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We report on the first demonstration of an optical waveguide amplifier in phospho-tellurite glass providing net gain at 1.5 μm. The device was fabricated using a high repetition rate femtosecond laser and exhibited internal gain across 100-nm bandwidth covering the entire C + L telecom bands.
©2010 Optical Society of America
1. Introduction
Active waveguides are key components for photonic circuits and networks. The femtosecond laser writing method is a promising technique for directly fabricating waveguides with 3D geometries in both passive and active glasses [1–3]. The method was first applied to write active waveguides in Nd-doped glass in 2000 [4] and then experimented on Er:Yb-doped phosphate glass to achieve operation at telecom wavelength [5] with the demonstration of net gain across the narrow C telecommunications band (1530-1565 nm) [6]. In an Er-doped bismuthate glass, femtosecond laser writing yielded an impressive 16-dB net gain at 1533-nm wavelength for 87-mm long waveguides [7].
The recent trend towards low-cost, coarse wavelength division multiplexing (CWDM) systems for photonic networks has required the use of longer wavelengths extending the amplification into the L band (1565-1625 nm). To broaden the emission bandwidth compared to phosphate glasses, tellurite-modified phosphates have been recently proposed and waveguides were femtosecond laser-written in Er-doped phospho-tellurite glass [8,9]. Researchers have also applied the femtosecond laser writing method to fabricate waveguides in zinc-modified tellurite glass with the purpose of producing active photonic devices for mid-IR applications [10]. Although the benchmark ion-exchange method is a fully developed technique, it is not suitable for fabricating waveguides in tellurite due to the surface damage induced by the wet etching step [11]. A rib waveguide with low losses (0.1 dB/cm) was recently fabricated using reactive sputtering and reactive ion etching [12], however this multistep method based on photolithography is more complex and not suitable for rapid prototyping of waveguide devices in tellurite. Unlike traditional photolithographic methods, femtosecond laser writing is a versatile and simple direct writing process, enabling single-step fabrication of waveguide devices in tellurite and other glasses without the need for a cleanroom environment.
In a previous paper [9] the present authors demonstrated an internal gain bandwidth of ~70 nm in a fs-laser written Er-doped phospho-tellurite glass, but the crucial achievement of a fiber-to-fiber (i.e. net) gain was still lacking, thus preventing a clear-cut demonstration of a real active device in this multi-component glass. Actually, the previous glass [9] led to high propagation loss (1.4 dB/cm) in the waveguide, as well as high coupling loss to standard silica fibers, due to the high Fresnel losses resulting from a high refractive index (1.95) of the glass base. Also, the active performance was limited by a relatively poor inversion of erbium ions, mainly caused by concentration quenching effects and non-optimized spectroscopical parameters. Finally, the lower P:Te ratio resulted in hygroscopicity of the glass which affected the stability of the waveguide performance over a long operation time. In this work, a new active phospho-tellurite glass is developed and waveguides are inscribed in it with a high repetition rate femtosecond laser. As compared to previous glass [9], we embedded fluorine and cerium to enhance the spectroscopical figures of merit and a new P source was adopted to increase the concentration of phosphate and reduce hygroscopicity. Also we introduced Yb-codoping to enhance pump absorption at 980 nm and exploit Yb3+ to Er3+ energy transfer mechanism to provide better inversion of Er3+ ions [13,14].
2. Fabrication and characterization of the glass sample
Transparent phospho-tellurite glasses were prepared by using high purity (>99.99%) TeO2, Na3P3O9, ZnO, ZnF2 doped with 1wt% Er2O3, 1wt% CeO2 and 2wt% Yb2O3. The chemicals were batched for required molar glass composition (50TeO2:20P2O5:20Na2O:5ZnO:5ZnF2) in dry air atmosphere to reduce moisture absorption. The batch was melted in an alumina crucible at 1150°C for 4 hours under dry oxygen atmosphere. The molten batch was casted and annealed at a temperature of 300°C for three hours, before being cooled to the room temperature at the rate of 0.5°C/min. From the energy dispersive x-ray (EDX) measurements we found that the loss of P2O5 in this phospho-tellurite glass prepared using Na3P3O9 to be less than 1wt%. This improvement of phosphate concentration might be due to the higher boiling point of Na3P3O9 (~620°C) and, as expected, it resulted in less hygroscopicity of the glass as compared to previous one [9]. The prepared glass was then cut and polished to optical quality for spectroscopic characterization and waveguide inscription.
The fluorescence decay lifetime of the glass was measured using a 980 nm diode laser which was modulated using a pulse generator. The measured decay lifetime [Fig. 1(a) ] of the fluorescence at 1535 nm corresponding to Er3+: 4I13/2 to 4I15/2 transition was 9.7 ms, which is 2.5-fold higher than the phospho-tellurite glass without fluorine studied previously [9]. There are several factors that may be responsible for the increased lifetime and these are currently under investigation. In high fluoride concentration glasses, the formation of -O-Er-F ionic bonds and O-Er-PO4 and F-Er-PO4 bonds may be responsible for a longer lifetime of the radiative transition and much reduced concentration quenching [15].
Fig. 1 (a) Fluorescence lifetime of the sample with 980nm wavelength excitation. (b) Absorption cross section of the 4I13/2 ←→ 4I15/2 erbium transition.
Another consequence of the fluorine addition is the reduction of OH- radicals in the glass, thus reducing the multiphonon relaxation process from Er: 4I13/2 level. On the other hand, the better solubility of rare earth ions in the reported glass due to the presence of phosphates reduces the fluorescence quenching by cross relaxation between Er3+ ions. The addition of Ce3+ to the glass is expected to decrease the excited state absorption from 4I11/2 through a cross-relaxation process [16]. The absorption cross section [Fig. 1(b)] of the glass is similar to the one previously studied [9]. The Raman spectrum shown in Fig. 2 was acquired with 633-nm laser excitation of the sample. As the tellurite glass network is modified in the presence of phosphate, the second glass former, the maximum phonon energy of the glass shifts towards the frequency of vibrations of PO4 tetrahedra from that of TeO4 bipyramids and TeO3 trigonal pyramids. The deconvoluted spectral peaks (Fig. 2, green curves) are identified and assigned to various optical phonon vibrations as follows, the first four vibration peaks within 300 - 500 cm−1 are assigned to the symmetrical stretching/bending vibrations of O-Te-O or Te-O-Te linkages [16, 17] the peak at 650 cm−1 is assigned to the antisymmetrical TeO4 vibration [18]. The main peak at 800 cm−1 corresponds to the TeO4 vibrations [19, 20] and the small peak at 790 cm−1 is due to the stretching vibrations of TeO3 [18]. Finally the two peaks at 1030 cm−1 and 1112 cm−1 are the contributions from the stretching vibrations of phosphate groups PO4 and PO2 respectively [21]. The increased structural connectivity between the phosphate (1030 cm−1) and tellurite (650 cm−1 and 780 cm−1) structural groups through bridging oxygen is evident from the apparent strength of the phonon bands in the 500 cm−1 in comparison with the sodium zinc tellurite structures [17]. Note that the Raman spectra of pure tellurite glass extend only up to 840 cm−1 but this phosphate modified tellurite glass has bands extending up to 1200 cm−1 thus increasing the average and maximum phonon energy of the base glass, which is expected to enhance the efficiency of the energy transfer from Yb3+ to Er3+ ions.
Fig. 2 Raman spectrum (red curve) at 20°C and deconvoluted vibrational spectra (green curves) of the Er3+-Yb3+ co-doped phospho-tellurite glass. The spectrum was deconvoluted using Gaussian fitting peaks and interpolation base correction. Initially the peak positions (selected by referring to the literature cited by considering the composition of the glass) were held constant, with amplitude and area taken as variables. Then the peak positions were varied by 1 cm−1 to find the best fit with R2~1. The Reduced Chi-square value was 1.92265 x 10−6.
3. Waveguide fabrication and passive characterization
3.1 Waveguide fabrication
The composition of glass reported here contains comparatively much reduced TeO2 concentration than the previously reported phospho-tellurite glass [9], the refractive index of glass was reduced from n = 1.95 to 1.66, which is more attractive for waveguide writing due to lower spherical aberration from the air-glass interface [22] and reduced self-focusing [23,24]. Further, the reduced refractive index is more suitable for coupling with conventional silica-based fiber optics due to reduced Fresnel losses and readily available index matching oils/gels at lower indices. Waveguides were written with a Yb:KYW femtosecond laser with 1-MHz repetition rate (model High Q femtoREGEN), 1040-nm wavelength, 400-fs pulse duration and 8-W average power. Such a laser is advantageous compared to conventional 1-kHz repetition rate Ti:Sapphire amplified lasers because at repetition rates greater than 200 kHz an interplay between heat accumulation and thermal diffusion can partially compensate for the asymmetric focal volume to yield waveguides with circular guided modes that couple efficiently to optical fibers [25].
The laser was focused 170 μm below the surface of the glass samples using a high numerical aperture (NA = 1.4) oil-immersion microscope objective. With moderate NA objectives (NA = 0.5), the resulting laser-formed structures were more asymmetric due to the greater offset between the depth of focus and spot size and from the increased self-focusing expected at lower NA where higher processing power is required. The sample was translated transversely relative to the laser with the polarization direction perpendicular to the scan axis. The trial windows of average power and translation speeds were 65 mW – 150 mW and 1 mm/s – 10 mm/s, respectively.
3.2 Passive waveguide characterization
Optical waveguides were first qualitatively characterized in terms of their morphology from overhead using white-light microscopy [Fig. 3(a) ]. The optimum waveguides, written with 1-MHz repetition rate, 130-nJ pulse energy and 4-mm/s scan speed, showed a uniform morphology with a bright contrast in the center. Figure 3(b) shows the cross sectional view of the waveguide, exhibiting a complex morphology with two dark circular regions along the direction of the writing laser beam (y-axes).
Fig. 3 (a) Overhead microscope image and (b) end facet cross-section of the optimum waveguide written with a speed of 4 mm/s and pulse energy of 130 nJ. The fs laser beam is incident along the y-axis and the sample is translated along the z axis.
The refractive index of the optimum waveguides were characterized quantitatively using a refracted near field (RNF) profilometer (Rinck Eletronik) [26]. Figure 4 shows the refractive index change relative to the bulk glass.
Fig. 4 (Color online) Refractive index profile by RNF of optimum waveguide written with 130-nJ pulse energy and 4-mm/s scan speed. A 3D representation of the refractive index change distribution is shown in (a). Slices along x and y axis are shown in (b) and (c), respectively.
A peak refractive index change of Δn = 2.5 × 10−3 was found, slightly higher than reported in a previous study in phospho-tellurite glass [9], but comparable to that reported in phosphate [27] and silicate glasses [25] using high repetition rate femtosecond laser writing. The refractive index morphology is symmetric about the transverse x-axis [Fig. 4(b)] as expected since the writing laser is a symmetric Gaussian beam. The RNF analysis revealed that the two dark regions observed in white-light microscopy (Fig. 3) corresponds to negative index change regions above and below a central guiding region of positive index change, as clearly visible in the index distribution along the laser writing direction (y-axis) in Fig. 4(c).
Waveguides were then characterized in terms of their guided intensity mode profiles at the pump (980 nm) and signal (1600 nm) wavelengths. Light was butt-coupled to the input facet of the optically polished waveguide sample using a standard single mode fiber (SMF).
An aspheric lens (60 × ) was aligned at the output facet to image the waveguide mode on a phosphor-coated CCD (Spiricon LBA-USB-SP620-1550). As a reference, the profiles of the fundamental mode of the SMF are shown in Fig. 5(a) , revealing expected circular profiles with mode field diameter (MFD, evaluated as 4 times the standard deviation of the energy distribution along the X and Y transverse cross-sections of the beam intensity profile) of 10.5 μm and 6.5 μm at 1600- and 980-nm wavelengths, respectively.
Fig. 5 (Color online) Guided mode profiles at 980 nm and 1600 nm wavelengths for (a) SMF, (b) waveguides written with 1-MHz repetition rate, 130-nJ pulse energy and scan speeds of 2, 4 and 6 mm/s. The total insertion loss of the waveguides is shown in the right column.
Figure 5(b) shows the mode profiles at pump and signal wavelengths for the optimum waveguides formed at 1-MHz repetition rate, 130-nJ pulse energy and scan speeds of 2, 4 and 6 mm/s. At 1600 nm, symmetric and well-confined fundamental modes were observed with MFD of 8.5 μm, 9.5 μm and 10.7 μm at speeds of 2, 4 and 6 mm/s, respectively. The trend of mode size increasing with scan speed indicates a decreasing refractive index change, which is expected as the net fluence (laser exposure) is reduced. At the 980-nm wavelength, multimode behaviour is found at 2 mm/s writing speed, where the highest index contrast is expected. Such a multimode behaviour will result in poor pump-signal overlap which will reduce the internal gain for active operation. At 4 mm/s (MFD 6.1 μm) and 6 mm/s (MFD 5.9 μm) writing speed, well-confined modes were found at 980-nm wavelength, showing good overlap with the signal mode at 1600 nm.
Shown in the right column of Fig. 5(b) is the insertion loss (IL) at 1650-nm wavelength, i.e. outside the absorption band of the glass. The IL (defined as the excess loss introduced in the transmission of a standard telecom fiber when it is cut and the waveguide is inserted in the path) was obtained by butt-coupling the waveguide with two standard telecom fibers, with index matching fluid to minimize Fresnel losses, and comparing the transmitted power to that measured with the same fibers directly butt-coupled to each other. The insertion loss, which includes coupling and propagation losses, was found to be lowest (2.3 dB) for the waveguide inscribed with a scan speed of 4 mm/s. After accounting for a coupling loss of 0.15 dB/facet (calculated from the fiber-waveguide mode overlap integral [28]), a propagation loss of 0.9 dB/cm was inferred for the 2.1-cm long waveguide. This propagation loss is an improvement on the previous best result of femtosecond laser writing in active phospho-tellurite glasses (1.4 dB/cm) [9].
4. Optical amplification in phospho-tellurite channel waveguide
The active gain performance of the waveguide was characterized by means of a standard set-up with the bi-propagating pump scheme (Fig. 6 ). The waveguide was butt-coupled to single mode optical fibers by means of precision 5-axes micro-positioning stages (Melles-Griot Nanomax). A fiber-pigtailed AlGaAs laser diode provided the pump radiation at 976 nm (220 mW maximum pump power) at both ends via a wavelength division multiplexer (WDM). The probe signal from a tunable laser source (Agilent 8164B) was fed in through the WDM along with the co-propagating 976-nm wavelength pump. The tunable laser is a low noise narrow bandwidth source operating in the wavelength range 1460-1640 nm. The probe signal was attenuated to −26 dB/nm input power level (monitored by a high precision power meter, Ando AQ2140) by means of a variable attenuator. An index matching fluid able to support high power density at the pump wavelength was also inserted between waveguide and fiber ends.
Fig. 6 (Color online) Experimental setup for active waveguide characterization in a bi-propagating pump scheme.
Using the set-up above described, the insertion loss, absorption spectrum, enhancement and internal gain of the waveguide were measured and the net gain was deduced from this data. A maximum absorption of 4.7 dB was measured at 1534 nm, as shown in Fig. 7(a) .
Fig. 7 (Color online) (a) Measured absorption spectrum (dotted line) of the 21-mm long active waveguide with enhancement (dashed line) and internal gain (solid line) at 400 mW total incident pump power. (b) Internal gain at 1534 nm (squares), 1550 nm (circles) and 1610 nm (triangles) as a function of incident pump power.
For gain measurements we adopted the ON-OFF technique and the spurious contribution given by the amplified spontaneous emission (ASE) to the signal output was taken into account by preliminary measurement of the ASE noise (integrated on the resolution bandwidth) according to the standard procedure reported in [29]. The enhancement spectrum of the waveguide (which is defined by the ratio between the output power level with pump ON and the output power level with pump OFF) shows a peak value of 7.9 dB at 1534 nm, spanning a bandwidth of 100 nm from 1530 nm to 1630 nm. This represents a ~50% improvement from the internal gain bandwidth of 70 nm reported in the previously studied glass [9]. Most importantly, a maximum internal gain value of 3.2 dB was achieved at the erbium peak, yielding for the first time a net gain (~1 dB), which is defined as the difference in internal gain and the insertion loss, enabling the waveguide to operate as an optical amplifier device. Figure 7(b) shows the small-signal internal gain as a function of incident pump power at 1534 nm, 1550 nm and 1610 nm wavelength. The saturating behavior of the curves indicates a good average inversion of active ions along the waveguide resulting in higher internal gain compared to the previous study [9].
Improvement of the insertion loss figure and optimization of the active ions doping level in a longer active substrate can eventually extend the operation of the optical waveguide amplifier as a C + L-band device.
It is worth noting that the same fs-laser written amplifier was repeatedly tested during several months and no degradation of the performance was observed, in agreement with the indication that the present glass exhibits poor hygroscopicity.
5. Conclusions
Femtosecond laser written active waveguides have been fabricated in an erbium-ytterbium-doped phospho-tellurite glass by means of a Yb-KYW femtosecond laser system. The newly optimized glass substrate allowed a significant improvement of the passive as well as active figure if merits of the written waveguides. Optimum waveguides showed low propagation loss (0.9 dB/cm) as well as low coupling losses to standard single mode fibers, with a large internal gain bandwidth of 100 nm spanning the entire C and L bands (1530-1630 nm), enabling for the first time (using any fabrication technology) fiber-to-fiber (i.e. net) optical amplification in an active-doped phospho-tellurite waveguide.
Acknowledgements
T. T. Fernandez and P. Laporta would like to acknowledge the fellowship from the Italian Ministry of University and Research (Prot. n.1039/, 11.06.2008) in the framework of the research co-operation between Politecnico di Milano and Mahatma Gandhi University, Kottayam (India). S. M. Eaton would like to thank the Natural Sciences and Engineering Research Council of Canada (NSERC) for a postdoctoral fellowship. The authors would also like to thank N. V. Unnikrishnan, School of Pure & Applied Physics, Mahatma Gandhi University.
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