阅读说明：本技术 一种采用相位调制器的双光束激光多普勒测速系统 (Double-beam laser Doppler velocity measurement system adopting phase modulator ) 是由 艾梦奇 孙国良 于 2018-07-05 设计创作，主要内容包括：本发明属于光学领域,公开了一种采用相位调制器的双光束激光多普勒测速系统,所述测速仪包括：红外半导体激光器,红光指示半导体激光器,耦合器,相位调制器,第一准直透镜,第二准直透镜,接收透镜,红外光探测器,其中,所述红外半导体激光器及红光指示半导体激光器分别通过光纤连接所述耦合器的一端,所述耦合器的另一端通过光纤连接所述相位调制器,所述相位调制器将传导的红外激光分成均匀的两路,并分别通过光纤传导至所述第一准直透镜及第二准直透镜前端,经移动物体反射的光由接收透镜聚焦至红外光探测器。本发明解决了原有的采用声光移频器进行移频的测速系统技术复杂、成本较高的问题。(The invention belongs to the field of optics, and discloses a double-beam laser Doppler velocimetry system adopting a phase modulator, wherein the velocimeter comprises: infrared semiconductor laser, ruddiness instruction semiconductor laser, coupler, phase modulator, first collimating lens, second collimating lens, receiving lens, infrared light detector, wherein, infrared semiconductor laser and ruddiness instruction semiconductor laser are respectively through optical fiber connection the one end of coupler, the other end of coupler passes through optical fiber connection the phase modulator, the phase modulator divides into the infrared laser of conduction into even two tunnel to conduct respectively through optical fiber extremely first collimating lens and second collimating lens front end, the light of through moving object reflection is focused to infrared light detector by receiving lens. The invention solves the problems of complex technology and higher cost of the original speed measuring system adopting the acousto-optic frequency shifter to carry out frequency shift.)

1. A dual beam laser doppler velocimetry system employing a phase modulator, said velocimeter comprising: infrared semiconductor laser, ruddiness instruction semiconductor laser, coupler, phase modulator, first collimating lens, second collimating lens, receiving lens, infrared light detector, wherein, infrared semiconductor laser and ruddiness instruction semiconductor laser are respectively through optical fiber connection the one end of coupler, the other end of coupler passes through optical fiber connection the phase modulator, the phase modulator divides into the infrared laser of conduction into even two tunnel to conduct respectively through optical fiber extremely first collimating lens and second collimating lens front end, the light of through moving object reflection is focused to infrared light detector by receiving lens.

2. A velocimetry system as claimed in claim 1, wherein said phase modulator is a lithium niobate phase modulator.

3. A velocimetry system as claimed in claim 1, wherein the output pigtail of said infrared semiconductor laser is encapsulated with an infrared polarization maintaining fiber.

4. The system according to claim 1, wherein said coupler input and output pigtails are packaged with infrared polarization maintaining fiber.

5. A velocimetry system as claimed in claim 1, wherein the infrared laser light is split evenly into two equal paths as it passes through said phase modulator, with its phase being modulated by a sawtooth modulation signal applied to the phase modulator.

6. The velocity measurement system according to claim 2, wherein said lithium niobate phase modulator is a Y-type lithium niobate phase modulator, and a lithium niobate crystal of said Y-type lithium niobate phase modulator is prepared into an optical waveguide structure.

7. A velocimetry system as claimed in claim 1, wherein the first collimating lens and the second collimating lens are arranged at an angle, the receiving lens is located between the two collimating lenses, and the infrared detector is located directly above the receiving lens; the first collimating lens, the second collimating lens and the receiving lens are biconvex lenses, plano-convex lenses, concave-convex lenses, achromatic lenses or aspherical lenses.

8. The velocity measurement system according to claim 5, wherein the sawtooth wave modulation signal is a sawtooth wave with a duty ratio of 0 or 1, and the modulation depth of the sawtooth wave is 0.5 pi.

Technical Field

The invention belongs to the field of optical measurement, and particularly relates to a dual-beam laser Doppler velocity measurement system which realizes frequency shift by adopting a phase modulator and realizes frequency detection by matching with fast Fourier transform so as to realize velocity measurement.

Background

The laser doppler shift is a difference between the frequency of the scattered light and the frequency of the incident light when the laser light is incident on the surface of a moving object and scattered by the moving object, and the difference is proportional to the moving speed of the object, so that the moving speed of the object can be detected by detecting the shift of the laser frequency.

Due to the high laser frequency (-10)¹⁴Hz) and therefore the laser frequency cannot be directly detected. Two laser beams having a frequency difference are combined, the light intensity will fluctuate periodically after the combination, and the fluctuation frequency of the light intensity is the frequency difference between the two laser beams. The detection of the laser Doppler shift can be realized by a heterodyne method.

The double-beam double-scattering light path structure is a light path structure commonly adopted in the existing laser speed measuring system, has a simple light path structure, receives a large light intensity signal, and can adapt to speed detection under most conditions. In order to realize the detection of positive and negative speeds, one path of emergent light or two paths of emergent light need to be subjected to frequency shift. Therefore, the speed measuring system can detect a frequency value when the measured object does not move and laser Doppler frequency shift is not generated. When the object moves, the frequency value detected by the speed measuring system is increased or decreased and respectively corresponds to the forward movement or backward movement of the object, so that the detection of positive and negative speeds is realized.

Most of the existing laser speed measuring systems adopt an acousto-optic frequency shifter to shift frequency. The acousto-optic frequency shifter is typically frequency shifted above tens of megahertz due to the crystal material size limitations. In the heterodyne detection system, if a frequency shift amount of several mhz or even lower needs to be generated, two acousto-optic frequency shifters are usually needed to shift the frequency of two paths of light respectively, so that the frequency shift amount of the two paths of light has a smaller difference, and the purpose of the frequency shift amount of several mhz or even lower is achieved. In addition, in order to improve the diffraction efficiency and the frequency shift bandwidth of the output light of the acousto-optic frequency shifter, the acousto-optic frequency shifter must work in a bragg diffraction mode, so a series of complicated technical measures such as increasing the bandwidth of a piezoelectric transducer, ultrasonic tracking, bandwidth impedance matching and the like need to be adopted, and the volume of the device is large. Meanwhile, the acousto-optic frequency shifter uses space light to propagate in the crystal, which is not beneficial to packaging optical fibers.

Disclosure of Invention

The invention aims to provide a dual-beam laser Doppler velocity measurement system adopting a phase modulator, so as to solve the problems of complex technology and higher cost of the original velocity measurement system adopting an acousto-optic frequency shifter to carry out frequency shift.

The technical scheme adopted by the invention is as follows:

a dual beam laser doppler velocimetry system employing a phase modulator, the velocimetry system comprising: the infrared laser device comprises an infrared semiconductor laser, a red light indicating semiconductor laser, a coupler, a phase modulator, a first collimating lens, a second collimating lens, a receiving lens and an infrared light detector, wherein the infrared semiconductor laser and the red light indicating semiconductor laser are respectively connected with one end of the coupler through optical fibers, the other end of the coupler is connected with the phase modulator through the optical fibers, the phase modulator divides conducted infrared laser into two uniform paths and conducts the two paths to the front ends of the first collimating lens and the second collimating lens through the optical fibers, and light reflected by a moving object is focused to the infrared light detector through the receiving lens.

When infrared laser passes through the lithium niobate phase modulator, the light wave is uniformly divided into two equal parts, and the phase of the light wave is modulated by a sawtooth wave modulation signal applied to the lithium niobate phase modulator. The light divided into two equal parts by the lithium niobate phase modulator is guided to a first collimating lens and a second collimating lens by an infrared polarization maintaining optical fiber respectively, the light reflected by the moving object is focused to an infrared light detector by a receiving lens, a light intensity signal is converted into an electric signal by the infrared light detector, the signal is subjected to Fourier frequency analysis after being collected by an AD (analog-to-digital) converter, so that beat frequency signals of two beams of light are obtained, and the moving speed of the object is obtained through linear operation.

Preferably, the phase modulator is a lithium niobate phase modulator.

Preferably, the output tail fiber of the infrared semiconductor laser is packaged by an infrared polarization maintaining fiber.

Preferably, the input and output tail fibers of the coupler are packaged by infrared polarization maintaining fibers.

Preferably, when the infrared laser passes through the phase modulator, the optical wave is uniformly divided into two equal paths, and the phase of the optical wave is modulated by a sawtooth wave modulation signal applied to the phase modulator.

Preferably, the lithium niobate phase modulator is a Y-type lithium niobate phase modulator, and the lithium niobate crystal of the Y-type lithium niobate phase modulator is prepared into an optical waveguide structure.

Preferably, the first collimating lens and the second collimating lens are arranged to form a certain included angle, the receiving lens is located between the two collimating lenses, and the infrared light detector is located right above the receiving lens; the first collimating lens, the second collimating lens and the receiving lens are biconvex lenses, plano-convex lenses, concave-convex lenses, achromatic lenses or aspherical lenses.

Preferably, the sawtooth wave modulation signal is a sawtooth wave with a duty ratio of 0 or 1, and the modulation depth of the sawtooth wave is 0.5 pi.

Compared with the prior art, the double-beam laser Doppler velocity measurement system adopting the phase modulator provided by the invention has the advantages that the phase modulator is adopted to replace an acousto-optic frequency shifter to realize laser frequency shift, and frequency offset at zero speed is provided for the laser velocity measurement system, so that the positive and negative speed detection of the velocity measurement system is realized. In addition, the cost of the lithium niobate phase modulator is far lower than that of the acousto-optic frequency shifter, the lithium niobate phase modulator is used for replacing the acousto-optic frequency shifter in the dual-beam laser Doppler velocity measurement system, a complex radio frequency driving technology is not needed to be used like the acousto-optic frequency shifter, and the technical complexity and the cost of the velocity measurement system can be greatly reduced.

Drawings

Fig. 1 is a schematic structural diagram of a dual-beam laser doppler velocity measurement system using a phase modulator according to an embodiment of the present invention.

Fig. 2 is a schematic diagram of a sawtooth signal applied to a phase modulator by a velocity measurement system according to an embodiment of the present invention.

Fig. 3 is a relative value of each frequency component in the beat signal received by the infrared detector when the modulation depth of the sawtooth wave in fig. 2 is different.

In the figure, 1-infrared semiconductor laser, 2-red light indication semiconductor laser, 3-coupler, 4-phase modulator, 5-first collimating lens, 6-second collimating lens, 7-receiving lens, 8-infrared light detector, 9-object to be measured and 10-optical fiber.

Detailed Description

The present invention will be described in further detail below with reference to the accompanying drawings, but the present invention is not limited thereto.

Referring to fig. 1, a dual-beam laser doppler velocity measurement system using a phase modulator disclosed in the embodiment of the present invention includes an infrared semiconductor laser 1, a red light indicating semiconductor laser 2, a coupler 3, a phase modulator 4, a first collimating lens 5, a second collimating lens 6, a receiving lens 7, an infrared light detector 8, and an optical fiber 10.

The infrared semiconductor laser 1 and the red light indication semiconductor laser 2 are respectively connected to one end of the coupler 3 through two optical fibers. The red light emitted by the red light indication semiconductor laser 2 is coupled into the lithium niobate phase modulator 4 through the coupler 3, and then is divided into two beams of light which are collimated into space beams through the first collimating lens 5 and the second collimating lens 6 respectively, and the area where the two beams are overlapped in space is the detection area of the speed measurement system. The light emitted by the infrared semiconductor laser 1 is used for calculating Doppler signals, the light emitted by the red light indicating semiconductor laser 2 is not used for resolving the laser Doppler signals, and is only used for indicating a detection area of a speed measuring system, so that the actual operation is facilitated. The coupler 3 transmits electric signals by taking light as a medium, and has good isolation effect on input and output electric signals. The coupler is arranged on the light path conducted by the laser, so that optical signals are transmitted in a single direction, the input end and the output end are completely electrically isolated, output signals have no influence on the input end, and the transmission efficiency is high.

The other end of the coupler 3 is connected with a phase modulator, and the phase modulator modulates light signals and divides infrared laser into two parts with equal power and two paths of uniform light. In this embodiment, the phase modulator is preferably a lithium niobate phase modulator, and more preferably, a Y-type lithium niobate phase modulator is selected, where the lithium niobate phase modulator is an electro-optical device. When an electric field is applied to the lithium niobate crystal, the refractive index of the lithium niobate crystal is changed, so that the phase of the light wave passing through the lithium niobate crystal is changed. The electro-optic response speed of lithium niobate crystals is very high, and thus the signal bandwidth thereof can be very large. Meanwhile, the frequency shifter has no requirement on the frequency of an input signal, can cover all frequency ranges, and is very suitable for frequency shifting application of the frequency range under dozens of megahertz. Due to the wide frequency adaptation range, the acoustic-optical frequency shifter does not need to use complex radio frequency driving technology like an acoustic-optical frequency shifter. In the embodiment of the invention, the lithium niobate crystal of the lithium niobate phase modulator is prepared into the optical waveguide structure, so that the lithium niobate crystal can be directly coupled with the optical fiber, and the optical fiber is very convenient to package.

The output end of the phase modulator 4 is connected to the first collimating lens 5, the second collimating lens 6 and the front end through two optical fibers, and a certain interval is reserved between the tail end of the optical fiber at the output end of the phase modulator 4 and the two collimating lenses to align the two collimating lenses. The first collimating lens 5 and the second collimating lens 6 form a certain included angle when being installed, so that two beams of space light beams passing through the two collimating lenses intersect at a certain distance in front of the speed measuring system. The receiving lens 7 is positioned between the two collimating lenses, and the infrared light detector 8 is positioned right above the receiving lens 7; the first collimating lens 5, the second collimating lens 6 and the receiving lens 7 may be a biconvex lens, a plano-convex lens, a meniscus lens, an achromatic lens or an aspherical lens.

In the embodiment of the invention, the optical paths among the infrared semiconductor laser 1, the coupler 3, the phase modulator 4 and the collimating lenses (5 and 6) are conducted by optical fibers, so that compared with a traditional direct laser scattering mode, the transmission speed is higher and more stable, and attenuation is not easy to occur.

In the embodiment of the present invention, the output pigtail of the infrared semiconductor laser 1 is packaged by using an infrared polarization maintaining fiber, and the input and output pigtails of the coupler 3 are packaged by using infrared polarization maintaining fibers. In the process of drawing the polarization maintaining optical fiber, the structural defect generated in the optical fiber can cause the reduction of polarization maintaining performance, namely when linearly polarized light is transmitted along one characteristic axis of the optical fiber, part of optical signals can be coupled into the other characteristic axis vertical to the characteristic axis, and finally the reduction of the polarization extinction ratio of emergent polarized light signals is caused. The polarization maintaining optical fiber can ensure that the linear polarization direction is unchanged, and the coherent signal-to-noise ratio is improved, so that high-precision measurement of physical quantity is realized.

The working principle of the double-beam laser Doppler velocity measurement system adopting the phase modulator to carry out physical velocity measurement disclosed by the embodiment of the invention is as follows:

light emitted by the semiconductor laser 1 enters the lithium niobate phase modulator 4 after passing through the coupler 3, the light is modulated by a modulation signal applied to the lithium niobate phase modulator and is divided into two equal parts with equal power, the modulated light waves respectively reach the first collimating lens 5 and the second collimating lens 6 after passing through the infrared polarization-maintaining optical fiber 10, and the light waves are changed into two spatial light beams with certain light spot size after entering the first collimating lens 5 and the second collimating lens 6. The first collimating lens 5 and the second collimating lens 6 form a certain angle when being installed, so that the two spatial light beams intersect at a certain distance in front of the speed measuring system. The intersecting area of the two spatial light beams is the detection area of the speed measurement system. When the object 9 to be measured is in the detection area, the two spatial light beams each generate scattered light with doppler shift on the surface of the object 9 to be measured. The scattered light passes through a receiving lens 7 and is focused on an infrared light detector 8. The infrared light detector 8 converts the beat frequency signal of the scattered light with doppler shift into an electrical signal, which is processed by a subsequent signal processing circuit to calculate a frequency value of the signal, which is proportional to the moving speed of the object 9 to be measured. The moving speed of the object 9 to be measured can be measured by simple conversion.

Where Δ f denotes the Doppler shift, V denotes the velocity, φ denotes the angle of the incident light from the normal as in 1, and in denotes the laser wavelength.

The signal applied to the lithium niobate phase modulator 4 is a sawtooth wave signal. When the amplitude of the sawtooth wave signal is in a certain range, the beat frequency signal of the scattered light of the two modulated light waves can only have 1 frequency multiplication component, so that the zero-speed detection is realized. Specifically, the sawtooth wave signal is recorded as s (t), the modulation depth is recorded as M, and the frequency is recorded as f_mAs shown in fig. 2. Thus, the two modulated light waves can be expressed as:

E₁＝E₁₀exp[i(kz-ωt+s(t))]

E₂＝E₂₀exp[i(kz-ωt-s(t))]

at zero velocity, there is no doppler shift, so the superimposed light of the scattered light can be expressed as:

E＝E₁+E₂

the converted electrical signal of the infrared light detector 8 is:

I＝EE^*

as known from the Fourier series decomposition theory, the signal detected by the infrared light detector 8 should exist in f_mThe frequency components of each level of the fundamental frequency are multiplied. However, the suppression of high-order frequency components can be realized by reasonably setting the sawtooth wave modulation depth M, so that the fundamental frequency f is accurately detected_mAnd zero-speed detection is realized. Fig. 3 shows relative values of frequency components in the beat signal received by the infrared detector 8 when the sawtooth modulation depth M is different. As can be seen from fig. 3, when the modulation depth of the sawtooth wave is 0.5 pi, the frequency doubling component 1 in the beat signal is the largest, and the frequency doubling components such as the frequency doubling 2, the frequency doubling 3, and the frequency doubling 4 are all 0, so in the embodiment of the present invention, when the zero-speed detection of the velocity measurement system is implemented, the modulation depth of the sawtooth wave is set to be 0.5 pi.

While the foregoing description shows and describes several preferred embodiments of the invention, it is to be understood, as noted above, that the invention is not limited to the forms disclosed herein, but is not intended to be exhaustive or to exclude other embodiments and may be used in various other combinations, modifications, and environments and may be modified within the scope of the inventive concept described herein by the above teachings or the skill or knowledge of the relevant art. And that modifications and variations may be effected by those skilled in the art without departing from the spirit and scope of the invention as defined by the appended claims.