Interferometry

Young interferometer

Conventional biochemical analyses are carried out in chemical laboratories with relatively large and expensive equipment; they are also time-consuming. Developing miniaturized biosensors, which are low cost and reliable to use is thus coming the motivation of our research. Since the technique of interferometry is very sensitive and such sensors are well suited to compact sizes, a Young interferometer structure was chosen to be used.

The integrated Young interferometer sensor is based on confining light in a waveguide structure. This waveguide structure guides the light inside the sensor and divides the light to go through a sample area and a reference area. Although light is confined inside the waveguide layer, a small part of it, called an evanescent wave, penetrates through the physical boundary of the waveguide into the sample/reference area. This interaction between the light and the sample causes a phase change in the propagating light, because the refractive index of the sample area differs from the refractive index of the reference area. When the light leaves the sensor, it is superimposed on an observation screen or a detector, where it creates an interference pattern. The shift of this interference pattern is directly proportional to the change in the phase of the light, and thus we can observe changes happening in the sample area. For example, an antibody-antigen binding event on the sample area can be detected from the interferogram shift.

In our lab we use UV-imprinting techniques to fabricate polymer-waveguide-based sensor chips with integrated Young interferometer. Micro-engineering and machining of polymer materials does not require complex processing techniques, thus greatly reducing the fabrication costs and satisfying the disposable needs of use.

A SEM cross-section image of an UV-imprinted inverted rib single-mode waveguide.



An integrated Young’s interferometer sensor based on inverted rib waveguides.


Schematics of the self-mixing interferometer.

Self-mixing interferometer

Self-mixing interferometry is a promising technique for a variety of measurement applications. Using a laser diode with an external cavity as interferometer, the technique offers several advantages over traditional interferometric configurations. The interferometer using this technique and built in our laboratory is based on a blue-light emitting GaN laser diode with a wavelength of 405 nm. Light is directed through an optical fiber from which a 1-cm section of cladding has been removed, and a cuvette for holding the sample is fixed around this part. Interference patterns, created in the laser cavity, are acquired with a computer-based data acquisition system and later processed using Matlab software. Since samples with different refractive indices create interference patterns with different phases, even small changes in sample concentrations can be measured.

Fiber-optic self-mixing biosensor capable for detection of certain molecules from a sample solution by using antibodies.

Sensors built by using this special technique are potential options in the future to perform non-invasive body fluid concentration measurements such as glucose. This kind of non-invasive method will hopefully someday be used in wireless healthcare systems to offer painless ways to replace daily or weekly blood samples for people having e.g. diabetes.

 

References

M. Wang, J. Hiltunen, C. Liedert, S. Pearce, M. Charlton, L. Hakalahti, P. Karioja, R. Myllylä “Highly sensitive biosensor based on UV-imprinted layered polymeric-inorganic composite waveguides,” Optics Express 20(18), 20309-20317 (2012).

M. Wang, S. Uusitalo, C. Liedert, J. Hiltunen, L. Hakalahti, R. Myllyllä, “Polymeric dual-slab waveguide interferometer for biochemical sensing applications,” Applied optics 51(12), 1886-1893 (2012).

M. Wang, J. Hiltunen, S. Uusitalo, J. Puustinen, J. Lappalainen, P. Karioja, R. Myllylä, "Fabrication of optical inverted-rib waveguides using UV-imprinting", Microelectron. Eng. 88(2), 175-178 (2011). [PDF]

M. Wang, S. Uusitalo, M. Määttälä, R. Myllylä, M. Känsäkoski, "Integrated dual-slab waveguide interferometer for glucose concentration detection in the physiological range", Proc. SPIE 7003, 70031N (2008). [PDF]

M. Määttälä, J. Lauri, M. Kinnunen, J. Hast, R Myllylä, “Fiber-optic biosensor based on self-mixing interferometry”, Proc. SPIE 7142, 71420I (2008). [PDF]

M. Määttälä, J. Hast, R. Myllylä, "Refractive index detection with self-mixing interferometry for biosensing applications", Proc. SPIE 6445, 64450V (2007). [PDF]

Last updated: 9.9.2016