Biomedical Engineering Reference
In-Depth Information
One
10 µm
Bloch line-
crosstie pair
on a Neel wall
Zero
Neel wall only
FIGURE 3.33
Permalloy array pattern.
FIGURE 3.34
Optical detection format.
This provides a rotation of approximately 2° with a permalloy thickness of
600 Å, at λ 0 = 830 nm. Thin film and multilayer dielectric polarizers supply a
3.5% change in transmitted intensity per degree of analyzer rotation with
the polarizer at 45° from extinction with respect to the input polarization.
A fixed insertion loss of 15% for the polarizers, and a loss due to absorption
in the permalloy of 2.7% for 600 Å thickness (α = 6 × 10 5 cm −1 ) along with a
50% loss due to initial polarizer rotation, provide a decrease in optical power
density form polarizer to detector of 65%. This level is decreased by 3.5%
polarizer transmittance per degree of rotation X2 = 7% due to the presence
of a crosstie. If we couple 3 mW of power into the system, the PDs will see
changes of 74 μW on a background of 1050 μW.
PDs can produce 25 mV μW −1 μs −1 integration time at 12 μm center-to-center
spacings. A power level of 1050 μW then provides a 650 mV signal using
25 ns integration time, with a 46 mV change in voltage for crosstie detection.
Excellent signal to noise ratios are thus possible for low false alarm prob-
abilities. The signal may be compared to a reference zero location electro-
optically switched simultaneously, and differentially amplified for further
signal processing. Such a system provides excellent high-speed detection for
Search WWH ::




Custom Search