Khay Ming Tan

In-fiber common-path optical coherence tomography using a conical-tip fiber

Common-path optical coherence tomography (CPOCT) is known to reduce group velocity dispersion and polarization mismatch between the reference and the sample arm as both arms share the same physical path. Existing implementations of CPOCT typically require one to incorporate an additional cover glass within the beam path of the sample arm to provide a reference signal. In this paper, we aim to further reduce this step by directly making use of the back-reflected signal, arising from a conical lens-tip fiber, as a reference signal. The conical lens, which is directly manufactured onto the optical fiber tip via a simple selective-chemical etching process, fulfils two functions acting as both the imaging lens and the self-aligning reference plane. We use a Fourier-domain OCT system to demonstrate the feasibility of this technique upon biological tissue. An in-fiber CPOCT technique may prove potentially useful in endoscopic OCT imaging.

Fiber probe based microfludic raman spectroscopy

We report a novel fiber probe based Raman detection system on a microfluidic platform where a split Raman probe is directly embedded into a polydimethylsiloxane (PDMS) chip. In contrast to previous Raman detection schemes in microfluidics, probe based detection offers reduced background and portability. Compared to conventional backscattering probe designs, the split fiber probe we used in this system, results in a reduced size and offers flexibility to modify the collection geometry to minimize the background generated by the fibers. Also our microfluidic chip design enables us to obtain an alignment free system. As a proof of concept we demonstrate the sensitivity of the device for urea detection at relevant human physiological levels with a low acquisition time. The development of this system on a microfluidic platform means portable, lab on a chip devices for biological analyte detection and environmental sensing using Raman spectroscopy are now within reach.

Toward the Development of Raman Spectroscopy as a Nonperturbative Online Monitoring Tool for Gasoline Adulteration

There is a critical need for a real-time, nonperturbative probe for monitoring the adulteration of automotive gasoline. Running on adulterated fuel leads to a substantive increase in air pollution, because of increased tailpipe emissions of harmful pollutants, as well as a reduction in engine performance. Consequently, both classification of the gasoline type and quantification of the adulteration content are of great significance for quality control. Gasoline adulteration detection is currently carried out in the laboratory with gas chromatography, which is time-consuming and costly. Here, we propose the application of Raman spectroscopic measurements for on-site rapid detection of gasoline adulteration. In this proof-of-principle report, we demonstrate the effectiveness of Raman spectra, in conjunction with multivariate analysis methods, in classifying the base oil types and simultaneously detecting the adulteration content in a wide range of commercial gasoline mixtures, both in their native states and spiked with different adulterants. In particular, we show that Raman spectra acquired with an inexpensive noncooled detector provides adequate specificity to clearly discriminate between the gasoline samples and simultaneously characterize the specific adulterant content with a limit of detection below 5%. Our promising results in this study illustrate, for the first time, the capability and the potential of Raman spectroscopy, together with multivariate analysis, as a low-cost, powerful tool for on-site rapid detection of gasoline adulteration and opens substantive avenues for applications in related fields of quality control in the oil industry.

Diagnosis of virus infection in orchid plants with high-resolution optical coherence tomography.

This work investigates the use of optical coherence tomography (OCT) to identify virus infection in orchid plants. Besides revealing the cross-sectional structure of orchid leaves, highly scattering upper leaf epidermides are detected with OCT for virus-infected plants. This distinct feature is not observable under histological examination of the leaf samples. Furthermore, the leaf epidermides of stressed but healthy plants, which exhibit similar visual symptoms as virus-infected plants, are not highly scattering and are similar to those of healthy plants. The results suggest that virus-infected orchid plants can be accurately identified by imaging the epidermal layers of their leaves with OCT. The OCT modality is suitable for fast, nondestructive diagnosis of orchid virus infection, which may potentially lead to significant cost savings and better control of the spread of viruses in the orchid industry.

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.

Resolving interparticle position and optical forces along the axial direction using optical coherence gating

This research is supported by grants SCS-BU0052 and RP C-015/2007 from Agency for Science, Technology and Research and grant RGM39/06 from the Ministry of Education, Singapore.

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.

Novel fiber Bragg grating sensor for temperature-insensitive displacement measurement

We report the design and development of a novel optical fiber Bragg grating based displacement sensor. A fiber Bragg grating is glued at a slant orientation onto the lateral side of a specially designed cantilever beam. It is found that the bandwidth of the FBG-based sensor changes linearly with the variation of displacement at the free end of the beam due to the displacement-induced strain gradient. Displacement sensing is realized by measuring the reflected optical power of the signal from the grating with a photodetector. A linear response of 37.9 mV/mm was obtained within a displacement range of 9.0 mm. This sensor is also cost effective due to the use of a simple demodulation method and is inherently temperature-insensitive; eliminating the need for temperature compensation.