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Optical functions by wavefront control
Various functions of optical elements such as lenses and prisms can also be explained as changes in the wavefront shape. This is because light rays can be defined as a line perpendicular to the wavefront, and most optical elements have a function that shifts the light ray direction to change the direction in which the light travels.
LCOS-SLM is capable of freely controlling the wavefront shape of the output light and so can deliver the same functions as a variety of optical elements. It also utilizes dynamic wavefront control to combine the functions of multiple optical elements or to change their functions. To actually apply this wavefront control, it is necessary to input a phase distribution pattern into the LCOS-SLM according to the purpose of the application.
Here are some examples of typical optical functions that can be obtained by controlling the wavefront of light using an LCOS-SLM.
Beam splitting (creating multiple beams)
A laser beam is usually focused onto a single point when a lens is used, but can also be split and focused onto multiple different positions using an LCOS-SLM. This feature allows simultaneous irradiation of laser beams onto various positions or “patterned irradiation” to irradiate a wide area in any desired shape. Laser processing utilizes this feature to simultaneously process multiple positions or large areas. When used with a microscope, it is also possible to scan and measure different areas simultaneously.
In the case of patterned irradiation, by displaying a computer-generated hologram (CGH) on the liquid crystal of the LCOS-SLM to match the desired shape of irradiation, a laser beam that is usually focused on a single point can be irradiated in a pattern of any desired shape.
Schematic diagram of beam splitting using LCOS-SLM
Example of patterned irradiation
Beam position and shape control
Adding a spherical wave to the laser beam incident on the LCOS-SLM moves the focus position along the optical axis, and adding an inclined wavefront moves it in a direction perpendicular to the optical axis. Combining these wavefronts also allows three-dimensional control of the focusing position. Furthermore, adding a conical phase distribution to the incident beam causes the focused shape to elongate along the optical axis. This focused shape will be utilized to form an elongated hole by laser processing. Adding a helical phase distribution creates a donut-like focused shape. In STED (stimulated emission depletion) microscopy, donut-shaped laser beams with a very small central pinhole are utilized to deliver resolution that greatly exceeds the resolution of ordinary optical systems.
Schematic diagram of beam shape control using LCOS-SLM
Example of beam shape control
Aberration correction
Optical elements usually deviate from their ideal state, so the light passing through them may lose its uniformity causing the wavefront to become distorted. This distortion is called an “aberration.” When an aberration occurs, the point where the light is focused by the lens might not be a distinct point, or the projected pattern might be blurred. However, the ideal wavefront state can be restored by correcting the wavefront with LCOS-SLM so as to cancel out the aberration. This feature enables high-precision and high-efficiency laser microfabrication and precise microscopic observation.
Schematic diagram of aberration correction using LCOS-SLM
Comparison before and after aberration correction in a side-viewed focused beam.
Comparison before and after aberration correction for fundus camera
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