A high-precision laser phase ranging system and method for a common-path optical path

Abstract

The present invention provides a high-precision laser phase ranging system and method with a common-path optical path. The system includes an arbitrary waveform generator, a semiconductor laser, a loop unit, an optical fiber collimation unit, a photodetector, a data collector, and a processor. The optical fiber collimation unit transmits a part of the laser signal to the target, collects the object light reflected by the target, and at the same time reflects a part of the laser signal to form a reference light. The object light and the reference light received by the optical fiber collimation unit are transmitted in a common path. The loop unit receives the object light and the reference light transmitted in the common path, and after being processed by the photodetector and the data collector, is transmitted to the processor. The processor calculates the phase difference based on the intensity signals of the interference light corresponding to the positions of the target before and after movement, and obtains the relative distance measurement value. The present invention reduces the measurement error by making the reference light and the object light share the same optical path, simplifies the optical path structure, reduces external interference, improves the stability and accuracy of ranging, and is beneficial to realizing high-resolution and high-precision displacement measurement.

Description

Technical Field

[0001] The present invention relates to the technical field of lidar, and more specifically, to a high-precision laser phase ranging system and method with a common-path optical path. Background Art

[0002] Lidar is an active detection technology that uses laser as the light source and adopts optoelectronic detection technology to measure the distance to the target. It has the advantages of high precision, high speed, and small volume, and is widely used in fields such as autonomous driving, unmanned aerial vehicles, intelligent robots, 3D modeling, and geodetic surveying. For example, lidar can help robots perform precise positioning and navigation in complex environments and update the environmental map in real time; during the manufacturing process, lidar can be used to detect changes in the size and shape of mechanical parts, monitor the position and movement of objects on the production line in real time, and ensure precise control of the production process. How to ensure the ranging accuracy of lidar has always been an important research topic in this field.

[0003] Existing lidar ranging methods include: First, the time-of-flight ranging method, which directly measures the distance by detecting the round-trip time of the pulsed light from the light source, reflected by the object surface, and back to the detector. This method has a simple structure, but is severely affected by background light interference. In a complex environment, the reflected signal may be affected by multipath interference; Second, the frequency-modulated continuous-wave ranging method, which indirectly measures the distance by detecting the beat signal between the reference light and the signal light reflected by the object surface. This method has anti-interference ability, but has strict requirements on the frequency modulation linearity of the light source. Due to the nonlinearity of frequency modulation, the measurement accuracy will be severely affected.

[0004] To further improve the ranging accuracy, Zhang Enyao et al. disclosed in the literature "Improvement of the Performance of Fiber Optic Heterodyne Interference Displacement Sensors" a dual-path fiber Fabry-Perot interferometer composed of a semiconductor laser (LD), an optical isolator, a fiber directional coupler, a self-focusing lens, etc. By using triangular wave current modulation and differential phase discrimination of the upper and lower edge beat signals, the measurement sensitivity and displacement response speed are improved. An auxiliary interferometer is used to detect the phase drift of the system, and feedback control is performed on the emission wavelength of the LD to improve the stability of the system. However, the optical path structure in this technology is complex, which affects the measurement efficiency. Moreover, it is necessary to use a coupler for beam splitting, and there is a large optical path difference between the reference light and the object light. External factors (such as air turbulence, mechanical vibration, etc.) will have different effects on the phases of the two beams of light, resulting in an increase in phase noise, thereby reducing the measurement stability and accuracy. Summary of the Invention

[0005] The purpose of the present invention is to provide a high-precision laser phase ranging system and method with a common-path optical path, which reduces the measurement error by making the reference light and the object light share the same optical path, simplifies the optical path structure, reduces external interference, improves the stability and accuracy of ranging, and is conducive to realizing high-resolution and high-precision displacement measurement.

[0006] To achieve the above object, the present invention provides the following solutions:

[0007] A high-precision laser phase ranging system with a common-path optical path, comprising an arbitrary waveform generator, a semiconductor laser, a loop unit, an optical fiber collimation unit, a photodetector, a data collector, and a processor;

[0008] The arbitrary waveform generator generates a frequency-modulated signal, and frequency-modulates the laser signal of the semiconductor laser by controlling the drive current of the semiconductor laser;

[0009] The output end of the semiconductor laser is connected to the loop unit through an optical fiber;

[0010] The loop unit is connected to the optical fiber collimation unit through an optical fiber, and is used for transmitting the laser signal to the optical fiber collimation unit;

[0011] The optical fiber collimation unit is used for transmitting a part of the laser signal to the target, collecting the object light reflected by the target, and reflecting a part of the laser signal to form a reference light. The object light and the reference light received by the optical fiber collimation unit are transmitted in a common path;

[0012] The loop unit receives the object light and the reference light returned by the optical fiber collimation unit, and transmits them to the photodetector;

[0013] The photodetector is used for converting the intensity signal of the interference light of the object light and the reference light into an electrical signal, and transmitting it to the data collector;

[0014] The data collector is used for receiving the electrical signal converted by the photodetector and transmitting it to the processor;

[0015] The processor is used for receiving the electrical signal, and calculating a phase difference based on the intensity signals of the interference light corresponding to the positions of the target before and after movement, obtaining a relative distance measurement value, and the relative distance measurement value is the distance of the target movement.

[0016] Further, the arbitrary waveform generator is further used for generating a synchronous electrical signal, transmitting it to the data collector, and the synchronous electrical signal has the same frequency as the frequency-modulated signal.

[0017] Further, the signal transmission mode between the loop unit and the photodetector includes one or more of optical fiber transmission, collimator conversion transmission, and free space optical transmission.

[0018] Further, the loop unit is provided with three interfaces, namely port a, port b, and port c. Among them, port a is used for receiving the laser signal from the DFB laser, port b is used for outputting the laser signal to the optical fiber collimation unit, and receiving the object light and the reference light, and port c is used for transmitting the object light and the reference light to the photodetector.

[0019] Further, the fiber collimation unit uses a fiber collimator with a PC-type interface, a lens with transmission and reflection functions, or a combined structure of an optical fiber and a collimating mirror.

[0020] Further, the loop unit uses an optical fiber circulator.

[0021] Further, the frequency-modulated signal is a symmetric triangular wave or a sawtooth wave, and the synchronous electrical signal is a square wave with a duty cycle of 50%.

[0022] The present invention also provides a high-precision laser phase ranging method for a common-path optical path, which is applied to the high-precision laser phase ranging system for the common-path optical path described above, and includes the following steps:

[0023] An arbitrary waveform generator generates a frequency-modulated signal and transmits it to a semiconductor laser to frequency-modulate the laser signal of the semiconductor laser;

[0024] The frequency-modulated laser signal is transmitted to the fiber collimation unit through the loop unit;

[0025] The fiber collimation unit transmits a part of the laser signal to the target, collects the object light reflected by the target, and simultaneously reflects a part of the laser signal to form a reference light;

[0026] The object light received by the fiber collimation unit and the reference light reflected from the end face of the fiber collimation unit are transmitted in a common path, and are transmitted to a photodetector through the loop unit;

[0027] The photodetector converts the intensity signal of the interference light of the object light and the reference light into an electrical signal and transmits it to a data collector;

[0028] The data collector receives the electrical signal converted by the photodetector and transmits it to a processor;

[0029] The processor receives the electrical signal, calculates the phase difference based on the intensity signals of the interference light corresponding to the positions where the target is located before and after moving, and obtains a relative distance measurement value, where the relative distance measurement value is the distance that the target moves;

[0030] Among them, the calculation formula for the relative distance measurement value is as follows:

[0031]

[0032] In the formula, represents the relative distance measurement value of the positions where the target is located before and after moving;

[0033] is the time difference corresponding to the optical path difference between the object light and the reference light when measuring the distance at the position where the target is located before moving;

[0034] The time difference corresponding to the optical path difference between the object light and the reference light when measuring the distance at the position where the target is located after moving;

[0035] The phase difference actually measured based on the intensity signals of the interference light corresponding to the positions where the target is located before and after moving;

[0036] The initial frequency of the laser signal; The speed of light.

[0037] Furthermore, calculating the phase difference based on the intensity signals of the interference light corresponding to the positions where the target is located before and after moving, and obtaining the relative distance measurement value specifically includes:

[0038] When measuring the distance at the position where the target is located before moving, the time-domain expression of the intensity of the difference-frequency signal light obtained after the interference of the object light and the reference light is:

[0039]

[0040]

[0041] In the formula, and are respectively the intensities of the rising edge and the falling edge of the difference-frequency signal obtained after the interference of the object light and the reference light when measuring the distance at the position where the target is located before moving; and are respectively the intensities of the reference light and the object light; is the time difference corresponding to the optical path difference between the object light and the reference light when measuring the distance at the position where the target is located before moving; is the frequency modulation rate, that is, the slope of the rising edge of the triangular wave, is the frequency modulation bandwidth, is the frequency modulation period;

[0042] Then, when measuring the distance at the position where the target is located before moving, the phase difference between the rising edge and the falling edge light intensities is ;

[0043] Similarly, assuming that the time difference corresponding to the optical path difference between the object light and the reference light when measuring the distance at the position where the target is located after moving is , then when measuring the distance at the position where the target is located after moving, the phase difference between the rising edge and the falling edge light intensities is ;

[0044] Calculating the phase difference based on the intensity signals of the interference light corresponding to the positions where the target is located before and after moving: ;

[0045] From , it is obtained that ;

[0046] Then, 。

[0047] Furthermore, ranges from 0 - When is

[0048]

[0049] then takes values in the range of 0 - wherein, is the wavelength of the laser signal.

[0050] According to the specific embodiments provided by the present invention, the present invention discloses the following technical effects: for the high-precision laser phase ranging system and method with a common-path optical path provided by the present invention, the object light received by the fiber collimation unit and the reference light reflected from the end face of the fiber collimation unit are transmitted in a common path; the processor calculates the phase difference based on the intensity signals of the interference light corresponding to the positions of the target before and after movement, and obtains the relative distance measurement value.

[0051] First, in the existing ranging system, in the case of a non-common-path optical path, the polarization state of light will change, and the difference in the polarization states of the object light and the reference light will seriously affect the signal contrast; while the present invention combines the characteristics of a common-path optical path, the object light received by the fiber collimation unit and the reference light reflected from the end face of the fiber collimation unit are transmitted in a common path and form interference light, which is transmitted to the photodetector through the loop unit. When ranging, based on the intensity signals of the interference light corresponding to the positions of the target before and after movement, the phase difference is calculated. Since the object light and the reference light are transmitted in a common path, the influence of external environments (such as temperature, pressure, vibration, etc.) on the object light and the reference light is the same. When calculating the phase difference, the external interference can be cancelled out by phase subtraction, maximizing the measurement accuracy.

[0052] Second, the present invention obtains the relative distance measurement value:

[0053]

[0054] The distance calculated by this formula is the distance that the target moves. Since it is not necessary to directly measure the distance between the target and the fiber collimation unit, there is no need to consider the transmission distance of the laser signal in the fiber optical path, avoiding the measurement error caused by this part of the transmission distance; at the same time, in the case of common-path optical path transmission, the influence of the polarization states of the object light and the reference light is the same and cancels each other out, improving the contrast of the interference light, thereby further improving the measurement accuracy.

[0055] This application uses the heterodyne method for measurement, avoiding the effective information being carried on the DC signal, which facilitates signal processing such as amplification and filtering. In summary, the present invention combines the strong anti-interference ability of the common-path optical path, the high precision of the phase method, and the convenience of heterodyne interference signal processing, comprehensively improving the laser ranging effect.

[0056] In addition, the present invention reduces the use of couplers. In the traditional method, multiple couplers are required for beam splitting and beam combining, which increases the complexity and cost of the system. By using the reflection of the fiber collimation unit to form the reference light, the use of these couplers can be reduced, the energy loss generated by the couplers can be lowered, and the overall efficiency of the system can be improved. At the same time, various errors introduced by the couplers (such as insertion loss, reflection loss, etc.) are reduced. Meanwhile, the object light received by the fiber collimation unit and the reference light are transmitted in a common path, reducing the non-common-path error and further improving the ranging accuracy. It can be seen that the method of using the collimator to reflect and form the reference light not only reduces the use of couplers, simplifies the system design, but also significantly improves the ranging accuracy.

[0057] In summary, the present invention calculates the phase difference based on the intensity signals of the interference light corresponding to the positions of the target before and after movement to obtain the relative distance measurement value and the ranging result. Compared with the traditional frequency difference method for ranging, the phase difference ranging method adopted by the present invention can achieve higher measurement accuracy, is very sensitive to small distance changes, can detect very subtle displacements, and is suitable for ranging applications that require extremely high resolution. BRIEF DESCRIPTION OF THE DRAWINGS

[0058] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for use in the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For those of ordinary skill in the art, other drawings can be obtained based on these drawings without creative efforts.

[0059] Figure 1 It is a schematic structural diagram of the high-precision laser phase ranging system with a common-path optical path of the present invention;

[0060] Figure 2 It is a schematic diagram of the triangular wave frequency modulation signal in the embodiment of the present invention;

[0061] Figure 3 It is a schematic structural diagram of a typical FMCW lidar ranging system in the prior art;

[0062] Figure 4 It is a beat frequency signal diagram obtained by a typical FMCW lidar ranging system in the prior art;

[0063] Figure 5The beat frequency signal diagram obtained after improving the optical path of the high-precision laser phase ranging system using the common optical path of the present invention;

[0064] Figure 6 Schematic diagram of the fiber collimation unit structure formed by the combination of the optical fiber and the collimating mirror;

[0065] Explanation of reference numerals:

[0066] 1. Arbitrary waveform generator; 2. Semiconductor laser; 3. Loop unit (divided into three ports a, b, and c, input from port a and output from port b, input from port b and output from port c); 4. Fiber collimation unit; 5. Photoelectric detector; 6. Data collector (two-way input); 7. Processor; 8. Position where the target was located before movement; 8'. Position where the target is located after movement;

[0067] 3-1. Tunable laser; 3-2. Isolator; 3-3. Coupler 1; 3-4. Circulator; 3-5. Lens; 3-6. Target object; 3-7. Coupler 2; 3-8. Balanced detector;

[0068] 6-1. Optical fiber; 6-2. Collimating mirror. Specific embodiments

[0069] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

[0070] The purpose of the present invention is to provide a high-precision laser phase ranging system and method with a common optical path, which reduces measurement errors by making the reference light and the object light share the same optical path, simplifies the optical path structure, reduces external interference, improves the stability and accuracy of ranging, is conducive to increasing the measurement range, and realizes high-resolution and high-precision displacement measurement.

[0071] To make the above objects, features, and advantages of the present invention more obvious and understandable, the present invention will be further described in detail below in conjunction with the accompanying drawings and specific embodiments.

[0072] As Figure 1 shown, the high-precision laser phase ranging system with a common optical path provided by the present invention includes an arbitrary waveform generator 1, a semiconductor laser 2, a loop unit 3, a fiber collimation unit 4, a photoelectric detector 5, a data collector 6, and a processor 7;

[0073] The arbitrary waveform generator 1 generates a frequency modulation signal, and by controlling the driving current of the semiconductor laser 2, the laser signal of the semiconductor laser 2 is frequency modulated;

[0074] The output end of the semiconductor laser 2 is connected to the loop unit 3 through an optical fiber;

[0075] The loop unit 3 is connected to the optical fiber collimation unit 4 through an optical fiber, and is used to transmit the laser signal to the optical fiber collimation unit 4;

[0076] The optical fiber collimation unit 4 is used to transmit a part of the laser signal to the target, collect the object light reflected by the target, and at the same time reflect a part of the laser signal to form a reference light. The object light and the reference light received by the optical fiber collimation unit 4 are transmitted in a common path;

[0077] The loop unit 3 receives the object light and the reference light returned by the optical fiber collimation unit 4 and transmits them to the photodetector 5;

[0078] The photodetector 5 is used to convert the intensity signal of the interference light of the object light and the reference light into an electrical signal and transmit it to the data collector 6;

[0079] The data collector 6 is used to receive the electrical signal converted by the photodetector 5 and transmit it to the processor 7;

[0080] The processor 7 is used to receive the electrical signal, and calculate the phase difference based on the intensity signals of the interference light corresponding to the positions of the target before and after movement, and obtain a relative distance measurement value. The relative distance measurement value is the distance of the target movement.

[0081] The ranging result described in the present invention refers to the difference between the positions of the target before and after movement, that is, the distance of the target movement. That is, the relative displacement of the target can be measured when the optical fiber collimation unit 4 is fixed. As Figure 1 in 、 respectively represent the distances from the position 8 where the target is located before movement and the position 8' where the target is located after movement to the optical fiber collimation unit 4, represents the distance of the target movement.

[0082] The arbitrary waveform generator 1 is further used to generate a synchronization electrical signal and transmit it to the data collector 6. The synchronization electrical signal has the same frequency as the frequency modulation signal, such as 1 kHz. Exemplarily, the frequency modulation signal is a symmetric triangular wave or a sawtooth wave, and other waveforms can also be used. The synchronization electrical signal is a square wave with a duty cycle of 50%. In the embodiment of the present invention, a triangular wave frequency modulation signal is taken as an example.

[0083] Exemplarily, the semiconductor laser 2 can be a narrow linewidth semiconductor laser, for example, a DFB (Distributed Feedback Laser) laser; the loop unit 3 can be an optical fiber circulator.

[0084] Exemplarily, the signal transmission mode between the loop unit 3 and the photodetector 5 includes one or more of fiber optic transmission, collimator conversion transmission, and free space optical transmission.

[0085] Specifically, the loop unit 3 is provided with three interfaces, namely port a, port b, and port c. Among them, port a is used to receive the laser signal from the DFB laser 2, port b is used to output the laser signal to the fiber collimation unit 4 and receive the object light and the reference light, and port c is used to transmit the object light and the reference light to the photodetector 5.

[0086] Exemplarily, the fiber collimation unit 4 can adopt a fiber collimator with a pc-type interface, a lens with transmission and reflection functions, or a combined structure of a fiber and a collimating mirror, etc. The combined structure of the fiber and the collimating mirror is as Figure 6 shown. The present invention does not limit the selection of the fiber collimation unit 4, as long as the fiber collimation unit 4 can achieve the functions of transmitting and reflecting the laser signal. The laser signal is reflected at the end face of the fiber collimation unit 4 to form the reference light. After the object light passes through the fiber collimation unit 4, it is transmitted to the photodetector 5 through the loop unit 3 with the reference light in a common optical path, making the measured optical path difference between the reference light and the object light closer to the distance to be measured and reducing the measurement error.

[0087] In FMCW laser ranging, generally a triangular wave is used as the modulation waveform to linearly modulate the frequency of the laser, as Figure 2 shown. The variation law of the laser frequency with time under the modulation of the periodic triangular wave. The solid trace represents the optical frequency of the reference light, and the dashed trace represents the optical frequency of the object light. Due to the time delay between them, there is a frequency difference when they are mixed. In a frequency modulation period, the frequency difference will jump, but since the change time is extremely short, the influence can be ignored. Therefore, the difference frequency signal obtained by mixing can be regarded as a sine wave with a fixed frequency.

[0088] After the modulation of the laser signal by the frequency modulation signal, the photodetector 5 receives a sine wave with a phase mutation within the T / 2 period, which is an AC signal and can be filtered and signal amplified by the processing method of the AC signal. If there is no modulation of the frequency modulation signal, at a measurement position, the photodetector 5 obtains a DC signal.

[0089] Under the modulation of the rising edge of the triangular wave, the instantaneous frequency of the reference light is expressed as:

[0090] ,

[0091] where represents the initial frequency of the laser, is the frequency modulation rate, that is, the slope of the rising edge of the triangular wave, is the FM bandwidth.

[0092] Within one FM period , the expression of the reference optical signal is:

[0093] ;

[0094] The expression of the object optical signal is: {E}_{r}\left ( {t} \right )={E}_{2}exp\left [ {2\pi {f}_{0}\left ( {t-\tau} \right )+\pi \alpha \left ( {t-\tau} \right )^{2}+{\varphi}_{0}} \right ] ;

[0095] Among them, is the initial phase of the reference optical signal, is the amplitude of the reference optical signal, is the amplitude of the object optical signal.

[0096] After mixing and interference, the time-domain expression of the difference-frequency signal intensity can be obtained as:

[0097] {I}_{rise}=\left [ {{E}_{e}\left ( {t} \right )+{E}_{r}\left ( {t} \right )} \right ]={I}_{1}+{I}_{2}+2\sqrt {{I}_{1}{I}_{2}}\cos {\left ( {2\pi \alpha \tau t+2\pi {f}_{0}\tau -\pi \alpha {\tau}^{2}} \right )}

[0098] Since the quadratic term is much smaller than the phase term, ignoring the quadratic term, we can get:

[0099]

[0100] And when it is the falling edge, the instantaneous frequency of the reference signal is expressed as , , similarly, we can get:

[0101]

[0102] According to the odd function property of the cos function, we can get:

[0103]

[0104] The synchronous square wave generated by the arbitrary waveform generator can be used at the data collector to separate the rising edge and falling edge signals of the triangular wave. From the expression, it can be known that the phase difference between the rising edge and the falling edge is .

[0105] Among them, represents the relative time delay of .

[0106] The ranging result of the present invention can only perform relative distance measurement because the phase difference will undergo phase wrapping when it is greater than one 2 π , and the specific phase value cannot be determined. In addition, when performing relative distance measurement, the phase corresponding to the change value of the distance between two measurements also needs to be less than 2 π .

[0107] Based on the above principle, the present invention provides a high-precision laser phase ranging method for a common-path optical path, which is applied to the high-precision laser phase ranging system for the common-path optical path described above, and includes the following steps:

[0108] The arbitrary waveform generator 1 generates a frequency-modulated signal, which is transmitted to the semiconductor laser 2 to perform frequency modulation on the laser signal of the semiconductor laser 2;

[0109] The frequency-modulated laser signal is transmitted to the fiber collimation unit 4 through the loop unit 3;

[0110] The fiber collimation unit 4 transmits a part of the laser signal to the target, collects the object light reflected by the target, and at the same time reflects a part of the laser signal to form a reference light;

[0111] The object light received by the fiber collimation unit 4 and the reference light are transmitted in a common path, and are transmitted to the photodetector 5 through the loop unit 3;

[0112] The photodetector 5 converts the intensity signal of the interference light of the object light and the reference light into an electrical signal, and transmits it to the data collector 6;

[0113] The data collector 6 receives the electrical signal converted by the photodetector 5 and transmits it to the processor 7;

[0114] The processor 7 receives the electrical signal, calculates the phase difference based on the intensity signals of the interference light corresponding to the positions of the target before and after movement, and obtains a relative distance measurement value, where the relative distance measurement value is the distance that the target moves.

[0115] Among them, the calculation formula for the relative distance measurement value is as follows:

[0116]

[0117] In the formula, Indicates the relative distance measurement value of the position where the target is located before and after movement;

[0118] When measuring the distance to the position where the target is located before movement, it is the time difference corresponding to the optical path difference between the object light and the reference light;

[0119] When measuring the distance to the position where the target is located after movement, it is the time difference corresponding to the optical path difference between the object light and the reference light;

[0120] It is the phase difference actually measured based on the intensity signal of the interference light at the positions where the target is located before and after movement;

[0121] Is the initial frequency of the laser signal; Is the speed of light.

[0122] Specifically, based on the intensity signal of the interference light corresponding to the positions where the target is located before and after movement, calculate the phase difference and obtain the relative distance measurement value, which specifically includes:

[0123] When measuring the distance to the position where the target is located before movement, the time-domain expression of the intensity of the difference-frequency signal light obtained after the interference of the object light and the reference light is:

[0124]

[0125]

[0126] In the formula, 、 Are respectively the light intensities of the rising edge and the falling edge of the difference-frequency signal obtained after the interference of the object light and the reference light when measuring the distance to the position where the target is located before movement; 、 Are respectively the light intensities of the reference light and the object light; Is the time difference corresponding to the optical path difference between the object light and the reference light when measuring the distance to the position where the target is located before movement; Is the frequency modulation rate, that is, the slope of the rising edge of the triangular wave, Is the frequency modulation bandwidth, Is the frequency modulation period;

[0127] Then, when measuring the distance to the position where the target is located before movement, the phase difference between the light intensities of the rising edge and the falling edge is ;

[0128] Similarly, assume that when measuring the distance to the position where the target is located after movement, the time difference corresponding to the optical path difference between the object light and the reference light is , and the time-domain expression of the intensity of the difference-frequency signal light obtained after the interference of the object light and the reference light is:

[0129]

[0130]

[0131] Wherein, and are respectively the light intensities of the rising edge and the falling edge of the beat frequency signal obtained after the interference of the object light and the reference light when measuring the distance at the position where the target is located after moving;

[0132] Then, when measuring the distance at the position where the target is located after moving, the phase difference between the light intensities of the rising edge and the falling edge is ;

[0133] Based on the intensity signals of the interference light at the positions where the target is located before and after moving, the combined phase difference is calculated:

[0134] ;

[0135] From , it is obtained that ;

[0136] Among them, because the phase difference between the light intensities of the rising edge and the falling edge is amplified by one time, the corresponding measured phase difference also needs to be amplified by one time, which is ;

[0137] Then, .

[0138] The range of is 0 - When

[0139]

[0140] Then The value range of is 0 - Among them, is the wavelength of the laser signal.

[0141] For phase difference ranging, its maximum ranging distance depends on the wavelength of the laser signal, and the ranging accuracy depends on the resolution of the phase difference.

[0142] The present invention further calculates the combined phase difference of the intensity signals of the interference light at the positions where the target is located before and after moving based on the phase difference between the light intensities of the rising edge and the falling edge, and uses the combined phase difference between the rising edge and the falling edge to obtain the distance difference. Compared with ranging by comparing the phase differences of the two rising edges, at the same phase difference resolution, the distance accuracy is doubled.

[0143] Among the above, based on the intensity signals of the interference light at the positions where the target is located before and after moving, the actually measured phase difference , specifically, it can be calculated using the existing technologies in the art, such as the cross-correlation method, the fast Fourier transform method, the Hilbert transform method, the zero-crossing method (interpolation method), etc. Cross-correlation method: Perform cross-correlation on two discrete signals, find the position of the correlation peak, calculate the time offset, and then convert it into a phase difference in combination with the sampling frequency. It is suitable for processing periodic signals and non-periodic signals. Fast Fourier transform (FFT) method: Perform FFT on the signal to obtain the phase information in the spectrum, and then calculate the phase difference between the same-frequency points of the two signals. This method is suitable for sinusoidal signals with stable frequencies. Hilbert transform method: Perform Hilbert transform on the signal to obtain the analytic signal (including amplitude and phase information), and then calculate the phase difference. Zero-crossing method (interpolation method): Find the zero-crossing points in the sampled data, accurately locate the time difference of the zero-crossing points through the interpolation method, and calculate the phase difference therefrom. It is suitable for data with high sampling rates.

[0144] In addition, the following embodiments can also be adopted for the calculation of the phase difference:

[0145] When performing signal processing, it is also possible to directly obtain the phase difference only by referring to the rising edge part when measuring the positions before and after the target movement, that is:

[0146]

[0147] From , it is obtained that ;

[0148] Similarly, the above-mentioned is obtained.

[0149] Such as Figure 3 is a typical FMCW lidar ranging system in the prior art. It is provided with two couplers, and the reference light and the object light do not form a common-path transmission in the fiber collimation unit. The beat signal obtained when this system performs ranging is as shown in Figure 4 . It can be seen that before the optical path of the present invention is improved, the beat signal does not have obvious sine wave characteristics, it is difficult to observe obvious sine wave signals from its signal, and its phase cannot be judged.

[0150] Figure 5 shows the beat signal obtained after the optical path of the high-precision laser phase ranging system with a common-path optical path of the present invention is improved. By comparing it with Figure 4 , it can be seen that the present invention can observe obvious parts with sine signal characteristics, and after fitting, it can be used to obtain phase information, and the waveform integrity is significantly improved compared with the signal before the optical path is improved. It can be seen that the present invention has better technical effects compared with the traditional FMCW lidar ranging system, and the technical improvement is realized.

[0151] In addition, compared with the typical FMCW lidar ranging system in the prior art, the present invention uses the reflection of the fiber collimation unit to form the reference light, which can reduce the use of couplers, reduce the energy loss generated by the couplers, and improve the overall efficiency of the system. At the same time, various errors introduced by the couplers are reduced. Meanwhile, the object light received by the fiber collimation unit and the reference light are co-propagated, reducing the non-common path error and further improving the ranging accuracy.

[0152] In summary, for the high-precision laser phase ranging system and method with a common path optical path provided by the present invention, compared with the traditional ranging optical path, a reflected light is generated at the end face of the fiber collimation unit as the reference light, avoiding the additional optical path introduced by the beam splitting scheme. The present invention combines the characteristics of the common path optical path. The object light received by the fiber collimation unit and the reference light reflected at the end face of the fiber collimation unit are co-propagated to form interference light, which is transmitted to the photodetector through the loop unit. When ranging, based on the intensity signals of the interference light corresponding to the positions of the target before and after movement, the phase difference is calculated. Since the object light and the reference light are co-propagated, the influence of the external environment (such as temperature, pressure, vibration, etc.) on the object light and the reference light is the same. When calculating the phase difference, the external interference can be mutually cancelled by phase subtraction, maximizing the measurement accuracy. What the present invention obtains is the distance of the target movement. Since there is no need to directly measure the distance between the target and the fiber collimation unit, there is no need to consider the transmission distance of the laser signal in the fiber optical path, avoiding the measurement error caused by this part of the transmission distance. The present invention uses the method of phase difference for measurement, greatly improving the measurement accuracy.

[0153] In summary, the present invention combines the strong anti-interference ability of the common path optical path, the high precision of the phase method, and the convenience of heterodyne interference signal processing, comprehensively improving the laser ranging effect.

[0154] In this article, specific examples are used to elaborate on the principle and implementation manner of the present invention. The description of the above embodiments is only used to help understand the method and its core idea of the present invention. At the same time, for those of ordinary skill in the art, according to the idea of the present invention, there will be changes in the specific implementation manner and application scope. In summary, the content of this specification should not be construed as a limitation to the present invention.

Claims

1. A high-precision laser phase ranging system for a common optical path, characterized in that: It comprises an arbitrary waveform generator (1), a semiconductor laser (2), a loop unit (3), an optical fiber collimation unit (4), a photodetector (5), a data acquisition device (6) and a processor (7); The arbitrary waveform generator (1) generates a frequency modulation signal, and performs frequency modulation on the laser signal of the semiconductor laser (2) by controlling the driving current of the semiconductor laser (2); The output end of the semiconductor laser (2) is connected to the loop unit (3) via an optical fiber; The loop unit (3) is connected to the optical fiber collimation unit (4) via an optical fiber, and is used to transmit the laser signal to the optical fiber collimation unit (4); The optical fiber collimation unit (4) is used to transmit a part of the laser signal to the target, collect the object light reflected by the target, and reflect a part of the laser signal to form reference light. The object light received by the optical fiber collimation unit (4) and the reference light are transmitted together. The loop unit (3) receives the object light and the reference light transmitted back by the optical fiber collimation unit (4), and transmits them to the photoelectric detector (5); The photoelectric detector (5) is used to convert the intensity signal of the interference light of the object light and the reference light into an electrical signal and transmit it to the data acquisition device (6); The data collector (6) is used to receive the electrical signal converted by the photoelectric detector (5) and transmit it to the processor (7); The processor (7) is used to receive the electrical signal and calculate the phase difference based on the intensity signal of the interference light corresponding to the position of the target before and after movement, so as to obtain a relative distance measurement value, wherein the relative distance measurement value is the distance moved by the target; The arbitrary waveform generator (1) is also used to generate a synchronous electrical signal, which is transmitted to a data collector (6), wherein the synchronous electrical signal has the same frequency as the frequency modulation signal.

2. The high-precision laser phase ranging system for common optical paths according to claim 1, characterized in that: The signal transmission mode between the loop unit (3) and the photoelectric detector (5) includes: one or more of optical fiber transmission, collimator conversion transmission, and free space optical transmission.

3. The high-precision laser phase ranging system for common optical paths according to claim 1, characterized in that: The loop unit (3) is provided with three interfaces, namely, port a, port b and port c, wherein port a is used to receive a laser signal from a DFB laser (2), port b is used to output the laser signal to a fiber collimation unit (4) and receive object light and reference light, and port c is used to transmit the object light and reference light to a photoelectric detector (5).

4. The high-precision laser phase ranging system for common optical paths according to claim 1, characterized in that: The optical fiber collimation unit (4) adopts an optical fiber collimator with a PC-type interface, a lens with transmission and reflection functions, or a combined structure of an optical fiber and a collimator.

5. The high-precision laser phase ranging system for common optical paths according to claim 1, characterized in that: The loop unit (3) adopts an optical fiber circulator.

6. The high-precision laser phase ranging system for common optical paths according to claim 1, characterized in that: The frequency modulation signal is a symmetrical triangle wave or a sawtooth wave, and the synchronous electrical signal is a square wave with a duty cycle of 50%.

7. A high-precision laser phase ranging method for a common optical path, applied to the high-precision laser phase ranging system for a common optical path according to any one of claims 1 to 6, characterized in that: The steps include: The arbitrary waveform generator (1) generates a frequency modulation signal and transmits it to the semiconductor laser (2), thereby frequency modulating the laser signal of the semiconductor laser (2); The frequency modulated laser signal is transmitted to the optical fiber collimation unit (4) through the loop unit (3); The optical fiber collimation unit (4) transmits a part of the laser signal to the target, collects the object light reflected by the target, and reflects a part of the laser signal to form a reference light; The object light received by the optical fiber collimation unit (4) and the reference light reflected at the end face of the optical fiber collimation unit (4) are transmitted together and transmitted to the photoelectric detector (5) via the loop unit (3); The photoelectric detector (5) converts the intensity signal of the interference light of the object light and the reference light into an electrical signal and transmits it to the data acquisition device (6); The data collector (6) receives the electrical signal converted by the photodetector (5) and transmits it to the processor (7); The processor (7) receives the electrical signal, calculates the phase difference based on the intensity signal of the interference light corresponding to the position of the target before and after the movement, and obtains a relative distance measurement value, wherein the relative distance measurement value is the distance the target moves; The calculation formula for the relative distance measurement value is as follows: In the formula, Δl represents the relative distance measurement value of the target's position before and after movement; τ1 is the time difference corresponding to the optical path difference between the object light and the reference light when measuring the distance at the position where the target was before moving; τ2 is the time difference corresponding to the optical path difference between the object light and the reference light when measuring the distance at the position where the target moves; The phase difference is actually measured based on the intensity signal of the interference light corresponding to the position of the target before and after movement; f0 is the initial frequency of the laser signal; c is the speed of light.

8. The high-precision laser phase ranging method for a common optical path according to claim 7, characterized in that: The phase difference is calculated based on the intensity signal of the interference light corresponding to the position of the target before and after the movement to obtain the relative distance measurement value, which specifically includes: When measuring the distance at the position where the target moves before, the time domain expression of the difference frequency signal intensity obtained after the interference of the object light and the reference light is: In the formula, I 上升 ,I 下降 are the light intensities of the rising and falling edges of the difference frequency signal obtained after the interference of the object light and the reference light when measuring the distance at the position where the target was before moving; I1 and I2 are the light intensities of the reference light and the object light respectively; τ1 is the time difference corresponding to the optical path difference between the object light and the reference light when measuring the distance at the position where the target was before moving; α=2B / T is the frequency modulation rate, that is, the slope of the rising edge of the triangular wave, B is the frequency modulation bandwidth, and T is the frequency modulation period; When measuring the distance at the position where the target was before it moved, the phase difference between the rising and falling edges is 4πF0τ1; Similarly, if the time difference corresponding to the optical path difference between the object light and the reference light is τ2 when the distance is measured at the position where the target moves, then the phase difference between the rising and falling edge light intensities is 4πf0τ2 when the distance is measured at the position where the target moves; Based on the intensity signal of the interference light corresponding to the position of the target before and after movement, the phase difference is calculated: 4πf0(τ1-τ2); but, 9. The high-precision laser phase ranging method for a common optical path according to claim 8, characterized in that: The range is 0-2π, when hour, Then the value range of Δl is 0-λ, where λ is the wavelength of the laser signal.

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