Coherent Free-Space Optical Communications

[IMAGE: Rooftop at dusk with two telescopes on tripods, pointing toward the horizon, surrounded by cabling and electronics enclosures]

Figure 1: Roof-top free-space optical test range on Building 1. The transmitter telescope (150 mm Cassegrain, centre) is mounted on a motorised gimbal with arc-second pointing resolution. The receiver assembly (background, under the weather shelter) uses a 200 mm Schmidt-Cassegrain with a Shack-Hartmann wavefront sensor for adaptive optics.

Overview

This Phase II programme develops coherent optical communication links for high-bandwidth data transmission through the atmosphere. Unlike direct-detection (intensity-modulated) systems, coherent detection recovers both amplitude and phase information, enabling higher receiver sensitivity and compatibility with advanced modulation formats. Target data rates exceed 1 Gbps over multi-kilometre horizontal paths with bit error rates below 10⁻⁹.

The programme is supported in part through a consultancy arrangement with the European Space Agency and has benefited from Phase II funding awarded in March 2003.

Technical Approach

Transmitter Subsystem

The transmitter uses the division's single-frequency Nd:YAG MOPA at 1064 nm, modulated by a fibre-coupled LiNbO₃ phase modulator for BPSK and QPSK formats. Output is collimated through a 150 mm Cassegrain telescope with a transmitted beam divergence of approximately 20 μrad (diffraction-limited). The telescope is mounted on a motorised gimbal providing ±45° azimuth and ±30° elevation coverage with arc-second step resolution.

Receiver Subsystem

A 200 mm Schmidt-Cassegrain telescope collects the downlink signal. Coherent detection is achieved through homodyne mixing with a local oscillator derived from a second, phase-locked Nd:YAG NPRO. A Shack-Hartmann wavefront sensor (32×32 lenslet array, 1 kHz frame rate) drives a 37-element deformable mirror for real-time atmospheric turbulence compensation. The adaptive optics loop achieves a Strehl ratio exceeding 0.6 under moderate seeing conditions (r₀ > 5 cm at 1064 nm).

[IMAGE: Block diagram showing signal flow from transmitter laser through atmosphere to receiver with adaptive optics and digital signal processing]

Figure 2: Block diagram of the coherent free-space optical link. Transmitter chain (left): laser, phase modulator, power amplifier, telescope. Atmospheric path (centre). Receiver chain (right): telescope, adaptive optics, homodyne mixer, balanced detector, clock recovery, decoder.

Phase-Locked Diode Laser Arrays

A parallel effort investigates coherent beam combining using phase-locked arrays of up to 16 diode lasers. The approach uses an external-cavity Talbot configuration to enforce mutual coherence across the array without requiring active phase control of each emitter. Combined output exceeding 5 W in a near-diffraction-limited beam has been demonstrated. [Tanaka et al., Appl. Opt. 2003]

Field Trials

Horizontal-path testing is conducted between the Building 1 roof platform and a retroreflector/transceiver station located on the Building 6 roof (path length approximately 180 m). Longer-range trials using a corner-cube retroreflector on a nearby hill (path length 2.8 km) are planned for mid-2004, pending regulatory approval for the remote station.

Related Publications

Tanaka A. et al. — "Phase-locked diode laser array for coherent beam combining." Appl. Opt. 42, 5637–5643 (2003).
Voss H., Tanaka A. — "Adaptive optics for horizontal-path coherent optical links." (In preparation, 2004.)

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