A high-precision laser ranging method based on two-photon time-gated balanced detection

Abstract

The application discloses a high-precision laser ranging method based on two-photon time-gated balanced detection, and belongs to the technical field of laser ranging. A pulse laser source is used to split into a reference pulse and a measurement pulse. After being reflected by a target, the measurement pulse is superimposed with the reference pulse in a beam combining path and irradiated to a photosensitive surface of a two-channel balanced detector. Only when the reference pulse and the measurement pulse are overlapped in space-time, two-photon absorption occurs on the photosensitive surface of the detection unit, a two-photon current signal related to the probability of the two pulses arriving at the same time is generated, and a two-photon balanced output signal is obtained through differential amplification. The pulse repetition frequency is scanned in a grid manner within a preset range, the two-photon time-gated window is periodically shifted relative to the measurement pulse flight time period, and a repetition frequency point meeting a predetermined criterion is formed on a repetition frequency axis. The frequency difference between adjacent repetition frequency points or the frequency offset of the same repetition frequency point in different sweep frequency periods is recorded, and the measurement pulse round-trip flight time and the target distance are solved in combination with the refractive index and the light speed constant.

Description

Technical Field

[0001] This invention belongs to the field of laser ranging technology, and specifically relates to a high-precision laser ranging method based on two-photon time-gated balanced detection. Background Technology

[0002] Laser ranging technology is one of the important foundations of modern precision measurement, and it is widely used in industrial manufacturing, engineering surveying, unmanned system navigation, space target monitoring, and 3D imaging. Based on different measurement principles, existing laser ranging methods can be broadly classified into laser interferometry, triangulation, phase measurement, and pulse-time-of-flight methods. Each method has its own advantages and disadvantages in terms of range, resolution, measurement speed, and system complexity. In practical engineering applications, a trade-off must usually be struck between measurement accuracy, measurement range, and anti-interference capability.

[0003] Existing laser ranging technologies still have significant shortcomings in balancing large range, high precision, high measurement speed, and strong anti-interference capabilities. In particular, there is a lack of a ranging scheme that, without requiring bulk nonlinear crystals and complex mode-locked lasers, can introduce two-photon time-gated control at the detector level to suppress background and linear noise, efficiently encode distance information on the repetition frequency axis, and possesses good potential for miniaturization and integration. Based on this, this invention proposes a high-precision laser ranging method based on two-photon time-gated balanced detection. By directly realizing the two-photon absorption process on the photosensitive surface of a dual-channel balanced detector, time gating and photoelectric conversion are integrated into the same device. Furthermore, by extracting the characteristic frequency points of the repetition frequency grid and the two-photon balanced output signal on it, the target distance information is mapped from the time domain to the frequency domain. This achieves high-speed, large-range, small dead-zone, and high-stability laser ranging while suppressing background noise and system drift. Summary of the Invention

[0004] The purpose of this invention is to provide a high-precision laser ranging method based on two-photon time-gated balanced detection, which addresses the problems mentioned above. It is expected to improve the existing laser ranging technology by addressing issues such as limited linear detection bandwidth, difficulty in integrating nonlinear crystals, and large ranging dead zones.

[0005] The technical solution adopted in this invention is as follows: A high-precision laser ranging method based on two-photon time-gated balanced detection, the high-precision laser ranging method comprising the following steps: Step S100: A pulsed laser source generates pulsed light, which is then divided into a reference pulse and a measurement pulse. The measurement pulse is reflected back from the target under test. Step S200: After the returned measurement pulse and the reference pulse are superimposed in the beam combining optical path, they are guided to the dual-channel balanced detector so that the two superimposed pulses respectively illuminate the photosensitive surfaces of the two semiconductor detection units of the dual-channel balanced detector. Step S300: The photosensitive surfaces of the two semiconductor detection units have two-photon absorption characteristics. Under the action of the pulse peak light intensity, the two-photon absorption process occurs only when the reference pulse and the measurement pulse overlap in time and space, generating a two-photon current signal related to the probability of the two pulses arriving simultaneously. The two two-photon current signals are input into a balanced differential amplifier circuit to obtain a two-photon balanced output signal that varies with the relative time delay of the reference pulse and the measurement pulse. Step S400: Within the preset repetition frequency scanning range, the repetition frequency of the pulsed laser source is changed by a preset step size, so that the relative arrival time of the reference pulse and the measurement pulse in the two-photon absorption region periodically drifts with the repetition frequency, thereby forming multiple repetition frequency points on the repetition frequency axis that make the two-photon balanced output signal meet the predetermined criteria. Step S500: Record the frequency value of the repetition frequency point corresponding to the predetermined criterion. Based on the frequency difference between adjacent repetition frequency points or the frequency offset of the same repetition frequency point in different scanning cycles, combined with the propagation speed of light in the medium under test, calculate the round-trip flight time of the measurement pulse, and calculate the distance between the target under test and the ranging system accordingly.

[0006] Furthermore, the pulsed laser source is used to output pulsed light with an adjustable repetition frequency; the pulsed light is split into two paths, a reference pulse and a measurement pulse, by the beam splitting and optical path control module, and the measurement pulse is emitted to the surface of the target under test and its reflected pulse is received, so that the reflected pulse and the reference pulse are superimposed in the beam combining optical path.

[0007] Furthermore, the dual-channel balanced detector includes two semiconductor photodetector units with two-photon absorption characteristics and a differential amplifier circuit; the photosensitive surfaces of the semiconductor photodetector units are respectively used to receive two superimposed pulses transmitted through the beam combining optical path; under the action of the pulse peak light intensity, a two-photon absorption process occurs on the photosensitive surface only when the reference pulse and the measurement pulse overlap in time and space, generating two two-photon current signals respectively related to the probability of simultaneous arrival of the two pulses; the differential amplifier circuit is used to differentially amplify the two two-photon current signals and output a two-photon balanced output signal.

[0008] Furthermore, at any fixed repetition frequency, the two-photon balanced output signal output by the dual-channel balanced detector exhibits a monotonically zero-crossing variation with the relative time delay between the reference pulse and the measurement pulse, forming a time-gated response; the effective time width of the time-gated response is jointly determined by the pulse width and the two-photon absorption response.

[0009] Furthermore, the beam splitting and optical path control module is equipped with an optical fiber delay line or an equivalent optical delay unit to introduce a fixed time bias in the optical path of the reference pulse or the measurement pulse. This allows the relative arrival time of the photosensitive surface of the dual-channel balanced detector of the reference pulse and the measurement pulse to oscillate back and forth around a fixed bias with the pulse interval as the period, forming a repetition frequency scale with a periodic distribution determined by the fixed time bias and the flight time of the measurement pulse on the repetition frequency axis.

[0010] Furthermore, in step S400, the frequency control module controls the repetition frequency of the pulsed laser source to vary within a predetermined range according to a preset sweep waveform, so as to construct a repetition frequency grid on the repetition frequency axis. The frequency control module controls the repetition frequency of the pulsed laser source using a coarse and fine sweep strategy. In the coarse sweep stage, a rapid scan is performed with a larger repetition frequency step size to locate at least one frequency interval in which the repetition frequency point that makes the two-photon balanced output signal meet the predetermined criteria is located. In the fine sweep stage, a fine scan is performed within the frequency interval with a smaller repetition frequency step size to collect multi-point data of the two-photon balanced output signal within the frequency interval. The data acquisition and processing module then accurately obtains the repetition frequency value that meets the predetermined criteria through interpolation or curve fitting.

[0011] Furthermore, the predetermined criterion is the zero intersection point of the two-photon balanced output signal or a preset amplitude threshold. The data acquisition and processing module performs linear or nonlinear fitting on the sampling points of the two-photon balanced output signal in the neighborhood of the zero intersection point or the preset amplitude threshold to estimate the corresponding repetition frequency position, and repeats the estimation in multiple repetition frequency scanning cycles, and statistically averages the multiple repetition frequency estimates.

[0012] Furthermore, the data acquisition and processing module is used to acquire response data between the two-photon balanced output signal and the repetition frequency on the repetition frequency grid, determine one or more repetition frequency points that make the two-photon balanced output signal meet a predetermined criterion, and calculate the distance between the target and the ranging system based on the repetition frequency or repetition frequency difference corresponding to one or more repetition frequency points.

[0013] Furthermore, the photosensitive surfaces of the two semiconductor photodetector units adopt semiconductor heterojunction, quantum well, or quantum dot structures with two-photon absorption characteristics. During the two-photon absorption process, two incident photons are absorbed simultaneously to generate electron-hole pairs and form a two-photon current. The dual-channel balanced detector differentially amplifies the two two-photon currents to suppress common-mode intensity noise and linear background light.

[0014] Furthermore, the frequency control module is used to generate sawtooth waves, triangular waves, or other periodic control signals to drive the pulsed laser source to achieve periodic frequency sweeping of the repetition frequency; and the data acquisition and processing module works synchronously with the frequency control module to acquire the complete response curve of the two-photon balanced output signal to the repetition frequency in each frequency sweep cycle.

[0015] In summary, due to the adoption of the above technical solution, the beneficial effects of the present invention are: 1. This invention achieves two-photon time gating by directly introducing two-photon absorption onto the photosensitive surface of the balanced detector. A significant response is only generated when the reference pulse and the measurement pulse overlap in time and space. This effectively suppresses linear background light and single-photon noise, making the ranging resolution mainly determined by the pulse width and the two-photon response.

[0016] 2. This invention utilizes repetitive frequency grid coding and feature frequency point extraction to map target distance information from the time domain to the frequency domain. This enables large-range measurement within a small time adjustment range, significantly reducing the ranging dead zone. At the same time, it balances ranging speed and ranging accuracy through a coarse and fine two-level frequency sweep strategy.

[0017] 3. The present invention adopts a dual-channel balanced differential structure to differentially amplify the two dual-photon current signals, which can suppress common-mode intensity fluctuations and part of the system drift, and improve the system's robustness to environmental changes and laser source power drift.

[0018] 4. This invention transfers the two-photon nonlinear process from the bulk crystal to the detector surface. Combined with fiber delay lines and a compact optical path layout, the system structure is simpler, easier to modularize and integrate, and is beneficial for miniaturizing and setting up high-stability engineering laser ranging devices. Attached Figure Description

[0019] Figure 1 This is a flowchart of the method of the present invention; Figure 2 A system schematic diagram of the high-precision laser ranging method based on two-photon time-gated balanced detection provided by the present invention; Figure 3 A graph showing the relationship between pulse width and balanced optical cross-correlation signal in the high-precision laser ranging method based on two-photon time-gated balanced detection provided by the present invention. Figure 4 The actual measured balanced optical cross-correlation signal diagram of the high-precision laser ranging method based on two-photon time-gated balanced detection provided by the present invention; Figure 5 A graph showing the relationship between the ranging accuracy and measurement time of the high-precision laser ranging method based on two-photon time-gated balanced detection provided by the present invention. Figure 6This is a schematic diagram of the pulse ranging structure of the high-precision laser ranging method based on two-photon time-gated balanced detection provided by the present invention.

[0020] Figure reference numerals: 101, frequency-agile pulsed laser; 201, beam splitter; 301, polarization delayer; 401, first two-photon detector; 501, second two-photon detector; 601, differential amplifier circuit.

[0021] 1. Continuous laser; 2. Electro-optic modulator; 3. Electro-pulse signal generator; 4. First optical amplifier; 5. Spectral broadening and pulse compression device; 6. Second optical amplifier; 7. Collimator; 8. Half-wave plate; 9. First quarter-wave plate; 10. First mirror; 11. First polarizing beam splitter; 12. Second quarter-wave plate; 13. Target object; 14. Second mirror; 15. First beam splitter; 16. Second beam splitter; 17. Second polarizing beam splitter; 18. Third mirror; 19. Fourth mirror; 20. First lens; 21. Second lens; 22. First detector; 23. Second detector; 24. Differential amplifier circuit; 25. Data acquisition card. Detailed Implementation

[0022] The present invention will now be described in detail with reference to the accompanying drawings.

[0023] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0024] Existing laser ranging methods, including but not limited to laser interferometry, triangulation, phase method, and pulse time-of-flight method, have the following shortcomings: Laser interferometry deduces optical path changes by measuring the phase or number of interference fringes, achieving ranging accuracy at the nanometer or even sub-nanometer level. However, traditional interferometry requires a stable coherent light source, a long reference optical path, and a highly stable optical platform, making it sensitive to environmental vibrations, temperature drift, and changes in air refractive index. Furthermore, the periodicity of the interference fringes introduces distance variations. Blurring usually requires the use of multi-wavelength synthesis, frequency scanning or auxiliary coding to expand the measurement range, which to some extent increases the structural complexity and calibration difficulty of the system, limiting its application in large-scale, high-dynamic environments.

[0025] Laser triangulation, based on the principles of geometric optics, deduces the distance to the object by measuring the position of a laser spot on an imaging device. It is characterized by its simple structure and low cost, making it suitable for short-distance, low-cost detection scenarios. However, the measurement accuracy of triangulation is closely related to the baseline length, optical magnification, and imaging device resolution. It is difficult to maintain a large measurement range while ensuring high resolution, and it is easily affected by non-uniform target reflectivity, surface roughness, and changes in ambient light, thus limiting its application in complex environments or long-distance high-precision measurements.

[0026] Phase-based laser ranging measures the phase of continuous or modulated light, encoding target distance information into the phase difference of the modulated signal. This can achieve sub-millimeter or even higher ranging accuracy, and the system structure is relatively compact. However, the phase-based method is sensitive to the stability of the modulation frequency, signal linearity distortion, and phase noise. Under conditions of strong background light or low-reflection targets, the signal-to-noise ratio of the echo signal is low, which can easily introduce large phase estimation errors. This typically requires a cooperative target or the use of a high-power laser and narrowband filters to improve measurement performance, increasing the complexity of system design.

[0027] Pulsed laser time-of-flight ranging (TOF) technology directly measures the time interval between the emitted and echo pulses. It boasts advantages such as relative simplicity, large measurement range, and strong adaptability, making it one of the most widely used ranging methods in engineering practice. However, the temporal resolution of TOF ranging is primarily limited by the bandwidth of the photodetector, the sampling rate of the electronic acquisition system, and overall system jitter. Typical commercial systems typically have a temporal resolution on the picosecond to nanosecond scale, corresponding to a distance resolution on the millimeter to centimeter scale. In scenarios requiring nanometer or submicron level precision, the pulsed time-of-flight method, relying solely on linear detection, is insufficient.

[0028] With the development of ultrashort pulse technology and optical frequency comb technology, high-precision laser ranging schemes based on coherent ultrashort pulses have gradually become a research hotspot. An optical frequency comb manifests as a series of equally spaced ultrashort pulse sequences in the time domain, corresponding to a comb-shaped spectrum with fixed intervals in the frequency domain. It can provide a highly coherent, high-repetition-frequency light source for interferometric measurements and cross-correlation ranging. By combining optical frequency combs with interferometry or cross-correlation techniques, researchers have proposed various large-scale, high-precision ranging schemes.

[0029] Dual-comb ranging technology uses two optical combs with slightly different repetition frequencies to map spatial distance information onto a beat frequency signal in the radio frequency domain, enabling high-speed, large-range, and high-precision ranging without introducing large-scale mechanical scanning. However, dual-comb systems typically require two highly stable mode-locked lasers or precisely synchronized light sources, resulting in complex system structures, large size, and sensitivity to temperature and vibration. Furthermore, the choice of repetition frequency difference affects the trade-off between unambiguous ranging range and measurement time, limiting its application in portable or highly disturbed environments.

[0030] To reduce system complexity, a cross-correlation ranging method based on a single optical frequency comb has been proposed. This type of method typically utilizes a femtosecond or picosecond pulse sequence generated by a single mode-locked laser. After beam splitting in the optical path, a reference pulse and a measurement pulse are formed. A nonlinear optical crystal is used to achieve sum-frequency or second-harmonic cross-correlation between the two pulses. A balanced probe structure is then used to suppress common-mode noise, thereby mapping time-of-flight information on nanosecond or even longer timescales onto a femtosecond or picosecond-level cross-correlation waveform. While this approach offers significant advantages in ranging accuracy, it still heavily relies on a mode-locked laser and a bulk nonlinear crystal, resulting in relatively high system size, power consumption, and environmental sensitivity.

[0031] In cross-correlation ranging schemes, to achieve higher time resolution and signal-to-noise ratio, bulk nonlinear crystals such as BBO, LBO, and PPKTP are often used to generate sum frequencies or second harmonics. Nonlinear crystals require strict phase-matching conditions and are sensitive to parameters such as the polarization, temperature, and incident angle of the incident beam. In practical systems, precise temperature control and optical path alignment mechanisms are often necessary. Furthermore, since nonlinear crystals are typically placed in the main optical path, further miniaturization is difficult, posing obstacles to system miniaturization, integrated packaging, and mass production.

[0032] On the other hand, existing high-precision ranging systems mostly use linear photodetectors to convert light intensity signals into electrical signals. For noise components such as strong background light, scattered light, and laser source intensity drift, they can only rely on optical filtering and circuit compensation to suppress them. Linear detection lacks intrinsic time gating capability and cannot fundamentally distinguish between "temporally overlapping signal light" and "background light in non-gated time periods." In complex environments or under strong background light conditions, the signal-to-noise ratio of the ranging signal is difficult to further improve, which is particularly prominent in long-distance measurement, non-cooperative target detection, and atmospheric turbulence environments.

[0033] In recent years, some schemes have attempted to achieve time-gated detection using two-photon absorption or other higher-order nonlinear effects. However, these schemes often employ the method of inserting bulk nonlinear crystals or waveguide devices into the main optical path, which still presents problems such as difficulty in aligning nonlinear elements, high losses, temperature sensitivity, and separation from the photoelectric detection circuit. These factors are detrimental to the compactness and modular integration of the system, while also increasing the complexity of maintenance and calibration.

[0034] Therefore, existing laser ranging technologies still have significant shortcomings in balancing large range, high precision, high measurement speed, and strong anti-interference capabilities. In particular, there is a lack of a ranging scheme that, without requiring bulk nonlinear crystals and complex mode-locked lasers, can introduce two-photon time-gated control at the detector level to suppress background and linear noise, efficiently encode distance information on the repetition frequency axis, and possess good potential for miniaturization and integration. Based on this, this invention proposes a high-precision laser ranging method based on two-photon time-gated balanced detection. By directly realizing the two-photon absorption process on the photosensitive surface of a dual-channel balanced detector, time gating and photoelectric conversion are integrated into the same device. Furthermore, by extracting the characteristic frequency points of the repetition frequency grid and the two-photon balanced output signal on it, the target distance information is mapped from the time domain to the frequency domain. This achieves high-speed, large-range, small dead-zone, and high-stability laser ranging while suppressing background noise and system drift.

[0035] Example 1 like Figures 1-5 As shown, one embodiment of the present invention is a high-precision laser ranging method based on two-photon time-gated balanced detection. The high-precision laser ranging method includes the following steps: Step S100: A pulsed laser source with an adjustable repetition frequency generates pulsed light and splits the pulsed light into two paths: a reference pulse and a measurement pulse. The measurement pulse is reflected back from the target under test. Step S200: After the returned measurement pulse and the reference pulse are superimposed in the beam combining optical path, they are guided to the dual-channel balanced detector so that the two superimposed pulses respectively illuminate the photosensitive surfaces of the two semiconductor detection units of the dual-channel balanced detector. Step S300: The photosensitive surfaces of the two semiconductor detection units have two-photon absorption characteristics. Under the action of the pulse peak light intensity, the two-photon absorption process occurs only when the reference pulse and the measurement pulse overlap in time and space, generating a two-photon current signal related to the probability of the two pulses arriving simultaneously. The two two-photon current signals are input into the balanced differential amplifier circuit 601 to obtain a two-photon balanced output signal that varies with the relative time delay of the reference pulse and the measurement pulse. Pulse-balanced ranging is a common method based on relative time delay measurement in the time domain. Its working principle is as follows: an ultrashort pulse sequence is injected into a Michelson interferometer optical path with unequal arm lengths. The reference pulse sequence is directed towards the scanning reference mirror, while the measurement pulse sequence is directed towards the target mirror. The two beams recombine at the beam splitter position and interfere. The interference signal is received by a photodetector and displayed on a data acquisition card. The expression for its intensity signal is:

[0036] in, The relative delay between the two pulses, For the time-domain light intensity of the reference light pulse, To detect the temporal intensity of light.

[0037] Based on cross-correlation signals The expression for the strength of the two pulses is the relative delay. The cross-correlation signal of the optical pulse is a function of the fundamental frequency light intensity and is also affected by fluctuations in the fundamental frequency light intensity. The cross-correlation signal of the optical pulse can be generated through the sum-frequency effect. To eliminate the influence of fundamental frequency light intensity fluctuations, a balanced cross-correlation method is used: by generating two cross-correlation processes, a fixed delay is introduced into one of them. As a reference delay, the two cross-correlation signals are subtracted in a balanced detector to cancel out the influence of fundamental frequency pulse intensity fluctuations on the cross-correlation signal. At this point, the intensity of the cross-correlation signal is determined only by the initial relative delay between the reference pulse and the measurement pulse, thus achieving accurate measurement of the time interval between the two pulses. Unlike traditional methods, this method uses an optical fiber delay line to adjust the time delay between the reference pulse and the measurement pulse, replacing traditional nonlinear optical elements. In this process, the fixed delay introduced by the optical fiber delay line... This reference delay ensures the accuracy and stability of the balanced cross-correlation signal. Ultimately, the strength of the balanced cross-correlation signal is determined solely by the time delay between the two pulses. The decision enabled the precise measurement of the pulse flight time.

[0038] After the measurement pulse and reference pulse are combined, the time delay of the pulses is precisely adjusted using an optical fiber delay line to generate two balanced cross-correlation signals. These two signals are received by a balanced detector, which converts them into electrical signals and performs differential amplification before outputting them. Through this process, the balanced cross-correlation signal output by the balanced detector can be expressed as:

[0039] in, These are the pulse widths of the reference pulse and the measurement pulse, respectively. These represent the peak light intensities of the two pulses, respectively. For nonlinear crystal thickness, It is a fixed delay generated by two pulses passing through the crystal, which is related to the length of the nonlinear crystal and the group velocity of the two pulses with perpendicular polarization states in the crystal; The relative time delay between the two pulses.

[0040] Step S400: Within the preset repetition frequency scanning range, the repetition frequency of the pulsed laser source is changed by a preset step size, so that the relative arrival time of the reference pulse and the measurement pulse in the two-photon absorption region periodically drifts with the repetition frequency, thereby forming multiple repetition frequency points on the repetition frequency axis that make the two-photon balanced output signal meet the predetermined criteria. Figure 2 This diagram illustrates the principle of the optical balanced cross-correlation method for frequency-agile pulsed lasers. Based on this principle, by changing the repetition frequency of the pulses, the time interval between the reference and measurement pulses is adjusted, thereby finding the point where the balanced cross-correlation signal intensity is zero. The pulse flight time of the measurement pulse is then... Lock to pulse interval The frequency of the pulsed light is an integer multiple of the given frequency. Assume the repetition frequency of the pulsed light is... The time-domain period of the corresponding pulse for:

[0041] Let the target distance be D. To measure the pulse flight time, then

[0042] Where c is the speed of light in vacuum, and n is the refractive index of the space to be measured. When the equilibrium cross-correlation signal intensity is zero, the flight time of the measurement pulse is... Locked to pulse time domain period Integer multiples of m, that is:

[0043] Where m is a positive integer, and combining this with the formula for the target distance D, we have:

[0044] For a given target distance, there is more than one repetition frequency that satisfies the condition that the balanced cross-correlation signal strength is zero, and the multiple m is related to the repetition frequency. One-to-one correspondence. This means that for the same target distance, there are multiple repetition frequencies. This makes the balanced cross-correlation signal zero, with each frequency corresponding to a specific integer multiple m. Assume the repetition frequencies of two adjacent zeros are respectively... and 2, then we have Therefore, we can obtain: ,in - 2 represents the difference in repetition frequency between two adjacent zeros.

[0045] Combining the formula for target distance D, the target distance is obtained as follows:

[0046] in, For the target distance, The speed of light in a vacuum. The refractive index of air, To balance the frequency difference between adjacent zero-point signals in cross-correlation.

[0047] The target distance can be accurately calculated by precisely measuring the repetition frequency difference between adjacent zeros of the balanced cross-correlation signal. During the frequency sweep of the frequency-agile pulsed laser 101, a large number of balanced cross-correlation signals are generated, which are recorded in real time by a data acquisition card. The time points when the intensity of adjacent balanced cross-correlation signals is zero are extracted from the data acquisition card, and the time difference between them is calculated to obtain the time interval Δt between adjacent zeros. Combining the frequency sweep parameters of the frequency-agile pulsed laser 101, the time interval Δt is converted into a frequency interval Δf, and thus the repetition frequency difference is obtained.

[0048] The frequency conversion formula is as follows:

[0049] In the formula This refers to the frequency change generated by the frequency sweep of the 101 frequency-agile pulsed laser. The time interval between adjacent zero points in the balanced cross-correlation signal sampled and displayed by the data acquisition card. The frequency sweep period (the reciprocal of the frequency sweep rate); Furthermore, the output signal of the balanced detector can be split into two paths: one connected to a data acquisition card, and the other connected to a frequency counter. In this way, the repetition frequency value corresponding to the zero-value balance cross-correlation signal can be directly read. By recording the repetition frequencies at two consecutive zero-point locations and calculating the difference, the repetition frequency difference between adjacent zero points can be obtained. .

[0050] Step S500: Record the frequency value of the repetition frequency point corresponding to the predetermined criterion. Based on the frequency difference between adjacent repetition frequency points or the frequency offset of the same repetition frequency point in different scanning cycles, combined with the propagation speed of light in the medium under test, calculate the round-trip flight time of the measurement pulse, and calculate the distance between the target under test and the ranging system accordingly.

[0051] The distance formula is as follows:

[0052] Figure 5 To demonstrate the ranging accuracy of the ranging method in this embodiment, the ranging accuracy of this method can reach nm. The magnitude of that, compared to the accuracy of traditional ranging methods ( (on the order of magnitude), an increase of three orders of magnitude. Figure 4 To demonstrate the high-speed measurement capability of this ranging method, the measurement speed can be less than [specified value]. The speed of this method is improved by seven orders of magnitude compared to traditional ranging methods (on the order of 1), thus offering advantages in both ranging accuracy and speed.

[0053] like Figure 2 As shown, the frequency-agile pulsed laser 101 outputs pulses with agile frequency, which generate cross-correlation signals through a crystal and are detected by a balanced detector. This avoids the influence of pulse intensity fluctuations and reduces unnecessary drift and common noise.

[0054] The output laser pulse width of the electro-optic modulated optical comb is 100 ps to 10 fs. Figure 3 For the two-photon balanced cross-correlation signals generated under different pulses, from Figure 3 It is known that the peak power is relatively high, thus it can generate a two-photon balanced cross-correlation signal with a higher peak value and a larger zero-point slope, thereby improving measurement accuracy and sensitivity.

[0055] The detector is a two-photon detector, including a first two-photon detector 401 and a second two-photon detector 501, and has a semiconductor heterojunction, quantum well or quantum dot structure with two-photon absorption characteristics.

[0056] Electro-optic modulation optical frequency combing with highly nonlinear fiber spectral broadening and single-mode fiber pulse compression achieves... Pulse light of magnitude.

[0057] Example 2 like Figure 6 As shown, another embodiment of the present invention is a laser ranging system, including a pulsed laser source with adjustable repetition frequency, a beam splitting and optical path control module, a dual-channel balanced detector, a frequency control module, and a data acquisition and processing module; the output light of the continuous laser 1 is input into an electro-optic modulator as seed light, and an electric pulse signal generator 3 generates an electrical narrow pulse with a repetition frequency agile. The agile electric pulse is input into the electro-optic modulator 2 to modulate the seed light, generating a frequency agile electro-optic comb. The electro-optic comb is optically amplified by a first optical amplifier 4, and the amplified light enters a spectral broadening and pulse compression device 5. At this time, the compressed light is a femtosecond-level optical pulse.

[0058] The femtosecond pulse light re-enters the second optical amplifier 6 for optical amplification. The output light enters the collimator 7 and is input into space. After being polarized by the half-wave plate 8, it is split into two paths by the first polarizing beam splitter 11. One path serves as the reference light and passes through the first quarter-wave plate 9 and the first reflector 10. The other path serves as the probe light and passes through the second quarter-wave plate 12 to hit the target object 13. Subsequently, the two beams are reflected and split by the first polarizing beam splitter 11 to hit the second reflector 14. After being split by the first beam splitter 15, the first path passes through the first lens 20 and enters the first detector 22, while the other path passes through the second beam splitter 16 and then through the second polarizing beam splitter 17 to separate the probe beam and the reference beam. These beams then strike the third reflector 18 and the fourth reflector 19, respectively, and their relative displacement is adjusted. The reflected beams then pass through the second beam splitter 16 and the second lens 21 and enter the second detector 23. The beams then combine with the signal received by the first detector 22 to generate two-photon signals. The two signals are differentially amplified to form a balanced two-photon signal, which finally enters the data acquisition card 25 for data and signal processing. It should be noted that the reference numerals "first + component" and "second + component" in the attached drawings only distinguish the same components in the drawings and do not represent a distinction in their grade or other similarities.

[0059] The pulsed laser source is used to output pulsed light with an adjustable repetition frequency; the beam splitting and optical path control module is used to split the pulsed light into two paths: a reference pulse and a measurement pulse, and to emit the measurement pulse to the surface of the target under test and receive its reflected pulse, so that the reflected pulse and the reference pulse are superimposed in the beam combining optical path.

[0060] The dual-channel balanced detector includes two semiconductor photodetector units with two-photon absorption characteristics and 601; the photosensitive surfaces of the semiconductor photodetector units are respectively used to receive two superimposed pulses transmitted through the beam combining optical path; under the action of the pulse peak light intensity, the two-photon absorption process occurs only when the reference pulse and the measurement pulse overlap in time and space, generating two two-photon current signals respectively related to the probability of simultaneous arrival of the two pulses; 601 is used to differentially amplify the two two-photon current signals and output a two-photon balanced output signal.

[0061] The frequency control module is used to control the repetition frequency of the pulsed laser source to vary within a predetermined range according to a preset sweep waveform, so as to construct a repetition frequency grid on the repetition frequency axis.

[0062] The data acquisition and processing module is used to acquire response data between the two-photon balanced output signal and the repetition frequency on the repetition frequency grid, determine one or more repetition frequency points that make the two-photon balanced output signal meet a predetermined criterion, and calculate the distance between the target under test and the ranging system based on the repetition frequency or repetition frequency difference corresponding to the one or more repetition frequency points.

[0063] At any fixed repetition frequency, the two-photon balanced output signal of the dual-channel balanced detector exhibits a monotonically zero-crossing change with the relative time delay between the reference pulse and the measurement pulse, forming a time-gated response. The effective time width of the time-gated response is determined by both the pulse width and the two-photon absorption response. The ranging resolution is determined by the time width and signal-to-noise ratio of the time-gated response, and is essentially independent of the actual measured distance.

[0064] The beam splitting and optical path control module is equipped with an optical fiber delay line or equivalent optical delay unit to introduce a fixed time bias in the optical path of the reference pulse or the measurement pulse. This allows the relative arrival time of the reference pulse and the measurement pulse at the photosensitive surface of the dual-channel balanced detector to oscillate back and forth around a fixed bias with the pulse interval as the period when the pulse repetition frequency is changed. This forms a repetition frequency scale with a periodic distribution determined by the fixed time bias and the flight time of the measurement pulse on the repetition frequency axis.

[0065] The frequency control module controls the repetition frequency of the pulsed laser source using a two-stage coarse and fine sweep strategy. In the coarse sweep stage, a rapid scan is performed with a larger repetition frequency step size to locate at least one frequency range containing a repetition frequency point that satisfies a predetermined criterion for the two-photon balanced output signal. In the fine sweep stage, a fine scan is performed within the frequency range with a smaller repetition frequency step size to collect multi-point data of the two-photon balanced output signal within the frequency range. The data acquisition and processing module then accurately calculates the repetition frequency value that satisfies the predetermined criterion through interpolation or curve fitting.

[0066] The predetermined criterion is the zero intersection point of the two-photon balanced output signal or a preset amplitude threshold. The data acquisition and processing module performs linear or nonlinear fitting on the sampling points of the two-photon balanced output signal in the neighborhood of the zero intersection point or the preset amplitude threshold to estimate the corresponding repetition frequency position. The estimation process is repeated in multiple repetition frequency scanning cycles. The multiple repetition frequency estimates are statistically averaged to reduce the impact of random noise and system jitter on the ranging results.

[0067] The photosensitive surfaces of the two semiconductor photodetector units adopt semiconductor heterojunction, quantum well or quantum dot structure with two-photon absorption characteristics. During the two-photon absorption process, two incident photons are absorbed simultaneously to generate electron-hole pairs and form a two-photon current. The dual-channel balanced detector differentially amplifies the two two-photon currents to suppress common-mode intensity noise and linear background light.

[0068] The frequency control module is used to generate sawtooth waves, triangular waves or other periodic control signals to drive the pulsed laser source to achieve periodic frequency sweeping of the repetition frequency. The data acquisition and processing module works synchronously with the frequency control module to obtain the complete response curve of the two-photon balanced output signal to the repetition frequency in each frequency sweeping cycle.

[0069] This embodiment achieves two-photon time gating by directly introducing two-photon absorption onto the photosensitive surface of the balanced detector. A significant response is only generated when the reference pulse and the measurement pulse overlap in time and space, which can effectively suppress linear background light and single-photon noise, so that the ranging resolution is mainly determined by the pulse width and the two-photon response.

[0070] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A high-precision laser ranging method based on two-photon time-gated balanced detection, characterized in that, High-precision laser ranging methods include the following steps: Step S100: A pulsed laser source generates pulsed light, which is then divided into a reference pulse and a measurement pulse. The measurement pulse is reflected back from the target under test. Step S200: After the returned measurement pulse and the reference pulse are superimposed in the beam combining optical path, they are guided to the dual-channel balanced detector so that the two superimposed pulses respectively illuminate the photosensitive surfaces of the two semiconductor detection units of the dual-channel balanced detector. Step S300: The photosensitive surfaces of the two semiconductor detection units have two-photon absorption characteristics. Under the action of the peak pulse light intensity, the two-photon absorption process occurs in their respective photosensitive surface regions only when the reference pulse and the measurement pulse overlap in time and space, generating a two-photon current signal related to the probability of the two pulses arriving at the same time. Two dual-photon current signals are input into a balanced differential amplifier circuit to obtain a dual-photon balanced output signal that varies with the relative time delay between the reference pulse and the measurement pulse. Step S400: Within the preset repetition frequency scanning range, the repetition frequency of the pulsed laser source is changed by a preset step size, so that the relative arrival time of the reference pulse and the measurement pulse in the two-photon absorption region periodically drifts with the repetition frequency, thereby forming multiple repetition frequency points on the repetition frequency axis that make the two-photon balanced output signal meet the predetermined criteria. Step S500: Record the frequency value of the repetition frequency point corresponding to the predetermined criterion. Based on the frequency difference between adjacent repetition frequency points or the frequency offset of the same repetition frequency point in different scanning cycles, combined with the propagation speed of light in the medium under test, calculate the round-trip flight time of the measurement pulse, and calculate the distance between the target under test and the ranging system accordingly.

2. The high-precision laser ranging method based on two-photon time-gated balanced detection according to claim 1, characterized in that, The pulsed laser source is used to output pulsed light with an adjustable repetition frequency. The pulsed light is split into two paths, a reference pulse and a measurement pulse, by the beam splitting and optical path control module. The measurement pulse is emitted to the surface of the target and the reflected pulse is received, so that the reflected pulse and the reference pulse are superimposed in the beam combining optical path.

3. The high-precision laser ranging method based on two-photon time-gated balanced detection according to claim 1, characterized in that, The dual-channel balanced detector includes two semiconductor photodetector units with two-photon absorption characteristics and a differential amplifier circuit; the photosensitive surfaces of the semiconductor photodetector units are respectively used to receive the two superimposed pulses transmitted through the beam combining optical path; Under the influence of the peak pulse light intensity, a two-photon absorption process occurs on the photosensitive surface only when the reference pulse and the measurement pulse overlap in time and space, generating two two-photon current signals with a probability related to the simultaneous arrival of the two pulses. The differential amplifier circuit is used to differentially amplify the two two-photon current signals and output a two-photon balanced output signal.

4. The high-precision laser ranging method based on two-photon time-gated balanced detection according to claim 3, characterized in that, At any fixed repetition frequency, the two-photon balanced output signal of the dual-channel balanced detector exhibits a monotonically zero-crossing change with the relative time delay between the reference pulse and the measurement pulse, forming a time-gated response; the effective time width of the time-gated response is jointly determined by the pulse width and the two-photon absorption response.

5. A high-precision laser ranging method based on two-photon time-gated balanced detection according to claim 2, characterized in that, The beam splitting and optical path control module is equipped with an optical fiber delay line or equivalent optical delay unit to introduce a fixed time bias in the optical path of the reference pulse or the measurement pulse. This allows the relative arrival time of the reference pulse and the measurement pulse at the photosensitive surface of the dual-channel balanced detector to oscillate back and forth around a fixed bias with the pulse interval as the period. This forms a repetition frequency scale with a periodic distribution determined by the fixed time bias and the flight time of the measurement pulse on the repetition frequency axis.

6. The high-precision laser ranging method based on two-photon time-gated balanced detection according to claim 1, characterized in that, In step S400, the frequency control module controls the repetition frequency of the pulsed laser source to vary within a predetermined range according to a preset sweep waveform, so as to construct a repetition frequency grid on the repetition frequency axis. The frequency control module controls the repetition frequency of the pulsed laser source using a coarse and fine sweep strategy. In the coarse sweep stage, a rapid scan is performed with a larger repetition frequency step size to locate at least one frequency interval containing a repetition frequency point that satisfies a predetermined criterion for the two-photon balanced output signal. In the fine sweep stage, a fine scan is performed within the frequency interval using a smaller repetition frequency step size to collect multi-point data of the two-photon balanced output signal within the frequency interval. The data acquisition and processing module then accurately calculates the repetition frequency value that satisfies the predetermined criterion through interpolation or curve fitting.

7. A high-precision laser ranging method based on two-photon time-gated balanced detection according to claim 6, characterized in that, The predetermined criterion is the zero intersection point of the two-photon balanced output signal or a preset amplitude threshold. The data acquisition and processing module performs linear or nonlinear fitting on the sampling points of the two-photon balanced output signal in the neighborhood of the zero intersection point or the preset amplitude threshold to estimate the corresponding repetition frequency position. The estimation is repeated in multiple repetition frequency scanning cycles, and the multiple repetition frequency estimates are statistically averaged.

8. A high-precision laser ranging method based on two-photon time-gated balanced detection according to claim 6, characterized in that, The data acquisition and processing module is used to acquire response data between the two-photon balanced output signal and the repetition frequency on the repetition frequency grid, determine one or more repetition frequency points that make the two-photon balanced output signal meet a predetermined criterion, and calculate the distance between the target and the ranging system based on the repetition frequency or repetition frequency difference corresponding to one or more repetition frequency points.

9. A high-precision laser ranging method based on two-photon time-gated balanced detection according to claim 3, characterized in that, The photosensitive surfaces of the two semiconductor photodetector units adopt semiconductor heterojunction, quantum well or quantum dot structures with two-photon absorption characteristics. During the two-photon absorption process, two incident photons are absorbed simultaneously to generate electron-hole pairs and form a two-photon current. The dual-channel balanced detector differentially amplifies the two two-photon currents to suppress common-mode intensity noise and linear background light.

10. A high-precision laser ranging method based on two-photon time-gated balanced detection according to claim 6, characterized in that, The frequency control module is used to generate sawtooth waves, triangular waves or other periodic control signals to drive the pulsed laser source to achieve periodic frequency sweeping of the repetition frequency; and the data acquisition and processing module works synchronously with the frequency control module to acquire the complete response curve of the two-photon balanced output signal to the repetition frequency in each frequency sweeping cycle.

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