Femtosecond laser technologies

Photonics - Femtosecond laser technologies
Whilst having relatively low average powers femtosecond lasers have exceptionally high peak powers that that last only for tens to hundreds of femtoseconds. When focussed the high peak pulse powers, with an energy density equivalent to that at the surface of the sun, enable materials to be modified at a fundamental level.

The Aston Institute of Photonic Technologies has three femtosecond laser systems covering a range of repetition rates, peak pulse energies and wavelengths. This variety of systems allows us to work effectively in a number of different areas some of which are described below.

Micro-machining of optical fibre

Fibre Bragg Grating:

In comparison with UV writing method, femtosecond laser inscription technique has the advantages of: 

  •  Material independent, all glass fibre and polymer fibre can be used while UV lasers require photosensitivity and Hydrogen loading
  • All inscription methods are applicable: phase mask technique , direct point-by-point inscription and holographic interfering
  • Strong thermal resistance, survivable up to 800˚C while UV laser inscribed grating decays quickly above 300 ˚C

 Microstructures inscribed in optical fibre

femtosecond laser inscription
fs inscription using adaptive optics
femtosecond laser inscription apparatus with adaptive optics
femtosecond laser inscription
Using the femtosecon laser to direct to inscribe structure in  the  optical  fiber,  there  would be distortion to the focus volume of the laser beam ]. This effect is caused by the cylindrical geometry  of  the  fiber  and  is  more  pronounced  as  the  focus  volume  moved  deeper  into  the fiber, limiting the depth of inscription. As Figure shows, the focused laser only left a track on the front part of its traveling path for a through line across the fiber. When it ran closer to the core, the focus volume was expanded, yielding a broader but shallower modification to the glass which gradually disappeared. The deepest distance where the fs exposure still worked depended on the energy of the laser pulse but generally it was limited to within only half of
the fiber’s diameter. To circumvent the cladding’s curvature, a glass slip was placed on top of the fibre and index-matched to the fiber with oil as Figure to the right discloses. The focus spot was then freely moved within the fiber without being distorted. Shown in Figure is a much morewell-defined through line made with this way.

Micro-structured devices

Using the above technique , a number of micro-structures can be created within the optical fibre, such as Microchannels, microslots etc. Of course, fibre Bragg grating is included.
fs laser inscribed waveguide

Planar waveguide

  • We developed the technique to overcome aberration of laser beam focus by the cylindrical geometry of optical fibre which hinders writing structures of high quality.
  • With a simple index-matching adaptive optical setup, the astigmation can be corrected to give clear inscription
  • Fs laser inscribed material present much improved etching rate (> 100 times faster), allowing for hollow structures of high aspect ratio being engraved in fibre.
  • Applications include high sensitive bio/chemical sensing and gas detection

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