A dynamic interferometric method for measuring wavefront aberrations of low-reflectivity optical systems

Abstract

The application discloses a low-reflection optical system wave aberration dynamic interferometry method, which comprises the following steps: firstly, a power and longitudinal mode number database of a single-frequency semiconductor laser at different temperature and current working points is established; then, a low-reflection optical system to be measured is evaluated to obtain a laser power threshold for obtaining a dynamic interference figure with good contrast under a vibration environment; and then, temperature and current working points above the threshold power and operating in a single longitudinal mode are dynamically selected from the power and longitudinal mode number database of the laser, so that the low-reflection optical system wave aberration dynamic interferometry is realized. The application can effectively solve the problem of dynamic measurement failure caused by the poor longitudinal mode and linewidth characteristics and low power of a light source when a dynamic interferometer measures the wave aberration of a low-reflection optical system.

Description

Technical Field

[0001] This invention belongs to the field of optical measurement, specifically relating to a dynamic interferometric measurement method for wave aberrations in low-reflectivity optical systems. Background Technology

[0002] With the rapid development of high-precision technologies such as ultra-large-scale integrated circuit manufacturing, astronomical observation, and high-power laser devices, the application of large-aperture, high-precision planar optical components is becoming increasingly widespread. These applications place new demands on the online inspection of optical components in production environments. In production environments with significant interference, the use of vibration-damping devices to suppress the impact of vibration on measurements is very limited, often making time-shifted interferometry impossible. To achieve interferometric measurements under unstable conditions, dynamic interferometers have become one of the research hotspots in the field of optical testing in recent years.

[0003] Most dynamic interferometers employ the principle of polarization interference, dynamically matching the power ratio of the reference light to the test light. Therefore, they are frequently used for measuring low-reflectivity optical components. This performance places high demands on the coherence and output power of the light source. The test light is formed by reflections from multiple optical components. Ordinary dynamic interferometers use weak lasers, requiring long exposure times for the camera to record interference fringes, which can easily lead to vibration-resistant failure in dynamic interferometry. Furthermore, ordinary dynamic interferometers have poor longitudinal mode characteristics, resulting in poor contrast of the interference fringes in the phase-shifted interferogram acquired by the camera, making it difficult for software to decompose and calculate the correct wavefront aberrations of the optical system. Therefore, the light source of a dynamic interferometer must possess high power and good multi-mode characteristics to meet the requirements of the optical path in dynamic interferometry for low-reflectivity optical systems.

[0004] In their paper "Design of Semiconductor Laser Driver Circuits," Ma Xiaoming et al. achieved constant power output of a laser using an automatic power control (APC) circuit, an automatic temperature control (ATC) circuit, and an operating parameter detection circuit. However, they did not pay attention to the multi-mode and linewidth characteristics of the laser. In their paper "Design and Implementation of High-Precision Semiconductor Laser Control System," Ma Chao et al. used an STM32 chip to achieve high-precision control of the laser drive current and operating temperature. However, they did not solve the problem of the influence of laser temperature and current operating point on power, multi-mode, and linewidth characteristics. In their paper "Drive and Temperature Control Design of Narrow Linewidth Semiconductor Lasers," Si Qi et al. used an ATLWS200MA103 driver module and a TECA1 series temperature control module to achieve laser output with a 5kHz linewidth and 0.1% optical power stability. However, the operating point power they selected was relatively low, making it difficult to apply in the dynamic interferometric measurement optical path of a low-reflectivity optical system. Summary of the Invention

[0005] This invention proposes a dynamic interferometric measurement method for wavefront aberrations in low-reflectivity optical systems. A temperature control module and a constant current drive module are added to a semiconductor laser in a dynamic interferometer to form a light source. A suitable operating point is dynamically selected from a database of characteristic parameters of the laser's temperature and current operating points. Simultaneously, low-contrast interferometric images captured by the dynamic interferometer are discarded, enabling dynamic interferometric measurement of wavefront aberrations in low-reflectivity optical systems. This solves the problem of dynamic measurement failure caused by poor longitudinal mode and linewidth characteristics and low power of the light source when measuring wavefront aberrations in low-reflectivity optical systems.

[0006] The technical solution for achieving this invention is: a dynamic interferometric measurement method for wavefront aberrations in a low-reflectivity optical system, comprising the following steps:

[0007] Step 1: Use a high-power single-frequency semiconductor laser as the light source for the dynamic interferometer. Add a temperature control module and a constant current drive module to the light source. Use the temperature control module to provide a constant temperature working environment for the light source, and use the constant current drive module to provide a low-noise drive current for the light source.

[0008] Step 2: Use the controlled variable method to measure the light source point by point to obtain the measurement parameters at each point. The measurement parameters include output power, number of longitudinal modes and frequency stability. Establish a database of characteristic parameters of laser temperature and current operating points.

[0009] Step 3: Construct a low-reflectivity optical system using a light source, a dynamic interferometer host, and a low-reflectivity test element.

[0010] Step 4: Evaluate the low-reflectivity optical system to obtain the laser power threshold that yields a good contrast dynamic interferogram under the vibration environment and interference cavity length.

[0011] Step 5: Based on the laser output power threshold, dynamically select a suitable operating point from the characteristic parameter database of laser temperature and current operating points to perform dynamic interferometric measurement of wavefront aberration of the low reflectivity optical system.

[0012] Compared with the prior art, the significant advantages of this invention are:

[0013] A controlled variable method was used to measure the light source of the dynamic interferometer point by point, establishing a database of characteristic parameters for the laser's temperature and current operating points. Based on the laser power threshold required to obtain a good contrast dynamic interferogram under vibration conditions, the optimal operating point was selected from the parameter database to perform dynamic interferometric measurements of the wavefront aberrations of the low-reflectivity optical system. This makes the dynamic interferometric measurement method for wavefront aberrations of the low-reflectivity optical system low-cost and widely applicable, reducing the design requirements of the light source, improving the substitutability of the light source component in the dynamic interferometric measurement optical path of the low-reflectivity optical system, and better adapting to measurement requirements in different scenarios. Attached Figure Description

[0014] Figure 1 This is a flowchart of a dynamic interferometric measurement method for wave aberrations in a low-reflectivity optical system according to the present invention.

[0015] Figure 2 A schematic diagram showing the connection of a semiconductor laser to a temperature control module.

[0016] Figure 3 A schematic diagram showing the connection of a semiconductor laser to a constant current drive module.

[0017] Figure 4 This invention relates to a device for testing the characteristic parameters of a semiconductor laser, including its temperature and current operating point.

[0018] Figure 5 This is a schematic diagram of the optical path for low-reflectivity dynamic interferometry measurement according to the present invention. Detailed Implementation

[0019] The technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention.

[0020] Combination Figure 1 A dynamic interferometric measurement method for wavefront aberrations in a low-reflectivity optical system, comprising the following steps:

[0021] Step 1: Use a high-power single-frequency semiconductor laser as the light source for the dynamic interferometer. Add a temperature control module and a constant current drive module to the light source. Use the temperature control module to provide a constant temperature working environment for the light source, and use the constant current drive module to provide a low-noise drive current for the light source.

[0022] Combination Figure 2A high-power single-frequency semiconductor laser with an output power greater than 5 milliwatts is selected to replace the He-Ne laser source in the traditional dynamic interferometer. This overcomes the shortcomings of He-Ne lasers, such as low power and difficulty in frequency stabilization, reducing camera exposure time and gain when measuring wavefront aberrations of low-reflectivity optical systems in the dynamic interferometer, and improving the measurement accuracy and vibration resistance of the dynamic interferometer. Temperature control module 1 provides a constant temperature operating environment for the semiconductor laser. Temperature sensor 4, located close to the core of semiconductor laser 2, converts the temperature of the core of semiconductor laser 2 into an analog signal and transmits it to temperature control module 1. The temperature control module 1, based on the received temperature signal, combined with the set operating temperature and PID adjustment algorithm, delivers positive and negative currents of different magnitudes to the thermoelectric cooler 5 to achieve different power cooling and heating of the thermoelectric cooler 5, so that the operating temperature of the core of the semiconductor laser 2 continuously approaches the set operating temperature; the semiconductor laser housing 6 seals the core of the semiconductor laser 2, the temperature sensor 4, the thermoelectric cooler 5 and the fiber optic grating 3 to form a closed structure, and due to the heat dissipation when the core of the semiconductor laser 2 is working, the external environment represented by the core of the semiconductor laser 2, the thermoelectric cooler 5, the fiber optic grating 3 and the semiconductor laser housing 6 eventually reach thermal equilibrium, realizing the constant temperature operation of the semiconductor laser.

[0023] Combination Figure 3 The constant current drive module 8 provides low-noise drive current for the semiconductor laser. The PCB board 7 of the constant current drive module 8 is housed within a metal casing to isolate it from external electromagnetic interference. The components 9 on the PCB board 7 are selected from high thermal stability devices to reduce the impact of temperature changes on the output characteristics of the semiconductor laser drive circuit and improve the constant current accuracy.

[0024] Step 2: Use the controlled variable method to measure the light source point by point to obtain the measurement parameters at each point. The measurement parameters include output power, number of longitudinal modes, and frequency stability. Establish a database of characteristic parameters of the laser's temperature and current operating points.

[0025] Combination Figure 4The temperature control module 1 and the constant current drive module 8 provide a constant temperature operating environment and low-noise drive current for the semiconductor laser 10. The semiconductor laser 10 outputs through a polarization-maintaining fiber 11, and the laser output from the polarization-maintaining fiber 11 is fed into the measuring device 12. The measuring device 12 includes a power measuring device and a multi-mode and linewidth characteristic measuring device. By adjusting the temperature control module 1 and the constant current drive module 8, the temperature operating point and current operating point of the semiconductor laser 10 are changed, and the power, multi-mode characteristic parameters, and frequency stability corresponding to each temperature operating point and current operating point are measured by the measuring device 12. When measuring the light source, the initial current operating point is set according to the oscillation threshold current of the semiconductor laser chip design, and is usually selected as a current point higher than the oscillation threshold. The initial temperature operating point is set according to the operating temperature range of the semiconductor laser chip design and the temperature control capability of the constant temperature control module, and is usually taken as the minimum value of the semiconductor laser's operating range under the temperature control capability. The measurement employs the controlled variable method. First, the current operating point is controlled at the initial current operating point. Then, the temperature operating point is increased in steps ΔT until the maximum value of the semiconductor laser's operating range under temperature control is reached. The output power, multi-mode characteristics, and linewidth of the semiconductor laser are measured at different temperature operating points. The value of the step ΔT depends on the control accuracy of the temperature control module and is typically higher than that of the temperature control module. Next, the current operating point is increased in steps ΔI, and the temperature operating point measurement process is repeated. The value of the step ΔI depends on the control accuracy of the constant current drive module and is typically higher than that of the constant current drive module. Finally, a database of characteristic parameters for the laser's temperature and current operating points is established.

[0026] At the same time, it is necessary to perform repeated measurements on the light source and average the measurement parameters to eliminate random errors caused by environmental factors.

[0027] Step 3: Construct a low-reflectivity optical system using a light source, a dynamic interferometer main unit, and a low-reflectivity test element:

[0028] Combination Figure 5 The semiconductor laser light source, consisting of temperature control module 1, constant current drive module 8 and semiconductor laser 10, together with dynamic interferometer 13 and low reflectivity test element 14, forms a low reflectivity dynamic interferometric measurement optical path.

[0029] Step 4: Evaluate the low-reflectivity optical system to obtain the laser power threshold that yields a good contrast dynamic interferogram under vibration conditions.

[0030] The power of the constructed low-reflectivity optical system was calibrated. A specific temperature and current operating point were selected from the established database of laser temperature and current operating points, at which point the laser's output power was P0. The power of the reference light and the test light in the constructed low-reflectivity optical system were measured respectively, and denoted as P0. r and P t The relationship between the power of the light source in the dynamic interferometer and the power of the reference and test lights can be expressed as:

[0031] P r =k r ×P0 (1)

[0032] P t =k t ×P0 (2)

[0033] Based on the relationship between amplitude and power, for any light source power P, under the same low-reflectivity optical system, the amplitude a of the reference light in the dynamic interferometer is... r and test light a t It can be represented as:

[0034]

[0035]

[0036] After power calibration, a simulation evaluation of the low-reflectivity optical system is performed. The required power threshold for the light source is assessed based on the reflectivity of the device under test, the interference cavity length, and the system's vibration resistance margin. In the low-reflectivity optical system, the expression for the light intensity I in the interferogram under the influence of vibration signals is:

[0037] I = a r 2 +(Ra t ) 2 +2a r (Ra t cos[δ+V(t)] (5)

[0038] Where R is the reflectivity of the element under test, V(t) is the effect of the seismic test margin of the entire system on the phase difference, and δ is the phase difference.

[0039]

[0040] Where λ1 is the output wavelength of the semiconductor laser source, and ΔL is the optical path difference.

[0041] The interferogram acquired by the CCD camera in the dynamic interferometer is related to the set exposure time T. It is the light intensity value obtained by integrating the incident interference light intensity within the exposure time T. The interference light intensity E1 obtained at time t1 is expressed as:

[0042]

[0043] Where A is the gain set by the CCD camera inside the dynamic interferometer host;

[0044] Due to the multi-mode nature of the light source, the light intensity E obtained by the CCD camera is ultimately expressed as the superposition of E1, E2, E3... obtained from λ1, λ2, λ3..., which is:

[0045]

[0046] Where i represents the current longitudinal mode number of the light source, and n represents the total number of longitudinal modes of the light source, which is determined by the center wavelength of the semiconductor laser light source and its own FSR parameters; then the intensity I' of the interference image obtained by the dynamic interferometer is expressed as:

[0047] I′=E×E * (9)

[0048] In the formula E * It is the conjugate of E. The contrast of the interference image can be expressed as:

[0049]

[0050] In the formula I′ M and I′ m These represent the maximum and minimum values ​​of the interference image intensity obtained by the dynamic interferometer, respectively.

[0051] The effects of the reflectivity of the device under test, the length of the interferometric cavity, and the system's seismic resistance margin on the final CCD camera interferogram imaging are simulated using formula (20). The light source power is changed until the interferogram light intensity approaches saturation and the interferogram contrast approaches the demodulation threshold K. th Then record the power P of the light source at this time. th This refers to the laser power threshold for obtaining a good contrast dynamic interferogram under vibration conditions.

[0052] Step 5: Based on the laser output power threshold, dynamically select a suitable operating point from the characteristic parameter database of laser temperature and current operating points to perform dynamic interferometric measurements on the wavefront aberration of the low-reflectivity optical system.

[0053] Combination Figure 5Based on parameters such as the reflectivity of the component under test in the low-reflectivity dynamic interferometry optical path, the influence of the system's vibration resistance margin on the phase difference, the optical path difference, and the exposure time of the dynamic interferometer, the laser power threshold for obtaining a good contrast dynamic interferogram under vibration conditions can be obtained. Suitable operating points are dynamically selected from the characteristic parameter database of laser temperature and current operating points based on the evaluation value. Then, dynamic interferometry is performed on the wavefront aberration of the low-reflectivity optical system. During dynamic interferometry, the interferometric images captured by the dynamic interferometer should be screened, and low-contrast images should be discarded. If the contrast K of the interferometric image... <K th If the image has low contrast, it should be discarded. The remaining interferometric images are then subjected to multiple average phase-shift demodulations to obtain the wavefront aberration of the low-reflectivity optical system.

Claims

1. A dynamic interferometric measurement method for wavefront aberrations in a low-reflectivity optical system, characterized in that, The steps are as follows: Step 1: Use a high-power single-frequency semiconductor laser as the light source for the dynamic interferometer. Add a temperature control module and a constant current drive module to the light source. Use the temperature control module to provide a constant temperature working environment for the light source, and use the constant current drive module to provide a low-noise drive current for the light source. Step 2: Use the controlled variable method to measure the light source point by point to obtain the measurement parameters at each point. The measurement parameters include output power, number of longitudinal modes and frequency stability. Establish a database of characteristic parameters of laser temperature and current operating points. Step 3: Construct a low-reflectivity optical system using a light source, a dynamic interferometer host, and a low-reflectivity test element; Step 4: Evaluate the low-reflectivity optical system to obtain the laser power threshold that yields a good contrast dynamic interferogram under the vibration environment and interference cavity length; Step 5: Based on the laser output power threshold, dynamically select a suitable operating point from the characteristic parameter database of laser temperature and current operating points to perform dynamic interferometric measurement of wavefront aberration of the low reflectivity optical system.

2. The dynamic interferometric measurement method for wavefront aberrations of a low-reflectivity optical system according to claim 1, characterized in that: In step 1, the output power of the high-power single-frequency semiconductor laser is greater than five milliwatts; the temperature control module is used to control the core temperature of the semiconductor laser, as well as the temperature control of components that can affect the multi-longitudinal mode and linewidth characteristics of the semiconductor laser output, including the semiconductor laser housing and fiber optic grating.

3. The dynamic interferometric measurement method for wavefront aberrations of a low-reflectivity optical system according to claim 1, characterized in that: In step 1, the constant current drive module has PCB board electromagnetic isolation and PCB board temperature control functions. The circuit components selected for the constant current drive module have a high thermal stability coefficient to reduce the influence of temperature changes on the output characteristics of the semiconductor laser drive circuit and improve the constant current accuracy.

4. The dynamic interferometric measurement method for wavefront aberrations of a low-reflectivity optical system according to claim 1, characterized in that: In step 2, when measuring the light source, the initial current operating point is selected as the current point higher than the oscillation threshold; the initial temperature operating point is taken as the minimum value of the semiconductor laser's operating range under temperature control capability.

5. The dynamic interferometric measurement method for wavefront aberrations of a low-reflectivity optical system according to claim 4, characterized in that: In step 2, the controlled variable method is used to measure the light source point by point, as follows: First, the current operating point is controlled at the initial current operating point. Then, the temperature operating point is increased by a step ΔT until the maximum value of the semiconductor laser's operating range under temperature control capability is reached. The output power, multi-mode, and linewidth characteristics of the semiconductor laser are measured at different temperature operating points. The value of the step ΔT is higher than the control accuracy of the constant temperature control module. Then, the current operating point is increased by a step ΔI, and the above temperature operating point measurement process is repeated. The value of the step ΔI is higher than the control accuracy of the constant current drive module.

6. The dynamic interferometric measurement method for wavefront aberrations of a low-reflectivity optical system according to claim 5, characterized in that: In step 2, the light source is repeatedly measured, and the measurement parameters are averaged to eliminate random errors caused by environmental factors.

7. The dynamic interferometric measurement method for wavefront aberrations of a low-reflectivity optical system according to claim 1, characterized in that, In step 3, a low-reflectivity optical system is built using a light source, a dynamic interferometer host, and a low-reflectivity test element. The light source is output through a polarization-maintaining fiber, and the output end of the polarization-maintaining fiber is connected to the dynamic interferometer host.

8. The dynamic interferometric measurement method for wavefront aberrations of a low-reflectivity optical system according to claim 1, characterized in that, In step 4, the low-reflectivity optical system is evaluated as follows: Power calibration was performed on the constructed low-reflectivity optical system. An arbitrary temperature and current operating point were selected from the established database of characteristic parameters for laser temperature and current operating points, at which point the laser's output power was P0. The power of the reference light and the test light in the constructed low-reflectivity optical system were measured respectively, and denoted as P. r and P t The relationship between the power of the dynamic interferometer light source and the power of the reference and test lights is expressed as follows: P r =k r ×P0 (1) P t =k t ×P0 (2) In the formula k r and k t For the reference light power ratio and the test light power ratio; according to the relationship between amplitude and power, for any light source power P, under the same low-reflectivity optical system, the reference light amplitude a of the dynamic interferometer is... r and test light a t Represented as: A simulation evaluation of a low-reflectivity optical system is performed; the power threshold required for the light source is evaluated based on the reflectivity of the component under test, the length of the interference cavity, and the system's vibration resistance margin; in a low-reflectivity optical system, the expression for the light intensity I of the interferogram under the influence of vibration signals is: I=a r 2 +(Ra t ) 2 +2a r (Ra t )cos[δ+V(t)] (5) Where R is the reflectivity of the element under test, V(t) is the effect of the seismic test margin of the whole system on the phase difference, and δ is the phase difference; Where λ1 is the output wavelength of the semiconductor laser source, and ΔL is the optical path difference; The interferogram acquired by the CCD camera in the dynamic interferometer is related to the set exposure time T. It is the light intensity value obtained by integrating the incident interference light intensity within the exposure time T. The interference light intensity E1 obtained at time t1 is expressed as: Where A is the gain set by the CCD camera inside the dynamic interferometer host; Due to the multi-mode nature of the light source, the light intensity E obtained by the CCD camera is ultimately expressed as the superposition of E1, E2, E3... obtained from λ1, λ2, λ3..., which is: Where i represents the current longitudinal mode number of the light source, and n represents the total number of longitudinal modes of the light source, which is determined by the center wavelength of the semiconductor laser light source and its own FSR parameters; then the intensity I' of the interference image obtained by the dynamic interferometer is expressed as: I′=E×E * (9) In the formula E * The conjugate of E; the contrast of the interference image is expressed as: In the formula I′ M and I′ m These represent the maximum and minimum values ​​of the interference image intensity obtained by the dynamic interferometer, respectively. The effects of the reflectivity of the device under test, the length of the interferometric cavity, and the system's seismic resistance margin on the final CCD camera interferogram imaging are simulated using formula (10). The light source power is changed until the interferogram light intensity approaches saturation and the interferogram contrast approaches the demodulation threshold K. th Then record the power P of the light source at this time. th This refers to the laser power threshold for obtaining a good contrast dynamic interferogram under vibration conditions.

9. The dynamic interferometric measurement method for wavefront aberrations of a low-reflectivity optical system according to claim 1, characterized in that: In step 5, when selecting the operating point, temperature and current operating points below the laser power threshold are excluded from the characteristic parameter database of laser temperature and current operating points.

10. A dynamic interferometric measurement method for wavefront aberrations of a low-reflectivity optical system according to claim 9, characterized in that: In step 5, if multiple points in the database of characteristic parameters of laser temperature and current operating points meet the power threshold requirements, then the temperature operating point and current operating point that can stably provide high frequency stability and single longitudinal mode output laser should be selected. When performing continuous dynamic interferometry measurements on wavefront aberrations of low-reflectivity optical systems, the interferometric images captured by the dynamic interferometer should be screened, and low-contrast images should be discarded; if the contrast K of the interferometric images... <K th If the image has low contrast, it should be discarded. The remaining interferometric images are then subjected to multiple average phase-shift demodulations to obtain the wavefront aberration of the low-reflectivity optical system.

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