1.1

Ruled gratings capabilities

HORIBA Scientific (formerly Jobin Yvon) designs and manufactures Ruled blazed diffraction gratings for more than 50 years in parallel to the holography development [1]. Several generations of ruling machines have been developed with continuous improvement of the gratings performances, metrology and production automation (Fig. 1).

Figure 1.

HORIBA Scientific Ruling machine picture.

The ruling machine principle is based on a mechanical ruling by a diamond which will scribe the grating groove profile line by line in a metallic layer. The grating groove profile will be naturally blazed with a triangular shape as described in Fig. 2. The ruling machine is calibrated and carefully aligned to manufacture the designed and targeted groove profile in terms of groove depth, blazed angle, roughness, homogeneity, …

Figure 2.

Ruled blazed grating groove profile measured with an Atomic Force Microscope (AFM) showing the triangular shaped groove profile.

Various types of substrate material can be used: standard glass, zerodur, metal, SiC.

1.2

TRL9 Space Qualification

The space qualification of the whole range of HORIBA Scientific diffraction gratings was performed long time ago during the Long Duration Exposure Facility (LDEF) mission. LDEF was deployed in orbit in 1984 and carried on-board a full set of diffraction gratings from Jobin Yvon listed below:

The optical performances of these three grating types, representative of our capabilities, were tested in space vacuum and environment exposure. Correlation with identical components, stored on ground under Air-Nitrogen pressure during all the experiment duration was made. Integrity, wavefront quality, diffraction efficiency and stray light were the four main parameters checked and compared in this mission.

The tested gratings were in space vacuum during a long exposure (69 months, about 34000 orbits) with thermal cycle/orbit from -30°C to +70°C and in space environment with cosmic dust and sun irradiation during ten months.

The LDEF mission results proved the space qualification with a Test Readiness Level (TRL) 9 of the different grating types manufactured by Jobin Yvon [2]. The ruled grating type was among the technology qualified for space missions.

1.3

Ruled gratings heritage

A major mission we were involved in manufacturing ruled gratings was the Juno mission (NASA/JPL) launched few years ago and in orbit close to Jupiter for more than 2 years. For the Jovian Infrared Auroral Mapper (JIRAM) instrument which is an image spectrometer that will explore Jupiter, carried by the Juno spacecraft, we have manufactured very low groove density ruled plane reflection gratings (Figure 3). These ruled gratings, ~30gr/mm groove density, were optimized for a large spectral range in the mid-IR from 2µm to 5µm, in order to probe the upper layers of Jupiter’s atmosphere.

Figure 3.

Optical Layout of the JIRAM spectrometer with the diffraction grating [3]

Thanks to a precisely optimized low blazed angle allowed by the ruling machine, a peak absolute efficiency of about 91% at ~2.75µm was obtained and an over whole efficiency from 2µm to 5µm higher than 50% given in the Figure 4 below:

Figure 4.

Computed absolute diffraction efficiency with the experimental groove profile of the low groove density 30gr/mm ruled plane grating, for unpolarized light, 1^st order (pink curve) and 2d order (yellow curve), from 1µm to 5µm.

The efficiency measurements at available wavelengths have shown a good agreement with the computed values. These gratings have been space qualified with extensive environmental tests (thermal cycling from -173°C to +70°C) and characterized to be installed in a spectrograph.

This successful heritage allowed us to continue the development of high performances ruled reflexion gratings for space flight mission, such as MAVEN.

2.1

MAVEN mission overview

The MAVEN mission is part of NASA’s Mars Scout program. Launched in Nov. 2013, entered orbit around Mars in Sept. 2014, the mission explores the Red Planet’s upper atmosphere, ionosphere and interactions with the sun and solar wind.

Scientists will use MAVEN data to determine the role that loss of volatiles from the Mars atmosphere to space has played through time, giving insight into the history of Mars’ atmosphere and climate, liquid water, and planetary habitability.

MAVEN’s instrument (Fig. 5) suite consists of eight sensors:

Figure 5.

Artistic view of the MAVEN satellite with Mars (NASA credit).

One of the mission results coupled with past observations data from NASA’s Mars Reconnaissance Orbiter and Mars Odyssey spacecraft suggested that there is not enough CO2 remaining on Mars to provide significant greenhouse warming were the gas to be put into the atmosphere; in addition, most of the CO2 gas is not accessible and could not be readily mobilized. As a result, terraforming Mars is not possible using present-day technology [4].

For the MAVEN Imaging UV spectrometer, we have designed and manufactured a low groove density ruled grating, specially tailored to work in two diffracted orders.

2.2

MAVEN UV Ruled Grating design

The MAVEN grating is optimized for the 1st order of diffraction at 280nm central wavelength and at 160nm central wavelength for the 2nd order. The blazed groove profile and blazed angle are carefully selected to have efficiency in this particular use configuration. To obtain a maximum efficiency in the two diffracted orders, the 276gr/mm MAVEN grating was designed with a blazed angle <3° which is very challenging to manufacture and have the needed homogeneity over the whole useful area.

The groove density was set to 276gr/mm, so the use of ruling machine to record the blazed profile was fully adapted. The grating size was chosen to quite large: H114 x L70mm with grooves parallel to the height and using a low thermal expansion coefficient glass.

Intensives runs of calibration and tests were performed on engineering models.

3.1

MAVEN UV Ruled Grating efficiency

The MAVEN UV ruled grating was measured in efficiency by using a scanning reflectometer system measuring the 1^st order and the 0^th order of diffraction from 190nm to 400nm (Fig. 6).

Figure 6.

Diffraction efficiency of the 1^st diffracted order (pink curve) and the 0^th order (blue curve) measured at 15° deviation angle, unpolarized light.

As we can see on the Figure 6, the efficiency is optimized in the UV part for the 1^st order and achieve about 70%, quite constant from 190nm to 280nm. In parallel, single wavelengths efficiency measurement (at 121nm and 161nm) under vacuum for the 2^nd order of diffraction exhibited an efficiency ranging from 40% to 47%. In order to enhance the diffraction efficiency in the FUV, an Aluminum with MgF2 overcoating was coated.

3.2

MAVEN UV Ruled Grating diffracted wavefront

The diffracted wavefront quality of the MAVEN UV ruled grating was measured by using a Phase-shift Fizeau interferometer, with 632nm wavelength. The PV and RMS values were obtained over the whole useful area to respectively 0.22lambda and 0.04lambda (Fig. 7).

Figure 7.

Diffracted wavefront of the 1^st order measured by interferometer. 0.22λ PV and 0.04λ RMS was obtained over the whole useful area, without power.

3.3

Low stray light performances of master Ruled UV Grating

Thanks to low grooves roughness <0.8nm RMS and highly accurate grooves positioning, the BRDF measurement performed at 325nm on UV master ruled grating exhibits a very good stray light performance without ghosts or “grass” effects (Fig. 8).

Figure 8.

BRDF measurement of a UV ruled master grating, at 325nm on two spots location (orange and brown curves)