Optically addressed spatial light modulator tolerant to high average power laser and method for preparing the same

Abstract

The present invention discloses an optically addressed spatial light modulator that can withstand high average power lasers and a preparation method thereof, and relates to the technical field of spatial light modulators. The spatial light modulator includes, from top to bottom, a first sapphire substrate, a first transparent conductive film layer ITO, a first PI orientation layer, a liquid crystal layer, a second PI orientation layer, a photoconductive layer, a second transparent conductive film layer ITO, and a second sapphire substrate, wherein the second sapphire substrate is combined with the photoconductive layer by bonding. Compared with traditional K9 glass substrates and quartz glass substrates, the sapphire structure in the present invention can quickly dissipate the deposited heat of the liquid crystal layer and the photoconductive layer, thereby improving the thermal conductivity and heat dissipation performance of the optically addressed spatial light modulator, optimizing the problem of thermal deposition when the device works under high-power continuous lasers or quasi-continuous lasers, and improving the tolerable power density of the optically addressed spatial light modulator.

Description

Technical Field

[0001] The invention relates to the technical field of liquid crystal spatial light modulators, in particular to an optically addressable spatial light modulator that can withstand high average power lasers. Background Art

[0002] With the rapid development of high-power laser systems, the demand for optically addressed spatial light modulators in many fields such as laser weapons, additive manufacturing, and laser communications is becoming more and more urgent. However, there is a problem of thermal deposition in the application of optically addressed spatial light modulators in the field of high-power lasers. The commonly used conductive film layer in the optically addressed spatial light modulator is indium tin oxide film, which has strong absorption in the infrared band and easily heats up after laser irradiation, causing heat to accumulate inside the liquid crystal layer. Since liquid crystals are extremely sensitive to operating temperature, it is difficult for optically addressed spatial light modulators to work stably for a long time or even fail under high-power laser irradiation.

[0003] The optically addressable spatial light modulator (OASLM) is mainly composed of a photoconductive layer, a liquid crystal layer, a glass substrate, a transparent electrode, an orientation film and other structures. At present, most common optically addressable spatial light modulators use K9 glass or fused quartz glass as the substrate of the liquid crystal module. However, the thermal conductivity of these materials is poor. In the absence of external heat dissipation measures, the heat generated by the conductive film absorbing the laser energy cannot be conducted out in time, so the temperature of the optically addressable spatial light modulator will increase. The general sapphire substrate OASLM does not have any heat dissipation measures in the photoconductive layer, which will cause serious heat sinking of the photoconductive layer during use, and will also cause the device performance to deteriorate after heating. This results in the optically addressable spatial light modulator still not being able to be used in higher power laser systems. Therefore, by bonding sapphire to the photoconductive layer, the heat dissipation efficiency of the photoconductive layer in the OASLM can be greatly improved, and the problem of heat deposition in the OASLM under high-power lasers can be improved. Summary of the invention

[0004] The purpose of the present invention is to overcome the shortcomings of the prior art, provide a bonded sapphire structure, increase the heat dissipation efficiency of the optical guide layer, and have a higher laser tolerance power, thereby solving the serious problem of heat sinking of the optical guide layer of a general optically addressed spatial light modulator.

[0005] The present invention is achieved through the following technical solutions:

[0006] An optically addressable spatial light modulator that tolerates high average power lasers, characterized in that it includes, from top to bottom, a first sapphire substrate, a first transparent conductive film layer, a first orientation layer, a liquid crystal layer, a spacer, a second orientation layer, a light guide layer, a second transparent conductive film layer, and a second sapphire substrate;

[0007] The sapphire substrate is plated with a first transparent conductive film layer and a first orientation layer; the second orientation layer and the second transparent conductive film are plated on both sides of the photoconductive layer respectively; the liquid crystal layer is encapsulated between the first orientation layer and the second orientation layer through a spacer, and the second sapphire substrate and the photoconductive layer are combined by hydroxide catalytic bonding;

[0008] The sapphire substrate is a double-sided polished substrate with a surface shape smaller than λ / 4.

[0009] The size of the sapphire substrate is larger than that of the light guide layer.

[0010] The method for preparing the optically addressable spatial light modulator capable of withstanding high average power laser light comprises the following steps:

[0011] Step 1. The first transparent conductive film layer ITO and the first PI orientation layer are sequentially plated on the first sapphire substrate; the second PI orientation layer and the second transparent conductive film layer ITO are respectively plated on both sides of the photoconductive layer; the liquid crystal layer is encapsulated between the first sapphire substrate plated with the conductive film and the orientation layer and the photoconductive layer plated with the conductive film and the orientation layer through a spacer.

[0012] Step 2. Clean and dry the lower surface of the photoconductive layer having the second transparent conductive film layer ITO;

[0013] Step 3. Clean and dry the upper surface of the second sapphire substrate;

[0014] Step 4. Configure Rich The bonding solution is dripped onto the lower surface of the photoconductive layer having the second transparent conductive film layer ITO, so that the second sapphire substrate and the photoconductive layer are pressed together for pre-bonding;

[0015] Step 5: Heat and anneal the pre-bonded parts to complete the bonding.

[0016] Compared with the prior art, the present invention has the following beneficial effects:

[0017] Without changing the basic structure of the optically addressed spatial light modulator, the present invention adopts sapphire as the device substrate, and bonds the sapphire substrate to the optical waveguide layer side. The bonding structure is fixed by chemical bonds, and has good light-guiding and thermal conductivity, thus solving the problems of low heat dissipation efficiency of the optical waveguide layer in the general OASLM, which easily heats up during use and causes device performance degradation.

[0018] Compared with the traditional structure, the structure based on sapphire substrate increases the heat dissipation area and heat dissipation rate of each structure of the device, and can timely remove the heat inside the device without affecting the quality of the light beam. This enables the optically addressed spatial light modulator to have a smaller temperature rise under higher power laser irradiation, and the power density that the device can withstand is increased from 7.5W / cm 2Increased to 75W / cm 2 , and can maintain stable performance during long-term operation. This makes it more widely used in high-power laser systems. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 The schematic diagram of the structure of the optically addressable spatial light modulator of the present invention that can withstand high average power laser is shown in the figure. In the figure, 1 is the first sapphire substrate, 2 is the first transparent conductive film layer, 3 is the first PI orientation layer, 4 is the liquid crystal layer, 5 is the second PI orientation layer, 6 is the light guide layer, 7 is the second transparent conductive film layer, 8 is the spacer, and 9 is the second sapphire substrate.

[0020] Figure 2 Schematic diagram of implementing amplitude modulation for a light-addressable spatial light modulator.

[0021] Figure 3 Figure 2 is the temperature distribution of each layer in the laser-induced model of the optically addressed spatial light modulator, where (a) is the temperature distribution of the bismuth silicate layer and the liquid crystal layer of the device based on the K9 substrate; (b) is the temperature distribution of the bismuth silicate layer and the liquid crystal layer of the device based on the sapphire substrate; (c) is the temperature distribution of the bismuth silicate layer and the liquid crystal layer of the device after bonding to the sapphire substrate.

[0022] Figure 4 Temperature variation curves of optically addressable spatial light modulators with different structures under high-power laser irradiation.

[0023] Figure 5 The light field distribution after the amplitude-type light-addressable spatial light modulator modulates the light beam, where (a) is the device based on the K9 substrate at 20W / cm 2 (b) The light field distribution after amplitude modulation under laser irradiation; (c) The device with double-sided sapphire substrate at 100W / cm 2 Light field distribution after amplitude modulation under laser irradiation. DETAILED DESCRIPTION

[0024] The present invention is further described below with reference to examples and drawings, but the protection scope of the present invention shall not be limited thereby:

[0025] Embodiment 1: As attached Figure 1 As shown, a light-addressable spatial light modulator that can withstand high average power lasers is characterized in that it includes, from top to bottom, a first sapphire substrate 1, a first transparent conductive film layer 2, a first PI orientation layer 3, a liquid crystal layer 4, a second PI orientation layer 5, a bismuth silicate single crystal 6, a second transparent conductive film layer 7, a spacer 8 and a second sapphire substrate 9.

[0026] First, the K9 substrate is replaced with a sapphire substrate 1, and a liquid crystal box is made according to the traditional method. At this time, it has played a certain role in heat dissipation, and the tolerable power density has been improved; in order to further improve the heat dissipation capacity, the sapphire substrate 9 is precisely ground and polished, and the surface shape is better than λ / 10; the surface of the sapphire substrate 9 and the bismuth silicate single crystal 6 on the liquid crystal box is cleaned to remove the impurity ions and grease on the surface. The cleaning liquid can be acetone or anhydrous ethanol, and then ultrasonic cleaning is carried out in deionized water for 10 minutes, and then blown dry with nitrogen; a rich The bonding solution is filtered and dripped onto the surface of the bismuth silicate single crystal 6 in a clean and dust-free environment. The sapphire substrate 9 and the bismuth silicate single crystal 6 are bonded together and gently squeezed for pre-bonding. The pre-bonded device is placed at room temperature for 24 hours, then placed in a vacuum heat treatment furnace and heated to 200°C, and then slowly cooled to room temperature. The heating and cooling process is maintained at 2°C / min. The bonded sample is taken out to complete the bonding.

[0027] like Figure 2 As shown, further wires are connected on both sides of the bonding part and voltage is applied, and the 470nm writing light 10 is reflected by the dichroic mirror 11 and irradiated onto the bonding part to achieve addressing. When the incident light passes through the optical addressing spatial light modulator, it will be amplitude modulated according to the information of the writing light, and the output light is the modulated output light.

[0028] The following model is used to analyze the laser-induced temperature rise. The transparent conductive film layer of the optically addressed spatial light modulator absorbs laser energy and converts it into heat after laser irradiation. A laser-induced temperature rise model is established for analysis. According to the Fourier heat conduction equation:

[0029]

[0030] Among them, ρ,, are the density, specific heat capacity and thermal conductivity of the material, Q is the body heat source, α is the absorption coefficient of the material to the laser, and I represents the intensity of the incident laser. The power density is 30W / cm 2 The laser is irradiated on the light-addressable spatial light modulator. The temperature distribution of each layer of the light-addressable spatial light modulator based on the K9 substrate is shown as follows: Figure 3 As shown in (a), the solid line represents the temperature of the conductive film layer on the bismuth silicate single crystal side, and the dotted line represents the temperature of the device liquid crystal layer, which means that the maximum temperature of the liquid crystal material has exceeded the clearing point, seriously affecting the device performance. The temperature distribution of each layer of the device after the improvement to the sapphire substrate is shown in Figure 3 As shown in (b), the result shows that the temperature of the liquid crystal layer has been significantly improved, with the highest temperature being only 33°C. However, the temperature of the conductive film layer on the bismuth silicate single crystal side is still high, resulting in a large temperature difference in the thickness direction of the device, close to 30°C. After the second sapphire substrate is bonded to the bismuth silicate crystal side, the temperature distribution of each layer of the device is shown in the figure below. Figure 3As shown in (c), the temperatures of the liquid crystal layer and the bismuth silicate crystal side are significantly improved, and the temperature difference in the thickness direction does not exceed 2°C.

[0031] The temperature rise of the optically addressable spatial light modulator under high-power laser irradiation was investigated. The device was irradiated with a high-power laser, and the laser intensity was Gaussian distributed. The temperature rise under different laser power densities was recorded by a thermal imager. Since the temperature of the liquid crystal layer cannot be directly detected, and the temperature difference between the glass substrate and the liquid crystal layer is small, the highest temperature of the glass substrate is used for characterization. The device temperature distribution under different laser power densities is shown in Figure 2. Figure 4 As shown in the figure, the temperature of the optically addressed spatial light modulator using the K9 substrate increases rapidly with the laser power density. 2 When the device temperature is around 36°C, and at 20W / cm 2 The clearing point of the liquid crystal is reached at 75W / cm. The temperature rise of the device using sapphire substrate is very slow. 2 The temperature rises to 36°C only under the laser power of 100W / cm2. The bismuth silicate curve represents the temperature rise curve of the bismuth silicate crystal under the single-sided sapphire substrate. The curve shows that the temperature rise of the bismuth silicate crystal is fast, which will cause a certain temperature difference in the thickness direction of the device. From the temperature rise curves of the first sapphire substrate and the second sapphire substrate, it can be concluded that the device with a double-sided sapphire structure improves the problem of large temperature gradient in the thickness direction. At the same time, at 100W / cm2, the temperature rise of the bismuth silicate crystal is very fast. 2 The maximum device temperature is only 36°C at the laser power density.

[0032] Example 2: Investigate the modulation performance of the light-addressable spatial light modulator under high-power laser irradiation. The implementation conditions are the same as those in Example 1, but a photodetector is used to record the light field distribution after the device is modulated. A black and white strip pattern is loaded on the amplitude-addressable spatial light modulator to record the light field distribution after the device is modulated. The light-addressable spatial light modulator based on the K9 substrate is irradiated at 20W / cm 2 The output light field under laser irradiation is as follows Figure 5 As shown in (a), the liquid crystal has reached the clearing point and the middle area has lost the modulation effect. 2 The modulation effect under laser irradiation is as follows Figure 5 As shown in (b), the results show that under the irradiation of high-power laser, the stripes of the output light spot still have a good contrast, and the modulation performance is in line with expectations. If the bonding process used is more mature, the laser tolerance power density of the optically addressed spatial light modulator can be further improved.

[0033] The embodiments described in the present invention are only for describing the preferred implementation of the present invention, and the protection scope of the present invention is not limited to such special statements and embodiments. Various modifications and improvements made by engineers and technicians in this field to the technical solution of the present invention should fall within the protection scope of the present invention.

Claims

1. An optically addressable spatial light modulator resistant to high average power laser, comprising, from top to bottom, a first sapphire substrate (1), a first transparent conductive film layer ITO (2), a first PI orientation layer (3), a liquid crystal layer (4) encapsulated by a spacer (8), a second PI orientation layer (5), a photoconductive layer (6) and a second sapphire substrate (9); characterized in that: The lower surface of the optical guide layer (6) is plated with a second transparent conductive film layer ITO (7) and is bonded to a second sapphire substrate (9).

2. The optically addressable spatial light modulator capable of withstanding high average power laser according to claim 1, characterized in that: The second sapphire substrate (9) and the photoconductive layer (6) are bonded by means of hydroxide catalytic bonding.

3. The optically addressable spatial light modulator capable of withstanding high average power laser according to claim 1, characterized in that: The first sapphire substrate (1) and the second sapphire substrate (9) are both double-sided polished substrates, and the surface shape is smaller than λ / 4.

4. The optically addressable spatial light modulator capable of withstanding high average power laser according to claim 1, characterized in that: The length and width of the first sapphire substrate (1) and the second sapphire substrate (9) are both greater than the length and width of the optical waveguide layer (6).

5. The optically addressable spatial light modulator capable of withstanding high average power laser according to claim 1 or 4, characterized in that: The optical guide layer (6) is bismuth silicate single crystal or bismuth germanate single crystal.

6. A method for preparing an optically addressable spatial light modulator that can withstand high average power laser, characterized in that: The steps include: Step 1. Sequentially depositing a first transparent conductive film layer ITO (2) and a first PI alignment layer (3) on the same side of a first sapphire substrate (1); A second PI orientation layer (5) and a second transparent conductive film layer ITO (7) are respectively plated on both sides of the light-conducting layer (6); The liquid crystal layer (4) is encapsulated between a first sapphire substrate (1) coated with a first transparent conductive film layer ITO (2) and a first PI orientation layer (3) and a photoconductive layer (6) coated with a second PI orientation layer (5) and a second transparent conductive film layer ITO (7) through a spacer (8); Step 2. Cleaning and drying the lower surface of the photoconductive layer (6) having the second transparent conductive film layer ITO (7); Step 3. Cleaning and drying the upper surface of the second sapphire substrate (9); Step 4. Configure Rich The bonding solution is dripped onto the lower surface of the optical guide layer (6) having the second transparent conductive film layer ITO (7), so that the second sapphire substrate (9) and the optical guide layer (6) are pressed together to perform pre-bonding; Step 5: Heat and anneal the pre-bonded parts to complete the bonding.

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