A high-sensitivity azimuthal ellipticity simultaneous measurement device and method

Abstract

This invention discloses a highly sensitive device for simultaneous measurement of azimuth angle and ellipticity. The device includes a laser emitting a laser beam. Along the optical path of the laser beam are sequentially arranged a linearly polarized light generation module, a sample to be measured, a half-wave plate, and a polarizing beam splitter. The polarizing beam splitter splits the laser beam in two, both of which are directed into the first and second channels of a balanced photodetector, performing differential measurement. The balanced photodetector demodulates the differential signals generated by the two channels via a lock-in amplifier and then transmits them to a computer to obtain the azimuth angle and ellipticity measurement results. Compared with conventional methods, this invention combines high-frequency modulation and differential detection methods to achieve simultaneous measurement of the azimuth angle and ellipticity of elliptically polarized light, while suppressing the effects of low-frequency technical noise and common-mode noise, thus improving the detection sensitivity by approximately one time.

Description

Technical Field

[0001] This invention relates to the field of optical detection, and in particular to a photoelastic modulation type highly sensitive azimuth ellipticity simultaneous measurement device and method. Background Technology

[0002] The interaction between polarized light and an optical medium can lead to changes in polarization parameters, and the magnitude of these changes reflects the properties of the medium. Therefore, the properties of optical media can be measured by measuring polarization parameters such as azimuth angle and ellipticity.

[0003] For example, in atomic magnetometers, the magnetic field is measured by the interaction between polarized light and the atomic ensemble. The azimuth angle of the polarized light reflects the magnitude of the atomic spin precession signal, while the ellipticity of the polarized light reflects important parameters such as optical pump rate and optical absorptivity. Highly sensitive measurements of the azimuth angle and ellipticity are required to reflect the intensity of the interaction between polarized light and the atomic ensemble. Therefore, the measurement of azimuth angle and ellipticity is of great significance.

[0004] Simultaneous measurement of azimuth and ellipticity can be achieved using equipment such as polarization analyzers, but their measurement accuracy and sensitivity are relatively low. Rotating analytical ellipsometers have simple optical configurations, but the measurement error is large when the ellipticity approaches 0 degrees. Rotating compensated ellipsometers have low-frequency mechanical rotating analyzers and compensators, limiting the measurement rate, and the system instability and beam drift caused by mechanical rotation are difficult to eliminate. Using multiple photoelastic modulators can achieve real-time measurement of azimuth and ellipticity across the entire range, but the required DC component is significantly affected by low-frequency technical noise, resulting in low measurement sensitivity; furthermore, the measurement system and process are complex and costly. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to realize a photoelastic modulation type azimuth ellipticity simultaneous measurement device and method, which can improve the measurement accuracy and measurement sensitivity by using optical element fine adjustment, modulation differential composite optical path and other technical solutions.

[0006] To achieve the above objectives, the technical solution adopted by the present invention is as follows: a highly sensitive device for simultaneous measurement of azimuth angle and ellipticity. The device has a laser that emits a laser beam. Along the optical path of the laser beam, a linearly polarized light generation module, a sample to be measured, a half-wave plate, and a polarizing beam splitter are arranged in sequence. The polarizing beam splitter splits the laser beam into two parts, which are then injected into the first and second channels of a balanced photodetector to complete differential measurement. The balanced photodetector demodulates the differential signals generated by the two channels through a lock-in amplifier and then transmits them to a computer to obtain the azimuth angle and ellipticity measurement results.

[0007] A photoelastic modulator with phase delay characteristics is positioned between a half-wave plate and a polarizing beam splitter. The photoelastic modulator modulates the azimuth angle and ellipticity to be measured to different frequency bands for simultaneous detection of ellipticity and azimuth angle information. The photoelastic modulator is connected to a lock-in amplifier via a photoelastic modulator controller. The photoelastic modulator controller is used to set the modulation frequency of the photoelastic modulator and to provide a reference frequency for the lock-in amplifier.

[0008] The linearly polarized light generating module consists of a polarizer and a quarter-wave plate arranged sequentially along the laser beam direction. The interaction between the linearly polarized light generating module and the sample under test converts the laser beam into elliptically polarized light. The laser beam split by the polarizing beam splitter is either directly injected into the first channel or reflected by a mirror and then injected into the second channel.

[0009] The fast axis of the quarter-wave plate forms an angle of 0° with the transmission axis of the polarizer, the fast axis of the half-wave plate forms an angle of 22.5° with the transmission axis of the polarizer, and the fast axis of the photoelastic modulator forms an angle of 45° with the transmission axis of the polarizer.

[0010] A highly sensitive method for simultaneous measurement of azimuth ellipticity includes the following steps:

[0011] Step 1: Block the reflected light from the mirror and rotate the polarizer until the output of the balanced photodetector is at its minimum value;

[0012] Step 2: Set the reference frequency of the lock-in amplifier to the modulation frequency of the photoelastic modulator, so that the lock-in amplifier only outputs the first and second harmonics of the input voltage signal from the balanced photodetector;

[0013] Step 3: Without placing the sample to be tested, change the direction of the fast axis of the quarter-wave plate and record the first harmonic voltage U1 output by the lock-in amplifier and the angle β between the fast axis of the quarter-wave plate and the transmission axis of the polarizer. QWP U1-β QWP By fitting half the slope of the straight line, we obtain k1;

[0014] Step 4: Without placing the sample to be tested, change the direction of the fast axis of the half-wave plate and record the first harmonic voltage U1, the second harmonic voltage U2, and the angle β between the fast axis of the half-wave plate and the transmission axis of the polarizer output by the lock-in amplifier. HWP The fitted U1-β HWP U2-β HWP The amplitudes of the sine curve are obtained as k2 and k4, respectively.

[0015] Step 5: Complete the calibration of k1, k2, k3 and k4 without placing the sample to be tested, where k3 = –k2k4 / k1;

[0016] Step 6: When placing the sample to be tested, ensure that the angle β between the fast axis of the quarter-wave plate and the transmission axis of the polarizer is [missing information].QWP It varies in the range of -14° to 14°, at which point sin(β) QWP ) is approximately β QWP Ellipticity ε varies with β QWP The linear change, U1 also changes with β QWP Linear change, U1-β QWP By fitting half the slope of the straight line, we obtain k1;

[0017] Step 7: When placing the sample to be tested, rotate the half-wave plate and make the azimuth angle α change linearly without changing the ellipticity ε. At this time, the first harmonic voltage U1 and the second harmonic voltage U2 output by the lock-in amplifier both change sinusoidally. The fitted U1-β HWP U2-β HWPP The amplitudes of the sine curve are obtained, k2 and k4 respectively;

[0018] Step 8: Complete the calibration of k1, k2, k3 and k4 under the condition of placing the sample to be tested, where k3 = –k2k4 / k1;

[0019] Step 9: Record the first harmonic voltage U1 and the second harmonic voltage U2 output by the lock-in amplifier before and after placing the sample to be tested. Then, use Δα = α 后 –α 前 , Δε=ε 后 –ε 前 The azimuth and ellipticity caused by the sample under test are calculated.

[0020] Suppose that after passing through the sample to be tested, elliptically polarized light with an azimuth angle of α and an ellipticity of ε is obtained, then the Jones vector E0 of this elliptically polarized light can be expressed as:

[0021]

[0022] The Jones matrix G of the half-wave plate HWP It can be represented as:

[0023]

[0024] Where, θ HWP This is the installation error angle of the half-wave plate.

[0025] The phase delay of the photoelastic modulator can be expressed as:

[0026] δ(t)=δ0sinωt+δ s

[0027] Where δ0 is the peak phase delay of the photoelastic modulator, ω is the modulation frequency of the photoelastic modulator, and δ s The static phase delay of the photoelastic modulator is given by the Jones matrix G of the photoelastic modulator.PEM It can be represented as:

[0028]

[0029] Where, θ PEM The installation error angle of the photoelastic modulator is given by E = G. The Jones vector of the emitted light from the photoelastic modulator can be expressed as E = G. PEM G HWP E0, then the intensity difference between the two beams of light received by the balanced photodetector is:

[0030]

[0031] in, * Indicates complex conjugate;

[0032] Finally, the first harmonic U1 and the second harmonic U2 output by the lock-in amplifier are:

[0033] U1=-2ηJ1(δ0)E0 2 cos(2θ PEM )[cosδ s sin(2ε)+sinδ s sin(2α+4θ HWP +2θ PEM [cos(2ε)]

[0034] U2=-2ηJ2(δ0)E0 2 cos(2θ PEM )[sinδ s sin(2ε)-cosδ s sin(2α+4θ HWP +2θ PEM [cos(2ε)]

[0035] Wherein, η is a coefficient related to the amplification of the balanced photodetector and the lock-in amplifier.

[0036] J1(δ0) and J2(δ0) are first-order and second-order Bessel functions, respectively.

[0037] The installation error angle θ of the half-wave plate and the photoelastic modulator HWP θ PEM If all values ​​are coupled to the azimuth angle α in a linear superposition manner, then the azimuth error α0 = 2θ HWP +θ PEM And cos(2θ) PEM This will be coupled into U1 and U2 as a scaling factor;

[0038] The first harmonic U1 and second harmonic U2 output by the lock-in amplifier can be expressed as:

[0039] U1=k1sin(2ε)+k2sin[2(α+α0)]cos(2ε)

[0040] U2=k3sin(2ε)+k4sin[2(α+α0)]cos(2ε)

[0041] Where k1 is an order of magnitude larger than k2, k4 is an order of magnitude larger than k3, and k3 = –k2k4 / k1.

[0042] After calibrating k1, k2, k3, and k4 before and after placing the sample to be tested, the first harmonic voltage and second harmonic voltage output by the lock-in amplifier before and after placing the sample to be tested are recorded respectively, and the distribution is obtained as U. 1前 U 2前 and U 1后 U 2后 Then the azimuth angle α in front of the sample to be tested 前 and ellipticity ε 前 for:

[0043]

[0044]

[0045] Similarly, the azimuth angle α after placing the sample to be tested can be obtained. 后 and ellipticity ε 后 Then the azimuth and ellipticity caused by the sample to be tested can be expressed by Δα = α 后 –α 前 , Δε=ε 后 –ε 前 Calculated.

[0046] The highly sensitive azimuth ellipticity simultaneous measurement device and method of the present invention has the following advantages:

[0047] 1. Polarization modulation is achieved by using a single photoelastic modulator, which effectively suppresses low-frequency technical noise interference in the signal under test and achieves effective noise suppression.

[0048] 2. The measurement error caused by optical components was analyzed, and a fine-tuning scheme for optical components was proposed to reduce the installation error angle. This error was then suppressed using a calibration scheme and methods for calculating azimuth and ellipticity. In this invention, the quarter-wave plate and half-wave plate were fine-tuned based on the first and second harmonics of the lock-in amplifier output voltage signal, respectively. A calibration scheme and methods for calculating azimuth and ellipticity were proposed to reduce optical component errors and improve measurement accuracy.

[0049] 3. Differential optical path is adopted. In this invention, the differential optical path greatly reduces the DC component in the input signal of the lock-in amplifier, thereby suppressing the fundamental frequency oscillation error and bias error, and the sensitivity is doubled compared with the non-differential optical path. Attached Figure Description

[0050] The following is a brief explanation of the content and markings in each of the accompanying drawings in this specification:

[0051] Figure 1 A schematic diagram of a highly sensitive device for simultaneous measurement of azimuth ellipticity;

[0052] Figure 2 The simulation results for U1-(α,ε) are shown in the figure.

[0053] Figure 3 The simulation results for U2-(α,ε) are shown in the figure.

[0054] Figure 1 The labels in the diagram are: 1. Laser; 2. Polarizer; 3. Quarter-wave plate; 4. Sample under test; 5. Half-wave plate; 6. Photoelastic modulator; 7. Polarizing beam splitter; 8. Mirror; 9. Balanced photodetector; 10. Lock-in amplifier; 11. Computer; 12. Photoelastic modulator controller.

[0055] Figure 2 In the diagram, the horizontal axis represents the azimuth angle α of the elliptically polarized light after passing through the sample, the vertical axis represents the ellipticity ε of the elliptically polarized light after passing through the sample, and the color intensity represents the magnitude of the first harmonic U1 of the demodulated signal. Figure 3 In the diagram, the horizontal axis represents the azimuth angle α of the elliptically polarized light after passing through the sample, the vertical axis represents the ellipticity ε of the elliptically polarized light after passing through the sample, and the color intensity represents the magnitude of the second harmonic U2 of the demodulated signal. Detailed Implementation

[0056] The following description, with reference to the accompanying drawings, details the specific implementation of the present invention, including the shape and structure of each component, the relative positions and connections between the parts, the function and working principle of each part, the manufacturing process, and the operation and use methods, to help those skilled in the art to have a more complete, accurate, and in-depth understanding of the inventive concept and technical solution of the present invention.

[0057] Highly sensitive simultaneous measurement device for azimuth ellipticity, such as Figure 1 As shown, the device includes a laser 1, a polarizer 2, a quarter-wave plate 3, a sample to be tested 4, a half-wave plate 5, a photoelastic modulator 6, a polarizing beam splitter 7, a mirror 8, a balanced photodetector 9, a lock-in amplifier 10, and a computer 11.

[0058] The laser beam emitted from laser 1 passes sequentially through polarizer 2, quarter-wave plate 3, sample under test 4, half-wave plate 5, photoelastic modulator 6, and polarization beam splitter 7 before entering balanced photodetector 9. Balanced photodetector 9 has two incident channels, namely the first channel and the second channel. Polarization beam splitter 7 splits the incident light into two and sends it into the first channel and the second channel. One of the light paths is refracted by mirror 8 and enters balanced photodetector 9.

[0059] A linearly polarized light generation module is composed of a laser 1, a polarizer 2, and a quarter-wave plate 3. After the linearly polarized light interacts with the sample 4, it becomes elliptically polarized light, and the azimuth angle and ellipticity parameters of the elliptically polarized light are to be measured. The elliptically polarized light is modulated by a photoelastic modulator 6. After passing through a polarizing beam splitter 7, it is split into two beams of linearly polarized light with mutually perpendicular polarization directions. One beam of linearly polarized light is reflected by a mirror 8 and then incident on a balanced photodetector 9, while the other beam of linearly polarized light is directly incident on the balanced photodetector 9. The balanced photodetector 9 outputs a voltage signal proportional to the intensity difference between the two beams. The balanced photodetector 9 performs differential analysis on the incident light. The differential signal is demodulated by a lock-in amplifier 10. The demodulated signal is processed by a computer 11 to obtain the azimuth angle and ellipticity measurement results.

[0060] The first and second harmonic components of the voltage signal are extracted by the lock-in amplifier 10, whose reference frequency is the modulation frequency of the photoelastic modulator 6, and then processed by the computer 11. The photoelastic modulator controller 12 is used to set the modulation frequency of the photoelastic modulator 6 and to provide a reference frequency for the lock-in amplifier 10. Utilizing the phase delay characteristics of the photoelastic modulator 6, the azimuth and ellipticity to be measured are modulated to different frequency bands to achieve simultaneous detection of ellipticity and azimuth information.

[0061] The angular relationships between the components in the optical path are as follows: the angle between the fast axis of the quarter-wave plate 3 and the transmission axis of the polarizer 2 is 0°, the angle between the fast axis of the half-wave plate 5 and the transmission axis of the polarizer 2 is 22.5°, and the angle between the fast axis of the photoelastic modulator 6 and the transmission axis of the polarizer 2 is 45°.

[0062] Based on the aforementioned highly sensitive simultaneous azimuth ellipticity measurement device, the simultaneous measurement of azimuth ellipticity is achieved, comprising the following steps:

[0063] a. Install laser 1, polarizer 2, polarizing beam splitter 7, reflector 8, and balanced photodetector 9. First, block the reflected light from reflector 8. Rotate polarizer 2 until the output of balanced photodetector 9 reaches its minimum value. The spot size must be determined by the optical components and the detection area of ​​balanced photodetector 9. Install quarter-wave plate 3, half-wave plate 5, and photoelastic modulator 6. Rotate quarter-wave plate 3 according to the fast axis position marked on the components until the angle between its fast axis and the transmission axis of polarizer 2 is 0°. Rotate half-wave plate 5 until the angle between its fast axis and the transmission axis of polarizer 2 is 22.5°. Rotate photoelastic modulator 6 until the angle between its fast axis and the transmission axis of polarizer 2 is 45°. Note that the laser wavelength of laser 1 needs to be matched with the optical components, especially with the settings of photoelastic modulator 6.

[0064] b. Set the reference frequency of the lock-in amplifier 10 to the modulation frequency of the photoelastic modulator 6, so that the output of the lock-in amplifier 10 balances the first and second harmonics of the voltage signal of the photodetector 9. First, fine-tune the quarter-wave plate 3 until the first harmonic is at its minimum value to achieve fine-tuning of the quarter-wave plate 3, and then fine-tune the half-wave plate 5 until the second harmonic is zero to achieve fine-tuning of the half-wave plate 5. Since the quarter-wave plate 3 changes both the azimuth angle and ellipticity of the polarized light, while the half-wave plate 5 only changes the azimuth angle of the polarized light, the fine-tuning order of the quarter-wave plate 3 and the half-wave plate 5 cannot be reversed.

[0065] c. Calibrate k1, k2, k3, and k4 when no sample 4 is available. Rotate the quarter-wave plate 3 so that the angle between its fast axis and the transmission axis of the polarizer 2 varies between -14° and 14°. To facilitate data reading, the specific rotation angle can be fine-tuned. Record the first harmonic voltage U1 output by the lock-in amplifier 10 and the angle β between the fast axis of the quarter-wave plate 3 and the transmission axis of the polarizer 2. QWP U1-β QWP Half the slope of the fitted straight line is k1. Rotate the half-wave plate 5 and record the first harmonic voltage U1 and the second harmonic voltage U2 output from the lock-in amplifier 10, as well as the angle β between the fast axis of the half-wave plate 5 and the transmission axis of the polarizer 2. HWP The fitted U1-β HWP U2-β HWP The amplitudes of the sine curve are k2 and k4. Once k1, k2, and k4 are obtained, k3 can be calculated using k3 = -k2k4 / k1. After calibrating k1, k2, k3, and k4,

[0066] Repeat the fine-tuning operation of quarter-wave plate 3 and half-wave plate 5 in step b; fine-tune quarter-wave plate 3 to the minimum value of the first harmonic of the demodulated signal to achieve fine-tuning of quarter-wave plate 3; fine-tune half-wave plate 5 to the zero value of the first harmonic of the demodulated signal to achieve fine-tuning of half-wave plate 5, and rotate quarter-wave plate 3 so that the angle between its fast axis and the transmission axis of polarizer 2 is between -14° and 14°.

[0067] d. Record the first harmonic voltage U1 and the second harmonic voltage U2 output by the lock-in amplifier 10 before and after placing the sample 4, respectively. Then, use Δα = α 后 –α 前 , Δε=ε 后 –ε 前 The azimuth and ellipticity caused by the test sample 4 were calculated. This completes the measurement of the azimuth and ellipticity caused by the test sample 4.

[0068] This invention, based on the functional relationship between the first and second harmonics of the output voltage and the azimuth and ellipticity, achieves the measurement of the azimuth and ellipticity caused by the sample under test 4. A fine-tuning scheme for the optical elements is proposed to reduce the installation error angle of the optical elements. The measurement error caused by the optical elements is analyzed, and a calibration scheme and a method for calculating the azimuth and ellipticity are proposed to suppress it, thereby improving the measurement accuracy. Through the combined application of modulation detection and differential detection methods, the sensitivity of this device is doubled compared to the non-differential optical path.

[0069] The calculation process is explained in detail below:

[0070] Suppose that after passing through the sample 4, we obtain elliptically polarized light with an azimuth angle of α and an ellipticity of ε. Then, the Jones vector E0 of this elliptically polarized light can be expressed as:

[0071]

[0072] Jones matrix G of half-wave plate 5 HWP It can be represented as:

[0073]

[0074] Where, θ HWP The installation error angle is for half-wave plate 5.

[0075] The phase delay of the photoelastic modulator 6 can be expressed as:

[0076] δ(t)=δ0sinωt+δ s (3)

[0077] Where δ0 is the peak phase delay of the photoelastic modulator 6, ω is the modulation frequency of the photoelastic modulator 6, and δ s This represents the static phase delay of the photoelastic modulator 6. Therefore, the Jones matrix G of the photoelastic modulator 6 is... PEM It can be represented as:

[0078]

[0079] Where, θ PEM Let be the installation error angle of the photoelastic modulator 6. The Jones vector of the light emitted from the photoelastic modulator 6 can be expressed as E = G.PEM G HWP E0, then the intensity difference between the two beams of light received by the balanced photodetector 9 is:

[0080]

[0081] in, * This represents the complex conjugate. Finally, the first harmonic U1 and second harmonic U2 output by the lock-in amplifier 10 are:

[0082]

[0083] Where η is a coefficient related to the amplification of the balanced photodetector 9 and the lock-in amplifier 10, and J1(δ0) and J2(δ0) are first-order and second-order Bessel functions, respectively. Therefore, the installation error angle θ of the half-wave plate 5 and the photoelastic modulator 6... HWP θ PEM All of these will be directly coupled to the azimuth angle α in a linear superposition, thus causing the azimuth angle error α0 = 2θ. HWP +θ PEM And cos(2θ) PEM This will be coupled into U1 and U2 as a scaling factor. The static phase delay δ of the photoelastic modulator 6 s It will not be directly coupled to the azimuth angle α and ellipticity ε, but it will still affect U1 and U2. In addition, the differential optical path used greatly reduces the DC component in the input signal of the lock-in amplifier 10, thereby suppressing the fundamental frequency oscillation error and bias error, and the sensitivity is doubled compared to the non-differential optical path.

[0084] From equation (6), we can see that the first harmonic U1 and the second harmonic U2 can be expressed as:

[0085]

[0086] Among them, due to δ s Since the order of magnitude is smaller, k1 is an order of magnitude larger than k2, and k4 is an order of magnitude larger than k3, and k3 = –k2k4 / k1. Figure 2 and Figure 3 The simulation results for U1-(α,ε) and U2-(α,ε) are shown below. It is easy to see that the first harmonic U1 is sensitive to the ellipticity ε, while the second harmonic U2 is sensitive to the azimuth angle α. Since cos(2θ) PEM ) and δ s These are all included in the coefficients k1, k2, k3, and k4. Therefore, by calibrating these four coefficients, the problem of cos(2θ) can be eliminated in the subsequent calculation of azimuth and ellipticity. PEM ) and δ s This reduces the errors caused by measurement errors, thereby improving measurement accuracy.

[0087] When the sample 4 is not placed, k1, k2, k3, and k4 are calibrated. Since the azimuth angle α and ellipticity ε are both small at this time, U1 = 2k1ε. In the experiment, the angle β between the fast axis of the quarter-wave plate 3 and the transmission axis of the polarizer 2 is set. QWP It varies in the range of -14° to 14°, at which point sin(β) QWP It can be approximated as β QWP Ellipticity ε varies with β QWP The linear change, U1 also changes with β QWP Linear change, U1-β QWP Half the slope of the fitted straight line is k1. By rotating the half-wave plate 5, the azimuth angle α can be changed linearly without changing the ellipticity ε. From equation (7), it can be seen that U1 and U2 both change sinusoidally at this time, and the angle β between U1, U2 and the fast axis of the half-wave plate 5 and the transmission axis of the polarizer 2 is... HWP The amplitudes of the fitted sine curves are k2 and k4, respectively, and k3 is calculated from k3 = –k2k4 / k1.

[0088] After completing the calibration of k1, k2, k3, and k4, record the first harmonic voltage and second harmonic voltage output by the lock-in amplifier 10 before and after placing the sample 4 under test, i.e., U. 1前 U 2前 and U 1后 U 2后 Then, from equation (7), we can know the azimuth angle α before placing the sample 4 to be tested. 前 and ellipticity ε 前 for:

[0089]

[0090] Similarly, the azimuth angle α after placing the sample 4 can be obtained. 后 and ellipticity ε 后 Then the azimuth and ellipticity caused by the sample 4 can be expressed by Δα = α 后 –α 前 , Δε=ε 后 –ε 前 The calculation yields α. 后 With α 前 Subtraction eliminates α0, thereby eliminating the azimuth error caused by the installation error angle of the half-wave plate 5 and the photoelastic modulator 6, and improving the measurement accuracy.

[0091] Based on this, the highly sensitive azimuth ellipticity simultaneous measurement device and method of the present invention utilizes the phase delay characteristics of the photoelastic modulator 6 to modulate the ellipticity and azimuth information to different frequency bands, respectively. The first and second harmonics of the demodulated signal reflect the ellipticity and azimuth of the polarized light, respectively. Compared with conventional methods, the present invention combines high-frequency modulation and differential detection methods to achieve simultaneous measurement of the azimuth and ellipticity of elliptically polarized light, while suppressing the influence of low-frequency technical noise and common-mode noise, thus improving the detection sensitivity by approximately one time.

[0092] The present invention has been described above by way of example with reference to the accompanying drawings. Obviously, the specific implementation of the present invention is not limited to the above-described manner. Any non-substantial improvements made using the inventive concept and technical solution of the present invention, or the direct application of the inventive concept and technical solution of the present invention to other occasions without modification, are all within the protection scope of the present invention.

Claims

1. A high sensitivity azimuthal ellipticity simultaneous measurement device, the device having a laser emitting a laser beam, characterized in that: Along the optical path of the laser beam, there are sequentially arranged a linearly polarized light generation module, a sample to be tested, a half-wave plate, and a polarizing beam splitter. The polarizing beam splitter splits the laser beam into two parts, which are then directed into the first and second channels of a balanced optical detector to complete the differential signal. The balanced optical detector demodulates the differential signals generated by the two channels through a lock-in amplifier and then sends them to a computer to obtain the azimuth and ellipticity measurement results. A photoelastic modulator with phase delay characteristics is positioned between a half-wave plate and a polarizing beam splitter. The photoelastic modulator modulates the azimuth angle and ellipticity to be measured to different frequency bands for simultaneous detection of ellipticity and azimuth angle information. The photoelastic modulator is connected to a lock-in amplifier via a photoelastic modulator controller. The photoelastic modulator controller is used to set the modulation frequency of the photoelastic modulator and to provide a reference frequency for the lock-in amplifier. The linearly polarized light generating module consists of a polarizer and a quarter-wave plate arranged sequentially along the laser beam direction. The interaction between the linearly polarized light generating module and the sample under test converts the laser beam into elliptically polarized light. The laser beam split by the polarizing beam splitter is either directly injected into the first channel or reflected by a mirror and then injected into the second channel. The fast axis of the quarter-wave plate forms an angle of 0° with the transmission axis of the polarizer, the fast axis of the half-wave plate forms an angle of 22.5° with the transmission axis of the polarizer, and the fast axis of the photoelastic modulator forms an angle of 45° with the transmission axis of the polarizer.

2. A measurement method based on the high-sensitivity azimuthal ellipticity simultaneous measurement device according to claim 1, characterized by, Includes the following steps: Step 1: Block the reflected light from the mirror and rotate the polarizer until the output of the balanced photodetector is at its minimum value; Step 2: Set the reference frequency of the lock-in amplifier to the modulation frequency of the photoelastic modulator, so that the lock-in amplifier only outputs the first and second harmonics of the input voltage signal from the balanced photodetector; Step 3: Without placing the sample to be tested, change the direction of the fast axis of the quarter-wave plate and record the first harmonic voltage U output by the lock-in amplifier. 1QWP前 The angle β between the fast axis of the quarter-wave plate and the transmission axis of the polarizer QWP前 U 1QWP前 -β QWP前 By fitting half the slope of the straight line, we obtain k1; Step 4: Without placing the sample to be tested, change the direction of the fast axis of the half-wave plate and record the first harmonic voltage U output by the lock-in amplifier. 1HWP前 Second harmonic voltage U 2HWP前 The angle β between the fast axis of the half-wave plate and the transmission axis of the polarizer HWP前 The U obtained by fitting 1HWP前 -β HWP前 U 2HWP前 -β HWP前 The amplitudes of the sine curve are obtained as k2 and k4, respectively. Step 5: Complete the calibration of k1, k2, k3 and k4 without placing the sample to be tested, where k3 = –k2k4 / k1; Step 6: When placing the sample to be tested, ensure that the angle β between the fast axis of the quarter-wave plate and the transmission axis of the polarizer is [missing information]. QWP后 It varies in the range of -14° to 14°, at which point sin(β) QWP后 ) is approximately β QWP后 Ellipticity ε varies with β QWP后 Linear change, the first-harmonic voltage U output by the lock-in amplifier 1QWP后 Also with β QWP后 Linear change, U 1QWP后 -β QWP后 By fitting half the slope of the straight line, we obtain k5; Step 7: When placing the sample to be tested, rotate the half-wave plate and make the azimuth angle α change linearly without changing the ellipticity ε. At this time, the lock-in amplifier outputs a first-harmonic voltage U. 1HWP后 Second harmonic voltage U 2HWP后 The U obtained by fitting the sinusoidal variation is... 1HWP后 -β HWP后 U 2HWP后 -β HWP后 The amplitudes of the sine curves were obtained, k6 and k8, respectively; Step 8, complete the calibration of k5, k6, k7, k8 in the state of placing the sample to be tested, wherein k 7=- k6k8 / k5; Step 9: Record the first harmonic voltage and second harmonic voltage output by the lock-in amplifier before and after placing the sample to be tested, respectively. Using Δα=α 后 –α 前 , Δε=ε 后 –ε 前 Calculate the azimuth and ellipticity caused by the sample, α. 前 ε is the azimuth angle before the sample to be tested is placed. 前 α is the ellipticity before the sample is placed. 后 ε is the azimuth angle after the sample to be tested is placed. 后 The ellipticity is the value after the sample to be tested is placed.

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