JP2000330079A - Polarization mode dispersion compensating device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a polarization mode dispersion compensating device capable of efficiently extending the compensation range of polarization mode dispersion by a small number of polarization controllers and optical delay units. SOLUTION: The delay time difference ratio of a 1st optical delay unit 201 to a 2nd optical delay unit 202 is made to 1:3, and signal light is made to pass through a 1st polarization controller 101, the 1st optical delay unit 201, a 2nd polarization controller 102, and the 2nd optical delay unit 202 in this order at an receiving end, respectively. An output of the 2nd optical delay unit 202 is branched into two through an optical branching unit 7, and one of them is inputted to an optical receiver 8 for demodulating the signal. The other is inputted to a polarization mode dispersion monitor 9, and a sum of the intensity of a 5 GHz component and that of a 2.5 GHz component in a base band spectrum of a received optical signal by an adder 16, and is adopted as a monitor signal 17. The 1st and 2nd polarization controllers 101, 102 are controlled by a controller 18 so that this monitor signal 17 always takes the maximum.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] 1. Field of the Invention [0002] The present invention relates to a polarization mode dispersion compensator for suppressing the influence of polarization mode dispersion of an optical fiber.

[0002]

2. Description of the Related Art In recent years, in an optical communication system using an optical fiber, a waveform deterioration of signal light due to polarization mode dispersion of an optical fiber or an optical device has become a problem due to an increase in a bit rate due to an increase in transmission speed. Is coming. Polarization mode dispersion is caused by accumulation of birefringence caused by distortion or stress of the core of an optical fiber, difference in optical length between two orthogonal eigen axes (eigen polarization modes) in an optical device, and the like. When the polarization mode dispersion exists in the optical transmission line, a propagation delay time difference occurs between two orthogonal polarizations of the signal light, and a deterioration such as a collapse of a reception waveform occurs to lower reception sensitivity.
In addition, since the polarization mode dispersion shows a temporal change due to a change in stress applied to the optical fiber or a change in temperature, it causes a change in reception sensitivity. Therefore, it is necessary to compensate for the polarization mode dispersion of the optical transmission line.

As a method of compensating for the polarization mode dispersion, for example, as shown in FIG. 7, a polarization maintaining fiber 201 having a propagation delay time difference substantially equal to the polarization mode dispersion of the optical fiber 3 and a polarization controller 101 The method of canceling the polarization mode dispersion by using the above is proposed by the inventor of the present invention (Japanese Patent No. 2739813). FIG. 8 shows a relationship between the polarization mode dispersion and the reception sensitivity deterioration when optical transmission is performed at a transmission speed of 10 Gb / s using a polarization maintaining fiber with a delay time difference of 40 ps in the configuration of FIG.
In the drawing, a curve a shows a case where the control of the polarization mode dispersion compensation is not performed, and a curve b shows a case where the control of the polarization mode dispersion compensation is performed. In the case where the transmission sensitivity is 10 Gb / s and the reception sensitivity degradation of 1 dB is allowed, it can be seen from the curve a in FIG. I understand.

On the other hand, when the polarization mode dispersion compensation is controlled, as shown by a curve b in FIG.
The reception sensitivity deteriorates from ps to 20ps, but 20p
When the time exceeds s, the deterioration of the reception sensitivity decreases and the deterioration becomes almost zero at 40 ps. When the time exceeds 40 ps, the deterioration of the reception sensitivity starts to increase again. When the time exceeds 60 ps, the characteristic exceeds the allowable value of 1 dB. It can be seen that it can be expanded up to 60 ps. Note that this residual degradation occurs when the polarization mode dispersion is 20 ps because the delay time difference of the polarization maintaining fiber is 40 ps, which results in an excessive compensation of 20 ps. Similarly, when the polarization mode dispersion is 60 ps, deterioration occurs because the 20 ps compensation is insufficient.
However, when the polarization mode dispersion is 0 ps, no degradation occurs because the signal light is incident so as to coincide with the eigenaxis of the polarization maintaining fiber.

A method of detecting the polarization mode dispersion itself or a waveform deterioration due to the polarization mode dispersion and controlling the polarization controller in accordance with the detected signal includes, besides the above-mentioned method, a method of detecting a received signal. A method of detecting the entire baseband spectrum and a specific component and controlling the polarization controller so that the sum of the respective intensities is maximized [F. Heismann, DAF
ishman, and DLWilson, "Automatic compensation of f
irst-order polarization mode dispersion in a 10 Gb
/ s transmission system ", ECOC'98,20-24 Sep.1998, Mad
rid, Spain] or a method of detecting a specific component in the baseband spectrum of a received signal and controlling the polarization controller to maximize the intensity of the component [T. Takahashi, T. Imai
and M.Aiki, "Automatic compensation technique for
timewisefluctuating polarisation mode dispersiion
in in-line amplifier systems ", IEE Electronics Lett
ers, Vol. 30, No. 4, 348-349 (1994)].

[0006]

However, in the above method, the range of the polarization mode dispersion that can be compensated is determined by the allowable receiving sensitivity deterioration amount. It is necessary to connect polarization mode dispersion compensators in multiple stages in order to widen the compensation range of polarization mode dispersion. In this case, since the number of optical delay units such as the polarization controller and the polarization maintaining fiber increases, there has been a problem that the apparatus is increased in size and the cost is increased. An object of the present invention is to provide a small number of polarization controllers and optical delay devices,
An object of the present invention is to provide a polarization mode dispersion compensator capable of efficiently expanding a polarization mode dispersion compensation range.

[0007]

In order to solve the above-mentioned problems, a polarization mode dispersion compensating apparatus according to the present invention provides a polarization mode dispersion compensating apparatus in which a signal light having two orthogonal polarizations is incident and the polarization state of the signal light is changed. And a signal controller connected to the output side of the polarization controller and passing the signal light emitted from the polarization controller, and a delay time difference between two orthogonal polarizations of the signal light. A polarization mode dispersion compensator group in which n (n is an integer of 2 or more) polarization mode dispersion compensators connected in series with an optical delay unit to be generated, and an n-th polarization mode dispersion compensator Control means for controlling the polarization controller so that the polarization mode dispersion of the signal light emitted from the optical modulator is minimized, wherein m (m is 2 or more and n or less) out of n optical delay devices (Integer) so that each delay time difference between the optical delay units has a predetermined proportional relationship. It characterized by.

In this case, one example of the configuration of the optical delay unit is a delay of m (m is an integer of 2 or more and n or less) out of n (n is an integer of 2 or more) optical delay devices. The time difference τi (i is an integer from 1 to m) is τi = ai · τ0, where ai = 3 ⁽ⁱ⁻¹⁾ τ0 has a predetermined delay time difference. Also,
Another configuration example of the optical delay unit is n (n is an integer of 2 or more).
Of the optical delay units (m is an integer from 2 to n), the delay time difference τi (i is an integer from 1 to m) is τi = ai · τ0, where ai = 2 ^{(i -1)} τ0 has a relationship of a predetermined delay time difference. Also,
As another configuration example of the optical delay device, m (m is an integer of 2 or more and n or less) out of n (n is an integer of 2 or more) optical delay devices
Τi (i is an integer from 1 to m)
Where τi = ai · τ0 where ai = 1 ^(i-1) τ0 has a relationship of a predetermined delay time difference. Also,
One configuration example of the predetermined delay time difference τ0 is characterized by having a range larger than 0 and not more than 0.8T (T is a 1-bit time width).

[0009]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention will be described below with reference to the drawings. First, a first embodiment of the present invention will be described. The polarization mode dispersion compensator according to the first embodiment of the present invention has n (n is 2) so that the polarization mode dispersion compensator can operate efficiently with a small number of stages.
Among the optical delay devices of the above, m (where m is an integer of 2 or more and n or less) delay time difference τi (i is an integer from 1 to m) is τi = ai · τ0, where ai = 3 ^(i-1) τ0 has a relationship of a predetermined delay time difference. Based on the above equation, the delay time differences τ1 to τm of the m optical delay units are set to 1 and the basic delay time difference τ0 is set to 1. Then, τ1: τ2: τ3: τ4: ...: τm = 1: 3: 9: 27: ...:
It is set so as to be a power-of-three ratio such as 3 ^(m-1) . The basic delay time difference τ0 is determined by the required allowable deterioration amount. In this embodiment, the delay time difference τ0 is set to 40 p in order to set the allowable deterioration amount to 1 dB.
s. Further, the number m of the optical delay units is set such that the compensable range of the polarization mode dispersion according to the present invention is wider than the fluctuation range of the polarization mode dispersion of the transmission line.

Here, a description will be given of an example in which there are two polarization mode dispersion compensators, which is the simplest configuration of the first embodiment of the present invention. This corresponds to n = m = 2. FIG. 1 is a block diagram showing the configuration of the first embodiment of the present invention. In FIG. 1, the polarization mode dispersion compensator 6 according to the first embodiment of the present invention includes an optical transmitter 1, an optical fiber 3, an optical amplifier 4, an optical filter 5, an optical filter 5, a polarization mode dispersion compensator 6, The receivers 8 are arranged on the optical transmission lines connected in this order. Here, the optical transmitter 1 is a transmitter that outputs a signal light 2 obtained by intensity-modulating a laser beam having a wavelength of 1.55 μm at 10 Gb / s.
This is a 55 μm band zero dispersion optical fiber. The optical amplifier 4 was an optical fiber amplifier, and the optical filter 5 was a bandpass filter having a center wavelength of 1.55 μm and a 3 dB optical band of 1 nm.

The polarization mode dispersion compensator 6 includes a first polarization controller 101, a first optical delay device 201, a second polarization controller 102, a second optical delay device 202, and an optical splitter 7. And a polarization mode dispersion monitor 9 and a controller 18, and has one optical input and one optical output. In this case, the first polarization controller 101 and the second polarization controller 102 each have one electrical signal input, one optical input, and one optical output, and have a first optical delay. The device 201 and the second optical delay device 202 each have one optical input and one optical output. The optical splitter 7 has one optical input and two optical outputs, and the polarization mode dispersion monitor 9 has one optical input and one electric signal output. The controller 18 has one electrical signal input and two electrical signal outputs.

Here, an optical input of the first polarization controller 101 is connected to an optical output of the optical filter 5 as an input of the polarization mode dispersion compensator 6. The optical output of the first polarization controller 101 is connected to the optical input of the first optical delay unit 201, and the optical output of the first optical delay unit 201 is connected to the optical input of the second polarization controller 102. It is connected to the. The optical output of the second polarization controller 102 is connected to the optical input of the second optical delay unit 202, and the optical output of the second optical delay unit 202 is connected to the optical input of the optical splitter 7. The first optical output of the optical splitter 7 is
The optical output of the polarization mode dispersion compensator 6 is connected to the optical input of the optical receiver 8. The second optical output of the optical splitter 7 is connected to the optical input of the polarization mode dispersion monitor 9. An electric signal output of the polarization mode dispersion monitor 9 is connected to an electric signal input of the controller 18. A first electrical signal output of the controller 18 is connected to an electrical signal input of a first polarization controller 101, and a second electrical signal output is connected to an electrical signal input of a second polarization controller 102. I have.

In this case, the first polarization controller 101 and the second
The polarization controller 102 uses a fiber squeezer-type polarization control element that changes the polarization state by applying a lateral pressure to the optical fiber, and converts an arbitrary incident polarization into an arbitrary output polarization. And an infinite tracking operation of the polarized wave is possible. For the first optical delay unit 201 and the second optical delay unit 202, a polarization maintaining fiber is used.
The ratio of the delay time difference between the optical delay device 201 and the second optical delay device 202 is 1: 3. In this case, the first optical delay device 2
The polarization maintaining fiber of No. 01 has a length with a delay time difference of 40 ps, and the polarization maintaining fiber of the second optical delay unit 202 has a length with a delay time difference of 120 ps. Since the polarization maintaining fiber has a polarization mode dispersion of 1.5 ps / m, the length of the polarization maintaining fiber actually used is 26.7 m for the polarization maintaining fiber of the first optical delay unit 201. The polarization maintaining fiber of the second optical delay unit 202 is 80
m. The optical splitter 7 uses an optical fiber coupler, and splits the incident signal light into two.

The polarization mode dispersion monitor 9 includes a photodetector 10
, An amplifier 11, a first electric filter 12, a first intensity detector 13, a second electric filter 14, a second intensity detector 15, and an adder 16. In this case, the photodetector 10 has one optical input and one electric signal output, and includes an amplifier 11, a first electric filter 12, a first intensity detector 13, a second electric filter 14, and a second electric filter. The two intensity detectors 15 each have one electric signal input and one electric signal output. The adder 16 has two electric signal inputs and one electric signal output.

Here, an optical input of the photodetector 10 is connected to a second optical output of the optical splitter 7 as an input of the polarization mode dispersion monitor 9. The electric output of the photodetector 10 is connected to the input of the amplifier 11, and the output of the amplifier 11 is connected to the input of the first electric filter 12 and the input of the second electric filter 14. The output of the first electric filter 12 is connected to the input of the first intensity detector 13, and the output of the first intensity detector 13 is connected to the first input of the adder 16. Second electric filter 14
Is connected to the input of the second intensity detector 15, and the output of the second intensity detector 15 is connected to the second output of the adder 16.
Connected to the input. The output of the adder 16 is connected to the electric signal input of the controller 18 as the output of the polarization mode dispersion monitor 9.

In this case, a pin photodiode is used for the photodetector 10 and a 10 Gb
An amplifier capable of amplifying a high-speed signal of / s to a level that can be used in the next stage circuit is used. As the first electric filter 12, a band-pass filter that passes a 5-GHz component is used, and as the second electric filter 14, a band-pass filter that passes a 2.5-GHz component is used. A square detector is used as the first intensity detector 13 and the second intensity detector 15, and a commonly used one is used as the adder 16.

The controller 18 has a built-in microprocessor and sequentially outputs two control signals 301 and 302 to apply a small perturbation to the polarization state of the signal light 2 to search for the maximum value of the input signal. Control of the law. Optical receiver 8
Is a receiver for demodulating the intensity-modulated signal light into an electric signal by a direct detection reception method.

Next, the operation of the polarization mode dispersion compensator according to the first embodiment will be described with reference to FIG. Optical transmitter 1
Wavelength 1.5 modulated by 10 Gb / s
A 5 μm signal light 2 is transmitted through an optical fiber 3 having polarization mode dispersion, and is input to an optical amplifier 4 at a receiving end and amplified. By passing the output through an optical filter 5 having a 3 dB optical band of 1 nm, unnecessary spontaneous emission optical noise is removed. The output of the optical filter 5 is sent to the first polarization controller 101, the first
, An optical delay device 201, a second polarization controller 102, and a second optical delay device 202 in this order. Next, the second optical delay unit 202
Is split into two by an optical splitter 7, and one is input to an optical receiver 8 to demodulate a signal. The other is input to the polarization mode dispersion monitor 9 to obtain a monitor signal 17 for controlling the first polarization controller 101 and the second polarization controller 102.

Here, the polarization mode dispersion monitor 9 amplifies the signal received by the photodetector 10 with the amplifier 11, and then measures the intensity of a half (5 GHz) component of the clock frequency in the baseband spectrum. The first electric filter 12 and the first
Are detected using the intensity detector 13 of (1), and a quarter component (2.
5 GHz) is detected using the second electric filter 14 and the second intensity detector 15. Next, the output of the first intensity detector 13 and the output of the second intensity detector 15 are respectively input to the adder 16, and a signal of the sum thereof is obtained from the output of the adder 16 and used as the monitor signal 17. . The controller 18 controls the first polarization controller 101 and the second polarization controller 102 so that the sum of the intensities always becomes the maximum. As this control method, a “hill-climbing method” for finding the maximum value of the monitor signal 17 by applying a small perturbation to the polarization state of the signal light 2 is used.

In such a configuration, as a result of measuring the transmission characteristics, the value of the polarization mode dispersion of the transmission line is almost 0 ps.
However, the reception sensitivity degradation was always suppressed to 1 dB or less, although the temporal change was in a range from to 150 ps (the average value was about 50 ps). When the polarization mode dispersion compensator having the configuration shown in FIG. 7 was used under the same conditions, the degradation was 1 dB or more in the time zone where the polarization mode dispersion of 60 ps or more occurred, and sometimes the signal could not be received at all. From the above results, the effectiveness of the present invention was confirmed.

Next, the operation principle of the polarization mode dispersion compensator according to the first embodiment of the present invention will be described. In the configuration shown in FIG. 1, the polarization mode dispersion of the transmission line is 0 ps.
, The first optical delay unit 201 (delay time difference 40p
s) and the second optical delay unit 202 (delay time difference 120p
In s), by injecting the signal light 2 so that the polarization state of the signal light 2 coincides with the characteristic axis of each optical delay device,
Each delay time difference has no effect. As a result,
When the polarization mode dispersion is 0 ps, the degradation becomes a minimum value.
When the polarization mode dispersion of the transmission line is 40 ps,
Only the first optical delay device 201 operates, and the second optical delay device 2
The delay time difference of 120 ps does not affect the signal light 02 by injecting the signal light 2 so that the polarization state of the signal light 2 coincides with the eigenaxis of the second optical delay unit 202. As a result, even when the polarization mode dispersion is 40 ps, the deterioration has a minimum value. Also, the polarization mode dispersion of the transmission line is 80 ps.
, The first optical delay 201 and the second optical delay 20
Since the respective polarization controllers 101 and 102 operate so that the two delay time differences are in directions that cancel each other out,
As a whole, it operates as an optical delay unit having a delay time difference of 80 ps. As a result, even when the polarization mode dispersion is 80 ps, the deterioration becomes a minimum value.

Similarly, when the polarization mode dispersion of the transmission line is 12
At 0 ps, only the second optical delay unit 202 operates, and the first optical delay unit 201 applies the signal light such that the polarization state of the signal light 2 coincides with the eigenaxis of the first optical delay unit 201. By injecting 2, the delay time difference of 40 ps has no effect. As a result, even when the polarization mode dispersion is 120 ps, the deterioration has a minimum value. 160ps polarization mode dispersion
In this case, both optical delay units 201 and 202 operate to realize almost perfect polarization mode dispersion compensation at each value. As a result, the polarization mode dispersion becomes 16
Also at 0 ps, the degradation is a minimum value. As described above,
The polarization mode dispersion of the transmission line is the basic delay time difference τ
When the value is an integral multiple of 0, the deterioration of the receiving sensitivity is minimized. Therefore, it is understood that the combination of the ratios of the delay time differences of the respective optical delay devices should be continuous integers.

In this case, the ratio of the delay time difference between the first optical delay unit 201 and the second optical delay unit 202 is set to 1: 3, so that the sum and difference of the numbers and the numbers are not used. Thus, continuous integers of 0, 1, 2, 3, and 4 can be created. As a result, as shown by a curve c in FIG. 2, the polarization mode dispersion of the transmission line is 0 ps, 40 ps, and 80 ps.
At s, 120 ps, and 160 ps, the reception sensitivity degradation is minimized, and the controllable range in which the reception sensitivity degradation is within 1 dB can be expanded to 180 ps. Here, FIG. 2 is a diagram illustrating the degradation of the receiving sensitivity with respect to the value of the polarization mode dispersion when optical transmission is performed at a transmission speed of 10 Gb / s in the configuration of FIG. In the same figure, a curve a shows a case where the polarization mode dispersion compensation is not controlled, and a curve c shows a case where the polarization mode dispersion compensation is controlled.

Similarly, in the case of a polarization mode dispersion compensator having a three-stage configuration, the delay time difference between the respective optical delay units is set to 1:
By setting the ratio to 3: 9, all the integers from 0 to 13 can be created by the operations of the respective numbers. As a result, the controllable range can be expanded to 540 ps. That is, the number of polarization controllers is three, and the ratio of the delay time difference is 1:
In the case of 3: 9, the compensation range is 27 times as large as the allowable range when no control is performed, and 9 times as large as the compensation range when the conventional polarization mode dispersion compensator is used. Can be expanded. By setting the delay time difference between the plurality of optical delay units to a power-of-three ratio, the amount of expansion of the compensation range can be tripled for each optical delay unit. Accordingly, the compensation range of the polarization mode dispersion can be efficiently expanded with a small number of polarization controllers and optical delay devices.

Next, a second embodiment of the present invention will be described. In the polarization mode dispersion compensator according to the second embodiment of the present invention, m (m is an integer of 2 or more and n or less) light out of n (n is an integer of 2 or more) optical delay devices. The delay time difference τi (i is an integer from 1 to m) of the delay unit is τi = ai · τ0, where ai = 2 ^(i-1) τ0 has a predetermined delay time difference, and is based on the above equation. When the delay time differences τ1 to τm of the m optical delay devices are set to 1 as the basic delay time difference τ0, τ1: τ2: τ3: τ4:...: Τm = 1: 2: 4: 8:. 2
It is set so as to be a power-of-two ratio as in ^(m-1) . The basic delay time difference τ0 is determined by the required allowable deterioration amount. In this embodiment, the delay time difference τ0 is set to 2 in order to set the allowable deterioration amount to 0.5 dB.
0 ps. Further, the number m of the optical delay units is set such that the compensable range of the polarization mode dispersion according to the present invention is wider than the fluctuation range of the polarization mode dispersion of the transmission line.

Here, a case where there are three polarization mode dispersion compensators will be described as an example. This corresponds to n = m = 3. FIG. 3 is a block diagram showing the configuration of the second embodiment of the present invention. In FIG. 3, the same reference numerals as those in FIG. 1 denote the same parts. In FIG. 3, a polarization mode dispersion compensator 36 according to a second embodiment of the present invention
The optical fiber 3, the optical amplifier 4, the optical filter 5, the polarization mode dispersion compensator 36, and the optical receiver 8 are arranged on an optical transmission line connected in this order. Here, an oscillator 19 is connected to the optical transmitter 21, and the signal light 2 is transmitted to the oscillator 1.
The optical transmitter 1 differs from the optical transmitter 1 of the first embodiment in that optical frequency modulation is performed with a low-frequency signal 20 of 5 kHz generated by the signal 9. The optical fiber 3, the optical amplifier 4, the optical filter 5, and the optical receiver 8
Since it is the same as the first embodiment, the description is omitted.

The polarization mode dispersion compensator 36 includes a first polarization controller 101, a first optical delay device 201, a second polarization controller 102, a second optical delay device 202, and a third polarization controller. Wave controller 103, third optical delay unit 203, and polarization maintaining optical splitter 21
And a polarization mode dispersion monitor 29 and a controller 38, and has one optical input and one optical output. In this case, the first polarization controller 101 and the second polarization controller 102
And the third polarization controller 103 has one electric signal input, one optical input, and one optical output, respectively.
The first optical delay device 201, the second optical delay device 202, and the third
The optical delay units 203 each have one optical input and one optical output. The polarization maintaining optical splitter 22 has one optical input and two optical outputs, and the polarization mode dispersion monitor 29 has one optical input and one electric signal output. The controller 38 has one electrical signal input and three electrical signal outputs.

Here, an optical input of the first polarization controller 101 is connected to an optical output of the optical filter 5 as an input of the polarization mode dispersion compensator 36. The optical output of the first polarization controller 101 is connected to the optical input of the first optical delay unit 201, and the optical output of the first optical delay unit 201 is connected to the optical input of the second polarization controller 102. It is connected to the. The optical output of the second polarization controller 102 is connected to the optical input of the second optical delay unit 202, and the optical output of the second optical delay unit 202 is connected to the optical input of the third polarization controller 103. Have been. The optical output of the third polarization controller 103 is connected to the optical input of the third optical delay unit 203, and the optical output of the third optical delay unit 203 is connected to the optical input of the polarization maintaining optical splitter 22. ing.

The first optical output of the polarization maintaining optical splitter 22 is:
As the optical output of the polarization mode dispersion compensator 36, the optical receiver 8
Connected to the optical input of In addition, the polarization maintaining optical splitter 2
The second optical output 2 is connected to the optical input of the polarization mode dispersion monitor 29. An electric signal output of the polarization mode dispersion monitor 29 is connected to an electric signal input of the controller 38. A first electrical signal output of the controller 38 is connected to an electrical signal input of a first polarization controller 101, a second electrical signal output is connected to an electrical signal input of a second polarization controller 102, The third electric signal output is connected to the electric signal input of the third polarization controller 103.

In this case, the first polarization controller 101 and the second
The polarization controller 102 and the third polarization controller 103 of
The same polarization controller as that used in the first embodiment is used. First optical delay device 201, second optical delay device 2
The same polarization preserving fiber as that of the first embodiment is used for the second optical delay device 203 and the third optical delay device 203, and the first optical delay device 201, the second optical delay device 202, and the third optical delay device 203 are used. Delay device 2
03 is 1: 2: 4. In this case, the polarization maintaining fiber of the first optical delay unit 201 has a length such that the delay time difference is 20 ps, and the second optical delay unit 202
The polarization maintaining fiber of the third optical delay unit 203 has a length such that the delay time difference becomes 80 ps.

Since the polarization maintaining fiber has a polarization mode dispersion of 1.5 ps / m, the length of the polarization maintaining fiber actually used is 13 μm for the polarization maintaining fiber of the first optical delay unit 201. .3 m, the polarization maintaining fiber of the second optical delay unit 202 is 26.7 m, and the third optical delay unit 20
3 polarization maintaining fiber is 53.3 m. The polarization-maintaining optical splitter 22 uses a polarization-maintaining optical fiber coupler, and splits the incident signal light into two.

The polarization mode dispersion monitor 29 includes a polarization splitter 23, a photodetector 24, an amplifier 11, and an electric filter 25.
And an intensity detector 13. In this case, the polarization separation element 23 has one optical input and two optical outputs. The photodetector 24 has two light inputs and one electric signal output, and the amplifier 11, the electric filter 25, and the intensity detector 13 have one electric signal input and one electric signal output, respectively. I have.

Here, as an input of the polarization mode dispersion monitor 29, the optical input of the polarization splitter 23 arranged so that the optical axis is inclined by 45 degrees with respect to the eigen axis of the polarization maintaining optical splitter 22 is given by: It is connected to the second optical output of the polarization maintaining optical splitter 22. Further, a first optical output of the polarization separation element 23 is connected to a first optical input of the photodetector 24, and a second optical output thereof is connected to a second optical input of the photodetector 24. . The electric output of the photodetector 24 is connected to the input of the amplifier 11, and the output of the amplifier 11 is connected to the input of the electric filter 25. The output of the electric filter 25 is the intensity detector 1
The output of the intensity detector 13 is connected to the electric signal input of the controller 38 as the output of the polarization mode dispersion monitor 29.

In this case, the polarization splitting element 23 splits the incident light into two polarization states orthogonal to each other and outputs the light. The photodetector 24 uses a dual pin photodiode, and constitutes a balanced optical receiver together with the amplifier 11. As the electric filter 25, a band-pass filter that passes a 5-kHz component is used. Intensity detector 1
3 is the same as in the first embodiment, and a description thereof will be omitted. The controller 38 has a built-in microprocessor and sequentially outputs three control signals 301 to 303 to apply a small perturbation to the polarization state of the signal light 2 to search for the minimum value of the input signal. Perform control.

Next, the operation of the polarization mode dispersion compensator according to the second embodiment will be described with reference to FIG. Optical transmitter 3
1 is connected to an oscillator 19, which generates a laser beam having a wavelength of 1.55 μm, which is optically frequency-modulated by a low-frequency signal 20 of 5 kHz generated by the oscillator 19, and generates 10 Gb /
The signal light 2 intensity-modulated by s is transmitted through an optical fiber 3 having polarization mode dispersion, and is input to an optical amplifier 4 at a receiving end and amplified. By passing the output through an optical filter 5 having a 3 dB optical band of 1 nm, unnecessary spontaneous emission optical noise is removed. The output of the optical filter 5 is supplied to a first polarization controller 10.
1, a first optical delay device 201, a second polarization controller 102,
The second optical delay unit 202, the third polarization controller 103, the third
Through the optical delay unit 203 in this order. Next, the third optical delay unit 2
03 is split into two by the polarization maintaining optical splitter 22,
One is input to the optical receiver 8 to demodulate the signal. The other is input to the polarization mode dispersion monitor 29 to obtain a monitor signal 17 for controlling the first polarization controller 101, the second polarization controller 102, and the third polarization controller 103.

Here, the polarization mode dispersion monitor 29 separates the other of the signal light 2 split by the polarization maintaining optical splitter 22 into two polarization states orthogonal to each other by a polarization splitter 23. Next, the two separated signal lights are respectively incident on two light receiving sections of a balanced optical receiver composed of the photodetector 24 and the amplifier 11, converted into electric signals, and output from the amplifier 11. Next, the output of the amplifier 11 is connected to the electric filter 2.
5 to extract only 20 components of the demodulated low-frequency signal of 5 kHz. The output of the electric filter 25 is input to the intensity detector 13 and the intensity of the low frequency signal 20 is detected and used as the monitor signal 17. The controller 38 controls the first polarization controller 101, the second polarization controller 102, and the third polarization controller 103 such that the intensity of the monitor signal 17 is always minimized. As this control method, the signal light 2
The "hill-climbing method" for finding the minimum value of the monitor signal 17 by adding a small perturbation to the polarization state of the signal is used.

In such a configuration, as a result of measuring the transmission characteristics, the value of the polarization mode dispersion of the transmission line is almost 0 ps.
(The average value is about 50 ps), but the minimum value of the polarization mode dispersion of the polarization maintaining fiber is set to 20 ps, so that the reception sensitivity degradation is always suppressed to 0.5 dB or less. I was able to. From the above results, the effectiveness of the present invention was confirmed.

In the second embodiment, the first optical delay 201, the second optical delay 202, and the third optical delay 20
By setting the ratio of the delay time difference of 3: 2 to 1: 2: 4,
0 to 7 by not using the sum of each number and the number
All integers up to can be made. As a result, as shown by a curve d in FIG. 4, the polarization mode dispersion of the transmission line is 0 ps, 20 ps, 40 ps, 60 ps, 80 ps, and 1 ps.
At the time of 00 ps, 120 ps, and 140 ps, the deterioration of the reception sensitivity is minimized, and the controllable range in which the reception sensitivity deterioration is within 0.5 dB can be expanded to 150 ps.
Here, FIG. 4 shows a transmission speed of 10 Gb in the configuration of FIG.
FIG. 7 is a diagram illustrating degradation of reception sensitivity with respect to the value of polarization mode dispersion when optical transmission is performed at / s. In FIG.
Shows a case where the polarization mode dispersion compensation is not controlled, and a curve d shows a case where the polarization mode dispersion compensation is controlled.

In the second embodiment, the ratio of the polarization mode dispersion of the optical delay unit is set to be a power of 2, so that the ratio of 3 which is the power of the first embodiment is selected. Although the controllable range per the number of optical delay units is smaller than in the case, the control stability is expected to be improved because all numbers can be created without using the difference in delay time difference due to cancellation. .

Next, a third embodiment of the present invention will be described. In the polarization mode dispersion compensator according to the third embodiment of the present invention, m (m is an integer of 2 or more and n or less) light out of n (n is an integer of 2 or more) optical delayers. The delay time difference τi (i is an integer from 1 to m) of the delay unit is τi = ai · τ0, where ai = 1 ^(i-1) and τ0 has a predetermined delay time difference. Τ1: τ2: τ3: τ4:...: Τm = 1: 1: 1: 1:..., When the delay time difference τ1 to τm of the m optical delay devices is set to 1 as the basic delay time difference τ0. : 1
It is set so as to be a power-of-one ratio as in ^(m-1) . That is, in the third embodiment, the delay time differences of the m optical delay units are all set to be the same. The basic delay time difference τ0 is determined by the required minimum deterioration amount. In this embodiment, the allowable delay amount is set to 0.5 dB.
s. Further, the number m of the optical delay units is set such that the compensable range of the polarization mode dispersion according to the present invention is wider than the fluctuation range of the polarization mode dispersion of the transmission line.

Here, an example in which the number of polarization mode dispersion compensators is eight will be described. This corresponds to n = m = 8. FIG. 5 is a block diagram showing the configuration of the third embodiment of the present invention. In FIG. 5, the same reference numerals as those in FIG. 1 denote the same parts. Referring to FIG. 5, a polarization mode dispersion compensator 46 according to a third embodiment of the present invention includes an optical transmitter 1, an optical fiber 3, an optical amplifier 4, an optical filter 5, an optical filter 5, a polarization mode dispersion compensator 46, The receivers 8 are arranged on the optical transmission lines connected in this order. Here, the optical transmitter 1, the optical fiber 3, the optical amplifier 4, the optical filter 5, and the optical receiver 8 are the same as those in the first embodiment, and the description is omitted.

The polarization mode dispersion compensator 46 includes first to eighth polarization controllers 101 to 108, first to eighth optical delay units 201 to 208, an optical splitter 7, and a polarization mode dispersion monitor 4.
9 and a controller 48, and has one optical input and one optical output. In this case, the first to eighth polarization controllers 101 to 108 each have one electric signal input and one electric signal input.
One optical input and one optical output,
Each of the optical delay units 201 to 208 has one optical input and one optical output. The optical splitter 7 has one optical input and two optical outputs, and the polarization mode dispersion monitor 49 has one optical input and one electric signal output. Controller 48 has one electrical signal input and eight electrical signal outputs.

Here, eight polarization mode dispersion compensators each composed of a polarization controller and an optical delay unit connected to the output side of the polarization controller are connected in series, and a first polarization controller is provided. An optical input of the wave controller 101 is connected to an optical output of the optical filter 5 as an input of the polarization mode dispersion compensator 46. The optical output of the eighth optical delay unit 208 is connected to the optical input of the optical branch unit 7. The first optical output of the optical splitter 7 is connected to the optical input of the optical receiver 8 as the optical output of the polarization mode dispersion compensator 46. The second optical output of the optical splitter 7 is connected to the optical input of the polarization mode dispersion monitor 49. The electric signal output of the polarization mode dispersion monitor 49 is
It is connected to the electrical signal input of the controller 48. Controller 4
8 is connected to the electric signal input of the first polarization controller 101, and the second to eighth electric signal outputs are similarly connected to the second to eighth polarization controllers 102. To 108 electrical signal inputs.

In this case, the first to eighth polarization controllers 101
As to, the same polarization controller as that used in the first embodiment is used. First to eighth optical delay units 20
The same polarization preserving fibers as those in the first embodiment are used for 1 to 208, and their delay time differences are set to be the same. In this case, each polarization preserving fiber is set to a length such that the delay time difference is 20 ps. Since the polarization maintaining fiber has a polarization mode dispersion of 1.5 ps / m, the length of the polarization maintaining fiber actually used is 13.3 m, respectively. The optical splitter 7 is the same as in the first embodiment, and a description thereof will be omitted.

The polarization mode dispersion monitor 49 includes the photodetector 1
0, an amplifier 11, an electric filter 26, and an intensity detector 13. In this case, the photodetector 10 has one optical input and one electric output, and the amplifier 11, the electric filter 26, and the intensity detector 13 respectively output one electric signal input and one electric signal output. Have. Here, the optical input of the photodetector 10 is a polarization mode dispersion monitor 49.
Is connected to the second optical output of the optical splitter 7 as an input. The electrical output of the photodetector 10 is connected to the input of an amplifier 11, and the output of the amplifier 11 is connected to an electrical filter 2.
6 inputs. The output of the electric filter 26 is connected to the input of the intensity
Is connected to the electrical signal input of the controller 48 as the output of the polarization mode dispersion monitor 49.

In this case, since the photodetector 10, the amplifier 11, and the intensity detector 13 are the same as in the first embodiment,
Description is omitted. As the electric filter 26, a band-pass filter that passes a frequency component from 2 GHz to 5 GHz is used. The controller 48 has a built-in microprocessor, sequentially outputs eight control signals 301 to 308, adds a small perturbation to the polarization state of the signal light 2, and searches for the maximum value of the input signal. Perform control.

Next, the operation of the polarization mode dispersion compensator according to the third embodiment will be described with reference to FIG. Optical transmitter 1
Wavelength 1.5 modulated by 10 Gb / s
A 5 μm signal light 2 is transmitted through an optical fiber 3 having polarization mode dispersion, and is input to an optical amplifier 4 at a receiving end and amplified. By passing the output through an optical filter 5 having a 3 dB optical band of 1 nm, unnecessary spontaneous emission optical noise is removed. After the output of the optical filter 5 is passed through eight polarization mode dispersion compensators connected in series, it is split into two by an optical splitter 7, one of which is input to an optical receiver 8 to demodulate a signal. Let it. The other is input to the polarization mode dispersion monitor 49, and a monitor signal 17 for controlling the first to eighth polarization controllers 101 to 108 is obtained.

Here, the polarization mode dispersion monitor 9 amplifies the signal received by the photodetector 10 by the amplifier 11, and then, from the baseband spectrum of the signal received by the electric filter 26 having the band-pass characteristic from 2 GHz. Frequency components up to 5 GHz are extracted and input to the intensity detector 13.
Thus, the intensity of the frequency component from 2 GHz to 5 GHz is obtained, and this is used as the monitor signal 17. The controller 48 controls the first to eighth polarization controllers 101 to 108 such that the intensity of the monitor signal 17 is always maximized. As this control method, a “hill-climbing method” for finding the maximum value of the monitor signal 17 by applying a small perturbation to the polarization state of the signal light 2 is used.

In such a configuration, as a result of measuring the transmission characteristics, the value of the polarization mode dispersion of the transmission line is almost 0 ps.
(The average value is about 50 ps), but the minimum value of the polarization mode dispersion of the polarization maintaining fiber is set to 20 ps, so that the reception sensitivity degradation is always suppressed to 0.5 dB or less. I was able to. From the above results, the effectiveness of the present invention was confirmed.

In the third embodiment, by making the delay time differences of the first to eighth optical delay units the same, the sum of the respective numbers and the use of the total number of You can make an integer. As a result, as shown by a curve e in FIG.
s, 20ps, 40ps, 60ps, 80ps, 100
At ps, 120 ps, 140 ps, and 160 ps, the reception sensitivity degradation is minimized, and the controllable range in which the reception sensitivity degradation is within 0.5 dB can be expanded to 170 ps. Here, FIG. 6 is a diagram illustrating the degradation of the receiving sensitivity with respect to the value of the polarization mode dispersion when the optical transmission is performed at the transmission speed of 10 Gb / s in the configuration of FIG. In the figure, a curve a shows a case where the control of the polarization mode dispersion compensation is not performed, and a curve e shows a case where the control of the polarization mode dispersion compensation is performed.

In the third embodiment, each optical delay unit 2
Since the delay time differences of 01 to 208 are the same, the polarization mode dispersion ratios are all 1, which is a power of 3 ratio according to the first embodiment or a power of 2 ratio according to the second embodiment. The controllable range per number of optical delay units is smaller than when a certain ratio is selected, but by connecting polarization mode dispersion compensators in multiple stages, fine control becomes possible and higher-order polarization mode Can compensate for dispersion. As a result, the remaining deterioration amount can be reduced.

In the first to third embodiments of the present invention, since 10 Gb / s signal light is used, 20 ps or 40 ps is used as the basic delay time difference τ0. It is necessary to change the time difference τ0. In this case, assuming that the time width of one bit is T, the deterioration amount can be suppressed to a predetermined value or less by selecting τ0 in a range from 0 to 0.8T or less. In addition,
T is also the reciprocal of the clock frequency used for data transmission. Therefore, in the case of 10 Gb / s, the time width of one bit is 100 ps, and 0.8T is 80 p of the delay time difference τ0.
s. The maximum deterioration amount in this case is about 5 dB. Therefore, the maximum deterioration amount can be suppressed to about 5 dB or less. However, the value of 5 dB varies depending on the characteristics of the transceiver used, and is not an absolute value.

Although the embodiments of the present invention have been described above, the present invention is not limited to these configurations, and various modifications are possible. For example, signal light 2
Is not limited to 1.55 μm which is usually used in optical communication, but may be another wavelength. The optical fiber 3 is not limited, and the present invention is effective for optical fibers of any kind and dispersion. In the embodiment of the present invention, the polarization maintaining fiber is used as the optical delay unit, but the present invention is not limited to this.
For example, after separating the signal light into two orthogonal polarizations using a polarization separation element and giving a delay time difference between the separated signal lights,
An optical delay unit that combines light with a polarization multiplexing element (same structure as a polarization splitting element) may be used. In this optical delay device,
A variable optical delay device having a structure capable of changing the optical path length difference using a movable mirror or the like and changing the polarization mode dispersion value may be used. Further, an optical component that generates polarization mode dispersion using birefringence in the photonic crystal may be used.

Further, although the result of the bit rate of 10 Gb / s has been described, the present invention is not limited to this, and may be faster or slower. Of course, it can be applied to wavelength division multiplexing (WDM) optical communication. Although optical preamplifier reception is used to improve reception sensitivity, any type of amplifier such as a semiconductor optical amplifier, a Raman amplifier, and a parametric amplifier can be used in addition to an optical fiber amplifier. Further, even if the optical preamplifier reception is not used, it does not affect the operation of the present invention.
As the photodetector, any photodetector other than the pin photodiode, such as an avalanche photodiode (APD), can be used.

Although the modulation / demodulation method uses the intensity modulation / direct detection reception method, the present invention is not limited to this. Any modulation method such as frequency modulation, phase modulation, polarization modulation, or duobinary modulation can be applied. It is possible. The same applies to the demodulation method, and any demodulation method such as a combination of an optical interferometer and direct detection can be applied. In the embodiment of the present invention, the polarization mode dispersion compensator has two stages,
Although the polarization mode dispersion compensator having a configuration in which three or eight stages are connected in series has been described, it is needless to say that the number of stages may be smaller or larger.

The positional relationship when the polarization controller and the optical delay unit are set as one set and they are connected in a plurality of stages is not related to the magnitude of the delay time difference between the respective optical delay units, but in the opposite order. Alternatively, the order may be changed only partially. Further, it is not necessary that the ratios of the delay time differences of the optical delay devices are all different from each other. For example, those having the same delay time difference such as 1: 3: 1 may be included.

In the embodiment of the present invention, 10 Gb
/ S signal light, the basic delay time difference τ0
Although 20 ps or 40 ps is used as the value, the value may be larger or smaller than this value, and varies depending on the allowable deterioration amount. For example, if degradation of about 5 dB is allowed, τ0 is 8
It can be set to about 0 ps. Also, 0.5dB
When it is desired to suppress the following deterioration, it is possible to achieve both a small deterioration amount and a wide controllable range by setting τ0 to 20 ps or less and increasing the number of stages of the polarization mode dispersion compensator.

As the polarization controller, any type of polarization controller other than the fiber squeezer type, such as an optical waveguide type, a wave plate rotating type, an optical fiber loop rotating type, and a liquid crystal type, can be used. In addition, the polarization controller and the optical delay device can be integrated on a lithium niobate substrate, a quartz substrate, an optical semiconductor substrate, an organic material substrate, or the like to reduce the size. As a control method of the polarization controller, the maximum value (or minimum value) control method "hill climbing method" was used, but the present invention is not limited to this. A control method in which an optimum point is searched for by synchronous detection is also applicable.

In the embodiment of the present invention, a plurality of polarization controllers are controlled by one controller in order to reduce the size of the apparatus. However, one controller is used for each polarization controller. You may do so. In this case, it is necessary to take measures such as changing the perturbation frequency for each controller. Further, in the embodiment of the present invention, as the polarization mode dispersion monitor,
A method for detecting the intensity of the baseband spectrum;
Although the method using the M-AM conversion has been described, the present invention is not limited to this, and any other monitor method such as a method of detecting the degree of polarization of signal light can be used. Further, the intensity detector is not limited to the square detector, and may be of any circuit type as long as it can detect the intensity. Further, not only the intensity of the baseband spectrum but also the amplitude of the baseband spectrum can be detected by using an amplitude detector and used for polarization control.

[0060]

As described above, according to the present invention, a polarization mode dispersion compensator capable of efficiently expanding the polarization mode dispersion compensation range with a small number of polarization controllers and optical delay devices. Can be provided.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a configuration of a first embodiment.

FIG. 2 is a diagram illustrating degradation of reception sensitivity with respect to a value of polarization mode dispersion in the configuration of FIG. 1;

FIG. 3 is a block diagram showing a configuration of a second embodiment.

FIG. 4 is a diagram illustrating degradation of reception sensitivity with respect to the value of polarization mode dispersion in the configuration of FIG. 3;

FIG. 5 is a block diagram showing a configuration of a third embodiment.

6 is a diagram illustrating degradation of reception sensitivity with respect to the value of polarization mode dispersion in the configuration of FIG. 5;

FIG. 7 is a block diagram showing a configuration of a conventional technique.

FIG. 8 is a diagram illustrating degradation of reception sensitivity with respect to a value of polarization mode dispersion according to the related art.

[Explanation of symbols]

1, 21: optical transmitter, 2: signal light, 3: optical fiber, 4
… Optical amplifier, 5… optical filter, 6, 36, 46, 56…
Polarization mode dispersion compensator, 7: optical splitter, 8: optical receiver, 9, 29, 49 ... polarization mode dispersion monitor, 10, 2
4: photodetector, 11: amplifier, 12, 14, 25, 26
... Electric filters, 13 and 15 ... Intensity detector, 16 ... Adder, 17 ... Monitor signal, 18,38,48,58 ... Controller, 19 ... Oscillator, 20 ... Low frequency signal, 22 ... Polarization preserving optical branch 23, a polarization splitting element, 101 to 108, a polarization controller, 201 to 208, an optical delay unit (polarization preserving fiber), 301 to 308, a control signal.

Claims (5)

[Claims] 1. A polarization controller that receives a signal light and controls a polarization state of the signal light, and is connected to an output side of the polarization controller and receives a signal from the polarization controller. N number of polarization mode dispersion compensators (n is 2) constituted by an optical delay device for transmitting the emitted signal light and generating a delay time difference between two orthogonal polarizations of the signal light.
A polarization mode dispersion compensator group connected in series with the polarization mode dispersion compensator, and the n-th polarization mode dispersion compensator is configured to minimize the polarization mode dispersion of the signal light emitted from the polarization mode dispersion compensator. A polarization mode dispersion compensator comprising: a controller for controlling a controller, wherein each of m (where m is an integer of 2 or more and n or less) optical delay devices among the n optical delay devices is provided. A polarization mode dispersion compensator, wherein the delay time difference has a predetermined proportional relationship. 2. A delay time difference τi (i is 1 to m) of m (m is an integer of 2 or more and n or less) optical delayers out of n (n is an integer of 2 or more) optical delayers. 2. The polarization mode dispersion compensator according to claim 1, wherein τi = ai · τ0 where ai = 3 ^{(i−1) and} τ0 is a predetermined delay time difference. 3. A delay time difference τi (where i is 1 to m) of m (m is an integer of 2 or more and n or less) optical delayers among n (n is an integer of 2 or more) optical delayers. 2. The polarization mode dispersion compensator according to claim 1, wherein τi = ai · τ0 where ai = 2 ^{(i−1) and} τ0 is a predetermined delay time difference. 4. A delay time difference τi (i is 1 to m) of m (m is an integer of 2 or more and n or less) optical delayers out of n (n is an integer of 2 or more) optical delayers. 2. The polarization mode dispersion compensator according to claim 1, wherein τi = ai · τ0, where ai = 1 ^{(i−1) and} τ0 is a predetermined delay time difference. 5. The method according to claim 2, wherein the predetermined delay time difference τ0 is in a range from 0 to 0.8T or less (T is a time width of 1 bit). The polarization mode dispersion compensator according to the above description.