JPH06186514A - Polarization controller - Google Patents

Abstract

PURPOSE:To produce a polarization controller with only one magneto-optical material or element and with simple constitution by using a Cotton-Mouton effect. CONSTITUTION:An arbitrary polarized state can be converted to a known polarized state (for example, linear polarization) by using only one magneto-optical material or element by using the Cotton-Mouton effect that is one of magneto- optical effects. In other words, a cylindrical magneto-optical material or element 1 is arranged in electromagnetic coils 2, 3 which generate two pairs of transverse magnetic fields in a plane perpendicular to the longitudinal axis. The size of magnetization M and an angle (phi) in the plane perpendicular to the advancing direction of light can be controlled by changing a current on the two pairs of electromagnetic coils 2, 3 intersecting orthogonally to each other, and incident light whose polarized state is unknown is converted to arbitrary prescribed polarized light. Therefore, only electric control can be applied, and no mechanical driving is required, which enables simple constitution to be taken by employing such constitution.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a polarization control device for converting incident light whose polarization state is unknown to any known predetermined polarization.

[0002]

2. Description of the Related Art It is known that the polarization state of the propagation mode of light passing through a single mode fiber becomes unstable due to the disturbance of the structure of the fiber and the environment.
This is an obstacle when applying an optical fiber to optical communication or optical measurement. This is a serious problem especially when the output light from the fiber is to be coupled to a waveguide device having a large polarization dependence. The polarization control device of the present invention is a device that solves these problems.

The following three elements are used to control the polarization state.
Two groups have been proposed (for details, see "Coherent Optical Communication Engineering" written by Takataka Ohgoshi and Kazuro Kikuchi, published by Ohmsha). 1. Type 1 (fiber pressurizer, electro-optic crystal) 1. Second class (rotary fiber coil, rotary phase plate, rotary fiber crank) 3. Class 3 (Faraday rotator) Among these, the one using the Faraday rotator which is an element utilizing the same magneto-optical effect as the present invention will be described.

FIG. 6 shows the structure and changes in the polarization state.
FIG. 7 shows the operation on the Poincare sphere. Two Faraday rotators 61 and 62 and one quarter wave plate 63 are used. The elliptically polarized light (A) incident on the first Faraday rotator 61 is converted into elliptically polarized light (B) with its principal axis upright by controlling the magnetic field with the electromagnet coil 64 there (see FIG. 6B). . This conversion corresponds to rotating point A to point B around the axis connecting the sphere center (O) and the north pole (N) on the Poincare sphere in FIG. On the Poincare sphere, the central axis for obtaining the polarization path when the Faraday effect is used is the axis connecting the spherical center (O) and the pole, and the central axis for obtaining the polarization path when the birefringence is used is the sphere. It is an axis connecting the center (O) and a point on the equator, and the center axis when the two are mixed is the axis connecting the sphere center (O) and the point on the equator and the point between the poles.

Next, when the light passes through the quarter-wave plate 63, the elliptically polarized light (B) becomes linearly polarized light (C) with its principal axis inclined (FIG. 6).
(See (b)). On the Poincare sphere of FIG. 7, the point B is π / with the axis connecting the point O and the point (D) representing horizontal polarization as the center.
It will rotate by 2.

Finally, the second Faraday rotator 62 converts it into a horizontally polarized wave (D) (see FIG. 6B), but on the Poincaré sphere of FIG. 7, the O point and the south pole point (S).
Corresponds to the rotation around the axis connecting and. As described above, the conventional magneto-optical polarization controller has a complicated structure including three optical elements.

There are also other problems such as the problem that two electro-optic crystals are used in the above-mentioned first class and the need for mechanical driving in the second class. .

Therefore, it is an object of the present invention to provide a polarization controller having a simple structure for converting incident light whose polarization state is unknown to arbitrary known predetermined polarization.

[0009]

In the polarization control device according to the present invention, the Cotton-Mouton effect, which is one of the magneto-optical effects, is used, so that only one magneto-optical material or element is used to obtain an arbitrary polarization state. Is converted to a known polarization state (for example, linearly polarized light). Furthermore, in the present invention, only electrical control is required, and mechanical drive is not required.

Specifically, it has a means for controlling the direction and strength of the magnetic field applied to the magneto-optical element in a plane perpendicular to the traveling direction of light, or an electromagnet for generating two sets of orthogonal magnetic fields, and a cotton. · A magneto-optical element having the Mouton effect and a feedback circuit for detecting a part of emitted light to control the ratio and intensity of the orthogonal magnetic field, or the magneto-optical element made of a magnetic garnet material or a magnetic semiconductor material. The magneto-optical element may have a waveguide structure in which magneto-optical thin films having different compositions are laminated.

In the optical communication system according to the present invention,
In the wavelength division multiplexing optical communication integrated node according to the present invention, the polarization control device and the single wavelength optical fiber are used, and the waveguide polarization control device and the waveguide wavelength filter are formed on the same substrate. It is characterized by having done.

The cotton-Mouton effect will be described below. The Cotton-Mouton effect is a magneto-optical effect when the traveling direction of light is perpendicular to the magnetic field. When there is no magnetization, birefringence does not occur in an isotropic material, but when magnetization M is present, uniaxial anisotropy is induced in the direction of M, and linearly polarized light (ordinary ray) vibrating in the M direction and perpendicular to M A birefringence occurs due to a difference in refractive index between the light vibrating in the direction of (extraordinary ray). This phenomenon is the Cotton-Mouton effect.

4 and 5 illustrate the principle of the present invention. In FIG. 4, consider a point (A) representing an arbitrary elliptical polarization and a point (D) representing horizontal polarization on the Poincare sphere. To directly convert from A point to D point, A point and D point
It suffices to rotate the point A on the spherical surface by an angle Φ about the axis connecting the point P, which is equidistant from the point and on the equator, and the center point (O) of the sphere. To this end, the angle between the axis OD and the axis OP is set to 2φ, and the main axis of birefringence is set to the angle φ from the horizontal direction.
Just tilt it.

In order to provide such birefringence by the Cotton-Mouton effect, the magnetization M may be tilted by an angle φ from the horizontal direction in a plane perpendicular to the traveling direction of light as shown in FIG. Further, assuming that the element length is l and the rotation angle per unit length due to the Cotton-Mouton effect is θ _M , FIG.
The rotation angle Φ at is given by Φ = θ _M l.

In order to convert an arbitrary elliptically polarized wave into a horizontally polarized wave, it is sufficient that Φ = π when the maximum magnetic field is applied. That is, any elliptical polarization can be converted into horizontal polarization as long as the rotation angle Φ is at maximum π. Therefore, the rotation angle per unit length due to the Cotton-Mouton effect under the maximum magnetic field is θ
_The element length required for _Mmax must be l = π / θ _Mmax . In the case of Bi-doped garnet single crystal (ferromagnetic material), θ _Mmax = 1000 degrees / cm, so that the device length is 0.18 mm.

A magneto-optical element satisfying the above conditions is shown in FIG.
When the elliptically polarized wave given at point A is entered, the outgoing light becomes horizontal polarized wave shown at point D. The magnitude of θ _M is the magnetization M
, And the magnitude and direction of the magnetization M of the magneto-optical material depends on the external magnetic field. Therefore, by controlling the magnitude and direction of the external magnetic field, the magnitude of θ _M and the direction of the birefringence axis can be changed, and arbitrary elliptical polarization can be converted into horizontal polarization.

[0017]

Embodiment 1 FIG. 1 shows a first embodiment of the present invention, (a) is a front view and (b) is a perspective view of a magneto-optical material 1. A cylindrical magneto-optical material or element 1 is installed in electromagnet coils 2 and 3 where two sets of orthogonal magnetic fields are generated in a plane perpendicular to the major axis thereof. The magnitude of the magnetization M and the angle φ in the plane perpendicular to the traveling direction of light are controlled by changing the currents flowing in the two sets of the electromagnet coils (Hy coil 3 and Hz coil 2) that are orthogonal to each other. Here, the magneto-optical material 1 has a long wavelength (1.
For 3 μm and 1.55 μm), a YIG (yttrium / iron / garnet) single crystal, which is one type of magnetic garnet, or a material in which Bi or the like is added to increase the magneto-optical constant is used. Short wavelength (0.83 μm, 0.
For 78 μm, 0.68 μm, 0.63 μm), a magnetic semiconductor single crystal (CdMnTe or ZnM) transparent in the visible region
nSe etc.) can be used.

In such a structure, the electromagnet coil 2,
The function described above can be achieved by controlling the current flowing through 3 to change the ratio and strength of the orthogonal magnetic field. For example, a detector that detects a part of the emitted light through a polarizing plate that passes only linearly polarized light in a predetermined direction, and to control the ratio and intensity of the orthogonal magnetic field so that the amount of light detected by this detector becomes maximum. Provide a feedback circuit.

[0019]

Second Embodiment FIG. 2 shows a waveguide type embodiment. In this embodiment, MBE (molecular beam epitaxial growth method) is used.
Alternatively, it is formed by MOCVD (Metal Organic Chemical Vapor Deposition). The process is as follows: GaAs substrate 1
On 1, laminating CdTe buffer layer _{12, Cd 1-x Mn x} Te lower cladding layer 13 and Cd _1-y Mn y Te core layer 14. The core layer 14 is left in a ridge shape by resist application, mask exposure and etching. After removing the resist remaining on the ridge of the core layer 14, Cd _1-x Mn _x
It is filled with the Te upper clad layer 15. The typical film thickness of each is 2 μm for the buffer layer, 2 μm for each of the upper and lower clad layers, and 1 μm for the core layer. Also, the core layer 14
Has a ridge width of 1 μm.

X> y in order to use the core layer 14 as an optical waveguide
Must. For example, for a wavelength of 0.83 μm, a composition of x = 0.3 and y = 0.1 is suitable.
Also, for 0.68 μm, x = 0.6, y = 0.
45 is suitable.

As a matter of course, since the optical waveguide must not have birefringence when the magnetic field H is not applied, the waveguide must be phase-matched between the TE wave and the TM wave. . The method of applying the external magnetic field H is the same as that in the first embodiment.

A waveguide type polarization controller using a magnetic garnet film is also possible. In this case, a magnetic garnet film having a different composition is formed on a GGG (gadolinium gallium garnet, non-magnetic) substrate by LPE (liquid layer epitaxy).
The layers are laminated by the method to form a waveguide structure.

[0023]

Third Embodiment FIG. 3 shows an example of a waveguide type polarization controller 22 integrated with a wavelength selector 21. The waveguide type polarization controller 22 has the same structure as that of the second embodiment. Here, a DFB in which a diffraction grating 23 is formed in the vicinity of the active layer 24 over the entire wavelength selector.
A (Distributed Feedback) type wavelength selector 21 is illustrated. The waveguide type wavelength selector 21 is designed to operate only in a specific polarized wave due to its structure. Therefore, the wavelength-multiplexed incident light is constantly polarized by the polarization controller 22 (for example, TE wave).
Need to fix.

In wavelength-division multiplexing communication, each wavelength (λ ₁ , λ ₂ , ..., λ _n ) emitted from a fiber (not shown)
Have different polarization states because the environments of the fibers emitted from different nodes and through which the respective lights have passed are different. In order to detect a signal having a specific wavelength (λ _i ) from such a wavelength group, the intensity of light passing through the wavelength selector 21 tuned to λ _i should be maximized. Then, the direction and strength of the magnetic field H of the polarization controller 22 are controlled. The wavelength selector 21 is tuned by controlling the injection current, the applied voltage and the like.

In addition to the above integration, a beam splitter (for example, a slit coupler formed in a waveguide structure) for separating monitor light, a photodetector (for example, a pin photodiode, a phototransistor, etc.), and photodetection. It is also possible to integrate FETs (field effect transistors) for controlling the electromagnet current (see Example 1) using the output from the device on the same substrate.

The integrated optical node described above can be used in a wavelength division multiplexing optical communication system.

[0027]

As described above, according to the present invention,
By using the Cotton-Mouton effect, it is possible to fabricate a polarization control device with a simple structure using only one magneto-optical material or element. If the polarization control device has a waveguide structure, it can be integrated with a wavelength selector, a photodetector, or the like.

Further, this apparatus is important when applying a polarization diversity type heterodyne receiver in coherent optical communication or a waveguide device having a strong polarization dependence to optical fiber communication.

[Brief description of drawings]

1A and 1B show a first embodiment of the present invention, in which FIG. 1A is a front view seen from a light incident direction, and FIG. 1B is a perspective view showing the operation of the present embodiment.

FIG. 2 is a perspective view of a second embodiment of a waveguide type polarization controller in which a CdMnTe waveguide is formed on a GaAs substrate.

FIG. 3 is a perspective view of an example of an integrated node for wavelength division multiplexing optical communication in which a polarization controller and a wavelength selector are formed on the same substrate.

FIG. 4 is a diagram showing the operation of the present invention (FIG. 1) on a Poincare sphere.

FIG. 5 is an explanatory diagram of the Cotton-Mouton effect.

FIG. 6 shows a polarization controller (conventional example) using a Faraday rotator, in which (a) is two Faraday rotators and 1/4.
The side view of what combined the wave plate, (b) is a figure which shows the polarization state in each point (A, B, C, D) of (a).

FIG. 7 is a diagram showing the operation of the polarization controller of FIG. 6 on a Poincare sphere.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 Magneto-optical element 2, 3, 64 Electromagnet coil 11 GaAa substrate 12 CdTe buffer layer 13, 15 CdMnTe clad layer 14 CdMnTe core layer 21 Waveguide type wavelength selecting part 22 Waveguide type polarization controlling part 23 Waveguide type wavelength selecting part Diffraction grating 24 Active layer of waveguide type wavelength selecting section 61, 62 Faraday rotator 64 1/4 wavelength plate

Claims (7)

[Claims] 1. A polarization control device for converting arbitrary elliptical polarization into a known polarization state by using magnetic birefringence due to the Cotton-Mouton effect. 2. The polarization controller according to claim 1, further comprising means for controlling the direction and intensity of a magnetic field applied to the magneto-optical element in a plane perpendicular to the traveling direction of light. 3. An electromagnet for generating two sets of orthogonal magnetic fields, a magneto-optical element having the Cotton-Mouton effect, and a feedback circuit for detecting a part of emitted light and controlling the ratio and intensity of the orthogonal magnetic fields. The polarization controller according to claim 1, wherein: 4. The polarization controller according to claim 2 or 3, wherein the magneto-optical element is made of a magnetic garnet material or a magnetic semiconductor material. 5. The polarization controller according to claim 2 or 3, wherein the magneto-optical element has a waveguide structure in which magneto-optical thin films having different compositions are laminated. 6. An optical communication system using the polarization control device according to claim 1 and a single-mode optical fiber. 7. An integrated node for wavelength division multiplexing optical communication, wherein the waveguide type polarization controller according to claim 5 and a waveguide type wavelength filter are formed on the same substrate.