JP6454288B2 - Electric field detector - Google Patents

Description

  The present invention relates to a position detection type electric field detection apparatus that measures electric field intensity using an electro-optic effect.

  The electric field detection device is used for measuring the electric field intensity using the electro-optic effect. The devices disclosed in Patent Document 1 and Non-Patent Document 1 include an optical crystal (electro-optic crystal) whose optical characteristics change according to changes in an external physical quantity such as an electric field, a magnetic field, and / or pressure. The light is incident on the inside, and the change in the physical quantity of the outside world is measured based on the change in the polarization state of the light.

  Further, the system disclosed in Patent Document 2 includes a polarization maintaining fiber that transmits probe light, a quarter-wave plate, an optical crystal (electro-optical crystal) having a reflecting means, a circulator, and a beam splitter. Prepare. In this system, the light passing through the polarization maintaining fiber enters the optical crystal through the quarter-wave plate, and then the return light reflected by the reflecting means passes through the same path as that at the time of incidence, It is detected via a beam splitter.

Japanese Patent No. 4875835 JP 2010-14579 A

Toko et al., “An electro-optic probe that enables more accurate electric field measurement”, NTT Technical Journal, June 2006, pp. 21-24

  The above-described apparatus and system can be configured without using a metal material, and can prevent electric field or / and magnetic field disturbance to be measured and detect electric field strength. However, there is a problem that measures against fluctuations in the outside air temperature are insufficient. Details will be described below.

  In an optical crystal (electro-optic crystal), the refractive index changes depending on the electric field applied to the optical crystal. When light is incident on this optical crystal in a specific polarization state, the birefringence of the crystal changes according to the applied electric field strength, so the polarization state of the light passing through the crystal changes. This state is schematically shown in FIG. Since the change in the polarization state due to the external electric field is extremely small, it is necessary to observe the polarization state with extremely high sensitivity. If this is not done, only the polarization state of almost the same shape can be observed.

  The following table shows the calculation results of the amount of phase change of light when passing through a crystal with wavelength λ = 1.55 μm and length L = 4000 μm from the physical constants of InP and ZnTe, which are representative materials of electro-optic crystals. It is shown.

  In the table below, for comparison, the calculation results for quartz (Silica), which is an optical fiber material having no electro-optic effect, are also shown.

In the above table, n0 = refractive index when there is no electric field, Δφ = (2π / λ) · Δn · L, Δn (E) = (1/2) · n ₀ ³ · r · E, Δn (T) = (dn / dT) · T, λ = 1.55 μm, L = 4000 μm.

In the above table, in the case of InP crystal, the coefficient of the electro-optic effect (Pockels effect) is 1.3 pm / V. Considering the case where an optical crystal is provided in an electric field having an electric field strength of 3 V / m, the phase changes by 1 × 10 ⁻⁶ [rad] compared to the case where there is no electric field. Similarly, even if a ZnTe crystal is used, the phase is about twice as large, but the number of phase digits is still the same as in the case of InP crystal and the phase is 1.9 × 10 ⁻⁶ [rad]. The two phase differences are extremely small, and even when these polarization states are viewed, it is necessary to detect the difference in polarization state corresponding to this phase change.

  On the other hand, when the outside air temperature at which the optical crystal is provided changes by 1 ° C., the phase changes 1.3 [rad] in InP and 2.6 [rad] in ZnTe. This amount is six orders of magnitude greater than when changing due to the field effect described above. Thus, the variation in phase change is considered to be greater due to the temperature dependence of the birefringence of the crystal. The refractive index has anisotropy, which at least affects the amount of change in polarization.

In addition, because the crystal length changes with temperature change due to the thermal expansion coefficient of the crystal material, the polarization state in the crystal changes. Since the linear thermal expansion coefficient of InP is about 4.60 × 10 ⁻⁶ , even if the length of a side is 4 × 10 ⁻³ m (4 mm), if it changes by 1 ° C., the length is 18 × 10 ⁻⁹ m (18 nm) Changes. Since the refractive index of InP is about 3.16, the length of 18 nm in vacuum corresponds to the length of 56 nm in the crystal. When viewed from 1.55 μm (1550 nm) light, a phase change of 0.23 [rad] occurs on the output side of the crystal.

  In other words, as discussed above, the phase change (polarization state change) due to changes in the outside air temperature is orders of magnitude higher than the phase change (polarization state change) induced by changes in the external electric field. It turns out that it becomes big.

  Moreover, although it is a polarization maintaining fiber, when the stress applied from the outside changes, the polarization state also changes. As shown in Non-Patent Document 1, the fiber is fixed using an adhesive or the like in the ferrule, but if the adhesive is unevenly attached at the time of fixing, the temperature changes. Further, the stress value of the adhesive in the non-uniform portion changes. For this reason, the polarization state of the light incident on the crystal also changes, and as a result, the input state can also change before the change in the polarization state due to the electric field fluctuation is detected.

  As a method for detecting only a change in a specific frequency under such a condition, for example, by performing lock-in detection, a change in light phase or polarization state due to a specific electric field change in an outside temperature fluctuation signal. However, as described above, the corresponding signal is extracted from an extremely part of the amount that changes due to the change in the outside air temperature.

  The present invention has been made in view of such a situation, and an object of the present invention is to provide an electric field detection device capable of performing stable measurement with high accuracy even if the temperature during measurement changes.

  In order to solve the above problems, the present invention provides an input waveguide, an output waveguide, a plurality of arm waveguides provided between the input waveguide and the output waveguide, A plurality of electro-optic crystals that are provided corresponding to the respective optical paths and receive and reflect optical signals propagating through the respective optical paths, each of the plurality of electro-optic crystals being perpendicular to the direction of application of the electric field The crystal orientation is different in advance so that an optical signal is incident on the crystal plane and the refractive index does not change with temperature.

  Here, the electric field detection device may be configured by a planar lightwave circuit, and the plurality of electro-optic crystals may be formed on the same side of the planar lightwave circuit.

  Any one of the plurality of arm waveguides may be provided with a groove having a phase difference, and the groove may be filled with a material having a negative temperature dependency coefficient of refractive index. Good.

  The present invention may further include a plurality of lenses optically coupled to each of the plurality of electro-optic crystals.

  The present invention may further include a waveguide for aligning the lens.

  According to the present invention, stable measurement can be performed with high accuracy even when the temperature during measurement changes.

It is a figure which shows schematic structure of the electric field detection apparatus produced in advance in relation to the electric field detection apparatus of embodiment. In the electric field detection device of FIG. 1, the light from the first arm waveguide is incident on the electro-optic crystal, reflected from the crystal end face, and propagated to the third arm waveguide. It is a figure for demonstrating the aspect in case the refractive index of an electro-optic crystal changes. It is a figure which shows the structural example of the electric field detection apparatus of 1st Embodiment. It is a figure which shows an example of the evaluation system used in order to evaluate the detection result of an electric field strength. It is a figure which shows the correspondence of electric field strength and signal strength. It is a figure which shows an example of the equivalent circuit of the electric field detection apparatus of 2nd Embodiment. It is a figure which shows the structural example of the electric field detection apparatus of 3rd Embodiment. It is a figure for demonstrating the positioning method of a lens in the electric field detection apparatus of 3rd Embodiment. It is a figure which shows an example of the alignment system used in order to position the lens of the electric field detection apparatus of 3rd Embodiment. It is a figure for demonstrating the alignment procedure of the lens for making the reflected light to a fiber become the maximum. It is a figure for demonstrating the operating point in which detection sensitivity becomes the maximum in the electric field detection apparatus of 4th Embodiment. It is a figure for demonstrating the structural example of the electric field detection apparatus of 4th Embodiment. It is a figure for demonstrating a transmission spectrum. It is a figure for demonstrating the change of the refractive index when it sees from each crystal plane of an electro-optic crystal.

<First Embodiment>
Hereinafter, the electric field detection device 1 according to the first embodiment of the present invention will be described.

  First, in relation to the electric field detection device 1, the configuration of the electric field detection device 100 manufactured in advance will be described with reference to FIGS. FIG. 1 is a diagram illustrating a configuration of the electric field detection device 100. FIG. 2 is a diagram for explaining how the return light that is reflected by the light incident on the electro-optic crystal of the electric field detection device 100 is emitted. FIG. 3 is a diagram for explaining an aspect when the refractive index of the electro-optic crystal changes.

  The electric field detection device 100 is configured as a Mach-Zehnder interferometer. As shown in FIG. 1, the electric field detection device 100 includes an input waveguide 11, 3 db couplers 20 a and 20 b, three arm waveguides 30, 31 and 32, and two output waveguides 51 and 52. These components 11, 20a, 20b, 30 to 32, 51, 52 are formed on a planar lightwave circuit (PLC) 101 formed on the substrate. In the example of FIG. 1, for example, a quartz material is used for the planar lightwave circuit 101.

  The coupler 20a is a two-branch coupler with one input and two outputs, and the coupler 20b is a two-input one-output coupler.

  The electro-optic crystal 41 that is optically coupled to the lens 40 enters the light from the first arm waveguide 30 via the lens 40, and the return light reflected by the end face 41a of the electro-optic crystal described later via the lens 40. Thus, the light is emitted to the third arm waveguide 32.

  The lens 40 is, for example, a cylindrical GRIN (Gradient Index Rod Lenses) lens mainly having a refractive index distribution in the radial direction, and is used to obtain optical adjustment called collimation. In FIG. 1, the lens 40 is provided because collimated light (parallel light) is incident on the electro-optic crystal 41, so that the loss is lower. However, the lens 40 is not necessarily provided (about the embodiments described later). The same).

  In the example of FIG. 1, the lenses 40 are formed to have a ¼ pitch with a diameter of 1 mm and a lens length of 2.63 mm, for example. The pitch represents the ratio of the length of the lens to one cycle of the light sine wave. The ¼ pitch is equal to ¼ of the sine wave, and collimated light can be obtained from the point light source on the lens surface.

  The electro-optic crystal 41 is a crystal having a zinc blende structure, for example. For example, this crystal is a ZnTe crystal having 100 faces on the upper surface. The electro-optic crystal 41 has, for example, a cubic structure with one side having a length of 1 mm.

In FIG. 1, the planar lightwave circuit 101 can be manufactured by the following procedure. First, a glass serving as a core layer through which light propagates is formed on a quartz substrate having a thickness of 1 mm. The core layer is set so that the refractive index is higher than the refractive index of quartz using GeO ₂ as a dopant. The thickness of the core layer is 6 μm. The refractive index of the core layer is adjusted so that the bending radius of the core layer is 2 mm.

  Next, using the standard photolithography technique and reactive ion etching, the core layer was patterned and processed so as to obtain a desired optical waveguide circuit.

  After that, the glass which becomes the over clad is deposited and made transparent using the FHD (flame-hydrolysis-deposit) method. The overcladding thickness is 20 μm.

  As shown in FIG. 2, in the electro-optic crystal 41, the return light reflected by the (01-1) plane after the light from the first arm waveguide 30 is incident on the (0-11) plane is the third. It is made to radiate | emit to the arm waveguide 32 of this. In the electro-optic crystal 41, the (0-11) plane is provided with an anti-reflection (AR) coating 41a made of a dielectric multilayer film, and the opposite (01-1) plane is highly reflective (HR: High Reflection) coat 41b is applied.

  In the electro-optic crystal 41, the refractive index changes according to the applied electric field. For example, the description will be given with reference to the example shown in FIG. When an electric field E is applied in a direction perpendicular to the (100) plane of the electro-optic crystal 41 and light d1 is incident from the (0-11) direction, the refractive index of the electro-optic crystal 41 is increased. It is known that the rate n is represented by the following formula (1).

In equation (1), n ₀ represents the refractive index when no electric field is applied, and r 41 represents the electro-optic (EO) coefficient of the crystal 41.

  In the electro-optic crystal 41 shown in FIG. 3, even if the light d2 is incident from the (0-1-1) direction instead of the (0-11) direction described above, the refraction represented by the formula (1) The only difference is the sign of the rate, and the refractive index n given according to the change in the electric field strength is the same as that in the equation (1).

  Further, in FIG. 3, if the electric field E is applied in a direction horizontal to the (100) plane, the change in the refractive index is not induced by the change in the electric field strength (001) direction, (00-1). ) Direction, (010) direction, and (0-10) direction, the electric field can be detected even if an electric field is applied from a direction from another angle. However, since efficiency is lowered, it is preferable that light is incident from a direction that is horizontal or close to the (0-11) direction or (0-1-1) direction.

  In the example of FIG. 1, the length of the electro-optic crystal 41 is L (for example, 1 mm in FIG. 1), and light incident on the electro-optic crystal 41 is reflected (that is, reciprocates through the electro-optic crystal). , The above-described refractive index changes over a length of 2L. From this, the amount of phase change in the electro-optic crystal 41 is given by the following equation (2).

  At this time, the output of the electric field detection device 100 as a Mach-Cender interferometer is given by the following equation (3).

  In Expression (3), φ0 represents an initial phase when no electric field is applied.

  As described above, the electric field detection apparatus 100 can finally detect the intensity of light according to the magnitude of the electric field intensity. However, in the electric field detection device 100 described above, when the outside air temperature changes, the phase difference between the upper and lower arms (the upper arm waveguides 30 and 32 and the lower arm waveguide 31) changes. This is because the refractive indexes of the arm waveguides 30 to 32 and the electro-optic crystal 41 made of a glass material vary depending on temperature dependency and expansion coefficient, and the optical path length also varies accordingly.

In addition, as described above, the degree of fluctuation varies in such a way that the number of phase digits is 6 digits higher than the phase change due to the electric field strength to be detected. For this reason, a detection component due to a weak electric field fluctuation is detected among fluctuations of a large noise component. Also, since the initial phase φ ₀ also changes with temperature, it is difficult to determine whether the change in light intensity is due to a change in electric field strength or a change in temperature. Therefore, it becomes difficult to measure with high accuracy, and stable measurement cannot be performed.

  However, the electric field detection device 1 of the present embodiment, which will be described later, is configured to perform stable measurement with high accuracy even when the outside air temperature changes.

  Hereinafter, the electric field detection device 1 of the first embodiment will be described. FIG. 4 is a diagram illustrating a configuration example of the electric field detection device 1.

  Unlike the electric field detection device 100 shown in FIG. 1, the electric field detection device 1 has electro-optic crystals 41 and 43 on the upper arm waveguides 30 and 32 side and the lower arm waveguides 31 and 33 side, respectively. Is provided. Other configurations are almost the same as those shown in FIG. In the following description of the present embodiment, the reference numerals and the like used in the description of the electric field detection device 100 shown in FIG.

  As shown in FIG. 4, the electric field detection device 1 includes an input waveguide 11, 3db couplers 20a and 20b, first, second, third, and fourth arm waveguides 30, 31, 32, and 33. Output waveguides 51 and 52. These components 11, 20a, 20b, 30 to 33, 51, 52 are formed on a planar lightwave circuit (PLC) 101 formed on a substrate.

  The total length of the upper arm waveguides 30 and 32 (hereinafter also referred to as “upper arm”) is equal to the total length of the lower arm waveguides 31 and 33 (hereinafter also referred to as “lower arm”). Is set to

  The electric field detection device 1 according to the present embodiment includes the electro-optic crystal 41 as in the case illustrated in FIG. 1, but also includes the electro-optic crystal 43 in addition to the electro-optic crystal 41. In this embodiment, the electro-optic crystals 41 and 43 are crystals having, for example, a zinc blende structure. For example, this crystal is a ZnTe crystal having a (100) plane on the top surface. The electro-optic crystals 41 and 43 have, for example, a cubic structure with one side having a length of 1 mm.

  The electro-optic crystals 41 and 43 are rotated 90 degrees around the vertical axis (the vector direction of the electric field E in FIG. 3) so that the optical path lengths are the same at the time of polishing. , 43 are polished at the same time so that the electro-optic crystals 41, 43 have the same length.

  In the electro-optic crystal 41, the light from the first arm waveguide 30 enters through the lens 40, and the return light reflected by the end face 41a of the electro-optic crystal passes through the lens 40 to the third arm waveguide 32. To exit. For example, in the electro-optic crystal 41, an electric field E is applied in a direction perpendicular to the (100) plane, and light enters from the (0-11) direction.

  In the electro-optic crystal 43, the light from the second arm waveguide 31 enters through the lens 42, and the return light reflected by the end face 43a of the electro-optic crystal passes through the lens 42 and passes through the fourth arm guide. The light is emitted to the waveguide 33. For example, in the electro-optic crystal 43, an electric field E is applied in a direction perpendicular to the (100) plane, and light enters from the (0-1-1) direction. The lens 42 is also a GRIN lens, for example, and is used to obtain optical adjustment called collimation.

  The electro-optic crystal 43 has a non-reflective (AR) coat 43a made of a dielectric multilayer film on the (0-1-1) plane, and a high-reflective (HR) coat 43b on the (011) plane. Yes.

  In the electric field detection device 1 of the present embodiment, the lengths of the upper arm waveguides 30 and 32 are equal to the lengths of the lower arm waveguides 31 and 33. Further, the length (thickness) along the light propagation direction in the lens 40 and the electro-optic crystal 41 is equal to the length (thickness) along the light propagation direction in the lens 42 and the electro-optic crystal 43. . For this reason, even if the outside air temperature changes and the refractive index of the arm waveguides 30 to 32 and the refractive index of the electro-optic crystals 41 and 43 change due to the temperature dependence and the expansion coefficient, the upper arm waveguide 30. , 32 and the lower arm waveguides 31, 33 have the same amount of change, and therefore no phase difference due to a change in outside air temperature occurs. That is, the electric field detection device 1 of the present embodiment can cancel the phase change due to the temperature change.

  On the other hand, as described above, the two electro-optic crystals 41 and 43 are arranged in a state in which the directions of the two electro-optic crystals 41 and 43 are rotated by 90 degrees. The amount of phase fluctuation when applied is given by the following equation (4).

  The amount of phase fluctuation shown in Expression (4) is twice the value shown in Expression (2) above. That is, in the electric field detection device 1 of the present embodiment, the detection sensitivity can be doubled compared to that shown in FIG.

  Therefore, in the electric field detection device 1 of the present embodiment, in addition to canceling the phase change due to the temperature change, the sensitivity can be improved.

[Evaluation system]
FIG. 5 illustrates an evaluation system 2 that is used to evaluate the detection result of the electric field detection device 1.

  In FIG. 5, the evaluation system 2 uses, for example, a C-band tunable laser as the light source 60. Light propagating from the light source 60 through the polarization maintaining fiber 61 is incident on the electric field detection device 1 in a TE polarized state. In the example of FIG. 5, the electric field detection device 1 is installed in an electric field application device 2 having two flat plates 81a and 81b. The electric field applying device 2 includes a voltage source 70 that applies a voltage to the flat plates 81a and 81b, and a function generator 71 that generates a synchronization signal s.

  The outputs of the two output ports (output waveguides 51 and 52) of the electric field detection device 1 are input to the EO conversion device 63 via the two-core single mode fiber 62. In the EO conversion device 63, differential reception of the signal intensity (light intensity) from the above-described output ports (output waveguides 51 and 52) is performed and detected by a lock-in amplifier 64 that amplifies them. This detection result is shown in FIG.

  FIG. 6A shows a detection result S1 of the electric field intensity applied to the electric field detection apparatus 1 by the electric field application apparatus described above and the amplitude of the signal intensity (light intensity). FIG. 6A also shows the detection result S2 of the electric field detection device 100 shown in FIG. 1 for comparison. From FIG. 6A, it was found that the signal intensity of the detection result S1 increased at a rate about twice that of the detection result S2. That is, the electric field detection device 1 of the present embodiment has a detection sensitivity that is about twice that shown in FIG.

  FIG. 6B shows a detection result S3 indicating the temporal stability of the signal intensity. FIG. 6B also shows a detection result S4 indicating temporal stability in the electric field detection device 100 shown in FIG. 1 for comparison. Looking at the detection result S3, the signal intensity was almost constant and stable. On the other hand, in the detection result S4, the signal intensity changed slightly. This is due to fluctuations in the temperature of the atmosphere, and it is considered that the operating point fluctuated due to temperature changes and the signal intensity was not stable.

  As described above, the electric field detection device 1 according to this embodiment includes a plurality of input waveguides 11, output waveguides 51 and 52, and a plurality of input waveguides 11 and output waveguides 51 and 52. Arm waveguides 30 to 33, and a plurality of electro-optic crystals 41 and 43 that are provided corresponding to the optical paths of the arm waveguides 30 to 33 and receive and reflect an optical signal propagating through each optical path. Here, the electro-optic crystals 41 and 43 are arranged in advance so that the directions of the crystals are different so that the refractive index does not change with temperature. Thereby, even if the temperature at the time of measurement changes, stable measurement can be performed with high accuracy.

Second Embodiment
Hereinafter, an electric field detection apparatus 1A according to a second embodiment of the present invention will be described. The configuration of the electric field detection device 1A is almost the same as that shown in FIG. In the following description of the present embodiment, the symbols and the like used in the description of the first embodiment are used as they are unless otherwise specified.

  FIG. 7 is a schematic diagram illustrating an example of the electric field detection device 1A of the present embodiment.

  This electric field detection device 1A is characterized in that the electro-optic crystals 41 and 43 of the electric field detection device 1 of the first embodiment are arranged on the same surface of the PLC 101.

  Other configurations are the same as those of the electric field detection device 1 of the first embodiment. That is, the electric field detection device 1A includes an input waveguide 11, 3db couplers 20a and 20b, four arm waveguides 30, 31, 32, and 33, and two output waveguides 51 and 52. These components 11, 20a, 20b, 30 to 33, 51, 52 are formed on the PLC 101 formed on the substrate.

  In the electro-optic crystal 41, light from the first arm waveguide 30 enters through the lens 40, and return light reflected by the end face 41a of the electro-optic crystal passes through the lens 40 to the third arm guide. The light is emitted to the waveguide 32. For example, in the electro-optic crystal 41, an electric field E is applied in a direction perpendicular to the (100) plane, and light enters from the (0-11) direction. In the electro-optic crystal 43, light from the second arm waveguide 31 enters through the lens 42, and return light reflected by the end face 43 a of the electro-optic crystal passes through the lens 42 and passes through the fourth arm waveguide 33. To exit. For example, in the electro-optic crystal 43, an electric field E is applied in a direction perpendicular to the (100) plane, and light enters from the (0-1-1) direction.

  The electric field detection device 1A can be manufactured by the same manufacturing method as the electric field detection device 1 of the first embodiment.

  In this electric field detection device 1A, the same detection results (FIGS. 6A and 6B) as those in the first embodiment were obtained. Therefore, also in the electric field detection device 1A, the detection sensitivity can be increased and the temporal stability of the signal intensity can be improved.

  In the electric field detection device 1A, the electro-optic crystals 41 and 43 are arranged on the same surface of the PLC 101. Therefore, it is only necessary to connect the fiber to the chip one side of the PLC 101. That is, the chip area is reduced.

  On the other hand, in the electric field detection device 1 of the first embodiment, since the electro-optic crystals 41 and 43 are arranged on both sides of the PLC 101, it is necessary to connect the fiber to both sides of the chip of the PLC 101. That is, the chip area increases. This increases the module size of the electric field detection device 1. In addition, the cost of members increases and the assembly cost increases.

  Therefore, the electric field detection device 1A of the present embodiment can be downsized compared to the electric field detection device 1 of the first embodiment. Moreover, the cost of a member and an assembly cost can be reduced.

  Furthermore, in the electric field detection device 1A of the present embodiment, the distance between the electro-optic crystals 41 and 43 is shorter than that of the electric field detection device 1 of the first embodiment, so that the spatial resolution can be improved. Spatial resolution corresponds to the distance between two electro-optic crystals.

<Third Embodiment>
Hereinafter, the electric field detection apparatus 1B which is 3rd Embodiment of this invention is demonstrated. The configuration of the electric field detection device 1B is almost the same as that shown in FIG. In the following description of the present embodiment, the symbols and the like used in the description of the first embodiment are used as they are unless otherwise specified.

  FIG. 8 is a schematic diagram showing an example of the electric field detection device 1B of the present embodiment.

  This electric field detection device 1B is different from the electric field detection device 1 of the first embodiment in that each of the alignment waveguides 56 and 57 for aligning the lenses 40 and 42 of the first embodiment is provided.

  Other configurations are the same as those of the electric field detection device 1 of the first embodiment. That is, the electric field detection device 1B includes an input waveguide 11, 3db couplers 20a and 20b, arm waveguides 30, 31, 32, and 33, and output waveguides 51 and 52. These components 11, 20a, 20b, 30 to 33, 51, 52 are formed on the PLC 101 formed on the substrate.

  In the electro-optic crystal 41, light from the first arm waveguide 30 enters through the lens 40, and return light reflected by the end face 41b of the electro-optic crystal passes through the lens 40 to the third arm guide. The light is emitted to the waveguide 32. For example, in the electro-optic crystal 41, an electric field E is applied in a direction perpendicular to the (100) plane, and light enters from the (0-11) direction. In the electro-optic crystal 43, light from the second arm waveguide 31 enters through the lens 42, and return light reflected by the end face 43 a of the electro-optic crystal passes through the lens 42 and passes through the fourth arm waveguide 33. To exit. For example, in the electro-optic crystal 43, an electric field E is applied in a direction perpendicular to the (100) plane, and light enters from the (0-1-1) direction.

  In the electric field detection device 1B of the present embodiment, as shown by the solid circle in FIG. 8, the two waveguides 32 and 56 intersect and the two waveguides 33 and 57 intersect. Therefore, generally, a cross loss occurs at a corresponding location. However, in the electric field detection device 1B according to the present embodiment, the layout of the electric field detection device 1B using the alignment waveguides 56 and 57 as interference paths, They are crossed so as to have a symmetrical loss. Thereby, the unbalance due to the crossing loss is eliminated, and a large extinction ratio can be obtained.

  In the interferometer, the extinction ratio can be deteriorated due to the loss imbalance between the two arm waveguides, so that a dummy intersection is provided so as to make the loss the same in the two arm waveguides. Is preferred.

  Next, an alignment method of the lenses 40 and 42 will be described with reference to FIG.

  FIG. 9 shows a case where the optical signal b1 from the alignment waveguide 57 is incident from the position of the lens central axis o. In the example shown in FIG. 9, the return light b2 reflected and returned by the end face 41b of the electro-optic crystal returns to the same position of the central axis o as that at the time of incidence.

  In FIG. 9, d1 and d2 are set as 100 μm, but d1 and d2 do not need to be the same value. Moreover, d1 and d2 should just be smaller than the radius of the lenses 40 and 42, and can be set to values other than 100 micrometers.

  The lenses 40 and 42 are aligned using the lens characteristics described above. Hereinafter, this alignment method will be described with reference to FIG.

  FIG. 10A illustrates a method for maximizing the intensity of the return light. At this time, a fiber is connected to a waveguide made of quartz, and light is introduced into the lenses 40 and 42. The lenses 40 and 42 are fixed to the fine movement table on the end face of the PLC 101, and alignment is performed. In this case, the lenses 40 and 42 are positioned so that the intensity of the reflected light d2 to the fiber is maximized.

  FIG. 10B illustrates the method for arranging the arm waveguides 30 to 33. For example, when light enters the lens 40 from the first arm waveguide 30 as an input waveguide provided at a position away from the center o by d1, as described above, the lens 40 is symmetrical with the center position as the origin. Has the property of condensing light. Therefore, the third arm waveguide 32 as the output waveguide may be provided at the position where the light is condensed. At this time, d1 = d2. The arm waveguides 31 and 33 are also arranged in the same manner.

  However, it may be difficult to obtain a coupled state of the two waveguides 30 and 32 via the lens 40. Therefore, in addition to the two waveguides 30 and 32, an alignment waveguide is provided, and light is propagated to the alignment waveguide, and the lens 40 is aligned by the method described with reference to FIG. To fix.

  FIG. 10C illustrates the method in the case of using the waveguide 2 for alignment. In this case, if the waveguide 2 is designed at a position where d1 = d2 due to the characteristics of the lens 40, optical coupling between the waveguide 30 and the waveguide 32 is obtained when the position of the center o is determined. It becomes possible.

  Note that the relationship between d1 and d2 is not necessarily d1 = d2. For example, when the electro-optic crystal 41 connected to the lens 40 is fixed at a specific angle θ shown in FIG. 10D, for example, d2 is a function of θ. This relationship can determine d2 according to the angle attached by arithmetic such as ray tracing. In this case, a decrease in detection sensitivity due to light reflection at the lens end and at both ends of the electro-optic crystal 41 can be suppressed. Thereby, detection with higher sensitivity can be realized.

  Also in the case of FIG. 10D, the alignment waveguide 2 shown in FIG. 10C is prepared, light is incident on this, and the lens 40 is aligned by the method described in FIG. 10A. Then, the light input from the position d1 shown in FIG. 10D is focused on the position d2 determined by θ. Therefore, optical coupling between the waveguides 30 and 32 can be realized. For the arm waveguides 31 and 33, optical coupling can be realized by the same method.

  FIG. 11 shows a system configuration used for aligning the lens 40.

  This system includes a power meter 86, a laser 87, and a circulator 88. At the time of alignment, the PLC 101 is installed in the aligner. Then, a laser beam of 1.55 μm is made incident on the aligning waveguide 57 via the single mode fiber by the laser 87, and after aligning the waveguide 57, the light is made incident on the PLC 101.

  Thereafter, active alignment of the lens 40 is performed with a jig using a stage that operates in three axes. At this time, the circulator 88 is connected to the input fiber, and the lens 40 is moved so that the intensity of the return light d2 when the light d1 is incident from the laser (LS: Laser) 87 is maximized. The intensity of the return light d2 is detected by the power meter 86. The lens 40 is fixed at the position of the lens 40 when the intensity of the return light reaches the maximum. As a fixing method, for example, a UV adhesive is applied and UV irradiation is used. The lens 42 is similarly fixed.

  After alignment using the alignment waveguide 57, light was incident from the arm waveguide 30 side in the electric field detection device 1B of the present embodiment, and the increase in loss due to the insertion of the lens 40 was calculated. As a result, the insertion loss per location was about 1 dB.

  According to the electric field detection device 1B of the present embodiment, the positioning of the lenses 40 and 42 described above can be realized by having the waveguides 56 and 57 for alignment. Further, the assembly throughput can be improved and the cost can be reduced.

<Fourth embodiment>
Hereinafter, an electric field detection device 1C according to a fourth embodiment of the present invention will be described.

  In the above-described first to third embodiments, the case where the electric field detection device is an isometric Mach-Cender interferometer has been exemplified. However, a phase difference may be provided between the interferometer lines composed of arm waveguides.

  First, an operating point of the electric field detection device will be described in order to realize the electric field detection device 1C.

  FIGS. 12A and 12B are diagrams for explaining such operating points. FIG. 12A illustrates an ideal case, and FIG. 12B illustrates the other cases. 12A and 12B, the horizontal axis indicates the electric field intensity, and the vertical axis indicates the light output.

  Explaining with the example shown in FIG. 12A, an ideal operating point is the initial of two outputs (solid line and broken line) of the electric field detection device 1C when no electric field is applied (when E = 0). Both phases are at π / 4.

  In FIG. 12A, even if the electric field intensity changes slightly, it is preferable that the point where the amount of change in the light intensity is the maximum is the operating point.

  For example, as shown in FIG. 12B, when the operating point is set, even if the electric field intensity changes, the amount of change in the light intensity decreases, and the detection sensitivity of the phase change deteriorates. That is, in the electric field detection device 1C, in order to obtain the maximum sensitivity, it is necessary to set the operating point when the initial phase is π / 4.

  Next, consider the case where the length L of the electro-optic crystals 41 and 43 and the design value are slightly shifted in the electric field detection devices of the first to third embodiments described above. This may occur due to polishing errors of the electro-optic crystals 41 and 43. If two electro-optic crystals are polished at the same time or the like, the polishing thickness of each electro-optic crystal can be made uniform. For example, one of the two electro-optic crystals 41 and 43 may be polished in another batch. Below, the influence by the difference in the thickness of such an electro-optic crystal is considered.

  In this case, for example, InP is used as the electro-optic crystal, and the refractive index of InP is set to about 3.16. For example, when the length of the electro-optic crystal is shifted by 0.123 μm from the design value for a wavelength of 1.55 μm, this shift is a length corresponding to the wavelength of λ / 4 in the electro-optic crystal having a refractive index of 3.16. It will be. Considering that light propagates back and forth in the electro-optic crystal, the above deviation eventually becomes a length of λ / 2.

  In other words, when the electro-optic crystal length of several millimeters deviates from the design value by 0.123 μm, it means that the output of the desired port is turned ON due to a manufacturing error even though the output of the electric field detection device is OFF. .

  In general, the interference state of the electric field detection device does not become as designed due to an error in the thickness of the electro-optic crystal, the thickness of the adhesive for fixing the electro-optic crystal, or the like. Rather, in an electric field detection device, it is almost impossible to perform an isometric operation as designed, and it is difficult to set an operating point as designed.

  From the above viewpoint, in the electric field detection device 1C of the present embodiment, a phase difference is provided in advance in the interferometer.

  FIG. 13 is a diagram illustrating a configuration example of the electric field detection device 1C. The configuration of the electric field detection device 1C is almost the same as that shown in FIG. In the following description of the present embodiment, the symbols used in the description of the second embodiment are used as they are unless otherwise specified.

  In FIG. 13, as in the second embodiment, the electric field detection device 1C includes an input waveguide 11, 3db couplers 20a and 20b, arm waveguides 30, 31, 32, and 33, output waveguides 51 and 52, and Is provided. These components 11, 20a, 20b, 30 to 33, 51, 52 are formed on the PLC 101 formed on the substrate.

  In the electro-optic crystal 41, light from the first arm waveguide 30 enters through the lens 40, and return light reflected by the end face 41a of the electro-optic crystal passes through the lens 40 to the third arm guide. The light is emitted to the waveguide 32. For example, in the electro-optic crystal 41, an electric field E is applied in a direction perpendicular to the (100) plane, and light enters from the (0-11) direction. In the electro-optic crystal 43, light from the second arm waveguide 31 enters through the lens 42, and return light reflected by the end face 43 a of the electro-optic crystal passes through the lens 42 and passes through the fourth arm waveguide 33. To exit. For example, in the electro-optic crystal 43, an electric field E is applied in a direction perpendicular to the (100) plane, and light enters from the (0-1-1) direction.

  On the other hand, unlike the second embodiment, in the electric field detection device 1C, the arm waveguide 30 has a plurality of grooves 301 (for example, ten grooves 301). The groove 301 is formed in order to realize athermalization (temperature independence). The width of the groove 301 is, for example, 12.5 μm. The groove 301 is filled with a resin (a resin whose main component is silicone).

  In the electric field detection device 1C of the present embodiment, when the temperature characteristics were evaluated, the spectrum did not change according to the temperature. In this respect, according to the electric field detection device 1C, athermalization is realized.

  Note that methods for athermalizing the Mach-Cender interferometer (temperature independence) are disclosed in International Publications WO2010 / 079761 and JP2012-014151A. In these examples, assuming that N division grooves are formed and the length of the i-th division groove is dLi, dLi is designed so as not to fluctuate due to temperature. That is, the temperature dependence of the refractive index of the material filled in the dividing groove satisfies the following formula (6).

  Hereinafter, the configuration of the above-described groove 301 will be described in detail.

  The wavelength in the 1.55 μm band corresponds to a spectral interval (FSR: Free Spectral Range) of transmitted light of 1.6 nm. In the present embodiment, a C-band tunable laser used for communication is used as the light source 60 (FIG. 5), and therefore, about 19 peaks are included in 30 nm of the C-band. .

  The optical path length difference is about 500 μm in a buried waveguide made of glass. FIG. 14A illustrates ideal spectral characteristics.

  In the state shown in FIG. 14A, for example, if two electro-optic crystals having slightly different lengths are provided in the interference path, the FSR will be shifted (in some cases, widened or narrowed). It is possible to observe how the light intensity changes from maximum to minimum.

  In this case, by selecting the wavelength of the probe light as the operating point, the initial phase can be set at the position of π / 4 shown in FIG. In this case, the output of the electric field detection device is as shown by output 1 in FIG.

On the other hand, when the phase difference (wavelengths λ ₀ , λ ₁ ) as shown in FIG. 14C is generated when the two interference paths are designed to have the same length without providing the phase difference by the groove 301 described above, The output of the electric field detection device is as shown by output 2 in FIG. That is, the output 2 of the electric field detection device cannot be set to the 3 dB point even if the operating wavelength is changed. That is, even if a wavelength tunable laser is used as the light source 60, an operating point may not be obtained in this variable range.

  As described above, by setting the phase difference in the interference path in advance, even if the operating point fluctuates due to errors during mounting or polishing of the electro-optic crystal, the wavelength of the probe light used by the electric field detector is changed. By changing, the operating point can be adjusted.

  However, by providing the above-described phase difference, if the temperature changes between the two interference paths, the phase difference between the interference paths changes. Here, if the difference in length between the two interference paths is ΔL and the wavelength is λ, the phase difference Δφ generated by ΔL when the temperature fluctuates by ΔT is expressed by the following equation (5).

  In the above formula (5), dn / dt represents a coefficient indicating the temperature dependence of the refractive index of the glass. When the phase difference Δφ shown in Expression (5) occurs, the spectrum shown in FIG. 14A moves in the wavelength axis direction, and as a result, the operating point varies.

  Therefore, in the electric field detection device 1C of the present embodiment, dn / dt (about the refractive index of the glass shown in the equation (5) is formed on the longer one of the two arm waveguides 30 and 32. A plurality of grooves 301 filled with a material having a negative refractive index temperature dependency (dnr / dt) having a sign different from that of the coefficient indicating temperature dependency is formed. In the example of FIG. 13, each groove 301 is formed so as to divide the core of the arm waveguide 30.

  In the present embodiment, the case where the grooves 301 shown in FIG. 13 are all the same size has been described, but the width of the groove 301 is set to be different for each of the grooves 301 if the above expression (5) is satisfied. You can also. As for the material filled in the groove 301, other materials can be used as long as they have the above-described temperature dependence (dnr / dt) of the negative refractive index.

  In the electric field detection device 1C of the present embodiment, since the groove 301 is provided in the arm waveguide 30, the operating point can be provided in the operating range of the light source 60 (for example, a tunable laser). Furthermore, since the groove 301 is filled with a material (resin) for temperature compensation, athermalization (temperature independence) can be realized. Therefore, in the electric field detection device 1C, even if the temperature changes, it is possible to realize a state where there is no spectrum fluctuation as shown in FIG. Therefore, the electric field can be detected stably even with respect to temperature changes. In addition, the electric field can be detected with high accuracy.

  In each of the embodiments described above, the electro-optic crystals 41 and 43 are not limited to the examples described in the embodiments as long as the crystals are arranged in different directions so that the refractive index does not change with temperature. For example, the electro-optic crystals 41 and 43 emit light in the crystal plane directions (001), (00-1), (010), or (0-10), or their isotopic directions, in which the refractive index does not change with temperature. It is also possible to arrange so that does not propagate. This will be described with reference to FIG. FIG. 15A illustrates a state when the electric field E is applied in a direction horizontal to the (100) plane. In this example, the case where light enters the (0-11) plane and the case where light enters the (0-1-1) plane are shown.

  For example, when light is incident on the above-described (0-11) plane (that is, the y direction shown in FIG. 15A), the light is viewed from the (0-11) plane direction as shown in FIG. 15B. The refractive index at the time changes as e1 → e2. In FIG. 15B, e1: a refractive index before application of the electric field E and e2: a refractive index before application of the electric field E are shown.

  On the other hand, for example, when light is incident on the (0-1-1) plane (that is, the x direction shown in FIG. 15A), as shown in FIG. 15C, (0-1- 1) The refractive index when viewed from the plane direction changes as e3 → e4. In FIG. 15C, e3: a refractive index before application of the electric field E and e4: a refractive index before application of the electric field E are shown.

  FIG. 15D shows the state of the refractive index that changes according to the light propagation direction k when the electric field E shown in FIG. 15A is applied to the electro-optic crystal.

  In FIG. 15D, as shown in FIG. 15B, when the light propagation direction k is the y direction, the sign of the change in the refractive index is positive, which is larger than before the application of the electric field (e1 → e2). On the other hand, as shown in FIG. 15C, when the light propagation direction k is the x direction, the sign of the change in the refractive index is negative, which is smaller than that before application of the electric field (e3 → e4).

  As described above, the refractive index changes as the light propagation direction k changes, but the light propagation direction k can be considered in addition to the examples shown in FIGS. 15B and 15C. For example, even if the light propagation direction k is not parallel to the x or y direction, the refractive index changes before and after applying the electric field, even if the maximum amount of change in refractive index cannot be obtained. In this case, the refractive index increases or decreases except when the light propagation direction k is a certain direction (a direction connecting the points OI) described later. O represents the origin, and I represents the intersection of the refractive index before application of the electric field (solid line in FIG. 4D) and the refractive index after application of the electric field (broken line in FIG. 4D).

  In view of the change in the refractive index, in each embodiment, light is propagated in a direction in which the change in the refractive index is positive (a direction in which it is negative) with respect to at least one of the two arm waveguides forming the interferometer. In addition, by propagating light in a direction in which the refractive index change is opposite to the above direction with respect to the other, the two electro-optic crystals are arranged so that the refractive index change becomes zero as a whole. That's fine.

1, 1A, 1B, 1C Electric field detection device 2 Electric field application device 11 Input waveguide 20a, 20b Coupler 30, 31, 32, 33 Arm waveguide 40, 42 Lens 41, 43 Electro-optic crystal 51, 52 Output waveguide 56, 57 Waveguide for alignment

Claims (5)

An input waveguide;
An output waveguide;
A plurality of arm waveguides provided between the input waveguide and the output waveguide;
A plurality of electro-optic crystals that are provided corresponding to the respective optical paths of the arm waveguides and that receive and reflect an optical signal propagating through the optical paths;
Each of the plurality of electro-optic crystals is arranged in advance so that an optical signal is incident on a crystal plane perpendicular to the direction of application of an electric field and the orientation of the crystals is different so that the refractive index does not change with temperature An electric field detection device characterized by that. The electric field detection device is composed of a planar lightwave circuit,
The electric field detection device according to claim 1, wherein the plurality of electro-optic crystals are formed on the same side of the planar lightwave circuit.   Any one of the plurality of arm waveguides is provided with a groove having a phase difference, and the groove is filled with a material having a negative coefficient of temperature dependency of a refractive index. The electric field detection device according to claim 1 or 2.   The electric field detection device according to claim 1, further comprising a plurality of lenses optically coupled to each of the plurality of electro-optic crystals.   The electric field detection device according to claim 4, further comprising a waveguide for aligning the lens.

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