Biomedical Engineering Reference
In-Depth Information
FIGURE 3.10
Fourier transform of a rectangular window with sinusoidal grating.
The first term is the zeroth order term which is not dependent on the RF
input, and the last two terms are the first order terms, the “−
f
” term being
the downward Doppler shift, and the “+
f
” term the upward shift. Figure 3.10
is an illustration of this.
3.3.2 Dynamic Range
The light intensity on the PD array is proportional to the square of the intensity:
(
) | (
)|
2
l x
=
U x
(3.5)
f
f
This illustrates Bragg cell technology's weakness: dynamic range. For exam-
ple, for a Bragg cell receiver to have 60 dB of RF dynamic range, the PD array
must have 120 dB. Dynamic range will be referred to with the RF signal as
the reference. One hundred and twenty decibel (60 dB RF) is beyond the per-
formance for most PDs. The PD limits the RF dynamic range of the system to
between 25 and 40 dB, depending on the type of PD used, but the Bragg cell
itself has a dynamic range between 50 and 70 dB [19].
The two categories of linear array PDs useful for AO signal processing are
the photodiode and the MOS depletion mode sensor [20].
One example of a commercially available randomly addressable photodi-
ode linear array is the Reticon CP1006 device. It consists of two interdigi-
tated rows of 256 photodiodes with a width of 50 μm. Its dynamic range
is 20 dB for noncoherent (conventional) detection processors. Another
commercially available array is the Reticon CP1023 device, with CCD
shift registers. It has 256 elements on an 18 μm pitch, with four indepen-
dent outputs of 64 elements each, capable of a 6 MHz data rate (10.6 μs
access time). Its specified dynamic range is 30 dB for noncoherent detec-
tion systems. A detector that has been built for AO Braggs cell receivers
has 140 elements on a 12 μm pitch, including an element for zeroth order
dumping. The measured dynamic range is 41 dB for 2 μs access time. A
MOS-CCD PD has been built to be used with an IO Bragg cell receiver
with four groups of 25 elements each. It is designed to be read out in four
parallel channels, or two parallel channels of 50 elements.
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