Reliability Concerns for a Space Flight Environment

Under current study are the failure and degradation modes of fiber laser components to further the flight readiness for current and future fiber laser designs.  Most systems are designed to function in the thermal range of 0ºC to 70ºC because they are based on commercially available components.  How much of a harsh environment a system can withstand will always depend on the most vulnerable component or components present in the system.  Since this study is currently underway, full reliability assessments that are not currently available, will be available in future reports.

Vibration and Vacuum Considerations:

It is expected that fiber lasers will be very robust against vibration since most of the components are linked together with fiber fusion splices.  However, different coupling methods require some bulk optic parts and additional study of the packaging methods for the bulk optical component alignment will be necessary.  In most cases there are very few parts that are coupled with bulk optics and are usually limited to the initial pump coupling if a side pumped method is used and the coupling to and from the nonlinear crystals if wavelength conversion is necessary.  To assure stability in a vacuum environment additional study is require for materials analysis on key components. 

Thermal Considerations:

Thermal transients in a vacuum environment may effect the performance and stability of components such as bragg gratings, and the wavelengths of the absorption and emission band of the amplifier fiber, for example.  For gratings the effects can be limited by thermally annealing the components during manufacturing.  The changes in the spectrum of the amplifier fiber can be characterized to see how much of an effect will be transferred to the system performance.  The double clad fiber has an external polymer cladding over a silica material causing a CTE mismatch and this too will be affected during thermal transients.  In spite of the fact that thermal transients may have an effect on the components, it is expected that these effects will produce minor performance changes.

Radiation Considerations:

In general, when Erbium fiber laser amplifiers were tested as a system in a radiation environment along with other passive components, the results showed that the majority of the damage was a result of color center formation in the Erbium fiber.  The pump power as well as the amplified signal were absorbed.[6]  In other studies, Erbium and Erbium/Ytterbium fiber were tested in a radiation environment but far less data exists on Ytterbium fiber.  In several cases of study, models that are typically used for telecommunications optical fiber were applied to extrapolate the high dose rate exposure data for the rare earth doped results to a lower dose space flight environment.  It was concluded that when this modeling was applied that the extrapolation results over estimated the expected absorption based losses and that other models were necessary.  Some of this has been documented.[7-8]  Another assumption applied to the testing and modeling of radiation effects on rare earth doped fiber is that as with telecommunications optical fiber, gamma radiation should provide a worse case scenario than proton exposure.  It may be the case that this assumption is also incorrect and further study is necessary.[8]. It is however, true of rare earth doped fiber as it is with telecommunications grade fiber, that the dopants do have an enormous effect on the radiation induced performance.  There are currently a wide variety of available optical fiber choices for usage in fiber amplifiers.  Due to the vast number of choices available, it will be necessary to study the effects of the most common types of fiber used since the dopants used among these different products can also contain wide variations.  Different Ytterbium fiber products in particular were shown to behave differently in a radiation environment based on the different dopants contained in each.[9]  Double clad rare earth fiber, which makes the injection of higher pump powers possible, have outer claddings comprised of fluorine doped polymers with low optical index.  Polymers are typically more sensitive to radiation in general than silica optical materials and fluorine doped fiber is typically more sensitive to radiation effects than fiber not containing fluorine.  The fact that polymers are used for this outer cladding will also make them more prone to embrittlement as a result of radiation exposure.  Pump laser sources will have to be characterized for proton induced displacement damage but in general are considered quite robust against typical environments.  More research is necessary to assess the other components of the fiber laser system and will be available in future reports.