Sensor and interference measurement system comprising same

Abstract

The utility model relates to a sensor 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] The present disclosure relates to a sensor and an interferometric measurement system including the same. Background Art

[0002] Optical fiber sensors have been widely used in various industries, such as oil, aviation, aerospace, medical, ocean, etc., and have shown excellent performance. At present, application requirements for simultaneous measurement of temperature and pressure parameters have been proposed in various fields.

[0003] An existing fiber optic F-P cavity pressure sensor with temperature self-compensation uses a fiber optic with a grating engraved thereon as a conduction fiber, and an optical F-P cavity as a pressure sensitive element. The fiber optic grating and the optical F-P cavity are connected by means of adhesive bonding or carbon dioxide laser welding. Such a sensor has the following problems: The composite sensor needs to be equipped with a fiber optic grating demodulation system and an F-P cavity cavity length demodulation system at the same time, increasing the time and space costs. Summary of the Utility Model

[0004] Therefore, the object of the present disclosure is to provide a sensor, which can realize the demodulation of a pressure sensing cavity and a temperature sensing cavity through the same demodulation device.

[0005] The above object is achieved by the sensor described below.

[0006] The present disclosure provides a sensor, including: a base, a pressure sensitive diaphragm is provided at a first end of the base, and an end face of a second end of the base opposite to the first end is 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, and the cavity length of the pressure sensing cavity changes according to the change of the pressure to be measured; a temperature sensing cavity is formed between the incident optical fiber in the hollow tube and the 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; 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.

[0007] In one embodiment, the reflection 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, it further includes a reflection optical fiber disposed in the hollow tube, the reflection optical fiber is fixed to the end face of the second end of the base, and the temperature sensing cavity is formed between the end face of the incident optical fiber and the end face of the reflection optical fiber.

[0009] In one embodiment, the thermal expansion coefficient 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 one local region of the pressure-sensitive diaphragm has a doping substance doped into the base material of the pressure-sensitive diaphragm to generate stress.

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

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

[0014] The present disclosure also provides a method for manufacturing a sensor, including: manufacturing a base having a cavity at a first end or a pressure-sensitive diaphragm having a cavity; bonding the pressure-sensitive diaphragm 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; fixing a hollow tube to a 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 disposed opposite to each other along the direction in which light exits from the incident optical fiber.

[0015] The present disclosure also provides an interferometric measurement system, including: a light source that emits coherent light; a sensor as described in any one of the above, 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 interference demodulation device configured to receive the modulated light and demodulate the modulated light to obtain an interference spectrum of the pressure sensing cavity and an interference spectrum of the 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° with respect to the first polarization direction.

[0017] The present 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 oppositely arranged along the incident optical fiber. The light of the incident optical fiber is incident into the temperature sensing cavity and the pressure sensing cavity in sequence, and a modulated light modulated by the pressure sensing cavity and the temperature sensing cavity is formed and output. 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, and 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 - The first end of the base 12 - The second end of the base 2 - Pressure sensitive diaphragm 3 - Pressure sensing cavity 4 - Temperature sensing cavity 5 - Incident optical fiber 51 - Reflection optical fiber 6 - Hollow tube 61 - The tail end of the hollow tube Description of the Drawings

[0019] Figure 1 is a schematic structural diagram of a sensor according to an embodiment of the present disclosure;

[0020] Figure 2 is a schematic structural diagram of a sensor according to an embodiment of the present disclosure;

[0021] Figure 3 is a schematic diagram of the interference spectrum of a sensor according to an embodiment of the present disclosure. Detailed Embodiments

[0022] In order to make the objectives, technical solutions and advantages of the technical solutions of the present disclosure clearer, the technical solutions of the embodiments of the present disclosure will be clearly and completely described below with reference to the drawings of the specific embodiments of the present disclosure. The same reference numerals in the drawings represent the same components. It should be noted that the described embodiments are part of the embodiments of the present disclosure, rather than all of the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the described embodiments of the present disclosure without creative efforts shall fall within the scope of protection of the present disclosure.

[0023] Unless otherwise defined, the technical terms or scientific terms used herein shall have the ordinary meanings as understood by those of ordinary skill in the art to which this disclosure pertains. The terms "first", "second" and similar terms used in the specification and claims of this patent application of the disclosure do not denote any order, quantity or importance, but are merely used to distinguish different components. Similarly, terms such as "a" or "an" do not necessarily denote a quantity limitation. Terms such as "comprising", "including" or "having" mean that the elements or items appearing before this term cover the elements or items listed after this term and their equivalents, without excluding other elements or items. Terms such as "connected" or "communicated" are not limited to the physical or mechanical connection or communication shown in the drawings, but may include equivalent connection or communication thereto, whether direct or indirect. Terms such as "upper", "lower", "left" and "right" are only used to indicate relative positional relationships, and when the absolute position of the object being described changes, the relative positional relationship may also change accordingly.

[0024] Reference is made below to Figures 1 to 2 describe in detail an embodiment of the sensor according to the present disclosure.

[0025] The present disclosure provides a sensor, comprising: a base 1, a pressure-sensitive diaphragm 2 is arranged 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 is 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 enclosed by the first end 11 of the base and the pressure-sensitive diaphragm 2, and the cavity length of the pressure sensing cavity changes according to the change of the pressure to be measured; 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, and the cavity length of the temperature sensing cavity changes 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.

[0026] The principle by which the sensor of the present disclosure can be demodulated in the same demodulation device is as follows: 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, so that the light exiting from the incident optical fiber 5 is sequentially incident on the temperature sensing cavity and the pressure sensing cavity. A part of the light generates a partial reflection signal through the reflecting end face of the temperature sensing cavity and is superimposed with a part of the incident light at the end of the incident optical fiber; a part of the light generates a reflection signal through the end face of the base forming the pressure sensing cavity 3, and a part of the light is reflected back by the pressure-sensitive diaphragm 2 to the end face of the base forming the pressure sensing cavity and is superimposed, forming and outputting a 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, the demodulation of the temperature sensing cavity and the pressure sensing cavity can be realized in the same optical path.

[0027] The temperature sensing cavity 4 and the pressure sensing cavity 3 are oppositely arranged along the direction in which light exits from the incident optical fiber 5. Preferably, the incident optical fiber 5 is directly opposite to the pressure sensing cavity 3, and the projection of the incident optical fiber 5 on the pressure-sensitive diaphragm 2 is within the cross-sectional range of the pressure sensing cavity, so that all the light exiting 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.

[0028] The reflective end face of the temperature sensing cavity can be the end face of the second end portion 12 of the base or the end face of the reflective optical fiber 51 disposed in the hollow tube. In one embodiment, as Figure 1 shown, the reflective end face of the temperature sensing cavity is the end face of the second end portion 12 of the base, and the temperature sensing cavity 4 is formed between the end face of the second end portion 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 the cost is low. In one embodiment, as Figure 2 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 further includes a reflective optical fiber 51, and the reflective optical fiber 51 is disposed in the hollow tube 6 and fixed to the end face of the second end portion 12 of the base, and 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.

[0029] There is no limit to the difference in cavity lengths between the pressure sensing cavity and the temperature sensing cavity. Preferably, the difference in cavity lengths between the temperature sensing cavity and the pressure sensing cavity is not less than 1 μm, so as to ensure that the demodulation signals of the pressure sensing cavity and the temperature sensing cavity do not overlap. Preferably, the difference in cavity lengths between the pressure sensing cavity and the temperature sensing cavity is less than 100 μm, so that the 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 to lower the cost. Realizing the demodulation of the temperature sensing cavity and the pressure sensing cavity in the same channel of the demodulation device means realizing the demodulation of the temperature sensing cavity and the pressure sensing cavity in the same optical path of the demodulation device.

[0030] The material of the base 1 is preferably made of glass, but other materials can also be selected, such as but not limited to single crystal silicon, silicon carbide, sapphire, etc., in order to achieve good light guiding performance. The thickness of the base can be selected to be 200 μm to 500 μm. The shape of the base is not limited.

[0031] The sensitivity of the temperature sensing cavity 4 is directly proportional to both the length of the hollow tube and the thermal expansion coefficient of the hollow tube. The sensitivity of the temperature sensing cavity 4 can be adjusted by adjusting the material and length of the hollow tube 6. The thermal expansion coefficient of the hollow tube 6 covering the incident optical fiber 5 is preferably 2*10 -6 / k - 30*10 -6 / k, the material of the hollow tube 6 can be, for example, glass, ceramic, stainless steel, etc. The longer the length of the hollow tube 6, the higher the sensitivity of the temperature sensing cavity. For the sensor of the present disclosure, pressure sensing cavities and temperature sensing cavities are respectively formed at two end portions of the base, and the temperature sensing cavity is formed by the hollow tube and the incident optical fiber disposed in the hollow tube. It can not only demodulate the double cavity lengths in the same demodulation device, but also improve the sensitivity of the temperature sensing cavity at the same time. 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 of 50 μm - 100 μm as an example, the sensitivity can be increased by 5 - 10 times.

[0032] The hollow tube 6 can be fixed to the second end portion 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 portion 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, the outer diameter of the incident optical fiber is 0.125 mm, and the inner diameter of the hollow tube 6 is 0.126 mm - 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.

[0033] The pressure sensing cavity 3 is disposed in the base 1, and the pressure sensing cavity 3 can also be machined in the pressure sensitive diaphragm 2. The material of the pressure sensitive diaphragm 2 includes but is not limited to single crystal silicon. The thickness of the pressure sensitive diaphragm 2 can be selected to be 1 μm to 5 μm. Generally, the pressure sensing cavity 3 is formed in a vacuum state, which can improve the stability of the pressure sensing cavity. The cavity of the pressure sensing cavity 3 can be a cavity with a circular cross-section but is not limited thereto. For a circular cavity, its diameter can be selected to be 80 μm to 300 μm.

[0034] As the pressure changes, the pressure sensitive diaphragm 2 can deform towards or away from the base 1, thereby changing the cavity length of the pressure sensing cavity, and further used to sense the pressure. The cavity length of the temperature sensing cavity changes with the temperature. When the temperature rises, the hollow tube 6 drives the incident optical fiber 5 to expand, the cavity length of the temperature sensing cavity increases, and the interference signal of the temperature sensing cavity moves to the right in the spectral position; when the temperature drops, the hollow tube 6 drives the incident optical fiber 5 to contract, the cavity length of the temperature sensing cavity decreases, and the interference signal of the temperature sensing cavity moves to the left in the spectral position.

[0035] Specifically, during measurement, the measurement light is introduced by the incident optical fiber 5. A part of the measurement light generates a partial reflection signal at the end face of the second end 12 of the base or the end face of the reflection optical fiber, and is superimposed with the partial outgoing light at the end of the incident optical fiber; a part of the light generates a reflection signal at the end face of the base forming the pressure sensing cavity 3, and a part of the light is reflected back by the pressure sensitive diaphragm 2 to the end face of the base forming the pressure sensing cavity and is superimposed. The change in the external pressure causes the pressure sensitive diaphragm 2 to deform, changing the cavity length of the pressure sensing cavity 3, thereby changing the optical path difference. The change in the external temperature causes the temperature sensing cavity to deform, changing the cavity length of the temperature sensing cavity, 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.

[0036] In one embodiment, at least one local region of the pressure sensitive diaphragm 2 has a doping substance doped into the base material of the pressure sensitive diaphragm to generate stress, which can reduce the temperature coefficient of the pressure sensing cavity, so that the cavity length of the pressure sensing cavity will not change due to temperature changes. The temperature coefficient of the pressure sensing cavity is low, so temperature correction is not required. Among them, the doping substance can be one or more of the following materials: P, B, As, Al, Ga, Sb, Ge, O, Au, Fe, Cu, Ni, Zn, Mg.

[0037] In one embodiment, the sensor further includes a reflection film, and the reflection film is disposed on the end face of the incident optical fiber and / or the reflection end face of the temperature sensing cavity. The material of the reflection film can be one of the following: Cr, Ti, Au, Ag, TaN, Al2O3, Ta2O5.

[0038] The sensor of the present disclosure includes a pressure sensing cavity formed between the first end of the base and the pressure sensitive diaphragm, and a temperature sensing cavity formed between the incident optical fiber in the hollow tube and the reflection end face of the temperature sensing cavity. The temperature sensing cavity is disposed opposite to the pressure sensing cavity along the direction of light exiting from the incident optical fiber. The light of the incident optical fiber is incident on the temperature sensing cavity and the pressure sensing cavity in sequence, forming and outputting a 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. The demodulation of the pressure sensing cavity and the temperature sensing cavity can be achieved by the same demodulation device, and the sensor structure and manufacturing process of the present disclosure are simple and the cost is low.

[0039] The present disclosure also provides a method for manufacturing a sensor, including: manufacturing a base having a cavity at a 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 enclosed 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 disposed opposite to each other along the direction in which light exits from the incident optical fiber. The manufacturing method of the present disclosure has a simple process and low cost.

[0040] In one embodiment, a method for manufacturing a sensor further includes the following step: fixing the tail end 61 of the hollow tube to the incident optical fiber 5.

[0041] In one embodiment, a method for manufacturing a sensor further includes the following step: fixing a reflective optical fiber in the hollow tube to the second end 12 of the base.

[0042] It should be noted that the steps listed above are the preferred steps for manufacturing the sensor described in the present disclosure, and the execution order of the operation steps is not limited. Based on the above content, those skilled in the art can also change or omit a specific operation according to the specific situation, add a specific operation, or adjust the order of one or more operation steps.

[0043] The present disclosure also provides an interference measurement system, including: a light source that emits coherent light; the sensor described above, 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 interference demodulation device, configured to receive the modulated light and demodulate the modulated light to obtain an interference spectrum of the pressure sensing cavity and an interference spectrum of the temperature sensing cavity.

[0044] In one embodiment, the interference demodulation device includes: a condenser configured to receive the 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° with respect to the first polarization direction.

[0045] The process of cavity length demodulation is as follows: The light emitted by a light source (the light source is, for example, a white light LED, a xenon lamp, or a halogen lamp) is coupled into an optical fiber and enters a coupler or an optical circulator. It is transmitted through the optical fiber to the sensor from the other end, and the optical signal reflected by the sensor enters the interference demodulation device after passing through the coupler again. The incident light beam forms a linearly polarized light beam after passing through the first polarizer and is incident on a birefringent optical wedge. Equal-thickness interference occurs on the upper and lower surfaces of the optical wedge, and the interference spectrum of the pressure sensing cavity is obtained at the position where the thickness of the optical 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 optical wedge is equal to the cavity length of the temperature sensing cavity. As described above, the demodulation of the temperature sensing cavity and the pressure sensing cavity is achieved in the same channel of the demodulation device, that is, the 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 optical signal reflected by the sensor and modulated by the pressure to be measured and the temperature to be measured passes through the same polarizer and birefringent element in the same optical fiber in sequence to obtain the interference spectrum of the pressure sensing cavity and the interference spectrum of the temperature sensing cavity.

[0046] When the pressure and / or temperature acting on the pressure-sensitive diaphragm changes, the cavity length of the pressure sensing cavity and / or the cavity length of the temperature sensing cavity changes, and the position of the interference spectrum of the pressure sensing cavity and / or the temperature sensing cavity changes, thereby calculating the magnitudes of the pressure and the temperature. As Figure 3 shown in the simulation results of the demodulation signal of the sensor based on the white light interference coherent demodulation method, it can be seen from the figure that the interference spectrum of the temperature sensing cavity and the interference spectrum of the pressure sensing cavity appear on the same interference spectrum diagram.

[0047] The interference measurement system of the present disclosure can achieve the demodulation of the pressure sensing cavity and the temperature sensing cavity through the same demodulation device, and has a simple structure and manufacturing process and low cost.

Claims

1. A sensor, characterized in that: include: A base, wherein a pressure-sensitive membrane is disposed at a first end of the base, and an end surface of a second end of the base opposite to the first end is fixedly connected to the 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, and the cavity length of the pressure sensing cavity changes according to the change of 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 changes according to the change of the temperature to be measured; The temperature sensing cavity and the pressure sensing cavity are arranged opposite to each other along the direction in which light is emitted 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 arranged in the hollow tube, the reflective optical fiber is fixed to the end face of the second end of the base, and the temperature sensing cavity is 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 partial region of the pressure-sensitive membrane has a doping substance doped into a base material of the pressure-sensitive membrane in order to generate stress.

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

9. An interferometric measurement system, characterized in that: The interferometric measurement system comprises: a light source, emitting coherent light; The sensor according to any one of claims 1 to 8, configured to receive the coherent light, and form and output modulated light modulated by the pressure to be measured and the temperature to be measured; The 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.

10. The interferometric measurement system according to claim 9, characterized in that: The interference demodulation device comprises: a light concentrator configured to receive the modulated light; a first polarizer, disposed downstream of the concentrator 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 is located between the first polarizer and the second polarizer and has an optical axis that is 45° or −45° to the first polarization direction.

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