Ultrahigh-Performance Ion-Etched Holographic Diffraction Gratings

Dr. John R. Gilchrist and Bruno Touzet

Demand for higher-resolution and longer-wavelength gratings drives novel holographic and ion-etched designs for use in applications from laser pulse compression to wavelength division multiplexing systems.

Up until the mid-1960s, mechanically ruled diffraction gratings with approximately 1800-g/mm ruling density were considered to be the state of the art in spectroscopy. By the late ’60s, scientists had developed diffraction gratings by forming a hologram ruling in photoresist. Soon they also were able to make aberration-corrected gratings using a holographically produced variable line-spacing structure. Such gratings have been available for almost 30 years for use in UV to visible spectroscopy, and in meeting the demands of synchrotron and spaceflight applications.

Subsequent developments in ion-etching techniques led to the combination of holographic gratings with ion-etched blazed profiles that were close in diffractive efficiency to those achieved by mechanical ruling, but with superb stray-light performance and no ghost artifacts. These gratings are often the best choice for ensuring that a spectrometer has the highest signal-to-noise ratio.

Today’s state-of-the-art diffraction gratings are employed in applications from soft x-ray spectroscopy to ultrahigh-energy pulsed laser experiments where operation up to the petawatt level is required. Scientists at Jobin Yvon recently demonstrated high-ruling-density transmission gratings engraved directly into fused silica for the Laser MegaJoule project, a French government initiative similar to the Lawrence Livermore National Laboratory’s National Ignition Facility in Livermore, Calif.

Each grating is 420 x 470 mm in area and performs the functions of wavelength separation, third harmonic at 351 nm from the fundamental at 1053 nm, and beam steering. It also focuses the beam as a lens to a small spot that is approximately 400 μm in diameter. Special holographic recording is used to produce the gratings with a high-aspect-ratio laminar profile. After recording, the photoresist is developed, leaving a mask through which ion etching can be made. With a 2500-g/mm-density laminar grating, groove depths up to 1 μm are achievable, resulting in grating efficiencies as high as 90 to 95 percent — essential for the transmission of these intense laser pulses.

By recording curved and nonequidistant grooves, the transmission grating can be adjusted to provide a focusing and a dispersive effect, allowing laser designers to minimize the number of optical elements in each beam line. Laser MegaJoule has 240 such beam lines that concentrate their light onto a fusion target to produce 500-TW pulses in 3 ns and sample temperatures as high as 10⁸ K. Diffractive optics is one of the key enabling technologies for this project.

In the same area of high-energy lasers, it’s possible to produce large pulse-compression gratings engraved into high-damage-threshold multilayer dielectric coatings. These new gratings offer important advantages in comparison with the more classical gold-coated pulse-compression gratings: higher efficiency, in the 93 to 98 percent range, along with a doubled laser-induced damage threshold.

Scientists continue to research grating materials, groove profiles and distributions, and manufacturing techniques for mastering gratings using holographic and etching techniques. In the process, we are seeing many exciting possibilities for designing spectrometer systems with high resolution, high efficiency and minimal stray light.

An important step was the development of spectrometers that allowed the grating to be the only optical element. In this case, the grating can be concave-spherical or -toroidal and aberration-corrected. This enables the design of instruments for the soft x-ray as well as for the UV to the infrared regions. Although this concept is not new, it has been developed to a very high level of performance for both monochromator and spectrograph applications. Indeed, convex gratings are commonly used in Offner spectrograph designs, which provide excellent spectral resolution and imaging and stray light performance for multitrack measurements such as multipoint process monitoring for pharmaceutical manufacturing.

Use of elements such as these toroidal and aberration-corrected diffraction gratings makes possible the design of instruments for the soft x-ray as well as for the UV to the IR regions.

In the next three years or so, the continual development of novel holographic designs and ion-etched techniques is expected to deliver high-efficiency gratings for laser-pulse compression, and spectrometers in the extreme-UV for synchrotron and space applications, and in the infrared for semiconductor analysis and dispersive gas sensing. With an upswing in the telecommunications industry, the requirement for higher-resolution and longer-wavelength gratings also will drive novel designs for use in wavelength division multiplexing systems.

The diffraction gratings market is steadily expanding as instrument designers move away from filter-based instruments and toward wavelength-dispersive instruments that can measure a far richer spectroscopic signal. Diverse market segments exist — from large laser gratings to small monochromator gratings — and run into many hundreds of thousands of gratings per year worldwide with prices per unit ranging from $100,000 and more to as low as $100. Development focus continues on the multilayer dielectric-type gratings and on holographic designs for near-infrared operation.

Meet the authors

John R. Gilchrist is director of the custom optics group at Jobin Yvon Inc. in Edison, N.J. He obtained his doctorate in applied physics from Strathclyde University.

Bruno Touzet is sales and marketing manager for the OEM and custom optics division of Jobin Yvon SAS in Longjumeau, France.