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Pulse shaping using a spatial light modulator- POSTER_Nicolene

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Pulse shaping using a spatial light modulator- POSTER_Nicolene Powered By Docstoc
					Pulse shaping using a spatial light modulator
N BOTHA1,2, L BOTHA1,2, A DU PLESSIS1,2, H SCHWOERER2 1CSIR National Laser Centre, PO Box 395, Pretoria, 0001 2 University of Stellenbosch, Private Bag X1, Matieland,7602 Email: nbotha@csir.co.za - www.csir.co.za

INTRODUCTION
Femtosecond pulse shaping can be done by different kinds of pulse shapers, such as liquid crystal spatial light modulators (LC SLM), acousto optic modulators (AOM) and deformable and movable mirrors. A few applications where pulse shaping is implemented are coherent control of molecules, communication systems, encoding and decoding and biomedical imaging.

Input Curved Mirror Curved Mirror

Output

SLM

f Plane Mirror Plane Mirror

f

Focus length necessary for a sufficient angular dispersion to illuminate the whole SLM: dλ nm n = grating groove density = m = light order = 1 dθ cos θ ∫ cos θ dθ = ∫ nm d λ
sin θ = nm ∆λ = (1200 × 10 )(44 × 10 ) θ = 3.03 25mm f = = 945.26 mm tan(θ / 2)
3 −9
25 mm
θ/2

THEORY OF PULSE SHAPING
In the time domain the output of the filter (shaper) is eout(t), and the input is ein(t). h(t) represents the input response function1.

Grating

Grating

Fig2. Compact design

f

eout (t ) = ein (t ) ∗ h(t ) = ∫ dt ' ein (t ')h(t − t ')
ein(t)
h(t)

PULSE SHAPING USING AN AOM
An acousto optic modulator uses sound wave to form a grating. The velocity of the sound wave is so much slower than that of the incoming light, that the grating appears stationary to the light. The two traces below was measured using a GRENOUILLE.

eout(t)

Impulse response In the frequency domain the filter is characterised by its frequency response H(ω). The output of the linear filter Eout(ω) is the product of the input signal and response function1.

Eout (ω) = Ein (ω)H (ω)
Ein(ω)
H(ω)

Eout(ω)

Frequency response eout(t), ein(t), h(t) and Eout(ω), Ein(ω), H(ω) respectively are Fourier transforms of each other.

Double pulse: separation 400 fs Chirped pulse: 26000 fs2 Double pulse – 400 fs separation The trace in the second figure should correspond to a double pulse but is slightly misaligned in this case.

LIQUID CRYSTAL SPATIAL LIGHT MODULATOR (LC SLM)
The most popular pulse shapers are the acousto optic modulator and the liquid crystal spatial light modulator.
63.7x103 µm

Normalised electric field

1 iωt iωt H (ω ) = ∫ dth(t )e , h(t ) = d ω H (ω )e ∫ 2π

SIMULATION: SHAPING WITH A SLM e (t) H(ω) - amplitude
1

in

x 10

13

0.8 0.6 0.4 0.2

4

2

Intensity
4.4 4.5 4.6 4.7 4.8 Tim e 4.9 5 5.1 5.2 x 10 5.3
-12

0 -0.2

0

-2
-0.4 -0.6 -0.8 -1 4.3

-4

-6 1.585 1.59 1.595 1.6 1.605 1.61 1.615 F requenc y (ω ) 1.62 1.625 1.63 x 10
17

H(ω) - phase
82

1 0.8

eout(t)

Normalised electric field

pixels
Phase

0.6
81

0.4 0.2 0 -0.2 -0.4 -0.6

10mm

80

79

78

Gap = 3.084 µm

96.52 µm

77

-0.8
0 1 2 3 4 Angular frequency 5 6 x 10
5

-1 4.65

4.7

4.75

4.8

PULSE SHAPER SETUP DESIGN
The pulse shape setup that is most commonly used is the 4f design (see figure 1). There are different variations of the 4f setup of which some are more compact2 (see figure 2). SLM Resolution: lens lens Grating λ spread 44 nm Grating = 3 length of SLM 63.7 ×10 µ m = 0.000691 nm / µ m f f Fig1. 4f Design f f

4.85 4.9 Tim e

4.95

5

5.05 x 10

5.1
-12

CONCLUSION
We are going to use the LC SLM for our pulse shaper. The compact design in figure 2 is preferred for our setup due to the smaller size and good accessibility to the optical components.

BIBLIOGRAPHY
1. 2. A. M. Weiner, “Femtosecond pulse shaping using spatial light modulators," Review of scientific instruments. 71, 1929-1960 (2000). A. Präkelt, M. Wollenhaupt, A. Assion, Ch. Horn, C. Sharpe-Tudoran, M. Winter and T. Baumert, “Compact, robust, and flexible setup for femtosecond pulse shaping,“ Review of scientific instruments. 75, 4950-4953 (2003).

0.000691 × length of pixel = 0.0667 nm / pixel


				
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