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Investigations on Excited-State Absorption in Pr3+
Doped Fluorozirconate Fibers
A. Saissy, E. Maurice, G. Monnom, D. Ostrowsky
To cite this version:
A. Saissy, E. Maurice, G. Monnom, D. Ostrowsky. Investigations on Excited-State Absorption in Pr3+ Doped Fluorozirconate Fibers. Journal de Physique III, EDP Sciences, 1996, 6 (2), pp.323-330.
�10.1051/jp3:1996125�. �jpa-00249457�
Investigations on Excited-State Absorption in Pr~+ Doped
Fluorozirconate Fibers
A. Saissy (*), E. Maurice, G. Monnom and D-B, Ostrowsky
Laboratoire de Physique de la MatiAre Condens6e (**), Parc Valrose, 06108 Nice Cedex 2, France
(Received 4 July 1995, accepted 24 November 1995)
PACS.42.81.-I Fiber optics PACS.78.45.+h Stimulated emission
Abstract. Excited state absorption (E.S.A.) in Pr~+ doped fluorozirconate fibers pumped at 1012 nm is investigated by the pump-probe beam method. Probe wavelengths are successively
830, 780 and 1310 nm, we measure both the attenuation of the probe beam when 1012 nm
pumping is applied and the modulation of the visible
or 1.3 /Lm fluorescence when the probe is lauched into the fiber. Modelisation of these measurements allows us deducing the magnitude
of E.S.A. crost-sections at 830, 780 and 1012 nm.
Introduction
Pr~+ doped fluorozirconate fibers are of interest in 1.31 ~tm optical amplifiers (I] useful for the second telecommunication window and visible upconversion fiber laser (2]. Compact de- vices can be realized with laser diode excitation around I ~tm, the wavelength of the ~H,-~G4
Ground State Absorption (G.S.A.) of Pr~+ ions, Figure I. As G-S-A- cross-section is weak the
efficiency of these devices is strongly related to the budget of Pr~+ excited ions. With 1.3 ~tm
amplifiers, spurious attenuation of the signal arises from ~G4-~D2 and ~H4-~F4 transitions, whereas visible upconversion lasers requires a strong Excited State Absorption (E.S.A.) phe-
nomenon to pump the ~Po upper laser level from ~G4 level. In Pr~+-Z.B.L.A. glasses, the
E-S-A- cross-sections associated to ~G,-~D2 and ~G4-~Po transitions have been indirectly obtained by Quimby [3] from fluorescence spectra or directly measured by Tropper (4] from experiments performed with two argon-ion-pumped titanium sapphir lasers. Therefore, when
we are interested only by a routine characterization of Pr~+ doped fibers, it seems interesting
to measure directly these cross-sections from
more compact pump sources. As the lifetime of the ~G4 excited state of Pr~+ ions in Z-B-L-A- glasses is short (l13 ~ts iii the measure of
E-S-A- spectrum by the popular pump-probe beam method is not easy with low power incoher- ent tunable probe beam. To overcome this difficulty, we propose to reduce the measurements to some discrete wavelengths and to use powerful laser diode as probe beam. This technique
presents the advantage to provide informations, not only from the attenuation of the probe
(*) Author for correspondence (e-mail: saissy©unice.fr) (**) CNRS URA 190
@ Les (ditions de Physique1996
324 JOURNAL DE PHYSIQUE III N°2
3p~
3 3p 780 nm
6 ~
3p
o
830 nm
1012 nm
3
F~ ~
~fll
3~
3
~fi~
3~
~
6 1012nm
H~
1320 nm 607 nm
~H~
Fig. I. Energy levels of Pr~+ showing G-S-A and E-S-A transitions.
beam when 1012 nm pumping is applied, but also from the modulation of the Pr~+ emission when the probe beam is lauched into the fiber. Therefore the E-S-A of Pr~+ in Z-B-L-A- glasses
can be strong enough to give rise to saturation of E-S-A- If we use powerful probe beam, such effects must be take into account in the pump-probe beam method.
The purpose of this paper is to determine -directly the E-S-A- cross-sections at 0.83, 0.78 and 1 ~tm in Pr~+ doped Z-B.L.A. fibers by a two laser diodes excitation method.
1. Experiment Results
The fiber used is a Pr~+ doped Z-B.L.A. fiber (2000 ppm concentration) fabricated by "Le Verre FluorA". Its cutoff wavelength is 1.18 ~tm for 4Ii 25 ~tm core /clad diameters, and typically a 2 m
length is investigated. A 65 mW C-W, 1012 nm (~H4-~G, G-S-A- transition) laser diode (QLM 95450, Spectra Diode Lab.) is coupled to one fiber end whereas on the other fiber end, we couple successively three probe diodes with characteristics: 25 mW at 830 nm (~G,-~I6+~Pi
E-S-A- transition), 10 mW at 780 nm (~G,-~P2 E-S-A- transition) and 0.5 mW at 1320 nm.
ua
i
( ,'
' ' "
480 580 ~(nm)
Fig. 2. Visible fluorescence spectrum of Pr~+ Z-B-L-A fiber pumped at 1012 and 830
nm. (~)
77 K (1 ua = 1 /LW), 300 K (1 ua = 250 nW).
1.1. 830 nm EXPERIMENT. As a result of 1012 nm excitation of the fiber, the attenuation of the 830 nm laser beam increases, the transmitted power experiences a 55% variation at room
temperature and 80% at 77 K.
The guided conter-propagating visible fluorescence, reflected by a dichroic-mirror on the 1012
nm beam, is recorded with an optical spectrum analyser at room and liquid nitrogen temper- ature, Figure 2. Although many fluorescence bands appear on this spectrum, we focus our attention on the main band at 640 nm (~Po-~F2 transition). Replacing the 830 nm laser diode
by a more powerful Ti: Sapphire laser, we study the intensity of the 640 nm fluorescence versus the pump power, Figure 4. From the fluorescence saturation, we deduce that the saturation
power is P~ re 40 mW.
On the other hand, we observe, Figure 3, that the 1.32 ~tm fluorescence (~G,-~H5 transition)
induced by 1012 nm excitation experiences a 32% decrease as the fiber is excited with 830 nm diode.
1.2. 780 nm EXPERIMENT. The visible fluorescence is also observed as the fiber is excited with the 780 nm (~G,-~P2 transition) laser diode. The transmitted 780 nm power experience
a 36% (resp. 15%) variation with 1012 nm pumping at 77 K (rep. 300 K): the efficiency of 780 nm E-S-A- band is less important than the 830 nm one. As the 830 and 780 nm radiations
experience no G-S-A, the absorption of these radiations is attributed to an E-S-A- process from the ~G4 level.
1.3. 1.3 ~tm EXPERIMENT. Although the 1012 nm excitation of the Pr~+ doped Z-B-L-A
fiber results'mainly in an 1.32 /1m emission, we also observe a weak upconversion emission
(~D2-~H,) at 607 nm. Excitation of Pr~+ ions
on the ~D2 level arises from sequential
326 JOURNAL DE PHYSIQUE III N°2
pw
Iw
8
#
$
~ ' -_ ",
50 '
,
.2 1.3
Fig. 3. Fluorescence spectrum at 1.3 /Lm. (~) 1012
nm excitation. 1012 + 830 nm
excitation.
ua I
o
( (
~ Q-S
o
~
0
0 loo 200 300 400 mw
pumppower
Fig. 4. Evolution of the 640 nm fluorescence of a Pr~+ Z-B-L-A fiber pumped at 1012 and 820 nm
versus the pump power at 820 nm.
absorption of1012 nm photons: ~H4-~G, ground state absorption, non-radiative relaxation
on ~F4 level and ~F4-~D2 excited state absorption. As the fiber is simultaneously excited with 1.012 and 1.3 /1m laser diodes, we observe, Figure 5, beside the amplification of1.3 pm
radiations Ii I dB) that the 607 nm emission is 3.4 times more efficient.
nw
I
©I 8
#
$
~ "
~ / '~
/~ ',
/ '
/ i
/ '
/ ',
'
557 607
Fig. 5. Fluorescence spectrum at 607 nm. (~) 1012 nm excitation. 1012 +1310 nm
excitation.
2. Discussion
A simplified energy level diagram of Pr~+ in Z-B-L-A- glass [6] is given in Figure 1. Levels of interest for the present study are labelled from the ground state. We consider a step-index fiber in the weakly guiding approximation with transverse linear polarized modes and an uniformly doped core.
2.1. 830 nm EXPERIMENT. The excitation radiations at 1012 nm (~i) and 830 nm (~2)
wavelengths are supposed to propagate respectively along the directions +Oz and -Oz on the fiber axis. In the weak excitation regime, the evolution of the spectral density power W (resp.
P) corresponding to the 1012 (resp. 830) signal satisfies the relation I (resp. relation 2)
~~
= -aiw with al
" a)°~ + ai7fiN and W(z)
= W(0) exp(-aiz) (1)
dz
a17 " G-S-A- cross-section, fi = fundamental mode overlapp coefficient with the radial distri-
bution of ion concentration (N).
$
= O(~~P + a7a~iI21~ ~ p
~ ~~~~ ~ ~~~ ~ ~ ~~~ ~
~~~
Z 1 + 12 ~ ~~~~~~ ~~~~~
Ps R12
" overlapp coefficient between the two excitation radiations, a7a
" E-S-A cross-section,
a(°)
= background absorption coefficient, r7 =~G4 level lifetime, W~, P~ saturation spectral density power of the 1012 and 830 nm signal.
In the hypothesis a2 " 0, the integration of equation (2) gives the relation
328 JOURNAL DE PHYSIQUE III N°2
Relation (3) allows us to relate the E-S-A- cross-section a7a to the relative variation of 830 nm transmitted probe power:
llil')
~a~h c IiI2 ~~ ~ ~
with W~b~ = ~~~~(l e~°~~)Wj0)
?7a "
~ ~ f j~~~jv
~~ iil' ~2il12~ ~l
~ ~ ~ ~
l' ~llil2Wabs From a17
" 5.7 x 10~~~ cm~ [4], L = 200 cm and N
= 3.9 x 10~~ ions/cm~ (2000 ppm), we
deduce that NLa17
= 4.4 so the background absorption can be neglected. As r7
" 113 /1s,
a = 2 pm, Wabs
" 25.5 mW (50% coupling efficiency, 85% absorption) and /hP/P
= 55%
we
deduce that a7a
" 0.67 x 10~~° cm~ at
room temperature where as a7a = 1.40 x 10~~° cm~ at 77 K. To deduce realistic values of a7a from excited state absorption at 830 nm, the saturation of the ~G4-~I6+~Pi transition has been taken into account through the parameter Ps.
~
The saturation power Ps associated to the ~G4-~I6+~Pi E-S-A- transition is P~ = ~~
~~~,
~ ~ , a
2 7 7a
from the room temperature value of a((~ we deduce that P~ = 39.5 mW.
The intensity I of the 640 nm fluorescence takes a constant value In~ as the pump power P becomes very large. We define a saturation power P~ by the relation I (P~) = 0.5 In~. From
Figure 4 we deduce that P~ = 40 mW which is consistent with the above value of 39.5 mW.
The decrease of the fluorescence power around ~3
" 1.3 /1m induced by the 830 nm excitation
is given by the simplified expression
/hS P
I
~ ~ ln(I + /hP/P)
~
~ 12/hP/P
The previously established value P~ = 39.5 mW allows to deduce a /hS/S
= 0.44 ratio, which is consistent with the 0.32 experimental value, and permit us to confirm the room temperature
value of a7a.
From the value a7a " 0.67 x 10~~° cm~, we can estimate the 640 nm fluorescence power S(0)
emitted at the input end of the fiber given by the expression:
~~~~ ~
~~~~~~
~)~
~ ~
ln(~+ /hP/P)
~
~ 12/hP/P
where a(~) is the 640
nm emission cross-section and B the noise power related to one photon
per mode in the emission bandwidth live. As we have a multimode fiber at 640 nm, we consider that the emission is uniformly distributed over the fiber cross-section. Assuming rg m 35 /1s,
a(~) m 1.8 x 10~~° cm~ (4] and a noise power B
= hue/hve m 1.5 /1w, we obtain Sth " 0.48 /1w at 300 K which is consistent ~vith the experimental value Sexp m 0.4 /1w.
Published values of a7a are 0.8 x 10~~° cm~ and 10~~° cm~ from visible and IR fluorescence measurements of Tropper (4] and 1.8 x 10~~° cm~ from calculation of Quimby (3]. Our results
are in agreement (20%) with Tropper's results, whereas the discrepancy (62%) is more higher
with Mccumber-Judd-Ofeld calculation of Quimby. The slight discrepancy between the present study and reference (4] can result from the difference in the optogeometrical parameters of the
fibers which must be taken into account by computing the various overlapp integrals.
2.2. 780 nm EXPERIMENT. The E-S-A- cross-section a7b relative to ~G4-~P2 transition is related to E-S-A cross-section a7a of the ~G4-~I6+~Pi transition by the relation
~a~h c /hP/P
~~~ ~ir7 Wabs
We deduce that a7b
" 0.13 x 10~~° cm~ at
room temperature and a7b
" 0.30 x 10~~° cm~ at
77 K. We found that the ratio a7bla7a is nearly temperature independent. The 780 nm E-S-A- remains higher than the 1012 nm G-S-A- but it is only 20% of 830 nm E-S-A- Indeed, a 780 nm upconversion pumping of Pr~+ doped Z-B-L-A- fiber requires a more powerful 780 nm laser diode to be efficient.
The increase of the cross-sections a7a and a7b as the temperature decreases can be related to variations in the thermal population of the Stark sublevels in ~H4 and ~G4 manifolds.
2.3. 1.3 ~tm EXPERIMENT. The increase of 607 nm upconversion emission induced by
1.3 /1m excitation is calculated in the weak excitation regime
/hS jr7 a7s ~ a16 ~~ a17N P(L)~(1 e~">L )~
~ T6 ?68 ?17 ~l al Wabs I e~~°'~ ~~~
Two sequential absorptions of1.012 and 1.3 /1m photons contribute to /hS/S. The first term arises from ~H4-~G4 (1 /1m) +~G4-~D2 (1.3 /1m) transitions, whereas the second term arises from ~H4-~F4 (1.3 /1m) +~F4-~D2 (1 /1m) transitions. The data of Quimby (3] (a161a17
"
0.02) and the experimental value of /hS/S are used to calculate the ratio a781a68. Previously,
we have established that aiL » 1, Wabs " 25.5 mW, P(L)
= 0.5 mW, r7 = 113 ps, ~i
"
/1m, ~2
= 1.3 /1m. We suppose that the non-radiative lifetime r6 of level ~F4 results from multiphonon relaxation. As the mean energy shift bet~veen ~F4 and ~F2+~H6 is 2300 cm~~,
we estimate from reference [6] that r6
" 16 /1s. From the relation (4) and the experimental
value /hS/S
= 2.4 we deduce that a781a68
" 4.I. The spectrum of a78 cross-section has
been established from fluorescence measurements [3]. At 1.31 /1m, the value of a78 is equal to 8 x 10~~~ cm~ which implie8 that a68 "1.9 x10~~~ cm~. This 1.3
/1m experiment allows us
establishing that the /1m excitation of Pr~+ doped Z-B-L-A- is characterized by a ~H4-~G4
cross-section (G.S.A.) 3 times higher than the ~F4-~D2 cross-section (E.S.A.). Higher values of /hS/S, can be obtained by tuniiig the probe wavelength to the maximum of the cross-section
a78 at 1.4 pm (~G4-~D2) or a16 at 1.44 /1m (~H4-~F4), [5].
Conclusion
E-S-A- in Pr~+ doped Z-B-L-A- fibers pumped at 1012 nm has been investigated by a pump- probe beam technique. When the probe is tuned to 830 nm, we establish that a value of 0.67 x 10~~°cm~ for the
room temperature E-S-A- cross-section is in agreement with measure- ments on 830 nm absorption, 640 nm fluorescence saturation, decrease of1.32 /1m fluorescence and the absolute value of the visible fluorescence power. A probe beam tuned to 780 nm allows
deducing a 0.13 x 10~~° cm~ value for the 780 nm room temperature E-S-A- cross-section. Us-
ing a 1.31 /1m probe beam leads to an increase of the 607 nm upconversion emission and allows deducing an 1012 nm E-S-A- cross-section 3 times lower than the G-S-A- one. Implementation
of the pump-probe beam method with convenient and powerful laser diodes has open up the
way for a lot of accurate measurements useful in a routine characterization of Pr~+ doped
Z-B-L-A, fibers.
330 JOURNAL DE PHYSIQUE III N°2
Acknowledgments
The authors express their thanks to G. Maze from "Le Verre FluorA" for kindly supplying the Pr-doped fluoride fiber.
References
[1] Ohishi Y., Kanamori T., Kitagawa T., Takahashi S., Snitzer E. and Sigel G-H- Jr, Pr~+- doped fluoride amplifier operating at 1-31 /1m, Opt. Lett. 16 (1991) 1747-1749; Miyajima Y., Sugawa T. and Fukasaku Y., 38.2 dB amplification at 1.31 /1m and possibility of
0.98 ~tm pumping in Pr~+-doped fluoride fibre, Electron. Lett. 27 (1991) 1706-1707.
[2] Allain J.Y., Monerie M- and Poignant H., Red upconversion Yb-sensitised Pr fluoride fibre laser pumped in 0.8 mm region, Electron. Lett. 27 (1991) l156-l15?; Piehler D-, Craven D., Kwong N- and Zarem H-, Laser-diode pumped red and green upconversion fibre lasers,
Electron. Lett. 29 (1993) 1857-1858.
[3] Quimby R-S- and Zheng B-, New excited-state absorption measurement technique and
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[4] Tropper A-C-, Carter J-N-, Lauder R-D-T-, Hanna D-C-, Davey S-T- and Szebesta D., Analysis of blue and red laser performance of the infrared-pumped praseodymium-doped
fluoride fiber laser, J. Opt- Soc. Am- 811 (1994) 886-893.
[5] Pedersen B., Miniscalco W-J- and Quimby R-S, Optimization of Pr~+. ZBLAN fiber amplifiers, Photon. Technoi- Lett- 4 (1992) 446-448.
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