Bradley Whitaker and Dr. Stephen Schultz, Department of Electrical and Computer Engineering
High power microwave (HPM) weapons create large electric and magnetic fields with the purpose of disrupting electronic equipment. In order to protect electronics, the interaction between the equipment and the electromagnetic fields must be characterized. This requires a noninvasive sensor that can detect large electromagnetic fields and the sensing area needs to be in close proximity to the target electronics. However, to serve as a reliable measurement tool, the sensor must neither cause electromagnetic interference of its own nor disrupt the electric field it measures. Finally, the sensor must not be sensitive to electric and magnetic field interference outside the small sensing area.
Researchers at BYU developed the optical fiber based Slab Coupled Optical Sensor (SCOS) to meet these needs. The two primary advantages of SCOS over other field-sensing technologies are that it is dielectric and small. Being completely dielectric, it can measure electromagnetic fields without interfering with them or creating fields of its own. Because the sensing area is 1.0 mm x 0.5 mm x 0.1 mm, it can measure fields in a localized area.
In order to use SCOS devices to characterize electromagnetic fields produced from a HPM source, two main problems must be overcome. First, the sensor must be properly calibrated in order to produce the greatest accuracy. Second, the sensed field must be filtered in order to extract any pertinent data.
SCOS calibration is best understood when taking into consideration the interrogation process. Figure 1 shows a diagram of a SCOS measurement. First, a laser is tuned to a device-specific wavelength and amplified with an erbium doped fiber amplifier (EDFA). The amplified laser signal is sent through the SCOS. Depending on the electric field surrounding the SCOS, a different signal is emitted by the SCOS device. This optical signal is converted to an electrical signal via a photo detector. The electrical signal is amplified using a transimpedance amplifier (TIA) and measured using equipment such as an oscilloscope or an electrical spectrum analyzer.
The first part of SCOS calibration is selecting a laser wavelength that will optimize SCOS performance. This process is straightforward and involves (1) applying a single frequency sinusoidal electric field to the SCOS, (2) tuning the laser to a particular wavelength, (3) measuring the SCOS signal, and (4) determining the amplitude of the sinusoidal SCOS voltage signal. These steps are repeated across the wavelength band of the laser and the wavelength that produces the strongest SCOS signal is chosen as the optimal wavelength. This project involved automating the laser calibration by creating a LabVIEW program to control the laser, measure the SCOS output, and compare the signal strength at each wavelength.
The second part of SCOS calibration is to measure the relationship between electric field strength and SCOS signal measurement. This relationship is known as the calibration factor. Since the sensitivity of the SCOS is affected by the laser power and the environmental conditions such as ambient temperature, the calibration factor should be measured before every test.
To determine the calibration factor, a function generator produces a sinusoidal voltage at a specific frequency. The voltage signal is amplified and connected to electrodes surrounding the SCOS. The amplitude of the amplified voltage is divided by the distance between the electrodes to determine the electric field applied to the SCOS. After the field is applied, an oscilloscope captures the SCOS signal, and the magnitude is recorded. The calibration factor is simply the ratio of the electric field and the measured SCOS signal, and is typically listed in units of mV/(kV/m).
After completing the calibration of the SCOS, the device can be used to measure electric fields, such as those produced from a HPM source. Figure 2 (a) shows the signal measured by the SCOS. This signal is almost completely hidden by noise. Part (b) shows the signal after applying the developed digital filter. Each of the eight pulses can be clearly seen. Part (c) shows a zoomed in view of one of the pulses, showing that the SCOS can detect the shape of each individual pulse, even at high microwave frequencies (~1.3 GHz).
Previously, SCOS had never been used to measure electric fields produced by a HPM source. This project produced the sensor calibration and measurement filtering that allowed SCOS to accurately detect high powered microwave frequency electric fields.
The author would like to acknowledge Dr. Stephen Schultz, Dr. Richard Selfridge, Jon Noren, Spencer Chadderdon, and Daniel Perry for their support in relation to this project.