Chia Meng Tay

Compact fluorescence detection using in-fiber microchannels—its potential for lab-on-a-chip applications

This paper reports a compact and practical fluorescence sensor using an in-fiber microchannel. A blue LED, a multimode PMMA or silica fiber and a mini-PMT were used as an excitation source, a light guide and a fluorescence detector, respectively. Microfluidic channels of 100 microm width and 210 microm depth were fabricated in the optical fibers using a direct-write CO(2) laser system. The experimental results show that the sensor has high sensitivity, able to detect 0.005 microg L(-1) of fluorescein in the PBS solution, and the results are reproducible. The results also show that the silica fiber sensor has better sensitivity than that of the PMMA fiber sensor. This could be due to the fouling effect of the frosty layer formed at the microchannel made within the PMMA fiber. It is believed that this fiber sensor has the potential to be integrated into microfluidic chips for lab-on-a-chip applications.

Inverse-Gaussian apodized fiber Bragg grating for dual-wavelength lasing

A fiber Bragg grating (FBG) with an inverse-Gaussian apodization function is proposed and fabricated. It is shown that such a FBG possesses dual-wavelength narrow transmission peaks and the wavelength spacing between the two peaks is easily controllable during fabrication. Incorporating such a FBG filter into a fiber laser with a linear cavity, we obtain stable dual-wavelength emission with 0.146 nm wavelength spacing. This arrangement provides a simple and low cost way of achieving dual-wavelength fiber laser operation.

High relative humidity sensing using gelatin-coated long period grating

A long-period grating (LPG) coated with gelatin was developed as a high relative humidity (RH) sensor. The resonance dip or coupling intensity of the LPG spectrum varies with humidity while the resonance wavelength remains constant. The principle of operation of the sensor is based on the effect of an external medium, with higher refractive index than that of silica or cladding, on the LPG spectrum. Experimental investigations on the sensor yield a sensitivity of 1.2dB/%RH with an accuracy of ±0.25%RH, and a resolution of ±0.00833%RH. The LPG RH sensor also offers repeatability, hysteresis and stability errors of less than ±0.877%RH, ±0.203%RH and ±0.04%RH respectively. In addition to the characterization of the LPG RH sensor, further studies were conducted to determine the effect of grating periodicities on the sensitivity. Results show that higher-order cladding modes from smaller grating periods enable the sensor to achieve higher sensitivity to humidity. This method is proposed to be more cost effective as compared to more complex spectroscopic methods based on wavelength detection. This sensor can also help to solve problems in measuring high humidity with existing relative humidity measurement systems.

High-resolution tunable fiber Bragg grating filter

High resolution tunable optical filters are important in dense wavelength division multiplexing (DWDM) applications as channel spacing in optical communications systems can be as low as 0.4nm. The bandwidth of the filter must be narrow, to prevent filtering neighbouring channels. In this paper, a simple, low cost technique for the tuning of the Bragg wavelength of the FBG filter with high resolution and good repeatability is demonstrated. A FBG was embedded in a triangular carbon fiber composite package and aluminium plates were used to clamp the wider end of the package, leaving the thinner end free, like a cantilever beam. A micrometer was placed under the thinner end of the package and the vertical displacement of the micrometer will bend the carbon composite. This bending will produce compression and tension forces on the FBG depending on which side of the package is used, which will result in a shift of the Bragg wavelength. The total tuning range of the FBG filter is 2nm with a resolution of 1pm. The repeatability error was found to be 0.4% over the whole tuning range. The 3dB bandwidth of the reflected spectra from the FBG is 0.235nm, much less than channel spacing of 0.4nm.

Simultaneous measurement of curvature and temperature for LPG bending sensor

For an embedded LPG bending sensor, in which the its resonance coupling strength changes with bending curvature, cross-talk issues between temperature and bending curvature arises if it is to be deployed in non-controlled environments. A 2 x 2 matrix method was thus employed for simultaneous measurement of bending curvature and temperature for the embedded LPG bending sensor. The matrix is made up of bending and temperature coefficients from 2 different fiber-types LPG; one is H2-loaded and the other is Bo/Ge co-doped. To find out the percentage error, a random test has to be carried out and the matrix was deployed for calculation. From the test results, the percentage error achieved for curvature measurement yields less than 6%. For temperature measurement, the percentage error fluctuates between 1.56% and 5.4%. The use of simultaneous measurement of both bending curvature and temperature enables researchers and engineers to measure bending of structures more accurately.

Bending measurement using embedded long-period fiber gratings

The use of LPG embedded in carbon-fiber composite laminates (ELPG), in a 4-3 configuration, for bending measurement has been demonstrated. With increased bending curvatures on the 4 layers side, the coupling strength of the cladding mode decreases while the resonance wavelength remains relatively constant. A reduction in coupling strength leads to a reduction of the resonance amplitude depth. From the bending test covering the range of curvatures from 0m-1 to 2m-1, the ELPG yields a sensitivity of 5.065dB/m-1 and a repeatability of 98.1%. In another investigation, the ELPG ability to determine direction of bend has also been demonstrated by applying bending at the 3 layers side of the laminate. Despite having a short curvature range between 0m-1 and ~0.626m-1, the test demonstrates an increase of the cladding mode coupling strength with an increase in bending curvature, thus showing the ELPG ability to differentiate bending directions. By exploiting the unique characteristics of ELPG, two ELPGs can be exploited for 2-axis measurement of structures. Hence the overall cost and complexity of the bending sensor system can be greatly reduced.

Compact fluorescence detection using in-fiber microchannels

Applications involving fluorescence detection in point-of-care systems are both interesting and challenging in nature. The applications usually require a simple, compact, robust, highly sensitive yet affordable system. As a result, the system needs to be efficient in fluorescence detection by using practical and easily fabricated, hence inexpensive sensors. In this paper, a fluorescence sensor using an in-fiber microchannel has been developed and tested successfully. A blue LED, multimode PMMA or silica fiber, mini-PMT and fluorescein in PBS pH 7.4 buffer solution were used as the excitation source, light guide, fluorescence detector and sample, respectively. Microfluidic channels of 100μm width and 1cm length were fabricated in the optical fibers using a direct write CO2 laser system. The channels in the fibers were examined using a SEM and an optical microscope. Experimental results show that the sensor is highly sensitive, being able to detect 0.1 μg/L of fluorescein in the PBS buffer solution, with good signal to noise ratio and the results are reproducible. The data obtained using silica fibers as sensors when compared with the results from PMMA fibers show that the silica fiber sensor has better sensitivity than the PMMA fiber sensor. This could be due to the fouling effect created by the frosty layer at the bottom of the microchannel made within the PMMA fiber. Our future work will integrate the fiber sensor into microfluidic chips for lab-on-a-chip applications.