Carbon nanotube coated fiber Bragg grating for photomechanical optic modulatorB. N. Shivananju, Ashish Suri, Sundarrajan Asokan, and Abha Misra

Citation: Review of Scientific Instruments 84, 095101 (2013); doi: 10.1063/1.4819742View online: https://doi.org/10.1063/1.4819742View Table of Contents: http://aip.scitation.org/toc/rsi/84/9Published by the American Institute of Physics

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REVIEW OF SCIENTIFIC INSTRUMENTS 84, 095101 (2013)

Carbon nanotube coated fiber Bragg grating for photomechanicaloptic modulator

B. N. Shivananju, Ashish Suri, Sundarrajan Asokan, and Abha Misraa)

Department of Instrumentation and Applied Physics, Indian Institute of Science, Bangalore 560012, India

(Received 28 May 2013; accepted 15 August 2013; published online 3 September 2013)

We have demonstrated novel concept of utilizing the photomechanical actuation in carbon nanotubes(CNTs) to tune and reversibly switch the Bragg wavelength. When fiber Bragg grating coated withCNTs (CNT-FBG) is exposed externally to a wide range of optical wavelengths, e.g., ultravioletto infrared (0.2–200 μm), a strain is induced in the CNTs which alters the grating pitch and re-fractive index in the CNT-FBG system resulting in a shift in the Bragg wavelength. This novel ap-proach will find applications in telecommunication, sensors and actuators, and also for real timemonitoring of the photomechanical actuation in nanoscale materials. © 2013 AIP Publishing LLC.[http://dx.doi.org/10.1063/1.4819742]

Carbon nanotubes (CNTs) have been shown to beexhibiting near-perfect black body characteristics,1 respond-ing to a wide range of optical wavelengths.1, 2 The sensi-tivity of CNTs towards infrared (IR), visible, and ultravio-let (UV) radiations has found many practical applications3

such as infrared thermal detectors,4, 5 pyroelectric sensor,6 so-lar energy collectors,7 optical modulators,8 sensors,9 transis-tors, and diodes.10 Upon illumination with light, CNTs showactuation due to changes in their physical properties suchas bond bending and stretching.11 When the light illumina-tion is removed, CNTs restore their initial position due totheir photo-elastic nature.11 Zhang and Iijima have shownthat single walled carbon nanotubes (SWCNTs) can undergophotomechanical changes when exposed to the visible lightusing halogen lamp (Olympus LGW, 150 W) and He-Nelaser (632.8 nm, 10 mW) as the light source.11 Thereafter,many researchers have explored the photomechanical actua-tion responses in pure CNTs,11 as well as its composites withpolymers.12 These studies have revealed that the photome-chanical actuation in CNT is dependent on intensity/power(the number of photons) and polarization of light.13 The align-ment and orientation of the CNT also influence the photome-chanical actuation.13 Further, researchers have speculated thatthe interplay between photo-elastic, electrostatic, polaronic,and thermal interactions give rise to the overall photomechan-ical responses in carbon nanotubes.13

The phenomenon of photo-induced strain has also beenobserved in polymers,14 chalcogenide glasses,15 and lead zir-conate titanate (PZT) ceramics.16 However, the usage of thesematerials is limited by the kinetics of contraction and relax-ation, lower induced strains, and high operating temperatures.All these limitations can be overcome by using CNTs as pho-tomechanical actuators.13

Fiber Bragg grating (FBG) sensors have been exploitedrecently, for a variety of sensing applications, due to theirmany desirable advantages such as high sensitivity, compactform, inherent multiplexing capability, multi-functionality,long term stability, immunity to electromagnetic interference,

a)Email: abha@isu.iisc.ernet.in

etc. A FBG sensor is fabricated by modulating periodicallythe refractive index of the core of a single mode optical fiber,by exposing it to intensity modulated UV light. When a broad-band light is guided through the core of a FBG, one particularwavelength (λB), which satisfies the following Bragg condi-tion as shown in Eq. (1) is reflected back and the remainingwavelengths are transmitted:

λb = 2neff �, (1)

where neff is the effective refractive index of the core and �

is the grating periodicity. The sensing property of a FBG isbased on measuring the shift in the Bragg wavelength, af-fected by an external perturbation, which causes a change in� or neff or both.

In addition to sensing, FBGs also find applications incommunication engineering such as fiber laser, fiber am-plifier, fiber Bragg filter, wavelength division multiplex-ers/demultiplexers, dispersion compensators, etc. in which itis necessary to tune the Bragg wavelength. The tuning of theBragg wavelength in FBGs is generally achieved by usingthe techniques such as acousto-optic,17, 18 electro-optic,19–21

and magneto-optic.22, 23 However, these techniques are of lim-ited use as they are mainly contact based (except magneto-optic systems) and generate noises resulting from electromag-netic interactions. Recently, Shao et al. have used a manuallywrapped thin sheet of SWCNT around the tilted FBG for afour-wave mixing.24 Here, IR wavelength is exposed inter-nally to the gratings (internal method), which reflects lightbeam onto the CNTs to tune the Bragg wavelength. However,this method is limited to the use of only single wavelength totune the Bragg wavelength. Also, in this work, the effect ofIR exposure on the bare tilted FBG is not explained, whichis crucial to induce a shift in the Bragg wavelength when ex-posed to bare FBG.25

The limitations associated with the existing techniquescould be overcome by using the photo induced actuation char-acteristic of the CNTs to tune the Bragg wavelength. In ourwork, the photomechanical actuation in CNT coated FBG(CNT-FBG system) allows the tuning of the Bragg wave-length using a wide range of optical wavelengths, e.g., UVto IR (0.2–200 μm)1, 2 upon exposing externally. It provides

0034-6748/2013/84(9)/095101/8/$30.00 © 2013 AIP Publishing LLC84, 095101-1

095101-2 Shivananju et al. Rev. Sci. Instrum. 84, 095101 (2013)

FIG. 1. (a) SEM image of the CNT, coated around circular surface of the FBG. (b) High-resolution image of vertically aligned CNT.

a very simple, compact, non-contact, and non-destructivemethod to tune the Bragg wavelength. The system responseis not only stable over external sources of noise but alsobe able to precisely tune the Bragg wavelength with elec-tromagnetic immunity. The photomechanical optical modu-lator system will find applications in the field of fiber opticsand optical communications, especially in the area of opticaladd-drop multiplexers (OADM),26, 27 filters,28, 29 lasers,30, 31

modulators,32 amplifiers,33 interrogators,34 etc.We have also demonstrated that CNT-FBG system can

also be used for a real time monitoring of the photomechani-cal actuation in nanoscale materials, for providing more pre-cise measurements than any other reported techniques, whichare mostly mechanical and complex in nature.12, 13

The FBG sensors35, 36 are fabricated on a photosensi-tive germania doped silica fiber (Fibercore, SM1500) with acore of refractive index 1.47 and a diameter of 4.2 μm withthe cladding of refractive index 1.44 having a diameter of125 μm. The Bragg gratings have been inscribed on the coreof this fiber using a phase mask37 (of pitch 1064 nm) by ir-radiating the core with a 3 mJ pulses from a 248 nm KrF ex-cimer laser with repetition rate of 200 Hz. The gratings areinscribed over a length of about 3 mm. The magnitude of thephoto-induced periodic modulation of refractive index insidethe core is in the order of 10−4.38 The grating periodicity pro-duced with this phase mask is approximately 532 nm giving abaseline Bragg wavelength around 1550 nm.

Vertically aligned multi-walled carbon nanotubes(MWCNT) have been coated onto the FBG using chemicalvapor deposition. This process involves the thermal decom-position of a hydrocarbon vapor (toluene) in the presenceof a metal catalyst (Ferrocene). The two precursors aremixed in a ratio of 0.02. The solution is pre-heated to itsvaporization temperature and then carried into the reactionzone using argon as the carrier gas at a flow rate of 800 sccm(standard cubic centimeter). The three-zone furnace is usedwhere the first zone is used for preheating the solution to itsvapor state and the middle is the reaction zone, where thechemical reaction takes place to synthesize CNTs on FBG.The temperature of 780 ◦C is maintained in the middle zone,while the end zones are set at room temperature.

Figure 1(a) shows the scanning electron micrograph(SEM) of the CNT-FBG system, where MWCNT are growndirectly around the cylindrical FBG surface along its radial

direction. The diameter of the CNT is measured between 10and 50 nm and the overall length of the CNT mat is ∼60 μm.Figure 1(b) shows a high-resolution image of the as-grownCNT where a vertical orientation of all CNT in the mat canbe observed.

The schematic of the experimental setup is shown inFigure 2(a). The CNT-FBG system is exposed to radiationsfrom different sources such as IR light (1550 nm, throughan optical fiber), visible light (514 nm, using an argon-ionlaser), and UV light (248 nm, using an UV excimer laser).The IR laser power is tunable from 17 to 34 mW (with a reso-lution of 1 mW); the visible laser is tunable from 1 to 70 mW(with a resolution of 1 mW); and the UV laser is tunable from1 to 950 mW (with a resolution of 1 mW). The light is ex-posed perpendicular to the axis of CNT-FBG system. In caseof visible and UV light, it is a free space delivery; whereasin case of IR light it is a fiber optic delivery. As the powerwill differ largely in case of fiber optic delivery, the CNT-FBG is kept as close as possible to the fiber end so that lossescan be minimized. Further, we have used an optical powermeter to check the power (33 mW) delivered to CNT-FBGsystem.

FBG interrogator (Micron Optics SM130) having awavelength resolution of 1 pm was used to monitor a shiftin the Bragg wavelength of the CNT-FBG (Figure 2(a)). Aschematic in Figure 2(b) shows the interaction of light withthe CNT-FBG system and a resulting shift in the Bragg wave-length.

As mentioned earlier, when a broad band light islaunched in the core of a FBG, the contributions of the re-flected light from each of the grating planes add construc-tively in the backward direction to form a back-reflectedpeak, which represents the Bragg wavelength.37 In CNT-FBG,light from an external source is made to be incident trans-versely and the Bragg wavelength is tuned using photome-chanical actuation phenomenon in CNTs, as can be seen inFigure 2(b). When CNTs are exposed to light, a strain isinduced11, 13 and the induced strain in the CNT alters the grat-ing pitch and refractive index in the CNT-FBG system, whichresults in shift in the Bragg wavelength; this shift is directlycorrelated with the strain induced in the CNTs due to the phe-nomenon of photomechanical actuation in CNTs.

Figure 3(a) shows a cyclic switching response of theBragg wavelength with time, when the CNT-FBG is exposed

095101-3 Shivananju et al. Rev. Sci. Instrum. 84, 095101 (2013)

FIG. 2. (a) The schematic diagram of the experimental setup used to tune the Bragg wavelength utilizing the phenomenon of photomechanical actuation inCNTs. (b) Exploded schematic diagram of FBG explaining the mechanism behind the observed Bragg wavelength shift.

to the IR wavelength. It can be seen that the Bragg wave-length is shifted from 1526.49 to 1527.58 nm with a largeshift of 1090 pm at 33 mW of incident IR power, due to thephenomenon of photomechanical actuation in CNTs. In thiscontext, it is interesting to note that a shift of only 250 pmis observed in bare FBG at 33 mW IR power, this is aboutfive times less than the shift observed in the CNT-FBG. Uponanalyzing the temperature-induced changes, it is found thatthe observed shift in Bragg wavelength in bare FBG is dueto the photothermal effects associated with the IR light.25Amaximum temperature change of 18 K (from the room tem-perature at 319 K) is recorded for the maximum available IRpower of 33 mW at the tip of the fiber optic IR laser. Thisthermal heating results in the shifting of Bragg wavelengthby 250 pm in the bare FBG, which is much lesser comparedto the large shift of 1090 pm observed in the Bragg wave-length in the CNT-FBG. Moreover, we have not observed anythermal heating effect during the exposure of visible light ascan be seen in Figure 3(b). Figure 3(b) shows a cyclic switch-ing response of Bragg wavelength in CNT-FBG due to the

visible light incidence. A shift of 410 pm in Bragg wave-length is observed when exposed to visible light (33 mW),which is ∼264% lesser than the IR exposure. The total shiftobserved in this case is only due to the photomechanical ac-tuation phenomenon, which confirms the observation that theBragg wavelength shift observed with IR exposure on bareFBG, is due to the photothermal effects.12

Figure 3(c) shows the cyclic response of the Bragg wave-length with UV light illumination. A shift in the Bragg wave-length is about 30 pm at 33 mW of UV light which is muchlower as compared to the shift in case of IR and visible lightexposures. Figure 3(d) shows that the observed Bragg wave-length shift increases with the increase in the wavelength oflight, which is also schematically depicted in the inset inFigure 3(d). This is in complete agreement with the absorp-tivity spectrum of the CNTs,1, 2 which illustrates that CNTsabsorb more IR light than visible and UV wavelengths.

Interestingly, the shift in the Bragg wavelength not onlydepends on the wavelength of the incident light but also on theintensity/power of the incident light.11 The photomechanical

095101-4 Shivananju et al. Rev. Sci. Instrum. 84, 095101 (2013)

FIG. 3. Bragg wavelength shift observed in CNT-FBG exposed to different wavelengths. (a) IR-source (wavelength, 1550 nm). (b)Visible-source (wavelength,514 nm). (c) UV-source (wavelength, 248 nm) and (d) the comparison of the Bragg wavelength shift observed with respect to different wavelengths of light.

actuation in CNTs has been shown to increase with theincrease in power of the light (number of photons).13

Figure 4(a) shows the increase in shift of the Bragg wave-length with an increase in IR power, which corresponds tothe increased strain in CNTs, as shown in previous studies.13

A maximum shift of 1090 pm is observed at the maximum IRpower of 33 mW. This observed shift in the Bragg wavelengthcan be fitted linearly with respect to the IR power, as is evi-dent from Figure 4(b). Additionally, a reversible tuning of thewavelength has been demonstrated by reducing the IR powerback to its original value. The overlap of the backward tuninglinear curve with the forward tuning linear curve is indicativeof photo elastic property of the CNTs, another photomechan-ical actuation characteristic that can be demonstrated usingthis technique. A similar trend is observed when the exper-iments are carried out with visible light source. Figure 4(c)shows the shift in the Bragg wavelength, indicating a max-imum shift of 770 pm at the maximum visible light powerof 70 mW. The backward and forward tuning curves are alsolinearly fitted as shown in Figure 4(d).

Our method of tuning the Bragg wavelength using exter-nally exposed light sources can find applications in OADM:a device extensively used in optical communication for wave-length division multiplexing (WDM). These devices are com-

posed of three important segments: optical multiplexer, opti-cal demultiplexer, and a method for reconfiguring the pathsbetween them and also between set of ports. Currently, FBGsor other optical switches are used for this reconfiguration. TheFBGs used in OADM are presently being tuned by electro-optic, acousto-optic, and piezoelectric techniques.26, 27

External method of tuning Bragg wavelength in FBG us-ing photomechanical actuation in CNTs is not only a newconcept but also is the non-contact technique. The insets inFigures 4(b) and 4(d) schematically depict the working ofone such CNT-FBG system supported OADM. In order toadd or drop a particular wavelength, the Bragg wavelengthis required to be shifted to a higher or to lower values, respec-tively. This is achieved by changing the power of the externalincident light, which is exposed onto the CNT-FBG system.

Similarly, CNT-FBG can find other practical applica-tions, which exploit the photomechanical characteristic ofCNTs. To demonstrate this, an optical filter has been devisedusing the CNT-FBG system as shown in Figure 5(a). Thesystem is swept from 17 to 34 mW with a resolution of 1mW of IR power and the corresponding shift is plotted inFigure 5(b).

The Bragg wavelength at 17 mW of IR power is mea-sured to be 1527.036 nm, and by increasing IR power by

095101-5 Shivananju et al. Rev. Sci. Instrum. 84, 095101 (2013)

FIG. 4. (a) The response of the Bragg wavelength to different IR powers. (b) The linear fit of the shift in Bragg wavelength with IR power. (c) The response ofBragg wavelength to different visible light powers. (d) The linear fit of the shift in Bragg wavelength with visible light power. The insets ((b) and (d)) represent anovel method of adding or dropping the Bragg wavelength in OADM, using the phenomenon of photomechanical actuation in CNTs using IR and visible light.

1 mW only, we could observe a shift of 10 pm in the Braggwavelength. This result clearly indicates that the CNT-FBGsystem is able to sense the photomechanical actuation inCNTs at a much higher resolution of IR light (<1 mW). De-

spite using different CNT geometries (axial and radial) andconfigurations (sheets, bulk, or suspended), there is no reportyet of achieving such a high resolution.13 Therefore, whena broadband source needs to be filtered with a pico-meter

FIG. 5. (a) Schematic representation of an optical filter based on CNT-FBG system, which is exposed to external light source. (b) The Bragg wavelength shiftwith IR power with a minimum resolution of 1 mW.

095101-6 Shivananju et al. Rev. Sci. Instrum. 84, 095101 (2013)

FIG. 6. (a) The comparison between different levels of modulation achieved with different IR powers. The cyclic response of Bragg wavelength at 34 mW ofIR power after different constant intervals of time at (b) 60 s, (c) 10 s, and (d) 1 s.

resolution, it can be achieved just by tuning the power of theexternal light source onto the CNT-FBG system.

In addition, the system has also been demonstrated as awavelength modulator to modulate a beam of light. When theCNT-FBG system is exposed externally to different IR pow-

ers (0–17 mW, 0–21 mW, 0–27 mW, 0–34 mW), it resultsin a corresponding shift (from 1526.5 nm) in Bragg wave-lengths (550 pm, 660 pm, 850 pm, 1090 pm). Figure 6(a)shows that by increasing the external light power we can in-crease/decrease the level of wavelength modulation, which

FIG. 7. Schematic representation of the wavelength modulator based on CNT-FBG system.

095101-7 Shivananju et al. Rev. Sci. Instrum. 84, 095101 (2013)

FIG. 8. (a) The reproducibility characteristics of photomechanical actuation of CNT-FBG system measured after one month. (b) Shift in Bragg wavelength withincrease in IR laser power.

means that the Bragg wavelength can be shifted back andforth by a specified value. In Figures 6(b)–6(d) the cyclicresponse of bare FBG is also given for comparison. In thepresent study, the cyclic response is also recorded after dif-ferent exposure times of 60, 10, and 1 s, respectively, inFigures 6(b)–6(d). It can be seen that the CNT-FBG registersa stable response on longer exposure. At lower exposures, therecovery is not complete, leading to unstable behavior.

Figure 7 shows schematically a wavelength modulator,where in a continuous input Bragg wavelength is modulatedinto pulsed Bragg wavelength when the CNT-FBG system isexposed to pulse light/photon source.

To assess the long-term stability of the CNT-FBG sys-tem, we have performed a reproducibility test during a gap ofone-month period between the first and second trials. The re-sults of this long-term reproducibility experiment are shownin Figure 8(a). Figure 8(b) shows the Bragg wavelength shiftversus IR laser power. It can be seen from Figures 8(a) and8(b) that there is no significant difference in the performanceof the sensor even after one month indicating a good stabilityof the photomechanical actuation of the CNT-FBG system.Both the trials show a good sensitivity in the whole rangeand the Bragg wavelength shift with respect to laser poweris found to be linear in both the trials.

We report a novel approach of utilizing the photome-chanical actuation in CNTs to tune and reversibly switch theBragg wavelength externally by exposing the CNT-FBG sys-tem to a wide range of wavelengths, namely, infrared, vis-ible, and ultraviolet. The shift in the Bragg wavelength notonly depended on the wavelength of the incident light, butalso on the intensity/power of the incident light. The presentstudy also demonstrates the usage of CNT-FBG system as anovel optical setup for studying the photomechanical actu-ation phenomenon in CNTs. Further, the application of thephotomechanical optic modulator system in telecommunica-tion as OADMs, filters, modulators, has also been demon-strated.

A.M. would like to acknowledge CiSTUP, IISc for thepartial funding. A part of this work is funded by DST, un-der the Centre for Strategic Initiatives and the Robert BoschCentre for Cyber Physical Systems, IISc.

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