Modern electronics are often shielded with metallic packaging to protect them from harmful electromagnetic
radiation. In order to determine the effectiveness of the electronic shielding, there is a need to perform non-intrusive
measurements of the electric field within the shielding. The requirement to be non-intrusive requires
the sensor to be all dielectric and the sensing area needs to be very small. The non-intrusive sensor is attained
by coupling a slab of non-linear optical material to the surface of a D shaped optical fiber and is called a slab
coupled optical fiber sensor (SCOS). The sensitivity of the SCOS is increased by using an organic electro-optic
We present progress on advanced optical antennas, which are compact, small size-weight-power units capable
to receive super wideband radiated RF signals from 30 MHz to over 3 GHz. Based on electro-optical
modulation of fiber-coupled guided wave light, these dielectric E-field sensors exhibit dipole-like azimuthal
omni directionality, and combine small size (<< λ<sub>RF</sub>) with uniform field sensitivity over wide RF received
signal bandwidth. The challenge of high sensitivity is addressed by combining high dynamic range photonic
link techniques, multiple parallel sensor channels, and high EO sensing materials. The antenna system
photonic link consists of a 1550 nm PM fiber-pigtailed laser, a specialized optical modulator antenna in
channel waveguide format, a wideband photoreceiver, and optical phase stabilizing components. The optical
modulator antenna design employs a dielectric (no electrode) Mach-Zehnder interferometer (MZI) arranged
so that sensing RF bandwidth is not limited by optical transit time effects, and MZI phase drift is bias
stabilized. For a prototype optical antenna system that is < 100 in<sup>3</sup>, < 10 W, < 5 lbs, we present test data on
sensitivity (< 20 mV/m-Hz<sup>1/2</sup>), RF bandwidth, and antenna directionality, and show good agreement with
This paper presents a means for creating optical fiber sensors that are capable of detecting electric fields. This
novel E-field sensor is formed as part of a contiguous fiber resulting in a flexible and small cross-section device
that could be embedded into electronic circuitry. The sensor is formed by partially etching out the core of a
D-shaped optical fiber and depositing an electro-optic polymer. Using PMMA and DR1 for proof of concept,
we demonstrate the operation of the first in-fiber hybrid waveguide electric field sensor with a sensitivity of less
than 100 V/m at a frequency of 2.9 GHz. Sensors optimized for low loss (~1dB) have an estimated <i>E</i>&pgr; of 222
MV/m. A sensor with an <i>E</i>&pgr; of 60 MV/m is also demonstrated with an insertion loss of 14.4 dB.
Based on the electro-optic (EO) polymer Mach–Zehnder interferometer (MZI) technology, IPITEK develops optical E-field sensor devices. As a receive antenna, the present device exhibits wide and flat bandwidth, up to 10 GHz. Testing the E-field sensor response was performed using a transverse electromagnetic (TEM) cell at frequencies from 0.2 to 1 GHz, and a set of 4 horn antennas at frequencies from 2.6 to 12 GHz. The minimum detectable E-field, Emin, was about 70 mV/(m) for an all-dielectric field sensor and was about 7 mV/(m) for a sensor with electrodes and a short wire loop antenna. A photonic down-conversion technique was developed to address bandwidth and receiving power limitations of the receiver photodetector. The down-conversion experimental results agree well with the theoretical heterodyne predictions. The EO polymer sensor sensitivity can be further improved by reducing the device optical insertion loss, optimizing the photodetector and detection circuitry, and using recently developed higher EO coefficients polymers.
Aimed at test and evaluation needs on high power microwave (HPM) weapons, we describe new developments on miniature all-dielectric optical field sensors with flat RF sensing response from ~ MHz to 12 GHz, with negligible field perturbation, good sensitivity (~70 mV/(mH√z), and >100dB dynamic range. Present devices use a 20 mm long sensing region in an integrated optical (IO) waveguide Mach-Zehnder interferometer (MZI) using electrooptic (EO) polymer for the waveguide. The fiber-coupled optical transmitter/receiver utilizes common optical communication technology. The incident HPM RF field induces an instantaneous change in the index of refractive of the polymer that is converted into an optical intensity modulation in the MZI device. The poled EO polymer requires no electrodes nor metallic antennas that can distort the field under test. We characterized the frequency response and polarization sensitivity of the field sensor, and both agree well with modeling predictions. Common fabrication limitations result in devices with sensitivity to thermal drift. New sensor designs are being developed with remote bias control that also can provide self-calibration. To further reduce the sensor size and insertion loss, beneficial for array applications, an "in-fiber" field sensor is being developed. The core of a D-shaped fiber is partially removed and replaced with EO polymer. Such a device may use polarization modulation sensing, or be configured in similar MZI structures as the IO waveguide sensors.
Fiber optic sensor technology offers the possibility of implementing low weight, high performance and cost effective health and damage assessment for infrastructure elements. Common fiber sensors are based on the effect of external action on the spectral response of a Fabry-Perot or a Bragg grating section, or on the modal dynamics in multimode (MM) fiber. In the latter case, the fiber itself acts as the sensor, giving it the potential for large range coverage. We were interested in this type of sensor because of its cost advantage in monitoring structural health. In the course of the research, a new type of a rugged modal filter device, based on off-center splicing, was developed. This device, in combination with a MM fiber, was found to be a potential single point-pressure sensing device. Additionally, by translating the pressing point along a MM sensing fiber with a constant load and speed, a sinusoidal intensity modulation was observed. This harmonic behavior, during load translation, is explained by the theory of mode coupling and dispersion. The oscillation period, L~0.43. mm, obtained at 980 nm in a Corning SMF-28 fiber, corresponds to the wavevector difference, Db, between the two-coupled modes, by L = 2p/Db. An additional outcome of the present research is the observation that the response of the loaded MM fiber is strongly dependent on the polarization state of the light traveling along the MM fiber due to different response of the modes to polarization active elements. Our main conclusions are that in MM fiber optic sensor design, special cautions need to be taken in order to stabilize the system, and that the sensitivity along a MM fiber sensor is periodic with a period of ~ 0.4 - 0.5 mm, depending on various fiber parameters and excited modes.
We added a control electrode to a phase-shifted Bragg grating filter in an electro-optic polymer waveguide to obtained voltage tunability. The waveguide grating transmission spectrum near 1.3 microns featured a 5 GHz passband with a peak transmission of 32% within a 2 nm wide, 12 dB deep blocking band. With the waveguide grating sandwiched between gold layers separated by ~10 microns, we were able to shift the transmission spectrum at a rate of 0.1 GHz/volt. Such filter tunability may be used in ultradense WDM channel selection or to compensate for detuning by environmental factors.
An integrated Mach-Zehnder interferometer made of electro-optic polymer, which has excellent broadband (>100 GHz) response, was fabricated as a mm-wave receive antenna. When an electric field is applied to the interferometer arm(s) made of EO material, a phase delay is generated which results in a net imbalance in the interferometer and thus a change in the output intensity. This output intensity change, which contains electric field strength and temporal profile information, is then read by a photodetector and processed. To test this antenna in free space, a micro-strip travelling electromagnetic cell, which has uniform electric field distribution in the 1 GHz range, was constructed. The test results show the antenna had good linear response over a 40 dB power range, at 1 GHz center frequency. The measured minimum detectable E-field strength was about 0.22 V/m (or 6.7 nW/cm<sup>2</sup>) at 1 kHz bandwidth with a laser power of 7.9 μWatt (-21dBm) measured after the sensor, which agrees with our theoretical calculations. The measured E-field signal increases with increasing laser power, which indicates that significant sensitivity improvement, can be easily obtained by lowering passive losses. The antenna was found to be thermally stable over a temperature range from -30 to 50 C. The antenna sensitivity can be further improved by lowering the device insertion loss, optimizing the photodetector and detection circuitry, and using EO polymers with higher electro-optic coefficients.