Patent Application
20230273323
The present disclosed technique relates to a laser radar device and a wind measurement method using the laser radar device.
A laser radar device called a light detection and ranging (LiDAR) device has been used for a long time in the fields of geology and meteorology, and in recent years, attracts attention also, for example, in the field of automatic driving of an automobile.
An operation principle of the laser radar device is to obtain information on a distance to a target and a speed of the target by irradiating the target with laser light and analyzing the light that has hit the target and bounced back. As indicated by the name, the laser radar device may be considered as a radar device using laser light. When it is desired to obtain a distance to the target, it is possible to obtain the distance by measuring time (time of flight, hereinafter referred to as “TOF”) from when the laser light is emitted to when the laser light is reflected and returned, and multiplying a speed of the laser light by the TOF. When it is desired to obtain a speed of the target, it is possible to obtain the speed by analyzing a frequency of the reflected light, obtaining a Doppler frequency, and using a relationship between the frequency of the emitted laser light and the Doppler frequency.
In a case of an application of simultaneously obtaining the distance to the target and the speed of the target, the TOF and the Doppler frequency are to be simultaneously obtained as described above. In order to directly obtain these, the laser radar device needs a sensor corresponding to two states of time and frequency. Regarding this, a technique for converting TOF into information on frequency using a frequency modulated continuous wave (hereinafter, referred to as “FMCW”) is disclosed. In particular, Non-Patent Literature 1 discloses a technique in which a frequency difference between transmitted light and received light caused by a distance to a target and a frequency difference between the transmitted light and the received light caused by a speed of the target are separately obtained by combining an up-chirp FMCW and a down-chirp FMCW (
Non-Patent Literature 1: D. Pierrottet et al. “Flight test performance of a high precision navigation Doppler Lidar”, Proceedings of SPIE Vol. 7323, 732311 (2009)
In the technique disclosed in Non-Patent Literature 1, the frequency of transmitted light constantly changes, and information on up-chirp and down-chirp is used for analysis. Non-Patent Literature 1 discloses the technique as an application of navigation of a lunar surface exploration mobile body, and a problem does not occur for such a target at a certain distance (referred to as a hard target).
However, in a case of wind measurement, usually, there is aerosol at a plurality of places in an irradiation direction of laser light, and light scattered by the aerosol at the plurality of places is received in an overlapping manner. Therefore, a waveform of received light is complicated. In an application of wind measurement, the laser radar device is required to have a mechanism for separating frequency information resulting from a distance to a target and frequency information resulting from a speed of the target by a simpler method.
An object of the present disclosed technique is to solve the above problem, and to provide a laser radar device including a mechanism for separating frequency information resulting from a distance to a target and frequency information resulting from a speed of the target by a simple method.
A laser radar device according to the present disclosed technique includes: a Light source to oscillate laser light in a continuous-wave manner or a quasi-continuous- wave manner; a frequency modulator to apply frequency modulation to the laser light oscillated by the Light source; a beam splitter to split the laser light modulated by the frequency modulator into transmission light and local oscillator light; a transmitting and receiving optics system to transmit the transmission light and to receive light reflected from a target; a receiver to receive the received light and the local oscillator light received by the transmitting and receiving optics system and to convert each of the received light and the local oscillator light into an electrical signal; and a receiving circuit to process the electrical signal converted by the receiver and to calculate distance information and speed information of the target, wherein the frequency modulation applied by the frequency modulator is modulation in which a stepwise change in which a frequency increases or decreases by a frequency difference F for each time width T is performed for at least one step, and the receiving circuit divides a frequency difference between the local oscillator light and the received light by the frequency difference F, and determines a frequency difference corresponding to a remainder or a shortage as a Doppler frequency.
Since the laser radar device according to the present disclosed technique has the above configuration, the frequency of emitted laser light has discrete values at intervals of the frequency difference F. Therefore, TOF corresponding to a distance to a target appears in units of the frequency difference F for each time T, and a Doppler frequency corresponding to a speed of the target appears as a finer change than the frequency difference F. As described above, the present disclosed technique provides a laser radar device including a mechanism for separating frequency information resulting from a distance to a target and frequency information resulting from a speed of the target by a simple method.
Embodiments according to the present disclosed technique will be apparent by the following description with reference to the drawings.
As the Light source 10, a laser light source having a narrow line width is used. A light source at a wavelength band of 1.5 μm is preferable from the viewpoint of eye safety. Note that there is no problem if the Light source 10 is in other wavelength bands. The narrower the line width of the laser light is, the better it is. However, the narrower the line width of the laser light is, the more expensive the laser light tends to be. The line width of the laser light is appropriately determined by design. The Light source 10 is preferably a light source in a continuous-wave manner, that is, a continuous wave (CW) light source, but may be a light source in a quasi-continuous-wave manner, that is, a quasi-CW (QCW)-operated light source in consideration of a case where an intermittent operation is performed due to a safety problem and the like.
Either the frequency modulator 20 or the phase modulator 20B may be used, but in this case, frequency modulation is applied to laser light emitted by the Light source 10. For the sake of simplicity, the description of the first embodiment will be given on the premise of using the frequency modulator 20. In addition, the frequency modulator 20 is controlled by the control signal circuit 200. Note that a reason why either the frequency modulator 20 or the phase modulator 20B may be used is that a relational formula represented by the following mathematical formula (1) is established.
dθ/dt=2πf (1)
in which θ represents a phase, f represents a frequency, π represents the circular constant, and t represents time.
The first beam splitter 30 splits the laser light modulated by the frequency modulator 20 into transmission light and local oscillator light. Usually, a beam splitter having an asymmetric branching ratio such as 9:1 is often used because the local oscillator light does not require so strong power. Note that the first beam splitter 30 may be a 5:5 beam splitter. In general, an optical fiber may be used as a beam splitter used in a laser radar device, and the constituent member may be referred to as another name such as an optical fiber coupler.
The amplifier 40 amplifies the transmission light out of the laser light split by the first beam splitter 30. The amplifier 40 may have a one-stage configuration or a multistage configuration. A fiber amplifier is often used for the amplifier 40, but an optical amplifier other than the fiber amplifier may be used. In a case of the multistage configuration, a fiber amplifier and an optical amplifier other than the fiber amplifier may be combined. In addition, in the case of the multistage configuration, a filter or an isolator for preventing return light may be inserted between an amplifier and an amplifier in order to remove amplified spontaneous emission (hereinafter, referred to as “ASE”).
The transmission light amplified by the amplifier 40 is sent to the transmitting and receiving optics system 60 via the circulator 50. In general, a circulator refers to a device that transmits light input from an optical path A to an optical path B, but outputs light input from the optical path B not from the optical path A but from another optical path C. In addition, a circulator in a narrow sense refers to a circulator using a Faraday rotator, but the circulator 50 may be simply a combination of a polarization beam splitter and a ¼ wavelength plate.
The transmitting and receiving optics system 60 adjusts a beam diameter of the transmission light, emits the transmission light into the atmosphere, and collects received light scattered by and returned from aerosol in the atmosphere. The transmitting and receiving optics system 60 includes a telescope and a beam expander. In addition, the transmitting and receiving optics system 60 may use a scanner in order to be able to observe a plurality of lines of sight.
The second beam splitter 70 interferes with the received light collected by the transmitting and receiving optics system 60 and the local oscillator light, and transmits two beams of interference light obtained by the interference to the balanced detector 80.
The balanced detector 80 includes two light receiving units, converts beams of light incident on the respective light receiving units into electrical signals, and outputs a difference between the two converted electrical signals. The balanced detector 80 may be an integrated light receiving element or may separately include a circuit that obtains a difference between photocurrents obtained from two photodiodes. The balanced detector 80 functions as a receiver, and therefore may be simply referred to as a receiver.
A role of the second beam splitter 70 is to obtain a frequency difference obtained by subtracting the frequency of the received light from the frequency of the local oscillator light. That is, this configuration utilizes a property that a beat of a difference between frequencies of two beams of light occurs as amplitude modulation when two beams of light having close frequencies interfere with each other. The balanced detector 80 is used to remove a DC component. Here, there is no problem if the frequency difference obtained by subtracting the frequency of the received light from the frequency of the local oscillator light is obtained by obtaining frequency components of the local oscillator light and the received light and then obtaining the frequency difference. In addition, since the frequency of the local oscillator light at each time is known, the frequency difference may be calculated simply by obtaining the frequency component of the received light in a time window in which the frequency of the local oscillator light is constant.
Note that the following paragraphs are based on the premise that the second beam splitter 70 and the balanced detector 80 are included.
The receiving circuit 210 processes the electrical signal converted by the balanced detector 80, and calculates distance information and speed information of a target. More specifically, the receiving circuit 210 processes the difference signal output from the balanced detector 80 and calculates wind speed information for each distance. The receiving circuit 210 includes an electric filter, an amplifier, an analog-digital converter, a computer, and the like.
An unnecessary frequency of the difference signal output from balanced detector 80 is removed by the pre-filter 300, and then amplified by the amplifier 310. The amplified signal is distributed for each frequency domain by the frequency filter 320 and transmitted to the receivers 331 to 333 depending on the frequency domain. In general, since an amplifier has band characteristics, the amplifier 310 may also function as the pre-filter 300. In addition, since a band is limited after the frequency filter 320, the amplifier 310 may be disposed after the frequency filter 320. In this case, a common filter may be used as the pre-filter 300 and the frequency filter 320, and the same number of amplifiers as the number of receivers are required. In the configuration example illustrated in
An operation of the laser radar device and a wind measurement method according to the first embodiment will be clarified by the following description with reference to
The frequency difference F in the stepwise change is sufficiently larger than the Doppler frequency due to movement of aerosol in the air. The Doppler frequency fd due to movement of aerosol in the air is represented by the following mathematical formula (2), in which the frequency of light is represented by ν0, the Doppler frequency is represented by fd, the wind speed is represented by V, and the light speed is represented by c.
f
d=−2ν0V/c (2)
Note that, as for the distance, a direction in which it goes away from the laser radar device serving as an origin is defined as a positive direction. In this case, also as for the speed, a direction in which it goes away from the laser radar device is considered as a positive speed. In this case, the wind speed has a positive value at the time of a tailwind, and has a negative value at the time of a headwind. The Doppler frequency fd may have either a positive value or a negative value corresponding to this. For example, when the frequency of light is 2 [THz] (≈wavelength 1.5 [μm]), the Doppler frequency fd is −1.33 [MHz] per 1 [m/s] of wind speed. Therefore, it can be said that the Doppler frequency fd when the frequency difference F is 100 [MHz] as illustrated in
An effect of the present disclosed technique will be clarified by the following specific example.
Transmission light of the laser radar device having the frequency characteristics illustrated in
For example, light scattered from aerosol at a position of 600 [m] from the laser radar device is received after a lapse of 4 [μs], which is time during which the light flies for a distance of 1200 [m] in going and returning ways. This is because the light speed is 300,000 kilometers per second. The local oscillator light and the received light received by the balanced detector 80 have frequency characteristics illustrated in
Here, the light scattered from the aerosol at the position of 600 [m] is received in a time-shifted manner after a lapse of TOF of 4 [μs]. Since the laser radar device according to the present disclosed technique uses stepwise frequency modulation, the information on the TOF is converted into information of a frequency difference that the frequency of the received light is lower than that of the local oscillator light by 200 [MHz]. Furthermore, since the received light is influenced by a Doppler shift, a relationship between the frequency of the received light and the frequency of the local oscillator light can be represented by the following mathematical formula (3).
(Frequency of received light)=(Frequency of local oscillator light)−200 [MHz]−fd (3)
The influence of the Doppler shift will be described by the following specific example. When the frequency of light is 2 [THz] (≈wavelength 1.5 [μm]), the Doppler frequency fd is −1.33 [MHz] per 1 [m/s] of wind speed. Therefore, when the frequency of the received light is lower than that of the local oscillator light by 202.66 [MHz], it can be seen that a wind condition at the distance of 600 [m] is +2 [m/s], that is, 2 [m/s] of a tailwind.
As described above, the laser radar device according to the present disclosed technique divides the frequency difference between the local oscillator light and the received light by the frequency difference F, and determines a frequency difference corresponding to a remainder as the Doppler frequency fd. In the specific example described above, the present disclosed technique divides the frequency difference 202.66 [MHz] between the local oscillator light and the received light by the frequency difference F=100 [MHz], and determines 2.66 [MHz] corresponding to a remainder as the Doppler frequency fd.
When the frequency difference between the received light and the local oscillator light is 197.34 [MHz], a remainder obtained by dividing the frequency difference by the frequency difference F=100 [MHz] is not considered as 97.34 [MHz]. A reference frequency is considered as 200 [MHz] close to 197.34 [MHz], that is, it is considered that the frequency is 2.66 [MHz] short, and it is considered that a wind condition at the distance of 600 [m] is −2 [m/s], that is, 2 [m/s] of a headwind.
In the present disclosed technique, there is no particular problem even if the TOF is not exactly an integral multiple of the time width T. The following specific example clarifies a way of thinking when the TOF is not exactly an integral multiple of the time width T.
Consider a case where the TOF is 3.5 [μs]. Similarly, when the received light is observed in a time window of 12 [μs] to 14 [μs] in which the frequency of the local oscillator light is 400 [MHz], the frequency of the received light is 200 [MHz] in a section of 12 [μs] to 13.5 [μs], and 300 [MHz] in a section of 13.5 [μs] to 14 [μs]. That is, the frequency difference F is 200 [MHz] in a section of 12 [μs] to 13.5 [μs], and 100 [MHz] in a section of 13.5 [μs] to 14 [μs].
Also consider a case where the TOF is 4.5 [μs]. Similarly, when the received light is observed in a time window of 12 [μs] to 14 [μs] in which the frequency of the local oscillator light is 400 [MHz], the frequency of the received light is 100 [MHz] in a section of 12 [μs] to 12.5 [μs], and 200 [MHz] in a section of 12.5 [μs] to 14 [μs]. That is, the frequency difference F is 300 [MHz] in a section of 12 [μs] to 12.5 [μs], and 200 [MHz] in a section of 12.5 [μs] to 14 [μs].
As described above, according to the present disclosed technique, the received light is observed in a time window in which the frequency of the local oscillator light is constant, and the TOF can be obtained from information on the two frequencies of the observed received light and information on a timing at which the frequency of the received light is switched. The information on the frequency of the received light is obtained by Fourier transform by the computer 340.
The laser radar device may obtain a wind condition for each certain distance range (hereinafter, referred to as “range”). In the first embodiment, the frequency filter 320 and the receivers 331 to 333 correspond thereto.
The following is a specific example of the frequency filter 320 according to the first embodiment. For example, the frequency filter 320 includes three types of band pass filters. Specifically, the frequency filter 320 includes a band pass filter of 50 to 150 [MHz], a band pass filter of 150 to 250 [MHz], and a band pass filter of 250 to 350 [MHz]. In addition, the frequency filter 320 is disposed so as to send frequency components of 50 to 150 MHz to the receiver 331, frequency components of 150 to 250 MHz to the receiver 332, and frequency components of 250 to 350 MHz to the receiver 333.
When the frequency filter 320 and the receivers 331 to 333 are included as described above, information on a wind condition near a distance of 300 [m] can be measured by the receiver 331, information on a wind condition near a distance of 600 [m] can be measured by the receiver 332, and information on a wind condition near a distance of 900 [m] can be measured by the receiver 333. The computer 340 can calculate a wind condition in each range by performing Fourier transform on an electrical signal measured by each of the receivers and obtaining a peak frequency.
A signal including a frequency component of 50 MHz or less corresponding to a distance of zero includes a signal generated by reception of light scattered by the optical system until the light is emitted into the atmosphere from the circulator 50 through the transmitting and receiving optics system 60 by the balanced detector 80. Such scattered light generated inside the device is generally stronger than scattered light generated by aerosol in the atmosphere. In order to solve this problem, by inclusion of the pre-filter 300 that prevents saturation of the amplifier 310, scattered light generated inside the device can be removed. As a result, SN in the receiving circuit 210 can be improved.
The laser radar device according to the first embodiment has the above configuration, and therefore includes a mechanism for separating frequency information resulting from a distance to a target and frequency information resulting from a speed of the target by a simple method.
In the first embodiment, the configuration in which frequency modulation is performed on laser light and then the laser light is split into transmission light and local oscillator light has been described, but the present disclosed technique is not limited to this configuration. A laser radar device according to a second embodiment splits laser light into transmission light and local oscillator light and then performs frequency modulation or phase modulation. The same reference signs are used for components common to those in the first embodiment, and redundant description is omitted.
Laser light emitted from a Light source 10 is split into transmission light and local oscillator light by the first beam splitter 30. The transmission light is transmitted to the first frequency modulator 21, and the local oscillator light is transmitted to the second frequency modulator 22, and frequency modulation is applied to each of the transmission light and the local oscillator light. Although description has been given in the first embodiment, a phase modulator may be used instead of the frequency modulator. That is, phase modulation may be applied to the transmission light and the local oscillator light by a first phase modulator 21B and a second phase modulator 22B, respectively. Both the first frequency modulator 21 and the second frequency modulator 22 are controlled by a control signal circuit 200, and perform modulation in synchronization. When the first phase modulator 21B and the second phase modulator 22B are used, similarly, the first phase modulator 21B and the second phase modulator 22B perform modulation in synchronization.
Advantages of using separate modulators for the transmission light and the local oscillator light will be clarified by the following specific example. As a specific example, the first phase modulator 21B and the second phase modulator 22B are used. Phase modulation applied by the first phase modulator 21B is represented by θ(t), and phase modulation applied by the second phase modulator 22B is represented by φ(t). If the same phase modulation is applied to the transmission light and the local oscillator light as in the first embodiment, θ(t) and φ(t) satisfy the following mathematical formula (4).
φ(t)=θ(t) (4)
The laser radar device according to the second embodiment applies modulation in which time is shifted between the phase modulation θ(t) of the transmission light and the phase modulation φ(t) of the local oscillator light. A relationship between θ(t) and φ(t) satisfies the following mathematical formula (5).
φ(t+t1)=θ(t) (5)
Formula (5) means that the phase modulation θ of the transmission light is delayed by time t1 as compared with the phase modulation φ of the local oscillator light.
Here, it is assumed that the phase modulation θ of the transmission light is phase modulation corresponding to frequency modulation as illustrated in
Similarly to the first embodiment, the frequency filter 320 includes a band pass filter of 50 to 150 [MHz], a band pass filter of 150 to 250 [MHz], and a band pass filter of 250 to 350 [MHz]. In addition, the band pass filters are arranged so as to send frequency components of 50 to 150 MHz to a receiver 331, frequency components of 150 to 250 MHz to a receiver 332, and frequency components of 250 to 350 MHz to a receiver 333. As a result, information on a wind condition near a distance of 150 [m] can be measured by the receiver 331, information on a wind condition near a distance of 450 [m] can be measured by the receiver 332, and information on a wind condition near a distance of 750 [m] can be measured by the receiver 333.
It is difficult to observe a range including a distance of zero because noise is large due to scattered light generated inside the device as described above. However, according to the laser radar device according to the second embodiment, a wind condition in a range close to a distance of zero can also be measured. The laser radar device according to the second embodiment exhibits the above effect in addition to the effect described in the first embodiment. Note that the positions of the first frequency modulator 21 and the second frequency modulator 24 are not limited to the positions illustrated in
A configuration of a laser radar device according to a third embodiment is similar to that of the second embodiment except for a receiving circuit 210. The same reference signs are used for components common to those in the first or second embodiment, and redundant description is omitted.
An operation of the laser radar device according to the third embodiment will be clarified by the following specific example. A second phase modulator 22B included in the laser radar device according to the third embodiment applies phase modulation φ(t) represented by the following formula (6).
φ(t−t2)=θ(t) (6)
Note that formula (6) is different from formula (5) of the second embodiment in positive and negative of the shift time. Formula (6) means that the phase modulation θ of the transmission light advances by time t2 as compared with the phase modulation φ of the local oscillator light.
Here, it is also assumed that the phase modulation θ of the transmission light is phase modulation corresponding to frequency modulation as illustrated in
The pre-filter 300 of the laser radar device according to the third embodiment is a band pass filter of 50 to 150 [MHz], and the receiver 330 is included at a later stage of the pre-filter 300. In this specific example, the receiver 330 can measure a range of 600 [m]. A range to be measured can be changed by changing t2. The laser radar device according to the third embodiment intensively measures one variable range.
In
In general, in light interference, a negative frequency and a positive frequency are measured as the same frequency whose phases are reversed in a processing system in which an interference fringe is measured by a balanced detector and an electrical signal is processed. Therefore, strong scattered light having a distance of zero is mixed in information on a range desired to be measured. In order to prevent this, by applying an offset to a frequency added to the local oscillator light or the received light, it is possible to prevent mixing of information on another range due to folding back of a negative frequency as described above. That is, by obtaining the following formula (7) by adding a term of an offset frequency to the left side of formula (6), it is possible to prevent a negative frequency from being folded back.
φ(t−t2)+2πfot=θ(t) (7)
in which fo represents an offset frequency. Applying an offset to the frequency is none other than movement of a plot of the local oscillator light or a plot of the received light in the vertical axis direction in
With the above configuration, the laser radar device according to the third embodiment can measure a plurality of ranges by switching a delay time of a modulation frequency with one receiver. In addition, as a range has a longer distance, measurement at a higher frequency is necessary, and with such a configuration, the laser radar device according to the third embodiment can also be expected to lower the measurement frequency. In general, design of a circuit is more difficult as a frequency is higher, but the present disclosed technique can perform similar measurement by lowering the frequency, and can be expected to reduce cost.
Fourth embodiment.
The specific examples described in the first to third embodiments have clarified a detection principle of wind conditions at the points of 300 [m], which is a half of a distance for which light moves with a time width T=2 [μs], and 600 [m], 900 [m], . . . , which are multiples of the distance. A laser radar device according to a fourth embodiment can easily obtain not only wind conditions at points of discrete distances determined by the time width T but also a wind condition at a point between the points of the discrete distances.
A problem to be solved by the laser radar device according to the fourth embodiment will be clarified by the following description with the same time width T=2 [μs] as the specific examples described in the above embodiments. In the frequency modulation, up-chirp signals are adopted as illustrated in
An object of the laser radar device according to the fourth embodiment is to systematically obtain a wind condition at a point close to a boundary between a range and a range.
In order to solve the above problem, the laser radar device according to the fourth embodiment includes two discrete modulation lidar systems, and applies inverse frequency modulations to the respective two systems. Beams of laser light of the two systems are emitted to the atmosphere, and the beams of received light reflected by aerosol are mixed by a receiving circuit of the laser radar device.
Note that, in the description of the technique according to the fourth embodiment, the same reference signs are used as much as possible for components common to those in the above embodiments, and redundant description is omitted appropriately.
The first Light source 11 is the same as the Light source 10 in the laser radar devices according to the first to third embodiments. The second Light source 12 is a Light source having a wavelength different from that of the first Light source 11. A reason why the wavelength of the second Light source 12 is different from that of the first Light source 11 is to easily couple respective beams of laser light to each other and to easily split the respective beams of laser light from each other. The second Light source 12 may have a different mode by performing different polarization instead of having a different wavelength from that of the first Light source 11.
The first frequency modulator 23 and the second frequency modulator 24 function in the same manner as the frequency modulator 20 of the laser radar devices according to the first to third embodiments. The first frequency modulator 23 and the second frequency modulator 24 may be replaced with a first phase modulator 23B and a second phase modulator 24B, respectively. Synchronism is important for modulation added by the first frequency modulator 23 and the second frequency modulator 24. Therefore, the laser radar device according to the fourth embodiment includes the control signal circuit 201, and the control signal circuit 201 controls the first frequency modulator 23 and the second frequency modulator 24.
The first beam splitter 31 and the second beam splitter 32 function in the same manner as the first beam splitter 30 of the laser radar devices according to the first to third embodiments. Beams of laser light output from the first Light source 11 and the second Light source 12 are modulated by the first frequency modulator 23 and the second frequency modulator 24, respectively, and are split into transmission light and local oscillator light by the first beam splitter 31 and the second beam splitter 32, respectively. Branching ratios of the first beam splitter 31 and the second beam splitter 32 are determined by design.
The transmission light split by the first beam splitter 31 and the transmission light split by the second beam splitter 32 are multiplexed by the channel multiplexer 91. When the second Light source 12 emits laser light having a wavelength different from that of the first Light source 11, the channel multiplexer 91 only needs to be formed by a wavelength filter that transmits the wavelength of the first Light source 11 and reflects the wavelength of the second Light source 12. In general, a wavelength filter formed of an optical fiber may be referred to as a WDM coupler. When polarization of the second Light source 12 is different from that of the first Light source 11, the channel multiplexer 91 may be a polarization beam splitter. In addition, the polarization beam splitter may be referred to as a polarization coupler or a polarization combiner.
The transmission light multiplexed by the channel multiplexer 91 is amplified by the amplifier 40. The amplifier 40 is similar to those of the laser radar devices according to the first to third embodiments. In the configuration example illustrated in
Output from the amplifier 40 is transmitted to the transmitting and receiving optics system 60 through the circulator 50. This operation is the same as those of the laser radar devices according to the first to third embodiments. It should be noted that, when beams of laser light having different wavelengths are used between the first Light source 11 and the second Light source 12, the transmitting and receiving optics system 60 is an optical system in which chromatic aberration is reduced.
The laser light emitted to the atmosphere is reflected by aerosol in the atmosphere, and is incident on the transmitting and receiving optics system 60 as received light. The incident received light is separated from the transmission light by the circulator 50 and is incident on the channel demultiplexer 92. The channel demultiplexer 92 is a component that functions inversely with the channel multiplexer 91. That is, the channel demultiplexer 92 emits, of the received light, a component corresponding to the wavelength of the first Light source 11 to the third beam splitter 71, and emits a component corresponding to the wavelength of the second Light source 12 to the fourth beam splitter 72. Note that there is no problem even if the wavelength of the second Light source 12 is mixed in the third beam splitter 71 or the wavelength of the first Light source 11 is mixed in the fourth beam splitter 72. The channel demultiplexer 92 may be constituted by a beam splitter. Note that, when the channel demultiplexer 92 is constituted by a beam splitter, light intensity is lost by 3 [db].
The component corresponding to the wavelength of the first Light source 11 emitted from the channel demultiplexer 92 and the local oscillator light split by the first beam splitter 31 interfere with each other in the third beam splitter 71. The two beams of interference light obtained in the third beam splitter 71 are emitted to the first balanced detector 81. Note that the same applies to the system from the second Light source 12, and thus description thereof is omitted here.
The receiving circuit 211 processes the signals obtained from the first balanced detector 81 and the second balanced detector 82 and calculates wind speed information for each distance. The receiving circuit 211 includes an electric filter, an amplifier, an analog-digital converter, and a computer.
A processing flow of the receiving circuit 211 of the laser radar device according to the fourth embodiment is the same as that of the first embodiment up to the first pre-filter 301, the first amplifier 311, and the first frequency filter 321. The receiving circuit 211 also includes the second pre-filter 302, the second amplifier 312, and the second frequency filter 322 in parallel in order to process a signal from the second balanced detector 82. The first frequency filter 321 distributes and emits signals emitted from the first amplifier 311 to the first mixer 351, the second mixer 352, and the third mixer 353 for each frequency.
The first mixer 351, the second mixer 352, and the third mixer 353 output a signal having a sum frequency or a difference frequency to the receivers 330 (331, 332, and 333) by multiplying the received signals by each other. In general, whether a sum frequency signal or a difference frequency signal is obtained by a mixer depends on phases of two input signals. Therefore, each of the first mixer 351, the second mixer 352, and the third mixer 353 may include a phase adjustment mechanism. A processing flow from the receivers 330 (331, 332, and 333) to the computer 340 is the same as that in the first embodiment.
An operation principle of the laser radar device according to the fourth embodiment will be clarified by the following specific example with reference to the drawings.
Each frequency modulation applied by the first frequency modulator 23 and the second frequency modulator 24 is modulation in which a stepwise change in which a frequency increases or decreases by a frequency difference F for each time width T is performed for at least one step. The transmission light of the laser radar device according to the fourth embodiment has stepwise frequency characteristics as illustrated in
The laser radar device according to the fourth embodiment is characterized in that two systems of beams of transmission light are generated, one of the systems is up-chirp, and the other of the systems is down-chirp. Each of the two systems of beams of transmission light is emitted to the atmosphere, scattered by aerosol in the atmosphere, and measured as received light. The received light interferes with the local oscillator light and is received by the balanced detector.
An effect of the laser radar device according to the fourth embodiment will be clarified by the following specific example. As a specific example, consider that there is aerosol at a point having a distance of 600 [m]. In this specific example, the received light is delayed by 4 [μs] as compared with the local oscillator light. The received light corresponding to the up-chirp stepwise local oscillator light has a frequency of −200 [MHz] +Doppler frequency fd as compared with the frequency of the local oscillator light. Conversely, the received light corresponding to the down-chirp stepwise local oscillator light has a frequency of +200 [MHz] +Doppler frequency fd as compared with the frequency of the local oscillator light.
The laser radar device according to the fourth embodiment includes the first frequency filter 321 and the second frequency filter 322 so as to perform processing for the two systems. The first frequency filter 321 and the second frequency filter 322 output frequency components from 50 [MHz] to 150 [MHz] to the first mixer 351, frequency components from 150 [MHz] to 250 [MHz] to the second mixer 352, and frequency components from 250 [MHz] to 350 [MHz] to the third mixer 353. As a result, also in the laser radar device according to the fourth embodiment, information on a wind condition near a distance of 300 [m] is intended to be measured by the receiver 331, information on a wind condition near a distance of 600 [m] is intended to be measured by the receiver 332, and information on a wind condition near a distance of 900 [m] is intended to be measured by the receiver 333.
The mixers 350 (351, 352, and 353) aim to obtain a sum frequency by multiplying two input signals by each other. In a specific example of aerosol at a point having a distance of 600 [m], a signal of “−200 [MHz]+Doppler frequency fd” and a signal of “200 [MHz]+Doppler frequency fd” are input to the second mixer 352. The second mixer 352 outputs a signal of a sum frequency having a frequency twice the Doppler frequency fd to the receiver 333.
Next, a specific example of aerosol at a point having a distance of 450 [m], to be solved by the laser radar device of the fourth embodiment, will be considered. Received light of an up-chirp system is observed as “−200 [MHz]+Doppler frequency fd” in the first half of the observation window and “−100 [MHz]+Doppler frequency fd” in the second half of the observation window. Received light of a down-chirp system is observed as “+200 [MHz] +Doppler frequency fd” in the first half of the observation window and “+100 [MHz]+Doppler frequency fd” in the second half of the observation window. Also in this specific example, the mixers 350 (351, 352, and 353) each output a signal of a sum frequency having a frequency twice the Doppler frequency fd.
In this case, the mixer 350 (351, 352, 353) each output a value obtained by adding a value obtained by multiplying the Doppler frequency fd by 2 to −100 MHz. In this manner, with the configuration of the modified example illustrated in
In other words, the laser radar device according to the fourth embodiment is a laser radar device including: the first balanced detector 81 that receives received light and first local oscillator light and converts each of the received light and the first local oscillator light into a first electrical signal; the second balanced detector 82 that receives received light and second local oscillator light and converts each of the received light and the second local oscillator light into a second electrical signal; and a receiving circuit that processes the first electrical signal and the second electrical signal converted by the first balanced detector 81 and the second balanced detector 82 and calculates distance information and speed information of a target, in which the frequency modulation applied by the first frequency modulator 23 is modulation in which a stepwise change in which a frequency increases or decreases by a frequency difference F for each time width T is performed for at least one step, the frequency modulation applied by the second frequency modulator 24 is modulation in which a frequency decreases or increases by the frequency difference F for each time width T, the modulation being reverse to that performed by the first frequency modulator 23, and a Doppler frequency fd is obtained by mixing the frequency of the received light for the first transmission light and the frequency of the received light for the second transmission light.
With the above configuration, the laser radar device according to the fourth embodiment can accurately measure a wind condition at any position regardless of how the range is taken in addition to exhibiting the effects of the laser radar devices according to the above embodiments.
A laser radar device according to a fifth embodiment is obtained by adding a new configuration to the configuration of the laser radar device according to the fourth embodiment. The laser radar device according to the fifth embodiment uses a first Light source 11 in accordance with a gas absorption line. Here, the gas refers to a gas in the atmosphere, an exhaust gas from a factory, a gas leaked from a pipe, or the like. In the description of the technique according to the fifth embodiment, the same reference signs are used as much as possible for components common to those in the above embodiments, and redundant description is omitted appropriately.
In other words, in the laser radar device according to the fifth embodiment, the first Light source 11 emits first laser light having a wavelength controlled to match an absorption line of a gas component to be measured, the second Light source 12 emits second laser light having a wavelength different from that of the first Light source 11, and the laser radar device further includes: the first gas concentration measurement receiver 371 that receives a component corresponding to the first laser light in the received light; and the second gas concentration measurement receiver 372 that receives a component corresponding to the second laser light in the received light, and can measure the concentration of the gas component in the atmosphere by decomposing the concentration for each distance.
The laser radar device according to the fifth embodiment can measure a wind condition by combination with the method described in the fourth embodiment. In the measurement of the wind condition, signals divided by the first signal splitter 361 and the second signal splitter 362 are input to a first mixer 351. The first mixer 351 can output a sum frequency to the receiver 331 and can calculate a Doppler frequency fd.
The laser radar device according to the fifth embodiment has the above configuration, and therefore can simultaneously measure a gas distribution in the atmosphere and a wind condition.
The laser radar device according to the present disclosed technique can be used for a device that measures a wind condition and a gas distribution in the atmosphere.
10: Light source, 11: First Light source, 12: Second Light source, 20: Frequency modulator, 20B: Phase modulator, 21 and 23: First frequency modulator, 21B and 23B: First phase modulator, 22 and 24: Second frequency modulator, 22B and 24B: Second phase modulator, 25: Local oscillator light frequency modulator, 25B: Local oscillator light phase modulator, 30 and 31: First beam splitter, 32: Second beam splitter, 40: Amplifier, 50: Circulator, 60: Transmitting and receiving optics system, 70: Second beam splitter, 71: Third beam splitter, 72: Fourth beam splitter, 80: Balanced detector, 81: First balanced detector, 82: Second balanced detector, 91: Channel multiplexer, 92: Channel demultiplexer, 200 and 201: Control signal circuit, 210 and 211: Receiving circuit, 220: Control device, 300: Pre-filter, 301: First pre-filter, 302: Second pre-filter, 310: Amplifier, 311: First amplifier, 312: Second amplifier, 320: Frequency filter, 321: First frequency filter, 322: Second frequency filter, 330, 331, 332, and 333: Receiver, 340: Computer, 350: Mixer, 351: First mixer, 352: Second mixer, 353: Third mixer, 360: Signal splitter, 361: First signal splitter, 362: Second signal splitter, 371: First gas concentration measurement receiver, 372: Second gas concentration measurement receiver
This application is a Continuation of PCT International Application No. PCT/JP2020/048332 filed on Dec. 24, 2020, which is hereby expressly incorporated by reference into the present application.
| Number | Date | Country | |
|---|---|---|---|
| Parent | PCT/JP2020/048332 | Dec 2020 | US |
| Child | 18143764 | US |
