JPH1090631A - Optical isolator and composite module using it - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a polarization independent type optical isolator suppressing a polarization dependent loss together with polarizing dispersion and preventing optical isolator inside reflection. SOLUTION: In the optical isolator constituted of at least one sheet or above of wedge shape birefringent crystals 1, 2, at least one sheet or above of 45 deg. Faraday rotators 8 and at least one sheet or above of planar birefringent crystals 3, the isolator is constituted so that the main section of the planar birefringent crystal 3 is orthogonally intersected with optical aces of at least one sheet of wedge shape birefringent crystals 1, 2, and the optical axis of the planar birefringent crystal 3 isn't parallel to the light passing surface of the planar birefringent crystal 3. Thus, the precise optical isolator is provided.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an optical isolator used for an optical fiber amplifier and an optical isolator used for an optical fiber amplifier. Further, the present invention relates to a composite module in which the optical isolator, the beam splitter, the WDM coupler, the bandpass filter, and the photodetector are appropriately arranged in one package.

[0002]

2. Description of the Related Art An optical isolator is an optical device having a function of passing light in one direction and blocking light in the opposite direction. An optical isolator is used in optical fiber communication to prevent reflected light returning from an optical component to a laser light source and to prevent occurrence of light resonance in an optical fiber amplifier. At present, in long-distance optical communication such as a submarine optical cable, an optical fiber amplifier having one or more optical isolators between optical cables is incorporated in multiple stages as an optical repeater, and an optical signal is directly amplified and transmitted. The polarization state of light passing through the optical fiber changes randomly due to external stress or bending. As an optical isolator used between optical fibers, a polarization-independent isolator that does not depend on the polarization state of light is used. As the polarization-independent type, an optical isolator having two wedge-shaped birefringent crystals, an optical element portion including a 45-degree Faraday rotator disposed between the wedge-shaped birefringent crystals, and lenses and fibers at both ends thereof is known. (See Japanese Patent Publication No. 61-58809), which is shown in FIG.
From the optical fiber A to the lens A in the forward direction from FIG.
The parallel light that has passed through is divided into light that receives the refractive index of ordinary light and two lights that receive the refractive index of extraordinary light when passing through the wedge-shaped birefringent crystal. The two lights are rotated in the polarization plane by the Faraday rotator by 45 degrees, and enter the wedge-shaped birefringent crystal B. At this time, the optical axes of the wedge-shaped birefringent crystal A and the wedge-shaped birefringent crystal B are 4
Since the positional relationship is shifted by 5 degrees, the light having the ordinary light refractive index in the wedge-shaped birefringent crystal A and the two lights having the extraordinary light refractive index in the wedge-shaped birefringent crystal B have the same relation. After receiving the refractive index of ordinary light and the refractive index of extraordinary light, the two lights exit the wedge-shaped birefringent crystal B as parallel light to the incident light incident on the wedge-shaped birefringent crystal A, and are converged by the lens B. The light enters the optical fiber B. In the opposite direction, the parallel light that has passed through the lens from the optical fiber B is separated by the wedge-shaped birefringent crystal B into light receiving the ordinary light refractive index and two lights receiving the extraordinary refractive index, and rotated by 45 degrees Faraday. After passing through the crystal, it enters the wedge-shaped birefringent crystal A. Since the two light beams are orthogonal to those in the forward direction due to the non-reciprocity of the 45-degree Faraday rotator, the light beam which has ordinary light in the wedge-shaped birefringent crystal B and the light which has received the refractive index of the extraordinary light is in a wedge shape. The birefringent crystal A receives and passes the refractive indexes of extraordinary light and ordinary light. Therefore, the light that has passed through the wedge-shaped birefringent crystal A does not become parallel light, but is emitted so as to spread, and is not coupled to the optical fiber A even after passing through the lens. This has the function of passing light from one direction and blocking the passage of light in the opposite direction.

[0003]

At present, in long-distance optical communications such as submarine optical cables, optical fiber amplifiers having one or more optical isolators as optical repeaters are incorporated in multiple stages between optical cables to directly amplify optical signals. Let me transmit. In forward optical signal transmission by the optical isolator, polarization dispersion occurs between two parallel lights after passing through the optical element due to the difference between the phase velocity of ordinary light and the phase velocity of extraordinary light due to the birefringent crystal. If the wedge-shaped birefringent crystal is rutile, the time delay difference is about 0.7 picoseconds. The shift of the optical signal transmission time due to the polarization dispersion is not a problem at the optical communication speed of 2.4 Gbps, but in the long-distance communication and the high-speed optical communication at 10 Gbps, an optical repeater needs to be several tens to one hundred and several tens. Because they are connected in series before and after the stage, the optical signal waveform is greatly disturbed by the polarization dispersion of the optical isolator, and accurate information transmission becomes difficult. The difference between the maximum value and the minimum value of the insertion loss due to the polarization state of the light is the polarization dependent loss. The forward polarization dependent loss of the optical isolator is two parallel light beams emitted from the optical element unit. And the optical characteristics of the lens. The shorter the separation distance of the parallel light after passing through the optical element unit, the smaller the polarization dependent loss. In the above-mentioned optical isolator, the polarization dependent loss is about 0.2 dB. However, in the case of the long-distance optical communication, the optical repeaters are connected in multiple stages in series. The S / N ratio of the signal is greatly deteriorated. Therefore, accurate information transmission becomes difficult. It is an object of the present invention to provide a polarization independent optical isolator that suppresses polarization dependent loss simultaneously with polarization dispersion and prevents internal reflection of the optical isolator.

[0004]

SUMMARY OF THE INVENTION The present invention has been made to solve these problems, and includes an optical fiber A as an incident side optical fiber, a lens A as an incident side lens, and an optical fiber as an exit side optical fiber. B, a lens B serving as an exit side lens, a wedge-shaped birefringent crystal A, a wedge-shaped birefringent crystal B, and a 45-degree Faraday rotator are arranged, and at least one plate-shaped birefringent crystal is replaced with the following four wedge-shaped birefringent crystals. Between B and lens B Between lens A and wedge-shaped birefringent crystal A Between 45-degree Faraday rotator and wedge-shaped birefringent crystal B Four types of polarization-independent light arranged between wedge-shaped birefringent crystal A and 45-degree Faraday rotator In the isolator, in the case of the above, the main cross section of the flat birefringent crystal is orthogonal to the optical axis of the wedge-shaped birefringent crystal B, and the optical axis of the flat birefringent crystal is optimally inclined with respect to the light passing surface,
In this case, the main cross section of the flat birefringent crystal is orthogonal to the optical axis of the wedge-shaped birefringent crystal A, and the optical axis of the flat birefringent crystal is optimally inclined with respect to the light passing surface. By controlling the thickness of the birefringent crystal, the polarization dispersion is suppressed in the forward direction and
The polarization-dependent loss was reduced by minimizing the separation distance of the parallel light. In the above-mentioned optical isolator, the plate-shaped birefringent crystal is disposed in the case, and in this case, the main cross section of the plate-shaped birefringent crystal is orthogonal to the optical axis of the wedge-shaped birefringent crystal B, and The optical axis is optimally inclined with respect to the light passing surface. In this case, the optical axis of the flat birefringent crystal is orthogonal to the optical axis of the wedge-shaped birefringent crystal A, and the optical axis of the flat birefringent crystal is By controlling the thickness and tilting it optimally with respect to the light passing surface, polarization dispersion is suppressed in the forward direction, and at the same time, polarization separation is minimized by minimizing the separation distance between two parallel lights incident on the lens B. By reducing the loss and inclining the flat birefringent crystal, it is possible to prevent reflected return light and multiple reflected light from the flat birefringent crystal from entering the optical fiber A and the optical fiber B. A and Hikari The optical axis deviation distance B was small. When light is incident on the optical element portion in the forward direction, the two lights separated by the wedge-shaped birefringent crystal receive the same refractive index in the wedge-shaped birefringent crystal A and the wedge-shaped birefringent crystal B (the ordinary light refractive index). (→ ordinary light refractive index, extraordinary light refractive index → extraordinary light refractive index), there is a phase velocity difference in the light emitted from the optical element. If there is a flat birefringent crystal whose optical axis is orthogonal to the exit side of the optical element,
The light that has undergone the ordinary light and extraordinary light refractive index in the wedge-shaped birefringent crystal A and the wedge-shaped birefringent crystal B receives the extraordinary light and ordinary light refractive index in the flat birefringent crystal. Therefore, by controlling the thickness of the plate-like birefringent crystal so that the above-mentioned phase velocity becomes 0, the phase difference between the two lights passing through the plate-like birefringent crystal is canceled out, and the polarization dispersion is eliminated. From FIG. 11, when two parallel lights emitted from the optical element section in the forward direction are condensed by the lens and made incident on the optical fiber, the incident angle to the optical fiber is changed due to the difference in the axis shift distance from the optical axis. different. The difference in the coupling loss to the optical fiber due to the difference in the incident angle causes the polarization dependent loss. Therefore, the polarization dependent loss can be reduced by reducing the separation distance between the two parallel lights incident on the lens. The main section is a plane including the propagation direction of incident light to the birefringent crystal and the optical axis of the birefringent crystal. FIG.
According to 3, when two parallel lights pass through the flat birefringent crystal, one receives the refractive index of ordinary light and the other receives the refractive index of extraordinary light. Only the light receiving the refractive index of the extraordinary light shifts in the main cross section due to the inclination angle of the optical axis, and is emitted as two parallel lights. In the case of a positive birefringent crystal, extraordinary light shifts along the direction of inclination of the optical axis. The separation distance between the two parallel lights can be reduced by controlling the inclination angle and the thickness of the flat birefringent crystal in the main section of the optical axis. When light is incident on the plate-like birefringent crystal, reflected light is generated at the entrance end face and the exit end face due to the difference in the refractive index at the interface. By inclining the plate-like birefringent crystal, a reflection angle is generated in the reflected light to prevent coupling of the reflected light to the incident optical fiber and the output optical fiber. Also,
From Snell's law, the optical axis shift distance of the output optical fiber with respect to the position of the input optical fiber is reduced.

[0005]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below with reference to the drawings. FIG. 1A is a sectional view showing one embodiment of the optical isolator according to the present invention. This optical isolator includes an optical fiber A4, a lens A6, a wedge-shaped birefringent crystal A1,
It is composed of a 45-degree Faraday rotator 8, a wedge-shaped birefringent crystal B2, a flat birefringent crystal 3, a lens B7, an optical fiber B5, and a magnet. However, the magnet is omitted in the figure. Further, if a 45-degree Faraday rotator 8 that does not require external application of a magnetic field is used, a magnet is not required. The wedge-shaped birefringent crystal A1, the wedge-shaped birefringent crystal B2, and the plate-shaped birefringent crystal 3 are, for example, rutile single crystal, YVO4, and the optical axis 10 of the wedge-shaped birefringent crystal A1 and the wedge-shaped birefringent crystal B2 is perpendicular to the incident light. It is cut out so that it is in the plane. The optical axis 10 of the plate-like birefringent crystal 3 is not parallel to the light passing surface, but is within the main section 11 so as to minimize the separation distance between two parallel lights emitted from the wedge-shaped birefringent crystal B2 in the forward direction. It is inclined. The thickness is controlled so that the polarization dispersion generated when the light passes through the optical element portion can be canceled and the two parallel lights have a minimum separation distance. Since the extraordinary refractive index received when light passes through the flat birefringent crystal 3 is different from the extraordinary refractive index received by the wedge-shaped birefringent crystal, the thickness of the flat birefringent crystal 3 is equal to the light passing through both wedge-shaped birefringent crystals. It does not add up to the thickness of the part. The thick part and the thin part of the wedge-shaped birefringent crystal A1 and the wedge-shaped birefringent crystal B2 are arranged to face each other, and the wedge surface is on the lens side at this time. As shown in FIG. 1B, the optical axis 10 of the wedge-shaped birefringent crystal A1 is at a position inclined 22.5 degrees clockwise with respect to the X axis, and the optical axis 10 of the wedge-shaped birefringent crystal B2 is rotated by 45 degrees Faraday. 22.5 counterclockwise with respect to the X axis due to the rotation of the polarization plane by the element 8.
Degrees. Therefore, the relative angles of the optical axes 10 of the wedge-shaped birefringent crystals A1 and B2 are shifted by 45 degrees. The main section 11 of the flat birefringent crystal 3 is inclined 22.5 degrees counterclockwise with respect to the Y axis, and the optical axis 10 is inclined in the main section 11. This optical isolator operation is shown in FIG.
And the Z direction in the figure is a forward direction. The states of the two lights in (a), (b), (c), and (d) shown in FIG. 1A are shown in (a), (b), (c), and (b) of FIG. (D). At this time, the rotation behavior of the polarization plane by the 45-degree Faraday rotator 8 is omitted. The light emitted from the optical fiber A4 becomes parallel light by the lens A6 and enters the wedge-shaped birefringent crystal A1. In the wedge-shaped birefringent crystal A1, the light is separated into two lights that receive the ordinary light refractive index and the extraordinary light refractive index, and then rotated 45 degrees counterclockwise by the 45 degree Faraday rotator 8,
It passes through the wedge-shaped birefringent crystal B2. At the time of passing, the light which has received the ordinary light refractive index and the extraordinary refractive index in the wedge-shaped birefringent crystal A1 also receives the ordinary light refractive index and the extraordinary refractive index in the wedge-shaped birefringent crystal B2. The parallel light is parallel to the light incident on the wedge-shaped birefringent crystal A1. The main section 11 of the plate-shaped birefringent crystal 3 is a wedge-shaped birefringent crystal B2.
Of the parallel light passing through the flat birefringent crystal 3, the light having the ordinary light refractive index in the wedge-shaped birefringent crystal receives the extraordinary refractive index, and The received light has an ordinary refractive index. Also, a wedge-shaped birefringent crystal B
The state of the two parallel lights before and after the two passes ((c) → (d)) is shown in FIG. 1 (c). After passing through the wedge-shaped birefringent crystal B2, the position of light receiving the ordinary refractive index is denoted by P1, the position of light receiving the extraordinary refractive index is denoted by P2, and the shift position of P1 after passing through the flat birefringent crystal 3 is denoted by P3. Was. Assuming that the separation distance a is P1 and P2, the minimum separation distance b between the two parallel lights is b = a × sin (22.5 deg). In order to obtain the minimum separation distance b, light is shifted from P1 to P3 by controlling the inclination angle of the optical axis 10 in the main cross section 11 of the plate-like birefringent crystal 3 and the thickness of the plate-like birefringent crystal 3. You can do it. Moving distance c at that time
Can be expressed as c = a × cos (22.5 deg). In an optical isolator using rutile as the wedge birefringent crystal, the parallel light separation distance when emitting the wedge-shaped birefringent crystal B2 is about 30 μm. Here, by controlling the optical axis 10 and the thickness of the flat birefringent crystal 3,
The separation distance of the parallel light could be made close to 11.5 μm, and the polarization dependent loss could be improved to 0.1 dB or more from FIG. In the opposite direction, the light is separated into two lights having the ordinary refractive index and the extraordinary refractive index by the flat birefringent crystal 3 and is incident on the wedge-shaped birefringent crystal B2. Wedge-shaped birefringent crystal B
When the two light beams pass through the plate 2, the light that has received the ordinary light refractive index in the flat birefringent crystal 3 receives the extraordinary light refractive index, and the light that has received the extraordinary light refractive index has the ordinary light refractive index. These two lights are incident on the wedge-shaped birefringent crystal A1 after passing through the 45-degree Faraday rotator 8. 45 degrees Faraday rotator 8 to rotate counterclockwise 4
Since the light rotates by 5 degrees, the light having received the ordinary light refractive index in the wedge-shaped birefringent crystal B2 receives the extraordinary light refractive index, and the light having received the extraordinary light refractive index receives the ordinary light refractive index. Thus, the wedge-shaped birefringent crystal A
The two outgoing lights from 1 are emitted so as not to become parallel lights but to spread. Therefore, even through the lens A6, the two lights are not focused on the incident fiber, and the light from the opposite direction is blocked. FIG. 2 shows a second embodiment according to the present invention. This is an optical isolator in which the flat birefringent crystal 3 shown in the first embodiment is disposed between the lens A6 and the wedge-shaped birefringent crystal A1. At this time, the main section 11 of the optical axis 10 of the flat birefringent crystal 3 is orthogonal to the optical axis 10 of the wedge-shaped birefringent crystal A1. FIG.
FIG. 9 shows a third embodiment according to the present invention. By tilting the plate-like birefringent crystal 3 shown in the first embodiment counterclockwise about the X-axis, the reflected light returning light generated by the plate-like birefringent crystal 3 and the multiple reflection light to the optical fiber Incident can be prevented. Also, according to Snell's law, by shifting the two parallel lights in the direction of the optical axis 8, the optical fiber B5 with respect to the optical fiber A4 can be reduced by the optical axis shift displacement α.
The outer diameter of the optical isolator can be reduced. Also,
The plate-shaped birefringent crystal 3 is composed of a lens A6 and a wedge-shaped birefringent crystal A1.
The same effect can be obtained by disposing them between them and inclining them similarly. FIG. 4 shows a fourth embodiment according to the present invention.
Even when the plate-like birefringent crystal 3 is arranged between the 45-degree Faraday rotator 8 and the wedge-shaped birefringent crystal B2, the polarization dispersion and the polarization-dependent loss can be reduced, and the light incident on the plate-like birefringent crystal 3 is incident. Due to the angle, the reflected return light and the multiple reflected light generated in the flat birefringent crystal 3 can be blocked. FIG. 5 shows a fifth embodiment according to the present invention. Even when the plate-shaped birefringent crystal 3 is arranged between the wedge-shaped birefringent crystal A1 and the 45-degree Faraday rotator 8, the polarization dispersion and the polarization-dependent loss can be reduced, and light incident on the plate-shaped birefringent crystal 3 is incident. Due to the angle, the reflected return light and the multiple reflected light generated in the flat birefringent crystal 3 can be blocked. FIG. 6 shows a sixth embodiment according to the present invention. By applying the bandpass filter film 13 to one of the light passing surfaces of the flat birefringent crystal 3 shown in the third embodiment, the bandpass filter module constituting the optical fiber amplifier can be omitted. FIG. 7 shows a seventh embodiment of the composite module according to the present invention. A wavelength multiplexing / demultiplexing element was arranged in the optical isolator shown in the third embodiment. FIG. 8 shows an eighth embodiment of the composite module according to the present invention. A beam splitter and a photodetector for monitoring signal light were arranged in the optical isolator shown in the third embodiment. FIG. 9 shows a ninth embodiment of the composite module according to the present invention. In the optical isolator shown in the third embodiment, a wavelength multiplexing / demultiplexing element, a beam splitter, and a photodetector for monitoring signal light were arranged.

[0006]

According to the present invention, the optical fiber A, the lens A6,
A wedge-shaped birefringent crystal B, a wedge-shaped birefringent crystal B, a 45-degree Faraday rotator, an optical fiber B, and a lens B are arranged. Between the lens A and the wedge-shaped birefringent crystal A Between the 45-degree Faraday rotator and the wedge-shaped birefringent crystal B In the four types of polarization-independent optical isolators arranged between the wedge-shaped birefringent crystal A and the 45-degree Faraday rotator, In this case, the main cross section of the flat birefringent crystal is orthogonal to the optical axis of the wedge-shaped birefringent crystal B, and the optical axis of the flat birefringent crystal is optimally inclined with respect to its light passing surface,
In this case, the main cross section of the flat birefringent crystal is orthogonal to the optical axis of the wedge-shaped birefringent crystal A, and the optical axis of the flat birefringent crystal is optimally inclined with respect to the light passing surface. By controlling the thickness of the birefringent crystal, the polarization dispersion is suppressed in the forward direction, and at the same time, the polarization-dependent loss is reduced by minimizing the separation distance between two parallel lights incident on the lens B. In the above-mentioned optical isolator, the plate-shaped birefringent crystal is disposed in the case, and in this case, the main cross section of the plate-shaped birefringent crystal is orthogonal to the optical axis of the wedge-shaped birefringent crystal B, and The optical axis is optimally inclined with respect to the light passing surface. In this case, the optical axis of the flat birefringent crystal is orthogonal to the optical axis of the wedge-shaped birefringent crystal A, and the optical axis of the flat birefringent crystal is By controlling the thickness to be optimally inclined with respect to the light passing surface, polarization dispersion is suppressed in the forward direction, and at the same time, the light is incident on the lens B.
The polarization-dependent loss is reduced by minimizing the separation distance of the parallel light of the book, and the return light from the flat birefringent crystal and the multiple reflected light are tilted by tilting the flat birefringent crystal to form an optical fiber. A, the incidence on the optical fiber B is prevented, and the outer diameter of the optical isolator can be reduced by reducing the optical axis deviation distance between the optical fiber A and the optical fiber B. In the above-mentioned optical isolator, by providing a band-pass filter film on one of the light-passing surfaces of the plate-like birefringent crystal, the band-pass filter module constituting the optical fiber amplifier can be omitted.

[Brief description of the drawings]

1A is a cross-sectional view of a first embodiment of an optical isolator according to the present invention, and FIG. 1B is a perspective view of a birefringent crystal used in the present invention in an optical axis direction and an operation diagram of light. And
(C) is an explanatory diagram for minimizing the separation distance between two lights.

2A is a sectional view of a second embodiment of the optical isolator according to the present invention, and FIG. 2B is a perspective view of a birefringent crystal used in the present invention in the optical axis direction and an operation diagram of light. It is.

FIG. 3 shows a sectional view of a third embodiment of the optical isolator according to the present invention.

FIG. 4A is a cross-sectional view of a fourth embodiment of the optical isolator according to the present invention, and FIG. 4B is a perspective view of a birefringent crystal used in the present invention in an optical axis direction and an operation diagram of light. It is.

FIG. 5A is a sectional view of a fourth embodiment of the optical isolator according to the present invention, and FIG. 5B is a perspective view of the birefringent crystal used in the present invention in the optical axis direction and an operation diagram of light. It is.

FIG. 6 is a sectional view of a sixth embodiment of the optical isolator according to the present invention.

FIG. 7 is a sectional view of a seventh embodiment of the composite module according to the present invention.

FIG. 8 shows a sectional view of an eighth embodiment of the composite module according to the present invention.

FIG. 9 shows a sectional view of a ninth embodiment of a composite module according to the present invention.

FIG. 10 shows a cross-sectional view of the prior art.

FIG. 11 is a schematic diagram showing a relationship between a separation distance of two parallel lights from an optical isolator and an incident angle to an optical fiber.

FIG. 12 is a graph showing a difference in coupling loss to an optical fiber depending on a distance between two parallel light beams.

FIG. 13 shows an operation diagram of an optical axis and light in a flat birefringent crystal.

[Explanation of symbols]

 Reference Signs List 1 wedge-shaped birefringent crystal A 2 wedge-shaped birefringent crystal B 3 flat birefringent crystal 4 optical fiber A 5 optical fiber B 6 lens A 7 lens B 8 45-degree Faraday rotator 9 optical axis 10 optical axis 11 main section 12 Y- Projection of main cross section onto Z plane 13 Band pass filter film 14 Optical isolator 15 Optical fiber 16 Lens 17 Light passing surface 18 Wavelength multiplexing / demultiplexing device 19 Beam splitter 20 Photodetector

Claims (11)

[Claims] At least one or more wedge-shaped birefringent crystals, at least one or more 45-degree Faraday rotator,
In an optical isolator composed of at least one flat birefringent crystal, a main cross section of the flat birefringent crystal is orthogonal to an optical axis of at least one wedge birefringent crystal,
An optical isolator, wherein an optical axis of the flat birefringent crystal is not parallel to a light passing surface of the flat birefringent crystal. 2. The optical isolator according to claim 1, wherein said flat birefringent crystal is inclined with respect to an optical axis. 3. The flat birefringent crystal is arranged at the rearmost position,
2. The optical isolator according to claim 1, wherein the main cross section is orthogonal to the optical axis of the wedge-shaped birefringent crystal immediately before the flat birefringent crystal. 4. The flat birefringent crystal is arranged at the forefront, and its main cross section is orthogonal to the optical axis of the wedge-shaped birefringent crystal immediately after the flat birefringent crystal. The optical isolator according to claim 1. 5. A 45-degree Faraday rotator, said plate-shaped birefringent crystal and a wedge-shaped birefringent crystal are arranged in this order, and a main cross section of said plate-shaped birefringent crystal has a wedge-shaped birefringence immediately after said plate-shaped birefringent crystal. 2. The optical isolator according to claim 1, wherein the optical isolator is orthogonal to the optical axis of the crystal. 6. A wedge-shaped birefringent crystal, the flat birefringent crystal and a 45-degree Faraday rotator are arranged in this order, and the main cross section of the flat birefringent crystal is a wedge-shaped birefringent immediately before the flat birefringent crystal. 2. The optical isolator according to claim 1, wherein the optical isolator is orthogonal to the optical axis of the crystal. 7. The optical isolator according to claim 1, wherein a band-pass filter film is provided on one of the light passing surfaces of the flat birefringent crystal. 8. The optical isolator according to claim 1, wherein at least two optical fibers and at least one lens are arranged. 9. The composite module according to claim 8, wherein at least one optical element having a wavelength multiplexing / demultiplexing function is arranged. 10. The composite module according to claim 8, wherein at least one beam splitter and at least one photodetector are arranged. 11. The optical isolator according to claim 8, wherein at least one optical element having a wavelength multiplexing / demultiplexing function, at least one beam splitter, and at least one photodetector are arranged. And a composite module.