Abstract:
A tunable, reconfigurable optical add/drop multiplexer comprises a first signal routing component and at least one wavelength selective switch having an input port and an output port. The input port is optically coupled to the first signal routing component. The wavelength signal selective switch is wavelength tunable, so as to allow a selected wavelength to be routed to the first signal routing component and the rest of the wavelengths to be routed to the output port. According to an embodiment of the present invention the first signal routing component is an optical circulator.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates generally to a reconfigurable optical add-drop multiplexer (ROADM) for WDM network applications that utilizes switches and a wavelength-tuning device.  
         BACKGROUND OF THE INVENTION  
         [0002]    The increasing demand for high-speed voice and data communications has led to an increased reliance on optical communications, especially optical fiber communications. The use of optical signals as a vehicle to carry channeled information at high speed is preferred in many instances to carrying channeled information at other electromagnetic wavelengths/frequencies in media such as microwave transmission lines, coaxial cable lines, and twisted copper pair transmission lines. Advantages of optical media include higher channel capacities (bandwidth), greater immunity to electromagnetic interference, and lower propagation loss. In fact, it is common for high-speed optical systems to have signal rates in the range of approximately several megabits per second (Mbit/s) to approximately several tens of gigabits per second (Gbit/s), and greater. However, as the communication capacity is further increased to transmit greater amounts of information at greater rates over fiber, maintaining signal integrity can be exceedingly challenging.  
           [0003]    The emergence of optical communications as a useful approach for short and long-haul data and voice communications has led to the development of a variety of optical amplifiers. One type of optical amplifier is the rare-earth element doped optical amplifier (i.e., an optical amplifier that has a host material doped with rare earth elements). One such rare-earth element optical amplifier is based on erbium-doped silica fiber. The erbium doped fiber amplifier (EDFA) has gained great acceptance in the telecommunications industry.  
           [0004]    Another type of optical amplifier is a Raman amplifier. Raman amplifiers utilize optical transmission fiber as their gain medium. Both erbium-doped fiber amplifiers (EDFA) and Raman fiber amplifiers are useful in a variety of optical communication systems.  
           [0005]    One way to more efficiently utilize available resources in the quest for high-speed information transmission is known as multiplexing. One particular type of multiplexing is wavelength division multiplexing (WDM). In WDM, several information streams (voice and/or data streams) share a particular transmission medium, such as an optical fiber. Each high-speed information channel is transmitted at a designated wavelength along the optical fiber. At the receiver end, the interleaved channels are separated (de-multiplexed) and may be further processed by electronics. (By convention, when the number of channels transmitted by such a multiplexing technique exceeds approximately four, the technique is referred to dense WDM or DWDM).  
           [0006]    Dense Wavelength Division Multiplexing (DWDM) has been widely accepted as the technology of choice for the next generation optical communication systems to meet the increasing demand for information bandwidth. In order to reduce network cost, future DWDM networks will need to route signals of specific wavelengths through the optical communication system without performing optical to electric and electric to optical conversions. The increasing demand for bandwidth attracted considerable interest in utilization of optical add/drop multiplexers (ADM) to implement add/drop functionality for DWDM networks.  
           [0007]    Considerable efforts have been made to design fixed (i.e. non-reconfigurable and/or non-tunable) wavelength optical add/drop multiplexers (OADMs) using various techniques. These fixed OADMs generally include a combination of circulators and gratings, Mach-Zehnders and gratings, or phasar and thermo-optic switches. However, such devices have no way of re-configuring the optical network based on changing need, short of replacing the existing OADMs with another set of OADMs. This is both time-consuming and expensive.  
           [0008]    One suggested approach to this problem is utilization of reconfigurable optical add/drop multiplexers (ROADMs). With ROADM, reconfigurable add/drop nodes can be implemented to form the basis of optical networks, in which customers can pay-on-demand. Such ROADM is described, for example, in the article entitled “A Hitless Reconfigurable Add/Drop Multiplexer for WDM Network Utilizing Planar Waveguide, Thermo-Optic Switches and UV-reducing Gratings,” by R. E. Scott et al, WHI, OFC&#39;1998. However, although the disclosed add/drop multiplexer is reconfigurable, it is not tunable. That is, the disclosed device can operate only on a predetermined set of wavelengths. This device allows someone to re-route a specific wavelength, but does not allow the device to be tuned so as to allow one to re-route a different wavelength with the same ROADM.  
         SUMMARY OF THE INVENTION  
         [0009]    According to one aspect of the present invention a tunable, reconfigurable optical add/drop multiplexer comprises a first signal routing component and at least one wavelength selective switch device having an input port and an output port. The input port is optically coupled to the first signal routing component. The wavelength signal selective switch is wavelength tunable, so as to allow a selected wavelength to be routed to the first signal routing component and the rest of the wavelengths to be routed to the output port. According to an embodiment of the present invention the first signal routing component is an optical circulator.  
           [0010]    According to one embodiment of the present invention, a wavelength tunable switching device comprises (a) an input port and an output port; (b) a first optical waveguide located between the input port and the output port; (c) a second optical waveguide located between the input port and the output port, the second optical waveguide having a wavelength tunable, wavelength selectable optical component; (d) a first switch selectively coupled to the first or the second optical waveguide for coupling signal light from the input port into one of the optical waveguides; and (e) a second switch selectively coupled to the first or the second optical waveguide for coupling the signal light from one of the first and second optical waveguides into the output port. 
       
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0011]    The invention is best understood from the following detailed description when read with the accompanying figures. It is emphasized that the various features are not necessarily drawn to scale. In fact, the dimensions may be arbitrarily increased or decreased for clarity of discussion.  
         [0012]    [0012]FIG. 1 is a schematic diagram of a reconfigurable add/drop multiplexer RADM.  
         [0013]    [0013]FIG. 2 is a schematic diagram of a tunable wavelength selective switch device incorporated in the RADM of FIG. 1.  
         [0014]    [0014]FIGS. 3A and 3B are schematic diagrams illustrating 2×2 switches of the selective, tunable switch device of FIG. 2.  
         [0015]    [0015]FIG. 4 is a schematic drawing of a tunable wavelength selective switch device of a second embodiment.  
         [0016]    [0016]FIG. 5 is a schematic diagram illustrating a multi-channel ROADM incorporating a wavelength selective switch device of FIG. 2. 
     
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS  
       [0017]    In the following detailed description, for purposes of explanation and not limitation, exemplary embodiments disclosing specific details are set forth in order to provide a thorough understanding of the present invention. However, it will be apparent to one having ordinary skill in the art that the present invention may be practiced in other embodiments that depart from the specific details disclosed herein. In other instances, detailed descriptions of well-known devices and methods may be omitted so as not to obscure the description of the present invention.  
         [0018]    [0018]FIG. 1 illustrates an exemplary reconfigurable optical add/drop multiplexer (ROADM)  10  that is also tunable. The tunable ROADM  10  is formed by two signal routing components, (for example circulators  14 ,  16 ) and a wavelength selective (tunable) switch WSS device  15  that includes either a fiber Bragg grating  18 , a dielectric filter, or another tunable wavelength filtering component. In this embodiment, the tunable ROADM  10  adds or drops a channel corresponding to the Bragg grating&#39;s wavelength of reflection. Because fiber Bragg grating  18  is tunable, the network provider can select which channel (i.e., a specific optical signal&#39;s wavelength) is being dropped or passed through by the ROADM  10 . This can be done by either remotely located network operator or via a routing algorithm. In the event of drop, another channel at the same wavelength may be added to the signal channels. Generally, the added signal would be provided to the WSS device  15  through its output port  15 B, for example, via the circulator  16 . As stated above, the add/drop channel corresponds to the specific reflection wavelength of the Bragg grating  18 . The Bragg grating  18  is tuned by temperature, or strain, or other means so as to select a different drop channel (corresponding to the reflected wavelength λ d ). Thus, active temperature tuning or stress tuning can be used to achieve wavelength selection. More specifically, the local refractive index of the grating  18  is modified, thereby shifting the Bragg wavelength (reflection wavelength λ d ) by stretching, compressing or heating of the optical fiber containing the Bragg grating  18 .  
         [0019]    The tunable ROADM  10  includes a pass-through path  25 , which enables non-interruptive reconfiguration and an alternative pass  23 . The term “non-interrupting” reconfiguration means that when a particular add/drop channel is switched between the two states (the two states being add/drop and pass-through states), the power of channels that are not being dropped or added is not impacted during switching. More specifically, the non-interruptive reconfiguration is accomplished by the tunable wavelength selective switch (WSS) device  15  of the ROADM  10  (see FIG. 1 and FIG. 2) as described below. The tunable WSS device  15  includes two synchronized 1×2 or 2×2 switches  22 A,  22 B. In this embodiment switches  22 A,  22 B are bending fiber coupler switches. The switches  22 A,  22 B vary the coupling ratio of the signal between the two paths  25  and  23 . When both switches  22 A,  22 B are in bar state, the WSS device  15  is off and the signal is directed through the alternative channel selection path  23  corresponding to an optical fiber  24  with Bragg grating  18 . Thus, the device operates in normal OADM configuration. When both switches  22 A,  22 B are in cross-bar state, the WSS device  15  is on and the signal is re-directed through a pass-through path  25  while the Bragg gratings  18  is being tuned to reflect the desired wavelength λ d . The pass-through path  25  corresponds to the optical fiber  28 . The optical fiber  28  may be transmission fiber, for example SMF-28™ fiber, available from Corning Inc. of Corning, NY.  
         [0020]    The bar state of the switch is the state of the switch when input  1  is routed to the output port # 1  and input  2  is routed to output port # 2 . This is illustrated in FIG. 3A. The cross-bar state of a switch is the state of the switch when input  1  is routed to the output port # 2  and input  2  is routed to the output port # 1 . This is illustrated in FIG. 3B. Thus, as described above, the two bending switches  22 A,  22 B couple the optical signal S either through the channel selection path or through the reconfiguration path and in conjunction with the grating  18  and the fibers  24 ,  28  form the tunable non-interferometric WSS switch device  15 . During the switching state of the WSS device  15 , when the signal light is switched from fiber  24  to  28 , the optical phase relation between the two paths  23 ,  25  is maintained in order to ensure that during switching the intensity of the output signal remains constant in each channel. Therefore, the optical transmittance of the optical signal will have no significant change during the transition state. That is, the optical transmittance of the WSS device  15  will stay below 10% and preferably at or below 5%. The optical transmittence is defined as P out /P in , where P out  is the optical output power of the device and P in  is the device&#39;s optical input power. It is preferable to operate at low switching speed (more than 10 msec) to fully benefit from active path length stabilization so as to avoid the noise (i.e. intensity variation at the output port due to phase mismatch during switching) generated during switching state. Minimal optical path length change and active compensation during the switching states (e.g. heating) maintain the optical phase difference between the two paths  23 ,  25 . Because the WSS device  15  is non-interferometric switch in static state, there is minimal penalty from phase variance induced noise. A static state is the state of the device operation when the WSS device  15  is not being switched.  
         [0021]    An alternative embodiment of a tunable WSS switch device  15  of the ROADM  10  is shown in FIG. 4. This tunable WSS switch device  15  is similar to the one shown in FIG. 2, but is in a planar configuration. More specifically, the planar WSS switch device  15  of FIG. 4 utilizes two 1×2 thermo-optic switches  22 A,  22 B and two optical waveguides  24 ′ and  28 ′, assembled in a Mach-Zehnder (MZ) configuration. One arm of the MZ contains a Bragg grating  18  that is tunable via a channel selector which includes a channel activator such as a heating electrode  30 A, for example. Similar electrode  30 B is located along the optical waveguide  24 ′ to keep the phase matching constant when the grating  18  is being tuned. Switching heaters  22 A,  22 B activate the thermo-optic switches  22 A,  22 B and the electrode  30 B keeps the WSS device  15  phase matched during switching.  
         [0022]    To achieve accurate channel selection, channel selector  30 ,  30 A is utilized through an active wavelength control of the Bragg grating  18  with a feedback loop  32 . This is shown in FIGS. 1, 2 and  4 . The channel selector in conjunction with a feed back loop  32  measures the reflectance wavelength of the Bragg grating  18 . The channel selector gives signal to an actuator to apply either less or more pressure, strain or heat to the Bragg grating  18 . The channel selector  30  may utilize, as an actuator, a heating coil or compression applying device. After the selection of add/drop channel the optical signal is switched back to the original add/drop route (path  23 ) without intensity interruption at the output port. Thus, the tunable ROADM can reconfigure an optical node to add/drop a variety of channels corresponding to different wavelength, without interruption of any service.  
         [0023]    Reconfiguration and tuning of the ROADM is accomplished in following three steps: (1) Turn on WSS device  15 . (put it in pass-through state), routing signal light along the path  25 , through fiber  28 . This can be done remotely, by a network operator or by a routing algorithm. (2) While the signal light is routed through the fiber  28 , tune grating  26  to select the drop channel a specific wavelength that is being dropped. (3) Turn off WSS device  15  (put in add/drop state), by routing the signal light through the channel selection path  23 . More specifically, the steps to tune and switch ROADM  10  are as follows:  
         [0024]    1. Put WSS device  15  in pass-through state by putting 2×2 switches ( 22 A,  22 B) in cross-state. This will route the signal S entering the input port  15 A along fiber  28  and out of the output port  15 B (see FIG. 2);  
         [0025]    2. Tune grating  18  to a desired spectral channel. For example, a fiber Bragg Grating may be thermally tuned, or alternatively tuned by tensioning or compression via a wavelength tuning actuator  20 . A planar Bragg grating waveguide may also be thermally tuned in a similar manner.  
         [0026]    3. Switch WSS device  15  to the add/drop state by putting 2×2 switches into bar state. This would route the signal S through the optical fiber  28  towards the grating  18 . The grating  18  is tuned to reflect a specific wavelength of light λ d  back towards the circulator  14 , and the signal corresponding to this wavelength λ d  is then dropped through the drop port  14 A of the circulator  14 . The rest of the signal wavelengths pass through the grating and the fiber  18  and enter the circulator  16 .  
         [0027]    The tunable ROADM architecture described here forms the basis of a simple all-fiber tunable add/drop module. Depending on the network requirements for tuning range and tuning speed, one can utilize a split-band configuration to achieve a wider channel selection range.  
         [0028]    The split-band configuration utilizes a plurality of tunable concatenated ROADMs (FIG. 5). Each WSS device  15 A′,  15 B′,  15 C′ can select a single channel within a certain band of wavelengths by tuning an individual grating. Thus, the applicants achieve a simple architecture for TOADM based non-interferometric switches and an active wavelength selector. The technique offers flexibility in implementing all-fiber, tunable add/drop function to a wide spectrum of OADM devices. The added functionality includes non-interruptive channel selection and wavelength stabilization. The architecture provides a tunable optical add/drop multiplexer and provides an important advantage of flexibility for future optical network applications.  
         [0029]    It will be apparent to those skills in the art that various modifications and variations can be made to the present invention without departing from the spirit or scope of the invention. Thus, it is intended that the present invention cover modifications and variations of this invention provided they come within scope of the appended claims and their equivalents.