The aim of the following experiment is to optically measure an electric field using the Whispering Gallery mode or WGM phenomena. This is achieved by exciting the optical modes of the dielectric microsphere, using a single mode optical fiber. As a second step, an external field is applied to the microsphere, which causes a shift in the WGM of the microsphere.
Next, the WGM shifts and the magnitude of the applied electric field are recorded on a pc. The results obtained show the relationship between the WGM shifts and the applied electric field. The main advantages of this technique are that the sensors much smaller, it consumes much less power, and there's significant improvement to measurement sensitivity.
It was inspired by the recent developments in optical communications technology, This method that can be used in a wider range of field, including oland, security, defense, lightening prediction, and the neuroscience Visual demonstration of this method is critical as achieving good light coupling with between the sphere and the fiber can be difficult to learn because of the need for precise position between the sphere and the fiber. Three types of microsphere sensors are studied here. To prepare a type one sphere, start with poly dimethyl Sloane or PDMS base and the curing agent S guard 1 8 4 silica and elastomer mix PDMS base and the curing agent with a volume ratio of 60 to one.
Next, use an optical stripper to strip a two centimeter strand of silica optical fiber of his plastic jacket. Heat one end of the fiber with a butane torch and stretch it to provide a stem end that is approximately 25 to 50 micrometers in diameter. At the tip to create a sphere, dip the stretched end of the fiber into the PDMS mixture to a depth of two to four millimeters, and then pull it out the final depth of the fiber in the mixture and the speed at which it is extracted, controls the size of the sphere, bury these two parameters to obtain sphere diameters in the 100 to 1000 micrometer range.
Finally, place the microsphere and stem assembly in an oven about 90 degrees centigrade for four hours to allow proper curing of the polymer material for the type two triple AO spheres. Begin with A-P-D-M-S microsphere sphere, type one under a lamina flow hood, and using a mask add between around 2%to around 10%volume of barium Titan eight nanoparticles to the 60 to one PDMS and curing agent mixture used to create the microsphere. This mixture will form the middle layer.
The quantity used will determine the dielectric properties of the sphere. Dip the PDMS microsphere into the PDS barium titanate mixture to coat it with a layer with a nominal thickness of approximately 10 micrometers. Next place the two layer sphere in an oven, about 90 degrees centigrade for four hours to allow proper curing of the second layer.
Once the two layer sphere is cured and cooled to room temperature, immerse it again in the 60 to one PDMS mixture to provide a third outer layer of about 10 micrometers. This outermost layer will serve as a spherical optical guide for the type three silica. PDMS microspheres.
Begin by preparing a bath of pure PDMS space. Start with a single mode silica optical fiber, and use an optical stripper to strip a three centimeter long section of its plastic buffer coating from its end. Use a torch to melt the end of the fiber, including the cladding and core using surface tension and gravity shape.
The sphere silica spheres ranging from 200 to 500 micrometers can be shaped from the melted tip after fabrication. The silica microsphere is immersed in the PDMS base to cover it with a coat of approximately 50 micrometers. This outer layer stays as a highly viscous yield stress fluid for the optical fiber preparation.
Again, start with the length of single mode optical fiber and use an optical stripper to remove three to four centimeters of its plastic buffer somewhere in the middle. Use a micro torch to heat the strip section of the fiber until the cladding and fiber core are molten. When the core is molten, pull one end of the optical fiber along its axis to form a tapered section that is about one to two centimeters long.
The diameter of the tapered region is determined by the duration of heating, the pulling speed and the distance of the pull. The range of diameters is between 10 and 20 micrometers. To probe the system, couple the output of tunable distributed feedback laser in the near infrared with nominal wavelength of 1.3 microns into one end of the prepared single mode optical fiber.
The other end should be terminated at a fast photo diode. The photo diod output is digitized and stored on a computer. Use a micro translation stage to bring one of the microspheres type one, two, or three in contact with the tapered section of the optical fiber to provide optical coupling between the two elements.
With a function generator output, a sawtooth voltage of about 600 millivolts amplitude and one kilohertz frequency. This is input to the DFB laser controller. An approximately uniform electric field is created using two two centimeter by two centimeter brass plates, each with a thickness of one millimeter.
These are arranged as a parallel plate capacitor and connected to a voltage supply. The sphere sensors are placed in the gap between the plates. A prolonged high electric field can also be used to increase the measurement sensitivity of the sensors.
To achieve this place, the spheres in an electric field of one mega volt per meter for two hours before measurements the shift in the wavelength of transmitted light. In the data shown here is evidence of a change in the type one sphere geometry in the presence of an electric field. This plot shows the Whispering Gallery mode shift of a type one sphere under harmonic field perturbation and the Whispering Gallery mode shift versus electric field amplitude for the same sphere.
Shown here is the Whispering Gallery mode shift of a type two sphere and a harmonic field perturbation, and the whispering gallery mode shift versus electric field amplitude for a type two sphere. Note that there is an increase in sensitivity compared to a type one sphere. Shown here is the Whispering Gallery mode shift of a type three sphere and a harmonic field perturbation.
And finally, the Whispering Gallery mode shift versus electric field amplitude. For the type three sphere shows this type has the greatest sensitivity. One master at this technique can be done in several hours, including sphere and fiber preparation if it is performed properly.
While performing this procedure, it's important not to get the sphere surface contaminated Following this procedure. Other measurement techniques based on WGM shifts can be developed such as those for magnetic field detection.
