Biomedical Engineering Reference
In-Depth Information
3.7 BulkversusIntegratedOpticBraggCells
Bulk type Bragg cell receivers now have the edge on dynamic range, band-
width of operation, high center frequency, and frequency resolution. This is
because every component of the system can be separately optimized, and the
light propagates through air from component to component. IO type Bragg
cells cannot have every component optimized, and compromises must be
made to integrate the system. For example, the geodesic lens focuses fairly
well, but a precision-ground conventional lens focuses more sharply. In IO
systems, the light is coupled into a waveguide, processed, and coupled out
to the PD array. The light is subject to waveguide losses of 0.5-1.0 dB cm
−1
,
coupling losses of 3 dB per coupling, and scattering caused by impurities in
the waveguide. Scattering causes the PD array to have a higher background
noise in frequency-adjacent pels, degrading the resolution. One advantage
of IO Bragg cells is their mechanical stability and ruggedness. Everything
moves together, eliminating any microphonic interference. This makes
IO receivers desirable for avionic and other size-and -weight-constraint
applications.
3.8 IntegratedOpticReceiverPerformance
Two IO Bragg cell receivers that have been developed will be discussed. The
first was proposed in 1977 by Hamilton et al. [27] and in 1978 by Barnoski
et al. [28]. It includes a semiconductor laser, a Y-cut LiNbO
3
substrate with
indiffused Ti to form the waveguide, geodesic lenses, interdigital SAW trans-
ducers, and a CCD PD array coupled at the output. Design parameters were
presented for a 400-800 MHz spectrum analyzer with a projected resolution
of 4 MHz, and 40 dB dynamic range.
This was in response to a number of devices and system objectives. First
was to identify the amplitude and frequency of emitters and to separate
the different emitter categories, such as narrow pulse, wide pulse, and CW.
Signals that met a specified detection criterion were to be sorted to allow
reduction in data handling, while maintaining a minimum intercept uncer-
tainty time constant. The device objectives were a monolithic integrated
circuit which could interface to external systems, which operated with low
noise and at high speed and low power consumption.
The first receiver built and demonstrated was by D. Mergerian et al. at
Westinghouse (see Figure 3.14) [29]. A 100 μW HeNe laser was end-fire cou-
pled into an x-cut LiNbO
3
crystal indiffused with Ti to 280 Å for a waveguide,
and a 140 pel self-scanned photodiode array butt coupled at a 45° angle to
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