JP3874697B2 - Beat noise detection device, communication device - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a beat noise detection device used to suppress the influence of beat noise caused by interference of optical signals from a plurality of slave stations on a signal in a subcarrier multiple access (SCMA) system using a passive optical network (PON). The present invention also relates to a communication device that suppresses the influence of beat noise.
[0002]
[Prior art]
In an SCMA system using PON, a plurality of slave stations transmit different subcarrier signals on different wavelengths of light (transmission light). After these transmission lights are combined at the PON, the master station receives the optical signals from the plurality of slave stations in a lump without separating them, and separates the signals after photoelectric conversion. For this reason, if the frequency band of noise (beat noise) caused by interference between transmission lights of a plurality of slave stations overlaps with the frequency band of the signal, it is difficult to receive the signal satisfactorily.
[0003]
Therefore, a method has been developed in which the wavelengths of transmitted light from a plurality of slave stations are separated so that beat noise does not affect the signal, and this is maintained. For example, an optical interference canceller (Seto et al., “Self-healing road-to-vehicle communication” ROF system (2) -Optical multiplexing by optical interference canceller- "2001 IEICE Communication Society Conference, B-10-59) has been proposed.
[0004]
In this method, the wavelengths of the transmission lights of a plurality of slave stations are separated in advance so that beat noise does not affect the signal, and the wavelength interval may fluctuate due to a temperature change or the like. The side detects the presence or absence of beat noise generated by the transmitted light.If beat noise is detected, it analyzes this and notifies the slave station that the wavelength of the transmitted light is too close. Is to instruct to release.
[0005]
[Problems to be solved by the invention]
In this method, the wavelength of the transmission light is controlled by detecting beat noise contained in an electrical signal obtained by photoelectrically converting the transmission light in the master station.
[0006]
Beat noise is generated by interference between two transmission lights, while interference between the two transmission lights decreases as the polarization approaches orthogonality, and does not interfere if the transmission waves are completely orthogonal. For this reason, the beat noise itself becomes small and the detection becomes difficult as the polarizations of the two transmission lights approach orthogonal, and the beat noise itself does not occur when the two transmission lights are completely orthogonal.
[0007]
However, since the polarization of the transmission light transmitted from a plurality of slave stations changes randomly every moment depending on conditions such as temperature, even if the polarizations of the two transmission lights are orthogonal at a certain time, another time Is not necessarily orthogonal.
[0008]
Therefore, even if the wavelengths of the two transmitted lights are close, beat noise is not generated while the polarizations are orthogonal, so that the approach of the wavelength cannot be detected, and suddenly after the wavelengths are very close. There is a possibility that the polarization noise becomes not orthogonal and beat noise affects the signal.
[0009]
Therefore, an object of the present invention is to provide an apparatus that can detect beat noise more reliably.
[0010]
[Means for Solving the Problems]
In order to solve the above-described problem, a beat noise detection apparatus according to the present invention converts a first optical signal into a first electric signal among optical signals branched into at least a first system and a second system. A photoelectric converter, a polarization-dependent loss element disposed in the path of the second optical signal, and a second optical signal output from the polarization-dependent loss element are converted into a second electrical signal. A second photoelectric converter; and a beat detector that detects beat noise components included in the first and second electric signals.
[0011]
The beat noise detection device according to the present invention includes a first polarization-dependent loss element arranged in a path of a first optical signal among optical signals branched into at least a first, a second, and a third system. The effective direction of the polarization axis is disposed in the path of the second optical signal, and the effective direction of the polarization axis of the first polarization-dependent loss element is an integer (including 0) times 90 degrees. And the first and second optical signals output from the first and second polarization-dependent loss elements having an angle difference excluding the first and second polarization-dependent loss elements, respectively. First and second photoelectric converters that convert electrical signals, a third photoelectric converter that converts the third optical signal into a third electrical signal, and the first, second, and third electrical converters A beat detector for detecting a beat noise component included in the signal.
[0012]
In the beat noise detection apparatus of the present invention, the first and second polarization-dependent loss elements are first and second polarizers, respectively, and the polarization axis of the first polarizer is effective. The direction is an angle difference of 30 degrees or more and 60 degrees or less from the effective direction of the polarization axis of the second polarizer, or the point that effectively corresponds to each of the polarization axes on the Poincare sphere, The angle formed with respect to the center of the Poincare sphere may be 60 degrees or more and 120 degrees or less.
[0013]
The beat noise detection apparatus of the present invention further includes a third polarization-dependent loss element through which the third optical signal passes before being input to the third photoelectric converter. The effective direction of the polarization axis of the wave-dependent loss element may be different from the effective direction of the polarization axis of the first and second polarization-dependent loss elements.
[0014]
In addition, the beat noise detection device of the present invention has a terminal for receiving the input of the fourth electric signal obtained by photoelectrically converting the third optical signal externally, instead of the third photoelectric converter. The beat detector may use a fourth electric signal instead of the third electric signal.
[0015]
The beat noise detection apparatus of the present invention includes an axially variable polarization dependent loss element in which an effective direction of a polarization axis to which an optical signal is input is variable, and an optical signal output from the axially variable polarization dependent loss element A photoelectric converter that converts the signal into an electric signal, a control unit that changes the effective direction of the polarization axis of the variable axis polarization dependent loss element, and a beat detection unit that detects beat noise included in the electric signal; Have
[0016]
Further, in the beat noise detection device of the present invention, the polarization dependent loss element is an axis variable polarization dependent loss element in which an effective direction of a polarization axis is variable, and further the axis variable polarization dependency A control unit that changes the effective direction of the polarization axis of the lossy element may be provided.
[0017]
The beat noise detection apparatus of the present invention includes a variable polarization conversion element that converts the polarization of an input optical signal, and a polarization dependent loss arranged in a path of the optical signal output from the variable polarization conversion element. An element, a photoelectric converter that converts an optical signal output from the polarization dependent loss element into an electrical signal, and a beat detector that detects a beat noise component included in the output signal of the photoelectric converter.
[0018]
The beat noise detection device of the present invention further includes a variable polarization conversion element that converts the polarization of the second optical signal, and the polarization dependent loss element includes the variable polarization conversion element. It is good also as inputting the 2nd optical signal output from.
[0019]
In addition, the beat noise detection device of the present invention has a terminal for receiving an input of a third electric signal obtained by photoelectrically converting the first optical signal externally, instead of the first photoelectric converter. The beat detector may use a third electrical signal instead of the first electrical signal.
[0020]
A communication apparatus according to the present invention is a communication apparatus corresponding to a master station of a subcarrier multiple access system connected to a plurality of slave stations through optical fibers, and at least first and second optical signals transmitted from the slave stations are transmitted. Branching means for branching into the first optical signal, a polarization-dependent loss element disposed in the path of the first optical signal, and the first optical signal output from the polarization-dependent loss element as a first A photoelectric converter for converting into an electric signal; an optical receiving means for converting the second optical signal into a second electric signal to extract a signal component; and the first electric signal and the second electric signal. A beat detection unit for detecting the included beat noise component.
[0021]
Further, in the communication device of the present invention, the polarization dependent loss element is an axis variable polarization dependent loss element in which an effective direction of a polarization axis is variable, and the axis variable polarization dependent loss element It is good also as providing the control part which changes the effective direction of these polarization axes.
[0022]
The communication apparatus of the present invention further includes a variable polarization conversion element that converts the polarization of the first optical signal, and the polarization dependent loss element is output from the variable polarization conversion element. The first optical signal may be input.
[0023]
A communication apparatus according to the present invention is a communication apparatus corresponding to a master station of a subcarrier multiple access system connected to a plurality of slave stations by optical fibers, and at least first, second and third optical signals transmitted from the slave stations. Branching means for branching into an optical signal of the first system, a first polarization-dependent loss element disposed in the path of the first optical signal, a path of the second optical signal, and a polarization axis A second polarization-dependent loss element whose effective direction has an angular difference excluding an integer multiple of 90 degrees from the effective direction of the polarization axis of the first polarization-dependent loss element; A photoelectric converter that converts the first and second optical signals output from the loss element into first and second electrical signals, respectively, and a signal component that converts the third optical signal into a third electrical signal. And an optical receiving means for extracting a beat included in the first, second and third electric signals. And a beat detector which detects a sound component.
[0024]
In general, even if the polarizations of two transmission lights whose wavelengths are close to each other are orthogonal to each other, they can be brought into an interference state by passing through a polarization-dependent loss element having an appropriate axis. Even when three or more transmission light wavelengths approach each other and cause interference, it can be considered that there are a plurality of “two sets of transmission light”, and therefore interference may be caused by two transmission lights.
[0025]
FIG. 3 shows an example in which a polarizer is used as the polarization dependent loss element. Hereinafter, in order to simplify the explanation, it is assumed that a polarizer is used as the polarization dependent loss element. One of the two transmission lights is a linear polarization as represented by the polarization 101 of the first transmission light, and the other transmission light is the polarization 102 of the second transmission light. These are linearly polarized waves as shown, which are orthogonal to each other. When an optical signal including these two transmission lights passes through a polarizer having a polarization axis 100, components corresponding to the projection of the polarization of the two transmission lights pass through the polarizer. Passed components are polarized in the same direction as the polarization axis 100, and these cause interference, so that beat noise can be detected.
[0026]
When one of the polarizations of two transmission lights whose wavelengths are close to each other is orthogonal to the polarization axis, the projection component of the polarization of the transmission light becomes 0, so that the two transmission lights also pass through the polarizer. Does not cause interference. However, in an actual optical communication system, each transmitted light undergoes random polarization conversion when propagating through an optical fiber, so the polarization of each transmitted light when reaching the receiving side is random, It is rare that the polarizations of two approaching transmission lights are orthogonal. In addition, even if it happens to be orthogonal, the polarization of either one of the two transmitted lights rarely coincides completely with the polarization axis. Therefore, if full reliability is not required, the configuration of the first invention of the present application is composed of two systems and at least one polarizer. Can be high enough.
[0027]
As described above, with a polarizer having one fixed axis, it is impossible to cause interference between two transmission lights in an arbitrary polarization state. Considering that interference basically occurs from two transmitted lights, it can be seen that at least three systems and two polarizers are required to reliably detect beat noise.
[0028]
This is because, in only two systems, two transmitted lights, one orthogonal to the polarization axis of the first polarizer of the first system and the other orthogonal to the polarization axis of the second polarizer of the second system. This is because the optical signal that has passed through both the first and second polarizers does not cause interference. For this purpose, a third system is necessary, but this system does not need to include a polarizer unless the polarization axis of the first polarizer and the polarization axis of the second polarizer are orthogonal to each other. This is because, if the two polarizers are not orthogonal, the polarizations of the two transmitted lights orthogonal to the two polarizers are also not orthogonal, and thus interfere even without a polarizer. Of course, a polarizer may be provided.
[0029]
From the above description, it is clear that complete reliability cannot be obtained if the direction of the polarization axis of the first polarizer and the direction of the polarization axis of the second polarizer are the same.
[0030]
The polarization axis and polarization are “same”, “orthogonal”, and “integer multiple of 90 degrees” are not about the absolute direction of the space, but when the polarization of the incident optical signal is fixed, That is, it is effectively “same”, “orthogonal”, and “integer multiple of 90 degrees”. For example, when the polarization axis of the first polarizer is in the x-axis direction and the polarization axis of the second polarizer is in the y-axis direction, it is spatially “orthogonal” but before the second polarizer. For example, when a half-wave plate having an axis at an angle of 45 degrees with respect to the x axis is provided, the first polarizer and the second polarizer are viewed with the polarization of the incident optical signal fixed. In some cases (effectively) have “same” axes. As described above, the spatial directions of the polarization axes of the two polarizers need to be determined in consideration of the polarization conversion received from when the optical signal is branched until it reaches the polarizer.
[0031]
When the wavelengths of the two transmission lights are approaching, a beat noise component is always included in any of the electrical signals obtained by photoelectrically converting each of these three systems of optical signals. In the beat noise detector, the presence / absence of beat noise is detected from each of these electric signals, or from an appropriately selected and synthesized signal. By adopting such a configuration, it is possible to reliably detect beat noise from an optical signal including two transmission lights of any polarization whose wavelengths are close to each other.
[0032]
As described above, in order to reliably detect beat noise, two polarizers and three systems are required. It is desirable that the effective direction of the polarization axis of the polarizer used can detect beat noise that is as large as possible even if the polarization of two light beams that are close in wavelength changes arbitrarily. It is particularly desirable that the intensity of beat noise detected by the three systems varies with the variation in polarization, but it is desirable that the minimum value of the variation range of the total beat noise intensity be as large as possible.
[0033]
When two polarizers are used, the minimum value of the total beat noise intensity is maximized because the effective direction of the first polarizer is 45 degrees from the effective direction of the second polarizer. This is the case. This can be explained as follows.
[0034]
Although a polarizer is not inserted into the third system, this state can be regarded as a virtual polarizer that completely transmits one of the two transmitted lights having wavelengths close to each other. The amount of beat noise is determined by the amount that the other light can pass through the virtual polarizer. Since one transmitted light always transmits through the virtual polarizer, the average amount of the resulting beat noise is larger than the other two systems in which the real polarizer is inserted.
[0035]
Accordingly, when the polarization changes, the total beat noise amount is minimized when the beat noise obtained by the third photoelectric converter is 0, that is, the polarizations of the two transmission lights are orthogonal. This is the case. At this time, when one polarization of the two lights is completely orthogonal to the axis of the first polarizer, the light transmitted through the first polarizer does not cause interference. That is, there is a polarization state in which no beat noise is detected from the electrical signals obtained by the third system and the first system.
[0036]
In order to make the beat noise detected by the second system as large as possible in such a case, it is sufficient that the polarization axis of the polarizer is just in the middle of the polarizations of the two transmission lights. One of the polarizations of the two transmission lights is a polarization orthogonal to the polarization axis of the first polarizer, and the polarization of the other transmission light is orthogonal to this, that is, the polarization of the first polarizer. Polarization in the same direction as the axis. Therefore, the middle is 45 degrees from the effective direction of the polarization axis of the first polarizer. Therefore, it is best that the effective direction of the polarization axis of the first polarizer has an angular difference of 45 degrees from the effective direction of the polarization axis of the second polarizer.
[0037]
Also, the angle at which the beat noise is approximately half the intensity (−3 db) when the minimum value is 45 degrees is 30 degrees and 60 degrees from the effective direction of the polarization axis of the first polarizer. The effective direction of the polarization axis of the first polarizer may be an angle difference of 30 degrees or more and 60 degrees or less from the effective direction of the polarization axis of the second polarizer.
[0038]
By making such an angle difference, the minimum value of the detected beat noise can be increased as much as possible even if the polarization changes.
[0039]
As described above, the third system may not include a polarizer. When the beat noise detector of the present application is installed in a master station of an SCMA system using a PON, the master station is always provided with an optical receiver for data for receiving an optical signal and converting it into an electrical signal. . Since the optical receiver for data must be able to receive any polarization of the optical signal, a polarizer is not usually inserted in front of the optical receiver.
[0040]
The reason why beat noise is detected in such a system is to prevent the beat noise from overlapping the data signal frequency by detecting that the wavelengths of the two transmission lights are close and separating them. Therefore, it is desirable to detect the beat noise while it is in a frequency band higher than the data signal frequency.
[0041]
In general, a data optical receiver used for SCMA has sufficient characteristics at a data signal frequency. When the frequency deviates from the data signal frequency, characteristics such as gain do not usually deteriorate suddenly, and characteristics gradually deteriorate at higher frequencies. Since the variation of the wavelength of the slave station is slow, it is sufficient that beat noise can be detected during a sufficiently long time on the order of seconds. Therefore, it does not require a high S / N ratio like data. Therefore, in the present application, an input terminal from the outside is provided so that the optical receiver for data of the master station can be used instead of the third photoelectric converter.
[0042]
By doing so, it is possible to save the space and parts necessary for the third photoelectric converter and to reduce the number of light branches.
[0043]
When a polarizer is inserted into the third system in addition to the first and second systems, its axis is different from the effective direction of the polarization axis of either the first or second polarizer. desirable. Furthermore, if the effective directions of the polarization axes of the first, second, and third polarizers are separated as much as possible, beats of the remaining systems can be obtained even in a polarization state in which beat noise becomes weak in other systems. Since noise increases, detection can be performed stably.
[0044]
Up to this point, beat noise was detected by converting it to an electrical signal after passing through a polarizer having an effectively different polarization axis for each of the optical signals branched into multiple systems. Even if the effective direction of the polarization axis is changed with time using a polarizer capable of changing the general direction, the same effect can be obtained.
[0045]
Specifically, an optical signal is input to the variable axis polarizer, the output is converted into an electric signal by a photoelectric converter, and beat noise is detected by a beat detector. When the effective direction of the polarization axis of the variable axis polarizer is changed with time, the magnitude of the beat noise changes corresponding to the polarization of the two transmitted lights whose wavelengths are close to each other. Even if beat noise does not occur at all at a certain moment due to the combination of the effective direction of the polarization axis and the polarization of the two transmitted lights, the beat noise is caused by the change in the effective direction of the polarization axis over time. Will occur.
[0046]
As a pattern for changing the effective direction of the polarization axis, a pattern in which the effective direction of the polarization axis is continuously rotated in one direction, or an effective direction of the polarization axis is reciprocated with an amplitude of 180 degrees. A pattern to be modulated and a pattern to be switched to three different directions as far apart as possible corresponding to the case where the detection is performed by branching into the above-described three systems are conceivable.
[0047]
Here, since the effective direction of the polarization axis of the polarizer is temporally changed, it is necessary to perform temporal averaging processing or sampling processing when detecting beat noise after photoelectric conversion.
[0048]
As the configuration of the variable axis polarizer, in addition to the configuration in which the polarizer (normal polarizer) whose polarization axis direction is fixed is rotated by a motor or the like, the polarization of the optical signal input to the polarizer is variable. A configuration in which the effective direction of the polarization axis of the polarizer is rotated by changing with a polarization converter may be used. An axial variable polarizer having a configuration using a variable polarization converter that can electrically convert polarization does not include a mechanical configuration, so that the durability of the entire beat detection device can be maintained.
[0049]
By doing in this way, it is possible to reliably detect beat noise caused by two transmission lights whose wavelengths are close to each other regardless of the polarization state of the optical signal. Further, only one system for inserting a polarizer is required, and the number of photoelectric converters can be reduced, so that there is an advantage that the configuration can be made compact.
[0050]
DETAILED DESCRIPTION OF THE INVENTION
(First embodiment)
Hereinafter, a first embodiment of the present invention will be described with reference to the drawings. FIG. 1 is an outline of a communication system using a beat noise detecting apparatus according to a first embodiment of the present invention and a communication apparatus incorporating the beat noise detecting apparatus.
[0051]
This communication system is an SCMA system using PON, in which a plurality of slave stations 9 and a master station 8 are connected by an optical fiber 2-2. A plurality of transmission lights having different wavelengths transmitted from each slave station 9 are transmitted to the master station 8 through the branching coupler 11 as mixed optical signals. The optical signal is distributed to the optical receiver 10 and the beat noise detector 1 by the distributor 3-2. The optical signal distributed to the optical receiver 10 is photoelectrically converted into an electric signal, and a signal from each slave station is extracted. Further, a part of the electric signal is branched and output in the optical receiver 10 and sent to the beat noise detection device via the high-pass filter 14. On the other hand, the optical signal distributed to the beat noise detection device 1 detects beat noise included in the optical signal.
[0052]
The beat noise detection apparatus (hereinafter, this apparatus) according to the present embodiment detects beat noise from a signal transmitted from the slave station 9 by being incorporated in the master station 8 in the communication system. Information for performing wavelength control of transmission light is provided.
[0053]
This apparatus includes an optical fiber 2-1 that inputs an optical signal in which transmission lights from a plurality of slave stations 9 are mixed, and a first polarization-dependent loss that gives a polarization-dependent loss element loss to the output light. The element 4-1, the first photoelectric converter 5-1 that photoelectrically converts the output light from the first polarization-dependent loss element 4-1 and outputs an electric signal, and the electricity obtained by photoelectric conversion. It has a beat detector 6 that detects beat noise from the signal and an output signal from the high-pass filter 14 that the master station 8 has, and a result output terminal 15 that outputs the detection result of the beat noise.
[0054]
The polarization-dependent loss element is an optical element that has first and second orthogonal polarization axes and has different losses with respect to the polarization corresponding to each of the two polarization axes. Polarization conversion that changes the component ratio between the polarization corresponding to the polarization axis and the polarization corresponding to the second polarization axis is generated. A polarizer is a kind of polarization-dependent loss element and has an infinite loss with respect to the polarization corresponding to one polarization axis. In the present embodiment, the first polarization-dependent loss element 4-1 is assumed to be a polarizer in order to simplify the description, but a general polarization-dependent loss element may be used.
[0055]
An optical signal from the slave station is input to the beat noise detection device 1 from the optical fiber 2-1. The optical signal is input to the first photoelectric converter 5-1 through the first polarization-dependent loss element 4-1, and is converted into an electric signal.
[0056]
Together with the high-pass filter 14 output signal, these two electrical signals are input to the beat detector 6 to detect the presence and content of beat noise using the electrical signal obtained by combining the two electrical signals. And output the result via the result output terminal 15.
[0057]
The beat detector 6 may use a method of selecting and using the one having the larger beat noise component of the two electrical signals as a method other than power combining. In this case, it is preferable to switch when the other beat noise component becomes larger due to the fluctuation of the beat noise component while one is used.
[0058]
An electrical signal obtained by photoelectrically converting an optical signal is typical (when the wavelengths of the two transmission lights are close and the polarizations of the two transmission lights are not orthogonal) as shown in FIG. Since the data signal appears on the low frequency side and beat noise appears on the high frequency side, it is possible to detect high frequency components corresponding to beat noise by using a high-pass filter. is there.
[0059]
Hereinafter, the operation when the first and second transmission lights from the two slave stations are input from the optical fiber 2-1 will be described for each polarization direction. For the sake of simplicity, it is assumed that the two transmission lights included in the optical signal are both linearly polarized waves.
[0060]
When the polarization direction of the first transmission light and the polarization direction of the second transmission light are not orthogonal, the first transmission light and the second transmission light cause interference. Therefore, if the angle between the two polarizations is sufficiently far from 90 degrees, the beat of a magnitude that can be detected is detected in the electrical signal obtained by photoelectrically converting the optical signal input to the optical receiver 10 described above. Since noise is included, beat noise can be detected.
[0061]
On the other hand, when the polarization direction of the first transmission light and the polarization direction of the second transmission light are orthogonal or the angle between the two polarizations is close to 90 degrees, the first transmission light and the second transmission light There is no interference with the transmitted light, or the degree of interference is weak. Therefore, the electric signal obtained by photoelectrically converting the optical signal input to the optical receiver 10 described above does not contain beat noise, or the included beat noise is weak, so that detection is difficult. .
[0062]
The first and second transmission lights branched into the first system are subjected to polarization conversion by the first polarization-dependent loss element 4-1. In the present embodiment, since the first polarization dependent loss element 4-1 is a polarizer, both the first transmission light 101 and the second transmission light 102 are in the direction of the polarization axis 100 as shown in FIG. Is converted into light having only the polarization component. As a result, in the optical signal output from the first polarization-dependent loss element 4-1, the first transmission light and the second transmission light have the same polarization and cause strong interference, which is photoelectrically converted. Since beat noise included in the electric signal obtained by photoelectric conversion in the device 5-1 becomes strong, detection becomes easy.
[0063]
The above is the case of linearly polarized waves, but beat noise can be detected from the optical signal input to the optical receiver when the general polarized waves are similarly orthogonal.
[0064]
In the present embodiment, a polarizer is used for the first polarization-dependent loss element 4-1, but the same applies to a general polarization-dependent loss element. This will be described with reference to FIG.
[0065]
FIG. 4 is a sphere called a Poincare sphere that represents the state of polarization. Various polarizations are mapped on the spherical surface of the Poincare sphere, and the polarization state of light can be expressed as one point on the spherical surface of the Poincare sphere. For example, every point on the circumference of the equator 401 of the Poincare sphere is linearly polarized, and points of the north pole 402 and the south pole 403 are circularly polarized waves. In addition, as indicated by points A and B, two polarizations represented by two points that are point-symmetric with respect to the origin O in the Poincare sphere are orthogonal to each other.
[0066]
The polarization axis of the polarizer is also expressed as one point on the Poincare sphere. The light that has passed through the polarizer becomes a polarized wave represented by a point representing the polarization axis. For example, when light having a polarization represented by a point A passes through a polarizer having a polarization axis represented by a point P2, the polarization of the output light becomes a point P2. That is, the point A is converted to the point P2 on the Poincare sphere. Similarly for point B, the converted polarization is represented by point P2.
[0067]
A general polarization-dependent loss element has two orthogonal polarization axes, which are represented by two points that are point-symmetric with respect to the origin O on the Poincare sphere, such as point P1 and point P2. In a general polarization-dependent loss element, conversion is performed so as to approach a point representing the polarization axis with the smaller loss on the Poincare sphere. For example, when the point P1 is a polarization axis with a large loss and the point P2 is a polarization axis with a small loss, the light having the polarization represented by the point A is converted so as to approach the point P2 and the polarization represented by the point C. Have a wave. Similarly, the point B is converted so as to approach the point P2 and has a polarization represented by the point D.
[0068]
As a result, the points A and B were point-symmetric with respect to the origin O and the two polarized waves were orthogonal to each other before the conversion, but the points C and D were not point-symmetric with respect to the origin O. The waves will not be orthogonal. That is, when the polarization directions of the first and second transmission lights are orthogonal, even if the polarization-dependent loss element 4-1 is a general polarization-dependent loss element, the optical fiber 2-1. The polarized waves become non-orthogonal by passing through the polarization dependent loss element 4-1, and beat noise can be easily detected.
[0069]
As described above, according to the present embodiment, beat noise can be detected even when the polarizations of two transmission lights are orthogonal, and beat noise can be detected more reliably than in the past.
[0070]
(Second Embodiment)
Hereinafter, a second embodiment of the present invention will be described with reference to the drawings. FIG. 5 shows a beat noise detection apparatus according to the second embodiment of the present invention. Since the outline of the configuration is similar to that of the beat noise detection apparatus according to the first embodiment, different parts will be mainly described.
[0071]
This apparatus includes an optical fiber 2-1 that inputs an optical signal in which light of a plurality of wavelengths is mixed, a distributor 3-1 that distributes the optical signal to three systems, and a first output from the distributor 3-1. A first polarization-dependent loss element 4-1 that gives polarization-dependent loss to light, and an output signal that photoelectrically converts output light from the first polarization-dependent loss element 4-1 and outputs an electrical signal. One photoelectric converter 5-1, a second polarization-dependent loss element 4-2 that gives a polarization-dependent loss to the second output light from the divider 3-1, and a second polarization dependency The first photoelectric converter 5-2 that photoelectrically converts the output light from the lossy element 4-2 and outputs an electrical signal, and the third output light from the distributor 3-1 photoelectrically converts the electrical signal. A third photoelectric converter 5-2 for outputting a beat, a beat detector 6 for detecting beat noise from three electric signals obtained by photoelectric conversion, and detection of beat noise And a result output terminal 15 for outputting the results.
[0072]
In the present embodiment, among the optical signals branched into the first to third systems, the first and second systems of optical signals undergo photoelectric conversion after passing through the polarization-dependent loss element, so that there are two polarization-dependent losses. By providing an angle between the polarization axes of the elements (an angle other than 0 degrees and 90 degrees), it is difficult to detect in the first embodiment, and the directions of polarization of the two transmitted lights are orthogonal to each other and transmitted. Even when the polarization direction of light is orthogonal to the polarization axis, beat noise can be detected. That is, beat noise can be detected in any polarization state.
[0073]
However, the minimum value of the beat noise that can be detected (assuming that each light intensity is constant) differs depending on the relative relationship between the directions of the polarization axes of the first and second polarization dependent loss elements. In an extreme example, if the difference between the axes of the first and second polarization-dependent loss elements is only once on the Poincare sphere, the minimum value is very small, and it is difficult to reliably detect beat noise. It is. Therefore, it is desirable to set the difference between the axes of the two polarization dependent loss elements so that the minimum value of the beat noise is as large as possible.
[0074]
When a polarizer is used as the first and second polarization dependent loss elements, it is desirable to set the angle of the two polarization axes to be 45 degrees. 45 degrees here is a spatial angle when the polarizations of the optical signals input to the two polarizers are the same. When this “45 degrees” is represented on the Poincare sphere, it becomes 90 degrees.
[0075]
When different polarization conversions occur before the optical signal branched into the first and second systems reaches the polarization dependent loss element, the minimum value of the beat noise is the maximum of the polarizer. The difference in polarization axis is not necessarily 45 degrees spatially. In such a case, the spatial angle is set to be “effectively 45 degrees” including the effect of polarization conversion.
[0076]
FIG. 6 shows detection by the first photoelectric converter and the second photoelectric converter twice the angle difference between the polarization axes of the two polarizers (this corresponds to the phase difference on the Poincare sphere). It is the graph which showed the relationship of the minimum value of the total power of beat noise. The vertical axis shows the relative value of the fluctuation of the minimum value of the beat noise total power when the polarization of one of the two lights that generate beat noise is arbitrarily changed and the other is always orthogonal to it. The axis is twice the angular difference between the polarization axes of the two polarizers.
[0077]
Looking at FIG. 6, the maximum is 90 degrees (45 degrees in terms of the polarization axis angle of the polarizer). Therefore, it can be seen that 90 degrees is the best on the Poincare sphere. On the Poincare sphere, half of the intensity at 90 degrees, that is, −3 db is 60 degrees (30 degrees in the polarization axis angle difference of the polarizer) and 120 degrees (60 degrees in the polarization axis angle difference of the polarizer). Therefore, even if the angle is not exactly 45 degrees, the two polarizers are arranged so that they have a phase difference of 60 to 120 degrees on the Poincare sphere (the polarization axis angle difference of the polarizer is 30 to 60 degrees). I think that it should be set.
[0078]
In the present embodiment, only the optical signal branched into the first and second systems passes through the polarizer, but the optical signal branched into the third system may also pass through the polarizer. In this case, the minimum value of the beat noise is maximized if the polarization axis angles of the three polarizers are spaced from each other by 60 degrees (120 degrees on the Poincare sphere).
[0079]
As described above, in this embodiment, an optical signal is branched into three or more systems, and a polarizer having a different effective direction of the polarization axis is passed through two or more systems among them. Therefore, detection is difficult in the first embodiment. Beat noise can be detected even when the polarization directions of transmission light from two slave stations are orthogonal and the polarization direction of transmission light is orthogonal to the polarization axis. That is, beat noise can be detected in any polarization state.
[0080]
(Third embodiment)
Hereinafter, a third embodiment of the present invention will be described with reference to the drawings. FIG. 7 shows a beat noise detecting apparatus according to the second embodiment of the present invention and a master station incorporating the beat noise detecting apparatus. Since the outline of the configuration is similar to that of the beat noise detection apparatus of the first and second embodiments, the description will focus on the different parts.
[0081]
The beat noise detection device of this embodiment has a structure using an optical receiver 10 as a substitute for the third photoelectric converter in the second embodiment.
[0082]
As shown in FIG. 2, beat noise is a high-frequency component compared to the signal, so that the beat noise component is extracted by passing the high-pass filter 14 for the electric signal obtained by photoelectric conversion by the optical receiver 10. can do. The extracted beat noise component is input to the beat detector 6, and the beat detector 6 combines power with the electrical signals from the first photoelectric converter 5-1 and the second photoelectric converter 5-2 to generate beat noise. Is detected.
[0083]
As described above, according to the present embodiment, three photoelectric converters are used in the beat detection device in the second embodiment. However, since this can be reduced to two, the device can be made compact, and Equipment manufacturing costs can also be reduced.
[0084]
(Fourth embodiment)
Hereinafter, a fourth embodiment of the present invention will be described with reference to the drawings. FIG. 8 shows a beat noise detecting apparatus according to a fourth embodiment of the present invention and a master station incorporating the beat noise detecting apparatus. Since the outline of the configuration is similar to that of the beat noise detection apparatus according to the first to third embodiments, different portions will be mainly described.
[0085]
The beat noise detection apparatus according to the present embodiment is different from the first embodiment in that an axially variable polarization dependent loss element 17 in which the effective direction of the polarization axis is variable is used instead of the polarization dependent loss element. . The variable axis polarization-dependent loss element 17 changes the direction of the polarization axis according to a signal from the control unit 19.
[0086]
The axis variable polarization dependent loss element 17 is such that, for example, a normal (fixed axis) polarization dependent loss element 17-1 as shown in FIG. 9A is mechanically rotated by a motor 17-2. Can be considered.
[0087]
The control unit 19 sends a control signal to the variable axis polarization dependent loss element 17 to change the effective direction of the polarization axis by 45 degrees at regular time intervals. The control signal transmitted by the control unit 19 is not limited to this pattern. For example, the polarization axis is continuously rotated, the control signal is changed by 60 degrees at regular time intervals, or a constant section (for example, 0 degree to 0 degree). 180 degrees) may be reciprocated. In short, it is only necessary to change the effective direction of the polarization axis so as to include two points that are not point-symmetric about the origin on the Poincare sphere.
[0088]
In the present embodiment, the amount of beat noise also changes with time with time change in the effective direction of the polarization axis of the variable axis polarization dependent loss element 17. For this reason, the beat detection unit 6 obtains beat noise by time averaging in order to stably detect beat noise.
[0089]
The beat detection unit 6 notifies the control unit 19 of the beat noise intensity at regular intervals, and the control unit 19 sets the polarization axis of the variable axis polarization dependent loss element 17 when the beat noise is equal to or less than a predetermined intensity. When it rotates at a constant speed and exceeds a predetermined intensity, the polarization axis of the variable axis polarization dependent loss element 17 is rotated to search for the angle at which the beat noise is detected most greatly and perform control to follow it. And even better.
[0090]
Although it is not essential to input an electric signal from the optical receiver 10 of the master station 8 to the beat detector 6 via the high-pass filter 14, in order to detect beat noise with certainty, the electric signal of the optical receiver 10 is used. When no signal is used, at least three effective directions of the polarization axis of the variable axis polarization dependent loss element 17 are required. However, by using the electric signal of the optical receiver 10, two directions are sufficient. There is.
[0091]
Further, in this embodiment, the variable axis polarization dependent loss element 17 is used. However, as shown in FIG. 10, the input light that has passed through the variable polarization converter 17-3 whose axis direction can be controlled by the control unit 19 is used. The configuration may be such that the first polarization dependent loss element 4-1 is input.
[0092]
As shown in FIG. 9B, the polarization axis can be effectively rotated by combining the normal polarization dependent loss element 17-1 and the variable polarization converter 17-3. When the variable polarization converter 17-3 and the polarization dependent loss element 17-1 are combined, the optical signal is polarized by the variable polarization converter 17-3 and then the polarization dependent loss element 17-1. To enter. By passing the variable polarization converter 17-3, an effect similar to that obtained by spatially rotating the polarization dependent loss element 17-1 is obtained.
[0093]
As the variable polarization converter 17-3, for example, there is one that rotates an anisotropic crystal with a motor, but other than this, the magnitude and direction of the magnetic field applied to the Faraday element is controlled to control the rotation amount of the polarization. A mechanical movable mechanism can be used if it can be changed, or if it uses a Bockels effect crystal to control the effective axis direction and phase difference between axes by applying an electric field from various directions. Therefore, a decrease in the reliability (durability) of the apparatus can be avoided. In the present embodiment, a combination of a variable polarization converter using a Faraday element and a polarizer is used as the variable axis polarization dependent loss element 17.
[0094]
As described above, according to this embodiment, since only one photoelectric converter dedicated to beat noise detection is required, the apparatus can be further compacted and the manufacturing cost of the apparatus can be reduced.
[0095]
【The invention's effect】
As described above, according to the present invention, an optical signal is converted into a polarization state including two points that are not point-symmetric about the origin on the Poincare sphere using a polarization dependent loss element, and then beat noise is detected. It becomes possible to reliably detect beat noise when the wavelengths of the two transmission lights included in the signal approach each other.
[Brief description of the drawings]
FIG. 1 is a schematic configuration diagram of a beat noise detection apparatus according to a first embodiment.
FIG. 2 is a spectrum example of a beat noise component and a signal component.
FIG. 3 is a diagram in which orthogonal linearly polarized waves are converted to the same polarization by a polarizer.
FIG. 4 is an explanatory diagram of a polarization state by a Poincare sphere.
FIG. 5 is a schematic configuration diagram of a beat noise detection device according to a second embodiment.
FIG. 6 shows the relationship between the angle difference of a polarizer and the minimum value of beat noise.
FIG. 7 is a schematic configuration diagram of a beat noise detection device according to a third embodiment.
FIG. 8 is a schematic configuration diagram of a beat noise detection device according to a fourth embodiment.
FIG. 9 is a schematic configuration diagram of an axially variable polarization dependent loss element according to a fourth embodiment.
FIG. 10 is a schematic configuration diagram illustrating a modification of the beat noise detection device according to the fourth embodiment.
[Explanation of symbols]
1 Beat detector
2-1, 2-2 Optical fiber
3-1, 3-2, 16 distributor
4-1, 4-2, 17-1 Polarization dependent loss element
5-1, 5-2, 5-3 Photoelectric converter
6 Beat detector
8 Master station
9 slave stations
10 Optical receiver
11 Branch coupler
12 Signal output terminal
14 High-pass filter
15 Result output terminal
17-axis variable polarization dependent loss element
17-2 Motor
17-3 Variable polarization converter
19 Control unit

Claims (14)

A first photoelectric converter that converts a first optical signal into a first electrical signal out of at least the first and second optical signals;
A polarization dependent loss element disposed in a path of the second optical signal;
A second photoelectric converter that converts the second optical signal output from the polarization dependent loss element into a second electrical signal;
A beat noise detection apparatus comprising: a beat detector that detects a beat noise component included in at least one of the first and second electric signals. Of the optical signals branched into at least the first, second, and third systems, a first polarization-dependent loss element disposed in the path of the first optical signal;
Arranged in the path of the second optical signal, the effective direction of the polarization axis has an angular difference excluding an integral multiple of 90 degrees from the effective direction of the polarization axis of the first polarization dependent loss element. A second polarization dependent loss element;
First and second photoelectric converters for converting the first and second optical signals output from the first and second polarization dependent loss elements into first and second electrical signals, respectively;
A third photoelectric converter for converting the third optical signal into a third electrical signal;
A beat noise detection apparatus comprising: a beat detector that detects a beat noise component included in at least one of the first, second, and third electric signals. The first and second polarization-dependent loss elements are first and second polarizers, respectively, and the effective direction of the polarization axis of the first polarizer is the polarization axis of the second polarizer. Or an angle difference of 30 degrees to 60 degrees with the effective direction of
Or
The beat noise detection according to claim 2, wherein an angle formed by a point that effectively corresponds to each of the polarization axes on the Poincare sphere with respect to the center of the Poincare sphere is 60 degrees or more and 120 degrees or less. apparatus. A third polarization-dependent loss element that passes before the third optical signal is input to the third photoelectric converter, and an effective polarization axis of the third polarization-dependent loss element; 3. The beat noise detection device according to claim 2, wherein the first direction is different from the effective direction of any polarization axis of the first and second polarization dependent loss elements. In claim 2 or claim 3,
Instead of the third photoelectric converter,
A terminal for receiving an input of a fourth electric signal obtained by photoelectrically converting the third optical signal outside;
The beat detector
A beat noise detecting apparatus using a fourth electric signal instead of the third electric signal. An axially variable polarization-dependent loss element in which the effective direction of the polarization axis to which an optical signal is input is variable;
A photoelectric converter that converts the optical signal output from the variable axis polarization dependent loss element into an electrical signal;
A beat noise detection apparatus comprising: a control unit that changes an effective direction of a polarization axis of the variable axis polarization dependent loss element; and a beat detection unit that detects beat noise included in the electrical signal. The polarization dependent loss element is an axis variable polarization dependent loss element whose effective direction of the polarization axis is variable,
The beat noise detection apparatus according to claim 1, further comprising a control unit that changes an effective direction of a polarization axis of the variable axis polarization dependent loss element. A variable polarization conversion element that converts the polarization of the input optical signal;
A polarization-dependent loss element disposed in a path of an optical signal output from the variable polarization conversion element;
A photoelectric converter that converts an optical signal output from the polarization-dependent loss element into an electrical signal;
A beat noise detecting device having a beat detector for detecting a beat noise component included in an output signal of the photoelectric converter. In claim 1,
Furthermore, it has a variable polarization conversion element that converts the polarization of the second optical signal,
The beat noise detecting device, wherein the polarization dependent loss element receives the second optical signal output from the variable polarization conversion element. In claim 7 or claim 9,
Instead of the first photoelectric converter,
A terminal for receiving an input of a third electric signal obtained by photoelectrically converting the first optical signal outside;
The beat detector
A beat noise detecting apparatus using a third electric signal instead of the first electric signal. A communication device corresponding to a master station of a subcarrier multiple access system connected to a plurality of slave stations by optical fibers,
Branching means for branching the optical signal transmitted from the slave station into at least first and second optical signals;
A polarization dependent loss element disposed in the path of the first optical signal;
A photoelectric converter that converts the first optical signal output from the polarization-dependent loss element into a first electrical signal;
Optical receiving means for converting the second optical signal into a second electrical signal and extracting a signal component;
A communication apparatus comprising: a beat detection unit that detects a beat noise component included in at least one of the first electric signal and the second electric signal. The polarization dependent loss element is an axis variable polarization dependent loss element whose effective direction of the polarization axis is variable,
The communication device according to claim 11, further comprising a control unit that changes an effective direction of a polarization axis of the variable axis polarization dependent loss element. Having a variable polarization conversion element for converting the polarization of the first optical signal;
12. The communication apparatus according to claim 11, wherein the first optical signal output from the variable polarization conversion element is input to the polarization dependent loss element. A communication device corresponding to a master station of a subcarrier multiple access system connected to a plurality of slave stations by optical fibers,
Branching means for branching the optical signal transmitted from the slave station into at least first, second and third optical signals;
A first polarization-dependent loss element disposed in the path of the first optical signal;
The effective direction of the polarization axis is disposed in the path of the second optical signal, and the effective direction of the polarization axis of the first polarization dependent loss element has an angular difference excluding an integral multiple of 90 degrees. Two polarization dependent loss elements,
A photoelectric converter that converts the first and second optical signals output from the polarization dependent loss element into first and second electric signals, respectively;
Optical receiving means for converting the third optical signal into a third electrical signal and extracting a signal component;
A communication apparatus comprising: a beat detection unit that detects a beat noise component included in at least one of the first, second, and third electric signals.

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