Multilayer Dielectric (MLD) Gratings for OMEGA EP

January 2011

Colorful spectral dispersion

Spectral dispersion is observed through diffraction from a multilayer-dielectric grating

High-energy, petawatt (HEPW)-class lasers depend upon the superlative performance, efficiency, and damage threshold for picosecond pulses that multilayer dielectric diffraction gratings offer for pulse compression. LLE has played a major role in the worldwide effort to realize the potential of these multilayer dielectric gratings. For OMEGA EP, a major limitation to performance is the damage threshold of the final grating in the pulse compressor. For dielectric materials, the sub-100-ps energy damage threshold is significantly lower than for nanosecond pulses. To strengthen the on-target energy, it is vital to boost the diffraction efficiency and damage threshold of the final grating. OMEGA EP uses diffraction gratings etched into an LLE-produced, high-reflecting multilayer dielectric coating with great success.

Gratings for pulse compression are fabricated with exacting specifications. The grating design requires the electron-beam deposition of a multilayer dielectric reflector coating (~23 layers) on an optical substrate. The devices must be constructed with extreme surface uniformity and perform under high vacuum. Since an excess of compressive stress will deform the substrate and an excess of tensile stress will result in coating crazing, stress must be carefully analyzed. In addition, laser damage thresholds must be high in the short pulse (0.5 to 10 ps) regime.

Traditional diffraction gratings for pulse compression applications are holographically recorded and coated with a thin metallic film. Such metalized gratings feature diffraction efficiencies that can exceed 92% over a range of wavelengths. These gratings typically use gold, silver, or aluminum as coatings. The disadvantage of metalized gratings is their inherently low damage threshold. With regard to laser-induced damage thresholds, gold-coated gratings typically tolerate 400 mJ/cm2 on the grating surface for nanosecond pulses and 250 mJ/cm2 for picosecond pulses and even lower fluences for shorter pulses or wavelengths. Today's high energy, short-pulse lasers are designed to operate at fluences of several joules per square centimeter.

Characterization of MLD gratings

Characterization of MLD gratings included atomic force and Nomarski microscopy over the entire grating aperture

For the OMEGA EP Laser Facility, MLD gratings have been fabricated onto a thick BK7 glass substrate (borosilicate crown optical glass with high homogeneity and low bubble and inclusions content) to prevent crazing and excessive bending prompted by tensile coating stress when operated in high vacuum. However, the high coefficient of thermal expansion (CTE) of the BK7 can produce wavefront distortions and changes in the period of the grating if the temperature is not well controlled during the grating writing process. In collaboration with Plymouth Grating Laboratory (PGL), Carver, Massachusetts, LLE has worked to optimize the grating process to deliver parts with improved short-pulse damage thresholds.

LLE contributions included:
  • coating the optics with a HfO2/SiO2 film using an electron-beam deposition process demonstrated to deliver high-damage threshold,
  • establishing grating cleaning parameters for the LLE coated optics, and
  • optimizing the duty cycle and pillar height requirements.

PGL established a grating writing process for large BK7 blanks; tuned their lithography steps to suit our coating process; and pushed their exposure process to manufacture low-duty cycle gratings with good fidelity. Diffraction efficiencies can exceed 97%. Witness data have shown MLD gratings to yield laser-damage thresholds at 10 ps under vacuum that are an order of magnitude higher than those of metalized gratings. In October 2010, LLE installed a full beamline of OMEGA EP size (470 x 430 mm aperture) PGL gratings into the Grating Compressor Chamber, and initial system results suggest an improvement in beam quality. Tests of damage performance will begin in 2QFY11 with the goal of providing increased energy for upcoming experiments in 3QFY11 and beyond.

Tiled-grating assembly

The grating inspection system (right) is shown being aligned to the tiled-grating assembly

Development work continues for MDL gratings in the area of coating deposition using plasma-ion-assisted-deposition, which makes the coating more compressive and dense allowing a potentially wider range of cleaning operations as well as a change of substrate material to fused silica. Other work focuses on optimizing the cleaning chemicals and lithography process to allow a consistent and reliable low-duty cycle, tall pillar height fabrication. The benefits of these processes can potentially be higher damage threshold gratings.