Frequency combs and new lasers measure up

Since its invention near the turn of the century, the optical frequency comb has heralded a new era in optical frequency measurements.

05 March 2018
Marcus Woo

A frequency comb is a spectrum with hundreds of thousands of lines at precise, evenly spaced intervals-forming what looks like the teeth of a comb. These teeth are like the markings of a ruler, enabling optical measurements with unprecedented precision.

Frequency combs, which garnered the 2005 Nobel Prize in Physics, have enabled optical atomic clocks and advances in everything from telecommunications to high-speed spectroscopy.

In the LASE Plenary session at SPIE Photonics West 2018, Ursula Keller, a physicist at ETH Zurich, discussed her group's new method for dual-comb spectroscopy using a single laser. In conventional laser spectroscopy, you take the spectrum of a gas by firing a laser through it. By steadily changing the laser's frequency and comparing its intensity to a reference laser, you can measure the absorption spectrum of the gas. But this method is slow. For example, current tunable diode laser absorption spectroscopy (TDLAS) can tune over about 5 nm using a distributed feedback interband cascade laser with a maximum scan rate of 125 microseconds for one scan.

Ursula Keller, ETH Zurich (Switzerland)Dual-comb spectroscopy, which uses two frequency combs, allows for more precise measurements that can also be done more quickly. The two frequencycombs, which are created by laser pulses, have slightly different frequency intervals, generating a beat frequency in the megahertz range that's accessible by photodetectors. When the dual combs shine through a gas, the changes in beat frequency reveal the absorption of the gas.

Traditionally, dual-comb spectroscopy requires two lasers for each comb. But because they both must be stabilized, the setup is often complex and expensive.

Keller's group, how ever, has developed a new method that needs only a single laser. The laser produces a comb that passes through an intracavity birefringent crystal, which splits the comb into two and generates a shift in the frequency interval.

The researchers used a mode-locked, integrated external-cavity surface-emitting laser, or MIXSEL. The laser is based on optically pumped semiconductor laser technology, which is an established commercial product for continuous-wave operation, Keller says, making the new technique simpler and inexpensive for industrial application.

"What I'm addressing here is not another world record in terms of accuracy," Keller told the Show Daily. "My approach here is to make it applicable for industry." The researchers first tested their technique on water vapor. More recently, they went beyond a simple proof-of-principle, expanding their measurable bandwidth ten-fold, and demonstrated their method with acetylene, a gas that's of interest in industry. They showed that they could make a measurement in just 20 microseconds.

In the future, Keller said, this tool could be used for fast, real-time moni- toring of methane leaks along a pipeline or factory emissions of gases like NO, CO, and CO2.

Hidetoshi Katori, University of Tokyo, and RIKEN (Japan)In the second plenary talk, physicist Hidetoshi Katori of the University of Tokyo and RIKEN discussed optical lattice clocks, a new type of atomic clock with a fractional uncertainty down to 10-18.

These clocks could be used to test fundamental physics, relativistic geodesy, and even redefining the second.

Lasers for all occasions
The session ended with Berthold Schmidt, the CTO of the business unit of Trumpf Laser Technology and CEO of Trumpf Photonics, who surveyed the progress of industrial lasers over the past 30 years, which, he said, is reflected in the development of Trumpf's laser technology.

One of the oldest workhorse lasers is the CO2 laser, used primarily for cutting. In 1985, citing the need for lasers with 1 kW of power, Trumpf produced their first CO2 laser. Investing in that technology has today led to a 30kW master-oscillator power-amplifier system for producing extreme ultraviolet light (EUV) for microlithography. The laser pulses vaporize droplets of tin, producing a mist. The laser then ionizes the mist, creating a hot plasma that radiates EUV light at a wave-length of 13.5 nm. This kind of UV light source can be used to produce next-generation microchips.

Over the past 15 years, however, CO2 lasers have given way to solid-state lasers, which are now the main laser technology used to process materials. Trumpf's solid-state lasers include thin-disk lasers, direct-diode lasers, fiber lasers, and slab lasers. Each has its own strengths that are suited for a range of industrial applications, giving the market the power of choice, Schmidt said.

Berthold Schmidt, TRUMPF Photonics (USA)A thin-disk short pulse laser can be used as a cavity dump. After maximizing the energy that can be stored in the resonator, you can then dump it all in a single pulse, producing high-energy lasers in the kW range. This concept is also the basis for regenerative amplifier lasers, which generate ultra-short and powerful pulses -- useful for precise drilling of materials, and cutting and welding of transparent materials.

You can boost the power of cavity-dumped oscillators or regenerative amplifiers even more by sending the pulses through multi-pass cells. By reflecting the pulses through multiple mirrors, passing them through the amplifying disk over and over again, multi-pass cells can increase the power by up to 25 times. This can open up applications that haven't even been explored yet, Schmidt said.

These high-powered pulses can be used for laser lift-off of ultra-thin flexible displays, laser-based rapid thermal processing of large-area architectural glass substrates, and potentially annealing, which transforms an amorphous material into a high-quality crystal. Another possible application is to shoot down lightning bolts, clearing the way for airplanes. A laser can create a line of plasma in the air, through which a lightning bolt could discharge.

Trumpf has also developed laser systems with smart sensors. "Being an industrial laser manufacturer today doesn't mean you only have a laser source," Schmidt said. The company is developing systems that constantly monitor the laser, allowing the incorporation of artificial in telligence software to automatically make adjustments on the fly. "It's making its own analysis, and then it's adjusting parameters and optimizing the process," he said. "There's no human involved anymore."

Lasers will continue to be a powerful industrial tool, and the company will continue to adapt and optimize laser technology of all kinds, he said. "There is not one laser technology that can serve all applications."

Marcus Woo is a freelance science journalist based in California. A version of this article appeared in the Photonics West Show Daily in February.

Related SPIE content:

Dual-comb modelocked lasers generated from a single source

Wideband mode-locked optical frequency combs

Octave-spanning semiconductor laser for frequency comb applications

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