Fiber components in high-power systems


In recent years we are seeing installations of high power, 0.5-10 Watt systems, and designs of 100 Watt to 1000-Watt fiber-based systems. These systems offer significant benefits, but testing, and safe handling standards at the higher powers need to be addressed.

When you stop to consider 1 Watt of power, it doesn't seem like much. After all, the nightlight in your bathroom is 5 times brighter than this. However, in fiber optics, this energy may be crammed into a physical region only 8 ?m in diameter. The resulting photon flux over the small cross-section of the fiber, equivalent to about 1.9 million W/cm2, is about 300 times greater than the photon flux (W/cm2) at the surface of the sun! So why doesn't everything melt? Very few of these photons are actually absorbed at any point in the single mode fiber, due to its high purity. But how will other components perform at the higher powers?

Modern low optical loss single mode fibers can reliably carry the signal at significant power. But as the signal moves out of the fiber core and into the optical components and connection points, there may be potential for failure. The photon intensity in 1W systems is enough to destroy optical materials, connectors, surfaces and coatings.


Where are the high power points
Understanding the issues of high power in components

Where are the high power points

The highest powers are generally evident in the pump laser paths and signal paths after amplification. In the pump path, multiple sources may be combined utilizing different wavelengths or polarization. After amplification, all the single path components such as connectors, taps and isolators experience the high power converted from the pump signal. Optical components for high power may include taps, isolators, wideband and narrowband WDMs, and polarization combining modules.

Understanding the issues of high power in components

The collimating process used in a typical bulk-optic component becomes a concern under high power. Collimating the light has the desirable effect of creating an expanded parallel beam, which is easier to manipulate. After traveling through various components such as a thin film filter, or an isolator core, the beam reverses itself through another collimator and is launched back onto the fiber. However, as the light is focused on and off the fiber tip the optical intensity is at its greatest, with the greatest potential for damage.

The larger collimated beam is 500 mm in diameter (compared to 8 mm in the fiber), resulting in much less intensity - 1/4000th of that the field in the fiber core. The large beam still poses problems for the optical components, though. Any dirt or other contamination on the surfaces can act as light absorber or a tiny lens. The dirt locally focuses the beam until the intensity burns a hole in the surface. Increased insertion loss is the result. Components assembled with epoxy have an additional mode for failure. Epoxies are made of long chains of organic polymers. Several years ago, epoxies under prolonged exposure to high optical intensity would yellow or darken. As the epoxy became darker and more opaque, the insertion loss rose. Today, epoxies are improved and more resistant to this type of breakdown. However, at higher optical powers, how will the epoxies react? Also, this is a longer-term failure mode. Small insertion losses tolerable at low optical power can lead to failures in high power optical systems. Knowing the input power, the insertion loss and calculating the power dissipated inside each device yielded the following:

Input power, mW

Insertion loss, dB

Dissipation, mW

1000

0.01

2.3

1000

0.1

22.8

1000

0.5

108.7

1000

1

205.6

1000

3

500

Dissipated optical power usually scattered/absorbed and can lead to device heating, providing positive feedback loop for further increase of dissipated power and finally catastrophic failure. Also poor connections in high power fiber systems can create parasitic optical resonators, further increasing probability of optical damage.