JP2015011075A - Optical attenuator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical attenuator that can be further downsized without additional significant cost increase.SOLUTION: An optical attenuator in which two directions orthogonal to an optical axis 50 in a back-and-forth direction are respectively set as an up-and-down direction and a right-and-left direction includes: a first collimator 20a for holding a first optical fiber 23a; a second collimator 20b for holding a second optical fiber 23a; and an electro-optic element 10 configured by forming electrodes (11, 13) on the up-and-down or right-and-left faces of an electro-optic material 12. The electro-optic element is incorporated in at least either the first or second collimator, and the electro-optic element is arranged such that optical paths (B1, B11) between collimator lens 22a are put through the electro-optic material from an opening end 26a of the first optical fiber, and when an electrical field is applied between the electrodes by an external drive circuit, refractive index distribution accompanied by a space charge restriction state is generated, and the optical path of light B0 made incident to the electro-optic material is deflected at an angle θ corresponding to electrical field intensity such that the coupling loss of light (B4, B14) made incident to the second optical fiber is changed.

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). This optical attenuator using an EO material generally applies a pair of collimators arranged in opposition to each other, an electro-optic element having electrodes formed on both sides of a flat EO material, a polarizer, an analyzer, and an electric field applied between the electrodes. The polarizer, the electro-optic element, and the analyzer are arranged in this order along the optical path from one collimator on the light input side to the other collimator on the light output side. It has an arranged structure. 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 drive circuit, a phase difference (retardation) occurs in the light transmitted through the EO substance, and the plane of polarization rotates by 90 ° in the direction of the electric field. As a result, incident light passes through the analyzer. Further, the intensity attenuation amount of light transmitted through the analyzer can be changed by controlling the inclination of the polarization plane 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.).

  However, 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. In addition, there exist the following patent documents 2 and nonpatent literature 1 as literature relevant to this invention.

WO2005 / 121876 JP 2011-118438 A

Nippon Telegraph and Telephone Corporation, “NTT Technology Journal”, [online], [Searched on May 10, 2013], Internet <URL: http://www.ntt.co.jp/journal/0712/files/jn200712050 .pdf>

  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. However, in a conventional optical attenuator using an EO material, it is essential to arrange a deflecting element on the optical path in order to use retardation. For this reason, if the rearward extension of the optical path is the front-rear direction, it is difficult to reduce the length in the front-rear direction. In addition, an increase in cost due to an increase in the number of parts becomes a problem. Further, although the above-described 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 extremely narrow in order to reduce the drive voltage to 15V. A condensing lens is also needed. Therefore, it is difficult to process the EO substance, the number of parts is further increased, and the manufacturing cost is increased.

  Incidentally, in an optical attenuator for optical communication using an optical fiber regardless of the operating principle, it is basically difficult to make the size extremely small in the first place. Specifically, in an optical attenuator, since it is necessary to couple light emitted from an input-side optical fiber to an output-side optical fiber, the collimator is an optical fiber collimator (hereinafter, referred to as a ferrule) that holds the optical fiber. The collimator is an essential configuration. The so-called “in-line type” and “pigtail type” are used. In the optical path from the input-side optical fiber to the output-side optical fiber, the optical elements constituting the optical attenuator are arranged between the input-side and output-side collimators. That is, it is theoretically difficult to reduce the length of the optical attenuator in the front-rear direction. Further, it is necessary to make the area of the light incident / exit surface in the optical element larger than the beam diameter of the parallel light between the collimators. For example, when an EO substance is used as an optical element, it is necessary to make the light incident / exit surface of the EO substance larger than the beam diameter. Since the direction of the electric field applied to the EO material is a direction crossing the optical path of the beam, the distance between the electrodes naturally increases unless a complicated configuration such as the optical attenuator described in Patent Document 1 is adopted. In principle, it is necessary to increase the drive voltage. If the diameter of the collimating lens is reduced in order to reduce the driving voltage, or if the shape of the EO substance is reduced after using a condenser lens as in the technique described in Patent Document 1, it is difficult to process the EO substance. In addition, the diameter of the beam passing through the optical element is reduced, and it is difficult to accurately adjust the position of the optical component constituting the optical attenuator. In any case, the cost increase is inevitable.

  Therefore, the main object of the present invention is to provide an optical attenuator that can be further reduced in size while using an EO substance as an optical element without greatly increasing the cost.

In order to achieve the above object, the present invention attenuates the intensity of light emitted rearward along the optical axis from the first optical fiber on the input side disposed in the front direction with the front-rear direction as the optical axis. An optical attenuator that emits from the second optical fiber on the output side,
Two directions perpendicular to the optical axis are defined as a vertical direction and a horizontal direction, respectively.
The first optical fiber is held in an integral sleeve, and a first collimating lens that receives the light emitted from the first optical fiber and emits it as parallel light is housed. 1 optical fiber collimator;
The second optical fiber is held in an integral sleeve, and a second collimating lens that couples the parallel light emitted from the first collimating lens to the second optical fiber is housed. A second optical fiber collimator,
An electro-optic element formed by facing electrodes on both upper and lower surfaces or both right and left surfaces of a three-dimensional body made of an electro-optic material having an electro-optic effect;
With
The electro-optic element is incorporated in at least one of the first and second optical fiber collimators,
The electro-optic element is disposed in an optical fiber collimator in which the electro-optic element is incorporated so that an optical path between the opening end of the optical fiber and the collimating lens passes through the electro-optic material, and is driven by an external drive circuit. When an electric field is applied between the electrodes facing each other, a refractive index distribution associated with a space charge limited state is generated in the electro-optical material, and an optical path of light incident on the electro-optical material is determined according to the intensity of the electric field. Changing the coupling loss of light incident on the second optical fiber by deflecting at an angle;
The optical attenuator is characterized by this.

Further, the optical attenuator is arranged in series in the front-rear direction so that the first optical fiber collimator and the second optical fiber collimator face each other.
The electro-optic element is incorporated only in the first optical fiber collimator,
The first optical fiber is a polarization maintaining optical fiber that maintains the direction of the polarization plane of the propagating light, and the direction of the polarization plane of the light propagating through the first optical fiber is the vertical or horizontal normal direction. It can be an optical attenuator that matches any one.

Alternatively, the first optical fiber collimator and the second optical fiber collimator are arranged in series so that both collimating lenses face each other,
The electro-optic element is built in both the first and second optical fiber collimators, the facing direction of the electrodes of the electro-optic element built in the first optical fiber collimator, and the second optical fiber collimator An optical attenuator in which the facing directions of the electrodes of the electro-optic element built in the optical fiber collimator are orthogonal to each other can also be used.

The first optical fiber collimator and the second optical fiber collimator are arranged in series at the front and back so that both collimating lenses face each other,
The electro-optic element is built in both the first and second optical fiber collimators, the facing direction of the electrodes of the electro-optic element built in the first optical fiber collimator, and the second optical fiber collimator The facing directions of the electrodes of the electro-optic element incorporated in the optical fiber collimator are parallel to each other,
A half-wave plate is inserted between the first optical fiber collimator and the second optical fiber collimator, and the optical axis of the half-wave plate is inclined 45 ° with respect to the vertical and horizontal directions. It is good also as an optical attenuator arrange | positioned so that it may become a direction.

The second optical fiber collimator also serves as the first optical fiber collimator, and the first optical fiber and the second optical fiber are formed on the optical fiber collimator in parallel on the left and right. Fiber optic cable is held,
The electro-optic element has electrodes formed so as to face in the vertical direction,
A quarter-wave plate and a reflector are arranged in this order behind the collimating lens,
The quarter-wave plate is disposed such that the optical axis is inclined by 45 ° with respect to the vertical and horizontal directions,
The reflector has two mirror surfaces with the same inclination angle so as to gradually become thicker in the left-right direction from the intersection with the straight line extending backward from the midpoint between the open ends of the first and second optical fibers. Then, when light traveling backward from the opening end of the first optical fiber is incident on one mirror surface, the light traveling forward toward the opening end of the second optical fiber is reflected forward through one slope. It can also be an optical attenuator.

  An optical attenuator in which the electro-optic element is incorporated in both the first and second optical fiber collimators is a core diffusion fiber in which the mode field diameter on the opening end side of the first and second optical fibers is expanded. Is possible.

In the optical attenuator according to any one of the above, the electro-optical material has the general formula K _{1- y My} Ta _1-x Nb _x O ₃ (where M is a monovalent metal, 0 <x <1, 0 ≦ y An optical attenuator that is a substance represented by <1) is more preferable.

  The optical attenuator of the present invention can be driven at a low voltage and has excellent attenuation characteristics. Further, it is possible to reduce the size while ensuring the ease of manufacturing, and it can be expected to provide it at a low cost.

It is a figure for demonstrating the space charge restriction | limiting state in an electro-optical substance. It is a figure which shows the relationship between the position in an electro-optical substance in a space charge restriction | limiting state, and various physical properties. It is a figure which shows the relationship between the voltage applied to an electro-optical substance, and the deflection angle of light. It is a figure for demonstrating the deflection | deviation effect | action of the light based on a space charge restriction | limiting state. 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 operation | movement of the optical attenuator based on 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 operation | movement of the optical attenuator based on the said 2nd Example. 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 structure of the optical attenuator which concerns on the 4th Example of this invention. It is a figure which shows the optical axis direction of the half-wave plate which comprises the optical attenuator which concerns on the said 4th Example. It is a figure which shows operation | movement of the optical attenuator based on the said 4th Example. It is a figure which shows the structure of the optical attenuator which concerns on the 5th Example of this invention. It is the figure which compared the coupling loss characteristic when the optical attenuator which concerns on the said 2nd Example and the optical attenuator which concerns on the said 5th Example are driven on predetermined conditions. It is a figure which shows the coupling loss characteristic when it drives on the other conditions of the optical attenuator which concerns on the said 5th Example. It is a figure for demonstrating the drive method of the optical attenuator which concerns on the Example of this invention.

=== Principle of operation of optical attenuator ===
As described above, the optical attenuator is required to have performance such as high-speed response, power saving, low voltage driving, and miniaturization. In order to realize further downsizing and low-voltage driving, the present inventor uses electric power of EO materials such as the above-mentioned commercial product “PLZT high-speed optical shutter” and an optical attenuator using the technique described in Patent Document 1 above. Considering to improve the optical attenuator using the retardation based on the optical effect, we have conducted extensive research.

  However, it has been found that it is extremely difficult to realize further downsizing and low power drive on the extension line of existing optical attenuators, including optical attenuators based on the conventional retardation as an operating principle. Therefore, the electro-optic effect of the EO material was reconsidered, and the feasibility of an optical attenuator with a completely new configuration was examined. Then, attention was paid to the light deflection action due to the refractive index distribution in the EO material caused by the gradient of the electric field intensity.

  FIG. 1 shows the principle of generating a refractive index distribution in the EO substance eo. FIG. 1 shows the transition of the distribution of electrons (e, e1 to e4) in the EO substance eo accompanying the application of an electric field in the order of (A) to (C). The electro-optic element (EO element) 10c using the EO substance eo has a configuration in which the EO substance eo is disposed between the opposing electrodes (Ad-Cd). The electrode and the electrodes (Ad, Cd) are made of a metal and a dielectric. When an EO substance eo is in ohmic contact, when a voltage is applied between the opposing electrodes (Ad-Cd), as shown in (A), the charge released from the cathode electrode Cd on the negative electrode side ( E) e is accelerated toward the anode electrode Ad which becomes a positive electrode. Here, when a voltage is continuously applied between the electrodes (Ad-Cd), as shown in (B), electrons e2 are emitted one after another from the cathode electrode Cd, and each electron e1 emitted earlier is The electric field Ee in the radial direction is generated so as to be directed to itself. A repulsive force acts between the electron e2 emitted one after another from the cathode electrode Cd and the electric field Ee generated by the electron e1 emitted earlier, and the electron e2 emitted later decelerates locally. The density of the emitted electrons e1 increases. When the density of the local electrons e1 becomes sufficiently high, as shown in (C), the electric field in the region e4 where the electrons e1 exist at a high density becomes strong, and some of the electrons e3 are moved to the cathode electrode Cd side. Reflected. As a result, electrons e accumulate as space charges in the EO material eo, and a gradient occurs in the density of the electrons e from the cathode electrode Cd to the anode electrode Ad in the EO material eo. That is, the electric field strength is not uniform at the position in the EO substance eo, and a so-called “space charge limited state” is obtained. FIG. 2 shows the relationship between various physical properties in the EO material eo and the distance x, where x is the distance from the boundary between the cathode electrode Cd and the EO material eo to the anode electrode Ad. 2A shows the relationship between the distance x and the electron density distribution N, and FIG. 2B shows the relationship between the distance x and the dielectric constant ε. The electro-optic effect of the EO material eo depends on the electric field strength. For example, the Kerr effect is proportional to the square of the electric field strength. Therefore, the refractive index in the EO material eo changes according to the position of the distance x, and as a result, the light incident on the EO material bends from a high charge density to a low charge in the EO material ( Deflect). Further, as shown in FIG. 3, the deflection angle θ also increases in accordance with the voltage V applied between the electrodes (Cd−Ad). This light deflection state continues until the space charge accumulated in the EO material eo is released as free charge.

FIG. 4 is a diagram showing a light deflection operation by the EO element 10c. In this figure, a locus (optical path) followed by the center of light (beam) passing through the inside and outside of the EO substance eo is shown. The EO material eo has birefringence, and the light B _in incident on the EO material eo has a TE component light (TE light) B _TE whose vibration direction is parallel to the direction of the electric field E, and the direction of the electric field E. It separates into orthogonal component light (TM light) _BTM . Since the EO material eo has a different refractive index with respect to the light (B _TE , B _TM ) of each component in the electric field applied state, when the EO material eo is in the space charge limited state, The light (B _TE , B _TM ) is deflected in different directions (θ _E , θ _M ) with respect to the traveling direction of the incident light Bin. In the example shown in FIG. 4, an EO substance eo that exhibits an electro-optic effect due to the Kerr effect and whose sign and magnitude in the Kerr constant differs depending on the TE light B _TE and the TM light B _TM is used. As an example, TE light B _TE and TM light B _TM are deflected in opposite directions with respect to incident light Bin. Then, the light (B _TE , B _TM ) of each component in the EO material eo is refracted at the inner and outer boundaries of the EO material eo, and finally becomes light (B _oE , B _oM ) with mutually orthogonal polarization planes. The light is emitted in different directions.

  The expression principle of the space charge limited state and the relationship between the space charge limited state and the refractive index distribution in the EO substance eo are described in detail in the above-mentioned Patent Document 2 and Non-Patent Document 1. . In these documents, an embodiment applied to a scanner for forming a latent image on a photoreceptor of a laser printer using a refractive index distribution in an EO substance associated with a space charge limited state is described.

  Here, the present inventor considered using the above-mentioned light deflection action in the EO material for an optical attenuator. However, the techniques described in Patent Document 2 and Non-Patent Document 1 are based on the assumption that linearly polarized light is incident on the EO material. In other words, the structure is the same as that of a conventional optical attenuator only in the operation principle, and it is not possible to sufficiently meet the demands for low power drive and miniaturization. Therefore, the present inventor decided to fundamentally review the structure of the optical attenuator itself. In general, when focusing on the structure of the collimator that constitutes the optical attenuator, the open end of the optical fiber from which light is emitted and combined and the collimating lens are space for securing the focal length of the collimating lens. It has become. Therefore, the present invention has been conceived by considering the effective use of this space.

=== Example according to the present invention ===
An optical attenuator according to an embodiment of the present invention has a basic configuration in which light emitted from an input-side optical fiber is shaped into parallel light by a collimator, and the parallel light is again coupled to the output-side optical fiber through the collimator. In addition, an EO element is built in the space of at least one collimator. By driving the EO element, the EO substance exhibits a light deflection effect based on the space charge limited state, and the intensity of the light beam coupled to the output side optical fiber by controlling the deflection angle of the light. Is variably controlled. In the following, some specific examples will be given as examples of the present invention according to the arrangement of various optical elements constituting the optical attenuator. In addition, in the following description, the same code | symbol may be attached | subjected to the same or similar part, and the overlapping description may be abbreviate | omitted.

=== First Embodiment ===
FIG. 5 shows a schematic structure of the optical attenuator 1 according to the first embodiment of the present invention. FIG. 5A shows the internal structure of the optical attenuator 1, and is a side view when a light path (optical path) passing through the optical attenuator 1 is viewed from the side. (B) shows the structure of the EO element 10 constituting the optical attenuator 1, and is an enlarged view of the EO element 10 in (A). The optical attenuator 1 includes a collimator (hereinafter referred to as an input side collimator) 20a that holds an input side optical fiber (hereinafter referred to as an input side fiber) 23a at both ends of a cylindrical casing 100, and an output side optical fiber (hereinafter referred to as an optical fiber). The collimator (hereinafter referred to as output-side collimator) 20b holding the output-side fiber) 23b is connected so as to face each other.

  As is well known, the collimator (20a, 20b) includes a ferrule (21a, 21b) that holds the end of the optical fiber (23a, 23b) and a collimator lens (22a, 22b) in a cylindrical sleeve (24a, 24b). In the optical attenuator 1 according to the first embodiment, the EO element 10 is disposed between the ferrule 21a and the collimator lens 22a in the input side collimator 20a. Here, the optical axis 50 direction of the collimating lenses (22a, 22b) on the input / output side is defined as the front-rear direction, and the input-side fiber 23a is defined as the front. Then, the vertical direction is defined as shown in the figure. The horizontal direction is defined by the direction when viewed from the front. That is, FIG. 5A is a side view when the optical attenuator 1 is viewed from the right side, and the depth direction in the drawing is the left direction.

As shown in an enlarged view in FIG. 5B, the EO element 10 includes an EO substance (also referred to as an EO layer) 12 between a substrate 11 also serving as one electrode and the other electrode 13 made of a thin metal. In this example, potassium niobate tantalate (KTa _1-x Nb _x O ₃₎ , which is an EO substance, is formed on a conductive substrate 11 mainly composed of strontium titanate (STO). , KTN), and a metal electrode layer 13 formed by laminating a titanium layer 13t and a platinum layer 13p is formed on the EO layer 12 as the other electrode. Yes. The KTN constituting the EO layer 12 is a colorless transparent crystal composed of potassium niobate (KNbO ₃ : KN) and potassium tantalate (KTaO ₃ : KT), and the crystal structure is rhombohedral when the temperature rises. It is known to change into four crystal phases, orthorhombic, tetragonal and cubic. Then, the dielectric constant ε increases remarkably as it approaches the phase transition temperature at which it transitions from a cubic state to a tetragonal state. That is, the Kerr effect is remarkably increased near the phase transition temperature. This induces a large refractive index change. Since the phase transition temperature varies depending on the composition (mixing ratio of KN and KT), a huge Kerr effect can be obtained at room temperature if a composition that undergoes phase transition near room temperature is selected. Further, the formula _{K 1-y M y Ta 1} -x Nb x O 3 (M is a monovalent metal, 0 <x <1,0 ≦ y <1) containing the KTN material represented by the excellent electrical It is also an environmentally friendly EO material that has optical properties and does not contain lead (Pb).

  As a manufacturing procedure of the EO element 10, first, a conductive substrate 11 made of STO to which 1 wt% of niobium is added is prepared. The initial shape of the conductive substrate 11 is a circular flat plate having a diameter of 1 inch, and the thickness ds is 650 μm. Further, the crystal orientation of the circular plane serving as the substrate surface 14 is the (100) plane.

  Then, KTN is crystal-grown on the substrate surface 14 by liquid phase epitaxy until the thickness d becomes 15 μm, thereby forming the EO layer 12. Further, a titanium layer 13t is formed on the EO layer 12 by sputtering, and a platinum layer 13p is formed on the titanium layer 13t to form the metal electrode layer 13. Finally, the laminate formed by laminating the EO layer 12 and the metal electrode layer 13 on the conductive substrate surface 14 having a diameter of 1 inch is cut so as to be large enough to be accommodated in the collimator 20a, and then driven. Electrode terminals (not shown) serving as circuits and connection paths are attached to the conductive substrate 11 and the metal electrode layer 13 to complete the EO element 10.

In the EO element 10 in the first embodiment, the relative dielectric constant of KTN is 20,000 at 25 ° C., and the overall size is 150 μm in the front-rear length Leo and 500 μm in the left-right direction. Further, the overall height h of the vertical direction, the conductive substrate 11 and the EO layer 12 and the thickness of each layer of the thin-film metal electrode layer _{13 (d s, d, d} m) becomes a the sum of the by sputtering the thickness _{d m} of the thin-film metal electrode layer 13 is formed very thin (e.g., 500 nm), substantially between about 665μm which is the sum of thickness of the conductive substrate 11 and the EO layer 12 is EO element 10 high It becomes h. The EO element 10 is built in the input-side collimator 20a so that the layer surfaces of the conductive substrate 11 and the metal electrode layer 13 are parallel to the optical axis 50. Here, the EO element 10 is disposed such that a conductive substrate (hereinafter also referred to as an electrode) 11 and a metal electrode layer (hereinafter also referred to as an electrode) 13 face each other in the vertical direction. That is, when a potential difference is generated between the electrodes (10-13) by an external drive circuit, an electric field is applied in a direction orthogonal to the light transmitted through the EO layer 12 along the optical axis 50 in the front-rear direction. become.

  By the way, as previously shown in FIG. 4, in the EO material, the polarization planes are separated into TE light and TM light which are orthogonal to each other. Therefore, in this first embodiment, one of TE light and TM light is separated. Only polarized light corresponding to light is used. Therefore, a polarization maintaining optical fiber is used for the input side fiber 23a, and the polarization plane of the light propagating through the input side fiber 23a is matched with the direction parallel or orthogonal to the electric field direction applied to the EO layer 12. The arrangement relationship between the input side fiber 23a and the EO element 10 is adjusted. In this example, because of the nature of the EO substance 12, TE light is more largely deflected than TM light. Therefore, the polarization plane of light propagating in the input side fiber 23a and the direction of the electric field applied to the EO element 10 are determined. They are parallel. That is, the light B0 emitted from the input side fiber 23a vibrates in the vertical direction.

<Operation>
Next, the operation of the optical attenuator 1 according to the first embodiment will be described. FIG. 6 shows the operation of the optical attenuator 1 according to the first embodiment. 6A shows an optical path of light (hereinafter also referred to as a beam) passing through the optical attenuator 1 when no electric field is applied to the EO element 10, and FIG. 6B shows a circle 200 in FIG. It is the figure which expanded the inside. (C) shows an optical path when an electric field is applied to the EO element 10. In addition, the optical path shown by the arrow in the figure corresponds to the locus of the center of the beams (B0 to B4). That is, as shown in (B), a beam B0 (also referred to as input light) emitted from the opening end 26a of the input side fiber 23a spreads conically toward the rear at an opening angle α. Further, the end faces (25a, 25b) of the actual ferrules (21a, 21b) are slightly inclined with respect to the optical axis 50. As a result, “return light” in which light traveling in the direction opposite to the intended direction is incident on the optical fiber (23a, 23b) from the outside is not generated. Therefore, strictly speaking, the beam B0 emitted from the input side fiber 23a is refracted at the end face 25a of the ferrule 21a and the inner and outer boundaries of the EO element 10. However, in the drawings showing the operations of the optical attenuators according to the following embodiments including FIGS. 6A and 6C, the beam trajectory (optical path) is simplified so that the operations can be easily understood.

  First, as shown in FIG. 6A, when no electric field is applied to the EO element 10, light that vibrates in the vertical direction from the input side fiber 23a that is a polarization maintaining optical fiber is incident on the EO element 10. To do. Since no electric field is applied to the EO layer 12, the beam B1 is emitted from the EO element 10 without being deflected in the EO layer 12. Then, the beam B2 enters the input side collimating lens 22a.

  As is well known, in the optical fiber collimators (20a, 20b), the opening end faces (26a, 26b) of the optical fibers (23a, 23b) are the focal positions of the collimating lenses (22a, 22b). When the beam B0 traveling backward at the opening angle α is incident on the collimating lens 22a, the beam B2 is emitted as parallel light B3 traveling along the optical axis 50. Then, the parallel light B3 emitted from the input-side collimating lens 22a is coupled to the focal position as a beam B4 collected by the output-side collimating lens 22b. If this coupling position is made to coincide with the position of the opening end 26b of the output side fiber 23b, the coupling loss is minimized, and the optical attenuator 1 is in an “on” state that transmits the incident light B0. In the optical attenuator 1 according to the first example, the diameter of the beam B0 emitted from the input side fiber 23a was 10 μm, and the coupling loss in the on state was 0.6 dB.

  On the other hand, as shown in (C), since the polarization direction of the beam B0 incident on the EO element 10 coincides with the facing direction (vertical direction) of the electrodes (11, 13) in the EO element 10, the distance between the electrodes ( When an electric field is applied to the EO substance 12 by applying a potential difference to 11-13), B0 is deflected in the angle θ direction as a beam B11 corresponding to TE light. Then, the light is refracted at the rear end of the EO element 10 in accordance with the difference in refractive index between the inside and outside of the EO material 12, and is emitted as a beam B 12 that travels in the angle direction of the angle β with respect to the optical axis 50. At this time, even if the beam is deflected by the EO element 10 in the input-side collimator 20a, the focal position of the input-side collimating lens 22a is not changed, so that the beam B12 incident on the input-side collimating lens 22a is parallel light B3 in the ON state. Toward the rear as parallel light B13 shifted in the vertical direction, and the parallel light B13 enters the output-side collimating lens 22b. As a result, the coupling position of the beam B14 emitted from the collimating lens 22b deviates from the position of the opening end 26b of the output side fiber 23b. As a result, the coupling loss increases. That is, the light intensity of the input light B0 is attenuated. The intensity of the light B14 incident on the output-side fiber 23b can be variably controlled by adjusting the voltage applied between the electrodes (11-13) of the EO element 10 to change the deflection angle θ.

  In the optical attenuator 1 according to the first example, when a voltage of 10 V was applied between the electrodes (11-13) of the EO element 10, a coupling loss of 30 dB was obtained, and the input light B0 was sufficiently blocked. “Off” state. From the above, it was confirmed that the optical attenuator 1 according to the first example had a practically sufficient attenuation characteristic. Of course, there is no need to arrange other optical components between the input-side and output-side collimating lenses (22a, 22b), and the focal lengths (f) of both the input-side and output-side collimating lenses (22a, 22b). It is only necessary to secure a distance of 2f that is added), and the size can be further reduced as compared with the conventional optical attenuator. In particular, the longitudinal length can be made extremely short.

=== Second Embodiment ===
In the optical attenuator 1 according to the first embodiment, a polarization maintaining optical fiber is used for the input side fiber 23a. However, a general optical transmission line is composed of a single mode optical fiber (SMF). Therefore, as a second embodiment, there is an optical attenuator that can cope with SMF even if the input side fiber 23a is the same as the output side fiber 23b. FIG. 7 shows the structure of the optical attenuator 2 according to the second embodiment. Here again, as in the first embodiment, the front, rear, up, down, left and right directions are defined. FIG. 7A is a side view of the optical attenuator 2 viewed from the right side, and FIG. It is a top view at the time. As shown in FIG. 7, in the optical attenuator 2 according to the second embodiment, EO elements (10a, 10b) are incorporated in both the input side collimator 20a and the output side collimator 20b. The formation surfaces of the electrodes (11, 13) of the EO element (input-side EO element) 10a in the input-side collimator 20a and the output-side EO element 10b in the output-side collimator 20b are orthogonal to each other. That is, in the EO elements (10a, 10b) on the input side and the output side, the directions of the electric field applied to the EO material 12 are orthogonal to each other.

  Regarding the operation of the optical attenuator 2, in the ON state, the light incident on the input side EO element 10a is not deflected as in the first embodiment, and the beam emitted backward from the input side EO element 10a is not input. It is coupled to the position of the opening end 26b of the output side fiber 23b through the side collimating lens 22a, the output side collimating lens 22b, and the output side EO element 10b. The beam diameter of the input light was 10 μm, and the coupling loss in the on state was 0.6 dB.

  FIG. 8 shows the operation of the optical attenuator 2 of the second embodiment in the off state. FIG. 8A shows an optical path when the optical attenuator 2 is viewed from the right side, and FIG. 8B shows an optical path when viewed from above. (C) shows the position of the beam when each position (P1, P2) on the optical path shown in (A) and (B) is viewed from the front. When a voltage is applied between the electrodes (11-13) of the input-side and output-side EO elements (10a, 10b), the inside of the EO substance 12 of each EO element (10a, 10b) is in a space charge limited state. When the input light emitted from the input-side fiber 23a enters the input-side EO element 10a, the light is separated into TE light B1e and TM light B1m whose vibration directions are orthogonal to each other in the EO material 12, and at different angles. Deflect in the direction of. Then, the TE light B1e and the TM light B1m are refracted at the rear end of the EO element 10a, and two beams (B2e, B2m) traveling in different angular directions from a position shifted from the optical axis 50 are rearward from the EO element 10. Exit toward. That is, two beams (B2e, B2m) having polarization dependency are emitted.

  The two beams (B2e, B2m) are individually incident on the output side collimator lens 22b as parallel light (B3e, B3m) by the input side collimator lens 22a, and refracted toward the respective coupling positions. Light (B4e, B4m) enters the output-side EO element 10b. An electric field having the same intensity is applied to the EO material 12 of the output-side EO element 10b in a direction orthogonal to the input-side EO element 10a. The beams (B4e and B4m) incident on the output-side EO element 10b have a plane of polarization. Proceeding backward while deflecting as the exchanged beams (B5m, B5e). Then, the two beams (B5m, B5e) are emitted from the output-side EO element 10b in a state where the phases are aligned, and the polarization dependency is eliminated. Therefore, the beam is incident on the opening end 26b of the output side fiber 23b with a coupling loss corresponding to the voltage applied between the electrodes (11-13) of the input side and output side EO elements (10a, 10b). . In the optical attenuator 2 according to the second embodiment, a coupling loss of 30 dB was obtained when a voltage of 10 V was applied between the electrodes (11-13) of the EO elements (10a, 10b) on the input side and the output side. . As described above, the optical attenuator 2 according to the second embodiment has a sufficiently practical attenuation characteristic even at a low voltage like the optical attenuator 1 according to the first embodiment while using a general-purpose SMF. A voltage may be applied between the electrodes (11-13) of the output side EO element 10b so as to coincide with the deflection angles of the TE light B1e and the TM light B1m in the input side EO element 10a. Is not the same between the input-side and output-side EO elements (10a, 10b), feedback control or the like is used, either the input-side and output-side EO elements (10a, 10b), or both What is necessary is just to adjust the voltage value applied between electrodes (11-13).

=== Third embodiment ===
In the second embodiment, in order to eliminate the polarization dependence of the beam when an electric field is applied to the input-side EO element 10a, the electric field directions in the input / output-side EO elements (10a, 10b) are orthogonal to each other. . That is, the formation surface of the electrodes (11, 13) was rotated 90 ° with respect to the optical axis 50. Therefore, the terminals attached to the electrodes (11, 13) are arranged in different directions between the input-side EO element 10a and the output-side EO element 10b, and the wiring from the terminals to the drive circuit can be complicated. There is sex. Therefore, as a third embodiment, there is an optical attenuator arranged so that the electrodes (11, 13) of the two EO elements (10a, 10b) are in the same direction.

  FIGS. 9A to 9C show the structure and operation of the optical attenuator 3 according to the third embodiment. FIG. 9A shows the structure when the optical attenuator 3 is viewed from the right side. Instead of arranging the electrodes (11, 13) of the EO elements (10a, 10b) on the input side and the output side in parallel. Further, the half-wave plate 30 is disposed between the collimating lenses (22a-22b). FIG. 9B is a diagram when the half-wave plate 30 is viewed from the front, and shows the direction of the optical axis 31 of the half-wave plate 30. The optical axis 31 of the half-wave plate 30 is normal to the surface on which the electrodes (11, 13) are formed in the EO elements (10a, 10b) when viewed from the front along the optical axis 50. It is set so as to intersect with the direction at an angle of 45 °. 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 electrodes (11-13).

  FIG. 9C shows the operation of the optical attenuator 3 according to the third embodiment in the off state. As shown in this figure, the optical path until the input light exits the input-side collimator 20a is the same as that of the optical attenuator 2 of the second embodiment, and two beams (B11e corresponding to the TE light and the TM light are used. , B11m) travels backward along the routes B11e → B12e → B13e and B11m → B12m → B13m, respectively. When the two beams (B12e, B13m) emitted from the input-side collimating lens 22a are incident on the half-wave plate 30, the beams (B14m, B13m) are obtained by rotating the polarization planes of the beams (B12e, B13m) by 90 °. B14e) is incident on the output-side collimating lens 22b, and the two beams (B15m, B15e) refracted by the output-side collimating lens 22b are incident on the output-side EO element 10b. An electric field having the same intensity as that of the input-side EO element 10a is applied to the output-side EO element 10b in the same direction. In the output-side EO element 10b, the beam (B16e, B16m) has its polarization plane and the direction of application of the electric field. And proceeding backward while being deflected in accordance with the following, and coupled at the rear end of the output-side EO element 10b. In other words, it is coupled at a position shifted in the vertical direction with respect to the optical axis 50. And the light of the intensity | strength according to the coupling position injects into the output side fiber 23b. In the optical attenuator 3 according to the third example, the beam diameter of the input light was 10 μm, and the coupling loss was 0.6 dB in the on state. The coupling loss was 30 dB when a voltage of 10 V was applied between the electrodes (11-13) of the EO elements (10a, 10b) on the input / output side to turn them off.

=== Fourth embodiment ===
The optical attenuators (1-3) according to the first to third embodiments are individually provided with an input-side collimator 20a and an output-side collimator 20b, and the collimators (20a, 20b) are aligned with the optical axis 50. In the face-to-face arrangement. That is, the transmission type optical attenuators (1 to 3) are configured. Therefore, a reflection type optical attenuator is cited as a fourth embodiment.

  FIG. 10 shows the structure of the optical attenuator 4 according to the fourth embodiment. FIG. 10A is a plan view when the optical attenuator 4 is viewed from above, and FIG. 10B is a side view when the same optical attenuator 4 is viewed from the right side. As shown in the figure, the optical attenuator 4 includes a single collimator 20, and a quarter wavelength plate 40 and a reflection plate 60 are arranged behind the collimator 20 in this order. As shown in (A), the ferrule 21 connected to the collimator 20 holds a two-core optical fiber 23, a so-called “panda fiber”, and one optical fiber of the panda fiber 23 is connected to the input side. The fiber 23a is the output side fiber 23b. One collimator 20 serves as both an input side and an output side collimator. Further, the mirror surface on the front side of the reflecting plate 60 is concave, and with respect to a straight line 51 (hereinafter referred to as a central axis) 51 extending rearward from the midpoint of the open ends (26a, 26b) of the input / output side optical fibers (23a, 23b). The two inclined surfaces (61a, 61b) are formed so as to be symmetrical. Further, the inclined surface 61a on the left side and the inclined surface 61b on the right side with respect to the central axis 51 are axes (hereinafter referred to as a first optical axis) 50a directed rearward from the opening end 26a of the optical fiber 23a on the input side. It is formed on each of the upper and lower axes (hereinafter referred to as second optical axis) 50b from the open end 26b of the output side fiber 23b. The two inclined surfaces (61a, 61b) are orthogonal to each other.

  The electrodes (11, 13) of the EO element 10 have an electric field in a direction (vertical direction) perpendicular to the parallel direction (left-right direction) of the two optical fibers (23a, 23b) as shown in (B). Is formed on the upper and lower surfaces of the EO material 12. Note that the size of the EO element 10 in the fourth embodiment is such that the lateral width W = 250 μm and the longitudinal length Leo = 250 μm. The thickness d of the EO material 12 is set to 20 μm. As for the arrangement of the quarter-wave plate 40, as shown in FIG. 11, when viewed from the front along the central axis 51, the optical axis 41 applies a voltage between the electrodes (11-13) of the EO element 10. It is set so as to intersect with the direction of the electric field generated at an angle of 45 °.

  FIG. 12 is a diagram illustrating the operation of the optical attenuator 4 according to the fourth embodiment, and FIG. 12A is a diagram when the optical path in the ON state is viewed from above, and FIGS. FIG. 5B is a view when the optical path in the off state is viewed from the right side, and FIG. 5B is an optical path of the beam B21e obtained by separating the input light from the input side fiber 23a as TE light in the EO element 10 (B21e → B22e → (B23e-> B25m-> B26m-> B27m), and (C) shows the optical path (B21m-> B22m-> B23m-> B25e-> B26e-> B27e) of the beam B21m separated as TM light in the EO element 10.

  In the optical attenuator 4 according to the fourth embodiment, as shown in FIG. 12A, when no electric field is applied to the EO material 12 of the EO element 10 and it is in the ON state, the light is emitted from the input side fiber 23a. The transmitted beam passes rearward along the first optical axis 50a through the EO element 10, the collimating lens 22, and the quarter wavelength plate 40 (B21 → B22 → B23), and one inclined surface 61a of the reflecting plate 60 is obtained. Is incident on. While the beams (B21 to B23) are separated into TE light and TM light within the EO element 10, a phase difference of ¼ wavelength is given by the ¼ wavelength plate 40 and converted into circular deflection, Since it is not deflected in the EO element 10, it follows the same optical path.

  Then, the beam B23 that has reached the reflecting plate 60 is bent in a U shape by the inclined surfaces (61a, 61b) on the optical axis 50a of the input side fiber 23a and reflected forward, and the second optical axis 50b. Are coupled to the output-side fiber 23b through the quarter-wave plate 40 and the collimating lens 22 along the optical fiber. Note that the coupling loss in the on state was 0.7 dB.

  On the other hand, as shown in FIGS. 12B and 12C, when an electric field is applied to the EO material 12, each of the TE light beam B21e and the TM light beam B21m separated by the EO element 10 has an electric field strength. The two beams (B22e, B22m) that are deflected in the vertical direction at an angle corresponding to the above and deviated from the first optical axis 50a travel backward as collimated light (B23e, B23m) through the collimator lens 22. Then, two parallel lights (B23e, B23m) shifted in the vertical direction with respect to the first optical axis 50a are incident on the quarter wavelength plate 40, respectively, and a phase difference corresponding to the quarter wavelength is given. The light is incident on the reflection plate 60 as circularly polarized light, and the optical path is shifted to the right (front side in the drawing) by the left inclined surface 61a (B23e → B24e, B23m → B24m), and is reflected again by the right inclined surface 61b. Then, forward reflected light is incident on the quarter-wave plate 40. As a result, the beam corresponding to each of the TE light and TM light traveling from the front to the rear is given a phase difference of ½ wavelength in the reciprocation, and the plane of polarization rotates by 90 °. Then, the two beams (parallel light: B25m, B25e) traveling backward from the quarter-wave plate 40 follow the optical path shifted from the second optical axis 50b, and again pass through the collimating lens 22 and the EO element, and are polarization dependent. In the state in which is eliminated, the light is emitted forward from the EO element 10 from the same position (B25m → B26m → B27m, B25e → B26e → B27e). In the optical attenuator 4 according to the fourth example, the coupling loss was 30 dB when the voltage between the electrodes (11-13) of the EO element 10 was 15V.

  As described above, the optical attenuator 4 according to the fourth embodiment can be driven with a low voltage while having practically sufficient optical attenuation characteristics as in the above embodiments. And compared with the 1st-3rd Example, it becomes possible to further shorten the full length of the front-back direction.

=== Fifth embodiment ===
By the way, in the EO element, when the voltage between the electrodes is the same, the electric field strength increases as the thickness between the electrodes decreases. In other words, a predetermined attenuation characteristic can be obtained even when driven at a lower voltage. However, if the optical fiber constituting the optical attenuator other than the EO element has a core diameter of about 10 μm, the thickness between the electrodes of the EO element is further required. In addition, when the thickness between the electrodes is brought close to the beam diameter, the relative position of the up / down / left / right method between the EO element and the open end of the optical fiber is adjusted with extremely high accuracy in order to advance the beam efficiently in the longitudinal direction of the EO element. Need to do. Specifically, the beam emitted from the optical fiber is not parallel light but emitted at a predetermined opening angle. In a general SMF, the beam diameter is about 10 μm at the time of emission, but in the optical fiber collimator, the beam diameter expands until reaching the collimating lens. The numerical aperture N / A of the optical fiber is larger as the core diameter is smaller. That is, the opening angle is increased.

  Therefore, it is necessary to consider so-called “vignetting” in the optical attenuator of the present invention which is premised on the arrangement of the EO element between the opening end of the optical fiber and the collimating lens. That is, the diameter of the beam incident on the EO element from the input side fiber gradually expands in the EO material, and the edge portion on the outer side in the radial direction of the beam does not come into contact with the end surface of the EO material so It is necessary to make it. For this purpose, it is necessary to set the thickness of the EO material in consideration of the beam diameter on the rear end side of the EO element.

  However, when the thickness of the EO material is increased in consideration of vignetting, a contradiction occurs that the electric field strength decreases. That is, it is necessary to drive the EO element with a high voltage. When the EO element is thick, it is difficult to reduce the vertical size of the optical attenuator. If the thickness of the EO element is reduced, in addition to vignetting, there arises a problem that an allowable adjustment error is reduced when the collimator is assembled. For example, when adjusting for an EO element having a thickness of 10 μm, which is equal to the beam diameter, even when the allowable assembly error is the same 1 μm, the thickness is 10 μm when adjusting for an EO element having a thickness of 100 μm. It is much more difficult to adjust the position of the EO element. Accordingly, as a fifth embodiment of the present invention, a configuration is provided in which the thickness of the EO element is increased so that the relative positional relationship between the optical fiber and the EO element in the collimator can be easily adjusted with high accuracy. An optical attenuator having sufficient attenuation characteristics even at a low voltage will be mentioned.

  The basic configuration of the optical attenuator according to the fifth embodiment is the same as that of the attenuator according to the second or third embodiment. However, it is characterized in that the optical fiber held by the collimator is not a general SMF, but a so-called “TEC fiber” in which the mode field diameter on the opening end side is enlarged. The operation of the optical attenuator according to the fifth embodiment is the same as that of the optical attenuator according to the second or third embodiment. The specific structure and characteristics of the optical attenuator according to the fifth embodiment will be described below.

  FIG. 13 shows the structure of the optical attenuator 5 according to the fifth embodiment and the size of each part. FIG. 13A is a side view when the optical attenuator 5 is viewed from the right side, and FIG. 13B is an enlarged view of the front portion of the optical attenuator 5. The optical attenuator 5 shown in FIG. 13 has the same basic configuration as the optical attenuator 2 according to the second embodiment. However, the difference is that a TEC fiber is used for the optical fibers (123a, 123b). Note that the TEC fibers (123a, 123b) have their open ends (126a, 126b) and the core in the vicinity thereof diffused, but the TEC fibers (123a, 123b) themselves are general-purpose ferrules as with general optical fibers. Can be inserted. In FIG. 13B, the shape of the open ends (126a, 126b) of the TEC fibers (123a, 123b) is changed for convenience in order to facilitate the explanation.

Further, in the optical attenuator 5 according to the fifth embodiment, the thickness d of the EO material 12 of the EO element 110a is set to the second embodiment in order to compare the characteristics with those of the optical attenuator 2 of the second embodiment. The EO element (10a, 10b) used in the optical attenuator 2 is set to 32 μm, which is twice or more. Further, the electrode 111 sandwiching the EO material 12 is a titanium thin film formed by sputtering so that the EO elements (110a, 110b) are vertically or horizontally symmetrical, and the total height h of the EO elements (110a, 110b) is substantially equal. This is equivalent to the thickness of the EO material 12. The front / rear length Leo need not be vignetted at the rear end of the EO element 110a, and may be set appropriately according to the drive voltage. For example, assuming that the length Leo when the beam diameter φ ₀ at the opening end 126a of the input side fiber 123a becomes the beam diameter φ ₁ = 2φ ₀ at the rear end of the EO element 110a, the Leo is the beam diameter. Using φ ₀ , the wavelength λ of the input light, and the refractive index n of the EO material 12, the following equation 1 can be used. If the EO substance 12 is KTN, n = 2.185. The wavelength λ of the input light may be set to the wavelength λ of light used in optical communication = 1550 nm.
Leo = πnφ ₀ ² {(d / 2φ ₀ ) ² −1} ^1/2 / λ (Formula 1)

The total length L of the optical attenuator 5 depends on the focal length f of the collimating lenses (22a, 22b), and the distance D1 between the collimating lenses on the input / output side (22a-22b) is the focal length. The distance D2 between the open ends (126a-126b) of the input / output side optical fibers (123a, 123b) is four times the focal length f. The size in the vertical and horizontal directions is set on the assumption that it is mounted on a rack board because optical components are systemized by mounting them on a rack board in the optical communication field. Specifically, the outer diameter phi ₂ of the cylindrical housing 100 and a 3.5 mm. In the optical attenuator 5 according to the fifth embodiment, the collimator (20a, 20b) including the EO elements (110a, 110b) is connected to the casing 100 having the diameter φ ₂ = 3.5 mm. . For the collimating lenses (22a, 22b) built in the collimators (20a, 20b), the working distance WD considering the focal length f, the diameter φ ₃ and the lens thickness is stored in the collimators (20a, 20b). Any numerical value is possible. Further, since the collimating lenses (22a, 22b) are also relatively expensive parts than other parts constituting the optical attenuator 5, a commercially available general-purpose aspheric lens that can be obtained at low cost is employed. Hereinafter, the attenuation characteristic of the optical attenuator 5 according to the fifth embodiment will be described.

<Characteristic evaluation>
In order to evaluate the characteristics of the optical attenuator 5 according to the fifth embodiment, the optical attenuator 5 (sample A) shown in FIG. 13 is used by using any one of three types of commercially available aspherical lenses having different focal lengths f. ) Were produced. Further, in order to compare with the characteristics of the optical attenuator 2 of the second embodiment, an optical attenuator (referred to as sample B) having the same configuration as the optical attenuator 5 of the fifth embodiment is the same except that the optical fiber is SMF. It was fabricated using any of three types of lenses. Note that the longitudinal length Leo of the EO elements (10a, 10b, 110a, 110b) is set according to the above equation 1, and φ ₀ in the equation 1 is φ ₀ = 15 μm for the sample A, and φ for the sample B ₀ = 5 μm.

  Table 1 below shows the focal length f and the working distance WD of the three types of lenses 1 to 3.

  Next, a voltage of 40.5 V was applied between the electrodes (111-111) of the EO elements (110a, 110b) for the three types of samples A and B produced using the lenses 1 to 3 shown in Table 1. The coupling loss was measured. FIG. 14 shows the coupling loss between Sample A and Sample B. In FIG. 14, the coupling loss of each sample is shown with the focal length f corresponding to the three types of lenses used for each sample as the horizontal axis. As shown in this figure, sample B using SMF did not reach 30 dB, which is a sufficiently practical off-time attenuation rate for optical communication applications, regardless of the type of collimating lens. Incidentally, in the optical attenuator 2 of the second embodiment having substantially the same configuration as that of the sample B, the thickness of the EO material 12 in the EO element (10a, 10b) is 15 μm, 1/2 of the sample B, 30 dB at 10V. Coupling loss is obtained.

  On the other hand, in the sample A using the TEC (123a, 123b) fiber, a coupling loss of 50 dB far exceeding the attenuation factor of 30 dB was obtained, which was practically “excess spec”. Therefore, for sample A, when the voltage applied to the EO elements (110a, 110b) was 28 V, a coupling loss of about 30 dB was obtained as shown in FIG. The longitudinal length Leo of the EO elements (110a, 110b) for obtaining a coupling loss of 30 dB at 28V is set to Leo = 370 μm according to the above equation 1.

  As described above, in the optical attenuator 5 according to the fifth embodiment, the NA can be reduced by using the TEC fiber (123a, 123b), and the beam emitted from the input side fiber 123a passes through the EO substance 12. In doing so, the beam does not expand significantly until reaching the rear end, and the thickness of the EO elements (110a, 110b) can be reduced. For this reason, the longitudinal length Leo of the EO elements (110a, 110b) can be made longer as long as it is within the focal length f of the collimating lenses (22a, 22b). Therefore, if the drive voltage is the same, the beam can be deflected more greatly.

  On the other hand, if the coupling loss is the same, the drive voltage can be lowered, and the total height h of the EO elements (110a, 110b) can be increased. Therefore, an allowable error when adjusting the relative positional relationship between the center of the optical fiber (123a, 123b) and the EO element (110a, 110b) is increased. For example, the EO element is included in a general-purpose sleeve used for an optical connector or the like. (110a, 110b) can be incorporated without an advanced adjustment process. That is, the EO element built-in collimator (20a, 20b) can be assembled simply by inserting the EO element (110a, 110b) into a commercially available sleeve. As a result, the optical attenuator 5 can be provided at a lower cost.

Note that the optical attenuator 5 according to the fifth embodiment, which is manufactured using the lens 2 with the focal length f = 1.81 mm described in Table 1 above, has an outer diameter φ ₂ of the casing 100 of 3.5 mm. The total length L was 10 mm, and it was possible to make it extremely compact.

=== Response speed and drive ===
The optical attenuator according to the embodiment of the present invention is assumed to be applied to the optical communication field. Therefore, for example, a response speed corresponding to a case where the optical attenuator is turned off instantaneously at the time of optical burst is required. Therefore, the response characteristics of the optical attenuator 5 according to the fifth embodiment were evaluated. The response characteristic can be evaluated by calculating the delay time by regarding the EO element as an RLC equivalent circuit.

First, the capacitance C of the EO element, the area S and thickness of the EO material d and EO dielectric constant material ε C = εS / d (= ε 0 ε r S / d) of the electrode (Equation 2)
It can be calculated by the following formula. Here, the left-right width W of the EO elements (110a, 110b) is the thickness d (= 32 μm) of the EO material 12, S = d × Leo, and C = εLeo. The relative dielectric constant ε _r of KTN is 20000. The front-rear length Leo is 370 μm as described above.

Here, assuming that the capacitance C obtained by the above equation 2 between the electrodes (111-111) is 100% and the time until the capacitance is changed from 10% to 90% is the response time t, the response time t is the electrode. Using the resistor R and the capacitance C from 111 through the terminal to the driving device, the following equation 3
t = RC (In0.1-In0.9) (Formula 3)
Sought by. Since R = 200Ω, the response time t is t = 28.8 ns. Since the response speed (response time) required for the optical burst in optical communication is about 1 μs, it has been confirmed that there is no problem in optical communication applications.

By the way, the above response time is premised on that the EO element is controlled in the space charge limited state. In the EO element, carriers that carry charge are electrons, but the movement speed of the electrons is slow, and the charge mobility is about 3.3 (cm ² / v · s). Therefore, in order to always drive with the response time, it is necessary to develop and maintain the space charge limited state. Specifically, as shown in FIG. 1C, the region e4 having a high electron density is unevenly distributed in the EO material eo, and the electrons e sequentially supplied from the cathode electrode Cd side to the EO material eo It is necessary to maintain a state in which the anode electrode Ad is sequentially reached and current continues to flow in the EO material eo.

  In order to keep the current flowing, for example, the sweep may be performed at a low frequency (for example, 40 Hz). In addition, a voltage that provides a predetermined coupling loss may be applied. Therefore, as a practical driving method of the optical attenuators 1 to 5 according to each of the above embodiments, a driving method (hereinafter referred to as trap charge control mode) in which charges are trapped in a level that is in a space charge control state is illustrated. To do. FIG. 16 shows a driving waveform of the EO element in the trap charge control mode. A drive signal ac is obtained by superimposing a low-frequency (for example, 40 Hz) AC modulation signal having an amplitude (Vs−Vdc) on the DC bias Vdc. For example, when the optical attenuator 5 according to the fifth embodiment is driven in the wrap charge control mode so as to obtain the coupling loss shown in FIG. 15, a DC bias of Vdc = 14V is applied to reduce the amplitude of 14V. A frequency signal is applied between the electrodes (111-111) of the EO elements (110a, 110b). As a result, the optical attenuator 5 is driven with the maximum potential difference Vs = 28V.

  The present invention can be applied to optical communication using an optical fiber network.

1-5 optical attenuator, 20, 20a, 20b optical fiber collimator,
21, 21a, 21b Ferrule 22, 22a, 22b Collimating lens,
23a, 23b, 123a, 123b optical fiber,
26a, 26b, 126a, 126b Optical fiber open ends,
10, 10a, 10b, 110a, 110b Electro-optical element (EO element),
11, 13, 111, Ad, Cd electrodes,
12, eo electro-optic material (EO material, EO layer), 30 1/2 wavelength plate,
40 1/4 wavelength plate, 50, 50a, 50b optical axis, 60 reflector

Claims (7)

Using the front-rear direction as the optical axis, the intensity of the light emitted backward from the first optical fiber disposed on the front side along the optical axis is attenuated and emitted from the second optical fiber on the output side. An optical attenuator that
Two directions perpendicular to the optical axis are defined as a vertical direction and a horizontal direction, respectively.
The first optical fiber is held in an integral sleeve, and a first collimating lens that receives the light emitted from the first optical fiber and emits it as parallel light is housed. 1 optical fiber collimator;
The second optical fiber is held in an integral sleeve, and a second collimating lens that couples the parallel light emitted from the first collimating lens to the second optical fiber is housed. A second optical fiber collimator,
An electro-optic element formed by facing electrodes on both upper and lower surfaces or both right and left surfaces of a three-dimensional body made of an electro-optic material having an electro-optic effect;
With
The electro-optic element is incorporated in at least one of the first and second optical fiber collimators,
The electro-optic element is disposed in an optical fiber collimator in which the electro-optic element is incorporated so that an optical path between the opening end of the optical fiber and the collimating lens passes through the electro-optic material, and is driven by an external drive circuit. When a voltage is applied between the facing electrodes, a refractive index distribution associated with a space charge limited state is generated in the electro-optical material, and an optical path of light incident on the electro-optical material is generated by the applied voltage. By changing the coupling loss of light incident on the second optical fiber by deflecting it in an angular direction according to the intensity of the electric field;
An optical attenuator characterized by that. In Claim 1, said 1st optical fiber collimator and said 2nd optical fiber collimator are arranged in series back and forth so that both collimating lenses may face,
The electro-optic element is incorporated only in the first optical fiber collimator,
The first optical fiber is a polarization maintaining optical fiber that maintains the direction of the polarization plane of the propagating light, and the direction of the polarization plane of the light propagating through the first optical fiber is the vertical or horizontal normal direction. Matches either
An optical attenuator characterized by that. In Claim 1, said 1st optical fiber collimator and said 2nd optical fiber collimator are arranged in series back and forth so that both collimating lenses may face,
The electro-optic element is built in both the first and second optical fiber collimators, the facing direction of the electrodes of the electro-optic element built in the first optical fiber collimator, and the second optical fiber collimator The facing directions of the electrodes of the electro-optic element built in the optical fiber collimator are orthogonal to each other,
An optical attenuator characterized by that. In Claim 1, said 1st optical fiber collimator and said 2nd optical fiber collimator are arranged in series back and forth so that both collimating lenses may face,
The electro-optic element is built in both the first and second optical fiber collimators, the facing direction of the electrodes of the electro-optic element built in the first optical fiber collimator, and the second optical fiber collimator The facing directions of the electrodes of the electro-optic element incorporated in the optical fiber collimator are parallel to each other,
A half-wave plate is inserted between the first optical fiber collimator and the second optical fiber collimator, and the optical axis of the half-wave plate is inclined 45 ° with respect to the vertical and horizontal directions. Arranged in a direction,
An optical attenuator characterized by that. 2. The first optical fiber collimator according to claim 1, wherein the second optical fiber collimator also serves as the first optical fiber collimator, and the first optical fiber and the second optical fiber are parallel to the optical fiber collimator in the left-right direction. The formed two-core optical fiber cable is held,
The electro-optic element has electrodes formed so as to face in the vertical direction,
A quarter-wave plate and a reflector are arranged in this order behind the collimating lens,
The quarter-wave plate is disposed such that the optical axis is inclined by 45 ° with respect to the vertical and horizontal directions,
The reflector has two mirror surfaces with the same inclination angle so as to gradually become thicker in the left-right direction from the intersection with the straight line extending backward from the midpoint between the open ends of the first and second optical fibers. Then, when light traveling backward from the opening end of the first optical fiber is incident on one mirror surface, the light traveling forward toward the opening end of the second optical fiber is reflected forward through one slope. Let
An optical attenuator characterized by that.   5. The optical attenuator according to claim 3, wherein the first and second optical fibers are core diffusion fibers having an expanded mode field diameter on the opening end side. 7. The electro-optical material according to claim 1, 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 ≦ An optical attenuator, which is a substance represented by y <1).

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