Subscriber terminal of adjusting intensity of optical signal by controlling attenuation, and a method thereof

A subscriber terminal connected to a central-office unit in an optical communication network includes a variable optical attenuator for attenuating an optical signal received from the central-office unit; an optical-electric converter for converting the optical signal received via the attenuator to a corresponding electric signal; a clock extractor for extracting a clock from the electric signal and producing a clock extraction information signal representing whether or not the clock is extracted stably; and a terminal controller. The controller includes a clock extraction decider for determining whether or not the extractor stably extracts the clock on the basis of the information signal, a receiving level adjuster for setting an attenuation value to a value between a minimum and a maximum value, and an attenuation controller for setting the attenuation amount for the attenuator to the set attenuation value.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a subscriber terminal, particularly for use in a passive optical communication network (PON) in which a plurality of subscriber terminals are connected for communication to a central-office unit. The invention also relates to a method for adjusting the intensity of an optical signal in such a subscriber terminal.

2. Description of the Background Art

As a typical example of a communication system between a central-office unit and a plurality of subscriber terminals, a passive optical network (PON) is known. The PON system includes a single central-office unit, a plurality of subscriber terminals and an optical splitter, which interconnects the central-office unit and the subscriber terminals with optical fibers. Applications where the PON system is collaborated with the code division multiplexing (CDM) scheme can enjoy various advantages inherent to the CDM scheme.

Telecommunications network systems utilizing the CDM scheme use a code common to both transmitter and receiver sides, thus accomplishing high security for communication. Additionally, the network system utilizing the CDM scheme allows transmission data from plural sources to be multiplexed in a single time slot. The use of the CDM scheme thus accomplishes a larger capacity of data communications with communication resources, such as time slots, saved as referred to in U.S. Pat. No. 7,630,642 B1 to Tamai et al., for example.

Optical fibers for use in the PON system bring an attenuation rate of about 0.5 dB/km. In such a case, a subscriber terminal residing at a distance from the optical splitter further than another subscriber terminal by 10 km, for example, may receive an optical signal transmitted by the central-office unit weaker in optical intensity than the other subscriber terminal by about 5 dB. If the subscriber terminals have the tolerable range thereof, i.e. dynamic or receivable range, equal to about 5 dB, some of the subscriber terminals distancing themselves from the optical splitter further than others by 10 km or more may receive optical signals out of the dynamic ranges, thus failing to properly receive the optical signals.

Particularly, when the PON system is powered on to perform a presence check for the subscriber terminals by the central-office unit, the subscriber terminals may receive a too strong intensity of optical signals transmitted by the central-office unit, thus failing to properly receive the signals. As a result, the central-office unit may not able to check the presence of the subscriber terminals.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a subscriber terminal for use in a passive optical communication network capable of receiving, when its presence is checked, an optical signal with its intensity kept within the dynamic range. It is also an object of the invention to provide such a method for adjusting the intensity of an optical signal.

In accordance with the present invention, a subscriber terminal for use in an optical communication network including a central-office apparatus comprises a variable optical attenuator, an optical-electric converter, a clock extractor, and a controller. The variable optical attenuator attenuates an optical signal received from the central-office apparatus. The optical-electric converter converts the optical signal received via the variable optical attenuator to a corresponding electric signal. The clock extractor extracts a clock from the electric signal and produces a clock extraction information signal representing whether or not the clock is extracted stably.

The controller controls the variable optical attenuator, and includes a clock extraction decider, a receiving level adjuster and an attenuation controller. The clock extraction decider determines whether or not the clock extractor stably extracts the clock on the basis of the clock extraction information signal. The receiving level adjuster sets an attenuation value to a value between a minimum value and a maximum value, which are predetermined. The attenuation controller sets an attenuation amount for the variable optical attenuator to the attenuation value.

Further in accordance with the invention, an optical communication network includes the above-described subscriber terminal.

Still further in accordance with the invention, a method for adjusting the intensity of a downstream optical signal received by the subscriber terminal in the optical communication network includes the following steps. First, an attenuation amount for a variable optical attenuator is adjusted to one predetermined extreme value, minimum or maximum. Next, it is determined whether or not a clock extractor included in the subscriber terminal stably extracts the clock. It is further determined, when determining that the clock is not extracted stably, whether or not the attenuation amount is equal to the other predetermined extreme value, maximum or minimum. When the attenuation amount is equal to the other extreme value, the attenuation amount is subsequently changed to the one extreme value. Otherwise, namely, when the attenuation amount is not equal to the other extreme value, the attenuation amount is subsequently changed by one stage toward the other extreme value. It is then determined whether or not the clock extractor stably extracts the clock.

According to the present invention, the subscriber terminal controls the attenuation amount for the optical signal received from the central-office apparatus, for example, when having its presence checked. That makes the intensity of the optical signal received by the subscriber terminal kept within the dynamic range to thereby ensure reception of a presence check request signal from the central-office apparatus.

DESCRIPTION OF THE PREFERRED EMBODIMENT

A preferred embodiment of the present invention will So far as the positional relationship of the components is concerned, the figures conceptually show them merely to the extent of understanding the invention. The preferred embodiment of the present invention will be described with the numerical conditions or the like are just exemplified. Therefore, the present invention is not understood restrictive to the specific embodiment. Those skilled in the art may change or modify the illustrative embodiment so as to accomplish the advantages of the present invention without departing from the scope and spirit of the present invention.

As an example of optical communication networks according to the embodiment of the invention, a passive optical network (PON) system100will be described with reference toFIG. 1. The PON system100includes a single central-office unit, or an optical line terminal (OLT),200, which is interconnected to an optical splitter400by an optical fiber410-0. The optical splitter400is further interconnected to a plurality (n) of subscriber terminals, or optical network units (ONUs),300-1to300-nby a corresponding plurality of optical fibers410-1to410-n, where n is an integer equal to or more than two. In the context, the optical network units300-1to300-nand the optical fibers410-1to410-n, may sometimes be designated simply with300and410, respectively.

The configuration of the optical line terminal200in the PON system100will be described with reference to FIG.2. The optical line terminal200generally includes a transmitter210, an electric-optical converter220, an optical multiplexer/demultiplexer230, an optical-electric converter240, a receiver250and a central-office controller260, which are interconnected as shown.

The transmitter210includes, for example, an encoder and an addition multiplexer, not specifically shown, and is adapted to receive transmission data, indicated by a thin arrow S261in the figure, from the central-office controller260to produce a code division multiplexed (CDM) signal S211. In the PON system100including the plurality (n) of optical network units300-1through300-n, the CDM signal S211has n channels of signals multiplexed. The transmitter210sends the generated CDM signal S211to the electric-optical converter220.

The electric-optical converter220is an electro-optical conversion device advantageously comprising, for example, a laser diode (LD) to convert the CDM signal S211in the form of electric signal to a corresponding CDM optical signal depicted with a thick arrow S221in the figure. The electric-optical converter220transfers the CDM optical signal S221to the optical multiplexer/demultiplexer230.

The optical multiplexer/demultiplexer230may advantageously comprise, for example, an optical circulator. The optical multiplexer/demultiplexer230is adapted to receive the CDM optical signal S221from the electric-optical converter220and transmit the received signal S231to the respective optical network units300. The optical multiplexer/demultiplexer230is also adapted to receive the CDM optical signal S301from the respective optical network units300and transfer the received signal S233to the optical-electric converter240.

The optical-electric converter240may advantageously comprise, for example, an avalanche photodiode (APD) adapted to convert the CDM optical signal S233to a corresponding CDM signal S241in the form of electric signal. The optical-electric converter240sends the CDM signal S241to the receiver250.

The receiver250may advantageously comprise, for example, a charge-coupled device (CCD) matched filter and a comparator, not shown. The CCD matched filter is adapted to calculate the convolution of the CDM signal S241with a code assigned to the CCD matched filter. The comparator is adapted to obtain received data S251from a correlation signal resultant from the convolution calculation by the CCD matched filter. The receiver250sends the received data S251to the central-office controller260.

The central-office controller260may advantageously comprise, for example, a field programmable gate array (FPGA). The central-office controller260may be designed to implement desired functions, such as generation of the transmission data S261, an analysis on the header of the received data S251and a presence check of each optical network unit300, which are necessary for communication over the PON system100by means of the CDM scheme.

Now, with reference toFIG. 3, description will be made on the configuration of the optical network unit300, which may be any of the optical network units300-1to300-n, which are the same in structure and function as each other. The optical network unit300generally includes a transmitter310, an electric-optical converter320, an optical multiplexer/demultiplexer330, an optical-electric converter340, a receiver350, a terminal controller360, a variable optical attenuator (VOA)370, a clock generator380and a clock extractor390, which are interconnected as depicted.

The variable optical attenuator370functions as attenuating an optical signal S371to be transmitted by the optical network unit300toward the optical line terminal200, often referred to as upstream optical signal, and another optical signal S201received by the optical network unit300from the optical line terminal200, often referred to as downstream optical signal by a common amount of attenuation.

The transmitter310may advantageously comprise, for example, an encoder adapted to encode transmission data S361received from the terminal controller360by a code assigned to that optical network unit300to thereby generate a code spread signal. The transmitter310sends the generated code spread signal S311to the electric-optical converter320.

The electric-optical converter320may advantageously comprise, for example, a laser diode (LD) adapted to convert the code spread signal S311in the form of electric signal to a corresponding code spread optical signal S321. The electric-optical converter320sends the code spread optical signal S321to the optical multiplexer/demultiplexer330.

The optical multiplexer/demultiplexer330may advantageously comprise, for example, an optical circulator, which is adapted to receive the code spread optical signal S321from the electric-optical converter320and send the received signal S333to the variable optical attenuator370. The code spread optical signal S371, when transmitted via the variable optical attenuator370toward the optical line terminal200, is combined by the optical splitter400,FIG. 1, with other code spread optical signals transmitted from other optical network units300to be converted to a CDM optical signal410-0. The optical line terminal200receives this CDM optical signal400-0as a signal from the respective optical network units300. Thus, signals are designated with reference numerals of connections on which they are conveyed.

The optical multiplexer/demultiplexer330is also adapted for receiving a CDM optical signal S373from the optical line terminal200via the variable optical attenuator370and sends the received signal S331to the optical-electric converter340.

The optical-electric converter340may advantageously comprise, for example, an avalanche photodiode (APD) adapted to convert the CDM optical signal S331to a corresponding CDM signal in the form of electric signal. The CDM optical signal goes onto two separate routes so as to be sent on one route S341to the receiver350and on another route S343to the clock extractor390.

The clock extractor390serves as extracting a clock signal from the CDM signal S343. The clock signal goes onto two separate routes so as to be sent on one route S391to the receiver350and on another route S393to the terminal controller360.

The clock extractor390also produces a clock extraction information signal S395representing whether or not a clock is extracted stably. The clock extraction information signal S395is transferred to the terminal controller360. The clock extractor390may advantageously comprise, for example, a clock data recovery (CDR) circuit, such as model ADN2812 manufactured by Analog Devices, Inc. This CDR circuit outputs a loss of lock (LOL) signal as the clock extraction information signal. The LOL signal takes its binary value “0” when the clock is stably extracted, and its binary value “1” when no clock is extracted.

The receiver350may advantageously comprise, for example, a CCD matched filter and a comparator. The CCD matched filter is adapted to use the clock signal S391received from the clock extractor390to calculate a convolution of the CDM signal S341with a code assigned to the CCD matched filter. The comparator is adapted for using a correlation signal resultant from a convolution calculation by the CCD matched filter to obtain received data S351. The receiver350sends the received data S351to the terminal controller360.

In the optical network unit300, the transmitter310and the receiver350is operative in response to the clock extracted from the optical signal received from the optical line terminal200for carrying out encoding and decoding and adjusting the received-signal intensity as described later on. However, where a clock that would have been transmitted from the optical line terminal200cannot be extracted, as with presence check, a clock generated by the clock generator380is used. The clock generator380may be similar one used in a general PON system. Note that a similar clock generator is also provided in the optical line terminal200although a description is refrained from.

The terminal controller360may advantageously comprise, for example, a field programmable gate array (FPGA). The terminal controller360may be designed to implement desired functions, such as generation of CDM frames and an analysis on the header of the received data, which are necessary for communication over the PON system100by means of the CDM scheme.

In the illustrative embodiment in accordance with the present invention, in order to adjust the received-signal intensity, the terminal controller360of the optical network unit300includes a clock extraction decider362, a receiving level adjuster364and an attenuation controller366, which may advantageously be implemented by the FPGA programmed as such, or software stored therein. The illustrative embodiment of the terminal controller360is depicted and described as configured by separate functional blocks as above. It is however to be noted that such a depiction and a description do not restrict the terminal controller360to an implementation only in the form of hardware but at least the controller360may partially or entirely be implemented by software, namely, by an FPGA or a computer, so programmed or having a computer program installed and functions, when executing the computer program, as part of, or the entirety of, the terminal controller360. In this connection, the word “circuit” maybe understood not only as hardware, such as an electronics circuit, but also as a function that may be implemented by software installed and executed on a computer.

The clock extraction decider362serves as using the clock extraction information signal S395to determine whether or not the clock extractor390stably extracts the clock. The receiving level adjuster364serves as adjusting a set value of an attenuation amount for the variable optical attenuator370. In the following description, the set value of an attenuation amount may often be referred to as an attenuation value or VOA value. The attenuation controller366is operative in response to a value set by the receiving level adjuster364to direct the variable optical attenuator370to change its attenuation amount.

Next, the presence check in the PON system100will be described. The presence check may be performed when the optical line terminal200and optical network units300are powered on, as in the case of starting up the PON system100. Each optical network unit200waits for, when powered on, a signal to be transmitted from the optical line terminal200. On the other hand, the optical line terminal200starts, when powered on, the presence check for the optical network units300involved in the PON system100.

In operation, the optical line terminal200converts a frame including a header and a data signal to a CDM optical signal to transmit the latter signal to the optical network units300. When performing a presence check, the line terminal200forms such a header including a presence check request. The optical network units300analyze the header of a received frame and checks whether or not the received signal includes the presence check request.

The optical network units300transmit, when finds the presence check request, a presence check request acknowledgement signal toward the optical line terminal200. Upon the optical line terminal200having received the presence check request acknowledgement signal or a predetermined period of time having elapsed without receiving a presence check request acknowledgement signal, the presence check finishes.

In the prior art, during the course of presence checking, the distance of an optical network unit from the optical line terminal may cause the optical network unit to fail to receive an optical signal transmitted from the line terminal with its intensity out of the receivable or dynamic range of that optical network unit. In such a case, the optical network unit cannot receive the presence check request from the optical line terminal.

In order to overcome such a situation that would be caused in the prior art, the illustrative embodiment of the optical network unit300is adapted to adjust or control the intensity of a received signal during the presence checking.

Now with reference toFIGS. 4 and 5, it will be described how the received-signal intensity is adjusted or controlled in the optical network unit300.FIG. 4is a flowchart useful for understanding the operation of attenuation control in the optical network unit300.FIG. 5is a timing chart useful for understanding how to adjust the received-signal intensity in the optical network unit300.FIG. 5shows how the VOA value changes during one optical network unit300proceeding to an ONU receiving level adjusting process.FIG. 5has its horizontal and vertical axes representing time (t) and the value of attenuation amount (VOA value) for the variable optical attenuator370, respectively.

First, in a step S10, the receiving level adjuster364,FIG. 3, sets an attenuation value to a predetermined minimum value MIN. The minimum value MIN may be determined depending on the specifications of the variable optical attenuator370and readably stored in a storage, not shown, included in the optical network unit300. The receiving level adjuster364reads out the minimum value from the storage to set the attenuation value to the minimum value, at time T0shown inFIG. 5.

The attenuation controller366sends information S362on the attenuation value set by the receiving level adjuster364to the variable optical attenuator370. The variable optical attenuator370changes its attenuation amount according to the received attenuation value S362. It is to be noted that the variable optical attenuator370is adapted to attenuate not only the optical signal S201or S231received by the optical network unit300but also the optical signal S371to be transmitted by the optical network unit300toward the optical line terminal200by the same attenuation value.

Next, in a step S20, it is decided whether or not the received signal is suitable. In the step S20, particularly, the clock extraction decider362decides whether or not the clock extractor390stably extracts the clock. This decision is performed by using the clock extraction information signal S395generated by the clock extractor390. Where the clock extraction information signal S395takes the form of LOL signal, the clock extraction decider362acts as detecting the negative-going edge, i.e. from “1” to “0”, of the LOL signal for the decision, at time T1shown inFIG. 5.

Meanwhile, if the decision were performed by detecting the negative-going edge only, a noise or the like would possibly cause a wrong decision. It is therefore preferable to determine whether or not the LOL signal keeps the state of binary “0” following the negative-going edge for a predetermined period of time.

When the deciding result from the step S20indicates “YES”, namely the clock is extracted stably, the adjusting process for the received-signal intensity is completed. Then, the optical network unit300analyzes the header of a frame included in the received signal S231and checks whether or not a presence check request is included therein. If included, then a presence check request acknowledgement signal will be transmitted toward the optical line terminal200.

By contrast, when the deciding result from the step S20indicates “NO”, i.e. a clock is not extracted stably, the processing advances to a step S30subsequently.

In the step S30, the receiving level adjuster364decides whether or not the attenuation amount is equal to a predetermined maximum value MAX. The receiving level adjuster364reads out the maximum value from the storage, not shown, included in the optical network unit300to compare the maximum value with the currently set attenuation value. The minimum value MAX may be determined depending on the specifications of the variable optical attenuator370and readably stored in the storage.

When the deciding result of the step S30indicates “YES” to decide that the attenuation value is equal to the maximum value, the processing returns to the step S10. In other words, the attenuation value for the variable optical attenuator370is changed to the minimum value (at time T1inFIG. 5).

On the other hand, the deciding result from the step S30indicates “NO”, namely the attenuation value is not equal to the maximum value, the processing advances to a step S40subsequently. In the step S40, the receiving level adjuster364increments the attenuation value by one or unit step of predetermined amount. The amount of one step by which the attenuation value is incremented or decremented may be preferably determined to an arbitrary value according to, for example, the variable range of the variable optical attenuator370. After the attenuation value is increased by one step, the processing returns to the step S20.

The adjustment of the received-signal intensity is repeated until the clock extraction decider362stably extracts a clock.

FIG. 5depicts a time interval Δt0in which the attenuation value is changed and which may be determined according to, for example, the response time of the variable optical attenuator370or a time during which the step S20requires for determining the binary state following a negative-going edge of an LOL signal as described above.

InFIGS. 6A and 6B, the horizontal and vertical axes represent time (t) and a received optical power, respectively. The adjustment of the received-signal intensity described with reference toFIG. 4uses an LOL signal to decide whether or not a clock is extracted to set the received-signal intensity accordingly. Therefore, until the receiving power reaches a proper level, the VOA value, or received optical power, is changed at the time interval Δt0determined according to, for example, the response time of the variable optical attenuator370. After the received optical power reaches the proper level for extracting the clock stably, the header is analyzed to check the reception of a presence check request signal. Thus, the presence check can be performed quickly for the period of time determined according to the response time of the variable optical attenuator370, as shown inFIG. 6A.

For example, instead of using the LOL signal, the header of the received signal may be analyzed to thereby check the reception of a presence check request signal. However, the use of the header analysis may require the time for presence check to depend upon not only the response time of the variable optical attenuator370but also the time necessary for analyzing the header. Therefore, the use of the header analysis renders the time interval Δt1,FIG. 6B, required for changing the attenuation value longer than the time interval Δt0required when using the LOL signal. The time required for the presence check is longer accordingly, as shown inFIG. 6B.

As described above, in accordance with the illustrative embodiment of the present invention, the optical network unit300controls, when having its presence checked, the intensity of an optical signal received from the optical line terminal200. That makes the intensity of the optical signal received by the optical network unit300kept within the dynamic range to thereby ensure that the presence check request signal is received from the optical line terminal200.

Additionally, when the attenuation amount is controlled by using an LOL signal from the CDR circuit, the header does not need to be analyzed during the adjustment of the received-signal intensity. Therefore, the time required for using an LOL signal to determine whether or not the clock exists is shorter than the time required for analyzing the header, thus completing the adjustment of the received-signal intensity for a shorter time.

The illustrative embodiment described above is directed to an exemplified case where the received-signal intensity is adjusted by changing the attenuation value from the minimum to the maximum value in order. However, the present invention is not to be understood as restrictive to such a case. The received-signal intensity may also be adjusted by changing the attenuation value from the maximum to the minimum value in order. In this sense, the minimum and maximum values may be referred to as extreme values which are opposite to. each other. Additionally, the present invention may not be restricted to the telecommunications system relying on the CDM scheme, but can also be applied to PON systems based upon other telecommunications schemes, such as TDM (Time Division Multiplexing) and WDM (Wavelength Division Multiplexing).

The entire disclosure of Japanese patent application No. 2009-164631 filed on Jul. 13, 20109, including the specification, claims, accompanying drawings and abstract of the disclosure, is incorporated herein by reference in its entirety.

While the present invention has been described with reference to the particular illustrative embodiment, it is not to be restricted by the embodiment. It is to be appreciated that those skilled in the art can change or modify the embodiment without departing from the scope and spirit of the present invention.