A system and method for real-time in-situ measurement of a laser far-field divergence angle

Abstract

The present application belongs to the field of measurement technology, and relates to a kind of laser far field divergence angle real-time in-situ measurement system and method.The system includes laser emission unit, detection imaging unit, detection control unit, data processing unit, beam monitoring unit.The system and method proposed by the present application adopt short-wave infrared camera with integrated range-gated function, can obtain distance information and spatial distribution of laser spot in real time without cooperative target or complex laboratory device, and solve the divergence angle of laser in real time.The present application is especially suitable for laser performance monitoring and evaluation under field, mobile platform and dynamic change conditions.

Description

Technical Field

[0001] This invention belongs to the field of measurement technology and relates to a real-time in-situ measurement system and method for the far-field divergence angle of a laser. Background Technology

[0002] The far-field divergence angle of a laser beam, as one of the core parameters for measuring the spatial transmission characteristics of a laser beam, is a key indicator for evaluating the performance of a laser system and the quality of the beam. In the field of laser communication, especially in high-speed space optical communication and space-to-ground and inter-satellite link systems, a smaller far-field divergence angle helps reduce the geometric spread loss of the beam during long-distance transmission, improving signal reception strength and communication stability. In lidar and precision ranging, the beam divergence angle directly affects spatial resolution and ranging accuracy. In high-tech applications such as industrial processing, remote sensing, optical guidance, laser interferometry, and directional energy transfer, there are clear and strict constraints on the far-field divergence characteristics of laser beams.

[0003] Currently, the measurement of far-field laser divergence angle mainly relies on two types of methods: one is based on theoretical calculations and constructs an optical imaging system in a laboratory environment to acquire and analyze the laser spot image using an imaging camera; the other is to set up cooperative targets such as reflective targets in practical application scenarios, acquire the laser spot image formed by laser irradiation at a known distance, and then obtain the divergence angle data through image processing. These methods are typically complex to operate, sensitive to environmental conditions, and difficult to perform dynamic real-time measurements without cooperative targets. Measurement data obtained in a laboratory environment often deviates significantly from theoretical expectations in actual long-distance application scenarios due to factors such as changes in air quality, atmospheric turbulence, and disturbances in other transmission media, thus rendering the divergence angle parameters calculated based on ideal models inaccurate and unreliable. Therefore, developing a real-time in-situ measurement technology and device for laser far-field divergence angle that can adapt to complex external environments, achieve rapid, accurate, and easily integrated systems, is of significant theoretical and engineering application value, especially in the absence of cooperative targets. Summary of the Invention

[0004] This invention provides a real-time in-situ measurement system and method for the far-field divergence angle of a laser, solving the problem in existing technologies where it is difficult to continuously, accurately, and frequently monitor the divergence angle of a laser beam in complex outdoor environments. By integrating a range-gated short-wave infrared camera and a signal delay generator, the system enables real-time, continuous, and in-situ measurement of the far-field laser divergence angle without interrupting laser emission or moving equipment, and without any cooperative targets.

[0005] The technical solution provided by this invention is as follows: A real-time in-situ measurement system for the far-field divergence angle of a laser, comprising:

[0006] A laser emitting unit, used to generate and emit a laser beam, includes a laser water cooling device, a laser power controller, a laser emitting head, and a two-dimensional oscillating mirror connected to the light output port of the laser emitting head;

[0007] The detection imaging unit is used to perform in-situ real-time imaging of the laser beam at the target distance. It includes a short-wave infrared camera with range gating function, a gimbal for adjusting the camera's pointing direction, a telescope, and a predetermined band filter placed in front of the telescope.

[0008] The detection and control unit, used to generate a distance gating control signal, includes a beam splitter arranged along the laser emission path, an APD detector arranged on the reflection path of the beam splitter, a discrimination circuit and a pulse conditioning circuit connected to the output of the APD detector, a delay generator, and an oscilloscope for monitoring the output signal of the APD detector. The beam splitter reflects part of the laser to the APD detector. The output of the APD detector is processed by the discrimination circuit and the pulse conditioning circuit and then input to the delay generator. The delay generator outputs a gating control signal to the external trigger interface of the short-wave infrared camera to control the camera to expose and image within a set distance gate.

[0009] The data processing unit, connected to the shortwave infrared camera, is used to process the acquired images and calculate the far-field divergence angle of the laser in real time.

[0010] The beam monitoring unit is used to monitor the laser emission direction in real time.

[0011] Furthermore, the pulse conditioning circuit is used to widen the pulse width and adjust the amplitude of the digital pulse output by the identification circuit.

[0012] Furthermore, the shortwave infrared camera and telescope are mounted together on a gimbal, and the wavelength of the filter is matched with the laser emission wavelength.

[0013] Furthermore, the two-dimensional swing mirror is used to adjust the laser emission azimuth and elevation angles.

[0014] Furthermore, the present invention provides a method for real-time in-situ measurement of the far-field divergence angle of a laser, comprising the following steps:

[0015] S1. Drive the laser emitting unit to generate a laser beam. After the laser beam is split by the beam splitter, the emission direction is controlled by the two-dimensional swing mirror.

[0016] S2. Part of the beam reflected by the beam splitter is received by the APD detector. One of its output signals is connected to the oscilloscope to monitor the laser status, and the other is converted into a digital pulse signal by the discrimination circuit and pulse conditioning circuit and then input to the delay generator.

[0017] S3. Set the channel delay and gate width parameters corresponding to the target distance in the delay generator, generate a gating control signal with preset delay and width, and send it to the shortwave infrared camera.

[0018] S4. The short-wave infrared camera focuses in continuous exposure mode, and after capturing the laser beam, it switches to external trigger mode, receives the gating control signal, and acquires the laser beam image within a set distance gate.

[0019] S5. The data processing unit processes the acquired image in real time, performs double integration on the light intensity distribution function within the selected spatial region, inverts the peak light intensity at the center of the beam, determines the pixel width corresponding to the beam waist radius in the image, calculates the beam waist radius of the spot in the actual space based on the correspondence between pixel size and spatial size, and calculates the far-field divergence angle of the laser by combining the transmission distance of the light column.

[0020] Furthermore, in step S4, a filter matching the laser band is installed in front of the shortwave infrared camera lens to suppress background noise.

[0021] Furthermore, in step S5, the data processing unit performs cumulative averaging on multiple frames of images and then extracts the spot size information.

[0022] Furthermore, it also includes a step of monitoring the laser emission direction in real time through a beam monitoring unit to ensure the safety of the laser transmission path.

[0023] Compared with existing technologies, the system and method proposed in this invention employ a short-wave infrared camera with integrated range gating function, enabling real-time acquisition of laser spot distance information and spatial distribution without the need for cooperative targets or complex laboratory equipment. This invention is particularly suitable for laser performance monitoring and evaluation in outdoor fields, on mobile platforms, and under dynamically changing conditions. Attached Figure Description

[0024] Figure 1 This is a hardware connection diagram of the real-time in-situ measurement system for the far-field divergence angle of a laser provided in this embodiment of the invention; in the diagram, 1 is a water cooler; 2 is a power controller; 3 is a laser emitter; 4 is a two-dimensional tilting mirror; 5 is a short-wave infrared camera; 6 is an electronically controlled pan-tilt unit; 7 is a telescope; 8 is a filter; 9 is a beam splitter; 10 is an APD detector; 11 is a discrimination circuit; 12 is a pulse conditioning circuit; 13 is a multi-channel digital signal delay generator; 14 is an oscilloscope; 15 is a host computer system; and 16 is a short-wave infrared night vision device.

[0025] Figure 2 This is a schematic diagram of the imaging after range gating;

[0026] Figure 3 It is a laser beam image captured by a short-wave infrared camera;

[0027] Figure 4 This is a schematic diagram of pixel group selection for the image after distance gating during divergence angle calculation;

[0028] Figure 5 This is a schematic diagram of pixel group selection for laser beam imaging during divergence angle calculation. Detailed Implementation

[0029] To facilitate understanding of the present invention, the specific embodiments of the present invention will be described in further detail below with reference to the accompanying drawings and specific examples. The following examples or drawings are used to illustrate the present invention, but are not intended to limit the scope of the present invention.

[0030] Example 1: The real-time in-situ measurement system for the far-field divergence angle of a laser provided in this example includes:

[0031] The laser emitting unit drives a high-power laser, modifies various laser parameters in real time, and projects the laser beam to any location. The detection and imaging unit performs in-situ, real-time detection and imaging of the laser beam at any distance. The detection and control unit sets the distance-gated parameters and connects to both the laser emitting unit and the detection and imaging unit. The data processing unit processes the laser beam image from the detection and imaging unit and calculates the laser's far-field divergence angle in real time. The beam monitoring unit uses a short-wave infrared night vision device to detect the laser beam's position throughout the measurement process.

[0032] like Figure 1 As shown, the laser emitting unit includes a water cooler 1, a power controller 2, a laser emitting head 3, and a two-dimensional tilting mirror 4. The water cooler 1 is connected to the laser emitting head 3 to adjust the temperature of the laser emitting head. The power controller 2 is used to regulate the power of the laser. The laser emitting head 3 is used to amplify and emit light. The two-dimensional tilting mirror 4 is connected to the light outlet of the laser emitting head to adjust the emission direction of the laser.

[0033] The detection and imaging unit includes a short-wave infrared camera 5 with range gating, an electronically controlled pan-tilt unit 6, a telescope 7, and a filter 8. The short-wave infrared camera 5 is used for in-situ, real-time monitoring of the target light beam. The electronically controlled pan-tilt unit 6 controls the left-right and tilt adjustments of the short-wave infrared camera 5 and the telescope 7. The telescope 7 is rigidly mounted on the high-precision electronically controlled pan-tilt unit 6 and is used for long-range detection. The specific-band filter 8 selectively allows light of the same wavelength as the laser-emitted light beam to pass through, reducing noise interference. The filter 8 is specifically designed for laser echo signals and is installed in the front sleeve of the short-wave infrared camera 5. The two are connected by a precision lens adapter ring, and then mechanically clamped and aligned with the optical path by the fixing cylinder at the eyepiece end of the telescope, forming a complete detection and imaging unit.

[0034] The detection and control unit includes a beam splitter 9, an APD detector 10, a discrimination circuit 11, a pulse conditioning circuit 12, a multi-channel digital signal delay generator (DG645) 13, and an oscilloscope 14. The beam splitter 9 is used to split the laser beam emitted from the laser. The APD detector 10 is used to detect a small portion of the reflected light split by the beam splitter, generating an analog pulse signal. The discrimination circuit 11 is used to receive the analog signal from the APD detector and perform level discrimination. The pulse conditioning circuit 12 is used to adjust the amplitude and pulse width of the digital pulse signal output by the discrimination circuit. The DG645 is used to perform corresponding delay and broadening based on the laser emission indication signal, ultimately setting the gating imaging parameters of the distance-gated shortwave infrared camera 5. The oscilloscope 14 is used to monitor the amplitude and width of the analog signal from the APD detector in real time, thereby determining the stability of the laser.

[0035] The data processing unit is integrated into the host computer system 15 and connected to the shortwave infrared camera 5. It is mainly responsible for the real-time processing and analysis of the information collected by the APD detector. It mainly has the functions of image display and storage, multi-frame cumulative averaging processing, real-time calculation of spot size and real-time analysis of beam divergence angle.

[0036] The beam monitoring unit refers to a short-wave infrared night vision device 16 used for a wide field of view, which monitors the laser emission beam to ensure safe transmission in the atmosphere and prevent it from being directed at pedestrians or buildings.

[0037] Example 2: This example provides a real-time in-situ measurement method for the far-field divergence angle of a laser, which is adapted to the measurement system of Example 1. Using this method, the far-field divergence angle of a laser can be measured at night. Real-time, in-situ measurements were performed on a 1064 nm laser beam at a distance of km that did not illuminate the cooperative target, and its divergence angle was calculated simultaneously. The specific steps are as follows:

[0038] (1) Start the laser water heater and, after it has stabilized and the temperature has dropped to the set range, turn on the power controller. The laser is first emitted in the form of oscillating light, pointing towards the atmosphere. Then, the energy is amplified step by step, and finally emitted with energy in the range of hundreds of millijoules. At the same time, a beam splitter at a 45° angle is placed between the laser output port and the two-dimensional oscillating mirror, and an APD detector is placed in the direction of reflection at a 45° angle. Throughout this process, the laser emission direction is monitored with the help of a short-wave infrared night vision device to ensure that it is accurately pointed towards the distant atmospheric region.

[0039] (2) After the output parameters of the high-power laser are debugged and optimized, the optical detection unit is integrated and assembled. In this embodiment, a telescope with a focal length of 650 mm is selected as the main optical receiving component and rigidly mounted on a high-precision electronically controlled two-dimensional gimbal. The gimbal has independent horizontal azimuth and pitch angle adjustment functions. Through the control of the host computer, the optical axis of the receiving system can be quickly and accurately pointed within a large spatial range, thereby flexibly aligning with the target to be measured at different azimuths and heights.

[0040] To clarify the spatial resolution and coverage capabilities of the system, the imaging field of view of the selected telescope was calculated. The imaging field of view is determined by the detector photosensitive surface size and the telescope focal length, and the field of view angle is calculated using the following formula:

[0041] Equation (1);

[0042] Equation (2);

[0043] in, and These are the horizontal and vertical dimensions of the photosensitive surface of the APD detector, respectively. This is the focal length of the telescope.

[0044] The actual imaging coverage of the system was then calculated, given that the horizontal distance between the target and the APD detector was known to be... km, then the horizontal width of the field of view coverage with vertical height The calculation can be performed according to equations (3) and (4):

[0045] Equation (3);

[0046] Equation (4);

[0047] In the formula, Given the imaging distance, the area that can be imaged is calculated as follows: × m 2 .

[0048] After assembling the telescope and gimbal and calculating the field of view, the integration of the imaging unit continues. This embodiment uses a short-wave infrared camera with range gating. The front end of the camera is equipped with a narrowband filter specifically designed for 1064 nm laser echo signals, which is mounted inside a sleeve. The two are connected via a precision lens adapter ring. Finally, the imaging unit is mechanically tightened and optically aligned with the fixing cylinder at the telescope eyepiece end, forming a complete detection imaging unit.

[0049] (3) Make overall adjustments to the detection imaging unit, set the shortwave infrared camera to internal trigger working mode, and adjust the exposure time and gain parameters in a coordinated manner. Gradually increase the exposure time and use an appropriate gain coefficient until the image of the transverse penetrating light pillar formed by the laser beam can be clearly observed in the camera's field of view. The light pillar should show a continuous and uniform energy distribution and penetrate the horizontal direction of the field of view.

[0050] Connect the output signal terminal of the APD detector next to the beam splitter to a 1-to-2 SMA cable, with one end connected to an oscilloscope to monitor its analog electrical pulse signal waveform in real time. While maintaining stable laser operation, slowly rotate the APD detector body to fine-tune its photosensitive surface to a certain angle with the reflected beam, thereby changing the beam's illumination position and coverage on the photosensitive surface, adjusting the effective light-gathering area, and thus controlling the detector's output voltage amplitude. When the oscilloscope displays an analog pulse signal with a peak voltage of approximately 2 V, a stable waveform, and good repeatability, the APD detector can be considered to have reached optimal alignment.

[0051] (4) After confirming the signal is correct using an oscilloscope, connect the other end of the 1-to-2 SMA cable to the level discrimination circuit. When the input signal exceeds the preset threshold of 2V, the circuit outputs a digital pulse, indicating that the signal has been effectively recognized. This pulse is then transmitted to the pulse conditioning circuit, where it is shaped to generate a standard digital square wave signal with an amplitude of 3.3V, a pulse width of 40ns, and an impedance of 50Ω. This square wave signal is input to the T0 port of the DG645 as the reference signal for the laser emission time. At the same time, the inherent delay of the hardware circuit is measured using an oscilloscope and should be deducted in subsequent data processing.

[0052] The round-trip flight time of a photon can be calculated using the following formula:

[0053] Equation (5);

[0054] In the formula, To measure distance, Using the speed of light as a reference, we can calculate the expected travel time of a photon. The delay of the DG645 was adjusted to be less than the expected photon flight time, and the gate width was configured to the camera's maximum parameter of 10 μs. At this point, the short-wave infrared camera should... km to Objects km away (observation distance of the light beam) It should be included arrive (between). Subsequently, the synchronous trigger signal output by the DG645 is connected to the external trigger interface of the shortwave infrared camera as the gate width control signal for range-gated imaging. During image acquisition, 1000 consecutive frames of images are accumulated and averaged to improve the signal-to-noise ratio and suppress random noise, thereby enhancing the target signal extraction capability. After completing the above parameter settings, the telescope's field of view is driven to scan from a large angle to a small angle along the parallel direction of laser emission by adjusting the left and right azimuth angles of the pan-tilt unit. At this time, the image features displayed on the host computer show that only the light column appears in the right third of the area, and there is no significant signal in the remaining area. Based on this, the start time parameter of range-gated imaging is fixed, and the end time of gating is gradually adjusted to control the imaging distance range by changing the gating gate width. This process is repeated until the image state is adjusted to Figure 2 As shown. The image captured by the host computer at this time is as follows. Figure 3 As shown, a strip of light with significantly higher brightness than the background is visible within the field of view. At this moment, the light column is... km to Images between km.

[0055] (5) The system saves the acquired light column images and transmits them to the host computer for real-time quantitative analysis. According to the aforementioned calculations, at a distance of... At the target location km, the instantaneous field of view area presented by a telescope with a focal length of 650 mm is: × m 2 The system's imaging detection unit is a short-wave infrared camera, whose imaging sensor contains [data / capabilities] in the horizontal direction. Valid pixels, containing in the vertical direction Each effective pixel constitutes a complete image acquisition matrix. By dividing the physical dimensions of the field of view in the horizontal and vertical directions by the number of pixels in the corresponding directions, the image at a specified distance can be accurately calculated. At a distance of km, the actual spatial geometric dimension corresponding to each pixel is called pixel resolution.

[0056] (6) When the data processing unit in the host computer processes the acquired image, it selects the effective bright area in the middle of the light column image and sums up the pixel values ​​in this area. The sum of these pixel values ​​represents the energy mapping relationship of the light intensity information of the beam obtained by the distance-gated imaging system on the image plane of the short-wave infrared camera under actual detection conditions. Figure 4 As shown, based on the target distance value determined by the range gating technique, and combined with the straight-line distance between the laser and the short-wave infrared camera, the spatial angle between the central axis of the light beam and the optical axis of the camera's field of view can be further calculated. Meanwhile, the data processing unit performs centroid calculations based on the acquired image data, and automatically extracts a line from the image using the calculated centroid as a reference. Figure 5 The pixel path shown has a specific tilt direction. The pixel values ​​of the pixels covered by this path are summed to obtain the total pixel value. The angle of inclination of this oblique line is the same as the angle between the light column and the optical axis of the camera's field of view, calculated previously. Maintain consistency. By Figure 4 and Figure 5 It can be seen that the cumulative value of all pixels along the diagonal line corresponds to the mapping of the total energy of the longitudinal slice of the light beam within the selected pass distance onto the camera's focal plane. Furthermore, assuming the laser beam propagates along the Z-axis, the intensity distribution in this coordinate system can be described as a function of the horizontal coordinate X and the vertical coordinate Z. Its specific form is shown in Equation 6:

[0057] Equation (6);

[0058] in, This indicates the peak light intensity at the center of the beam. Let be the beam waist radius, expressed by equation (7). The radius of the light spot propagating to position Z is expressed by equation (8):

[0059] Equation (7);

[0060] Equation (8);

[0061] In equation (8) The wavelength of the laser; This represents the coordinate position along the direction of light propagation.

[0062] After obtaining the theoretical model, the data processing unit inverts the model parameters based on the acquired image data through integral operations. Subsequently, the light intensity distribution function... Perform a double integral over a specific spatial region until it equals the sum of the pixel values ​​within the corresponding region in the image. As shown in equation (9):

[0063] Equation (9).

[0064] according to Figure 4 The imaging geometry shown corresponds to a longitudinal slice within the gated distance range, with the integration interval along the Z direction being determined by the tilt angle of the light beam in space. Decision. The starting distance for strobing is... The termination distance is Then the lower limit of integration is The maximum number of points is This correction factor The angle between the optical column axis and the camera's field of view ensures that the integration path aligns with the actual longitudinal extension direction of the optical column. The integration interval along the X-direction is from negative infinity to positive infinity.

[0065] By solving the integral equation represented by equation (9), the peak light intensity at the center of the beam can be obtained by inversion. The calculated By substituting into equation (7), we can determine the result. The corresponding pixel width in the image represents the projected size of the beam waist on the detector image plane. Based on the previously calculated correspondence between pixel size and spatial size, the beam waist radius of the light spot in the actual space can be calculated in real time. Combined with the transmission distance of the light beam, the divergence angle of the laser in the far field can be calculated.

Claims

1. A real-time in-situ measurement system for the far-field divergence angle of a laser, characterized in that, include: A laser emitting unit, used to generate and emit a laser beam, includes a laser water cooling device, a laser power controller, a laser emitting head, and a two-dimensional oscillating mirror connected to the light output port of the laser emitting head; The detection imaging unit is used to perform in-situ real-time imaging of the laser beam at the target distance. It includes a short-wave infrared camera with range gating function, a gimbal for adjusting the camera's pointing direction, a telescope, and a predetermined band filter placed between the camera and the telescope. The detection and control unit, used to generate a distance gating control signal, includes a beam splitter arranged along the laser emission path, an APD detector arranged on the reflection path of the beam splitter, a discrimination circuit and a pulse conditioning circuit connected to the output of the APD detector, a delay generator, and an oscilloscope for monitoring the output signal of the APD detector. The beam splitter reflects part of the laser to the APD detector. The output of the APD detector is processed by the discrimination circuit and the pulse conditioning circuit and then input to the delay generator. The delay generator outputs a gating control signal to the external trigger interface of the short-wave infrared camera to control the camera to expose and image within a set distance gate. The data processing unit, connected to the shortwave infrared camera, is used to process the acquired images and calculate the far-field divergence angle of the laser in real time. The beam monitoring unit is used to monitor the laser emission direction in real time.

2. The system according to claim 1, characterized in that, The pulse conditioning circuit is used to widen the pulse width and adjust the amplitude of the digital pulse output by the identification circuit.

3. The system according to claim 1, characterized in that, The shortwave infrared camera and telescope are mounted together on a gimbal, and the wavelength of the filter is matched with the laser emission wavelength.

4. The system according to claim 1, characterized in that, The two-dimensional swing mirror is used to adjust the laser emission azimuth and elevation angles.

5. A method for real-time in-situ measurement of the far-field divergence angle of a laser using the system described in any one of claims 1-4, characterized in that, Includes the following steps: S1. Drive the laser emitting unit to generate a laser beam. After the laser beam is split by the beam splitter, the emission direction is controlled by the two-dimensional swing mirror. S2. Part of the beam reflected by the beam splitter is received by the APD detector. One of its output signals is connected to the oscilloscope to monitor the laser status, and the other is converted into a digital pulse signal by the discrimination circuit and pulse conditioning circuit and then input to the delay generator. S3. Set the channel delay and gate width parameters corresponding to the target distance in the delay generator, generate a gating control signal with preset delay and width, and send it to the shortwave infrared camera. S4. The short-wave infrared camera focuses in continuous exposure mode, and after capturing the laser beam, it switches to external trigger mode, receives the gating control signal, and acquires the laser beam image within a set distance gate. S5. The data processing unit processes the acquired image in real time, performs double integration on the light intensity distribution function within the selected spatial region, inverts the peak light intensity at the center of the beam, determines the pixel width corresponding to the beam waist radius in the image, calculates the beam waist radius of the spot in the actual space based on the correspondence between pixel size and spatial size, and calculates the far-field divergence angle of the laser by combining the transmission distance of the light column.

6. The method according to claim 5, characterized in that, In step S4, a filter matching the laser band is installed in front of the shortwave infrared camera lens to suppress background noise.

7. The method according to claim 5, characterized in that, In step S5, the data processing unit performs cumulative averaging on multiple frames of images and then extracts the spot size information.

8. The method according to claim 5, characterized in that, It also includes steps to monitor the laser emission direction in real time through a beam monitoring unit to ensure the safety of the laser transmission path.

Images