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Frequency upconversion in Er3+- and Yb3+Er3+ -doped silica fibers
Optics Communications88 (1992) 441-445 North-Holland
OPTICS COMMUNICATIONS
Frequency upconversion in Er3+- and yb3+/Er3+-doped silica fibers Yi-Min Hua, Qu Li, Ying-Li Chen a n d Yi-Xin Chen Department of Applied Physics, Shanghai Jiao Tong University, ShanghaL 200030, China Received 25 June 1991;revised manuscript received 3 October 1991
Frequency upconversionhas been demonstrated in Era+- and yb3+/Er3--dopedsilica optical fibers pumped by 1064 nm radiation. IR to blue upconversionin Er3+-dopedfibers and IR to blue, green and red upconversion in yba+/Era+-doped fibers have been observed. Ground state absorption, excited state absorption, two-photonabsorption and energytransfer processesare proposed to account for the experimental observations.
1. Introduction Upconversion in rare-earth ions such as Nd 3+- [ 1 ], Ho 3+- [2], Tm 3+- [3,4] and y b 3 + / T m 3+- [5] doped fibers recently draws much attention mainly for the possibility to develop visible fiber superfluorescence sources and laser sources pumped in the near infrared. The upconversion process, as first investigated by Auzel [ 6 ], involves stepwise excitation and energy transfer between rare-earth ions in solids. Because of the multiphoton nature of the upconversion process, high pump intensity is required. Optical fibers having the core doped with the rare-earth ions of interest, offer a particularly attractive approach, as even for quite modest power, high pump intensity can be achieved and maintained over long length. In this paper, we report the observation of frequency upconversion at wavelengths of 466 (468) nm in Er 3+/GeO2/SiO2 (Er 3+/AI203/GeO2/SiO2) optical fibers, and of 467, 546 and 667 nm in y b 3 + / Er3+/Ge02/Si02 optical fibers pumped by a cw (continuous wave) Nd 3÷ : YAG laser at 1064 nm. In this experiment, a cw N d : Y A G laser at 1064 nm was used as pumping source. The Er 3+- and yb3+/Er3+-doped silica optical fibers used in this experiment were fabricated by the solution doping technique [ 7 ]. The Er 3+ and Yb 3+/Er a+ concentration and other parameters of the fibers are listed in table 1. A simplified Er 3+ and Yb 3+ energy level dia-
gram is presented in fig. 1. The fibers to be tested here were kept from exposure of high peak power infrared pumping laser, so as to minimize the intensities of second-harmonic generation (SHG) and third-harmonic generation ( T H G ) in the fibers under the excitation of cw infrared laser [ 8 ], because the fibers, having been prepared with high peak power ( > l kW) IR pulses, will generate efficient SHG [9] and THG [10], which makes the upconversion pump mechanism complex. The frequency upconversion spectra were recorded by a 0.5 m spectrometer and an S-20 photomultiplier. And the powers of upconverted fluorescence light were measured at the output end of the fibers by a calibrated power meter. From a simple rate equation model [ 11 ], it can be deduced that for any upconversion mechanism, visible upconverted fluorescence intensity Iup will be proportional to some power n of the infrared excitation intensity/~x, i.e. Iupat:I~x, where n (2, 3, ...) is the number of infrared photons absorbed per upconverted visible photon emitted. The number n can easily be determined from the slope of curve log(/~p)-log(/~x). In order to find the power dependence of upconversion intensities correctly, sidelight intensities of all upconverted visible bands were measured as a function of pump power. Since the optical attenuation at radial direction in the fiber core is extremely low, even near the peak of an absorption
0030-4018/92/$05.00 © 1992 Elsevier Science Publishers B.V. All rights reserved.
441
Volume 88, number 4,5,6
OPTICS COMMUNICATIONS
1 April 1992
Table l Parameters of Era+- and Yb3+/Er a÷-doped silica fibers. Fiber No.
Core dopants Era+ (ppm)
1 2
25
230 30 790 900
3 4 5
Yba+ (ppm)
AI3+ (Wt%)
-
-
2700
0.3 1.0 -
2a (pm)
An
5.7 5.2 6.8 8.9 8.0
0.0126 0.0125 0.0075 0.0043 0.0045
b a n d the intensity of fluorescence light would not be distorted by the ground state reabsorption. 30
__ E n e r g y -- ( 10acre-' )
25
---
G7 K,5/% II
149/2
-22 00
-~'--
712 . H11/2
-
15
10
--
S
a
a/2
4F9/2 9/2 1112
~
- Blue 1312 5 -0
'" R e d
~-
15/2
E r ~+
30 ~
Energy 1103cm
-1 )
A~
r-
25r-
T
|
-
,
E.T. 20
2. Er3*-doped
fibers
F 3/2
J
~"
L
~ - Z ~
E
i
,v.."
'
i l
i
ta ~a
2G7/2 /~/2K1512 --4 -1112
G~,~
Upconverted fluorescence light ( m a i n l y in the LPol m o d e ) had been observed in Er3+-doped fibers p u m p e d by a cw N d : Y A G laser at 1064 nm. A blue fluorescence b a n d together with a very weak red fluorescence b a n d was detected (fig. 2), when the cw IR p u m p power was 60 m W in a 60 cm long Er 3+doped fiber (fiber no. 3). N o S H G a n d / o r T H G was detected, the blue fluorescence intensities, wavelengths a n d b a n d w i d t h s ( f w h m ) of fibers with v a r ious Er a+ doping were measured a n d shown in table 2. First, it can be seen that the more Er a+ d o p a n t t h e stronger upconverted blue fluorescence intensity. For example,
,.,,,
~ --
6/2 7/2 2 4HI1/2
FLUORESCENCE 4G
s~=
912
INTENSITY(A.U,)
-- 4 11312 4 6 ~
i ~i
1"1 ii,!, --B'ue
4~'9/2
-- 4115/2 65Ohm
,
0 Yb
3+
Er
3+
Fig. 1. A simplified energy level diagram of (a) Er3+ and (b) Era+/yb3+" 442
350
450
550
650
750
WAVELENGTh)
Fig. 2. Upconverted spectra of Er3+ doped silica fiber (No. 3) pumped at 1064 nm.
Volume 88, number 4,5,6
OPTICS COMMUNICATIONS
Table 2 Measured upconverted intensities, wavelengthsand bandwidths (fwhm) of Er3+-doped silica fibers. Fiber No.
Upconverted spectra
1 2 3 4
intensity (~tW)
wavelength (rtrn)
bandwidth (nm)
1.1 : 10.1 2.3 10.2
466 466 468 468
13.9 14.1 12.3 12.8
BANDWIDTH(nrn)
14 13 " 12 11 10
. . . .
0
,
1 O0
,
,
,
•
.
.
.
.
.
200
.
.
.
300
.
.
.
400
I:~Jlvl:)(mW)
Fig. 3. IVleasuredbandwidth (fwhm) of emitted radiation (fiber No. 3 ) at 468 nm as a function of pump power. 230 p p m Er 3+ (fiber no. 2) 25 p p m Er 3+ (fiber no. 1 )
1 April 1992
the blue upconverted fluorescence band was also investigated and shown in fig. 4. It can be seen that the blue fluorescence intensity has an approximately cubic (slope 2.9) dependence on excitation intensity. In other words, this band is the result of a three-photon upconversion process. The blue emission band at 466 nm (468 nm with AI3+ ) corresponds to the 4 G 9 / 2 - - - ) 4 1 1 3 / 2 transition. The process o f excitation of 4G9/2 can be explained as follows. The first step is a two-photon (1064 nm) absorption process, which populates the tail of 2H11/2. The second step is a one-photon (1064 n m ) excited state (2H~/2) absorption process, with which the 4G9/2 state is excited. The fact that the blue emission band has a cubic dependence on IR pump intensity and the fact that energy matching conditions are satisfied for both two absorption processes, support above explanation. And another experimental evidence of the 4G9/2---)4113/2 transition is that a very weak upconverted fluorescence band at 650 nm was detected (fig. 2), which corresponds to 4F9/2--.4115/2 transition. Since the one-IR-photon excited state (2H1~/2) absorption process is quasiresonant, most of Er 3+ in 2H11/2 are excited t o 4G9/2, so that the green upconverted fluorescence light is too weak to be detected in our experiment. The population oPFg/2 can only result through the one-IR-pump-photon excited state absorption process, exciting the Er a+ in 4113/2 metastable level (populated through 4G9/2--*4113/2 transition) to 4F9/2.
10.1 ~tW (/up of fiber no. 2) 1.1 I~W (Iupoffiberno. 1) ' FLUORESCENCE INTENSITY(A,U.}
in Er3+-doped fibers no. 1 and no. 2, which have almost same geometric parameters. Second, adding A13+ to Er3+-doped fibers (fibers no. 3 and no. 4), makes the wavelength of blue emission peak shift from 466 to 468 nm and narrows the bandwidth. Although the experimental phenomenon can be partly explained by energy level shift caused by AI3+ doping [ 12 ], it remains a problem to be investigated. The dependence of blue fluorescence bandwidth on IR p u m p power was then investigated (fiber no. 3) and shown in fig. 3. As the IR p u m p power increases from 10 to 380 mW, the bandwidth narrows from 13.8 to 11.5 rim. This behavior is typical of amplified spontaneous emission. Power dependence of
10-2
1 0 -1
10 °
101
10 2
10 3
F:'t_llvlP(r'nW)
Fig. 4. Power dependence of emitted radiation (fiber No. 3) at 468 nm on the excitation power at 1064 nm. 443
Volume 88, number 4,5,6 3.
yb3+/Er3+-doped
OPTICS COMMUNICATIONS
fibers
Upconverted fluorescence light (mainly in the LPo~ mode) has also been observed in Yb 3+/Er3+-doped fibers pumped by a cw N d : Y A G laser at 1064 nm. Three visible bands were detected (fig. 5). The power of the upconverted fluorescence bands at 467, 546 and 667 nm were 4, 100 and 300 ~tW, respectively, when the cw pumping power was I W. No SHG a n d / or T H G was detected either. Power dependence of the three upconverted fluorescence bands were then investigated and shown in fig. 6. It can be seen that the blue fluorescence intensity has an approximately cubic (slope 2.9) dependence on excitation intensity, and that the green and red fluorescence intensities have approximately quadratic (slope 1.9) deF~uorescence int ensity(a.u,) 4S3/2-
4115/2
546nm
I g9/2 - 115/2 66~nm
4 G 9 / 2 __4f13/2 / l
350
450
550 650 Wavelength(rim)
7~0
Fig. 5. UpconvertedspectraofYb3+/Er3+-dopedsilicafiber (No. 5) pumped at 1064 rim. FLI,JORESCENCE INTENSITY(A.U.) •
546r~n
D
~
o
•
•
~..Te c¢C"
10
10 2 PUMP (mW)
Fig. 6. Power dependence of emitted radiation (fiber No. 5 ) at 467,546 and 667 nm bands on the excitation power at 1064 rim. 444
1 April 1992
pendence on excitation intensity. Under higher pump powers, the curves will saturate, especially for those of blue and red bands, which are caused by the depletion of ground and intermediate states populations. The green emission at 546 nm band corresponds to the 453/2---~4Ii5/2 transition. The process of excitation of 453/2 can be explained as follows [ 6 ]. First, the absorption of a 1064 nm photon by a Yb 3÷ leads to the 2F7/2 ---~2F6/2transition. Then, the excitation of an Er 3+ in the 4Ill/2 is realized by the energy transfer (ET) process of excited Yb 3+ to Er 3+. Finally, either the same Yb 3+ which absorbs a second 1064 nm photon or another nearby Yb 3+ being still in the 2F5/2 state, transfers its energy to the same Er 3+, and the process that the Er 3+ in the excited state 4111/2 absorbs a 1064 nm photon is also possible. The Er 3+ reaches 453/2 with phonon assistance. This explanation derives from the facts that the green output follows a quadratic law (fig. 6), indicating that two IR photons are involved, and that energy matching conditions are satisfied for both energy transfer process and excited state absorption [6]. Another experimental evidence is that the y b 3 + / Er3+-doped fiber was measured to have an energy transfer efficiency of 37% for zero population inversion, and an estimated efficiency of 20% for an inversion of 50% [ 13 ]. In addition, the small emission peak at 526 nm corresponds to the 2H11/2--, 4I 15/2 transition, 2Hl 1/2 being thermally populated from 453/2 [14]. The red emission band at 667 nm is attributed to the 4F9/2-,4115/2 transition. There are two main excitation routes proposed for red emission in the yb3+/Er3+-doped fibers. Route A: this route corresponds to the deexcitation of 4 5 3 / 2 towards 4F9/2 through multiphonon interaction. Obviously, the red emission versus pump intensity should follow a quadratic law. Route B: the excited 4Ill/2 relaxes to 4It3/2 through multiphonon interaction, then 4F9/2 is excited by the process of excited state (4113/2) absorption. This route also involves two IR photons, which is very similar to route A, because the energy gaps involved are of the same order. The facts of a high phonon emitting rate in silica and the quadratic law of the red output (fig. 6) confirmed above explanation. Since the intensity of the blue emission band is weak, Ihe route we have dealt with in Er 3+-
Volume 88, number 4,5,6
OPTICS COMMUNICATIONS
doped fiber is neglected here. The blue emission band at 467 n m corresponds to the 4G9/2---,4113/2 transition. One main way o f population Er 3+ in 4G9/2 is through 1064 nm two-photon absorption of Er 3÷ followed by one 1064 n m photon excited state absorption, which is confirmed in the Er3+-doped fibers. The other is through one 1064 nm photon excited states (453/2) absorption or energy transfer processes (Yb 3+ in 2F5/2 to Er a+ in 483/2). Thus the blue emission band has a cubic dependence on excitation power (fig. 6). Comparing the measured upconverted fluorescent spectra o f Er 3+- and yb3+/Er3+-doped fibers, we find that the main upconverted fluorescence is blue in Er3+-doped fibers, and green in y b 3 + / E r 3 + - d o p e d fibers. And the red (blue) fluorescence in y b 3 ÷ / Er3÷-doped fibers is stronger (weaker) than that in Er3+-doped fibers. These phenomena might arise from the difference o f main pumping routes, which is caused by different dopants. The two-photon absorption and excited state absorption are predominant in Er3+-doped fibers. Adding Yb 4+ ions to Er 3+doped silica fibers changes the pumping mechanism in two aspects. One is that the excited Yb 3+ to Er 3+ energy transfer process plays an important role in upconversion. Another is that the slight shifts o f Er 3÷ energy levels in Yb 3+/Er3+-doped fibers might make the quasiresonance o f one-IR-photon excited 2HI i/2 state absorption poor. Further quantitative theoretical analysis is being developed. In summary, we have observed a blue upconventional fluorescence band in Er3÷-doped silica fibers and three visible (blue, green and red) upconverted fluorescence bands in y b 3 + / E r 3 + - d o p e d silica optical fibers, which have the characteristics o f amplified spontaneous emission. The upconverted fluorescence bands are caused by the ground state
1 April 1992
absorption, excited state absorption, two-photon absorption and energy transfer processes.
Acknowledgements The authors wish to thank J.Q. Lin and K.Q. Dai of China Building Materials Academy for providing the Er a +- and Er a + / Y b 3+-doped silica optical fibers, and W.Y. Zhu and Y. Yan o f Shanghai Jiao Tong University for helpful discussions. Funding for this work was provided by the National Science Foundation o f Shanghai Jiao Tong University.
References [1]T.F. Carruthers, I.N. Duling III, C.M. Shaw and E.J. Friebele, Appl. Phys. Lett. 54 (1989) 875. [2 ] J.Y. Allain, M. Monerie and H. Poignant, Electron. Lett. 26 (1990) 261. [ 3 ] J.Y. Allain, M. Monerie and H. Poignant, Electron. Lett. 26 (1990) 166. [4 ] A.S.L.Gomes and Cid B. de Aratijo, CLEO'90 (1990) paper CME2. [ 5] D.C. Hanna, R.M. Percival, I.R. Perry, R.G. Smart, J.E. Townsend and A.C. Tropper, Optics Comm. 78 (1990) 187. [6] F.E. Auzei, Proc. IEEE 61 (1973) 758. [7] B.J. Ainslie, 14th Eur. Conf. Opt. Comm. IEE 292 (1988) 621. [ 8 ] Yi-min Hua, Qu Li, Ying-li Chert and Yi-Xin Chen, Acta Optical Sinica (Chinese) 12 (1992), to be published. [9 ] U. Osterberg and W. Margulis, Optics Lett. 12 (1987) 57. [ 10] U. Osterberg, Electron. Lett. 26 (1990) 103. [ 11 ] M.A. Chamarro and R. Cases, J. Lumin. 46 (1990) 59. [ 12 ] B.J. Ainslie, S.P. Craig, S.T. Davey and B. Wakefield, Mater. Lett. 6 (1988) 139. [13]W.L. Berries, S.B. Poole, J.E. Townsend, L. Reelde, D.J.Taylor and D.N. Payne, J. LightwaveTechnol. 7 (1989) 1461. [14] H. Berthou and C.K. J0rgensen, Optics Lett. 15 (1990)
1100.
445
OPTICS COMMUNICATIONS
Frequency upconversion in Er3+- and yb3+/Er3+-doped silica fibers Yi-Min Hua, Qu Li, Ying-Li Chen a n d Yi-Xin Chen Department of Applied Physics, Shanghai Jiao Tong University, ShanghaL 200030, China Received 25 June 1991;revised manuscript received 3 October 1991
Frequency upconversionhas been demonstrated in Era+- and yb3+/Er3--dopedsilica optical fibers pumped by 1064 nm radiation. IR to blue upconversionin Er3+-dopedfibers and IR to blue, green and red upconversion in yba+/Era+-doped fibers have been observed. Ground state absorption, excited state absorption, two-photonabsorption and energytransfer processesare proposed to account for the experimental observations.
1. Introduction Upconversion in rare-earth ions such as Nd 3+- [ 1 ], Ho 3+- [2], Tm 3+- [3,4] and y b 3 + / T m 3+- [5] doped fibers recently draws much attention mainly for the possibility to develop visible fiber superfluorescence sources and laser sources pumped in the near infrared. The upconversion process, as first investigated by Auzel [ 6 ], involves stepwise excitation and energy transfer between rare-earth ions in solids. Because of the multiphoton nature of the upconversion process, high pump intensity is required. Optical fibers having the core doped with the rare-earth ions of interest, offer a particularly attractive approach, as even for quite modest power, high pump intensity can be achieved and maintained over long length. In this paper, we report the observation of frequency upconversion at wavelengths of 466 (468) nm in Er 3+/GeO2/SiO2 (Er 3+/AI203/GeO2/SiO2) optical fibers, and of 467, 546 and 667 nm in y b 3 + / Er3+/Ge02/Si02 optical fibers pumped by a cw (continuous wave) Nd 3÷ : YAG laser at 1064 nm. In this experiment, a cw N d : Y A G laser at 1064 nm was used as pumping source. The Er 3+- and yb3+/Er3+-doped silica optical fibers used in this experiment were fabricated by the solution doping technique [ 7 ]. The Er 3+ and Yb 3+/Er a+ concentration and other parameters of the fibers are listed in table 1. A simplified Er 3+ and Yb 3+ energy level dia-
gram is presented in fig. 1. The fibers to be tested here were kept from exposure of high peak power infrared pumping laser, so as to minimize the intensities of second-harmonic generation (SHG) and third-harmonic generation ( T H G ) in the fibers under the excitation of cw infrared laser [ 8 ], because the fibers, having been prepared with high peak power ( > l kW) IR pulses, will generate efficient SHG [9] and THG [10], which makes the upconversion pump mechanism complex. The frequency upconversion spectra were recorded by a 0.5 m spectrometer and an S-20 photomultiplier. And the powers of upconverted fluorescence light were measured at the output end of the fibers by a calibrated power meter. From a simple rate equation model [ 11 ], it can be deduced that for any upconversion mechanism, visible upconverted fluorescence intensity Iup will be proportional to some power n of the infrared excitation intensity/~x, i.e. Iupat:I~x, where n (2, 3, ...) is the number of infrared photons absorbed per upconverted visible photon emitted. The number n can easily be determined from the slope of curve log(/~p)-log(/~x). In order to find the power dependence of upconversion intensities correctly, sidelight intensities of all upconverted visible bands were measured as a function of pump power. Since the optical attenuation at radial direction in the fiber core is extremely low, even near the peak of an absorption
0030-4018/92/$05.00 © 1992 Elsevier Science Publishers B.V. All rights reserved.
441
Volume 88, number 4,5,6
OPTICS COMMUNICATIONS
1 April 1992
Table l Parameters of Era+- and Yb3+/Er a÷-doped silica fibers. Fiber No.
Core dopants Era+ (ppm)
1 2
25
230 30 790 900
3 4 5
Yba+ (ppm)
AI3+ (Wt%)
-
-
2700
0.3 1.0 -
2a (pm)
An
5.7 5.2 6.8 8.9 8.0
0.0126 0.0125 0.0075 0.0043 0.0045
b a n d the intensity of fluorescence light would not be distorted by the ground state reabsorption. 30
__ E n e r g y -- ( 10acre-' )
25
---
G7 K,5/% II
149/2
-22 00
-~'--
712 . H11/2
-
15
10
--
S
a
a/2
4F9/2 9/2 1112
~
- Blue 1312 5 -0
'" R e d
~-
15/2
E r ~+
30 ~
Energy 1103cm
-1 )
A~
r-
25r-
T
|
-
,
E.T. 20
2. Er3*-doped
fibers
F 3/2
J
~"
L
~ - Z ~
E
i
,v.."
'
i l
i
ta ~a
2G7/2 /~/2K1512 --4 -1112
G~,~
Upconverted fluorescence light ( m a i n l y in the LPol m o d e ) had been observed in Er3+-doped fibers p u m p e d by a cw N d : Y A G laser at 1064 nm. A blue fluorescence b a n d together with a very weak red fluorescence b a n d was detected (fig. 2), when the cw IR p u m p power was 60 m W in a 60 cm long Er 3+doped fiber (fiber no. 3). N o S H G a n d / o r T H G was detected, the blue fluorescence intensities, wavelengths a n d b a n d w i d t h s ( f w h m ) of fibers with v a r ious Er a+ doping were measured a n d shown in table 2. First, it can be seen that the more Er a+ d o p a n t t h e stronger upconverted blue fluorescence intensity. For example,
,.,,,
~ --
6/2 7/2 2 4HI1/2
FLUORESCENCE 4G
s~=
912
INTENSITY(A.U,)
-- 4 11312 4 6 ~
i ~i
1"1 ii,!, --B'ue
4~'9/2
-- 4115/2 65Ohm
,
0 Yb
3+
Er
3+
Fig. 1. A simplified energy level diagram of (a) Er3+ and (b) Era+/yb3+" 442
350
450
550
650
750
WAVELENGTh)
Fig. 2. Upconverted spectra of Er3+ doped silica fiber (No. 3) pumped at 1064 nm.
Volume 88, number 4,5,6
OPTICS COMMUNICATIONS
Table 2 Measured upconverted intensities, wavelengthsand bandwidths (fwhm) of Er3+-doped silica fibers. Fiber No.
Upconverted spectra
1 2 3 4
intensity (~tW)
wavelength (rtrn)
bandwidth (nm)
1.1 : 10.1 2.3 10.2
466 466 468 468
13.9 14.1 12.3 12.8
BANDWIDTH(nrn)
14 13 " 12 11 10
. . . .
0
,
1 O0
,
,
,
•
.
.
.
.
.
200
.
.
.
300
.
.
.
400
I:~Jlvl:)(mW)
Fig. 3. IVleasuredbandwidth (fwhm) of emitted radiation (fiber No. 3 ) at 468 nm as a function of pump power. 230 p p m Er 3+ (fiber no. 2) 25 p p m Er 3+ (fiber no. 1 )
1 April 1992
the blue upconverted fluorescence band was also investigated and shown in fig. 4. It can be seen that the blue fluorescence intensity has an approximately cubic (slope 2.9) dependence on excitation intensity. In other words, this band is the result of a three-photon upconversion process. The blue emission band at 466 nm (468 nm with AI3+ ) corresponds to the 4 G 9 / 2 - - - ) 4 1 1 3 / 2 transition. The process o f excitation of 4G9/2 can be explained as follows. The first step is a two-photon (1064 nm) absorption process, which populates the tail of 2H11/2. The second step is a one-photon (1064 n m ) excited state (2H~/2) absorption process, with which the 4G9/2 state is excited. The fact that the blue emission band has a cubic dependence on IR pump intensity and the fact that energy matching conditions are satisfied for both two absorption processes, support above explanation. And another experimental evidence of the 4G9/2---)4113/2 transition is that a very weak upconverted fluorescence band at 650 nm was detected (fig. 2), which corresponds to 4F9/2--.4115/2 transition. Since the one-IR-photon excited state (2H1~/2) absorption process is quasiresonant, most of Er 3+ in 2H11/2 are excited t o 4G9/2, so that the green upconverted fluorescence light is too weak to be detected in our experiment. The population oPFg/2 can only result through the one-IR-pump-photon excited state absorption process, exciting the Er a+ in 4113/2 metastable level (populated through 4G9/2--*4113/2 transition) to 4F9/2.
10.1 ~tW (/up of fiber no. 2) 1.1 I~W (Iupoffiberno. 1) ' FLUORESCENCE INTENSITY(A,U.}
in Er3+-doped fibers no. 1 and no. 2, which have almost same geometric parameters. Second, adding A13+ to Er3+-doped fibers (fibers no. 3 and no. 4), makes the wavelength of blue emission peak shift from 466 to 468 nm and narrows the bandwidth. Although the experimental phenomenon can be partly explained by energy level shift caused by AI3+ doping [ 12 ], it remains a problem to be investigated. The dependence of blue fluorescence bandwidth on IR p u m p power was then investigated (fiber no. 3) and shown in fig. 3. As the IR p u m p power increases from 10 to 380 mW, the bandwidth narrows from 13.8 to 11.5 rim. This behavior is typical of amplified spontaneous emission. Power dependence of
10-2
1 0 -1
10 °
101
10 2
10 3
F:'t_llvlP(r'nW)
Fig. 4. Power dependence of emitted radiation (fiber No. 3) at 468 nm on the excitation power at 1064 nm. 443
Volume 88, number 4,5,6 3.
yb3+/Er3+-doped
OPTICS COMMUNICATIONS
fibers
Upconverted fluorescence light (mainly in the LPo~ mode) has also been observed in Yb 3+/Er3+-doped fibers pumped by a cw N d : Y A G laser at 1064 nm. Three visible bands were detected (fig. 5). The power of the upconverted fluorescence bands at 467, 546 and 667 nm were 4, 100 and 300 ~tW, respectively, when the cw pumping power was I W. No SHG a n d / or T H G was detected either. Power dependence of the three upconverted fluorescence bands were then investigated and shown in fig. 6. It can be seen that the blue fluorescence intensity has an approximately cubic (slope 2.9) dependence on excitation intensity, and that the green and red fluorescence intensities have approximately quadratic (slope 1.9) deF~uorescence int ensity(a.u,) 4S3/2-
4115/2
546nm
I g9/2 - 115/2 66~nm
4 G 9 / 2 __4f13/2 / l
350
450
550 650 Wavelength(rim)
7~0
Fig. 5. UpconvertedspectraofYb3+/Er3+-dopedsilicafiber (No. 5) pumped at 1064 rim. FLI,JORESCENCE INTENSITY(A.U.) •
546r~n
D
~
o
•
•
~..Te c¢C"
10
10 2 PUMP (mW)
Fig. 6. Power dependence of emitted radiation (fiber No. 5 ) at 467,546 and 667 nm bands on the excitation power at 1064 rim. 444
1 April 1992
pendence on excitation intensity. Under higher pump powers, the curves will saturate, especially for those of blue and red bands, which are caused by the depletion of ground and intermediate states populations. The green emission at 546 nm band corresponds to the 453/2---~4Ii5/2 transition. The process of excitation of 453/2 can be explained as follows [ 6 ]. First, the absorption of a 1064 nm photon by a Yb 3÷ leads to the 2F7/2 ---~2F6/2transition. Then, the excitation of an Er 3+ in the 4Ill/2 is realized by the energy transfer (ET) process of excited Yb 3+ to Er 3+. Finally, either the same Yb 3+ which absorbs a second 1064 nm photon or another nearby Yb 3+ being still in the 2F5/2 state, transfers its energy to the same Er 3+, and the process that the Er 3+ in the excited state 4111/2 absorbs a 1064 nm photon is also possible. The Er 3+ reaches 453/2 with phonon assistance. This explanation derives from the facts that the green output follows a quadratic law (fig. 6), indicating that two IR photons are involved, and that energy matching conditions are satisfied for both energy transfer process and excited state absorption [6]. Another experimental evidence is that the y b 3 + / Er3+-doped fiber was measured to have an energy transfer efficiency of 37% for zero population inversion, and an estimated efficiency of 20% for an inversion of 50% [ 13 ]. In addition, the small emission peak at 526 nm corresponds to the 2H11/2--, 4I 15/2 transition, 2Hl 1/2 being thermally populated from 453/2 [14]. The red emission band at 667 nm is attributed to the 4F9/2-,4115/2 transition. There are two main excitation routes proposed for red emission in the yb3+/Er3+-doped fibers. Route A: this route corresponds to the deexcitation of 4 5 3 / 2 towards 4F9/2 through multiphonon interaction. Obviously, the red emission versus pump intensity should follow a quadratic law. Route B: the excited 4Ill/2 relaxes to 4It3/2 through multiphonon interaction, then 4F9/2 is excited by the process of excited state (4113/2) absorption. This route also involves two IR photons, which is very similar to route A, because the energy gaps involved are of the same order. The facts of a high phonon emitting rate in silica and the quadratic law of the red output (fig. 6) confirmed above explanation. Since the intensity of the blue emission band is weak, Ihe route we have dealt with in Er 3+-
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OPTICS COMMUNICATIONS
doped fiber is neglected here. The blue emission band at 467 n m corresponds to the 4G9/2---,4113/2 transition. One main way o f population Er 3+ in 4G9/2 is through 1064 nm two-photon absorption of Er 3÷ followed by one 1064 n m photon excited state absorption, which is confirmed in the Er3+-doped fibers. The other is through one 1064 nm photon excited states (453/2) absorption or energy transfer processes (Yb 3+ in 2F5/2 to Er a+ in 483/2). Thus the blue emission band has a cubic dependence on excitation power (fig. 6). Comparing the measured upconverted fluorescent spectra o f Er 3+- and yb3+/Er3+-doped fibers, we find that the main upconverted fluorescence is blue in Er3+-doped fibers, and green in y b 3 + / E r 3 + - d o p e d fibers. And the red (blue) fluorescence in y b 3 ÷ / Er3÷-doped fibers is stronger (weaker) than that in Er3+-doped fibers. These phenomena might arise from the difference o f main pumping routes, which is caused by different dopants. The two-photon absorption and excited state absorption are predominant in Er3+-doped fibers. Adding Yb 4+ ions to Er 3+doped silica fibers changes the pumping mechanism in two aspects. One is that the excited Yb 3+ to Er 3+ energy transfer process plays an important role in upconversion. Another is that the slight shifts o f Er 3÷ energy levels in Yb 3+/Er3+-doped fibers might make the quasiresonance o f one-IR-photon excited 2HI i/2 state absorption poor. Further quantitative theoretical analysis is being developed. In summary, we have observed a blue upconventional fluorescence band in Er3÷-doped silica fibers and three visible (blue, green and red) upconverted fluorescence bands in y b 3 + / E r 3 + - d o p e d silica optical fibers, which have the characteristics o f amplified spontaneous emission. The upconverted fluorescence bands are caused by the ground state
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absorption, excited state absorption, two-photon absorption and energy transfer processes.
Acknowledgements The authors wish to thank J.Q. Lin and K.Q. Dai of China Building Materials Academy for providing the Er a +- and Er a + / Y b 3+-doped silica optical fibers, and W.Y. Zhu and Y. Yan o f Shanghai Jiao Tong University for helpful discussions. Funding for this work was provided by the National Science Foundation o f Shanghai Jiao Tong University.
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