Method for simultaneously measuring light absorptivity and thermal diffusivity of optical film based on phase-locked thermal imaging

Abstract

The invention discloses a method for simultaneously measuring the light absorptivity and the thermal diffusivity of an optical thin film based on phase-locked thermal imaging, which comprises the following steps of: focusing high-power laser subjected to light intensity sinusoidal modulation to form a point light source to excite an optical element, and forming a point heat source which is in same-frequency sinusoidal change along with time on the surface of the optical element through light absorption of the thin film; a same-frequency thermal radiation dynamic image signal emitted by the temperature rise of the sample is sampled and shot by the infrared CCD and is transmitted to the computer through the frame capturing device; a laser modulation signal is used as a reference signal, digital phase locking operation is carried out on discrete time domain signals of all pixel points of the CCD in different frames, and an amplitude image and a phase image can be extracted from noise; the numerical values of the light absorptivity and the thermal diffusivity of the optical thin film can be respectively obtained by analyzing the amplitude image data and the phase image data. The method can provide a non-contact, lossless, quantitative, rapid and online light absorption and thermal diffusion parameter quantitative measurement method for the intense laser optical film.

Description

Technical Field

[0001] The present invention relates to the field of optical element testing, and particularly to a non-contact, non-destructive, and quantitative optical measurement method for the light absorption and thermal diffusion properties of optical thin films. Background Art

[0002] In various lasers and their application systems, a large number of optical elements are used, especially thin-film optical elements. Due to the existence of thin-film absorption loss, when the thin-film optical element is used, the internal temperature rises due to laser irradiation, resulting in thermal distortion on the surface of the optical element. In severe cases, catastrophic damage to the optical element will occur, ultimately leading to the collapse of the entire system. Therefore, accurately measuring the absorption loss of optical elements can be used to reduce the absorption loss of optical elements by optimizing the coating design and process, improve the performance of optical elements, and enhance the overall performance and stability of lasers and application systems.

[0003] Thermal imaging technology is a non-contact, non-destructive, and fast optical characterization and measurement method. Compared with passive DC thermal infrared imaging, active lock-in thermography based on AC optical excitation can reduce the thermal diffusion length by increasing the modulation frequency, thereby significantly improving the signal-to-noise ratio resolution of thermal infrared images. The present invention utilizes the quantitative correlation characteristics between the amplitude and phase images of active lock-in thermography and the light absorption and thermal diffusion properties of optical thin films to achieve non-contact, non-destructive, and quantitative optical measurement of the light absorption rate and thermal diffusion rate of optical thin films. Summary of the Invention

[0004] The present invention proposes a method for simultaneously measuring the light absorption rate and thermal diffusion rate of an optical thin film based on lock-in thermography. The system includes a function generator 1, an excitation laser 2, a beam focusing system 3, an optical element sample 4 to be characterized, an infrared CCD 5, a long-pass filter 6, and a computer 7. It is characterized in that:

[0005] The signal generated by the function generator 1 is used to modulate the laser 2 so that it emits a laser with a sinusoidally varying light intensity over time. After passing through the beam focusing system 3, it forms a point light source to excite the optical element 4 and generates a point heat source with a sinusoidally varying frequency over time on its surface. The dynamic image signal of the co-frequency thermal radiation emitted by the temperature rise of the sample is sampled and photographed by the infrared CCD 5. The long-pass filter 6 placed in front of the CCD 5 lens filters out the excitation light emitted by the laser 2 and allows infrared thermal radiation to enter the CCD 5. The captured dynamic image data is transmitted into the computer 7 with a frame grabber. Taking the laser modulation signal as the reference signal, the computer 7 performs digital lock-in operations on the discrete time-domain signals of all pixel points of the CCD 5, and extracts the amplitude image and phase image from the noise; by analyzing the amplitude image and phase image data, the numerical values of the light absorption rate and thermal diffusion rate of the optical thin film can be obtained respectively.

[0006] The frequency f of the modulation signal generated by the function generator 1 should be such that the sample satisfies the semi-infinite approximation, i.e., the AC thermal diffusion length L of the sample D =(D / πf) 1 / 2 is much smaller than the sample size, where D is the thermal diffusivity of the sample.

[0007] The laser optical power emitted by the laser 2 should be high enough so that the temperature rise generated in the weakly absorbing optical thin film can be detected by the infrared CCD 5.

[0008] After the laser beam emitted from the laser 2 passes through the beam focusing system 3, the spot size on the surface of the optical element 4 should be much smaller than the AC thermal diffusion length L D so that the point heat source approximation is satisfied.

[0009] The CCD 5 has the function of external triggering, its frame rate is adjustable, and it should satisfy the Nyquist sampling theorem, that is, the frame rate is greater than twice the laser modulation frequency; the shooting trigger sequence signal of the CCD 5 is generated by the computer 7, and the synchronous control of the entire system is realized by the computer 7.

[0010] The overall duration of the image sequence obtained by the CCD 5 should be an integer multiple of the laser modulation period. Generally, when the laser modulation frequency is set as an integer multiple of 1 Hz and the overall duration of the image sequence is selected as an integer multiple of 1 s, this condition is naturally satisfied.

[0011] The computer 7 takes the laser modulation signal as the reference signal and performs digital phase-locked operation on the discrete time-domain signals of each pixel point of the image sequence. The amplitude and phase values of each pixel point obtained therefrom constitute the phase-locked amplitude and phase images of the optical element at this modulation frequency.

[0012] Using the formula A = C * R * A0, the light absorption rate A of the thin film can be calculated from the amplitude image, where C is the system calibration factor, R is the ratio of the maximum value of the amplitude images of the sample to be measured and the calibration sample, and A0 is the light absorption rate of the calibration sample; using the formula D = πf / a 2 the thermal diffusivity D of the thin film can be calculated from the phase image, where a is the slope of the thermal wave phase change with the radial distance.

[0013] The beneficial effects of the present invention are: by using the quantitative correlation characteristics between the amplitude and phase images of active phase-locked thermography and the light absorption and thermal diffusion properties of the optical thin film, a non-contact, non-destructive, and quantitative optical measurement of the light absorption rate and thermal diffusivity of the optical thin film is realized. Description of the Drawings

[0014] Figure 1Schematic diagram of the experimental system of the present invention, where 1 is a function generator, 2 is an excitation laser, 3 is a beam focusing system, 4 is an optical element sample to be characterized, 5 is an infrared CCD, 6 is a long-pass filter, and 7 is a computer.

[0015] Figure 2 Schematic diagram of the system synchronous trigger sequence signal.

[0016] Figure 3 Schematic diagram of the quantitative imaging result of a certain optical element. (a) is the amplitude image at a modulation frequency of 20 Hz, (b) is the phase image, and (c) is the curve of the thermal wave phase varying with the radial distance. Specific implementation manner

[0017] Next, in combination with Figures 1-3 Specifically describe a method for simultaneously measuring the optical absorption rate and thermal diffusivity of an optical thin film based on lock-in thermography proposed by the present invention. However, it should be understood that the provision of the drawings is only for better understanding the present invention and should not be construed as a limitation to the present invention. The specific implementation steps are as follows:

[0018] (1) Build an experimental system. Build an experimental system for simultaneously measuring the optical absorption rate and thermal diffusivity of an optical thin film based on lock-in thermography as shown in Figure 1 , including a function generator 1, an excitation laser 2, a beam focusing system 3, an optical element sample 4 to be characterized, an infrared CCD 5, a long-pass filter 6, and a computer 7.

[0019] a. Connect the function generator to the laser, and based on the drive signal data provided in the laser instruction manual, set the safe range of the output signal amplitude of the function generator.

[0020] b. Select a continuous laser with a wavelength of 808 nm and a peak power of 45 W as the excitation laser.

[0021] c. Adjust the beam focusing system 3 so that the spot size on the surface of the optical element 4 is much smaller than the alternating current thermal diffusion length, so that the point heat source approximately satisfies.

[0022] d. Place a long-pass filter in front of the infrared CCD to completely filter out the stray excitation light and only pass the thermal radiation signal generated by the temperature rise of the optical element.

[0023] e. Adjust the CCD lens so that its focal plane is at the sample plane position.

[0024] (2) Lock-in thermography of the optical element. Based on the above experimental system, carry out lock-in thermography, and finally obtain the lock-in amplitude and phase image data through four steps: modulation signal selection and setting, absolute light intensity measurement and calibration, synchronous control software writing, and dynamic image data acquisition.

[0025] a. According to the basic thermal and geometric information of the sample to be measured, set the modulation frequency generated by the function generator to 20 Hz to meet the semi-infinite approximation of the sample.

[0026] b. Measure the incident light power and spot area at the sample position, and calculate the absolute excitation light intensity.

[0027] c. Use LABVIEW to write the synchronous control software of the system. After the computer sends the main trigger signal, the function generator starts to modulate the laser to emit light. At the same time, four times the 20-Hz modulation frequency is used as the trigger signal for the CCD, meeting the Nyquist sampling theorem. The exposure time of the camera is selected as 16.6 ms, which is much smaller than the time scale corresponding to the frame rate. Figure 2 As shown, after the computer sends the main trigger signal, the function generator starts to modulate the laser to emit light. At the same time, four times the 20-Hz modulation frequency is used as the trigger signal for the CCD, meeting the Nyquist sampling theorem. The exposure time of the camera is selected as 16.6 ms, which is much smaller than the time scale corresponding to the frame rate.

[0028] d. According to the aforementioned system control scheme, the CCD starts to continuously capture the image sequence. To ensure excellent signal-to-noise ratio, the image signals of 100 modulation cycles are continuously recorded.

[0029] (3) Image data processing and parameter measurement. Based on the image sequence data obtained above, the light absorption rate and thermal diffusivity of the optical element can be calculated.

[0030] a. Taking the optical modulation signal as the reference signal, perform digital phase-locked operation on the discrete time-domain sequence of each pixel point, and the amplitude and phase of each pixel point can be calculated. The calculation formula is as follows

[0031]

[0032] where S0 and S 90 are the in-phase and quadrature signals respectively, N is the number of points of the time-domain signal, f(t j ) is the value of a certain pixel point at time t j , ω r is the angular frequency of the reference signal, and A and are the amplitude and phase respectively.

[0033] b. Process the amplitude and phase data of each pixel point to obtain the amplitude and phase images at the 20-Hz modulation frequency respectively, as shown in Figure 3 (a) and (b).

[0034] c. Using the formula A = C * R * A0, the light absorption rate A of the thin film can be calculated from the amplitude image, where C is the system calibration factor, R is the ratio of the maximum value of the amplitude images of the sample to be measured and the calibration sample, and A0 is the light absorption rate of the calibration sample; using the formula D = πf / a 2 , the thermal diffusivity D of the thin film can be calculated from the phase image, where a is the slope of the thermal wave phase change with the radial distance, as shown in Figure 3 (c).

[0035] The present invention utilizes the quantitative correlation characteristics between the amplitude and phase images of active lock-in thermography and the light absorption and heat diffusion properties of an optical thin film to achieve non-contact, non-destructive, and quantitative optical measurement of the light absorption rate and heat diffusion rate of the optical thin film.

Claims

1. A method for simultaneously measuring the optical absorption rate and thermal diffusivity of an optical thin film based on lock-in thermography, characterized in that: The signal generated by the function generator 1 is used to modulate the laser 2 so that the laser emits laser light with a sinusoidally varying light intensity over time. After passing through the beam focusing system 3, a point light source is formed to excite the optical element 4 and generate a point heat source on its surface that varies sinusoidally with the same frequency over time. The dynamic image signal of the thermal radiation emitted by the sample temperature rise is sampled and captured by the infrared CCD 5. The long-pass filter 6 placed in front of the CCD 5 lens filters out the excitation light emitted by the laser 2 and allows the infrared thermal radiation to enter the CCD 5. The captured dynamic image data is transmitted into the computer 7 with a frame grabber. Using the laser modulation signal as a reference signal, the computer 7 performs digital phase-locked operations on the discrete time-domain signals of all pixel points of the CCD 5, and extracts the amplitude image and the phase image from the noise; by analyzing the amplitude image and the phase image data, the numerical values of the light absorption rate and the thermal diffusivity of the optical thin film can be obtained respectively.

2. A method for simultaneously measuring the optical film light absorption rate and thermal diffusivity based on lock-in thermography according to claim 1, characterized in that: The frequency f of the modulation signal generated by the function generator 1 should be such that the sample satisfies the semi-infinite approximation, i.e., the AC thermal diffusion length L D =(D / πf) 1 / 2 is much smaller than the sample size, where D is the thermal diffusivity of the sample.

3. A method for simultaneously measuring the optical film light absorption rate and thermal diffusivity based on lock-in thermography according to claim 1, characterized in that: The laser power emitted by the laser 2 should be high enough so that the temperature rise generated in the weakly absorbing optical thin film can be detected by the infrared CCD 5.

4. A method for simultaneously measuring the optical absorption rate and thermal diffusivity of an optical thin film based on lock-in thermography according to claim 1, characterized in that: After the laser beam emitted from the laser 2 passes through the beam focusing system 3, the spot size on the surface of the optical element 4 should be much smaller than the AC thermal diffusion length L D , so that the point heat source approximately satisfies the condition.

5. A method for simultaneously measuring the optical film light absorption rate and thermal diffusivity based on lock-in thermography according to claim 1, characterized in that: The CCD 5 has the function of external triggering, its frame rate is adjustable, and it should satisfy the Nyquist sampling theorem, that is, the frame rate is greater than twice the laser modulation frequency; the shooting trigger sequence signal of the CCD 5 is generated by the computer 7, and the synchronous control of the entire system is realized by the computer 7.

6. The simultaneous measurement method of the optical film light absorption rate and thermal diffusivity based on lock-in thermography according to claim 1, characterized in that: The overall duration of the image sequence obtained by the CCD 5 should be an integer multiple of the laser modulation period. Generally, when the laser modulation frequency is set to an integer multiple of 1 Hz and the overall duration of the image sequence is selected as an integer multiple of 1 s, this condition is naturally satisfied.

7. A method for simultaneously measuring the optical absorption rate and thermal diffusivity of an optical thin film based on lock-in thermography according to claim 1, characterized in that: The computer 7 uses the laser modulation signal as a reference signal and performs digital phase-locked operations on the discrete time-domain signals of each pixel point of the image sequence. The amplitude and phase values of each pixel point obtained thereby form the phase-locked amplitude and phase images of the optical element at this modulation frequency.

8. A method for simultaneously measuring the optical absorption rate and thermal diffusivity of an optical thin film based on lock-in thermography according to claim 1, characterized in that: Using the formula A = C * R * A0, the light absorption rate A of the thin film can be calculated from the amplitude image, where C is the system calibration factor, R is the ratio of the maximum value of the amplitude image of the sample to be measured to that of the calibration sample, and A0 is the light absorption rate of the calibration sample.

9. A method for simultaneously measuring the optical film light absorption rate and thermal diffusivity based on lock-in thermography according to claim 1, characterized in that: Using the formula D = πf / a 2 , the thermal diffusivity D of the thin film can be calculated from the phase image, where a is the slope of the thermal wave phase change with the radial distance.

Images