ISSUE 114 AUTUMN 2024 BEAM DELIVERY


THE LASER USER


MINIATURE OPTICAL DELIVERY HEADS


FOR RESTRICTED ACCESS AREAS MIKE O'KEY


We can all probably think of situations where we would like to use laser machining but the physical or safety implications of access are too restrictive. This may be cleaning or repair inside a jet engine, or the complex pipework within a chemical facility or nuclear reactor. Historically the only option in situations such as these has been partial or full disassembly which can be time consuming, costly, and in some cases hazardous.


Traditionally lasers have been delivered to the processing area either in free space, within an articulated arm or through a fibre. Although fibre-delivered lasers provide a very flexible means of delivery, they can be too bulky for many applications due to the fibre protection jackets and collimator diameters.


Fibre delivery is routinely used in medical scenarios but the manipulation of the beam at the distal end is very limited, and conventional techniques such as galvo scanners, moving optics or motion stages are just not practical.


After some successful feasibility studies and process parameter development at the Rolls- Royce University Technology Centre, University of Nottingham [1], Rolls-Royce approached OpTek Systems to develop a fibre delivered solution for a particular in-engine repair procedure. To address this Optek developed a fibre delivered laser system with a miniature beam manipulation head, more details of which are discussed later in this article.


OpTek has developed heads ranging from 8 mm to 25 mm in diameter, capable of handling average laser powers of 10 to 100 W, peak powers of tens of kW and mJ pulse energies from a 1064 nm pulsed ns laser source. These have been successfully tested within complex geometries, such as through the various stages of a gas-turbine aero-engine, and shown to be capable of cutting and surface-processing difficult high-temperature materials such as aerospace alloys. This article describes two of these devices.


Challenges of high power fibre delivery 24


It is well known that kW of power can be delivered down an optical fibre - fibre laser manufacturers have demonstrated this. However, the core diameter will probably be too large, the coupling schemes too complicated and the delivery head too big for Optek's applications.


The two main hurdles to overcome are efficiently launching light into the fibre (proximal end) and managing and manipulating the light from the output (distal end).


The numerical aperture (determined by the relative refractive indices of the core and cladding) is a measure of the acceptance angle of the fibre. It determines how strongly a fibre guides light, and so how resistant it is to bend- induced losses. This and the core diameter will determine the design of the coupling optics used to launch light into the fibre.


Getting this coupling wrong can result in several undesirable consequences including damage to the fibre or coupling into the cladding. Light in the cladding results in losses, uncontrolled output at the distal end and, in the worst case, leaking of light into the outer coating.


A higher numerical aperture is better for guiding in the fibre core but is not necessarily desirable for the application described here because it also largely determines the spread angle of the light emitted from the distal end. Ideally this must be as low as possible as managing the highly divergent beam within the small head cross sections OpTek is trying to achieve can be problematic. A trade-off is therefore required when selecting the appropriate fibre.


Optical delivery heads


To be able to manipulate the beam from the distal end to achieve controlled laser machining, OpTek's application requires very small actuators. Off-the-shelf solutions are very limited and so alternative small scale manipulation techniques required development.


Micro-electro-mechanical system (MEMS)


Rolls-Royce and University of Nottingham initially approached OpTek Systems to help develop a mirror mounted on a MEMS device (granted patent US-10207367-B2).


For one of the smallest heads, OpTek worked with a MEMS manufacturer to develop a system which could provide X/Y angular motion (and indeed a degree of z (focus) adjustment) and handle average powers in excess of 20 W. The mirror must be low mass to be supported by the MEMS actuators but robust enough to withstand the laser power requirements. The larger the mirror, the lower the optical loading, improving the peak-power-handling capability. But the angular range of the mirror deflection must also


be adequate to provide a usable machining area, and achieving high angles favours a smaller mirror. Once again, a trade-off is necessary.


The reflectivity of the mirrors needs to be very high as the MEMS support structure provides only a limited thermal conduction path for any energy which is absorbed in the mirror. This would translate to a low average-power handling capability, and hence inconveniently long machining process times. Several mirror substrate materials were tested with high reflectivity coatings, and silicon was found to give the best performance in terms of low mass and coating performance.


For use at 45 degrees incidence, elliptical silicon mirrors were laser cut from a wafer for mounting on the MEMS platform. Figure 1 shows the MEMS prior to mounting in the head. The mirror can clearly be seen in the centre. The mirror measures 3mm across the major axis giving an indication of the small scale of the device.


The MEMS system shown in Figure 1 was built in to an 8 mm diameter head, a schematic of which is shown in Figure 2. The head can be attached to a manual endoflex manipulation arm or as a robot end effector. The laser delivery fibre, collimation and focusing optics and MEMS


Figure 1: MEMS unit with mirror


Figure 2: 8 mm outside-diameter MEMS head. Input fibre shown in turquoise on right.


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