Maser technology has been somewhat neglected in defence, however, its ability to detect and produce radio signals with very little noise shows promise.
You have almost certainly heard of lasers and seen them. They are everywhere. From the supermarket checkout to tailoring, eye surgery to welding, our world would be very different without them. Yet you may not have heard of masers, which is a shame because their invention predated lasers by seven years. Like the latter, maser is an acronym, although translated as microwave amplification by stimulated emission of radiation. As with laser, this anacronym is now in everyday use. Despite the age of the technology, military interest in maser technology could increase in the coming years. Masers are interesting because they can be used to generate microwaves, and militaries need microwaves for communications and sensing.
Back in the ‘50s and ‘60s
The first maser was built at New York’s Colombia University in 1953. One year earlier a trio of physicists – Russia’s Nikolay Basov and Alexander Prokhorov, together with America’s Joseph Weber – articulated the principle of the maser. Dr Weber did this during the Electron Tube Conference held that year in Ottawa. Half a world away in the Soviet Union, Professors Basov and Prokhorov made a similar presentation. Theirs’ took place during the USSR’s Academy of Sciences’ May 1952 All-Union Conference on Radio Spectroscopy. Another three American physicists – Charles Townes, James Gordon and Hebert Zeiger – would build the first maser in 1953. Professors Townes, Basov and Prokhorov would later win the 1964 Nobel Prize for Physics for their work on masers. Professor Townes would also work with fellow American physicist Arthur Schawlow to lay the theoretical groundwork for the laser.
Masers have since been employed for several tasks. The Harvard and Smithsonian Centre for Astrophysics in Cambridge, Massachusetts says masers can be used to measure the structure and size of the Milky Way. The National Radio Astronomy Observatory in Charlottesville, Virginia notes that masers are used in atomic clocks. Another employment has been space communications. On 28 November 1964, the NASA’s Mariner-IV was sent into space from Cape Canaveral, Florida onboard a Convair/General Dynamics Atlas-Agena rocket en route to Mars. Mariner-IV reached the Red Planet on 14 July 1965, performing a flypast which concluded on 15 July and taking 21 full pictures of the Martian surface. At one point, the craft was just 9,846 km (5,316 NM) above the planet. NASA’s official history discusses the use of masers to provide a link between the probe and Earth. The technology was very much in its infancy at the time.
Masers and the spectrum
Before we consider how the maser works, it is worth refreshing our basic physics knowledge. Atoms have a nucleus containing protons and neutrons, which is surrounded by electrons moving at various distances from the nucleus. Radio waves harness electrons to create signals. Radio frequency (RF) radiation was initially produced using vacuum tubes and in the centre is a cathode. When heated by an electric current the cathode emits a cloud of electrons. At one end of the tube is a positively charged electric plate known as an anode. Electrons emitted by the cathode rush towards the anode. A metallic wired grid is placed between the cathode and anode, and a current passed through it. The grid’s voltage can be controlled, and it can be negatively or positively charged. A negatively charged grid repels electrons, reducing the flow between the cathode and anode, and vice versa. The vacuum inside the tube is essential as it lets electrons move more freely than they would do in the atmosphere. These generated electrons will form the basis of the signal.
In 1940, British physicists John Randall and Harry Boot, working at the University of Birmingham in central England invented the cavity magnetron. Cavity magnetrons represented a major step forward in radio engineering as they could produce microwave radiation, including RF radiation. The term ‘microwave’ encompasses electromagnetic radiation stretching from frequencies of 300 MHz to 3 THz. While definitions differ, some classify microwave radiation as occurring in frequencies between 1 GHz and 1 THz.
Cavity magnetrons use a similar cathode-anode arrangement to the vacuum tube. However, in the former, there is a hollow space between the anode and cathode. However, cavity magnetron, The surrounding anode has cavities (known as ‘resonating cavities’) cut at regular intervals along its internal circumference. Magnets are positioned either end of the hollow space enclosing this, with the anode and cathode in a cylinder. The magnets create a magnetic field inside the hollow space and force the electrons to follow a curved path when inside the space, like people walking up a spiral staircase. As electrons pass by each cavity, this causes electrons within the grooves to move back and forth (known as oscillation). This in turn causes the grooves to alternate between positively charged and negatively charged states. Each cavity is charged with the opposite polarity from its neighbour in a process that causes inductor-capacitor (LC) oscillation, a process by which a direct current voltage is changed into an alternating current (AC). A radio wave is derived from an AC source; a metal loop extracts the oscillation from a single cavity, which is sent to an antenna and transmitted in the form of a radio wave. Oscillations are sustained because the electrons are retained in the magnetron, continually transferring their energy into the cavities and stimulating the LC oscillation.
The problem with streams of electrons, whether generated by vacuum tubes or cavity magnetrons, is that they produce ‘noise’. These devices create heat which causes the agitation of free electrons, a process that increases with temperature. Known as thermal noise, this process is largely unavoidable. Thermal noise is problematic because it accompanies an RF signal. Natural and human-made RF noise is also in the atmosphere. These factors can risk drowning out a weak signal as the noise may be too loud, with the strength of the signal compared to the noise, known as the signal-to-noise ratio, being a fundamental part of radio engineering. Noise can be overcome to an extent by amplifying a weak signal, but the process of amplification creates some thermal noise. Thus, the signal may not be as ‘pure’ as would be desired.
To refresh our memory, atoms comprise protons and neutrons in their nucleus and are surrounded by electrons. These electrons are not at a uniform distance from the nucleus – some are closer than others. The farther away an electron is from the nucleus, the more energy it has and vice versa. Electrons can be persuaded to change their proximity to the nucleus. If they move away from the nucleus, they will use up energy, absorbing a photon, but will lose energy if they move towards it, emitting a photon. Photons comprise light, and indeed are the quanta of the electromagnetic spectrum. Light enters the gain medium, to stimulate the electrons within, causing them to lose energy which is released in the form of photons. This process has a cascading effect: Photons triggered by the stimulated electrons hit other electrons, causing them to lose energy and release more photons, and so on.
Masers and radio waves
Going back to the heady days of 1953 and the first artificial maser created by Professors Townes, Gordon and Zeiger, their efforts focused on using ammonia molecules to provide the gain medium. Usefully, Ammonia molecules have a resonant frequency of 23.79GHz. Resonance occurs when the molecules are subjected to an external force which matches that frequency; in the case of Ammonia, a 23.79 GHz microwave signal. When experiencing this matching external force, the molecules will start resonating at 23.79 GHz, albeit with a higher amplitude compared to the original amplitude of the external force. Put simply, the first maser used a beam of excited Ammonia molecules, pushed through a focuser to separate excited from ground state molecules, with the excited molecules continuing into the resonant cavity. The resonant cavity will have internal dimensions mirroring the resonant frequency of the incoming molecules or atoms; in this case, the wavelength of 23.79 GHz is 12.6 mm. The incoming excite ammonia molecules then decay into their low-energy state, emitting photons at a frequency of 23.75 GHz – which is within the microwave band (300 MHz to 300 GHz). These can be released through an outlet as a coherent microwave signal.
It is worth noting that photons emitted under stimulated emission are of the same frequency as those moving past them, allowing the creating of a very low-noise signal. Some of these photons bounce back and forth within the chamber, inducing other Ammonia molecules to give up their photons; in sufficient quantities, this induces an oscillating electromagnetic field within the cavity. The beauty of the maser is that unlike the vacuum tube and the cavity magnetron, it produces very little noise, as it is not reliant on a stream of electrons. It is noteworthy that quartz oscillators and solid-state electronics which can be used to generate RF energy still produce noise at differing levels.
Defence applications
Signal-to-noise ratio (SNR) is a consideration in all aspects of radio engineering, including defence applications. Radars, radio communications and satellite navigation all depend on the transmission and reception of RF signals. The SNR measures the relative strengths of prevailing electromagnetic radiation and the desired signal. The less noise there is, the clearer the signal will be. Likewise, masers can amplify a weak signal with potential benefits for military RF-dependent systems. As noted above, the problem with amplification is that it brings noise; thermal noise from the electrons, and prevailing natural and artificial noise in the ether.
David Stupples, Professor of Electronic and Radio Engineering at London’s City University, says that the ability of masers to amplify very weak signals could pay dividends for militaries. Radios and radars increasingly use low probability of interception/deception (LPI/D) techniques to hide signals in the ether. Several articles could be written on the intricacies of LPI/D techniques, but in essence, what they try to do is use the background electromagnetic noise of their environment to camouflage their signals. Electromagnetic noise is ever-present on our planet and indeed the universe at large. Levels of electromagnetic noise vary depending on the environment. For example, a crowded, busy city will have more noise than the sparsely populated countryside: There will be far more people using cell phones, which employ radio waves, in a city compared to rural areas. The prevailing, local level of electromagnetic radiation is known as the ‘noise floor’, which can be likened to the hubbub of a crowded room. When you walk into the room, you will notice that the different conversations merge into a continuous sound – this is the noise floor. You will not hear someone whispering very quietly to someone else in another part of the room, for example. LPI/D systems try to maintain the strength of their signals below that of the noise floor. Systems can be designed to use techniques to filter out noise to detect LPI/D signals, but there will always be a limit to the faintness of a signal that they can detect. We have the same problem that the system’s own activity will produce noise, like electromagnetic tinnitus.
Professor Stupples explained that the advent of LPI/D RF techniques is prompting a renewed interest in masers, particularly for weak signal amplification. As the employment of masers for communications with Mariner-IV showed, the technology has applications for satellite communications too. Signals coming from, or going to, space must travel long distances. Signals are like long-distance runners; the further the run, the more energy they lose. For example, Position, Navigation and Timing (PNT) signals transmitted from space to Earth by Global Navigation Satellite System constellations are very weak by the time they arrive. PNT signals can be as low as -130 dBm (decibels-per-milliwatt) when they reach Earth; Decibels being a standard measure of signal strength. Figures produced by the US Federal Communications Commission, America’s spectrum regulator, say that signals from the US Global Positioning System constellation can have a strength of circa 56.5 dBm when they leave the satellite.
Attenuation is less of a concern for masers: “There is so much attenuation for lasers in the atmosphere, but much less so for masers,” according to Prof. Stupples. All-in-all, maser technology has a great deal of potential in the military RF world. The technology has not had the uptake in the defence environment that one might expect, arguably because high-powered radios and radars already work well. Nonetheless, as militaries increasingly embrace LPI/D technology, we may see a greater adoption. After all, as Professor Stupples noted, maser technology “has a great deal of potential.”
Dr Thomas Withington