Chin B. Su

Measurement of the refractive index of liquids at 1.3 and 1.5 micron using a fibre optic Fresnel ratio meter

A fibre optic technique based on the Fresnel reflection from the fibre tip is used for measurements of the refractive indices of various liquids at wavelengths of 1310 and 1551 nm. Reflection signals from liquid?fibre interface are compared with reflection signals from air?fibre interface to obtain the refractive index. Values of refractive indices for distilled water measured by this technique compare very well with known values at both wavelengths only if the fibre effective waveguide index is used. Applying the double-pulse measurement technique, it is shown that a measurement resolution of about 2.5 ? 10?5 can be achieved.

Electrospinning of silica nanochannels for single molecule detection

We have fabricated silica nanochannels with inner diameter as small as 20nm using a scanned coaxial electrospinning and demonstrated their application for single molecule detection. A coaxial jet, with the use of motor oil as the core and silica sol-gel solution as the shell, is extruded through a coaxial source and deposited on the rotating collector as oriented nanofibers. They are then annealed to cross-link silica and eliminate motor oil, thereby forming nanochannels. Subsequently, a fluorescent dye was injected into the individual nanochannels via a capillary force and single molecule detection was performed by monitoring the photon signals from 5-Iodoacetamidofluorescein.

Fiber optic reflectometer for velocity and fraction ratio measurements in multiphase flows

A technique based on the coherent mixing of scattered signal with Fresnel reflection signal from the tip of an optical fiber is used to demonstrate the feasibility of measuring the velocity and fraction ratio of solid particles and gas bubbles or liquid droplets in a liquid or gas flow. If the liquid or gas flow is seeded with small neutrally buoyant particles, the technique is then capable of measuring the velocity as well as the fraction ratio of all three phases of the flow at a given point. The method is briefly described as follows. An optical signal derived from a diode laser driven by a constant current is launched into a single-mode optical fiber and transmitted, through a fiber coupler, to the signal fiber inserted into the test fluid. The diode laser used is a multilongitudinal mode device that has a low coherence length of about 200 μm. The coherently mixed signal propagates back to the signal fiber, through the fiber coupler, and detected by a detector. By analyzing the signal, the velocity and fraction ratio of each phase can be obtained. Using water seeded with small solid particles and air bubbles, it is demonstrated that the technique is capable of measuring the velocity in the direction parallel to the fiber. Since the only intrusion to the fluid is the tiny fiber probe (a dimension of 125 μm in diameter), the disturbance to most fluid flows is negligible, therefore, the technique is nearly nonintrusive.

Explanation of low‐frequency relative intensity noise in semiconductor lasers

For the first time, the enhanced low‐frequency relative intensity noise characteristics of semiconductor lasers is explained. It is shown, by multimode rate equation analysis, that the enhanced low‐frequency noise is caused by coupling between longitudinal modes which can renormalize the resonance frequency of the individual modes to very low values. It is further shown that a single‐mode laser will also exhibit enhanced low‐frequency noise unless the side‐mode suppression is high.

Nonlinear gain caused by cavity standing wave dielectric grating as an explanation of the relationship between resonance frequency and damping rate of semiconductor diode lasers

Calculation using the recently proposed nonlinear gain caused by the cavity standing wave induced gain and index grating shows that the resonance frequency square is very nearly proportional to the modulation damping factor irrespective of cavity dimensions in semiconductor diode lasers. This is in agreement with experimental observation. Furthermore, the magnitude of the predicted damping factors is compared with measurements of V‐groove lasers and mass‐transport lasers. Excellent agreement is also obtained. This gives an alternative explanation of the origin of nonlinear gain other than spectral hole burning.

A fibre optic Fresnel ratio meter for measurements of solute concentration and refractive index change in fluids

A new and simple normalization technique that greatly enhances the measurement resolution of conventional fibre-optic reflectometry based on Fresnel reflection from the tip of a fibre is used for demonstrating the feasibility of measuring solute concentrations and index changes in fluids to very high precision. The amplitude of pulses originating from reflection from the fibre?fluid interface is compared in real time with the amplitude of reference pulses from a fibre?air interface such that errors caused by pulse amplitude fluctuations and slightly varying detector responses are corrected. Using solutions of sodium chloride and water, it is demonstrated that the technique is capable of measuring index changes of 2 ? 10?5 corresponding to a NaCl concentration of 0.02%.

Theory and experiment of the parasitic‐free frequency response measurement technique using facet‐pumped optical modulation in semiconductor diode lasers

The letter describes the theory and experiment of the newly developed parasitic‐free frequency response measurement technique using facet‐pumped optical modulation on diode lasers. It was found that the modulation damping rate depends on the relative position of the pump laser wavelength with respect to the Fabry–Perot resonance wavelength of the laser under study. This effect is fully explained in terms of the carrier‐induced index change. For multilongitudinal mode lasers, it is shown experimentally and theoretically that the damping rate and frequency response measured by this technique give the intrinsic frequency response corresponding to direct current modulation only if the cavity lengths of pump and probed lasers are equal, and then only if the pump laser wavelength coincides with the Fabry–Perot transmission maximum or minimum of the probed laser.

Simultaneous measurement of spontaneous emission rate, nonlinear gain coefficient, and carrier lifetime in semiconductor lasers using a parasitic‐free optical modulation technique

An optical modulation technique is used to determine three important parameters for 1.3 μm InGaAsP diode lasers: the rate of spontaneous emission into the guided modes, the nonlinear gain coefficient, and the carrier lifetime at threshold. These results are unaffected by electrical parasitics, and are essential to understanding the noise and modulation properties of diode lasers.

Observation of positive and negative nonlinear gain in an optical injection experiment: proof of the cavity standing-wave-induced nonlinear gain theory in 1.3 μm wavelength semiconductor diode lasers

The origin of nonlinear gain in 1.3 μm InGaAsP semiconductor diode lasers is studied by measuring the magnitude and phase of a probe laser’s frequency response to optical injection with orthogonally polarized light from a pump laser. The sign and magnitude of the gain nonlinearity induced by optical injection depend on wavelength separation between pump and probe laser. The maximum magnitude of the nonlinear gain parameter is about 1.5×10−15 cm2. These results are consistent with the recently proposed theory that nonlinear gain is caused by the feedback from the dielectric grating induced by the standing wave in the laser cavity.

Bubble velocity, diameter, and void fraction measurements in a multiphase flow using fiber optic reflectometer.

A fiber optic reflectometer (FOR) technique featuring a single fiber probe is investigated for its feasibility of measuring the bubble velocity, diameter, and void fraction in a multiphase flow. The method is based on the interference of the scattered signal from the bubble surface with the Fresnel reflection signal from the tip of the optical fiber. Void fraction is obtained with a high accuracy if an appropriate correction is applied to compensate the underestimated measurement value. Velocity information is accurately obtained from the reflected signals before the fiber tip touches the bubble surface so that several factors affecting the traditional dual-tip probes such as blinding, crawling, and drifting effects due to the interaction between the probe and bubbles can be prevented. The coherent signals reflected from both the front and rear ends of a bubble can provide velocity information. Deceleration of rising bubbles and particles due to the presence of the fiber probe is observed when they are very close to the fiber tip. With the residence time obtained, the bubble chord length can be determined by analyzing the coherent signal for velocity determination before the deceleration starts. The bubble diameters are directly obtained from analyzing the signals of the bubbles that contain velocity information. The chord lengths of these bubbles measured by FOR represent the bubble diameters when the bubble shape is spherical or represent the minor axes when the bubble shape is ellipsoidal. The velocity and size of bubbles obtained from the FOR measurements are compared with those obtained simultaneously using a high speed camera.