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Humidity sensors using nanofilms deposited on hollow core fibers

Coating the surface of a fiber optic with polymeric nanofilms proves a means to highly sensitive, rapid-response humidity sensors.
16 August 2006, SPIE Newsroom. DOI: 10.1117/2.1200608.0347

Hollow core fibers, or HCFs, transmit light through a central air hole. Recent developments make HCF technology suitable for detecting moisture, and we are using it to develop relative humidity sensors. HCFs are especially attractive because one can deposit on them nanofilms that make it possible to modulate the so-called evanescent—that is, highly localized—electromagnetic field as a function of the external medium. In our work, we coat the fibers with a humidity-sensitive nanofilm whose thickness is the same order of magnitude as that of the evanescent field penetration length (typically in the 100nm range).

Several appropriate techniques are available for depositing thin-layer films, such as spin coating, dip coating, physical and thermal evaporation, and electrostatic self-assembly (ESA). ESA is based on attraction between oppositely charged species and has the advantage of providing both self-assembly and molecular control of the structure. Alternating layers of ionic and cationic aqueous solutions are deposited on the substrate, in our case, the HCF external surface (see Figure 1). In contrast to other deposition techniques, substrates of any size and shape can be coated. In addition, the resulting nanomaterial is optically homogeneous, with low scattering loss and long-term stability.1 Another advantage is that ESA does not require sophisticated materials. Moreover, it can be carried out under standard temperature and pressure conditions, and is also easily automated. The main advantage of ESA, however, is that it allows nanometer precision control of film thickness and deposition of azimuthally symmetric coatings on the HCF cylindrical surface.

Figure 1. Shown is a hollow core fiber nanostructure for relative humidity sensing, including (a) a photograph of the structure, (b) a functional scheme, (c) projections of the input light, (d) an intermediate HCF section, and (e) the sensor output light.

HCFs may consist of a central air hole, a doped silica ring core, and a silica cladding, or a central air hole and a silica cladding of different diameters.2 For the sake of simplicity, here we discuss only one type of tubular waveguide consisting of a central air hole and a silica cladding with inner and outer diameters of 50 and 150μm, respectively.3 This design is intended to spread the light into the cladding to achieve a higher evanescent field ratio with respect to the total optical power transmitted.

Figure 1(a) shows a photograph of our HCF-based structure, which is portrayed schematically in Figure 1(b). The structure consists of a short HCF segment (10–20mm), spliced between two standard (50/125μm) multimode fibers (MMFs). The HCF jacket must be removed prior to assembly. When the HCF and MMF are spliced together using appropriate electric arc conditions, the HCF collapses, forming a tapered solid fiber at the interface between both fibers. In these devices, the tapered region allows the input light that is guided in the core of the lead-in MMF—see Figure 1(c)—to be coupled to the HCF cladding instead of being confined in the air core, as illustrated in Figure 1(d). When the light reaches the lead-out MMF, it is again coupled to the silica core, as shown in Figure 1(e).

Once the fiber structure has been fabricated, it is coated using the ESA procedure. For our experiments we selected hydrophilic Poly R-478 as the anionic electrolyte and hydrophobic poly(diallyldimethyl ammonium chloride, PDDA) as the cationic solution. The optical fiber structure was first cleaned and treated to create a charged surface. Then it was alternately dipped into the cationic and anionic polymer solutions to create a multilayer thin film. The final multilayer acted as a homogeneous optical medium. Furthermore, the deposition of alternating hydrophobic and hydrophilic materials trapped the water molecules at the surface and prevented them from being absorbed inside the coating.

To evaluate the response of these devices, we exposed the sensor head to rapid changes in relative humidity. Human breath contains more water vapor than normal air. Accordingly, the sensor was placed 3cm from a subject's mouth. The results we obtained are shown in Figure 2. The observed rise response time was around 300ms. The fall time was less than a couple of seconds.

Figure 2. This graph illustrates the experimental response to human breathing using a 20mm-long hollow core fiber sensor.

We have used HCFs to show that ambient humidity can vary the output optical power in a humidity-sensing device. The materials deposited on the sensors through ESA, namely, Poly R-478 and PDDA, showed rapid response with good sensitivity and reproducibility. Our work demonstrates that nanostructures of this kind, based on modulating evanescent fields, are appropriate for moisture-sensing applications. They may also find use in other new sorts of chemical and biosensing technologies.

Ignacio R. Matias, Jesús M. Corres, Francisco J. Arregui, Javier Bravo
Fiber Optic Nanosensors Laboratory, Public University of Navarra
Pamplona, Navarra, Spain 
Ignacio R. Matias is a professor at the Public University of Navarra, Spain. He received his MS (1992) and PhD (1996) in electrical and electronic engineering from the Polytechnic University of Madrid. He has coauthored more than 200 book chapters, journal and conference papers related to optical fiber sensors and optical devices. He currently serves as associate editor of IEEE Sensors Journal.
Jesús M. Corres received his MS (1996) and PhD (2003) in electrical engineering from the Public University of Navarra. He has been involved in different projects with industry. His main research interests include optical fiber sensors and nanostructured materials.
Francisco J. Arregui received his MS in electrical engineering (1994) from the Catholic University of Navarra in San Sebastian, Spain, and his PhD (2000) from the Public University of Navarra, where he is currently a professor. His main research interests include optical fiber sensors, sensor materials, and nanostructured materials. He is an associate editor of IEEE Sensors Journal, and a member of SPIE.
Javier Bravo received his MS in electrical engineering (2003) from the Public University of Navarra. He is currently working toward his PhD in the communications program of the Electrical and Electronic Engineering Department, also at Navarra. His research interests are mainly in fiber-optic sensors.