Advisor: Professor Andrei
M. Shkel
Projects:
NUCLEAR MAGNETIC RESONANCE
(NMR) GYROSCOPE
Certain noble gas nuclei possess an inherent magnetic
moment. When subjected
to a static magnetic field, B0, these nuclei
will precess about the magnetic field lines at the Larmor precession
frequency:
ωL = γB0
where γ is the gyromagnetic ratio. If a collection of nuclei are confined in a cell together with an alkali metal vapor, the net magnetization of the nuclear spins is initially zero since about half of the spins point in a direction parallel to the magnetic field and the other half is antiparallel, as illustrated in (a). However, a technique known as spin-exchange optical pumping can be employed in order to align the nuclear spins and create a non-zero net magnetization, M, as shown in (b). Furthermore, if an oscillating magnetic field with a frequency equal to the Larmor frequency of the nuclear spins is applied along an axis perpendicular to B0, the individual nuclear spins will become phase-coherent and a precessing magnetization vector is obtained in (c). This magnetization vector forms an inertial reference frame for the NMR gyroscope.
If the precession of the nuclear spins is observed in a coordinate frame that rotates about the z-axis at an angular rate ωR, an apparent change in the Larmor frequency is observed:
ω = γB0 - ωR
Assuming that γ and B0 are known, the angular rate can thus be measured by monitoring the Larmor frequency. Circularly polarized probe light is transmitted along an axis perpendicular to the z-axis in (c) and a signal modulated at the Larmor frequency is received on the photodetector. This readout technique is similar to the optical pumping used to align the nuclear spins.
Our current effort deals
with the miniaturization of an NMR gyroscope, and our long-term goal
is to develop a device on the order of a cubic centimeter
in size and with an angle random walk of 0.001°/h1/2 and a bias drift of 0.01°/h. Several components are required
for this project, including coils, heater, gas confinement
chamber, light source, photodetectors, polarizers, and magnetic
shielding.
PIEZORESISTIVE ACCELEROMETER
FOR HIGH-G APPLICATIONS
High-G piezoresistive accelerometers for impact testing are
designed and fabricated using an SOI fabrication process.
Crash testing of cars, structural monitoring of spacecrafts,
and munitions testing are some of the possible applications.
The accelerometers are expected to offer good linearity, high
sensitivity, and stable thermal performance.
The piezoresistive effect causes a change in resistance when
a conductive material is subjected to stress. Deflection of
the accelerometers proof mass leads to bending around the
centrally located suspension beam, in turn stretching one
of the piezoresistors and compressing the other. This leads
to increased resistance in the stretched piezoresistor and
decreased resistance in the compressed piezoresistor. By detecting
the change in resistance, the acceleration can be measured.
An assembly technique is also developed. By
etching a cavity in SOI wafers, a three-dimensional package
can be obtained. In addition to allowing for mounting a sensor
perpendicular to a surface, this process potentially enables
post-fabrication assembly of three-axis accelerometers.
OPTICAL ACCELEROMETER
BASED ON FABRY-PEROT INTERFEROMETRY
The design of a navigation-grade silicon accelerometer is
investigated. By utilizing an optical detection system, a
small sensor with high sensitivity and wide bandwidth can
be created. The sensor is designed to be highly resistant
to RFI and EMI, as well as temperature fluctuations, making
it ideal for applications in harsh environments, e.g. aerospace,
automotive, nuclear, industrial, etc.
A Fabry-Perot interferometer (FPI), consisting of two parallel
plates with reflective inner surfaces, serves as the main
building block of the accelerometer. One of the FPI's transparent
plates is fixed and the other is suspended and can move due
to acceleration.
Inside the FPI, two parallel mirrors form a cavity with
an optical resonance that depends on the distance between
them. At resonant wavelengths, all of the incident light energy
is transmitted through the FPI, and intensity peaks occur.
If the space between the mirrors is changed, e.g. due to acceleration,
the wavelengths of the intensity peaks will change. Thus,
by detecting the shift in wavelength of the transmitted light,
the acceleration can be acquired.
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