JP4443557B2 - Optical phase modulation evaluation device - Google Patents

Description

  The present invention relates to an optical phase modulation evaluation apparatus that evaluates an optical phase modulation signal in which an optical carrier wave in a coherent optical communication system is phase modulated (for example, DPSK, DQPSK, etc.) by a data signal. It is related with the optical phase modulation evaluation apparatus which can perform evaluation of correctly.

  Conventionally, as an optical phase modulation evaluation apparatus that evaluates an optical phase modulation signal in which an optical carrier wave is phase-modulated by a data signal, optical phase detection of the optical phase modulation signal is performed by a bit delay interferometer and the phase of the optical phase modulation signal is detected. Some have evaluated modulation characteristics (see, for example, Patent Document 1).

  FIG. 8 shows a schematic configuration of this type of optical phase modulation evaluation apparatus. The optical phase modulation signal, which is the light to be measured by the optical phase modulation evaluation apparatus, is generated by phase-modulating an optical carrier wave with a data signal having a predetermined symbol rate, and a bit delay interferometer 20 as an optical phase detector. Is input. The bit delay interferometer 20 is composed of a Mach-Zehnder interferometer using an optical waveguide. The optical phase modulation signal input from the port 20a and the light passing through the arm 20c by the branching unit 20b and the arm 20d (bit delay). The light passing through the arm 20c and the light passing through the arm 20d are multiplexed and interfered by the multiplexing unit 20e. Thereby, the phase change of the optical phase modulation signal is converted into a change in light intensity, and two light intensity conversion signals whose phases are different from each other by 180 ° (π) are output from the two ports 20g and 20h, respectively. The bit delay unit 20f has an optical path length of the arm 20d corresponding to the delay amount so that the delay amount (delay time difference) between the two arms becomes a delay amount (time) corresponding to one bit of the symbol rate. Is longer than the optical path length of the arm 20c.

The light receiver (PD) 21 photoelectrically converts the light intensity conversion signal output from the port 20g of the bit delay interferometer 20 and outputs an electrical signal. The signal processing unit 22 demodulates the data signal from the electrical signal output from the light receiver 21, and performs error rate measurement and waveform observation using the demodulated waveform obtained thereby to evaluate the optical phase modulation signal. .
JP-A-6-21891

  Recently, phase modulation schemes such as DPSK and DQPSK, which have high frequency efficiency, are attracting attention because of problems of fiber nonlinearity and dispersion accompanying an increase in communication traffic capacity. Depending on the method, different symbol rates are used. For example, in 40 Gbps-DPSK, the symbol rate is 40 Gbps, and in 40 Gbps-DQPSK, the symbol rate is 20 Gbps. As described above, in a situation where different symbol rates are used depending on the system, a single optical phase modulation evaluation apparatus can support a plurality of symbol rates as an optical phase modulation evaluation apparatus for evaluating an optical phase modulation signal. It is desired.

  However, in the above conventional optical phase modulation evaluation apparatus, since the delay amount corresponding to one bit of the symbol rate set by the bit delay unit 20f is fixed with respect to the specific symbol rate, the optical phase When the symbol rate of the modulation signal is different from 20 Gbps or 40 Gbps as described above, there is a problem that only the optical phase modulation signal of one of the symbol rates can be evaluated.

  The present invention enables a bit delay unit of a bit delay interferometer to set a delay amount corresponding to each of a plurality of symbol rates of an optical phase modulation signal and corresponding to one bit of those symbol rates, An object of the present invention is to solve these problems and provide an optical phase modulation evaluation apparatus capable of accurately evaluating optical phase modulation signals having different symbol rates.

In order to solve the above-mentioned problem, the optical phase modulation evaluation apparatus according to claim 1 of the present invention includes an optical phase modulation signal obtained by phase-modulating an optical carrier wave with a data signal, and includes a first mirror (3). A first beam splitter (1) that demultiplexes the first light that passes through the first optical path constituted by the second light and the second light that passes through the second optical path constituted by the second mirror (4). And the first beam and the second beam are combined to interfere with each other, and a second beam splitter (6) for outputting a light intensity conversion signal obtained by converting the optical phase modulation signal, and the first beam A Mach-Zehnder type bit delay interferometer (10) having an optical path or a bit delay (2) disposed in the second optical path;
An optical phase modulation evaluation apparatus comprising: an optical waveform measurement unit (11) that converts the optical intensity conversion signal output from the bit delay interferometer into an electrical signal and observes the optical phase modulation signal;
The bit delay unit can set a delay amount corresponding to one bit of the symbol rate corresponding to each of the plurality of symbol rates of the optical phase modulation signal;
Delay amount setting means (12) is provided for setting a delay amount of the bit delay unit based on a symbol rate specifying signal for specifying the symbol rate of the optical phase modulation signal.

  Also, in the optical phase modulation evaluation apparatus according to claim 2 of the present invention, in the optical phase modulation evaluation apparatus according to claim 1 described above, the bit delay unit provides a delay amount corresponding to one bit of the symbol rate. A parallel plate (2a, 2b) of a transparent material having a thickness, comprising a plurality of parallel plates each having a thickness corresponding to each of the plurality of symbol rates, and switching these to delay corresponding to the symbol rate The amount can be set.

The optical phase modulation evaluation apparatus according to claim 3 of the present invention is the optical phase modulation evaluation apparatus according to claim 1 described above, wherein the bit delay unit includes a first parallel plate (2c ₁ , 2e ₁ ) and a second parallel plate (2c ₁ , 2e ₁ ). Of parallel plates (2c ₂ , 2e ₂ ) of a pair of transparent plates made of a translucent material, each having a thickness corresponding to each of the plurality of symbol rates. A plurality of pairs are provided, and the pair of parallel plates can be switched to set a delay amount corresponding to the symbol rate.

  The optical phase modulation evaluation apparatus according to claim 4 of the present invention is the optical phase modulation evaluation apparatus according to claim 3 described above, wherein an incident angle in the first parallel plate and an emission angle in the second parallel plate are The parallel plate pair is arranged on the optical path so as to be equal.

The optical phase modulation evaluation apparatus according to claim 5 of the present invention is the optical phase modulation evaluation apparatus according to claim 1 described above, wherein the bit delay units are wedge-shaped transmissions whose transmission surfaces are opposed to each other. A pair of fields (2f ₁ , 2f ₂ ), the optical path length is adjusted by moving at least one of the transmission bodies, and the delay amount corresponding to each of the plurality of symbol rates can be set. did.

  Further, in the optical phase modulation evaluation apparatus according to claim 6 of the present invention, any one of the first and second optical paths of the bit delay interferometer in the optical phase modulation evaluation apparatus according to any one of claims 1 to 5 described above. An optical phase delay device (5) that is provided on either side and can arbitrarily set a delay amount that can be changed by one period of the optical phase difference between the two optical paths, and sets the delay amount of the optical phase delay device Phase control means (13).

  The optical phase modulation evaluation apparatus according to claim 7 of the present invention is the optical phase modulation evaluation apparatus according to claim 6, wherein the optical phase retarder is a parallel plate (5 a) made of a transparent material, An arbitrary delay amount can be set by changing the incident angle of light to the parallel plate by rotating the parallel plate.

Further, in the optical phase modulation evaluation apparatus according to claim 8 of the present invention, in the optical phase modulation evaluation apparatus according to claim 6 described above, the optical phase delay devices are two pieces of transparent material arranged in a square shape. A pair of parallel plates (5b ₁ , 5b ₂ ), and at least one of the two parallel plates is rotated to change the incident angle of the light to the parallel plate, thereby setting an arbitrary delay amount I was able to do it.

  The optical phase modulation evaluation apparatus according to claim 9 of the present invention is the optical phase modulation evaluation apparatus according to claim 6, wherein the optical phase delay is the first and second mirrors of the bit delay interferometer. Any one of these mirrors and a piezoelectric element (5c) provided on the mirror can be set to an arbitrary delay amount by moving the mirror with the piezoelectric element.

  In the optical phase modulation evaluation apparatus according to claim 10 of the present invention, in the optical phase modulation evaluation apparatus according to any one of claims 1 to 9, the bit delay interferometer is output from the second beam splitter. In order to give a delay time difference between the two light intensity conversion signals, an optical output signal delay device (7) for changing the optical path length of one of the optical paths through which the two light intensity conversion signals pass is provided.

  In the optical phase modulation evaluation apparatus according to the first to fifth aspects of the present invention, the bit delay unit of the bit delay interferometer corresponds to each of a plurality of symbol rates of the optical phase modulation signal and corresponds to one bit of those symbol rates. Since the delay amount to be set can be set, the waveforms of the optical phase modulation signals having different symbol rates can be accurately observed. Accordingly, it is possible to accurately evaluate the phase modulation state of the optical phase modulation signal in DPSK, DQPSK, or multilevel modulation with different symbol rates with one optical phase modulation evaluation apparatus.

  In the optical phase modulation evaluation apparatus according to claims 6 to 9 of the present invention, one of the first and second optical paths of the bit delay interferometer can be changed by one period of the optical phase difference between the two optical paths. Since an optical phase retarder that can arbitrarily set a delay amount is provided, the optical phase difference between the two optical paths can be adjusted to π / 2 radians or 0 even when environmental conditions such as temperature change. Thereby, it is possible to set an arbitrary relative phase difference between bits.

  In the optical phase modulation evaluation apparatus according to the tenth aspect of the present invention, the bit delay interferometer is configured to provide a relative delay time for the two light intensity conversion signals output from the second beam splitter. Since the optical output signal delay device for changing the delay time of the optical path is provided in one of the optical paths through which the two light intensity conversion signals pass, for example, the two optical output signals of the bit delay interferometer are balancedly received. In this case, the phase difference between the modulation signals of the two light intensity conversion signals can be accurately matched.

  Embodiments of the present invention will be described below.

[First Embodiment]
The configuration of the optical phase modulation evaluation apparatus according to the first embodiment of the present invention is shown in FIG. An optical phase modulation signal, which is light to be measured by the optical phase modulation evaluation apparatus, is generated by phase-modulating an optical carrier wave with a data signal having a predetermined symbol rate, and is input to the bit delay interferometer 10. The bit delay interferometer 10 is composed of a Mach-Zehnder interferometer using a spatial propagation path. The input optical phase modulation signal is a beam splitter (BS) 1 and includes a bit delay 2 and a mirror 3. The first light passing through the first optical path is split into the first light passing through the first optical path configured and the second light passing through the second optical path configured including the mirror 4. And the second light passing through the second optical path are combined by a beam splitter (BS) 6 to interfere with each other. Thereby, the phase change of the optical phase modulation signal is converted into a change in light intensity, and two light intensity conversion signals whose phases are different from each other by 180 ° (π) are output from the beam splitter 6.

The bit delay unit 2 described above has a delay amount so that a delay amount (delay time difference) between the two first and second optical paths becomes a delay amount (time) corresponding to one bit of the symbol rate. The optical path length of the first optical path is made longer than the optical path length of the second optical path, and the thickness d ₁ of the transparent material (for example, glass, silicon, etc.) is parallel to correspond to the two types of symbol rates. It has become a parallel plate 2b of the plate 2a and the thickness d ₂ be switched by moving in a direction perpendicular to the optical path. Switching between the parallel plate 2 a and the parallel plate 2 b is performed based on a command from the delay amount setting means 12.

The thickness d of the parallel plate with respect to the symbol rate is obtained as follows. That is, when the symbol rate is SR [bps], the delay time t [sec] corresponding to 1 bit is t = 1 / SR. The thickness d of the parallel plate is d = (c × t) / (n−1). Here, n is the refractive index of the parallel plate (transparent material), and c is the speed of light. A specific example when glass (n = 1.5) is used for the parallel plate will be t = 25 ps and d = 15 mm when SR = 40 Gbps. When SR = 20 Gbps, t = 50 ps and d = 30 mm. Accordingly, when the symbol rate is 40 Gbps, the glass parallel plate 2a with d ₁ = 15 mm may be inserted into the optical path, and when the symbol rate is 20 Gbps, the glass parallel plate 2b with d ₂ = 30 mm may be inserted into the optical path. Here, a case where two types of symbol rates are supported has been described, but the same applies even if the number of symbol rates increases.

  The above-described mirror 3 reflects the first light output from the beam splitter 1 and input through the bit delay device 2 and inputs the first light to the beam splitter 6. Further, the mirror 4 described above reflects the second light input from the beam splitter 1 and causes the beam splitter 6 to input the reflected second light.

  Next, the optical waveform measurement unit 11 converts one of two light intensity conversion signals output from the bit delay interferometer 10 and having a phase difference of 180 ° (π) into an electrical signal (O / E conversion). Then, data processing of the obtained electric signal waveform is performed to observe the waveform of the optical phase modulation signal, measure the phase modulation characteristic, and the like. An optical sampling oscilloscope can be used as the optical waveform measurement unit 11. FIG. 1 shows the case where one of the two light intensity conversion signals is input to the optical waveform measuring unit 11, that is, the case of single reception, but both the two light intensity conversion signals are input. Also good. In that case, the optical waveform measuring unit 11 can perform balanced reception, and the light receiving sensitivity of the measurement can be improved as compared with the case of single reception.

  The delay amount setting means 12 receives a symbol rate designation signal (designates the symbol rate of the optical phase modulation signal) input from an operation unit or the like (not shown), and based on this symbol rate designation signal, the bit delay unit 2 Set the delay amount. Specifically, when the symbol rate designation signal designates a symbol rate of 40 Gbps, the parallel plate 2a (d1 = 15 mm), and when the symbol rate 20 Gbps is designated, the parallel plate 2b (d2 = 30 mm) is controlled so that the bit delay unit 2 is inserted into the optical path.

  The bit delay unit 2 of the first embodiment may be the one shown in FIGS. 2 and 3 in addition to the one shown in FIG.

That is, in the bit delay unit 2 shown in FIG. 2, a transparent material (for example, glass) having a thickness d ₃ for giving a predetermined delay amount corresponding to the symbol rate so as to correspond to two types of symbol rates. Two parallel plates 2c ₁ , 2c ₂ made of silicon, etc.) and two parallel plates 2e ₁ , 2e ₂ made of a transparent material having a thickness d ₄ as a pair. The parallel plates 2c ₁ , 2c ₂ and the parallel plates 2e ₁ , 2e ₂ of the pair of C are shaped like a letter C so that the incident angle and the outgoing angle of light are equal and a delay amount corresponding to one bit of the symbol rate is given. Placed in the mold. The two pairs corresponding to the two types of symbol rates are moved and switched in a direction orthogonal to the optical path based on a command from the delay amount setting means 12. As described above, the two parallel plates in each pair are arranged in a C shape, so that reflection from the bit delay unit 2 to the beam splitter 1 can be prevented, and the light beam at the time of switching the delay amount can be prevented. Movement (beam shift) can be suppressed. When the beam shift is large, the interference intensity is affected. Here, a case where two types of symbol rates are supported has been described, but the same applies even if the number of symbol rates increases.

In the bit delay device 2 shown in FIG. 3, a pair of transmission bodies 2f ₁ and 2f ₂ (for example, glass, silicon, etc.) formed with inclined surfaces facing each other are arranged orthogonal to the optical path. . Then, based on a command from the delay amount setting means 12, at least _{one of} the transmissive bodies 2f ₁ and 2f ₂ is moved in a direction orthogonal to the optical path, and a delay amount corresponding to each of a plurality of symbol rates. Can be set. When the pair of transmission bodies 2f ₁ and 2f ₂ are moved in conjunction with each other so that they are always symmetrical with respect to the optical path, the beam shift at the time of setting the delay amount can be suppressed. .

[Second Embodiment]
FIG. 4 shows the configuration of the optical phase modulation evaluation apparatus according to the second embodiment of the present invention. The first embodiment shown in FIG. 1 is a delay amount that can change the optical phase difference between the first optical path and the second optical path by one period in the second optical path of the bit delay interferometer 10 described above. The only difference is that an optical phase delay 5 that can be arbitrarily set is provided and the delay amount of the optical phase delay 5 is controlled by the phase control means 13. Therefore, the optical phase delay device 5 will be mainly described.

Optical phase delay device 5, arranged as shown in FIG. 4 in the optical path, formed by a parallel plate 5a of the transparent material (e.g. glass, silicon, etc.) with a thickness d ₅ for giving a predetermined delay amount Yes. Based on a command from the phase control means 13, the parallel plate 5a is rotated to change the incident angle of light to the parallel plate 5a, so that an arbitrary delay amount can be set. In the case of this configuration, a beam shift is generated by the rotation of the parallel plate 5a, but the amount thereof is small on the order of wavelength and hardly affects the interference intensity. In FIG. 4, the optical phase delay 5 is provided in the second optical path, but may be provided in the first optical path.

  The optical phase retarder 5 of the second embodiment may be the one shown in FIGS. 5 and 6 in addition to the one shown in FIG.

That is, in the optical phase retarder 5 shown in FIG. 5, two parallel plates 5b ₁ and 5b ₂ made of a transparent material (for example, glass, silicon, etc.) having a thickness d ₆ for giving a predetermined delay amount are provided. As a pair, the parallel plates 5b ₁ and 5b ₂ of the pair are arranged in a square shape so that the incident angle and the outgoing angle of light are substantially equal. Then, based on a command from the phase control means 13, two rotating at least one of the parallel plates 5b _1, 5b ₂ by changing the incident angle of light to this parallel plate 5b _1, 5b ₂ An arbitrary delay amount can be set. The parallel plates 5b ₁ and 5b ₂ in a pair are arranged in a square shape so that the incident angle and the outgoing angle of light are equal, and the parallel plates 5b ₁ and 5b ₂ are interlocked to be opposite to each other. In the case of rotation, it is possible to suppress reflection and to suppress beam shift when setting the delay amount.

  In the optical phase retarder 5 shown in FIG. 6, the optical phase retarder 5 includes the above-described mirror 4 and a piezoelectric element 5 c (for example, a piezo element) provided on the mirror 4. Then, based on a command from the phase control means 13, the mirror 4 is moved in the direction of the arrow (see the figure) by the piezoelectric element 5c (the optical path length between the beam splitter 1 and the mirror 4 is changed), so that an arbitrary delay amount is obtained. It can be set. In the case of this configuration, a beam shift is generated by the movement of the mirror 4, but the amount thereof is small on the wavelength order and hardly affects the interference intensity. In FIG. 6, the piezoelectric element 5 c is provided on the mirror 4, but the optical phase retarder 5 may be configured by providing the piezoelectric element 5 c on the mirror 3.

[Third Embodiment]
The configuration of the optical phase modulation evaluation apparatus according to the third embodiment of the present invention is shown in FIG. The second embodiment shown in FIG. 4 is different from the above two light intensity conversions in one of the optical paths through which the two light intensity conversion signals output from the beam splitter 6 of the bit delay interferometer 10 described above pass. The only difference is that an optical output signal delay device 7 for changing the optical path length of the one optical path for providing a relative delay time to the signal is provided. Therefore, the optical output signal delay device 7 will be mainly described.

  As shown in FIG. 7, the optical output signal delay device 7 is composed of four mirrors 7a, 7b, 7c, 7d and a movable part (not shown). Of these, the two mirrors 7a, 7b form a corner mirror, and are movable. The part moves the corner mirror in the direction of the arrow. Then, the phase difference between the two light intensity conversion signals can be adjusted by moving the corner mirror and varying the optical path length between the mirror 7b and the mirror 7c and between the mirror 7a and the mirror 7d.

The figure which shows the structure of 1st Embodiment of this invention. The figure for demonstrating another structure of the bit delay device of this invention The figure for demonstrating another structure of the bit delay device of this invention The figure which shows the structure of 2nd Embodiment of this invention. The figure for demonstrating another structure of the optical phase delay device of this invention The figure for demonstrating another structure of the optical phase delay device of this invention The figure which shows the structure of 3rd Embodiment of this invention. The figure which shows schematic structure of a prior art example

Explanation of symbols

1,6 ... beam splitter (BS), 2,20f ··· bit delay _{unit, 2a, 2b, 2c 1,} 2c 2, 2e 1, 2e 2, 5a, 5b 1, 5b 2 ··· parallel plate 2f ₁ , 2f ₂ ... Transmissive body, 3, 4, 7a, 7b, 7c, 7d... Mirror, 5... Optical phase retarder, 5c. Signal delay unit, 10, 20 ... Bit delay interferometer, 11 ... Optical waveform measurement unit, 12 ... Delay amount setting means, 13 ... Phase control means, 20a, 20g, 20h ... Port , 20b ... demultiplexing unit, 20c, 20d ... arm, 20e ... multiplexing unit, 21 ... light receiver (PD), 22 ... signal processing unit.

Claims (10)

The first light and the second mirror (4) passing through the first optical path configured to include the first mirror (3) upon receiving the optical phase modulation signal in which the optical carrier wave is phase-modulated with the data signal The first beam splitter (1) that demultiplexes into the second light that passes through the second optical path that includes the first light, the first light, and the second light are combined and interfered with each other. A Mach-Zehnder type having a second beam splitter (6) for outputting a light intensity conversion signal obtained by converting a phase modulation signal and a bit delay (2) arranged in the first optical path or the second optical path. A bit delay interferometer (10) of
An optical phase modulation evaluation apparatus comprising: an optical waveform measurement unit (11) that converts the optical intensity conversion signal output from the bit delay interferometer into an electrical signal and observes the optical phase modulation signal;
The bit delay unit can set a delay amount corresponding to one bit of the symbol rate corresponding to each of the plurality of symbol rates of the optical phase modulation signal;
An optical phase modulation evaluation apparatus comprising delay amount setting means (12) for setting a delay amount of the bit delay unit based on a symbol rate specifying signal for specifying the symbol rate of the optical phase modulation signal .   The bit delay unit is a parallel plate (2a, 2b) made of a transparent material having a thickness that gives a delay amount corresponding to one bit of the symbol rate, and has a thickness corresponding to each of the plurality of symbol rates. 2. The optical phase modulation evaluation apparatus according to claim 1, wherein a plurality of the parallel plates are provided, and a delay amount corresponding to the symbol rate can be set by switching the parallel plates. The bit delay unit is a pair of two parallel plates made of a transparent material in which a first parallel plate (2c ₁ , 2e ₁ ) and a second parallel plate (2c ₂ , 2e ₂ ) are arranged in a square shape. A plurality of pairs of parallel plates each having a thickness corresponding to each of the plurality of symbol rates is provided, and a delay amount corresponding to the symbol rate can be set by switching the pair of parallel plates. The optical phase modulation evaluation apparatus according to claim 1.   4. The light according to claim 3, wherein the pair of parallel plates is arranged on the optical path so that an incident angle in the first parallel plate and an outgoing angle in the second parallel plate are equal. Phase modulation evaluation device. The bit delay unit is a pair of wedge-shaped transmission bodies (2f ₁ , 2f ₂ ) arranged so that the transmission surfaces face each other, and moves at least one of the transmission bodies to increase the optical path length. The optical phase modulation evaluation apparatus according to claim 1, wherein the delay amount corresponding to each of the plurality of symbol rates can be adjusted and set. An optical phase delay device that is provided on any one of the first and second optical paths of the bit delay interferometer and can arbitrarily set a delay amount that can be changed by one period of the optical phase difference between the two optical paths. (5) and
6. The optical phase modulation evaluation apparatus according to claim 1, further comprising phase control means (13) for setting a delay amount of the optical phase delay device.   The optical phase retarder is a parallel plate (5a) made of a transparent material, and can be set to an arbitrary delay amount by rotating the parallel plate and changing the incident angle of light to the parallel plate. The optical phase modulation evaluation apparatus according to claim 6. The optical phase retarder is a pair of _two parallel plates (5b ₁ , 5b ₂ ) made of a transparent material arranged in a square shape, and rotates at least one of the two parallel plates. The optical phase modulation evaluation apparatus according to claim 6, wherein an arbitrary delay amount can be set by changing an incident angle of light to the parallel plate.   The optical phase delay device is configured by one of the first and second mirrors of the bit delay interferometer and a piezoelectric element (5c) provided on the mirror, and the mirror is formed by the piezoelectric element. The optical phase modulation evaluation apparatus according to claim 6, wherein an arbitrary delay amount can be set by moving the optical phase modulation evaluation apparatus.   In order for the bit delay interferometer to give a delay time difference between the two light intensity conversion signals output from the second beam splitter, the optical path length of one of the two optical paths through which the light intensity conversion signals pass is set. 10. The optical phase modulation evaluation apparatus according to claim 1, further comprising an optical output signal delay device (7) to be changed.

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