Abstract:
An optical add/drop multiplexer to be tuned locally or remotely, to compensate for drift in operation, and to facilitate installation or replacement at remote terminals.

Description:
BACKGROUND 
     1. Field of the Invention 
     The invention generally relates to optically-based products and more particularly to communications over optical transmission networks. 
     2. Description of Related Art 
     Synchronous Optical Networks (SONET) have been used for inter alia, commercial telephone and data distribution services. Most of the implementations of optical data transmission for commercial use are SONET based architectures. These networks consist of an optical ring emanating from a central office (CO) terminating either at end offices or remote terminal (RT) boxes. 
     SONET solves the need to deploy voice and some business data service to an existing group of subscribers. However, the bandwidth for SONET was architected for traditional 64 kb/s voice services and is based on Time Domain Multiplexing (TDM). Most of the implementations were installed in the early to mid 1990s and were designed to support on the order of 10,000 subscribers per optical carrier 3 (OC3) optical ring. With business subscribers utilizing for example, T 1  connections, the number of subscriber service lines is further limited. 
     In the context of residential broadband service, Asymmetric Digital Subscriber Line (ADSL) or other sorts of broadband access technologies operate at a bandwidth typically on the order of 100 times that of voice. Thus even though SONET rings appear to have a significant bandwidth, there is a considerable amount of oversubscription that occurs from a data perspective when trying to provide broadband telephony on a commercial scale. Gigabit Ethernet technology is a native packet transport scheme which is more efficient than a traditional TDM structure. As such it is better suited to operate in a data domain than is a TDM SONET. 
     Most of the existing SONET rings are built on 1310 nanometer (nm) optics. A typical OC3 ring includes a 1310 nm. laser, a photodetector that is optimized for the 1310 nm. band of light, a single mode fiber and optical splicing components. Basically, in such systems, light is transmitted from a Central Office (CO) to remote terminal # 1 , to remote terminal # 2 , to remote terminal # 3 , and so on, with add/drop multiplexers at each interim node. 
     To add another wavelength of light over the same channel (e.g., to combine a 1550 nm signal with a 1310 nm. signal in a channel), one must typically employ wave division multiplexing (WDM) hardware. The WDM hardware typically includes a Fiber Bragg Grating (FBG) or other nonlinear crystal and associated optical fibers. An erbium doped optical fiber ring can transmit both the 1310 nm. and the 1550 nm. wavelengths of light. A standard Erbium Doped Fiber Amplifier (EDFA) typically may have a bandwidth wide enough to handle either band, or anywhere in-between for that matter. 
     Transmission problems are seen where it is desired to use more than the 1310 nm and 1550 nm specific wavelengths. Examples include four channel add/drop multiplexers, or taking a 1310 nm., and adding four channels of 15xx wavelength hardware on top of it. For example, a four channel 1550 nm wave division multiplexor (WDM) wavelength, typically operates at 1510, 1520, 1530, and 1540 nm. Four channel 1550 operations usually have windows at each of those stages. The limitations is that if one is only adding or dropping one channel at a particular node one must have a laser that is tuned to that particular frequency, and a photodetector that looks at that particular frequency. Which means that each node has to be preset to that particular frequency. This means that the telephony installation technician has to have optics components for each wavelength at his disposal to service each different terminal. Each wavelength in which the telephony system is capable of transmitting data must have add/drop multiplexers that are tuned to that wavelength and labeled accordingly. This results in much overhead distributed among the stock room, the CO, and the other service technicians who are required to have all hardware variations with them. Additionally, the technicians must know which multiplexers to use when they try to replace terminal hardware. In other words they have to make sure the replacement for the particular multiplexer he is replacing matches the wavelength for the bad module so he does not plug in a module tuned to the wrong frequency and thus interfere with a signal of a different wavelength of some other remote terminal in the ring. 
     FIG. 1 is a representation of an optical ring. Central Office (CO)  100  is connected to the remote terminal equipment by fiber optic loop  500 . Incumbent equipment includes a SONET OC3 ring. Due to large deployment rates in broadband, there is not enough bandwidth in the traditional SONET OC3 optical ring. Data congestion could be alleviated by a virtual point to point connection to each remote terminal without having individual fibers connect each of these sites. If this were a traditional OC3 bandwidth ring, all the bandwidth of the OC3 ring is shared over the entire ring. A parallel path could be built using the exiting fiber in the ring by using WDM. Add/drop multiplexers may be used to use put each of these Remote Terminals (RT) at different wavelengths. Assume existing SONET uses 1310 nm. Assigning RT # 1  to wavelength 1510 nm., RT # 2  at 1530 nm., and RT # 3  at 1550 nm. These wavelengths of light are overlaid on an existing SONET ring, but use different wavelengths of light. Using traditional techniques each site would have a fixed-wavelength add/drop multiplexer and laser tuned at that particular wavelength. This would require three specific models of gear as well as deployment of all this gear at the various sites. This invention is one add/drop multiplexing module that is capable of operating at all of the wavelengths transmitted over an optical fiber and yet tunable to any specific wavelength desired at the time of deployment or replacement. 
     A lot of progress has been made on tunable laser technology. For example inter alia, how to tune a laser to get exactly the frequency desired. Add/drop technology in the transmit, direction is unnecessary, all that is needed is an optical combiner. 
     Photodiodes are generally not tunable to a particular frequency, nor do they have enough quality or selectivity to be able to isolate its own tunable band. A photo diode that is tuned to the 1300 band is likely going to respond to a signal in the 1500 band as well. A filter in front of the device is required. 
     Wavelength Add/drop modules are such that they will only split off a particular wavelength of light while passing remaining wavelengths. Typically the majority of the selected wavelengths energy will be split off and 99% of the other frequencies of light will pass through, making them advantageous where multiple nodes must share a single fiber run. 
     The difficulty with optical filters is how to make an optical filter that can be set to one of a variety of wavelengths in the field. Something that is scaleable to deployment in the field. Small enough to fit in an optical interface, and cheap enough to deploy on a mass basis. That is where that tunable wavelength filter is beneficial. 
     An improved add/drop multiplexer that is affordable and small enough to be deployed commercially is needed. 
     SUMMARY 
     In one embodiment, a scheme for tuning an optical add/drop multiplexer may be used to maintain focus on the wavelength of interest, and avoid drift in signal capture due to misuse or environmental conditions and pass undesired wavelengths through to other nodes. In one example, an apparatus configured for use in the present scheme includes one add/drop multiplexing module that is tunable to the frequencies desired at the time of deployment or replacement. In this manner an existing SONET ring may employ course wave division multiplexing (two channel, potentially four channel, optical add/drop multiplexing) to add Gigabit Ethernet services on top of traditional SONET services (e.g., voice). Such a scheme may allow for an increase in useful bandwidth of approximately 10 times the bandwidth of traditional OC3. 
     Additional features, embodiments, and benefits of this scheme will be evident in view of the figures and detailed description presented herein. 
    
    
     BRIEF DESCRIPTION 
     The present invention is illustrated by way of example, and not limitation, in the figures of the accompanying drawings, in which: 
     FIG. 1 is a schematic illustration of an embodiment of an optical ring network. 
     FIG. 2 is a schematic illustration of one embodiment of a drop function configured in accordance with the present invention. 
     FIG. 3 is a schematic illustration of one embodiment of an add function configured in accordance with the present invention. 
     FIG. 4 is a schematic illustration of one embodiment of a drop side wavelength selector and locker/detector configured in accordance with the present invention. 
     FIG. 5 is a schematic illustration of one embodiment of a drop side wavelength selector and locker/detector configured in accordance with the present invention. 
     FIG. 6 is a flow diagram representation of one method of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A tunable add/drop multiplexer is disclosed herein. According to one embodiment, a tunable optical filter component is configured to act as an add/drop multiplexer on an optical fiber communications loop. 
     In the following description, for the purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the invention. It will be apparent, however, to one of ordinary skill in the art that the present invention may be practiced without some of these specific details. The following description and accompanying drawings provide various examples for the purposes of illustration. However, these examples should not be construed in a limiting sense as they are merely intended to provide examples of the invention rather than to provide an exhaustive list of all possible implementations of the invention. 
     Reference will now be made to drawings wherein like structures will be provided with like reference designations. In order to show the structures of the invention most clearly, the drawings included herein are diagrammatic representations of integrated circuit structures. Thus, the actual appearance of the fabricated structures, for example in a photomicrograph, may appear different while still incorporating the essential structures of the invention. Moreover the drawings show only the structures necessary to understand the invention. Additional structures known in the art have not been included to maintain the clarity of the drawings. 
     FIG. 1 illustrates an optical ring communication system. In one embodiment, Central Office (CO)  100  is connected to any number of remote terminals (RT)  200 ,  300  and  400  through an Erbium Doped Fiber Amplifier (EDFA)  500 . A virtual point to point optical connection can be made from the Central Office to each Remote Terminal  250 ,  350  and  450  respectively along the existing EDFA. In this embodiment, the Central Office uses 1510 nm. light to communicate with Remote Terminal # 1 , 1530 nm. light to communicate with Remote Terminal # 2  and 1550 nm. light to communicate with Remote Terminal # 3 . 
     FIG. 2 is a schematic illustration of one embodiment of the drop function incorporated in the present add/drop multiplexer. Fiber Ring West  1  carrying all relevant colors of light is received into a tunable wavelength filter  10  where the frequency of interest λ x  is extracted through drop path  3 . Fiber Ring East  2  then provides all colors of light, except the frequency of interest that was dropped at the filter  10 , to the next element in the ring. The component of the transmitted light at the frequency of interest travels along the drop path  3  to a beam splitter  11  that separates the majority of the signal, ˜99%, from a minority of the signal ˜1%. Following this separation, the majority component of the signal travels along an optical fiber to a photodiode  13  that converts the light signal into an electrical signal that represents the extracted data from the original carrier signal (i.e., the data that was modulated at the selected wavelength of interest). The minority component of the data signal  4  that was split off enters a wavelength detector  12  and is used in an optical feedback loop where it is combined with the wavelength selected from optical path  8  in an optical mixer  14  to obtain a control wavelength to control the selection window of the filter  10 . 
     FIG. 3 is a schematic illustration of one embodiment of an add function for the present multiplexer. The data to be inserted in fiber ring west  28  is represented by an electrical signal that is provided to tunable laser  29  along electrical path  20 . The tunable laser transforms the electrical signal into a light signal at a wavelength of interest λ x . The light signal travels along optical path  21  to a beam splitter  30  that provides the majority component of the light signal, ˜99%, along optical path  26  to optical mixer  33 . Optical mixer  33  combines this majority component of the light signal at the wavelength of interest with the other light signals of different wavelengths traveling along fiber ring east  27  into fiber ring west  28 . 
     A minority component of the light signal at the wavelength of interest λ x leaves beam splitter  30  along optical path  22  bound for wavelength detector  31 . The output of wavelength detector  31  is summed with the light signal selected from optical path  24  in optical mixer  32  to create a feedback control loop for tunable laser  29 . 
     FIG. 4 illustrates an embodiment of a drop side wavelength selector and locker/detector configured in accordance with one embodiment. A light signal made up of a group of light signals of various wavelengths traveling along source fiber  40  enters collimator  57  and impinges on positionable prismatic element  46 . In one embodiment, the prismatic element is a Bragg grating. 
     The prismatic element has a fixed point and a piezoelectric actuator  54  that allows the prismatic element to move relative to the fixed positions of collimators  41 ,  42 ,  43 ,  44  and  45 . A group of light signals of various wavelengths exiting collimator  57  and striking prismatic element  46  will undergo diffraction based on the course frequency of the light. The light leaving prismatic element  46  will spread out in a plane according to wavelength. 
     Motion of the prismatic element relative to the fixed positions of the collimators  41 ,  42 ,  43 ,  44  and  45  allows steering of the refracted light leaving the prismatic element. Steering the refracted light from prismatic element  46  allows the wavelength of interest λ x  to strike collimator  43 . Collimator  43  is the collimator for the drop line for the wavelength of interest. Collimators  42  and  44  pick up the side lobes of the wavelen gth of interest that do not strike collimator  43 , and are used by the electrical feedback loop to steer the prismatic element. The wavelength of interest λ x . strikes collimator  43  and is transformed into an electrical signal by photodiode # 1   48 . The electrical signal is the data signal extracted by the drop function from source fiber  40 . 
     FIG. 4 also shows the feedback circuit used to steer the refracted light of prismatic element  46 . The side lobes of the light signal at the wavelength of interest striking collimator  43  also strike collimators  42  and  44 . Light striking collimators  42  and  44  is converted into electrical signals by photodetectors  49  and  50 , respectively, and these electrical signals are amplified by differential amplifiers  51  and  52  and combined in mixer  53 . The output of mixer  53  is the automatic frequency control voltage which is summed with the source tuning voltage in mixer  56 , amplified by amplifier  55  and used as the excitation voltage to adjust the piezoelectric actuator  54  that moves the prismatic element  46 . 
     FIG. 4 further shows what happens to the remaining components of a light signal that are not at the wavelength of interest in source fiber  40 . On either side of the three inside collimators  42 ,  43  and  44  are collimators  41  and  45 . Collimators  41  and  45  collect any light that is not at the wavelength of interest and combines this light in combiner  47  before transmitting the light out through the destination fiber  58 . So then destination fiber  58  carries all wavelengths of light that left source fiber  40  except the wavelength of interest λ x . 
     One embodiment of the system illustrated in FIG. 4 could be a four-band wave division multiplexer based on the 1550 nm. light band. In that embodiment, collimator  41  would have a 40 nm. window, say from 1510 nm. to 1550 nm. Collimator  45  would have a 40 nm. window from 1520 nm. to 1560 nm. Collimator  43  would have a 10 nm. window, for example 1525 nm. to 1535 nm. In this embodiment the wavelength of interest, λ x  would be  1530  nm. In another embodiment, one might apply all the colors of a light signal but one through collimator  41  and only one color through collimator  43 . In another embodiment, one might apply all of the colors of a light signal but one through collimator  45 , and only one color through collimator  43 . In still another embodiment, one might apply some colors of a light signal through collimator  41 , other colors of the light signal through collimator  45  and one color of the light signal through collimator  43 . 
     FIG. 5 is an illustration of an embodiment of a multiplexer, which uses a diffraction wavelength filter to enable a drop function. Source fiber  70  carries all the colors of the light signal in the fiber ring, and emits them through collimator  61 . Destination fiber  71  carries all of the colors of the light signal in the fiber ring except the wavelength of interest λ x , having received them through collimator  62 . 
     All light from collimator  61  impinges on temperature dependent diffraction filter  63 . Diffractor  63  reflects the entire light incident thereon, except the wavelength of interest λ x . Diffractor  63  is of such size that by heating and cooling it can be tuned to transmit any of the potential wavelengths of interest transmitted over source fiber  70 . A heating element  69  is positioned to maintain diffraction filter  63  at the appropriate temperature to pass the wavelength of interest λ x . Then, light at the wavelength of interest λ x , is collected by collimator  64  and transmitted to photodiode  65 , which turns the light into an electrical signal that represents the extracted data. 
     A temperature sensor  66  observes the temperature of diffraction filter  63 . The signal from temperature sensor  66  is mixed with a tuning voltage in mixer  67  and amplified to act as a feedback control loop to maintain the diffraction filter  63  at an appropriate temperature for transmitting the wavelength of interest λ x . 
     FIG. 6 is a flow diagram representing one method of the present invention. At step  610 , the operation of the prismatic element is monitored. The observation of the prismatic element is converted into an electrical signal in part  620 , by for example, the side-lobe radiation in collimators  42  and  44  in FIG. 4 that is converted to an electrical signal, or the temperature of the diffraction wavelength filter  63  in FIG. 5 measured by temperature sensor  66 . The electrical signal is amplified in a differential amplifier in part  630 . The amplified signal is summed with a source signal in part  640 . The summed signal is used to keep the prismatic element tuned to a particular wavelength of light in part  650 . 
     In another embodiment, the whole module is fixed, and a prismatic element refracts the source light into all used colors of light. Each color of light will, without interference, then go back into an individual collimator thence into a combiner, and then return to the destination fiber. One color of light will enter a beam splitter where a majority of the light will travel on to the destination fiber, but some of the light will be used to tune the prismatic element to adjust for vibrations or expansion of the module. A drop fiber placed in an appropriate slot in the module will intercept a defined color of light and disrupt that color from returning to the destination fiber. 
     In another embodiment, choosing the wavelength of light selected by the drop is done by actuating one or more dip switches on the module. There is a one-source fiber entering the module, and one destination fiber leaving the module, as well as one-drop fiber. Actuating the dipswitches selects the color of light that will be dropped at that module. The rest of the colors of light will continue on through the destination fiber. 
     In the preceding detailed description, the invention is described with reference to specific embodiments thereof. It will, however, be evident that various modifications and changes may be made thereto without departing from the broader spirit and scope of the invention as set forth in the claims. The specification and drawings are, accordingly, to be regarded in an illustrative rather than a restrictive sense.