ULTRASHORT LASER PULSE PROPAGATION IN HIGHLY

SCATTERING MEDIA

A.P. Popov

What is the task?• Pulse structure

• Output pulse duration τ(ti)

• Photon scattering multiplicity• Absorption, scattering,

transmission• Change of medium parameters• Effect of stranger particles on skin

properties

2

Modelled setup3

3

4

2 1

5

6 7

8

1 – Cr: forsterite laser, 2 – attenuator, 3 – “blind” mirrors, 4 – beam splitter, 5 –power meter, 6 – vessel with scattering medium, 7 – streak-camera, 8 – CCD-camera

3

Scattering particle

Detector

Absorbing particle

DetectorIncident laser beam

Scheme of the modelled experiment

4

0 1 2 3 4 5 6 7 8 9 10

0,1

1

10

100

g = 0.98

s = 85 mm-1

a = 0.6 mm-1

l* = 0.43 mm

L = 0.1 mm L = 0.2 mm L = 0.4 mm

Rel

. n

um

ber

of

ph

oto

ns,

%

Registration time, ps

Forward detected photons vs registration time

5

0 1 2 3 4 5 6 7 8 9 10

0

1

2

3

4

Rel

. n

um

ber

of

ph

oto

ns,

%

Registration time, ps

g = 0.98

s = 85 mm-1

a = 0.6 mm-1

l* = 0.43 mm

L = 0.6 mm L = 0.8 mm L = 1.0 mm

Forward detected photons vs registration time6

0 100 200 300 400 500 600 700 800 900 10001E-5

1E-4

1E-3

0,01

0,1

1

Thickness, m

Unscattered part Tc

Scattered part Ts

Beer's law

g = 0.9

s = 10 mm-1

a = 0.01 mm-1

n = 1.37

Tra

nsm

issio

n, re

l. u

nit

s

Behavior of scattered and unscattered parts of radiation at various sample thickness

7

0 10 20 30 40 50 60

0

2

4

6

8

10

12

g = 0.98

s = 85 mm-1

a = 0.6 mm-1

L = 0.1 mm L = 0.2 mm L = 0.4 mm

Rel

. n

um

ber

of

ph

oto

ns,

%

Number of scattering acts

Forward detected photons vs their scattering multiplicity

8

0 50 100 150 200 250 300 350

0,0

0,2

0,4

0,6

0,8

g = 0.98

s = 85 mm-1

a = 0.6 mm-1

L = 0.6 mm L = 0.8 mm L = 1.0 mm

Rel. n

um

ber

of

ph

oto

ns, %

Number of scattering acts

Forward detected photons vs their scattering multiplicity

9

0,0 0,2 0,4 0,6 0,8 1,00

20

40

60

80

100

Y = A + B * XA=-1,61096+/-0,73594

B=95,69863+/-1,21262 mm-1

g = 0.98

s = 85 mm-1

a = 0.6 mm-1

Max

imum

of

scat

teri

ng m

ultip

licity

Medium thickness, mm

Maximum of scattering multiplicity vs medium thickness10

0,0 0,2 0,4 0,6 0,8 1,0

0

5

10

15

20

25

30

Depth, mm

g = 0.95

a = 1 mm-1

L = 1 mm

s = 95 mm-1

s = 75 mm-1

Rel. n

um

ber

of

scatt

eri

ng

, %

Radiation scattering within medium11

0 10 20 30 40 50

0,00

0,02

0,04

0,06

0,08

g = 0.85

s = 35 mm-1

L = 1 mm

a = 0.1 mm-1

a = 0.4 mm-1

a = 0.8 mm-1

Rel. n

um

ber

of

ph

oto

ns, %

Registration time, ps

Forward detected photons vs registration time12

Forward detected photons vs registration time

0 10 20 30 40 50

0

50

100

150

200

250

300

350

g = 0.76

s = 20 mm-1

à = 0.076 mm-1

n = 1.39

Nu

mb

er

of

ph

oto

ns

Registration time, ps

ti = 1 ps - 20 times

ti = 5 ps - 4 times

ti = 10 ps - 2 times

13

0 5 10 15 20 25 30 35 40 45 50

0

50

100

150

200

250

300

Nu

mb

er

of

ph

oto

ns

g << 1

s = 35 mm-1

a = 0.6 mm-1

L = 1 mm

ti = 0.1 ps - 100 times

ti = 1.0 ps - 10 times

ti = 5.0 ps - 2 times

ti = 10.0 ps - no

Registration time, ps

Forward detected photons vs registration time14

Determination of optical parameters of skin

1 – incident beam, 2, 3 – integrating spheres, dealing with reflectance and transmission respectively, 4, 5 – photodetectors, 6 – sample

1

2 3

4 56

15

Skin structure Stratum corneum

Epidermis

Dermis - 1

Dermis - 2

Dermis

Dermis - 3

Subcutaneous fat

16

Parameters of epidermis with substancesSkin type μa,

mm-1

μs,

mm-1

g Diffuse reflectance R,

%

TransmissionT, %

AbsorptionA, %

Skin 0.15 20 0.8 20.4 75.2 4.4

Skin + 0.25хFluo

1.00 20 0.8 14.3 64.1 21.6

Skin + 1хTiO2

0.15 30 0.8 26.5 68.7 4.8

Skin + 2хTiO2

0.15 40 0.8 31.2 63.6 5.2

Skin + 0.25хFluo + 1хTiO2

1.00 30 0.8 18.6 57.1 24.3

Skin + 0.25хFluo + 2хTiO2

1.00 40 0.8 22.3 51.7 26.0

17

Ratio of radiation components for epidermis with various substances

1 2 3 40

10

20

30

40

50

60

70

80

90

100

Inte

nsit

y, %

Skin type

Dif. reflectance Transmission Absorption

18

Layer withTiO2

Rest of stratum corneum

Incident radiation

Transmitted radiation

Stratum corneum with particles of TiO2 19

0 1 2 3 4 5 6 7 80,0

0,1

0,2

0,3

0,4

0,5

0,6

0,7R

el. n

um

ber

of

ph

oto

ns, %

Thickness, m

Whole stratum corneum Rest stratum corneum layer with particles of TiO

2

Absorption in different parts of stratum corneum20

0,08 0,10 0,12 0,14 0,16 0,18 0,20 0,22 0,24

0,0

0,2

0,4

0,6

0,8

1,0

1,2

1,4

1,6

1,8

2,0

g = 0.8

1

s = 150 mm-1

2

s = 100 mm-1

a = 0.1 mm-1

n = 1.5

ti = 0.01 ps

ti = 0.05 ps

ti = 0.10 ps

Rel. n

um

ber

of

ph

oto

ns, %

Registration time, ps

Pulses of various durations passed through stratum corneum with scattering substance (TiO2)

21

Summary• Ballistic, snake- and diffusion

components registration• Scattering multiplicity, time detection of

pulse maximum depends linear on μs, L

• Decreasing of μa makes detected pulses longer and increase their amplitude

• There exists maximum of scattered and absorbed radiation under the medium surface

22

Summary• Multiple scattering makes short

pulses longer• Scattering substance injection

causes absorption and reflectance increase in upper skin layers, transmission decrease

23