JPS6257014B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a waveguide Faraday rotation element in the field of optical communications.

Non-reciprocal elements that utilize the Faraday effect, such as optical isolators and optical circulators, are used in the field of optical communications to stabilize light sources, suppress line echoes,
It is an important element used for purposes such as two-way communication. Conventional devices of this kind have used bulk Faraday rotators, which are made by cutting a single crystal of yttrium-iron-garnet (YIG) into an appropriate size and applying an external magnetic field. For this reason,
While semiconductor lasers, optical fibers, and the like are waveguide elements, Faraday rotators are not waveguide elements, so they must be used through a coupling lens. Furthermore, since an external magnetic field of several hundred Oe is required to be applied, a special magnetic circuit configuration is required, resulting in disadvantages such as an increase in the size of the entire device.

As an attempt to make the Faraday rotator waveguide,
The magneto-optic modulator using meandering electrodes by PKTien et al. is famous (Journal of Applied
(See Physics Vol. 45, p. 3059, 1974). In addition, the Kotton-Mouton effect (also known as magnetic birefringence or Voigt effect, an effect where when a magnetic field is applied perpendicular to the traveling direction of light, the electric field has a different refractive index for polarized light parallel to the magnetic field and polarized light perpendicular to the magnetic field) is used. Its operation has been confirmed in the optical isolator developed by G. Hepner et al.
(See Physica Vol. 89B, p. 264, 1977).

However, since these elements can guide several modes, they have the disadvantage that they cannot be efficiently coupled directly to other elements. For devices operating in single mode, a device in which anisotropic crystals are brought into contact has been proposed by J. Warner (IEEE Transaction MTT,
(see MTT-vol. 23, p. 70, 1975), but its operation has not yet been confirmed because optical adhesion and film thickness control are difficult.

An object of the present invention is to provide a waveguide Faraday rotation element that operates in a single mode, can be directly coupled to other waveguide elements, requires only a very small external magnetic field to be applied, and is suitable for optical isolators and the like.

The present invention will be described in detail below with reference to the drawings.

FIG. 1 shows the structure of a two-layer epitaxial garnet film which is the basis of the present invention, where 1 is a waveguide layer, 2 is an intermediate layer, and 3 is a GGG (gadolinium gallium garnet) substrate. For the waveguide layer 1 and the intermediate layer 2, substituted YIG (yttrium/iron/garnet), such as Sc or Ga substituted YIG, is used, and LPE is used.
(liquid phase epitaxial). Here, Sc is substituted to make the lattice constant of the epitaxial film larger than that of the GGG substrate 3, to provide in-plane magnetization, and to exhibit optical anisotropy. Ga is substituted to control the refractive index. Middle layer 2 is Sc
It is assumed that the misfit of the lattice constant with the GGG substrate 3 is made zero by adjusting the amount of Ga. The amount of Ga in the waveguide layer 1 is slightly reduced compared to the intermediate layer 2, and a lattice constant misfit of 0.03% is introduced. Note that the refractive index of the waveguide layer 1 is 2.206, and the refractive index of the intermediate layer 2 is 2.188. Furthermore, waveguide layer 1
is different from TE mode due to lattice constant misfit.
It has an optical anisotropy of 2.0×10 ^-4 for TM mode.

In Fig. 2, the solid line indicates the calculation result of the conversion rate F between the TE mode and the TM mode when the film thickness D1 of the waveguide layer 1 is changed in the film configuration shown in Fig. 1, and the 〇 mark indicates the actual measured value. . Here, Ws is a single mode film thickness at which the TE1 mode is cut off and only the zero-order mode is guided. The mode conversion rate F becomes 100% when the propagation constants of the TE0 mode and the TM0 mode completely match, but according to Figure 2, when the film thickness Ws≒3 μm, F=100%, and at this time both It can be seen that the mode propagation constants match perfectly.

The solid line in FIG. 3 shows the calculated result of the dependence of the rotation of the plane of polarization on the propagation distance when TE0 mode light is incident, and the circles indicate actual measured values. Here, the film thickness
D1 was approximately 3 μm. At this time, a characteristic with a Faraday rotation angle of 210 degrees/cm was obtained as shown in the figure.
In the film configuration of the Faraday rotation element of the present invention,
Since the propagation constants of the TE0 mode and the TM0 mode match well, when linearly polarized light is incident on the Faraday rotation element of the present invention, the output light after waveguide also becomes linearly polarized light. On the other hand, if the propagation constants for both modes do not match as in the conventional case,
The light after being guided becomes elliptically polarized light with a rotated plane of polarization.
Therefore, in conventional thin-film waveguides, the propagation constants of the TE mode and TM mode are different, so it was necessary to operate the Faraday rotator while always taking into account the polarization plane of the light incident on the Faraday rotator. In contrast, in the present invention, since the propagation constants of both are the same, any polarized light can be incident, and the polarization plane rotation between the incident light and the output light is exactly the same as that of a bulk crystal. can be handled. According to Figure 2, the propagation distance obtained by rotating the plane of polarization by 45° is 2.
Figures 4A and 4B show the dependence of the TE mode and TM mode outputs on the magnetic field H, respectively, when light with such a polarization plane of 45° (the same amount of TE mode and TM mode) is incident. As can be seen from the figure, TE mode or TM mode can be obtained depending on the direction of the magnetic field. Further, the magnetic field H required for magnetization reversal was approximately 0.5 Oe.

Next, specific examples of various elements according to the present invention using the basic configuration of the present invention described above will be explained with reference to FIGS. 5 to 7.

FIG. 5 shows the structure of an optical isolator for a semiconductor laser according to the present invention. Generally, optical isolators are
It consists of a 45° Faraday rotator and two polarizers placed on its input side and output side. However, since semiconductor lasers normally operate in TE mode, when connecting a semiconductor laser and an optical isolator, the semiconductor laser is affected by the return light of the TE mode, but not by the return light of the TM mode. It is known that the polarizer on the semiconductor laser side can be omitted by taking advantage of the fact that it is not affected. In the present invention, taking advantage of this point, a polarizer, for example, a metal-clad polarizer 4 is arranged on the output side of a 45° Faraday rotator in the form of a two-layer epitaxial garnet film consisting of two layers 1 and 2 as shown in the first figure. Then, the TE mode light from the semiconductor laser 5 is made incident on a 45° Faraday rotator.

When the propagation distance is 2 mm, the two-layer epitaxial garnet films 1 and 2 according to the present invention have a polarization plane of 45° (the amount of light in the TE mode and TM mode is the same in the Faraday rotator) as shown in Fig. 3. Therefore, as shown in FIG. 5, the polarization plane of the semiconductor laser 5 is rotated by 45 degrees with respect to the plane of the two-layer epitaxial garnet films 1 and 2. Then, when a magnetic field H in the direction of the arrow is applied to the waveguide layer 1, the propagating light is converted to the TE mode. The metal-clad polarizer 4 transmits the TE mode but absorbs the TM mode. Therefore, in this example, the plane of polarization of the traveling wave is shown by a solid line, and the plane of polarization of the backward wave is shown by a dotted line.

The size of the light-emitting surface of a semiconductor laser varies depending on the laser structure, but in a single-mode laser used for optical communication, the light-emitting surface is at most 2 to 3μ in both directions.
m, which is approximately the same size as the thickness of the garnet film waveguide layer 1. Therefore, the semiconductor laser 5 and the waveguide layer 1 can be directly coupled. Further, since the core diameter of a single mode optical fiber is about 10 μm, direct coupling between such an optical fiber and the waveguide layer 1 is easy. As shown in Figure 4, the external magnetic field H required to saturate the magnetization is extremely small, less than 1 Oe, and is several hundred times smaller than the magnetic field strength required for bulk optical isolators. 1
is sufficient, and extremely compact size is possible.

FIG. 6 shows another embodiment of the present invention, in which an external magnetic field for controlling the direction of magnetization is applied using a current, and can be used as a magneto-optic modulator. In FIG. 6, the waveguide layer 1 of the two-layer epitaxial garnet films 1 and 2 is covered with a buffer layer 6 made of a material having a suitable refractive index smaller than that of the waveguide layer 1, and then The electrode 7 is attached by vapor deposition or the like. The buffer layer 6 is for preventing light absorption by the metal electrode 7. Alternatively, a separately produced current circuit may be placed in close proximity to the waveguide layer 1. Here, the current I(A) is defined as the width d
Magnetic field H (Oe) generated when flowing through electrode 7 of (mm)
is given by H=I/2d×4π (1). When the sheet current density J is defined as J (A/mm) = I/d (2), a magnetic field of 6.3 Oe is generated at J = 1 A/mm. In this example, when the light propagation distance is set to 2 mm to obtain a rotation angle of 45° of the polarization plane, and 45° polarized light 8 is incident on the waveguide layer 1, this light 8 will change depending on the direction of the current I. is converted into TE mode or TM mode, and the converted light 9 is polarized by a polarizer (not shown).
When passed through , an intensity modulated output is obtained. In this case,
Since the magnetic field H at which magnetization is saturated is 0.5 Oe, the required current of 0.16 A is sufficient from equation (1).

FIG. 7 shows still another embodiment of the present invention, in which the rotation of the plane of polarization of light sent through the optical fiber 10 is controlled as desired, and the thin film waveguide 1
1 to convert it into light with an appropriate plane of polarization. When coupling an optical fiber to a thin-film waveguide, a high-quality single-mode optical fiber transmits light while maintaining linear polarization well, whereas a thin-film waveguide
Since most of the devices operate only in either the TE mode or the TM mode, it is necessary to align the planes of polarization at the connection point between the two modes. For this purpose, it is possible to use the Faraday rotation element of the present invention as shown in Figure 7. can. That is, the output end face of the optical fiber 10 is coupled to the input end face of the waveguide layer 1 of the two-layer epitaxial garnet films 1 and 2 according to the present invention, and the input end face of the thin film waveguide 11 is coupled to the output end face. 13 in the figure is a substrate of the thin film waveguide 11.

When coupling the optical fiber 10 and the thin film waveguide 11 using the Faraday rotation element of the present invention, it is necessary to rotate the plane of polarization by any desired angle. Since the Faraday rotation θ _F is proportional to the component of the applied magnetic field in the forward direction of the light, as shown in Figure 7, the magnetic fields Hx and Hy perpendicular to the plane are transferred to the waveguide layer using a coil or the like, similar to magnetic bubble memory. 1, and by appropriately controlling the strength of both magnetic fields, the direction of the combined magnetic field can be set in a desired direction. In this way, the Faraday rotation θ _F changes the magnetization M
and when the angle between magnetization M and Hy is θ,
It becomes proportional to Mcosθ. The propagation distance of light
If the length is 2 mm, which gives a rotation of the plane of polarization by 45 degrees,
A polarization plane rotation of +45° to -45° is obtained, and no matter what polarization plane of linearly polarized light is sent from the optical fiber 10, it can be matched with the polarization plane in which the thin film waveguide 11 operates. In order to always maintain the best matching condition, a part of the light is taken out in the thin film waveguide 11 using a directional coupler or the like, and the amount of light in the TE mode is detected through a TE mode pass filter, for example, and returned to the magnetic field generation section. The amount of light should be maximized.

As explained above, according to the Faraday rotation element of the present invention, the propagation constants of the TE mode and the TM mode match well, so no matter what plane of polarization linearly polarized light is guided, the linear polarization property remains the same. Almost preserved. Therefore, although the eigenmodes of the propagating light are different between the normal bulk crystal and the waveguide layer of the present invention, the polarization plane rotations of the incident light and the output light can be handled in exactly the same manner. Also,
Since the Faraday rotation element of the present invention operates in a single mode, it has the advantage that it can be used in direct connection with other optical circuit elements. Further, since the Faraday rotary element of the present invention can sufficiently operate with a magnetic field several hundredths of the strength of a normal bulk type element, the element can be made smaller and more economical.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the structure of a two-layer epitaxial garnet film according to the present invention, and FIG. 2 is a diagram showing its TE-
FIG. 3 is a characteristic curve diagram showing the dependence of the TM mode conversion rate on the waveguide layer film thickness, and FIG. 4 is a characteristic curve diagram showing the propagation distance dependence of the polarization plane rotation in the present invention.
and B are characteristic curve diagrams showing the magnetic field dependence of the TE and TM mode outputs when light with a polarization plane of 45° is incident, respectively. Figure 5 is the configuration of an optical isolator for a semiconductor laser as an embodiment of the present invention. FIG. 6 is a perspective view showing an example of the configuration of a modulator as an embodiment of the present invention, and FIG. 7 is a perspective view showing an example of the configuration of a polarization plane rotation element as an embodiment of the present invention. It is a diagram. DESCRIPTION OF SYMBOLS 1... Waveguide layer, 2... Intermediate layer, 3... GGG substrate, 4... Metal clad polarizer, 5... Semiconductor laser, 6... Buffer layer, 7... Electrode, 8... Incident light, 9...Emission light, 10...Optical fiber, 11
... Thin film waveguide, 12 ... Substrate.

Claims (1)

[Claims] 1. A gadolinium-gallium-garnet substrate, on which a waveguide layer made of substituted yttrium-iron-garnet is applied via an intermediate layer made of substituted yttrium-iron-garnet. The refractive index of the waveguide layer is larger than the refractive index of the intermediate layer, and the waveguide layer is formed so as to exhibit optical anisotropy, and a magnetic field is input to the waveguide layer while a magnetic field is applied to the waveguide layer. A waveguide type Faraday rotation element, characterized in that the polarization plane of the output light from the waveguide layer is rotated according to the propagation distance of the input light in the waveguide layer. 2. A gadolinium/gallium/garnet substrate is provided, and a waveguide layer of substituted yttrium/iron/garnet is formed on the substrate via an intermediate layer of substituted yttrium/iron/garnet, and the refractive index of the waveguide layer is is larger than the refractive index of the intermediate layer, and the waveguide layer is formed to exhibit optical anisotropy, and input light is made to enter the waveguide layer while a magnetic field is applied to the waveguide layer. The plane of polarization of the output light from the waveguide layer is rotated by 45 degrees depending on the propagation distance of the input light in the waveguide layer, and a metal-clad polarizer is provided on the upper surface of one end of the waveguide layer. A waveguide type Faraday rotation element characterized in that it is arranged to operate as an optical isolator. 3. A gadolinium/gallium/garnet substrate is provided, and a waveguide layer made of substituted yttrium/iron/garnet is formed on the substrate via an intermediate layer made of substituted yttrium/iron/garnet, and the refractive index of the waveguide layer is is larger than the refractive index of the intermediate layer, and the waveguide layer is formed to exhibit optical anisotropy, and input light is made to enter the waveguide layer while a magnetic field is applied to the waveguide layer. The plane of polarization of the output light from the waveguide layer is rotated by 45 degrees depending on the propagation distance of the input light in the waveguide layer, and a current is caused to flow close to the waveguide layer. An external magnetic field generated by an electric current is applied to the waveguide layer to control the direction of magnetization thereof, and light incident on the waveguide layer is magneto-optically modulated in accordance with the electric current. Waveguide type Faraday rotation element. 4 A gadolinium/gallium/garnet substrate is provided, and a waveguide layer made of substituted yttrium/iron/garnet is formed on the substrate via an intermediate layer made of substituted yttrium/iron/garnet, and the refractive index of the waveguide layer is is larger than the refractive index of the intermediate layer, and the waveguide layer is formed to exhibit optical anisotropy, and input light is made to enter the waveguide layer while a magnetic field is applied to the waveguide layer. The polarization plane of the output light from the waveguide layer is rotated according to the propagation distance of the input light in the waveguide layer, and external magnetic fields are applied to the waveguide layer in two mutually orthogonal directions. A waveguide type Faraday rotation element, characterized in that the polarization plane of light propagated through the waveguide layer is rotated by a predetermined angle.