Micro-pulse time division multiplexing differential laser radar transmitter

Abstract

A micropulse time division multiplexing differential laser radar transmitter provided by the present invention comprises a near-field unit and a far-field unit, and the near-field unit and the far-field unit each comprise a laser diode, a first self-focusing lens, a band-pass filter, a servo motor, an achromatic triplet lens, a second self-focusing lens, a cylindrical lens, a 45-degree total reflection mirror and a telescope primary mirror. The servo motor is used for driving the band-pass optical filter to rotate back and forth within a certain angle range, and the center of the 45-degree total reflection mirror is provided with a middle hole, so that the divergence angle is reduced, the parallelism is higher, the device has the advantages of simple structure, low cost and high reliability, large-batch deployment can be realized, and the application range is wide. And a powerful guarantee is provided for constructing a near-earth boundary layer thermodynamic monitoring network with national coverage.

Description

Technical Field

[0001] The present invention relates to the field of laser radar technology, and in particular to a micro-pulse time-division multiplexing differential laser radar transmitter. Background Art

[0002] A nationwide near-Earth boundary layer thermodynamic monitoring network is urgently needed to fill the critical gaps that currently hinder weather model forecasting capabilities.

[0003] Radiosondes remain the standard means of obtaining true water vapor values ​​worldwide. These stations are sparsely distributed, usually hundreds of kilometers apart. Due to practical conditions such as air traffic control, most weather service stations only launch radiosondes twice a day. The existing radiosonde network does not provide sufficient temporal and horizontal resolution to fully sample the rapidly changing water vapor distribution, especially in the atmospheric boundary layer (ABL). The key to improving horizontal resolution is to find a reliable, low-cost measurement device that can be deployed in large numbers.

[0004] Active remote sensing technologies for measuring water vapor include differential absorption lidar and Raman lidar. Raman lidar uses weak inelastic scattering from atmospheric water vapor molecules to infer water vapor mixing ratio. Raman lidar has been shown to provide accurate water vapor mixing ratio profiles with good temporal and vertical resolution. Currently, only a few of these instruments are operating worldwide. These expensive, research-grade instruments require powerful laser transmitters (usually unsafe for the eyes) and large receiving apertures to detect weak inelastic backscattering. Raman lidar also requires independent, companion radiosondes for calibration. In addition, Raman lidar exhibits a certain degree of residual overlap factor, which may cause large deviations in low-altitude water vapor mixing ratio. There are also several schemes for differential absorption lidar under development and trial operation. The differential absorption lidar transmitter introduced here has high component maturity, easy to obtain supply guarantee, and good reliability and stability. At present, it is necessary to find a reliable, low-cost lidar that can be deployed in large quantities to provide strong support for the construction of a nationwide near-ground boundary layer thermodynamic monitoring network. Summary of the invention

[0005] The problem to be solved by the present invention is to provide a micro-pulse time-division multiplexing differential laser radar transmitter, which can reduce the cost of use and facilitate large-scale deployment.

[0006] To solve the above technical problems, a micro-pulse time-division multiplexing differential lidar transmitter provided by the present invention includes a near-field unit and a far-field unit. Both the near-field unit and the far-field unit include a laser diode, a first self-focusing mirror, a band-pass filter, a servo motor, an achromatic triplet lens, a second self-focusing mirror, a cylindrical lens, a 45° total reflection mirror, and a telescope primary mirror. The servo motor is used to drive the band-pass filter to rotate back and forth within a certain angle range, and a central hole is provided at the center of the 45° total reflection mirror.

[0007] Preferably, the peak power of the laser diode is above 150W, the central wavelength is between 905 - 945nm, and the spectral width is within 6nm.

[0008] Preferably, the laser pulse repetition frequency range of the laser diode is 1KHZ - 10KHZ, and the pulse time range is 150ns - 600ns.

[0009] Preferably, the diameter of the telescope primary mirror of the near-field unit is 150 ± 20m, and the diameter of the telescope primary mirror of the far-field unit is 300 ± 50m.

[0010] Preferably, the band-pass filter is placed in the free optical space formed by the first self-focusing mirror and the second self-focusing mirror, and an achromatic triplet lens is configured.

[0011] Preferably, the band-pass filter reciprocates with precise angles and stable frequencies under the drive of the servo motor.

[0012] Preferably, it further includes a driver, and the driver includes a switched current source and a temperature controller. The switched current source is used to provide pulsed current to the laser diode, and the temperature controller is used to control the temperature of the laser diode.

[0013] The beneficial effects of the present invention are as follows: The present invention provides a micro-pulse time-division multiplexing differential lidar transmitter. The laser beam emitted by the laser diode enters the first self-focusing lens for collimation. After that, the divergence angle of the beam becomes smaller, and the nearly parallel beam passes through the band-pass filter. A large part of the laser power is intercepted by the band-pass filter, and only the laser with a wavelength within the passband range of the band-pass filter can pass through the band-pass filter, which reflects the gating characteristic of the band-pass filter. Driven by the servo motor, the plane of the band-pass filter rotates around the symmetry axis. The rotating shaft of the motor rotates back and forth with a specific angular displacement and a stable frequency, so that the filter alternately presents two states of "central wavelength and passband", and the transmitter can alternately emit two kinds of beams called "online" and "offline". The beam passing through the band-pass filter enters the achromatic triplet lens. The combined lens compensates for the astigmatism of the beam and compensates for the radial offset of the beam caused by the oblique incidence on the band-pass filter. After that, the beam enters the second self-focusing lens and the cylindrical lens, and the beam converges at the focus of the telescope primary mirror. After that, the laser beam emitted from the focus passes through the central hole of the 45° total reflection mirror and finally emits from the telescope primary mirror, and its divergence angle becomes smaller and the parallelism is higher. This lidar transmitter has the advantages of simple structure, low cost and high reliability, and can be deployed in large quantities, providing a strong guarantee for the construction of a national near-surface boundary layer thermodynamic monitoring network. Description of the Drawings

[0014] Figure 1 Illustrates the structural schematic diagram of the emission optics and the telescope of the present invention.

[0015] Figure 2 Illustrates the structural schematic diagram of the optical unit of the present invention.

[0016] Figure 3 Illustrates the schematic diagram of the driver of the present invention.

[0017] Figure 4 Illustrates the normalized water vapor absorption cross-section, normalized offline and online laser spectral power distribution diagrams (solid line for far field, dashed line for near field) of the present invention at 296K and 1atm.

[0018] Figure 5 Illustrates an example of the original emission power spectrum distribution of the laser diode of the present invention (the vertical axis is the relative unit power density, not a limited value).

[0019] Figure 6 Illustrates an example of the offline spectral distribution of the ultimate emission of the far-field unit of the present invention (the vertical axis is the relative unit power density, not a limited value).

[0020] Figure 7Example of the online spectral distribution of the ultimate emission of the far-field unit of the present invention (the vertical axis is relative to the unit power density, non-limiting value)

[0021] Figure 8 Example of the offline spectral distribution of the ultimate emission of the near-field unit of the present invention (the vertical axis is relative to the unit power density, non-limiting value)

[0022] Figure 9 Example of the online spectral distribution of the ultimate emission of the near-field unit of the present invention (the vertical axis is relative to the unit power density, non-limiting value)

[0023] Explanation of the reference numerals in the drawings: Laser diode 1, first self-focusing mirror 2, band-pass filter 3, servo motor 4, achromatic triplet lens 5, second self-focusing mirror 6, cylindrical lens 7, 45° total reflection mirror 8, telescope primary mirror 9. Detailed implementation manners

[0024] To make the objectives, technical solutions, and advantages of the embodiments of the present disclosure clearer, the technical solutions of the embodiments of the present disclosure will be clearly and completely described below with reference to the accompanying drawings of the embodiments of the present disclosure. Obviously, the described embodiments are some but not all of the embodiments of the present disclosure.

[0025] All other embodiments obtained by those of ordinary skill in the art based on the described embodiments of the present disclosure without creative efforts shall fall within the scope of protection of the present disclosure.

[0026] Reference Figures 1-9 .

[0027] The present invention provides a micro-pulse time-division multiplexing differential lidar transmitter, which includes a near-field unit and a far-field unit. Both the near-field unit and the far-field unit include a laser diode 1, a first self-focusing mirror 2, a band-pass filter 3, a servo motor 4, an achromatic triplet lens 5, a second self-focusing mirror 6, a cylindrical lens 7, a 45° total reflection mirror 8, and a telescope primary mirror 9. The servo motor 4 is used to drive the band-pass filter 3 to rotate back and forth within a certain angle range, and a central hole is provided in the center of the 45° total reflection mirror 8.

[0028] Based on the above embodiment, the peak power of the laser diode 1 is above 150 W, the central wavelength is between 905 - 945 nm, and the spectral width is within 6 nm.

[0029] Based on the above embodiment, the laser pulse repetition frequency range of the laser diode 1 is 1 KHZ - 10 KHZ, and the pulse time range is 150 ns - 600 ns. The energy of each pulse fluctuates within a certain range (5%), so the energy of each emitted pulse is monitored.

[0030] Based on the above embodiments, the diameter of the telescope primary mirror 9 of the near-field unit is 150 ± 20 m, and the diameter of the telescope primary mirror 9 of the far-field unit is 300 ± 50 m.

[0031] Based on the above embodiments, the band-pass filter 3 is placed in the free optical space formed by the first self-focusing mirror 2 and the second self-focusing mirror 6, and an achromatic triplet lens 5 is configured.

[0032] Based on the above embodiments, the band-pass filter 3 reciprocally swings at a precise angle and a stable frequency under the drive of the servo motor 4.

[0033] Based on the above embodiments, a driver is further included. The driver includes a switched current source and a temperature controller. The switched current source is used to provide pulsed current to the laser diode, and the temperature controller is used to control the temperature of the laser diode.

[0034] The lidar consists of a near-field unit and a far-field unit; the diameter of the telescope primary mirror of the near-field unit is φ150 ± 20 m, and the diameter of the telescope primary mirror of the far-field unit is φ300 ± 50 m; each measurement unit uses its own transmitter, as Figure 1 shown. The respective transmitting units alternately transmit for 5 seconds to avoid echo crosstalk between the two units.

[0035] The online laser beam and the offline laser beam are successively emitted with a common aperture, and their atmospheric backscattering is also collected by the common aperture. The transmitting aperture and the receiving aperture share the same telescope. The parallel beam from the transmitting optical path is emitted from the central circular area of the telescope; the outer peripheral circular area of the telescope collects the backscattered light, which is deflected by the circular planar 45° mirror and enters the echo detection optical path.

[0036] The single-pulse energy of the micropulse lidar is small, greater than 1 μJ and not exceeding 1000 μJ; the single-pulse energy of the far-field unit is greater than that of the near-field unit. The single-pulse repetition frequency of the micropulse lidar is high, greater than 1 kHz and less than 10 kHz. The energy of each pulse fluctuates within a range of 5%, so the energy of each emitted pulse is monitored.

[0037] The differential absorption lidar emits two kinds of beams. One kind has a relatively large absorption cross-section for water vapor molecules and is called the online beam, and the other kind has a very small or even negligible absorption cross-section for water vapor molecules and is called the offline beam, as Figure 4 shown. At regular intervals, the transmitter alternately emits the online beam and the offline beam successively. This emission method can be called the online / offline time-division multiplexing emission method.

[0038] As Figure 2As shown, the cross-section of the laser beam emitted by the laser diode 1 is an elliptical beam (diffraction effect). Therefore, the divergence angles of the beam in two orthogonal directions are quite different. Thus, in the final stage of beam emission, the cylindrical lens 7 is required to shape the beam, compressing the beam in the long axis direction of the ellipse to make the beam cross-section tend to be circular. The selection of the curvature radius of the cylindrical lens 7 is determined by the ellipticity e of the laser beam of the laser diode 1.

[0039] As Figure 1 shown, the beam emitted by the laser diode 1 (whose full spectral distribution is as Figure 5 shown and the spectrum is very wide) encounters the first self-focusing lens 2, and the beam is collimated and becomes closer to a parallel beam. Such a beam passes through the band-pass filter 3, and a large part of the energy is isolated by the band-pass filter 3. Only the light that conforms to the passband of the band-pass filter 3 can pass through, as Figure 6 and Figure 7 , Figure 8 and Figure 9 . After passing through a section of free space, the beam passes through the achromatic triplet lens 5 (the achromatic triplet lens 5 can use Steinheil's TRS064-010-A), is compensated by the achromatic triplet lens 5, and enters the second self-focusing lens 6 and the cylindrical mirror 7. Thereafter, the beam converges at the focus of the telescope primary mirror 9. The laser beam emitted from this focus, after passing through the telescope primary mirror 9, is further collimated and enters the atmosphere with a smaller divergence angle.

[0040] If λ normal is the central wavelength of the passband of the band-pass filter 3 for normal incidence (θ = 0°), λ θ is the central wavelength of the passband when the incident beam deviates from the normal direction of the band-pass filter 3 by an angle θ, and n is the refractive index of the substrate material of the band-pass filter 3. Among them, there is a quantitative relationship between the peak wavelength λ θ of the band-pass filter 3 and the incident angle θ:

[0041]

[0042] Based on the switching of the incident beam's angle with respect to the normal of the band-pass filter 3, the bandwidth and central wavelength of the transmitted beam are changed, as Figure 6 and Figure 7 , as Figure 8 and Figure 9 shown, which are the offline and online emission spectra respectively. According to the pre-determined time sequence, when the transmitter needs to switch the emission wavelength, the motor rotates at the pre-determined angle.

[0043] As Figure 3As shown, the driver of the transmitter includes a switched current source and a temperature controller. The frequency-stabilized oscillator generates a sine wave with a frequency of 1 kHz - 10 kHz. This sine wave triggers the pulse generator to generate switching pulses with a width of 150 - 600 ns. The pulse generator switches a current source on the order of dozens of amperes, and the pulsed current drives the diode to emit pulsed laser light.

[0044] As Figure 3 shown, based on the difference between the signal output by the temperature-sensitive diode and the set signal, the TEC device is adjusted to control the temperature of the laser diode.

[0045] The above embodiments are only descriptions of the preferred embodiments of the present invention and do not limit the scope of the present invention. Without departing from the design spirit of the present invention, various deformations and improvements made by those of ordinary skill in the art to the technical solutions of the present invention shall fall within the protection scope determined by the claims of the present invention.

Claims

1. A micro-pulse time-division multiplexing differential laser radar transmitter, characterized in that: It comprises a near-field unit and a far-field unit, each of which comprises a laser diode, a first self-focusing mirror, a band-pass filter, a servo motor, an achromatic triplet lens, a second self-focusing mirror, a cylindrical lens, a 45° total reflection mirror, and a telescope primary mirror. The servo motor is used to drive the band-pass filter to rotate back and forth within a certain angle range, and a central hole is provided at the center of the 45° total reflection mirror.

2. A micro-pulse time-division multiplexing differential laser radar transmitter according to claim 1, characterized in that: The peak power of the laser diode is above 150W, the central wavelength is between 905-945nm, and the spectrum width is within 6nm.

3. A micro-pulse time-division multiplexing differential laser radar transmitter according to claim 2, characterized in that: The laser pulse repetition frequency interval of the laser diode is 1KHZ-10KHZ, and the pulse time interval is 150ns-600ns.

4. A micro-pulse time-division multiplexing differential laser radar transmitter according to claim 3, characterized in that: The diameter of the telescope primary mirror of the near-field unit is 150±20m, and the diameter of the telescope primary mirror of the far-field unit is 300±50m.

5. A micro-pulse time-division multiplexing differential laser radar transmitter according to claim 4, characterized in that: The bandpass filter is placed in the free light space formed by the first self-focusing mirror and the second self-focusing mirror, and is configured with the achromatic triplet lens.

6. A micro-pulse time-division multiplexing differential laser radar transmitter according to claim 5, characterized in that: The bandpass filter is driven by the servo motor to reciprocate at a precise angle and a stable frequency.

7. The micro-pulse time-division multiplexing differential laser radar transmitter according to claim 1, characterized in that: It also includes a driver, which includes a switching current source and a temperature controller. The switching current source is used to provide a pulse current to the laser diode, and the temperature controller is used to control the temperature of the laser diode.

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