Crystal Fibre: the fibre of the future?

17 Jun 2002

Danish start-up company Crystal Fibre believes that its technology will revolutionize the telecommunications industry. Nadya Anscombe visits the firm to find out about its products and plans for the future.

From Opto & Laser Europe December 2001

When researchers launch a company, they normally have a clear idea of the applications of their products. However, Danish firm Crystal Fibre's products are still so new that not even the company's founders know exactly what their uses will be.

Crystal Fibre makes photonic-crystal fibre (PCF) - optical fibre that has periodical air holes running along its length. The arrangement of these holes can give the fibre a variety of interesting properties - for example, it might be endlessly singlemode, have a double core or guide light through an air core - which could mean that it has a range of uses. Michael Kjær, CEO of the young start-up, said: "The full applications potential has yet to be identified, but the market could be immense."Crystal Fibre already has a number of customers who use its fibres in a range of applications. Researchers at the University of Amsterdam in the Netherlands, for example, use the fibres as broadband light sources for optical coherence tomography.

At the European Southern Observatory, Crystal Fibre's large-mode-area PCF is used to deliver more than 10 W of continuous-wave laser light at 589 nm with diffraction-limited quality. This generates an artificial star in the mesosphere which is used as a reference to correct atmospheric turbulence effects. The mode-field area of the fibre is several times as large as that of a standard singlemode fibre at this wavelength. This increases the nonlinear and damage thresholds of the fibre, enabling high power to be transmitted easily.

Another of the company's customers uses the fibres to produce large bandwidth pulses from diode-pumped femtosecond lasers and is planning to launch a product by the end of next year.

Crystal Fibre's main target, however, will be the telecommunications market. PCFs can convert light from one colour to another, which makes them useful in optical switches. By far the biggest potential market for PCFs, however, is their use as transmission fibres. According to René Kristiansen, sales manager at Crystal Fibre, his company's fibres have several advantages over conventional fibres.

Kristiansen said: "We can make singlemode fibres with mode field areas that are three or four times as large as conventional fibres. The nonlinear threshold of our fibres is also higher [than that of conventional fibre] and we can tailor the dispersion to the desired value. This means that our fibres can either carry more information over the same distance, or take the same amount of information further."

However, because of the intricacies of fabricating PCF, Kristiansen admits that further scientific advances and improvements in manufacturing technology will be necessary before Crystal Fibre's products can be made in large enough quantities to be useful in transmission applications.

The main stumbling block, according to Kristiansen, is losses. "Our fibres have high losses owing to the simple manufacturing process that we use. The water content in the preform and the drawn fibre is too high," he explained. "We are working on the problem, but because most of our current customers only buy short pieces of fibre, the issue of losses is not important to them. It is only important when it comes to long-distance applications."

Another drawback is that PCF is more difficult to fabricate than standard fibre. Although both types of fibre are drawn in the same way, their preforms and manufacturing requirements are very different.

To fabricate PCF, several thin capillary tubes are stacked together to make a preform. This allows a high level of design flexibility, because the size and shape of the core and the index profile throughout the cladding region can be controlled.

To make a fibre with a solid core, for example, the central capillary tube is simply replaced with a silica rod. This preform is then drawn on a standard drawing tower, while making sure that the air holes remain intact along the entire length of the fibre. Once the finished fibre is coated it becomes as robust as a standard fibre, capable of being both striped and cleaved using standard tools.

Crystal Fibre is currently concentrating on the manufacture of high-index fibres. However, it is also developing low-index fibres in which the fibre's bandgap effect confines light to an air core. This is the most difficult type of PCF to manufacture, but it could solve the problem of losses. "Photonic bandgap fibre could transmit light through a vacuum core and give even lower losses," said Kristiansen.

Crystal Fibre's Jes Broeng was one of the first people to make air-core PCF during his time at the University of Bath in the UK. The Bath group has since launched a company - Blaze Photonics - which is Crystal Fibre's main competitor in Europe.

"The main difference between us and Blaze Photonics is that we have the backing of a sizeable company - NKT Holdings of Denmark," said Kristiansen. "Our parent firm has a large amount of knowledge of starting up businesses and many years of industrial experience."

Crystal Fibre is a spin-off from the Danish Technical University's Centre for Communication, Optics and Materials. It is now wholly owned and funded by NKT, but three of the COM's research scientists - Anders Bjarklev, Stig Barkou and Jes Broeng - have warrants in the company, which allow them to take future joint stakes in Crystal Fibre.

The company recently invested in new production equipment, including a draw tower and cleanroom facilities. The draw tower is of the standard type for the production of conventional optical fibres, but with some modifications for crystal fibres. Kjær said: "We haven't had our own production facility before, so until now we've only been able to manufacture in small quantities. For the first couple of years we will be selling to a niche market, principally comprising research institutions, but after that we will target the telecoms industry."

Even with increased production capacity, however, Crystal Fibre's founders admit that it will be a few years before the firm no longer has to depend on its parent. "Selling many short pieces of fibre is not a sustainable business model in the long term," said Kristiansen. "Our business has also been affected by the current economic climate. Fortunately, NKT is a patient investor and our company keeps growing."

Crystal Fibre currently has 14 employees and is looking to expand further next year. Kjær is unconcerned about whether he will find enough markets to sustain growth. He said: "It is not a problem finding applications for our products - it's a problem choosing which ones to aim for."