FIFTH CANADIAN CONFERENCE ON NONDESTRUCTIVE ... - IAEA

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in practice the central fringe is selected by using a circular hole at the
lens focus (at the expense of a much higher complexity a mask pattern matching
the fringes can be used). Taking a maximum path difference of A/4, it follows
a limit aperture 654 « >J A/4Ad. In order to be able to detect ultrasound in
the range 1 to 10 MHz, the free spectral range Av (the bandwidth can be
defined as Av/2) should not be too large: taking Av = 25 MHz yields Ad = 6 in
(even if folding mirrors are used, the system will be large and bulky) and
8j4 = 2 10~ rd for A = 1.06 pm. Using an entrance aperture of 10 cm in
diameter (and mirrors of this size), the system has an étendue of = 10" mm .
This is much larger than the optical heterodyne systems described in section
II and III, but not sufficient to use all the light scattered by an area of
1/4 inch in diameter, 2 m away from a 10 cm aperture: the required étendue is
- 0.06 mm . We present below a way to increase the étendue.
The time response of this interferometer can be understood by considering that
the path length from the laser source is changed by 6(t) for one arm and by
6(t-T) for the other (27). Then, when the laser frequency is properly
adjusted to the zero crossing, the detected signal is given by (6