WO2024087891A1

WO2024087891A1 - Frequency modulated continuous wave radar and self-detection method for longitudinal mode state of light source

Info

Publication number: WO2024087891A1
Application number: PCT/CN2023/117011
Authority: WO
Inventors: 汪洋; 向少卿
Original assignee: 上海禾赛科技有限公司
Priority date: 2022-10-24
Filing date: 2023-09-05
Publication date: 2024-05-02
Also published as: CN117970357A

Abstract

Embodiments of the present invention provide a frequency modulated continuous wave radar and a self-detection method for a longitudinal mode state of a light source. The frequency modulated continuous wave radar comprises: a light source suitable for transmitting a frequency-modulated optical signal in a preset period, wherein at least one period is a self-detection period, and the frequency-modulated optical signal transmitted by the light source in the self-detection period is a self-detection signal; and a detection module suitable for detecting the self-detection signal so as to determine the longitudinal mode state of the light source. By using the solution, the detection of the longitudinal mode state of the light source of the frequency modulated continuous wave radar can be realized without an external device, so that the detection cost is reduced, and the detection efficiency is improved.

Description

FMCW radar and self-check method of longitudinal mode state of light source

Technical Field

The embodiments of the present invention relate to the field of radar technology, and in particular to a frequency modulated continuous wave radar and a method for self-checking the longitudinal mode state of a light source.

Background technique

Narrow linewidth tunable light source is a key component of laser radar, especially frequency modulated continuous wave (FMCW) laser radar. FMCW laser radar emits frequency modulated continuous laser as detection light. There is a certain frequency shift between the echo signal reflected by the obstacle and the corresponding detection light signal. By measuring the frequency shift, the distance and speed information of the obstacle can be obtained, thereby realizing space detection.

At present, the narrow linewidth light source commonly used in FMCW LiDAR is usually an external cavity laser (ECL). Among them, lasers with short external cavities are not easy to meet the linewidth requirements; while lasers with long external cavities have narrow linewidth, but small mode spacing, and are easily affected by changes in environmental conditions such as temperature to produce mode hopping, multi-longitudinal mode competition and other states. Among them, the longitudinal mode refers to the light wave mode that oscillates stably along the axis of the resonant cavity, and the linewidth refers to the frequency (or wavelength) range corresponding to when the light intensity drops to half of the maximum value. Multi-longitudinal modes refer to the existence of multiple relatively stable light wave modes in the resonant cavity. In the multi-longitudinal mode state, multi-longitudinal mode competition will occur, that is, the light source outputs lasers of multiple frequencies (or wavelengths), the detection unit cannot obtain normal beat frequency signals, and the FMCW LiDAR cannot operate normally. Therefore, it is necessary to detect the longitudinal mode state of the light source of the FMCW LiDAR.

However, the existing longitudinal mode state detection usually requires the use of external equipment such as FP scanning interferometer or spectrometer, which is not only expensive, but can only be detected when the laser radar is not working, and the detection efficiency is low. This detection method is not applicable to many laser radar application scenarios, such as vehicle-mounted laser radar systems. Therefore, it is necessary for those skilled in the art to realize the detection of the longitudinal mode state of the light source of the frequency modulated continuous wave radar without the help of external equipment.

Summary of the invention

In view of this, an embodiment of the present invention provides a frequency modulated continuous wave radar and a self-detection method for the longitudinal mode state of a light source, which can detect the longitudinal mode state of the light source of the frequency modulated continuous wave radar without the aid of external equipment, thereby reducing the detection cost and improving the detection efficiency.

An embodiment of the present invention provides a frequency modulated continuous wave radar, including:

A light source, adapted to emit a frequency-modulated light signal in a preset period, wherein at least one period is a self-test period, and the frequency-modulated light signal emitted by the light source in the self-test period is a self-test signal;

The detection module is adapted to detect the self-detection signal to determine the longitudinal mode state of the light source.

Optionally, the self-test signal is a fixed-frequency modulated optical signal.

Optionally, the modulation frequency of the fixed-frequency modulation makes the selfie signal frequency between at least two longitudinal modes of the light source smaller than the bandwidth of the detection module, and the selfie signal frequency between different sidebands of the same longitudinal mode greater than the bandwidth of the detection module.

Optionally, the frequency modulated continuous wave radar does not transmit an optical signal to the external space during the self-test period, and the detection module is suitable for completing the detection of the longitudinal mode state within one self-test period.

Optionally, the detection module is adapted to determine that the light source is in a multi-longitudinal mode state when a Selfie signal of the self-test signal is detected.

Optionally, the light source is adapted to emit a frequency modulated light signal at a preset period, wherein at least two adjacent periods are self-test periods;

The detection module is suitable for completing the detection of the longitudinal mode state within at least two adjacent self-detection cycles.

Optionally, the modulation frequencies of the self-test signals emitted in at least two adjacent self-test cycles are different.

Optionally, the detection module is adapted to determine that the light source is in a multi-longitudinal mode state when a Selfie signal is detected in at least two self-timer cycles and the frequencies of the Selfie signals in different self-timer cycles are different.

Optionally, the light source further has a detection period, and the frequency modulated light signal emitted by the light source during the detection period is a detection signal, which is suitable for detecting the external space of the frequency modulated continuous wave radar.

Optionally, the detection signal is a linear frequency modulated optical signal or a nonlinear frequency modulated optical signal.

Optionally, the light source comprises:

A laser emitting unit, adapted to emit laser light;

The modulation unit is suitable for modulating the laser to obtain a frequency modulated optical signal.

Optionally, the frequency modulated continuous wave radar further includes: a first light splitting unit, adapted to split the frequency modulated light signal into a local oscillator light signal and an outgoing signal, wherein the local oscillator light signal is transmitted to the detection module.

Optionally, the FMCW radar further comprises: an isolation unit, the isolation unit comprising at least Three ports, wherein the first port is suitable for receiving the outgoing signal; the second port allows the outgoing signal to be emitted to the external space and receives the echo signal of the outgoing signal reflected by an obstacle; and the third port is coupled to the detection module so that the echo signal is transmitted to the detection module.

Optionally, the detection module includes:

A detection unit, adapted to receive the local oscillator optical signal, or the local oscillator optical signal and the echo signal, and convert the optical signal into an electrical signal;

The processing unit is adapted to receive the electrical signal and obtain the longitudinal mode state of the light source based on the electrical signal.

Optionally, the FMCW radar further includes: a control module, adapted to trigger the processing unit to obtain the longitudinal mode state of the light source based on a self-check start instruction.

The embodiment of the present invention further provides a method for self-checking the longitudinal mode state of a light source, comprising:

The light source emits a frequency modulated light signal in a preset period, wherein at least one period is a self-test period, and the light source emits a self-test signal in the self-test period;

Acquiring the self-test signal;

The self-test signal is detected to determine the longitudinal mode state of the light source.

Optionally, the self-test signal is a fixed-frequency modulated optical signal.

Optionally, the detecting the self-test signal to determine the longitudinal mode state of the light source includes:

When a Selfie signal of the self-test signal is detected within the self-test period, it is determined that the light source is in a multi-longitudinal mode state.

Optionally, the frequency modulated continuous wave radar does not transmit light signals to the external space during the self-test period.

When a Selfie signal of the self-test signal is detected in at least two adjacent self-test cycles, it is determined that the light source is in a multi-longitudinal mode state.

Optionally, the modulation frequencies of the self-test signals of the at least two adjacent self-test cycles are different, wherein when the frequencies of the self-test signals detected in at least two adjacent self-test cycles are different, it is determined that the light source is in a multi-longitudinal mode state.

By using the frequency modulated continuous wave radar in the embodiment of the present invention, a light source emits at least one period of a frequency modulated light signal as a self-test signal, and a detection module detects the self-test signal. The longitudinal mode state of the light source can be accurately determined without the aid of any external equipment, thereby effectively reducing the detection cost. In addition, the detection process of the longitudinal mode state of the light source is performed during radar detection. Therefore, the longitudinal mode state of the light source can be quickly determined without affecting the normal operation of the radar, and the detection efficiency is high.

Furthermore, by setting the modulation frequency of the fixed-frequency modulation so that the frequency of the self-timer signal between at least two longitudinal modes of the light source is less than the bandwidth of the detection module, and the frequency of the self-timer signal between different sidebands of the same longitudinal mode is greater than the bandwidth of the detection module, on the one hand, the self-timer signal between the longitudinal modes of the light source can be frequency-reduced, that is, the frequency of the self-timer signal between multiple longitudinal modes of the light source is lower, so that the detection module can detect the self-timer signal between the longitudinal modes of the light source even when the bandwidth is small, and determine the longitudinal mode state of the light source based on the self-timer signal, thereby reducing the design difficulty and hardware cost of the radar; on the other hand, it can prevent the self-timer signal between different sidebands of the same longitudinal mode from interfering with the self-timer signal between multiple longitudinal modes of the light source, avoid misjudging different sidebands of the same longitudinal mode as multiple longitudinal modes, and thus obtain accurate detection results of the longitudinal mode state of the light source.

Furthermore, since the FMCW radar does not transmit optical signals to the external space during the self-test period, no echo signals reflected from the external space are generated during the detection process, which can effectively avoid interference signals during the detection process, thereby improving the accuracy of the detection result.

Furthermore, when the detection module detects the Selfie signal of the self-test signal, it can determine that the light source is in a multi-longitudinal mode state. Not only does it not require the aid of any external equipment, but the radar's own detection system can also be used to detect the longitudinal mode state of the light source without the need for hardware changes, thereby reducing the detection cost.

Furthermore, the light source is suitable for emitting a frequency-modulated light signal in a preset period, wherein at least two adjacent periods are self-test periods, and the detection module is suitable for completing the detection of the longitudinal mode state in at least two adjacent self-test periods. On the one hand, by comparing the Selfie signals obtained in the two self-test periods, the longitudinal mode state of the light source can be determined. On the other hand, since the detection module obtains the detection results of the longitudinal mode state of the light source in multiple self-test periods, it can avoid interference caused by reflected echoes from external space obstacles and improve the accuracy of detection.

Furthermore, by setting the modulation frequencies of the self-test signals emitted in at least two adjacent self-test cycles to be different, it is possible to accurately determine whether the self-test signal is a self-test signal between longitudinal modes, thereby further improving the detection efficiency. The accuracy of the test results.

Furthermore, when the detection module detects a Selfie signal in at least two self-check cycles and the Selfie signal frequencies in different self-check cycles are different, it can be determined that the light source is in a multi-longitudinal mode state, thereby avoiding interference caused by reflection echoes from external space obstacles and improving the accuracy of the detection result. The entire detection process does not require the use of any external equipment, and the detection of the longitudinal mode state of the light source can be achieved using the radar's own detection system, without the need for hardware changes, so the detection cost can be reduced.

Furthermore, the light source also has a detection cycle, and the frequency-modulated light signal emitted by the light source during the detection cycle is a detection signal, which is suitable for detecting the external space of the frequency-modulated continuous wave radar. Therefore, the longitudinal mode state of the light source can be detected at any time without affecting the normal operation of the radar, and the longitudinal mode state of the radar light source can be monitored in real time.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to more clearly illustrate the embodiments of the present application or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are merely embodiments of the present application. For ordinary technicians in this field, other drawings can be obtained based on the provided drawings without paying any creative work.

FIG1 shows a schematic structural diagram of a frequency modulated continuous wave radar in an embodiment of the present invention;

FIG2a is a schematic diagram showing an emission signal of a single longitudinal mode light source in an embodiment of the present invention;

FIG2b shows the modulation spectrum of the transmission signal shown in FIG2a;

FIG2c is a schematic diagram showing a transmission signal of a multi-longitudinal mode light source in an embodiment of the present invention;

FIG2d shows a modulation spectrum of the transmission signal shown in FIG2c according to an embodiment of the present invention;

FIG3a is a schematic diagram showing the spectrum of an outgoing signal and an echo signal of a fixed-frequency modulated optical signal in an embodiment of the present invention;

FIG3 b shows a schematic diagram of the spectrum of an outgoing signal and an echo signal of another fixed-frequency modulated optical signal in an embodiment of the present invention;

FIG4 shows a timing diagram of longitudinal mode state detection of a light source based on a single-cycle self-test signal in an embodiment of the present invention;

FIG5 shows a timing diagram of longitudinal mode state detection of a light source based on a multi-cycle self-test signal in an embodiment of the present invention;

6a to 6c are schematic diagrams showing the spectrum of the detection signal in the embodiment of the present invention;

FIG7 shows a schematic diagram of the structure of another FMCW radar in an embodiment of the present invention;

FIG8 shows a schematic diagram of a specific structure of a frequency modulated continuous wave radar in an embodiment of the present invention;

FIG. 9 is a schematic diagram showing the steps of a method for self-checking the longitudinal mode state of a light source of a FMCW radar according to an embodiment of the present invention.

Detailed ways

As described in the background technology, the narrow linewidth light source commonly used in current frequency modulated continuous wave lidar is usually an external cavity laser, among which the laser with a short external cavity is not easy to meet the linewidth requirements; and although the linewidth of the laser with a long external cavity is narrow, the mode spacing is small, and it is easy to produce mode hopping, multi-longitudinal mode competition and other conditions due to changes in environmental conditions such as temperature. The detection unit cannot obtain the normal beat frequency signal, and the frequency modulated continuous wave lidar cannot operate normally.

When the light source of the FMCW radar is in a multi-longitudinal mode state, the laser output by the light source contains multiple longitudinal modes (frequencies or wavelengths). If the response bandwidth of the detection unit is large enough, the selfie signal between the longitudinal modes of the light source can be detected. The lowest selfie signal frequency is the longitudinal mode interval. Taking the FMCW radar using an external cavity laser ECL with a cavity length of 30 mm as an example, its longitudinal mode interval is about 3.3 GHz. If the selfie signal between its longitudinal modes is to be obtained, the response bandwidth of the FMCW radar detection unit is at least 3.3 GHz, and the sampling rate of the analog-to-digital converter is at least 6.6 GS/s. However, such a high-frequency detection circuit and its supporting components are difficult to design and costly, and are difficult to implement in laser radar. Therefore, the existing FMCW radar usually needs to use external equipment such as FP (Fabry–Pérot) scanning interferometer or spectrometer to detect the longitudinal mode state, which has high detection cost and low efficiency.

In view of the above problems, an embodiment of the present invention provides a frequency modulated continuous wave radar, which transmits at least one period of frequency modulated light signal as a self-test signal through a light source, and detects the self-test signal through a detection module. The longitudinal mode state of the light source can be accurately determined without the aid of any external equipment, thereby effectively reducing the detection cost. In addition, the detection process of the longitudinal mode state of the light source is carried out during radar detection, so the longitudinal mode state of the light source can be quickly judged without affecting the normal operation of the radar, and the detection efficiency is high.

In order to enable those skilled in the art to better understand and implement the embodiments of the present invention, the concepts, schemes, principles and advantages of the embodiments of the present invention are described in detail below with reference to the accompanying drawings and through specific application examples.

First, an embodiment of the present invention provides a frequency modulated continuous wave radar. Referring to a schematic diagram of a frequency modulated continuous wave radar structure shown in FIG. 1 , the frequency modulated continuous wave radar LA includes: a light source A1 and a detection module A2, wherein:

The light source A1 is suitable for emitting a frequency-modulated light signal in a preset period, wherein at least one period is a self-test period, and the frequency-modulated light signal emitted by the light source A1 in the self-test period is a self-test signal.

The detection module A2 is adapted to detect the self-detection signal to determine the longitudinal mode state of the light source.

The frequency modulated continuous wave radar provided in the embodiment of the present invention transmits a self-test signal within a self-test period. When the light source is in a single longitudinal mode state and a multi-longitudinal mode state, the detection module detects the self-test signal and can obtain different detection information, thereby determining the longitudinal mode state of the light source.

By adopting the above-mentioned frequency modulated continuous wave radar LA, the light source A1 emits at least one period of frequency modulated light signal as a self-test signal, and the self-test signal is detected by the detection module A2. The longitudinal mode state of the light source A1 can be accurately determined without the aid of any external equipment, thereby effectively reducing the detection cost. In addition, the detection process of the longitudinal mode state of the light source A1 is carried out during the detection period of the radar LA. Therefore, the longitudinal mode state of the light source A1 can be quickly determined without affecting the normal operation of the radar LA, and the detection efficiency is high.

In order to enable those skilled in the art to better understand and implement the embodiments of the present invention, some specific examples are given below for the specific implementation method of determining the longitudinal mode state of the light source in the embodiments of the present invention.

In order to avoid the interference of the modulation signal itself on the detection result, the self-test signal can be modulated at a fixed frequency, so that a single-frequency longitudinal mode Selfie signal can be obtained, so that the frequency modulated continuous wave radar can obtain accurate detection results of the longitudinal mode state of the light source.

It is understandable that the embodiment of the present invention does not limit the self-test signal to a fixed-frequency modulated optical signal. If the frequency interval between the longitudinal mode sidebands of the light source is small, the self-test signal may also be a non-fixed-frequency modulated optical signal.

In a specific implementation, when the self-test signal is modulated at a fixed frequency, the modulation frequency of the fixed frequency modulation can be set to meet the following conditions: the self-test signal frequency between at least two longitudinal modes of the light source A1 is less than the bandwidth of the detection module A2, and the self-test signal frequency between different sidebands of the same longitudinal mode is greater than the bandwidth of the detection module A2.

By adopting the above-mentioned embodiment, on the one hand, the Selfie signal between the longitudinal modes of the light source can be frequency-reduced, that is, the frequency of the self-timer signal between the multiple longitudinal modes of the light source is relatively low, so that the detection module can detect the self-timer signal between the longitudinal modes of the light source even when the bandwidth is small, and determine the longitudinal mode state of the light source based on the self-timer signal, thereby reducing the design difficulty and hardware cost of the radar; on the other hand, it can prevent the self-timer signal between different sidebands of the same longitudinal mode from interfering with the self-timer signal between the multiple longitudinal modes of the light source, avoid misjudging different sidebands of the same longitudinal mode as multiple longitudinal modes, thereby obtaining accurate detection results of the longitudinal mode state of the light source.

As a specific example, referring to Figures 2a to 2d, Figure 2a shows a schematic diagram of a transmission signal of a single longitudinal mode light source (unmodulated), wherein the single longitudinal mode light source transmits a longitudinal mode n with a light frequency of ω _n,0 , and Figure 2b shows a modulation spectrum of the transmission signal shown in Figure 2a. As shown in Figure 2b, the transmission signal generates sidebands such as ω _n,-1 , ω _n,+1 , and ω _n,+2 after modulation. If the modulation frequency of the fixed frequency modulation is appropriate, the frequency of the self-timer signal between different sidebands of the longitudinal mode n can be large, which is not within the response bandwidth of the detection module of the frequency modulated continuous wave radar, and the detection module cannot obtain the self-timer signal between different sidebands of the longitudinal mode n.

Referring to the schematic diagram of the transmission signal of a multi-longitudinal mode light source (unmodulated) shown in FIG2c, the multi-longitudinal mode light source transmits two longitudinal modes n and m with optical frequencies of ωn _,0 and ωm _,0 respectively, and FIG2d shows the modulation spectrum of the transmission signal shown in FIG2c. As shown in FIG2d, each longitudinal mode generates multi-order sidebands after modulation. If the modulation frequency of the fixed frequency modulation is appropriate, the frequencies of the different sidebands of the longitudinal modes n and m can be close, so that the beat signal frequency between the longitudinal modes n and m can enter the response bandwidth of the detection module of the frequency modulated continuous wave radar. As shown in FIG2d, the carrier ωm _,0 of the longitudinal mode m and the +1-order sideband ωn _,+1 of the longitudinal mode n.

The following specifically describes the method for determining the modulation frequency. Based on the different characteristics of the modulation results of the signals emitted by the above-mentioned different mode light sources, it can be assumed that the light source A1 is in a multi-longitudinal mode state, and its longitudinal mode interval is ω ₀ . Take any two longitudinal modes n and m, the optical frequencies are ω _n and ω _m respectively, and set their modulation frequency to Ω(t). The sideband frequencies of the longitudinal modes n and m after modulation are as follows:

K-order sideband of longitudinal mode n: ω _n,K = ω _n + KΩ(t) = nω ₀ + KΩ(t) (1)

G-order sideband of longitudinal mode m: ω _m,G = ω _m + GΩ(t) = mω ₀ + GΩ(t) (2)

Where K and G are arbitrary integers. The selfie signal frequency between longitudinal modes n and m is as follows:
Δω(t)＝| _ωn - _ωm +(KG)Ω(t)|＝|(nm) _ω0 +(KG)Ω(t)| (3)

It can be seen from formula (3) that when the modulation frequency Ω(t) is not a constant, if and only if K=G, Δω(t) is a constant, and its minimum non-zero value is ω ₀ , and there is no frequency reduction effect.

Therefore, in order to obtain a single-frequency self-timer signal between longitudinal modes and avoid interference from the modulation signal, the modulation frequency can be fixed: Ω(t) = Ω ₀ . The response bandwidth of the FMCW radar detection module A2 is B. To ensure that the detection module A2 can obtain the self-timer signal between longitudinal modes of the light source A1, that is, Δω(t) < B, Ω ₀ needs to meet the following conditions:

On the other hand, to prevent interference between self-timer signals of different sidebands of the same longitudinal mode, the frequency of the self-timer signal between the sidebands (ie, Ω ₀ ) needs to be greater than the response bandwidth of the detection module A2:
Ω ₀ >B (5)

Combining equations (4) and (5), the range of Ω ₀ can be determined as follows:

From the above, it can be seen that the frequency modulated light signal obtained by modulating the light emitted by the light source with a frequency within the range of formula (6) can be used as a self-test signal, satisfying that the self-test signal frequency between at least two longitudinal modes is less than the bandwidth of the detection module, and the self-test signal frequency between different sidebands of the same longitudinal mode is greater than the bandwidth of the detection module. If there are multiple longitudinal modes in the light emitted by the light source, the frequency difference between different sidebands of the multiple longitudinal modes falls within the response bandwidth of the detection module, that is, the detection module of the frequency modulated continuous wave radar can detect the self-test signal of the self-test signal, and at this time it can be determined that the light source is in a multi-longitudinal mode state. On the contrary, if the self-test signal of the self-test signal is not detected, it can be determined that the light source is in a single longitudinal mode state.

In a specific implementation, in order to avoid interference signals during the detection process, the FMCW radar LA may be configured not to emit optical signals to the external space during the self-test period, and the detection module A2 completes the detection of the longitudinal mode state within one self-test period.

According to the above embodiment, since the FMCW radar does not transmit optical signals to the external space during the self-test period, no echo signals reflected from the external space are generated during the detection process, which can effectively avoid interference signals during the detection process, thereby improving the accuracy of the detection result.

As a specific example, refer to the frequency spectrum diagram of the output signal and the echo signal of the fixed-frequency modulated optical signal shown in Figures 3a and 3b. As shown in Figure 3a, the solid line represents the fixed-frequency modulated optical signal emitted by the frequency-modulated continuous wave radar to the external space, and the dotted line represents the echo signal of the fixed-frequency modulated optical signal reflected by an obstacle at a certain distance. The distance of the obstacle only causes a time delay of the optical signal, but does not cause a frequency change. The beat signal between the output signal and the echo signal is zero. Therefore, the fixed-frequency modulated optical signal cannot detect the time delay caused by the distance. However, as shown in Figure 3b, the solid line represents the fixed-frequency modulated optical signal emitted by the frequency-modulated continuous wave radar to the external space, and the dotted line represents the echo signal generated by the fixed-frequency modulated optical signal reflected by a moving obstacle at a certain distance. It can be seen that the echo signal reflected by the moving obstacle produces a Doppler frequency shift relative to the emitted fixed-frequency modulated optical signal, and the beat signal frequencies of the two are also within the response bandwidth of the detection module. The beat signal of the emitted fixed frequency modulated light signal and the echo signal can be detected. Therefore, if the detection module A2 detects the echo signal of the moving obstacle while detecting the self-test signal, it will interfere with the detection and cause the misjudgment of the longitudinal mode state of the light source.

In order to eliminate the interference of the echo signal of the moving obstacle on the detection process, the channel for the frequency modulated continuous wave radar LA to transmit the light signal to the external space can be closed to ensure that the frequency modulated continuous wave radar LA does not transmit the light signal to the external space during the self-test period.

In some embodiments of the present invention, the channel through which the FMCW radar LA transmits light signals to the external space may also be blocked, so that the FMCW radar LA cannot transmit light signals to the external space during the self-test period.

It is understandable that the embodiment of the present invention does not limit the specific closing or shielding method, as long as the FMCW radar LA does not transmit light signals to the external space during the self-test period.

In a specific implementation, the detection module A2 can determine that the light source is in a multi-longitudinal mode state when detecting the self-timer signal of the self-test signal. Specifically, the frequency modulated continuous wave radar does not transmit light signals to the external space during the self-test period, and there is no beat signal interference of the echo signal. The self-timer signal detected by the detection module corresponds to the self-timer signal between the sidebands of multiple longitudinal modes of the light source. At this time, the multi-longitudinal mode state of the light source can be determined according to the detection results within one self-test period. Not only does it not require the use of any external equipment, but the detection system of the radar itself can also be used to detect the longitudinal mode state of the light source without the need for hardware changes, so the detection cost can be reduced.

As a specific example, referring to a timing diagram of a longitudinal mode state detection of a light source based on a single-cycle self-test signal shown in FIG4 , as shown in FIG4 , the detection period N is a self-test period, the light source A1 transmits a self-test signal in the detection period N, and the frequency modulated continuous wave radar LA does not transmit a light signal to the external space, which is represented by the light emission state OFF. If the self-timer signal is detected in the detection period N, it can be determined that the light source is in a multi-longitudinal mode state.

Specifically, taking 1/2 cycle as the frequency detection segment, each cycle includes two frequency detection segments, and the beat signal frequency detected by each frequency detection segment forms a frequency group. The beat signal frequency group corresponding to each 1/2 cycle is recorded as {f _k } _i , (k=1,2), i represents the i-th detection cycle, i=..., N-1, N, N+1... Then the detection cycle N includes two frequency groups {f ₁ } _N , {f ₂ } _N . If an obvious self-beating signal appears in the detection cycle N, the frequencies in the beat signal frequency groups {f ₁ } _N , {f ₂ } _N detected by the detection module A2 are the same, and it is determined that the light source is in a multi-longitudinal mode state. Otherwise, it is determined that the light source is in a single longitudinal mode state.

In some embodiments of the present invention, in order to improve the accuracy of detection, the light The source A1 emits a self-test signal of at least two adjacent cycles, and the detection module A2 needs to complete the detection of the longitudinal mode state within at least two adjacent self-test cycles.

By adopting the above embodiment, on the one hand, the longitudinal mode state of the light source can be determined by comparing the self-timer signals obtained in two self-test cycles; on the other hand, since the detection module A2 obtains the detection results of the longitudinal mode state of the light source in multiple self-test cycles, it can avoid the interference caused by the reflected echo from external space obstacles and improve the accuracy of detection.

In a specific implementation, in order to further improve the accuracy of the detection result, the modulation frequencies of the self-test signals emitted in at least two adjacent self-test cycles may be set to be different.

By adopting the above embodiment, it is possible to accurately determine whether the selfie signal is a selfie signal between longitudinal modes, thereby further improving the accuracy of the detection result.

As a specific example, since the self-timer signal frequency between longitudinal modes is related to the modulation frequency, if the modulation frequencies of the self-timer signals emitted in different self-test cycles are different, the self-timer signal frequencies between longitudinal modes detected in different cycles are also different. However, the Doppler frequency shift generated by a moving obstacle is, on the one hand, not related to the modulation frequency; on the other hand, it is limited by acceleration in a short time (μs level), so it can be considered that the speed of the obstacle is almost unchanged, so the Doppler frequency shift generated by the moving obstacle is also almost unchanged. In other words, the Doppler frequency shift detected in different self-test cycles is the same. From the above, it can be seen that by detecting whether the beat signal frequencies obtained in different self-test cycles are the same, it can be accurately judged whether the beat signal is the Doppler frequency shift generated by the moving obstacle or the Selfie signal between longitudinal modes, thereby improving the accuracy of the detection result.

In a specific implementation, the detection module A2 can determine that the light source is in a multi-longitudinal mode state when it detects a self-timer signal in at least two self-check cycles and the frequencies of the self-timer signals in different self-check cycles are different, thereby avoiding interference caused by reflection echoes from external space obstacles and improving the accuracy of the detection results. The entire detection process does not require the use of any external equipment, and the detection of the longitudinal mode state of the light source can be achieved using the radar's own detection system without hardware changes, thereby reducing the detection cost; further, there is no need to control the frequency modulated continuous wave radar LA not to emit light signals to the external space during the self-check cycle, thereby reducing the complexity of detection.

As a specific example, referring to a timing diagram of longitudinal mode state detection of a light source based on a multi-cycle self-test signal as shown in FIG5 , as shown in FIG5 , two consecutive detection cycles N and detection cycle N+1 are self-test cycles, and the light source A1 emits a self-test signal in the detection cycle N and the detection cycle N+1, and And emit light signals to the external space, and the light emission state is ON. The self-test signals emitted in detection cycle N and detection cycle N+1 are fixed-frequency modulated, and the modulation frequencies are different. If self-timer signals are detected in detection cycle N and detection cycle N+1, and the Selfie signal frequencies are different, it can be determined that the light source is in a multi-longitudinal mode state.

Specifically, taking 1/2 cycle as the frequency detection segment, each cycle includes two frequency detection segments, and the beat signal frequency detected by each frequency detection segment forms a frequency group. The beat signal frequency group corresponding to each 1/2 cycle is recorded as {f _k } _i , (k＝1,2), i represents the i-th detection cycle, i＝..., N-1, N, N+1... Then the detection period N includes two frequency groups {f ₁ } _N and {f ₂ } _N , and the detection period N+1 includes two frequency groups {f ₁ } _N+1 and {f ₂ } _N+1 . If the frequencies in the beat signal frequency groups {f ₁ } _N and {f ₂ } _N are the same, the frequencies in the beat signal frequency groups {f ₁ } _N+1 and {f ₂ } _N+1 are the same, and the frequencies between {f ₁ } _N and {f ₂ } _N and {f ₁ } _N+1 and {f ₂ } _N+1 are different, it is determined that the light source is in a multi-longitudinal mode state; otherwise, it is determined that the light source is in a single longitudinal mode state.

In a specific implementation, if the frequencies in the beat signal frequency groups {f ₁ } _N , {f ₂ } _N and {f ₁ } _N+1 , {f ₂ } _N+1 are the same, it can be determined that the same frequency is the Doppler frequency shift caused by the moving obstacle.

It should be noted that in the embodiments shown in Figures 4 and 5, 1/2 of the detection period is used as the frequency statistical duration in the frequency group. In specific applications, the beat signal frequency detected within a detection period can be statistically calculated using other durations to form a frequency group. The embodiments of the present invention do not make specific limitations on this.

In some embodiments of the present invention, the light source A1 may also have a detection period, and the frequency modulated light signal emitted by the light source A1 during the detection period is a detection signal, which is suitable for detecting the external space of the frequency modulated continuous wave radar LA. Therefore, the frequency modulated continuous wave radar LA can detect the longitudinal mode state of the light source A1 at any time without affecting the normal operation of the radar, and monitor the longitudinal mode state of the radar light source A1 in real time.

In a specific implementation, the detection signal may be a linear frequency modulated optical signal or a nonlinear frequency modulated optical signal.

As a specific example, refer to the schematic diagrams of the detection signal spectrum shown in Figures 6a to 6c, in which the solid line represents the curve of the frequency change of the optical signal emitted by the frequency modulated continuous wave radar over time, and the dotted line represents the curve of the frequency change of the echo signal over time. The detection signal can be a linear frequency modulated optical signal, as shown in Figures 6a and 6b; wherein the linear frequency modulated optical signal shown in Figure 6a includes two adjacent frequency sweeping segments, namely an upper frequency sweeping segment whose frequency increases over time and a lower frequency sweeping segment whose frequency decreases over time, and the linear frequency modulated optical signal shown in Figure 6b includes an upper frequency sweeping segment, a non-frequency sweeping segment (the frequency of the optical signal does not change over time) and a lower frequency sweeping segment. In other embodiments, the detection signal can also be a nonlinear frequency modulated optical signal, that is, the frequency of the optical signal changes nonlinearly with time. Linear change, as shown in Figure 6c.

It is understandable that the present invention does not limit the modulation form of the detection signal, as long as it can detect the distance and speed of the obstacle.

In some embodiments of the present invention, referring to another FMCW radar structure schematic diagram shown in FIG. 7 , the FMCW radar LA1 includes: a light source B1 and a detection module B2, wherein the light source B1 includes: a laser emitting unit B11 and a modulation unit B12, wherein:

The laser emitting unit B11 is suitable for emitting laser.

The modulation unit B12 is suitable for modulating the laser to obtain a frequency modulated optical signal.

The detection module B2 is suitable for detecting the longitudinal mode state of the light source.

The frequency modulated continuous wave radar LA1 in the above embodiment is used, and at least one cycle of laser is emitted by the laser emitting unit B11, and the laser is modulated by the modulation unit B12, and the obtained frequency modulated light signal is used as a self-test signal. The detection module B2 obtains the longitudinal mode state of the light source B1 based on the self-timer signal of the self-test signal without the help of any external device. The detection of the longitudinal mode state of the light source is shared with the detection system of the frequency modulated continuous wave radar itself for obstacle detection, without any hardware modification, which can effectively reduce the detection cost; in addition, the detection process of the longitudinal mode state of the light source B1 is carried out during the radar detection period, so the longitudinal mode state of the light source can be quickly judged without affecting the normal operation of the radar, and the detection efficiency is high.

In a specific implementation, continuing to refer to FIG. 7 , the FMCW radar LA1 may further include: a first light splitting unit B3 adapted to split the FMC light signal into a local oscillator light signal and an outgoing signal, wherein the local oscillator light signal is transmitted to the detection module B2 .

In some other embodiments of the present invention, continuing to refer to Figure 7, the frequency modulated continuous wave radar LA1 may also include: an isolation unit B4, the isolation unit B4 includes at least three ports, wherein the first port is suitable for receiving the outgoing signal; the second port allows the outgoing signal to be emitted to the external space, and receives the echo signal of the outgoing signal reflected by an obstacle; the third port is coupled to the detection module B2 so that the echo signal is transmitted to the detection module B2.

The detection module B2 includes: a detection unit B21 and a processing unit B22, wherein:

The detection unit B21 is adapted to receive the local oscillator optical signal, or the local oscillator optical signal and the echo signal, and convert the optical signal into an electrical signal.

The processing unit B22 is adapted to receive the electrical signal and obtain the longitudinal mode state of the light source based on the electrical signal.

In some embodiments of the present invention, the frequency modulated continuous wave radar does not emit an outgoing signal to the external space during the self-test period, and therefore does not receive an echo signal during the self-test period. At this time, the detection unit B21 receives the local oscillation light signal of the self-test signal transmitted by the isolation unit B4, obtains the Selfie signal of the local oscillation light signal, and converts it into an electrical signal. The processing unit B22 detects the longitudinal mode state of the light source based on the electrical signal.

In other embodiments of the present invention, the frequency modulated continuous wave radar transmits an outgoing signal to the external space during the self-test period, so an echo signal reflected by an obstacle may be received during the self-test period. At this time, the detection unit B21 receives the local oscillator light signal and the echo signal transmitted by the isolation unit B4, obtains the beat signal, and converts it into an electrical signal. The processing unit B22 detects the longitudinal mode state of the light source based on the electrical signal.

In a specific implementation, continuing to refer to FIG. 7 , the FMCW radar LA1 may further include: a control module B5 adapted to trigger the processing unit B22 to obtain the longitudinal mode state of the light source B1 based on a self-check start instruction.

In order to enable those skilled in the art to better understand and implement the FMCW radar in the embodiment of the present invention, a specific FMCW radar structure is provided below.

In combination with FIG. 7 and referring to a specific structural diagram of a frequency modulated continuous wave radar shown in FIG. 8 , as shown in FIG. 8 , the frequency modulated continuous wave radar LA2 includes: a light source C1, a detection module C2, a first light splitting unit C3 and an isolation unit C4, and a control module C5; the light source C1 includes: a laser C11 and a modulator C12, wherein:

The laser C11 is suitable for emitting laser. In some embodiments of the present invention, the laser C11 is an external cavity laser.

The modulator C12 is coupled to the laser C11 and is suitable for modulating the laser to obtain a frequency modulated optical signal.

The first light splitting unit C3 is coupled to the modulator C12, and is adapted to split the frequency modulated light signal into a local oscillator light signal and an output signal, wherein the local oscillator light signal is transmitted to the detection module C2.

As a specific example, the first light splitting unit C3 may be a coupler.

As a specific example, the isolation unit C4 may be a circulator, which includes three Ports, wherein the first port C41 is coupled to the first splitter unit C3 and is suitable for receiving the outgoing signal; the second port C42 allows the outgoing signal to be emitted to the external space and receives the echo signal of the outgoing signal reflected by an obstacle; the third port C43 is coupled to the detection module C2 so that the echo signal is transmitted to the detection module C2.

In some embodiments of the present invention, the isolation unit C4 may also be a polarization beam splitter.

The detection module C2 includes: a detection unit and a processing unit, wherein:

The detection unit is adapted to receive the local oscillator optical signal, or the local oscillator optical signal and the echo signal, and convert the optical signal into an electrical signal.

As a specific example, the detection unit includes a second light splitting unit C21 and a photodetector. In a specific implementation, the second light splitting unit C21 can be a coupler. The photodetector includes a detector C22 and a detector C23. The second light splitting unit C21 divides the received optical signal into two paths, which are transmitted to the detectors C22 and C23 respectively. The detectors C22 and C23 convert the optical signal into an electrical signal respectively, and transmit it to the processing unit.

As a specific example, the processing unit is a signal processing unit (Signal Processing Unit, SPU).

As a specific example, the control module C5 may be a main processor (MPU), which is adapted to trigger the processing unit to obtain the longitudinal mode state of the external cavity laser B11 based on a self-check start instruction. At the same time, the control module C5 is also adapted to control the light source C1 to emit a self-check signal during the self-check cycle.

The frequency modulated continuous wave radar in the above embodiment is used, and at least one cycle of laser is emitted by the laser C11, and the laser is modulated by the modulator C12, and the obtained frequency modulated light signal is used as the self-test signal. The self-test signal is divided into a local oscillator light signal and an outgoing signal through the coupler C3, and the local oscillator light signal and/or the echo signal of the local oscillator light signal and the outgoing signal reflected by the obstacle are transmitted to the detection module C2 by the isolation unit C4, wherein the coupler C21 performs light splitting and transmits them to the detector C22 and the detector C23 respectively, and the two detectors generate electrical signals with the same DC and opposite AC phases, and the processing unit C24 receives and processes the electrical signals to obtain the longitudinal mode state of the laser C11. Not only does it not require any external equipment, but the detection of the longitudinal mode state of the light source is also consistent with the self-test signal of the frequency modulated continuous wave radar. It can be shared with the detection system used for obstacle detection without any hardware modification, which can effectively reduce the detection cost. In addition, the detection process of the longitudinal mode state of laser C11 is carried out during radar detection, so the longitudinal mode state of laser C11 can be quickly judged without affecting the normal operation of the radar, and the detection efficiency is high.

The embodiment of the present invention further provides a method for self-checking the longitudinal mode state of a light source of a frequency modulated continuous wave radar. Referring to the schematic diagram of the steps of the method for self-checking the longitudinal mode state of a light source of a frequency modulated continuous wave radar shown in FIG. 9 , the following steps can be used to perform self-checking the longitudinal mode state of the light source:

In step A, the light source emits a frequency-modulated light signal in a preset period, wherein at least one period is a self-test period, and the light source emits a self-test signal in the self-test period.

Step B, obtaining the self-test signal.

Step C: detecting the self-test signal to determine the longitudinal mode state of the light source.

By adopting the above-mentioned self-detection method for the longitudinal mode state of the light source, by controlling the light source to emit at least one period of frequency-modulated light signal as a self-detection signal, and detecting the self-detection signal, the longitudinal mode state of the light source can be accurately determined without the aid of any external equipment, thereby effectively reducing the detection cost; in addition, the detection process of the longitudinal mode state of the light source is carried out during radar detection, so the longitudinal mode state of the light source can be quickly judged without affecting the normal operation of the radar, and the detection efficiency is high.

In a specific implementation, in order to avoid interference of the modulation signal itself on the detection result, the self-test signal can be modulated at a fixed frequency, so as to obtain a single-frequency longitudinal mode self-timer signal, so that the frequency modulated continuous wave radar can obtain accurate detection results of the longitudinal mode state of the light source.

In a specific implementation, when the self-test signal is modulated at a fixed frequency, the modulation frequency of the fixed frequency modulation can be set to meet the following conditions: the frequency of the Selfie signal between at least two longitudinal modes of the light source is less than the bandwidth of the detection module, and the frequency of the self-timer signal between different sidebands of the same longitudinal mode is greater than the bandwidth of the detection module. The specific implementation example of determining the modulation frequency of the fixed frequency modulation can be referred to the specific example of the frequency modulated continuous wave radar mentioned above, which will not be repeated here.

By adopting the above embodiment, on the one hand, the Selfie signal between the longitudinal modes of the light source can be frequency-reduced, that is, the frequency of the self-timer signal between the multiple longitudinal modes of the light source is low, so that the detection module can detect the self-timer signal between the longitudinal modes of the light source even when the bandwidth is small, and determine the longitudinal mode state of the light source based on the Selfie signal. Thereby reducing the design difficulty and hardware cost of the radar; on the other hand, it can prevent the selfie signals between different sidebands of the same longitudinal mode from interfering with the selfie signals between multiple longitudinal modes of the light source, and avoid misjudging different sidebands of the same longitudinal mode as multiple longitudinal modes, thereby obtaining accurate detection results of the longitudinal mode state of the light source.

In a specific implementation, when the Selfie signal of the self-test signal is detected within the self-test cycle, it can be determined that the light source is in a multi-longitudinal mode state. Specifically, if there are multiple longitudinal modes in the light emitted by the light source, the frequency difference between different sidebands of the multiple longitudinal modes falls within the response bandwidth of the detection module, that is, the detection module of the frequency modulated continuous wave radar can detect the self-timer signal of the self-test signal. At this time, it can be determined that the light source is in a multi-longitudinal mode state. Conversely, if the self-timer signal of the self-test signal is not detected, it can be determined that the light source is in a single longitudinal mode state. Not only does it not require the aid of any external equipment, but the detection system of the radar itself can also be used to detect the longitudinal mode state of the light source without the need for hardware changes, so the detection cost can be reduced.

In a specific implementation, in order to avoid interference signals during the detection process, the frequency modulated continuous wave radar can be set not to transmit light signals to the external space during the self-test period, and the detection module completes the detection of the longitudinal mode state within one self-test period. The specific implementation example of setting the frequency modulated continuous wave radar not to transmit light signals to the external space during the self-test period can be referred to the specific example of the frequency modulated continuous wave radar mentioned above, which will not be repeated here.

In some embodiments of the present invention, when a Selfie signal of the self-test signal is detected within at least two adjacent self-test cycles, it can be determined that the light source is in a multi-longitudinal mode state.

By adopting the above-mentioned embodiment, on the one hand, the longitudinal mode state of the light source can be determined by comparing the self-timer signals obtained in two adjacent self-test cycles; on the other hand, since the detection results of the longitudinal mode state of the light source are obtained in multiple self-test cycles, the interference caused by the reflected echo from external space obstacles can be avoided, thereby improving the accuracy of detection.

In a specific implementation, in order to further improve the accuracy of the detection results, the modulation frequencies of the self-test signals emitted in at least two adjacent self-test cycles can be set to be different. When the frequencies of the self-test signals detected in at least two adjacent self-test cycles are different, it can be determined that the light source is in a multi-longitudinal mode state.

By adopting the above embodiment, it is possible to accurately determine whether the Selfie signal is a Selfie signal between longitudinal modes, thereby further improving the accuracy of the detection result. In addition, since the entire detection process does not require the aid of Any external device is not required, and the detection system of the radar itself can be used to detect the longitudinal mode state of the light source without hardware modification, so the detection cost can be reduced; further, there is no need to control the frequency modulated continuous wave radar not to emit light signals to the external space during the self-test period, thereby reducing the detection complexity.

Although the embodiments of the present invention are disclosed above, the present invention is not limited thereto. Any person skilled in the art may make various changes and modifications without departing from the spirit and scope of the present invention. Therefore, the protection scope of the present invention shall be subject to the scope defined by the claims.

Claims

A frequency modulated continuous wave radar, characterized in that it comprises:

A light source, adapted to emit a frequency-modulated light signal in a preset period, wherein at least one period is a self-test period, and the frequency-modulated light signal emitted by the light source in the self-test period is a self-test signal;

The detection module is adapted to detect the self-detection signal to determine the longitudinal mode state of the light source.
The frequency modulated continuous wave radar according to claim 1, characterized in that the self-test signal is a fixed frequency modulated optical signal.
The frequency modulated continuous wave radar according to claim 2 is characterized in that the modulation frequency of the fixed frequency modulation makes the selfie signal frequency between at least two longitudinal modes of the light source less than the bandwidth of the detection module, and the selfie signal frequency between different sidebands of the same longitudinal mode is greater than the bandwidth of the detection module.
The FMCW radar according to claim 1 is characterized in that the FMCW radar does not transmit optical signals to the external space during the self-test period, and the detection module is suitable for completing the detection of the longitudinal mode state within one self-test period.
The FMCW radar according to claim 4 is characterized in that the detection module is adapted to determine that the light source is in a multi-longitudinal mode state when a Selfie signal of the self-test signal is detected.
The frequency modulated continuous wave radar according to claim 1, characterized in that the light source is suitable for emitting frequency modulated light signals at a preset period, wherein at least two adjacent periods are self-test periods;

The detection module is suitable for completing the detection of the longitudinal mode state within at least two adjacent self-detection cycles.
The FMCW radar according to claim 6 is characterized in that the modulation frequencies of the self-test signals emitted in at least two adjacent self-test cycles are different.
The frequency modulated continuous wave radar according to claim 7 is characterized in that the detection module is suitable for detecting a Selfie signal in at least two self-test cycles, and when the frequencies of the Selfie signals in different self-test cycles are different, determining that the light source is in a multi-longitudinal mode state.
The FMCW radar according to claim 1 is characterized in that the light source further has a detection period, and the FMCW light signal emitted by the light source during the detection period is a detection signal, which is suitable for detecting the external space of the FMCW radar.
The frequency modulated continuous wave radar according to claim 9, characterized in that the detection signal It is a linear frequency modulated optical signal or a nonlinear frequency modulated optical signal.
The FMCW radar according to claim 1, wherein the light source comprises:

A laser emitting unit, adapted to emit laser light;

The modulation unit is suitable for modulating the laser to obtain a frequency modulated optical signal.
The frequency modulated continuous wave radar according to claim 1 is characterized by further comprising: a first splitting unit, adapted to split the frequency modulated light signal into a local oscillator light signal and an outgoing signal, wherein the local oscillator light signal is transmitted to the detection module.
The FMCW radar according to claim 12 is characterized in that it also includes: an isolation unit, the isolation unit including at least three ports, wherein the first port is suitable for receiving the outgoing signal; the second port allows the outgoing signal to be emitted to the external space and receives an echo signal of the outgoing signal reflected by an obstacle; and the third port is coupled to the detection module so that the echo signal is transmitted to the detection module.
The FMCW radar according to claim 13, wherein the detection module comprises:

A detection unit, adapted to receive the local oscillator optical signal, or the local oscillator optical signal and the echo signal, and convert the optical signal into an electrical signal;

The processing unit is adapted to receive the electrical signal and obtain the longitudinal mode state of the light source based on the electrical signal.
The FMCW radar according to claim 14 is characterized by further comprising: a control module, adapted to trigger the processing unit to obtain the longitudinal mode state of the light source based on a self-test start instruction.
A method for self-checking the longitudinal mode state of a light source of a frequency modulated continuous wave radar, characterized by comprising:

The light source emits a frequency modulated light signal in a preset period, wherein at least one period is a self-test period, and the light source emits a self-test signal in the self-test period;

Acquiring the self-test signal;

The self-test signal is detected to determine the longitudinal mode state of the light source.
The self-test method according to claim 16 is characterized in that the self-test signal is a fixed-frequency modulated optical signal.
The self-test method according to claim 17 is characterized in that the modulation frequency of the fixed-frequency modulation makes the self-timer signal frequency between at least two longitudinal modes of the light source smaller than the bandwidth of the detection module, and the Selfie signal frequency between different sidebands of the same longitudinal mode greater than the bandwidth of the detection module.
The self-test method according to claim 18, characterized in that the detecting the self-test signal to determine the longitudinal mode state of the light source comprises:

When a Selfie signal of the self-test signal is detected within the self-test period, it is determined that the light source is in a multi-longitudinal mode state.
The self-test method according to claim 19 is characterized in that the frequency modulated continuous wave radar does not emit light signals to the external space during the self-test period.
The self-test method according to claim 18, characterized in that the detecting the self-test signal to determine the longitudinal mode state of the light source comprises:

When a Selfie signal of the self-test signal is detected in at least two adjacent self-test cycles, it is determined that the light source is in a multi-longitudinal mode state.
The self-test method according to claim 21 is characterized in that the modulation frequencies of the self-test signals of at least two adjacent self-test cycles are different, wherein when the frequencies of the self-test signals detected in at least two adjacent self-test cycles are different, it is determined that the light source is in a multi-longitudinal mode state.