Liquid-crystal devices promise high performance

Cindana Cornwell

Richard Albert

Like other optical solutions to optical-network problems, liquid-crystal technology promotes development of new applications to solve the costly problem of optical-electrical conversion when transmitting and processing signals. Unlike legacy optomechanical technologies, there are no mechanical moving parts to slow down devices and reduce their reliability. Components are smaller and can integrate multiple functions in a single package if necessary. Furthermore, their transparency to network protocols such as SONET, ATM, and IP, as well as the bit-rate parameter, can "future-proof" the networks and provide long-term cost relief.

Specifically, the optical characteristics provided by liquid-crystal materials could support, for the first time, a family of component products for wavelength selection and fiber dispersion compensation. These functions must be supported economically and reliably to meet the demands of high-bitrate time-division-multiplexing and dense wavelength-division-multiplexing (DWDM) optical networks-architectures that now support transmission speeds of 2.5 and 10 Gbit/s and will need to support 40 Gbit/s and higher in the future.

GOING ALL OPTICAL

Liquid-crystal device operation is based on polarizations: one polarization component reflects off surfaces, the second transmits through surfaces. A typical liquid-crystal device will include both passive and active elements. The passive element, a beamsplitter, splits the light into two polarization components before the signal reaches the active element.

Depending on whether a voltage is applied or not, the active cell either changes the polarization state of the incident light beam or leaves it unaltered (see Fig. 1). The active liquid-crystal element either rotates or does not rotate the light`s polarization as the signal travels through the element. With an electrical field present, the polarized light travels through the active cell without change. When there is no electrical field, the light is rotated. Parallel polarization becomes perpendicular polarization and vice versa. The beam combiner then directs the beam to the desired output port (see Fig. 2).

Liquid-crystal materials have the largest electro-optic coupling constant of all active materials-millions of times larger than lithium niobate or active planar waveguides-making them one of the most efficient classes of materials for processing light. Several other characteristics make liquid-crystal materials uniquely suited for passive optical devices. For example, devices can be tailored to address a significant number of optical processing applications. In essence, products using liquid-crystal materials can operate in either digital or analog mode-on/off like a switch function or partially on/partially off as with an attenuating function. These substances can switch light, select wavelengths for DWDM, attenuate signal intensity, and even store information. Thousands of liquid-crystal material chemistries are available, and a vendor can combine a number of these recipes to meet particular environmental requirements or operational modalities.

Another benefit is that devices using liquid-crystal materials can withstand harsh and rugged environments. The mean-time-between-failure for liquid-crystal displays in military and commercial aerospace cockpits has reportedly exceeded 28 years. Liquid-crystal materials also feed the huge flat-panel-display industry, which means that materials, fabrication infrastructure and processes, and vendor networks are in place to supply large quantities of liquid-crystal cell elements (see Fig. 3).

FUTURE NETWORKS

All-optical devices based on liquid-crystal materials will comprise a significant portfolio of devices offering low insertion loss, polarization-dependent loss (PDL), polarization mode dispersion (PMD), and crosstalk, along with the fast switching speeds, high reliability, and low cost required for high-bit-rate optical networks and systems of the future.

Several manufacturers are developing optical components using liquid-crystal materials. With devices such as a 1 × 2 optical switch, quoted insertion losses range from less than 1 dB to less than 1.5 dB, and crosstalk from 35 to 50 dB, with minimal back reflection, PDL, and PMD. Switching speeds are fast-sub-millisecond in some cases, with future devices promising nanosecond switching-because only molecules move. Alternative technologies, however, often must move comparatively massive fibers, mirrors, or molecular planes (see "Switching alternatives," p. 36). Liquid-crystal devices are controlled by an on-board microprocessor, which maintains optimal device performance over the entire temperature range, while outputting both the switching state and the operating conditions of the device.

Liquid-crystal materials were crucial to the computer industry by enabling the production of thin, small, low-power, reliable screens. These versatile materials now offer a next-generation technology to the telecommunications industry.

FIGURE 1. A typical liquid-crystal device will include both passive and active elements. The two passive elements first split, then recombine the light into two polarization components. With an applied voltage, the active element changes the polarization characteristic of the light.

FIGURE 3. Liquid-crystal all-optical components can be manufactured in large volumes with wafer-processing techniques.

Switching alternatives

Other switching technologies in existence or under development include optomechanical, micro-optoelectromechanical, planar waveguides, polymer waveguides, lithium niobate, and semiconductor optical amplification. The most mature and inexpensive of these technologies, optomechanical switching, uses moving parts such as solenoids or stepper motors to move fibers or rotate mirrors to redirect light from one exit port to another. These switches operate over a wide wavelength range with typical switching times of 10 to 25 ms.

Micro-optoelectromechanical switches (MOEMs) use piezoelectric materials to change the orientation of small mirrors or the placement of metallic membranes to redirect the light from one fiber to another. The membranes delay light as it travels in an optical waveguide. Employing silicon semiconductor technology, MOEM technology has the potential of supporting N × M arrays with sub-millisecond switching times.

Planar waveguides are based on silica-on-silicon, silica-on-silica, sol/gel, or polyimide-on-substrate technologies. Active planar waveguides exhibit nonlinear behavior similar to Kerr or Pockel`s cells, which rapidly change optical properties with an applied electric field.

Polymer waveguides alter optical properties with heat or an electric field to change the refractive index or to switch light signals. The polyimide waveguide with added chromospheres dopant is a Kerr medium.

Lithium niobate, a potentially high-speed electro-optic material, becomes optically birefringent with an electric field, altering the refractive index. Lithium niobate responds to electrical impulses in nanoseconds, making it fast enough for WDM. However, lithium niobate devices tend to be very polarization dependent.

The semiconductor optical amplifier switch based on gallium arsenide indium phosphide operates in the optical domain with no introversion of photons to electrons. These switches are very fast, reaching sub-nanosecond response times.