An absorption tomography instrument that is capable of acquiring 100 projections
of 100 elements each in less than 200 ns is described. The instrument uses
time-multiplexed, fiber-optic fan-beam sources that are sequentially activated
in groups to reduce greatly the total number of detectors required for achieving
a given resolution. The quantitative details required to tailor this instrument
to a particular application are presented. A single-fiber prototype was used to
verify the design and establish its sensitivity. The sensitivity is limited by
laser-speckle noise. The fiber-optic fan-beam generator can produce an
interdetector correlation of the projection noise, reducing the effect of this
noise on the reconstruction. The noise is measured as a function of optic-fiber
stability and size, laser bandwidth and mode stability, and detector size.
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Low-intensity linear absorption coefficient from Ref. 33.
Low-intensity transmission through 2-m length of fiber.
High-intensity linear absorption coefficient calculated assuming a factor
of 10 decrease in transmission at 193 nm and a factor of 3.5 decrease at
248 and 308 nm.
Below these intensities, nonlinear two-photon absorption is not
important.
Ref. 34.
Energy, fluence, and intensity calculations are based on 0.85 of these
values.
One-half breakdown value for uniform illumination. Use of one half of
these values for beams with Gaussian profiles.
Asymptotic values of Eq.
(10) for 193 and 248 nm.
Table III
Null Tomogram Noise Study (400-μm diam fiber, 60° fan)
Standard deviation of percent of projection noise calculated across the
fan is presented in parentheses.
The ratio of measured reconstruction noise to predicted uncorrelated
reconstruction noise.
Peaked; see Fig. 10.
Table IV
Ratio of Reconstruction Noise in Unsmoothed Data to that in Smoothed Data
Low-intensity linear absorption coefficient from Ref. 33.
Low-intensity transmission through 2-m length of fiber.
High-intensity linear absorption coefficient calculated assuming a factor
of 10 decrease in transmission at 193 nm and a factor of 3.5 decrease at
248 and 308 nm.
Below these intensities, nonlinear two-photon absorption is not
important.
Ref. 34.
Energy, fluence, and intensity calculations are based on 0.85 of these
values.
One-half breakdown value for uniform illumination. Use of one half of
these values for beams with Gaussian profiles.
Asymptotic values of Eq.
(10) for 193 and 248 nm.
Table III
Null Tomogram Noise Study (400-μm diam fiber, 60° fan)
Standard deviation of percent of projection noise calculated across the
fan is presented in parentheses.
The ratio of measured reconstruction noise to predicted uncorrelated
reconstruction noise.
Peaked; see Fig. 10.
Table IV
Ratio of Reconstruction Noise in Unsmoothed Data to that in Smoothed Data
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