Optical characterization of Er3+and Yb3+ co-doped barium ...

non-radiatively to the 4 I13/2 level which give rise to the transition
within the manifold 4 I13/2- 4 I15/2 corresponds to emission at
1571nm. At the same time, the populated 4 I11/2 level is further
excited to the 4 F7/2 state by ESA of a second photon. From 4 F7/2 level
another 980 nm photon can be absorbed to an equally similar excited
level. As explained earlier these excited state processes can equally
happen under high photon density through nonlinear absorption
processes such as two photon or three photon absorption. After these
excited state processes, de-excitation accumulates electron in different
excited states and the intensity of emission depends on the
electron density at that particular level as well as the nonradiative
contribution to the emission band. The two photon processes that
populate the 4 F7/2 level is so efficient that it can give very efficient
green emission transition from 2 H 11/2 due to the fast multiphonon
non-radiative decay of 4 F7/2 level to 2 H11/2. Because of several closely
spaced levels in between 2 H 11/2 and 4 S 3/2 multiphonon relaxation
results in decaying the 2 H11/2 level to 4 S3/2 yielding the strongest
emission band at 547 nm. Further, since the energy gap between
4 S3/2 and 4 F9/2 is 3080 cm 1 multiphonon relaxation is unlikely to
happen and hence the 4 S 3/2- 4 F 9/2 transition could be radiative. Part
of the population decaying to level 4 F9/2 can result in the red emission
at 670 nm through the 4 F 9/2- 4 I 15/2 transition. The other possible
major process that can populate the 4 F9/2 level of Er 3þ is
the energy transfer process from Yb according to the equation
2 F5/2(Yb 3þ )þ 4 I13/2(Er 3þ )- 2 F7/2(Yb 3þ )þ 4 F9/2(Er 3þ ). In order to
establish the presence of these nonlinear processes, the pump power
dependence of the emission bands on the emission intensity was
measured and the relationship obtained is shown in the inset of
Fig. 3. The slope obtained for 533, 547 and 670 nm emission bands
are respectively 2.3, 2.2 and 2.1, which shows that two photon
processes contribute to these emissions.
Fig. 5 presents the NIR emission spectra of Yb 3 þ ions singly
doped and Yb 3 þ /Er 3 þ ions co-doped glasses. A 2 fold decrease
in the peak emission intensity of Yb 3 þ : 2 F5/2- 2 F7/2 transition at
1006 nm in the co-doped sample clearly indicates the occurrence
of energy transfer. The left inset of Fig. 5 shows the decay profiles
for the 1006 nm emission of Yb 3 þ ions in the singly doped and codoped
sample fitted by single exponential function with a
Emission Intensity (a.u)
10.0 x10 6
8.0 x10 6
6.0 x10 6
4.0 x10 6
2.0 x10 6
0
M. Pokhrel et al. / Journal of Luminescence 132 (2012) 1910–1916 1913
Yb: 2 F 5/2-> 2 F 7/2
Log Photon Counts (a.u)
λemi = 1006nm
YbEr; τf = 152μs
Yb; τf = 634μs
correlation factor of 0.99998, and the right inset shows the decay
profile for 1571 nm of Er 3þ ions in the co-doped sample also fitted
by single exponential function with a correlation factor of 0.99999.
From the measured lifetimes of Yb 3þ transitions at 1006 nm in the
singly doped (Yb 3þ )andco-dopedglass(Yb 3þ /Er 3þ ), the energy
transfer rate and efficiency can be calculated using the following
expressions [32]:
WET ¼ 1 1
ð1Þ
t
Z ET ¼ 1
t0
t
, ð2Þ
t0
where t is the lifetime of donor, Yb 3 þ ions in the presence of
acceptor, Er 3 þ ions and t 0 is the lifetime of donor, Yb 3 þ ions in
the absence of acceptor, Er 3 þ ions. The energy transfer rate from
(Yb 3 þ ) 2 F 5/2-(Er 3 þ ) 4 I 11/2 in the present glass is found to be
5001.6 s 1 showing an energy transfer efficiency of 76%.
3.3. Cross section and gain coefficient
Emission cross-section (se) and the FWHM are important
parameters in an optical amplifier’s achieving broadband and
high-gain amplification. The bandwidth properties of the optical
amplifier can be evaluated from the product FWHM s e. The
larger the product is, the better are the performance of the
amplifier. Tables 3 and 4 list the parameters corresponding to
1571 nm emission for comparison such as FWHM, se, and
FWHM s e of Er 3 þ in various Er 3 þ /Yb 3 þ doped tellurite and
oxide glasses. It is seen that FWHM se in present glass composition
80TeO 2þ15(BaF 2þBaO)þ3La 2O 3 is larger than in other tellurite
glasses reported in the literature [17,27], but less than the
reported in the literature [33,34] system, which indicates that
present glass composition is better among the composition based
on TeO 2þBaF 2, TeO 2þZnO, germinate [35], and silicate [36] for a
broadband amplifier.
A long upper state lifetime in a gain medium enables a
significant population inversion with a relatively low pump
400 800 120016002000 1200 2400 3600 4800
Time (micro sec)
Yb
YbEr
λ emi = 1571nm
YbEr;τ f = 2693 μs
1000 1125 1250 1375 1500 1625
Wavelength (nm)
Fig. 5. Emission spectra of Yb 3þ ion singly doped and Yb 3þ /Er 3þ ion co-doped tellurite glasses. Insets show the decay profiles for the emissions.
Er: 4 I 13/2-> 4 I 15/2