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Ion-exchanged planar lossless splitter at 1.5µm

P.Camy, J.E.Román, F.W.Willems,  M.Hempstead, J.C.van der Plaats , C.Prel, A.Béguin,
A M J Koonen, J S Wilkinson, and C Lerminiaux

Corning Europe Inc., 7 bis, Avenue de Valvins, 77210 Avon, France
Optoelectronics Research Centre
AT&T Network Systems, P.O.Box 18, 1270 AA Huizen, The Netherlands

Abstract

We demonstrate the first ion-exchanged, planar lossless splitter at 1.5 μm. The 1 × 2 device was fabricated by thallium ion exchange in an Er/Yb codoped borosilicate glass, and achieved lossless splitting over the wavelength range of 1534-1548nm when pumped with a 980 nm laser diode.

Introduction
The growing need for cost-effective extension of broadband digital and CATV 1.5 μm optical networks from the curb to business premises - or even the home - has made the development of planar lossless splitters highly desirable. Such devices must combine splitting ratios of up to 1×16 with noise figures as low as 5dB [1]. These demanding specifications require a low-loss waveguide fabrication technology suitable for mass reproduction at low cost. Such technology must also allow the realisation and integration of passive splitters with amplifying sections capable of achieving net gains of between 3 and 13 dB using laser diode pumping. To date, thin-film sputtering [2], flame-hydrolysis [3, 4], and ion-exchange [5, 6] have been employed as fabrication techniques to realise erbium-doped planar waveguide amplifiers at 1.5 μm. Of these technologies, ion-exchange is particularly attractive because it is well-developed and has been used in the fabrication of reliable commercial passive devices. Furthermore, net gains of 6dB [5] and 7 dB [6] have already been reported in Er and Er/Yb codoped ion-exchanged glass waveguides, respectively. Recently, we demonstrated the potential of the ion-exchange technology to realise planar devices for broadband fibre-to-the-home telecommunication systems by realising a lossless splitter [7]. The configuration of this device was similar to one realised by flame-hydrolysis [3], and comprised a 1 × 2 splitter fabricated by thallium ion exchange in an Er/Yb codoped silicate glass. In this letter, we report on the performance of an improved, fibre-pigtailed device.

Gain measurements
In order to evaluate the prospects for lossless splitting, we measured the gain of a prototype waveguide amplifier to be used in the splitter. The prototype amplifier section was a straight waveguide fabricated by thallium-sodium exchange in a borosilicate glass uniformly codoped with 5% wt Yb2O3 and 3% wt Er2O3. The fabrication process included a second exchange to bury the waveguide. The resulting waveguide was 3.9 cm long and had full-width modal intensity dimensions of 4.2 × 3.3 μm2 at 980 nm and 6.5 × 5 μm2 at 1480 nm. Using a single-mode 980/1550nm fibre WDM for input coupling, we measured insertion losses of 3 dB and 3.6 dB at 980 nm and 1400 nm, respectively. Fig. 1 shows the measured net gain versus pump power for this waveguide using a Ti:sapphire laser tuned to 978 nm as the pump source. The net gain was obtained by measuring the signal throughput with and without the device, taking care to subtract any amplified spontaneous emission. A net gain of 9 dB at 1537 nm was achieved with 130 mW of pump power for a gain coefficient of 2.3 dB/cm and a gain efficiency of 0.07 dB/mW. In principle, this gain is sufficient to achieve 1 × 8 lossless splitting, and the power required is close to that available from commercial, fibre-pigtailed 980 nm diodes.


Figure 1Measured net gain in straight Er/Yb codoped channel waveguide.

1×2 splitter
Once the straight waveguide amplifier was characterised, we proceeded to fabricate the 1×2 splitter using the same Er and Yb concentrations and fabrication procedure. In this case, the photolithographic mask used contained a straight amplifying section followed by a 1×2 splitting section. In addition to the standard fabrication process described above, the input and output ends of the device were thermally tapered over approximately 0.5 cm in order to reduce the insertion losses of the device at 1.5 μm whilst maintaining tight mode confinement in the majority of the gain section, thus avoiding reduction in the net gain. The resulting 3.5 cm-long device, shown in fig. 2, comprised a 1.6 cm-long straight section followed by a 1.9-cm long splitting section. The tapering process increased the 1/e2 modal intensity dimensions up to 8×7 μm2 from the original dimensions of 6×5 μm2, and brought the insertion losses at 1400 nm down to 2.2 dB from the original value of 3.6 dB. Finally, the tapered input and output ends of the device were pigtailed to 980/1550 nm fibre WDMs.

Figure 2Planar 1×2 lossless splitter.

To measure the net gain and noise figure (NF) of the lossless splitter, a tunable laser diode was used as the 1.5 μm signal and a fibre-pigtailed 980 nm laser diode as the pump source. The fibre-pigtailed pump and signal sources were spliced to the input fibre WDM of the device. At the device output, the signal from each splitter arm was separated from the pump by the fibre 980/1550 nm WDMs and detected. The net gain was measured in a similar manner to that described previously. Fig. 3 shows the measured net gain and noise figure versus wavelength for one of the two arms of the device. The noise figure was measured using the polarisation-nulling technique [8] with copropagating pump. In these measurements, the pump power at the fibre output of the 980 nm pigtailed diode was 115 mW and the signal power was -11 dBm. As the figure shows, lossless splitting with an 8 dB noise figure was achieved over the wavelength range of 1534-1548 nm. The other output arm, not shown in the figure, also achieved lossless splitting, but with a lower gain.


Figure 3Measured net gain and noise figure vs. wavelength for one of the splitter arms.

Discussion
The 2.3 dB/cm gain coefficient and 0.07 dB/mW gain efficiency achieved with 130 mW in the 3.9 cm long straight waveguide amplifier are comparable with those achieved in the planar amplifiers fabricated by thin-film sputtering (2.2 dB/cm and 0.07 dB/mW) [2] and flame hydrolysis (0.34 dB/cm and 0.12 dB/mW) [4] using the same pump power, and is better than that achieved in an ion-exchanged waveguide amplifier realised in an Er/Yb co-doped phosphate glass [6] with similar Er and Yb concentrations.

In the lossless splitter, the active and passive waveguiding regions were not separated. Consequently, the pump power in each arm of the splitter suffered a decrease of roughly 3 dB, thus limiting the gain achievable in each arm. In addition, the waveguides were multimoded at 980 nm, which led to different pumping conditions and thus different gains at the two outputs. Furthermore, although tapering of the waveguide ends reduced the coupling losses at 1.5 μm, it would be more advantageous to have the tapered sections outside the erbium-doped region. These problems can be circumvented in future devices by separating the active and passive sections. We have already developed substrates containing separate active and passive regions by fusing doped and undoped glasses, and are currently working on implementing amplifying waveguides integrated with passive splitters and tapered sections in these substrates. Since we have already measured a net gain of 9 dB at 1537 nm with 130 mW of pump power, these improvements should in principle allow the demonstration of 1×8 lossless splitting under laser diode pumping. These improvements are also expected to reduce the noise figure of the device, bringing it closer to the design target of 5 dB.

Conclusion
We have demonstrated the first ion-exchanged, planar lossless splitter at 1.5 μm. The 1 × 2 device achieved lossless splitting with an 8 dB noise figure over the wavelength range of 1534-1548 nm when pumped with 115 mW from a 980 nm laser diode. Although the present device is aimed at digital applications [1], it is expected that improvements in the Er:Yb concentration ratio and circuit layout will lead to 1×8 lossless splitters with good noise figures, which would be ideal for analog CATV applications. These results demonstrate that ion-exchange technology is a strong contender to yield active planar components for broadband telecommunication systems.

Acknowledgements
This work was supported by the RACE Programme as part of project R2109 LIASON. The Optoelectronics Research Centre is an Interdisciplinary Research Centre supported by the UK EPSRC.
P Camy, A Béguin, C Prel, C Lerminiaux (Corning Europe Inc, 7 bis, Avenue de Valvins, 77210 Avon, France)
J E Román,  M Hempstead, J S Wilkinson (Optoelectronics Research Centre, University of Southampton, Southampton SO17 1BJ, United Kingdom)
F W Willems, J C van der Plaats, and A M J Koonen (AT&T Network Systems, P.O.Box 18, 1270 AA Huizen, The Netherlands)

References
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Electronics Letters (1996) Vol.32(4) pp.321-323

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