liquid crystal modulators (original) (raw)

Acronym: LC modulators

Definition: optical modulators which are based on liquid crystals

Category: photonic devices

• optical modulators
  • acousto-optic modulators
  • electro-optic modulators
  • Pockels cells
  • electroabsorption modulators
  • liquid crystal modulators
  • intensity modulators
  • phase modulators

Related: optical modulatorsliquid crystal displaysadaptive opticsmodulator drivers

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Contents

What are Liquid Crystal Modulators?

Liquid crystal modulators are a kind of optical modulators for intensity, phase or polarization, which are based on liquid crystals. They are mostly used for displays, but also for various other purposes, as discussed in the section on applications below.

Operation Principle

Liquid crystals are substances which are liquid but not optically isotropic — which is unusual for liquids. They contain long molecules which have a tendency to get aligned with each other, and that preferential orientation leads to anisotropic optical properties. In particular, there is birefringence: the refractive index depends on whether the optical electric field is in the direction of the molecules or perpendicular to them. In most cases, the birefringence is of uniaxial type, i.e., there is an optical axis such that the refractive index becomes independent of polarization direction for light propagating along that axis.

The orientation of those molecules can be manipulated with an applied electric field: they tend to get aligned in a direction parallel to the field lines. Another relevant aspect is that one can make suitably prepared surfaces (e.g. a glass surface with a polyimide coating which is brushed in a certain direction), which pull molecules towards some preferred orientation.

TN Modulators

A typical implementation of a liquid crystal modulator — the TN = twisted nematic type — is based on a twisted nematic liquid crystal material. That material is contained in a small chamber between two glass plates prepared for a preferential orientation of the liquid crystal molecules; the plates are oriented such that their preferential orientations are perpendicular to each other. In the zero voltage state, this leads to a twisted orientation pattern of the molecules: while those molecules close to one surface are oriented according to its preferred direction, on the way towards the other glass surface the orientation is rotated increasingly towards the preferred molecule orientation of that surface.

The optical effect of that twisted configuration is that when incident light starts with a polarization corresponding to the molecule orientation on the input side, during the passage through the liquid crystal material the polarization is rotated to the perpendicular direction; the light polarization is “dragged along” by the variable orientation of the molecules. That, however, can be prevented by applying an electric field via two transparent electrodes at the opposite glass surfaces. For a strong enough electric field, the molecule orientation is getting nearly perpendicular to the glass surfaces, and the effect of polarization rotation vanishes. For medium values of the electric field, the polarization rotation is only reduced to some extent.

The liquid crystal cell can then be enclosed by two polarizers with crossed orientations. Without an electric voltage applied to the liquid crystal cell, light which is transmitted by the input polarizer can also be largely transmitted by the output polarizer. By applying a voltage (with some driver electronics), one can reduce the transmission. Usually, a few volts are sufficient, and hardly any electric current needs to be provided.

When applying a DC voltage over longer times, the performance of such a cell would usually be degraded by a kind of electrolysis process. That can be prevented by applying alternating voltage (AC), typically with a rectangular shape and a moderate frequency. Such an alternating voltage can easily be generated e.g. with CMOS electronics.

The described TN modulator configuration is often used in liquid crystal displays. They appear gray without an applied voltage, and can be made darker (nearly black) with a voltage.

IPS Modulators: In-plane Switching

A modified kind of modulator is based on in-plane switching. Here, interdigitated electrodes are patterned on one glass substrate so that the applied electric field lies primarily along the glass surface rather than perpendicular to it. The liquid crystal is typically a homogeneous (untwisted) nematic layer whose directors rotate within the plane under the lateral field. IPS cells are usually used with crossed polarizers in a normally-black configuration: Transmission is very low with no applied field and increases as the field rotates the directors, producing the bright state.

Other Configurations

Particularly for other applications than displays, one may use further modified configurations. For example, one can omit the input polarizer when working with polarized laser light as an input. One may even work without any polarizer for realizing a phase modulator. One can choose the modulator design such that the polarization state is not modified, but only the change in optical phase. Tentatively, the modulator designs are simpler for non-display applications, since one often requires operation only with narrow wavelength ranges and/or restricted angular ranges.

There is also the technology of liquid crystal on silicon (LCoS). Here, a typically quite small two-dimensional array of liquid crystal modulators is fabricated on a silicon backplane, which in addition to the modulators contains CMOS electronics for controlling the pixels. Between the electronics and the liquid crystal modulators, there is a reflective layer; such devices need to be used in reflection. They are suitable for projection displays, where the projected image area is typically far larger than the active chip area. Three different LCoS chips may be used for red, green and blue color components. However, there are also single-panel LCoS color displays, e.g. for use in miniature projectors (pico-projectors). There are also other fields of applications, for example programmable beam steering and pulse shaping.

Performance Figures of Liquid Crystal Modulators

Depending on the application (see below), different performance figures of modulators can be relevant. The most important of them are:

• Maximum transmission: This is important for display applications, for example, for achieving high image brightness with limited illumination power.
• Modulation contrast: Modulators differ quite substantially in terms of contrast, which can be important for displays, for example.
• Switching speed: This is limited by the time required by the molecules for changing their orientation. Such processes are relatively slow, typically requiring a few milliseconds. (Typically, the recovery into the zero voltage state takes more time than the adjustment to a non-zero voltage state.) Liquid crystal modulators belong to the optical modulators with lowest speed. For some applications, one needs to optimize the speed, for example by realizing quite thin liquid crystal cells, using materials with low viscosity or applying overdrive signals.
• Wavelength range: The performance details can be somewhat wavelength-dependent, limiting the usable wavelength range.
• Damage threshold: Liquid crystals are more sensitive to intense light than e.g. optical glasses. Particularly at short wavelengths, optical damage can be a problem.
• Power consumption: A specific advantage of liquid crystal modulators e.g. for passive displays is that they have a quite low power consumption. In active displays, however, power consumption is dominated by the backlight, and that depends on the maximum transmission of the modulators.

Various further aspects can be relevant, depending on the application. For example, it can be disturbing that some LC displays exhibit substantial image distortions when being touched; this should be avoided particularly for touch screen displays. Another aspect can be the temperature range in which a modulator can properly work.

Applications of Liquid Crystal Modulators

Displays

Most liquid crystal modulators are used in displays. They are often produced in large volumes and technologically optimized to a very high degree.

See the article on liquid crystal displays for more details.

Laser and Optics Applications

Liquid crystal modulators in the form of intensity modulators or phase modulators have found some applications in laser and optics technology where their quite limited switching speed can be tolerated.

Single-element modulators can be used to modulate a whole laser beam; relatively large areas can be realized for beams with high optical power. Such variable optical attenuators can also used as electrically controlled beam shutters, for example. Also, they can be used for noise eaters, although their bandwidth tends to be lower than desirable for such an application. Further, there are electrically controllable waveplates, also called active retarders.

Spatial light modulators (SLM) can be made which contain many liquid crystal cells in a one-dimensional or two-dimensional array. (A display panel can actually also be considered as a 2D spatial light modulator.) One-dimensional SLM are used for certain types of Fourier transform pulse shapers, for example, or for tunable optical filters, e.g. based on spatial separation of spectral components with diffraction gratings or on a Lyot filter design.

Two-dimensional devices find applications in signal processing and holography, for example. One can also use them for wavefront correction, i.e., in adaptive optics. Arrays of phase modulators can also be used for programmable beam steering.

Other Applications

Some cars contain an interior rear view mirror containing a liquid crystal device. One can automatically reduce the reflectivity of the mirror in situations where the driver would be irritated by too intense light from the headlights of a following car.

Frequently Asked Questions

What is a liquid crystal modulator?

A liquid crystal modulator is an optical modulator that uses liquid crystals to alter the intensity, phase, or polarization of light. The principle is based on controlling the orientation of anisotropic molecules within the liquid crystal using an electric field.

How does a twisted nematic (TN) modulator work?

In a TN modulator, a liquid crystal is placed between two specially prepared surfaces that force its molecules into a 90° twisted structure. This structure rotates the polarization of light passing through it, an effect which can be eliminated by applying an electric voltage.

What is in-plane switching (IPS)?

In-plane switching is a liquid crystal modulator technology where structured electrodes on a single glass plate generate electric fields parallel to the surface. This field reorients the liquid crystal molecules within that plane to modulate light.

What are the main applications of liquid crystal modulators?

What limits the speed of liquid crystal modulators?

Their speed is limited by the time it takes for the molecules to physically change their orientation, which is a relatively slow process typically requiring a few milliseconds. For this reason, they are among the slowest types of optical modulators.

What is liquid crystal on silicon (LCoS)?

LCoS is a technology where a miniature array of liquid crystal modulators is fabricated on a silicon backplane that also contains the control electronics. These reflective devices are used in applications like projection displays and for pulse shaping.

Why are AC voltages used to drive liquid crystal modulators?

Applying a constant DC voltage over long periods can cause degradation of the liquid crystal material through an electrolysis-like process. Using an alternating (AC) voltage prevents this degradation while still effectively controlling the molecule orientation.

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