Daniel Perry and Dr. Stephen Schultz, Electrical Engineering
High powered microwave weapons use electric fields to overload electronics. We developed a non-intrusive sensor using a technology based on slab coupled optical sensing (SCOS). Each sensor detects the electric field component normal to the surface of the slab. By mounting two of these sensors orthogonally to each other, a more complete image of the electrical field can be obtained. One of the major hurdles of creating a multi-axial SCOS is keeping the size of the sensor small. The size is limited by (1) the size of the sensing material and (2) the ability to package the sensor to maintain its structural integrity and orientation. Good sensitivity is attained with SCOS with a length less than 3 mm and the D-fiber platform has a small core which allows for much less bending loss than standard single mode fiber. We have developed a mounting system that is heat resistant and structurally robust to protect the sensor that is extremely small when compared to traditional electric field sensors.
Electric field sensors have been developed based on the coupling of light from an optical fiber to nonlinear optical slab waveguide. This technology is called slab coupled optical fiber sensing (SCOS). This sensing technology does not perturb the electric fields because it is nonmetallic and has a small cross-sectional area. The complexity of electric fields within shielded electronics packaging cannot be accurately measured with a sensor that is only capable of detecting the field in a single direction. The existing SCOS technology can only detect an electric field in one direction, which is not sufficient in many situations.
In order to fabricate a device that is capable of detecting multiple axis of an electric field the following is required. (1) A sensor that is capable of detecting an electric field in a single direction without picking up electric fields along the other two axes and (2) a way to mount two of these sensors perpendicular to each other. If the two sensors are correctly aligned, the sensor is able to accurately detect the electric field in two dimensions. We developed a new way of detecting high powered microwave (HPM) and electromagnetic fields in two axes by using multiple fiberbased sensors mounted at orthogonal angles to each other. With a correctly aligned two dimensional electromagnetic field sensor, the mapping of an electromagnetic field is more accurate and complete.
The sensor is based on a D-fiber platform which an electro-optic slab placed on it. The ligh in the D-fiber will interact with the slab because of the extension of the evanescent field into the slab waveguide causing coupling between the two materials. In order for this coupling to occur, the effective index of one of the slab waveguide modes must be equal to the effective index of the D-fiber mode. This mode matching occurs for the mth slab waveguide mode when the wavelength is given by
where t and no are respectively the thickness and refractive index of the overlay material, nef is the effective index of the fiber mode, and m is the mode number. The coupling causes steep dips in the transmission spectrum of the SCOS, where each transmission dip corresponds to the coupling wavelength given in Eq. 1.
The incident electric field causes a change in the refractive index of the slab waveguide resulting in a wavelength shift in the transmission dips. If a laser with a narrow wavelength linewidth is transmitted through the SCOS then the wavelength shift will be converted into a change in power of the transmitted light. The relationship between the electric field and the power change of the light depends on various parameters such as the material parameters of slab waveguide, the dimension of the slab waveguide, and the separation between the slab waveguide and the Dfiber core.
Packaging of the SCOS sensor has repeatedly been refined. Figure 1 illustrates the new form of packaging that protects the sensor more than the previous glass mounts, makes the sensor easier to mount when detecting electric fields in multiple dimensions, and is heat resistant. The packaging is made of epoxy glass and fiberglass both of which have very high heat tolerances and more elasticity than the previous glass mounts. An approximately 2 mm wide strip of the epoxy glass it cut. The middle of the strip is removed using a mill creating a U-shaped trough down the middle. The sensor is then placed in the trough and covered with a low index epoxy to hold it in place and preserve the coupling between the electro-optic slab and etched D-fiber. To protect the fragile optical fiber from the abrupt ends of the epoxy glass trough fiberglass tubing comes out the ends of the trough. Finally, rigid glue is applied over the entire trough to hold everything together and to protect the sensor.
In order to attain a two-axis electric field sensor, two SCOS need to be packaged in such that their optic axes are perpendicular. This is accomplished by aligning the optic axis of crystal to be either parallel or perpendicular to the mounting substrate. Figure 2 shows the equipment used to attain the required alignment. In this process the mounting substrate is filled with UV cure epoxy. The SCOS is then placed into the epoxy filled substrate and placed between two electrodes with an applied sinusoidal voltage. The optical fiber is then rotated until the signal measured by the SCOS is at a minimum or maximum.
Previous sensors using this technology only were able to detect electric fields in a single direction. When the SCOS sensor is not orthogonal to the electric field, the sensitivity of the sensor diminishes. This becomes apparent when the sensor is placed in an electric field and then rotated. Figure 2 shows a graph of the signal output of a multi-axial SCOS. This shows how the sensor can detect the field when it is orthogonal to the field (at about 90 degrees) but when it is parallel to the field (at 0 degrees) no output is detected. The second sensor detects the fields at a different angle. When two sensors are mounted at orthogonal the device is capable of detecting two dimensions of the entire field. Two sensors are required to measure both the x and y components of the electric field because each individual sensor can only detect the field in one direction. By combining the measurements of the two sensors, a two dimensional reading of the electric field can be measured.