Optical Components for Laser Applications › fileadmin › user_upload › pdf › ... · 2019-10-02 · Optical Components for Laser Applications Günter Toesko - Laserseminar BLZ

Optical Components for Laser Applications

Günter Toesko - Laserseminar BLZ im Dezember 2009 1



AberrationsAberrations

An optical aberration is

a

distortion

in the image formed by

an

optical

system

compared

to

the

original. It can arise for

a

number

of

reasons due to the

limitations

of

optical components

such as

lenses

and

mirrors.



Spherical aberration

occurs

in a

spherical lens or mirror because these

do

not focus

parallel

rays

to a

point,

but instead along

a line. Therefore, off-axis rays are brought

to a

focus closer

to

the lens or mirror than are

on-axis rays.



Spherical aberration



Astigmatism

occurs

in

lenses because

a lens

has different

focal lengths for rays

of

different

orientations,

resulting in a distortion

of

the

image. In

particular,

rays

of light

from

horizontal and

vertical lines

in a plane on

the object are not

focused

to

the same

plane on

the edges of

the

image.



Astigmatism



Astigmatism



Distortionis caused because the transverse

magnification may be

a

function

of

the off-axis

image distance.

Distortion is

classified

as positive (so-called pincushion distortion),

or

negative (so-

called barrel distortion



Field curvature

results because the focal

plane

is actually not planar,

but spherical.



Field curvature



Astigmatism, Distortion, Field curvature



Chromatic aberration occurs in

lenses because

lenses

bring different

colors

of

light to a

focus

at different points

as

the refractive index changes with the wavelength.

V = Abbe number = measure of a material‘s dispersion

f1 * V1 + f2 * V2 = 0 1/f = 1/f1 + 1/f2



Chromatic aberration –

single lens



Chromatic aberration

-

achromat



Coma

occurs because

off-axis rays

no

not quite converge

at

the focal

plane.



Coma



Fiber Fiber collimatorcollimator

•

to be used with

an optical fiber

to provide

a collimated beam

•

focal length depending on the

NA and the required collimated beam diameter

•

single or multi-element system depending

on fiber core diameter



Collimator

-

Basics

focal length f

θ D

)sin(#2

1Θ=

⋅=

FNA

DfF =# NAfD ⋅⋅=⇒ 2



Collimator -

single lens or multi element

NA=0,14, f=80 mmD=?



Collimator

-

single lens or multi element



Afocal telescopesAfocal telescopes•

to provide

a collimated beam with

a certain

diameter (magnified

or

de-magnified)•

at least 2 lens elements–

Galilei > no internal

focus

–

Kepler > spacial

filter

possible

•

beam waist radius scales invers to divergence angle

•

wavefront maintanance•

lens

material

depending

on

the wavelength

•

adjustable divergence



Afocal Telescopes

• wavelength 355 nm • input aperture 10 mm• magnification 3.0• fully diffraction limited• adjustable divergence



AfocalAfocal

Zoom Zoom TelescopesTelescopes•

to provide

a collimated beam with a certain diameter

achieved by a variable (de-)magnification factor

•

at least 3 lens elements

depending

on requirements

–

Galilei > no

internal focus

–

Kepler >

spacial filter possible

•

beam waist radius scales invers to

divergence

angle

•

wavefront maintanance

•

lens

material

depending

on

the wavelength

•

adjustable divergence



Zoom

Beam

Expander

• input aperture 18 mm• magnification 1.5 – 2.5• fully diffraction limited• total length remains constant



FF--theta scan lenstheta scan lens•

y‘ = f *

theta

[rad]

•

flat field

at the

image plane•

while standard focusing lenses deliver

a

focused spot

to

only one

point,

scan lenses deliver

a

focused spot

to

many points

on a

scan

field or workpiece. •

typical applications:–

laser materials Processing, e.g. marking, plastics welding, trimming, structuring

of

thin

film solar cells

–

rapid manufacturing, e.g. laser sintering, rapid tooling

–

..................................



F-theta scan lens –

single element

XY Scanner



F-theta scan lens

–

single element




multi element for fixed focus



F-theta scan lens

–

multi element for fixed focus




a real one

cover glass

Y mirrorX mirror

lens elements



F-theta scan lens

–

a real

one



Telecentric F-theta lens

• round spot shape • typical deviation from telecentricity in a XY scan system: <1° • smallest spot size variation in the image field• large fields > large lens diameters > costs



Focal

diameter

-

Basics

focal diameter (1/e²) =

•

D = 1/e² diameter prior focussing•

k = scale factor

–

4/π = 1.27 for an

unclipped beam (k/D ≅

0.25)clear aperture

diameter

≅

2 * 1/e² beam

diameter

truncation loss approx. 0.03%–

1.41 (k/D ≅

0.21)

clear aperture

diameter

≅

1.5 * 1/e²

beam

diameter truncation loss approx. 1%

–

1.83 (k/D ≅

0.18)1/e² beam

diameter

= clear aperture

diameter, truncation loss

approx. 13.5%

DMkfλ 2•••



Focal diameter

-

Basics 2

•

fiber: NA = 0.14, f = 80 mm ⇒ D = 22.4 mm

•

BPP = M²*Lambda/Pi = NA * fiber core radius

•

e.g. fiber core radius

100 µm ⇒ M² ≅

41 (1.064nm)M² = beam quality factor

= 1 for a perfect Gaussian beam

> 1 for

real lasers



Focal diameter

(1/e²) -

Example

•

fiber: NA = 0.14, f = 80 mm ⇒ D = 22.4 mm•

fiber core diameter 200 µm

•

f-theta scan lens with

f = 160 mm

•

spot size:–

using

M²=41 (1.064nm) ⇒ 400 µm

–

ratio of focal lengths 160/80 = 2

X fibre core diameter ⇒ 400 µm



Focal diameter

(1/e²) -

Example



Focal diameter

-

Example

image width 500 µm image width 200 µmfor M²=1 (single mode fiber)aberration free spot 30 µm



GhostsGhosts!!

• unwanted back-reflections can destroy scan mirrorsor lens elements

• low M² values can result in diffraction limited ghosts



Color Color corrected lens corrected lens systemssystems

•

online inspection

•

different wavelengths in one system



Color corrected lens systems –

Beamexpander




Fused silica focussing lens

spot @ 1,064 nm




fused silica focussing lens

• ideal lens, i.e. no aberrations

• CCD lens with f=120 mm

• CCD 6.4 mm x 4.8 mm (½“)

• visible range

CCD

• fused silica focussing lens f=120mm

• diffracton limited @ 1,064 nm

• magnification –1 , i.e. FOV ½“




fused silica focussing lens

original ups....




the solution!

spot @ 1,064 nm



• lens system with a mix of lens materials• diffraction limited focus for 1,064 nm• very good image quality even for small CCD pixels (here 10 µm)


the solution!




f-theta lens

CCD objective

F-Theta objective




f-theta lens

•color corrected for 532/1,064nm

•focal length 254 mm•dual AR

coating

•beam diameter 15 mm




f-theta lens

originalcenter of scan field

LED illumination 532 nm +/- 10 nm




f-theta lens

corner of scan field> small lateral color error




f-theta lens

• lens system with a mix of lens materials• diffraction limited focus for 1,064 nm and 532 nm• LED bandwidth of 20 nm at 532 nm acceptable• very good image quality even for small CCD pixels (here 10 µm)

color correction pays off!



It is over nowIt is over now......

Optical Components for Laser Applications › fileadmin › user_upload › pdf › ... · 2019-10-02 · Optical Components for Laser Applications Günter Toesko - Laserseminar BLZ

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