In contrast to fibre-optic communications, optical communications in free space are affected by the medium of propagation. Atmospheric turbulence reduces the communication flow because of scintillation and the degradation of the wavefront. Adaptive optics reduces this phenomenon, thus permitting the deployment of temporary optical networks (Campus Area Network) as well as optical connections on the last mile.

Fast deformable mirrors, large strokes and low power consumption.

The constraints encountered in the field of wireless optical communications for deformable mirrors are very similar to those in the world of astronomy. Indeed, the goal of deformable mirrors is to compensate for atmospheric turbulence.

These constraints are:

• Large strokes (10-20µm)
• A high operating frequency (~1kHz)
• Linearity in order to reduce the complexity of the terminals.

Moreover, integrating deformable mirrors into the terminals requires compactness and low power consumption. The range High-Speed Magnetic Deformable Mirror Series fulfils this set of requirements.

The Hi-Speed ALPAO deformable mirrors range offers very large strokes both for the inter-actuator stroke and to compensate for low-order aberrations. Figure 1 shows the deformation typically obtained by applying an electrical current to a 3x3 actuator zone. Figure 2 shows the inter-actuator stroke (by pushing and pulling alternately on each actuator).

Figure 1

Figure 2

 

Figure 3

In addition, these mirrors offer excellent temporal performance. Figure 3 represents the temporal response obtained during the generation of a 30µm defocalisation (Z4=30µm amplitude). Thanks to a stabilisation time of 1 millisecond, it is possible to close the loop with a very large frame rate, thus permitting effective correction of atmospheric turbulence.

The compactness of deformable mirrors permits easy integration into the terminals. So, for example, there are 52 actuators in a 9.0mm mirror. The power consumption typically necessary to compensate for atmospheric turbulence is of the order of 100mW. In addition, thanks to the choice of magnetic technology, a high electrical output is achieved, in contrast to other technologies which require intermediate amplification stages.

A drive electronics, scalable to 1024 channels, have been specially designed in order to exploit all the potential of deformable mirrors. Thanks to the control of the current applied to each actuator, the stability achieved is unparalleled.

The two tables hereafter summarize the main characteristics of the ALPAO Hi-Speed DM.

1.5mm between two actuators

2.5mm between two actuators

(1) -> in closed loop

(2) -> first resonance of the membrane

(3) -> All ALPAO mirrors can be coated with different metallic materials (silver, aluminium, gold,...).

 

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Open and flexible A.O. software

The structure of adaptive optics systems for communication depends very strongly on their application: a system to communicate between a satellite and the earth differ from an optical system within the framework of a temporary campus (Campus Area Network). In addition, the control method of the deformable mirror will be completely different in the case of a strong perturbation regime (scintillation problem) and the case of optical communication between a satellite and a terrestrial station.

The ALPAO Core Engine (ACE) architecture is the perfect solution to this situation thanks to its open and modular architecture (represented in figure 1). Indeed, it is an adaptive optics toolbox for Matlab. All the data are availably and the user can customize the built-in examples to develop an optimized adaptive optics system. Therefore, it speeds up the development of free space optics system using adaptive optics.

So, for example, the user can easily construct an optical communication system by using the effective signal coming from a photo-diode directly (aceCam module) or using an ultra-rapid wavefront sensor (aceWFS module). The user has access to all data in real-time, thus facilitating performance analysis and allowing parameter optimisation.

For users looking for a ready out of the box product, ALPAO offers two products (described below):

• AOS-0: system optimised for teaching and R&D
• AOS-1: system optimised for rapid turbulence and low luminous flux applications such as are encountered in optical communication between a satellite and a terrestrial station.

Please contact ALPAO to use an embedded system.

 

ALPAO AOS-0: Open and flexible system for teaching and R&D

This plug-and-play system has been specially designed for:

1. engineers and scientists studying adaptive optics, the laws of control and real-time processing,
2. the teaching of adaptive optics.

The AOS-0 workbench includes all the components necessary to simulate a complete adaptive optics system:

• a Hi-Speed ALPAO DM52-15 mirror (including the control electronics)
• a wavefront sensor
• an imaging camera
• a rotating turbulence screen
• an optical source
• the opto-mechanical parts
• Matlab® software to control the system.
• a Matlab® licence (if the user doesn't have one).

Thanks to the ALPAO CORE ENGINE (ACE) architecture and the AOS-0 workbench, it is possible to develop control methods going from the simple law of integration right up to the most advanced solutions such as Kalman filtering. The real-time access to all data (residual errors, wavefront, camera images) makes developing your project easier.

 

Figure 2: Optimisation of the coupled power in a mono-mode fibre (the cost function represents the inverse of the coupled power).

The flexibility of this system is demonstrated by the possibility of using the signal coming from the photo-diode directly to close the loop. Thanks to the ACE's modular approach, you will thus be able to very rapidly develop an optimal control method to, for example, compensate for uncommon aberrations or those not seen by the sensor. ALPAO Core Engine is compatible with operating frequencies up to the kilohertz range (Microsoft Windows® and some versions of Linux®).

The choice of Matlab® allows the user to benefit from the support of the very large user community and the numerous forums associated with it.

Thanks to the AOS-0, you don't have to be a specialist to do adaptive optics, but you can become one if you want to!

For more information you can contact us via the site (click here) or by telephone or you can download the information brochure in the Downloads section.

 

AOS-1: high-performance adaptive optics loop

The AOS-1 system is based on:

• a Hi-Speed DM241-25 Deformable Mirror with 241 actuators spread across a diameter of 37.5mm (17 x 17 grid).
• An ultra-sensitive sensor manufactured by ALPAO and based on an EMCCD camera (16x16 sub-pupils for the Fried configuration)
• A real-time computer operating at 500 fps and based on the ALPAO Core Engine (ACE).

For more information you can contact us via the site (click here) or you can download the information brochure in the Downloads section.

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