Basic Optics for the Laser Safety Officer

LIA Practical Applications SeminarSan Francisco, CA

March 20, 2007

Leonard Migliore

Coherent, Inc.

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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 =c/f

Wavelength and Frequency• varies from 104 to 10-10 meters

• The wavelength of visible light ranges from4 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

θ

θ'

TRANSPARENT MEDIUM:INDEX OF REFRACTION = n

AIR (n=1)

INCIDENT BEAM REFLECTED BEAM

sin θsin θ'

= nIntensity of reflected beamis given by Fresnel's lawsand is dependent on θ andon the polarization of theincident beam.As θ goes to zero, the reflectanceis:

n-1n+1( )

2

Total Internal Reflection

TRANSPARENT MEDIUM:INDEX OF REFRACTION = n

AIR (n=1)

INCIDENT BEAM

If sin >

REFLECTED BEAM

1n

then the light will betotally reflected atthe interface

Lensmaker’s Formula

R1

R2

F

1f

= (n-1) 1R1

1R2( )-

The Lens Equation

LENSFOCAL LENGTH = f

OBJECTHEIGHT = H

IMAGEHEIGHT = H'

do di

FOCUSING MAGNIFICATION

do di f1 1 1+ = do

di

HH'

=

But why do real lenses look like this?

Spherical aberrationAstigmatismComaField curvatureDistortionColor correction

To correct for:

Zeiss Planar

Spherical aberration

Rays from the outer part of the lens focus at a differentpoint than rays from the inner part of the lens

Focusing can also be accomplishedwith 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 d0 and their divergence θ

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 TEM00. This mode diverges less

than any other mode of equal diameter and wavelength

• Lasers may operate in higher order modes (for example, TEM67) or in

combinations of modes (“multimode” operation)

• The descriptor “M2” applied to laser beams denotes the divergence of that beam compared to that of a TEM00 beam with the same waist

diameter. The descriptor “K”, its inverse, is also used.

Laser Beam Characteristics

BEAM WAISTDIAMETER D0

LASERBEAM

BEAM DIAMETER Dz

z

BEAMCENTERLINE

FAR FIELDDIVERGENCE θ

Laser Beam Focusing

LENS OF FOCAL

LENGTH f

Z1 Z2

BEAM WAIST D01 NEW BEAM WAIST D02

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

DIRECTION OF LIGHT

PLANE OF POLARIZATION

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 m are easily reached with CO2 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-θ Lenses• Most scanner lenses incorporate distortion that makes the spot

location proportional to the input beam angle

Scanning through an f-θ 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 CO2

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 m; used for visible and near-infrared 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 m; it is used for CO2

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 CO2 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

CO2, Nd:YAG and fiber

lasers to cut a wide

variety of materials

• Most cutters use free-

space optics to obtain the

smallest spot sizes

• Some cutters send the

beam over long

distances, requiring a

low-divergence beam

CO2 laser cutting system processing

stainless steel sheet

Welding

• Most laser welding is

done on metals; some

plastic is also welded

• CO2, 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

CO2 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 are 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 kW CO2 laser