Before a multi-link GRACE mission can be realised, more analysis and testing will be required. In addition to the tests outlined in Section 7.3, alternate multi-link implement- ations should be investigated and the optical head(s) should be designed and tested. In this section, some of the work needed to take multi-link interferometry from concept through to mission architecture is discussed. This includes additional tests of the core challenges, investigating some variations on the multi-link interferometry architecture, and continuing the work that has started on the optical head.
8.2.1 Additional tests of core challenges
In Section 7.1 the results of the three experiments presented in this thesis were discussed. From this discussion a number of additional tests were highlighted that need to be ad- dressed before a multi-link GRACE can be considered viable. Further work on the multi- link GRACE concept must address the following:
Experimental verification of 1 fW tracking
In Chapter 4 it was predicted that with pre-stabilised lasers a 1 kHz bandwidth could be used to track a 1 fW signal with a cycle slip rate less than 10−6 cycle slips per second. This was not tested as part of this thesis due to the un- availability of frequency stabilised lasers, however it needs to be experimentally verified.
Verify GRACE TDI combination
Although it has been demonstrated in a benchtop experiment as described in Chapter 5, if the tone-assisted TDIR experiment developed as part of the LEGOP project is tested on GRACE-FO this will help to verify TDI as a viable frequency suppression technique in a multi-link GRACE. This will also confirm TDI can be used with large relative spacecraft motion.
Test multi-link concept with two optical head arrays
As an extension of the experiment described in Chapter 6 the multi-link inter- ferometer should be tested with two modeled spacecraft, each with three optical heads. This will allow the cancellation of rotation-to-pathlength coupling to be tested with 18 links. In addition to confirming the rotation of both spacecraft can be canceled in post processing, this can also be used to test both the link and DEHI code acquisition that will be needed in a multi-link GRACE.
8.2.2 Investigate alternate multi-link configurations
In addition to these core tests, alternate multi-link configurations should also be investig- ated. The multi-link GRACE presented in this thesis is an example implementation that has been used to explain the multi-link architecture. The multi-link interferometer concept could however be implemented in a number of ways. Other implementations should be considered in order to determine the optimum implementation.
8.2 Further work
At its core the multi-link architecture uses multiple interspacecraft displacement meas- urements to synthesise a centre of mass displacement in post-processing. While the im- plementation discussed in this thesis used time delay interferometry to suppress the laser frequency displacement noise, digitally enhanced heterodyne interferometry to multiplex signals and did not use a dedicated acquisition strategy, there are alternate implementa- tions. For example the multi-link technique could also work with offset phase locked lasers, using different heterodyne frequencies and separate detectors in each link, to multiplex. Figure 8.1 shows such an implementation. In the figure, spacecraft 2 is shown. In each optical head path an acousto-optic modulator (AOM) is used to shift the frequency. Each AOM on the spacecraft would shift the light by a different frequency to allow the links to be distinguished on the distant spacecraft. Each path has a separate detector to avoid double passing the AOMs. This could also be achieved using a single detector in the re- turn path, however separating the 9 beatnotes on the detector may be more difficult. The displacement measured along one of the links is used to phase lock the spacecraft 2 laser to the pre-stabilised spacecraft 1 laser, minimising the impact of laser frequency noise in the displacement measurements.
AOM AOM +f2A AO M AOM x2:AA(t) x2:BA(t) x2:CA(t) OH2A OH2B OH2C ADC C controller laser 2 frequency reference p h a se l o ck z x y FPGA PM interspacecraft links PM PM AD C AD C AD C Esig,2:AA(t-τ) (f1A) Esig,2:BA(t-τ) (f1B) Esig,2:CA(t-τ) (f1C) D AC ELO,2:A(t) (f2A) c. m re fe re n ce p la n e detector line-of-sight analog-to-digital converter digital-to-analog converter acousto-optic modulator
Figure 8.1: An alternate multi-link GRACE implementation. Time delay interferometry and digitally enhanced heterodyne interferometry can be replaced with phase locking on spacecraft 2 and acousto-optic modulators with separate detectors in each link to multiplex the optical head signals.
Using multiple detectors and separate return paths for each optical head path may avoid some of the complications with a TDI/DEHI based multi-link interferometer. This ap- proach however is hardware intensive as the number of detectors per spacecraft will scale with the number of links. Relying on different heterodyne frequencies in each arm to distinguish between links on the distant spacecraft may also be complicated by doppler shifts due to relative spacecraft motion. Fortunately, the doppler will shift the frequency in each arm by approximately the same amount, therefore it should be possible to use an FFT based acquisition to determine the frequency of each beatnote.
This is one example of an alternate implementation. Other modifications could include using digitally enhanced homodyne interferometry [30] for multiplexing, link acquisition similar to the GRACE-FO scheme [13, 14], or even using a larger number of optical heads per spacecraft to sense additional degrees of freedom. These alternate implementations should be considered in the further design of a multi-link GRACE.
Chapter 8 Conclusions, further work and other applications
8.2.3 Building an optical head
In Chapter 6, the suppression of rotation-to-pathlength coupling was investigated using a optical head approximated with traditional optics. An alternative to positioning multiple optical heads on each spacecraft is to have a compact, multi-element device attached to one side of the spacecraft. This would allow the spacecraft integration to be simplified further. The signal-to-noise ratio of the synthesised centre of mass measurement will however suffer, as discussed in Section 3.5.4.
In parallel to the work described in this thesis a compact 3 element optical head has been developed in collaboration with OptoFab at Macquarie University using 3D laser writing in a glass waveguide chip [144].
Figure 8.2 shows microscope images of the input and output interfaces of the chip. The waveguide chip is used to remap a 1-Dimensional array of fibres (a) into a 2-dimensional configuration of emitters (b). In addition to the triangular configuration that has been considered throughout this thesis, additional emitters have been added to allow different combinations to be tested.
To use a waveguide chip in a multi-link GRACE, since the optical head is both an emitter and receiver, the output interface needs to be collimated. In Figure 8.3 the waveguide optical head prototype is shown. A v-groove array of fibres and a 30µm microlens array have been bonded to the waveguide chip using UV curing glue. The waveguide assembly has also been mounted on a glass slide to provide strain relief to the fibres.
The waveguide technology is promising as a way to miniaturise the multi-link interfero- meter but it needs testing to determine if it is robust for launch. Although early testing has shown comparable suppression of rotation-to-pathlength coupling to that reported in Chapter 6 further testing is required. This prototype multi-link GRACE optical head is also very premature and it is envisioned that more of the interferometer architecture could be incorporated into the waveguide chip. At current the EOMs needed for the pseudo ran- dom noise modulation are in fibre, with fibre splitters used to split the laser between the different optical head paths. A more compact optical head design could do the splitting and phase modulation in the glass [145].