical 1×2 coupler is connected such that a single input port receives a DWDM signal and two output ports provide the sliced DWDM signal into odd and evenly spaced channels respectively for further slicing and/or processing. Such a coupler may also have its connections reversed so as to perform a multiplexing operation. Unfortunately, such devices do not have a constant gain transfer function in the passbands when functioning as a slicer. That is, as the individual wavelengths are separated from the incoming signal, they lose signal quality because the optimal rectangular response desired in such device is not realized. One attempt to improve this condition is to pass the incoming DWDM signal through multiple stages of Mach-Zhender devices to achieve a flatter and more rectangular response. This approach is not always effective because of the inherently varied manufacturing tolerances of each devices and is generally inopposite to the concept of having integrated optical techniques for signal processing. 
   SUMMARY OF THE INVENTION 
   These and other deficiencies of the prior art are addressed by the present invention of an apparatus for filtering an optical input signal. The apparatus is a filter having at least one coupler and at least one arrayed waveguide grating connected to the coupler and having a phase shifting region. In one embodiment, the coupler is a single 2×2 coupler having a first input port, a second input port, a first output port and a second output port. The first output port is connected to a filter lower arm and a reflector and the second output port is connected to a filter upper arm and the arrayed waveguide grating. The phase shifting region is a back wall of a free space region of the arrayed waveguide grating and includes an image surface and a reflector surface arranged concentrically. An array formed by a plurality of radial waveguides is located between an image surface and a reflector surface. The array is divided into alternating regions or segments of a first set radial waveguides and a second set of radial waveguides. The first set of radial waveguides has greater optical length than the second set of radial waveguides. In an alternate embodiment, the back wall of the free space region has a curved image surface and a flat reflector surface spaced apart from each other with a plurality of curved waveguides between image surface and a reflector surface. 
   In a second embodiment of the invention, the filter includes a first 2×2 coupler and a second 2×2 coupler. A first arrayed waveguide grating and a second arrayed waveguide grating are connected to each other between an output port of the first 2×2 coupler and an input port of the second 2×2 coupler. The first arrayed waveguide grating and the second arrayed waveguide grating are connected to each other via a waveguide array. The array is made up of alternating regions of varying optical length of the individual waveguides. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The teachings of the present invention can be readily understood by considering the following detailed description in conjunction with the accompanying drawings, in which: 
       FIG. 1  depicts a high level block diagram of an optical add-drop filter according to an embodiment of the present invention; 
       FIG. 2  depicts a detailed view of one embodiment of a back wall portion of the filter of  FIG. 1 ; 
       FIG. 3  depicts a detailed view of a second embodiment of a back wall portion of the filter of  FIG. 1 ; and 
       FIG. 4  depicts a high level block diagram of an alternate embodiment of the filter of the present invention. 
   

   To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures. 
   DETAILED DESCRIPTION OF THE INVENTION 
   The subject invention will be primarily described within the context of an optical add-drop filter which may be used in wavelength-division multiplexed (WDM) and dense WDM (DWDM) optical communications systems. However, it will be appreciated by those skilled in the art that the invention may be advantageously employed in any optical communications system in which it is desirable to maintain a substantially rectangular transfer function in the relevant passbands. 
     FIG. 1  depicts a high level block diagram of an optical add-drop filter according to an embodiment of the present invention. Specifically, the filter  100  of  FIG. 1  comprises a coupler  102 , a waveguide grating router  108 , a reflector  120  and a plurality of circulators  114  and  115 . The filter  100  receives an input signal Ps, illustratively a WDM or DWDM optical signal comprising N optical signals having respective wavelengths λ 1 -λ N  and transported via an optical fiber. The filter  100  responsively produces a corresponding output signal Pout at a specific output port of the coupler  102  (depending upon the phase characteristics as explained below). The filter  100  operates to controllably pass through, insert (add) and/or extract (drop) optical communications signals having specified wavelengths to implement thereby the known add-drop function. 
   Specifically, the input optical signal Ps is applied to one of a first input port  121  or a second input port  122  of the coupler  102 .  FIG. 1  depicts the optical input signal Ps being applied to the first input port  121  via first circulator  114 . The coupler  102  has two output ports, each of which are connected to an arm. For example, a first output port  126  is connected to a lower arm  104  and a second output port  128  is connected to an upper arm  106 . The lower arm  104  is connected to a reflector  120 . The upper arm  106  consists of a waveguide grating router  108  combined with a reflector arrangement  124 . Both the lower arm  104  and the upper arm  106  are approximately of the same optical length; however, they are designed to have different reflection coefficient properties. Particularly in one embodiment, the lower arm  104  has a reflection coefficient of ρ and the upper arm  106  has a reflection coefficient of ±ρ (where ±ρ varies periodically as a function of the wavelength of the input signal Ps). Lastly, circulators  115  and  114  respectively are provided at each of the first input port  121  and the second input port  122 . The circulators  114  and  115  permit the input signal Ps to be applied to the filter  100  and have a reflected output signal also appear at the same input port, yet do not interfere with one another. That is, the circulator passes the reflected output signal Pout to a separate branch of the circulator than the applied signal Ps. 
   In operation, the input signal Ps is applied to either of the input ports  121  or  122  of the coupler  102 . The coupler  102  then splits the input signal Ps into two substantially equal component signals Ps/2 which are respectively provided to the upper arm  106  and lower arm  104 . Each respective component signal Ps/2 is then reflected back with substantially equal magnitude in its respective arm  104 ,  106 . The filter  100  is so configured that when the reflected component signals are in phase, the resultant output signal Pout is provided to the input port  121  or  122  to which the input signal Ps was applied and when the reflected component signals are out of phase, the resultant output signal Pout is provided to the input port to which the input signal Ps was not applied. Regardless of where the output signal is seen, such port displays a substantially rectangular transfer function characteristic. 
   The configuration of the waveguide grating router (WGR)  108  is directly linked to the sign duality of the reflection coefficient (±ρ) and the resultant output signal Pout routing to the first or second input ports  121  and  122 . Specifically, the waveguide grating router  108  comprises a first free space region  110 , a second free space region  112  and a plurality of arrayed waveguides  130  disposed therebetween on the upper arm  106 . This arrangement transforms the component signal Ps/2 of the upper arm  106  into a set of output images I (a result of a series of convergent rays) produced on a back wall (or image circle)  124  of the second free space region  112 . The output images I are physically equally spaced apart by an arc distance Ω and their locations on the image circle  124  are approximately linear functions of the wavelength of the input signal Ps. By using a suitable reflector, all of the images I are reflected back with negligible loss. A suitable circular reflector producing a constant reflection along a circle can be realized by for example cutting and polishing a portion of a the wafer along a circle. 
   The back wall or image circle  124  of the second free space region  112  is not a single reflector device, but a dual reflector device. Specifically, the back wall  124  is a segmented arrangement made up of a plurality of first reflective regions  116  (denoting an original wavelength characteristic) and a plurality of second reflective regions  118  (denoting a phase-shifted wavelength characteristic). The first reflective regions  116  and second reflective regions  118  are interspersed segments of width Ω/2 along the back wall  124 . The second reflective regions  118  are configured to impart a predetermined phase shift π to the input signal Ps with respect to that imparted by the adjacent first reflective regions  116 . Accordingly, the reflection coefficient of the upper arm  106  is characterized by ±ρ, which is governed by the phase shift π between the first reflective regions  116  and the second reflective regions  118 . In other words, since the input signal Ps comprises various signal components (λ n ) of variable wavelengths, the resultant image I will linearly vary along the back wall  124  of the second free space region  112 . Dependant upon such wavelength, the image will either (1) appear in the first reflective region  116 , maintain its phase with respect to the input signal Ps and have a reflection coefficient of +ρ; or (2) appear in the second reflective region  118 , have its phase shifted by a value of π and have a reflection coefficient of −ρ. If the reflection coefficient in the upper arm is +ρ, the output signal Pout will appear at the first input port  121  via the circulator  114  (as this is the port that the input signal Ps was initially applied. If the reflection coefficient in the upper arm is −ρ, the output signal Pout will appear at the second input port  122  via second circulator  115 . 
   It should be noted that unavoidable losses in the waveguide grating of the upper arm  106  will cause the magnitude of the reflected signals to be appreciably less than unity. Therefore, a similar loss must be included in the lower arm  104 , for instance by forming the reflector  120  at a small angle with respect to the waveguide, or by including a loss in the waveguide  104 . Ideally, the magnitude of the reflected signals in both the upper arm  106  and the lower arm  104  should be substantially equal for optimum performance of the filter  100 . 
     FIG. 2  depicts a detailed view of one embodiment of a back wall formed at the image circle  124  of the second free space region  112  of the WGR  108 . Specifically, the back wall  124  is two concentric surfaces, namely, an image surface  202  and a reflector surface  204 . A radial array of waveguides  206  and  208  are disposed between the image surface  202  and the reflector surface  204 . That is, a proximal end  210  of the radial array of waveguides communicates with the image surface  202  and a distal end  212  of the radial array of waveguides communicates with the reflector surface  204 . A first set of the radial array of waveguides  206  corresponds to the first reflective region  116  and a second set of the radial array of waveguides  208  corresponds to the second reflective region  118 . 
   In one embodiment, all waveguides in the array are of equal physical length. Thus, some other feature must be altered so as to establish in the phase shift π in alternating regions. The phase shift π is realized by altering a width W+ of the distal ends of the radial array of waveguides in the first reflective region  116  in comparison to the width W− of the distal ends in the second reflective region  118  to establish a periodic array of radial waveguides. Optical theory provides, typically, that the wider a given waveguide is shaped, the longer the optical path for a signal propagating through such waveguide. Typically, W− must be small enough so that only a fraction of the signal power propagates inside waveguides in the second reflective region  118  whereas W+ must be large enough to cause a substantially larger fraction of the signal power to propagate within waveguides in the first reflective region  116  . For instance, W+ may be chosen to be twice W−, or wider. One skilled in the art will realize that the widths W+ and W− can be altered in any manner suitable to achieve the desired results. Notice, for any given W+ and W−, the appropriate phase shift can be realized by appropriately choosing the waveguide length in the end section. 
     FIG. 3  depicts a detailed view of a second embodiment of a back wall  124  of the filter  100 . Specifically, the back wall  124  is one concave surface (the image surface  202 ) and one flat reflector surface  302 . Instead of a radial array of physically equal length waveguides as in  FIG. 2 , an array of curved waveguides  306  and  308  are disposed between the image surface  202  and the flat reflector surface  302 . A proximal end  310  of the curved waveguides is at the image surface  202  and a distal end  312  of the curved waveguides is at the flat reflector surface  302 . As in the embodiment of  FIG. 2 , a first set of curved waveguides  306  corresponds to the first reflective region  116  and a second set of the curved waveguides  308  corresponds to the second reflective region  118 . In this case, by using curved waveguides, the appropriate phase shift π between adjacent regions (segments)  306  and  308  is realized by choosing different physical lengths for the waveguides of the two regions. As such, and to summarize, the phase shift π is created by changing the optical path length of adjacent regions  206 ,  208  or  306 ,  308 . The optical path length is altered by changing the physical width or physical length of the desired waveguide. A combination of altering length and width is also within the scope of the invention. 
     FIG. 4  depicts a high level block diagram of an alternate embodiment of the filter of the present invention. Specifically, a filter  400  that provides the same results as that of the two circulator filter  100  is shown, but without the use of circulators. The use of circulators is avoided in the filter  400  of  FIG. 4  by using two couplers  402 ,  404  and two gratings  412 ,  416  arranged symmetrically about an additional waveguide array  414 . This arrangement consists of two identical halfs separated by a symmetry line S. For example, by cutting the first half along the symmetry line S, and including a flat reflector along the cut, one obtains the reflective arrangement considered previously, in the arrangement of FIG.  3 . 
   First coupler  402  is arranged and performs essentially in the same manner as coupler  102  of  FIG. 1  by splitting the input signal Ps applied to a first coupler input port  406  into two equal component signals Ps/2 at first and second output ports  408  and  410  respectively. A lower arm  418  of the filter  400  passes one of the equal component signals Ps/2 to a first input port  420  of a second coupler  404 . This lower arm  418  has the same transmission coefficient characteristics ρ as the lower arm  104  of filter  100 . 
   An upper arm  420  of the filter  400  has a different construction and arrangement than that of filter  100 , but the two arrangements are equivalent. That is, the transmission coefficient of filter  400  has essentially the same behavior ±ρ as the reflection coefficient of the upper arm  108  of filter  100 . In detail, upper arm  420  has a first arrayed waveguide grating  412  connected to the first output port  408  of the first coupler  402 . The first arrayed waveguide grating  412  has a rear free space region  430  (similar to the second free space region  112  of filter  100 ) and a back wall  426  of the rear free space region  430 . A second arrayed waveguide grating  416  mirrors the first arrayed waveguide grating  412  and is connected to a second input port  422  of the second coupler  404 . The second arrayed waveguide grating  416  has a front free space region  432  with a front wall  428 . 
   A waveguide array  414  is disposed between the first arrayed waveguide grating  412  and second arrayed waveguide grating  416 . Specifically, the waveguide array  414  is connected to the back wall  426  and front wall  428 . This arrangement duplicates the physical conditions of the curved waveguide array of  FIG. 3  about either side of symmetry line S. That is, a series of images I are formed at a back wall  426  of the rear free space region  430  as a result of the applied component signal Ps/2. Certain ones of these images appear at a + transmissive region (denoted by + about symmetry line S) and certain ones of these images appear at a − transmissive region (denoted by − about symmetry line S) as they propagate through the waveguide array  414 . Each region is formed by a segmented array with different optical length in adjacent segments. Thus, all images produced at a particular wavelength recombine in phase with transmission coefficient ±ρ at the output  422  of the second arrayed waveguide grating  416 . Images having a +ρ transmissive coefficient on upper arm  420  will recombine with Ps/2 of the lower arm  418  and the resultant signal Ps will appear at first output port  434  of second coupler  404 . Similarly, for images having a −ρ transmission coefficient on upper arm  420 , the resultant signal Ps will appear at second output port  424  of second coupler  404 . Thus, the function of the filter  400  is the same as filter  100  without the need for circulators. As indicated above with respect to filter  100 , filter  400  also realizes a substantially rectangular transfer function and extremely low loss (theoretically close to zero) by virtue of the port characteristics of the couplers  402  and  404 . 
   A method of processing optical signals (either separating or multiplexing) is also realized by the subject invention. Specifically, an input signal is applied to an optical device to split the input signal into two substantially equal components. The component input signals are propagated through respective arms that are connected to the optical splitting device. A first arm has non-phase shifting characteristics and a second arm has phase-shifting characteristics so as to alter the phase of the component input signals relative to each other. As a result of propagating through the first arm, the component input signal applied to the first arm is imparted with a first reflection coefficient. As a result of propagating through the second arm, the component input signal applied to the second arm is imparted with a second reflection coefficient. The first reflection coefficient may be substantially the same as the second reflection coefficient or substantially opposite to the second reflection coefficient. The component signal imparted with the first reflection coefficient (in the first arm) and the component signal imparted with the second reflection coefficient (in the second arm) are received at an optical device for combining the component signals. If the component signal of the first arm is in phase with the component signal of the second arm, an output signal is provided at a first output port of the optical combining device. If the component signal of the first arm is out of phase with the component signal of the second arm, an output signal is provided at a second output port of the optical combining device. In the embodiment of  FIG. 1  it is understood that the first output port for the output signal is the branch of first circulator  114  and the second output port the output signal is the branch of second circulator  115 . 
   Although various embodiments that incorporate the teachings of the present invention have been shown and described in detail herein, those skilled in the art can readily devise many other varied embodiments that still incorporate these teachings.