Zongjie Bi, Songlin Yin, Yanchao Zhang,2, Zihao Cui,2, Zhaoshuo Tian,2 and Shiyou Fu,2*

(1.Information Optoelectronics Research Institute, Harbin Institute of Technology(Weihai), Weihai 264209, Shandong, China;2. Shandong Institute of Shipbuilding Technology, Weihai 264209, Shandong, China)

Abstract: This paper presents a novel laser-induced fluorescence(LIF)Lidar system for marine oil spilling detection.A bifurcated Y-type optical fiber and an optical collimating lens compose a coaxial configuration transceiver for this LIF-Lidar system.This LIF-Lidar system was further applied to measure the excitation spectra from floating oil slicks with different thicknesses on top of seawater at different distances.The system presents several advantages such as compact structure, stable optical path, and convenient operation, which offers a wide application prospect in ocean exploration.

Keywords: LIF-Lidar; Y-type optical fiber; common transceiver; oil spill detection

0 Introduction

In recent years, with the exploitation of petroleum resources and the prosperity of oil industry, the incidents of oil spills have occurred frequently which are mainly caused by offshore drilling, oil tanker collisions, and leakages from pipelines and ships[1-2].Oil spill pollution results in not only serious detrimental implications on marine environment, but also decade-long threats to marine biota and human health[3-6].Therefore, some reliable detection instruments and techniques are necessary for emergency response and oil quantity assessment in oil spilling incidents[7].Laser-induced fluorescence(LIF)is considered as a promising method since it can rapidly discriminate and detect the fluorescent substances(Polycyclic Aromatic Hydrocarbons-PAHs)in petroleum[8-10].By employing light detection and ranging(LIDAR)operation with the advantages of no sampling, non-contact, and non-destructive detection, LIF-Lidar has been studied for a long time for oil-polluted monitoring[11-14].In general, two types of Lidar optical collimating configurations are used for transmitting and receiving signals, namely the biaxial transceiver and coaxial transceiver[15].In the biaxial transceiver, the emitting laser is not coaxial with the receiving field of view(FOV)center of echo signals.The incomplete overlap between the transmitter divergence and receiver’s FOV in Lidar leads to the ineffective capture of the return-light signals during the measurement[16-17].In theory, this issue would be resolved by employing the advanced lengthy alignment procedures with complex setups.However, some unavoidable factors, such as mechanical shocks and vibrations in the observation platform[18], would simply result in a misalignment during the actual mobile measurement.On the other hand, in the coaxial transceiver, in order to capture the return-light signals effectively, the transmitting laser beam is usually launched along the axis of the receiver FOV[19]by building an optical system using reflective mirrors and auxiliary optical components including beam splitter, light filter, and wave plate.However, these optical components result in high complexity and cost of the optical system.Moreover, they also lead to a central obstruction.This obstruction affects the cross section area around the optical axis, thus reducing the collection efficiency of echo optical signals[15].

In this paper, a novel LIF-Lidar system was designed for oil spilling detection in the seawater.In this system, the common transceiver consisted of a bifurcated “8+1” Y-type fiber and one collimating lens.Based on this design, the laser source and the collection FOV of excited spectral signals were under coaxial condition.The practicability of this system was proved by a series of experiments detecting the spectrum from the oil slicks with different thicknesses on top of seawater.Compared with the traditional biaxial/coaxial transceiver optical systems, this LIF-Lidar system offers several advantages such as simple operation, compact structure, small volume, and effective spectral signal capture.

1 Development of LIF-Lidar System

1.1 Principle of Lidar Optical Structure

Y-type optical fiber is one of the major optical devices in the transmissive LIF-Lidar system that we designed.Generally, Y-type optical fiber, which can split and combine light signals, is composed of a common detecting port and two legs corresponding to transmitting and receiving ends as shown in Fig.1.This special structure enables its different applications such as in biosensors[20-23].Therefore, Y-type optical fiber is also referred to as the “fluorescence probe”[24].Depending on the types of optical fiber adopted in the receiving end, Y-type optical fiber can be further classified as split and bifurcated structures.As shown in Fig.1(a), in the split structure, both the transmitting and receiving ends adopted the single-core optical fibers.These two fibers were fused together and extended to detecting port by fused taper.On the other hand, in the bifurcated structure shown in Fig.1(b), the transmitting end adopted the single-core type fiber and the receiving end employed an eight-core fiber bundle group.This eight-core fiber bundle group would greatly improve the collecting efficiency of total light signals.At the detecting port, the single transmitting fiber and the eight-core receiving fibers were bundled into an “8-around-1” fiber cable.In order to increase the delivering efficiency, the eight fibers from the receiving end were arranged as the ring circling the central single fiber from the transmitting end.Compared with the split fiber structure, the bifurcated fiber one offers many advantages such as high collecting efficiency of optical signals, convenient productive process, low cost, and simple maintenance.

Fig.2 presents the schematic diagram of the designed transmissive common transceiver LIF-Lidar optical system based on a bifurcated “8-around-1” Y-type fiber.As is shown, a laser beam propagating to the detecting port was firstly coupled into the fiber through the emitting end by a lens.The laser beam emitting from the detecting port was then collimated by another lens.The focal point of the collimation lens and the position of the detecting port were coincident through adjustment.This collimated laser beam was used to excite the distant target.Meanwhile, the generated optical signals from the target were collected by the same optical system.These collected signals finally propagated to the spectral detection system through the receiving end.This common transceiver optical structure offered a simpler operation process since only one-dimensional alignment was necessary in this optical structure.

Fig.2 Schematic diagram of transmissive common transceiver optical system using a bifurcated Y-type fiber

1.2 Operation of the LIF-LIDAR

Employing the optical system described above, Fig.3 shows the schematic depiction of an LIF-Lidar that we developed for marine oil spill detection.The common transceiver is composed of the Y-type bifurcation fiber and collimating optical lens.The fiber length of each leg was 25 cm from the bifurcation.The emitting port(A in Fig.3)consisted of a single-core type quartz fiber with a core diameter of 400 μm, and the receiving end(B in Fig.3)was bundled with 8 quartz fibers with a 200 μm-diameter core.The detecting port(C in Fig.3)of the “8-around-1” bundled fiber cable was arranged at the optical axis of the collimating lens and close to the focus point.The lens was a double convex lens with an 8 cm diameter and 12 cm focal length.

In the LIF-Lidar detection process, the equipped semiconductor laser source, which has a central wavelength at 405 nm and the maximum emitting power of 800 mW, was operated in pulse modulation mode.The laser beam was focused and coupled to the emitting end of the Y-type fiber by a coupling lens.After being output from the common detection port of the fiber, the collimated laser beam irradiated at a distant target.Meanwhile, the generated optical signals collected by the collimating lens propagated to the Lidar detection system through the fiber receiving end.A long-pass filter with the cut-off wavelength of 410 nm was employed to block the elastic laser radiations to the detection system.The detection system consisted of a grating spectrometer and an intensified charge-coupled device(ICCD)camera, which had a resolution of 656×492.The operating frequency and timegate(800 ms integral time)of ICCD can be modulated.A digital generator with the pulse generation and time delay functions was used to synchronously trigger ICCD and laser source.Each laser pulse yielded a set of spectral data after subtracting ambient light.The broadband coverage from 418 to 816 nm with a spectral resolution of 0.15 nm and the signals from the CCD camera were recorded and processed by PC.The used LIF-Lidar operational software was developed by LabVIEW instrument control package(National Instruments)and the data processing was performed using Origin Pro 9.0 software.

2 Experiment and Results

By using the developed LIF-Lidar system, the seawater with and without oil slicks were measured separately at Weihai Port in Shandong Province.Fig.4(a)shows the schematic diagram of experimental set-up used in the experiments.The horizontal 405 nm laser beam from the Lidar system was reflected by a 45-degree mirror and vertically incident to the tested sample.The spectral signals generated by this excitation laser were then collected by the Lidar optical system through the same mirror.The photographs of the installed measurement set-up on the experimental ship are presented in Fig.4(b).The LIF-Lidar system with the adjustable height and horizontal position was driven by the DC 12 V battery.A 50 cm×50 cm observation window on the stern deck and the mirror was set one meter above.Besides, the laser beam and the excited fluorescence of seawater were visible in the experiments.

Fig.4 (a)Schematic diagram of LIF-Lidar experimental set-up(b)Installation pictures of spectral measurement on the experimental ship

Fig.5 presents the excited inelastic spectrum from clean seawater received by the developed LIF-Lidar system.As is shown, the received spectrum mainly included three peaks corresponding to the Raman, organic soluble, and chlorophyll fluorescence peaks, respectively.The Raman peak centering at 470 nm had the highest intensity since the incident laser mainly interacted with the water molecules[25].The centers of emission bands from Chromophoric Dissolved Organic Matter(CDOM)and chlorophyll in seawater were located at 520 nm and 685 nm, respectively.By changing the sensing distance from the lidar system to the seawater surface from 10 to 2 m respectively, the intensity of the spectral signals were gradually amplified with the distance increase.

Fig.5 The excited spectrum of clean seawater at diverse detection distances

In addition, the spectra of diesel oil slicks spreading on the seawater surface with different thicknesses were measured and the detection range was fixed at 6 m.To prepare the test samples, certain volume 0# diesel oil from Sinopec Group was firstly released into the seawater with the transfer pipette.The average thickness of oil slicks(except those sunk into water)was then calculated based on the overall volume and area of the slicks on the seawater surface.All the thickness calculations and spectral measurements were performed when the floating oil was stable.As shown in Fig.6, by using the developed LIF-Lidar system, the excitation spectrum of both seawater and oil films were obtained simultaneously.The diesel oil slick presents stronger fluorescence spectra distributed in the range between 420 nm and 550 nm.Meanwhile, due to the low viscosity and good transparency[12]of the refined diesel oil, the intensity of Raman peak and chlorophyll peak from the seawater can also be observed.Furthermore, as the thickness of the oil flicks increased from 25 μm to 200 μm, the intensity of fluorescence spectrum from the oil flicks increased significantly.These results demonstrate that the LIF-Lidar system developed in this work can be used to assess pollution level according to fluorescence intensity of oil slicks.

Fig.6 The excited spectrum of clean seawater and spreading diesel oil films with diverse thicknesses

Raman scattering light is a special property of water.The position of Raman peak in the fluorescence spectrum of sea water was fixed under a certain wavelength of laser.Raman scattering light will be attenuated according to the law of Lambert Beer absorption due to the absorption of oil film.Using the inhibition of oil film thickness on the surface of seawater to calculate the thickness of oil film on the surface of seawater is a method of measuring oil film thickness by using Raman scattering technology.The formula can be simplified as follows:

(1)

whereAis the extinction coefficient,R′ is the intensity part of the Raman peak higher than the fluorescence curve when there is oil film, andRis the intensity part of the Raman peak that is higher than the fluorescence curve when there is no oil film.Then the logarithm of the Raman intensity value and the oil film thickness value were extracted in the above curve for numerical fitting, and the resulting relationship is shown in Fig.7.

Fig.7 Relationship between Raman intensity and oil film thickness

It can be seen from the figure that the logarithm of Raman intensity value is approximately linear with the value of oil film thickness, which is consistent with the theory.It can be used to measure and analyze the oilfilm thickness.

3 Conclusions

In this paper, the design, development, and application of the novel LIF-Lidar system are presented, which combines the receiving and transmitting parts based on the bifurcated “8-around-1” Y-type fiber.Compared with traditional transceiver, this system offers simple operations in optical collimation and adjustment.This system was then employed to measure the fluorescence spectra from the oil slicks with different thicknesses on top of the seawater.Compared with the spectrum from the clean seawater(without oil slicks on top), the contribution from the oil slicks can be clearly observed.Meanwhile, this contribution became more pronounced as the oil slick thickness increased.These experimental results demonstrate that this novel LIF-Lidar system not only detects marine in-situ oil leakages, but also supplies information for oil spill quantity.