Introduction to the Vortex Beam
Over 60 years ago, a scientific breakthrough was the catalyst for the discovery of the first laser. Since then, the use of laser devices has been growing in research applications, and in recent years, industrial and commercial applications have been utilizing this technology as well. Lately, that growth trend has rapidly increased thanks to laser availability and a sharp decrease in laser price-per-watt.
A typical laser source generates a near Gaussian beam profile with the maximum energy at the center and a moderate energy decay towards the edges. This intensity profile is not ideal in many applications, and so many research and development engineers seek beam shaping technologies to achieve their desired laser beam shape and intensity profile.
One such desired shape is a ring-like donut shape with controlled ring intensity and an energy hole in the center. This is the output shape of a vortex beam.
An optical vortex beam can be best achieved using a single diffractive vortex phase plate, also known as a Vortex Lens (VL).
Diffractive vortex phase plate
Vortex beam generation can be performed using a diffractive optical vortex phase plate, which is a unique optical element composed of a helical (spiral phase or “staircase” step structure). Hence its other name “Spiral Phase Plate” or SPP. This sort of vortex phase structure adds orbital angular momentum to the beam, effectively “pushing” the energy out from the center. This effect is similar to water spinning around a sinkhole that generates a whirlpool that pushes water to the outside. The vortex beam results in the generation of a donut mode, regardless of incoming beam polarization, merely through the orbital angular momentum of the light. This is in contrast to liquid crystal-based vortex phase plates, which manipulate the spin angular momentum.
The diffractive vortex plate requires a high-quality collimated TEM00 Gaussian input beam and converts it to a TEM01 axially symmetric mode.
More than a single staircase structure can be fabricated on the diffractive surface, with the number of such staircases corresponding to the topological charge m. Each such staircase structure’s etch depth is a function of the design wavelength and the substrate’s optical index.
The main effects of the different topological charges are:
- The higher the topological charge, the higher the angular moment of the vortex beam by a factor of m.
- The higher the topological charge, the larger the donut ring intensity profile by a factor of m.
Advantages of vortex phase plates
Vortex phase plates have several advantages compared to other beam shaping methods:
- An optical vortex beam is self-healing. Thus, it is not sensitive to clipping, requiring clear apertures similar to a Gaussian beam. This is not the case for beam shapers, where often the required clear aperture is twice the beam diameter.
- The vortex phase works for all input beam sizes. The resulting donut is scaled with input beam size, in a manner similar to the Gaussian beam. Therefore, there is no need for a well-known input beam diameter.
- When scanning vortex laser beams, the integrated energy profile across the scan line (that is the trench energy profile for ablation) is flat- top, unlike the case with a Gaussian laser beam. This is true even for low topological charges, including m=1. This enables relatively narrow scanned lines with good uniformity across the line profile.
Vortex beam applications
Vortex laser beams are typically used in research applications mainly in the life science disciplines. One such application where vortex beams pose a distinct enabling advantage is super resolution imaging with STED microscopy. In this application, the depletion laser is shaped into a vortex beam, which is then focused with the excitation laser by the objective.
Another application is optical tweezers. A highly focused laser beam can hold and move microscopic and sub-microscopic objects like atoms, nanoparticles and droplets. The donut-shaped vortex beam provides the advantage of trapping the particle inside the donut hole at the focused beam waist.
Other known applications using vortex lenses include:
- Solar coronagraphs (astronomy)
- Quantum optics
Holo/Or offers a wide variety of vortex phase plate DOEs. To calculate the ring and hole diameter of the vortex plate, Holo/Or created a optical vortex calculator to aid our customers in choosing the parts fitting their needs.
For more information about this or other Holo/Or products, contact Holo/Or.