Optical Components for Laser Applications. Günter Toesko - Laserseminar BLZ im Dezember

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1 Günter Toesko - Laserseminar BLZ im Dezember

2 Aberrations 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. Günter Toesko - Laserseminar BLZ im Dezember

3 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. Günter Toesko - Laserseminar BLZ im Dezember

4 Spherical aberration Günter Toesko - Laserseminar BLZ im Dezember

5 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. Günter Toesko - Laserseminar BLZ im Dezember

6 Astigmatism Günter Toesko - Laserseminar BLZ im Dezember

7 Astigmatism Günter Toesko - Laserseminar BLZ im Dezember

8 Distortion is 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 (socalled barrel distortion Günter Toesko - Laserseminar BLZ im Dezember

9 Field curvature results because the focal plane is actually not planar, but spherical. Günter Toesko - Laserseminar BLZ im Dezember

10 Field curvature Günter Toesko - Laserseminar BLZ im Dezember

11 Astigmatism, Distortion, Field curvature Günter Toesko - Laserseminar BLZ im Dezember

$light to a focus at different points as the refractive index changes with the$ wavelength.

12 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 Günter Toesko - Laserseminar BLZ im Dezember

13 Chromatic aberration single lens Günter Toesko - Laserseminar BLZ im Dezember

14 Chromatic aberration - achromat Günter Toesko - Laserseminar BLZ im Dezember

15 Coma occurs because off-axis rays no not quite converge at the focal plane. Günter Toesko - Laserseminar BLZ im Dezember

16 Coma Günter Toesko - Laserseminar BLZ im Dezember

17 Fiber collimator 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 Günter Toesko - Laserseminar BLZ im Dezember

18 Collimator - Basics focal length f θ D NA = 1 2 F # = sin( Θ) f F # = D = 2 f NA D Günter Toesko - Laserseminar BLZ im Dezember

19 Collimator - single lens or multi element NA=0,14, f=80 mm D=? Günter Toesko - Laserseminar BLZ im Dezember

20 Collimator - single lens or multi element Günter Toesko - Laserseminar BLZ im Dezember

21 Afocal 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 Günter Toesko - Laserseminar BLZ im Dezember

22 Afocal Telescopes wavelength 355 nm input aperture 10 mm magnification 3.0 fully diffraction limited adjustable divergence Günter Toesko - Laserseminar BLZ im Dezember

23 Afocal Zoom Telescopes 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 Günter Toesko - Laserseminar BLZ im Dezember

24 Zoom Beam Expander input aperture 18 mm magnification fully diffraction limited total length remains constant Günter Toesko - Laserseminar BLZ im Dezember

25 F-theta 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... Günter Toesko - Laserseminar BLZ im Dezember

26 F-theta scan lens single element XY Scanner Günter Toesko - Laserseminar BLZ im Dezember

27 F-theta scan lens single element Günter Toesko - Laserseminar BLZ im Dezember

28 F-theta scan lens multi element for fixed focus Günter Toesko - Laserseminar BLZ im Dezember

29 F-theta scan lens multi element for fixed focus Günter Toesko - Laserseminar BLZ im Dezember

30 F-theta scan lens a real one cover glass lens elements X mirror Y mirror Günter Toesko - Laserseminar BLZ im Dezember

31 F-theta scan lens a real one Günter Toesko - Laserseminar BLZ im Dezember

32 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 Günter Toesko - Laserseminar BLZ im Dezember

33 focal diameter (1/e²) = Focal diameter - Basics 2 λ f k M D 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% Günter Toesko - Laserseminar BLZ im Dezember

34 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 Günter Toesko - Laserseminar BLZ im Dezember

35 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 Günter Toesko - Laserseminar BLZ im Dezember

36 Focal diameter (1/e²) - Example Günter Toesko - Laserseminar BLZ im Dezember

37 Focal diameter - Example image width 500 µm image width 200 µm for M²=1 (single mode fiber) aberration free spot 30 µm Günter Toesko - Laserseminar BLZ im Dezember

38 Ghosts! unwanted back-reflections can destroy scan mirrors or lens elements low M² values can result in diffraction limited ghosts Günter Toesko - Laserseminar BLZ im Dezember

39 Color corrected lens systems online inspection different wavelengths in one system Günter Toesko - Laserseminar BLZ im Dezember

40 Color corrected lens systems Beamexpander Günter Toesko - Laserseminar BLZ im Dezember

41 Color corrected lens systems Fused silica focussing lens 1,064 nm Günter Toesko - Laserseminar BLZ im Dezember

$Color corrected lens systems fused silica focussing lens CCD ideal lens, i.e. no aberrations fused silica focussing lens f=120mm CCD lens with f=120 mm diffracton limited @ 1,064 nm CCD 6.$

42 Color corrected lens systems fused silica focussing lens CCD ideal lens, i.e. no aberrations fused silica focussing lens f=120mm CCD lens with f=120 mm diffracton 1,064 nm CCD 6.4 mm x 4.8 mm (½ ) visible range magnification 1, i.e. FOV ½ Günter Toesko - Laserseminar BLZ im Dezember

43 Color corrected lens systems fused silica focussing lens original ups... Günter Toesko - Laserseminar BLZ im Dezember

44 Color corrected lens systems the solution! 1,064 nm Günter Toesko - Laserseminar BLZ im Dezember

45 Color corrected lens systems the solution! 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) Günter Toesko - Laserseminar BLZ im Dezember

46 Color corrected lens systems f-theta lens F-Theta objective CCD objective Günter Toesko - Laserseminar BLZ im Dezember

47 Color corrected lens systems f-theta lens color corrected for 532/1,064nm focal length 254 mm dual AR coating beam diameter 15 mm Günter Toesko - Laserseminar BLZ im Dezember

48 Color corrected lens systems f-theta lens original center of scan field LED illumination 532 nm +/- 10 nm Günter Toesko - Laserseminar BLZ im Dezember

49 Color corrected lens systems f-theta lens corner of scan field > small lateral color error Günter Toesko - Laserseminar BLZ im Dezember

50 Color corrected lens systems 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! Günter Toesko - Laserseminar BLZ im Dezember

51 It is over now... Günter Toesko - Laserseminar BLZ im Dezember

Lecture 2: Geometrical Optics. Geometrical Approximation. Lenses. Mirrors. Optical Systems. Images and Pupils. Aberrations.

Lecture 2: Geometrical Optics. Geometrical Approximation. Lenses. Mirrors. Optical Systems. Images and Pupils. Aberrations. Lecture 2: Geometrical Optics Outline 1 Geometrical Approximation 2 Lenses 3 Mirrors 4 Optical Systems 5 Images and Pupils 6 Aberrations Christoph U. Keller, Leiden Observatory, keller@strw.leidenuniv.nl