Patent Description:
There has been known a Passive Optical Network (PON) system that is an optical communication system. The PON system includes an optical communication device (referred to also as a master station device) installed in a station of a telecommunications carrier and a plurality of optical communication devices (referred to also as slave station devices) on the subscribers' side (referred to also as slave stations' side). The master station device is referred to as an Optical Line Termination (OLT). The slave station device is referred to as an Optical Network Unit (ONU).

The number of devices included in a PON system is increasing. Thus, it is being required to increase the distance between the OLT and each ONU and increase the number of branches in the network. Satisfying such requirements leads to non-constant distance between the OLT and each ONU. Besides, the OLT has to receive packet signals having great signal intensity variations. Further, in a PON system, realizing an increase in the transmission rate is being requested. Furthermore, in a PON system, realizing a new system capable of accommodating the existing system is being requested.

The OLT receives optical signals from the ONUs. Here, a device for receiving optical signals has been proposed (see Patent Reference <NUM>). This device includes a photodiode, a pre-amplifier and a reception circuit, for example. The photodiode transduces an optical signal transmitted via an optical fiber into an electric signal. The pre-amplifier amplifies the electric signal. The reception circuit includes a main amplifier and an SR-type flip-flop circuit.

The main amplifier limits an output signal outputted from the pre-amplifier to a constant voltage amplitude signal. The SR-type flip-flop circuit outputs a Signal Detect (SD) signal. The SD signal may be regarded as a signal indicating that an optical signal as a main signal has been detected. Here, detecting an optical signal is referred to as SD detection. Further, a circuit like the SR-type flip-flop circuit is referred to as an SD circuit.

Even in a state in which no optical signal is inputted to the photodiode as in a no-signal interval, the pre-amplifier outputs a signal based on random noise. The noise can generally be represented by the normal distribution. Further, due to the amplification of the signal based on the noise by the main amplifier, the SD circuit erroneously outputs the SD signal.

As mentioned above, the reception circuit is required to have the function of detecting the reception of an optical signal. Here, this function is referred to also as an SD function. The SD function may be regarded as "not erroneously outputting the SD signal when noise is acquired" and "outputting the SD signal when an optical signal within a predetermined power range is received". Incidentally, the power range is -<NUM> dBm to -<NUM> dBm, for example. Especially, achieving both "not erroneously outputting the SD signal when noise is acquired" and "outputting the SD signal when an optical signal at power lowest in the power range is received" is necessary. Incidentally, the lowest power is -<NUM> dBm, for example.

In order to realize such achievement of both conditions, the Patent Reference <NUM> describes the following features, for example: In the first feature, a difference is caused between Direct Current (DC) voltages of a normal phase of a signal and a reverse phase of the signal, and the SD signal is outputted due to intersection of the normal phase and the reverse phase. Incidentally, the difference in the DC voltage is referred to also as an offset voltage. The second feature is that the frequency of erroneously outputting the SD signal when noise is acquired (hereinafter referred to as an SD false detection frequency) can be reduced by the offset voltage. For example, the SD false detection frequency can be reduced by increasing the offset voltage.

However, increasing the offset voltage also increases the amplitude at the time of detecting an optical signal, and thus the normal phase and the reverse phase based on an optical signal having power close to minimum reception sensitivity cannot intersect with each other. Consequently, the outputting of the SD signal will never occur. Therefore, in the third feature, the reception circuit includes a counter circuit in order to further reduce the SD false detection frequency. When a count counted by the counter circuit exceeds a predetermined number, the SD circuit outputs the SD signal. Incidentally, when the count is less than the number, the SD circuit does not output the SD signal.

Patent Reference <NUM> discloses a receving circuit for receiving a burst signal that detects received optical signal other than noise.

Incidentally, the Signal-to-Noise Ratio (SNR) in an OLT included in a PON system performing high-speed transmission is lower than the SNR in an OLT included in a PON system performing low-speed transmission due to restriction on frequency characteristics of the photodiode. Therefore, it is necessary to reduce the offset voltage. However, reducing the offset voltage leads to a higher SD false detection frequency. Thus, "outputting the SD signal when an optical signal within a predetermined power range is received" cannot be realized.

Here, it is possible to consider a method of reducing the SD false detection frequency by using a counter circuit. First, the cycle of the intersection of the normal phase and the reverse phase in a case where no counter circuit exists and noise is acquired is assumed to be A seconds. The SD false detection frequency in this case is "<NUM>/A". Here, the denominator of "<NUM>/A" is referred to as a false detection time. Next, a case where the counter circuit exists and noise is acquired will be considered.

Here, the condition for outputting the SD signal is set as N intersections of the normal phase and the reverse phase. The time it takes for N intersections of the normal phase and the reverse phase is "N × A" seconds. Thus, the SD false detection frequency in this case is "<NUM>/(N × A)". As above, by increasing the false detection time to N times, the SD false detection frequency is decreased to "<NUM>/N" times.

As described above, by employing the counter circuit, the false detection time is increased without reducing the offset voltage. Here, in the case where the false detection time is increased to N times, the count increases to N times. Due to the increase in the count to N times, an SD detection time, as a time for which the SD detection is performed erroneously, also increases.

For example, the offset voltage necessary for the SD detection is assumed to be B [mV]. The false detection time in this case is assumed to be <NUM> seconds. When the false detection time is desired to be set to <NUM> year (approximately <NUM> × <NUM><NUM> seconds) or longer, the count N1 is <NUM> × <NUM><NUM> (= <NUM> × <NUM><NUM>/<NUM>). Here, the SD detection time when the count is <NUM> is assumed to be <NUM> ns (nanosecond). If this count is multiplied by <NUM> × <NUM><NUM>, the SD detection time is <NUM> × <NUM>-<NUM> seconds (<NUM> milliseconds).

A time range that is considered to be permissible as the SD detection time is a microsecond. Accordingly, <NUM> milliseconds is not included in the permissible time range. Therefore, it is difficult to lower the SD false detection frequency by the method increasing the false detection time to N times by using the counter circuit.

An object of the present invention is to lower the SD false detection frequency.

A reception device according to an aspect of the present invention is provided. The reception device includes a measurement unit that measures a first number of times for which a first phase and a first reverse phase based on a differential signal obtained by amplifying a signal based on noise intersect with each other, the first reverse phase being a reverse phase of the first phase, an oscillator that transmits a first signal, a comparison unit that compares the first number of times with a predetermined first reference value, and a signal output unit that outputs a second signal indicating that an optical signal has been received when the first number of times and the first reference value coincide with each other. The measurement unit resets the first number of times when the first signal is received.

According to the present invention, the SD false detection frequency can be lowered.

Embodiments will be described below with reference to the drawings. The following embodiments are just examples and a variety of modifications are possible within the scope of the present invention.

<FIG> is a functional block diagram showing the configuration of a reception device in a first embodiment. The reception device <NUM> includes a reception unit <NUM>, a photodiode <NUM>, a pre-amplifier <NUM>, a reference value selection unit <NUM> and a reset signal generation unit <NUM>. Incidentally, the reference value selection unit <NUM> may be represented as a count reference value selection circuitry. Further, the reset signal generation unit <NUM> may be represented as a reset signal generation circuitry. Furthermore, the reception device <NUM> may be regarded as an OLT. The reception device <NUM> may be referred to as a false detection frequency reducing device.

First, the photodiode <NUM> and the pre-amplifier <NUM> will be described below. The photodiode <NUM> is referred to also as a photoelectric transducing element. The photodiode <NUM> receives an optical signal. The photodiode <NUM> outputs an electric signal corresponding to the optical signal. The pre-amplifier <NUM> is referred to also as a Trance Impedance Amplifier (TIA). Further, the pre-amplifier <NUM> is referred to also as a preamp. The pre-amplifier <NUM> converts a current signal into a voltage signal. The voltage signal obtained by the conversion is amplified by the reception unit <NUM>. The reception unit <NUM> outputs the amplified voltage signal to an external circuit. There are cases where the pre-amplifier <NUM> outputs a signal based on noise.

Next, the reception unit <NUM> will be described below. The reception unit <NUM> includes a differential amplifier <NUM>, a buffer amplifier <NUM>, a first generation unit <NUM>, a second generation unit <NUM>, a measurement unit <NUM>, an oscillator <NUM>, a comparison unit <NUM> and a signal output unit <NUM>.

Here, the first generation unit <NUM> may be represented as Alternating current (AC) coupling capacitance. The second generation unit <NUM> may be represented as a variable bias circuitry. The measurement unit <NUM> may be represented as a crossing count counter circuitry. The comparison unit <NUM> may be represented as a comparator.

The signal output unit <NUM> may be represented as a DFF-type SD circuitry. As above, the first generation unit <NUM>, the second generation unit <NUM>, the measurement unit <NUM>, the comparison unit <NUM> and the signal output unit <NUM> can be implemented by processing circuitries. Incidentally, the measurement unit <NUM> may be referred to as a counter.

The differential amplifier <NUM> may be regarded as a low-noise high-frequency differential amp. The differential amplifier <NUM> receives a signal based on the optical signal received by the photodiode <NUM> via the pre-amplifier <NUM>. Further, the differential amplifier <NUM> receives the signal based on the noise from the pre-amplifier <NUM>.

The function of the differential amplifier <NUM> will be described in detail below. The differential amplifier <NUM> amplifies the voltage signal outputted by the pre-amplifier <NUM>. The differential amplifier <NUM> outputs a differential signal obtained by amplifying the voltage signal. Here, the differential signal is a signal obtained by amplifying the signal based on the optical signal, or a signal obtained by amplifying the signal based on the noise, for example. Here, the differential signal is referred to as a first differential signal. Incidentally, a part of the first differential signal outputted by the differential amplifier <NUM> is inputted to the first generation unit <NUM>. The buffer amplifier <NUM> regulates the first differential signal at constant amplitude.

Next, the first generation unit <NUM>, the second generation unit <NUM>, the measurement unit <NUM>, the oscillator <NUM>, the comparison unit <NUM> and the signal output unit <NUM> will be described below by using <FIG>. <FIG> is a diagram mainly showing the configuration of the measurement unit in the first embodiment. The first generation unit <NUM> includes capacitors <NUM> and <NUM>. The capacitors <NUM> and <NUM> let through high frequency components only. Here, the first differential signal includes a signal of a certain phase and a signal of a reverse phase as inversion of the phase. For example, the signal of the phase is inputted to the capacitor <NUM>. The signal of the reverse phase is inputted to the capacitor <NUM>.

Here, the phase based on the first differential signal obtained by amplifying the signal based on the noise is referred to also as a first phase. The first phase may be represented as a first normal phase. The reverse phase based on the first differential signal obtained by amplifying the signal based on the noise is referred to also as a first reverse phase. The first reverse phase is the reverse phase of the first phase.

Further, the phase based on the first differential signal obtained by amplifying the signal based on the optical signal is referred to also as a second phase. The second phase may be represented as a second normal phase. The reverse phase based on the first differential signal obtained by amplifying the signal based on the optical signal is referred to also as a second reverse phase. The second reverse phase is the reverse phase of the second phase.

The first generation unit <NUM> removes a DC component included in the first differential signal. The first differential signal from which the DC component has been removed is referred to also as a second differential signal. As above, the first generation unit <NUM> generates the second differential signal by removing the DC component included in the first differential signal.

The second generation unit <NUM> includes a power supply and a plurality of resistors. Resistors R1 and R2 form a voltage dividing circuit. Resistance values of the resistors R1 and R2 can be changed from the outside. A power supply voltage is divided for resistors R3 and R4. The second generation unit <NUM> generates a third differential signal by applying a predetermined voltage to the second differential signal. The third differential signal may be represented as a detection-dedicated differential signal.

The measurement unit <NUM> includes flip-flop circuits <NUM>, <NUM>, <NUM> and <NUM>. The measurement unit <NUM> measures the number of times of intersection of the phase and the reverse phase based on the third differential signal. This sentence may also be expressed as follows: The measurement unit <NUM> measures the number of times of intersection of the signal of the phase and the signal of the reverse phase included in the third differential signal.

Put another way, the measurement unit <NUM> measures the number of times of intersection of a waveform of the phase and a waveform of the reverse phase included in the third differential signal. Here, the number of times measured by the measurement unit <NUM> is referred to also as a first number of times or a second number of times.

The flip-flop circuit <NUM> outputs an output signal Out1 when the phase and the reverse phase have intersected twice. The flip-flop circuit <NUM> outputs an output signal Out2 when the phase and the reverse phase have intersected four times. The flip-flop circuit <NUM> outputs an output signal Out3 when the phase and the reverse phase have intersected eight times. The flip-flop circuit <NUM> outputs an output signal Out4 when the phase and the reverse phase have intersected sixteen times.

The comparison unit <NUM> is capable of detecting the number of times of intersection of the phase and the reverse phase by receiving the output signals outputted by the measurement unit <NUM>. The number of times detected by the comparison unit <NUM> is the same as the number of times measured by the measurement unit <NUM>. The number of times measured by the measurement unit <NUM> will hereinafter be referred to as a count number.

The oscillator <NUM> periodically transmits a signal for resetting the count number. This signal is referred to also as a first signal or a third signal.

Here, concrete examples of the case where the count number is reset will be shown below. <FIG> shows a concrete example (No. <NUM>) of the case where the count number is reset in the first embodiment. The concrete example shown in <FIG> is a case where an optical signal is inputted to the photodiode <NUM>. <FIG> indicates that the third differential signal based on the optical signal is inputted to the measurement unit <NUM>. Further, <FIG> indicates that the measurement unit <NUM> outputs the output signal Out1, the output signal Out2, the output signal Out3 and the output signal Out4.

<FIG> indicates that the oscillator <NUM> transmits the signal for resetting the count number. For example, the measurement unit <NUM> resets the count number when the signal transmitted by the oscillator <NUM> is High. As above, when the measurement unit <NUM> receives the signal transmitted by the oscillator <NUM>, the measurement unit <NUM> sets the count number at <NUM>. After the interval in which the signal transmitted by the oscillator <NUM> is High ends, the measurement unit <NUM> starts the measurement.

<FIG> shows a concrete example (No. <NUM>) of the case where the count number is reset in the first embodiment. The concrete example shown in <FIG> is a case where the pre-amplifier <NUM> outputs the signal based on the noise. <FIG> indicates that the third differential signal based on the noise is inputted to the measurement unit <NUM>. Further, <FIG> indicates that the measurement unit <NUM> outputs output signals such as the output signal Out1.

As above, by the transmission of the signal by the oscillator <NUM>, the count number is reset. As explained earlier, the count number is the same as the number of times detected by the comparison unit <NUM>. Thus, when the count number turns to <NUM>, the number of times detected by the comparison unit <NUM> turns to <NUM>. For example, when the count number has turned to <NUM>, the measurement unit <NUM> transmits a signal indicating that the count number is <NUM> to the comparison unit <NUM>. When the comparison unit <NUM> receives the signal, the comparison unit <NUM> sets the detected number of times at <NUM>.

The above description has been given of the case where the measurement unit <NUM> sets the count number at <NUM> when the measurement unit <NUM> receives the signal transmitted by the oscillator <NUM>. The measurement unit <NUM> may set a value close to <NUM> as the count number. For example, the measurement unit <NUM> sets <NUM> as the count number. Returning to <FIG>, the function of the comparison unit <NUM> will be described below.

The comparison unit <NUM> acquires a count reference value selected by the reference value selection unit <NUM>. The count reference value is a predetermined reference value. The count reference value is referred to also as a first reference value. The comparison unit <NUM> compares the number of times detected by the comparison unit <NUM> with the count reference value.

In other words, the comparison unit <NUM> compares the count number with the count reference value. When the number of times detected by the comparison unit <NUM> and the count reference value coincide with each other, the comparison unit <NUM> outputs a signal indicating the coincidence to the signal output unit <NUM>. When the number of times detected by the comparison unit <NUM> and the count reference value do not coincide with each other, the comparison unit <NUM> performs nothing.

When receiving the signal from the comparison unit <NUM>, the signal output unit <NUM> outputs an SD signal. Here, the SD signal is a signal indicating that an optical signal has been received. The SD signal is referred to also as a second signal. Here, the reset signal generation unit <NUM> will be described below. The reset signal generation unit <NUM> generates a reset signal as a signal for stopping the outputting of the SD signal. When the signal output unit <NUM> receives the reset signal from the reset signal generation unit <NUM>, the signal output unit <NUM> stops the outputting of the SD signal.

Further, the signal output unit <NUM> outputs the SD signal when an optical signal close to the minimum reception sensitivity is inputted to the photodiode <NUM> and the signal output unit <NUM> receives the signal from the comparison unit <NUM> according to the above description, for example. By this operation, the condition "outputting the SD signal when an optical signal within a predetermined power range is received" is realized.

Here, when the number of times measured by the measurement unit <NUM> (i.e., the number of times detected by the comparison unit <NUM>) by using the third differential signal based on the noise and the count reference value coincide with each other and the signal output unit <NUM> outputs the SD signal due to the coincidence, that means that the signal output unit <NUM> erroneously outputs the SD signal.

The time it takes until the number of times measured by the measurement unit <NUM> by using the third differential signal based on the noise and the count reference value coincide with each other may be regarded as the false detection time. The count number is reset due to the signal transmitted by the oscillator <NUM>. Thus, the time it takes until the number of times measured by the measurement unit <NUM> by using the third differential signal based on the noise and the count reference value coincide with each other increases. Namely, the false detection time increases. The SD false detection frequency is represented as "<NUM>/(false detection time)". The increase in the false detection time serves as the lowering of the SD false detection frequency. Thus, according to the first embodiment, the reception device <NUM> is capable of lowering the SD false detection frequency.

Further, even after the count number is reset by the oscillator <NUM> as shown in <FIG>, the signal output unit <NUM> outputs the SD signal when the signal output unit <NUM> receives the signal from the comparison unit <NUM>. Accordingly, the reception device <NUM> is capable of realizing "lowering the SD false detection frequency" and "outputting the SD signal when an optical signal within a predetermined power range is received".

The above description has been given of the case where the measurement unit <NUM> measures the count number by using the third differential signal. It is also possible for the measurement unit <NUM> to measure the count number by using the first differential signal. Namely, the measurement unit <NUM> may measure the number of times of intersection of the phase and the reverse phase as the reverse phase of the phase based on the first differential signal. As above, the reception device <NUM> does not necessarily have to include the first generation unit <NUM> and the second generation unit <NUM>. With such a configuration, the cost for the reception device <NUM> is reduced.

Next, a second embodiment will be described below. The following description of the second embodiment will be given mainly of features different from those in the first embodiment, and the description is omitted for features in common with the first embodiment. <FIG> are referred to in the second embodiment.

In the first embodiment, the description is given of the case where there is only one transmission rate of the optical signals. In the second embodiment, a description will be given of a case where there are a plurality of transmission rates of the optical signals. <FIG> is a functional block diagram showing the configuration of a reception device in the second embodiment. Each component in <FIG> that is the same as a component shown in <FIG> is assigned the same reference character as in <FIG>. The reception device <NUM> further includes a control unit <NUM>.

<FIG> is a diagram mainly showing the configuration of the measurement unit in the second embodiment. The control unit <NUM> notifies the oscillator <NUM> of information indicating the transmission rate of an optical signal scheduled to be received by the reception device <NUM> among the plurality of transmission rates. Incidentally, it is also possible for the reception device <NUM> to acquire the information by means of an input operation performed by a user. Here, when the count reference values corresponding to the plurality of transmission rates are set at the same value, it is necessary to vary the time for resetting the count number in regard to each of the transmission rates. Therefore, when the oscillator <NUM> receives the notification, the oscillator <NUM> transmits a signal at a frequency corresponding to the transmission rate of the optical signal scheduled to be received by the reception device <NUM>. This signal may be regarded as the first signal or the third signal.

The control unit <NUM> may notify the reference value selection unit <NUM> of information indicating the transmission rate of the optical signal scheduled to be received by the reception device <NUM>. When the reference value selection unit <NUM> receives the notification, the reference value selection unit <NUM> selects a count reference value corresponding to the transmission rate of the optical signal scheduled to be received by the reception device <NUM> from the plurality of count reference values. The reference value selection unit <NUM> provides the comparison unit <NUM> with the selected count reference value.

The control unit <NUM> may optimize the offset voltage in the second generation unit <NUM> depending on the transmission rate of the optical signal scheduled to be received by the reception device <NUM>. For example, the control unit <NUM> increases the offset voltage with the increase in the value of the transmission rate. According to the second embodiment, the reception device <NUM> is capable of lowering the SD false detection frequency even in a case where the reception device <NUM> is a device that receives optical signals at a plurality of transmission rates.

Next, a third embodiment will be described below. The following description of the third embodiment will be given mainly of features different from those in the first embodiment. In the third embodiment, the description is omitted for features in common with the first embodiment. <FIG> are referred to in the third embodiment. In the third embodiment, a description will be given of a case where a no-signal interval is detected. Incidentally, the no-signal interval means an interval in which no optical signal is inputted to the photodiode <NUM>.

<FIG> is a functional block diagram showing the configuration of a reception device in the third embodiment. The reception unit <NUM> further includes an intersection measurement unit <NUM>, a connection control unit <NUM>, a connection control unit <NUM> and a relaying unit <NUM>. Incidentally, the relaying unit <NUM> may be referred to as a logical sum circuit. The intersection measurement unit <NUM>, the connection control unit <NUM>, the connection control unit <NUM> and the relaying unit <NUM> may be implemented by processing circuitries.

Next, the intersection measurement unit <NUM>, the connection control unit <NUM>, the connection control unit <NUM> and the relaying unit <NUM> will be described in detail below. <FIG> is a diagram for explaining in detail components such as the intersection measurement unit in the third embodiment. The measurement unit <NUM> includes the flip-flop circuit <NUM>, <NUM> and <NUM>. The intersection measurement unit <NUM> measures the number of times of intersection of the phase and the reverse phase of the phase based on a differential signal obtained by amplifying the signal based on the optical signal. Specifically, when the third differential signal is inputted, the intersection measurement unit <NUM> measures the number of times of intersection of the phase and the reverse phase of the phase based on the third differential signal. The intersection measurement unit <NUM> outputs the output signal Out1 to the comparison unit <NUM> each time the phase and the reverse phase intersect twice. Incidentally, this number of times may be referred to as the second number of times.

The connection control unit <NUM> and the connection control unit <NUM> perform processing based on the signal outputted by the signal output unit <NUM>. The functions of the connection control unit <NUM> and the connection control unit <NUM> will be described in detail later. The relaying unit <NUM> relays signals.

First, a case where an optical signal is received will be described below. In other words, a case where the SD signal is outputted will be described below. In a state before the SD signal is outputted, the signal output unit <NUM> is not outputting the SD signal. The signal output unit <NUM> outputs a signal (hereinafter referred to as a low signal) indicating that the signal output unit <NUM> is not outputting the SD signal.

When the low signal is detected by the connection control unit <NUM>, the connection control unit <NUM> electrically connects the intersection measurement unit <NUM> and the measurement unit <NUM> together via the connection control unit <NUM>. Further, when the low signal is detected by the connection control unit <NUM>, the connection control unit <NUM> electrically connects the oscillator <NUM> and the measurement unit <NUM> together via the connection control unit <NUM>. When the low signal is detected by the connection control unit <NUM>, the connection control unit <NUM> electrically connects the comparison unit <NUM> and the signal output unit <NUM> via the connection control unit <NUM>.

When the low signal is detected by the reference value selection unit <NUM>, the reference value selection unit <NUM> selects a count reference value. The reference value selection unit <NUM> notifies the comparison unit <NUM> of the count reference value.

When an optical signal is inputted to the photodiode <NUM>, the intersection measurement unit <NUM> and the measurement unit <NUM> measure the number of times of intersection of the phase and the reverse phase of the phase based on the third differential signal (i.e., the count number).

The comparison unit <NUM> compares the count number with the count reference value. When the count number and the count reference value coincide with each other, the comparison unit <NUM> outputs the signal indicating the coincidence to the signal output unit <NUM> via the connection control unit <NUM>. When the signal is received by the signal output unit <NUM>, the signal output unit <NUM> outputs the SD signal. When the reset signal is received by the signal output unit <NUM>, the signal output unit <NUM> stops the outputting of the SD signal. Incidentally, the reset signal is a signal outputted by the reset signal generation unit <NUM> via the relaying unit <NUM>.

Further, when the measurement unit <NUM> receives the signal transmitted by the oscillator <NUM>, the measurement unit <NUM> sets the count number at <NUM>. Namely, the count number is reset. When the count number is reset, the measurement unit <NUM> transmits the signal indicating that the count number is <NUM> to the comparison unit <NUM>. When the comparison unit <NUM> receives the signal indicating that the count number is <NUM>, the comparison unit <NUM> sets the count numbers at <NUM>.

Namely, the comparison unit <NUM> not only sets the count number based on the output signal Out2, the output signal Out3 and the output signal Out4 at <NUM> but also sets the count number based on the output signal Out1 at <NUM>. As above, the reception device <NUM> is capable of implementing the first embodiment.

Next, the detection of a no-signal interval will be described below. In a state before a no-signal interval is detected, the signal output unit <NUM> is outputting the SD signal. In other words, the signal output unit <NUM> is outputting a high signal.

When the SD signal is detected by the connection control unit <NUM>, the connection control unit <NUM> electrically connects the intersection measurement unit <NUM> and the oscillator <NUM> together via the connection control unit <NUM>. Further, when the SD signal is detected by the connection control unit <NUM>, the connection control unit <NUM> electrically connects the measurement unit <NUM> and the oscillator <NUM> together via the connection control unit <NUM>. Incidentally, when the SD signal is detected by the connection control unit <NUM> may be expressed as when the SD signal is outputted. When the SD signal is detected by the connection control unit <NUM>, the connection control unit <NUM> electrically connects the comparison unit <NUM> and the signal output unit <NUM> together via the connection control unit <NUM> and the relaying unit <NUM>.

When the SD signal is detected by the reference value selection unit <NUM>, the reference value selection unit <NUM> selects a no-signal interval detection reference value. The no-signal interval detection reference value is a predetermined reference value. The no-signal interval detection reference value is referred to also as a second reference value. Incidentally, the no-signal interval detection reference value will be described in detail later. The reference value selection unit <NUM> notifies the comparison unit <NUM> of the no-signal interval detection reference value.

When the intersection measurement unit <NUM> receives the signal transmitted by the oscillator <NUM>, the intersection measurement unit <NUM> sets the count number at <NUM>. Namely, the count number is reset.

To the measurement unit <NUM>, the signal transmitted by the oscillator <NUM> is inputted. The measurement unit <NUM> measures the number of times of transition of the signal. For example, the number of times of transition is the number of times the signal transitions upward or downward. As explained earlier, the count number measured by the intersection measurement unit <NUM> is reset. Thus, the measurement by the measurement unit <NUM> is measurement of an interval in which the intersection measurement unit <NUM> does not output the output signal Out1 (i.e., an interval in which no optical signal is inputted to the photodiode <NUM>). The measurement of the interval in which no optical signal is inputted to the photodiode <NUM> is synonymous with measurement of the no-signal interval.

The comparison unit <NUM> compares the number of times of transition with the no-signal interval detection reference value. Here, an example of a method of determining the no-signal interval detection reference value will be described. It is desirable to determine the no-signal interval detection reference value so as to discriminate between a case where "<NUM>" continues due to Non Return to Zero when an optical signal is inputted to the photodiode <NUM> and a case where "<NUM>" as a signal value when there is no signal continues.

First, the time for which "<NUM>" continues when an optical signal is inputted to the photodiode <NUM> (hereinafter referred to as a same sign duration time) has been stipulated by a standard or specification. The same sign duration time is assumed to be XCIDA. The unit is second. Further, the cycle of the transmission of the signal by the oscillator <NUM> is assumed to be XFREQ. The unit is second.

The number measured by the measurement unit <NUM> increases by <NUM> every XFREQ seconds. The measured number is assumed to be NA. The unit is time(s). Let XNON represent the no-signal interval, the relationship between the no-signal interval XNON, the cycle XFREQ and the measured number NA is represented by the following expression (<NUM>): Incidentally, the unit of the no-signal interval is second.

When the measured number NA (i.e., the number of times of transition) and the no-signal interval detection reference value coincide with each other, the comparison unit <NUM> outputs a signal indicating that a no-signal interval has been detected. Thus, it is desirable to set the no-signal interval detection reference value so that the comparison unit <NUM> outputs the signal when "no-signal interval XNON > same sign duration time XCIDA" holds. As described above, the comparison unit <NUM> outputs the signal indicating that a no-signal interval has been detected when the number of times of transition and the no-signal interval detection reference value coincide with each other.

When the signal is received by the signal output unit <NUM>, the signal output unit <NUM> outputs a signal indicating that a no-signal interval has been detected. In other words, the signal output unit <NUM> outputs the signal indicating that a no-signal interval has been detected when the number of times of transition and the no-signal interval detection reference value coincide with each other. The signal output unit <NUM> may output information indicating the no-signal interval (in other words, information indicating how many seconds the no-signal interval is). Further, the signal output unit <NUM> can use the signal as the reset signal.

According to the third embodiment, the reception device <NUM> is capable of detecting the no-signal interval.

In a modification of the third embodiment, a case where the second embodiment and the third embodiment are combined with each other will be described.

<FIG> is a functional block diagram showing the configuration of a reception device in the modification of the third embodiment. The reception device <NUM> further includes the control unit <NUM>.

<FIG> is a diagram mainly showing the configuration of the measurement unit in the modification of the third embodiment. Processing performed by the control unit <NUM> is the same as the processing performed by the control unit <NUM> in the second embodiment. Thus, description of the function of the control unit <NUM> is left out.

Advantages of the modification of the third embodiment are the same as the advantage of the second embodiment and the advantage of the third embodiment.

Claim 1:
A reception device (<NUM>) comprising:
- a measurement unit (<NUM>) that is configured to measure a first number of times for which a first phase and a first reverse phase based on a differential signal obtained by amplifying a signal based on noise intersect with each other, the first reverse phase being a reverse phase of the first phase;
- an oscillator (<NUM>) that is configured to transmit a first signal;
- a comparison unit (<NUM>) that is configured to compare the first number of times with a predetermined first reference value; and
- a signal output unit (<NUM>) that is configured to output a second signal indicating that an optical signal has been received when the first number of times and the first reference value coincide with each other,
- wherein the measurement unit (<NUM>) is configured to reset the first number of times when the first signal is received.