JP2005352469A - Variable light attenuator - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a variable light attenuator which is manufactured at a low cost, capable of being controlled for attenuation with high precision so that a predetermined light attenuation is available in a short time, and keeping a performance even when environment changes. <P>SOLUTION: The variable light attenuator is for transmitting signal light including information by attenuating, and is provided with: a first optical member having a first light transmission path from which the light is emitted; a second optical member having a second light transmission path to which the light emitted from the first optical member is made incident; a moving means which moves the position of the spot formed by the light on the incident end face of the second optical member at least in a first direction; a light detection means which detects the light incident to a light receiving face via the incident end face of the second optical member; and a control means for performing negative feed back control to the moving means so that the spot position moving with the moving means is directed to a position at which the light intensity incident to the second light transmission path becomes a predetermined set intensity on the basis of the detected result by the light detection means. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention is used as an attenuator for attenuating and adjusting light intensity in an optical communication system or the like, and more particularly to a variable optical attenuator used for flattening amplification characteristics of an optical amplifier.

  In general, in an optical communication system or an optical measurement system, an optical attenuator for adjusting an optical transmission level is necessary. As the optical attenuator, a variable optical attenuator and a semi-fixed optical attenuator are known. A variable optical attenuator is generally capable of continuously adjusting an attenuation of 30 dB or more, and is used as a measuring instrument for adjusting a level of a system transceiver or measuring characteristics of an optical device. The semi-fixed optical attenuator is mainly used for adjusting the section transmission loss of the optical transmission line.

  Examples of conventional variable optical attenuators include (1) optical loss caused by deforming part of an optical fiber and adjusting optical attenuation (see Patent Document 1 below), (2) optical fiber Adjusting the light attenuation by adjusting the distance between a pair of end faces obtained by cutting (see Patent Document 2 below), (3) Between a pair of end faces obtained by cutting an optical fiber (4) The optical attenuation is adjusted by inserting a variable attenuation filter between a pair of end faces obtained by cutting the optical fiber. (5) A lens is inserted between a pair of end faces obtained by cutting the optical fiber, and the lens is moved in the optical axis direction to reduce the light. Adjusting the amount (see Patent Document 5 below), (6) providing a Faraday rotator between a pair of end faces obtained by cutting an optical fiber, and adjusting the optical attenuation by the magneto-optic effect (described below) Patent Document 6).

JP 2000-75222 A Japanese Patent Laid-Open No. 04-133005 JP 04-166803 A Japanese Patent Laid-Open No. 03-225312 JP 2002-221675 A Japanese Patent Laid-Open No. 10-90639

  In general, since the core diameter of an optical fiber is several μm, it is necessary to position each optical component including the optical fiber with high accuracy. Therefore, in each of the above Patent Documents 1 to 6, each optical component is firmly fixed by welding or using an adhesive, thereby maintaining a state of being positioned with high accuracy. However, the variable optical attenuators described in the above Patent Documents 1 to 6 do not assume various environmental changes such as changes in mechanical conditions such as vibration, ambient temperature changes, and changes with time. Therefore, when the relative positional relationship between the optical components is deviated due to the environmental change, it is not possible to correct the amount of light attenuation caused by the misalignment. Taking the variable optical attenuator described in Patent Document 1 as an example, there is a risk of disconnection of the optical fiber due to deterioration over time. In addition, even with the variable optical attenuator described in documents other than Patent Document 1, misalignment between components due to deterioration of the adhesive accompanying environmental changes may occur.

  Furthermore, the variable optical attenuators described in Patent Documents 1 to 4 adjust the optical attenuation amount by deforming or driving a long optical fiber or the variable attenuation filter itself. For this reason, it has been pointed out that the efficiency of the mechanism such as a large load on the mechanism related to deformation and driving, a small change in the amount of light attenuation with respect to the deformation amount and the driving amount, and a long time for attenuation control is pointed out.

  When the lens between the optical fibers is moved as in the variable optical attenuator described in Patent Document 5, the change in optical attenuation with respect to the movement of the lens per unit amount in the optical axis direction is small. That is, the dynamic range becomes small, and the efficiency is poor as in the above-mentioned Patent Documents 1 to 4. In general, the configuration in which the lens moves in a plane orthogonal to the optical axis has a larger change in the amount of light attenuation with respect to the amount of movement of the lens than the configuration in which the lens moves in the optical axis direction. For this reason, when the lens is displaced in a plane orthogonal to the optical axis, the displacement greatly affects the change in the amount of light attenuation, and it is pointed out that a highly accurate attenuation control cannot be achieved. .

  The thing using the magneto-optical effect like patent document 6 has the problem that it is expensive.

  In view of the above circumstances, the present invention can be manufactured at low cost, can perform attenuation control with high accuracy so as to obtain a predetermined amount of light attenuation in a short time, and can maintain performance even when there is an environmental change. It is an object of the present invention to provide a variable optical attenuator that can be used.

  In order to achieve the above object, a variable optical attenuator according to the present invention is a variable optical attenuator for attenuating and transmitting a signal light including information, and a first optical transmission in which light is emitted. A first optical member having a path; a second optical member having a second optical transmission path on which light emitted from the first optical member is incident; and a position of a spot formed by light on an incident end face of the second optical member Based on the detection result of the light detecting means, the moving means for moving the light in at least the first direction, the light detecting means for detecting the light incident on the light receiving surface via the incident end face of the second optical member, Control means for performing negative feedback control on the moving means so that the position of the moving spot is directed to a position where the intensity of the light incident on the second optical transmission line becomes a preset set intensity. It is characterized by.

  According to the first aspect of the present invention, an expensive component that uses the magneto-optical effect is not used as the variable optical attenuator disposed in the optical communication device, so that it can be configured at a low cost. In addition, since the light passing through the incident end face of the second optical member is detected and negative feedback control is performed, a very high-precision light attenuation process is performed in a short time. In addition, even when there is an environmental change such as a change with time, the signal light can be controlled to always have a predetermined light intensity.

  When the light detection means detects the light intensity, the second optical member is an optical fiber having a clad and a core as a second optical transmission line, and a step of a predetermined dimension is formed on the core surface and the clad surface at the incident end face. It is desirable that the spot has a diameter larger than the core diameter and smaller than the clad diameter. As a result, a diffraction phenomenon occurs at the incident end face, so that a change in light intensity can be clearly detected.

  According to the variable optical attenuator described in claim 3, the predetermined dimension is set to a value smaller than about λ / (4n) where λ is the wavelength of light and n is the refractive index of the medium. More preferably, the predetermined dimension is set to approximately λ / (8n).

  According to the variable optical attenuator described in claim 5, the incident end face is configured such that the end face of the core and the end face of the clad are in a substantially parallel relationship.

  According to the variable optical attenuator according to claim 6, the light receiving surface of the light detection means is divided into two parts separated by a boundary line extending in a direction corresponding to the second direction that is not parallel to the first direction. Having a light detection area, the control means can drive and control the movement means based on outputs from the two light detection areas to move the spot position in the first direction.

  In the case of adopting the configuration according to claim 6, the control means sets the moving means so that the output difference between the two light detection areas substantially matches the reference output difference obtained when the light intensity is the set intensity. The drive can be controlled (claim 7).

  Further, in the case of adopting the configuration according to claim 6, the control means uses the reference light obtained when the light intensity distribution generated based on the outputs from the two light detection areas is the set intensity. The moving means may be driven and controlled so as to substantially coincide with the intensity distribution.

  Note that, when the configuration according to any one of claims 6 to 8 is adopted, the light receiving surface is in the relationship with the incident end surface, and the light passing through the surface center of the second transmission path at the incident end surface is the center of the boundary line. It is positioned so as to enter the vicinity (claim 9).

  According to the variable optical attenuator according to claim 10, the light receiving surface of the light detecting means extends in a direction corresponding to each of the first direction and the second direction non-parallel to the first direction. The control means has four light detection areas divided by the one boundary line and the second boundary line, and the control means drives and controls the movement means based on the output from the four light detection areas, and adjusts the position of the spot. It can be moved in the first direction and the second direction.

  When the configuration according to claim 10 is adopted, the control means outputs the sum of the outputs of the two photodetection areas on one side and the outputs of the two photodetection areas on the other side with respect to the first boundary line. The first output difference, which is the difference from the sum of the two, substantially matches the reference first output difference obtained when the light intensity is the set intensity, and is located on one side with respect to the second boundary line. Reference second output obtained when the second output difference, which is the difference between the sum of the outputs of the two light detection areas and the sum of the outputs of the two light detection areas on the other side, is the set light intensity. The moving means is driven and controlled so as to substantially match the difference.

  When the configuration according to claim 10 is adopted, the control means uses the reference light intensity obtained when the light intensity distribution generated based on the output from each light detection area is the set intensity. The position of the spot may be moved in the first direction by controlling the moving means so as to substantially match the distribution (claim 12).

  Note that, when the configuration according to any one of claims 10 to 12 is adopted, the light receiving surface is in a relationship with the incident end surface, and the light passing through the surface center of the second transmission path at the incident end surface is the first boundary. It is positioned so as to be incident near the intersection of the line and the second boundary line (claim 13).

  Furthermore, according to the variable optical attenuator according to claim 14, the light receiving surface of the light detection means is composed of a single light detection area, and the control means sets the output from the light detection area and the light intensity. The moving means is driven and controlled so as to substantially match the reference output obtained when the intensity is, and the position of the spot is set in the first direction and the first direction at a predetermined frequency lower than the frequency of the light transmission band. You may change periodically in the 2nd direction orthogonal to a direction.

  In this case, the second optical member has a deflecting member that deflects a part of the light incident on the second optical transmission line via the incident end face to the outside of the second optical member, and the light detecting means is provided by the deflecting member. It is desirable to receive a part of the light deflected to the outside (claim 15). For example, the second optical member can be configured as an optical fiber having a core as a second optical transmission line, and the deflecting member can be configured as a half mirror disposed in the core.

  According to the variable optical attenuator according to claim 17, the moving means has the third optical member installed between the first optical member and the second optical member, and drives the third optical member. Thus, the position of the spot on the incident end face can be moved.

  For example, the third optical member is a condensing lens that condenses the light emitted from the first optical member on the incident end surface of the second optical member.

  In the variable optical attenuator according to claim 19, the second optical member is configured so that the light emitted from the third optical member is retroreflected at the incident end face, and retroreflected at the incident end face. The optical system may further include a deflecting unit that deflects the emitted light in a direction deviating from the optical path of the light incident on the incident end surface and guides the light to the light amount detecting unit. Preferably, the deflecting means includes a polarizing beam splitter and a quarter wave plate.

  The third optical member may be a rotatable galvanometer mirror or a vertex angle variable prism that can freely change the vertex angle.

  The first optical member may be an optical fiber. Alternatively, the first optical member may be an optical waveguide (claim 24).

  As described above, according to the present invention, since light passing through the incident end face of the second optical member is detected and negative feedback control is performed, a highly accurate light attenuation process is performed. Further, by adopting a configuration in which the light attenuation is adjusted by moving the position of the spot formed on the incident end face by the light, a predetermined light attenuation can be obtained in a short time. According to the variable optical attenuator configured to perform negative feedback control in this way, light intensity suitable for optical communication can always be obtained without being influenced by environmental changes, changes with time, etc. It becomes possible to maintain performance.

  Further, according to the variable optical attenuator according to the present invention, high-precision and stable attenuation control can be realized at a low cost by not using expensive components as in the configuration described in Patent Document 6.

  FIG. 1 is a diagram showing a configuration of a variable optical attenuator 11 as a first embodiment of the present invention. The variable optical attenuator 11 is used as an attenuator that attenuates and adjusts the light intensity in an optical communication system or the like.

  The variable optical attenuator 11 of this embodiment includes a first optical fiber 1, a condenser lens 2, and a second optical fiber 3, a photodetector 4, a controller 5, and an actuator 6. FIG. 2 is an enlarged view of the vicinity of a surface (incident end surface) 3 a on which light from the first optical fiber 1 is incident in the second optical fiber 3. On the incident end face 3a of the second optical fiber 3, the surface of the clad 3b and the surface of the core 3c appear. In the present specification, for convenience of explanation, it is assumed that the center of the core 3 c coincides with the center of the second optical fiber 3.

  The first optical fiber 1, the condensing lens 2, and the second optical fiber 3 are configured such that light emitted from the first optical fiber 1 is incident on the incident end surface 3 a of the second optical fiber 3 through the condensing lens 2. It is positioned with high accuracy so as to be incident on the entire surface of the core 3c. And the 1st optical fiber 1 and the 2nd optical fiber 3 are being firmly fixed by fixing means, such as an adhesive agent, in the state positioned with high precision. The condenser lens 2 is fixed so as to be slightly movable by the actuator 6 with reference to a state (initial position state) positioned with high accuracy.

  As shown in FIG. 2, the light emitted from the first optical fiber 1 is collected by the condenser lens 2 so that the spot diameter r1 formed on the incident end face 3a is slightly larger than the core diameter r2. Focused. For this reason, even when light is incident at the highest light transmission efficiency, that is, at the center of the core 3c, the spot peripheral edge of the light is incident on the cladding 3b.

  Here, the condenser lens 2 is in a state of being movable in one axial direction (X direction) within a plane orthogonal to the optical axis by the actuator 6 under the control of the controller 5. The spot moves in the X ′ direction on the incident end surface 3 a corresponding to the direction in which the condenser lens 2 is driven.

  In this specification, the positional relationship and direction in the variable optical attenuator 11 will be described with reference to the moving direction of the spot on the incident end face 3a. In the configuration of the present embodiment, when facing the incident end face 3a from the first optical fiber 1, the left direction is the X ′ (−) direction, the right direction is the X ′ (+) direction, and the upper direction is based on the position of the core 3c. The direction is the Y ′ (+) direction, and the downward direction is the Y ′ (−) direction. Therefore, the direction of the condensing lens 2 is the X (−) direction for the left direction, the X (+) direction for the right direction, and the Y (+) direction for the right direction when facing the condensing lens 2 from the first optical fiber 1. The direction and the downward direction are defined as the Y (−) direction.

  As shown in FIG. 2, the incident end face 3a has a step formed by projecting the surface of the core 3c in a direction substantially perpendicular to the surface of the cladding 3b (the optical axis direction of the second optical fiber 3). . The incident end face 3a is processed so that the protruding core 3c surface and the clad 3b surface are substantially parallel. In the present embodiment, the incident end face 3a is processed into the above shape by using a photolithography technique. The size of the step is set to a value smaller than λ / (4n), more preferably λ / (8n) when light is incident on both the protruding core 3c surface and the cladding 3b surface. Where λ is the wavelength of incident light and n is the refractive index of the medium. By setting the dimension of the step as described above, when the light beam condensed so that the spot diameter r1 is slightly larger than the core diameter r2 is incident so as to straddle both the core 3c and the clad 3b on the incident end face 3a. A diffraction phenomenon occurs. In the present embodiment, the medium is assumed to be air, and the light intensity difference obtained by the light incident on the core 3c and reflected (zero-order light) and the light incident on the cladding 3b and reflected (primary light). The predetermined dimension is set to approximately λ / 8 so that is maximized.

The reflected light from the incident end face 3a enters the photodetector 4 and forms a diffraction pattern on the light receiving face 4a. In general, the spot size formed by the light emitted from the first optical fiber 1 at the incident end face 3a is 1 / e ² of the maximum intensity in the intensity distribution of the light (where e is a natural logarithm). Bottom) It is defined as a range having the above strength. Further, it is known that the diffraction pattern is more prominently formed by light having a greater intensity. However, a diffraction pattern can be formed on the light receiving surface 4a even if it is a portion having an intensity smaller than 1 / e ² of the maximum intensity of the incident light beam.

  That is, as the spot diameter formed on the incident end face 3a is increased, the amount of reflected light increases and the diffraction pattern appears clearly on the light receiving surface 4a, but the amount of light incident on the optical fiber 3 decreases. The coupling efficiency between the optical fibers 1 and 3 deteriorates. Conversely, as the diameter of the spot formed on the incident end face 3a is reduced, the diffraction pattern on the light receiving surface 4a becomes blurred, but the coupling efficiency can be increased.

  In view of the above circumstances, in order to balance the clarity of the diffraction pattern and the effectiveness of the coupling efficiency, in this embodiment, r1 shown in FIG. 2 is set to 11 μm and r2 is set to 10 μm. Thus, even when light is incident on the incident end face 3a at the highest light transmission efficiency, that is, when entering the core 3c, the spot peripheral edge of the light is incident on the clad 3b. However, the present invention can be implemented even if r1 <r2 is set, that is, the spot is contained in the core 3c on the incident end face 3a.

  In the present embodiment, a two-divided photodetector is used as the photodetector 4. Specifically, as shown in FIG. 3, the light receiving surface 4a is divided into two lattice-like light detection areas α and β by a boundary line 4b passing through the center O of the light receiving surface 4a. The photodetector 4 outputs a voltage signal corresponding to the light received in each of the light detection areas α and β to the controller 5.

  The photodetector 4 is arranged so that the extending direction of the boundary line 4 b substantially coincides with a direction (Y ″ direction) corresponding to a direction (here, the Y direction) orthogonal to the moving direction of the condenser lens 2. Further, in the relationship with the other optical members 1 to 3, the photodetector 4 is incident on the light receiving surface 4 a by light emitted from the first optical fiber 1 and reflected by the incident end surface 3 a of the second optical fiber 3. As a result, the signal levels output from the light detection areas α and β are fixed while being positioned to be equal, and the signal levels output from the light detection areas α and β are equal to each other. This means that the light component reflected at the center is incident on the center O of the light receiving surface 4a, and the incident end surface 3a and the light detector 4 emit light reflected from the incident end surface 3a from the condenser lens 2. Spot formed on the incident end face 3a For example, the light detector 4 may be disposed at a position sufficiently away from the incident end surface 3a, or the reflected light may be reflected. A predetermined optical system (not shown) that enhances the divergence may be arranged.

  Note that the variable light attenuator 11 of the first embodiment is configured to reflect in a direction different from the incident direction when the control light emitted from the condenser lens 2 is reflected by the incident end face 3a. Thereby, the light receiving surface 4a of the photodetector 4 can be arranged on the optical path of the light reflected by the incident end surface 3a without disturbing the optical path of the light incident on the incident end surface 3a.

  FIG. 3 is a view showing the light receiving surface 4a of the photodetector 4 according to the first embodiment. FIG. 3 shows a state where the light receiving surface 4a is viewed from the back side for convenience of explanation. As shown in FIG. 3, the direction of the light receiving surface 4 a is the direction corresponding to the X ′ (+) direction, that is, the light detection area α side with respect to the position of the center O when the light receiving surface 4 a is viewed from the back side. The direction corresponding to the X ″ (+) direction and the X ′ (−) direction, that is, the light detection area β side is defined as the X ″ (−) direction. Similarly, the direction corresponding to the Y ′ (+) direction is defined as the Y ″ (+) direction, and the Y ′ (−) direction is defined as the Y ″ (−) direction.

  Hereinafter, the principle of the light attenuation process performed by the controller 5 will be described in detail. 4A to 4C show the incident position of the light from the first optical fiber 1 on the incident end face 3a and each light when the light from the first optical fiber 1 is at each incident position. It is the figure which showed the light intensity distribution of the X direction based on the output from detection area (alpha) and (beta). In the incident end face 3a shown in each figure, the left side of the drawing is the X '(-) direction, and the right side is the X' (+) direction. FIG. 4A shows a state in which the light from the first optical fiber 1 is incident in the X ′ (−) direction from the core 3c, and the light intensity distribution of the light from the first optical fiber 1 in this state. Indicates. FIG. 4B shows a state in which the incident position of light from the first optical fiber 1 substantially coincides with the core 3c and the light intensity distribution of light from the first optical fiber 1 in this state. FIG. 4C shows a state in which the light from the first optical fiber 1 is incident in the X ′ (+) direction from the core 3c and the light intensity distribution of the light from the first optical fiber 1 in this state. Indicates. That is, immediately after each optical member 1 to 3 is positioned and fixed, the light intensity distribution shown in FIG. 4B is obtained.

  As shown in FIGS. 4A to 4C, the light intensity distribution generated based on the output from the photodetector 4 differs depending on the position of the spot on the incident end face 3a. That is, the light intensity distribution changes corresponding to the deviation between the spot formed on the incident end face 3a and the surface of the core 3c. Therefore, a light intensity distribution is acquired in advance when the light is attenuated by a predetermined light attenuation, in other words, the intensity of light transmitted through the core 3c is a predetermined intensity. In optical communication, a light (hereinafter referred to as a reference light intensity distribution) corresponding to an intensity (hereinafter referred to as a set intensity) set as light transmitted through the core 3c is suitable for optical communication, and a photodetector The position of the spot is moved so that the light intensity distributions generated based on the actual output from 4 match. Thereby, the intensity of light used for optical communication is adjusted, in other words, an optical attenuation process is performed.

  The controller 5 performs negative feedback to adjust the position of the spot on the incident end face 3a so that the current light intensity distribution created based on the outputs from the light detection areas α and β substantially matches the reference light intensity distribution. Take control. The reference light intensity distribution of the present embodiment corresponds to the distribution shown in FIG. 4B when the attenuation rate is suppressed to the lowest (the light intensity required for optical communication is high). When increasing the amount of light attenuation to keep the light intensity required for optical communication low, the distributions shown in FIGS. 4A and 4C correspond to the reference light intensity distribution.

  The signal output from each of the light detection areas α and β (that is, the amount of light received in each of the light detection areas α and β) is proportional to the integral value of the generated light intensity distribution. Therefore, instead of setting the intensity distribution itself as a comparison target, signal outputs from the respective light detection areas α and β may be set as comparison targets.

  That is, as shown in FIG. 4A, when the incident position (spot) formed by the light from the first optical fiber 1 on the incident end face 3a is shifted in the X ′ (−) direction from the core 3c, The output of the light detection area β (hereinafter referred to as the output in the X ″ (−) direction) is larger than the output of the light detection area α (hereinafter referred to as the output in the X ″ (+) direction). On the contrary, as shown in FIG. 4C, when the incident position of the light from the first optical fiber 1 is shifted in the X ′ (+) direction from the core 3c, the output in the X ″ (−) direction. Also, the output in the X ″ (+) direction is larger.

  4B, when the incident position of light from the first optical fiber 1 substantially coincides with the core 3c, the light from the first optical fiber 1 is the second optical fiber 3. After being reflected at the center of the light, it enters the light receiving surface 4a of the photodetector 4. Accordingly, the outputs of the light detection areas α and β are substantially equal, that is, the deviation amount is zero.

  FIG. 5 shows a value obtained by subtracting the output in the X ″ (+) direction from the output in the X ″ (−) direction (hereinafter referred to as an output difference in the X ″ direction) and the X ′ direction of the spot on the incident end face 3a with respect to the core 3c. 6 is a graph showing the relationship between the deviation amount (that is, the distance between the core center and the spot center position). As shown in FIG. 5, the output difference in the X ″ direction is caused by the action of the diffraction phenomenon on the incident end face 3a. 'It changes to a sharp S-shape with respect to the amount of deviation in the direction. As shown in FIG. 5, if the output difference in the X ″ direction is obtained, the shift amount in the X ′ direction is inevitably determined.

  FIG. 6 is a graph showing the relationship between the amount of light attenuation and the amount of deviation in the X ′ direction. As shown in FIG. 6, if a predetermined light attenuation amount (predetermined light intensity) is determined, the shift amount in the X ′ direction is inevitably determined. As described above, the light attenuation process can also be performed by moving the spot so that the output difference in the X ″ direction substantially matches the output difference in the X ″ direction corresponding to the set intensity (hereinafter referred to as the reference output difference). Understand.

  The light attenuation process using the output difference between the light detection areas α and β will be described in detail. First, the controller 5 determines a reference output difference corresponding to an optical attenuation amount (or set intensity) set from the outside based on a table or function as shown in FIGS. Then, the controller 5 obtains the output difference in the X ″ direction based on the output from the photodetector 4. Then, the controller 5 moves the actuator 6 so that the output difference in the X ″ direction matches the reference output difference. Then, the condensing lens 2 is driven in the X direction to move the incident position on the incident end face 3a in the X ′ (+) direction or the X ′ (−) direction.

  Here, as shown in FIGS. 5 and 6, there are two types of output differences in the X ″ direction corresponding to an arbitrary light attenuation amount (except for the light attenuation amount 0). In the embodiment, among the positive and negative output differences corresponding to the set light attenuation amount, the controller 5 sets the reference output difference that is closer to the output difference in the X ″ direction immediately after execution of the light attenuation process.

  The controller 5 performs negative feedback control so that the output difference in the X ″ direction matches the reference output difference as described above, so that the light from the first optical fiber 1 has light intensity suitable for optical communication. High-precision positioning of the spot with respect to the core 3c on the incident end face 3a can be performed in a short time so as to be incident on the second optical fiber 3. Moreover, fluctuations in optical attenuation due to changes over time can be suppressed by negative feedback control. be able to.

  Strictly speaking, when the amount of deviation in the X ′ direction changes, the position of the spot formed on the light receiving surface 4 a by the light reflected by the incident end face 3 a also changes. However, the area of the light receiving surface 4a is much larger than that of the incident end surface 3a, and the spot formed on the light receiving surface 4a is much larger in diameter than the spot formed on the incident end surface 3a. Accordingly, the change in the position of the spot on the light receiving surface 4a corresponding to the change in the shift amount in the X ′ direction is very small, and the intensity change due to the diffraction action on the incident end surface 3a as shown in FIG. 5 is affected. Absent.

  The above is the description of the optical attenuation process in the variable optical attenuator 11 of the first embodiment. Next, the variable optical attenuator 11 of the second embodiment will be described. The variable optical attenuator 11 of the second embodiment is shown in FIG. In the variable optical attenuator 11 of the first embodiment, the light attenuation process is performed by moving the spot formed on the incident end face 3a in the X ′ direction. The variable optical attenuator 11 of the second embodiment is configured to be able to move a spot formed on the incident end face 3a in the X ′ direction and the Y ′ direction. That is, the condenser lens 2 is configured to be movable in the X direction and the Y direction by the actuator 6.

  In the second embodiment, a quadrant photodetector is used as the photodetector 4. FIG. 7 is a diagram showing the light receiving surface 4a of the photodetector 4 according to the second embodiment. FIG. 7 shows a state where the light receiving surface 4a is viewed from the back side for convenience of explanation. Specifically, as shown in FIG. 7, the light receiving surface 4a is divided into four lattice-like light detection areas A to D by boundary lines 4b and 4c that intersect at the center O of the light receiving surface 4a. The photodetector 4 has a direction (X ″ direction, Y ″ direction) in which the extending direction of the two boundary lines 4b, 4c corresponds to a direction (X direction, Y direction) in which the condenser lens 2 can move. The light components that are substantially coincident and are emitted from the first optical fiber 1 and reflected at the center of the core 3c of the incident end face 3a are arranged so as to enter the center O of the light receiving surface 4a.

  The above is the configuration unique to the variable optical attenuator 11 of the second embodiment. In the variable optical attenuator 11 of the second embodiment, the description of the same configuration as that of the first embodiment is omitted here.

  In the variable optical attenuator 11 of the second embodiment, the optical attenuation processing can be performed by moving the spots formed on the incident end face 3a in the X ′ direction and the Y ′ direction. That is, the variable optical attenuator 11 of the second embodiment performs the optical attenuation process using not only the output difference in the X ″ direction but also the output difference in the Y ″ direction. In the second embodiment, the output difference in the X ″ direction is the sum of outputs of the light detection areas B and C (output in the X ″ (−) direction) and the output sum of the light detection areas A and D (X ″ (+ In the second embodiment, the output difference in the Y ″ direction is the output sum of the light detection areas C and D (output in the Y ″ direction (−)) and the light. It is defined as the difference from the output sum of the detection areas A and B (output in the Y ″ direction (+)).

  FIG. 8 is a table showing the light intensity distributions in the X ″ and Y ″ directions detected by the photodetector 4 for each position of the spot formed on the incident end face 3a with respect to the core. When the spot position formed on the incident end face 3a is deviated from the core 3c in one or both of the X ′ direction and the Y ′ direction, the output from each of the light detection areas A to D is different. The distribution is not symmetrical between the + direction and the − direction, that is, in the second embodiment, the relationship between the output difference in the Y ″ direction and the amount of deviation in the Y ′ direction, and the amount of optical attenuation and the amount of deviation in the Y ′ direction 5 and 6 are the same as those in the graphs shown in Fig. 5 and Fig. 6. Therefore, the light generated by moving the spot formed on the incident end face 3a in the X 'direction as described above in the first embodiment. By executing the same process as the attenuation process in the Y ′ direction, it is possible to achieve light attenuation control with higher accuracy than in the first embodiment.

In the case of the variable optical attenuator 11 of the second embodiment, since the spot formed on the incident end face 3a is movable in two axes, the first optical fiber 1, the condensing lens 2, the second The relative positioning when the optical fiber 3 is fixed by the fixing means may be coarser than in the first embodiment. Then, after fixing each member 1 to 3 (however, the condenser lens 2 is movably fixed by an actuator), the first optical fiber 1 is moved by moving the condenser lens 2 in the X and Y directions. The position may be adjusted so that the light emitted from the light enters the entire surface of the core 3c of the incident end face 3a of the second optical fiber 3 via the condenser lens 2. Therefore, the variable optical attenuator 11 of the second embodiment has an advantage that the burden during manufacturing is reduced.
.

  The variable optical attenuator that executes the optical attenuation process by moving the spot formed on the incident end face 3a in each of the X ′ and Y ′ directions may be configured as in the third embodiment described below. good.

  A variable optical attenuator 11W of the third embodiment is shown in FIG. Unlike the variable optical attenuator 11 of the first and second embodiments, the variable optical attenuator 11W has a step formed between the core 3c and the cladding 3b on the incident end face 3a of the second optical fiber 3. Absent. That is, the incident end face 3a is configured as a plane. The second optical fiber 3 is formed with a half mirror portion 3d for taking out a part of light transmitted through the core to the outside of the fiber 3. The light receiving surface 4a of the photodetector 4 is configured as a single light detection area.

  Since the other configuration of the variable optical attenuator 11W is the same as that of the variable optical attenuator 11, the same reference numerals as those of the variable optical attenuator 11 shown in FIG.

  In the variable optical attenuator 11W of the third embodiment, the controller 5 causes the condenser lens 2 to vibrate slightly with a constant period and constant amplitude in the X direction and the Y direction, that is, in a plane orthogonal to the optical axis of the condenser lens 2. . Thereby, the position of the spot formed on the incident end face 3a by the light emitted from the condenser lens 2 is slightly vibrated in the X ′ direction and the Y ′ direction. Then, a part of the light transmitted through the core 3 c is reflected and guided to the light receiving surface 4 a of the photodetector 4.

  In the present specification, the operation of causing the spot formed on the incident end face 3a to vibrate slightly with a constant period and a constant amplitude in the X direction or the Y direction will be referred to as “wobbling” hereinafter. In the variable optical attenuator 11W of the third embodiment, wobbling is performed by vibrating the condenser lens 3 in the X or Y direction with a constant period and a constant amplitude. The wobbling frequency is lower than the frequency of the transmission band of light modulated by the signal. Therefore, the two signals can be effectively separated by adding an appropriate electrical frequency filter. Therefore, the controller 13 can extract the light intensity change due to wobbling even during optical communication.

  FIGS. 10A to 10C are diagrams illustrating a wobbling operation of the spot S on the incident end face 3 a of the second optical fiber 3. In the following, including FIG. 10, for convenience of explanation, the wobbling operation is only performed in the Y ′ direction. However, the following description is also applied in common to a case where a wobbling operation is performed in the X ′ direction and the Y ′ direction. 10A to 10C, the hatched area indicates the spot S, and the white arrow line indicates the direction of wobbling.

  FIG. 10C shows a case where the spot S before wobbling substantially coincides with the core 3c, and FIG. 10B shows that the spot S before wobbling slightly shifts in the Y ′ (+) direction with respect to the core 3c. FIG. 10A shows a case where the spot S before wobbling is displaced in the Y ′ (+) direction with respect to the core 3c by a larger amount than in the case of FIG. 10B. 10 (a) to 10 (c), symbol P1 (and symbol P3) is the center position of the spot S when there is no movement due to wobbling, and symbol P2 is the most negative side in the Y ′ direction due to wobbling. The center position of the spot S, the symbol P4, indicates the center position of the spot S when it is swung most positively in the Y ′ direction due to wobbling. That is, the center position of the spot S moves in the order of P1, P2, P3, P4, and P1 by wobbling, and this operation is repeated as one cycle.

  FIGS. 11A to 11C show changes in the output from the photodetector 4 when the position of the spot S in FIGS. 10A to 10C changes due to wobbling. FIG. 11A shows an output change when wobbling is performed in the state shown in FIG. 10A, and FIG. 11B shows a case where wobbling is performed in the state shown in FIG. 10B. FIG. 11 (c) shows the output change when wobbling is performed in the state shown in FIG. 10 (c). 11A to 11C, the horizontal axis indicates time, and the vertical axis indicates output.

  As shown in FIGS. 11A to 11C, the output from the photodetector 4 changes at a higher level as the wobbling operation is performed near the center of the core 3c, that is, the output average becomes higher. I understand. In general, the intensity of light transmitted through the core 3c and the signal level output from the photodetector 4 corresponding to the light incident through the half mirror portion 3d change substantially linearly. Therefore, the controller 5 obtains an output average (hereinafter referred to as a reference output average) obtained when the light of the set intensity is transmitted during the core 3c transmission, so that the output average actually obtained by the photodetector 4 becomes the reference output average. Negative feedback control.

  The above is the embodiment of the present invention. The variable optical attenuator according to the present invention is not limited to the above-described configuration, and for example, effects similar to the above can be obtained by the following modifications.

  In the above embodiment, it has been described that the X direction and the Y direction, which are the movable directions of the condenser lens 2, are configured to be orthogonal to each other. This configuration has the advantage of reducing the processing load of the controller 5 most. However, this configuration is merely an example, and the present invention is not limited to this. In the variable optical attenuator 11 according to the present invention, the X direction and the Y direction may be non-parallel. In the present invention, the extending directions of the boundary lines 4b and 4c are not necessarily orthogonal.

  The variable optical attenuator 11 according to the present invention can perform optical attenuation processing with high accuracy even by processing other than the above embodiment. For example, in the above embodiment, as an optimal embodiment for simplifying and speeding up the processing, the extending direction of the boundary line 4b coincides with the X direction, and the extending direction of the boundary line 4c is the Y direction. The photodetector 4 is cast so as to match. In the variable optical attenuator of the present invention, the extending direction of the boundary line and the movable direction (X direction, Y direction) of the condensing lens 2 do not necessarily match. For example, in the second embodiment, as shown in FIG. 12, the photodetector 4 is arranged so that the extending directions of the boundary lines 4 b and 4 c divide the angle formed by the X ″ direction and the Y ″ direction into substantially equal parts. You may arrange. For example, in the configuration of the above embodiment, the photodetector 4 can be arranged so that the angle formed by the extending direction, the X ″ direction, and the Y ″ direction is 45 °. In this case, the controller 5 obtains the shift amount in the X ′ direction based on the outputs of the light detection areas A and C that are in a symmetric position with respect to the center O, in other words, diagonally, and detects the light detection area B. And the amount of deviation in the Y ′ direction is obtained based on the outputs of D.

  Further, the positioning process described above can be executed even with a configuration other than the variable optical attenuators 11 and 11W. FIGS. 13 to 17 show variable optical attenuators 12 to 16 that have the same configuration as the variable optical attenuator 11 but can execute the same processing as described above. FIG. 13 shows the variable optical attenuator 12 using the second optical fiber 3 ′ cut along a plane other than the plane orthogonal to the extending direction of the second optical fiber 3. The variable optical attenuator 12 has the characteristics that the coupling efficiency of the optical fiber is higher than that of the variable optical attenuator 11 and the manufacture is easy. In the variable optical attenuator 12, the optical fiber 3 ′ is arranged so that the optical axis of the fiber is inclined at a predetermined angle with respect to the optical axis of the condenser lens 2 in consideration of the refraction phenomenon at the incident end face 3a. Has been.

  FIG. 14 shows a variable optical attenuator 13 in which the first optical fiber 1, the condensing lens 2, and the second optical fiber 3 are arranged on a common axis. The variable optical attenuator 13 is characterized by being smaller than the variable optical attenuator 11 and easy to manufacture. In the variable optical attenuator 13, the reflected light from the incident end face 3a passes through the same optical path as the incident optical path. For this reason, a deflecting unit including a quarter-wave plate 7 and a beam splitter 8 is disposed between the condenser lens 2 and the second optical fiber 3 so that the reflected light is deflected and guided to the photodetector 4. Is done. According to the variable optical attenuator 13, since the reflected light is in a polarization state by the quarter wavelength plate 7 and the beam splitter 8, the reflected light does not return to the first optical fiber 1, and almost totally reflected light is detected. The light enters the container 4. Therefore, efficient light detection is possible. The variable optical attenuator 14 shown in FIG. 15 has almost the same configuration as the variable optical attenuator 13. However, the variable optical attenuator 14 is different from the variable optical attenuator 13 in that the quarter wavelength plate 7 and the beam splitter 8 are disposed between the first optical fiber 1 and the condenser lens 2. Further, a collimator lens 9 can be disposed between the first optical fiber 1 and the deflecting means 7 and 8 as in the variable optical attenuator 15 shown in FIG.

  In the above embodiment, since the light incident on the incident end face 3a of the second optical fiber 3 is diffracted, the core 3c protrudes from the cladding 3b at the incident end face 3a. The same effect can be obtained even with a configuration other than the above, for example, a configuration in which the core 3c is recessed by λ / (4n), λ / 8 by following the above embodiment, rather than the cladding. Further, the diameter of the step may be slightly larger than the diameter of the core to include a part of the clad.

  Moreover, in the said embodiment, the light attenuation process is performed by moving the spot which the light from the 1st optical fiber 1 forms in the incident end surface 3a by driving the condensing lens 2. FIG. The spot moving means may have another configuration.

  In addition, as shown in FIG. 17, the position of the spot on the incident end face 3 a can be moved by arranging a galvano mirror 10 on the optical path of the light emitted from the first optical fiber 1. The light emitted from the first optical fiber 1 and passing through the condenser lens 2 is reflected by the galvanometer mirror 10 and enters the incident end face 3 a of the second optical fiber 3. The galvanometer mirror 10 is configured to swing under the control of the controller 5 in the direction indicated by the white arrow X1 in FIG.

  Further, as the spot moving means, it is possible to arrange a variable apex angle prism having the structure described below on the optical path of the light emitted from the first optical fiber 1. FIG. 18 is a cross-sectional view showing an example of the variable apex angle prism 20. The variable apex angle prism 20 has two parallel glass plates 21 and 22 and an elastic bellows-like cover 23 sealed by the two glass plates 21 and 22. The cover 23 is filled with a liquid such as silicone oil. Each glass plate 21 and 22 is hold | maintained by glass holding | maintenance part 24a-24d. The glass holding portion 24 a is screwed to a lead screw 26 that is rotatable by the motor portion 25. Therefore, as the lead screw 26 rotates, the glass holding portion 24a moves forward and backward in the α direction (the extending direction of the lead screw 26) in the drawing. Thereby, the magnitude | size of the vertex angle (theta) made by each glass plate 21 and 22 changes, and the optical axis of the light which permeate | transmits the vertex angle variable prism 20 can be moved (from a broken line to a continuous line in the figure). Accordingly, the apex angle adjusting unit 27 including the motor unit 25 and the lead screw 26 described above is disposed so that the moving direction of the optical axis corresponds to the X ′ direction and the Y ′ direction. It can be used as a spot moving means.

  Moreover, in the said embodiment, the 1st optical fiber 1 is used as an optical member in which the light which injects into the 2nd optical fiber 3 via the condensing lens 2 is inject | emitted. Here, the optical member that can be used in the variable optical attenuator according to the present invention is not limited to the optical fiber such as the first optical fiber 1 but may be an optical waveguide.

It is a figure showing schematic structure of the variable optical attenuator of 1st, 2nd embodiment of this invention. It is an enlarged view of the incident end face vicinity in the optical fiber of 1st, 2nd embodiment of this invention. It is the figure which showed the light-receiving surface of the photodetector of 1st embodiment of this invention. It is the figure which showed the light intensity distribution of the X "direction based on the incident position in the incident end surface of the light from a 1st optical fiber, and the output from each photon detection area. It is a graph showing the relationship between the output difference of a X "direction, and the deviation | shift amount of the X 'direction with respect to the core of the incident position in an incident end surface. It is a graph showing the correlation data of the variation | change_quantity of the distance between core center-spot positions, and optical attenuation change. It is the figure which showed the light-receiving surface of the photodetector of 2nd embodiment of this invention. It is a figure showing the relationship between the incident position of the reflected light on a light-receiving surface, and the light intensity distribution of each direction of X ", Y". It is a figure showing schematic structure of the variable optical attenuator of 3rd embodiment of this invention. It is a figure which shows the wobbling operation | movement of the spot in the entrance-end surface of a 2nd optical fiber of 3rd embodiment. It is a figure which shows the change of the output from a photodetector, when the position of a spot changes by wobbling. The other arrangement | positioning drawing of the light-receiving surface of a photodetector is shown. The modification of the variable optical attenuator of embodiment of this invention is shown. The modification of the variable optical attenuator of embodiment of this invention is shown. The modification of the variable optical attenuator of embodiment of this invention is shown. The modification of the variable optical attenuator of embodiment of this invention is shown. The modification of the variable optical attenuator of embodiment of this invention is shown. The modification of the moving means in the variable optical attenuator of embodiment of this invention is shown.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 1st optical fiber 2 Condensing lens 3 2nd optical fiber 3a Incident end surface 3b Cladding 3c Core 4 Photodetector 4a Light-receiving surface 5 Controller 6 Actuator 7 1/4 wavelength plate 8 Beam splitter 9 Collimator lens 10 Galvano mirror 11 -16 Variable optical attenuator 20 Vertical angle variable prism

Claims (24)

A variable optical attenuator for attenuating and transmitting signal light including information,
A first optical member having a first optical transmission path through which light is emitted;
A second optical member having a second optical transmission line on which light emitted from the first optical member enters;
Moving means for moving the position of the spot formed by the light on the incident end face of the second optical member in at least a first direction;
A light detection means for detecting light incident on the light receiving surface via the incident end surface of the second optical member;
Based on the detection result of the light detection means, the position of the spot moved by the movement means moves to a position where the intensity of the light incident on the second optical transmission line becomes a preset set intensity. And a control means for performing negative feedback control on the moving means. The variable optical attenuator according to claim 1, wherein
The second optical member is an optical fiber having a clad and a core as a second optical transmission line, and a step having a predetermined dimension is formed on the core surface and the clad surface at the incident end face.
The variable optical attenuator, wherein the spot has a diameter larger than the core diameter and smaller than the clad diameter. The variable optical attenuator according to claim 2, wherein
The variable optical attenuator is characterized in that the predetermined dimension is set to a value smaller than about λ / (4n) where λ is the wavelength of the light and n is the refractive index of the medium. The variable optical attenuator according to claim 2 or 3,
The variable optical attenuator is characterized in that the predetermined dimension is set to approximately λ / (8n). The variable optical attenuator according to any one of claims 2 to 4,
The variable optical attenuator is characterized in that the incident end face has a substantially parallel relationship between the end face of the core and the end face of the clad. The variable optical attenuator according to any one of claims 1 to 5,
The light receiving surface of the light detection means has two light detection areas divided by a boundary line extending in a direction corresponding to a second direction non-parallel to the first direction,
The variable light attenuator, wherein the control means drives and controls the moving means based on outputs from the two light detection areas to move the position of the spot in the first direction. The variable optical attenuator according to claim 6,
The control means drives and controls the moving means so that an output difference between the two light detection areas substantially matches a reference output difference obtained when the light intensity is the set intensity. A variable optical attenuator. The variable optical attenuator according to claim 6,
The control means, so that the light intensity distribution generated based on the outputs from the two light detection areas substantially matches the reference light intensity distribution obtained when the light intensity is the set intensity, A variable optical attenuator characterized by drivingly controlling the moving means. The variable optical attenuator according to any one of claims 6 to 8,
The light receiving surface is positioned so that light passing through the center of the surface of the second transmission path at the incident end surface is incident near the center of the boundary line in relation to the incident end surface. Variable optical attenuator. The variable optical attenuator according to any one of claims 1 to 5,
The light receiving surface of the light detection means includes a first boundary line and a second boundary line extending in directions corresponding to the first direction and a second direction not parallel to the first direction, respectively. Has four light detection areas divided by
The control means drives and controls the moving means based on outputs from the four light detection areas to move the spot position in the first direction and the second direction. Variable optical attenuator. The variable optical attenuator according to claim 10,
The control means is a first difference which is a difference between a sum of outputs of two light detection areas on one side and a sum of outputs of two light detection areas on the other side with respect to the first boundary line. The output difference is substantially equal to the reference first output difference obtained when the light intensity is the set intensity, and the outputs of the two light detection areas on one side with respect to the second boundary line The second output difference, which is the difference between the sum of the outputs and the sum of the outputs of the two light detection areas on the other side, substantially matches the reference second output difference obtained when the light intensity is the set intensity. The variable optical attenuator is characterized in that the moving means is driven and controlled. The variable optical attenuator according to claim 10,
The control means is configured so that a light intensity distribution generated based on an output from each light detection area substantially matches a reference light intensity distribution obtained when the light intensity is the set intensity. A variable optical attenuator characterized by drivingly controlling a moving means. The variable optical attenuator according to any one of claims 10 to 12,
In the relationship between the light receiving surface and the incident end surface, light passing through the center of the surface of the second transmission path at the incident end surface is incident near the intersection of the first boundary line and the second boundary line. A variable optical attenuator characterized by being positioned on the surface. The variable optical attenuator according to claim 1, wherein
The light receiving surface of the light detection means is composed of a single light detection area,
The control means drives and controls the moving means so that an output average from the light detection area substantially matches a reference output average obtained when the intensity of the light is the set intensity. The variable optical attenuation is characterized in that the position of is periodically changed at a predetermined frequency lower than the frequency of the light transmission band in the first direction and in a second direction orthogonal to the first direction. vessel. The variable optical attenuator according to claim 14,
The second optical member has a deflection member that deflects a part of the light incident on the second optical transmission line through the incident end face to the outside of the second optical member,
The variable light attenuator, wherein the light detection means receives a part of the light deflected to the outside by the deflection member. The variable optical attenuator according to claim 15,
The second optical member is an optical fiber having a core as a second optical transmission line,
The variable optical attenuator, wherein the deflecting member is a half mirror disposed on the core. The variable optical attenuator according to any one of claims 1 to 16,
The moving means has a third optical member installed between the first optical member and the second optical member, and drives the third optical member to position the spot on the incident end face. A variable optical attenuator characterized by being moved. The variable optical attenuator according to claim 17,
The variable optical attenuator, wherein the third optical member is a condensing lens that condenses the light emitted from the first optical member on an incident end face of the second optical member. The variable optical attenuator according to claim 18,
The second optical member is configured such that the light emitted from the third optical member is retroreflected at the incident end surface,
A variable optical attenuator further comprising deflecting means for deflecting light retroreflected at the incident end face in a direction deviating from an optical path of light incident on the incident end face and guiding the light to the light quantity detecting means. The variable optical attenuator according to claim 19,
The variable optical attenuator, wherein the deflecting unit includes a polarizing beam splitter and a quarter-wave plate. The variable optical attenuator according to claim 17,
The variable optical attenuator, wherein the third optical member is a rotatable galvanometer mirror. The variable optical attenuator according to claim 17,
The variable optical attenuator, wherein the third optical member is a vertex angle variable prism capable of freely changing the vertex angle. The variable optical attenuator according to any one of claims 1 to 22,
The variable optical attenuator, wherein the first optical member is an optical fiber. The variable optical attenuator according to any one of claims 1 to 22,
The variable optical attenuator, wherein the first optical member is an optical waveguide.

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