We have made and characterized a new, erbium-doped tellurite glass that has high glass transition temperature. Addition of phosphate is found to increase the phonon energy. The peak emission cross section is 6 × 10-21 cm2 at 1537 nm and the fluorescence lifetime of the 4I13/2-4I15/2 transition is 4.1 ms. We have written 2-D channel waveguides in this glass using focused, 45-fs pulses from an amplified Ti:sapphire laser at different laser energies and writing speeds. Migration of atoms towards the periphery of the waveguides occurs, leading to refractive index changes. Channels show waveguiding at 1310 nm which is promising for the fabrication of integrated lasers and broadband amplifiers.
©2006 Optical Society of America
Micromachining within glass substrates by means of femtosecond laser pulses has potential application in the fabrication of optical integrated circuits . The ready availability of high power and low pulse-width laser technology has led to increasing interest in fabricating channel waveguides by this technique . Such laser writing opens new vistas for fabrication of 3-D waveguides inside transparent glass substrates, which is not possible using conventional ion-exchange and photolithographic processes . Channel waveguides written using ultrafast lasers in erbium-doped phosphate glasses for integrated amplifiers and lasers operating in the C-band have been demonstrated [4,5]. Extension of the method to telluritebased glasses has been less successful: there is one report on longitudinal writing of relatively short waveguides and positive refractive index change in niobium tellurite glasses using 130 fs pulses . In contrast, it has also been reported that only negative refractive index change is induced by ultrafast laser irradiation of tellurite glass, making waveguiding impossible . Moreover, surface damage during photolithographic processing has hindered fabrication of ion-exchanged channel waveguides in Er-doped tellurite glass . On the other hand, tellurite glasses that are doped by rare earth elements like thulium and erbium are strong candidates for broadband amplification in the S-C-L bands . Tellurite, as a host of Er-doped glass, has the disadvantage of having a small phonon energy, which enhances the upconversion process and decreases gain at 1550 nm under 980 nm pumping. Addition of P2O5 in TeO2 increases the phonon energy as well as the glass transition temperature in a phospho tellurite (PT) host doped with erbium .
In this paper we report waveguide writing in an erbium-doped tellurite glass modified by phosphate and aluminum addition. The physical and spectroscopic properties of this glass are studied to assess its suitability for 1550 nm amplification under 980 nm excitation. Centimetre-long channels have been written by us inside the glass using ultrashort (45 fs) laser pulses. Migration of La and P atoms occurs from the center towards the periphery of such channels; the resulting modulation of Te:P ratio across the irradiated region may be responsible for a net positive refractive index change across each channel.
2. Experimental procedure
Tellurite glass doped with 0.25 mole% erbium was made by us using the melt quenching technique. High-purity chemicals like TeO2, P2O5, Al2O3, La2O3 and Er2O3 were used for batch melting. A two-step melting process at ~1100°C was performed before quenching. The melt obtained after the second step was quenched at ~350°C to form the transparent glass. The glass was annealed for 1 hour below the glass transition temperature, and slowly cooled down to room temperature. Polished glass was used for spectroscopy, refractive index measurement and waveguide writing using a femtosecond laser.
The refractive index was measured using a prism coupler (Metricon 2010) at 633 nm (Table 1). The erbium ion concentration (N) in the glass was calculated from the density and the weight percentage of the Er2O3 used initially for batch melting. The glass transition temperature (Tg) was measured using a differential scanning calorimeter (Perkin Elmer DSC-7) at a heating rate of 10 °C/min. The glass transition temperature Tg obtained from the measurement is indicated in Table 1.
The absorption spectrum in the wavelength range 300–1100 nm was recorded using a dualbeam spectrophotometer (Perkin Elmer Lambda 25) with 0.1 nm resolution. Room temperature fluorescence spectrum was recorded in an optical spectrum analyzer (Agilent 86142B) using a cw 980 nm laser diode (JDS Uniphase) as the excitation source. The fluorescence decay of the glass (from Er3+:4I13/2 level) was measured using an InGaAs detector and a 200 MHz digital storage oscilloscope (Tektronics TDS 2024) by modulating the laser diode at ~ 10 Hz.
The spectral broadening due to self phase modulation of the laser is observed on transmitting unfocused laser light, of energy 343 µJ, through the glass sample. The spectrum was recorded by a fiber-coupled spectrometer (Ocean Optics) over the wavelength range 750–880 nm. Using this data the nonlinear index of refraction, n2, of the glass was deduced.
The waveguide writing experiment used a Ti:sapphire laser operating at 806 nm wavelength with a pulse duration of 45 fs and 1 kHz repetition rate. The laser was tightly focused inside the transparent tellurite glass using a microscope objective (25X, NA=0.45), and the glass was translated transversely to the focused beam at three different writing speeds.
3. Results and discussion
3.1 Bulk glass properties
The composition of the glass that we prepared is 67TeO2-30P2O5-1Al2O3-1.75La2O3-0.25Er2O3 (all in mole %); the physical and spectroscopic properties that we measured are summarized in Table 1. The refractive index is higher compared to that of phosphate glasses. The glass transition temperature is seen to have increased compared to the phospho tellurite glass reported by us earlier . Addition of Al2O3 and La2O3 into the glass matrix is responsible for this enhancement. The absorption spectrum in the UV-VIS region is shown in Fig. 1. The absorption peaks are labeled with respect to transitions from the ground level (4I15/2) to various upper levels of erbium. Oscillator strengths of these transitions were used to calculate the JO intensity parameters (Ωt) of the erbium doped glass [11, 12]. The parameter Ω2 shows a decrease compared to the earlier reported glass , indicative of the formation of more covalent bonding upon addition of aluminum and lanthanum oxides.
The emission cross-section of the glass was obtained from the emission spectrum using standard procedures . The radiative lifetime obtained from the JO analysis was used in the calculation of the spectrum. A typical emission cross-section spectrum is depicted in Fig. 2(a) and yields a cross-section of 6×10-21 cm2 at 1537 nm. The fluorescence decay, shown in Fig. 2(b), yields a lifetime of 4.1 ms.
In order to deduce the nonlinear refractive index, n2, of the glass we used the self phase modulation (SPM) phenomena that generates spectral broadening in the 750–880 nm wavelength region. SPM was induced using an unfocussed, 45 fs laser pulse of energy 343 µJ. The peak wavelength of the incident laser pulse was measured to be 806 nm, with 26 nm bandwidth. After passing through a glass sample, the peak laser wavelength shifted to 805 nm and bandwidth increased to 31 nm. The broadening (Fig. 3) allows us to deduce  the value of n2 to be 6×10-20 m2/W, about an order of magnitude less than that reported for tellurite glass . We note that there is an overlap of the broadened light output (Fig. 3) with the 4I9/2 state (Fig. 1). However, the product of sample length (0.1 cm) and absorption coefficient at 800 nm for the 4I9/2 state (0.2 cm-1) is small enough to ensure that this overlap did not seriously affect the n2 value that is deduced by us.
3.2 Channel waveguides
Channels were fabricated in the glass using procedures described in the experimental section. Figure 4(a) shows the top view of three channels in a typical glass sample photographed using optical microscope. Each of these channels was written at 3µJ laser energy and constant writing speed of 0.01 cm/s (top channel), 0.02 cm/s (middle channel), and 0.03 cm/s (bottom channel).. Many other writing experiments were performed using different laser energies and writing speeds, but the essential morphological features remained the same as shown in Fig. 4(a).
We used the transverse writing geometry without any beam shaping. The channels show good continuity under microscope inspection and are separated by ~50 µm. They are formed at a distance of 500–800 µm below the surface of the glass. The width of the channels lies in the 4–8 µm range and each channel is 1 cm long. We have imaged the cross section of the channels under white light illumination and a slight ellipticity is noted. While transverse micromachining puts no limits on the length of the waveguide, the cross-section of the waveguide in this geometry is expected to be elliptical because the structural modification is localized over the confocal (Rayleigh) range. To minimize ellipticity necessitates a transverse circular focal volume and this can only be achieved if the laser beam is shaped using either beam-shaping optics or slits [16, 17]. No beam shaping was carried out in the present work.
We also noted that there was a certain amount of ablation of the glass at the channel end faces. It has been reported that explosive expansion of ionized material (microexplosion) can take place in a tellurite glass under femtosecond laser irradiation . However, in the present glass the lowering of the n2 value (increase in the bandgap) compared to previously studied tellurite glasses has resulted in lower nonlinear absorption. Moreover, the higher glass transition temperature and the structural modification introduced by phosphate addition seems to favor waveguide formation. These facets are indicated in the side images of the waveguides that we depict in Fig. 4(b). The images shown in Figs. 4(a) and 4(b) are indicative of a refractive index profile that is quite complex. More detailed studies are clearly warranted.
The channel written glass was edge polished for efficient light coupling; light was launched into the channels by butt-coupling the fiber output from a 1310 nm laser source. Optical guidance was clearly observed. In Fig. 4(c) we show a mode-profile image (through a 45X microscope objective) of the guided 1310 nm laser beam through the channel laserwritten at a speed of 0.02 cm/s. The 3-D intensity profile of this channel output, shown in Fig. 4(d), indicates a net positive change in the refractive index, contrary to a previously reported result in tellurite glasses . We made loss measurements at 1310 nm; estimated propagation losses are < 2 dB cm-1. We also measured atomic concentrations of different components across the channels that we wrote, using scanning electron microscopy with X-ray microanalysis (LEO 1430VP). We find a decrease in La and P atomic concentrations at the center of the channel and a migration of these atoms towards the periphery of the channels, resulting in a modulation of Te:P ratio across the irradiated region. This modulation can lead to a positive refractive index change  across each channel.
An erbium-doped tellurite glass with new composition has been designed, fabricated and its physical and spectroscopic properties characterized. Channels have been written inside this glass with focused, 45 fs laser pulses at 806 nm wavelength, using different energies and by varying substrate translation speeds. Our laser-written channels guide light of 1310 nm wavelength with estimated propagation losses of < 2 dB cm-1. We believe this to be the first report of waveguiding in ultrashort-laser-written channels in erbium-doped tellurite glass. We attribute the positive index change under laser irradiation to the modification of the glass upon addition of phosphate. This introduces significant increase in the glass transition temperature and decrease of the nonlinear index of refraction. The results obtained in present work are promising for the fabrication of erbium-doped integrated amplifiers and lasers based on tellurite glasses.
GJ acknowledges financial support from Department of Science & Technology, New Delhi under the Fast-Track scheme. JAD thanks the Homi Bhabha Fellowship Council for financial support.
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