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A Clear View of Fused Silica Transmission Gratings

Photonics Spectra
Sep 2005
As fused silica transmission gratings have evolved to offer high efficiency, low polarization dependence and high power handling capabilities, they have begun to displace reflection gratings in various applications.

Kristian J. Buchwald, Ibsen Photonics A/S

Diffraction gratings have been manufactured for more than 200 years. Their development was driven initially by the requirements of spectroscopy, where they have been applied since Joseph von Fraunhofer’s work in the early 1800s.

For many years, grating production remained the work of high-precision instrument makers, who built one-of-a-kind, complex ruling machines that manually scratched material away from master gratings. Because this technology is time-consuming and expensive, commercial gratings mainly have been replicated: gratings that are copied in large quantities from a master form into a polymer.

This technology allows only for reflection gratings because the polymer is of a low optical quality and unsuitable for transmission, and because groove profiles for high-efficiency transmission gratings are not possible by the ruling method. Metal-coated reflection gratings thus have “ruled” the world for centuries.

Since the advent of the laser, holographic patterning has superseded ruling technology in the production of gratings for applications that demand high quality. This has improved grating performance in terms of important figures of merit such as resolution and stray light.

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Advanced etching techniques enable the optimization of a fused silica transmission grating’s performance to the application.

Over the past decades, semiconductor manufacturing technology has appeared and matured, providing techniques applicable to transmission gratings and bringing with them the high-volume/low-cost benefits of semiconductor technology. By combining recent strides in production-oriented holographic patterning technology with semiconductor materials and etching technologies, it is now possible to produce transmission gratings that rival reflection gratings in all aspects.

Pattern transfer

Holography typically has been characterized by its superb, noise-free quality, but it has been plagued with an Achilles heel in reproducibility. A new holographic stepper overcomes this limitation by combining the holographic principle with the stepper approach of semiconductor manufacturing. Besides offering a reproducible, high-yield process, this highly automated technique is suitable for use in high-volume production.

The stepper is the main contributor to the quality of the gratings and is supportive of the high-volume, low-cost production process. The holographic patterning, however, produces only an intermediate product in photoresist, which — like any polymer — is both environmentally and thermally unstable, so the ideal diffraction grating thus is polymer-free. (Replicated reflection gratings maintain polymer material, as do volume holographic transmission gratings.)

What is needed is a means of transferring the high-quality grating pattern from the polymer into a stable material. This is where the semiconductor manufacturing technologies come into play. The grating pattern, exposed by the holographic stepper, is transferred into bulk fused silica by a semiconductor etch process.

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Production of fused silica transmission gratings is based on scalable semiconductor manufacturing technology.

The technique provides close to 100 percent diffraction efficiency even for high grating resolutions. Although most manufacturers of reflection gratings benchmark diffraction efficiency against a metal mirror, which inevitably absorbs a few percent of the illumination, transmission gratings in fused silica have fundamentally no absorption over a very broad illumination bandwidth, stretching from the deep-UV to the near-IR.

The stepper system has been utilized to produce gratings with resolutions of 450 to 3000 lines per millimeter. Gratings thus manufactured in fused silica are both metal- and polymer-free. Fused silica is widely recognized and used for its environmental stability (including against harsh temperature and humidity variations) as well as for its energy and power handling capabilities. As such, it is the ideal material for optical components, including diffraction gratings.

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Fused silica gratings have superior stability and power handling capabilities.

Spectroscopy was the initial grating application, and it remains one of the prime markets for these optical elements. Using fused silica transmission gratings as a replacement for reflection gratings, spectroscopy can maintain the valued characteristics of high diffraction efficiency, high repeatability and low cost of replicated gratings, and it also benifits from a greater freedom in optical design.

Spectral dispersion

Optimal efficiency for a grating requires close to Littrow illumination. Unfortunately for reflection gratings, this means that the output plane is very close to the input plane, which results in challenges for optical design. With transmission gratings, the output plane is on the opposite side. The grating can be used simultaneously as a dispersion-generating and a beam-folding element, allowing compact designs.

Moreover, the metallic coating on a reflection grating interacts with the light to cause spectral anomalies (often present in just one polarization). The nonmetallic transmission gratings are inherently free of these problems.

Table_Ibsen.jpgA specialized spectroscopic grating is used in telecommunications applications. In dense wavelength division multiplexing, free-space optical modules use diffraction gratings as demultiplexers in demultiplexing units and as the initial component in reconfigurable optical add/drop modules, dynamic gain-flattening filters and dispersion-compensation modules. These applications require high dispersion within a narrow bandwidth (1525 to 1575 nm) and are extremely demanding in terms of diffraction efficiency and polarization independence.

The fused silica grating manufacturing technology has been applied to this application and has produced components with a diffraction efficiency of 95 percent and a polarization-dependent loss of 0.05 dB. Being 100 percent dielectric, these gratings also are ideal with respect to Telcordia requirements, as they contain no polymers, glues or epoxies.

Pulse compression gratings

Another type of optical component based on dispersion is the pulse compression grating used in femtosecond lasers for pulse stretching and/or compression. As with spectroscopy, this application traditionally has utilized reflection gratings. As laser powers increase, however, metallic reflection gratings are breaking down under the extreme energy levels. This does not occur with fused silica gratings.

The grating applications described above utilize the dispersive property of gratings. Because gratings also have the property that a discrete wavelength illumination will be diffracted into several discrete diffraction orders according to the grating equation, beamsplitting applications also abound.

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Transmission grating production at Ibsen Photonics A/S takes place in a Class 10 cleanroom environment.

The high power handling capabilities and environmental stability of transmission gratings, together with the fact that they can have grating periods down to 200 nm, make fused silica transmission gratings ideal for space, astronomy and scientific applications.

A well-known example of a beamsplitting grating is the phase mask, a diffractive tool used in the manufacture of fiber Bragg and waveguide gratings. The transmission properties of UV-grade fused silica further facilitate the use of 193- and 248-nm light sources.

The way has been paved

Based on advances in holographic stepper technology and semiconductor materials processing, fused silica transmission gratings have begun to displace reflection gratings in applications new and old. They meet or even surpass the performance of reflection gratings, and they establish new standards for reliability, stability and power handling.

The way has been paved for higher-power, higher-efficiency optical modules, thanks to fused silica transmission diffraction gratings.

Meet the author

Kristian J. Buchwald is product manager of gratings at Ibsen Photonics A/S in Copenhagen, Denmark; e-mail: kristian.j.buchwald@ibsen.dk.


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