Passive mode-locking in semiconductor lasers in a Fabry–Perot configuration with a bandgap blueshift applied to the saturable absorber (SA) section has been experimentally characterized. For the first time a fully post-growth technique, quantum well intermixing, was adopted to modify the material bandgap in the SA section. The measurements showed not only an expected narrowing of the pulse width but also a significant expansion of the range of bias conditions generating a stable train of optical pulses. Moreover, the pulses from lasers with bandgap shifted absorbers presented reduced chirp and increased peak power with respect to the nonshifted case

Pusino, Vincenzo

Sorel, Marc

Strain, Michael J.

English

Passive mode-locking in semiconductor lasers in a Fabry-Perot configuration with a bandgap blueshift applied to the saturable absorber (SA) section has been experimentally characterized. For the first time a fully post-growth technique, quantum well intermixing, was adopted to modify the material bandgap in the SA section. The measurements showed not only an expected narrowing of the pulse width but also a significant expansion of the range of bias conditions generating a stable train of optical pulses. Moreover, the pulses from lasers with bandgap shifted absorbers presented reduced chirp and increased peak power with respect to the nonshifted case

University of Strathclyde Institutional Repository

Strathprints Institutional RepositoryPusino, Vincenzo and Strain, Michael J. and Sorel, Marc (2014) Passive mode-locking in semiconductor lasers with saturable absorbers bandgap shifted through quantum well intermixing. Photonics Research, 2 (6). pp. 186-189. ISSN 2327-9125 , http://dx.doi.org/10.1364/PRJ.2.000186This version is available at http://strathprints.strath.ac.uk/54324/Strathprints is  designed  to  allow  users  to  access  the  research  output  of  the  University  of Strathclyde. Unless otherwise explicitly stated on the manuscript, Copyright © and Moral Rights for the papers on this site are retained by the individual authors and/or other copyright owners. Please check the manuscript for details of any other licences that may have been applied. You may  not  engage  in  further  distribution  of  the  material  for  any  profitmaking  activities  or  any commercial gain. You may freely distribute both the url (http://strathprints.strath.ac.uk/) and the content of this paper for research or private study, educational, or not-for-profit purposes without prior permission or charge. Any  correspondence  concerning  this  service  should  be  sent  to  Strathprints  administrator: strathprints@strath.ac.ukPassive mode-locking in semiconductor lasers with saturable absorbers bandgap shifted through quantum well intermixing Vincenzo Pusino,1,* Michael J. Strain,1,2and Marc Sorel1 1School of Engineering, University of GlasgowRankine Building, Oakfield Avenue, Glasgow, G128LT, U.K. 2Institute of Photonics, University of Strathclyde,Glasgow G4 0NW, U.K. *Corresponding author: Vincenzo.Pusino@glasgow.ac.uk Received July X, 2014; revised Month X, XXXX; accepted Month X, XXXX; posted Month X, XXXX (Doc. ID XXXXX); published Month X, XXXX Passive mode-locking in semiconductor lasers in a Fabry-Pérot configuration with a bandgap blue-shift applied to the saturable absorber section has been experimentally characterized. For the first time a fully post-growth technique, quantum well intermixing, was adopted to modify the material bandgap in the saturable absorber section. The measurements showed not only an expected narrowing of the pulse width, but also a significant expansion of the range of bias conditions generating a stable train of optical pulses. Moreover, the pulses from lasers with bandgap shifted absorbers presented reduced chirp and increased peak power with respect to the non-shifted case.  © 2014 Optical Society of America OCIS Codes: (140.4050) Mode-Locked Lasers, (250.5960) Semiconductor Lasers, (130.3120) Integrated Optics Devices.  http://dx.doi.org/10.1364/OL.99.099999  The generation of high frequency, high power optical pulses is of importance for a number of applications, from frequency conversion based on nonlinear phenomena to regeneration of optical pulse trains, as well as THz generation [1-2]. Monolithic semiconductor mode-locked lasers (SMLLs), based on the so-called split-contact Fabry-Pérot cavity configuration [3] and including a saturable absorber (SA) as trigger for pulsed operation, are ideal candidates for generation of pulses with high repetition rates because of their very short cavity allowing pulse train frequencies of several tens of GHz or higher. However, major drawbacks of SMLLs which are effectively limiting their widespread diffusion and use in commercial applications are the limited average output powers and the narrow biasing parameter range over which stable ML operation occurs [4]. The analysis in [5] suggested that such limitations are caused by the wavelength-dependent properties of the absorbing section. Starting from a situation of stable and complete ML (as defined in [6]), an increase of the gain section current directly translates into an increase of the SA photocurrent, with the associated Joule heating shifting the SA bandedge away from that of the gain section, and the optimal conditions for ML ceasing.  Based on this qualitative analysis, a blue bandgap shift applied to the SA has the potential to shift the range of bias conditions providing stable ML to higher currents. Previous research [7-8] on bandgap shifted absorber ML showed a pulse width reduction. Here we show that applying a blue detuning to the SA of a split-contact passively mode-locked semiconductor laser in a Fabry-Pérot configuration not only narrows the pulse, but also reduces the chirp and leads to a great expansion of the range of bias conditions that provide stable ML operation.  The bandgap of the SA region was blue-shifted through Quantum Well Intermixing (QWI). Unlike the techniques previously employed to achieve blue-detuning of the SA [7-8], QWI is fully post-growth, therefore it provides a simple and low-cost way to effectively induce an area-selective blue detuning on several MQW, quantum dot and quantum dash material systems [9]. The devices characterized were fabricated at the University of Glasgow, exploiting the facilities of the James Watt Nanofabrication Centre. The material used was a MQW AlGaInAs/InP wafer structure, and a detailed description of the epitaxial layers can be found in [10]. This particular material was chosen because its mode-locking behavior is well understood and documented, thanks to extensive characterization carried out previously [6]. Besides the insertion of a preliminary QWI step, the fabrication followed a standardized laser fabrication process, whose details can be found elsewhere [11]. The applied bandgap blue-shift through QWI had to be carefully chosen; given the lack of extensive published data on the behavior of monolithic SMLLs with blue-detuned absorbers. In [7] a pulse width reduction by more than a factor of two in comparison with a non-detuned SA was reported, from 2.6 to 1.2 ps with a 24 nm blue-detuning. The interpretation given to this result was that, due to the increased transparency of a blue-detuned absorber, a larger reverse bias is required, and therefore the SA has  a faster loss recovery. However, excessive blue detuning translates into the requirement for a large SA reverse bias, which might eventually cause a junction breakdown because of the high electric field. In view of the previous considerations, the bandgap shift applied to the SA was chosen to be 10 nm. Fig. 1. Schematic of the fabricated device geometry. The geometry of the fabricated structures is depicted in Fig. 1. The sample contained lasers with intermixed SAs and lasers whose absorbers were not blue- shifted. For all fabricated SMLLs the length of the absorber section LSA varied between 1 and 15% of the total cavity length Ltot, which was chosen to be 1250 Ǎm in order to produce optical pulses with a repetition rate of quasi-40 GHz. A 10 Ǎm-wide gap g between the SA and gain section provided electric insulation and the lasers waveguide width d was chosen to be 2 Ǎm to ensure single-mode propagation. Fig. 2. Schematic of the setup used to test the obtained bandgap shift (a) and bandedge comparison (b) between non-intermixed and intermixed case for 2 Ǎm-wide waveguides Prior to the characterization of the SMLLs, the obtained bandgap shift was verified through wavelength scan measurements of a specifically designed waveguides array on chip, using the setup depicted in Fig. 2a. The bandgap shift check confirmed the value of 10 nm which had been targeted, as Fig. 2b shows. For the characterization of the devices, the SMLLs were temperature controlled and kept at 20 ºC throughout the whole of the experiments, and their output was collected from the gain section side with an optical fiber lens and simultaneously distributed through fused fiber splitters between an intensity auto-correlator (IAC) and an optical spectrum analyzer (OSA), with a power splitting ratio of 90% and 10%, respectively. Before coupling into the IAC, the optical signal was amplified by means of an Erbium Doped Fiber Amplifier (EDFA), whose gain was kept constant at 15 dB and whose output polarization was controlled to maximize the IAC output. A LabView code was used for automating the data collection, for simultaneous acquisition of the IAC and OSA traces, and for characterization of an extensive range of gain section currents and SA reverse biases. Fig. 3. . Comparison between pulses with FWHM of 1.4 ps (a) and 2.7 ps (b) for a SMLL whose blue-detuned SA is 7 % of the total cavity length. Devices with SA lengths ranging from 1 to 7% of the total cavity length Ltot all produced stable pulse trains, independently of the absorber being blue-shifted or not. The IAC data were post-processed in order to extract the Full Width at Half Maximum (FWHM) of the pulse. This parameter was used to assess the interval of currents and voltages providing stable ML, with a chosen 2.5 ps boundary value since wider pulses were not pedestal-free and displayed dynamical instabilities. A comparison between pulses inside and outside the optimal ML region is presented in Fig. 3. The extension of parameter space producing stable ML increased proportionally to the absorber length, as Fig. 4 illustrates by showing the comparison of the regions where pulses are found to have FWHM of less than 2.5 ps for SMLLs with different SA lengths (1 and 2 % of the total cavity length). The SMLL with absorbing length of 7% was the one producing narrow pulses (below 2.5 ps) over the largest range of operating conditions, for both intermixed and non-intermixed SAs. For lasers with longer absorbing sections, Q-switching instability regimes were dominant, in line with expectations [12]. Although the following analysis will focus on the device with a 7% absorber, it is worth mentioning that the differences due to QWI which will be discussed have been observed also for shorter SAs. Fig. 4. The color maps show the FWHM of the IAC pulses emitted by the SMLLs with intermixed SAs for absorbers whose length is 1% of the cavity length (left) and 2% (right). The effect of blue-detuning of the SA can be clearly observed from the juxtaposition of the color maps shown in Fig. 5. The maps have been plotted for the same current and voltage intervals, and on the same color scale for ease of comparison. It can be seen that the SMLL with intermixed SA (Fig. 5b) produces pulses under 2.5 ps to currents nearing 300 mA, whereas the non-intermixed emits pulses narrower than 2.5 ps only up to 215 mA (Fig. 5a). The shortest measured FWHM of the pulses also displays a net improvement, with 0.7 ps obtained for the intermixed case, and 1 ps for the non-intermixed one. As for the range of currents and voltages providing the narrowest pulses, there is a shift of 0.5 V towards more negative biases for the QWI case, in line with expectations. The FWHM of the optical spectrum was also extracted through data post-processing, and the results were combined with the IAC FWHM data to provide the time-bandwidth product (TBP) of the pulses. This quantity tells how close a pulse is to its transform limit, i.e. the narrowest possible spectrum for a given pulse duration. Fig. 5. The color maps show the areas where the FWHM of the IAC pulses is lower than 2.5 ps for (a) one SMLL with non-intermixed 7% SA and (b) a device with a blue-detuned 7% SA; (c) and (d) show an enlarged section of the maps for currents between 250 mA and 350 mA. The TBP of a transform-limited sech2 pulse, the temporal shape that better fits mode-locked emission from the MQW material used, is 0.315 [13]. Higher values of TBP indicate a chirped pulse. The comparison of the TBP data for the non-intermixed and QWI case indicate that the QWI device exhibits lower chirp, despite both devices being far from the transform limit (Fig. 6). It is thought the chirp reduction is a consequence of complex dispersion and nonlinearities dynamics in both the SA and the gain section, although to confirm this further investigation is required. The total power vs. current (LI) measurements, carried out with a broad area detector, showed the same level of average power between intermixed and non-intermixed devices fabricated on the same chip. This together with the narrower pulses lead the intermixed devices to exhibit higher peak power, with 215 mW achieved vs. only 182 mW obtained from the non-intermixed lasers. Fig. 6. The color maps show the TBP of the pulses for (a) one SMLL with non-intermixed 7% SA, and (b) a device with a blue-detuned 7% SA, for currents between 250 mA and 350 mA. In summary, we have demonstrated how a bandgap blue detuning applied to the SA of a SMLL can improve the laser performance in terms of pulse width, optical chirp and peak power. The devices whose absorbers had undergone QWI emit pulses whose FWHM is 30% lower than that obtained from standard SMLLs fabricated on the same MQW platform. The temporal narrowing exhibited by the pulses also contributed to an improvement in the pulses peak power, which also experienced an increase between the devices compared. Moreover, the TBP data clearly suggest that a blue-detuned absorber is beneficial in reducing the amount of chirp experienced by the pulses. However, the most interesting feature is the expansion of the region in which stable ML is achieved, with a ~100 mA current range difference between a detuned absorber and a standard one. The extension of the ML region is in line with the analysis shown in [5], since starting from a shifted SA bandedge delays the point where Joule effects become detrimental to the pulse formation mechanism. The narrower pulses and the chirp reduction  are thought to be linked to a shorter recovery time exhibited by an intermixed absorber, further experiments will be carried out to confirm this hypothesis. The authors acknowledge the support from the technical staff of the James Watt Nanofabrication Centre at Glasgow University, and discussions with Salvador Balle. V.P. acknowledges partial financial support from the GRPE program and EPSRC under grant agreement EP/P504937/1.    1. E. A. Avrutin, J. H. Marsh, and E. L. Portnoi, Monolithic and multi-gigahertz mode-locked semiconductor lasers: constructions, experiments, models and application. Optoelectronics, IEE Proceedings, Vol. 147, No. 4, pp. 251-278, 2000. 2. D. Burghoff, T. Kao, N. Han, C. I. Chan, X. Cai, Y. Yang, D. J. Hayton, J. Gao, J. L. Reno and Q. Hu, Terahertz laser frequency combs, Nature Photonics, Vol. 8, No. 6, pp. 462-467, 2014 3. Y. L. Kam, C. Harder, J. S. Smith and A. Yariv, Passive mode-locking of buried heterostructures lasers with nonuniform current injection. Applied Physics Letters, Vol. 42, No. 9, pp. 771-774, 1983. 4. K. Yvind, D. Larsson, L. J. Christiansen, C. Angelo, L. K. Oxenlwe, J. Mrk, D. Birkedal, J. M. Hvam and J. Hanberg, Low jitter and high-power 40-ghz all active mode-locked lasers. Photonics Technology Letters, IEEE, Vol 16, No. 4, pp. 975-977, 2004. 5. J. Javaloyes and S. Balle, Detuning and thermal effects on the dynamics of passively mode-locked quantum-well lasers. Quantum Electronics, IEEE Journal of, Vol. 48, No. 12, pp. 1519-1526, 2012. 6. P. Stolarz, J. Javaloyes, G. mezĞsi, L. Hou, C. N. Ironside, M. Sorel, A. C. Bryce and S. Balle, Spectral dynamical behavior in passively mode-locked semiconductor lasers, IEEE Photonics Journal, Vol. 3, No. 6, pp. 1067-1082, 2011. 7. D. Kunimatsu, S. Arahira, Y. Kato and Y. Ogawa, Passively mode-locked laser diodes with bandgap-wavelength detuned saturable absorbers. Photonics Technology Letters, IEEE, Vol. 11, No. 11, pp. 1363-1365, 1999. 8. R. Scollo, H.-J. Lohe, J. F. Holzman, F. Robin, H. Jäckel, D. Erni, W. Vogt, and E. Gini, Mode-locked laser diode with an ultrafast integrated uni-traveling carrier saturable absorber, Optics Letters, Vol. 30, No. 20, pp. 2808-2810, 2005. 9. B. S. Ooi, K. McIlvaney, M. W. Street, A. S. Helmy, S. G. Ayling, A. C. Bryce, J. H. Marsh, and J. S. Roberts, Selective quantum-well intermixing in GaAs-AlGaAs structures using impurity-free vacancy diffusion. Quantum Electronics, IEEE Journal of, Vol. 33, No. 10, pp. 1784-1793, 1997. 10. L. Hou, M. Haji, J. Akbar, B. Qiu, and A. C. Bryce, Low divergence angle and low jitter 40GHz AlGaInAs/InP 1.55 Ǎm mode-locked lasers. Optics Letters, Vol. 36, No. 6, pp. 966968, 2011. Available online: http://ol.osa.org/abstract.cfm?URI=ol-36-6-966. 11. M. J. Strain, P. Stolarz, and M. Sorel, Passively mode-locked lasers with integrated chirped Bragg grating reflectors, Quantum Electronics, IEEE Journal of, Vol. 47, No. 4, pp. 492-499, 2011.  12. J. Javaloyes and S. Balle, Mode-locking in semiconductor Fabry-Pérot lasers. Quantum Electronics, IEEE Journal of, Vol. 46, No. 7, pp. 1023-1030, 2010. 13. C. Huang, H. C. Kapteyn, J. W. McIntosh and M. M. Murnane, Generation of transform-limited 32-fs pulses from a self-mode-locked Ti:sapphire laser. Optics letters, Vol. 17, No. 2, pp. 139-141, 1992. 

Passive mode-locking in semiconductor lasers with saturable absorbers bandgap shifted through quantum well intermixing

Enlighten

    Pusino, V., Strain, M. J., and Sorel, M. (2014) Passive mode-locking in semiconductor lasers with saturable absorbers bandgap shifted through quantum well intermixing. Photonics Research, 2(6), pp. 186-189.    Copyright © 2014 Chinese Laser Press     Version: Published    http://eprints.gla.ac.uk/106494    Deposited on:  21 May 2015              Enlighten – Research publications by members of the University of Glasgow http://eprints.gla.ac.uk  Passive mode-locking in semiconductor lasers withsaturable absorbers bandgap shifted throughquantum well intermixingVincenzo Pusino,1,* Michael J. Strain,1,2 and Marc Sorel11School of Engineering, University of Glasgow—Rankine Building, Oakfield Avenue, Glasgow G128LT, UK2Institute of Photonics, University of Strathclyde, Glasgow G4 0NW, UK*Corresponding author: Vincenzo.Pusino@glasgow.ac.ukReceived August 14, 2014; revised October 23, 2014; accepted October 23, 2014;posted November 5, 2014 (Doc. ID 216715); published November 26, 2014Passive mode-locking in semiconductor lasers in a Fabry–Perot configuration with a bandgap blueshift applied tothe saturable absorber (SA) section has been experimentally characterized. For the first time a fully post-growthtechnique, quantum well intermixing, was adopted to modify the material bandgap in the SA section. The mea-surements showed not only an expected narrowing of the pulse width but also a significant expansion of the rangeof bias conditions generating a stable train of optical pulses. Moreover, the pulses from lasers with bandgap shiftedabsorbers presented reduced chirp and increased peak powerwith respect to the nonshifted case. © 2014 ChineseLaser PressOCIS codes: (140.4050) Mode-locked lasers; (250.5960) Semiconductor lasers; (130.3120) Integrated opticsdevices.http://dx.doi.org/10.1364/PRJ.2.0001861. INTRODUCTIONThe generation of high frequency, high power optical pulses isof importance for a number of applications, from frequencyconversion based on nonlinear phenomena to regenerationof optical pulse trains, as well as terahertz generation [1,2].Monolithic semiconductor mode-locked lasers (SMLLs),based on the so-called split-contact Fabry–Perot cavity con-figuration [3] and including a saturable absorber (SA) as a trig-ger for pulsed operation, are ideal candidates for generation ofpulses with high repetition rates because of their very shortcavity allowing pulse train frequencies of several tens of giga-hertz or higher. However, major drawbacks of SMLLs whichare effectively limiting their widespread diffusion and use incommercial applications are the limited average outputpowers and the narrow biasing parameter range over whichstable mode-locking (ML) operation occurs [4].The analysis in [5] suggested that such limitations arecaused by the wavelength-dependent properties of the absorb-ing section. Starting from a situation of stable and completeML (as defined in [6]), an increase of the gain section currentdirectly translates into an increase of the SA photocurrent,with the associated Joule heating shifting the SA band edgeaway from that of the gain section, and the optimal conditionsfor ML ceasing.Based on this qualitative analysis, a blue bandgap shift ap-plied to the SA has the potential to shift the range of bias con-ditions providing stable ML to higher currents. Previousresearch [7,8] on bandgap-shifted absorber ML showed apulse width reduction. Here we show that applying a bluedetuning to the SA of a split-contact passively mode-lockedsemiconductor laser in a Fabry–Perot configuration not onlynarrows the pulse but also reduces the chirp and leads to agreat expansion of the range of bias conditions that providestable ML operation.The bandgap of the SA region was blueshifted throughquantum well intermixing (QWI). Unlike the techniques pre-viously employed to achieve blue detuning of the SA [7,8],QWI is fully post-growth; therefore it provides a simple andlow-cost way to effectively induce an area-selective bluedetuning on several multi-quantum well (MQW), quantumdot, and quantum dash material systems [9].2. DEVICE DESIGN AND FABRICATIONThe devices characterized were fabricated at the University ofGlasgow, exploiting the facilities of the James Watt Nanofab-rication Centre. The material used was a MQW AlGaInAs/InPwafer structure, and a detailed description of the epitaxiallayers can be found in [10]. This particular material waschosen because its ML behavior is well understood anddocumented, thanks to extensive characterization carriedout previously [6]. Besides the insertion of a preliminaryQWI step, the fabrication followed a standardized laser fabri-cation process, whose details can be found elsewhere [11].The applied bandgap blueshift through QWI had to be care-fully chosen, given the lack of extensive published data onthe behavior of monolithic SMLLs with blue-detuned absorb-ers. In [7] a pulse width reduction by more than a factor of twoin comparison with a nondetuned SAwas reported, from 2.6 to1.2 ps with a 24 nm blue detuning. The interpretation given tothis result was that, due to the increased transparency of ablue-detuned absorber, a larger reverse bias is required,and therefore the SA has a faster loss recovery. However,excessive blue detuning translates into the requirement fora large SA reverse bias, which might eventually cause a186 Photon. Res. / Vol. 2, No. 6 / December 2014 Pusino et al.2327-9125/14/060186-04 © 2014 Chinese Laser Pressjunction breakdown because of the high electric field. In viewof the previous considerations, the bandgap shift applied tothe SA was chosen to be 10 nm.The geometry of the fabricated structures is depicted inFig. 1. The sample contained lasers with intermixed SAsand lasers whose absorbers were not blueshifted. For allfabricated SMLLs the length of the absorber section LSA variedbetween 1% and 15% of the total cavity length Ltot, which waschosen to be 1250 μm in order to produce optical pulses with arepetition rate of quasi-40 GHz. A 10 μm wide gap g betweenthe SA and gain section provided electric insulation, and thelaser’s waveguide width d was chosen to be 2 μm to ensuresingle-mode propagation.Prior to the characterization of the SMLLs, the obtainedbandgap shift was verified through wavelength scan measure-ments of a specifically designed waveguide array on a chip,using the setup depicted in Fig. 2(a). The bandgap shift checkconfirmed the value of 10 nm which had been targeted, asFig. 2(b) shows.3. EXPERIMENTAL RESULTS ANDDISCUSSIONFor the characterization of the devices, the SMLLs were tem-perature controlled and kept at 20°C throughout the whole ofthe experiments, and their output was collected from the gainsection side with an optical fiber lens and simultaneously dis-tributed through fused fiber splitters between an intensity au-tocorrelator (IAC) and an optical spectrum analyzer (OSA),with power splitting ratios of 90% and 10%, respectively.Before coupling into the IAC, the optical signal was amplifiedby means of an erbium-doped fiber amplifier, whose gainwas kept constant at 15 dB and whose output polarizationwas controlled to maximize the IAC output. A LabView codewas used for automating the data collection, for simultaneousacquisition of the IAC and OSA traces, and for characteriza-tion of an extensive range of gain section currents and SAreverse biases.Devices with SA lengths ranging from 1% to 7% of the totalcavity length Ltot all produced stable pulse trains, independ-ently of the absorber being blueshifted or not. The IAC datawere post-processed in order to extract the full width at half-maximum (FWHM) of the pulse. This parameter was used toassess the interval of currents and voltages providing stableML, with a chosen 2.5 ps boundary value since wider pulseswere not pedestal-free and displayed dynamical instabilities.A comparison between pulses inside and outside the optimalML region is presented in Fig. 3. The extension of parameterspace producing stable ML increased proportionally to theabsorber length, as Fig. 4 illustrates by showing the compari-son of the regions where pulses are found to have FWHM ofless than 2.5 ps for SMLLs with different SA lengths (1% and2% of the total cavity length). The SMLL with absorbing lengthof 7% was the one producing narrow pulses (below 2.5 ps)over the largest range of operating conditions, for both inter-mixed and nonintermixed SAs. For lasers with longer absorb-ing sections, Q-switching instability regimes were dominant,in line with expectations [12]. Although the following analysiswill focus on the device with a 7% absorber, it is worthmentioning that the differences due to QWI which will bediscussed have been observed also for shorter SAs.The effect of blue detuning of the SA can be clearlyobserved from the juxtaposition of the color maps shownin Fig. 5. The maps have been plotted for the same currentand voltage intervals and on the same color scale for easeof comparison. It can be seen that the SMLL with intermixedSA [Fig. 5(b)] produces pulses under 2.5 ps to currents nearing300 mA, whereas the nonintermixed SA emits pulses narrowerthan 2.5 ps only up to 215 mA [Fig. 5(a)]. The shortestmeasured FWHM of the pulses also displays a net improve-ment, with 0.7 ps obtained for the intermixed case and 1 psfor the nonintermixed one. As for the range of currents andFig. 1. Schematic of the fabricated device geometry.Fig. 2. (a) Schematic of the setup used to test the obtained bandgapshift and (b) band edge comparison between nonintermixed andintermixed cases for 2 μm wide waveguides.Fig. 3. Comparison between pulses with FWHM of (a) 1.4 ps and(b) 2.7 ps for a SMLL whose blue-detuned SA is 7% of the total cavitylength.Fig. 4. Color maps show the FWHM of the IAC pulses emitted by theSMLLs with intermixed SAs for absorbers whose length is (left) 1% ofthe cavity length and (right) 2%.Pusino et al. Vol. 2, No. 6 / December 2014 / Photon. Res. 187voltages providing the narrowest pulses, there is a shift of0.5 V toward more negative biases for the QWI case, in linewith expectations.The FWHM of the optical spectrum was also extractedthrough data post-processing, and the results were combinedwith the IAC FWHM data to provide the time–bandwidthproduct (TBP) of the pulses. This quantity tells how close apulse is to its transform limit, that is, the narrowest possiblespectrum for a given pulse duration.The TBP of a transform-limited sech2 pulse, the temporalshape that better fits mode-locked emission from the MQWmaterial used, is 0.315 [13]. Higher values of TBP indicate achirped pulse. The comparison of the TBP data for thenonintermixed and QWI case indicate that the QWI deviceexhibits lower chirp, despite both devices being far fromthe transform limit (Fig. 6). It is thought the chirp reductionis a consequence of complex dispersion and nonlinearitydynamics in both the SA and the gain section, although toconfirm this further investigation is required.The total power versus current measurements, carried outwith a broad area detector, showed the same level of averagepower between intermixed and nonintermixed devices fabri-cated on the same chip. This together with the narrowerpulses leads the intermixed devices to exhibit higher peakpower, with 215 mW achieved versus only 182 mW obtainedfrom the nonintermixed lasers.4. CONCLUSIONIn summary, we have demonstrated how a bandgap blue de-tuning applied to the SA of a SMLL can improve the laser per-formance in terms of pulse width, optical chirp, and peakpower. The devices whose absorbers had undergone QWIemit pulses whose FWHM is 30% lower than that obtainedfrom standard SMLLs fabricated on the same MQW platform.The temporal narrowing exhibited by the pulses also contrib-uted to an improvement in the pulses’ peak power, which alsoexperienced an increase between the devices compared.Moreover, the TBP data clearly suggest that a blue-detunedabsorber is beneficial in reducing the amount of chirp expe-rienced by the pulses. However, the most interesting feature isthe expansion of the region in which stable ML is achieved,with a ∼100 mA current range difference between a detunedabsorber and a standard one. The extension of the ML regionis in line with the analysis shown in [5], since starting from ashifted SA band edge delays the point where Joule effects be-come detrimental to the pulse formation mechanism. The nar-rower pulses and the chirp reduction are thought to be linkedto a shorter recovery time exhibited by an intermixedabsorber; further experiments will be carried out to confirmthis hypothesis.ACKNOWLEDGMENTSThe authors acknowledge the support from the technical staffof the James Watt Nanofabrication Centre at Glasgow Univer-sity and discussions with Salvador Balle. V. Pusino acknowl-edges partial financial support from the GRPE program andEPSRC under grant agreement EP/P504937/1.REFERENCES1. E. A. Avrutin, J. H. Marsh, and E. L. Portnoi, “Monolithic andmulti-gigahertz mode-locked semiconductor lasers: construc-tions, experiments, models and application,” IEE Proc. Optoe-lectron. 147, 251–278 (2000).2. D. Burghoff, T. Kao, N. Han, C. I. Chan, X. Cai, Y. Yang, D. J.Hayton, J. Gao, J. L. Reno, and Q. Hu, “Terahertz laser frequencycombs,” Nat. Photonics 8, 462–467 (2014).3. Y. L. Kam, C. Harder, J. S. Smith, and A. Yariv, “Passivemode-locking of buried heterostructures lasers with nonuniformcurrent injection,” Appl. Phys. Lett. 42, 771–774 (1983).4. K. Yvind, D. Larsson, L. J. Christiansen, C. Angelo, L. K.Oxenlwe, J. Mrk, D. Birkedal, J. M. Hvam, and J. Hanberg,“Low jitter and high-power 40-GHz all active mode-lockedlasers,” IEEE Photon. Technol. Lett. 16, 975–977 (2004).5. J. Javaloyes and S. Balle, “Detuning and thermal effects on thedynamics of passively mode-locked quantum-well lasers,” IEEEJ. Quantum Electron. 48, 1519–1526 (2012).6. P. Stolarz, J. Javaloyes, G. Mezősi, L. Hou, C. N. Ironside, M.Sorel, A. C. Bryce, and S. Balle, “Spectral dynamical behaviorin passively mode-locked semiconductor lasers,” IEEE Photon.J. 3, 1067–1082 (2011).7. D. Kunimatsu, S. Arahira, Y. Kato, and Y. Ogawa, “Passivelymode-locked laser diodes with bandgap-wavelength detunedsaturable absorbers,” IEEE Photon. Technol. Lett. 11, 1363–1365 (1999).8. R. Scollo, H.-J. Lohe, J. F. Holzman, F. Robin, H. Jäckel, D. Erni,W. Vogt, and E. Gini, “Mode-locked laser diode with an ultrafastintegrated uni-traveling carrier saturable absorber,” Opt. Lett.30, 2808–2810 (2005).9. B. S. Ooi, K. McIlvaney, M. W. Street, A. S. Helmy, S. G. Ayling,A. C. Bryce, J. H. Marsh, and J. S. Roberts, “Selective quantum-well intermixing in GaAs-AlGaAs structures using impurity-freevacancy diffusion,” IEEE J. Quantum Electron. 33, 1784–1793(1997).Fig. 5. Color maps show the areas where the FWHM of the IACpulses is lower than 2.5 ps for (a) one SMLL with nonintermixed7% SA and (b) a device with a blue-detuned 7% SA; (c) and (d) showan enlarged section of the maps for currents between 250 and 350 mA.Fig. 6. Color maps show the TBP of the pulses for (a) one SMLL withnonintermixed 7% SA and (b) a device with a blue-detuned 7% SA, forcurrents between 250 and 350 mA.188 Photon. Res. / Vol. 2, No. 6 / December 2014 Pusino et al.10. L. Hou, M. Haji, J. Akbar, B. Qiu, and A. C. Bryce, “Lowdivergence angle and low jitter 40 GHz AlGaInAs/InP 1.55 μmmode-locked lasers,” Opt. Lett. 36, 966–968 (2011).11. M. J. Strain, P. Stolarz, and M. Sorel, “Passively mode-lockedlasers with integrated chirped Bragg grating reflectors,” IEEEJ. Quantum Electron. 47, 492–499 (2011).12. J. Javaloyes and S. Balle, “Mode-locking in semiconductorFabry–Perot lasers,” IEEE J. Quantum Electron. 46, 1023–1030 (2010).13. C. Huang, H. C. Kapteyn, J. W. McIntosh, and M. M. Murnane,“Generation of transform-limited 32-fs pulses from a self-mode-locked Ti:sapphire laser,” Opt. Lett. 17, 139–141 (1992).Pusino et al. Vol. 2, No. 6 / December 2014 / Photon. Res. 189

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Pusino, V., Strain, M. J., and Sorel, M. (2014) Passive mode-locking in 
semiconductor lasers with saturable absorbers bandgap shifted through 
quantum well intermixing. Photonics Research, 2(6), pp. 186-189. 
 
 
 
Copyright © 2014 Chinese Laser Press 
 
 
 
 
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Passive mode-locking in semiconductor lasers with
saturable absorbers bandgap shifted through
quantum well intermixing
Vincenzo Pusino,1,* Michael J. Strain,1,2 and Marc Sorel1
1School of Engineering, University of Glasgow—Rankine Building, Oakfield Avenue, Glasgow G128LT, UK
2Institute of Photonics, University of Strathclyde, Glasgow G4 0NW, UK
*Corresponding author: Vincenzo.Pusino@glasgow.ac.uk
Received August 14, 2014; revised October 23, 2014; accepted October 23, 2014;
posted November 5, 2014 (Doc. ID 216715); published November 26, 2014
Passive mode-locking in semiconductor lasers in a Fabry–Perot configuration with a bandgap blueshift applied to
the saturable absorber (SA) section has been experimentally characterized. For the first time a fully post-growth
technique, quantum well intermixing, was adopted to modify the material bandgap in the SA section. The mea-
surements showed not only an expected narrowing of the pulse width but also a significant expansion of the range
of bias conditions generating a stable train of optical pulses. Moreover, the pulses from lasers with bandgap shifted
absorbers presented reduced chirp and increased peak powerwith respect to the nonshifted case. © 2014 Chinese
Laser Press
OCIS codes: (140.4050) Mode-locked lasers; (250.5960) Semiconductor lasers; (130.3120) Integrated optics
devices.
http://dx.doi.org/10.1364/PRJ.2.000186
1. INTRODUCTION
The generation of high frequency, high power optical pulses is
of importance for a number of applications, from frequency
conversion based on nonlinear phenomena to regeneration
of optical pulse trains, as well as terahertz generation [1,2].
Monolithic semiconductor mode-locked lasers (SMLLs),
based on the so-called split-contact Fabry–Perot cavity con-
figuration [3] and including a saturable absorber (SA) as a trig-
ger for pulsed operation, are ideal candidates for generation of
pulses with high repetition rates because of their very short
cavity allowing pulse train frequencies of several tens of giga-
hertz or higher. However, major drawbacks of SMLLs which
are effectively limiting their widespread diffusion and use in
commercial applications are the limited average output
powers and the narrow biasing parameter range over which
stable mode-locking (ML) operation occurs [4].
The analysis in [5] suggested that such limitations are
caused by the wavelength-dependent properties of the absorb-
ing section. Starting from a situation of stable and complete
ML (as defined in [6]), an increase of the gain section current
directly translates into an increase of the SA photocurrent,
with the associated Joule heating shifting the SA band edge
away from that of the gain section, and the optimal conditions
for ML ceasing.
Based on this qualitative analysis, a blue bandgap shift ap-
plied to the SA has the potential to shift the range of bias con-
ditions providing stable ML to higher currents. Previous
research [7,8] on bandgap-shifted absorber ML showed a
pulse width reduction. Here we show that applying a blue
detuning to the SA of a split-contact passively mode-locked
semiconductor laser in a Fabry–Perot configuration not only
narrows the pulse but also reduces the chirp and leads to a
great expansion of the range of bias conditions that provide
stable ML operation.
The bandgap of the SA region was blueshifted through
quantum well intermixing (QWI). Unlike the techniques pre-
viously employed to achieve blue detuning of the SA [7,8],
QWI is fully post-growth; therefore it provides a simple and
low-cost way to effectively induce an area-selective blue
detuning on several multi-quantum well (MQW), quantum
dot, and quantum dash material systems [9].
2. DEVICE DESIGN AND FABRICATION
The devices characterized were fabricated at the University of
Glasgow, exploiting the facilities of the James Watt Nanofab-
rication Centre. The material used was a MQW AlGaInAs/InP
wafer structure, and a detailed description of the epitaxial
layers can be found in [10]. This particular material was
chosen because its ML behavior is well understood and
documented, thanks to extensive characterization carried
out previously [6]. Besides the insertion of a preliminary
QWI step, the fabrication followed a standardized laser fabri-
cation process, whose details can be found elsewhere [11].
The applied bandgap blueshift through QWI had to be care-
fully chosen, given the lack of extensive published data on
the behavior of monolithic SMLLs with blue-detuned absorb-
ers. In [7] a pulse width reduction by more than a factor of two
in comparison with a nondetuned SAwas reported, from 2.6 to
1.2 ps with a 24 nm blue detuning. The interpretation given to
this result was that, due to the increased transparency of a
blue-detuned absorber, a larger reverse bias is required,
and therefore the SA has a faster loss recovery. However,
excessive blue detuning translates into the requirement for
a large SA reverse bias, which might eventually cause a
186 Photon. Res. / Vol. 2, No. 6 / December 2014 Pusino et al.
2327-9125/14/060186-04 © 2014 Chinese Laser Press
junction breakdown because of the high electric field. In view
of the previous considerations, the bandgap shift applied to
the SA was chosen to be 10 nm.
The geometry of the fabricated structures is depicted in
Fig. 1. The sample contained lasers with intermixed SAs
and lasers whose absorbers were not blueshifted. For all
fabricated SMLLs the length of the absorber section LSA varied
between 1% and 15% of the total cavity length Ltot, which was
chosen to be 1250 μm in order to produce optical pulses with a
repetition rate of quasi-40 GHz. A 10 μm wide gap g between
the SA and gain section provided electric insulation, and the
laser’s waveguide width d was chosen to be 2 μm to ensure
single-mode propagation.
Prior to the characterization of the SMLLs, the obtained
bandgap shift was verified through wavelength scan measure-
ments of a specifically designed waveguide array on a chip,
using the setup depicted in Fig. 2(a). The bandgap shift check
confirmed the value of 10 nm which had been targeted, as
Fig. 2(b) shows.
3. EXPERIMENTAL RESULTS AND
DISCUSSION
For the characterization of the devices, the SMLLs were tem-
perature controlled and kept at 20°C throughout the whole of
the experiments, and their output was collected from the gain
section side with an optical fiber lens and simultaneously dis-
tributed through fused fiber splitters between an intensity au-
tocorrelator (IAC) and an optical spectrum analyzer (OSA),
with power splitting ratios of 90% and 10%, respectively.
Before coupling into the IAC, the optical signal was amplified
by means of an erbium-doped fiber amplifier, whose gain
was kept constant at 15 dB and whose output polarization
was controlled to maximize the IAC output. A LabView code
was used for automating the data collection, for simultaneous
acquisition of the IAC and OSA traces, and for characteriza-
tion of an extensive range of gain section currents and SA
reverse biases.
Devices with SA lengths ranging from 1% to 7% of the total
cavity length Ltot all produced stable pulse trains, independ-
ently of the absorber being blueshifted or not. The IAC data
were post-processed in order to extract the full width at half-
maximum (FWHM) of the pulse. This parameter was used to
assess the interval of currents and voltages providing stable
ML, with a chosen 2.5 ps boundary value since wider pulses
were not pedestal-free and displayed dynamical instabilities.
A comparison between pulses inside and outside the optimal
ML region is presented in Fig. 3. The extension of parameter
space producing stable ML increased proportionally to the
absorber length, as Fig. 4 illustrates by showing the compari-
son of the regions where pulses are found to have FWHM of
less than 2.5 ps for SMLLs with different SA lengths (1% and
2% of the total cavity length). The SMLL with absorbing length
of 7% was the one producing narrow pulses (below 2.5 ps)
over the largest range of operating conditions, for both inter-
mixed and nonintermixed SAs. For lasers with longer absorb-
ing sections, Q-switching instability regimes were dominant,
in line with expectations [12]. Although the following analysis
will focus on the device with a 7% absorber, it is worth
mentioning that the differences due to QWI which will be
discussed have been observed also for shorter SAs.
The effect of blue detuning of the SA can be clearly
observed from the juxtaposition of the color maps shown
in Fig. 5. The maps have been plotted for the same current
and voltage intervals and on the same color scale for ease
of comparison. It can be seen that the SMLL with intermixed
SA [Fig. 5(b)] produces pulses under 2.5 ps to currents nearing
300 mA, whereas the nonintermixed SA emits pulses narrower
than 2.5 ps only up to 215 mA [Fig. 5(a)]. The shortest
measured FWHM of the pulses also displays a net improve-
ment, with 0.7 ps obtained for the intermixed case and 1 ps
for the nonintermixed one. As for the range of currents and
Fig. 1. Schematic of the fabricated device geometry.
Fig. 2. (a) Schematic of the setup used to test the obtained bandgap
shift and (b) band edge comparison between nonintermixed and
intermixed cases for 2 μm wide waveguides.
Fig. 3. Comparison between pulses with FWHM of (a) 1.4 ps and
(b) 2.7 ps for a SMLL whose blue-detuned SA is 7% of the total cavity
length.
Fig. 4. Color maps show the FWHM of the IAC pulses emitted by the
SMLLs with intermixed SAs for absorbers whose length is (left) 1% of
the cavity length and (right) 2%.
Pusino et al. Vol. 2, No. 6 / December 2014 / Photon. Res. 187
voltages providing the narrowest pulses, there is a shift of
0.5 V toward more negative biases for the QWI case, in line
with expectations.
The FWHM of the optical spectrum was also extracted
through data post-processing, and the results were combined
with the IAC FWHM data to provide the time–bandwidth
product (TBP) of the pulses. This quantity tells how close a
pulse is to its transform limit, that is, the narrowest possible
spectrum for a given pulse duration.
The TBP of a transform-limited sech2 pulse, the temporal
shape that better fits mode-locked emission from the MQW
material used, is 0.315 [13]. Higher values of TBP indicate a
chirped pulse. The comparison of the TBP data for the
nonintermixed and QWI case indicate that the QWI device
exhibits lower chirp, despite both devices being far from
the transform limit (Fig. 6). It is thought the chirp reduction
is a consequence of complex dispersion and nonlinearity
dynamics in both the SA and the gain section, although to
confirm this further investigation is required.
The total power versus current measurements, carried out
with a broad area detector, showed the same level of average
power between intermixed and nonintermixed devices fabri-
cated on the same chip. This together with the narrower
pulses leads the intermixed devices to exhibit higher peak
power, with 215 mW achieved versus only 182 mW obtained
from the nonintermixed lasers.
4. CONCLUSION
In summary, we have demonstrated how a bandgap blue de-
tuning applied to the SA of a SMLL can improve the laser per-
formance in terms of pulse width, optical chirp, and peak
power. The devices whose absorbers had undergone QWI
emit pulses whose FWHM is 30% lower than that obtained
from standard SMLLs fabricated on the same MQW platform.
The temporal narrowing exhibited by the pulses also contrib-
uted to an improvement in the pulses’ peak power, which also
experienced an increase between the devices compared.
Moreover, the TBP data clearly suggest that a blue-detuned
absorber is beneficial in reducing the amount of chirp expe-
rienced by the pulses. However, the most interesting feature is
the expansion of the region in which stable ML is achieved,
with a ∼100 mA current range difference between a detuned
absorber and a standard one. The extension of the ML region
is in line with the analysis shown in [5], since starting from a
shifted SA band edge delays the point where Joule effects be-
come detrimental to the pulse formation mechanism. The nar-
rower pulses and the chirp reduction are thought to be linked
to a shorter recovery time exhibited by an intermixed
absorber; further experiments will be carried out to confirm
this hypothesis.
ACKNOWLEDGMENTS
The authors acknowledge the support from the technical staff
of the James Watt Nanofabrication Centre at Glasgow Univer-
sity and discussions with Salvador Balle. V. Pusino acknowl-
edges partial financial support from the GRPE program and
EPSRC under grant agreement EP/P504937/1.
REFERENCES
1. E. A. Avrutin, J. H. Marsh, and E. L. Portnoi, “Monolithic and
multi-gigahertz mode-locked semiconductor lasers: construc-
tions, experiments, models and application,” IEE Proc. Optoe-
lectron. 147, 251–278 (2000).
2. D. Burghoff, T. Kao, N. Han, C. I. Chan, X. Cai, Y. Yang, D. J.
Hayton, J. Gao, J. L. Reno, and Q. Hu, “Terahertz laser frequency
combs,” Nat. Photonics 8, 462–467 (2014).
3. Y. L. Kam, C. Harder, J. S. Smith, and A. Yariv, “Passive
mode-locking of buried heterostructures lasers with nonuniform
current injection,” Appl. Phys. Lett. 42, 771–774 (1983).
4. K. Yvind, D. Larsson, L. J. Christiansen, C. Angelo, L. K.
Oxenlwe, J. Mrk, D. Birkedal, J. M. Hvam, and J. Hanberg,
“Low jitter and high-power 40-GHz all active mode-locked
lasers,” IEEE Photon. Technol. Lett. 16, 975–977 (2004).
5. J. Javaloyes and S. Balle, “Detuning and thermal effects on the
dynamics of passively mode-locked quantum-well lasers,” IEEE
J. Quantum Electron. 48, 1519–1526 (2012).
6. P. Stolarz, J. Javaloyes, G. Mezősi, L. Hou, C. N. Ironside, M.
Sorel, A. C. Bryce, and S. Balle, “Spectral dynamical behavior
in passively mode-locked semiconductor lasers,” IEEE Photon.
J. 3, 1067–1082 (2011).
7. D. Kunimatsu, S. Arahira, Y. Kato, and Y. Ogawa, “Passively
mode-locked laser diodes with bandgap-wavelength detuned
saturable absorbers,” IEEE Photon. Technol. Lett. 11, 1363–
1365 (1999).
8. R. Scollo, H.-J. Lohe, J. F. Holzman, F. Robin, H. Jäckel, D. Erni,
W. Vogt, and E. Gini, “Mode-locked laser diode with an ultrafast
integrated uni-traveling carrier saturable absorber,” Opt. Lett.
30, 2808–2810 (2005).
9. B. S. Ooi, K. McIlvaney, M. W. Street, A. S. Helmy, S. G. Ayling,
A. C. Bryce, J. H. Marsh, and J. S. Roberts, “Selective quantum-
well intermixing in GaAs-AlGaAs structures using impurity-free
vacancy diffusion,” IEEE J. Quantum Electron. 33, 1784–1793
(1997).
Fig. 5. Color maps show the areas where the FWHM of the IAC
pulses is lower than 2.5 ps for (a) one SMLL with nonintermixed
7% SA and (b) a device with a blue-detuned 7% SA; (c) and (d) show
an enlarged section of the maps for currents between 250 and 350 mA.
Fig. 6. Color maps show the TBP of the pulses for (a) one SMLL with
nonintermixed 7% SA and (b) a device with a blue-detuned 7% SA, for
currents between 250 and 350 mA.
188 Photon. Res. / Vol. 2, No. 6 / December 2014 Pusino et al.
10. L. Hou, M. Haji, J. Akbar, B. Qiu, and A. C. Bryce, “Low
divergence angle and low jitter 40 GHz AlGaInAs/InP 1.55 μm
mode-locked lasers,” Opt. Lett. 36, 966–968 (2011).
11. M. J. Strain, P. Stolarz, and M. Sorel, “Passively mode-locked
lasers with integrated chirped Bragg grating reflectors,” IEEE
J. Quantum Electron. 47, 492–499 (2011).
12. J. Javaloyes and S. Balle, “Mode-locking in semiconductor
Fabry–Perot lasers,” IEEE J. Quantum Electron. 46, 1023–
1030 (2010).
13. C. Huang, H. C. Kapteyn, J. W. McIntosh, and M. M. Murnane,
“Generation of transform-limited 32-fs pulses from a self-
mode-locked Ti:sapphire laser,” Opt. Lett. 17, 139–141 (1992).
Pusino et al. Vol. 2, No. 6 / December 2014 / Photon. Res. 189


Passive mode-locking in semiconductor lasers with saturable absorbers bandgap shifted through quantum well intermixing

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