Free Space Optical (FSO) Communications is now a proven solution for high-speed broadband access, especially for overcoming the “last mile” bottleneck. These systems use laser beams transmitted through the atmosphere and have advanced to the point of mass production. Key benefits of FSO technology include:

• Low setup and operational costs

• Rapid deployment

• High bandwidth comparable to fibre

• Flexible architectures

• Wide application compatibility

Despite being easy to deploy, FSO transceivers are complex, requiring careful system engineering to balance performance variables. Understanding these engineering considerations helps buyers make informed decisions when evaluating FSO products.

Which Wavelength for FSO?

Current FSO systems typically operate at either 800–980 nm or 1550 nm wavelengths. While vendors promote their chosen wavelength as superior, the real determining factor is “real-world link margin,” not marketing claims.

• 1550 nm systems are often promoted for:

  • Laser eye safety (but offset by wider apertures in 980 nm systems)

  • Reduced solar background noise (negated by less sensitive detectors)

  • “Infrastructure compatibility” (invalid due to required O-E-O conversion)

• 980 nm systems (e.g. from CableFree):

  • Use enhanced silicon detectors with higher sensitivity

  • Offer better real-world performance and link margin

  • Are more suitable and cost-effective for today’s needs (E1/T1 to Gigabit under 4km)

• The true advantage of 1550 nm lies in:

  • EDFAs (optical amplifiers for high-power transmission)

  • DWDM capabilities (for future multi-channel, high-capacity systems)

However, 1550 nm’s higher costs make it less practical for typical short- to mid-range FSO deployments today.

FSO and Eye-Safety

Laser light between 400-1400 nm (including 800-980 nm) can penetrate the eye and focus on the retina, posing potential eye safety risks. Wavelengths above 1400 nm (e.g. 1550 nm) are absorbed by the cornea and lens, making them inherently safer for the eye.

Although 1550 nm lasers are allowed to operate at up to 50 times higher power levels under eye safety standards, this benefit is often misrepresented. In practice, a large-aperture 980 nm system can be safer than a small-aperture 1550 nm system, an important detail frequently overlooked by 1550 nm advocates.

FSO and Atmospheric Attenuation

Carrier-class FSO systems must be designed to accommodate heavy atmospheric attenuation, particularly by fog. Although longer wavelengths are favoured in haze and light fog, under conditions of very low visibility this long-wavelength advantage does not apply. The argument is about usable link margin, NOT wavelength.

FSO Optical Receivers

The effectiveness of the receiver in a Free Space Optics (FSO) system depends on several key factors:

1. Detector Type:

• Two main types: PIN (no internal gain, low cost) and APD (with internal gain, higher sensitivity, higher cost).

• APDs offer approx. 4x better sensitivity (6 dB gain) than PINs, which can translate to 4x less transmit power or 2x the distance for the same performance.

2. Receiver Optics:

• Larger receiver apertures help reduce scintillation effects (intensity fluctuations due to atmospheric turbulence).

• A large aperture averages out surges and fades better than multiple smaller apertures.

• Multiple transmitters taking slightly different atmospheric paths also help mitigate scintillation.

3. Operating Wavelength:

• 800–980 nm and 1550 nm receivers have similar quantum efficiencies, but 1550 nm detectors tend to have higher noise floors, resulting in lower sensitivity.

4. System-Level Trade-offs:

• To truly assess performance, focus on “usable real-world link margin” rather than just specs.

• If a system uses cheaper components (e.g. PIN over APD), it must compensate elsewhere (e.g. with higher power or shorter distances).

FSO Commercial Infrastructure

The 1550 nm wavelength is not the most common for terrestrial fibre communications. In fact, 850 nm is widely used in LAN ports and newer automotive fibre systems due to its lower cost and suitability for shorter links. While 1550 nm is preferred for long-haul fibre because of lower dispersion, this advantage does not apply to FSO. In FSO systems, there’s no dispersion benefit at 1550 nm, making 850 nm devices a more cost-effective and practical choice for short to medium-range applications.

FSO Performance: Transmit Power & Receiver Sensitivity

The performance of FSO systems is primarily determined by four key parameters (for a given data rate):

1. Total Transmitted Power

   High transmitted power can be achieved via EDFA amplifiers or combining multiple low-cost lasers.

2. Transmitting Beamwidth

   Narrow beamwidth improves performance (higher antenna gain). Fixed units must allow for building sway/wind, while actively pointed systems (with angle tracking and steering mirrors) can achieve much narrower beams for better focus.

3. Receiving Optics Area

   Larger apertures collect more light, but increase cost, size, and weight.

4. Receiver Sensitivity

   Improved by using low-capacitance photodetectors, compensation circuits, or APDs (which offer 5-10 dB better sensitivity than PINs, at higher cost and complexity).

These factors collectively improve link range and fog penetration.

A figure of merit (FOM) can be used to compare competing systems, based on
the basic physics of this equation:

Figure of Merit = (Power*Diameter2)/(Divergence2*Sensitivity); where:

Power = Laser power in milliwatts
Diameter = effective diameter in cm (excluding any obscuration losses)
Divergence = beam divergence in millirad
Sensitivity = receiver sensitivity in nanowatts

An additional consideration is scintillation tolerance, where FSO receivers must handle signal fluctuations caused by atmospheric turbulence. This requires high dynamic range and fast response in the receiver front end to avoid constant background error rates and maintain zero-error performance.

FSO Fixed-Pointing or Active Tracking/Pointing?

A key consideration in FSO system design is pointing stability- maintaining precise alignment between transceivers. There are two main approaches:

1. Active Pointing Stabilisation

• Uses tracking systems (e.g. mirrors, sensors) to keep beams aligned.

• Effective but adds cost, complexity, and reliability concerns.

2. Fixed-Pointed Systems

• Suitable for shorter ranges and lower data rates.

• Achieves stability by broadening the transmitted beam and receiver field of view, allowing for loose alignment tolerances.

• Reduces sensitivity to building movement.

• Can remain unattended for years after initial alignment.

Practical experience shows that systems with beamwidths less than 5 milli-radians are unreliable without tracking, as buildings move more than that; systems above 5 mrad are reliable without tracking.

FSO and Network Protocols – Transparency is best

Carriers face a key challenge in ensuring interoperability between legacy and next generation network systems. Most FSO systems are physical layer devices, similar to fibre optics, and can work with all protocols while not being limited to any of them, in a protocol ‘transparent’ approach which enhances deployment flexibility and reduces costs. If additional switching functionality is needed, customers can integrate standard switches, which are compatible with physical layer devices.

Reliability in FSO products

Customers, especially in outdoor or industrial settings, prioritise equipment reliability and low failure rates. Achieving long system lift requires high-quality, telecom-grade components, low-stress electronics, and maintaining optimal internal conditions. This includes rugged, sealed housings, effective heating/cooling systems, and features like laser power reduction in clear weather and active laser cooling. When well-engineered with these factors, a system can achieve a high mean time before failure (MTBF).

Qualification & Testing

To ensure safety and performance, FSO systems must meet key certifications for Laser Safety (CFR, ANSI, IEC), Electrical Safety (CSA, UL, EN), and Electromagnetic Compatibility (EMC). Laser eye safety is classified by the IEC, with Class 1M indicating eye-safe transmitters. Beyond mandatory certifications, extensive environmental and reliability testing should be conducted- such as rain, wind, pressure, vibration, salt-corrosion, and underwater immersion tests- with units operating during testing to verify functionality.

Accelerated life and subsystem tests in extreme conditions help ensure long-term reliability. EMC testing includes electromagnetic emissions and susceptibility assessments to confirm that the system won’t interfere with or be disrupted by nearby equipment. Additional tests for harmonic currents, magnetic fields, voltage fluctuations, electrostatic discharge, and lightning surges further ensure robust, dependable operation in harsh environments.

FSO Field Tests & Availability

The final proof of the viability of any broadband access approach, including optical wireless, is the successful conclusion of rigorous field-tests. Ideally, such field tests should include operation 24 hours per day, 7 days per week. The most convincing tests are those in which weather conditions vary widely during the tests, and include periods of steady drizzle, heavy driving rain, snow, and various degrees of fog.

Independent Tests

Another way to evaluate the field-readiness of FSO hardware is through independent testing, focusing on factors like performance, ease of installation, and reliability. Testers may simulate real-world challenges, such as introducing obstacles to transmission, to gauge usability and robustness. Reliability can also be measured using MTBF (mean time before failure). Telcordia/Bellcore standards are among the most rigorous, providing benchmarks for carrier-grade equipment. These independent tests help validate manufacturers’ claims and ensure the system meets industry expectations.

Free Space Optics Installation

FSO installation hardware should be easy to set up and align, while also being rugged and stable enough to maintain alignment over time and in harsh conditions. A typical setup might use a yoke mount on a vertical pole for quick installation, with coarse alignment via sighting scopes and fine adjustment using push-pull screws viewed through a PC or voltmeter. Fixed systems should have a wide enough beam to tolerate building sway and wind, while active-pointed systems need to respond quickly to sudden movements like strong winds or earthquakes.

FSO Network Management & Monitoring

Modern carrier networks require network management and monitoring with a user-friendly graphical interface. Since many FSO systems are physical layer devices, management is typically handled via a separate IP-addressable CAT5 or RS-232 connection. Some carriers prefer integrating management with the optical data stream, using a separate channel or SONET data bits, which may require additional Layer 3 equipment. Regardless of the method, the SNMP interface should provide detailed monitoring of component health to detect and address issues before they lead to outages.

Useful status indicators include:

• Received signal strength
• Transmitter power settings (bias, modulation currents) for each laser
• Temperature of each of the lasers
• Temperature of key interior locations
• Interior humidity
• Four power supply voltages and currents
• TE cooler controller currents
• Clock recovery status
• Network signal status
• Extensive historical logging capability

Product mix – one size does not fit all

The FSO market is defined by three main factors: cost, data rate, and link length. Due to cost sensitivity, a single product cannot effectively serve all market segments without becoming too expensive for simpler applications. To deliver the best value, manufacturers should offer a range of products tailored to specific application needs, providing only the necessary features without adding extra cost for capabilities that aren’t required. In short, a “one size fits all” approach is inefficient- targeted solutions offer better value and performance.

Production Capacity and Volume

For carriers, high-volume manufacturing is essential due to the large scale of network deployments. FSO production facilities should be purpose-built, as FSO differs from other wireless or fibre technologies. Production lines should be optimised by product type (e.g. fixed vs. active systems), and just-in-time procurement practices should be used to streamline supply chains. Operating in multiple shifts maximises asset use and allows flexibility to meet market demand.

Cost – Is the Price Right?

While cost is important in telecom purchases, buyers often seek the best value in the medium to low cost range. Delivering high performance at minimal extra cost offers strong value. Key cost drivers include smart system design, reduced manual labour (especially for optical alignment), and efficient volume manufacturing. Applying these principles consistently leads to high performance gains with low additional cost.

References – Sites and Customers

Prospective buyers gain confidence from credible references such as case studies, demo sites, and contacting customers. The most valuable proof of performance is provided by FSO companies with a global base, especially those showcasing carrier-class, mission-critical links. Thorough field-testing programmes further demonstrate how well devices perform in real-world conditions.

Conclusions

Free-space optical (FSO) networking is gaining popularity as a reliable solution for broadband access and communication bottlenecks. Careful evaluation of design factors is crucial when choosing FSO systems. Well-engineered, feature-rich, and thoroughly tested systems deliver the best performance and value.

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