Patent Abstract:
A method and system for controlling extinction ratio in an optical network is disclosed. A first optical transceiver sends modulated light to a second optical transceiver and a digital measurement of a signal parameter reflecting the optical power levels of the received modulated light is taken. The modulated light sent by the first optical transceiver is adjusted in accordance with the digital measurement.

Full Description:
[0001]     The application claims the benefit of U.S. Provisional Patent Application No. 60/485,077 filed Jul. 3, 2003. 
     
    
     TECHNICAL FIELD  
       [0002]     This invention relates to optical fiber networks.  
       BACKGROUND  
       [0003]      FIG. 1  shows optical power as a function of current for an optical transmitter over time. In general, digital optical communication systems transmit binary data using two levels of optical power, where the higher power level represents a binary 1 and the lower power level represents a binary 0. These two power levels can be represented as P 1  and P 0 , where P 1 &gt;P 0  and the units of power are watts. The difference between P 1  and P 0  is an average power P avg .  
         [0004]     In optical transmitters, electrical current is converted to optical power and in optical receivers optical power is converted back to electrical current. The electrical currents I 1  and I 0  are proportional to the corresponding optical power levels and are controlled by the limit on modulation (I mod ) and bias (I bias ) currents of the transmitter&#39;s laser diode.  
         [0005]     The ratio between the high level and the low level shown in the equation below is defined as the “extinction ratio” and is represented by the symbol r e .  
         r   e     =         I   1       I   0       =       P   1       P   0             
 
 In an ideal transmitter, P 0  would be zero and thus r e  would be infinite. In most practical optical transmitters, however, the laser must be biased so that P 0  is in the vicinity of the laser threshold, meaning that a finite amount of optical power is emitted at the low level and thus P 0 &gt;0. This increase in transmitted power due to non-ideal values of extinction ratio is called the “power penalty”. As the extinction ratio is degraded below its ideal value of infinity, the average power transmitted must be increased in order to maintain a constant Bit Error Rate (BER). 
 
         [0007]     Seemingly small changes in extinction ratio can make a relatively large difference in power required to maintain a constant BER. The effect is especially acute for extinction ratios less than seven, where a change of one in extinction ratio value translates to an approximate 10% change in required average power. This additional required power is aptly termed the “power penalty”, as nothing is gained by this increase in power other than the unnecessary privilege of operating at a reduced extinction ratio.  
         [0008]     As illustrated in  FIG. 1 , the slope of a laser diode&#39;s current to optical power transfer characteristics changes as a function of process, increasing temperature and age (e.g. curves T 1  and T 2 ). The slope variation can affect the extinction ratio, and therefore the BER, during the operational lifetime of an optical transmitter.  
       SUMMARY  
       [0009]     In one aspect, a method of controlling extinction ratio in an optical network configured for transmitting and receiving network data is provided. The extinction ratio can be controlled by providing a first optical transceiver configured for sending modulated light, a second optical transceiver configured for receiving modulated light, taking a digital measurement of at least one signal parameter reflecting the optical power levels of the received modulated light, and adjusting the modulated light sent by the first optical transceiver in accordance with the digital measurement.  
         [0010]     Aspects of the invention can include one or more of the following features.  
         [0011]     The measured signal parameter can include the high and low power levels of the received modulated light. The measured signal parameter can be the difference between high and low power levels of the received modulated light. The measured signal parameter can be the average power level of the received modulated light.  
         [0012]     The digital measurement can be stored in memory. The average power levels of the received modulated light can be computed using the measured high and low power levels. The difference between measured high and low power levels can also be computed.  
         [0013]     Data of a measured signal parameter can be transmitted from the second optical transceiver to the first optical transceiver. Network data can also be transmitted from the second optical transceiver to the first optical transceiver and the data of the digital measurement can be multiplexed into the network data.  
         [0014]     A predetermined extinction ratio can be transmitted from the second optical transceiver to the first optical transceiver, or otherwise provided to the first optical transceiver. The predetermined signal parameter can be extinction ratio. The predetermined signal parameter can be average optical power. The predetermined signal parameter can be compared with the measured signal parameter.  
         [0015]     Adjusting the modulated light sent by the first optical transceiver can include adjusting its extinction ratio. The average optical power of the modulated light sent by the fist optical transceiver can also be adjusted. Adjusting the extinction ratio of the sent modulated optical power can include adjusting the modulation current supplied to a laser diode in the first optical transceiver. The bias supplied to the laser diode can also be adjusted to adjust the average optical power of the sent modulated light.  
         [0016]     Predetermined threshold values of bias and/or modulation current can be provided. The predetermined values of bias and/or modulation current can be compared with the adjusted bias and modulation current to determine whether the threshold values have been exceeded. If the threshold values have been exceeded, a visual indication can be provided.  
         [0017]     Trace histories of the bias current adjustments and/or modulation current adjustment can be stored. The end of life of the laser diode can be predicted on the basis of the stored trace histories of the bias current adjustments and/or modulation current adjustments.  
         [0018]     A visual indication of the time to end of life can be provided.  
         [0019]     In another aspect, an optical network for transmitting and receiving network data is disclosed. The optical network can include a first optical transceiver configured for sending modulated light, a second optical transceiver configured for receiving modulated light, an optical fiber coupling the first optical transceiver to the second optical transceiver. The second optical transceiver can be configured to perform a digital measurement of at least one signal parameter reflecting optical power levels of the received modulated light. The first optical transceiver can be configured to adjust the modulated light sent by the first optical transceiver in accordance with the digital measurement.  
         [0020]     Aspects of the invention may include one or more of the following features.  
         [0021]     The signal parameter can include the high and low power levels, the difference between the high and low power levels and/or the average power level of the received modulated light.  
         [0022]     The network can include a memory configured to store the digital measurement and a communication logic configured to compute the average power level and/or the difference between the high and low power levels of the received modulated light using the measured high and low power levels.  
         [0023]     The second optical transceiver can be configured to transmit data of the measured signal parameter to the first optical transceiver. The data of the measured signal parameter can be multiplexed into the network data.  
         [0024]     The second optical transceiver can be configured to transmit a predetermined signal parameter to the first optical transceiver. The predetermined signal parameter can include a predetermined extinction ratio and/or a predetermined average optical power. The fist optical transceiver can be configured to compare a predetermined signal parameter to the measured signal parameter.  
         [0025]     The first optical transceiver can be configured to receive a predetermined signal parameter and compare the predetermined signal parameter to the measured signal parameter. The predetermined signal parameter can include a predetermined extinction ratio and/or a predetermined received average optical power.  
         [0026]     Adjusting the modulated light sent by the first optical transceiver can include adjusting an extinction ratio and/or an average transmitted optical power of the sent modulated light. The first optical transceiver can include a laser diode and adjusting the extinction ratio of the sent modulated light can include adjusting the range of the modulation current supplied to the laser diode. The first optical transceiver can include a laser diode and adjusting the average transmitted optical power of the sent modulated light can include adjusting the bias current supplied to the laser diode.  
         [0027]     The network can include a memory configured to store a predetermined threshold value of a range of a modulation current. The network can include a communication logic configured to compare the predetermined threshold value of a range of a modulation current to the adjusted modulation current supplied to a laser diode. If the adjusted range of modulation current exceeds the threshold value, a visual indication can be provided.  
         [0028]     The network can include a memory configured to store a predetermined threshold value of bias current. The network can include a communication logic configured to compare the predetermined threshold value of bias current to the adjusted bias supplied to a laser diode. If the adjusted bias current exceeds the threshold value, a visual indication can be provided.  
         [0029]     The network can include a memory configured for storing trace histories of the modulation and/or bias current adjustments. The network can include communication logic configured to predict the end of life of a first optical transceiver&#39;s laser diode on the basis of the trace histories of the modulation and/or bias current adjustments.  
         [0030]     The network can include communication logic configured to provide a visual indication reflecting a predicted time to end of life.  
         [0031]     Advantages of the invention can include one or more of following. Aspects of the invention enable the control of extinction ratio in optical fiber networks without the use of ancillary detectors such as photodiodes dedicated exclusively for extinction ratio monitoring. This allows extinction ratio to be controlled with fewer components than conventional systems. Moreover, aspects of the invention accurately control extinction ratio by using optical transceivers capable of accurately detecting high and low power levels in the data signal. Further, aspects of the invention provide for an efficient way to maintain an optical network over time as components reach their end of life.  
         [0032]     The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims. 
     
    
     DESCRIPTION OF DRAWINGS  
       [0033]      FIG. 1  shows optical power as a function of current for an optical transmitter over time.  
         [0034]      FIG. 2  shows an optical fiber network.  
         [0035]      FIG. 3  shows a block diagram of a passive optical fiber network.  
         [0036]      FIG. 4  is a flow diagram showing a method of controlling extinction ratio in an optical network. 
     
    
       [0037]     Like reference symbols in the various drawings indicate like elements.  
       DETAILED DESCRIPTION  
       [0038]      FIG. 2  shows a high-level fiber optic data network  50 . The network includes a first transceiver  200  in communication with a second transceiver  201  via a fiber  208 . The first transceiver  200  and the second transceiver  201  include transmitter circuitry (Tx)  234 ,  235  to convert electrical data input signals into modulated light signals for transmission over the fiber  208 . In addition, the first transceiver  200  and the second transceiver  201  also include receiver circuitry (Rx)  233 ,  236  to convert optical signals received via the fiber  208  into electrical signals and to detect and recover encoded data and/or clock signals. First transceiver  200  and second transceiver  201  may contain a micro controller (not shown) and/or other communication logic and memory  231 ,  232  for network protocol operation. Although the illustrated and described implementations of the transceivers  200 ,  201  include communication logic and memory in a same package or device as the transmitter circuitry  234 ,  235  and receiver circuitry  233 ,  236 , other transceiver configurations may also be used.  
         [0039]     First transceiver  200  transmits/receives data to/from the second transceiver  201  in the form of modulated optical light signals via the optical fiber  208 . The transmission mode of the data sent over the optical fiber  208  may be continuous, burst or both burst and continuous modes. Both transceivers  200 ,  201  may transmit a same wavelength (e.g., the light signals are polarized and the polarization of light transmitted from one of the transceivers is perpendicular to the polarization of the light transmitted by the other transceiver). Alternatively, a single wavelength can be used by both transceivers  200 ,  201  (e.g., the transmissions can be made in accordance with a time-division multiplexing scheme or similar protocol).  
         [0040]     In another implementation, bi-directional wavelength-division multiplexing (WDM) may also be used. Bi-directional WDM is herein defined as any technique by which two optical signals having different wavelengths may be simultaneously transmitted bi-directionally with one wavelength used in each direction over a single fiber. In yet another implementation, bi-directional dense wavelength-division multiplexing (DWDM) may be used. Bi-directional DWDM is herein defined as any technique by which more than two optical signals having different wavelengths may be simultaneously transmitted bi-directionally with more than one wavelength used in each direction over a single fiber with each wavelength unique to a direction. For example, if wavelength division multiplexing is used, the first transceiver  200  may transmit data to the second transceiver  201  utilizing a first wavelength of modulated light conveyed via the fiber  208  and, similarly, the second transceiver  201  may transmit data via the same fiber  208  to the first transceiver  200  utilizing a second wavelength of modulated light conveyed via the same fiber  208 . Because only a single fiber is used, this type of transmission system is commonly referred to as a bi-directional transmission system. Although the fiber optic network illustrated in  FIG. 2  includes a first transceiver  200  in communication with a second transceiver  201  via a single fiber  208 , other implementations of fiber optic networks, such as those having a first transceiver in communication with a plurality of transceivers via a plurality of fibers (not shown), may also be used.  
         [0041]     Electrical data input signals (Data IN  1 )  215 , as well as any optional clock signal (Data Clock IN  1 )  216 , are routed to the transceiver  200  from an external data source (not shown) for processing by the communication logic and memory  231 . Communication logic and memory  231  process the data and clock signals in accordance with an in-use network protocol. Communication logic and memory  231 , 232  provides management functions for received and transmitted data including queue management (e.g., independent link control) for each respective link, demultiplexing/multiplexing and other functions as described further below. The processed signals are transmitted by the transmitter circuitry  234 . The resulting modulated light signals produced from the first transceiver&#39;s  200  transmitter  234  are then conveyed to the second transceiver  201  via the fiber  208 . The second transceiver  201 , in turn, receives the modulated light signals via the receiver circuitry  236 , converts the light signals to electrical signals, processes the electrical signals using the communication logic and memory  232  (in accordance with an in-use network protocol) and, optionally, outputs the electrical data output signals (Data Out  1 )  219 , as well as any optional clock signals (Data Clock Out  1 )  220 .  
         [0042]     Similarly, the second transceiver  201  receives electrical data input signals (Data IN  1 )  223 , as well as any optional clock signals (Data Clock IN)  224 , from an external data source (not shown) for processing by the communication logic and memory  232  and transmission by the transmitter circuitry  235 . The resulting modulated light signals produced from the second transceiver&#39;s  201  transmitter  235  are then conveyed to the first transceiver  200  using the optical fiber  208 . The first transceiver  200 , in turn, receives the modulated light signals via the receiver circuitry  233 , converts the light signals to electrical signals, processes the electrical signals using the communication logic and memory  231  (in accordance with an in-use network protocol), and, optionally, outputs the electrical data output signals (Data Out  1 )  227 , as well as any optional clock signals (Data Clock Out  1 )  228 .  
         [0043]     Fiber optic data network  50  may also include a plurality of electrical input and clock input signals, denoted herein as Data IN N  217 / 225  and Data Clock IN N  218 / 226 , respectively, and a plurality of electrical output and clock output signals, denoted herein as Data Out N  229 / 221  and Data Clock Out N  230 / 222 , respectively. The information provided by the plurality of electrical input signals may or may not be used by a given transceiver to transmit information via the fiber  208  and, likewise, the information received via the fiber  208  by a given transceiver may or may not be outputted by the plurality of electrical output signals. The plurality of electrical signals denoted above can be combined to form data plane or control plane bus(es) for input and output signals respectively. In some implementations, the plurality of electrical data input signals and electrical data output signals are used by logic devices or other devices located outside (not shown) a given transceiver to communicate with the transceiver&#39;s communication logic and memory  231 ,  132 , transmit circuitry  234 ,  235 , and/or receive circuitry  233 , 236 .  
         [0044]      FIG. 3  illustrates an implementation of a passive optical network (PON)  52 , where the functions described above associated with the first transceiver  200  and the second transceiver  201  of  FIG. 2 , are implemented in an optical line terminator (OLT)  350  and one ore more optical networking units (ONU)  355 , and/or optical networking terminals (ONT)  360 , respectively. PON(s)  52  may be configured in either a point-to-point network architecture, wherein one OLT  350  is connected to one ONT  360  or ONU  355 , or a point-to-multipoint network architecture, wherein one OLT  350  is connected to a plurality of ONT(s)  360  and/or ONU(s)  355 . In the implementation shown in  FIG. 3 , an OLT  350  is in communication with multiple ONTs/ONUs  360 ,  355  via a plurality of optical fibers  352 . The fiber  352  coupling the OLT  350  to the PON  52  is also coupled to other fibers  352  connecting the ONTs/ONUs  360 ,  355  by one or more passive optical splitters  157 . All of the optical elements between an OLT and ONTs/ONUs are often referred to as the Optical Distribution Network (ODN). Other alternate network configurations, including alternate implementations of point-to-point and point-to-multipoint networks are also possible.  
         [0045]     A receiver RX  236  of a transceiver  201  receives optical data transmissions from another transceiver  200  in the form of modulated light. The receiver RX  236  is capable of digitally measuring the received optical power of the data transmissions. The digital measurements include the received optical power for the high and the low data transmission and/or the difference between the optical high and the optical low data transmissions. The Communication Logic &amp; Memory  232  of transceiver  201  stores the digital measurement(s) for eventual transmission back to the transmitting transceiver  200 . Additionally the Communication Logic &amp; Memory  232  may compute and store, an average of the stored high, low and/or difference values for eventual transmission back to the transmitting transceiver  200 . The Communication Logic &amp; Memory  232  may also compute and store the difference between a desired value and the stored values for eventual transmission back to the transmitting transceiver  200 . The Communication Logic &amp; Memory  232  can include volatile and/or non-volatile memory, registers, buffers, or other circuitry for storing data. The transmission of the digital measurement(s) is accomplished by multiplexing a message containing the digital measurement(s) into the user data, management and/or control traffic of the network protocol in-use.  
         [0046]     Various events can trigger the transceiver  201  to begin measuring and/or storing data about the extinction ratio and average received power of the received modulated light. For example, the transceiver  201  can perform the measurements automatically at predetermined intervals. The transceiver  201  can also receive a message to measure extinction ratio and/or average power from some other transceiver in the fiber optical network. This message can come from the transmitting transceiver  200 , or from some upstream transceiver, for example, a transceiver that can transmit to transceiver  201 .  
         [0047]     Transmitting transceiver  200  may have prior knowledge of receiving transceiver&#39;s  201  desired received extinction ratio and desired received average optical power. Alternatively, receiving transceiver  201  may transmit its desired received extinction ratio and desired received average optical power with the digital measurement(s). Once transmitting transceiver  200  receives the digital measurement(s) and/or the any of the stored values described above, the extinction ratio and average transmitting optical power of transmitter Tx  234  may be adjusted. The adjustment of the average transmitting power is accomplished by changing the I bias  current to the laser diode contain in transmitter Tx  234  appropriately to match receive transceiver&#39;s  201  desired received optical power based on the digital measurement(s). The adjustment of the extinction ratio is accomplished by changing the range of the I mod  current to the laser diode contain in transmitter Tx  234  appropriately to match the receive transceiver&#39;s  201  desired received extinction ratio based on the digital measurement(s).  
         [0048]      FIG. 4  is a flow chart diagram showing a method of controlling extinction ratio. First a receiving transceiver measures the optical power highs and lows of a received data signal  410 . Next, the average received optical power, the difference between the high and low power level, and the extinction ratio are calculated  420 . This information or a subset thereof is then transmitted through the network to the transmitting transceiver  430 . The measured values and/or calculated values are then compared with predetermined values for extinction ratio and average transmitted power  440 . The bias and modulation current of the laser diode in the transceiver&#39;s transmitter are then adjusted such that the average power and extinction ratio of the data signal received at the receiving transceiver match the predetermined values  450 .  
         [0049]     With a trace history of changes to a transceiver&#39;s extinction ratio and/or average transmitted power (e.g. I bias  and I mod  current changes) or with knowledge of present I bias  current value and range of I mod  current, a prediction can be made of a period of time before “end of life” of the transceiver&#39;s laser diode. The trace history may be stored at the transceiver, for example in the communication logic and memory, or at a network entity operating at an application layer in the protocol in-use according to the Open Systems Interconnection (OSI) 7 layer reference model (hereby included by reference). Alternatively, the transceiver may also have a predetermined thresholds for I bias  and I mod  currents to predict the “end of life” of its laser diode. Once the I bias  and I mod  currents pass or cross the thresholds the transceiver may give a visual indication of having reached the predetermined prediction period or period before “end of life”. In either cases, the transceiver may declare by means of a visual indication of having reached the period before “end of life” e.g., light an LED, change an LED&#39;s color or generate a message to a network entity operating at an OSI application layer via the protocol in-use resulting in a visible report. The comparing and declaration functions can be implemented in the communication logic.  
         [0050]     Once a transceiver is not able to adjust its extinction ratio to meet a desired extinction ratio then the laser diode within the transceiver is declared to have reached its “end of life”. Alternatively declaring “end of life” may be triggered by detecting I bias  and I mod  currents passing or crossing a predetermined threshold wherein the laser diode consumers too much power to maintain a desired extinction ratio or average transmitted power. In either case, the transceiver may declare by means of a visual indication of having reached “end of life” e.g., light an LED, change an LED&#39;s color or generate a message to a network entity operating at an OSI application layer via the protocol in-use resulting in a visible report.  
         [0051]     Although the invention has been described in terms of particular implementations, one of ordinary skill in the art, in light of this teaching, can generate additional implementations and modifications without departing from the spirit of or exceeding the scope of the claimed invention. Accordingly, it is to be understood that the drawings and descriptions herein are proffered by way of example to facilitate comprehension of the invention and should not be construed to limit the scope thereof. Accordingly, other embodiments are within the scope of the following claims.

Technology Classification (CPC): 7