Project Cyclops, A Design... - Department of Earth and Planetary ...

Siliconphotodetectors with about80%quantum
efficiencyare availableat Ip and provideinternal
avalanche multiplication. Sincetheycanbecooledto
giveverylow darkcurrents,we haveusedphoton
detectionto allowfull advantage to betakenof an
incoherent receivingantennaarray.Thearrayconsists of
400mirrors,each5 m (200 in.) in diameter, giving an
equivalent clear aperture of 100 m.
This system provides the example for required
photon count given in the last section (p.47) where an
of 9.7 was found to be adequate. The product of the
transmission of two atmospheres and the detector
quantum efficiency gives an overall efficiency of
r/ = 0.7XO.7X0.8 = 0.4 with no allowance for interstellar
absorption or optical surface losses. The range limit was
calculated using equation (51).
System B. Present neodymium lasers are capable of
about 300 W CW output with a spectral linewidth of
about 100 GHz. For optical system B we assume a 1/1
laser with 3000 times the power output, or 100 kW, and
a linewidth decrease of 3×104to 3 MHz. Clearly, this
system is far beyond the state of the art.
The assumed bandwidth of 3 MHz allows for some
drift but nevertheless implies a frequency stability of a
part in 108 . This narrow receiver bandwidth may be
achievable using parallel plate Fabry-Perot filters at the
collimated outputs of each receiving telescope. With 5-m
primary mirrors and a magnification of 50×, the exit
pupil will be 10 cm in diameter. Pointing errors and
image blur due to atmospheric turbulence will produce
a range of input angles for the filter of about
-+0.6" X 50 = 30". At this angle the resonant frequency
will shift about I part in I0 s and the "walk-off" of the
mode will amount to about 7 cm, so the operation is a
little marginal. Nevertheless with a cavity l-m long and
mirrors having 99.4% reflectance, 0.1% absorption and
0.5% transmission, the required Q of 108 can be
realized. Two cascaded filters and a mop-up interference
filter would be needed to produce a single pass-band.
The transmission of these would probably not exceed
25%, so r/ is reduced to 0.1.
Although the bandwidth is 0.003 as wide as in optical
system A, the peak power is only 10-7 times as large. As
a result, the solar background is increased by 3× 104 to a
value b. = 36. Other characteristics are the same as
optical A.
The necessary expected signal photon count (292)
was calculated from equation (50) with n b = 0, and the
range was calculated from equation (51).
Infrared
Systems
System A. Here we assume a CO2 laser capable of CW
operation at 100 kW with a line width of 3 kHz. Higher
power CO2 lasers have no doubt been built, but it is
unlikely that the spectral power density we specify has
been exceeded.
Self-oscillating lasers at this power level exhibit much
broader spectral lines because of turbulent changes in
index of refraction in the gas, mode jumping due to
schlieren effects and cavity vibration. We assume a
master oscillator power amplifier system so that these
effects cause phase and amplitude, rather than frequency
and mode, perturbations.
At lO.6/.t no avalanche detectors are known. Quantum
efficiencies of 80% appear feasible but, without
avalanche multiplication, shot noise in the following
amplifier dominates. The best known solution is to use
photoelectric mixing to provide substantial power gain,
and a noise level approaching hvB. Each antenna must
deliver a spatially coherent signal, and the receiver
antenna element size is thus limited to about 2-1[4-m
diameter, so that 1975 units are needed for a 100-m
array.
At lO.6/.t we assume the atmosphere has 50% transmission
at 45 ° elevation angle (on a clear night) so that
r/= 0.5X0.5X 0.8 = 0.2. Again we have made no allowance
for interstellar absorption or surface losses.
At 10.6# thermal radiation from the (lossy) atmosphere
produces only about 2° K increase in the
system noise temperature. We find (hulk) + 2 = 1360 °.
The range was calculated from equation (40) using
rl = nsn r = 3000 X 1975 and ignoring
System B. All out for spectral purity! We assume the
sky's not the limit and postulate a 100 kW CO2 laser
with a line width less than 1 Hz. This implies frequency
stabilities of about a part in I 0 _4 and is hardly practical
in view of Doppler rates (see Chap. 6). But given this
fantastic stability we could construct a complete coherent
receiver with synchronous detection. However,
we can now use only one 2-1/4-m antenna for our
receiver. Equation (27) gives the range limit for this
system and for the microwave systems to follow.
Microwave
Systems
System A. This is a state-of-the-art system using two
100-m antennas, I00 kW of power, a 20° K system noise
temperature, and a I-Hz bandwidth obtained by
synchronous deteclion. The beamwidth of this system is
over one minute of arc, so no pointing difficulties should
b,.
49