Centre for Optics Manufacturing
The TNO Centre for Optics Manufacturing provides state-of-the-art optical components for space and astronomy instruments, the semiconductor industry, medical applications, and many scientific research fields.
There is an ongoing trend towards the use of aspheric and free form optics in advanced instrumentation and sensors. lt is almost impossible to manufacture aspheres and free form optics with conventional manufacturing technologies, During the past 15 years TNO has invested in deterministic machining processes, such as Single Point Diamond Turning (SPDT) and Computer Controlled Polishing (CCP). The available processes enable optical finishing of a large range of materials, including Fused Silica, Zerodur, Silicon Carbide, Calcium Fluoride and Aluminium. Accurate validation of the surface form of free forms is very challenging. Therefore TNO has developed a non-contact measuring machine for aspheric and free form optics (NANOMEFOS). ln addition to computer assisted machining, our skilled professionals can produce dìfficult flat and spherical optics by hand, including applications that require optical contacting. A comprehensive optical coating facility enables the design an application of complex, custom designed coatings. This means that everything from design to final figuring and coating on 0.5 m class optical components can be done under one roof.
Diamond turning of TROPOMI free form mirrors
The TROPOspheric Monitoring lnstrument (TROPOMI) is an advanced absorption spectrometer for Earth observation, developed in the Netherlands under contract to NSO in the frame of the ESA Copernicus Earth Observation programme. TROPOM1 is a push broom instrument that combines a very large field of view of 2600 km with a 7 km resolution and a spectral range encompassing UV, UVIS, NIR and SWIR bands. It is scheduled for launch in 2016 on the Sentinel-5P mission. The telescope has an aperture of f/9 x f/10 and is capable of imaging the entire 1080, swath with a resolution nominally better than 0.020. This corresponds to a sub-km spot size on earth. The unprecedented viewing angle and resolution are realised with an optical system consisting of only two free form mirrors, which correct the geometric aberrations and maintain the resolution over the entire viewing angle.
At TNO, we are involved in the total chain of design, tolerance analysis, manufacture and test, including free form designs. The TROPOMI design is based on heritage from the Sciamachy and OMI instruments Envisat and NASA-AURA respectively. A telescope with two conventional spherical mirrors was applied in OMl. It has a comparable field of view and a nominal resolution of circa O.50. TROPOMI could only be realised with a telescope that has an order of magnitude improvement in the nominal resolution. One notable change in the mirror shape is the application of toroidal mirrors, i.e. mirrors with different radii of curvature in two perpendicular directions. This enables the realisation of an anamorphic telescope (i.e. a telescope with a different focal length in the spatial and spectral direction), in which the spatial and spectral resolution performance can be optimized separately. The free form mirrors cannot be manufactured using conventional tools. They are both non-spherical and have no axis of symmetry. The sag of the non-rotational symmetric terms varies on the order of 1 mm. ln addition to requirements on resolution, which dictates the form and the tolerances on the surface shape, other issues to deal with are throughput and stray light, dictating requirements on reflectivity and roughness, respectively.
The material of choice for the free form mirrors is Nickel Phosphorous plated Aluminium-6061. lt can be machined by diamond turning, and it allows the realisation of an athermal design. Manufacturing is done on a Precitech 700A single point diamond turning (SPDT) machine. The work piece is rotated and translated, the diamond turning chisel moving relatively in the perpendicular direction to follow the contour of the surface. ln view of the large deviations from spherical shape, the diamond turning machines could not be used in fast servo mode but rather in slow servo mode. Slow servo mode allows larger deviations at the cost of longer processing times: with a rotation speed of 20 to 50 revolutions per minute, a processing time of up to 30 hours was necessary for these free form surfaces. Long processing times adversely affect the form accuracy because of e.g. thermal and mechanical stability of the machine setup. ln addition to the rotation speed, the step over distance per revolution (groove distance) is a tunable parameter to optimise the manufacturing process. Large step over gives shorter processing time (and consequently better figure accuracy) yet increased surface roughness, Small step over leads to better surface roughness at the cost of form accuracy, as well as tool wear. Balancing surface figure and roughness, the telescope mirrors were turned at an intermediate step over of circa 10 micron. A final manual polishing step reduced the roughness to 1 nm while maintaining surface figure. The surface roughness of the mirrors was measured on a Wyko NT9300, using phase shifting interferometry and the surface error was measured with NANOMEFOS. SPDT has been used at TNO for over ten years. A large number of projects have benefitted from our aspheric and free form manufacturing capability, including free form mirrors for the SCUBA2 instrument on the James Clerk Maxell Telescope in Hawaii, The ESO VLTI Star Separators in Chile and the airborne NASA/DLR SOFIA telescope.
Computer controlled polishing of VLT laser guide star lenses
The image quality of ground based astronomical telescopes can be significantly improved with the use of Adaptive Optics (AO). The AO system corrects the wavefront distortions introduced by the turbulent atmosphere, resulting in much sharper images. ESO (the European Southern Observatory) is currently upgrading one of its Very Large Telescopes (VLT) in Chile with a four Laser Guide Star AO facility. This system creates four artificial stars by exciting Sodium atoms in the mesosphere with a powerful 589 nm laser. TNO has built the Laser Launch Telescopes (LLT) for the four VLT Laser Guide Stars.
The Laser Launch Telescopes are 20x Galilean beam expanders, expanding a Ø15 mm,25W CW, 589 nm input beam to a steerable Ø300 mm output beam. The exit lens (12) is a Ø380 mm convex asphericai collimation lens. The final corrective polishing of the L2 was done at TNO. Since 2002 TNO has been developing free form optics related technologies, which are now being successfully applied for various projects, one of which are the LLT L2 aspheric lenses. Key is the iterative loop between the manufacturing (deterministic polishing or diamond turning) and the absolute metrology of the NANOMEFOS machine. This allows for rapid and efficient manufacturing, since no dedicated measurement setup has to be made. All equipment is capable of handling optics ranging from flat to free form and from convex to concave, up to Ø500 mm. The LLT L2 aspheres cannot be polished conventionally due to the large departure from the best fit sphere. Computer Controlled Deterministic local polishing is therefore applied. A process optimisation was performed:
– to minimise mid-spatial generation
– to minimise roughness (1 nm Rq achieved)
– to optimise the process predictability
The convex aspherical surface of the L2 is particularly difficult to measure. Usually this is done by assembling and aligning the system, measure its transmitted wavefront, remove the lens and correct the errors in several iterations. The NANOMEFOS machine allows for measuring the surface form directly, making the production much more flexible and efficient. lts characteristics are:
– Universal (from flat to free form, convex to concave)
– Large measurement volume (Ø5OO x 125 mm)
– High accuracy (30 nm, 2σ)
– Fast (minutes)
The L2 has a convex aspherical side with a radius of 637.381 mm and a conic constant of -0.447, resulting ¡n a departure from the best-fit-sphere of 320 um. lt was polished in six runs from 1200 nm rms (incl the radius error), to 24 nm rms. The mid-spatial content is approximately 14 nm rms, contributing about 7 nm rms to the final wavefront error. The concave spherical side has a radius of 6876,981 mm, and was conventionally polished to 1OO nm PV and 18 nm rms. Note that the NANOMEFOS machine can easily measure this surface, while interferometric techniques would require a 7 m long test bed. The TNO Coating Facility designed and applied a custom 589 nm coating, resulting in a measured reflectivity of less than 0.2% for both sides. The optical performance of the Laser Launch Telescopes was measured using an interferometer and reference flat mirror. The final wavefront error is about 17 nm rms, which was as expected based on the individual measurements of the optical surfaces. All four Laser Launch Telescopes have been delivered to ESO and are currently being integrated in the 4LGSF facility. First light of the new VLT Laser Guide Star Facility is expected by mid 2015.
Aspherical and free form optics enable significant improvements in the performance of optical instruments. Free form optics result in fewer optical aberrations and considerably reduce the number of components needed, which leads to better optical transmission, smaller dimensions and a lower instrument mass. lnnovative designs are also made possible. Classical methods of measuring are not suitable for measuring these complex, asymmetric and strongly curved surfaces. To solve this problem TNO, in close cooperation with TU Eindhoven and the Van Swinden Laboratory, developed NANOMEFOS (Nanometer Accuracy Non-contact Measurement of Free form Optical Surfaces), a device that can measure large free form optics quickly and without contact at a measurement uncertainty of just a few nanometers. The measurement volume is Ø500 x 125 mm. The object to be measured (which may also have non-rotational symmetry or an interrupted surface) is placed on an air bearing spindle over which an optical probe moves with scanning speeds of up to 1.5 m/s. This enables very short measurement times compared to conventional CMM metrology.
The position of the probe and the spindle are measured by an interferometry system and capacitive sensors relative to a silicon carbide metrology frame, thereby compensating for drift as well as static and dynamic position errors. The absolute accuracy of the surface error measurements is 30 nm (2σ). The point spacing is programmable (typically 1 mm) and can be adapted even to pm level, if necessary (e.g.to enable the measurement of mid spatials). Another advantage of NANOMEFOS is the ability to derive the radius of the optical component from the measurement data. Since every asphere or free form (convex and concave) can be programmed and measured without the requirement of a dedicated computer-generated hologram, NANOMEFOS is a very flexible, advanced measuring machine and unique in its kind. NANOMEFOS signifies a revolution for the manufacturing of free form optics. After completion of an extensive calibration programme in 2010, NANOMEFOS is fully operational and is also available for external parties.
Silicon carbide optics for GAIA basic angle monitoring system The GAIA mission will create an extraordinarily precise three-dimensional map of more than one billion stars in our Galaxy. Part of ESA’S Cosmic Vision programme, the GAIA spacecraft is being built by EADS Astrium. TNO has developed the Basic Angle Monitoring system for this mission. The astrometrical measurements performed with Gaia will be accurate to 24 microarcsec, comparable to measuring the diameter of a human hair at a distance of a 1000 kilometres. To achieve sufficient stability at a minimum operational temperature of 100K, the entire Gaia Payload Module is made out of sintered Silicon Carbide (SiC). ln order to achieve the required astrometric accuracy, the mission requires a Basic Angle Monitoring (BAM)
system to measure the line of sight fluctuations, caused by small thermal variations. The BAM consists of a double Michelson laser ¡interferometer and has a resolution of 1.2 picometer. Ultra-stable mounting of optical components is required to achieve the severe beam pointing, beam tilt, OPD and WFE requirements, both after vibration and shock loads and over a wide temperature range of more than 190K. The worst case number of optical surfaces that are passed by one of the BAM beams amounts up to 18, each surface adding to the total beam tilt and wave front error and making alignment a very precise job. The TNO Optical Manufacturing department made an important contribution to the BAM metrology instrument. Hand and computer controlled polishing was used to finish the optical and mechanical components of the BAM. ln order to achieve the required tilt stability of ≤ 2 urad per component, the mirror mounts on the optical bench were hand polished to submicron flatness. The folding mirrors were hand polished to a surface error of <2 nm rms (over an aperture of 9 mm). The mirror mounts are integrated and in the same plane as the mirror, allowing accurate and stable mirror positioning on the optical bench. The mirrors are mounted with a high spring preload, resulting in light optical contacting, contributing to the overall stability. The laser beams are injected into the interferometer via a fibre. A strongly curved off-axis parabolic SiC mirror collimates the beam. The main difficulty of this collimator mirror, beside that it is made of Silicon Carbide, is it small radius of curvature (R = 50.17 mm) over an effective aperture of only 10 mm, maintaining a surface shape error of ≤ 12.5 nm rms and a surface roughness of Rq ≤ 6 nm. With this unique combination of requirements, worldwide no suppliers could be found that could guarantee the delivery of such a state-of-the-art component in time. ln close cooperation between TNO and the Leibniz lnstitute of Surface Modification (l0M), a manufacturing process of iterative robot machine pollshing and Plasma Jet Machining was developed, which resulted in the realisation of flight mirrors with a surface error in the range of 4.4-7.2 nm rms and a surface roughness better than 6 nm rms. The transmissive beam splitters also require a tilt stability of ≤ 2 urad. But the CTE mismatch between glass and SiC makes this a tricky design, especially for use at cryogenic temperature. Optical contacting of Fused Silica plates with a beam splitter coating and a stress free mount was used to make an ultra stable beam splitter (no adhesive used). Tuning of the thickness and tilt angle of the SiC alignment shims is executed as part of the alignment process. The collimator mirror is aligned with a single shim ¡n tip/tilt, piston and lateral position with a single SiC shim. This shim is hand polished and iteratively adjusted to the required dimension. Several other shims required submicron accuracy, which could be achieved thanks to the availability of NANOMEFOS. The successful performance of the BAM has been verified at subsystem and at payload level, during an extensive test campaign at the Centre Spatiale de Liege. Gaia was launched on December 19, 2013.
Polarisation scrambters for TROPOMI
The UVN module of the TROPOMI instrument consists of the telescope – which is shared by the UVN and the SWIR – and the 3 UVN spectrometer channels (UV, VIS and NIR) each equipped with individual detector units. The telescope has a very wide field-of-view of 108 degrees. A polarisation scrambler is placed in the optical path to make the measurements insensitive to the polarisation state of the incoming light.
This results in a much more reliable and more accurate radiometric calibration. The polarisation scrambler is a so-called Dual Babinet Compensator Pseudo depolariser, consisting of a stack of four alternating birefringent Magnesium Fluoride (MgF2) and Crystalline Quartz (SiO2) wedges. Each birefringent wedge acts similarly to a quarter wave plate but then with a varying phase shift, to create a continuum of polarisation states after the scrambler, thus pseudo-scrambling the incident polarisation. The orientations of the wedge shapes and crystal optic axes are specifically selected to optimise the depolarisation power and geometric aberrations. The wedge top angles are in the range 0.60 to 1.350. These are tuned to minimise chromatic aberrations. Each wedge has a central thickness of 2 mm. The wedges are assembled pairwise by optical contacting, with a gap in between the two pairs. The scrambler stack is positioned close to the pupil. The dimensions of the scrambler are approximately 25 mm diameter with a clear aperture of 11 x 13 mm. One of the most difficult parts of the scrambler are the optical coatings. The polarisation scrambler has to work in a very wide UV-VIS band (270-495 nm), a NIR band (675-775 nm) and a short wave infrared band (2305-2385 nm). The separation between the VIS and NlR bands is relatively small. The combination of the large wavelength range and the narrow separation between the VIS and NIR bands result in a very complex AR coating with a relatively large number of layers (20), and a large variation in layer thickness (10-200 nm). Consequently, the design is sensitive to small manufacturing variations. These challenges are increased by the fact that the transmission requirements at 270nm limits the choice of materials. The TROPOMI instrument is scheduled for launch on the Sentinel 5P mission in 2015.
Immersed grating for TROPOMI
The mass and dimensions of lR spectrometers can be drastically reduced with the use of Silicon immersed gratings. SRON and TNO have developed this innovative type of grating for the TROPOMI SWIR module onboard ESA’s Sentinel 5P spacecraft. An immersed grating is a grating that is illuminated from the inside: a specially developed combination of a prism and an optical grating. ln an immersed grating the light first enters a medium with an index of refraction n. After dispersion by the grating surface, the light exits the immersion medium and acquires an extra dispersion due to refraction. An immersed grating delivers a factor n more diffraction for the same geometry. This trick results in an n times smaller grating and an instrument n3 times smaller. The TROPOMI Silicon immersed grating has a refractive index of 3.42, resulting in a forty-fold volume reduction for the SWIR module (e.g. compared with the Sciamachy instrument on the European Envisat satellite). This makes the immersed gratings also ‘enabling technology’ for future scientific space missions.
The gratings are lithographically produced. By using anisotropic etching, arbitrary blaze angles can be obtained, enabling optimisation of the diffraction efficiency. The etched reflecting facets are very smooth suppressing stray light. Silicon is transparent for wavelengths above 1.2 um, therefore the grating has a wide application range. The TROPOMI SWIR grating has a line spacing of 2.5 um, a 54.70 blaze angle and is operated in sixth order. The total grating area is 50 x 60 mm. An efficiency of 60% is obtained. The prisms are made out of a Silicon block after etching of the grating. This makes the handling and production of the prism a very delicate job. The grating surface needs special protection during cutting and polishing. The prism entry surface is hand polished to a surface error of 50 nm PV. A special cleaning procedure was developed to avoid damage of the delicate grating structure. During an 18-month program the SRON and TNO researchers have perfected the technique and qualified the gratings for use in space. TROPOMI is the first space instrument to make use of these gratings, but it will certainly not be the last!. Immersed gratings will be used in other space instruments and are considered for the METIS instrument on the new European Extremely Large Telescope (E-ELT). SRON has already built an immersed grating demonstration model for METIS, which is even larger than the TROPOMI grating.
SRON has delivered the TROPOMI immersed gratings to Surrey Satellite Technology Ltd. (SSTL), which builds the TROPOMI SWIR module under contract from Dutch Space in Leiden. TROPOMI is being developed in a partnership between KNMI, SRON, TNO and Dutch Space, on behalf of the Netherlands Space Office (NSO) and ESA.