JP7133307B2 - Performance evaluation device for optical deflection element - Google Patents

Description

The present invention relates to a performance evaluation device for optical deflection elements.

As an element that deflects light, MEMS (Micro Electro Mechanical Systems) based on a mechanical operating principle has already been developed. On the other hand, an optical phased array is rapidly gaining attention as a deflecting element that performs an electrically deflecting operation without a mechanical operation (see, for example, Patent Document 1).

Unlike conventional mechanical deflecting elements, the optical phased array can freely converge and diverge light in addition to deflecting the generated light beams. On the other hand, in order to deflect, converge, and diverge the light, a phase shifter is mounted for each individual channel optical waveguide (0ptical channel waveguide, abbreviated as channel or optical waveguide and waveguide ) of the optical phased array. , feedback control must be performed by performing phase correction (see Patent Document 2, for example).

Furthermore, in order to perform phase control of the optical phased array, it is necessary to perform electrical phase control of light for each channel constituting the optical phased array. Means for electrically modulating the phase of light include liquid crystal, thermo-optic effect, and electro-optic (EO) effect. Among them, the EO effect is suitable as a phase shifter for an optical phased array because it has an ultra-high-speed response of 10 GHz or more.

On the other hand, there is an electro-optic constant as a performance index of devices using the EO effect. For example, in the case of an EO polymer as a material that exhibits the EO effect, the electro-optic constant (EO constant: Pocketless constant) r, which indicates the magnitude of the change in the refractive index n due to the electro-optic effect, is given by the following equation ( 1).

また、電場Eを印加したときの屈折率n(E)は、さらに次式（２）で近似することができる。

Further, the refractive index n(E) when the electric field E is applied can be further approximated by the following equation (2).

電気光学定数ｒは、通常ｒ₁₃、ｒ₃₃などの表記などテンソル量として記述される。

The electro-optic constant r is usually written as a tensor quantity, such as the notation r ₁₃ , r ₃₃ .

Conventional methods for measuring the EO constant include the phase detection method, which detects the phase difference between two optical paths, such as the Michelson interferometry and the Mach-Zehnder interferometry, and the Senarmont method. It is broadly classified into a phase difference detection method that detects the phase difference due to an applied electric field between two linearly polarized light beams that are orthogonal to each other like the method (see, for example, Patent Document 3).

In the Senarmont method, a retarder such as a Babinet-Soleil compensator or a quarter-wave plate is used to compensate for polarization while detecting the phase difference. The electro-optic constant, which is the figure of merit of the EO effect, is calculated from the voltage characteristics (see, for example, Patent Document 3).

Phase difference evaluation by the Senarmont method can also be considered as a kind of ellipsometry method. In particular, the Teng-Man method, developed by C.C. Teng and H.T. Man, is a method for measuring electro-optical constants using reflection ellipsometry, which is useful for the preparation of the sample to be measured and the simple measurement method. (see, for example, Non-Patent Document 1).

In this method, a sample of glass substrate/transparent electrode/EO material/metal electrode is prepared, and a laser beam polarized at 45° is incident on the sample from the glass substrate side. Here, the EO material is assumed to be a material having an electro-optical effect, such as EO polymer. At that time, in order to eliminate the influence of light reflection from the glass substrate surface that does not include the signal from the inside of the sample, an aperture is used to spatially block the reflected light from the glass substrate surface. , which improves the measurement accuracy.

On the other hand, Yamada et al.'s group recently developed an electro-optical constant measurement method that uses transmitted light through the sample instead of the reflection type by the Teng-Man method and has high measurement accuracy without providing an aperture. (For example, see Non-Patent Document 2).

This transmissive method is characterized by not having an aperture and is not affected by multiple reflections between the glass substrate and the sample or multiple reflections inside the sample. It is possible to calculate EO constants with accuracy.

These calculation methods are effective means for measuring the material-specific performance of EO materials. has been used to determine the EO constant for a device called Although these slab-type and bulk-type optical modulation devices have been put to practical use, in recent years, many devices with an optical waveguide structure that can be expected to operate at high speed, especially those with an optical waveguide core diameter of about 2 μm, have been developed. has become

The simplest optical modulation element of the optical waveguide type is the Mach-Zehnder type element, which has a basic structure of a device composed of two optical waveguides. As typified by optical modulators using optical interference between devices, the electro-optical constants can be obtained directly from the output characteristics of the optical output using the interference effect. In other words, since the phase difference component between the two waveguides is directly reflected in the optical output intensity, the half-wave voltage (minimum drive voltage that gives a phase difference of π) Vπ By obtaining , the electro-optical constants can be obtained easily and with high accuracy.

On the other hand, devices using multi-optical waveguides having two or more optical waveguides have recently been developed. This technology is attracting attention for its high controllability of the output light beam, such as the ability to control not only the deflection direction of the light output beam but also the shape of the light beam, as typified by, for example, an optical phased array.

In the case of an optical deflection element using multiple optical waveguides, the optical output characteristic is different from the simple interference effect between two waveguides, and a combined beam of light propagated through multiple optical waveguides is output. Therefore, it is required to obtain the initial phase of each individual waveguide and the electro-optical constant of the phase shifter (EO element) embedded in each waveguide. Until now, a method to calculate the initial phase and electro-optic constant of each optical waveguide by measuring the output shape of the synthesized beam of multiple optical waveguides while controlling the applied voltage to each optical waveguide individually. is proposed. According to this technique, while monitoring the shape of the combined beam output from multiple optical waveguides, the phases of the individual optical waveguides are controlled so that the main beam (main lobe) can be efficiently formed through total optimization. It is said that feedback control can be established.

Japanese Patent Publication No. 2016-508235 Japanese translation of PCT publication No. 2015-509207 JP-A-1-074434

C. C. Teng and H. T. Man; Simple reflection technique for measuring the electro-optic coefficient of poled polymers Appl. Phys. Lett. 56, 1734 (1990) Toshiki Yamada, and Akira Otomo. Transmission ellipsometric method without an aperture for simple and reliable evaluation of electro-optic properties under various ambient conditions Optics Express Vol. 21, No. 24, 29240.

Many optical phased arrays using multiple optical waveguides with a core diameter of the optical waveguide of about 2 μm have been developed. The conventional method of calculating the electro-optic constant is based on the evaluation of the combined beam of light output from the output ends of the multi-optical waveguides. The output of the optical region is also based on the premise that the light beam is in the Fraunhofer region where the combined light beam can be observed. Bulk type phase modulation elements and Mach-Zehnder type elements detect the light beam after phase modulation, and calculate the electro-optic constant from the output characteristics.

Also, regarding the formation of light beams from multiple optical waveguides, the evaluation method of electro-optic constants reported so far is to control the phase of individual optical waveguides while monitoring the shape of the combined beam output from multiple optical waveguides. By doing so, feedback control is applied such that a main beam (main lobe) is efficiently formed by overall optimization. Therefore, as the number of optical waveguides increases, the number of control parameters that must be individually controlled increases dramatically.

Specifically, the light intensity at the coordinates (x, y) on the monitor of the light beam shape analyzer is measured, and in the case of N multi-waveguides, the n-th (1 ≤ n ≤ N) optical waveguide While changing the applied voltage from the minimum applied voltage Vmin to the maximum applied voltage Vmax, the light intensity at the coordinates (x, y) of the monitor is monitored. At that time, by changing the voltage applied to each optical waveguide, it is optimized so that the desired main beam (main lobe) is formed by phase matching, and the individual optical waveguides are adjusted until the error from the design value is minimized. It is necessary to apply feedback control for the applied voltage many times.

Another problem is that it is essential to measure (understand) the initial phase of the multi-waveguide. Compared to the number of input channels of two or less in the bulk-type phase-modulating element and the Mach-Zehnder-type element, in this method, the initial phase variation has a great influence on the output characteristics. In other words, in a multi-optical waveguide, multi-stage Y-branching is used to split light from a single optical input into multiple optical waveguides, and multi-mode interference (MMI) couplers are inserted into the input section. It is necessary to drive the device considering the initial phase of the wave path. Especially in multi-channel optical waveguides using MMI, as the number of channels increases, the initial phase difference between the channels at the center and the edges of the multi-channel optical waveguide increases (from -π to π If the initial phase cannot be determined, the optical phased array cannot be driven by the multi-optical waveguides having phase-matched outputs. Also, the initial phase of the multi-optical waveguide is conventionally calculated while monitoring the shape of the composite beam output from the multi-optical waveguide. In other words, while controlling the phase of individual optical waveguides and performing feedback control for overall optimization, the separation of the EO constant and the initial phase component is performed while applying feedback control many times until the error from the design value is minimized. had to go and determine the initial phase.

Accordingly, it is an object of the present invention to provide an optical deflection element performance evaluation apparatus capable of directly measuring the initial phase and electro-optic constant of each optical waveguide in an optical deflection element having an optical phased array using multiple optical waveguides.

A performance evaluation apparatus for an optical deflection element according to an embodiment of the present invention comprises: a laser light source; a polarizer that polarizes the laser light emitted from the laser light source; Light in an optical waveguide of an optical deflection element having a plurality of light exit ends, using a multi-optical waveguide having an entrance end and one light entrance end and a plurality of light exit ends for deflecting incident light by an electro-optic effect. a first optical fiber having an incident end and a first light emitting end optically coupled in a near field; a second optical fiber having a second light input end that is optically coupled and a second output end that outputs laser light input from the second light input end; and the second optical fiber of the second optical fiber. A wave plate that cancels out a phase difference generated while the laser light emitted from the output end is incident and propagates in the optical waveguide, and the wave plate that satisfies a crossed Nicols relationship with the polarizer. an analyzer arranged on the output side of the analyzer, a photodetector that detects the laser beam emitted from the analyzer, an output detector that detects the output of the laser beam detected by the photodetector, and the light A voltage control unit for controlling a voltage for phase modulation applied individually to each optical waveguide in the multi- optical waveguide of the deflection element; a phase of the laser light emitted from the wave plate; a calculation unit for detecting the amount of change in the phase of the laser light in the optical waveguide due to the application of the voltage for phase modulation from the applied phase difference.

It is possible to provide an optical deflection element performance evaluation apparatus capable of directly measuring the initial phase and electro-optic constant of each optical waveguide in an optical deflection element having an optical phased array using multiple optical waveguides.

FIG. 3 is a diagram showing an optical deflection element 100; 1 is a diagram showing a performance evaluation device 200 for an optical deflection element; FIG. 1 is a diagram showing a performance evaluation device 200 for an optical deflection element; FIG. FIG. 2 is a diagram showing part of a performance evaluation apparatus 200 for an optical deflection element; FIG. ₁₀ is a diagram showing the characteristics of the amplitude value Im of the output current I of the PD 270 when the amplitude value _Vm of the modulation voltage V is changed to various values; It is a figure which shows 200 A of performance evaluation apparatuses of the optical deflection element by the modification of embodiment.

Embodiments to which a performance evaluation apparatus for an optical deflection element of the present invention is applied will be described below.

<Embodiment>
FIG. 1 is a diagram showing an optical deflection element 100. FIG. In FIG. 1, an XYZ coordinate system is defined. FIG. 1A shows a plan view (XY plan view) of the optical deflection element 100, and FIG. 1B shows a perspective view.

The optical deflection element 100 includes a lower electrode 110 , a clad layer 120 , a core layer 130 , a clad layer 140 and an upper electrode 150 .

The lower electrode 110 is formed on the surface of a silicon (Si) wafer substrate (not shown) on which a silicon oxide (SiO ₂ ) layer is formed. The substrate is not limited to a silicon substrate, and may be, for example, a substrate made of an insulating material such as resin.

The bottom electrode 110 is an electrode formed over the entire surface of the substrate. The lower electrode 110 serves as a reference potential point when applying voltage to the upper electrode 150 to control the refractive index of the core layer 130 .

The lower electrode 110 has the same shape as the upper surface of the substrate, and is provided on the entire lower surface (the surface on the Z-axis negative direction side) of the cladding layer 120 when a substrate having a rectangular upper surface is used as an example. A rectangular bottom electrode 110 is shown. For the lower electrode 110, for example, a chromium (Cr) layer may be formed on the entire surface of the substrate by sputtering. The thickness of the chromium layer is 100 nm as an example.

Here, a mode in which the lower electrode 110 is a chromium layer will be described. Just do it. Therefore, the lower electrode 110 includes a metal electrode composed of a metal layer other than a chromium layer, an indium oxide/zinc oxide In ₂ O ₃ —ZnO (IZO: Indium Zinc Oxide) thin film, and a tin oxide-added indium oxide In ₂ O 3 oxide. It may be an O ₃ —SnO (ITO: Indium Tin Oxide) thin film.

The cladding layer 120 is arranged on the surface of the lower electrode 110 on the Z-axis positive direction side. The cladding layer 120 has a rectangular shape in plan view, and has the same size as the lower electrode 110 in plan view. The clad layer 120 has a thickness of 0.5 μm, for example.

The cladding layer 120 may be made of a polymer material or inorganic material having a lower refractive index than the core layer 130 . For the cladding layer 120, for example, a polymer thin film is formed by spin coating a polymer material, and silicon oxide (SiO2), which is a low-refractive inorganic material, is coated in the positive Z-axis direction of the lower electrode 110 by a sol-gel method or a sputtering method. formed on the top surface of the side.

The core layer 130 is made of an electro-optic polymer (EO polymer) and has a waveguide 130A, a branch portion 130B, and branch waveguides 131-138. The core layer 130 is a multi-mode interference (MMI) type waveguide superimposed on the clad layer 120 .

The waveguide 130A is a single optical waveguide that has a light incident end 130A1 at the end on the Y-axis negative direction side and connects the light incident end 130A1 and the branch portion 130B. The branch portion 130B has a rectangular shape in plan view, and connects one light incident end 130A1 and eight branch waveguides 131-138. The branching section 130B divides the light incident from the light incident end 130A1 into eight and outputs them to the branching waveguides 131-138. The branch waveguides 131 to 138 extend from the branch portion 130B to the end on the Y-axis positive direction side, and emit eight lights from eight light emitting ends. The branch waveguides 131 to 138 are bent from the middle in the Y-axis direction so that the pitch between them becomes narrower. Eight lights emitted from the output ends of the branch waveguides 131 to 138 are synthesized into one light beam after being emitted.

The waveguide 130A and the branch waveguides 131 to 138 are arranged so that the waveguide 130A is positioned at the center of the branch waveguides 134 and 135 in the X-axis direction. The waveguide 130A extends parallel to the Y-axis, and the branch waveguides 131 to 138 are arranged at equal intervals in the X-axis direction and extend along the Y-axis. Therefore, the central axis parallel to the Y-axis of the waveguide 130A passes through the centers of the branch waveguides 134 and 135 in the X-axis direction. The branch waveguides 131 to 134 and the branch waveguides 135 to 138 are arranged line-symmetrically about the central axis parallel to the Y-axis of the waveguide 130A.

The thickness of the core layer 130 is, for example, 1 μm. Core layer 130 is composed of an EO polymer material. As the EO polymer material, for example, tricyanofuran (TCF), which is a nonlinear optical polyimide with a refractive index of n=1.7 and an electrooptic constant of r=70 pm/V, can be used. As the material of the core layer 130, any other material may be used as long as it is an EO material having a refractive index higher than that of the clad layers 120 and 140. FIG.

After laminating the core layer 130 on the cladding layer 120, a photoresist was applied to prepare a waveguide pattern. The crosslinkable EO polymer becomes insoluble in solvents after light irradiation or heat treatment, and is resistant to photoresist solvents and developing solutions. can. Next, by reactive ion etching (RIE), the core layer 130 where there is no photoresist is scraped off, the photoresist is peeled off, and the cladding layer 140 is applied to form a channel waveguide with the branch waveguides 131 to 138. be.

Core layer 130 is covered by clad layers 120 and 140 . Of the core layer 130, only the light incident end 130A1 and the light emitting ends (eight pieces) of the branch waveguides 131 to 138 are exposed on the outer surface of the optical deflection element 100, and the other surfaces are the clad layers 120. and 140.

The clad layer 140 is stacked on the core layer 130 on the Z-axis positive direction side. Since the cladding layer 140 has the same planar size as the cladding layer 120, the portion of the surface of the cladding layer 120 on the Z-axis positive direction where the core layer 130 does not exist is directly bonded to the cladding layer 120. be done. The clad layer 140 is formed, for example, by spin coating a thin film of a polymer material for the clad layer. As long as the clad layer 140 has a lower refractive index than the core layer, the clad layer 140 may be made of an EO polymer like the core layer.

Moreover, the total thickness d of the clad layer 120, the core layer 130, and the clad layer 140, which are set to the thicknesses described above, is 2 μm.

The upper electrode 150 has electrodes 151 to 158 and wirings 151A to 158A. The electrodes 151 to 158 are respectively superimposed on the branch waveguides 131 to 138 (on the Z-axis positive direction side) in plan view. As the upper electrode 150, an IZO thin film having a thickness of 200 nm was formed by sputtering, and an electrode pattern was formed by dry etching.

The electrodes 151-158 are refractive index control electrodes for controlling the refractive indices of the branch waveguides 131-138, respectively. Wirings 151A to 158A are connected to the electrodes 151 to 158, respectively. Different voltages can be applied to the electrodes 151-158. By controlling the voltage applied to the electrodes 151-158 and controlling the phase of the light propagating through the branch waveguides 131-138, the synthetic light of the eight beams emitted from the branch waveguides 131-138 is deflected. can do. Therefore, the portions of the branch waveguides 131 to 138 that overlap the electrodes 151 to 158 are called phase shifters.

Such an upper electrode 150 is formed, for example, by forming an IZO thin film on one surface of the cladding layer 140 on the Z-axis positive direction side, and forming unnecessary portions (electrodes 151 to 158 and wirings 151A to 158A on the Y-axis). (the portion on the positive direction side) is formed by dry etching. Incidentally, the thickness of the electrodes 151 to 158 is 200 nm as an example.

Furthermore, after the upper electrode 150 is formed, poling treatment for orienting the EO polymer molecules in one direction is performed as a technique for exhibiting the EO effect. In this technique, the light deflection element 100 is heated to a temperature near the glass transition temperature (Tg) of the EO polymer, and a voltage is applied between the upper electrode 150 and the lower electrode 110 to apply a unidirectional electric field to the EO polymer. By returning to normal temperature, the dye molecules of the EO polymer are oriented in the Z-axis direction, thereby exhibiting the EO effect.

In the optical deflection element 100 configured as described above, the branch portion 130B of the core layer 130 does not have a mechanism for aligning the phases of the light beams output to the branch waveguides 131 to 138. A phase difference occurs in the light split into 131-138. In addition, regardless of the MMI, even in the case of branch waveguides using a star coupler such as a Y branch, since the shape processing of the optical waveguide is generally accurate, a phase difference occurs to some extent. Therefore, the phase difference that occurs at the output ends of the branch waveguides 131 to 138 is added to the phase difference that occurs when branching to the branch waveguides 131 to 138 of the MMI 130B. It is also affected by the phase difference of the generated light.

Note that the optical deflection element 100 can be used for a distance measuring device, a stereoscopic display, a laser lidar (LiDAR: Light Detection and Ranging), and the like.

2 and 3 are diagrams showing a performance evaluation apparatus 200 for an optical deflection element. FIG. 4 is a diagram showing a part of the performance evaluation device 200 for the optical deflection element. FIG. 4 shows a photograph obtained with an optical microscope. Below, the performance evaluation device 200 of the optical deflection element is referred to as the performance evaluation device 200 .

FIG. 2 shows the optical deflection element 100 together with a performance evaluation apparatus 200 for the optical deflection element. FIG. 2 shows a modulation voltage source 160 that supplies a voltage to the upper electrode 150 of the optical deflection element 100, and switches 170A and 170B provided between the modulation voltage source 160 and the wirings 151A to 158A. Modulated voltage source 160 independently outputs sinusoidal voltages to any of electrodes 151-158 of upper electrode 150 via switches 170A and 170B. Thereby, the branch waveguides 131 to 138 can be individually phase-controlled. The switch 170A is provided between the modulated voltage source 160 and the wirings 151A-154A, and the switch 170B is provided between the modulated voltage source 160 and the wirings 155A-158A. The switches 170A and 170B are switches that independently connect between the modulation voltage source 160 and the wirings 151A to 158A. By using the switches 170A and 170B, the voltage output from the modulation voltage source 160 can be applied independently to the electrodes 151-158 via the wirings 151A-158A.

However, when detecting the figure of merit of the optical deflection element 100, any one of the electrodes 151 to 158 (of the branch waveguides 131 to 138 whose figure of merit is to be detected) is selected via the switch 170A or 170B. A voltage is supplied to the electrode corresponding to any one of them. Such control, including control of the switches 170A and 170B, is performed by the control section 290. FIG.

The performance evaluation apparatus 200 includes a laser light source (Laser) 210, a polarizer 220, an optical fiber 230, an optical fiber 240, a retarder 250, an analyzer 260, a PD (Photo Diode) 270, a lock-in amplifier 280, and a controller 290. .

The laser light source 210 is a laser light source with an arbitrary wavelength from visible light to infrared light, and the size of the waveguide to be manufactured changes in proportion to the wavelength. Here, as an example, the wavelength of the laser light is assumed to be 1.55 μm.

The polarizer 220 is provided between the laser light source 210 and the optical fiber 230, and is set in a direction that forms an angle of 45° with the Z-axis to convert the linearly polarized component of the laser light incident from the laser light source 210 to To Penetrate. The polarizer 220 is configured to satisfy the crossed Nicols relationship with the analyzer 260 .

The optical fiber 230 is an example of a first optical fiber and has a lens portion 231 and an incident end 232 . The optical fiber 230 is a fiber with a lens that can maintain the polarization of incident light, and is a spherical PMF (Polarization Maintaining Fiber). The lens portion 231 is an example of a first light emitting end optically coupled to the light incident end 130A1 of the waveguide 130A of the optical deflection element 100 in the near-field, and the incident end 232 is connected to the light emitting end of the polarizer. It is an example of the 1st light-incidence end connected.

The optical fiber 240 is an example of a second optical fiber and has a lens portion 241 and an emission end 242 . Similar to the optical fiber 230, the optical fiber 240 is a lensed fiber that can hold the polarization of incident light, and is a spherical PMF.

The lens portion 241 is an example of a second light incident end that is optically coupled in the near-field with a light exiting end of any one of the branch waveguides 131 to 138 of the optical deflection element 100 . Here, to optically couple the lens portion 241 to any one of the light emitting ends of the branch waveguides 131 to 138 in the near-field means that light emitted from the lens portion 241 and the branch waveguides (131 to 138) In addition to the light that is sufficiently smaller than the distance between the ends of the branch waveguides 131 to 138 (for example, 6 μm) and is incident on the lens unit 241 from the branch waveguides to be measured, other , the light from the branch waveguide is not incident. Regarding the conditions for near-field optical coupling to prevent light from entering other branch waveguides, the distance between the light emitting ends is d (6 μm in the above example), and the distance from the tip of the lens portion to the light emitting end It is preferable that the following equation (3) is satisfied when the distance to is p.

FIG. 4 shows, as an example, a state in which the lens portion 241 is optically coupled to the light emitting end of the branch waveguide 133 in the near-field. The emission end 242 is an example of a second emission end that emits laser light incident from the second light incidence end, and is connected to the retarder 250 .

The optical fiber 240 is moved and positioned by a moving device (not shown) so that the lens portion 241 is optically coupled to the light emitting end of any one of the waveguides 131 to 138 in the near field. A waveguide (one of 131 to 138) having a light output end to which lens portion 241 is coupled has an electro-optic constant r, which is a figure of merit, detected by performance evaluation device 200 .

Retarder 250 is an example of a waveplate or phase compensator provided between optical fiber 240 and analyzer 260 . The retarder 250 is an optical element capable of varying the retardation from 0 to 2π, and a liquid crystal variable retarder or a Babinet soleil compensator can be used. The retarder 250 is in a state where no voltage is applied to the electrode (corresponding one of 151-158) on top of the waveguide (one of 131-138 whose figure of merit is to be detected). A phase difference is imparted to the propagating light so as to cancel out the phase difference generated while the laser light propagates through the waveguide. The phase difference imparted by the retarder 250 is obtained by measuring the phase difference imparted to the laser light by the waveguides 131 to 138 while no voltage is applied to each of the waveguides 131 to 138 from the electrodes 151 to 158, and calculating the phase. It is sufficient to set the phases to cancel each other.

The analyzer 260 is provided between the retarder 250 and the PD 270 and transmits the linearly polarized light component of the laser beam incident from the retarder 250 . The analyzer 260 is configured to satisfy the crossed Nicols relationship with the polarizer 220 .

The PD 270 is provided on the output side of the analyzer 260, photoelectrically converts the laser light output from the analyzer 260, and outputs a current having a magnitude corresponding to the intensity of the laser light. A current output from the PD 270 is input to the lock-in amplifier 280 .

Lock-in amplifier 280 detects the current output from PD 270 in synchronization with the sine wave voltage output from modulated voltage source 160 . Lock-in amplifier 280 is controlled by control section 290 , and data representing the detected current value is input to control section 290 . Lock-in amplifier 280 is an example of an output detector.

The control section 290 controls the modulation voltage source 160 and the lock-in amplifier 280 . More specifically, the control section 290 controls the voltage value output by the modulated voltage source 160 and controls the lock-in amplifier 280 to take in the current output by the PD 270 . The control unit 290 also calculates the initial phase and electro-optical constants. The control unit 290 generates data representing the characteristics of the amplitude value of the current output from the PD 270 with respect to the amplitude value of the voltage value output from the modulated voltage source 160, and calculates the electro-optic constant from the slope of this characteristic. A portion of the control section 290 that controls the voltage value output by the modulated voltage source 160 is an example of a voltage control section. Also, the portion of the control section 290 that calculates the initial phase and the electro-optic constant is an example of a calculation section.

Next, how to obtain the figure of merit will be described. The electric field vector E of the light incident on the multi-optical waveguide can be expressed by the following equation (4). Here, E ₀ is the initial value of the electric field vector E, K is the propagation vector, 2π/λ (where λ is the wavelength of the laser light) is the scalar of the wave number, and the vector p is the position vector.

ここで、Ｙ軸方向に伝搬する光を考え、光の進行方向をY軸の負から正の方向にとると、K・ｐ＝ωNｙ/ｃとなる。Nは複素屈折率であって、屈折率ｎと消光係数κを用いて、N＝ｎ+iκと表す。

Considering the light propagating in the Y-axis direction, K·p=ωNy/c when the traveling direction of the light is taken from the negative direction to the positive direction of the Y-axis. N is a complex refractive index, which is expressed as N=n+iκ using the refractive index n and the extinction coefficient κ.

At that time, using a polarizer 220 as shown in FIGS. 2 and 3, light linearly polarized in a direction inclined by 45° from the Z-axis is guided through an optical fiber 230 to construct an optical phased array of the optical deflection elements 100. It enters the wave path 130A. The incident light propagates through the waveguide 130A, passes through the branch portion 130B (MMI), and passes through the phase shifters of the branch waveguides 131-138 (where the electrodes 151-158 are located). At the output end of one of the branch waveguides 131 to 138 whose figure of merit is to be detected, the light passes through the optical fiber 240 and passes through the retarder 250 . The light emitted from the optical deflection element 100 is elliptically polarized, passes through the analyzer 260 in which the polarization component is orthogonal to the incident direction, and is converted into light intensity (current value I) by the PD 270 . Here, Δφ is the static phase difference of the light emitted from the optical deflection element 100 when no voltage is applied to the phase shifter, and the phase change amount caused by applying the voltage by the phase shifter of the optical deflection element 100 is Assuming that (dynamic phase difference) is Δθ, the light intensity (current value I) output from the PD 270 is expressed by the following equation (5).

Note that the static phase difference Δφ is obtained when no voltage is applied to the electrodes 151 to 158 and linearly polarized light forming an angle of 45° with the Z direction is incident on the light deflection element 100 using the polarizer 220. , is the relative phase difference between the X-polarized light and the Z-polarized light produced by the light deflection element 100 and the retarder 250 . Thus, the static phase difference Δφ includes the phase difference Δφ _r imparted to the light by the compensation by the retarder 250 (Babinet soleil plate). In addition, the dynamic phase difference Δθ is defined as, among the electrodes 151 to 158, when a voltage is applied to one electrode corresponding to the branch waveguide (one of 131 to 138) for which the figure of merit is to be detected. , is the relative phase difference between the X-polarized light and the Z-polarized light generated by the electro-optical effect (EO effect) in the optical deflection element 100 . This phase difference differs depending on which one of the branch waveguides 131 to 138 is passed.

また、式（５）において、Ｉ_０は、偏光子２３０、光偏向素子１００、リターダ２５０、検光子２６０の全ての素子を透過した光の強度が最大値となるように、偏光子２３０と検光子２６０の角度を調整し、光出力を最大とするときのＰＤ２７０を流れる電流値である。また、Δφ₀は、静的位相差Δφからリターダ２５０による補償によって光に付与される位相差Δφ_rを引いた位相差（初期位相）である。すなわち、位相差Δφ₀は、電極１５１～１５８に電圧を印加しない場合に、偏光子２２０を用いてＺ方向と４５°の角度をなす直線偏光を電極１５１～１５８に電圧を印加しない場合に、光偏向素子１００の出射光とに生じるＸ偏光とＺ偏光の相対的な位相差である。この位相差は、分岐導波路１３１～１３８のいずれを通るかで異なる。また、式（５）から分かるように、ＰＤの出力は、Ｘ偏光とＺ偏光の位相差の半分の角度だけ偏光面が回転した直線偏光の成分を検出していることになる。

Further, in equation (5), _I0 is added to the polarizer 230 and the detector so that the intensity of the light transmitted through all of the polarizer 230, the optical deflection element 100, the retarder 250, and the analyzer 260 becomes maximum. It is the value of current flowing through the PD 270 when the angle of the photon 260 is adjusted to maximize the light output. Δφ ₀ is the phase difference (initial phase) obtained by subtracting the phase difference Δφ _r imparted to light by compensation by the retarder 250 from the static phase difference Δφ. That is, the phase difference Δφ ₀ is obtained when no voltage is applied to the electrodes 151 to 158 and the linearly polarized light that forms an angle of 45° with the Z direction using the polarizer 220 is applied to the electrodes 151 to 158. It is the relative phase difference between the X-polarized light and the Z-polarized light generated in the output light of the optical deflection element 100 . This phase difference differs depending on which one of the branch waveguides 131 to 138 is passed. Also, as can be seen from Equation (5), the output of the PD detects a linearly polarized component whose plane of polarization is rotated by half the angle of the phase difference between the X-polarized light and the Z-polarized light.

Here, when performing modulation using the electro-optic constants _r13 and _r33 of the phase shifter of the optical deflection element 100 using EO polymer, the dynamic phase difference Δθ can be expressed by the following equation (6). can.

ここで、ｎ_０は常光に対する屈折率、ｎ_ｅは異常光に対する屈折率、ｄは上部電極１５０と下部電極１１０との間で特にEOポリマーの厚みに相当する等価距離、Ｖは電極（１５１～１５８）において、位相シフタに印加する電圧のうち、特にEOポリマーに印加される電圧である。ここで、式（５）および（６）中の（V/d）はEOポリマーに印加される電界強度を表しているので、EOポリマーを含むポリマー材料からなるクラッド/コア/クアッド構造の場合には、比抵抗がクラッドとコアとでが同等と見なせるため、等価距離ｄを上部電極１５０と下部電極間１１０の間の距離とし、電圧Vを電極（１５１～１５８）に印加する電圧とすることができる。さらに、式（５）および（６）中ｒ_３３とｒ_１３は、電気光学定数である。ここで、ｎ_０＝ｎ_ｅ＝ｎ、及び、ｒ_３３＝３ｒ_１３が成立することとすると、動的位相差Δθは、次式（７）で表すことができる。

Here, n ₀ is the refractive index for ordinary light, n _e is the refractive index for extraordinary light, d is the equivalent distance between the upper electrode 150 and the lower electrode 110, especially corresponding to the thickness of the EO polymer, and V is the electrode (151 to 158), among the voltages applied to the phase shifters, is the voltage applied to the EO polymer. Here, (V/d) in equations (5) and (6) represents the electric field strength applied to the EO polymer, so in the case of a clad/core/quad structure made of polymer materials including EO polymer, Since the specific resistance of the cladding and the core can be regarded as equivalent, the equivalent distance d is the distance between the upper electrode 150 and the lower electrode 110, and the voltage V is the voltage applied to the electrodes (151 to 158). can be done. Furthermore, _r33 and r13 _in equations (5) and (6) are electro-optical constants. Here, assuming that n ₀ =n _e =n and r ₃₃ =3r ₁₃ hold, the dynamic phase difference Δθ can be expressed by the following equation (7).

ただし、lは、位相シフタの電極（１５１～１５８）の分岐導波路（１３１～１３８）に沿った（Ｙ軸方向の）長さ、ｄは、上部電極１５０と下部電極１１０との間の等価距離、λは、光偏向素子１００に使用するレーザ光の波長である。

where l is the length (in the Y-axis direction) of the phase shifter electrodes (151-158) along the branch waveguides (131-138), and d is the equivalent between the upper electrode 150 and the lower electrode 110. The distance λ is the wavelength of the laser light used for the light deflection element 100 .

Here, the optical bias is applied by adjusting the compensation amount of the retarder 250 so that the following equation (8) holds in equation (5). The optical bias is the sum of the phase difference Δφ ₀ produced in the optical deflection element 100 when no voltage is applied to the phase shifter and the phase difference Δφ _r imparted to the light by compensation by the retarder 250 .

式（８）が成立するようにするには、位相シフタに電圧を印加しない状態はΔθ＝０にできるため、リターダ２５０（バビネソレイユ板）の位相の補償量を調整し（実際は、バビネソレイユ板の位相調整つまみを回転させ）、リターダ２５０の出力が０（ゼロ）になる位置（Δφ₀＋Δφ_ｒ＝0になる位置）と、さらに次に出力が０（ゼロ）になる位置（Δφ₀＋Δφ_ｒ＝2πになる位置）とを計測する。そして、これら２つの位置の中間位置にリターダ２５０の位相調整つまみを調整すれば、リターダ２５０の最大出力が得られる位置（Δφ₀＋Δφ_ｒ＝πが成立する位置）を求めることができる。さらに、出力が０（ゼロ）になる位置と、最大出力が得られる位置との中間位置にリターダ２５０の位相調整つまみを調整すれば、式（８）が成立するようにリターダ２５０を調整することができる。

In order to make equation (8) hold, Δθ can be set to 0 when no voltage is applied to the phase shifter. ), the position where the output of the retarder 250 becomes 0 (the position where Δφ ₀ +Δφ _r = 0) and the position where the output next becomes 0 (Δφ ₀ + Δφ The position where _r = 2π) is measured. By adjusting the phase adjustment knob of the retarder 250 to an intermediate position between these two positions, the position where the maximum output of the retarder 250 is obtained (the position where Δφ ₀ +Δφ _r =π is established) can be obtained. Furthermore, by adjusting the phase adjustment knob of the retarder 250 to an intermediate position between the position where the output is 0 (zero) and the position where the maximum output is obtained, the retarder 250 can be adjusted so that the formula (8) holds. can be done.

Also, when the formula (8) holds, the following formula (9) holds, so the approximation of the formula (10) holds.

また、光偏向素子１００の位相シフタの変調電圧Ｖは、次式（１１）で表すことができる。変調電圧Ｖは正弦波状の交流電圧であり、V_mは変調電圧Ｖの最大振幅値である。変調電圧Ｖは、電極１５１～１５８のうち、性能指数の検出対象となる分岐導波路（１３１～１３８のうちの１つ）に対応する１つの電極に個別に印加する電圧である。

Also, the modulation voltage V of the phase shifter of the optical deflection element 100 can be expressed by the following equation (11). The modulation voltage V is a sinusoidal AC voltage, and V _m is the maximum amplitude value of the modulation voltage V. FIG. The modulation voltage V is a voltage applied individually to one of the electrodes 151 to 158 corresponding to the branch waveguide (one of 131 to 138) whose figure of merit is to be detected.

変調電圧Ｖとして用いる正弦波電圧の角速度をωとすると、式（５）、（７）、（１０）、（１１）から、ＰＤ２７０の出力する電流値Iの振幅値Ｉ_ｍは、次式（１２）で与えられる。

Assuming that the angular velocity of the sinusoidal voltage used as the modulation voltage V is ω, the amplitude value Im of the current value I output from the PD ₂₇₀ is given by the following equation ( 12).

また、光偏向素子１００の出射光に生じた位相差Δφ_０と、リターダ２５０による補償によって光に付与された位相差Δφ_ｒとが、Δφ_０＋Δφ_ｒ＝πを満たすようにリターダ２５０（バビネソレイユ）の補償量を設定しているので、電極（１５１～１５８）から位相シフタに電圧を印加しなければ、Δθ＝０となるため、式（５）から電流値Ｉ_０を求めることができる。

Further, the _{retarder 250} ( _{Babinet soleil} ) is set, the current value _I0 can be obtained from the equation (5) because Δθ=0 if no voltage is applied from the electrodes (151 to 158) to the phase shifter.

Further, in equation (12), the electro- _optic constant _r33 can be obtained from the slope of the amplitude value Im of the current value I output by the PD 270 when the amplitude value _Vm of the modulation voltage V is changed to various values. can. This will be described later with reference to FIG.

Also, the phase difference Δφ ₀ produced in the light emitted from the optical deflection element 100, the phase difference Δφ _r imparted to the light by the compensation by the retarder 250, and the compensation amount of the retarder 250 are adjusted so that Δφ ₀ +Δφ _r =0. From the compensation amount of the retarder 250 when adjusted, the phase difference occurring at each output end of the branch waveguides 131 to 138 can be individually calculated.

FIG. 5 is a diagram showing the characteristics of the amplitude value _Im when the amplitude value _Vm of the modulation voltage V is changed to various values. FIG. 5 shows V _m -I _m characteristics obtained by changing the amplitude value V _m in four optical deflection elements with different electro-optic constants r ₃₃ . As shown in FIG. 5, the electro-optical constant r ₃₃ is determined to be 10 pm/V, 25 pm/V, 48 pm/V, and 78 pm/V from the slope of the V _m -I _m characteristic. Thus, the electro-optic constant r ₃₃ can be obtained from the slope of the V _m -I _m characteristic obtained by changing the amplitude value V _m of the modulation voltage V to various values.

Here, the control unit 290 substitutes the data representing the current value detected by the lock-in amplifier 280 into the equation (5), performs the calculation using the equations (6) to (12), and further modulates the voltage V The controller 290 can obtain the electro-optical constant _r33 by obtaining the slope of the amplitude value _Im when the amplitude value _Vm of is changed to various values.

The electro-optic constant obtained in this manner is obtained by optically coupling the optical fiber 230 and the optical fiber 240 to the light incident end 130A1 of the optical deflection element 100 and each of the branch waveguides 131 to 138, respectively, in the near field. It is calculated using the phase difference Δφ ₀ and the static phase difference Δφ and the phase difference Δφ _r of the retarder 250 . When optical fiber 240 is optically coupled to each output end of branch waveguides 131 to 138, a centering device is used to align lens portion 241 of optical fiber 240 with each output end of branch waveguides 131 to 138. may be positioned so as to be optically coupled to .

Therefore, it becomes possible to measure the electro-optical constants with high sensitivity and high accuracy. Therefore, according to the embodiment, the optical deflection element performance evaluation apparatus 200 can directly measure the initial phase and the electro-optic constant of each optical waveguide in the optical deflection element 100 having an optical phased array using multiple optical waveguides. can be provided.

In order to realize driving that satisfies the deflection control and convergence of the optical phased array, it is necessary to satisfy the phase matching condition of the light beams formed by the optical phased array. At that time, the detection of the initial phase and the electro-optical constant of each channel is essential to realize the drive that satisfies the matching condition.

Conventionally, the initial phase and electro-optic constants of an optical phased array are measured from the shape of the optical interference pattern of outputs from a plurality of channels from the optical phased array. In this method, since the optical interference pattern is formed by the interference of the outputs of a plurality of channels, it is necessary to back calculate the initial phase and electro-optical constant of each channel from the interference pattern.

At that time, the interference pattern predicted (on the way) in the calculation process and the actual measured value of the interference pattern (composite beam) are measured repeatedly until they fall within the error range (until consistency is achieved). I had to repeat the feedback. Since this conventional technique indirectly predicts the electro-optical constants and the initial phase, there is a drawback that the number of channels in the optical phased array increases dramatically.

On the other hand, in the performance evaluation apparatus 200 of the optical deflection element of the embodiment, the initial phase and the electro-optic constant are individually measured for each channel. Therefore, as a method of detecting the initial phase and the electro-optical constant of each channel, since the phase change amount of each channel can be directly measured, rapid measurement is possible.

In addition, by using polarization-maintaining spherical fibers (optical fibers 230 and 240) for the output light of each channel, the amount of phase change due to the electric field can be directly measured for each channel. As a result, the measurement time can be reduced compared to the conventional method, and the measurement accuracy can be improved by performing this measurement with lock-in detection (synchronization detection).

In addition to the EO polymer, an inorganic EO material such as LiNbO ₃ can be used as the clad layer 120 , the core layer 130 and the clad layer 140 . For such an inorganic EO material, the electro-optic constant can be similarly measured by changing the definition of the dynamic phase difference Δθ shown in Equation (6) according to the symmetry of the crystal.

For example, in isotropic materials such as cubic crystals (symmetry: 23, 4 bars, 3 m) and thermoelectric effects, the dynamic phase difference Δθ can be defined by the following equation (13).

また、一軸性性結晶（対称性：3ｍ）でr₂₂=-r₁₂の関係を有する結晶では、動的位相差Δθを次式（１４）のように定義すればよい。

Further, in a uniaxial crystal (symmetry: 3 m) having a relationship of r ₂₂ =−r ₁₂ , the dynamic phase difference Δθ can be defined by the following equation (14).

このように、位相差を生じる材料の対称性を考慮して、適宜動的位相差Δθの定義を変えることで、熱電光学材料や無機結晶材料についても適用することが可能となる。

In this way, by appropriately changing the definition of the dynamic phase difference Δθ in consideration of the symmetry of the material that produces the phase difference, it can be applied to thermoelectric optical materials and inorganic crystal materials.

Further, the optical deflection element performance evaluation apparatus 200 shown in FIGS. 2 and 3 may be modified as shown in FIG. FIG. 6 is a diagram showing a performance evaluation apparatus 200A for an optical deflection element according to a modification of the embodiment. The optical deflection element performance evaluation apparatus 200A has a configuration in which an optical chopper 295 and an optical chopper controller 295A are added to the optical deflection element performance evaluation apparatus 200 shown in FIGS.

Optical chopper 295 is provided between laser light source 210 and polarizer 220 and is controlled by optical chopper controller 295A. The optical chopper controller 295A allows the laser light emitted from the laser light source 210 to pass through the optical chopper 295 when the lock-in amplifier 280 performs a synchronous operation (lock-in operation) based on the current value input from the PD 270 . The optical chopper controller 295A performs a synchronous operation at the modulation frequency of the phase shifter to allow the laser light emitted from the laser light source 210 to pass through.

Using such an optical chopper 295 improves the SN ratio of the current value detected by the PD 270, thereby further improving the measurement accuracy.

Although the optical deflection element performance evaluation apparatus according to the exemplary embodiments of the present invention has been described above, the present invention is not limited to the specifically disclosed embodiments, and the Various modifications and changes may be made without departing from the description.

REFERENCE SIGNS LIST 100 optical deflection element 110 lower electrode 120 clad layer 130 core layer 130A waveguide 130A1 light incident end 130B branching portion 131-138 branch waveguide 140 clad layer 150 upper electrode 151-158 electrode 160 modulation voltage source 170A, 170B switch 200, 200A Optical Deflection Element Performance Evaluation Apparatus 210 Laser Light Source 220 Polarizer 230 Optical Fiber 231 Lens Section 232 Incident End 240 Optical Fiber 241 Lens Section 242 Output End 250 Retarder 260 Analyzer 270 PD
280 lock-in amplifier 290 control unit 295 optical chopper 295A optical chopper controller

Claims (4)

a laser light source;
a polarizer that polarizes the laser light emitted from the laser light source;
a first light input end connected to the light output end of the polarizer ; a first optical fiber having a first light output end optically coupled near-field to the light input end of the optical waveguide of the optical deflection element having a light output end of
a second light incident end that is optically coupled near-field to one of the plurality of light emitting ends of the optical deflection element; and a second emitting end that emits the laser light incident from the second light incident end a second optical fiber having an end;
a wave plate for canceling a phase difference generated while the laser light emitted from the second emission end of the second optical fiber is incident and the laser light propagates through the optical waveguide;
an analyzer arranged on the output side of the wave plate so as to satisfy the relationship between the polarizer and the crossed Nicols;
a photodetector that detects laser light emitted from the analyzer;
an output detection unit that detects the output of the laser light detected by the photodetection unit;
a voltage control unit for controlling a voltage for phase modulation applied individually to each optical waveguide in the multi- optical waveguide of the optical deflection element;
A change amount of the phase of the laser light in the optical waveguide due to the application of the voltage for phase modulation is detected from the phase of the laser light emitted from the wave plate and the phase difference given to the laser light by the wave plate. A performance evaluation device for an optical deflection element, comprising: an arithmetic unit; 2. The optical deflection element performance evaluation apparatus according to claim 1, wherein said computing unit derives an electro-optic constant from a slope of a characteristic of the output of said laser light with respect to said voltage for phase modulation. 3. The optical deflection element performance evaluation apparatus according to claim 1, wherein said output detector detects the output of said laser light in synchronization with said laser light emitted from said laser light source. 4. The optical chopper according to any one of claims 1 to 3, further comprising an optical chopper provided between the laser light source and the polarizer and operating in synchronization with the output of the laser light detected by the output detector. A performance evaluation device for an optical deflection element.

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