Photodarkening is an
optical effect observed in the interaction of laser radiation with amorphous media (glasses) in
optical fibers. Until now, such creation of color centers was reported only in glass fibers.
[ ] Photodarkening limits the density of excitations in
fiber laser
A fiber laser (or fibre laser in British English) is a laser in which the active gain medium is an optical fiber doped with rare-earth elements such as erbium, ytterbium, neodymium, dysprosium, praseodymium, thulium and holmium. They are related t ...
s and amplifiers. The experimental results suggest that operating at a saturated regime helps to reduce photodarkening.
[N. Li; S. Yoo; X. Yu; D. Jain; J. K. Sahu (201]
“Pump Power Depreciation by Photodarkening in Ytterbium-Doped Fibers and Amplifiers”
'' IEEE Photonics Technology Letters'', Vol. 26, Issue 2, pp.115-118
Definition
One could expect the term photodarkening to refer to any process when any object becomes non-transparent (dark) due to illumination with light. Formally, the darkening of the photo-emulsion also could be considered as photodarkening. However, recent papers use this term meaning reversible creation of absorbing color centers in optical fibers. One may expect that the effect is not specific for fibers; therefore, the definition should cover wide class of phenomena, excluding, perhaps, non-reversible darkening of photographic emulsions.
According to the Encyclopedia of Laser Physics and Technology,
photodarkening is the effect that the optical losses in a medium can grow when the medium is irradiated with light at certain wavelengths. We may also define photodarkening as reversible creation of absorption centers in optical media at the illumination with
light.
Photodarkening rate
The inverse of the timescale at which photodarkening occurs can be interpreted as photodarkening rate.
Color centers
Usually, photodarkening is attributed to creation of
color centers
An F center or Farbe center (from the original German ''Farbzentrum'', where ''Farbe'' means ''color'' and ''zentrum'' means center) is a type of crystallographic defect in which an anionic vacancy in a crystal lattice is occupied by one or more un ...
due to resonant interaction of electromagnetic field with an active medium.
Possible mechanisms of photodarkening
The phenomenon, similar to photodarkening in fibers, was recently observed in chunks of Yb-doped ceramics and
crystals. At the high concentration of excitations, the
absorption
Absorption may refer to:
Chemistry and biology
* Absorption (biology), digestion
**Absorption (small intestine)
*Absorption (chemistry), diffusion of particles of gas or liquid into liquid or solid materials
*Absorption (skin), a route by which ...
jumps up, causing the
avalanche of the broadband
luminescence.
Increase of absorption can be caused by formation of
color center
The colour centre is a region in the brain primarily responsible for visual perception and cortical processing of colour signals received by the eye, which ultimately results in colour vision. The colour centre in humans is thought to be located ...
s by
electrons in the
conduction band
In solid-state physics, the valence band and conduction band are the bands closest to the Fermi level, and thus determine the electrical conductivity of the solid. In nonmetals, the valence band is the highest range of electron energies in w ...
, created by several neighboring excited ions. (The energy of one or two excitations is not sufficient to pop an
electron into the
conduction band
In solid-state physics, the valence band and conduction band are the bands closest to the Fermi level, and thus determine the electrical conductivity of the solid. In nonmetals, the valence band is the highest range of electron energies in w ...
). This explains, why the rate of darkening is strong function of the intensity of the exciting beam (as in the case with optical fibers discussed above). In the experiments,
the
thermal effects are important; therefore only the initial stage of the
avalanche
can be interpreted as photodarkening, and such interpretation is not yet confirmed. Recent work
pointed out the role of
thulium contamination. Through
laser pump and signal
absorption
Absorption may refer to:
Chemistry and biology
* Absorption (biology), digestion
**Absorption (small intestine)
*Absorption (chemistry), diffusion of particles of gas or liquid into liquid or solid materials
*Absorption (skin), a route by which ...
, and energy transfer from
ytterbium;
thulium is able to emit UV light, known to create color centers in silica glass. Although the actual mechanism of photodarkening is still unknown, a reliable setup to test the photodarkening properties of different types of fibers has been recently reported.
References
{{reflist
Optical materials
Laser gain media
Laser science