1. An Optical Receiver for Interplanetary Communications Jeremy Bailey

2. JPL Concept for Mars Link 10m Telescope Laser transmitter Q switched Nd:YAG Laser (1064nm) 1W average power 10cm telescope 2 AU Si APD detector 256-PPM modulation 30 kb/s 646 photons/pulse (Ortiz et. al. 2000, TMO Prog. Rep. 42-142) MARS EARTH

3. Problems with JPL Scheme 30kb/s is comparable to current radio systems Sensitivity falls far short of quantum limit –Problems with analogue APD detection. –Large detector size required to match telescope. –Background from daylight sky and Mars. 10m telescopes are expensive - and need several of them. Operating when Mars is near the Sun impossible.

4. Detection Schemes Analogue Direct Detection with APDs - –Measure the output current of the diode with a low noise preamplifier. –Standard technique in fibre optic communication. –Limited by thermal noise and excess noise. –Best performance - 40 photons/bit in ESA SILEX system - more typically ~200 photons per/bit for Si detectors, >1000 photons/bit for InGaAs detectors.

5. Coherent Detection Mix input signal with local oscillator and detect at the beat frequency. Quantum limited performance - e.g. 4.5 photons/bit demonstrated in ESA experiments. Not suitable for large ground based telescopes. Good for space to space systems.

6. Photon Counting Detectors Essentially noise free provided signal is well above dark count. Quantum limited performance (0.4 photons/bit demonstrated in 256-PPM) – Katz,1982, TDA Prog. Rep. 42-70 Dead time (~50ns) prevents high speed operation. –Can’t detect a narrow pulse. –Max count rate ~10 7 photons/sec

7. Solution - Multiple Photon Counters 1 photon counter n photon counters dead time

8. Multi-Telescope Telescope 10m effective aperture, made from 25 individual telescopes mounted together. Each telescope feeds a photon counter via optical fibres. Individual telescopes are f/5, matched to fibres.

9. Advantages Overall telescope is compact. No need to make a large mirror. No problems matching detector size. No need for active control systems. Long (f/5) tube assemblies can be baffled to allow operation within ~10 degrees of Sun. Redundancy. Scalable to any size you want.

10. Daylight Operation Essential for system to be able to operate in daylight. –To follow Mars throughout its orbit. –Background from Mars itself will be at daylight levels. Need narrow band filter with width ~10 –6 –Tunable to follow orbital motion of spacecraft –Can be achieved by Fabry-Perot and probably other technologies.

11. Performance 532nm 1W transmitter at 2AU (3 x 10 8 km) 20cm transmission telescope - 0.55 arc sec beam 800km beam width at Earth Received power in 10m telescope = 1.6 x 10 –10 W = 4.2 x 10 8 photons/sec With 20% efficiency and 5 photons/bit this will support communication at 16 Mb/s (uncoded). Receiver with 25 detectors can handle 50 Mb/s (with more detectors could go up to ~500Mb/s)

12. Transmitter RSEncoderConvolutionalEncoder Laser Electro-optic cell switched between +/–  /4 Transmitter Telescope Polarization Modulated beam Data Input 532nm Nd: YAG Laser

13. Receiver Data Processing APD Modules Photon Timing Computer System TimeSeriesreconstructionDemodulation ConvolutionaldecodingRSDecoding Photon pulses Data Output Fibres From telescope Photon arrival times

14. Conclusions An optical communication reciever built around multiple APD photon counters can provide: –Quantum limited operation. –Data rates 100-1000 times greater than JPL’s concept (and existing radio systems). –Simplification of many large-telescope design issues. –Scalability to any size telescope. –Data rates up to ~500Mb/s.