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book reviews
books on lasers and photonicsreviewed by T. Nelson |
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book reviews
books on lasers and photonicsreviewed 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 don't regret the error at all.
Reviewed by T. Nelson
In this 4th edition, Robert Alfano, who along with Stuart L. Shapiro invented supercontinuum lasers way back in 1969, finally updates the best (and only) book on this important subject. It's 47% bigger than the third edition. Two old chapters have been dropped and eight new ones added. So it's no longer an outdated multi-author book on some obscure topic. It's now up to date, and it's now fascinating.
Simply put, supercontinuum (SC) is a white laser that is so intense it focuses itself in air to produce a filament. It's produced when a fast pulsed laser impacts a nonlinear medium. There are four processes that produce it.
(1) Self-phase modulation (SPM), where the pulse changes the refractive index (RI) of the medium. This change is time-dependent, so the phase of the pulse also changes, which means the pulse gets chirped. By using a filter, you can make the pulse even shorter.
(2) Induced phase modulation (IPM) where a weaker pulse propagates in the distorted medium whose RI is changed by a strong pulse.
(3) Cross phase modulation, which causes stimulated Raman scattering.
(4) Four-photon parametric generation.
This means there are two different physical effects going on: amplification of dispersive waves, which extends the pulse into the blue, and multiple soliton self-frequency shifts, which spread it into the infrared. Depending on how the beam is generated, it can be in a normal mode or an anomalous dispersion mode, where you get soliton fission, where high-order solitons break up into fundamental solitons.
The book has this amazing picture of what happens to solitons, which helps explain why the output is so noisy:
Computer-generated spectrogram from the chapter by Govind P. Agrawal (p. 154). The author says the blue lines across the top show the output pulse train, the tracing at right shows the spectrum, and the round objects are solitons. The original source is in Cumberland et al., Optics Express 16, 5954, 2008.
What are solitons? Solitons are small “clumps” of light or “light bullets,” as one author calls them, that propagate separately from each other. Different solitons shift their spectra by different amounts because Raman-induced frequency shift (RIFS) depends on pulse width. As a soliton shifts its spectrum, it slows down due to a process called anomalous group velocity dispersion (GVD). Solitons collide with each other, changing their velocity even more. Soliton fission makes these sources noisy and incoherent, so the idea is to use the normal GVD regime where solitons can't form.
An SC source in soliton mode is so unstable that it's necessary to average several thousand pulses to measure its spectrum. As you might imagine, this makes them challenging to use for communications. The one short chapter on telecommunications keeps a stiff upper lip and makes a case for wavelength division multiplexing. The challenges are formidable, but these guys, like most of the authors, are academics, and more challenges means fewer competitors, which means they have the funding all to themselves, so they are happy as clams.
A revolution came with the invention of photonic crystal fibers, or PCFs, which are silica fibers with tiny air holes. With a PCF it's even possible to generate a supercontinuum from a CW laser. The authors say that if solitons are tolerable, neon-filled tapered PCFs can theoretically reach wavelengths as short as 90 nm [p.162] and hollow-core fibers would be spatially coherent over the entire range.
Unfortunately, filling the tiny holes in an optical fiber with gas is a right pain, and the intrinsic noise of solitons gives poor compressibility and coherence. Later studies clarified that the lasers could be in three operating regimes, depending on the relationship between the wavelength of the mode-locked laser pump and the zero dispersion wavelength (ZDW) of the PCF:
λ of the ZDW < λ of the pump—soliton-Cherenkov radiation, no SC.
λ of the ZDW = λ of the pump—incoherent SC regime, wide bandwidth.
λ of the ZDW > λ of the pump—fully coherent SC regime, but narrow bandwidth.
So at the moment, people are trying different kinds of fiber to improve the performance in regime #3.
Maybe the most interesting phenomenon is superfluidic light, where laser light starts to act according to the same equations as a Bose-Einstein condensate. This happens with twisted laser pulses that form a “light spring” or helical intensity distribution due to mode superposition. In superfluidic light, which happens so far only in Kerr media, diffraction disappears. Close to the optical axis, superfluidity collapses, so there's a moving horizon between normal and superfluid. So you have regions of superfluid, regions of supercontinuum, and regions of both.
A Kerr medium is material with a high third-order dispersion χ(3). The ac (optical) Kerr effect occurs in when the light induces nonlinear polarization caused by intensity-dependent refractive index. The dc Kerr effect is used in Kerr cells, which were once used as polarizers.
With all these challenges, is supercontinuum really useful or is it still an academic curiosity? People are interested: One vendor now sells a Supercontinuum Generation Kit for a mere $9,881.57 . . . laser not included . . . and a mid-IR SC laser, price available for those who are sitting down before they read it. The authors in this book mention optical coherence tomography, coherent nonlinear microscopy, frequency metrology, environmental sensing, and nonlinear frequency amplification, but it seems that even 55 years after discovery the technology is still experimental. Nevertheless, an attosecond pulse is ideal for probing atom-sized things, and an ultra-high-power per pulse, which easily reaches 1 terawatt/cm2, theoretically can reach 1014 watts per square centimeter and, one author predicts, someday will reach 1022W/m2, has got to be good for something.
SC is still way too noisy for tomography, and even though the military wouldn't care about soliton noise, the need for a fragile engineered fiber would limit the power too much for their needs, so the book leaves us wondering. We need a broader treatment of the difference between SC and ordinary soliton lasers. If history is a guide, someday an SC laser will be able to do something, like cut a hole in something or propagate outside a lab, and everybody will have one.
aug 21, 2024