Abstract:
An optical add/drop multiplexer may be formed using ring resonators. In some embodiments, ring resonators may be used instead of Bragg gratings in a Mach-Zehnder interferometer configuration. One or more wavelengths may be added or dropped or a band pass of wavelengths may be added or dropped in a wavelength division multiplexed system.

Description:
BACKGROUND 
   This invention relates generally to optical add/drop multiplexers (OADMS) that may be used in wavelength division multiplexed networks to either add a channel or to drop a channel from the network. 
   Conventionally, optical networks may consist of carriers that carry a large number of channels, each channel being of a different wavelength. At stations along the network, additional channels may be added or channels may be dropped. Typically, an optical add/drop multiplexer is used to either add or withdraw such channels. The most conventional form of OADM includes a Mach-Zehnder interferometer including Bragg gratings. 
   The Mach-Zehnder interferometer with photo-induced Bragg gratings is an attractive device as a wavelength-selective OADM circuit. As an example, a Mach-Zehnder interferometer-based fiber grating may include identical Bragg gratings photo-imprinted in the two arms of a Mach-Zehnder interferometer. The Bragg gratings act as distributed-feedback reflection mirrors. A wavelength division multiplexed signal launched into the designated input port of the Mach-Zehnder interferometer is split evenly by a first 3 deciBel (dB) coupler, provided that the interferometer includes two 3 dB couplers having the same coupling ratio and the same arm path lengths. 
   The wavelength division multiplexed signal, except the Bragg-resonant wavelength, propagates along each arm to the second 3 dB coupler, where the wavelength division multiplexed signal is coherently recombined to emerge from the output port. 
   The signal of the Bragg-resonant wavelength is reflected back by the Bragg gratings located symmetrically in the two arms. The reflected Bragg-resonant wavelength appears from the drop port rather than the input port, because of the double half-a-n (n/2) phase shift arising at the 3 dB coupler. Owing to the merit of the symmetrical structure of the device, another signal of the Bragg wavelength inserted from the add port can be guided to the output port. 
   One problem with Bragg gratings is that, in some cases, they involve the use of sophisticated ultraviolet interference patterns and phase grating masks. The generation of these devices may be complex and their tuning can sometimes be awkward. 
   Thus, there is a need for an optical add/drop multiplexer with improved characteristics. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic depiction of one embodiment of the present invention; 
       FIG. 2  is a partial, enlarged depiction of the embodiment shown in  FIG. 1 , focusing on the ring resonator in accordance with one embodiment of the present invention; 
       FIG. 3  is a greatly enlarged cross-sectional view taken generally along the line  3 — 3  in  FIG. 2 ; 
       FIG. 4  is a schematic depiction of another embodiment of the present invention; 
       FIG. 5  is a schematic depiction of another embodiment of the present invention; 
       FIG. 6  is a schematic depiction of another embodiment of the present invention; 
       FIG. 7  is a schematic depiction of another embodiment of the present invention; and 
       FIG. 8  is a schematic depiction of another embodiment of the present invention. 
   

   DETAILED DESCRIPTION 
   Referring to  FIG. 1 , an optical add/drop multiplexer  10  may use a symmetrical Mach-Zehnder interferometer  11 . The interferometer  11  includes an upper arm  28   a  and a lower arm  28   b . The upper arm  28   a  includes an input  12 , a heater  16 , and an add port  22 . The lower arm  28   b  includes a drop port  14 , a heater  18 , and an output port  24 . Between the upper and lower arms  28  is a ring resonator  20 . 
   In one embodiment of the present invention, the multiplexer  10  may be formed as a planar light circuit in a semiconductor substrate. The planar light circuit includes a substrate in which are formed the arms  28   a  and  28   b , the heaters  16  and  18 , the ports  12 ,  14 ,  22 , and  24 , and the ring resonator  20  in one embodiment. 
   Referring to  FIG. 2 , the ring resonator  20  may include a ring waveguide  26  formed in the substrate. The ring waveguide  26  may be positioned proximately to the upper arm  28   a  and the lower arm  28   b .    
   As shown in  FIG. 3 , the ring waveguide  26  may include an upper cladding layer  32 , a lower cladding layer  34 , and a substrate  36 . In one embodiment, the substrate  36  may be a silicon substrate, the cladding layers  32  and  34  may be made of silicon dioxide, and the core  30  may be formed of SiON. The ring waveguide  26  may be formed in the silicon substrate  36  using plasma-enhanced chemical vapor deposition. 
   Light is coupled between the straight portions of the arms  28  and the ring waveguide  26  by way of evanescent wave interaction. The resonator  20  has a transmissivity spectrum including multiple sharp resonance peaks as a function of wavelength reminiscent of a cone. 
   In the embodiment shown in  FIG. 1 , the Mach-Zehnder interferometer  11  is symmetrical and the heaters  16  and  18  are not used. The wavelength division multiplexed signal is launched into the input port  12 , including wavelengths from 0 through N. The wavelength division multiplexed signal is split evenly by the first 3 dB coupler  13   a  and is coherently recombined after passing through the second 3 dB coupler  13   b . When the signal reaches the optical ring resonator  20 , the resonant wavelength, e.g. λ i , is coupled into the ring waveguide  26  from the lower arm  28   b  and subsequently coupled into the upper arm  28   a.    
   The resonant wavelength λ i  satisfies the following resonance relationship: λ i =2πrn e /m where, r is the ring radius, n e  is the effective index of the ring waveguide  26 , and m is an integer. Owing to the symmetrical nature of the Mach-Zehnder interferometer  11 , the coupled wavelength into the upper arm  28   a  emerges at the drop port  14 . Similarly, another signal of wavelength λ i  coming from the add port  22  can be coupled into the ring resonator  20  to show up at the output port  24 . 
   Referring to  FIG. 4 , in the OADM  10   a , either the heater  16  or the heater  18  is turned on. As a result, the add port  22  and output ports  24  are interchanged between the arms  28   a  and  28   b  (compared to the OADM  10  shown in  FIG. 1 ) because of a switching feature of the Mach-Zehnder interferometer  11 . 
   Referring to  FIG. 5 , the Mach-Zehnder interferometer  11   a  is asymmetrical and the heaters  16  and  18  are turned off. In this case, the output port  24  is in the upper arm  28   a  and the add port is in the lower arm  28   b.    
   Similarly, in the embodiment shown in  FIG. 6 , the Mach-Zehnder interferometer  11   a  is asymmetrical. Either the heater  16  or the heater  18  is turned on, and the add port  22  is in the upper arm  28   a  while the output port  24  is in the lower arm  28   b.    
   Referring next to  FIG. 7 , an arrayed optical add/drop multiplexer matrix  10   c  includes input ports  12   a  and  12   b , output ports  24   a  and  24   b , drop ports  14   a  and  14   b , and add ports  22   a  and  22   b . The matrix  10   c  includes arms  28   a  and  28   b , as well as arms  28   c  and  28   b . The ring resonators  20   a  and  20   b  are included between pairs of arms  28 . 
   An asymmetrical Mach-Zehnder interferometer  11   a  includes heaters  40 , while a symmetrical Mach-Zehnder interferometer  11  also includes heaters  40 . The arms  28   c  and  28   b  cross at  42 . The arrayed optical add/drop multiplexer matrix  10   c  may include a number of additional arms not shown in FIG.  7 . 
   The ring resonators  20  are not necessarily of identical resonant wavelengths. Therefore, the matrix  10   c  is able to add or drop multiple wavelengths simultaneously, adding significant flexibility to communication system applications. 
   Referring to  FIG. 8 , a bandpass optical add/drop multiplexer  10   d  allows adding or dropping a sub-band or a plurality of channels of different wavelengths out of a larger group of channels. Multiple ring resonators  20  may be utilized, with each ring resonator  20  tuned to one wavelength. Alternatively, one ring resonator  20  may have a bandpass characteristic. The ring resonator  20  can be a normal ring with fine structures such as a ring with lithographically written gratings. 
   While the present invention has been described with respect to a limited number of embodiments, those skilled in the art will appreciate numerous modifications and variations therefrom. It is intended that the appended claims cover all such modifications and variations as fall within the true spirit and scope of this present invention.