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Menlo takes frequency combs to the masses

04 Sep 2008

How do you convert Nobel Prize winning research into a robust product suitable for everyday use in a range of applications? Nadya Anscombe asks the optical frequency comb experts at Menlo Systems in Germany to share the secrets of their success.

Starting up a company in the photonics industry is usually a long and risky business. Finding investors and customers can be a struggle and it can take a long time before the company is profitable. Not so for Menlo Systems.

The company – the first ever spin-off from the renowned Max-Planck Institute for Quantum Optics in Garching, Germany – was set up in 2001 by Theodor Hänsch, Michael Mei and Ronald Holzwarth. Soon after, Bruno Gross, Alex Cable and the Max-Planck Society joined. They each put their own money into the company, which already had two customers, and it made a profit from the start. Their magic product? An optical frequency comb synthesizer.

The product, which produces a comb of frequency lines separated by the laser repetition rate, promised to revolutionize many areas of research and enable measurements with precision that was otherwise unobtainable at the time.

"Starting up was relatively straightforward," Mei told OLE. "Before we had even set up we had several enquiries about the frequency comb synthesizers that we had developed. Before long we had orders from two national metrology institutes in Austria and Italy. It was as if the industry had been waiting for this technology."

Frequency-comb basics
A frequency comb provides a direct link between the optical and the microwave frequency regimes and can be used in either direction (see figure 1 and box). Phaselocked to a radio-frequency reference, any unknown optical frequency can be measured by simply comparing its frequency to that of the nearest tooth of the stabilized frequency comb. The accuracy of the frequency measurement is only limited by that of the reference.

"Measuring the optical frequency of a laser used to require several people and three labs full of equipment," said Mei. Researchers used cumbersome frequency chains, but the Garching group's optical frequency comb was more precise and less cumbersome, and also more flexible.

In the late 1990s this technique caught the attention of John Hall from the University of Colorado's JILA laboratory. He soon became an ardent evangelist for "this goofy technique that makes everything obsolete that we have worked on for so long". Hänsch admits that "increasingly heated competition" between his Garching group and Hall's group in Boulder, Colorado, US, accelerated the development of the technology. In 2005 he and Hall shared the Nobel Prize for physics for their part in the invention of the optical frequency comb.

But, in 1997, Hänsch realized the importance of his research. In that year he wrote a confidential six page proposal for a universal optical frequency comb synthesizer and asked two colleagues to witness and sign every page because he thought this might become important for patent applications.

The Max-Planck Institute was later granted patents for the technology and Menlo now has exclusive rights to them.

"Those were exciting times," said Mei. "And the area is still exciting. Back then, the accuracy with which we were able to measure frequency, distance or time was unprecedented. Now, the exciting bit is that our systems are more operable and the technology is available to everyone."

Looking to the skies
Menlo's products have been used in groundbreaking research as well as new and emerging industrial applications.

For example, when an international group of astronomers suggested that one of the fundamental physical constants may have changed over the past seven billion years, physicists were presented with a problem. How do you measure such minute changes objectively? Researchers using an optical frequency comb have shown that, so far, there is no indication that these constants have changed over time.

Another way in which comb technology is being used by astronomers and physicists is in distance measurements between satellites. The European Space Agency is investigating using the technology on its satellite systems, which fly in formation and need intersatellite distances to be measured within a few microns (see figure 2). There is currently no other technology that can measure distances of several hundred metres with an accuracy of several microns, or possibly even nanometres.

In theory, an optical system that can measure long and short distances for this application would be extraordinarily complex because it needs to generate and detect a broad range of wavelengths and would require several lasers to be installed on each satellite. But the optical frequency comb technology has changed all that.

"For many years the optical frequency combs were in our heads, in a few years from now we hope to see them flying over our heads," said Mei. "To make this a reality, advancements in technology and funding have to go hand in hand."

Because an optical frequency comb can be seen as 500 000 continuous-wave lasers (see figures 3 and 4) all emitting at slightly different wavelengths simultaneously, the list of possible applications is endless. "Many applications simply have not been thought of yet," commented Mei. "Particularly in the areas of chemical analysis and biotechnology."

Telecoms calling
But one area where the benefits of the technology are clear is in the telecoms industry. Comb technology would be ideal for wavelength-division multiplexing where data is sent down a fibre using several wavelengths at the same time. The more densely packed the wavelengths, the more data can be sent.

"Telecoms applications were in our original business plan," explained Mei. "But since the telecoms crash, we have concentrated on other application areas. Also there is still a lot of work to do before optical frequency comb technology can be used in the telecoms industry. It's all very well being able to send a huge amount of data down a fibre, but you need to be able to deconstruct it at the end. Also, you need to be able to individually modulate each line of the comb and that is a huge challenge."

In theory, comb technology would be ideal for use in the telecoms industry because its centre wavelength is in the C-band (1530–1565 nm). The fibre that is currently in the ground would be able to cope with the technology, but new components and a new system architecture would be required in order to take full advantage of what comb technology could offer.

"As a first step in this direction, we have established extremely precise fibre links between optical frequency combs separated by many hundred kilometres," explained Mei. "This allows us to show the power of the comb technology when applied to optical fibre."

Menlo's other offerings
Optical frequency combs are not the company's only product. It also sells femtosecond lasers, which it has built using its experience in developing the comb technology. The company's most recent is a femtosecond fibre laser called Orange. It is based on ytterbium-doped fibre and produces record-breaking values of 200 mW average power in 100 fs pulses at a repetition rate of 100 MHz. Soon, the Orange product will be complemented by an ytterbium-doped amplifier to boost the peak power while keeping the pulses short. This, claims the company, opens up new possibilities in microfabrication, cell manipulation, multiphoton excitation and spectroscopy.

These are the types of technological advances you might expect from a large company with a big R&D budget, but today Menlo employs only 35 people and has recently opened a US office. Located in Cambridge, Massachusetts, this will help the company support its US and Canadian clients and also provide a base for a collaboration with MIT scientists James Fujimoto, a pioneer of optical coherence tomography, and Franz K√§rtner, an expert in ultrafast science and few-cycle pulse generation.

"We are hoping that this collaboration will help us develop many new and exciting applications for optical frequency combs," said Mei. "We've come a long way since starting up and it has been an exciting time, but I am sure that so far we have just caught a glimpse of how powerful the optical frequency comb technology can be."

• This article originally appeared in the September 2008 issue of Optics & Laser Europe magazine.

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