High-peak-power pulse generation from a monolithicmaster oscillator power amplifier at 1.5 μm

P. Adamiec,* B. Bonilla, A. Consoli, J. M. G. Tijero, S. Aguilera, and I. EsquiviasUniversidad Politécnica de Madrid, ETSI Telecomunicación-CEMDATIC, Ciudad Universitaria, Madrid 28040, Spain

*Corresponding author: pawel.adamiec@tfo.upm.es

Received 6 June 2012; revised 4 September 2012; accepted 6 September 2012;posted 7 September 2012 (Doc. ID 170116); published 11 October 2012

We present an experimental study on the generation of high-peak-power short optical pulses from a fullyintegrated master-oscillator power-amplifier emitting at 1.5 μm. High-peak-power (2.7 W) optical pulseswith short duration (100 ps) have been generated by gain switching themaster oscillator under optimizeddriving conditions. The static and dynamic characteristics of the device have been studied as a function ofthe driving conditions. The ripples appearing in the power-current characteristics under cw conditionshave been attributed to mode hopping between the master oscillator resonant mode and the Fabry–Perotmodes of the entire device cavity. Although compound cavity effects have been evidenced to affect thestatic and dynamic performance of the device, we have demonstrated that trains of single-mode shortoptical pulses at gigahertz frequencies can be conveniently generated in these devices. © 2012 OpticalSociety of AmericaOCIS codes: 140.3070, 140.5960, 320.5550.

1. Introduction

The requirement of eye safety for important applica-tions like lidar or free space optical (FSO) communi-cations makes the generation of high-power shortoptical pulses at 1.5 μm specifically interesting.Moreover, a short rise time of such pulses would re-duce the distance error and the measurement time inapplications involving pulsed time-of-flight (TOF)measurements, as range finders, three-dimensional(3D) scanners, or traffic velocity controls [1].

The master oscillator power amplifier (MOPA)architecture is a rather natural choice for high-powerpulse generation because a large and fast change inthe output power can be obtained with a relativelysmall change in the master oscillator (MO) current.In this respect, in addition to the well-known advan-tages of semiconductor lasers (i.e., compactness, ef-ficiency, and reduced cost), monolithically integratedMOPAs offer the possibility of a versatile selectionof the pulse repetition frequency by using the

gain-switching (GS) technique to drive theMO. Thusfar, hybrid semiconductor MOPAs consisting of twoseparate devices coupled through additional opticshave been preferred over monolithically integratedMOPAs because of the possibility of an independentoptimization of each section. Recently, a 1060 nm hy-brid MOPA based on a tapered power amplifier (PA)delivering pulses with 50 W peak power and 80 pspulse width has been reported [2].

Monolithically integrated MOPAs are devices ofreduced size and mechanical complexity with respectto their counterparts, hybrid MOPAs. They havedemonstrated 10 W at 977 nm in cw together witha nearly diffraction limited beam and narrow emis-sion bandwidth [3]. At an eye-safe wavelength closeto 1.5 μm, 700 mW cw from a single-mode fiber mod-ule and 50 ns pulses with 10W peak power have beenreported [4].

Although in an integrated MOPA, the laser andthe amplifier are ideally independent devices,different studies have shown that they are proneto compound cavity effects because of the residualreflectance of the amplifier front facet [5,6]. Theseeffects have been theoretically analyzed [5,7–9],

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and the results indicate that they produce modalinstabilities, such as self-pulsations and coupledcavity mode competition. These effects are morerelevant at low injection levels of the MO, producingripples in the power-current (P-I) characteristics as aconsequence of longitudinal mode hopping [5].

The dynamic response of monolithic MOPAs undermodulation of the oscillator section has been studiedby Dzurko et al. [10]. They reported 2.2 GHz modula-tion bandwidth in 944 nm flared amplifier MOPAs,corresponding to the bandwidth of the distributedBragg reflector master oscillator. As far as we know,there are no previous reports on the response ofmonolithic MOPAs when the MO is driven in GS con-ditions and, a priori, it is not clear whether com-pound cavity effects affect their temporal response.

In this paper, we report on the generation andcharacterization of high-power short optical pulsesfrom a monolithic MOPA emitting around 1.5 μm.The pulses are generated in the MO by GS techni-ques, whereas the power amplifier is driven in cwconditions. The temporal response of the MOPA asreflected in the pulse characteristics is analyzed asa function of the driving conditions. We show thatstable and powerful pulses can be generated at theoscillator wavelength by properly choosing the GSparameters.

This paper is organized as follows. In Section 2, thedevice geometry and some relevant details of the ex-perimental setup are briefly described. Section 3 ex-amines and discusses the P-I characteristics, thegeneration of different type of pulses as a functionof the GS parameters, and the analysis of the opti-mum conditions for generating short- and high-power pulses. Conclusions are drawn in Section 4.

2. Experimental Techniques

The measured device is a commercially availableMOPA with emission wavelength close to 1.55 μm(QPC Lasers 4715-0000). It consists of a distributedfeedback (DFB) MO and a tapered power amplifier.The total device length is around 2.5 mm, and itsoutput facet width is 250 μm. The device is p-upmounted on a c-mount.

The output power under cw operation of both sec-tions wasmeasured using a thermal detector (GentecUP19K-H) placed close to the output facet. A slighttilting of the thermal detector was necessary to avoidoptical feedback. The optical pulses were obtainedby GS the DFB laser while driving the amplifier withcw current. The DFB laser was fed through a bias-tee with a cw bias current and a sinusoidal RF signal(Agilent 8648B), which was amplified up to 23 dBm.A microstrip circuit was used to supply the RF signalto the DFB laser to decrease the electrical parasitics.All the measurements were performed at a constanttemperature of 20°C.

In pulsed conditions, a fraction of the optical powerwas collected by a lensed optical fiber. The pulseshapes were then measured with the optical moduleof a 20 GHz digital sampling oscilloscope (Tektronix

DSA8000), whereas the optical spectra weresimultaneously measured with an optical spectrumanalyzer (OSA) ANDO AQ-6315B (50 pm resolution).The absolute power of the optical pulses was cali-brated by comparing the average power measuredwith the thermal detector in pulsed conditions withthe mean value of the pulse profile measured in theoscilloscope. We ensured that the measured temporalprofiles were repetitive and independent of the posi-tion of the fiber with respect to the laser facet.

3. Results and Discussion

Figure 1 shows the output power Pout versus thepower amplifier current (IPA) for different currentsof the master oscillator (IMO) in cw operation. Atlow oscillator injection (IMO ≤ 10 mA), the outputpower is negligible; for higher oscillator injection(IMO � 30 mA and IMO � 50 mA), ripples in thePout-IPA characteristic are apparent. Similar (butnot identical) ripples were previously reported bySpreeman [5] in 1060 nmMOPAs. The origin of theseripples is analyzed in the following paragraphs. Athigh oscillator current (IMO ≥ 100 mA), the ripplesdisappear. The Pout-IPA characteristics are affectedby thermal rollover, as it was confirmed by the mea-sured redshift of the lasing wavelength when in-creasing the amplifier current. The thermalrollover is more pronounced for low oscillator injec-tion (compare IMO � 30 mA with IMO � 500 mA inFig. 1). This is due to the change in the balancebetween the total electrical power supplied to thedevice and the output optical power: At high IMO,the output power is higher than the output powerat low IMO, and therefore the dissipated electricalpower is lower, thus reducing the chip temperatureand the rollover effects.

The inset in Fig. 1 shows as an example the opticalspectrum measured at a high output power, Pout �1.3 W (IMO � 500 mA and IPA � 4 A). The distancebetween secondary modes, ∼0.38 nm, is attributedto the resonances of the DFB cavity, which is around900 μm long. The FWHM is ∼50 pm, corresponding

Fig. 1. (Color online) Continuous wave optical power versus am-plifier current for different master oscillator currents (0, 10, 30, 50,100, 200, and 500 mA, from bottom to top). Inset, optical spectrumat IPA � 4 A and IMO � 500 mA.

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to the OSA resolution. The main peak shows twoshoulders, attributed to secondary modes, indicatinga side mode suppression ratio (SMSR) of about 28 dBin these driving conditions. The spectra measuredat a constant high oscillator injection for differentamplifier currents are similar to that of Fig. 1,showing only a slight redshift caused by self-heating(∼90 pm ∕A at IMO � 500 mA).

At low oscillator injection, mode hopping wasobserved. Figure 2 shows the spectra measured ata constant IMO � 30 mA and three closely spacedvalues of the amplifier current, corresponding toconsecutive valley, peak, and valley regions of theripples in the Pout-IPA characteristics (see Fig. 1).At the peaks of the ripples, the lasing wavelengthjumps 20 nm with respect to the laser wavelengthat the previous valley, from ∼1547 nm down to∼1527 nm, and again it jumps back to its initial va-lue when increasing the current up to the value cor-responding to the next valley. This is interpreted asthe result of mode competition between the DFB re-sonance at ∼1547 nm and the Fabry–Perot (FP)modes of the complete MOPA cavity. In detail, whenthe device is working as a real MOPA, the oscillationof the DFB, at around 1547 nm, is amplified by thetapered gain section, and this is the lasing wave-length. The residual front facet reflectance of thepower amplifier, which cannot be totally suppressed,may make the entire MOPA device behave as an FPtwo-section tapered laser, with a resonator length gi-ven by the total MOPA cavity. In this case, the lasingtakes place in an FP mode at the maximum of thegain spectrum at 1527 nm (as shown in Fig. 2). Infact, the wavelength separation between secondarymodes at low oscillator current is 0.14 nm, whichcorresponds to the total cavity length of 2.5 mm.The relatively large wavelength shift is due to thelarge detuning of the DFB wavelength from the wa-velength of maximum gain. A rich spectral dynamicsis also observed at low oscillator injection, includingpulsations at 18 GHz as evidenced in the RF spectra

(not shown). This frequency corresponds to theround-trip time of the complete cavity, and it there-fore is attributed to longitudinal mode beating. Thistype of spectral dynamics has been studied pre-viously in lasers with active optical feedback [11].

A similar behavior, periodic ripples in the Pout-IPAcharacteristics correlated with mode hopping hasbeen observed in 1060 nm DFBMOPAs [5]. Its originhas been clearly explained and theoretically repro-duced [5,9] as arising from compound cavity effectsand thermal tuning of the mode wavelength. The wa-velength jump was much smaller than in our experi-ments and corresponded to the spacing between theneighboring modes of the compound cavity. In ourcase, we attribute the periodicity of the ripples andthe corresponding mode hops to the different thermalshifts of the compound and DFB cavity modes whenincreasing the amplifier current. This produces aperiodicity of the coincidence of both frequencycombs, which promotes lasing at the compound cav-ity mode closest to the maximum gain. A more de-tailed analysis of the emission spectra is underprogress and will be further reported.

The characteristics of the pulses obtained by GSthe DFB laser were studied for different drivingconditions, that is, RF amplitude and repetitionfrequency, and oscillator and amplifier currents.Figures 3(a) and 3(b) show the pulses obtained byGS the oscillator section with an RF nominal powerof 23 dBm at 1 GHz repetition rate and varyingoscillator and amplifier currents, respectively. Theincrease of the oscillator current at constant ampli-fier current shortens the turn-on time and producesfaster relaxation oscillations, thus reducing the dura-tion of the output pulse arising from the first relaxa-tion oscillation spike [see Fig. 3(a)]. This is thestandard behavior in single-section gain-switchedsemiconductor lasers when the bias current is in-creased [12,13]. The peak power increases up to amaximum level at IMO � 30 mA. At this bias, the sec-ond spike of relaxation oscillations manifests as apulse tail. At higher IMO, the second peak increasesand the power of the first peak decreases, leading

Fig. 2. (Color online) Optical spectra for a master oscillator cur-rent of 30 mA and different amplifier currents IPA, correspondingto a valley of the ripples in Fig. 1 (top), to a peak (center), and to theconsecutive valley (bottom). The curves have been verticallyshifted for clarity.

Fig. 3. (Color online) Evolution of optical pulses with the (a) oscil-lator and (b) amplifier currents at a repetition frequency of 1 GHz.

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eventually to a direct modulation regime at oscillatorcurrents higher than 100 mA.

The evolution of the pulses as a function of the am-plifier current at constant oscillator current, shownin Fig. 2(b), indicates the absence of linear amplifica-tion of the pulses generated by the oscillator, as wewill discuss later. For a low current in the amplifiersection (1.5 A), the pulses generated have a longduration (220 ps) and low peak intensity (0.3 W).As the amplifier current is increased, shorter pulseswith higher peak power are obtained up to 4 A. Athigher currents, the pulse broadens and its peakpower decreases, which is attributed to thermaleffects.

The peak-power and duration of the pulse obtainedas a function of the oscillator and amplifier currentswith an RF nominal power of 23 dBm at 1 GHzrepetition rate are respectively shown in Figs. 4(a)and 4(b). The increase of the bias current in theDFB section leads to pulses of higher intensity andshorter duration, until long-tailed pulses appear,because of the second spike of the relaxation oscil-lations. The most powerful (2.7 W) and shortestpulses (100 ps) are obtained for IPA � 4 A andIMO � 30 mA.

The dependence of the pulse amplitude and dura-tion on the amplifier current shows a similar beha-vior for the three values of the oscillator currentshown in Fig. 4(b). The pulse duration decreasesfrom more than 200 ps down to around 100 ps, whilethe peak power has a maximum at around IPA � 4 A.If the optical pulse were linearly amplified, the peakpower would be expected to increase as a functionof the amplifier bias, with no variation of thepulse shape and duration [14]. Contrarily, in our

experiments, the pulse duration decreases rapidlyfor IPA between 1.5 A and 3 A. Therefore, we inter-pret this behavior as a consequence of a compoundcavity effect. Namely, the optical pulse is generatedby the complete cavity (DFB plus amplifier) behavingmore as a two-section tapered laser than as an idealMOPA. In fact, a similar dependence of the pulsewidth on the tapered section current has been re-ported in 1060 nm two-section tapered lasers [15].

In the range of currents from 3 to 4.5 A, the pulseduration is almost constant, as expected in MOPAdevices [14]. We attribute the power degradationand the slight pulse broadening for amplifier cur-rents above 4 A to thermal effects, as previously com-mented. The rollover of the pulse power as a functionof the amplifier current is more pronounced than inthe cw regime [compare Fig. 4(b) with Fig. 1] becauseof the lower optical power leaving the device inpulsed conditions. This result suggests that higherpeak-power levels could be reached if the amplifiersection were driven with a pulsed current with lowduty cycle, thus reducing the pulse degradation be-cause of internal heating.

Figure 5 shows the pulse train generated by GSobtained at IPA � 4 A and IMO � 30 mA. The peakpower is 2.7 W, and FWHM is about 100 ps. Its risetime, 78 ps, is an interesting value for TOF applica-tions, in which the rise time is the main parameterand the pulse tail is not relevant. The pulses arestable and repetitive in shape and output power.

The spectrum of the pulses in Fig. 5 is shown inFig. 6 where it has been plotted in logarithmicscale in a large wavelength range and in a linearscale around the emission peak (inset). It shows asingle longitudinal mode close to the DFB emissionwavelength ∼1547 nm. This indicates that underthese driving conditions the amplifier is amplifyingthe pulses generated by the gain-switched DFB. Thiswas not always the case, however, and under somedriving conditions, especially at low repetition fre-quencies, longitudinal mode hopping was observed,and also the temporal profile of the pulses was not

Fig. 4. (Color online) Peak power and FWHM of the pulses as afunction of the (a) oscillator and (b) amplifier currents at a repeti-tion frequency of 1 GHz.

Fig. 5. Pulse train with highest peak power and lowest FWHMmeasured at 1 GHz and IPA � 4 A and IMO � 30 mA.

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the standard in gain-switched single emitters, indi-cating again compound cavity effects.

The spectral width of the pulse in Fig. 6 is 80 pmFWHM. The broadening is attributed to the well-known chirp of gain-switched laser diodes [13],indicating a relatively low chirp level. The smallshoulder at ∼15 dB at the high wavelength side ofthe pulse (see the inset of Fig. 6) is attributed to asecondary compound cavity mode. The SMSR is∼10 dB, with the main secondary mode at the semi-conductor gain maximum. These results indicatethat the pulses generated by GS the DFB oscillatorof a tapered MOPA are promising candidates fora broad range of applications requiring high-peak-power short pulses at eye-safe wavelengths.

4. Conclusion

We have demonstrated for the first time, to the bestof our knowledge, the generation of high-powerpulses at 1.5 μm by GS the MO of a fully integratedMOPA device. 2.7 W peak-power pulses with 100 psduration (FWHM) and 78 ps rise time have beenattained. Compound cavity effects have been evi-denced, and their influence on the device perfor-mance on cw and GS operation has been studied.At low oscillator current in cw operation, the deviceshows competition between FP modes of the com-plete cavity and the amplified DFB resonance. Inspite of these compound cavity effects, we have de-monstrated that it is possible to drive the devicein GS operation to produce stable single-frequencypulses. The procedure has demonstrated to be aneasy, versatile, and low-cost method for obtaininghigh-power pulses in the picoseconds range at con-trolled repetition rates from a fully integrated com-mercial MOPA. Such pulses may find application intechnologies requiring high-power short-rise-timepulses at low cost in the eye-safe spectral range.

The authors gratefully acknowledge the support ofthe Ministerio de Ciencia e Innovación (Spain)

through project TEC2009-14581 and European pro-gram PEOPLE—Marie Curie Actions IEF, projectNo. E100945552.

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Fig. 6. Spectrum of the pulses in Fig. 5. Inset, spectrum in linearscale and small wavelength range.

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