High-precision terahertz optical comb ranging system and method based on balance cross-correlation

Abstract

The invention provides a high-precision terahertz optical comb distance measuring system and method based on balance cross correlation, and the system comprises a repetition frequency tunable terahertz optical comb generation module, one output end of which is connected with a terahertz photoconductive antenna to excite the photoconductive antenna to generate terahertz pulses, and the other output end of which is connected with the terahertz optical comb generation module; the other path of output end is connected with the input end of an optical frequency multiplication module, and the optical frequency multiplication module outputs a first sampling light pulse and a second sampling light pulse, and the first sampling light pulse and the second sampling light pulse are input into a balance cross-correlation electro-optical sampling module; the flight time ranging module comprises an off-axis parabolic mirror, a beam splitter, a target mirror and a reference mirror; terahertz pulses generated by a photoconductive antenna focus light beams through an off-axis parabolic mirror, divergent light is converted into parallel terahertz pulses, and the parallel terahertz pulses are divided into two paths of terahertz optical comb pulses through a beam splitter; one path is reflected by a beam splitter, enters a measuring light path, is vertically reflected by a target mirror, then is transmitted out of the beam splitter, and enters a balanced cross-correlation electro-optical sampling module as a terahertz pulse to be measured; the other path is transmitted by a beam splitter, enters a reference light path, is reflected by a reference mirror and then enters a balanced cross-correlation electro-optical sampling module as a reference terahertz pulse; the output end of the balance cross-correlation electro-optical sampling module is connected with the input end of the optical balance detector; and the signal acquisition and processing module is connected with the output end of the optical balance detector. According to the invention, nano-scale measurement precision can be realized.

Description

Technical Field

[0001] The present invention belongs to the technical field of laser precision ranging, and in particular relates to a high-precision terahertz optical comb ranging system and method based on balanced cross-correlation. Background Art

[0002] Laser ranging is a very important component in the field of modern precision measurement. Compared with other ranging methods, laser ranging has the advantages of high sensitivity, high resolution, strong directionality, good monochromaticity and strong anti-interference. It is widely used in the aerospace field, military field and advanced manufacturing field.

[0003] In recent years, due to the emergence of femtosecond optical frequency combs, which appear as a series of ultrashort pulses with pulse widths in the femtosecond order in the time domain and as a series of single chromatographic lines with equal frequency intervals and extremely wide spectral range in frequency, and have the advantages of high peak power and narrow pulse width, they are widely used in the field of laser ranging, bringing great changes to the field of precision measurement.

[0004] In high-precision optical frequency comb time-domain ranging technology, time-of-flight ranging is a more common distance measurement method in laser ranging. Its principle is to convert time information into distance information by measuring the round-trip flight of light pulses in the measurement optical path, thereby obtaining the measurement result.

[0005] The time-of-flight ranging method has the characteristics of strong anti-interference ability and large measurement range, but its measurement accuracy is limited by the response bandwidth of the detection device, and its time resolution can only reach the picosecond level. In 2009, Coddington of the National Institute of Standards and Technology of the United States first proposed the dual-comb asynchronous sampling method, which realizes the measurement of absolute distance. When combined with the time-of-flight measurement, its measurement accuracy reaches the micron level.

[0006] Terahertz waves (THz) are electromagnetic waves with a frequency of 0.1-10THz. In the low-frequency THz band, due to its position in the electromagnetic spectrum close to the microwave band, its measurement accuracy is relatively low; in the high-frequency THz band, due to its shorter wavelength, the accuracy is relatively high, and it is more suitable for distance measurement. The measurement accuracy of the time-of-flight method depends on the accuracy of the time interval measurement. Due to the performance of the photodetector, it can only distinguish the time in the picosecond level. And because the pulse width of the terahertz wave is in the picosecond level, if the terahertz wave is used as a distance measurement electromagnetic wave, its measurement accuracy can only be in the micrometer level. Summary of the invention

[0007] The purpose of the present invention is to propose a high-precision terahertz optical comb time-of-flight ranging system and method based on balanced cross-correlation, in view of the fact that it is difficult to achieve nanometer-level measurement accuracy when using terahertz waves for ranging due to the limitations of time domain pulse width and wavelength. The technical solution adopted is as follows:

[0008] High-precision terahertz optical comb time-of-flight ranging system based on balanced cross-correlation, including:

[0009] Repetition rate tunable terahertz optical comb generation module, time-of-flight ranging module, balanced cross-correlation electro-optical sampling module, and signal acquisition and processing module.

[0010] The repetition frequency tunable terahertz optical comb generation module comprises a near-infrared electro-optical frequency comb, a terahertz photoconductive antenna, and an optical frequency doubling module.

[0011] The repetition rate tunable electro-optical frequency comb has a repetition rate tuning range covering the kHz-GHz range and has two output ports.

[0012] One path is connected to the input end of the terahertz photoconductive antenna, and is used to excite the terahertz photoconductive antenna to generate a terahertz light comb, whose repetition frequency is consistent with the near-infrared electro-optical frequency comb;

[0013] The other path passes through the optical frequency doubling module, and two beams of light with equal wavelengths are generated as sampling light through optical frequency doubling.

[0014] The time-of-flight ranging module comprises an off-axis parabolic mirror, a beam splitter, a delay line, a target mirror, and a reference mirror.

[0015] The generated terahertz light comb pulse converts the divergent light into parallel terahertz pulses through an off-axis parabolic mirror, and the beam splitter splits the terahertz light comb pulse into two terahertz light comb pulses.

[0016] One path is reflected by the beam splitter, reflected by the target mirror and then transmitted out of the beam splitter as the terahertz pulse to be measured; the other path is transmitted through the beam splitter, reflected by the reference mirror and used as the reference terahertz pulse.

[0017] The target mirror is used as a reflector to be measured, and the flight time of the terahertz pulse to be measured is changed by moving the position of the target mirror. The terahertz pulse to be measured and the reference terahertz pulse are respectively reflected by the off-axis parabolic mirror and transmitted in opposite collinear directions, and converge at the same focus.

[0018] The balanced cross-correlation electro-optical sampling module comprises an optical delay line, a polarization beam splitter, a quarter wave plate, an off-axis parabolic mirror with a hole, an electro-optical crystal, and a balanced photoelectric detector.

[0019] The two sampling lights are respectively injected into the electro-optical crystal together with the measured and reference terahertz light comb pulses, and the electro-optical crystal is placed at the focus.

[0020] The optical delay line is used to adjust the time delay between the sampling light and the terahertz light comb pulse, wherein the reference terahertz pulse and the sampling pulse spatial delay are completely synchronized, and the spatial delay between the terahertz pulse to be measured and the sampling pulse is adjusted by the delay line.

[0021] The off-axis parabolic mirror with holes is used to converge the terahertz signal and allow the sampling light to pass through the parabolic mirror with holes, the 1 / 4 wave plate is used to adjust the polarization state of the sampling light, the polarization beam splitter is used to perform polarization beam splitting on the sampling light, and the balanced photodetector is used to detect the sampling light after being split by the polarization beam splitter and perform intensity balance detection.

[0022] The signal acquisition and processing module includes a data acquisition card.

[0023] The data acquisition card is connected to the output end of the balanced detector.

[0024] The data acquisition card performs high-speed digital signal analysis and processing on the optical balanced cross-correlation signal, thereby obtaining a distance measurement result.

[0025] A high-precision terahertz optical comb time-of-flight ranging method based on balanced cross-correlation comprises the following steps:

[0026] Step 1: Use a near-infrared electro-optical comb as a pump light source to generate terahertz optical comb pulses with tunable repetition frequency.

[0027] The near-infrared electro-optic comb has the characteristics of continuously tunable repetition frequency, large adjustable range of repetition frequency, low noise and good anti-interference ability.

[0028] The near-infrared electro-optical frequency comb outputs an optical pulse with a central wavelength of 1560 nm and a pulse width better than 100 fs.

[0029] According to the working parameters of the terahertz photoconductive antenna, the repetition frequency tuning range of the near-infrared electro-optic comb is set to 20MHz-250MHz.

[0030] One of the output ends of the electro-optical frequency comb is connected to a photoconductive antenna through an optical fiber, and the photoconductive antenna generates terahertz optical comb pulses with a spectrum covering 0.05-4THz and a pulse width of about 1.5ps.

[0031] The other output end of the near-infrared electro-optical frequency comb is connected to an optical frequency doubling module to frequency-double the laser light with a central wavelength of 1560 nm into a sampling light with a wavelength of 780 nm.

[0032] Step 2: Use free-space electro-optic sampling technology to detect terahertz pulses.

[0033] Free-space electro-optic sampling technology is an indirect terahertz coherent detection technology with high signal-to-noise ratio and wide spectrum response.

[0034] When the terahertz pulse generated in step 1 and the sampling light pulse act on the electro-optical crystal at the same time and in a collinear manner, the terahertz electric field causes the electro-optical crystal to produce a transient birefringence effect, thereby changing the polarization state of the sampling light pulse. The refractive index of the electro-optical crystal changes with the intensity of the terahertz pulse electric field, so the polarization state of the sampling light pulse is modulated by the terahertz electric field.

[0035] In order to detect the polarization state of the sampling pulse, a 1 / 4 wave plate and a polarization beam splitter are added after the electro-optical crystal to measure the ellipticity of the sampling pulse. After passing through the 1 / 4 wave plate and the polarization beam splitter, the polarization beam splitter will divide the polarization of the sampling light pulse into s polarization (vertical polarization) and p polarization (horizontal polarization), where the light intensity difference between s polarization and p polarization is proportional to the terahertz electric field intensity, and the terahertz electric field intensity at different time delays is different. Therefore, the detected sampling pulse intensity (the intensity of s light) is a quantity linearly related to the terahertz time delay, which can be used to reflect the time delay characteristics of the sampling pulse and the transient terahertz electric field.

[0036] Step 3: Use the balanced cross-correlation method to convert the measurement of the optical signal into the measurement of the electrical signal.

[0037] The two sampling pulses modulated by the terahertz electric field are split by a polarization beam splitter and input into the two ends of a balanced detector respectively. Two mutually matched photodiodes are used inside the balanced detector for photoelectric conversion. By differentially amplifying the current signals output by the two photodiodes, a cross-correlation signal can be obtained. The intensity of this cross-correlation signal is not affected by the signal intensity fluctuation and is only related to the relative intensity of the two sampling optical pulse signals.

[0038] When two terahertz pulses with different flight times act on the electro-optical crystal together with the sampling pulse, the sampling pulses are modulated to different degrees, so light with different intensities is input into the balanced detector, thereby generating an intensity signal related to the flight time.

[0039] The specific process is as follows: by introducing a delay line into the transmission optical path of the terahertz pulse to be measured, the delay line is adjusted so that the relative delay between the reference terahertz pulse and the terahertz pulse to be measured is zero or the pulses are synchronized;

[0040] Specifically, let's take terahertz pulse as an example:

[0041] synchronization between the Nth reference terahertz pulse and the Nth second sampling light pulse;

[0042] By adjusting the delay line in the sampling pulse optical path, the delay between the reference terahertz pulse and the terahertz pulse to be measured and the corresponding sampling light pulse is zero (Δt0=0). When the two terahertz pulses pass through the electro-optical crystal, the polarization modulation of the sampling pulse by the two terahertz pulses is maximum and equal. After passing through the optically balanced detector, a cross-correlation signal with an intensity of 0 is output.

[0043] When distance measurement is performed, the time delay between the reference terahertz pulse and the sampling pulse (the second sampling light pulse) remains unchanged (0). When there is a time delay Δt between the terahertz pulse to be measured and its corresponding sampling pulse (the first sampling light pulse), the overlap between the terahertz pulse and the sampling pulse in the time domain decreases, resulting in a decrease in the polarization modulation of the sampling light, which reduces the sampling pulse signal input into the balanced detector. After passing through the optical balanced detector, a cross-correlation signal with a non-zero intensity is output, such as Figure 1 shown.

[0044] At this time, the repetition frequency of the terahertz pulse is changed, and the specific calculation formula is as follows:

[0045] Assume the repetition frequency of the near-infrared electro-optical frequency comb is f r , then the pulse repetition period t s =1 / f r , through the time-of-flight method, we know that the distance L to be measured satisfies:

[0046]

[0047] Where c is the speed of light, n is the atmospheric refractive index, t T It is the time it takes for the terahertz pulse to be emitted by the terahertz antenna, reflected by the target mirror and enter the detector.

[0048] like Figure 2 As shown, the end to be measured and the point to be measured both refer to the target mirror 5. When the relative delay between the reference terahertz pulse and the terahertz pulse to be measured is 0 (including the two cases where the delay of the same-beam pulse and the delay of the different-beam pulse is 0), the two paths generate cross-correlation signals of the same intensity. After subtracting them from the optical balanced detector, the output cross-correlation signal intensity is zero, and a zero point appears in the relationship diagram between the balanced cross-correlation signal size and time.

[0049] The flight time at this time is t T Locked to the pulse train time interval t s An integer multiple of (m), that is, t T =mt s

[0050] at this time:

[0051]

[0052] Among them, about t T=mt s , explained as follows:

[0053] Flight time T is the time it takes for light to travel from the terahertz photoconductive antenna to the optical balanced detector in free space; the pulse sequence time interval t s is the time interval of the pulse train emitted by the terahertz photoconductive antenna;

[0054] When the optical balance cross-correlation signal appears zero, it indicates that the delay between the terahertz pulse to be measured and the first sampling light pulse is zero. At this time, the flight time t T The pulse sequence time interval t can be used as an integer multiple s express;

[0055] The optical frequency comb is scanned by adjusting the repetition frequency of the optical frequency comb so that the optically balanced detector outputs adjacent zero points with a cross-correlation intensity of 0 again.

[0056] Specifically, the repetition rate of the near-infrared electro-optical frequency comb 1 is set, and the balanced photodetector 17 outputs a zero point by changing the delay line. After the target mirror 5 moves, the repetition rate of the near-infrared electro-optical frequency comb 1 is set to perform continuous frequency scanning, and the balanced light photodetector 17 will produce several signals with balanced cross-correlation of 0.

[0057] In the above signals, let the repetition frequency of a corresponding zero point be f r1 , the repetition frequency f of an adjacent zero point r2 , where Δf = f r2 -f r1 ;

[0058] Since the repetition frequency corresponds to the integer multiple m of the pulse period, we have So the distance to be measured L is

[0059]

[0060] Thus, in the distance measurement, the measurement of the round-trip flight time in the time-of-flight method is converted into the measurement of the repetition frequency difference in the optical balanced cross-correlation.

[0061] Since the optical balanced detector outputs the relationship curve between the balanced cross-correlation signal intensity and time, the time interval τ1 when the adjacent balanced cross-correlation signals collected by the data acquisition card in the information processing module are zero needs to be calculated by the formula Where T is the frequency sweep period, f s is the frequency variation of the near-infrared electro-optic frequency comb sweep.

[0062] Among them, about The explanation is as follows:

[0063] In linear frequency sweep, the actual frequency change formula is:

[0064]

[0065] Where f0 is the starting frequency, f max is the end frequency, t is the time from the start of the sweep to the current time, and T is the sweep period.

[0066] set up

[0067]

[0068]

[0069] Let f s =f max -f0, τ1=t2-t1, Δf=f(t2)-f(t1), we get Where T is the sweep period of the near-infrared electro-optical frequency comb, f s is the frequency variation of the near-infrared electro-optical frequency comb sweep, that is, the set sweep range, and τ1 is the time interval between two adjacent zero points of the balanced cross-correlation signal.

[0070] By formula Δf in the figure is the repetition frequency difference in the RF domain after amplification by the balanced detector, and the time amplification factor is It can be seen that the accuracy of distance measurement is determined by the repetition frequency accuracy of the terahertz pulse before amplification. The repetition frequency of the terahertz pulse is consistent with the repetition frequency of the near-infrared electro-optical frequency comb. Therefore, the ranging accuracy is determined by the repetition frequency adjustment accuracy of the near-infrared electro-optical frequency comb.

[0071] Since the repetition frequency adjustment accuracy of the near-infrared electro-optical frequency comb is in the millihertz level and the repetition frequency adjustment range is in the range of 20MHz-250MHz, the distance measurement accuracy is in the nanometer level.

[0072] Therefore, the ranging accuracy of the high-precision terahertz optical comb time-of-flight ranging device based on balanced cross-correlation can reach the nanometer level.

[0073] in, The specific derivation process is as follows: Since the time delay between the terahertz pulse to be measured and the first sampling pulse is 0, and the time delay between the reference terahertz pulse and the second sampling pulse is 0, the intensity of the balanced detector output is 0, and the flight time t T is locked in the terahertz pulse period t s An integer multiple of m, then:

[0074] t T =mt s ;

[0075] therefore,

[0076]

[0077] t s =1 / f r

[0078]

[0079] Among them, f r1 This is the repetition frequency of the near-infrared electro-optical frequency comb at this time;

[0080] It can be seen from the above formula that m and fr correspond one to one, so there are many zero points of the balanced cross-correlation signal.

[0081] Assume that the repetition frequencies of two adjacent zero points of the cross-correlation signal are fr1 and fr2, and the corresponding number of pulses is m and m+1; therefore,

[0082]

[0083]

[0084]

[0085] Δf=f r2 -f r1 , put it into formula (3), and we get

[0086] For the calculation of accuracy, It can be seen that the distance measurement accuracy can reach the nanometer level

[0087] The time magnification factor

[0088] This method has higher measurement accuracy than traditional time-of-flight ranging method. The ranging accuracy of traditional time-of-flight ranging method is affected by pulse width, system intensity noise, and time jitter. The optical balanced cross-correlation system can eliminate the influence of system intensity noise on the ranging results and overcome the influence of pulse signal intensity disturbance.

[0089] The intensity of the S-shaped balanced cross-correlation signal output by the optical balanced cross-correlation method is linearly related to the time delay. Its signal intensity is only determined by the time delay between pulses, not by the intensity of the pulses. Therefore, the measurement accuracy can be improved to the nanometer level.

[0090] Compared with the prior art, the advantages of the present invention are:

[0091] (1) By combining the time-of-flight measurement method, free-space electro-optical sampling and the optical balanced cross-correlation system, the measurement of the time interval in the time-of-flight method is transformed into the measurement of the intensity of the optical balanced cross-correlation signal. By measuring the intensity of the cross-correlation signal, the position information of the distance to be measured is obtained, so that the measurement accuracy is not limited by the terahertz pulse width, so that its measurement accuracy can reach the nanometer level. Specifically:

[0092] (2) Using a near-infrared femtosecond electro-optic comb with tunable repetition frequency as the light source, the generated terahertz optical comb has the characteristics of rapid change of repetition frequency, large adjustable range of repetition frequency, low noise and good anti-interference ability.

[0093] (3) The optical balanced cross-correlation technology eliminates the influence of system intensity noise on the distance measurement accuracy and overcomes the influence of terahertz pulse signal intensity disturbance. In actual measurement, the distance information to be measured can be obtained by simply observing the repetition frequency difference corresponding to two adjacent zero points of the balanced cross-correlation signal. The optical balanced cross-correlation technology not only improves the anti-interference ability of the distance measurement system, but also improves the distance measurement accuracy, raising the measurement accuracy from the micrometer level to the nanometer level. BRIEF DESCRIPTION OF THE DRAWINGS

[0094] Figure 1 Schematic diagram of the effect of the time delay between the terahertz signal and the sampling pulse on the size of the balanced cross-correlation signal;

[0095] Figure 2 This is a schematic diagram of the balanced cross-correlation terahertz time-of-flight ranging principle of the present invention;

[0096] Figure 3 Schematic diagram of balanced cross-correlation signal. DETAILED DESCRIPTION

[0097] The high-precision terahertz optical comb time-of-flight ranging method based on balanced cross-correlation of the present invention will be described in more detail below in conjunction with a schematic diagram, wherein a preferred embodiment of the present invention is shown, and it should be understood that those skilled in the art can modify the present invention described herein while still achieving the advantageous effects of the present invention. Therefore, the following description should be understood as being widely known to those skilled in the art, and not as a limitation to the present invention.

[0098] like Figure 2 The high-precision terahertz optical comb time-of-flight ranging system based on balanced cross-correlation includes: a terahertz optical comb generation module with tunable repetition rate, a time-of-flight ranging module, a balanced cross-correlation electro-optical sampling module, and a signal acquisition and processing module.

[0099] The repetition rate tunable terahertz optical comb generation module is a near-infrared electro-optical frequency comb 1, whose center frequency is 1560nm.

[0100] One of the output ends is connected to the terahertz photoconductive antenna 2 to excite the photoconductive antenna 2 to generate a terahertz pulse. The repetition frequency of the terahertz pulse is consistent with the repetition frequency of the near-infrared electro-optical frequency comb and is input into the time-of-flight ranging module.

[0101] The other output end is connected to the input end of the optical frequency doubling module 7. The optical frequency doubling module 7 outputs two first sampling optical pulses and a second sampling optical pulse with a central wavelength of 780nm. Both the first sampling optical pulse and the second sampling optical pulse are input into the balanced cross-correlation electro-optical sampling module.

[0102] The time-of-flight ranging module comprises an off-axis parabolic mirror 3, a beam splitter 4, a target mirror 5, and a reference mirror 6.

[0103] The terahertz pulse generated by the photoconductive antenna 2 is focused by the off-axis parabolic mirror 3 to convert the divergent light into parallel terahertz pulses. The parallel terahertz pulses are split into two terahertz light comb pulses by the beam splitter 4.

[0104] One path is reflected by the beam splitter 4 at a reflection angle of 45°, enters the measurement optical path, and is vertically reflected by the target mirror 5 before being transmitted out of the beam splitter 4, and enters the balanced cross-correlation electro-optical sampling module as the terahertz pulse to be measured;

[0105] The other path is transmitted through the beam splitter 4, enters the reference light path, is reflected by the reference mirror 6 at a reflection angle of 45°, and then enters the balanced cross-correlation electro-optical sampling module as a reference terahertz pulse after being reflected by the reference mirror 6.

[0106] Balanced cross-correlation electro-optical sampling module, including:

[0107] A first delay line 8, a second delay line 16, a first polarization beam splitter 9, a second polarization beam splitter 15, a first quarter wave plate 10, a second quarter wave plate 14, a first off-axis parabolic mirror with a hole 11, and a second off-axis parabolic mirror with a hole 13;

[0108] The first sampling optical pulse output by the optical frequency doubling module 7 is connected to the input end of the first delay line 8. The second sampling optical pulse output by the optical frequency doubling module 7 is connected to the input end of the second delay line 16;

[0109] The balanced photodetector 17 has a first input end receiving the first sampling light pulse reflected by the second polarization beam splitter 15 , and a second input end receiving the second sampling light pulse reflected by the first polarization beam splitter 9 .

[0110] The electro-optic crystal 12 is placed at the focus of the first off-axis parabolic mirror 11 with a hole and the second off-axis parabolic mirror 13 with a hole.

[0111] The first sampling light pulse passes through the first delay line 8 and is horizontally incident on the first polarization beam splitter 9 and is transmitted through the first polarization beam splitter 9. After passing through the first 1 / 4 wave plate 10, the polarization state of the first sampling light pulse changes from linear polarization to circular polarization. The first sampling light pulse meets the terahertz pulse to be measured at the first off-axis parabola with holes 11, and they are jointly incident on the electro-optical crystal 12.

[0112] Since the electro-optic crystal 12 generates transient birefringence under the action of the terahertz electric field, the polarization state of the first sampling light pulse in the electro-optic crystal 12 changes from circular polarization to elliptical polarization, and the polarization state of the first sampling light pulse is modulated by the terahertz electric field to be measured.

[0113] The first sampling light pulse is output from the electro-optical crystal 12 , passes through the second off-axis parabolic mirror 13 with holes, and then enters the second polarization beam splitter 15 after passing through the second quarter wave plate 14 . After being reflected by the second polarization beam splitter 15 , it enters the first input end of the optical balanced detector 17 .

[0114] The second quarter wave plate 14 and the second polarization beam splitter 15 detect the polarization state of the first sampling pulse.

[0115] The second sampling light pulse is horizontally incident on the second polarization beam splitter 15 after passing through the second delay line 16 and is transmitted through the second polarization beam splitter 15. After passing through the second 1 / 4 wave plate 14, the polarization state of the second sampling light changes from linear polarization to circular polarization. The second sampling light pulse meets the reference terahertz pulse at the second off-axis parabola with holes 13, and they act on the electro-optic crystal 12 together in a collinear manner. Since the electro-optic crystal 12 generates transient birefringence under the action of the terahertz electric field, the polarization state of the second sampling pulse in the electro-optic crystal 12 changes from circular polarization to elliptical polarization.

[0116] The first sampling light pulse passing through the electro-optic crystal 12 will also meet the reference terahertz pulse at the second off-axis parabola with holes 13. Similarly, the second sampling light pulse passing through the electro-optic crystal 12 will also meet the terahertz pulse to be measured at the first off-axis parabola with holes 11, but they do not affect each other.

[0117] The second sampling light pulse is output from the electro-optical crystal 12 , passes through the first off-axis parabolic mirror 11 with a hole, and then enters the first polarization beam splitter 9 after passing through the first 1 / 4 wave plate 11 , and is reflected by the first polarization beam splitter 9 to enter the second input end of the optical balanced detector 17 .

[0118] In addition, when the first sampling light pulse and the terahertz pulse to be measured are injected into the electro-optical crystal 12 , the second sampling light pulse and the reference terahertz pulse are not injected into the electro-optical crystal 12 .

[0119] Similarly, when the second sampling light pulse and the reference terahertz pulse are injected into the electro-optical crystal 12 , the first sampling light pulse and the terahertz pulse to be measured are not injected into the electro-optical crystal 12 .

[0120] In other embodiments, when the first sampling light pulse and the terahertz pulse to be measured are injected into the electro-optical crystal 12, the second sampling light pulse and the reference terahertz pulse are injected into the electro-optical crystal 12. The reasons are as follows:

[0121] Since the direction of light transmission and the direction of the external electric field (terahertz pulse) are the same, that is, it is longitudinal electro-optical modulation at this time, but the directions of the terahertz pulse to be measured and the first sampling light pulse, the reference terahertz pulse and the second sampling light pulse entering the electro-optical crystal are different, they will not affect each other.

[0122] The optical balanced detector 17 consists of two photodiodes and a differential amplifier and a filter.

[0123] Firstly, by adjusting the first delay line 8 and the second delay line 16, the relative delay between the reference terahertz pulse and the terahertz pulse to be measured is made zero or the pulses are synchronized.

[0124] After electro-optical sampling, the first sampling pulse and the second sampling pulse are input into the positive and negative ends of the optical balanced detector 17 respectively.

[0125] Since the two sampling optical pulse signals have no time delay difference with the terahertz pulse, the polarization modulation of the two terahertz pulses on the sampling pulse is maximal and equal, resulting in the cross-correlation signal intensity output after being input into the balanced detector 17 being zero.

[0126] Due to the distance measurement, the target mirror 5 moves, and the flight time of the terahertz pulse returning after passing through the target mirror 5 changes, resulting in a time delay of Δt between the terahertz pulse to be measured and the first sampling light pulse, causing the temporal overlap of the terahertz electric field and the first sampling pulse in the electro-optical crystal 12 to decrease, and the modulation of the polarization of the first sampling light by the terahertz electric field to decrease, resulting in a decrease in the ellipticity of the first sampling pulse measured by the second 1 / 4 wave plate 14 and the second polarization beam splitter 15.

[0127] However, the reference mirror 6 does not move, and the delay between the reference terahertz pulse and the sampling pulse remains unchanged. At this time, the polarization modulation of the second sampling pulse by the terahertz electric field remains maximum, so the optical cross-correlation signal output by the balanced detector 17 is not zero.

[0128] By adjusting the repetition frequency of the near-infrared electro-optical frequency comb 1 and performing continuous frequency sweeping, the balanced cross-correlation signal generates a zero point again, and the repetition frequencies of adjacent zero points that satisfy the balanced cross-correlation are subtracted, and the formula is used: The distance to be measured can be obtained. Figure 3 As shown, it represents the time difference between two adjacent zero points.

[0129] The signal acquisition and processing module includes a data acquisition card 18 .

[0130] The input end of the data acquisition card 18 is connected to the output end of the balanced detector 17. The data acquisition card 18 performs high-speed digital signal analysis and processing on the optical balanced cross-correlation signal, thereby obtaining a distance measurement result, that is, the distance to be measured after being divided by the time amplification factor.

[0131] The above is only a preferred embodiment of the present invention and does not limit the present invention in any way. Any technician in the relevant technical field, without departing from the scope of the technical solution of the present invention, makes any form of equivalent replacement or modification to the technical solution and technical content disclosed in the present invention, which does not depart from the content of the technical solution of the present invention and still falls within the protection scope of the present invention.

Claims

1. A high-precision terahertz optical comb ranging system based on balanced cross-correlation, characterized in that: include: A terahertz optical comb generation module with tunable repetition rate, wherein one output end is connected to a terahertz photoconductive antenna (2) to excite the photoconductive antenna (2) to generate a terahertz pulse, and another output end is connected to an input end of an optical frequency doubling module (7), and a first sampling optical pulse and a second sampling optical pulse output by the optical frequency doubling module (7) are both input into a balanced cross-correlation electro-optical sampling module; A time-of-flight ranging module comprises an off-axis parabolic mirror (3), a beam splitter (4), a target mirror (5), and a reference mirror (6); The terahertz pulse generated by the photoconductive antenna (2) is focused by an off-axis parabolic mirror (3), and the divergent light is converted into parallel terahertz pulses. The parallel terahertz pulses are split into two terahertz light comb pulses by a beam splitter (4); One path is reflected by the beam splitter (4), enters the measurement optical path, is vertically reflected by the target mirror (5), and then transmits out of the beam splitter (4), and enters the balanced cross-correlation electro-optical sampling module as a terahertz pulse to be measured; The other path is transmitted through the beam splitter (4), enters the reference optical path, and after being reflected by the reference mirror (6), enters the balanced cross-correlation electro-optical sampling module as a reference terahertz pulse; The balanced cross-correlation electro-optical sampling module has an output end connected to an input end of an optical balanced detector (17); and a signal acquisition and processing module connected to the output end of the optical balance detector (17).

2. The high-precision terahertz optical comb ranging system based on balanced cross-correlation according to claim 1, characterized in that: The balanced cross-correlation electro-optical sampling module comprises: A first delay line (8), a second delay line (16), a first polarization beam splitter (9), a second polarization beam splitter (15), a first quarter wave plate (10), a second quarter wave plate (14), a first off-axis parabolic mirror with a hole (11), and a second off-axis parabolic mirror with a hole (13); The first sampling light pulse output by the optical frequency doubling module (7) is connected to the input end of the first delay line (8), and the second sampling light pulse output by the optical frequency doubling module (7) is connected to the input end of the second delay line (16); A first input end of the balanced photodetector (17) receives a first sampling light pulse reflected by the second polarization beam splitter (15), and a second input end thereof receives a second sampling light pulse reflected by the first polarization beam splitter (9); and an electro-optical crystal (12) placed at the focus of the first off-axis parabolic mirror with a hole (11) and the second off-axis parabolic mirror with a hole (13).

3. A high-precision terahertz optical comb ranging method based on balanced cross-correlation, based on the high-precision terahertz optical comb ranging system based on balanced cross-correlation according to claim 2, characterized in that: The following steps are involved: Step 1, adjusting the second delay line (16) so that the reference terahertz pulse and the second sampling light pulse are synchronized; Step 2: adjusting the first delay line (8) so that the terahertz pulse to be measured is synchronized with the first sampling pulse; Step 3: Move the target mirror (5) along the measurement optical path, scan the repetition frequency of the terahertz optical comb generation module with tunable repetition frequency, read the repetition frequency corresponding to two adjacent zero points appearing in the balanced photodetector (17), and use it as the final repetition frequency f of the terahertz optical comb generation module with tunable repetition frequency. r1 、f r2 ; Step 4: Calculate the distance L to be measured.

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