JPH0534632A - Optical isolator - Google Patents

Abstract

PURPOSE:To provide an optical isolator highly suited to the high degree and the large capacity of optical fibers communication, having no polarization dispersion and being little loss. CONSTITUTION:A first polarization crystal 12, a rotary polarization crystal 13, a second polarization crystal 14, a Faraday element 15, a first analyzing crystal (double refraction element) 17 are successively arranged from the light incident side and the first and the second analyzing crystals 16, 17 are characterized by being composed of a parallel plate-shaped double refraction material.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical circuit element, and more particularly to a polarization-independent optical isolator suitable for optical fiber communication, optical fiber measurement and the like.

[0002]

2. Description of the Related Art Conventionally used optical isolators are
In order to make the characteristics independent of the polarization of the incident light, the birefringent plate as shown in FIG. 1 was used and the structure was as shown in FIG. However, in such a conventional optical isolator, a birefringent plate is used as the polarization separation means,
The incident light beam is split into polarized waves by a birefringent plate, and after performing a predetermined operation, it is configured to combine polarized waves again.
Since the difference in the optical path length between the ordinary ray and the extraordinary ray of the birefringent plate is not 0 but a finite numerical value, the time during which the light passes through the isolator is not constant as the polarization of the incident light flux changes.
There was a problem that it changed. This is a particular problem because it is difficult to perform system compensation in an optical communication system of high capacity. This is called polarization dispersion.

The structure shown in FIG. 1 shows the principle of the polarized light separating means. When the incident light C is incident, it is separated into an ordinary ray and an extraordinary ray and passes therethrough, and the ordinary ray A and the extraordinary ray B are passed. And separated by the separation width d. In addition, what is indicated by an arrow in the illustrated crystal represents the optical axis of the crystal, that is, the C axis. The thickness of this crystal is t. Usually, the separation width d is about t / 10.

Further, in the conventional optical isolator shown in FIG. 2, an unpolarized (that is, randomly polarized) incident light 1 shown in FIG. 2 enters a birefringent crystal 2 and passes through a crystal 2 having a thickness t _1. , And then becomes a light beam with the polarization property shown in
To the birefringent crystal 4 having the thickness t ₂ , and then to the birefringent crystal 5 having the thickness t ₃ and incident on the birefringent crystal 5 having the thickness t _3. ,again,
The light is combined into a light beam having the polarization property shown in. The ratio of each thickness, t ₁ : t ₂ : t ₃ , is √2: 1: 1.
However, √ indicates a square root, and so on. The polarizability of each ray is different in the forward direction and the reverse direction, and as shown in the figure, the light in the reverse direction, that is, the light reflected and returned from the outside has different polarizability. That is, in the configuration of the conventional optical isolator, one birefringent crystal, an optical rotatory crystal, and two birefringent crystals are sequentially arranged along the order of the optical path, and the birefringent crystal has a parallel plate-shaped birefringence. Material is used.

[0005]

SUMMARY OF THE INVENTION The present invention has been made to solve the above problems, and in particular, eliminates the drawbacks of the conventional methods as described above, has no polarization dispersion, and An object of the present invention is to provide an optical isolator suitable for high-capacity, high-capacity optical fiber communication with low loss. Another object of the present invention is to provide an optical isolator that can be manufactured easily and can reduce costs because it uses parallel plate components.

[0006]

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned technical problems, and includes a first polarizing crystal, an optical rotatory crystal, in order from a light incident side to a light emitting side. A second polarizing crystal, a Faraday element, a first analyzing crystal and a second analyzing crystal are arranged, and the first and second analyzing crystals are composed of parallel plate birefringent materials. An optical isolator is provided. Further, the first polarizing crystal and the second polarizing crystal have the same thickness, and the thickness of the first analyzing crystal and the second analyzing crystal is √2 of the thickness of the polarizing crystal. A value that is (square root of 2) times is preferable.
Further, the optical axis of the first polarizing crystal and the optical axis of the second polarizing crystal are opposed to each other, and the optical axis of the first analyzing crystal and the optical axis of the second analyzing crystal are opposed to each other. It is suitable.

FIG. 1 is a perspective view schematically showing how, when a ray C is incident on a birefringent crystal, it is separated into an ordinary ray A and an extraordinary ray B at a separation distance d = about t / 10. Is. Then, as shown in FIG. 2, FIG. 2 shows the first port 1
The light incident on is separated into an ordinary ray and an extraordinary ray by the birefringent crystal 2 having a thickness t ₁ , and the ordinary ray goes straight, but the extraordinary ray component moves in the vertical direction by about d. And
Ordinary rays are incident on the non-reciprocal optical rotatory crystal 3 and, in the optical rotatory crystal (Faraday element) 3, (counterclockwise 45
Degree, the polarization direction is rotated), the polarization direction is rotated, and the polarization of the extraordinary ray moved by about d is also rotated by 45 °. Both rays then enter the birefringent crystal 4. And, the two light rays emitted from the birefringent crystal 4 are
The light enters the birefringent crystal 5, is recombined, and is emitted.

FIG. 3 shows an ellipsoid of the refractive index when a light ray is incident on the birefringent crystal 2 (calcite), and the portion sandwiched by the end faces 2a and 2b of the birefringent crystal 2 becomes the birefringent crystal. . The ellipsoid represents the refractive index in each direction, and n
_e is the refractive index in the Z-axis direction, and n ₀ is the refractive index in the X-axis direction. Further, θ is an angle forming an end face with the optical axis (that is, the Z axis), and γ is a separation angle between an ordinary ray and an extraordinary ray. And A shows an ordinary ray and B shows an extraordinary ray.

FIG. 4 shows that when the thickness of the birefringent crystal is replaced by l, the separation width of the ordinary ray and the extraordinary ray is d, and the separation angle is γ, the geometrical length l _e of the optical path of the extraordinary ray is , L _e = l / cos γ. By strictly considering such an optical path difference, the optical path difference (dispersion) between the ordinary ray and the extraordinary ray in the birefringent crystal can be known.

FIG. 5 shows the structure of the present invention, in which the light beam 11 is sequentially provided from the incident side to the light emitting side to form the first polarizing crystal 1.
2, optical rotatory crystal 13, second polarizing crystal 14, Faraday
Element 15, first analysis crystal 16 and second analysis crystal 17
Through. Then, the light in the opposite direction shifts in position, travels in the opposite direction, and does not return to the position 11. The arrow on the wall surface of the crystal represents the optical axis (c-axis) of the crystal. Among them, the solid line indicates that it is on the surface shown, and the broken line indicates that it is not on that surface.

That is, sequentially, as shown in the lower half of FIG. 5, the light incident from the first port 11 is a randomly polarized light beam as shown by, and the birefringent crystal 12 of thickness t
The ordinary ray and the extraordinary ray are separated, and the ordinary ray goes straight, but the extraordinary ray component moves in the vertical direction by about d. That is,
The polarized light is shown in. And both rays are
The light passes through the optical rotatory crystal 13 having the thickness t ₁ to become a polarized light ray shown by, and further, the optical rotatory crystal 14 having the thickness t _2.
And becomes a polarized light ray indicated by, and in the magneto-optical crystal (Faraday element) 15 (45 degrees counterclockwise, the polarization direction is rotated), the polarization is rotated, and the polarization of the extraordinary ray The sex is also rotated 45 °. That is, the polarized light beam shown in Next, both of the light rays pass through the birefringent crystal 16 having the thickness t ₃ to become the polarized light rays shown by, and further enter the birefringent crystal 17 having the thickness t ₄ , are recombined and emitted. The outgoing ray is also randomly polarized and does not depend on the polarization state of the incoming ray.

On the other hand, the light rays incident in the opposite direction (from 18) are as shown in the lower half of the lower half of FIG. Therefore, the light rays in the opposite directions are well separated, and each light ray can be separated (removed).

That is, the thickness ratio of the illustrated polarization crystal, optical rotatory crystal and birefringent crystal 12, 14, 16 and 17 is t ₁ : t.
_{2: t 3: t 4 =} 1: 1: √2: a √2. The optical rotatory crystal is a crystal that rotates incident light by 90 °.
Further, the magneto-optical crystal (15) is a crystal which causes non-reciprocal optical rotation at 45 °. The symbol H in FIG. 5 indicates a magnetic field.

FIG. 6 is a schematic sectional view for better explaining the above-mentioned principle of the optical isolator of the present invention. That is, the optical isolator of the present invention comprises the first polarization crystal 12, the optical rotatory crystal 13, the second polarization crystal 14, the Faraday element 15, and the first polarization crystal in order from the light incident side to the light emission side.
The birefringent crystal 16 and the second birefringent crystal 17 are arranged. Then, the incident light ray 11 travels in the parallel plate-shaped birefringent material according to the present invention as indicated by a solid arrow. Therefore, at point 18, the traveling waves are the same, there is no difference due to the path, and the rays reflected from the outside are
As shown by the broken line arrow, when the light travels and exits the optical isolator of the present invention, it is separated as shown. Therefore, each light ray can be separated. Further, the thickness ratio of the illustrated polarization crystals 12, 14, 16, 17 is t ₁ :
t ₂ : t ₃ : t ₄ = 1: 1: √2: √2.

According to the structure of the present invention, it is possible to obtain a polarization-independent isolator in which the transit time of light does not change even if the polarization of the incident light flux changes, that is, there is no polarization dispersion. That is, in the optical isolator of the present invention, among the three birefringent crystals of the conventional isolator, instead of the polarizing crystal (first birefringent crystal) having a difference in the optical path length between the ordinary ray and the extraordinary ray, Using two polarizing crystals with a thickness of half,
The optical axes of the birefringent crystals are opposed to each other, and a 90-degree optical rotatory crystal is disposed between the polarizing crystals to make the difference in optical path length between the ordinary ray and the extraordinary ray of the birefringent crystal zero. Therefore, the dispersion due to the incident polarized light of the isolator is set to zero.

According to the structure of the present invention, a polarization-independent optical isolator is obtained in which the light transit time does not change even if the polarization of the incident light beam to the optical isolator changes, that is, there is no polarization dispersion. To be Therefore, an optical isolator that does not depend on polarized light and has good separation and coupling can be realized with high accuracy.

Next, the optical isolator of the present invention will be specifically described by way of examples with reference to the drawings, but the present invention is not limited thereto.

[0018]

EXAMPLE FIG. 1 is a schematic perspective view showing how, when a randomly polarized light beam is incident on a calcite 1 as a birefringent plate, the light beam is divided into two polarized light beams, an ordinary ray and an extraordinary ray, which are orthogonal to each other. is there. At this time, the traveling directions of the two light fluxes are referred to as a separation angle and are denoted by γ. If a birefringent crystal is used as the optical isolator, the larger the separation angle γ, the better. In calcite, it is customary to use "cleavage" to create a birefringent crystal, but the angle of cleavage at this time is close to the ideal as a birefringent crystal, so the separation angle γ is It depends on the angle.

The angle of the cleavage is represented by θ. That is, the angle θ between the optical axis and the cleavage plane is 45 ° 24 ′, and the light beam is made to enter perpendicularly to the cleavage plane. At this time, due to the birefringence of calcite, the light beam is divided into "ordinary rays" and "extraordinary rays" in the crystal.

The separation angle γ at this time is such that the refractive index of calcite is
As n ₀ = 1.663 and n _e = 1.477 (for incident light wavelength λ = 1.55 μm), γ = ψ-tan ^-1 {(n ₀ / n _e ) ² × tan ψ} (However, ψ is the complementary angle of θ, and ψ = 90 ° −θ.). Therefore, γ =
5.72 °, or 5 ° 43'19 "(wavelength λ of incident light
= 1.55 μm).

Further, the refractive index for extraordinary rays corresponds to the length of the radius of the polarization azimuth of the extraordinary rays forming the index ellipsoid. Therefore, if n _e 'is the refractive index for extraordinary rays, then n _e ' = (n ₀ × n _e ) / √ (n _e ² × sin ² ψ + n _e ² × cos ² ψ) (where √ is its square root) Therefore, n _e 'is
Calculated as 1.5502.

When the thickness of calcite (that is, the length through which light rays pass) is l, the air-converted length l ₀ of the optical path of ordinary rays is l ₀ = l / n ₀ . On the other hand, the air-converted length l _e of the optical path of the extraordinary ray is given by l _e = l / n ′ ₀ / cosγ.

Therefore, regarding the configuration of the present invention, FIG.
Calculated with reference to 3 and 4, calcite thickness l is 1 m
When m is n ₀ = 1.633 and n _e = 1.477, l ₀ = 1 / 1.633 = 0.6124 (mm), and l _e = 1 / n ′ ₀ / cosγ = 1/1 0.5502 / 0.
It becomes 995 = 0.6483 (mm). Therefore, Δl =
| L ₀ −l _e | = 0.0359 (mm).

FIG. 4 shows the thickness of a birefringent crystal or calcite,
When replaced by l, if the separation width of the ordinary ray and the extraordinary ray is d and the separation angle is γ, the geometric length l _e of the optical path of the extraordinary ray is l _e = l / cosγ.

FIG. 5 is an explanatory view conceptually showing the structure of the optical isolator of the present invention. That is, the first polarization crystal 12, the optical rotatory crystal 13, and the second polarization crystal are sequentially arranged from the incident side to the light emitting side.
The parallel plate-shaped birefringent material including the polarizing crystal 14, the Faraday element 15, the first analysis crystal 16 and the second analysis crystal 17. With respect to the above configuration, the traveling paths of the respective light rays therein are shown in detail in FIG.

In the optical isolator of the present invention, as described above and as shown in the drawings, the light propagates through the parallel plate optical element. Furthermore, in the optical isolator of the present invention, all the optical components that are formed are parallel plane plates, so that assembly is very easy. For example, each of the birefringent plates to be used may be made by aligning the axes and polishing it into parallel planes. For example, in the case of calcite, only the cleaved surface may be polished. Then, as the optically active crystal to be used, for example, a quartz optical rotator may be used, and it may be simply polished into a plane parallel to a length giving a desired rotation angle.

Therefore, since the parts and the crystal device are all parallel flat plates, they can be assembled by assembling the surfaces and bonding them together with an optical adhesive. When producing a large amount, a birefringent crystal or an optical rotatory crystal can be adhered as a large plate (while having a large size), and then cut into small pieces.

[0028]

As described above, the optical isolator of the present invention has the above-mentioned effects. Summarizing them, the following remarkable technical effects are obtained. That is, firstly, it is possible to realize an optical isolator suitable for high-capacity optical fiber communication, which has a small insertion loss, has no polarization dependence, and has no polarization dispersion.

Secondly, the optical isolator can be constructed at low cost and with high precision, and all the optical parts are parallel plane plates, and only the transmitted light of the parallel plate parts is used.
Thirdly, since the composite polarizing crystal, which is the main component, is a parallel plate having the same size, a large number of optical isolator elements can be adjusted by one adjustment by axially adhering and fixing the large plate birefringent crystal and then cutting. The optical isolator can be manufactured easily, and the cost can be reduced.

[Brief description of drawings]

FIG. 1 is a schematic configuration diagram illustrating an optical path of calcite as an ordinary birefringent plate.

FIG. 2 is a schematic configuration diagram showing a technique of a conventional optical isolator.

FIG. 3 is an explanatory diagram showing a uniaxial crystal used in the optical isolator of the present invention; an index ellipsoid of calcite.

FIG. 4 is an explanation for calculating an optical path difference between an ordinary ray and an extraordinary ray used in the optical isolator of the present invention.

FIG. 5 is a schematic perspective view showing the configuration of an example of the optical isolator of the present invention.

FIG. 6 is a schematic diagram illustrating the principle of the optical isolator of the present invention.

[Explanation of symbols]

12 First polarizing crystal 13 Optical rotation crystals 14 Second polarizing crystal 15 Faraday element 16 First detection crystal 17 Second detection crystal 18 reflection points

Claims (3)

[Claims] 1. A first polarizing crystal, an optical rotatory crystal, a second polarizing crystal, a Faraday element, a first analyzing crystal (birefringent element) and a second polarizing crystal in order from the light incident side to the light emitting side. An optical isolator, in which analyzer crystals (birefringent elements) are arranged, and the first and second analyzer crystals are made of a parallel plate birefringent material. 2. The first polarizing crystal and the second polarizing crystal have the same thickness, and the first analyzing crystal and the second analyzing crystal have the same thickness. 2. The optical isolator according to claim 1, wherein the optical isolator is √2 (square root of 2) times. 3. The optical axis of the first polarizing crystal and the optical axis of the second polarizing crystal face each other, and the optical axis of the first analyzing crystal faces the optical axis of the second analyzing crystal. The optical isolator according to claim 1 or 2, characterized in that.