| The soliton
system here in the magnetics lab at CSU is used to investigate
one of the most fascinating entities in modern nonlinear science;
the soliton, or solitary wave. Solitons are formed in nonlinear
dispersive media due to a balance between nonlinear compression
and dispersive spreading. These nonlinear solitary waves have
been observed in many physical systems, such as optical fibers,
water, and electrical circuits to name a few. Here at Colorado
State University we study microwave magnetic envelope solitons
in thin magnetic films. The low loss characteristics of single
crystal yttrium iron garnet (YIG) thin films make it possible
to study a great number of nonlinear spin wave phenomena, including
soliton formation and propagation. Thin YIG films are distinguished
from other media, like optical fibers, in that their nonlinear
and dispersive properties may be varied through experimentally
variable parameters. In order to realize the formation and propagation
of solitons it is necessary to introduce and receive microwave
pulses in these films. To this end a simple microstrip delay
line is used. A microstrip delay line consists of an input and
output antenna with a YIG film placed face down on the antennas.
A simple explanation of the typical soliton
experiment is as follows. First a microstrip delay line is
placed in an external magnetic field. Next, its transmission
vs frequency characteristic is measured for some fixed external
field. From the transmission characteristic one may choose
an operating point frequency. These rrow rectangular pulses
are then applied to the input of the delay line at that carrier
frequency. Through careful manipulation of the experimentally
variable parameters, such as antenna separation, carrier frequency
input pulse width and power, soliton formation and propagation
is realizable. A soliton formed in a thin (5 - 10 mm) YIG
film has the following typical characteristics: a carrier
frequency of 4 - 8 GHz, a temporal width of 10 - 20 ns, a
carrier wave number of 100 - 200 rad/cm and is formed from
an input pulse of 25 - 30 ns at a power level below 1 W.
A subsystem of the soliton system in the magnetics
lab at CSU is the inductive magnetodynamic probe (IMP) system.
This system operates under the very simple idea of Faraday's
law. When a spin wave propagates within a thin magnetic film,
due to the continuity of B and H at the surface of the film,
there is a microwave magnetic field near the surface of the
film. Through the use of a carefully constructed inductive
probe loop it is possible to probe these fields near the surface
of the film. With this system it is possible to construct
spatial and temporal contours of pulsed and cw excitations
within the film. The IMP system here at CSU has a spatial
and temporal resolution of better than 50 um and 1 ns, respectively.
This makes it possible to investigate both linear and nonlinear
pulse and cw propagation characteristics directly with a very
good accuracy. The resolution of the probe is many orders
of magnitude smaller than that of the excitations which it
probes.
Besides the soliton system, the IMP system
also works in tandem with the BLS system. While both the BLS
and IMP systems have comparable resolutions, there are important
differences between them. The most important difference is
in the wave vector selectivity of the BLS system. However,
while the IMP system has no wave vector selectivity, it does
allow one to measure all characteristics of the waves in the
film which are accessible to modern microwave measurement
techniques. These include phase and frequency capabilities
as well as many others which are inaccessible to the BLS system.
Another important advantage of the IMP system, is its abillity
to probe planar microwave devices which are inaccessible to
the BLS system. The IMP system combined with the soliton system
and the BLS system make it possible to measure a wide variety
of linear and nonlinear processes in thin ferrite films as
well as device characterization capabilities.
Key instruments used for this system include:
- Agilent Infiniium Oscilloscope (1.5 GHz,
8 GSa/s)
- HP 8161A Pulse Generator
- HP 83650A Synthesizer Sweeper (10 MHz -
50 GHz)
- HP 83623B Synthesizer Sweeper (10 MHz -
20 GHz)
- HP 8510C Network Analyzer (45 MHz - 50
GHz)
- HP 8593E Spectrum Analyzer (9 kHz - 26.5
GHz)
- HP 70820a Microwave Transition Analyzer
(DC - 40 GHz)
other systems
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