Beam Shaping Optics

There are many different types of beam shaping optics that we diamond turn for our customers. For some applications we can provide additional services including optical design, testing, material growth, polishing, and coating. Some common applications for beam shaping optics include the following:

  1. Beam diverging and converging optics in one or two axes
    a. Cylinders
    b. Toroids
  2. Uniform intensity generation optics
    a. Faceted beam integrators
    b. Gaussian to flat top beam convertors
  3. Ring mode optics
    a. Axicon lenses
    b. Vortex lenses
    c. Alvarez-type lenses

 

Beam diverging and converging optics in one or two axes

Cylinders

Cylindrical optics can be used to converge or diverge a laser beam in one direction. Cylindrical lenses are typically polished conventionally. Cylindrical mirrors, however, are often diamond machined, particularly if they are made from copper or aluminum. Depending on the allowable form tolerances, metal cylinders can be produced using a diamond flycut process where the flywheel is angled relative to the motion of the machine axis. This actually produces an elliptical cylinder, but over a given aperture, the form deviation from a perfect cylinder may be acceptable. Depending on the machine axis length, several or more cylinders can be flycut in one pass. Aspheric cylinders or cylinders with more demanding figure tolerances can be diamond turned using Slow Tool Servo (STS) technology. Aspheric cylinders are turned one at a time like a regular asphere on the face of a diamond turning lathe, except the Z-axis of the machine traverses the contours of the cylinder as the part rotates. This is a slower process so aspheric cylinders tend to be more expensive than regular cylinders.

Toroids

Toroids are similar to cylinders except they can be used to converge or diverge a laser beam in two directions. Toroids have a radius of curvature along the X direction and another radius of curvature along the Y direction. Toroids can be diamond turned in multiples when positioned on the lathe at the radius of revolution while the other radius of curvature is programmed into the controller. If the radius of revolution is too large (typically > 350mm) it will exceed the swing capacity of the diamond turning machine. When it becomes impossible or impractical to diamond turn toroids off-axis, they can be brought on-axis and turned using STS.


Uniform intensity generation optics

Faceted beam integrators

Diamond turning of a beam-shaping optic.

Faceted beam integrators create a rectangular-shaped uniform intensity distribution (sometimes referred to as a flat top or top hat profile) at focus, and they are often used in cladding and heat treating applications. They can be designed using geometric ray tracing techniques to work either in transmission or in reflection. These optics can be diamond turned using a Fast Tool Servo (FTS) actuated toolpost which has the ability to traverse sharp changes in surface geometry very quickly and accurately. For reflective beam integrators, STS is used in tandem with the FTS to create the underlying geometry, which is typically an off-axis segment of an ellipse or parabola.
Beam integrators are widely used with multi-mode fiber lasers. We typically produce faceted beam integrator lenses using multi-spectral ZnS material, but reflective versions in copper are also common. There are limitations to how well beam integrators work due to the diffractive effects associated with stacking rectangles (or hexagons) of coherent light at a common focus. A spiky diffraction pattern can result when using a beam integrator with a modern CO2 laser, so for that application we recommend a Gaussian to flat top beam converter.

Gaussian to flat top beam convertors

Beam shaping optics

Gaussian to flat top beam convertors are designed by mapping the Gaussian beam distribution at the optic to a uniform intensity distribution at the focus. The distribution can be circular or rectangular. Beam converters are sensitive to beam quality (M2 < 1.2 is recommended), as well as the location and size of the beam waist. Circular flat top beam converters have the advantage of being rotationally symmetric, so they can be diamond turned at high rotational speeds, but rectangular flat top beam converters are not, so they must be turned at a slower speed.

 

Ring mode optics

Axicon lenses

Some axicon lenses are used to create a ring at focus. This is achieved by designing an ideal aspheric lens with a slope term superimposed on the surface. The slope term spoils the focus by a controlled amount and is useful for cutting thick material with fiber lasers. The disadvantage of axicon lenses is their sensitivity to alignment. A hexagonal array of axicon lenses, which can be produced with FTS technology, is insensitive to alignment and produces a similar ring at focus.
Ring modes can be generated in collimated space as well. This can be achieved through pairs of precisely aligned axicon lenses or by a double-sided monolithic axicon lens. The ring can be tailored to a specific size by controlling the slope, additional aspheric terms, and distance between axicon surfaces.

Vortex lenses

Vortex lenses, which have some number of spiral or helical steps on the surface, are designed to produce total destructive interference at the center of the beam throughout the entire depth of focus. The step height must be designed to produce an optical path difference equal to a multiple of the laser wavelength for the interference effect to work properly. The properties of the ring can be customized by specifying various parameters of the vortex. The vortex surface can be diamond turned using FTS technology.

Alvarez-type lenses

One challenge with ring mode generation is creating a variable ring. Highyag, a subsidiary of II-VI, has patented (US 9261702 B2) a novel variable ring mode generator borrowing from the principle of how Alvarez lenses work. Two identical freeform lenses opposing each other can be moved laterally relative to each other resulting in a variable conical wavefront. We can diamond turn these variable axicon lenses using STS technology.

Based on your laser beam configuration, we can design and manufacture custom beam-shaping optics.  A common application is for converting a Gaussian distribution to rectangular flat-top profile.