AOTFs in space
Gooch & Housego has over many years built a strong reputation for the design of innovative acousto-optic devices, including Tunable Filters (AOTFs) . Adherence to rigorous quality-control procedures has attracted the attention of customers for whom performance reliability in the harshest of environments is of paramount importance. One such customer is the Belgian Institute for Space Aeronomy (IASB-BIRA) and G&H has worked with IASB-BIRA (and by extension ESA) for many years on tailoring AOTFs for spectroscopic applications.
One of the outcomes of the AOTF development work is the supply of two G&H designs for incorporation into the NOMAD instrument, part of the ExoMars mission with an expected launch-date of 2016. NOMAD is a 3-channel instrument (UV/Vis + 2xIR) which aims to further characterise the Martian atmosphere. The AOTF development and supply has been funded by the Belgian Federal Science Policy Office (Belspo). It will be part of the payload of a satellite due to enter Mars orbit in late 2016.
Both AOTFs are fundamentally similar in concept with each acting as an order-sorter in a diffraction-grating based spectrometer; that is they choose the order (and thus the wavelength-window) at which the grating diffracts. The first AOTF has a 5mm aperture and operates in the IR (2-4µm) wavelength range. It is used on the “SO” (solar occultation) IR channel.
The second AOTF is essentially a larger version of the “SO” AOTF and draws upon G&H’s unique ability to routinely grow good quality large boules of single crystal tellurium dioxide (TeO2) at the G&H (Ohio) facility. The increased (15mm) aperture, 3 times the size of the SO AOTF, in principle enables a nine-fold increase in the throughput. This is a critical factor for the instrument design and hence performance. It will be used on the “LNO” (limb, nadir and occultation) IR channel.
AO technology inherently offers the fundamental advantages of delivering electronically controlled solid-state components that are rugged with no moving-parts, compatible with operation in a vacuum. These AOTFs are noteworthy due to the attention to design and manufacture in order to meet the demanding requirements of use in space. There has been significant development in order to engineer them to satisfy these requirements, particularly the extended temperature range in which they have to survive and operate.