Salik Research Group


Department of Physics and Astronomy at Cal Poly Pomona
3801 W Temple Ave, 8-218
Pomona, CA 91768, U.S.A.

Voice: 909-869-3981
Fax: 909-869-5090


Tapered Fiber Optic Biosensors

Biosensors have great potential to address numerous needs in medical diagnosis, food safety, environmental monitoring, and biodefense. An important class of biosensing technologies is fiber optic biosensing, which has numerous attractive features: biocompatibility, ability for multiplexing, remote sensing capability, immunity to electro-magnetic interference, minimal invasiveness, low cost, small size, light weight, hence portability. Fiber optic biosensors can be used with fluorescent labels or in an unlabeled fashion. Our research focuses on unlabeled tapered fiber optic biosensors, which were demonstrated for detection of bacteria, toxins, proteins, and nucleic acids.

We fabricate our fiber tapers by heating standard optical communication fibers (cost: < 5 ยข per sensor) while they are pulled under tension. In this special tapered fiber geometry, light waves are strongly affected by the medium just outside the tapered fiber region (e. g. within < 1 m) while any change in the bulk medium is not detected at all. Binding events between target analyte and the biorecognition molecules (e. g. antibodies) will modify the transmission through the tapered region with no need of any fluorescent labels. Our biosensor works based on phase difference created between two propagation modes in the tapered region, which makes it a modal interferometer. This project is currently funded by the CSU Agricultural Research Institute to develop biosensing against E. coli O157:H7, which is a public health threat and one of the five big foodborne pathogens that cost ~$6.9 billion/year to our economy. Our collaborators are Dr. Wei-Jen Lin (Biological Sciences) and Dr. Shelton Murinda (Animal and Veterinary Science).

Singlemode-Multimode-Singlemode (SMS) Fiber Sensors

SMS fiber structures can be used for various sensing applications, including strain, temperature, and refractive index (RI) sensing. SMS sensors are very attractive because they deliver similar sensitivity to fiber Bragg grating–based sensors, yet they are very simple and easy to manufacture. A given length of multimode fiber (MMF) is spliced between two single-mode fibers (SMF). Multiple modes excited in the MMF interfere as they couple with the single mode of the lead-out SMF, giving rise to a modal interferometer. In our SMS sensors, we use a Few-Mode Fiber (FMF) instead of MMF, which gives rise to the interesting critical wavelength behavior.

When the FMF in the SMS structure is subjected to tension or temperature variation, the length of the FMF changes, but more importantly, the RI of the cladding and the core material is modified. As a result, the phase difference between the two modes excited in the FMF is modified leading to a change in the transmission spectrum. However, this typical behavior is guaranteed only if difference of propagation constants hows a monotonic variation with wavelength. If, on the other hand, the phase difference shows a quadratic behaviour with wavelength then there exists a critical wavelength at which there is no peak or dip to shift. Rather, transmission at this special wavelength becomes higher and lower in response to strain or temperature variation. In addition, sensitivity to temperature and strain increases for the peaks near the critical wavelength.