JPH07191281A - Fiber type optical isolator - Google Patents

Abstract

PURPOSE:To provide the fiber type optical isolator which consists of a small number of components, and is reducible in size, wide in application, and large in utilization value. CONSTITUTION:The fiber type optical isolator is constituted by removing part of the clad 3 of a single-mode optical fiber 1 installed while bent having curvature and loading the removal part with a magnetooptic material 4, and then a magnetic field is applied at right angles to the propagation direction of light to cause a nonreciprocal phase shift by magnetooptic effect in the core 2 of the single-mode optical fiber 1; and the relation of loss due to the bending of the optical fiber 1 with the value of a propagation constant is utilized to vary an attenuation quantity in the opposite direction from the forward direction. Consequently, a lens is not necessary and there is no polarizer, etc., so the need for optical alignment is eliminated to decrease the number of components; and the optical fiber itself is used as an optical isolator to reduce the size, and there is no incidence/projection surface present halfway, so reflected light from the inside of the optical isolator is reduced. Further, the isolator can be added directly to an optical fiber for transmission, so the fiber type optical isolator which is wide in application and large in utilization value is obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical isolator used in the fields of optical communication and optical measurement, and more particularly to a fiber type optical isolator having a fiber structure as a whole.

[0002]

2. Description of the Related Art Semiconductor lasers (hereinafter referred to as LDs) used for optical measurement and optical communication are LDs whose reflected light returns.
When the light enters the active layer, the oscillation wavelength and output fluctuate, and accurate signal transmission and measurement cannot be performed. An optical isolator is used to block this reflected light.
In particular, an optical isolator used for an arbitrary optical fiber or for a fiber amplifier is called an in-line type or pigtail type optical isolator. FIGS. 6A and 6B show a configuration example of a conventional in-line type optical isolator. Reference numeral 7 is an incident light ray, and 8 is a birefringent plate, for example, a uniaxial crystal cut out so that its optical axis is inclined to the surface and polished into parallel flat plates. To separate. Reference numeral 9 denotes a Faraday rotator formed of YIG or the like, to which a magnetic field is applied from the outside in parallel with the light propagation direction. The Faraday rotator 9 rotates the light by 45 ° about the plane of polarization about the direction of the magnetic field. Reference numeral 10 is an optical rotator that rotates the plane of polarization by 45 ° about the light propagation direction. Reference numeral 11 denotes a birefringent plate, which is installed so that the optical axis of the birefringent plate 8 and the optical axis of 11 are parallel to each other with the light propagation direction as an axis. 1
Reference numerals 2 and 13 are lens systems for coupling light into a fiber. The direction from the lens 12 to the lens 13 is the forward direction, and the opposite is the reverse direction. As shown in FIG. 6A, two polarization components orthogonal to each other in the forward direction are separated by the birefringent plate 8 and their polarization planes are rotated separately, and then are combined and propagated by the birefringent plate 11. . The light in the opposite direction has polarization components orthogonal to each other due to the birefringence plate 11 as shown in FIG.
The light is separated into two lights, and the polarization planes are rotated separately, and then the lights are further separated by the birefringent plate 8 and thus blocked. This prevents the reflected light from returning.

[0003]

However, the above-described in-line type optical isolator uses a large number of parts and is therefore expensive. In addition, it is necessary to adjust the position of each component, and in particular, optical coupling between the lens and the optical fiber is troublesome. Further, the reflected light from the incident surface of many optical elements has been a cause of deterioration of isolation.

[0004]

SUMMARY OF THE INVENTION The present invention provides a method for directly adding an optical isolator function to an ordinary optical fiber for transmission in order to solve the above problems.

Specifically, a part of the cladding of the optical fiber for transmission which is bent and installed so as to have a curvature is removed,
A magneto-optical material is loaded on the part. By applying a magnetic field to the magneto-optical material in a direction perpendicular to the traveling direction of light, a non-reciprocal phase shift is caused in the originally isotropic fiber, and the attenuation due to bending of the optical fiber varies depending on the propagation constant. However, it makes a difference in the light loss in the forward and backward directions.

Here, the non-reciprocal phase shift of the magneto-optical waveguide will be briefly described with reference to the perspective schematic view of FIG.
In the waveguide mode of light in the three-dimensional waveguide 6 composed of the waveguide layer made of the magneto-optical material 4 and the substrate 14, the Ex mode in which the main components of the electromagnetic field are Ex and Hy and the main component of the electromagnetic field are It is roughly divided into Ey mode, which is Ey and Hx. These modes are hybrid modes, but for the sake of simplicity, set Ex mode to Ey = 0, TE mode, Ey
The mode can be approximated to the TM mode by setting Hy = 0. E (Ex, Ey, Ez) is an electric field vector, and H (Hx, Hy, Hz) is a magnetic field vector. In the conventional non-reciprocal waveguide formed of the magneto-optical material 4, TM is applied by applying a magnetic field in the X direction of the waveguide.
A non-reciprocal phase shift occurs in the mode. This is a phenomenon in which the propagation constants of light propagating in the + Z direction and light propagating in the -Z direction are different. If the propagation constant of the TM mode when no magnetic field is applied is β0, the propagation constant when the magnetic field is applied is in the + Z direction. For the forward direction, βf = β0 + Δβ, and for the -Z direction in the reverse direction, βb = β0 -Δβ. Δβ is the nonreciprocal phase shift generated by the magneto-optical effect, and βf −βb is called the nonreciprocal phase shift amount.

[0007]

1 shows a schematic sectional view of a first embodiment of the present invention. The quartz single mode optical fiber 1 is bent and installed on the substrate 14, a part of the clad 3 is removed, and a Ce-substituted YIG single crystal which is the magneto-optical material 4 is loaded on the part. That is, the fiber type optical isolator 5 is constructed by utilizing the non-reciprocal phase shift generated by loading the magneto-optical material 4 on the single mode optical fiber 1 and the loss caused by bending the single mode optical fiber 1.

In the past, in order to generate the magneto-optical effect in the waveguide, the waveguide layer itself in which light mainly exists was formed of a magneto-optical material. Here, there are some problems such as difficulty in formation, optimal control of refractive index difference, and problems of coupling between waveguide and fiber. However, the electromagnetic field of the propagating light exists as an evanescent wave to the outside of the waveguide layer or the core, and the magneto-optical material acts on the evanescent wave to propagate through the originally isotropic optical fiber. A magneto-optical effect can be produced in light.

FIG. 2 is a schematic view of the structure of FIG. 1 for analysis, and shows a sectional view perpendicular to the light propagation direction. The core 2 of the single-mode optical fiber 1 is originally circular, but is approximate to a square. Further, by removing a part of the clad 3 and loading the magneto-optical material 4, the X direction and the Y direction can be changed.
Since the symmetry is different depending on the direction, the waveguide modes are considered to be similar to TE and TM as in the rectangular waveguide. The magnetic field is applied in the X direction. FIG. 3 shows the core 2 and the magneto-optical material 4 in the structure of FIG.
By changing the distance H of the non-reciprocal phase shift amount Φ = β f −β
b is calculated by the finite element method. Magneto-optical material 4
Thickness d = 0.4 μm, the core diameter is 6 μm, the refractive index of the core 2 of the optical fiber 1 is 1.504, the refractive index of the cladding 3 is 1.5, and the refractive index of the Ce-substituted YIG is 2.23. The magneto-optical constant γ is set to 7.7 × 10 -3. The non-reciprocal phase shift amount changes depending on H, but it is about 1 at 0 to 3 μm.
It shows 6.5 rad / cm. Thus, even if the core 2 is an isotropic material, it is possible to cause a non-reciprocal phase shift.

In order to construct a fiber type optical isolator 5 using this fiber type non-reciprocal component, the loading portion of the magneto-optical material 4 of the single mode optical fiber 1 is bent as shown in FIG. By this bending, the electromagnetic field is further distributed outside the bending, that is, in the portion close to the magneto-optical material, and the non-reciprocity is increased. The loss due to bending increases as the propagation constant decreases. As a result of the non-reciprocal phase shift, the forward propagation constant> the backward propagation constant, and further, the bending causes the forward loss <the backward loss, and the isolator operation can be exhibited.

The above principle will be briefly described with reference to the graph of FIG. The distance H between the fiber core 2 and the magneto-optical material 4 is taken horizontally, and the propagation constant β is taken vertically. Below the cutoff value, the emission mode is a continuous mode, and light is emitted to the cladding 3 and is not confined in the core. Cut-off condition is core 2
Is determined by the difference in refractive index between the cladding 3 and the cladding 3, and the relationship between the propagation constant β 0 and the cutoff when no magnetic field is applied is the bending, stress, physical properties of the material loaded in the single mode optical fiber 1, core It depends on the distance between 2 and the substance to be loaded. By applying a magnetic field, the propagation constant becomes forward βf and backward βb. In Example 1 of FIG. 1, β 0 and the cutoff value can be made close to each other by bending, and only the backward propagation constant β b can be configured to overlap with the radiation mode (shaded portion) to cause loss.

Since the non-reciprocal phase shift occurs in the TM mode, the fiber type optical isolator 5 operates in the forward direction with TE mode transmission and TM mode transmission, and in the reverse direction with TE mode transmission and TM mode cutoff.

FIG. 7 shows a fiber type optical isolator 5 of FIG. 1 for adjusting the propagation constant and increasing the attenuation of the emitted light.
5 is a schematic cross-sectional view showing a second embodiment of the present invention in which a metal film 15 of aluminum, silver, copper or the like is formed on the magneto-optical material 4 of FIG. Metal has a very low dielectric constant (negative) with respect to light, which causes a larger loss. Therefore, it is possible to improve the isolation by adjusting the area and the thickness of the metal film 15 to obtain the optimum propagation constant for constructing the fiber type optical isolator 5 and by giving a large attenuation to the emitted light. is there.

The propagation constant of light can also be controlled by the stress on the waveguiding layer. Therefore, in FIGS. 1 and 7, by applying stress from the vertical direction, the propagation constant is controlled and TE
It is possible to separate the propagation constants so that the modes and the TM modes are not easily converted, to correct the dimensional tolerance during manufacturing, and to obtain a more efficient isolator operation. FIG. 8 is a schematic perspective view showing a third embodiment of the fiber type optical isolator of the present invention. The fiber type optical isolator 5 of the first example shown in FIG. It is a perspective schematic diagram showing an example installed in series. Since the TM mode and the TE mode are interchanged in the two fiber type optical isolators 5, TE and TM mode transmission in the forward direction and TE and TM mode cutoff in the opposite direction can be realized and a polarization independent optical isolator can be constructed. The fiber type optical isolator 5
May be that of the second embodiment shown in FIG.

By processing an arbitrary isotropic fiber in this manner, non-reciprocity is added to an arbitrary position, and an optical isolator can be constructed very easily and inexpensively.

[0016]

As described above, according to the fiber type optical isolator of the present invention, a part of the clad of the optical fiber bent and installed so as to have a curvature is removed, and the removed portion is loaded with the magneto-optical material. Then, a magnetic field is applied in a direction perpendicular to the light propagation direction to cause a non-reciprocal phase shift due to the magneto-optical effect in the core of the optical fiber, and the loss due to bending of the fiber is related to the magnitude of the propagation constant. By changing the amount of attenuation in the forward and reverse directions, a metal film is further formed on the magneto-optical material to adjust the propagation constant and increase the amount of attenuation of radiated light. Is unnecessary and there is no need for optical alignment because there is no polarizer, etc., the number of parts is small, the optical fiber itself is an optical isolator, so the size is small. Reduces the reflected light from the internal data, and because that can be added directly to any transmission fiber, high applicability, it becomes larger utility value.

[Brief description of drawings]

FIG. 1 is a schematic sectional view showing a first embodiment of the present invention.

FIG. 2 is a cross-sectional view perpendicular to the traveling direction of light in FIG.

FIG. 3 is a graph showing non-reciprocal phase shift.

FIG. 4 is a graph showing the operating principle of the fiber type isolator.

FIG. 5 is a schematic perspective view showing a magneto-optical waveguide.

FIG. 6 is a diagram showing a conventional configuration example of an in-line type isolator.

FIG. 7 is a schematic cross-sectional view showing a second embodiment of the present invention.

FIG. 8 is a schematic perspective view showing a third embodiment of the present invention.

[Explanation of symbols]

1: Single-mode optical fiber 7: Incident light beam 2: Core 8, 11: Birefringent plate 3: Clad 9: Faraday rotator 4: Magneto-optical material 10: Optical element 5: Fiber type optical isolator 12, 13:
Lens 6: Three-dimensional waveguide 14: Substrate

Claims (3)

[Claims] 1. A part of the clad of an optical fiber bent to have a curvature is removed, and a magneto-optical material is loaded on the removed part to construct a magnetic field in a direction perpendicular to the light propagation direction. Is applied to cause a nonreciprocal phase shift in the core of the optical fiber due to the magneto-optical effect, and the fact that the loss due to bending of the optical fiber is related to the magnitude of the propagation constant is used to change the attenuation in the forward and reverse directions. A fiber-type optical isolator that features 2. The fiber type optical isolator according to claim 1, wherein a metal film is formed on the magneto-optical material to adjust the propagation constant and increase the attenuation of the emitted light. 3. A fiber-type optical isolator according to claim 1 and / or 2, wherein two fiber-type optical isolators, one of which is rotated by 90 degrees about the traveling direction of light, are provided in series. A polarization-independent fiber type optical isolator characterized by.