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Abstract
The Uplink Wavefront Corrector System (UWCS) is a pathfinder instrument to demonstrate the uplink correction by Adaptive Optics techniques; this novel application can be directly usable in two fields: Free-Space Optical Communications and the generation of Laser Guide Stars. A Rayleigh LGS is propagated to the sky while the atmospheric wavefront aberrations are measured by a Shack-Hartmann WFS with 12 x 12 sub-apertures using a Natural Guide Star as a reference. The laser upwards propagation path is then pre-compensated by a 97-actuator deformable mirror. A scoring camera is attached to the finder telescope, next to the main aperture, in order to show the overall result, which is assessed in terms of beam power concentration. Present paper described the design process of the UWCS and its integration and testing in the Optical Ground Station telescope, at Teide Observatory (Spain).
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Free-Space Optical communication system consists of a line-of-sight technology which transmits a modulated laser beam through the medium for broadband communications,performed from satellite to satellite, from ground to ground, or from satellite to ground and vice versa [1]. The research presented in this paper focuses on the last scenario: satellite-ground and ground-satellite FSO communications, known from now on as downlink and uplink, respectively.
Downlink and uplink use the atmosphere as the communication channel. If this channel were ideal, the transmitted data would reach the receiver at maximum rate; hence, the most the atmosphere looks like to an ideal media, the better the communication performance would be, and this one is the aim of the proposed Adaptive Optics system: characterising the channel as much as possible in order to use this information to extract the atmospheric contribution from the links and make them propagating through a media the most ideal the better.
By applying AO techniques to the optical communications links, beam wavefront aberrations are suppressed to the possible extent; [2] and [3] demonstrated through simulations up to a 6dB improvement in SNR when using adaptive optics in daytime with high background environment for downlink communications.
There are some references in literature about correcting atmospheric turbulence on optical communication links, although most of them try to minimize its effect by modelling statistically the aberrations in a way that the beam receptor has some predefined information in order to reduce data loss when the fading occurs [4] [5] [6] [7]. Other studies [8] focus on reinforcing the communication by placing multiple receptor devices, which could compensate each other in case of link intensity loss, although the non-broadcast nature of the FSO links makes this option not the most suitable one, as one emitter shall be entirely dedicated to send the signal to the relay. Another research group [9] proposes the use of hybrid links: optical links operated together with RF.
All previous studies concentrate upon “static” corrections: either they formulate the expected response of the links through the atmosphere or they use a back-up solution in case of fading and data loss (multiple receptors or hybrid link) and thereby, the turbulence influence on the communication is reduced. However, they are not properly measuring atmospheric wavefront and correcting it “on-the-fly”; related to this idea, few examples can be found in bibliography: [1] presents one system that controls aligning, tracking and position of the laser beam in closed-loop by dividing the beam into two, one for measuring scintillation with a photo-diode and the other one for quantifying the variations in the angle-of-arrival. In the same line of dynamically correcting the atmosphere, [10] and [11] analyse the possibility of using a holographic wavefront sensor within the FSOC systems.
[12] propose the utilization of a modal zernike wavefront sensor to measure and correct the aberrations in optical communications; the operation consisted of applying a positive and a negative bias to the input signal and by subtracting both outputs, the result was related to the amplitude of the modal content of the incoming wavefront, allowing aberration correction by phase conjugation method.
Regarding the downlink correction, this scenario resembles conventional astronomical observations when applying Adaptive Optics techniques: the light is originated in space and it travels downwards through the atmosphere to the receiver (where the AO system would be placed), whereas the uplink needs to be corrected before existing the launching telescope by measuring the atmospheric wavefront with an a-priori unknown reference source. Examples of downlink AO systems can be found in [13].
As it is has been previously stated, the uplink pre-compensation entails a scientific and technological challenge. In this regard, lately [14] and [15] have built an AO-box to pre-compensate upwards propagated beams by AO techniques, demonstrating the viability of this instrument over a 1-kilometre horizontal path. Their research focuses on using the downlink signal as a reference source for wavefront sensing.
The Uplink Wavefront Corrector System (UWCS) is presented as a proof-of-concept instrument to demonstrate the advantages of applying Adaptive Optics techniques to Earth-Satellites optical communications (uplink), or the so-called upwards propagation path of a laser beam, and furthermore, to Laser Guide Star generation in conventional AO systems, by creating more focused and hence, brighter spots in the Na layer or in the upper parts of the atmosphere. This goal is achieved by the uplink atmospheric pre-compensation of the laser beam.
The UWCS consists of an Adaptive Optics system which introduces on-purpose added aberrations onto the laser by creating certain shapes in a deformable mirror, which will counteract the effects of the atmospheric turbulence when travelling through it. The research gathered in this paper becomes a novel understanding of the Adaptive Optics systems themselves, opening up a vast field of new applications for AO, as those techniques are usually applied to downlink beams (from sky to Earth) as opposed to the studies presented in the following sections.
2. Design
The UWCS was designed for its integration at the Coudé focus of the Optical Ground Station (OGS): the European Space Agency telescope whose main functions are the space debris tracking and the testing of optical communications with on-board terminals in GEO and LEO satellites.
In previous works [16], computer simulations were carried out with the purpose of getting further knowledge about the wavefront correction in the uplink propagation direction and the possible solutions for the arisen operational problems. Results from the isoplanatic study outlined the need of propagating a Laser Guide Star to the expected uplink location in order to perform an optimal uplink pre-compensation, instead of using the downlink signal (Satellite-Earth communication link) as a reference source for the wavefront sensing.
The UWCS was devised as a previous phase to the FSOC link pre-compensation, by propagating only one laser, which would play the role of Rayleigh Laser Guide Star as well as of the laser beam to be corrected.
The UWCS Rayleigh LGS is created by combining the Coherent Verdi V18, a Diode-Pumped Solid-State laser with a CW Output of 18W at 532nm, with an optical choppers to work in pseudo-pulsed operation.
The laser is propagated through the whole telescope aperture, therefore the secondary mirror that obscures the primary mirror could introduce significant central vignetting losses. The upwards propagation of several types of beam profiles (obstructed Gaussian, wide ring, narrow ring and top-hat) was simulated by Fresnel propagation, without taking into account the atmospheric contribution, but only the light diffraction effects. After analysing the light fraction over an area at 20-kilometre height, simulations point out that the ring-shaped laser beams perform better, achieving light fractions closed to 1 by propagating ring-shaped laser beams through the whole telescope aperture. Hence, the laser needs to be properly shaped before exiting the telescope. An axicon-scheme was devised for efficient coupling of the laser to the obscured launching telescope.
Axicons are both afocal refractive and reflective optical elements with a flat front surface and a conical rear surface. The rays near the edge of the beam entering the axicon get located at the inside edge of the annular beam when exiting. Likewise, the rays at the centre of the incident beam get located around the edge of the annular beam when exiting [17]. For the launching system, a pair of refractive axicons was selected. The axicons were designed in the way the resulting ring-shaped intensity distribution matches the ratio between the primary and secondary mirror in order to minimize coupling losses. Two identical axicons (Fig. 1) are used in the launch system, one positive and one negative, in order to create a ring shaped with a collimated input beam that keeps collimated after the second axicon. Due to the fact that the two identical axicons are properly aligned, the wavefront quality of the laser beam is not affected, as the difference in the optical paths of the rays after the first axicon is compensated by the second one.
The ratio between the secondary and the primary mirrors at the OGS which has to be matched by the axicon, is $D_{secondary}/D_{primary} = 288.912 mm/1016 mm = 0.28$. With an entrance beam diameter of 10 mm and an axicon with physical angle 20$^{\circ }$, if the negative axicon is placed at 45 mm from the positive one, the diameter of the output ring ($d_r$) will be 14.5 mm with a 5-millimetre thickness, producing a footprint on the primary mirror whose ratio between obscured and filled area will be 0.30. Therefore, the laser beam does not suffer from clipping losses due to the telescope central obstruction.
The CILAS SAM97 deformable mirror is selected as the corrector element for the UWCS after performing the corresponding stroke budget calculations. This DM is based on the Stacked Array Mirror (SAM) technology; it is made of a 11x11 piezo-electric actuators (97 useful actuators) with an 8-millimetre spacing and a coated optical aperture of 100 mm diameter. However, after properly performing interferometry measurements on the DM, a central area of 55mm was decided to be used as the DM presented certain curvature along one of its axis; therefore, in order to minimised the stroke needed to flatten this area, only the central 55-millimetre would be illuminated by the UWCS laser beam.
The UWCS wavefront sensor consists of a Shack-Hartmann built with the OCAM2k camera and a lenslet array that was customised for the specific OGS set-up. Based on SNR estimations, a 12x12 lenslet array was selected to be the optimum one for the UWCS WFS. In order to measure the wavefront in the uplink position, the WFS shall be located on the same launching axis, implying the shared-path operation between WFS and LGS; as both laser launch and photon return follow a common optical path to a certain extent, not only the LGS system needs an optical chopper, but also the wavefront sensor system; an optical chopper placed right before the WFS avoids the sensor saturation when the laser is propagating (the light intensity during the laser launch is much higher than the atmosphere backscatter) and hence, it allows the direct measurement of the Rayleigh return at the desired atmospheric height.
The UWCS final design is illustrated in Fig. 2. The UWCS has five subsystems: the laser launch system, the sensor and corrector system (WFS+DM), the injection system (flat mirrors to fold the beam and inject it onto the telescope axis) and the calibration system for the interaction matrix computation. The main UWCS elements and their technical details are gathered in Table 1.
Fig. 2. UWCS final design. Launch and return path are represented in green and blue. Full layout above; Zoom section with main elements from the laser launch system to the M6 mirror in the telescope Coudé path, below.
Table 1. UWCS main elements.
The Durham Adaptive Optics Real-Time Controller (DARC) [18] is selected as the RTC for the UWCS; DARC is a real-time control system (RTCS) for AO that was initially developed to be used with the CANARY on-sky multi-object AO technology demonstrator, due to a demand for DARC to be used with other instruments, an improved version of DARC was released to the public using an open source GNU General Public License.
3. Assembly and integration at the Optical Ground Station
The UWCS optical bench is installed at the Coudé focus in the OGS dome, as the telescope Coudé room (located on the floor below) was not available due to recoating operations.
3.1 UWCS set-up
The final UWCS set-up is shown in Fig. 3: in red the calibration system optical path; in green solid line, the laser launch optical path, and in green dashed line, the path followed by the return light from the sky to the WFS.
Fig. 3. The light from the Verdi laser follows the solid line through the optical elements, the deformable mirror and the folded mirrors which inject the laser beam to the telescope tube; on the return path (dashed line), the light coming from the sky follows the exact same optical path until the 50/50 beam splitter which divides the beam and leads it to the WFS. In red, the calibration system.
As it has been previously stated, the LGS is planned to be launched by creating a ring shape in the laser beam with the purpose of avoiding the secondary mirror losses. The laser ring is propagated throughout the optical set-up to the telescope tube and ultimately, to the sky.
Figure 4 presents the ring generation on the OGS dome. The transmission losses in the launch system were measured by installing a powermeter right after it; less than 10$\%$ in power loss was detected, whereas propagating a Gaussian laser through the secondary mirror would imply power losses of around 30$\%$. Therefore, it has been demonstrated the successful use of this device as launching system in reflecting telescopes.
3.2 UWCS closed-loop operation in synchronisation mode
The main function of the optical choppers in the UWCS is allowing the shared-path operation. Both optical choppers are integrated in the optical bench by taking into account the beam size at the chopper position: the smaller the beam diameter, the faster it would be chopped. Hence, the optical choppers need to be located one at the laser output where the beam had the minimum divergence, and the other one, at the focus position immediately before the WFS.
However, due to the mechanical nature of the optical choppers and the limited chopping speed, neither the laser beam nor the return light from the Rayleigh backscattering were chopped abruptly in time, but in several microseconds, causing delays in the reception which were translated into a reduction on the received backscattered return.
Initially, the UWCS design was thought to use the Rayleigh return from the launched Laser Guide Star as the reference source for the wavefront sensing and therefore, the close loop operation. Nevertheless, based on the insignificant Rayleigh return captured by the camera without the microlenses array, the decision of using a Natural Guide Star instead to close the AO loop, had to be made in order to guarantee the experiment success.
Theoretical results indicated that the expected Rayleigh return would be such that the SNR would be high enough for the measurement. Regardless, the calculations did not take into account the already described chopping problem and another encountered difficulty: some residual light entering the wavefront sensor when operating the laser at maximum power, which, after carefully analysing the UWCS set-up, was found to be a fluorescence light coming from the beam splitter and lasting around 3ms. Therefore, a NGS is used to close the UWCS AO loop.
4. On-sky performance validation
The OGS has a 20 cm finder telescope, attached and aligned to the main telescope. An ANDOR iXon3 888 with 1024x1024 pixels is attached to this finder telescope as scoring camera, in order to get a large field-of-view image and ease the localization and spot size analysis of the LGS. The launch geometry is shown in Fig. 5.
Fig. 5. Launch geometry of the UWCS: the laser is propagated through the 1-meter aperture and the laser plume is imaged by the Andor camera attached to the finder telescope.
The AO control loop was closed with Arcturus as Natural Guide Star at 300 Hz and the wavefront correction in the laser uplink was evaluated by analysing the detected light in the scoring camera. The reason why this operation frequency is selected, was the fact that both optical choppers could not work at higher frequencies without becoming unstable in their synchronization.
The reference centroids, imposed as the true value with refer to which the wavefront error was minimised, were computed by averaging the NGS centroids positions over 1000 frames at 150Hz rate.
Figure 6 shows two long exposure frames from the ANDOR camera to easily detect the pre-compensation effect (integration time 500 ms), before and after closing the loop, demonstrating the successful pre-compensation of the LGS upwards propagated path, whose plume gets brighter at its focus position when closing the AO loop.
Fig. 6. Scoring camera images of the laser plume before and after closing the AO loop with Arcturus as NGS, also in the image. Notice the increase in brightness when the laser wavefront is being pre-compensated.
The ANDOR images of the laser beam have been analysed by selecting a region of the plume and within it, 5 columns in the image to study the intensity profile shape (Fig. 7).
Fig. 7. Selected region of the plume; the light distribution along the red lines has been selected for further analysis.
The beam profiles along the selected lines in the laser plume are represented in Fig. 8; the top graph corresponds to the least focused positions in the LGS plume (first red line on the left in Fig. 7), and the bottom graph to the most focused one (last red line on the left in Fig. 7).
Fig. 8. From top to bottom light distribution corresponding to the selected lines in the laser plume (red lines from left to right in Fig. 7); in red open-loop operation, in blue, close-loop operation.
The analysis has been repeated for a case in which the AO loop was closed without the prior DM flattening (Fig. 9), with the purpose of studying the effect of ignoring the best flat configuration of the deformable mirror, as its interferometric analysis showed a defective 1-micron curvature along one of its axis in the selected aperture, which could have some implication in the reference centroids computation for the AO loop.
Fig. 9. From top to bottom light distribution corresponding to the selected lines in the laser plume (red lines from left to right in Fig. 7); in red open-loop operation, in blue, close-loop operation without performing the DM flattening.
When closing the loop with the optimal parameters and after initialising and calibrating the system properly, an increase of 22$\%$ in mean the laser intensity is achieved at the most focused position; and the beam profile becomes narrower with a 25$\%$ decrease in the FWHM (Fig. 10(a)); whereas, without performing the DM flattening (Fig. 10(b)), the mean light gain is around 20$\%$, but the FWHM only diminishes by a 9$\%$. This result indicates that the FWHM in open loop is mostly due to the atmospheric aberrations, as the DM flattening only becomes noticeable when applying the wavefront correction, in which case, the optimum performance of the AO system is achieved when the deformable mirror has been previously flattened. In both scenarios, the beam shifts one respect to the other (when closing the loop), although without flattening the DM, this shift is much more noticeable (the number of pixels of the shift is double in this last case, with respect to the scenario where the proper DM flattening was performed). This large shift in the operation without prior DM flattening, is due to a miscalculation of the reference centroids, caused by the uncorrected DM curvature.
Fig. 10. Laser intensity distribution corresponding to the most focused point the laser plume (last red line in Fig. 7); in red open-loop operation, in blue, close-loop operation with (a) and without (b) performing the DM flattening.
5. Conclusion
The UWCS was successfully integrated in the OGS telescope by installing an optical bench inside the dome and aligning the optical set-up with the telescope axis. The Rayleigh Laser Guide Star was propagated using the whole primary mirror; the axicon launch system allowed the avoidance of the central obscuration due to the secondary mirror and hence, the clipping losses in the laser beam. No previous record of propagating a laser with this ring-shape generator has been found, therefore it is considered a novel approach in laser launch systems, which has been completely accomplished in the UWCS. The axicon absorption in terms of laser power was measured and proved to be less than the clipping losses for a Gaussian beam.
Due to the limitations associated to the mechanical nature of the optical choppers and an unexpected fluorescence coming from the beam splitter, it was not possible to use the Rayleigh return from the LGS as the reference for the wavefront sensor. However, the Adaptive Optics loop was successfully closed with a Natural Guide Star, demonstrating the uplink pre-compensation of the laser beam.
Funding
European Space Agency (400011732416NLBJzk); Instituto de Astrofísica de Canarias (IAC).
Disclosures
The authors declare that there are no conflicts of interest related to this article.
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