Photodarkening and Photobleaching of an Ytterbium-doped Silica
Double-clad LMA fiber
J. Boullet, I. Manek-Hönninger
CELIA-PALA (UMR 5107), Université Bordeaux1, 351 cours de la Libération, F-33405Talence, France
T. Cardinal, F. Guillen
ICMCB – CNRS (UMR 9048), Université Bordeaux I, 87 av. A. Schweitzer, F-33608 Pessac, France
M. Podgorski, S. Ermeneux, R. Bello Doua, F. Salin
EOLITE, 6 allée du doyen George Brus, F-33600 Pessac, France
Abstract: We studied the temporal behaviour of photodarkening in an Yb-doped LMA fiber and
show photobleaching of the same fiber. The absorption spectra and the influence on the lasing
properties are shown.
©2007 Optical Society of America
OCIS codes: (140.3510) Lasers, fiber; (060.2290) Fiber materials
Yb-doped fibers are very often used for fiber lasers and amplifiers . In the recent years, big progress has been
made, for example in the generation of high peak power pulses in Q-switched operation . When generating
such pulses new challenges are facing the experiments as dopage concentrations become higher and fiber
lengths are getting shorter. Photodarkening turned out to be one of the limiting factors in high peak power fiber
lasers as the laser output power decreases gradually as the photodarkening occurs. The phenomenon has been
investigated for other rare-earth doped fibers such as Terbium and Thulium doped fibers [3, 4], and recently has
shown up as well for Yb-doped fibers [5, 6].
We demonstrate for the first time the temporal evolution of the photodarkening process in an Yb-doped fiber as
well as photobleaching of the same fiber. Moreover, we show the influence of the photodarkening and
photobleaching on the lasing properties when this fiber is used in a cw laser oscillator.
The used fiber is a commercially available microstructured large-mode-area (LMA) double-clad Yb-doped
silica fiber, the inner active core has a diameter of 22µm, whereas the pump cladding has a diameter of 265µm.
In order to study the temporal behaviour of the photodarkening effect in this fiber we measured the absorption
spectra of the fiber after a variable exposure time to pumping light at the absorption peak wavelength of 980 nm
by 45 W pump power delivered from a fiber-coupled diode laser. The spectra were taken by injection of a fiber-
coupled white-light source (OH-2000-BAL MICROPACK) and were recorded by a spectrometer (SPECTRA
PRO 500I). We applied the cut-back method for all absorption measurements to ensure that there was no
influence caused by the injection alignment.
Photobleaching has been reported for Thulium-doped fibers using visible light  to restore the fiber
characteristics. We achieved photobleaching of the Yb-doped fiber by a short exposure time to UV light at 355
nm delivered by a frequency-tripled Nd:YVO4 laser.
Fig.2: Transmission signal of a HeNe laser as a function of
Fig.1: Absorption spectra of a new Yb-doped LMA fiber (black)
and of the same fiber at different states of photodarkening (upper
curves) and after successive photobleaching (red).
Figure 1 (left) shows the absorption spectra of a new fiber (black line), and of the same fiber after different
exposure times to pump light at 980 nm as well as the spectrum of the photobleached fiber (red line). One
clearly sees the dramatic increase of the absorption in the visible range with increasing pump power
(photodarkening) which is continuing to longer wavelengths up to 1100 nm. These changes in the absorption
spectra might be the signature of the formation of color centres. After photobleaching by UV light the
absorption spectrum gets almost back to the original one (see Fig.1). From the spectra at different states of
photodarkening one can see that the absorption at the lasing wavelength of 1030 nm increases. Even though the
increase is much weaker in the infrared part of the spectrum than in the visible, there is a crucial influence on
the lasing properties as we will show below.
In Fig. 2 the transmission of a HeNe laser as a function of exposure time to the pump light is depicted. The
decrease in transmission at the wavelength of 633 nm is in complete agreement with the increase in absorption
in the visible as can be seen from Fig. 1. For exposure times exceeding ≈40 minutes the photodarkening effect
saturates, and thus the transmission loss at 633 nm stays almost constant. This means that the photodarkening is
an effect caused by the pump light and saturating after about an hour. Note that photodarkening on this time
scale was provoked by pumping the fiber with excitation light around 980 nm without lasing and not in
continuous wave laser configuration where we saw no photodarkening effect even after several hours of cw
laser operation. This confirms the hypothesis that the photodarkening depends on the population inversion as
reported in reference 6. Moreover, one has to notice that during pumping cooperative luminescence was
observed in the visible indicating the presence of Yb3+-Yb3+ pairs.
In order to study the influence of the photodarkening and the photobleaching on the laser properties a laser
oscillator was built up with a 1 m long piece of the Yb-doped fiber that was pumped via a dichroic mirror by a
fiber-coupled laser diode (Φ=400µm, NA=0.22). The cavity was formed by a high reflecting end mirror and by
the cleaved end of the fiber that served as output coupler. Figure 3 shows the slope efficiency of the cw fiber
laser oscillator as a function of absorbed pump power for different states of photodarkening.
The slope efficiency decreases considerably when a photodarkened fiber has been used as active laser medium.
Moreover, the ratio between output power to pump power does not stay linearly but shows a behavior that could
be interpreted by an additional saturable loss within the cavity as it is the case by reabsorption at the lasing
wavelength of 1030 nm. This is in excellent agreement with the evolution of the absorption spectra shown in Fig.
1. Thermal effects could be excluded. Note that the slope efficiency is the same for the laser using a new and a
photobleached fiber. We tested the stability of the laser after photobleaching of the fiber, and we did not see any
difference between a new and a photobleached fiber.
In conclusion, we investigated the temporal evolution of the photodarkening phenomenon in an Yb-doped
double-clad silica fiber and demonstrate photobleaching by UV exposure of the same fiber. Moreover, the
influence caused by the photodarkening on the lasing properties of this fiber are shown. By photobleaching the
lasing characteristics can be fully restored. Our results in lasing operation are in total agreement with the
interpretation of the white-light absorption spectra of the fiber. Correlation with cooperative luminescence and
an inversion population effect could be shown.
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Fig. 3: Slope efficiency of a fiber laser that uses the same Yb-doped LMA fiber: new or 0 minutes of photodarkening (black squares),
after different times of pump light exposure and photobleached (red dots).