Optical Fiber Communication Systems and Laser Safety
Jane E. Ehrgott
The communications industry was revolutionized in the 1980s with the advent of optical fiber communications systems (OFCS) consisting of diode lasers emitting infrared light signals at wavelengths chosen for transmission with minimal loss in high-purity glass fibers. The high bandwidth and low noise of optical systems led to their rapid adoption in undersea as well as landline communications networks. In the 1990s, optical amplifiers called erbium-doped fiber amplifiers (EDFA) began replacing electro-optical systems for long-distance OFCS signal regeneration and amplification. EDFAs are further advancing OFCS technology by enabling simultaneous transmission of multiple signals of different wavelength in each optical fiber.
Today, new technologies like dense wave division multiplexing (DWDM) are being deployed in OFCS. These systems can carry high-powered, optically amplified signals in the Class 3B and, eventually, Class 4 laser power ranges. Over the past several years, the laser safety standards community has been working on updating the standards so as to address this emerging technology.
International Electrotechnical Commission (IEC) standard 60825-2 Safety of Laser Products Part 2: Safety of Optical Fibre Communication Systems (1993) and its Amendments 1 and 2 provide requirements and specific guidelines for the safe use of OFCS. IEC recognizes that an OFCS is technically a Class 1 laser system because, under intended operation, the optical radiation is totally enclosed. However, IEC also recognizes that such a classification may not reflect accurately all potential hazards.
In the event of a fiber disconnect or break, IEC defines the "hazard level" as the potential optical hazard at any accessible location within an OFCS, based on the level of the radiant energy that could become accessible. Hazard levels are assigned values from 1 to 4 based on the customary laser accessible emission limits (AELs); for example, Hazard Level 1 does not exceed the AEL for laser Class 1, etc. If automatic power reduction (APR) is used, the normal level of power in the fiber and the speed of the APR system determine the hazard level. For the increasing powers of DWDM systems, the laser Maximum Permissible Exposure (MPE) levels in Part 1 of the IEC standard must not be exceeded if any person can be exposed to laser radiation emerging from the port (or fracture) from the instant of disconnection (or break) until the power reaches an acceptable level.
Engineering and administrative controls are specified by IEC for the hazard level and type of environments in which the OFCS may operate. Hazard Level 4, analogous to a Class 4 laser, is not permitted in any location. This means that any OFCS carrying Class 4 optical power levels needs to incorporate control mechanisms to reduce the power to an acceptable hazard level in the case of a reasonably foreseeable event that would permit access to the radiant energy from a fiber or cable.
American National Standards Institute (ANSI) Z136.2 American National Standard for Safe Use of Optical Fiber Communication Systems Utilizing Laser Diode and LED Sources (1997) is intended to be used by manufacturers of OFCS and by installation and service personnel, for example, network operators. It recognizes that, under normal operation, an OFCS is completely enclosed and that no optical energy is accessible to the operator or incidental personnel (inherently Class 1). However, it also recognizes that, during service operations, optical connectors are removed and there is a risk of exposure to optical energy if the system is energized. To address such situations, this standard recommends that an appropriate "service group" be assigned based on the potential radiant energy hazard and specifies appropriate controls for each service group. The service group hazard classification scheme considers the special viewing conditions when cables are severed or disconnected. ANSI Z136.2 is expected to be harmonized with the IEC standard within the next 18 months.
New optical communication activities include an emerging trend in fiber-less optical point-to-point communicationsoften referred to as "free space" optical communications. This technology is gaining popularity in the communications industry based on various advantages, including the fact that this portion of the spectrum is not regulated by the FCC and frequency coordination is not required (does not produce radiofrequency interference). Both ANSI and IEC are currently proposing that user and manufacturer safety standards be developed for these systems.
Author: Jane E. Ehrgott works at Lucent Bell Laboratories in Murray Hill, New Jersey. She received a BA from Rutgers University, an MS from the University of Michigan in health physics, and a Masters of Technology Management from Stevens Institute of Technology. Jane is an active participant in OFCS laser safety standards development for IEC and ANSI.