Gratings

Multi-layer Dielectric Gratings for Pulsed Compression

Multi-layer Dielectric Gratings for Pulsed Compression

The rapid development of intense sources for picosecond and femtosecond light pulses and in particular pulse compression techniques has prompted the need for new ultra-high performance, high damage threshold, diffraction gratings. HORIBA Jobin Yvon has been a leading supplier of gold coated pulse compression gratings since the development of the technique. Today HORIBA Jobin Yvon is developing unique MLD gratings1 with higher damage threshold for very high power laser chirped pulse compression.

Traditional diffraction gratings for pulse compression applications are holographically recorded and coated with a gold metallic film. Metalized gratings have many useful features including diffraction efficiencies that can exceed 92% over a broad range of wavelengths. The groove profile as well as the optical properties of the metal coating determine the properties of the grating.

As far as laser induced damage threshold is concerned, gold coated gratings typically present the following values:

  • 400 mJ/cm2 on the grating surface for nanosecond pulses
  • 250 mJ/cm2 on the grating surface for picosecond pulses

and lower fluences for shorter pulses or shorter wavelengths.

Typical multi-layer dielectric mirror
Typical multi-layer dielectric mirror, L: low index layer, H: high index layer

For many years multi-layer dielectric (MLD) structures composed of alternating high and low index layers have been well known to be highly reflecting. At each interface between a low and high index pair about 4% of the light is reflected. Summing all of the light from the many layers gives an optic that can approach close to complete reflection. Since MLD structures are insulators they lack the conduction electrons that make metals good reflectors and thus can have intrinsically higher damage thresholds.

The manufacture of MLD gratings requires control of the stack of dielectric films, each of a predefined thickness, uniform coating of photoresist and very precise generation of the holographic pattern that defines the groove shape and distribution. The latent image in the photoresist is transferred permanently into the dielectric stack by ion etching.

Multi-layer dielectric grating, grooves engraved into the low index MLD upper layer
Multi-layer dielectric grating, grooves engraved into the low index MLD upper layer

Current Achievements

HORIBA Jobin Yvon is presently developing MLD gratings exclusively with 1740 l/mm groove density and to work at 1.053 micron.

We have produced a series of MLD gratings with following specifications:

  • groove density: 1740 l/mm
  • central wavelength: 1053 nm
  • bandpass: 30 nm
  • substrate dimensions: 120x140x20 mm

    • 210x420x50 mm
    • 335x485x50 mm
    • 430x470x100 mm

  • efficicency at 1053 nm: higher than 92%
  • wavefront quality: λ/4

Example of 210x420 mm, 1740 l/mm, MLD Grating efficiency map
Example of 210x420 mm, 1740 l/mm, MLD Grating efficiency map
1740 l/mm, optimised 1053 nm, MLD Grating bandpass

NOTE:
This curve is for reference only and is not meant to be a specification

NOTE our damage threshold conversions:

Influence of the incident beam angle: if 1.7 J/cm2 fluence on the grating surface has been measured for 10 picosecond pulses, it may correspond to different beam fluences.

For example:
for 61° incidence, 1.7 J/cm2 fluence on the grating surface, will be equal to 3.5 J/cm2 beam fluence (cos61° = 0.48);
and for 72° incidence, 1.7 J/cm2 fluence on the grating surface will be equal to 5.5 J/cm2 beam fluence (cos 72°=0.31).
Consequently, designs with higher incidence angle on the grating at the output of the compressor are expected to be favorable for damage threshold.

NOTE concerning 800 nm MLD pulse compression gratings:
To compress titanium sapphire laser pulses to extremely short pulses such as 100 fs or less, the compressor gratings have to support very large wavelength bandpass typically 100 nm or more. The development of multi-layer dielectric coatings for this application has not yet led yet to any commercially viable solution. For these applications, we propose gold coated, pulse compression gratings that work around 800 nm, with large bandpass.

1Sold in the US under license of Patent # 5,907,436