Opinion Physics World  June 2017

Why the laser industry needs physicists

Lucian Hand argues that physicists’ multi-disciplinary training makes them ideally suited for a career in the laser industry, where the challenges and opportunities vary almost daily

(UAB EKSPLA)

We’ve come a long way in the decades since the laser was invented, but in many ways the photonics industry is still nascent, reminiscent of where the electronics industry was in the 1960s. Lasers are nearly ubiquitous, but if you look inside the fancy consumer packaging you’ll still find a lot of duct tape holding things together (sometimes literally), and companies that operate out of the owner’s garage are effectively competing with giant international firms. In this environment, the inherently multidisciplinary nature of physics training means that people with a physics background have the opportunity to thrive. On any given day I may be called on to tackle problems in chemistry, biology, computer science, mechanical engineering, materials science or fluid dynamics – and even, on occasion, physics and optics.

Chemistry problems bedevil most laser systems. Photochemistry can lead to material changes (such as photodarkening in optical fibres) and ion mobility can cause colour centres and other localization problems, but the most common headaches are optical contamination and damage. The causes are impressively varied. In one particularly frustrating case we traced an ongoing optics contamination problem to sewer gas entering the lab from an unused floor drain. Volatile organics love to condense on optics, and the source of these compounds can range from oils or lotion on a user’s hands, to Scotch tape residue, to outgassing from wire insulation. We learned the hard way that optical coatings may be damaged by ozone, which is generated when UV light from the laser interacts with air; it turns out that purging closed laser systems to remove the air really is necessary at higher power levels. Then again, the nitrogen gas used to perform purges will interact with certain coatings, and some coatings behave differently in zero humidity, so you’d better be careful how you purge.

In the laser itself, material coatings such as anodization or paint may interact with different wavelengths of light, with outcomes ranging from discolouration to off-gassing and film deposition on optics. And finally, in water-cooled lasers one must consider the potential for corrosion, bimetallic or galvanic interactions between components, or carbon dioxide from the air forming carbonic acid and “eating” a bushing in the water pump (to list just a few examples).

Multiple obstacles

Speaking of cooling, though, remember that where there is water, there is life. Unfortunately, “life” in a laser cooling system means algae and biofilms, which cause various sorts of mischief. In a solid-state laser this slimy stuff may coat the flow tube or active element (such as the YAG/YLF rod), blocking the pump light and thereby reducing output energy. Good chemical knowledge will help you solve the problem – but remember that “simple” solutions such as bleach may damage seals and tubing, so you have to consider carefully and understand all of the materials used in the system and their chemical interactions.

Electrical engineering is everywhere in laser science, and sometimes it brings fascinating and unexpected challenges. Of course every laser designer needs to know some electrical engineering in order to design power and control electronics. But electrical engineering expertise is also necessary for understanding and mitigating electromagnetic interference (EMI), whether it comes from high voltages generated within the laser system itself by things like Pockels cells (which typically require a few kilovolts, with switching times of a few nanoseconds) or from the customer’s other equipment. For some customers this “external” EMI can be huge; spare a thought for the people designing electronics to operate near, say, the 350 TW pulse generator at Sandia National Laboratory in the US.

Another example of how I’ve put my electrical engineering know­how into action involved configuring ~20 kW mains for a laser power supply so that the laser can be manufactured in Europe, integrated into a large system in the US and finally installed in Israel. This task required us to take into account each country’s electrical standards. I’ve also learned about corona discharge in high-voltage flashlamp leads (4 kV) and how to mitigate this so that the discharge won’t ionize air and break down the leads’ rubber insulation.

Applying knowledge

As lasers move into mainstream applications, users demand computerized controls and diagnostics. This requires knowledge of software and computer engineering. With computerized control comes the need (or ability) for the laser to interface with other equipment – whether it be detectors in a spectroscopy system, motion control in a micromachining system, or something else entirely. New capabilities lead to new applications, and to conversations with customers that include the phrase “Great, but could you also…”. This, in turn, leads to further software and computer engineering challenges/opportunities (“opportunity” and “challenge” should always be viewed as synonymous).

The applications of mechanical engineering, materials science and fluid dynamics to laser science are probably most evident in thermal management problems, such as extracting heat from the laser and maintaining stability across varying ambient temperatures. If we want to answer questions like “Why did that YAG rod crack?” or “Why is one mirror mount more stable than another, and how can we make mounts even more stable?” or “How can we transfer heat from where it is generated to where it can be dumped safely?” then it is necessary to have a good understanding of thermal conductivity and thermal coefficients of expansion for various materials. Otherwise it will not be possible to build a laser system that is stable across a “reasonable” temperature range (where “reasonable” is defined by not-always-reasonable customer requirements).

That explains why mechanical engineering and materials science are important, but fluid dynamics? Well, when designing cooling systems one must consider the relationships between tubing diameter and flow resistance, and understand why turbulent flow is generally essential for efficient heat transfer from a surface to a cooling fluid. Building a stable laser is thus a fascinating interplay of mechanical engineering, materials science and fluid dynamics.

And finally, physics

Optics and physics are common to everything we do, but not always in the ways you might expect. Ray tracing and elementary optics principles are, obviously, employed for designing the laser, but they are also necessary for delivering the laser output beam to the target, often with rather complex geometries. Lenses and mirrors may seem simple enough, but as one involves articulated arms and/or long path lengths, the problem becomes difficult indeed.

Conservation of energy is a fundamental principle of physics and it is also fundamental to troubleshooting laser problems. Often the first manifestation of a laser problem is low (or no) energy at the output. So, is there energy going in? (Check mains power.) Is there pump light to the active element? (Measure the optical power at test points.) At each stage, if there is energy going in, then there must be energy coming out, with accommodation for normal losses.

Moving on to 20th century physics, a working knowledge of the uncertainty principle provides a basis for understanding limits on frequency bandwidth and pulse duration (especially for pulses in the picosecond range and shorter). The Kerr effect, Raman effects and a host of related nonlinear effects make it possible to generate very short pulses (tens of femtoseconds) and are used in numerous spectroscopic applications, but they can also confound our efforts if they crop up when not wanted. For example, stimulated Brillouin scattering limits power transmission in fibre networks and nonlinear self-focusing limits peak power in laser amplifiers. Entropy is elementary to any physics education and manifests in various ways within laser systems – in particular, it explains why dust gets everywhere. And finally, there is Murphy’s law, which predicts that any speck of dust or other contamination will settle on (and damage) the most expensive optic in the system.

In any customer-facing position, you are likely to face all of the above opportunities/challenges, plus more of the same, as you work with customers to understand how they want to use the laser, what challenges they face and what they really need to accomplish their work. A laser that stably produces light with the specified parameters is a good start, but true success only happens when the user achieves the desired effect – whether it be fabricating a component or making a measurement leading to new scientific discoveries.

On any given day

Lasers don’t taste good and aren’t much use as protection from the weather, so we must monetize our creations in order to have food and shelter. Here again, physics training can serve you well. As Milton Chang asserts in his book Toward Entrepreneurship, physicists’ skills are easily portable to the business environment. In part, this is because physics training incorporates the notion of “widgets” – sets of tools and principles that can be applied to various systems in various frames of reference. We’ve already seen that conservation of energy is useful for laser troubleshooting, but it is also essentially the same thing as accounting. Whether you are accounting for units of energy or counting units of money, the principles are the same. Multidisciplinary product knowledge, combined with a grasp of basic accounting, is a solid foundation on which to build success, whether you are leading a product line or an entire company.

On any given day, laser scientists may be using nano, pico and femtosecond pulse durations to reach giga, tera and petawatt peak powers to interact with materials on micron to nanometre distance scales or femtosecond and shorter time scales. I joke that our jobs should be subtitled “Rarely within nine orders of zero”! Physics students tend to work with the small and the large, from subatomic particles to supernovae, so fluidly moving across 20 orders of magnitude is “all in a day’s work”.

Of course there is room for specialization, particularly in larger companies, but there are still many positions within the laser industry that are inherently multidisciplinary. A physics training, multidisciplinary in nature, provides a solid basis for developing specialization in any number of fields, or being versatile enough to work on and solve a wide range of problems.