Pulse Compression Gratings - Instructions for ordering -
order
When ordering a grating, please use the following example format below, or
choose from the standard
PC grating stock list...
PC 1200 W x H x Thk 800 nm (TM/-1) constant deviation 8°
1. PC stands for pulse compression
2. 1200 is the groove density (groove frequency) in grooves/mm
3. W is the blank dimension in mm parallell with the grating grooves
4. H is the blank dimension in mm perpendicular to the grating grooves
5. Thk is the blank thickness in mm
6. 800 nm is the desired optimized wavelength. A range or range with peaked
wavelength can also be specified
7. (TM/-1) is desired polarization state and diffraction order the grating
should be optimized for. TE and average (TM+TE)/2 can also be specified
8. Constant deviation 8° is the configuration the grating should be optimized
for. Constant incidence angle can also be specified
Standard tolerances on W, H, Dia: ± 0.2 mm Thk ± 0.5 mm
CA > 90 % of blank size
Standard Pulse Compression gratings Type PC, specified by combining from:
|
Grooves/mm |
Sizes |
Wavelength range |
Configuration |
|
0600
0900 1200 1400 1500 1600 1700 1800 2000 2100 2200 2300 |
25 x 25 x 6 mm
30 x 30 x 6 mm 30 x 64 x 10 mm 30 x 75 x 16 mm 30 x 110 x 16 mm 50 x 50 x 10 mm 50 x 110 x 16 mm 58 x 58 x 10 mm 64 x 64 x 10 mm 90 x 90 x 16 mm 110 x 110 x 16 mm 120 x 140 x 20 mm |
NIR 750-1064 nm
@ 790 nm @ 800 nm @ 920 nm @ 1032 nm @ 1047nm @ 1053 nm @ 1064 nm |
Typical near Littrow |
Other specifications available on request.
Note: The absolute efficiency curves shown are only representative for the
stated geometry and wavelength(s) and can vary depending on use geometry and
measurement technique *)
Pulse Compression Gratings
These gratings are especially suited for use in laser pulse compression
experiments. High diffraction efficiency, in combination with good spectral
quality and high damage resistance makes these gratings useful in all kinds of
laser pulse applications; both pulse compression using optical fibre grating
pairs, and for amplification of pulses with chirped pulse amplification.
Fibre grating compressor
When a short laser pulse is transmitted through an optical fibre, the pulse will
be stretched, or "chirped" due to nonlinear effects (selfphase modulation). The
group velocity dispersion in the fibre results in that the front of the pulse
will have a longer wavelength than the tail. By using a pair of gratings one can
arrange so that the long wavelength pulse will travel a longer path than the
short wavelength pulse, with the result that, after the grating pair, they
arrive at the same time. The grating pair not only compensates for the pulse
broadening in the fibre, but makes the pulse even shorter than the input; up to
90 times compression can be achieved.
Chirped pulse amplification
Very short pulses (100 femtoseconds) can be produced by some types of mode
locked lasers. For many applications, these pulses have too low peak power. The
technique of chirped pulse amplification (CPA) can be used for amplifying such
pulses, to achieve peak powers in the order of Terawatts.
The amplifier is basically a laser crystal inside a resonator. To avoid strong
nonlinear effects which would destroy the crystal, the input pulse is stretched
in time, so that the peak power is decreased. This chirped pulse is then
amplified, and subsequently compressed to obtain a high power pulse with a
duration nearly equal to the input pulse.
Stretching and compression with grating pairs
Both stretching and compressing utilize grating pairs, arranged in subtractive
dispersive mode; so that the angular dispersion of the first grating is
cancelled by the second grating. Two parallel beams of different wavelengths,
incident on the first grating, are still parallel when they leave the second
grating, but they have travelled different distances.
configuration
A grating pair arranged parallel as in fig.A, will introduce a negative group
velocity dispersion, i.e. pulses of long wavelength arrive later than short wave
pulses.
In order to achieve a positive dispersive delay, a more complicated arrangement
is necessary. Fig.B shows such an arrangement, normally used in the stretcher
stage. An afocal lens system (telescope) is inserted between the gratings. The
telescope reverses the sign of the angles so that the beams will hit the second
grating at the same angle as they leave the first one.
Both stretcher and compressor are normally used in double pass. The advantages
are twofold: the dispersion is doubled, and all wavelength components of the
beam emerge colinear, not linearly translated as shown in the figure for single
pass.
Extremely low stray light
The gratings are holographically recorded with two highly collimated, clean and
homogeneous beams, which give straight and equispaced grooves. The diffracted
light is free from "ghosts" and give very low levels of randomly scattered
light.
High efficiency and low loss
The groove profile of the pulse compression gratings is optimized to give
maximum efficiency for light polarized perpendicularly to the grooves (TM
polarization).
efficiency
Absolute efficiency of a pulse compresslon grating, 1800 gr/mm, gold coated, in
Littrow configuration
The efficiency depends on the wavelength and configuration, but frequently an
absolute efficiency of more than 90% for each grating is achieved. The figure
shows the efficiency of a gold coated grating with 1800 grooves/mm, in a Littrow
configuration.
Not only the diffraction efficiency is important. In order to avoid thermal
effects, the absorption in the grating surface should be as small as possible.
Therefore, a coating material with high reflectance should be used. In the NIR
region, gold coated gratings have the best performance.
Flat diffracted wavefront
The combination of a flat grating surface and a holographic exposure setup of
high optical quality, gives a flat diffracted wavefront. This gives the
possibility to focus the laser pulse to high intensities.