Books on lasersreviewed by T. Nelson
Reviewed by T. Nelson
You might think the military's main interest in lasers is to do “remote cutting and welding,” as DH Titterton puts it. But in fact, despite the abundance of powerful CO2 lasers in industry, their use as weapons is still mostly in the future.
In today's military, the biggest use of lasers is probably as pointers in PowerPoint presentations. But there are many other uses: communication, missile defense, range finding, laser ring gyroscopes, and remote sensing, to name a few.
High-power lasers are still struggling to create enough power to damage anything other than financial balance sheets and the coatings on their mirrors, but there are some amazing new technologies. One of the most powerful is the free electron laser, which is not only tunable, but potentially able to create megawatt pulses. Unfortunately, it requires a synchrotron, it is ginormous, and its cost is astronomical, even by DoD standards.
The optimal wavelength for a laser weapon is in the mid- to far-IR where fog and air turbulence are less of a problem. Almost all are above 1.4 μm because using lasers on the battlefield that could potentially blind is banned by the Vienna Protocol. This is a good fit for CO2 lasers, which have a dominant emission line at 10.9 μm. Other promising types are oxygen-iodine (50 kW at 1.315 μm) and quantum cascade lasers (low power, 3–10 μm). To counteract air turbulence, adaptive optics are needed.
Size and efficiency (which means heat dissipation) are also big challenges. Some of these lasers, with their complex tracking mechanisms requiring accuracy measured in microradians (1 μrad = 0.206 arcsec), are huge: they weigh several tons and would probably do as much damage by falling on you as by zapping you with their beam. Additional complexity comes from the need to use altitude-azimuth mounts, which are by their nature unable to track anything at the zenith. And because targets move very fast, it's essential to provide a way of inhibiting the beam when the target moves behind something important, like the ship's bridge.
Another interesting device is the super-continuum laser, which produces femtosecond pulses in the terawatt range. A titanium-sapphire laser emitting in the near-infrared at around 800 nm can create a peak-pulse irradiance above 1013 watts per sq. cm., which is enough to produce harmonics well into the UV down to 230 nm. These lasers produce a fundamentally new type of light: a self-focusing beam that is so energetic it ionizes atmospheric nitrogen, creating a “filament” of white light (350 nm–9 μm) that extends for several kilometers. They have a beam divergence of less than 0.2–0.3 millirad (0.011 to 0.017 degrees). Compare that to 25 degrees for a common laser diode, which is hardly coherent at all. These ultra-high-power pulses also create ionized plasma, which limits the power that can be transmitted, suggesting that there may be a maximum laser intensity attainable within Earth's atmosphere.
An important use of low-power lasers is jamming of infrared seekers in heat-seeking missiles. About 80% of the aircraft lost in the last twenty years have been due to heat-seeking missiles. Early missiles used rotating infrared optics with a fixed radial chopper to produce a signal whose frequency or amplitude depended on how far off target the missile was. These can be easily jammed by using an infrared laser to beam a custom on-off pattern toward the missile to make it think it's off-target.
Jamming missiles with modern imaging seekers will be a bit harder, but it's possible that countermeasures have already been found. Unfortunately, most of this newer stuff is secret. Titterton is at the UK Defence Academy, and it's likely that countries with bigger defense budgets have already solved these problems.
Detecting incoming missiles is usually done with ultraviolet cameras instead of infrared, since very few other sources of short wavelength UV are out there in the battlefield.
Titterton also proposes a number of rules of thumb. Most are technical, but some show his wry engineer's humor:
Although topics like Q-switching are covered, unlike in Siegman's textbook there's no atomic physics, no quantum mechanics, and no wave optics equations or any other equations. A background in optics would be helpful, but not essential. The graphs were created in color but printed in grayscale, which makes some of them unintelligible. There is a big chapter on safety. X-ray lasers and target characteristics such as reflectivity are not described.
feb 16, 2019
Correction An earlier version of this article said the free electron laser was ‘gigantic’. This has been corrected to ‘ginormous.’ We regret the error.