尚念泽，程熠，敖申，姑力米热，李梦文，王晓愚，洪浩，李泽晖，张晓艳,＊，符汪洋,＊，刘开辉,5,＊，刘忠范,＊

1北京大学前沿交叉学科研究院，人工微结构与介观物理国家重点实验室，北京 100871

2北京石墨烯研究院(BGI)，北京 100095

3北京大学化学与分子工程学院，纳米化学中心，北京 100871

4清华大学材料学院，北京 100084

5北京大学物理学院，纳光电子前沿科学中心，量子物质协同创新中心，北京 100871

1 Introduction

Distributed sensing and monitoring, generalized referring to simultaneous detection and control to a wide range of spatial area during a long period of time, was first came up in the field of optical fibers under the occasions of intrusion detection.The concept can be extrapolated to other purposes such as environmental monitoring which requires exactly such widespatial-range sensing ability.Compared to electro-chemical sensors designed for single-spot sensing and complicated sensors array, optical fiber sensors offer an elegant and simple approach to achieve distributed sensing configuration within lower system complicity1-3.

The fact that optical fiber acts as both propagation medium and sensing medium makes it an ideal platform to realize distributed measurements, and relevant application can be realized through Optical Time-Domain Reflectometry (OTDR)4technique.In such configurations, Back-propagating light caused by Rayleigh or inelastic scattering is collected and analyzed for the modulation information brought by the sensing targets, where the signals from different distances are separated in time domain.The combination to optical Fiber Bragg Gratings(FBGs) also provides another method to achieve spatial resolutionviafrequency domain separation5.Besides that, the application of Time Division Multiplexing (TDM) and Wavelength Division Multiplexing (WDM) and other techniques, have led to substantial improvement of the accuracy,spatial range and resolution of optical fiber distributed sensing systems6-8.However, due to the confinement nature of optical fibers, such distributed sensors are limited to the field of temperature, intruding detection and specific sensing targets related to refractive index changes, which leads to its incommensurability to environmental sensing requirements3,8-10.

On the other hand, sensitive detection of harmful gases or liquids by optical fibers has been investigated for decades.Evanescent field plays a central role in the build-up of these fiber sensors.Furtherly, combining special optical fiber structure to specific functional materials, one can achieve strong interaction between the evanescent field of optical fibers and the target materials11.Special optical fiber structures with high evanescent field intensity are usually chosen in such configurations, like microfiber, D-shaped fiber and U-bent fiber12-19.Assisted by Surface Plasmon Resonance (SPR) techniques, such configuration can achieve supremely high sensibility to refractive index change caused by gas or liquid molecules20-24.Two-dimensional materials are found to be excellent candidate as coating materials due to their high specific surface area, which guarantees a large sensing response and, at the same time,minimizes any side effects by suppressing the propagating mode of optical fibers25-29.However, due to the obstacle from optical fiber engineering and device fabrication, the abovementioned functional 2D sensors are still limited to sample-scale fabrication and face the bottleneck of mass-production.In a word, due to the incommensurability between traditional fiber systems and environmental sensing purpose, there is still vacancy for a solution towards distributed environmental sensingviaoptical fibers.

In this work, we propose a new configuration of griddistributed optical fiber environmental sensing by introducing low-pressure chemical vapor deposition (LPCVD) grown graphene photonic crystal fiber (PCF) into the optical fiber sensing system.We successfully synthesized monolayer and/or bilayer graphene in the air holes of PCF30.Then by fusing the Graphene PCF (Gr-PCF) to single mode optical fiber, we build up an all-optical-fiber sensing system.Gr-PCF acts as sensing tips in which the evanescent field interacts with graphene in the innermost air holes, and the intensity (only light intensity information is required in our design, substantially compressed the complicity of the system) of reflected light from the tip end is recorded.After immersed into the analysts, we obtained expected sensing responses based on light intensity difference,which can be attributed to the doping effect from adsorbing molecules (for gas, when the refractive index is negligible) or refractive index changes (for liquid, when it prevails).Our allfiber sensing system is proved to be feasible under typical scenarios, and moreover, combining with the mass-production capability of CVD growth, and WDM, TDM techniques, it provides an important possibility to realize grid-distributed optical fiber sensing towards environmental issues, which is beyond the capability of all well-established fiber sensing systems.

In summary, by introducing two-dimensional graphene in to PCFs, and also introducing electro-chemical mechanism to optical fiber sensing systems, we achieved a fiber sensing system with detectivity and promising potential for environmental sensing.The design we propose also offers new routines and opportunities to realize distributed optical fiber environmental monitoring.

2 Methods

2.1 Gr-PCF fluid sensor setup

An Amplified Spontaneous Emission (Shanghai Connet) light source was used to generate 1520-1610 nm infrared laser.An acoustic optical modulator (Bandwidth 250 MHz) was used to apply modulation to the light signal.Single-mode fiber with cutoff wavelength of 1310 nm was chosen to carry the light.All the optical fiber devices were connected by FC/APC ports.A balanced amplified detector (Thorlabs PDB480C-AC) was used to collect reflected signal intensity.The readout of the balanced amplified detector was then received and amplified by a Stanford SR830 lock-in amplifier.By a NI DAQ-device we collected the signal from lock-in.

2.2 Gr-PCF sample characterization

A Cobalt solid-state laser generated 532 nm CW laser as incident light.The back-scattered Raman signal was collected by HORIBA iHR330 spectrometer.The electrodes in the Gr-PCF electrical devices were fabricated by DUPONT quick-drying silver glue.TheVds-Vgtransfer curve measurements were carried out by a Zurich HF2LI lock-in amplifier with pre-current amplifier.

2.3 NO2 sensing test

5 mg·L-1NO2 (N2 as background gas) and pure N2 are inserted to a gas distribution instrument equipped with mass flowmeter,diluted NO2gas with certain controllable thus flows into a homebuilt gas chamber.The chamber has an inlet and exit on each side and an aperture on the top for the PCF-SMF sample’s insertion.We started recording output signal with no gas supply, then we switched on the gas supply and kept it until the recording process ended.

Using a gas distribution instrument equipped with a mass flowmeter, N2and NO2gases with different concentrations flow through the self-designed gas chamber.Through the aperture on the chamber’s top, the PCF-SMF was inserted into the gas ambient, and at the same time, the exhausts were collected through activated carbon and eventually put into the atmosphere.

3 Results and discussion

A fiber structure with a strong evanescent field is chosen in our design, where total internal reflection photonic crystal fiber(TIR-PCF) turns to be a good choice.Periodic arranged air holes outside the core of TIR-PCF act as low index claddings,effectively confining the propagation mode in the core area.This endows PCF strong evanescent field upon the interfaces of core and adjacent air holes.By LPCVD growth method, we fabricated Gr-PCF samples with length of ～1 cm30, where uniform graphene films are proved to be formed in the air holes.

Then, we designed the fiber sensing system as illustrated in Fig.1.First, the Gr-PCF was fused with a Single-Mode Fiber(SMF), whose cut-off wavelength is 1310 nm, to prevent unwanted back-scattering light from the PCF-SMF joint.An amplified spontaneously emission light source was applied to generate C + L bands (1530-1625 nm) CW laser, which went through the 40 : 60 WDM module and split into two parts.One part of the light passed through a circulator into the fused SMFPCF fiber to interact with the graphene inside the air holes of PCF.As the cut-end of the PCF was controlled to be sharp and perpendicular to the fiber, back-propagating light reflected at the end of Gr-PCF could go back to the circulator.The collected back-reflected light and the other part of light divided by the WDM are recorded and detected by the balanced amplified detector.The target molecules can be detected when they diffused into the air holes of the Gr-PCF.Their adsorption on the monolayer graphene would modulate the interaction between the evanescent field and the graphene layer, leading to the modulation of the intensity of back-reflected light.The output voltage amplitude (an A.C.signal) of the balanced detector thus is proportional to the recorded light intensity differences, which can be extracted by lock-in technique.

Fig.1 The schematic of Gr-PCF based optical fiber sensing system.

Owing to the excellent electrical properties and chemical inertness, graphene-based field-effect transistors (FET) sensors displayed excellent sensitivity towards a wide range of chemical analysts, including pH, heavy metal ions, and harmful gas such as ammonia, nitrogen dioxide, toluene and so on31-39.Similar to the mechanism in the field-effect sensors, we attribute the sensing ability of Gr-PCF system to the interaction of graphene and target molecules, which modulates the Fermi-level of graphene and leads to both the regulated conductance and the light absorption change of graphene, rather than the mode scattering caused by refractive index change usually prevailing in optical fiber sensors.First, we characterized the quality of Gr film via optical methods.Raman spectroscopy showed uniform characteristic peaks across the external surface regions and hole regions of Gr-PCF, where the G peaks at 1580 cm-1and 2D peaks at 2680 cm-1held an intensity ratio of 2D/G ≈ 2 (Fig.2b),indicating the uniform monolayer graphene on both the inner and outer surfaces.Additionally, we examined the electrical properties of the Gr-PCF samples by fabricating Gr-FET devices(Fig.2c).To ensure that the only conducting channel is the continuous graphene on the inner surfaces of air holes, we removed the externally grown graphene by using oxygen plasma and metalized the drain and source electrodes near both ends.By injecting buffer solution into the air holes, we applied gate voltageVgviaan Ag/AgCl reference electrode and at the same time recorded the electrical conductivity of the Gr-PCF.The measured ～102kΩ resistivity between source and drain,confirms that the graphene films are continuous.The minimum conductivity located atVg= 0.45 V represents the lowest carrier concentration state and the Dirac point of graphene (Fig.2d).The existence of continuous atomic-layer thick graphene in the hole of PCF ensures the interaction between light and matter.

Fig.2 Characterization to the Gr-PCF.

Here, we applied the Gr-PCF to detect the typical pollutive gas nitrogen dioxide (NO2) for the first time.The home-made test equipment is shown in Fig.3a.The reflected light intensity changes caused by adsorption-induced-doping of graphene in different PCF-SMF samples under different gas conditions were measured.We compared the balanced detector readout in time domain (recorded as Relative Voltage) for Gr-PCF and Bare-PCF (PCF without graphene grown on the surface) under pure N2flow and 500 μg·L-1(1 μg·L-1= 1 ppb) NO2flow (N2as background), and only Gr-PCF under NO2flow showed distinguishable signal mutation when the gas flow was switched on, and from this we can conclude the gas flow caused vibration of the fiber and related signal change is negligible (Fig.3c).Considering the refractive index change induced by 500 μg·L-1NO2is negligible, the propagation mode of light is thus barely affected by the refractive index.And the signal to the modulation of light absorption is attributed to the NO2molecules adsorbed at the interfaces of graphene.NO2 molucules are diffused into the air holes of the Gr-PCF and naturally experience ad adsorption process.Given that NO2is oxidzing gas, the molecules adsorbed on graphene will attract the eletrons in graphene and cause the additional hole-doping of graphene in the inner air-holes39.Such additional hole-doping of graphene then leads to the the variation of graphene’s light absorption.In a more intuitive way, for a beam of monochromatic light with photon energy of ħω, pristine graphene with Fermi level higher than -ħω/2 will have strong absorption to the light because the transition process on the Dirac cone is permitted.However, when the Fermi level of graphene is lower than -ħω/2, the same transition process is then forbidden by Pauli blocking due to the ground states has been occupied by holes.Such evidential light absorption difference of graphene then leads to the obvious intensity contrast of the back-reflected light, which is then recorded by the detector.Further, we tested the sensitivity of our Gr-PCF sensor under different gas concentration, and the response of Gr-PCF under series of NO2 flow concentrations from 30 μg·L-1to 500 μg·L-1(Fig.3e) were collected, the flow rate was kept the same for different concentrations.It should be noticed that our Gr-PCF sensor showed comparable limit of detection (ppb level) to traditional FET sensors.Then we calculated the output voltage signal changes ΔVby subtraction ΔV = Vstat- V0, whereVstat is the average voltage readout after the switch on of the gas supply, fromt= 20 s tot= 120 s.andV0is the initial value of the voltage.The dependence of ΔVto NO2concentration was also presented (Fig.3d), which shows a clear logarithmic dependence, consistent to graphene FET sensors reported previously39.

Fig.3 Gas sensing experiment for NO2.

Besides the capability to sense gas molecules, Gr-PCF sensor is also capable to distinguish different liquid analytes.However,as the refractive index change brought by the liquid is far more obvious compared to the gas and cannot be neglected, it will prevail as the main cause of the signal change over adsorption induced light absorption change of graphene.The strategy for liquid sensing is to insert the PCF-SMF sample into different solutions and record the readout of the balanced detector (Fig.4a).As shown in Fig.4b, when the PCF-SMF samples are contacted with solution, the solution will soon be siphoned into the air holes of PCF by capillary effect.This changes the refractive index of the effective cladding, which will significantly alter the fiber modes and the evanescent field, and eventually modulate the total transmission loss (including the absorption of graphene) of light before collection.Compared to random signal evolution of Bare-PCF, the signal evolutions of the Gr-PCF sample were more stable and displayed distinguishable patterns when they were regularly put in and out of three different solutions: water, NaCl, and ethanol (Fig.4ce).The difference of readout voltage step changeΔVof three different solutions can be explained by the difference of capillary depth, together with the difference of refractive index.Consequentially, the mechanism of liquid sensing for our fiber system would be rather complex and require further analysis.

Fig.4 Liquid sensing experiment of Gr-PCF.

4 Conclusions

In conclusion, we designed and developed an optical fiber sensing system that is capable of environmental sensing and monitoring at room-temperature.By introducing chemical sensing mechanism to optical fibers, we can achieve sensing ability beyond traditional optical fiber configurations which only detect refractive index change.By an atomically-thin graphene photonic crystal fiber sensor we realized selective detection to NO2 with ultralow limit of detection (50 μg·L-1, 1 μg·L-1= 1 ppb), attributing to the interaction of graphene and target molecules.The sensor can be hopefully extended to other kinds of gases and liquids considering the affinity of graphene to molecules.In view of practical optical sensors, our design is compatible with the MUX/DEMUX techniques of optical fiber communication systems.Combining the ability of CVD synthesis to realize mass-production, the design we proposed shall be one of the answers to the distributed-optical-fiberenvironmental-sensors.