JP2016009108A - Optical attenuator and multi-channel optical attenuator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical attenuator capable of low-voltage drive and having excellent optical loss characteristic.SOLUTION: The present invention is an optical attenuator 101 created by arranging a polarization unit 110a including two birefringent elements (10a, 30a), along an axis z in order from the front, between optical fiber collimators (40a-40b) facing back and forth in an xyz orthogonal coordinate system in which a direction z is an optical axis 50, an electro-optical element 20 for imparting, to a linear polarization light from the front, a prescribed phase difference that corresponds to a voltage between electrodes (22-22) facing in a direction Y and emitting the light from the rear, and a light detection unit 110b including two birefringent elements (30b, 10b), wherein the polarization unit emits linear polarization light from first and second optical paths that separate input light from the front in parallel to an axis x, and the light detection unit makes light having traced the first and second optical paths from the EO element joined to the optical fiber Fb of the optical fiber collimator at the rear when the EO element is imparting either a 0° or a 180° phase difference.

Description

  The present invention relates to an optical attenuator for controlling the transmission and blocking of incident light using the electro-optic effect of a substance, and for variably controlling the attenuation of incident light.

  As an optical attenuator, for example, there is one using a substance having an electrooptic effect (hereinafter referred to as an electrooptic substance or an EO substance) such as PLZT (lead lanthanum zirconate titanate). In general, an optical attenuator using an EO material is an electro-optical element (hereinafter also referred to as an EO element) having a pair of collimators arranged opposite to each other and a plate-like EO substance sandwiched between two electrodes facing each other. A polarizer, an analyzer, and a drive circuit for applying an electric field between the electrodes, and the polarizer is directed from one collimator on the light input side toward the other collimator on the light output side. The EO element and the analyzer are arranged in this order along the optical path. The polarizer and the analyzer are arranged to face each other so that their optical axes are orthogonal or parallel to each other. The direction of the electric field formed between the two electrodes holding the EO material is inclined 45 ° with respect to the optical axis of the polarizer.

  When no electric field is applied between the electrodes, the linearly polarized light transmitted through the polarizer enters the analyzer while maintaining its vibration direction. At this time, light is blocked when the optical axis of the analyzer is orthogonal to the optical axis of the polarizer, and light is transmitted when the optical axis is parallel. On the other hand, when an electric field is applied between the electrodes by the drive circuit, a phase difference (retardation) occurs in the light transmitted through the EO substance, and the vibration direction of the incident linearly polarized light is rotated by 90 ° at the maximum. As a result, incident light is transmitted through or blocked by the analyzer. Further, the intensity attenuation amount of the light transmitted through the analyzer can be changed by controlling the inclination of the vibration direction according to the electric field strength.

  Incidentally, in the optical attenuator using the above-mentioned EO substance, it is necessary in principle to extremely increase the electric field strength in the EO substance. That is, it is necessary to increase the voltage between the electrodes. Therefore, in order to lower the voltage between electrodes (drive voltage), an optical attenuator that adopts a multilayer structure in which a structure in which an EO substance is sandwiched between electrodes is used and operates effectively even at a voltage of about 70 V has been put into practical use. . This optical attenuator is actually provided as a commercial product (for example, “PLZT high-speed optical shutter” manufactured by Furuuchi Chemical Co., Ltd .: see Non-Patent Document 1).

  However, since the above-mentioned commercially available “PLZT high-speed optical shutter” uses an optical path between layers of a multi-layer structure, there is a problem that diffraction or light scattering or reflection by electrodes occurs, and loss in an on state where light is transmitted increases. For this reason, in Patent Document 1 below, the EO material is thinned, and the EO material is arranged in the vicinity of the focal point by using a condenser lens, thereby reducing the distance between the electrodes and reducing the voltage to about 15V. In this way, various optical problems derived from the multilayer structure described above are solved.

International Publication No. 2005/121876

Furuuchi Science Co., Ltd., “PLZT high-speed light shutter”, [online], [searched on May 27, 2014], Internet <URL: http://www.furuchi.co.jp/measurement/shutter.html>

  The optical attenuator is not limited to a form using retardation based on the electro-optic effect of the EO substance, and there are forms using various principles and substances. For example, optical elements such as MEMS (Micro Electro Mechanical Systems), Faraday elements, and liquid crystals are used. Here, considering the use in optical communication, which is the main field of application of optical attenuators, a method using MEMS or liquid crystal as an optical element has a slow response speed and is difficult to employ. In the method using the Faraday element, the structure for applying the magnetic field is complicated, and it is difficult to reduce the size if peripheral structures other than the element are included. In addition, an electromagnet is used to variably control the magnetic field, and a relatively large current is required to drive the electromagnet, making it difficult to save power.

  On the other hand, an optical attenuator using retardation by an EO substance is excellent in high-speed response and can be driven by simply applying an electric field, so that it is easy to miniaturize, and because it is controlled by electric field strength, almost no current flows and power is saved. But there is. Therefore, it can be said that an optical attenuator using an EO substance as an optical element is more advantageous for power saving and miniaturization than an optical attenuator using another element.

  On the other hand, although the above-mentioned commercially available optical attenuator can be driven at a lower voltage than the previous optical attenuator, it still needs to be driven at a high voltage of 70V. In the optical attenuator described in Patent Document 1, the distance between the electrodes is gradually reduced from the light incident surface toward the light emission surface in order to reduce the drive voltage to 15V. The distance between the electrodes on the exit surface side is extremely narrow as 30 μm. Furthermore, a condensing lens is also required to pass the light beam between the narrow electrodes. Therefore, the shape of the EO material becomes complicated, it is difficult to process the EO material and to form an electrode on the EO material having a complicated shape, and the number of parts is further increased. Therefore, the manufacturing cost increases.

  Therefore, a main object of the present invention is to provide an optical attenuator that can be driven at a lower voltage and has excellent optical loss characteristics.

In order to achieve the above object, according to the present invention, a front first optical fiber collimator holding a first optical fiber and a rear second optical fiber collimator holding a second optical fiber are arranged to face each other. In addition, a plurality of optical elements having light incident / exit surfaces on the optical axis are arranged on the optical axis with the z axis extending in the front / rear direction as the optical axis, and the intensity of the input light from the first optical fiber Is an optical attenuator that attenuates and outputs to the second optical fiber,
Two axes orthogonal to the z-axis and orthogonal to each other are defined as an x-axis and a y-axis,
As the optical element, a polarizing unit, an electro-optical element unit, and a light detecting unit are arranged in this order from the first optical fiber collimator to the second optical fiber collimator.
In the polarizing section, the first birefringent element and the second birefringent element are arranged in this order from the front to the rear in such a relationship that the azimuth of the optical axis on the xy plane is rotated by 90 ° around the z axis. Including the configuration arranged in
In the first birefringent element, the azimuth of the optical axis on the xy plane is inclined 45 ° in a predetermined direction around the z axis with respect to the x axis, and the input light is divided into two straight lines of ordinary light and extraordinary light. The azimuths of the optical axes on the yz plane and the zx plane are set so that the two linearly polarized lights are emitted from positions separated from each other in the direction of 45 ° with respect to the x axis on the xy plane.
The second birefringent element emits the two linearly polarized lights emitted from the first birefringent element on a first optical path and a second optical path that are separated in a direction parallel to the x-axis,
In the electro-optic element portion, an electro-optic material having an electro-optic effect is sandwiched between electrodes facing each other with the y-axis as a normal direction, and a voltage is applied between the facing electrodes by an external driving circuit. A predetermined phase difference is given to each of the light incident along the first and second optical paths and emitted from the rear,
In the light analyzing section, the third birefringent element and the fourth birefringent element, which have a relationship in which the orientation of the optical axis on the xy plane is rotated by 90 ° around the z axis, are arranged in this order from the front to the rear. Including the configuration arranged in
When the electro-optic element portion gives a phase difference of 0 ° or 180 ° based on the electro-optic effect to the light incident on the electro-optic element portion, the x-trace follows the first and second optical paths. Two linearly polarized light beams that are orthogonal to each other while being separated in the axial direction are incident on the front surface of the third birefringent element, and the fourth birefringent element converts the two linearly polarized light beams into the light of the second optical fiber collimator. Coupled to the fiber,
The optical attenuator is characterized by this.

The polarizing section is formed by arranging a first half-wave plate between the first birefringent element and the second birefringent element, and the first half-wave plate is an optical axis. Is set to be parallel to the x-axis or y-axis,
The light analyzing unit includes a second half-wave plate disposed between the third birefringent element and the fourth birefringent element, and the second half-wave plate is xy. The azimuth of the optical axis on the surface is set to be a direction that bisects the azimuth of the optical axis on the xy plane of the third and fourth birefringent elements,
It is good also as an optical attenuator characterized by this.

The electro-optic material is made of a ferroelectric material having a birefringence effect due to a predetermined residual phase difference in a state where an electric field is not applied,
A quarter-wave plate is disposed between the polarizing unit and the electro-optical element unit, or between the electro-optical element unit and the light detecting unit,
The quarter-wave plate has an azimuth of the optical axis on the xy plane that is inclined by 45 ° around the z-axis with respect to the x-axis, and is emitted from the polarizing section onto the first and second optical paths. Compensating for the residual phase difference so that two linearly polarized lights inclined by an angle α around the z axis with respect to the orientation of the two linearly polarized lights in the xy plane are incident on the detection unit;
The azimuth of the optical axis on the xy plane of the third birefringent element coincides with one of the two linearly polarized lights emitted from the quarter wavelength plate.
An optical attenuator characterized by this can also be obtained.

Alternatively, the electro-optic material is made of a ferroelectric having a birefringence effect due to a predetermined residual phase difference in a state where an electric field is not applied,
An optical rotator comprising a half-wave plate is disposed immediately before the third birefringent element,
A quarter-wave plate is disposed between the polarizing unit and the electro-optical element unit, or between the electro-optical element unit and the optical rotation unit,
The quarter-wave plate has an azimuth of the optical axis on the xy plane that is inclined by 45 ° around the z-axis with respect to the x-axis, and is emitted from the polarizing section onto the first and second optical paths. Compensating for the residual phase difference so that two linearly polarized lights inclined by an angle α around the z axis with respect to the orientation of the two linearly polarized lights in the xy plane are incident on the optical rotation unit;
The azimuth of the optical axis on the xy plane of the third birefringent element coincides with the azimuth of the optical axis on the xy plane of the second birefringent element, or is inclined by 90 ° around the z axis,
The azimuth of the optical axis on the xy plane of the optical rotator is the azimuth of the vibration direction on the xy plane of the two linearly polarized light inclined by the angle α incident from the front and the xy plane of the third birefringent element. Is a direction in which the angle of intersection with the azimuth of the optical axis is divided at equal angles,
It is good also as an optical attenuator characterized by this. The electro-optical material comprising the ferroelectric formula _{K 1-y M y Ta 1} -x Nb x O 3 ( where, M is a monovalent metal, 0 <x <1,0 ≦ y <1) with If it is a substance represented, it is preferable.

  A multi-channel optical attenuator characterized in that the optical attenuator according to any one of the above is used as an optical attenuator for one channel, and a structure corresponding to a configuration in which a plurality of channels are arranged on the left and right for the one channel is integrally provided. Is also within the scope of the present invention.

The multi-channel optical attenuator is
The plurality of collimating lenses constituting the first and second optical fiber collimators constituting the optical attenuators for the plurality of channels may be integrated as a microlens array arranged side by side in the x-axis direction. Good.

  Further, at least one of the optical elements disposed between the first and second optical fiber collimators is formed to extend from side to side so as to cross the optical paths for a plurality of channels formed by the optical attenuators for the plurality of channels. A multi-channel optical attenuator can be provided.

  The optical attenuator of the present invention can be driven at a low voltage and has excellent optical attenuation characteristics. In addition, since it is easy to manufacture with a simple configuration, it can be expected to be provided at low cost. Other effects will be clarified in the following description.

It is a figure which shows the structure of the optical attenuator which concerns on a general example. It is a figure which shows operation | movement of the birefringent element which functions as a walk-off prism. It is a figure which shows the structure of the optical attenuator which concerns on the comparative example of this invention. It is a figure which shows operation | movement of the said comparative example. It is a figure which shows the optical characteristic of the optical fiber collimator which comprises an optical attenuator. It is a figure which shows the size of each site | part in the said comparative example. It is a figure which shows the structure of the optical attenuator which concerns on 1st Example of this invention. It is a figure which shows the optical axis direction of the birefringent element which comprises the said 1st Example. It is a figure which shows operation | movement of the said 1st Example. It is a figure which shows the structure of the optical attenuator which concerns on 2nd Example of this invention. It is a figure which shows the optical axis azimuth | direction of the birefringent element and 1/2 wavelength plate which comprise the said 2nd Example. It is a figure which shows the optical path of the said 2nd Example in operation | movement. It is a figure which shows the position and polarization state of the light beam in the said 2nd Example in operation | movement. It is a figure which shows the extinction ratio characteristic of the electro-optical element using a ferroelectric. It is a figure which shows the structure of the optical attenuator which concerns on the 3rd Example of this invention. It is a figure which shows the optical axis azimuth | direction of the birefringent element and the quarter wavelength plate which comprise the said 3rd Example. It is a figure which shows the principle of operation of the said 3rd Example. It is a figure which shows the optical path of the said 3rd Example in operation | movement. It is a figure which shows the position and polarization state of the light beam in the said 3rd Example in operation | movement. It is a figure which shows the size of each site | part in the optical attenuator which concerns on the said 3rd Example and its modification. It is a figure which shows the optical loss characteristic of the said comparative example, 3rd Example, and a modification. It is a figure which shows the structure of the optical attenuator which concerns on the 4th Example of this invention. It is a figure which shows an example of the optical axis direction of the 1/2 wavelength plate which comprises the said 4th Example. It is a figure which shows the optical path of the said 4th Example in operation | movement. Shows the position and polarization state of the light beam in the fourth embodiment in operation It is a figure which shows an example of the optical path formed in the general optical attenuator provided for practical use.

  Embodiments of the present invention will be described below with reference to the accompanying drawings. Note that in the drawings used for the following description, the same or similar parts may be denoted by the same reference numerals and redundant description may be omitted. In some drawings, unnecessary symbols may be omitted in the description.

  The optical attenuator according to the embodiment of the present invention employs a phase difference based on the electro-optic effect of the EO material as an operation principle in order to meet the demands for high-speed response, power saving, miniaturization, and the like. It has excellent optical loss characteristics while enabling driving at a lower voltage. Hereinafter, first, a general configuration of an optical attenuator (hereinafter referred to as an optical attenuator) using a phase difference based on an electro-optic effect of an EO material will be described, and an operation principle of the optical attenuator according to the general configuration will be described. Then, the problems of the conventional optical attenuator will be described.

=== Principle of operation of optical attenuator ===
FIG. 1 is a diagram for explaining the operation principle of the optical attenuator. FIG. 1A is a diagram showing a configuration of a general optical attenuator 1, and FIG. 1B shows directions of optical axes (3a, 3b) of the polarizer 2a and the analyzer 2b included in the configuration. FIG. As shown in FIG. 1 (A), a general optical attenuator (hereinafter also referred to as General Example 1) has an optical fiber collimator (facing in the front-rear direction) where light B0 is incident from the front to the rear. When the optical axis 50 is set between them (not shown), the general example 1 has a structure in which the polarizer 2a, the EO element 20, and the analyzer 2b are arranged along the optical axis 50 in this order from the front.

  The EO element 20 has a structure in which an EO substance 21 having front and rear surfaces with the optical axis 50 as a normal direction as a light incident / exit surface is sandwiched between two electrodes (22-22) facing each other. The two electrodes (22-22) are arranged so as to apply an electric field E in a direction perpendicular to the light. Here, for convenience, the application direction of the electric field E is defined as the vertical direction, and the electric field E is defined as being applied from the upper side to the lower direction. Further, a direction orthogonal to the vertical direction when viewed from the front-rear direction is defined as the left-right direction, and the left and right directions are defined by the direction when the front is viewed from the rear.

  The polarizer 2a and the analyzer 2b are applied to the EO element 20, as shown in FIG. 1B, with the optical axes (3a, 3b) as the light transmission axes. It is inclined 45 ° with respect to the electric field E direction. The optical axes (3a, 3b) of the polarizer 2a and the analyzer 2b are orthogonal or parallel to each other, and here, a case where they are orthogonal is shown. That is, the illustrated general example 1 is a so-called normally-off type in which the EO element 20 is in a dark state when an electric field E is not applied (hereinafter also referred to as an initial state). In the case of being parallel to each other, the initial state is a bright state and a normally-on type.

When a voltage is applied between the electrodes (22-22) of the EO element 20 by the drive circuit V, an electric field E is generated in the EO material 21. The EO material 21 vibrates in a direction perpendicular to the electric field direction and light (hereinafter referred to as TE light) B _TE that vibrates the incident linearly polarized light B1 in a direction parallel to the electric field E direction due to the electro-optic effect based on the electric field E. The light (hereinafter referred to as TM light) B _TM is separated into a _TM, and there is a phase difference between the TE light B _TE and the TM light B _{TM according} to the applied electric field E and the optical path length until the incident light is emitted. Arise. Then, elliptically polarized light B2 corresponding to the phase difference is emitted backward. If a phase difference of π (rad) = 180 ° occurs between the TE light B _TE and the TM light B _TM , the incident linearly polarized light B2 is emitted backward as light B2 rotated by 90 ° around the optical axis 50. It will be.

  When the light B2 is incident from the EO element 20, the analyzer 2b blocks a component in a direction perpendicular to the optical axis 3b of the light B2, and linearly polarized light B3 having a component coinciding with the direction of the optical axis 3b. Is output backwards. Thereby, the intensity of the light B0 incident on the polarizer 2 is attenuated and emitted from the optical attenuator 1. When the electric field E is applied to the EO material 21 of the EO element 20 and a phase difference of 180 ° is generated, the linearly polarized light B2 obtained by rotating the incident linearly polarized light B1 by 90 ° around the optical axis 50 becomes the analyzer 2b. Is incident on. If it is a normally-off type, this linearly polarized light B2 coincides with the optical axis 3b of the analyzer 2b, and the light transmittance is maximized. If it is a normally-on type, light is blocked and the device is turned off.

=== Problems of conventional optical attenuators ===
By the way, the general example 1 shown in FIG. 1 is a principle model for more simply explaining the operation of the optical attenuator, and polarizing plates are used for the polarizer 2a and the analyzer 2b. However, since the polarizing plate absorbs vibration components other than the optical axis direction of the incident light in order to emit linearly polarized light, the intensity of light transmitted light is halved even with an ideal polarizing plate. That is, the transmitted light intensity in the ON state cannot be made larger than 50% of the incident light. Therefore, it is not possible to increase the light loss that is the light intensity ratio between the on state and the off state. Therefore, in order to use incident light without waste, it is conceivable to use a known birefringent element made of a rutile crystal or the like for a polarizer or an analyzer. As is well known, when non-polarized light is incident on the birefringent element, it is separated into two linearly polarized light beams of ordinary light and extraordinary light whose vibration directions are orthogonal to each other, and the two linearly polarized light beams can be combined. In this case, incident light can be used without waste. However, when a birefringent element is used for a polarizer and an analyzer, there is a problem that it is difficult to achieve both power saving by low voltage driving and improvement of optical loss characteristics due to the path of light passing through the optical attenuator. Occur. This problem will be specifically described below.

  FIG. 2 is a diagram showing an optical path and a polarization state of light incident on the birefringent element 10. Also in FIG. 2, the front, rear, up, down, left and right directions are defined in the same manner as in FIG. 2A is a perspective view when the birefringent element 10 is viewed from above and rear, and FIG. 2B is a plan view when viewed from the rear. As shown in this figure, the birefringent element 10 has a rectangular parallelepiped shape having a light incident / exit surface with the optical axis 50 extending in the front-rear direction as a normal direction. The birefringent element 10 used in the optical attenuator is also called a “walk-off prism”. When the birefringent element 10a is used as a polarizer and an analyzer of the optical attenuator, the birefringent element when viewed from the front-rear direction. The azimuth of the ten optical axes 11 is inclined 45 ° with respect to the direction of the electric field applied to the EO element (the vertical direction). Further, when the optical axis 11 is viewed from the left-right direction, it is inclined obliquely from the front to the rear. In this example, it is inclined upward.

  When light B0 in a non-polarized state is incident on the birefringent element 10 having the azimuth of the optical axis 11 from the front surface, the light B0 oscillates in a direction perpendicular to the optical axis 11 when viewed from the front-rear direction and the optical axis. 11 is separated into extraordinary light e that vibrates in a direction parallel to 11. The extraordinary light e is refracted in the direction of the optical axis 11 from the front to the rear. Therefore, when collimated light is incident on the front surface of the birefringent element 10, two linearly polarized lights (B1o and B1e) that are separated from each other in the direction of 45 degrees with respect to the vertical direction are emitted from the rear surface. Thereby, two linearly polarized light (B1o, B1e) follows each optical path toward back.

By the way, when the optical attenuator is configured using the above-described birefringent element 10 as a polarizer and an analyzer, the EO element has a vertical height for transmitting two linearly polarized lights separated from each other emitted from the polarizer composed of the birefringent element 10. Is required. The EO substance is proportional to the electric field intensity E applied between the electrodes or its square E ² depending on the type of electro-optical effect (Bockels effect, Kerr effect) that is manifested in itself. A large electro-optic effect is exhibited. However, as described above, when a birefringent element is used for the polarizer and the analyzer in order to increase the light utilization efficiency, it is necessary to form two linearly polarized light paths separated in the vertical direction in the EO element. It becomes difficult to shorten the distance between the electrodes facing each other in the vertical direction.

Further, the two linearly polarized light cannot be shortened to a dark cloud so as not to interfere with each other (to reduce crosstalk) (hereinafter also referred to as a walk shift amount wf). In order to obtain the walk shift amount wf in which crosstalk does not occur, it is necessary to increase the longitudinal length L _p of the birefringent element 10. However, when the optical attenuator is installed at a node of the optical communication network, the optical elements of the polarizer, the EO element, and the analyzer are disposed between the optical fiber collimators facing in the front and rear directions. The distance from the incident optical fiber collimator (hereinafter also referred to as the incident-side collimator) to the outgoing-side optical fiber collimator (hereinafter also referred to as the outgoing-side collimator) is defined based on a well-known Rayleigh distance. That is, there is an appropriate distance (Rayleigh region) between the opening end faces of the input and output side optical fibers, and if the beam deviates from the Rayleigh region, two beams corresponding to the ordinary light o and the abnormal light e are emitted from the output side optical fiber Insertion loss (excess loss) occurs. If an optical element is arranged in the Rayleigh region while ensuring the walk shift amount Wf, the front-rear length of the EO element must be shortened. When the longitudinal length of the EO element is shortened, the generated phase difference is reduced when the electric field strength is the same. Therefore, in order to obtain a phase difference of 180 °, it is necessary to apply a larger electric field.

  As described above, when a birefringent element is used for a polarizer and an analyzer in order to improve light utilization efficiency, various problems caused by separation into ordinary light and extraordinary light (walk-off) are complicatedly related. Accordingly, it becomes difficult to drive the EO element at a low voltage.

=== Comparative Example ===
Prior to the description of the optical attenuator according to the embodiment of the present invention, first, the configuration and operation of a conventional optical attenuator using a polarizer composed of a birefringent element and an analyzer will be described. The optical design conditions of the optical attenuator will also be described.

<Configuration>
FIG. 3 is a diagram showing a configuration of a conventional optical attenuator (hereinafter also referred to as Comparative Example 100). 3 defines the front, rear, up, down, left, and right directions as in FIGS. 1 and 2, and FIG. 3A is a perspective view of the configuration of the comparative example 100 as viewed from above and below. B) and (C) show the azimuth of the optical axis 11a of the polarizer 10a and the azimuth of the optical axis 11b of the analyzer 10b when looking forward from the rear. As shown in FIG. 3, in the comparative example 100, the polarizer 10a, the EO element 20, and the analyzer 10b are arranged in this order from the front to the rear between two optical fiber collimators (40a-40b) facing each other in the front and rear directions. The arrangement is different from that of the general example 1 in that a birefringent element is used for the polarizer 10a and the analyzer 10b. The comparative example 100 shown in FIG. 3 is a normally-on type, and when viewed from the rear, the optical axis (11a, 11b) direction of the polarizer 10a and the analyzer 10b is parallel, and the horizontal direction or the vertical direction. When viewed from above, the optical axes (11a, 11b) of each other are mirror images of the front and rear.

  Here, it is assumed that the light B1 emitted from the front incident side collimator 40a travels toward the rear emission side collimator, and the position where the emitted light is incident on the polarizer is the origin, and from this origin in the front-rear direction. The optical axis 50 is set to extend. In addition, the y-axis is set in the vertical direction and the x-axis is set in the horizontal direction with respect to the origin. The azimuth (θ, φ) of the optical axes (11a, 11b) of the polarizer 10a and the analyzer 10b is defined by a counterclockwise angle with the right direction being 0 °. Therefore, in the comparative example 100 shown here, the orientations of the optical axes (11a, 11b) of the polarizer 10a and the analyzer 10b on the xy plane are −45 ° and + 45 °. And in the comparative example 100 of the said structure, the light (henceforth input light) radiate | emitted from the optical fiber Fa of the front entrance side collimator 40a is shaped by the collimating lens Ca into parallel light (henceforth also called a light beam). The light beam passes through the polarizer 10a, the EO element 20, and the analyzer 10b, and enters the opening end of the optical fiber Fb of the output collimator 40b on the rear side. The ratio of the intensity of the input light to the intensity of the incident light (hereinafter also referred to as output light) is a light loss. The operation of Comparative Example 100 will be described below.

<Operation>
FIG. 4 is a diagram for explaining the operation of the comparative example 100. FIG. 4A shows the optical path in the on or off state when the comparative example 100 is viewed from the top, and FIG. 4B shows the optical path when viewed from the left. 4C to 4H are views showing the position of the light beam and the polarization state of the light when viewed from the rear, and FIGS. 4C and 4D are front and rear views of the polarizer 10b. The position of the light beam and the polarization state are shown. 4E and 4F show the position and polarization state of the light beam on the front surface and the rear surface of the analyzer 10b in the on state, and FIGS. 4G and 4H show the analyzer 10b in the off state. The position and polarization state of the light beam on the front surface and the rear surface are shown. 4C to 4H, the position of the light beam is indicated by a white circle, and the vibration direction is indicated by a double arrow.

  First, the unpolarized input light B0 incident on the polarizer 10a from the incident-side collimator 40a is two linearly polarized light of ordinary light B11 whose vibration direction is orthogonal to the optical axis 11a and parallel extraordinary light B12 in the polarizer 10a. To separate. In the example shown here, the optical axis 11a of the polarizer 10a when viewed from the left-right direction is a direction extending upward at an angle β with respect to the z-axis from the front to the rear, and the azimuth angle on the xy plane. θ = −45 °. Therefore, as shown in FIGS. 4C and 4D, the ordinary light B11 travels straight through the polarizer 10a and exits, and the extraordinary light B12 is separated by a walk shift amount wf in the 45 ° direction when viewed from the rear. Emanates from. Then, two linearly polarized lights (B21 and B22) emitted from the polarizer 10a enter the EO element 20.

  When the electric field E is not applied to the EO element 20, as shown in FIG. 4E, the linearly polarized light (B21, B22) incident on the EO element 20 is emitted while maintaining the polarization state. When the electric field E is applied to the EO element 20, the incident light (B21, B22) emits elliptically polarized light according to the phase difference between the TE light and the TM light due to the birefringence effect according to the electric field E. When the phase difference is 180 °, the linearly polarized light (B21, B22) incident on the EO element 20 is emitted by rotating the vibration direction by 90 °.

The azimuth angle φ of the optical axis 11b of the analyzer 10b on the xy plane is −45 °, and the direction of the optical axis 11b when viewed from above and below and from the left and right is a direction that is symmetric with respect to the optical axis 10b of the polarizer 10a. It is. Therefore, in a state where the electric field E is not applied to the EO material 21 of the EO element 20, the linearly polarized light (B31 ₀ , B32 ₀ ) emitted from the EO element is transmitted through the EO element 20 without changing the vibration direction. Two linear polarized light (B31 ₀ and B32 emitted from EO element 20 follows the respective two optical path at a distance of walk shift amount wf each other corresponding to each of the polarizers 10a in ordinary B1 and extraordinary light B2 ₀ ) Behaves as ordinary light B41o and extraordinary light B41e even in the analyzer 10b. That is, the ordinary light B41o travels straight in the analyzer 10b, and the extraordinary light B42e is refracted in the lower right direction along the optical axis 11b. Then, as shown in FIG. 4F, the ordinary light B41o and the refracted abnormal light B42e that have traveled straight at the rear end of the analyzer 10b are emitted from the same position on the z-axis. The outgoing light B5 enters the output side collimator 40b and is coupled to the outgoing side optical fiber Fb by the outgoing side lens Cb. As a result, the light enters the on state.

In the EO element 20, when the electric field E is applied to the EO material 21 so that the phase difference based on the electro-optic effect is 180 °, as shown in FIG. The linearly polarized light B31 _E that oscillates in a direction that coincides with the azimuth of the optical axis 11b of the analyzer 10b along the axis, and the straight line B32 _E that oscillates in a direction orthogonal to the azimuth of the optical axis 11b along the optical path that is separated from the linearly polarized light. Will be incident. The linearly polarized light B31 _E incident on the analyzer 10b from the z-axis, acts as extraordinary light B41e within the analyzer 10b, the linearly polarized light B32 _E incident spaced by linearly polarized light B31 _E and walk shift wf is In the analyzer 10b, it behaves as ordinary light B42o. Therefore, the ordinary light B42o separated from the z axis travels straight, and the extraordinary light B41e on the z axis is refracted so as to be further separated from the ordinary light B42o along the optical axis 11b. Then, as shown in FIG. 4H, the two linearly polarized lights B52 and B51 corresponding to the ordinary light B42o and the extraordinary light B41e are emitted from positions separated from each other in the direction of 45 ° in the opposite direction from the z axis. In other words, the light is not coupled to the emission side optical fiber Fb, and the coupling loss is maximized. When a phase difference of less than 180 ° is generated in the EO element 20, elliptically polarized light is incident on the analyzer 10b from each of the optical paths separated from each other. Therefore, the elliptically polarized light is further separated into an ordinary light component and an abnormal light component in the analyzer 10b. Then, a part of the incident light is coupled to the emission side optical fiber Fb. Next, the design conditions of the optical elements constituting the comparative example 100 will be described.

<Optical design conditions>
FIG. 5 shows the optical characteristics of the fiber collimator used in Comparative Example 100. Here, the positional relationship and the characteristics of collimated light when only the optical fiber collimators (40a, 40b) on the input / output side in the comparative example 100 are arranged in a vacuum are shown. As is well known, the optical fiber collimators (40a, 40b) are held so that the single mode optical fibers (SMF: Fa, Fb) and the collimating lenses (Ca, Cb) are coaxial in the casings (41a, 41b). It is a structured. In the optical fiber collimator (40a, 40b) used here, the optical fiber (Fa, Fb) has a mode field diameter W ₀ = 10.4 μm, and the collimating lens (Ca, Cb) has a refractive index distribution constant of 1.087 mm. ⁻¹ , the on-axis refractive index is 1.4528, and the lens length L _L = 1.489 mm. The incident-side collimator 40a is designed to emit a light beam having a beam radius = 25 μm (beam spot diameter W ₁ = 50 μm).

The collimated light Bc emitted from the front collimating lens (hereinafter referred to as the incident side lens) Ca converges most at the position of the Rayleigh distance _Zr0 , and thereafter increases in diameter toward the rear. The optical fiber collimators (40a, 40b) designed as described above have a Rayleigh distance Z _r0 = 2 mm in a medium having a refractive index n = 1 (such as in the air), and between the collimating lenses facing each other in the front and rear (Ca-Cb). The distance (hereinafter also referred to as the inter-lens distance L _C0 ) is L _C0 = 4 mm. In an actual optical attenuator, a material having a refractive index larger than 1 such as a birefringent element or an EO material is disposed between the collimating lenses (Ca, Cb). The distance (hereinafter also referred to as the inter-lens distance L _c ) is larger than the inter-lens distance L _c0 (= 4 mm) in the space of the refractive index n = 1 shown in FIG.

Next, physical properties (refractive index, etc.) and sizes of various optical elements (10a, 20, 10b) arranged between the collimating lenses (Ca-Cb) will be described. As described above, an optical path composed of a light beam having the maximum beam spot diameter W ₁ = 50 μm is formed between the input and output side collimators (40a-40b). In addition, since the light beam is separated into two by the polarizer 10a, the two light beams need to be separated so as not to interfere with each other. Here, when two separated light beams are coupled to an optical fiber using an optical fiber collimator, if a coupling loss of 35 dB or more is obtained, it is considered that the two are separated sufficiently, and the coupling loss of 35 dB is The resulting walk shift amount Wf was 71 μm. And the walk shift amount Wf is birefringent element (polarizer 10a, analyzer 10b) constituting the optical attenuator direction and its longitudinal length of the optical axis (11a, 11b) (hereinafter, also referred to as element length _{L p)} and Can be adjusted by. In Comparative Example 100, the polarizer 10a has an element length L _p = 756.9 μm, and the angle β at which the z axis intersects the optical axis 11a in the yz plane shown in FIG. 4B is 5.63 °. With this setting, the walk shift amount Wf = 71 μm.

Regarding the thickness of the EO material 21 in the EO element 20 in the vertical direction (hereinafter also referred to as an interelectrode distance d), two light beams having a spot diameter W ₁ = 50 μm are directed to the EO material 21 in the 45 ° direction on the xy plane. The above-mentioned walk shift amount wf = 71 μm is transmitted while being separated. For this reason, the EO substance 21 needs to have an inter-electrode distance d that allows these two light beams to enter and exit so that no vignetting occurs on the light entrance / exit surface. In Comparative Example 100, d = 102.4 μm. Yes.

Longitudinal length of the EO devices 20 (hereinafter, also referred to as element length _{L eo)} for, EO material 21 in the polarizer 10a, EO devices 20 to be inserted between the collimator lens (Ca-Cb), each refraction analyzer 10b It can be set based on the rate n and the element length L _{p of} the polarizer 10a and the analyzer 10b.

The polarizer 10a and the analyzer 10b have a refractive index n = 2.568, and the polarizer 10a and the analyzer 10b have an element length L _p = 756.9 μm. The element length L _eo of the EO element 20 was set to 2.15 mm. FIG. 6 shows the sizes of the optical elements (10a, 20, 10b) constituting the comparative example 100. The distance L _M between the rear surface of the front collimating lens Ca and the front surface of the polarizer 10a, and the rear surface of the analyzer 10b and the front surface of the rear collimating lens Cb is a margin in consideration of ease of assembly. Here, L _M = 0.4 mm. Also provided space _{L S} of 0.05mm as margin between the polarizer 10a and the analyzer 10b adjacent to its front and rear with EO element 20. The element length (L _p , L _eo , L _p ) of each optical element (10a, 20, 10b) is L _p = 756.9 μm as described above. As for the element length L _eo of the EO element 20, the gap between the polarizer 10a and the analyzer 10b may be adopted for the EO element 20. In any case, the actual inter-lens distance _L polarizer from the _c a distance excluding the margin _{L M} 10a, EO element 20, the analyzer 10b may if arranged.

=== First Embodiment ===
As described above, in the comparative example 100, it is necessary to enter the EO element 20 with two light beams separated in the direction of 45 degrees with respect to the x axis. Therefore between the electrodes of the EO devices 20 (22-22), the required distance obtained by adding a distance corresponding to two optical beams in each beam spot diameter _{W 1} to walk shift amount Wf sine (Wf × sin 45 °) As a result, it was necessary to apply a high voltage between the electrodes (22-22) in order to obtain a sufficient electric field strength. However, if it is possible to enter the two light beams to EO element 20 in a state arranged in the x-axis direction, the thickness d between the electrodes (22-22), if any substantially only the beam spot size W ₁ Well, the thickness of the EO material 21 can be reduced, and a sufficient electric field can be applied to the EO material 21 even at a low voltage. Therefore, as a first embodiment of the present invention, an optical attenuator having a basic configuration for causing two light beams arranged in the x direction to enter the EO element 20 will be described.

<Configuration of the first embodiment>
FIG. 7 shows the configuration of an optical attenuator (hereinafter also referred to as the first embodiment 101) according to the first embodiment. In the drawings shown below, the front, rear, top, bottom, left, and right directions and the x, y, and z axes are defined in the same manner as the comparative example 100 described above. The first embodiment 101 is different from the comparative example 100 in that birefringence elements (30a, 30b) are arranged before and after the EO element 20 in addition to the polarizer 10a and the analyzer 10b as shown in FIG. ing. Further, the EO element 20 has a shorter inter-electrode distance d than the comparative example 100, and is thin in the vertical direction.

  Here, the embodiment of the present invention including the first embodiment 101 includes a total of four birefringent elements (10a, 30a, 30b, 10b) including the polarizer 10a and the analyzer 10b. The polarizer 10a is first, and the birefringent element 30a disposed between the polarizer 10a and the EO element 20 in the order from the polarizer 10a to the rear is referred to as a second birefringent element 30a. The birefringent element 30b disposed between the EO element 20 and the analyzer 10b will be referred to as a third birefringent element 30b. The configuration including the two birefringent elements (10a, 30a) in front of the EO element 20 is referred to as a polarizing unit 110a for convenience, and the two birefringent elements (30b, 10b) in the rear of the EO element are referred to. ) Is referred to as a light detection unit 110b.

8A to 8D show the optical axes (11a, 31a, 4a) of the four birefringent elements (10a, 30a, 30b, 10b) constituting the first embodiment 101 when viewed from the rear to the front. The orientations 31b and 11b) are shown. The optical axes of the polarizer 10a and the second birefringent element 30a on the xy plane are in an orientation rotated by 90 degrees around the z axis, and the counterclockwise angle with respect to the x axis when viewed from the rear is “+ ”, The polarizer 10a has an azimuth angle θ ₁ = −45 °, and the second birefringent element 30a has an azimuth angle θ ₂ = + 45 °. Further, as shown in FIG. 8, the orientations of the optical axes (11a, 30a) of the polarizer 10a and the second birefringent element 30a on the yz plane are the same. In this example, the front is upward and the upward is upward. Yes. Accordingly, the optical axis 10a of the polarizer 10a is oriented from the lower right front to the upper left rear, and the optical axis 31a of the second birefringent element 30a is oriented from the lower left front to the upper right rear.

The optical axes (31b, 11b) of the two birefringent elements (30b, 10b) constituting the analyzer 110b are also rotated by 90 ° around the z axis. In the illustrated example, the third birefringence is shown. The optical axis 31b of the element 30b has an azimuth angle φ ₁ = + 45 ° on the xy plane, and the analyzer 10b has an azimuth angle φ ₂ = −45 °. The optical axes of the polarization unit 110a and the light detection unit 110b are mirror images of each other in the front-rear direction, and form a normally-on type optical attenuator as in the comparative example 100.

<Operation>
FIG. 9 is a diagram illustrating the operation of the first embodiment 101. FIG. 9A shows the optical path in the on or off state when the first embodiment 101 is viewed from the top, and FIG. 9B shows the optical path when viewed from the left. FIGS. 9C to 9J are views showing the position of the light beam and the polarization state of the light beam when viewed from the rear. FIGS. 9C and 9D show the rear surface s1 of the polarizer 10b. And the position and polarization state of the light beam on the rear surface s2 of the second birefringent element 30a are shown. FIGS. 9E to 9G show the positions and deflection states of the light beams on the rear surface s3 of the EO element 20, the rear surface s4 of the third birefringent element 30b, and the rear surface s5 of the analyzer 10b in the on state. Is shown. 9H to 9J show the positions and deflection states of the light beams on the surfaces s3 to s5 in the off state. 9C to 9J, the position and vibration direction of the light beam at each position are indicated by white circles and double arrows.

First, the operation in the on state will be described. The non-polarized input light B0 incident on the polarizer 10a from the incident-side collimator 40a is converted into two linearly polarized lights of ordinary light B111 whose vibration direction is orthogonal to the optical axis 11a and parallel extraordinary light B112 in the polarizer 10a. To separate. Here, the azimuth angle θ = −45 °. Therefore, as shown in FIG. 9 (C), ordinary light B1o emits straight through the polarizer 10a, the extraordinary light B1e is emitted from a position spaced 45 ° direction when viewed from the rear only walk shift amount wf ₀ To do. The operation so far is the same as that of the comparative example 100.

The two linearly polarized light (B121, B122) emitted from the polarizer 10a is incident on the second birefringent element 30a. The orientation of the optical axis 31a on the xy plane of the second birefringent element 30a is rotated by 90 ° around the z axis with respect to the optical axis 11a of the polarizer 10a. The azimuth of the optical axis on the yz plane is a direction from the lower side to the upper side toward the rear as in the polarizer 10a. Therefore, the ordinary light B111 and the extraordinary light B112 in the polarizer 10a behave as the extraordinary light B131 and the ordinary light B132 in the second birefringent element 30a, respectively. Therefore, the light B121 incident in correspondence with the ordinary light B111 in the polarizer 10a is refracted in the direction of the azimuth angle θ = + 45 ° of the optical axis 30a on the xy plane of the first birefringent element 30a, and FIG. As shown in FIG. 2, two linearly polarized lights (B121, B122) emitted from the polarizer 10a with a spacing of 45 ° are emitted side by side in the x-axis direction on the rear surface s2 of the first birefringent element 30a. In the first embodiment, the distance between the optical paths of the two linearly polarized lights (B141, B142) emitted from the second birefringent element 30a is made to coincide with the walk shift amount Wf in the comparative example 100. Desired walk shift when ie the input light B0 is passed through the two birefringent elements of the same element length L _p polarizer 10a and the second birefringent element 30a (polarizer 10a, a second birefringent element 30a) The amount Wf is set.

Two linearly polarized light beams (B141 and B142) emitted from the polarizing unit 110a configured by the polarizer 10a and the first birefringent element 30a enter the EO element 20. When the electric field E is not applied to the EO element 20, as shown in FIG. 9E, the two linearly polarized light (B141, B142) are emitted while maintaining the polarization state, and this emitted light ( B151 ₀ and B152 ₀ ) are incident on the third birefringent element 30b. Note that the two linearly polarized lights (B141, B142) incident on the EO element 20 enter the EO element 20 along an optical path that is above the optical axis 50 and that is separated from the left and right in parallel with the x axis. Therefore, the EO material 21 in the EO element 20 only needs to have a thickness d that is equal to or larger than the beam spot diameter of two linearly polarized light (B151 ₀ , B152 ₀ ), and the EO element 20 can be made thinner than the comparative example 100. . And in order to make these two linearly polarized light (B141, B142) inject into the EO substance 21 reliably, the EO element is arrange | positioned in the position shifted from the optical axis 50 upwards.
The orientation of the optical axis 31b of the third birefringent element 30b on the xy plane is the same as that of the second birefringent element 30a, and the orientation on the yz plane is symmetric with respect to the second birefringent element 30a. It has become. Therefore, as shown in FIG. 9F, the ordinary light B132 and the extraordinary light B131 in the second birefringent element 30a behave as the ordinary light B162o and the extraordinary light B161e in the third birefringent element 30b, and the extraordinary light B161e is changed. The light is refracted in the third birefringent element 30b. That is, on the rear surface s4 of the third birefringent element 30b, light (B171e, B172o) having the same polarization state is emitted from the same position as the light (B121, B122) incident on the second birefringent element 30a.

  The orientation of the optical axis 11b of the analyzer 10b on the xy plane is an orientation rotated by 90 ° around the z axis with respect to the third birefringent element 30b, and the orientation on the yz plane is the same direction. That is, in this example, the orientation on the xy plane is the same and the orientation on the yz plane is anteroposteriorly symmetrical with respect to the optical axis 11a of the polarizer 10a. Further, as described above, the light (B171e, B172o) having the same polarization state is emitted from the rear surface s4 of the third birefringent element 30b from the same position as the light (B121, B122) incident on the second birefringent element 30a. To do. Therefore, as shown in FIG. 9G, the light B181o acting as ordinary light in the analyzer 10b travels straight on the z-axis so that the ordinary light B11 and the anomalous B12 separated by the polarizer 10a follow opposite optical paths. The light B182e that behaves as extraordinary light is refracted toward the z axis, and the ordinary light B181o and the extraordinary light B182e are emitted from the z axis on the rear surface s5 of the analyzer 10b. Then, the outgoing light B190 is coupled to the optical fiber of the outgoing side collimator 40b and is turned on.

On the other hand, in the off state, as shown in FIG. 9H, the two linearly polarized lights (B141 and B142) incident on the EO element 20 are emitted with the oscillation direction rotated by 90 °. Then, in the emitted light (B151 _E , B152 _E ), the relationship between ordinary light and extraordinary light in the third birefringent element 30b and the analyzer 10b is reversed from the on state. That is, as shown in FIGS. 9A, 9B, 9I, and 9J, the light B132 that behaves as ordinary light in the second birefringent element 10a is extraordinary light in the third birefringent element 30b. It behaves as B162e, and behaves as ordinary light B182o in the analyzer 10b. The light B131 that behaves as extraordinary light in the second birefringent element 10a behaves as ordinary light B161o in the third birefringent element 30b, and behaves as extraordinary light B181e in the analyzer 10b. Thus light B132 which behaved as ordinary light in the second birefringent element 10a _{is, B142, B152 E, B162e,} B172e, emitted from the surface s5 after analyzer 10b as light B192 away from the z-axis through the optical path of B182o. Light B131 which behaved as extraordinary light in the second birefringent element 10a _{is, B141, B151 E, B161o,} B171o, while spaced from the z-axis through the optical path of the B181e, and other light B192 that light path emitted by tracing the The light B191 symmetric about the z axis on the xy plane is emitted from the rear surface s5 of the analyzer 10b.

  Thus, in the first embodiment 101, the two light beams that pass through the EO element 20 are arranged in parallel to the x-axis. Therefore, in principle, the distance between the electrodes (22-22) of the EO element 20 may be the same as the beam spot diameter of one light beam. Therefore, if the value of the walk shift amount Wf is the same, the distance between the electrodes (22-22) can be shortened with respect to the comparative example 100. That is, even if the voltage between the electrodes (22-22) is lowered, the electric field having the same intensity can be applied to the EO substance 21, and the same level of optical loss characteristics can be obtained at a lower voltage.

Specifically, the element lengths L _p of the four birefringent elements (10a, 30a, 30b, 10b) constituting the polarizing unit 110a and the analyzing unit 110b are set to the element lengths of the polarizer 10a and the analyzer 10b of the comparative example 100. If the length is 1 / √2 with respect to L _p = 756.9 μm, the same walk shift amount Wf as in Comparative Example 100 can be secured. For example, the inter-electrode distance d of the EO element 20 only has to be greater than the beam spot diameter in principle, and even if “vignetting” is considered, the EO element 20 can be made sufficiently thinner than the comparative example 100. Even if the voltage applied between the electrodes (22-22) is lowered, a loss equivalent to that of the comparative example 100 can be obtained.

=== Second Embodiment ===
In the first embodiment, by using four birefringent elements, two light beams arranged parallel to the x-axis can be incident on the EO element, thereby reducing the inter-electrode distance d of the EO element 20. I was able to. However, since the optical paths of the two light beams are formed above the x-axis, vignetting occurs when the thin EO elements 20 are vertically symmetrical with respect to the optical axis 50. Therefore, in the first embodiment 101, the EO element 20 is disposed at a position shifted upward with respect to the optical axis 50. That is, the first embodiment 101 requires a housing having a complicated structure for holding the EO element 20 at a position shifted upward with respect to the optical axis 50. Therefore, as a second embodiment of the present invention, an optical attenuator having a configuration in which two light beams passing through the EO element 20 are arranged on the x axis will be described.

<Configuration>
FIG. 10 shows the structure of an optical attenuator (hereinafter also referred to as a second embodiment 102) according to the second embodiment. The second embodiment 102 is different from the first embodiment 101 in that a half-wave plate (between the two birefringent elements (10a-30a and 30b-10b) included in each of the polarization unit 110a and the light detection unit 110b) 60a and 60b) are different. In the following description, a half-wave plate (hereinafter also referred to as a λ / 2 plate) in the polarization unit 110a is referred to as a first λ / 2 plate 60a, and the λ / 2 plate 60b in the light detection unit 110b is referred to as a second λ. / 2 plate 60b.

  11A to 11F show the polarizer 10a, the first λ / 2 plate 60a, the second birefringent element 30a, the third birefringent element 30b, the second constituting the second embodiment 102, respectively. The azimuth | direction in the xy plane of the optical axis of 2 (lambda) / 2 board 60b and the analyzer 10b is shown. As shown in FIG. 10, the orientations of the optical axes (11a, 31a, 31b, 11b) of the four birefringent elements (10a, 30a, 30b, 10b) are the same as those in the first embodiment, and are normally-on type. An optical attenuator is configured. The first λ / 2 plate 60a, which is an optical element added to the first embodiment 101, has optical axes (11b, 31b) on the xy plane of the polarizer 10a and the second birefringent element 30a. It is in a direction that bisects the crossing angle equally. That is, it coincides with the x-axis or y-axis direction. In the illustrated example, it coincides with the x-axis direction. The same applies to the second λ / 2 plate 60b. That is, the optical axis 61b of the second λ / 2 plate 60b is an x-axis that equally bisects the azimuths of the optical axes (31b, 11b) of the third birefringent element 30b and the analyzer 10b in the xy plane. Or it corresponds to the y-axis direction. Here, the optical axis 61b coinciding with the x-axis direction is illustrated. Next, the operation of the second embodiment 102 will be described.

<Operation of Second Embodiment>
12 and 13 are diagrams showing the operation of the second embodiment 102. FIG. FIG. 12A shows the optical path in the on or off state when the second embodiment 101 is viewed from the top, and FIG. 12B shows the optical path when viewed from the left. 13A to 13K are views showing the position of the light beam and the polarization state of the light when viewed from the rear. FIGS. 13A, 13B, and 13C respectively show the position of the light beam on the rear surface s1 of the polarizer 10b, the rear surface s6 of the first λ / 2 plate, and the rear surface s2 of the second birefringent element. The polarization state is shown. FIGS. 13D to 13G show each of the rear surface s3 of the EO element 20, the rear surface s4 of the third birefringent element, the rear surface s7 of the second λ / 2 plate 60b, and the rear surface s5 of the analyzer in the on state. The position of the light beam on the surface and the deflection state are shown. FIGS. 13H to 13K show the positions and deflection states of the light beams on the surfaces s3, s4, s7, and s5 in the off state. 13A to 13K, the position of the light beam and the vibration direction at each position are indicated by white circles and double arrows.

First, the operation in the on state will be described. As shown in FIGS. 12A, 12B, and 13A, the unpolarized input light B0 incident on the polarizer 10a from the incident-side collimator 40a has a vibration direction within the polarizer 10a. The light beam is separated into two linearly polarized light beams, that is, ordinary light B211 orthogonal to the optical axis 11a and parallel abnormal light B212. Here, the azimuth angle θ = −45 °. Therefore, ordinary B211 emits straight through the polarizer 10a, the extraordinary light B212 is emitted from a position spaced 45 ° direction when viewed from the rear only walk shift amount wf _0. The operations so far are the same as those in the first embodiment 101. Here, the optical path followed by the ordinary light B211 in the polarizer 10a is defined as a first optical path, and the optical path followed by the extraordinary light B212 is defined as a second optical path.

  The two linearly polarized lights (B221 and B222) emitted from the polarizer 10a are incident on the first λ / 2 plate 60a. As is well known, the λ / 2 plate is rotated so that the vibration direction of the incident linearly polarized light is rotated so as to be symmetric with respect to its own optical axis. Therefore, as shown in FIG. 13B, the first λ / 2 plate 60a emits the incident linearly polarized light (B221, B222) by rotating the vibration directions on the xy plane by 90 °, respectively. . The linearly polarized light is oscillated in the direction of 45 degrees so that it is symmetric with respect to its own optical axis. Then, light (B231, B232) emitted from the first λ / 2 plate 60a is incident on the second birefringent element 30a.

  The orientation of the optical axis 31a on the xy plane of the second birefringent element 30a is rotated by 90 ° around the z axis with respect to the optical axis 11a of the polarizer 10a. The azimuth of the optical axis on the yz plane is a direction from the lower side to the upper side toward the rear as in the polarizer 10a. Then, since the vibration direction on the xy plane of the light (B221, B222) emitted from the polarizer 10a is rotated by 90 ° from the first λ / 4 plate 60a, the ordinary light B211 and the extraordinary light B212 in the polarizer 10a. The light (B231 and B232) that has followed the first and second optical paths behaves as ordinary light B241 and extraordinary light B242 in the second birefringent element 30a. Accordingly, the light B232 incident from the first λ / 2 plate 60a corresponding to the extraordinary light B212 in the polarizer 10a has an azimuth angle θ = + 45 ° of the optical axis 30a with respect to the xy plane of the second birefringent element 30a. Refracts in the direction. As a result, as shown in FIG. 13C, two linearly polarized light (B251, B252) arranged on the left and right on the x-axis are emitted from the second birefringent element 30b, and the first and second optical paths are light beams. Are arranged so as to be spaced apart at an interval of the walk shift amount Wf on the x-axis.

Next, two linearly polarized lights (B251 and B252) emitted from the polarization unit 110a enter the EO element 20. Since these two linearly polarized light (B251, B252) follow the optical paths aligned on the x axis, the EO element 20 is arranged so as to be vertically symmetrical with respect to the optical axis 50 as shown in FIG. can do. The thickness d incident light EO material 21 (B251, B252) never greater if shading occurs than the beam spot diameter _{W 1} of the. When the electric field E is not applied to the EO element 20, as shown in FIG. 13D, the two linearly polarized light (B251, B252) are emitted while maintaining the polarization state, and this emitted light ( B261 ₀ and B262 ₀ ) are incident on the third birefringent element 30b.

In the second embodiment 102, the polarizing part 110a and the light detecting part 110b are optical axes (11a, 61a, 31a and 31b, 61b, 10b) of the optical elements (10a, 60a, 30a and 30b, 60b, 10a) constituting them. Are mirror images of each other, and the light (B251, B252) emitted from the second birefringent element 30a disposed at the rear end of the polarization unit 110a and the third disposed at the front end of the light detection unit 110b. The light (B261 ₀ , B262 ₀ ) incident on the birefringent element 30b has the same optical path position and deflection state on the xy plane. Therefore, the light (B261 ₀ and B262 ₀ ) incident on the light detection unit 110b is an optical path (B271o, B281o, B291o, B301o and B272e, B282e, B292e) in which the optical path in the forward direction from the front to the rear of the polarization unit 110a is a mirror image. , B302e), the light B310 emitted from the rear surface s5 of the analyzer 10b along the z-axis is coupled to the optical fiber of the emission-side collimator 40b and is turned on.

On the other hand, in the off state, as shown in FIG. 13H, the two linearly polarized lights (B251 and B252) incident on the EO element 20 are emitted with the vibration direction rotated by 90 °. Accordingly, as shown in FIGS. 13I to 13K, the outgoing light (B261 _E , B262 _E ) from the EO element 20 has the relationship between the normal light and the abnormal light in the on state reversed, and the first optical path The third birefringent element 30b is refracted in the direction of 45 ° on the xy plane so that the light traveling along the second optical path is further separated from the light traveling straight along the second optical path, and in the analyzer 10b, − Refracts in the direction of 45 °. Therefore, the light that has followed the first optical path (B271e, B281e, B291e, B301e) and the light that has followed the second optical path (B272o, B282o, B292o, B302o) are x-axis on the rear surface s5 of the analyzer 10b. The light is emitted as two lights (B311 and B312) at positions symmetrical with respect to the z-axis, and the loss is maximized.

  As described above, in the second embodiment, since the light passing through the EO element 20 is arranged in the left-right direction as in the first embodiment, the thickness of the EO element 20 in the vertical direction can be reduced. Further, a λ / 2 plate (60a, 60b) that rotates the linearly polarized light incident between the two birefringent elements (10a, 30a, 30b, 10b) in the polarizing unit 110a and the analyzing unit 110a by 90 ° is interposed. The first and second optical paths formed in the EO element 20 are arranged on the x-axis. As a result, the EO element can be arranged vertically symmetrically with respect to the optical axis 50, and the EO element 30 can be easily aligned and held while maintaining the same optical loss characteristic as in the first embodiment 101. Can be realized. Further, all the optical elements can be arranged symmetrically with respect to the optical axis 50, and the size in the vertical direction including the housing can be reduced.

=== Third embodiment ===
In order to drive the optical attenuator at a low voltage, it is effective to reduce the inter-electrode distance d of the EO element 20. In the first and second embodiments (101, 102), the walk shift amount wf is the same. If so, the inter-electrode distance d can be reduced to 1 / √2 compared to the comparative example 100. In principle, it has been shown that the distance d between the electrodes can be reduced to the beam spot diameter of the light beam passing through the EO element 20.

  On the other hand, there is a limit to reducing the beam spot diameter, and there is a limit to the effect of reducing the driving voltage by reducing the inter-electrode distance d. Moreover, it becomes difficult to make the EO material 21 very thin. Therefore, conventionally, the EO element 20 has been configured using an EO material 21 made of a ferroelectric material having an excellent electro-optic effect such as PLZT.

  However, if an electric field is applied even once to the EO element 20 in a ferroelectric material, it will be in the same state as when a weak electric field is applied due to residual charge, and due to the birefringence effect even in the initial state where no electric field is applied. A phase difference occurs. Even when a sufficient electric field is applied, the effect of birefringence in the initial state remains, and the incident linearly polarized light is emitted as elliptically polarized light. Therefore, there is a problem that it is difficult to use in applications that require a larger light loss in the off state. Therefore, as a third embodiment of the present invention, the EO element 20 is reduced in thickness on the same principle as the first and second embodiments (101, 102), and the residual phase difference in the ferroelectric material is canceled out. An optical attenuator having extremely excellent optical loss characteristics will be described.

<About residual phase difference>
FIG. 14 schematically shows extinction ratio characteristics when linearly polarized light is incident on the EO element 20. This figure shows the extinction ratio characteristic when the EO substance 21 has a residual phase difference, and a straight line incident on the EO element 20 in a state where no electric field is applied to the EO element 20 (E = 0). The relationship between the rotation angle of polarized light and the extinction ratio of the light emitted from the EO element 20 is shown. Here, the rotation angle of linearly polarized light is expressed as an angle with respect to the vertical direction. The characteristic shown in FIG. 14 is that the EO substance 21 has a general formula K _{1- y My} Ta _1-x Nb _x O ₃ (where M is a monovalent metal, 0 <x <1, 0 ≦ y < This is the EO element 20 using the substance represented by 1) (hereinafter referred to as KTN). KTN is known as a ferroelectric having a large relative dielectric constant εr at room temperature, and unlike PLZT, which has been well known as an EO material in the past, KTN is an environmentally friendly EO material. Therefore, KTN is regarded as promising as an EO substance 21 that replaces PLZT. The EO element 20 used here has an element length L _eo = 2.55 mm and an interelectrode distance d = 94 μm.

  As shown in FIG. 14, the extinction ratio becomes maximum when the rotation angle of the incident linearly polarized light is in the vertical direction, that is, parallel to the application direction of the electric field E, or in the horizontal direction (perpendicular to the electric field E). , Minimum when tilted 45 °. However, since there is a residual phase difference, the minimum value of the extinction ratio is not about 0, and in this example, it is 6 dB, and the optical attenuator according to each of the above embodiments (using the EO element 20 made of a ferroelectric material) 101, 102), it can be seen that the light emitted from the EO element 20 is always elliptically polarized light having two components orthogonal to each other. Therefore, in the first or second embodiment (101, 102), when a ferroelectric is used as the EO material 21, the voltage for setting the phase difference based on the electro-optic effect to 180 ° can be further reduced. However, it becomes difficult to further improve the optical loss characteristics. In other words, if the residual phase difference in the EO element 20 can be canceled out, the optical loss characteristic can be further improved while enabling low voltage driving.

<Configuration of Third Embodiment>
FIG. 15 shows a schematic structure of an optical attenuator according to the third embodiment (hereinafter also referred to as third embodiment 103). The third embodiment 103 shown here is a normally-on type. The third embodiment 103 is different from the first embodiment in that a quarter-wave plate (hereinafter also referred to as a λ / 4 plate 70) is disposed immediately after the EO element 20, and the light detection unit 110b. Is different from the z-axis by a predetermined angle α. FIG. 16 shows xy when the polarizer 10a, the second birefringent element 30a, the λ / 4 plate 70, the third birefringent element 30b, and the analyzer 10b constituting the third embodiment 103 are viewed from the rear. The orientation of the optical axis (11a, 31a, 71, 31b, 10b) on the surface is shown. As shown in FIGS. 16A and 16B, the orientations of the optical axes (11a, 31a) of the two birefringent elements (10a, 30a) constituting the polarizing unit 110a are the same as those in the first embodiment. . FIGS. 16C and 16D show the azimuth of the optical axis 71 of the λ / 4 plate 70 which is a configuration added to the first embodiment 101, and in the application direction of the electric field E in the EO element 20. FIGS. On the other hand, the azimuth angle ω on the xy plane is ± 45 °, and any orientation shown in FIGS. 16C and 16D may be used. In other words, the orientation may be such that the x axis is 45 ° counterclockwise around the z axis (ω = + 45 °) or 45 ° clockwise (ω = −45 °) when viewed from the rear. Then, as shown in FIGS. 16E and 16F, the orientations of the optical axes (11a, 31a) of the two birefringent elements (10b, 30b) constituting the light detecting unit 110b are the same as those in the first embodiment. The optical part 110b is inclined by an angle α in the same direction around the z axis.

<Residual phase difference cancellation principle>
FIG. 17 shows the principle of canceling the residual phase difference according to the configuration of the third embodiment 103. Here, for the sake of simplicity of explanation, a light having a configuration in which a λ / 4 plate 70 is disposed immediately after the EO element 20 in the comparative example 100 in which the birefringent element is composed of only the polarizer 10a and the analyzer 10b. An attenuator is taken as an example. The EO element 20 uses KTN having a longitudinal length L _eo = 94 μm and an interelectrode distance d = 94 μm as the EO material 21 and a walk shift amount Wf = 64 μm. Then, using the residual phase difference δ (deg) as a parameter, the relationship between the azimuth angle φ of the optical axis 11b of the analyzer 10b and the optical loss (dB) when the electric field E is not applied to the EO element 20 is shown. Show. As shown in FIG. 17, by rotating the analyzer 10b appropriately around the optical axis 50 regardless of the value of the residual phase difference δ, the loss is practically sufficient for optical communication applications from 0 dB. It is variable up to a value larger than 30 dB. This means that, in an initial state where no electric field is applied to the EO element 20, there is an azimuth angle φ that has a loss of 0 dB for the normally-on type and a loss of 30 dB or more that is practically sufficient for the normally-off type.

  The first embodiment 101 is different from the comparative example 100 in that the second birefringent element 30a for arranging two optical paths separated in the x-axis direction by the polarizer 10a in the 45 ° direction on the xy plane is arranged. Only the third birefringent element 30b for returning the optical path so as to be separated in the direction of 45 degrees on the xy plane is added, and the vibration direction of the light following the optical path does not change. Therefore, in the third embodiment shown in FIG. 15, the residual phase difference δ can be canceled based on the principle shown in FIG. That is, there is an angle α for canceling the residual phase difference.

<Operation of the third embodiment>
18 and 19 show the operation of the third embodiment 103 shown in FIG. FIG. 18A shows the optical path in the on or off state when the third embodiment 101 is viewed from the top, and FIG. 18B shows the optical path when viewed from the left. FIGS. 19A to 19J are views showing the position of the light beam and the polarization state of the light beam when viewed from the rear. FIGS. 19A and 19B show the rear surface s1 of the polarizer 10b. And the position and polarization state of the light beam on the rear surface s2 of the second birefringent element are shown. 19C to 19F show the light on each surface of the rear surface s3 of the EO element 20, the rear surface s8 of the λ / 4 plate 70, the rear surface s4 of the third birefringent element, and the rear surface s5 of the analyzer in the ON state. The beam position and deflection state are shown. FIGS. 19H to 19J show the positions and deflection states of the light beams on the surfaces s4 to s6 in the off state.

As shown in FIGS. 18, 19A and 19B, the optical path and polarization state of light from the incident-side collimator 40a to the polarizer 10a, the second birefringent element 30a, and the EO element 20 are the same as those in the first embodiment. 101. In FIG. 18 and FIG. 19, the light from the input light B0 to the light (B141, B142) incident on the EO element 20 is indicated by the same reference numerals as in FIG. However, in the third embodiment 103, the EO element 20 lambda / 4 plate 70 is disposed between the third birefringent elements 30b, deflection of light _{(B351 0,} B352 0) emitted from EO element 20 The state and the subsequent optical path to the output side collimator 40b and the light deflection state are different from those of the first embodiment 101. Also in this case, the light path followed by the light that was ordinary light B111 in the polarizer 10a is defined as a first optical path, and the light path that is followed by the light that was abnormal light B112 is defined as a second optical path.

First, the operation in the on state will be described. The EO material 21 of the EO element 20 has a residual phase difference δ, and when the electric field E is not applied to the EO element 20, as shown in FIG. each of the two linearly polarized light incident on the EO device 20 (B141, B142) is emitted to the vibration direction of itself as elliptically polarized light extinction ratio according to the residual phase difference δ with the long axis _{(B351 0,} B352 0) . lambda / 4 plate 70 is elliptically polarized light from the EO devices 20 _{(B351 0,} B352 0) is incident, as shown in FIG. 19 (D), the extinction ratio of each of the elliptical polarization _{(B351 0,} B352 0) The linearly polarized light (B361 ₀ , B362 ₀ ) that oscillates in the combined vector direction of the major axis direction and the minor axis direction according to is emitted. The vibration direction of the linearly polarized light (B361 ₀ , B362 ₀ ) on the xy plane is inclined by an angle α with respect to the linearly polarized light (B141, B142) incident on the EO element 20. In the illustrated example, it is inclined clockwise by an angle α.

The third azimuth angle phi ₁ of the optical axis 31b of the birefringent element 30b is to 45 ° the direction counter-clockwise in a clockwise angle α oblique angle (phi _{1 =} 45 ° -α when viewed from the rear ). With linearly polarized light B361 ₀ to behave as extraordinary light B371e oscillates the optical axis 31b in a direction parallel to itself incident following the first optical path in the order third birefringent elements 30b, perpendicular to the optical axis 31b Thus, linearly polarized light B362 ₀ that behaves as ordinary light B372o enters the second optical path. Then, 19 first linear polarization B361 ₀ to follow the optical path incident on the third birefringence element 30b is straight as shown in (E), a third birefringent element following the second light path linearly polarized light B362 ₀ incident on 30b is emitted from the rear surface s4 refracted to the optical axis direction, these outgoing light (B381e, B382o) is incident on the analyzer 10b.

The azimuth phi ₂ in the xy plane of the optical axis 11b of the analyzer 10b is perpendicular to the optical axis 31b of the third birefringent elements 30b (φ ₂ = -45 °-.alpha.). In addition, the orientation on the yz plane coincides with that of the third birefringent element 30b. Therefore, the ordinary light B372o and the extraordinary light B371e in the third birefringent element 30b become the extraordinary light B392e and the ordinary light B391o in the analyzer 10b. Accordingly, as shown in FIG. 19F, the light B381e refracted and emitted by the third birefringent element 30b goes straight through the optical axis 11b and is emitted from the rear surface s5. The light B382o that travels straight through the third birefringent element 30b is refracted along the optical axis 11b so as to be close to the light B391o that travels straight through the analyzer 10b as ordinary light. As a result, light (B401, B402) whose vibration directions in the xy plane are orthogonal to each other is emitted from the rear surface s5 of the analyzer 10b in a narrow region B400 near the z-axis, and these light (B401, B402) are emitted. The operation of a normally-on type optical attenuator that is coupled to the side optical fiber Fb and is turned on when the electric field E is not applied to the EO element 20 is performed.

The above is the operation when the electric field E is not applied to the EO element 20. The operation in the off state in the on state will be described. As shown in FIG. 19 (G), each of the two linearly polarized light (B141, B142) incident on the EO element 20 is extinguished according to the residual phase difference δ, with the direction orthogonal to its own vibration direction as the major axis. It is emitted as elliptically polarized light ratio _{(B351 E,} B352 E). The lambda / 4 plate 70 is elliptically polarized light _{(B351 E,} B352 E) is incident from the EO element 20, as shown in FIG. 19 (H), the extinction of each elliptically polarized light _{(B351 E,} B352 E) Linearly polarized light (B361 _E , B362 _E ) that oscillates in the combined vector direction of the major axis direction and the minor axis direction according to the ratio is emitted. The vibration direction of the linearly polarized light (B361 _E , B362 _E ) on the xy plane is tilted by an angle α from the direction rotated by 90 ° with respect to the linearly polarized light (B141, B142) incident on the EO element 20. . The third azimuth angle phi ₁ of the optical axis 31b of the birefringent element 30b is to 45 ° the direction counter-clockwise in a clockwise angle α oblique angle (phi _{1 =} 45 ° -α when viewed from the rear ). Therefore, linearly polarized light B362 _E that vibrates in the direction parallel to its own optical axis 31b and behaves as extraordinary light B372e enters the third birefringent element 30b along the second optical path and is orthogonal to the optical axis 31b. Thus, linearly polarized light B361 _E acting as ordinary light B371o enters the first optical path. Then, as shown in FIG. 19I, the linearly polarized light B361 _E incident on the third birefringent element 30b along the first optical path travels straight, and the second birefringent element follows the second optical path. The linearly polarized light B362 _E incident on 30b is refracted in the direction of the optical axis 31b and emitted from the rear surface s4, and these emitted lights (B381o, B382e) enter the analyzer 10b.

The orientation of the optical axis 11b of the analyzer 10b on the xy plane is orthogonal to the optical axis 31b of the third birefringent element 30b (φ ₂ = −45 ° −α). In addition, the orientation on the yz plane coincides with that of the third birefringent element 30b. Therefore, the ordinary light B372o and the extraordinary light B371e in the third birefringent element 30b become the extraordinary light B392e and the ordinary light B391o in the analyzer 10b. Accordingly, as shown in FIG. 19F, the light B381e refracted and emitted by the third birefringent element 30b goes straight through the optical axis 11b and is emitted from the rear surface s5. The light B382o that travels straight through the third birefringent element 30b is refracted along the optical axis 11b so as to be close to the light B391o that travels straight through the analyzer 10b as ordinary light. As a result, light (B401, B402) whose vibration directions in the xy plane are orthogonal to each other is emitted from the rear surface s5 of the analyzer 10b in a narrow region B400 near the z-axis, and these light (B401, B402) are emitted. Coupled to the side optical fiber Fb and turned on.

On the other hand, in the off state, the electric field E is applied to the EO material 21 so that the phase difference based on the electro-optic effect in the EO element 20 becomes 180 °. In this case, two elliptically polarized light major axis directions due to the residual phase difference δ are orthogonal _{(B351 E,} B352 E) is emitted from the EO element 20, as shown in FIG. 19 (G) . Then, the two elliptical polarization _{(B351 E,} B352 E) is elliptically polarized light in the ON state _{(B351 0,} B352 0) and the long axis are orthogonal to each other. Therefore, in FIG. 19 (H), the outgoing light (B361 _E , B362 _E ) from the λ / 4 plate 70 has the relationship between ordinary light and abnormal light in the on state reversed, and FIG. 18 and FIGS. 19 (I) (J) As shown in FIG. 4, the light B361 _E incident on the third birefringent element 30b along the first optical path travels straight as the ordinary light B372o, and the light B362 _E incident along the second optical path becomes the extraordinary light B372e. The light is refracted along the optical axis 31b so as to be separated from the ordinary light B371o. In the analyzer 10b, the light B382e that has followed the second optical path travels straight as the ordinary light B392o, and the light B381o that has followed the first optical path further separates from the ordinary light B392o along the optical axis 11b as the abnormal light B391e. Refracts like so. As a result, two light beams (B411, B412) separated in the direction of the angle α with respect to the x axis are emitted from the rear surface s5 of the analyzer 10b, and the loss is maximized.

  Thus, in the third embodiment, elliptically polarized light caused by the residual phase difference δ in the EO element 20 is returned to linearly polarized light using the λ / 4 plate 70. Then, the analyzer 110b including two birefringent elements (30b, 10b) having optical axes (31b, 11b) in a direction that coincides with and orthogonal to the vibration direction on the xy plane of the linearly polarized light is a λ / 4 plate 70 It is arranged behind Thereby, a part of the light in the on state maintains the same vibration state as the off state, and the coupling strength does not decrease. Similarly, even in the off state, a part of the light incident on the light detector 110b maintains the same vibration state as the on state, and part of the light is not coupled to the emission side optical fiber Fb. Accordingly, the third embodiment 103 includes an EO element employing an EO material made of a ferroelectric material and four birefringent elements (10a, 30a, 30b, 10b), and can be driven at a low voltage, resulting in an optical loss. Both have excellent optical loss characteristics in terms of on / off ratio. As a matter of course, the third embodiment 103 can be changed to a configuration similar to that of the second embodiment 102 in which the λ / 2 plates (60a and 60b) are provided in the polarization section 110a and the light detection section 110b.

<Optical loss characteristics>
In the third embodiment 103, when a ferroelectric is used for the EO material 21 of the EO element 20, the light transmission characteristic in the on state and the off state are compensated by compensating the residual phase difference δ in the EO material 21. This improves the optical loss characteristics. Therefore, the voltage for causing the 180 ° phase difference based on the electro-optic effect in the EO element 20 is equivalent to that in the first embodiment 101 and the second embodiment 102. Therefore, an optical attenuator having the configurations of the comparative example 100 and the third example 103 was produced as a sample. Also, an optical attenuator (hereinafter also referred to as a modified example) obtained by adding the configuration of the second example 102 to the third example 103 was produced as a sample. In all samples, KTN was adopted as the EO substance 21, and electrodes 22 made of Pt thin films were formed on the upper and lower surfaces of the KTN to form the EO element 20. The KTN used here has a relative dielectric constant εr = 15000, a refractive index n = 2.185 at a wavelength λ = 1550 nm, electrooptic constants g ₁₁ = 0.0809 m ⁴ / c ² , g ₁₂ = 0. 0023 m ⁴ / c ² .

FIG. 20 shows the sizes of the optical elements constituting the sample corresponding to the third example 103 and the sample corresponding to the modification example 102b. The sizes of the optical elements constituting the comparative example 100 are shown in FIG. The sizes of the optical elements (10a, 30a, 20, 30b, 10b, 60a, 60b, 70) shown in FIG. 20 are determined based on the optical design conditions of the optical fiber collimators (40a, 40b) shown in FIG. _Is determined based on the inter-lens distance L _c0 in the space of the refractive index n = 1, the walk shift amount Wf considering the crosstalk employed in the comparative example 100, and the refractive index n of each optical element. Yes.

As described above, in the comparative example 100, the element length L _p of the birefringent elements (10a, 10b) is 756.9 μm. The element length L _p of the birefringent elements (10a, 30a, 30b, 10b) in the third embodiment 103 and the modified example 103b is L _p corresponding to 1 / √2 of the element length L _p = 756.9 μm in the comparative example 100. = 539 μm.

The λ / 4 plate 70 used in the third embodiment 103 and the modification 103b is made of flat crystal and has a refractive index n = 1.53. Further, the thickness Lq at the front and rear is 100 μm, and the thickness is designed so that a phase difference of 3/4 wavelength is generated before and after light is transmitted. The λ / 2 plate 80 used in the modification 103b has a longitudinal length Lh = 80 μm. The margin _{L M} and the actual including space _{L S} between the optical element lens distance _{L c} of the optical element (10a, 30a, 20,30b, 10b , 60a, 60b, 70) EO element 20 so as to fill in Arranged. As a result, the element lengths L _eo of the EO elements 20 in the comparative example 100, the third example 103, and the modification example 103b were 2.15 mm, 2.05 mm, and 1.845 mm, respectively. The inter-electrode distance d of the EO element 20 is d = 102.4 μm in the comparative example as described above, and the beam spot diameter W1 (= 50 μm) may be the same in the third example 103 and the modified example 103b. Was set to d = 70 μm.

FIG. 21 shows the optical loss characteristics of each sample. In the sample having the configuration of Comparative Example 100, when a voltage of 22.8 V was applied between the electrodes (22-22) of the EO element 20, the phase difference based on the electro-optic effect in the EO element 20 was 180 °. In the sample having the configuration of the third embodiment 103 and the modification 103b, when the voltages of 14.0 V and 14.7 V were applied between the electrodes (22-22) of the EO element 20, respectively, the electro-optic in the EO element 20 was obtained. The phase difference based on the effect was 180 °. The excess loss in the on state is zero, and the optical loss in the off state is improved. Note that the voltage for turning off the sample of the modification 103b was higher than that of the sample of the third embodiment 103, despite the fact that the number of optical elements was higher than that of the other samples. This is because the margin L _M and the space L _s were ensured in the same manner as in the above sample, and the element length Leo of the EO element 20 was shortened due to the limitation of the Rayleigh distance Z ₀ described above.

=== Fourth embodiment ===
In the third embodiment, an excellent light loss characteristic is obtained by rotating the analyzer 10b. The configuration in which the analyzer 10b is rotated has the effect that the λ / 4 plate 70 converts elliptically polarized light caused by the residual phase difference δ into linearly polarized light and rotates the vibration direction of the linearly polarized light (hereinafter, for convenience). The residual phase difference δ is compensated using the optical rotation effect). For example, in the first and second embodiments, the residual phase difference is set by setting the azimuth angle of the optical axis 11b of the analyzer 10b so as to coincide with or orthogonal to the vibration direction of linearly polarized light due to the optical rotation effect of the λ / 4 plate 30. Is offset.

  However, in the first and second embodiments, it is necessary to precisely adjust the relative rotation angle of the polarizer 10a and the analyzer 10b around the optical axis 50. In particular, when the above-described walk-off prism made of the parallel rutile birefringent crystal is used for the polarizer 10a and the analyzer 10b, the polarizer 10a and the analyzer 10b have a three-dimensional shape of a rectangular parallelepiped shape. It is necessary to hold the analyzer 10b in a state of being rotated around the optical axis 50 with respect to the polarizer 10a. This complicates the structure of the housing that houses various optical elements including the analyzer 10b. Further, by rotating the analyzer 10b with respect to the polarizer 10a, the vertical and horizontal sizes of the analyzer 10b are substantially increased with respect to the polarizer 10a. This is a factor that hinders downsizing of the optical attenuator. Of course, it is conceivable to cut out the birefringent crystal so that the optical axis 11b is inclined by the above-mentioned α to produce the analyzer 10b. However, if there is an individual difference in the residual phase difference δ of the EO element 20 to be used, the individual difference An analyzer 10b corresponding to the above must be prepared. Further, when the optical axes of the polarizer and the analyzer are arranged at an angle other than parallel or orthogonal to each other, as shown in FIG. 18 and FIG. And the extraordinary light are not emitted from a completely coincident position, and in principle, a coupling loss occurs. The same phenomenon as so-called polarization dependent loss (PDL) occurs.

  Therefore, as a fourth embodiment of the present invention, the orientation of the optical axes (31b, 11b) of the birefringent elements (30b, 10a) in the xy plane in the analyzer 110b is 45 ° with respect to the x axis. Even if the optical attenuator is disposed in the optical attenuator, an optical attenuator having a more excellent optical loss characteristic by reliably canceling the residual phase difference δ of the EO element 20 will be mentioned.

<Configuration>
FIG. 22 shows the configuration of an optical attenuator according to the fourth embodiment (hereinafter also referred to as the fourth embodiment 104). The fourth embodiment 104 shown here is a normally-on type. FIG. 22A is a perspective view when the configuration of the fourth embodiment 104 is viewed from above and behind. The fourth embodiment 104 is a half-wave plate (hereinafter, referred to as the third embodiment 103). (also referred to as a λ / 2 plate 80) is disposed immediately after the λ / 4 plate 70, and the optical axes (31b, 11b) of the two birefringent elements (30b, 10b) constituting the light detector 110b. The difference is that the orientation is the same as in the first embodiment. FIG. 22B shows the orientation of the optical axis 81 on the xy plane when the λ / 2 plate 80, which is a configuration added to the third embodiment 103, is viewed from the rear.

  In the fourth example 104, the orientations of the optical axes (11a, 31a, 31b, 11b) of the four birefringent elements (10a, 30a, 30b, 10b) constituting the polarizing unit 110a and the analyzing unit 110b are the first implementation. Similar to Example 101, the azimuth of the optical axis 71 of the λ / 4 plate 70 is the same as that in the third example 103, and is the direction obtained by rotating the x axis around the z axis by + 45 ° or −45 °.

The orientation of the optical axis 81 of the λ / 2 plate 80 is set based on the vibration direction on the xy plane of the linearly polarized light emitted from the λ / 4 plate in the third embodiment 103. Specifically, as shown in FIG. 22B, the λ / 4 plate 70 is orthogonal to each other corresponding to the ordinary light and the extraordinary light separated by the polarizer 10a and has an azimuth angle of ± 45 °. Two linearly-polarized lights P obtained by tilting the two linearly-polarized lights by an angle α are emitted. Here, when viewed from the rear, the angle α rotates clockwise (minus direction) around the z axis. The azimuth angle ± 45 ° −α of the linearly polarized light P and the optical axis 31b of the third birefringent element 30b. The azimuthal bisector of the azimuth angle φ ₁ or the azimuth angle orthogonal to the azimuth angle φ ₁ is the azimuth of the optical axis 71 of the λ / 2 plate 70. The configuration shown in FIG. 22B shows an example in which the azimuth angle ω ₂ of the optical axis 81 of the λ / 2 plate 80 is set to ω ₂ = 45 ° −α / 2. Of course, ω ₂ = −45 ° −α / 2 may be set as shown in FIG. In the fourth embodiment 104, as in the second embodiment 102, λ / 2 plates (60a, 60b) for shifting the optical path to the vicinity of the optical axis 50 are provided in the polarization section 110a and the light detection section 110b. Also good.

  In the fourth embodiment 104 having the above-described configuration, the λ / 2 plate 80 is rotated so that the vibration direction of the linearly polarized light P incident from the λ / 4 plate 70 is symmetric with respect to its own optical axis 81. And exit. Therefore, when the electric field E is not applied to the EO element 20, or when the electric field E is applied to the EO element 20 and the phase difference due to the electro-optic effect is 180 °, the light is emitted from the λ / 4 plate 30. If the azimuth of the optical axis 71 of the λ / 2 plate 70 is set so that the vibration direction of the linearly polarized light P and the azimuth of the optical axis 11b of the analyzer 10b are orthogonal or parallel to each other, the analyzer 10b is substantially placed on the optical axis 50. It is the same as rotating around the angle α. The operation of the fourth embodiment 104 will be described below.

<Operation>
As shown in FIG. 22, the fourth embodiment 104 has the same configuration as the third embodiment from the incident-side collimator 40a to the λ / 4 plate 70 toward the rear. That is, in the fourth embodiment 104, the operation until the light incident on the polarizer 10a is emitted from the λ / 4 plate is the same as that of the input light B0 in the operation of the third embodiment 103 shown in FIGS. This is the same as the change in the optical path and polarization state to the light (B361 ₀ , B362 ₀ , or B361 _E , B362 _E ) emitted from the plate 70. Therefore, in the following, as the operation of the fourth embodiment 104, changes in the optical path and the deflection state in the optical element behind the λ / 4 plate 70 will be mainly described.

  The operation of the fourth embodiment 104 is shown in FIGS. FIG. 24A shows the optical path in the on or off state when the fourth embodiment 104 is viewed from the top, and FIG. 24B shows the optical path when viewed from the left. FIG. 25 is a diagram showing a characteristic operation in the fourth embodiment 104. FIG. 25 shows changes in the optical path from the EO element 20 to the analyzer 10b and the deflection state when viewed from the rear. 25A to 25E show the back surfaces (s3, s8, s9) of the EO element 20, the λ / 4 plate 70, the λ / 2 plate 80, the third birefringent element 30b, and the analyzer 10b in the ON state. , S4, s5) show the position and polarization state of the light beam, and FIGS. 25F to 25J show the position of the light beam on each of the surfaces (s3, s8, s9, s4, s5) in the off state. And the polarization state.

Then, as shown in FIGS. 24A and 24B, the optical path of light from the incident-side collimator 40a through the polarizer 10a, the second birefringent element 30a, and the EO element 20 until the light is emitted from the λ / 4 plate 70. The polarization state is the same as that of the third embodiment 101. Therefore, in FIGS. 24 and 25, the light (B361 ₀ , B362 ₀ , or B361 _E , B362 _E ) from the input light B0 to the light emitted from the λ / 4 plate 70 is denoted by the same reference numerals as those in FIGS. Hereinafter, the optical path and polarization state of light from the rear surface s3 of the EO element 20 to the rear surface s5 of the analyzer 10b will be specifically described for each of the on state and the off state.

First, the operation in the ON state will be described. As shown in FIG. 25A, the two linearly polarized lights (B141 and B142) incident on the EO element 20 have a long oscillation direction of the incident light (B141 and B142). while the shaft according to the residual phase difference δ extinction ratio of the elliptically polarized light _{(B351 0,} B352 0) is emitted as. lambda / 4 plate 70 is elliptically polarized light from the EO devices 20 _{(B351 0,} B352 0) is incident, an extinction ratio of 25, as shown (B), the respective elliptical polarization _{(B351 0,} B352 0) The linearly polarized light (B361 ₀ , B362 ₀ ) that oscillates in the combined vector direction of the major axis direction and the minor axis direction according to is emitted. The vibration direction of the linearly polarized light (B361 ₀ , B362 ₀ ) on the xy plane is inclined by an angle α with respect to the linearly polarized light (B141, B142) incident on the EO element 20.

In the λ / 2 plate 80, the vibration direction in the xy plane of the two linearly polarized light (B361 ₀ , B362 ₀ ) incident from the λ / 4 plate 70 is the azimuthal bisector with the azimuth of the optical axis 81 of itself. Rotate so that the orientation is the same. That is, as shown in FIG. 25C, the incident linearly polarized light (B361 ₀ and B362 ₀ ) is rotated so that the azimuth angles are −45 ° and 45 °, which are the same as the vibration directions in the xy plane. . The position of the optical path and the polarization state of the light (B471 ₀ and B472 ₀ ) emitted from the λ / 2 plate 80 are the same as the light incident on the third birefringent element 30b when in the on state in the first embodiment 101. The same. Therefore, thereafter, the optical path and polarization state are the same as those in the first embodiment 101, and as shown in FIGS. 25D and 25E, the light B471 ₀ , B481e, B491e, B501o, and B501o that follow the first optical path, and The light B472 ₀ , B481o, B491o, and B501e that follow the second route pass through the two light paths on the rear surface of the analyzer 10b, and the light B510 is coupled to the emission side optical fiber Fb. Is turned on.

On the other hand, when the electric field is applied and the phase difference based on the electro-optic effect is 180 °, the two linearly polarized lights (B141 and B142) incident on the EO element 20 are as shown in FIG. the incident light (B 141, B 142) is emitted as elliptically polarized light having a long axis direction of vibration in the direction rotated 90 ° _{(B351 E,} B352 E). lambda / 4 plate 70, as shown in FIG. 25 (G), vibrates in the long axis and the short axis of the combined vector direction corresponding to the extinction ratio of the incident elliptically polarized light _{(B351 0,} B352 0) Linearly polarized light (B361 _E and B362 _E ) is emitted. The vibration direction of the linearly polarized light (B361 _E , B362 _E ) on the xy plane is also inclined by the angle α with respect to the linearly polarized light (B141, B142) incident on the EO element 20. Then, it vibrates in a direction rotated by 90 ° with respect to the vibration direction of the emitted light (B361 ₀ , B362 ₀ ) in the ON state shown in FIG.

As shown in FIG. 25H, the λ / 2 plate 80 changes the vibration direction in the xy plane of the two linearly polarized lights (B361 _E and B362 _E ) incident from the λ / 4 plate 70 by its own optical axis. The azimuth 81 is rotated so as to have an azimuth bisector. That is, two linearly polarized lights that vibrate in a direction rotated by 90 ° with respect to the vibration direction of the outgoing light (B471 ₀ , B472 ₀ ) from the λ / 2 plate 80 in the ON state shown in FIG. The position of the optical path and the polarization state of the light (B471 _E and B472 _E ) emitted from the λ / 2 plate 80 are the same as the light incident on the third birefringent element 30b in the off state in the first embodiment 101. The same. Accordingly, the optical path and polarization state are the same as those in the first embodiment 101, and the light B471 _E , B481o, B491o, B501e, and B501e that follow the first optical path, as shown in FIGS. The light B472 _E , B481e, B491e, and B501o that follow the second route are emitted from the rear surface s5 of the analyzer 10b. And the emitted light (B511, B512) from the rear surface s5 is emitted from positions separated from each other so as to be symmetric with respect to the z axis on the x axis. As a result, the outgoing light (B511, B512) is not coupled to the outgoing-side optical fiber Fb, and enters an off state where the maximum optical loss occurs.

  Thus, in the fourth embodiment 104, in the ON state, an optical path is formed in which the light separated by the polarizer 10a is emitted from the same position on the rear surface of the analyzer 10b. Correspondingly, the high-intensity light can be transmitted with low loss. In addition, the polarizer 10a and the analyzer 10b can be arranged so that the directions of the optical axes (11a, 11b) coincide or are orthogonal to each other, and the analyzer 10a is rotated around the optical axis 50 as in the third embodiment 103. There is no need to let them. In the case of the flat λ / 2 plate 40, the direction of the optical axis can be easily adjusted by simply rotating the λ / 2 plate 40 about the z axis with the z axis direction as the normal.

  The relative orientations of the optical axes (11a, 31a, 31b, 11b) of the birefringent elements (10a, 30a, 30b, 10b) having a three-dimensional shape in the polarizing unit 110a and the analyzing unit 110b are strictly determined. No need to adjust. Assembling of the optical attenuator is facilitated, and the structure of the housing for holding the optical element can be simplified. All the birefringent elements (10a, 30a, 30b, 10b) can be made of substantially the same element, and the types of components can be reduced. In the fourth embodiment 104, similarly to the second embodiment 102 and the modification 103, a λ / 2 plate for arranging two optical paths in the x-axis direction may be provided in the polarization section 110a and the light detection section 110b. .

=== Other Embodiments ===
In the third and fourth embodiments (103, 104), the λ / 4 plate 70 is disposed immediately after the EO element 20, but the λ / 4 plate 70 only compensates for the residual phase difference δ in the EO element 20. Therefore, even if the λ / 4 plate 70 is arranged immediately before the EO element 20, the same effect can be obtained. That is, the light incident on the EO element 20 is given in advance a phase difference that compensates for the residual phase difference δ, and the EO element 20 causes the incident light to vibrate on the xy plane by ±± due to its residual phase difference δ. The light is emitted from the 45 ° direction as light inclined by the angle α.

  In the first to fifth embodiments 101 to 104, various optical elements (10a, 10b, 20, 30a, 30b, 60a, 60b, 70, 80) are arranged between a pair of optical fiber collimators (40a-40b) facing in the front-rear direction. And configured an optical attenuator. These optical attenuators may be for one channel of a multi-channel optical attenuator. That is, the optical attenuators 101 to 104 of the embodiments may be used for one channel of the multi-channel optical attenuator.

  When a multi-channel optical attenuator is actually configured, the configuration is substantially the same as that of a plurality of collimators by using a tape core wire and a microlens array that are held in a state where a plurality of optical fibers are arranged. can do. In addition, various optical elements (10a, 10b, 20, 30a, 30b, 60a, 60b, 70, 80) may be arranged on the left and right by the same number as the channel, or at least one optical element is provided between the optical paths for a plurality of channels. It may be an integral shape extending in the left-right direction so as to be interposed. Of course, all of the optical elements may be formed to be interposed between the optical paths for a plurality of channels.

  In an optical communication network, an optical attenuator is often installed in a place where there is little temperature change, such as indoors. Therefore, in each of the above-described embodiments (101 to 104), the configuration considering the ambient temperature change was not provided. . However, particularly in the third and fourth embodiments (103, 104) including the EO element 20 using a ferroelectric material, the residual phase difference δ may change depending on the ambient temperature. Therefore, a temperature adjusting mechanism may be added to these optical attenuators. Alternatively, a bias voltage may be applied in advance between the electrodes of the EO element 20 (22-22) so as to generate a predetermined retardation in the EO substance and maintain the retardation. Of course, by applying a bias voltage, the change of the optical loss characteristic can be moderated with respect to the voltage change, and the characteristics can be improved to be more “linear”. This case is also applicable to the first and second embodiments (101, 102).

  In an optical attenuator having various optical elements arranged between optical fiber collimators facing in the front and rear, the end face of the optical fiber is usually polished in one direction in order to prevent light from entering the optical fiber. ing. That is, the end face is inclined with respect to the optical axis. Therefore, in an optical attenuator for practical use, when a line connecting the end faces of optical fibers facing in the front and rear directions is set as the optical axis, the outgoing light from the incident side fiber collimator is slightly inclined with respect to the optical axis. Therefore, in general, the front and rear surfaces of the EO element are polished so as to be inclined with respect to the optical axis so that the optical path is symmetric. FIG. 26 shows the structure of the optical attenuator when the end face of the optical fiber (Fa, Fb) is polished. In this figure, optical elements unnecessary for explanation are omitted, and the polishing direction of the end face of the optical fiber (Fa, Fb), the inclination direction of the light incident / exit surface of the EO element 20, and between the optical fibers (Fa− Only the optical path of Fb) is schematically shown. As shown in FIG. 26, the facing optical fibers (Fa, Fb) have end faces in opposite directions. The EO element 20 is formed in a trapezoidal shape when viewed from the left-right direction, and the light incident / exit surface and the end surfaces of the optical fibers (Fa, Fb) opposed thereto are inclined in opposite directions. In this figure, light is emitted obliquely upward with respect to the optical axis 50 from the incident side optical fiber. This light is refracted so as to follow an optical path parallel to the optical axis 50 in the EO element, and the light emitted from the EO element passes through an optical path obliquely downward to reach the rear optical fiber. Therefore, an optical path symmetrical with respect to the EO element is formed in the optical attenuator.

1,100,101,102,103,103b, 104 optical attenuator,
1a polarizer (polarizing plate), 1b analyzer (polarizing plate), 10 birefringence element,
10a Polarizer (first birefringent element), 10b Analyzer (fourth birefringent element),
11, 11a, 11b, 31a, 31b The optical axis of the birefringent element,
20 electro-optic element (EO element), 21 electro-optic substance (EO substance, KTN),
22 electrode (Pt thin film electrode), 30a second birefringent element, 30b third birefringent element, 40a, 40b optical fiber collimator, 50 optical axis,
60a, 60b, 80 1/2 wavelength plate (λ / 2 plate),
61a, 61b, 81 Optical axis of a half-wave plate, 70 1/4 wave plate (λ / 4 plate),
71 1/4 wavelength plate optical axis, Ca, Cb collimating lens,
Fa, Fb optical fiber (single mode optical fiber)

Claims (8)

The front first optical fiber collimator holding the first optical fiber and the rear second optical fiber collimator holding the second optical fiber are opposed to each other in the front-rear direction, and the z-axis extending in the front-rear direction is As the optical axis, a plurality of optical elements having light incident / exit surfaces on the front and rear are arranged on the optical axis, and the intensity of the input light from the first optical fiber is attenuated and output to the second optical fiber. An optical attenuator that
Two axes orthogonal to the z-axis and orthogonal to each other are defined as an x-axis and a y-axis,
As the optical element, a polarizing unit, an electro-optical element unit, and a light detecting unit are arranged in this order from the first optical fiber collimator to the second optical fiber collimator.
In the polarizing section, the first birefringent element and the second birefringent element are arranged in this order from the front to the rear in such a relationship that the azimuth of the optical axis on the xy plane is rotated by 90 ° around the z axis. Including the configuration arranged in
In the first birefringent element, the azimuth of the optical axis on the xy plane is inclined 45 ° in a predetermined direction around the z axis with respect to the x axis, and the input light is divided into two straight lines of ordinary light and extraordinary light. The azimuths of the optical axes on the yz plane and the zx plane are set so that the two linearly polarized lights are emitted from positions separated from each other in the direction of 45 ° with respect to the x axis on the xy plane.
The second birefringent element emits the two linearly polarized lights emitted from the first birefringent element on a first optical path and a second optical path that are separated in a direction parallel to the x-axis,
In the electro-optic element portion, an electro-optic material having an electro-optic effect is sandwiched between electrodes facing each other with the y-axis as a normal direction, and a voltage is applied between the facing electrodes by an external driving circuit. A predetermined phase difference is given to each of the light incident along the first and second optical paths and emitted from the rear,
In the light analyzing section, the third birefringent element and the fourth birefringent element, which have a relationship in which the orientation of the optical axis on the xy plane is rotated by 90 ° around the z axis, are arranged in this order from the front to the rear. Including the configuration arranged in
When the electro-optic element portion gives a phase difference of 0 ° or 180 ° based on the electro-optic effect to the light incident on the electro-optic element portion, the x-trace follows the first and second optical paths. Two linearly polarized light beams that are orthogonal to each other while being separated in the axial direction are incident on the front surface of the third birefringent element, and the fourth birefringent element converts the two linearly polarized light beams into the light of the second optical fiber collimator. Coupled to the fiber,
An optical attenuator characterized by that. In claim 1,
The polarizing section is formed by arranging a first half-wave plate between the first birefringent element and the second birefringent element, and the first half-wave plate is an optical axis. Is set to be parallel to the x-axis or y-axis,
The light analyzing unit includes a second half-wave plate disposed between the third birefringent element and the fourth birefringent element, and the second half-wave plate is xy. The azimuth of the optical axis on the surface is set to be a direction that bisects the azimuth of the optical axis on the xy plane of the third and fourth birefringent elements,
An optical attenuator characterized by that. In claim 1 or 2,
The electro-optic material is made of a ferroelectric material having a birefringence effect due to a predetermined residual phase difference in a state where an electric field is not applied,
A quarter-wave plate is disposed between the polarizing unit and the electro-optical element unit, or between the electro-optical element unit and the light detecting unit,
The quarter-wave plate has an azimuth of the optical axis on the xy plane that is inclined by 45 ° around the z-axis with respect to the x-axis, and is emitted from the polarizing section onto the first and second optical paths. Compensating for the residual phase difference so that two linearly polarized lights inclined by an angle α around the z axis with respect to the orientation of the two linearly polarized lights in the xy plane are incident on the detection unit;
The azimuth of the optical axis on the xy plane of the third birefringent element coincides with one of the two linearly polarized lights emitted from the quarter wavelength plate.
An optical attenuator characterized by that. In claim 1 or 2,
The electro-optic material is made of a ferroelectric material having a birefringence effect due to a predetermined residual phase difference in a state where an electric field is not applied,
An optical rotator comprising a half-wave plate is disposed immediately before the third birefringent element,
A quarter-wave plate is disposed between the polarizing unit and the electro-optical element unit, or between the electro-optical element unit and the optical rotation unit,
The quarter-wave plate has an azimuth of the optical axis on the xy plane that is inclined by 45 ° around the z-axis with respect to the x-axis, and is emitted from the polarizing section onto the first and second optical paths. Compensating for the residual phase difference so that two linearly polarized lights inclined by an angle α around the z axis with respect to the orientation of the two linearly polarized lights in the xy plane are incident on the optical rotation unit;
The azimuth of the optical axis on the xy plane of the third birefringent element coincides with the azimuth of the optical axis on the xy plane of the second birefringent element, or is inclined by 90 ° around the z axis,
The azimuth of the optical axis on the xy plane of the optical rotator is the azimuth of the vibration direction on the xy plane of the two linearly polarized light inclined by the angle α incident from the front and the xy plane of the third birefringent element. Is a direction in which the angle of intersection with the azimuth of the optical axis is divided at equal angles,
An optical attenuator characterized by that. 5. The electro-optical material according to claim 3, wherein the electro-optic material has a general formula K _{1- y My} Ta _1-x Nb _x O ₃ (where M is a monovalent metal, 0 <x <1, 0 ≦ y <1). An optical attenuator characterized by being a substance represented by   6. The optical attenuator according to claim 1 as an optical attenuator for one channel, and a configuration corresponding to a configuration in which a plurality of channels are arranged on the left and right for the one channel are integrally provided. Multi-channel optical attenuator. The multi-channel optical attenuator according to claim 6,
The plurality of collimating lenses constituting the first and second optical fiber collimators constituting the optical attenuators for the plurality of channels are integrated as a microlens array arranged in the x-axis direction. Multi-channel optical attenuator.   8. The multi-channel optical attenuator according to claim 6 or 7, wherein at least one of optical elements disposed between the first and second optical fiber collimators is formed by the optical attenuator for the plurality of channels. A multi-channel optical attenuator, wherein the multi-channel optical attenuator is formed to extend to the left and right so as to cross an optical path for a plurality of channels.

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