CN215894939U - Frequency modulation continuous wave laser imaging system based on Fourier domain mode locking fiber laser - Google Patents
Frequency modulation continuous wave laser imaging system based on Fourier domain mode locking fiber laser Download PDFInfo
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Abstract
The utility model relates to a frequency modulation continuous wave laser imaging system based on a Fourier domain mode locking fiber laser, and belongs to the field of laser ranging and imaging. The transmitting end is a Fourier mode locking fiber laser, the Fourier mode locking fiber laser is connected with the beam splitter, laser reaches three parts after being split, the three parts are a measuring light path, an auxiliary reference light path and a gas pool calibration light path respectively, the laser emits out of the measuring light path, the auxiliary reference light path and the gas pool calibration light path and then passes through a photoelectric detector, the photoelectric detector is a bridge connecting the measuring light path, the auxiliary reference light path, the gas pool calibration light path and a data acquisition system, an optical signal emitted into the laser emitting out of the measuring light path can be transmitted to a focusing synthetic aperture imaging system, and the laser finally reaches the data acquisition system. The utility model improves the stability and the resolution ratio of a radar system, realizes long-distance accurate measurement, can simultaneously realize the functions of distance measurement and imaging, and can realize the distance measurement imaging sensor with low cost and miniaturization.
Description
Technical Field
The utility model relates to the field of laser ranging and imaging, in particular to a frequency modulation continuous wave laser imaging system based on a Fourier domain mode locking fiber laser.
Background
As is known, the existing laser imaging system has low accuracy and poor stability and resolution in long-distance measurement, and cannot simultaneously realize the ranging and imaging functions.
Disclosure of Invention
In order to overcome the defects of the prior art, the utility model provides a frequency modulation continuous wave laser imaging system based on a Fourier domain mode locking fiber laser, and aims to solve the problems of poor resolution and lower accuracy of the conventional imaging system.
The technical scheme adopted by the utility model for solving the technical problems is as follows: a frequency modulation continuous wave laser imaging system based on a Fourier domain mode locking fiber laser is characterized in that a transmitting end is the Fourier mode locking fiber laser, the Fourier mode locking fiber laser is connected with a beam splitter, laser reaches three parts after beam splitting and respectively comprises a measuring light path, an auxiliary reference light path and a gas pool calibration light path, the laser can pass through a photoelectric detector after being emitted out of the measuring light path, the auxiliary reference light path and the gas pool calibration light path, the photoelectric detector is a bridge connecting the measuring light path, the auxiliary reference light path, the gas pool calibration light path and a data acquisition system, an optical signal emitted into the measuring light path of the laser can be emitted to a focusing synthetic aperture imaging system, and the laser finally reaches the data acquisition system.
The utility model can also be realized by the following measures:
the Fourier mode locking fiber laser is an annular cavity structure FDML frequency sweeping fiber laser, a pumping source of the Fourier mode locking fiber laser is a semiconductor optical amplifier, laser is transmitted in an annular structure and passes through a coupler after passing through a fiber isolator, part of laser exists in the annular cavity structure, the other part of laser is transmitted to a detector, the detector is connected with a data acquisition end and is used for measuring parameters of the laser coupled by the coupler, the laser left in the annular structure reaches an FDML scanning frequency device and a polarization controller through a dispersion fiber and reaches the semiconductor optical amplifier again through the fiber isolator to complete the whole annular transmission, the FDML scanning frequency device is connected with the amplifier, and laser data are transmitted to the data acquisition end after being amplified to measure laser parameters.
The laser in the measuring light path is respectively emitted into the circulator and the coupler through the beam splitter, the laser part passing through the circulator returns to the coupler, and part of the laser can be emitted into the collimator for outputting optical signals.
The laser in the auxiliary reference light path is injected into the beam splitter, part of the laser directly reaches the coupler, and the other part of the laser is injected into the coupler after the nonlinear effect is corrected by the auxiliary optical fiber.
And laser in the gas cell calibration light path passes through the hydrogen cyanide gas cell and is subjected to length calibration on the auxiliary optical fiber by using an absorption spectrum line.
The pyramid prism in the focusing synthetic aperture imaging system is arranged on the movable guide rail, the laser of the movable target detection object returns after reaching the position, and the laser interferometer is used for measuring the position of the pyramid prism at the front end.
The utility model has the advantages that the light path adopts an all-fiber loop, thereby improving the stability and the resolution of the radar system and realizing long-distance accurate measurement; the Fourier mode-locked fiber laser is adopted as a light source for modulation, so that the functions of distance measurement and imaging can be realized at the same time; and the distance measurement imaging sensor with low cost and miniaturization can be realized by adopting an all-fiber micro structure.
Drawings
The utility model is further illustrated with reference to the following figures and examples.
FIG. 1 is a schematic diagram of the present invention.
Fig. 2 is a schematic diagram of a fourier mode locked fiber laser.
In the figure, 10 is a Fourier mode-locked fiber laser, 11 is a semiconductor optical amplifier, 12 is a fiber isolator, 13 is a coupler, 14 is a detector, 15 is a data acquisition end, 16 is a dispersive fiber, 17 is an amplifier, 18 is an FDML scanning frequency device, 19 is a polarization controller, 20 is a beam splitter, 30 is a measurement light path, 31 is a beam splitter, 32 is a circulator, 33 is a coupler, 34 is a collimator, 40 is an auxiliary reference light path, 41 is a beam splitter, 42 is an auxiliary fiber, 43 is a coupler, 50 is a gas cell calibration light path, 51 is a hydrogen cyanide gas cell, 60 is a photoelectric detector, 70 is a focusing synthetic aperture imaging system, 71 is a pyramid prism, 72 is a moving guide rail, 73 is a laser interferometer, and 80 is a data acquisition system.
Detailed Description
In the figure, the left end, i.e. the integral transmitting end, of the present invention is a fourier mode-locked fiber laser 10, and the fourier mode-locked fiber laser 10 is an annular cavity structure FDML swept-frequency fiber laser. The whole ring is connected by an optical fiber, a pumping source is a Semiconductor Optical Amplifier (SOA)11, laser is transmitted in a ring structure and passes through an optical fiber Isolator (ISO)12 and then a coupler (OC)13, wherein part of the laser exists in the ring cavity structure, the other part of the laser is transmitted to a detector (PD)14, and the detector (PD)14 is connected with a data acquisition terminal (DAQ)15 and used for measuring parameters of the laser coupled by the coupler 13. The laser light remaining in the ring structure continues through the dispersive fiber 16 to the FDML scanning frequency 18 and the Polarization Controller (PC)19, passes through the fiber isolator 12, and again to the semiconductor optical amplifier 11, completing the entire ring propagation. The FDML scanning frequency device 18 is connected with an Amplifier (AMP)17, and laser data are transmitted to a data acquisition end after being amplified to measure laser parameters.
The fourier mode-locked fiber laser 10 passes through the beam splitter 20 rightward, and the laser reaches three parts after being split, namely a measurement optical path 30, an auxiliary reference optical path 40 and a gas cell calibration optical path 50.
In the measuring beam path 30, the laser light is emitted by the beam splitter 31 into the circulator 32 and the coupler 33, respectively, and part of the laser light passing through the circulator returns to the coupler, and part of the laser light is emitted into the collimator 34 for outputting the optical signal.
The optical signal entering the collimator 34 is transmitted to a focusing synthetic aperture imaging system 70, in which a pyramid prism 71 is placed on a moving guide 72, the moving target probe laser reaches the moving target probe laser and returns, and a laser interferometer 73 is used to measure the position of the front end pyramid prism 71.
In the auxiliary reference optical path 40, the laser light enters the beam splitter 41, part of the laser light directly reaches the coupler 43, and part of the laser light enters the coupler 43 after the nonlinear effect is corrected by the auxiliary optical fiber 42.
In the gas cell calibration light path 50, the laser passes through the hydrogen cyanide gas cell 51 and performs length calibration on the auxiliary optical fiber by using an absorption spectrum line.
The laser emitted from the measurement light path 30, the auxiliary reference light path 40 and the gas cell calibration light path 50 passes through the photoelectric detector 6, the photoelectric detector 60 is a bridge connecting the measurement light path 30, the auxiliary reference light path 40, the gas cell calibration light path 50 and the data acquisition system 80, and the laser finally reaches the data acquisition system 80, i.e. the system can be used for imaging and ranging the object to be measured.
The specific structure of the fourier mode-locked laser 10, namely a ring cavity structure FDML swept fiber laser, is shown in fig. 2, and comprises a Semiconductor Optical Amplifier (SOA)11, an optical fiber Isolator (ISO)12, a coupler (OC)13, a detector (PD)14, a data acquisition terminal (DAQ)15, a dispersive fiber 16, an Amplifier (AMP)17, an FDML swept frequency device 18 and a Polarization Controller (PC) 19. The FDML mechanism is a stable laser mode of operation with high repeatability. The stable operating conditions of the FDML swept fiber laser are that the drive signal frequency of the sweep filter matches the local oscillator frequency of the laser resonator length, i.e.:
wherein L iscavityFor the effective cavity length of the laser, v is the speed of light in the fiber, N is a positive integer, and f is the scanning filter drive signal frequency. The basic principle of FDML is to make the photons cycle one turn for the same time or an integer multiple of the scan period of the scan filter, i.e., each photon after passing through the scan filter cycles back in the cavity just enough to pass through the scan filter again without loss. The method can simultaneously start all longitudinal modes in the scanning frequency (or wavelength) range,and the time domain can follow the driving signal to output in sequence. Because the process of continuously transforming the longitudinal mode and then reestablishing the stability does not exist, the Fourier mode-locked laser 10 can realize ultra-high-speed frequency sweep in the frequency modulation continuous wave laser imaging system based on the Fourier domain mode-locked fiber laser. All optical fibers in the laser resonant cavity of the device are connected by adopting an optical fiber fusion splicer so as to reduce the cavity loss to the greatest extent.
A Semiconductor Optical Amplifier (SOA)11 is used as a pumping source in the fourier mode-locked laser 10 to excite a laser working substance and pump active particles from a ground state to a high energy level, so as to realize population inversion.
And an optical fiber Isolator (ISO)12 for ensuring that the laser is transmitted in the same direction in the optical fiber.
The coupler (OC)13 is installed at the position where the optical fiber is divided into two branches, and the coupling ratio thereof is 90%: and 10%, feeding 90% of laser energy back into the resonant cavity for oscillation and amplification, and using 10% of laser output for testing.
And a detector (PD)14 is arranged between the coupler and the data acquisition terminal 15 and is used for receiving the output 10% laser light, so that the test is convenient.
And a data acquisition terminal (DAQ)15 serving as a detection terminal is arranged on an external part of the Fourier mode-locked laser 10.
The dispersive fiber 16 is mounted in the fiber loop of the fourier mode-locked laser 10. Since FDML swept-fiber lasers typically require long delay fibers to store photons, material dispersion is a factor that must be considered. Appropriate dispersion compensation can improve the performance of the laser and thus the accuracy of the measurement or sensing of the fourier-mode locked laser 10.
An Amplifier (AMP)17 is installed after the scan filter 18 for oscillation and amplification of the laser light in the resonant cavity.
The FDML scan filter (FFP-TF)18 belongs to a new generation of high speed narrow bandwidth scan filter, driven by PZT for scan filtering. For the FDML swept fiber laser, the sweep frequency of the laser is only dependent on the sweep rate of the sweep filter because the FDML swept fiber laser is not limited by the laser setup time, and the PZT is driven by a sinusoidal signal in high-frequency modulation.
A Polarization Controller (PC)19 is installed in the fiber loop for boosting the output power.
The beam splitter 20 is an optical device that can split one light beam into two or more light beams, and is generally formed of a metal film or a dielectric film.
The measurement light path 30 is the front end of the focusing aperture combination imaging system 70, and forms a dual-light path distance measurement system with the auxiliary reference light path 40, mainly responsible for distance measurement and imaging, and is composed of a beam splitter 31, a circulator 32, a coupler 33, and a collimator 34. The distance to be measured of the dual optical path ranging system can be expressed as:
where c is the speed of light propagation in vacuum; l is the length of the delay optical fiber in the auxiliary optical path; n is the refractive index of the fiber. And obtaining distance information under the condition that the length L of the delay optical fiber is known and the peak position point K is known. The auxiliary signal peak-valley value is used for resampling the measurement signal, and according to the sampling law, the auxiliary signal frequency is at least twice of the measurement signal frequency, namely the optical path of the auxiliary optical path is at least twice of the distance to be measured, and the optical path is generally 4-5 times in practical application. According to the formula, in the double-optical-path FMCW laser ranging system, the distance to be measured is in direct proportion to the length of the auxiliary optical-path delay optical fiber, and the ranging precision of the system is closely related to the calibration precision of the length of the auxiliary optical fiber.
The beam splitter 31 is installed inside the measurement optical path 30, and splits the laser light entering the measurement optical path.
The circulator 32 is installed in the laser light path split by the beam splitter 31, and can unidirectionally transmit a high-frequency energy signal to control the laser light to be transmitted in a certain annular direction. The circulator has the characteristics of small volume, wide frequency band, small insertion loss and the like.
The coupler 33 is also called a connector, and is a photoelectric converter that transmits electrical signals through light as a medium, and combines optical signals in the measurement optical path 30.
The collimator 34 is used for outputting an optical signal, and has a simple structure in which divergent light from an optical fiber is converted into parallel light (gaussian light beam) by a similar convex lens arranged in front. Its purpose is to maximize the efficient coupling of optical signals into the focused aperture imaging system 70.
The auxiliary reference light path 40 and the measurement light path 30 form a dual-light-path ranging system, and the auxiliary reference light path and the measurement light path 50 form an auxiliary optical fiber calibration system. Mainly composed of a beam splitter 41, an auxiliary fiber 42, and a coupler 43 installed behind both.
The beam splitter 41 is installed behind the beam splitter 20 to assist beam splitting inside the reference optical path 40.
The auxiliary optical fiber 42 is used to correct for the nonlinear effects of the auxiliary reference optical path 40.
A coupler 43 is mounted in the auxiliary reference optical path for coupling the laser light in the auxiliary reference optical path.
The gas cell calibration optical path 50 is a method for calibrating an auxiliary optical fiber by using a gas cell, and mainly comprises a hydrogen cyanide gas cell 51.
The hydrogen cyanide gas cell 51 can absorb specific wavelengths, and the length of the auxiliary optical fiber is calibrated by using the absorption spectral line. H13C14The N gas cell optical fiber calibration method is composed of an auxiliary reference optical path and a gas cell calibration optical path. After the laser is emitted, the laser enters the two optical path parts through the coupler respectively. The tunable laser passes through the gas cell to calibrate the optical path due to H13C14The absorption characteristic of N gas pool to specific wavelength can be obtained to obtain absorption peak spectrum diagram, the beat signal of auxiliary optical path can be produced by mixing frequency of laser local oscillator signal and delay signal, and its phase position can be formed from local oscillator signal
And delayed signal phase
Is subtracted to represent, pair
Taylor expansion, ignoring second and higher order terms, is expressed as:
and the phase of each sampling point can be obtained by performing Hilbert transform on the beat frequency signal and then performing phase solution
Warp H13C14Comparing the calibration signal obtained by the N gas cell with the absorption spectrum, the corresponding absorption wavelength lambda at the position k of the absorption peak can be knownk. Because the beat frequency signal and the calibration signal are simultaneously acquired by the data acquisition card, the same acquisition position point of the two signals is the same time signal, and the phase at the position of k in the beat frequency signal is found
At the same time, wavelength λkCorresponding beat frequency phase
Conversion of wavelength into frequency fkTo obtain
And (4) corresponding relation. The beat frequency has a linear function of phase and frequency, and
the correspondence is fit to a linear function by the least squares method, and is expressed as:
a is the slope of the linear function after fitting, the formula above is combined, and the delay time of the auxiliary fiber can be expressed as:
the conversion of the delay time to the fiber length is expressed as:
thus, the auxiliary fiber length calibration value is obtained.
The photodetector 60 is a bridge connecting the measurement optical path 30, the auxiliary reference optical path 40, and the gas cell calibration optical path 50 with the data acquisition system 80. The principle is that the electrical conductivity of the irradiated material is changed due to radiation, and an optical signal can be converted into an electrical signal. Facilitating the data acquisition system 80 to better acquire the data measured at the front end.
The focused synthetic aperture imaging system 70 provides 1550 nm-centered linear chirp for imaging starting from the actively stabilized fourier-mode-locked fiber laser 10; hydrogen cyanide cells provide an absolute frequency reference for chirp. The constituent members are a pyramid prism 71, a moving guide 72, and a laser interferometer 73.
The pyramid prism 71 is placed on a moving guide rail 72, the movement corresponds to the detection of the target object, and the laser returns after reaching the position and beats with the residual 10% of local oscillation light at the 50/50 coupler.
The moving guide 72 provides a space for the movement of the corner cube 71, so that the observation is more convenient.
The laser interferometer 73 is used for measuring the position of the corner cube 71, and the relative distance value measured by the laser interferometer is used as the standard value of the distance measuring system.
The data acquisition system 80 is responsible for collecting data of each component, and measuring a final actual distance through specific data acquisition, heavy sample acquisition and dispersion compensation.
The specific principle of the utility model is as follows:
the frequency of the laser emitted by the frequency-modulated continuous wave light source is theoretically a linear function changing with time, but actually, the laser frequency does not change strictly linearly, but is interfered by a nonlinear term, so the frequency of the emitted laser can be expressed as:
fL(t)＝f0ten μ t + ve(t) (1)
In the formula (1) f0Is the initial frequency, μ is the frequency modulation speed, ve(t) is a non-linear interference term. The phase of the exiting laser light is represented as:
the optical signal of the outgoing laser light is expressed as:
the amplitude of the emergent light signal is expressed by A in the formula (3), the emergent laser enters a measuring light path and passes through an optical circulator and a collimator, and the echo signal generated from the pyramid prism is expressed as follows:
in the formula (4) < tau >s(t) is the time delay generated by the distance to be measured, and tau can be determined because the laser dispersion in the air can be ignoreds(t) as a constant. The phase of the beat frequency generated by the echo signal and the frequency modulation continuous wave light source signal is expressed as follows:
in the formula (5)
Representing the beat phase error caused by the non-linear term. Similarly, the beat frequency signal generated when the frequency modulated continuous wave light source signal enters the reference light path is expressed as:
in the formula (6) < tau >r(t) is the time delay of the laser light through a reference fiber of known length, τ due to the effect of the fiber dispersionr(t) is a variable that varies with time,
the echo signal is coupled with the emergent signal to generate a beat signal, and the beat signal is expressed as follows after normalization:
in order to solve the problem of light source nonlinearity, a Mach-Zehnder interference light path with known optical fiber length is added in an original system as an auxiliary light path, and the obtained beat frequency signal is expressed as follows after normalization:
τr′the time delay generated by the auxiliary optical path delay optical fiber. The data acquisition card acquires a measurement light path and an auxiliary light path beat frequency signal, and extracts a peak-valley position point of the auxiliary light path beat frequency signal, wherein the peak-valley position is expressed as:
resampling the measurement beat frequency signal by using the peak-valley position to obtain a sampling interval as follows:
performing FFT operation on the resampled measurement beat frequency signal to obtain a spectrogram, wherein a frequency value corresponding to a peak point position K is required frequency information, and the frequency is as follows:
as can be seen from the above formula, the obtained single frequency is independent of nonlinearity, and the influence caused by the nonlinearity is eliminated by resampling. The relationship between the frequency and the distance to be measured is expressed as:
c is the speed of light propagation in vacuum; l is the length of the delay optical fiber in the auxiliary optical path; n is the refractive index of the fiber. And obtaining distance information under the condition that the length L of the delay optical fiber is known and the peak position point K is known. The auxiliary signal peak-valley value is used for resampling the measurement signal, and according to the sampling law, the auxiliary signal frequency is at least twice of the measurement signal frequency, namely the optical path of the auxiliary optical path is at least twice of the distance to be measured, and the optical path is generally 4-5 times in practical application. In the double-optical-path FMCW laser ranging system, the distance to be measured is in direct proportion to the length of the auxiliary optical-path delay optical fiber, and the ranging precision of the system is closely related to the calibration precision of the length of the auxiliary optical fiber.
The maximum error between the range finding value and the interferometer measuring value within the range of 60-80 m of the accuracy is less than 1 mm.
Claims (6)
1. A frequency modulation continuous wave laser imaging system based on a Fourier domain mode locking fiber laser is characterized in that a transmitting end is the Fourier mode locking fiber laser, the Fourier mode locking fiber laser is connected with a beam splitter, laser reaches three parts after beam splitting and respectively comprises a measuring light path, an auxiliary reference light path and a gas pool calibration light path, the laser can pass through a photoelectric detector after being emitted out of the measuring light path, the auxiliary reference light path and the gas pool calibration light path, the photoelectric detector is a bridge connecting the measuring light path, the auxiliary reference light path, the gas pool calibration light path and a data acquisition system, an optical signal emitted into the measuring light path of the laser can be emitted to a focusing synthetic aperture imaging system, and the laser finally reaches the data acquisition system.
2. The frequency modulated continuous wave laser imaging system based on the Fourier domain mode locked fiber laser of claim 1, it is characterized in that the Fourier mode-locked fiber laser is an annular cavity structure FDML frequency-swept fiber laser, the pumping source is a semiconductor optical amplifier, laser is transmitted and then transmitted in an annular structure, passes through an optical fiber isolator and then passes through a coupler, wherein part of laser exists in the annular cavity structure, the other part of laser is transmitted to a detector, the detector is connected with a data acquisition end, the laser device is used for measuring parameters of laser coupled by the coupler, the laser left in the annular structure reaches the FDML scanning frequency device and the polarization controller through the dispersive optical fiber, and then reaches the semiconductor optical amplifier again through the optical fiber isolator to complete the whole annular transmission, the FDML scanning frequency device is connected with the amplifier, and laser data are transmitted to the data acquisition end after being amplified to measure the laser parameters.
3. The system according to claim 1, wherein the laser beam in the measurement path is emitted into the circulator and the coupler through the beam splitter, and the laser beam passes through the circulator and the coupler, and the laser beam is partially emitted into the collimator for outputting the optical signal.
4. The frequency-modulated continuous wave laser imaging system based on the Fourier domain mode-locked fiber laser as claimed in claim 1, wherein the laser light in the auxiliary reference optical path is injected into the beam splitter, part of the laser light directly reaches the coupler, and the other part of the laser light is injected into the coupler after the nonlinear effect is corrected by the auxiliary optical fiber.
5. The system according to claim 1, wherein the laser in the calibration path of the gas cell passes through a hydrogen cyanide gas cell and is calibrated for length by absorption lines.
6. The frequency-modulated continuous wave laser imaging system based on the fourier domain mode-locked fiber laser as claimed in claim 1, wherein the pyramid prism in the focusing synthetic aperture imaging system is disposed on a moving guide rail, the moving target probe laser is returned after reaching the moving target probe laser, and the laser interferometer is used to measure the position of the front-end pyramid prism.
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