3.1

Thin film Synthesis

Broad band functions constituted with more than one hundred multidielectric layers combined with coloured glasses are necessary to be in adequacy with the required specifications as well for the maximum transmission value or spectral centring as for the out-of-band rejection value.

The band-pass functions are based on multiple half wave Fabry-Perot structures and rejection is ensured by a combination of blocking multilayer stacks (typically high pass and low pass functions) deposited either on Bk7G18 substrate or on coloured glass substrate.

We present such an example in “Fig.2.” that leads to an 18.3nm band pass and 2.10^-3 level of rejection with a mean transmission higher than 89%.

Fig. 2.

Theoretical spectral responses

3.2

Uniformity optimization

The filters specifications concerning spectral bandwidth and wavelength centring require an accurate monitoring of deposited thicknesses and a good master of thickness uniformity within the deposition chamber. Indeed, a difference of uniformity between two separate locations on the samples-holder leads to a spectral shift in the optical response.

A multidielectric coating is typically composed with layers of alternative high and low index materials with different thicknesses that give the filter transmittance shape. At first order, if we neglect the index dispersion, a change of each layer thickness by the same factor leads to a similar spectral response but spectrally shifted.

Substrates are generally mounted on a rotating substrates-holder and the distance with the target material moves in a periodic way. The quantity of deposited material maybe considered as nearly proportional to the square of the substrate-target distance. So, as the substrates have generally a flat shape, all the points of the filter cannot be strictly at the same distance from the source and then will exhibit non-uniformity. Such non-uniformity is generally quite small and one will only obtain a shifted spectral response from one point of the filter to another. It is thus necessary to predict and control the uniformity curve in the deposition chamber.

Spectral measurements of the filter along X and Y axis permit to obtain a cartography of the central filter wavelength variations (“Fig.3.”) and to adjust the substrates position inside the chamber in order to minimize the non-uniformity.

Fig. 3.

Uniformity cartography

In the case presented above, a ± 0.17% non-uniformity over 40mm diameter is obtained that leads for example to a spectral shift of about 2.5nm at 740nm wavelength.

3.3

Coating deposition

The technology used for filters manufacturing is Dual Ion Beam Sputtering (D.I.B.S.). This technology involves one ion beam gun for sputtering a target and a second one for compacting the sputtered material. This technology leads to very dense layers and high quality coatings and is intensively used for manufacturing very narrow band filters for optical telecommunication market. The density of deposed layer is very close to the bulk material and enables the filter to be nearly insensitive to environmental parameters [4].

It enables to have very smooth interfaces too, that lead to very low scattering levels [5]. This is a privilegied technology for space applications as there is no measurable spectral shift between air and vacuum and versus temperature variations, and as coatings show very low ageing effects. The second ion beam is also used for cleaning the substrate before coating, leading to better adhesion.

The overall process is very stable and enables, together with an in-situ optical monitoring system carried out directly on one of the final parts to reach the required performances on all the samples.

Fig. 4.

Dual Ion Beam Sputtering principle

On “Fig.5.”, a comparison between theoretical spectral responses and filter measurements after deposition shows a good agreement in coating manufacturing with DIBS technology and in-situ real-time optical monitoring.

Fig. 5.

Comparison between theoretical and measured spectral responses

Fig. 6.

A 49mm diameter double face coated filter

For each channel of the filters wheel in the Ocean Imager instrument, three filters have been manufactured together at all stages: a Flight Model, a Spare Model and a Qualification Model.

4.1

Gamma radiation

The transmission degradation under Gamma radiations has been measured for each type of uncoated substrate involved.

On the following figure (“Fig.7.”), we present the spectral responses measured on uncoated 3mm-thick coloured glasses substrates before and after irradiation tests. We can notice transmission degradation after 100krads dose which corresponds to cumulated End-of-Life dose.

Fig. 7.

Degradation of coloured glasses under Gamma radiations

4.2

Air/Vacuum spectral measurement

Spectral measurements in transmission have been realized under normal atmosphere and in vacuum, with the following parameters:

On the graph “Fig.8.”, it is shown that the spectral shift of DIBS filters between air and vacuum is very low, and less than the measurement precision (0.5nm).

Fig. 8.

Spectral measurement under normal atmosphere and in vacuum

4.3

Humidity and Thermal cycling

Environmental tests have been performed according to the following list :

Visual inspection and spectral measurements have been performed on each component before and after the tests that showed no physical degradation and no variation in the spectral responses.

Moreover such tests have been successfully performed on Qualification Models including gluing in a blackened barrel.