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Flexible photonic integrated circuit technology is an emerging field expanding the usage possibilities of photonics, particularly in sensor applications, by enabling the realization of conformable devices and introduction of new alternative production methods. Here, we demonstrate that disposable polymeric photonic integrated circuit devices can be produced in lengths of hundreds of meters by ultra-high volume roll-to-roll methods on a flexible carrier. Attenuation properties of hundreds of individual devices were measured confirming that waveguides with good and repeatable performance were fabricated. We also demonstrate the applicability of the devices for the evanescent wave sensing of ambient refractive index. The production of integrated photonic devices using ultra-high volume fabrication, in a similar manner as paper is produced, may inherently expand methods of manufacturing low-cost disposable photonic integrated circuits for a wide range of sensor applications.
© 2016 Optical Society of America
1. Introduction
Photonic integrated circuits (PICs) are an optical counterpart for integrated electrical circuits, where several functionalities are integrated onto a single platform to perform a wide variety of optical functions. The research on PICs has been mostly motivated by telecommunication and short-range interconnection applications [1–3]. PICs are also highly applicable in sensor usage, since they enable convenient implementation of waveguide-based signal routing between a light source, sensing area and detection point, as well as multiparameter sensing [4,5]. Silicon-based PICs have become particularly popular due to their compatibility with mature complementary metal–oxide–semiconductor (CMOS) technologies [1]. Due to the diversity of the implemented functionalities, a large variety of other materials has also been used to realize PICs including indium phosphide [6,7], glass [8] and ceramics [9,10]. On the other hand, polymeric materials are also used in PICs and as interconnection busses between PICs due to their versatile processing [11–13].
Polymers are dominant in flexible photonic platforms [14]. Flexible photonics is an emerging field expanding the usage possibilities of photonics in several applications, such as displays, interconnections, solar cells and sensors [15–19]. This is because flexibility allows the realization of conformable and diverse-shaped photonic devices and systems. To date, flexible photonics has been mostly produced in batches by fabricating functional devices on rigid substrates and assembling separately produced devices on a deformable carrier typically made of plastic [20]. For example, photonic crystals [21], light-emitting diodes (LEDs) [15] and PICs [16] have been integrated on deformable platforms using the device transfer based method. In order to decrease the complexity and the cost of the manufacture, the additional assembly step is preferably avoided. Therefore, flexible photonic devices and systems have been produced also by fabricating functional parts in batches directly on deformable carriers. Particularly, the development of organic light emitting diodes (OLEDs) [22] initiated the vast field of research on flexible organic electronics [23,24] and photonics [20,25–28] resulting in the monolithic fabrication of active and passive devices directly on deformable substrates including light emitting devices [29,30], polymer solar cells [31] and waveguide devices [17].
To date, independent of the choice of material, realization of PICs has relied on batch processing, however. The increasing need to minimize the production costs of photonics has, however, initiated the researchers to seek alternative methods for batch-based fabrication. Especially disposable photonics, such as optical sensors for point-of-care diagnostics, are very difficult to realize by wafer or sheet-level processing due to the large footprint, typically several square centimeters, required for different functionalities and handling as well as due to cost strains. To offer an alternative for traditional batch production, roll-to-roll (R2R) processing has been introduced to produce photonics and electronics in a similar manner as paper is produced [32,33]. In R2R processing, different surface structures and materials are patterned onto a continuously moving carrier. Diffractive optics [34], slab light guiding [35], surface enhanced Raman spectroscopy (SERS) sensor structures [36], and solar cells [37,38] have already been demonstrated in the roll format. However, PICs with signal routing and transducing functionalities have not been produced by ultra-high volume R2R processing.
Micro- and nanoscale replication methods suitable for high-throughput production have attracted significant research and development efforts since the introduction of nanoimprint lithography (NIL) [39,40]. The operation principle of NIL is simple where a mould with a surface pattern is replicated to another surface by stamping and curing the material to be replicated. Depending on the curing method, NIL or imprinting can be divided into thermal embossing and UV-imprinting. In thermal embossing the replication tool is heated above the glass transition temperature of the thermoplastic polymer to be replicated. During the stamping, thermoplastic material softens and the surface pattern of the mould is replicated due to applied pressure. In UV-imprinting, resin to be replicated is in liquid phase when the stamping tool is brought into the contact. By applying UV-light, the resin is cured and the surface structure is replicated. Both, thermal and UV-based methods have been successfully transferred from static wafer- or sheet-level processing to continuous R2R processing with demonstrations to produce sub-µm structures [33,40,41]. As our previous work has shown, sheet-level UV-imprint method can be used to fabricate optical single-mode waveguides [42–44], the UV-curing method was chosen in this work to investigate whether R2R scale processing can be also applied in production of single-mode waveguides.
PICs can be used to implement evanescent wave sensors, such as Young interferometers (YIs) [45]. Integrated YIs are based on waveguides and they utilize an evanescent wave field to sense refractive index (RI) changes at or near the waveguide surface. Sensor chips have been realized by using inorganic materials such as silicon [46], glass [47,48], and tantalum pentoxide [49]. More recently, polymeric YI sensor chips have been implemented based on slab [50], inverted ridge [51] and slot [52] waveguides. Polymeric YIs have been reported to have limit of detections (LODs) 10−5-10−6 RIU [51,52] whereas inorganic YIs have achieved LODs 10−6-10−8 RIU [46,48,49] placing them among the most sensitive ones among integrated interferometric sensors [45].
In this paper, we demonstrate that polymeric single-modal PICs for disposable sensors can be manufactured on rolls at lengths of hundreds of meters by R2R processing. This opens a new avenue for the utilization of PICs by enabling their wide spread usage as disposable platforms. We assess the single-modal operation of the waveguides, and analyze attenuation properties of hundreds of individual waveguide samples. The results show that waveguide devices with good and repeatable performance were fabricated. Then we demonstrate the applicability of the PICs for evanescent wave sensing of ambient RI changes utilizing them as integrated YI chips.
The organization of this paper is as follows: In Section 2 we describe the sensor design and the sensing principle of integrated YIs. Section 3, Materials and setups, discusses the determination of the RIs of the used materials, optical setups, and sample solutions used in the sensing experiments as well as their actuation. In Section 4 the stamp fabrication and the R2R processing of PICs are described. Section 5 contains the experimental results and discussion of the single-modality of the waveguides, the waveguide attenuation measurements, and the ambient RI sensing experiments. Finally, our conclusions are given in Section 6.
2. Sensor design and sensing principle
A roll of PICs is shown in Fig. 1(a) and a sensor chip with PIC sensors cut from the roll is shown in Fig. 1(b). A schematic of the YI sensor chip layout used in this paper is shown in Fig. 1(c). The sensor chip input waveguide has three Y-junctions splitting the waveguide into four parallel waveguides forming two YIs, named YI1 and YI2, both consisting of a reference and a measurement waveguide. The distance between the measurement and reference waveguides is increased at the measurement window enabling the patterning of the necessary overcladding layer without lithographic precision. The exact sensor chip design used in this paper, is discussed in detail in Section 5.3.
Fig. 1 (a) Roll of sensor PICs, (b) sensor chip cut from the roll, and (c) illustration of the YI sensor layout with a captured interferogram.
During YI experiments, laser light was end-fire coupled into the input waveguide. The outcoupled diverging light beams overlapped and interfered, and the interferogram was imaged onto a camera detector by using a microscope objective. Imaging was done sufficiently close to the end of the chip, and thus the beams interfered only with the nearest beam forming two, 2-beam interferograms. In addition, the separation between the microscope objective and the camera was chosen so that the interferograms were imaged simultaneously onto a single camera detector as shown in Fig. 1(c).
The sensing method is based on the analysis of the sample-induced shifts of the interferogram fringes. When RI in the measurement window on the measurement waveguide is changed by the sample, the effective RI of the waveguide is changed due to the interaction of the sample with the evanescent wave of the light propagating in the waveguide. Since the reference waveguide is passivated by an overcladding layer, the mutual optical path length difference between the measurement and reference waveguides changes that is seen as a shift of the interferogram fringes. More detailed discussion of the designing principles of integrated YIs and the measurement technique can be found in the literature [53,54].
3. Materials and setups
3.1 Determination of refractive indices
Refractive indices, n, of the used materials including undercladding (Nalax2, Nanocomp), core material (Epocore, Micro resist technology) and overcladding (OP-4-20632, Dymax) were determined by using the prism coupling method and applying the first order Sellmeir equation of
to interpolate the RIs for the wavelength of 975 nm, i.e. for the wavelength used in the experiments. In Eq. (1) λ is the wavelength of light, and A and B are the Sellmeir coefficients.A prism coupler (2010, Metricon) was equipped with lasers of 633, 829, 1320 and 1552 nm wavelengths. The obtained Sellmeir coefficients for each material are listed in Table 1. The Sellmeir RI dispersion curves and measured RIs at discrete wavelengths are shown in Fig. 2. It is worth noting that the maximum observed deviation between the experimentally obtained RI value and the dispersion curve is 0.001. Calculated RIs at the wavelength of 975 nm are shown in Table 1.
Table 1. Obtained Sellmeier coefficients for the used materials and the RIs calculated at the wavelength of 975 nm.
Fig. 2 Obtained Sellmeier dispersion curves and measured RIs at discrete wavelengths for the used materials.
3.2 Optical setups
The measurement setup for the attenuation measurements is illustrated in Fig. 3(a). Laser light from a source (QFBGLD-980-5, QPhotonics) emitting at 975 nm was coupled into the waveguides from a tapered polarization maintaining (PM) input fiber forming a spot about 2.8 µm in diameter. The operational wavelength of 975 nm was chosen in this work allowing the usage of silicon-based cameras and the single-modal waveguide operation that is discussed in Section 5.1. When investigating modal intensity distribution, the waveguide output facet was imaged onto a camera (UI-3240CP-NIR-GL, IDS Imaging Development Systems) through a 100× microscope objective. During intensity measurements, the transmitted light was collected with another tapered fiber. An autoalign station (Newport) was used during measurements to align the fibers into their optimal transmission positions.
Fig. 3 (a) Setup for investigating the modal intensity distribution and for the measurement of attenuation values. The input fiber was moved in the x-direction in the intensity distribution experiments. The white arrows point to the edges of the overcladding area related to the modal mismatch loss discussed in Section 5.1. (b) Optical setup for ambient RI sensing experiments. PM = polarization maintaining.
A schematic of the measurement setup for YI experiments is shown in Fig. 3(b). The light source, the incoupling optics and the camera were the same as used for the attenuation measurements. TM polarized light was used in the YI experiments due to its higher sensitivity compared to TE polarization [51]. Polarization state was confirmed with an external polarizer prior to the measurements. The interference patterns were imaged onto the camera using a 40× microscope objective. The distance between the camera detector and the outcoupling end of the chip was about 17 cm. The interferograms were imaged at a distance of ~250 µm and the interval between the captured interferograms was 2 s.
Interferograms were analyzed by a two-dimensional fast-Fourier transform (FFT) algorithm yielding the phases of the fringes [53]. The areas of the interferograms for the FFT analysis were selected for each measurement and for both of the interferometers individually.
3.3 Sample solutions for ambient refractive index sensing experiments and their actuation
For ambient RI measurements, D-Glucose (Sigma-Aldrich) solutions were prepared in ultrapure water (MilliQ Academic, Merck Millipore) at the following concentrations: 0.006, 0.01, 0.03, 0.1 and 0.5 weight % (wt.%) with the following calculated RI differences to pure water: 8 × 10−6, 1.4 × 10−5, 4.2 × 10−5, 1.4 × 10−4 and 7.0 × 10−4 RIU, respectively. The RI differences were calculated by using a polynomial [55] fitted to the tabulated RI values of aqueous glucose solutions [56]. Solutions were stored at room temperature to make sure that samples and measurement setup had the same temperature.
For the sample actuation during the sensing experiments, a flow cell was assembled on top of the interferometer chip, and sealed with a seal ring. The outlet of the flow cell was connected to a syringe pump (Nexus 3000, Chemyx) and the inlet to a sample vial by a tube. The syringe pump was operated in withdraw mode at a constant flow rate of 100 µl/min. To reduce the effect of water absorption into the waveguides during the experiments [50], the flow cell was filled with water at least a day before the measurements. During the experiments the sample and flushing solutions were sequentially pipetted into the sample vial.
4. Roll-to-roll fabrication
4.1 Nickel stamp fabrication
A nickel (Ni) stamp was used as a stamping tool to produce the waveguide grooves on the rolls. The fabrication and usage of the Ni tool is illustrated in Fig. 4(a). First, a contact lithography process with an aligner (MA6, Karl Suss) with an i-line filter was applied to fabricate original waveguide groove structures in a positive tone resist layer (Ultra-I 123, Dow) that was spin-coated on a silicon (Si) wafer. In order to ease the release of the nanoimprint mould from the cured structures, tilted sidewalls were produced into the grooves by applying an additional reflow step.
Fig. 4 (a) Fabrication of the Ni nanoimprint stamping tool and its usage to produce waveguide grooves into an acrylate layer. AFM scans from (b) the Ni stamp, and (c) the replicated waveguide groove. The numbers displayed in white lettering in figures b and c are the obtained RMS roughness values for the stamp and replica in the ridge and groove regions, respectively. d) Perpendicular scan from an AFM image of a waveguide groove. The inset shows slight ripple at the sidewall of the groove.
An actual UV-nanoimprint stamp with a ridge structure was fabricated by electroforming a Ni plate from the lithographically patterned grooves. Ni plates were then laser welded to an embossing reel. Atomic force microscopy (AFM) scans (Dimension 3100, Veeco) from the Ni-stamp and from an R2R produced waveguide groove in UV-cured Nalax2 (undercladding) are shown in Fig. 4(b) and 4(c), respectively. Rather similar root-mean-square (RMS) surface roughness values of 1.1 nm and 1.2 nm were obtained for both the stamp and replicated structure, respectively, when 1.5 µm x 1.5 µm areas from the top of the imprint stamp and from the bottom of the waveguide grooves were analyzed. The values were obtained by averaging five AFM measurements from five different spots of the sample.
Figure 4(d) shows a single perpendicular scan from an AFM image. Depth of the replicated waveguide groove is about 700 nm. In the inset, slightly observable ripple at the sidewall of the groove is attributed to the lithography phase of the stamping tool fabrication, where interference effects during the UV-exposure results in sidewalls with waviness.
4.2 Roll-to-roll chip fabrication
The processing of the inverted ridge waveguides for the attenuation measurements and the sensor chips was done in two phases: in the first phase the waveguide grooves were fabricated, and in the second phase the grooves were filled. The length of the processed roll was about 200 m and the width 15 cm, while sensor chips about 3.5 cm long were used in the YI sensing experiments.
The R2R process to fabricate inverted ridge waveguides is depicted in Fig. 5(a) and 5(c). The continuous UV-nanoimprinting method was used to pattern waveguide grooves into the acrylate based under-cladding material (Nalax2). As illustrated in Fig. 5(a), the processing of the under-cladding layer was initiated by unwinding the blank polycarbonate (PC) web and removing a protective foil. During the processing, the web was run at the constant speed of 7 m/min. The gravure coating method was utilized to deposit the undercladding material [57]. The groove pattern was transferred by pressing a nickel stamp (temperature 30 °C) onto the uncured undercladding resin, and applying UV-light through the transparent PC web resulting in grooved surface structures whose shape and dimensions were defined by the stamp. The grooves were manufactured with strongly tilted sidewalls to facilitate the release of the stamp from the UV-cured waveguide grooves. The thickness of the replicated under-cladding material was about 6 µm being rather thick compared to the waveguide groove thickness of about 0.7 µm. Therefore, it is expected that modal field is well isolated from the PC web as illustrated in Fig. 6(a). An additional foil was applied on the structured surface before rewinding the roll to protect the waveguide grooves against mechanical defects. The top view image of the waveguide groove with a Y-junction structure is shown in Fig. 5(b).
Fig. 5 a) R2R unit for waveguide groove fabrication, b) waveguide groove with a Y-junction, c) R2R unit for waveguide groove filling, and d) cross-sectional scanning electron microscope image of an inverted ridge waveguide.
Fig. 6 (a) Theoretical TM-mode field profiles of the waveguides with and without the overcladding, and (b) imaged intensity distributions for TM-modes with and without the overcladding layer.
The rolls with the waveguide grooves were then processed in another R2R unit to fill the grooves with an epoxy-based waveguide core material (Epocore). During the processing, the roll was running at a constant speed of 1 m/min. As illustrated in Fig. 5(c), the protective foil was removed after the unwinding of the roll. As in the coating of the undercladding layer, the gravure printing method was utilized to deposit the core material that was diluted in propylene glycol monomethyl ether acetate (PGMEA) with the weight ratio of 1:3.5. The printing was done at room temperature. The solvent was evaporated in three sequential ovens (length of each 1 m, temperature 80 °C) before UV-curing of the epoxy resin. It is worth noting that no replication stamp was applied in this step, because no surface pattern was required. Finally, the roll with the inverted ridge waveguide structures was protected with foil, and the processed roll was rewound. A cross-sectional image of the inverted ridge waveguide is shown in Fig. 5(d). Strongly tilted sidewalls are due to a reflow step applied during the stamp preparation in order to ease the release of the stamp from the UV-cured under-cladding layer.
The thickness variation of the waveguide groove processed in the under-cladding material was determined from AFM scans to be less than 40 nm. The groove filling with the core material using the gravure printing method resulted in bending of the surface as observable in Fig. 5(d). According to the AFM measurements, the variation in the bending was below 20 nm. This observation is consistent with the previously reported values for gravure printed layers showing thickness variations of few tens of nanometers [58]. In addition, the thickness variations were smooth when lossless adiabatic changes to the propagating modes are expected.
The waveguide chips were cut from the roll by first pre-cutting the web with scissors leaving the waveguide facet areas intact. The waveguide facets were formed by cooling the plastic samples with liquid nitrogen and cleaving the intact areas. As it can be seen in Fig. 5(d), the facet of the cleaved core material is smooth while the surface of the undercladding material is granular. As the waveguide facet quality affects the results of attenuation measurements, a large number of samples (640) were characterized to obtain reliable data. The potential variation in the facet quality does not affect significantly the operation of interferometric sensor used in this work, since the operation is based on the phase difference occurring during the light propagation in the waveguides. In the sensor measurements, only the visibility of the interference pattern is decreased if the facet quality varies between the reference and measurement waveguides.
5. Experiments and discussion
5.1 Simulations and experiments of single-modality
The single-modality of the fabricated waveguides was first assessed by fully vectorial finite-element method (FEM) simulations and experiments. Theoretical mode field profiles of the waveguides were calculated with commercial waveguide simulation software (Fimmwave, Photon Design). The waveguide geometry for the simulation was delivered from the scanning electron microscope image of the cross-section of the fabricated waveguide shown in Fig. 5(d). In the simulations, RI values shown in Table 1 for the waveguide undercladding and core material were used.
According to simulations, the waveguides without the overcladding layer can support two TM-modes at the wavelength of 975 nm (i.e. at the polarization state and the wavelength used in sensor experiments). On the other hand, simulations suggested that by utilizing an overcladding with a RI of 1.541, only one propagating TM-mode is supported. The theoretical TM-mode field profiles are shown in Fig. 6(a). The simulations showed that besides the difference in the number of propagating modes, there is also a notable difference in the intensity location: while most of the intensity remains inside the waveguide core layer, when no overcladding is applied, modes are strongly pushed towards the overcladding layer in case one is present on the structure.
As illustrated in Fig. 3(a), area with the width of 2 mm was left uncoated with the overcladding layer at the edges of the chips to ensure intact waveguide facets and to facilitate the alignment of the input fiber and the input waveguide in the experiments. Thus every waveguide had two interfaces between areas with and without the overcladding as indicated by the white arrows in Fig. 3(a). It was simulated that a loss of about 2.2 dB occurs at each of these interfaces due to the modal mismatch.
The simulation results were confirmed by experiments: with the waveguides without the overcladding layer, it was possible to observe a double-peaked intensity distribution by moving the input fiber laterally, as shown in Fig. 6(b). However, such excitation of higher-order modes could not be confirmed with the waveguides with the overcladding layer being well in accordance with the simulation results of single-modality.
5.2 Waveguide attenuation measurements
The attenuation measurements were carried out for the waveguide samples with and without the overcladding layer. For attenuation measurements, chips lengths of 10, 18, 26 and 34 mm were cut from the roll at each of the four sampling areas that were about 10 m apart from one another. This resulted in 16 sample chips. Each chip contained 40 parallel straight waveguides, and thus a total of 640 individual waveguides were characterized.
The attenuation value histograms, each based on 160 measurements, and the line fitted to the median attenuation values are shown in Fig. 7 for the waveguides without and with the overcladding. Rather similar attenuation values of 1.7 and 1.5 dB/cm were obtained with and without the overcladding layer, respectively. It is worth noting that the attenuation values were comparably well localized in the vicinity of the median value. As can be seen from the histograms of Fig. 7, 80% of the samples had attenuation values at the maximum, about 1 dB higher than the related median value. The observations from attenuation measurements confirm that the R2R method can be applied to produce integrated optics with repeatable performance. The excess loss from 2.2 dB to 6.3 dB in the presence of an overcladding layer can mostly be explained by the theoretical modal mismatch loss of 2.2 dB at each of the overcladding interfaces near the waveguide input and output facets. The length of the waveguides in Fig. 7(b) is the area with the overcladding layer being 4 mm shorter than the overall length of waveguides.
Fig. 7 Attenuation value histograms, each of which is based on measurements from 160 different waveguides a) without, and b) with an overcladding layer. Median attenuation values at different waveguide lengths are shown as well as line fitted to values. The red dashed lines show the attenuation values below which 80% of the measured values lie.
Obtained attenuation values are higher than the recently reported value of 0.19 dB/cm for polymeric single-mode waveguides produced on rigid silicon wafers with static nanoimprint processing [44]. However, since typical sensor chip lengths are in the centimeter range, the waveguides are suitable for sensors and applications that are not highly sensitive to power budget. For example, the waveguide length of 3.5 cm, corresponding to roughly the length of the sensor waveguides in this work, results in about 5.3 dB propagation loss that is acceptable in many applications.
The losses in optical waveguides can be generally associated with radiation, absorption and scattering [59]. Losses due to radiation are typically negligible in straight waveguides with well confined modes. The absorption is determined by the material properties, while scattering can be due to inhomogeneity of the material or surface roughness. In this work, the main selection criteria for the materials were their refractive indices, R2R processability and compatibility when layering different materials. Refractive index contrast between the core and cladding materials is obviously required to form waveguide structures. In the used R2R process, the requirement for the undercladding material (Nalax2) and the core material (Epocore) was that they could be coated with a good wettability by gravure printing method. While the under-cladding material had to be UV-imprintable, the requirement for the core material was the formation of smooth surface during drying and UV-curing without applying a moulding tool. Furthermore, since solvent (PGMEA) was utilized to tune the viscosity of the core material, the underlying under-cladding with imprinted groove structures had to tolerate the used solvent. Enormous amount of research efforts has been made over the time to develop polymeric materials with low absorption and volume scattering for the optical waveguide applications [60]. Providing that new materials meeting the above mentioned criteria for processing and refractive indices are developed, the attenuation of R2R processed waveguides might be decreased. Furthermore, the optimization of the fabrication process to produce smoother waveguide surfaces might be another approach to decrease the attenuation value. This process optimization can cover both the mould tool processing with lithography and electroplating steps together with the actual R2R processing.
5.3 Ambient refractive index sensing experiments
To demonstrate the applicability of the waveguides for evanescent wave sensing, sensor responses to ambient RI changes were determined with two chips by using aqueous glucose solutions to modify the RI in the measurement window. The chips were first cut from the roll for post-processing. An optical adhesive (OP-4-20632) was used to pattern the overcladding layer leaving 10 mm of the measurement waveguide of YI2 uncoated to form a measurement window for the sample interaction. Illustration of the sensor structure with dimensions is shown in Fig. 8.
Each concentration was measured four times by exposing the sensor surface to the respective glucose solution for five minutes leading to a 500 µl sample volume. This was followed by flushing with water until the end of the experiment. The timing of the glucose solutions and water in the flow cell are indicated in Fig. 9(a). The phase change curves were first calculated from the interferograms and were baseline corrected based on the values between 0 and 1.1 min. Phase change curves of YI2 are shown in Fig. 9(a). It can be seen that ambient RI changes were detected at the level of 10−6 RIU, thus showing comparable or better performance than reported earlier for polymeric YIs [51,52].
Fig. 9 (a) Phase change curves of YI2 of ambient RI experiments measured with various glucose concentrations. Two chips were used in the experiments and every concentration was measured four times with each of the chips. The RI difference of the glucose solution to water, Δn, and the glucose concentration are indicated next to the curves. Timing of the solutions in the flow cell is indicated by the background color. Calculated responses and lines fitted to the data points are based on (b) the phase change step height, and (c) the slope of the rising edge of the phase change curves. Responses of the three lowest concentrations are also shown in the inset figures.
Responses of YI2 were determined by 1) calculating the height of the phase change step by taking the average of the phase change values within 5.0-5.3 min, and 2) by determining the slope of the phase change curves within 2.0-2.8 min. Responses are shown Fig. 9(b) and 9(c) including lines fitted to the data points. It can be seen that, in general, the responses increase with increasing glucose concentration quantitatively. Thus, at the two lowest concentrations the measurement cannot be considered quantitative since the response values overlap significantly. The slope-based method enables faster signal quantification than the method based on the phase change step height representing an advantage for rapid diagnostics. The results of these experiments demonstrate that the YI sensor chips with R2R manufactured waveguides are capable of sensing ambient RI changes.
6. Conclusion
In conclusion, we demonstrated that disposable polymeric PICs can be produced by ultra-high volume, large-area, and low-cost fabrication methods in the roll format for sensor applications in lengths of hundreds of meters. The attenuation properties of hundreds of waveguides were investigated showing that the attenuation values were well localized in the vicinity of the median value indicating that waveguides with repeatable performance were produced. The attenuation of the single-mode waveguides was about 1.7 dB/cm that is high in comparison to requirements of telecommunication applications, but can be well suitable in sensor usage. We also demonstrated that the waveguides were suitable for evanescent wave sensing of ambient RI.
The fabrication of integrated photonic devices using ultra-high volume fabrication, in a similar manner as paper is produced, may inherently open up ways of manufacturing low-cost disposable photonic integrated circuits for a wide range of applications including point-of-care diagnostics, food safety applications and environmental monitoring.
Acknowledgments
The authors gratefully acknowledge financial support by the European Commission via the Seventh Framework Programme under the grant agreement No. 263382 “PHOTOSENS”. M. Hiltunen, J. Hiltunen and P. Karioja gratefully acknowledge financial support by Academy of Finland grant No. 284907. The authors want to acknowledge J. Ollila, VTT, for his assistance in preparing the sensor chips.
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