CN106019259B - Laser frequency discrimination device and frequency discrimination method based on Mach-Zehnder interferometer - Google Patents

Abstract

The invention discloses a laser frequency discrimination device and a frequency discrimination method based on a Mach-Zehnder interferometer. It includes three mirrors, four special prisms of different shapes and sizes, a quarter-wave plate, a triangular prism, a one-dimensional piezoelectric stage, two Wollaston polarizers, two converging lenses, four Composed of unit detectors, it can divide the single-longitudinal-mode or multi-longitudinal-mode parallel laser beams incident twice successively with a small frequency difference into four beams of outgoing light whose relative energy varies according to the frequency of the incident light, and then use four unit detectors At the same time, the energy of each beam of outgoing light is detected, and the frequency difference between the two incident lights can be retrieved by detecting the energy of the four channels corresponding to the two incident lights. The advantages of this system are: compact structure, optical path difference can be coarsely adjusted or finely scanned, high detection accuracy, can use multi-longitudinal mode laser incidence, and has low requirements for detectors, especially suitable for vehicle-mounted, airborne and other mobile platforms Various Doppler lidar frequency discrimination systems.

Description

Laser frequency discrimination device and frequency discrimination method based on Mach-Zehnder interferometer

technical field

The invention relates to a laser frequency identification system, in particular to a laser frequency identification device and a frequency identification method based on a Mach-Zehnder interferometer.

Background technique

Doppler lidar is often used to measure wind speed or the moving speed of hard targets. It is widely used in atmospheric physics, meteorological remote sensing, and military weapons. The Doppler frequency shift device used to measure light is one of the most important applications in these applications. An essential core part. Currently, there are two types of detection methods used by Doppler lidar worldwide, coherent detection and non-coherent detection. Incoherent detection is also called direct detection. Direct detection is divided into edge detection technology and fringe imaging technology. Edge detection uses narrow-band filters to identify the frequency shift of the laser caused by wind speed or moving targets. Fringe imaging uses F-P interferometer, Fizeau Interferometer, Michelson interferometer or Mach-Zehnder (Mach-Zehnder) interferometer's interference fringe and frequency correspondence to identify the frequency difference between the transmitted light and the received echo to invert the wind speed or the moving speed of the moving target. Both technologies can work normally under normal working conditions and each has its own advantages. However, once the system is to be placed on a mobile platform such as a vehicle or even spaceborne, the two technologies are both too complex and bulky, and the structure is not stable enough. Photodetection is a single longitudinal mode laser, and some devices require constant temperature conditions and other issues. In view of this situation, the present invention designs a laser that requires very low frequency stabilization accuracy, is insensitive to temperature changes, does not require the laser light source to be incident in a single longitudinal mode, and once the components are fixed, the laser frequency discrimination device will be very stable. A laser frequency discrimination device based on Mach-Zehnder interferometer. The Mach-Zehnder interferometer here is not a fringe-imaging Mach-Zehnder interferometer, but a four-channel non-stripe-imaging Mach-Zehnder interferometer, which is based on a method proposed by Zhao YanLiu and Takao Kobayashi in 1995. A device for detecting frequency shift in wind laser radar is an improvement on the original device. Compared with the original device, the structure is more compact, which makes it possible for the airborne or even spaceborne laser frequency discrimination device. The optical path difference can be adjusted to facilitate Adjustment, data acquisition and inversion of laser frequency, can receive multi-longitudinal mode incident laser and expand the application range of laser frequency discrimination device. Since the laser frequency discrimination device and frequency discrimination method do not utilize fringe imaging technology, they can be classified as an edge detection technology.

Contents of the invention

The purpose of the present invention is to provide a laser radar frequency discrimination device and a frequency discrimination method that can detect frequency shifts, so that Doppler laser radar wind field detection and hard target moving speed detection have a method that can accept multi-longitudinal mode incident laser light and be affected by temperature. It is a stable, reliable and compact frequency discrimination device with little influence and low requirement on laser frequency stabilization accuracy.

In order to achieve the above-mentioned purpose, the laser frequency discriminating device among the present invention is by the first reflection mirror 1, the second reflection mirror 4, the third reflection mirror 7, the first special prism 2, the second special prism 3, the third special prism 8, Fourth special prism 9, triangular prism 6, one-dimensional piezoelectric displacement stage 5, quarter wave plate 10, first Wollaston polarizer 11, second Wollaston polarizer 12, first converging lens 13. The second converging lens 14, the first unit detector 15, the second unit detector 16, the third unit detector 17, and the fourth unit detector 18 are jointly composed.

The heights of the first reflecting mirror 1, the second reflecting mirror 4, the third reflecting mirror 7, the triangular prism 6 and the quarter wave plate 10 are not higher than the first special prism 2, the second special prism 3, the third special prism Half of the lowest height among the special prism 8 and the fourth special prism 9.

A one-dimensional piezoelectric stage 5 for precisely controlling and scanning the optical path difference of the laser frequency discrimination device is installed under the triangular prism 6 .

The parallel incident reference laser or the tested laser beam is reflected by the first reflector 1 placed at an angle of 45° to the incident optical axis, and then enters the first special prism 2 perpendicularly, and the second special prism 3 next to the first special prism 2 The light beam containing 50% of the light energy of the reference laser or the detected laser passes through this surface and reaches the quarter-wave plate 10 and passes through the quarter-wave plate 10. , while the light containing 50% light energy of the reference laser or the detected laser is reflected back inside the first special prism 2, and is reflected again on another surface parallel to the surface partially coated with the semi-transparent and semi-reflective film, The reflected light passes through the first special prism 2 and the second special prism 3, reaches the second reflector 4 placed at an angle of 45° with the optical axis here and is reflected to the triangular prism 6, and experiences twice in the triangular prism 6 After internal reflection, it goes out to the third reflector 7 placed at 45° with the optical axis here, and the light reflected by the third reflector 7 encounters and passes through the third special prism 8 to reach the fourth special prism 9. The light beam that contains 50% of the reference laser or the energy of the detected laser emitted by the one-third wave plate 10 is parallel to the beam that also contains 50% of the energy of the reference laser or the energy of the detected laser emitted from the third special prism, and they are perpendicularly incident on the fourth The special prism 9 is lifted by the fourth special prism 9 to a certain height and then emerges along the incident light axis. The exit direction is opposite to the incident direction. The highest height of the second reflecting mirror 4 and the third reflecting mirror 7 is lower than the highest height of the third special prism 8, the second special prism 3, and the first special prism 2, so the two beams of light are emitted through the fourth special prism 9. One beam will directly reach the second special prism 3 through the top of the quarter wave plate 10, and the other beam will pass through the third special prism 8 to reach the second special prism 3; The light beam of the second special prism 3 passes through the second special prism 3 and reaches the surface of the first special prism 2 partially coated with a semi-transparent and semi-reflective film, and then is split again, containing 25% of the energy of the reference laser or the tested laser. After passing through this surface and passing through the first special prism 2, it reaches the second Wollaston polarizer 12, and is divided by the second Wollaston polarizer 12 into two beams of parallel light with different outgoing directions and containing different polarization components. One beam is obliquely upward and the other is obliquely downward. The two parallel lights are focused to two points on the focal plane through the second converging lens 14, and the second unit detector 16 and the fourth unit detector 18 aligned up and down receive the focal plane. Spot energy on the surface; on the surface of the first special prism 2 partially coated with a semi-transparent and semi-reflective film, the light containing 25% of the energy of the reference light or the tested light is reflected from the surface and passes through the second special prism 3, It is reflected by another surface parallel to the surface next to the first special prism 2 and passes through the second special prism 3 again, before it is emitted to the first Wollaston polarizer 11, and is absorbed by the first Wollaston polarizer 11 Divided into two beams of parallel light with different polarization components that exit obliquely, these two beams of light are converged by the first converging lens 13 and reach the focal plane of the first converging lens 13, and the light spot falls on the first unit detector that is aligned up and down 15 and the third unit detector 17; the light beam that reaches the second special prism 3 through the third special prism 8 passes through the second special prism 3, at the other side of the first special prism 2 parallel to the joint surface of the two special prisms Reflect once and reach the surface of the first special prism 2 coated with a semi-transparent and semi-reflective film, on which the light containing 25% of the light energy of the reference laser or the detected laser passes through this surface and reaches the surface parallel to the joint surface of the two prisms. One side of the second special prism 3 is reflected, the reflected light beam exits the second special prism 3 and reaches the first Wollaston polarizer 11, it will also be divided into two oblique outgoing parallel light beams with different directions and containing different polarized light components and On the first unit detector 15 and the third unit detector 17 focused on its focal plane by the first converging lens 13; there is also a reference laser light at the surface of the first special prism 2 coated with a semi-transparent and half-reflective film. Or the light of 25% of the light energy of the detected laser is reflected and exits the first special prism 2 to reach the second Wollaston polarizer 12, and is divided into two beams with different directions and different polarization components by the second Wollaston polarizer 12 The obliquely emitted parallel light beams pass through the second converging lens 14 and respectively reach the second unit detector 16 and the fourth unit detector 18 on the focal plane thereof.

In the described laser frequency discrimination device based on the Mach-Zehnder interferometer, a one-dimensional piezoelectric displacement stage 5 is installed below the triangular prism 6 to accurately control or scan the optical path difference L of the laser frequency discrimination device. The difference is the difference in the optical path between a beam of parallel light that enters the triangular prism 6 and passes through the third special prism 8 twice and a beam of parallel light that passes through the quarter-wave plate 10 once, and the former optical path is the first The light beam containing 50% of the incident light energy reflected at the interface between the special prism 2 and the second special prism 3 goes through the first special prism 2, the second special prism 3, the second reflector 4, the triangular prism 6, and the third special prism. Reflector 7, the 3rd special prism 8, the 4th special prism 9, the 3rd special prism 8, the second special prism 3, the optical path at the aforementioned interface again after the first special prism 2, the latter optical path is in A beam of parallel light transmitted at the aforementioned interface and containing 50% of the energy of the incident light passes through the quarter-wave plate 10, the fourth special prism 9, and the second special prism 3 to reach the optical path of the aforementioned interface again, all of the above The optical distance in the prism is multiplied by the refractive index of the prism material to calculate the total optical distance.

During single longitudinal mode laser frequency discrimination, a reference single longitudinal mode laser beam of known frequency is first emitted to the laser frequency discrimination device, and the four unit detectors obtain the voltage values corresponding to the four light intensities, and then send The laser frequency discrimination device emits a beam whose frequency differs from that of the reference light by less than one free spectral range of the laser frequency discrimination device The inspected single longitudinal mode laser beam, where c is the speed of light in vacuum, L is the optical path difference of the laser frequency discrimination device, and the voltage values corresponding to the four light intensities are obtained by four unit detectors. During the detection process of the four reference light intensities, the frequency of the reference light is scanned, and the four reference light intensity voltage values will form four sinusoidal curves with a phase interval of π/2 from each other. According to the reference light obtained by the four detectors and the detected The relative positional relationship of the eight voltage values corresponding to the photodetection in the four sinusoidal curves can be used to calculate the light frequency difference between the two, and then the frequency of the photodetected light can be obtained. In multi-longitudinal-mode laser frequency discrimination, when the incident reference laser and the detected laser are multi-longitudinal-mode lasers, if the free spectral range FSR _laser of this multi-longitudinal-mode laser is the same as the FSR _sys of this laser frequency discrimination device, it can still be This device is used to detect the frequency shift of multi-longitudinal mode laser. If the free spectral range of the multi-longitudinal mode laser is different from the FSR _sys of the laser frequency discrimination device, the optical path difference of the laser frequency discrimination device can be changed by adjusting the piezoelectric displacement stage, so that the FSR _sys becomes the same as the FSR _laser , so that the laser frequency discrimination device can still be used for frequency discrimination.

Described first special prism 2 is a straight pentagonal prism, and its upper and lower bases are a pentagon, wherein two non-adjacent sides with light transmission or reflection are parallel to each other, and the shorter one of the parallel sides corresponds to The side of the pentagonal prism is coated with a total reflection film, and in the longer side, the corresponding part of the side where it intersects with the center line of the first reflector 1 and the center of the quarter wave plate 10 is coated with a semi-transparent and semi-reflective film, and the rest of the side Part of the anti-reflection coating is coated, and the side of the pentagonal prism where the laser is incident is coated with an anti-reflection coating.

Described second special prism 3 is a straight hexagonal prism, and its upper and lower bottom surfaces are a hexagon, wherein the side corresponding to the face close to the first special prism 2 and the opposite side of this side are in parallel relationship, and the second A special prism 2 is coated with an anti-reflection coating on the surface next to it, and its opposite side is coated with a total reflection coating; the two sides of the light incident and outgoing second special prism 3 are coated with an anti-reflection coating, and these two sides are also parallel. A parallel plane is perpendicular to the direction of light incident or exiting itself.

The described laser frequency discrimination device based on Mach-Zehnder interferometer, wherein the third special prism 8 is a cuboid, and its two sides intersecting with the optical axis are coated with an anti-reflection coating, and the third special prism 8 It only exists to increase the optical path difference of the laser frequency discrimination device. If the laser frequency discrimination device does not require a large optical path difference, the third special prism 8 can be removed.

The fourth special prism 9 is a straight pentagonal prism. The shape of the upper and lower bottom surfaces of this prism is the same as the shape of the three-dimensional upper and lower bottom surfaces spliced by the pentagonal prism and the triangular prism. Anti-reflection coatings are coated on the sides of light incident and outgoing, and the other sides Coating with total reflection coating; the fourth special prism 9 can be replaced by a triangular prism with two right-angled sides corresponding to side coating with total reflection coating and hypotenuse corresponding side coating anti-reflection coating in the upper and lower bottom surfaces.

The optical axes of the first Wollaston polarizer 11 and the second Wollaston polarizer 12 are all parallel or perpendicular to the fast axis direction of the quarter-wave plate 10, which can be replaced by other polarization splitting elements For the two polarizers, it is only necessary to change the positions of the converging lens and the unit detector at the respective rear ends accordingly.

The three sides of the triangular prism 6 need to be coated, the sides corresponding to the right-angled sides in the upper and lower bottom surfaces are coated with a total reflection coating, and the sides corresponding to the hypotenuse are coated with an anti-reflective coating.

Described triangular prism 6 below is equipped with in order to accurately control and scan the one-dimensional piezoelectric displacement platform 5 of optical path difference of laser frequency discriminating device, and its movement direction is consistent with the light beam direction incident to one-dimensional piezoelectric displacement platform 5, scanning system The optical path difference can be used to draw the spectrum of the light intensity on the four unit detectors changing with the optical path difference, and the control system optical path difference is to meet the requirements of adjusting the optical path difference in some specific applications, such as It is used to find the operating point with the highest signal-to-noise ratio of the system.

The heights of the first reflecting mirror 1, the second reflecting mirror 4, the third reflecting mirror 7, the triangular prism 6 and the quarter wave plate 10 are not higher than the first special prism 2, the second special prism 3, the third special prism Half of the lowest height among the special prism 8 and the fourth special prism 9.

The laser frequency discrimination device based on the Mach-Zehnder interferometer can use other polarization beam splitters instead of the Wollaston polarizer to separately detect the horizontally polarized light component and the vertically polarized light component of the light incident therein.

The third special prism 8 exists only to increase the optical path difference of the laser frequency discrimination device. If a large optical path difference is not required in a specific application, this prism can be removed.

The steps of the frequency discrimination method of the laser frequency discrimination device are as follows:

First inject a reference laser beam into the laser frequency discrimination device, record the corresponding voltage values I ₁₅ , I ₁₆ , I ₁₇ , and I ₁₈ on the four unit detectors, then stop injecting the reference laser beam and inject another beam The frequency difference from the reference light is less than one laser frequency discrimination device free spectral range FSR _sys The detected laser beam is sent to the laser frequency discrimination device, and the corresponding voltage values I ₁₅ ' and I ₁₆ ' on the four unit detectors are recorded again , I ₁₇ ', I ₁₈ ', calculate the optical path difference L when the reference laser is incident according to the following cotangent function, where c is the speed of light in vacuum, and υ is the frequency of incident light:

After obtaining L, take L as known, and substitute I ₁₅ ′, I ₁₆ ′, I ₁₇ ′, and I ₁₈ ′ into the corresponding positions of I ₁₅ , I ₁₆ , I ₁₇ , and I ₁₈ in the above formula to obtain the detected laser frequency υ, thereby obtaining the frequency difference of the two lasers; the above formula (1) is the formula for solving the laser frequency applicable when the fast axis of the quarter wave plate is parallel to the direction of the optical axis of the Wollaston prism. When the fast axis of the one-wave plate is perpendicular to the direction of the optical axis of the Wollaston prism, use the following formula (2) to calculate the laser frequency:

Description of drawings

Fig. 1 is a diagram of the structure of the device of the present invention, and the labels in the figure are: 1-the first reflector, 2-the first special prism, 3-the second special prism, 4-the second reflector, 5-one-dimensional piezoelectric displacement stage, 6-triangular prism, 7-third mirror, 8-third special prism, 9-fourth special prism, 10-quarter wave plate, 11-first Wollaston polarizer, 12-second Wollaston polarizer, 13-first converging lens, 14-second converging lens, 15-first unit detector, 16-second unit detector, 17-third unit detector, 18-fourth unit detector.

Fig. 2 is a three-dimensional view of the laser frequency discrimination device without the one-dimensional piezoelectric displacement stage, two converging lenses and four unit detectors.

FIG. 3 is a schematic diagram of part of the optical path of the incident light beam on the bottom layer of the device before it enters the fourth special prism 9 in a top view.

FIG. 4 is a schematic diagram of the optical path of the light beam being "lifted" in the fourth special prism 9 at a horizontal viewing angle.

FIG. 5 is a schematic diagram of part of the optical path before the light beam emitted from the fourth special prism 9 reaches the two Wollaston polarizers 11 and 12 in a top view.

FIG. 6 is a three-dimensional view of the positional relationship between the triangular prism 6 and the one-dimensional piezoelectric displacement stage 5 .

Fig. 7 is a schematic diagram of the positional relationship of each Wollaston polarizer, its corresponding converging lens at the rear end, and two detectors in a flat view.

Fig. 8 is a graph showing the relationship between the intensity of received light on the four detectors and the frequency of the incident light without considering various errors.

detailed description

Fig. 1 is an example of a laser frequency discrimination device based on a Mach-Zehnder interferometer described in a top view. Fig. 2 is the spatial three-dimensional view of all components except the converging lens 13,14, the unit detectors 15,16,17,18, and the one-dimensional piezoelectric displacement stage 5, which clearly reflects the relative size and positional relationship of the main components. With the layer where the first reflector 1 is located as the bottom layer, the bottom layer also has a second reflector 2, a third reflector 3, a triangular prism 6, and a quarter-wave plate 10, with the first Wollaston polarizer 11, the second The layer where two Wollaston polarizers 12, the first converging lens 13, and the second converging lens 14 are located is the top layer, and the first unit detector 15 and the second unit detector 16 are at a higher position in the top layer, and the third unit The detector 17 and the fourth unit detector 18 are at the lower positions in the top floor. When observed from a top view, the positions of the first unit detector 15 and the third unit detector 17 are coincident, and the positions of the second unit detector 16 and the fourth unit detector 18 are coincident; the first special prism 2, the second special prism 3, the third special prism The heights of the prism 8 and the fourth special prism 9 cover the bottom layer and the top layer, that is, their height is about twice that of the mirrors 1, 2, 3, the triangular prism 6, and the quarter wave plate 10.

A beam of reference laser light or the laser light to be inspected is incident on the first reflector 1 at 45° on the bottom layer, the forward direction is changed from the upward direction in Fig. 1 to the right direction, and it is vertically incident on the first special prism 2. The transmission of the bottom layer in the two special prisms 3 is as shown in Figure 3, and the sides of the two prisms are all processed through coating so that the incident light is split at the interface (specifically, the first special prism 2 in the joint surface is coated with A part of the semi-transparent and semi-reflective film for light splitting), the light transmission joint surface containing 50% of the light energy of the reference laser or the tested laser and the second special prism 3, and the light containing 50% of the light energy of the reference laser or the tested laser It is reflected on the joint surface and is reflected on the other side of the first special prism 2, and then passes through the first special prism 2 and the second special prism 3. Thereafter, two beams of light containing 50% of the energy of the first incident reference laser or the detected laser that are emitted from the second special prism 3 advance in parallel to the right at the bottom, and one beam of light reaches the fast axis in the horizontal direction of the system (vertical direction in the top view) ) of the quarter-wave plate 10, the polarization component in the direction of the slow axis of the light beam is obtained The phase retardation, arrives at the 4th special prism 9 through the quarter-wave plate 10; Make a beam of light advance to the right and be reflected to the triangular prism 6 by the second reflector 4 that becomes 45 ° obliquely with the optical axis here, this The sides corresponding to the right-angle sides in the upper and lower bottom surfaces of the triangular prism 6 are coated with a total reflection coating, and the hypotenuse is coated with an anti-reflection coating, so that the upwardly incident light is reflected to make it advance downwards, reaching the third reflection at an angle of 45° to the direction of light advancement. The mirror 7 is turned to advance to the right, is vertically incident and exits the third special prism 8 in the shape of a cuboid, and reaches the fourth special prism 9 . Here, the two beams of light transmitted and reflected from the interface of the first special prism 2 and the second special prism 3 will reach the lower layer of the fourth special prism 9 through different optical paths. The optical path in the special prism 9 is shown in Fig. 4 . The upper part of the fourth special prism is a standard pentagonal prism, and the lower part is a triangular prism. The angles of the sides of the fourth special prism 9 that they form together make the light incident at the bottom level can be at the top level. shoot. In the top view of the system in Fig. 1, of the two beams of light emitted from the fourth special prism 9 to the left, the upper beam directly reaches the second special prism 3, and the lower beam passes through the third special prism 8 to reach the second special prism 3, and then the two beams The optical paths of the beam light in the first special prism 2 and the second special prism 3 are shown in Figure 5, both of which are split again at the junction of the two prisms, and finally four beams of light are emitted from the two prisms, but these four The beams of light overlap in pairs, and the spatial distribution is still two beams of light. These two beams of light pass through the first Wollaston polarizer 11 and the second Wollaston polarizer 12 respectively, wherein the respective P polarization component and S polarization component are converged by the first converging lens 13 and the second converging lens 14 To the first unit detector 15, the third unit detector 17, the second unit detector 16, and the fourth unit detector 18, as shown in FIG. 7 .

So far, a beam of tested laser light or reference laser light is split at the joint surface of the first special prism 2 and the second special prism 3 for the first time after it is incident, and the two beams of light are transmitted through different paths at the joint surface of the aforementioned two prisms. When they meet again (spatial meeting), the difference in the optical path (including in the air and in the prism material) traveled by the two beams of light is the optical path difference L of the system.

The described laser frequency discrimination device based on Mach-Zehnder interferometer is characterized in that: the first special prism 2 is a straight pentagonal prism, and its upper and lower bases are a pentagon, two of which have light transmission Or the non-adjacent sides of the reflection are parallel to each other, the side of the corresponding pentagonal prism corresponding to the shorter side in the parallel sides is coated with a total reflection film, and the longer side is connected to the center of the first reflector 1 and the quarter wave plate 10. The side of the part corresponding to the intersecting position of the center line is coated with a semi-transparent and semi-reflective film, the rest of the side is coated with an anti-reflective coating, and the side of the pentagonal prism where the laser is incident is coated with an anti-reflective coating.

The described laser frequency discrimination device based on Mach-Zehnder interferometer is characterized in that: the second special prism 3 is a straight hexagonal prism, and its upper and lower bases are hexagonal, and the first special prism 3 is 2. The side corresponding to the adjacent surface is parallel to the opposite side of this side. The surface adjacent to the first special prism 2 is coated with an anti-reflection coating, and the opposite side is coated with a total reflection coating; light incident and exit the second special prism The two sides of 3 are coated with anti-reflection coating, and these two sides are also in parallel relationship, and these two parallel surfaces are perpendicular to the direction of incident or outgoing light.

The described laser frequency discrimination device based on Mach-Zehnder interferometer is characterized in that: the third special prism 8 is a cuboid, and its two sides intersecting with the optical axis are plated with an anti-reflection coating, and the third The special prism 8 only exists to increase the optical path difference of the laser frequency discrimination device, if the laser frequency discrimination device does not require a large optical path difference, the third special prism 8 can be removed.

The described laser frequency discrimination device based on Mach-Zehnder interferometer is characterized in that: the fourth special prism 9 is a straight pentagonal prism, the shape of the upper and lower bottom surfaces of this prism is spliced with a pentagonal prism and a triangular prism The shape of the three-dimensional upper and lower bottom surfaces is the same, and anti-reflection coatings are coated on the sides of light incident and outgoing, and total reflection coatings are coated on other sides; the fourth special prism 9 can be coated with total reflection coatings on the sides corresponding to two right-angled sides in a bottom surface, and the hypotenuses correspond to Triangular prism replacement with AR-coated sides.

The described laser frequency discrimination device based on Mach-Zehnder interferometer is characterized in that: the optical axes of the first Wollaston polarizer 11 and the second Wollaston polarizer 12 are all parallel or vertical In the direction of the fast axis of the quarter-wave plate 10, the two polarizers can be replaced by other polarization splitting elements, as long as the positions of the converging lens and the unit detector at the respective rear ends are changed correspondingly.

Described a kind of laser frequency discriminating device based on Mach-Zehnder interferometer is characterized in that: the three sides of described triangular prism 6 need to be coated with film processing, and the side corresponding to right-angled side in its upper and lower bottom surfaces is coated with total reflection film, oblique The side corresponding to the edge is coated with anti-reflection coating.

The described laser frequency discrimination device based on Mach-Zehnder interferometer is characterized in that: a one-dimensional piezoelectric stage 5 for precisely controlling and scanning the optical path difference of the laser frequency discrimination device is installed under the triangular prism 6 , its motion direction is consistent with the light beam direction incident to the one-dimensional piezoelectric displacement stage 5, and the optical path difference of the scanning system can be used to draw the spectrum of the light intensity on the four unit detectors changing with the optical path difference, and the control system The optical path difference is to meet the requirements of adjusting the optical path difference in some specific applications, for example, it can be used to find the operating point with the highest signal-to-noise ratio of the system.

The laser frequency discrimination device based on the Mach-Zehnder interferometer can use other polarization beam splitters instead of the Wollaston polarizer to separately detect the horizontally polarized light component and the vertically polarized light component of the light incident therein.

In the described laser frequency discrimination device based on Mach-Zehnder interferometer, the third special prism 8 exists only to increase the optical path difference of the laser frequency discrimination device. Remove this prism.

In Fig. 5, the light incident on the bonding surface of the first special prism 2 and the second special prism 3 from the right is due to the thin air gap between the two prisms, and the reflected light is actually coated by a part of the first special prism. The surface reflection of the semi-transparent and semi-reflective medium film is an external reflection, and the reflected light has a phase loss of π, and the quarter-wave plate 10 introduces the light component in the direction of the slow axis of the wave plate Phase loss, these two additional phase delays finally make the light intensity signals detected on the four unit detectors can be expressed by the following formula:

where I ₀ is the incident beam intensity, λ is the wavelength of incident light, L is the optical path difference of the system, and c is the speed of light in vacuum. The schematic diagram of these four intensity signals changing with Δ is shown in Figure 8 (the sequence of the four signals is related to the fast axis direction of the quarter-wave plate and the optical axis direction of the Wollaston polarizer, where the two axes are parallel to each other) Show. The period of the periodic intensity signal in the figure with respect to the frequency is This is also the free spectral range FSR _sys of the system. The difference between the two incident frequencies is less than one FSR _sys . The four intensity signals detected by this device are different. According to the corresponding relationship between intensity and frequency, the frequency of the two incident lights can be retrieved. According to the frequency difference, the wind speed detected by the lidar and the moving speed of the hard target can be retrieved. The inversion method can use the look-up table method, or the mathematical method proposed in some existing literature, that is:

then there is

Thus, the one-to-one mapping relationship between the incident light wavelength (frequency) and the signals detected by the four detectors can be obtained, and the signals detected by the detectors on the four channels corresponding to the reference laser (the electrical signal represents the light intensity signal) are substituted into (9 ) formula to obtain the optical path difference L, and then substitute the signals detected by the detectors on the four channels corresponding to the tested laser light and the calculated L into the formula (9) to obtain the wavelength (frequency) of the tested laser light. The above formula (9) is consistent with formula (1), and it is the formula for solving the laser frequency when the fast axis of the quarter wave plate is parallel to the direction of the optical axis of the Wollaston prism. When the quarter wave plate When the fast axis of the prism is perpendicular to the direction of the optical axis of the Wollaston prism, use formula (2) to calculate the laser frequency.

In practice, any laser has broadening. At this time, the signal intensity detected by the detector is the convolution of the incident light spectrum broadening and I _i , i=15, 16, 17, 18, which will affect the detection accuracy of the system, and the spectral broadening The smaller the value, the higher the detection accuracy.

For wind speed detection, the signal detected by the detector of the device is the convolution of the spectrum of the incident light and the theoretical light intensity transmittance I _i /I ₀ of each channel, i=15, 16, 17, 18. The incident light twice successively is the laser light not emitted into the atmosphere and the atmospheric echo received by the telescope, and if the incident light is the atmospheric scattering echo, it will be broadened by Rayleigh scattering and Meter scattering, which will affect the detected The contrast of light intensity (ie, the relative intensity of I ₁₅ , I ₁₆ , I ₁₇ , and I ₁₈ ), because the spread of Rayleigh scattering is much larger than that of meter scattering, it is a constant after convolution with the light intensity transmittance of each channel , to bring the same DC bias to each channel signal, at this time the relative intensity of the light intensity signal obtained by each channel detector is only related to the meter scattering, that is, the device only uses the atmospheric meter scattering when detecting the laser frequency shift light signal.

A one-dimensional piezoelectric displacement stage is placed under the triangular prism 6 as shown in Figure 6. The vibration direction of the piezoelectric displacement stage is the same as the incident light direction of the triangular prism, and its four corners are fixed with screws. The screw holes are "U" shaped, which allows Or roughly adjust the optical path difference of the system. There is a control cable at the bottom of the piezoelectric stage, which is connected to its control circuit. The introduction of the piezoelectric displacement stage can make this device have the ability of optical path difference control and scanning, which can more easily obtain the intensity spectrum on the four unit detectors, invert the frequency difference, and also help to find the highest signal-to-noise of the system than work.

Under the condition of multi-longitudinal-mode laser incidence, as long as the position of the piezoelectric displacement stage is adjusted to adjust the optical path difference L of the system, the free spectral range FSR _sys of the device is the same as the free spectral range FSR _laser of the incident multi-longitudinal-mode laser. Let the intensity spectra of all longitudinal modes overlap as shown in Figure 8 (and the "order" difference of adjacent longitudinal mode intensity spectra is 1), so that the combined intensity spectrum of each longitudinal mode is still the same as the intensity spectrum line shape of a single longitudinal mode , does not affect our inversion frequency difference.

Claims (8)

1. a kind of laser frequency discrimination device based on Mach-Zehnder interferometer, including the first speculum (1), the second speculum (4), the Three speculums (7), the first special prism (2), the second special prism (3), the 3rd special prism (8), the 4th special prism (9), Triangular prism (6), one-dimensional piezoelectric position moving stage (5), quarter-wave plate (10), the first Wollaston polarizer (11), second is fertile Lars polarizer (12), the first convergent lens (13), the second convergent lens (14), first module detector (15), second is single First detector (16), third unit detector (17) and the 4th single-element detector (18)；It is characterized in that：

Described the first speculum (1), the second speculum (4), the 3rd speculum (7), triangular prism (6), quarter-wave plate (10) height is no more than the first special prism (2), the second special prism (3), the 3rd special prism (8), the 4th special prism (9) half of minimum altitude in；

One-dimensional piezoelectric position to accurate control and scanning laser frequency discrimination device optical path difference is housed below described triangular prism (6) Moving stage (5)；

Parallel incident reference laser or tested laser beam pass through to be reflected with the first speculum (1) of incident light axis placement at 45 ° Vertical incidence enters the first special prism (2) afterwards, in the adjacent second special prism (3) of the first special prism (2) and by portion Divide and be plated with the face of semi-transparent semi-reflecting film, the light beam containing reference laser or the light energy of tested laser 50% is through four points through this face One of wave plate (10) and through quarter-wave plate (10), and the light containing reference laser or the light energy of tested laser 50% in addition It is reflected back that the first special prism (2) is internal herein, and in parallel another face in the face with being partly plated with semi-transparent semi-reflecting film again Secondary to be reflected, reflected light passes through the first special prism (2) and the second special prism (3) again, and arrival is put with the angle at 45 ° of optical axis herein The second speculum (4) for putting simultaneously is reflected to triangular prism (6), is emitted in triangular prism (6) after experience internal reflection twice With the 3rd speculum (7) of the placement at 45 ° of optical axis herein, met with by the light that the 3rd speculum (7) reflects and to pass through the 3rd special Prism (8) reaches the 4th special prism (9), at this moment contains 50% reference laser or tested from quarter-wave plate (10) outgoing The light beam of laser energy and the same light beam containing 50% reference laser or tested laser energy from the 3rd special prism outgoing Be parallel to each other, they impinge perpendicularly on the 4th special prism (9), by after the 4th special prism (9) lifting certain altitude along incidence Optical axis is emitted, and exit direction is with incident direction on the contrary, the two-beam position of outgoing is higher than quarter-wave plate (10), the first reflection Mirror (1), the second speculum (4) and the 3rd speculum (7) maximum height and less than the 3rd special prism (8), the second special edge Mirror (3), the maximum height of the first special prism (2), thus two-beam is wherein a branch of after the 4th special prism (9) outgoing will be logical The top for crossing quarter-wave plate (10) directly reaches the second special prism (3), and another beam will undergo the 3rd special prism (8) and arrive Up to the second special prism (3)；The light beam that the second special prism (3) is incided above from quarter-wave plate (10) is special through second The quilt that different prism (3) reaches the first special prism (2) afterwards is divided again after partly having plated the face of semi-transparent semi-reflecting film, contains reference The light of the energy of laser or tested laser 25% is through this face and undergoes the first special prism (2) to reach the second Wollaston again inclined Shake device (12), and it is different flat containing different polarization component to be divided into two beam-emergence directions by the second Wollaston polarizer (12) Row light, obliquely, two directional lights are focused onto on its focal plane a branch of beam another obliquely by the second convergent lens (14) , the hot spot energy on focal plane is received with the second unit detector (16) of consistency from top to bottom and the 4th single-element detector (18) at 2 points Amount；It is coated with the part of the first special prism (2) on the face of semi-transparent semi-reflecting film, the energy containing reference light or tested light 25% After light reflects from the face, the second special prism (3) is undergone, by its another parallel with the face that the first special prism (2) is adjacent Face is reflected and undergoes the second special prism (3) again, before being emitted to the first Wollaston polarizer (11), by the first Wollaston Polarizer (11) is divided into the directional light containing different polarization component that two beams are tiltedly emitted, and this two-beam is by the first convergent lens (13) Converge on the focal plane up to the first convergent lens (13), hot spot fall consistency from top to bottom place first module detector (15) and On third unit detector (17)；The light beam for undergoing the 3rd the second special prism (3) of special prism (8) arrival is special through second Prism (3), once arrival first is special for reflection at the another side parallel with two special prism composition surfaces of the first special prism (2) The part of different prism (2) is coated with the face of semi-transparent semi-reflecting film, and reference laser or the light energy of tested laser 25% are contained on this face Light reach the one side of the second special prism (3) parallel with two prisms composition surface through this face and reflected, the reflected beams go out Penetrate the second special prism (3) and reach the first Wollaston polarizer (11), it is different containing difference that it can also be divided into two beam directions The oblique outgoing collimated light beam of polarized light component and the first module detector that its focal plane is focused on by the first convergent lens (13) (15) and on third unit detector (17)；It is coated with to also have at the face of semi-transparent semi-reflecting film in the part of the first special prism (2) and contains There are reference laser or the light of the light energy of tested laser 25% to be reflected and be emitted the first special prism (2) and reach the second Wollaston Polarizer (12), the different oblique outgoing containing different polarization component in two beam directions are divided into by the second Wollaston polarizer (12) Directional light, this two beams directional light arrive separately at the second unit detector (16) on its focal plane through the second convergent lens (14) again On the 4th single-element detector (18)；

The optical path difference L of laser frequency discrimination device be into triangular prism (6) and transmit twice the light beam of the 3rd special prism (8) with By the optical path difference between the light beam of quarter-wave plate (10) once, the former light path is the first special prism (2) and second The light beam containing 50% incident light energy reflected at special prism (3) composition surface undergoes the first special prism (2), second successively It is special prism (3), the second speculum (4), triangular prism (6), the 3rd speculum (7), the 3rd special prism (8), the 4th special Arrived again at after prism (9), the 3rd special prism (8), the second special prism (3), the first special prism (2) at foregoing composition surface Light path, the latter's light path at foregoing interface transmit a branch of directional light containing 50% incident light energy, undergo four points One of wave plate (10), the 4th special prism (9), the second special prism (3) arrive again at light path at foregoing composition surface, above-mentioned institute The refractive index that the light path for having in the prism will be multiplied by prism material is included in total optical path；

During single longitudinal mode laser frequency discrimination, launch the reference single longitudinal mode laser of a branch of given frequency to described laser frequency discrimination device first Beam, the magnitude of voltage as corresponding to four single-element detectors obtain four light intensity；Then it is a branch of to described laser frequency discrimination device transmitting With less than one laser frequency discrimination device Free Spectral Range of reference light frequency phase-differenceTested single longitudinal mode laser beam, Wherein c is the light velocity in vacuum, the magnitude of voltage as corresponding to four single-element detectors four light intensity of acquisition；In four reference light light intensity In detection process, scanning refers to light frequency, and this four reference light light intensity magnitudes of voltage can form four and be separated by the four of pi/2 phase 8 magnitudes of voltage are in four sine curves corresponding to bar sine curve, the reference light obtained according to four detectors and tested light Relative position relation, the light frequency that can be extrapolated between the two is poor, and then obtains the frequency of tested light；Multilongitudianl-mode laser frequency discrimination When, when incidence reference laser and tested laser be multilongitudianl-mode laser when, if the Free Spectral Range of this multilongitudianl-mode laser FSR_laserWith the FSR of this laser frequency discrimination device_sysIt is identical, then the frequency displacement of multilongitudianl-mode laser, frequency discrimination still can be detected with the present apparatus Method is constant, if the FSR of the Free Spectral Range of this multilongitudianl-mode laser and this laser frequency discrimination device_sysDifference, then it can pass through tune Piezoelectric position moving stage is saved to change the optical path difference L of laser frequency discrimination device, so that FSR_sysBecome and FSR_laserIt is identical, so still This laser frequency discrimination device frequency discrimination can be used.

A kind of 2. laser frequency discrimination device based on Mach-Zehnder interferometer according to claim 1, it is characterised in that：It is described The first special prism (2) be a straight pentagonal prism, bottom surface is a pentagon thereon, and two of which has light transmission or reflection Non-conterminous side it is parallel to each other, the side plating total reflection film of pentagonal prism corresponding to shorter one side, longer in parallel side The surface for the position correspondence that its in one side intersects with the first speculum (1) center and quarter-wave plate (10) line of centres Semi-transparent semi-reflecting film is plated, this side remainder plating anti-reflection film, the side plating anti-reflection film of the laser light incident of pentagonal prism.

A kind of 3. laser frequency discrimination device based on Mach-Zehnder interferometer according to claim 1, it is characterised in that：It is described The second special prism (3) be straight six prism, bottom surface is a hexagon thereon, wherein with the first special prism (2) Opposite side corresponding to adjacent face while with this is parallel relation, plates anti-reflection film with the adjacent face of the first special prism (2), its is right Side plating total reflection film corresponding to side；Light is incident and anti-reflection film, and the two are plated in two sides of the second special prism (3) of outgoing Side is also parallel relation, the two parallel faces with it is incident or be emitted the light of itself direction it is vertical.

A kind of 4. laser frequency discrimination device based on Mach-Zehnder interferometer according to claim 1, it is characterised in that：It is described The 3rd special prism (8) be a cuboid, its two side intersected with optical axis plating anti-reflection film, when laser frequency discrimination device not When needing very big optical path difference, the 3rd special prism (8) can be removed.

A kind of 5. laser frequency discrimination device based on Mach-Zehnder interferometer according to claim 1, it is characterised in that：It is described The 4th special prism (9) be a straight pentagonal prism, shape and the pentagonal prism and triangular prism of the upper bottom surface of this prism are spelled The three-dimensional bottom shape up and down connect is identical, and anti-reflection film, other side plating total reflection films are plated in the side that light is incident and is emitted；Or The special prism (9) of person the 4th is that two right-angle side corresponding side surface plating total reflection films, the plating of hypotenuse corresponding side surface increase in a upper bottom surface The triangular prism of permeable membrane.

A kind of 6. laser frequency discrimination device based on Mach-Zehnder interferometer according to claim 1, it is characterised in that：It is described The first Wollaston polarizer (11) and the second Wollaston polarizer (12) can be substituted with polarization beam splitter.

A kind of 7. laser frequency discrimination device based on Mach-Zehnder interferometer according to claim 1, it is characterised in that：It is described Triangular prism (6) two right-angle sides corresponding to side plating total reflection film, corresponding to hypotenuse side plate anti-reflection film.

A kind of a kind of 8. laser frequency mirror of laser frequency discrimination device based on Mach-Zehnder interferometer based on described in claim 1 Frequency method, it is characterised in that method and step is as follows：First incident a branch of reference laser light beam records to described laser frequency discrimination device Corresponding magnitude of voltage I on four single-element detectors₁₅、I₁₆、I₁₇、I₁₈, then stop incident reference laser beam and re-shoot a branch of With less than one laser frequency discrimination device Free Spectral Range FSR of reference light frequency phase-difference_sysTested laser beam swash to described Light frequency discrimination device, corresponding magnitude of voltage I on four single-element detectors is recorded again₁₅’、I₁₆’、I₁₇’、I₁₈', more than following Optical path difference L when function calculates reference laser incidence is cut, wherein c is the light velocity in a vacuum, and υ is incident light frequency：

Try to achieve after L again using L as, it is known that by I₁₅’、I₁₆’、I₁₇’、I₁₈' substitute into I in above formula₁₅、I₁₆、I₁₇、I₁₈Relevant position, ask Laser frequency υ must be detected, so as to obtain the difference on the frequency of two kinds of laser；Above formula (1) be quarter-wave plate fast axle and When the direction of Wollaston prism optical axis is parallel be applicable solutions laser frequency formula, when quarter-wave plate fast axle and irrigate When the direction of Lars prism optical axis is vertical, laser frequency is calculated using following formula (2)：

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