A laser radar light source based on external cavity stimulated raman amplification

Abstract

The application discloses a kind of based on external cavity stimulated raman amplification laser radar light source, including continuous laser, laser pulse modulation module, first optical amplifier, optical coupling isolation module, pump light source, pump combiner.The application increases pump light source and pump combiner, so that the pulsed light source of laser radar atmospheric detection and the laser output by pump light source are co-propagating in the same path in atmosphere, can further amplify pulsed light source, improve signal-to-noise ratio, extend laser radar detection distance.The laser light source of the application is used for laser radar atmospheric detection, when transmitting in atmosphere, beam is not constrained by optical fiber, nonlinearity in atmosphere is very weak, the power of pump light source can be very large (up to hundreds of watts), and the output signal light signal is not affected by the nonlinearity of optical fiber transmission.

Description

Technical Field

[0001] This invention relates to the field of laser technology, and in particular to a lidar source based on external cavity stimulated Raman amplification. Background Technology

[0002] LiDAR (Light Detection and Ranging) is an active, modern optical remote sensing technology, a product of the combination of traditional radar technology and modern laser technology. Lasers possess characteristics of high brightness, high directionality, high coherence, and high peak power. Therefore, lidar offers advantages such as high angular resolution, high range resolution, high temporal resolution, high measurement accuracy, long detection range, multi-target detection, and strong anti-interference capabilities. By using lasers as the information carrier, lidar can convey information through amplitude, frequency, phase, and polarization. Therefore, it can not only accurately measure distance but also accurately measure frequency shift, angle, attitude, and depolarization. Following microwave radar, lidar has increased the frequency of its radiation source to the optical frequency, four orders of magnitude higher than millimeter waves, enabling the detection of tiny natural targets, such as aerosols and molecules in the atmosphere. With the development of laser technology and optoelectronics technology, lidar has become an important remote sensing tool.

[0003] However, the inventors of this invention discovered through research that when using lidar for atmospheric detection, the backscattered signal from atmospheric particles is very weak, which requires a high transmission power from the lidar. The weak backscattered signal also leads to a low signal-to-noise ratio of the lidar, causing a series of problems for effective signal extraction. Summary of the Invention

[0004] To address the aforementioned problems, this invention provides a lidar light source based on external cavity stimulated Raman amplification.

[0005] This invention is implemented as follows:

[0006] A lidar source based on external cavity stimulated Raman amplification includes a continuous laser, a laser pulse modulation module, a first optical amplifier, an optical coupling and isolation module, a pump source, and a pump beam combiner; wherein:

[0007] A continuous laser is used to output a continuous laser signal of a predetermined wavelength; the output end of the continuous laser is connected to the input end of a laser pulse modulation module.

[0008] The laser pulse modulation module is used to modulate the input continuous laser signal into a pulse signal;

[0009] The first optical amplifier is used to amplify the signal output from the laser pulse modulation module;

[0010] The first optical amplifier is used to amplify the input signal;

[0011] The input terminal of the optical coupling isolation module is connected to the output terminal of the first optical amplifier, and the co-directional output terminal of the optical coupling isolation module is connected to the pump combiner; the optical coupling isolation module is used to transmit the positive pulse signal emitted by the first optical amplifier to the pump combiner, and to isolate the reverse echo signal returned from the pump combiner and output it from the reverse output terminal.

[0012] The pump light source is used to generate continuous pump light;

[0013] The pump beam combiner is used to combine the positive pulse signal output from the optical coupling isolation module and the pump beam and output it to the telescope of the lidar, and to input the reverse echo signal received by the telescope back to the optical coupling isolation module.

[0014] Furthermore, it also includes a first fiber optic coupler, which has an input end and two output ends. The first fiber optic coupler is used to split the input signal according to a preset ratio and output it through the two output ends respectively. The input end of the first fiber optic coupler is connected to the output end of the continuous laser, and one of the output ends is connected to the input end of the laser pulse modulation module.

[0015] Furthermore, the laser module is used for coherent lidar, and the other output of the first fiber coupler is used as the beat frequency of the reverse echo signal output by the local oscillator and the optical coupling isolation module.

[0016] Furthermore, the optical coupling isolation module is a fiber optic circulator, which includes an input end, a co-directional output end, and a reverse output end. The input end of the fiber optic circulator is connected to the output end of the first optical amplifier. The co-directional output end of the fiber optic circulator is connected to one input end of the pump combiner, and the reverse output end of the fiber optic circulator is used to output the reverse echo signal received by the pump combiner.

[0017] Furthermore, the continuous laser is a fiber laser; the first optical amplifier is a rare-earth element-doped fiber amplifier corresponding to the output laser wavelength of the continuous laser; the first optical amplifier is one or more of erbium-doped fiber amplifiers, ytterbium-doped fiber amplifiers, and erbium-ytterbium co-doped double-clad fiber amplifiers; the laser pulse modulation module is an acousto-optic modulator, an electro-optic modulator, or a magneto-optic modulator.

[0018] Furthermore, the medium to be tested is the atmosphere, including one or more of air, aerosols, and vacuum.

[0019] Furthermore, the optical coupling isolation module includes an isolator and a second fiber optic coupler, which are connected by an optical fiber, wherein:

[0020] The isolator is connected to the output of the first optical amplifier to allow the positive pulse output by the first optical amplifier to pass through and to isolate the reverse echo signal; the input of the isolator is connected to the second fiber optic coupler.

[0021] The second fiber optic coupler is a 2*1 type fiber optic coupler, including two input ends and one output end; one input end is connected to the isolator, the output end is connected to the pump combiner, and the other input end is used to output the reverse echo signal.

[0022] Furthermore, the wavelength of the pump light source is shorter than the signal wavelength output by the continuous laser, and the power output by the pump light source is greater than the signal power output by the first optical amplifier.

[0023] Furthermore, the power of the pump light source is 2W~1000W; the pump light source and the pump combiner are connected by optical fiber; the wavelength difference between the pump light source and the signal wavelength output by the continuous laser is 70~100nm.

[0024] Furthermore, the pump combiner is one or more of wavelength division multiplexing devices, fiber optic couplers, and polarization combiners.

[0025] In summary, this invention provides a lidar source based on external cavity stimulated Raman amplification, comprising a continuous laser, a laser pulse modulation module, a first optical amplifier, an optical coupling isolation module, a pump source, and a pump combiner. By adding a pump source and a pump combiner, this invention ensures that the pulsed light source used for lidar atmospheric detection and the laser output from the pump source propagate along the same path and in the same direction in the transmission medium (atmosphere), further amplifying the pulsed light source and extending the lidar detection range. When the laser source of this invention is used for lidar atmospheric detection, the beam is not constrained by the optical fiber during atmospheric transmission, the nonlinearity in the atmosphere is very weak, and the power of the pump source can be very high (up to several hundred watts), and the output signal is not affected by the nonlinearity of optical fiber transmission. By placing the optical coupling isolation module before the pump combiner, when the detection pulsed light signal returns from the atmosphere, it propagates in the opposite direction along the same path as the pump source, further amplifying the returned light. Therefore, it can receive and detect returned light signals from farther distances, thereby further improving the lidar signal strength and extending the detection range. Attached Figure Description

[0026] To more clearly illustrate the technical solutions and advantages in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0027] To more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings used in the following description of the embodiments will be briefly introduced. Obviously, the drawings described below are only embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0028] Figure 1 This is a structural block diagram of a lidar light source based on external cavity stimulated Raman amplification provided in an embodiment of the present invention;

[0029] Figure 2 This is another structural block diagram of a lidar light source based on external cavity stimulated Raman amplification provided in an embodiment of the present invention;

[0030] Figure 3 Another structural block diagram of a lidar source based on external cavity stimulated Raman amplification provided in an embodiment of the present invention;

[0031] Figure 4 Another structural block diagram of a lidar light source based on external cavity stimulated Raman amplification provided in an embodiment of the present invention;

[0032] Figure 5 The schematic diagram of a lidar source based on external cavity stimulated Raman amplification provided in an embodiment of the present invention. Detailed Implementation

[0033] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the protection scope of the present invention.

[0034] Example:

[0035] Figure 1 This is a structural block diagram of a lidar source based on external cavity stimulated Raman amplification, provided as an embodiment of the present invention. (See diagram below.) Figure 1 As shown, a lidar source based on external cavity stimulated Raman amplification includes a continuous laser 1, a laser pulse modulation module 2, a first optical amplifier 3, an optical coupling and isolation module 4, a pump source 5, and a pump beam combiner 6; wherein:

[0036] The continuous laser 1 is used to output a continuous laser signal of a predetermined wavelength; the output end of the continuous laser 1 is connected to the input end of the laser pulse modulation module 2.

[0037] The laser pulse modulation module 2 is used to modulate the input continuous laser signal into a pulse signal;

[0038] The first optical amplifier 3 is used to amplify the signal output by the laser pulse modulation module 2;

[0039] The first optical amplifier 3 is used to amplify the input signal;

[0040] The input terminal of the optical coupling isolation module 4 is connected to the output terminal of the first optical amplifier 3, and the co-directional output terminal of the optical coupling isolation module 4 is connected to the pump combiner 6. The optical coupling isolation module 4 is used to transmit the positive pulse signal emitted by the first optical amplifier 3 to the pump combiner 6, and to isolate the reverse echo signal returned from the pump combiner 6 and output it from the reverse output terminal.

[0041] The pump light source 5 is used to generate continuous pump light;

[0042] The pump beam combiner 6 is used to combine the positive pulse signal output by the optical coupling isolation module 4 and the pump beam and output it to the telescope 7 of the lidar, and input the reverse echo signal received by the telescope 7 back to the optical coupling isolation module 4.

[0043] The telescope 7 of the lidar, also called the beam expander, is used to emit lasers into the atmosphere and receive the atmospheric back echo signal.

[0044] The lidar in this invention is a lidar for atmospheric detection.

[0045] Specifically, the continuous light output from the continuous laser 1 is modulated into pulsed laser light by the laser pulse modulation module 2, and then amplified to form larger pulsed light. This pulsed light then enters the pump combiner 6 through the circulator's co-directional output end (port 2), and is output from the pump combiner 6 to the telescope 7 for beam expansion before entering the test medium (such as the atmosphere). Simultaneously, the high-energy continuous laser light generated by the pump source 5 also enters the pump combiner 6 and is output from its output end. Thus, the high-peak-power pulsed light source and the high-energy pump source 5 propagate along the same path and in the same direction. During propagation in the medium, the power of both light sources changes synchronously. The pulsed light source absorbs energy from the pump source 5, with minimal attenuation, and can even be further amplified, achieving the goal of increasing the testing distance. Furthermore, some weak return light travels along the same path and in the opposite direction to the pump source 5, and is further amplified during its return journey. This also achieves the goal of receiving and detecting return light from a greater distance, thereby increasing the lidar's detection range. Finally, the amplified return light and the local oscillator light enter the detector together to extract target information carried in the signal (such as atmospheric wind speed, aerosols, gas concentration, etc.).

[0046] This invention utilizes the principle of Raman amplifiers to amplify signals used for atmospheric detection in lidar. A Raman amplifier is an optical amplifier based on Raman gain, derived from stimulated Raman scattering. The Raman-active medium is typically optical fiber, but can also be a crystal, a waveguide structure in a photonic integrated circuit, or a gas or liquid medium. The signal light, either in the same direction or opposite to the pump beam, is amplified, and its wavelength is typically tens of nanometers larger than the pump light. Therefore, the amplification medium is the transmission medium itself. For example, unlike EDFAs, Raman fiber amplifiers do not require specially doped optical fibers as the amplification medium; their amplification medium is the transmission fiber itself. One type is a distributed Raman fiber amplifier, which uses relatively long optical fibers, typically tens of kilometers, and reduces the pump source power to several hundred milliwatts, primarily assisting EDFAs in improving the performance of DWDM communication systems.

[0047] In the field of optical communication, because the fiber core is about 10µm and the distance is tens of kilometers, it inherently exhibits strong nonlinearity, so the pump power only needs to be a few hundred milliwatts. The laser source in this paper is used for atmospheric detection in lidar. When transmitting through the atmosphere, the beam is not constrained by the optical fiber, and the nonlinearity in the atmosphere is very weak, so it is necessary to increase the pump power to several hundred watts.

[0048] Specifically, the wavelength of the pump light source 5 in this invention is shorter than the signal wavelength output by the continuous laser 1, and the power output by the pump light source 5 is greater than the signal power output by the first optical amplifier 3. In principle, as long as the output power of the pump light source 5 is greater than the signal power output by the first optical amplifier 3, it is acceptable. Considering the special characteristics of atmospheric detection scenarios, the inventors of this invention have found that when the power of the pump light source 5 is between 2W and 1000W, the lidar atmospheric detection effect is better, especially for detecting signals at distances greater than 30 kilometers in the atmosphere; in this case, a power of 100W to 1000W is optimal.

[0049] In one embodiment, the pump light source 5 and the pump combiner 6 are connected via optical fiber. The pump light source 5 and the pump combiner 6 can be connected via optical fiber, or they can simply transmit data spatially. However, optical fiber connection offers better transmission efficiency.

[0050] The continuous laser 1, also known as a seed laser or seed light source, is capable of providing a continuous laser signal of a predetermined wavelength.

[0051] In one embodiment, the emission wavelength of the continuous laser 1 is 0.9 micrometers to 2.5 micrometers; the linewidth of the laser spectrum output by the continuous laser 1 is less than 15 kHz; and the pulse width output by the laser pulse modulation module 24 is 0.1 nanoseconds to 2000 nanoseconds.

[0052] In a preferred embodiment, the wavelength generated by the continuous laser 1 is preferably in the C-band (1520nm to 1570nm) and L-band (1570nm to 1610nm) of optical communication. In the C-band and L-band, the influence of the Rayleigh signal from atmospheric molecules is negligible. Due to the rapid development and maturity of optical communication devices, these devices are stable and reliable. Furthermore, this band has a high eye safety factor, allowing operation in densely populated areas such as cities, airports, and weather stations, enabling miniaturized, convenient, and eye-safe detection. In contrast, existing technologies widely using near-infrared and short-wave bands for atmospheric measurements cannot achieve full fiber optic integration, resulting in large and non-compact systems.

[0053] In one embodiment, the wavelength difference between the pump light source 5 and the signal wavelength output by the continuous laser 1 is 70nm~100nm.

[0054] Raman scattering itself is a nonlinear effect that is harmful to the system, but this invention cleverly utilizes it to transfer the pump light energy to the light signal that is transmitted in the atmosphere, thereby amplifying the light signal.

[0055] By appropriately selecting the emission wavelength of the pump light, its amplification range can fall within the desired wavelength region. In this invention, the wavelength difference between the pump light source 5 and the signal wavelength output by the continuous laser 1 is 70~100nm.

[0056] For example, when the output wavelength of continuous laser 1 is 1310nm, the emission wavelength of the pump light is 1240nm; when the pump light wavelength is selected as 1450nm, the optical signal in the C-band with a wavelength of 1550nm can be amplified; when the pump light emission wavelength is selected as 1480nm, the optical signal in the L-band with a wavelength of 1550nm can be amplified.

[0057] The pump combiner 6 is one or more of wavelength division multiplexing devices, fiber couplers, and polarization combiners. The pump combiner 6 can also be made of two tapered optical fibers, or it can be a glass slide, achieved by coating on the glass slide.

[0058] Preferably, when the pump combiner 6 is a wavelength division multiplexer (WDM), preferably a 2*1 type WDM, it has two inputs and one output. The inventors of this invention have discovered that the detection efficiency is highest when the pump combiner 6 is a WDM.

[0059] Optionally, the laser pulse modulation module 2 is an acousto-optic modulator, an electro-optic modulator, or a magneto-optic modulator, wherein the acousto-optic modulator is further used to generate a preset frequency shift in the input laser signal.

[0060] The acousto-optic modulator (AOM) module has a specific frequency shift, which is taken as an example in this invention with a frequency shift of 80MHz. The main control circuit board of the laser controls the switching of the acousto-optic modulator and the opening threshold width by controlling the radio frequency signal input to the acousto-optic modulator module, thereby cutting the continuous laser into pulsed lasers of a certain width (such as a few nanoseconds). At this time, the opening time will be recorded as a timing reference.

[0061] Of course, electro-optic modulators, magneto-optic modulators, and mechanical modulators can all convert continuous laser signals into pulsed light signals. As long as they can achieve the function of modulating the input continuous laser signal into a pulsed light signal for output, they are all applicable to this invention.

[0062] The continuous laser 1 is a fiber laser; the first optical amplifier 3 is a fiber device; the continuous laser 1, the laser pulse modulation module 2, the first optical amplifier 3, and the optical coupling isolation module 43 are all connected by optical fibers.

[0063] The test medium is the atmosphere, including one or more of air, aerosols, and vacuum. The test medium is the same as that used in lidar.

[0064] The continuous laser 1 is a fiber laser, and the first optical amplifier 3 is a fiber amplifier doped with rare earth elements corresponding to the laser wavelength. The first optical amplifier 3 is one or more of erbium-doped fiber amplifiers, ytterbium-doped fiber amplifiers, and erbium-ytterbium co-doped double-clad fiber amplifiers. For example, the first optical amplifier 3 is an erbium-doped or erbium-ytterbium co-doped fiber amplifier.

[0065] In one embodiment, the amplification factor of the first optical amplifier 3 is between 100 and 10,000, i.e., 20 dB to 40 dB.

[0066] In one embodiment, the laser spectral linewidth output by each optical amplifier is the limit of the Fourier transform, indicating that the present invention can achieve very good beam quality. The laser single-pulse energy output by each optical amplifier is 0.1 microjoules to 2000 microjoules.

[0067] In particular, in weak signal detection applications (such as atmospheric detection, wind speed detection, aerosol detection, and gas detection), the laser single pulse energy output by each optical amplifier module is 0.1 microjoules to 150 microjoules.

[0068] In one embodiment, such as Figure 2As shown, the optical coupling isolation module 4 is a fiber optic circulator 41, also called a circulator. The fiber optic circulator 41 includes an input end, a co-directional output end, and a reverse output end. The input end of the fiber optic circulator 41 is connected to the output end of the first optical amplifier 3; the co-directional output end of the fiber optic circulator 41 is connected to one input end of the pump combiner 6; and the reverse output end of the fiber optic circulator 41 is used to output the reverse echo signal received by the pump combiner 6.

[0069] In one embodiment, the sum of the lengths of the input and output pigtails of the fiber optic circulator 41 is 0.8 meters to 1.5 meters. For example, the total length of the input and output pigtails of the fiber optic circulator 41 is 1 meter.

[0070] In one embodiment, such as Figure 3 As shown, the optical coupling isolation module 4 includes an isolator 42 and a second fiber optic coupler 43, which are connected by an optical fiber.

[0071] The isolator 42 is connected to the output of the first optical amplifier 3 to allow the positive pulse output by the first optical amplifier 3 to pass through and to isolate the reverse echo signal; the input of the isolator 42 is connected to the second fiber optic coupler 43.

[0072] The second fiber optic coupler 43 is a 2*1 type fiber optic coupler, including two input ends and one output end; one input end is connected to the isolator 42, the output end is connected to the pump combiner 6, and the other input end is used to output the reverse echo signal.

[0073] Specifically: In one embodiment, the second fiber coupler 43 is a 2*1 type, and the splitting ratio can be set to 50:50.

[0074] The forward-transmitted signal, after being amplified by a multi-stage optical amplifier and outputting into isolator 42, enters the second fiber coupler 43 from its first input end, with 50% of the signal outputting from the output end. It then enters the pump combiner 6 for amplification. Here, 50% of the energy is lost, resulting in a 3dB attenuation. The amplified light returns from the output end in the reverse direction, with 50% of the light output from the first input end. This reversed light is blocked and lost after entering isolator 42. The remaining 50% of the light is output from the second input end. In coherent lidar applications, the output signal and 850% of the light from the first fiber coupler enter a balanced detector for frequency matching.

[0075] An optical fiber coupler, also known as an optical fiber splitter, has multiple input and output terminals and can split and couple optical beams.

[0076] In one embodiment, the laser module of the present invention is used for coherent lidar.

[0077] Coherent lidar generates a difference frequency signal by combining the echo signal from a polarization-maintaining pulsed laser emitted by a light source with the local oscillator light of the local system. Simultaneously measuring this amplified difference frequency signal allows for relatively easy determination of atmospheric parameters such as radial wind speed. The light source consists of a high-power pulsed laser (output light) and a narrow-linewidth continuous-wave laser (local oscillator light). A small portion of the output light is mixed with the local oscillator light, while the majority is emitted into the atmosphere and scattered by aerosols or atmospheric molecules, generating an echo signal. This echo signal is received by telescope 7 and mixed with the local oscillator light. The detection section includes two detectors: one to detect the frequency f0 of the output light and the frequency f of the local oscillator light. LO One is the mixing signal, and the other is used to detect the echo signal f0+Δf and the local oscillator signal f. LO The mixed frequency signal.

[0078] like Figure 4 As shown, in the application scenario of coherent lidar, a first fiber coupler 8 is also included. The first fiber coupler 8 includes an input end and two output ends. The first fiber coupler 8 is used to split the input signal according to a preset ratio and output it through the two output ends respectively. The input end of the first fiber coupler 8 is connected to the output end of the continuous laser 1, and one of the output ends is connected to the input end of the laser pulse modulation module 2.

[0079] The laser module is used for coherent lidar, and the other output of the first fiber coupler 8 is used as the beat frequency of the reverse echo signal output by the local oscillator and the optical coupling isolation module 4.

[0080] In one embodiment, a second optical amplifier and a third optical amplifier are also included; the second optical amplifier and the third optical amplifier are disposed between the first optical amplifier 3 and the pump combiner 6.

[0081] In a preferred embodiment, the first optical amplifier 3, the second optical amplifier, and the third optical amplifier are connected sequentially. The third optical amplifier is connected to both the second optical amplifier and the optical coupling isolation module 4; the optical coupling isolation module 4 is connected to the pump combiner 6. Since the echo signal is very weak, and optical amplifiers typically enhance the amplification capability of weak signals, placing the optical coupling isolation module 4 between the second optical amplifier and the pump combiner 6 amplifies the echo signal only once. Because the reflected light is very weak, it does not consume excessive pump energy, thus amplifying the echo signal without affecting the normal forward amplification signal.

[0082] The second and third optical amplifiers are rare-earth element-doped fiber amplifiers corresponding to the output laser wavelength of the continuous laser 1; the second and third optical amplifiers are one or more of erbium-doped fiber amplifiers, ytterbium-doped fiber amplifiers, and erbium-ytterbium co-doped double-clad fiber amplifiers; the laser pulse modulation module 2 is an acousto-optic modulator, an electro-optic modulator, or a magneto-optic modulator.

[0083] The acousto-optic modulator module described in the example is an acousto-optic modulator module with a specific frequency shift; in this invention, a frequency shift of 80MHz is used as an example. By controlling the radio frequency signal input to the acousto-optic modulator module, the switching on and off of the acousto-optic signals and the on / off threshold width are controlled, thereby truncating continuous laser light into nanosecond pulsed laser light.

[0084] The principles and implementation of this invention will be explained in detail below from a specific application scenario, namely the application scenario of coherent lidar.

[0085] Figure 5 This is a schematic diagram of a lidar source based on external cavity stimulated Raman amplification according to an embodiment of the present invention. Figure 5 As shown, a portion of the continuous light source from the pulsed light source is the output local oscillator light, which enters the detection system. The main output is a high-energy pulsed light source, which enters the pump combiner 6 and is combined with the pump light source 5, and finally output together into the medium under test. During the transmission and return of the pulsed light source in the medium under test, it is amplified by the stimulated Raman scattering effect of the pump light source 5. Finally, the returned light enters the pump combiner 6 in reverse and enters the detection system through the circulator in the pulsed light source, realizing the comparison and extraction of the frequency components of the two signals.

[0086] Figure 4 This is a specific structure of the pulsed light source, pump light source 5, and pump combiner 6. Combined with... Figure 4 According to an embodiment of the present invention, a lidar source based on external cavity stimulated Raman amplification operates at a wavelength of around 1.5 μm and is used for coherent lidar. It is a narrow linewidth nanosecond pulsed fiber laser.

[0087] As shown in the diagram above, a continuous-wave laser 1 (CW) seed source with an emission wavelength of 1548 nm provides the original signal light for the entire system. Continuous-wave laser 1 generates a narrow-linewidth polarization-maintaining continuous-wave laser, such as a linewidth of 5 kHz and a power of 50 mW.

[0088] The first fiber coupler 8 is used to seed a continuous (CW) laser with an emission wavelength of 1.5μm. The first fiber coupler 8 is a specially designed all-fiber passive device with one fiber at the input end and two fibers at the output end. It is used to divide the continuous laser with an emission wavelength of 1.5μm into two parts, such as 10:90.

[0089] The laser output from the continuous laser 1 passes through the first fiber coupler 8 and outputs about 5mW as the local oscillator light. The remaining about 45mW is used as the signal light and enters the pulse modulation module (such as an acousto-optic modulator AOM) to form a pulse waveform. The output laser parameters are: linewidth 5kHz, power 90uW, repetition frequency 20kHz, pulse width 200ns, and single pulse energy 4nJ.

[0090] An acousto-optic modulator (AOM) is used to truncate a portion of a continuous signal light into pulsed laser light and to generate a fixed frequency shift. An AOM is a fiber optic acousto-optic modulator with a specific frequency shift. In certain applications, such as coherent lidar for measuring atmospheric wind speed, the frequency shift generated by the AOM is required. This invention uses an 80MHz frequency shift as an example. The laser's main control circuit board controls the switching and on / off threshold width of the acousto-optic modulator module by controlling the radio frequency signal input to the module, thereby truncating the continuous laser light into nanosecond pulsed laser light. The on-time is recorded as a timing reference. Of course, in some specific applications, the AOM may not generate a frequency shift and may simply function as a pulse modulation device to generate pulsed light.

[0091] The laser output parameters after three stages of amplification by the first optical amplifier 3, the second optical amplifier, and the third optical amplifier are 300uJ and 6W. Continuing through the circulator input (port 1), the laser output parameters from the circulator's in-direction output (port 2) are 250uJ and 5W. After amplification by the pump combiner 6, the output laser parameters are 200uJ and 4W. At this point, the 5500W pump light source (1480nm, output fiber is GDF-50 / 400) passes through the pump combiner 6 and, together with the pulse light source, is incident on the air field from a collimation system such as the telescope 7. The pump combiner 6 can be of type (1+1)*1, with the same signal input and output fiber, GDF-25 / 300-M, and the pump input fiber is GDF-50 / 400. During transmission, the pulse light source absorbs the energy from the pump light source 5, amplifying it as it is transmitted, and gradually losing energy as the distance increases. The test showed that the farthest signal range was 40km. Therefore, the farthest signal is amplified during transmission and return. When the energy returning to the laser source exceeds the threshold of the balanced detector, the detection range can be increased from 20km to 40km under the influence of this invention. This greatly improves the detection distance and the ability to detect weak signals.

[0092] In summary, the present invention has at least the following beneficial effects:

[0093] (1) By adding a pump source and a pump combiner, the present invention enables the pulse source and the laser output from the pump source used for atmospheric detection of lidar to propagate in the same direction and along the same path in the transmission medium (atmosphere), which can further amplify the pulse source and extend the detection range of lidar.

[0094] Because this invention adds a pump source that propagates in the same direction and along the same path after the laser output, and the wavelength of this pump source is different from the wavelength of the pulse signal, it will not interfere with the signal and can amplify the pulse signal during the detection process. Overall, the pulse energy gradually increases with distance in the initial stage, and gradually decreases with pump power loss in the later stage, but the rate of decrease is significantly lower than when there is no pump source. Assuming a 40dB gain from the co-propagating Raman amplification effect, a 2W signal light output and a 20nW return signal can measure atmospheric targets at a distance of 20km, thus increasing the detection range to 80dB. Upgrading to 120dB atmospheric detection capability, the estimated increase to a detection distance of 30km can be calculated, thus significantly improving the signal strength and detection range of lidar atmospheric detection.

[0095] (2) For atmospheric detection, atmospheric echo signals are usually weak, resulting in a very low signal-to-noise ratio. When atmospheric loss is constant, to improve the signal-to-noise ratio and measurement range of the lidar, the single-pulse energy or peak power of the emitted light is usually increased. Although this can improve the signal-to-noise ratio to some extent, when the optical power exceeds the threshold of stimulated Brillouin scattering, it will excite strong stimulated Brillouin scattering, reduce the laser power, and damage the fiber optic devices. Therefore, this limits the detection range of the lidar.

[0096] For fiber optic devices, strong stimulated Brillouin scattering (SBS) occurs when the input signal power reaches or exceeds the threshold for stimulated Brillouin scattering. SBS converts most of the input power into a reverse Stokes wave, which consumes transmitted laser energy, causing a significant loss of forward-transmitted laser power and resulting in jitter in the transmitted signal. More seriously, the strong reverse Stokes wave can damage the laser itself, thereby harming the entire optical system.

[0097] The laser light source of this invention is used for atmospheric detection in lidar. When transmitted through the atmosphere, the beam is not constrained by optical fiber, the nonlinearity in the atmosphere is very weak, the power of the pump light source can be very high (up to several hundred watts), and the output signal light signal is not affected by the nonlinearity of optical fiber transmission.

[0098] (3) For lidar, we need to capture atmospheric backscattered signals, also known as backscattered signals or atmospheric echo signals. In this invention, the optical coupling isolation module is placed before the pump combiner. When the pulse light signal used for detection returns from the atmosphere, it is transmitted in the opposite direction along the same path as the pump light source. This can further amplify the back-transmitted light, thus enabling the detection of back-transmitted light signals from farther away, thereby further improving the signal strength of the lidar and extending the detection range.

[0099] In addition, by placing the optical coupling isolation module before the pump combiner and modifying the matching according to the type of the input and output fibers of the front and rear amplification stages, the reflected light is very weak after the output light returns and does not consume too much pump energy. Therefore, it does not affect the normal forward amplification signal during reverse amplification.

[0100] The return optical amplification is a small-signal amplification with a gain significantly greater than 40dB. Therefore, it can also increase the distance by 10km during the return process. Thus, the combined effects of forward and reverse amplification are expected to double the detection range, thereby greatly improving the atmospheric detection range of the lidar.

[0101] (4) When the optical coupling isolation module of the present invention is a fiber optic circulator, the circulator is placed before the last stage optical amplifier, which greatly reduces the maximum power that the circulator needs to withstand, thereby greatly reducing the cost of the circulator. For example, if the amplification factor of the last stage is 100 times (20dB), the maximum power that the circulator can withstand can be reduced from 2 watts to only about 20mw, reducing the cost of the circulator by about half.

[0102] (5) The emitted light of the present invention does not need to be output directly through the circulator pigtail, which can effectively improve the nonlinear threshold of the amplifier.

[0103] Since the total length of the input and output pigtails of the circulator is about one meter, the circulator of this invention is placed before the pump combiner rather than after it. This is equivalent to directly reducing the fiber transmission distance by one meter, thereby greatly reducing the stimulated Brillouin scattering effect of the laser module. As a result, the pulse peak power can be increased by about 50%, which in turn greatly improves the signal strength and detection distance.

[0104] This invention, through the ingenious design of an optical coupling isolation module, amplifies the reflected light exponentially before detection and signal extraction without increasing existing costs, thus significantly improving the detection range. In existing technologies, increasing the detection range and signal-to-noise ratio typically involves increasing the power of the emitted laser. This invention optically amplifies the echo signal before analysis, greatly increasing its intensity and allowing for the extraction of weaker optical signals, thereby significantly expanding the detection range of the lidar.

[0105] The above description is merely a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in the present invention should be included within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. A lidar light source based on external cavity stimulated Raman amplification, characterized in that, Includes a continuous laser, a laser pulse modulation module, a first optical amplifier, an optical coupling and isolation module, a pump source, and a pump beam combiner; wherein: A continuous laser is used to output a continuous laser signal of a predetermined wavelength; the output end of the continuous laser is connected to the input end of a laser pulse modulation module. The laser pulse modulation module is used to modulate the input continuous laser signal into a pulse signal; The first optical amplifier is used to amplify the signal output from the laser pulse modulation module; The first optical amplifier is used to amplify the input signal; The input terminal of the optical coupling isolation module is connected to the output terminal of the first optical amplifier, and the co-directional output terminal of the optical coupling isolation module is connected to the pump combiner; the optical coupling isolation module is used to transmit the positive pulse signal emitted by the first optical amplifier to the pump combiner, and to isolate the reverse echo signal returned from the pump combiner and output it from the reverse output terminal. The pump light source is used to generate continuous pump light; the wavelength of the pump light source is smaller than the signal wavelength output by the continuous laser, and the power output by the pump light source is greater than the signal power output by the first optical amplifier. The pump beam combiner is used to combine the positive pulse signal output from the optical coupling isolation module and the pump beam and output it to the telescope of the lidar, and to input the reverse echo signal received by the telescope back to the optical coupling isolation module.

2. A lidar source based on external cavity stimulated Raman amplification according to claim 1, characterized in that, It also includes a first fiber optic coupler, which has an input end and two output ends. The first fiber optic coupler is used to split the input signal according to a preset ratio and output it through the two output ends respectively. The input end of the first fiber optic coupler is connected to the output end of the continuous laser, and one of the output ends is connected to the input end of the laser pulse modulation module.

3. A lidar source based on external cavity stimulated Raman amplification according to claim 2, characterized in that, The laser module is used for coherent lidar, and the other output of the first fiber coupler is used as the beat frequency of the reverse echo signal output by the local oscillator and the optical coupling isolation module.

4. A lidar source based on external cavity stimulated Raman amplification according to claim 1 or 2, characterized in that, The optical coupling isolation module is a fiber optic circulator, which includes an input end, a co-directional output end, and a reverse output end. The input end of the fiber optic circulator is connected to the output end of the first optical amplifier. The co-directional output end of the fiber optic circulator is connected to one input end of the pump combiner, and the reverse output end of the fiber optic circulator is used to output the reverse echo signal received by the pump combiner.

5. A lidar source based on external cavity stimulated Raman amplification according to claim 1, characterized in that, The continuous laser is a fiber laser; the first optical amplifier is a rare-earth element-doped fiber amplifier corresponding to the output laser wavelength of the continuous laser; the first optical amplifier is one or more of erbium-doped fiber amplifiers, ytterbium-doped fiber amplifiers, and erbium-ytterbium co-doped double-clad fiber amplifiers; the laser pulse modulation module is an acousto-optic modulator, an electro-optic modulator, or a magneto-optic modulator.

6. A lidar source based on external cavity stimulated Raman amplification according to claim 1, characterized in that, The optical coupling isolation module includes an isolator and a second fiber optic coupler, which are connected by an optical fiber, wherein: The isolator is connected to the output of the first optical amplifier to allow the positive pulse output by the first optical amplifier to pass through and to isolate the reverse echo signal; the input of the isolator is connected to the second fiber optic coupler. The second fiber optic coupler is a 2*1 type fiber optic coupler, including two input ends and one output end; one input end is connected to the isolator, the output end is connected to the pump combiner, and the other input end is used to output the reverse echo signal.

7. A lidar source based on external cavity stimulated Raman amplification according to claim 1, characterized in that, The wavelength of the pump light source is shorter than the signal wavelength output by the continuous laser, and the power output by the pump light source is greater than the signal power output by the first optical amplifier.

8. A lidar source based on external cavity stimulated Raman amplification according to claim 1, characterized in that, The power of the pump light source is 2W~1000W; the pump light source and the pump combiner are connected by optical fiber; the wavelength difference between the pump light source and the signal wavelength output by the continuous laser is 70~100nm.

9. A lidar source based on external cavity stimulated Raman amplification according to claim 1, characterized in that, The pump combiner is one or more of wavelength division multiplexing devices, fiber couplers, and polarization combiners.

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