Two-Color-Interferometerand Coherent Focusing
FRED project course, LTH
Kathrin Klünder
Motivation: HHG in a Two-Color-Field
• High-order harmonic generation (HHG) by focusing an intense laser field (800nm) into an atomic gas
• Properties of the harmonics determined by fundamental driving field
• Adding a second field to the generation process by frequency doubling a fraction of the fundamental laser frequency =>now wavelengths of 800nm and 400nm
• Ability of changing properties of harmonics and pulse train itself
• Standard procedure in our lab
red only
red and blue
Experiment Theory
Setup: HHG in a Two-Color-Field
doubling crystal
Brewster window (55.47˚ for 800nm)
HHG generation (gas inlet)
probe beam
focusing mirror
interferometer
delay plate
Ti:Sapphire, 800nm, 35fs, 4mJ
•Changing the relative phase by rotation of delay plate
The FRED project
Procedure for aligning interferometer: • two beams overlap spatially and in time directly after passing interferometer• No further check after propagation though rest of the setup
=> Works fine, BUT what are the possible problems or what should one keep in mind?
Use FRED to model influence of delay plate and investigatestandard procedure when aligning/working with the Two-Color-Interferometer.
Our setup in FRED…
Defining the Light Source
desired properties: • coherent• polarized• pulsed (35fs)
=> in FRED not possible
• define spectral bandwidth of ≈ 30nm
Properties of the Delay Plate
new materials can added from a huge range of cataloged materials
Using FRED Scripting
create variables
ray-tracing and drawing the rays
define and open output file #1
access delay plate coordinates
close output file #1
get infos about ray j for angle i
if rays ended up on right detector: print infos in #1
built-in functions to get ray informations and control optical components:
Performance of Interferometer
delay plate
analysis surface
170µm
340µm
510µm
thickness of delay plate
delay of red arm as a function of angle
red arm
blue arm
Interferometer works fine.
Influence of Brewster Window
• Brewster window introduces dispersion for both colors– relative delay of the two arms changes– temporal spread of the two pulses; bigger influence on blue
pulse (approx. 10fs for spectral bandwidth)
focusing mirror (f=75cm)
Brewster window (1mm)
vacuum
red arm
blue arm
delay of red arm as a function of angle
Influence of Brewster Window
foci positions red and blue beams:– 3µm offset in direction of beam propagation– 6µm offset in Y-direction
no Brewster window: with Brewster window:
offset 6µm
beam in focal plane:
Some Words about the Focusing
• focus size and astigmatism depends on incoming angle• for better focusing conditions spatial offset gets more
important • perturbation of wave fronts due to focusing mirror
decrease distance x:
spherical focusing mirror x
in focusbefore focus
some other bad example…
In the End
FRED is...– easy to get started with– convenient to model small questions– good to get an idea what happens in the setup
