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Spectroscopic analysis of Er3+ transitions in lithium niobate
JOURNAL OF
LUMINESCENCE EISEVIER
Journal
of Luminescence
69 (1996) 17.-26
Spectroscopic analysis of Er3 + transitions in lithium niobate J. Amin”? *, B. Dussardierb, T. Schweizer”, M. Hempstead” bLahorataire
aOptoelectronics Research Centre, University of Southampton. Southampton, SOI 7 IBJ, UK de Physique de la MatiPre Condenske. Universit2 de Nice-Sophia Antipolis, Part Valrose. F-06108 Nice Cede.r 2. France Received
7 March
1996; accepted
2 May
1996
Abstract
We report on the results of a spectroscopic analysis of the transition strengths of Er-doped LiNb03. The line strengths of several transitions from the ground state to excited state manifolds are evaluated from measured polarised absorption spectra and analysed using Judd-Ofelt theory, taking into account the crystal anisotropy. The measured and calculated oscillator strengths at 300 K compare favourably within the typical uncertainties associated with the Judd-Ofelt theory. The Judd-Ofelt parameters obtained have been used to evaluate the excited-state-absorption (ESA) transition strengths from the 4111,~ level, which have important implications for pumping Er: LiNb03 devices. Keywords: Er-doped LiNb03; spectroscopy
1. Introduction The development of erbium-doped fibre lasers and amplifiers has recently generated an interest in rare-earth-doped integrated optical devices in various host materials for applications in optical communications, high resolution spectroscopy and optical sensing. In particular, Er-doped LiNb03 is extremely attractive as it permits a high degree of integration through a combination of the mature optical waveguiding techniques, optical gain introduced by the active Er3+ ions and good intrinsic electro-optic and acousto-optic properties of the host. Several sophisticated devices have recently been demonstrated in this material system, taking advantage of the above attributes [l, 23. * Corresponding Broadway.
author. Presently at NIST, Boulder, CO 80303, USA.
Div. 815.04, 325
0022-23 13/96/$15.00 0 1996 Elsevier Science B.V. All rights reserved PII SOO22-23 13(96)00063-4
To exploit this material system fully, it is important to know the spectral characteristics of the dopant-host combination and to quantify radiative and non-radiative transitions between different excited levels. To this end, a great deal of research has been undertaken recently to determine the spectroscopic properties of Er:LiNb03. Studies by GSg et al. [3] and Milori et al. [4], using X-ray standing wave analysis and electron-spin resonance techniques, respectively, have shown that Er3+ ions in LiNb03 exist in positions close to Lit sites, but slightly shifted along the --z direction, with a C3 point symmetry. Low-temperature absorption spectroscopy has been carried out to determine the Stark level splitting for the ground state t41, 5,2) and the upper laser level (4113,2) of the Er3+ ion in the crystalline LiNb03 host, and room-temperature absorption and emission cross-sections have been determined through the use of McCumber theory
18
J. Amin et al. / Journal of Luminescence
and the density matrix formalism [S-7]. The lowtemperature absorption spectra have revealed the existence of at least two sites for the Er3+ ion in the host lattice. This has been corroborated by siteselective spectroscopy, which indicates the existence of several non-equivalent sites [8,9]. However, the non-equivalent sites are generally believed to be Li+ sites, with the Er3+ ions at slightly different off-centred positions around the Li+ location. The optical 4f transitions of rare-earth ions in glasses and crystals are generally characterised using Judd-Ofelt (J-O) theory [lo, 111, which allows key laser parameters, such as fluorescence transition probabilities, radiative lifetimes, and excited-state-absorption (ESA) transition strengths to be calculated from measured absorption intensities. Despite the great interest in the Er:LiNbO, system, a thorough Judd-Ofelt analysis has, to our knowledge, not yet been carried out. The only published Judd-Ofelt analysis for Er:LiNbO, was carried out using unpolarised light propagating perpendicularly to the crystalline z-axis, and assuming the absorption spectra thus obtained to be an average of the two polarisations [12]. However, due to the uniaxial nature of LiNb03, there are strong absorption and emission differences between extraordinary(Ellz) and ordinary(El_z) polarised modes for rare-earth ions situated at sites with a C3 local symmetry along the trigonal axis (z-axis), and we have found that the Judd-Ofelt parameters differ for unpolarised and polarised spectra considerably. In this paper, therefore, we present a detailed Judd-Ofelt analysis for Er:LiNb03 using measured polarised absorption spectra and calculating the line strengths by summation over both polarisations, as described in Ref. [13]. The averaged J-O parameters obtained from this polarisationdependent analysis have been used to obtain values for (ESA) transition strengths. Decay rates for transitions to the ground state have been measured at room temperature for four different excited levels, and the results are compared to radiative transition rates calculated from the J-O theory. The results are discussed in comparison with published data for a variety of erbium-doped glasses.
69 (1996) 17-26
2. Experimental Room-temperature, polarisation-resolved absorption experiments were carried out on a congruent, 0.95 mm thick, x-cut sample of LiNb03, provided by Deltronics. The sample was doped with Er3+ ions in the bulk, to a concentration of 1.5 x 1Or9cmm3. The absorption spectra from 300 to 1600 nm were recorded on a Perkin Elmer Lambda 9 spectrophotometer, using an undoped sample of similar dimensions in the reference arm. The resolution of the spectrophotometer for these measurements was better than 0.5 nm. Fluorescence-lifetime measurements were carried out on a 1 mm thick z-cut Er:LiNbO, with concentration of sample a dopant 5 x 1019 cmp3, supplied by the University of Strathclyde. This higher dopant level was chosen to allow collection of a significant fluorescence signal whilst keeping shortening of the lifetimes due to cross-relaxation and up-conversion effects to a minimal level. Four cw laser sources, at 488, 660, 975 and 1480 nm, were used to excite the ions to the 4S3,2, 4F9,z, 411,,z and 4113,z levels respectively, and the decay from each of these levels to the thereby 4115,2 ground state was monitored, allowing the true fluorescence lifetimes of these states to be observed. The excitation powers were kept to a low level ( < 50 mW) to reduce shortening of the lifetime due to up-conversion. The pump laser signal was in each case focused at the very edge of the sample and side fluorescence was collected from the same edge, in order to minimise lifetime lengthening effects due to reabsorption in the three-level erbium system. Discrimination of the pump and fluorescence was obtained by measuring the decay signals through a 300 mm Bentham monochromator with a 750 nm blazed grating. Table 1 summarises the energy levels evaluated, together with the wavelength of the analysed fluorescence and the excitation and detection systems used.
3. Theory The Judd-Ofelt analysis, used to determine the probability of radiative transitions of the rare-earth
J. Amin et al. 1 Journal of Luminescence 69 (1996) 17- 26 Table 1 Experimental
conditions
for fluorescence
Level
Excitation
source
Air-cooled
19
detection Excitation
Fluorescence
wavelength
(nm)
(nm)
488
559
Detector
wavelength
Photo-multiplier
Ar + -laser 4Fg z
Dye laser
Y I,,2
InGaAs
‘+I13>2
GaInAsP
laser diode laser diode
660
666
Photo-multiplier
915
981
SI-APD
1480
1530
InCaAs
ions, has been well documented since the development of the technique by Judd and Ofelt [S-15], so we present only a brief summary, as relevant to this study. The theory allows, through the phenomenological Judd-Ofelt parameters R,, the determination of intra-shell radiative transition probabilities within the ion configuration. Once determined these parameters may be used to calculate the strength of any radiative transition for the particular dopant-host combination. Moreover, radiative lifetimes and fluorescence branching ratios, which may be derived using the transition probabilities, are particularly important in the determination of the efficiency of lasers and amplifiers, and may also be used to determine stimulated emission cross-sections through the measurement of emission spectra. The analysis in this paper has been carried out in the MKS (SI) unit system. According to the J-O theory, the electric-dipole (ed) oscillator strengths, in an isotropic material, from level a to h can be expressed by
where L + 2s is the md operator, h is the Planck’s constant, m is the mass of an electron, and L’is the speed of light in vacuum. The host-independent matrix elements of L + 29 and U@) have been numerically evaluated for Er3+ ion transitions and are tabulated in Ref. [16]. The corresponding transition strength from level a to h can be expressed in terms of Sed and $,,d as follows:
i
+ X&$!!
,
(3)
where I is the mean wavelength of the transition between the two manifolds, and J, is the total angular momentum of level a according to the Russel-Saunders notation. The degeneracy of the level is defined by 25, + 1. X,d and Xmd are the local field corrections due to the crystalline host material which, for an isotropic material of refractive index n, are given by Xed
=
(no-l2+ a2
and
9n(i)
(1)
where (ali U”‘llb) are doubly reduced matrix elements of the tensor operator UC’),independent of the host material, and e is the electronic charge. The host dependence in the above equation is carried in the three Q parameters. The line strength for magnetic-dipole (md) transitions between J manifolds is
X.d+ r
Xmd = n(/?)
For uniaxial crystals such as LiNb03, the local field corrections may be made by averaging over the extraordinary (7~)and ordinary (B) polarisations [13] so that X ed(md)
Xed(mdLn =
+
2Xed(md).n
3
The experimental transition strengths can be found by measurement of the integrated absorption
20
J. Amin et al. 1 Journal of Luminescence 69 (1996) 17-26
coefficient for transitions between the ground state and upper levels, and then using the expression f(a, b),,,,
=
4mEoc2
ln( 10). OD(;1) d2
Ne2d(;Z)2s
- ln(lO)*OD(&]
+*exp[
-ln(lO)~OD(Q,J}d;l.
(7)
The theoretical oscillator strength, given by Eq. (3) can then be fitted to the experimental one, given by Eqs. (6) and (7), using least-squares fitting, allowing the extraction of the J-O parameters 52,. The Einstein A coefficient, which gives the rate of spontaneous emission between two levels a and b, is given by A(a+b)
=
16n3n2 3hso(2J, + 1);i”
(Xed&d
+
XmdSmd).
(8)
The radiative lifetime of level a may be extracted from the emission probability, using the following expression:
where the summation is carried out over all final levels b. The fluorescence branching ratio for the transition a + b is defined as the ratio of the relaxation rate for this transition to the total sum of all spontaneous emission probabilities from level a, i.e. B(a, b) = r,A(a, b).
(10)
The measured lifetime of the fluorescence decay from level a combined with the radiative lifetime calculated using J-O theory allows the calculation
efficiency through
the
(11)
(6)
where N is the number of active ions per unit volume, OD(n) is the measured optical density, defined as [ - log,, T(1)], where T is the transmission through the material, and d is the thickness of the material. For an anisotropic material, two measurements are required for the optical density, one along each of the extraordinary and ordinary axes, with polarised beams, thus giving OD(l), and OD(l),. To simulate an isotropic material, the integral in Eq. (6) may then be replaced by [13] Jln{iexp[
of the radiative quantum relation
4. Results and discussion (a) Measured absorption spectra: Fig. 1 shows the room-temperature absorption spectra for nine different transitions from the ground state to excited levels, measured as detailed above. The spectra for both the ordinary and extraordinary polarisations are shown in the figures, and the relevant transitions from the ground state have been marked. The absorption spectra for adjacent energy levels do not overlap, thereby allowing each transition to be analysed separately. (b) Transition strengths: ‘,Table 2 shows the measured oscillator strengths, obtained from the absorption spectra using Eqs. (6) and (7). The mean of the transitions from the wavelengths 41ls,2 ground state are shown in the table. The measured transition strengths have a systematic error of the order of 18%, due to uncertainties in the concentration of Er in the sample and the thickness of the sample, assumed to be of the order of + 0.15 x 10” cme3 and + 0.05 mm respectively_ The estimated eriors in column 4 of Table 2 do not include this systematic error, and take account only of uncertainties in the integration of the absorption spectra due to noise in the measurements and interpretation of the peak values and the limits of the spectra. (c) Judd-Ofelt parameters: All the transitions, except for the 41,,,2 + 41i3i2 transition, were assumed to be electric dipole in nature. The magneticdipole contribution to the transition strength for the 41ls,z -+ 4I13,2 transition was calculated to be 67 x 10m8, and was subtracted from the measured transition strength to give the purely electric dipole transition strength. Values of the host-independent matrix elements as tabulated for Er3’ ions in Ref. [16] were used, and Xed and Xmd were calculated using the wavelength-dependent Sellmeier equation for the refractive index of LiNb03 at room temperature [17]. Using Eq. (3), the Judd-Ofelt
21
J. Amin et al. / Journal of Luminescence 69 (199~5) 17- 26
‘h2 -
a-polarised
-
cpolarised
- -
x-polarised
- -
n-polarised
-0.002 -I 1440
1480
950
1600
1560
1520
960
970
Wavelengthlnm 0.016
0.08 %/2
0.014
990
1000
1010
i
1020
I
ZHIv2
0.07
__ --
0.06
0.012
$
980
Wavelengthlnm
a-polarised n-polarised
0.010
3
0.008
i 2
0.006 0.004 0.002 0.000
I
-0.01 ,, ,, ,,, ,, ,, ,, 640
650
670
660
680
690
460
,/,,(
480
,,,,
500
Wavelengthlnm
-,
,.
520
,,,
540
__I
560
Wavelength/nm
_
I
i
0.08
1
‘G11n
0.06 5d g 0.04
I
-
o-polarisation
--
n-polarisation
-
2
1....,...,,,‘.,,,..,,.“‘,“.‘,‘.“1 350
360
370
360
390
400
410
420
Wavelengthlnm
Fig. I. Polarised absorption transitions from the ground
spectra of Er: LiNb03, doped to a concentration state. 4115,2, to the levels indicated.
of 1.5 x 1Ol9 cmm3. The spectra
shown are for absorption
22
J. Amin et al. / Journal of Luminescence
69 (1996) 17-26
Table 2 Measured and calculated oscillator strengths. All transitions are from the 41,5,1 level to the levels indicated. All the levels have been analysed separately in the J-O calculations Level
411312 41I I,,? ‘+Fg,z % 3/2 *H II,2 4F~,z ‘Hw 4G 1I,2 ‘Gw
Average wavelength (nm) 1522 982 661 548 525 488 410 381 369
Estimated error in fk.%llO- 8 280 84 430 68 2100 450 140 4050 390
19 5 10 9 110 46 22 110 34
parameters were adjusted to give the best fit to the measured transition strengths. The calculated J-O intensity parameters are as follows: Q2 = (7.29 & 1.50) x 10-20 cm-‘, Q4 = (2.24 + 0.48) x 10-20 cm-‘,
The value of Q2 is large compared to the other two parameters, possibly due to the rare-earth ions being in a highly polarised environment [l&19]. In the case of erbium, the transitions to the 2H1 i,2 and the 4G 1l,2 levels are hypersensitive, i.e . UC2)is large, and, reflecting the fact that Q2 is also large, these transitions are very strong compared to the other ground state absorptions [20]. Table 2 shows the calculated transition strengths compared with the measured ones, and also gives the deviation between the two values. The transition strengths given in the table are the total strengths, resulting from both ed and md contributions. The rms deviation, which allows the validity of the intensity parameters obtained by the fit to be evaluated, is given by 6*Ins=
C(deviations)2 no. of levels - no. of parameters
112 f (14
300 120 460 89 2070 410 150 4060 370
8
Deviation fcalE-“&../10-
8
20 36 30 21 - 30 - 40 10 10 - 20
For the above parameters, 6,,, is found to be 31 x lo-‘, which is within the typical uncertainties associated with these calculations [14]. Our J-O intensity parameters are significantly different from those in Ref. [12]. However, we believe this to be due to unpolarised light being used to measure the absorption cross-sections in Ref.
WI.
fib = (1.27 + 0.35) x 1O-2o cm-2.
I&/10-
(d) Transition rates and quantum ejjkiencies: Table 3 summarises the calculated total radiative lifetimes of the various energy levels and the branching ratios for each level. The measured lifetimes for the 4113,2,4111j2,4Fs,2 and 4S3,z levels are also indicated in the table. The decay characteristics of the four measured levels were all single exponential in nature. The presence of dominant non-radiative decay mechanisms is clearly indicated by the measured lifetime being considerably lower than that calculated for the 4111,2, 4Fs,2 and 4S3,2 levels. However, the reason for the anomalous behaviour of the measured 4113,2 lifetime, where the calculated radiative lifetime of 2.3 ms is much shorter than the experimentally measured value of 3.6 ms, is not known at present. Reported measured lifetimes of the 1500 nm fluorescence vary from 2.6 ms [21] to 7 ms [22], and it is also known that the lifetime of the 4I13iz level can vary significantly depending upon the conditions of excitation due to radiation trapping [23-251. However, as mentioned previously, care has been taken
J. Amin et al. / Journal of Luminescence
Table 3 Lifetimes Level
and branching
ratios
Calculated radiative (ms)
41 1J2 ‘+IIL 2 4FW 4sW ZH 1L/Z ‘V? 2 2Hq,2 4G 11:z 2GW
2.3 + 0.4 1.7 + 0.3 0.17 + 0.03 0.172 + 0.036 0.028 + 0.005 0.06 k 0.01 0.074 * 0.008 0.006 1_ 0.001 0.009 & 0.00 1
of the various
excited levels. Band 0 is the ground
Measured lifetime (ms)
Band no.
3.6 +_ 0.2 0.2 * 0.01 0.0018 k 0.0002 0.028 + 0.005
1 2 3 4 5 6 7 8 9
in this work to ensure that the pump was focused at the edge of the sample and that the side fluorescence was detected from the same edge, thus ensuring that trapping effects were kept at a minimum. We also believe that effects due to up-conversion and ion-ion interactions are negligible, due to the low concentration of the sample (5 x 10” cmP3) and the low excitation power used ( < 50 mW). Although the J-O analysis has an inherent inaccuracy of 2&30%, which has not been taken into account in our calculations, this can only be partially responsible for the difference in measured and calculated lifetimes. We are therefore currently investigating this discrepancy further. Assuming though that the non-radiative decay from the 41,3,, is negligible, the quantum efficiency of this level can be taken to be close to loo%, and that of the 4111,2, 4Fg,z and 4S3,2 levels is calculated, from Eq. (11) to be ll%, 1% and 16.3%, respectively. The non-radiative multiphonon decay rate W,, for these levels may be given by 1 1 w,, = ___ - r meaS r,
23
69 (1996) 17-26
state, 41,,,,
Branching
Ratios (X)
0
1
2
3
4
5
6
I
8
100 86 91 69 96 16 39 90 16
14 5 26 1 15 45 7 73
3 2 1 6 11 0 5
0 0 0 1 2 4
0 0 0 0 0
0 2 0 0
0 0 1
0 0
0
expressed as W,, = Cexp(
-a AE),
(14)
where AE is the energy difference between the excited level and the next level below this, and C and a are positive constants characteristic of the host. In Fig. 2, AE for the levels 4S3,2, 4Fg,, and 4111129 as measured in Ref. [26], are plotted against the multiphonon emission rate W,,. From this
le+6
,
\
-1
(13)
where ra is the radiative lifetime as calculated from Eq. (9). The empirical “energy gap law” [14] may then be used to determine the exponential dependence of the multiphonon emission rate on the energy gap between consecutive levels. This can be
2500
3000
3500
4oCw
Energy gap AWcm-’
Fig. 2. Multiphonon emission rates of Er: LiNb03 against AE, the energy gap to the next lowest level, at room temperature.
24
J. Amin et al. / Journal of Luminescence
curve, we obtain c=4.4x10”s-’ and c( = 5.5 x 1O-3 cm-‘. These parameters may then be used to determine the non-radiative decay rates for any other excited level. (e) Excited-state absorption: Er-doped integrated optical devices in LiNb03 reported thus far have almost exclusively been pumped at 1480 nm, mainly due to the reduced photorefractive damage at this wavelength, and also due to the fact that the waveguides can readily be made single-moded at both pump and signal wavelength. However, amplification measurements in Er-doped fibres have shown that pumping at 980 nm provides the best performance in terms of gain, gain efficiency (in dB/mW) and signal-to-noise ratios [27]. Pumping at 980 nm is also advantageous in that it readily allows the fabrication of compact WDMs for independent routing of the pump and signal light on the same chip, as has been demonstrated in a glass waveguide circuit [28]. However, pumping at 980 nm in Er: LiNb03 is complicated by the long lifetime of the 4111,2 level (compared to common silicate and phosphate glasses) and a two-step resonant excited-state-absorption (ESA) from the pump level, 4111,z, to the 4F7,2 level, as shown in Fig. 3. In the ESA process, the ions decay through rapid non-radiative processes from the 4F,jz level to the 4S3,z level, and from there to the ground state through a combination of radiative and nonradiative processes, as evidenced by a bright green fluorescence from this level. In a material system where ESA at 980 nm can be ignored, a long lifetime of the pump level leads to “bottlenecking”, with the limited rate of decay of ions to the metastable level resulting in decreased population inversion and lower gain. However, in a material such as Er:LiNbO, where there is a higher energy level resonant with the pump transition, the relatively large population of ions in the 4111,2 state absorb incoming photons and thus decrease the amplifier efficiency at high excitation levels [29]. As a first step towards a study of ESA in this system, we have used the calculated J-O parameters to evaluate the ESA transition strengths from the 4111,2level (Table 4). The peak wavelength for each transition has been calculated using the difference in average energy of each level (Table 2), and may therefore be different from that observed
69 (1996) i7-26
‘5,*
ESA 980 nm
Fluorescence 1530 nm
GSA 1480 nm
GSA 980 nm
la
Fig. 3. Energy level structure of E?+ions, mechanism at 980 nm.
showing the ESA
Table 4 ESA oscillator strengths from the 4111i, level Level
.M10-s
Wavelength (nm)
‘+F,,, % 3/z 2H ll,Z 4F~,, ‘H,,, ‘Yi Il,Z ‘%z
223 19 117 182 183 46 408
2022 1239 1128 968 703 623 590
experimentally. Table 5 compares the J-O parameters, the GSA (ground-state-absorption) and ESA transition strengths and the lifetime for the 4z 11,2 level of Er: LiNb03 and various glasses doped with erbium [14,30]. To our knowledge, of the material systems included in Table 5, the only ones which have been pumped efficiently at 980 nm are the phosphate and silicate glass systems [31]. In
J. Amin et al. 1 Journal
Table 5 Oscillator
strengths
Material
ZBLAN” ZBLAb Fluorophosphate (high F) Fluorophosphate (low F) Phosphate” Silicate” LiNbO,
for the Er 4115,2 + 41, 1,2 GSA transition
qf Luminescence
25
69 (1996) 17-26
and the 41, ,:2 + 4F,,,
ESA transition
-%
fes*
(10m20 cm2)
(10-y
in various
host materials 4l I I.‘2 lifetime
%
04
(lOmzo cm2)
(lo-20
2.51 2.54 2.15
1.41 1.39 1.69
0.956 0.965 1.15
0.414 0.418 0.491
0.655 0.651 0.785
1.58 1.56 1.6
3.39
1.79
1.2
0.526
0.830
1.58
-8
5.42 4.26 1.29
1.55 0.8 1 2.24
0.98 0.46 1.27
0.498 0.277 1.16
0.717 0.371 1.82
1.44 1.34 1.57
<3 - IO - 200
cm2)
- 7200 - 7000 . 550
* Data from Refs. [29] and 1301. b Data from Ref. [14].
these systems the ratiof,,, :fgsais only slightly lower than for the other materials in Table 5, and the main distinction appears to be the substantially lower pump level lifetime. In pumping an Er-doped device at 980 nm, therefore, a low pump-level lifetime is desirable to maximise the pumping efficiency. Er : Ti : LiNb03 waveguide amplifiers pumped around 980 nm have recently been reported, where a total gain, or signal change over the peak absorption at 1532 nm, of - 10.7 dB has been obtained [32]. However, it is not evident from the results given in Ref. [32] whether there is any net gain in the waveguide, and a comparison between the gain efficiencies in dB/mW of amplifiers pumped at 1480 nm [ 1,2] and 980 nm is therefore difficult to make. A more thorough analysis of ESA in Er: LiNb03 is required, with experimental measurements of the actual ESA transition wavelength, the shape of the ESA spectrum and values for cross-sections, in order to be able to evaluate suitable wavelengths for pumping around 980 nm and predict the saturation pump powers at these wavelengths.
strengths for the same transitions and the results are found to be within the limits of the accuracy of this type of analysis. Fluorescence lifetimes were measured for four excited levels and were used to evaluate non-radiative multiphonon decay rates. The exponential dependence of the multiphonon decay rates on the energy gap supports the accuracy of our lifetime measurements. The J-O parameters were used to calculate ESA transition strengths from the 4111,z level, and the results of this preliminary analysis coupled with the 0.2 ms lifetime of the 4111,2 level lead us to believe Er : LiNb03 devices may not be pumped efficiently at a wavelength of 980 nm.
Acknowledgements
The authors would like to thank Dr J.S. Wilkinson of the Optoelectronics Research Centre for useful discussions. J. Amin would like to thank GEC-Marconi and EPSRC for support under a CASE award. The Optoelectronics Research Centre is an Interdisiplinary Research Centre, partly supported by a grant from the UK EPSRC.
5. Conclusions We have presented a complete polarised Judd-Ofelt spectroscopic evaluation of the Er : LiNbOJ system. Measured oscillator strengths at 300 K have been compared with calculated
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26
J. Amin et al. / Journal of Luminescence
[2] H. Suche, Erbium-doped integrated lasers and amplifiers, in: Proc. 7th ECIO, Session ThA4, 565, Delft (April 1995). [3] Th. Cog, M. Griebenow and G. Materlik, X-ray standing wave determination of the lattice location of Er diffused into LiNbO,, Phys. Lett. A 181 (1993) 417. [4] D.M.B.P. Milori, LJ. Moraes, A.C. Hernandes, R.R. de Souza, M. Siu Li, and M.C. Terrile, Optical and ESR study of Er3+ in LiNbO,, Phys. Rev. B 51 (1995) 3206. [S] C.-H. Huang, L. McCaughan and D.M. Gill, Evaluation of absorption and emission cross-sections of Er-doped LiNbO, for application to integrated optic amplifiers, J. Lightwave Technol. 12 (1994) 803. [6] M. Dinand and W. Sohler, Theoretical modeling of optical amplification in Er-doped Ti: LiNbO, waveguides, J. Quantum Electronics, 30 (1994) 1267. [7] N.M. Puscas, D.M. Grobnic, D. Scarano, M. Guidi, G. Perrone, and I. Montrosset, Absorption and emission cross-sections evaluation of Er3 +-doped LiNbO, waveguides, Optoelectronics, to appear. [S] D.M. Gill, J.C. Wright, and L. McCaughan, Site characterisation of rare-earth-doped LiNbO, using total site selective spectroscopy, Appl. Phys. Lett., 64 (1994) 2483. [9] 0. Witte, H. Stolz and W. von der Osten, Upconversion and site selective spectroscopy in erbium doped LiNbO,, J. Phys. D, to appear. [lo] B.R. Judd, Optical absorption intensities of rare-earth ions, Phys Rev. 127 (1962) 750. [l l] G.S. Ofelt, Intensities of crystal spectra of rare-earth ions, J. Chem. Phys. 37 (1962) 511. [12] G. Dominiak-Dzik, S. Golab, I. Pracka and W. RybaRomanowski, Spectroscopic properties and excited-state relaxation dynamics of Ers+ in LiNbO,, Appl. Phys. A 58 (1994) 551. [13] T.S. Lomheim and L.G. DeShazer, Optical-absorption intensities of trivalent neodymium in the uniaxial crystal yttrium orthovanadate, J. Appl. Phys. 49 (1978) 5517. [14] M.D. Shinn, W.A. Sibley, M.G. Drexhage and R.N. Brown, Optical transitions of Er3+ ions in fluorozirconate glass, Phys. Rev. B 27 (1983) 6635. [15] E. Desurvire, Erbium Doped Fibre Amplifiers: Principles and Applications, (Wiley, New York, 1994). [16] M.J. Weber, Probabilities for radiative and nonradiative decay of Er 3f in LaF,, Phys. Rev. 157 (1967) 262. [17] Physical and optical properties of LiNbO,, Crystal Tech, Inc. data sheet.
69 (1996) I7-26
[18] C.K. Jorgenson and B.R. Judd, Hypersensitive pseudoquadrupole transitions in lanthanides, Mol. Phys. 8 (1964) 281. [19] R.D. Peacock, The intensities of lanthanide fttf transitions, Struct. Bond. 22 (1975) 83. [20] S. Edvardson, M. Wolf and J.O. Thomas, Sensitivity of optical-absorption intensities for rare-earth ions, Phys. Rev. B 45 (1992) 10918. [21] R. Brinkmann, University of Paderborn, personal communication. [22] D.M. Gill, A. Judy, L. McCaughan, and J.C. Wright, Method for the local incorporation of Er into LiNbO, guided wave optic devices by Ti indiffusion, Appl. Phys. Lett. 60 (1992) 1067. [23] C. Li, C. Wyon and R. Moncorge, Spectroscopic properties and fluorescence dynamics of Er’+ and Yb3+ in Y,SiO,, J. Quantum Electronics 28 (1992) 1209. [24] D.S. Sumida and T.Y. Fan, Impact of radiation trapping on fluorescence lifetime and emission cross section measurements in solid-state laser media, Optics Lett. 19 (1994) 1348. [25] M.P. Hehlen, Effects of radiation trapping on measured excited-state lifetimes, Technical digest of the conference on Advanced Solid State Lasers 1996, San Francisco, Paper FC-8 (1996) 311. [26] V.T. Gabrielyan, A.A. Kaminski and L. Li, Absorption and luminescence spectra and energy levels of Nd3 * and Er3+ ions in LiNbO, crystals, Phys. Stat. Sol. A 3 (1970) K37. [27] W.J. Miniscalco, Erbium-doped glasses for fibre amplifiers at 1500 nm, J. Lightwave Technol. 9 (1991) 234. [28] K. Hattori, T. Kitagawa, M. Oguma, Y. Ohmori, and M. Horiguchi, Erbium-doped silica-based waveguide amplifier integrated with a 980/1530nm WDM coupler, Electronics Lett. 30 (1994) 856. [29] R.S. Quimby, Output saturation in a 980nm pumped erbium-doped fiber amplifier, Appl. Optics 30 (1991) 2546. [30] R.S. Quimby, W.J. Miniscalco, and B. Thompson, Excited state absorption at 980 nm in erbium doped glass, SPIE Vol. 1581, Fiber Laser Sources and Amplifiers III (1991) 72. [31] W.J. Miniscalco, GTE Labs. Inc., personal communication. [32] C.H. Huang, and L. McCaughan, Er-indiffused Ti : LiNbO, channel waveguide optical amplifiers pumped at 980 nm, Electronics Lett. 32 (1996) 215.
LUMINESCENCE EISEVIER
Journal
of Luminescence
69 (1996) 17.-26
Spectroscopic analysis of Er3 + transitions in lithium niobate J. Amin”? *, B. Dussardierb, T. Schweizer”, M. Hempstead” bLahorataire
aOptoelectronics Research Centre, University of Southampton. Southampton, SOI 7 IBJ, UK de Physique de la MatiPre Condenske. Universit2 de Nice-Sophia Antipolis, Part Valrose. F-06108 Nice Cede.r 2. France Received
7 March
1996; accepted
2 May
1996
Abstract
We report on the results of a spectroscopic analysis of the transition strengths of Er-doped LiNb03. The line strengths of several transitions from the ground state to excited state manifolds are evaluated from measured polarised absorption spectra and analysed using Judd-Ofelt theory, taking into account the crystal anisotropy. The measured and calculated oscillator strengths at 300 K compare favourably within the typical uncertainties associated with the Judd-Ofelt theory. The Judd-Ofelt parameters obtained have been used to evaluate the excited-state-absorption (ESA) transition strengths from the 4111,~ level, which have important implications for pumping Er: LiNb03 devices. Keywords: Er-doped LiNb03; spectroscopy
1. Introduction The development of erbium-doped fibre lasers and amplifiers has recently generated an interest in rare-earth-doped integrated optical devices in various host materials for applications in optical communications, high resolution spectroscopy and optical sensing. In particular, Er-doped LiNb03 is extremely attractive as it permits a high degree of integration through a combination of the mature optical waveguiding techniques, optical gain introduced by the active Er3+ ions and good intrinsic electro-optic and acousto-optic properties of the host. Several sophisticated devices have recently been demonstrated in this material system, taking advantage of the above attributes [l, 23. * Corresponding Broadway.
author. Presently at NIST, Boulder, CO 80303, USA.
Div. 815.04, 325
0022-23 13/96/$15.00 0 1996 Elsevier Science B.V. All rights reserved PII SOO22-23 13(96)00063-4
To exploit this material system fully, it is important to know the spectral characteristics of the dopant-host combination and to quantify radiative and non-radiative transitions between different excited levels. To this end, a great deal of research has been undertaken recently to determine the spectroscopic properties of Er:LiNb03. Studies by GSg et al. [3] and Milori et al. [4], using X-ray standing wave analysis and electron-spin resonance techniques, respectively, have shown that Er3+ ions in LiNb03 exist in positions close to Lit sites, but slightly shifted along the --z direction, with a C3 point symmetry. Low-temperature absorption spectroscopy has been carried out to determine the Stark level splitting for the ground state t41, 5,2) and the upper laser level (4113,2) of the Er3+ ion in the crystalline LiNb03 host, and room-temperature absorption and emission cross-sections have been determined through the use of McCumber theory
18
J. Amin et al. / Journal of Luminescence
and the density matrix formalism [S-7]. The lowtemperature absorption spectra have revealed the existence of at least two sites for the Er3+ ion in the host lattice. This has been corroborated by siteselective spectroscopy, which indicates the existence of several non-equivalent sites [8,9]. However, the non-equivalent sites are generally believed to be Li+ sites, with the Er3+ ions at slightly different off-centred positions around the Li+ location. The optical 4f transitions of rare-earth ions in glasses and crystals are generally characterised using Judd-Ofelt (J-O) theory [lo, 111, which allows key laser parameters, such as fluorescence transition probabilities, radiative lifetimes, and excited-state-absorption (ESA) transition strengths to be calculated from measured absorption intensities. Despite the great interest in the Er:LiNbO, system, a thorough Judd-Ofelt analysis has, to our knowledge, not yet been carried out. The only published Judd-Ofelt analysis for Er:LiNbO, was carried out using unpolarised light propagating perpendicularly to the crystalline z-axis, and assuming the absorption spectra thus obtained to be an average of the two polarisations [12]. However, due to the uniaxial nature of LiNb03, there are strong absorption and emission differences between extraordinary(Ellz) and ordinary(El_z) polarised modes for rare-earth ions situated at sites with a C3 local symmetry along the trigonal axis (z-axis), and we have found that the Judd-Ofelt parameters differ for unpolarised and polarised spectra considerably. In this paper, therefore, we present a detailed Judd-Ofelt analysis for Er:LiNb03 using measured polarised absorption spectra and calculating the line strengths by summation over both polarisations, as described in Ref. [13]. The averaged J-O parameters obtained from this polarisationdependent analysis have been used to obtain values for (ESA) transition strengths. Decay rates for transitions to the ground state have been measured at room temperature for four different excited levels, and the results are compared to radiative transition rates calculated from the J-O theory. The results are discussed in comparison with published data for a variety of erbium-doped glasses.
69 (1996) 17-26
2. Experimental Room-temperature, polarisation-resolved absorption experiments were carried out on a congruent, 0.95 mm thick, x-cut sample of LiNb03, provided by Deltronics. The sample was doped with Er3+ ions in the bulk, to a concentration of 1.5 x 1Or9cmm3. The absorption spectra from 300 to 1600 nm were recorded on a Perkin Elmer Lambda 9 spectrophotometer, using an undoped sample of similar dimensions in the reference arm. The resolution of the spectrophotometer for these measurements was better than 0.5 nm. Fluorescence-lifetime measurements were carried out on a 1 mm thick z-cut Er:LiNbO, with concentration of sample a dopant 5 x 1019 cmp3, supplied by the University of Strathclyde. This higher dopant level was chosen to allow collection of a significant fluorescence signal whilst keeping shortening of the lifetimes due to cross-relaxation and up-conversion effects to a minimal level. Four cw laser sources, at 488, 660, 975 and 1480 nm, were used to excite the ions to the 4S3,2, 4F9,z, 411,,z and 4113,z levels respectively, and the decay from each of these levels to the thereby 4115,2 ground state was monitored, allowing the true fluorescence lifetimes of these states to be observed. The excitation powers were kept to a low level ( < 50 mW) to reduce shortening of the lifetime due to up-conversion. The pump laser signal was in each case focused at the very edge of the sample and side fluorescence was collected from the same edge, in order to minimise lifetime lengthening effects due to reabsorption in the three-level erbium system. Discrimination of the pump and fluorescence was obtained by measuring the decay signals through a 300 mm Bentham monochromator with a 750 nm blazed grating. Table 1 summarises the energy levels evaluated, together with the wavelength of the analysed fluorescence and the excitation and detection systems used.
3. Theory The Judd-Ofelt analysis, used to determine the probability of radiative transitions of the rare-earth
J. Amin et al. 1 Journal of Luminescence 69 (1996) 17- 26 Table 1 Experimental
conditions
for fluorescence
Level
Excitation
source
Air-cooled
19
detection Excitation
Fluorescence
wavelength
(nm)
(nm)
488
559
Detector
wavelength
Photo-multiplier
Ar + -laser 4Fg z
Dye laser
Y I,,2
InGaAs
‘+I13>2
GaInAsP
laser diode laser diode
660
666
Photo-multiplier
915
981
SI-APD
1480
1530
InCaAs
ions, has been well documented since the development of the technique by Judd and Ofelt [S-15], so we present only a brief summary, as relevant to this study. The theory allows, through the phenomenological Judd-Ofelt parameters R,, the determination of intra-shell radiative transition probabilities within the ion configuration. Once determined these parameters may be used to calculate the strength of any radiative transition for the particular dopant-host combination. Moreover, radiative lifetimes and fluorescence branching ratios, which may be derived using the transition probabilities, are particularly important in the determination of the efficiency of lasers and amplifiers, and may also be used to determine stimulated emission cross-sections through the measurement of emission spectra. The analysis in this paper has been carried out in the MKS (SI) unit system. According to the J-O theory, the electric-dipole (ed) oscillator strengths, in an isotropic material, from level a to h can be expressed by
where L + 2s is the md operator, h is the Planck’s constant, m is the mass of an electron, and L’is the speed of light in vacuum. The host-independent matrix elements of L + 29 and U@) have been numerically evaluated for Er3+ ion transitions and are tabulated in Ref. [16]. The corresponding transition strength from level a to h can be expressed in terms of Sed and $,,d as follows:
i
+ X&$!!
,
(3)
where I is the mean wavelength of the transition between the two manifolds, and J, is the total angular momentum of level a according to the Russel-Saunders notation. The degeneracy of the level is defined by 25, + 1. X,d and Xmd are the local field corrections due to the crystalline host material which, for an isotropic material of refractive index n, are given by Xed
=
(no-l2+ a2
and
9n(i)
(1)
where (ali U”‘llb) are doubly reduced matrix elements of the tensor operator UC’),independent of the host material, and e is the electronic charge. The host dependence in the above equation is carried in the three Q parameters. The line strength for magnetic-dipole (md) transitions between J manifolds is
X.d+ r
Xmd = n(/?)
For uniaxial crystals such as LiNb03, the local field corrections may be made by averaging over the extraordinary (7~)and ordinary (B) polarisations [13] so that X ed(md)
Xed(mdLn =
+
2Xed(md).n
3
The experimental transition strengths can be found by measurement of the integrated absorption
20
J. Amin et al. 1 Journal of Luminescence 69 (1996) 17-26
coefficient for transitions between the ground state and upper levels, and then using the expression f(a, b),,,,
=
4mEoc2
ln( 10). OD(;1) d2
Ne2d(;Z)2s
- ln(lO)*OD(&]
+*exp[
-ln(lO)~OD(Q,J}d;l.
(7)
The theoretical oscillator strength, given by Eq. (3) can then be fitted to the experimental one, given by Eqs. (6) and (7), using least-squares fitting, allowing the extraction of the J-O parameters 52,. The Einstein A coefficient, which gives the rate of spontaneous emission between two levels a and b, is given by A(a+b)
=
16n3n2 3hso(2J, + 1);i”
(Xed&d
+
XmdSmd).
(8)
The radiative lifetime of level a may be extracted from the emission probability, using the following expression:
where the summation is carried out over all final levels b. The fluorescence branching ratio for the transition a + b is defined as the ratio of the relaxation rate for this transition to the total sum of all spontaneous emission probabilities from level a, i.e. B(a, b) = r,A(a, b).
(10)
The measured lifetime of the fluorescence decay from level a combined with the radiative lifetime calculated using J-O theory allows the calculation
efficiency through
the
(11)
(6)
where N is the number of active ions per unit volume, OD(n) is the measured optical density, defined as [ - log,, T(1)], where T is the transmission through the material, and d is the thickness of the material. For an anisotropic material, two measurements are required for the optical density, one along each of the extraordinary and ordinary axes, with polarised beams, thus giving OD(l), and OD(l),. To simulate an isotropic material, the integral in Eq. (6) may then be replaced by [13] Jln{iexp[
of the radiative quantum relation
4. Results and discussion (a) Measured absorption spectra: Fig. 1 shows the room-temperature absorption spectra for nine different transitions from the ground state to excited levels, measured as detailed above. The spectra for both the ordinary and extraordinary polarisations are shown in the figures, and the relevant transitions from the ground state have been marked. The absorption spectra for adjacent energy levels do not overlap, thereby allowing each transition to be analysed separately. (b) Transition strengths: ‘,Table 2 shows the measured oscillator strengths, obtained from the absorption spectra using Eqs. (6) and (7). The mean of the transitions from the wavelengths 41ls,2 ground state are shown in the table. The measured transition strengths have a systematic error of the order of 18%, due to uncertainties in the concentration of Er in the sample and the thickness of the sample, assumed to be of the order of + 0.15 x 10” cme3 and + 0.05 mm respectively_ The estimated eriors in column 4 of Table 2 do not include this systematic error, and take account only of uncertainties in the integration of the absorption spectra due to noise in the measurements and interpretation of the peak values and the limits of the spectra. (c) Judd-Ofelt parameters: All the transitions, except for the 41,,,2 + 41i3i2 transition, were assumed to be electric dipole in nature. The magneticdipole contribution to the transition strength for the 41ls,z -+ 4I13,2 transition was calculated to be 67 x 10m8, and was subtracted from the measured transition strength to give the purely electric dipole transition strength. Values of the host-independent matrix elements as tabulated for Er3’ ions in Ref. [16] were used, and Xed and Xmd were calculated using the wavelength-dependent Sellmeier equation for the refractive index of LiNb03 at room temperature [17]. Using Eq. (3), the Judd-Ofelt
21
J. Amin et al. / Journal of Luminescence 69 (199~5) 17- 26
‘h2 -
a-polarised
-
cpolarised
- -
x-polarised
- -
n-polarised
-0.002 -I 1440
1480
950
1600
1560
1520
960
970
Wavelengthlnm 0.016
0.08 %/2
0.014
990
1000
1010
i
1020
I
ZHIv2
0.07
__ --
0.06
0.012
$
980
Wavelengthlnm
a-polarised n-polarised
0.010
3
0.008
i 2
0.006 0.004 0.002 0.000
I
-0.01 ,, ,, ,,, ,, ,, ,, 640
650
670
660
680
690
460
,/,,(
480
,,,,
500
Wavelengthlnm
-,
,.
520
,,,
540
__I
560
Wavelength/nm
_
I
i
0.08
1
‘G11n
0.06 5d g 0.04
I
-
o-polarisation
--
n-polarisation
-
2
1....,...,,,‘.,,,..,,.“‘,“.‘,‘.“1 350
360
370
360
390
400
410
420
Wavelengthlnm
Fig. I. Polarised absorption transitions from the ground
spectra of Er: LiNb03, doped to a concentration state. 4115,2, to the levels indicated.
of 1.5 x 1Ol9 cmm3. The spectra
shown are for absorption
22
J. Amin et al. / Journal of Luminescence
69 (1996) 17-26
Table 2 Measured and calculated oscillator strengths. All transitions are from the 41,5,1 level to the levels indicated. All the levels have been analysed separately in the J-O calculations Level
411312 41I I,,? ‘+Fg,z % 3/2 *H II,2 4F~,z ‘Hw 4G 1I,2 ‘Gw
Average wavelength (nm) 1522 982 661 548 525 488 410 381 369
Estimated error in fk.%llO- 8 280 84 430 68 2100 450 140 4050 390
19 5 10 9 110 46 22 110 34
parameters were adjusted to give the best fit to the measured transition strengths. The calculated J-O intensity parameters are as follows: Q2 = (7.29 & 1.50) x 10-20 cm-‘, Q4 = (2.24 + 0.48) x 10-20 cm-‘,
The value of Q2 is large compared to the other two parameters, possibly due to the rare-earth ions being in a highly polarised environment [l&19]. In the case of erbium, the transitions to the 2H1 i,2 and the 4G 1l,2 levels are hypersensitive, i.e . UC2)is large, and, reflecting the fact that Q2 is also large, these transitions are very strong compared to the other ground state absorptions [20]. Table 2 shows the calculated transition strengths compared with the measured ones, and also gives the deviation between the two values. The transition strengths given in the table are the total strengths, resulting from both ed and md contributions. The rms deviation, which allows the validity of the intensity parameters obtained by the fit to be evaluated, is given by 6*Ins=
C(deviations)2 no. of levels - no. of parameters
112 f (14
300 120 460 89 2070 410 150 4060 370
8
Deviation fcalE-“&../10-
8
20 36 30 21 - 30 - 40 10 10 - 20
For the above parameters, 6,,, is found to be 31 x lo-‘, which is within the typical uncertainties associated with these calculations [14]. Our J-O intensity parameters are significantly different from those in Ref. [12]. However, we believe this to be due to unpolarised light being used to measure the absorption cross-sections in Ref.
WI.
fib = (1.27 + 0.35) x 1O-2o cm-2.
I&/10-
(d) Transition rates and quantum ejjkiencies: Table 3 summarises the calculated total radiative lifetimes of the various energy levels and the branching ratios for each level. The measured lifetimes for the 4113,2,4111j2,4Fs,2 and 4S3,z levels are also indicated in the table. The decay characteristics of the four measured levels were all single exponential in nature. The presence of dominant non-radiative decay mechanisms is clearly indicated by the measured lifetime being considerably lower than that calculated for the 4111,2, 4Fs,2 and 4S3,2 levels. However, the reason for the anomalous behaviour of the measured 4113,2 lifetime, where the calculated radiative lifetime of 2.3 ms is much shorter than the experimentally measured value of 3.6 ms, is not known at present. Reported measured lifetimes of the 1500 nm fluorescence vary from 2.6 ms [21] to 7 ms [22], and it is also known that the lifetime of the 4I13iz level can vary significantly depending upon the conditions of excitation due to radiation trapping [23-251. However, as mentioned previously, care has been taken
J. Amin et al. / Journal of Luminescence
Table 3 Lifetimes Level
and branching
ratios
Calculated radiative (ms)
41 1J2 ‘+IIL 2 4FW 4sW ZH 1L/Z ‘V? 2 2Hq,2 4G 11:z 2GW
2.3 + 0.4 1.7 + 0.3 0.17 + 0.03 0.172 + 0.036 0.028 + 0.005 0.06 k 0.01 0.074 * 0.008 0.006 1_ 0.001 0.009 & 0.00 1
of the various
excited levels. Band 0 is the ground
Measured lifetime (ms)
Band no.
3.6 +_ 0.2 0.2 * 0.01 0.0018 k 0.0002 0.028 + 0.005
1 2 3 4 5 6 7 8 9
in this work to ensure that the pump was focused at the edge of the sample and that the side fluorescence was detected from the same edge, thus ensuring that trapping effects were kept at a minimum. We also believe that effects due to up-conversion and ion-ion interactions are negligible, due to the low concentration of the sample (5 x 10” cmP3) and the low excitation power used ( < 50 mW). Although the J-O analysis has an inherent inaccuracy of 2&30%, which has not been taken into account in our calculations, this can only be partially responsible for the difference in measured and calculated lifetimes. We are therefore currently investigating this discrepancy further. Assuming though that the non-radiative decay from the 41,3,, is negligible, the quantum efficiency of this level can be taken to be close to loo%, and that of the 4111,2, 4Fg,z and 4S3,2 levels is calculated, from Eq. (11) to be ll%, 1% and 16.3%, respectively. The non-radiative multiphonon decay rate W,, for these levels may be given by 1 1 w,, = ___ - r meaS r,
23
69 (1996) 17-26
state, 41,,,,
Branching
Ratios (X)
0
1
2
3
4
5
6
I
8
100 86 91 69 96 16 39 90 16
14 5 26 1 15 45 7 73
3 2 1 6 11 0 5
0 0 0 1 2 4
0 0 0 0 0
0 2 0 0
0 0 1
0 0
0
expressed as W,, = Cexp(
-a AE),
(14)
where AE is the energy difference between the excited level and the next level below this, and C and a are positive constants characteristic of the host. In Fig. 2, AE for the levels 4S3,2, 4Fg,, and 4111129 as measured in Ref. [26], are plotted against the multiphonon emission rate W,,. From this
le+6
,
\
-1
(13)
where ra is the radiative lifetime as calculated from Eq. (9). The empirical “energy gap law” [14] may then be used to determine the exponential dependence of the multiphonon emission rate on the energy gap between consecutive levels. This can be
2500
3000
3500
4oCw
Energy gap AWcm-’
Fig. 2. Multiphonon emission rates of Er: LiNb03 against AE, the energy gap to the next lowest level, at room temperature.
24
J. Amin et al. / Journal of Luminescence
curve, we obtain c=4.4x10”s-’ and c( = 5.5 x 1O-3 cm-‘. These parameters may then be used to determine the non-radiative decay rates for any other excited level. (e) Excited-state absorption: Er-doped integrated optical devices in LiNb03 reported thus far have almost exclusively been pumped at 1480 nm, mainly due to the reduced photorefractive damage at this wavelength, and also due to the fact that the waveguides can readily be made single-moded at both pump and signal wavelength. However, amplification measurements in Er-doped fibres have shown that pumping at 980 nm provides the best performance in terms of gain, gain efficiency (in dB/mW) and signal-to-noise ratios [27]. Pumping at 980 nm is also advantageous in that it readily allows the fabrication of compact WDMs for independent routing of the pump and signal light on the same chip, as has been demonstrated in a glass waveguide circuit [28]. However, pumping at 980 nm in Er: LiNb03 is complicated by the long lifetime of the 4111,2 level (compared to common silicate and phosphate glasses) and a two-step resonant excited-state-absorption (ESA) from the pump level, 4111,z, to the 4F7,2 level, as shown in Fig. 3. In the ESA process, the ions decay through rapid non-radiative processes from the 4F,jz level to the 4S3,z level, and from there to the ground state through a combination of radiative and nonradiative processes, as evidenced by a bright green fluorescence from this level. In a material system where ESA at 980 nm can be ignored, a long lifetime of the pump level leads to “bottlenecking”, with the limited rate of decay of ions to the metastable level resulting in decreased population inversion and lower gain. However, in a material such as Er:LiNbO, where there is a higher energy level resonant with the pump transition, the relatively large population of ions in the 4111,2 state absorb incoming photons and thus decrease the amplifier efficiency at high excitation levels [29]. As a first step towards a study of ESA in this system, we have used the calculated J-O parameters to evaluate the ESA transition strengths from the 4111,2level (Table 4). The peak wavelength for each transition has been calculated using the difference in average energy of each level (Table 2), and may therefore be different from that observed
69 (1996) i7-26
‘5,*
ESA 980 nm
Fluorescence 1530 nm
GSA 1480 nm
GSA 980 nm
la
Fig. 3. Energy level structure of E?+ions, mechanism at 980 nm.
showing the ESA
Table 4 ESA oscillator strengths from the 4111i, level Level
.M10-s
Wavelength (nm)
‘+F,,, % 3/z 2H ll,Z 4F~,, ‘H,,, ‘Yi Il,Z ‘%z
223 19 117 182 183 46 408
2022 1239 1128 968 703 623 590
experimentally. Table 5 compares the J-O parameters, the GSA (ground-state-absorption) and ESA transition strengths and the lifetime for the 4z 11,2 level of Er: LiNb03 and various glasses doped with erbium [14,30]. To our knowledge, of the material systems included in Table 5, the only ones which have been pumped efficiently at 980 nm are the phosphate and silicate glass systems [31]. In
J. Amin et al. 1 Journal
Table 5 Oscillator
strengths
Material
ZBLAN” ZBLAb Fluorophosphate (high F) Fluorophosphate (low F) Phosphate” Silicate” LiNbO,
for the Er 4115,2 + 41, 1,2 GSA transition
qf Luminescence
25
69 (1996) 17-26
and the 41, ,:2 + 4F,,,
ESA transition
-%
fes*
(10m20 cm2)
(10-y
in various
host materials 4l I I.‘2 lifetime
%
04
(lOmzo cm2)
(lo-20
2.51 2.54 2.15
1.41 1.39 1.69
0.956 0.965 1.15
0.414 0.418 0.491
0.655 0.651 0.785
1.58 1.56 1.6
3.39
1.79
1.2
0.526
0.830
1.58
-8
5.42 4.26 1.29
1.55 0.8 1 2.24
0.98 0.46 1.27
0.498 0.277 1.16
0.717 0.371 1.82
1.44 1.34 1.57
<3 - IO - 200
cm2)
- 7200 - 7000 . 550
* Data from Refs. [29] and 1301. b Data from Ref. [14].
these systems the ratiof,,, :fgsais only slightly lower than for the other materials in Table 5, and the main distinction appears to be the substantially lower pump level lifetime. In pumping an Er-doped device at 980 nm, therefore, a low pump-level lifetime is desirable to maximise the pumping efficiency. Er : Ti : LiNb03 waveguide amplifiers pumped around 980 nm have recently been reported, where a total gain, or signal change over the peak absorption at 1532 nm, of - 10.7 dB has been obtained [32]. However, it is not evident from the results given in Ref. [32] whether there is any net gain in the waveguide, and a comparison between the gain efficiencies in dB/mW of amplifiers pumped at 1480 nm [ 1,2] and 980 nm is therefore difficult to make. A more thorough analysis of ESA in Er: LiNb03 is required, with experimental measurements of the actual ESA transition wavelength, the shape of the ESA spectrum and values for cross-sections, in order to be able to evaluate suitable wavelengths for pumping around 980 nm and predict the saturation pump powers at these wavelengths.
strengths for the same transitions and the results are found to be within the limits of the accuracy of this type of analysis. Fluorescence lifetimes were measured for four excited levels and were used to evaluate non-radiative multiphonon decay rates. The exponential dependence of the multiphonon decay rates on the energy gap supports the accuracy of our lifetime measurements. The J-O parameters were used to calculate ESA transition strengths from the 4111,z level, and the results of this preliminary analysis coupled with the 0.2 ms lifetime of the 4111,2 level lead us to believe Er : LiNb03 devices may not be pumped efficiently at a wavelength of 980 nm.
Acknowledgements
The authors would like to thank Dr J.S. Wilkinson of the Optoelectronics Research Centre for useful discussions. J. Amin would like to thank GEC-Marconi and EPSRC for support under a CASE award. The Optoelectronics Research Centre is an Interdisiplinary Research Centre, partly supported by a grant from the UK EPSRC.
5. Conclusions We have presented a complete polarised Judd-Ofelt spectroscopic evaluation of the Er : LiNbOJ system. Measured oscillator strengths at 300 K have been compared with calculated
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