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17 Jun 2002
Companies around the globe are competing to develop the de factooptical switches to be used in future all-optical communications networks. Nadya Anscombe reports on one of the lesser-known developments in switching technology - electrically switchable holograms.
From Opto & Laser Europe September 2001
Time and again, the latest developments in optical switching technology have been hailed as the key to the all-optical network of the future. Microelectromechanical systems, for example, are currently receiving a lot of attention, with a plethora of start-up companies racing to be the first to market.
However, two start-ups - Israeli company Trellis Photonics and US firm Digilens - have opted to try something completely different. They are working on electrically switchable holographic technology, which uses holograms activated by a voltage to reflect light beams. As with microelectromechanical systems (MEMS), electrically switchable holograms eliminate the need for converting photonic signals into electronic ones. Unlike MEMS, however, electrically switchable holograms contain no moving parts. They are also wavelength selective, have fewer alignment problems and are potentially much faster.
Digilens uses holographic polymer-dispersed liquid-crystal
technology, a material system that is relatively well understood; Trellis, on the other hand, uses a
proprietary and unique crystal-based technology. Trellis was founded by two Israeli
scientists, Aharon Agranat and Elon Littwitz. Agranat is an electroholography expert from the Hebrew
University of Jerusalem. Littwitz was the founder of Ornet Data Communications Technology, a
company that developed switches for local-area networks before being acquired by Siemens in 1995.
Trellis's research, development and manufacturing facilities are in Yokneam and Jerusalem, Israel, while
sales, marketing and communications are handled out of the company's office in Columbia,
US. Agranat told OLE: "Because our technology is unique, we sometimes have some
difficulty in persuading people of its advantages. As far as optical switching technologies are concerned,
we are completely off their radar screens. Our technology demands a conceptual change in the process
by which you do switching." Trellis's technology is based on electrically activated holograms
written in a crystal of potassium lithium tantalum niobate. Having achieved switching times of mere
nanoseconds, the company claims to have the fastest and most cost-effective switching technology in the
world. In the absence of a voltage, the crystal is transparent and light passes through it. When a
voltage is applied, however, the hologram becomes reflective and bounces incoming light to an output
fibre. A hologram's wavelength response is chosen when it is written into the crystal. An optical
cross-connect is made by arranging the holographic crystals into a matrix of rows and columns in which
the columns correspond to a series of output fibres. Trellis claims that the design allows any wavelength
to be routed to any fibre. Agranat lists many advantages to his technology: it is solid-state; it is
capable of adjusting the power of the output signals; it can remotely test any wavelength in the entire
switch at a protocol level; it ensures balanced power levels for all wavelengths being delivered to a fibre;
it features instantaneous signal restoration; and it is fully scalable with a modular
architecture. Potential problems with the technology could include the 200 V that are needed to
activate the hologram fully, making the electronics complicated; and the reflection efficiency of the
holograms depends on temperature, which means that the crystals have to be
temperature-stabilized. Agranat responds to these points: "The high voltage is not a problem for
the user - it is incorporated into the electronics. And our devices simply need to be kept at room
temperature, which is relatively easy." Trellis has developed a proof-of-concept prototype, but its
plans to launch products this year have been postponed due to the current climate in the
industry. Despite winning funding last year and continuing to expand its production facility in
Israel, the firm has not been immune to the current downturn in the optical communications market, and
recently made several redundancies at its Columbia site. Of the company's 150 employees, however,
only 10 were affected, including its CEO, Timothy Cahall, who stepped down in July. While Digilens' technology is based on
the same idea as Trellis's, the two companies use very different materials; Digilens' electrically
switchable Bragg grating (ESBG) is based on holographic polymer-dispersed liquid crystals. The gratings
are created by mixing monomer and liquid crystals in a free-standing cell or waveguide. Two interfering,
coherent laser beams are used to polymerize this mixture. The interfering laser beams create a fringe
pattern in the monomer and the liquid-crystal mixture, instigating a photo-initiated polymerization process
in which the liquid crystal droplets preferentially form in the dark fringes of the interference
pattern. The diffusion process causes phase separation of the liquid crystal into two regions of
differing refractive indices. One region contains only polymer, while the other, in the dark fringes,
contains a mixture of polymer and liquid crystal. The resulting combination is essentially a photo induced
phase volume, or Bragg, hologram. To exploit the electro-optic properties of the liquid crystal,
electrodes are deposited onto the cell or waveguide walls. An applied AC voltage orients the
liquid-crystal molecular optical axis so that its refractive index now matches that of the polymer. This
makes the material transparent, and thus enables light to pass through it. Digilens has been using
this technology to make microdisplays for some time, and it is now moving into the telecoms
market. Ashmead said: "Our switching times are around 100 ms or less - our material system will
never achieve nanosecond switching times. Switching speeds are not a big issue at the moment, but they
will become increasingly important when we finally achieve the all-optical network." Digilens'
technology has a number of other advantages over Trellis's: it is more compatible with today's
manufacturing techniques; it requires a lower switching voltage; it is based on a well characterized
material system; and it is compatible with planar waveguide geometry. Digilens licenses
intellectual property rights for the ESBG from the Science Applications International Corporation (SAIC),
an independent R&D company in the US that owns Telcordia Technologies. SAIC has a 10% stake in
the business, and is Digilens' biggest investor. Digilens also shares an investor with Trellis: Crescendo
Ventures has a stake in both companies. Despite their similar technologies and markets, Digilens
and Trellis do not see one another as competitors. They have a much larger, mutual competitor that they
must tackle before their own technologies will be accepted on the market-place - MEMS.
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