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

Outline of Presentation

• Background / Introduction to project

• Project Objective

• Relevance of Project to SKA/KAT

• Methodology & Results

• Conclusions

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

Why Optical fibre?

Very high bandwidth -> greater sensitivityLow attenuation -> signal travel further

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

Optical terminal equipment manufacturers

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

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)

Small note about phase and propagation delay

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

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.

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

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

Tools 1 – Measuring system

Tools 2 - Labview

Tools 3 – Optical Tx and Rx

Tools 4 – Overall setup

Results 1 – Group delay changes @ SUNRISE

Results 2 – Phase changes @ 1.5GHz during SUNRISE

Results 3 – Gain changes @ 1.5GHz during SUNRISE

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

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

Results 5 – Phase vs Elevation position (mean removed)

Results 6 – GAIN @ 1.5GHz vs Elevation position

Results 7 – GAIN vs Azimuth position

Results 8 - Gain [dB] vs Azimuth

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

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.