WO2010041334A1

WO2010041334A1 - Received light monitoring device and balanced optical receiver provided with the same

Info

Publication number: WO2010041334A1
Application number: PCT/JP2008/068473
Authority: WO
Inventors: 隆宏藤本; 森　和行
Original assignee: 富士通オプティカルコンポーネンツ株式会社
Priority date: 2008-10-10
Filing date: 2008-10-10
Publication date: 2010-04-15

Abstract

An optical receiver for an optical transmission system employing the phase modulation method, wherein the need for providing an OSNR monitoring device separately is eliminated. A received light monitoring device is built in a balanced optical receiver having a delay interferometer (10) that branches a light signal into the two and outputs the complementary light signals demodulated to the strength modulation level corresponding to the phase information the light signal has by making both of the branched light signals interfere with each other after delayed waveguide of one of the branched light signals, and a photoelectric conversion section (20) to perform differential photoelectric conversion detection using two photoelectric converter elements (21, 22) that receive each of the complementary light signals output from the delay interferometer (10) to provide the current corresponding to the signal strength. The received light monitoring device comprises measurement sections (30, 31) each of which measures the current Ipd1, Ipd2 flowing through the photoelectric converter elements, and computing sections (40, 41, 42) to detect variation in quantity of the spontaneous emission light included in the light signal on the basis of the current values measured by the measurement sections (30, 31).

Description

Received light monitoring apparatus and balanced optical receiver having the same

The present invention relates to an optical receiver in an optical transmission system that transmits signal light by a phase modulation method.

The introduction of next-generation 40 Gbit / s optical transmission systems is urgently needed as home appliances and PCs (Personal Computers) become more sophisticated and the Internet rapidly spreads. As a method for realizing this, an optical transmission system that transmits signal light by a multiphase phase modulation method such as DPSK (Differential Phase Shift Keying) or DQPSK (Differential Quadrature Phase Shift Shift Keying) has attracted attention.

In such a multi-phase phase modulation type optical transmission system, a balanced optical receiver including a delay interferometer and a photoelectric conversion unit as disclosed in Patent Document 1 is used at a receiving station. The delay interferometer is configured by, for example, a Mach-Zehnder interferometer that splits input light into two, and the branched light that passes through one of the branched waveguides is used as a phase reference. The other branching waveguide is formed such that the branched light passing through the branching waveguide becomes an optical path that is delayed by one symbol (or a plurality of symbols) with respect to the reference light. Both branched lights that have passed through these branched waveguides are multiplexed at the output section and then branched into two to be output, whereby complementary light of normal phase light and reverse phase light is output from the delay interferometer. Note that a heater for adjusting the temperature in order to adjust the phase of interference can be provided in the branching waveguide on the delay side. The complementary light output from the delay interferometer becomes intensity-modulated light corresponding to the phase information of the received signal light.

The photoelectric conversion unit receives complementary light output from the delay interferometer and performs differential photoelectric conversion detection (Balanced Detection). The two photoelectric conversion elements having the same characteristics and the photodiode An electric amplifier that amplifies the output is used. In the twin photodiode, complementary lights from the delay interferometer are received and converted into electric signals.

JP 2007-060583 A

By the way, in a general optical transmission system, an optical amplifier such as an EDFA (Erbium Doped Fiber Amplifier) is installed at an appropriate position of the optical transmission line in order to compensate for the transmission line loss. As this optical amplifier adds spontaneous emission light (ASE) as shown in FIG. 5, it is known that the OSNR (Optical Signal to Noise) Ratio of signal light deteriorates. That is, the optical power is recovered before and after the optical amplifier, but the OSNR is deteriorated. Due to this, in particular, in a WDM (Wavelength Division Multiplexing) optical transmission system, the spontaneous emission light generated by the optical amplifier has a wavelength dependency, so that the OSNR is different for each wavelength (channel) of the WDM light. Since different results are brought about, it is necessary to monitor the OSNR for each channel at the receiving station in operation.

In order to monitor this OSNR, in the conventional WDM optical transmission system, a monitoring device separate from the optical receiver is provided for each WDM optical channel in the receiving station, and a part of the optical signal received by the optical receiver is provided. The monitor light is branched and given to the monitoring device, and the OSNR of the monitor light is monitored (for example, SKShin, KJPark, and YCChung, "A Novel Optical Signal-to-Noise Ratio Monitoring Technique for WDM Networks", OFC2000, WK6-1). However, when such a monitoring device is provided, a part of the received light is used as the monitor light for OSNR monitoring, so that the reception power decreases and the reception sensitivity decreases, Since a dedicated device is provided, there arises a problem such as a large cost increase.

These problems can occur in the same way when a multiphase phase modulation method is applied to each channel of WDM light, and problems to be solved in realizing an optical transmission system of a polyphase phase modulation method It has become one of the. Accordingly, an object of the present invention is to propose a technique that eliminates the need to provide a monitoring device separate from the optical receiver.

In order to solve the above-described problem, here, the phase-modulated signal light propagating through the optical transmission path is branched into two, and after one of the branched lights is delayed and guided, both branched lights are made to interfere with each other. A delay interferometer that outputs complementary light demodulated to intensity modulation corresponding to the phase information of the signal light, and a complementary light output from the delay interferometer, and a current corresponding to the intensity 2 Proposed is a received light monitoring device in a balanced optical receiver comprising a photoelectric conversion unit that performs differential photoelectric conversion detection using two photoelectric conversion elements. The received light monitoring apparatus detects a change in spontaneous emission light included in the signal light based on a measurement unit that measures currents flowing through the photoelectric conversion elements and a current value measured by the measurement unit. And an arithmetic unit.

The received light monitoring device according to this proposal is a balanced optical receiver used in a phase modulation type optical transmission system, and measures the received light by measuring the current of the photoelectric conversion element incorporated in the photoelectric conversion unit. The OSNR can be monitored. That is, since the received light monitoring device uses an electric circuit for measuring the current of the photoelectric conversion element, it is not necessary to branch a part of the received light as the monitor light, and thus the reception power is not reduced. Does not affect the reception sensitivity. In addition, since it is an electric circuit that measures current and performs arithmetic processing, it can be integrated with the photoelectric conversion unit, and a separate expensive optical monitoring device as in the prior art is unnecessary. That is, even when a monitoring device is provided, the cost of the balanced optical receiver can be suppressed and can be mounted compactly.

1 is a circuit diagram showing a first embodiment of a received light monitoring apparatus. The horizontal axis represents the heater voltage of the delay interferometer, and the vertical axis represents the light receiving current of the photoelectric conversion element, showing that the light receiving current changes due to the OSNR variation. The circuit diagram which showed 2nd Embodiment of the received light monitoring apparatus. The figure explaining the spontaneous emission light addition by the optical amplifier in an optical transmission line.

Explanation of symbols

10 Delay Interferometer 20 Photoelectric Conversion Units 21, 22 Photodiode (Photoelectric Conversion Element)
23

Electric amplifier

30, 31 Measuring unit 40 Adder circuit (calculation unit)
41 Subtraction circuit (arithmetic unit)
42 Division circuit (calculation unit)

FIG. 1 shows a first embodiment of the received light monitoring apparatus. The balanced optical receiver having the received light monitoring device of this embodiment is provided in the receiving station of the phase modulation type optical transmission system. When the signal light to be transmitted is WDM light, each wavelength is received. Is provided. Such a balanced optical receiver includes the delay interferometer 10 and the photoelectric conversion unit 20.

The delay interferometer 10 of the present embodiment is configured by a Mach-Zehnder interferometer that splits input light into two. When the phase-modulated signal light propagating through the optical transmission line is incident on the input waveguide 11, the incident light is bifurcated at the branching section 12, and is sent to the delay branching waveguide 13 and the reference branching waveguide 14. Sent. Of the bifurcated light, the branched light passing through the reference branching waveguide 14 becomes the phase reference. Then, the branched light passing through the other delayed branching waveguide 13 is delayed by one symbol (or a plurality of symbols) with respect to the branched light passing through the reference branching waveguide 14. Both branched lights that have passed through the

branched waveguides

13 and 14 are multiplexed at the multiplexing unit 15, and the reference light and the delayed light interfere with each other. The combined light is branched into two

output waveguides

16 and 17, and normal phase light is output from one output waveguide 16, and reverse phase light is output from the other output waveguide 17.

The delay branch waveguide 13 of the delay interferometer 10 is provided with a temperature adjusting heater 18 so that the amount of delay given to the branched light passing through the branch waveguide 13 can be adjusted. . That is, by controlling the voltage applied to the heater 18 and adjusting the temperature of the branching waveguide 13 to change the optical path length, the delay interferometer 10 can be optimized so as to obtain optimal normal phase light and reverse phase light. Temperature control is performed.

The normal phase and reverse phase complementary light output from the delay interferometer 10 becomes light demodulated by intensity modulation according to the phase information of the received signal light, and is input to the photoelectric conversion unit 20 in the next stage. The photoelectric conversion unit 20 receives the complementary light and performs differential photoelectric conversion detection. The photoelectric conversion unit 20 includes photodiodes 21 and 22 as two photoelectric conversion elements with uniform characteristics. The photodiodes 21 and 22 of the first embodiment are reverse-biased by connecting their cathode electrodes to a common power supply Vpd via a measurement unit, which will be described later, and one of the photodiodes 21 is a positive phase light. And the other photodiode 22 receives the reverse phase light and outputs an electric signal corresponding to the received light power to the electric amplifier 23. The output signal amplified by the electric amplifier 23 is sent to a subsequent identification circuit (not shown) or an analog-digital conversion circuit (in the case of digital processing). Since these circuits after the photoelectric conversion unit 20 can be used, a description thereof will be omitted.

When the twin photodiodes 21 and 22 operate, currents Ipd1 and Ipd2 flow in accordance with the received light intensity, respectively. The received light monitoring apparatus of this embodiment includes measuring

units

30 and 31 that measure the currents Ipd1 and Ipd2, respectively. The

measurement units

30 and 31 are configured using differential amplifiers, and measure an average current (a DC component or a low frequency component sufficiently lower than the phase modulation speed) for the currents Ipd1 and Ipd2. The measurement results of the currents Ipd1 and Ipd2 by the measuring

units

30 and 31 are shown in FIG.

In the graph of FIG. 2, the vertical axis represents the values (μA) of the currents Ipd1 and Ipd2, and the horizontal axis represents the square value of the voltage applied to the heater 18 of the delay interferometer 10 (the heater 18 is driven by power). Since the delay control for the interference is performed by adjusting the temperature of the heater 18 as described above, the intensity of the complementary light output from the delay interferometer 10 changes according to the heater voltage, and the complementary light is received accordingly. The currents Ipd1 and Ipd2 of the photodiodes 21 and 22 to be changed also change. Normally, the heater is equivalent to a voltage at which the current difference between the current Ipd1 due to the positive phase light and the current Ipd2 due to the reverse phase light is optimal, in other words, 6 to 7V ² or 16 to 17V ² at which the current difference is maximum. Voltage is used.

2A, the graph of (A) shows a case where the OSNR of the light (received light) input to the delay interferometer 10 is relatively good at 25 dB, and the graph of (B) shows that the OSNR of the received light is 6. This shows a relatively bad case of 7 dB. As can be seen by comparing these graphs, the current difference Ipd1−Ipd2 varies with the OSNR of the received light. The received light monitoring apparatus of the present embodiment monitors the OSNR of the received light based on this current difference Ipd1-Ipd2. The reason why the current difference Ipd1−Ipd2 changes in accordance with the OSNR is that the signal components of the branched lights when the branched lights that have passed through the branched

waveguides

13 and 14 interfere with each other in the multiplexing unit 15 in the delay interferometer 10. This is because interference occurs due to the correlation, but spontaneous emission light (noise due to the optical amplifier in the optical transmission line) on the branched light does not cause any interference because there is no correlation.

However, since the current difference Ipd1−Ipd2 also changes according to the fluctuation of the overall input power of the complementary light to the photoelectric conversion unit 20, it is necessary to consider the influence of this input power. Therefore, the OSNR information is extracted by normalizing the current difference Ipd1−Ipd2 with the input power. Specifically, since the input power of the complementary light can be expressed by the sum Ipd1 + Ipd2 of both currents, it is normalized by (Ipd1−Ipd2) / (Ipd1 + Ipd2) (Equation 1). By this equation 1, that is, by dividing the difference between the current values measured by the measuring

units

30 and 31, and the sum, the increase or decrease of the spontaneous emission light included in the signal light, that is, the signal component and the spontaneous emission light component in the received light. And contrast can be detected. In operation, the received light having a different OSNR is given to the delay interferometer 10 in advance to calculate Equation 1 at several points to obtain a function α representing the relationship between the calculated value of Equation 1 and contrast, and α [(Ipd1 −Ipd2) / (Ipd1 + Ipd2)] (Equation 2) is preferably used to detect the increase or decrease in spontaneous emission light.

In the present embodiment, an adder circuit 40, a subtractor circuit 41, and a divider circuit 42 are provided as arithmetic units for performing the above-described arithmetic operations. The adder circuit 40 adds the output values of the measurement units 30 and 31 (Ipd1 + Ipd2), and the subtraction circuit 41 subtracts the output values of the measurement units 30 and 31 (Ipd1−Ipd2). Then, the division circuit 42 divides the addition value by the addition circuit 40 and the subtraction value by the subtraction circuit 41 (Equation 1), and outputs the detection result.

The detection result output from the calculation unit can be used for notification to an operator or bias control of the photoelectric conversion unit 20 as OSNR information obtained by monitoring spontaneous emission light in the signal light. When the bias control of the photoelectric conversion unit 20 is performed, the operating points of the photodiodes 21 and 22 are adjusted according to the calculation unit detection result. For example, the operating point adjustment can be realized by providing a control unit that controls the input bias of the electric amplifier 23 connected to the photodiodes 21 and 22 according to the detection result of the arithmetic unit. The reason for bias control is to use twin photodiodes with uniform characteristics, but it is actually difficult to match the characteristics of both, so it is difficult to fine-tune the characteristic difference by bias control. It is to do.

The

measurement units

30 and 31 and the

calculation units

40, 41, and 42 are electric circuits, and can be integrated on the same circuit board or IC chip as the photoelectric conversion unit 20. That is, it is not necessary to provide a monitoring device separate from the balanced optical receiver. In addition, since no optical element is used for monitoring, there is no decrease in received light and no significant increase in cost.

FIG. 3 shows a second embodiment. In the following, description of portions common to the first embodiment will be omitted.

The second embodiment shown in FIG. 3 differs in the connection form of the twin photodiodes. That is, in the case of the first embodiment, each anode electrode of the photodiodes 21 and 22 is individually connected to the differential input terminal of the electric amplifier 23. In the case of the third embodiment, the photodiode 121 is connected. Are connected to each other in series, whereby the

twin photodiodes

121 and 122 are connected in series to each other, and the connection point between the

photodiodes

121 and 122 is input to the electric amplifier 23. It is connected. Since the

photodiodes

121 and 122 are connected in series, a positive voltage Vpd1 is applied to the cathode electrode of the photodiode 121 via the measurement unit 30 and the measurement unit 31 is applied to the anode electrode of the photodiode 122. A negative voltage Vpd2 is applied through the

photodiodes

121 and 122, and the

photodiodes

121 and 122 are reverse-biased. Even in this case, the relationship of the current difference Ipd1−Ipd2 with respect to the OSNR as described in FIG. 2 is not changed. Therefore, also in this embodiment, the

measurement units

30 and 31 and the addition circuit 40, the subtraction circuit 41, and the division circuit 42 that constitute the calculation unit are the same as those in the first embodiment.

Claims

According to the phase information of the signal light, the phase-modulated signal light propagating through the optical transmission path is branched into two, and after one of the branched lights is delayed and guided, both the branched lights interfere with each other. A delay interferometer that outputs complementary light demodulated to intensity modulation;
A photoelectric conversion unit that performs differential photoelectric conversion detection using two photoelectric conversion elements that each receive complementary light output from the delay interferometer and pass a current corresponding to the intensity thereof;
A received light monitoring device for a balanced optical receiver comprising:
A measurement unit for measuring each of the currents flowing through the photoelectric conversion elements;
Based on the current value measured by the measurement unit, a calculation unit that detects increase / decrease in spontaneous emission light included in the signal light,
A received light monitoring apparatus comprising:
The received light monitoring apparatus according to claim 1,
The received light monitoring apparatus, wherein the arithmetic unit performs division of a difference and a sum of current values respectively corresponding to the photoelectric conversion elements measured by the measurement unit.
According to the phase information of the signal light, the phase-modulated signal light propagating in the optical transmission path is branched into two, and after one of the branched lights is delayed and guided, both the branched lights interfere with each other. A delay interferometer that outputs complementary light demodulated to intensity modulation;
A photoelectric conversion unit that performs differential photoelectric conversion detection using two photoelectric conversion elements that each receive complementary light output from the delay interferometer and pass a current corresponding to the intensity thereof;
A balanced optical receiver comprising:
A measurement unit for measuring each of the currents flowing through the photoelectric conversion elements;
Based on the current value measured by the measurement unit, a calculation unit that detects increase / decrease in spontaneous emission light included in the signal light,
Comprising a received light monitoring device comprising:
A balanced optical receiver in which an operating point of the photoelectric conversion element is adjusted according to a detection result of the arithmetic unit.
The balanced optical receiver according to claim 3, wherein
The photoelectric conversion unit has an electric amplifier that amplifies the output of the photoelectric conversion element,
A balanced optical receiver in which an input bias of the electric amplifier is controlled according to a detection result of the arithmetic unit.
The balanced optical receiver according to claim 4, wherein
The photoelectric conversion unit is a balanced optical receiver in which each anode electrode of the photoelectric conversion element is individually connected to an input terminal of the electric amplifier.
The balanced optical receiver according to claim 4, wherein
A balanced optical receiver in which the photoelectric conversion elements of the photoelectric conversion unit are connected in series to each other, and a connection point between the photoelectric conversion elements is connected to an input terminal of the electric amplifier.