JP2001215446A - Polarization independent dispersion type optical module, optical isolator and optical circulator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical isolator which is small in size, is reduced in insertion loss and is extremely small in polarization dispersion. SOLUTION: The optical module arranged with at least >=1 birefringent crystal plates between the end faces of at least >=2 fibers has the two birefringent crystal plates which are equal in each other's thicknesses and is reverse in a shift direction of a beam as the polarization dispersion compensation means of the optical module.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical module such as an optical isolator and an optical circulator for use in optical communication and optical measurement, and more particularly to an optical module of a polarization independent type and a polarization free type.

[0002]

2. Description of the Related Art A polarization-independent optical isolator is an optical component having a nonreciprocal function of allowing an optical signal to pass in only one direction, and has been used for an input / output unit of an optical fiber amplifier.

FIG. 6 is a diagram showing the configuration of a conventional optical isolator 30 (Japanese Patent Publication No. 60-51960). A first birefringent crystal 31, a first magneto-optical crystal 34, a second birefringent crystal 32, and a third birefringent crystal 33 are arranged in this order, and lenses 35 and optical fibers 36 and 37 are arranged on both sides thereof. You.

The birefringent crystals 31, 32, and 33 have a function of separating and combining light according to the polarization direction of the light. here,
Assuming that the thickness of the first birefringent crystal 31 is t, the second and third
The thicknesses of the birefringent crystals 32 and 33 are equal, and are configured with a thickness of t / √2.

The direction of separation and synthesis is determined by the direction of the crystal axis of the birefringent crystal. The crystal axis of the second birefringent crystal 32 is at an angle of 45 degrees with respect to the crystal axis of the first birefringent crystal 31. And a third birefringent crystal 33
Is arranged so as to have an angle of 90 degrees with the crystal axis of the second birefringent crystal 32. Magneto-optical crystal 34
Has a thickness that rotates the plane of polarization of light having a predetermined wavelength at a saturation magnetic field strength by 45 °, and is disposed in a ring-shaped magnet via a component holder (not shown).

The light emitted from the optical fiber 36 or 37 is condensed by the lens 35 with a spot radius W of a Gaussian beam having substantially the same size. At this time, the spot radius W and the thickness t of the birefringent crystal 31 determine the isolation characteristics of the optical isolator, and are usually designed to obtain sufficient isolation. Equation (1) is an equation showing the relationship between W, t, and isolation Is.

Is = −10 × log (exp (− (t / 10) ² / W ² ) (1) As can be seen from equation (1), t is increased in order to increase the isolation Is. Alternatively, there is a method of reducing W. In general, the birefringent crystal has a problem that the cost increases as the thickness t increases, the size of the product increases, and the mounting cost increases. A method of realizing high isolation characteristics by reducing the size is generally used.

FIGS. 7 and 8 show the optical path and the polarization direction in a conventional optical isolator. In the drawing, a straight line indicates the polarization direction of light, and a circle indicates a beam position.

First, light traveling in the forward direction from the optical fiber 36 will be described with reference to FIG. The light incident from the optical fiber 36 becomes a condensed beam by the first lens 35 and enters the first birefringent crystal 31 (A). Light is separated into ordinary light and extraordinary light by the first birefringent crystal 31 (B). By passing through the first magneto-optical crystal 34, the polarization directions of the ordinary light and the extraordinary light are rotated counterclockwise by -45 degrees and are incident on the second birefringent crystal 32 (C). One of the two lights incident on the second birefringent crystal 32 becomes extraordinary light in the birefringent crystal 32 and is shifted (D). Then the third
Incident on the birefringent crystal 33 of the second
The light not shifted in step 2 becomes extraordinary light and is shifted and combined (E). The combined light is used as the second lens 35
And emitted from the optical fiber 37.

Next, the light traveling in the opposite direction, which is incident from the optical fiber 37, will be described with reference to FIG. The light incident from the optical fiber 37 is condensed by the second lens 35 and is incident on the third birefringent crystal 33 (E). The light is separated into ordinary light and extraordinary light by the third birefringent crystal 33 (D). Of the two lights incident on the second birefringent crystal 32, the ordinary light becomes extraordinary light in the birefringent crystal 32 and is shifted (C). Next, by passing through the first magneto-optical crystal 34, the polarization directions of the ordinary light and the extraordinary light are rotated clockwise by 45 degrees, and enter the first birefringent crystal 31 (B). Then the first
One of the lights incident on the birefringent crystal 31 is abnormal light and is shifted (A), and the separated light is not synthesized by the second lens 35 and is not emitted from the optical fiber 36.

As described above, the optical fiber 36
Is emitted from the optical fiber 37 irrespective of its polarization state, and the light incident from the optical fiber 36 is
An optical isolator function of not emitting light from the optical fiber 37 regardless of its polarization state can be provided.

An optical isolator having a configuration in which the polarization dispersion of the optical isolator is improved has also been proposed. Polarization dispersion refers to a characteristic in which the group velocity of light propagating in an optical module changes depending on the polarization direction, and is an important characteristic that determines the spread of transmitted optical pulses, that is, transmission capacity. In general, an optical module used for a high-speed and high-density transmission system requires a design with zero polarization dispersion.

In the optical isolator shown in FIG. 6, polarization dispersion occurs because the optical path lengths of two separated and combined lights are different. More specifically, since the second birefringent crystal and the third birefringent crystal have the same thickness and their optic axes are orthogonal to each other, the polarization dispersion generated by these two birefringent crystals is zero. is there. Therefore, the difference between the group velocities of ordinary light and extraordinary light in the first birefringent crystal 31 becomes polarization dispersion.

In order to compensate for this dispersion, a single birefringent crystal for dispersion compensation is arranged on the optical path.

FIG. 9 is a diagram showing a configuration of a conventional non-polarization dispersion type optical isolator 39. As shown in FIG. 6, the polarization compensating birefringent crystal 38 is arranged on the optical path in the optical isolator shown in FIG. 6, and unlike the other birefringent crystals, the dispersion compensating birefringent crystal 38 does not separate and combine light. They are arranged so that the group velocity differs depending on the polarization direction of light. Specifically, other birefringent crystals that separate and combine light are configured such that the optical axis is inclined by 48 degrees with respect to the light incident direction (Z-axis direction) in order to maximize the light separation angle. Birefringent crystal for polarization dispersion compensation 3
8 is an optical axis at an angle of 90 degrees with respect to the light incident direction,
In addition, they are arranged at an angle of 90 degrees with respect to the optical axis of the birefringent crystal 31.

[0016]

However, in the conventional optical isolator 30 shown in FIG. 6, the ordinary light and the extraordinary light are separated and combined and emitted from the optical fiber 37. At this time, the ordinary light and the extraordinary light have different optical path lengths. There is a problem that polarization dispersion occurs.

In the conventional non-polarization polarization dispersion type optical isolator 39 shown in FIG. 9, since the birefringent crystal generally has a different refractive index depending on the incident angle of the extraordinary ray, the birefringent crystal is inserted into the condensed beam. The extraordinary ray does not converge at one point, causing aberration. Further, here, the aberration generated in the birefringent crystal 31 for beam separation is different from the aberration generated in the birefringent crystal 38 for polarization dispersion compensation.
When the beams are combined and coupled to the optical fiber 37, the polarization dispersion is compensated, but the aberration is not compensated and the insertion loss of the optical module increases.

The phenomenon of this difference in aberration is that the crystal axis of the birefringent crystal 31 for beam separation is set at 48 degrees with respect to the light incident direction (Z-axis direction), whereas the polarization Since the optical axis of the refraction crystal 38 is set at an angle of 90 degrees with respect to the light incident direction (Z-axis direction), the refractive index and the refraction angle are different for extraordinary rays having the same incident angle.

When the diameter of the condensing spot is increased, the angle of incidence of the light beam is reduced, so that the aberration is reduced and the insertion loss is reduced. However, in order to obtain a sufficient isolation characteristic,
It is necessary to increase the thickness of the birefringent crystal, so that the cost increases and there is a problem that the apparatus becomes larger.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned conventional problems, and an object of the present invention is to provide an optical module having a reduced external size, a reduced insertion loss, and no polarization dispersion. .

[0021]

SUMMARY OF THE INVENTION The present invention has been made to solve these problems, and it is an object of the present invention to provide an optical module in which at least one birefringent crystal plate is disposed between at least two fiber end faces. As the wave dispersion compensating means, two birefringent crystal plates having the same thickness and the opposite beam shifting directions are used.

Further, according to the present invention, two orthogonal polarization components in an incident light beam are separated between at least two or more fiber end faces, and two polarization components incident on different optical paths are combined into the same optical path. An optical isolator comprising at least two or more light beam separating / combining means, a polarization rotating means disposed between the light beam separating / combining means, and a polarization dispersion compensating means, wherein the polarization dispersion compensating means has a mutual thickness. It is characterized by comprising two birefringent crystal plates having the same and opposite beam shift directions.

Furthermore, the present invention separates two orthogonal polarization components in an incident light beam between at least two fiber end faces, and combines two polarization components incident on different optical paths into the same optical path. At least two or more light beam splitting / combining means, and an optical path determining means disposed between the light beam splitting / combining means for advancing an incident light beam in different directions according to the polarization direction thereof; In an optical circulator provided with a polarization rotating unit and a polarization dispersion compensating unit disposed between the optical path determining unit and the polarization dispersion compensating unit, the polarization dispersion compensating units may have the same thickness and opposite beam shift directions. Characterized by comprising two birefringent crystal plates.

[0024]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, an optical module according to the present invention will be described.

FIG. 1 is a configuration diagram showing an embodiment of an optical isolator 10 which is an example of the optical module of the present invention. The light beam emitted from the optical fiber 1 passes through the lens 3 and
Birefringent crystal 4, first and second dispersion-compensating birefringent crystals 5, 6, magneto-optical crystal 7, third and fourth birefringent crystals 8,
9 and enters the optical fiber 2.

Here, a ray that follows the ordinary law of refraction is called an ordinary ray, and a ray that does not obey it is called an extraordinary ray. A polarized ray parallel to the direction of the crystal axis on the XY plane is an extraordinary ray, and a perpendicular polarized ray is an extraordinary ray. Acting as an ordinary ray, the extraordinary ray shifts in the direction of the crystal axis.

The arrows shown on the birefringent crystals 4, 5, 6, 8, and 9 indicate the directions of the crystal axes, and the crystal axes of all the birefringent crystals are set to have the maximum beam separation angle.
If the angle between the light incident direction (Z-axis direction) and the crystal axis is θ, the angle between the beam shift direction of the extraordinary ray and the crystal axis is φ, the ordinary refractive index is no, and the extraordinary refractive index is ne, then θ and φ Is in the relationship shown in equation (2).

Tan φ = (no ² / ne ² ) × tan θ (2) From equation (2), the beam separation angle is the maximum, that is, (θ−φ)
The crystal axis direction in which is maximized is determined. For example, in the case of a rutile crystal, no = 2.45 and ne = 2.71;
From the equation (2), it can be calculated that the crystal axis direction θ at which the beam separation angle (θ−φ) is the maximum is 47.8 degrees. At this time, the beam separation angle (θ−φ) = 5.7 degrees.

Next, the crystal axis direction of each birefringent crystal on the XY plane will be described. The first birefringent crystal 4 has a crystal axis in the Y-axis direction.
The crystal axis of the dispersion-compensating birefringent crystal 6 is shifted by 90 degrees from the crystal axis of the first birefringent crystal 4. And,
The second dispersion-compensating birefringent crystal 6 has a crystal axis direction in a state where the first dispersion-compensating birefringent crystal 5 is rotated by 180 degrees about the X axis. By arranging the crystal axis directions in this manner, the beam shift directions of the first birefringent crystal for dispersion compensation 5 and the second birefringent crystal for dispersion compensation 6 are opposite.

Next, the second birefringent crystal 8 is arranged so as to be shifted by 45 degrees with respect to the crystal axis of the first birefringent crystal 4, and the third birefringent crystal 9 is positioned with respect to the second birefringent crystal 8. They are shifted by 90 degrees. By arranging the crystal axes in this way, the beam separated by the first birefringent crystal 4 is converted into the second, birefringent crystal 4
The third birefringent crystals 8 and 9 are synthesized.

Next, the thickness of each crystal will be described. First
, The thickness of the first and second birefringent crystals for polarization dispersion compensation 5 and 6 is equal to each other, and
Let it be 2. The thicknesses of the third and fourth birefringent crystals 8 and 9 are equal to each other and are set to t / √2. In addition, the thickness of the magneto-optical crystal 7 is set to a thickness at which polarized light rotates by 45 degrees in a saturation magnetic field.

FIGS. 2 and 3 are views showing the optical path and the polarization direction in the optical isolator according to the present invention. In the drawing, a straight line indicates the polarization direction of light, and a circle indicates a beam position.

First, the light traveling in the forward direction from the optical fiber 1 will be described with reference to FIG. Optical fiber 1
Incident on the first birefringent crystal 4 becomes a condensed beam by the first lens 3 and enters the first birefringent crystal 4 (A). The light is separated into ordinary light and extraordinary light by the first birefringent crystal 4 (B).
In the first birefringent crystal 5 for polarization dispersion compensation, ordinary light in the first birefringent crystal 4 becomes extraordinary light and shifts (C). Further, in the second polarization-dispersion compensating birefringent crystal 6, the beam is shifted as extraordinary light, but the shift direction is opposite to that of the first polarization-dispersion compensating birefringent crystal, and the beam position and the polarization direction are changed. Is the same as the state (B) transmitted through the first birefringent crystal 4,
The optical path lengths of the ordinary light and the extraordinary light become equal, and the polarization dispersion generated in the birefringent crystal 4 is compensated (D). Thereafter, by passing through the first magneto-optical crystal 7, the polarization directions of the ordinary light and the extraordinary light are rotated counterclockwise by −45 degrees, and enter the second birefringent crystal 8 (E). 2 incident on the second birefringent crystal 8
One of the lights of the book becomes extraordinary light in the birefringent crystal 8 and is shifted (F). As for the light incident on the third birefringent crystal 9, the light not shifted by the third birefringent crystal 8 becomes an extraordinary light and is shifted, and the two lights are combined (G). The combined light is condensed by the second lens 3 and emitted from the optical fiber 2.

Next, referring to FIG. 3, the light traveling from the optical fiber 2 and traveling in the opposite direction will be described. Optical fiber 2
Incident on the second birefringent crystal 9 is converted into a condensed beam by the second lens 3 and is incident on the third birefringent crystal 9 (G). The light is separated into ordinary light and extraordinary light by the third birefringent crystal 9 (F).
Of the two lights that have entered the second birefringent crystal 8, ordinary light becomes extraordinary light in the birefringent crystal 8 and is shifted (E). Next, by passing through the first magneto-optical crystal 7, the polarization directions of the ordinary light and the extraordinary light are rotated clockwise by 45 degrees, and are incident on the second polarization dispersion compensating birefringent crystal 6 (D). In the second polarization-dispersion compensating birefringent crystal 6, the extraordinary light in the second birefringent crystal 8 becomes extraordinary light and shifts (C). Further, in the first polarization-dispersion compensating birefringent crystal 5, the beam is shifted as extraordinary light, but the shift direction is opposite to that of the second polarization-dispersion compensating birefringent crystal 6, so that the beam position and the polarization The direction is the same as the state (D) incident on the second polarization-dispersion compensating birefringent crystal 6, and the direction is incident on the first birefringent crystal 4 (B). After that, one of the light incident on the first birefringent crystal 4 becomes an extraordinary light and is shifted (A), and the separated light is the first light.
And is not emitted from the optical fiber 1.

As described above, the light incident from the optical fiber 1 is emitted from the optical fiber 2 irrespective of its polarization state, and the light incident from the optical fiber 2 is irrespective of its polarization state. 1 can have the function of an optical isolator that does not emit light. Also, by inserting the birefringent crystals for dispersion compensation 5 and 6 on the optical path,
The optical path lengths of the two beams to be separated and combined become equal, and a non-polarization dispersion type optical isolator in which polarization dispersion is compensated is realized.

FIG. 4 is a diagram for explaining the shape of an aberration spot generated in a birefringent crystal. The arrow in the figure indicates the direction of the crystal axis of the birefringent crystal.

In FIG. 4, the spot shape at the focal position when the condensed beam 11 passes through the birefringent crystal 4 and the birefringent crystals 5 and 6 related to polarization dispersion compensation is shown as (A). ) (B), (C) and (D). (A) shows a condensed beam having a Caussian distribution, and the spot shape of (B) shows a case where only the birefringent crystal 4 is inserted into the condensed beam 11 of a Gaussian distribution. When the birefringent crystal 4 and the polarization-dispersion compensating birefringent crystal 5 are inserted, the spot shape of (D) is such that the birefringent crystal 4 and the polarization-dispersion compensating birefringent crystals 5 and 6 are inserted into the condensed beam 11. Represents the spot shape in the case. The spot shape was observed with an infrared camera.

The Gaussian beam (A) incident on the birefringent crystal plate 4 is separated into ordinary light and extraordinary light. Here, since the refractive index of the extraordinary light changes depending on the crystal axis of the birefringent crystal and the traveling direction of the light beam, the refraction direction differs depending on the incident angle of the beam. Z
A non-uniform focal point is formed in the axial direction. That is, in the case of a condensed beam that is condensed at an angle, after passing through the birefringent crystal 4, astigmatism occurs without converging to one point, resulting in an elliptical beam shape (B). . On the other hand, since ordinary light does not have a different refractive index depending on the direction in which it travels through the birefringent crystal, astigmatism does not occur and the beam shape remains a perfect circle (B). Next, the birefringent crystals 5 and 6 for polarization dispersion compensation have their crystal axes shifted by 90 degrees with respect to the birefringent crystal 4 so that the ordinary light becomes extraordinary light and the extraordinary light becomes ordinary light. Further, the total thickness of the polarization-dispersion compensating birefringent crystals 5 and 6 is equal to the birefringent crystal 4.
And the crystal axes are arranged opposite to each other. Therefore, the exit position of the light transmitted through the polarization-dispersion compensating birefringent crystals 5 and 6 is the same as the incident position, and the extraordinary light is 9 times larger than the beam shape of the extraordinary light generated in the birefringent crystal 4.
It becomes an elliptical shape rotated by 0 degrees (D).

As described above, the two beams transmitted through and separated from the birefringent crystal 4 and the birefringent crystals 5 and 6 for compensating for polarization dispersion generate astigmatism in directions shifted by 90 degrees from each other. Are combined, the aberrations cancel each other out and are compensated for, resulting in a beam free of astigmatism. Therefore, the coupling efficiency with the optical fiber is improved, and an optical module with a small insertion loss is realized.

Next, another embodiment of the present invention will be described.
FIG. 5 is a configuration diagram showing an embodiment of the optical circulator 20 which is an example of the optical module of the present invention.

Condensing lenses 15 and 16 are disposed between the two optical fibers 12 and 14 and one optical fiber 13, and a birefringent crystal and a magneto-optical crystal are disposed between the condensing lenses 15 and 16. Is inserted. Since the non-reciprocal portion 17 has a function of non-reciprocally shifting the beam, the light output from the optical fiber 12 is input to the optical fiber 13, and the light output from the optical fiber 13 is returned to the optical fiber 12 without returning to the optical fiber 12. By being input to the fiber 14, the function of a three-port optical circulator can be obtained. The optical coupling system composed of the optical fibers 12 and 14 and the condenser lens 15 and the optical coupling system composed of the optical fiber 13 and the condenser lens 16 have the same spot diameter.

Next, the non-reciprocal portion 17 will be described. The non-reciprocal portion 17 includes first and second birefringent crystals 18 and 19, a composite birefringent crystal 21 in which two birefringent crystals are bonded, first and second magneto-optical crystals 22 and 25, The first and second birefringent crystal plates 23 and 24 for polarization dispersion compensation are provided.

The first and second birefringent crystals 18 and 19 have a function of separating and combining an incident light into an ordinary light and an extraordinary light.
Functions as a beam separation / combination unit. Synthetic birefringent crystal 20
Has a function of shifting an incident light beam as extraordinary light, and functions as an optical path determining means. The birefringent crystal 18 and the birefringent crystal plate 19 are arranged with the crystal axes in opposite directions,
Further, the synthetic birefringent crystal 21 is arranged so that the crystal axes of the two birefringent crystals are opposite to each other and the bonding plane and the crystal axis are horizontal.

The thickness of the synthetic birefringent crystal 21 in the Z-axis direction is represented by t
Then, the thickness of the birefringent crystals 18 and 19 is t / √2 times the thickness. Birefringent crystal 18, synthetic birefringent crystal 2
1. Between the birefringent crystals 19, between the birefringent crystals 18 and 19, a magneto-optical crystal 22 whose thickness is such that the polarization rotation direction rotates clockwise by 45 degrees with respect to the Z axis, and the polarization rotation direction is A magneto-optical crystal 25 having a thickness that rotates −45 degrees counterclockwise with respect to the Z axis is arranged. Further, the polarization dispersion compensating birefringent crystals 23 and 24 having a function of compensating the optical path length difference between the ordinary light and the extraordinary light are disposed after the combined birefringent crystal 21.

The polarization dispersion compensating birefringent crystals 23 and 24 have their crystal axes shifted by 90 degrees with respect to the combined birefringent crystal 21 so that ordinary light becomes extraordinary light and extraordinary light becomes ordinary light. And a second polarization dispersion compensating birefringent crystal 24.
Is the crystal axis direction in a state where the first polarization dispersion compensation birefringent crystal 23 is rotated by 180 degrees about the X axis. By arranging the crystal axis directions in this way, the first birefringent crystal 23 for polarization dispersion compensation and the second birefringent crystal 24 for polarization dispersion compensation have opposite beam shift directions. Further, the total thickness of the polarization dispersion compensating birefringent crystals 23 and 24 is equal to the thickness of the combined birefringent crystal 21.

Therefore, the outgoing position of the light transmitted through the polarization-dispersion compensating birefringent crystals 23 and 24 is not different from the incident position.
Further, the optical path length difference between the ordinary light and the extraordinary light generated in the synthetic birefringent crystal 21 becomes zero, and a non-polarization dispersion type optical circulator in which polarization dispersion is compensated is realized.

According to the optical circulator 20 described above, an optical circulator having zero polarization dispersion and no astigmatism caused by a birefringent crystal and having a reduced insertion loss is realized.

The above-described polarization dispersion compensating means includes:
Without limitation to the configuration of the optical isolators and optical circulators, a polarization combiner / splitter having a function of separating and combining two polarizations, a composite module in which an excitation light input port and a monitor port are combined with an optical isolator, and the like.
The present invention can be applied to an optical module having a birefringent crystal plate that causes polarization dispersion, and the same effects as described above can be obtained.

[0049]

EXAMPLE A three-port optical circulator shown in FIG. 5 was experimentally manufactured as an example of the optical module of the present invention. The product and configuration used for each part will be described.

The optical fibers 12, 13, and 14 are single mode optical fibers having a spot diameter of 10 μm,
One in which the optical fibers 12 and 14 are fixed to an m-pitch two-core ferrule and the light input / output end face is polished, and one in which the optical fiber 13 is fixed to a single-core ferrule and the light input / output end face is polished. Using. As the condenser lenses 15 and 16, an aspherical lens with a magnification of 2 was used, and the optical coupling system composed of the lens and the optical fiber was composed of a condenser beam having a spot right = 20 μm.

Each birefringent crystal 18, 19,
Rutile crystal is used for 21, 23 and 24, magneto-optical crystal 2
Bismuth-substituted garnet crystals were used for Nos. 2 and 25.
As specific design values, the interval between the optical fibers 12 and 14 = 125 μm, the magnification of the optical coupling system = 2, and the non-reciprocal portion 1
7 was designed with a beam shift amount of 250 μm. In general, the beam separation width of a rutile crystal is 1/10 of its thickness t. Therefore, in order to realize a beam shift amount of 250 μm, the thickness of the rutile crystals 18 and 19 is about 1.8 mm, and the thickness of the synthetic rutile crystal 21 is The thicknesses of the polarization dispersion compensating rutiles 23 and 24 were designed to be approximately 2.5 mm and equal to each other and approximately 1.25 mm.

As a result of trial production and evaluation of the optical circulator in this embodiment, the insertion loss from the optical fiber 12 to the optical fiber 13 and the insertion loss from the optical fiber 13 to the optical fiber 14 are both about 0.4 dB. There is an effect of reducing the insertion loss by 0.3 dB or more as compared with the conventional polarization dispersion compensation system having a single-layer structure.

Further, the optical fiber 13 to the optical fiber 12
And the isolation from the optical fiber 14 to the optical fiber 13 were both 50 dB or more, and very good characteristics as a three-port optical circulator could be obtained. Further, the polarization dispersion was 0.01 ps, which was a result sufficiently satisfying the generally required polarization dispersion value <0.1 ps.

Although the lens used in this embodiment is an aspherical lens, the same function can be obtained with a ball lens, a hemispherical lens, a rod lens, and the like. In addition, as a material used as a birefringent crystal in the non-reciprocal part,
Examples include calcite, rutile, LN crystals, yttrium vanadide, and the like. When rutile is used, for example, the thickness of the crystal is about ten times the desired beam separation width. Further, bismuth-substituted garnet crystal, YIG crystal, or the like is used as the magneto-optical crystal. The thickness of the magneto-optical crystal is set so that the polarization plane of the incident light is rotated by 45 degrees when a saturation magnetic field is applied in the traveling direction of the incident light. As a means for applying a saturation magnetic field, a permanent magnet, for example, an SmCo magnet or the like is usually used (not shown). Further, when a self-biased bismuth-substituted garnet crystal or the like is used as the magneto-optical crystal, no magnet is required, and the size can be further reduced.

[0055]

According to the present invention, as the polarization dispersion compensating means of an optical module in which at least one birefringent crystal plate is disposed between at least two optical fiber end faces,
Providing two birefringent crystal plates having the same thickness and the opposite beam shift directions enables extremely small polarization dispersion and low insertion loss.

[Brief description of the drawings]

FIG. 1 is a configuration diagram showing an embodiment of an optical isolator of the present invention.

FIG. 2 is a diagram showing an optical path and a polarization direction in the optical isolator of the present invention.

FIG. 3 is a diagram showing an optical path and a polarization direction in the optical isolator of the present invention.

FIG. 4 is a diagram illustrating an aberration spot shape generated in a birefringent crystal.

FIG. 5 is a configuration diagram showing an embodiment of the optical circulator of the present invention.

FIG. 6 is a diagram illustrating a configuration of a conventional optical isolator.

FIG. 7 is a diagram showing an optical path and a polarization direction in a conventional optical isolator.

FIG. 8 is a diagram showing an optical path and a polarization direction in a conventional optical isolator.

FIG. 9 is a diagram showing a configuration of a conventional polarization-free dispersion type optical isolator.

[Explanation of symbols]

1, 2, 12, 13, 14, 36, 37: Optical fiber 3, 15, 16, 35: Lens 4, 8, 9, 18, 19, 31, 32, 33: Birefringent crystal 21: Synthetic birefringent crystal 5, 6, 23, 24, 38: Birefringent crystal for polarization dispersion compensation 7, 22, 25, 35: Magneto-optical crystal 11: Light beam 10, 30, 39: Optical isolator 20: Optical circulator 17: Non-reciprocal part

Claims (3)

[Claims] 1. An optical module in which at least one or more birefringent crystal plates are arranged between end faces of at least two or more optical fibers, two optical fibers having equal thicknesses and opposite beam shift directions. A non-polarization dispersion type optical module comprising a birefringent crystal plate as polarization dispersion compensation means. 2. An optical system comprising: an end face of at least two optical fibers for separating two orthogonal polarization components in an incident light beam and combining two polarization components incident on different optical paths into the same optical path; In an optical isolator including the above-described light beam separation / combination means, a polarization rotation means disposed between the light beam separation / combination means, and a polarization dispersion compensation means, the polarization dispersion compensation means has a thickness of each other. An optical isolator comprising two birefringent crystal plates having equal and opposite beam shift directions. 3. An at least two optical fiber between end faces of at least two optical fibers for separating two orthogonal polarization components in an incident light beam and combining two polarization components incident on different optical paths into the same optical path. The above-described light beam separation / combination means, an optical path determination means disposed between the light beam separation / combination means, and for making incident light travel in different directions according to the polarization direction thereof, and the light beam separation / combination means and the light path determination means In the optical circulator provided with the polarization rotating means and the polarization dispersion compensating means disposed between the two, the polarization dispersion compensating means may include two sheets having the same thickness and the opposite beam shifting directions. An optical circulator comprising a birefringent crystal plate.