JPH09321708A - Method for receiving polarization plane modulated signal light and polarized optical diversity receiver using the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent the deterioration of noise proofing ratio and to attain a low cost in a receiver by detecting the carrier wave level of polarization plane modulated signal light which is received by means of respective optical receivers, adding demodulated signals outputted from the respective optical receivers through a signal demodulator in proportion to the carrier wave level and cancelling the addition concerning the demodulated signal with the low carrier wave level at this time. SOLUTION: The carrier wave level of an optical strength fluctuating signal which is outputted from the optical receivers 1 is detected by carrier wave level detectors 4. At the time of the high carrier wavelevel, the attenuation quantity of a variable attenuator 5 is set small by a control signal to be outputted to the attenuator 5. The lower the carrier wave level becomes, the larger the attenuation quantity of the attenuator 5 is successively set. When the carrier wave level becomes less than a set value, the attenuation quantity of the variable attenuator 5 is set to the max., the control signal is outputted to a change-over equipment 6 and the change-over equipment 6 is changed-over to be in a service interrupting state. Then, a base band signal to be inputted is not outputted to an adder 7 so that only another base band signal is outputted through the adder 7.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is used in a polarization plane modulated signal optical communication system that modulates the polarization plane of light and transmits information, and a receiver for appropriately receiving the polarization plane modulated signal light. The present invention relates to a method and a polarization diversity optical receiver using the method.

[0002]

2. Description of the Related Art In a basic polarization plane modulation signal optical communication system, a carrier wave is modulated by a baseband signal carrying information (AM modulation, FM modulation, PM modulation, etc.) on the transmission side.
Then, the modulated signal is applied to the light output from the light emitting element such as an LD to modulate its polarization plane, and this polarization plane modulated signal light is transmitted by an optical fiber or the like. A signal light is received by a light receiving element such as a PD through an analyzer, and a light intensity fluctuation signal (electrical signal) output from the light receiving element is demodulated by a demodulator to obtain a baseband signal. In this optical communication system, the polarization plane modulated signal light propagating through the optical fiber undergoes irregular polarization plane fluctuations due to external pressure applied to the optical fiber. It is necessary to compensate for irregular fluctuations in the wavefront by some means. The polarization diversity optical receiver is one of optical receivers capable of receiving such polarization plane modulated signal light.

The polarization diversity optical receiver splits the polarization plane modulated signal light transmitted to the optical fiber into two or more polarization plane modulated signal lights having different polarization components, and receives each of them individually by the optical receiver. Even if the signal cannot be received by one optical receiver due to the state of the polarization plane, the other remaining optical receivers can receive the signal and receive the polarization-modulated signal light. It is the one. Conventionally, such a polarization diversity optical receiver has been shown in FIGS.

The polarization diversity optical receiver of FIG. 3 splits the input polarization plane modulated signal light into two signal lights having different polarization states in the polarization splitter A, and divides the two signal lights into optical signals. While receiving individually by the receiver B, the carrier level of the signal received by each optical receiver B is detected by the detector C, the comparator D compares the levels of both carrier levels, and the signal demodulator F
Of the two demodulated signals demodulated in step 1, the switch E selects and outputs the demodulated signal in which the higher carrier level is detected.

In the polarization diversity optical receiver shown in FIG. 4, the input polarization plane modulated signal light is split into two signal lights having different polarization states by the polarization splitter A, and the two signal lights are optically split. The receiver B individually receives the signal, and the carrier level of the signal received by each optical receiver B is detected by the detector C, and the attenuation amount of the variable attenuator G is adjusted according to the carrier level to demodulate the signal. The level of the demodulated signal demodulated by the device F is adjusted, and the demodulated signal whose level is adjusted is mixed by the adder H and output. In this optical receiver, when the detected carrier level is high, the attenuation amount of the attenuator G is set small, and when the carrier level is low, the attenuation amount is set large. Therefore, the demodulated signal is proportional to the carrier level. The C / N is superior to that of the optical receiver shown in FIG. 3, which is weighted and added, and only one of the demodulated signals is selected and output.

In the polarization diversity optical receiver shown in FIG. 5, the input polarization plane modulated signal light is split into two signal lights having different polarization states by the polarization splitter A, and the two signal lights are optically split. The receiver B individually receives the carrier wave level and noise level of the signal received by each optical receiver B, and the detector C
And the detector I, and the C / N calculator J and the C / N ratio calculator K adjust the attenuation amount of the variable attenuator G according to the carrier wave and noise level, and the signal demodulator F demodulates. The level of the demodulated signal thus adjusted is adjusted, and the demodulated signal whose level has been adjusted is mixed by the adder H and output. In this optical receiver, the carrier level and the noise level are detected, and they are processed and judged based on an appropriate algorithm, so that only the carrier level is detected and the attenuation amount of the attenuator G is adjusted. It is possible to receive signals with a better C / N than the optical receiver of.

The polarization diversity optical receiver shown in FIGS. 6 (a) and 6 (b) is described in Japanese Patent Laid-Open No. 6-276153. While splitting into two signal lights having different polarization states, the two signal lights are individually received by the optical receiver B, and
The signal received by each optical receiver B is demodulated by the signal demodulator F,
A noise level is detected for each demodulated signal, and the receiving sensitivity of the optical receiver B is individually adjusted according to the noise level,
The demodulated signal from the signal demodulator F is output.

The polarization diversity optical receiver shown in FIG. 7 is described in Japanese Patent Laid-Open No. 1-293328.
Similar to other optical receivers, the two optical receivers B individually receive the separated two signal lights, and the IF signal strength detector L detects the signal strength of the IF signal output from each optical receiver B. It detects and feeds it back to keep the signal strength of each IF signal constant, and the gain-adjusted two IF
The signal is demodulated by the signal demodulator F, and the signal demodulated by the signal demodulator F in proportion to the square of the IF signal strength detected by the IF signal strength detector L is adjusted by the gain adjuster M (variable attenuator G). (Corresponding to) and the gain is adjusted by the combining circuit N (corresponding to the adder H) and output.

[0009]

As described above, although there are various configurations of the polarization diversity optical receiver, there are the following problems. In the optical receiver shown in FIG. 3, noise is generated when only the one having a higher carrier level among the signals received by the two optical receivers B is switched and output by the switch E, so that the reception of the analog signal is performed. Can be used for, but is not suitable for receiving digital signals.

The optical receiver shown in FIGS. 4 to 7 has two optical receivers B.
Although the methods of adding the signals received in step 2 are different from each other, there is commonality in that the two received signals are always mixed and output by the adder H or the combining circuit N. In these optical receivers, even when one of the optical receivers B receives no signal, they are mixed by the adder H or the combining circuit N. When the carrier level of the added signal is low or the noise level is high, the level is lowered by adjusting the attenuation amount of the variable attenuator G, adjusting the gain of the gain adjuster M, or adjusting the light receiving sensitivity of the optical receiver B. However, since the signal level cannot be reduced to 0, noise is actually added. Therefore, there is a problem that the noise-to-noise ratio deteriorates in such a case.

Further, in the optical receiver of FIG. 5 which detects the carrier level and the noise level to add the signals, a highly sensitive measuring circuit is required to detect the noise level, which complicates the receiving circuit. However, it requires a computer with high computing power to calculate the addition rate of signals from the detected carrier level and noise level, which also leads to circuit complexity and cost increase.

An object of the present invention is to provide a polarization plane modulated signal light receiving method and a polarization diversity optical receiver using the method for solving the above problems.

[0013]

As shown in FIG. 1, the polarization plane modulated signal light according to claim 1 of the present invention is converted into two or more polarization plane modulated signal lights having different polarization components. The optical receivers 1 split the light and individually detect it, detect the carrier level of the signal received by each optical receiver 1, and output the demodulated signal output from each optical receiver 1 through the signal demodulator 2. The addition is performed in proportion to the carrier level, and at this time, the addition of the demodulated signal having a low carrier level is canceled.

In this method, even if a signal is not received by one optical receiver 1 and a noise signal is output from the signal demodulator 2 of the same optical receiver 1, the same signal (noise signal) is not added. It is possible to prevent the noise signal that deteriorates the noise ratio from being mixed into the demodulated signal output from the signal demodulator 2 of the optical receiver 1 receiving the signal.

As shown in FIG. 1, the polarization plane modulated signal light according to a second aspect of the present invention is formed by splitting the polarization plane modulated signal light into two or more polarization plane modulated signal lights having different polarization components. The optical receiver 1 individually receives the light, detects the carrier level of the signal received by each optical receiver 1, and outputs the demodulated signal output from each optical receiver 1 through the signal demodulator 2 to the square of the carrier level. The addition is performed in proportion to each other, and the addition of the demodulated signal having a low carrier level is canceled at this time.

According to this method, the carrier level can be detected even with an FM-modulated or PM-modulated signal whose carrier-level cannot be detected simply by detection, and the FM-modulated or PM-modulated signal can also be received.

The polarization diversity optical receiver according to claim 3 of the present invention is, as shown in FIG. 1, a polarization splitter which splits a polarization plane modulated signal light into two or more polarization plane modulated signal lights having different polarization components. 3, two or more optical receivers 1 that receive the separated polarization-modulated signal light, a signal demodulator 2 that obtains a demodulated signal from the signal output from each optical receiver 1, and each optical receiver 1
From the signal output from the detector 4, the variable attenuator 5 that attenuates the demodulated signal output from each signal demodulator 2, and the addition that adds the signals output from each variable attenuator 5 ON / O the demodulation signal that is provided between the adder 7 and each variable attenuator and the adder 7 and is input to the adder 7.
A switch 6 for FF, adjusting the amount of attenuation of each variable attenuator 4 according to the carrier level detected by the detector 4,
The ON / OFF control of each switch 6 is performed.

In this optical receiver, if any one of the two or more optical receivers 1 does not receive a signal, the signal demodulator 2 of the optical receiver 1 outputs a noise signal, which is variable. Even if the attenuator 5 does not completely extinguish, the noise output from the variable attenuator 5 can be cut by the switch 6, and the signal demodulator 2 of the other optical receiver 1 input to the adder 7 can be cut. It is possible to prevent the mixing of noise that deteriorates the noise ratio in the demodulated signal of.

In the polarization diversity optical receiver according to claim 4 of the present invention, as shown in FIG. 1, the attenuation amount of each variable attenuator 4 is adjusted according to the square of the carrier level of the detector 4. The ON / OFF control of each switch 6 is performed.

In this optical receiver, since the detector 4 adjusts the attenuation amount of each variable attenuator 4 and controls the ON / OFF of each switch 6 according to the square of the carrier level,
As described in claim 2, it is also possible to receive an FM-modulated or PM-modulated signal.

[0021]

Embodiment 1 FIG. 1 shows an embodiment of a polarization diversity optical receiver that realizes a method for receiving polarization-modulated signal light of the present invention. Polarization-modulated signal light received by this optical receiver. Is the baseband signal carrying the information
It is M-modulated, and the plane of polarization of the light output from the light source is vibrated by this AM-modulated signal to modulate the plane of polarization.

The polarization splitter 3 of this optical receiver splits the incident polarization plane modulated signal light into two polarization plane modulated signal lights and outputs them as signal lights having different polarization states. Then, the two optical receivers 1 respectively convert the polarization plane modulated signal lights output from the polarization demultiplexer 3 into optical intensity fluctuation signals, and the two signal demodulators (AM signal demodulators) 2 The optical intensity fluctuation signals output from the respective units are demodulated into baseband signals. The AM signal demodulator 2 down-converts the frequency of the carrier wave in the light intensity fluctuation signal and performs DC conversion to demodulate the baseband signal carrying information. The two variable attenuators 5 appropriately attenuate the baseband signal output from the AM signal demodulator 2 on the basis of the attenuation amount control signal, and the two switchers 5 convert the baseband signal output from the variable attenuator 5 into one. Based on the switching control signal, they are turned on or off respectively, and the adder 7 mixes the two baseband signals input from the switch 6 and outputs the mixed signals to the outside.

The carrier level detector (detector) 4 of this optical receiver is for detecting the carrier level of the optical intensity fluctuation signal output from the optical receiver 1, and is variable when the detected carrier level is high. With the control signal output to the attenuator 5, the attenuation amount of the attenuator 5 is set small, the attenuation amount of the variable attenuator 5 is gradually increased as the carrier level decreases, and the set value with the carrier level is set to a certain value. When it falls below the maximum value, the attenuation amount of the variable attenuator 5 is set to maximum, and a control signal is output to the switch 6 to switch the switch 6 from the conductive state to the non-conductive state. When the switch 6 is in the non-conductive state, the input baseband signal is not output to the adder 7, and in this case, only the other baseband signal is output to the outside through the adder 7.

In this polarization diversity optical receiver, the level of the baseband signal added by the adder 7 is proportional to the magnitude of the carrier level, and when the carrier level falls below a certain set value, the baseband signal of the baseband signal is dropped. Since the addition is cancelled, even if no signal is received by one of the two optical receivers 1 and a baseband signal of only noise is output, it is cut and is not added by the adder 7,
The deterioration of the noise-to-noise ratio is prevented.

[0025]

Embodiment 2 FIG. 2 shows another embodiment of a polarization diversity optical receiver that realizes the method of receiving polarization-modulated signal light according to the present invention. Polarization modulation received by this optical receiver. The signal light is obtained by subjecting a carrier wave to FM modulation with a baseband signal carrying information, and oscillating the polarization plane of the light output from the light source with this FM modulation signal to perform polarization plane modulation.

The polarization splitter 3 of this optical receiver splits the incident polarization plane modulated signal light into two polarization plane modulated signal lights and outputs them as signal lights of different polarization states. Then, the two optical receivers 1 respectively convert the polarization plane modulated signal lights output from the polarization demultiplexer 3 into optical intensity fluctuation signals, and the two signal demodulators (FM signal demodulators) 2 are the optical receivers 1. The optical intensity fluctuation signals output from the respective units are demodulated into baseband signals. The two variable attenuators 5 are the FM signal demodulators 2
The baseband signals output from the respective attenuators are appropriately attenuated based on the attenuation amount control signal, and the two switching devices 5 are the variable attenuator 5
The baseband signal output from the switch is turned on or off based on the switching control signal, and the adder 7 mixes the two baseband signals input from the switch 6 and outputs it to the outside.

The carrier level detector (detector) 4 of this optical receiver detects the carrier level of the optical intensity fluctuation signal output from the optical receiver 1, and the squarer 10 that follows detects the carrier level. Square (the carrier level is squared because the level of the baseband signal is constant regardless of the carrier level). The squarer 10 outputs an attenuation amount control signal to the variable attenuator 5 to change the amount of attenuation of the variable attenuator 5, and makes the level of the baseband signal passing through the variable attenuator 5 proportional to the square of the carrier level. Increase or decrease. The squarer 10
Outputs a control signal to the switching device 6 to switch the switching device 6 from the conducting state to the non-conducting state when a value obtained by squaring the carrier wave level falls below a set value. When the switch 6 is in the non-conductive state, the input baseband signal is not output to the adder 7, and in this case, only the other baseband signal is output to the outside through the adder 7.

In this polarization diversity optical receiver, the baseband signals added by the adder 7 are weighted and added in proportion to the square of the carrier level, and the squared carrier level is below the set value. Since the addition of the baseband signal and the baseband signal is canceled, even if the signal is not received by one of the two optical receivers 1 and a baseband signal of only noise is output, it is cut and is not added, and noise resistance is increased. Ratio deterioration is prevented.

In the optical receiver, the polarization plane modulated signal light in which the carrier wave is FM-modulated by the baseband signal is used. However, this optical receiver can also receive the AM modulation or PM modulated polarization plane modulated signal light. it can.

[0030]

Comparative Example In the present invention, the carrier level is detected and the demodulated signal (baseband signal) is weighted and added, and the noise level is not detected. In a 2-port polarization diversity optical receiver, the carrier level of each port is set to C ₁ , C ₂ ,
The noise level is N ₁ and N _2, and the output of each port is a, b
When the two signals are combined by matching the phases, the signals are in-phase and voltage-added, and noise is random and uncorrelated with each other, resulting in power addition. In this case, the general formula after synthesis can be written as in Equation 1.
Various combinations can be made by taking the above a and b. In the weighted addition method by carrier level in the present invention, a = C
_{With 1} and b = C ₂ , C / N can be expressed by Equation 2. On the other hand, in the weighted addition method based on the noise level, C / N can be expressed by Equation 3 with a = C ₁ / N ₁ and b = C ₂ / N ₂ . FIG. 8 shows the calculation results of the carrier level weighted addition method and the noise level weighted addition method, but both are exactly the same, and it is possible to perform sufficiently good signal synthesis only by the carrier level. Recognize.

(Equation 1)

(Equation 2)

(Equation 3)

Further, in the present invention, it is also possible to extend and apply to a three-port polarization diversity optical receiver in which polarization plane modulated signal light is split into three or more signal lights.

[0032]

The method of receiving a polarization-modulated signal light of the present invention and the polarization diversity optical receiver using the method have the following effects. 1. Since the demodulated signals of the ports where no signal is received are cut and not added, the noise ratio does not deteriorate. 2. Even if you don't measure the noise level, you can get almost the same performance as an optical receiver that measures the noise level.
The structure of the receiving circuit does not become complicated, and a computer with high computing power is not required, so that the cost of the receiver can be reduced. 3. Since the addition method and the switching method are used together, noise due to intermittent signals does not occur, it can also be used for digital signal transmission, and the imperfections of the attenuator (attenuator) are also complemented to improve the noise ratio. Deterioration can be prevented.

[Brief description of drawings]

FIG. 1 is an explanatory diagram showing an embodiment of a polarization diversity optical receiver of the present invention.

FIG. 2 is an explanatory diagram showing another embodiment of the polarization diversity optical receiver of the present invention.

FIG. 3 is an explanatory diagram showing a first example of a conventional polarization diversity optical receiver.

FIG. 4 is an explanatory diagram showing a second example of a conventional polarization diversity optical receiver.

FIG. 5 is an explanatory diagram showing a third example of a conventional polarization diversity optical receiver.

6A and 6B are explanatory views showing a fourth example of the conventional polarization diversity optical receiver.

FIG. 7 is an explanatory diagram showing a fifth example of a conventional polarization diversity optical receiver.

FIG. 8 is an explanatory diagram of an addition signal by a carrier level weighted addition method and a noise level weighted addition method.

[Explanation of symbols]

 1 Optical receiver 2 Signal demodulator 3 Polarization separator 4 Detector 5 Variable attenuator 6 Switch 7 Adder

Claims (4)

[Claims] 1. A polarization plane modulated signal light is split into two or more polarization plane modulated signal lights having different polarization components, each of which is individually received by an optical receiver (1), and each of the optical receivers (1) receives it. The carrier level of the received signal is detected, and the demodulated signals output from each optical receiver (1) through the signal demodulator (2) are added in proportion to the carrier level. At this time, the demodulated signal having a low carrier level is added. Is a method of receiving polarization-modulated signal light, characterized in that the addition is canceled. 2. A polarization plane modulated signal light is split into two or more polarization plane modulated signal lights having different polarization components, each of which is individually received by an optical receiver (1), and each of the optical receivers (1). The carrier level of the received signal is detected, and the demodulated signals output from the optical receivers (1) through the signal demodulators (2) are added in proportion to the square of the carrier level. A method for receiving polarization-modulated signal light, characterized in that addition of demodulated signals is canceled. 3. A polarization splitter (3) for splitting a polarization-modulated signal light into two or more polarization-modulated signal lights having different polarization components, and two or more optical receivers for receiving the separated polarization-modulated signal light. (1), a signal demodulator (2) for obtaining a demodulated signal from the signal output from each optical receiver (1), and detection for detecting the carrier level from the signal output from each optical receiver (1) (4), a variable attenuator (5) that attenuates the demodulated signal output from each signal demodulator (2), and an adder (7) that adds the signals output from each variable attenuator (5) And a switch (6) provided between each variable attenuator (5) and the adder (7) to turn on / off the demodulated signal input to the adder (7), and the detector ( 4) Adjusting the attenuation of each variable attenuator (4) and turning on each switch (6) according to the carrier level detected by Polarization diversity optical receiver and performing OFF control. 4. Adjustment of the amount of attenuation of each variable attenuator (4) according to the detector (4) squared of the carrier level,
The polarization diversity optical receiver according to claim 3, wherein ON / OFF control of each switch (6) is performed.