Transmitting/Local-Oscillator Lasers and Magic Frequencies

Radobs 16

This document is concerned with some thoughts about the selection of
suitable local-oscillator lasers, and the identification of the "magic
frequencies" in the visible and near-infrared region of the spectrum.  This
supplements information given towards the end of the OPTICAL SETI SURVEY
(RADOBS.7).

It is unnecessarily restrictive to require that the receiving local-
oscillator laser be the same type as the transmitting laser, even if were
proposing to do CETI (Communications with Extraterrestrial Intelligences)
and reply to any received messages.  Because the laser power requirements
may differ by a factor of 1 billion or more, the technologies are bound to
be very different.  While it is conceivable that transmitting and receiving
telescopes could be of similar size, and for the comparative analysis we
have assumed symmetrical systems, it is unlikely that the same or similar
lasers would be used, even if a transmitter and receiver shared the same
telescope optics.  It is important to note this point, because otherwise it
would unnecessarily constrain our thinking about suitable lasers and laser
frequencies.

Admittedly, aliens could bleed off one billionth of the power of a 1 GW
transmitting laser for receiver local oscillator applications, but would
probably find that the quality of the local-oscillator beam, e.g. intensity
noise, was no where as good as obtained from a dedicated local-oscillator
laser.  Anyway, if simultaneous operation of the transmitter and receiver is
required when multiplexing different targets, sharing a laser would be
unnecessarily restrictive; preventing independent chirping and offsetting of
the frequencies, even assuming that external modulators were used for the
transmitter.  This restriction would apply even if the transmitter consisted
of a relatively low power laser-oscillator and a series of high power
quantum amplifiers.  ETIs wouldn't spend their equivalent of billions or
trillions of dollars, to save a pittance on the local-oscillator.  Thus, we
shall assume that the ETIs have developed very special powerful transmitting
lasers for the visible or infrared region of the spectrum.  It is up to us
to find suitable low-power lasers to act as local-oscillators to their
transmission frequency or frequencies.  Of course, first of all it would
help if we had an idea as to what these frequencies are.

If we haven't any preconceived ideas about visible "magic frequencies", the
use of dye local-oscillator lasers gives us the flexibility to tune over the
entire visible part of the spectrum and a part of the near-infrared.  If we
can narrow down the likely ETI transmission frequencies, other choices of
local-oscillator laser present themselves.  There are major technical
difficulties, e.g., changing dyes, if we wish to deploy a (liquid-state) dye
laser in space.  Hence my previous suggestion that a space-based telescope
employing such a laser, is probably more compatible with Space Station
Freedom activities.

There is no shortage of suitable gas lasers for optical local-oscillators. 
For instance, Argon, Argon-Ion, Argon/Krypton, Krypton, and Krypton-Ion
lasers have suitable frequencies close to the H-beta (486.1342 nm)
Fraunhofer absorption line; this line being about 467 GHz wide.  For some
years now, JPL has been developing optical communications technology for the
next generation of deep space probes.  The most favored form of technology
is the solid-state Nd:YAG (Neodymium: Yttrium Aluminum Garnet) laser pumped
by LEDs or semiconductor lasers, with the Nd:YAG output being frequency-
doubled.  This leads to a very efficient and reliable device with very good
TEMoo Gaussian beam shape, with the additional advantage of very small
linewidth if coherent communications is preferred.  Nd:YAG Lasers are
normally thought of as being very inefficient, but when they are pumped by
LEDs or laser-diodes, their efficiency increases dramatically.

As has been previously stated, it is generally felt that the Carbon Dioxide
wavelength of 10,600 nm can be labelled with the tag "magic wavelength", but
what about other wavelengths, particularly ones is the visible part of the
electromagnetic spectrum?  Perhaps the most well-known Fraunhofer lines in
the visible spectrum are the Hydrogen lines:

H-alpha                                  H-beta
Wavelength = 656.2808 nm                 Wavelength = 486.1342 nm
Frequency  = 457,121.40 GHz              Frequency  = 617,113.55 GHz
Linewidth  = 0.4020 nm (280.0 GHz)       Linewidth  = 0.3680 nm (467.2 GHz)

H-gamma                                  H-delta
Wavelength = 434.0475 nm                 Wavelength = 410.1748 nm
Frequency  = 691,168.59 GHz              Frequency  = 731,395.49 GHz
Linewidth  = 0.2855 nm (454.6 GHz)       Linewidth  = 0.3133 nm (558.7 GHz)

I would like to nominate the H-alpha line as a possible "magic wavelength",
a wavelength that has been very important to solar astronomers over the
years for observations of our nearest star (Sol).  On those philosophical
grounds alone, H-alpha might be an "obvious" frequency.

 -------------------------------------------------------------------------- 
|                       MAGIC FREQUENCY  = 457,123 GHz                     |
|                       FRAUNHOFER LINEWIDTH = 280 GHz                     |
|                                                                          |
|                               or equivalent                              |
|                                                                          |
|                       MAGIC WAVELENGTH = 656.2808 nm                     |
|                      FRAUNHOFER LINEWIDTH = 0.4020 nm                    |
|                                                                          |
|                                                                          |
| In binary form this "magic frequency" is:                                |
|                                                                          |
|           1100111111011111111011110101010101000111000000000 Hz           |
 -------------------------------------------------------------------------- 

When the H-alpha line wavelength was originally selected for modelling my
visible laser system, it was done because in was a principal (wide and deep)
Fraunhofer line in the visible spectrum, and the quantum efficiencies of
photodetectors are reasonably high at this wavelength.  It was not selected
because it was thought that it was a preferred ETI transmission wavelength. 
Indeed, since my Optical SETI rationale assumed diffraction limited
receiving telescopes, the requirement to operate in a Fraunhofer line wasn't
that strong.

When considering possible "magic frequencies", we should bear in mind that
the maximum local receiver Doppler shifts will be of the order of
+/-80 GHz, while the maximum local receiver Doppler drift rate will be of
the order of +/-60 kHz/s (assuming ground-based receivers).  It is difficult
to judge whether ETIs will remove the Doppler shift (+/-30 GHz) due to our
respective radial stellar velocities, typically 20 km/s, or leave it to us
to do this.  This is not very important since the frequency offset is common
to both the signal and the starlight.  We note that the local transmitter
Doppler shifts are comparable to the effective linewidths of typical
Fraunhofer lines.  Only the Calcium CaII lines at 393.3682 nm and
396.8492 nm have the much larger effective linewidths of 3930 GHz and
2950 GHz, respectively.

It is assumed that ETIs will remove (de-chirp) the local transmitter Doppler
drifts, and may remove some or all of the local transmitter Doppler shift
(frequency offset).  The ETI transmitter will probably be in its own orbit
about its star, but however it is deployed, it will have differential
(local) Doppler shifts and chirps with respect to its star.  If they are
aiming to work within a Fraunhofer line, they may adjust their apparent
transmission frequency so that it appears to the target to remain centered
within the confines of the Fraunhofer line.  Whatever local Doppler shift or
chirp we introduce at our end of the link, is of course, common to both the
signal and the radiation from its star.

Here are a few examples of commercial solid-state sources of laser radiation
that can produce coherent light at or around certain Fraunhofer lines:

Lightwave Electronics manufactures a CW Single Frequency Nd:YAG device
called the Series 122 Non-Planar Ring Laser.  This state-of-the-art laser
has a short-term linewidth of 5 kHz, may be thermally-tuned in a linear
fashion over an 18 GHz frequency range without mode hops, and over 100 GHz
or more with mode hops.  It is perhaps the most coherent solid-state laser
available today, and has a linewidth which is much smaller than many gas
lasers, and comparable to that of a Helium-Neon at 632.8 nm.  It would be
very compatible with ETI modulation bandwidths greater than one hundred
times the linewidth, i.e., bandwidths in excess of 0.5 MHz.

The laser is also available at several near-infrared wavelengths, such as
1064 nm and 1319 nm, and at powers up to 300 mW.  The output of these lasers
may be frequency-doubled using a second-harmonic generating crystal (SHG). 
The frequency-doubled 1064 nm laser-line produces a wavelength of 532 nm,
which is very close to the 532.8051 FeI Fraunhofer line, and frequency-
doubling the 1319 nm laser produces a wavelength of 659 nm, which is near to
the 656 nm H-alpha line.

By coincidence, the location of the company is just down the road from the
SETI Institute!  It should be noted that some of the work that went into
developing this technology came about through NASA (Ames) SBIRs.  Prices of
the laser systems range from about $6,400 for the low power (4mW) models to
$25,000 for the high power (300 mW).

Quantronix Corporation manufactures a Nd:YLF (Neodymium: Yttrium Lithium
Fluoride) laser operating at 1313 nm.  If this is frequency-doubled to
656.50 nm, a wavelength is obtained that is not so very different to the
center of the 280 GHz wide H-alpha line at 656.2808 nm.  In fact, they are
within 153 GHz of each other.

The strategy for a modest initial series of visible light Optical SETI
observations, might involve starting at a wavelength of 656 nm, using a
frequency-doubled laser-pumped Nd:YAG as the local-oscillator source.  Later
will shall examine near-infrared Fraunhofer lines for suitable "magic
frequencies".

January 13, 1991
RADOBS.16
BBOARD No. 318

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* Dr. Stuart A. Kingsley                       Copyright (c), 1991        *
* AMIEE, SMIEEE                                                           *
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