Light modulation device and method for controlling spatial light modulator

Abstract

To provide a light modulating device that can reduce deviation of an actual phase modulation amount from a target phase modulation amount.SOLUTION: A light modulating device 1A comprises a light source 3, a control unit 7, and a spatial light modulator 10. The light source 3 outputs a laser beam La having an intensity according to a preset intensity. The spatial light modulator 10 has a plurality of pixel electrodes, a liquid crystal layer, a driving unit, and a cooling unit. The liquid crystal layer modulates the phase of the laser beam La according to the size of an electric field formed by the pixel electrodes. The driving unit applies voltage to the plurality of pixel electrodes. The cooling unit cools the liquid crystal layer so that the temperature of the liquid crystal layer becomes closer to a preset temperature. The control unit 7 determines the preset temperature for the cooling unit based on the preset intensity of the laser beam La.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present disclosure relates to a light modulation device and a method for controlling a spatial light modulator. [Background technology]

[0002] Patent Document 1 discloses a spatial light modulation device and a spatial light modulation method. The spatial light modulation device modulates the phase of incident light for each of multiple pixels arranged one-dimensionally or two-dimensionally. The spatial light modulation device includes a liquid crystal layer, a temperature sensor, multiple pixel electrodes, and a voltage generation unit. The liquid crystal layer modulates the phase of incident light according to the magnitude of an applied electric field. The temperature sensor generates a temperature signal corresponding to the temperature of the liquid crystal layer. The pixel electrodes are provided for each pixel and apply a voltage to the liquid crystal layer that generates an applied electric field. The voltage generation unit provides voltages to the multiple pixel electrodes. The voltage generation unit includes a memory means. The memory means pre-stores one or more coefficients included in a function representing the correlation between the temperature change amount of the liquid crystal layer relative to a reference temperature and the fluctuation amount of phase modulation in the liquid crystal layer. The voltage generation unit performs a calculation to correct the magnitude of the voltage using the temperature indicated in the temperature signal provided by the temperature sensor and the one or more coefficients. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Patent Publication No. 2021-168391 Summary of the Invention [Problem to be solved by the invention]

[0004] A conventional technique for modulating the phase of light using a spatial light modulator (SLM) is known. A liquid crystal spatial light modulator includes a liquid crystal layer and multiple pixel electrodes arranged along the liquid crystal layer. When a voltage is applied to the pixel electrodes, the liquid crystal molecules rotate according to the magnitude of the voltage, changing the birefringence of the liquid crystal. When light enters the liquid crystal layer, the phase of the light changes within the liquid crystal layer, and light with a phase difference relative to the incident light is emitted to the outside. The phase modulation characteristics of a spatial light modulator represent the relationship between the magnitude of the applied voltage and the phase difference of the emitted light before and after the voltage application, i.e., the phase modulation amount. In the phase modulation characteristics, the relationship between the phase modulation amount and the applied voltage is nonlinear. To easily convert such a nonlinear relationship, a lookup table (LUT) is used, which shows the correspondence between the grayscale value representing the phase modulation amount and the applied voltage.

[0005] When the temperature of the liquid crystal layer changes, the relationship between the amount of phase modulation and the applied voltage fluctuates. That is, even when a certain voltage is applied, the amount of phase modulation varies depending on the temperature of the liquid crystal layer at that time. In the spatial light modulation device and spatial light modulation method disclosed in Patent Document 1, the magnitude of the applied voltage is corrected using the temperature of the liquid crystal layer detected by a temperature sensor, based on the correlation between the amount of temperature change of the liquid crystal layer relative to a reference temperature and the amount of fluctuation in the amount of phase modulation in the liquid crystal layer.

[0006] However, in many cases, the temperature sensor is located far from the liquid crystal layer, for example, on the back surface of the spatial light modulator, opposite the light incident surface. Therefore, various components, such as a substrate, are interposed between the liquid crystal layer and the temperature sensor, causing the temperature detected by the temperature sensor to deviate from the temperature of the liquid crystal layer. In particular, as the intensity of light incident on the liquid crystal layer increases, the energy of the light significantly increases the temperature of the liquid crystal layer, increasing the temperature deviation between the liquid crystal layer and the temperature sensor. A large temperature deviation between the liquid crystal layer and the temperature sensor makes it difficult to accurately correct the magnitude of the applied voltage using the temperature detected by the temperature sensor. As a result, the actual phase modulation amount deviates from the target phase modulation amount. This phenomenon can cause serious problems depending on the application of the spatial light modulator. For example, in laser processing, when laser light output from a laser light source is irradiated onto a workpiece via a spatial light modulator, deviations in the phase modulation amount significantly affect processing accuracy. Furthermore, when a spatial light modulator is used in a microscope or ophthalmoscope, depending on the operating temperature, a useful observation image may not be obtained.

[0007] An object of the present disclosure is to provide an optical modulation device and a method for controlling a spatial light modulator that can reduce the deviation of an actual phase modulation amount from a target phase modulation amount. [Means for solving the problem]

[0008] [1] A first light modulation device according to an embodiment of the present disclosure includes a light source, a spatial light modulator, and a control unit. The light source outputs light having an intensity according to a set intensity. The spatial light modulator includes a plurality of pixel electrodes, a liquid crystal layer, a driving unit, and a cooling unit. The plurality of pixel electrodes are arranged one-dimensionally or two-dimensionally. The liquid crystal layer modulates the phase of the light according to the magnitude of an electric field formed by each pixel electrode. The driving unit applies a voltage to the plurality of pixel electrodes. The cooling unit cools the liquid crystal layer so that the temperature of the liquid crystal layer approaches a set temperature. The control unit determines a set temperature for the cooling unit. The control unit determines the set temperature based on the set intensity.

[0009] [2] A first control method for a spatial light modulator according to an embodiment of the present disclosure is a method for controlling a spatial light modulator. The spatial light modulator has a plurality of pixel electrodes, a liquid crystal layer, a driving unit, and a cooling unit. The plurality of pixel electrodes are arranged one-dimensionally or two-dimensionally. The liquid crystal layer modulates the phase of light according to the magnitude of an electric field formed by each pixel electrode. The driving unit applies a voltage to the plurality of pixel electrodes. The cooling unit cools the liquid crystal layer so that the temperature of the liquid crystal layer approaches a set temperature. The first control method includes a determining step of determining a set temperature for the cooling unit and a light input step of inputting light having an intensity according to the set intensity into the liquid crystal layer. In the determining step, the set temperature is determined based on the set intensity.

[0010] As described above, when the intensity of light incident on the liquid crystal layer increases, the energy of the light significantly increases the temperature of the liquid crystal layer, increasing the temperature difference between the liquid crystal layer and the temperature sensor. Therefore, in the first optical modulation device and the first control method, the control unit (or the determining step) determines the set temperature of the cooling unit based on the set intensity of the light when bringing the temperature of the liquid crystal layer closer to a predetermined temperature. The set temperature of the cooling unit is determined based on the set intensity of the light incident on the liquid crystal layer. This allows the temperature of the liquid crystal layer to approach a constant value regardless of the intensity of the light incident on the liquid crystal layer. This enables the applied voltage to be set correctly, and the actual phase modulation amount to accurately approach the target phase modulation amount. Therefore, the first optical modulation device and the first control method can reduce the deviation of the actual phase modulation amount from the target phase modulation amount. In the first optical modulation device and the first control method, the relationship between the phase modulation amount and the applied voltage is set based on, for example, a predetermined temperature, rather than a set temperature.

[0011] [3] In the first light modulation device of [1] above, the control unit may previously store a data table showing the relationship between the set intensity and the set temperature, and may determine the set temperature using the data table. [4] Alternatively, in the first light modulation device of [1] above, the control unit may previously store a calculation formula showing the relationship between the set intensity and the set temperature, and may determine the set temperature using the calculation formula. In either of these cases, the control unit can easily determine the set temperature according to the set intensity. Similarly, [5] in the determination step of the first control method of [2] above, the set temperature may be determined using a data table showing the relationship between the set intensity and the set temperature. [6] Alternatively, in the determination step of the first control method of [2] above, the set temperature may be determined using a calculation formula showing the relationship between the set intensity and the set temperature. In either of these cases, the determination step can easily determine the set temperature according to the set intensity.

[0012] [7] In any one of the first light modulation devices [1], [3], and [4] above, the set temperature may be proportional to the set intensity. [8] Similarly, in any one of the first control methods [2], [5], and [6] above, the set temperature may be proportional to the set intensity. According to the inventor's findings, the temperature of the liquid crystal layer deviates from the temperature detected by the temperature sensor in proportion to the intensity of light input to the liquid crystal layer. By determining the set temperature of the cooling unit so as to be proportional to the set intensity of light, it is possible to make the temperature of the liquid crystal layer closer to a constant value regardless of the intensity of light incident on the liquid crystal layer, and the actual phase modulation amount can be made closer to the target phase modulation amount with greater accuracy. Therefore, the deviation of the actual phase modulation amount from the target phase modulation amount can be further reduced.

[0013] [9] A second light modulation device according to an embodiment of the present disclosure includes a light source, a spatial light modulator, and a control unit. The light source outputs light having an intensity corresponding to a set intensity. The spatial light modulator includes a plurality of pixel electrodes, a liquid crystal layer, a driving unit, and a cooling unit. The plurality of pixel electrodes are arranged one-dimensionally or two-dimensionally. The liquid crystal layer modulates the phase of the light according to the magnitude of an electric field formed by each pixel electrode. The driving unit applies a voltage to the plurality of pixel electrodes. The cooling unit cools the liquid crystal layer so that the temperature of the liquid crystal layer approaches a set temperature. The control unit determines the magnitude of the voltage applied to the plurality of pixel electrodes. When setting the phase of each pixel of the liquid crystal layer to a predetermined phase, the control unit determines the magnitude of the voltage so that the greater the set intensity, the greater the voltage corresponding to the predetermined phase.

[0014]

[10] A second control method for a spatial light modulator according to an embodiment of the present disclosure is a method for controlling a spatial light modulator. The spatial light modulator includes a plurality of pixel electrodes, a liquid crystal layer, a driving unit, and a cooling unit. The plurality of pixel electrodes are arranged one-dimensionally or two-dimensionally. The liquid crystal layer modulates the phase of light according to the magnitude of an electric field formed by each pixel electrode. The driving unit applies a voltage to the plurality of pixel electrodes. The cooling unit cools the liquid crystal layer so that the temperature of the liquid crystal layer approaches a set temperature. The second control method includes a determining step of determining the magnitude of a voltage to be applied to each pixel electrode, and a light input step of inputting light having an intensity according to the set intensity into the liquid crystal layer. In the determining step, when setting the phase of each pixel of the liquid crystal layer to a predetermined phase, the magnitude of the voltage is determined so that the greater the set intensity, the greater the voltage corresponding to the predetermined phase.

[0015] As described above, as the intensity of light incident on the liquid crystal layer increases, the energy of the light significantly increases the temperature of the liquid crystal layer, increasing the temperature difference between the liquid crystal layer and the temperature sensor. Furthermore, since the refractive index of liquid crystal decreases with increasing temperature, the amount of phase modulation by the liquid crystal layer when a certain voltage is applied decreases with increasing temperature of the liquid crystal layer (i.e., with increasing intensity of light input to the liquid crystal layer). Therefore, in the second optical modulation device and the second control method, when the control unit (or the determining step) sets the phase of each pixel in the liquid crystal layer to a predetermined phase, the magnitude of the voltage is determined so that the greater the set intensity of light input to the liquid crystal layer, the greater the voltage corresponding to the predetermined phase. This allows the actual phase modulation amount to accurately approach the target phase modulation amount, even if the temperature of the liquid crystal layer fluctuates due to fluctuations in the intensity of light incident on the liquid crystal layer. Therefore, the second optical modulation device and the second control method can reduce the deviation of the actual phase modulation amount from the target phase modulation amount.

[0016]

[11] In the second light modulation device of [9] above, the control unit may have correlation data indicating a correlation between the set intensity and the total number of gradations used in a lookup table showing the relationship between the magnitude of the voltage and each of a plurality of gradation values. The control unit may use the correlation data to determine the total number of gradations used in the lookup table based on the set intensity. In this case, the control unit can easily determine the magnitude of the voltage according to the set intensity.

[12] Similarly, in the determination step of the second control method of

[10] above, correlation data indicating a correlation between the set intensity and the total number of gradations used in the lookup table showing the relationship between the magnitude of the voltage and each of a plurality of gradation values. In the determination step, the correlation data may be used to determine the total number of gradations used in the lookup table based on the set intensity. In this case, it is easy to determine the magnitude of the voltage according to the set intensity in the determination step.

[0017]

[13] In the second light modulation device of

[11] above, the control unit may have reference data indicating the correspondence between the amount of phase modulation by the liquid crystal layer and each gradation value in the lookup table, and may input an input value indicating the amount of phase modulation from outside the light modulation device and convert the input value into a gradation value using the reference data. In this case, a person using the second light modulation device can freely set the amount of phase modulation.

[14] Similarly, in the determination step of the second control method of

[12] above, an input value indicating the amount of phase modulation from outside the light modulation device may be input and converted into a gradation value using reference data indicating the correspondence between the amount of phase modulation by the liquid crystal layer and each gradation value in the lookup table. In this case, a person using the second control method can freely set the amount of phase modulation.

[0018]

[15] In the second light modulation device of [9] above, the control unit may determine, based on the set intensity, a lookup table to be used from among a plurality of lookup tables each showing a relationship between a voltage magnitude and each of a plurality of gradation values ​​and each having a different total number of gradations, using correlation data showing the correlation between the total number of gradations and the set intensity. In this case, the control unit can easily determine the voltage magnitude according to the set intensity.

[16] Similarly, in the determination step of the second control method of

[10] above, the control unit may determine, based on the set intensity, a lookup table to be used from among a plurality of lookup tables each showing a relationship between a voltage magnitude and each of a plurality of gradation values ​​and each having a different total number of gradations, using correlation data showing the correlation between the total number of gradations and the set intensity. In this case, the determination step can easily determine the voltage magnitude according to the set intensity.

[0019]

[17] In the second light modulation device of

[15] above, the control unit may have a plurality of reference data indicating the correspondence between the amount of phase modulation by the liquid crystal layer and each gradation value in each of a plurality of lookup tables, and may input an input value indicating the amount of phase modulation from outside the light modulation device and convert the input value into a gradation value using the reference data corresponding to the lookup table to be used from among the plurality of reference data. In this case, a person using the second light modulation device can freely set the amount of phase modulation.

[18] Similarly, in the determination step of the second control method of

[16] above, an input value indicating the amount of phase modulation from outside the light modulation device may be input and converted into a gradation value using the reference data corresponding to the lookup table to be used from among the plurality of reference data indicating the correspondence between the amount of phase modulation by the liquid crystal layer and each gradation value in each of a plurality of lookup tables. In this case, a person using the second control method can freely set the amount of phase modulation. [Effects of the Invention]

[0020] According to the present disclosure, it is possible to provide a light modulation device and a method for controlling a spatial light modulator that can reduce the deviation of the actual phase modulation amount from the target phase modulation amount. [Brief explanation of the drawings]

[0021] [Figure 1] FIG. 1 is a diagram schematically illustrating a configuration of an optical modulation device according to a first embodiment of the present disclosure. [Figure 2] FIG. 2 is a diagram schematically showing the cross-sectional structure of the light modulation element. [Figure 3] FIG. 3 is a diagram showing a plurality of pixel electrodes viewed from the normal direction of the light incident and exit surfaces. [Figure 4] FIG. 4 is a perspective view showing the appearance of a container that houses the spatial light modulator. [Figure 5] FIG. 5 is a perspective view showing the configuration of a spatial light modulator. [Figure 6] FIG. 6 is a diagram illustrating an example of a lookup table. [Figure 7]FIG. 7 is a diagram showing a schematic configuration of a test device for examining the relationship between the phase modulation amount of the light modulation element and the voltage applied to the pixel electrode. [Figure 8] FIG. 8 is a graph showing an example of the relationship between the gray scale value corresponding to the amount of phase change and the light intensity measured by the power meter. [Figure 9] FIG. 9 is a graph showing the results of measuring the relationship between the grayscale value input to the light modulation element and the amount of phase modulation. [Figure 10] FIG. 10 is a graph showing the relationship between the gray level and the amount of phase modulation. [Figure 11] FIG. 11 is a graph showing the relationship between the grayscale value and the amount of phase change when the temperature of the liquid crystal layer is controlled to 20° C. and 25° C. [Figure 12] FIG. 12 is a graph showing an example of the relationship between the intensity of the test light and the maximum phase modulation amount of the optical modulation element when the set temperatures of the cooling unit are 20° C. and 25° C. [Figure 13] FIG. 13 is a graph showing an example of the relationship between the set intensity of the laser light and the set temperature of the cooling unit, which allows the amount of phase modulation to be constant. [Figure 14] FIG. 14 is a flowchart showing a method for controlling the spatial light modulator according to the first embodiment. [Figure 15] FIG. 15 is a graph showing an example of the relationship between the gradation value and the phase modulation amount. [Figure 16] FIG. 16 is a graph showing an example of correlation data. [Figure 17] FIGS. 17(a) and 17(b) are diagrams showing examples of reference data. [Figure 18] FIG. 18 is a flowchart showing a method for controlling a spatial light modulator according to the second embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0022] Hereinafter, embodiments of a light modulation device and a method for controlling a spatial light modulator according to the present disclosure will be described in detail with reference to the accompanying drawings. Note that in the description of the drawings, the same elements are given the same reference numerals, and duplicated explanations will be omitted. [First embodiment]

[0023] FIG. 1 is a diagram schematically illustrating a configuration of a light modulation device 1A according to a first embodiment of the present disclosure. As illustrated in FIG. 1, the light modulation device 1A of this embodiment includes a light source 3, a mirror 4, an imaging optical system 5, a condenser lens 6, a control unit 7 (controller), and a spatial light modulator 10. In laser processing, for example, the light modulation device 1A of this embodiment irradiates a laser beam La output from the light source 3 onto an object B, which is a workpiece, via the spatial light modulator 10. Alternatively, in a microscope such as a fluorescence microscope, for example, the light modulation device 1A of this embodiment irradiates a laser beam La output from the light source 3 onto an object B, which is an object to be observed, via the spatial light modulator 10.

[0024] The light source 3 outputs laser light La. The light source 3 is configured to output, for example, single-wavelength, linearly polarized laser light La. The light source 3 is also configured to repeatedly output laser light La having a pulse waveform with an extremely short time width, for example, on the order of picoseconds or femtoseconds. The wavelength of the laser light La is, for example, 1030 nm. The pulse width of the laser light La is, for example, 10 picoseconds. The pulse repetition frequency of the laser light La is, for example, 1 MHz. The light source 3 is configured to be able to change the intensity of the laser light La, and receives a signal indicating a set intensity from outside the optical modulation device 1A or from the control unit 7, and outputs laser light La having an intensity corresponding to the set intensity. The intensity here refers to the peak intensity of each pulse. Alternatively, the intensity here may be the time average value of the light intensity within a certain period.

[0025] The mirror 4 is disposed opposite the light incident / exit surface 11a of the spatial light modulator 10. The mirror 4 is, for example, a prism-type mirror having two flat reflecting surfaces 41 and 42. The reflecting surface 41 is disposed on the optical path between the light source 3 and the light incident / exit surface 11a of the spatial light modulator 10, optically coupling the light source 3 and the light incident / exit surface 11a. The laser light La output from the light source 3 is reflected by the reflecting surface 41 and reaches the light incident / exit surface 11a. The incident direction of the laser light La on the light incident / exit surface 11a is inclined with respect to the normal direction of the light incident / exit surface 11a. The reflecting surface 42 is disposed on the optical path between the light incident / exit surface 11a and the imaging optical system 5, optically coupling the light incident / exit surface 11a and the imaging optical system 5. The modulated laser light Lb output from the light incident / exit surface 11a is reflected by the reflecting surface 42 and reaches the imaging optical system 5. The emission direction of the laser beam Lb from the light incident / emitting surface 11a is inclined with respect to the normal to the light incident / emitting surface 11a in the opposite direction to the laser beam La. In one example, a portion of the optical axis between the light source 3 and the reflecting surface 41 that is closer to the reflecting surface 41 is positioned on the same straight line as a portion of the optical axis between the reflecting surface 42 and the imaging optical system 5 that is closer to the reflecting surface 42.

[0026] The imaging optical system 5 optically couples the light incident / exit surface 11a of the spatial light modulator 10 and the condenser lens 6. The imaging optical system 5 includes, for example, a pair of lenses 51 and 52. The lenses 51 and 52 are arranged side by side on the optical axis between the reflecting surface 42 and the condenser lens 6. The lenses 51 and 52 form, for example, a double-telecentric optical system.

[0027] The condenser lens (objective lens) 6 condenses the modulated laser light Lb output from the light incident / exit surface 11a of the spatial light modulator 10 toward the target B. The condenser lens 6 is optically coupled to the light incident / exit surface 11a via the imaging optical system 5 and the reflecting surface 42.

[0028] The spatial light modulator 10 includes a light modulation element 11, a temperature adjustment unit 16, and an external control board 17 (control unit). The light modulation element 11 has a plurality of pixels arranged one-dimensionally or two-dimensionally, and modulates the phase of laser light La input to a light incident / exit surface 11a for each pixel to generate modulated laser light Lb. Although the light modulation element 11 of this embodiment has a reflective configuration, the light modulation element 11 may also have a transmissive configuration.

[0029] Fig. 2 is a diagram schematically showing the cross-sectional structure of the light modulation element 11. As shown in Fig. 2, the light modulation element 11 has a glass substrate 112, a transparent conductive film 113, a plurality of pixel electrodes 114, a liquid crystal layer 115, a transparent layer (phase shift layer) 116, a dielectric multilayer film (reflective layer) 117, a light-shielding layer 118, and a drive circuit layer 119 (controller). Note that the transparent layer 116 does not necessarily have to be provided.

[0030] The glass substrate 112 has a front surface 112a and a back surface 112b. The front surface 112a of the glass substrate 112 forms the light incident / emitting surface 11a of the light modulation element 11. The glass substrate 112 transmits the laser light La incident from the light incident / emitting surface 11a (front surface 112a) into the interior of the light modulation element 11. The transparent conductive film 113 is provided on the back surface 112b of the glass substrate 112, and is mainly composed of a conductive material (e.g., ITO) that transmits the laser light La.

[0031] As shown in FIG. 3 (a view of the pixel electrodes 114 viewed from the normal direction of the light incident / emitting surface 11a), the pixel electrodes 114 are arranged one-dimensionally or two-dimensionally to form the pixels of the light modulation element 11. Each pixel electrode 114 is made of a metal material such as aluminum, and its surface is processed to be flat and smooth. The pixel electrodes 114 are driven by an active matrix circuit (drive unit) provided in the drive circuit layer 119. The active matrix circuit is provided between the pixel electrodes 114 and the silicon substrate 110 and applies a voltage to each pixel electrode 114, the magnitude of which corresponds to the amount of phase modulation required for each pixel. The active matrix circuit includes, for example, a first driver circuit that applies a voltage to each pixel column aligned in the X-axis direction and a second driver circuit that applies a voltage to each pixel column aligned in the Y-axis direction. Voltages are applied to the pixel electrodes 114 of pixels designated by both driver circuits.

[0032] The liquid crystal layer 115 is disposed between the transparent conductive film 113 and the pixel electrodes 114 and modulates the phase of the laser light La according to the magnitude of the electric field formed by each pixel electrode 114. That is, when a voltage is applied to a pixel electrode 114 by the active matrix circuit, an electric field is formed between the transparent conductive film 113 and the pixel electrode 114. This electric field is applied to the dielectric multilayer film 117 and the liquid crystal layer 115 at a rate according to their respective thicknesses. The alignment direction of the liquid crystal molecules changes according to the magnitude of the electric field applied to the liquid crystal layer 115. When the laser light La passes through the glass substrate 112 and the transparent conductive film 113 and enters the liquid crystal layer 115, the laser light La is phase-modulated by the liquid crystal molecules while passing through the liquid crystal layer 115, reflected by the dielectric multilayer film 117, and then phase-modulated again by the liquid crystal layer 115 before being extracted as laser light Lb.

[0033] In this embodiment, the liquid crystal layer 115 includes alignment films 115a and 115b. The alignment films 115a and 115b are formed on both end surfaces of the liquid crystal layer 115, and align the liquid crystal molecules in a certain direction. The alignment films 115a and 115b are made of a polymer material such as polyimide, and the surfaces that come into contact with the liquid crystal layer 115 are subjected to rubbing treatment or the like.

[0034] The transparent layer 116 shifts the phase at the interface with the liquid crystal layer 115 so that the peaks of the laser light La incident from the glass substrate 112 side and the laser light La reflected by the dielectric multilayer film 117 do not overlap with each other. The optical film thickness of the transparent layer 116 is set to be equal to or greater than the wavelength of the incident light. Alternatively, the optical film thickness of the transparent layer 116 is set to be equal to or greater than (τ×c) / 30, where τ is the pulse width of the incident light and c is the speed of light. The constituent materials of the transparent layer 116 mainly contain, for example, SiO2 and / or Nb2O5. As mentioned above, the transparent layer 116 does not necessarily have to be provided.

[0035] The dielectric multilayer film 117 is provided between the liquid crystal layer 115 and the light-shielding layer 118 (between the transparent layer 116 and the light-shielding layer 118 when the transparent layer 116 is provided). The dielectric multilayer film 117 reflects the laser light La with a high reflectance of, for example, more than 99%. The constituent material of the dielectric multilayer film 117 may be, for example, an alternate stack of SiO2 and TiO2, but is not particularly limited to this, and may be changed as appropriate, for example, by using HfO2 instead of TiO2, or MgF2 instead of SiO2.

[0036] The light-shielding layer 118 is disposed between the dielectric multilayer film 117 and the pixel electrodes 114, and is formed directly on the surfaces of the plurality of pixel electrodes 114. The light-shielding layer 118 suppresses the occurrence of light leakage.

[0037] FIG. 4 is a perspective view showing the appearance of a container 20 that houses the spatial light modulator 10. The container 20 shown in FIG. 4 includes a housing 21, a heat sink 22, and a fan 23. The housing 21 is a hollow container having a substantially rectangular parallelepiped or cubic shape and is made of, for example, metal. The housing 21 has an optical window 21c on each of a pair of side surfaces 21a and 21b aligned in a certain direction D1 (only the optical window 21c on the side surface 21a is shown in the figure). The optical window 21c on the side surface 21a faces the optical window 21c on the side surface 21b in the direction D1. The housing 21 has a pair of side surfaces 21d and 21e aligned in a direction D2 that intersects with the direction D1. The heat sink 22 and the fan 23 are attached to one of the side surfaces 21d and 21e, the side surface 21d. Heat sink 22 has a plurality of protrusions on its outer surface and is fixed in a state in which the protrusions are inserted into rectangular opening 21f formed in side surface 21d. Fan 23 is disposed so as to cover the outer surface of heat sink 22 and is fixed to side surface 21d. Fan 23 blows air onto the outer surface of heat sink 22 to cool it.

[0038] 5 is a perspective view showing the configuration of the spatial light modulator 10. The spatial light modulator 10 is housed in a container 20 together with the above-mentioned mirror 4. The spatial light modulator 10 shown in FIG. 5 has a light modulation element 11, a substrate 12, a metal block 13, a temperature sensor 14, and a cooling unit 15.

[0039] The substrate 12 has a first surface 12a and a second surface 12b. The first surface 12a faces the mirror 4. The second surface 12b faces away from the first surface 12a and faces the heat sink 22. The optical modulation element 11 is mounted on the first surface 12a of the substrate 12 so that the light incident / exit surface 11a (see FIG. 2) faces the mirror 4.

[0040] A substantially rectangular opening 12c is formed in the substrate 12. When viewed from the normal direction of the first surface 12a and the second surface 12b, the opening 12c overlaps with the light modulation element 11. A back surface 11b (the surface opposite to the light incident / exit surface 11a) of the light modulation element 11 is exposed from the opening 12c. A metal block 13 is disposed in the opening 12c and is thermally coupled to or in contact with the back surface 11b of the light modulation element 11. The metal block 13 is, for example, a copper block.

[0041] The temperature sensor 14 and the cooling unit 15 constitute the temperature adjustment unit 16 shown in FIG. 1. The temperature sensor 14 detects the ambient temperature of the liquid crystal layer 115 (see FIG. 2) of the light modulation element 11. The temperature sensor 14 is, for example, a thermistor, and changes its resistance value according to the ambient temperature of the liquid crystal layer 115. In the illustrated example, the temperature sensor 14 is disposed inside the metal block 13, but the temperature sensor 14 may be disposed anywhere near the light modulation element 11, and may also be disposed outside the metal block 13 (for example, on the substrate 12). Note that in the present disclosure, the temperature of the liquid crystal layer 115 is not limited to the actual temperature of the liquid crystal layer 115 but also includes the ambient temperature of the liquid crystal layer 115.

[0042] The cooling unit 15 is disposed at a position where the metal block 13 is sandwiched between the cooling unit 15 and the light modulation element 11. The cooling unit 15 includes, for example, a Peltier element. The heat absorption surface of the Peltier element is thermally coupled to or in contact with the metal block 13. The heat dissipation surface of the Peltier element is thermally coupled to or in contact with the inner surface of the heat sink 22 (the surface opposite to the outer surface on which the multiple protrusions are provided). The cooling unit 15 cools the light modulation element 11 (particularly the liquid crystal layer 115). The cooling unit 15 is controlled by, for example, a control driver circuit provided on the substrate 12, and cools the liquid crystal layer 115 through the metal block 13 so that the temperature detected by the temperature sensor 14 approaches the set temperature. As a result, the actual temperature of the liquid crystal layer 115 also approaches the set temperature.

[0043] Laser light La output from the light source 3 (see FIG. 1) passes through the optical window 21c on the side surface 21b of the housing 21 and reaches the reflecting surface 41 of the mirror 4. The laser light La is reflected by the reflecting surface 41 and enters the light incident / exit surface 11a of the light modulation element 11. The light modulation element 11 modulates the phase of the laser light La for each pixel. The modulated laser light Lb output from the light modulation element 11 is reflected by the reflecting surface 42 of the mirror 4, passes through the optical window 21c on the side surface 21a of the housing 21, and is output to the outside of the housing 21 (the imaging optical system 5 shown in FIG. 1).

[0044] Referring again to FIG. 1 , the control unit 7 of this embodiment will be described. The control unit 7 may be physically configured as a typical computer including a processor (CPU), main storage devices such as ROM and RAM, input devices such as a keyboard, mouse, and touch screen, output devices such as a display (including a touch screen), and auxiliary storage devices such as a hard disk. The control unit 7 is electrically connected to the light source 3, the light modulation element 11, and the temperature adjustment unit 16. The control unit 7 provides information A1 regarding the set intensity of the laser beam La to the light source 3 or receives information A1 regarding the set intensity of the laser beam La from the light source 3 via an external control board 17. In addition, the control unit 7 provides information A2 regarding the gradation value indicating the target amount of phase modulation for each of the multiple pixels of the light modulation element 11 to the light modulation element 11 via the external control board 17. In addition, the control unit 7 provides information A3 regarding the set temperature of the cooling unit 15 to the control driver circuit of the temperature adjustment unit 16 via the external control board 17.

[0045] Generally, the phase modulation amount of each pixel of the light modulation element 11 has a nonlinear relationship with the magnitude of the voltage applied to each pixel electrode 114. Therefore, the external control board 17 has a lookup table for converting the grayscale values ​​input from the control unit 7 into voltage magnitudes, and uses this lookup table to convert the grayscale values ​​into analog voltage values ​​and input them to the drive circuit layer 119. FIG. 6 is a diagram showing an example of the lookup table. The grayscale values ​​are digital signals with a total number of grayscale levels N (0 to N-1), where N=256 in this embodiment. The pixel input values ​​of all N grayscale levels, from 0 to N-1, indicate the phase modulation amount for one cycle from 0 to 2π. Although FIG. 6 also shows the phase modulation amount for reference, the drive circuit layer 119 does not have information regarding the phase modulation amount. The drive circuit layer 119 applies a voltage to the pixel electrode 114 of each pixel based on the analog voltage value input for each pixel. As will be described later, the refractive index of the liquid crystal layer 115 decreases as the temperature increases. Therefore, the amount of phase modulation by the liquid crystal layer 115 when a certain voltage is applied decreases as the temperature of the liquid crystal layer 115 increases. Therefore, the external control board 17 has a different lookup table for each temperature of the liquid crystal layer 115. The drive circuit layer 119 may also have the lookup table. In this case, the drive circuit layer 119 converts the grayscale values ​​input from the control unit 7 into analog voltage values ​​using the lookup table. The drive circuit layer 119 then applies a voltage to the pixel electrode 114 of each pixel based on the converted analog voltage values. Alternatively, the control unit 7 may have the lookup table. In this case, the control unit 7 converts the grayscale values ​​into analog voltage values ​​using the lookup table. The drive circuit layer 119 then applies a voltage to the pixel electrode 114 of each pixel based on the analog voltage values ​​input from the control unit 7.

[0046] 7 is a diagram schematically showing the configuration of a test apparatus 2 for examining the relationship between the phase modulation amount of the light modulation element 11 and the voltage applied to the pixel electrode 114. The test apparatus 2 includes a laser light source 31, a half-wave plate 32, a polarizing beam splitter 33, a light absorber 34, a beam expander 35, a dielectric mirror 36a, a dielectric mirror 36b, a homogenizer 37, a polarizing beam splitter 38, a light absorber 39, an aperture 40, and a power meter 43.

[0047] The laser light source 31 outputs test light Lc simulating laser light La. The test light Lc passes through a half-wave plate 32 and then enters a polarizing beam splitter 33. The polarization direction of the test light Lc is adjusted by the half-wave plate 32, and only a component of the test light Lc having a predetermined polarization direction passes through the polarizing beam splitter 33. The remaining components of the test light Lc excluding this component are reflected by the polarizing beam splitter 33 and absorbed by the light absorber 34. The half-wave plate 32 and the polarizing beam splitter 33 rotate the polarization direction of the test light Lc by 45° with respect to the orientation direction of the liquid crystal in the liquid crystal layer 115 of the light modulation element 11.

[0048] The beam expander 35 expands the beam diameter of the test light Lc to match the size of the modulation surface of the light modulation element 11 (e.g., 16 mm × 12.8 mm). The beam diameter of the test light Lc before expansion is, for example, 7 mm. When the size of the modulation surface of the light modulation element 11 is, for example, 16 mm × 12.8 mm, the beam diameter of the test light Lc after expansion is, for example, 11 mm. The beam expander 35 may be configured, for example, by a combination of a concave lens and a convex lens. The test light Lc after its beam diameter expansion is guided to the homogenizer 37 by dielectric mirrors 36a and 36b. The homogenizer 37 causes the test light Lc to have a uniform light intensity distribution within an area of ​​a specific shape (e.g., circular).

[0049] The mirror 4 contained in the container 20 is positioned on the optical axis between the homogenizer 37 and the polarizing beam splitter 38. The test light Lc output from the homogenizer 37 is reflected by the mirror 4 and input to the optical modulator 11. At this time, the polarization direction of the modulated test light Ld rotates according to the amount of phase modulation provided by the optical modulator 11. The modulated test light Ld output from the optical modulator 11 is reflected by the mirror 4 and reaches the polarizing beam splitter 38. A component of the test light Lc with a predetermined polarization direction passes through the polarizing beam splitter 38 and then passes through the aperture 40 to be input to the power meter 43. The opening diameter of the aperture 40 is, for example, 6 mm. Components of the test light Lc other than the predetermined polarization component are reflected by the polarizing beam splitter 38 and absorbed by the optical absorber 39. The power meter 43 measures the light intensity of the test light Lc that has passed through the aperture 40. In this test device 2, the amount of light of the polarized component of the test light Lc that can pass through the polarizing beam splitter 38 and the aperture 40 changes depending on the amount of phase modulation provided by the optical modulation element 11, so the amount of phase change in the optical modulation element 11 is observed as an amount of intensity change in the power meter 43.

[0050] 8 is a graph showing an example of the relationship between the gray scale value corresponding to the amount of phase change and the light intensity measured by the power meter 43. The light intensity I measured by the power meter 43 is expressed by the following formula (1). Note that θ is the amount of phase modulation, and I n is a constant.

number

[0051] FIG. 9 is a graph showing the results of measuring the relationship between the gradation value input to the light modulation element 11 and the phase modulation amount using test light Lc with a pulse width of 10 picoseconds, a wavelength of 1030 nm, and a repetition frequency of 1 MHz. The horizontal axis represents the gradation value, and the vertical axis represents the phase modulation amount (rad). As shown in FIG. 9, the gradation value and the phase modulation amount have an approximately linear relationship. FIG. 10 is a graph showing the relationship between the gradation value and the phase modulation amount, with the horizontal axis representing the phase modulation amount (rad) and the vertical axis representing the gradation value. By approximating the graph shown in FIG. 10 with a polynomial, it is possible to determine the gradation value required to obtain a target phase modulation amount. For example, by approximating the graph shown in FIG. 10 with a linear function, the following equation (2) is obtained.

number

[0052] Here, the temperature of the liquid crystal layer 115 fluctuates with changes in the ambient temperature or irradiation with the laser light La. The refractive index of the liquid crystal layer 115 decreases as the temperature of the liquid crystal layer 115 increases. Therefore, the amount of phase modulation by the liquid crystal layer 115 when a certain voltage is applied decreases as the temperature of the liquid crystal layer 115 increases. FIG. 11 is a graph showing, as an example, the relationship between the grayscale value and the amount of phase change when the temperature of the liquid crystal layer 115 is controlled to 20°C and 25°C. As shown in FIG. 11, the higher the temperature of the liquid crystal layer 115, the smaller the amount of phase change for a certain grayscale value. Therefore, the cooling unit 15 keeps the temperature of the liquid crystal layer 115 constant by bringing the temperature of the liquid crystal layer 115 closer to a set temperature. This allows the amount of phase modulation to be controlled with high precision.

[0053] However, in this embodiment, the temperature sensor 14 is disposed inside the metal block 13. Thus, in many cases, the temperature sensor 14 is disposed at a position distant from the liquid crystal layer 115. Therefore, various components, such as the silicon substrate 110, the dielectric multilayer film 117, and the drive circuit layer 119, are interposed between the liquid crystal layer 115 and the temperature sensor 14, causing the temperature detected by the temperature sensor 14 to deviate from the temperature of the liquid crystal layer 115. In particular, when the intensity of the laser light La incident on the liquid crystal layer 115 increases, the energy of the laser light La significantly increases the temperature of the liquid crystal layer 115, increasing the temperature difference between the liquid crystal layer 115 and the temperature sensor 14. When the temperature difference between the liquid crystal layer 115 and the temperature sensor 14 is large, it becomes difficult to correctly set the magnitude of the applied voltage even if the temperature of the liquid crystal layer 115 detected by the temperature sensor 14 is used. As a result, the actual phase modulation amount deviates from the target phase modulation amount.

[0054] Therefore, the control unit 7 of this embodiment determines the set temperature of the cooling unit 15 based on the set intensity of the laser light La. That is, when the temperature of the liquid crystal layer 115 approaches a predetermined temperature, the control unit 7 determines the set temperature of the cooling unit 15 based on the set intensity of the laser light La such that the set temperature of the cooling unit 15 becomes lower than the predetermined temperature as the set intensity of the laser light La increases. In other words, when the temperature of the liquid crystal layer 115 approaches a predetermined temperature, the control unit 7 determines the set temperature of the cooling unit 15 based on the set intensity of the laser light La such that the set temperature of the cooling unit 15 becomes lower than the predetermined temperature and the difference between the set temperature of the cooling unit 15 and the predetermined temperature becomes larger as the set intensity of the laser light La increases. Note that the lookup table shows the relationship between the amount of phase modulation and the magnitude of the applied voltage not at the set temperature of the cooling unit 15 but at a predetermined temperature such as room temperature (or at the set temperature of the cooling unit 15 when it is assumed that the set intensity of the laser light La is zero).

[0055] Specifically, the control unit 7 stores in advance a data table showing the relationship between the set intensity of the laser light La and the set temperature of the cooling unit 15, and determines the set temperature of the cooling unit 15 by referring to the data table based on the known set intensity of the laser light La. Alternatively, the control unit 7 stores in advance a calculation formula (for example, formula (3) described below) showing the relationship between the set intensity of the laser light La and the set temperature of the cooling unit 15, and determines the set temperature of the cooling unit 15 by applying the known set intensity of the laser light La to the calculation formula. Note that storing a calculation formula means storing coefficients included in the calculation formula.

[0056] FIG. 12 is a graph showing an example of the relationship between the light intensity of the test light Lc and the maximum phase modulation amount of the light modulation element 11 when the set temperature of the cooling unit 15 is 20°C and 25°C. In FIG. 12, the horizontal axis represents the light intensity (W) of the test light Lc, and the vertical axis represents the maximum phase modulation amount (πrad). Plot P1 in the figure shows the measured value of the maximum phase modulation amount when the set temperature of the cooling unit 15 is 20°C, and Q1 is its approximate straight line. Plot P2 in the figure shows the measured value of the maximum phase modulation amount when the set temperature of the cooling unit 15 is 25°C, and Q2 is its approximate straight line. Note that the figure also shows the approximate straight line Q3 when the set temperature of the cooling unit 15 is 23°C.

[0057] As is clear from FIG. 12, under the condition that the set temperature of the cooling unit 15 is constant, the greater the light intensity of the test light Lc, the smaller the maximum phase modulation amount of the light modulation element 11. The light intensity of the test light Lc and the maximum phase modulation amount of the light modulation element 11 have a substantially linear relationship. Furthermore, under the condition that the light intensity of the test light Lc is constant, the greater the set temperature of the cooling unit 15, the smaller the maximum phase modulation amount. For example, in this example, when the set temperature is 25°C and the light intensity of the test light Lc is 10 W, the maximum phase modulation amount is 2.105π (rad). When the set temperature is 25°C and the light intensity of the test light Lc is 75 W, the maximum phase modulation amount is 2.066π (rad). When the set temperature is 20°C and the light intensity of the test light Lc is 10 W, the maximum phase modulation amount is 2.193π (rad). When the set temperature is 20°C and the light intensity of the test light Lc is 75 W, the maximum phase modulation amount is 2.167π (rad). This shows that even when the light intensity of the laser light La fluctuates, it is possible to correct the maximum phase modulation amount of the optical modulator 11 by controlling the set temperature of the cooling unit 15. That is, by complementing the gaps between the approximated straight lines Q1 to Q3 corresponding to each temperature and changing the set temperature of the cooling unit 15 in response to changes in the set intensity of the laser light La, it is possible to stabilize the phase modulation amount even when the set intensity of the laser light La is changed.

[0058] FIG. 13 is a graph showing an example of the relationship between the set intensity of laser light La and the set temperature of the cooling unit 15, which allows the amount of phase modulation to be constant. In the graph, the horizontal axis represents the set intensity (W) of laser light La, and the vertical axis represents the set temperature (°C) of the cooling unit 15. Plot P4 in the graph shows the actual measured value of the set temperature versus the set intensity when the amount of phase modulation is constant. Q4 in the graph is its approximate line. The approximate line Q4 can be obtained, for example, by the following formula (3). Note that T is the set temperature (°C) of the cooling unit 15, and I is the set intensity (W) of the laser light La.

number

[0059] The degree of temperature rise of the liquid crystal layer 115 with respect to the set intensity of the laser light La differs depending on the conditions of the laser light La (such as the pulse width, wavelength, and repetition frequency). Therefore, the degree of variation in the amount of phase modulation with respect to the set intensity of the laser light La also differs depending on the state of the laser light La. Therefore, multiple data tables showing the relationship between the set intensity of the laser light La and the set temperature of the cooling unit 15 or multiple calculation formulas showing the relationship between the set intensity of the laser light La and the set temperature of the cooling unit 15 may be prepared according to the number of combinations of conditions for the laser light La.

[0060] 14 is a flowchart showing a method for controlling the spatial light modulator 10 according to this embodiment. First, the control unit 7 determines the set temperature of the cooling unit 15 based on the set intensity of the laser light La such that, when bringing the temperature of the liquid crystal layer 115 closer to a predetermined temperature, the set temperature becomes lower than the predetermined temperature as the set intensity of the laser light La increases (determination step S11). At this time, the control unit 7 determines the set temperature of the cooling unit 15 based on the known set intensity of the laser light La while referring to a data table showing the relationship between the set intensity of the laser light La and the set temperature of the cooling unit 15. Alternatively, the control unit 7 determines the set temperature of the cooling unit 15 by applying the known set intensity of the laser light La to a calculation formula showing the relationship between the set intensity of the laser light La and the set temperature of the cooling unit 15.

[0061] Subsequently, the laser light La having an intensity according to the set intensity is input to the liquid crystal layer 115 of the light modulation element 11 (light input step S12). The laser light La is phase-modulated for each pixel in the liquid crystal layer 115, and the modulated laser light Lb is output from the light modulation element 11.

[0062] The advantages of the light modulation device 1A according to this embodiment and the control method for the spatial light modulator 10 according to this embodiment, as described above, can be summarized as follows. In the light modulation device 1A according to this embodiment and the control method for the spatial light modulator 10 according to this embodiment, when the temperature of the liquid crystal layer 115 approaches a predetermined temperature, the control unit 7 (or in the determination step S11) determines the set temperature of the cooling unit 15 based on the set intensity of the laser light La input to the liquid crystal layer 115 so that the set temperature of the cooling unit 15 is lower than the predetermined temperature as the set intensity of the laser light La input to the liquid crystal layer 115 increases. This allows the temperature of the liquid crystal layer 115 to approach a constant value regardless of the intensity of the laser light La input to the liquid crystal layer 115. This makes it possible to correctly set the magnitude of the applied voltage even when a common lookup table is used, and allows the actual phase modulation amount to accurately approach the target phase modulation amount. Therefore, the light modulation device 1A according to this embodiment and the control method for the spatial light modulator 10 according to this embodiment can reduce the deviation of the actual phase modulation amount from the target phase modulation amount. Furthermore, unlike the second embodiment described later, the reference data (see FIG. 17) indicating the correspondence between the phase modulation amount and the gradation value can be commonly used regardless of the intensity of the laser light La.

[0063] As in the present embodiment, the control unit 7 may previously store a data table showing the relationship between the set intensity of the laser beam La and the set temperature of the cooling unit 15, and determine the set temperature using the data table. Alternatively, the control unit 7 may previously store a calculation formula showing the relationship between the set intensity of the laser beam La and the set temperature of the cooling unit 15, and determine the set temperature using the calculation formula. In either of these cases, the control unit 7 can easily determine the set temperature of the cooling unit 15 according to the set intensity of the laser beam La. Similarly, in the determination step S11, the set temperature of the cooling unit 15 may be determined using a data table showing the relationship between the set intensity of the laser beam La and the set temperature of the cooling unit 15. Alternatively, in the determination step S11, the set temperature of the cooling unit 15 may be determined using a calculation formula showing the relationship between the set intensity of the laser beam La and the set temperature of the cooling unit 15. In either of these cases, in the determination step S11, the set temperature of the cooling unit 15 can easily be determined according to the set intensity of the laser beam La. Note that the function of the control unit 7 may be provided in the external control board 17 or the drive circuit layer 119 of the light modulation element 11.

[0064] As in this embodiment, the set temperature of the cooling unit 15 may be proportional to the set intensity of the laser light La. According to the inventor's findings, the temperature of the liquid crystal layer 115 deviates from the temperature detected by the temperature sensor 14 in proportion to the intensity of the laser light La input to the liquid crystal layer 115. By determining the set temperature of the cooling unit 15 so that it is proportional to the set intensity of the laser light La, it becomes possible to make the temperature of the liquid crystal layer 115 closer to a constant value regardless of the intensity of the laser light La input to the liquid crystal layer 115, and the actual phase modulation amount can be made closer to the target phase modulation amount with greater accuracy. Therefore, it is possible to further reduce the deviation of the actual phase modulation amount from the target phase modulation amount. [Second embodiment]

[0065] Next, a second embodiment of the present disclosure will be described. In this embodiment, the control unit 7 determines the magnitude of the voltage to be applied to the plurality of pixel electrodes 114, instead of determining the set temperature of the cooling unit 15. That is, when setting the phase of each pixel of the liquid crystal layer 115 to a predetermined phase, the control unit 7 determines the magnitude of the voltage to be applied to the plurality of pixel electrodes 114 so that the greater the set intensity of the laser light La, the greater the voltage corresponding to the predetermined phase.

[0066] FIG. 15 is a graph showing an example of the relationship between the grayscale value and the phase modulation amount. The horizontal axis represents the grayscale value, and the vertical axis represents the phase modulation amount (π rad). The figure shows an approximate line Q5 that shows the relationship when the light intensity of the laser light La is 10 W, and an approximate line Q6 that shows the relationship when the light intensity of the laser light La is 100 W. As is clear from FIG. 15, for a given grayscale value, the phase modulation amount decreases as the light intensity of the laser light La increases. This is because, as described in the first embodiment, the higher the light intensity of the laser light La, the more the temperature of the liquid crystal layer 115 deviates from the set temperature of the cooling unit 15 and increases, thereby correspondingly decreasing the refractive index of the liquid crystal layer 115. Therefore, in this embodiment, the higher the set intensity of the laser light La, the higher the applied voltage corresponding to a given phase is, thereby maintaining the phase modulation amount constant regardless of fluctuations in the set intensity of the laser light La.

[0067] Specifically, the control unit 7 stores in advance correlation data indicating the correlation between the total number of gradations used in a lookup table (see, for example, FIG. 6 ) held by the external control board 17, which indicates the relationship between the magnitude of the voltage applied to the pixel electrode 114 and each of a plurality of gradation values, and the set intensity of the laser light La. FIG. 16 is a graph showing an example of such correlation data. In the graph, the horizontal axis represents the light intensity of the laser light La, and the vertical axis represents the total number of gradations. In the graph, plot P7 indicates the actual measured value of the gradation value versus the set intensity of the laser light La when the phase modulation amount is constant. Q7 in the graph is an approximate straight line. As shown in FIG. 16, when the phase modulation amount is constant, the set intensity of the laser light La and the gradation value have an approximately linear relationship. Using this correlation data, the control unit 7 determines the total number of gradations used in the lookup table held by the external control board 17 based on the set intensity of the laser light La.

[0068] The control unit 7 also has reference data indicating the correspondence between the phase modulation amount by the liquid crystal layer 115 and each gradation value in the lookup table. (a) and (b) of FIG. 17 are diagrams showing examples of such reference data. The reference data shown in (a) of FIG. 17 indicates the correspondence relationship between each value of the phase modulation amount 0 (rad) to 2.1π (rad) and each value of the gradation value 0 to 244 when the total number of gradations is 245. The reference data shown in (b) of FIG. 17 indicates the correspondence relationship between each value of the phase modulation amount 0 (rad) to 2.1π (rad) and each value of the gradation value 0 to 250 when the total number of gradations is 251. The control unit 7 refers to the correlation data shown in FIG. 16, and when the set intensity of the laser light La is 70 W, the control unit 7 sets the total number of gradations to 245 and uses the reference data shown in (a) of FIG. 17. Furthermore, the control unit 7 refers to the correlation data shown in FIG. 16, and when the set intensity of the laser light La is 140 W, the total number of gradations is set to 251, and the reference data shown in FIG. 17(b) is used.

[0069] In this embodiment, the control unit 7 receives an input value indicating a target phase modulation amount from outside the light modulation device 1A. The control unit 7 converts the input value into a gradation value using reference data shown in FIG. 17. The control unit 7 provides the converted gradation value to the light modulation element 11. The light modulation element 11 determines an application voltage corresponding to the gradation value using a lookup table, and applies the application voltage to the pixel electrode 114.

[0070] 18 is a flowchart showing a method for controlling the spatial light modulator 10 according to this embodiment. First, when setting the phase of each pixel of the liquid crystal layer 115 to a predetermined phase, the control unit 7 determines the magnitude of the voltage to be applied to each pixel electrode 114 so that the voltage corresponding to the predetermined phase increases as the set intensity of the laser light La increases (determining step S21). At this time, the control unit 7 determines the total number of gradations used in a lookup table (see, for example, FIG. 6) showing the relationship between the magnitude of the applied voltage and each of a plurality of gradation values, based on the set intensity of the laser light La, using correlation data (see, for example, FIG. 16) showing the correlation between the total number of gradations used in the lookup table. Furthermore, the control unit 7 has reference data (see, for example, FIG. 17) showing the correspondence between the amount of phase modulation by the liquid crystal layer 115 and each gradation value in the lookup table. The control unit 7 also receives an input value indicating the amount of phase modulation from outside the light modulation device 1A and converts the input value into a gradation value using the reference data.

[0071] Subsequently, the laser light La having an intensity according to the set intensity is input to the liquid crystal layer 115 of the light modulation element 11 (light input step S22). The laser light La is phase-modulated for each pixel in the liquid crystal layer 115, and the modulated laser light Lb is output from the light modulation element 11.

[0072] The effects obtained by the light modulation device 1A according to this embodiment and the control method for the spatial light modulator 10 according to this embodiment described above can be summarized as follows. As described above, when the intensity of the laser light La incident on the liquid crystal layer 115 increases, the energy of the laser light La significantly increases the temperature of the liquid crystal layer 115, and the temperature difference between the liquid crystal layer 115 and the temperature sensor 14 increases. Furthermore, since the refractive index of liquid crystal decreases as the temperature increases, the amount of phase modulation by the liquid crystal layer 115 when a certain voltage is applied decreases as the temperature of the liquid crystal layer 115 increases (i.e., as the intensity of the laser light La input to the liquid crystal layer 115 increases). Therefore, in the light modulation device 1A according to this embodiment and the control method for the spatial light modulator 10 according to this embodiment, the control unit 7 (or in determining step S21) determines the magnitude of the voltage to be applied to each pixel electrode 114 when setting the phase of each pixel of the liquid crystal layer 115 to a predetermined phase so that the voltage corresponding to the predetermined phase increases as the set intensity of the laser light La increases. This makes it possible to accurately bring the actual phase modulation amount closer to the target phase modulation amount even if the temperature of the liquid crystal layer 115 fluctuates due to fluctuations in the intensity of the laser beam La. Therefore, the light modulation device 1A according to this embodiment and the control method for the spatial light modulator 10 according to this embodiment can reduce the deviation of the actual phase modulation amount from the target phase modulation amount.

[0073] As in the present embodiment, the control unit 7 (or in the determination step S21) may use correlation data indicating the correlation between the total number of gradations used in the lookup table, which indicates the relationship between the magnitude of the voltage applied to each pixel electrode 114 and each of the multiple gradation values, and the set intensity of the laser light La, to determine the total number of gradations used in the lookup table based on the set intensity of the laser light La. In this case, the control unit 7 (or in the determination step S21) can easily determine the magnitude of the applied voltage according to the set intensity of the laser light La.

[0074] As in this embodiment, the control unit 7 (or in decision step S21) may receive an input value indicating the amount of phase modulation from outside the light modulation device 1A and convert the input value into a gradation value using reference data indicating the correspondence between the amount of phase modulation by the liquid crystal layer 115 and each gradation value in the look-up table. In this case, a person using the light modulation device 1A (or its control method) can freely set the amount of phase modulation. Note that, in this embodiment as well, the function of the control unit 7 may be provided in the external control board 17 or the drive circuit layer 119 of the light modulation element 11. [Variations]

[0075] In the second embodiment described above, the control unit 7 determines the total number of gradations based on the set intensity of the laser light La using correlation data indicating the correlation between the total number of gradations used in the lookup table and the set intensity of the laser light La. In contrast, in this modification, the control unit 7 holds multiple lookup tables with different total numbers of gradations. Note that the control unit 7 has correlation data indicating the correlation between the total number of gradations and the set intensity of the laser light La, just like in the second embodiment. The control unit 7 of this modification uses the correlation data to determine which of the multiple lookup tables to use based on the set intensity of the laser light La, and provides the lookup table to the external control board 17. Then, as in the second embodiment, the control unit 7 receives an input value indicating the amount of phase modulation from outside the light modulation device 1A. The control unit 7 holds multiple reference data indicating the correspondence between the amount of phase modulation by the liquid crystal layer 115 and each gradation value in each of the multiple lookup tables, and converts the input value into a gradation value using the reference data corresponding to the lookup table to be used.

[0076] As in this modified example, the control unit 7 (or in the determination step S21) may determine which lookup table to use from among a plurality of lookup tables having different total numbers of gradations based on the set intensity of the laser light La, using correlation data that indicates the correlation between the total number of gradations and the set intensity of the laser light La. In this case, the control unit 7 (or in the determination step S21) can easily determine the magnitude of the applied voltage according to the set intensity of the laser light La.

[0077] As in this modification, the control unit 7 (or in the decision step S21) may receive an input value indicating the amount of phase modulation from outside the light modulation device 1A, and convert the input value into a gradation value using reference data corresponding to the lookup table to be used, among a plurality of reference data indicating the correspondence between the amount of phase modulation by the liquid crystal layer 115 and each gradation value in each of a plurality of lookup tables. In this case, a person using the light modulation device 1A (or this control method) can freely set the amount of phase modulation. Note that, in this modification as well, the function of the control unit 7 may be provided in the external control board 17 or the drive circuit layer 119 of the light modulation element 11.

[0078] The light modulation device and the method for controlling a spatial light modulator according to the present invention are not limited to the above-described embodiments, and various other modifications are possible. For example, the control unit of the light modulation device may have both the functions of the control unit 7 according to the first embodiment and the functions of the control unit 7 according to the second embodiment. That is, the control unit may simultaneously lower the set temperature of the cooling unit 15 as the set intensity of the laser light La increases, and increase the applied voltage corresponding to a predetermined phase as the set intensity of the laser light La increases.

[0079] Furthermore, in the first embodiment described above, the control unit 7 determines the set temperature of the cooling unit 15 using a data table or a calculation formula that indicates the relationship between the set intensity of the laser light La and the set temperature of the cooling unit 15, but the method for determining the set temperature of the cooling unit 15 is not limited to this. Furthermore, in the first embodiment described above, the set temperature of the cooling unit 15 is proportional to the set intensity of the laser light La, but the relationship between the set temperature of the cooling unit 15 and the set intensity of the laser light La is not limited to a proportional relationship.

[0080] Furthermore, in each of the above-described embodiments, a Peltier element is used as an example of the cooling unit, but the cooling unit may have other configurations (for example, a water-cooling system using a water-cooled plate, etc.). [Explanation of symbols]

[0081] 1A...optical modulation device, 2...test device, 3...light source, 4...mirror, 5...imaging optical system, 6...condenser lens, 7...control unit, 10...spatial light modulator, 11...optical modulation element, 11a...light incident / exit surface, 11b...rear surface, 12...substrate, 12a...first surface, 12b...second surface, 12c...opening, 13...metal block, 14...temperature sensor, 15...cooling unit, 16...temperature adjustment unit, 17...external control board, 20...container, 21...housing, 21a, 21b, 21d, 21e...side surface, 21c...optical window, 21f...opening, 22...heat sink, 23...fan, 31...laser light source, 32...half-wave plate, 33, 38...polarized beam splitter 115a, 115b...alignment film, 116...transparent layer, 117...dielectric multilayer film, 118...light-shielding layer, 119...drive circuit layer, B...object, D1, D2...direction, La, Lb...laser light, Lc, Ld...test light, Q1 to Q6...approximate straight line.

Claims

1. a light source that outputs light having an intensity according to a set intensity; a spatial light modulator having a plurality of pixel electrodes arranged one-dimensionally or two-dimensionally, a liquid crystal layer that modulates the phase of the light in accordance with the magnitude of an electric field formed by each pixel electrode, a driving unit that applies a voltage to the plurality of pixel electrodes, a temperature sensor that detects the ambient temperature of the liquid crystal layer, and a cooling unit that cools the liquid crystal layer so that the temperature detected by the temperature sensor approaches a set temperature; a control unit that determines the set temperature of the cooling unit; Equipped with The control unit determines the set temperature based on the set intensity.

2. The optical modulation device according to claim 1 , wherein the control unit holds in advance a data table showing a relationship between the set intensity and the set temperature, and determines the set temperature using the data table.

3. The optical modulation device according to claim 1 , wherein the control unit holds in advance a formula that indicates the relationship between the set intensity and the set temperature, and determines the set temperature using the formula.

4. 4. The optical modulation device according to claim 1, wherein the set temperature is proportional to the set intensity.

5. a light source that outputs light having an intensity according to a set intensity; a spatial light modulator having a plurality of pixel electrodes arranged one-dimensionally or two-dimensionally, a liquid crystal layer that modulates the phase of the light in accordance with the magnitude of an electric field formed by each pixel electrode, and a driving unit that applies a voltage to the plurality of pixel electrodes; a control unit that determines the magnitude of a voltage applied to the plurality of pixel electrodes; Equipped with the control unit determines the magnitude of the voltage using a lookup table that indicates a relationship between the magnitude of the voltage and each of a plurality of gradation values ​​when setting the phase of each pixel of the liquid crystal layer to a predetermined phase; The control unit determines the total number of gradations used in the lookup table based on the set intensity.

6. A light source that outputs light having an intensity according to a set intensity; a spatial light modulator having a plurality of pixel electrodes arranged one-dimensionally or two-dimensionally, a liquid crystal layer that modulates the phase of the light in accordance with the magnitude of an electric field formed by each pixel electrode, and a driving unit that applies a voltage to the plurality of pixel electrodes; a control unit that determines the magnitude of a voltage applied to the plurality of pixel electrodes; Equipped with the control unit determines the magnitude of the voltage such that, when setting the phase of each pixel of the liquid crystal layer to a predetermined phase, the voltage corresponding to the predetermined phase increases as the set intensity increases; the control unit has correlation data indicating a correlation between the set intensity and a total number of gradations used in a lookup table indicating a relationship between the magnitude of the voltage and each of a plurality of gradation values, The control unit uses the correlation data to determine the total number of gradations used in the lookup table based on the set intensity.

7. 7. The optical modulation device of claim 6, wherein the control unit has reference data indicating the correspondence between the amount of phase modulation by the liquid crystal layer and each gradation value in the lookup table, and inputs an input value indicating the amount of phase modulation from outside the optical modulation device and converts the input value into the gradation value using the reference data.

8. A light source that outputs light having an intensity according to a set intensity; a spatial light modulator having a plurality of pixel electrodes arranged one-dimensionally or two-dimensionally, a liquid crystal layer that modulates the phase of the light in accordance with the magnitude of an electric field formed by each pixel electrode, and a driving unit that applies a voltage to the plurality of pixel electrodes; a control unit that determines the magnitude of a voltage applied to the plurality of pixel electrodes; Equipped with the control unit determines the magnitude of the voltage such that, when setting the phase of each pixel of the liquid crystal layer to a predetermined phase, the voltage corresponding to the predetermined phase increases as the set intensity increases; the control unit determines, based on the set intensity, a lookup table to be used from among a plurality of lookup tables each indicating a relationship between the magnitude of the voltage and each of a plurality of gradation values ​​and each having a different total number of gradations, using correlation data indicating a correlation between the total number of gradations and the set intensity.

9. 9. The optical modulation device according to claim 8, wherein the control unit has a plurality of reference data each indicating a correspondence between the amount of phase modulation by the liquid crystal layer and each gradation value of the plurality of lookup tables, and inputs an input value indicating the amount of phase modulation from outside the optical modulation device, and converts the input value into the gradation value using the reference data among the plurality of reference data corresponding to the lookup table to be used.

10. A method for controlling a spatial light modulator having a plurality of pixel electrodes arranged one-dimensionally or two-dimensionally, a liquid crystal layer that modulates the phase of light in accordance with the magnitude of an electric field formed by each pixel electrode, a driving unit that applies a voltage to the plurality of pixel electrodes, a temperature sensor that detects the ambient temperature of the liquid crystal layer, and a cooling unit that cools the liquid crystal layer so that the temperature detected by the temperature sensor approaches a set temperature, a determining step of determining the set temperature of the cooling unit; a light input step of inputting light having an intensity according to a set intensity into the liquid crystal layer; Including, The method for controlling a spatial light modulator, wherein the setting temperature is determined based on the setting intensity in the determining step.

11. A method for controlling a spatial light modulator having a plurality of pixel electrodes arranged one-dimensionally or two-dimensionally, a liquid crystal layer that modulates the phase of light in accordance with the magnitude of an electric field formed by each pixel electrode, and a driving unit that applies a voltage to the plurality of pixel electrodes, a determining step of determining the magnitude of a voltage to be applied to each pixel electrode; a light input step of inputting light having an intensity according to a set intensity into the liquid crystal layer; Including, In the determining step, when setting the phase of each pixel of the liquid crystal layer to a predetermined phase, the magnitude of the voltage is determined using a lookup table that indicates a relationship between the magnitude of the voltage and each of a plurality of gradation values; In the determining step, the total number of gradations used in the lookup table is determined based on the set intensity.

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