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Optical Design & Engineering

IR Optics Advance - Today's single-point diamond-turning machines can produce toroidal, cylindrical, and spiral lenses.

From oemagazine October 2005
30 October 2005, SPIE Newsroom. DOI: 10.1117/2.5200510.0010

More than 15 years ago, a consultant for a laser manufacturer told us that high power yttrium aluminum garnet lasers were going to put carbon dioxide (CO2) lasers out of business.

His not-so-subtle point was that we in the CO2 laser optics business should look for other markets. Less than four years ago at a small conference on diode and fiber lasers, I was told that the market for IR optics for high power lasers was going to be a thing of the past because the "new" lasers (i.e., fiber and diode lasers) do not need optics. Both of these predictions have yet to come true. Not only do CO2 lasers constitute one of the largest markets for IR optics, but the optics also find strong demand in other markets, including near-IR (NIR) and mid-IR laser and vision systems. The IR optics market is healthy and vibrant.

The majority of IR optics today are produced using conventional grinding and polishing machines. Multiple parts can be mounted on wax or pitch laps and polished to a satisfactory surface figure and finish. Polishing times per lap vary greatly by optic type, size, and material, but they are usually measured in hours. One lap can polish many optics.

During the last 25 years, new technologies have come along to expand the manufacturing capability of IR optics. High speed grinding and polishing machines, for example, allow manufacturers to produce parts in tens of minutes instead of hours. The main advantage of these machines is that they are easily programmable and their setup can be changed quickly for short runs of optics. For flat optics and spherical lenses, these tools are the workhorses of the industry. Their primary disadvantage is that they can produce only one optic at a time.

Getting to the Point

An alternative technology is single-point diamond turning (SPDT; see oemagazine, July 2004, p. 26). Standard two-axis diamond-turned aspheric lenses have been used in multi-kilowatt CO2 lasers for many years. Free-form-type diamond-turned optics have surfaces that are cosmetically equal to, or better than, 40/20 scratch-dig. Surface damage and stress are not visible in cross polarizer tests and so they should not limit the power handling capability of the optics.

SPDT can be used to economically produce interesting non-rotationally ymmetric (free-form) optics, such as faceted lenses or mirrors (see figure 1). Notice that the facets on the mirror in the figure are rectangular, which will produce a square focus. They are arranged tangentially to a radius, so the laser power in each facet is reflected to the image plane where the reflected beamlets overlap. The imaged spot has the same dimensions as the facets, factoring in the 45° angle of incidence.


Figure 1. The power or energy incident on each facet of this mirror (left) is directed, unfocused, to a common image plane. In the case of incoherent laser beams, it produces a flat-top intensity profile (right).

For very high power lasers with poor coherence, these mirrors produce a uniform intensity at the image plane; as a result, they are sometimes called faceted beam integrators. Faceted mirrors are used for heat-treating and cladding, or for other applications that require the conversion of a large, high-order beam mode to a uniform intensity.

Faceted mirrors date back to approximately 212 BC and the siege of Syracuse by the Roman fleet.1 According to legend, Archimedes used a large mirror constructed of smaller facets to focus the rays of the sun onto the Roman ships, thus setting them afire. Modern-day faceted mirrors for the laser industry have also been constructed using individual mirrors attached to a curved substrate. Today, however, a faceted mirror can be produced quickly and easily in one cutting using advanced SPDT technology.

Free-form SPDT methods now make it possible to machine long-radius, off-axis parabolic segments. The off-axis segment is typically machined at its radial distance from the spin axis of the parent parabola. Standard SPDT machines usually have spindle diameters of 600 or 700 mm maximum, which limits working distances to less than 300 mm in the manufacture of off-axis parabolic mirrors. Newer SPDT machines can achieve parabolic working distances of greater than 300 mm, allowing them to fabricate long-radius, off-axis segments on the spin axis. Spinning the mirror on its mechanical axis introduces significantly less vibration and wobble than spinning the mirror at a long off-axis position; thus, the new technique offers the unexpected bonus of producing parts with very good surface figure.

An additional advantage to using SPDT for the fabrication of parabolas or other types of mirrors is that it permits variations on the surface shape. It is possible, for example, to add a conical term to an off-axis parabolic surface so that the focused spot is a ring shape. At II-VI, we have produced this type of parabola and tested it in a low power CO2 laser.

The Shape of Things to Come

Biconic (toroidal) mirrors and lenses are also machinable with the same SPDT technology. These types of optics have different radii of curvature along each orthogonal axis in the plane of the surface. In the past, one or more cylindrical lenses were used in systems requiring anamorphic beam shaping or expansion. Today it is possible to machine biconic surfaces (mirrors and lenses) directly onto materials such as copper, aluminum, germanium, zinc selenide, and zinc sulfide.

One application for these types of components is producing a line focus. The optics can also be used in anamorphic beam expanders or condensers, replacing two lenses with one. For high power laser users who wish to add some optical power to their 45° beam-delivery mirrors, biconic surfaces provide a solution. In principle, this type of mirror is capable of replacing the telescope mirrors commonly found in most high power laser systems. Because SPDT ensures the perpendicularity of the curves during machining, biconic mirrors can be aligned easily in a beam delivery system.

Diamond-turned optical arrays are also something new for IR optics. Now it is possible to machine arrays of lenses on a single substrate as large as 300 mm (see figure 2). The individual elements of these types of arrays usually have a common shape (spherical, biconic, parabolic, etc.); within limits, it is possible to integrate multiple radii on a surface. The technology can produce arrays of elements, each with up to 70 µm of sag. It is also possible to produce hybrid arrays of lenses and mirrors. Of course, the substrate must be transparent and needs to be coated in two coating runs. One coating run would involve masking the mirror surfaces and covering the lens elements with an antireflection coating. A second coating run would involve masking the lens elements and applying a totally reflective coating on the mirror elements.


Figure 2. The given lens array machined into a 60-mm-diameter copper substrate by SPDT has 12-mm-diameter, 60-µm-deep concave lenslets. The technology can achieve consistent quality, as this interferogram of 1.5-µm-deep lenslets shows.

Let's consider one more example of a free-form element that can be machined using the new diamond-turning technology - a spiral-step surface. Spiral or vortex lenses have been around for some time now. They exist most often in the form of diffractive optical elements (DOEs) used in the visible or NIR spectral regions. DOEs are expensive and, if a variety of spiral surface types are needed, the nonrecurring engineering and mask fees become very high. SPDT can produce these spiral shapes in mirrors or IR-transmissive materials easily and quickly. The method requires no special engineering or tooling, only a small reprogramming of the SPDT controller.

The spiral or vortex lens has the unique ability to produce a spiraling phase at the focus.2 The spiral phase creates a null field in the center of the focus; in other words, it creates a focused ring-mode beam. The parabola with a conical term men-tioned earlier is used to produce large (greater than 1-mm diameter) focused rings. The vortex lens can produce very small ring foci (see figure 3) and is one of the few lenses that can achieve the desired results for applications requiring tightly focused ring modes.

Figure 3. SPDT machined a spiral pattern into the spherical surface of this zinc selenide substrate (top) to produce a focused ring mode. Such a lens produces a spiral phase at focus (middle) and a beam caustic (bottom); color contours indicate intensity levels.

Most IR lenses produced today are simple spherical and aspheric designs used to focus a laser beam to a Gaussian or airy disk focus. New technologies are being developed to create simple but unusual foci, however, such as faceted mirrors, conical parabolas, and spiral lenses. These new lens and mirror designs are being driven largely by user requests and are made possible by recent advances in diamond-turning technology.

If your IR optics application requires an unusual shape or intensity pattern, don't hesitate to ask your optics vendor about new technologies. Chances are good that he or she will be able to manufacture an optic to meet your needs. oe

References

1. Greek Mathematical Works vol. II (I. Thomas, translator), Harvard University Press, Cambridge, MA (1968).

2. D. Rozas, C. Law, and G. Swartzlander Jr., J. Opt. Soc. Amer. B 14, p. 3054 (1997).


Gary Herritt
Gary Herrit is a senior optical design engineer at II-VI Inc., Saxonburg, PA.