Transmitting/Local-Oscillator Lasers and Magic Frequencies
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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