Del Mar Photonics

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
dev 8, (TM/-1)



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.
 

Pulse compression grating for Yb-based femtosecond laser