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Abstract
We generate a macro-pulsed chaotic laser based on pulse-modulated laser diode subject to free space optical feedback, and demonstrate the performance of suppressing backscattering interference and jamming in turbid water. The macro-pulsed chaotic laser with a wavelength of 520 nm as a transmitter is used with a correlation-based lidar receiver to perform an underwater ranging. At the same power consumption, macro-pulsed lasers have higher peak power than in the continuous-wave form, enabling the former to detect longer ranging. The experimental results show that a chaotic macro-pulsed laser has excellent performance of suppressing the backscattering of water column and anti-noise interference compared with traditional pulse laser, especially by multiple accumulations about 10∼30 times, and target position can still be determined when SNR is -20 dB.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Airborne ocean lidar has been studied widely in bathymetry, which has the advantages of high efficiency, resolution and detection accuracy near the coast compared with shipboard sonar [1–4]. Lidar bathymetry is an advanced detection technology which integrates laser, communication, signal processing, target recognition and electronics. Typical commercial laser bathymetry systems include the Coastal Zone Mapping and Imaging Lidar (CZMIL) and RIEGL VQ series producing high-resolution 3D environmental data of the beach, water column, and seafloor [5–7].
The performance of bathymetric lidar is constrained by two types of factors: (1) Optical parametric properties of water, which include absorption coefficient and scattering coefficient of water body [8–10]; (2) the type of laser source. The complexity of underwater channel brings a severe challenge to the underwater detection of lidar [11,12]. The absorption and scattering of light by molecules of water and particles in the seawater make it difficult to extract underwater echo signals. Absorption of light by water can be minimized by using blue wavelengths in the open ocean and green wavelengths in the coastal waters. However, due to the backscattering of water column, especially turbid water, the ability to improve the detection depth by increasing the laser power is limited.
The key of lidar bathymetry is to obtain and extract the echo signal of the bottom of water. The echo signal can be submerged in the backscattering noise for turbid water. There are three main ways to solve the backscattering of water and the extraction of weak signal, which are traditional linear detection, single photon detection and intensity modulation detection, respectively.
The linear detection uses short pulses with range-gated receivers which provide a way to reject backscattered light by timing the receiver to open only when target-reflected light is anticipated to be received, thus a priori knowledge of the underwater depth range is required. Furthermore, high-speed optical receivers must be used to obtain high range accuracy and resolution [13,14].
Single photon detection uses high repetition frequency, low-energy laser with highly sensitive single photo detector to count the number of echo photons. The time of flight is discovered by the correlation of photo cumulative count and underwater target. This technology can detect long distance with low transmit power and small receiving optical aperture. However, the accumulation times are many and the anti-interference performance is poor [15,16].
Intensity modulation detection is an intensity-modulated approach on continuous-wave (CW) source and pulse source. The range is determined by calculating the phase of the detected RF envelope relative to the modulation signal using a RF coherent receiver. The technique has the ability to distinguish target light from backscattered light. After the non-scattered and forward scattered light are reflected by the target, the returned light should carry the modulated frequency of the initial signal, however, the returned backscattered light distributed between the light source and the target destructively interferes with each other, so that the backscattered light in the high frequency band is eliminated. Therefore, the target component with initial modulated frequency can be separated from backscattered light at the receiver by a band-pass filter with the center frequency of the initial signal [17–19].
Chaotic lidar can be considered as a special intensity modulation detection technology, which uses a noise-like, wideband signal, and a cross-correlating receiver to perform ranging with high ranging resolution and no range ambiguity [20–23]. Chaotic laser have researched on CW source and chaos-modulated pulse source in fiber [24,25]. CW chaotic lidar has been studied on underwater ranging and imaging, which has demonstrated the potential of suppressing backscattering in turbid water because of its inherent high-frequency intensity modulation [26,27].
In this paper, we generate chaotic macro-pulsed green laser beam by pulse-modulating a 520 nm laser diode subject to free space optical feedback, and then perform underwater ranging experiment to analyze the ability of suppressing backscattering interference and jamming in turbid water. Compared with CW form, macro-pulsed lasers have higher peak power to detect longer ranging.
2. Experimental setup
The experimental setup of bathymetry using macro-pulsed chaotic laser subject to free space optical feedback is shown in Fig. 1. A 520 nm SM LD of maximum output power of 70 mW and polarization of 100:1 placed in the Mounts (THORLABS LDM56/M) with a RF interface from 100 kHz to 600 MHz is modulated to pulse laser by SG with square-wave modulation. The injection current and temperature of LD are controlled by DC source with a maximum output of 1 A and TC with a power of 12 W, respectively. The output beam of LD is collimated and shaped by CL with a focal length of 8 mm. The free space optical feedback cavity generating macro-pulsed chaotic laser is composed of a half-wave plate, polarization beam splitter with an extinction ratio of 1000:1, and mirror with a reflectivity of 95.8% at 520 nm. As shown in Fig. 1, the cavity length is just 132 mm with l1 = 20 mm, l2 = 30 mm and l3 = 36 mm. The compactness ensures a low loss and realizes a miniature laser. By precisely adjusting the feedback strength controlled by HWP combined with PBS in some injection current, the LD is perturbed by the pulse laser in the cavity returned by the mirror, and then results in the generation of macro-pulsed chaotic green laser with the output power of 30 mW and the wide bandwidth of 1.2 GHz.
Fig. 1. The bathymetry system composed of macro-pulsed chaotic laser subject to free space optical feedback. SG: Signal generator; SM LD: Single-mode laser diode; DC: Direct current; TC: Temperature controller; CL: Collimating lens; HWP: Half-wave plate; PBS: Polarization beam splitter; M: mirror; BS: Beam splitter; PD: Photodetector; APD: Avalanche photodiode; T: Target
The macro-pulsed chaotic laser is then split by a 10:90 beam splitter (BS) before entering the water tank. 10% of the beam energy is focused directly onto a PD as the reference signal, and the remaining 90% is transmitted into the water bank. The reflected light beam on the target (T) passes back out of the tank, and is focused by collection lens onto the APD which can receive fainter light than PD. The cross-correlation between the reference and return signals is performed that lag is directly proportional to the difference between the reference and probe path lengths. The peak time lag is converted into range by L = (ΔT+Δτ) c / 2ncosθ, which ΔT is the peak time lag, Δτ is the time of system deviation calibration, c is the speed of light in vacuum, n is the refractive index for water, θ is the acute angle between reflected light and the vertical incident light at target.
3. Temporal and correlation characteristics of macro-pulsed chaos
As the name suggests, macro-pulsed chaotic laser is a continuous chaotic laser loaded on top the pulse as shown in Fig. 2(a), and the characteristics are influenced by the feedback intensity, injection current, modulation voltage, duty cycle, and modulation frequency (MF) on the pulses which have been investigated in detail [28]. Actually, the noise-like chaotic state is excited by perturbing the laser diode with free space optical feedback. In this setup, the optimum chaotic states are chosen with feedback intensity of 11.7%, injection current of 75 mA, duty cycle of 50% and MF of 15 MHz for obtaining as much output power as possible. The thumbtack autocorrelation trace is shown in Fig. 2(b), the full width at half maximum (FWHM) determining the ranging resolution is about 0.27 ns and the peak side-lobe level (PSL) evaluating the signal-to-noise ratio (SNR) is -8.3 dB.
Fig. 2. The characteristics of macro-pulsed chaotic laser generated by modulating laser diode subject to free space optical feedback. Here, the waveform of macro-pulsed chaotic signal (a), with the modulation frequency of 15 MHz and duty cycle of 50%, the thumbtack autocorrelation trace (b), with FWHM of 0.27 ns and PSL of -8.3 dB.
4. Results and analysis of underwater ranging
4.1 System calibration
Calibrate the system deviation when the turbidity of water in the tank is 1.1 NTU (piped water) and the target is 1.420 m away from the front glass surface of the tank. In this case, the echo and reference signals are shown in Fig. 3(a), and cross-correlation curve is shown in Fig. 3(b) that the peak time lag is 15.70 ns. According to the L = (ΔT+Δτ) c / 2ncosθ, the time of system deviation calibration (Δτ) is -3.11 ns (c = 3 × 108 m/s, n = 1.33, θ = 0°). The echo and reference signals are acquired by DC-coupled PD and AC-coupled APD, respectively. During cross-correlation operation, both the useful parts of chaotic signals are used, as shown in Fig. 3(a).
4.2 Underwater ranging experiment for chaotic macro-pulse laser
Move the target position to 1.320 m away from the front glass surface of the tank, add 30 mL, 20 mL and 10 mL orange juice in turn into the clear water with a volume of 147 cm × 38 cm × 12 cm in the tank, measure the turbidities of the water are respectively 2.3 NTU, 3.6 NTU and 4.5 NTU after the diffusion is uniform. The echo and reference signals under three turbidities are shown in Fig. 4(a), Fig. 4(c) and Fig. 4(e), respectively, and the corresponding cross-correlation curves are shown in Fig. 4(b), Fig. 4(d) and Fig. 4(f). The peak time lags are 14.80 ns, 14.9 ns and 14.1 ns, respectively, and the calculated distances are 1.318 m, 1.330 m and 1.230 m. The differences between the single ranging result and true value are respectively 2 mm, 10 mm and 90 mm. It can be seen from the Fig. 4 that the echo signal is gradually weak with the increase of water turbidity, indicating that the absorption and scattering of water are becoming more and more serious. In Fig. 4(d), the single ranging error of 10 mm is mainly caused by water refractive index. With the increase of the water turbidity, the refractive index should increase from 1.33 to about 1.34. In this case, if the refractive index uses 1.337 in the turbidity of 3.6 NTU, the single error is 2.7 mm. In Fig. 4(f), the peak of cross-correlation is not the lag time of target, because the backscattering of water column is too serious and the echo signal is submerged.
The above-mentioned is single measurement result. Next, we discuss multiple cumulative measurements. In the water turbidity of 3.6 NTU, the cross-correlation curves with different cumulative numbers are shown in Fig. 5. With the increase of accumulation times, the ratio of the main peak to other secondary peaks gradually increases, indicating that multiple accumulation can improve the SNR. Thus, in order to obtain the actual position of the target under the condition of enhanced scattering, it is necessary to accumulate the cross-correlation curve calculated from a single measurement for many times. In the water turbidity of 4.5 NTU (Fig. 4(f)), single measurement result cannot determine the target position by the maximum lag time. In order to obtain the correct lag time of the target under the condition of enhanced scattering, we accumulate the cross-correlation curves for 30 times, as shown in Fig. 6, and the maximum peak is the lag time of the target.
Fig. 6. The cross-correlation curve with cumulative numbers for 30 times under the turbidity of 4.5 NTU.
4.3 Comparison of ranging between chaotic and traditional pulse laser
In order to compare the ranging performance of pulse chaotic laser with traditional pulse laser, the cavity feedback mirror (M) in Fig. 1 is removed, and the output laser is a common pulse laser, as shown in Fig. 7. It can be seen that the reference signal is relatively flat compared with chaotic signal, while the echo signal still fluctuates, which may be caused by the backscattering in the water. The common laser ranging uses half of the rising edge as the judgment, so the time differences between the reference and echo signals are 14.0 ns, 13.7 ns and 17.4 ns / 12.6 ns, respectively, and the calculated distances L are 1.228 m (n = 1.33), 1.188 m (n = 1.337) and 1.603 m / 1.065 m (n = 1.337). The differences between the single ranging result and the true value are 92 mm, 132 mm, and -283 mm / 255 mm, indicating that the backscattering in turbid water seriously affects the accuracy of common laser ranging, especially it is difficult to determine the time of the half rising edge in the turbidity of 4.5 NTU, maybe 17.4 ns or 12.6 ns, while chaotic laser could suppress the backscattering of particles in water.
4.4 Analysis of suppressing noise for chaotic pulse laser
In order to analyze the anti-noise performance of chaotic macro-pulse laser, we add Gaussian white noise to the returned chaotic laser (Fig. 8(a)). When the intensity of Gaussian white noise is 10 times that of the chaotic macro pulse, the target distance information can still be extracted from the cross-correlation curve with noise (Fig. 8(c)). It shows that distance information can still be extracted for chaotic macro-pulse laser under negative SNR (-20 dB).
Fig. 8. The analysis of anti-noise of chaotic macro-pulse laser (Target distance: 1.420 m, Turbidity: 1.1 NTU).
Because the bandwidth of chaotic laser can be as high as GHz or even higher, and the mutual interference between backscattering of water column causes its corresponding noise in the low frequency region, the broadband characteristics of chaotic pulse laser suppress the narrow-band channel noise, and the chaotic signal has good orthogonal correlation characteristics, making it possible to extract the underwater weak echo signal under the condition of negative SNR.
4.5 Resolution and relative errors of the ranging under different parameters
The ranging resolution of macro-pulsed chaotic lidar is determined by FWHM of cross-correlation curve between reference signal and echo signal, which is expressed as cτFWHM/2n. The FWHM (τFWHM) and ranging resolution are shown in Table 1 under different turbidies. It can be seen that the performance of ranging resolution decreases with the increase of turbidity, which is due to the enhancement of water volume scattering, resulting in the broadening of the FWHM.
Table 1. The resolution and relative errors of the ranging under different turbidies
The relative error is expressed as |Ltrue-Lmea|/Ltrue, which Ltrue is true ranging value, Lmea is measured ranging value. As shown in Table 1, the relative error of macro-pulsed chaotic laser ranging is little, but when the turbidity is 4.5 NTU, the cross-correlation peak is not the target peak (Fig. 4(f)), and the relative error is significantly increased. After multiple cumulation, the correct target peak is identified (Fig. 6), and the relative error is significantly reduced.
5. Conclusion
We have generated macro-pulsed chaotic laser by pulse modulating a 520 nm laser diode with free space optical feedback, and analyzed the performance for underwater ranging application. The results show that chaotic macro-pulsed laser has the ability to suppress the backscattering of water, especially the ability to extract the correct target lag time of cross-correlation curve under high turbidity water by multiple cumulative measurements, due to the wide bandwidth and good orthogonality of chaotic signals. The analysis of anti-noise capability shows that the target position information can still be extracted when the SNR is -20 dB.
Future work will include using high-power pulsed chaotic laser to explore a longer ranging. When the chaotic pulse energy reaches 3 mJ, the optical receiving aperture is 200 mm, the bottom reflectance is 10%, and the diffuse attenuation coefficient of water body is 0.08 m-1, according to the bathymetric lidar equation and the ranging ability of chaotic pulse laser with negative SNR, the measuring depth can reach more than 70 m. Simultaneously, it can also verify the performance of chaotic pulse laser to suppress the stronger backscattering generated in a longer ranging.
Funding
China National Funds for Distinguished Young Scientists (61705007, 61705160); Guangxi Innovation-Driven Development Project (GuiKe AA 18118038).
Acknowledgments
Thanks to Professor Y. C. Wang for help identifying collaborators for this work.
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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