Tunable laser structures with nanosecond switching time between wavelength channels and low-power injection locking are demonstrated on a low-cost platform. These lasers are suitable as source or slave lasers in WDM passive optical access networks

Barry, Liam P.

Corbett, Brian

Donegan, John Francis

Ellis, A.D.

Engelstaedter, P.

Lambkin, Paul

Mondal, S.K.

Peters, F.H.

Phelan, Richard

Roycroft, Brendan

Smyth, Frank

English

B. Roycroft

S. K. Mondal

P. Lambkin

P. Engelstaedter

B. Corbett

F. H. Peters

F. Smyth

L. Barry

R. Phelan

J. F. Donegan

A. D. Ellis

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Fast Switching Tunable Laser Sources for Wavelength Division Multiplexing in Passive Optical Access Networks

DCU Online Research Access Service

2007 International Conference on Indium Phosphide and Related Materials 12:00 - 12:15Conference Proceedings FrB2-519th IPRM 14 - 18, May 2007 Matsue, JapanFAST SWITCHING TUNABLE LASER SOURCES FOR WAVELENGTHDIVISION MULTIPLEXING IN PASSIVE OPTICAL ACCESSNETWORKSB. Roycroft (l)*, S. K. Mondal (1), P. Lambkin (1), P. Engelstaedter (1), B. Corbett (1), F. H.Peters (2) F. Smyth(3), L. Barry (3), R. Phelan(4), J. F. Donegan (4), A. D. Ellis(5)(1), CTVR, Photonics Group, Tyndall National Institute, Cork, Ireland(2), CTVR, Physics Department, University College Cork, Cork, Ireland(3), CTVR, School of Electronic Engineering, Dublin City University, Dublin, Ireland(4), CTVR, School of Physics, Trinity College Dublin, Dublin, Ireland(5), Photonics Systems Group, Tyndall National Institute, Cork, Ireland(* Corresponding author e-mail: Brendan.Roycroftgtyndall.ie)AbstractTunable laser structures with nanosecond switching time between wavelength channels andlow-power injection locking are demonstrated on a low-cost platform. These lasers aresuitable as source or slave lasers in WDMpassive optical access networks.I. Introduction p-type InP cladding layer, capped with a 0.2 ptm p+Telecoms lasers are typically fabricated with DFB InGaAs layer. Using standard optical lithography atechnology for single frequency, or use Sampled 3 ptm wide slotted ridge was etched down 1.95 ptmGrating-DBR lasers for tunability. Both to a buried etch stop layer. The slots weretechnologies require overgrown, lithographically nominally 0.85 ptm long. The etch depth went to thedefined gratings. However, single frequency top of the waveguide layers, and did not go throughoperation can be more simply achieved by the active region in order to avoid leakage currentsintroducing a small number of modal perturbations and excessive losses in the waveguide. To produce(slots) in a ridge laser [1-3]. We have termed this a multichannel laser, we cleaved a 1.1 mm longlaser the slotted Fabry-Perot (SFP) laser. Here we laser with a slot configuration of three slots spacedpresent a tunable SFP laser, where we have at 89.5 ptm from each other and from the facet atcontrolled the spectral modes by varying the etch one end of the laser, and three slots spaced 107.4depth, the number of slots present in the laser and ptm from each other and from the facet at the otherthe number of contacts to the laser. As is the case end of the laser. These slots give reflectivity spectrafor slotted single frequency lasers, this allows for a that implement the Vernier effect with a periodicitydramatic cost reduction and so, such tunable lasers of about 400 GHz. The laser used two electricalmay become applicable to a wide range of contacts, and one of the slots also electricallyapplications including Wavelength Division isolated the contacts from each other. For injectionMultiplexed passive optical access networks locking, we used a single contact SFP laser that was(WDM-PON). We discuss applications in a tunable 607 ptm long with eight slots equally spaced at 37WDM-PON system, where the lasers can play two ptm separation from each other and 237 ptm fromroles - firstly as the initial single-frequency the cleave. The lasers were mounted p-side updownstream laser located at the head end requiring without facet coatings on a ceramic test board.a high side mode suppression ratio and fastswitching time for rapid provisioning, and secondly III. Experimentas injection locked lasers in the remote optical The 1.1 mm long SFP laser was temperaturenetwork unit [4,5] which will require low injection controlled by a Peltier stage, and was firstpowers. Slotted lasers fulfil the requirements both characterised in DC operation with combinedfor the tunable single mode laser and for the currents in the range 40 - 150 mA. The laser outputinjection locked laser. was fibre coupled and the emission measured on anoptical spectrum analyser. Selected spectra areII. Device structure shown in Fig. 1, which show the single modeLaser material was grown as a standard p-i-n spectra on the designed channel spacing of 3.2 nm,heterostructure with lattice matched AlInGaAsP on which corresponds to a frequency of 400 GHz.an InP substrate. It had a standard laser layerstructure, with five QWs emitting at 1550 nm,embedded in a 540 unm waveguide, and a 1.75 pim1-4244-0875-X/07/$20.OO ©2007 IEEE. 606Authorized licensed use limited to: DUBLIN CITY UNIVERSITY. Downloaded on January 19, 2009 at 10:59 from IEEE Xplore.  Restrictions apply.1 0-DOPS~~~EDFA~~~~~~~~~~~~~~~~~~~~~~~~~TI I VA1- 1SwitchingPIG OsEilloscope32Gbft-1 40---- -External cavity------eb iurn-dopedELI PhO6todetectbrFig. 3. Experimental configuration used to-30 ------- determine switching time of tunable laser module.DCPS=DC power supply, TL= tunable laser,-.40-ECL=External cavity laser, EDFA=erbium doped1530 1535 1540 1545 1550 15 156 1565 1570 fibre amplifier, VOA=variable optical amplifier,Waveleiig h (iuii) 0 aEopto to electronic photodetector, LPF= lowFig. 1. Overlay of selected single mode spectra pass filterwith SMSR > 28 dB showing 7 wavelengthchannels spaced by 3.2 nm separation of 4.24 THz). The output was combinedwith the output of a separate tunable external cavity120 ------------------------------------laser (ECL) which was detuned from the SFP laserby approximately 1 GHz and detected using a high100-----I--------------------------------speed detector, a low pass filter with a 1.87 GHzcut-off frequency and an oscilloscope. An EDFASo L-- -- ----- -r ~~ ~~~11 ~~~~r--- -- ---0 k and Variable Optical Attenuator are used to ensureE GOL < ;> that the power from the Tunable Laser and the_ Ipower from the ECL are equalised to give a strong40 r - - - z- - v . zw beat signal. When the SFP laser and the ECL laserI v ', v vare less than 1.87 GHz apart in frequency, the beat20------------------------------- signal appears as a high frequency oscillation onI___ ____ ____ I L the oscilloscope, and when the SFP was far from0 400 800 1200 1600 2000 2400 2600 the ECL frequency the beat signal is beyond theFrequency shift (GHz) bandwidth of the detection electronics, so only theFig. 2 : Full 2D map of single mode spectra as a DC power component is displayed. Thesefunction of the two bias currents. Triangles measurements demonstrated a typical switchingrepresent spectra with SMSR between 20 and 30 time between channels of 1.5 ns as shown in Fig. 4.dB, while squares represent SMSR greater than 30dB. A clear tendency for hopping at 400 GHz is time at Xi ts,vtch tune at X2shown. Frequency values are taken relative to the a...lasingpeak at 1559 nm.Wvhile selected spectra are plotted in Fig. 1, allsingle mode data is summarised in Fig 2. Thecurrent in section 1 was varied from 0 to 60 mAand the current in section 2 was varied from 0 to125 mA. The current ranges are proportional to thelength of each section, which had a ratio of about 2to 1. A clear tendency to hop at a frequency spacingof 400 GHz is presented, showing good agreementbetween the model and experiment.To determine the switching speeds of these lasers,we use a heterodyne technique [6] that involves the Fig. 4: Switching time of 1.5 ns demonstrateduse of an external cavity laser (ECL) set to a between two wavelength channelsfrequency 1 GHz away from channel 1 emittedfrom the tunable laser, Fig. 3. A 770 pim longmulti-channel SFP laser was used for this A further set of experiments characterised the laserexperiment. One section was biased using a bias tee as a slave laser as may be required in a remoteand a high speed pulse pattern generator while the optical network unit (ONU). In our implementationsecond section was driven DC. With apeak-to-peak of this function, the laser needs to lock to thetuning current of 20 mA, the laser switched injection wavelength and to amplify the injectedbetween two single mode channels which were signal. The 607 pim long SFP laser was pumpedsepratd i waelegthby 3.4nm afequncywith an Ando tunable laser source in thear ed n engh 3 (afreqencyconfiguration shown in Fig. 5.607Authorized licensed use limited to: DUBLIN CITY UNIVERSITY. Downloaded on January 19, 2009 at 10:59 from IEEE Xplore.  Restrictions apply.-10Slotted ECa 20laser l -5r m t C r p c l W n mer sour. wvculator IsolaterTLS f 1 0 - =8IPM ,v.1 0/90 splitter 5 _Suppression of the free-running peak (black, downThe laser has a threshold current of 37 mA, and was triangles) is shown, while the power of the lockedbiased at a fixed current of 45 mA. The free- mode (blue upwards triangles) increases slightly.running wavelength in this case is 1536.82 unm, as Powers in all other modes stay low, and evenshown in Fig. 6(a) and has an output power of decreases slightly during locking.about ±4 dBm. At an estimated injection power of -16 dBm, it was found that when the injection was at Finally, in Fig. 7 we show how the peak power inany frequency in a band of 25 GHz around the the free running peak is suppressed as the injectednatural frequencies of the SFP laser thesfreewavelength is variedfaoss one0 to the cnne.. .. wavelength 1S var1eSuacrossionof theferungpak(lck,annelrunningeemissionwasesuppressed,andothe7injecte wavelengths. In this case, the channel at 1528.18wavelength became the dominant wavelength nmois chosn forlo es injecting tunshown n Fig.6(a) ad has n outpt powe of d crasesnslighlyodurng.locking. tgtualwithout adjustment of the SFP bias current Fg.naua frqece of th SFP1laser tefe- wavelength iS detuned acro somo the chan elent3w(o-). The laser required only -25 dBm Of CO-polari.ze injet ed onlo w25 dBm oMc and is swept from 1528.06 nm to 1528.22 nm. Thepolarized injected power to lock with 25 dB SMSRinetdpwrsetmadat20Bmniethto any of these channels, which is very low injected power is estimated at -20 dBm inside thecompared with previously reported systems using slotted laser for this set of data. As the injectionFP lasers which typically required between -6 to - wavelength approaches the channel wavelength, the14 dBm injected power for significant locking [4- free-running peak denoted by the black downward5]. triangles drops in power. As the injectingwavelength sweeps through 1528.18 ±0.02 nm, the(a) power in the other channels also drops slightly,--------,, showing that the injected power is contributing to(b) the stimulated emission. The best SMSR in thiscase is 29 dBm. Locking width is taken at an SMSRof 25 dBm, and for the -20 dBm injected power(C) ~~used in this example a locking width of 9.0 GHz is(d) measured.AAIV. Conclusions(e) We have shown that monolithic, single-stageepitaxy lasers based on the SFP platform can have(D) several functions: wavelength-selection of channelsseparated by 400 GHz, rapid switching between1520 1530 1540 1550 156different channels in the nanosecond regime, andWavelength (nm) these lasers can be designed to be easily injectionFig. 6: Injection locking of different channels. (a) locked. These functions should be useful in passivefree-running spectrum, (b-J) locked spectra with optical network access applications.injected power of -20 dBm at the dominantwavelength. AcknowledgmentsThis work was supported by Science FoundationIreland, under the project Centre forTelecommunications Value-chain-driven Researchand under Grant 03/IN.1/1340.608Authorized licensed use limited to: DUBLIN CITY UNIVERSITY. Downloaded on January 19, 2009 at 10:59 from IEEE Xplore.  Restrictions apply.References[1] B. Corbett and D. McDonald, Electron. Lett.,vol. 31, pp 2181-2182, 1995[2] P. Lambkin et al, J. Quantum Electron., vol. 40,pp 10-17, 2004[3] A. Klehr et al, J. Appl. Phys., 90, pp 43-47,2001[4] L. Y. Chan, C. K. Chan, D. T. K. Tong, F. Tongand L. K. Chen, Electron. Lett., vol. 38, pp. 43-44,2002.[5] W. Hung, L. K. Chen, and F. Tong, IEEEPhoton. Tech. Lett., vol. 15, pp. 1476-1478, 2003.[6] H. Joseph, D. Sadot, IEEE Photon. Tech. Lett.,vol 16, No 8, pp. 1921-1923, 2004.609Authorized licensed use limited to: DUBLIN CITY UNIVERSITY. Downloaded on January 19, 2009 at 10:59 from IEEE Xplore.  Restrictions apply.

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Fast switching tunable laser sources for wavelength division multiplexing in passive optical access networks

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Fast switching tunable laser sources for wavelength division multiplexing in passive optical access networks

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