FEATURE
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Transmission revamp

Stephen Mounsey looks at some of the less well known applications of speciality fibre optics

Optical fibres are one of the most important components of the digital age. Most consumers will recognise the importance of these long, thin pieces of glass or silica in broadband communications, and many professionals in the field of photonics will also be familiar with the fibre lasers that have revolutionised the materials-processing market. Aside from these headline markets there are many less well-known applications, in which standard and speciality fibres have facilitated entirely new possibilities.

A perfect match

In fibre lasers, the doped fibres that form the gain media have always been carefully designed, but now a new approach to the transmission fibres within a fibre laser is allowing manufacturers to achieve more stability at high powers than has previously been possible. Bryce Samson, vice president for business development at US-based fibre and fibre laser manufacturer NuFern (part of the Rofin Sinar group), explains the purpose of these transmission fibres: ‘Inside a fibre laser, there is a whole chain of fibres that take the diode pump light into the active fibre via a series of components and gratings. Over the last six months or so, we’ve realised the importance of optimising the whole series of fibres in order to work together at higher power levels, and that this can improve the laser performance at high powers.’

Getting the fibres to work together, he adds, is a matter of carefully matching several properties across all components, including the index profiles, the mode fields, cladding sizes, numerical aperture, and splicing. ‘It’s stuff that we now look at and say “well, of course we have to do that”, but until the penny dropped, the industry didn’t really realise how important these details were to the performance at higher levels,’ he says, explaining that some of the company’s fibre laser manufacturing clients had struggled to solve reliability issues at high powers, before realising that mismatched fibres were responsible.

‘In a kilowatt fibre laser, even a few per cent loss at a mismatched splice can lead to failure, as well as decreasing the efficiency of the system. Instead of 70 per cent slope off, the laser could actually run at 50 per cent, so customers are effectively throwing away all that pump power, having paid for the extra diodes.’

NuFern started by offering matched component sets for the fibre lasers at the kilowatt level, as these tend to be where the limitations of the previous generation would appear, but Samson states that even lower powered lasers can lose efficiency due to mismatching. ‘Now that we’ve figured out the key parameters, we intend to broaden our offering to cover other laser platforms.’

Remote control

Spectroscopy applications can use specialised fibre optics to transmit light from a hazardous area to a spectrometer some distance away. Matching of fibres is important here too, as Ken Norris of Glen Spectra (a division of Horiba) explains: ‘In spectrometry, the fibre exit is matched to the input of their spectrometer, so that whatever the user is measuring is presented to the spectrometer in the correct manner. Rather than doping and coating, this is usually a matter of getting the geometry of the fibres correct,’ he says, adding that, although fiddly, getting these geometries right is within the capabilities of most customers. ‘The real challenge is getting the light in and out of the sample with low loss,’ he says.

FiberLogix is a UK-based company also offering fibre optics for extreme sensing applications. Company director Shafiq Parwaz explains that a range of coatings (gold, copper or other metals) allow the fibre probes to function across a temperature range from -40 to +750°C, alongside extreme pressures and vibrations. ‘One particular application for this is in the oil and gas industry, where a fibre optic probe can allow temperature profiling of an oil well to a depth of four or five kilometres,’ he explains.

Temperature profile is important in wells containing more viscous oil, as steam is sometimes injected in order to facilitate extraction. ‘In this application, the whole fibre becomes one continuous sensor, with the properties of scattered light changing depending on the temperature of the fibre.’

Spectroscopy specialist Avantes also promotes the use of fibre optics for monitoring processes at extreme temperatures, and is able to offer probes with temperature tolerances right up to 700°C, thanks to coatings of a copper alloy dubbed CuBALL. Avantes also has customers in the petrochemical industries, where the probes are used to remotely monitor gas levels, or naptha crackers and fractional distillation columns. Klaas Otten, a technical sales manager at Avantes, explains that in vacuum applications, the materials used must not out-gas, and so in many applications additional consideration must be given to the glues and protective sleevings used, as well as the properties of the fibre itself.

Staying in pole position

Polarised laser light is required for any applications in which non-linear optics are to be used. Producing polarisation is not difficult using solid-state lasers; producing it using a fibre laser and subsequently maintaining that polarisation state within a transmission fibre is a little more challenging. Andrew Robertson, senior vice president at Gooch & Housego, explains the importance of the polarisation-maintaining (PM) fibres and components that the company produces: ‘Any non-linear optics requires polarisation maintenance. These applications include changing the wavelength of light or creating coupled photon pairs. Lasers produced by crystals are polarised already, as are diode lasers, but fibre lasers are generally not polarised. If you want to frequency double or create longer wavelengths, you need a polarised laser.’

Additionally, he explains, in some sensing applications, the polarisation state of the light used can be important. When using optical coherence tomography to produce images of the retina for ophthalmic purposes, for example, 90 per cent of patients’ eyes will produce a good image regardless of polarisation state, but clinicians have found that 10 per cent of patients have eyes that need polarised light in order to be imaged properly. ‘In sensing applications,’ Robertson explains, ‘PM fibres are used because, for whichever method the customer is using, the polarisation of the light has a big enough effect to need to be controlled. Polarisation has an effect in most things, but if it’s not a big effect it can be ignored. ‘

Standard optical fibres rely on the different refractive indices between the glass of the core and that of the bulk material. Light in the core is confined by a process of total internal reflection, bouncing off the junction between the materials and staying within the core. To make the fibres, Robertson explains, the manufacturers cut the middle out of a large cylinder of one glass, and then fill it with the core glass; a subsequent drawing process produces thin fibres with the same cross-sectional make-up.

‘In making PM fibres, instead of boring just the one hole, we bore two extra holes on either side, which are filled with a third kind of glass to become what we call stress members. When the glass is pulled, you’ll see the core in the middle with these two circular stress rods either side,’ says Robertson. These stress members produce an optical axis, which allows designers to tune the birefringence of the fibre.

Some PM fibres, Robertson notes, are called panda fibres, because their cross section resembles a panda’s face, and others resemble a bow tie, with triangular stress members either side of a circular core. Fibercore is a UK-based company producing PM fibres, particularly in the bow tie profile. Chris Emslie, chief executive, says that the company is manufacturing and shipping more PM fibres than anybody else, at more than a million metres a month. ‘Ninety-five per cent of that PM fibre goes into fibre optic laser gyroscopes, which are probably the most successful PM fibre-based components to have hit the market so far,’ he says.

A fibre optic laser gyroscope is similar to a ring laser gyroscope (RLG) – a Sagnac effect interferometer, with at least three coils of fibre (one for each axis of motion). The use of PM fibre reduces fading in the signal, Emslie explains, and this allows increased precision compared to non-PM fibre.

Gyroscopes are at the core of inertial measurement units (IMUs), used in aviation applications. The most accurate IMUs built in this way are accurate to 0.1 degrees per hour, Emslie says, and may use up to 1km of fibre per axis. Ultra-high-accuracy models may have up to 5.5km of fibre per axis, as used in specialist applications such as maintaining the orientation of a satellite in orbit.

Hollow ideas

Regular readers of Electro Optics will be familiar with photonic crystal fibres (PCFs) – a specialised non-linear fibre, in which a pattern of closely-spaced holes runs through the glass of fibre. PCFs are being used to commercial success as the enabling component in supercontinuum lasers, which provide broadband light white light sources, particularly well suited to use in biophotonics applications such as fluorescence microscopy.

Danish firm NKT Photonics produces PCFs and supercontinuum lasers. Kim Hansen, shipping and logistics coordinator for the company’s flexible products, explains how refractive indices are controlled in PCFs and related fibres: ‘The holes in PCFs have a purpose. There are no dopants in these fibres – they’re made from pure silica. Instead of having an up-doped core and down-doped cladding, we have un-doped core, and effectively down-doped cladding, because there are a lot of holes in it – the effective refractive index of the cladding is a lot lower, because it’s diluted by air.’

By tuning the hole size and pitch, and using air as one of the materials, NKT and other PCF designers can achieve immense flexibility in terms of tuning the properties of the fibre. ‘We can shift the wavelengths around very precisely,’ he says.

Hansen also notes that two other types of specialised fibres are being developed by NKT: large mode area fibres and hollow core fibres. ‘Large mode area fibres look a lot like non-linear PCF fibres, but they’re larger,’ he explains. ‘They contain air holes in the classic, hexagonal close-packed design, with five or 10 rings around a core. These are useful because they can transmit single mode light at all wavelengths, meaning that we can use the same fibre with a blue laser and a 1,064nm laser, and it will guide both of them equally well. These fibres are also well-suited to transmission of broadband sources, such as our supercontinuum light.’ These designs, he adds, can feature very large cores while still maintaining the single-mode character of the transmitted light, whereas this can be difficult when conventionally-doped fibres are used.

While PCFs can induce non-linear effects, Hansen explains that the company’s hollow-core fibres can eliminate non-linearity where it is not required. ‘Normal fibres guide light by the process of total internal reflection, but hollow core fibres have a completely different principle of operation. The hollow core sets up a photonic band gap between core and cladding, analogous to the electronic band gap present in semiconductors. It’s a glass-air matrix, with very large holes, and the band gap can be as wide as 100-200nm. A defect in such a structure can be the only place the light can go, and the hollow core in the middle serves as this defect.’ Because only two per cent of light in a hollow core fibre is guided in glass, and the remaining 98 per cent in air, a hollow core fibre offers a great reduction in non-linear effects, he adds.

The hollow cores, however, need not stay hollow; by filling the space with a liquid or a different gas, researchers are able to produce a spectrometer with an interaction length as long as they require, improving signal quality. Hansen adds that these fibres also have applications in fibre optic laser gyroscopes, but these devices have long development cycles, and it may be 10 years or more before commercial products are ready.

While it may be some time before the more exotic technologies find their way into mainstream applications, Robertson from G&H believes that focusing on the more technically complex fibre components is the best way for European companies to stay competitive: ‘We made a conscious decision 10 years ago to look at lower volume, higher value applications. PM fibres are important to us, because we can’t generally compete with Eastern companies producing low cost telecoms components.’ In these markets, it seems success depends on out-innovating the competition, rather than cutting costs.