Narrowband interrogation of plasmonic optical fiber biosensors based on spectral combs

Optics and Laser Technology 96 (2017) 141–146

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Narrowband interrogation of plasmonic optical fiber biosensors based on spectral combs Álvaro González-Vila ⇑, Damien Kinet, Patrice Mégret, Christophe Caucheteur Electromagnetism and Telecommunication Department, University of Mons, Boulevard Dolez 31, 7000 Mons, Belgium

a r t i c l e

i n f o

Article history: Received 11 April 2017 Accepted 10 May 2017

Keywords: Optical fiber sensor Tilted fiber Bragg grating Surface Plasmon resonance Fiber Bragg grating Sensor interrogation Refractive index sensing

a b s t r a c t Gold-coated tilted fiber Bragg gratings can probe surface Plasmon polaritons with high resolution and sensitivity. In this work, we report two configurations to interrogate such plasmonic biosensors, with the aim of providing more efficient alternatives to the widespread spectrometer-based techniques. To this aim, the interrogation is based on measuring the optical power evolution of the cladding modes with respect to surrounding refractive index changes instead of computing their wavelength shift. Both setups are composed of a broadband source and a photodiode and enable a narrowband interrogation around the cladding mode that excites the surface Plasmon resonance. The first configuration makes use of a uniform fiber Bragg grating to filter the broadband response of the source in a way that the final interrogation is based on an intensity modulation measured in transmission. The second setup uses a uniform fiber grating too, but located beyond the sensor and acting as a selective optical mirror, so the interrogation is carried out in reflection. Both configurations are compared, showing interesting differential features. The first one exhibits a very high sensitivity while the second one has an almost temperature-insensitive behavior. Hence, the choice of the most appropriate method will be driven by the requirements of the target application. Ó 2017 Elsevier Ltd. All rights reserved.

1. Introduction Since the rise of fiber Bragg grating (FBG) sensors over the last decades, many configurations have been proposed to study a wide set of phenomena. Temperature, strain, pressure, magnetic field or acoustic sensing are just some examples of achievable platforms in which these devices provide some improvements or advantages. Aside from physical phenomena, FBGs have also been used as chemical sensors, especially for detecting hazardous compounds in harsh environments [1]. Within this group, refractometric sensors remain one of the most extended solutions to sense the characteristics of both gaseous and liquid media. Refractometry can be defined as the technique that measures the speed of light in a given substance with respect to the speed of light in air, a ratio known as refractive index, mathematically represented by the letter n, that gives information about the composition or purity of the medium. Several optical refractometric platforms exist [2,3]. Those based on fiber gratings can be obtained with etched FBGs [4–6] or long period gratings (LPGs) [7,8] due to the possibility of coupling light to an external medium. ⇑ Corresponding author. E-mail address: [email protected] (Á. González-Vila). http://dx.doi.org/10.1016/j.optlastec.2017.05.015 0030-3992/Ó 2017 Elsevier Ltd. All rights reserved.

Tilted fiber Bragg gratings (TFBGs) constitute an additional platform to couple light to the medium surrounding the fiber [9], featuring some advantages as keeping the fiber physical integrity and providing a high Q-factor interrogation. A TFBG is a periodic modulation of the refractive index of an optical fiber core that is slightly tilted with respect to the perpendicular to the fiber longitudinal axis. Similarly to FBGs, the part of the light that satisfies the Bragg condition gets reflected backwards when it reaches a TFBG. However, the tilt angle of the grating planes causes the coupling of a whole set of modes into the cladding of the optical fiber, each of them propagating with a corresponding effective refractive index. These modes are confined in the cladding while the total internal reflection condition is satisfied (i.e. while the refractive index of the surrounding medium is lower than the refractive index of the modes). But in the reverse case, the modes tend to couple to the external medium, producing a distinctive response on the TFBG transmitted spectrum [10], which has been used for the development of lots of different sensors [11–15]. Another feature of TFBGs is their ability to excite a surface Plasmon wave when being coated with a thin metallic film. A surface Plasmon is a collective oscillation of electrons at the interface between a metal and a dielectric, widely studied in the wellknown Krestchmann prism configuration [16]. In the case of TFBGs,

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when properly polarized modes propagating through the fiber cladding reach the metal coating, some of them are propagated to the surrounding medium and this propagation can be appreciated on the transmitted spectrum as a characteristic surface Plasmon resonance (SPR) signature [17]. Due to the optical properties of biological tissues in terms of refractive index [18] and to the high sensitivity obtained with these configurations, plasmonic TFBG sensors have been specially applied for biosensing applications [19,20]. The change in the transmitted spectrum due to a change in the refractive index of the medium surrounding a plasmonic TFBG sensor can be demodulated by different means. The tracking of the wavelength shift of the most sensitive cladding mode or the interrogation based on the polarization properties of the sensor are techniques that have proven to exhibit high sensitivity and resolution [21]. However, they both rely on spectral measurements (most often over a wavelength range of a few tens of nanometers) that imply the use of quite expensive and cumbersome optical equipment. As this can be a handicap when using these sensors in practical situations such as in clinical settings [22], the aim of this work is to study cost-effective configurations to interrogate plasmonic TFBG biosensors. The shift exhibited by the cladding mode resonances implies that at a fixed wavelength the optical power also changes as a function of the external refractive index. That is why the following interrogation setups will be based on optical power measurements, which will allow to significantly reduce the cost of the required equipment. 2. Study into the modes behavior To start analyzing the response of plamonic TFBG sensors in terms of the optical variation in their modes, several samples were produced and a special attention was paid to obtain good conditions for surface Plasmon excitation by measuring their transmitted spectrum. As the centerpieces of the sensors, TFBGs were photo-inscribed in the core of a hydrogen-loaded PS-1250 photosensitive optical fiber from FiberCore. Light emitted by a continuous-wave (CW) frequency-doubled fiber laser at 244 nm was properly guided to reach a 1063 nm pitch silica phase-mask. Given the period of the phase-mask, a tilt angle of 6° for the grating planes was chosen. In doing so, it was possible to get a Plasmon signature in the spectral region around 1550 nm when immersing the sensor in aqueous media exhibiting similar refractive index values to the ones of biological tissues. On the photo-inscription setup, the phase-mask was tilted within the plane perpendicular to the laser beam to obtain the aforementioned angle. The TFBGs were then introduced in an oven set to 85 °C for about 18 h to remove the residual hydrogen molecules still present in the fiber and after that, a gold coating with a thickness of 50 nm was deposited by a sputtering process [23]. The sensors were immersed into a solution to perform the first characterization shown in Fig. 1A, obtained by interrogating them in transmission with an optical vector analyzer (OVA). Between the optical source and the sensor, a polarizer was introduced to select a radial (P) polarization and thus be able to excite a surface Plasmon resonance at the interface between the gold and the surrounding dielectric medium [24]. The arrows in the figure point to the regions of interest for the present study. Beginning from the right, the Bragg mode is the one that propagates confined in the core of the optical fiber, also known as core mode and that can serve as a temperature reference for spectral measurements. Next to it the ghost mode is the cladding mode featuring the highest effective refractive index and the entire resonance comb located on the lower part of the spectrum propagate along the fiber cladding. The SPR excitation involves a characteristic signature that is also highlighted and that is produced by the set of cladding modes that

Fig. 1. (A) Transmitted spectrum of a gold-coated TFBG immersed in solution with arrows pointing to its resonances of interest and (B) wavelength shift due to a variation in the surrounding refractive index.

satisfy the conditions to be transferred to the outer medium as an evanescent wave. Finally, the rest of the cladding modes remain confined into the fiber cladding due to their reflection against the gold film. The modes that exhibit the highest sensitivity for refractive index sensing are the ones located near the SPR signature on the left part of the spectrum. Following the convention implemented on previous studies [17], these modes are numbered taking the one with the highest peak-to-peak amplitude as the ‘‘mode 0” and subsequently assigning incremental indices towards the right. The analysis of the modes behavior will be carried out on the modes ranging from the indices +1 and +7, both included. The refractive index of the solution in which the sensors were immersed was increased by 7.5  10 3 refractive index unit (RIU) in a controlled manner and the spectra before and after this modification can be seen in Fig. 1B. The wavelength shift of the modes can be easily appreciated and at the same time a difference in insertion loss occurs at fixed wavelengths. Thus, the most relevant information comes from studying the evolution of the optical power along the whole refractive index range. To do so, the previously used broadband source was substituted by a tunable laser source (TLS) set to emit at the specific wavelengths of the cladding modes of interest [25]. The optical detector consisted of a photodiode and the refractive index of the solution was measured by a digital refractometer with a resolution of 10 4 RIU. With this schema, the corresponding cladding mode modulates the optical power from the source in a similar way as in edge filtering interrogation [26–30]. As a result, a graphical evolution of the optical power of the seven modes can be obtained, as depicted in Fig. 2. It is worth mentioning that the optical power changes do not correspond to the optical power differences of the modes peaks, but to the actual difference at fixed wavelengths, due to the nature of the TLS. According to the graph, it is easy to determine that the evolution of the transmitted power is not linear but follows the characteristic shape of a negative sigmoid function. This means that the response can be considered linear but just in reduced refractive index regions. In other words, for each refractive index region a different mode should be chosen in order to perform a linear refractometric measurement. As an example, the‘‘mode + 2” exhibits a linear behavior for relative increments until 1.5  10 3 RIU but it is clear that this mode is not appropriate at all to measure changes higher than 4.5  10 3 RIU. The investigated refractive index relative variation was divided into five different increments of 1.5  10 3 RIU and a numerical

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3. Results and discussion 3.1. Interrogation in transmission

Fig. 2. Power variation at the wavelengths matching certain cladding modes when the refractive index around the sensor changes.

quantification of the sensitivity of each mode in each region was carried out, as shown in Table 1. Three different perspectives can exist in view of these data to explain the final conclusion in the same way. First of all, looking at the data row by row it is possible to see that there is a region of maximum sensitivity for each mode and this region conforms an absolute maximum. The sensitivity of the neighboring regions tends to decrease the more they get away from the maximum. Secondly, looking to the data column by column, a similar behavior can be observed but between modes. In general, for a fixed refractive index region there is a mode that exhibits the maximum sensitivity (in bold) and the sensitivity of the neighboring modes decreases the more they get away from the maximum. Finally, if the maximum sensitivity for each of the refractive index regions is computed, the result is that the higher the refractive index gets, the higher is the mode order that has to be chosen in order to obtain the maximum sensitivity. In particular, for the selected intervals of 1.5  10 3 RIU, each time that the refractive index changes, the maximum sensitive mode order increases by one. The final conclusion is similar to the one obtained by analyzing the wavelength shift of the modes with respect to surrounding refractive index variations [23]. There is not a mode that could be chosen for measuring the whole refractive index region, but each mode exhibits its maximum sensitivity within a certain reduced region. This has sense also in terms of SPR excitation, since if the refractive index increases the Plasmon wave is excited by the cladding modes located into higher wavelength regions. In the case of the plasmonic TFBG sensors studied in this work, the maximum sensitivity exceeds 2000 dB/RIU, which is extremely high taking into consideration the small refractive index variations occurring in most biosensing applications [31,32].

From the results obtained in the previous section, it was decided to fix a tiny refractive index increment of 1.5  10 3 RIU as the total region of study. More precisely, this region corresponds to the third one in the previous section, from 3.0 to 4.5  10 3 RIU, which is the one where the ‘‘mode + 4” exhibited a maximum sensitivity of 2326.5 dB/RIU. Although this value of sensitivity is really high and the previous setup is perfectly valid for interrogating this kind of sensors, the use of a TLS significantly increases the price of the interrogating system. As this work claims to develop costeffective interrogation schemes, it is desirable to substitute the previous source by a simpler alternative without really altering the efficient interrogation principle described before. The first step was to replace the narrowband source by a broadband LED source with a flat response in the C + L bands. However, this source is not valid for the purpose of measuring optical power variations in a single mode because of its broadband nature: the photodiode integrates a broad spectral region and not only the modes of interest, masking the power change. In order to filter the source response and produce a narrowband Gaussian-shaped profile, a uniform fiber Bragg grating was introduced by means of an optical circulator, as shown in Fig. 3A. In doing so, the portion of light reaching the sensor is established by the reflection spectrum of the FBG. At this point, several options result interesting to create a tunable FBG filter so to be able to point to the specific cladding mode of interest. Solutions based on temperature [33], pressure [34] or piezoelectric strain [35] applied to the FBG have already been studied, as well as advanced filtering techniques like the ones used for single-longitudinal-mode (SLM) fiber lasers [36]. However, the fact of precisely knowing where the cladding mode resonances are going to be present from the fabrication parameters of the TFBG or the usual working operation in media with similar refractive index to the one of tissues, e.g. phosphate-buffered saline (PBS), serum or blood, make the use of a tunable source unnecessary. The FBG was fabricated through UV irradiation emitted by an ArF excimer laser at 193 nm in the core of a BendBright-XS fiber from Draka. The phase-mask was chosen to have a period such that the central wavelength of the grating would be the nearest to the mode of interest as possible. From then on, a calibrated motorized platform was used to regulate the strain between the clamps holding the fiber during the photo-inscription so that a fine tune of the Bragg wavelength was achieved. For this interrogation setup the fabrication parameters of the FBG are of utmost importance. On one hand, it is desirable to create a strongly reflective grating in order to maximize the optical power entering the photodiode. Nevertheless, from a certain threshold the spectral width of the main lobe increases with the reflectivity, widening the response of the filter and thus widening the portion of light that reaches the plasmonic TFBG sensor too. In addition, the side lobes of the FBG are

Table 1 Absolute value in dB/RIU of the sensitivity exhibited by the modes within different refractive index regions. Dn is expressed in 10

Mode + 1 Mode + 2 Mode + 3 Mode + 4 Mode + 5 Mode + 6 Mode + 7

3

RIU.

0.0  Dn  1.5

1.5  Dn  3.0

3.0  Dn  4.5

4.5  Dn  6.0

6.0  Dn  7.5

941.1 1473.4 661.4 593.2 7.9 89.4 12.6

337.5 948.6 1439.0 1326.3 347.3 203.9 46.7

174.5 318.4 1802.0 2326.5 1133.5 681.0 44.7

131.5 263.5 1022.9 1695.7 2125.2 1465.8 334.1

203.9 16.9 28.3 146.3 1761.2 1935.0 813.0

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desirable to be as low as possible in order to reduce the unwanted optical signal out of the filter range, with the possibility of using apodized FBGs as an alternative. All of this defines a trade-off between the amount of power reaching the plasmonic sensor and the quality of the FBG-based filter used as part of the source. The FBG that was used for the experiments was optimized to have a full width at half maximum (FWHM) of 80 pm and a difference of 14.6 dB between the main and secondary lobes. After connecting it to the rest of the elements, the end of the FBG was immersed into adapting refractive index gel to avoid undesired Fresnel reflections that would also reach the sensor under test. The proposed interrogation setup was tested by modifying the refractive index of the solution in which the plasmonic sensor was immersed, obtaining the results in Fig. 3B. As expected from the study carried out before, the system exhibits a linear response with a good overlap between the experimental data and the linear adjustment. Concerning the quantitative response, this configuration features an absolute sensitivity of 982.5 dB/RIU with a resolution of 1.0  10 6 RIU within the studied refractive index region. The obtained sensitivity is slightly lower than half the value obtained with the TLS configuration due to the intrinsic characteristics of the elements employed. As mention before, the photodiode measures the incoming optical power integrated along a wide region of the spectrum. Thus, while the TLS has a narrow response from the noise level to the peak, the FBG presents a set of side lobes that interferes in the measurements. With regard to the resolution, it is clear that it directly depends on the photodiode chosen, so the selection of one or another will be established by the requirements of the specific application. Finally, in spite of the mention differences between the two configurations, the response of the proposed schema is still high enough for most applications in which these sensors are required.

Light Source Sensor + Receptor

A Polarizer

Au-TFBG

3

PD

2 FBG

B

Broadband Narrowband

0.1

−0.2

−0.5

−0.8

−1.1 Experimental data Linear fit: -982.5 dB/RIU

−1.4

−1.7 2.8

3.1

3.4

3.7

4.0

4.3

n (RIU)

4.6 x 10-3

Fig. 3. (A) Setup used to interrogate the sensor in transmission and (B) sensitivity obtained with this configuration.

Source + Detector Sensor + filter

A Polarizer

3.2. Interrogation in reflection

Circulator Au-TFBG

2

1

LED

FBG

3 Broadband Narrowband

PD

B

0.02

−0.01

−0.04

P (dB)

The previous interrogation setup results highly sensitive but has the counterpart of requiring the access to both ports of the plasmonic sensor, due to the fact that the interrogation is carried out in transmission. Due to their reduced size, several works propose to introduce optical fiber biosensors inside the human body for in vivo detection [37,38]. Because of the target for noninvasive operation, there is not enough physical space to curve the fiber and extract the sensor response to an external receptor. It is the reason why having a setup based on interrogating these sensors in reflection results very important especially for implementations out of the laboratory environment. In order to keep the system as simple as possible, the same elements employed for the interrogation in transmission were reused, so the interrogation in reflection could be performed without needing any additional element. This time, light from the LED source is properly polarized, coupled by the optical circulator and directly reaches the sensor, as shown in Fig. 4A. The narrowband filtering is then carried out by the FBG located beyond the plasmonic TFBG sensor, so the reflected signal is centered on the cladding mode of interest and is propagated backwards to the photodiode. With this configuration, the couple formed by the plasmonic TFBG sensor and the FBG can be located far away from the measurement equipment and even the distance between the two can be adjusted if needed. The results obtained with this setup can be seen in Fig. 4B, featuring an absolute sensitivity of 77.6 dB/RIU and a resolution of 1.3  10 5 RIU. At first sight, these values seem not to match with the results obtained before, but the different response of this setup can be easily explained. In the two previous configurations the refractive index changes produced a variation in a single peak of the optical spectrum received by the photodiode, being the peak

Circulator 1

LED

P (dB)

144

−0.07

Experimental data −0.10

−0.13 2.8

Linear fit: -77.6 dB/RIU

3.1

3.4

3.7

n (RIU)

4.0

4.3

4.6 x 10-3

Fig. 4. (A) Setup used to interrogate the sensor in reflection and (B) sensitivity obtained with this configuration.

of the TLS source and the one of the FBG filter respectively. However, now the photodiode receives two peaks: the one of the FBG filter located at the end of the fiber and the one corresponding to the Bragg mode of the TFBG. In order to get high peak-to-peak amplitude on the cladding mode resonance set and maximize the

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power coupled to the outer medium, TFBGs have to be strongly photo-inscribed in the optical fiber core. That involves that the reflectivity of the Bragg mode of the TFBG is high enough to predominate over the peak produced by the FBG filter, partially attenuated by the sensor. As the Bragg mode does not exhibit any variation in the presence of a refractive index change, its contribution when it reaches the photodiode masks the real measurement of interest, corresponding to the FBG filter. The portion of optical power that reacts to refractive index changes of the medium surrounding the sensor is thus significantly reduced with respect to the two previous configurations. Nevertheless, this setup provides a competitive solution for measurements in reflection, specially taking into consideration the following remarks. As mentioned before, it only requires the access to one side of the optical path, so it enables to develop a compact single-port ‘‘plug&play” interrogator, useful in case of sensor commercialization. In addition, it is fully compatible with self-referencing configurations designed to improve and stabilize the measurements in the presence of potential power fluctuations along the optical path [39]. And finally a great advantage of this setup with respect to the ones discussed before is its stability with respect to temperature variations. The cladding mode resonances of TFBGs are not just sensitive to refractive index variations of the outer medium but to strain and temperature effects too. As well as the Bragg mode in uniform FBGs, they exhibit a wavelength shift of around 10 pm/°C. In the setup we can find a cladding mode of a TFBG being filtered by a FBG, so if temperature changes, both modes will exhibit a wavelength shift of approximately the same value. The power variations registered by the photodiode will then be only due to refractive index variations of the medium under test and will not be influenced by the temperature on each step. To verify this behavior, both the plasmonic sensor and the FBG filter were immersed in a solution whose refractive index remained constant. Several temperature cycles were performed from room temperature to 90 °C to test the response of the system. The temperature was not increased over 90 °C not to enter into the thermal decay region of the gratings [40]. The response obtained showed a linear behavior with a slope of 2  10 3 dB/°C, which suggests that the wavelength shift ratio of both gratings is not strictly the same. However, the contribution to the optical power measured by the photodiode is so small that the interrogation system can be considered as temperature-independent for practical situations. 4. Conclusion The behavior of the most sensitive modes of a plasmonic TFBG sensor in terms of optical power change with respect to refractive index variations in the surrounding medium has been studied. In view of the experimental results, it was concluded that the choice of a particular mode directly depends on the refractive index region under study. Choosing the mode and region of highest sensitivity, two interrogation setups have been proposed. The first one constituted a configuration to carry out measurements in transmission. A broadband source was filtered by means of a FBG and the narrowband peak of the latter reached the sensor before being measured by a photodiode. This setup resulted very sensitive and suitable for situations when the sensor under test is accessible from its two ends. In the opposite case, when measurements in reflection are needed, a second configuration was presented. This time, the FBG filter was located beyond the sensor acting as a selective mirror, keeping both the broadband source and the photodiode from the previous setup. While the first configuration exhibits a higher sensitivity, the second one can be considered almost immune to temperature changes, making both setups suitable solutions for interrogating this kind of sensors in a cost-effective manner.

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Acknowledgements Á. González-Vila is supported by the Belgian F.R.S.-FNRS through a FRIA grant. C. Caucheteur is Associate Researcher of the F.R.S.-FNRS. The authors would also like to thank the financial support from the ERC (European Research Council) Starting Independent Researcher Grant PROSPER (grant agreement No. 280161 – http://www.umons.ac.be/erc-prosper). References [1] X. Wang, O.S. Wolfbeis, Fiber-optic chemical sensors and biosensors (2008– 2012), Anal. Chem. 85 (2012) 487–508, http://dx.doi.org/10.1021/ac303159b. [2] K. Bremer, B. Roth, Fibre optic surface plasmon resonance sensor system designed for smartphones, Opt. Express. 23 (2015) 17179–17184, http://dx. doi.org/10.1364/OE.23.017179. [3] A.N. Castro Martinez, M. Komanec, T. Nemecek, S. Zvanovec, S. Khotiaintsev, Fiber optic refractometric sensors using a semi-ellipsoidal sensing element, Appl. Opt. 55 (2016) 2574–2579, http://dx.doi.org/10.1364/AO.55.002574. [4] A. Mukherjee, D. Munsi, V. Saxena, R. Rajput, P. 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