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Femtosecond laser technologies

Photonics - Femtosecond laser technologies

Femtosecond laser has been used in chemistry, metrology, biology, eye surgery and fabrication of photonic devices. The highly nonlinear absorption by a tightly focused laser pulse can induce strong material modification in a subwavelength scale, enabling an unprecedented freedom for highly precise microfabrication using lasers.

Key research areas 

  • Direct inscription of optical waveguide of 3D profiles
  • Microfluidic for medical/clinic applications
  • Fibre gratings made with fs laser survives up to 800° C
  • Microslots for highly sensitive bio/chemical sensing
  • Microstructures for calibration of Optical Coherent 
  • Topology (OCT)
  • Micromachining of materials

Advantages of micro-fabrication with femtosecond laser

  • Flexible 3D-positional control of highly precise energy deposition with sub-µm resolution.
  • Wide choice of materials, virtually all dielectrics.
  • High thermal and mechanical stability of the structures. 
  • Enormous, controllable change to materials
  • Compatibility with complementary techniques, particularly laser-based, such as interference lithography or micromachining; or applications. 
  • Unique possibility of micromachining of microchannels and voids in the bulk of materials

Possible applications

fs laser insribed fibre Bragg grating
fibre Bragg grating inscribed with femtosecond laser
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
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. 
femtosecond laser inscription
fs inscription using adaptive optics
femtosecond laser inscription apparatus with adaptive optics
femtosecond laser inscription
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.

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

 

fs laser inscribed waveguide

Experimental  activity  on direct  femtosecond inscription   in   lithium   niobate (LN) crystals has been conducted under the framework of Leverhulme Trust research grant RPG-278.

The work involved the mapping of the inscription parameters suitable for producing uniform waveguides in order  to optimise transmission loss and refractive index contrast.

Extensive trials resulted in refractive index (RI) values up to ten times greater than achieved by other groups working in this area. Induced refractive   index  contrasts  between the modified and intact volumes of lithium niobate up to -0.02 have been demonstrated.

This  development  has resulted  in  promising designs for low loss micro-structured waveguides operating  at wavelengths up to 3.5 microns.

An agreement has recently been reached with Birmingham-based Arden Photonics, who are now selling OCT calibration phantoms designed and fabricated by researchers at Aston University.

OCT is a rapidly expanding measurement technique especially in the medical field with many applications including the measurement of eyes for clinical purposes.

One issue with this technique is the lack of available calibration sources allowing the user to quickly validate the performance of the system and ensure that it is still working optimally.

The development work carried out at Aston was in collaboration with the National Physics Laboratory. Three-dimensional calibration test phantoms were designed and fabricated as part of the project.

The phantoms being sold by Arden Photonics are fabricated in fused silica and can be used to measure sensitivity, distortion, spatial resolution, and the point spread function of a system.

Femtosecond lasers can be used to directly inscribe phase masks in fused silica substrates, both on the surface and sub-surface.

This has some advantages over commercial masks in terms of cost and turn-around time. In addition, when the pattern is below the surface it becomes very robust.

To compensate for the size of the beam the masks generate second and third order Bragg gratings instead of a conventional first order Bragg gratings. This is not in itself a major drawback, but the effective laser induced “etch-depth” of the mask needs to be optimised with respect to the UV wavelength used for the Bragg grating inscription.

FBGs with reflectivity greater than 90% have been written in hydrogen loaded SMF28 with masks produced in this way.

Similar patterns can be inscribed on the end faces of fibre pigtails – diffracting the light as it exits the fibre. Some of this work was carried out with funding for a short-term scientific mission from the COST Action TD1001: Novel and Reliable Optical Fibre Sensor Systems for Future Security and Safety Applications (OFSeSa).

Interest in Mid Infra-Red (Mid-IR) photonics has exploded over the last decade. We have been working on experimental and numerical studies of femtosecond laser inscription and the characterisation of waveguiding structures in crystals for applications in this area.

Waveguides sculptured in bulk glass or crystal offer many advantages since such an approach allows the natural integration of multiple components within the same optical motherboard. Inscribed waveguide devices have been fabricated and characterised in RbPb2Cl5 (RPC) and β-BaB2O4 crystals.

This work has been performed in collaboration with Prof A. Okhrimchuk in the framework of the Leverhulme Professorship hosted by AIPT.