The U.S. Department of Defense has concerns employing laser up and down links. Defense necessities must be balanced against economic feasibility of system procurement. Every procurement program is judged on its merits, and that usually begins with an advantages-disadvantages list. This section of the manuscript describes simply that. From the U.S. Department of Defense’s perspective, what are the advantages and disadvantages of employing laser communications uplinks and downlinks.
4.1. Advantages—Throughput, Power, Information Protection
The warfighters like that laser crosslinks provides from ten to one-hundred times higher data rates than traditional radio frequency [3
]; that laser crosslinks can be protocol independent and able to support multiple platforms and interfaces [35
]; and that laser crosslinks has a low probability of detection, of interception, and of exploitation [36
The programming and budget personnel like that: laser crosslinks exists in an radio frequency band that is currently license-free; that laser crosslinks components tend to have smaller mass, power, and volume requirements [97
]; and that laser crosslinks has a low probability of detection, of interception, and of exploitation. Programmers also like that laser crosslinks systems are easier to install (less funding for labor) and have lower cost per bit ratios (i.e., lower signal operating costs) [35
The primary advantage of using the small wavelengths of laser crosslinks is the orders of magnitude-increased throughput - laser crosslinks provides higher bit rates with lower bit error rate. Where a microwave link at geosynchronous orbit ranges can support data rates in the tens of mega-bits per second, at the same ranges, laser crosslinks can support in the tens of giga-bits per second [52
]. Laser crosslinks has the potential to support in the terabits per second range—that is akin to streaming two-hundred thousand high-definition movies simultaneously [97
]. Laser crosslinks also supports employing the entire bandwidth of the signal [46
], providing the opportunity for full duplex operations [62
Auspiciously, the power requirements for laser crosslinks are much lower (than traditional radio frequency) while the efficiency is higher. As a rule-of-thumb, laser crosslinks can send ten times more data using ten times less power [97
]. Additionally, the smaller signal power requirement allows for smaller collecting antenna, an advantage seen in a smaller size, weight, and power [53
]. During the early days of human spaceflight, the U.S. national air and space administration’s Apollo spacecraft communicated with the Earth using seven-foot antennas. In 2009, the lunar reconnaissance orbiter launched by the U.S. national air and space administration used two-and-a-half-foot antenna. Meanwhile spacecraft laser terminals can be a mere four inches [42
Laser crosslinks is advantageous in both ensuring the message gets from sender to receiver, and to assuring that the receiver is confident the message is true and undisturbed. In addressing warfighting information protection, the extremely small beam divergence both minimizes signal loss and increases security by making jamming problematic [7
]. The narrow beam makes both in-path interception difficult as well as interference both to and from adjacent satellites [7
]. Additionally, using well-selected optical signals makes for immunity from electromagnetic interference, while quantum key distribution adds additional security.
“Quantum key distribution (QKD) uses individual light quanta in quantum superposition states to guarantee unconditional communication security between distant parties. However, the distance over which QKD is achievable has been limited to a few hundred kilometers, owing to the channel loss that occurs when using optical fibres or terrestrial free space that exponentially reduces the photon transmission rate. Satellite-based QKD has the potential to help to establish a global-scale quantum network, owing to the negligible photon loss and decoherence experienced in empty space. Here we report the development and launch of a low-Earth-orbit satellite for implementing decoy-state QKD-a form of QKD that uses weak coherent pulses at high channel loss and is secure because photon-number-splitting eavesdropping can be detected. We achieve a kilohertz key rate from the satellite to the ground over a distance of up to 1,200 kilometres. This key rate is around 20 orders of magnitudes greater than that expected using an optical fibre of the same length. The establishment of a reliable and efficient space-to-ground link for quantum-state transmission paves the way to global-scale quantum networks.”
4.2. Disadvantages—Acquisition, Tracking, and Pointing; and the Atmosphere
If not appreciated from above, acquisition, tracking, and pointing between a fixed ground station and a ‘slow moving’ geosynchronous orbiting satellite is difficult on the best of days. The challenge of introducing two moving objects (e.g., transmitting from an unmanned aerial vehicle in flight to a geosynchronous orbiting satellite) is akin to “…trying to hit a bullet with a smaller bullet whilst wearing a blindfold, riding a horse” [99
Acquisition, tracking, and pointing requirements expand to include the need for highly accurate ephemeris data, the point-ahead angle, pointing within an error budget, and choosing to not employ a guidance beacon (pointing aid) to maintain low probabilities of detection. While the above requirement for an acquisition, tracking, and pointing accuracy of one microradian was done as both sample mathematical calculation and as hyperbole, potential beam widths of twenty microradians may require pointing accuracies of ten microradian or less [7
There is one reprieve to acquisition, tracking, and pointing attributed to laser crosslinks using optical links. As most satellites employ solar panels to generate electrical power, those same panels (the photovoltaic panel or other photo-detector components) can serve as a target for an uplink signal. The large surface area panels greatly simplify the requisite precision needed for uplink acquisition, tracking, and pointing [100
If the difficulties in mastering two moving target acquisition, tracking, and pointing fails to warrant pause, then the effects of transmitting optical links through the atmosphere must. Near and in-band visible light waves are notoriously fickle for having energy absorbed by the atmosphere. Additionally, atmospheric non-uniformity leads to beam bending and scattering; and if strong enough, beam break-up due to loss of coherence.
However, as evidenced by very recent progress [101
], determined engineers partnered with a tenacious defense funding programmer will overcome the disadvantages to reap the advantages and make U.S. Department of Defense laser crosslinks a reality.
4.4. Civil and Commercial Sector Adoption
As footnoted above, the motivations for the civil and commercial sectors differ—one for science, the other for finance. And while their ends may be different, their means and ways are often similar.
The civil and commercial sectors’ motivation for adopting laser communications falls mainly to balancing throughput versus cost. Radio frequency links are costly due to size (ground stations and transmitter power), access and licensing (congested spectrum), and the control of intellectual property. Laser crosslink systems have demonstrated more efficient and Size, weight, and power -shrewd operations including scalable technologies (e.g., CubeSat has the capability to maintain one-hundred mega-bits per second transmissions from fifteen-hundred kilometers from low-earth orbit to ground, and is in development to scale to giga-bits per second rates [136
]), with development backing from a wide range of sources—academia (e.g. Massachusetts institute of technology), civil space agencies (e.g. European and Japanese space agencies), and even internet titans (e.g. Google, Facebook) [137
]. The work of each leads to production of smaller, more effective, and less expensive data transmission systems employing cutting-edge laser communications.
Lower costs will come from smaller systems hosted on numerous vehicles including small satellites, or even as independent nanosatellites. TESAT has completed miniaturization actions to supply a host with a three kilogram, one-hundred mega-bits per second to ten giga-bits per second, laser communications system [138
]. More immediate, the U.S. national air and space administration laser communications experiment uses a semiconductor laser downlink weighing under twenty-two kilogram and consuming only 81 watts of power [97
]. To shrink the laser source and transmitter further will require the further development of short pulse laser light—something like an attosecond in length—and then be able to generate that pulse in something easily manufactured like a fiber optic cable [54
]. One attosecond is a billionth of a billionth of a second.
“Optical communication is becoming more prevalent in orbit due to the need for increased data throughput. Nanosatellites, which are satellites that typically weigh less than 10 kg, are also becoming more common due to lower launch costs that enable the rapid testing of technology in a space environment. Nanosatellites are cheaper to launch than their larger counterparts and may be a viable option for communicating beyond Earth’s orbit, but have strict Size, Weight, and Power requirements. The Miniature Optical Communication Transceiver (MOCT) is a compact optical transceiver designed to provide modest data rates to size, weight, and power constrained platforms, like nanosatellites. This manuscript will cover the optical amplifier characterization and simulated performance of the MOCT amplifier design that produces 1 kW peak power pulses and closes three optical links which include Low Earth Orbit (LEO) to Earth, LEO to LEO, and Moon to Earth. Additionally, a benchtop version of the amplifier design was constructed and was able to produce amplified pulses with 1.37 W peak power, including a 35.7% transmit optics loss, at a pump power of 500 mW. Finally, the modulator, seed laser, amplifier, receiver, and time-to-digital converter were all used together to measure the Bit Error Ratio (BER), which was 0.00257 for a received optical peak power of 176 nW.”
In conjunction with smaller and lighter, making for abundant opportunities to host in orbit, is ubiquity, making for data transmission and reception capability everywhere. In development by the public-private partnership of the European space agency and Airbus is the space data highway, an “optical fiber network in the sky” using geosynchronous orbiting satellites linked to ground stations and transmitting at one-point-eight giga-bits per second (that is an uncompressed six gigabyte high-definition movie in twenty-eight seconds, not one truncated for streaming). The Airbus and European space agency’s space data highway has completed in excess of twenty-thousand laser connections in last two-plus years, downloading more than a petabyte of data, with a reliability rate exceeding ninety-nine percent [101
]. Considering the reliability of the components used including the source, optical pump, and external modulators and so on…a reliability of 0.9998 over ten years of operation without degradation in space has been achieved [52
A proposed laser communication between the Moon and Earth system would consist of eight solid lasers, each one-hundred twenty-five milliwatts, at eight-hundred ten nanometers with five-hundred to fifteen hundred microradian divergence. Four of the communication lasers would form one six hundred mega-bits per second channel, transmitting with right-hand circular polarization, and the other four would form the other channel, transmitting with left-hand circular polarization. The total of one-point-two giga-bits per second would be transmitted [60
Research and development efforts to bring the proposed idea to fruition will depend strongly on six key factors for optimization of the optical link operating at ten gigabits per second duplex at forty-thousand kilometers distance with a bit error rate of one nano bit error per second [52
A five-to-eighteen-meter large dish (and associated facility) radio frequency ground segment system costs more than one million dollar. A laser transmitter and steering system cost fifteen thousand dollars (e.g., Massachusetts institute of technology’s nanosatellite optical downlink experiment, a thirty-centimeter astronomy telescope costing forty-thousand dollars [137
The European space agency’s secure and laser communication technology supports the research, development, and evolution of laser communications technologies, and provides flight opportunities for their in-orbit verification [102