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Abstract
To meet the urgent need for surveying and mapping using remote sensing instruments, a hyperspectral imaging lidar using a supercontinuum laser is proposed. This novel lidar system can solve the problem of the mismatching of the traditional lidar retrieved elevation data and hyperspectral data obtained by passive imaging instruments. The optical design of the lidar receiving system is described, developed, and tested in this study. An off-axis parabolic mirror is used as the receiving telescope of the system, and a transmissive grating is used to split the received hyperspectral light to each detection channel. A fiber array equipped with a micro-lens is used to guide the split light to the detectors. In practice, several fibers can be coupled to one detector according to the wavelength sensitivity of different objects. A reference laser is used to monitor the possible energy jitter of each transmitted laser pulse in real time. A spectrum calibration of the receiving system is accomplished in the laboratory, and radiation calibration is applied by receiving the backscattered light reflected by a standard white board. The spectral resolution of a single fiber is approximately 3 nm. An outdoor 500-m distance experiment was carried out for green and yellow leaves in day and evening settings. During the experiment, the wavelength of the laser was 460-900 nm. The reflection spectra collected by the lidar system in day and evening were consistent, indicating that the design of the optical receiving system is reliable and can be used for airborne hyperspectral imaging lidar.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
The simultaneous acquisition of the elevation and hyperspectral information of a target is a future research area in remote sensing surveying and mapping technology [1,2]. Among the current main technical methods, lidar can acquire the elevation information of the targets, and passive imaging instruments can obtain hyperspectral information [3,4]. However, neither of these methods can achieve the simultaneous acquisition of the co-located elevation and spectral information [5,6]. By combining the advantages of both technologies, many pioneering researchers have carried out studies in this area [7–12]. Lidar technology has developed from “one-wavelength” to “multi-spectral” and “hyperspectral” [13–16]. However, the development of hyperspectral imaging lidar is limited because of the feasibility of a supercontinuum laser and the synchronous detection and acquisition of a multi-channel return signal [17,18].
Traditional lidar technology uses a single-wavelength laser emission [19] or multi-beam laser emission [20]. However, the hyperspectral imaging lidar was adopted for supercontinuum laser pulse transmitting and multi-channel receiving, which poses several challenges to lidar detection. First, a requirement for the accurate splitting and efficient coupling of the return signal [21]. Second, the nonlinear effect of the supercontinuum laser source is the cause of the uneven laser energy distribution. In particular, the laser energy of the hyperspectral lidar in the short-wave band is much weaker than the existing traditional Mie scattering lidar for atmosphere detection [22], and its return signal intensity even reaches the level of a single photon. The increasing number of spectral detection bands and the detection of weak signals when applying lidar mean that the bandwidth, sensitivity, and gain of traditional line and plane array detectors cannot meet the data processing requirements of the return signals. Therefore, new designs and methodologies are needed to solve these problems. At the same time, the light source used in hyperspectral imaging lidar is a high repetition frequency laser. The laser is limited by its own luminescence mechanism, which causes the spectral energy of each laser pulse emitted to change [23]. In traditional ground-based imaging lidar, the same target can be scanned several times. In general, the method of taking the mean value after many accumulations is adopted. To eliminate the jitter of the laser pulse energy, the return signal is accumulated several times to obtain the mean value. For airborne hyperspectral imaging lidar, it is difficult to obtain the mean value of the same target after scanning multiple laser pulses.
In this study, a hyperspectral imaging lidar receiving system is designed, which can realize the synchronous acquisition of hyperspectral and multiple detection channels. The experimental test of the ground object at a distance of 500 m was completed. The coupling method of an optical fiber array with a micro-lens array is adopted to improve the efficiency of the return signals to each channel. To receive a weak return signal within short-wave bands, the system adopts a single high-gain detector for detection. The advantage of a single detector rather than a linear array or plane array, has the responsiveness and gain of each channel can be adjusted independently. To record the energy change of each pulse of the continuous spectrum laser, a reference laser is used to monitor the energy of each pulse in real time and eliminate the influence of the energy jitter of the laser itself on the data processing.
2. Materials and methods
As shown in Fig. 1, we completed the development of a hyperspectral imaging lidar receiving system. The main tasks are focusing on optical receiving and multi-channel synchronous detection and acquisition. To improve the signal-to-noise ratio, particularly during daytime, background light suppression is carefully treated. By receiving the laser return signal scattered by the target object, the distance and spectral information of the target can be obtained simultaneously. Based on grating spectrometer splitting and fiber array coupling technology, high sensitivity APD (Avalanche Photo Diode) and PMT (Photo Multiplier Tube) detectors are used to detect the return signals. An advantage of this design, the central wavelength and bandwidth of different detection channels can be optimized, and different numbers of optical fibers can be grouped into the corresponding detectors, which shows the flexibility of the chosen central wavelength and bandwidth of different detection channels.
2.1 Overall design of the system
Figure 2 shows the overall design block diagram of the optical receiving system. According to the hyperspectral imaging lidar equation, the reflected return signal of the object surface is simulated and calculated, and the main technical parameters of the lidar receiving system can be optimized and confirmed according to the results of the simulation. The design of the receiving system includes a telescope, grating spectrometer, and micro-lens-fiber array. The design and development of the system, spectral calibration, radiation calibration, and performance testing of the system are needed.
2.2 Optical design
As shown in Fig. 3, the hyperspectral imaging lidar receiving system includes several parts, such as a scanning mirror, receiving telescope, grating spectrometer, micro-lens-fiber array, and reference laser. The laser beam emitted by the reflection of the rotating mirror and the backscattered light is guided by the receiving system by this mirror. The rotation of the rotating mirror drives the change in the direction of the laser beam to form a one-dimensional scanning track on the ground, which is combined with the flight of an aircraft to form a two-dimensional covering of the ground objects. A small portion of the transmitted laser is used as a reference, which is coupled to the receiving system through the optical fiber. The energy of each laser pulse was monitored during every measurement.
An off-axis parabolic mirror is used as a receiving telescope to receive full-band laser return signals reflected from the ground objects, and the receiving field angle of the telescope is restricted through a small aperture to suppress the background light signals entering the grating spectrometer. The return signals passing through the aperture are split through the grating, and the return signals of different bands are then coupled to the corresponding detection channels.
2.2.1 Receiving telescope
Off-axis parabolic mirrors are widely used in astronomy and laser optics and have almost no aberration along the direction of the parabolic axis [24]. An off-axis parabolic mirror is used as the receiving telescope of the system. The parabolic design can effectively reduce the dispersion of light spots on the focal plane, and the off-axis design can avoid the central blocking of the reflecting system and improve the effective optical receiving area of the receiving telescope.
According to the simulation results, the effective aperture of the receiving telescope is 170 mm in our design, which not only meets the need for a return signal detection, it also reduces the size and weight of the telescope. To make the receiving unit more compact, the focal length of the receiving telescope is designed to be 600 mm and the off-axis angle is 15°. The dielectric film was coated on the optical reflector of the telescope such that the average optical reflectivity was greater than 97% within the required spectral range.
2.2.2 Grating spectrometer
A grating spectrometer is a spectroscopic device typically used in hyperspectral imaging lidar [25], which mainly includes four parts: a small aperture, a collimating mirror, a grating, and a telecentric focusing lens. A circular aperture with a diameter of 0.3 mm is used to limit the field angle of the receiving telescope, and the receiving field angle of the system is 0.5 mrad. Both sides of the aperture are black to reduce the scattered light from the aperture surface. The light passing through the aperture is collimated by a collimating lens with a focal length of 350 mm. An off-axis parabolic mirror is also used as the collimating mirror of the grating spectrometer. The off-axis parabolic mirror can collimate the full-band return signal well and suppress the excessively large dispersion spot at the focal plane of the grating caused by insufficient collimation light, thus improving the spectral resolution of the system.
Under the premise of satisfying the spectral resolution of the system, the lidar system needs to be as compact as possible, and thus a transmission grating is selected as the spectroscopic device of the grating spectrometer, and the line density of the grating is 360 lp/mm. The focusing system behind the grating uses a transmissive-type telecentric focusing lens with a focal length of 500 mm. This design can reduce the aberrations of each channel focusing spot to improve the spectral resolution of the system. In addition, the focusing mirror system adopts a telecentric structure, which ensures that the main light of each spectral channel and perpendicular to the surface of the micro-lens-fiber, effectively matching the numerical aperture of the receiving system, and improving the optical coupling efficiency of the system.
2.2.3 Micro-lens-fiber array
For a hyperspectral imaging lidar, the return signal received by each channel is extremely weak, particularly within the short-wave bands. To achieve the detection of a full-spectrum return signal, a high gain and low noise detector is needed. The optical coupling efficiency of the lidar system should also be improved as much as possible. To match the design of the grating spectrometer, an array of 168 fibers is used to guide the return signals of different wavelengths in the focal plane of the grating spectrometer into different detectors. Figure 4 shows the layout of the fiber array. The core diameter of the fiber was 400 um, and the coating layer was 440 um. To ameliorate the gap between adjacent fibers, a micro-lens array is installed in front of the fiber array to focus the optical signal incident to the corresponding area through the micro-lens and then coupling into the fiber, which reduces the duty ratio between adjacent fibers and improves the optical coupling efficiency of the system. The size of a single micro-lens is 700 um × 570 um, arranged into a 1 × 168 one-dimensional matrix. One end of the micro-lens corresponds to the focal spot at the focal plane of the grating spectrometer, and the other end is directly coupled to the 1 × 168 fiber array.
In the absence of a micro-lens, the filling rate of the spot is 38.7%. After the micro-lens is installed in front of the optical fiber array, the simulation results show that after the light spots transmitted by the telecentric lens pass through the micro lens, the light spots are more concentrated on the optical fiber end face, which is equivalent to improving the filling rate of the optical fiber array. With the addition of a micro-lens, the energy of the spot is mainly concentrated in the center of the end face of the fiber. On the one hand, in the design process of the lidar system, we ensure that the numerical aperture of each optical component is consistent. On the other hand, we also simulated the coupling efficiency of the system directly by software. Approximately 90% of the energy of the spot is contained within a range of ∼100 um at the center of the fiber. Therefore, the coupling efficiency of the fiber array can be increased from 38.7% to more than 90% after the use of the micro-lens array.
3. Spectral calibration and radiation calibration
Compared with traditional single-wavelength lidar, an important measurement goal of hyperspectral lidar is the backscattered spectral reflection response of the object surface. However, it is necessary to calibrate the spectral intensity of the lidar system before applying the spectral reflection response measured by the hyperspectral lidar system to tasks such as ground object classification. The calibration of the system is extremely important for the application of hyperspectral lidar data [26].
In general, the return signal strength of the lidar is mainly affected by three aspects. First, the return signal of the lidar is generated by the interaction between the laser beam and target. For a hyperspectral imaging lidar, this is the reflectivity of the target on the intensity of the return signal. Second, the medium between the lidar and the target on the return signal, such as the attenuation of the return signal by the atmosphere, also has an influence. Finally, the return signal strength of the lidar is also related to the configuration and technical factors of the system itself.
The multiple channel hyperspectral lidar adopts the working mechanism is single pulse emission and multi-channel receiving. According to the actual working mode of the hyperspectral lidar system, the return signal power received by the multiple channel hyperspectral lidar can be described by the following LIDAR formula [27]:
Therefore, we determine the center wavelength and bandwidth for each channel through the spectral calibration of the lidar system. Further, through the radiation calibration of the lidar system, the most realistic ground target reflection response is obtained, that is, ${\beta _T}(\lambda )$ in the lidar equation.
3.1 Spectral calibration
The bandwidth obtained by spectral calibration is the spectral resolution of the system, which is usually described by the half-peak width. The narrower the spectral resolution of the system, the more precise the response of ground objects can be distinguished and recognized by the lidar system. The spectral resolution of the system is different from that of the traditional spectroscopic resolution instrument, which has a uniform spectrum distribution in each channel. The system adopts grating splitting followed by a fiber array so that the spectral resolution of each channel can be designed according to the application requirements. Therefore, the spectral resolution of each channel of the system must be calibrated separately.
The light source used for the spectral calibration of the system is the supercontinuum monochromator. The wavelength accuracy of the monochromator and the spectral resolution of the monochromator were both 0.2 nm. Therefore, the error in the accuracy of the spectral calibration results of the lidar system is less than 1 nm. During the actual calibration process, to improve the calibration accuracy of the system as much as possible, the increases in the wavelength used in the spectral calibration of the system were set to a step size of 0.2 nm.
As shown in Fig. 5, the collimating light from the monochromator is directly incident to the surface of the telescope window. After passing through the grating spectrometer of the system, light of different wavelengths is incident onto the corresponding fiber. A computer-controlled monochromator steps through each channel at interval wavelengths of 0.2 nm, and the other end of the optical fiber is directly coupled to the detector. The acquisition card is controlled by a computer. At the same time, the acquisition card transmits the acquisition signal to the computer for storage.
As shown in Table 1, the monochromator was used to calibrate the spectrum of the receiving system, and the central wavelength and bandwidth of 40 channels of the lidar system were given. The results show that the spectral resolution of most receiving channels in the system is approximately 9 nm. During this calibration process, the optical signals of the three optical fibers are coupled to a detector, and the spectral resolution of each fiber is approximately 3 nm. During actual airborne flight tests, wavelengths can be selected according to the actual situation of the ground object [28], and the bandwidth and central wavelength of each channel can be flexibly set. The spectral resolution of a single 3-nm fiber can accurately classify most of the ground object targets.
Table 1. Results of spectral calibration
3.2 Radiation calibration
We completed the radiation calibration of the system according to the hyperspectral imaging lidar equation [29,30], the hyperspectral lidar equation is shown in Eq. (1). To accurately obtain the most realistic reflection spectrum ${\beta _G}(\lambda )$ of the ground object, it is necessary to comprehensively consider the various factors that affect the ground object reflectivity.
Among them, the energy variation of each pulse of the high-frequency laser is an extremely important factor. The light source used in hyperspectral imaging lidar is a supercontinuum laser with high frequency repetition. The energy of each laser pulse variable on command [31]. During the data processing, if the impact of the energy jitter of the laser itself cannot be accurately deduced, it will cause the spectral distortion of the ground targets collected by the lidar.
Therefore, during the process of data processing of return signals to accurately obtain the spectrum of ground object targets, we need to monitor the energy spectrum of each laser pulse. By coupling the laser to an optical fiber (50 m), approximately 0.01% of the laser energy is used as a reference. The laser passes through the focusing lens and penetrates through the hole of the reflector mirror, reaching the collimator mirror. The energy of each pulse of the laser is then monitored.
After the reference laser is split by the grating spectrometer of the system, the optical power received by each detection channel of the system is${P_{Re f}}(\lambda )$.
The radiation calibration method applied in the current study requires us to calibrate the system using a white board before each test. We can also calibrate each constant of the system directly in the laboratory using a standard light, and finally obtain the absolute calibration coefficient of each channel. However, the influence of the atmosphere on the measurement results cannot be eliminated in the process of laboratory calibration, and it is necessary to further complete the calibration of atmosphere in the airborne test, which needs to be conducted in the future.
4. Results and discussion
We conducted an outdoor field experiment to test the performance of the lidar system, and the distance was set to 500 m where there was a dome building, as shown in Fig. 6. During the field test, a supercontinuum laser with high frequency repetition within a spectrum range of 460–900 nm was selected. The single pulse energy of the laser is 3.6uJ, the pulse width is 4 ns, and the laser frequency is 63kHz.
Figure 7 shows an overall flow chart of the test procedure. Before testing, we used an angle reflector to ensure that the optical axis of the laser was parallel to the optical axis of the telescope. Green and yellow leaves were selected for the field experiment. We placed the green and yellow leaves on the second floor of the dome building 500 m away and pointed the incident laser beam from the lidar onto the surface of the leaves. The photons reflected from the leaves returned to the lidar system and were received by the telescope and detected by the PMTs. The data acquisition system completes the multi-channel synchronous recording apparatus.
According to the radiation calibration principle and Eq. (5) of this system, it has been introduced in Section 3.2. We measured the spectra of the green and yellow leaves, and at the same time, we conducted a spectral test on the white board. The spectrum obtained using the whiteboard as the target object was compared with the spectrum of leaves, and then the spectrum of leaves was measured through this system.
As shown in Fig. 8, we obtained the reflection spectrum of green and yellow leaves at a range of 500 m during the day and evening settings, respectively. The reflection spectra collected by the lidar system in day were compared with the spectra in the evening, and the results showed that the spectra collected by the system were basically consistent in day and evening settings. The results verified that the performance of the system was both reliable and consistent [32].
The correlation analysis was made for the test results of yellow and green leaves respectively, as shown in Fig. 9. This slight difference may be caused by the wavelength-dependent attenuation of the atmosphere to the return signal. There is also a time difference between the lidar system measures the spectrum of the white board and leaves. During that period, we were required to change the testing targets. With respect to the weather conditions on the experiment day, the visibility was approximately 3 km, which is not very high.
5. Conclusions
We designed and developed an optical receiving system for a hyperspectral imaging lidar. The coupling of an optical fiber array focal plane splitter with a micro-lens is adopted to improve the detection ability of the system for weak signals. We have completed the spectral calibration of the system in the laboratory, and the spectral resolution of the single fiber of the system can reach 3 nm. The system has the advantages of optimizing the central wavelength and bandwidth of the detection channel by coupling one or more optical fibers to the corresponding detector, which indicates the flexibility of our lidar system. In this paper, the radiation calibration method of the hyperspectral lidar system is also presented. Finally, a preliminary experiential outdoor test is carried out, the results of which show good performance and the reliability of the system.
The optical receiving system will be used for airborne flight measurements, and will provide an unprecedented dataset for remote sensing surveying and mapping research. Additional stability tests such as vibration and electromagnetic compatibility (EMC) will be carried out to fit the aircraft flight environment. Meanwhile, the data analysis theory and further algorithmic developments will also need to be elaborated on in the future. We believe this novel hyperspectral imaging lidar technology will open a new era for cultivating vegetation, surface classification, mapping, and other research fields.
Funding
National Key Research and Development Program of China (2018YFB0504503).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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