Coherent Detection
9006-009
9006-009a
The Carrier-To-Noise Ratio (CNR) in a coherent (heterodyne) detection system with a
100% mixing efficiency, is given by:
(MRi)2PrPo
CNR = ---------------------------------------------------------
[e{Ri(Pr + NbBo + Po) + Id}M(2+x) + 2(MRi)2NbPo + 2kTF/RL]Be
^ ^ ^ ^ ^ ^
. . . . . .
. . . . . .
. . . . . .
Signal . LO . Background .
Quantum Noise . Shot Noise . Shot Noise .
. . .
. . .
Background Dark Current Thermal
Noise Shot Noise Noise
9006-009b
where:
M = avalanche gain (1 for PIN detector),
x = excess noise factor (0 for a PIN photodetector),
Ri = current responsivity (0.26 mA/mW),
Pr = received signal power (1.60 x 10-15),
Po = local oscillator power (1 mW),
Nb = background radiation (= 0),
e = electronic charge (1.6 x 10-19 C),
Id = dark current at unity gain (0.3 nA),
k = Boltzmann's constant (1.38 x 10-23 J/K),
T = temperature (300 K),
F = noise figure of amplifier (6 dB),
RL = effective load resistance (10 kohm),
Bo = optical bandwidth (1 Hz),
Be = electrical output bandwidth (BI.F. = 1 Hz).
9006-009c
Because of the "gain" of coherent receivers, we assume a PIN photodetection
system (9008-029). There is no point in using the more
expensive APD device as this would only reduce sensitivity and dynamic range, and would be
difficult to implement in two dimensional array form. For a small PIN photodetector (M =
1, x = 0, Id ~ 0) and Po >> Pb and Pr,
shot noise limited detection is reached when the shot noise due to left hand side of the
denominator exceeds the thermal noise of the right hand side of the denominator, viz
2kTF
Po > ----
eRiRL
For the 10 kohm front-end with with the values given above in parentheses:
Po > 100 µW
9006-009d
As a rule of thumb for optical receivers, we need about a milliwatt of power on the
photodetector to ensure shot-noise limited detection for a 1 k load. At that point the CNR
will be 3 dB less than the ultimate.
Using the parameter values given in parentheses for a 1 kW SETI transmitter at 10
light years distance, the received power Pr = 1.60 x 10-15 W, i.e.,
-118 dBm, we find that:
CNR = 34 dB re 1 Hz
This is significantly greater (by more than 27 dB) than is possible with power-
starved incoherent receivers. The other very important thing to notice here is that the
Planck spectral density in starlight from distances greater than several light years is
very low (Pb << Po). Even if the entire spectrum of the star
falls upon the photodetector, Pb (NbBo) does not affect
the shot-noise level. Contrast this with the case of incoherent detection, where an
optical bandwidth Bo = 1.6 kHz would cause Pb = Pr, and
essentially double the shot-noise level (if x = Id = 0). Indeed, in the
coherent system, the Planck spectrum from many stars could be allowed to simultaneously
fall onto the photodetector and still not increase the noise floor.
9006-009e
Only the 2(MRi)2NbPo component of the Planck
continuum spectrum that falls within the electrical output (I.F.) bandwidth Be
is important. This noise is produced by the background beating with the local oscillator.
This is independent of the optical bandwidth. Noise in the optical spectrum covering a
bandwidth 2Be will be down converted to an output bandwidth Be, so
the effective Planck noise spectral density referred to the output is twice (3 dB) as
large as indicated in the spectral graphs. Note that as we increase the electrical output
bandwidth Be (BI.F.), the quantum noise floor increases at the same
rate as the Planck radiation (background) noise, since both sources of noise power are
proportional to Be. Thus, as long as the Planck noise spectral density is less
than the quantum noise-floor, the Planck noise is not detected.
Whatever the value of Planck continuum spectral density, as long as Pb
<< Po, the continuum cannot affect the recovered SNR. If all other
sources of noise can be eliminated, i.e., excess photodetector noise, dark current noise
and background radiation noise is negligible, a perfect quantum (shot) noise limited
optical heterodyne receiver has a Carrier-To-Noise Ratio given by:
9006-009f
Pr
CNR = ----
hfBe
This is 3 dB more than obtainable with an ideal incoherent (direct detection) optical
receiver. To obtain near 100% mixing efficiency and this value, the signal and local
oscillator beams must be colinear and overlap on the photodetector surface (9008-030).
The CNR for optical homodyne receivers is another 3 dB greater. For practical
considerations, e.g., the large Doppler shifts and chirp, heterodyne reception would be
preferable as homodyne receivers have a zero Intermediate Frequency. Also, heterodyne
detection allows for the use of existing parallel multiplexing radio frequency techniques
to reduce the "search" time. Thus the subject of homodyne detection has been
ignored in this study. This is not to imply that homodyne detection is ruled out. Indeed,
it might be found that the ability to frequency lock the local oscillator to the received
signal after "detection" of the signal, may be desirable. This will be dependent
on the modulation format chosen by the aliens.
The Columbus Optical SETI Observatory
Copyright (c), 1990
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