Optical Transmission Of NTSC/PAL Over 10 L.Y.
We have seen how difficult it is to use microwaves to transmit standard NTSC or PAL TV signals over 10 light years - we require tremendous antenna areas to get a decent signal. Let us now see what can be done in the optical regime via directed transmissions, using the same type of signal (modulation format) that is downlinked to TVRO (Television Receive Only) systems from geostationary terrestrial satellites in the C/Ku-Band.
We have shown that a symmetrical 10-meter diameter telescope system can produce a Carrier-To-Noise ratio (CNR) of 34 dB re 1 Hz bandwidth at a wavelength of 656 nm, over a 10 light year range, with just 1 kW of mean transmitted power. This corresponds to the detection of 2640 photons per second. If we up the alien transmitter power to 1 GW, something that an advanced technical civilization might find quite easy, the carrier-to-noise ratio increases by six orders of magnitude, i.e., to 94 dB, and the received power increases from 1.6 x 10-15 W to 1.6 x 10-9 W. Assuming for the moment no Planckian background, if we now expand the detection I.F. bandwidth to 30 MHz, then the CNR falls by 75 dB, i.e. to 19 dB. Since the F.M. threshold is 7 to 8 dB, a CNR = 19 dB is more than enough for high-quality TV pictures. For the 1 GW transmitter, the number of signal photons detected per second is about 2.6 billion.
One only needs to believe that it would be possible for an advanced technical civilization to put out the equivalent energy of a terrestrial-type power-station from a laser and telescope! Since the area of a 10-meter diameter transmitting telescope is 78.5 m2, a 1 GW beam will produce a transmitted energy density of 1.3 kW/cm2.
Assuming high-efficiency (high-Q dielectric interference) mirrors with only 0.1% of the energy turned into heat at the source, the main mirror would have to dissipate about 1.3 W/cm2. While this may not be a problem for the large mirror, smaller mirrors would get very hot unless there was a very efficient heat-dissipation mechanism.
For a 1 kW transmitter at 656 nm, the Signal-To-Planck ratio (SPR) was calculated to be 65.8 dB re 1 Hz bandwidth. For a 30 MHz bandwidth this ratio would fall to -84 dB, since the detected noise due to the Planckian noise power is proportional to Be2. Even when we increase the transmitter power to 1 GW, the SPR only rises to -24 dB, and hence the SPR drastically limits the CNR.
The signal thus does not stand out from the continuum in this bandwidth. We would have to employ diffraction-limited space telescopes to improve the rejection of Planckian starlight by steering the alien star into a polar response null, and rely on an improvement in the SPR by the aliens selecting a laser wavelength that falls within a Fraunhofer absorption line.
Conventional high-resolution incoherent optical filters or spectrographic gratings have resolutions of 1 part in 100,000. At 656 nm, this resolution is equivalent to a spectral width of 0.0066 nm or 4.6 GHz. With this excess bandwidth, the SPR would fall by a further 44 dB to -66 dB. Thus, it is clear that for a post-detection bandwidths of 30 MHz or 4.6 GHz, this signal is way below the Planckian noise floor and cannot be detected. However, if the output noise was integrated (post-detection), the presence of the signal could be detected.
Of course, an incoherent optical detection receiver would not be able to demodulate the frequency modulation directly imposed on the optical carrier. So if we wished to use incoherent detection, we could only do so on a video signal that intensity or polarization-modulated the optical carrier.
We can also approach this very speculative high-power scenario from a stellar magnitude viewpoint. Since we have already calculated that a 10 light year distant 1 kW transmitter has an apparent stellar magnitude of +23, if we up the transmitter power by a factor of 1 million to 1 GW, the apparent stellar magnitude will increase in brightness by a factor of 15, i.e., to 8. This is just a little too dim to be seen by the naked eye. The alien G-type star will appear as a 2nd magnitude object, so it is brighter than the transmitter by a factor of about 250.
The HST Goddard High Resolution Spectrograph has a sensitivity range of between 11th to 17th magnitude and a resolution of 1 part in 100,000. It would be able, after some signal integration, to easily "see" the spectral line corresponding to the 1 GW transmitter. However, this spectrometer only covers the wavelength range of 105-320 nm.
The Faint-Object Spectrograph has a sensitivity range of between 19th and 26th magnitude, and covers the broader wavelength range of 110-800 nm, but its resolution is limited to 1 in 250 or 1 in 1300. This range of resolution is such as to make it doubtful that the 1 GW transmitter would be noticed in a scanned "search" unless the transmitter and alien star were spatially resolved so that the Planckian radiation received was substantially reduced.
Clearly, if the aliens have knowledge of our TV and modulation formats, they could send us high-quality "real-time" (delayed by only 10 years) color TV signals. Thus, the optical transmission technique demonstrates how compact transmitters and receivers can allow for very high data-rate transmissions over huge distances.
Although a 1 GW transmitter may be taken as an extreme, it is apparent that transmitter powers below 1 MW are not likely to be detected by conventional terrestrial optical telescopes by casual spectrographic analysis. So the fact that so far such optical signals have not been discovered by accident, does not imply that they are not there. Only a systematic study with high resolution detectors can prove this, one way or the other.