LASER 2003: Diffractive optics aids design

30 May 2003

Diffractive optics promises a new design freedom for engineers thanks to smaller, cost-effective optical parts.

From LASER 2003. World of Photonics Visitor Magazine

As photonics continues to penetrate the world of consumer electronics and telecommunications, there is an increasing need for custom-designed optical parts that are low cost, compact and lightweight. Recent advances in materials and manufacturing technology suggest that the answer may lie with diffractive optical elements (DOEs), often also referred to as micro-optics.

"More and more products are using semiconductor lasers, LEDs and new display technologies," explained Jyrki Saarinen, the co-founder of Heptagon, a Finnish-Swiss developer of DOE technology. "A common feature of these products is that price, quality and size are critical and these are all characteristics that ideally suit diffractive optics."

Put simply, DOEs use microstructures that are embossed or etched into a suitable optical substrate, typically glass or a polymer, to create elements that can manipulate light beams in a desired way. The technology can also be integrated with conventional optics to create a monolithic hybrid design - a single compact optical component that combines several functions.

"We are producing a small glass part that contains a mirror, prism and two diffractive elements for Leica Geosystems," commented Saarinen. "The prism is a conventional optic, the mirror is made by evaporation on one side and the diffractive elements are replicated directly onto the prism."

Over the past couple of years, well-known consumer and telecoms equipment manufacturers have started to use DOEs in their products. For example, Canon has incorporated a hybrid lens into one of its cameras and several commercial designs of CD/DVD pickup optical systems now use DOEs and hybrid lenses. JVC also uses a diffractive pattern-generator for an autofocus system in a camcorder, while Agilent has incorporated a DOE into a fibre-optic transceiver.

Rather than simply focusing light to a point like a lens, DOEs are ideal for more demanding tasks such as optical pattern generation for bar-code scanners and displays. Aside from growing consumer applications, the most important markets for DOEs to date are high-performance fine-pitch gratings for spectroscopy and security holograms.

Another area that's becoming increasingly important is the efficient collection of light from an array of emitters, such as a cluster of LEDs. For example, a micro-optic featuring tiny lenslets can route light signals from several optical fibres into a multichannel data receiver for telecoms. A slightly different design can take the light from numerous LEDs and "smear" it to create uniform illumination. The latter could prove useful when it is not desirable to see the individual LEDs, for example in brakelights for cars, and white LED lighting.

Not only can micro-optics perform sophisticated imaging tasks that are difficult with bulk optical parts, but they also offer the potential for cheap mass production. Replication technologies that rely on injection moulding or embossing could now enable the manufacture of polymer micro-optics in their millions for a unit price of less than a dollar.

Over the past 10 years, these replication technologies have been refined and there are now ultraviolet (UV) embossing techniques that allow the wafer-scale fabrication of double-sided polymer micro-optics.

Mirror mirror The first and most expensive step of the UV embossing fabrication process is to make a master mould that is the mirror image of the part to be made. This mould, which is usually made by etching a piece of glass or metal, is then used to "stamp" or emboss a UV-curable polymer/epoxy substrate. During the embossing process the polymer is cured with ultraviolet light to create a rigid part. The mould is then removed, leaving a replica of the part. There have also been some very promising results with sol-gel composites recently. These materials can be cured to produce glass-like films that are hard and have good temperature stability.

Double-sided replicas may be created by processing both sides of the substrate simultaneously or in turn. By combining wafer-scale moulds and accurate dicing it is possible to manufacture a large number of individual parts very quickly.

According to Saarinen, two factors have so far hindered micro-optics' market penetration. The first problem has been finding suitable polymer materials that offer the required environmental and optical properties. Until recently, most micro-optics had to be made in fused silica to achieve the necessary optical quality, and this is expensive because the parts have to be individually etched. Secondly, there is a lack of customer awareness about the technology and what it offers. Saarinen believes that both these barriers are beginning to fall.

"We see that the future is in special polymers and plastics. We now have technologies and materials that are 'glass-like' but that are also compatible with replication production techniques," he said. "The key thing for the success of diffractive optics is that you can make high-quality parts in high volume, at a price that is compatible with consumer products. It just takes time for industry to learn about the possibilities of micro-optics."