Abstract:
In an optical telecommunication system in which an intensity of an arriving optical signal is different for each packet, detected is an optical intensity for each packet with little error. For this purpose, contrived is to detect an average optical intensity across header parts for each packet by focusing on the fact that the header part comprising the preamble and delimiter of a packet is in a bit pattern which includes approximately the same numbers of “0” and “1”.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to an optical monitor for use in a reception unit of an optical line terminal (OLT) in a high speed passive optical network (PON). 
         [0003]    2. Description of the Related Art 
         [0004]      FIG. 1  is a diagram describing a PON telecommunication system. 
         [0005]    The PON telecommunication system is one in which an optical star coupler  10  receives an optical signal from each of clients ONU # 1  through # 3  and transmits it to an OLT  11 . An optical signal from each of the clients ONU# 1  through # 3  is in a packet form. Only the optical star coupler  10  is installed between the clients ONU# 1  through # 3  and the OLT  11 , and an active apparatus such as an optical amplifier is not installed. The individual clients ONU# 1  through # 3  send out optical packet signals at timings determined between themselves and the OLT  11 . 
         [0006]    Here, an optical packet signal sent from each of the clients ONU# 1  through # 3  is input to the optical star coupler  10  with a different loss caused by each transmission path because the characteristic and/or length of an optical transmission path connecting between each of the clients ONU# 1  through # 3  and the optical star coupler  10  are different. That is, an optical packet signal propagating through a transmission path with a low loss remains as a high optical power when it is mixed with other optical packet signals at the optical star coupler  10 , while one propagating through a transmission path with a high loss remains as a low optical power when it is mixed with other optical packet signals at the optical star coupler  10 . Therefore, optical signals sent from the optical star coupler  10  to the OLT  10  are burst signals with different optical powers for each of the packet signals. 
         [0007]    In such a burst telecommunication, since optical powers of optical packet signals from individual client ONU # 1  through # 3  are different, it is possible to obtain information of the path from each client in real time if an optical power of each optical packet signal can be monitored for each packet. Therefore, it is desired to measure an optical power for each packet in the above described PON telecommunication system. 
         [0008]    A conventional power monitor is mainly the function of monitoring an average optical power of an optical signal over a predetermined time period and outputting it, whereas a function of monitoring an optical power for each packet is a technique that is not conventionally available. 
         [0009]    A patent document 1 has noted a technique for receiving a preamble bit preceding a burst signal and controlling an amplifier for a PON system. 
         [0010]    [Patent document 1] Japanese Patent Application Publication No. 2000-151290 
         [0011]    Since an optical signal intensity-modulated causes a difference of optical power between a “0” sign and a “1” sign, an input optical power is specified by an intermediate optical power between the “0” and “1”. However, if long bits of the same sign are included in a data part of an optical packet, the average optical power is biased toward an optical power of the “0” or “1”, hence making impossible to evaluate an intermediate value during such a period. 
       SUMMARY OF THE INVENTION 
       [0012]    The object of the present invention is to provide an optical power monitor capable of correctively measuring an optical power for each packet for use in an optical telecommunication system in which an optical power is different in each packet. 
         [0013]    According to the present invention, an optical power monitor for use in an optical telecommunication system in which an optical signal is transmitted and received in a packet format comprises an integration unit for integrating an electrical signal corresponding to an intensity of an optical signal with a mark ratio at the head of a packet being shorter time constant than a fixed time length; and an obtaining unit for obtaining an integration value of the integration unit to make it an optical intensity of a measurement target packet according to an instruction of the integration unit. 
         [0014]    The present invention is contrived to enable a high accuracy monitoring of an optical power for each packet and a real time obtainment of information of a path connecting between each client, which is the transmitter of the packet, and an OLT in the OLT for use in a PON telecommunication system. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]      FIG. 1  is a diagram describing a PON telecommunication system; 
           [0016]      FIG. 2  is a diagram describing an embodiment of the present invention; 
           [0017]      FIG. 3  is a configuration block diagram of an optical reception unit according to an embodiment of the present invention; 
           [0018]      FIG. 4  is a diagram showing an extraction of a configuration only pertaining to a power monitor and its periphery according to an embodiment of the present invention; 
           [0019]      FIG. 5  is a flow chart showing an overall operation flow of an embodiment of the present invention; and 
           [0020]      FIG. 6  exemplifies a circuit configuration of a power monitor circuit. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0021]      FIG. 2  is a diagram describing an embodiment of the present invention. 
         [0022]    The header part of a packet includes a time zone of a fixed length called “preamble” and “delimiter” in which “1” and “0” signs appear approximately equally. The “preamble” is a field for synchronizing data (i.e., adjusting a threshold so as to enable a correct detection of the signal values “1/0”) and extracting a clock, while the “delimiter” is a field indicating a start of a frame in the head of a packet. The delimiter is followed by a payload part in which the appearance of “1/0” signs depends on a data content. And a measurement of an average optical power across the field obtains a measurement value of an approximate intermediate value between “1” and “0” of an optical packet signal in the field, such as preamble and delimiter, in which “1/0” signs appear approximately equally. Accordingly, a monitoring of an optical power only within the time period of the header part comprising the preamble and delimiter and a retaining of an optical power in the payload part avoid a bias (i.e., an error) in a measurement value of an optical power occurring at the payload part. This in turn enables a correct measurement of the optical power for each packet as an intermediate value of 1/0 signs. 
         [0023]      FIG. 3  is a configuration block diagram of an optical reception unit according to the embodiment of the present invention. 
         [0024]    An optical packet signal is input to an optical/electrical conversion circuit  15  for being converted into an electrical signal. The optical/electrical conversion circuit  15  transmits a packet which is converted into an electrical signal to a power monitor circuit  16 . A reset signal generation/output unit  19  transmits a Reset signal to the power monitor circuit  16  and a clock counter  20 . The power monitor circuit  16  and clock counter  20  are reset by the Reset signal which is the one inserted between packets without exception, and the one for indicating the completion of a packet, on the system side. The Reset signal is the one resetting a receiver at every completion of packet and for making the receiver ready. The clock counter  20  transmits a monitor hold signal to the power monitor circuit  16 . The monitor hold signal is output approximately at the end of the delimiter. Having received a Reset signal, the clock counter  20  starts counting, detects the end position of the delimiter by a count value and generates a monitor hold signal. The power monitor circuit  16  outputs a monitor value at the current time to a power monitor arithmetic operation circuit  17  which then calculates an optical power from the output of the power monitor circuit  16  and stores it in an external output memory  18 . 
         [0025]      FIG. 4  is a diagram showing an extraction of a configuration only pertaining to a power monitor and its periphery according to the embodiment of the present invention. 
         [0026]    With a Reset signal inserted between packets as trigger, initiated are a resetting of the power monitor circuit  16  and a starting of counting the clock. The clock counter  20  outputs a hold signal at a predetermined time length after detecting a Reset signal. That is, the configuration is in a manner to output the hold signal before the clock count value passes the end of a delimiter, since the clock count value from a Reset signal to the end of a delimiter is predetermined. The power monitor circuit  16  is configured to rise in the time range in the preamble zone and delimiter zone, and have a time constant so as to obtain an average power between the preamble and delimiter. For example, it is configured to have a time constant of 0.8t where “t” is the time of a zone combining the preamble and delimiter. 
         [0027]      FIG. 5  is a flow chart showing an overall operation flow of the embodiment of the present invention. 
         [0028]    First, before inputting a packet signal, the Reset signal generation/output unit  19  generates and outputs a Reset signal. Therefore, the step S 10  is to wait for a Reset signal being generated and output. Then in the step S 11 , with the Reset signal as trigger, a hold output value of the power monitor circuit  16  is reset and the clock counter  20  starts counting. The time constant of the power monitor circuit  16  is designed so as to stabilize an output between packet header parts, the power is stabilized at the clock counter which generates and outputs a hold signal before the end of the header part and holds the output of the optical power monitor circuit  16 . That is, in the step S 12 , waiting for the clock counter  20  counting up to a predetermined count value and upon counting up, the power monitor circuit  16  holds the power monitor value, in the step S 13 . The above described operation continues during the operation of the telecommunication system. 
         [0029]      FIG. 6  exemplifies a circuit configuration of a power monitor circuit. 
         [0030]    A current mirror  25  is connected to a power supply voltage Vdd, and a photo diode PD 27  is connected to one terminal of the current mirror  25 . The photo diode PD 27  converts a received optical signal into an electrical signal. The electrical signal is intensity-modulated, and a current Ipd proportional to an optical intensity input to the photo diode PD 27  flows therein. The current Ipd which has flown in the photo diode PD 27  is connected to the ground, while it is omitted in the delineation of the drawings. The current Ipd becomes the one converted into an electrical signal from the optical signal. The signal converted into the electrical signal by the photo diode PD 27  is amplified by a trans-impedance amplifier (preamplifier)  26  and transmitted to a signal processing circuit at a later stage. Meanwhile, the current which has flown in the photo diode PD 27  is duplicated by the current mirror  25  to appear in another terminal which is connected to an integrator  28 . The integrator  28  has a time constant possessing a predetermined value of a time from a Reset signal to the end of a delimiter as described before, and integrates, at the time constant, an electrical signal corresponding to an intensity of light transmitted from the current mirror  25 . A sample hold circuit  29  samples, and holds, an integration value of the integrator  28  at the timing before the end of the delimiter. This configuration obtains an integration value corresponding to an average optical intensity between “1” and “0” of an optical signal. The signal value held by the sample hold circuit  29  is an analog value and therefore an analog-digital converter  30  converts the analog value into a digital value and outputs it.