Radiation-resistant Fibers

Author: the photonics expert Dr. Rüdiger Paschotta

In certain applications of optical fibers and fiber-based components, the fibers may be exposed to substantial amounts of radiation:

• Space satellites on various kinds of Earth orbits are exposed to galactic and extra-galactic cosmic rays. In addition, there are energetic solar particles, which may directly arrive or are trapped in the magnetic field of Earth. In effect, there is a wide range of radiation, including protons and electrons, neutrons, heavy ions, gamma rays (<$\gamma$> rays) and X-rays. Some of it has very high quantum or particle energies. This can affect technology for optical inter-satellite communications and LIDAR (for Earth exploration), such as fiber amplifiers and passive fibers. The dose rate is normally not very high, but during more than 10 years of service time the cumulative radiation dose may be substantial.
• To a lower extent, fibers in airplanes are affected, for example in fiber-optic links and gyroscopes.
• In high-energy physics experiments, e.g. involving particle accelerators or laser-induced nuclear fusion, fibers are used for optical fiber communications, transferring large amounts of data. The fibers may be exposed e.g. to synchrotron radiation (particularly intense at wigglers and undulators), bremsstrahlung or directly to particle radiation.
• Nuclear power plants and sites for the storage of nuclear waste may be equipped with communications fibers, and also with distributed fiber-optic sensors, some of which may be used for monitoring conditions around nuclear reactors in areas exposed to high levels of radiation. For example, intense neutron irradiation with extremely high lifetime doses occur near fission reactors. Similar challenges would arise in future fusion reactors.
• Medical imaging and cancer therapies can involve X-rays or proton radiation, for example, and optical fibers for various purposes.

Fibers which are substantially less affected by radiation than others are called radiation-resistant fibers, even if they are not completely immune against radiation effects. They may be used in space-qualified lasers, for example.

Types of Fiber Degradation

In such applications, problems may arise from the degradation of fibers (mostly silica fibers) by the received radiation:

• The primary problem is usually radiation-induced attenuation (RIA), i.e., an increase of the propagation losses by induced absorption and possibly Rayleigh scattering of light. However, much of those losses may disappear after the irradiation e.g. within a couple of minutes or an hour, i.e., the fiber can to some extent exhibit a spontaneous recovery. Also, there are cases where the introduced absorption initially rises monotonically but then saturates with further irradiation or is even reduced.
• In some cases, radiation-induced luminescence (or radiation-induced emission) or Cerenkov light guided in a fiber core is problematic, e.g. disturbing measurements with fiber-optic sensors. Such luminescence may be strong during the irradiation, but there can also be long-lived phosphorescence.

• Further, there can be problems with radiation-induced changes of refractive index – particularly relevant for fiber Bragg gratings as used in certain fiber-optic sensors. A typical observation is that the Bragg wavelength, which may be essential for measurements, is modified. Note that refractive index changes are usually associated with changes of ultraviolet absorption via Kramers–Kronig relations.
• Additional fiber properties may be affected which are relevant for certain fiber-optic sensors – for example, the properties related to Brillouin scattering.

Generally, such effects can lead to a reduction in performance or even to the complete loss of essential functions. In extreme cases, a single nanosecond-long intense radiation pulse in a physics experiment can introduce propagation losses beyond 1000 dB/m in standard telecom fibers. Therefore, radiation-resistant fibers, a kind of specialty fibers, are needed. They are also called radiation-hardened (particularly when specific measures have been applied) or radiation-tolerant.

Note that other technologies such as electronics and optoelectronics are also to some extent radiation-sensitive; fibers are by no means unique in terms of radiation-induced degradation. For example, the electronics of robots used for inspection or repair purposes in high-radiation areas are known to fail after a relatively short time of operation. Optical fibers also have substantial advantages in terms of their immunity to non-ionizing radiation; for example, even extremely intense RF noise cannot disturb the operation of fiber-optic communication links.

There are also cases where radiation-induced effects in fibers are utilized for measuring accumulated doses and/or real-time dose rates.

Note that problems with radiation-induced effects in different applications can be quantitatively and qualitatively very different due to several reasons:

• The type and amount of radiation to which fibers are exposed can be very different. It can also make a big difference whether a certain radiation dose is slowly accumulated over time or occurs by an intense radiation flash.
• Different lengths of fiber are used in different cases, between a few centimeters and many kilometers.
• The transmitted light can be in different wavelength regions, which can be very differently affected by the radiation. Sensitivities also arise from different requirements e.g. on measurement accuracies.
• The used fibers are of different types, optimized for different applications, and can exhibit very different radiation sensitivity. For example, active fibers are generally much more radiation-sensitive than passive fibers.
• In some applications, light in the fiber causes helpful photo-bleaching (see below), which reduces the problem.

Simulations of Fiber Degradation

When an active fiber in a fiber amplifier, for example, develops increased propagation losses, for example, an important question is how much that will impact the overall system performance. Such things can be simulated with the RP Fiber Power software, which is a very flexible tool for such work.

Physical Mechanisms Behind Radiation-induced Degradation

The main challenge is usually radiation-induced attenuation. This can be caused by the generation of microscopic defects by the radiation. There are also effects on already existing defects (precursors):

• Some defects have been present in the fiber preform already. For example, the fiber glass naturally does not have a perfectly regular microscopic structure like a single crystal, but rather a more complicated random structure which involves various kinds of defects. For example, in silica fibers there can be a substantial density of nonbridging oxygen hole centers (NBOHC), per-oxy linkages (oxygen interstitials) and oxygen vacancies. The concentration of certain defects can be strongly affected by small deviations from perfect stoichiometry. Also, there are defects related to impurities like hydroxyl (OH) ions, chlorine (Cl) and various metals. Radiation can transform such defects such that new absorption bands arise.
• Further defects may be created by mechanical stress during the fiber drawing process. This is particularly the case for fibers drawn at high speed, although details of the fiber drawing tower can substantially modify the results.
• Radiation can ionize existing defects, but also create new defects. In particular, neutron radiation is known to cause substantial structural defects.

• It is known that various frequently used dopants – in the fiber core, but sometimes also in the fiber cladding – severely increase radiation-induced losses. Examples are germania, phosphorus, aluminum, and particularly certain rare earth ions (e.g. Yb³⁺, Er³⁺, Tm³⁺ and Ho³⁺) as used in active fibers for fiber amplifiers and lasers as well superluminescent sources. In addition two rare earth ions, one often dopes with substances like alumina in order to improve the incorporation of the rare earth ions (avoid clustering), but such additional dopants can further increase the radiation sensitivity.

The arising defects often act as color centers, absorbing light in a wide range of wavelengths. Frequently, they cause strong ultraviolet absorption, with substantial tails into the visible spectral region, but much less into the infrared. Nevertheless, some amount of infrared absorption can be induced, partly with the underlying physical details not being clearly identified yet.

The relative importance of different contributions to radiation-induced attenuation depends very much on the type of fiber.

The self-healing of radiation-induced loss is largely based on thermally activated processes. Therefore, it may be strongly accelerated (by orders of magnitude) if the temperature is substantially increased. At the same time, however, detrimental effects involved in the build-up of induced absorption may also be accelerated.

In some applications, there are photo-bleaching effects, i.e., the reduction of radiation-induced absorption caused by light which is sent through the fiber during operation. This can already have a substantial impact for milliwatt-level optical powers, of course with a substantial dependence of optical wavelength.

Radiation may also induce luminescence, again involving certain microscopic defects which can be electronically excited by radiation, followed by spontaneous emission of light.

Methods for Improving Radiation Resistance of Fibers

There is no general method for achieving perfect resistance to radiation, i.e., for avoiding any level of radiation-induced degradation in fibers. However, with various measures one can achieve substantial improvements:

Avoiding Non-essential Impurities

It can be helpful to avoid certain non-essential extrinsic impurities such as chlorine. This is not always easy, however, e.g. because preform fabrication processes involving chlorine have been worked out to reduce hydroxyl content, and new processes then need to be developed to achieve that with other means. Nevertheless, for example radiation-hardened pure silica has been developed.

Dopants

Further, one may try to avoid otherwise intentionally introduced dopants like germania and alumina; their functions then need to be replaced in some ways. For example, instead of the refractive index increase in the fiber core by germanium doping, one may use a pure silica core in conjunction with a fluorine-doped (index-depressed) cladding.

Some dopants have been found to increase radiation resistance. For example, fluorine doping appears to be generally helpful; one may thus have a lightly fluorine-doped fiber core surrounded by a more highly doped fluorine-doped cladding [10]. Both multimode and single-mode fibers can be made with such designs. Also, there are nitrogen-doped fibers with substantially improved radiation hardness.

Another example is the addition of cerium to erbium-doped fibers [14]. That way, even active fibers can be made which can better tolerate radiation.

Hydrogen, Deuterium or Oxygen Loading

It has been found that loading a fiber with hydrogen gas (H₂) or with deuterium is generally reducing the radiation sensitivity [2, 3]. For that, the fiber is exposed to high-pressure hydrogen gas at an elevated temperature for some period of time, allowing some hydrogen to diffuse into the glass. Unfortunately, even at room temperature the hydrogen may also diffuse out of the glass again later on within a short time, and common polymer coatings cannot prevent that. Therefore, certain metallic coatings have been developed which prevent that effect.

It has also been found that irradiation of a fiber directly after hydrogen loading creates a permanent beneficial effect, even if the hydrogen can diffuse out of the glass later on.

Loading a fiber with oxygen (O₂) has also been found to be beneficial. In that case, the diffusion constant at room temperature is extremely small, so that a later oxygen loss is normally not occurring to a relevant extent within several years.

Fiber Drawing

To some limited extent, one can also achieve improvements by optimizing the fiber drawing process, which causes some amount of built-in mechanical stress, which itself has an effect on the density of certain defects.

Hollow-core Fibers

Hollow-core fibers, where most of the guided light propagates in a hollow region, i.e., in air, naturally are less sensitive to radiation effects [18]. This is just also a promising route as far as other requirements on the fibers can also be satisfied.

Side Effects of Radiation Hardening

Methods of improving the radiation resistance of fibers (e.g. applying additional dopants) are often called radiation hardening. Some of these measures may compromise other performance features of fibers, but may be acceptable as part of a trade-off. Note that effectivity of certain measures of radiation hardening can very much depend on the application context.

Additional Measures

Depending on the situation, there may be other possible measures to mitigate the problem:

• One can sometimes shield fibers against the radiation, e.g. in extreme radiation environments such as nuclear reactors, where effective radiation shielding can be indispensable. Shielding may be particularly effective for fiber coils, but less practical for elongated fibers in fiber-optic links or distributed sensors.
• One may optimize system architectures such that the sensitivity to radiation effects is reduced, for example by using shorter fibers.
• One may incorporate additional operational margins into system designs, e.g. use enhanced data transmitter powers or enhanced receivers such that higher propagation losses can be tolerated.

Prediction of Performance

Particularly for delicate applications like space missions or in nuclear facilities, a reliable prediction of the effects of radiation is highly desirable. Unfortunately, for various reasons this is generally quite hard to accomplish. Therefore, one can often only achieve relatively uncertain estimates and requires substantial safety margins to avoid serious problems. Here, it obviously helps to have fibers which have been subject to effective radiation hardening procedures.

Quantifying Radiation Doses

The received radiation dose is commonly quantified in units of Gray = joules of absorbed radiation energy per kilogram of the receiving material (here typically silica). This is not just a measure for how much radiation is in the environment because it depends on the absorption properties of the fiber material, which themselves depend on the type and energy of the relevant radiation. Estimating the radiation dose of a fiber during a space mission of a certain duration, for example, is not trivial; one needs to add up the contributions of different types and energy ranges of radiation, considering their absorption in the fiber material.

In the case of irradiation with particles like neutrons, one instead uses fluence values – the number of particles received per unit area. The particle type and energy need to be specified in addition.

A dose rate is the dose received per unit time, for example in units of Gy/s or Gy/d (Gray per second or day).

There can be substantial uncertainties concerning the radiation dose to be expected, for example if fibers are used in nuclear facilities, where possible nuclear accidents and their consequences cannot be reliably predicted.

Even if the radiation dose is exactly known, it also needs to be taken into account that the same dose from different types of radiation can have different effects in the fibers. Therefore, measured effects for one type of radiation may not allow one to fully predict the reaction to other radiation. (Unfortunately, it is hardly practical to do a wide range of irradiation tests, for example with gamma radiation in different quantum energy bands, and all this for different fiber types.)

Furthermore, it matters whether a certain radiation dose is slowly accumulated over time or occurs by intense radiation within a short while, or as radiation flashes with certain time intervals for recovery in between.

Note that different applications involve very different radiation levels. For example, space satellites experience rather low dose rates of e.g. below 10⁻³ Gy/h, although often with strong time dependence, with exposure over possibly many years and with no realistic option of replacing degraded fibers. On the other hand, certain high-energy physics experiments can lead to extreme dose rates of more than 1 MGy/s, while the lifetime dose may not be larger than in a space mission. Therefore, certain fibers may be considered as very radiation-tolerant concerning one application, while being clearly unsuitable for another one.

Dynamics of Radiation Effects

As mentioned already, radiation can initiate complicated dynamics in fibers. For example, certain microscopic defects can be generated, while others can be transformed into forms with different optical effects. The competition of different processes can lead to curious types of response, for example induced attenuation which initially rises and then decreases again. At the same time, thermally activated processes can take place. That also introduces a substantial dependence of the ambient temperature, which particularly in the case of space missions can vary strongly.

For reliable predictions based on physical modeling, one would need to develop rather sophisticated models, taking into account a wide range of material data (some of which are quite subtle and difficult to reliably assess) as well as the time-dependent dose rate and temperature for which the evolution is investigated. Only for a limited range of cases, kinetic models have been developed with a substantial predictive power. Some simple models can be analytically solved, leading to dose-dependent attenuation e.g. with a power law or saturating exponential functions.

Consistency of Fiber Properties

Unfortunately, it may not always be guaranteed that fiber properties including those related to radiation resistance are completely consistent, for example from batch to batch. This is because such properties can depend on minor concentrations of impurities, which may not be perfectly constant in the used raw materials. Besides, it is not simple and fast to regularly check rates of radiation-induced attenuation, for example.

Therefore, particularly critical applications, for example space missions with no realistic chance of correcting mistakes later on, particularly rely on carefully designed and diligently observed protocols and practices in fiber fabrication and quality control.

More to Learn

Encyclopedia articles:

• fibers
• specialty fibers
• photodarkening
• space-qualified lasers