CN106094071B - FM-CW laser ranging dual-beam quick focusing method based on liquid lens - Google Patents

Abstract

The invention discloses a liquid-lens-based frequency modulation continuous wave laser ranging double-beam rapid focusing method. In the process of target point positioning, the rapid focusing of the red guiding laser is realized through a three-step search algorithm of rough, medium adjustment and fine adjustment. In the wave measurement laser automatic focusing, the compensation parameters are determined according to the relationship between the parameters of the red guiding laser and the frequency-modulated continuous wave laser, and thus the driving current value of the liquid lens when the red guiding laser is focused in the stage of target point positioning can be used to calculate the position. Frequency-modulated continuous wave measurement of the driving current value of the laser focus, without using other infrared sensing devices to search for the minimum focus spot position of the frequency-modulated continuous-wave laser in the infrared band, greatly reducing system complexity and saving frequency-modulated continuous wave measurement The focusing time of the laser can realize the direct and fast focusing of the frequency-modulated continuous wave laser after the stage is turned from the positioning stage to the measurement stage.

Description

Double-beam fast focusing method for frequency-modulated continuous wave laser ranging based on liquid lens

technical field

The invention relates to a liquid lens-based fast focusing method of a frequency-modulated continuous wave laser ranging beam.

Background technique

Frequency-modulated continuous wave laser ranging is an interferometric large-scale laser absolute ranging technology, which has the characteristics of high measurement accuracy, absolute measurement, and direct measurement of diffuse reflection targets. It has broad application prospects in large-scale geometric measurement, major equipment manufacturing, military technology, space technology and other fields.

The frequency-modulated continuous wave laser ranging system uses the red guiding laser to locate the position of the target point to be measured, and uses the frequency linearly modulated high-frequency laser to measure the absolute distance of the target point. In order to realize the position positioning and distance measurement of the target point to be measured, it is necessary to couple the visible light and the frequency-modulated continuous wave laser into the same optical fiber and then focus it on the target point to be measured by the zoom system. However, due to the large difference between the wavelength of visible light and frequency-modulated continuous wave laser, the light of the two wavelengths cannot be focused on one point at the same time due to the existence of chromatic aberration after passing through the zoom system.

There are two traditional solutions: the first method only focuses on the red guide laser, and the frequency-modulated continuous wave laser will remain in a converging state after passing through the zoom system within a certain range, but it cannot be fully focused at the target point to be measured. One point, but a spot with a diameter less than a certain value, as shown in Figure 2, this method is based on the premise of losing the frequency modulation continuous wave laser focusing effect to achieve the purpose of beam focusing at the same time, but there are two disadvantages in this method: the first , the frequency-modulated continuous wave laser used for ranging cannot be completely converged at one point at the target point to be measured, which will reduce the echo power and ranging accuracy of the system; second, this method is only suitable for a small ranging range. After this range, the frequency-modulated continuous wave laser will be in a divergent state when the red guiding laser can be focused, as shown in Figure 3, the divergence of the beam will reduce the ranging accuracy and echo power of the system, and with the increase of the distance to be measured, it will affect will become more serious. The second method adopts the method of time-sharing focusing, because the red guide laser is only used for the positioning of the target point, and the frequency-modulated continuous wave laser is only used for the measurement of the distance of the target point. , so it is possible to only focus on the red guide laser in the positioning stage without considering the focus of the frequency-modulated continuous wave laser, and in the measurement stage to focus only on the frequency-modulated continuous wave laser without considering the focus of the red guide laser, which not only ensures the accuracy of positioning and guarantee the accuracy of distance measurement. However, this method has a disadvantage. During the focusing process of the red-guided laser, camera-assisted feedback can be used to adjust the driving current of the liquid lens to achieve automatic focusing of the red-guided laser. However, the frequency-modulated continuous wave laser is invisible light and cannot be assisted by a camera. Focusing requires the use of photodetectors to detect the frequency-modulated continuous wave laser echo energy to find the best focus point, that is, the liquid lens driving current when the echo energy is maximum. The addition of a photodetector will increase the complexity of the system. At the same time, since the echo signal will be interfered by various noises when it propagates in the air, the echo energy detected by the photodetector is not accurate, which will affect the focus of the frequency-modulated continuous wave laser. Effect.

Contents of the invention

Aiming at the deficiencies of the existing frequency-modulated continuous wave laser ranging system for guiding and positioning lasers and measuring laser focusing methods, the present invention proposes a dual-beam fast focusing method for frequency-modulated continuous wave laser ranging systems based on liquid lenses. , the search steps are simplified during the positioning process by using the red guided laser, and the frequency-modulated continuous wave measurement laser auto-focus is directly calculated through the mathematical model of the liquid lens established by the present invention to obtain the corresponding driving current value of the liquid lens, and the present invention does not need to use other infrared sensing devices Searching for the minimum focus spot position of the frequency-modulated continuous wave laser in the infrared band greatly reduces the system complexity and the data processing requirements for the hardware system, and saves the focusing time of the frequency-modulated continuous wave laser, enabling the positioning stage Direct fast focusing of the frequency-modulated CW laser after the steering measurement phase.

In order to solve the above technical problems, the present invention proposes a two-beam rapid focusing method based on a liquid lens for frequency-modulated continuous wave laser ranging. The red guiding laser and the frequency-modulated continuous wave measuring laser are coupled into the same optical fiber to synthesize a beam of light, and the beam enters the A liquid lens. An aperture stop is used in the liquid lens to limit the light beam to the paraxial region of the optical system. The red guiding laser and the frequency-modulated continuous wave measurement laser are used for time-sharing focusing, and finally automatic focusing is realized. Specific steps are as follows:

Step 1. The red guide laser automatically focuses to realize the positioning of the target point to be measured, including

1-1. Coarse adjustment of liquid lens driving current:

The driving current of the liquid lens is decremented with the step size I _coarse , and the driving current value of the nth time is set to be

I _n =I _n-1 -I _coarse (1)

In the formula (1), In is the nth liquid lens driving current value in the liquid lens driving current adjustment process, and I _{n -1} is the n-1 liquid lens driving current value in the liquid lens driving current adjustment process, I _coarse is the adjustment step size of the liquid lens drive current;

Comparing the size S _n of the focused spot of the red-guided laser in the images captured by the camera under different driving currents of the liquid lens, the size of the focused spot of the red-guided laser will first decrease and then increase with the decrease of the driving current of the liquid lens The trend of , when S _n1-1 >S _n1 , S _n1+1 >S _n1 appears from a certain point n1, the liquid lens drive current stops decreasing, at this time, the lens drive current is I _n1 , where S _n1-1 , S _n1 , S _n1+1 are respectively the size of the focus spot of the red guiding laser when the liquid lens driving current value is adjusted for the n1-1th, n1th, n1+1th time;

1-2. Medium adjustment of liquid lens driving current:

The driving current of the liquid lens starts from I _n1 -I _coarse and decreases with the step size I _mid . I _mid <I _coarse . When S _n2-1 >S _n2 and S _n2+1 >S _n2 appear from a certain point n2, The driving current of the liquid lens stops decreasing, and at this moment, the driving current of the lens is _In2 ;

1-3. Liquid lens drive current fine-tuning:

The driving current of the liquid _{lens starts} from _{I n2 -I mid} and _decreases with the step size I _{fine .} The liquid lens driving current stops decreasing, and at this time, the lens driving current is _{In3; the liquid lens driving current I n3 is} output to the liquid lens;

Step 2: Frequency modulated continuous wave measurement of the liquid lens drive current I' when the laser is best focused,

I'=I _n3 +A (2)

In formula (2), A is a constant value, the wavelength of the red guided laser is 658nm, and the frequency-modulated continuous wave measures the laser wave

When the length is 1550nm, A=32.4nm;

Step 3: Output the liquid lens drive current I' determined in step 2 to the liquid lens to realize automatic focusing of the frequency-modulated continuous wave measurement laser.

Compared with prior art, the beneficial effect of the present invention is:

The liquid-lens-based frequency-modulated continuous-wave laser ranging double-beam fast focusing method of the present invention performs time-sharing focusing on the red guiding laser and the frequency-modulating continuous wave measuring laser, and uses three steps of coarse adjustment, middle adjustment, and fine adjustment for the red guiding laser in the positioning stage. Focusing algorithm simplifies the search process and saves focusing time; in the stage of target point distance measurement, the beam is limited to the paraxial region through the aperture diaphragm, and the liquid The mathematical model of the lens, according to the mathematical model of the liquid lens, the mathematical relationship between the corresponding liquid lens driving currents when the red guiding laser and the frequency-modulated continuous wave measuring laser reach the best focus state at the same distance from the target point can be calculated. The driving current value of the liquid lens when the frequency modulation continuous wave measurement laser is focused at this position and reaches the best focusing state can be calculated according to the driving current value of the liquid lens when the red guiding laser reaches the best focusing state during the target point positioning stage. The current value It can be calculated and obtained by software program, so as to realize the rapid focusing of the frequency-modulated continuous wave measurement laser after the transition from the positioning stage to the measurement stage, eliminating the complicated process of using the search algorithm to search for the best focus point position again and the use of other infrared sensing devices. It greatly reduces the system complexity and the data processing requirements of the hardware system, and saves the focusing time of the frequency-modulated continuous wave laser. The focusing time is fast, and it can realize the direct and fast focusing of the frequency-modulated continuous wave laser after the positioning stage turns to the measurement stage.

Description of drawings

Fig. 1 is the flow chart of the double-beam fast focusing method of the frequency-modulated continuous wave laser ranging system based on liquid lens in the present invention;

Figure 2 is a schematic diagram of the simultaneous convergence of the red guiding laser and the frequency-modulated continuous wave measuring laser;

Fig. 3 is a schematic diagram of red guided laser convergence and frequency modulated continuous wave measurement of laser divergence;

Fig. 4 is the relation curve between the spot size and the driving current in the red guiding laser auto-focusing process in the focusing method of the present invention;

Fig. 5 is a structural schematic diagram of a focusing device used in the focusing method of the present invention;

Fig. 6 is a schematic diagram of the connection relationship between the optical fiber and the liquid lens unit shown in Fig. 5;

Fig. 7 is a diagram of spot changes during the focusing process of the red guiding laser according to the embodiment of the present invention;

Fig. 8 is a focus spot diagram when the FM continuous wave measurement laser of the example of the present invention reaches the best focus state.

In the picture:

1-Tunable laser 2-Red laser 3-Fiber coupler

4-liquid lens unit 5-half mirror 6-camera

7-computer spot discrimination unit 8-lens driving circuit 9-surface of the object to be measured

10-aperture diaphragm 11-compensated plano-convex lens 12-liquid lens

13-FC/PC fiber optic adapter plate 14-threaded adapter 15, 17-gasket

16-C Thread Extension Tube 18-Housing

detailed description

The technical solution of the present invention will be further described in detail below in conjunction with the accompanying drawings and specific embodiments, and the described specific embodiments are only for explaining the present invention, and are not intended to limit the present invention.

The present invention is a liquid-lens-based frequency-modulated continuous-wave laser ranging double-beam fast focusing method, which is to couple the red guiding laser and the frequency-modulated continuous wave measurement laser into the same optical fiber to synthesize a beam of light, the beam enters a liquid lens, and is placed in the liquid lens The internal aperture diaphragm is used to limit the beam to the paraxial region of the optical system, and the red guiding laser and the frequency-modulated continuous wave measuring laser are used for time-sharing focusing, and finally automatic focusing is realized. The specific implementation steps are as follows:

As shown in Figure 1, all equipment in the FM continuous wave measurement laser system is preheated after being powered on, the system is initialized, and the liquid lens enters the DC mode and DC input and output mode.

After the equipment is warmed up and initialized, it enters the red-guided laser automatic focusing stage to realize the positioning of the target point to be measured, as shown in Figure 4. The specific steps are as follows:

1-1. Coarse adjustment of liquid lens driving current:

The driving current of the liquid lens is decremented with the step size I _coarse , and the driving current value of the nth time is set to be

I _n =I _n-1 -I _coarse (1)

In the formula (1), In is the nth liquid lens driving current value in the liquid lens driving current adjustment process, and I _{n -1} is the n-1 liquid lens driving current value in the liquid lens driving current adjustment process, I _coarse is the adjustment step size of the liquid lens drive current;

Comparing the size S _n of the focused spot of the red-guided laser in the images captured by the camera under different driving currents of the liquid lens, the size of the focused spot of the red-guided laser will first decrease and then increase with the decrease of the driving current of the liquid lens The trend of , when S _n1-1 >S _n1 , S _n1+1 >S _n1 appears from a certain point n1, the liquid lens drive current stops decreasing, at this time, the lens drive current is I _n1 , where S _n1-1 , S _n1 , S _n1+1 are respectively the size of the focus spot of the red guiding laser when the liquid lens driving current value is adjusted for the n1-1th, n1th, n1+1th time;

1-2. Medium adjustment of liquid lens driving current:

The driving current of the liquid lens starts from I _n1 -I _coarse and decreases with the step size I _mid . I _mid <I _coarse . When S _n2-1 >S _n2 and S _n2+1 >S _n2 appear from a certain point n2, The driving current of the liquid lens stops decreasing, and at this moment, the driving current of the lens is _In2 ;

1-3. Liquid lens drive current fine-tuning:

The driving current of the liquid _{lens starts} from _{I n2 -I mid} and _decreases with the step size I _{fine .} The liquid lens driving current stops decreasing. At this time, the lens driving current is _{In3; the liquid lens driving current I n3 is} output to the liquid lens, so far, the red-guided laser auto-focusing process ends;

After the positioning process of the target point to be measured is completed, the frequency-modulated continuous wave laser ranging system is transferred to the target point distance measurement stage, and the final stable driving current value I _n3 of the liquid lens determined according to the above-mentioned red-guided laser automatic focusing process and taking into account the correction and compensation parameters are obtained. When the frequency modulation continuous wave measurement laser reaches the best focus state at the target point distance, the driving current value of the liquid lens, when the liquid lens is driven by this liquid lens driving current value, the frequency modulation continuous wave measurement laser focuses on the target point to be measured , the liquid lens drive current I' when measuring the best focus of the laser with frequency modulated continuous wave,

I'=I _n3 +A (2)

In formula (2), A is a constant value, is 658nm at red guide laser wavelength, and when frequency modulation continuous wave measures laser wavelength is 1550nm, A=32.4nm;

The liquid lens driving current I' determined above is output to the liquid lens to realize automatic focusing of the frequency-modulated continuous wave measurement laser.

To sum up, when the red guiding laser is automatically focused to realize the positioning of the target point to be measured, after the red guiding laser reaches the best focusing state, the driving current value of the liquid lens at the best focusing point can be calculated according to the above formula (2) The driving current value of the liquid lens when the frequency-modulated continuous wave laser reaches the best focus state at this position can realize the direct and fast focusing of the frequency-modulated continuous wave measurement laser when the positioning stage turns to the measurement stage.

Applications:

The device for implementing the liquid lens-based frequency-modulated continuous wave laser ranging double-beam rapid focusing method of the present invention, as shown in Figure 5, the device includes a tunable laser 1, a red laser 2, a fiber coupler 3, a liquid lens unit 4, A half-mirror 5, a camera 6 and a computer spot discrimination unit 7.

The tunable laser 1 is used to generate narrow-linewidth frequency-modulated continuous wave laser; the red laser 2 is used to generate red guide laser; the fiber coupler 3 is used to combine the laser generated by the tunable laser 1 with the The laser light generated by the red laser 2 is coupled into the same optical fiber all the way, and the laser light exits from the optical fiber and enters the liquid lens unit 4 .

The liquid lens unit 4 includes a housing 18 and a lens driving circuit 8. An aperture stop 10, a compensating plano-convex lens 11 and a liquid lens 12 are sequentially arranged in the housing 18 along the laser direction, and the lens driving circuit 8 can drive The liquid lens 12 moves in the axial direction.

The half mirror 5 is arranged on the outgoing light side of the liquid lens unit 4, the camera 6 is arranged on the transmitted light side of the half mirror 5, and the target point 9 to be measured is arranged on the The reflected light side of the half-mirror 5; the half-mirror 5 reflects a part of the laser light back to the ranging system and forms a beat frequency with the emission signal of the tunable laser 1 for the calculation of the distance of the target point 9 to be measured, Another part of the laser light is transmitted into the camera 6 .

The camera 6 is used to capture the focused spot image of the red guiding laser, and transmit the focused spot image to the computer spot discrimination unit 7 .

The computer facula discrimination unit 7 is connected with the camera 6 through a data interface, and the computer facula discrimination unit 7 is connected to the lens drive circuit 8 through a USB interface; The drive current value of the liquid lens 12 when the focus spot in the spot image is the smallest; the lens drive circuit 8 keeps outputting the drive current value, so that the liquid lens 12 maintains a constant focal length.

Figure 6 shows an embodiment of the connection structure between the liquid lens unit and the optical fiber connected to the optical fiber coupler 3 in the present invention. The end of the optical fiber to the housing 18 is sequentially provided with a FC/PC optical fiber adapter plate 13, a threaded adapter Connector 14 , washer 15 , C thread extension pipe 16 , washer 17 , the external thread of the C thread extension pipe 16 is connected with the internal thread of the housing 18 . In the present invention, the optical fiber liquid lens unit is connected with the liquid lens unit through the FC/PC optical fiber adapter plate 13, the thread adapter 14, the C thread extension tube 16, the gasket, and the FC/PC optical fiber adapter plate 13 is connected to the screw thread Adapter 14, thread adapter 14 is connected to C thread extension tube 16, C thread extension tube is connected to liquid lens 18, gasket 15 and gasket 17 are respectively located on thread adapter 14, C thread extension tube 16, and C thread extension tube 16 Between the liquid lens 18, it is used to fine-tune the length of the connection structure, which connects the optical fiber connector and the liquid lens together, eliminating the complicated process of alignment adjustment between the optical fiber connector and the liquid lens, and increasing the stability of the device It improves the anti-seismic ability of the device. A plano-convex compensation lens 11 and an aperture stop 10 are added to the liquid lens unit, wherein the plano-convex compensation lens 11 reduces the overall focusing range of the liquid lens 12, and the aperture stop 10 limits the incident laser light to the paraxial area, reducing the impact of spherical aberration.

Using the above-mentioned device and according to the method of the present invention, the light beam is automatically focused on the target point to be measured at a position of about 10m. The red guide laser uses red light with a wavelength of 658nm. The spot formed by the laser at the position of the target point to be measured rapidly changed from large to small, and finally remained at the smallest spot. The changing process of the spot is shown in Figure 7. The minimum spot diameter is 1.341mm, and the focusing process takes less than 1s. After shifting from the positioning stage to the measurement stage, use an infrared beam analyzer to observe the focus of the frequency-modulated continuous wave measuring laser with a center wavelength of 1550nm. It is observed that the frequency-modulated continuous wave measuring laser rapidly focuses on one point, and the image of the focused spot is shown in Figure 8. The time is on the order of milliseconds, and the diameter of the focused spot is 1.344mm. Through the above examples, it is verified that the double-beam rapid focusing method of the liquid lens-based frequency-modulated continuous wave based light ranging system proposed by the present invention can realize fast and fully automatic focusing of the red guiding laser and the frequency-modulated continuous wave measuring laser, with small focusing spots and fast focusing time .

Although the present invention has been described above in conjunction with the accompanying drawings, the present invention is not limited to the above-mentioned specific embodiments, and the above-mentioned specific embodiments are only illustrative, rather than restrictive. Under the enlightenment of the present invention, many modifications can be made without departing from the gist of the present invention, and these all belong to the protection of the present invention.

Claims (1)

1. A two-beam fast focusing method for frequency-modulated continuous wave laser ranging based on liquid lens, characterized in that: red guide laser and frequency-modulated continuous wave measurement laser are coupled into the same optical fiber to synthesize a beam of light, and the beam enters a liquid lens unit , the liquid lens unit includes an aperture diaphragm, which limits the light beam to the paraxial region of the optical system, uses the red guiding laser and the frequency-modulated continuous wave to measure the time-sharing focusing of the laser, and finally realizes automatic focusing; the specific steps are as follows: Step 1. The red guide laser automatically focuses to realize the positioning of the target point to be measured, including: 1-1. Coarse adjustment of liquid lens driving current: The driving current of the liquid lens is decremented with the step size I _coarse , and the driving current value of the nth time is set to be I _n =I _n-1 -I _coarse (1) In the formula (1), In is the nth liquid lens driving current value in the liquid lens driving current adjustment process, and I _{n -1} is the n-1 liquid lens driving current value in the liquid lens driving current adjustment process, I _coarse is the adjustment step size of the liquid lens drive current; Comparing the size S _n of the focused spot of the red-guided laser in the images captured by the camera under different driving currents of the liquid lens, the size of the focused spot of the red-guided laser will first decrease and then increase with the decrease of the driving current of the liquid lens The trend of , when S _n1-1 >S _n1 , S _n1+1 >S _n1 appears from a certain point n1, the liquid lens drive current stops decreasing, at this time, the lens drive current is I _n1 , where S _n1 - ₁ , S _n1 , S _n1+1 are respectively the size of the focus spot of the red guiding laser when the liquid lens driving current value is adjusted for the n1-1th, n1th, n1+1th time; 1-2. Medium adjustment of liquid lens driving current: The driving current of the liquid lens starts from I _n1 -I _coarse and decreases with the step size I _mid , I _mid <I _coarse , when S _n2-1 >S _n2 and S _n2+1 >S _n2 appear from a certain point n2, The driving current of the liquid lens stops decreasing, and at this moment, the driving current of the lens is _In2 ; 1-3. Liquid lens drive current fine-tuning: The driving current of the liquid lens starts from I _n2 -I _mid and decreases with the step size I _fine , I _fine < I _mid , when S _n3-1 > S _n3 and S _n3+1 > S _n3 appear from a certain point n3, The liquid lens driving current stops decreasing, and at this time, the lens driving current is _{In3; the liquid lens driving current I n3 is} output to the liquid lens; Step 2: Frequency modulated continuous wave measurement of the liquid lens drive current I' when the laser is best focused, I'=I _n3 +A (2) In formula (2), A is a constant value, is 658nm at red guide laser wavelength, and when frequency modulation continuous wave measures laser wavelength is 1550nm, A=32.4nm; Step 3: Output the liquid lens drive current I' determined in step 2 to the liquid lens to realize automatic focusing of the frequency-modulated continuous wave measurement laser.