Optics shape new applications

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David Robson explores developments in lenses, optis and coatings.

Consider laser eye surgery. What are the most important features of this delicate and precise procedure? A red diode laser beam maps out the contours of the eyeball. A detector sends the information to a computer, which will then precisely control a cool, ultraviolet beam of light from an excimer laser. But the layman may be forgiven for forgetting possibly the most essential components of all – the optics.

Lenses and prisms are used to control the shape and position of the beam to the high level of accuracy necessary for perfect vision. A window is used within the excimer laser to contain the gas while transmitting the beam with no significant loss of power. And a beamsplitter may be used to deflect a small portion of the beam into a detector that can check that the frequency and power of the beam are within suitable levels for the patient’s safety. Optics feature in every possible application involving the manipulation of light, and yet their importance is sometimes easy to forget.

To anyone but the optical engineer, who must design the shape and combination of components to control the beam in the desired way, they can seem deceptively simple. ‘Optics are just pieces of polished material until you apply the coating,’ says Dave Collier, president of Alpine Research Optics.

The ‘polished material’, or substrate, is usually fused silica, although there are hundreds to choose from. The coatings are determined by the power, wavelength and polarisation of the light that must be transmitted or reflected. ‘The small diode lasers used in CD and DVD players would not have an impact on the coating design,’ says Collier, ‘but in large lasers, such as those used at the National Ignition Facility (NIF), the energy is of an extreme nature, and the optics need coatings that can handle the power.’ The NIF currently has the largest laser under construction, and uses lasers to investigate high-energy and nuclear physics.

Mirrors from Alpine Research Optics

Antireflection coatings are commonly used to reduce the amount of energy lost during transmission, typically from around 30 per cent to fewer than one per cent total loss. There are many ways to apply coatings: sputtering is a common technique for thin film coatings. JDSU has recently developed a new magnetron sputtering platform. ‘The UCP-1 is a leap forward,’ says Carla Feldman, the marketing manager of custom optics at JDSU.

The new platform is fully automated, maintaining the desired pressure throughout the manufacturing process. This has reduced processes from 48 hours to as little as 10 hours, with fewer defects, adding a number of new products to JDSU’s product line.

Biomedical instrumentation in particular has benefited from the greater control over layer deposition. New fluorescence filters have been produced with higher optical densities in specific spectral regions. This gives an excellent signal-to-noise ratio that allows greater precision and detection, even with the weak fluorescent signals typical to biomedical research.

Of course, the effect optics can have is not limited to the coating. The shape of the optics, which can be cylindrical, spherical, or flat, determines the function of the component. According to Gregory Fales, product line manager for optics at Edmund Optics, the development of aspherical lenses has been the most exciting development in this area. ‘They used to be the designer’s dream, and the manufacturer’s nightmare.’

Aspherical lenses can eliminate several spherical lenses, reducing the weight, assembly time and reflection of optical systems. The reduction in weight makes them especially useful for military applications, such as night-vision equipment worn by soldiers. The focusing of laser diode light, and low-light-level imaging are also common applications.

In the past, aspherical lenses were moulded. The high temperatures involved caused shrinkage when the lenses cooled, reducing the surface accuracy. Instead, Edmund Optics uses a Magneto Rheological Finishing (MRF) machine to shape the lenses. In an MRF machine, magnets finely control a fluid that polishes and machines the glass. In addition to lowering the production temperature, this method gives much greater precision, providing a surface accuracy of one twentieth of a wavelength.

Although aspheres are now an attractive option for optical designers, the traditional kinds of optics are still widely used. Resolve Optics produces high-resolution scanning lenses to finely focus beams of laser light. The lenses are built in clean rooms to prevent dust from entering the lens, which could cause defects that could alter the beam shape and size as it scans across the lens.

In addition to focusing light, some optics, such as mirrors and prisms, are used to alter the direction of the laser beam, around a corner for example. Prisms provide higher accuracy control, especially for low-power applications. Flat optics sometimes have problems with the alignment of the beam, caused by the light penetrating a few millimetres below the surface before reflection or transmission. On the other hand, prisms transmit light rather than simply reflecting it, which can result in a loss of energy. In addition, they require at least two coatings, which can make them more expensive.

Prisms can also be used as beamsplitter, where the coatings applied will split up the different wavelengths and send them in different directions. This could be used in fluorescence microscopy, sending the output light into the eyepiece and the source light away from it, giving an excellent contrast. Like all beamsplitters, prisms can also split light according to its polarisation using dielectric coatings.

Chris Varney, managing director of Laser Components, which make such prisms, explains how the type of laser light can affect the manufacturing of these lenses: ‘With shorter wavelengths, the surface quality must be better, whereas longer wavelengths are not so exacting. IR beams will not “see” defects, whereas a UV beam may diffract or refract more, depending on the nature of the mark.

‘In addition, the power density of UV beams is quite demanding. The coatings of optical surfaces need more attention in these cases, otherwise they will be damaged by ablation, burning, or cracking, which would result in poor performance.’

Power density seems to be one of the few limiting factors that may affect the future of the optics market. The need for high power thresholds has been a running theme for all the experts consulted for this article. ‘In general, there is a higher demand for optics that can deal with higher power densities. There may be a point where we can’t produce an optic that won’t be blown apart,’ says Varney.

Princeton Instruments/Acton Research specialises in making such robust coatings. ‘All kinds of industries asked for this, including laser machining, laser surgery and the semiconductor industry,’ says Mark Dykstra, business unit manager for optics at PI/Acton. ‘To achieve this, we developed cleaner processes, better substrate materials, and more dense coatings.’

As far as the direction of the market is concerned, there are differing views. Varney believes that niche markets may be forming, where each company will have its own market due to a patented product or engineering expertise.

This is not a view shared by everyone. Dave Collier, president of Alpine Research Optics, says: ‘There are probably fewer companies to choose from than in the past: the bigger companies are swallowing the smaller companies to produce larger portfolios.’ Whatever direction it is heading, one thing is certain: optics are everywhere, and the market is set to expand even further in the foreseeable future.

The secret of adaptive optics

Few would believe that mirrors could have been the topic of much secrecy during the cold war. However, adaptive optics made from deformable mirrors were thought to be of such importance in communication and the tracking of satellites that their development only finally became public in 70s, following 20 years of research.

Since then, they have gone from strength to strength. They are made of tiny mirrors that can move to correct aberrations and distortions in a wide variety of applications, including medicine and entertainment. Many now make use of MEMS technology, as was covered in the last issue of Electro Optics.

A deformable mirror from OKOTech with 109 actuators.

Gleb Vdovin, director of OKOTech, explains an innovative use of these devices in modifying pulsed laser signals, which are used in spectroscopy, chemistry and biology: ‘The pulses are used as probes to excite processes through an instantaneous interaction – rather like tapping a bell, which happens in an instant, and then analysing the ringing afterwards.’

When the duration of the pulses is decreased, the spectrum of light can increase to a range of 500 to 1,000nm. The different wavelengths will then be propagated at different speeds, and arrive at the target at different times. This will ultimately increase the duration of the pulse at the receiving end. The mirrors are used to shift the different parts of the spectrum to compress the pulses to possible femtosecond periods.

Deformable mirrors controlled by a computer processor are an example of adaptive linear optics. However, this is by no means the limit of adaptive optics. Non-linear adaptive optics are achieved through physical phenomena, and are very efficient in correcting aberrations in lasers.

In an example of this, laser light is shone on a tank full of liquid or gas, which causes a sound wave in the medium. This will then cause an exact reflection of the light. The reflected and emitted waves are then superimposed, cancelling the distortions and correcting the aberrations.