A linear sweep laser generating device

Abstract

The application relates to a linear sweep frequency laser generating device, which comprises a semiconductor laser, a driving circuit, a laser beam splitter, a laser frequency discriminator unit, a radio frequency signal generating unit and a laser frequency shift unit; the semiconductor laser is controlled by the driving circuit to generate sweep frequency laser, and is divided into two beams by the laser beam splitter and enters the laser frequency discriminator unit and the laser frequency shift unit respectively; the laser frequency discriminator unit extracts a nonlinear sweep frequency component of the sweep frequency laser and generates a control signal to control the radio frequency signal generating unit; the radio frequency signal generating unit generates a radio frequency signal with a frequency opposite to that of the nonlinear sweep frequency component according to the control signal to drive the laser frequency shift unit; the laser frequency shift unit modulates the frequency of the radio frequency signal on the input light frequency to offset the nonlinear sweep frequency component of the sweep frequency laser and output linear sweep frequency laser. Compared with the prior art, the application has the advantages of high sweep frequency rate, large sweep frequency range and linear sweep frequency laser.

Description

Technical Field

[0001] This invention relates to the field of laser technology, and in particular to a linear sweeping laser generator. Background Technology

[0002] Linearly swept lasers are light waves whose instantaneous frequency changes linearly with time. Utilizing their frequency sweeping characteristics, swept lasers can be used to realize a variety of optical sensing and measurement systems, such as optical frequency domain reflectometers, frequency-modulated continuous wave lidar, fiber optic grating sensors, and spectral measurement systems.

[0003] From a performance perspective, sensing and measurement systems using frequency-sweeping lasers as the detection source can achieve performance levels that are difficult to match by similar systems using single-frequency, pulsed, or broadband light sources in certain key areas. For example, in distributed fiber optic sensing technology, optical frequency domain reflectometers using linearly swept-frequency lasers as the light source can achieve centimeter-level or even higher spatial resolution, far exceeding the meter-level spatial resolution achievable by optical time domain reflectometers based on pulsed laser sources. Frequency-modulated continuous-wave lidar using linearly swept-frequency continuous light as the light source exhibits a significantly higher signal-to-noise ratio than pulsed lidar, thus significantly improving measurement distance and accuracy. Phase-shifting fiber optic grating sensors and heterodyne interferometric spectral measurement systems based on linearly swept-frequency lasers demonstrate wavelength resolution significantly superior to classic grating spectrometers. Therefore, linearly swept-frequency lasers hold significant value for high-performance optical sensing and measurement systems.

[0004] Semiconductor lasers, with their excellent performance and mature technology, are an important and common type of laser generator. The output frequency of a semiconductor laser can be modulated by a driving current. When driven by a time-varying current, a semiconductor laser can output a laser with a frequency that changes over time, i.e., a swept-frequency laser. Compared to other swept-frequency light source implementations such as mechanical tuning, piezoelectric tuning, external modulator tuning, and temperature tuning, current-tuned semiconductor swept-frequency laser sources offer flexible control, moderate cost, and the ability to simultaneously achieve high sweep rates and wide sweep ranges. Furthermore, because they do not rely on moving mechanical parts, they exhibit strong resistance to environmental vibrations, stable operation, and a long lifespan. Therefore, current-tuned semiconductor swept-frequency laser sources are widely used in various optical sensing and measurement systems.

[0005] However, current current-tuned semiconductor swept-frequency laser sources still suffer from insufficient sweep frequency linearity. This is because, on the one hand, the time-varying drive current not only directly modulates the laser's output frequency but also causes instantaneous changes in the laser core temperature, which in turn introduces additional and unpredictable disturbances to the output frequency. On the other hand, due to the rapid change in drive current over time, the laser's active region, drive current circuit, and temperature control circuit are all in an unstable state, resulting in a phase noise level in the laser output light that is far below its nominal value, exhibiting significant frequency jitter. Both of these factors disrupt the linear sweep frequency state of the laser output, thus reducing its sweep frequency linearity. The sweep frequency linearity of the light source directly determines the spatial resolution and limiting sensing distance of optical frequency domain reflectometers / frequency-modulated continuous-wave lidar, as well as the measurement resolution and accuracy of interferometric spectral measurement systems. Therefore, it can be argued that for some high-performance optical sensing and measurement systems, the insufficient sweep frequency linearity of semiconductor swept-frequency laser sources becomes a major bottleneck restricting further improvements in the system's core performance indicators. Summary of the Invention

[0006] The purpose of this invention is to provide a linear sweep laser generator that can effectively compensate for the nonlinear sweep components of a semiconductor sweep laser, thereby obtaining a linear sweep laser with good sweep linearity.

[0007] The objective of this invention can be achieved through the following technical solutions:

[0008] A linear sweeping laser generator, comprising:

[0009] Semiconductor laser, driving circuit, laser beam splitter, laser frequency discriminator, radio frequency signal generation unit, laser frequency shifting unit;

[0010] The semiconductor laser is controlled by a driving circuit to generate a swept laser, which is then split into two beams by a laser beam splitter, and enters the laser frequency discrimination unit and the laser frequency shifting unit respectively.

[0011] The laser frequency discrimination unit extracts the nonlinear sweeping component of the sweeping laser and generates a control signal based on the nonlinear sweeping component to control the radio frequency signal generation unit.

[0012] The radio frequency signal generating unit generates a radio frequency signal whose frequency change is opposite to the nonlinear sweep frequency component according to the control signal, and drives the laser frequency shifting unit.

[0013] The laser frequency shifting unit modulates the frequency of the radio frequency signal onto the input optical frequency, canceling the nonlinear sweeping component of the sweeping laser and outputting a linear sweeping laser.

[0014] The input to the semiconductor laser is a time-varying driving current generated by the driving circuit, and the output is a laser beam, the frequency of which is modulated by the driving current.

[0015] The laser frequency discrimination unit includes an optical input terminal and a control signal output terminal. The optical input terminal receives the frequency sweep laser. The laser frequency discrimination unit internally acquires the time-frequency change of the frequency sweep laser in real time, separates the nonlinear frequency sweep component from the time-frequency change of the frequency sweep laser in real time, and outputs a control signal based on the nonlinear frequency sweep component.

[0016] The frequency-time variation of the swept laser is a function of the frequency-time of the swept laser.

[0017] The nonlinear sweep frequency component is a frequency-time function that is the sum of all components of the sweep frequency laser time-frequency change, excluding the constant term and the first term.

[0018] Inside the laser frequency discrimination unit, the acquisition of the time-frequency change of the swept laser and the separation of the nonlinear swept components are achieved by a digital signal processor through calculation; the calculation method is real-time calculation.

[0019] The control signal is an analog electrical signal or a digital electrical signal.

[0020] The radio frequency signal generating unit includes a control signal input terminal and a radio frequency signal output terminal; the radio frequency signal generating unit outputs a radio frequency signal whose frequency changes with time according to the input control signal; the radio frequency signal is output to the laser frequency shifting unit.

[0021] The frequency-time function of the radio frequency signal is a fixed frequency constant minus the nonlinear sweep frequency component.

[0022] The laser frequency shifting unit includes an optical input terminal, a radio frequency signal input terminal, and an optical output terminal. The laser frequency shifting unit applies a frequency shift equal to the frequency of the input radio frequency signal to the input sweeping laser and then outputs a linear sweeping laser through the optical output terminal.

[0023] Compared with the prior art, the present invention has the following beneficial effects:

[0024] (1) The present invention obtains a linear frequency sweep laser by compensating the nonlinear frequency sweep component of the current-tuned semiconductor frequency sweep laser. This method retains the advantages of the current-tuned semiconductor frequency sweep laser source, such as high frequency sweep rate and large frequency sweep range, flexible control, strong resistance to environmental vibration, stable operation and long life cycle, while eliminating the defect of its insufficient frequency sweep linearity.

[0025] (2) The structure of the nonlinear sweep frequency compensation part in this invention is based on a branch-merge open-loop feedforward structure, replacing the traditional closed-loop feedback structure based on a loop, which can achieve higher system performance. The closed-loop feedback structure is based on the technical solution of locking the laser frequency feedback to the resonant frequency of a fixed optical resonant cavity, which is ideal for single-frequency laser frequency stabilization. However, for sweep frequency lasers, a reference resonant cavity with a resonant frequency that can be linearly scanned is required, which is technically difficult to implement (especially when the linear scanning rate or range of the resonant frequency is high). In addition, considering that the laser frequency tuning rate of the closed-loop feedback structure is determined by the sum of the delays of all devices in the loop, its upper limit of feedback bandwidth is low (especially when the loop contains a semiconductor laser), which is not conducive to the realization of the system designer's desired "both accurately correcting the frequency deviation of the laser relative to the resonant cavity and enabling the laser to quickly follow the resonant cavity scan". The open-loop feedforward structure used in this invention solves the above challenges better. First, the laser frequency discrimination unit in the scheme only needs its operating frequency band to cover the sweep frequency range of the light source, without active scanning. Furthermore, the laser frequency tuning rate is determined solely by the response time of the RF signal generation unit and the laser frequency shifting unit, thus enabling the achievement of higher feedforward bandwidth.

[0026] (3) This invention selects to characterize the time-frequency variation function of the swept laser in a digital form within a digital signal processor and performs separation operations on the nonlinear swept components. The time-frequency variation function of the swept laser, in addition to the non-ideal frequency jitter component with a certain amplitude, also has a linear frequency variation component with a much larger amplitude superimposed on it. This indicates that the dynamic range of the time-frequency variation function of the swept laser is extremely large, and the digital quantity can arbitrarily expand the numerical precision by increasing the number of effective bits, thus completely characterizing the function; in addition, the operation of digital signals does not generate any additional noise. In contrast, if the time-frequency variation function of the swept laser is characterized by an analog voltage signal, and the signal operation is performed by an analog circuit, the limited dynamic range of the analog signal (equal to the ratio of the maximum value to the noise level) and the existence of intrinsic noise in the analog operation circuit will inevitably lead to a decrease in system performance.

[0027] (4) The present invention decouples the light source part from the control part in terms of structure, which reduces the design difficulty of the drive circuit. At the same time, all the optical and electronic components required for the control part can be selected from conventional standard components in the fields of optical communication technology and electronic information technology, such as fiber optic couplers, acousto-optic frequency shifters, balanced photodetectors, digital-to-analog / analog-to-digital converters, field-programmable gate arrays (FPGAs), voltage-controlled oscillators (VCOs), electro-optic intensity modulators, etc., which has strong feasibility. Attached Figure Description

[0028] Figure 1This is a schematic diagram of the linear sweep frequency laser generator of the present invention;

[0029] Figure 2 This is a schematic diagram of the structure of a laser frequency discrimination unit according to an embodiment of the present invention;

[0030] Figure 3 This is a signal processing flowchart of a laser frequency discrimination unit according to an embodiment of the present invention;

[0031] Figure 4 This is a schematic diagram of the structure of a laser frequency shifting unit according to an embodiment of the present invention;

[0032] Figure 5 The graph shows the actual effect test data of the embodiment of the present invention. Detailed Implementation

[0033] The present invention will now be described in detail with reference to the accompanying drawings and specific embodiments. These embodiments are based on the technical solution of the present invention and provide detailed implementation methods and specific operating procedures. However, the scope of protection of the present invention is not limited to the following embodiments.

[0034] This embodiment provides a linear sweeping laser generator, such as... Figure 1 As shown, the system includes: a semiconductor laser 11, a driving circuit 12, a laser beam splitter 13, a laser frequency discriminator 14, an RF signal generation unit 15, and a laser frequency shifting unit 16. The output of the driving circuit 12 is connected to the input of the semiconductor laser 11; the optical output terminal of the semiconductor laser 11 is connected to the optical input terminal of the laser beam splitter 13; the two optical output terminals of the laser beam splitter 13 are respectively connected to the optical input terminals of the laser frequency discriminator 14 and the laser frequency shifting unit 16; the signal output terminal of the laser frequency discriminator 14 is connected to the signal input terminal of the RF signal generation unit 15; the RF output terminal of the RF signal generation unit 15 is connected to the RF input terminal of the laser frequency shifting unit 16; the optical output terminal of the laser frequency shifting unit 16 serves as the system's optical output terminal, outputting a linearly swept frequency laser.

[0035] Specifically, the structure and settings of each part are as follows:

[0036] (1) Semiconductor laser 11

[0037] The semiconductor laser 11 is controlled by the driving circuit 12 to generate a swept laser.

[0038] In this embodiment, the semiconductor laser 11 is a distributed feedback semiconductor laser with a nominal output linewidth of 600kHz. The laser has a drive current input terminal and a built-in thermistor and semiconductor cooler.

[0039] (2) Drive circuit 12

[0040] The driving circuit 12 generates a time-varying driving current to drive the semiconductor laser 11. The frequency of the laser output by the semiconductor laser 11 can be modulated by the driving current.

[0041] In this embodiment, the driving circuit 12 generates a sawtooth wave current, which is output to the driving current input terminal of the semiconductor laser 11. The amplitude and repetition period of the sawtooth wave current are determined based on the operating current range of the selected semiconductor laser and the desired frequency repetition frequency of the linear sweep laser. In this embodiment, the amplitude range of the sawtooth wave current is I. min =35mA to I max =120mA, based on the current frequency tuning coefficient C of the selected semiconductor laser. I =1.43GHz / mA, therefore the sweep frequency range of the sweep laser directly output by the laser is f. sw =(I max -I min ) / C I =122GHz. The repetition period of the sawtooth wave current is T. sw =2.0ms, corresponding to a sweep laser repetition frequency of 500Hz.

[0042] The driving circuit 12 also performs temperature control on the semiconductor laser 11. Based on the resistance value of the built-in thermistor of the laser, and with a fixed preset temperature as the target, it performs PID control on the built-in semiconductor cooler of the laser to achieve temperature control of the laser. The laser temperature determines the absolute frequency of its output laser. In this embodiment, the target temperature is selected as 32.0 degrees Celsius. When the selected laser operates at a temperature of 32.0 degrees Celsius and a driving current of 35 mA, the measured center wavelength of the output light is 1550.20 nm, corresponding to an optical frequency of ν0 = 193.390 THz. The above temperature control scheme is a conventional engineering technique and will not be described in detail further.

[0043] Based on the above description, the semiconductor laser 11 directly outputs a frequency-sweeping laser with a sweeping frequency range of 122 GHz and a scanning repetition frequency of 500 Hz, which can be considered as an approximately linear frequency-sweeping laser. Its average sweeping rate is K. sw =f sw / T sw = 61.1 THz / s. During the operation of the semiconductor laser, any single drive current value equal to I... min When the current is 35mA, let t=0 be the time. Then the frequency-time function ν of the swept laser is... in (t) can be characterized as:

[0044] ν in (t)=ν0+K sw t+f NL (t), 0≤t <T sw

[0045] In the formula, the term ν0 on the right represents the absolute frequency of the laser at the initial moment, K sw The t term represents the linear sweep frequency component, f NL The term (t) represents the nonlinear sweep frequency component. K sw t and f NL The terms (t) are all frequency-time functions.

[0046] (3) Laser beam splitter 13

[0047] The swept laser generated by the semiconductor laser 11 is split into two beams by the laser beam splitter 13, which enter the laser frequency discrimination unit 14 and the laser frequency shifting unit 16 respectively.

[0048] In this embodiment, a fiber optic coupler is used as the laser beam splitter to split the input light into two beams according to a fixed power ratio. In this embodiment, the splitting ratio of the selected fiber optic coupler is 95:5, that is, 95% of the optical output branch of the fiber optic coupler is connected to the laser frequency discriminator 14, and 5% of the optical output branch is connected to the laser frequency shifter 16.

[0049] (4) Laser frequency discrimination unit 14

[0050] The laser frequency discrimination unit 14 includes an optical input terminal and a control signal output terminal. The optical input terminal receives the frequency sweep laser. The unit internally acquires the time-frequency change of the frequency sweep laser in real time, separates the nonlinear frequency sweep component from the time-frequency change of the frequency sweep laser in real time, and generates a control signal based on the nonlinear frequency sweep component to control the radio frequency signal generation unit 15.

[0051] In this embodiment, the frequency-time variation of the swept laser is a frequency-time function of the swept laser; the nonlinear swept component is a frequency-time function of the sum of all components of the frequency-time variation of the swept laser except for the constant term and the first term.

[0052] In this embodiment, the laser frequency discrimination unit is selected as follows: Figure 2 The implementation scheme shown includes: a first fiber coupler 141, a delay fiber 142, an acousto-optic frequency shifter 143, a second fiber coupler 144, a balanced photodetector 145, an analog-to-digital converter 146, a field-programmable gate array (FPGA) 147, and a digital-to-analog converter 148.

[0053] The first fiber coupler 141, the delay fiber 142, the acousto-optic frequency shifter 143, the second fiber coupler 144, and the balanced photodetector 145 constitute the frequency discrimination optical path of the laser frequency discrimination unit 14; the analog-to-digital converter 146, the FPGA 147, and the digital-to-analog converter 148 constitute the frequency discrimination circuit of the laser frequency discrimination unit 14.

[0054] In this embodiment, the acquisition of the time-frequency change of the swept laser and the separation of the nonlinear swept components within the laser frequency discrimination unit are achieved by a digital signal processor (i.e., FPGA) through calculation; the calculation method is real-time calculation.

[0055] The first fiber coupler 141 has one input end and two output ends, with a splitting ratio of 50:50. In this embodiment, it is not necessary to distinguish between the two output ends of the first fiber coupler.

[0056] The length of the delay fiber 142 is l d =4.00m, the input end is connected to one of the output ends of the first fiber optic coupler 141.

[0057] The frequency shifting frequency of the acoustic-optical frequency shifter 143 is f aom =80.0MHz, the input is connected to the other output of the first fiber optic coupler 141.

[0058] The second fiber optic coupler 144 has two inputs and two outputs with a splitting ratio of 50:50. In this embodiment, it is not necessary to distinguish between the two inputs and two outputs of the second fiber optic coupler. One of its inputs is connected to the output of the delay fiber 142, and the other input is connected to the output of the acousto-optic frequency shifter 143.

[0059] The optical input end of the balanced photodetector 145 is connected to a pair (two) output ends of the second fiber optic coupler 144. Its analog bandwidth is 120MHz and the output coupling method is AC coupling.

[0060] The frequency discrimination optical path of the aforementioned laser frequency discrimination unit 14 constitutes a time-delayed self-heterodyne interferometer, and the voltage signal V output by the balanced photodetector 145... in (t) represents the beat frequency signal of the swept laser after passing through the interferometer, which can be characterized as:

[0061] V in (t)=V bpd (t)cos(2πf aom t+φ in (t))

[0062] In the formula V bpd The term (t) represents the envelope of the beat frequency signal, which can be regarded as collimated flux and its effect on the signal phase can be ignored; the 2πf in the cos(·) function aom The term t represents the center frequency of the beat frequency signal introduced by the acousto-optic frequency shifter; φ within the cos(·) function in The term (t) represents the phase shift of the beat frequency signal.

[0063] φ in The term (t) carries the instantaneous frequency information of the swept laser, and its formula is as follows:

[0064]

[0065] In the formula τ d =nl d / c0 = 20ns, where n is the refractive index of the fiber and c0 is the speed of light in vacuum. The right side of the equation is the sum of three terms, which, from left to right, represent: A. the initial phase of the beat frequency signal determined by the absolute frequency of the laser; B. the phase change of the beat frequency signal introduced by the linear sweep component; and C. the phase change of the beat frequency signal introduced by the nonlinear sweep component.

[0066] The above describes the frequency discrimination optical path of the laser frequency discrimination unit 14 in this embodiment.

[0067] The frequency discrimination circuit of the laser frequency discrimination unit 14 is described below.

[0068] The sampling rate of the analog-to-digital converter 146 is f s =250MS / s, 14-bit resolution, for the analog voltage signal V output by the balanced photodetector 145. in (t) is sampled and converted into a discrete sequence v[k]:

[0069] v in [k]=V(k·T s ), k = 0, 1, ..., T sw / T s

[0070] Where T s =1 / f s = 4.00ns, which is the sampling period of the digital signal.

[0071] The FPGA147 acts as a digital signal processor, based on the digital signal V. in [k], execute as follows Figure 3 The signal processing flow shown includes the following steps:

[0072] S1: Down-conversion mixer;

[0073] S2: Phase extraction and defolding;

[0074] S3: Generate linear sweep frequency components;

[0075] S4: Separate the nonlinear sweep frequency component;

[0076] S5: Construct the output signal.

[0077] The following describes each step of the signal processing flow.

[0078] S1: Down-conversion mixing. In this step, the digital signal processor internally generates a frequency that is strictly equal to f. aomThe cosine and sine signals are respectively compared with the input digital signal v. in After multiplying [k] and performing a low-pass filter, we obtain the down-conversion mixing signals i[k] and q[k], as shown in the following equation:

[0079]

[0080] Where LPF{·} represents the filtering operation, which is implemented by a digital low-pass filter. In this embodiment, the selected filter is a 64th-order FIR filter with a cutoff frequency of 12MHz. The multiplication operation is implemented by a digital multiplier. The sine / cosine signals used for mixing can be generated based on the ROM lookup table method or the Direct Digital Synthesis (DDS) method, both of which are classic methods in digital signal processing technology.

[0081] S2: Phase Extraction and Defolding. In this step, the down-conversion mixer signals i[k] and q[k] are first treated as the real and imaginary parts of a complex signal, respectively, and their principal argument signals are calculated. Then, the principal argument signals are defolded to obtain the foldless beat frequency phase signal φ[k], as shown in the following equation:

[0082] φ[k]=Unwrap{arctan(q[k] / i[k])}=φ in (k·T s )

[0083] Here, arctan(·) is the arctangent operation, which can be implemented by the CORDIC algorithm, and Unwrap{·} is the phase signal defolding operation; both the CORDIC algorithm and the phase defolding (unwrap) operation are classic methods in digital signal processing technology, and will not be described in detail here. The φ[k] obtained in this step is the φ that characterizes the phase shift of the beat frequency signal. in Discrete sampling of (t) in the digital domain.

[0084] S3: Generate linear sweep frequency components. In this step, the digital signal processor generates linear sweep frequency components based on a pre-loaded known constant K. sw τ d T s ν0 generates a linearly varying phase component φ L [k], as shown in the following formula:

[0085]

[0086] Since all constants in the formula are known, this step does not depend on the real-time input signal v[k]. In fact, φ L [k] represents the linear sweep component of the sweep laser in the real-time signal φ[k]. Since φ L[k] is a linear function, so it can be directly generated using a digital accumulator with a fixed increment.

[0087] S4: Separate the nonlinear sweep frequency component. In this step, first subtract the signal φ obtained in S3 from the signal φ[k] obtained in (S2). L [k], extract the nonlinear sweep frequency component corresponding to the sweep frequency laser in φ[k]; then normalize the result to obtain the nonlinear sweep frequency component f. NL [k], as shown in the following formula:

[0088]

[0089] The condition for the last step of the formula to approximate the relationship is: the equivalent linewidth of the swept laser is much smaller than τ. d The reciprocal of τ, in this embodiment 1 / τ d =50MHz meets this condition. This step completes the calculation of the nonlinear sweep frequency component, where the subtraction and multiplication operations can be implemented by digital subtractors and digital multipliers, respectively.

[0090] S5: Construct the output signal. In this step, the nonlinear sweep frequency component f is used. NL [k] Generate a control signal. Depending on the type of RF signal generation unit selected, the control signal can be an analog electrical signal or a digital electrical signal. In this embodiment, since the RF signal generation unit is a voltage-controlled oscillator (VCO), the control signal must be an analog signal V. out [k], as shown in the following formula:

[0091]

[0092] Where V0 is the center operating voltage of the VCO, k vco is the frequency tuning coefficient of the VCO.

[0093] The above description of the S5 step for constructing the output signal is merely illustrative of this example. In other embodiments, the control signal can be a digital signal. For example, in embodiments where a direct digital frequency synthesizer (DDS) is selected as the radio frequency signal generation unit, the control signal can be a digital signal carrying frequency tuning words (FTW).

[0094] The above describes the signal processing flow executed internally by the digital signal processor in this embodiment, corresponding to the digital logic master clock f. dsp =250MHz, equal to the analog-to-digital converter sampling rate f s .

[0095] It should be noted that in this embodiment, an FPGA is selected as the digital signal processor to implement the digital logic of the signal processing flow in a pipelined manner. Pipelining refers to a digital logic circuit where, for each input at any given moment, the corresponding calculation can be completed after several master clock cycles. In this embodiment, the input V... in [k] is in data stream form, therefore the intermediate signal φ[k] and the nonlinear sweep frequency component f represent the time-frequency variation of the swept laser. NL [k], and output V out [k] represents real-time data streams. Therefore, the calculation of the time-frequency variation of the swept laser and the nonlinear swept components in this example is highly real-time; there is no need to wait for a complete V segment. in After [k] completes the data collection, then start collecting data on φ[k] and f. NL [k]、V out [k] is calculated point by point in sequence.

[0096] The digital adders / subtractors, digital multipliers, FIR operations, DDS operations, CORDIC operations, and digital accumulators involved in the signal processing flow all have pipelined digital logic implementation methods based on FPGA. These are conventional engineering technologies in the field of digital signal processing, so they will not be described in detail.

[0097] The digital-to-analog converter 148 converts the digital control signal V provided by the FPGA 147 into digital control signal V. out [k], converted to analog signal V in real time out (t), which serves as the output of the laser frequency discrimination unit 14. In this embodiment, the digital-to-analog converter with a sampling rate of f is used. dac =250MS / s, resolution is 16 bits.

[0098] The above describes the frequency discrimination circuit of the laser frequency discrimination unit 14 in this embodiment, outputting signal V. out (t) is connected to the control signal input terminal of the radio frequency signal generation unit 15.

[0099] (5) Radio frequency signal generation unit 15

[0100] The radio frequency signal generation unit 15 includes a control signal input terminal and a radio frequency signal output terminal. Based on the input control signal, it generates a radio frequency signal with a frequency change opposite to the nonlinear sweep frequency component and outputs it to the laser frequency shifting unit 16 to drive the laser frequency shifting unit 16.

[0101] In this embodiment, the frequency-time function of the radio frequency signal is a fixed frequency constant minus the nonlinear sweep frequency component.

[0102] In this embodiment, the radio frequency signal generation unit 15 selects a low-noise narrowband VCO with a center operating voltage V0 = 5.0V, a center operating frequency f0 = 2.70GHz, and a frequency tuning coefficient k near the center operating voltage. vco =20.5MHz / V. Subject to control signal V out (t) During modulation, the frequency of its output radio frequency signal is:

[0103] f vco (t)=f0+k vco (V out (t)-V0)=f0-f NL (t)

[0104] That is, the frequency of the radio frequency signal output by the radio frequency signal generation unit carries a frequency change that is opposite to the nonlinear sweeping component of the sweeping laser.

[0105] (6) Laser frequency shifting unit 16

[0106] The laser frequency shifting unit 16 includes an optical input terminal, an radio frequency input terminal, and an optical output terminal. Based on the real-time frequency of the input radio frequency signal, it applies a frequency shift equal to the frequency of the input radio frequency signal to the input laser, thereby canceling the nonlinear frequency sweeping component of the frequency sweeping laser, and outputs a linear frequency sweeping laser through the optical output terminal.

[0107] The laser frequency shifting unit 16 in this embodiment is implemented as follows: Figure 4 As shown, it includes: QPSK-MZI electro-optic modulator 161, bias voltage output circuit 162, RF signal amplifier 163, and RF 90-degree bridge 164.

[0108] In this embodiment, the selected QPSK-MZI electro-optic modulator 161 has a radio frequency bandwidth of 10 GHz, and the input / output terminals of the electro-optic modulator are the input / output terminals of the laser frequency shifting unit 16; the QPSK-MZI electro-optic modulator 161 also includes three bias control input terminals and a pair (two) radio frequency modulation input terminals.

[0109] The bias voltage output circuit 162 generates three DC voltage signals, which are respectively input to the three bias control terminals of the QPSK-MZI electro-optic modulator 161, so that the QPSK-MZI electro-optic modulator 161 operates in the positive first-order single-sideband modulation mode.

[0110] The input terminal of the radio frequency signal amplifier 163 serves as the radio frequency signal input terminal of the laser frequency shifting unit 16, amplifying the power of the input radio frequency signal to reduce the insertion loss of the electro-optic modulator.

[0111] The input of the RF 90-degree bridge 164 is connected to the output of the RF signal amplifier 163, splitting the input RF signal into two orthogonal RF signals with a phase difference of 90 degrees (π / 2 radians) to match the positive first-order single-sideband modulation mode of the QPSK-MZI electro-optic modulator 161. In this configuration, the output of the QPSK-MZI electro-optic modulator 161 is a single-frequency laser with an optical frequency ν. out (t) is equal to the sum of the input optical frequency and a factor of one (positive two) of the radio frequency signal frequency, that is:

[0112] ν out (t)=ν in (t)+f vco (t)

[0113] =ν0+K sw t+f NL (t)+f0-f NL (t)

[0114] =ν0+f0+K sw t

[0115] According to the derivation in the formula, from ν in K in (t) sw t is retained; at the same time ν in f in (t) NL (t) Items and from f vco -f in (t) NL (t) Partial cancellation. This means that the nonlinear sweeping component of the frequency-varying laser directly output from the semiconductor laser 11 is compensated by the radio frequency signal generated by the feedforward branch formed by the laser frequency discriminator 14 and the radio frequency signal generation unit 15, which carries a frequency change opposite to the nonlinear sweeping component. This ultimately results in a highly linear sweeping laser. Furthermore, the fixed frequency offset f0 introduced by the center operating frequency of the radio frequency signal generation unit 15 is a known fixed value and is much smaller than the initial absolute laser frequency ν0; therefore, it does not affect the use of the linear sweeping laser generator.

[0116] For the linear sweep frequency laser generator of this embodiment of the invention Figure 5 The results curve of the output frequency sweep laser measured by the time-delay self-zero difference method is given; the horizontal axis of the curve is the laser frequency sweep frequency, and the vertical axis is the short-time frequency increment during the laser frequency sweep process, that is, the short-time frequency of the beat frequency signal of the time-delay self-zero difference method.

[0117] Figure 5(a) is the result curve of the delay self-zero difference test of the frequency sweep laser directly output by the semiconductor laser 11 in this embodiment. It can be seen that its short-time frequency increment is centered at about 66MHz, and the fluctuation range reaches ±10MHz, indicating that its frequency sweep linearity is still not ideal and cannot fully meet the requirements of some high-performance optical sensing and measurement systems for the frequency sweep linearity of the light source. Figure 5 (b) shows the result curve of the linear frequency-sweeping laser output in this embodiment. The short-time frequency increment is centered at 66.1MHz, but the fluctuation range is significantly reduced to within ±0.1MHz, an improvement of more than two orders of magnitude. The measurement results show that the present invention can effectively further improve the frequency-sweeping nonlinearity of the output frequency-sweeping laser from the semiconductor laser, resulting in a more ideal linear frequency-sweeping laser. This is to further improve the core performance of high-performance optical sensing and measurement systems such as optical frequency domain reflectometers, frequency-modulated continuous wave lidar, fiber optic grating sensors, and spectral measurement systems.

[0118] The preferred embodiments of the present invention have been described in detail above. It should be understood that those skilled in the art can make numerous modifications and variations based on the concept of the present invention without creative effort. Therefore, all technical solutions that can be obtained by those skilled in the art based on the concept of the present invention through logical analysis, reasoning, or limited experimentation on the basis of existing technology should be within the scope of protection defined by the claims.

Claims

1. A linear sweep laser generating apparatus characterized by comprising: include: Semiconductor laser, driving circuit, laser beam splitter, laser frequency discriminator, radio frequency signal generation unit, laser frequency shifting unit; The semiconductor laser is controlled by a driving circuit to generate a swept laser, which is then split into two beams by a laser beam splitter, and enters the laser frequency discrimination unit and the laser frequency shifting unit respectively. The laser frequency discrimination unit extracts the nonlinear sweeping component of the sweeping laser and generates a control signal based on the nonlinear sweeping component to control the radio frequency signal generation unit. The radio frequency signal generating unit generates a radio frequency signal whose frequency change is opposite to the nonlinear sweep frequency component according to the control signal, thereby driving the laser frequency shifting unit; The laser frequency shifting unit modulates the frequency of the radio frequency signal onto the input optical frequency, canceling the nonlinear frequency sweeping component of the frequency sweeping laser and outputting a linear frequency sweeping laser. The laser frequency discrimination unit includes an optical input terminal and a control signal output terminal. The optical input terminal receives the frequency sweeping laser. The laser frequency discrimination unit internally acquires the time-frequency change of the frequency sweeping laser in real time, separates the nonlinear frequency sweeping component from the time-frequency change of the frequency sweeping laser in real time, and outputs a control signal based on the nonlinear frequency sweeping component. The laser frequency shifting unit includes an optical input terminal, an radio frequency signal input terminal, and an optical output terminal. The laser frequency shifting unit applies a frequency shift equal to the frequency of the input radio frequency signal to the input sweeping laser and then outputs a linear sweeping laser through the optical output terminal.

2. The linear sweep laser generator of claim 1, wherein, The input to the semiconductor laser is a time-varying driving current generated by the driving circuit, and the output is a laser beam, the frequency of which is modulated by the driving current.

3. The linear sweep laser generator of claim 1, wherein, The frequency-time variation of the swept laser is a function of the frequency-time of the swept laser.

4. The linear sweep laser generator of claim 1, wherein, The nonlinear sweep frequency component is a frequency-time function that is the sum of all components of the sweep frequency laser time-frequency change, excluding the constant term and the first term.

5. A linear sweeping laser generator according to claim 1, characterized in that, Inside the laser frequency discrimination unit, the acquisition of the time-frequency change of the swept laser and the separation of the nonlinear swept components are achieved by a digital signal processor through calculation; the calculation method is real-time calculation.

6. The linear sweep laser generator of claim 1, wherein, The control signal is an analog electrical signal or a digital electrical signal.

7. The linear sweep laser generator of claim 1, wherein, The radio frequency signal generating unit includes a control signal input terminal and a radio frequency signal output terminal; the radio frequency signal generating unit outputs a radio frequency signal whose frequency changes with time according to the input control signal; the radio frequency signal is output to the laser frequency shifting unit.

8. The linear sweep laser generator of claim 1, wherein, The frequency-time function of the radio frequency signal is a fixed frequency constant minus the nonlinear sweep frequency component.

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