ULTRAFAST LASERS FEATURE
Toptica’s new
Femto Fiber Ultra series permits femtosecond pulses with up to 5W output power at 789nm or 1,050nm
the devices’ manufacturers. ‘For us it’s one of the four main areas that our ultrafast lasers are used in,’ Chui explained, the others being medicine, micromachining and high-energy physics. Spectra-Physics predominantly provides femtosecond lasers for life science applications, with picosecond lasers more commonly being used for industrial micromachining applications, according to Chui. While bio-imaging isn’t a new application
The beams in a sum frequency generation
spectroscopy module
Physics. ‘The primary purpose in this case is to study the brain’s structure and function, which could lead to a solution of solving neuro degenerative diseases.’ According to Chui, if a continuous-wave
laser were to be used in a standard confocal microscopy setup, fluorescence would come from everything above and below the focal point of the laser, as the excitation wouldn’t be localised, which generates background noise in the imaging process. For a multiphoton process using ultrafast lasers, the only place where the excitation could occur is extremely localised, due to the photons having to converge at an exact location, therefore preventing background noise and enabling clearer imaging. Ultrafast lasers are the only way to
achieve this type of imaging, which provides a very significant opportunity for
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for ultrafast lasers, new techniques are being developed in the field to increase its capability. ‘There’s a lot of development focused on it,’ confirmed Chui. ‘For the multiphoton techniques, people are moving from using two-photon excitation to three-photon excitation, which requires three photons to be in the exact same place and time to produce an excitation. With that you get even more localised excitations and can image at higher depths into tissue.’ With two-photon excitation, depths of up to approximately 1mm can be imaged in live tissue. However, by switching to three-photon excitation, researchers could potentially reach depths of over a millimetre, which in a mouse brain would enable the imaging of the emotional and memory centres. ‘This is one very interesting direction that has developed in the last year or two,’ Chui commented. To aid in increasing the depth of multiphoton imaging, manufacturers are now working to offer ultrafast lasers with longer wavelengths. While traditional imaging is performed using green fluorescent protein, which gets excited at around 900nm in a two-photon setup, the light can become scattered at this wavelength as it goes deeper in the tissue. When a longer wavelength is used, however, scattering drops off proportionally and a larger depth of imaging can be reached. ‘There’s a lot of work in moving to longer wavelengths with the two-photon technique.
Sources that both we and others have developed are moving out to 1.3µm and are producing 100fs pulses,’ said Chui. ‘With three-photon techniques, people are looking to move further out from 1.3µm to 1.7µm in the future.’ Increasing the power of these longer
wavelength lasers is also important, in order to improve their imaging quality. However, manufacturers must do this within the limitations of live tissue, as a significant increase in power could lead to it being damaged when examined. Additionally, because of the large amounts of water in live tissue cells, care must also be taken when choosing higher wavelengths to select those that aren’t significantly absorbed by water, to ensure that imaging can be performed effectively.
“There’s a lot of work in moving to longer wavelengths with the two-photon technique. Sources that we and others have developed are moving out to 1.3μm”
Earlier this year Spectra-Physics released its Insight X3 ultrafast laser for multi- photon imaging, a single-source device with a tuning range between 680nm to 1,300nm, nearly double that of Ti:sapphire ultrafast lasers. Suited to delivering higher powers at longer wavelengths – more than 2W at 900nm and 1.4W at 1,200nm – the Insight X3 fits within the limitations of live tissue imaging, and has apparently been well received by the company’s research customers, according to Chui. Higher power ultrafast lasers can also be
coupled with optical parametric amplifiers (OPAs) to deliver suitable wavelengths for multiphoton imaging. Back in June, Spectra-Physics’ new 100W Spirit 1030-100
g December 2017/January 2018 Electro Optics 45
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