The closest approach to bending light - laser materials processing using diffractive optical elements

TWI Bulletin, January/February 1999

Paul Hilton is Technology Manager - Lasers in the Arc Laser & Sheet department. His prime responsibilities are for strategic development of laser materials processing at TWI and business development.

For 30 years laser materials processing has used simple mirrors and lenses to produce small, round focused spots. Diffractive or holographic optical techniques, which make use of the phase coherence of laser beams, can now be used to 'tailor' the laser beam energy distribution to exactly that required for a particular process. As Paul Hilton reports this opens up the possibility of processing operations with lasers which are simply not possible using conventional focusing of the laser beam. Three examples of such processes are described.

Almost all high power lasers used for materials processing applications use a focused laser beam, invariably produced either by refractive optical techniques, eg simple lenses, or reflective optical techniques eg paraboloidal mirrors. These techniques produce the high intensity, symmetric energy distributions used in a wide range of laser welding and cutting applications. Energy distributions other than symmetric spots are not easy to produce using reflective and refractive optical systems. However, the use of diffractive or holographic optical techniques enables (in principle) any required energy distribution in the beam to be reproduced from the surface of either reflective or transmissive optics. This paper describes the work of a recent LINK Surface Engineering project, whose objectives were to produce diffractive optical elements for use with high power CO ₂ lasers and use these to demonstrate materials processing applications.

Key elements in the production of diffractive optical elements for use with high power lasers are the computation of the diffraction patterns, the efficiency of the process and the optic manufacturing technique.

The type of computer generated hologram used for this work is the kinoform. A kinoform transforms the phase of the illuminating beam and has the ability to diffract all illuminating radiation into a single diffraction image or 'order'. Kinoforms, therefore have an ideal diffraction efficiency of 100%. The kinoform pattern is calculated from a knowledge of the desired laser beam energy distribution, and the intensity profile of the incoming beam. Figure 1 shows a kinoform phase pattern designed to produce a uniform rectangular intensity profile.

Fig.1. A kinoform phase pattern 24 x 35mm designed to reconstruct a uniform intensity rectangular profile. In this case the kinoform phase has been quantised to 256 levels which are represented by variations in grey level across the film surface. This type of pattern is developed on 35mm negative film.

In order to improve the diffraction efficiency, multiple level techniques in the diffraction patterns and diffractive surfaces have been used. Techniques have been developed which avoid the very expensive 'step and repeat' method of using multiple diffraction patterns laid on top of one another, in favour of a single pattern containing sufficient information for the photo-lithographic manufacturing process to produce, in a single operation, a surface structure which can manipulate the phase of the incident light.

This culminates in a high efficiency and high accuracy transformation of the input laser beam energy by the diffractive optic. The process results in a diffractive pattern in photoresist laid on the top surface of a flat silicon 'mirror'. The diffractive elements produced for this work all operate reflectively, phase changes being imposed on the incident beam by spatial variations in depth across the photoresist. Onto the photoresist surface is then evaporated a thin metallic layer reflective to the CO ₂ laser wavelength, which protects the photoresist from the incident laser beam. The resultant mirror is then used as the final focusing element in the optical beam delivery system. The optics produced have been tested at laser powers up to 4kW for extended periods without sustaining damage.

Results

Lasers are not efficient as bulk heating devices, however, they are efficient at heating discrete areas very rapidly and are therefore suitable for certain transformation hardening applications. Use of a de-focused laser beam for transformation hardening, with its Gaussian distribution of energy produces a very lenticular shaped transformed area, the case depth being largest in the centre where the applied power density is usually the highest which will prevent surface melting. A diffractive optic has been manufactured to produce a line of uniform energy with peaks of higher energy superimposed at each end of the line. Using this distribution, hardened tracks ~8mm wide have been produced with a very uniform case depth. The uniformity has been produced by a correct balance between the energy in the peaks at the extremes of the track and that in the centre. Figure 2 shows a 'spinning needle' laser beam analyser scan of the energy distribution used. The case hardened track produced by this optic is shown in Fig.3.

Fig.2. Spinning needle laser beam analyser scan of the 'twin peaked' energy distribution after reflection from the diffractive optic.

Fig.3. CO 2 laser transformation hardening of 0.4%C steel using the 'twin peaked' diffractive optic. Track width 7.5mm.

A large potential application for diffractive optics is for repetitious laser welding or soldering of electronic components with complex leg arrangements to circuit boards. The advantage of a diffractive optic is that the laser beam can be split into a pattern of discrete spots to perform this operation without movement of either component or laser beam. A 16 point printed circuit board has been used as a test component to demonstrate discrete point laser soldering. A diffractive optic, designed to split the laser beam into 16 even intensity, but asymmetrically distributed spots were produced and successfully used to solder the leads to the board after manual application of solder paste to each pin. The potential for soldering a whole component in a single laser pulse has obvious advantages for mass production. Figure 4 shows the circuit board used for these trials while Fig.5 shows the spinning needle laser beam analyser scan of the beam used for soldering. Fig.6 shows a close up of a single connection after soldering.

Fig.4. Circuit board before the addition of solder paste. Note the asymmetric distribution of connections.

Fig.5. Spinning needle laser beam analyser scan of the beam used for soldering the 16 connections shown in Fig.4.

Fig.6. Close up of a single connection after soldering.

Fig.7. Cut seal joint in 0.1mm thick polypropylene sheet produced using a diffractive optic. Weld width ~2.5mm.

The benefits of welding and cutting plastics using lasers largely stem from the process speeds available and the nature of the process. For high speed cutting the laser beam is used at or near focus, while for welding applicationsit is common to work in a defocused condition to produce a wide weld. Indeed by correct positioning of the focused spot, simultaneous cut seal joints can be produced, but these are generally not particularly strong due to the narrowweld zone produced. A third diffractive optic has been designed and manufactured to produce simultaneously a wide low intensity energy distribution to make a weld, with sharp centre peak of intensity to produce a cut. When this opticwas used to cut and weld two sheets of 0.1mm thick polypropylene, a consistent weld some 2.5mm wide was produced on each side of the cut. A cross section of the weld produced can be seen in Fig.7.

Conclusions

A potentially cost effective technique for the design and fabrication of high efficiency reflective diffractive optical elements for CO ₂ laser materials processing has been demonstrated. Further work is anticipated which will extend the computation and fabrication techniques to laser wavelengths which will include Nd:YAG and diode lasers.

Diffractive optical techniques have been successfully used to produce tailored energy distributions which have been applied to industrial applications in the areas of surface engineering, plastics processing and printed circuitboard soldering. In all cases, the processes concerned would have been impossible or difficult using conventional reflective or refractive CO ₂ laser beam focusing systems.

Acknowledgements

This work was performed within the recent LINK Surface Engineering Programme, supported by the Department of Trade and Industry and the EPSRC. The author would like to thank his colleagues at the University of Loughborough whomanufactured the diffractive optics, in particular Dr John Tyrer, Ms Sara Noden and Mr Colin Cole, and the industrial partners in the LINK Project who have given permission for some of the results of the project to be reported here.