## 5 Interplanetary Laser Communications  
Of the proposed applications, high bandwidth interplanetary laser communications over distances of up to 40 AU (e.g., Pluto) place the greatest engineering burden on the instrument. In this paper, we will consider the case of On-Off Keying (OOK), a simple communication code which transmits a series of ones (detection) and zeros (no detection). If we assume a detector with single-photon sensitivity, the probability of NOT detecting a \"one\" pulse is given by P(ND) = 1 \(-\) exp (\(-n\)) where \(n\) is the number of photons incident on the receive detector in that bit. From Figure 3, we see that for \(n\) = 7, 14, and 21, respectively, the probability of not correctly detecting a \"one\" is one in a thousand (KHz), one in a million (MHz), and one in a billion (GHz). Having an expected multiphoton response for a \"one\" also helps to reject false alarms for a \"zero\", due to either internal receiver noise or low-level background light passed through a narrowband spectral filter, by allowing the imposition of a threshold count below which the bin is considered a \"zero\". In OOK mode, the laser pulse train could be either a rapidly modulated CW beam or alternatively a repetitively pulsed Q-switched or mode-locked laser whose pulsewidths are shorter than the desired bit interval, e.g., <1 \(\upmu\)s for 1 MHz systems or <1 ns for 1 GHz systems. Other modulation approaches are certainly possible, such as the pulse position modulation (PPM) used in the highly successful 622 MHz LLCD mission between the Earth and Moon mentioned previously. Early space-based laser communications efforts in the 1970s and beyond were mostly directed toward near-Earth applications to satellites. They dealt with a wide range of issues such as device technology, including lasers, detectors, optical antennas, precision beam pointing mechanisms, modulation methods, etc. Because of the relatively short propagation paths, a wide range of laser sources were considered spanning the infrared and visible wavelengths. The best papers on these subjects published before 1991 were collected and published by the SPIE Optical Engineering Press in a book entitled \"Selected Papers on Free-Space Laser Communications\" edited by [PERSON] [15]. A more recent book on the subject has been compiled by [PERSON], a former researcher at the Jet Propulsion Laboratory (JPL) [16]. The link equations allow us to estimate the laser power and telescope apertures required to achieve the desired data rates between Earth and the planet under study. Fortunately, the orbits of the planets about the Sun are coplanar (with the exception of Pluto) and highly circular. In performing the link analyses, we will use the average Earth-planet distance, \(R_{ave}\), which under the assumption of quasi-circular orbits is equal to the orbital radius of the planet about the Sun. The actual range can vary within \(R_{ave}-AU\) and \(R_{ave}+AU\). However, as mentioned previously, at or near the minimum interplanetary distance, the Earth is viewed against the extremely bright background of the Sun, while at or near the maximum distance, the Sun is physically blocking the optical path between the Earth and the planet. We will also ignore any transmission or scattering losses in the atmosphere since these can be largely mitigated by utilizing an astronomical telescope on a mountaintop or alternatively a large orbiting telescope, such as NASA's 2.4 m Hubble Telescope, which in polar orbit would not experience interruptions in service due to Earth rotation. As for the planetary terminal, the need for high bandwidth communications would suggest a satellite in a near-polar orbit outside the planetary atmosphere and oriented for maximum visibility to the Earth station. Figure 3: A plot of the number of received photons per bit vs. the probability that a “one” will not be properly detected in a wideband OOK communications system equipped with single-photon sensitive detectors.