6030
OPTICS LETTERS / Vol. 39, No. 20 / October 15, 2014
Graphene-based D-shaped fiber multicore mode interferometer for chemical gas sensing Y. Wu,1,4 B. C. Yao,1 A. Q. Zhang,1 X. L. Cao,1 Z. G. Wang,2 Y. J. Rao,1,5 Y. Gong,1 W. Zhang,1 Y. F. Chen,2 and K. S. Chiang1,3 1
2
Key Lab of Optical Fiber Sensing & Communications (Education Ministry of China), University of Electronic Science & Technology of China, Chengdu, Sichuan 610054, China
State Key Laboratory of Electronic Thin Films and Integrated Devices, University of Electronic Science and Technology of China, Chengdu 610054, China 3
Department of Electronic Engineering, City University of Hong Kong, Kowloon, Hong Kong, China 4 5
e-mail: [email protected] e-mail: [email protected]
Received July 14, 2014; revised September 9, 2014; accepted September 9, 2014; posted September 17, 2014 (Doc. ID 216932); published October 15, 2014 In this Letter, a graphene-coated D-shaped fiber (GDF) chemical gas sensor is proposed and demonstrated. Taking advantage of both the graphene-induced evanescent field enhancement and the in-fiber multimode interferometer, the GDF shows very high sensitivity for polar gas molecule adsorptions. An extinction ratio of up to 28 dB within the free spectrum range of ∼30 nm in the transmission spectrum is achieved. The maximum sensitivities for NH3 and H2 O gas detections are ∼0.04 and ∼0.1 ppm, respectively. A hybrid sensing scheme with such compactness, high sensitivity, and online monitoring capabilities may pave the way for others to explore a series of graphene-based lab-on-fiber devices for biochemical sensing. © 2014 Optical Society of America OCIS codes: (060.2370) Fiber optics sensors; (310.2790) Guided waves; (120.3180) Interferometry. http://dx.doi.org/10.1364/OL.39.006030
Graphene has attracted worldwide interest for its exceptional electronic and photonic properties [1–3]. Due to its unique band structure, graphene has tunable complex permittivity, as determined by its Fermi level [4,5]. Accordingly, the complex refractive index of the graphene as determined by its permittivity could be modulated via gating, laser pumping, doping, or external molecular adsorptions [6–10]. Hence, graphene shows promise in its use in photonic sensors to detect photocurrents, gas molecules, and biological parameters [11–13]. Moreover, as graphene is only an atom in thickness, it is highly flexible, which allows it to be incorporated into all-fiber structures by several modes, including depositing it onto the fiber ends [14], covering it along polished fibers [15,16], and wrapping it around microfibers [7,17]. Currently, both the graphene–metal-based surface plasmonic resonance (SPR) chemical sensors [13,18,19] and the graphene–microfiber-based gas sensors are reported to have impressive detection sensitivities [20,21]. However, the microfiber-based sensors are fragile, unstable, and cross-sensitive, thus limiting their applications greatly. In this Letter, by wet transferring a layer of graphene onto a D-shaped fiber (DF), we propose an online graphene-coated DF (GDF)-based all-fiber gas sensor. By taking advantage of the graphene-enhanced multimode interference on the surface of the GDF, maximum sensitivities of 0.04 ppm for NH3 gas and 0.1 ppm for H2 O vapor are obtained. Figures 1(a) and 1(b) show the GDF schematically. The DF was fabricated by burnishing a standard MMF (Corning Inc.) with a core diameter of Dcore 50 μm, a cladding diameter of Dcladding 125 μm, a burnished depth h ∼45 μm, and a burnished (D-shaped) area length LD ∼8 mm. First, the monolayer graphene was grown on Cu foil by the chemical vapor deposition (CVD) method [22]. Then, the monolayer 0146-9592/14/206030-04$15.00/0
graphene was transferred onto a polymethyl methacrylate (PMMA) layer. Finally, we cut the graphene–PMMA hybrid film to the length of ∼5 mm, deposited it on the polished surface of DF, then removed the PMMA by acetone. Figure 1(c) shows the sensing principles. Basically, along the MMF, evanescent fields are excited by polarized broadband light. Within the D-shaped area, graphene enhanced the interference between the surface modes with the core modes. When gas molecules were adsorbed by the graphene film, the permittivity of the graphene film is changed, which induced the refractive index to be altered on the sensing region, so that the interference spectrum shifts could be measured according to the refractive index variation [12,20], and the gas concentration changes could be detected.
Fig. 1. (a) Schematic diagram of the GDF and (b) its crosssectional view. (c) Enhanced evanescent mode for gas sensing. © 2014 Optical Society of America
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Fig. 2. (a) Optical micrograph of the GDF. (b) Strong scattering on the graphene-coated area (on the left side of the whitedashed line). (c) Cross-sectional SEM of the D-fiber. (d) Raman spectrum of the graphene of the GDF. All bars in (a), (b), and (c) represent 50 μm.
Figures 2(a) and 2(b) present the optical micrographs of the GDF. Here, the graphene-covered area is viewable on the left side of the white-dashed line, as shown in Fig. 2(a). In Fig. 2(b), with a 633 nm light guided through the DF, the graphene-coated area was clearly visible from the scattered light, due to a significant enhancement of the evanescent fields that were guided along the polished surface of the DF. Figure 2(c) shows the scanning electron micrograph (SEM) of the D-fiber (crosssection). Figure 2(d) provides the Raman spectrum of the coated monolayer graphene, in which the G-to-2D intensity ratio is ∼0.45. The experimental setup for chemical gas sensing is schematically shown in Fig. 3(a). A tunable laser (sweep range of 1510–1590 nm with the maximum resolution of 0.1 pm) with a power of 12 dBm was used as the light source (81960 A, Agilent, USA), and an optical multimeter with spectral measurement software (8163B, 81636B, Agilent, USA) was used to observe the transmission spectra. The light was launched into the GDF and, at the output end, light was collected by the OSA. In order to reduce the insertion loss in front of the GDF, a polarization controller (PC) was adopted to adjust the input light to that of TE polarization, as the TE modes have smaller transmission loss along the GDF [15]. Finally, the GDF was placed in a sealed gas chamber. To see how the graphene cladding influences the light transmission along the DF structure, we investigated the transmission spectra with the launched broadband light of the multimode fiber (MMF), the DF, and the GDF, respectively, as shown in Fig. 3(b). Here the transmission losses of the MMF, the DF, and the GDF are ∼ − 1.5, ∼ − 18, and < − 20 dB, respectively. Due to the graphene-induced
Fig. 3. (a) Experimental setup. (b) Spectra of the MMF, DF, and GDF. (c) Simulated mode distributions of the MMF, DF, and GDF.
surface mode enhancement, polarization-dependent absorption, and phase perturbations, the multicore mode interference occurred between the LP0n modes and the LP1n modes. By adjusting the polarization of light, we observed significant interference on the GDF’s transmission spectrum, which achieved a maximum extinction ratio of up to ∼28 dB with the free spectrum range of up to 30 nm. In comparison, the maximum extinction ratio of the DF (blue curve) is 2500 ppm, it can be assumed that the GDF sensor could produce nonlinear response to gas concentration changes above 2500 ppm. Figure 5(c) shows all GDF spectral shifts when detecting 1000 ppm NH3 , H2 O, and xylene. It indicates that the phase-based GDF sensor is selective, which is better used for polar gas sensing. This result is also supported by Refs. [19,21]. Figure 5(d) provides the recoverability of the GDF. By exposing the GDF in 100 ppm NH3 ∕dry air and H2 O∕dry air circularly, and stabilizing the initial phases, polarizations, and losses, the spectral shifts are well recoverable. It also can be observed that
Fig. 4. Spectral shifts of the GDF exposed in (a) NH3 gas, (b) H2 O vapor, and (c) xylene gas.
1000 ppm, the dip locations were at 1552.8, 1553.4, 1553.8, and 1554.2 nm. Correspondingly, the depths of the dips were −46.5, −45.3, −44.3, and −43.0 dB, as shown in Fig. 4(a). Second, when the GDF was exposed to H2 O vapor with concentrations of 0, 200, 500, and 1000 ppm, the dips were located at 1552.8, 1553.0, 1553.2, and 1553.4 nm. Correspondingly, the dip’s depths were −46.5, −46.1, −45.6, and −44.8 dB, as shown in Fig. 4(b). Finally, Fig. 4(c) displays the GDF’s spectral alterations when the GDF was exposed to xylene gas with concentrations of 0, 200, 500, and 1000 ppm. It can be observed that the spectral shift in response to 200 ppm xylene would be >0.1 nm, but the adsorption of xylene molecules would dramatically deteriorate the resonant spectrum, and this may be relevant to the group polymerization of xylene. Thus, for xylene sensing, it is difficult to achieve a wide dynamic range with high resolution. Figure 5(a) shows the correlation of gas concentration and the spectral shift, and Fig. 5(b) shows the
Fig. 5. (a) Correlation of the gas concentration and spectral shift. (b) Correlation of the gas concentration and the dip intensity increase. (c) GDF’s sensitivities for three types of gases. (d) Recoverability of the GDF. In (a), (b), and (d), the red cubes represent NH3 gas, while the blue dots represent H2 O vapor.
October 15, 2014 / Vol. 39, No. 20 / OPTICS LETTERS
the response time of this sensor is
OPTICS LETTERS / Vol. 39, No. 20 / October 15, 2014
Graphene-based D-shaped fiber multicore mode interferometer for chemical gas sensing Y. Wu,1,4 B. C. Yao,1 A. Q. Zhang,1 X. L. Cao,1 Z. G. Wang,2 Y. J. Rao,1,5 Y. Gong,1 W. Zhang,1 Y. F. Chen,2 and K. S. Chiang1,3 1
2
Key Lab of Optical Fiber Sensing & Communications (Education Ministry of China), University of Electronic Science & Technology of China, Chengdu, Sichuan 610054, China
State Key Laboratory of Electronic Thin Films and Integrated Devices, University of Electronic Science and Technology of China, Chengdu 610054, China 3
Department of Electronic Engineering, City University of Hong Kong, Kowloon, Hong Kong, China 4 5
e-mail: [email protected] e-mail: [email protected]
Received July 14, 2014; revised September 9, 2014; accepted September 9, 2014; posted September 17, 2014 (Doc. ID 216932); published October 15, 2014 In this Letter, a graphene-coated D-shaped fiber (GDF) chemical gas sensor is proposed and demonstrated. Taking advantage of both the graphene-induced evanescent field enhancement and the in-fiber multimode interferometer, the GDF shows very high sensitivity for polar gas molecule adsorptions. An extinction ratio of up to 28 dB within the free spectrum range of ∼30 nm in the transmission spectrum is achieved. The maximum sensitivities for NH3 and H2 O gas detections are ∼0.04 and ∼0.1 ppm, respectively. A hybrid sensing scheme with such compactness, high sensitivity, and online monitoring capabilities may pave the way for others to explore a series of graphene-based lab-on-fiber devices for biochemical sensing. © 2014 Optical Society of America OCIS codes: (060.2370) Fiber optics sensors; (310.2790) Guided waves; (120.3180) Interferometry. http://dx.doi.org/10.1364/OL.39.006030
Graphene has attracted worldwide interest for its exceptional electronic and photonic properties [1–3]. Due to its unique band structure, graphene has tunable complex permittivity, as determined by its Fermi level [4,5]. Accordingly, the complex refractive index of the graphene as determined by its permittivity could be modulated via gating, laser pumping, doping, or external molecular adsorptions [6–10]. Hence, graphene shows promise in its use in photonic sensors to detect photocurrents, gas molecules, and biological parameters [11–13]. Moreover, as graphene is only an atom in thickness, it is highly flexible, which allows it to be incorporated into all-fiber structures by several modes, including depositing it onto the fiber ends [14], covering it along polished fibers [15,16], and wrapping it around microfibers [7,17]. Currently, both the graphene–metal-based surface plasmonic resonance (SPR) chemical sensors [13,18,19] and the graphene–microfiber-based gas sensors are reported to have impressive detection sensitivities [20,21]. However, the microfiber-based sensors are fragile, unstable, and cross-sensitive, thus limiting their applications greatly. In this Letter, by wet transferring a layer of graphene onto a D-shaped fiber (DF), we propose an online graphene-coated DF (GDF)-based all-fiber gas sensor. By taking advantage of the graphene-enhanced multimode interference on the surface of the GDF, maximum sensitivities of 0.04 ppm for NH3 gas and 0.1 ppm for H2 O vapor are obtained. Figures 1(a) and 1(b) show the GDF schematically. The DF was fabricated by burnishing a standard MMF (Corning Inc.) with a core diameter of Dcore 50 μm, a cladding diameter of Dcladding 125 μm, a burnished depth h ∼45 μm, and a burnished (D-shaped) area length LD ∼8 mm. First, the monolayer graphene was grown on Cu foil by the chemical vapor deposition (CVD) method [22]. Then, the monolayer 0146-9592/14/206030-04$15.00/0
graphene was transferred onto a polymethyl methacrylate (PMMA) layer. Finally, we cut the graphene–PMMA hybrid film to the length of ∼5 mm, deposited it on the polished surface of DF, then removed the PMMA by acetone. Figure 1(c) shows the sensing principles. Basically, along the MMF, evanescent fields are excited by polarized broadband light. Within the D-shaped area, graphene enhanced the interference between the surface modes with the core modes. When gas molecules were adsorbed by the graphene film, the permittivity of the graphene film is changed, which induced the refractive index to be altered on the sensing region, so that the interference spectrum shifts could be measured according to the refractive index variation [12,20], and the gas concentration changes could be detected.
Fig. 1. (a) Schematic diagram of the GDF and (b) its crosssectional view. (c) Enhanced evanescent mode for gas sensing. © 2014 Optical Society of America
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Fig. 2. (a) Optical micrograph of the GDF. (b) Strong scattering on the graphene-coated area (on the left side of the whitedashed line). (c) Cross-sectional SEM of the D-fiber. (d) Raman spectrum of the graphene of the GDF. All bars in (a), (b), and (c) represent 50 μm.
Figures 2(a) and 2(b) present the optical micrographs of the GDF. Here, the graphene-covered area is viewable on the left side of the white-dashed line, as shown in Fig. 2(a). In Fig. 2(b), with a 633 nm light guided through the DF, the graphene-coated area was clearly visible from the scattered light, due to a significant enhancement of the evanescent fields that were guided along the polished surface of the DF. Figure 2(c) shows the scanning electron micrograph (SEM) of the D-fiber (crosssection). Figure 2(d) provides the Raman spectrum of the coated monolayer graphene, in which the G-to-2D intensity ratio is ∼0.45. The experimental setup for chemical gas sensing is schematically shown in Fig. 3(a). A tunable laser (sweep range of 1510–1590 nm with the maximum resolution of 0.1 pm) with a power of 12 dBm was used as the light source (81960 A, Agilent, USA), and an optical multimeter with spectral measurement software (8163B, 81636B, Agilent, USA) was used to observe the transmission spectra. The light was launched into the GDF and, at the output end, light was collected by the OSA. In order to reduce the insertion loss in front of the GDF, a polarization controller (PC) was adopted to adjust the input light to that of TE polarization, as the TE modes have smaller transmission loss along the GDF [15]. Finally, the GDF was placed in a sealed gas chamber. To see how the graphene cladding influences the light transmission along the DF structure, we investigated the transmission spectra with the launched broadband light of the multimode fiber (MMF), the DF, and the GDF, respectively, as shown in Fig. 3(b). Here the transmission losses of the MMF, the DF, and the GDF are ∼ − 1.5, ∼ − 18, and < − 20 dB, respectively. Due to the graphene-induced
Fig. 3. (a) Experimental setup. (b) Spectra of the MMF, DF, and GDF. (c) Simulated mode distributions of the MMF, DF, and GDF.
surface mode enhancement, polarization-dependent absorption, and phase perturbations, the multicore mode interference occurred between the LP0n modes and the LP1n modes. By adjusting the polarization of light, we observed significant interference on the GDF’s transmission spectrum, which achieved a maximum extinction ratio of up to ∼28 dB with the free spectrum range of up to 30 nm. In comparison, the maximum extinction ratio of the DF (blue curve) is 2500 ppm, it can be assumed that the GDF sensor could produce nonlinear response to gas concentration changes above 2500 ppm. Figure 5(c) shows all GDF spectral shifts when detecting 1000 ppm NH3 , H2 O, and xylene. It indicates that the phase-based GDF sensor is selective, which is better used for polar gas sensing. This result is also supported by Refs. [19,21]. Figure 5(d) provides the recoverability of the GDF. By exposing the GDF in 100 ppm NH3 ∕dry air and H2 O∕dry air circularly, and stabilizing the initial phases, polarizations, and losses, the spectral shifts are well recoverable. It also can be observed that
Fig. 4. Spectral shifts of the GDF exposed in (a) NH3 gas, (b) H2 O vapor, and (c) xylene gas.
1000 ppm, the dip locations were at 1552.8, 1553.4, 1553.8, and 1554.2 nm. Correspondingly, the depths of the dips were −46.5, −45.3, −44.3, and −43.0 dB, as shown in Fig. 4(a). Second, when the GDF was exposed to H2 O vapor with concentrations of 0, 200, 500, and 1000 ppm, the dips were located at 1552.8, 1553.0, 1553.2, and 1553.4 nm. Correspondingly, the dip’s depths were −46.5, −46.1, −45.6, and −44.8 dB, as shown in Fig. 4(b). Finally, Fig. 4(c) displays the GDF’s spectral alterations when the GDF was exposed to xylene gas with concentrations of 0, 200, 500, and 1000 ppm. It can be observed that the spectral shift in response to 200 ppm xylene would be >0.1 nm, but the adsorption of xylene molecules would dramatically deteriorate the resonant spectrum, and this may be relevant to the group polymerization of xylene. Thus, for xylene sensing, it is difficult to achieve a wide dynamic range with high resolution. Figure 5(a) shows the correlation of gas concentration and the spectral shift, and Fig. 5(b) shows the
Fig. 5. (a) Correlation of the gas concentration and spectral shift. (b) Correlation of the gas concentration and the dip intensity increase. (c) GDF’s sensitivities for three types of gases. (d) Recoverability of the GDF. In (a), (b), and (d), the red cubes represent NH3 gas, while the blue dots represent H2 O vapor.
October 15, 2014 / Vol. 39, No. 20 / OPTICS LETTERS
the response time of this sensor is
