A team of researchers at the Council for Scientific and Industrial Research (CSIR) in South Africa have developed the world’s first ‘digital laser’, whereby the laser mode can be altered as required and in real time. The device uses a spatial light modulator (SLM) as one of its mirrors, which can be controlled digitally to change the properties of the laser. Customised laser modes can therefore be generated on demand, by changing only a picture written to the laser mirror. The research has recently been published in Nature Communications.
‘The main advantage is that, rather than having a custom diffractive optic for every beam shape you want, now, with a relatively inexpensive LCD screen, you can get all of these shapes,’ explained Professor Andrew Forbes, who led the research team at CSIR’s National Laser Centre.
In conventional lasers the laser mode is either not controlled at all, or a single shape is selected by expensive optics. Alternatively, researchers would shape the laser light after exiting the laser using an SLM. The CSIR team’s approach, whereby the light is shaped inside the laser cavity, is a significant advance over the traditional approaches to laser beam control, which require costly optics and realignment of the laser for every beam change. ‘In that sense, it’s [a digital laser] far, far cheaper, because you need one laser instead of many lasers [to change laser modes],’ commented Prof Forbes.
He sees this technology having potential uses in additive manufacturing, optical communications, and in health and the medical field. The obvious application choice, he said, would be additive manufacturing, whereby a laser beam is scanned across layers of powdered material to fuse it together and build up a 3D structure. ‘This [digital] laser could be programmed in advance to output a circle when you want a circle and a square when you want a square. A movie is playing on the SLM in sync with the shape you’re trying to create. Much like the way CNC machines are programmed to machine away at material, the laser could be programmed to create that structure,’ he said.
Another potential use would be in communications, for instance in mode division multiplexing, which involves switching light signals travelling down optical fibres on and off very quickly. The idea is to use the spatial modes of the fibre as a way to encode information. Being able to fit 10 modes in a fibre would increase the bandwidth by a factor of 10. The digital laser creates modes of light on demand, which could potentially be used here. ‘Possibly, one would see the digital laser as being the input source to a mode division multiplexing communication system, using the patterns of light to carry information,’ commented Prof Forbes.
The team at CSIR demonstrated the power of the digital laser by programming the SLM to play a video of a selection of images representing a variety of desired laser modes. The result was that the laser output changed in real-time from one mode shape to another.
The SLM used in the demonstration version is not cheap, but Professor Forbes said, in principle, it doesn’t have to be a commercial SLM. He said the digital laser could be built very cheaply with the LCD displays used in projector technology, for instance. In addition, the technique only supposes knowledge of creating images and thus makes laser beam control accessible to a very wide audience.
The device as it stands is a low-power Nd:YAG laser, operating at 1µm. The group wants to take this work forward by developing the technology for higher powers, different wavelengths, and also investigating the frequency of the LCD screen, since basing a pulsed laser on this technique would require altering the beam shape very quickly.
Prof Forbes commented that there are many obstacles to overcome to engineer a digital laser, primarily around the losses of the device and the polarisation sensitivity. ‘The first time we tried this, it didn’t work,’ he said. ‘We’d been working with digital holography for a long time and we’ve been working on building custom lasers for a long time, and there aren’t many groups that do both. We have the skills to overcome the problems and the perseverance paid off.
‘I think now we’ll see many people making them [digital lasers],’ he added. The group is now working to make the laser more robust and to develop it for a particular application in combination with commercial partners.
Greg Blackman is the editor for Electro Optics, Imaging & Machine Vision Europe, and Laser Systems Europe.
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