Navy

Simultaneous spatial light modulator (SLM) beam steering with system aberration correction

Technology facilitates the use of lower-cost, lower-quality optics while still maintaining a diffraction-limited beam in a wide-angle steering system

Photonics Software & Information Technology

SLMs are routinely used to vary a waveform of a beam of light and they find application in presentation projectors, holographic storage, holographic display, and optical tweezers. Optics engineers would benefit from the ability to use SLMs for beam steering as an alternative to mechanical mirror-based gimbals, galvanometer mechanisms, and other mechanical systems. However, SLMs have a tendency to introduce optical aberrations into a device and to date there has not been an approach to correct for these irregularities. This limits optimization of the laser power in wide field-of view-optics. As a result, methods that utilize an SLM as a steering device, but which do not correct for system aberrations impose tight tolerance and lens design requirements on the system’s optical elements in order to maintain a tight, focused beam.

In response, Navy researchers have developed a device which pre-corrects for optical aberrations in an SLM beam steering configuration. The apparatus includes a spatial light modulator, a wide-field optical system (this contains optical aberration), and a camera. The wide-field optical system collimates a light beam toward the camera. The camera communicates with the SLM via a feedback loop that performs the correction. Correction is done by an intensity point spread function (PSF or impulse response) of the light beam recorded on the camera and a prior impulse response is set as the previous maximum impulse response. Each pixel of a Fresnel zone plate on the SLM is perturbed randomly and simultaneously and another intensity PSF of the light beam is recorded on the camera.

Whether the intensity sharpness is greater for this PSF than for the previous is compared. The comparison includes adding a new phase aberration to the system phase corrections on the SLM, when the intensity sharpness of another intensity PSF is greater than a previous maximum intensity PSF. The comparison further includes subtracting the new phase aberration from the system phase corrections on the SLM, when the intensity sharpness of subsequent intensity PSF is less than the previous maximum. The random and simultaneous perturbation of each pixel of the Fresnel zone plate on the SLM, the recording of another intensity PSF of the collimated light beam on the camera, and the comparison of whether an intensity sharpness is greater for succeeding intensity PSF than for the previous intensity PSF, is repeated until an aggregate pixel intensity is maximized.

A notable benefit of this invention is the simplicity of its design and operation. Beam steering is accomplished solely with an SLM and a wide-field optical system, which could be as simple as a single lens. No additional mechanical devices or mirrors are necessary. In addition to the SLM acting as a steering device, it also simultaneously acts as a wavefront corrector, reducing the requirement of high-precision, costly optics that minimize aberrations in low f-number optical systems. Whereas high-precision optical systems require multiple lenses to correct for the optical aberrations, this device reduces that requirement, allowing for a smaller, lighter system.

Applications for this invention include microscopy raster scanning, directed energy, and optical communications.

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