# Towards NGGM: Laser Tracking Instrument for the Next Generation of Gravity Missions
## 3 LTI Units
### Optical Bench Assembly (OBA)  
The optical bench hosted the majority of the free space interferometer optics. It incorporated the fiber-to-free space interface of the laser, optical elements for beam clean-up, shaping, routing and splitting of the laser light, as well as the FSM and two-quadrant photo receivers. To achieve the required highly stable interferometric measurement between the local oscillator and the received beam from the other spacecraft, the optical bench had several key features, including:  
Figure 6: Left: cubic ULE spacer with 5 cm length. Right: titanium mounting frame with the first gold-coated heat shield, holding the ULE spacer in place via a tetrahedral mounting arrangement (image credit: NPL London, UK).  
Figure 7: Photo of the thermally controlled vacuum system containing the cavity (image credit: Airbus DS).  
**Beam steering:** The OBA fine steering mirror allowed the LTI to counter any satellite pointing error with microradian accuracy. Control of the mirror was performed on the basis of the DWS error signal with the ICU.  
**Beam walk compensation and ranging noise compensation:** The OBA entrance aperture and the steering mirror surface were imaged onto the detector plane of the quadrant photoreceivers (QPRs), thereby avoiding any beam walk and aperture diffraction effects on the quadrant photoreceivers. Furthermore, a \"compensation plate\" was introduced to significantly reduce angular coupling into the ranging measurement.  
Equally important were a very good absolute wavefront planarity of \(\lambda/12\) (\(p\)-\(v\)) over two times the Gaussian beam diameter, a high multipath suppression and a high pointing stability. To achieve this, special care was put into the CTE (coefficient of thermal expansion)-compensated low-stress mounts, optical surfaces vertical to the beam were avoided and critical optics have well-defined, non-zero wedge angles (see Figure 8). For all but one (the second beam splitter) internal optical components, no fine adjustment was needed, significantly reducing the integration effort and increasing the robustness of the structure. The approach achieved a high thermal noise suppression (due to the high thermal mass enclosing the components) and high beam pointing stability (due to the symmetry of the titanium body with respect to the plane containing the beam). The optical components were mostly out of BK/G18, which matched the CTE to titanium well, further improving the thermo-optical stability of the OBA. The only elements that required precise alignment were the beam collimators, called fiber injector assemblies (FIAs), which were ultra-stable fully monolithic designs, in which the fiber was directly spliced to the collimator lens and the quadrant photoreceivers. The FIAs were aligned in an angle with shim rings and in a lateral position with micrometer screws (which were the ground support equipment). The QPRs were aligned in a longitudinal position with shim rings and in a lateral position again with micrometer screws, which were the ground support equipment and were later removed.  
The difference between the GRACE-FO OBA and the NGGM OBA(s) was related mainly to the evolution from an experimental interferometer to the main instrument, leading to a required increase in lifetime, reliability and mechanical loads. Figure 9 shows the current CAD design status of the NGGM OBA with redundant FIAs and an updated mechanical interface.  
Figure 8: Optical bench of GRACE FO. (**a**) Cross-sectional view of the optical bench; (**b**) optical bench flight spare.  
The optical bench for the NGGM LTI was based on the design developed for the GRACE-FO LRI, which has been successfully operating in orbit for more than 4 years by now. The heritage of the design was considered high, and the focus on the current development activities lay on the redundant FIAs and the verification of the suitability of the updated mechanical interface.