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applications optical coherence tomography


combine it with multiphoton tomography to develop novel functional capabilities enabling “morphofunctional” performance,’ explains Peter Andersen, from the Technical University of Denmark, and FUN OCT coordinator. ‘These methodologies will enable unprecedented non-invasive detection of depth- resolved physiological, metabolic and molecular specific tissue information.’


FUN OCT consortium members have made considerable progress in a number of areas including polarisation-sensitive OCT, Doppler OCT and light source technology. However Andersen highlights two areas of the project that he thinks are particularly exciting. ‘Researchers at the Ludwig- Maximilians University (LMU) in Munich, Germany, have used a newly developed laser source – the Fourier-domain mode-locked (FDML) laser – to obtain amazing images of the eye,’ says Andersen. ‘Also, researchers in Austria have combined photoacoustic tomography with OCT to give high- resolution 3D imaging of vascular structures to depths of up to 5mm.’


a new laser source


The FDML laser was invented by Robert Huber while he was working with OCT pioneer James Fujimoto at the Massachusetts Institute of Technology. It is fundamentally different to standard tunable lasers. In an FDML laser the cavity length is increased up to several kilometres. Huber, who is now at the LMU in Munich, Germany, says: ‘We developed the FDML in order to overcome the tuning speed limitations of standard lasers. We use a semiconductor optical amplifier as our gain medium and a fibre spool is used to increase the cavity length. The challenge is to synchronise the optical round trip time with the tuning rate of the filter so that by the time light with a certain wavelength arrives back at the filter, the filter has been tuned over exactly one wavelength sweep.’


www.electrooptics.com


The FDML laser was developed for use in swept-source OCT, where each sweep of the laser generates one depth scan, or A-scan. Unlike other swept sources that are currently on the market, the sweep rate of the FDML laser is only limited by the speed of the filter and not by fundamental laser physics. This has enabled Huber


and his colleagues to achieve sweep repetition rates of up to 5MHz, which makes this system more than 50 times faster than current commercial instruments. ‘Tuning speed in standard setups is limited by the time required to build up lasing from amplified spontaneous emission background at each new spectral position to


which the laser is tuned,’ says Huber. ‘Our system is only limited by how fast we can drive the fibre Fabry-Perot tunable filter, which is used as the optical bandpass filter. It is really surprising that the filter can be controlled precisely enough so that it can be driven at such high speeds with sufficient repeatability for synchronisation. This enables us ➤


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