Optical fuse: threshold-triggered, switch-off passive component
Almost every electrical system is protected by an over-current switch, namely, a fuse. Although having no functional role in the system, the fuse is considered an essential part of it. However, optical systems lack such a device and are therefore unprotected from excessive input power. Unfortunately, while working in the lab or field, many of us have experienced damage to optical equipment or components due to over-power. In recent years, we have witnessed a significant increase of power applied in optical fibres. That includes DWDM telecommunications systems with a large number of channels, high-power lasers and amplifiers, or applications in the medical or industrial laser fields. Some existing high-power systems include sophisticated protection schemes, mainly based on software-controlled feedback loops.
The optical fuse is a totally passive component, designed to protect against excessive power transmission. The optical fuse is immediately switched off as a result of an over-power event.
It has a single input and output port. When the input power is lower than a certain threshold level, the fuse is transparent. But when the power exceeds the threshold level, the fuse becomes permanently opaque.
Experimental results are shown in Figure 1. Here, the output power increases with the input power up to the threshold power, in which the output power decreases dramatically. In the specific example, the threshold power is approximately 30 dBm (1 W). However, the threshold power is currently selectable from 25 to 32 dBm (300 mW to 1.5 W), and fuses with lower threshold powers are now under development. Also, the output power dropped by about 25 dB at the threshold, reducing the output power to approximately 0.3% of its original power at the time threshold power was exceeded (typical drop values are between 10 and 20 dB—1-10%).
To protect against over-power, the optical fuse can be placed either at the input of a sensitive optical device such as a detector or at the output of a high-power device such as a laser or an optical amplifier or integrated within an optical device, for example, between different stages of an optical amplifier. The normal operation power of the system should be up to 3 dB (50%) below the threshold power. At these operating conditions, the insertion loss of the fuse is between 0.5 and 1 dB (80-90% transmission), and it has low polarisation loss and high return loss (low reflectance).
The lack of simple protection devices has urged the development and usage of sophisticated methods, including software-controlled feedback loops, to prevent over-power. Moreover, these methods need fast response time, generally using expensive fast detectors, switches, or attenuators, to ensure damage prevention from power spikes or short pulses that may occur in high-power erbium-doped fibre amplifiers or Raman amplifiers.
The optical fuse is fast, as can be seen in Figure 2. Here, the input is a high-power pulse, marked in red. As evident, the output pulse (blue) is blocked after the input pulse exceeds the threshold power. Here, the response time is shorter than 5 µsec. Even shorter response times were measured for more powerful input pulses.
Like other passive components, the optical fuse is transparent to both modulated and unmodulated signals. It is currently available in a fibre design and operates in the C- and L-bands using standard singlemode fibres. However, the fuse is broadband and can operate in other spectral regions such as 980, 1060, or 1300 nm, using other fibres. Also, the fuse can be connected using either connectors or a splice. A photograph of an optical fuse is shown (see Figure 3).
Introducing optical fuses into test setups in the laboratory or into networks ensures that over-power events will result in damage to the optical fuse itself and not to other components or subsystems. An activated (burned) fuse blocks the forward power without increasing the reflected power and does not induce damage to its surroundings. Thus, aside from preventing the costs associated with damaged equipment, by replacing the fuse, the system downtime can be minimal, without the problem of locating damaged equipment.
The optical fuse is a passive, customer-replaceable component that provides protection from failure of major subsystems and components due to exposure to over-power. Although this concept has been used for many years in the electric and electronic fields, the introduction of the optical fuse allows for new protection schemes in optical test equipment and networks.
Tom Marrapode is director of marketing, Fiber Optics Division, Molex Inc. (Lisle, IL). Dr. Ram Oron is chief technology officer at KiloLambda Technologies Ltd. (Tel Aviv).