Programmable time-delay dual optical comb picosecond ultrasonic film thickness measurement system

Abstract

This invention discloses a programmable time-delay dual-comb picosecond ultrasonic thin film thickness measurement system, relating to the fields of ultrafast optical detection, picosecond ultrasonic thin film measurement, and electro-optical sampling technology. The system includes a digital phase-locked loop (PLL) time programming control module for obtaining the real-time phase difference between two pulse trains through digital phase detection; a dual-comb light source module for outputting pump pulses and probe pulses; a pump-probe optical path module for focusing the pump pulses and probe pulses onto the thin film under test; a data acquisition module for receiving the probe light and converting it into a probe electrical signal; and a data processing module for associating each probe value or group of probe values ​​with the corresponding acquisition time, real-time repetition rate, and real-time repetition rate difference. Based on the equivalent picosecond time axis, the system extracts acoustic feature time from the probe signal and calculates the thin film thickness in conjunction with material parameters. This system can improve the measurement speed of the target acoustic time window and solve the reconstruction distortion of the equivalent picosecond time axis.

Description

Technical Field

[0001] This invention relates to the fields of ultrafast optical detection, picosecond ultrasonic thin film measurement and electro-optical sampling technology, and in particular to a programmable time-delay dual-comb picosecond ultrasonic thin film thickness measurement system. Background Technology

[0002] Picosecond ultrasound is a thin-film detection method that utilizes ultrashort pulse lasers to excite and probe high-frequency acoustic waves. In a typical pump-probe structure, after the pump pulse irradiates the sample, the sample absorbs the light energy and undergoes transient thermoelastic expansion, thereby exciting a longitudinal acoustic pulse within the thin film. The probe pulse arrives at the sample at different delay times, and by detecting changes in reflectivity, phase, or polarization state, information about the propagation, reflection, and transmission of the acoustic pulse within the thin film can be obtained. By combining the material's sound velocity with acoustic characteristic time, the thin film thickness can be calculated.

[0003] Traditional picosecond ultrasonic measurement systems typically employ mechanical delay lines to alter the optical path of either the pump or probe optical path, thereby scanning the time delay between the pump and probe pulses. This approach is limited by the displacement platform's travel, mechanical response speed, and reciprocating motion frequency, and may introduce beam pointing drift, spot position changes, and vibrations. Therefore, it is not conducive to high-speed repetitive measurements, system miniaturization, and online detection.

[0004] Asynchronous optical sampling (ASOPS) uses two pulse trains with similar but unequal repetition frequencies as pump and probe pulses. Due to the slight difference in the repetition periods of the two pulse trains, the relative delay of adjacent pulse pairs varies in fixed steps, thus eliminating the need for mechanical delay lines to complete the equivalent time scan.

[0005] Fixed repetition frequency difference (ASOPS) typically scans cyclically within an equivalent time delay of approximately one pulse repetition period. Even when the effective acoustic response of the thin film occupies only a small portion of this range, the system still acquires a large number of time delay intervals that do not contain the target echo. For example, at a repetition frequency of approximately 50 MHz, the pulse repetition period is approximately 20 ns, while the effective response of the thin film under test may be concentrated within a window much shorter than this period, thus limiting the effective sampling percentage of the full-cycle scan.

[0006] Existing window scanning or electro-optical sampling schemes can use phase detectors, controllers, and PZT or EOM actuators to change the laser cavity length or refractive index, causing the relative delay to vary back and forth within a defined window. However, if the equivalent time axis is directly estimated using control voltage, ideal modulation waveform, preset repetition rate difference, or constant time axis scaling factor, the nonlinearity, hysteresis, bandwidth limitation, and drift of the actuator will cause the real-time repetition rate and repetition rate difference to deviate from the set values, resulting in a nonlinear mapping between the acquisition time and the equivalent time delay, thus causing time axis distortion.

[0007] Therefore, it is necessary to read the repetition frequency of the reference optical comb and the repetition frequency difference between the two optical combs in real time during the window scanning process, and calculate the dynamic time axis scaling factor at each time step accordingly, and accumulate or integrate it along the acquisition time to reconstruct the acquisition time into an equivalent picosecond time axis; at the same time, the acquisition time, real-time repetition frequency and real-time repetition frequency difference are bound to the detection data, and the measured frequency quantity rather than the ideal modulation waveform or the nominal constant scaling factor is used as the basis for time axis reconstruction. Summary of the Invention

[0008] The technical problem to be solved by the present invention is how to provide a programmable time-delay dual-comb picosecond ultrasonic thin film thickness measurement system that can improve the measurement speed of the target acoustic time window, reduce the proportion of the no-signal area in the overall measurement range, and solve the equivalent picosecond time axis reconstruction distortion caused by the nonlinearity of repetition modulation.

[0009] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is: a programmable time-delay dual-comb picosecond ultrasonic thin film thickness measurement system, the system comprising:

[0010] The digital phase-locked time programming control module is used to synchronously receive two repetition frequency radio frequency signals corresponding to the pump pulse train and the probe pulse train, obtain the real-time phase difference between the two pulse trains through digital phase detection, compare the real-time phase difference with the target phase difference, and adjust the repetition frequency of the probe mode-locked laser according to the error between the two; read the repetition frequencies of the pump comb and the probe comb in real time, calculate the repetition frequency difference between the two, and provide the real-time repetition frequency, the real-time repetition frequency difference, and the corresponding trigger signal or timestamp to the data acquisition module and the data processing module.

[0011] The dual optical comb light source module is used to output pump pulses and probe pulses. The pump pulses are used to excite coherent acoustic pulses in the thin film, and the probe pulses are used to detect the transient optical response caused by the propagation of the acoustic pulses.

[0012] The pump-probe optical path module is used to transmit, shape, combine, or spatially register pump pulses and probe pulses, focus them onto the thin film under test, and separate the probe light carrying the transient optical response from the sample reflected light.

[0013] The data acquisition module is used to receive the detection light and convert it into a detection electrical signal, sample the detection electrical signal according to the acquisition trigger signal, and record the acquisition time corresponding to each detection value or each group of detection values.

[0014] The data processing module is used to associate each detected value or each group of detected values ​​with the corresponding acquisition time, real-time repetition frequency, and real-time repetition frequency difference; calculate the time axis scaling factor that changes with time based on the real-time repetition frequency and real-time repetition frequency difference, convert and accumulate each acquisition time interval into an equivalent time delay to obtain an equivalent picosecond time axis; based on the equivalent picosecond time axis, extract the acoustic feature time from the detected signal, and calculate the film thickness in combination with material parameters.

[0015] The beneficial effects of adopting the above technical solution are as follows: The system modulates the range of the relative time delay scanning window, so that the actual scanning is concentrated in the target acoustic time window, reducing the proportion of the no-signal area in the overall measurement range and increasing the proportion of effective signal, thereby shortening the single measurement time and increasing the measurement speed; at the same time, the repetition frequency and repetition frequency difference are read in real time to obtain the time axis scaling factor that changes with time, and the acquisition time is reconstructed into an equivalent picosecond time axis based on the real-time scaling factor, thereby eliminating or suppressing the time axis distortion caused by the nonlinearity of the repetition frequency modulation, improving the consistency of repeated measurements and the reliability of thin film thickness inversion. The above effects do not depend on the assumption of uncalibrated actuator linearity, ideal modulation waveform or constant scaling factor. Attached Figure Description

[0016] The present invention will now be described in further detail with reference to the accompanying drawings and specific embodiments.

[0017] Figure 1 This is a general principle block diagram of the measurement system described in the embodiment of the present invention;

[0018] Figure 2 This is a schematic diagram comparing the traditional ASOPS full-cycle scan (left) and the time-programmed effective window scan (right).

[0019] Figure 3 These are typical data graphs obtained from probing the metal film (a) and the transparent dielectric film (b) according to embodiments of the present invention;

[0020] Figure 4 This is a schematic block diagram of the digital phase-locked loop time programming control module in the system described in the embodiment of the present invention;

[0021] Figure 5 This is a schematic block diagram of the first pump-probe optical path module in the system described in the embodiment of the present invention;

[0022] Figure 6 This is a schematic block diagram of the second type of pump-probe optical path module in the system described in the embodiment of the present invention;

[0023] Figure 7 This is a schematic diagram of the third type of pump-probe optical path module in the system described in this embodiment of the invention.

[0024] The module includes: 1. Digital phase-locked loop timing programmable control module; 2. Dual optical comb light source module; 3. Pump-probe optical path module; 4. Data acquisition module; 5. Data processing module.

[0025] 1001, First photodetector; 1002, Second photodetector; 1011, First analog-to-digital converter; 1012, Second analog-to-digital converter; 1020, Time axis parameter generation unit; 1031, First quadrature mixer; 1032, Second quadrature mixer; 1040, NCO digital local oscillator unit; 1051, First low-pass filter; 1052, Second low-pass filter; 1061, First phase calculation unit; 1062, Second phase calculation unit; 1071, First phase unwrapping unit; 1072, Second phase unwrapping unit; 1081, First subtractor; 1082, Second subtractor; 1090, Target phase generation unit; 1100 11. PI controller; 1110. Digital-to-analog converter; 1120. High-voltage amplifier; 211. Probe pulse; 212. Pump pulse; 31. First reflecting mirror; 311. First focusing lens; 312. Second focusing lens; 313. First half-wave plate; 314. Second half-wave plate; 315. Third focusing lens; 316. Third half-wave plate; 317. Fourth focusing lens; 318. Fifth focusing lens; 319. Sixth focusing lens; 32. Sample stage; 33. Frequency doubling crystal; 331. First polarization beam splitter; 332. Second polarization beam splitter; 333. Third polarization beam splitter; 34. Second reflecting mirror; 35. Dichroic mirror; 36. Quarter-wave plate. Detailed Implementation

[0026] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0027] Many specific details are set forth in the following description in order to provide a full understanding of the invention. However, the invention may also be practiced in other ways different from those described herein, and those skilled in the art can make similar extensions without departing from the spirit of the invention. Therefore, the invention is not limited to the specific embodiments disclosed below.

[0028] Overall, such as Figure 1 As shown in the figure, an embodiment of the present invention discloses a programmable time-delay dual-comb picosecond ultrasonic thin film thickness measurement system. The system includes: a digital phase-locked time programming control module 1, a dual-comb light source module 2, a pump-probe optical path module 3, a data acquisition module 4, and a data processing module 5.

[0029] The digital phase-locked time programming control module 1 obtains the real-time phase difference between two pulse trains through digital phase detection; it compares the real-time phase difference with the target phase difference, and adjusts the repetition frequency of the probe mode-locked laser according to the error between the two, so that the real-time phase difference follows the change of the target phase difference, thereby controlling the relative time delay between the pump pulse and the probe pulse. The digital phase-locked time programming control module 1 also reads the repetition frequencies of the pump comb and the probe comb in real time, calculates the repetition frequency difference between the two, and provides the real-time repetition frequency, the real-time repetition frequency difference, and the corresponding trigger signal or timestamp to the data acquisition module 4 and the data processing module 5.

[0030] The dual-comb light source module 2 outputs two ultrashort pulse lasers. As a non-limiting example, the nominal repetition frequency of the two lasers can be approximately 50 MHz, and the pulse width can be 100–200 fs; the specific wavelength, pulse width, repetition frequency, and average power are selected based on the absorption, damage threshold, and detection sensitivity of the material under test. Methods for forming mode-locked pulses or optical combs include, but are not limited to, nonlinear polarization rotation mode-locking, nonlinear amplifying ring mirror mode-locking, saturable absorber mode-locking, electro-optic modulation optical combs, and microring resonator optical combs.

[0031] The pump-probe optical path module 3 focuses the pump pulse and probe pulse onto the sample surface. After the pump pulse is absorbed by the sample, it generates transient thermoelastic stress and excites a coherent acoustic pulse; the reflected light from the probe pulse carries the reflectivity, phase, or polarization changes caused by the propagation of the acoustic pulse and interface reflection.

[0032] The data acquisition module 4 can use a PIN photodiode, avalanche photodiode, or balanced photodetector for photoelectric conversion, and the detection signal is recorded by an analog-to-digital acquisition card.

[0033] The data processing module 5 calculates the dynamic time axis scaling factor point by point based on the acquisition time, real-time repetition frequency and real-time repetition frequency difference recorded synchronously with the actual detection signal, and accumulates or integrates the acquisition time interval to map the actual detection signal to the reconstructed equivalent picosecond time axis; then, it performs detrending, mean filtering and peak fitting on the reconstructed detection signal to obtain the picosecond ultrasonic time response.

[0034] By employing closed-loop tracking of the target phase trajectory, the actual relative time delay is scanned within a preset effective window. By binding the acquisition time, real-time repetition rate, and real-time repetition rate difference to the detection data, and reconstructing the equivalent picosecond time axis point-by-point according to the dynamic time axis scaling factor, the use of only PZT or EOM control quantities, ideal modulation waveforms, or nominal repetition rate differences as time coordinates is avoided. These technical features collectively form a closed-loop data chain from time delay closed-loop control and real-time frequency reading to ultrasonic time axis reconstruction.

[0035] Figure 2This system is used to compare the full-cycle scanning of traditional ASOPS with the effective window scanning of the system described in this embodiment. Traditional asynchronous optical sampling maintains a constant repetition frequency difference Δf between two pulse trains, causing their relative timing phase to continuously accumulate. Fixing Δf allows the relative delay to traverse the entire repetition cycle at an approximately constant speed. In contrast, the system described in this embodiment does not use a constant Δf as the sole time reference. Instead, it employs a digital phase-locked programmable delay method to generate a target phase trajectory based on the target delay window. By changing the repetition frequency difference between the controlled optical comb and the reference optical comb, the relative delay is made to scan back and forth only within the target window from T0 to T0+T1. During the forward scanning phase, the repetition frequency of the controlled optical comb is adjusted to f. rep -Δf; During the reverse scan phase, adjust to f rep +Δf, thereby reversing the rate of change of phase difference and the direction of change of time delay. The reference repetition frequency f is continuously read during the scan. rep (t) and frequency repetition rate difference Δf rep (t) is used to determine the time axis scaling factor for each acquisition moment based on the real-time ratio of the two rather than the nominal constant Δf, and the acquisition time is reconstructed into an equivalent picosecond time axis so that the probe pulse data corresponds to the equivalent time delay within the required time delay window. It should be noted that the scanning range and scanning speed of the time delay window can be modulated using various waveforms such as triangular waves, sine waves, and sawtooth waves.

[0036] Figure 3 The left side shows a typical echo curve for a metallic film (Au). When the time interval Δt between adjacent effective acoustic echoes on the reconstructed equivalent picosecond time axis corresponds to the round-trip propagation time of the acoustic pulse within the film, the film thickness is:

[0037] ;

[0038] In the formula, v is the longitudinal sound velocity of the thin film material, and d is the thickness of the thin film.

[0039] Figure 3 The right side shows a typical Brillouin oscillation curve for a transparent dielectric film (SiO2). The start and end times of the Brillouin oscillation, the phase abrupt change points, or the propagation times corresponding to interfacial acoustic features can be determined based on the equivalent picosecond time axis reconstructed by dynamic scaling of the real-time repetition rate and the real-time repetition rate difference. This propagation time corresponds to the one-way propagation time of the acoustic pulse within the film; therefore, the film thickness can be directly calculated using the longitudinal wave velocity and the Brillouin duration.

[0040] Figure 4A specific embodiment of a digital phase-locked loop (PLL) time-programmable control module 1 is illustrated. As shown, a first photodetector 1001 and a second photodetector 1002 receive optical signals from a pump pulse train and a probe pulse train, respectively, and convert them into corresponding repetition frequency radio frequency signals. The output of the first photodetector 1001 is split into two paths after passing through a first analog-to-digital converter 1011. The first path is connected to one input of a first quadrature mixer 1031, and the second path is connected to one input of a time axis parameter generation unit 1020. The output of the second photodetector 1002 is split into two paths after passing through a second analog-to-digital converter 1012. The first path is connected to one input of a second quadrature mixer 1032, and the second path is connected to the other input of the time axis parameter generation unit 1020. The output of the time axis parameter generation unit 1020 is connected to the control terminal of a data processing module 5. The two signal outputs of an NCO digital local oscillator unit 1040 are connected to the two inputs of the first quadrature mixer 1031. The other two signal output terminals of 0 are connected to the two input terminals of the second quadrature mixer 1032; the output terminal of the first quadrature mixer 1031 is connected to one input terminal of the first subtractor 1081 after passing through the first low-pass filter 1051, the first phase calculation unit 1061 and the first phase unwrapping unit 1071 in sequence; the output terminal of the second quadrature mixer 1032 is connected to the other input terminal of the first subtractor 1081 after passing through the second low-pass filter 1052, the second phase calculation unit 1062 and the second phase unwrapping unit 1072 in sequence; the output terminal of the first subtractor 1081 is connected to one input terminal of the second subtractor 1082, the output terminal of the target phase generation unit 1090 is connected to the other input terminal of the second subtractor 1082, and the output terminal of the second subtractor 1082 is connected to the control terminal of the dual optical comb light source module 2 after passing through the PI controller 1100, the digital-to-analog converter 1110 and the high-voltage amplifier 1120 in sequence.

[0041] The first analog-to-digital converter 1011 and the second analog-to-digital converter 1012 use the same sampling clock to synchronously sample two repetition frequency radio frequency signals. Let the digital sampled signals obtained by the first analog-to-digital converter 1011 and the second analog-to-digital converter 1012 be:

[0042] ;

[0043] The time axis parameter generation unit 1020 records the pump comb repetition frequency fpump(t) and probe comb repetition frequency fprobe(t) in real time, and calculates the repetition frequency difference Δfrep(t) = fpump(t) - fprobe(t), providing data support for subsequent time axis reconstruction. The NCO digital local oscillator unit 1040 generates mutually orthogonal digital cosine signals and digital sine signals, namely:

[0044] ;

[0045] The NCO digital local oscillator unit 1040 provides mutually orthogonal digital cosine signals c and sine signals s to the first quadrature mixer 1031 and the second quadrature mixer 1032, respectively. The first quadrature mixer 1031 and the second quadrature mixer 1032 multiply the corresponding RF sampling signals with the digital cosine and sine signals, respectively. Since fNCO is close to fpump and fprobe, after filtering out the sum-frequency components by the first low-pass filter 1051 and the second low-pass filter 1052, the in-phase components and quadrature components Ipump, Qpump, Iprobe, and Qprobe corresponding to the pump light and probe light are obtained, i.e.:

[0046] ;

[0047] The first phase calculation unit 1061 and the second phase calculation unit 1062 calculate the phase respectively based on the corresponding I and Q components using a four-quadrant arctangent operation:

[0048] ;

[0049] in pump and probe Represented as the enveloping phase of the pump pulse train and the probe pulse train, its output range is only... When the true phase continuously crosses or At that time, the package phase will appear close to The numerical jump is caused by the continuous unwrapping of the wrapped phase through the first phase unwrapping unit 1071 and the second phase unwrapping unit 1072, resulting in the real-time phase φ of the pump pulse train. pump and the real-time phase φ of the probe pulse train probe The first subtractor 1081 subtracts the two continuously unwrapped phase signals to obtain the actual phase difference between the two pulse repetition frequency radio frequency signals:

[0050] ;

[0051] The target phase generation unit 1090 generates the target phase difference Δφ based on the user-defined target delay window and scanning trajectory. ref And a trigger signal, which controls the start of the data acquisition module 4. The second subtractor 1082 compares the target phase difference with the real-time actual phase difference to obtain the phase error signal:

[0052] ;

[0053] The PI controller 1100 generates a digital control quantity based on the phase error signal. The digital-to-analog converter 1110 converts the digital control quantity into an analog control voltage, which is then amplified by the high-voltage amplifier 1120 and applied to the PZT of the probe mode-locked laser to adjust the repetition frequency of the probe mode-locked laser, so that the real-time actual phase difference follows the target phase difference, thereby forming a digital phase-locked loop. The digital phase-locked loop is used to control the relative time delay scanning trajectory; the time axis reconstruction uses the synchronously read real-time repetition frequency and real-time repetition frequency difference, without using the target phase trajectory, PZT control voltage, or nominal constant repetition frequency difference as a fixed time reference.

[0054] During data acquisition, the pump optical comb is used as the reference optical comb, and the reference repetition frequency f is defined. rep (t)=f pump (t), defining the frequency repetition rate difference Δf rep (t)=f pump (t)-f probe (t), and record the acquisition time t of the i-th probe data. i Under dual-comb asynchronous optical sampling conditions, the time axis scaling factor κ(t) is calculated from the real-time repetition rate and the real-time repetition rate difference; its sign represents the forward or reverse scanning direction.

[0055] ;

[0056] ;

[0057] In the formula, τ eq (t i ) represents the i-th acquisition time t i The corresponding equivalent picosecond delay, τ eq (t0) represents the delay time corresponding to the initial acquisition time t0. For t0 to t i For any small acquisition time interval dt between t0 and t1, the equivalent delay increment is κ(t)dt; these equivalent delay increments are accumulated from t0 to t1. i and with the initial delay τ eq Adding (t0) together yields τ. eq (t i The lower limit of integration is set to t0 because timeline reconstruction begins from the initial acquisition time; the upper limit of integration is set to t. i This is because what we are looking for is the cumulative equivalent delay corresponding to the i-th acquisition time. τ eq (t0) can be determined by the zero-delay overlap time of the pump pulse and the probe pulse or other known time delay indicators. Since κ(t) is determined by the measured f at each time step... rep (t) and Δf rep(t) Recalculation: The nonlinearity of the repetition modulation is incorporated into the time mapping process, thereby eliminating or suppressing the equivalent picosecond time axis distortion caused by using a nominal constant scaling factor.

[0058] Figure 5 This is a schematic block diagram of the first pump-probe optical path module in the measurement system described in this embodiment of the invention; as follows: Figure 5 As shown, the pump light pulse 212, after being focused by the second focusing lens 312, is incident on the surface of the thin film to be tested on the sample stage 32. It is used to excite the transient thermoelastic expansion of the sample surface and generate a coherent acoustic pulse propagating along the thickness direction inside the thin film. The probe light pulse 211, after being focused by the first focusing lens 311, irradiates the same test area or its adjacent area on the sample surface with a different spatial incident path than the pump light pulse 212. It is used to detect the change in the reflectivity of the sample surface caused by the propagation, reflection, and interface echo of the acoustic pulse.

[0059] This application employs a spatial separation method to separate the pump light and the probe light. The pump light pulse 212 and the probe light pulse 211 propagate along different spatial paths before reaching the sample stage 32, and are focused by the second focusing lens 312 and the first focusing lens 311, respectively. Due to the different incident directions of the two beams, the pump light and probe light reflected from the sample surface naturally separate in space. The reflected probe light can enter the data acquisition module 4 along a predetermined direction, while the reflected pump light will not significantly overlap with the probe signal. The data acquisition module 4 acquires the transient reflectivity change signal carried by the reflected probe light and transmits the acquisition results to the data processing module 5. The data processing module 5 reconstructs the probe signal at different delay times based on the delay information provided by the digital phase-locked time programming control module 1, obtains the pump-probe time response curve, and further calculates the film thickness based on the acoustic echo time interval.

[0060] It should be noted that in actual implementation, the pump light and the probe light can be focused by two independent lenses, or they can be incident on and focused onto the sample surface by different spatial regions of the same lens. As long as the two beams can be ensured to overlap or nearly overlap on the sample surface and have a sufficient spatial separation angle after reflection, the spatial separation pump-probe measurement described in this embodiment can be realized.

[0061] Figure 6 This is a schematic block diagram of the second type of pump-probe optical path module in the measurement system described in this embodiment of the invention; as follows: Figure 6As shown, the probe light pulse 211 first passes through the first half-wave plate 313. By adjusting the optical axis direction of the first half-wave plate 313, the probe light pulse 211 is given a preset polarization state. The pump light pulse 212, after passing through the second half-wave plate 314, is also adjusted to a different polarization state than the probe light pulse 211. The pump light pulse 212 and the probe light pulse 211 are adjusted to mutually orthogonal linear polarization states so that they can be combined and separated by a polarization beam splitter. After the pump light pulse 212 is polarized by the second half-wave plate 314, its propagation direction is changed by the first reflector 31, and it is incident on the first polarization beam splitter 331. After the probe light pulse 211 is polarized by the first half-wave plate 313, it is incident on the first polarization beam splitter 331 from another direction. The first polarization beam splitter 331 transmits the probe light and reflects the pump light according to the difference in polarization states of the two beams, thereby combining the pump light pulse 212 and the probe light pulse 211. The combined beam is focused by the third focusing lens 315 and obliquely incident on the surface of the film under test on the sample stage 32. The pump pulse 212 excites the transient thermoelastic response of the sample surface, generating a coherent acoustic pulse propagating along the thickness direction within the film under test. The probe pulse 211 detects the transient reflectivity change of the sample surface caused by the propagation, reflection, and interface echo of the acoustic pulse. The beam reflected from the sample stage 32 enters the second polarization beam splitter 332. The second polarization beam splitter 332 separates the polarization of the pump and probe beams based on the difference in polarization states, allowing the probe beam carrying information about the transient reflectivity change of the sample to enter the data acquisition module 4, while simultaneously suppressing the pump beam from entering the detection channel. The data acquisition module 4 performs photoelectric conversion and sampling of the probe beam signal and transmits the acquired time-domain signal to the data processing module 5. The data processing module 5 reconstructs the detection signal at different delay times based on the delay information provided by the digital phase-locked time programming control module 1, obtains the pump-detection time response curve of the sample, and further calculates the thickness of the film under test based on the acoustic echo time interval.

[0062] It should be noted that in this embodiment, the polarization state of the pump light 212 is set to be reflected from the polarization beam splitter, and the polarization state of the probe light 211 is set to be transmitted through the polarization beam splitter. In actual implementation, the polarization state of the pump light 212 can also be set to be transmitted through the polarization beam splitter, and the polarization state of the probe light 211 can be reflected from the polarization beam splitter. As long as the polarization states of the pump light and the probe light can be guaranteed to be orthogonal and completely separated by the polarization beam splitter, the polarization separation pump-probe measurement described in this embodiment can be realized.

[0063] Figure 7This is a block diagram illustrating the principle of the third type of pump-probe optical path module in the measurement system described in this embodiment of the invention. In this embodiment, the probe light pulse 211 passes sequentially through the third half-wave plate 316 and the third polarization beam splitter 333. The third half-wave plate 316 is used to adjust the polarization direction of the probe light pulse 211, enabling it to be completely transmitted through the third polarization beam splitter 333. The pump light pulse 212 then enters the frequency doubling optical path, first being focused by the fourth focusing lens 317 into the frequency doubling crystal 33. A second harmonic conversion occurs within the frequency doubling crystal 33, converting the pump light from the fundamental wavelength to a frequency-doubled wavelength. For example, when the fundamental wavelength is 1032 nm, the pump light can be converted into approximately 516 nm green light after frequency doubling. After being collimated by the fifth focusing lens 318, the frequency-doubled pump light is then redirected by the second reflecting mirror 34 and incident on the dichroic mirror 35. The dichroic mirror 35 is used to combine pump and probe light of different wavelengths. Specifically, the probe light pulse 211 is incident horizontally onto the dichroic mirror 35 after passing through the third polarization beam splitter 333; the frequency-doubled pump light pulse 212 is deflected by the second reflecting mirror 34 and incident from another direction onto the dichroic mirror 35. Since the pump light and probe light have different wavelengths, the dichroic mirror 35 can transmit one beam and reflect the other according to its coating characteristics, thus combining the pump light pulse 212 and the probe light pulse 211 into a coaxially propagating pump-probe beam. The combined pump light and probe light pass sequentially through the quarter-wave plate 36 and the sixth focusing lens 319, and are focused onto the surface of the thin film under test on the sample stage 32. The pump light pulse 212 is absorbed by the sample surface and generates transient thermoelastic expansion, thereby exciting a coherent acoustic pulse inside the thin film; the probe light pulse 211 is used to detect the transient reflectivity change caused by the propagation, reflection, and transmission of the acoustic pulse within the thin film. The beam reflected back from the sample surface returns along the original optical path. Since the pump light and probe light have different wavelengths, the dichroic mirror 35 can selectively separate the pump light and probe light in the reflected light, thereby suppressing the pump light from entering the probe channel. The reflected probe light passes through the quarter-wave plate 36 again, and its polarization state changes relative to its incident state. After passing through the third polarization beam splitter 333, it is guided to the data acquisition module 4. The data acquisition module 4 performs photoelectric conversion and sampling on the probe light signal carrying information about the transient reflectivity change of the sample, and transmits the acquisition results to the data processing module 5. The data processing module 5 reconstructs the probe signal under different time delays based on the delay scan information provided by the digital phase-locked time programming control module 1, obtains the pump-probe time response curve, and calculates the thickness of the film under test based on the time interval of the acoustic echo.

[0064] It should be noted that in practical applications, the frequency of the probe can be doubled to keep the pump light at its fundamental wavelength, depending on the laser output power, sample absorption characteristics, and detection sensitivity. Alternatively, other wavelength combinations that meet the 35° dichroic mirror splitting condition can be selected. Considering the pump light isolation, ease of optical path adjustment, and picosecond ultrasonic signal intensity, this embodiment preferably doubles the pump light to green light and keeps the probe light at its near-infrared fundamental frequency, thereby achieving high-isolation wavelength-separated pump-probe measurement.

[0065] Furthermore, in this document, relational terms such as "first" and "second" are used merely to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Moreover, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or terminal device that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or terminal device. Without further limitations, an element defined by the phrase "comprising..." or "including..." does not exclude the presence of additional elements in the process, method, article, or terminal device that includes said element. Furthermore, in this document, "greater than," "less than," "exceeding," etc., are understood to exclude the stated number; "above," "below," "within," etc., are understood to include the stated number.

[0066] Although the above embodiments have been described, those skilled in the art, once they understand the basic inventive concept, can make other changes and modifications to these embodiments. Therefore, the above descriptions are merely embodiments of the present invention and do not limit the scope of patent protection of the present invention. Any equivalent structural or procedural transformations made using the content of the present invention's specification and drawings, or direct or indirect applications in other related technical fields, are similarly included within the scope of patent protection of the present invention.

Claims

1. A programmable time-delay dual-comb picosecond ultrasonic thin film thickness measurement system, characterized in that... include: The digital phase-locked time programming control module (1) is used to synchronously receive two repetition frequency radio frequency signals corresponding to the pump pulse train and the probe pulse train, obtain the real-time phase difference between the two pulse trains through digital phase detection, compare the real-time phase difference with the target phase difference, and adjust the repetition frequency of the probe mode-locked laser according to the error between the two; read the repetition frequency of the pump comb and the probe comb in real time, calculate the repetition frequency difference between the two, and provide the real-time repetition frequency, the real-time repetition frequency difference and the corresponding trigger signal or timestamp to the data acquisition module (4) and the data processing module (5). The dual optical comb light source module (2) is used to output pump pulses and probe pulses. The pump pulses are used to excite coherent acoustic pulses in the thin film, and the probe pulses are used to detect the transient optical response caused by the propagation of the acoustic pulses. The pump-probe optical path module (3) is used to transmit, shape, combine or spatially register pump pulses and probe pulses, focus them onto the thin film to be tested, and separate the probe light carrying transient optical response from the sample reflected light. The data acquisition module (4) is used to receive the detection light and convert it into a detection electrical signal, sample the detection electrical signal according to the acquisition trigger signal, and record the acquisition time corresponding to each detection value or each group of detection values. The data processing module (5) is used to associate each detection value or each group of detection values ​​with the corresponding acquisition time, real-time repetition rate and real-time repetition rate difference; The time axis scaling factor is calculated based on the real-time repetition frequency and the real-time repetition frequency difference, and each acquisition time interval is converted and accumulated into an equivalent time delay to obtain an equivalent picosecond time axis. Based on the equivalent picosecond time axis, the acoustic feature time is extracted from the detection signal, and the film thickness is calculated in combination with the material parameters.

2. The programmable time-delay dual-comb picosecond ultrasonic thin film thickness measurement system as described in claim 1, characterized in that: The dual-comb light source module (2) includes two lasers with a nominal repetition frequency of 50 MHz and a pulse width of 100-200 fs.

3. The programmable time-delay dual-comb picosecond ultrasonic thin film thickness measurement system as described in claim 1, characterized in that: The digital phase-locked time programming control module (1) includes a first photodetector (1001) and a second photodetector (1002), which are used to receive the optical signals of the pump pulse train and the probe pulse train, respectively, and convert them into corresponding repetition frequency radio frequency signals; the output of the first photodetector (1001) is divided into two paths after passing through the first analog-to-digital converter (1011), the first path is connected to one input of the first quadrature mixer (1031), and the second path is connected to one input of the time axis parameter generation unit (1020); the second photodetector... The output of the detector (1002) is split into two paths after passing through the second analog-to-digital converter (1012). The first path is connected to one input of the second quadrature mixer (1032), and the second path is connected to the other input of the time axis parameter generation unit (1020). The output of the time axis parameter generation unit (1020) is connected to the control terminal of the data processing module (5). The two signal outputs of the NCO digital local oscillator unit (1040) are connected to the two inputs of the first quadrature mixer (1031). The other two signal output terminals of the first quadrature mixer (1040) are connected to the two input terminals of the second quadrature mixer (1032); the output terminal of the first quadrature mixer (1031) is connected to one input terminal of the first subtractor (1081) after passing through the first low-pass filter (1051), the first phase calculation unit (1061), and the first phase unwrapping unit (1071); the output terminal of the second quadrature mixer (1032) is connected to the second low-pass filter (1052), the second phase calculation unit (1062), and the second phase unwrapping unit (1071) in sequence. (1072) is then connected to the other input terminal of the first subtractor (1081); the output terminal of the first subtractor (1081) is connected to one input terminal of the second subtractor (1082), the output terminal of the target phase generating unit (1090) is connected to the other input terminal of the second subtractor (1082), and the output terminal of the second subtractor (1082) is connected to the control terminal of the dual light comb light source module (2) after passing through the PI controller (1100), the digital-to-analog converter (1110) and the high voltage amplifier (1120) in sequence.

4. The programmable time-delay dual-comb picosecond ultrasonic thin film thickness measurement system as described in claim 1, characterized in that: The pump-probe optical path module (3) includes a first focusing lens (311) and a second focusing lens (312). The pump light pulse (212) is focused by the second focusing lens (312) and incident on the surface of the thin film to be tested on the sample stage (32) to excite the transient thermoelastic expansion of the sample surface and generate a coherent acoustic pulse propagating along the thickness direction inside the thin film. The probe light pulse (211) is focused by the first focusing lens (311) and irradiates the same area to be tested or its adjacent area on the sample surface with a different spatial incident path than the pump light pulse (212) to detect the change in the reflectivity of the sample surface caused by the propagation, reflection and interface echo of the acoustic pulse.

5. The programmable time-delay dual-comb picosecond ultrasonic thin film thickness measurement system as described in claim 1, characterized in that: The pump-probe optical path module includes a first half-wave plate (313) and a second half-wave plate (314). The probe light pulse (211) first passes through the first half-wave plate (313). By adjusting the optical axis direction of the first half-wave plate (313), the probe light pulse (211) is made to have a preset polarization state. After the pump light pulse (212) passes through the second half-wave plate (314), it is also adjusted to a different polarization state than the probe light pulse (211). The pump light pulse (212) and the probe light pulse (211) are adjusted to mutually orthogonal linear polarization states. 212) After the polarization is adjusted by the second half-wave plate (314), the propagation direction is changed by the first reflecting mirror (31) and it is incident on the first polarization beam splitter (331). The probe light pulse (211) is polarized by the first half-wave plate (313) and is incident on the first polarization beam splitter (331) from another direction. The first polarization beam splitter (331) transmits the probe light and reflects the pump light according to the difference in polarization state of the two beams, and combines the pump light pulse (212) and the probe light pulse (211). The combined beam passes through the third focusing lens (315). The light beam is focused and obliquely incident on the surface of the thin film to be tested on the sample stage (32); wherein, the pump light pulse (212) is used to excite the transient thermoelastic response of the sample surface, generating a coherent acoustic pulse propagating along the thickness direction inside the thin film to be tested; the probe light pulse (211) is used to detect the transient reflectivity change of the sample surface caused by the propagation, reflection and interface echo of the acoustic pulse; the beam reflected back from the sample stage (32) enters the second polarization beam splitter (332); the second polarization beam splitter (332) polarizes the pump light and the probe light according to the difference in polarization state between the reflected light and the probe light. Vibration separation allows the probe light carrying information about the transient reflectivity change of the sample to enter the data acquisition module (4), while suppressing the pump light from entering the probe channel; the data acquisition module (4) performs photoelectric conversion and sampling on the probe light signal, and transmits the acquired time domain signal to the data processing module (5); the data processing module (5) reconstructs the probe signal at different delay times according to the delay information provided by the digital phase-locked time programming control module (1), obtains the pump-probe time response curve of the sample, and further calculates the thickness of the film under test according to the acoustic echo time interval.

6. The programmable time-delay dual-comb picosecond ultrasonic thin film thickness measurement system as described in claim 1, characterized in that: The pump-probe optical path module includes a third half-wave plate (316). The probe light pulse (211) passes through the third half-wave plate (316) and the third polarization beam splitter (333) in sequence. The third half-wave plate (316) is used to adjust the polarization direction of the probe light pulse (211) so that it can be completely transmitted through the third polarization beam splitter (333). The pump light pulse (212) enters the frequency doubling optical path and is first focused into the frequency doubling crystal (33) by the fourth focusing lens (317). Second harmonic conversion occurs in the frequency doubling crystal (33), which converts the pump light from the fundamental wavelength to the frequency doubling wavelength. After the frequency doubling pump light is collimated by the fifth focusing lens (318), the propagation direction is changed by the second reflecting mirror (34) and it is incident on the dichroic mirror (35). The dichroic mirror (35) is used to combine pump light and probe light of different wavelengths. The probe light pulse (211) is incident horizontally onto the dichroic mirror (35) after passing through the third polarization beam splitter (333); the pump light pulse (212) after frequency doubling is refracted by the second reflecting mirror (34) and incident on the dichroic mirror (35) from another direction. The pump light and the probe light have different wavelengths. The dichroic mirror (35) transmits one of the beams and reflects the other beam according to its coating characteristics, combining the pump light pulse (212) and the probe light pulse (211) into a coaxially propagating pump-probe beam; the combined pump light and probe light pass through the quarter-wave plate (36) and the sixth focusing lens (319) in sequence, and are focused onto the surface of the thin film to be tested on the sample stage (32); the pump light pulse (212) is used to be absorbed by the sample surface and generate transient thermoelastic expansion, thereby exciting a coherent acoustic pulse inside the thin film; The probe light pulse (211) is used to detect the transient reflectivity change caused by the propagation, reflection and transmission of the acoustic pulse in the thin film. The beam reflected back from the sample surface returns along the original optical path. Since the pump light and the probe light have different wavelengths, the dichroic mirror (35) can selectively separate the pump light and the probe light in the reflected light, and suppress the pump light from entering the probe channel. After the reflected probe light passes through the quarter-wave plate (36) again, its polarization state changes relative to the incident state. After passing through the third polarization beam splitter (333), it is guided to the data acquisition module (4). The data acquisition module (4) performs photoelectric conversion and sampling on the probe light signal carrying the transient reflectivity change information of the sample, and transmits the acquisition results to the data processing module (5). The data processing module (5) reconstructs the probe signal under different time delays according to the delay scan information provided by the digital phase-locked time programming control module (1), obtains the pump-probe time response curve, and calculates the thickness of the thin film under test according to the time interval of the acoustic echo.

7. The programmable time-delay dual-comb picosecond ultrasonic thin film thickness measurement system as described in claim 1, characterized in that: The data acquisition module (4) uses a PIN photodiode, avalanche photodiode or balanced photodetector for photoelectric conversion, and the detection signal is recorded by an analog-to-digital acquisition card.

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