FEATURE
Issue: 

Red or violet, resolution is the answer

Rob Coppinger finds greater precision will provide a clearer picture for UV and IR photonics

While ultraviolet technology is not about to be used for infrared applications, the two wavelengths’ photonic technologies, including optics, loom large over the markets – and improvements in both are driving applications for the microelectronics and defence industries among many others.

‘Most materials that transmit UV will transmit in IR but not the opposite,’ explains Craig Hanson, product line manager at CVI Laser Optics, who specialises in UV optics. ‘IR does not transmit the UV at all. So there are no applications that are driven by both requirements – one or the other, but not both. There are visible and IR applications in defence but not for UV and IR.’

For Hanson, contamination is the big issue: ‘The big thing is particulates. Contamination in the optics is the biggest issue and it is not something that requires new technology – it is just a matter of process control,’ he says. The things companies do to avoid contamination include careful handling and the use of class-100 clean room environments, which Hanson says ‘are for anything in the deep UV range, for example 193nm or lower’.

What Hanson and others are trying to avoid is any kind of absorption contaminant that can create scatter in the optic train – the series of optics the laser light must pass through. ‘If there is contaminant in the coating layer, or between the layer and the optic itself, the absorption will cause damage through heat or scattering,’ says Hanson.

To stop this, process control is all important: ‘You can’t have contaminants when putting optics into the optic chamber.’

For controlling coatings with highly accurate surface finishes, sputtering is often used but for UV optics it isn’t suitable for working with material that operates below the 240nm wavelength needed for UV. ‘Sputtering is not usually used for any deeper UV coatings, but it can go from 400 to 240nm. There are systems that use 248 to 266nm and they can both be used for that kind of coating,’ explains Hanson. When the wavelengths needed get down to 193 to 213nm or 157nm, companies can’t use the same sputtering process – they have to use an electron beam or ion assisted deposition type process. ‘Usually it’s an e-beam gun-type system for the deeper UV stuff,’ says Hanson. Beyond the coatings to the optic itself, process control for particulates and homogeneity of the fused silica are all important: ‘The manufacturing process isn’t different to other wavelengths. The difference is the material for the UV region. It uses different grades of silica, so there are different levels of homogeneity and inclusions and impurities. The higher the grade, the less impurities in the glass,’ he adds. Steps in the manufacturing processes and the post-processes that are used to verify that optics products meet the specifications are spread across two stages.

‘There is a way we can look at the coating itself and do tests to ensure minimal particulates. So there are two steps, as the part goes from polishing to coating, and then to final inspection. We do have in-house people who are specially trained to handle this and use different technology to evaluate the quality of the product,’ says Hanson. CVI does not always use 100 per cent inspection. Its process control uses the acceptable quality level (AQL) methodology. 

 ‘It is done by AQL, which is anything from 10 to 25 per cent. The manufacturing process has been vetted, so we don’t have to do 100 per cent inspection because we already have enough process control. One hundred per cent is not necessary, it is overkill.’

For Richard Harvey, sales engineer at Hamamatsu for IR optics, the challenge is all in the resolution. ‘We joined the market at the lower resolution end with a first product two years ago, a 64 x 64 pixel array. The idea is to slowly increase the pixel number. 128 x 128 is the highest resolution,’ says Harvey.

Hamamatsu has its 64 x 64 pixel product, with a 50μm pixel pitch, and then the 128 x 128 pixel sensor comes in two versions with a 50μm pixel pitch and a pitch at 20μm.

‘That is the newest thing; it is higher resolution versions with smaller pixel pitches. We have products with a pixel pitch of 20μm coming out soon and, because of that reduction in pixel spacing, you can use less material, which lowers costs,’ says Harvey.

The sensors Harvey is talking about are made of Indium Gallium Arsenide, or InGaAs. They are based on CMOS technology, employing a back-illuminated photodiode array on a CMOS read out circuit that has a timing generator. ‘You can use offset compensation to eliminate the amplifier offset voltage in each pixel.’

According to Harvey, the general feedback from customers indicates that they want to move away from linear arrays for imaging. ‘The feedback is that they thought it would be more expensive than it is,’ he adds.

‘Two years ago we offered two dimensional InGaAs arrays. Usually we offer one dimensional arrays for spectroscopy, but there is a demand now for 2D for imaging.

‘Typically these sorts of sensors have been very expensive and are difficult to get hold of. They aren’t suitable for all applications, but they do lend themselves to laser beam profiling and foreign object detection,’ Harvey explains.

Hamamatsu also offers additional accessories. ‘We offer a driver camera head; you can attach a lens to it. It is an OEM solution rather than an end-user solution – though some end-users are taking it.

‘There is Hamamatsu software available and users can quite quickly evaluate the camera. Some just use the chip and design their own driver,’ says Harvey.

‘Some would take driver head itself, which is cheaper than is available in bulk in the market now.’

Hamamatsu offers sensors that operate in the 0.95 to 1.7μm spectral range and different types of InGaAs products, such as pin photodiodes, including one that can be used for wavelengths down to 500nm.

Harvey says: ‘At the moment we are focusing strongly on the InGaAs market.’ Greater purity in fused silica, sophisticated alloys such as InGaAs, increased pixel density, the technologies used for IR and UV photonics including optics, are pushing back the frontiers of what can be done with these wavelengths and how different industries can benefit from their application.