A Measurement Method of Laser-Excited Photoacoustic Pressure Electrothermal Diffusivity by Barker Code Modulation

Abstract

The present invention discloses a method for measuring the thermal diffusivity by photoacoustic piezothermic effect with Barker code modulated laser excitation. The method uses a Barker code modulated signal to modulate the laser intensity to form a Barker code modulated laser. The Barker code modulated laser excites the sample to be measured and generates a photoacoustic signal based on the photoacoustic effect. The photoacoustic signal is detected by a piezoelectric transducer pasted on the back surface of the sample. The photoacoustic signal is related to the thermal diffusivity of the sample, the thickness of the sample, and the characteristics of the Barker code modulated laser. When the characteristics of the Barker code modulated laser and the thickness of the sample are determined, by performing a correlation operation on the theoretical photoacoustic signals of samples with different thermal diffusivities and the measured photoacoustic signal, the thermal diffusivity corresponding to the maximum value of the correlation peak is the thermal diffusivity of the sample. This method uses a correlation processing method and a Barker code with better correlation detection performance as the laser modulation signal, providing a non-destructive, fast, sensitive, and accurate measurement method for the thermal diffusivity of materials.

Description

Technical Field

[0001] The present invention relates to the field of detecting thermal physical properties of solid materials, and particularly to a non-destructive, fast, sensitive, accurate, and quantitative characterization method for measuring the thermal diffusivity by Barker code modulated laser-excited photoacoustic piezothermal effect. Background Art

[0002] Thermophysical properties are one of the important properties of materials. Among them, the thermal diffusivity characterizes the speed of heat diffusion in materials and is an important characteristic parameter of the thermophysical properties of materials. In the fields of materials, microelectronics, bioengineering, energy, aerospace, medicine, metallurgy, power, etc., it not only determines the usage scenarios, scopes, and durations of materials, components, and systems, but even affects the usage safety and economic benefits. Therefore, it is very important to achieve non-destructive, fast, quantitative, and accurate measurement of the thermal diffusivity of materials.

[0003] Measuring the thermal diffusivity based on the photoacoustic and photothermal effects of materials is an important way to detect the thermal diffusivity of materials. Among them, the photoacoustic piezoelectric technology is based on the photoacoustic effect and piezoelectric effect of substances. An intensity-modulated laser is used to irradiate a sample. The sample absorbs light energy and is converted into heat energy through a non-radiative relaxation process, forming a periodic heat source and diffusing in the sample. Due to the thermoelastic effect, the heat wave causes periodic changes in strain and stress in the sample, generating sound waves. The sound signal is detected by a piezoelectric transducer coupled to the back of the sample. Since the photoacoustic signal carries information about the interaction between light and the sample, the thermal diffusivity of the sample can be inversely calculated by analyzing the sound signal, which has the characteristics of non-destructive, quantitative, high sensitivity, and strong specificity.

[0004] At present, the photoacoustic piezoelectric technology for measuring the thermal diffusivity of materials can be divided into a frequency-domain measurement method and a time-domain measurement method. The traditional photoacoustic piezoelectric method is a frequency-domain measurement method. The measurement requires multi-frequency measurement and uses phase-locked detection. The amplitude and phase information of photoacoustic piezoelectricity at multiple frequency points are measured by phase-locking to obtain the amplitude-frequency and phase-frequency curves for inverse calculation of the thermal diffusivity, which is time-consuming and the system is complex, and it is not conducive to engineering applications. The time-domain measurement methods based on square wave modulation and chirp signal modulation overcome the problem of measurement speed, but the measurement sensitivity and accuracy are far less than those of the frequency-domain measurement method. Therefore, it is an urgent need in the engineering application of material thermophysical property detection and the field of photoacoustic and photothermal to develop a photoacoustic piezoelectric technology for measuring the thermal diffusivity with non-destructive, fast, high sensitivity, and high accuracy. Summary of the Invention

[0005] The problem to be solved by the present invention is: how to overcome the deficiencies of the existing traditional frequency-domain photoacoustic piezothermal thermal diffusivity measurement method, such as slow measurement speed and complex system, and the time-domain photoacoustic piezothermal thermal diffusivity measurement method, whose measurement sensitivity and accuracy are inferior to those of the frequency-domain measurement method, and provide a new photoacoustic piezoelectric measurement method for thermal diffusivity to achieve non-destructive, fast, sensitive, and accurate measurement of the thermal diffusivity of materials.

[0006] The technical problem proposed by the present invention is solved as follows: A Barker code modulated laser excited photoacoustic piezothermal diffusivity measurement method is proposed. The system includes a function generator, a laser, a beam splitter mirror, a total reflection mirror, a focusing lens, a sample to be measured, a piezoelectric transducer, a photodetector, a data acquisition card, and a computer. It is characterized in that: The computer controls the function generator to generate a Barker code signal and modulate the laser to form a laser beam with intensity Barker code modulation. After the Barker code modulated laser beam passes through the beam splitter mirror, one beam of light is reflected by the total reflection mirror, focused by the focusing lens, and then used to excite the sample to be measured to generate a photoacoustic signal, which is detected by the piezoelectric transducer pasted on the back surface of the sample; at the same time, the other part of the modulated laser beam split by the beam splitter mirror is focused by the focusing lens and then received by the photodetector as a real-time monitoring signal of the excitation light. The data acquisition card collects the photoacoustic signal and the real-time monitoring excitation light signal and transmits them to the computer. The computer, based on the real-time monitored excitation light signal and the frequency-domain photoacoustic theory model, under the condition that the characteristics of the Barker code modulated laser and the sample thickness are determined, obtains the theoretical photoacoustic signals with different thermal diffusivities under the excitation of the Barker code modulated laser, and performs a correlation operation with the measured photoacoustic signal collected. Then, the thermal diffusivity corresponding to the maximum value of the correlation peak is the thermal diffusivity of the sample.

[0007] The frequency of the Barker code signal used for intensity modulation of the excitation light is low frequency, that is, it satisfies the quasi-steady state approximation of elastodynamics f << c / L, where f is the frequency of the generated sound wave, c is the sound speed in the sample to be measured, L is the sample size, and it satisfies the thermal thickness condition of the sample.

[0008] The Barker code sequence of the Barker code signal used for intensity modulation of the excitation light is any one of 7 typical code patterns of 2, 3, 4, 5, 7, 11, and 13 bits (as shown in the following table).

[0009]

[0010] The Barker code signal used for intensity modulation of the excitation light has two types: without carrier and with carrier. The carrier of the with-carrier type is a sine signal, a triangular wave signal, a square wave signal, and a chirp signal; when using sine, triangular wave, and square wave as the carrier, its period is 1 / n or equal to the symbol width, where n is an integer; when using the chirp signal as the carrier, the chirp duration is 1 / n or equal to the symbol width, where n is an integer.

[0011] The specific steps for the computer to generate a series of theoretical photoacoustic signals under the excitation of the Barker code modulated laser are as follows: Perform a time-frequency transformation on the Barker code modulated excitation light signal detected by the photodetector in real time to obtain the frequency-domain excitation light signal, then multiply the frequency-domain excitation light signal by the frequency-domain photoacoustic response function H(ω, D) corresponding to different thermal diffusivities to obtain a series of complex vectors, and after normalization processing, obtain a series of theoretical photoacoustic signals under the excitation of the Barker code modulated laser. H(ω, D) is shown in the following formula:

[0012]

[0013] Where ω = 2πf is the angular frequency, D is the thermal diffusivity of the sample, and l is the thickness of the sample.

[0014] The specific steps for inverting the thermal diffusivity are as follows: Correlation operations are performed between the theoretical photoacoustic signals under a series of Barker code modulated laser excitations and the measured photoacoustic signals respectively, and the thermal diffusivity corresponding to the theoretical photoacoustic signal with the largest correlation peak value is taken as the thermal diffusivity of the sample.

[0015] The beneficial effects of the present invention are as follows: The present invention is a time-domain photoacoustic piezoelectric measurement method, which overcomes the deficiencies of traditional frequency-domain photoacoustic piezothermal diffusivity measurement methods such as slow measurement speed and complex system. At the same time, the present invention uses Barker code signals with better correlation detection performance to modulate the laser, improving the signal-to-noise ratio of correlation detection, enhancing the measurement sensitivity and accuracy of the existing time-domain photoacoustic piezothermal diffusivity measurement method, and realizing non-destructive, fast, sensitive, and accurate measurement of the thermal diffusivity of materials. Description of the Drawings

[0016] Figure 1 It is a schematic diagram of the system of the present invention, including a function generator, a laser, a beam splitter, a total reflector, a focusing lens, a sample to be measured, a piezoelectric transducer, a photodetector, a data acquisition card, and a computer.

[0017] Figure 2 It is a 7-bit Barker code modulation signal with a sine carrier and a symbol width equal to the carrier period.

[0018] Figure 3 It is a block diagram of the algorithm for signal correlation processing and quantitative measurement of thermal diffusivity. Where r(t) is the excitation optical signal, s(t) is the measured photoacoustic signal, FFT and IFFT are Fourier transform and inverse transform, Z* is taking the complex conjugate, and H(D) is the frequency-domain photoacoustic response function. Detailed Embodiments

[0019] The following Figure 1 , Figure 2 , Figure 3 are used to describe a method for measuring photoacoustic piezothermal diffusivity with Barker code modulated laser excitation proposed by the present invention. However, it should be understood that the provision of the drawings is only for better understanding of the present invention and should not be construed as a limitation to the present invention. The specific implementation steps are as follows:

[0020] As Figure 1 shown, the computer controls the function generator to generate a Barker code modulation signal with a sine carrier and adjustable frequency as follows:

[0021]

[0022] where \(T_0\) is the symbol width, \(N\) is the number of Barker code bits, \(\omega_0 = 2\pi / T_0\) is the carrier angular frequency, and \(C\) n is the Barker code sequence (1 or -1).

[0023] The Barker code modulated signal (for example, a 7-bit Barker code modulated signal with a sine carrier is as shown in Figure 2 ) modulates the intensity of the light emitted by the laser to generate a Barker code modulated laser beam. After passing through the beam splitter mirror, one beam of light is reflected by the total reflection mirror, focused by the focusing lens, and then incident on the front surface of the sample to be measured. The sample absorbs the light energy and generates an acoustic signal, which is detected by the piezoelectric transducer coupled to the back surface of the sample. At the same time, the other part of the modulated laser beam split by the beam splitter mirror is focused by the focusing lens and received by the photodetector as the real-time monitoring signal of the excitation light. The data acquisition card collects the experimental photoacoustic signal and the real-time monitoring excitation light signal and transmits them to the computer. Based on the real-time monitored excitation light signal and the frequency-domain photoacoustic theory model, when the characteristics of the Barker code modulated laser and the sample thickness are determined, the theoretical photoacoustic signals with different thermal diffusivities under the excitation of the Barker code modulated laser are obtained, as shown in Figure 3 The steps are as follows:

[0024] (1) First, perform a Fourier transform on the real-time monitored excitation light signal \(r(t)\) to obtain its spectrum \(R(\omega)\);

[0025] (2) Multiply \(R(\omega)\) point by point with the frequency-domain photoacoustic piezoelectric response function \(H(\omega, D)\) corresponding to different thermal diffusivities based on the photoacoustic piezoelectric model. The expression of \(H(\omega, D)\) is as follows:

[0026]

[0027] where \(\omega = 2\pi f\) is the angular frequency, \(D\) is the thermal diffusivity of the sample, and \(l\) is the sample thickness

[0028] (3) Normalize a series of complex vectors obtained by multiplying \(R(\omega)\) with \(H(\omega, D)\) corresponding to different thermal diffusivities respectively, so that their two-norms are all 1, thereby obtaining a series of normalized frequency-domain theoretical photoacoustic signals.

[0029] The computer performs a Fourier transform on the collected photoacoustic signal \(s(t)\), takes the complex conjugate of its spectrum, multiplies it with the frequency-domain theoretical photoacoustic signals with different thermal diffusivities under the excitation of the Barker code modulated laser respectively, and then performs an inverse Fourier transform to obtain a series of time-domain correlation signals. The thermal diffusivity corresponding to the maximum value of the correlation peak is the thermal diffusivity of the sample, as shown in Figure 3 .

[0030] A method for measuring the photoacoustic piezothermal diffusivity by modulating laser excitation with Barker code proposed by the present invention uses Barker code signals with better correlation detection performance to modulate laser excitation of the sample to be measured to generate photoacoustic signals. The signal acquisition, signal processing, and thermal diffusivity inversion are simultaneously completed through a correlation processing method. This Barker code modulation correlation demodulation method improves the signal-to-noise ratio, sensitivity, and accuracy of photoacoustic piezoelectric correlation detection, providing a non-destructive, fast, sensitive, and accurate measurement method for thermal diffusivity measurement.

Claims

1. A Barker code modulation laser-excited photoacoustic piezothermal diffusivity measurement method, characterized in that: The computer-controlled function generator generates a Barker code signal and modulates a laser to form a laser beam with intensity-modulated Barker code. After the Barker code-modulated laser beam passes through a dichroic mirror, one beam of light is reflected by a total reflector mirror, focused by a focusing lens, and then used to excite the sample under test to generate a photoacoustic signal, which is detected by a piezoelectric transducer attached to the back surface of the sample. At the same time, another part of the modulated laser beam split by the dichroic mirror is focused by a focusing lens and then received by a photodetector as a real-time monitoring signal of the excitation light. The data acquisition card collects the photoacoustic signal and the real-time monitoring excitation light signal and transmits them to the computer. Based on the real-time monitored excitation light signal and the frequency-domain photoacoustic theory model, and with the characteristics of the Barker code-modulated laser and the sample thickness determined, the theoretical photoacoustic signals with different thermal diffusivities under the excitation of the Barker code-modulated laser are obtained. Then, a correlation operation is performed with the measured photoacoustic signal collected. The thermal diffusivity corresponding to the maximum value of the correlation peak is the thermal diffusivity of the sample. The Barker code signals used for intensity modulation of the excitation light have two types: with carrier and without carrier. The carrier of the type with carrier is a sine signal, a triangular wave signal, a square wave signal, or a chirp signal. When using sine, triangular wave, or square wave as the carrier, its period is 1 / n of the symbol width or equal, where n is an integer. When using a chirp signal as the carrier, the chirp duration is 1 / n of the symbol width or equal, where n is an integer. The specific steps for the computer to generate a series of theoretical photoacoustic signals under the excitation of the Barker code-modulated laser are as follows: perform a time-frequency transformation on the Barker code-modulated excitation light signal detected in real time by the photodetector to obtain the frequency-domain excitation light signal, then multiply the frequency-domain excitation light signal by the frequency-domain photoacoustic response function H(ω, D) corresponding to different thermal diffusivities to obtain a series of complex vectors, and after normalization, obtain a series of theoretical photoacoustic signals under the excitation of the Barker code-modulated laser. H(ω, D) is shown as follows: In the formula, ω = 2πf is the angular frequency, D is the thermal diffusivity of the sample, and l is the sample thickness. The specific steps for thermal diffusivity inversion are as follows: the computer generates a series of theoretical photoacoustic signals under the excitation of the Barker code-modulated laser and performs a correlation operation with the measured photoacoustic signal respectively. The thermal diffusivity corresponding to the theoretical photoacoustic signal with the largest peak value of the correlation peak is the thermal diffusivity of the sample.

2. The method for measuring the photoacoustic piezothermal diffusivity by Barker code modulation laser excitation according to claim 1, wherein: The frequency of the Barker code signal used for intensity modulation of the excitation light is low frequency, that is, it satisfies the quasi-steady state approximation of elasticity mechanics and the thermal thickness condition of the sample.

3. The Barker code modulation laser-excited photoacoustic piezothermal diffusivity measurement method according to claim 1, wherein: The Barker code sequences of the Barker code signals used for intensity modulation of the excitation light have 7 typical Barker code patterns with 2, 3, 4, 5, 7, 11, and 13 bits.

Images