Biomedical subjects

Kin Seng Chiang

Publications and source records attributed to Kin Seng Chiang.

7 recordsLinked to original sources

PubMed2006

Condition for the realization of a temperature-insensitive long-period waveguide grating.

We analyze the condition for achieving a temperature-insensitive resonance wavelength of a long-period grating formed in a channel waveguide. We find that by controlling the waveguide cladding thickness, zero temperature sensitivity can be achieved with core and cladding materials that have significantly different thermo-optic coefficients. To verify our finding, we design a polymer long-period waveguide grating (LPWG) according to the zero-sensitivity condition, where the thermo-optic coefficient of the core is twice that of the cladding. The temperature sensitivity of the fabricated grating is within +/-0.15 nm/ degrees C over a temperature range of approximately 15 degrees C, which is more than an order of magnitude lower than those of previously reported LPWGs fabricated with the same materials.

Journal Article↗

PubMed2006

Widely tunable optical bandpass filter by use of polymer long-period waveguide gratings.

We report the design and fabrication of a widely tunable optical bandpass filter based on using two identical long-period gratings formed along a polymer channel waveguide with a section of the waveguide core removed. The peak transmission of the passband of the fabricated filter is approximately 60% and the bandwidth of the passband is approximately 6 nm. The filter is thermally tunable over the C + L band (1520-1610 nm) for both TE and TM polarizations with a temperature control of approximately 30 degrees C.

Journal Article↗

PubMed2005

Microbend-induced mode coupling in a graded-index multimode fiber.

We present a theoretical analysis of the mode coupling effects at a microbend along a graded-index (GI) multimode fiber (MMF). By matching the incident and excited mode fields at the microbend, we obtain the coupling coefficients among the guided modes at different microbending conditions. The theoretical results compare well with the experimental results from near-field measurements on a GI MMF subject to microbending. The usefulness of our theory is demonstrated with two applications: (i) an explanation of the operation principle of a wavelength-switchable fiber laser that contains a fiber Bragg grating in a GI MMF and (ii) quantification of the function of a microbend-based mode scrambler.

Journal Article↗

PubMed2005

Fiber-Bragg-grating force sensor based on a wavelength-switching actively mode-locked erbium-doped fiber laser.

A fiber-Bragg-grating (FBG) transverse-force sensor based on a wavelength-switching actively mode-locked erbium-doped fiber laser is proposed, in which a FBG is used as both the sensing element and the wavelength-selection element of the laser. When a force is applied to the FBG, the induced birefringence in the FBG causes the laser to emit pulses at two close wavelengths, whose separation is proportional to the applied force. To suppress the interference between the two wavelengths, the laser is made to emit at the two wavelengths alternately by use of a polarization-switching technique. The wavelength separation is converted into a time difference by transmission of the laser pulses through a dispersive single-mode fiber, so the wavelength measurement is replaced by the less-expensive time measurement. The output of the sensor is insensitive to temperature and axial strain changes along the FBG. To interrogate similar FBG sensing elements connected in series it is necessary only to change the modulating frequency of an electro-optic modulator to select the corresponding laser cavity. The practicability of this approach was demonstrated experimentally with two multiplexed sensing elements.

Journal Article↗

PubMed2005

Generation of picosecond pulses at five close wavelengths by use of a self-seeded Fabry-Perot laser diode and a spectrum-split fiber Bragg grating.

We have demonstrated simultaneous generation of stable picosecond laser pulses at five close wavelengths at a pulse repetition rate of approximately 2 GHz by using a self-seeding configuration that consisted of a gain-switched Fabry-Perot laser diode (FPLD) with an external cavity formed by a tunable spectrum-split fiber Bragg grating (FBG). The FBG selected only one of the modes of the FPLD and, at the same time, filtered the selected FPLD mode such that the reflection from the FBG, tapped with a directional coupler, provided an output of dual-wavelength pulses and the transmission through the FBG provided an output of three-wavelength pulses. We could change the intensities and the wavelength separations of the pulses by adjusting the modulating radio frequency and the temperature of the FPLD and the separation of the FBG reflection peaks. In our experiments the highest side-mode suppression ratios of the pulses obtained were 44.5 dB for the dual-wavelength output and 25.0 dB for the three-wavelength output. Also, the laser was demonstrated to minimize pulse intensity imbalance and produce equally separated wavelengths (with a wavelength separation of 0.2 nm). This simple laser offers considerable flexibility for various applications.

Evaluation Studies as Topic↗

PubMed2003

Holey optical fiber with circularly distributed holes analyzed by the radial effective-index method.

We analyze a holey fiber that consists of a circular distribution of air holes by the radial effective-index method. By this method, we show that the holey fiber is a leaky structure and its extended single-mode operation is governed by the differential leakage loss between the first two modes of the fiber. The effects of the hole size and the hole separation on the leakage losses of the first two modes are calculated. The leakage loss of the fundamental mode of the fiber is found to be comparable to that of a conventional holey fiber that has a hexagonal distribution of air holes.

Journal Article↗

PubMed2002

Long-period gratings in planar optical waveguides.

We present a theoretical analysis of light propagation in a four-layer planar waveguide that consists of a long-period grating (LPG) having a period of the order of 100 microm. By means of the coupled-mode theory, we show that such a structure is capable of coupling light from the fundamental guided mode to the cladding modes at specific wavelengths (resonance wavelengths) and thus results in sharp rejection bands in the transmission spectrum of the waveguide. Our numerical results show that the resonance wavelengths as well as the transmission spectrum can be significantly changed with the waveguide and grating parameters. A waveguide-based LPG should provide a useful approach to the design of a wide range of integrated-optic devices, including wavelength-tunable filters, switches, and environmental sensors.

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