CN109212551B - Optical fiber Doppler velocimeter without upper speed limit and capable of distinguishing direction and velocity measuring method thereof - Google Patents

Abstract

The invention relates to a speed-upper-limit-free direction-distinguishable optical fiber Doppler velocimeter and a speed measuring method thereof, which comprise a single-mode laser, an optical fiber isolator, a 1 x 3 optical fiber beam splitter, an optical fiber circulator, an acousto-optic frequency shifter, a 1 x 2 optical fiber beam splitter, a 2 x 1 optical fiber beam combiner, an optical fiber collimator and a photoelectric detector, wherein laser output from the single-mode laser is divided into 3 beams by the 1 x 3 optical fiber beam splitter after passing through the optical fiber isolator, a second beam vertically irradiates the surface of a moving object through the optical fiber circulator and the collimator, is divided into two beams after carrying motion information and reflecting the motion information back to the collimator, and the two beams of signal light are respectively interfered with a first beam of intrinsic light of the 1 x 3 optical fiber beam splitter and a third beam of reference light passing through the acousto-optic frequency shifter and are respectively detected and received by the two detectors. The invention adopts the homodyne method to obtain the Doppler frequency shift quantity, and combines the heterodyne method to judge the direction, thereby breaking through the upper speed limit of the conventional heterodyne method Doppler velocity measurement technology while distinguishing the speed direction.

Description

A fiber-optic Doppler velocimeter with no velocity upper limit and direction-distinguishing method and its velocity measurement method

technical field

The invention belongs to the technical field of optical measurement, and in particular relates to an optical fiber Doppler velocimeter without a speed upper limit and a direction-discriminating method thereof.

Background technique

Laser Doppler velocimetry technology is a new measurement technology that accompanies the birth of lasers. It uses the Doppler effect of lasers to measure the velocity of fluids or solids. The principle of laser Doppler speed measurement is to use the optical Doppler effect, that is, when the laser irradiates a moving object, the laser is scattered by the particles moving with the object, and the frequency of the scattered light will change. The difference between it and the frequency of the incident laser is called Doppler frequency difference or Doppler beat frequency. This frequency difference is proportional to the speed, so the speed can be obtained by measuring the Doppler frequency difference. The traditional laser Doppler velocimetry technology has slow response, low test accuracy and poor reliability. The optical fiber Doppler velocity measurement system combined with laser Doppler velocity measurement technology and optical fiber devices has a simple and flexible optical path structure, and the whole system is more compact and compact, so it has great development potential.

Typical detection methods in all-fiber Doppler velocity measurement technology include homodyne method and heterodyne method. When the homodyne method is used, the light output by the single-mode laser is divided into two beams by a 1×2 fiber beam splitter after passing through the fiber isolator. After the moving object returns, as the signal light carries motion information, the local oscillator light and the signal light are combined and interfered to obtain the absolute value of the Doppler frequency shift, which can only determine its size but not its direction; use the heterodyne method When the light output by the single-mode laser passes through the fiber isolator, it is divided into two beams by a 1×2 fiber beam splitter. One beam is passed through an acousto-optic frequency shifter to form a reference beam, and the other beam is emitted through a fiber optic collimator. After arriving at the moving object, it returns as the signal light carrying motion information. The reference light and the signal light undergo heterodyne interference and are detected and received by the photodetector to obtain the difference between the frequency shift amount of the acousto-optic frequency shifter and the Doppler frequency shift amount. The absolute value can realize the measurement of the velocity and the discrimination of the direction at the same time, but because the maximum value of the Doppler frequency shift cannot exceed the fixed frequency difference between the reference light and the initial signal light without Doppler frequency shift, its velocity The measurement range is limited by the amount of frequency shifting by the acousto-optic frequency shifter.

Contents of the invention

In view of this, the present invention aims at the problem that the speed measurement range of the optical fiber Doppler velocimetry system using the heterodyne method is limited by the frequency shift amount of the acousto-optic frequency shifter, and proposes a fiber optic Doppler velocimeter with no velocity upper limit and direction Its speed measurement method can simultaneously realize speed measurement and direction discrimination, and there is no upper limit of speed measurement.

In order to solve the problems existing in the prior art, the technical solution of the present invention is: a fiber optic Doppler velocimeter with no velocity upper limit and direction discrimination, including a single-mode laser, an optical fiber isolator, an optical fiber circulator, an optical fiber collimator, The acousto-optic frequency shifter, the first 2×1 fiber combiner and the first photodetector are characterized in that: it also includes a 1×3 fiber splitter, a 1×2 fiber splitter, a second 2×1 fiber A beam combiner and a second photodetector; the output end of the single-mode laser is connected to the input end of the fiber isolator, the output end of the fiber isolator is connected to the input end of the 1×3 fiber splitter, and the 1×3 fiber splitter The second output port of the fiber optic circulator is connected to the first port of the fiber optic circulator, the second port of the fiber optic circulator is connected to the fiber collimator to align with the moving target to be tested, the third port of the fiber optic circulator is connected to the 1×2 fiber beam splitter, and the 1×2 fiber splitter One output port of the beamer and the first output port of the 1×3 fiber splitter are connected to the two input ports of the second 2×1 fiber combiner, and the output port of the second 2×1 fiber combiner is connected to the second photodetector The third output port of the 1×3 fiber splitter is connected to the acousto-optic frequency shifter, the output port of the acousto-optic frequency shifter is connected to the first 2×1 fiber combiner with the other output port of the 1×2 fiber splitter The two input ends of the first 2×1 fiber combiner are connected to the first photodetector at the output end.

The continuous single-frequency laser output by the single-mode laser is divided into three beams by a 1×3 fiber beam splitter, and the output light from the second output port of the 1×3 fiber beam splitter is input to a port of the fiber optic circulator, and is fed by The two ports of the fiber optic circulator output to the fiber collimator, irradiate vertically to the surface of the moving object to be measured, and after being reflected by the moving object, the signal light carrying the motion information of the measured object is output by the three ports of the fiber optic circulator and divided into two One beam combines with the local oscillator light at the first output port of the 1×3 fiber beam splitter, and is detected and received by the second photodetector, and the other beam is combined with the third port of the 1×3 fiber beam splitter The reference light frequency-shifted by the acousto-optic frequency shifter performs beam combination interference, and is detected and received by the first photodetector.

The light splitting ratio of the 1×3 fiber optic beam splitter is 1:1:1, and the light splitting ratio of the 1×2 fiber optic beam splitter is 1:1.

The local oscillator light output from the first output port of the 1×3 optical fiber beam splitter performs homodyne interference with the signal light returned with motion information after vertically irradiating the moving object to obtain the absolute value of the Doppler frequency shift |f _d |, The reference light shifted by the acousto-optic frequency shifter and the signal light returned with motion information after vertically irradiating the moving object are subjected to heterodyne interference to obtain the absolute value of the difference between the frequency shift amount of the acousto-optic frequency shifter and the Doppler frequency shift amount |f _l -f _d |, the size of the Doppler frequency shift can be obtained from |f _d |, combined with |f _l -f _d | to determine the direction, and then the magnitude and direction of the speed can be obtained at the same time.

Compared with prior art, advantage of the present invention is as follows:

The invention constitutes an all-fiber Doppler measurement optical path, which performs heterodyne interference on signal light and reference light through homodyne interference on local oscillator light and signal light, and simultaneously uses homodyne method and heterodyne method to determine Doppler Frequency shift, and then determine the magnitude and direction of the speed of the measured moving object;

Compared with the all-fiber Doppler speed measuring technology using the homodyne method, the present invention can realize the discrimination of the speed direction on the basis of the unrestricted measurement range;

Compared with the all-fiber Doppler speed measuring technology using the heterodyne method, the present invention can realize the speed measurement in a wider range under the premise of judging the speed direction, and is no longer limited by the frequency shifting amount of the acousto-optic frequency shifter.

Description of drawings

Fig. 1 is the structural representation of the embodiment of the present invention;

Fig. 2 is a schematic structural diagram of a traditional heterodyne velocity measuring device.

Marking description: 1-single-mode laser, 2-fiber isolator, 3-1×3 fiber splitter, 4-fiber circulator, 5-fiber collimator, 6-moving object, 7-first 2×1 Fiber combiner, 8-first photodetector, 9-1×2 fiber splitter, 10-acousto-optic frequency shifter, 11-second 2×1 fiber combiner, 12-second photodetector .

Detailed ways

The following will clearly and completely describe the technical solutions in the embodiments of the present invention with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only some, not all, embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by persons of ordinary skill in the art without creative efforts fall within the protection scope of the present invention.

The invention belongs to an all-fiber Doppler velocity measuring method, adopts a homodyne method and a heterodyne method for measurement at the same time, and proposes an optical fiber Doppler velocity measuring instrument with no velocity upper limit and direction discrimination. The signal light and the local oscillator light are combined and interfered, and the amount of Doppler frequency shift is obtained by the homodyne method. The signal light and the reference light shifted by the acousto-optic frequency shifter are combined and interfered by the heterodyne method. direction, which can break through the upper limit of the speed measurement range of the heterodyne all-fiber Doppler speed measurement technology while distinguishing the speed direction.

Embodiment: as shown in Figure 1:

A fiber optic Doppler velocimeter with no speed upper limit and direction discrimination, including a single-mode laser 1, a fiber isolator 2, a 1×3 fiber beam splitter 3, a fiber circulator 4, a fiber collimator 5, and a moving object 6 , the first 2×1 fiber combiner 7 and the first photodetector 8, the 1×2 fiber splitter 9, the acousto-optic frequency shifter 10, the second 2×1 fiber combiner 11 and the second photodetector device 12; the output end of the single-mode laser 1 is connected to the input end of the fiber isolator 2, and the output end of the fiber isolator 2 is connected to the input end of the 1 × 3 fiber beam splitter 3 with a splitting ratio of 1:1:1, The second output port of the 1×3 optical fiber beam splitter 3 is connected to the first port of the optical fiber circulator 4, the second port of the optical fiber circulator 4 is connected to the optical fiber collimator 5 to align with the moving target to be measured, and the third port of the optical fiber circulator 4 is connected to the splitting ratio 1:1 1×2 fiber splitter 9, one output port of 1×2 fiber splitter 9 and the first output port of 1×3 fiber splitter 3 are connected to the second 2×1 fiber combiner 11 two input terminals, the output terminal of the second 2×1 optical fiber beam combiner 11 is connected to the second photodetector 12, the third output port of the 1×3 optical fiber beam splitter 3 is connected to the acousto-optic frequency shifter 10, the acousto-optic frequency shifter The output port of frequency converter 10 and the other output port of 1 × 2 optical fiber beam splitter 9 are connected to the two input ends of the first 2 × 1 optical fiber beam combiner 7, and the output terminals of the first 2 × 1 optical fiber beam combiner 7 are connected to the first 2 × 1 optical fiber beam combiner 7 A photodetector 8 .

The continuous single-frequency laser output by the single-mode laser 1 is divided into three beams by the 1×3 fiber beam splitter 3, and the output light of the second output port of the 1×3 fiber beam splitter 3 is input to a port of the fiber circulator 4, and Output from the two ports of the fiber optic circulator 4 to the fiber optic collimator 5, irradiate vertically to the surface of the moving object to be measured, and after being reflected by the moving object, carry the motion information of the measured object as signal light and output it from the three ports of the fiber optic circulator 4 and divided into two beams, one of which is combined and interfered with the local oscillator light at the first output port of the 1×3 optical fiber beam splitter 3, detected and received by the second photodetector 12, and the other beam is split with the 1×3 optical fiber The reference light frequency-shifted by the acousto-optic frequency shifter 10 at the third port of the beamer 3 undergoes beam combination interference, and is detected and received by the first photodetector 8 .

The method of using the device of the present invention to measure speed is: first align the optical fiber collimator 5 vertically with the target moving object 6 . The continuous single-frequency laser output from the single-mode laser 1 in the device is equally divided into 3 beams by the 1×3 fiber beam splitter 3, one of which is output to the optical fiber by the second output port of the 1×3 fiber beam splitter 3 One port of the circulator 4 is output to the optical fiber collimator 5 by the second port, and the surface of the moving target object 6 is irradiated vertically by the optical fiber collimator 5, and the reflected light is used as signal light to carry the motion information of the target object by the optical fiber circulator 4 three-port output, the signal light output by the three ports is connected to the 1×2 optical fiber beam splitter 9 and divided into two beams, one of which is frequency-shifted light and the local oscillator light output by the first output port of the 1×3 optical fiber beam splitter 3 The beams are combined by the second 2×1 fiber beam combiner 11 for heterodyne interference, and are detected and received by the second photodetector 12, and the light output from the third port of the 1×3 fiber beam splitter 3 passes through the acousto-optic frequency shifter 10 is frequency-shifted and combined with another frequency-shifted light beam from the 1×2 fiber beam splitter 9 to be combined by the first 2×1 fiber beam combiner 8 for heterodyne interference, and detected and received by the first photodetector 8 .

Assuming that the local oscillator frequency of the laser is f _o , the frequency shift amount of the AO frequency shifter 10 is f _l , and the frequency of the light beam output from the AO frequency shifter 10 is f ₀ +f _l , the multiplicity of the measured target object The Puler frequency shift is f _d , the output beam frequency of the three ports of the optical fiber circulator 4 is f ₀ +f _d , the local oscillator light and the output beams of the three ports of the optical fiber circulator 4 are mixed by the second 2×1 optical fiber combiner 11 Received by the second photodetector 12 after the frequency, the frequency shift at this time is |f _d |, the light beam coming out from the acousto-optic frequency shifter 10 and the outgoing light beam of the three ports of the optical fiber circulator 4 pass through the first 2×1 After the optical fiber beam combiner 7 mixes the frequency, it is received by the first photodetector 8, and the frequency shift at this time is |f _l -f _d |.

When the laser beam irradiates the surface of the moving object and is reflected, because the object is in motion, according to the Doppler effect, the reflected light will produce a Doppler frequency shift f _d compared with the incident light, and the moving speed is v, the distance between the laser and the moving direction of the object The included angle is θ, then the Doppler frequency shift of the laser is:

At the photodetector 12, |f _d | can be obtained, at the photodetector 8, |f _l -f _d | can be obtained, and the value of f _l is known. According to theoretical analysis, there are the following three situations:

1. If f _l +|f _d |=|f _l -f _d |, then f _d is a negative value;

2. If |f _d |-f _l ＝|f _l -f _d |, then f _d is a positive value and f _d is greater than f _l ;

3. If f _l -|f _d |=|f _l -f _d |, then f _d is a positive value and f _d is smaller than f _l .

By demodulating, analyzing and comparing the above two detection signals, f _d can be determined, and then the magnitude and direction of the speed can be obtained.

Another traditional speed measurement method that only uses heterodyne method, as shown in Fig. 2, the laser beam with frequency f _o generated from laser 1 is divided by 1×2 fiber beam splitter 3 after passing through fiber isolator 2 Two beams of light. One of the beams is connected to the collimator 5 through the optical fiber circulator 4 and is vertically aligned with the measured object 6, and returns to the collimator 5 with motion information. Due to the Doppler effect, the beam is used as a signal light at this time, and the frequency is f _o + f _d ; another beam of light is used as a reference light after passing through the acousto-optic frequency shifter 10, and the frequency is f _o + f _l . The reference light and signal light are mixed by a 2×1 fiber combiner and then reach the photodetector. The frequency of the mixed beam is |f _l -f _d |. The quantity measured by the heterodyne method is the difference between the Doppler frequency shift f _d and the reference optical frequency shift f _l , i.e. |f _l -f _d |, f cannot be obtained from the formula |f _l -f _d | The actual size of _d , the absolute value of the default Doppler frequency shift is smaller than the absolute value of the reference frequency shift, that is, the measured value of the heterodyne method is f _l -f _d , and the range of the Doppler frequency shift can be known from this formula is [-f _l ,+f _l ], the measurable range of the corresponding velocity v is

In the above two measurement methods: when only the traditional heterodyne method is used, the measurement range of the Doppler frequency shift is limited by the size of the reference frequency shift; The Le speed measurement method can realize that the speed measurement range is not limited by the reference frequency shift amount, and then realize a wider range of speed measurement and direction judgment.

The above descriptions are only preferred embodiments of the present invention, and are not intended to limit the protection scope of the present invention.

Claims (4)

1. A fiber optic Doppler velocimeter with no velocity upper limit and identifiable direction, including a single-mode laser (1), a fiber optic isolator (2), a fiber optic circulator (4), a fiber optic collimator (5), an acousto-optic Frequency shifter (10), first 2×1 fiber beam combiner (7) and first photodetector (8), are characterized in that: also include 1×3 fiber beam splitter (3), 1×2 optical fiber beam splitter (9), the second 2×1 fiber beam combiner (11) and the second photodetector (12); the output end of the single-mode laser (1) is connected to the input end of the optical fiber isolator (2) , the output end of the optical fiber isolator (2) is connected to the input end of the 1 × 3 optical fiber splitter (3), and the second output port of the 1 × 3 optical fiber splitter (3) is connected to a port of the optical fiber circulator (4) , the two ports of the fiber optic circulator (4) are connected to the fiber collimator (5) to align with the moving target to be tested, the three ports of the fiber optic circulator (4) are connected to the 1×2 fiber beam splitter (9), and the 1×2 fiber beam splitter One output port of the device (9) and the first output port of the 1×3 fiber splitter (3) are connected to the two input ends of the second 2×1 fiber combiner (11), and the second 2×1 fiber bundle The output terminal of the device (11) is connected with the second photodetector (12), the third output port of the 1×3 optical fiber beam splitter (3) is connected with the acousto-optic frequency shifter (10), and the acousto-optic frequency shifter (10) outputs The port and the other output port of the 1×2 fiber splitter (9) are connected to the two input ports of the first 2×1 fiber combiner (7), and the output ports of the first 2×1 fiber combiner (7) Connect to the first photodetector (8). 2. A kind of optical fiber Doppler velocimeter without speed upper limit and direction discernible according to claim 1, characterized in that: the continuous single-frequency laser output by the single-mode laser (1) is divided by 1×3 optical fiber The beam splitter (3) is divided into 3 bundles, and the output light of the second output port of the 1×3 fiber optic beam splitter (3) is input to a port of the optical fiber circulator (4), and is transmitted by the second port of the optical fiber circulator (4) Output to the optical fiber collimator (5), irradiate vertically to the surface of the moving object to be measured, and after being reflected by the moving object, it is used as signal light to carry the motion information of the object to be measured, output by the three ports of the optical fiber circulator (4) and divided into two One of them combines and interferes with the local oscillator light at the first output port of the 1×3 optical fiber splitter (3), and is detected and received by the second photodetector (12), and the other is split with the 1×3 optical fiber The reference light frequency-shifted by the acousto-optic frequency shifter (10) at the third port of the beamer (3) performs beam combining and interference, and is detected and received by the first photodetector (8). 3. according to claim 1 and 2 described a kind of optical fiber Doppler velocimeter without velocity upper limit discernible direction, it is characterized in that: the splitting ratio of described 1 * 3 optical fiber beam splitter (3) is 1: 1:1, the splitting ratio of the 1×2 optical fiber beam splitter (9) is 1:1. 4. the method for measuring speed of a kind of optical fiber Doppler velocimeter without speed upper limit according to claim 1, is characterized in that: the local oscillator output of the first output port of 1 * 3 optical fiber beam splitter (3) Homodyne interference is carried out between the light and the signal light returned with motion information after irradiating the moving object vertically, and the absolute value |f _d | of the Doppler frequency shift is obtained, and the frequency shifted reference light and After the moving object is irradiated vertically, the signal light returned with motion information is subjected to heterodyne interference to obtain the absolute value |f _l -f _d | of the difference between the frequency shift amount of the acousto-optic frequency shifter and the Doppler frequency shift amount _. |Get the size of the Doppler frequency shift, combine |f _l -f _d | to determine the direction, and then obtain the size and direction of the speed at the same time.

Images