Adaptive Optics


Adaptive optics (AO) are optical devices or technologies used to correct wavefront distortions on a fast timescale (100 – 1kHz). Adaptive optics are different from active optics, which work on a slower timescale (~1 Hz). AOs are used in astronomy, in laser communication systems like FSO and fiber-optics, in microscopy, and in retinal imaging to fix aberrations and distortions in incident waves. AOs can also be used to “pre-correct” a wavefront before it is transmitted; this is especially useful for ground-to-space communications through atmospheric turbulence.


Adaptive optics remove the distortion from atmospheric turbulence by correcting the wavefront with a deforming mirror (DM) or actuator mirror array. The goal is to match the wavefront’s profile with the DM so that the reflected plane waves are flat, removing aberrations and distortions. The distortions from the incident light are measured using a wavefront sensor, which passes information to a computer which calculates the perfect mirror shape to remove the distortions, and then the DM is shaped to account for the calculations. Often, the lower-order distortions, i.e., the large distortions or those with great magnitude, will be corrected by a single tip-tilt mirror and the higher-order distortions are corrected by a DM.

The modulation technique also determines how effectively a signal’s power is concentrated on to the detector. Typically, deep-space optical communication bits are determined by what time slot a pulse arrives in (pulse position modulation) as opposed to turning a laser on and off to send a signal (on-off-keying, or OOK). In 2008, a silicon avalanche photodiode (Si-APD) and a 64-bit pulse-position modulation (64-PPM) scheme was able to achieve a 6 dB gain in signal receiver using a 31-actuator mirror array. A gain closer to 3 dB was achieved using an 11-actuator mirror array. 64-PPM exceeded OOK and 16-PPM gains by 0.3 to 0.5 dB in total. [1]

Practical Applications

For FSO purposes, adaptive optics are built into the on-the-ground system and are used to pre-correct the transmitter beam before it is sent into the atmosphere. The on-the-ground system measures the pre-transmitted wavefront of the signal, converts it into actuator commands that adjusts the beam’s wavefront, and then the signal is transmitted. This approach, in part, mitigates the effect of atmospheric turbulence.

When the coherence length of the atmosphere changes, as is common with atmospheric turbulence or other limitations, the atmospheric seeing phenomenon occurs. This phenomenon manifests as perturbations in the optical beam and blurring of the received signal. Turbulence in the atmosphere will distort the received laser beam wavefront, leading to spot motion jitter or image dancing at the receiver’s focal plane. However, the effects of atmospheric seeing can be limited by AO or array detectors.  The effects of angle-of-arrival fluctuations can also be compensated using AO. [2]

A study done by researchers in South Australia used AO to assist with the design of an on-the-ground system by calculating the uplink/downlink mitigation achieved. The paper showed that, for a low-altitude orbit, boundary layer turbulence dominates downlink, which poses a challenge for AO in terms of coupling efficiency at the receiver (Grant, et. al).