Content uploaded by Peter Petrik
Author content
All content in this area was uploaded by Peter Petrik on May 20, 2015
Content may be subject to copyright.
MeV Energy N+-Implanted Planar Optical
Waveguides in Er-Doped Tungsten-Tellurite
Glass Operating at 1.55 µm
Volume 4, Number 3, June 2012
I. Bányász
S. Berneschi
M. Bettinelli
M. Brenci
M. Fried
N. Q. Khanh
T. Lohner
G. Nunzi Conti
S. Pelli
P. Petrik
G. C. Righini
A. Speghini
A. Watterich
Z. Zolnai
DOI: 10.1109/JPHOT.2012.2194997
1943-0655/$31.00 ©2012 IEEE
MeV Energy Nþ-Implanted Planar Optical
Waveguides in Er-Doped Tungsten-Tellurite
Glass Operating at 1.55 m
I. Ba´nya´sz,1S. Berneschi, 2;3M. Bettinelli, 4M. Brenci,3M. Fried, 5
N. Q. Khanh,5T. Lohner, 5G. Nunzi Conti, 3S. Pelli,3P. Petrik, 5
G. C. Righini,2;3A. Speghini, 4A. Watterich, 1and Z. Zolnai5
1Department of Crystal Physics, Wigner Research Centre for Physics,
Hungarian Academy of Sciences, 1525 Budapest, Hungary
2Museo Storico della Fisica e Centro di Studi e Ricerche BEnrico Fermi,[
00184 Roma, Italy
3MDF-Lab, BNello Carrara[Institute of Applied Physics, IFAC-CNR,
50019 Sesto Fiorentino (FI), Italy
4Laboratory of Solid State Chemistry, Department of Biotechnology, University of Verona, and INSTM,
University of Verona, 37134 Verona, Italy
5Research Institute for Technical Physics and Materials Science, Hungarian Academy of Sciences,
1525 Budapest, Hungary
DOI: 10.1109/JPHOT.2012.2194997
1943-0655/$31.00 Ó2012 IEEE
Manuscript received March 12, 2012; revised April 3, 2012; accepted April 5, 2012. Date of publication
April17, 2012; date of current version May 4,2012. This work was supported by Hungarian NationalResearch
Fund (OTKA-NKTH and OTKA) underprojectsK 68688 and K 101223, as well as fromthe bilateral 2010-2012
CNR/MTA project. Corresponding author: I. Ba´nya´ sz (e-mail: banyasz@sunserv.kfki.hu).
Abstract: We report on the fabrication and characterization of planar waveguides in an
Er-doped tungsten-tellurite glass by implantation of 3.5 MeV Nþions. Implantations were
carried out in a wide fluence range of 1 1016 81016 ions/cm2. Waveguides were
characterized by m-line spectroscopy and spectroscopic ellipsometry. Irradiation-induced
refractive index modulation saturated around a fluence of 8 1016 ions/cm2. Waveguides
operating at 1550 nm were obtained in that material using 3.5 MeV Nþion implantation.
Index Terms: Integrated optics, planar waveguide, ellipsometry, m-line spectroscopy.
1. Introduction
Ion beam techniques are among the best methods for optical waveguide fabrication in crystalline
and amorphous materials. The first report on waveguide fabrication in fused silica using proton
beam was published by Schineller et al. in 1968 [1]. An important monograph of this field was
written by Townsend et al. [2]. A detailed review of ion-implanted waveguides in optical materials
has recently been published by Chen et al. [3].
Tellurite glasses have gained a widespread attention because of their potential as hosts of rare-
earth elements for the development of fiber and integrated optic amplifiers and lasers covering all
main telecommunication bands [4]–[6]. Er3þ-doped tellurite glasses in particular are very attractive
materials for the fabrication of broadband amplifiers in wavelength division multiplexing (WDM)
around 1.55 m, as they exhibit large stimulated cross sections and broad emission bandwidth.
Furthermore, tellurite glasses have low process temperature and nonlinear properties. Fabrication
and optical properties of bulk and planar waveguides obtained in such materials have been
extensively studied [7]–[9]. Only a few papers over the past few years reported on the possibility to
fabricate channel waveguides in these activated glassy materials by means of femtosecond laser
Vol. 4, No. 3, June 2012 Page 721
IEEE Photonics Journal N
+
Implanted Planar Optical Waveguides
writing [10] and reactive ion etching (RIE) process in pure TeO2thin films [11], [12]. In spite of the
relevant advancements in performance achieved by these approaches, still significant problems
persist toward the full exploitation of this material. In fact, RIE still lacks the necessary quality output
to allow the fabrication of waveguides fully embedded in the glass, which would enable more
flexibility in the optimization of the waveguides and of the pumping schemes. On the other hand,
laser writing does not induce high index contrast between waveguide and substrate, which would
permit a better field confinement and, consequently, a more efficient performance of the device.
Research interest in the field of Er3þ-doped tellurite materials is still going on, with the aim of
discovering the best combination of tellurite glass formulation and fabrication process in order to
reduce the cost and increase the performance of the devices so obtained. In this context, ion
implantation presents some unique advantages compared with other fabrication methods. It proved
to be a universal technique for producing waveguides in many optical materials and has good
controllability and reproducibility [3].
We have recently reported fabrication of channel waveguides in an Er-doped tungsten-tellurite
(Er:Te) glass via implantation of MeV Nþions [13]. Based on our experiences with the 2-D
waveguide structures, we carried out a systematic study of the implantation parameters for planar
waveguides, in order to better understand the nature of the guiding structure and realize waveguide
operation in the 1550-nm telecommunication band. We increased the ion energy from the 1.5-MeV
value used in those experiments to 3.5 MeV to achieve this goal by obtaining a broader guiding
layer. Nþions implantation has already been long demonstrated as a means to fabricate low loss
waveguides in other materials, hence our choice for the tests described in this paper [14], [15].
Moreover, we decided to use Nþions for the fabrication of the waveguides because of two reasons:
on the one hand ion mass in the accelerator was limited by ion mass energy selector magnet. On
the other hand, the higher the atomic mass the bigger the ion-solid interaction. If compared with He
ions, in the case of N, we expected three times bigger electronic stopping and more than ten times
bigger nuclear stopping in the MeV energy range. In fact, we succeeded in achieving the same
refractive index modulation in a Pyrex glass with Nþions of a dose of one order of magnitude lower
than with Heþions of the same energy [16].
2. Waveguide Fabrication
The composition of the glass developed for our experiments was 60 TeO2-25 WO3- 15 Na2O-0:5 Er2O3
(mol. %) with a density of 5.75 g/cm3. We fabricated planar waveguides in the Er:Te glass samples via
implantation of Nþions with 3.5-MeV energies. Irradiations were carried out with a collimated beam from a
Van de Graaff accelerator at the Research Institute for Particle and Nuclear Physics, Budapest, Hungary,
with normal incidence on the glass samples. Lateral homogeneity of the irradiation was assured by
defocusing the ion beam with a magnetic quadrupole and by scanning the sample under a 2 mm 2 mm
beam. Useful size of the implanted waveguides was 6 mm 6 mm. Irradiation was carried out at fluences
of 1, 2, 4, and 8 1016 ions/cm2.
3. SRIM Simulations and Ellipsometric Measurements
Structure of the ion implanted planar waveguides is determined mainly by the energy and fluence of
the implanted ions. Distribution of the implanted ions or that of the collision events along the depth
of the implanted sample can serve as a rough estimation of the refractive index profile of the
implanted waveguide. We used SRIM 2008 code (Stopping and Range of Ions in Matter) [17] to
simulate the fabrication of the waveguides. Full-cascade SRIM simulations were performed to
estimate the ion and damage depth distributions in the target. Maximum of the distribution of the
implanted Nþions was 2.5 m below the surface of the Er:Te glass sample in case of 3.5-MeV ion
energy, with a longitudinal straggling of 0.5 m. Maximum of the collision events (vacancy
production) distribution roughly coincides with that of the ion distribution, but it extends considerably
toward sample surface.
The ion-implanted waveguides were measured with a WOOLLAM M-2000DI spectroscopic
ellipsometer ð¼193ÿ1690 nmÞ. A three-layer optical model was applied in the evaluation of the
IEEE Photonics Journal N
+
Implanted Planar Optical Waveguides
Vol. 4, No. 3, June 2012 Page 722
spectroscopic ellipsometry (SE) data. The first layer, which is adjacent to the substrate, represents
the stopping region. The second layer is the region that the implanted ions traverse before they stop.
The third layer is a surface roughness film taken into account on basis of effective medium
approximation [18]. Dielectric functions of the first and second layers were described by the Cauchy
dispersion relation. Parameters of the Cauchy dispersion relations and layer thicknesses were
considered as free parameters. We applied the evaluation software WVASE32 created by J. A.
Woolam, Inc. [19] for the analysis of the spectroellipsometric data. The evaluation yielded 7.9
0.1 nm for the thickness of the surface roughness layer and 2.019 0.001 for the refractive
index of the nonimplanted glass substrate at ¼635 nm.
A graphical comparison of implanted Nþdistribution obtained by SRIM and barrier layer bound-
aries obtained by SE is shown in Fig. 1. Center of the barrier layer is slightly beyond the projected
range, while its width increases with the implanted fluence, as expected.
4. M-line Spectroscopic Measurements
For the characterization of the planar waveguides implanted in the tungsten-tellurite glass, we used
a semiautomatic m-line spectroscopic instrument (BCOMPASSO[), which was developed at IFAC.
Accuracy of the instrument is generally 110ÿ4and 410ÿ4on the effective refractive index and
bulk refractive index, respectively. In the case of the Nþ-implanted planar waveguides in the Er:Te
glass, due to the lower contrast in the measurement, the accuracy was lower: about 510ÿ4and
110ÿ3, respectively.
Due to the high refractive index of the bulk glass (around 2.0 at 635 nm), we used a rutile prism to
couple the light in the irradiated regions. Table 1 summarizes the values obtained from the m-line
measurements performed at 635 and 1550 nm. Each value is an average of three measurements.
Bulk refractive index was 2:041 110ÿ3. We observed modes with effective refractive index
below the corresponding value of the bulk refractive index, a clear indication of leaky modes.
Fabrication of the Er:Te glass waveguides with 3.5-MeV energy Nþions resulted in waveguides
able to support modes also at 1550 nm wavelength. We present here two examples of the results
obtained. The m-line spectrum (reflected relative intensity versus effective refractive index) with five
TE modes for the highest implanted fluence, 8 1016 ions/cm2, measured at 635 nm, is presented in
Fig. 2(a). Proof that the same waveguide supports modes at 1550 nm is shown in Fig. 2(b).
To analyze how the implantation parameters influence the characteristics of the waveguides, we
have carried out numerical fits based on the m-lines measurements. The model upon which the
numerical fit was based was rather simple but effective: The waveguide has been considered a
Fig. 1. SRIM simulation and SE fit of the waveguide structure.
IEEE Photonics Journal N
+
Implanted Planar Optical Waveguides
Vol. 4, No. 3, June 2012 Page 723
three layer step structure and the leaky nature of the waveguide has been neglected, using the
barrier layer as the substrate of the waveguide. Indeed, to justify the latter assumption it must be
stressed that, though propagation losses are obviously strongly influenced by how well the mode is
confined by the leaky structure or, in other terms, how well the leaky structure resembles a truly
confined mode, values of the effective indices are much less dependent from this factor, and the
approximation can be applied to the numerical fit used to assess only the physical characteristics of
the structure.
The availability of data at several wavelengths (635, 980, 1310, and 1550 nm) has allowed us to
obtain a more accurate and flexible data processing. Actually, assuming a Sellmeier-like law to
account for chromatic dispersion for all layers:
n2¼1þ2
A2þB(1)
Fig. 2. M-line spectra of waveguide. Fluence ¼81016 ions/cm2, E ¼3:5 MeV. (a) At 635 nm and (b) at
1550 nm.
TABLE 1
Effective refractive indices (average values) versus ion fluence measured at 635 and 1550 nm for Er:Te
glass. Error of the refractive indices is 510ÿ4
IEEE Photonics Journal N
+
Implanted Planar Optical Waveguides
Vol. 4, No. 3, June 2012 Page 724
and using it in the fit process, it was possible to obtain the thickness of the guiding layer and the
parameters A and B for the guiding and the barrier layers in a broad wavelength range.
As shown in Fig. 3 for the sample irradiated with 8 1016 ions/cm2, the fit process allowed us to
model through the A and B parameters the wavelength dependent refractive index of both the
guiding (nf) and barrier layers (ns) by means of the A and B parameters and (1). Moreover, the
effective indices of the modes were calculated with the values of the numerical regression results
and the same assumptions used in the fit process. The agreement between the experimental data
(dots in Fig. 3) and the calculated effective indices is very good. Thickness of the guiding layer was
assessed to be 2.2 m, in agreement with SRIM simulations [17].
Fig. 4 shows how the difference of the refractive indices of the guiding and barrier layers is
affected by the fluence, as obtained by the same fit procedure at 635 nm. The guiding layer index
shows a slight increase with higher fluences, whereas the barrier layer exhibits a saturating larger
decrease; combination of both effects produces the guiding properties of the structure.
Fig. 3. Reconstruction of the refractive index of the guiding and barrier layers as a function of the
wavelength. Sample irradiated with 8 1016 ions/cm2Calculated effective indices of the modes are also
shown.
Fig. 4. Refractive index difference of the guiding and barrier layers versus fluence at 635 nm.
IEEE Photonics Journal N
+
Implanted Planar Optical Waveguides
Vol. 4, No. 3, June 2012 Page 725
5. Discussion and Conclusion
In conclusion, planar waveguides were fabricated by irradiation with 3.5 MeV Nþions in a rare-
earth-doped tellurite glass. Fluences of the implanted ions ranged from 1 1016 to 8 1016 ions/cm2.
The experimental results and modeling reported here demonstrate the optical barrier nature of the
waveguide due to the nuclear interactions among the bombarding and target ions. Therefore, light is
confined in the layer comprised between the end of range or optical barrier (of lower refractive
index) and the surface of the glass. Nevertheless, with an increase of the ion fluence, we observed
an increase of effective indices of the leaky modes toward the bulk refractive index value. This
seems to partially justify the assumption, formulated in a previous work [13], for which some
ionization phenomenon along the path of the nitrogen ions induces a positive refractive index
change ðn90Þlocally in the region between the surface of the glass and the end of range. All
samples supported guided modes from 635 nm to 1550 nm. Saturation of the refractive index
change occurred in the 5 1016 1017 ions/cm2range of the implanted fluence. The fact that these
waveguides supported propagation also in the 1.55 m band envisages their application in the
telecom field to the fabrication of integrated optical amplifiers capable of exploiting the good
spectroscopic properties offered by using tellurite glasses as hosts for the rare earth dopants.
Refractive indices of the barrier and guiding layers and the thickness of the guiding layer have
also been characterized using the effective indices of the waveguide modes. These measurements
have evidenced that the waveguides obtained by ion implantation exhibit a large refractive index
contrast ðnffi0:09Þ, which will enable the fabrication of waveguides with small field size, provided
the barrier formation process is optimized.
Agreement with the experimental data and thickness assessment by SRIM modeling and
ellipsometric measurements was good. Next steps will include the fabrication of channel waveguides
with optimized irradiation and structural parameters based on the results of the characterization
described in this paper.
References
[1] E. R. Schineller, R. P. Flam, and D. W. Wilmot, BOptical waveguides formed by proton irradiation of fused silica,[J. Opt.
Soc. Amer., vol. 58, no. 9, pp. 1171–1173, Sep. 1968, DOI: 10.1364/JOSA.58.001171.
[2] P. D. Townsend, P. J. Chandler, and L. Zhang, Optical Effects of Ion Implantation. Cambridge, U.K.: Cambridge Univ.
Press, 1994.
[3] F. Chen, X.-L. Wang, and K.-M. Wang, BDevelopment of ion-implanted optical waveguides in optical materials: A
review,[Opt. Mater., vol. 29, no. 11, pp. 1523–1542, Jul. 2007, DOI: 10.1016/j.optmat.2006.08.001.
[4] M. Yamada, A. Mori, K. Kobayashi, H. Ono, T. Kanamori, K. Oikawa, K. Nishida, and Y. Ohishi, BGain-flattened tellurite-
based EDFA with a flat amplification bandwidth of 76 nm,[IEEE Photon. Technol. Lett., vol. 10, no. 9, pp. 1244–1246,
Sep. 1998.
[5] G. Nunzi Conti, S. Berneschi, M. Bettinelli, M. Brenci, B. Chen, S. Pelli, A. Speghini, and G. C. Righini, BRare-earth
doped tungsten tellurite glasses and waveguides: Fabrication and characterization,[J. Non-Cryst. Solids, vol. 345/346,
pp. 343–348, 2004.
[6] R. Rolli, K. Gatterer, M. Wachtler, M. Bettinelli, A. Speghini, and D. Ajo, BOptical spectroscopy of lanthanide ions in
ZnOÿTeO2glasses,[Spectrochim. Acta A, Mol. Spectrosc., vol. 57, no. 10, pp. 2009–2017, Sep. 2001.
[7] A. Narazaki, K. Tanaka, K. Hirao, and N. Soga, BInduction and relaxation of optical second-order nonlinearity in tellurite
glasses,[J. Appl. Phys., vol. 85, no. 4, pp. 2046–2051, Feb. 1999.
[8] Z. Jihong, T. Haizheng, C. Yu, and Z. Xiujian, BStructure, upconversion and fluorescence properties Er3þ-doped
Te02ÿTi02ÿLa2O3tellurite glass,[J. Rare Earth, vol. 25, no. 1, pp. 108–112, Jun. 2007.
[9] Y. Yang, B. Chen, C. Wang, G. Ren, Q. Meng, X. Zhao, W. Di, X. Wang, J. Sun, L. Cheng, T. Yu, and Y. Peng,
BSpectroscopic properties ofEr3þ-doped xGeO2–(80-x)TeO2–10ZnO–10BaO glasses,[J. Non-Cryst. Solids, vol. 354,
no. 31, pp. 3747–3751, Aug. 2008.
[10] T. Toney Fernandez, S. M. Eaton, G. Della Valle, R. Martinez Vazquez, M. Irannejad, G. Jose, A. Jha, G. Cerullo,
R. Osellame, and P. Laporta, BFemtosecond laser written optical waveguide amplifier in phospho-tellurite glass,[
Opt. Exp., vol. 18, no. 19, pp. 20 289–20 297, Sep. 2010.
[11] S. J. Madden and K. T. Vu, BVery low loss reactively ion etched Tellurium Dioxide planar rib waveguides for linear and
non-linear optics,[Opt. Exp., vol. 17, no. 20, pp. 17 645–17 651, Sep. 2009.
[12] K. Vu and S. Madden, BTellurium dioxide Erbium doped planar rib waveguide amplifiers with net gain and 2.8 dB/cm
internal gain,[Opt. Exp., vol. 18, no. 18, pp. 19 192–19 200, Aug. 2010.
[13] S. Berneschi, G. Nunzi Conti, I. Ba´nya´ sz, A. Watterich, N. Q. Khanh, M. Fried, F. Pa´szti, M. Brenci, S. Pelli, and
G. C. Righini, BIon beam irradiated channel waveguides in Er3þ-doped tellurite glass,[Appl. Phys. Lett., vol. 90,
no. 12, pp. 121136-1–121136-3, Mar. 2007, DOI: 10.1063/1.2717085.
IEEE Photonics Journal N
+
Implanted Planar Optical Waveguides
Vol. 4, No. 3, June 2012 Page 726
[14] I. K. Naik, BLow-loss integrated optical waveguides fabricated by nitrogen ion implantation,[Appl. Phys. Lett., vol. 43,
no. 6, pp. 519–520, Sep. 1983.
[15] A. B. Faik, P. J. Chandler, P. D. Townsend, and R. Webb, BRefractive index changes formed by Nþimplants in silica,[
Rad. Effects, vol. 98, no. 1–4, pp. 233–241, 2006.
[16] I. Ba´ nya´ sz, M. Fried, C. Du¨cso¨ , and Z. Ve´ rtesy, BRecording of transmission phase gratings in glass by ion implantation,[
Appl. Phys. Lett., vol. 79, no. 23, pp. 3755–3758, Dec. 2001.
[17] J. F. Ziegler, BSRIM-2003,[Nucl. Instrum. Methods Phys. Res. B, vol. 219, pp. 027–036, 2004. [Online]. Available:
http://www.srim.org.
[18] D. E. Aspnes, BOptical properties of thin films,[Thin Solid Films, vol. 89, no. 1, pp. 249–262, 1982.
[19] Woollam Co., Inc., Lincoln, NE. [Online]. Available: http://www.jawoollam.com
IEEE Photonics Journal N
+
Implanted Planar Optical Waveguides
Vol. 4, No. 3, June 2012 Page 727