JP2012191424A - Polarization multiplexed phase modulated light evaluation method and device thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a small and low cost evaluation device without accuracy degradation.SOLUTION: A polarization multiplexed phase modulated light evaluation device includes a step (S1) in which a polarization multiplexed phase modulated light signal is converted to intensity modulated light the intensity of which varies according to a phase change between symbols, a step (S2) in which the obtained intensity modulated light is separated into first polarization axis component light aligned to a first polarization axis as a separation reference and second polarization axis component light aligned to a second polarization axis orthogonal to the first polarization axis, a step (S3) in which polarization directions of polarization components of the light signal are aligned to the first polarization axis and the second polarization axis respectively by controlling a direction of incidence polarization of the light signal to maximize a component of the differential intensity between the separated first polarization axis component and the second polarization axis component, a step (S4) in which the light signal is separated into each of the polarization components and is converted to intensity modulated light the intensity of which is varied according to a phase change between symbols, a step (S5) in which the obtained intensity modulated light for each polarization component is sampled, and a step (S6) a quality evaluation of the light signal is performed based on the sampled data.

Description

  The present invention relates to a technique for reducing the size, cost, and accuracy of an apparatus for evaluating polarization multiplexed phase-modulated light obtained by phase-modulating two orthogonal polarization components.

  Recently, along with the increase in capacity of optical communication, long distance transmission of 100 Gbps per channel is required.

  In such long-distance transmission, attention has been paid to transmission systems with high tolerance to dispersion and wavelength filtering of optical fibers as transmission media, and frequency-efficient multilevel modulation schemes in which multiple bits of data are placed on one symbol. As one of the technologies, research on digital coherent technology has been actively conducted.

For example, DP-QPSK (Dual-Polarization) which is polarization multiplexed QPSK that combines orthogonal two-polarization multiplexing and quaternary modulation QPSK (Quadrature Phase Shift Keying)
The QPSK method and the CP-QPSK (Coherent Polarization multiplexed QPSK) method can perform 100 Gbps transmission with 25 [G / symbol] signals, and digital coherent technology is also adopted in these methods.

  To put these DP-QPSK and CP-QPSK systems into practical use, it is necessary to evaluate polarized multiplexed phase-modulated light obtained by the modulation system.

  Since the polarization multiplexed phase-modulated light according to the above method is signal light that is phase-modulated and multiplexed into two orthogonal polarization components, conventionally, in order to perform the evaluation, the input signal light is converted into two orthogonal polarization components. Separation (polarization diversity), the separated polarization components are mixed with local light by a 90-degree hybrid (optical mixer), the signal obtained by the mixing is digitally processed, and signal demodulation is performed. We are evaluating.

  More specifically, as shown in FIG. 14, the signal light Px enters the polarization beam splitter (PBS) 11 and is separated into two orthogonal polarization components Pp and Ps, and each is a 90-degree hybrid (optical mixer) 13. , 14.

  Further, the local light Ploc is branched into two by the beam splitter 12 and is incident on the 90-degree hybrids 13 and 14 respectively, and the output light is incident on the balanced receivers 15 to 18 so that the symbol data (Ip, Qp) of the polarization component Pp is obtained. ) And the symbol data (Is, Qs) of the polarization component Ps, convert them into digital values by the high-speed A / D converters 21 to 24, and provide them to the signal processing unit 25 made of DSP for signal demodulation. The demodulated signal is evaluated.

  An example of the configuration of the polarization multiplexed phase modulated light receiving system is disclosed in, for example, the following Patent Document 1.

JP 2010-098617 A

  However, the device configuration for the above evaluation is substantially the same as the device configuration used in the DP-QPSK and CP-QPSK transmission devices, and in addition to local light emission and 90-degree hybrid, ultra high-speed data is used. Sampling (A / D converter) and digital signal processing (DSP) are required, and there is a problem that the apparatus becomes very large and cost increases.

  An object of the present invention is to provide an evaluation method and apparatus capable of solving the above-described problems and performing evaluation of polarized multiplexed phase-modulated light with a small and inexpensive configuration without reducing accuracy.

In order to achieve the above object, the polarization multiplexed phase-modulated light evaluation method according to claim 1 of the present invention comprises:
A step (S1) of converting signal light in which polarization components orthogonal to each other are phase-modulated and multiplexed in a predetermined format and a predetermined symbol rate into intensity-modulated light whose intensity changes in accordance with a phase change between symbols (S1);
From the intensity-modulated light, first polarization axis component light along a first polarization axis that serves as a reference for separation and second polarization axis component light along a second polarization axis perpendicular to the first polarization axis are separated. Stage (S2);
The incident polarization direction of the signal light is controlled so that the amplitude of the difference component of the intensity of the first polarization axis component light and the second polarization axis component light is maximized, and the polarization direction of each polarization component of the signal light is determined. Making the first and second polarization axes coincide with each other (S3);
A separation process of each polarization component of the signal light in a state where the respective polarization directions coincide with the first polarization axis and the second polarization axis, and a conversion process to intensity-modulated light whose intensity changes in accordance with a phase change between symbols. Performing step (S4);
Sampling intensity-modulated light for each polarization component obtained by the separation process and the conversion process to obtain sampling data (S5);
And (S6) performing a quality evaluation of the signal light based on the acquired sampling data.

Moreover, the polarization multiplexed phase-modulated light evaluation apparatus according to claim 2 of the present invention includes:
A light incident part (31) for making the signal components multiplexed by polarization modulation orthogonal to each other in a predetermined format and a predetermined symbol rate;
A polarization controller (32) for changing the polarization direction of the signal light incident on the light incident portion;
The signal light that has passed through the polarization controller is received, and the signal light and delayed light obtained by delaying the signal light by an amount corresponding to one symbol are combined and interfered, and the intensity changes according to the phase change between the signal light symbols. A first modulation conversion unit (35) for emitting intensity-modulated light to be
The first polarization axis component light along the first polarization axis that is a reference for separation from the intensity modulation light that is received from the first modulation conversion unit, and orthogonal to the first polarization axis A first polarization beam splitter (36) for separating the second polarization axis component light along the second polarization axis;
A differential component amplitude detector (37) for detecting an amplitude of a differential component of the intensity of the first polarization axis component light and the second polarization axis component light;
The polarization controller is controlled so that the amplitude detected by the differential component amplitude detector is maximized, and the polarization direction of each polarization component of the signal light emitted from the polarization controller is set to the first polarization axis and the first polarization axis. A control unit (40) for matching the two polarization axes,
The first polarization axis and the second polarization axis with respect to the signal light emitted from the polarization controller in a state where the polarization direction of each polarization component coincides with the first polarization axis and the second polarization axis under the control of the control unit. A separation / conversion processing unit (50) for performing a component separation process along the lines and a modulation conversion process to intensity-modulated light whose intensity changes in accordance with a phase change between symbols;
A sampling unit (60) for sampling the intensity-modulated light for each polarization component obtained by the separation / conversion processing unit and obtaining the sampling data;
An evaluation unit (70) for performing quality evaluation of the signal light based on the sampling data acquired by the sampling unit.

A polarization multiplexed phase-modulated light evaluation apparatus according to claim 3 of the present invention is the polarization multiplexed phase-modulated light evaluation apparatus according to claim 2,
The separation processing and modulation conversion processing by the separation / conversion processing unit are performed by the first modulation conversion unit and the first polarization beam splitter,
It is characterized in that the intensity-modulated light for each polarization component separated by the first polarization beam splitter is emitted to the sampling unit via branching means (58, 59).

According to a fourth aspect of the present invention, there is provided the polarization multiplexed phase-modulated light evaluation apparatus according to the third aspect.
An optical switch (52 ') that selectively makes one of the intensity-modulated light components of each polarization component separated by the first polarization beam splitter and branched by the branching unit enter the sampling unit; It is characterized by being.

The polarization multiplexed phase-modulated light evaluation apparatus according to claim 5 of the present invention is the polarization multiplexed phase-modulated light evaluation apparatus according to claim 2,
A branching unit (34) for splitting the light emitted from the polarization controller into two, causing one of the light to enter the first modulation conversion unit, and emitting the other to the separation / conversion processing unit;
The separation / conversion processing unit
A second polarization beam having an optical axis in the same direction as the first polarization axis and the second polarization axis, receiving the signal light incident through the branching means, and separating both polarization components of the signal light A splitter (51);
The polarization component separated by the second polarization beam splitter and delayed light obtained by delaying the polarization component by an amount corresponding to one symbol are combined and interfered, and the intensity changes according to the phase change between the symbols of the polarization component. And a second modulation conversion unit (53) that emits intensity-modulated light.

A polarization multiplexed phase-modulated light evaluation apparatus according to claim 6 of the present invention is the polarization multiplexed phase-modulated light evaluation apparatus according to claim 5,
An optical switch (52) for selectively making any one of the polarization components separated by the second polarization beam splitter incident on the second modulation conversion unit is provided.

The polarization multiplexed phase-modulated light evaluation apparatus according to claim 7 of the present invention is the polarization multiplexed phase-modulated light evaluation apparatus according to claim 5,
The second modulation conversion unit of the separation / conversion processing unit is:
A branching means (54) for splitting the input light into two;
A first delay interference section (55) for combining and interfering one branched light branched by the branching means and a delayed light obtained by giving a delay equivalent to one symbol and a delay of a predetermined phase to the branched light;
A second light beam that causes the other branched light beam branched by the branching unit and a delayed beam beam having a delay equivalent to one symbol to the branched beam beam and a delay having a difference of 90 degrees with respect to the predetermined phase; A delay interference unit (56),
The evaluation unit calculates the phase of each polarization component of the signal light from sampling data for the outgoing light of the first delay interference unit and the outgoing light of the second delay interference unit, and evaluates based on the phase value It is characterized by performing.

  As described above, in the present invention, signal light multiplexed by polarization modulation orthogonal to each other in a predetermined format and a predetermined symbol rate is converted into intensity-modulated light whose intensity changes in accordance with a phase change between symbols. Then, from the intensity-modulated light obtained thereby, the first polarization axis component light along the first polarization axis serving as a reference for separation and the second polarization axis component along the second polarization axis perpendicular to the first polarization axis Each polarization of the signal light is controlled by controlling the incident polarization direction of the signal light so that the difference component of the intensity of the separated first polarization axis component light and the second polarization axis component light is maximized. The polarization directions of the components are made to coincide with the first polarization axis and the second polarization axis, respectively. Then, in this state, separation processing of each polarization component of the signal light and conversion processing to intensity-modulated light whose intensity changes according to the inter-symbol phase change, and intensity-modulated light for each polarization component obtained thereby And the quality of the signal light is evaluated based on the sampling data.

  For this reason, it is possible to evaluate polarized multiplexed phase-modulated light with a small and inexpensive configuration without reducing accuracy.

The flowchart which shows the procedure of the evaluation method of the polarization multiple phase modulation light of this invention The figure which shows the state where the polarization direction has shifted The figure which shows the state where the polarization direction matched Diagram showing the relationship between polarization direction deviation and waveform Overall configuration diagram of an embodiment of the present invention The figure which shows the structural example of a difference component amplitude detection part. The figure which shows the structural example of a sampling part The figure which shows the structural example of a sampling part Diagram showing the relationship between phase delay and composite light amplitude Diagram showing an example of phase distribution The figure which shows another structural example of a 2nd modulation | alteration conversion part. Diagram showing an example of phase distribution The figure which shows another structural example of a separation / conversion processing part Configuration diagram of conventional equipment

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
FIG. 1 is a flowchart showing the procedure of the polarization multiplexed phase-modulated light evaluation method of the present invention.

  Hereinafter, the polarization multiplexed phase-modulated light evaluation method of the present invention will be described based on this flowchart.

  This polarization multiplexed phase-modulated light evaluation method evaluates the modulation state of signal light multiplexed by phase modulation of polarization components orthogonal to each other in a predetermined format and a predetermined symbol rate. The polarization components orthogonal to each other included in the signal light input in the polarization state are correctly separated, and the phase modulation characteristics of each separated polarization component are obtained by converting the phase modulated light into intensity modulated light. is there.

  That is, first, the signal light P to be evaluated is converted into intensity-modulated light Pa whose intensity changes in accordance with the phase change between the symbols (S1). This conversion process can be realized by using a delay interference technique described later.

  Next, from the converted intensity-modulated light Pa, the first polarization axis component light Pax along the first polarization axis X as a reference for separation and the second polarization axis component light Pay along the second polarization axis Y orthogonal thereto. Are separated (S2). This separation process can be realized by using a separation action by a polarization beam splitter described later.

  Subsequently, the incident polarization direction of the signal light is controlled so that the difference in intensity between the first polarization axis component light Pax and the second polarization axis component light Pay is maximized, and the polarization components Pp and Ps of the signal light P are controlled. The polarization directions are made to coincide with the first polarization axis X and the second polarization axis Y, respectively (S3). In other words, by adjusting the polarization direction of the signal light incident in an unknown polarization state, the intensity-modulated light corresponding to both polarization components included in the signal light is correctly set as the first polarization axis component light and the second polarization axis component light. It is in a state where it can be separated.

  For example, as shown in FIG. 2, both polarization components Pap and Pas of the intensity-modulated light Pam corresponding to the signal light P are shifted by a certain angle θ with respect to the first polarization axis X and the second polarization axis Y, which are separation references. Then, the first polarization axis component light Pax includes a component Papx along the X axis of the polarization component Pap and a component Pasx along the X axis of the polarization component Pas. Includes a component Pasy along the Y-axis of the polarization component Pas and a component Papy along the Y-axis of the polarization component Pap.

And if cos θ = α, sin θ = β,
Papx = Pap ・ α
Papy = Pap · β
Pasx = Pas · α
Pasy = Pas · β
Because
Pax = Papx + Pasx = Pap · α + Pas · β
Pay = Pasy + Papy = Pas / α + Pap / β
It becomes.

The difference ΔP in the intensity of the components along both polarization axes is
ΔP = Pax ² −Pay ²
= (Pap · α + Pas · β) ² − (Pas · α + Pap · β) ²
= (Pap · α) ² + 2Pap · Pas · α · β + (Pas · β) ²
− {(Pas · α) ² +2 (Pas · Pap · α · β) + (Pap · β) ² }
= (Pap ² -Pas ² ) (α ² -β ² )
= (Pap ² -Pas ² ) (2α ² -1)
It becomes.

Here, since both polarization components Pap and Pas are signals that are uncorrelated with each other and fluctuate in their respective amplitude ranges over time, the difference component of the intensity (Pap ² −Pas ² ) is the amplitude of both signals. The amplitude range corresponding to the sum is randomly varied. Further, since the range in which alpha ² takes is between 0~1, (2α ² -1) takes a range of -1 to 1 in accordance with the theta, which is the difference component of the intensity (Pap ² -Pas ²⁾ Is maximized when (2α ² −1) = ± 1 where θ = 0 or an integral multiple of π / 2.

  Accordingly, when the polarization direction of the incident signal light P is rotated so that the signal amplitude of the intensity difference ΔP is maximized, the polarization directions of the two polarization components Psp and Pas are, for example, as shown in FIG. A state in which the polarization axis X and the second polarization axis Y coincide with each other, only the polarization component Pap appears on the first polarization axis X, and only the polarization component Pas appears on the second polarization axis Y can be obtained. (Separation is also possible when π / 2 is rotated from the state of FIG. 3).

  FIG. 4 shows the polarization separation in a waveform so that it can be understood intuitively. The amplitudes of the polarization components Pap and Psa of the intensity-modulated light Pa are as shown in (a) and (b), respectively. In the state where the polarization direction is changed and the polarization direction is deviated by, for example, θ = π / 4 from the first polarization axis X and the second polarization axis Y (α = β), FIGS. ), The component Papx along the first polarization axis X of the polarization component Pap and the component Papy along the second polarization axis Y appear with substantially the same amplitude, and similarly, the first polarization axis of the polarization component Pas. The component Pasx along X and the component Pasy along the second polarization axis Y appear with substantially the same amplitude.

  Therefore, the component Pax along the first polarization axis X and the component Pay along the second polarization axis Y axis are signals including the same components, and the signal amplitude of the difference ΔP in intensity is substantially zero ( In the above formula, α = β to ΔP = 0).

  On the other hand, when the polarization direction of the incident light is controlled and the amplitude of the intensity difference ΔP is maximized as described above, along the first polarization axis X, as shown in FIGS. The component Pax is only the polarization component Pap, the component Pay along the second polarization axis Y is the polarization component Pas, and both polarization components can be correctly separated.

  In this way, with the polarization state of the incident light aligned with the first polarization axis X and the second polarization axis Y as the separation reference, the separation processing of the polarization components Pp and Ps of the signal light P and the intersymbol phase change are performed. If conversion processing to intensity-modulated light whose intensity changes according to the above, information necessary for evaluating the modulation characteristics for each polarization component can be obtained (S4).

  As a system for obtaining information necessary for this evaluation, the conversion process into the intensity-modulated light used for controlling the polarization direction and the separation process of the polarization components Pp and Ps can be used in combination.

  That is, as described above, the first polarization axis component light Pax and the second polarization axis separated from the intensity-modulated light Pa in a state where the incident polarization direction of the signal light coincides with the first polarization axis X and the second polarization axis. Since the component light Pay is obtained by converting the phase change between the polarization components Pp and Ps of the signal light P, which are phase-modulated, into an intensity change, the characteristics of the phase modulation are determined from the amplitudes of these component lights Pax and Pay. Can be evaluated.

  Therefore, the intensity-modulated light Pax and Pay for each polarization component obtained by the separation process and the conversion process are sampled to obtain the sampling data (S5), and the signal light is obtained based on the obtained sampling data. Quality evaluation regarding phase modulation is performed for each polarization component of P (S6).

  With such a method, it is possible to evaluate polarization multiplexed phase-modulated light with a small and inexpensive configuration without reducing accuracy.

FIG. 5 is an overall configuration diagram of a polarization multiplexed phase modulation light evaluation apparatus 30 to which the above method is applied.
In FIG. 5, the polarization multiplexed phase-modulated light evaluation device 30 is a signal modulated by the orthogonal two-polarization multiplexed phase modulation method to be evaluated in which polarized components orthogonal to each other are phase-modulated and multiplexed in a predetermined format at a predetermined symbol rate. The light P is received by a light incident portion 31 formed of an optical connector or the like, and is incident on a polarization controller 32 for changing the polarization state of the signal light P.

  The polarization controller 32 is, for example, a combination of a λ / 2 plate and a λ / 4 plate, and rotates the polarization direction of the incident signal light P to be emitted under the control of the control unit 40 described later.

  The signal light P ′ that has passed through the polarization controller 32 is incident on the optical branching unit 34 via the variable wavelength bandpass filter 33 that selectively passes only the wavelength component of the signal light P ′ to be evaluated, and is branched into two. The The variable wavelength bandpass filter 33 may be inserted between the light incident unit 31 and the polarization controller 32, and may be omitted when the wavelength of the optical signal to be evaluated is limited.

  One of the branched signal lights P ′ is incident on the first modulation converter 35. The first modulation / conversion unit 35 multiplexes and interferes the incident signal light P ′ and the delayed light obtained by delaying the signal light P ′ by one symbol, and responds to the phase change between the signal light symbols. Thus, the intensity-modulated light Pa whose intensity changes is emitted.

  As schematically shown in FIG. 5, the structure of the first modulation conversion unit 35 divides incident light into two by a branching path 35a, and one of the lights is delayed by one symbol by a delay unit 35b. Thus, a delay interferometer can be used in which the delayed light and the other light are combined and interfered by the optical waveguide 35c.

  The intensity modulated light Pa emitted from the first modulation conversion unit 35 is incident on the first polarization beam splitter 36. The first polarization beam splitter 36 performs polarization separation of incident light, and serves as a reference for separation of the first polarization axis X and the second polarization axis Y that are orthogonal to each other in a plane orthogonal to the incident optical axis. The first polarization axis component light Pax along the first polarization axis X and the second polarization axis component light Pay along the second polarization axis Y are separated from the incident intensity-modulated light Pa. Exit.

  The first polarization axis component light Pax and the second polarization axis component light Pay are incident on a difference component amplitude detector 37 that detects the amplitude of the signal component of the intensity difference. For example, as shown in FIG. 6, the differential component amplitude detector 37 receives each component light Pax, Pay by a pair of photodiodes (37a, 37b) connected in series in the forward direction, and is connected to the connection point. The difference component Vd of the intensity change of each component light Pax and Pay appearing at both ends of the load resistor 37c is input to the amplitude detector 37e via the amplifier (buffer) 37d, and the amplitude Av is detected.

  The control unit 40 sets the pass wavelength of the variable wavelength bandpass filter 33 and controls the polarization controller 32 so that the difference component amplitude Av detected by the difference component amplitude detection unit 37 is maximized. The polarization directions of the polarization components Pp and Ps of the signal light P ′ emitted from the polarization controller 32 are made to coincide with the first polarization axis X and the second polarization axis Y, respectively.

  On the other hand, the other signal light P ′ branched by the light branching unit 34 enters the separation / conversion processing unit 50. The separation / conversion processing unit 50 controls the polarization component Pp of the signal light P ′ incident under the control of the control unit 40 in a state where the polarization directions of the polarization components Pp and Ps coincide with the first polarization axis X and the second polarization axis Y. , Ps are subjected to polarization separation processing of components along the first polarization axis X and the second polarization axis Y, and modulation conversion processing into intensity-modulated light whose intensity changes according to the inter-symbol phase change.

  The separation / conversion processing unit 50 converts the signal light P ′ incident through the optical branching unit 34 into a second polarization beam splitter having optical axes in the same direction as the first polarization axis X and the second polarization axis Y. 51, the two polarization components Pp and Ps of the signal light P ′ are separated.

  The separated polarization components Pp and Ps are incident on the optical switch 52, and one of them is selectively emitted to the second modulation conversion unit 53.

  The second modulation conversion unit 53 multiplexes and interferes with the delayed light obtained by delaying the incident polarization component by one symbol, and emits intensity-modulated light whose intensity changes according to the phase change between the polarization component symbols. Similar to the first modulation conversion unit 35, the incident light is branched into two by the branch path 53a, and one of the lights is delayed by one symbol by the delay unit 53b. It is configured by a delay interferometer that multiplexes and interfers light with the optical waveguide 53c, but here, for convenience of evaluation, a phase delay device 53d that gives a predetermined phase delay to one of the light before multiplexing. Has been inserted.

  The polarization component converted into intensity modulated light by the second modulation conversion unit 53 is incident on the sampling unit 60. The sampling unit 60 samples the incident intensity-modulated light at a speed (or higher) synchronized with the symbol rate of the signal light P, and acquires the sampling data.

  The configuration of the sampling unit 60 may be either an optical sampling method or an electrical sampling method.

  FIG. 7 shows an optical sampling method, in which incident light is amplified by an optical amplifier 601 and incident on an optical gate circuit 602. The optical gate circuit 602 allows incident light to pass only while receiving sampling light emitted from the short pulse light source 603 at a predetermined period T. This period T is set equal to or equal to an integer of the sampling period Ts for data acquisition.

  The light that has passed through the optical gate circuit 602 enters the light receiver 604, is converted into an electric signal having a magnitude corresponding to the intensity, and is input to the A / D converter 605. The A / D converter 605 samples the input signal in synchronization with the sampling clock signal C output from the sampling clock generator 606 at the cycle Ts, converts it to a digital value, and outputs it.

  On the other hand, the sampling light emitted from the short pulse light source 603 at the period T is incident on the light receiver 607, and the received light signal is input to the sampling clock generator 606. The sampling clock generator 606 generates a clock signal C having a period Ts synchronized with the light reception signal and supplies the clock signal C to the A / D converter 605.

  FIG. 8 shows an electrical sampling method, in which incident light is incident on a light receiver 608, the received light signal is incident on an A / D converter 605, and a clock signal output from the sampling clock generator 606 at a cycle Ts. Sampling is performed at a timing synchronized with C.

Data obtained by such sampling is input to the evaluation unit 70.
The evaluation unit 70 acquires the data sampled by the sampling unit 60 after the polarization component of the signal light P is correctly separated by the control unit 40, obtains the amplitude distribution thereof, and determines the quality of the phase-modulated light. Evaluate.

  For example, when the polarization components Pp and Ps of the signal light P are phase-modulated by the quaternary phase modulation method, the phase of the light for each symbol is 0 (reference phase), π / 2, π, 3π / 2. If the phase difference between adjacent symbols is 0 (in-phase), the maximum amplitude Vmax is obtained when converted into intensity-modulated light. Further, if the phase difference between adjacent symbols is ± π (reverse phase), the minimum amplitude Vmin (theoretically 0) is obtained when converted into intensity-modulated light.

  If the phase difference between adjacent symbols is ± π / 2, a value Va between Vmax and Vmin is obtained when converted into intensity-modulated light.

  That is, in this case, the amplitude of the intensity-modulated light theoretically changes between three points according to the phase change, but the actual modulation characteristics vary in the vicinity of the amplitude values of these three points. If the modulation accuracy is poor, the variation becomes large.

  Therefore, the probability distribution of the amplitude at the timing matched with the symbol rate of the intensity modulated light is obtained, and the phase modulation of the signal light can be evaluated from the characteristics of the distribution (average value difference, standard deviation, etc.)

  The above description is for the case where the delay of the phase delay unit 53d is 0. However, when this delay amount is set to π / 4, the amplitude value theoretically taken by the intensity-modulated light can be reduced to two.

  That is, even if the phase difference between adjacent symbols of the original input light is 0 (in-phase), as shown in FIG. Since the light B is shifted by π / 4, the symbol lights A and B ′ having a π / 4 phase difference are combined, and the amplitude of the combined light Q1 becomes V1 smaller than the maximum amplitude Vmax. Further, the amplitude of the combined light Q2 obtained by combining the symbol light A by shifting the phase of the symbol light C shifted by 3π / 2 with respect to the symbol light A by π / 4 is also equal to V1.

  On the other hand, as shown in FIG. 9B, the combined light Q3 obtained by combining the symbol light D, whose phase is shifted by π / 2 with respect to the symbol light A, by further shifting by π / 4. The amplitude of the combined light Q4 obtained by combining the symbol light A by shifting the phase of the symbol light E shifted by π / 4 from the symbol light A by π / 4 is also equal to V2. .

  Thus, by adding a phase delay of π / 4 by the phase delay unit 53d, the amplitude of the intensity-modulated light is distributed around the two values V1 and V2, as shown in FIG.

Using the average phases μ1, μ2 and standard deviations σ1, σ2 of this distribution, a value Q representing the phase modulation quality is
Q = | μ2−μ1 | / (σ2 + σ1)
And the modulation quality of the signal light P can be evaluated from the value Q. This evaluation is performed for both polarization components Pp and Ps by switching the optical switch 52.

  Note that the phase modulation quality may be evaluated from the distribution of the three amplitude values Vmax, Va, and Vmin by setting the delay amount by the phase delayer 53d to 0 or deleting the phase delayer 53e.

  As described above, the polarization multiplexed phase-modulated light evaluation device 30 according to the embodiment converts the signal light P, which is obtained by phase-modulating polarization components orthogonal to each other into a predetermined format and a predetermined symbol rate, according to a phase change between symbols. The intensity-modulated light Pa is converted into intensity-modulated light Pa. From the intensity-modulated light a, the first polarization axis component light Pax along the first polarization axis X, which is a reference for separation, is orthogonal to the first polarization axis X. The second polarization axis component light Pay along the polarization axis Y is separated, and the incident polarization direction of the signal light P is controlled so that the signal amplitude of the difference in the intensity of the separated component light is maximized. The polarization directions of the P polarization components Pp and Ps are made to coincide with the first polarization axis X and the second polarization axis Y, respectively.

  Then, in this state, separation processing of each polarization component Pp, Ps of the signal light P and conversion processing to intensity-modulated light whose intensity changes according to the inter-symbol phase change are performed, and each polarization component obtained thereby The intensity-modulated lights Pap and Pas are sampled, and the quality of the signal light is evaluated based on the sampling data.

  In other words, in addition to the local light emission and 90-degree hybrid used in conventional transmission devices, ultra-high-speed data sampling and digital signal processing are not required, so polarization can be achieved with a small and inexpensive configuration without reducing accuracy. Multiple phase modulated light can be evaluated.

  In the embodiment, the second modulation conversion unit 53 is configured by a set of delay interference units. However, as illustrated in FIG. One of them is incident on the first delay interference unit 55, the other is incident on the second delay interference unit 56, and the delay amount of the phase delay device 55d of the first delay interference unit 55 is set to a reference value (for example, 0 degree), and by making the delay amount of the phase delay unit 56d of the second delay interference unit 56 different by 90 degrees with respect to the reference value, the intensity modulated light Papi, Papq ( Alternatively, Pasi, Pasq) can be obtained by switching the optical switch 52, and each of them can be sampled at the same timing by the sampling unit 60 to obtain the waveform information, which can be given to the evaluation unit 70.

In the case of this configuration, signal components I and Q having phases different from each other by 90 degrees can be obtained, and a phase θ = tan ⁻¹ (Q / I) obtained from the components I and Q is calculated to obtain a distribution thereof. In FIG. 11, reference numerals 55a and 56a denote branch paths, 55b and 56b denote delay elements corresponding to one symbol, and 55c and 56c denote multiplexing waveguides.

  FIG. 12 shows the phase distribution in the case of quaternary phase modulation, and the average phases μ1 to μ4 and the standard deviations σ1 to σ4 of the distribution centered on each phase 0, π / 2, π, 3π / 2 are shown. Based on this, the signal light P can be evaluated by obtaining the Q value between the phases.

  In the embodiment, the separation / conversion processing unit 50 includes the second modulation conversion unit 53 and the second polarization beam splitter 51 independent of the first modulation conversion unit 35 and the first polarization beam splitter 36. However, as shown in FIG. 13, the separation and conversion processing of the separation / conversion processing unit 50 may be performed by the first modulation conversion unit 35 and the first polarization beam splitter 36. it can. That is, the order of the polarization separation process and the modulation conversion process may be reversed.

  In this case, the intensity-modulated light for each polarization component separated by the first polarization beam splitter 36 is branched by the optical branching units 58 and 59, respectively, one of which is supplied to the differential component amplitude detection unit 37, and the other is the optical switch. 52 '.

  As described above, first, the polarization direction of the signal light P is aligned with the first polarization axis X and the second polarization axis Y, and then the sampling process for the intensity-modulated light for each polarization component Pp, Ps of the signal light P And the evaluation process.

  In this case, a phase delay unit 35d is provided in the first modulation conversion unit 35, and the amount of delay is set to 0 in the polarization control by the control of the control unit 40, and in the evaluation, Similarly to the above, by giving a delay of π / 4, a distribution centered on the two amplitudes V1 and V2 can be obtained, and the evaluation value Q can be obtained from the distribution.

  The apparatus according to this embodiment requires only one set of the delay interference unit and the polarization beam splitter, which makes the configuration simpler and further reduces the cost.

  DESCRIPTION OF SYMBOLS 30 ... Polarization multiple phase modulation light evaluation apparatus, 31 ... Light incident part, 32 ... Polarization controller, 33 ... Variable wavelength band pass filter, 34 ... Optical branching part, 35 ... First modulation conversion part, 36... First polarization beam splitter 37. Difference component amplitude detection unit 40. Control unit 50. Separation / conversion processing unit 51. Second polarization beam splitter 52 and 52 ′ Optical switch 53... Second modulation conversion unit 54... Optical branching unit 55... First delay interference unit 56... Second delay interference unit 60. Part

Claims (7)

A step (S1) of converting signal light in which polarization components orthogonal to each other are phase-modulated and multiplexed in a predetermined format and a predetermined symbol rate into intensity-modulated light whose intensity changes in accordance with a phase change between symbols (S1);
From the intensity-modulated light, first polarization axis component light along a first polarization axis that serves as a reference for separation and second polarization axis component light along a second polarization axis perpendicular to the first polarization axis are separated. Stage (S2);
The incident polarization direction of the signal light is controlled so that the amplitude of the difference component of the intensity of the first polarization axis component light and the second polarization axis component light is maximized, and the polarization direction of each polarization component of the signal light is determined. Making the first and second polarization axes coincide with each other (S3);
A separation process of each polarization component of the signal light in a state where the respective polarization directions coincide with the first polarization axis and the second polarization axis, and a conversion process to intensity-modulated light whose intensity changes in accordance with a phase change between symbols. Performing step (S4);
Sampling intensity-modulated light for each polarization component obtained by the separation process and the conversion process to obtain sampling data (S5);
A polarization multiplexed phase modulation light evaluation method including a step (S6) of evaluating the quality of the signal light based on the acquired sampling data. A light incident part (31) for making the signal components multiplexed by polarization modulation orthogonal to each other in a predetermined format and a predetermined symbol rate;
A polarization controller (32) for changing the polarization direction of the signal light incident on the light incident portion;
The signal light that has passed through the polarization controller is received, and the signal light and delayed light obtained by delaying the signal light by an amount corresponding to one symbol are combined and interfered, and the intensity changes according to the phase change between the signal light symbols. A first modulation conversion unit (35) for emitting intensity-modulated light to be
The first polarization axis component light along the first polarization axis that is a reference for separation from the intensity modulation light that is received from the first modulation conversion unit, and orthogonal to the first polarization axis A first polarization beam splitter (36) for separating the second polarization axis component light along the second polarization axis;
A differential component amplitude detector (37) for detecting an amplitude of a differential component of the intensity of the first polarization axis component light and the second polarization axis component light;
The polarization controller is controlled so that the amplitude detected by the differential component amplitude detector is maximized, and the polarization direction of each polarization component of the signal light emitted from the polarization controller is set to the first polarization axis and the first polarization axis. A control unit (40) for matching the two polarization axes,
The first polarization axis and the second polarization axis with respect to the signal light emitted from the polarization controller in a state where the polarization direction of each polarization component coincides with the first polarization axis and the second polarization axis under the control of the control unit. A separation / conversion processing unit (50) for performing a component separation process along the lines and a modulation conversion process to intensity-modulated light whose intensity changes in accordance with a phase change between symbols;
A sampling unit (60) for sampling the intensity-modulated light for each polarization component obtained by the separation / conversion processing unit and obtaining the sampling data;
A polarization multiplexed phase-modulated light evaluation apparatus comprising: an evaluation unit (70) that evaluates the quality of the signal light based on the sampling data acquired by the sampling unit. The separation processing and modulation conversion processing by the separation / conversion processing unit are performed by the first modulation conversion unit and the first polarization beam splitter,
The intensity-modulated light for each polarization component separated by the first polarization beam splitter is configured to be emitted to the sampling unit via branching means (58, 59). Polarized multiple phase modulation light evaluation device.   An optical switch (52 ') that selectively makes one of the intensity-modulated light components of each polarization component separated by the first polarization beam splitter and branched by the branching unit enter the sampling unit; The polarization multiplexed phase-modulated light evaluation apparatus according to claim 3. A branching unit (34) for splitting the light emitted from the polarization controller into two, causing one of the light to enter the first modulation conversion unit, and emitting the other to the separation / conversion processing unit;
The separation / conversion processing unit
A second polarization beam having an optical axis in the same direction as the first polarization axis and the second polarization axis, receiving the signal light incident through the branching means, and separating both polarization components of the signal light A splitter (51);
The polarization component separated by the second polarization beam splitter and delayed light obtained by delaying the polarization component by an amount corresponding to one symbol are combined and interfered, and the intensity changes according to the phase change between the symbols of the polarization component. The polarization multiplexed phase-modulated light evaluation apparatus according to claim 2, further comprising: a second modulation conversion unit (53) that emits intensity-modulated light.   6. An optical switch (52) for selectively making any one of the polarization components separated by the second polarization beam splitter incident on the second modulation conversion unit. The polarization multiplexed phase-modulated light evaluation apparatus described. The second modulation conversion unit of the separation / conversion processing unit is:
A branching means (54) for splitting the input light into two;
A first delay interference section (55) for combining and interfering one branched light branched by the branching means and a delayed light obtained by giving a delay equivalent to one symbol and a delay of a predetermined phase to the branched light;
A second light beam that causes the other branched light beam branched by the branching unit and a delayed beam beam having a delay equivalent to one symbol to the branched beam beam and a delay having a difference of 90 degrees with respect to the predetermined phase; A delay interference unit (56),
The evaluation unit calculates the phase of each polarization component of the signal light from sampling data for the outgoing light of the first delay interference unit and the outgoing light of the second delay interference unit, and evaluates based on the phase value The polarization multiplexed phase-modulated light evaluation apparatus according to claim 5, wherein:

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