JP2003202531A - Pulse delay quantity corrector and polarization mode dispersion compensator using the corrector - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a pulse delay amount corrector which can be manufactured at a low cost and miniaturized and a polarization mode dispersion compensator using the corrector. <P>SOLUTION: A polarization controller 12 and the pulse delay amount corrector 13 are formed on the same ferroelectric substrate 14 to obtain the polarization mode dispersion compensator 11. The corrector 13 is constituted by arraying TE-TM mode conversion parts 20 to 23 for converting a TE mode to a TM mode or vice versa along an optical waveguide 15 in series so as to convert the delayed TE mode into the TM mode and convert the advanced TM mode into the TE mode by using any one of the mode conversion parts 20 to 23. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a pulse delay amount compensator for compensating a group delay time difference caused by polarization mode dispersion in an optical transmission system, and a polarization mode dispersion compensator using this pulse delay amount compensator. Regarding vessels.

[0002]

2. Description of the Related Art In optical fiber communication, in order to cope with an explosive increase in communication volume, wavelength multiplexing communication (W) in which optical signals of many wavelengths are multiplexed and transmitted in one optical fiber.
DM) method is used. Furthermore, in order to cope with the increase in the amount of information, the modulation speed is changed from 2.5 GHz to 10 GHz.
The speed has been increased to GHz, and in the future 40 GHz, 8
Further increase in modulation speed is going to progress to 0 GHz.

When such a high-speed modulated optical signal is incident on an optical fiber for transmission, polarization mode dispersion (PMD:
Polarization Mode Dispersion) is known to occur. Usually, since the structure (cross-sectional shape) of the transmission optical fiber is not a perfect circle, birefringence of about 10 ⁻⁷ to 10 ⁻⁵ occurs. Polarization mode dispersion is a phenomenon in which two orthogonal polarization states are degenerated due to birefringence occurring in a transmission optical fiber, and the group velocity is different for each polarization. Although this difference in group velocity is very small, it is known that transmission characteristics are adversely affected by high-speed modulation (high-speed communication) or long-distance transmission.

At each optical frequency, there exists an orthogonal polarization state that minimizes the optical frequency dependence of the output orthogonal polarization state. This state is the PSP (Principal State of Polarizat).
ion) is called. This orthogonal polarization state changes every moment depending on the state of the optical fiber through which the optical signal propagates, the change in the outside temperature, the vibration, and the like. Further, since the group velocities are different between the two orthogonal polarizations, the arrival times of the two pulses (polarizations) are different from each other, and the waveform of the received signal is rounded. This waveform rounding of the received signal causes a decrease in bit error rate (BER).

Strictly speaking, the first-order PMD is a differential time delay (DGD) between two orthogonal polarizations at the output PSP.
Priority Group Delay). Since the PSP changes due to a disturbance such as a temperature change, the PMD value changes with time. Further, it is known that high-order (for example, second-order) PMD occurs when the PSP has frequency dependence.

The large time delay that occurs between different polarization components in an optical signal results in a large pulse spread when the optical signal is received. This is particularly remarkable in a system that transmits at a bit rate of 10 Gbs or more per transmission wavelength channel. For example, the spread of the pulse due to the group delay difference of about 20 ps causes closure of about 0.5 dB in the eye diagram of the received electric signal, resulting in a decrease in bit error rate (BER).
For high-speed long-distance transmission, a polarization mode dispersion compensator (PMDC: Polarization M
ode DispersionCompensator) is an urgent task.

As a method of processing a signal failure due to PMD of an optical fiber, an electrical equivalent method of signal distortion caused by PMD (IEEE. Photonics Technology Lett. Vol. 2, No.
8,519,1990) and the electrical equivalent method of the differential time delay in the received electric signal (IEEE.Photonics Technology Lett. Vol.
9, No1, 121, 1997). Such an electrical equivalent method
Only a relatively small delay time difference can be corrected. In addition, an expensive high-speed circuit device is required to execute these electrical equivalent methods.

On the other hand, a method of optically correcting the delay time difference has also been proposed. FIG. 7 is a block diagram showing a configuration for optically correcting the delay time difference. This configuration
It is disclosed in Japanese Patent Laid-Open No. 11-196046. 1
The light output from the 0 GB / s optical transmitter 1 is transmitted by the optical fiber 2 and input to the compensator 3. The compensator 3 corrects the delay time difference of the input light and outputs it. The corrected light is converted into an electric signal by the 10 GB / s optical receiver 4.

The compensator 3 comprises an automatic polarization transformer 5, a variable differential time delay line (pulse delay amount corrector) 6, an optical tap 7, and a distortion analyzer 8. The automatic polarization transformer 5 adjusts the two polarization directions of the PSP so that they coincide with the polarization main axis of the variable differential time delay line 6.
The variable differential time delay line 6 separates the output signal of the automatic polarization transformer 5 into two orthogonal polarization states for the two PSPs of the optical fiber 2 and delays the two polarization states by a variable amount τc. , The delay time difference generated in the optical fiber 2 is corrected.

Here, the operating principle of the variable differential time delay line 6 will be described. The signal light incident on the variable differential time delay line 6 has two orthogonal polarization components having a time difference τ with each other due to polarization mode dispersion. In the automatic polarization transformer 5, the polarization direction of the signal light is controlled so that the polarization direction of the polarization component arriving earlier than the time difference τ coincides with the fast axis direction of the variable differential time delay line 6. The light incident on the variable differential time delay line 6 receives the polarization separation prism 81.
Are separated into two polarized lights by the two, and the two separated polarized lights are guided to different optical paths 85 and 86, respectively. Time difference τ
The polarization components that arrive later are propagated along the optical path 85. On the other hand, the polarization component that arrives earlier by the time difference τ is
Propagates along the optical path 86, is reflected by the movable mirror 82, and propagates again along the optical path 86. The movable mirror 82 is configured so that its position can be moved by a motor (not shown).
The optical path 86 is adjusted by adjusting the position of the movable mirror 82.
It is possible to delay the time difference .tau.
The two polarization components propagating through the optical paths 85 and 86 are combined by the polarization combining prism 83. The time difference between the two polarization components of the multiplexed signal light is zero.
In this way, the variable differential time delay line 6 corrects the delay time difference of the input light.

Further, since the PSP of the optical fiber 2 varies with time, the two PSPs are adjusted according to the variation of the PSP.
Need to be corrected automatically. A part of the light emitted from the variable differential time delay line 6 is monitored by the optical tap 7 and given to the strain analyzer 8. Strain analyzer 8
Measures the distortion of the photocurrent and generates an electric output converted into a voltage Vc proportional to the distortion.

Assuming here that the respective delay time differences between the optical fiber 2 and the compensator 3 are τf and τc, the total delay time difference is expressed by the following equation (1). Τ total = r ・ √ (τf ^ 2 + τc ^ 2 + 2τf ・ τc ・ cos (2θ))… (1)

Here, 2θ is the PSP of the optical fiber 2.
And the so-called Stokes vector angle with respect to the PSP of the compensator 3 directly controlled by the automatic polarization transformer 5. When the angle 2θ is adjusted to a numerical value of ± π, so-called high-speed PSP and low-speed PS of the optical fiber 2
When P and the so-called high-speed PSP and low-speed PSP of the compensator 3 respectively match, the total delay time difference τ
It is clear that total is the minimum of | τf−τc |.

Thus, the adjustment of the angle θ is achieved by adjusting the automatic polarization transformer 5 in accordance with the value of the feedback voltage Vc supplied via the feedback path 9 so that the voltage Vc becomes the relative maximum value. be able to.
Similarly, the delay time difference τc in the compensating device 3 is such that τc is substantially equal to τf of the optical fiber 2, and thus
It can be adjusted according to the value of the feedback voltage Vc so that total becomes 0. As described above, if the direction and amount of the delay time difference generated by the automatic polarization transformer 5 and the variable difference time delay line 6 are adjusted by the above-described method, the distortion level of the optical signal output from the compensator 3 is minimized. Becomes

[0015]

In the above-mentioned conventional technique, the compensator 3 connects the automatic polarization transformer 5 and the variable differential time delay line (pulse delay amount corrector) 6 which are respectively configured as separate parts. Is configured. Therefore, there are problems that the manufacturing cost is high and the structure is large.

Further, in the variable differential time delay line 6, it is necessary to spatially separate the signal light into two polarization components and then finally combine them, which makes the optical axis adjustment and the like troublesome.
Further, in the variable differential time delay line 6, since the optical path length is mechanically changed, there is a mechanical sliding portion, there is a problem in high speed / reliability, and an optical path length adjusting device without a mechanical sliding portion is provided. Development is desired.

An object of the present invention is to provide a pulse delay compensator which can be manufactured at low cost and can be miniaturized, and a polarization mode dispersion compensator using the same.

[0018]

According to the present invention, an optical waveguide is formed on a ferroelectric substrate, and a plurality of mode converters for switching TE mode and TM mode are arranged in series along the optical waveguide. The TE mode delayed by any one of the plurality of mode conversion units is converted into the TM mode, and the advanced TM mode is converted into the TE mode.
A pulse delay compensator characterized in that a delayed TM mode and a advanced TE mode are output from the output end of the optical waveguide at the same timing by utilizing the characteristic of propagating slower than the M mode. .

According to the present invention, the light passing through the optical waveguide of the ferroelectric substrate utilizes the characteristic that the TE mode propagates slower than the TM mode, so that the delay time difference of the incident light, that is, T
The delay time difference between the E mode and the TM mode is corrected. It is known that the propagation speed of the TE mode is slower than that of the TM mode in the propagated light. Therefore, in the present invention, the TE mode and the TM mode are switched by converting the polarization state of the incident light at an appropriate portion of the optical waveguide. As a result, the delayed TE mode becomes the TM mode, the advanced TM mode becomes the TE mode, and the TM mode propagates fast and the TE mode propagates slowly while passing through the subsequent optical waveguide. The TE mode and the TM mode are output from the output end at the same timing. That is, if the position for switching the modes is selected based on the delay time difference between the TE mode and the TM mode in the incident light and the length of the optical waveguide, the TE mode and the TM mode are output from the output end of the optical waveguide at the same timing. Can be made.

Further, according to the present invention, each of the plurality of mode conversion parts is formed by arranging a plurality of domain regions along the light traveling direction in the optical waveguide, and the polarization direction is 180 between adjacent domain regions. And a field-applying means for applying an electric field to the optical waveguide in the periodic 180-degree domain-inverted structure in the direction orthogonal to the traveling direction. An electric field is applied to the optical waveguide in any one of the plurality of mode conversion units.

According to the present invention, the mode conversion section applies the electric field to the periodic 180 ° polarization inversion structure formed on the ferroelectric substrate and the optical waveguide in the periodic 180 ° polarization inversion structure. And means. Therefore, the pulse delay compensator can be realized with a relatively simple structure, can be manufactured at low cost, and can be miniaturized.

Further, according to the present invention, two optical waveguides are formed in parallel, a plurality of mode conversion parts are arranged in each optical waveguide, and the incident light is separated into a TE mode and a TM mode. Separating means for guiding to the optical waveguide,
It has a multiplexing means for multiplexing and outputting the TE mode and TM mode from the two optical waveguides, and the TE mode and TM
It is characterized by controlling the mode independently.

According to the invention, the incident light is TM mode and T
It is separated into E mode and given to two optical waveguides respectively, and after mode conversion is performed separately, they are multiplexed and output. Therefore, the position where the TM mode is converted to the TE mode and the position where the TE mode is converted to the TM mode can be set to different positions, and fine correction can be performed. As a result, the correction accuracy of the pulse delay amount corrector can be improved.

Further, in the present invention, the electric field applying means includes a pair of electrodes arranged on the main surface of the ferroelectric substrate at positions facing each other with the optical waveguide in the mode converting section interposed therebetween. Is characterized by.

According to the present invention, an electric field is applied to the optical waveguide by disposing, for example, a plate electrode on the main surface of the ferroelectric substrate. Therefore, the pulse delay compensator can be easily manufactured with a relatively simple structure.

Further, according to the present invention, the above-mentioned pulse delay amount corrector and the pulse delay amount corrector are arranged in front of the pulse delay amount corrector, and the polarization state of the incident light coincides with the main axis direction of the pulse delay amount corrector. A polarization mode dispersion compensator having a polarization controller, wherein the pulse delay compensator and the polarization controller are formed on the same ferroelectric substrate.

According to the present invention, the pulse delay compensator and the polarization controller are formed on the same ferroelectric substrate,
A polarization mode dispersion compensator is formed. Therefore, since the two configurations can be integrated on one substrate, the polarization mode dispersion compensator can be downsized.

In the present invention, the polarization controller is
A periodic 180-degree domain-inverted structure in which a plurality of domain regions are arranged along the light traveling direction in the optical waveguide, and the polarization directions are inverted by 180 degrees between adjacent domain regions, and the periodic 180-degree domain-inverted structure. The optical waveguide in the degree polarization inversion structure includes an electric field applying unit that applies an electric field in a direction orthogonal to the traveling direction, and the periodic 180
The polarization state of light passing through the optical waveguide is changed by applying an electric field to the optical waveguide in the polarization inversion structure.

According to the present invention, the polarization controller includes a periodic 180-degree polarization inversion structure formed on a ferroelectric substrate and an electric field application for applying an electric field to an optical waveguide in the periodic 180-degree polarization inversion structure. And means. Therefore, the polarization controller can be realized with a relatively simple structure, can be manufactured at low cost, and can be miniaturized. This makes it possible to reduce the manufacturing cost of the polarization mode dispersion compensator and reduce the size of the structure.

According to the present invention, the electric field applying means is arranged on the main surface of the ferroelectric substrate at a position opposed to each other with the optical waveguide in the periodic 180 ° polarization inversion structure interposed therebetween. It is characterized by including the electrode of.

According to the present invention, an electric field is applied to the optical waveguide by disposing, for example, a plate electrode on the main surface of the ferroelectric substrate. Therefore, the polarization controller can be easily manufactured with a relatively simple structure, can be manufactured at low cost, and can be miniaturized.
This makes it possible to reduce the manufacturing cost of the polarization mode dispersion compensator and reduce the size of the structure.

In the present invention, the polarization controller is
Further, it is characterized in that it comprises a phase adjusting means arranged before the periodic 180-degree domain-inverted structure and adjusting the phase of incident light.

According to the present invention, the optical signal incident on the periodic 180-degree domain-inverted structure is subjected to TE by the phase adjusting means.
The phase difference between the two modes, mode and TM mode, is adjusted. For example, the phase difference between the TE mode and the TE mode can be adjusted to be ± π / 2. As a result, the polarization state can be adjusted efficiently.

The present invention is also characterized in that the ferroelectric substrate is made of lithium niobate crystal.

According to the present invention, a high performance pulse delay compensator or polarization mode dispersion compensator can be realized.

[0036]

BEST MODE FOR CARRYING OUT THE INVENTION In order to correct a large DGD, it is necessary to form a pulse delay compensator using a material having a large birefringence so that a large differential delay time can be obtained. Generally, an x-axis cut y-axis propagation type or an x-axis cut z-axis propagation type lithium niobate crystal substrate is
Used in the polarization controller. Since the polarization controller using the x-axis cut z-axis propagation type lithium niobate crystal substrate does not feel birefringence, it is difficult to integrate it with a pulse delay compensator requiring large birefringence.
On the other hand, a polarization controller that uses an x-axis cut y-axis propagation type lithium niobate crystal substrate propagates an optical signal in the direction in which a large birefringence is felt, so it can be integrated with a pulse delay compensator. Is.

The polarization mode dispersion compensator of the present invention is realized by configuring the polarization controller and the pulse delay compensator on the same ferroelectric substrate. The polarization controller forms a periodic 180-degree polarization inversion structure (details will be described later) on a ferroelectric substrate, and opposes the main surface of this periodic 180-degree polarization inversion structure across an optical waveguide. A pair of flat plate-shaped electrodes are arranged at a position to perform the TE-TM mode conversion by applying an electric field between the pair of flat plate upper electrodes to control the polarization state. The pulse delay compensator comprises a plurality of periodic 180-degree domain-inverted structures on a ferroelectric substrate, and opposes the optical waveguide on the main surface of each periodic 180-degree domain-inverted structure. A pair of flat plate-shaped electrodes are arranged at respective positions, and any one of the periodic 18
TE-TM mode conversion is performed by applying an electric field between a pair of flat plate electrodes arranged in a 0-degree polarization inversion structure, and two orthogonal polarizations are exchanged to correct the time difference between two polarization components.

FIG. 1 is a perspective view showing the configuration of a polarization mode dispersion compensator 11 according to the first embodiment of the present invention. The polarization mode dispersion compensator 11 is realized by configuring the polarization controller 12 and the pulse delay compensator 13 on a ferroelectric substrate 14 made of an x-axis cut lithium niobate crystal. On the main surface of the x-axis cut ferroelectric substrate 14, an optical waveguide 15 having a single axis at a wavelength of 1.55 μm and having a y-axis as a propagation direction is formed by a titanium diffusion method.

The polarization controller 12 includes a phase adjusting unit 16
And a TE-TM mode conversion section 17 at least one unit. The phase adjusting unit 16 includes a pair of flat plate-shaped electrodes 18 a, 1 on the main surface of the ferroelectric substrate 14.
8b is arranged, and a pair of flat plate-shaped electrodes 18a, 18 is formed so that an electric field is applied to the optical waveguide 15 in the z-axis direction.
One electrode 18b of b is arranged immediately above the optical waveguide 15. The TE-TM mode conversion unit 17 forms a periodic 180-degree domain-inverted structure on the ferroelectric substrate 14, and a pair of flat plate-shaped electrodes 19a and 19b is formed on the main surface of this periodic 180-degree domain-inverted structure. The optical waveguide 15 is arranged and configured.
The pair of flat plate-shaped electrodes 19a and 19b are arranged so as to face each other with the optical waveguide 15 interposed therebetween so that an electric field is applied to the x-axis direction. What is a periodic 180 degree polarization inversion structure?
This is a structure in which a plurality of domain regions D1 and D2 are arranged along the light traveling direction in the optical waveguide 15, and the polarization direction is inverted by 180 degrees between adjacent domain regions. The polarization direction is the z-axis direction of the ferroelectric substrate 14 made of lithium niobate crystal, and is set in the direction orthogonal to the light traveling direction.

The phase adjustment unit 16 and the TE-TM mode conversion unit 17 are treated as one unit, and a polarization controller normally composed of one unit requires several reset cycles during use. However, this will damage the communication data. In order to avoid such a situation and to be reset-free, as shown in US Pat. No. 4,966,431, a unit including a phase adjusting unit 16 and a TE-TM mode converting unit 17 is arranged in a plurality of stages in series. For simplicity, only one stage is used.

By the way, the polarization state of the optical signal input from the optical fiber to the polarization controller 12 varies from moment to moment. Therefore, it is necessary to rotate the polarization state of the optical signal in accordance with this variation in the fast axis and slow axis directions of the pulse delay amount corrector 13 in the subsequent stage. At this time, in order to completely rotate the polarization state,
It is necessary to adjust the phase by the phase adjusting unit 16. In order to adjust the phase of the optical signal, it is necessary to detect the polarization state of the input optical signal moment by moment. Therefore, normally, a polarization analyzer (not shown) for detecting the polarization state is provided in front of the polarization mode dispersion compensator 11 (polarization controller 12).
Is provided. For the polarization analyzer, see "DW
DM optical measurement technology "
B., 2001).

The polarization analyzer detects the polarization state of the passing optical signal without changing the polarization state. The polarization analyzer can detect the directions of the two principal axes of polarization and their phase differences. The phase adjustment unit 16 can adjust the phase by feeding back the phase difference signal from the polarization analyzer to the phase adjustment unit 16 of the polarization controller 12 in accordance with the polarization state which changes every moment. The necessity of the phase adjusting unit 16 is described in USP 4384760.
The E-TM mode converter 17 will be described.

In the ferroelectric substrate 14, the refractive index in the x-axis direction and the y-axis direction becomes the refractive index no of the ordinary ray, and the refractive index in the polarization axis direction (z-axis direction) is the refractive index ne of the extraordinary ray.
However, both refractive indices have different values. Therefore,
Regarding the effective refractive index felt by the optical waveguide 15, the light polarized in the x-axis direction, that is, the ordinary ray (TE mode) is
Light having a refractive index nTE in the x-axis direction and polarized in the z-axis direction, that is, an extraordinary ray (TM mode), has a refractive index nTM in the z-axis direction. That is, since the light propagates in the direction in which the birefringence is strongly felt, the propagation velocities (phase velocities) of the two when propagating through the optical waveguide 15 are different, which causes a phase difference between the two. If this phase difference remains generated, it is not possible to obtain a change in the polarization state described later, so it is necessary to match the phase velocities of the two and the group velocities in the case of pulses (phase matching). The principle of phase matching will be described below.

X throughout the ferroelectric substrate 14
A uniform electric field Ex is generated only in the axial direction (Ex ≠ 0,
Ey = Ez = 0). Due to the electro-optic effect generated by this electric field Ex, in the domain regions D1 and D2, as shown in FIG.
As shown in FIGS. 2A and 2B, the ferroelectric substrate 1
The principal axis direction of the index ellipsoid of 4 rotates in the xz plane.
Here, in the domain area D1 and the domain area D2,
Since the crystal axes of both are inverted by 180 degrees, the rotation directions of the main axes due to the electric field application are opposite to each other, and the x-axis direction components of the main axes due to this rotation are additively coupled. This rotation angle θ is given by the following known equation. θ = (1/2) tan ⁻¹ {2 × r51 × Ex / (ne-2-no-2)} (2) (where r51 is an electro-optic constant)

Although the rotation of this main shaft is usually slight,
The lithium niobate crystal has a relatively large nonlinear optical constant r.
Since it has 51, a large rotation angle can be realized. That is, when the principal axis of the index ellipsoid is rotated by the applied electric field Ex, two light waves (light waves in the x-axis direction and the z-axis direction) whose polarization planes are orthogonal to each other are coupled, whereby the TE-TM mode is obtained. The conversion will take place. This coupling coefficient κ is approximately represented by the following equation. κ = (π / λ) × n3 × r51 × Ex (3) (where λ is the wavelength of the propagating optical signal)

Here, in the domain region D1 and the domain region D2, since the crystal axes of both are inverted by 180 degrees, the rotation directions of the main axes due to the application of the electric field are opposite to each other, and the main axes due to this rotation are in the x-axis direction. The components are combined additively.

Both light waves whose polarization planes are orthogonal to each other have different propagation constants nTE and nTM in the optical waveguide, so that the interaction length between them is short and the coupling between them is only slight. However, by alternately repeating the domain regions D1 and D2 whose polarization directions are inverted by 180 degrees and arranging them in the propagation direction of the optical signal, phase matching can be achieved by adjusting the repetition period. . Such a repeating periodic structure functions as a grating because the refractive index is also modulated when a voltage is applied. As a result, both polarized lights can be combined at 100%.

The power transfer rate (ratio of phase matching) η between both polarized lights by the grating is represented by the following known equation. η = sin ² {(| κ | ² + Δ ² ) (1/2) × L} / {1+ (Δ ² / κ ² )} (4) where Δ = π (nTE-nTM) / λ- (π / Λ) (5), where Λ is the grating period.

From the above equations (4) and (5), when Δ = 0 at the wavelength λ, 100% power transfer is performed, and the wavelength λp at that time is λp = Λ (nTE-nTM) (6) ).

That is, the power transfer between the two polarized lights has a wavelength band that can be realized by the equation (4) centering on the wavelength λp with respect to the wavelength of the design wavelength λp, and is 100 at the wavelength λp only. % Power transfer is possible.

As a result, as long as the optical signal has a wavelength of λ, both polarized light beams make a power transition with each other, and the complete coupling length Lp at that time is Lp = π / 2 |
κ ｜…
(7), and when propagating this length, complete power (intensity) transition, that is, phase matching occurs.

For example, when the optical signal is linearly polarized light having a plane of polarization in the z-axis direction (TM mode) and the incident light wavelength is λp (if the above expression (6) is satisfied), the electric field E is obtained.
When there is no x, the linearly polarized light in the z-axis direction (TM
Mode) is emitted, but when the electric field Ex increases, the emitted light is gradually coupled, and starts to rotate from the polarized wave in the z-axis direction to the x-axis while maintaining the linear polarized wave. When the condition that satisfies the expression (7) is satisfied, the emitted light is converted into linearly polarized light (TE mode) having a polarization plane in the x-axis direction, and 1
00% coupling (phase coupling). When the optical signal is linearly polarized light having a polarization plane in the x-axis direction (TE mode), the emitted light is linearly polarized light having a polarization plane in the z-axis direction (TM).
Mode).

Therefore, the light incident on the polarization controller 12 may have any polarization regardless of the polarization, and the phase adjusting unit 16 and the TE-TM mode converting unit 1 of the polarization controller 12 may be used.
7 makes the polarization direction of the linearly polarized light coincident with the z-axis direction and the x-axis direction of the crystal. Polarization controller 1
The light passing through 2 is incident on the pulse delay amount corrector 13 at the subsequent stage.

The pulse delay compensator 13 has four TE-
The TM mode converters 20, 21, 22, and 23 are connected to the optical waveguide 1
5 are arranged in series in series. The TE-TM mode converters 20 to 23 have a pair of flat plate-shaped electrodes 24a, 24b; 25a, 2 on the main surface of the periodic 180-degree domain-inverted structure.
5b; 26a, 26b; 27a, 27b are arranged, and a pair of flat plate-shaped electrodes 24a, 24b; 25a, 25 are arranged so that an electric field is applied to the optical waveguide 15 in the x-axis direction.
b; 26a, 26b; 27a, 27b are arranged at positions facing each other with the optical waveguide 15 in between.

Since the ferroelectric substrate 14 has birefringence in the propagation direction, the pulse delay compensator 13 has different propagation velocities between the TE mode polarized light and the TM mode polarized light (Δτ to 0.2 ps / mm). ). Therefore, 4 TE
The TE mode polarized light and the TM mode polarized light can be switched by applying an electric field to one of the -TM mode converters 20 to 23, for example, the plate electrodes 25a and 25b in FIG. . That is, when light passes through the TE-TM mode converter 21 to which an electric field is applied, TE mode polarized light is converted into TM mode polarized light and TM mode polarized light is converted into TE mode polarized light. 4 TE
-The distances between the TM mode converters 20 to 23 and the output end of the optical waveguide 15 are different from each other.
By selecting the TM mode converter, the optical waveguide 1
At the output of 5, the delay time difference between the two polarizations can be adjusted.

As a method of selecting the TE-TM mode converter to be operated, various methods can be adopted. For example, the optimum TE-TM mode conversion unit may be selected by detecting the delay time between the TE mode and the TM mode of the incident light, or the optical signal output from the pulse delay amount corrector 13 may be detected. It is also possible to convert a part of the signal into an electric signal, analyze a so-called eye pattern of the electric signal, and select a TE-TM mode converter to be operated so that the waveform distortion is minimized.

Here, a series of operating characteristics will be described in the pulse state. Pulses transmitted through an optical fiber 30,
40 is output from the optical fiber in a polarization state (SOP) orthogonal to each other due to birefringence of the optical fiber. There is a delay time τ between these pulses 30, 40. These pulses 30 and 40 are generated by the polarization controller 12 in the first stage and are output from the pulse delay amount corrector 13 in the second stage.
It is controlled so as to match the two polarization main axis directions (x-axis direction and z-axis direction). Here, the pulse 30 is controlled at the output end of the polarization controller 12 to the pulse 31 of the TE polarization mode in the x-axis direction, and the pulse 40 is controlled to the pulse 41 of the TM polarization mode in the z-axis direction. These pulses 31,
Reference numeral 41 is incident on the pulse delay compensator 13 and applies a voltage to, for example, the flat plate-shaped electrodes 25a and 25b to generate TE-TM.
By operating the mode converter 21, the TE
The polarization states are switched at the end of the -TM mode converter 21. That is, the TE polarization mode and the TM polarization mode are switched. After that, the pulse 32 that became the TE polarization mode
Propagates in the optical waveguide 15 at a slower speed than the pulse 42 in the TM polarization mode. Therefore, at the end of the optical waveguide 15, the pulse 32 and the pulse 42 are simultaneously output. Therefore, by appropriately selecting the TE-TM mode converter to be operated, the delay time between two pulses can be corrected to 0 at the end of the optical waveguide 15.

As described above, according to this embodiment, since the polarization controller 12 and the pulse delay compensator 13 are formed on the same ferroelectric substrate 14, the polarization mode dispersion compensation is low in cost and small in size. The device 11 can be realized.

Further, by configuring the polarization controller 12 and the pulse delay compensator 13 on the same ferroelectric substrate 14 so as to correspond to different wavelengths, the multi-channel polarization mode can be produced at a lower cost. A dispersion compensator can be realized. If the wavelength channels are closely spaced, the polarization mode dispersion compensator 11 will be affected by temperature. It is desirable to control the temperature of the ferroelectric substrate 14 in order to minimize changes in characteristics due to such temperature fluctuations. The temperature control is preferably performed by mounting the ferroelectric substrate 14 on a copper plate arranged on the Peltier temperature control element and using the Peltier temperature control element, for example.

In the embodiment shown in FIG. 1, the pulse delay compensator 13 has four TE-TM mode converters 20 to 23.
In this configuration, the DGD delay time can be controlled in four stages. When actually using the DG
It is most desirable that the adjustment amount of the D delay time can be controlled continuously, and if it is controlled stepwise, it is desirable that it can be controlled as finely as possible. In order to finely control the DGD delay time, it is necessary to increase the number of TE-TM mode conversion units. However, an increase in the number of TE-TM mode conversion units will increase the length of the pulse delay amount corrector 13. Further, in order to finely control the DGD delay time with a constant length without increasing the length of the pulse delay amount corrector 13, it is necessary to increase the voltage applied to the TE-TM mode converter. This is because mode conversion can be performed in a short distance when the applied voltage is large. However, it is desirable that the driving voltage is as low as possible.

Therefore, if the TE polarization mode and the TM polarization mode are separated and the modes are independently converted, it is possible to control the fine delay time at a low voltage without increasing the configuration (length). is there.

FIG. 3 is a perspective view showing the configuration of the polarization mode dispersion compensator 51 according to the second embodiment of the present invention. The polarization mode dispersion compensator 51 of the present embodiment includes a polarization controller 52, a pulse delay amount corrector 53, and a waveguide type polarization demultiplexer interposed between the polarization controller 52 and the pulse delay amount corrector 53. It is realized by configuring the device 54 and the waveguide-type multiplexer 55 arranged in the subsequent stage of the pulse delay compensator 53 on the ferroelectric substrate 56 made of an x-axis cut lithium niobate crystal. On the main surface of the x-axis cut ferroelectric substrate 56, two optical waveguides 57a and 57b which have a y-axis as a propagation direction and are in a single mode with respect to a wavelength of 1.55 μm are formed by a titanium diffusion method. It is formed in parallel.

The polarization controller 52 includes a phase adjusting section 58.
And a TE-TM mode conversion section 59. The phase adjuster 58 includes a pair of flat plate-shaped electrodes 60 a, 6 on the main surface of the ferroelectric substrate 56.
0b are arranged, and the pair of flat plate-shaped electrodes 60a and 6a are arranged so that an electric field is applied to the optical waveguide 57a in the z-axis direction.
One electrode 60b of 0b is arranged directly above the optical waveguide 27a.

The TE-TM mode converter 59 has a periodic 180-degree domain-inverted structure on the ferroelectric substrate 56.
A pair of flat plate-shaped electrodes 61a and 61b are arranged on the main surface of this periodic 180-degree domain-inverted structure.
The pair of flat plate-shaped electrodes 61a and 61b are arranged at positions facing each other with the optical waveguide 57a interposed therebetween so that an electric field is applied to the 7a in the x-axis direction. The optical waveguide 57b is not particularly used.

Vertical polarization (TM mode) perpendicular to the main surface of the ferroelectric substrate 56 by the polarization controller 52.
The optical pulse whose polarization direction is adjusted to horizontal polarized light (TE mode) is incident on the waveguide type polarization separator 54. The waveguide type polarization separator 54 is realized by, for example, a polarization beam splitter (PBS), separates an optical pulse into a TE mode and a TM mode, and converts the TE mode into an optical waveguide 62a.
Then, the TM mode is propagated to the optical waveguide 62b.
The optical waveguides 62a and 62b are formed in the same manner as the optical waveguides 57a and 57b, and are formed in parallel with the y-axis direction of the ferroelectric substrate 56 as the propagation direction.

The pulse delay compensator 53 includes an optical waveguide 62.
a and 62b each have a plurality of TE-TM mode converters 6
3a, 64a, 65a; 63b, 64b, 65b are arranged in series.

Each TE arranged along the optical waveguide 62a
The -TM mode converters 63a, 64a, 65a are located on the main surface of the ferroelectric substrate 56 having the periodic 180-degree domain-inverted structure and are opposed to each other with the optical waveguide 62a interposed therebetween so that an electric field is generated in the x-axis direction. A pair of flat plate electrodes 66a, 6a
6m; 67a, 67m; 68a, 68m. In addition, each TE-TM mode conversion unit 63b, 64b, 65b arranged along the optical waveguide 62b is also
Ferroelectric substrate 56 having periodic 180 degree polarization inversion structure
On the main surface of the optical waveguide 6 so that an electric field is generated in the x-axis direction.
A pair of flat plate-shaped electrodes 66 are provided at positions facing each other with 2b interposed therebetween.
b, 66m; 67b, 67m; 68b, 68m. The flat plate electrodes 66m, 67m,
68 m is shared by the optical waveguide 62 a and the optical waveguide 62 b.

The TE mode passing through the optical waveguide 62a is
The TM mode is converted by applying an electric field to one of the TE-TM mode converters 63a, 64a and 65a, for example, the plate electrodes 66a and 66m. on the other hand,
The TM mode passing through the optical waveguide 62b is converted into the TE mode by applying an electric field to one of the TE-TM mode converters 63b, 64b, 65b, for example, the plate electrodes 68b, 69m. Since there is a difference in propagation speed between the TE mode and the TM mode, T
The delay time can be controlled by appropriately selecting the E-TM mode converter.

The converted TE mode and TM mode are combined by the waveguide type multiplexer 55, and the optical waveguide 6
It is output from the output terminal of 9a. Although the optical waveguide 69b is formed in parallel with the optical waveguide 69a, this optical waveguide 69b is not particularly used.

Also in this embodiment, the same effect as in the first embodiment can be obtained. Further, since the TE mode and the TM mode separated by the waveguide type separator 54 are converted into the TM mode and the TE mode, respectively, at independent positions, finer DGD correction is possible.

FIG. 4 is a perspective view showing the configuration of the polarization mode dispersion compensator 71 which is the third embodiment of the present invention. Since this embodiment is similar to the above-described second embodiment, the same components are designated by the same reference numerals, and detailed description thereof will be omitted. Further, it goes without saying that the same effects as those of the second embodiment can be obtained in this embodiment as well.

The feature of this embodiment is that the periodic 180-degree domain-inverted structure is intermittently formed at regular intervals in the above-described second embodiment, but is uniformly continuous in this embodiment. That is what I configured. As a result, the number of TE-TM mode converters that can be formed for the same optical waveguide length can be increased as compared with the second embodiment. In the polarization mode dispersion compensator 71, the two optical waveguides 62a, 6a
4th TE-TM mode conversion parts 72a and 72b are formed for 2b, respectively.

Further, in the polarization mode dispersion compensator 71, one strip-shaped common electrode 73 is arranged between the optical waveguides 62a and 62b, and the flat plate-shaped electrodes 66a, 67a, 68a are sandwiched between the optical waveguides 62a. , 74a are placed and TE
-TM mode converters 63a, 64a, 65a, 72a are constituted, and plate electrodes 66b, 67b, 68b, 74b are arranged at positions sandwiching the optical waveguide 62b to form a TE-TM.
The mode converters 63b, 64b, 65b, 72b are configured. As a result, the number of electrodes to be formed can be reduced and the manufacturing cost can be reduced.

Since the operating principle of this embodiment is the same as that of the second embodiment, its explanation is omitted.

In this embodiment, as compared with the second embodiment,
If the performance (adjustment amount) is the same, the configuration can be downsized, and if the size is the same, the number of TE-TM mode conversion units can be increased, so the performance (adjustment amount) is improved. Is possible.

FIG. 5 is a plan view showing the structure of the waveguide type polarization separator 54. Crossing length L of two optical waveguides 57a, 57b; 62a, 62b configured with an opening angle of angle θ1
When c is changed, the polarization characteristic as shown in FIG. 6 is obtained. Therefore, in order to configure the waveguide type polarization separator 54, the distance Lc1 at which TE is maximum and TM is minimum may be set as the cross length. The waveguide multiplexer 55 may be formed according to the same principle as that of the waveguide polarization separator 54.

[0077]

As described above, according to the present invention, the light passing through the optical waveguide of the ferroelectric substrate utilizes the characteristic that the TE mode propagates slower than the TM mode. The delay time difference between the TE mode and the TM mode is corrected. That is, TE mode and TM in incident light
If the position for switching the modes is selected based on the delay time difference from the mode and the length of the optical waveguide, the TE mode and the TM mode can be output from the output end of the optical waveguide at the same timing.

Further, according to the present invention, the mode conversion section has an electric field for applying an electric field to the periodic 180 ° polarization inversion structure formed in the ferroelectric substrate and the optical waveguide in the periodic 180 ° polarization inversion structure. The pulse delay compensator can be realized with a relatively simple structure because it is composed of an applying means, and can be manufactured at low cost and can be miniaturized.

Further, according to the present invention, the position for converting the TM mode into the TE mode and the position for converting the TE mode into the TM mode can be set at different positions, and fine correction can be performed. As a result, the correction accuracy of the pulse delay amount corrector can be improved.

Further, according to the present invention, an electric field is applied to the optical waveguide by disposing, for example, a plate electrode on the main surface of the ferroelectric substrate, so that the pulse delay amount can be easily achieved with a relatively simple structure. A compensator can be manufactured.

Further, according to the present invention, the pulse delay compensator and the polarization controller can be integrated on one ferroelectric substrate, so that the polarization mode dispersion compensator can be miniaturized.

Further, according to the present invention, the polarization controller includes an electric field for applying an electric field to the periodic 180 ° polarization inversion structure formed on the ferroelectric substrate and the optical waveguide in the periodic 180 ° polarization inversion structure. The polarization controller can be realized with a relatively simple structure, can be manufactured at low cost, and can be miniaturized. This makes it possible to reduce the manufacturing cost of the polarization mode dispersion compensator and reduce the size of the structure.

Further, according to the present invention, an electric field is applied to the optical waveguide by disposing, for example, a plate electrode on the main surface of the ferroelectric substrate, so that the polarization controller can be easily constructed with a relatively simple structure. Can be manufactured at a low cost, and the size can be reduced. This makes it possible to reduce the manufacturing cost of the polarization mode dispersion compensator and reduce the size of the structure.

Further, according to the present invention, the phase difference between the two modes of the TE mode and the TM mode of the optical signal incident on the periodic 180 degree polarization inversion structure is adjusted by the phase adjusting means. The polarization state can be adjusted well.

Further, according to the present invention, a pulse delay amount corrector or a polarization mode dispersion compensator having good performance can be realized.

[Brief description of drawings]

FIG. 1 is a perspective view showing a configuration of a polarization mode dispersion compensator 11 which is a first embodiment of the present invention.

FIG. 2 is a diagram showing rotation of a refractive index ellipsoid in domain regions D1 and D2 in a main axis direction.

FIG. 3 is a perspective view showing a configuration of a polarization mode dispersion compensator 51 which is a second embodiment of the present invention.

FIG. 4 is a perspective view showing a configuration of a polarization mode dispersion compensator 71 which is a third embodiment of the present invention.

FIG. 5 is a plan view showing a configuration of a waveguide type polarization separator used in the second and third embodiments.

FIG. 6 is a graph showing polarization characteristics when the cross length Lc of two optical waveguides 57a, 57b; 62a, 62b configured with an opening angle of θ1 is changed.

FIG. 7 is a block diagram showing a configuration for optically correcting a delay time difference.

[Explanation of symbols]

11, 51, 71 Polarization mode dispersion compensator 12, 52 Polarization controller 13, 53 Pulse delay amount corrector 14, 56 Ferroelectric substrate 15, 57a, 57b, 62a, 62b, 69a, 69
b Optical Waveguide 16, 58 Phase Adjusting Section 17, 20-23, 59, 63a-65a, 72a, 6
3b to 65b, 72b TE-TM mode converters 18a, 19a, 24a to 27a, 60a, 61a, 6
6a to 68a, 74a, 18b, 19b, 24b to 27
b, 60b, 61b, 66b to 68b, 74b, 66
m, 67 m, 68 m flat plate electrode 54 waveguide type polarization separator 55 waveguide type multiplexer 73 common electrode D1, D2 domain region

Claims (9)

[Claims] 1. An optical waveguide is formed on a ferroelectric substrate, and a plurality of mode converters for switching the TE mode and the TM mode are arranged in series along the optical waveguide. The TE mode delayed by any one of the modes is converted into the TM mode, and the advanced TM mode is converted into the TE mode, and the TE mode uses the characteristic of propagating slower than the TM mode. Then, the delayed TM mode and the advanced TE mode are output from the output end of the optical waveguide at the same timing. 2. The plurality of mode conversion parts are each formed by arranging a plurality of domain regions along the light traveling direction in the optical waveguide, and the polarization directions are inverted by 180 degrees between adjacent domain regions. A periodic 180 degree polarization inversion structure, and an electric field applying means for applying an electric field to the optical waveguide in the periodic 180 degree polarization inversion structure in a direction orthogonal to the traveling direction, respectively. The pulse delay compensator according to claim 1, wherein an electric field is applied to an optical waveguide in any one of the mode conversion units. 3. The two optical waveguides are formed by forming two optical waveguides in parallel and disposing a plurality of mode converters in each optical waveguide, and further separating incident light into a TE mode and a TM mode. And a multiplexing unit that multiplexes and outputs the TE mode and the TM mode from the two optical waveguides, and independently controls the TE mode and the TM mode. The pulse delay amount corrector according to claim 1 or 2. 4. The electric field applying means includes a pair of electrodes arranged on the main surface of the ferroelectric substrate at positions facing each other with an optical waveguide in the mode converting section interposed therebetween. The pulse delay amount corrector according to claim 2 or 3. 5. The pulse delay amount corrector according to any one of claims 1 to 4, and the pulse delay amount corrector arranged before the pulse delay amount corrector and adapted to change the polarization state of incident light. A polarization controller that matches the main axis direction of the pulse delay amount corrector and the polarization controller,
A polarization mode dispersion compensator formed on the same ferroelectric substrate. 6. The polarization controller is formed by arranging a plurality of domain regions along a traveling direction of light in the optical waveguide, and the polarization direction is inverted by 180 degrees between adjacent domain regions. A 180 degree polarization inversion structure, and an electric field applying means for applying an electric field to the optical waveguide in the periodic 180 degree polarization inversion structure in a direction orthogonal to the traveling direction. The polarization mode dispersion compensator according to claim 5, wherein the polarization state of light passing through the optical waveguide is changed by applying an electric field to the optical waveguide in the structure. 7. The electric field applying means includes a pair of electrodes arranged on the main surface of the ferroelectric substrate at positions facing each other with an optical waveguide in the periodic 180 ° polarization inversion structure interposed therebetween. 7. The polarization mode dispersion compensator according to claim 6, further comprising: 8. The polarization controller further includes a phase adjusting unit that is arranged in front of the periodic 180-degree polarization inversion structure and that adjusts the phase of incident light. 7. The polarization mode dispersion compensator according to 7. 9. The pulse delay compensator or the polarization mode dispersion compensator according to claim 1, wherein the ferroelectric substrate is made of a lithium niobate crystal.