The intensity and frequency chirp of picosecond pulses from a self-seeded gain-switched Fabry-Perot laser diode have been directly measured using the technique of frequency-resolved optical gating. Measurements over an output sidemode suppression ratio (SMSR) range of 15-35 dB show that higher SMSR's are associated with an increasingly linear frequency chirp across the output pulses. This complete pulse characterization allows the conditions for optimum pulse compression to be determined accurately, and indicates that transform-limited, pedestal free pulses can be obtained at an SMSR of 35 dB

Barry, Liam P.

Dudley, John M.

Harvey, John D.

Thomsen, Benn C.

English

Name not available

IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 10, NO. 7, JULY 1998 935Characterization of 1.55- m Pulses from aSelf-Seeded Gain-Switched Fabry–Perot LaserDiode Using Frequency-Resolved Optical GatingL. P. Barry, Member, IEEE, B. C. Thomsen, J. M. Dudley, and J. D. Harvey, Member, IEEEAbstract— The intensity and frequency chirp of picosecondpulses from a self-seeded gain-switched Fabry–Perot laser diodehave been directly measured using the technique of frequency-resolved optical gating. Measurements over an output sidemodesuppression ratio (SMSR) range of 15–35 dB show that higherSMSR’s are associated with an increasingly linear frequencychirp across the output pulses. This complete pulse characteri-zation allows the conditions for optimum pulse compression tobe determined accurately, and indicates that transform-limited,pedestal free pulses can be obtained at an SMSR of 35 dB.Index Terms—Optical fiber communication, optical fiber dis-persion, optical pulses, semiconductor lasers, ultrafast optics.I. INTRODUCTIONTHE GENERATION and measurement of picosecondpulses at wavelengths around 1.55 m is very importantfor the development of high-capacity optical communicationssystems. A convenient technique for single-mode picosec-ond pulse generation is the self-seeding of gain-switchedFabry–Perot (FP) laser diodes, and many experimentalschemes using this technique have been reported [1]–[6].With self-seeding, an external cavity containing a wavelength-selective element reinjects a small fraction of the output lightback into the gain-switched laser at only one longitudinalmode frequency. Provided that reinjection occurs during thepulse buildup time in the FP laser, gain is suppressed inall, but the reinjected mode, and the laser produces a stabletrain of single-mode pulses. This technique is an effectiveway to produce wavelength-tunable 10–100-ps pulses withlow pulse-to-pulse timing jitter [3], and recent experimentshave also demonstrated multiwavelength output [5] suitablefor application in wavelength-division-multiplexed (WDM)networks.An intrinsic problem associated with pulse generation usinggain-switching is the presence of a large frequency chirp acrossthe pulses. This arises from variations in carrier density in thegain region during pulse buildup [4], and is also present inself-seeded gain-switched lasers. For practical applications ofgain-switched pulses, chirp compensation is usually employed,using dispersion-compensating fiber (DCF) [4], [7]–[9] orchirped fiber Bragg gratings (FBG’s) [10]. In these experi-ments, however, the absence of a priori chirp characterizationManuscript received January 30, 1998; revised March 16, 1998.The authors are with the Department of Physics, University of Auckland,Auckland, New Zealand.Publisher Item Identifier S 1041-1135(98)04707-7.Fig. 1. Experimental setup used for pulse generation and measurement.results in the conditions for optimum chirp compensationbeing obtained using an approximate pulse compression modelassuming linearly chirped Gaussian pulses [4], [7], [8], or bytrial and error using different lengths of DCF [9].In this letter, we use the technique of frequency-resolvedoptical gating (FROG) [11] to measure the intensity and thefrequency chirp of pulses from a self-seeded, gain-switchedFP laser, and we study in particular the variation in the chirpas a function of the external cavity reinjection level. Thiscomplete pulse characterization allows the optimal conditionsfor pulse compression to be determined using numerical sim-ulations without any assumptions about the pulse intensity orchirp characteristics. We use the simulations to determine theoptimal laser operation conditions which result in transform-limited pulses after compression.II. EXPERIMENTAL SETUPFig. 1 shows our experimental setup. The FP laser was atemperature-controlled 1.5- m p-side down InGaAsP device,with a longitudinal mode spacing of 1.1 nm, a threshold currentof 27 mA, and a 10-GHz modulation bandwidth. With the laserbiased below threshold at 5 mA, gain-switching was carriedout around 500 MHz using a step-recovery diode to generatean electrical pulse train of 13-V amplitude and 80-ps full-width at half-maximum (FWHM) [2]. The external cavity forreinjection was all-fiber based [6], containing a polarizationcontroller (PC), a 3-dB coupler, and a FBG with centralreflection wavelength of 1552 nm and bandwidth of 0.3 nm.Gain-switching without reinjection resulted in the generationof 12-ps pulses (FWHM) with a multimode spectrum. Self-seeded gain-switched operation was achieved by first adjustingthe FP laser temperature so that an oscillating longitudinal1041–1135/98$10.00  1998 IEEEAuthorized licensed use limited to: DUBLIN CITY UNIVERSITY. Downloaded on January 19, 2009 at 07:05 from IEEE Xplore.  Restrictions apply.936 IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 10, NO. 7, JULY 1998Fig. 2. Measured pulse spectra for gain-switched self-seeded laser operationcorresponding to observed SMSR values of (a) 15 dB, (b) 25 dB, and (c)35 dB.laser mode coincided with the central reflection wavelengthof the fiber grating. Then, the gain-switching frequency wastuned to 496.76 MHz, corresponding to the 26th harmonicof the external cavity. The amount of reinjection from theexternal cavity could be adjusted by varying the polarizationof the reflected signal using the PC.The laser output was taken from the 3-dB coupler shownin Fig. 1. Pulse characterization was carried out using anoptical spectrum analyzer, a 25-GHz detector and digitaloscilloscope, and a FROG measurement setup using secondharmonic generation. The FROG measurement yields a two-dimensional (2-D) time-frequency spectrogram of the pulsefrom which the time-dependent intensity and phase can beobtained using phase-retrieval techniques [11]. Specifically, ifthe pulse electric field is written, the FROG technique allows the recovery of and. The instantaneous frequency chirp (in gigahertz)across the pulse is obtained from the phase by. To obtain good signal-to-noise ratio inthe FROG measurements, an erbium-doped fiber amplifier(EDFA) amplified the laser pulses to a peak power of around1 W. At this power level, amplification was purely linear andintroduced no intensity or chirp distortion.III. RESULTS AND DISCUSSIONThe pulse characteristics from the self-seeded gain-switchedlaser were examined as a function of reinjection level. Thehighest sidemode suppression ratio (SMSR) in the output spec-trum for stable pulse generation was 35 dB corresponding toan estimated reinjection level of 6%. With reduced reinjection,the SMSR decreased, and stable output pulses were obtainedover a SMSR range of 35–15 dB. At 5-dB increments overthis range, the pulses were characterized by monitoring thepulse spectra and by performing FROG measurements. Withinexperimental error, the average output power was measured ataround 0.17 mW for all SMSR values. At estimated reinjectionFig. 3. Output pulse intensity (solid line, left axis) and frequency chirp(dashed line, right axis) for observed SMSR values of (a) 15 dB, (b) 25 dB, and(c) 35 dB. The corresponding simulation results after optimal compression areshown in (d), (e), and (f), respectively. Note the different timescales between(a)–(c) and (d)–(f).levels greater than 6%, pulse deformation and instabilitieswere observed [12], although a comprehensive analysis of thisoperating regime was not carried out in our experiments.Fig. 2(a)–(c) shows measured output spectra for estimatedreinjection levels of 0.1, 0.8, and 6% corresponding to SMSR’sof 15, 25, and 35 dB respectively. It is clear that increasedreinjection results in improved SMSR, reduced spectral width,and increased spectral symmetry. Additional physical insightis obtained from the FROG characterization, and Fig. 3(a)–(c)shows the corresponding retrieved pulse intensity and fre-quency chirp . As expected, the chirp blue-shifts the pulseleading edge and red-shifts the trailing edge [1]. It is also clearthat an increased SMSR is associated with increased pulseduration and reduced chirp, consistent with the spectral datain Fig. 2. Significantly, the FROG measurements show thatwhilst the chirp is highly nonlinear at low SMSR, it becomesincreasingly linear with increasing SMSR. The reduction in thechirp magnitude with increasing reinjection has been notedpreviously [12], [13] and arises since the presence of thereinjected signal reduces the peak inversion reached duringthe pulse emission process. To the best of our knowledge,the results presented here are the first direct measurements ofthe change in chirp across the pulse with reinjection level,and the first to demonstrate improved linearity with increasedreinjection.With the complete intensity and chirp characterization of thelaser pulses in Fig. 3(a)–(f), the optimal conditions for pulsecompression can be investigated by numerically simulatingthe propagation of the pulses in DCF [9]. By examining theintensity and chirp characteristics as a function of propagationdistance, the required fiber length for optimum compressioncan be obtained accurately. In our simulations, we modeledpulse propagation neglecting nonlinear propagation effects atAuthorized licensed use limited to: DUBLIN CITY UNIVERSITY. Downloaded on January 19, 2009 at 07:05 from IEEE Xplore.  Restrictions apply.BARRY et al.: CHARACTERIZATION OF 1.55- m PULSES 937TABLE ISUMMARY OF RESULTS FROM EXPERIMENTS AND SIMULATIONSthe peak power levels in our experiments, and assuming anormal dispersion of 16 ps/nm km in the DCF.Fig. 3(d)–(f) shows simulation results for the optimally com-pressed pulses corresponding to Fig. 3(a)–(c), respectively.In all cases, the chirp is close to zero across the pulsecenter, but the nonlinear chirp of the input pulses at thelower SMSR values shown in Fig. 3(a) and (b) results inimperfect compression and subsidiary pulses on the outputsshown in Fig. 3(d) and (e). At the highest SMSR 35 dB,however, the linear chirp of the input pulse in Fig. 3(c)results in the compressed output pulse in Fig. 3(f), whichdoes not possess any significant substructure in the wings.Table I summarizes our results. For each SMSR value, wetabulate the pulse spectral and temporal FWHM, and the pulsechirp is characterized by its average slope across thepulse FWHM. We also tabulate the temporal FWHM andtime-bandwidth product of the compressed pulses, and therequired fiber length from the simulations. As expected, thelinear chirp associated with higher SMSR’s leads to improvedtime-bandwidth products after compression. Indeed, the mostlinearly chirped input pulse at an SMSR 35 dB is well-fitted by a Gaussian after compression, and is essentiallytransform-limited with 0.44.As well as using numerical simulations as describedabove, the direct retrieval of the pulse chirp slopefrom the FROG measurements allows the conditions foroptimal compression to be obtained using the result[7], [8]. Here is the optimal length of fiberwith dispersion parameter . Using 16 ps/nm km asabove, the required fiber lengths obtained using this techniqueare also summarized in Table I and we note that the resultsare in good agreement with simulations even for low SMSR’swhen there is appreciable nonlinear chirp and asymmetricpulse spectra.IV. CONCLUSIONThe FROG technique has been used to characterize theintensity and chirp of pulses from a gain-switched self-seededFP laser. The pulse chirp has been found to decrease andto become increasingly linear with increasing SMSR. At thehighest SMSR of 35 dB, simulations indicate that transform-limited pulses are obtained after compression. The optimumconditions for pulse compression can also be obtained accu-rately using the average chirp slope across the pulse FWHM.We anticipate that FROG measurements will allow furtheroptimization and improved physical understanding of gain-switched laser diodes.REFERENCES[1] M. Cavelier, N. Stelmakh, J. M. Xie, L. Chusseau, J. M. Lourtioz,C. Kasmierski, and N. Bouadama, “Picosecond (<2.5 ps) wavelengthtunable (20 nm) semiconductor laser pulses with repetition rates upto 12 GHz,” Electron. Lett., vol. 28, pp. 224–226, 1992.[2] L. P. Barry, R. F. O’Dowd, J. Debeau, and R. Boittin, “Tunabletransform limited pulse generation using self-injection locking of a FPlaser,” IEEE Photon. Technol. Lett., vol. 5, pp. 1132–1134, 1993.[3] M. Schell, W. Utz, D. Huhse, J. Kassner, and D. Bimberg, “Lowjitter single mode pulse generation by a self-seeded, gain-switchedFabry–Perot semiconductor laser,” Appl. Phys. Lett., vol. 65, pp.3045–3047, 1994.[4] Y. C. Lee and C. Shu, “Wavelength-tunable nearly transform-limitedpulses generated by self-injection seeding of a laser diode at an arbitraryrepetition rate,” IEEE Photon Technol. Lett., vol. 9, pp. 590–592,1997.[5] C. Shu and S. P Yam, “Effective generation of tunable single- andmultiwavelength optical pulses from a Fabry–Perot laser diode,” IEEEPhoton Technol. Lett., vol. 9, pp. 1214–1216, 1997.[6] Y. Zhao and C. Shu, “Single-mode operation characteristics of a self-injection seeded Fabry–Perot laser diode with distributed feedback froma fiber grating,” IEEE Photon Technol. Lett., vol. 9, pp. 1436–1438,1997.[7] H.-F. Liu, Y. Ogawa, and S. Oshiba, “Generation of an extremely shortsingle-mode pulse (2 ps) by fiber compression of a gain-switched pulsefrom a 1.3 m distributed-feedback laser diode,” Appl. Phys. Lett., vol.59, pp. 1284–1286, 1991.[8] K. A. Ahmed, H-F Liu, N. Onodera, P. Lee, R. S. Tucker, and Y.Ogawa, “Nearly transform-limited pulse (3.6 ps) generation from gain-switched 1.55 m distributed feedback laser by using fiber compressiontechnique,” Electron. Lett., vol. 29, pp. 54–56, 1993.[9] L. Chusseau and C. Kasmierski, “Optimum linear pulse compression ofa gain switched 1.5 m DFB laser,” IEEE Photon. Technol. Lett., vol.6, pp. 24–26, 1994.[10] B. J. Eggleton, P. A. Krug, L. Poladian, K. A. Ahmed, and H. F. Liu,“Experimental demonstration of compression of dispersed optical pulsesby reflection from self-chirped optical fiber Bragg gratings,” Opt. Lett.,vol. 19, pp. 877–879, 1994.[11] R. Trebino, K. W. DeLong, D. N. Fittinghoff, J. N. Sweetster, M. A.Krumbu¨gel, and B. A. Richman, “Measuring ultrashort laser pulses inthe time-frequency domain using frequency-resolved optical gating,”Rev. Sci. Instrum., vol. 68, pp. 3277–3295, 1997.[12] D. S. Seo, H. Feng Liu, D. Y. Kim, and D. D. Sampson, “Injectionpower and wavelength dependence of an external-seeded gain-switchedFabry-Perot laser,” Appl. Phys. Lett., vol. 67, pp. 1503-1505, 1997.[13] Y. Matsui, S. Kutsuzawa, S. Arahira, and Y. Ogawa, “Generation ofwavelength tunable gain-switched pulses from FP MQW lasers withexternal injection seeding,” IEEE Photon. Technol. Lett., vol. 9, pp.1087–1089, 1997.Authorized licensed use limited to: DUBLIN CITY UNIVERSITY. Downloaded on January 19, 2009 at 07:05 from IEEE Xplore.  Restrictions apply.

Characterization of 1.55-μm pulses from a self-seeded gain-switched Fabry-Pérot laser diode using frequency-resolved optical gating

L.P. Barry

B.C. Thomsen

J.M. Dudley

J.D. Harvey

Crossref

Characterization of 1.55-μm pulses from a self-seeded gain-switched Fabry-Perot laser diode using frequency-resolved optical gating

The intensity and frequency chirp of picosecond pulses from a self-seeded gain-switched Fabry-Perot laser diode have been directly measured using the technique of frequency-resolved optical gating. Measurements over an output sidemode suppression ratio (SMSR) range of 15-35 dB show that higher SMSR\u27s are associated with an increasingly linear frequency chirp across the output pulses. This complete pulse characterization allows the conditions for optimum pulse compression to be determined accurately, and indicates that transform-limited, pedestal free pulses can be obtained at an SMSR of 35 dB

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Characterization of 1.55-μm pulses from a self-seeded gain-switched Fabry-Pérot laser diode using frequency-resolved optical gating

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