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Polarization-dependent characteristics of a photon-counting laser ranging system
Optics Communications 456 (2020) 124597
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Polarization-dependent characteristics of a photon-counting laser ranging system Dongsong Shi a,b , Ming Li a ,∗, Genghua Huang a , Rong Shu a a b
Key Laboratory of Space Active Opto-electronics Technology, Shanghai Institute of Technical Physics, Chinese Academy of Sciences, Shanghai 200083, China University of Chinese Academy of Sciences, Beijing 100049, China
ARTICLE
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Keywords: LIDAR Photon counting Scattering Polarization Geiger-mode Avalanche photodiodes
ABSTRACT We have built a photon-counting dual-polarization-sensitive time-division laser ranging system. The laser source can emit linearly polarized light with both horizontal and vertical polarizations, and the polarizationsensitive receiving module counts the echo photons in the two polarization states separately. By analyzing the scattering characteristics of target surfaces and counting the numbers of echo photons with different polarizations, metallic and nonmetallic surfaces can be distinguished. The proposed photon-counting system takes advantage of its high detection sensitivity and polarization-sensitive detection to obtain information on the target surface. Together, these two features improve the detection efficiency of the system as a whole and enable secure target detection.
1. Introduction In contrast to traditional microwave radar, lidar has a low transmission power, a high angular resolution and a fast processing speed when detecting targets [1]. In recent years, a qualitative leap in lidar performance has been achieved. In addition to providing distance, speed and other basic information, current multibeam imaging lidar systems can accurately capture a variety of characteristic parameters, such as the sizes and shapes of the target objects, and can even enable the establishment of three-dimensional models [2–6]. In the field of deep space exploration, lidar is continually being used for new tasks and subjected to new demands [7–9]. First, the required detection distances are increasing. It is necessary not only to accurately locate targets within a dozen kilometers in the atmospheric environment but also to detect and identify tiny satellites or space debris within a range of thousands of kilometers in the space exploration field [10]. However, as the detection distance increases, the received echo signal becomes extremely weak; consequently, further research on high-resolution, high-sensitivity detectors is needed. The photoncounting system takes advantage of its high detection sensitivity. It can convert the echo amplitude detection based on thousands of photons in linear mode into counting single photon. The application of statistical optical theory to the distance ranging system can make full use of each photon in the echo signal. The high sensitivity and high detection efficiency of the photon-counting system can reduce the requirements on laser power consumption and telescope aperture. Since the nature of photon-counting system is probabilistic detection, the poor target
characteristics and working environment will only affect its detection probability, and the target will not be completely undetectable. Therefore, photon-counting detection has extremely high reliability. In addition, the recognition of target materials and the differentiation of different targets have also received considerable attention. For such purposes, it is important to analyze the photon polarization characteristics of the detected target echo signals. In addition to the polarization measurements of underwater targets [11,12], polarization lidar can also be used for volcanic ash measurements [13,14], ice measurements [15–17], atmospheric measurements [18,19], and detection of flying honey bees. [20]. All of the above schemes rely on polarization-sensitive reception at the detector end. However, no systematic research has been carried out regarding the laser emission side. The instability of the elliptically polarized light emitted by the probe lasers also affects the polarization statistics on the receiving end. Starting with the laser emission, we have reconceptualized a lidar system based on multiple wavelengths and multiple polarization states and carried out simulation-based design [21]. To verify the feasibility of the designed system, we started by designing a desktop verification device based on a single wavelength and two polarization states. In this device, a light source that can produce both horizontal and vertical linear polarization is used at the transmission end, and the light source is controlled by means of a timedivision scheme to explore the polarization characteristics of the target surface.
∗ Corresponding author. E-mail address: [email protected] (M. Li).
https://doi.org/10.1016/j.optcom.2019.124597 Received 23 May 2019; Received in revised form 27 August 2019; Accepted 18 September 2019 Available online 20 September 2019 0030-4018/© 2019 Elsevier B.V. All rights reserved.
D. Shi, M. Li, G. Huang et al.
Optics Communications 456 (2020) 124597
2. Scattering characteristics of the target surface
Thus, one can obtain the reflectance for the photoelectric field of the S light: √ √ √ (𝑛1 cos 𝜃 − 𝑢2 )2 − 𝑣2 2 | | (7) 𝑅𝑠 = |𝑟𝑠 | = √ (𝑛1 cos 𝜃 + 𝑢2 )2 − 𝑣22
When photons propagate through the interface between two media, the polarization of the photons change in accordance with the law of refraction and Fresnel’s law of reflection [22], as shown in Eq. (1). For nonmetallic media, the electric field of the photons after the refraction or reflection of S light (perpendicular- or senkrecht-polarized light) and P light (parallel-polarized light) may change in direction and intensity, but the polarization direction of the photons does not change. ⎧ ⎪𝐸 ′ ⎪ 0𝑠 ⎪ ⎪𝐸 ′′ ⎪ 0𝑠 ⎨ ⎪𝐸 ′ ⎪ 0𝑝 ⎪ ⎪𝐸 ′′ ⎪ 0𝑝 ⎩
= = = =
𝑛1 cos 𝜃 − 𝑛2 cos 𝜃 ′′ sin(𝜃 − 𝜃 ′′ ) 𝐸 =− 𝐸 𝑛1 cos 𝜃 + 𝑛2 cos 𝜃 ′′ 0𝑠 sin(𝜃 + 𝜃 ′′ ) 0𝑠 ′′ 2𝑛1 cos 𝜃 2 sin 𝜃 cos 𝜃 𝐸 = 𝐸 𝑛1 cos 𝜃 + 𝑛2 cos 𝜃 ′′ 0𝑠 sin(𝜃 + 𝜃 ′′ ) 0𝑠 𝑛2 cos 𝜃 − 𝑛1 cos 𝜃 ′′ tan(𝜃 − 𝜃 ′′ ) 𝐸 = 𝐸 𝑛2 cos 𝜃 + 𝑛1 cos 𝜃 ′′ 0𝑝 tan(𝜃 + 𝜃 ′′ ) 0𝑝 ′′ 2𝑛1 cos 𝜃 2 sin 𝜃 cos 𝜃 𝐸 = 𝐸 𝑛2 cos 𝜃 + 𝑛1 cos 𝜃 ′′ 0𝑝 sin(𝜃 + 𝜃 ′′ ) cos(𝜃 − 𝜃 ′′ ) 0𝑝
the change in the phase angle of the s-field: tan 𝛿𝑠 =
tan 𝛿𝑝 = 2𝑛1 𝑛22 cos 𝜃
𝑛̃ = 𝑛 − 𝑖𝑘𝜅
(3)
2𝜅𝑢2 − (1 − 𝜅 2 )𝑣2 𝑛42 (1 + 𝜅 2 )2 cos 2𝜃 − 𝑛21 (𝑢22 + 𝑣22 )
(10)
The light reflected from a metallic interface can be represented as ) (| | ( ′) ( ) 𝑟 𝐴 𝐸𝑠 𝐴 𝑠 𝑟𝑠 𝛿𝑠 | 𝑠 | 𝑠 (12) = = 𝑒 | | 𝐸𝑝′ 𝐴 𝑝 𝑟𝑝 |𝑟𝑝 | 𝐴𝑝 𝑒𝛿 | | where 𝛿 = 𝛿𝑝 − 𝛿𝑠 . When non-S- and non-P-polarized light is reflecting from a flat metal surface, the main factors affecting the polarization state of the system are the reflectances and phase differences for S- and P-polarized light. This will cause the linearly polarized light to become elliptically polarized after being reflected. In short, the effect of reflection from a metal surface on linear polarization usually manifests as bidirectional attenuation and phase delay [23], and any roughness of the metal surface will cause polarized laser light to diffusely reflect and become depolarized [24–26]. The reflectances of a metal surface for both S and P light components are usually much larger than the surface reflectance of a nonmetallic material. The higher the magnetic metal content of the metal is, the more pronounced the depolarization effect of photons [27,28]. By additionally exploiting the high sensitivity of single-photon detection, we can count the number of received echo photons with each of the two linear polarizations to distinguish whether a target surface is metallic or nonmetallic and even to distinguish among different metal surfaces.
(4)
where 𝜃̃′′ is the complex refractive index angle, 𝑟𝑠 is the reflection coefficient for the S wave, and 𝑟𝑝 is the reflection coefficient for the P wave. Let the following hold:
3. Dual -polarization-sensitive time-division laser ranging system 3.1. Experimental setup The proposed dual-polarization-sensitive laser ranging system is shown in Fig. 1. A pulsed laser (Teem Photonics, SNG-20F) is used as the photon source. The laser emits pulses at a wavelength of 532 nm with a full width at half maximum (FWHM) of 550 ps and a repetition rate of 21 kHz, and the reflected light passes through an attenuator to enter the main wave detector. The polarization direction of the transmitted photons is modified to horizontal by a linear polarizer. Then, a 50:50 beam splitter (BS) divides the horizontally polarized light into two beams of approximately equal energy. One beam is transmitted through a polarizing beam splitter (PBS) and transmitted; the other horizontally polarized beam passes through a half-wave plate (HWP), thus changing its polarization to vertical. These photons are then emitted through the reflecting surface of another PBS.
(5)
The law of refraction 𝑛1 sin 𝜃 = 𝑛̃2 sin 𝜃̃′′ and Eq. (3) together can be used to solve for the values of 𝑢2 and 𝑣2 . Substituting Eq. (5) into Eq. (4) yields 𝑛1 cos 𝜃 − (𝑢2 − 𝑖𝑣2 ) 𝑛1 cos 𝜃 + (𝑢2 − 𝑖𝑣2 )
𝑛̃2 cos 𝜃 − 𝑛1 (𝑢2 − 𝑖𝑣2 ) | | = |𝑟𝑝 | 𝑒𝛿𝑝 = 2 | | 𝑛̃22 cos 𝜃 + 𝑛1 (𝑢2 − 𝑖𝑣2 )
(9)
1 2
For a flat metal surface, the polarization directions of the S and P waves remain unchanged, and the amplitude and phase changes do not affect the detection of the polarization state. Linear polarization in other polarization directions can be represented by means of a Jones matrix: ( ) ( ) ⃖⃖⃗ = 𝐸𝑠 = 𝐴𝑠 𝐽 (11) 𝐸𝑝 𝐴𝑝
where 𝑛 is the real part of the refractive index and 𝜅 is the extinction coefficient. When a photon is incident from a medium with a refractive index of 𝑛1 at an angle 𝜃 on a metal surface with a complex refractive index 𝑛̃2 , the Fresnel formula applies, as shown in Eq. (4):
= ||𝑟𝑠 || 𝑒𝛿𝑠 =
2
and the change in the phase angle of the p-field,
(1)
(2)
⎧ ⎪𝑟 𝑠 ⎪ ⎪ ⎪𝑟 ⎨ 𝑝 ⎪ ⎪ ⎪= ⎪ ⎩
1 2
2
⃖⃖⃗ + ̃ ⃖⃖⃗ = 0 ∇2 𝐸 𝑘2 𝐸 √ 𝜀. where ̃ 𝑘 = 𝑤 𝜇̃ The index of refraction is also complex,
𝑛̃2 cos 𝜃̃′′ = 𝑢2 − 𝑖𝑣2
(8)
𝑢22 + 𝑣22 − 𝑛21 cos 2𝜃
the reflectance for the photoelectric field of the S light: √ √ 2 [𝑛 (1 − 𝜅 2 ) cos 𝜃 − 𝑛1 𝑢2 ]2 + [2𝑛22 𝜅 cos 𝜃 − 𝑛1 𝑣2 ]2 | | √ 𝑅𝑝 = |𝑟𝑝 | = √ 2 | | [𝑛2 (1 − 𝜅 2 ) cos 𝜃 − 𝑛 𝑢 ]2 + [2𝑛2 𝜅 cos 𝜃 + 𝑛 𝑣 ]2
where 𝐸0𝑠 and 𝐸0𝑝 represent the amplitudes of the incident S and ′ and 𝐸 ′ represent the amplitudes of the S P light, respectively; 𝐸0𝑠 0𝑝 ′′ and 𝐸 ′′ represent the and P light, respectively, after reflection; 𝐸0𝑠 0𝑝 amplitudes of the S and P light, respectively, after refraction; 𝑛1 and 𝑛2 are the refractive indices of media 1 and 2, respectively; and 𝜃, 𝜃 ′ , and 𝜃 ′′ are the incidence, reflection, and refraction angles, respectively, of the light. When photons propagate at a metallic interface, the part of the light field propagating within the metal material satisfies the Helmholtz equation:
⎧ 𝑛 cos 𝜃 − 𝑛̃2 cos 𝜃̃′′ ⎪𝑟 𝑠 = 1 ⎪ 𝑛1 cos 𝜃 + 𝑛̃2 cos 𝜃̃′′ ⎨ 𝑛̃2 cos 𝜃 − 𝑛1 cos 𝜃̃′′ ⎪ ⎪𝑟 𝑝 = 𝑛̃2 cos 𝜃 + 𝑛1 cos 𝜃̃′′ ⎩
𝑛1 cos 𝜃 − (𝑢2 − 𝑖𝑣2 )
(6)
[𝑛22 (1 − 𝜅 2 ) cos 𝜃 − 𝑛1 𝑢2 ] − 𝑖[2𝑛22 𝜅 cos 𝜃 − 𝑛1 𝑣2 ] [𝑛22 (1 − 𝜅 2 ) cos 𝜃 − 𝑛1 𝑢2 ] − 𝑖[2𝑛22 𝜅 cos 𝜃 + 𝑛1 𝑣2 ] 2
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Optics Communications 456 (2020) 124597
Fig. 1. Optical schematic of the dual-polarization-sensitive laser ranging system.
roughness of 6.30. The metal plate was tilted at an angle of 45 degrees to the incident surface. We first blocked the vertically polarized photons, and allowed the horizontally polarized light to illuminate the target metal plate to obtain the point cloud data shown in Fig. 3(a). The point cloud data shown in Fig. 3(b) were similarly obtained by blocking the horizontally polarized photons. These point cloud data were generated by fusing the signals received from vertical channel and horizontal channel. Despite the use of polarization-sensitive detection, we can only extract distance information based on point cloud data. However, by performing polarization-sensitive detection on the echo photons, we can obtain multiple sets of different photon data to further analyze the polarization characteristics of the target. First, let us analyze the echo photons detected in the two channels with horizontally polarized light incident on the target. As shown in Fig. 4(a), the GM-APD of vertical channel receives only vertically polarized noise photons, and no distance information can be extracted. By contrast, in Fig. 4(b), the distance information is clearly displayed. The echo photon information obtained in the two channels is essentially consistent with expectations. Compared to the case of a nonmetallic target, more noise data are received in horizontal channel when the target is metallic. Fig. 4(c) and (d) shows the echo photons detected in the two channels with vertically polarized light incident on the target. As seen in Fig. 4(c), the GM-APD of vertical channel receives vertically polarized photons containing distance information. However, as seen in Fig. 4(d), although horizontal channel is expected to receive only horizontally polarized noise photons, but it also receives intermittent information about the distance-related photons. The aluminum plate was tilted at an angle of 45 degrees to the incident surface. Due to the rough surface of the aluminum plate, both probes reflected non-S- and non-P-polarized light from the surface of the aluminum plate. When polarized light is incident on the metallic plane at a large angle, the reflectance in the P direction is greater than the reflectance in the S direction [27]. This makes the depolarization effect of vertically polarized photons more obvious. During the experiment, when the direction or incident angle of the target changes, Fig. 4(a) also shows that the vertical channel receives obviously distance-related signal photons. From the abscissa of the point cloud diagram in Fig. 2(b) and (c), it can be known that the number of photons received by a single photon detector during a probe is approximately 1000. However, from the abscissa of the point cloud diagram in Fig. 4(b) and (c), it can be known that the number
The target can be rotated to change different angles of incidence. After the emitted photons are reflected by the target object, they are received by the optical system. The echo photons are filtered by a 532 nm narrowband filter (NBF) to remove stray light and are then passed through a PBS. The PBS separates the photons into those that are horizontally polarized and those that are vertically polarized. The photons received by the receiving telescope are coupled into two separate Geiger-mode avalanche photodiodes (GM-APDs) (Excelitas, SPCM-AQRH-13, dead time 22 ns, dark count 250 Hz, 50%@532 nm) through fibers. The output of each GM-APD is connected to the ‘‘Stop’’ pin of a time-to-digital converter (TDC) (Agilent, U1051A), while the main wave detector signal is connected to the ‘‘Start’’ pin. The TDC module can simultaneously measure the time information of six channels and has a time resolution of 50 ps. The vertically linearly polarized light enters vertical channel (channel 2) of the TDC, and the horizontally linearly polarized light enters the horizontal channel (channel 3). 3.2. Experimental results and analysis A nonmetallic detection experiment was carried out with a wall as the target. The incident surface was inclined at an angle of 45 degrees to the wall. We first blocked the vertically polarized photons and allowed the horizontally polarized light to illuminate the wall. Then, the horizontally polarized photons were shielded, and the vertically polarized light was allowed to emerge. The point cloud data obtained with the horizontally and vertically polarized light are shown in Fig. 2(a)–(d), where Fig. 2(a) and (b) show the photons received in vertical channel and horizontal channel, respectively, with horizontally polarized light incident on the target, in which case distance information can be obtained only from horizontal channel, and Fig. 2(c) and (d) show the photons received in vertical channel and horizontal channel, respectively, with vertically polarized light incident on the target, in which case distance information can be obtained only from vertical channel. The photon counts shown in Fig. 2(a)–(d) that are not at range are the combined effects of the dark count of the detector and the scattered light in the environment. As demonstrated here, for the detection of nonmetallic objects, the polarization information of the received echo photons is well preserved, with no obvious changes. Next, a metal target was detected. We chose 2A12 (aluminum) as the metal target, and its surface was black anodized with a surface 3
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Optics Communications 456 (2020) 124597
Fig. 2. Point cloud data for nonmetallic target detection.
The system was calibrated based on the horizontal polarization case. Based on this calibration, the results obtained with the two probes differ by 0.195 m and 0.210 m. As seen from the optical schematic in Fig. 1, the vertically polarized light is deflected by 90◦ from the horizontally polarized light through an HWP and then reflected by a PBS. Thus, compared with the horizontally polarized light, the vertically polarized light is subjected to an additional polarization conversion process in the current system. This discrepancy can be corrected. All of the above experiments were carried out at an angle of 45 degrees to the incident surface between the wall and the aluminum plate. When the incident angle of metal surface is changed, the depolarization effect of the echo photons will also change correspondingly. Fig. 6(a)–(c) shows the point cloud data for the detection of the aluminum plate at an incident angle of 0, 30 and 45 degrees. Where the laser source is incident with vertically polarized light and receives photons with horizontal channel. Fig. 6(c) is the same Fig. 4(d), and the depolarization effect of photons is relatively obvious. Fig. 6(a) shows that when the incident angle is 0 degrees, depolarized photons are barely detected in the horizontal channel. As shown in Section 2, when polarized laser light is incident on a metallic surface, the ratio of the light reflected in the S and P directions changes. This phenomenon causes linearly polarized light to become elliptically polarized after being diffusely reflected by the rough surface of a metal target. The elliptically polarized light thus generated can be decomposed into two linear polarized light components with different intensities that are perpendicular to each other. Thus, we can distinguish whether the surface of a target is metallic or nonmetallic according to the numbers of echoes observed in vertical channel and horizontal channel, and we can even distinguish among different metal surfaces. To distinguish between different metallic surfaces, it is necessary to ensure that the detection distance and the angle of incidence are approximately the same. The magnetic content of different metallic surface is different, and the depolarization effect of echo photons is also different. So we can take advantage of the photon-counting laser ranging system to obtain the number of corresponding photons in the effective distance range of horizontal channel and vertical channel respectively. Different metallic surfaces have different ratios of depolarized photons (𝑛𝐻𝑉 , 𝑛𝑉 𝐻 ) to polarized photons (𝑛𝐻𝐻 , 𝑛𝑉 𝑉 ). Where 𝑛𝐻𝑉 is the number of photons transmitted in the horizontal polarized direction and received in the vertical channel; 𝑛𝑉 𝐻 is the number of photons transmitted in the vertical polarized direction and received in the horizontal channel; 𝑛𝐻𝐻 is the number of photons transmitted in the horizontal polarized direction and received in the horizontal channel; 𝑛𝑉 𝑉 is the number of photons transmitted in the vertical polarized direction and received in the vertical channel. By calculating 𝑛𝐻𝑉 ∕𝑛𝐻𝐻 and 𝑛𝑉 𝐻 ∕𝑛𝑉 𝑉 , different metallic surface can be distinguished.
Fig. 3. Point cloud data for metallic target detection. (a) with horizontally polarized incident light, (b) with vertically polarized incident light.
of photons received by the single photon detector during a probe is about 2500. Both probes are performed at the same distance in the same laboratory environment, indicating that the reflectivity of the aluminum plate is much higher than the wall surface. By combining the depolarization effect of the metallic surface and the reflectivity of the surface of different materials, we can distinguish whether the surface of a target is metallic or nonmetallic. Fig. 5(a)–(d) shows histogram of the polarization statistic for the photon information received during the two detections. The measured distance information is represented on the abscissa, and the number of photons received at the corresponding distance is represented on the ordinate. Statistics presented in different colors represent photon information received in different channels. For the detection of the nonmetallic targets, a measured distance of 6.825 m can be obtained from the photon information collected in horizontal channel in the case of horizontally polarized light emission. In the case of vertically polarized light emission, a distance of 7.020 m can be obtained from the photon statistics of vertical channel. In each case, the other channel shows only a sporadic noise distribution. For the detection of the metal target, a measured distance of 6.840 m can be obtained from the photon information collected in horizontal channel in case of horizontally polarized light emission. In the case of vertically polarized light emission, vertical channel can clearly yield a distance measurement of 7.050 m. 4
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Optics Communications 456 (2020) 124597
Fig. 4. Echo photon received from different channels. (a) vertical channel with horizontally polarized incident light, (b) horizontal channel with horizontally polarized incident light, (c) vertical channel with vertically polarized incident light, (d) horizontal channel with vertically polarized incident light.
Fig. 5. Histograms of polarization statistics. (a) nonmetallic target detection with horizontally polarized incident light, (b) nonmetallic target detection with vertically polarized incident light, (c) metallic target detection with horizontally polarized incident light, (d) metallic target detection with vertically polarized incident light.
4. Conclusion
could be obtained, thereby enabling secure anti-interference detection. This concept also suggests some new possibilities for future
We have built a polarization-sensitive photon-counting system based on dual linear polarized time-division-emitted laser light. By combining the advantages of the high sensitivity of single-photon detection with the benefits of dual linear polarization detection for identifying objects, we can obtain highly precise distance and polarization information for a target surface. If a multilinearly polarized light array could be transmitted and received, then a composite three-dimensional image containing polarization information regarding the target object
developments in lidar technology. In our future work, we will further optimize the experimental device by adding a 1064 nm laser source and modifying the receiving polarization detection module to recognize 4 types of light, thus further expanding the application scenario to multiple polarization states at multiple wavelength for secure target detection. 5
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Optics Communications 456 (2020) 124597
Fig. 6. Depolarization of echo photons at different incident angles. (a) 0 degrees, (b) 30 degrees, (c) 45 degrees.
Acknowledgment
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6
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Optics Communications journal homepage: www.elsevier.com/locate/optcom
Polarization-dependent characteristics of a photon-counting laser ranging system Dongsong Shi a,b , Ming Li a ,∗, Genghua Huang a , Rong Shu a a b
Key Laboratory of Space Active Opto-electronics Technology, Shanghai Institute of Technical Physics, Chinese Academy of Sciences, Shanghai 200083, China University of Chinese Academy of Sciences, Beijing 100049, China
ARTICLE
INFO
Keywords: LIDAR Photon counting Scattering Polarization Geiger-mode Avalanche photodiodes
ABSTRACT We have built a photon-counting dual-polarization-sensitive time-division laser ranging system. The laser source can emit linearly polarized light with both horizontal and vertical polarizations, and the polarizationsensitive receiving module counts the echo photons in the two polarization states separately. By analyzing the scattering characteristics of target surfaces and counting the numbers of echo photons with different polarizations, metallic and nonmetallic surfaces can be distinguished. The proposed photon-counting system takes advantage of its high detection sensitivity and polarization-sensitive detection to obtain information on the target surface. Together, these two features improve the detection efficiency of the system as a whole and enable secure target detection.
1. Introduction In contrast to traditional microwave radar, lidar has a low transmission power, a high angular resolution and a fast processing speed when detecting targets [1]. In recent years, a qualitative leap in lidar performance has been achieved. In addition to providing distance, speed and other basic information, current multibeam imaging lidar systems can accurately capture a variety of characteristic parameters, such as the sizes and shapes of the target objects, and can even enable the establishment of three-dimensional models [2–6]. In the field of deep space exploration, lidar is continually being used for new tasks and subjected to new demands [7–9]. First, the required detection distances are increasing. It is necessary not only to accurately locate targets within a dozen kilometers in the atmospheric environment but also to detect and identify tiny satellites or space debris within a range of thousands of kilometers in the space exploration field [10]. However, as the detection distance increases, the received echo signal becomes extremely weak; consequently, further research on high-resolution, high-sensitivity detectors is needed. The photoncounting system takes advantage of its high detection sensitivity. It can convert the echo amplitude detection based on thousands of photons in linear mode into counting single photon. The application of statistical optical theory to the distance ranging system can make full use of each photon in the echo signal. The high sensitivity and high detection efficiency of the photon-counting system can reduce the requirements on laser power consumption and telescope aperture. Since the nature of photon-counting system is probabilistic detection, the poor target
characteristics and working environment will only affect its detection probability, and the target will not be completely undetectable. Therefore, photon-counting detection has extremely high reliability. In addition, the recognition of target materials and the differentiation of different targets have also received considerable attention. For such purposes, it is important to analyze the photon polarization characteristics of the detected target echo signals. In addition to the polarization measurements of underwater targets [11,12], polarization lidar can also be used for volcanic ash measurements [13,14], ice measurements [15–17], atmospheric measurements [18,19], and detection of flying honey bees. [20]. All of the above schemes rely on polarization-sensitive reception at the detector end. However, no systematic research has been carried out regarding the laser emission side. The instability of the elliptically polarized light emitted by the probe lasers also affects the polarization statistics on the receiving end. Starting with the laser emission, we have reconceptualized a lidar system based on multiple wavelengths and multiple polarization states and carried out simulation-based design [21]. To verify the feasibility of the designed system, we started by designing a desktop verification device based on a single wavelength and two polarization states. In this device, a light source that can produce both horizontal and vertical linear polarization is used at the transmission end, and the light source is controlled by means of a timedivision scheme to explore the polarization characteristics of the target surface.
∗ Corresponding author. E-mail address: [email protected] (M. Li).
https://doi.org/10.1016/j.optcom.2019.124597 Received 23 May 2019; Received in revised form 27 August 2019; Accepted 18 September 2019 Available online 20 September 2019 0030-4018/© 2019 Elsevier B.V. All rights reserved.
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Optics Communications 456 (2020) 124597
2. Scattering characteristics of the target surface
Thus, one can obtain the reflectance for the photoelectric field of the S light: √ √ √ (𝑛1 cos 𝜃 − 𝑢2 )2 − 𝑣2 2 | | (7) 𝑅𝑠 = |𝑟𝑠 | = √ (𝑛1 cos 𝜃 + 𝑢2 )2 − 𝑣22
When photons propagate through the interface between two media, the polarization of the photons change in accordance with the law of refraction and Fresnel’s law of reflection [22], as shown in Eq. (1). For nonmetallic media, the electric field of the photons after the refraction or reflection of S light (perpendicular- or senkrecht-polarized light) and P light (parallel-polarized light) may change in direction and intensity, but the polarization direction of the photons does not change. ⎧ ⎪𝐸 ′ ⎪ 0𝑠 ⎪ ⎪𝐸 ′′ ⎪ 0𝑠 ⎨ ⎪𝐸 ′ ⎪ 0𝑝 ⎪ ⎪𝐸 ′′ ⎪ 0𝑝 ⎩
= = = =
𝑛1 cos 𝜃 − 𝑛2 cos 𝜃 ′′ sin(𝜃 − 𝜃 ′′ ) 𝐸 =− 𝐸 𝑛1 cos 𝜃 + 𝑛2 cos 𝜃 ′′ 0𝑠 sin(𝜃 + 𝜃 ′′ ) 0𝑠 ′′ 2𝑛1 cos 𝜃 2 sin 𝜃 cos 𝜃 𝐸 = 𝐸 𝑛1 cos 𝜃 + 𝑛2 cos 𝜃 ′′ 0𝑠 sin(𝜃 + 𝜃 ′′ ) 0𝑠 𝑛2 cos 𝜃 − 𝑛1 cos 𝜃 ′′ tan(𝜃 − 𝜃 ′′ ) 𝐸 = 𝐸 𝑛2 cos 𝜃 + 𝑛1 cos 𝜃 ′′ 0𝑝 tan(𝜃 + 𝜃 ′′ ) 0𝑝 ′′ 2𝑛1 cos 𝜃 2 sin 𝜃 cos 𝜃 𝐸 = 𝐸 𝑛2 cos 𝜃 + 𝑛1 cos 𝜃 ′′ 0𝑝 sin(𝜃 + 𝜃 ′′ ) cos(𝜃 − 𝜃 ′′ ) 0𝑝
the change in the phase angle of the s-field: tan 𝛿𝑠 =
tan 𝛿𝑝 = 2𝑛1 𝑛22 cos 𝜃
𝑛̃ = 𝑛 − 𝑖𝑘𝜅
(3)
2𝜅𝑢2 − (1 − 𝜅 2 )𝑣2 𝑛42 (1 + 𝜅 2 )2 cos 2𝜃 − 𝑛21 (𝑢22 + 𝑣22 )
(10)
The light reflected from a metallic interface can be represented as ) (| | ( ′) ( ) 𝑟 𝐴 𝐸𝑠 𝐴 𝑠 𝑟𝑠 𝛿𝑠 | 𝑠 | 𝑠 (12) = = 𝑒 | | 𝐸𝑝′ 𝐴 𝑝 𝑟𝑝 |𝑟𝑝 | 𝐴𝑝 𝑒𝛿 | | where 𝛿 = 𝛿𝑝 − 𝛿𝑠 . When non-S- and non-P-polarized light is reflecting from a flat metal surface, the main factors affecting the polarization state of the system are the reflectances and phase differences for S- and P-polarized light. This will cause the linearly polarized light to become elliptically polarized after being reflected. In short, the effect of reflection from a metal surface on linear polarization usually manifests as bidirectional attenuation and phase delay [23], and any roughness of the metal surface will cause polarized laser light to diffusely reflect and become depolarized [24–26]. The reflectances of a metal surface for both S and P light components are usually much larger than the surface reflectance of a nonmetallic material. The higher the magnetic metal content of the metal is, the more pronounced the depolarization effect of photons [27,28]. By additionally exploiting the high sensitivity of single-photon detection, we can count the number of received echo photons with each of the two linear polarizations to distinguish whether a target surface is metallic or nonmetallic and even to distinguish among different metal surfaces.
(4)
where 𝜃̃′′ is the complex refractive index angle, 𝑟𝑠 is the reflection coefficient for the S wave, and 𝑟𝑝 is the reflection coefficient for the P wave. Let the following hold:
3. Dual -polarization-sensitive time-division laser ranging system 3.1. Experimental setup The proposed dual-polarization-sensitive laser ranging system is shown in Fig. 1. A pulsed laser (Teem Photonics, SNG-20F) is used as the photon source. The laser emits pulses at a wavelength of 532 nm with a full width at half maximum (FWHM) of 550 ps and a repetition rate of 21 kHz, and the reflected light passes through an attenuator to enter the main wave detector. The polarization direction of the transmitted photons is modified to horizontal by a linear polarizer. Then, a 50:50 beam splitter (BS) divides the horizontally polarized light into two beams of approximately equal energy. One beam is transmitted through a polarizing beam splitter (PBS) and transmitted; the other horizontally polarized beam passes through a half-wave plate (HWP), thus changing its polarization to vertical. These photons are then emitted through the reflecting surface of another PBS.
(5)
The law of refraction 𝑛1 sin 𝜃 = 𝑛̃2 sin 𝜃̃′′ and Eq. (3) together can be used to solve for the values of 𝑢2 and 𝑣2 . Substituting Eq. (5) into Eq. (4) yields 𝑛1 cos 𝜃 − (𝑢2 − 𝑖𝑣2 ) 𝑛1 cos 𝜃 + (𝑢2 − 𝑖𝑣2 )
𝑛̃2 cos 𝜃 − 𝑛1 (𝑢2 − 𝑖𝑣2 ) | | = |𝑟𝑝 | 𝑒𝛿𝑝 = 2 | | 𝑛̃22 cos 𝜃 + 𝑛1 (𝑢2 − 𝑖𝑣2 )
(9)
1 2
For a flat metal surface, the polarization directions of the S and P waves remain unchanged, and the amplitude and phase changes do not affect the detection of the polarization state. Linear polarization in other polarization directions can be represented by means of a Jones matrix: ( ) ( ) ⃖⃖⃗ = 𝐸𝑠 = 𝐴𝑠 𝐽 (11) 𝐸𝑝 𝐴𝑝
where 𝑛 is the real part of the refractive index and 𝜅 is the extinction coefficient. When a photon is incident from a medium with a refractive index of 𝑛1 at an angle 𝜃 on a metal surface with a complex refractive index 𝑛̃2 , the Fresnel formula applies, as shown in Eq. (4):
= ||𝑟𝑠 || 𝑒𝛿𝑠 =
2
and the change in the phase angle of the p-field,
(1)
(2)
⎧ ⎪𝑟 𝑠 ⎪ ⎪ ⎪𝑟 ⎨ 𝑝 ⎪ ⎪ ⎪= ⎪ ⎩
1 2
2
⃖⃖⃗ + ̃ ⃖⃖⃗ = 0 ∇2 𝐸 𝑘2 𝐸 √ 𝜀. where ̃ 𝑘 = 𝑤 𝜇̃ The index of refraction is also complex,
𝑛̃2 cos 𝜃̃′′ = 𝑢2 − 𝑖𝑣2
(8)
𝑢22 + 𝑣22 − 𝑛21 cos 2𝜃
the reflectance for the photoelectric field of the S light: √ √ 2 [𝑛 (1 − 𝜅 2 ) cos 𝜃 − 𝑛1 𝑢2 ]2 + [2𝑛22 𝜅 cos 𝜃 − 𝑛1 𝑣2 ]2 | | √ 𝑅𝑝 = |𝑟𝑝 | = √ 2 | | [𝑛2 (1 − 𝜅 2 ) cos 𝜃 − 𝑛 𝑢 ]2 + [2𝑛2 𝜅 cos 𝜃 + 𝑛 𝑣 ]2
where 𝐸0𝑠 and 𝐸0𝑝 represent the amplitudes of the incident S and ′ and 𝐸 ′ represent the amplitudes of the S P light, respectively; 𝐸0𝑠 0𝑝 ′′ and 𝐸 ′′ represent the and P light, respectively, after reflection; 𝐸0𝑠 0𝑝 amplitudes of the S and P light, respectively, after refraction; 𝑛1 and 𝑛2 are the refractive indices of media 1 and 2, respectively; and 𝜃, 𝜃 ′ , and 𝜃 ′′ are the incidence, reflection, and refraction angles, respectively, of the light. When photons propagate at a metallic interface, the part of the light field propagating within the metal material satisfies the Helmholtz equation:
⎧ 𝑛 cos 𝜃 − 𝑛̃2 cos 𝜃̃′′ ⎪𝑟 𝑠 = 1 ⎪ 𝑛1 cos 𝜃 + 𝑛̃2 cos 𝜃̃′′ ⎨ 𝑛̃2 cos 𝜃 − 𝑛1 cos 𝜃̃′′ ⎪ ⎪𝑟 𝑝 = 𝑛̃2 cos 𝜃 + 𝑛1 cos 𝜃̃′′ ⎩
𝑛1 cos 𝜃 − (𝑢2 − 𝑖𝑣2 )
(6)
[𝑛22 (1 − 𝜅 2 ) cos 𝜃 − 𝑛1 𝑢2 ] − 𝑖[2𝑛22 𝜅 cos 𝜃 − 𝑛1 𝑣2 ] [𝑛22 (1 − 𝜅 2 ) cos 𝜃 − 𝑛1 𝑢2 ] − 𝑖[2𝑛22 𝜅 cos 𝜃 + 𝑛1 𝑣2 ] 2
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Optics Communications 456 (2020) 124597
Fig. 1. Optical schematic of the dual-polarization-sensitive laser ranging system.
roughness of 6.30. The metal plate was tilted at an angle of 45 degrees to the incident surface. We first blocked the vertically polarized photons, and allowed the horizontally polarized light to illuminate the target metal plate to obtain the point cloud data shown in Fig. 3(a). The point cloud data shown in Fig. 3(b) were similarly obtained by blocking the horizontally polarized photons. These point cloud data were generated by fusing the signals received from vertical channel and horizontal channel. Despite the use of polarization-sensitive detection, we can only extract distance information based on point cloud data. However, by performing polarization-sensitive detection on the echo photons, we can obtain multiple sets of different photon data to further analyze the polarization characteristics of the target. First, let us analyze the echo photons detected in the two channels with horizontally polarized light incident on the target. As shown in Fig. 4(a), the GM-APD of vertical channel receives only vertically polarized noise photons, and no distance information can be extracted. By contrast, in Fig. 4(b), the distance information is clearly displayed. The echo photon information obtained in the two channels is essentially consistent with expectations. Compared to the case of a nonmetallic target, more noise data are received in horizontal channel when the target is metallic. Fig. 4(c) and (d) shows the echo photons detected in the two channels with vertically polarized light incident on the target. As seen in Fig. 4(c), the GM-APD of vertical channel receives vertically polarized photons containing distance information. However, as seen in Fig. 4(d), although horizontal channel is expected to receive only horizontally polarized noise photons, but it also receives intermittent information about the distance-related photons. The aluminum plate was tilted at an angle of 45 degrees to the incident surface. Due to the rough surface of the aluminum plate, both probes reflected non-S- and non-P-polarized light from the surface of the aluminum plate. When polarized light is incident on the metallic plane at a large angle, the reflectance in the P direction is greater than the reflectance in the S direction [27]. This makes the depolarization effect of vertically polarized photons more obvious. During the experiment, when the direction or incident angle of the target changes, Fig. 4(a) also shows that the vertical channel receives obviously distance-related signal photons. From the abscissa of the point cloud diagram in Fig. 2(b) and (c), it can be known that the number of photons received by a single photon detector during a probe is approximately 1000. However, from the abscissa of the point cloud diagram in Fig. 4(b) and (c), it can be known that the number
The target can be rotated to change different angles of incidence. After the emitted photons are reflected by the target object, they are received by the optical system. The echo photons are filtered by a 532 nm narrowband filter (NBF) to remove stray light and are then passed through a PBS. The PBS separates the photons into those that are horizontally polarized and those that are vertically polarized. The photons received by the receiving telescope are coupled into two separate Geiger-mode avalanche photodiodes (GM-APDs) (Excelitas, SPCM-AQRH-13, dead time 22 ns, dark count 250 Hz, 50%@532 nm) through fibers. The output of each GM-APD is connected to the ‘‘Stop’’ pin of a time-to-digital converter (TDC) (Agilent, U1051A), while the main wave detector signal is connected to the ‘‘Start’’ pin. The TDC module can simultaneously measure the time information of six channels and has a time resolution of 50 ps. The vertically linearly polarized light enters vertical channel (channel 2) of the TDC, and the horizontally linearly polarized light enters the horizontal channel (channel 3). 3.2. Experimental results and analysis A nonmetallic detection experiment was carried out with a wall as the target. The incident surface was inclined at an angle of 45 degrees to the wall. We first blocked the vertically polarized photons and allowed the horizontally polarized light to illuminate the wall. Then, the horizontally polarized photons were shielded, and the vertically polarized light was allowed to emerge. The point cloud data obtained with the horizontally and vertically polarized light are shown in Fig. 2(a)–(d), where Fig. 2(a) and (b) show the photons received in vertical channel and horizontal channel, respectively, with horizontally polarized light incident on the target, in which case distance information can be obtained only from horizontal channel, and Fig. 2(c) and (d) show the photons received in vertical channel and horizontal channel, respectively, with vertically polarized light incident on the target, in which case distance information can be obtained only from vertical channel. The photon counts shown in Fig. 2(a)–(d) that are not at range are the combined effects of the dark count of the detector and the scattered light in the environment. As demonstrated here, for the detection of nonmetallic objects, the polarization information of the received echo photons is well preserved, with no obvious changes. Next, a metal target was detected. We chose 2A12 (aluminum) as the metal target, and its surface was black anodized with a surface 3
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Optics Communications 456 (2020) 124597
Fig. 2. Point cloud data for nonmetallic target detection.
The system was calibrated based on the horizontal polarization case. Based on this calibration, the results obtained with the two probes differ by 0.195 m and 0.210 m. As seen from the optical schematic in Fig. 1, the vertically polarized light is deflected by 90◦ from the horizontally polarized light through an HWP and then reflected by a PBS. Thus, compared with the horizontally polarized light, the vertically polarized light is subjected to an additional polarization conversion process in the current system. This discrepancy can be corrected. All of the above experiments were carried out at an angle of 45 degrees to the incident surface between the wall and the aluminum plate. When the incident angle of metal surface is changed, the depolarization effect of the echo photons will also change correspondingly. Fig. 6(a)–(c) shows the point cloud data for the detection of the aluminum plate at an incident angle of 0, 30 and 45 degrees. Where the laser source is incident with vertically polarized light and receives photons with horizontal channel. Fig. 6(c) is the same Fig. 4(d), and the depolarization effect of photons is relatively obvious. Fig. 6(a) shows that when the incident angle is 0 degrees, depolarized photons are barely detected in the horizontal channel. As shown in Section 2, when polarized laser light is incident on a metallic surface, the ratio of the light reflected in the S and P directions changes. This phenomenon causes linearly polarized light to become elliptically polarized after being diffusely reflected by the rough surface of a metal target. The elliptically polarized light thus generated can be decomposed into two linear polarized light components with different intensities that are perpendicular to each other. Thus, we can distinguish whether the surface of a target is metallic or nonmetallic according to the numbers of echoes observed in vertical channel and horizontal channel, and we can even distinguish among different metal surfaces. To distinguish between different metallic surfaces, it is necessary to ensure that the detection distance and the angle of incidence are approximately the same. The magnetic content of different metallic surface is different, and the depolarization effect of echo photons is also different. So we can take advantage of the photon-counting laser ranging system to obtain the number of corresponding photons in the effective distance range of horizontal channel and vertical channel respectively. Different metallic surfaces have different ratios of depolarized photons (𝑛𝐻𝑉 , 𝑛𝑉 𝐻 ) to polarized photons (𝑛𝐻𝐻 , 𝑛𝑉 𝑉 ). Where 𝑛𝐻𝑉 is the number of photons transmitted in the horizontal polarized direction and received in the vertical channel; 𝑛𝑉 𝐻 is the number of photons transmitted in the vertical polarized direction and received in the horizontal channel; 𝑛𝐻𝐻 is the number of photons transmitted in the horizontal polarized direction and received in the horizontal channel; 𝑛𝑉 𝑉 is the number of photons transmitted in the vertical polarized direction and received in the vertical channel. By calculating 𝑛𝐻𝑉 ∕𝑛𝐻𝐻 and 𝑛𝑉 𝐻 ∕𝑛𝑉 𝑉 , different metallic surface can be distinguished.
Fig. 3. Point cloud data for metallic target detection. (a) with horizontally polarized incident light, (b) with vertically polarized incident light.
of photons received by the single photon detector during a probe is about 2500. Both probes are performed at the same distance in the same laboratory environment, indicating that the reflectivity of the aluminum plate is much higher than the wall surface. By combining the depolarization effect of the metallic surface and the reflectivity of the surface of different materials, we can distinguish whether the surface of a target is metallic or nonmetallic. Fig. 5(a)–(d) shows histogram of the polarization statistic for the photon information received during the two detections. The measured distance information is represented on the abscissa, and the number of photons received at the corresponding distance is represented on the ordinate. Statistics presented in different colors represent photon information received in different channels. For the detection of the nonmetallic targets, a measured distance of 6.825 m can be obtained from the photon information collected in horizontal channel in the case of horizontally polarized light emission. In the case of vertically polarized light emission, a distance of 7.020 m can be obtained from the photon statistics of vertical channel. In each case, the other channel shows only a sporadic noise distribution. For the detection of the metal target, a measured distance of 6.840 m can be obtained from the photon information collected in horizontal channel in case of horizontally polarized light emission. In the case of vertically polarized light emission, vertical channel can clearly yield a distance measurement of 7.050 m. 4
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Optics Communications 456 (2020) 124597
Fig. 4. Echo photon received from different channels. (a) vertical channel with horizontally polarized incident light, (b) horizontal channel with horizontally polarized incident light, (c) vertical channel with vertically polarized incident light, (d) horizontal channel with vertically polarized incident light.
Fig. 5. Histograms of polarization statistics. (a) nonmetallic target detection with horizontally polarized incident light, (b) nonmetallic target detection with vertically polarized incident light, (c) metallic target detection with horizontally polarized incident light, (d) metallic target detection with vertically polarized incident light.
4. Conclusion
could be obtained, thereby enabling secure anti-interference detection. This concept also suggests some new possibilities for future
We have built a polarization-sensitive photon-counting system based on dual linear polarized time-division-emitted laser light. By combining the advantages of the high sensitivity of single-photon detection with the benefits of dual linear polarization detection for identifying objects, we can obtain highly precise distance and polarization information for a target surface. If a multilinearly polarized light array could be transmitted and received, then a composite three-dimensional image containing polarization information regarding the target object
developments in lidar technology. In our future work, we will further optimize the experimental device by adding a 1064 nm laser source and modifying the receiving polarization detection module to recognize 4 types of light, thus further expanding the application scenario to multiple polarization states at multiple wavelength for secure target detection. 5
D. Shi, M. Li, G. Huang et al.
Optics Communications 456 (2020) 124597
Fig. 6. Depolarization of echo photons at different incident angles. (a) 0 degrees, (b) 30 degrees, (c) 45 degrees.
Acknowledgment
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