100 GHz spaced 10 Gbit/s (NRZ, PRBS 2(31)-1) WDM transmission is demonstrated with an uncooled DBR laser. The wavelength of the laser was stabilised within 2 GHz from 10 degrees C to 70 degrees C using a predicting algorithm. (C) 2004 Optical Society of America

Cannard, P

Fice, MJ

Lealman, I

Renaud, CC

Rivers, L

Seeds, AJ

English

UCL Discovery

OW176.pdf100 GHz Spaced 10 Gbit/s WDM over 10 °C to 70 °C usingan uncooled DBR laserC. C. Renaud, M. J. Fice, I. Lealman*, P. Cannard*, L. Rivers*, A. J. SeedsDept. ofElectronic & Electrical Eng., University College London, Torrington Place, London, WCJE 7JE, UnitedKingdom.Tel: +44 (0)20 7679 7928, Fax: +44(0)20 7388 9325,email:a.seeds@ee.ucl.ac.uk* CentreforIntegratedPhotonics, Martlesham, Ipswich, IP5 3RE, UnitedKingdomAbstract: 100 GHz spaced 10 Gbit/s (NRZ, PRBS 23'-1) WDM transmission is demonstrated with anuncooled DBR laser. The wavelength ofthe laser was stabilised within 2 GHz from 10 'C to 70 'C using apredicting algorithm.©02004 Optical Society ofAmericaOCIS codes: (140.3600) Lasers, tunable; (060.4510) Optical communications1. IntroductionIn a more and more cost driven telecommunications industry the cooling system for the lasers is a prime target asshown by the strong growth in coarse wavelength division multiplex (CWDM) [1-2] network standards [3]. Thistechnology uses uncooled lasers, widely spaced channels (20 nm) and simple filters. Thus the cost of WDMnetworks with modest channel number is greatly reduced. The success of CWDM has started a growing interest inuncooled lasers for closer channel spacing using programmed control loops monitoring the laser wavelength.Recently, uncooled operation with high side mode suppression ratio (SMSR) ofa sampled-grating-distributed Braggreflector (SG-DBR) laser from 10 °C to 60 °C was demonstrated for standard WDM application with 100 GHzspacing where the optical filter components are still relatively inexpensive [4].This paper reports the first demonstration of 10 Gbit/s transmission over the commercially important 10 °C to 70 °Ctemperature range using an uncooled laser in a control system which by monitoring the temperature of the laseronly, stabilises its output wavelength with an error <2 GHz. The control loop was based on a predicting model ofthelaser behaviour using as input the measured temperature and the desired wavelength. The uncooled laser was adistributed Bragg reflector (DBR) laser with a long phase section to allow for multiple axial mode cavity lengthtuning. This laser was used to demonstrate 100 GHz spacing WDM transmission at 10 Gbit/s, 231-1 Pseudo RandomBit Sequence (PRBS), over 40 km offibre with comparable results to a temperature controlled laser.PDFabry-Pero Laser 2.... , ~ ............... Fa r-eo*Peltier.X\~~~~~~~~~~~~~~~~40kmofSMFPeltier-- ~~~ CurrentCurrent sourcesource1:/\ r1_. ))lerTherm sensor Oscilloscope PRBS 231 1 lOGbit/s Sampling scope| | * | 111|11_11 t * | |~~~~~~~~~Channell1 AWGLaser1 .or2. ........ Channel2 SpacingFig. 1: Left: Uncooled laser system where the phase and DBR sections are controlled using apredicting algorithm. The laser stability is measured using a Fabry-Perot interferometer (125MHz resolution). Right: Transmission system at lOGbit/s. Laser 1 and Laser 2 can both beoperated uncooled system or with standard TEC control.2. Experimental systemsFigure 1 shows the experimental system used to test the stabilised laser. The stabilisation is made by measuring thetemperature and correlating this measured value with the desired wavelength and the operating currents ofthe phaseand grating sections. The correlation is derived from an algorithm based on basic rate equations and coupledequations for the thermal behaviour ofthe different sections [5]. To calibrate the algorithm the linear behaviour ofOW176.pdfthe device as a function of temperature and current is measured by checking the peak wavelength of the Bragggrating when the temperature or the current varies. The laser gain section current was fixed at 50 mA to give anoutput of2 dBm at 10 °C. The two other sections (the grating and phase sections) were driven by a multiple channelGPIB controlled current source. The current source and the thermal sensor embedded in the laser package wereconnected to the computer containing the stabilisation programme. In order to change the laser temperature over alarge range, the Peltier cooler in the packaged laser was connected to a separate current source, allowingindependent control ofthe laser temperature.Wavelength stability monitoring was carried out using a Fabry-Perot interferometer set at half the maximumtransmission for the desired wavelength to offer a frequency sensitivity of 125 MHz. To test the uncooled lasersystem for transmission we used two lasers which were either an uncooled DBR laser with stabilisation enabled ornot or conventional temperature controlled (TEC) tuneable laser. The two lasers wavelengths were spaced by100 GHz. Laser 1 was then externally modulated with an NRZ, 10 Gbit/s PRBS 231-1 signal generated by a LiNbO3Mach-Zehnder modulator. Its output was combined with the laser 2 output and amplified to up to 0 dBm opticalpower per channel with an erbium doped fibre amplifier (EDFA). The signal was then transmitted over 40 km ofstandard single mode fibre (SMF) and each channel was separated by an arrayed waveguide grating (AWG-100 GHz spacing). The resulting eye diagram for channel 1 (laser 1) was then displayed on a sampling oscilloscope.3. Experimental resultsN'400 2 30 46-2> -3-u4-510 20 30 40 50 60 70Temperature (Degree C)Fig. 2: Wavelength stability ofthe laser output as a function ofthe laser temperature. The measurementwas made using a Fabry-Perot interferometer with a resolution of 125 MHz.Figure 2 shows the stability performance of the DBR laser from 10 °C to 70 °C (laser chip temperature). Thefrequency deviation from the desired wavelength remained within +/- 1 GHz which is comparable to a laser withTEC (typically 2.5 GHz precision of the peak wavelength). Over this range the SMSR remained constant withdetailed performance similar to that described in [4].Figure 3 shows three sets of eye diagrams recorded after transmission of the two channels over 40 km of SMF.Figure 3a shows the eye diagram when channel 1 (Laser 1) is selected, and Lasers 1 is operated uncooled but withthe stabilisation circuit enabled, laser 2 is operated with conventional TEC control. Laser 1 temperature was variedfrom 10 °C to 70 °C. Little degradation ofthe received signal is observed over the full temperature range. The eyediagram was still clearly open when the laser temperature was 70 °C. The extracted Q-factor varies from Q=7.1(10 °C) to Q=6.4 (70 °C) (the back to back measurement on Laser 1 with TEC or uncooled gave Q-8). Thedegradation was caused by the fact that at this high temperature the laser output power had dropped by 5 dBm andthe EDFA being close to saturation this resulted in a small drop in transmitted power. This could be compensated byintegrating a semiconductor amplifier at the output ofthe laser to equalize the output power.Figure 3b shows the eye diagram when channel 1 (Laser 1) is selected, and Laser 1 is operated uncooled but withoutthe stabilisation circuit and Laser 2 is operated with conventional TEC control. Laser 1 temperature was varied from10 °C to 18 °C. As expected in this case, as the temperature increases the peak wavelength drifts and when this driftis sufficient (>100GHz) no light is transmitted through the filter and the signal is lost.OW176.pdfFinally, Figure 3c shows the eye diagram when channel 1 (Laser 1) is selected, and Laser 1 is operated withconventional TEC control and Laser 2 is operated without the stabilisation circuit. Laser 2 temperature was variedfrom 10 °C to 18 °C with Laser 2 set to the shorter wavelength channel. The eye diagram ofthe TEC controlled laseris identical to the one for the uncooled system (Figure 3a) both have an extracted Q-7, and as expected whenLaser 2 drifts sufficiently it starts to interfere with channel 1 and the eye diagram starts to close. When the drift isclose to 100 GHz (-18 °C) the eye is completely closed.As expected from the stability measurements the uncooled laser gives comparable performance to a TEC controlledlaser for transmission over 40 km ofSMF when its temperature was varied from 10 °C to 70 °C..........=X *7A.m-r%0°j uncooled op .....o.. 0 un ed -p-~~~~~~~~~~~~~~~~..=....=__ ___ .........................................................................................................................................................Laser° 1 shcown). Bottom()opLaser 1 TE C controlled, Laser 2 uncontrolled (temperature of Laser 2shown).i.=-x.E-z=o-.u.E ..... .= too"_ M-40 @2| 0- m-204. ConclusionThe operation of an uncooled stabilised DBR laser was demonstrated from 10 °C to 70 °C. The control ofthis laserwas performed using for the first time a predicting algorithm based on basic rate equations and simple calibrationmeasurements. This stabilised uncooled laser offered similar performance in stability to a TEC controlled laser(2 GHz frequency stability). As expected it also demonstrated similar performance to a TEC controlled laser whenmodulated at 10 Gbit/s PRBS 23'1- in a 100 GHz spaced WDM system and transmitted over 40 km over SMF over alaser temperature range of 10 °C to 70 °C. Such stabilised lasers offer a route to low cost uncooled dense WDMoperation.Future work will concentrate on extending the predicting algorithm to more complicated tuneable lasers such as anSG-DBR, and studying direct modulation ofthe DBR laser with an integrated (faster) control system which will alsocompensate for the direct modulation.[1] J. A. Richard, Photonics Spectra, vol. 37, pp. 62-64, 2003[2] K. Kincade, Laser Focus World, vol. 39, pp.91-0to, 2003[3] ITU G 694.2[4] C. C. Renaud et al., IEE electron. Letters, vol 41, pp.a127-128, 2005[5] M. Teshima, IEEE J. ofQuantum Electron., vol. 31, pp. 1389-1400, 1995

100 GHz Spaced 10 Gbit/s WDM over 10 degrees C to 70 degrees C using an uncooled DBR laser

Abstract

Similar works

Full text

Available Versions

UCL Discovery