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850 nm Upconversion Lasing in Er+3 Doped Z.B.L.A.
Fibers
A. Saissy, E. Maurice, G. Monnom, S. Staroske, G. Baxter
To cite this version:
A. Saissy, E. Maurice, G. Monnom, S. Staroske, G. Baxter. 850 nm Upconversion Lasing in Er+3 Doped Z.B.L.A. Fibers. Journal de Physique III, EDP Sciences, 1995, 5 (3), pp.297-306.
�10.1051/jp3:1995126�. �jpa-00249311�
Classification Physics Abstracts
42.81 78.45
850 nm Upconversion Lasing in Er~~ Doped Z.B.L.A.
Fibers
A. Saissy, E. Maurice, G. Monnom, S, Staroske and G. Baxter
Laboratoire de Physique de la Mati~re Condens£e, CNRS-U.A. 190, Universit£ de Nice, Sophia Antipolis, Parc Valrose, 06108 Nice Cedex, France
(Received 25 April 1994, revised lst September and 14 December 1994, accepted 19 December 1994)
Abstract. Upconversion losing at 850 nm is studied in Er~~ doped Z-B.L.A. fibers. Modelling of excited state absorption and upconversion emission from basic principles allows determining the laser oscillation condition in the fiber. Values of excited state absorption cross-sections are
deduced from laser oscillation and spontaneous emission data and discussed.
Introduction
Erbium doped optical fibers are currently used to realise 1.55 pm optical amplifiers and lasers useful to long haul optical communication. Using fluorozirconate fibers these possibilities can be extended to shorter wavelengths as a result of upconversion processes : 850 nm (first tele- communications window) and 550 nm (visible laser). Efficient detectors and powerful diode lasers are already available in the first telecommunications window. Moreover, the realization of 850nm fiber amplifiers opens many possibilities in the field of short distance networks and related applications. Experiments on upconversion pumped erbium doped fluorozirconate fiber amplifiers and lasers operating at 850 nm have been reported by Millar [I] and Whit- ley [2].
Amplification of 850nm signals by upconversion-pumped erbium doped fiber amplifiers
have been analysed theoreticaly by Urquhart [3]. Upconversion emission processes result
mainly from sequential excitation of Er~~ ions via
pump excited state absorption. In Er~~
doped Z-B-L-A- fiber, the 850 nm optical gain relative to the ~ S~~-~ I~~~ transition results from the sequential absorption of two 800 nm pump photons (Fig. 4). In a first step, ground
state ions are excited on the ~ I~~~ level and relax on the ~ I~~~ and ~ I~~~ metastable lev-
els. These ions are then excited by pump photons into the ~F~~~ and ~ H~~~~ levels and
finally nonradiative relaxations provide the population of the ~S~~ level. The efficiency of
© Les Editions de Physique 1995
upconversion processes is described by the introduction of excited state absorption (E.S.A.) cross-sections whose numerical values must be determined experimentaly. Therefore, any ex- periments providing some numerical data on E-S-A cross-sections are welcome.
Upconversion amplification of spontaneous emission becomes important at pump powers commonly used in active fibers as the result of the enhancement of optical gain through mul-
tiple reflections at the fiber ends. As the pump power necessary to reach the oscillation
threshold depends on the E-S-A- cross-sections, we propose to measure the value of this
threshold to determine the E-S-A- cross-sections. Altematively the cross-section associated to
E-S-A- transition can be deduced from the spontaneous fluorescence spectrum and a Mc
Cumber analysis, as proposed by Miniscalco [8]. Comparison between the E-S-A- cross-
section values deduced from these two methods informs us about the accuracy of these
values.
The purpose of this paper is, in a first step, to measure the 850 nm laser-threshold under 790 nm-pumping. We then present a simplified model of the upconversion behaviour that al-
lows deducing both pump and signal E-S-A- cross-sections. In a second step, we confirm
these results by analysing the spontaneous emission spectrum in the 790-850 nm range
under 980 nm excitation.
Experiments and Results
The fiber studied was an Er~ ~ doped Z-B-L-A- fiber (1 000 ppm concentration) performed by
Le Verre Fluord. Its cutoff wavelength was 2.2 pm for 11/125 pm core/clad diameters and
1.45 pm for 6.5/125 pm core/clad diameters. Typically 2 m lengths were investigated with fiber ends carefully cleaved.
The excitation source was a c-w- Ti AI~O~ laser tuned on 790nm corresponding to
the ~I~~~~ -~
I~~~ absorption band of the Er~~ ion. The maximum beam power
was 900 mW.
Coupling was realized with X 20 microscope objective with an efficiency of 2596 to 3596.
The emission spectrum at the fiber exit was recorded with a optical spectrum analyser
lo.I nm resolution) at room and liquid nitrogen temperatures. Complementary measurements on fluorescence spectrum were also performed by exciting with a 980 nm laser diode.
Although the absorption of 790 nm pump photons in erbium doped fluorozirconate results
in many fluorescence bands [10], this study is focussed on the 850nm emission due
to ~ S~~~ -~
S~~~~ transition. Emission spectra for 750 mW and 900 mW incident pump powers
are shown in Figures I and 2 for a temperature of 77 K. The optical spectrum analyser resol- ution was respectively 2 and 0.I nm in Figures I and 2, also only the more powerful emiss-
ion bands are recorded in Figure 2. Whereas the room temperature emission spectrum of
Er~~ doped Z-B-L-A- fiber is rather flat from 840 to 850
nm, cooling the fiber increases the
emission around 850nm. When pumped with 750mW incident power a narrowing of the
850nm emission spectrum is observed due to the stimulation of spontaneous emission. In-
creasing the incident pump power to 900mW gives rise to sharp (0.2nm) and intense ( 500 pW ) spikes in the spectra : an oscillatory state between fibers ends is established. Such
an oscillatory state was also observed, at room temperature with a fibre of 6 pm core diam- eter as shown in Figure 3.
Discussion
A. LASER OSCILLATION.-A simplified energy level diagram of Er~~ in Z-B-L-A- [10] is
given in Figure 4, levels of interest for the present study are labelled I to 9 from the ground
Power 50
25
850 ~nm
Fig. 1. Emission spectrum of Er~~ doped Z-B-L-A- fiber (11 pm core diameter) with 750 mW incident
pump power at 790 nm and T = 77 K. Spectrum analyser resolution : 2 nm.
Power 5oo
250
850 852 l~ nm
Fig. 2. Emission spectrum of Er~~ doped Z-B-L-A- fiber (11 pm) core diameter) with 900 mW inci-
dent pump power at 790 nm and T = 77 K. Spectrum analyser resolution : o-I nm.
state. Non radiative desexcitations and optical transitions induced by the spectral density pow-
ers P (pump) and S (emission) are reported in Figure 4. The population densities on each of the nine levels shown in Figure 4 are derived in Appendix A from stationnary solutions of the
Power
2
MO 850 l~nm
Fig. 3. Emission spectrum of Er~~ doped Z-B-L-A- fiber (6 pm core diameter) with 600 mW incident pump power at 790 nm and room temperature. Spectrum analyser resolution 0.I nm.
rate equations. As the main interest is the threshold determination, we will only investigate the low pumping regime, in which pump and emitted signals are independant. In such a situation, the ground state population, N~, is quite equal to the volumic concentration of Er~ ~ ions N.
Following the above hypotheses the forward and backward emitted powers are decoupled,
which allows us treating independantly the forward signal S. As the fiber supports many propa- gation modes at 790 and 850 nm, it is assumed that the excitation and emission are uniform
over the fiber core.
A-I- Pump propagation. The evolution of the pump spectral density power P involved in the transitions I to 4, 2 to 7 and 3 to 9 satisfies the equation
Using the simplified population densities N~, N~, N~ of Appendix A we write
ff=-~X~~~P-flP~ (2)
The total linear absorption coefficient can be separated into a contribution of the erbium ions,
a([~N, and a background losses coefficient m)~~ such as m~~~ = nj~~+a((~N. The qua-
dratic pump absorption coefficient fl is such that
~ ~
Y22
~~~44
Y66
~~V~
~~~~ ~~~ ~~~~ ~ ~~~~~~ ~~
4
F 9
5-3/2
2
H ~
11/2 4
s 3/2
4
F wp 5
9/2
4
I
9/2
4
11/2
4
2 13/2
4
i
15/2
Fig. 4. Energy level diagram for Er~~ ions in Z.B.L.A. glass with
pump (P) and emission (S) transi- tions.
We introduce the absorption cross-section a~~~, the total relaxation rate y,,(s~~ ) of level I and the
y~~ relaxation rate from level I to j. Integration of equation (2) gives
~
P( 0) exp(- ~x~~~ z)
~~~
l +")(~(l -exp(- ~x~~~z))
A.2. Emission propagation. -Assuming that the spectral density power S radiated at 850 nm in the forward direction is unsaturated, the propagation of S is described by the equation
~=fS+gB with f=-m~~~+g.
The total absorption coefficient can be separated into a contribution of the excited state
m((~~, and the background losses coefficient, m)~~, such as m~~~
= m)~~+mfj~.
The excited state absorption and the differential gain g are such that
~~~A ~i~~ ~2 ~ ~~~~ ~3' ~ ~~~ ~6 (~)
The spectral density power B, a constant expressed in the same units as S, is related to the spontaneous noise and corresponds to an energy density 1/2 hv~ in each propagation mode. If
we take in consideration the reflection factors R~ and R~ respectively corresponding to the input and output ends of the fiber, equation (3) yields
S(co) =BT(L) j~g(v)exp ~ I(u)dudv
z
lz
v
I + KR~ exp f( u ) du g( v) exp f( u ) du dv
T(L " (1 R2) ~
z ,
K "
I )
I R~ R~ exp 2 f( u ) du g( v) exp f( u du dv
v
Analytic expressions of f and g are deduced from the population densities given in Appendix
A and from the expression of P(z).
A.3. Laser oscillation. The laser oscillation in the fiber will result from an asymptotic situ- ation defined by the relation
~ f(u) du
= Ln (@~). The differential gain g and the
~~~
o
ESA coefficient m~~~ are such that
q a$~
~ Y~~a~~~ + y~~a~~~~
g( z)
= ~
flP mf~~ = flP
~~~ ~~ ~~
~66 Ha hvp Y32 ~27 ~ ~22 ~39
q is the relaxation probability from the ~ H~~~~ to the ~ S~~~ energy level at thermal equilib-
rium. In the weakly pumping approximation, P(0) « m~~~/fl, the laser oscillation condi-
tion becomes
G=tY)~~Z-Ln(~/~i~2 )~
j(s)
I + L
L ~ W fl3~a[j~ail~ P( o)
Lj2 fl~~
a[j~<~~~ p( o L
2 d~~~ ~ Y22 Y66 na~
hv~ Y22 (~~~ na~ hv~ ~~~
We have introduced the dimensionless parameters :
(e)
~ ~~~~ ~~~~~~'~ ~~ ~R~~l'p
j(s) Y32 ~i~~ ~ ~22 ~~~~~ j(P)
~
£Y~~~ ~32 ~32~i~~
Y32 ~~~~ ~ Y22~~~ ~~ ~ ~~~~ ~~32 ~32 ~~~~ ~ ~22 ~~~ ~~
where fl~~ is the branching ratio of the ~I~~~~-~
I~~~~ transition. The Figure 5 represents G
versus P(0) in a realistic situation where L=4~~~ =1, 4~~~ «Q,
fl~~ =0.ll and
q= I at 77K and for different values of R=a~~la~~. The laser oscillation relation (5)
with tY~=0 and ~/R~R~
= 0.04 (reflection on fibers ends) will be satisfied at a pump
power P~~( 0)
= 280 mW inside the fiber with a~~ = 0.6 X 10~ ~~ cm~,
a~~ = 8 X 10~ ~~ cm~, a~~la~~ = 0,I and i~~ = 0.5 ms [10].
G
R
o
o-1
6 o,15
~RiR2 0.2
0.01
4
0.04
2
0 200 400 mW P(0)
Fig. 5. Computed gain G as a function of coupled pump power P(0) and ratio R = a~~la~~,
a~~ = 0.6 x 10~ ~~ cm~, a~~
= 8 x 10~ ~~ cmi
i~~ = o-S ms, fl~~ = 0.11. Horizontal lines corre- spond to G
= Ln ( ~$).
Some particular pump powers have been introduced in our modelization :
p
~h ~26
p(I)
~$ p(2 tX~~~
~~ ~66 ~27 ~62 ~ ~33 ~14 ~ fl
From the above cross-sections, we deduce that the threshold power P~~ required to reach a
positive gain G is 78 mW, the saturation power of the ground state pump absorption P~~~ is
479 mW with a~~ = 0.7 X 10~ ~~ cm~ [5],
i~~ = 7 ms [10] and P~~~
= 322 mW. As
P~~ < (P~~~,P~~~) the hypotheses of our model
are justified. In general, however, this is not always true. For example, in the case of Tm~ ~ doped materials, a lower value of P~~~
can
be found due to photon avalanche phenomenon [9].
Above the threshold, the pump and signal powers become important and require an accu- rate resolution of rate equations. Although it refers mainly to the amplification of a 850 nm signal, the Uquhart [3] modelization could be used to describe the laser behaviour in this
situation.
In conclusion we have experimentaly determinated that a 270mW-lauched power (3096
launching efficiency, 900 mW incident pump power) was required to establish a laser oscilla- tion between the free ends of our fiber. A similar result can be obtained from a modelization of the upconversion pump process and fumish respectively the 0.6X10~~~cm~ and
0.8 X10~~~ cm~ values for the
a~~ and a~~ E.S.A. cross-sections. As these values are de- duced from a simplified modelization, it is interesting to valid them by the results deduced of the fluorescence measurements.
B. FLuoREscENcE ANALYSIS. Excited state absorption cross-sections a~~~
are related to the corresponding emission cross-sections a~~~ through the relation (6)
a)/~ = a)/~ exp( hv e )/kT (6)
e is the net free energy required to excite one Er~~ ion from the lower to the upper level at temperature T and can be calculated from the ratib of the equilibrium populations of upper
and lower levels e = kTLn (N~/N~), [8]. In order to estimate e we suppose that the Stark
components of manifolds ~ S~~~ and ~I~~~~ are equally spaced, 200 and 50 cm~~
respec-
tively. An energy gap E~ between the lowest components of the manifold ~ S~~ and ~ I~~~~
equal to 11 869 cm~ leads to calculate a ratio a~~ la~~ = 0.I at 77 K by using the relation (6). Following the same procedure, we compute a~~ from the emission spectrum around
788nm (~H~~~~-~I~~~~ ) at 77K recorded under 980nm excitation; we deduce that
a~~ la~~ = 0.075, a~~ = 7.5 X 10~~~ cm~ and
a~~ = 0.56 X 10~ ~~ cm~. We estimate
a
a~~ = 8 X lO~~~ cm~ value from
a Fuchtbauer-Ladenburg analysis, [11], of the fluoresc-
ence spectrum using i~~=O.sms, fl~~
= 0.30 and Al
= lsnm the width of the 850nm
emission spectrum recorded under 980 nm excitation at 77 K. We conclude that the values of emission and absorption cross-sections deduced from spontaneous emission are consistent with the study of the 850 nm laser oscillation condition (5).
Under 790 nm-pumping, the ratio a~~ la~~ relative to the Er~~ doped fibers
are respectively 3.2 with AMP silica [4, 5] and 4.3 in our study, showing that sequential pumping efficiency is
therefore improved by the use of Z-B-L-A- glass host. The accuracy of the measurements could be improved from an experimental and conceptual point of view either by the use of
800 nm monomode fiber either by comparison of 850 and 550 nm [6, 7] upconversion lasing
in a fiber with optimised optogeometrical parameters.
Conclusion
We have presented experimental results relative to 850nm upconversion lasing in Er~~
doped Z-B-L-A- fibers. Modeling of the laser oscillation condition allows us deducing the E-S-A- cross-sections : O.6 X lO~ ~~ cm~ and O.8
X lO~~~ cm~ for the excited state ~
I~~~~
involved in 790nm excitation and 850nm emission respectively. These cross-sections are
also deduced from a Mc Cumber analysis of spontaneous emission spectrum at 790 and
850 nm under 980 nm excitation, consistency with the above values is verified. The method
used to measure E-S-A- cross-sections presented here is rather general and can be applied to
other transitions of Er~ ~
or to other rare earths (such as Tm~~
or Ho~ ~) doped fibers. If we consider the first telecommunication window, where large band superfluorescent sources and high gain amplifiers are of interest, upconversion emission in Er~~ doped Z.B.L.A. fibers
provides an altemative to the use of 8 lo nm emission in Tm~ ~ doped Z-B.L.A, fiber pumped by one photon process [12]. Improvement on the laser cavity design, through the use of high reflectivity mirrors instead of the free fiber ends, would result in a lower oscillation threshold which is very promising for a pulsed operation of an 850 nm Er~~ doped fiber laser. Beside technological interest for the first telecommunication window this study of the 850 nm emiss- ion provides a comprehensive investigation of the upconversion process involved in rare
earth doped Z.B.L.A. fibers.
Appendix A
To describe the population densities N~, of the nine levels shown in Figure 4 we use a rate
equations formalism. We simplify our analysis by assuming that only the two optical fields at
790 and 850nm have sufficient powers to modify the population densities. Following this
hypothesis we write :
~~9
~39 ~3 ~98 ~ ~93 ~9
~~8
~98 ~9 (~87 ~ ~84) ~8
~7 ~27 ~2 ~ ~87 ~8 ~76 ~ ~72~ ~7
~~6~~26~2~~76~7~~~61~~62~~63~~7
~4 ~14 ~l ~ ~68 ~8 ~41 ~ ~43 ~4
~~3
~43 ~4 ~ ~63 ~6 ~ ~93 ~9 ~31 ~ ~32 ~ ~39) ~3
~~2
~32 ~3 ~ ~62 ~6 ~ ~72 ~7 ~21 ~ ~26 ~ ~27) ~2
~~l~~21~2~~31~3~~41~4~~61$ ~14~l'
In the above equations the coefficients
y,~ are nonradiative desexcitation rates. As the excita- tion and the emission are supposed uniform over the fiber core we have :
~~f( + ) ~ ~~f(- ~~f( + ~ ~~f(-
a: =
alf~
~ and e.
= + a[i~
~ " ~
Ha hv ~ Z;y " Ha hv
where W~ ~ ~, W~~
are the forward, backward powers involved in the transition ij with spon-
taneous emission probability y,y = I/z,y In the weakly pumped fiber approximation used in
our analysis we write
e,~ = and a,~ = aj~~ ~~
,
the population densities involved in
Z,y Ha hv~
equations (I) and (4) will be
Y43 N
=
~~~N2
Ni " N, N2 )) ~~
'
~~ ~
~33 ~" ~~~
~ ~ ~~~
a~~ and a~~ belongs respectively to ~I~~~ -~I~~~ and ~I~~~~ -~
H~~~~ transitions induced by
the pump power P(Z ).
Acknowledgments
The authors express their thanks to G. Maze of Le Verre Fluord for supplying the fiber for this work.
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