Phase mask (transmission grating)

A phase mask is a special type of optical transmission grating . It usually consists of quartz glass into which linear depressions have been etched periodically at a defined distance.

If this structure is illuminated with coherent light, the light is diffracted , as is usual with a grating : the light is deflected in several directions. If the illumination takes place with a light beam that has a certain diameter, the diffracted light beams overlap in an area under the phase mask. As they spread out in different directions, there is a striped interference pattern .

Typical configurations

"+ 1 / -1" configuration

The simplest option is lighting perpendicular to the surface of the mask. To a large extent, the light is deflected into the orders +1 and −1. Thanks to a special manufacturing design, the zeroth order is largely suppressed, as it has a disruptive effect here. Higher orders can also occur. The diffraction angle of the first order (measured perpendicular to the mask surface) is calculated as

{\ displaystyle \ theta = \ sin ^ {- 1} {\ frac {\ lambda} {d}}}

.

Here are the wavelength of the light used and the line spacing of the grating. ${\ displaystyle \ lambda}$ ${\ displaystyle d}$

The period of the resulting stripe pattern is half the line spacing of the grating.

"0 / -1" configuration

In this configuration, the mask is illuminated at an angle. In addition to the non-diffracted beam, the first order occurs on one side. If the light wavelength is greater than the line spacing , then no second order can exist, as the diffraction angle would be greater than 90 °. In this configuration, disruptive orders can be avoided. For this, the mask must be illuminated at a specified angle of incidence, which can be calculated ${\ displaystyle \ lambda}$ ${\ displaystyle d}$

{\ displaystyle \ theta = \ sin ^ {- 1} {\ frac {\ lambda} {2d}}}

.

The period of the resulting stripe pattern is here equal to the line spacing of the grating.

Applications

The resulting stripe pattern allows a phase mask to be used for the photolithographic production of periodic structures. Instead of using a “mechanical” mask, in which the areas to be illuminated are defined by openings in the mask and the rest remains shaded, this “optical” mask uses an interference effect to periodically illuminate it.

A common application is the manufacture of fiber Bragg gratings .