Filtering in optics

Spatial light modulators (SLM)

The inconvenient principal of the photo snapshots is the time relatively long necessary for the chemical treatment.

It is preferable to use the electro-optical properties of certain crystals to create or register in real time the given optics that intervene in the system destined to the optical treatment of the signal.

Two categories of SLM are distinguished :

  • SLM at electric addressing, used if the information is collected by the optoelectronic components (photo diodes, camera CCD, numeric simulation)

  • SLM at optical addressing, used if the information is under the optical form (exit from a video monitor, exit from no matter what imaging system).

In all the cases, by definition, the exit (of the SLM component) is always optical. For ample information concerning this subject, the reader can consult the reference []

Usages of the SLM

At the origin, the spatial modulators of light were developed for a usage in optical processors such as :

  1. Converting an incoherent image into a coherent image

  2. Amplifying a weak image

  3. Converting the wavelengths (pass from the I.R. into the visible)

  4. Modifying the spatial filter used in the Fourier plane (spectral)

  5. ...

But, it is the development of the modulators for big public applications like video projectors that permitted the research work on the optical processors of recognition of forms that we know today.

Properties of liquid crystals

The use of liquid crystals is widespread (digital display, screens...). The applied tension to the electrodes provokes a variation in the intensity of the transmitted or applied light by the display unit. The liquid crystals can be seen as composed of ellipsoidal molecules. These molecules regroup between those of different means forming two classes (or phases) of liquid crystals: nematic, smectic, and the cholesteric (chiral).

Figure I-9 : Molecular arrangements for different types of crystal liquids. When it is necessary, the layers have been separated for more clarity
Figure I-9 : Molecular arrangements for different types of crystal liquids. When it is necessary, the layers have been separated for more clarity [zoom...]

The SLM principally use the nematic crystals (NLC) and a special class of smectic (C*) called the “Ferroelectric Liquid Cristal” (FLC).

Properties of the NLC :

It is possible to impose limiting conditions to orient nematic crystal liquids by polishing the surfaces of the layers of alignment in a given direction. The axis of the molecules in contact with the inner wall have a tendency to align with the little scratches of polishing at the level of the surface. To keep a continuity at the level of alignment of the axis, we can apply a torsion to the cell like is shown in figure I-10(a). By applying an electric field, a electric dipole is introduced in each molecule. The big axis of the molecule (or the dipole appears) aligns itself with the electric field (figure I-10(b)).

Figure I-10 : (a) Molecular arrangements in a cell of crystal liquids when the polishing directions of the alignment layers are perpendicular. (b) Alignment of the molecular axis with the electric field.
Figure I-10 : (a) Molecular arrangements in a cell of crystal liquids when the polishing directions of the alignment layers are perpendicular. (b) Alignment of the molecular axis with the electric field. [zoom...]

Les propriétés optiques des NLC

  • The elongated molecules provoke an anisotropy inducing an important birefringence. The variation of indication is elevated which permits us to have thicknesses at the level of the relatively weak cells ( ne following the axis of the molecule and n0 perpendicular to the axis)

  • If the molecules are disposed in a helical way (figure I-10 (a)) we have an important rotary power.

By combining these two properties, one can see modulations of light intensity.

Example of function :

In the absence of an electric field (figure I-11), the polarized light at 45°on the axis xOy meets the axis of the molecule oriented in the vertical.

Figure I-11 : In the absence of an electric field, the intensity of the reflected light by the cell is zero
Figure I-11 : In the absence of an electric field, the intensity of the reflected light by the cell is zero [zoom...]

It sees therefore the big as well as the small axis of the molecule. The components of the luminous field following x and y undergo a different phase difference (following ne and no ). At the level of the mirror at the back of the cell, the light will be polarized circularly if one arranges it so that the phase difference is equal to . This is achieved by choosing d a thickness of suitable cell. After the round trip (reflection on the mirror) the light will be polarized rectangularly and oriented at 90°of the incident light. The light meets the polarizer P placed at the entrance in a crossed position. The intensity will be minimal at the exit of the cell.

On the other hand and in the presence of a sufficient electric field, the molecule will be aligned with the applied electric field. This time, the components on the x and y of the light meet the small axis of the molecule (see figure I-12). There is no phase difference between the components of the luminous field. The light is polarized rectangularly by reflecting itself on the mirror and stays parallel to itself arriving at the polarizer P after the round trip. The exiting luminous intensity is maximal.

Figure I-12 : In the presence of a sufficient electric field, the intensity of the reflected light by the cell is maximal
Figure I-12 : In the presence of a sufficient electric field, the intensity of the reflected light by the cell is maximal [zoom...]

If the electric field is not sufficient to align all the molecules in the cell, a partial reflection results.

Typical technical characteristics :

  • Tension to apply : 5 – 10 V

  • Thickness of the cells :

  • Response time of alignment of the molecules  :

  • Response time of relaxation of the molecules : 20 ms

The number of pixels or of cells in a SLM (typically 600x800, commonly called and at high resolution) can vary depending on the expected application.

It is necessary to note that even though spatial modulators of light have been developed for use in optical processors, it is the development of modulators for application like video projectors that permitted the numerous research on the optical processors of recognition of forms over the last 15 years.

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