Free-space optics takes off

Since the 1980s free-space optics (FSO) has been adopted by enterprises (particularly to connect LANs), which accounted for 65% of sales by 2001. However, adoption by carriers has been slower due to doubts about FSO achieving the required five-nines (99.999%) link availability (see panel below).

Most analysts predicted carrier-driven sales growth from USD100m in 2000 to USD2–4bn by 2005/6. Unfortunately, many competitive carriers — the earliest adopters — have since gone bust, followed by incumbent operators hitting financial trouble, particularly in the US. By March 2002 Frost & Sullivan cut its already- conservative 2005 forecast from USD395m to USD215m, with most sales expected to come from Europe, the Middle East, and Africa.

Among FSO suppliers, in late 2002 the UK's PAV Data Systems underwent a management buyout, in January Ottawa's Plaintree Systems said that continuation might not be viable, then in late February San Diego's AirFiber (which had raised USD92m: USD50m from Nortel) closed down. Even Seattle's Terabeam (which in 2000 raised USD526m, including USD450m from Lucent, and employed 530) last July cut staffing by another 70 to 294 after supplementing its original 1Gbit/s Magna with a smaller 100Mbit/s Elliptica system.

But FSO suppliers' fortunes may be looking up as carrier trials yield more deployments. After in October 2001 claiming the first contract with a major carrier to integrate FSO (Sweden's Utfors), in January Terabeam announced the first contract in China for fibre extension: at least 50 links for China Railcom. San Diego's LightPointe (founded in 1998, with USD51.5m in funding: USD33m from Cisco) is now sole supplier to Telkom South Africa, "reconfirming demand for FSO by major telecom carriers". Also, last October it agreed a distribution deal with Century Man Communications Equipment Co Ltd, which is deploying its Flight FSO in tandem with Cisco's Catalyst 6500 Series and ONS 15454 multi-service provisioning platform in China.

At March's CeBIT expo LightPointe announced Flight distribution agreements with Siemens (a HiPath Ready Net reseller agreement, via its global sales channel), and Westcon Group Inc's Comstor and Voda One divisions (for North America).

Such adoption is helped by launches in the last few years of hybrid optical wireless systems that, when FSO is blinded by fog, automatically fail-over to either parallel or integrated radio frequency back-up links — which don't work in heavy rain but do in fog — providing link availability up to carrier-class (99.999%).

In May 2000 California's MRV Communications Inc acquired Astro Terra Corp and Israel's Jolt Ltd, which had developed hybrid systems, and formed the subsidiary Optical Access Inc. By last May it had installed nearly 5,000 links, including hybrid TeraScope Fusion systems.

Terabeam, last July, bought Harmonix Corp, whose GigaLink 60GHz millimetre-wave equipment complements its FSO systems.

At CeBIT 2002 Münster's Communication by light (CBL, founded in 1991) demonstrated its AirLaser IP100 system, which has an integrated microwave back-up and transmits at up to 622Mbit/s.

In May 2002, AirFiber launched its Hybrid Free space optic/Radio (HFR) system, which integrated 60GHz millimetre-wave wireless, coupled by a Redundant Link Controller, operating at up to 1.25Gbit/s over 1km.

Also providing hybrid RF/FSO systems is IRLan, which is based in Yokneam, Israel.

At CeBIT 2003 integration was announced of Vancouver-based fSONA's SONAbeam systems — which operate at up to 1.5Gbit/s over 5km — with Alcatel's high-capacity broadband urban wireless access systems under Alcatel's network management platform. These are branded as the high-capacity Alcatel 9600 FSO SDH/SONET and low-capacity Alcatel 9400 FSO PDH systems.

Also, LightPointe announced interoperability of its hybrid RF/FSO gear with ADC's Digivance radio-over-fibre solution, and is working with a US wireless carrier for initial deployment.

For example, after raising USD51m for FSO, Corning Cable Systems launched four-beam FreeLink Optical Transmission Equipment, which operates at 2.5Gbit/s over 1000m or 155Mbit/s over 4000m.

At CeBIT 2003 CBL launched its four-beam AirLaser IP1000, which operates at 1.25Gbit/s over several hundred metres. fSONA was founded in 1997 with an exclusive global license for 1550nm receiver technology from BT Research Labs. fSONA's 1250-AT also has AutoTrack technology, which senses motion and actively steers the beam to keep it on the line-of-sight. Last month, a further USD9.5m from a private group brought fSONA's total funding to USD51.5m.

At last year's CeBIT Pasadena-based Holoplex Technologies — founded in 1993 and backed by Samsung — launched Enhanced FSO. An ultra-collimated (30 microradian) 1550nm laser beam and PinPoint automatic active tracking raise available link margin to 42dB and reach to 4km for data rates of 155–622Mbit/s. Also, after trials with Czech Telecom, Prague-based Miracle Networks launched its 1.55µm Miracle 4020 system, which has self-guidance focusing for 155–622Mbit/s transmission up to 2km. Also, California-based 1550nm FSO supplier AOptix Technologies, which uses adaptive optics (AO) to dynamically correct signal distortions, closed a USD18m Series B funding round, bringing its total to USD28m.

Meanwhile, this February Terabeam's pointing and tracking technology was awarded a patent for automatically aligning its narrow laser beams, which transmit at up to 1Gbit/s. Its Elliptica system provides carrier-grade connectivity optimised for Fast Ethernet (100Mbit/s) and OC-3/STM-1 (155Mbit/s) links.

Also incorporating auto-tracking is Canon's 785nm Canobeam Optical Beam Transceiver.

Further technology developments were also prevalent at this year's CeBIT. Based on its fixed multi-beam FlightSpectrum 850nm system, LightPointe launched its FlightStrata line for carriers, said to be the first multiple-beam array tracking system, which can steer four laser beams and a receive lens array and incorporates automatic power control — to provide higher fade margins over shorter distances — as well as optical beam shaping.

Together with weather modelling tools, which enable users to manage networks and plan the correct link distance for the locale and desired availability, such technology developments are increasingly making FSO more compliant with carrier-grade requirements.

The barrier to delivering high-data-rate services is not technology, but the logistics and expense. CapEx cuts have constrained fibre trenching to mainly the metro core rather than access and edge have created a last-mile bottleneck. Wire and copper (cable modem, T1s or DSL) is limited to bandwidth of 2–3Mbit/s. Fixed-wireless or radio frequency technology (e.g. LMDS) scales to 622Mbit/s and offers long reach, but can need spectrum licensing. FSO can offer low-cost high-speed links over short reach.

Developed in the 1960s for secure military communications, FSO is an optical wireless broadband technology that requires a clear line of sight between transceivers containing an LED- or laser-based infrared transmitter and a receiver to provide full-duplex Layer 1 transmission, including via WDM (not possible with fixed-wireless/RF), through several kilometres of air.

FSO's advantages include:

• no interference, spectrum licensing or rights-of-way issues, security from interception, and protocol/topology/architecture transparency;
• small footprint, allowing deployment in hours and even indoors behind windows (simplifying cabling);
• portability and flexible re-deployment;
• low power consumption;
• low cost (a third to a fifth of laying fibre);
• bandwidth (from 10Mbit/s to 2.5Gbit/s).

Applications of free-space optics links include:

• the extension of fibre networks from the metro core to the network edge and access;
• digital video transport for cable TV networks;
• back-haul of data-intensive 2.5G/3G mobile traffic between wireless base-stations and network switches;
• redundant links for fibre back-up;
• disaster recovery (used by Merrill Lynch after September 11th).

But unfortunately FSO can incur outside disturbances, including:

• fog, which can scatter light and attenuate beam power by over 20dB/km, reducing reach for the required link availability;
• building vibration/sway or air turbulence, which breaks alignment, causing link loss at high data rates (2.5Gbit/s) and range (>1000m);
• obstructions such as birds, which can interrupt transmission.

Also, eye-safety for lasers emitting at 785, 850 or 910nm wavelengths requires beam divergence of milliradians to reduce intensity, limiting range to 500m. For 1550nm lasers eye-safe power is 50 times greater, allowing greater beam collimation and power density and therefore reach/link availability. Also, 1550nm technology allows laser modulation rates of up to 2.5 rather than 1.25Gbit/s.

Finally, since 1550nm is the wavelength most commonly used for fibre communications, the supporting infrastructure is vast. However, high data rates necessitate auto-tracking to maintain alignment of the narrower beam (offered by suppliers such as Canon, Terabeam, LightPointe and Holoplex).

The University of Glasgow in February announced that it had secured four further awards in the latest round of Scottish Enterprise's "Proof of Concept" grants. One of these relates to research applicable to free-space optics.

According to principal investigator professor Miles Padgett, a Royal Society research fellow, "It could be argued that the barrier to delivering high-data-rate services is no longer a lack of new and better technology but the logistics and expense of installation that restrict uptake, particularly in cities. Free-space optics offers a way to reduce the total expense for completion of high-speed data links over short distances."

His team of physicists, in collaboration with the University of Strathclyde, has recently demonstrated how certain quantum states — specifically orbital-angular-momentum (OAM) eigenstates — can be distinguished with high efficiency. In principle, the same amount of light required for one bit of information encoded in polarisation could transmit multiple bits of information encoded in OAM, resulting in a faster optical link. In addition, OAM quantum states do not affect the properties of light, so can be used with traditional techniques, such as polarisation and wavelength.

The technique is able to distinguish individual photons in different OAM states, possibly enabling quantum cryptography.

The current lab-based system needs to be developed to prove its capability in robust, maintenance-free deployment. It must be proven suitable for incorporation in free-space optics systems, necessitating system compatibility with engineering and atmospheric transmission challenges.

In parallel, a potential improvement in the method of sorting the OAM states using more robust components will be pursued. If it can be implemented successfully, the development could substantially increase the manufacturing viability and commercial value of the technology. The commercial potential of the project would result from developing a robust method of increasing data density, say the researchers.