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Side-polished fiber (SPF) with a graphene sheet on a polished area can enhance the interaction between graphene and a strong evanescent field of optical fiber. Graphene sheet is utilized due to its saturable absorption property. To investigate the response characteristics of the optical transmitted power (λ = 1550 nm) in the SPF, violet light (λ = 450 nm) as pump source is employed to illuminate the graphene sheet. Experimental results reveal that different pump power can change the optical transmitted power of the SPF. When the pump light changes from 0 mW to 11 mW, the optical transmitted power of the SPF varies ~3.4 dB. In this increasing process of the pump power, the linear correlation between relative transmitted power and violet power is ~97.1% with the sensitivity of ~0.29 dB/mW; while the linear correlation is ~96.5% with sensitivity of ~0.25 dB/mW in the decreasing process of the pump power. These results indicate that violet light (order of mW) can manipulate the optical transmitted power of the SPF coated with graphene sheet, which signifies the device can potentially be applied as all optically controllable devices.
© 2016 Optical Society of America
1. Introduction
Fiber-optic device with graphene is one of the research hotspots in the domain of optical communication and sensing in recent years [1,2]. Graphene is a two-dimensional material with thickness of single carbon atoms that consisted by the hexagonal honeycomb structure. It is also a new carbon material with combination of semiconductor and metal properties, possessing excellent nonlinear optical characteristic. The ultrafast dynamics of Dirac electron and Pauli blocking exist in its tapered energy band structure, therefore, graphene possesses the ultrafast saturable absorption properties [3]. Because of its extraordinary electron transport characteristics, excellent optical and electrical properties [4,5], graphene has attracted considerable attentions in the field of electronic and optical applications [6,7].
Graphene has been widely utilized in ultrafast mode-locked laser as nonlinear optical components (saturable absorber) embedded in the laser cavity [8–12]. Combining the optical properties of graphene with the fiber microstructure to achieve light modulation has been studied. Liu et al adopted to couple the transmitted light and the pump light through microfiber to fabricate broadband all-optical modulation using a graphene-covered-microfiber [13]; Yu et al used strong pump light to modulate weak signal light to obtain ultrafast optical modulation in microfiber with coated graphene [14]. Since its structure is not easy to be preserved and the Van der Waals force exists, microfiber is sensitive to the ambient temperature and humidity. Furthermore, functional microfiber device needs a relatively robust substrate to enable its operation stability.
To overcome the inconvenience of microfiber in certain applications, the side polished fiber (SPF) will be a suitable candidate. It can be manufactured by optical micro-processing technology with removing a part of fiber cladding, leads to enable evanescent light escape from fiber core to the polished surface, hence, strong interaction between evanescent light and the external environment can be achieved. Comparing with other special optical structured fibers, the SPF has some advantages like low cost, evanescent field controllability and low insertion loss. In recent years, the SPF has turned out to be a platform for constructing fiber-optic devices such as the wide spectrum polarizer [15], optical fiber temperature and humidity sensors [16–18], adjustable fiber comb filter [19] and optical fiber phase modulator [20] were demonstrated.
In this letter, the SPF is proposed with graphene sheet deposited onto the polished surface. The violet light as pump source is illuminated onto the graphene sheet to demonstrate the light-control-light feature of the device. The experimental results demonstrate the optical transmitted power can be tuned by violet light. In addition, the optical tuning of this device is relatively easy to control with order of mW violet light.
2. Fabrication of device
The used SPF is manufactured by wheel side-polishing technique. The polishing depth along the fiber is given in Fig. 1(a). The length of relative flat region, the polishing depth and the residual cladding thickness are ~1.8 cm, ~56.5 µm and ~2 µm respectively, these parameters can be adjusted yield a compromise between low insertion loss and sufficiently evanescent field of the SPF. And the diameters of single-mode fiber (SMF) and fiber core are 125 µm and ~8 µm respectively.
A basin on a glass slide shown in Fig. 1(b) is utilized in the deposition procedure of graphene onto the polished surface of SPF in order to contain liquid graphene solution. The Ultra-Violet glue is used to fasten the SPF onto a glass slide while the polished region is surrounded by a 30*10*5 mm3 basin. The available graphene solution is treated by ultrasonication for 30 minutes to uniformly distribute the graphene solution which can avoid the agglomeration. The 1.5 ml of graphene solution which is added into the basin is placed in ambient temperature for evaporation.
During the deposition process of graphene solution, a 1550 nm DFB laser (light source) is utilized to monitor the optical transmitted power of the SPF as displayed in Fig. 2. The optical transmitted power of the SPF in air is about −15.2 dBm, this is changed to be about −11.0 dBm at ~35 min. After the evaporation of the ethanol, the optical transmitted power gradually decreases to about −24.2 dBm at ~99 min. Before the formation of graphene sheet, the transmitted power changes from −24.2 dBm to −21.6 dBm. After ~103 min, the power changes from −21.6 dBm to −35.6 dBm and is stabilized at about −35.6 dBm finally.
The morphology of graphene sheet deposited onto the SPF is shown in the SEM images as Fig. 3(a) and 3(b), Fig. 3(b) presents an enlarged view for the region where is marked by dotted line. It can be observed there are many stacked graphene sheets on the surface of the SPF, and the size of the graphene sheets deposited on SPF is about a few microns with the average thickness of ~210nm, which are not uniformly distributed in the polished surface.
The Raman spectral was performed and the result is shown as Fig. 4. The 2 peaks of Raman spectroscopy are named D and G band at 1358 and 1590.5 cm−1 respectively, which are usually considered as a random vibration peak of graphene and generates in the surface of SP2 carbon atoms.
3. Experimental results and discussions
The experimental setup is composed of a 1550 nm DFB laser, a thermocouple, a pump laser (λ = 405 nm) and an optical power meter as shown in Fig. 5 [21]. The optical transmitted power of the device is recorded by an optical power meter which is connected to the output of DFB laser through the device under test. The experiments are performed with violet power ranging from 0 mW to 11 mW. The pump light which is focused by the cylindrical lens is placed ~10 cm above the sample. At the same time, the temperature change of the device is monitored by thermocouple. The experimental humidity is fixed in ~40% RH.
The violet light (405 nm) used for controlling transmission of the graphene-coated SPF due to the graphene has a high absorption than the other visible light. The experimental results for the SPF without graphene sheets reveal the optical transmitted power of SPF changes only approximately of −0.11 dB and no obvious tendency as illustrated in Fig. 6(a) with adjustable violet laser (405 nm) ranging from 0 mW to 11 mW, which means the illumination of violet light onto SPF can almost not obviously influence the optical transmitted power.
Fig. 6 (a) Optical transmitted power change with different pump power of bare SPF versus time; (b) Transmitted power of the SPF coated with graphene with different illuminated violet power.
The experimental results shown in Fig. 6(b) indicate that optical transmitted power of the SPF with graphene sheet has a relative variation of ~3.4 dB with respect to the increase of adjustable violet laser (405 nm) from 0 mW to 11 mW, which gives a significant difference from the SPF without graphene sheet. While the optical transmitted power has a relative variation of ~2.9 dB with respect to the decrease of adjustable violet laser from 11 mW to 0 mW. The difference of contrallable transmitted power between the increase and the decrease of violet light may be explained as followed: Since the characterizations were performed in ambient condition, after the increase step of illumination of violet light, the humidity and temperature of the graphene sheet were changed, this may contribute to the difference for transmitted optical light.
As the focused violet light induces heating effect, the thermocouple is used to measure the temperature variation as shown in Fig. 5. The maximum variation of temperature in the area of the violet light illumination is ~4 °C. To analyze the effect of temperature contribution to the experiment, the SPF with deposited graphene sheet is laid inside the chamber with variable temperature changing as the same variation of Fig. 7(a). The corresponding optical transmitted power changes as given in Fig. 7(b). Under the condition of the device without violet light radiation, the transmtted power changes ~0.15 dB as temperature increasing from 22 °C to 26 °C. Comparing with the variation of ~3.4 dB introduced by the illumination of violet light, the temperature change can be almost neglected.
Fig. 7 (a) Temperature change in the irradiation area of violet light; (b) Transmitted power of the SPF with deposited graphene sheet when temperature changes.
By analyzing the means of transmitted power for different step of violet power (Fig. 8), the linear correlation coefficient (LCC) can be obtained both in the increasing and decreasing process of violet light power. The black dashed line is linear fitting curve for the increasing process with LCC of ~0.971. While the red dashed line is linear fitting curve for the decreasing process with LCC of ~0.965. The slope of linear fitting curve is defined as sensitivity of the SPF coated with graphene to pump light, the response sensitivity for increasing violet power is of ~0.29 dB/mW, while ~0.25 dB/mW for decreasing violet power.
The controllable characteristics of SPF with graphene sheet may be explained as bellow: When violet power increases, saturable absorption in graphene sheet occurred due to the Pauli blocking of the electrons and holes for occupation of energy levels in the conduction and valence bands that are resonant when the incident photons [11], the concentration of excited electrons-holes increases [22]. Therefore, the Fermi-Dirac distribution of electrons in graphene sheet changes correspondingly, which reduces the real part of dynamic conductivity. The real part of conductivity determines the light absorption produced by intraband and interband transition in graphene sheet [23]. Hence, with the increase of illuminating violet power, the dynamic conductivity of graphene sheet is reduced which decreases the light absorption. Meanwhile, the transmitted loss of the SPF with graphene sheet is decreased while the transmitted optical power is increased. Consequently, the controllable functionality of violet light power can be achieved.
4. Conclusion
The light-control-light properties of the SPF coated with graphene sheet is demonstrated in this letter which is based on the enhanced interaction between evanescent field of the SPF and graphene sheet. The experimental results show when the pump power increases from 0 mW to 11 mW, the maximum change of the optical transmitted power for the SPF integrated graphene sheet is ~3.4 dB, while this value is of ~2.9 dB when the pump power decreases from 11 mW to 0 mW. In increasing process of violet light power, the linear correlation between relative transmitted power and pump power is ~97.1% with sensitivity of ~0.29 dB/mW; while the linear correlation of decreasing process is ~96.5% with the sensitivity of ~0.25 dB/mW. Therefore, the violet light with order of mW can manipulate the absorption of graphene sheet and the optical transmitted power of the device, which pave the capabilities of application in all optical controllable devices.
Acknowledgments
This work is supported by National Natural Science Foundation of China (No. 61177075; No. 61275046; No.61361166006; No. 61475066; No. 61405075; No. 61505069; No.61575084; No. 61401176), Guangdong Natural Science Funds for Distinguish Young Scholar(2015A030306046), Natural Science Foundation of Guangdong Province (No. 2014A030313377, No. 2014A030310205, No.2015A030313320), the Core Technology Project of Strategic Emerging Industries of Guangdong Province (2012A032300016;2012A080302004), Special Funds for major science and technology projects of Guangdong Province (2014B010120002,2014B010117002,2015B010125007), Special Funds for Discipline Construction of Guangdong Province (2013CXZDA005), Planned Science & Technology Project of Guangzhou under Grant (201506010046), Excellent Young Teachers Program of Guangdong High Education (YQ2015018).
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