Adaptive Optics for Free-Space Optical Communication (FSO)

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NASA’s Low-Cost Optical Terminal, Photo: NASA

The data collection is feasible during both nighttime and daytime.

However, unlike the radio frequency (RF) commonly used for telecommunications, FSO is restricted by the weather, like the presence of fog for instance. In contrast with RF communication, FSO operates in a license-free spectrum, immune to microwave/RF interference and other FSO links due to the narrow beam divergence and transmits high data rates up to 100 Gbps.

In FSO, high frequency wavelength is privileged because of the resistance of the signal propagation to fog (reduced scattering in light fog). In addition, 1.55 µm wavelength are usually used for safety reasons, as our cornea acts as an optical band-pass filter passing wavelengths ranging from 0.4 to 1.4 µm, longer wavelengths with high power do not impact the retina.

Atmospheric turbulence, a major obstacle for FSO

In the atmosphere, the photons may encounter diverse obstacles such as absorption by molecular constituents as water vapor, fog etc., or scattering, background noises, and the effect of turbulence. Within the troposphere, the concentration of particles is higher near the Earth surface and it decreases with increasing altitude up through to the ionosphere.
When the Sun heats the ground, the air immediately above it warms up and begins to rise. This warm air, buoyed by Archimedes ‘principle, then collides with and mixes with the cooler surrounding atmosphere, giving rise to the phenomenon we call “atmospheric turbulence”. This manifests as a variation of index of refraction due to temperature inhomogeneity, atmospheric pressure and wind velocities. The optical signals temporal intensity is thus fluctuating a lot, referred as scintillation or fading. A classic illustration of this phenomenon is the ‘twinkling’ of stars, where atmospheric turbulence distorts the incoming starlight.
The sensitivity of the receiver telescope is defined as the minimum amount of optical power needed to achieve a specified level performance, and is quantified by the bit error rate (BER). However, atmospheric turbulence can significantly degrade the BER by inducing wavefront aberrations and pointing errors. For context, while an ideal target BER is on the order of 10⁻⁹, severe turbulence can degrade this performance to as high as 10⁻³
During daytime operations, optical downlinks experience stronger background solar radiation and elevated ground-level turbulence. Consequently, spectral filtering via an ultra-narrowband optical transmission filter, combined with adaptive optics correction, is required to minimize the focal spot size of the downlink beam.

How adaptive optics can be the solution?

The effect of the atmospheric turbulence is non negligible, especially if one has to couple into a SMF at the output of a telescope, with a tiny core of about 10 µm. Coupling a distorted wavefront into a SMF leads to a significant reduction in coupling efficiency, resulting in substantial signal attenuation.

To correct the disturbed incoming light, we rely on adaptive optics (AO) principle. For an object from space, the wavefront is disturbed and not planar (flat). To retrieve a diffraction limit like image, we first analyze the incoming wavefront with a wavefront sensor and with a controller, apply the inverse shape of the wavefront to compensate the deformation with a deformable mirror. Correction is efficient if the loop runs as fast as the speed of the turbulence. The AO principle is illustrated in the figure below, using the image of Neptune from ESO as a reference.

The solution we offer at Imagine Optic: CIAO SWIR

At Imagine Optic, we offer an AO solution adapted for FSO application which is the CIAO SWIR device, a compact AO system that can be plugged at the output of the telescope and runs up at 3.5 kHz. It operates in the 1.0 to 1.7 µm, adapted for up to 1 meter class telescope with an input f# of f/9 to f/12. At the output of it, you can directly inject into a SMF and optimize the coupling efficiency by having control over the Non-Common Path Aberrations accessible via the WaveSky software, delivered with the device.

The output port of CIAO can be reconfigured to be adapted to your own coupler, collimated or converging beams are possible.

Several filter holders in the optical path are accessible by the user to add extra wavelength filtering in the case CIAO is used as a pre-compensator for the uplink

Results we achieved with CIAO SWIR:

Without AO, the turbulence causes strong losses in the coupling efficiency (up to -15 dB in the best cases, which do no occur often, as shown by the height of the right-most red bar).

With AO turned on, the average coupling efficiency is much better (-7.6 dB without -24 dB without AO), and the “spread” in coupling efficiency (how wide the distribution is) is much smaller. Meaning we are able to reach good coupling efficiency reliably.

PSF without AO (left) compared to PSF with AO (right)

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