A wedge-shaped birefringent device phase delay measurement optical path and measurement method

Abstract

The present invention provides an optical path and method for measuring the phase retardation of a wedge-shaped birefringent device. The optical path structure includes a beam splitter, a half-wave plate, a first focusing lens, a reflector, a beam reduction unit, a quarter-wave plate, and a second focusing lens. Incident light is divided into transmitted light and reflected light after passing through the beam splitter. A first polarizer and a first photodetector are sequentially arranged along the direction of the reflected light, and a second polarizer and a second photodetector are sequentially arranged in the opposite direction of the reflected light. The beam reduction unit, the half-wave plate, the birefringent device to be measured, the first focusing lens, the quarter-wave plate, the second focusing lens, and the reflector are sequentially arranged along the direction of the transmitted light. The present invention can measure the phase retardation of wedge-shaped structures and birefringent devices with varying wedge angles, inclined surfaces, curved surfaces, or other structures where the output surface is not parallel to the incident surface. The optical path structure is simple, and the measurement method is simple and convenient.

Description

Technical Field

[0001] The present invention relates to the technical field of phase delay measurement of a birefringent device, in particular to an optical path and a method for measuring the phase delay of a wedge-shaped birefringent device. Background Art

[0002] Polarization optical systems have extensive and important applications in a variety of disciplines and engineering fields, including mechanics, optics, biology, medicine, and physics. Birefringent optical devices, such as wave plates, polarizing prisms, and liquid crystals, are crucial components in polarization optical systems. Their parameters directly impact the overall performance of the system, with phase retardation being a key parameter for these birefringent components.

[0003] Currently, methods for measuring the phase retardation of birefringent devices include ellipsometry, extinction, electro-optical / magneto-optical modulation, interferometry, compensation, and optical feedback resonance. Ellipsometry uses an ellipsometer to measure the phase retardation of a device; the extinction method measures the phase retardation by rotating a polarization element and detecting the extreme value or change in the output light intensity; the electro-optical / magneto-optical modulation method generally inserts an electro-optical or magneto-optical modulator into the optical path to modulate the polarization state of the light beam, and then determines the phase retardation by determining the extinction position; the interferometry method generally measures the phase retardation by analyzing the intensity of the interference signal under different polarization states; the compensation method typically uses a phase compensator to modulate the total phase retardation of the system to measure the phase retardation; and the optical feedback resonance method uses a laser and an external reflector to form an external resonant cavity. The birefringent device under test is inserted into the optical path and the phase retardation is determined by analyzing the functional relationship between the signal and the device phase retardation in the resonant state. Many of these methods require the device under test to rotate, rotate, or move, and are generally only applicable to devices with parallel surfaces, such as wave plates. However, in actual polarization optical system applications, some birefringent components have two surfaces that are not parallel but rather have a certain wedge angle between them. Examples include wedge-shaped liquid crystal cells, wedge components of polarization prisms such as Babinet compensators and Wollaston prisms, and optical wedges made of birefringent crystal materials. Future optical systems may also feature birefringent components with varying wedge angles, curved surfaces, or other structures where the output surface is not parallel to the input surface. When the two surfaces of a device are not parallel (with a wedge angle), existing methods for measuring phase delay can either fail directly or have overly complex measurement systems or signal processing methods. Furthermore, when the situation on one of the surfaces is more complex, such as when the wedge angles vary at different locations or when the surface is curved, the measurement becomes even more difficult. This poses a challenge to measuring the phase delay.

[0004] Patent document CN100529715C discloses a method and apparatus for measuring the phase delay and fast axis direction of a birefringent optical device. If the device under test is a wedge-shaped or other birefringent device, the reflected light, after returning to the wedge-shaped or other birefringent device and entering the device, cannot overlap with the previous beam. Consequently, the position of the returned light differs from the position at the time of its incident moment, making it impossible to measure the phase delay. Only when the wedge angle is very small can the returned light be approximately considered to overlap with the previous beam. Therefore, this patented device can only measure birefringent devices with two parallel planes or wedge-shaped birefringent devices with a very small wedge angle. Summary of the Invention

[0005] To overcome the shortcomings of the prior art, the present invention provides an optical path and method for measuring the phase delay of a wedge-shaped birefringent device. The optical path and method can measure the phase delay of birefringent devices with parallel surfaces, such as wave plates, as well as wedge-shaped structures and birefringent devices with inclined surfaces, curved surfaces, or other structures where the output surface is not parallel to the incident surface. Furthermore, the measurement is accurate, the optical path structure is simple, and the measurement method is simple and convenient.

[0006] The technical solution adopted by the present invention to solve its technical problem is:

[0007] The present invention first provides a wedge-shaped birefringent device phase delay measurement optical path, comprising a beam splitter, a half-wave plate, a first focusing lens, a reflector, a beam reduction unit, a quarter-wave plate and a second focusing lens;

[0008] The incident light is divided into transmitted light and reflected light after passing through the spectroscope. A first polarizer and a first photodetector are arranged in sequence along the direction of the reflected light, and a second polarizer and a second photodetector are arranged in sequence in the opposite direction of the reflected light. The beam reduction unit, the half-wave plate, the birefringent device to be measured, the first focusing lens, the quarter-wave plate, the second focusing lens, and the reflector are arranged in sequence along the direction of the transmitted light.

[0009] As a further preferred embodiment, the incident light includes P-polarized light and S-polarized light whose polarization directions are perpendicular to each other, and there is a frequency difference between the P-polarized light and the S-polarized light.

[0010] As a further preferred embodiment, the beam reduction unit is a Galilean beam reduction optical path or a Keplerian beam reduction optical path.

[0011] As a further preferred embodiment, the position where the light beam emerges from the birefringent component to be measured is located at the front focus of the first focusing lens, and the reflecting mirror is located at the back focal plane of the second focusing lens.

[0012] As a further preferred embodiment, the angle between the fast axis of the quarter wave plate and the polarization direction of the initial P and S polarized light is 45°.

[0013] The present invention also provides a method for measuring the phase delay of a wedge-shaped birefringent device, which uses the above-mentioned wedge-shaped birefringent device phase delay measurement optical path for measurement, comprising the following steps:

[0014] S1. The birefringent device to be tested is placed between the half-wave plate and the first focusing lens;

[0015] S2. A beam containing two linearly polarized components (P and S) with a certain frequency difference and perpendicular polarization directions is used as the incident light. After passing through the beam splitter, it is split into a transmitted part and a reflected part.

[0016] The reflected light passes through the first polarizer and is received by the first photodetector to form a reference signal;

[0017] The transmitted light passes through the beam reduction unit, the half-wave plate, the birefringent device to be measured, the first focusing lens, the quarter-wave plate, the second focusing lens and the reflector in sequence, and then returns to the original path, and is then received by the second photodetector to form a measurement signal;

[0018] The half-wave plate is rotated to obtain the maximum and minimum values of the phase difference between the measurement signal and the reference signal, and the phase delay of the birefringent device to be measured at the position where the light beam passes is calculated based on the maximum and minimum values.

[0019] As a further preferred embodiment, the expression of the reference signal is:

[0020] Expression 1 I1=I 01 cos[2πΔft+(Φ p01 -Φ s01 )]

[0021] Among them, I1, I 01 are the light intensity of the reference signal and the maximum light intensity, Φ p01 , Φ s01 are the initial phases of the P and S components in the reference signal optical path, t is time, and Δf is the frequency difference between P-polarized light and S-polarized light;

[0022] The expression of the measurement signal is:

[0023] Expression 2 I2=I 02 cos[2πΔft+(Φ p02 -Φ s02 )+ΔΦ]

[0024] Among them, I2, I 02 are the light intensity and maximum light intensity of the measurement signal, Φ p02 , Φ s02are the initial phases of the P and S components in the measurement optical path, respectively; ΔΦ is the phase difference change of the measurement signal relative to the reference signal when the half-wave plate rotates;

[0025] Assume that the Jones vector of the incident P polarized light component is After the transmitted light returns to the second polarizer, the Jones vector of the P-polarized light component becomes:

[0026] Expression three

[0027] Assume that the Jones vector of the incident S polarization component is After the transmitted light returns to the second polarizer, the Jones vector of the S-polarized light component becomes:

[0028] Expression 4

[0029] in:

[0030] P1=|sinτsin(2(2ψ-θ))|;

[0031]

[0032] δ=atan(tanτcos(2(θ-2ψ)));

[0033] ψ is the angle between the fast axis of the half-wave plate and the direction parallel to the paper plane, τ and θ are the phase delay and fast axis azimuth of the birefringent device to be measured, respectively;

[0034] According to Expression 3 and Expression 4, the phase difference between the P and S components in the measurement signal is obtained as:

[0035] Expression 5 Φ = 2atan{tanτcos[2(θ-2ψ)]}

[0036] The change ΔΦ of the phase difference Φ is the phase difference change in Expression 2. The maximum and minimum values of the phase difference change ΔΦ are obtained by rotating the half-wave plate. The phase delay and fast axis azimuth of the birefringent device to be measured are calculated according to Expression 5.

[0037] The present invention has the following positive effects: Utilizing a first focusing lens, a quarter-wave plate positioned at 45 degrees, a second focusing lens, and a reflector, the present invention ensures that when light of different polarization states is reflected back into the birefringent device under test, the beams entering the device coincide with those entering the device. Therefore, the present invention can measure the phase delay of birefringent devices with parallel surfaces, such as wave plates, as well as wedge-shaped birefringent devices. Furthermore, the present invention can measure the phase delay of other birefringent devices with varying wedge angles, curved surfaces, or other structures where the output surface is not parallel to the input surface. Compared to existing methods for measuring the phase delay of wedge-shaped birefringent devices, the present invention's optical path and method are simpler and more efficient. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] Figure 1 is a light path diagram of the measuring light path of the present invention;

[0039] Figure 2a is a structural diagram of the Galileo type beam reduction unit according to an embodiment of the present invention;

[0040] Figure 2b 4 is a structural diagram of the Kepler type beam shrinking unit described in an embodiment of the present invention. DETAILED DESCRIPTION

[0041] The preferred embodiments of the present invention are described in detail below with reference to the accompanying drawings.

[0042] Reference Figure 1 The preferred embodiment of the present invention provides a wedge-shaped birefringent device phase delay measurement optical path, comprising a beam splitter 101, a half-wave plate 105, a first focusing lens 107, a reflector 110, a beam reduction unit 104, a quarter-wave plate 108 and a second focusing lens 109;

[0043] After passing through the spectrometer 101, the incident light is divided into transmitted light and reflected light. Along the direction of the reflected light, a first polarizer 102 and a first photodetector 103 are sequentially arranged. Along the opposite direction of the reflected light, a second polarizer 111 and a second photodetector 112 are sequentially arranged. Along the direction of the transmitted light, the beam reduction unit 104, the half-wave plate 105, the birefringent device to be measured 106, the first focusing lens 107, the quarter-wave plate 108, the second focusing lens 109, and the reflector 110 are sequentially arranged.

[0044] Specifically, the beam splitter is a 50:50 beam splitter; and the incident light is an incident light beam containing two P and S dual-frequency linear polarization components with a certain frequency difference and polarization directions perpendicular to each other.

[0045] The purpose of the beam reduction unit is to reduce the beam diameter and improve the accuracy of measurement results. Generally speaking, the diameter of the beam output by a laser ranges from 1.2 mm to 7.8 mm. Because the two surfaces of birefringent devices such as wedges are not parallel, the beam travels different distances inside the device when it enters the device at different locations, resulting in different phase delays at each location. If the light beam were a line without thickness, the measurement result would be the exact phase delay at the location where the light passes through the wedge-shaped birefringent device. However, because the beam spot has a certain diameter, the actual measurement result is actually the average phase delay at the device location covered by the beam spot. Therefore, a beam reduction unit is required to minimize the beam diameter. The more accurately the device location the light passes through, the more accurate the measurement result. Without a beam reduction unit, the measurement result would be equivalent to treating the exit surface of the device covered by the beam spot as a structure parallel to the incident surface, and the resulting phase delay measurement result would be less accurate.

[0046] Figure 2a 、 Figure 2b There are two typical beam reduction unit structures, one is a Kepler type beam reduction optical path composed of two convex lenses, and the other is a Galilean type beam reduction optical path composed of one convex lens and one concave lens.

[0047] To ensure that the light beam returns to its original path, the position where the light beam emerges from the birefringent component to be measured is located at the front focus of the first focusing lens, and the reflector is located at the back focal plane of the second focusing lens.

[0048] The fast axis of the quarter-wave plate forms an angle of 45° with the polarization direction of the initial P and S polarized light. The light beam passes through the quarter-wave plate twice, which is equivalent to passing through a half-wave plate, and its function is to interchange the polarization normal states of the two polarized lights.

[0049] Specifically, in this embodiment, the birefringent device 106 to be tested is a wedge-shaped liquid crystal cell. The entire optical propagation process is as follows: a beam of incident light containing two linearly polarized P and S components with mutually perpendicular polarization directions, with a frequency difference of 3 MHz, a wavelength of 632 nm, and a beam diameter of 2 mm, passes through a 50:50 beamsplitter 101 and is split into transmitted and reflected light of equal intensity, both containing P and S components. The reflected light passes through a first polarizer 102 and is received by a first photodetector 103. The transmitted light passes through a beam reduction unit 104 composed of two convex lenses, where its diameter is reduced to 0.2 mm. It then passes through a rotatable half-wave plate 105 and is incident on the wedge-shaped liquid crystal cell to be tested, which has a wedge angle of 5°. Due to the different refractive indices of the P and S light beams in the birefringent device, the emitted P and S light beams are refracted at different angles, forming a slight angle between them. With the bisector of this angle as the optical axis, a first focusing lens 107 with a focal length of 25 mm, a quarter-wave plate 108, a second focusing lens 109 with a focal length of 40 mm, and a plane mirror 110 are placed in the direction of beam propagation. The beam exit point on the wedge-shaped liquid crystal cell is located at the front focal point of the first focusing lens 107. The fast axis of the quarter-wave plate 108 forms an angle of 45° with the direction parallel to the paper. The plane mirror 110 is located at the back focal plane of the second focusing lens 109. The P and S lights emitted from the wedge-shaped liquid crystal cell at different deflection angles are converted into parallel beams by the first focusing lens 107, then pass through the quarter-wave plate 108 and are focused by the second focusing lens 109 and incident on the plane reflector 110. After being reflected, they are respectively returned to the birefringent device to be measured 106 along the previous incident paths of S and P, and are recombined into a beam and returned to the spectroscope 101. After passing through the second polarizer 111, they are received by the second photodetector 112.

[0050] This embodiment further provides a method for measuring the phase delay of a wedge-shaped birefringent device, which uses the above-mentioned wedge-shaped birefringent device phase delay measurement optical path for measurement, including the following steps:

[0051] S1. The birefringent device to be tested 106 (a wedge-shaped liquid crystal cell in this embodiment) is placed between the half-wave plate 105 and the first focusing lens 107;

[0052] S2. A beam containing two linearly polarized components, P and S, with a certain frequency difference and perpendicular polarization directions, is used as the incident light. After passing through the beam splitter 101, it is divided into two parts, namely, the transmitted and reflected parts.

[0053] After the reflected light passes through the first polarizer 102, interference fringes are formed in the direction of the transmission axis thereof, and are received by the first photodetector 103 to form a beat frequency interference signal as a reference signal;

[0054] The transmitted light passes through the beam reduction unit 104, the half-wave plate 105, the birefringent element to be measured 106, the first focusing lens 107, the quarter-wave plate 108, the second focusing lens 109 and the reflector 110 in sequence, and then returns to the original path. Finally, it passes through the second polarizer 111, and also forms interference fringes in the direction of its transmission axis. The interference fringes are received by the second photodetector 112 to form a beat frequency interference signal as a measurement signal.

[0055] The phase difference information between the measurement signal and the reference signal is obtained by a phase measuring instrument, and the half-wave plate 105 is rotated to obtain the phase difference change, and its maximum and minimum values are recorded, thereby obtaining the phase delay of the wedge-shaped birefringent device 106 to be measured.

[0056] Specifically, the expression of the reference signal is:

[0057] Expression 1 I1=I 01 cos[2πΔft+(Φ p01 -Φ s01 )]

[0058] Among them, I1, I 01 are the light intensity of the reference signal and the maximum light intensity, Φ p01 , Φ s01 are the initial phases of the P and S components in the reference signal optical path, t is time, and Δf is the frequency difference between the P polarized light and the S polarized light (a pair of linearly polarized lights with mutually orthogonal polarization directions: the frequency is f p , P light parallel to the paper and frequency f s , the S light perpendicular to the paper has a certain frequency difference Δf. );

[0059] The expression of the measurement signal is:

[0060] Expression 2 I2=I 02 cos[2πΔft+(Φ p02 -Φ s02 )+ΔΦ]

[0061] Among them, I2, I 02 are the light intensity and maximum light intensity of the measurement signal, Φ p02 , Φ s02 where ΔΦ is the initial phase of the P and S components in the measurement optical path, respectively. ΔΦ is the phase difference change of the measurement signal relative to the reference signal when the half-wave plate rotates. This phase difference change contains information about the phase delay of the birefringent device 106 to be measured, which can be measured by a phase measurement instrument and analyzed according to a calculation formula to determine the phase delay of the birefringent device 106 to be measured.

[0062] Assume that the Jones vector of the incident P polarized light component is After the transmitted light returns to the second polarizer, the Jones vector of the P-polarized light component becomes:

[0063] Expression three

[0064] Assume that the Jones vector of the incident S polarization component is After the transmitted light returns to the second polarizer, the Jones vector of the S-polarized light component becomes:

[0065] Expression 4

[0066] in:

[0067] P1=|sinτsin(2(2ψ-θ))|;

[0068]

[0069] δ=atan(tanτcos(2(θ-2ψ)));

[0070] ψ is the angle between the fast axis of the half-wave plate and the direction parallel to the paper plane, τ and θ are the phase delay and fast axis azimuth of the birefringent device to be measured, respectively;

[0071] According to Expression 3 and Expression 4, the phase difference between the P and S components in the measurement signal is obtained as:

[0072] Expression 5 Φ = 2atan{tanτcos[2(θ-2ψ)]}

[0073] The change ΔΦ of the phase difference Φ is the phase difference change in Expression 2. The maximum and minimum values of the phase difference change ΔΦ are obtained by rotating the half-wave plate. The phase delay and fast axis azimuth of the birefringent device to be measured are calculated according to Expression 5.

[0074] There are other existing methods that can measure the phase delay of wedge-shaped devices, but unlike the present invention, the optical path results and measurement method of the present invention are simpler and faster. The key difficulty in measuring birefringent devices such as wedges is that when light exits the birefringent device, the light beam deviates from the previous optical axis. Some existing methods require rotating the device under test, which changes the direction of the subsequent light beam, making it difficult to place the subsequent optical path and detector. Some existing methods, like the present method, also use light of different polarization states. Because light of different polarization states has different refractive indices in the birefringent device, they have different refraction angles when exiting the wedge or other surface, making the subsequent optical path configuration very difficult. For example, if the optical path structure is reflective, after the light returns to the birefringent device such as the wedge under test and enters the device, its beam often does not overlap with the previous beam. In this case, the position of the returning light is different from the position at the time of the previous incident light, making it impossible to measure. (This problem does not exist for devices with two parallel surfaces, as the phase delay at different positions is the same.)

[0075] The key point of the present invention lies in the use of a first focusing lens, a quarter-wave plate positioned at 45 degrees, a second focusing lens, and a reflector, which ensure that light of different polarization states returns to the birefringent device under test so that the beams entering the device overlap with those before. In summary, to address the problem of measuring and calibrating the phase delay of wedge-shaped birefringent devices, the present invention proposes a relatively simple measurement method and optical path. This method utilizes dual-frequency linearly polarized light to split into two paths: one path is a reference signal, and the other path passes through a series of components and the device under test, returning to form a measurement signal. By measuring the phase difference between the two signals, the phase delay of the device under test at the position where the light beam passes can be calculated. The present invention is applicable not only to wedge-shaped birefringent devices, but also to measuring the phase delay of other birefringent devices with varying wedge angles, curved surfaces, or other structures where the output surface is not parallel to the input surface.

[0076] The above are only preferred embodiments of the present invention. It should be understood that the description of the above embodiments is only used to help understand the method and core ideas of the present invention, and is not used to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, etc. made within the ideas and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A method for measuring the phase retardation of a wedge-shaped birefringent device, comprising: utilizing a wedge-shaped birefringent device phase retardation measurement optical path for measurement, the wedge-shaped birefringent device phase retardation measurement optical path comprising a beam splitter, a half-wave plate, a first focusing lens, a reflector, a beam reduction unit, a quarter-wave plate, and a second focusing lens; incident light is divided into transmitted light and reflected light after passing through the beam splitter; a first polarizer and a first photodetector are sequentially provided along the direction of travel of the reflected light; and a second polarizer and a second photodetector are sequentially provided in the opposite direction of travel of the reflected light; and the beam reduction unit, the half-wave plate, the birefringent device to be measured, the first focusing lens, the quarter-wave plate, the second focusing lens, and the reflector are sequentially provided along the direction of travel of the transmitted light; The measuring method comprises the following steps: S1. The birefringent device to be tested is placed between the half-wave plate and the first focusing lens; S2. A beam containing two linearly polarized components (P and S) with a certain frequency difference and perpendicular polarization directions is used as the incident light. After passing through the beam splitter, it is split into a transmitted part and a reflected part. The reflected light passes through the first polarizer and is received by the first photodetector to form a reference signal; The transmitted light passes through the beam reduction unit, the half-wave plate, the birefringent device to be measured, the first focusing lens, the quarter-wave plate, the second focusing lens and the reflector in sequence, and then returns to the original path, and is then received by the second photodetector to form a measurement signal; Rotating the half-wave plate to obtain the maximum and minimum values of the phase difference between the measurement signal and the reference signal, and thereby calculating the phase delay of the birefringent device to be measured at the position where the light beam passes; The expression of the reference signal is: Expression 1 I1=I 01 cos[2πΔft+(Φ p01 -Φ s01 )] Among them, I1, I 01 are the light intensity of the reference signal and the maximum light intensity, Φ p01 , Φ s01 are the initial phases of the P and S components in the reference signal optical path, t is time, and Δf is the frequency difference between P-polarized light and S-polarized light; The expression of the measurement signal is: Expression 2: I2 = I 02 cos[2πΔft + (Φ p02 - Φ s02 ) + ΔΦ] Among them, I2, I 02 are the light intensity and maximum light intensity of the measurement signal, Φ p02 , Φ s02 are the initial phases of the P and S components in the measurement optical path, respectively; ΔΦ is the phase difference change of the measurement signal relative to the reference signal when the half-wave plate rotates; Assume that the Jones vector of the incident P polarized light component is After the transmitted light returns to the second polarizer, the Jones vector of the P-polarized light component becomes: Expression three Assume that the Jones vector of the incident S polarization component is After the transmitted light returns to the second polarizer, the Jones vector of the S-polarized light component becomes: Expression 4 in: P1=|sinτsin(2(2ψ-θ))|; δ=atan(tanτcos(2(θ-2ψ))); ψ is the angle between the fast axis of the half-wave plate and the direction parallel to the paper plane, τ and θ are the phase delay and fast axis azimuth of the birefringent device to be measured, respectively; According to Expression 3 and Expression 4, the phase difference between the P and S components in the measurement signal is obtained as: Expression 5 Φ = 2atan{tanτcos[2(θ-2ψ)]} The change ΔΦ of the phase difference Φ is the phase difference change in Expression 2. The maximum and minimum values of the phase difference change ΔΦ are obtained by rotating the half-wave plate. The phase delay and fast axis azimuth of the birefringent device to be tested are calculated according to Expression 5.

2. The method for measuring the phase delay of a wedge-shaped birefringent device according to claim 1, wherein: The incident light includes P-polarized light and S-polarized light whose polarization directions are perpendicular to each other, and there is a frequency difference between the P-polarized light and the S-polarized light.

3. The method for measuring the phase delay of a wedge-shaped birefringent device according to claim 1, wherein: The beam reduction unit is a Galilean beam reduction optical path or a Keplerian beam reduction optical path.

4. The method for measuring the phase delay of a wedge-shaped birefringent device according to claim 1, wherein: The position where the light beam emerges from the birefringent component to be measured is located at the front focus of the first focusing lens, and the reflector is located at the back focal plane of the second focusing lens.

5. The method for measuring the phase delay of a wedge-shaped birefringent device according to claim 1, wherein: The angle between the fast axis of the quarter wave plate and the polarization direction of the initial P and S polarized light is 45°.

Images