JP2015172540A - Laser doppler velocimeter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a scan laser Doppler velocimeter (LDV) smaller than the conventional one.SOLUTION: A sensor includes: first and second array waveguide type diffraction gratings having a phase variable region in which path lengths of waveguides adjacent to a part of an array waveguide are substantially equal to each other; N sets of output ports configured by arranging, in two columns, N first grating couplers connected to N (N is integer of two or more) light output waveguides of the first array waveguide type diffraction grating and N second grating couplers connected to N light output waveguides of the second array waveguide type diffraction grating; and a receiver for receiving scattered light. Each of the array waveguide type diffraction gratings includes (N-1) tapered liquid crystal layers that are formed across the phase variable region and function as an upper cladding layer of the waveguide. A laser Doppler velocimeter including the sensor is also provided.

Description

  The present invention relates to a laser Doppler velocimeter, and more particularly to a laser Doppler velocimeter using an arrayed waveguide grating.

  A laser Doppler velocimeter (LDV: Laser Doppler Velocimeter) is a non-contact type velocity meter with high spatial resolution and high accuracy. Among them, the scanning LDV that can scan the measurement point can measure the velocity distribution in the flow path with high accuracy, and therefore is expected to be used in the medical field for noninvasive measurement of the blood flow velocity distribution in the blood vessel. Has been.

  Here, the measurement mechanism of the conventional scanning LDV 500 will be described with reference to FIG.

  In the scanning LDV 500, the laser light output from the laser 501, which is a light source, is cut out only in one direction by the polarizer 502, then split into two lights by the splitter 503 and enters the lens optical system 504. The two lights that have passed through the lens optical system 504 are incident on a mechanically movable portion that includes the rotating mirrors 505a and 505b.

  In the mechanically movable portion, the optical path is changed so that the two lights intersect in the flow path 507 which is a measurement field. Here, in the scanning LDV 500, the driver 506 controls the rotation of the rotating mirrors 505a and 505b to change the optical path of the two lights, thereby realizing scanning in the depth direction. In the flow path 507, an interference fringe is generated at a measurement point where two lights intersect, and scattered light emitted from the tracer particles passing through the interference light is received by the light receiving element 508. The signal processing unit 509 obtains the flow velocity at the measurement point by measuring the frequency of the beat signal of the scattered light received by the light receiving element 508.

  However, as shown in FIG. 12, there is a problem that the apparatus becomes large when the incident optical system of the probe light is constructed by a spatial optical system including a mechanical movable part.

  In this regard, Non-Patent Document 1 discloses a scanning LDV that does not use a mechanical movable part in a probe incident optical system. Here, based on FIG. 13, the measurement mechanism of the scanning LDV 600 of Non-Patent Document 1 will be described.

  As shown in FIG. 13A, in the scanning LDV 600, the laser light output from the wavelength tunable laser 601 that is a light source is only polarized in one direction by a polarizer 602 and is incident on an optical switch 603. An optical fiber array 604 composed of a plurality of polarization maintaining optical fibers is connected to the optical switch 603, and the driver 609 controls the optical switch 603 to select an output port.

  The output end of each optical fiber constituting the optical fiber array 604 is connected to the sensor unit 610, and the light incident from each optical fiber is divided into two lights in the sensor unit 610, and then is a measurement field. It is configured to intersect within the flow path 605. Here, in the scanning LDV 600, the output ends of the optical fibers constituting the optical fiber array 604 are juxtaposed in the horizontal direction of the flow path 605, and the optical switch 603 switches the output port, thereby scanning in the horizontal direction. Is possible.

  On the other hand, FIG. 13B shows an enlarged view of the sensor unit 610 of the scanning LDV 600. As shown in FIG. 13B, in the sensor unit 610, light incident from one optical fiber 604 (n) constituting the optical fiber array 604 is reflected by the mirror 612 and is converted into two lights by the splitter 613. Divided. The two divided lights are incident on the diffraction gratings 611a and 72b, respectively. The light incident on the diffraction gratings 611a and 72b is diffracted in the direction corresponding to the wavelength by the diffraction angle wavelength dependency. As a result, the two diffracted lights intersect in the flow path 605. The light receiving element 614 receives scattered light emitted from the measurement point in the flow path 605, and the signal processing unit 607 obtains the flow velocity at the measurement point by measuring the frequency of the beat signal of the scattered light received by the light receiving element 614. .

  In the scanning LDV 600, scanning in the depth direction is realized by changing the output wavelength of the wavelength tunable laser 601 to change the diffraction angles of the two lights in the sensor unit 610. By switching the output port, two-dimensional scanning of the measurement field is realized.

  In the above-described mechanism, the optical path is changed using the diffraction angle wavelength dependency of the diffraction grating, and the apparatus can be miniaturized as the mechanical movable part is unnecessary. However, the above-described mechanism still requires a space for constructing a spatial optical system, and thus there is a limit to downsizing.

Koichi Maru, Takahiro Hata “Two-dimensional scanning laser Doppler velocimeter using wavelength tunable laser and fiber lay”, The 60th JSAP Spring Meeting, Proceedings Proceedings p.03-069, 2013

  If the scanning LDV can be made even smaller than before, there is a way to apply the scanning LDV to the measurement of blood flow velocity distribution in various situations ranging from basic research to clinical diagnosis. It is expected to bring about significant improvements in the diagnosis and treatment of diseases.

  The present invention has been made in view of the above-described problems in the prior art, and an object of the present invention is to provide a scanning LDV that is further downsized than the conventional one.

  As a result of intensive studies on the configuration of a scanning LDV that has been further reduced in size compared to the prior art, the present inventors have conceived the following configuration and have reached the present invention.

  That is, according to the present invention, a wavelength tunable laser having two or more output wavelengths and two probe beams obtained by dividing the laser beam emitted from the wavelength tunable laser are emitted to a measurement point and generated from the measurement point. A sensor unit having a sensor substrate for outputting a detection signal of scattered light, a control means for variably controlling the output wavelength and a voltage applied to the sensor substrate, and a flow velocity based on the detection signal of the scattered light A signal processing unit for measuring, and the sensor substrate includes an input port for inputting the laser light, and two array conductors each receiving two lights divided at the input port. 1st and 2nd arrayed-waveguide-type diffraction gratings having a phase-variable region having substantially the same path length of waveguides adjacent to a part of the arrayed-waveguide, N first grating couplers connected to N (N is an integer of 2 or more) optical output waveguides of the arrayed waveguide grating and N lights of the second arrayed waveguide grating N sets of output ports configured by arranging N second grating couplers connected to the output waveguide in two rows, and a light receiving unit for receiving the scattered light, The arrayed waveguide grating includes tapered (N-1) liquid crystal layers formed across the phase variable region and functioning as an upper clad layer of the waveguide. The output port for emitting the probe light having the wavelength λ is switched by applying a positive or negative voltage individually to the liquid crystal layer to change the equivalent refractive index of each waveguide constituting the phase variable region. Laser Doppler velocimeter Provided.

  As described above, according to the present invention, a scanning LDV that is further reduced in size than the conventional one is provided.

The schematic block diagram of the laser Doppler velocimeter of this embodiment. The top view of the sensor board | substrate in this embodiment. The conceptual diagram for demonstrating the structure of the port for input in this embodiment. The conceptual diagram for demonstrating the structure of the port for input in this embodiment. The conceptual diagram for demonstrating the function of the grating coupler used in embodiment. The conceptual diagram for demonstrating the measurement mechanism of the laser Doppler velocimeter of this embodiment. The conceptual diagram for demonstrating the measurement mechanism of the laser Doppler velocimeter of this embodiment. The figure which shows the liquid-crystal layer formed in a phase variable area. The conceptual diagram for demonstrating the refractive index change of a liquid-crystal layer. The figure which shows the relationship between the pattern of the voltage application to a liquid crystal layer, and the inclination of a wave front. The conceptual diagram for demonstrating the control sequence which the driver of the laser Doppler velocimeter of this embodiment performs. The figure which shows the conventional scanning laser Doppler velocimeter. The figure which shows the conventional scanning laser Doppler velocimeter.

  Hereinafter, the present invention will be described with reference to embodiments shown in the drawings, but the present invention is not limited to the embodiments shown in the drawings. In the drawings referred to below, the same reference numerals are used for common elements, and the description thereof is omitted as appropriate.

  FIG. 1 shows a schematic configuration diagram of a laser Doppler velocimeter 10 according to an embodiment of the present invention. As shown in FIG. 1A, the laser Doppler velocimeter 10 of this embodiment includes a wavelength tunable laser 11 as a light source, and laser light output from the wavelength tunable laser 11 is unidirectionally transmitted by a polarizer 12. After being polarized, only the polarized light propagates through the optical fiber 13 and is input to the sensor unit 14. A sensor substrate 100 is mounted on the sensor unit 14, and the optical fiber 13 is optically connected to the sensor substrate 100.

  FIG. 1B shows a side view of the sensor substrate 100 mounted on the sensor unit 14. The light propagating through the optical fiber 13 is input to the input port 20 of the sensor substrate 100, then propagates through the arrayed waveguide type diffraction grating 30 formed on the sensor substrate 100, and flows from the output port 50 to the flow path 15. To be emitted as probe light. Here, the output port 50 is composed of a plurality of ports, and the driver 19 as a control means applies a control voltage to the arrayed waveguide type diffraction grating 30 via the signal line 18 to output probe light. The port is configured to be selected alternatively.

  On the other hand, scattered light generated at a measurement point in the flow path 15 is received by a light receiving unit (not shown) formed on the sensor substrate 100 and converted into an electric signal by a light receiving element 70 referred to as a photodiode or the like. The The converted electric signal is output to the signal processing unit 17 through the signal line 16. The signal processing unit 17 obtains the flow velocity at the measurement point by measuring the frequency of the beat signal of the scattered light input from the sensor substrate 100.

  The schematic configuration of the laser Doppler velocimeter 10 of the present embodiment has been described above. Next, the sensor substrate 100 will be described.

FIG. 2 is a top view of the sensor substrate 100 in the present embodiment. In the present embodiment, the sensor substrate 100 is formed as an optical circuit substrate including an optical waveguide composed of a Si core and a SiO ₂ clad by silicon photonics.

  As shown in FIG. 2, the sensor substrate 100 is formed with two equivalent array waveguide type diffraction gratings (hereinafter referred to as AWGs) facing each other. The first AWG 30a includes an optical input waveguide 32a connected to the input port 20, a slab waveguide 34a connected to the optical input waveguide 32a, and a slab guide connected to five optical output waveguides 38a. The waveguide 37a includes an arrayed waveguide 36a composed of a plurality of waveguides connecting the emission side of the slab waveguide 34a and the incident side of the slab waveguide 37a. Similarly, the second AWG 30b is connected to an optical input waveguide 32b connected to the input port 20, a slab waveguide 34b connected to the optical input waveguide 32b, and five optical output waveguides 38b. Slab waveguide 37b, and an arrayed waveguide 36b composed of a plurality of waveguides connecting the emission side of the slab waveguide 34b and the incident side of the slab waveguide 37b. Here, in the example shown in FIG. 2, the slab waveguide 34a and the slab waveguide 34b, and the slab waveguide 37a and the slab waveguide 37b are integrally formed.

  A grating coupler 50a that functions as an output port is connected to the tip of each of the five optical output waveguides 38a of the first AWG 30a. Similarly, the five optical output waveguides 38b of the second AWG 30b A grating coupler 50b that functions as an output port is connected to each tip.

  In the present embodiment, a row of five grating couplers 50a and a row of five grating couplers 50b are arranged in parallel so as to face each other, and the scattered light is received between the two rows. Are arranged. In the present embodiment, a large-area grating coupler 60 is disposed between two rows in order to efficiently receive scattered light from a measurement point and ensure an S / N ratio. Here, the scattered light received by the grating coupler 60 is photoelectrically converted by the light receiving element 70 optically connected to the grating coupler 60 and output to the signal processing unit 17 as an electric signal.

  The configuration of the sensor substrate 100 has been outlined above. Next, a process will be described until light input to the input port 20 is demultiplexed and output by the AWGs 30a and 30b.

  In the present embodiment, light input to the sensor substrate 100 is split at the input port 20 and is branched into the optical input waveguide 32a and the optical input waveguide 32b. Here, the input port 20 in the present embodiment can be configured using, for example, a grating coupler and an optical branching unit.

  Specifically, as shown in FIG. 3A, the output end of the optical fiber 13 is arranged at a predetermined angle on the back side of the sensor substrate 100 so that light enters the grating coupler from the back side of the substrate. After the configuration, any optical branching means (Y branch, multimode interferometer, directional coupler, etc .: see FIG. 4) is optically connected downstream of the grating coupler. In this case, the light incident from the back side of the sensor substrate 100 travels toward the downstream optical branching unit through the grating coupler.

  In the present embodiment, as shown in FIG. 3B, the output end of the optical fiber 13 is arranged at a predetermined angle on the front side of the sensor substrate 100, and light enters the grating coupler from the front side of the substrate. You may comprise. Also, as shown in FIG. 3C, the optical fiber 13 and the Si core of the optical waveguide are optically connected by arranging the output end of the optical fiber 13 so as to face the end surface of the sensor substrate 100. Also good (end face bonding method).

  Here, the grating coupler used in the present embodiment will be described with reference to FIG.

  When the diffraction grating formed at the end of the waveguide satisfies the following formula (1), the light propagating through the waveguide is reflected in the diffraction grating at the end as shown in FIG. The light is emitted with a wavefront having an inclination angle θ with respect to the surface.

In the above formula (1), m represents the diffraction order (integer), λ represents the wavelength of light propagating through the waveguide, n _eq represents the equivalent refractive index of the waveguide, and Λ ₁ represents the grating period. .

  Similarly, when the diffraction grating formed at the end of the waveguide satisfies the above formula (1), the light incident at the wavefront having the inclination angle θ with respect to the grating surface of the diffraction grating at the end is Due to the reciprocity, as shown in FIG. 5B, the light is coupled to the light propagating in the waveguide connected to the diffraction grating.

  The grating coupler used in the present embodiment utilizes the above-described properties of the diffraction grating, and the grating coupler connected to the waveguide is a coupler that emits light propagating from the waveguide at an emission angle corresponding to the wavelength. It has a function of propagating light of a predetermined wavelength, which is emitted from the surface and incident at a predetermined incident angle from the coupler surface, into the waveguide.

  Returning again to FIG. 2, the description will be continued. When wavelength multiplexed light is input to the input port 20, one branched light (wavelength multiplexed light) is introduced into the slab waveguide 34a through the optical input waveguide 32a. The light introduced into the slab waveguide 34a is spread by the diffraction effect and enters each waveguide constituting the arrayed waveguide 36a.

  Thereafter, the light propagates through the arrayed waveguide 36a and is introduced into the slab waveguide 37a. The light introduced into the slab waveguide 37a is condensed by the diffraction effect.

  Here, each waveguide constituting the arrayed waveguide 36a is formed so that the path lengths of adjacent waveguides are different from each other by ΔL, so that the phase of the light in each waveguide is propagated in the process of propagating through the arrayed waveguide 36a. Deviation occurs. In the AWG 30a, the wavefront of the focused light is tilted according to the phase shift amount, and the condensing position on the emission side of the slab waveguide 37a is determined according to the tilt angle.

  At this time, since the diffraction angle of the light incident on the slab waveguide 37a from the arrayed waveguide 36a differs depending on the wavelength λ of the light, the light is condensed at different positions for each wavelength. On the other hand, on the exit side of the slab waveguide 37a, the entrances of the five waveguides constituting the light output waveguide 38b are formed at the condensing position of each wavelength light. The light enters each of the waveguides.

  When the light incident on the five waveguides constituting the optical output waveguide 38a reaches the grating coupler 50a connected to each of the output ends, the light is output according to the wavelength as described above with reference to FIG. The light is emitted from the coupler surface at the corner in the out-of-plane direction of the sensor substrate 100.

  As described above, the AWG 30a has the ability to demultiplex wavelength-multiplexed light into five types of wavelength light when input from the input port 20, and the center of the light demultiplexed by the AWG 30a. The wavelength is proportional to the path length difference (ΔL) of each waveguide constituting the arrayed waveguide 36a and its equivalent refractive index.

  The contents described so far are the same in the AWG 30b having a configuration equivalent to the AWG 30a. The AWG 30b demultiplexes the wavelength multiplexed light into five types of wavelength light when it is input from the input port 20. The five grating couplers 50b emit light in the out-of-plane direction of the sensor substrate 100.

  The process until the light input to the input port 20 is demultiplexed and output by the AWGs 30a and 30b has been outlined above. In the laser Doppler velocimeter 10 of the present embodiment, the AWG demultiplexing function described above is used. To switch the output port of the probe light. Here, the measurement mechanism of the laser Doppler velocimeter 10 including the switching will be described again. In the following description, FIG. 1 will be referred to as appropriate.

In the laser Doppler velocimeter 10, when the laser light having the wavelength λ ₁ output from the wavelength tunable laser 11 is input to the input port 20 of the sensor substrate 100 through the optical fiber 13, a pair of grating couplers 50 a disposed to face each other. the light of wavelength lambda ₁ from each 50b is emitted to the out-of-plane direction of the sensor substrate 100. Here, the pair of grating couplers 50a and 50b are positioned and arranged so that the two optical paths intersect when light of the same wavelength is emitted from each of them, as shown in FIG. coupler 50a, the light of the wavelength lambda ₁ emitted from each of 50b causes the interference fringes intersect at the measurement point.

  Scattered light from the tracer particles passing through the interference fringes generated at the measurement point is received by the grating coupler 60 disposed between the pair of grating couplers 50a and 50b, as shown in FIG. The scattered light received by the grating coupler 60 is converted into an electrical signal by the light receiving element 70 optically connected thereto, and then output to the signal processing unit 17 via the signal line 16. The signal processing unit 17 obtains the flow velocity at the measurement point by measuring the frequency of the beat signal of the scattered light input from the light receiving element 70.

Similarly, when laser light having a wavelength λ ₂ is output from the wavelength tunable laser 11, light having a wavelength λ ₂ is emitted from each of the grating couplers 50a and 50b, and interference fringes are generated at measurement points. At this time, the light emitted from the grating couplers 50a and 50b has its emission angle changed according to the wavelength as shown in FIG. 6B. In this embodiment, this is utilized to switch the output wavelength of the wavelength tunable laser 11 to achieve scanning in the depth direction as shown in FIG.

  However, as shown in FIG. 7, in a normal AWG, the demultiplexed light is output only at a position fixed according to the wavelength. Therefore, when the output wavelength of the wavelength tunable laser 11 is switched, different wavelength light is emitted. Only the output port is changed and the light is emitted, and in this state, neither horizontal scanning nor depth scanning can be realized.

Here, in order to realize two-dimensional scanning, a configuration for emitting all wavelengths (λ ₁ , λ ₂ , λ ₃ , λ ₄ , λ ₅ ) at each of the five output ports is required. . With respect to this point, in the present embodiment, two-dimensional scanning is realized by emitting all wavelengths from each output port by applying a voltage to the liquid crystal layer 40 formed on the arrayed waveguide 36. Hereinafter, a mechanism for emitting all wavelengths from each output port will be described.

  As shown in FIG. 7, the arrayed waveguides (36a, 36b) in the present embodiment have a region in which a plurality of linear waveguides are arranged in parallel to each other at the central portion. Hereinafter, this region is referred to as a phase variable region. In this phase variable region, the path lengths of adjacent waveguides are substantially equal. In this embodiment, the liquid crystal layers 40a and 40b are formed so as to cross a plurality of waveguides constituting this phase variable region. ing.

  FIG. 8A shows an enlarged view of the liquid crystal layer 40a formed in the phase variable region of the arrayed waveguide 36a. As shown in FIG. 8A, the liquid crystal layer 40a includes two liquid crystal layers 40 (a) having an isosceles triangle shape that widens from the shorter path length to the longer path length of the arrayed waveguide 36a and the arrayed waveguide 36a. The liquid crystal layer 40 (b) and the liquid crystal layer 40 (b) are alternately arranged in the light propagation direction. Is formed. The shape of the liquid crystal layer 40 is not limited to an isosceles triangle, but may be a taper shape that widens in the transverse direction of the arrayed waveguide 36a.

  FIG. 8B shows a cross-sectional view taken along line X-X ′ in FIG. As shown in FIG. 8B, the liquid crystal layers 40 (a) and (b) constitute an upper clad layer of the arrayed waveguide 36a located immediately below the liquid crystal layers 40 (a) and (b). A transparent electrode (ITO) is formed on the upper layer of each of the liquid crystal layers 40 (a) and (b) (the transparent electrode is not shown in other drawings). In the present embodiment, the four liquid crystal layers 40 (a), (b), (c), and (d) are configured so that voltages can be individually applied through electrodes disposed in the respective liquid crystal layers 40 (a), (b), (c), and (d). ing.

Here, the liquid crystal molecules constituting the liquid crystal layer 40 are a substance having a birefringence called refractive index anisotropy, and the refractive index of the liquid crystal molecules is shown in FIG. as such, changes to any of the extraordinary refractive index n _{e (highest} refractive index state) or the ordinary refractive index n _{o (most} refractive index is low).

That is, in this embodiment, as shown in FIG. 9B, a positive charge is applied to the liquid crystal layer 40 to _tilt the liquid crystal molecules by + θ _tilt from the alignment direction (shown by a broken line). The refractive index of the liquid crystal layer 40 is decreased by applying a negative charge to the liquid crystal layer 40 and tilting the liquid crystal molecules by −θ _tilt from the alignment direction (shown by a broken line). ing. That is, in the present embodiment, by applying a positive or negative voltage to the liquid crystal layer 40, the refractive index of the liquid crystal layer 40 transitions between two states (high refractive index and low refractive index). It is configured.

  In the present embodiment, in response to the change in the refractive index of the liquid crystal layer 40, the equivalent refractive index (phase) of each waveguide having the liquid crystal layer 40 as an upper cladding layer changes. Here, referring again to FIG. 8, the liquid crystal layer 40 is formed in a tapered shape so as to cross a plurality of waveguides constituting the arrayed waveguide 36a, so that the liquid crystal layers 40 (a), (b) ), The length of the liquid crystal layer loaded as the upper clad layer (hereinafter referred to as the liquid crystal layer loaded length) becomes longer as it goes to the outside (the longer path length side), and the phase changes toward the outside. Will appear greatly. On the other hand, immediately below the liquid crystal layers 40 (c) and (d), the liquid crystal layer loading length becomes longer as it goes inward (the shorter path length side), and as a result, the phase change becomes larger as it goes inward. .

  FIG. 10 shows the relationship between the pattern of voltage application to the liquid crystal layer 40 and the wavefront slope produced thereby. First, consider a case where a negative voltage is applied to all of the four liquid crystal layers 40 (a) to (d) shown in FIG. In this case, the refractive index of the arrayed waveguide 36 immediately below the liquid crystal layers 40 (a) and (b) decreases toward the outside (the side with the longer path length), and immediately below the liquid crystal layers 40 (c) and (d). The refractive index of the arrayed waveguide 36 decreases as it goes inward (the shorter path length side). In this case, as a result of canceling out both effects, the phase change does not occur in the phase variable region, and the wavefront does not change, so that the predetermined wavelength light is emitted from the output port which is the original output position. This also applies to the case where a positive voltage is applied to all four liquid crystal layers 40 (a) to (d).

  Next, as shown in FIG. 10 (b), a positive voltage is applied to the liquid crystal layer 40 (b), and a negative voltage is applied to the remaining liquid crystal layers 40 (c), (b), and (d). Consider the case where a voltage is applied. Here, since the liquid crystal layers 40 (c) and (b) cancel out the effects, attention is paid to the liquid crystal layers 40 (a) and (d). In this case, the refractive index of the arrayed waveguide 36 immediately below the liquid crystal layer 40 (A) increases toward the outside and the phase is delayed, and the arrayed waveguide 36 directly below the liquid crystal layer 40 (D) is refracted toward the inside. The rate decreases and the phase advances. As a result, the wavefront tilts as shown in FIG. In this case, the predetermined wavelength light is emitted from another output port shifted by +1 with respect to the original output position.

  Next, as shown in FIG. 10 (c), a positive voltage is applied to the liquid crystal layers 40 (a) and (b), and a negative voltage is applied to the remaining liquid crystal layers 40 (c) and (d). Consider the case where a voltage is applied. In this case, the refractive index of the arrayed waveguide 36 immediately below the liquid crystal layers 40 (a) and (b) increases toward the outside and the phase lags, so that the array waveguides directly below the liquid crystal layers 40 (c) and (d). As the waveguide 36 goes inward, the refractive index decreases and the phase advances. As a result, the wavefront tilts as shown in FIG. In this case, the predetermined wavelength light is emitted from another output port shifted by +2 with respect to the original output position.

  Next, as shown in FIG. 10 (d), a positive voltage is applied to the liquid crystal layer 40 (c), and a negative voltage is applied to the remaining liquid crystal layers 40 (a), (b), and (d). Consider the case where a voltage is applied. Here, since the liquid crystal layers 40 (B) and (D) cancel out the effects, attention is paid to the liquid crystal layers 40 (A) and (C). In this case, the refractive index of the arrayed waveguide 36 directly below the liquid crystal layer 40 (a) decreases and the phase advances toward the outside, and the refractive index of the arrayed waveguide 36 directly below the liquid crystal layer 40 (c) increases toward the inside. The phase is delayed. As a result, the wavefront tilts as shown in FIG. In this case, the predetermined wavelength light is emitted from another output port shifted by −1 with respect to the original output position.

  Next, as shown in FIG. 10 (e), a negative voltage is applied to the liquid crystal layers 40 (b) and (b), and a positive voltage is applied to the remaining liquid crystal layers 40 (c) and (d). Consider the case where a voltage is applied. In this case, the refractive index of the arrayed waveguide 36 immediately below the liquid crystal layers 40 (a) and (b) decreases and the phase advances toward the outside, and the array waveguides directly below the liquid crystal layers 40 (c) and (d). In the waveguide 36, the refractive index increases and the phase is delayed toward the inner side. As a result, the wavefront tilts as shown in FIG. In this case, the predetermined wavelength light is emitted from another output port shifted by -2 with respect to the original output position.

As described above, the relationship between the pattern of voltage application to the liquid crystal layer 40 and the inclination of the wavefront generated thereby has been described. In the present embodiment, a ferroelectric liquid crystal (FLC) is used as the liquid crystal layer 40. It is preferable to do. FLC is on the refractive index difference is large as _{0.143 (n e = 1.653, n} O = 1.51), has excellent response speed (several tens [.mu.s]), in addition, memory property (last voltage by spontaneous polarization In other words, the power consumption for controlling the liquid crystal can be reduced.

  Next, on the premise of the contents described so far, a control sequence executed by the driver 19 to realize two-dimensional scanning will be described with reference to FIG. In the following description, FIG. 1 will be referred to as appropriate.

  In the present embodiment, the driver 19 repeatedly executes the following steps 1 to steps for measurement.

First, in step 1, and controls the output wavelength of the tunable laser 11 to the wavelength lambda _1, in the period, the shift amount of the output port is [-2] → [-1] → [0] → [+1 ] → [+2] The voltage applied to the liquid crystal layers 40a and 40b is controlled so as to transit in the order (see FIG. 10 for the voltage application pattern corresponding to each shift amount, the same applies hereinafter). As a result, the output port of the light of wavelength λ ₁ is switched in the order of [4] → [5] → [1] → [2] → [3].

In subsequent step 2, to control the output wavelength of the tunable laser 11 to the wavelength lambda _2, in the period, the shift amount of the output port is [-2] → [-1] → [0] → [+1] → The voltage applied to the liquid crystal layers 40a and 40b is controlled so as to transit in the order of [+2]. As a result, the output ports of the wavelength lambda ₂ of light is switched in the order of [5] → [4] → [3] → [2] → [1].

In the subsequent step 3, the output wavelength of the wavelength tunable laser 11 is controlled to the wavelength λ ₃ and the shift amount of the output port is changed from [−2] → [−1] → [0] → [+1] during the period. → The voltage applied to the liquid crystal layers 40a and 40b is controlled so as to transit in the order of [+2]. As a result, the light output port of the wavelength lambda ₃ is switched in the order of [1] → [2] → [3] → [4] → [5].

In the subsequent step 4, the output wavelength of the wavelength tunable laser 11 is controlled to the wavelength λ ₄ and the shift amount of the output port is changed from [−2] → [−1] → [0] → [+1] during the period. → The voltage applied to the liquid crystal layers 40a and 40b is controlled so as to transit in the order of [+2]. As a result, the output port of the light of wavelength λ ₄ is switched in the order [2] → [3] → [4] → [5] → [1].

In the subsequent step 5, the output wavelength of the wavelength tunable laser 11 is controlled to the wavelength λ ₅ and the shift amount of the output port is changed from [2] → [−1] → [0] → [+1] → The voltage applied to the liquid crystal layers 40a and 40b is controlled so as to transition in the order of [+2]. As a result, the light output port of the wavelength lambda ₅ is switched in the order of [3] → [4] → [5] → [1] → [2].

  As is apparent from FIG. 11, in the present embodiment, the measurement point moves in the horizontal direction in synchronization with the control timing of the voltage applied to the liquid crystal layers 40a and 40b, and the output wavelength of the wavelength tunable laser 11 changes. The measurement point moves in the depth direction in synchronization with the switching timing. That is, according to the sequence control shown in FIG. 11, two-dimensional scanning is realized in which the horizontal direction is the main scanning direction and the depth direction is the sub-scanning direction. It should be easily understood that by adopting another sequence, two-dimensional scanning in which the depth direction is the main scanning direction and the horizontal direction is the sub-scanning direction can be realized.

  Heretofore, the description has been made based on the embodiment in which the five types of wavelength light are switched in the five sets of output ports (output channels). However, this is merely an exemplary aspect for explanation, and in the present invention. It goes without saying that any number of output channels that are two or more can be configured to switch. For example, in the minimum case where two output channels are switched, one liquid crystal layer 40 may be formed for each AWG. When N output channels (N is an integer of 3 or more) are switched, the first liquid crystal layer that widens from the shorter path length to the longer path length and the shorter path length from each path to each AWG. It is only necessary to form (N−1 / 2) second liquid crystal layers that widen toward each other.

  Finally, a method for designing the liquid crystal layer 40 will be described.

  In the present embodiment, the output port is shifted by changing the diffraction order. Here, when the diffraction order is changed to the first order, the output wavelength is switched by FSR (free spectral range). The following formula (2) shows the relational expression of FSR.

  In the above equation (2), N represents the number of output channels, and Δλ represents the channel wavelength interval.

In the present embodiment, the optical path difference n _eff · ΔL ₁ necessary for switching the Np port is obtained by the following equation (3).

In the above equation (3), λ represents the center wavelength, n _eff represents the equivalent refractive index of the waveguide, n _eff · ΔL represents the required optical path difference, and N p represents the number of switching ports.

Next, the liquid crystal loading length for obtaining the obtained optical path difference with the liquid crystal is calculated. The optical path created by ± θ _tilt of the liquid crystal with the alignment direction as the center can be expressed by the following formula (4).

In the above formula (4), L represents the liquid crystal loading length for obtaining the required optical path difference, and n ± θ _eff represents the equivalent refractive index of the waveguide during liquid crystal operation.

  When the above formula (4) is arranged for L, the following formula (5) is obtained.

  In the present embodiment, the liquid crystal loading length L for obtaining the required optical path difference can be obtained from the above formula (5), and the size of the tapered liquid crystal loading region is obtained by multiplying L by an integer for each arrayed waveguide. Can be designed. Specifically, the number N of arrayed waveguides is expressed by the following formula (6).

In the above equation (6), D is an input / output waveguide interval, and X _FSR is a free space interval represented by the following equation (7).

In the above equation (7), f is the radius of curvature of the slab waveguide, n _s is the equivalent refractive index of the slab waveguide, and d is the arrayed waveguide interval at the connection between the arrayed waveguide and the slab connection.

  In the present embodiment, the size of the tapered liquid crystal layer 40 may be designed so that the liquid crystal loading lengths of adjacent waveguides in the arrayed waveguide are different by L and the maximum length is N × L.

  As described above, according to the present invention, a scanning laser Doppler velocimeter that is further downsized than the conventional one is provided. Since the laser Doppler velocimeter of the present invention employs optical wiring based on silicon photonics, it is not necessary to adjust the optical axis, and in principle there is no optical axis misalignment, and it has excellent impact resistance. In addition, the ultra-small probe can be realized by the strong optical confinement of the silicon waveguide, and can be easily mass-produced by forming the waveguide using the CMOS process. In addition, since switching control is performed by using the change in the refractive index of the liquid crystal due to the electric field, the power consumption is remarkably low compared with the conventional configuration using the thermo-optic effect and current injection, and between elements. Therefore, it is possible to cope with high density integration.

  The laser Doppler velocimeter of the present invention can be applied to clinical medicine such as a non-invasive measurement of blood flow velocity distribution by attaching a small probe in the vicinity of a blood vessel using a wavelength tunable laser emitting light in the near-infrared region with high biological tissue permeability. The application can be greatly expected. However, it goes without saying that the present invention is not intended to limit its application.

  As described above, the present invention has been described with the embodiments. However, the present invention is not limited to the above-described embodiments, and other functions and effects of the present invention are within the scope of embodiments that can be considered by those skilled in the art. As long as it plays, it is included in the scope of the present invention.

  Each function of the above-described embodiment can be realized by a device-executable program written in C, C ++, C #, Java (registered trademark) or the like, and the program of this embodiment includes a hard disk device, a CD-ROM, an MO It can be stored in a device-readable recording medium such as a DVD, a flexible disk, an EEPROM, or an EPROM, and can be distributed, and can be transmitted via a network in a format that other devices can.

DESCRIPTION OF SYMBOLS 10 ... Laser Doppler velocimeter 11 ... Wavelength variable laser 12 ... Polarizer 13 ... Optical fiber 14 ... Sensor part 15 ... Flow path 16 ... Signal line 17 ... Signal processing part 18 ... Signal line 19 ... Driver 20 ... Input port 30 ... Array waveguide type diffraction grating 32 ... Optical input waveguide 34 ... Slab waveguide 36 ... Array waveguide 37 ... Slab waveguide 38 ... Optical output waveguide 40 ... Liquid crystal layer 50 ... Output port (grating coupler)
60 ... Grating coupler 70 ... Light receiving element 100 ... Sensor substrate 500 ... Scanning LDV
DESCRIPTION OF SYMBOLS 501 ... Laser 502 ... Polarizer 503 ... Splitter 504 ... Lens optical system 505 ... Rotating mirror 506 ... Driver 507 ... Flow path 508 ... Light receiving element 509 ... Signal processing part 600 ... Scanning LDV
DESCRIPTION OF SYMBOLS 601 ... Tunable laser 602 ... Polarizer 603 ... Optical switch 604 ... Optical fiber array 605 ... Flow path 607 Signal processing part 609 ... Driver 610 ... Sensor part 611 ... Diffraction grating 612 ... Mirror 613 ... Splitter 614 ... Light receiving element

Claims (12)

A tunable laser having two or more output wavelengths;
A sensor unit including a sensor substrate for emitting two probe lights formed by dividing the laser light emitted from the wavelength tunable laser to a measurement point and outputting a detection signal of scattered light generated from the measurement point;
Control means for variably controlling the output wavelength and the voltage applied to the sensor substrate;
A signal processing unit for measuring a flow velocity based on the detection signal of the scattered light;
Including
The sensor substrate is
An input port for inputting the laser beam;
Two arrayed-waveguide-type diffraction gratings each having two light beams divided at the input port as inputs, and having a phase variable region having substantially the same path length of a waveguide adjacent to a part of the arrayed waveguide And first and second arrayed waveguide gratings,
N first grating couplers connected to N (N is an integer of 2 or more) optical output waveguides of the first arrayed waveguide grating and the second arrayed waveguide gratings N sets of output ports configured by arranging N second grating couplers connected to N optical output waveguides in two rows;
A light receiving portion for receiving the scattered light;
Including
Each of the arrayed waveguide gratings is
Including tapered (N-1) liquid crystal layers formed across the phase variable region and functioning as an upper cladding layer of the waveguide;
The control means includes
The output port that emits probe light having a wavelength λ by applying an individual positive or negative voltage to each liquid crystal layer to change the equivalent refractive index of each waveguide constituting the phase variable region Switch
Laser Doppler velocimeter.
The light receiving unit includes a third grating coupler disposed between the first and second grating couplers.
The laser Doppler velocimeter according to claim 1. For each of the phase variable regions, a first liquid crystal layer that widens from the shorter path length of the arrayed waveguide to a longer one and a second liquid crystal layer that widens from the longer path length to the shorter one of the array waveguides, respectively. (N−1 / 2) pieces are formed (N is an integer of 3 or more),
The laser Doppler velocimeter according to claim 1 or 2. The control means includes
(N-1) By applying voltages to the N liquid crystal layers in N patterns, the position of the output port that emits the probe light having the wavelength λ is switched to N ways.
The laser Doppler velocimeter as described in any one of Claims 1-3. The control means includes
While switching and controlling the N output wavelengths, the measurement points are switched by switching the voltage application to the (N−1) liquid crystal layers in N patterns during the period in which the output wavelength is controlled to the wavelength λ. Scan in two dimensions,
The laser Doppler velocimeter according to claim 4. The liquid crystal layer is a ferroelectric liquid crystal layer;
The laser Doppler velocimeter according to any one of claims 1 to 5. The first liquid crystal layer and the second liquid crystal layer are alternately formed.
The laser Doppler velocimeter according to any one of claims 3 to 5. A sensor for laser Doppler flow velocity measurement,
An input port for inputting laser light;
Two arrayed-waveguide-type diffraction gratings each having two light beams divided at the input port as inputs, and having a phase variable region having substantially the same path length of a waveguide adjacent to a part of the arrayed waveguide And first and second arrayed waveguide gratings,
N first grating couplers connected to N (N is an integer of 2 or more) optical output waveguides of the first arrayed waveguide grating and the second arrayed waveguide gratings N sets of output ports configured by arranging N second grating couplers connected to N optical output waveguides in two rows;
A light receiving unit for receiving scattered light generated from the measurement point;
Including
Each of the arrayed waveguide gratings is
Tapered (N-1) liquid crystal layers formed across the phase variable region and functioning as an upper cladding layer of the waveguide;
Electrodes for applying a positive or negative voltage individually to each of the liquid crystal layers;
including,
Sensor. The light receiving unit includes a third grating coupler disposed between the first and second grating couplers.
The sensor according to claim 8. For each of the phase variable regions, a first liquid crystal layer that widens from the shorter path length of the arrayed waveguide to a longer one and a second liquid crystal layer that widens from the longer path length to the shorter one of the array waveguides, respectively. (N−1 / 2) pieces are formed (N is an integer of 3 or more),
The sensor according to claim 8 or 9. The first liquid crystal layer and the second liquid crystal layer are alternately formed.
The sensor according to claim 10. The liquid crystal layer is a ferroelectric liquid crystal layer;
The sensor as described in any one of Claims 8-11.