High LIDT optical components for ultrafast applications - what are the specific points
As research is ongoing, many researchers often require optics that, when compared to standard catalogue optics, require some customisation. This often leads to a drastic increase in price and delivery time which then jeopardises the research project.
Manx Precision Optics Ltd. ('MPO') is adressing this issue by offering a wide range of catalogue optics and, furthermore, offering competitive prices and delivery times where customisations are required. In the following, a number of standard components are being described.
High LIDT Mirrors
The choice of substrate is crucial for the achievement of a high laser-induced damage threshold ('LIDT'). Hence, MPO uses Fused Silica substrates as standard as these give superior surface roughness. All substrates are inspected to laser grade quality under a bright halogen light against a black background to show even the smallest of scratches, digs or coating impurities. The MPO polishing process is also designed to deliver an excellent RMS roughness even on standard production substrates with RMS values only in a range of a few Angstroms. The standard flatness is lambda/10 @ 633nm.
Substrates are usually polished in economic batches with the batch size depending on diameter, material and finished specifications. Uncoated substrates of sizes from 1/2" diameter to 6" diameter are held in stock to allow competitive prices for smaller quantities. As MPO has got all shaping, grinding and polishing facilities in-house, any substrate in stock can be customised (for example, a wedge can be added to a surface) and where a customisation is not possible, the large stock of raw material often comes to assist.
The MPO coating department is equipped with 2 modern electron-beam coating chambers and one sputter chamber. This allows the company to utilise the best coating technology for any specific requirement.
When compared to standard optical components, components for ultrafast applications not only require a high reflectivity and a high LIDT, but also a low group delay dispersion ('GDD') to ensure that the pulse shape does not get distorted.
From a coating design point of view this represents additional challenges. When using so-called 'single-stack' designs, i.e. pairs of coating layers with high and low refractive index with a thickness of lambda/4 at the desired wavelength (at 0deg angle of incidence) the breadth of the reflectivity band is determined by the ratio of the refractive indices of the two coating materials used. At the same time, the choice of high index coating materials that also display a high LIDT is somewhat limited.
To expand this 'naturally defined' reflectivity bandwidth available options are the use of more than two coating materials, the use of an optimised broadband design or a combination of both.
Whatever coating design is used, it is important to have the relevant metrology available. Apart from checking the reflectivity / transmission of the optical components it is also necessary to check the GDD to ensure a fully compliant product.
Depending on pulse width and resulting required bandwith, polarisation and GDD requirement, spectral ranges of 750-850 up to 700-900nm can be covered with a large number of components available from stock and purchase on the website.
Roof Mirrors
Where a light beam has to be reflected back parallel to itself, the common solution is to use two generally rectangular mirrors and mount them at a right angle to each other. The disadvantage of this setup is the time that needs to be spent adjusting the mirrors. MPO has developed roof mirrors where the two mirrors are optically contacted together at a right angle to each other (see picture 1). This not only makes the setup much more stable, but also reduces the time that needs to be spend regularly to aign the mirrors. This is just one example of taylor-made solutions available from MPO.
Picture 1: Optically contacted roof mirror with protected silver coating
Broadband Polarisers
Broadband Plate Polarisers
In the field of optical components the typical plate polariser is a so-called Brewster Polariser, i.e. a plate polariser used at Brewster's angle. Variations on this type of polariser include plate polarisers used at 45º angle of incidence. These types of polarisers have in common that they only cover a very small spectral bandwidth, typically a few nm.
The reason for this lies in the coating design. When looking at a 'single stack' coating design (see picture 2) it is noticeable that there is a spectral area at wavelengths longer than the centre wavelength where s-polarised light is still reflected while p-polarised light is already transmitted. Any coating design for a plate polariser is essentially an optimised design utilising the same principle and making use of this 'spectral window'. 
Picture 2: Reflectivity of s-POL and p-POL at 45º
As the breadth of the spectral window is ultimately governed by the refractive index ratio of the coating materials, the achievable polarisation bandwidth faces a natural limit.
Whle a small bandwidth is sufficient for many single wavelength lasers, it is not broad enough for ultrafast applications. If a plate polariser is required, it is then necessary increase the angle of incidence as this also widens the spectral window usable for polarisation.
At an angle of incidence of 72º it is possible to cover 700-900nm, but unfortuantely this goes at the expense of achievable extinction ratio, which drops, depending on specific requirement to about 5:1! Furthermore, when using a substrate at 72º angle of incidence, the useful aperture decreases due to the oblique angle. For example, a standard 25.4mm diameter, 6.35mm thick substrate the useful aperture at 72º angle of incidence is only 5mm! To achieve a useful aperture of 20mm, the substrate diameter (maintaing also a thickness sufficient to achieve lambda/10 flatness) needs to be increased to almost 100mm.
The MPO catalogue specification for these components is
>90%R (avg.) s-POL & >90%T (avg.) p-POL @ 700-900nm /72º
This, however, can be adjusted to improve the performance in s-POL, at the expense of p-POL performance, or vice versa.
Cube Polarisers
To be suitable for use in ultrafast applications, cube polarisers need some specific adjustments when compared to conventional cube polarisers. A cemented cube polariser would be unsuitable as the optical cement is generally not able to withstand the required power levels. Hence, it is essential that the prisms constituting the cube polariser are optically contacted.
Conventional cube polarisers consisting of two right angle prisms can, like plate polarisers, only cover a relatively narrow spectral bandwidth. One alternative is the use of high-index glass prisms such as SF2 or SF6. For ultrafast applications this is, however, not a viable option as these glasses are difficult to optically contact and, most importantly, the cosmetic appearance and hence the LIDT are inferior to substrate materials such as Fused Silica.
The solution is similar to the one applied to plate polarisers - an increase of the angle of incidence. In a standard cube polariser one of the right angle prisms is coated with the polalrising coating which is immersed between the two prisms. The angle of incidence on the polarising coating is 45º.
By using slightly elongated prisms which increases the angle of incidence it is again possible to achieve a high extinction polarisation over 700-900nm. A typical specification of such a cube is:
Fused Silica Cube polariser, optically contacted, 1" x 1" x 1.38" (25.4mm x 25.4mm x 35.0mm), lambda/8 transmitted wavefront distortion, 10-5 scratch-dig, polarising coating immersed, outside faces coated AR (<0.5%R) @ 700-900nm/0º
Polarising performance: >99.5%R s-POL & >95%T p-POL @ 700-900nm/0º
Due to the angle of incidence on the polarising coating not being 45º, the two exit beams from the cube are not at 90º angle to each other. This can be corrected, if required, by contacting a wedged window to the exit surface of the p-polarised beam. The GDD of such components is usually low and they represent a compact and easy to align alternative to plate polarisers.
We are aware that the selection fo the most suitable optical component very often represents a challenge and look forward to assisting you to find the best solution for your application. MPO and its employees are not only dedicated to quality, but are also driven by innovation. Being a privately-owned company we are able to take a long-term view. Our enthusiasm, dedication and curiosity towards new technologies, combined with our craftsmanship, enable us to find the best solutions our customers' applications.
Please do not hesitate to contact us:
Manx Precision Optics Ltd., 4 Albert Street,
Douglas, Isle of Man, IM1 2QA
Tel: +44 -(0)1624 -620 800, sales@mpo.im or visit www.mpo.im
