Interstellar communication X: The colors of optical SETI

Brief

Working from the laser-engineering priority ranking of Narusawa et al. (2018), Hippke evaluates five candidate OSETI wavelengths, 1064.1 nm, 532.1 nm, 393.8 nm, 656.5 nm, and 589.1 nm, against interstellar extinction modeled with Draine (2003) dust curves, atmospheric transmission from the Cerro Paranal SkyCalc model (R = 10^6), scintillation variance, and stellar/sky noise across M-dwarf through G2V backgrounds. For strong signals (S >> N), optical wavelengths are competitive out to roughly 1 kpc, but Nd:YAG at 1064 nm stays within 50% of peak received flux under all tested conditions and all distances out to 3 kpc. Fraunhofer spectral lines provide reduced stellar background but are dominated by other loss terms in almost every scenario. A narrow 0.1 nm filter centered at 1064.14 nm at nanosecond cadence could, in principle, cover approximately 3% of one sky hemisphere in a single fast survey pass.

Metadata

Category

Search

Venue

Journal of Astrophysics and Astronomy

Type

Peer-reviewed

Year

2018

Authors

Michael Hippke

arXiv

1804.01249

Access

Open access

Length

2.7 M

Programs

Breakthrough Starshot, NIROSETI

Instruments

VLT / Cerro Paranal (SkyCalc model), Jacobus Kapteyn Telescope (La Palma)

Data sources

SkyCalc / Cerro Paranal Advanced Sky Model, Draine (2003) dust extinction curves, Leinert et al. (1998) sky radiance catalog

Tags

SETI, optical-SETI, technosignature, laser-communication, interstellar-communication, NIROSETI

Key points

• Five 'magic wavelengths' ranked by laser-engineering viability: 1064.1 nm (Nd:YAG R2→Y3), 532.1 nm (SHG-YAG), 393.8 nm (near CaK), 656.5 nm (near Hα), and 589.1 nm (near NaD2), with interstellar extinction fractions S_E at 100 pc of 0.94, 0.84, 0.77, 0.88, and 0.86, respectively.p.2
• Optical SETI is extinction-limited beyond roughly 300 pc (S_E < 0.5 in the visual); near-infrared searches are preferred beyond 1 kpc, and mid-IR (5–8 μm) is optimal toward the galactic center where 550 nm attenuation reaches a factor of 10^-18.p.3
• Nd:YAG at 1064 nm remains within 50% of maximum received flux for all atmospheric conditions and all distances out to 3 kpc, making it the single most robust broadband choice.p.8
• For S/N-limited searches toward M-type stars, wavelengths 320–485 nm are optimal out to ~1 kpc; Nd:YAG at 1064 nm is never competitive against M-dwarf backgrounds because the stellar flux peaks near 1 μm, raising the noise floor at that wavelength.p.9
• Fraunhofer spectral lines (CaK, Hα, NaD2) are largely irrelevant for OSETI: their stellar-background noise reduction of up to 50% is overpowered by atmospheric transmission, extinction, and diffraction losses in almost all use cases.p.9
• Breakthrough Starshot-class laser beamers, optimized for propulsion rather than communication, would produce beams brighter and tighter than any previous OSETI assumption, appearing as naked-eye stars at kpc distances.p.1
• A 0.1 nm narrow-band survey at 1064.14 nm at nanosecond cadence with a 1 m telescope could cover approximately 3% of one sky hemisphere while keeping sky-background photon counts below 10^9 per second, enabling fast, repeated all-sky scans.p.7
• Atmospheric transparency at 393.8 nm varies from above 70% under best conditions to below 35% at median water vapor (2.5 mm precipitable water at Cerro Paranal), making it the most condition-sensitive of the five candidate lines.p.3

Verbatim

• “These are primarily the Nd:YAG line at 1 , 064 nm and its second harmonic (532.1 nm).”

  p.1

• “Fraunhofer spectral lines, while providing lower stellar background noise, are irrelevant in most use cases, as they are overpowered by other factors.”

  p.1

• “Such beamers would appear as naked eye stars out to kpc distances.”

  p.1

• “Towards the galactic center, extinction increases to large values, E ( B − V ) ≈ 3 at A ( V ) > 44 mag at 550 nm (Porquet et al. 2008; Fritz et al. 2011), an attenuation by a factor of 10 − 18 .”

  p.3

Most interesting

• Earth's strongest lasers produce approximately 2 MJ in a 5 ns pulse, outshining a Sun-like G2V star at 100 pc by a factor of 10^4 during that pulse, independent of distance.
• Nd:YAG laser linewidths can be narrower than 1 Hz, but Earth's rotation and orbital motion impose a practical minimum observable bandwidth of roughly 0.1 nm, setting a hard floor on how narrow an OSETI filter can usefully be.
• Superconducting nanowire single-photon detectors (SSPDs) achieve ~93% quantum efficiency in the IR with 0.15 ns timing jitter, enabling single-photon detection near the 1064 nm Nd:YAG line with negligible dead time.
• A 10 m transmitting aperture at 1 μm wavelength produces a beam only 25 mas wide, narrowing to a 1 AU cone at approximately 40 pc, tight enough to target a single planet.
• Modern lunar laser ranging uses 80 MHz Nd:YAG at exactly 1064 nm and 532 nm, providing a terrestrial engineering precedent that directly validates the 'magic wavelength' hypothesis for interstellar communication.
• Toward the galactic center, mid-IR wavelengths between 5–8 μm are the only practical communication window; both water-ice absorption at 3.1 μm and silicate absorption at 10 and 18 μm must be avoided.