Sensor, manufacturing method thereof and interference measurement system comprising sensor

Abstract

The invention relates to a sensor, a manufacturing method thereof and an interference measurement system comprising the same. The sensor comprises a base part, the first end part of the base part is provided with a pressure sensitive diaphragm, and the end face of the second end part, opposite to the first end part, of the base part is fixedly connected with a hollow pipe; the pressure sensing cavity is formed between the first end part of the base part and the pressure sensitive diaphragm and is sealed by the first end part of the base part and the pressure sensitive diaphragm, and the cavity length of the pressure sensing cavity is changed according to the change of the pressure to be measured; the temperature sensing cavity is formed between an incident optical fiber in the hollow tube and a reflection end face of the temperature sensing cavity, and the cavity length of the temperature sensing cavity changes according to the change of the temperature to be measured; and the temperature sensing cavity and the pressure sensing cavity are oppositely arranged along the emergent direction of light from the incident optical fiber.

Description

Technical Field

[0001] This disclosure relates to sensors, methods of manufacturing the same, and interferometric measurement systems incorporating the same. Background Technology

[0002] Fiber optic sensors have been widely used in various industries, such as petroleum, aviation, aerospace, medical, and marine fields, and have demonstrated excellent performance. Currently, various fields are demanding applications that simultaneously measure temperature and pressure parameters.

[0003] An existing fiber optic FP cavity pressure sensor with temperature self-compensation uses an optical fiber with an etched grating as the transmission fiber and an optical FP cavity as the pressure-sensitive element. The fiber grating and the optical FP cavity are connected by adhesive bonding or CO2 laser welding. This type of sensor has the following problems: the composite sensor requires both a fiber grating demodulation system and an FP cavity longitude demodulation system, increasing time and space costs. Summary of the Invention

[0004] Therefore, the object of this disclosure is to provide a sensor that can demodulate a pressure sensing chamber and a temperature sensing chamber using the same demodulation device.

[0005] The above objective is achieved by means of the sensor described below.

[0006] This disclosure provides a sensor, comprising: a base, a pressure-sensitive diaphragm disposed at a first end of the base, and an end face of a second end of the base opposite to the first end fixedly connected to a hollow tube; a pressure sensing cavity formed between the first end of the base and the pressure-sensitive diaphragm, and closed by the first end of the base and the pressure-sensitive diaphragm, the cavity length of the pressure sensing cavity varying according to the change in the pressure to be measured; and a temperature sensing cavity formed between an incident optical fiber in the hollow tube and a reflective end face of the temperature sensing cavity, the cavity length of the temperature sensing cavity varying according to the change in the temperature to be measured; the temperature sensing cavity and the pressure sensing cavity are disposed opposite to each other along the direction in which light exits from the incident optical fiber.

[0007] In one embodiment, the reflective end face of the temperature sensing cavity is the end face of the second end of the base, and the temperature sensing cavity is formed between the end face of the second end of the base and the end face of the incident optical fiber.

[0008] In one embodiment, the device further includes a reflective optical fiber disposed in the hollow tube, the reflective optical fiber being fixed to the end face of the second end of the base, and the temperature sensing cavity being formed between the end face of the incident optical fiber and the end face of the reflective optical fiber.

[0009] In one embodiment, the coefficient of thermal expansion of the hollow tube is 2*10. -6 / k-30*10 -6 / k.

[0010] In one embodiment, the length of the hollow tube is not less than 0.5 mm.

[0011] In one embodiment, at least a localized region of the pressure-sensitive diaphragm has a dopant material incorporated into the base material of the pressure-sensitive diaphragm to generate stress.

[0012] In one embodiment, the difference between the length of the temperature sensing cavity and the length of the pressure sensing cavity is not less than 1 μm.

[0013] In one embodiment, the difference between the length of the temperature sensing cavity and the length of the pressure sensing cavity is less than 100 μm.

[0014] This disclosure also provides a method for manufacturing a sensor, comprising: manufacturing a base having a cavity at a first end or a pressure-sensitive diaphragm having a cavity; combining the pressure-sensitive diaphragm with the first end of the base, such that the cavity is closed by the pressure-sensitive diaphragm and the base to form a pressure sensing cavity; fixing a hollow tube to the second end of the base, and inserting an incident optical fiber into the hollow tube to form a temperature sensing cavity, such that the temperature sensing cavity and the pressure sensing cavity are arranged opposite to each other along the direction in which light exits from the incident optical fiber.

[0015] This disclosure also provides an interferometric measurement system, comprising: a light source for emitting coherent light; a sensor as described in any of the preceding claims, configured to receive the coherent light and form and output modulated light modulated by a pressure to be measured and a temperature to be measured; and an interferometric demodulation device configured to receive the modulated light and demodulate the modulated light to obtain an interference spectrum of a pressure sensing cavity and an interference spectrum of a temperature sensing cavity.

[0016] In one embodiment, the interference demodulation device includes: a condenser configured to receive modulated light; a first polarizer disposed downstream of the condenser and having a first polarization direction; a second polarizer disposed downstream of the first polarizer and having a second polarization direction perpendicular or parallel to the first polarization direction; and a birefringent element located between the first polarizer and the second polarizer and having an optical axis at 45° or -45° to the first polarization direction.

[0017] This disclosure has the following advantages: the first end and the second end of the base form a pressure sensing cavity and a temperature sensing cavity, respectively. The temperature sensing cavity and the pressure sensing cavity are arranged opposite to each other along the incident optical fiber. The light from the incident optical fiber is sequentially incident into the temperature sensing cavity and the pressure sensing cavity, forming and outputting modulated light modulated by the pressure sensing cavity and the temperature sensing cavity. The modulated light can carry optical path information corresponding to the cavity length of the pressure sensing cavity and the cavity length of the temperature sensing cavity. The demodulation of the pressure sensing cavity and the temperature sensing cavity can be realized by the same demodulation device. Moreover, the sensor structure and manufacturing process are simple and the cost is low.

[0018] 1-Base 11-First end of the base 12-Second end of the base 2-Pressure-sensitive diaphragm 3-Pressure sensing cavity 4-Temperature sensing cavity 5-Incident optical fiber 51-Reflecting optical fiber 6-Hollow tube 61-Tail end of the hollow tube Attached Figure Description

[0019] Figure 1 This is a schematic diagram of the structure of a sensor according to an embodiment of the present disclosure; Figure 2 This is a schematic diagram of the structure of a sensor according to an embodiment of the present disclosure; Figure 3 This is a schematic diagram of the interference spectrum of a sensor according to an embodiment of this disclosure. Detailed Implementation

[0020] To make the objectives, technical solutions, and advantages of this disclosure clearer, the technical solutions of the embodiments of this disclosure will be clearly and completely described below with reference to the accompanying drawings. The same reference numerals in the drawings represent the same components. It should be noted that the described embodiments are only some, not all, of the embodiments of this disclosure. All other embodiments obtained by those skilled in the art based on the described embodiments of this disclosure without creative effort are within the scope of protection of this disclosure.

[0021] Unless otherwise defined, the technical or scientific terms used herein shall have the ordinary meaning as understood by one of ordinary skill in the art to which this disclosure pertains. The terms “first,” “second,” and similar terms used in this patent application specification and claims do not indicate any order, quantity, or importance, but are merely used to distinguish different components. Similarly, the terms “an” or “a” and similar terms do not necessarily indicate a quantity limitation. The terms “comprising,” “including,” or “having,” and similar terms mean that the element or object preceding the word encompasses the element or object listed following the word and its equivalents, without excluding other elements or objects. The terms “connected” or “connected,” and similar terms are not limited to the physical or mechanical connection or connection shown in the drawings, but may include equivalent connections or connections, whether direct or indirect. The terms “upper,” “lower,” “left,” and “right,” etc., are used only to indicate relative positional relationships, which may change accordingly when the absolute position of the described object changes.

[0022] The following is for reference. Figures 1 to 2 A detailed description of embodiments of the sensor according to this disclosure is provided.

[0023] This disclosure provides a sensor, comprising: a base 1, a pressure-sensitive diaphragm 2 disposed at a first end 11 of the base, and an end face of a second end 12 of the base opposite to the first end 11 fixedly connected to a hollow tube 6; a pressure sensing cavity 3 formed between the first end 11 of the base and the pressure-sensitive diaphragm 2, and closed by the first end 11 of the base and the pressure-sensitive diaphragm 2, the cavity length of the pressure sensing cavity changing according to the change of the pressure to be measured; and a temperature sensing cavity 4 formed between an incident optical fiber 5 in the hollow tube 6 and a reflecting end face of the temperature sensing cavity 4, the cavity length of the temperature sensing cavity changing according to the change of the temperature to be measured; the temperature sensing cavity 4 and the pressure sensing cavity 3 are arranged opposite to each other along the direction in which light exits from the incident optical fiber 5.

[0024] The principle by which the sensor disclosed herein can achieve demodulation in the same demodulation device is as follows: the temperature sensing cavity 4 and the pressure sensing cavity 3 are arranged opposite each other along the direction in which light exits from the incident optical fiber 5, so that the light exiting from the incident optical fiber 5 is sequentially incident into the temperature sensing cavity and the pressure sensing cavity. Part of the light generates a partial reflection signal through the reflective end face of the temperature sensing cavity, which is superimposed with part of the incident light at the end of the incident optical fiber; part of the light generates a reflection signal through the end face forming the base of the pressure sensing cavity 3, and part of the light is reflected back to the end face forming the base of the pressure sensing cavity through the pressure-sensitive diaphragm 2 and superimposed, forming and outputting modulated light modulated by the pressure sensing cavity and the temperature sensing cavity. The modulated light carries optical path information corresponding to the cavity length of the pressure sensing cavity and the cavity length of the temperature sensing cavity. Therefore, demodulation of the temperature sensing cavity and the pressure sensing cavity can be realized in the same optical path.

[0025] The temperature sensing cavity 4 and the pressure sensing cavity 3 are arranged opposite each other along the direction in which light exits from the incident optical fiber 5. Preferably, the incident optical fiber 5 is directly opposite the pressure sensing cavity 3, and the projection of the incident optical fiber 5 onto the pressure-sensitive diaphragm 2 is within the cross-sectional area of ​​the pressure sensing cavity, so that all the light emitted from the incident optical fiber 5 is incident into the pressure sensing cavity. Preferably, when the pressure sensing cavity 3 is a circular cavity, the diameter of the pressure sensing cavity 3 is not less than the diameter of the incident optical fiber 5.

[0026] The reflective end face of the temperature sensing cavity can be the end face of the second end 12 of the base or the end face of the reflective optical fiber 51 disposed in the hollow tube. In one embodiment, such as Figure 1 As shown, the reflective end face of the temperature sensing cavity is the end face of the second end face 12 of the base. The temperature sensing cavity 4 is formed between the end face of the second end face 12 of the base and the end face of the incident optical fiber 5. The sensor structure and manufacturing process of this embodiment are simple and low in cost. In one embodiment, as... Figure 2 As shown, the reflective end face of the temperature sensing cavity is the end face of the reflective optical fiber 51 disposed in the hollow tube. In this embodiment, the sensor also includes a reflective optical fiber 51, which is disposed in the hollow tube 6 and fixed to the end face of the second end 12 of the base. The temperature sensing cavity 4 is formed between the end face of the incident optical fiber 5 and the end face of the reflective optical fiber 51.

[0027] The difference in cavity length between the pressure sensing cavity and the temperature sensing cavity is not limited. Preferably, the difference in cavity length between the temperature sensing cavity and the pressure sensing cavity is not less than 1 μm, which ensures that the demodulated signals of the pressure sensing cavity and the temperature sensing cavity do not overlap. Preferably, the difference in cavity length between the pressure sensing cavity and the temperature sensing cavity is less than 100 μm, so that demodulation of the temperature sensing cavity and the pressure sensing cavity can be realized in the same channel of a demodulation device, and the range of the demodulation device can be reduced, thereby reducing costs. Demodulation of the temperature sensing cavity and the pressure sensing cavity in the same channel of the demodulation device means demodulation of the temperature sensing cavity and the pressure sensing cavity in the same optical path of the demodulation device.

[0028] The base 1 is preferably made of glass, but other materials may also be used, such as, but not limited to, monocrystalline silicon, silicon carbide, sapphire, etc., to achieve good light guiding performance. The thickness of the base may be selected from 200μm to 500μm. The shape of the base is not limited.

[0029] The sensitivity of the temperature sensing cavity 4 is directly proportional to both the length of the hollow tube and its coefficient of thermal expansion. The sensitivity of the temperature sensing cavity 4 can be adjusted by changing the material and length of the hollow tube 6. The coefficient of thermal expansion of the hollow tube 6 covering the incident optical fiber 5 is preferably 2*10⁻⁶. -6 / k-30*10 -6The hollow tube 6 can be made of materials such as glass, ceramic, or stainless steel. The longer the hollow tube 6, the higher the sensitivity of the temperature sensing cavity. The sensor disclosed herein forms a pressure sensing cavity and a temperature sensing cavity at its two ends, respectively, and uses a hollow tube and an incident optical fiber disposed within the hollow tube to form the temperature sensing cavity. This not only allows for demodulation of the dual cavity lengths in the same demodulation device but also simultaneously improves the sensitivity of the temperature sensing cavity. Preferably, the length of the hollow tube 6 is not less than 0.5 mm. Taking the cavity length of a common temperature Fabry-Perot sensor (50 μm-100 μm) as an example, the sensitivity can be improved by 5-10 times.

[0030] The hollow tube 6 can be fixed to the second end 12 of the base by, for example, bonding or laser welding. The inner diameter of the hollow tube 6 is smaller than the size of the base to facilitate fixing the hollow tube 6 to the second end 12 of the base. The inner diameter of the hollow tube 6 is slightly larger than the outer diameter of the incident optical fiber 5. In one embodiment, the incident optical fiber is a multimode optical fiber with an outer diameter of 0.125 mm, and the inner diameter of the hollow tube 6 is 0.126 mm to 0.13 mm. The incident optical fiber 5 can be fixed to the tail end 61 of the hollow tube 6 by bonding or laser welding.

[0031] The pressure sensing cavity 3 is disposed in the base 1, and can also be fabricated in the pressure-sensitive diaphragm 2. The pressure-sensitive diaphragm 2 is made of materials including, but not limited to, single-crystal silicon. The thickness of the pressure-sensitive diaphragm 2 can be selected from 1 μm to 5 μm. Typically, the pressure sensing cavity 3 is formed in a vacuum state, which can improve the stability of the pressure sensing cavity. The cavity body of the pressure sensing cavity 3 can be a cavity with a circular cross-section, but is not limited to this. For a circular cavity, its diameter can be selected from 80 μm to 300 μm.

[0032] As pressure changes, the pressure-sensitive diaphragm 2 deforms toward or away from the base 1, thereby changing the cavity length of the pressure sensing cavity and thus sensing pressure. The cavity length of the temperature sensing cavity changes with temperature. When the temperature rises, the hollow tube 6 causes the incident optical fiber 5 to expand, increasing the cavity length of the temperature sensing cavity and shifting the interference signal of the temperature sensing cavity to the right in the spectral position. When the temperature decreases, the hollow tube 6 causes the incident optical fiber 5 to contract, decreasing the cavity length of the temperature sensing cavity and shifting the interference signal of the temperature sensing cavity to the left in the spectral position.

[0033] Specifically, during measurement, the measurement light is introduced through the incident optical fiber 5. A portion of the measurement light is partially reflected by the end face of the second end 12 of the base or the end face of the reflecting optical fiber, and this reflected signal is superimposed on a portion of the outgoing light at the end of the incident optical fiber. Another portion of the light is reflected by the end face of the base forming the pressure sensing cavity 3, and a portion of the light is reflected back to the end face of the base forming the pressure sensing cavity by the pressure-sensitive diaphragm 2 and superimposed. Changes in external pressure cause deformation of the pressure-sensitive diaphragm 2, altering the cavity length of the pressure sensing cavity 3, thereby changing the optical path difference. Changes in external temperature cause deformation of the temperature sensing cavity, altering its cavity length, thereby changing the optical path difference. By detecting the optical signal transmitted back through the incident optical fiber 5, the cavity lengths of the pressure sensing cavity and the temperature sensing cavity can be obtained through demodulation.

[0034] In one embodiment, at least a localized region of the pressure-sensitive diaphragm 2 has a dopant material incorporated into the base material of the pressure-sensitive diaphragm to generate stress. This reduces the temperature coefficient of the pressure sensing cavity, thus ensuring that the cavity length remains unaffected by temperature changes. The low temperature coefficient of the pressure sensing cavity eliminates the need for temperature correction. The dopant material can be one or more of the following materials: P, B, As, Al, Ga, Sb, Ge, O, Au, Fe, Cu, Ni, Zn, and Mg.

[0035] In one embodiment, the sensor further includes a reflective film disposed on the end face of the incident optical fiber and / or the reflective end face of the temperature sensing cavity. The material of the reflective film can be one of the following: Cr, Ti, Au, Ag, TaN, Al2O3, Ta2O5.

[0036] The sensor disclosed herein includes a pressure sensing cavity formed between a first end of a base and a pressure-sensitive diaphragm, and a temperature sensing cavity formed in a hollow tube between an incident optical fiber and a reflective end face of a temperature sensing cavity. The temperature sensing cavity and the pressure sensing cavity are arranged opposite to each other along the direction in which light exits from the incident optical fiber. The light from the incident optical fiber is sequentially incident into the temperature sensing cavity and the pressure sensing cavity, forming and outputting modulated light modulated by the pressure sensing cavity and the temperature sensing cavity. The modulated light carries optical path information corresponding to the cavity lengths of the pressure sensing cavity and the temperature sensing cavity. Demodulation of the pressure sensing cavity and the temperature sensing cavity can be achieved by the same demodulation device. Moreover, the sensor structure and manufacturing process of this disclosure are simple and the cost is low.

[0037] This disclosure also provides a method for manufacturing a sensor, comprising: manufacturing a base having a cavity at its first end or a pressure-sensitive diaphragm 2 having a cavity; combining the pressure-sensitive diaphragm 2 with the first end 11 of the base, such that the cavity is sealed by the pressure-sensitive diaphragm 2 and the base 1 to form a pressure sensing cavity 3; fixing a hollow tube 6 to the second end 12 of the base, and inserting an incident optical fiber 5 into the hollow tube 6 to form a temperature sensing cavity 4, such that the temperature sensing cavity 4 and the pressure sensing cavity 3 are arranged opposite to each other along the direction in which light exits from the incident optical fiber. The manufacturing method of this disclosure is simple and low-cost. In one embodiment, a method for manufacturing a sensor further includes the step of fixing the tail end 61 of the hollow tube to the incident optical fiber 5.

[0038] In one embodiment, a method for manufacturing a sensor further includes the step of fixing a reflective optical fiber in a hollow tube to a second end 12 of the base.

[0039] It should be noted that the steps listed above are preferred steps for manufacturing the sensor described in this disclosure, and the order in which the operation steps are performed is not limited. Based on the above description, those skilled in the art can also modify or omit a specific operation, add a specific operation, or adjust the order of one or more operation steps according to specific circumstances.

[0040] This disclosure also provides an interferometric measurement system, comprising: a light source that emits coherent light; the aforementioned sensor configured to receive the coherent light and to form and output modulated light modulated by the pressure and temperature to be measured; and an interferometric demodulation device configured to receive the modulated light and demodulate the modulated light to obtain the interference spectrum of the pressure sensing cavity and the interference spectrum of the temperature sensing cavity.

[0041] In one embodiment, the interferometric demodulation device includes: a condenser configured to receive modulated light; a first polarizer disposed downstream of the condenser and having a first polarization direction; a second polarizer disposed downstream of the first polarizer and having a second polarization direction perpendicular or parallel to the first polarization direction; and a birefringent element located between the first polarizer and the second polarizer and having an optical axis at 45° or -45° to the first polarization direction.

[0042] The cavity length demodulation process is as follows: Light emitted from a light source (such as a white LED, xenon lamp, or halogen lamp) is coupled into an optical fiber and enters a coupler or optical circulator. It is then transmitted from the other end of the fiber to the sensor. The light signal reflected back from the sensor passes through the coupler again and enters the interference demodulation device. The incoming light beam, after passing through the first polarizer, forms a line beam, which is incident on a birefringent wedge. The line beam undergoes equal-thickness interference on the upper and lower surfaces of the wedge. The interference spectrum of the pressure sensing cavity is obtained at the position where the thickness of the wedge is equal to the cavity length of the pressure sensing cavity, and the interference spectrum of the temperature sensing cavity is obtained at the position where the thickness of the birefringent wedge is equal to the cavity length of the temperature sensing cavity. As mentioned above, demodulation of the temperature sensing cavity and the pressure sensing cavity is achieved in the same channel of the demodulation device, that is, demodulation of the temperature sensing cavity and the pressure sensing cavity is achieved in the same optical path of the demodulation device. For example, the light signals reflected back from the sensor, modulated by the measured pressure and the measured temperature, pass sequentially through the same polarizer and birefringent element in the same optical fiber to obtain the interference spectra of the pressure sensing cavity and the temperature sensing cavity.

[0043] When the pressure and / or temperature change applied to the pressure-sensitive diaphragm, the length of the pressure-sensing cavity and / or the temperature-sensing cavity changes, altering the position of the interference spectrum of the pressure-sensing cavity and / or the temperature-sensing cavity, thereby calculating the magnitude of the pressure and temperature. For example... Figure 3 The figure shows the simulation results of sensor demodulation signal based on white light interferometric coherent demodulation method. As can be seen from the figure, the interference spectrum of the temperature sensing cavity and the interference spectrum of the pressure sensing cavity appear on the same interference spectrum.

[0044] The interferometric measurement system disclosed herein can demodulate the pressure sensing cavity and the temperature sensing cavity using the same demodulation device, and its structure and manufacturing process are simple and low in cost.

Claims

1. A sensor, characterized in that, include: The base has a pressure-sensitive diaphragm at its first end and a second end face of the base opposite to the first end fixedly connected to a hollow tube. A pressure sensing cavity is formed between the first end of the base and the pressure-sensitive diaphragm, and is closed by the first end of the base and the pressure-sensitive diaphragm. The length of the pressure sensing cavity varies according to the change in the pressure to be measured. A temperature sensing cavity is formed between the incident optical fiber in the hollow tube and the reflective end face of the temperature sensing cavity, and the cavity length of the temperature sensing cavity varies according to the change of the temperature to be measured. The temperature sensing cavity and the pressure sensing cavity are positioned opposite each other along the direction in which light exits from the incident optical fiber.

2. The sensor according to claim 1, characterized in that, The reflective end face of the temperature sensing cavity is the end face of the second end of the base, and the temperature sensing cavity is formed between the end face of the second end of the base and the end face of the incident optical fiber.

3. The sensor according to claim 1, characterized in that, It also includes a reflective optical fiber disposed in the hollow tube, the reflective optical fiber being fixed to the end face of the second end of the base, and the temperature sensing cavity being formed between the end face of the incident optical fiber and the end face of the reflective optical fiber.

4. The sensor according to claim 1, characterized in that, The thermal expansion coefficient of the hollow tube is 2*10. -6 / k-30*10 -6 / k.

5. The sensor according to claim 1, characterized in that, The length of the hollow tube is not less than 0.5 mm.

6. The sensor according to claim 1, characterized in that, At least one local region of the pressure-sensitive diaphragm has a dopant material incorporated into the base material of the pressure-sensitive diaphragm to generate stress.

7. The sensor according to claim 1, characterized in that, The difference between the length of the temperature sensing cavity and the length of the pressure sensing cavity is not less than 1 μm.

8. The sensor according to claim 1, characterized in that, The difference between the length of the temperature sensing cavity and the length of the pressure sensing cavity is less than 100 μm.

9. A method for manufacturing a sensor, characterized in that, include: Manufacture a base with a cavity at the first end or a pressure-sensitive diaphragm with a cavity; The pressure-sensitive diaphragm is joined to the first end of the base, such that the cavity is enclosed by the pressure-sensitive diaphragm and the base to form a pressure-sensing cavity; The hollow tube is fixed to the second end of the base, and the incident optical fiber is inserted into the hollow tube to form a temperature sensing cavity, such that the temperature sensing cavity and the pressure sensing cavity are arranged opposite each other along the direction in which light is emitted from the incident optical fiber.

10. An interferometric measurement system, characterized in that, The interferometric measurement system includes: A light source that emits coherent light; The sensor as described in any one of claims 1-8 is configured to receive the coherent light and to generate and output modulated light modulated by the pressure to be measured and the temperature to be measured. An interference demodulation device is configured to receive the modulated light and demodulate the modulated light to obtain the interference spectrum of the pressure sensing cavity and the interference spectrum of the temperature sensing cavity.

11. The interferometric measurement system according to claim 10, characterized in that, The interference demodulation device includes: A concentrator configured to receive modulated light; A first polarizer is disposed downstream of the concentrator and has a first polarization direction; A second polarizer is disposed downstream of the first polarizer and has a second polarization direction perpendicular or parallel to the first polarization direction; and A birefringent element, the birefringent element being located between the first polarizer and the second polarizer and having an optical axis at 45° or -45° to the first polarization direction.

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