Laser Material Processing

About

The Aston Institute for Photonic Technologies has an accomplished record of utilising UV lasers to create in-fibre gratings, with particular successes in exploiting photosensitive Ge-doped or Boron-doped optical fibre with different geometries. These include single mode, multimode, D-shaped and multicore fibres.

Recent research has advanced fabrication technology to fabricate gratings with complex design. The use of femtosecond fabrication has opened up new possibilities in device design. Work also continues in the areas of tilted grating and gratings in novel optical fibre. Together this research unlocks a host of potential applications in fields such as optical sensing, fibre laser technology, and optical communications.

The successful microfabrication of new photonic devices requires lasers capable of ultrashort pulse durations as well as high peak power. In the Laser Material Processing group, we utilise femtosecond (fs) lasers with these characteristics to fabricate photonic devices at the micro-scale. These produce unique interactions with matter, including two-photon absorption, index modification and direct ablation.

We have a number of UV and femtosecond-capable facilities at Aston. They have been used in a range of applications, including:

Fibre grating microfabrication
Optical waveguide in nonlinear optical materials (e.g. LiNbO3)
Hydrophobic surface nanostructures
Microstructures within glass material
Two photon polymerisation in polymers.

These photonic devices can be used in a wide range of applications, including sensing, food safety, agriculture, healthcare and medicine.

Our Projects

Single mode optical waveguide in LiNO3

Integrated Photonic Circuits (IPC) have been widely used in information processing and optical communication. Particularly, Lithium niobate (LiNbO3) offers incredible versatility when used as a substrate for IPC owing to its wide range of transparency, high second-order nonlinearity and commercial availability. Waveguide in LiNbO3 are conventionally made with lithographically. This approach includes a number of processing steps such as proton exchange, titanium in-diffusion, iron implantation and dry etching, and is costly and the optical mode has large mismatch with that of optical fibre.

In this research, we used direct fs inscription of waveguide into nonlinear LiNbO3, offering improved versatility in the choice of processed materials and greater flexibility in creating micro-structures in the three-dimensional space.

We fabricated and optimised optical-lattice-like depressed cladding waveguides in z-cut LiNbO3. The waveguides featured low propagation loss for the TE polarization state and moderate loss in the TM polarization. Fundamental mode guidance was observed across a wide portion of the spectrum from visible to near infrared. The low attenuation and high thermal stability of the waveguides suggest promising applications in telecommunications, nonlinear integrated optics, and harsh environment sensors.

Femtosecond laser inscribed advanced calibration phantom

Optical coherence tomography (OCT) has developed rapidly and is widely used in different fields such as biomedicine and optometry. The characterization and calibration of OCT systems is essential when testing the system and during normal use to ensure that there is no misalignment or distortion that could affect clinical decisions. Imaging distortion is a significant challenge for OCT systems when viewing through non-planar surfaces. In collaboration with Arden Photonics, we have developed a new multi-purpose plano-convex OCT phantom which is designed to be used for OCT characterization and calibration as well as to validate the post-processing algorithm for the imaging distortion of the OCT systems.

A femtosecond laser direct writing technique is used to fabricate this phantom which consists of a landmark layer with radial lines at a 45-degree angular spacing inscribed at 50μm in apparent depth (AD) underneath the planar surface. Below that there are a further 8 layers of a spherical inscription pattern which has a 150μm (in AD) separation between each layer. The first spherical layer is located at 150μm (in AD) underneath the planar surface.

The spherical pattern overcomes orientation issues seen with existing calibration phantoms. The landmark layer means that we easily tell the exact location when scanning which will also benefit the image distortion correction process. Overall, the work developed new phantom designs with circular geometry that overcome the issues when imaging under an OCT system.

Two photon polymerisation

Multiphoton direct writing is a photonic technology which is based on femtosecond laser pulses application and enables the fabrication of any designed 3D micro-devices with features on submicron-relevant length scales (i.e. 100 nm-scale precision). This unique method is used for additive manufacturing of polymer-based micro-optical and micro-fluidic elements with free-form 3D topographies that may be utilised for the development of precise, micro-sized and low-cost micro-sensors.

The fabrication system for multi-photon lithography (based on the Chromacity fs laser) has been designed and assembled at AIPT to fabricate micro-optical elements directly at the end-face of optical fibres. Combination of the mature technology of fibre optics and high potential of modern nano-photonics will offer new opportunities in the fabrication of low-cost and high precision sensors. The system will be used to design novel fibre-edge based gas sensors and fabricate them using a combination of 2PP lithography and femtosecond laser processing. We will apply the technique of femtosecond laser structuring of photosensitive materials recently developed at Aston to a variety of acrylate and epoxy materials to develop fibre-based gas sensors (for CO2, CH4, NH3, and other gases).

Direct 3D laser writing will provide us with many interesting possibilities for the design of novel optical systems and efficient light transformations. The new techniques for the flexible manufacturing of micro-optical and microfluidic elements for innovative devices and systems will be of wider interest in gas diagnostics research beyond food, agri-tech and biomedical applications.

Micro/nano surface showing wettability

As a result of the ultrashort duration of femtosecond laser pulses a surface can be irradiated with a laser beam for a variety of microfabrication processes the nature of which depend on the processing parameters. For example, the surface can be made to exhibit features like nanostructures and nanospikes. These surface nanostructures exhibit unique physical properties including the wettability and much reduced reflection.

Fs lasers are capable of processing a variety of materials including metals, semiconductors and dielectrics such as glass and ceramics, giving rise to superhydrophobic surface, which repels water, or a super-hydrophilic, which spread the water droplet to the maximised area. Both effects are very useful for industrial applications: the superhydrophobic surface can be used for self-clean surface while the superhydrophilic surface can be potentially used for remove oil.

In the industrial applications focused project ETICC a surface of stainless steel was process with femtosecond laser at AIPT demonstrating improved hydrophobic behaviour.

UV Inscription of tilted fibre grating

AIPT developed tilted fibre grating (TFG) technology, a type of fibre grating devices, which can couple light to cladding modes and radiation modes, presenting unique optical properties depending on the tilting angle and exploited them for many applications, including optical fibre sensing and fibre laser.

The large angle TFGs, coupling light to high order cladding mode which possess higher proportional of evanescent wave, show higher sensitivity to external refractive index and have been used for bio-chemical sensing. In a recent research of AIPT, such an effect was strengthened and used for ultrafast humidity sensing and human breath monitoring by coating of a large angle TFG with graphene oxide (GO) which has a high surface-area-to-volume ratio and strong absorption property in a wide wavelength range.

45° TFGs, showing excellent polarization depend radiation mode coupling and used as in-fibre polariser, were utilised to implement Loyts filter and Solc filter. These filters consist of pairs of 45°TFG and sand-witched polarization maintaining (PM) fibre shows prominent merits such as extremely simple and robust structure, narrow bandwidth, spectral flexibility, and low cost is highly desirable for in-fiber laser, spectral imaging, optical communication and sensing systems.