Abstract:
This invention provides method and apparatus for ancillary data in a wavelength division multiplexed (WDM) system. According to the invention, a low bit rate channel is provided over a amplitude modulated sub-carrier that is in turn used to amplitude intensity modulate an optical data signal that is output from a transmitter in the network. Data carried by the low bit rate channel can by used by another network element (NE) to determine the identity of the channel source, thereby allowing the NE to verify its connectivity to that source via the network. This invention is particularly useful in metropolitan optical networks (MON) where inexpensive methods of determining network connectivity are required.

Description:
FIELD OF THE INVENTION 
     This invention relates to wavelength division multiplexed (WDM) optical systems, and more particularly to method and apparatus for a dense WDM (DWDM) optical system. 
     BACKGROUND OF THE INVENTION 
     In optical transmission systems the capability to determine the connectivity of network elements (NE) is important in performing equipment inventory management, fault isolation, and automated provisioning of the system. In a DWDM system, which typically carries 33 or more multiplexed channels of information over a single fiber, this capability would normally require an optical demultiplexer in each NE in order to access the information carried by the channels. This information would then be used to determine the connectivity of the NE. However, optical demultiplexers are relatively expensive components, and therefore, to include them in an NE solely for determining the connectivity of the NE where they otherwise would not be required is undesirable. It appears then, that an alternative technique of providing the capability to determine the connectivity of an NE would be useful. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide improved method and apparatus for ancillary data in a WDM optical transmission system. 
     The present invention provides a low bit rate data channel for carrying ancillary data between a channel wavelength source and an optical NE in a WDM system. The low bit rate data channel, hereinafter called a WaveID, is carried by a sub-carrier frequency signal that has been modulated by the ancillary data. The modulated sub-carrier frequency signal is used to modulate an optical data; signal. The WaveID of each channel is orthogonal to other WaveIDs. That is, for an optical signal that is comprised of a plurality of modulated optical data signals that are each of a different channel wavelength, each WaveID can be detected independently from other WaveIDs. 
     According to one embodiment of the present invention each WaveID can carry ancillary data that includes channel source identification information, such as a unique channel identifier that uniquely identifies the wavelength source. Further to this end, the WaveID could carry such ancillary data as: Internet protocol (IP) address of the source, physical location identifier of the source, working or protection channel identification, payload format and bit rate identifiers, and other such information as dictated by future requirements. 
     The WaveIDs can be detected by an NE equipped with a tap coupler, a photo detector, and a WaveID detector. By tapping an incoming optical signal: and detecting the set of WaveIDs present, an NE can determine the wavelength sources to which it is connected via the network. This allows the NE to verify or discover its connectivity to the network. This capability is particularly attractive for Metro Optical Networks (MON) where inexpensive techniques of optical connectivity verification are required. 
     An advantage of the present invention is that it does not require optical demultiplexing of the channel wavelengths in order to identify the channel wavelength sources. Consequently, adding optical demultiplexors in order to perform channel source identification is not necessary. This leads to a relatively inexpensive technique of channel source identification that is useful in determining the connectivity of an NE in a DWDM system. 
     According to an aspect of the present invention there is provided an apparatus for ancillary data in a wavelength division multiplexed system comprising: a modulator for modulating a sub-carrier frequency signal with the ancillary data; an intensity modulator for amplitude intensity modulating an optical data signal: with the modulated sub-carrier frequency signal; al tap coupler for tapping a portion of the amplitude intensity modulated optical signal; an opto-electronic convertor for converting the tapped portion of the optical signal to an electrical signal; and a detector for detecting the modulated sub-carrier frequency signal from the electrical signal, and for detecting the ancillary data from the modulated sub-carrier frequency signal. 
     According to another aspect of the present invention there is provided an encoder for ancillary data in a wavelength division multiplexed system comprising: a modulator for modulating a sub-carrier frequency signal with the ancillary data; and an intensity modulator for amplitude intensity modulating an optical data signal with the modulated sub-carrier frequency signal. 
     According to another aspect of the present invention there is provided a decoder for extracting ancillary data from an optical data signal that has been amplitude intensity modulated by a modulated sub-carrier frequency signal that has been modulated by the ancillary data in a wavelength division multiplexed system, comprising: a tap coupler for tapping a portion of the modulated optical data signal; an opto-electronic convertor for converting the tapped portion to an electrical signal; and a detector for detecting the modulated sub-carrier frequency signal from the electrical signal, and for detecting the ancillary data from the modulated sub-carrier frequency signal. 
     According to yet another aspect of the present invention there is provided a method of identifying channel sources in a wavelength division multiplexed system comprising the steps of: at a first node in the system, modulating a sub-carrier frequency signal with the ancillary data; at the first node, amplitude intensity modulating an optical data signal with the modulated sub-carrier frequency signal; at the first node, transmitting the modulated optical data signal onto an optical fiber; at a second node in the system, tapping a portion of the modulated optical data signal from the optical fiber; at the second node, converting the tapped portion to an electrical signal; at the second node, detecting the modulated sub-carrier frequency signal from the electrical signal; and at the second node, detecting the ancillary data from the modulated sub-carrier frequency signal. 
     According to still another aspect of the present invention there is provided an amplitude intensity modulated optical signal for conveying ancillary data in a wavelength division multiplexed system comprising: 
     pulse modulated light of a constant wavelength that has been modulated at a first bit rate by a first sequence of data symbols; and an amplitude intensity modulation of the pulse modulated light at a sub-carrier frequency that is less than the bit rate of the optical data signal divided by two and where the sub-carrier frequency has been modulated by ancillary data represented by a second sequence of data symbols, wherein the second sequence of data symbols has a bit rate that is at least eight orders of magnitude lower than the first bit rate. 
     According to still another aspect of the present invention there is provided an apparatus for ancillary data in a wavelength division multiplexed system comprising: a data source for providing the ancillary data wherein the ancillary data includes channel source identifying information; and means for including the ancillary data in an optical data signal, wherein the optical data signal is amplitude intensity modulated by a signal that has been modulated by the ancillary data. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be better understood from the following description with reference to the drawings in which: 
     FIG. 1 is a block diagram of a MON in accordance with an embodiment of the present invention; 
     FIG. 2 is a block diagram of the WaveID insertion block of FIG. 1; 
     and 
     FIG. 3 is a block diagram of the WaveID detection block of FIG.  1 . 
    
    
     DETAILED DESCRIPTION 
     In FIG. 1 there is illustrated in a block diagram a MON  10  in accordance with an embodiment of the present invention. The MON  10  includes optical transmission equipment at a first site  20 , at a second site  40 , and at a third site  50 . The equipment at the first site  20  includes a  2 R transceiver  22 , a 3:1 multiplexor  24  coupled to the transceiver  22  via an optical fiber  14 , and an 11:1 multiplexor  26  coupled to the multiplexor  24  via an optical fiber  16 . The transceiver  22  is equipped with WaveID insertion apparatus  28 . 
     In operation, an optical data signal s 1  is input to the transceiver  22  via an optical fiber  12 . The WaveID insertion apparatus  28  inserts the WaveID into the signal s 1 , and a modulated optical data signal s 2  is output on the optical fiber  14 . The multiplexor  24  multiplexes the signal s 2  with two other signals (not shown), which are similar to the signal s 2  in format but are of different channel wavelengths, and outputs a multiplexed signal s 3  onto the optical fiber  16 . The multiplexor  26  multiplexes the signal s 3  with ten other multiplexed signals (not shown), which are similar to the multiplexed signal s 3  in format but are again of different channel wavelengths, then outputs a multiplexed signal s 4  onto the optical fiber  18 . Consequently, the multiplex signal s 4  is composed of thirty-three signals, each signal of a different channel wavelength, and the WaveID of each channel uniquely identifying its respective channel source. 
     The third site  50  includes an optical add drop multiplexor (OADM)  52 . The OADM  52  is connected to the multiplexor  26  of the first site  20  via the optical fiber  18  and to the second site  40  via an optical fiber  38 . 
     The OADM  52  is capable of extracting, or dropping, channels from the signal s 4 . Further, the OADM  52  can add channels in the place of dropped channels, or it can simply copy information carried by any of the channels and thereby allow the signal: s 4  to effectively passthrough the OADM  52 . 
     For simplicity, FIG. 1 shows the signal s 4  passing; through the OADM  52 . 
     The second site  40  comprises a 1:11 demultiplexor  42  connected to the third site  50  via the optical fiber  38 , a 1:3 demultiplexor  44  connected to the demultiplexor  42  via an optical fiber  36 , and a  2 R transceiver  46  connected to the demultiplexor  44  via an optical fiber  34 . The transceiver  46  is equipped with WaveID detection apparatus  48   a . The demultiplexors  42  and  44  are also shown equipped with WaveID detection apparatus  48   b  and  48   c , respectively. 
     In operation, the multiplexed signal s 4  is input to the demultiplexor  42  on the optical fiber  38 . The signal s 3  is output from the demultiplexor  42  on the optical fiber  36  and input to the demultiplexor  44 . The modulated optical data signal s 2  is output from the demultiplexor  44  on the optical fiber  34  and input to the transceiver  46 . The WaveID detection apparatus  48   a ,  48   b  , and  48   c  can be used in order to detect the WaveIDs on their respective input signals s 2 , s 3 , and s 4 , and thereby to determine wavelength source information of the channels. The transceiver  46  outputs an optical data signal s 1 ′ onto an optical fiber  32 . The signal s 1 ′ includes the information contained in the signal s 1  and the WaveID of the signal s 2 . 
     It should be noted, that there would be a plurality of transceivers  28  and  46  at the first and second sites, respectively, however these transceivers have been omitted for clarity. Further, it should be noted that the WaveID detection apparatus could be located in any of the NEs  22 ,  24 ,  26 ,  52 ,  42 ,  44 , and  46  shown in FIG.  1 . This would allow each NE to determine its connectivity to the network by determining the wavelength source information from the WaveIDs of the channels that it is receiving. In addition, a piece of equipment for managing the network, often referred to as a network manager, which is in communication with each NE in the network, could determine the connectivity of the network from the wavelength source information of the channels that each NE is receiving. 
     In FIG. 2 there is illustrated in a block diagram the WaveID insertion apparatus  28  of FIG.  1 . The WaveID insertion apparatus  28  includes a WaveID data source  74 , a WaveID modulator  70  coupled to the WaveID data source  74  via a link  73  and having an, input for a sub-carrier frequency signal of frequency f 1 , and an intensity modulator  72  coupled to the WaveID modulator  70  via a link  75 . An optical fiber  71  is connected to the input of the intensity modulator  72 , and at its output there is connected an optical fiber  77 . A network manager  100  is shown communicatively coupled to the WaveID data source  74  via a link  102 . 
     In operation, ancillary data to be carried by the WaveID is provided by the WaveID data source  74 , and optionally the network manager  100 . The ancillary data includes channel source identification information such as: IP address of the source, physical location of the source, working or protection channel identification, payload format and bit rate, and other such information as dictated by future requirements. The ancillary data is provided to the WaveID modulator  70 . The WaveID modulator  70  modulates the sub-carrier frequency signal of frequency f 1  with the ancillary data at a bit rate that is at least eight orders of magnitude (10 8 ) lower than the bit rate of the optical data signal s 1 . The sub-carrier frequency signal is amplitude modulated by the data to a given modulation depth m. The sub-carrier frequency signal has a frequency f 1  that less that than bit rate of the optical data signal s 1  divided by two. That is, the sub-carrier frequency signal is an in-band sub-carrier with respect to the optical data signal si. The resultant signal mw(t) is input to the intensity modulator  72  via the link  75 . The optical data signal s 1 , which can be represented by (1+d(t))Pavg, where d(t) is the time varying data and Pavg represents the average optical power, is input to the intensity modulator  72  via the optical fiber  71 . The optical data signal s 1  is intensity amplitude modulated according to the signal mw(t) by the intensity modulator  72 . The resulting modulated optical data signal s 2 , which can be represented by (1+wm(t))(1+d(t))Pavg, is output on the optical fiber  77 . Thus, the signal s 2  includes data of the type described earlier, the data being carried by the WaveID. Typically, the data would have a bit rate in the range of one to ten bits per second, and would be repeated over time. 
     It should be noted, that while amplitude modulation of a sub-carrier by the ancillary data to be communicated has been described, other modulation formats that would provide orthogonal WaveIDs are possible. These formats include such formats as frequency or phase modulation of a sub-carrier, frequency or phase shift keying of a sub-carrier, code division multiple access (CDMA), frequency division multiple access (FDMA), and other formats known in the art. Furthermore, the intensity modulator  72  for modulating the optical data signal s 1  at a SONET transceiver could be an optical attenuator or a circuit for controlling laser bias current of the transceiver. In addition, the WaveID is controlled to a target minimum modulation depth that both minimizes any undesirable effects that the modulation imposes on the optical data signal s 1  and allows the WaveID to be reliably detected at optical multiplexed interfaces in the network. 
     In FIG. 3 there is illustrated in a block diagram the WaveID detection apparatus  48   a  of FIG.  1 . The WaveID detection apparatus comprises a tap coupler  82 , a PIN photo detector  84  coupled to the tap coupler  82  via an optical fiber  85 , an electrical amplifier  86  connected to the PIN photo detector  84  via a link  87 , a WaveID Detector  80  connected to the amplifier  86  via a link  89  and having input signals of sub-carrier frequencies f 1  to fn. The network manager  100  is shown communicatively coupled to the WaveID detector  80  via a link  104 . The modulated optical data signal s 2  is input to the tap coupler  82  via an optical fiber  81 , and the signal s 1 ′ is passed through the tap coupler  82  and output on an optical fiber  83 . The input signals of sub-carrier frequencies f 1  to fn are provided by the transceiver  46 , where n is the maximum number of channels carried by a signal in the network; in this case n is thirty-three. The sub-carrier frequencies f 1  to fn could range from f 1 =20 kHz to fn=660 kHz with an increment of 20 kHz between adjacent frequencies. 
     In operation, the modulated optical data signal s 2  is input to the tap  10  coupler  82 , and a portion s 2 ′ of the modulated optical data signal s 2  is tapped and output on the fiber  85 , while the remainder is output on the optical fiber  83 . Typically, five percent of the optical power of the input signal is tapped by the tap coupler  82 . The portion s 2 ′ is input to the PIN. photo detector  84  that converts the portion s 2 ′ to an electrical signal se 2 . This signal has a low power level and is typically noisy. However, because of the low data rate of each WaveID, which would generally be in the range of 1-10 bps, the data carried by each WaveID can be reliably received despite the noise present on the electrical signal se 2 . The electrical signal se 2  is input to the amplifier  86  that outputs an amplified electrical signal se 2 ′. This amplified electrical signal se 2 ′ is input to the WaveID detector  80 , and the WaveIDs carried on sub-carrier frequency signals of frequencies f 1  to fn are detected. This detection can be done in a number of ways that are known in the art. One technique would be to provide a high Q filter for each sub-carrier frequency signal, with each filter having a center frequency that is equal to the frequency of the respective sub-carrier frequency signal. The respective amplitude modulated WaveID could then be envelope detected at the output of each filter using a simple envelope detector. The data detected from each WaveID would be available to the NE equipped with the WaveID detection apparatus  48   a  and may be sent to the Network Manager  100  over the link  104 . This data includes information that identifies the wavelength source of each channel, and therefore, the data can be used to determine the connectivity of the NE to the network. 
     It should be noted, that although the above discussed the detection of WaveIDs carried by amplitude modulated sub-carrier frequency signals, other techniques of WaveID detection are possible, and such techniques would need to be compatible with the WaveID modulation format used. 
     Examples of such formats were given earlier in connection with the WaveID insertion apparatus  28 . A particularly flexible WaveID detector  80  would include a high speed analog to digital convertor (A/D) and a digital signal processor (DSP). This type of WaveID detector would have the flexibility of detecting different types of low bit rate WaveID modulation formats, and the detector would have enough processing power to decode in parallel the information contained in the WaveIDs. 
     The WaveIDs could be detected at any optical NE where there is a benefit in doing so. Examples include at the output of optical multiplexors and at the input of optical demultiplexors for determining the connectivity to the network of each, and at the input of optical amplifiers for associating a set of transmitters to a chain of optical amplifiers, which is useful in equalizing the optical power of the channels. 
     It should be noted that a WaveID is not limited to carrying only channel wavelength source and related information. Any ancillary data for which the bit rate of the WaveID is suitable could be carried over the WaveID. 
     Numerous modifications, variations, and adaptations may be made to the particular embodiments of the invention described above without departing from the scope of the invention, which is defined in the claims.