4.1

Improved Analysis Methods

For the first experiment using real turbulence, infrared cameras were chosen for analysis, enabling easier alignment and facilitating easier measurement small changes to the PAA between the terminals. The limited dynamic range of the camera presented a drawback of this approach and the dramatic difference between measurements with and without compensation lead to overexposure as has been documented⁸. As an alternative analysis method and developmental step towards the application scenario, we have adapted the infrastructure to allow for AO analysis with fiber coupling systems and power meters instead of cameras in both the GTB and STB. This removes the constraints of the cameras’ dynamic range and allows for future integration in existing digital communication infrastructure.

4.2

Mechanical Solutions for the GTB

The universal nature of the AO-box is achieved by the conjugation of all AO elements and the telescope aperture to the entrance pupil of the box. This offers itself to coupling with any telescope, provided the correct choice of interface optics and mechanics. We were provided the opportunity to replace the GTB telescope, an 8” Meade LX200-ACF F/10 telescope with a 12-inch ASA Astrograph N200 Newton telescope, courtesy of Astro Systeme Austria GmbH (ASA). The F/3.6 telescope is of much higher quality and increases the receiving aperture of the downlink beam while increasing the theoretical resolution of the focussed uplink beam.

Motorized alignment of critical system components reduces the necessity of human contact with the box allowing for faster and more stable alignment. As an initial step, we made a replacement of manual mounts which were most prone to losing their stability with high quality motorized ones. This allows for a more stable system and facilitates operation of the system remotely, reducing the risk of accidental misalignment. The ASA Astrograph 12” F/3.6 was provided with the Direct Drive DDM85 Standard Mount with motorized right ascension and declination axes and Autoslew software, facilitating high accuracy pointing and tracking and for our purposes, enabled the motorized alignment of the AO-box and telescope. Additionally, the periscope in the telescope to AO-box interface optics (Mirrors M1 and M2) was also motorized for tip and tilt which allowed remote fine alignment of the complete telescope and AO-box optics via the use of the two new cameras, one for the point spread function (PSF) and one for the collimated beam. The Standa 8MBM24 motorized mirror mount were selected for this purpose, with a resolution of less than 1 arcsecond and a large tilt range of +/- 4:5o. This almost completely removes the need for physical human contact during alignment of the interface optics, the one exception being the need to introduce and remove the flip mount mirrors which have been added to the original optic design in order to divert the beams to the alignment cameras.

The point ahead mirror (PAM) mount was also replaced with the motorized mirror mount Standa 8MKVDOM-1 to allow for a larger positioning range of an effective +/- 7 mrad at the ASA telescope aperture.

The 5 degrees of freedom stage used to support the wavefront sensor (WFS) is necessary for precise alignment, but suboptimal mechanical loading of the element in the original mechanical design lead to loss of alignment. This was amended by a slight change to the optic design which allowed for hanging of the stage from the edge of the AO-box breadboard and consequently a better weight distribution.

4.3

STB Redesign

The new STB design addresses operational difficulties of the previous design, such as back reflections, long term mechanical stability and allows for measurement of large PAAs with variable displacement of the transmitting aperture and receiving aperture by up to 50 cm. For a 1 km separation of the GTB and STB, this allows for measurement of up to 0.5 mrad. The STB has been split into two subsystems - the STB DL which included the transmitting downlink laser and facilities for measurements of the uplink without a PAA, and the STB UL for measurement of the uplink signal under a PAA.

4.4

The STB DL

In the original STB design, the downlink beam was provided by a collimated laser beam with a polarizer and analysis of the uplink beam was made using a 10:1 telescope and an infrared PSF camera. The downlink and uplink channels were separated via a polarizing beam splitting cube with an anti-reflection coating which lead to on-axis back reflections of the downlink in the uplink analysis camera. These reflections exceeded the uplink signal in intensity and could not be excluded via apertures. Changes to the optical design have avoided the use of a polarizing beamsplitting cube instead using a plate, enabling the separation of the back reflections from the optical axis and providing the opportunity to use a field stop. Additional optics have been included to allow for variable divergence of the downlink beam and the rough alignment of the entire STB and GTB has been simplified by the inclusion of a VIS camera for greater pointing accuracy.

4.5

The STB UL

In order to measure large PAA values, a second aperture with variable position was necessary. This is achieved using a mirror which is mounted on a motorized linear stage diverting the uplink signal into the STB UL box via a second mirror. In order to correct the internal angular error which arises from changing PAA, the first two mirrors in the path are motorized for tip and tilt. For analysis of received light, we integrated a powermeter (Thorlabs PM100 with S122C Sensor).

4.6

Environmental Insulation

In our previous measurement campaign, experienced temperatures were in the range of -4ο C to +10ο C and unsettled conditions such as rain, wind and fog were frequent. Both terminals must be equipped for relatively high exposure. It was seen that the laser diode temperature controllers could not maintain a temperature of 15ο C when the environmental temperature was below approximately 5ο C, leading to an unstable laser source. In order to most efficiently solve this problem and to avoid overheating of the atmosphere surrounding the terminals contributing to additional artificial turbulence, we used smaller heating elements and 3D printed insulation boxes specifically for use with the laser diodes.

The STB container was not bolted down, and therefore relatively unstable. It was our experience that a small redistribution in weight, such as when a person in the container stood up, lead to a loss of alignment. Though these conditions are specific to this measurement site, it was further motivation for the remote control of the optical setup. Remote control also permits for isolation of the optics with a more robust dust and water proof housing, allowing for greater choice in the location of future measurement campaigns.

5.1

Measurement Campaign

The most recent measurement campaign took place in Germany in late 2017. It was performed on a horizontal 1 km path, over a largely green landscape. Local weather conditions were recorded including air temperature, wind, air pressure and humidity. Measurement times were typically in the evening in the absence of daylight.

The coarse alignment of the system was aided significantly by the integration of the VIS camera in the STB as well as the motorised and controlled ASA telescope in the GTB. Overall mechanical stability was satisfactory enough that the coarse alignment needed to be performed at most once per day. The time required for fine alignment of the GTB was also reduced through the use of the motorised periscope optics and additional beam and PSF cameras. The overall mechanical stability at the GTB was greatly improved on that of November 2016, so that realignment over the course of a measurement day was largely unnecessary.

The scientific goal of the measurement campaign was to investigate pre-compensation under a PAA. Here, we present a first look at a sample measurement sequence for a PAA of 0.273 mrad, corresponding to a displacement of the transmission aperture and receiving apertures of approximately 27 cm at the STB. We compare consecutive measurements with and without compensation at both the GTB and STB. Post-compensation and the general functioning of the AO is analysed using a Zernike analysis of the reconstructed wavefront as measured at the GTB. Pre-compensation is analysed using powermeter measurements from the STB UL. At the time of the reported measurement weather conditions were cold (2.5 o C) and calm (1007 hPa) with little wind (< 3 km/h).

5.2

Downlink analysis

The downlink analysis has been made by performing a Zernike polynomial deconstruction of the reconstructed wavefront as recorded in the AO-box of the GTB. Figure 3 presents the results of this analysis for a sample measurement sequence with (blue) and without (orange) AO compensation. The black line represents a derived theoretical value for D/r0 equal to 1⁹. A best fit of the measurement without AO compensation suggests a value of D/r₀ of 5.8, corresponding to a r₀ value in space of 5.2 cm. The blue curve corresponds to a D/r₀ value of 0.4, indicating that the use of AO improved the effective D/r₀ by approximately 5.3. It can be seen from this measurement that the AO was particularly effective in the compensation of modes 5 to 8, astigmatism and coma.

Figure 3

The wavefront as measured with the WFS was first reconstructed and then deconstructed as part of a Zernike annular polynomial analysis. The results were then compared to theory as indicated in black for D/r₀ = 1⁹. In blue, the compensated wavefront analysis has a best fit D/r0 equal to 0.4 whereas the uncompensated wavefront in orange has a best fit D/r0 equal to 5.8.

5.3

Uplink Analysis

In Figure 4, measurements of the power received at the STB under a PAA of 0.273 mrad are plotted. The orange curve represents the measurement in the absence of pre-compensation and was made simultaneously to the measurement in Figure 3. The blue curve represents the measurements taken with pre-compensation, and likewise corresponds to the blue curve in Figure 3. The mean value of each curve is depicted with a broken black line. The time scale is relative to each measurement, as they were not performed simultaneously.

Figure 4

A sample sequence with AO pre-compensation (blue) and without (orange) for a PAA of 0.273 mrad, corresponding to the same measurement sequence as is depicted in Figure 3. The mean value of each data set is depicted with a broken black line. Pre-compensation has increased the mean power measured from 58.6 +/- 40.2 μW to 136.6 +/- 32.8 μW.

With the application of pre-compensation, the overall mean power is increased from 58.6 μW to 136.6 μW or 3.7 dB. The absolute standard deviation of the measurement is also reduced from 40.2 μW to 32.8 μW. There has been an overall increase in the received power and a measurable increase in the stability of the signal.

This is a single sample measurement sequence at a large PAA of 0.273 mrad. During the measurement campaign, measurements were performed with a PAA of up to 0.32 mrad, at which no further benefit of pre-compensation could be measured. While these results are preliminary, they are very promising for the application of pre-compensation and moreover, they support the hypothesis of using focussed uplink beams in horizontal turbulence scenarios for the sake of increased PAA.