JPH1123849A - Fiber type optical isolator and its manufacture - Google Patents

Abstract

PROBLEM TO BE SOLVED: To actualize the superior optical isolator which has simple constitution, good productivity, and small loss of connection with a fiber for transmission without specially using a material, a magnet, etc., having nonreciprocal optical effect such as magnetooptics effect. SOLUTION: A diffraction grating 12 is formed in the core 11 of an ordinary single-mode optical fiber 1 for communication by making use of fiber Bragg diffraction grating technologies, nearly a half area of the clad 13 is formed of resin 14 which has a smaller refractive index than it, and the angle θ of the diffraction grating 12 to the optical axis of a wave front vector is set as 0<θ<90 deg.. Light transmitted in the core of the optical fiber is not diffracted forward, but diffracted backward and emitted to the clad to have a large difference in the transmissivity of the optical fiber between the forward and backward directions, so that the optical fiber can be constituted as the optical isolator.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical isolator formed of an optical fiber, and more particularly to an optical isolator formed without using an optically non-reciprocal material such as a magneto-optical material.

[0002]

2. Description of the Related Art With the spread of the Internet and multimedia communication in recent years, the demand for communication over the long distance has become greater than 10,000 km using optical fiber amplifiers in response to the vigorous expansion of communication demands in society. The development of technologies for increasing the capacity of optical communication systems has been more advanced than before, such as the development of a system to apply a broadband digital service integration system including HDTV by applying optical fiber transmission technology to the subscriber end. It is being carried out vigorously. One of the optical components indispensable for realizing such a system is an optical element such as an optical isolator having a directional characteristic having a strong light transmission characteristic.
That is, for example, when transmitting laser light emitted from a semiconductor laser as a transmission light source through an optical fiber, return light from an optical fiber connection point or the like re-enters the semiconductor laser. It is used as a component to prevent the reduction of S / N and unstable operation caused by the above.

As an example of such an optical isolator conventionally known, a magneto-optical crystal exhibiting a Faraday effect, which is a non-reciprocal light-rotating effect, is used as a 45-degree polarization rotator, and this polarization rotator is used as a light transmission element. Are sandwiched between two polarizers whose polarization directions are shifted by 45 degrees, and are used in many practical applications. That is, the laser light incident from one of the polarizers is polarized by the polarizer, and its polarization direction is rotated by 45 degrees when transmitted through the magneto-optical crystal. And
The light is emitted through the other polarized light. On the other hand, light incident in the opposite direction is polarized by the other polarizer, and the polarization direction is rotated by 45 degrees in one direction when transmitting through the magneto-optical crystal. Therefore, the polarizer is orthogonal to one polarizer, and cannot pass through the one polarizer. Therefore, by arranging this optical isolator on the emission side of the semiconductor laser, return light is prevented from re-entering the semiconductor laser. Further, as another technique using a magneto-optical material, there is a technique described in JP-A-7-64023.

[0004]

However, an optical isolator composed of such a magneto-optical crystal or polarizer requires a minimum of four components including a magnet in addition to the components described above. A large number of man-hours are required for processing and assembling, and there is a problem that reliability and stability are poor. In addition, in order to be able to insert and use the optical fiber in the middle of an optical fiber whose polarization state is not necessarily determined at the time of transmission, it is necessary to build a configuration in which element characteristics do not depend on polarization. A complicated optical path configuration is required.

An object of the present invention is to provide an optical isolator which is simple in construction and manufacture without using a special material such as a magneto-optical crystal, and a method of manufacturing the same.

[0006]

An optical isolator according to the present invention has a cross section including an optical axis of an optical fiber having a configuration in which a core is surrounded by a cladding, with respect to a straight line drawn close to the core. Having a dielectric cladding having a different refractive index, wherein the core has a wave number vector in a plane formed by a light guide direction of the core and a normal established on a boundary surface of the dielectric having a different refractive index, and Angle θ with optical axis of wave number vector
Have a diffraction grating satisfying 0 ° <θ <90 °. The dielectric cladding having a different refractive index has a configuration in which a part of the cladding of the optical fiber is deleted, and a resin having a refractive index different from that of the cladding is bonded to the deleted part. Also,
When the refractive indices of the cladding, core, and cladding having different refractive indices are n1, n2, and n3, respectively, n1>n2>
n3.

Further, according to the method of manufacturing an optical isolator of the present invention, an optical fiber whose core is surrounded by a cladding is irradiated with two-wave interference light from a side surface along an optical axis to change the refractive index of the core. Forming a diffraction grating comprising a phase grating, and removing the clad by polishing a surface perpendicular to the plane formed by the wave vector and the optical axis of the diffraction grating to a region close to the side surface of the core; and Bonding a resin having a lower refractive index than the core to the deleted region.

By setting the angle θ between the optical axis of the wave vector of the diffraction grating and 0 ° <θ <90 °, light transmitted in the core of the optical fiber can be diffracted in the forward direction. Instead, it is diffracted in the opposite direction and radiated to the cladding. As a result, the transmittance of the optical fiber differs greatly between the forward direction and the reverse direction, and the optical isolator characteristics of the optical fiber can be obtained.

[0009]

Next, embodiments of the present invention will be described with reference to the drawings. 1A and 1B are a side sectional view and an end sectional view of an optical isolator according to an embodiment of the present invention. Hereinafter, this optical isolator will be described in the order of its manufacturing process. For example, a silica-based single mode optical fiber 1 for ordinary optical communication is prepared, and K
When the ultraviolet light (up to 240 nm) of the rF excimer laser light is irradiated with two light beams interfering with each other, the core 11 of the optical fiber 1 is irradiated.
In addition, a change in the refractive index caused by Ge included in the core 11 occurs, and the phase grating corresponding to the interference fringes, that is, the diffraction grating 1
2 are formed. As a method for forming the diffraction grating 12, a method for forming a fiber Bragg diffraction grating is used.
At this time, the direction of the wave number vector of the diffraction grating 12 and the optical axis of the optical fiber have a finite angle θ. FIG.
In the arrangement of (a), ultraviolet light is irradiated vertically toward the paper surface, and the diffraction grating 12 is formed by the interference fringes.

After forming the diffraction grating 12, FIG.
As shown in (b), the area above the clad 13 of the optical fiber 1 is removed by polishing a surface perpendicular to the surface formed by the wave vector of the diffraction grating 12 and the optical axis, and the optical region is brought close to the cylindrical surface of the core 11. Grind. Thereafter, the dusting surface is covered with a resin 14 having a smaller refractive index than the quartz clad 13 of the optical fiber 1 such as a fluorinated resin, and a resin clad 14 is formed. In this embodiment, the resin 14 is formed in a thin thin plate shape, and the resin 14 is adhered to a portion that has been polished and removed. At this time, the refractive indices of the core 11, the quartz clad 13, and the resin clad 14 are nl,
If n2 and n3 are set, these are set to nl>n2> n3.

The optical fiber 1 configured as described above is, for example, as shown in FIG.
The laser light emitted from the semiconductor laser 2 is made incident on the optical fiber 1 and transmitted through the optical fiber 1. At this time, the transmission loss of the laser light incident from the right end face RF of the optical fiber 1 and the left end face L on the opposite side.
Since the transmission loss at the time of incidence from F is significantly different, a so-called isolator characteristic is obtained. As a result, the laser light incident on the optical fiber 1 from the semiconductor laser 2 and transmitted is transmitted through the optical fiber 1 in the opposite direction. The transmitted light is prevented from re-entering the semiconductor laser 2.

The behavior of the optical isolator can be understood from the following operation principle. FIG. 3 shows the conditions of diffraction of fiber-guided light by a diffraction grating.
It is a figure showing the matching state between wave number vectors of cladding radiation light, 20 is a wave number vector of guided light, 21 is a diagram of wave number of radiation light to quartz clad 13, and 22 is a resin clad 14 having a smaller refractive index than quartz. Respectively show diagrams of the synchrotron radiation. FIG. 3A shows diffraction conditions for guided light incident from the left end face LF of the optical fiber 1. The incident guided light (wave number vector 20 a) is diffracted by the diffraction grating 12 into radiation 24 of quartz cladding. The size of the wave vector 23 of the diffraction grating 12 is adjusted so that these three wave vectors close and match the triangle.
That is, the pitch and the angle β of the diffraction grating 12 are set in advance. If the change in the refractive index and the working length of the diffraction grating 12 are sufficiently set, the diffraction efficiency from the guided light to the quartz clad radiation can be made approximately 100%. On the other hand, the light once emitted from the core 11 to the tarad 13 is radiated out of the optical fiber without being recombined into the core. Light incident from the left end of the optical fiber is diffracted by the grating and radiates all its energy to the cladding.

On the other hand, FIG. 3B shows the state of the optical fiber 1 with respect to the guided light incident from the right end face RF. The wave vector 20b of the guided light passes through the diffraction grating as it is without being diffracted by the diffraction grating 12. This is because there is no emitted light that closes the triangle between the wave number vector 20b and the vector 23 of the diffraction grating 12 and satisfies the matching condition. This is because the upper part of the core 11 is covered with the resin clad 14 having a smaller refractive index than quartz, that is, a dielectric. As a result, light incident from the right end face RF is guided without loss.

Therefore, in the configuration shown in FIG. 2, light emitted from the semiconductor laser 2 and incident from the right end face RF of the optical fiber 1 is guided without loss, and conversely, the optical fiber 1 The laser beam incident from the left end face LF is diffracted by the diffraction grating 12 and all its energy is radiated to the clad. Thus, an irreversible optical isolator function of transmission characteristics is realized only by changing a part of the configuration of the optical fiber.

As a numerical example of the optical isolator of the present invention, if the grating period is Λ and the light wavelength is λ, the refractive index nl of the core 11 and the refractive index n2 of the quartz cladding 13 are extremely close to each other. The wave number K of the grating is approximately K = 2π / Λ == 2 × 2π × n2 × cos θ / λ.

For example, when the light wavelength λ is 1.3 μm, the refractive index n2 of quartz is 1.445, and the angle θ of the phase grating is 20 degrees, the period Λ is about 0.45 μm. This value has a slightly longer period than that of a so-called ordinary fiber diffraction grating (wavelength filter) that reflects a light wave of a specific wavelength as guided light, and such a grating is manufactured in the same manner as when an ordinary fiber-type wavelength filter is made. it can. Further, if the production length L of the diffraction grating is about 1 mm, the Klein = Cook parameter Q = (2πLλ /
Λ ² ) becomes extremely large, and becomes a so-called Bragg-type diffraction phenomenon due to a thick grating. For this reason, the diffraction efficiency by the grating becomes πΔnL / Λ = 2π,
Refractive index change Δn that guided light receives by the grating
Is appropriate, the diffraction efficiency will be 100%.

[0017]

As described above, according to the present invention, a normal single-mode optical fiber for communication is used, a diffraction grating is formed in the core thereof, and almost half of the cladding has a refractive index higher than that of the single-mode optical fiber. The light transmitted in the core of the optical fiber is diffracted in the forward direction by forming the angle θ between the optical axis of the wave vector of the diffraction grating and the optical axis by setting the angle θ to 0 ° <θ <90 °. It is diffracted in the opposite direction and emitted to the cladding. As a result,
The transmittance of the optical fiber differs greatly between the forward direction and the reverse direction, and the optical fiber can be configured as an optical isolator. Thereby, the configuration is simple and excellent in productivity without using a material or a magnet having a non-reciprocal light effect such as the conventional magneto-optical effect, and the connection loss with the transmission fiber is small because of the fiber type. An excellent optical isolator can be realized.

[Brief description of the drawings]

FIG. 1 is a side sectional view and an end sectional view of an embodiment of the present invention.

FIG. 2 is a diagram showing a use form of the optical isolator of FIG.

FIG. 3 is a diagram for explaining an isolation characteristic of the optical isolator of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Optical fiber 2 Semiconductor laser 11 Core 12 Diffraction grating 13 Quartz clad 14 Resin clad 20a, 20b Wave vector of guided light 21 Diagram of emitted light to quartz clad 22 Diagram of emitted light to resin clad 23 Wave vector of diffraction grating

Claims (4)

[Claims] In a cross section including an optical axis of an optical fiber having a configuration in which a periphery of a core is surrounded by a cladding, a dielectric cladding having a different refractive index is provided at a boundary of a straight line drawn close to the core. The core has a wave number vector in a plane formed by the optical waveguide direction of the core and a normal set on the boundary surface of the dielectric having a different refractive index, and an angle θ between the wave axis and the optical axis of the wave vector, A fiber type optical isolator having a diffraction grating satisfying 0 ° <θ <90 °. 2. The dielectric cladding having different refractive indices,
2. The fiber-type optical isolator according to claim 1, wherein a part of the clad of the optical fiber is deleted, and a resin having a refractive index different from that of the clad is adhered to the deleted part. 3. The relationship of n1>n2> n3, where n1, n2 and n3 are the refractive indices of the cladding, core and cladding having different refractive indices, respectively.
The fiber-type optical isolator according to 1. 4. A diffraction grating comprising a phase grating whose refractive index is changed is formed on the core by irradiating two-wave interference light from a side surface along an optical axis to an optical fiber having a core surrounded by a clad. And removing the clad by polishing a surface perpendicular to the plane formed by the wave vector and the optical axis of the diffraction grating to a region close to the side surface of the core, the resin having a smaller refractive index than the clad and the core. Adhering to the deleted region.