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High Precision Wide Dynamic Range Nonlinear Reflectivity Measurement - - PowerPoint PPT Presentation

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Diploma Thesis High Precision Wide Dynamic Range Nonlinear Reflectivity Measurement Christian Walther October 2004March 2005 Supervised by Deran Maas, Rachel Grange, and Markus Haiml Ultrafast Laser Physics Group Prof. Dr. Ursula Keller

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SLIDE 1

High Precision Wide Dynamic Range Nonlinear Reflectivity Measurement

Christian Walther October 2004–March 2005 Diploma Thesis

Supervised by Deran Maas, Rachel Grange, and Markus Haiml Ultrafast Laser Physics Group

  • Prof. Dr. Ursula Keller

Institute of Quantum Electronics ETH Zurich

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SLIDE 2

Setup 2: Cube Setup 1: AOM Theory Motivation Conclusion Remaining Challenges Results Measurement

Outline

High Precision Wide Dynamic Range Nonlinear Reflectivity Measurement

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SLIDE 3

SESAM reflectivity

small modulation depths ∆R < 1% are desired

10 10

1

10

2

10

3

Rlin Fsat Rns !Rns !R 100% Pulse Energy Fluence [µJ/cm2] Reflectivity

Setup 2: Cube Setup 1: AOM Theory Motivation Conclusion Remaining Challenges Results Measurement

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SLIDE 4

Previous measurement setup

incident and reflected power are measured separately

Setup 2: Cube Setup 1: AOM Theory Motivation Conclusion Remaining Challenges Results Measurement

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SLIDE 5

Errors

OUT = c1 P R ±5% c1 P (1–R) ±10% IN = c2 P ±5% c2 P ±10% c1/c2 R ±7% c1/c2 (1–R) ±14% R = 0.99

δR = 7% · 0.99

= 0.069

δR = δ(1–R) = 14% · 0.01

= 0.0014

Setup 2: Cube Setup 1: AOM Theory Motivation Conclusion Remaining Challenges Results Measurement

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SLIDE 6

Basic setup

100 MHz pulsed laser source beam splitter high reflector L L + !L power on detector sample lock-in amplifier

SIG REF

t

no interference

Setup 2: Cube Setup 1: AOM Theory Motivation Conclusion Remaining Challenges Results Measurement

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SLIDE 7

Fourier transform

S(t) = P(t) ∗ (D(t) + R · D(t − τ))

t t t t T S(t) P(t) D(t) D(t – !) !

= +

R

· *

ˆ S( f ) = ˆ P( f ) ·

  • ˆ

D( f ) + R · e2πi f τ · ˆ D( f )

  • = ˆ

P( f ) · ˆ D( f ) ·

  • 1 + R · e2πi f τ

f P(f) D(f) D(f) e2!if" FT

·

f 1/T f

+

R

· ·

1/" f Re Im FT FT P(f) D(f) 1 + R·e2!if" f f

· ·

f f0 f0 Re 1 R Im 1/"

=

1 – R

Setup 2: Cube Setup 1: AOM Theory Motivation Conclusion Remaining Challenges Results Measurement

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SLIDE 8

Lock-in amplifier output

x y sample arm signal reference arm signal combined signal with shifted variation small variation

power ↔ amplitude scaling delay ↔ phase rotation

exact adjustment of power ratio and delay is not needed Setup 2: Cube Setup 1: AOM Theory Motivation Conclusion Remaining Challenges Results Measurement

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SLIDE 9

Setup design

laser Faraday rotator AOM glass wedge beamsplitter detector

0th order 1 s t

  • r

d e r

high reflector sample high reflector polarizing cubes

!⁄2

Beam diameters at specific places [mm]: 1.3 1.6 0.3 6.5 0.03 7.0 0.15

f = 40 cm f = 12.5 cm

14 mm

f = 30 cm 0.99 1.32 0.99 3 . 5 5 3 . 3 2 . 6 2.66 2.80

1 1 2 2 3 3 4 4 5 5 6 6 7 7

position along beam [m]

Design considerations include:

  • fitting beams through apertures without clipping, errors by beam

displacement

  • effect of lenses on beam size, adjustment simulated in flexible

Matlab program Setup 2: Cube Setup 1: AOM Theory Motivation Conclusion Remaining Challenges Results Measurement

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SLIDE 10

Setup using an AOM

  • wide dynamic range attenuation by half wave plate and polarizer
  • isolator to prevent destabilizing feedback into the laser
  • focus in AOM
  • AOM splits beam into 0th and 1st diffraction order
  • separation of beams by a mirror
  • glass wedge beamsplitter deflects reflected light onto detector

Failure because of

  • too wide beam and too small diffraction angle
  • 50/50 beam splitting ratio difficult to stabilize

laser Faraday rotator AOM glass wedge beamsplitter detector

0th order 1 s t

  • r

d e r

high reflector sample high reflector polarizing cubes

!⁄2

Setup 2: Cube Setup 1: AOM Theory Motivation Conclusion Remaining Challenges Results Measurement

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SLIDE 11

Setup using a cube beamsplitter

  • large mode area holey fiber (single-mode) for beam cleaning
  • fiber acts as λ/5 plate because of bend-induced birefringence
  • cube beamsplitter, in contrast to coated glass plate beamsplitter,

is completely symmetric for accurate 50/50 beam splitting

  • lens creates focus on reference arm mirror
  • wide beam on detector makes additional attenuation

unnecessary

  • move sample and focusing lens to adjust arm length difference
  • move high reflector to adjust power ratio on detector surface by

changing beam diameter

laser fiber isolator detector beamsplitter cube high reflector sample

!⁄2

f = 21 mm 22 µm 0.16 mm 3.2 mm 5.6 mm 20 µm 5.8 mm f = 38.1 mm f = 75 mm 0.30 0.42 1.08 1.17 1.10 2.60

Setup 2: Cube Setup 1: AOM Theory Motivation Conclusion Remaining Challenges Results Measurement

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SLIDE 12

Reproducing sample position

  • lateral movement of the beam (≈ sample displacement) is

converted to angular movement by lens, which can be amplified by distance

  • reflex off the lens enables tilt control
  • not independent, but still uniquely identify sample orientation
  • collimating lens controls spot size

positional reproduction accuracy of 20 µm (Rayleigh range: 300 µm)

collimating lens f = 30 cm f = 12.5 cm sample folding mirror laser diode crosshair target (tilt) crosshair target (position)

1.5 m

Setup 2: Cube Setup 1: AOM Theory Motivation Conclusion Remaining Challenges Results Measurement

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SLIDE 13

Lock-in amplifier coherent pickup

  • small part of reference signal is picked up at measurement input

and added to the measured signal

  • not constant, has to be measured for every point
  • computer-controllable motorized beam blocker needed
  • no unused motor controller available – build our own, controlled

by the lock-in amplifier’s auxiliary output Setup 2: Cube Setup 1: AOM Theory Motivation Conclusion Remaining Challenges Results Measurement

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SLIDE 14

Beam blocker motor

Implementation problems:

  • current through relay affected lock-in amplifier measurement
  • moving away did not help
  • transistors were damaged by induction voltage

Setup 2: Cube Setup 1: AOM Theory Motivation Conclusion Remaining Challenges Results Measurement

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SLIDE 15

Attenuation calibration

1.0x10-3 0.8 0.6 0.4 0.2 0.0 Signal [V]

  • 30
  • 20
  • 10

10 20 Waveplate Angle [°]

  • 8
  • 7
  • 6
  • 5
  • 4
  • 3

log10(Signal [V])

  • 30
  • 20
  • 10

10 20 Waveplate Angle [°]

  • attenuation to zero is not possible
  • measurement by adaptive algorithm
  • fit to model function

Setup 2: Cube Setup 1: AOM Theory Motivation Conclusion Remaining Challenges Results Measurement

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SLIDE 16

Measurement

  • record lock-in amplifier x and y signal for a logarithmically spaced

set of powers

  • 3 measurement runs:
  • SESAM
  • no sample (reference arm only)
  • high reflector

Setup 2: Cube Setup 1: AOM Theory Motivation Conclusion Remaining Challenges Results Measurement

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SLIDE 17

Evaluation

For both SESAM and HR:

  • sample arm signal is reconstructed by subtracting reference

signal from combined signal

  • reference signal and reconstructed sample signal are projected
  • nto their main axes to get scalar values
  • division yields values proportional to reflectivity

x y reconstructed sample arm signal measured reference arm signal measured combined signal

  • = –c2 · P
  • = c1 · P · R

c1 c2 R = – ●

  • – ●
  • = –

= 1 – ●

  • = 1 – c1

c2 R ≈1 Setup 2: Cube Setup 1: AOM Theory Motivation Conclusion Remaining Challenges Results Measurement

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SLIDE 18

ES134 (∆R = 7%)

Setup 2: Cube Setup 1: AOM Theory Motivation Conclusion Remaining Challenges Results Measurement

10

!1

10 10

1

10

2

88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 Fluence [µJ/cm2] ± 11.5 % Reflectivity [%] HR SESAM 1 SESAM 2 previous

0.3%

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SLIDE 19

ES160 (∆R = 0.3%)

Setup 2: Cube Setup 1: AOM Theory Motivation Conclusion Remaining Challenges Results Measurement

10 10

1

10

2

99.2 99.3 99.4 99.5 99.6 99.7 99.8 99.9 100 Fluence [µJ/cm2] ± 11.5 % Reflectivity [%] HR 1 SESAM 1 SESAM 2 HR 2

0.03%

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SLIDE 20

ES160 (∆R = 0.3%)

Setup 2: Cube Setup 1: AOM Theory Motivation Conclusion Remaining Challenges Results Measurement

10 10

1

10

2

99.2 99.3 99.4 99.5 99.6 99.7 99.8 99.9 100 Fluence [µJ/cm2] ± 11.5 % Reflectivity [%] HR 1 SESAM 1 SESAM 2 HR 2 previous

0.02%

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SLIDE 21

Remaining challenges

  • nonlinearity

in amplitude and phase

  • reproducibility
  • nly intermittently achieved

Setup 2: Cube Setup 1: AOM Theory Motivation Conclusion Remaining Challenges Results Measurement

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SLIDE 22

Conclusion

precise SESAM measurements achieved

  • dynamic range of 4–5 orders of magnitude
  • accuracy below 0.1% over 2–3 orders of magnitude

experimental difficulties overcome

  • precise calibration of wide dynamic range attenuation
  • beam quality improvement by fiber
  • sample positioning reproducibility
  • coherent pickup correction by motorized beam blocker

Setup 2: Cube Setup 1: AOM Theory Motivation Conclusion Remaining Challenges Results Measurement

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SLIDE 23