JP2014197147A - Optical attenuator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To inexpensively provide a small-sized and high-performance optical attenuator.SOLUTION: In an optical attenuator 1, a light deflecting unit is constituted by arranging, along an optical path, one or more sets of light deflectors 10 in each of which two prism elements (11a and 11b) provided with triangular electrodes (13a and 13b) plane-symmetrical to each other on both of right and left surfaces (16bL-16bR and 16aL-16aR) of a solid comprising electrooptic materials (12a and 12b) are arranged in a longitudinal direction with a phase element 20 therebetween, and a first light deflecting unit, an etalon 30, and a second light deflecting unit are interposed along the optical path in this order, and drive circuits (14a, 14b, and 35) are provided to variably control intensities of electric fields among the triangular electrodes and among electrodes (32 to 34) of the etalon. The electrooptic materials have right and left surfaces parallel to each other, and an optical axis of the phase element is inclined at 45 degrees relative to a normal direction of right and left surfaces of the prism element when viewed from the front, and the triangular electrodes of two front and rear prism elements in each light deflector are arranged so as to be point-symmetric in the longitudinal direction.

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 EO substance) such as PLZT (lead lanthanum zirconate titanate). The optical attenuator using this EO material is generally composed of an electro-optic element having electrodes formed on both sides of a flat EO material, a polarizer, an analyzer, and a drive circuit for applying an electric field between the electrodes, It has a structure in which a polarizer, an electro-optic element, and an analyzer are arranged in this order along the optical path. The polarizer and the analyzer are arranged so as to face each other so that their optical axes are orthogonal to each other and to be parallel to each other. The direction of the electric field formed between the two electrodes arranged on both sides of 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 polarization plane, and the light is blocked. On the other hand, when a sufficient electric field is applied between the electrodes by the driving circuit, the plane of polarization rotates in the direction of the electric field, and the incident light passes through the analyzer. In this example, light has two states, transmission (ON) and blocking (OFF), and among the “optical attenuators”, it is classified as “optical shutter”. The optical shutter can also change the intensity attenuation amount of the light transmitted through the analyzer by controlling the inclination of the polarization plane according to the electric field intensity. In addition, the “optical shutter” is defined as an on (or off) state in which the amount of attenuation is equal to or greater than (or less than) a predetermined attenuation amount, and it can be said that the “optical shutter” is an “optical attenuator” in a broad sense. The present invention is thus directed to the “optical attenuator” including the optical shutter.

  In the optical attenuator using the EO material described above, since the electric field between the electrodes needs to be extremely high, the voltage becomes high, and a commercially available optical attenuator has a structure in which the EO material is sandwiched between the electrodes. Some adopt a multi-layered structure and operate effectively even at a voltage of about 70 V (for example, “PLZT high-speed optical shutter” manufactured by Furuuchi Chemical Co., Ltd.).

  In addition, since the above-mentioned commercially available “PLZT high-speed optical shutter” uses an optical path between layers of a multilayer structure, there is a problem that diffraction or light scattering or reflection by electrodes occurs, resulting in a large transmission loss. In Patent Document 1, while the EO material is made thin, the EO material is arranged in the vicinity of the focal point by using a condensing lens, thereby reducing the distance between the electrodes and realizing driving at a low voltage of about 15V. This solves various optical problems derived from the above-mentioned multilayer structure.

WO2005 / 121876

  The optical attenuator is not limited to the form using the electro-optic effect of the EO substance, and there are forms using various principles and substances. For example, there are MEMS (Micro Electro Mechanical Systems), Faraday elements, liquid crystals, and the like. Here, considering the use in optical communication, which is the main field of application of optical attenuators, the response speed is slow in a method using MEMS or liquid crystal, and it is difficult to adopt. 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 that uses the electro-optic effect in an EO material has excellent high-speed response and can be driven simply by applying an electric field. Therefore, the optical attenuator is easy to miniaturize. It is also power. Therefore, it can be said that an optical attenuator using an EO element is most promising.

  In addition, when the technique described in Patent Document 1 is applied to an optical attenuator, the driving voltage can be lowered, and the entire configuration of the optical attenuator including the driving circuit can be reduced in size. However, in the technique described in the document, the distance between the electrodes sandwiching the EO substance is narrowed to about several tens of μm. That is, the EO material is processed into a thin film shape, or an electrode is formed on the thin EO material. For this reason, the manufacturing process becomes complicated and extremely high processing accuracy is required. Therefore, the optical attenuator becomes expensive.

  Furthermore, the optical attenuator needs to have a wider adjustment range of attenuation and a performance capable of adjusting to a desired attenuation with extremely high accuracy. Of course, even if it is high performance, there is a problem in practical use if it is large or expensive. And the technique of patent document 1 assumes the application to the optical shutter which switches two states of ON and OFF, and a diffraction, scattering, etc. in the commercially available optical switch which employ | adopted the low voltage drive and the multilayer electrode structure Therefore, it can be applied to optical attenuators where performance of variably controlling attenuation and adjusting attenuation with high accuracy is important. difficult. That is, the technique described in Patent Document 1 cannot be directly used for an optical attenuator.

  Accordingly, an object of the present invention is to provide a high-performance optical attenuator that is easy to manufacture and can be downsized while utilizing the electro-optic effect of the EO substance.

The present invention for achieving the above object is an optical attenuator that attenuates and outputs the intensity of light of a predetermined wavelength component incident along the optical axis from the front with the front-rear direction as the optical axis,
Two directions perpendicular to the optical axis are defined as a vertical direction and a horizontal direction, respectively.
Two prism elements formed so that triangular electrodes are symmetrical on both the left and right sides of a three-dimensional body made of an electro-optic material having an electro-optic effect are arranged back and forth via a phaser made of a half-wave plate. An optical deflector formed by arranging one or more sets of optical deflectors arranged along the optical path;
An etalon in which electrodes and mirrors are arranged on both front and rear surfaces of a flat electro-optic material;
A drive circuit for variably controlling the electric field strength between the electrodes formed on the left and right surfaces of each of the two prism elements;
A drive circuit for variably controlling the electric field strength between the electrodes formed on the front and rear surfaces of the etalon,
On the optical path until the incident light is output, the first light deflection unit, the etalon, and the second light deflection unit are interposed in this order,
The three-dimensional shape of the electro-optic material in the prism element is such that the left and right surfaces are parallel to each other,
The optical axis of the phaser is inclined by 45 ° with respect to the normal direction of the left and right surfaces of the prism element when viewed from the front along the optical axis.
The optical deflector is disposed so that the triangular electrodes are symmetric in the front-rear direction.
The optical attenuator is characterized by this.

  The etalon has a transparent electrode and a dichroic mirror laminated in this order from the front to the rear of the flat electro-optic material, and a metal mirror that also serves as an electrode is laminated on the rear surface. The etalon is arranged to output the light incident from the front forward, and the etalon is disposed behind one of the light deflection parts serving as the first light deflection part and the second light deflection part. The incident light may be an optical attenuator that passes through the light deflecting unit twice before and after reflection by the metal mirror and then exits forward.

  The prism element may be a rectangular parallelepiped having front and rear surfaces orthogonal to the optical axis, and the triangular electrode may be an optical attenuator having an isosceles triangle having a base parallel to the optical axis. In the attenuator, it is more preferable that the vertex angle of the isosceles triangle is an obtuse angle. The light incident surface of the etalon may be an optical attenuator that is inclined with respect to the optical axis.

  According to the optical attenuator of the present invention, it is possible to adjust a large optical attenuation amount with extremely high accuracy. In addition, miniaturization is possible. Manufacturing is also easy and can be provided at low cost.

It is a figure which shows the structure and operation | movement of an etalon. It is a figure which shows the operation | movement principle of the Fabry-Perot resonator using an etalon. It is a figure which shows the usage example of the optical attenuator which concerns on the Example of this invention. It is a block diagram of the optical attenuator based on the Example of this invention. It is a figure which shows operation | movement of the optical attenuator based on the said Example. It is a figure which shows the refractive state of the light within the prism element which comprises the optical attenuator which concerns on the said Example. It is a figure which shows the characteristic of the optical attenuator based on the said Example.

=== Process up to the present invention ===
As described above, the optical attenuator has high-speed response, power saving, low-voltage drive, downsizing, performance that can be variably controlled over a wide range of attenuation, and performance that can be precisely adjusted to the desired attenuation. And asked. Therefore, the present inventor has given up pursuing the technical idea of the conventional optical attenuator such as the above-mentioned commercial product “PLZT high-speed optical shutter” and the optical attenuator using the technology described in Patent Document 1 above, and the EO substance. Reconsidering the electro-optic effect of the optical attenuator, the feasibility of a completely new optical attenuator was investigated. First, attention was paid to a Fabry-Perot resonator using an etalon that is often used as a spatial modulator (SLM).

  FIG. 1 is a diagram for explaining the configuration and characteristics of the etalon E. 1A shows the configuration and operation of a general etalon E, and FIG. 1B shows the characteristics of the etalon E. As shown in (A), in the etalon E shown here, a material having a thickness t and a predetermined refractive index is a solid S, and the solid S is between a pair of mirrors (MM) facing each other. It is a sandwiched configuration. This is a so-called “solid etalon”.

  The etalon E operates as a Fabry-Perot resonator, resonates the light Lin incident on itself while being reflected by the mirror M, and emits the transmitted light Lout. At this time, the mirror M selectively transmits when the wavelength of light is λ and the thickness t of the solid is an integral multiple of λ / 4. Therefore, as shown in (B), the transmitted light intensity of the light Lout emitted from the etalon E has a maximum value at a wavelength that is an integral multiple of λ / 4. Also. The sharpness of the wavelength-dependent characteristic curve of the transmitted light intensity changes according to the reflectance of the mirror M.

  Here, when an EO substance is used for the solid of the etalon and a transparent electrode is formed on the inner surface side of the mirror M, or the mirror M itself is formed of a metal and the mirror M is also used as an electrode, The refractive index of the substance changes according to the electric field applied between the electrodes, and the same effect as that of the optical change of the thickness of the solid S of the etalon E is obtained. That is, when an electric field is applied between the electrodes, the refractive index of the portion sandwiched between these electrodes in the EO material layer (hereinafter referred to as the EO layer) changes due to the electro-optic effect. This is the same action as the thickness t of the solid S between the mirrors (MM) facing each other in the etalon E in FIG. Then, as shown in FIG. 2, when light having a wavelength component of peak wavelength λ1 is incident on the etalon, if the refractive index of the EO layer is changed, the wavelength component of the initial peak wavelength λ1 (indicated by the solid line in the figure). The curve shifts from the curve) to the wavelength component of the peak wavelength λ2 (dotted curve in the figure). That is, the transmitted light intensity at the wavelength λ1 changes from the initial I1 to I2. When the wavelength λ1 is incident and the electric field applied between the electrodes is controlled, the intensity of the light having the wavelength λ1 can be variably controlled. Of course, a single etalon alone cannot provide a sufficient amount of attenuation, and a practical optical attenuator cannot be obtained. However, the Fabry-Perot resonator using an etalon has an advantage that it can output light with high sharpness by adjusting the waveform (fringe) of incident light.

  Next, if the present inventor can amplify the light attenuating action in the etalon, that is, the action of changing the refractive index of the solid in the etalon and changing the thickness of the solid in a pseudo manner, the light attenuation can be greatly varied. I thought it was possible to However, if the etalon is driven at a low voltage and the thickness of the solid is set to millimeters in consideration of manufacturability, the variable range of the refractive index becomes extremely small, and light attenuation over a wide range It becomes difficult to adjust the amount. Therefore, if the etalon alone is not used to construct an optical attenuator, the idea can be changed and the solid thickness can be variably controlled over a wider range by principles other than the electro-optic effect. I thought that would be obtained.

  An EO substance is sandwiched between electrodes having a triangular planar shape, and an electric field is applied between the electrodes so that the EO substance between the electrodes substantially operates as a prism. Focused on the element. A configuration similar to this optical element (hereinafter referred to as “prism element” for convenience) is described in, for example, Japanese Patent Application Laid-Open No. 2006-154145 (Patent Document 2). An optical switch that switches incident light from an incident port to an arbitrary output port by arbitrarily deflecting light using a prism element is described. Since the optical switch described in Patent Document 2 cannot obtain a large refraction angle with a single prism element, a plurality of prism elements are arranged along the optical path, and finally the ports are switched by integrating the refraction angles. The deflection angle is obtained. Therefore, even if a prism element is used alone, it is difficult to reduce the size of the optical component even if an optical device such as an optical switch or an optical attenuator is to be constructed.

  Thus, etalon and prism elements alone cannot function as a practical optical attenuator, and their main applications are waveform shaping, peak wavelength shift, or light deflection. However, paying attention to the principle of artificially increasing or decreasing the solid thickness of the etalon, even if optical elements that do not function alone and have different main uses are combined skillfully, practical light I thought that an attenuator could be constructed. In other words, the present inventor has tried to make a leap in the idea that an optical attenuator is configured by using optical elements having completely different operations and uses, such as an etalon and a prism element. Schematically, if an etalon is arranged after the prism element, the incident angle of light on the etalon changes, and the optical path length in the solid of the etalon increases or decreases depending on the angle. That is, the thickness of the solid varies in a pseudo manner. And, by the synergistic effect of the change in the deflection angle of light by this prism element and the change in the refractive index of the etalon solid, the shift amount of the wavelength of the incident light is increased, and as a result, a large attenuation is obtained. .

  Based on the above idea, an optical attenuator using an etalon and a prism element was examined, and it was confirmed that even if the etalon and the prism element are simply arranged on the optical path, they do not operate as an effective optical attenuator. For example, in the electro-optic effect in which the refractive index of a substance changes depending on an applied electric field, the refractive index is changed for incident mode of TM mode light (hereinafter referred to as TM polarized light) having a component parallel to the electric field direction. The refractive index for TE mode light (hereinafter, TE polarized light) having a component perpendicular to the electric field direction cannot be changed greatly. That is, in the EO layer, the light incident on the region functioning as a pseudo triangular prism due to the electro-optic effect is effectively refracted with respect to the TM polarized light but hardly refracted with respect to the TE polarized light. Further, the optical attenuator is required to have a contradictory performance of controlling the attenuation amount with high precision as well as a large attenuation amount.

  The inventor has intensively studied to realize a practical and high-performance optical attenuator that includes an etalon and a prism element as main optical components, and has arrived at the inventor.

=== Configuration and operation of optical attenuator ===
<Usage form>
FIG. 3 is a diagram illustrating a usage form of the optical attenuator according to the embodiment of the present invention. As shown in FIG. 3, the light propagating through the transmission line L of the optical communication network is branched by the optical circulator C, and the optical attenuator 1 is connected via the collimator 2 connected to the tip of the optical fiber FB at the branch destination. Is incident on. The optical attenuator 1 that is the subject of the present invention attenuates the intensity of the incident light to an appropriate level and outputs the attenuated light. The optical attenuator 1 shown here is of a reflective type, and light whose intensity is attenuated by the optical attenuator 1 is emitted in the incident direction. The collimator 2 emits light from the optical fiber FB, converts the light emitted from the port 3 for entering the optical fiber FB, and the light emitted from the port 3 into parallel light, and causes the light to enter the optical attenuator 1. Or a collimating lens 4 for coupling the light emitted from the optical attenuator 1 to the port 3.

  Therefore, in the usage mode shown in FIG. 3, the light emitted from the optical attenuator 1 follows the optical path opposite to the incident light, is coupled to the port 3 by the collimating lens 4, and finally passes through the optical fiber FB. It is returned to the transmission line L via C. Hereinafter, a specific configuration and operation of the optical attenuator 1 will be described.

<Configuration>
FIG. 4 is a schematic structural diagram of the optical attenuator 1 according to the embodiment of the present invention. In FIG. 4, the light 50 traveling from the front to the rear is incident on the optical attenuator 1, and the direction in which the path of the incident light 50 is extended is defined as the optical axis 80. In this drawing, the light 50 traveling from the left side to the right side of the paper is incident on the optical attenuator 1 with the left side of the paper as the front. In addition, two predetermined directions perpendicular to each other when viewed from the front are defined as the vertical and horizontal directions. FIG. 4A is a side view when the optical attenuator 1 is viewed from the right side, and FIG. 4B is a plan view when viewed from above.

  The optical attenuator 1 according to this embodiment shown here has left and right surfaces (16aL-16aR, 16bL) of a rectangular parallelepiped EO material (12a, 12b) whose front and rear surfaces are square with the direction of the optical axis 80 as a normal line. −16bR), two prism elements (11a, 11b) formed by forming triangular electrodes (hereinafter referred to as triangular electrodes: 13a, 13b), a phaser 20 composed of a half-wave plate, and a Fabry-Perot resonator. The etalon 30 that operates and a drive circuit (14a, 14b, 35) for operating the optical attenuator 1 are configured.

  The two prism elements (11a, 11b) are arranged in series from the front to the rear, and the etalon 30 is arranged further rearward of the rear prism element 11b. A phaser 20 is disposed between the two front and rear prism elements (11a, 11b).

  In each prism element (11a, 11b), the triangular electrodes (13a, 13b) are symmetrical with respect to two surfaces (here, the left surface 16aL-16aR and the right surface 16bL-16bR) parallel to the front-rear direction and facing each other. A drive circuit (14a, 14b) for applying an electric field is connected between the triangular electrodes (13a-13a, 13b-13b). The triangular electrodes (13a, 13b) in the front and rear prism elements (11a, 11b) are congruent, and are the same surface (left surface 16aL and 16bL or right surface 16aR and 16bR) of the rectangular parallelepiped EO material (12a, 12b). The triangular electrodes (13a, 13b) formed in the above are inverted in the front and rear prism elements (11a, 11b). That is, they are in a relation of being rotated by 180 ° with respect to the normal direction of the surfaces (16aL-16aR, 16bL-16bR) on which the triangular electrodes (13a, 13b) are formed.

  The phaser 20 interposed between the front and rear prism elements (11a, 11b) is a flat plate having front and rear surfaces parallel to each other, and this surface serves as a light incident / exit surface. Here, the normal direction of the front and rear surfaces of the phase shifter 20 coincides with the optical axis 80. The optical axis of the phaser 20 is 45 ° with respect to the normal direction of the surface on which the triangular electrodes are formed in the two prism elements (11a, 11b) when viewed from the front along the optical axis 80. It is set to intersect at an angle of. That is, it is set so as to intersect at 45 ° with the direction of the electric field generated when a voltage is applied between the triangular electrodes. And the structure which consists of this phaser 20, the two prism elements (11a, 11b) arranged in series before and behind that, and each prism element (11a, 11b) is the optical deflector 10, and two When the prism elements (11a, 11b) are driven by the drive circuits (14a, 14b), the optical polarizer 10 emits light incident from the front or rear in a direction bent with respect to the traveling direction.

  Behind the optical deflector 10 is arranged an etalon 30 in which a flat EO material is a solid 31 and mirrors (32, 34) and electrodes (33, 34) are arranged on both sides of the solid 31. The etalon 30 is also connected with a drive circuit 35 for applying an electric field between the electrodes (32-34). The etalon 30 is driven by the drive circuit 35 so as to shape the waveform of the light incident on the etalon 30 and also according to the electric field strength applied between the electrodes (32-34) by the drive circuit 35. It functions as a variable Fabry-Perot resonator that shifts the peak wavelength of the incident light. The above is the schematic structure of the optical attenuator 1. Below, the structure of each optical component (11a, 11b, 20, 30) which comprises the said optical attenuator 1 is demonstrated in more detail.

  First, the prism elements (11a, 11b) use a rectangular parallelepiped PLZT as the EO substance (12a, 12b), and in the rectangular parallelepiped, the two left and right surfaces (16aL-16aR, 16bL-16bR) facing each other have a planar shape. It has a structure in which triangular electrodes (13a, 13b) that are isosceles triangles are arranged to face each other. The bases of the isosceles triangles forming the triangular electrodes (13a, 13b) are parallel to the front-rear direction. Therefore, the triangular electrodes (13a, 13b) of the two front and rear prism elements (11a, 11b) are arranged so that the apex angles of the isosceles triangles are inverted up and down.

On the other hand, the etalon 30 has a configuration in which the solid 31 is made of flat plate-like PLZT, and electrodes (33, 34) and mirrors (32, 34) are arranged on both front and rear surfaces of the solid 31. The front mirror 32 is a dichroic mirror 32 using a multilayer dielectric film. For example, a Bragg mirror formed by laminating SiO ₂ and Ta ₂ O ₅ thin films is used as one dielectric thin film, and the dielectric thin film is used as the dielectric thin film. Furthermore, what laminated | stacked several layers is employable. A transparent electrode 33 made of ITO (Indium Tin Oxide) or the like is formed on the rear surface of the dichroic mirror 32. The rear mirror 34 is made of a metal such as Au or Ag, and is a metal mirror 34 that also serves as an electrode. The etalon 30 is configured such that light incident on itself is multiple-reflected in a solid 31 made of an EO substance to shape the waveform, shift the wavelength, and emit from the front.

<Operation>
Next, the operation of the optical attenuator 1 of this embodiment will be described. 5 and 6 are diagrams illustrating the operation of the optical attenuator 1 of the present embodiment. FIG. 5 is an optical path diagram when the optical attenuator 1 is viewed from the left, and FIG. 6 is a diagram for explaining the light deflection operation by the prism elements (11a, 11b). 6A shows a light refraction state in the front prism element 11a, and FIG. 6B shows a light refraction state in the rear prism element 11b.

  First, it is assumed that light 50 having a peak at a predetermined wavelength (for example, 1550 nm) is incident on the front prism element 11a from the front. At this time, if an electric field is applied between the triangular electrodes (13a-13a), the refractive index of the region between the triangular electrodes (13a-13a) (hereinafter referred to as a prism region) changes due to the electro-optic effect. That is, a pseudo triangular prism is formed inside the EO material 12a. The EO substance (1) which is the main body of the prism elements (11a, 11b) is a rectangular parallelepiped, and its front and rear surfaces are orthogonal to the optical axis 80. Therefore, the light 50 incident on the front prism element 11a travels straight, The light 51 traveling straight enters the prism area. In the prism element 11a made of PLZT, when an electric field is applied between the triangular electrodes (13a-13a), the refractive index in the prism area decreases. Therefore, the light 51 incident on the prism area is made of a material having a high refractive index. It will be incident on a low material. Since the triangular electrode 13a of the front prism element 11a is an isosceles triangle with the apex angle at the top, the incident light 51 is refracted upward in the prism region, and obliquely upward from the front prism element 11a. It is emitted as the light 60 directed to. However, the TE polarized light of the incident light 51 is hardly refracted by the electro-optic effect and is emitted as a minute amount of refracted light 70.

More specifically, as shown in FIG. 6A, assuming that the apex angle of the isosceles triangle forming the triangular electrode 13a is 2a, the light 51 incident on the prism region from the front is the triangular electrode 13a. Is incident on the prism region at an angle a with respect to the hypotenuse 82 of the triangle forming the triangle, that is, the perpendicular 81 from the interface 82 between the outside and inside of the prism region, and the interface between the outside and inside of the prism region in this prism element 11a 82, the light is refracted sequentially at angles b, c, and d at the interface 83 between the inside and outside of the prism region and the interface 84 between the prism element and the outside, respectively. These angles b to d can be expressed by a well-known Fresnel equation if the incident angle a to the prism region and the refractive indices n _{1 to} n _{3 of} the substances facing each other across the interfaces (82 to 84) are known. Can be based on.

  However, in the prism element 11a using the EO substance 12a, only the TM polarization 52 of light is refracted at the inner and outer interfaces 82 of the prism region. Therefore, the TE polarization 53 is not refracted even when entering the prism region. Will go straight along the optical axis 80. In other words, in the front prism element 11a, TM polarized light (52, 54) that proceeds obliquely upward with respect to the optical axis 80 while being refracted backward, and TE polarized light (53, 54) that travels straight along the optical axis 80. 55) and two modes of light (60, 70) having different traveling directions are emitted from the front prism element 11a.

  TM polarized light 60 and TE polarized light 70 emitted from the front prism element 11a and having different path directions are incident on the phase shifter 20, respectively. As is well known, the phase shifter 20 rotates the plane of polarization of light by 90 °, so that the polarization planes of TE polarized light and TM polarized light (60, 70) whose vibration directions are orthogonal to each other rotate 90 °. 20 exits backward. That is, TM polarized light 60 traveling obliquely upward toward the rear is converted to TE polarized light 61, and TE polarized light 70 traveling straight along the optical axis 80 is converted to TM polarized light 71. The light (61, 71) from the phase shifter 20 enters the rear prism element 11b.

  As shown in detail in FIG. 6B, the triangular electrode 13b of the rear prism element 11b has the same shape as the front prism element 11a and has an inverted relationship rotated by 180 ° about the horizontal direction. Therefore, the TM polarized light 71 incident along the optical axis 80 is vertically symmetrical with respect to the optical path (57 to 59) of the front prism element 11a. That is, the TM polarized light 71 incident along the optical axis 80 from the front surface 85 travels straight and enters the prism region at an angle a. Then, the light is refracted at angles b, c, and d at the interface 82 with the prism region, the interface between the inside and outside of the prism region, and the interface 84 between the inside and outside of the prism element 11b, respectively.

  On the other hand, the TE polarized light 61 incident obliquely upward from the front is not refracted in the prism region, and is refracted only at the inner and outer interfaces (85, 84) of the prism element 11b. That is, the TE polarized light 61 incident on the rear prism element 11b is refracted at the interface 85 separating the outer side and the inner side of the prism element 11b, and then the refracted light 56 travels straight in the prism region as it is, inside and outside the prism element 11b. The light is refracted again at the interface 84 and emitted backward. Thereby, the TE polarized light 62 emitted from the rear prism element 11 b enters the etalon 30 at the same angle d as the incident light 61.

  As shown in FIG. 5, the mirrors (32, 34) of the etalon 30 have normals in the direction of the optical axis 80, and the TE polarized light 62 and the TM polarized light 72 from the rear prism element 11b are respectively The light enters the etalon 80 obliquely at the same angle above and below the optical axis 80. The oblique incidence of light into the etalon 30 means that the optical path length passing through the solid 31 of the etalon 30 becomes long, and has the same effect as the solid 31 becomes substantially thick. Therefore, the peak wavelength of the light (62, 72) incident on the etalon 30 is greatly shifted.

  The light (62, 72) incident on the etalon 30 is multiple-reflected in the etalon 30, the waveform is shaped, and the peak wavelength is shifted, and then emitted as TE polarized light 63 and TM polarized light 73 heading forward. To do. And these outgoing lights (63, 73) reverse the optical path of each incident light (62, 72) (in order of 62 → 61 → 60 → 51 → 50, 72 → 71 → 70 → 51 → 50). Then, it is coupled to the port 3 of the optical fiber FB via the collimating lens 4 shown in FIG. At this time, light having a larger wavelength shift is incident on the port 3 of the optical fiber FB with respect to the incident light 50 directed toward the optical attenuator 1. Based on the principle shown in FIG. The intensity at the peak wavelength of the initial light 50 incident on the optical attenuator 1 is attenuated.

<Characteristic>
Here, based on FIG. 6, conditions for improving the characteristics of the optical attenuator 1 of the present embodiment and various conditions in the actually produced optical attenuator 1 will be described. First, in the optical attenuator 1 of this embodiment, a larger attenuation can be obtained by increasing the angle of the TE polarized light (60, 72) emitted from each of the front and rear prism elements (11a, 11b). Become. That is, the angle d in FIG. 6 is increased. For this purpose, it is necessary to make the emission angle Δc from the prism region smaller. That is, it is necessary to make the apex angle 2a of the isosceles triangle forming the triangular electrodes (13a, 13b) larger. However, if the angle 2a is too large, the light (51, 57) incident on the prism region is totally reflected, and the incident angle a reaches a critical angle. Therefore, in order to obtain a larger attenuation, the apex angle 2a is set so that the incident angle a is less than the critical angle and closer to the critical angle.

Next, in producing the actual optical attenuator 1, the manufacturing and driving conditions for the prism elements (11 a, 11 b) are considered in consideration of manufacturability of the prism elements (11 a, 11 b) and low voltage driving. Were determined. First, in view of ease of processing of the EO substance (12a, 12b) made of PLZT which is the main body of the prism element (11a, 11b), between the triangular electrodes (13a-13a, 13b-13b) shown in FIG. The distance d was set to d = 250 μm. The drive voltage V was set to 0 ≦ V ≦ 15V. As the conditions related to the electro-optic effect, the refractive index n ₁ = 1 of the space where the optical component is installed, the refractive index n ₂ = 2.2 of PLZT, and the Bockels coefficient r = 41.5 pm / V of PLZT are adopted. .

Then, the refractive index n ₃ in the prism region can be obtained by the above conditions relating to the electro-optic effect and the following formulas 1 and 2.
n ₃ = n ₂ + Δn Equation 1
Δn = −n ₂ ³ rE / 2 Formula 2

From the above formulas 1 and 2, the refractive index n ₃ in the prism region is minimum at V = 15 V when the driving voltage is maximized, and at that time, the refractive index n ₃ is n ₃ ≈2.999987. Therefore, the refractive index n ₃ of the prism region is 2.2 ≧ n ₃ ≧ 2.199987 in the driving voltage range 0 ≦ V ≦ 15 V of the prism elements (11a, 11b). Further, the critical angle θ _c when n ₃ ≈2.199987 was obtained from the well-known relationship of θ _c = arcsin (n ₃ / n ₂ ), and θ _c = 89.80303 ° was obtained. Therefore, the apex angle 2a of the triangular electrodes (13a, 13b) may be 2a <2θ _c ≈179.6061 °. However, when the apex angle 2a is close to a straight line (2a = 180 °), the base 15 of the triangular electrodes (13a, 13b) becomes very long, and the longitudinal length w1 of the prism elements (11a, 11b) also becomes long. This makes it difficult to reduce the size of the optical attenuator. Therefore, 2a = 175 ° was set, and the height h2 from the base 15 of the isosceles triangle was made the same as the height h1 of the prism elements (11a, 11b) (h1 = h2). As a result, the length w2 of the base 15 was sufficiently short w2≈11.45 mm. The longitudinal length w1 of the prism elements (11a, 11b) is also the base length w2 (= w1).

Next, as manufacturing conditions of the etalon 30, the thickness before and after the solid 31 made of PLZT was set to 10 mm, and a metal mirror 34 having a reflectance of 97% was formed behind the solid 31 by sputtering or the like. In addition, a transparent electrode 33 made of ITO having a transmittance of 92.3% is similarly formed on the front surface of the solid 31 by sputtering or the like, and SiO ₂ and Ta ₂ O ₅ are successively deposited in that order in front of the transparent electrode 33. A dichroic mirror 32 made of a multilayer dielectric film having a reflectivity of 97% was formed by further stacking one layer of a Bragg mirror formed by, for example. The film thicknesses of SiO ₂ and Ta ₂ O ₅ constituting the Bragg mirror are set so that the peak wavelength λ is λ / 4 with respect to light having a wavelength of 1550 nm. Accordingly, the etalon 30 is a reflection type that outputs light (62, 72) incident from the front to the front. Then, the two prism elements (11a, 11b), the phaser 20, and the etalon 30 are arranged in a space so that the normal direction of each light incident / exit surface coincides with the optical axis 80. An example optical attenuator 1 was constructed.

Next, in order to evaluate the performance of the optical attenuator 1, light having a peak wavelength λ = 1550 nm and a finesse of 103 is guided to a collimator by a single mode optical fiber, and has a focal length of 1.3 mm and an effective diameter of 256 μm. It was made into parallel light through. The parallel light is incident on the front prism element 11a along the optical axis 80 as incident light 50, between the triangular electrodes (13a-13a, 13b-13b) of the two prism elements (11a, 11b), and A voltage of 0V or 15V was selectively applied between the electrodes of the etalon 30 (33-34) by the respective drive circuits (14a, 14b, 35), and the optical deflector 10 and the etalon 30 were individually driven. In various driving states, the spectral intensity of the incident light 50 and the output light was measured, and the ratio of the spectral intensity of the incident and outgoing light was used as the optical loss to determine the wavelength dependence characteristics of the optical loss. When the optical deflector 10 is driven, the same voltage is applied between the triangular electrodes (13a-13a, 13b-13b) of the two prism elements (11a, 11b), and the two prism elements (11a, 11b) are driven. refractive index n ₃ of the prism region of 11b) is set to be the same.

  FIG. 7 shows the wavelength dependence characteristics of the optical loss according to the driving state of the optical attenuator 1. A characteristic curve 101 in the figure is an optical loss characteristic in an off state in which neither the optical deflector nor the etalon 30 is driven. In the off state, a peak value of loss of about −40 db was shown at the wavelength λ1. A curve 102 is a characteristic when only the optical deflector 10 is driven without driving the etalon 30, and a curve 103 is a characteristic when both the optical deflector 10 and the etalon 30 are driven.

  Next, in these curves (101 to 103), from the curves 101 and 102, when only the optical deflector 10 is driven, the peak wavelength λ1 of the loss is large and the short wavelength while maintaining the waveform of the curve 101 in the off state. Shifted to the side. When both the optical deflector 10 and the etalon 30 are driven (full driving state), the characteristic of the curve 103 is obtained, and the peak wavelength is finally shifted from λ1 to λ2. However, looking at the curves 102 and 103, it can be seen that the peak wavelength of the loss is not shifted. That is, it has been found that when only the etalon 30 is driven, a large attenuation cannot be obtained. In any case, in this optical attenuator 1, the peak wavelength λ1 in the off state can be shifted by the wavelength difference Δλ = 0.00031 nm up to the peak wavelength λ2 in the entire driving state. . The loss at the wavelength λ1 in the off state is about −40 db, whereas it is −2.3 db in the entire drive state. In the optical attenuator 1 of this embodiment, this loss difference becomes the variable range of attenuation. It was found that a large amount of attenuation was obtained.

  By the way, from the curves 102 and 103, the etalon 30 seems to hardly contribute to the light attenuation operation. However, the optical attenuator 1 is required to obtain a large attenuation amount and to precisely control the attenuation amount. Here, when the curves 101 and 102 are seen again, if the drive voltage of the optical deflector 10 is variably controlled from 0 V to 15 V, the loss peak wavelength is changed from the peak wavelength λ1 of the curve 101 to the peak wavelength of the curve 102. Can be set within a wide wavelength range. However, this means that even if the voltage change is extremely small, the peak wavelength of the loss is greatly shifted and the attenuation amount is greatly changed. Therefore, when only the optical deflector 10 is driven, it is difficult to set the attenuation amount accurately.

  On the other hand, as can be seen from the curves 102 and 103, the etalon 30 does not greatly shift the peak wavelength of loss even when the drive voltage is changed greatly. In other words, the loss peak wavelength can be shifted by an extremely short wavelength with a large voltage difference, and the attenuation adjustment amount can be set very finely. Therefore, if the driving voltages of the optical deflector 10 and the etalon 30 are individually changed, the peak wavelength of loss is precisely determined in the wavelength region from the peak wavelength λ1 in the curve 101 to the peak wavelength λ2 in the curve 103 shown in the figure. Variable control is possible. That is, according to the optical attenuator 1 of the present embodiment, it is possible to achieve both acquisition of a large attenuation amount and precise attenuation amount control.

  Further, the optical attenuator 1 of this embodiment also has an advantage that various optical components constituting the optical attenuator 1 are arranged in the space. For example, if the optical attenuator is configured by a PLC (Planar Lightwave Circuit), a structure corresponding to various optical components is formed on the substrate, so that the arrangement of the optical components cannot be changed. That is, it is impossible to adjust the optical arrangement relationship between the optical components constituting the optical attenuator. On the other hand, in the optical attenuator 1 of the present embodiment, the degree of freedom of installation of optical components is high, and the optical characteristics can be adjusted by adjusting the arrangement of each optical component.

  Furthermore, in the optical attenuator 1 of the present embodiment, the prism elements (11a, 11b) constituting the optical deflector 10 have a rectangular parallelepiped shape, and triangular electrodes (13a, 13b) between the front and rear prism elements (11a, 11b). ) Is flipped up and down. Therefore, TM polarized light 72 and TE polarized light 62 are emitted from the optical deflector 10 symmetrically with respect to the optical axis 80, and the optical deflector 10 and the etalon 30 can be arranged linearly. That is, it has a structure and configuration suitable for downsizing, particularly for suppressing the height in the vertical direction.

=== Other Embodiments ===
In the optical attenuator 1 of the above embodiment, the etalon 30 is a reflection type, and the longitudinal length of the optical attenuator 1 itself can be almost halved with respect to the optical path length from the incident light to the emitted light, so that it is extremely compact. Has been successful. Of course, the etalon may be transmissive. In this case, the rear mirror and electrode in the etalon may be formed of the same dichroic mirror and transparent electrode as in the front, and the optical deflector may be arranged in front and rear with respect to the etalon. That is, one set of optical deflectors may be arranged before and after the etalon so that the optical path is the target before and after the etalon.

  In the above embodiment, the apex angle 2a of the triangular electrodes (13a, 13b) is set to an angle close to a straight line, and a large refraction angle is obtained with one prism element. On the other hand, the front-to-back length L of the EO material is increased to about 40 times the left-right and top-bottom width. Of course, even if the apex angle 2a of the triangular electrodes (13a, 13b) is not close to a straight line, if a plurality of optical polarizers are continuously arranged on the optical path, finally a large refraction angle can be obtained. Therefore, the apex angle of the triangular electrode of the prism element is sharpened to shorten the longitudinal length, and a pair of optical deflectors comprising the two prism elements (11a, 11b) and the phaser 20 are continuously arranged in the front-rear direction. May be. In this case, although the number of parts is increased, it is not necessary to form a triangular electrode having a very vertical apex that is almost flat, and thus the manufacturing cost can be reduced with a single prism element. In any case, an optical attenuator may be configured by using a configuration including one or more sets of optical polarizers (hereinafter referred to as an optical deflection unit) and arranging the optical deflection unit, the etalon, and the optical deflection unit in this order on the optical path.

  In the above embodiment, the prism elements (11a, 11b) have a rectangular parallelepiped shape, and the front and rear surfaces are orthogonal to the optical axis 80. Of course, the orientation of the front and rear surfaces of the prism element can be set arbitrarily. However, if the traveling direction of the light incident on the prism element and the traveling direction of the light emitted from the prism element are twisted, the arrangement relationship between the optical components becomes complicated. It suffices if one set of both the left and right sides is a plane object. For example, the projected shape from the vertical direction or the horizontal direction is a triangle, a trapezoid, a parallelogram, or the like.

  In the above embodiment, the triangular electrodes (11a, 11b) are isosceles triangles, and the base 15 thereof is parallel to the optical axis 80. However, the triangular electrodes are not limited to isosceles triangles, The side may be inclined with respect to the optical axis 80. In the above embodiment, the optical axis 80 and the light incident surface of the etalon 30 are orthogonal to each other. However, if the optical axis 80 is disposed obliquely with respect to the optical axis 80, the light incident angle is further increased, resulting in a substantial solid. The thickness of the film becomes thicker, and a larger attenuation can be obtained. However, if the etalon is tilted too much, the path of the incident light to the etalon and the path of the emitted light from the etalon are greatly different, and there is a possibility that it will not be coupled to the port of the optical fiber via the collimating lens. Further, the beam shape of the light emitted from the etalon becomes an ellipse. Furthermore, in optical communication applications, the influence of a known group delay in which the delay of a pulsed optical signal differs depending on the wavelength component cannot be ignored. Therefore, when tilting the etalon, it is necessary to tilt it at least to the extent that the influence of group delay can be ignored.

  In any case, the optical attenuator of the present invention is configured based on a technical idea different from the principle of the conventional optical attenuator that increases the angle of light incident on the etalon by the optical deflector. The idea makes it possible to achieve both extremely large attenuation and more precise attenuation control. Furthermore, it is possible to provide a small and high-performance optical attenuator at a lower cost because the degree of freedom of arrangement of each optical component constituting the optical attenuator is high. Of course, not only the relative positional relationship among the optical components in the optical attenuator but also the relative positional relationship with other optical components such as collimating lenses disposed in the front and rear stages of the optical attenuator can be easily performed. Can be adjusted.

  The present invention is suitable for an application for attenuation to an appropriate light intensity while adjusting the waveform of an optical signal through an optical transmission line in optical communication.

1 optical attenuator, 2 collimator, 3 port, 4 collimating lens,
10 optical deflector, 11a, 11b prism element,
12a, 12b, 31 Electro-optical material (EO material), 13a, 13b Triangular electrode,
14a, 14b Prism element drive circuit, 20 phaser, 30, E etalon,
S etalon solid, M mirror, 32 dichroic mirror,
33 Transparent electrode, 34 Metal mirror, 35 Etalon drive circuit 50 Incident light,
80 optical axis, C optical circulator, FB optical fiber, L optical transmission line

Claims (5)

An optical attenuator that attenuates and outputs the intensity of light of a predetermined wavelength component that is incident along the optical axis from the front with respect to the front-rear direction,
Two directions perpendicular to the optical axis are defined as a vertical direction and a horizontal direction, respectively.
Two prism elements formed so that triangular electrodes are symmetrical on both the left and right sides of a three-dimensional body made of an electro-optic material having an electro-optic effect are arranged back and forth via a phaser made of a half-wave plate. An optical deflector formed by arranging one or more sets of optical deflectors arranged along the optical path;
An etalon in which electrodes and mirrors are arranged on both front and rear surfaces of a flat electro-optic material;
A drive circuit for variably controlling the electric field strength between the electrodes formed on the left and right surfaces of each of the two prism elements;
A drive circuit for variably controlling the electric field strength between the electrodes formed on the front and rear surfaces of the etalon,
On the optical path until the incident light is output, the first light deflection unit, the etalon, and the second light deflection unit are interposed in this order,
The three-dimensional shape of the electro-optic material in the prism element is such that the left and right surfaces are parallel to each other,
The optical axis of the phaser is inclined by 45 ° with respect to the normal direction of the left and right surfaces of the prism element when viewed from the front along the optical axis.
The optical deflector is disposed so that the triangular electrodes are symmetric in the front-rear direction.
An optical attenuator characterized by that.   2. The metal mirror according to claim 1, wherein the etalon includes a transparent electrode and a dichroic mirror laminated in this order on the front surface of the flat electro-optic material from the front to the rear, and also serving as an electrode on the rear surface. Are stacked so that light incident from the front is output forward, and the etalon is located behind one of the light deflectors serving as the first, light deflector, and second light deflector. The optical attenuator is characterized in that the incident light is emitted forward after passing through the light deflecting unit twice before and after reflection by the metal mirror.   3. The prism element according to claim 1, wherein the prism element has a rectangular parallelepiped shape having a front and a back surface perpendicular to the optical axis, and the triangular electrode is an isosceles triangle having a base parallel to the optical axis. Optical attenuator.   4. The optical attenuator according to claim 3, wherein an apex angle of the isosceles triangle is an obtuse angle.   5. The optical attenuator according to claim 3, wherein the light incident surface of the etalon is inclined with respect to the optical axis.

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