Holographic adaptive-optics system removes speed barriers

Optical engineers face a continual battle to remove aberrating effects from optical beams. The presence of even small distortions on light wavefronts will reduce image quality and focusing capabilities. Adaptive optics is a now-standard technique for correcting phase errors occurring in light beams transmitted through aberrating media such as the atmosphere. It typically works by determination of the phase errors using a wavefront sensor and passing that information to a deformable device to apply the appropriate phase correction. In conventional systems, the bottleneck occurs in the wavefront-sensor computations that characterize the phase errors. Even with dedicated, custom-made circuitry, the fastest adaptive-optics systems are bulky and typically work at speeds of 10kHz or less. For applications such as directed-energy weaponry or planned extremely large telescopes, higher speeds may be desirable.

A new system developed in the Laser and Optics Research Center at the US Air Force Academy offers a solution by removing the computation requirement from the process.^1,2 The Holographic Adaptive Laser Optics System (HALOS) replaces the conventional wavefront sensor with a multiplexed hologram and an array of photodetectors. Removing the computer from the loop reduces cost and complexity, as well as mass and volume. The result is a simpler system.

The holographic wavefront sensor uses an optically written hologram and a basic deformable mirror (DM) for wavefront correction. Its operation begins with application of a full stroke to a single actuator to get an object beam with the maximum phase error. A hologram is recorded between this object beam and a focused reference beam directed at some distant point ‘A.’ A second hologram is then recorded on the same medium, but with the minimum phase error and a focused beam directed to a different point ‘B.’ This multiplexing is a further property of holograms that allows storage of large amounts of information in a single element.

If the hologram is then illuminated with a beam characterized by an arbitrary phase error at the particular actuator location, there will be two focal spots reconstructed at A and B, since the object beam is not a perfect match for either recording condition. The ratio of the two spots' brightnesses is directly related to the absolute phase error, so an intensity-ratio measurement translates directly into a phase measure.

By repeating this process to create a pair of holograms for each actuator on the DM, the wavefront phase can be completely characterized simply by reading outputs from photodetectors. Since no calculations are required, this can be achieved at rates up to the GHz level if light permits. Even more significant is that the speed is virtually independent of the actuator number, since the hologram acts as an all-optical parallel processor. As a result, the sensing is equally fast with one or 10,000 actuators. Figure 1 shows a schematic of how this would work for four actuators.

Figure 1. A pair of spatially separated beams is created for each actuator. The focal-spot intensity ratio for each pair gives the phase-error measure for that portion of the wavefront. DM: Deformable mirror.

HALOS goes further once a one-off calibration is made. Using this information, a simple feedback circuit can be designed so that the detector voltages directly control the actuators for closed-loop adaptive-optics correction. This leads to a number of new applications, including directed-energy weaponry, image- and laser-beam correction, and extreme-adaptive-optics applications in astronomy.

The first working prototype of the HALOS system included software control in place of circuitry. In that experiment, we successfully demonstrated closed-loop correction of seven actuators in a micro-electromechanical system DM (see Figure 2). My current work aims at completely removing the computer from the loop to create a fully autonomous system. The design goal is to construct a working 10kHz, 32-actuator system by the end of the year.

Figure 2. Image of a 14× multiplexed hologram used for the Holographic Adaptive Laser Optics System prototype.

This work is funded by the Air Force Office of Scientific Research and the Joint Technology Office.