CN109212550B - Frequency modulation continuous wave laser ranging method of integrated FP etalon - Google Patents

Frequency modulation continuous wave laser ranging method of integrated FP etalon Download PDF

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CN109212550B
CN109212550B CN201811067690.7A CN201811067690A CN109212550B CN 109212550 B CN109212550 B CN 109212550B CN 201811067690 A CN201811067690 A CN 201811067690A CN 109212550 B CN109212550 B CN 109212550B
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时光
黑克非
王文
郑磊珏
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Hangzhou Dianzi University
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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
    • G01S17/00—Systems using the reflection or reradiation of electromagnetic waves other than radio waves, e.g. lidar systems
    • G01S17/02—Systems using the reflection of electromagnetic waves other than radio waves
    • G01S17/06—Systems determining position data of a target
    • G01S17/08—Systems determining position data of a target for measuring distance only
    • G01S17/32—Systems determining position data of a target for measuring distance only using transmission of continuous waves, whether amplitude-, frequency-, or phase-modulated, or unmodulated

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Abstract

The invention discloses a frequency modulation continuous wave laser ranging method of an integrated FP etalon. The optical fiber Mach-Zehnder interferometer is used as an auxiliary interference system to perform equal optical frequency interval resampling, a long optical fiber is used as a measurement reference, the stability is poor, and a dispersion phenomenon exists. In the invention, an FP etalon is added in a frequency modulation continuous wave laser ranging system as an auxiliary measuring system to resample the sampling signal at equal optical frequency intervals, and then the resample signal is processed by using a Hilbert transform algorithm to obtain the absolute distance of the measured target. The invention takes the cavity length of the FP etalon instead of the long optical fiber as the distance measuring reference, overcomes the influence of the long optical fiber on the environmental sensitivity and the optical fiber dispersion on the distance measuring precision, and can realize the measurement with higher precision under the condition that the modulation range of the laser is smaller.

Description

Frequency modulation continuous wave laser ranging method of integrated FP etalon
Technical Field
The invention belongs to the technical field of laser three-dimensional distance measurement, and particularly relates to a frequency modulation continuous wave laser distance measurement method of an integrated FP etalon.
Background
In recent years, frequency modulated continuous wave laser ranging receives more and more attention in the field of laser ranging, and has the advantages of high measurement precision, capability of realizing absolute ranging, capability of directly measuring a diffuse reflection target and the like. The measurement accuracy of frequency modulated continuous wave laser ranging is affected by the scanning range and the scanning linearity of the laser, so that the obtained interference signal needs to be processed off-line by using an equal optical frequency interval resampling method to improve the measurement accuracy. At present, in order to use the equal optical frequency interval resampling method, a set of optical fiber Mach-Zehnder interference system is generally added in a frequency modulation continuous wave laser ranging system to realize equal optical frequency interval resampling. However, because a long optical fiber is included as a measurement reference, the stability is poor, and dispersion phenomenon exists, so that the measurement accuracy is not ideal.
Disclosure of Invention
The invention aims to provide a frequency modulation continuous wave laser ranging method integrated with an FP etalon aiming at the defects of the prior art. In the invention, an FP etalon is added in a frequency modulation continuous wave laser ranging system as an auxiliary measuring system to resample the sampling signal at equal optical frequency intervals, and then the resample signal is processed by using a Hilbert transform algorithm to obtain the absolute distance of the measured target.
The method comprises the following steps:
step 1, a frequency modulation continuous wave laser ranging system integrated with an FP etalon is built, in the laser ranging system, a tunable laser controller controls a tunable laser to emit narrow-linewidth laser with optical frequency linearly modulated, the laser sequentially passes through an optical isolator, a collimating lens group and a half wave plate, then is divided into two paths through a polarization beam splitter prism, and respectively enters a measurement interference system and an FP interference system, and one path of interference signal is respectively obtained in the measurement interference system and the FP interference system. Wherein the FP etalon of the FP interferometric system is placed in a vacuum chamber.
And 2, synchronously acquiring data of interference signals of the measurement interference system and the reference interference system by two channels of the high-speed data acquisition system respectively to obtain signals sig1 and sig 2. The first peak point of a signal sig2 after a modulation start signal of the tunable laser is sent out is used as a starting point, the last peak point of a signal sig2 before a modulation stop signal of the tunable laser is sent out is used as a stop point, and the signal sig2 comprises k peak points in total between the starting point and the stop point.
And 3, performing secondary sampling on the signal sig1 at each peak point position of the signal sig2 to obtain a resampled signal sig 3.
Step 4, recording the number M of maximum values and minimum values of the signal sig1 between two adjacent peak points of the signal sig2i(i-1, 2, … …, k-1) to obtain MiHas an average value of Mmean。
And 5, solving the instantaneous phase of the resampled signal sig3 by using Hilbert transform, and unwrapping the obtained instantaneous phase to obtain an increasing curve. Then, the curve is linearized by means of a least squares methodFitting, and obtaining the slope of the straight line after fitting as l. Let fI＝l/(2π)，fII.e. the frequency of resampled signal sig 3.
And 6, calculating the distance L of the measured target by using the following formula.
Figure BDA0001798714570000021
Wherein M is MmeanInteger part of (d)FPIs the cavity length of the FP etalon, ngIs the refractive index of air.
Further, the tunable laser adopts an external cavity type narrow linewidth tunable laser with the central wavelength of 633nm and the modulation range of 1 nm.
The step 1 is further described as follows: in the measuring interference system, laser is divided into two paths by a non-polarizing beam splitter prism, is respectively reflected by a measuring retro-reflection prism and a reference retro-reflection prism, then is converged on a semi-transparent semi-reflection surface of the non-polarizing beam splitter prism, enters an optical fiber through a collimating lens, is divided into two paths through a 50:50 optical fiber coupler, and respectively enters two detection ports of a balance detector to form interference signals.
The step 1 is further described as follows: in the FP interference system, laser enters the FP etalon after sequentially passing through a reflector and a first convex lens; and the emergent laser of the FP etalon passes through a second convex lens and is focused on the surface of the photoelectric detector to obtain an interference signal.
Furthermore, the laser enters the FP etalon along the direction of the central axis of the FP etalon by adjusting the angles of the polarization beam splitter prism and the reflecting mirror, and the laser is focused at the center of the FP etalon by adjusting the position of the first convex lens.
Compared with the prior method for processing frequency modulation continuous wave laser ranging signals (by using an optical fiber Mach-Zehnder interferometer as an auxiliary interference system to perform equal optical frequency interval resampling), the method has the beneficial effects that: in the frequency modulation continuous wave laser ranging system, an FP etalon is added as an auxiliary measuring system, the sampled signal is resampled at equal optical frequency intervals, and then the resampled signal is processed by using a Hilbert transform algorithm to obtain the absolute distance of a measured target; the cavity length of the FP etalon is used as a distance measuring reference instead of a long optical fiber, so that the influence of the environmental sensitivity of the long optical fiber and the dispersion of the optical fiber on the distance measuring precision is overcome, and the high-precision measurement can be realized under the condition that the modulation range of a laser is smaller.
Drawings
FIG. 1 is a schematic diagram of a frequency modulated continuous wave laser ranging system incorporating an FP etalon according to the present invention;
FIG. 2 is a schematic diagram of a signal segment acquired by the high-speed data acquisition system of the present invention;
FIG. 3 is a schematic diagram of resampled signal sig 3;
FIG. 4 is a graph of the instantaneous phase of the resampled signal sig3 using the Hilbert transform;
fig. 5 is a phase diagram of the resampled signal sig3 after instantaneous phase unwrapping;
in the figure: 1. the device comprises a tunable laser, 2, an optical isolator, 3, a collimating lens group, 4, a half wave plate, 5, a polarization splitting prism, 6, a measurement retro-reflection prism, 7, a reference retro-reflection prism, 8, a non-polarization splitting prism, 9, a fiber collimating lens, 10, 50:50 fiber couplers, 11, a balance detector, 12, a reflector, 13, a first convex lens, 14, an FP etalon, 15, a vacuum cavity, 16, a second convex lens, 17, a photoelectric detector, 18, a tunable laser controller, 19, a high-speed data acquisition system, 20 and a computer.
Detailed Description
The invention is further illustrated by the following figures and examples.
The frequency modulation continuous wave laser ranging method of the integrated FP etalon comprises the following specific steps:
step 1, as shown in fig. 1, a frequency modulation continuous wave laser ranging system integrated with an FP etalon is built, in the laser ranging system, a tunable laser controller 18 controls a tunable laser 1 to emit optical frequency linear modulation laser with central wavelength of 633nm and modulation range of 1nm, the laser sequentially passes through an optical isolator 2, a collimating lens group 3 and a half wave plate 4, then is divided into two paths through a polarization beam splitter prism 5, and then respectively enters a measurement interference system and the FP interference system. In the measuring interference system, laser is divided into two paths by a non-polarizing beam splitter prism 8, is respectively reflected by a measuring retro-reflecting prism 6 and a reference retro-reflecting prism 7, then is converged on a semi-transparent semi-reflecting surface of the non-polarizing beam splitter prism 8, enters an optical fiber through a collimating lens 9, is divided into two paths through a 50:50 optical fiber coupler 10, and respectively enters two detection ports of a balance detector 11 to form interference signals. In the FP interference system, laser enters an FP etalon 14 after sequentially passing through a reflector 12 and a first convex lens 13, and the FP etalon 14 is placed in a vacuum cavity 15; by adjusting the angles of the polarization beam splitter prism 5 and the reflector 12, the laser enters the FP etalon 14 along the direction of the central axis of the FP etalon 14; the laser is focused at the center of the FP etalon by adjusting the position of the first convex lens 13. The emergent laser of the FP etalon passes through a second convex lens 16 and then is focused on the surface of a photoelectric detector 17 to form an interference signal.
Step 2, the first channel and the second channel of the high-speed data acquisition system 19 respectively perform synchronous data acquisition on the interference signals of the measurement interference system and the FP interference system to obtain signals sig1 (solid line part in fig. 2) and sig2 (dotted line part in fig. 2). Storing the acquired signals sig1 and sig2 into the computer 20 for processing; taking a first peak point of a signal sig2 after a modulation start signal of the tunable laser is sent out as an initial point; before a modulation termination signal of the tunable laser is sent out, the last peak point of the signal sig2 is used as a termination point, and the signal sig2 comprises k peak points in total of the start point and the termination point between the start point and the termination point.
And 3, performing secondary sampling on the signal sig1 at each peak point position of the signal sig2 to obtain a resampled signal sig3, as shown in fig. 3.
Step 4, respectively recording the maximum value and minimum value quantity of the signal sig1 between every two adjacent peak points of the signal sig2 as Mi(i-1, 2, … …, k-1) to obtain MiHas an average value of Mmean。
And 5, solving the instantaneous phase of the resampled signal sig3 by using Hilbert transform, as shown in FIG. 4. For the obtained transientsAfter phase unwrapping, an incremental curve is obtained, as shown in fig. 5. Then, the curve is linearly fitted by the least square method, and the slope of the straight line after fitting is obtained as l. In fig. 5, the open circles are instantaneous phase data points, and the straight lines are the least squares fit straight lines. Let fI＝l/(2π)，fII.e. the frequency of resampled signal sig 3.
And 6, calculating the distance L of the measured target by using the following formula.
Figure BDA0001798714570000041
Wherein M is MmeanInteger part of (d)FPIs the cavity length of the FP etalon, ngIs the refractive index of air.

Claims (5)

1. The frequency modulation continuous wave laser ranging method of the integrated FP etalon is characterized in that: the method comprises the following specific steps:
step 1, building a frequency modulation continuous wave laser ranging system integrated with an FP etalon, wherein a tunable laser controller controls a tunable laser to emit narrow-linewidth laser with optical frequency linearly modulated, the laser sequentially passes through an optical isolator, a collimating lens group and a half wave plate, then is divided into two paths through a polarization beam splitter prism, and respectively enters a measurement interference system and an FP interference system, and one path of interference signal is respectively obtained in the measurement interference system and the FP interference system; the FP etalon of the FP interference system is placed in a vacuum cavity;
step 2, synchronously acquiring data of interference signals of the measurement interference system and the reference interference system by two channels of the high-speed data acquisition system respectively to obtain signals sig1 and sig 2; taking the first peak point of a signal sig2 after a modulation start signal of the tunable laser is sent out as an initial point, taking the last peak point of a signal sig2 as an end point before a modulation end signal of the tunable laser is sent out, and setting the total k peak points of the initial point and the end point of the signal sig2 between the initial point and the end point;
step 3, performing secondary sampling on the signal sig1 at each peak point position of the signal sig2 to obtain a resampled signal sig 3;
step 4, recording the sum of the maximum value and the minimum value of the signal sig1 between two adjacent peak points of the signal sig2, and recording the sum as Mi(i-1, 2, … …, k-1) to obtain MiHas an average value of Mmean；
Step 5, solving the instantaneous phase of the resampled signal sig3 by using Hilbert transform, and obtaining an increasing curve after unwrapping the obtained instantaneous phase; then, carrying out linear fitting on the curve by using a least square method, and solving the slope of a straight line after fitting as l; let fI＝l/(2π)，fII.e. the frequency of the resampled signal sig 3;
step 6, calculating the distance L of the measured target by using the following formula;
Figure FDA0002532520400000011
wherein M is MmeanInteger part of (d)FPIs the cavity length of the FP etalon, ngIs the refractive index of air.
2. The frequency modulated continuous wave laser ranging method of the integrated FP etalon of claim 1, wherein: the tunable laser adopts an external cavity type narrow linewidth tunable laser with the central wavelength of 633nm and the modulation range of 1 nm.
3. A frequency modulated continuous wave laser ranging method of an integrated FP etalon according to claim 1 or 2, wherein: the step 1 is further described as follows: in the measuring interference system, laser is divided into two paths by a non-polarizing beam splitter prism, is respectively reflected by a measuring retro-reflection prism and a reference retro-reflection prism, then is converged on a semi-transparent semi-reflection surface of the non-polarizing beam splitter prism, enters an optical fiber through a collimating lens, is divided into two paths through a 50:50 optical fiber coupler, and respectively enters two detection ports of a balance detector to form interference signals.
4. A frequency modulated continuous wave laser ranging method of an integrated FP etalon according to claim 1 or 2, wherein: the step 1 is further described as follows: in the FP interference system, laser enters the FP etalon after sequentially passing through a reflector and a first convex lens; and the emergent laser of the FP etalon passes through a second convex lens and is focused on the surface of the photoelectric detector to obtain an interference signal.
5. The frequency modulated continuous wave laser ranging method of the integrated FP etalon of claim 4 wherein: the laser enters the FP etalon along the direction of the central axis of the FP etalon by adjusting the angles of the polarization beam splitter prism and the reflecting mirror, and is focused at the center of the FP etalon by adjusting the position of the first convex lens.
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