1. Phase and Gain stability of optical fibre link used in MeerKAT Author : Roufurd Julie Supervised by : Prof Michael Inggs SKA HCD bursary conference Dec 2008

2. Outline of Presentation Background / Introduction to project Project Objective Relevance of Project to SKA/KAT Methodology & Results Conclusions

3. Background / introduction to project The meerKAT project consists of 80 antennas configured as an interferometer Optical fibre is used to transport the RF signal Interferometry requires a GAIN and PHASE stable RF path

4. Why Optical fibre? Very high bandwidth -> greater sensitivity Low attenuation -> signal travel further

5. Why analogue optical modulation? Advantages Simpler and cheaper Geographically separate the noisy digital backend from sensitive RF front end Disadvantages Needs much better S/N than digital Limited dynamic range (at this stage!) Signal travels further before being digitised, and is susceptible to amplitude and phase effects

6. Optical terminal equipment manufacturers

7. Background / introduction to project The meerKAT project consists of 50+ antennas configured as an interferometer Optical fibre is used as the signal medium Interferometry requires a GAIN and PHASE stable RF path

8. Defining PHASE and GAIN stability Stability in this research, means some parameter change in terms of time GAIN stability is GAIN change over time Similarly PHASE stability is PHASE change over time. Ideally, GAIN and PHASE should be constant with time (all other things being equal)

9. Small note about phase and propagation delay

10. Relevance of project to KAT / SKA / other Phase instability has a deleterious effect on the GAIN of the synthesised beam i.e. % decorrelation Not good for Imaging dynamic range GAIN stability has an influence on the sensitivity of the radio telescope These effects need to quantified as it influences how often system needs to be calibrated

11. Project objectives Investigate properties of the optical fibre path that affects phase and gain stability Measure and report on performance of optical link using a commercial Tx and Rx pair.

12. Some quick facts Ambient temperature around cable affects phase stability Loose-tube cabled optical fibre has been shown to have the lower of the delay/kelvin coefficients Bending of fibre causes attenuation which affects gain stability Laser Tx power is affected by temperature, thus these units need to be kept at constant temperature Units used were Miteq & Photonics, 1550nm, DFB laser with external modulation

13. Methodology Find out phase requirements from the meerKAT Imaging team. (Not complete success yet) Study physical properties of components in the Optical fibre chain that contribute to phase and amplitude instabilities Create engineering tools to measure the stability Quantify VNA and optical terminal equipment stabilities Study the HartRAO optical fibre link installation

14. Tools 1 – Measuring system

15. Tools 2 - Labview

16. Tools 3 – Optical Tx and Rx

17. Tools 4 – Overall setup

18. Results 1 – Group delay changes @ SUNRISE

19. Results 2 – Phase changes @ 1.5GHz during SUNRISE

20. Results 3 – Gain changes @ 1.5GHz during SUNRISE

21. Phase @ 1.5GHz vs temperature. 2 hr snapshot during SUNSET

22. Results 4 – Phase @ 1.5GHz vs Azimuth position (mean removed)

23. Results 5 – Phase vs Elevation position (mean removed)

24. Results 6 – GAIN @ 1.5GHz vs Elevation position

25. Results 7 – GAIN vs Azimuth position

26. Results 8 - Gain [dB] vs Azimuth

27. Conclusions This optical path will have an effect on the amplitude and phase performance of the system I measured the performance of the HartRAO links. Movement of the dish affects GAIN more, while Temperature affects PHASE more Data analysis still in progress

28. Acknowledgements SKA HCD for bursary funding Mike Inggs and Venkat for their daily inputs Mike Gaylard for his many hours of trying to explain interferometry to me KAT project office for usage of measuring equipment KAT and HartRAO staff for their various bits and pieces along the way.