transmission gratings (original) (raw)

Definition: diffraction gratings which are used in transmission

Categories: general optics, photonic devices

• diffractive optics
  • diffraction gratings
    * transmission gratings
    * Bragg gratings

Related: diffractiondiffractive opticsdiffraction gratings

Opposite term: reflection gratings

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DOI: 10.61835/6rl Cite the article: BibTex BibLaTex plain textHTML Link to this page! LinkedIn

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Contents

What are Transmission Gratings?

Operation Principle

Light Transmission Through Materials

Fresnel Reflections

Limited Angular Range

Achieving Strong Enough Phase Modulation

Avoiding Absorption in Metals

Optical Performance of Diffraction Gratings

Alignment Properties

Practical Aspects

Special Forms

Typical Applications

Frequently Asked Questions

Summary:

This article provides an overview of transmission gratings, a type of diffraction grating used in transmission rather than reflection. It details their operation principle, emphasizing the need for transparent materials and the challenges of achieving strong phase modulation and suppressing Fresnel reflections.

Key advantages are highlighted, including the avoidance of absorption in metals (leading to a high laser-induced damage threshold) and significantly lower alignment sensitivity compared to reflection gratings. The article also covers the high optical performance possible, with diffraction efficiencies often exceeding 98%.

Special forms like grisms and two-dimensional gratings are mentioned, alongside typical applications such as compact spectrometers, pulse compressors for ultrashort pulses, beam splitters, and pattern generators.

(This summary was generated with AI based on the article content and has been reviewed by the article’s author.)

What are Transmission Gratings?

Transmission gratings are diffraction gratings which are used in transmission. This is in contrast to the more common reflection gratings, which reflect all incident light, as far as it is not absorbed or scattered.

Transmission gratings can be volume Bragg gratings or surface relief gratings. The latter have a diffractive structure at or near one of their surfaces. See the article on diffraction gratings concerning fabrication methods.

Operation Principle

The operation principle of all diffraction gratings, including transmission gratings, is based on the diffraction of light. However, for transmission gratings the application of that principle differs in some respects from the situation of reflection gratings:

Light Transmission Through Materials

A transmission grating must be fabricated from a well transparent medium, i.e., with low absorption and scattering losses. This is an easily met condition for visible and near-infrared light, but means stringent limitations for the choice of optical materials in other spectral regions such as the far ultraviolet and the far infrared.

In many cases, one fabricates transmission gratings based on robust optical materials (mostly glasses) with high optical quality and well-established processing methods. A particularly prominent example is fused silica, which also exhibits a very wide spectral transmission range.

Fresnel Reflections

It is also usually necessary to suppress Fresnel reflections, e.g. with an anti-reflection coating. Unfortunately, such coatings can have a high performance only in a limited angular and spectral range. Therefore, one sometimes uses uncoated gratings, accepting a certain amount of reflection losses.

Limited Angular Range

While a high reflectance of a reflection grating, e.g. containing a metallic reflector, can be achieved over a wide range of angles of incidence, it is more difficult to obtain low reflection losses of a transmission grating over a large angular range. This is one of the reasons why transmission gratings are often designed with a lower line density to limit the angular range of the involved beams. Of course, the applications need to be adapted accordingly.

As long as the beam angles relative to the surface are close to perpendicular, a low polarization sensitivity can also be achieved.

Achieving Strong Enough Phase Modulation

For transmission gratings, it is more difficult to achieve a strong enough modulation of optical phase of the grating structure. This is easily understood by considering that all involved beams are essentially co-propagating, and accumulated phase changes are determined not by geometrical path length differences, but by the product of a difference in refractive index and the propagation length. Here, the involved refractive index difference is often well below 1. In contrast, a relief grating structure operated in reflection leads to phase differences which are proportional to twice the relief height, corresponding to contributions from a forward and backward pass of light e.g. through air. Therefore, surface relief structures for transmission gratings typically require a several times higher aspect ratio (depth / line spacing) for achieving a similar diffraction efficiency.

Avoiding Absorption in Metals

On the other hand, problems related to the use of metallic reflecting structures are avoided — in particular, the increasing absorption at short optical wavelengths. Transmission gratings may be fabricated entirely from one or more dielectric materials with very low absorption coefficients. These materials and also exhibit a much higher threshold for laser-induced damage.

Optical Performance of Diffraction Gratings

Despite the challenges explained above, it has become possible to produce transmission gratings with very high performance — particularly with holographic techniques, in optimized forms allowing for structures with extreme aspect ratios. They can achieve a very high diffraction efficiency, defined as the optical power sent into the desired diffraction order divided by the incident power. This is particularly important for certain applications like pulse compressors for ultrashort pulses, where losses at compressor gratings reduce the available output power and at the same time may introduce thermal problems. Diffraction efficiencies far above 90% and very low stray light are possible in wide wavelength regions, and even efficiencies beyond 98% are sometimes achieved.

At the same time, low levels of stray light can be obtained, and the threshold for laser-induced damage can be pretty high. Compared with holographic volume gratings, the temperature sensitivity is typically much lower.

Alignment Properties

Concerning the alignment sensitivity, apparatuses based on transmission gratings are often far more forgiving than those using reflection gratings. It is easy to see that the zero-order (non-diffracted) beam has the same direction as the input beam, if the grating device has parallel surfaces. Any angular alignment error can only lead to a slight displacement of that beam. The diffracted beams are also affected by misalignment only to a small extent.

In comparison, the output angle of the zero-order beam of a reflection grating is changed by twice the angular shift of the grating, just as for an ordinary mirror, and the diffracted beams are also strongly affected.

Of course, a detailed comparison also needs to take into account possibly quite different line spacings of the gratings.

Practical Aspects

An often important practical aspect of operating a grating in transmission is that the diffracted light is better separated from the incoming light. It thus becomes much easier, for example, to mount a light detector (e.g. a focal plane array) without obstructing the path of the incident light, and the resulting device design, e.g. of a spectrometer, may become more compact.

Using a transmission grating, it is even possible to convert various types of imaging instruments into spectrometers, since the beam path is not profoundly altered.

Special Forms

Most transmission gratings have a structure of equidistant lines on a plane surface. However, it is also possible to make gratings on curved surfaces and/or to vary the line spacing.

For some applications e.g. in spectroscopy and pulse compressors, it is convenient to use a so-called grisms, which is a prism equipped with a diffractive structure on one of its surfaces.

It is also possible to fabricate two-dimensional gratings (2D transmission gratings) which can diffract beams into two dimensions. Some of those are used, for example, as optical pattern generators.

Typical Applications

Some typical applications of transmission gratings are briefly discussed in the following:

• There are compact spectrometer designs based on transmission gratings, e.g. in combination with a focal plane array of high spatial resolution. A high diffraction efficiency is important for a high measurement sensitivity.
• Pulse compressors for ultrashort pulses are used e.g. in chirped-pulse amplification systems. They particularly profit from very high diffraction efficiencies as are possible with optimized gratings.
• Transmission gratings can also be used as beam splitters for monochromatic light sources like lasers.
• Some transmission gratings are used as optical pattern generators in one or two dimensions.

Frequently Asked Questions

This FAQ section was generated with AI based on the article content and has been reviewed by the article’s author (RP).

What is a transmission grating?

A transmission grating is a type of diffraction grating that works by diffracting light which passes through it, in contrast to the more common reflection gratings which reflect the light.

What are the main advantages of transmission gratings over reflection gratings?

Key advantages include a much higher threshold for laser-induced damage as they can be made entirely of dielectric materials, and a significantly lower sensitivity to misalignment. They can also enable more compact device designs.

What are the main challenges in fabricating high-performance transmission gratings?

A primary challenge is achieving a strong enough modulation of the optical phase, which often requires surface relief structures with a high aspect ratio. Another challenge is the suppression of Fresnel reflections over a wide angular and spectral range.

How efficient can transmission gratings be?

Modern transmission gratings, particularly those made with holographic techniques, can achieve very high diffraction efficiency, often well above 90% and sometimes even beyond 98%, while also offering low levels of stray light.

What are transmission gratings used for?

What is a grism?

A grism is a special optical element which is a prism that has a diffractive grating structure on one of its surfaces. It combines the properties of both components.

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Bibliography

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