TRAPPING AND SENSING OF A SINGLE PARTICLE WITH A DIODE LASER

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This research proposed a technique of measuring micron size particle in liquid with laser trapping and self-mixing interferometer using a laser diode. By laser trapping it is possible to fix and manipulate a single particle in liquid. The idea is to measure the displacement of the trapped particle simultaneously using the backscattered laser light from the particle. The feedback effect of the light to the original laser cavity provides a sensitive mean to get the displacement of the particle in nanometer order.In the research we have also developed a method to measure the spring constant of the particle-laser system by using external modulation. The better precision of measuring the spring constant can be used to recognize particles in liquid, such as blood cells, viruses. We also developed a technique to determine exactly the displacement using the feedback effect of laser diode by digital FFT method and an analog lock-in amplifier. The displacement can be measured in a precision of about 10 nanometers, and further improvement of the measurement has be proposed using a reference reflector.The feedback effect of a laser diode is also applied to measure the blood flow. The pulse counting method and correlation method has been developed to analyze the speckle signal of the laser output due to the random feedback effect of the blood.