Wilhelm Kaenders. [Image: OSA]
For its May 2020 print article “The Laser at 60,” OPN interviewed a range of OSA Fellows to get their insights on some particularly interesting horizons for laser research today. We’re presenting a selection of those interviews online. Below is an edited version of our interview with Wilhelm Kaenders, the cofounder and chief technology officer of Toptica Photonics, Germany. Toptica celebrated the 22nd anniversary of its founding this past February.
It’s now 60 years since Theodore Maiman demonstrated the first working laser. Can you give me your thoughts on the two or three areas where you think laser technology is apt to see particularly active development in the next few years?
As far as the laser as “the ideal light source” is concerned, the 60 years have pushed the technology from being just a small, first dim laser light to many extremes. Just to name a few examples, we see today the electromagnetic spectrum spanned with laser light from the MHz frequency band, generated by down-converted laser light, all the way to the EUV or even X-ray spectral range. We see laser linewidths realized of a few mHz, but also random-laser approaches with little to no spectral signatures. We see the specifics of photon statistics being explored, as well as laser-based super-resolution overcoming traditional limits.
Enabled by the realization of optical frequency combs, which was honored in the 2005 Nobel Prize, a new laser field has developed to provide, and has made increasingly accessible, laser-based tools not only for basic research but also for applications. Today, this field of laser technology can be considered more like a high-frequency extension of the much-longer-established RF field, than a directed light source alone.
Another remarkable achievement is the extension of lasers to (Most materials can today be addressed for material processing with a suitable laser source; some new examples are cutting/ablating with UV wavelengths, welding copper with blue/green diodes or bioprinting with IR wavelengths (3 µm). All material combinations can be inspected with the right laser devices—for example, paint analysis with THz, gas analysis in the IR, semiconductors in UV.
Quantum technology is an up-and-coming area. What kinds of application needs will lasers have to meet as quantum technology develops?
Photonics will remain an invaluable tool for any type of quantum technology. In many approaches to quantum computers, lasers are an indispensable tool to prepare, initialize and interrogate quantum states. In addition, when we start to link quantum processors together over a larger distance to quantum networks, we either have to go free-space or via fiber. Both approaches will need laser light, no question.
Again, it helps that most atoms and ions used in some approaches to quantum technology are nicely addressable by lasers today—even with tunability and optical phase control when needed. The challenges for the future are more like how to get these lasers out of lab—ultra-compact and transportable but still high performing.
Is there additional development in laser technology itself—in terms of reduced noise, power, size, new wavelengths or other parameters—that will be necessary to drive their use in quantum technology?
We do see increasing activities to coherently connect together the relevant visible or UV wavelength ranges that are needed to drive atoms and ions into quantum behavior, with the [infrared] wavelength ranges utilized for telecom fiber transport. This connection has to happen in a phase-coherent process, and such solutions are still is not fully there.
So far, the question of overall power efficiency starts to come up for quantum technology as soon as people start to think about scaling from a few qubits to a larger number. It will come up strongly when “quantum computers as a service” become available, most likely as part of delocalized next-generation data centers. Power per qubit is an issue, and people will see big numbers challenging realistic and needed scaling.
Let’s move to some other areas of laser technology—such as the use ultrafast fiber lasers in applications that have been dominated by Ti:sapphire. What is making fiber lasers a competitive technology here?
Commercial Ti:sapphire lasers have been around for more than three decades, and are still the backbone of scientific research. They are a very mature technology, though, with very few prospects for another technological breakthrough. Blue or green diode pumping might change the field slightly, but to me it seems clear that they will not be able to drive the transition from research to relevant applications.
Only ultrafast fiber lasers have this potential—and maybe, at some later stage, mode-locked semiconductor solutions. The transition from research into clinical applications, just to name one example, will require fiber lasers for their size, convenience and power efficiencies, even allowing fiber delivery to the patient as part of the design.
Another example is microscopy. In linear (one-photon) microscopy, multi-laser engines with four to six colors had already replaced widely tunable CW lasers 20 years ago. The same will happen in two-photon microscopy, where one-to-four-color single-purpose solutions—which are cost-effective, compact and easy to integrate and operate—will replace widely tunable Ti:sapphire or OPO [optical parametric oscillator] solutions.
What will be the defined “new standard” wavelengths still needs to be decided. But 780, 920, 1030 nm already seems the ideal starting trio, with something around 1300 and 1700 as hot candidates for additional colors, allowing good tissue penetration and matching to existing fluorescent markers.
Are there other areas of “traditional” laser technology where fiber lasers can make inroads?
Well, the ubiquitous gas lasers that were among the technologies that started the laser field are still able to defend certain market niches. They are highly developed, most of them at a single spatial and temporal frequency, and are absolutely frequency stabilized by the nature of their laser process. But wherever other arguments—like compactness, efficiency and cost of ownership, and intelligent integration—come into play, their niches are eaten up by either diode-pumped solid state lasers or by direct diode or fiber lasers.
The multi-laser engines are a good example of such a process. Whereas formerly, they needed a cubic meter of space and multiple kW of electrical power, they can now be served out of a shoebox-sized device with up to seven laser colors, including direct modulation schemes, serving state-of-the-art fluorescence live-cell microscopes.
The advent and growth of diode lasers and diode-pumped solid-state lasers has had a huge impact on the laser business. Could you comment some of the things those lasers are enabling now?
Laser diode technology has always been driven by consumer volume applications—like data or telecommunications, or optical data storage (CD, DVD and Blu-ray), where each of these produced their individual favorite semiconductor material system. Diode-pumping was the key for the solid-state laser development, but also for fiber lasers, where raw optical diode power is efficiently converted into highly brilliant sources.
The quest for direct diode laser solutions, due to their even higher efficiency, is on, but it is still a big challenge. Today, these solutions find markets where there is no alternative, like in the blue region for working on copper
Finally, it has been just over 22 years since Toptica was founded, which is a fairly large slice of the overall history of laser technology. Are there things about the laser’s development over that time that have really surprised or particularly impressed you?
I look back at the boom-and-bust of telecom in the late 1990s and early 2000s as having a major impact on laser technology—but maybe in a somewhat unconventional way. At that time, so much funding was pouring into a suddenly stalling market, and several companies were left behind with enough cash to go on for years, but with no real sizable market anymore. They turned around and addressed the next-smaller market opportunity left, leading to an impressive development of fiber laser technology. As unusual as that time was for businesses development, it was the birthplace for many companies that dominate the industrial-laser marketplace today.
In hindsight, there are two key developments for the commercial success of lasers that especially impressed me: the establishment of the erbium-doped fiber laser amplifier, and the creation of blue LEDs and laser diodes. The first is the driver for today’s globally connected world (and not fully recognized yet). The second one, honored with the 2014 Nobel Prize in physics, revolutionizes the way we light up the world, and been saving the world’s energy in an unprecedented way.