Telescopes, for optical com, are the equivalent to the huge radio dishes in radio frequency com- both are buckets to collect signal photons and concentrate them on detectors, and both are used to direct transmitted photons efficiently towards a distant receiver – the only reason the two buckets look so different is the wavelength of the signal light they need to collect and transmit.

For optical com we are not concerned with using the telescope to get pretty pictures, but many of the telescope requirements are the same as those for astronomy. For communications our goal is to collect as many signal photons as possible from a point source (a laser transmitter on a spacecraft) and concentrating them on a tiny detector (often the signal detector diameter is measured in 10′s of microns). So ideally we would ‘image’ a point source into a point at the image plane of the telescope… unfortunately this is impossible, we can only try to get as close to perfect as possible.

In reality the finite size of the telescope itself sets a fundamental limit to how small a point source can be focused (The spot size will scale as wavelength of the light divided by the diameter of the telescope- the larger the diameter the smaller the spot- the smallest sized spot a telescope imaging a point source can have at its focus is determined by this diffraction limit). We also have to consider the impact of the turbulent atmosphere- for larger telescopes this is what degrades the performance the most.

You can visualize the light wavefronts from a distant point source as being flat planes travelling towards you – when these flat planes hit the atmosphere the turbulence in the air forces these flat planes to travel through pockets of denser air or air of different temperature- distorting the flat wavefront into a warped ‘potato chip’. This prevents the light received by a telescope from being focused to the telescope’s diffraction limit.

For many optical com applications you need to get the received light into a single mode optical fiber that has a diameter of only 12 microns, your telescope may have a diffraction limited spot size of 10 microns- meaning it could couple all the received light from a point source into the fiber- BUT in reality turbulence could blur this spot to 30 microns, meaning you can only get a small fraction of the received signal into the smaller fiber.

The only way a large telescope in the earth’s atmosphere to approach its diffraction limit is to make use of Adaptive Optics (AO). An AO system measures how the incoming light is distorted by the atmosphere, and then bounces the distorted light off a mirror that is distorted in just the right way to remove the atmospheric distortion. The deformable mirror is a reflecting membrane with 100′s of micro-actuators to precisely warp it to invert the distortion caused by the atmosphere. Since the atmospheric turbulence varies with time rapidly, the AO system has to measure the distortion, calculate the correction, and control all the actuators at a rate of 100′s or 1000′s of times a second. These AO systems are very costly and complex. Easily costing more than the telescope and mount.

Further complicating the task of receiving the signal it the problem of background light- the signal is detected by sensitive photon counting detectors- able to detect a single photon. You want to be able to pick out the signal photons and detect them day or night, even if the transmitter is near the sun in the sky. This means using very precise optical filters that block out all light EXCEPT the light that has the exact wavelength of the signal.

For the Lunar Laser Communications Demonstration the space terminal at the moon was transmitting ~0.5 Watts of laser light, the intensity of this signal at the Earth was measured in nano-Watts per square meter. The intensity of sun light at the earth’s surface is ~1000 Watts per square meter- so to detect the signal without being swamped by noise from background light requires very good filters…