CN114167392A - FMCW laser ranging light source nonlinear correction system and method - Google Patents

FMCW laser ranging light source nonlinear correction system and method Download PDF

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CN114167392A
CN114167392A CN202111455678.5A CN202111455678A CN114167392A CN 114167392 A CN114167392 A CN 114167392A CN 202111455678 A CN202111455678 A CN 202111455678A CN 114167392 A CN114167392 A CN 114167392A
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port
frequency
modulation
soa
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袁国慧
王卓然
李俊锋
赵浩浩
张海
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Yangtze River Delta Research Institute of UESTC Huzhou
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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
    • G01S7/00—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00
    • G01S7/48—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00 of systems according to group G01S17/00
    • G01S7/497—Means for monitoring or calibrating
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
    • G01S17/00—Systems using the reflection or reradiation of electromagnetic waves other than radio waves, e.g. lidar systems
    • G01S17/02—Systems using the reflection of electromagnetic waves other than radio waves
    • G01S17/06—Systems determining position data of a target
    • G01S17/08—Systems determining position data of a target for measuring distance only
    • G01S17/32—Systems determining position data of a target for measuring distance only using transmission of continuous waves, whether amplitude-, frequency-, or phase-modulated, or unmodulated
    • G01S17/34—Systems determining position data of a target for measuring distance only using transmission of continuous waves, whether amplitude-, frequency-, or phase-modulated, or unmodulated using transmission of continuous, frequency-modulated waves while heterodyning the received signal, or a signal derived therefrom, with a locally-generated signal related to the contemporaneously transmitted signal

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Abstract

The invention discloses a system and a method for nonlinear correction of an FMCW laser ranging light source, belongs to the fields of laser radar, laser ranging, laser three-dimensional scanning and the like, and solves the problems that the prior art cannot accurately extract a target position, so that the detection and the identification of a target are influenced and the like. The method comprises the steps of firstly initializing a semiconductor laser LD, an SOA and an FPGA; collecting sawtooth wave or triangular wave light source signals amplified by the SOA by using a photoelectric detector PD, transmitting the sawtooth wave or triangular wave light source signals to an FPGA main control unit for processing, and outputting power feedback control signals to the SOA drive controller after processing; collecting beat frequency signals generated by a Mach-Zehnder interferometer in the system by using a PD (pulse-Width modulation) and transmitting the beat frequency signals to an FPGA (field programmable gate array) main control unit for processing, and outputting iterative sawtooth wave or triangular wave modulation signals to correct LD nonlinearity after processing. The invention is used for application scenes such as high-precision laser ranging, laser radar, three-dimensional imaging and the like.

Description

FMCW laser ranging light source nonlinear correction system and method
Technical Field
A system and a method for correcting nonlinearity of an FMCW laser ranging light source are used for high-precision laser ranging, a laser radar and three-dimensional imaging application scenes, and effectively solve the problems of power imbalance and frequency modulation nonlinearity caused by large-range frequency sweeping, so that the measurement error caused by laser tuning is reduced, the ranging precision is improved, and the system and the method belong to the fields of laser radar, laser ranging and laser three-dimensional scanning.
Background
In the field of laser ranging, a pulse method and a phase method are commonly used at present, but the measurement resolution of the pulse method ranging is limited by the bandwidth of an electronic device, the resolution is only millimeter magnitude, the phase method ranging has the 2 pi winding ambiguity problem, the frequency modulation continuous wave laser absolute ranging can effectively solve the 2 pi winding ambiguity problem, and the precision can reach micron magnitude. In practical applications, usually, FMCW laser ranging requires an ideal linear frequency sweep to invert the distance, but when the coverage of the frequency sweep is enlarged, it is affected by the strong nonlinearity and thermal effect of the laser itself, so that the linearity is obviously distorted in a narrow frequency range, and the power is synchronously changed along with the tuning of the laser, which in turn leads to distortion of the ranging results due to the non-linear nature of the laser's frequency modulation curve, the frequency of the beat signal may change over time, the spectral peak region corresponding to the beat signal is severely extended, resulting in a large error introduced in extracting the beat frequency, thus, the measurement accuracy is reduced, and when the laser is operated under the condition of high-bandwidth and high-speed frequency sweep, the nonlinearity is more obvious, so that the corresponding frequency of the target cannot be accurately extracted, the detection and identification of the laser radar on the target are influenced, and the light source is limited in practical application. The invention provides a nonlinear correction system scheme and a nonlinear correction method, aiming at solving the problem that the existing laser radar signal measurement method for distance measurement based on FMCW technology is low in measurement accuracy in the technology of extracting beat frequency signal frequency.
On the other hand, the laser adopts a linear frequency modulation form to measure distance, and the output distance measuring signal inevitably has optical power fluctuation, and the optical power fluctuation influences the distance measuring precision and the distance resolution. Therefore, the SOA-based optical power stabilization control system researched by the invention aims to stabilize the optical power of the output optical signal after modulation without changing the modulation information carried by the modulation signal, thereby improving the measurement accuracy and distance resolution of the FMCW system.
In the aspect of laser light source selection, the conventional typical swept-frequency light source mostly adopts external cavity grating mechanical modulation, and although the frequency modulation range is large, the complex mechanical structure is not beneficial to integration.
Disclosure of Invention
Aiming at the problems of the research, the invention aims to provide a system and a method for correcting nonlinearity of an FMCW laser ranging light source, and solves the problems that when a laser in the prior art works under the conditions of high broadband and high-speed frequency sweep, nonlinearity is more obvious, so that the target position and corresponding frequency cannot be accurately extracted, detection and identification of a target are influenced, the ranging is carried out in a frequency modulation mode, the output ranging signal inevitably has optical power fluctuation, the optical power fluctuation influences the ranging precision and distance resolution, and the frequency sweep speed, coherence and reliability which are mechanically modulated by adopting an external cavity grating are limited by a complex mechanical structure.
In order to achieve the purpose, the invention adopts the following technical scheme:
an FMCW laser ranging light source nonlinear correction system comprises a semiconductor laser LD, an isolator ISO, an optical attenuator VOA and an optical amplifier SOA which are sequentially connected with the output end of the semiconductor laser LD, wherein the output end of the optical amplifier SOA is connected with a coupler in a ratio of 80: 10, and a first port of the coupler outputs 80% of light serving as a measuring path signal and is connected with a circulator;
the photoelectric detector PD and the spectrum analyzer are sequentially connected with the output end of the circulator;
the second port and the third port of the coupler respectively output light of 10% of a reference path signal;
the second port is sequentially connected with the photoelectric detector PD and the FPGA system;
the third port is sequentially connected with a Mach-Zehnder interferometer, a photoelectric detector PD and an FPGA system which are composed of a first coupler with the ratio of 50: 50, a delay fiber and a second coupler with the ratio of 50: 50;
the output end of the SOA drive controller is respectively connected with the semiconductor laser LD and the optical amplifier SOA;
the Mach-Zehnder interferometer divides an optical signal into two paths of signals from one path of signal through a first coupler of 50: 50, and one path of signal passes through a delay optical fiber with a fixed length difference and then is combined with the other path of signal into one path of signal through a second coupler of 50: 50.
The invention also provides a nonlinear correction method for the FMCW laser ranging light source, which comprises the following steps:
step 1: firstly, initializing a semiconductor laser LD, an optical amplifier SOA and an FPGA system;
step 2: after initialization, a modulated optical signal output by an optical amplifier SOA in the system is collected by using a photoelectric detector PD and transmitted to an FPGA main control unit of the FPGA system for processing, and a feedback type control signal is output to an SOA drive controller after processing;
and step 3: if the optical power output from the optical amplifier SOA is stabilized within a given cell, executing the step 4, otherwise, repeating the step 2;
and 4, step 4: the beat frequency signal generated by the Mach-Zehnder interferometer in the system is collected by the photoelectric detector PD and transmitted to the FPGA main control unit of the FPGA system for processing, and the processed sawtooth wave or triangular wave periodic signal is used for correcting the nonlinearity in the sweep frequency light source of the semiconductor laser LD.
Further, the step 1 specifically comprises:
step 1.1: setting the initial temperature of an optical amplifier SOA to be 25 ℃, the initial current to be 350mA, and storing a direct current signal in a ROM of an FPGA system to serve as an initial control signal, wherein the initial control signal is output through a DA1 port of the FPGA system, and a DA1 port is connected to a modulation port of the SOA drive controller, and the direct current signal is processed by the system to obtain a feedback control signal;
step 1.2: setting the initial temperature of the semiconductor laser LD to be 25 ℃, the initial current to be 350mA, and an initial sawtooth wave or triangular wave periodic signal, storing the initial sawtooth wave or triangular wave periodic signal in a ROM of an FPGA system, outputting the initial sawtooth wave or triangular wave periodic signal through a DA2 port of the FPGA system, and connecting a DA2 port to a modulation port of a laser diode driving controller of the semiconductor laser LD.
Further, the step 2 specifically comprises:
step 2.1: after initialization, collecting optical power output by an optical amplifier SOA in the system through a photoelectric detector PD, connecting an electric signal obtained after power conversion of the photoelectric detector PD into an AD module input port of the FPGA system, and transmitting the collected electric signal to an FPGA main control unit of the FPGA system through the AD module input port;
step 2.2: the electric signals transmitted to the FPGA main control unit are processed through a balance algorithm, the processed output result is transmitted to a DA1 port, and the DA1 port outputs the result to a modulation port of the SOA drive controller to control the optical power in an optical path in real time.
Further, the processing of the electric signal of the FPGA main control unit in step 2.2 by the equalization algorithm specifically includes: firstly, open-loop control is carried out through a pre-correction algorithm, and then closed-loop control is carried out through an incremental PID negative feedback algorithm;
the pre-correction algorithm is specifically as follows:
through testing the SOA amplification characteristic of the optical amplifier, measuring the functional relation between the SOA characteristic of the optical amplifier and the magnitude of the modulation current, and recording as K (i)1)，K(i1) The modulation current of the output of the optical amplifier SOA to the modulation port of the SOA drive controller is represented as i1The magnification of time;
optical power P before entering optical amplifier SOAinAnd the actual output optical power PoutThe sizes are respectively as follows:
Figure BDA0003379172260000031
where P (t) represents the optical power output from the semiconductor laser LD at time t, and σ is the optical power output before entering the optical amplifier SOAAttenuation coefficient of the optical path, Pout(t)，Pin(t)，K(i1(t)) are the optical powers P respectively output by the optical amplifier SOA at time toutInput optical power P of optical amplifier SOAinAnd magnification, i1(t) represents the pre-correction current, i.e. the modulation current, output to the modulation port of the SOA drive controller at time t;
under ideal conditions, there are
Pout(t)≡Ps (1-2)
Wherein, PsThe ideal power is represented, namely the power is constant under the condition of stable power;
obtaining a pre-correction current i output to a modulation port of the SOA drive controller at the time t1The formula (t) is as follows:
Figure BDA0003379172260000041
obtaining a pre-correction current i1(t) applying a pre-correction current i for the entire period1(t) write once into the dual port RAM of the FPGA system, the FPGA system reads out the pre-correction current i1(t), outputting to a DA module, and performing the next incremental PID negative feedback algorithm;
in the incremental PID negative feedback algorithm, a control object is an optical amplifier SOA, the controlled quantity is optical power, and the formula is as follows:
Figure BDA0003379172260000042
where Δ i denotes the pre-correction current i1(t) amount of change, i.e. negative feedback current, e (k) is tkPower P (t) at the sampling instantk) And the ideal power PsDifference of (d), tkRepresenting the sampling instant of the kth discrete sample, A, B, C being three adjustable fixed parameters, P (t)k) Namely the size of the electric signal transmitted to the FPGA main control unit;
and after the negative feedback current is synchronized with the pre-correction current signal, the negative feedback current and the pre-correction current signal are added through an adder in the FPGA system, and after the addition, the result is output to a modulation port of the SOA drive controller through a DA2 port of the DA module, so that the optical power is stabilized.
Further, the step 4 specifically includes:
step 4.1: beat frequency signals generated by a Mach-Zehnder interferometer composed of a first coupler in a ratio of 50: 50, a delay fiber and a second coupler in a ratio of 50: 50 are collected by a photoelectric detector PD, electric signals subjected to power conversion by the photoelectric detector PD are connected to an AD module input port of the FPGA system, and the AD module input port transmits the collected electric signals to an FPGA main control unit of the FPGA system;
step 4.2: the FPGA main control unit processes the electric signal through a nonlinear iterative correction algorithm, the processed output result is transmitted to a DA2 port, and a DA2 port outputs the result to a modulation port of a laser diode driving controller of the semiconductor laser LD to correct the nonlinearity in the sweep frequency light source of the semiconductor laser LD.
Further, the processing of the electric signal by the FPGA main control unit in the step 4.2 through the nonlinear iterative correction algorithm specifically includes:
the instantaneous frequency of the laser light of the semiconductor laser LD in the scanning period is expressed as:
ω(t)＝ω0+KLD[i2(t)]·i2(t) (2-1)
wherein, ω is0Is the initial optical frequency, i, of the semiconductor laser LD2(t) is the modulation current at the modulation port of the laser diode drive controller of the semiconductor laser LD at time t, KLD[i2(t)]Refers to the modulation current i in the non-linear frequency modulation response of the semiconductor laser LD2(t) gain or modulation current i2(t) frequency modulation factor;
according to the formula (2-1), the relationship between the instantaneous frequency of the laser of the semiconductor laser LD and the beat frequency signal frequency collected by the photodetector PD is obtained as follows:
Figure BDA0003379172260000051
wherein, ω isPDReferring to the beat frequency signal frequency collected by the photodetector PD, ω (t) is the instantaneous frequency of the laser light output by the semiconductor laser LD, τ represents the delay of the mach-zehnder interferometer MZI, and from the above two equations (2-1) and (2-2), the following equation is obtained:
Figure BDA0003379172260000052
wherein, ω isPD(t) denotes the beat signal frequency of the photodetector PD at time t,
Figure BDA0003379172260000053
is the sweep rate of the semiconductor laser LD, Fdist(i2) Representing a non-linear function, i2Represents a modulation current of a modulation port of a laser diode drive controller of the semiconductor laser LD;
obtaining the nonlinear function F from equation (2-3)dist(i2) As follows:
Figure BDA0003379172260000054
under ideal conditions, there are
ωPD(t)≡ωdesired (2-5)
Wherein, ω isdesiredWhich means that the ideal frequency, i.e. in case of a stable signal, the frequency is constant in magnitude, constant,
then there are:
Figure BDA0003379172260000061
reducing the scanning of the semiconductor laser LD by the current obtained from the formula (2-6)Non-linear, but Fdist(i2) Is unknown, therefore, F is solved by substituting the frequency of the beat signal acquired by the photodetector PD under the modulation of the initial sawtooth wave or triangular wave periodic signal into the formula (2-7)dist(i2) Substituting the formula (2-8) to obtain a new current to reduce the nonlinearity of the laser frequency sweep, and repeating the process, i.e. iterating n times, until the frequency sweep light output by the laser of the semiconductor laser LD approaches to the ideal linearity, i.e. obtaining an iterative equation set of the current through the formula (2-4) and the formula (2-6), as shown in the formula (2-7) and the formula (2-8):
Figure BDA0003379172260000062
Figure BDA0003379172260000063
substituting equations (2-7) into (2-8), the final iteration equation is as follows:
Figure BDA0003379172260000064
further, iteration ωPD(t)n-1The method comprises the following specific steps:
step 4.2.1: before the 1 st iteration, interference signals generated by a semiconductor laser LD are acquired by a photoelectric detector PD to obtain AD data, the AD data pass through a band-pass filter to obtain a signal 1, wherein the AD data are electric signals input into an FPGA main control unit;
step 4.2.2: respectively passing the signal 1 through a phase discrimination filter and a delayer to obtain a signal 2 and a signal 3;
step 4.2.3: inputting the signal 2 and the signal 3 into an arc tangent module to obtain a phase signal 4;
step 4.2.4: inputting the phase signal 4 into a differentiator to obtain a frequency signal 5;
step 4.2.5: inputting the frequency signal 5 into a low-pass filter to obtain a frequency signal 6 subjected to noise reduction;
step 4.2.6: inputting the frequency signal 6 into an iterator according to a formula (2-9) to obtain a DA modulation signal after one iteration, namely a processed iteration signal, generating a beat frequency signal by an FMCW system by the iteration signal, collecting the beat frequency signal by a photoelectric detector PD to be used as an input signal of the next iteration of the photoelectric detector PD until n-1 times of iteration is performed, and collecting omega by the photoelectric detector PDPD(t)n-1。
Compared with the prior art, the invention has the beneficial effects that:
the system scheme provided by the invention comprises a power balancing system and a nonlinear correction system, and the sweep frequency light source adopts a current modulation mode, so that the system has the obvious advantages of simple and compact structure, high cost performance, high frequency modulation speed, capability of accurately extracting a target position, corresponding frequency and the like, and is an ideal mode for realizing a linear sweep frequency light source;
secondly, an optical power stability control system based on an optical amplifier SOA is built for optical power fluctuation in frequency modulation, the purpose is to stabilize the optical power of the modulated output optical signal, modulation information carried by the modulation signal is not changed, the power fluctuation degree is reduced to 0.12 from 2.52, and therefore the measurement precision and the distance resolution of the FMCW system are improved;
the invention builds a double-light-path frequency modulation continuous wave laser light source correction system aiming at the frequency modulation nonlinearity, carries out mathematical derivation and establishes an applicable mathematical model by using the interference principle of double-light-path laser and the nonlinear generation mechanism, and carries out simulation verification on power balance and nonlinear correction in MATLAB (matrix laboratory), thereby proving that the light source is greatly improved by multiple iterations and power open-close loop control;
the correction effect of the iterative algorithm in the invention obviously improves the frequency modulation nonlinearity of the current tuning semiconductor laser, is equivalent to adding a plurality of digital filters for denoising in an FPGA system in the acquisition of instantaneous frequency, reduces the performance requirement and cost of a photoelectric detector PD, effectively inhibits noise, enables a sampling result to be clearer and more accurate, and performs repeatability test to prove that the iterative correction algorithm has good repeatability and stability, thereby simplifying the system and improving the system precision.
Drawings
FIG. 1 is a schematic diagram of the system of the present invention;
FIG. 2 is a schematic diagram of an iteration of the present invention;
FIG. 3 is an effect diagram of the FPGA main control unit before the electric signal is processed by the equalization algorithm;
FIG. 4 is an effect diagram of the electric signal of the FPGA main control unit processed by the equalization algorithm in the invention;
FIG. 5 is an effect diagram of the electric signals of the FPGA main control unit before passing through a nonlinear iterative correction algorithm in the invention;
FIG. 6 is an effect diagram of an electric signal of the FPGA main control unit iterating once through a nonlinear iterative correction algorithm;
FIG. 7 is an effect diagram of the electric signal of the FPGA main control unit iterated twice through a nonlinear iterative correction algorithm in the invention;
FIG. 8 is a frequency spectrum diagram of an electric signal of an FPGA main control unit before iteration through a nonlinear iterative correction algorithm in the invention;
fig. 9 is a frequency spectrum diagram of an electric signal of the FPGA main control unit after being iterated twice by a nonlinear iterative rectification algorithm in the present invention.
Detailed Description
The invention will be further described with reference to the accompanying drawings and specific embodiments.
At present, most of laser radars in the market use a 'time of flight-TOF' technology, and the detection means of direct detection causes the problems of poor anti-interference performance, short detection distance and the like, so that the requirements of laser radars of the level above an automatic driving L3 are difficult to meet. The proposal firstly builds a double-light path frequency modulation continuous wave laser light source correction system, establishes an applicable mathematical model by using the interference principle of double-light path laser and the nonlinear generation mechanism, carries out simulation verification on power balance and nonlinear correction in MATLAB, proves that multiple iterations and power open-close loop control greatly improve the light source, and finally takes an experiment as a core, the correction effect of the iterative algorithm is verified to obviously improve the frequency modulation nonlinearity of the current-tuned semiconductor laser, the cost of the photoelectric detector is reduced on the acquisition of instantaneous frequency, the denoising of a plurality of digital filters based on an FPGA platform is increased, the noise is effectively inhibited, the sampling result is clearer and more accurate, the repeatability test is carried out, and the iterative correction algorithm is proved to have good repeatability and stability, so that the system is simplified, and the system precision is improved.
An FMCW laser ranging light source nonlinear correction system comprises a semiconductor laser LD, an isolator ISO, an optical attenuator VOA and an optical amplifier SOA which are sequentially connected with the output end of the semiconductor laser LD, wherein the output end of the optical amplifier SOA is connected with a coupler in a ratio of 80: 10, and a first port of the coupler outputs 80% of light serving as a measuring path signal and is connected with a circulator;
the photoelectric detector PD and the spectrum analyzer are sequentially connected with the output end of the circulator;
the second port and the third port of the coupler respectively output light of 10% of a reference path signal;
the second port is sequentially connected with the photoelectric detector PD and the FPGA system;
the third port is sequentially connected with a Mach-Zehnder interferometer, a photoelectric detector PD and an FPGA system which are composed of a first coupler with the ratio of 50: 50, a delay fiber and a second coupler with the ratio of 50: 50;
the output end of the SOA drive controller is respectively connected with the semiconductor laser LD and the optical amplifier SOA;
the Mach-Zehnder interferometer divides an optical signal into two paths of signals from one path of signal through a first coupler of 50: 50, and one path of signal passes through a delay optical fiber with a fixed length difference and then is combined with the other path of signal into one path of signal through a second coupler of 50: 50.
A non-linear correction method for an FMCW laser ranging light source comprises the following steps:
step 1: firstly, initializing a semiconductor laser LD, an optical amplifier SOA and an FPGA system;
the method specifically comprises the following steps:
step 1.1: setting the initial temperature of an optical amplifier SOA to be 25 ℃, the initial current to be 350mA, and storing a direct current signal in a ROM of an FPGA system to serve as an initial control signal, wherein the initial control signal is output through a DA1 port of the FPGA system, and a DA1 port is connected to a modulation port of the SOA drive controller, and the direct current signal is processed by the system to obtain a feedback control signal;
step 1.2: setting the initial temperature of the semiconductor laser LD to be 25 ℃, the initial current to be 350mA, and the initial sawtooth wave or triangular wave periodic signal, storing the initial sawtooth wave or triangular wave periodic signal in a ROM of an FPGA system, outputting the initial sawtooth wave or triangular wave periodic signal through a DA2 port of the FPGA system, and connecting a DA2 port to a modulation port of a laser diode driving controller of the semiconductor laser LD, wherein the semiconductor laser LD can be a 1550nm semiconductor laser LD.
Step 2: after initialization, a modulated optical signal output by an optical amplifier SOA in the system is collected by using a photoelectric detector PD and transmitted to an FPGA main control unit of the FPGA system for processing, and a feedback type control signal is output to an SOA drive controller after processing;
the method specifically comprises the following steps:
step 2.1: after initialization, collecting optical power output by an optical amplifier SOA in the system through a photoelectric detector PD, connecting an electric signal obtained after power conversion of the photoelectric detector PD into an AD module input port of the FPGA system, and transmitting the collected electric signal to an FPGA main control unit of the FPGA system through the AD module input port;
step 2.2: the electric signals transmitted to the FPGA main control unit are processed through a balance algorithm, the processed output result is transmitted to a DA1 port, and the DA1 port outputs the result to a modulation port of the SOA drive controller to control the optical power in an optical path in real time.
The processing of the electric signals of the FPGA main control unit through an equalization algorithm specifically comprises the following steps: firstly, open-loop control is carried out through a pre-correction algorithm, and then closed-loop control is carried out through an incremental PID negative feedback algorithm;
the pre-correction algorithm is specifically as follows:
through testing the SOA amplification characteristic of the optical amplifier, measuring the functional relation between the SOA characteristic of the optical amplifier and the magnitude of the modulation current, and recording as K (i)1)，K(i1) The modulation current of the output of the optical amplifier SOA to the modulation port of the SOA drive controller is represented as i1The magnification of time;
optical power P before entering optical amplifier SOAinAnd the actual output optical power PoutThe sizes are respectively as follows:
Figure BDA0003379172260000091
where P (t) represents the optical power output from the semiconductor laser LD at time t, σ is the attenuation coefficient of the optical path before entering the optical amplifier SOA, and Pout(t)，Pin(t)，K(i1(t)) are the optical powers P respectively output by the optical amplifier SOA at time toutInput optical power P of optical amplifier SOAinAnd magnification, i1(t) represents the pre-correction current, i.e. the modulation current, output to the modulation port of the SOA drive controller at time t;
under ideal conditions, there are
Pout(t)≡Ps (1-2)
Wherein, PsThe ideal power is represented, namely, in the case of stable power, the power is constant and is a constant value:
obtaining a pre-correction current i output to a modulation port of the SOA drive controller at the time t1The formula (t) is as follows:
Figure BDA0003379172260000101
obtaining a pre-correction current i1(t) applying a pre-correction current i for the entire period1(t) write once into the dual port RAM of the FPGA system, the FPGA system reads out the pre-correction current i1(t), outputting to a DA module, and performing the next incremental PID negative feedback algorithm;
in the incremental PID negative feedback algorithm, a control object is an optical amplifier SOA, the controlled quantity is optical power, and the formula is as follows:
Figure BDA0003379172260000102
where Δ i denotes the pre-correction current i1(t) amount of change, i.e. negative feedback current, e (k) is tkPower P (t) at the sampling instantk) And the ideal power PsDifference of (d), tkRepresenting the sampling instant of the kth discrete sample, A, B, C being three adjustable fixed parameters, P (t)k) Namely the size of the electric signal transmitted to the FPGA main control unit;
and after the negative feedback current is synchronized with the pre-correction current signal, the negative feedback current and the pre-correction current signal are added through an adder in the FPGA system, and after the addition, the result is output to a modulation port of the SOA drive controller through a DA2 port of the DA module, so that the optical power is stabilized.
And step 3: if the optical power output from the optical amplifier SOA is stabilized within a given cell, executing the step 4, otherwise, repeating the step 2;
and 4, step 4: the beat frequency signal generated by the Mach-Zehnder interferometer in the system is collected by the photoelectric detector PD and transmitted to the FPGA main control unit of the FPGA system for processing, and the processed sawtooth wave or triangular wave periodic signal is used for correcting the nonlinearity in the sweep frequency light source of the semiconductor laser LD.
The method specifically comprises the following steps:
step 4.1: beat frequency signals generated by a Mach-Zehnder interferometer composed of a first coupler in a ratio of 50: 50, a delay fiber and a second coupler in a ratio of 50: 50 are collected by a photoelectric detector PD, electric signals subjected to power conversion by the photoelectric detector PD are connected to an AD module input port of the FPGA system, and the AD module input port transmits the collected electric signals to an FPGA main control unit of the FPGA system;
step 4.2: the FPGA main control unit processes the electric signal through a nonlinear iterative correction algorithm, the processed output result is transmitted to a DA2 port, and a DA2 port outputs the result to a modulation port of a laser diode driving controller of the semiconductor laser LD to correct the nonlinearity in the sweep frequency light source of the semiconductor laser LD.
The processing of the electric signals by the FPGA main control unit through a nonlinear iterative correction algorithm specifically comprises the following steps:
the instantaneous frequency of the laser light of the semiconductor laser LD in the scanning period is expressed as:
ω(t)＝ω0+KLD[i2(t)]·i2(t) (2-1)
where ω is the initial light frequency of the semiconductor laser LD, i2(t) is the modulation current at the modulation port of the laser diode drive controller of the semiconductor laser LD at time t, KLD[i2(t)]Refers to the modulation current i in the non-linear frequency modulation response of the semiconductor laser LD2(t) gain or modulation current i2(t) frequency modulation factor;
according to the formula (2-1), the relationship between the instantaneous frequency of the laser of the semiconductor laser LD and the beat frequency signal frequency collected by the photodetector PD is obtained as follows:
Figure BDA0003379172260000111
wherein, ω isPDReferring to the beat frequency signal frequency collected by the photodetector PD, ω (t) is the instantaneous frequency of the laser light output by the semiconductor laser LD, τ represents the delay of the mach-zehnder interferometer MZI, and from the above two equations (2-1) and (2-2), the following equation is obtained:
Figure BDA0003379172260000121
wherein, ω isPD(t) denotes the beat signal frequency of the photodetector PD at time t,
Figure BDA0003379172260000122
is the sweep rate of the semiconductor laser LD, Fdist(i2) Representing a non-linear function, i2Represents a modulation current of a modulation port of a laser diode drive controller of the semiconductor laser LD;
obtaining the nonlinear function F from equation (2-3)dist(i2) As follows:
Figure BDA0003379172260000123
under ideal conditions, there are
ωPD(t)≡ωdesired (2-5)
Wherein, ω isdesiredWhich means that the ideal frequency, i.e. in case of a stable signal, the frequency is constant in magnitude, constant,
then there are:
Figure BDA0003379172260000124
the scanning nonlinearity of the semiconductor laser LD is reduced by the current obtained from the formula (2-6), but Fdist(i2) Is unknown, therefore, F is solved by substituting the frequency of the beat signal acquired by the photodetector PD under the modulation of the initial sawtooth wave or triangular wave periodic signal into the formula (2-7)dist(i2) Substituting the formula (2-8) to obtain new current to reduce the nonlinearity of laser frequency sweep, repeating the process, i.e. iterating for n times, until the frequency sweep light output by the laser of the semiconductor laser LD is close to the ideal linearity, i.e. obtaining the iteration method of the current through the formula (2-4) and the formula (2-6)The equation set is shown as formulas (2-7) and (2-8):
Figure BDA0003379172260000131
Figure BDA0003379172260000132
substituting equations (2-7) into (2-8), the final iteration equation is as follows:
Figure BDA0003379172260000133
to obtain omegaPD(t)n-1The method comprises the following specific steps:
step 4.2.1: before the 1 st iteration, interference signals generated by a semiconductor laser LD are acquired by a photoelectric detector PD to obtain AD data, the AD data pass through a band-pass filter to obtain a signal 1, wherein the AD data are electric signals input into an FPGA main control unit;
step 4.2.2: respectively passing the signal 1 through a phase discrimination filter and a delayer to obtain a signal 2 and a signal 3;
step 4.2.3: inputting the signal 2 and the signal 3 into an arc tangent module to obtain a phase signal 4;
step 4.2.4: inputting the phase signal 4 into a differentiator to obtain a frequency signal 5;
step 4.2.5: inputting the frequency signal 5 into a low-pass filter to obtain a frequency signal 6 subjected to noise reduction;
step 4.2.6: inputting the frequency signal 6 into an iterator according to a formula (2-9) to obtain a DA modulation signal after one iteration, namely a processed iteration signal, generating a beat frequency signal by an FMCW system by the iteration signal, collecting the beat frequency signal by a photoelectric detector PD to be used as an input signal of the next iteration of the photoelectric detector PD until n-1 times of iteration is performed, and collecting omega by the photoelectric detector PDPD(t)n-1。
The laser adopted by the invention is the semiconductor laser LD, the output light wavelength of which is changed by changing the intensity of the injected current, so as to change the frequency of the output signal.
The above are merely representative examples of the many specific applications of the present invention, and do not limit the scope of the invention in any way. All the technical solutions formed by the transformation or the equivalent substitution fall within the protection scope of the present invention.

Claims (8)

1. The non-linear correction system for the FMCW laser ranging light source is characterized by comprising a semiconductor laser LD, an isolator ISO, an optical attenuator VOA and an optical amplifier SOA, wherein the isolator ISO, the optical attenuator VOA and the optical amplifier SOA are sequentially connected with the output end of the semiconductor laser LD;
the photoelectric detector PD and the spectrum analyzer are sequentially connected with the output end of the circulator;
the second port and the third port of the coupler respectively output light of 10% of a reference path signal;
the second port is sequentially connected with the photoelectric detector PD and the FPGA system;
the third port is sequentially connected with a Mach-Zehnder interferometer, a photoelectric detector PD and an FPGA system which are composed of a first coupler with the ratio of 50: 50, a delay fiber and a second coupler with the ratio of 50: 50;
the output end of the SOA drive controller is respectively connected with the semiconductor laser LD and the optical amplifier SOA;
the Mach-Zehnder interferometer divides an optical signal into two paths of signals from one path of signal through a first coupler with the ratio of 50: 50, and one path of signal passes through a delay optical fiber with a fixed length difference and then passes through a 50: the second coupler of 50 is a single signal.
2. A non-linear correction method for an FMCW laser ranging light source is characterized by comprising the following steps:
step 1: firstly, initializing a semiconductor laser LD, an optical amplifier SOA and an FPGA system;
step 2: after initialization, a modulated optical signal output by an optical amplifier SOA in the system is collected by using a photoelectric detector PD and transmitted to an FPGA main control unit of the FPGA system for processing, and a feedback type control signal is output to an SOA drive controller after processing;
and step 3: if the optical power output from the optical amplifier SOA is stabilized within a given cell, executing the step 4, otherwise, repeating the step 2;
and 4, step 4: the beat frequency signal generated by the Mach-Zehnder interferometer in the system is collected by the photoelectric detector PD and transmitted to the FPGA main control unit of the FPGA system for processing, and the processed sawtooth wave or triangular wave periodic signal is used for correcting the nonlinearity in the sweep frequency light source of the semiconductor laser LD.
3. The non-linear correction method for FMCW laser ranging light source as claimed in claim 2, wherein: the step 1 specifically comprises the following steps:
step 1.1: setting the initial temperature of an optical amplifier SOA to be 25 ℃, the initial current to be 350mA, and storing a direct current signal in a ROM of an FPGA system to serve as an initial control signal, wherein the initial control signal is output through a DA1 port of the FPGA system, and a DA1 port is connected to a modulation port of the SOA drive controller, and the direct current signal is processed by the system to obtain a feedback control signal;
step 1.2: setting the initial temperature of the semiconductor laser LD to be 25 ℃, the initial current to be 350mA, and an initial sawtooth wave or triangular wave periodic signal, storing the initial sawtooth wave or triangular wave periodic signal in a ROM of an FPGA system, outputting the initial sawtooth wave or triangular wave periodic signal through a DA2 port of the FPGA system, and connecting a DA2 port to a modulation port of a laser diode driving controller of the semiconductor laser LD.
4. The FMCW laser ranging light source nonlinearity correction method as set forth in claim 3, wherein: the step 2 specifically comprises the following steps:
step 2.1: after initialization, collecting optical power output by an optical amplifier SOA in the system through a photoelectric detector PD, connecting an electric signal obtained after power conversion of the photoelectric detector PD into an AD module input port of the FPGA system, and transmitting the collected electric signal to an FPGA main control unit of the FPGA system through the AD module input port;
step 2.2: the electric signals transmitted to the FPGA main control unit are processed through a balance algorithm, the processed output result is transmitted to a DA1 port, and the DA1 port outputs the result to a modulation port of the SOA drive controller to control the optical power in an optical path in real time.
5. The FMCW laser ranging light source nonlinearity correction method as set forth in claim 4, wherein: the step 2.2 of processing the electric signal of the FPGA main control unit through the equalization algorithm specifically comprises: firstly, open-loop control is carried out through a pre-correction algorithm, and then closed-loop control is carried out through an incremental PID negative feedback algorithm;
the pre-correction algorithm is specifically as follows:
through testing the SOA amplification characteristic of the optical amplifier, measuring the functional relation between the SOA characteristic of the optical amplifier and the magnitude of the modulation current, and recording as K (i)1)，K(i1) The modulation current of the output of the optical amplifier SOA to the modulation port of the SOA drive controller is represented as i1The magnification of time;
optical power P before entering optical amplifier SOAinAnd the actual output optical power PoutThe sizes are respectively as follows:
Figure FDA0003379172250000021
where P (t) represents the optical power output from the semiconductor laser LD at time t, σ is the attenuation coefficient of the optical path before entering the optical amplifier SOA, and Pout(t)，Pin(t)，K(i1(t)) are the optical powers P respectively output by the optical amplifier SOA at time toutInput optical power P of optical amplifier SOAinAnd magnification, i1(t) represents the pre-correction current, i.e. the modulation current, output to the modulation port of the SOA drive controller at time t;
under ideal conditions, there are
Pout(t)≡Ps (1-2)
Wherein, PsThe ideal power is represented, namely the power is constant under the condition of stable power;
obtaining a pre-correction current i output to a modulation port of the SOA drive controller at the time t1The formula (t) is as follows:
Figure FDA0003379172250000031
obtaining a pre-correction current i1(t) applying a pre-correction current i for the entire period1(t) write once into the dual port RAM of the FPGA system, the FPGA system reads out the pre-correction current i1(t), outputting to a DA module, and performing the next incremental PID negative feedback algorithm:
in the incremental PID negative feedback algorithm, a control object is an optical amplifier SOA, the controlled quantity is optical power, and the formula is as follows:
Figure FDA0003379172250000032
where Δ i denotes the pre-correction current i1(t) amount of change, i.e. negative feedback current, e (k) is tkPower P (t) at the sampling instantk) And the ideal power PsDifference of (d), tkRepresenting the sampling instant of the kth discrete sample, A, B, C being three adjustable fixed parameters, P (t)k) Namely the size of the electric signal transmitted to the FPGA main control unit;
and after the negative feedback current is synchronized with the pre-correction current signal, the negative feedback current and the pre-correction current signal are added through an adder in the FPGA system, and after the addition, the result is output to a modulation port of the SOA drive controller through a DA2 port of the DA module, so that the optical power is stabilized.
6. The FMCW laser ranging light source nonlinearity correction method as set forth in claim 5, wherein: the step 4 specifically comprises the following steps:
step 4.1: beat frequency signals generated by a Mach-Zehnder interferometer composed of a first coupler in a ratio of 50: 50, a delay fiber and a second coupler in a ratio of 50: 50 are collected by a photoelectric detector PD, electric signals subjected to power conversion by the photoelectric detector PD are connected to an AD module input port of the FPGA system, and the AD module input port transmits the collected electric signals to an FPGA main control unit of the FPGA system;
step 4.2: the FPGA main control unit processes the electric signal through a nonlinear iterative correction algorithm, the processed output result is transmitted to a DA2 port, and a DA2 port outputs the result to a modulation port of a laser diode driving controller of the semiconductor laser LD to correct the nonlinearity in the sweep frequency light source of the semiconductor laser LD.
7. The FMCW laser ranging light source nonlinearity correction method as set forth in claim 6, wherein: the specific step of processing the electric signals by the FPGA main control unit in the step 4.2 through the nonlinear iterative correction algorithm is as follows:
the instantaneous frequency of the laser light of the semiconductor laser LD in the scanning period is expressed as:
ω(t)＝ω0+KLD[i2(t)]·i2(t) (2-1)
wherein, ω is0Is the initial optical frequency, i, of the semiconductor laser LD2(t) is the modulation current at the modulation port of the laser diode drive controller of the semiconductor laser LD at time t, KLD[i2(t)]Refers to the modulation current i in the non-linear frequency modulation response of the semiconductor laser LD2(t) gain or modulation current i2(t) frequency modulation factor;
according to the formula (2-1), the relationship between the instantaneous frequency of the laser of the semiconductor laser LD and the beat frequency signal frequency collected by the photodetector PD is obtained as follows:
Figure FDA0003379172250000041
wherein, ω isPDReferring to the beat frequency signal frequency collected by the photodetector PD, ω (t) is the instantaneous frequency of the laser light output by the semiconductor laser LD, τ represents the delay of the mach-zehnder interferometer MZI, and from the above two equations (2-1) and (2-2), the following equation is obtained:
Figure FDA0003379172250000042
wherein, ω isPD(t) denotes the beat signal frequency of the photodetector PD at time t,
Figure FDA0003379172250000043
is the sweep rate of the semiconductor laser LD, Fdist(i2) Representing a non-linear function, i2Represents a modulation current of a modulation port of a laser diode drive controller of the semiconductor laser LD;
obtaining the nonlinear function F from equation (2-3)dist(i2) As follows:
Figure FDA0003379172250000044
under ideal conditions, there are
ωPD(t)≡ωdesired (2-5)
Wherein, ω isdesiredWhich means that the ideal frequency, i.e. in case of a stable signal, the frequency is constant in magnitude, constant,
then there are:
Figure FDA0003379172250000051
the scanning nonlinearity of the semiconductor laser LD is reduced by the current obtained from the formula (2-6), but Fdist(i2) Is unknown, therefore, F is solved by substituting the frequency of the beat signal acquired by the photodetector PD under the modulation of the initial sawtooth wave or triangular wave periodic signal into the formula (2-7)dist(i2) Substituting the formula (2-8) to obtain a new current to reduce the nonlinearity of the laser frequency sweep, and repeating the process, i.e. iterating n times, until the frequency sweep light output by the laser of the semiconductor laser LD approaches to the ideal linearity, i.e. obtaining an iterative equation set of the current through the formula (2-4) and the formula (2-6), as shown in the formula (2-7) and the formula (2-8):
Figure FDA0003379172250000052
Figure FDA0003379172250000053
substituting equations (2-7) into (2-8), the final iteration equation is as follows:
Figure FDA0003379172250000054
8. the FMCW laser ranging light source nonlinearity correction method as set forth in claim 7, wherein: iteration omegaPD(t)n-1The method comprises the following specific steps:
step 4.2.1: before the 1 st iteration, interference signals generated by a semiconductor laser LD are acquired by a photoelectric detector PD to obtain AD data, the AD data pass through a band-pass filter to obtain a signal 1, wherein the AD data are electric signals input into an FPGA main control unit;
step 4.2.2: respectively passing the signal 1 through a phase discrimination filter and a delayer to obtain a signal 2 and a signal 3;
step 4.2.3: inputting the signal 2 and the signal 3 into an arc tangent module to obtain a phase signal 4;
step 4.2.4: inputting the phase signal 4 into a differentiator to obtain a frequency signal 5;
step 4.2.5: inputting the frequency signal 5 into a low-pass filter to obtain a frequency signal 6 subjected to noise reduction;
step 4.2.6: inputting the frequency signal 6 into an iterator according to a formula (2-9) to obtain a DA modulation signal after one iteration, namely a processed iteration signal, generating a beat frequency signal by an FMCW system by the iteration signal, collecting the beat frequency signal by a photoelectric detector PD to be used as an input signal of the next iteration of the photoelectric detector PD until n-1 times of iteration is performed, and collecting omega by the photoelectric detector PDPD(t)n-1。
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Cited By (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CN115396018A (en) * 2022-07-26 2022-11-25 西安空间无线电技术研究所 System and method for testing gain and noise coefficient of optical fiber amplifier
CN115480514A (en) * 2022-09-28 2022-12-16 山东大学 System for integrating light intensity feedback and light trap quantification based on FPGA
CN116626696A (en) * 2023-07-20 2023-08-22 北京摩尔芯光半导体技术有限公司 Frequency modulation continuous wave laser range unit

Cited By (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CN115396018A (en) * 2022-07-26 2022-11-25 西安空间无线电技术研究所 System and method for testing gain and noise coefficient of optical fiber amplifier
CN115480514A (en) * 2022-09-28 2022-12-16 山东大学 System for integrating light intensity feedback and light trap quantification based on FPGA
CN116626696A (en) * 2023-07-20 2023-08-22 北京摩尔芯光半导体技术有限公司 Frequency modulation continuous wave laser range unit

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