We report on highly efficient transmission gratings in fused silica with a grating period of 800 nm generated by electron-beam lithography. At a wavelength of 1060 nm, 95% diffraction efficiency is achieved under Littrow conditions. The damage threshold, extremely enhanced compared with conventional gold-coated diffraction gratings, makes these gratings the key elements in high average power (>100 W) femtosecond fiber chirped-pulse amplification systems.

Highly efficient transmission gratings in fused silica for chirped-pulse amplification systems

Fiber- or rod-based optical source featuring a large-core, rare-earth-doped photonic-crystal device for generation of high-power pulsed radiation and method

Spectrally beam-combined (SBC) laser systems, wherein multiple laser outputs are spectrally multiplexed into a single high-quality beam, are rapidly advancing the power scaling frontier for high-average-power beam-combined fiber lasers with near-perfect beam quality. We describe two- and three-channel SBC fiber lasers featuring 93% power-combining efficiency, near-diffraction-limited beam quality, average output powers in excess of 500 W, and excellent prospects for additional power scaling. To our knowledge, this level of optical performance represents the highest combination of beam quality and average power obtained so far for a beam-combined fiber laser system.

Spectrally Beam-Combined Fiber Lasers for High-Average-Power Applications

Anomalous refraction is a form of extreme waveform manipulation that can be realized with artificially structured nanomaterials, such as metamaterials or metasurfaces. While this phenomenon has been previously demonstrated for select input and output angles, its generalization to arbitrary angles with high efficiencies remains a challenge. In this study, we show that periodic dielectric metasurfaces can support ultra-high-efficiency anomalous refraction for nearly arbitrary combinations of incident and outgoing angles (>90% efficiency for angles up to 50°). Both polarization-dependent and polarization-independent device configurations can be realized, and the achieved metrics exceed the capabilities of conventional metasurfaces by a large margin. Many of the devices studied here utilize dielectric nanostructures that support strong near-field optical interactions with neighboring structures and complex optical mode dynamics. We envision that these concepts can be integrated with practical applications in ...

Ultra-High-Efficiency Anomalous Refraction with Dielectric Metasurfaces

High efficiency, broad-band TE-polarization diffraction over a wavelength range centered at 800 nm is obtained by high index gratings placed on a non-corrugated mirror. More than 96% efficiency wide band top-hat diffraction efficiency spectra, as well as more than 1 J/cm(2) damage threshold under 50 fs pulses are demonstrated experimentally. This opens the way to high-efficiency Chirped Pulse Amplification for high average power laser machining by means of all-dielectric structures as well as for ultra-short high energy pulses by means of metal-dielectric structures.

High-efficiency, broad band, high-damage threshold high-index gratings for femtosecond pulse compression

We report on reflection gratings produced entirely of dielectric materials. This gives the opportunity to enhance the laser damage threshold over that occurring in conventional metal gratings used for chirped-pulse-amplification, high-power lasers. The design of the system combines a dielectric mirror and a well-defined corrugated top layer to obtain optimum results. The rules that have to be considered for the design optimization are described. We optimized the parameters of a dielectric grating with a binary structure and theoretically obtained 100% reflectivity for the -1 order in the Littrow mounting for a 45° angle of incidence. Subsequently we fabricated gratings by structuring a low-refractive-index top layer of a multilayer stack with electron-beam lithography. The multilayer system was fabricated by conventional sputtering techniques onto a flat fused-silica substrate. The parameters of the device were measured and controlled by light scatterometer equipment. We measured 97% diffraction efficiency in the -1 order and damage thresholds of 4.4 and 0.18 J/cm2 with 5-ns and 1-ps laser pulses, respectively, at a wavelength of 532 nm in working conditions.

High-efficiency dielectric reflection gratings: design, fabrication, and analysis.

We investigate a cavity ring down setup that offers the possibility to measure the spectrally resolved reflectivities of high reflectivity mirrors. The setup consists of a resonator (ring down cavity) and an intensified CCD camera system combined with a spectrograph for spectral resolution. A commercial supercontinuum laser (350-1750 nm) is applied as a compact excitation source. It is based on a photonic crystal fiber that is pumped by a q-switched microchip laser (1.6 ns pulse duration, 25 kHz repetition rate). This combination allows simultaneously recording the transmittance of the cavity over a wide wavelength range determined by the excitation source and the spectral sensitivity of the detector. The photon lifetimes inside the cavity (ring down times) are measured with high spectral resolution by means of an intensified camera system. Subsequently shifting the “gate” of the image intensifier from short to long delay times after the excitation pulse allows calculation of the reflectivity spectrum of the mirrors. Comparison of these results with measurements using a conventional setup (laser diode 675 nm and photomultiplier tube) clearly shows the high potential of the method due to the multichannel excitation and the detection scheme.

Spectrally resolved cavity ring down measurement of high reflectivity mirrors using a supercontinuum laser source

The key technologies for modern production processes with enhanced spatial resolution, require high performance DUV- excimer laser optics with enhanced optical properties. Major challenges imposed onto the requested new generation of optical elements are concentrated on lowest absorption and scattering as well as stability against highest pulse number throughput. These targets are the driving force within the German Joint Research Project 'OPUS II', which is dedicated to the development of high quality optical components for the DUV spectral range. As a major contribution to these investigations, sets of reflecting stacks with four different numbers of layer pairs of LaF3/MgF2 were produced by 6 partners of the consortium and characterized in respect to their optical performance and structural properties. The characterization includes spectrophotometric measurements from the VUV up tot eh mid RI range. calorimetric absorption measurements at 193 nm, and a comparative study in total scatter behavior at 193 nm, which was performed by three laboratories within the project. Also, besides the intrinsic stress and the surface topography of the layers, the non-linear absorption behavior of selected samples have been determined. The results are presented and discussed with respect to possible applications.

Hartmut Heyer

Papers

High-efficiency dielectric reflection gratings: design, fabrication, and analysis.

Spectrally resolved cavity ring down measurement of high reflectivity mirrors using a supercontinuum laser source

Properties of fluoride DUV excimer laser optics: influence of the number of dielectric layers