Biomedical Engineering Reference
In-Depth Information
3.12.3 Potential Optical Detection Scheme
Optical detection could supplant the magnetoresistance detection approach,
which limits access times to approximately 500 ns. This is too slow for many
primary memory applications. The optical technique will afford read-write
cycle times that are competitive with the fastest available technologies on the
market. In fact, the crosstie memory access times may prove to be the fastest
achievable, with the optical addressing scheme described herein.
The detection circuit for the optical detection (read) scheme requires sim-
pler circuitry; so there will be not only performance gain but also significant
improvement in cost and producibility. Additional limitations of the magne-
toresistance effect include the requirement to heat the substrate to increase
crystallite size and to provide the necessary magnetoresistance ratios, along
with temperature limitations. These limitations are eliminated with optical
access and detection using the Faraday magneto-optic effect.
Magneto-optic Faraday rotation occurs when linearly polarized light is
transmitted through a material parallel to the direction of a magnetic field
in the material. The amount of rotation φ is proportional to the magnetic
field
M
and the path length
t
, such that φ =
VHt
cos θ, where
V
is the Verdet
constant (rotation per unit length per unit magnetic field), and θ is the angle
between the magnetic field and direction of light propagation. In the pro-
posed crosstie memory detection configuration, the presence or absence of
a crosstie wall is used to produce a change in the transmitted optical power
density through crossed polarizers that are rotated from extinction by an
angle equal to the amount of Faraday rotation produced. Differences in
transmitted light intensity through different memory locations thus provide
detection of stored information. The Neel wall state does not have a magnetic
field perpendicular to the permalloy surface while the crosstie state does.
Figure 3.33 shows the permalloy pattern to be used to write information
on the memory device. Creation of crossties is performed by the application
of coincident currents flowing through conductor strips insulated by nitride
layers. Windows should be added to the nitride and conductor layers to
define the spot size of the transmitted light for optical detection of the cross-
ties. Polarizers can be either grown or bonded to the device as wafer process-
ing permits. Laser radiation will be applied through a particular memory
block or array accessed via electro-optic switching.
Detection of transmitted light intensity is accomplished with a silicon PD
array. Random access or parallel addressed read may also be accomplished
through the detector array, rather than electro-optic switching. Current lev-
els produced by changing light intensities resulting from partial extinction
of Faraday rotated light are amplified and coupled to a comparator with sen-
sitivity controls allowing identification of logic levels. The device configura-
tion is illustrated at Figure 3.34.
Assuming a crosstie magnetic field equal to the saturation magnetization,
we have a specific Faraday rotation in permalloy equal to 1-2 × 10
5
deg cm
−1
.
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