Basic Optics for the Laser Safety Officer LIA

Basic Optics for the Laser Safety Officer LIA Practical Applications Seminar San Francisco, CA March 20, 2007 Leonard Migliore Coherent, Inc.

Electromagnetic Radiation • Electromagnetic Radiation is generated by motion of a charge • Maxwell’s Equations describe the behavior of electromagnetic radiation • Light, radio, X-rays, microwaves are all electromagnetic radiation • All ER travels at the speed c • Wavelength l=c/f

Wavelength and Frequency • l varies from 104 to 10 -10 meters • The wavelength of visible light ranges from 4 X 10 -7 to 7 X 10 -7 meters. This corresponds to frequencies of 4 X 1014 to 7. 5 X 1014 Hz • Ultraviolet light ranges from 10 -8 meters up to the visible band with frequences up to 3 X 1016 Hz • Infrared light goes from 7 X 10 -7 to 10 -3 meters, corresponding to frequencies of 3 X 1011 Hz • Electronic and molecular vibrations occur at frequencies of 1012 - 1016 Hz

Lasers in the Electromagnetic Spectrum

Properties of Laser Light • Monochromatic • Coherent • Collimated

Properties of Laser Light • Monochromatic

Properties of Laser Light • Coherent

Properties of Laser Light • Collimated

Geometrical Optics Light travels in a straight line in a homogeneous medium Light is refracted and reflected at interfaces between materials

$Reflection & Refraction$

Reflection & Refraction

Total Internal Reflection q q

Lensmaker’s Formula

The Lens Equation

But why do real lenses look like this? To correct for: Spherical aberration Astigmatism Coma Field curvature Distortion Color correction Zeiss Planar

Spherical aberration Rays from the outer part of the lens focus at a different point than rays from the inner part of the lens

Focusing can also be accomplished with reflective optics

Laser Beam Propagation • Light emitted from a stable resonator must satisfy some mathematical contraints • The only allowable distributions of energy are those which do not change as they propagate • These distributions are called modes • Output modes may be fully described by their waist diameter d 0 and their divergence q

Laser Modes • The standard terminology for describing laser modes is TEMmn where TEM stands for “transverse electric and magnetic” and m and n are the number of phase changes across the beam in the horizontal and vertical directions • The fundamental gaussian mode is TEM 00. This mode diverges less than any other mode of equal diameter and wavelength • Lasers may operate in higher order modes (for example, TEM 67) or in combinations of modes (“multimode” operation) • The descriptor “M 2” applied to laser beams denotes the divergence of that beam compared to that of a TEM 00 beam with the same waist diameter. The descriptor “K”, its inverse, is also used.

Laser Beam Characteristics BEAM DIAMETER Dz BEAM WAIST DIAMETER D 0 FAR FIELD DIVERGENCE q LASER BEAM CENTERLINE z

Laser Beam Focusing LENS OF FOCAL LENGTH f BEAM WAIST D 01 NEW BEAM WAIST D 02 Z 1 Z 2

Laser Beam Focusing Equations

Polarization • The output from many lasers is polarized • Light is a transverse electromagnetic wave. The plane of vibration of this wave is the plane of polarization PLAN E OF POLA R IZATI ON DIRE CTIO N OF LIGH T

P- and S-Polarization • Any given direction of polarization may be characterized as P (parallel), S (senkrecht, German for perpendicular) or some combination of the two with respect to the surface on which the light impinges P-POLARIZATION S-POLARIZATION

The Effect of Polarization • The reflection of light from a surface is highly dependent on the polarization of the light • Metals behave quite differently from dielectrics with respect to their reflectivity for differing polarization states DIELECTRIC REFLECTION METALLIC REFLECTION

Laser Beam Delivery • Laser output must be delivered to the work area for processing to be done • Free space beams are handled with lenses and mirrors • Lasers may also be delivered through fibers

Focusing Lenses • The low divergence of laser beams allows a lens to concentrate all of the beam’s power in a small spot • Spot sizes of 100 mm are easily reached with CO 2 lasers • Ultraviolet lasers can be focused to spots only a few microns in diameter

Scanning Lenses • Galvanometer scanners have special requirements for focusing to correlate scan location to mirror rotation Scanning through a singlet lens: scan distance is not proportional to scan mirror rotation

F-q Lenses • Most scanner lenses incorporate distortion that makes the spot location proportional to the input beam angle Scanning through an f-q lens: Spot location follows mirror rotation. Field is generally flatter than with singlet

Telecentric Lenses • With additional elements, the exit beam can be made to remain perpendicular to the work over the entire field • This requires the front element of the lens to be larger than the scan field With a telecentric scan lens, the beam always strikes the work at normal incidence, keeping drilled holes vertical

Turning Mirrors • Free-space laser beams are manipulated with flat mirrors • Industrial mirror housings contain the beam and can incorporate interlocks to prevent access • Mirrors for large CO 2 lasers are water-cooled for stability SCHEMATIC OF TURNING MIRROR ASSEMBLY MIRROR (IN GREEN) IS CONTAINED IN ITS HOUSING

Beam Expanders • The product of a beam’s diameter and its divergence is constant, so if you double a beam’s diameter, you halve its divergence • Expansion allows the beam to propagate further without significant change and facilitates the generation of smaller focused spots • Beam expanders have either a negative and positive lens (Galilean) or two positive lenses (Keplerian) BEAM EXPANDING TELESCOPE FOCUSING LENS

Fiber Optics • Fibers allow beams to be sent long distances • Fiber optic delivery makes attaching lasers to robots easier • Fiber lasers generate the beam within a fiber in the first place

Fiber Optics • For free-space lasers, the beam must be focused into the fiber • The spot size must be less than the fiber diameter • The divergence must be less than the cone of acceptance

Using Fiber Optics • The end effector couples the fiber into free space • The lenses image the fiber end onto the work • Spot diameter is usually the same as the fiber diameter but can be somewhat smaller

Transmissive Optical Materials • Glass is transparent from 400 nm to 1. 5 mm; used for visible and nearinfrared lasers • UV-grade silica is transparent to 250 nm; used for ultraviolet lasers and also instead of glass because of its ruggedness • Zinc Selenide is transparent from 600 nm to 12 mm; it is used for CO 2 laser optics • Most transmissive optics are coated to reduce reflection. Uncoated glass reflects 4% per surface. Reflection losses for high-index materials such as zinc selenide are much higher (20% per surface) without coatings • Many other materials are used for special purposes.

Mirror Substrates • Glass is commonly used for UV, visible and near-IR mirrors because it is easy to polish to a flat surface. Dielectric coatings are used to provide reflectivity at the desired wavelengths • Fused silica is also used where its low expansion helps to maintain the mirror figure • Silicon, copper, tungsten and molybdenum are used in high-power CO 2 lasers because of their high thermal conductivity. Silicon must be coated to reflect mid-infrared while the metals noted above have sufficient reflectivity without any coatings.

Industrial Applications • Lasers are used to cut, drill, weld and mark metals and non-metals • Most processes favor high power and high speed • Having the correct focused spot size on the work is usually critical to the success of the process

Cutting • Laser cutting systems use CO 2, Nd: YAG and fiber lasers to cut a wide variety of materials • Most cutters use freespace optics to obtain the smallest spot sizes • Some cutters send the beam over long distances, requiring a lowdivergence beam CO 2 laser cutting system processing stainless steel sheet

Welding • Most laser welding is done on metals; some plastic is also welded • CO 2, Nd: YAG and fiber lasers are used. Fiber beam delivery is common for Nd: YAG welders • Lasers are used for high production rates; most systems are highly automated CO 2 laser transmission welding system with 2 work stations

Drilling • Laser drilling is done on metals, plastics, composites, leather, paper, silicone and other materials • All laser types are used Robotic drilling of automotive carpet

Other Processes • There a wide variety of laser processes. An understanding of optics is vital for anyone dealing with them. Bearing journal ready for laser overlay Journal being processed with 5 k. W CO 2 laser