High temperature operation of athermal widely tuneable laser with simplified wavelength control for WDM-PON systems Lalitha Ponnampalam,1,* Cyril Renaud,1 Martyn Fice,1 Rosie Cush,2 Richard Turner,2 Paul Firth,2 Mike Wale,2 and Alwyn Seeds1 1

Department of Electronic and Electrical Engineering, University College London, Torrington Place, London WC1E 7JE, UK 2 Oclaro Technology Ltd., Caswell, Towcester, Nothamptonshire NN12 8EQ, UK * [email protected]

Abstract: A simplified control system is described which uses only three point calibration to maintain the wavelength of the ITU channels of an uncooled DS-DBR laser, spaced at 50GHz, over the full C-band. Wavelength is controlled mode-hop free over a temperature range of 45° to 80°C. ©2014 Optical Society of America OCIS codes: (140.3600) Lasers, tunable; (060.2330) Fiber optics communications.

References and links 1. 2. 3. 4. 5. 6. 7.

C. C. Renaud and A. J. Seeds, “Coolerless tuneable semiconductor laser operated over 32, 100GHz-spaced channels with less than 0.1nm thermal drift,” Electron. Lett. 41(3), 127–128 (2005). S. H. Lee, A. Wonfor, R. V. Penty, I. H. White, G. Busico, R. Cush, and M. Wale, “Athermal colourless C-band optical transmitter for passive optical networks,” in Proceedings of European Conference on Optical Communication, Torino, Italy (2010), paper Mo.1.B.2. L. Ponnampalam, C. C. Renaud, R. Cush, R. Turner, M. J. Wale, and A. J. Seeds, “Simplified Wavelength Control of Uncooled Widely Tuneable DSDBR Laser for Optical Access Networks,” Proceeding of the European Conference on Optical Communications, London 2013. N. D. Whitbread, A. J. Ward, B. de Largy, M. Q. Kearley, B. Aplin, P. J. William, and M. J. Wale, “AlGaInAsInP C-Band Tunable DS-DBR Laser for Semi-Cooled Operation at 55°C,” Proceeding of the European Conference on Optical Communications, Brussels, Belgium 2008. A. J. Ward, D. J. Robbins, G. Busico, E. Barton, L. Ponnampalam, J. P. Duck, N. D. Whitbread, P. J. Williams, D. C. J. Reid, A. C. Carter, and M. J. Wale, “Widely Tuneable DS-DBR Laser with Monolithically Integrated SOA:Design and Performance,” IEEE J. of Sel. Top. in Qunatum Electron. 11(1), 149–156 (2005). L. Ponnampalam, D. J. Robbins, A. J. Ward, N. D. Whitbread, J. P. Duck, G. Busico, and D. J. Bazley, “Equivalent Performance in C- and L-bands of Digital Supermode Distributed Bragg Reflector Lasers,” IEEE J. Quantum Electron. 43(9), 798–803 (2007). M. Teshima, “Dynamic Wavelength Tuning Characteristics of the 1.5μm Three-Section DBR Lasers: Analysis and Experiment,” IEEE J. Quantum Electron. 31(8), 1389–1400 (1995).

1. Introduction There has been an explosion in the demand for user bandwidth, and WDM PON technology could be an ideal solution in next generation future-proof optical access networks. However, for widespread adoption it is vital that the WDM PON system cost is comparable to the currently available TDM access networks. Although high performance tuneable DBR lasers provide system flexibility, their cost, power consumption and stringent wavelength control requirements have previously made them unsuitable for access networks. As a large proportion of both cost and power consumption come from the use of Peltier based cooling for wavelength control, use of an uncooled solution could pave the way to WDM sources for access. Good wavelength stability for uncooled lasers by monitoring the temperature of the laser has already been demonstrated [1, 2]. However in these systems the control parameters were derived by calibrating each of the channels at several temperature points or by

#216424 - $15.00 USD Received 15 Jul 2014; revised 18 Sep 2014; accepted 20 Sep 2014; published 29 Sep 2014 (C) 2014 OSA 6 October 2014 | Vol. 22, No. 20 | DOI:10.1364/OE.22.024405 | OPTICS EXPRESS 24405

generating the full tuning maps over a range of temperatures. Such a lengthy calibration procedure would adversely affect the cost of the transmitter. In this paper we describe a simplified open loop algorithm to control the wavelength of an uncooled, widely tuneable digital supermode distributed Bragg reflector (DSDBR) laser. In this wavelength control process, due to the correlation of the temperature dependence of the tuning currents between channels, all the parameters of all channels at any temperature point are derived from characterizing only three channels at three different temperature points. This enables us to significantly simplify the wavelength calibration process. It is essential to maintain the operating wavelength of the laser without any longitudinal mode hops for error free transmission [2]. Hence the currents to the rear and the phase sections of the laser are tuned as a function of laser temperature to achieve a wide mode-hop free tuning range. A mode-hop free tuning range of 15°C to 40°C has been achieved using this simplified algorithm on lasers based on InGaAsP material [3]. However in access systems, we will need to operate at much higher temperatures, typically from 45°C to 80°C. In this paper, we demonstrate that this can be achieved using this simplified wavelength control algorithm on InAlGaAs based lasers [4], which also validates the robustness of the process. Further, we demonstrate an error-free 10Gb/s transmission system with a channel spacing of 50GHz using a cooler-less laser where the wavelength of the laser is maintained by the same control process. 2. Experimental arrangement The work in this paper is based on DSDBR lasers mounted in 26-pin packages. The laser consists of gain, phase, rear grating (with seven equal strength equally spaced reflection peaks) and multi-contact chirped front grating sections. In order to support laser operation at elevated temperatures, these lasers are based on InAlGaAs material. Wavelength tuning over the full C band is achieved by activating adjacent contact pairs of the front grating to access each of the rear reflection peaks that correspond to each of the supermodes. The laser is integrated with a semiconductor optical amplifier (SOA) that can be used to amplify or blank the optical output from the laser [5]. The control algorithm was tested over a wide temperature range by varying the laser temperature using a thermoelectric cooler, with the temperature monitored by a thermistor within the laser package. The laser current controller was computer controlled via GPIB interface and the wavelength was monitored using an optical spectrum analyser with a resolution of 0.07nm (Fig. 1).

Fig. 1. Experimental Arrangement (** used to vary the temperature over a wide range).

3. Athermal wavelength control A typical tuning map of the DSDBR laser is shown in Fig. 2. Here, the front grating peak is tuned by injecting a fixed current of 5mA to the first grating and the current to the second #216424 - $15.00 USD Received 15 Jul 2014; revised 18 Sep 2014; accepted 20 Sep 2014; published 29 Sep 2014 (C) 2014 OSA 6 October 2014 | Vol. 22, No. 20 | DOI:10.1364/OE.22.024405 | OPTICS EXPRESS 24406

grating is varied from 0 to 5mA. The current into the front grating pair is injected in such a way that the front and the rear grating peaks are tracked together as the laser is tuned across the full C band. This leads to the generation of simple 3-section DBR like tuning maps in the phase-rear plane, along the centre solid line in each of the supermodes [6] as shown in Fig. 2. The laser is pre-calibrated to operate at the ITU channel frequencies, spaced at 50GHz at 70°C by selecting the operating points in these phase-rear planes in the centre of the corresponding longitudinal modes as shown in Fig. 2. When the laser is not temperature controlled, the ITU channel frequency is maintained by varying both the phase and the rear current such that the operating point is tracked along the centre of the corresponding mode. This is done using the thermal model derived in [7] to which polynomial functions are fitted to derive the temperature dependent tuning currents that require three temperature points. Further, due to the simplicity of the tuning map in the phase – rear plane as in Fig. 2, an analytical solution is derived that requires three calibration points for which, channels with low, medium and high tuning currents are selected for the full calibration. By this 3-channel, 3-temperature point calibration method, calibration of the other channels at any temperature points are achieved, thereby greatly simplifying the athermal control and calibration process.

Fig. 2. Wavelength tuning map of the laser at 25°C. Vertical dashed lines correspond to front grating pair switch (FG: Front grating)[The phase and rear current scales are related to a polynomial function].

4. Results The developed control algorithm was applied to all the 84 ITU channels spaced at 50GHz, calibrated over the C band (1528nm-1564nm) and Fig. 3 shows the mode-hop free temperature tuning range of each of the channels. It can be seen that about 72 channels tune mode-hop free over 25°C temperature range, while 38 channels tune mode-hop free over 30°C temperature range. Here, the gain section current also was adjusted to extend the modehop free tuning range. The reason for some channels to have relatively narrower mode-hop free tuning range is that they lie at the ends of the supermodes in the tuning map, with either smaller rear current (50mA). These channels will require supermode switch to maintain the wavelength at the ITU point and will need more detailed characterization in addition to having cavity mode jumps while the rear current is swept over a wide range. This can be eliminated by modifying the DBR laser design to increase the wavelength overlap between the supermodes so that all ITU channels are calibrated with rear currents in the 3mA to 50mA range. This will enable all channels to tune mode-hop free over a wider temperature range.

#216424 - $15.00 USD Received 15 Jul 2014; revised 18 Sep 2014; accepted 20 Sep 2014; published 29 Sep 2014 (C) 2014 OSA 6 October 2014 | Vol. 22, No. 20 | DOI:10.1364/OE.22.024405 | OPTICS EXPRESS 24407

Further, operation at higher temperatures (>73°C) were limited by insufficient gain in the cavity (Fig. 6), as all lasers tested had relatively short gain section (350μm). This upper temperature limit can be extended by increased cavity gain, making it suitable for many applications.

Fig. 3. Mode-hop free tuning range of 84 channels spaced at 50GHz.

Figure 4(a) shows the typical wavelength deviation of the channels spanning over the C band for a temperature range of 45°C to 80°C. The wavelength remains within ± 0.1nm (12.5GHz) for the full range, indicating the trade-off between the wavelength accuracy and the degree of simplification in the algorithm. Figure 4(b) shows a typical variation in the side mode suppression ratio (SMSR) of these channels. The algorithm aims to maintain side-mode suppression better than 35dB over the entire temperature range.



Fig. 4. (a) Typical wavelength deviation of the ITU channels (b) Side mode suppression ratio over the entire tuning range.

The algorithm was successfully tested under slowly varying temperature environment and also by deliberately varying the laser temperature over a wide temperature range using the LABVIEW platform. In order to maintain the wavelength of the laser over a wide temperature range, it is important that the variation of the chip temperature be slower than the time constant of the control electronics, which could typically be in the sub-millisecond timescale for a firmware based control system. Hence, it is envisaged that this control process is feasible in typical access modules, such as T-XFP/T-SFP + , due to its relatively stable environment. Further, as integrated current sources and temperature controllers have already been developed around the T-XFP/T-SFP + pluggable transceiver module footprints, this control process can be realized in access equipment.

#216424 - $15.00 USD Received 15 Jul 2014; revised 18 Sep 2014; accepted 20 Sep 2014; published 29 Sep 2014 (C) 2014 OSA 6 October 2014 | Vol. 22, No. 20 | DOI:10.1364/OE.22.024405 | OPTICS EXPRESS 24408

Figure 5 shows the experimental arrangement to measure the bit error ratio (BER) over a back-to-back transmission system. Here, a 27 – 1 pseudo random binary sequence (PRBS) generator at 10Gb/s data rate was used as the data source to both modulators, and the athermal laser was set to operate at one of the arrayed waveguide grating (AWG) channel frequencies. The second laser was tuned to the adjacent channel spaced at 50GHz and its performance was analysed when the temperature of the athermal laser was deliberately varied over a wide temperature range with the wavelength control switched on and off. Figure 6 shows the measured BER in the adjacent channels where no error was measured over the full temperature range when the wavelength of the athermal laser was controlled. However, the BER in the adjacent channels increased rapidly when the temperature of the DS-DBR laser varied by more than ± 2.5°C from the nominal 70°C when the wavelength control algorithm was switched off.

Fig. 5. Back-to-back transmission experiment arrangement.

Fig. 6. Comparison of bit error ratio on adjacent channels spaced at 50GHz with wavelength control on and off. The wavelength of the athermal laser (Ch 2) was set at 1548.735nm.

Figure 7 shows eye diagrams for the adjacent channels at various temperatures of the athermal laser when its wavelength was controlled and not controlled. When the wavelength control algorithm was switched on, the eye remains open (E in Fig. 7) over the full temperature range on both channels either side. When the wavelength control algorithm was switched off, the eyes of adjacent channels towards which the frequency of the athermal laser drifts starts to close (A and B in Fig. 7). The eyes of the channels are unaffected when the athermal laser drifts away from those channels (C and D in Fig. 7).

#216424 - $15.00 USD Received 15 Jul 2014; revised 18 Sep 2014; accepted 20 Sep 2014; published 29 Sep 2014 (C) 2014 OSA 6 October 2014 | Vol. 22, No. 20 | DOI:10.1364/OE.22.024405 | OPTICS EXPRESS 24409

Fig. 7. Measured eye diagrams at different temperatures with the wavelength control off (A to D) and on (E) [time base: 50ps/div]. A – Channel 3 when athermal laser temperature was at 72°C (wavelength control off) B – Channel 1 when athermal laser temperature was at 66°C (wavelength control off) C – Channel 1 when athermal laser temperature was >66.5°C (wavelength control off) D – Channel 3 when athermal laser temperature was

High temperature operation of athermal widely tuneable laser with simplified wavelength control for WDM-PON systems.

A simplified control system is described which uses only three point calibration to maintain the wavelength of the ITU channels of an uncooled DS-DBR ...
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