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Reduced graphene oxide (rGO) sheet wrapped on the tapered region of microfiber is demonstrated to enhance the interaction between rGO and strong evanescent field of optical fiber. The 405 nm and 980 nm lasers are employed to illuminate the rGO to investigate the response characteristics of the optical transmitted power (λ = 1550 nm) in the MF. The transmitted optical power of the MF with rGO changes with ~1.7 dB relative variation when the violet light is ranging from 0 mW to 12 mW (~0.21dB/mW) in the outside-pumped experiment. And in the inside-pumped experiment, the change of the 980 nm laser power from 0 mW to 156.5 mW makes ~6 dB relative variation power of the transmitted optical powers of the MF with rGO. These results indicate the optical transmitted power of the MF with wrapped rGO can be manipulated by the 405 and 980 nm light (order of mW), which signifies the device can potentially be applied as all optically and versatilely controllable devices.
© 2017 Optical Society of America
Corrections
10 March 2017: A correction was made to the author listing.
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]. Following the abundant researches of graphene, the reduced graphene oxide (rGO) has also received great attentions. The rGO with two-dimensional structure is one of the widely used nanomaterial in the field of physics, chemistry, materials science and biotechnology [3]. As a competitive and alternative to graphene, rGO has the advantages of the properties of graphene while being easier to fabricate. rGO has attracted much attention due to its superior electronic mobility, remarkable optical transmittance, excellent chemical stability, and high specific surface area [4], conferring it for a rich variety of applications such as catalysis [5], sensing [6], lithium-ion batteries [7], fuel cells [8], super capacitors [9], and wastewater purification [10]. Gao et al. [11] have proposed a Watt-level, all-fiber, ultrafast Er/Yb-codoped double-clad fiber laser mode-locked by rGO interacting with photonic crystal fiber ultra-weakly. Lee et al. [12] have proposed a fiber-optic polarization beam splitter incorporating rGO as a metallic interlayer material. Thus, combining the fiber-optic device and the rGO presents tantalizing interests.
Micro fiber (MF) is an optical fiber with diameters of several to over 10 micrometers with high-index contrast between the MF material and the surrounding [13]. MF offers strong near-field interaction between the guided light and the surroundings because of the strong evanescent fields and shows low optical loss, outstanding mechanical flexibility, tight optical confinement and large fractional evanescent fields [14]. Thus, MF is used as a novel miniaturized platform for exploring fiber-optic technology on the micro or nanoscale, such as Mach-Zehnder interferometers [15], optical ring resonators [16], all-optical tunable resonators [1], compact filters [17], and optical sensors [18]. Combining the MF with the optical properties of graphene to achieve light modulation has been studied. Tong et al. [19] have demonstrated ultrafast optical modulation using a single 1-μm-diameter graphene-decorated MF with a peak power threshold down to 1.75 W. And they have further demonstrated that a graphene-clad MF all optical modulator can achieve a modulation depth of 38% and a response time of ∼2.2 ps [20]. Liu et al. [21] have demonstrated all-optical modulation based on ultrafast saturable absorption in graphene-covered-MF. Chen et al. [22] have demonstrated an in-line, all-optical fiber modulator based on a stereo graphene-MF structure with a modulation depth of 7.5 dB (2.5 dB) and a modulation efficiency of 0.2 dB/mW (0.07 dB/mW) for two polarization states. Furthermore, all-optical modulation is demonstrated at 1550 nm wavelength with a modulation depth of 12% by using multilayer graphene coated MF [23]. As mentioned above, taking the advantage of the mature platform of fiber optics, MF can be used as a miniaturized platform for exploring all fiber optical modulation.
In this paper, MF with rGO coated onto the tapered region is demonstrated for its light controlling. The 405 nm (outside-pumped) and 980 nm (inside-pumped) laser are illuminated (pumped) onto the coated rGO on the MF to demonstrate the light-control-light feature of the device. And the enhanced interaction between strong evanescent light of the MF and rGO results in a stronger controllable optical functionality. The experimental results demonstrate the optical transmitted power can be tuned by 405 nm and 980 nm light. And the optical tuning of this device can be controlled with order of mW.
2. Fabrication
The graphene oxide (GO) powder is prepared by oxidizing graphite powder based on a modified Hummers method [24]. The purified GO powder is collected by centrifugation and then air dried after removing residual salts and acids by dialysis method. The as-prepared GO powder is suspended into ultrapure water and dealing with by ultrasonic for 3 hours to get GO nanoplatelets. Hydrazine hydrate is then added to the GO dispersion solution (0.1 mg/mL) after adjusting pH to 11 by a 5% ammonium hydroxide. The resulting mixture is heated at 95-100 °C for 2 hours under a water-cooled condenser and then cooled to room temperature. The rGO is isolated by filtration over a medium fritted glass funnel and washed copiously with water (500 mL) and methanol (500 mL). The stand-by suspension can be obtained by mixing the rGO powder into high pure alcohol (10 g rGO per 100 ml alcohol).
As shown in Fig. 1(a), the Raman spectrum of rGO flakes on the waist region of the fiber excited by a 514.5 nm laser is measured with LabRAM HR Evolution (HORIBA JY, France). The rGO displays two prominent peaks at 1600 and 1356 cm−1 corresponding to the G and D bands, respectively. The G peak corresponds to the E2g mode observed for sp2 carbon domains, while the D peak indicates edges and/or defects of the sp2 domains. The D/G intensity ratio increases compared to that of GO, indicating the successful reduction of GO [25]. The absorption spectrum of rGO measured by an UV-VIS spectrophotometer (UV-2600, SHIMADZU) is shown in Fig. 1(b). The rGO has strong light absorption in the wavelength range from 200 to 450 nm, and relatively weak light absorption in the wavelength range from 500 to 800 nm, resulting in broadband operation rGO-based devices [26–28].
X-Ray Diffraction (XRD) pattern of rGO is shown in Fig. 2 which shows a broad diffraction peak at 2θ = 24.88 ° corresponding to the (002) interlayer spacing of 0.359 nm. This value is slightly larger than that of graphene, which is mainly due to the residual functional groups that may exist between the rGO layers [29].
Figure 3(a) shows the schematic diagram of the basin and the MF. The MF is manufactured from a standard single mode fiber (SMF, with a core diameter of 8 μm and a cladding diameter of 125 μm from Corning Inc.). The “flame-brushing” technique is used to fabricate MF and relies on a small flame moving under an optical fiber which is being stretched at a drawing speed of 0.2 mm/s. The MF with a diameter of ~8.6 μm in the uniform waist region is fabricated as shown in Fig. 3(b). Then the waist region is immobilized onto a glass slide. As shown in Fig. 3(a), a basin (15 mm × 4 mm × 1 mm) is constituted by using the UV adhesive (Loctite 352, Henkel Loctite Asia Pacific) to contain the rGO solution. And the UV adhesive basin is cured by a UV light source (365 nm, USHIO SP7-250DB) for 3 minutes illumination.
Fig. 3 (a) Schematic of basin used in deposition of rGO and configuration of a fixed MF on glass slide; (b) ideal enlarge view of MF.
The rGO alcohol suspension is processed by ultra-sonication for ~25 minutes to avoid agglomeration. Then, the suspension is dropped into the basin and evaporated for about 4 hours in ambient surrounding. Thus, the rGO is covered the tapered region of the fiber. A appropriate waist of the MF taper is important to demonstrate light-control-light characteristics. In order to obtain the appropriate value of the waist, a 1550 nm distributed feedback (DFB) laser as a light source has been used to monitor the transmitted optical power of the MF during the deposition process of rGO. The large waist of MF taper shows small variation of transmitted optical power, which indicates that the interaction between rGO and the evanescent wave of MF is insufficient. And the small waist of MF taper shows large variation of transmitted optical power, but the transmitted optical power may be too small which means the detected light of optical power meter is insufficient. Thus, the waist of MF taper should not be too large or too small. As shown in Fig. 4, the optical transmitted power of the MF (with diameter of ~8.6 μm) is about −2 dBm at the beginning without instillation of the rGO solution. About 9 minutes later, the optical power is decreased abruptly, which indicates that rGO solution starts to become as rGO film onto the MF. The decrease of the transmitted power is due to the inhomogeneous deposition of rGO sheets and the coexistence of dry and wet rGO pieces. The power remained at −35 dBm when the evaporation is finished, which denotes the deposition of rGO film is completed. The ~8.6 μm waist with ~33 dB transmitted optical power variation is just appropriate to reach a compromise between the light interaction (rGO and evanescent light of MF) and the loss.
The SEM images of the rGO coated MF (rGOCMF) are shown in Fig. 5. The surface of MF is partially covered with stacked rGO film. And the rGO is not uniformly distributed.
3. Experimental results and discussions
The light-control-light characteristics of rGO coated on the MF with mW pumping laser are demonstrated in this paper. By comparison, the visible light (405 nm) and near-infrared (NIR) light (980 nm) are chosen to pumped the rGO coated on the MF. In the range of visible light (~400 nm- 800 nm), according to the absorption spectra of rGO, the 405 nm is more absorptive and therefore more suitable for pumping among the visible light as shown in Fig. 1(b). Since the standard single mode fiber (SMF: 1550 nm) is used in the light-control-light experiments, the in-fiber wavelength division multiplex (WDM) for coupling the NIR 980 nm and 1550 nm is relatively common, low-cost and adequate for in-fiber pumping. Thus, the 980 nm is chosen for pumping among the NIR light. Most of the semiconductor 405 nm lasers with order of mW emit spatial light, and are not so compatible for coupling and transmission in SMF. Hence, the rGO coated on the MF is out-fiber pumped by 405 nm laser. The invisible 980 nm light is relatively suitable for propagation in fibers. Therefore, the rGO coated on the MF is in-fiber pumped by 980 nm laser..
The outside-pumped experimental setup consists of a 1550 nm DFB laser, a thermocouple, a 405 nm pump laser and an optical power meter as shown in Fig. 6. The output of the DFB laser through the sample is connected to an optical power meter, which monitors the optical transmitted power of the rGOCMF. The experiments are performed with violet power ranging from 0 mW to 12mW. The pump light is placed ~10 cm above the rGOCMF and focused by the cylindrical lense. The experimental humidity is ~38% RH.
As shown in Fig. 7 (a), the optical transmitted power of the MF without rGO varies only approximately of 0.07 dB with adjustable violet laser (405 nm) ranging from 0 mW to 12 mW. The experimental results reveal no obvious tendency. Thus, the optical transmitted power can’t obviously be influenced by the illumination of violet light onto the MF.
Fig. 7 (a) Optical transmitted power change with different pump power of bare MF versus time; (b) Transmitted power of the rGOCMF with different illuminated violet power.
The optical transmitted power of the MF with rGO has a relative variation of ~1.7 dB with respect to the increase of adjustable violet laser (405 nm) from 0 mW to 12 mW as shown in Fig. 7(b). And the optical transmitted power has a relative variation of ~1.4 dB with respect to the decrease of adjustable violet laser from 12 mW to 0 mW.
The rGO may be not absolutely pure, during the illumination of violet light in increase process, the residual GO can be reduced and partially become to rGO. This transition may contribute slightly to the different results between the increase pumping and the decrease pumping. In addition, the temperature of the rGO sheet may be higher after the increase step of illumination of violet light. As the temperature increases, the concentration of thermally excited electrons–holes increases. Consequently, the Fermi–Dirac distribution of electrons in the rGO changes, which in turn reduces the real part of the dynamic conductivity to decrease the 1550 nm light absorption. Thus, this is another factor resulting difference of controllable transmitted power between the increase and the decrease of violet light power [30].
The analysis of the means of transmitted power for different step of violet power is shown in Fig. 8. The sensitivity of the rGOCMF to pump light can be obtained by the slope of linear fitting curve. The response sensitivity for increasing violet power is of ~0.21 dB/mW, while ~0.28 dB/mW for decreasing violet power.
The inside-pumped experimental setup consists of an optical spectrum analyzer (OSA, Yokogawa-AQ6370D), a 980 nm pump laser, a 1550 nm DFB laser, a WDM (980 nm/ 1550 nm) and the rGOCMF as shown in Fig. 9. The light of the 1550 nm DFB laser and 980 nm pump laser are multiplexed into a single optical fiber by the WDM, and then pass through the rGOCMF sample. The OSA is used to monitor the optical transmitted light of the rGOCMF. The experimental humidity is ~38% RH. The optical power of 980 nm laser is sequentially changed from 0 mW, to 23 mW, 48.5 mW, 72.3 mW, 97 mW, 126 mW, and 156.5 mW, respectively.
The transmitted optical power of the MF without rGO with adjustable 980 nm laser ranging from 0 mW to 156.5 mW is shown in Fig. 10(a). The transmitted optical power of the MF remains almost unchanged. This implies the 980 nm light cannot influence the transmitted optical power of the MF without rGO. As shown in Fig. 10(b), the transmitted optical powers of the rGOCMF responds with a relative variation power of ~6 dB (λ = 1550 nm) when the adjustable 980 nm laser changes from 0 mW to 156.5 mW. The transmitted optical power increases with the increasing 980 nm laser power.
Fig. 10 Optical transmitted power of the MF without rGO (a) and rGOCMF with different 980 nm laser power (b).
As shown in Figs. 7 and 10, the rGO wrapped on the tapered MF reduces the transmission of the light. In our opinion, the light scattering and absorption of the rGO contribute to this relatively high transmission loss. When the rGO is not uniformly distributed onto the MF as shown in Fig. 5, the light will be transported to the rGO film from MF and subsequently be scattered by rGO, which results in transmission reduction. And the absorption of the rGO can also decrease the transmission of light. The transmission loss will be larger with the increased absorption of rGO, this absorption depends on the length of the MF coated rGO, the diameter of the MF and the rGO quality. The small diameter and long length of the MF coated rGO will increase transmission loss with more light scattered and absorbed by rGO. The imperfect quality of the rGO will further increase the optical loss. In order to reduce the optical loss, the parameters of MF and quality of rGO should be improved. Optimizing the diameter and the length of the MF coated rGO can reduce the optical loss and ensure the interaction enhancement between evanescent light and rGO. Optimizing the rGO coating technology can improve the uniformity. Besides, improving the production quality of rGO to decrease impurities is an effective way of reducing optical loss.
The results displayed in Fig. 11 indicate that the 980nm laser can change the transmitted power of rGOCMF. The red dashed line is linear fitting curve of the transmitted power for the 980 nm power increasing process. The response sensitivity of rGOCMF for increasing 980nm power is of ~0.04 dB/mW, and the response sensitivity of MF without rGO is of ~0.00034 dB/mW. The sensitivity of rGOCMF is enhanced about 118 times compared to that of the bare MF. Hence, the sensitivity is greatly improved by the rGO.
As shown in Figs. 8 and 11, the experimental results demonstrate the optical transmitted power can be tuned by 405 nm and 980 nm light. The 1550nm transmitted optical power increases with the increasing pumped laser powers. And the response sensitivity (~0.28 dB/mW) of rGOCMF with the 405 nm pumped laser is higher than that (~0.04 dB/mW) with the 980 nm pumped laser. On the other hand, the out-fiber pump may be more effective to control light characteristic of rGO than the in-fiber pump.
The controllable characteristics of MF with rGO may be explained as follow: as illuminating laser power increases, electrons and holes produced by the 405 nm and 980 nm light will occupy energy levels in the conduction and valence bands, which causes saturable absorption in rGO [31]. Hence, the Fermi-Dirac distribution of electrons in rGO sheet changes correspondingly, resulting in a real part reduction of dynamic conductivity. The real part of conductivity determines the light absorption produced by intraband and interband transition in rGO [32]. Therefore, the light absorption is decreased because of the dynamic conductivity reduction of rGO following the increase of pumped power. Meanwhile, the transmitted loss of the MF with rGO is decreased while the transmitted optical power is increased. Consequently, the controllable functionality of pumped light power can be achieved.
4. Conclusion
In summary, the light-control-light properties of the MF coated with rGO is demonstrated in this paper. The outside-pumped experimental results show when the pump power increases from 0 mW to 12 mW, the maximum change of the optical transmitted power for the rGOCMF is ~1.7 dB, while this value is ~1.4 dB when the 405 nm power decreases from 12 mW to 0 mW. The response sensitivity for increasing violet power is of ~0.28 dB/mW, while ~0.21 dB/mW for decreasing violet power. In the in-fiber pumped experiment, when the adjustable 980 nm laser changes from 0 mW to 156.5 mW, the rGOCMF shows a relative variation power of ~6 dB (λ = 1550 nm). Therefore, the 405 nm and 980 nm light with order of mW can manipulate the optical transmitted power of the rGOCMF. Further optimizations of the geometric configuration for the MF and quality improvement of the rGO deposition will promote its application for controllable functionality devices.
Funding
National Natural Science Foundation of China (No. 61475066; No. 61405075; No. 61505069; No. 61675092; No. 61177075; No. 61275046), National Major Project of China (22104001, 22117001), Guangdong Natural Science Funds for Distinguish Young Scholar (2015A030306046), Natural Science Foundation of Guangdong Province (No. 2016A030310098; No. 2014A030313377; No. 2014A030310205; No.2015A030313320), Planned Science & Technology Project of Guangzhou under Grant (201607010134, 201506010046, 201506010046).
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