Research Details

Fiber-optic gyroscope (FOG). Our group has been a world leader in academia in research on this commercially most successful of fiber sensors. It relies on the relativistic Sagnac effect, by which light traveling in a circular path suffers a phase shift when the path is rotated. In a FOG this phase shift, which is proportional to the rotation rate, is measured with a Sagnac (two-wave) interferometer consisting of one or more kilometers of fiber wrapped in a small coil. A key novelty in our work is that we interrogate the interferometer with a laser broadened by modulating its phase externally with noise to destroy the laser's temporal coherence. This approach reduces the noise, while improving the absolute precision of the measurement. This concept has recently led to the first laser-driven FOG with tactical-grade performance that meets the Federal Aviation Administration's requirements for the enormous market of aircraft navigation. Another novel avenue we are exploring is replacing the conventional fiber by a photonic-bandgap fiber to eliminate other residual sources of instability. The objective is to reach the extremely demanding strategic-grade performance for submarine and satellite applications, and earthquake monitoring.

Ship-scale SiN gyroscopes. We are studying a second kind of gyrsocope that consists of a ring (multi-wave) interferometer the size of a thumb nail fabricated on a silicon-nitride chip. This resonant device offers the potential of lower rotation sensitivity but in a considerably smaller device, of interest for cell phones, gaming, and drones. Like the FOG, this research involves in-depth experimental and theoretical studies of the fundamental properties of optical waveguides, in particular the origin of backscattering and polarization properties, as well as advanced electronic control. We just exceeded for the fist time the world record for a gyro this small (a capability to measure close to the Earth's spin). The next goal is to reduce this value ten fold and to improve the long-term stability.

Optical MEMS sensors. We are spearheading a new class of acoustic and acceleration sensors utilizing a suspended diaphragm fabricated in our cleanrooms using SIO wafers. When subjected to an acoustic wave or acceleration, the diaphragm vibrates, and this dynamic displacement is probed interferometrically with light delivered by an optical fiber. This technology has produced the world's most sensitive optical microphone. One objective is to fabricate on a single silicon wafer hundreds of identical sensors that can be multiplexed together on a single fiber to assemble large-scale sensor arrays for the oil extraction industry and geophysics applications. These sensors are also being considered to detect trace gases (hydrogen, volatile organic compounds) at the ppb level for hydrogen harvesting, fuel cells, and the food industry.

Slow-light fiber sensors. Another important aspect of our research is the implementation of slow and fast light in practical optical structures to produce sensors with unprecedented sensitivity. In one particular implementation, we use a strong fiber Bragg grating to generate very sharp resonances (linewidth of a few pm) that we use as references to measure exceedingly small perturbations such as a strain or a temperature change.

Exceptional-point sensors. Our group has been engaged in theoretical research on the use of exceptional points in coupled resonators to produce more sensitive gyroscopes. At an exceptional point, which occurs for example when two resonators are coupled at the right rate (or strength), the two fundamental modes of the coupled structure are fully degenerate—they have exactly the same frequency and linewidth. Applying a rotation lifts this degeneracy, leading to a frequency splitting of the two resonances proportional to the square root of the rotation rate. In the limit of zero rotation rate, the sensitivity to rotation becomes infinite. This high-payoff, high-risk project, of interest in particular to NASA, is also highly controversial: the main question that we are addressing, in keeping with our endeavor to remain practical, is developing practical method to measure this frequency splitting.

Optically cooled fiber lasers. There has been a strong push to improve fiber lasers in terms of reducing their frequency noise, increasing their power above the 100-kW level in a single-mode output, and reduing their size, wight, and power consumption. This effort has led to a recent renaissance in the exploration of exciting laser physics and the development of new materials. A most promising alternative solution that we have been exploring is anti-Stokes fluorescence (ASF) cooling. The laser ions (Yb3+) are pumped at a wavelength longer than their mean fluorescence wavelength. The fluorescence photons then have a greater mean energy than the pump photons, energy that they acquired from the host’s phonon bath, and when the fluorescence photons radiate out of the fiber, the fiber cools. This ongoing work has lead to remarkbale results in the last three years, including the cooling of the first silica fibers. Future work aims at exploring a wealth of new compositions to improve the cooling efficiency and scale the current cooled fiber lasers (~200 mW) to the 100-W level.

Some of the breakthroughs and records from our group over the past decades:

Smallest strain resolution (~200 attostrains)
Slowest light in a fiber (22 km/s)
Lowest noise in a laser-driven fiber-optic gyroscope (~300 µdeg/√Hz)
Smallest temperature resolution in a fiber Bragg grating (~0.5 mK)
Lowest minimum detectable rotation rate per area in a chip-scale gyro (~80 deg/h/√Hz)
Best pressure resolution in a diaphragm-based fiber microphone (sub µPa/√Hz)
Best acceleration resolution in a diaphragm-based fiber sensor (~100 ng/√Hz)
First laser-cooled fiber silica fiber
First radiation-balanced fiber laser
First radiation-balanced fiber amplifier