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
In comparison with UV writing method, femtosecond laser inscription technique has the advantages of:
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.
Dr. Kate Sugden, Dr. Vladimir Mezentsev,Dr. Mykhaylo Dubov, Dr. Kaiming Zhou