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TECHNICAL DIGESTSummariesof paperspresentedat the

Conference on Lasers and Electro-Optics

Conference Edition

.---Moscone Convention Center

San Francisco, California

May 7-12,2000

CLEO~ 2000 SPONSORED BY

IEEE/Lasers and Electro-Optics Society

OSA-Optical Society of AmericaIn Cooperation with:

Quanrum Electronics and Optics Division of the European PhysiCal SocietyJapanese Quantum Electronics Joint Group

CThM30

Pulse distortion In a quantum dotoptical amplifier

F. Romstad, P. Borri, J. M0rk, J. Hvam,

f. Hcinrichsdorff,. M.-H. Mao,.D.Bimberg,. Res.CrT.COM, DTU Build. 349,DK-2800Lrngby,Denmark;E-mail:jr@com.dtu.dk

Semiconductoroptical amplifiers are essentialdements in all-optical signal processing due totheirhigh gain and high bandwidth.l With theincreasingbit rate demand in OTDM systems,subpicosecond pulse dynamics becomes im-portant. The superiority of optical amplifiersbased on quantum dot (QD) gain materials\\ith respect to temperature stability and lowdrive current makes these devices especiallyinteresting to study.2 Experimental results onpulse propagation in an electrically-pumpedQD amplifier using a novel phase-sensitivepulse characterization technique are pre-sented.

The QD device is a P-I-N structure grownby MOCVD. The active region consists ofthree layers of binary/ternary lnAs/lnGaAsQDs separated by 2I-nm-thick GaAs barri-ers, in the center of a I20-nm-thick GaAslayer.Two Al GaAs cladding layers and ridgestructure of 8-m width and 400-m lengthprovide the optical confinement and

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CI'hM30 Fig. 1. Device transmission for 0, 4.IOmA.

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J I , thebulkdevice.

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Time [fs]250

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waveguiding. Tilted facets inhibit laser ac-tion. Ground state dot emission occurs at1.08 m wavelength.

Pulse amplitude and phase were measuredusing a XFROG (cross-frequency-resolved op-tical gating) technique based on the principleof sum frequency cross correlation. The pulseof interest is cross-correlated with a knownreference pulse and the upconverted signal isspectrally analyzed for different time-delaysbetween the reference and the output pulse. Analgorithm is then used to retrieve the ampli-tude and phase of the electric field of the out-put pulse from the experimental traces.) Thelaser source isthe idler of an optical parametricamplifier providing 150-fs pulses at 300-kHzrepetition rate.

In Fig. 1 the transmission of the device isshown as a function of the input pulse en-ergy. In the small signal regime a transmis-sion from - 3.35 dB to 1.85 dB is measuredwith bias currents ranging from 0 to 10 mA.For pulse energies above few pJ, nonreso-nant two-photon absorption leads to astrong transmission decrease. At 5 mA,pulses with 1 fl, 1 pJ an.d 40 pJ energies afterpropagation through the QD device areshown in Fig. 2(a) (see also arrows in Fig. I).For small energies the pulse experience onlyweak distortion. The constant instantaneousfrequency over the pulse indicates a nearly-transform-limited pulse. For increasing in-tensities the pulse broadens and finally at 40pI a weak breakup of the pulse is observedtogether with a nonlinear frequency chirp.This behavior is characteristic for all bias

. currents. The results are compared to thedistortion observed at material transparencyin a 250-m bulk amplifier [see Figure 2(b».The pulse narrowing seen in the bulk ampli-fier is not observed in the QD amplifier,indicating a strong effect of the dimension-ality of the active medium in the measuredpulse distortion.

Modeling of the processes behind the ob-served distortion (self-phase modulation andgain-dispersion) will be presented.

.. Tech. Univ. Berlin, Gemumy

1. S. Diez, et al., "Gain-transparent SOA-switchfor high-bitrate OTDM add/dropmultiplexing," IEEE Photon Techno!.Len. 11,60 (1999).

2. D. Bimberg, .et al., "InGaAs-GaAsQuantum-Dot Lasers." IEEE Journal Sel.Topics Quantum Electr. 3,196 (1997).

3. S. Linden, et al., "XFROG-a new methodfor amplitude and phase characterizationof weak ultrashort pulses," phys. stat. sol. b206,119 (l998).

4. F. Romstad, et al., "Sub-picosecond pulsedistortion in a InGaAs optical amplifier,"Proc. ECOC'99, 11-28(l999).

CThM31

Compact photoconductlve-basedsampling system with electronicsampling delay

Wei-lou Cao, Min Du, Chi H. Lee,Nicholas G. Paulter,. Univ. ofMaryland,DepartmentofElectricaland ComputerEngineering,CollegePark,Maryland, USA;

As electronic signals move to higher fre-quencies and wider bandwidths, there isneed for new methods of measuring thesehigh-frequency/high-speed (tens of GHz)and/or high bit rate (tens ofGBs/s) signals.'.2In this work, critical technical issues associ-ated with the design of a rugged, compact,"real-time" sampling system using photo-conductive switches as the test signal genera-tor and sampler were investigated. The de-sign concept is based upon an optoelectronicequivalent time sampling principle andoptical-microwave signal mixing. It involvesfirst phase locking of the periodic input sig-nal to be measured to the periodic opticalpulses from a mode-locked laser and subse-quent sampling of the locked signal by theoptical pulses. A photoconductive switch isused for the optical-microwave mixer andanother photoconductor for the sampler.The optical pulses we use were provided by100-fs pulses from a Ti-Sapphire laser. Theoptical-microwave intermixing process gen-erates a low-frequency replica of the high-frequency jnput signal. The ratio of the rep-etition rate of the input signal to its low-frequency replica is the time expansionfactor. The repetition rate of the low-frequency signal provides the offset fre-quency for the equivalent time sampling.Because there is no electro-mechanical mov-ing parts required to acquire a waveform, thesampling is done at a fast rate, and acquisi-tion times of 10 ms or less are possible. Thesuccess of this technique depends criticallyon the stability and reliability of the opticalmicrowave phase-locked loop (OMPLL),which locks the phase of the signal genera-tor's trigger to the optical pulses.

There are two methods for phase locking,one is phase ~nsitive and the other is fre-quency sensitive. This work shows that thephase-sensitive method yields the more desir-able result, that is, stable locking is achieved inthe presence of amplitude and phase noise.However, the requirement for the phase-

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sensitive detector is very stringent: the inter-mediate frequency signal used by the phasedetector, which is generated by OM mixing,must have exactly a 50% duty cycle in order toachieve the full locking range. Toward this goalwe discovered that a flip-flop circuit servedexactly this purpose, resulting in almost per-fect phase locking.

The phase-locked microwave signal wasthen used to drive a step r:ecoverydiode (SRD)in order to generate high-frequency signals fortesting. The measured replica waveform [Fig.1(a)] iscompared to the waveform obtained byusing a 20-GHz sampling oscilloscope [Fig.1(b)]. The traces in Fig. 1 clearly show that theQM sampling technique has a faster response.The 10%-to-90% rise time of the measured

OM-sampling-system-acquired waveformwas determined to be 7 ps, limited by the band-widthoftheinputsignal. -

The spectrum of the SRD output signal andthat of the replica were measured. The maxi-mum harmonics of the signal from SRD ex-tend beyond 30 GHz, which is beyond the

.bandwidth of the 20-GHz sampler. The low-frequency replica, on the other hand, had itsharmonics extending only to 18 MHz, which iswell within the bandwidth of the 20-GHz sam-pler and most digitizing waveform recorders.It is dear that information above 20 GHz is lost

using the 20-GHz sampling oscilloscope whileits replica preserves all information. The ex-perimental detail and results will be reported.It NIST -Gaithersburg, USA

1. CMOS phase-locked loops: 74HC(T)4046AI7046A & 74HCf9046A, Page 17 HCMOSDesigners Guide-Advance Information, Re-vised Edition: June 1995,PHIUPS.

2. S.L. Huang, C.H. Lee, H.-L.A. Hung,"Broadband and Real Time WaveformSamplingUsingOptic-MicrowavePhase-Locking,"IEEEMTT-5 InternationalMi-crowave Symposium, Paper QQ-l, At-lanta, Georgia,June 1993.

CThM32

1O-Gb~/. 81~ptlcal3R regenerationand format conversion uslne a pin-.wltched DFBlaser

M. Owen, V. Saxena, R.V. Penty, I.H. White,Ctr. for Comm. Res. Univ. of Bristot Queen'sBuilding, University Walk, Bristot BS81TR,United Kingdom; E-rrulil:Mark. Owen@bristoLac.uk

The regeneration of optical data signals is likelyto be an important requirement ofWDM andOTDM networking schemes" All-optical re-generators are currently complicated and re-quire several optical components togetherwith complex control. As a result there hasbeen interest in developing robust and simplesingle chip scQemes, whic~ allow regenera-tion.2 Here we demonstrate for the first time toour knowledge simultaneous all optical 3R re-generation and format conversion in a singleDFB laser. An NRZ data signal at 10 Gbitslswith poor Q is reamplified, retimed and re-shaped by the laser. The key to this regenera-tion process is to gain switch the DFB with theextracted clock signal in order to retime theconverted signal. This process also simulta-neously converts the input NRZ format to anoutput RZ data format and results in a signalwhose optical power and extinction ratio areconsiderably improved by the regenerationprocess.

The DFB laser used in these experiments is astrain-compensated multiple quantum welldevice, which has been optimized for low jittergain switched operation at 10 GHZ.3The de-vice threshold is approximately 8 mA and lasesat a wavelength of 1558.8 nm. The experimen-tal setup used to demonstrate 3R regenerationin this device is shown in Fig. 1. An integratedMactr.:Zehnder laser source at 1555nm (AI) ismodulated with a NRZ 27-1 10-Gbitls PRBS.

. The signal is transmitted down 80 km of stan-dard fiber then amplified, filtered and polar-ization controlled before being injected into aDFB laser. The coupled input power after am-plification is estimated to be -10 dBm. Abias-Tis used to apply the gain-switched drivesignal to the laser, which consists of a DC biasof 50 mA and a lO:-GHzlocal ~ock electricallyamplified to an RF power of 30 dBm. Afterf1ltering the input signal, the wavelength-converted signal at 1558.8 nm is detected on a

Integrated MZ/laser

source at AI

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CfhM32 Fig. 2. Eye diagrams ShO

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back-to-back NRZ signal with Q = 15.3. ) Re.ceived eye after 80 Ian of standard fiber , Q =6.6. (c) 3R regenerated RZ signal with m.rrCMdQ =9.2. I

32-GHz pin photodiode and analyzed ~th a50-GHz oscilloscope.

Figure2 shows the receivedeye~before and after transmission down 80lkm ofstandard fiber together with the 3R i

ener-ated optical signal. There can be ob cd .considerable improvement in eye quali. after

regeneration. This may be quantified bYI~Q1-lating the Q factor in each case as shown~ Fig.2. It may be observed that there is a Q factor

improvement of 2.6 after regeneration.l,It should be noted that a local clOflChas

been used here to simplify the experimentalsetup, a system with true remote clock qxtrac-tion will be presented at the conferen«, to-gether with BER measurements for the 3Rre-generator system.

Simultaneous NRZ to RZ format cOnver-

sion and 3R regeneration has been achievedusing one compact device at 10 Gbitl~ Theregenerated signal exhibits a Q factor improve-ment of 2.6 dB between input and outfut ofthe 3R regenerator.

1. S.J.B. Yoo, "Wavelength conversion tech-nologies for WDM network applicatJons..IEEEJournal of Lightwave TechnolOgy14,955-965 (1996).

2. M.F.C. Stephens, R.V. Penty, I.H. Whitt,"All-optical regeneration and wavelengthconversion in an integrated semiconduc-tor optical amplifier/distributed-fee~back

DFBlaser Tuncablc EDFAat ~ bandpa.ufilter

CThM32 Fig. 1. Experimental setup used to demonstrate 3R all-optical regeneration in a gain-switched DFBlaser.

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CThM31 Fig. 1. (a) OM sampled replicawaveform; (b) Microwave waveform.