Bornholdt, C.

Dekker, R.

Driessen, A.

Geuzebroek, D.H.

Klein, E.J.

Niehusmann, J.

English

When pulse lengths are shorter than the roundtrip time of a microring resonator filter, interference effects in the spectral response as shown with cw signals (both through and drop) are not that obvious. In order to clarify the behavior, a number of measurements have been performed on different $SiO_2/Si_3N_4$ microring resonators: a cw measurement, a measurement where the pulse length (1.8 ps) is in the order of the roundtrip time (1.9 ps) and finally a measurement where the pulse length (200 fs) is much shorter than the roundtrip time (4 ps). In all cases the spectral and time domain responses are presented

University of Twente Research Information

Behaviour of picosecond and femtosecond pulses inSiO2/Si3N4 microring resonator filtersD.H. Geuzebroek(1), R. Dekker(1), E.J. Klein(1), C. Bornholdt(2), J Niehusmann(3), andA. Driessen(1).(1) University of Twente, MESA+, Integrated Optical Microsystems, Enschede the Netherlands.(2) Fraunhofer Institute, Heinrich-Hertz-Institute, Berlin, Germany.(3) RWTH Aachen, Institut fr Halbleitertechnik, Aachen, Germany.When pulse lengths are shorter than the roundtrip time of a microring resonator filter,interference effects in the spectral response as shown with cwsignals (both through anddrop) are not that obvious. In order to clarify the behavior,a number of measurementshave been performed on different SiO2/Si3N4 microring resonators: a cw measurement, ameasurement where the pulse length (1.8 ps) is in the order of the roundtrip time (1.9 ps)and finally a measurement where the pulse length (200 fs) is much shorter than theroundtrip time (4 ps). In all cases the spectral and time domain responses are presented.IntroductionThe fundamental limit for filtering very high speed signals with integrated optic microringresonators (MRs) is reached when the duration of the pulse becomes smaller than theroundtrip time of the MR cavity. Since the duration of the pulse is so short, no interferenceof multiple roundtrips of the MR cavity can be observed and the MR can not be describedin a CW modus but exhibits only a transient response behaviour.General specifications of ps pulsesMeasurements have been performed with ps pulses to observe th influence of the MR onthe time and frequency behaviour of the pulses. With the use of a 40 GHz modelocked4 8 12 16Time (ps)Amplitude 5 mW/div1549.79 1555.79 1561.79Amplitude 10 dBm/divWavelength (nm)Figure 1: Measured wavelength spectrum and shape of detected 1.8 ps pulse of the gen-erated pulse train with a repetition rate of 40 GHz.ringlaser, a pulse train was generated with pulses having a FWHM of 1.8 ps and a 25 psinterval as is shown in Figure 1. In the right picture in this figure the ps pulse measuredProceedings Symposium IEEE/LEOS Benelux Chapter, 2005, Mons51by an optical autocorrelator is shown. The individual pulses have a duration in the orderof the roundtrip-time (τr ) of the microring which is 1.9 ps according to:τr =2πRngc(1)with a groupindex ng of 1.82 following from the measured 4.2 nm FSR as described in[1]. The measured wavelength spectrum of the generated pulse-train is shown in the leftpicture of Figure 1. It has a broad spectrum with a FWHM of 3.5 nmaccording to [2]FWHMλ ≈ λ20TBP·ngFWHMtc(2)with FWHMλ and FWHMt the 3-dB width of the wavelength spectrum and the pulse intime, respectively. The Time-Bandwidth Product (TBP) of a Gaussian pulse is≈ 0.44 [2].The spikes in the wavelength spectrum are the resonances of the ringlaser. Alternatively,they can be considered as a consequence of the pulse train with a repetition rate of 40GHz, being exactly the distance between the spikes in the wavlength spectrum.ps pulses filtered by microring resonatorsWhen a pulse train as described above is filtered by the mentioned MR there are twostates that can be distinguished: ’ON’ and ’OFF’ resonance.Since the pulse durationis a little shorter than the roundtrip time, the definition ofresonance conditions for theCW case can not be used. When the MR is tuned such that the main part of the pulsetrains spectrum does not overlap the normal resonance peaks, the tate is called ’OFF’resonance. When the center of the pulse train coincides with the resonance wavelength inthe CW case, the state is called ’ON’ resonance. The measured SiO2/Si3N4 MRs have a4 8 12 16Time (ps)Amplitude 2.34 mW/div1551.79 1555.79 1559.79Amplitude 10 dBm/divWavelength (nm)FSRFigure 2: Measured wavelength spectrum and detected response of the drop port of theMR in ’OFF’ condition.radius of 50µm and are described in detail in [3]. The MRs are themally tunable by meansof a chromium heater on top of the MR, enabling to switch between ’ON’ and ’OFF’resonance. The MRs were used to analyze their influence on the puls train describedabove. The measured spectrum and recovered pulse, measuredt the drop port, in theBehaviour of picosecond and femtosecond pulses in SiO2/Si3N4 microring resonatorfilters52’OFF’ state are shown in Figure 2. The pulse is altered into a pulse train with a separationaccording the roundtrips of the pulses. The distance between the pulses is 2.4 ps, a littlemore that the roundtrip time calculated above. This can be explained by the additionaldispersion a pulse has due to the tight radius of the ring. Finite-Difference Time Domain(FDTD) simulations confirm this. The exponential slope of the envelope of the pulse trainis a measure for the coupling constants and the loss inside the ring. However, since theincoming pulse has a tail broader than the roundtrip time, thpulses from the outgoingpulse train are not completely independent. This causes theenv lope to rise and so it isnot possible to determine the coupling ratio by this means.This can better be seen when looking at the measurements in the ’ON’ state, shown inFigure 3. The wavelength components of the pulse now exhibita longer delay, since thetails of the outgoing pulses interfere. This causes the indiv dual pulses from the pulse trainthe melt together in one long pulse with higher intensity as is hown in the right pictureof Figure 3. The spectrum confirms this as the FWHM is shortenedas is to be expectedfrom the pulse broadening in time. Also the wavelength respon e of the MR drop port inCW case, with a FSR of 4.2 nm, can be seen superposed on the pulsetrain pectrum.4 8 12 16Time (ps)Amplitude 2.34 mW/div1551.79 1555.79 1559.79Amplitude 10 dBm/divWavelength (nm)FSRFigure 3: Measured wavelength spectrum and detected response in the drop port of theMR in ’ON’ condition.fs pulses filtered by microring resonatorsTo see the behaviour of a pulse that has a FWHM in time that is much shorter than theroundtriptime of the ring, fs pulses where used to characterize the MR. The pulses havea FWHM of 200 fs and a repetition rate of 80 MHz and are generatedby an OpticalParametric Oscillator from Spectra Physics. The racetrackMR has a radius of 100µmand a coupling section of 40µm and is laterally coupled to the waveguides. The Si3N4waveguides with dimensions of 2.5 x 0.14µm were placed on a SiO2 buffer and an airtop cladding. This ring has a roundtrip time of≈4 ps. The measured spectrum of thedrop and through port are shown in Figure 4. This Figure also show the CW responseof the through port of the ring. Both in the drop and through ports the pulse spectrumcan be seen which is around 12 nm wide, superposed with the MR CWresponse of theappropriate port.Proceedings Symposium IEEE/LEOS Benelux Chapter, 2005, Mons53Wavelength [nm]1530 1540 1550 1560 1570Transmission[-]0.000.250.500.751.00Though port - CWThough port - 200fs pulsesDrop port - 200fs pulsesFigure 4: Measured wavelength spectrum after propagation of CW and 200 fs pulsesthrough MR with roundtrip time of≈4ps.ConclusionThese measurements show some interesting behaviour of ps and sub-ps pulses in resonantfilters (MR). These few measurements already show intriguingresults, stimulating addi-tional measurements and demonstrating the fundamental speed limitations for high speedfiltering by microring resonators. When even shorter pulses can be used with a durationclearly shorter than the roundtrip time, better answers canbe given to the behaviour of thepulses and additional phenomena like ballistic transport can be observed [5].AcknowledgementThis work was supported by EC funded project Next-generation Active Integrated-opticSubsystems (IST-2000-28018), Dutch government sponsoredFreeband BBPhotonics (BSIK03025, http://bbphotonics.freeband.nl) and European network of excellence ePIXnet.References[1] C.K. Madsen and J.H. Zhao.Optical Filter Design and Analysis. Wiley, 1999.[2] K. H. Kim, H. K. Lee, S. Y. Park, and E-H Lee.Calculation os dispersion and nonlinear effect limitedmaximum TDM and FDM bit rates of transformlimited pulses in single-mode optical fibers. Journ.Lightwave. Technol., 13(8):15971605, 1995.[3] E.J. Klein, D.H. Geuzebroek, H. Kelderman, G. Sengo, N. Baker, A. Driessen, (2005).Reconfigurableoptical add-drop multiplexer using microring resonators, Accepted for publication in IEEE photonicstechnol. lett. November 2005.[4] D.H. Geuzebroek,Flexible optical network components based on densely integrat d microring res-onators, PhD Thesis, University of Twente, Oktober 2005.[5] H. Gersen, D.J.W. Klunder, J.P. Korterik, A. Driessen, Van Hulst N.F., and L. Kuipers.The propagationof a femtosecond pulse in a microresonator visualized in time.Optics Letters, 29:12911293, 2004.Behaviour of picosecond and femtosecond pulses in SiO2/Si3N4 microring resonatorfilters54

Behaviour of picosecond and femtosecond pulses in SiO2/Si3N4 microring resonator filters

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Behaviour of picosecond and femtosecond pulses in SiO<sub>2</sub>/Si<sub>3</sub>N<sub>4</sub> microring resonator filters

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