JP4900288B2 - Optical access system and method for determining phase modulation frequency in optical access system - Google Patents

Description

  The present invention relates to a technique for improving reflection tolerance in an optical access system using a reflective semiconductor optical amplifier in an optical line terminating device installed in a subscriber's house.

  In a WDM-PON (Wavelength Division Multiplexing-Passive Optical Network) system, which is a type of optical access system, the purpose is to reduce the size and cost of an optical line termination unit (hereinafter referred to as ONU. ONU: Optical Network Unit). , And a system using a reflective semiconductor optical amplifier (hereinafter referred to as RSOA. RSOA: Reactive Semiconductor Optical Amplifier) has been proposed (for example, see Non-Patent Document 1).

  An ONU using an RSOA re-modulates and amplifies a downstream optical signal from an optical terminal device (hereinafter referred to as an OLT: OLT: Optical Line Terminal) installed on the communication station side, by using the RSOA. This is an optical signal. However, in the optical access system using RSOA, the upstream optical signal and the downstream optical signal have the same wavelength. For example, when reflection occurs in the optical transmission line as shown in FIG. There is a problem that reception characteristics in the OLT and ONU deteriorate.

  Non-Patent Document 2 discloses that when the amplitude-modulated downstream optical signal is further phase-modulated by a sine wave signal and the optical spectrum is expanded, the reflection resistance is improved. In this case, the beat noise interference components of the direct light (signal light) and the reflected light are as shown in number (1).

Here, P _d (t) and P _i (t) are the powers of direct light and reflected light, w _d and w _i are the optical frequencies of the direct light and reflected light, and φ _d (t) and φ _i (T) is an initial phase of direct light and reflected light, and Φ cos (2πft) and Φ cos (2πf (t−τ _i )) are phase modulation components of direct light and reflected light. Further, τ _i is a round trip delay time from the ONU to the reflection point.

Masamichi Fujiwara, et al. “Impact of Backreflection on Upstream Transmission in WDM Single-Fiber Loopback Access Network”, JOURNAL OF LIGHTWAVE TECHNOLOGY. 24, no. 2. February 2006 Peter J. et al. Legg, et al. “Solution Paths to Limit Interferometric Noise Induced Performance Degradation in ASK / Direct Detection Lightwave Networks”, JOURNAL OFLIGHTV. 14, no. 9, September 1996

  From the formula (1), when the phase relationship between the direct light and the reflected light is reversed, the effect of the optical phase modulation is maximized to improve the reflection resistance, and when the phase relationship between the direct light and the reflected light is in phase, the reflection is performed. It can be seen that the yield strength is reduced. The phase relationship between the direct light and the reflected light is determined by the round-trip delay time to the reflection point. However, since the position of the reflection point differs for each optical line, the optimum phase modulation frequency differs for each optical line. Further, since the initial phase is affected by the chirp parameter of the light source and the like, the optimum phase modulation frequency differs between the upstream direction and the downstream direction. In other words, even if the downstream optical signal transmitted by the OLT is subjected to phase modulation at an optimum frequency on the optical line, the upstream transmitted by the ONU using the RSOA that reuses the phase-modulated downstream optical signal. For optical signals, the worst phase modulation frequency may occur. Therefore, in determining the phase modulation frequency to be used, it is necessary to consider the balance in both the upstream and downstream directions.

  SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide an optical access system system capable of setting an optimum phase modulation frequency for each optical line and an optimum phase modulation frequency determination method in an optical access system including an ONU using RSOA. And

According to the optical access system of the present invention,
An optical terminal device that modulates the intensity of the optical signal with the data to be transmitted, phase-modulates the modulated optical signal with a sine wave signal, and transmits the optical signal received from the optical terminal device again with the data to be transmitted. An optical access system comprising an optical line termination device for intensity-modulated transmission, wherein the optical line termination device is configured such that an error rate measured based on fixed data transmitted by the optical terminal device is a predetermined value. A means for adjusting the received optical power and transmitting the received optical power when the error rate becomes a predetermined value to the optical terminal device as a downlink measurement value is transmitted from the optical terminal device. Means for adjusting the received optical power so that the error rate measured based on the fixed data becomes a predetermined value, and obtaining an uplink measured value that is the received optical power when the error rate becomes the predetermined value; , Means to get downlink measurement values, and generation The frequency of the sine wave signal is changed at predetermined intervals, the uplink measurement value and the downlink measurement value for each frequency are acquired, and the magnitude of the uplink measurement value and the downlink measurement value are compared to determine the sine wave signal used for communication. And a means for determining a frequency.

According to another embodiment of the optical communication device of the present invention,
The means for determining the frequency is equal to or higher than the first frequency when the magnitude relationship between the uplink measurement value and the downlink measurement value at the first frequency and the second frequency higher than the first frequency by the predetermined interval is opposite. It is also preferable to determine the frequency of the sine wave signal used for the communication from the second frequency or lower.

According to another embodiment of the optical communication device of the present invention,
If the absolute value of the difference between the uplink measurement value and the downlink measurement value at the first frequency is equal to or less than the absolute value of the difference between the uplink measurement value and the downlink measurement value at the second frequency, the means for determining the frequency It is also preferable to determine the frequency of 1 as the frequency of the sine wave signal used for the communication, and otherwise determine the second frequency as the frequency of the sine wave signal used for the communication.

According to the method of the present invention,
An optical terminal device that modulates the intensity of the optical signal with the data to be transmitted, phase-modulates the modulated optical signal with a sine wave signal, and transmits the optical signal received from the optical terminal device again with the data to be transmitted. A method for determining a frequency of a sine wave signal used for communication in an optical access system including an optical line terminator that transmits an intensity-modulated signal, wherein the frequency of the sine wave signal is set to a first frequency. In each of the optical line terminal device and the optical terminal device, a first step of measuring the received optical power when the error rate becomes a predetermined value as a downlink measurement value and an uplink measurement value, and the frequency of the sine wave signal The second frequency, which is changed from the first frequency by a predetermined amount, is set, and the received optical power when the error rate becomes a predetermined value in each of the optical line terminating device and the optical terminal device is measured as the downlink measurement value and the uplink frequency. As measured value A second step of measuring, a third step of comparing a magnitude relationship between the downlink measurement value and the uplink measurement value in the first step, a magnitude relationship between the downlink measurement value and the uplink measurement value in the second step, When the magnitude relationship between the downlink measurement value and the uplink measurement value in the first step is opposite to the magnitude relationship between the downlink measurement value and the uplink measurement value in the second step, the first frequency, the second frequency, or And a fourth step of selecting a frequency of the sine wave signal from a frequency between the first frequency and the second frequency.

  If the phase modulation frequency is changed in the presence of reflection, the influence of reflection changes, and thus the optical power required to achieve the same error rate. This low optical power indicates that the influence of reflection is small. However, since this frequency is different between the upstream direction and the downstream direction, it is necessary to consider the balance in both directions. If the optical power to achieve the same error rate is almost the same in both directions, the influence of reflection can be considered to be almost the same in both directions. The frequency of the optimal sinusoidal signal to use can be determined.

  The best mode for carrying out the present invention will be described in detail below with reference to the drawings. FIG. 1 is a schematic configuration diagram of an OLT of an optical access system according to the present invention, and FIG. 2 is a schematic configuration diagram of an ONU. 1 and 2 show only the parts necessary for the description of the present invention.

  According to FIG. 1, the OLT uses a DATA unit 1 that transmits a fixed pattern, a laser diode (LD) 2 that outputs intensity-modulated light based on input data, and a sine wave signal from an oscillator 5 to output light from the LD 2. A phase modulator 3 for phase modulating the signal, an optical filter 6 for removing ASE (Amplified Spontaneous Emission) noise of the optical signal received from the ONU, and an attenuation amount for adjusting the optical power of the optical signal received from the optical filter 6 A variable optical attenuator 7, an optical power measuring unit 9 for measuring the optical power of the optical signal from the optical attenuator 7, and an optoelectric converting unit 10 for converting the optical signal from the optical attenuator 7 into an electrical signal; While the ONU is transmitting the fixed pattern, the error measuring unit 11 that measures the error rate, and each part of the OLT are controlled, and the ONU control unit 16 is controlled. And a control unit 12 for instructing each operation. Reference numeral 4 denotes an optical circulator, and reference numeral 8 denotes an optical coupler.

  According to FIG. 2, the ONU adjusts the optical power of the optical signal received from the OLT, the optical attenuator 17 having a variable attenuation, and the optical power measuring unit 21 that measures the optical power of the optical signal from the optical attenuator 17. A photoelectric conversion unit 19 that converts an optical signal from the optical attenuator 7 into an electrical signal, an error measurement unit 20 that measures an error rate while the OLT transmits a fixed pattern, and a fixed pattern. A DATA unit 15, a control unit 16 that controls each unit of the ONU, an RSOA 14 that receives an optical signal from the OLT, performs intensity modulation and amplification by a signal from the control unit 16 or the DATA unit 15, and transmits to the OLT It has. Reference numerals 13 and 21 are optical couplers.

  Next, the operation of the optical access system according to the present invention will be described. FIG. 3 is a flowchart of a method for determining a phase modulation frequency according to the present invention. With this method, the control unit 12 determines a phase modulation frequency that is optimum for the optical line, and uses the phase modulation frequency used in actual communication. This value is set in the oscillator 5 as the frequency. First, the control unit 12 of the OLT sets the frequency F of the sine wave signal generated by the oscillator 5 to a predetermined initial value, and further sets the increase / decrease direction to an initial value, here “increase”. The increase / decrease direction is a setting that indicates the direction in which the frequency F is changed, that is, whether the frequency F is increased or decreased. The frequency F is increased when the frequency is increased, and the frequency is decreased when the frequency is decreased. (S31). Subsequently, in this state, the control unit 12 causes the DATA unit 1 to transmit fixed pattern data. The ONU control unit 16 adjusts the attenuation amount of the optical attenuator 17 according to an instruction from the OLT control unit 12, and the error rate measured by the error measurement unit 20 becomes a predetermined value, for example, 1E-12. The optical power is measured by the optical power measuring unit 21, and this value is output to the RSOA 14 and transmitted to the control unit 12 of the OLT 10 by the upstream optical signal. This measured value corresponds to the reception sensitivity of the downlink signal. Similarly, the ONU control unit 16 causes the DATA unit 15 to transmit fixed pattern data, the OLT control unit 12 adjusts the attenuation amount of the optical attenuator 7, and the error rate measured by the error measurement unit 11 is increased. The optical power at the predetermined value, for example, 1E-12, is measured by the optical power measuring unit 9 (S32). This measured value corresponds to the reception sensitivity of the upstream signal.

  Subsequently, the OLT control unit 12 increases or decreases the frequency F of the sine wave signal generated by the oscillator 5 by a predetermined value α, for example, 100 kHz, according to the current setting in the increase / decrease direction, and performs the same measurement as S32 ( S34) The latest measurement result and the previous measurement result are compared by the method described later (S35). From this comparison result, when the optimum frequency F can be determined by the method described later, the value is set in the oscillator 5, and when it cannot be determined, the setting of the increase / decrease direction is changed according to the comparison result ( In step S37, the frequency F of the sine wave signal is changed by a predetermined amount α according to the set increase / decrease direction (S38), and the comparison with the previous measurement result is repeated until the optimum frequency F can be determined (S38). S36).

Next, the process from S35 to S37 in FIG. 3 will be described. Among the previous measurement results in S35, referred to the light power measured by the ONU side D _p, the optical power measured by the OLT and U _p, of the latest measurement result, the light measured by the ONU The power is measured as D _c and the optical power measured on the OLT side is called U _c .

When the magnitude relationship between D _p and U _{p and} the magnitude relationship between D _c and U _c are reversed (Case 1), the absolute value of the difference between D _p and U _p and the absolute value of the difference between D _c and U _c If the absolute value of the difference between D _p and U _p is small, the frequency F when D _p and U _p are measured is the optimum frequency, and if the absolute value of the difference between D _c and U _c is small, D _c And the frequency F when _Uc is measured is set as the optimum frequency. That is, in Case 1, the result of S36 in FIG. 3 is “yes”, and the process ends.

When the magnitude relationship between D _p and U _{p and} the magnitude between D _c and U _c are not reversed, the result of S36 in FIG. 3 is “no”. At this time, if the absolute value of the difference between D _c and U _c is smaller than the absolute value of the difference between D _p and U _p (Case 2-1), the setting of the increase / decrease direction is not changed in S37, and the current value Leave. On the other hand, if the absolute value of the difference between D _c and U _c is greater than the absolute value of the difference between D _p and U _p (Case 2-2), the setting of the increase / decrease direction is reversed in S37, that is, the current value is “ If it is “Increase”, it is changed to “Decrease”, and if the current value is “Decrease”, it is changed to “Increase”.

When the absolute value of the difference between D _c and U _c is the same as the absolute value of the difference between D _p and U _p , when D _c and U _c are smaller than D _p and U _p (Case 3-1) , in S37, without changing the setting of the decrease direction, _{D c} and _{U c} is, when greater than _{D p} and _{U p} to (Case3-2), in S37, the inversion settings increase or decrease direction.

  In the case where the measurement in a certain frequency range is repeated without obtaining the Case 1 state, the frequency F in which the average value of the upstream and downstream optical power becomes the smallest among the measured frequency ranges is set as the optimum frequency in FIG. End the process.

  FIG. 4 is a diagram for explaining the processing from S35 to S37 in FIG. Changing the phase modulation frequency in the presence of reflection will change the effect of reflection, and thus the optical power required to achieve the same error rate. This low optical power indicates that the influence of reflection is small. However, since this frequency is different between the upstream direction and the downstream direction, it is necessary to consider the balance in both directions. When the optical power that achieves the same error rate is almost the same in both directions, the influence of reflection is almost the same in both directions, that is, in a balanced state, so the optical power measured on the OLT side and the ONU side It is only necessary to find a phase modulation frequency at which the measured optical power is almost the same.

Case 1 is a case where a line connecting D _{p and} D _c intersects a line connecting U _p and U _c as shown in FIG. 4A. In this case, the measurement of D _p and U _p is performed. There is an optimum phase modulation frequency between the phase modulation frequency used and the phase modulation frequency used to measure D _c and U _c . Therefore, the phase modulation frequency used for the measurement with the smaller difference in optical power necessary to achieve a predetermined error rate is determined to an optimum value. Of course, other values between the phase modulation frequency used to measure D _p and U _p and the phase modulation frequency used to measure D _c and U _c , for example, a line connecting D _{p and} D _c , _The phase modulation frequency corresponding to the intersection of the lines connecting _{p 1} and U _c may be set as the optimum phase modulation frequency.

The Case2-1, for example, as shown in FIG. 4 _{(b), D p,} and the line connecting the _{D c,} U p, a line connecting the _{U c} is not intersect, by changing the phase modulation frequency, This is the case where the difference between the connecting lines is small, and it is considered that the intersection can be found by changing the phase modulation frequency in the same direction as the current increase / decrease direction. . On the other hand, Case 2-2 is a case where, as shown in FIG. 4C, for example, a difference in connecting lines is increased by changing the phase modulation frequency. Since it is considered that the intersection can be found by changing in the opposite direction, the setting of the increase / decrease direction is reversed.

The Case3-1, for example, as shown in FIG. 4 _(d), a line connecting the _D p, _{D c,} is parallel to a line connecting the U p, _{U c,} by changing the phase modulation frequency, This is a case where the optical power required to achieve a predetermined error rate is small in both the upstream and downstream directions. In this case, by changing the phase modulation frequency in the same direction as the current increase / decrease direction, the required optical power becomes smaller in both the upstream and downstream directions, so the setting of the increase / decrease direction is left as it is. On the other hand, the Case3-2, for example, as shown in FIG. 4 _(e), the line connecting the _D p, _{D c,} is parallel to a line connecting the U p, _{U c,} changing the phase modulation frequency In this case, the optical power necessary to achieve a predetermined error rate increases in both the upstream and downstream directions. In this case, by changing the phase modulation frequency in the direction opposite to the current increase / decrease direction, the required optical power decreases in both the upstream and downstream directions, so the setting of the increase / decrease direction is reversed.

  As described above, in the optical access system according to the present invention, the optical power at a predetermined error rate in each of the upstream and downstream directions is measured while changing the phase modulation frequency for expanding the spectrum, and the upstream and downstream directions. The optimum phase modulation frequency is determined by searching for a measurement point where the magnitude relationship of the optical power of the light is reversed. As a result, it is possible to set an optimum phase modulation frequency in consideration of both the upstream and downstream directions for each optical line, and the reflection resistance can be improved. Note that the measurement may be performed first in a predetermined range of frequencies, and after all measurements are completed, a measurement point where the magnitude relationship is reversed is searched from the measurement result.

It is a schematic block diagram of OLT of the optical access system by this invention. It is a schematic block diagram of ONU of the optical access system by this invention. Fig. 2 is a flow diagram of a method according to the present invention. It is a figure explaining a judgment process.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1, 15 DATA part 2 Laser diode 3 Phase modulator 4 Optical circulator 5 Oscillator 5
6 Optical filter 7, 17 Optical attenuator 8, 13, 21 Optical coupler 9, 21 Optical power measurement unit 10, 19 Photoelectric conversion unit 11, 20 Error measurement unit 12, 16 Control unit 14 RSOA

Claims (4)

An optical terminal device that modulates the intensity of the optical signal with the data to be transmitted, phase-modulates the modulated optical signal with a sine wave signal, and transmits the optical signal received from the optical terminal device again with the data to be transmitted. An optical access system comprising an optical line termination device for intensity-modulated transmission,
Optical line termination equipment
The received optical power is adjusted so that the error rate measured based on the fixed data transmitted by the optical terminal equipment becomes a predetermined value, and the received optical power when the error rate becomes the predetermined value is used as the downlink measured value. Means for transmitting to the device;
The optical terminal equipment
Means for adjusting the received optical power so that the error rate measured based on the fixed data transmitted by the optical line terminator becomes a predetermined value, and obtaining the uplink measured value that is the received optical power when the error rate becomes the predetermined value When,
Means for obtaining downlink measurement values from the optical line termination device;
The frequency of the sine wave signal to be generated is changed at a predetermined interval, the uplink measurement value and the downlink measurement value for each frequency are obtained, and the magnitude of the uplink measurement value and the downlink measurement value are compared, so that the sine wave signal used for communication Means for determining the frequency of
Having optical access system. The means for determining the frequency is:
When the magnitude relationship between the uplink measurement value and the downlink measurement value at the first frequency is opposite to the magnitude relationship between the uplink measurement value and the downlink measurement value at the second frequency that is higher than the first frequency by the predetermined interval, A frequency of a sine wave signal used for the communication is determined from a frequency equal to or higher than the second frequency and equal to or lower than a second frequency.
The optical access system according to claim 1. The means for determining the frequency is:
If the absolute value of the difference between the uplink measurement value and the downlink measurement value at the first frequency is less than or equal to the absolute value of the difference between the uplink measurement value and the downlink measurement value at the second frequency, the first frequency is used for the communication. Determining the frequency of the sinusoidal signal to be used; otherwise, determining the second frequency to be the frequency of the sinusoidal signal used for the communication;
The optical access system according to claim 2. An optical terminal device that modulates the intensity of the optical signal with the data to be transmitted, phase-modulates the modulated optical signal with a sine wave signal, and transmits the optical signal received from the optical terminal device again with the data to be transmitted. A method of determining a frequency of a sine wave signal used for communication in an optical access system comprising an optical line termination device for intensity-modulated transmission,
The frequency of the sine wave signal is set to the first frequency, and the received optical power when the error rate becomes a predetermined value is measured as a downlink measurement value and an uplink measurement value in each of the optical line termination device and the optical terminal device. A first step to:
The frequency of the sine wave signal is set to a second frequency that is changed from the first frequency by a predetermined amount, and the received optical power when the error rate becomes a predetermined value in each of the optical line terminal device and the optical terminal device is determined. A second step of measuring as a downlink measurement value and an uplink measurement value;
A third step of comparing the magnitude relationship between the downlink measurement value and the uplink measurement value in the first step, and the magnitude relationship between the downlink measurement value and the uplink measurement value in the second step;
When the magnitude relationship between the downlink measurement value and the uplink measurement value in the first step is opposite to the magnitude relationship between the downlink measurement value and the uplink measurement value in the second step, the first frequency, the second frequency, or A fourth step of selecting a frequency of the sinusoidal signal from a frequency between the first frequency and the second frequency;
Including methods.

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