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Supercontinuum source boxed up and ready to go

13 Jul 2005

Push-button-operated supercontinuum sources that combine ultrafast laser pulses with microstructured fibre were a big hit at LASER 2005. World of Photonics in June in Germany. James Tyrrell talks booth-side with developers Fianium and Koheras.

Supercontinuum devices that generate an intense beam of broadband light at the flick of a switch are now hitting the market, thanks to advances in microstructured fibre and ultrafast lasers. Shoebox-sized and with an output spanning from the ultraviolet (UV) to the near-infrared (IR), the sources could displace high-end xenon bulbs and SLEDs, and replace multiple lasers for applications such as spectroscopy, detector characterization, confocal microscopy and even LIDAR.

Ultrafast fibre-laser developer Fianium, based in Southampton, UK, installed its first supercontiuum device around seven months ago, inspired by a request to develop an intense broadband light source.

"To take our amplified pulsed laser source and then look at generating a supercontinuum using photonic crystal fibre was [actually] quite a natural move for us," John Clowes, Fianium's research and development manager, told OLE. "We have three main building blocks: an [ultrafast] master oscillator [modelocked fibre laser]; high-brightness pumps, which are essential for amplifying short pulses; and our own custom-built fibre amplifiers."

A photonic crystal fibre (PCF) converts the fibre-amplified, ultrashort pulses into an intense white light source (see "Device specifications" box) by a nonlinear process of extreme spectral broadening. The characteristics of the resulting supercontinuum depend on fibre dispersion, pulse wavelength, duration and peak power.

Koheras, part of the NKT Photonics group, was also showing a PCF-based supercontinuum source at LASER 2005. The Danish firm has been providing the spectroscopy market with narrow linewidth sources for the past eight years. "If you can get all of the wavelengths from the one source, you reduce your servicing and stock management costs," Jakob Skov, Koheras's CEO, told OLE. "We see a lot of interest [in supercontinuum sources] from general spectroscopy."

The Birkerød-based company recently acquired German solid-state laser expert LG Laser Technologies, and has opted to use a Q-switched YAG laser as its pulse source.

"We can keep costs down by using a passively Q-switched nanosecond laser and address industrial applications that have been limited in the past," said Chuong Tran, Koheras' sales and business development manager. "The [supercontinuum] source is an order of magnitude higher in intensity than a traditional xenon lamp."

Both Tran and Skov feel that pumping know-how is key to maximizing the unit's microstructured fibre. "Anyone can buy photonic bandgap fibre - the trick is how to pump it correctly," said Tran. "We have a dual-wavelength pumping scheme that allows you to reach down to shorter wavelengths."

Fianium has been perfecting its seed laser. "Ultimately, the stability and quality of the supercontinuum comes from the quality of the master source ," said Clowes. "It is essential to start off with a master oscillator where every single pulse is stable."

With approximately 0.5 W going into the PCF, thanks to a Q-switched YAG laser, Koheras has chosen not to install a fibre amplifier in its supercontinuum product. But the sources from Fianium and Koheras do have one thing in common: microstructured photonic crystal fibre supplied by Danish firm Crystal Fibre. Like Koheras, Crystal Fibre is part of the NKT Photonics group, and Skov revealed that the two firms have been working together to identify potential applications for photonic bandgap material.

Looking at the market for supercontinuum sources, simple-to-use, push-button units are the ideal candidates to replace multiple sources and tunable lasers for low-power quasi-continuous-wave applications. With nothing to set up or tune, operation is straightforward. "You get all the wavelengths in an instant," commented Skov. "Our customers can [simply] take a bandwidth filter and select the wavelength they want."

However, ease of use is not the only advantage. The supercontinuum source is packaged as a robust unit with a small footprint, which makes it attractive to product designers. "We can squeeze it down to half the size [of a tunable source]," said Skov. "Looking at industrial applications, you are now down to what I would call a compact OEM unit."

By driving down cost and tailoring device features, Skov is confident that such an OEM module will open up many opportunities.

As Clowes explains, some of the applications have come as a surprise, such as detector calibration. With output ranging from the UV to the IR, the high-brightness source has caught the attention of production managers. Online monitoring of food is another application that could benefit from an all-in-one solution.

Looking ahead, Clowes sees chromatic confocal microscopy as a big application area. "[Here] different spectral components of the source are focused to various [sample] depths and the reflected spectrum gives a non-invasive, 3D visualization of the sample," he explained. "When confocal microscopy reaches its own volume markets, issues such as the power, size and cost of the source are going to become very important in driving down the complexity of the detector."

Xenon lamps provide a low-cost white light solution, but as Tran noted, a supercontinuum source has a spectral density that is thousands of times higher and is likely to unlock more demanding applications. Other alternatives, such as multiple laser sources, are not only expensive, but can introduce artefacts such as image speckle.

A supercontinuum source would also suit imaging techniques like optical coherence tomography, and metrology systems based on white-light interferometry. The unit's wide bandwidth enhances resolution.

Cost is another factor that will influence the market. "We are some way away from there [high-end xenon sources] currently, but it is foreseeable that we can drive it down into that [price] range," said Skov. "Maybe not all the way down, but you [will] get a much more intensive product."

Clowes is confident that, in sufficient quantities, the supercontinuum source will be able to challenge the market. "One of the benefits of fibre lasers is that, as you go to volume production, the price can come down significantly because of the ease of assembly," he said. "Ultimately, it is splicing pieces of fibre together."

Fianium has its manufacturing base in the UK, and is expanding its US sales office with an applications lab, primarily as a demonstration facility for potential customers.

"We are constantly exploring the limits of fibre lasers, pushing the peak power, reducing nonlinearity and generally improving the product platform," said Clowes. "As we move to higher powers, the supercontinuum will move to higher powers."

"One of the applications that triggered me at Koheras was LIDAR," added Skov. "This is really long-term, but if we can bring it [supercontinuum] into some different power schemes, it is the obvious source here."

For more information, please visit www.koheras.dk and www.fianium.com.

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