Thermally switchable and discretely tunable comb filter with a linearly chirped fiber Bragg grating.

We propose and experimentally demonstrate a novel switchable and discretely tunable comb filter based on a thermally induced linearly chirped fiber Bragg grating. Experimentally we achieved a thermally induced optical bandpass filter that has eight switchable passband peaks with peak separations of 1.6 and 3.2 nm, a very narrow bandwidth (as small as 10 pm) of each peak, a tunable range of 16.5 nm, and a greater than 25 dB rejection ratio. Two spectral peaks separated by only 0.8 nm were also obtained with this comb filter. This filter provides the unique advantages of an all-fiber structure, switchable spectral peaks, independent tuning of the center wavelength and wavelength spacing of the spectral peaks, low polarization sensitivity, simple structure, ease of fabrication, and low cost.

Thermally tunable narrow-bandpass filter based on a linearly chirped fiber Bragg grating.

We propose a new method for the development of a tunable optical bandpass filter (TOBF) based on a linearly chirped fiber Bragg grating (LCFBG). A NiCr wire heater is used to heat the LCFBG at a small point to introduce a narrow passband within the stop band of the LCFBG. The central wavelength of the passband is tuned by scanning the wire heater along the LCFBG. As an example demonstrating the effectiveness of the proposed method, we demonstrate a TOBF with a very small 3-dB bandwidth of approximately 7 pm, a tuning range of 16.4 nm, and a rejection ratio of more than 25 dB. Compared with previously reported tunable-fiber-based bandpass filters, this method provides the advantages of a large tuning range, continuous tunability, a switchable passband, a simple tuning mechanism, low cost, and narrow bandwidth.

Recording compliance of dental splint use in obstructive sleep apnoea patients by force and temperature modelling.

Fibre-optic sensors are used to monitor the force and temperature of dental splints worn by patients suffering from sleep apnoea. Owing to the small size of the sensors, they can be easily embedded within the splint in a way that does not affect the effectiveness of the splint, and, at the same time, are able to indicate whether the splint has been properly worn by the patient. The overall dimensions of the sensor are approximately 0.375 mm thickness, 1 cm length and 3 mm width. The force and temperature sensors are calibrated and found to have sensitivities of better than 0.5 N and 0.1 degrees C, respectively. Trials performed on patients show that the measurement of pressure and temperature is an effective way of monitoring the proper usage of the dental splint by the patients.

Continuous cardiac output monitoring system.

A continuous cardiac output monitoring system has been developed in the laboratory to allow the real-time measurement of the cardiac output. This form of continuous cardiac output measurement allows the doctor to view the beat-to-beat cardiac output and can be employed to measure artery constrictions as well. The sensor comprises a laser Doppler velocimeter and an impedance measurement unit. The laser Doppler velocimeter is capable of measuring bi-directional blood flow within the vessel while the impedance measurement unit determines the cross-sectional area of the vessel. In laboratory tests, it was demonstrated on a heart-lung machine that the product of the two parameters measured is proportional to the actual flow volume of up to 6 lmin(-1) with a mean percentage error of 12.4% and a mean square error of 0.09 (using the lmin(-1) scale) were obtained. This is significantly more accurate than the measurement made using the thermodilution cathether.

In vivo Doppler shift measurements using multimode fiber-optic catheters.

A new fiber-optic catheter for in vivo blood-flow measurements has been developed. The catheter is designed to measure blood flow in both the forward (toward the catheter tip) and reverse (away from the catheter tip) flow directions. It consists of two multimode optical fibers with core diameter of 50 microns and cladding diameter of 125 microns. One fiber transmits the laser beam into blood and the other receives the backscattered light from the erythrocytes within the probe volume. In the flow experiment, it was found that the flow within the boundary layer is indeed laminar and, hence, the relationship between the Doppler shift frequencies and the flow velocities is linear, thereby making the linear calibration possible for predicting the free stream flow velocity. Plots of the maximum shift frequency (frequency at which the Doppler spectrum disappeared into the noise spectrum) against the flow velocities are found to be more linear in both the forward and reverse flow directions than that of the dominant shift frequency (frequency with the highest amplitude). These results were reaffirmed by the numerical flow simulation along the catheter side wall.

Investigation into the effects of haematocrit and temperature on the resistivity of mammalian blood using a four-electrode probe.

Haematocrit and temperature effects on resistivity are investigated using the electrical impedance method. Measurements are made extensively for pig's blood. The experimental set-up basically involves four ring electrodes being placed around a wooden probe that is subsequently immersed into a syringe containing pig's blood. The syringe is then submerged in water maintained at a constant temperature while measurements are taken. The resistivity of blood is found to increase linearly by approximately 2.9% as the haematocrit level increases from 18% to 49% at a fixed temperature of 37 degrees C. Furthermore, the resistivity is found to decrease linearly by approximately 22% with temperature increasing from 33 degrees C to 42 degrees C for all practical levels of haematocrit.