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Liquid-core fibers improve sensor sensitivity

18 Oct 2007

New research is improving the performance of sensors based on fluid-core optical fibers, while also making them easier to fabricate. Tim Hayes reports.

Microstructured optical fibers have long held promise as sensitive and robust sensors for real-world applications such as bacteria detection and remote sensing, since their structure allows significant interaction between the light guided along the fiber and any chemical species filling the holes.

As a result they offer an attractive alternative to planar waveguides — where the sensing distance is usually limited to a few centimeters of free-space — and to standard fibers, where the guided mode is normally insensitive to the external environment.

"Photonic crystal fibers (PCFs) can offer the best of both worlds," Cristiano Cordeiro of the University of Campinas, Brazil, explained to optics.org. "They can be very sensitive sensors, but also robust and very long. And their small hole diameters make it possible to fill a meter of fiber with only a few tens of nanoliters of fluid."

Practical difficulties remain though. The analyte must be able to enter the fiber in a way that does not damage the fiber's integrity, and the laser light must be as close to single-mode as possible to achieve the required sensitivity.

Single-mode fiber

Cordeiro's Brazilian team, which also included researchers from Universidade Presbiteriana Mackenzie, has therefore developed a new method to limit the number of guided modes inside the fiber. "Any interferometric measurement that is phase-sensitive will be difficult if several modes are present, since in a multimode fiber each mode travels a different path," noted Cordeiro. (Measurement Science and Technology, 18, 3075.)

The team simultaneously and selectively fills the core of a PCF with one liquid, and the cladding microstructure with a different liquid. Careful selection of the fluids allows the refractive index difference between core and cladding to be controlled, and so limits the number of guided modes inside the fiber. "To the best of our knowledge this selective filling of a PCF to limit the guided modes has not been demonstrated before," Cordeiro said.

The filling technique involves first closing the cladding holes at one end of the fiber with an electric arc using a fusion splicer. Reduced air pressure then draws a UV-curable polymer into the core of the fiber at the opposite end, where it is cured into a plug. The result is a fiber with the core blocked at one end and the cladding pores blocked at the other.

Syringes are then used to manually introduce the chosen fluids into the core and the cladding from the appropriate ends. Trimming leaves a length of PCF with liquid-filled cladding and a liquid-filled core.

Cordeiro's team compared the behavior of PCFs having water-filled and air-filled cladding in an interferometer, using a 633 nm He-Ne source with the reference arm in free space. The core in both fibers was filled with a water-glycerin mixture. Tilting the reference beam produced interference fringes that indicated the extent of phase variance in the fiber. In the water-filled fiber the phase remained constant within the whole core area, suggesting the guidance of just the fundamental laser mode, while the air-filled fiber showed numerous fringe splittings.

Making PCF-based chemical or biological sensing practical for use by non-specialists, for example in a hospital environment, is the team's main challenge. "So far the process of inserting the two liquids is manual via syringes," explained Cordeiro. "Further development is needed to make it more practical for mass production and daily use. A second problem is evaporation of the liquids, which can empty the PCF in minutes. We're working on these issues."

Once the manufacture is optimized the team is looking at several future applications for fluid-filled PCFs, including all-fiber acetylene cells to be used as simple and cheap reference cells, bacteria detection, and characterization of liquid evaporation in PCFs.

Open access

Another key issue that researchers have been working on is making holes in the PCF through which the analyte can be introduced. "Lateral holes can be a major improvement in making PCFs into practical fluid sensors," Cordeiro said. "So far there is no standard way of creating these access points, and each process has its own advantages and disadvantages."

The Brazilian team has used a focused ion beam (FIB) to locally mill holes through a fiber's external silica jacket, without compromising the fiber guidance or robustness. "We were the first group to propose an FIB for this purpose, which gives us full control over the size, shape and position of all lateral ports," Cordeiro said.

The team milled several 20 x 5 µm rectangles in the side of a PCF by using an FIB operating at 30 kV and 20 nA to open up access to the fiber interior. By finely controlling the milling time and FIB current it is possible to open up lateral access without disturbing the fiber core. Alternatively, by applying the technique to a fiber in which the cladding holes have been locally blocked using a fusion splicer, access to just the core of a hollow-core PCF can be achieved, allowing selective filling if desired.

Cordeiro is also part of a project involving researchers from the University of Sydney developing fibers in which one of the cladding holes is open to the environment over long lengths of fiber, exposing the core in what is claimed to be a unique optical fiber design.

The slotted structure presents advantages for sensing applications, since the time taken for chemicals to diffuse into the evanescent sensing region is minimal and the length of the sensing region is no longer limited. (Optics Express, 15, 11843.)

But the novel design could have other applications too. "The key point is that we can bring anything into contact with the fiber core, not just liquids for chemical sensing," explained Felicity Cox of the University of Sydney. "We can evaporate metal coatings directly onto the core for plasmonic effects, since there is line-of-sight access to the core. We also want to try writing gratings without interference from the microstructure."

Fabricating the fibers required little deviation from standard fiber manufacture, involving only the drilling of holes into the side of the intermediate fiber perform before drawing to cane. The holes, carefully made so as to intersect with a single air hole and not impact the core, form slots along the fiber's length after drawing, with dimensions controlled by the size of the original hole. According to the team, the slot induced no additional losses to the fiber when in use.

The team is working towards using the sensors in practical environments where the fiber's robustness will be essential. "The exposed core provides an excellent sensing platform, although the sensitivity may be reduced by external contaminants being able to reach the core," said Cox. "But this is less of a problem in our slotted fiber than in existing evanescent wave sensors, where large areas of the core have to be deliberately exposed so as to make contact with the sample." In addition, the slotted fiber is made from plastic via a very simple process, so any sensors based on them could be disposable items, an additional practical benefit.

Further work is now planned to investigate whether slotted fibers with hollow cores can be made, which could then be used in remote gas sensing. Biosensing applications could also benefit, as many of them involve substrate-bound sensing.

"The slotted fiber is truly a sensor, rather than a probe, since it can deliver real-time information," commented Cox. "It could also be a quasi-distributed sensor, because the analyte could enter the fiber anywhere along its length. But the most exciting thing is that we have a robust fiber with an accessible core, and chemical sensing is just one of many applications possible."

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