Background

The increased demand for high-power laser sources with diffraction-limited beam quality has led to significant scaling in output power of laser systems. Attempts at scaling single-output lasers to the 100's of kilowatts has revealed issues with removing waste heat, maintaining beam quality, and avoiding optics damage at the higher powers.

A Spectral Beam Combining (SBC) system that can effectively combine diffraction-limited beams of varying wavelengths into a single beam with broad gain bandwidth provides a straightforward approach to power scaling. Until the development of an extreme-power, ultra-low-loss dispersive grating optics by LLNL, the output power of SBC systems was limited by the ability of the beam combiner to minimize power loss. LLNL's SBC grating optics are precisely designed and fabricated surface-relief grating structures embedded into the topmost layer of a highly reflective, ultra-low loss, multilayer dielectric thin film.

The Diffractive Optics Group of the NIF Directorate at LLNL is the preeminent supplier of large-aperture, high damage-threshold diffraction gratings and other diffractive optics to laser institutions around the world. Presently, the LLNL group is the only supplier of polarization insensitive gratings SBC grating optics. These gratings, designed for polarization independent operation at nominally 1µm and with a Littrow out-of-plane (conical) mounting, have been shown to have >98% diffraction efficiency over >40 nm bandwidth.

Description

Livermore Lab's SBC grating optics benefit from the combination of the following key technologies:

  • LLNL proprietary optical coating designs utilizing >100 thin film layers – enables ultra-low-loss, ppm transmission levels through the coating, high diffraction efficiency, and large bandwidth.
  • LLNL proprietary dispersive surface relief structure design – perfectly impedance matched to the thin film stack for optimum optical performance.
  • Ability to fabricate dispersive surface relief structure and advanced optical thin film coating on superior thermally conductive materials such as silicon and silicon carbide.
  • LLNL proprietary processing techniques permitting the fabrication of optimum optical design.
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Advantages

Presently, the LLNL's Diffractive Optics Group is the only supplier of polarization insensitive gratings SBC grating optics. These gratings, designed for polarization independent operation at nominally 1µm and with a Littrow out-of-plane (conical) mounting, have been shown to have >98% diffraction efficiency over >40 nm bandwidth.

LLNL's new generation of high-power SBC grating optics has numerous advantages over the current baseline configuration being provided, such as:

  • Improved MLD grating mirror stack absorption by over 60X over current configuration, current grating mirror stack ~60 ppm - new grating mirror stack <1 ppm
  • Lower mirror stack absorption translates to smaller heating loading and less optics distortion.
  • More robust and higher laser damage threshold.
Potential Applications

Spectral beam combining of lasers or laser emitters.

Development Status

LLNL demonstrated first hafnia oxide (HfO2) spectral beam combining (SBC) grating optic with ultra-low absorption and 2.8MW/cm2 laser damage performance. Testing showed that the HfO2 SBC grating optics are 2X less heating when illuminated with 2.8MW/cm2 and 6X higher laser damage threshold than the current baseline configuration when contaminated with carbon particles.

LLNL has also filed for patent protection on this invention.

Reference Number
42919
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