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  Capabilities / Diamond-Turned Specialty Optics
Diamond-Turned Specialty Optics
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Diamond-Turned Custom Optics: non-rotational symmetries

Most optics exhibit rotational symmetry. They are used in the vast majority of existing applications. Yet optics exhibiting non-rotational symmetries often possess numerous advantages over their more traditional, rotationally symmetric counterparts.

Examples include biconic lenses and mirrors, which combine two surface radii on a single substrate; faceted lenses and mirrors, which combine multiple plano surfaces onto a single substrate; and optical arrays -- both reflective and transmissive -- which combine multiple curved surfaces onto a single substrate. Additional non-rotationally symmetric optics include long working distance off-axis parabolas, ring-focus parabolas, and rooftop beamsplitters.

BICONIC LENSES

Biconic Lens
Biconic Lens

Biconic lenses have two different radii on one surface. It is possible to make a biconic lens with spherical curves or aspheric curves, depending on the application and need to eliminate aberrations. Biconic lenses are used to produce an elliptically shaped focus or line focus. These lenses are also used in anamorphic beam expanders to reduce astigmatism in the laser beam. Many waveguide-type lasers produce astigmatic beams. Since most laser applications require symmetric Gaussian beams, astigmatic beams must be corrected.

The usual type of optic used in anamorphic beam expanders and elliptical focus lenses is the cylinder lens. For the beam expander application and some focusing applications, it is necessary to use two cylinders, resulting in difficult alignment procedures. The biconic lens can reduce the number of elements used in this application and, more importantly, reduce alignment headaches.

  • Biconic optical power can be placed on one surface.
  • Easy to align. Perpendicularity of the curves is ensured by machining.
  • Useful in anamorphic beam expanders.
  • As a focusing lens, it will produce elliptically shaped spots.
  • Curvatures can be spherical or elliptical.

BICONIC MIRRORS

Biconic Mirror
Biconic Mirror

In many applications, spherical mirrors, cylindrical mirrors, and parabolic mirrors help shape the laser beam. Biconic mirrors -- or the more general toroidal mirrors -- can combine two separate optics into one.

Biconic mirrors have two different radii on one surface. It is possible to make a biconic mirror with spherical curves or aspheric curves, depending on the application and need to eliminate aberrations. When appropriately designed, they can replace common 90° bending mirrors to recollimate a laser beam in a long delivery path.

  • Biconic optical power can be placed on one surface.
  • Curves can be designed to produce diffraction-limited focus at 45º AOI.
  • Useful in anamorphic beam expanders.
  • As a focusing mirror at 0º AOI, it will produce elliptically shaped spots.
  • Curvatures can be spherical or aspherical.

 

TRANSMISSIVE BEAM INTEGRATORS

Faceted Lens
Faceted Lens

Transmissive beam integrators are used with laser applications requiring a relatively large, focused flat-top intensity. Faceted integrators focus a high-power beam to a relatively flat-top beam with a size and shape that is equivalent to the individual facet size and shape. Traditionally, it has been extremely difficult to produce transmissive faceted integrators. Today, however, these faceted integrator lenses are made using advanced diamond-turning techniques. Although the primary substrate material for faceted integrator lenses is ZnSe, it is possible to produce this surface on Ge or any other diamond-turnable material.

Faceted lenses are a good alternative to the faceted mirror. Facets are arranged on the lens surface in almost any shape or form. There are some practical limits to the size of the facets that are machined, but typical facet sizes of 2 to 8 mm are possible on mirror blanks up to 100 mm in diameter.

  • Transmissive beam integrators produce relatively flat intensity profiles.
  • Integrated beams can be square or rectangular.
  • Focused beam sizes are relatively large -- 2 mm and above -- and are ideal for welding and heat treating.
  • Degree of integration will depend on noncoherence of the laser beam.
  • Work best with laser beams having poor coherence.

REFLECTIVE BEAM INTEGRATORS

Faceted Mirror
Faceted Mirror

Reflective beam integrators are widely used in high-power lasers for welding, cladding, and heat treating applications. Faceted integrators focus a high-power beam to a relatively flat-top beam with a size and shape that is equivalent to the individual facet size and shape. Traditionally, reflective integrator optics are produced by making individual faceted mirrors and then arranging them on a curved substrate. Today, however, these faceted integrator mirrors are made using advanced diamond-turning techniques. The tedious and time-consuming job of arranging individual facets on a substrate is no longer required, allowing the additional advantage of the mirror being directly water cooled.

Facets are arranged on the mirror in almost any shape or form. There’s some practical limits to the size of the facets that are machined, but typical facet sizes of 2 to 8 mm are easily possible on mirror blanks up to 75 mm in diameter. Integrators work best with laser beams having poor coherence.

  • Reflective beam integrators produce relatively flat intensity profiles.
  • Integrated beams can be square, rectangular, or circular.
  • Mirrors are made of copper and are ideal for high-power lasers.
  • Focused beam sizes are relatively large -- 2 mm and above -- and ideal for welding and heat treating.
  • Degree of integration depends on noncoherence of laser beam.
  • Work best with laser beams having poor coherence.

FOCUSED FLAT-TOP DOUBLETS

Flat-Topped Doublet
Flat-Topped Doublet
II-VI designs a simple form lens to convert a Gaussian mode to a flat-top intensity profile.

Converting one beam mode to another type is always a difficult process. There are different products to address this problem, including diffractive lenses, special beam integrators, combinations of aspheric lenses, and phase plates. As with many design types, it’s desirable to use the simplest form. The II-VI aspheric form is one of the simplest types.

The method used to convert a Gaussian beam to a flat-top at focus is determined somewhat by the required focused beam size. A faceted beam integrator is necessary for large spot sizes (see above). However, when it’s necessary to focus a laser beam to a flat-top intensity with a spot size of 100µm, it’s also necessary to go to more sophisticated aspherics or diffractives. II-VI accomplishes this with a simple aspheric form. Depending on focal length, this lens is produced as a singlet or doublet.
  • Focal lengths from 25 mm and up.
  • Unit may consist of one or two lenses, depending on desired spot size.
  • Requires Gaussian input beams with M2 values < 1.1 for best results.
  • Applications for drilling and materials processing.

LONG WORKING DISTANCE OFF-AXIS PARABOLAS

Long WD Off-Axis Parabola
Long WD Off-Axis Parabola

In the past, the working distance (WD) of off-axis parabolic mirrors was limited to the two-axis diamond-turning lathe’s swing diameter. Today, II-VI routinely produces long working distance parabolas with any turning angle using slow tool servo technology.

Like standard working distance off-axis parabolic mirrors, long working distance mirrors are made from copper substrates (either tilted or flat) which withstand extremely high laser powers and industrial environments. These mirrors provide diffraction-limited focusing when properly mounted and aligned. Also, copper mirrors are coated to provide greater reflectivity.

II-VI designs parabolic mirrors to reflect and focus the laser beam through 90° (standard) or any other convenient angle. Custom-designed features, such as water cooling and nonstandard mounting configurations, are available upon request.

  • Working distances that exceed standard capabilities of two-axis machines.
  • Excellent figure accuracy < 0.5µm.
  • Excellent surface roughness < 0.6 nm.
  • Large-diameter optics up to 250 mm.

OPTICAL ARRAYS

Optical Array
Optical Array

Certain optical system designs require multiple optical elements to be positioned accurately as an array. In the past, individual optics were produced and connected to a common substrate, which posed significant position and alignment challenges. Now, with II-VI’s advanced diamond-turning techniques, it is possible to machine monolithic optical arrays directly on a substrate, with II-VI’s fast tool servo technology. Typical substrate materials include ZnSe and Ge, and metals including Cu and Al.

A common application for this optical design is a focusing lens array with lenslets having identical focal lengths. However, it is not necessary to produce only lenslets with equal focal lengths on one substrate. Individual elements may have different focal lengths, including a mixture of positive and negative elements. It is also possible to combine lenses and mirrors.

Monolithic optical arrays provide the designer with one more tool in the design bag for producing small, complex, optical elements for advanced applications.

  • Monolithic optical arrays provide unique, compact optical solutions.
  • Lenslet arrays are easily machined and provide multifocus arrays.
  • Combinations of lenses, mirrors, or other optical elements are possible on one substrate.

RING-FOCUS OFF-AXIS PARABOLAS

Ring-Focus Off-Axis Parabola
Ring-Focus Off-Axis Parabola

The ring-focus off-axis parabola is an optic that combines the properties of a 90° parabolic focusing mirror with an axicon focusing optic. Typically, ZnSe lenses with a conical term are used to create a ring focus. The ring-focus off-axis parabola eliminates the transmissive optic by combining the axicon with an off-axis parabolic mirror. The resulting geometry is a free-form surface which II-VI generates using slow tool servo technology.

This approach offers versatile design specifications for working distance, ring diameter, and turning angle. For high-power applications, a direct-cooled copper substrate design can be employed.

  • One optic performs the work of two.
  • Usable in high power laser systems.
  • Produced from standard off-axis substrate.
  • Excellent RMS roughness < 6nm.
  • Easily designed to produce any desired ring diameter at focus.

ROOFTOP BEAMSPLITTERS

Rooftop Beamsplitter
Rooftop Beamsplitter

Prisms and transmissive beamsplitters are common optical elements used to split laser beams into two separate beams. These devices are common at visible and the IR wavelengths. For very high laser powers in the IR (1 to 10.6 µm), most prisms and beamsplitters are not useful because they suffer from thermal lensing. This occurs especially in CO2 lasers at CW power levels > 500 W. For these high powers, it is possible to split the beam using a metal rooftop prism.

Rooftop beamsplitters, made from copper, are direct water cooled. This allows use at laser powers in excess of 6 kW. A 90° rooftop mirror is used to physically split the beam into two working beams. These two beams will travel 180° from each other. With some simple mirrors, the beams are used in welding and heat treating applications.

The rooftop mirror is made from a single substrate with two precision-aligned mirrors. Each mirror surface is flycut to achieve figure and finish. The angle between each mirror’s face is controlled to within 10 arc seconds (if required).

  • Prism beamsplitter is used to split very high-power IR laser beams into two working beams.
  • Mirrors are made of copper or aluminum.
  • Copper mirrors can be direct water-cooled for use at very high laser power of > 5.0 kW.

VORTEX LENSES

Vortex Lens
Vortex Lens

The vortex lens is unique because it has spiral-phase steps machined into the curved surface. This spiral pattern controls the phase of the transmitted beam. When the spiral steps are machined into a curved lens surface, they produce a focused beam with zero energy or power in the middle. In other words, the vortex lens produces a ring focus. One other focused beam feature is that the phase is spiraling as the beam propagates; therefore, it’s sometimes called a spiral lens.

Traditionally, these lens types were produced using diffractive elements. Now they are machined directly with diamond-turning techniques. The result is a precision spiral step or vortex lens that can produce a ring focus.

Vortex lenses are made from any type of diamond-turnable material. For use at 10.6µm, this includes materials such as ZnSe and Ge. It is also possible to put this surface on a reflective mirror such as Cu or Al.

  • Provides a unique optical surface for producing a spiral-phased focused beam.
  • Spiral phase at focus produces a ring mode.
  • Can be used in ring-focus applications.
 
           
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