JP7540982B2 - Lidar Equipment - Google Patents

Description

The present invention relates to a LiDAR (Light Detection and Ranging) device that uses Frequency Modulated Continuous Wave (FMCW).

An FMCW lidar device has been proposed that measures the distance to an object by irradiating the object with light and detecting the reflected light from the object. Specifically, the FMCW lidar device generates a transmitted light by linearly increasing the frequency f of the continuous light generated by the light source with time t, such as f = K (t - τ). K is an increase coefficient, and τ is a predetermined reference timing. The FMCW lidar device causes the reflected light from the object to interfere with the transmitted light. For example, if the reflected light received at a certain time t1 is the transmitted light transmitted at time t2 (<t1), the FMCW lidar device causes the reflected light of the transmitted light transmitted at time t2 to interfere with the transmitted light at time t1. The frequency of the transmitted light at time t2 is f2 = K (t2 - τ), and the frequency of the transmitted light at time t1 is f1 = K (t1 - τ). Because the frequency of the interference light is the difference between frequency f1 and frequency f2, the frequency of the interference light is constant at K(t1-t2). In other words, by measuring the frequency of the interference light, the round trip time (t1-t2) from the FMCW lidar device to an object can be detected, and therefore the distance to the object can be measured.

However, the above discussion is valid under the condition that the phase noise of a light source such as a laser can be ignored. In other words, the above discussion is valid when the phase noise at time t1 and the phase noise at time t2 can be considered to be equal. In order to consider the phase noise at time t1 and the phase noise at time t2 to be equal, the time difference between time t1 and time t2 must be short. The upper limit of the allowable time difference between time t1 and time t2 is based on the linewidth of the light source, that is, the amount of phase noise. For this reason, the upper limit of the allowable (t1-t2) value, that is, the maximum distance that can be measured, is limited by the linewidth of the light source. Non-Patent Document 1 discloses that a light source with a linewidth of about 100 kHz must be used to measure distances of about several tens of meters. On the other hand, Non-Patent Document 2 discloses an FMCW lidar device that can suppress the effects of phase noise.

A. Martin, et al. , "Photonic Integrated Circuit-Based FMCW Coherent LiDAR", in Journal of Lightwave Technology, vol. 36, no. 19, pp. 4640-4645, October 1, 2018 M. Pu, et al. , "Dual-Heterodyne Mixing Based Phase Noise Cancellation for Long Distance Dual-Wavelength FMCW Lidar", in Optical Fiber Com communication Conference (OFC) 2020, OSA Technical Digest (Optical Society of America, 2020), paper Th1K. 2.

However, the configuration in Non-Patent Document 2 is complex and expensive, as it requires a frequency comb.

The present invention provides a lidar device that can suppress the effects of phase noise with a simple configuration.

According to one aspect of the present invention, a lidar device includes a generating means for generating transmitted light including frequency-modulated light of a first polarization and continuous light of a second polarization orthogonal to the first polarization, a transmitting means for transmitting the transmitted light generated by the generating means, a receiving means for receiving reflected light of the transmitted light transmitted by the transmitting means at an object, a first detecting means for detecting a Stokes parameter of the transmitted light generated by the generating means, a second detecting means for detecting a Stokes parameter of the reflected light received by the receiving means, and a determining means for determining a round -trip time of the transmitted light between the object based on the Stokes parameter of the transmitted light detected by the first detecting means and the Stokes parameter of the reflected light detected by the second detecting means, wherein the frequency-modulated light of the first polarization and the continuous light of the second polarization are generated based on continuous light generated by the same light source .

The present invention makes it possible to suppress the effects of phase noise with a simple configuration.

FIG. 1 is a configuration diagram of a lidar device according to an embodiment.

The following embodiments are described in detail with reference to the attached drawings. Note that the following embodiments do not limit the invention according to the claims, and not all combinations of features described in the embodiments are necessarily essential to the invention. Two or more of the features described in the embodiments may be combined in any desired manner. In addition, the same reference numbers are used for the same or similar configurations, and duplicate descriptions are omitted.

Figure 1 is a configuration diagram of an FMCW lidar device according to this embodiment. A light source 1 generates continuous light and outputs the continuous light to a polarizing beam splitter (PBS) 2. The PBS 2 performs polarization separation of the continuous light, outputs the X-polarized continuous light to a frequency modulation unit 3, and outputs the Y-polarized continuous light orthogonal to the X-polarized light to a polarizing beam combiner (PBC) 4. For example, the PBS 2 can be arranged so that the amplitudes of the X-polarized and Y-polarized continuous lights output by the PBS 2 are equal. The frequency modulation unit 3 performs frequency modulation of the X-polarized continuous light and changes the frequency of the X-polarized continuous light over time. In this embodiment, the frequency modulation unit 3 generates the transmission light by linearly increasing the frequency of the X-polarized continuous light as f = K (t - τ). Note that K is an increase coefficient and τ is a predetermined reference timing. The upper and lower limits of the frequency are predetermined, and when the frequency reaches the upper limit, the frequency modulation unit 3 increases it again from the lower limit over time. The frequency modulation unit 3 outputs the X-polarized frequency-modulated light to the PBC 4.

The PBC 4 generates the transmission light by combining the frequency-modulated light of X-polarization from the frequency modulation unit 3 and the continuous light of Y-polarization from the PBS 2. The PBC 4 outputs the transmission light to the coupler 5. The coupler 5 splits the transmission light into two, outputting one to the transmitter 6 and the other to the Stokes receiver 8. The transmitter 6 emits the transmission light toward the object. The receiver 7 receives the light reflected from the object and outputs the reflected light to the Stokes receiver 9.

The Stokes receiving unit 8 outputs the Stokes parameters _S2 and _S3 of the input transmitted light to the measuring unit 10. Similarly, the Stokes receiving unit 9 outputs the Stokes parameters _S2 and _S3 of the input reflected light to the measuring unit 10.

For example, assume that at time t1, reflected light of transmitted light at time t2 (t2<t1) is received. If the phase noise at time t1 is φ1, the Jones vector of the transmitted light at time t1 is ( ^{ej{2πK(t1-τ)+φ1}t} , ^ejφ1t }. If the phase noise at time t2 is φ2, the Jones vector of the reflected light received at time t1, that is, the transmitted light at time t2, is ( ^{ej{2πK(t2-τ)+φ2}t} , ^ejφ2t }. Note that the relationship between the Stokes parameters _S2 and _S3 of light whose Jones vector is (Ex, Ey) and the values Ex and Ey is as follows:
S ₂ =2Re[Ex,Ey ^* ]
S ₃ =2Im[Ex,Ey ^* ]
Here, Ey ^* is the complex conjugate of Ey, and Re and Im represent the real part and the imaginary part, respectively.

Therefore, the Stokes parameters S ^t1 ₂ and S ^t1 ₃ of the transmitted light at time t1 output by the Stokes receiver 8 to the transmitter 10 at time t1 are respectively the real part and the imaginary part of 2e ^{j{2πK(t1-τ)}} . Similarly, the Stokes parameters S ^t2 ₂ and S ^t2 ₃ of the reflected light at time t1 output by the Stokes receiver 8 to the transmitter 10 at time t1, that is, the transmitted light at time t2, are respectively the real part and the imaginary part of 2e ^{j{2πK(t2-τ)}} . As is clear from the above equations, in the Stokes parameters of the transmitted light, the phase noise φ1 at time t1 is cancelled out, and in the Stokes parameters of the reflected light, the phase noise φ2 at time t2 is cancelled out.

The measurement unit 10 can obtain a complex amplitude E ₁ =e ^{j{2πK(t1-τ)} of the transmitted light at time t1 based on the Stokes parameters S t1 2 and S t1 3 of the transmitted light at time t1. Similarly, the measurement unit 10 can obtain a complex amplitude E 2 =e j{2πK(t2-τ)} of} _the ^reflected _{light at} time t1 based on the Stokes parameters S ^t2 ₂ and S ^t2 ₃ of the reflected light at time t1. Therefore, the measurement unit 10 can obtain a beat component B=e ^{j{2πK(t2-t1) }} of the transmitted light at time t1 and the reflected light at time t1. Since the frequency of the beat component is proportional to the round trip time (t2-t1) to the object, the measurement unit 10 can obtain the distance to the object.

As described above, according to this embodiment, the lidar device irradiates an object with transmitted light including frequency-modulated light of a first polarization and continuous light of a second polarization that is orthogonal to the first polarization. Since the amount of phase noise contained in the first polarization component of the transmitted light is equal to the amount of phase noise contained in the second polarization, the effect of phase noise can be suppressed with a simple configuration.

In this embodiment, the PBS 2 performs polarization separation of the continuous light generated by the light source 1 so that the amplitudes of the X-polarized continuous light and the Y-polarized continuous light are equal, but the present invention is not limited to such a form, and the amplitudes of the X-polarized continuous light and the Y-polarized continuous light output by the PBS 2 may be different. In that case, however, the amount of phase noise contained in the X-polarized wave and the amount of phase noise contained in the Y-polarized wave will also differ, and the phase noise will be offset by taking into account the ratio between the amplitudes of the X-polarized continuous light and the Y-polarized continuous light output by the PBS 2.

In addition, in this embodiment, the frequency modulation unit 3 linearly increases the frequency from the lower limit value to the upper limit value, but the present invention is not limited to such a configuration, and the frequency can be changed in any manner that satisfies the condition that the frequency of the beat component B differs depending on the round trip time between the object. For example, the frequency can be linearly decreased from the upper limit value to the lower limit value.

Furthermore, in this embodiment, the transmitted light is split into two by the coupler 5, but a Stokes receiver 8 can be provided at the position of the coupler 5 in Figure 1, and the Stokes receiver 8 can output the transmitted light from the PBC 8 to the transmitter 6 and transmit the Stokes parameters S ^t1 ₂ and S ^t1 ₃ of the transmitted light to the measurement unit 10.

The invention is not limited to the above-described embodiment, and various modifications and variations are possible within the scope of the invention.

The above configuration makes it possible to provide a lidar device that can suppress the effects of phase noise with a simple configuration. This makes it possible to contribute to Goal 9 of the United Nations-led Sustainable Development Goals (SDGs), which is to "build resilient infrastructure, promote sustainable industrialization and foster innovation."

1: Light source, 2: PBS, 3: Frequency modulation section, 4: PBC, 6: Transmitter, 7: Receiver, 8, 9: Stokes receiver, 10: Measurement section

Claims (8)

a generating means for generating a transmission light including a frequency-modulated light of a first polarization and a continuous light of a second polarization orthogonal to the first polarization;
a transmitting means for transmitting the transmission light generated by the generating means;
a receiving means for receiving the reflected light of the transmitted light from the transmitting means at an object;
a first detection means for detecting the Stokes parameters of the transmission light generated by the generation means;
a second detection means for detecting the Stokes parameters of the reflected light received by the receiving means;
a determination means for determining a round trip time of the transmitted light between the object and the object based on the Stokes parameters of the transmitted light detected by the first detection means and the Stokes parameters of the reflected light detected by the second detection means;
Equipped with
A lidar device, characterized in that the frequency-modulated light of the first polarization and the continuous light of the second polarization are generated based on continuous light generated by the same light source . The first detection means detects Stokes parameters _S2 and _S3 of the transmitted light,
2. The lidar device according to claim 1, wherein the second detection means detects Stokes parameters _S2 and _S3 of the reflected light. The lidar device according to claim 1 or 2, characterized in that the determination means determines the round trip time by calculating the frequency difference between the transmitted light and the reflected light based on the Stokes parameters of the transmitted light and the Stokes parameters of the reflected light. The generating means includes:
a modulation means for frequency-modulating the continuous light of the first polarization to generate frequency-modulated light of the first polarization;
a multiplexing means for multiplexing the frequency-modulated light of the first polarization and the continuous light of the second polarization to generate the transmission light;
The lidar device according to any one of claims 1 to 3, further comprising: a splitter that splits the continuous light generated by the light source into a first polarized continuous light and a second polarized continuous light;
5. The lidar device according to claim 4, further comprising: The lidar device according to claim 4 or 5, characterized in that the modulation means generates the frequency-modulated light of the first polarized wave by repeatedly changing the frequency of the continuous light of the first polarized wave over time between a first frequency and a second frequency higher than the first frequency. The lidar device according to claim 6, characterized in that the modulation means generates the frequency-modulated light of the first polarization by linearly increasing the frequency of the continuous light of the first polarization from the first frequency to the second frequency over time. The lidar device according to claim 6, characterized in that the modulation means generates the frequency-modulated light of the first polarization by linearly decreasing the frequency of the continuous light of the first polarization from the second frequency to the first frequency over time.

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