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Fairest of them all

Faster lasers require ever more sophisticated mirrors. Rob Coppinger reflects

Lasers are getting faster and more powerful – and mirror developments are reflecting those trends, both figuratively and physically.

Those trends include reflecting laser light that is not made up of one wavelength, but requires mirrors with negative dispersion to align the reflection of different wavelengths. Positive dispersion sees wavelengths slowed, relative to their accompanying wavelength, as they are reflected. Negative dispersion techniques re-align these wavelengths. It is a challenge that ultrafast lasers are presenting, and one that REO is rising to. ‘Most historical work has been with titanium sapphire lasers, which operate at about 800nm,’ says REO’s chief technology officer, Trey Turner. ‘A typical requirement has been a little bit of negative dispersion to compensate for some positive dispersion from the transmissive elements.’

The trend of lasers being scaled to higher power levels and experimented with other gain media and wavelength ranges has led to higher positive dispersion in the cavity. Turner explains: ‘The recent challenge has been to find clever ways to create higher levels of negative dispersion effect while scaling the power, and using different wavelengths for some applications.’ For the industry this issue is an outcome of the classic design and manufacturing challenges companies haver always faced. ‘We are investing in additional design capabilities and additional analysis capabilities to more rapidly optimise the increasingly complex coating designs,’ explains Turner. This is because the coatings are getting quite complex, to achieve the higher dispersion values needed. It is not just good design though; those classic manufacturing challenges mean ‘investing in design capabilities and better algorithms for optimisation while on the other hand developing better manufacturing capability to actually be able to produce those designs,’ in Turner’s view. This demands that industry looks at different monitoring techniques and ion sputtering technologies. ‘The most repeatable coating technology and metrology capabilities are not a trivial matter,’ says Turner.

Metrology is another challenge because, according to Turner, companies can’t use a traditional spectrometer measurement to characterise the reflection and transmission characteristics. ‘We have to use special techniques to measure actual dispersion properties of mirrors,’ he adds. All of this will advance ultrafast lasers in the fields of life sciences, and multiphoton microscopy in particular. However, for Turner, the application that is really driving the new advances in the optics technology is, ‘the industrial application, micromachining; that is where we’re looking for higher powers and other wavelengths that are a better match for different materials.’

As Turner pointed out, a key technology is sputtering. Sputtering has become an important process to meet the needs of lasers. Gregg Fales is senior product line manager at Edmund Optics. ‘Previously I would have said most of the market used metal coated mirrors, simple aluminium or silver coatings. With sputtering technology, broadband dielectric coatings are becoming more popular,’ says Gregg. He sees a lot of people wanting broadband dielectric coated mirrors rather than metals. As he explains, this is because, ‘it covers a wide spectrum including the infrared. They are only 93 to 96 per cent reflective but, if you’re bouncing the laser a few times, each time you’re losing quite a bit.’ To counter this, Fales sees a market developing where there are a lot more highly reflective dielectric coatings. These coatings are designed to cover the wavelengths 350 to 1100nm, which also covers the full range of common YAG laser wavelengths. Dielectric can be 99 per cent more reflective throughout that wavelength range. One application area Fales is seeing is life sciences. Where life science researchers are using UV and IR spectral ranges dielectric mirrors, rather than metal reflectors, are being ordered. ‘These cover the visible full spectrum of your typical silicon sensor and you can get 99 per cent reflective across that range and end up with higher damage threshold,’ adds Fales.

In Fales’ view the dielectric stack’s advantage is a higher damage threshold, with products 100 times less susceptible to laser damage than conventional mirrors. Another advantage is that mirrors with sputtered coatings are more resilient to the types of cleaning chemicals that can be used. Fales identifies metal coatings that can be lost due to cleaning as a drawback to traditional mirror technology.

Coatings is one way of altering the properties of a mirror but, for microscopy, astronomy and laser communication, the mirror itself can be deformed to enhance performance. Michael Feinberg is sales director at Boston Micromachines. His company makes microelectromechanical (MEMs) deformable mirrors. ‘Compared to traditional piezoelectric deformable mirror technology, we’re cheaper and lower-power,’ says Feinberg. Traditional adaptive optics has been used with large-scale technology such as astronomical observatories. ‘Ground-based astronomy uses it to compensate for distortions caused by the atmosphere,’ explains Feinberg. ‘You need to take out the aberrations caused by the atmosphere.’ According to Feinberg, traditional adaptive optics use wavefront sensors to detect how light is being abberated and then use the data to alter the mirror to compensate for the aberration. ‘We have found wavefront sensor adapted optics have drawbacks, it can be slow because it is computationally intensive. It takes a lot of computer power to work out what is going on with the atmosphere,’ says Feinberg.

Boston Micromachine’s technology doesn’t use a wavefront sensor. Instead, Boston’s technology uses a method called metric base optimisation. Feinberg explains: ‘I am saying give me your light and tell me something about it – and I will optimise it.’ That optimisation can be the light’s intensity or it could be the contrast or sharpness of an image. ‘We make two versions,’ says Feinberg. ‘One is image based and one is laser based. You bounce the lasers off of a mirror and, through a medium, you focus into a detector and use an algorithm.’ The algorithm is used to optimise the position of the actuators on the deformable mirror to increase the intensity of the light going through a pin hole to the detector, for example for astronomy.

‘With traditional adaptive optics the goal is to characterise a wavefront with a sensor, so you need enough light in the field of view to determine what that wavefront is doing,’ says Feinberg. ‘If you don’t have enough light you can correct the distorted wavefront. With our technology it doesn’t care if that wavefront is doing it is just going to optimise that property.’ Feinberg also says that some users don’t care about the state of the wavefront. This is because, for example, with two photon microscopy users are fluorescing particles and there is no concern about the wavefront for that activity.  The goal is simply to obtain the strongest signal.

Whether it is fluorescing biological samples in a laboratory or imaging organic molecules in deep space, mirrors that are either deformed by actuators or coated for negative dispersion effects will continue to play a key role in photonics.