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Definition: the phenomenon that the optical power losses in a medium can grow when the medium is irradiated with light at certain wavelengths
Various transparent optical media such as optical fibers, laser crystals or nonlinear crystal materials exhibit photodarkening (sometimes also called photochromic damage or photo-induced absorption) when they are irradiated with light at certain wavelengths. This means that the transmission losses resulting from absorption or scattering grow with time. The involved mechanisms and the characteristic parameters (e.g. the maximum amount of losses, its spectral shape, the dependence on light intensity and duration of irradiation, and the reversibility) can vary greatly, depending on the material. In many cases, the photodarkening mechanism involves the formation of color centers or other microscopic structural transformations in the medium. Photodarkening can lead to serious performance degradations and lifetime limitations of optical devices, and its minimization may require the optimization of materials and operation conditions.
In the following sections, some technologically important situations with photodarkening are discussed.
Photodarkening Caused by Ultraviolet Light
Silica fibers can transmit ultraviolet light, however with propagation losses which are significantly higher than in the visible or near-infrared spectral region. In addition, a rapid further increase of losses can be induced by the ultraviolet light. To a significant extent, this UV-induced photodarkening can be reduced with certain dopants (e.g. fluorine) or other treatments. Fibers with high hydroxyl (OH) content are often found to be superior over low-OH fibers, even though the latter have better transmission in the near infrared. However, there are also low-OH fibers with good UV transmission and resistance. Also, hydrogen-impregnated fibers can exhibit drastically improved resistance against irradiation with excimer laser light at 193 nm [14], even though hydrogen loading is in other cases used for increasing the photosensitivity of fibers for UV writing of fiber Bragg gratings. Certainly, the power of such methods for improved UV transmission and resistance strongly depends on the involved wavelengths.
Even blue light can cause photodarkening in silica-based fibers, when they are doped with germanium [1]. This is caused by a two-photon excitation process.
Photodarkening in Ytterbium-doped Silica Fibers
Ytterbium-doped optical fibers can exhibit severe transmission losses, which are strongest at short wavelengths (e.g. in the visible spectral range) and much weaker e.g. in the 1-μm spectral region, where ytterbium-doped fiber lasers and amplifiers operate. Such losses can occur in new fibers [7] but can also grow during operation of a fiber laser or amplifier [15]. The rate with which these losses grow appears to be proportional to the seventh power of the density of excited ytterbium ions [15]. This means that a fast degradation of such fibers can result from operation with high fractional ytterbium excitation density (as can occur e.g. in core-pumped fiber amplifiers), particularly for fibers with high doping concentration and poor homogeneity (→ formation of ion clusters). On the other hand, the strong dependence on the excitation density suggests that many devices can be designed to operate in a relatively safe operation regime where long device lifetimes can be expected. Difficult cases are those where a high doping concentration is required (e.g. in order to mitigate nonlinear effects via a reduced fiber length), or where high excitation levels are inevitable (e.g. fiber lasers operating around 975 nm).
The photodarkening effect normally appears to be permanent, although it has been demonstrated that it can be reversed by heating the fiber [16] or by irradiation with ultraviolet light [17]. Further studies will hopefully reveal the detailed physical mechanism and the dependence on the chemical composition of the fiber core.
Note that similar photodarkening can result from irradiation with gamma rays. This is relevant e.g. for space applications of fiber devices.
Photodarkening in Thulium-doped Fibers
There are reports of photodarkening occurring in silica fibers [2], more precisely in phosphosilicate and germanosilicate glasses, where a broadband loss (particularly short wavelengths) occurs following irradiation with high peak powers, e.g. from a mode-locked laser. A crucial ingredient is the excitation of high-lying energy levels of the Tm3+ ions. This phenomenon may not be technologically serious, since thulium-doped silica fibers are used for continuous-wave infrared lasers where high pump intensities and high concentrations of highly excited thulium ions are not required.
The situation is different for thulium-doped fluoride fibers as used mainly for upconversion fiber lasers, generating blue light when being pumped with infrared light e.g. around 1140 nm. Although this three-step upconversion scheme could in principle be very efficient [8], the actual device performance can be severely degraded by photodarkening [5,6]. The excitation of thulium (Tm3+) ions to high-lying energy levels again appears to be crucial for the photodarkening, and cannot be avoided in that case. As for silica fibers, it is believed that photodarkening is related to the formation of color centers, which becomes possible if ions are excited to energies above the bandgap of the host glass. Absorption of blue light (e.g. via upconversion lasing) has been shown to at least temporarily reduce the induced absorption, probably by removing color centers. In this sense, the photodarkening is partially reversible. The physical details are very complicated and have so far not been fully understood.
Photodarkening effects have also been reported for other rare-earth-doped fibers, e.g. with dopants like praseodymium (Pr3+), thulium (Tm3+), cerium (Ce3+), and terbium (Tb3+).
Induced Losses in Nonlinear Crystals
Some nonlinear crystal materials exhibit induced absorption when being irradiated with light having short wavelengths in the visible or ultraviolet spectral region. Most well-known is the effect of reversible green-induced infrared absorption (acronym GRIIRA) in materials like lithium niobate (LiNbO3), lithium tantalate (LiTaO3), and potassium titanyl phosphate (KTiOPO4, KTP). Particularly in the case of KTP, this is often called gray tracking; a gray line (track) is observed where the crystal has been irradiated.
Such effects are often reversible, but may also lead into permanent photochromic damage when the device is operated for a longer time. They are observed e.g. in frequency doublers which are pumped with 1-μm lasers (e.g. YAG lasers), or in optical parametric oscillators pumped at shorter wavelengths, and similar effects are observed for blue light (→ blue-induced infrared absorption, BLIIRA). For high-power devices, the resulting heating can seriously disturb the phase matching and also lead to thermal lensing. Particularly the former effect can make the conversion process quite unstable.
In niobates, the induced absorption appears to be related to the formation of polarons (electrons trapped by antisite niobium ions), also called color centers, which can result e.g. from two-photon absorption. It can be greatly reduced by doping the material with magnesium oxide (MgO) [11], and/or by using stoichiometric material, which contains fewer of the intrinsic niobium antisite defects. At the same time, both measures reduce the tendency for photorefractive beam distortions, as they increase the ionic conductivity.
In KTP, the induced absorption is also attributed to polarons; carriers may in that case be captured e.g. by Ti4+ ions or by Fe3+ impurities. An important difference to niobates is that induced absorption appears to occur only for pulses with sufficiently high peak intensity. Gray tracking usually disappears within a few hours after irradiation. Its strength depends on the crystal quality and thus on the fabrication method. Other important parameters are the pulse durations and pulse repetition rates.
Bibliography
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| [6] | P. Laperle and A. Chandonnet, "Photoinduced absorption in thulium-doped ZBLAN fibers", Opt. Lett. 20 (24), 2484 (1995) |
| [7] | R. Paschotta et al., "Lifetime quenching in Yb-doped fibres", Opt. Commun. 136, 375 (1997) |
| [8] | R. Paschotta, P. R. Barber, A. C. Tropper, and D. C. Hanna, "Characterization and modeling of thulium:ZBLAN blue upconversion fiber lasers", J. Opt. Soc. Am. B 14 (5), 1213 (1997) |
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| [10] | C. G. Akins, "Photodarkening and photobleaching in fiber optic Bragg gratings", IEEE J. Lightwave Technol. 15 (8), 1363 (1997) |
| [11] | Y. Furukawa et al., "Green-induced infrared absorption in MgO doped LiNbO3", Appl. Phys. Lett. 78 (14), 1970 (2001) |
| [12] | L. B. Glebov, "Linear and nonlinear photoionization of silicate glasses", Glass Sci. Technol. 75, C2 (2002) |
| [13] | S. Ferwana et al., "All-silica fiber with low or medium OH-content for broadband applications in astronomy", Proc. SPIE 5494, 598 (2004) |
| [14] | M. Oto et al., "Fluorine doped silica glass fiber for deep ultraviolet light", J. of Non-Crystalline Solids 349, 133 (2004) |
| [15] | J. J. Koponen et al., "Measuring photodarkening from single-mode ytterbium doped silica fibers", Opt. Express 14 (24), 11539 (2006) |
| [16] | J. Jasapara et al., "Effect of heat and H2 gas on the photo-darkening of Yb3+ fibers", paper CTuQ5 at Optical Fiber Communications (OFC) 2006 |
| [17] | I. Manek-Hönninger et al., "Photodarkening and photobleaching of an ytterbium-doped silica double-clad LMA fiber", Opt. Express 15 (4), 1606 (2007) |
| [18] | S. Yoo et al., "Photodarkening in Yb-doped aluminosilicate fibers induced by 488 nm irradiation", Opt. Lett. 32 (12), 1626 (2007) |
See also: fibers, laser crystals, nonlinear crystal materials
This encyclopedia is authored by Dr. Rüdiger Paschotta, the founder and executive of RP Photonics Consulting GmbH. Contact this distinguished expert in laser technology, nonlinear optics and fiber optics, and find out how his technical consulting services (e.g. product designs, problem solving, independent evaluations, or staff training) could become very valuable for your business!
