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Safety of Free-Space Optical Communications Systems

Editor’s Note: The paper on which this article is based was originally presented at the 2019 IEEE International Symposium on Product Safety Engineering in San Jose, CA. It is reprinted here with the gracious permission of the IEEE. Copyright 2018 IEEE.

With wireless communication spectrum being at a premium these days, and with optical fiber systems often expensive and difficult to deploy, an attractive alternative for point‑to‑point telecommunications is free‑space optical communication (FSOC) systems.

Such systems would be expected to comply with IEC 62368‑1, Ed. 3.0 (or IEC 62368‑1, Ed. 2.0, with IEC 60950‑22 for outdoor considerations). The optical capabilities would have to meet IEC 60825‑1 for laser systems, as well as IEC 60825‑12 for free‑space optical communications system deployment.

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An additional consideration is whether the optical output of FSOC systems can interfere with the operation of aircraft by the light visibly startling the pilot. Rules safeguarding aviation pilots from outdoor optical systems are in place globally.

This article introduces the reader to free‑space optical communication systems and discusses the specific safety considerations that have to be met with these types of communication systems.


FSOC Safety Considerations

What is FSOC?

A free‑space optical communication system is a point‑to‑point optical communication system between transceivers, using free space as the transmission medium instead of optical fiber. They are an alternative to wireless communication network equipment or buried fiber optic cable systems.

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They use communication lasers and lensing to transform a communication signal to a light beam that has all of the qualities necessary so that communications can be established through the atmosphere under most or all expected atmospheric conditions.

Figure 1 illustrates a free‑space optical communication system. An internal optical beam is typically diffused using a telescope (Figure 2). The telescope serves to lower the intensity of the laser light and may lower the laser classification of the transceiver.

Figure 1: Free-space optical communication system

 

Figure 2: Illustration of diffusion of laser beam using a telescope


Laser Classification per IEC 60825‑1

In a typical optical‑fiber communication system, the laser classification of the system follows that of the laser modules installed in the system. For example, a system with Class 3 lasers installed would likely also be classified as a Class 3 system per IEC 60825‑1.[1]

However, since lensing is likely involved to convert the light beam to be suitable for propagation through the atmosphere, the FSOC transceiver would need to be tested and classified per IEC 60825‑1 as an end product.

One would generally expect the classification optical output of an FSOC transceiver to be eye‑safe, such as Class 1 or Class 1M per IEC 60825‑1. However, IEC 60825‑12 permits higher classifications if located in restricted access locations as defined in the standard. These considerations are described in more detail herein.

Lensing

Lensing is likely used to diffuse a higher‑powered laser beam to one suitable to transmit across distances through the atmosphere and be detected easily at some distance away. As shown in Figure 2, this is usually done with a telescope.

In Figure 2, if the telescope is lowering the output optical signal, or making the optical output meet one of the safer laser classifications, then the telescope would be a radiation safeguard in accordance with IEC 62368‑1.

IEC 62368‑1 Compliance

The outdoor enclosure of an FSOC transceiver will have a window or lens made from glass or a hard, translucent polymeric material. Considering that the FSOC will likely have to comply with IEC 62368‑1 if the window is glass, concern has to be given to constraints for the use of glass as part of an enclosure acting as a safeguard. This is discussed in more
detail herein.

Aviation Safety

Aviation regulatory authorities, such as the United States Federal Aviation Administration (FAA) regulate the use of outdoor lasers in aircraft flight paths and prohibit visible lasers or high‑powered non‑visible lasers from being aimed at aircraft pilots. The visible laser beams can startle the pilot and cause him to lose control of the aircraft, while high‑powered lasers could cause eye damage. The aviation safety impacts on FSOC transceivers are discussed in more detail herein


Laser Classification and Evaluation

As mentioned above, FSOC transceivers have to comply with IEC 60825‑1 as a system because the light beam is transformed via lensing to successfully propagate through the atmosphere.

The FSOC transceiver is tested for the desired laser classification per IEC 60825‑1 under worst‑case, single‑fault conditions.

Radiation Energy Level Reduction Through Lensing

In a typical optical‑fiber communication system, the laser classification of the system follows that of the laser modules installed in the system. For example, a system with Class 3 lasers installed would likely also be classified as a Class 3 system per IEC 60825‑1.[1]

FSOC transceivers likely use lensing to magnify the laser beam for the beam to be suitable for transmission across free‑air space. An internal telescope may be employed for this.

The net effect of the lensing may be that light generated by higher‑classified lasers may be diffused going through the lensing, thereby possibly reducing the laser classification of the output light beam.


IEC 60825‑12 Considerations

The objective of IEC 60825‑12 is to:

  • Provide information to protect people from potentially hazardous optical radiation produced by free space optical communication systems (FSOCS) by specifying engineering controls and requirements, administrative controls and work practices according to the degree of the hazard;
  • Specify requirements for manufacturing, installation, service and operating organizations to establish procedures and provide written information so that proper precautions can be adopted.[2]

In general, IEC 60825‑12 serves to determine what installation locations are safe for unskilled persons, and what locations have to be made restricted access, based on the classification of the laser employed.


IEC 62368‑1 Compliance Considerations

Glass Windows in Outdoor Enclosure

The optical window of an FSOC transceiver will likely form part of the enclosure system: electrical, fire, and/or outdoor, and could also be made of glass.

The optical window may be small, perhaps not much larger than 50‑75 mm in diameter, but if glass, the safeguard robustness requirements for glass have to be balanced with the hazards inside of the enclosure.

According to IEC 62368‑1, glass that is accessible to an ordinary person or an instructed person:

  • having a surface area exceeding 0.1 m2; or
  • having a major dimension exceeding 450 mm; or
  • that prevents access to class 3 energy sources other than PS3

shall be subjected to the glass impact test.[3] For FSOC transceivers, the first two bullets are unlikely, but the third bullet might apply if ES3 voltages or RS3 optical radiation is present.

Thus, the options for complying with the safeguard robustness requirements for optical windows include the following:

1. Make sure there are no class 3 energy sources other than PS3 affected by the enclosure or enclosure compartment protected by the glass window.

Internal AC power should be housed in a separated enclosure than the optics if used. Else, ‑48 V dc telecommunication voltage sources and Power over Ethernet (PoE) inputs are ES1 (if insulated and not exposed, as expected).[3]

Also, the voltage used to power micro-electro-mechanical system (MEMS) mirrors may exceed the ES2 voltage threshold of 120 V dc. If possible, MEMS power sources should be current‑limited to ES2 levels (25 mA maximum).[3]

Note that Class 3R (invisible) Class 3B, and Class 4 lasers were considered RS3 sources under IEC 62368‑1, Ed. 2.0.[4]

2. If the above cannot be achieved, surround the glass optical window with a visor‑like guard that will reduce or eliminate impact energy from the steel ball of the Impact Test.

Note that these systems are likely fixed for horizontal beam propagation, so it is likely that only the pendulum option for the Impact Test is applicable.

3. Use a hardened glass or transparent polymeric window that can withstand the Impact Test.

4. With respect to Item 1 above, provide separated electrical and outdoor enclosures for ES3 and RS3 energy sources, so that if the glass window is cracked during Impact Testing, the ES3 and RS3 sources’ safeguards are not affected.


Aviation Safety Considerations

Laser light illuminated into the atmosphere, particularly visible light, is considered a hazard to aircraft pilots. The pilots can be startled by the laser light, possibly endangering the lives of all flying on the aircraft. In many countries, it is against the law to aim laser lights at aircraft.

Consideration would have to be given to what type of aircraft could enter the optical transmission path. For instance, airplanes may enter the optical transmission path of transceivers mounted on high towers, buildings, or mountains, in addition to take‑off and landing in airstrips. Helicopters are more versatile than airplanes for being able to intercept the optical transmission, because of their vertical flight and landing ability.

Major theme parks in the United States that have laser shows coordinate the illumination of the lasers with the presence of aircraft in the area, and turn off the lasers when aircraft are approaching. [5]

Free‑space optical communication systems, however, are generally not considered hazards to aircraft, and some industry standards that apply to aviation laser safety do not include FSOC systems in their scope.

Nonetheless, consideration must be given to the laser classification of the free‑space optical communication transceiver, its transmission path, the classification of the location per IEC 60825‑12, and whether piloted aircraft could be in the optical transmission path.


Conclusions

Free‑space optical communication systems are a unique means of establishing wireless point‑to‑point communications.

They have a unique set of safety concerns that are somewhat different than fiber optic communication equipment or radio systems. The transformation and lensing of laser light, along with the unique safety requirements of optical systems potentially in sight of unskilled persons requires FSOC transceiver safety compliance somewhat unique.

Aviation safety considerations have to be taken note of, but in general, these rules are not applied to FSOC systems.

References

  1. Safety of laser products – Part 1: Equipment classification and requirements,” IEC 60825‑1, Edition 3.0, 2014‑05.
  2. “Safety of laser products –Part 12: Safety of free space optical communication systems used for transmission of information,” Edition 1.0, 2004‑02.
  3. “Audio/video, information and communication technology equipment –Part 1: Safety requirements,” IEC 62368‑1, Edition 3.0, 2018‑10.
  4. “Audio/video, information and communication technology equipment –Part 1: Safety requirements,” IEC 62368‑1, Edition 2.0, 2014‑02.
  5. P. Murphy, “Lasers and Aviation Safety,” 2009‑09, International Laser Display Association.


author gies-donDon Gies
is a senior product compliance engineer, specializing in telecommunication equipment safety. Gies is also an adjunct professor at New Jersey Institute of Technology (NJIT), Newark, New Jersey USA, teaching graduate-level courses, “Transients in Power Systems,” and “Economic Control of Interconnected Power Systems.” Gies graduated from Rutgers University as an electrical engineer. He is a senior member of the IEEE, a member of the Board of Governors of the IEEE Product Safety Engineering Society, chair of the Telecom Safety Technical Committee, and chair of the US/Canada Technical Harmonization Committee for IEC 62368. He can be reached at
ddgies@verizon.net.

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