FEATURE PHOTONIC CRYSTAL FIBRES


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hole pattern is maintained in both fibres, while still producing a strong enough join. Similarly, fibre amplifiers require that pump light, which propagates in the cladding, be removed by a process called mode-stripping. It can be difficult to do this without affecting the fibre core. In addition, joining standard fibre to PCF presents challenges because of the different relationship between physical geometry and optical behaviour, making mode matching difficult. Gooch & Housego holds a patent


for splicing specialised fibre, which is applicable to PCF. ‘PCF is similar to the first commercial


lasers: it’s a solution looking for a problem,’ said Marvet. But their microstructuring offers infinite opportunities, where standard fibres are limited by the properties of the solid medium.


Making connections French optics and laser technology centre Alphanov offers a comprehensive line of connectors for joining hollow-core and large area PCF. It, too, partners with leading PCF supplier NKT Photonics, and addresses the interfacing needs of NKT’s entire product line as well as other manufacturers. ‘It’s not easy to find a process to cleave and splice and add an end cap, without jeopardising the microstructure and beam quality of the laser going through the fibre,’ said Alexandre Loulier, Alphanov engineer. Alphanov helps clients to reach


innovative solutions for specific applications. As an example, the centre has developed a connector for hollow core PCF, which can handle the powerful pulses of a


Hollow core fibre


femtosecond laser. The fibre may be useful for beam delivery during micromachining with ultrashort pulse lasers; but connectors are not available off-the-shelf. Alphanov also develops adaptors for splicing fibres with different mode field diameters, such as injecting a single mode fibre laser (10μm) into a 40μm PCF. ‘This is how you play with PCF to build what you want,’ said Loulier. These tools have led them to develop a fibre amplifier using a mode fill adapter to improve efficiency. The larger core allows for more power to propagate at a single mode with no loss. When PCF came along some 20 years


ago, it was not industrial-grade, but was made especially for scientists. Today, NKT offers an industrial line of PCF that can be used with more powerful lasers at different wavelengths. ‘Alphanov transforms any PCF into a patch-cable that is useable for everyone, for every industry, as any standard fibre,’ added Loulier.


State-of-the-art sensors While high power lasers are a prominent PCF application, the fibres have garnered interest in passive sensing applications. They take advantage of the large index contrast between core and cladding. ‘The PCF looks similar, superficially. But sensing fibres will not contain any active materials, like fluorescent material that you can pump and lase at certain wavelength. And they’ll have smaller core areas,’ said Fraunhofer’s Eilenberger. In recent decades, photonic biosensors


have attracted attention because they can characterise components in a biological sample, based on label-free optical tools. Now, scientists are combining PCF technology with plasmonic science to achieve the highest sensitivity and best detection accuracy. Surface plasmon resonance (SPR) uses


Cross-section view of proposed PCF biosensor


24 Electro Optics November 2017


the property that a thin layer of metal (gold, silver, copper, aluminium, etc) on a high refractive index glass surface can absorb laser light, producing electron waves or


PCF connectors


‘surface plasmons’ on the plasmonic metal surface. If a mobile molecule – an analyte – binds to a receptor on the metal surface, the refractive index of the metal film changes and can be detected in light reflected from the surface. Depositing the metal film onto PCF, instead of conventional optical fibre, offers a more flexible design because of a large index contrast between core and cladding. Here, SPR phenomenon can be easily observed. In a collaboration between the Australian


National University and the University of Birmingham, Rajib Ahmed and Ahmmed Rifat proposed a simple PCF biosensor


“It’s not easy to find a process to cleave and splice and add an end cap, without jeopardising the microstructure”


design in which chemically stable gold is deposited on the outer surface of the PCF. ‘External metal coated PCF sensors are more feasible from the fabrication point of view, compared to the internal metal coated sensors,’ said Rifat. The unknown analyte is simply flowed across the outer surface of metal layer, and detected by the resonance wavelength shift (either red or blue shift). Ahmed and Rifat are developing a sensor


in the refractive index range common to most biological molecules (1.33 to 1.37). The proposed design will reduce fabrication complexity compared to existing sensors, and models suggest it can achieve superior sensitivities. When fabricated, they hope it will find the broad range of applications, such as medical diagnostics, biochemical, environmental monitoring and food safety. PCF is one of the most active fields of


current optics research. While yet to find a foothold in the mainstream industry, these specialised optical fibres offer a diverse and growing range of applications that will drive future markets. EO


@electrooptics | www.electrooptics.com


Rifat et al, J Nanophotonics 12(1), 012503 (2017)


Fraunhofer IOF


Alphanov


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