Real-time measurement analyzer for laser polarization state

Abstract

The invention relates to the technical field of optical polarization state measurement, in particular to a laser polarization state real-time measurement analyzer which comprises collimators, a non-polarization beam splitter, a 1 / 4 wave plate, a compensation module and a polaroid. The first non-polarization beam splitter divides two light beams into a second non-polarization beam splitter, a 1 / 2 wave plate and a third non-polarization beam splitter respectively, and the second non-polarization beam splitter divides two light beams into a second collimator and a third collimator respectively through a first polaroid and a second polaroid; the third non-polarization beam splitter divides two paths of light beams into a fourth collimator and a 1 / 4 wave plate through a third polaroid, the fourth polaroid enters a fifth collimator, the second collimator, the third collimator, the fourth collimator and the fifth collimator are connected to an oscilloscope through four photoelectric detectors, and the upper computer collects data of the oscilloscope and calculates the real-time polarization state of the laser. The method is simple, high in precision and low in cost, and realizes real-time measurement of the laser polarization state.

Description

Technical Field

[0001] This invention belongs to the field of optical polarization state measurement technology, specifically relating to a real-time laser polarization state measurement and analysis instrument. Background Technology

[0002] When an electron in an atom absorbs energy and transitions from a lower energy level to a higher energy level, and then falls back down from the higher energy level, the released energy is emitted as photons, generating a beam of photons called a laser. Lasers exhibit highly consistent photonic optical properties, resulting in better monochromaticity and directionality, and higher brightness compared to ordinary light sources. Due to their excellent coherence and directionality, lasers are widely used in many fields such as communication, measurement, imaging, processing, and medicine.

[0003] A laser is a device or apparatus that generates laser radiation. It mainly consists of a laser working medium, an excitation (pumping) system, and an optical resonant cavity. Its principle is stimulated emission, which amplifies electromagnetic waves. Polarization, as one of the important intrinsic properties of lasers, has a significant and far-reaching impact on laser propagation characteristics and application effects. Real-time and accurate measurement of the laser polarization state can not only effectively assess system stability and monitor external disturbances, but also provide early warning and diagnosis of system anomalies, which is of great significance for optimizing optical systems and improving system stability.

[0004] Currently available high-precision commercial polarization analyzers have certain advantages, but also some drawbacks. Examples include Keysight's N7781C, Thorlabs' IPM5300, and Novoptel's PM1000. Among these, the N7781C and IPM5300 can both achieve a maximum resolution of [missing information - likely a resolution value]. The PM1000 can measure polarization states, and even more importantly, it can achieve a maximum resolution of [missing information - likely a value]. Polarization state measurement and recording in one go These points, while enabling high-precision measurement of polarization states and intuitive observation of the evolutionary trajectory on the Poincaré sphere, all suffer from the inability to perform real-time measurement of laser polarization states.

[0005] The existing patent publication number CN106500844A discloses a six-channel amplitude-splitting high-speed Stokes polarimeter and its parameter measurement method. This polarimeter, based on the amplitude-splitting method, has a nanosecond-level time response. While it can achieve accurate and high-speed measurement of polarization states, it suffers from drawbacks such as numerous optical path branches, difficulty in optical path adjustment, and high system cost. In this system, the half-wave plate, in conjunction with a polarizing beam splitter, enables the measurement of the S2 component of the Stokes vector.

[0006] Another existing patent document, CN107271041A, discloses a polarization state measuring instrument based on the amplitude splitting method. Although it can accurately measure and intuitively display the polarization state, it still suffers from drawbacks such as multiple optical path branches, difficulty in optical path adjustment, and high system cost. In this system, the half-wave plate, in conjunction with a polarizing beam splitter, splits the incident light into polarization components in the 60° and 150° directions, thereby measuring and characterizing the polarization state.

[0007] Both of the above patents are based on the amplitude division method to measure the polarization state. The main drawbacks are that "there are many optical path divisions, which makes adjustment difficult, and many optical components such as polarization beam splitters are used, resulting in high system costs".

[0008] The patent “In-line optical polarimeter based on integration of free-space optical elements” (US6836327B1) discloses an online optical polarization analyzer based on the amplitude division method. Although it achieves real-time measurement of polarization state while reducing the number of optical path branches, it has the drawback that the polarization state of the reflected beam is easily affected by non-ideal optical elements, resulting in low measurement accuracy and precision.

[0009] Another high-speed real-time response polarization state measurement and analysis instrument, published under CN111272285A, achieves high-speed measurement of polarization state and has only four branch optical paths. It has the advantages of low difficulty in optical path adjustment and low system cost, but it still has the disadvantage that the measurement results are easily affected by the performance of non-ideal depolarization beam splitter, which leads to low measurement accuracy and precision.

[0010] The polarization measurement and imaging instrument based on a superconducting nanowire single-photon detector disclosed in CN114279580A divides the optical path into four branches and combines a polarizer and a quarter-wave plate to measure the polarization state. However, it also suffers from the shortcomings of the measurement results being easily affected by the non-ideal non-polarized beam splitting cubic performance, resulting in low measurement accuracy and precision.

[0011] The main drawback of the three existing patents is that the results are easily affected by non-polarizing beam splitters, resulting in low measurement accuracy and precision.

[0012] In summary, existing commercial polarization analyzers and related patent documents mainly suffer from drawbacks such as "inability to measure laser polarization state in real time, difficulty in optical path adjustment, high system cost, and low measurement accuracy and precision." These are also technical challenges that urgently need to be addressed in the further development of laser polarization analyzers. Summary of the Invention

[0013] In view of the above situation, the present invention provides a laser polarization state real-time measurement and analysis instrument, which has a simpler optical path, lower adjustment difficulty and system cost, but higher measurement accuracy and precision, and can realize real-time measurement of laser polarization state, which is of great significance for solving the above technical problems in the prior art.

[0014] To achieve the above objectives, the present invention adopts the following technical solution: A real-time laser polarization state measurement and analysis instrument includes a laser, an optical path system, a photodetector group, an oscilloscope, and a host computer; wherein, the optical path system includes three non-polarizing beam splitters, four polarizers, five collimators, a quarter-wave plate, and a compensation module; The laser beam to be tested emitted by the laser enters the optical path system through the first collimator, and is split into two beams by the first unpolarized beam splitter. One beam enters the second unpolarized beam splitter, and the other beam enters the third unpolarized beam splitter through the compensation module. The beam entering the second unpolarized beam splitter splits into two beams again. One beam passes through the first polarizer and enters the second collimator, while the other beam passes through the second polarizer and enters the third collimator. The beam entering the third unpolarized beam splitter splits into two beams again. One beam passes through the third polarizer and enters the fourth collimator, while the other beam passes through the quarter-wave plate and the fourth polarizer in sequence and then enters the fifth collimator. The photodetector group includes four photodetectors; wherein, the second collimator, the third collimator, the fourth collimator and the fifth collimator are respectively connected to the four photodetectors and then connected to the four channels of the oscilloscope; the host computer is used to collect oscilloscope data and calculate the real-time polarization state of the laser.

[0015] Furthermore, the three non-polarizing beam splitters have the same specifications and parameters, and their beam splitting ratio is 50:50, that is, the optical power of the two beams split by the non-polarizing beam splitter accounts for 50% of the amount of laser light entering it.

[0016] Furthermore, the four polarizers have the same specifications and parameters, wherein the transmission axis of the first polarizer is parallel to the light transmission direction; the transmission axis of the second polarizer is perpendicular to the light transmission direction and is used to measure the S0 and S1 components; the transmission axis of the third polarizer is parallel to the light transmission direction and is used to measure the S2 component; and the transmission axis of the fourth polarizer is at 135° to the light transmission direction.

[0017] Furthermore, the compensation module is a half-wave plate with the fast axis direction at 22.5° to the light transmission direction, used to avoid the influence of an imperfect non-polarizing beam splitter on the measurement accuracy and precision.

[0018] Furthermore, the fast axis of the quarter-wave plate is parallel to the optical transmission direction, and is used in conjunction with the fourth polarizer to measure the S3 component.

[0019] Based on the angles of the polarizer, the quarter-wave plate, and the half-wave plate in the compensation module, the ideal relationship between the output light intensity values ​​of the second, third, fourth, and fifth collimators and the polarization state of the light under test (i.e., the Stokes vector of the light under test) can be obtained as follows:

[0020] The output light intensity values ​​of the second, third, fourth, and fifth collimators are denoted as I1, I2, I3, and I4, respectively; the light intensity value of the light to be measured is denoted as... I in The Stokes vector of the light to be measured is denoted as ( S 0, S 1, S 2, S 3) T .

[0021] The second, third, fourth, and fifth collimators are connected to the first, second, third, and fourth channels of the oscilloscope via four identical photodetectors, respectively. Finally, the host computer simultaneously acquires the voltages of the four channels of the oscilloscope and calculates the real-time polarization state (i.e., the Stokes vector) of the laser under test. Ideally, the formula for calculating the Stokes vector of the laser under test is as follows:

[0022] The voltage values ​​of the first channel, the second channel, the third channel, and the fourth channel are denoted as V1, V2, V3, and V4, respectively.

[0023] Furthermore, the five collimators have the same specifications and parameters.

[0024] Furthermore, the four photodetectors have identical specifications and parameters. Depending on the wavelength of the laser under test, either a silicon-based detector or an indium gallium arsenide (IGaAs)-based detector can be selected. When the wavelength of the laser under test is in the range of 400–1100 nm, a silicon-based detector is preferred; when the wavelength of the laser under test is in the range of 900–1700 nm, an IGaAs-based detector is preferred.

[0025] Furthermore, the center wavelength of the laser under test is in the range of 400~1700 nm, and conforms to the effective operating wavelength range of the collimator, unpolarized beam splitter, polarizer, quarter-wave plate and compensation module. At the same time, the repetition frequency of the laser is within the maximum bandwidth range of the photodetector and oscilloscope.

[0026] The present invention also includes other components that enable its normal use, all of which are conventional means in the art. In addition, devices or components not limited in the present invention, such as lasers, collimators, non-polarizing beam splitters, polarizers, quarter-wave plates, half-wave plates used in compensation modules, oscilloscopes and host computers, all adopt existing technologies in the art.

[0027] The beneficial effects of this invention are as follows: 1. This laser polarization state real-time measurement and analysis instrument uses three non-polarizing beam splitters to divide the optical path into four paths. Combined with four polarizers, a quarter-wave plate, and a compensation module, it realizes real-time measurement of laser polarization state while reducing the difficulty of optical path adjustment and system cost. It specifically solves the technical problems of existing polarization analyzers that "cannot measure laser polarization state in real time, have great difficulty in optical path adjustment, and have high system cost".

[0028] 2. This laser polarization state real-time measurement and analysis instrument avoids the influence of imperfect unpolarized beam splitters on the measurement results by adding a compensation module between the first and second unpolarized beam splitters, thereby significantly improving the measurement accuracy and precision of the instrument; it effectively solves the technical problem of "low measurement accuracy and precision" of existing polarization analyzers. Attached Figure Description

[0029] Figure 1 This is a schematic diagram of the optical path system structure of the laser polarization state real-time measurement and analysis instrument in this invention; Figure 2 This is a schematic diagram showing the overall composition and connection of the laser polarization state real-time measurement and analysis instrument in this invention; Figure 3 This is a diagram showing the measurement results display interface of the laser polarization state real-time measurement and analysis instrument in this invention.

[0030] In the diagram: 1. Laser, 2-1. First collimator, 2-2. Second collimator, 2-3. Third collimator, 2-4. Fourth collimator, 2-5. Fifth collimator, 3-1. First unpolarized beam splitter, 3-2. Second unpolarized beam splitter, 3-3. Third unpolarized beam splitter, 4. Compensation module, 5-1. First polarizer, 5-2. Second polarizer, 5-3. Third polarizer, 5-4. Fourth polarizer, 6. Quarter-wave plate, 7. Photodetector, 8. Oscilloscope, 9. Host computer. Detailed Implementation

[0031] The technical solution of the present invention will be clearly and completely described below with reference to specific embodiments. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments.

[0032] Example 1 like Figure 1-2As shown, a real-time laser polarization state measurement and analysis instrument includes five collimators, three unpolarized beam splitters, four polarizers, a quarter-wave plate, a compensation module, four photodetectors 7, a laser 1, an oscilloscope 8, and a host computer 9.

[0033] The laser beam emitted by the laser 1 enters the real-time polarization state measurement system through the first collimator 2-1. The first unpolarized beam splitter 3-1 splits the beam into two beams, which enter the second unpolarized beam splitter 3-2, the compensation module 4, and the third unpolarized beam splitter 3-3, respectively. The second unpolarized beam splitter 3-2 splits the beam into two beams, which enter the second collimator 2-2 through the first polarizer 5-1 and the third collimator 2-3 through the second polarizer 5-2. The third unpolarized beam splitter 3-3 splits the beam into two beams, which enter the fourth collimator 2-4 through the third polarizer 5-3 and the fifth collimator 2-5 through the quarter-wave plate 6 and the fourth polarizer 5-4, respectively. The second, third, fourth, and fifth collimators are connected to four photodetectors 7 and then to the four channels of the oscilloscope 8. The host computer 9 collects the oscilloscope data and calculates the real-time polarization state of the laser.

[0034] The three non-polarizing beam splitters have the same specifications and parameters, and the beam splitting ratio is 50:50; that is, the optical power of the two beams split by the non-polarizing beam splitter accounts for 50% of the amount of laser light entering it.

[0035] The four polarizers have the same specifications and parameters. The transmission axis of the first polarizer is parallel to the light transmission direction, the transmission axis of the second polarizer is perpendicular to the light transmission direction, and it is used to measure the S0 and S1 components. The transmission axis of the third polarizer is perpendicular to the light transmission direction and it is used to measure the S2 component. The transmission axis of the fourth polarizer is at 135° to the light transmission direction and it is used to measure the S3 component in conjunction with a quarter-wave plate.

[0036] The compensation module is a half-wave plate with a fast axis at 22.5° to the light transmission direction, used to avoid the influence of an imperfect non-polarizing beam splitter on the accuracy of polarization state measurement results.

[0037] The fast axis of the quarter-wave plate is parallel to the direction of light transmission and is used in conjunction with the fourth polarizer to measure the S3 component.

[0038] Based on the angles of the half-wave plate, quarter-wave plate, and polarizer, the ideal relationship between the output light intensity values ​​of the second, third, fourth, and fifth collimators and the polarization state of the input light, i.e., the four light intensity values ​​and the Stokes vector of the input light, can be expressed as:

[0039] The output light intensity values ​​of the second, third, fourth and fifth collimators are denoted as I1, I2, I3 and I4, respectively.

[0040] The second, third, fourth, and fifth collimators are connected to the first, second, third, and fourth channels of the oscilloscope, respectively, via four identical photodetectors. The host computer acquires the voltages of the four channels of the oscilloscope. Ideally, the Stokes vector of the input light can be calculated using the following formula:

[0041] The voltage values ​​of the first channel, the second channel, the third channel, and the fourth channel are denoted as V1, V2, V3, and V4, respectively.

[0042] The five collimators have the same specifications and parameters.

[0043] The four photodetectors have the same specifications and parameters, and either silicon-based detectors or indium gallium arsenide-based detectors can be selected according to the wavelength of the laser to be tested.

[0044] The center wavelength of the laser to be tested should be in the range of 400~1700 nm, and limited by the effective operating wavelength of the collimator, polarizer, quarter-wave plate and compensation mode. At the same time, the repetition frequency of the laser should be limited to the maximum bandwidth range of the photodetector and oscilloscope.

[0045] Before measurement, to eliminate the effects of uneven beam splitting ratio of the unpolarized beam splitter and the losses of the polarizer, half-wave plate and quarter-wave plate on the measurement accuracy and precision, a polarizer and a quarter-wave plate are added between the first collimator 2-1 and the first unpolarized beam splitter 3-1 to calibrate the polarization analyzer.

[0046] The error of the real-time laser polarization analyzer will be determined based on the azimuth angle and ellipticity during measurement. The azimuth angle is denoted as... The ellipticity is denoted as The calculation formula is as follows:

[0047] S0, S1, S2, and S3 are the four components of the input light Stokes vector.

[0048] The following measurement examples were performed based on the real-time laser polarization state measurement and analysis instrument provided in Example 1: Example 1: After system calibration using polarizers and a quarter-wave plate, a 30° linearly polarized beam was generated using a CW light source with a center wavelength of 1560 nm, combined with a polarizer whose transmission axis is at 30° to the light transmission direction and a quarter-wave plate whose fast axis is at 30° to the light transmission direction. The measured polarization state was calculated by the host computer. At this point, the azimuth error is 0.046° and the ellipticity error is 2.611°, indicating that the polarization analyzer can measure linearly polarized light with relatively small errors.

[0049] Example 2: After system calibration using polarizers and a quarter-wave plate, left-handed circularly polarized light was generated using a CW light source with a center wavelength of 1560 nm, combined with a polarizer whose transmission axis is at 90° to the light transmission direction and a quarter-wave plate whose fast axis is at 45° to the light transmission direction. The measured polarization state was obtained through calculation by the host computer. Since the azimuth angle of circularly polarized light is arbitrary, and the error of the azimuth angle is not considered, the error of the ellipticity is 6.419°. This indicates that the polarization analyzer can measure circularly polarized light with a small error.

[0050] Example 3: After system calibration using polarizers and a quarter-wave plate, elliptically polarized light was generated using a CW light source with a center wavelength of 1560 nm, combined with a polarizer whose transmission axis is at 135° to the light transmission direction and a quarter-wave plate whose fast axis is at 30° to the light transmission direction. The measured polarization state was obtained through calculation by the host computer. At this point, the azimuth error is 2.097° and the ellipticity error is 1.791°, indicating that the polarization analyzer can measure elliptically polarized light with relatively small errors.

[0051] The laser polarization state real-time measurement and analysis instrument provided by this invention can measure arbitrarily fixed continuous linearly polarized, circularly polarized, and elliptically polarized light with minimal error. Furthermore, this instrument can also perform real-time measurement of the polarization state of pulsed lasers with a center wavelength within the effective operating wavelength range of the collimator, polarizer, quarter-wave plate, compensation module, and photodetector, and a repetition frequency within the maximum bandwidth range of the photodetector and oscilloscope. The real-time polarization state measurement results for a pulsed laser with an arbitrary center wavelength of 1563.84 nm and a repetition frequency of 28.63 MHz are shown below. Figure 3 As shown.

[0052] The technical solutions of the present invention are not limited to the specific embodiments described above. Without departing from the scope and spirit of the described embodiments, many modifications and changes will be obvious to those skilled in the art. Any technical modifications made within the spirit and principles of the present invention will fall within the protection scope of the present invention.

Claims

1. A real-time laser polarization state measurement and analysis instrument, comprising a laser, an optical path system, a photodetector group, an oscilloscope, and a host computer, characterized in that: The optical path system includes three unpolarized beam splitters, four polarizers, five collimators, a quarter-wave plate, and a compensation module. The laser beam to be tested emitted by the laser enters the optical path system through the first collimator, and is split into two beams by the first unpolarized beam splitter. One beam enters the second unpolarized beam splitter, and the other beam enters the third unpolarized beam splitter through the compensation module. The beam entering the second unpolarized beam splitter splits into two beams again. One beam passes through the first polarizer and enters the second collimator, while the other beam passes through the second polarizer and enters the third collimator. The beam entering the third unpolarized beam splitter splits into two beams again. One beam passes through the third polarizer and enters the fourth collimator, while the other beam passes through the quarter-wave plate and the fourth polarizer in sequence and then enters the fifth collimator. The photodetector group includes four photodetectors; wherein, the second collimator, the third collimator, the fourth collimator and the fifth collimator are respectively connected to the four photodetectors and then connected to the four channels of the oscilloscope; the host computer is used to collect oscilloscope data and calculate the real-time polarization state of the laser.

2. The laser polarization state real-time measurement and analysis instrument according to claim 1, characterized in that: The three unpolarized beam splitters have the same specifications and parameters, and their beam splitting ratio is 50:

50.

3. The laser polarization state real-time measurement and analysis instrument according to claim 1, characterized in that: The four polarizers have the same specifications and parameters. The transmission axis of the first polarizer is parallel to the light transmission direction; the transmission axis of the second polarizer is perpendicular to the light transmission direction and is used to measure the S0 and S1 components; the transmission axis of the third polarizer is parallel to the light transmission direction and is used to measure the S2 component; and the transmission axis of the fourth polarizer is at 135° to the light transmission direction.

4. The laser polarization state real-time measurement and analysis instrument according to claim 1, characterized in that: The compensation module is a half-wave plate with its fast axis at 22.5° to the optical transmission direction, used to avoid the influence of an imperfect non-polarizing beam splitter on measurement accuracy.

5. The laser polarization state real-time measurement and analysis instrument according to claim 1, characterized in that: The fast axis of the quarter-wave plate is parallel to the direction of light transmission and is used in conjunction with the fourth polarizer to measure the S3 component.

6. The laser polarization state real-time measurement and analysis instrument according to claim 1, characterized in that: The five collimators have the same specifications and parameters.

7. The laser polarization state real-time measurement and analysis instrument according to claim 1, characterized in that: The four photodetectors have the same specifications and parameters.

8. The real-time laser polarization state measurement and analysis instrument according to claim 1, characterized in that: The wavelength range of the laser under test is 400~1700 nm, and it conforms to the effective operating wavelength range of the collimator, unpolarized beam splitter, polarizer, quarter-wave plate and compensation module. At the same time, the repetition frequency of the laser is within the maximum bandwidth range of the photodetector and oscilloscope.

9. A real-time laser polarization state measurement and analysis instrument according to claim 8, characterized in that: When the wavelength of the laser to be measured is between 400 and 1100 nm, the photodetector is a silicon-based detector.

10. A real-time laser polarization state measurement and analysis instrument according to claim 8, characterized in that: When the wavelength of the laser to be measured is in the range of 900~1700 nm, the photodetector is an indium gallium arsenide-based detector.

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