1054 IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. 41, NO. 8, AUGUST 2005

Influence of Optical Feedback Time-Delayon Power-Drops in Vertical-Cavity

Surface-Emitting LasersYanhua Hong and K. Alan Shore, Senior Member, IEEE

Abstract—The influence of optical feedback time delay on theaverage period of low-frequency fluctuations (LFF) in externalcavity vertical-cavity surface-emitting lasers (VCSELs) is studiedexperimentally. The regularity of power dropouts is shown toincrease with increased external cavity length. Coexistence of LFFand stable operation is observed when the external cavity length isclose to the VCSEL coherence length.

Index Terms—Optical feedback, surface-emitting lasers.

I. INTRODUCTION

OPTICAL feedback is inevitable in many applications ofsemiconductor lasers including, for example, in optical

communications where coupling the lasers to optical fibers canintroduce some feedback. As semiconductor lasers are verysensitive to optical feedback, extensive studies have been madeof the properties of semiconductor lasers subject to opticalfeedback. Such work has revealed the complex dynamicalproperties which may arise in that configuration. A typicalform of unstable dynamics in semiconductor lasers subject tooptical feedback is low-frequency fluctuations (LFF) wherethe lasers exhibit sudden power dropouts followed by gradualrecovery of the light emission. Vertical-cavity surface-emittinglasers (VCSELs), as a particular type of semiconductor laser,have many advantages compared to edge-emitting semicon-ductor lasers, such as low threshold current, single-longitudinalmode operation, circular output-beam profile and wafer-scaleintegrability. Near the lasing threshold the output of VCSELsis usually linearly polarized along one polarization direction.When the bias current is increased, the polarization oftenswitches to the orthogonal polarization. Despite their high facetreflectivity VCSELs also exhibit sensitivity to optical feedback.A salient feature of VCSELs subjected to optical feedback istheir tendency to exhibit polarization instability. Therefore, po-larization effects need also to be considered in the investigationof dynamics in VCSELs. There are some reports of LFF inVCSELs [1]–[9]. Most of the studies investigated the effect ofthe laser parameters on LFF dynamics [2]–[4], [7]–[9]. In [2], itwas theoretically predicted that the value of the relaxation rate

strongly affects the dynamics of VCSELs. Numerical studyalso shows that a transition from one type of LFF to the other is

Manuscript received December 17, 2004; revised April 13, 2005. This workwas supported by the U.K. EPSRC under Grant GR/S22936/01.

The authors are with the School of Informatics, University of Wales, BangorLL57 1UT, Wales, U.K. (e-mail: yanhua@informatics.bangor.ac.uk; alan@in-formatics.bangor.ac.uk).

Digital Object Identifier 10.1109/JQE.2005.850698

Fig. 1. Experimental setup.

dependent on the values of the linear anisotropies of VCSELs in[3]. A detailed theoretical and experimental investigation con-firms that the dichroism is an important parameter determiningthe behavior of the two orthogonal polarization componentsin the LFF regime [4], [8], [9]. The influence of the laser biascurrent and the optical feedback level on LFF in VCSELs hasalso been reported in previous papers [2], [5], [8], [9]. However,the dynamics of external cavity VCSELs is strongly influencedby three main parameters—the laser bias current, the opticalfeedback strength and the optical feedback time delay. Thereare no reports in the literature of the effect of optical feedbacktime delay on LFF power dropouts in VCSELs. The presentpaper reports an experimental investigation of the effect ofoptical feedback time delay on power dropouts in VCSELs. Wefind that the power dropout occurs more regularly with longerexternal cavities. We have also measured the average period ofLFF in VCSELs as a function of the laser bias current and theoptical feedback strength and compared these to the previousresults [2], [5], [8], [9].

II. EXPERIMENTAL SETUP

The experimental setup is shown in Fig. 1. A commercialsingle-mode VCSEL (Avalon Photonics, V-850-GMP) is usedin our experiments. Our measurements indicate that the deviceexhibits single-mode operation with side-mode suppression ofmore than 20 dB up to at least three times the threshold current.The experiments reported here mainly concern near-thresholdoperation where signal-mode operation of the solitary VCSELis maintained. Our measurements further have shown that thecoherence length of the single-mode device is of order 40 cm.The VCSELs were driven by low-noise current source and theirtemperatures were controlled within 0.01 C. The threshold

0018-9197/$20.00 © 2005 IEEE

HONG AND SHORE: INFLUENCE OF OPTICAL FEEDBACK TIME-DELAY ON POWER-DROPS 1055

Fig. 2. Polarization-resolved time series in the VCSEL.

current of the solitary VCSEL is 0.95 mA. Near threshold, theVCSEL lased in one polarization direction ( -polarization).When the current is increased to 2.7 mA, the polarizationabruptly switched to the orthogonal polarization ( -polariza-tion), whereas with decreasing current the polarization switchedback to the -direction at 1.9 mA. This indicates a bistabilityregime in the VCSEL. The laser output is collimated usingantireflection coated laser diode objective lens and is reflectedback by an external mirror (M1). The feedback is controlledby a tunable neutral density filter (NDF). A half-wave plate(HWP) and a polarization beamsplitter (PBS) are used todirect the orthogonal polarization components of the VCSELto detectors D1 and D2, which have a 6-GHz bandwidth. Theoutputs from the detectors are stored in a 4-GHz bandwidthdigital oscilloscope. Optical isolators (ISO1 and ISO2) withmore than 40-dB isolation are used to prevent light feedbackfrom the detectors into the VCSEL.

III. EXPERIMENTAL RESULTS

We define the optical feedback ratio as the ratio of the opticalfeedback power to the laser output power. The feedback powerwas measured just before the light returned to the laser diode ob-jective (in front of the VCSEL). When the VCSEL bias current,the external cavity length and the feedback ratio were 0.96 mA,124 cm, and 9 dB, respectively, the time traces of two orthog-onal polarization components of the VCSEL are as shown inFig. 2. The time trace of the -polarization has been shifted upfor clarity. The figure shows that the -polarized mode under-goes LFF cycles, however the -polarized mode has not beenexcited, which means that the VCSEL has very high value ofdichroism [4], [8], [9]. In the following, we only consider thedynamics of the -polarized mode.

In the experiment, the average period of LFF was obtained bycalculating the average of at least 1000 time intervals of powerdropout events from -polarized time-traces. For each externalcavity length, we carefully realigned the setup to obtain max-imum feedback coupling. Fig. 3 shows the average period ofLFF as a function of the external cavity length with the sameoperating parameters as that in Fig. 2. The figure shows thatthe average period of LFF increases essentially linearly withthe external cavity length, which is similar to theoretical predic-tions made for edge-emitting semiconductor lasers [10]. How-ever, with decreasing external cavity length, the power dropoutsbecome more and more frequent and when the external cavitylength was 24 cm, LFF cannot be distinguished. The VCSEL

Fig. 3. Average period of LFF as a function of the external cavity length.

Fig. 4. Normalized standard deviation (NSD) as a function of the externalcavity length.

Fig. 5. Time series of x-polarized mode with 44.5-cm external cavity length.

exhibits irregular fluctuations in the output power, which indi-cate that the VCSEL is in the coherence collapse regime. Thisphenomenon indicates the need for a theoretical study of LFFin VCSELs with short external cavity length. The figure alsoshows the fluctuation of the average period of LFF for externalcavity lengths between 40 and 60 cm. This will be explainedbelow.

In order to examine the regularity of LFF, a calculation hasbeen made of the normalized standard deviation (NSD) of theLFF period—defined as the standard deviation divided by theaverage period of LFF. Fig. 4 shows the NSD of the LFF pe-riod as a function of the external cavity length. The figure showsthat there is a sharp increase of the NSD for external cavitylengths between 40 and 50 cm, which is about the coherencelength of the VCSEL. The time series of -polarized compo-nent for 44.5 cm external cavity length in Fig. 4 is shown inFig. 5. The figure shows that the VCSEL has entered a regime ofcoexistence of LFF and stable emission—as has been found inedge-emitting semiconductor lasers [11], [12], DFB lasers [13],and VCSELs [1], when the external cavity length is near thecoherence length of the laser. For the present VCSEL the dura-tion of the stable emission state can be as long as 1 s, whichis more than 15 times longer than the average period of LFF

1056 IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. 41, NO. 8, AUGUST 2005

Fig. 6. Average period of LFF versus feedback ratio.

with the same parameters. The fluctuations of the average pe-riod of LFF between 40- and 60-cm external cavity length inFig. 3 can be related to the change of dynamical regime in theVCSEL. The transition of the dynamics regime in the VCSELby tuning the external cavity length can be understood by themodel proposed in [11], [14]–[16]. The origin of LFF [14], [15]is the chaotic itinerary which develops along the so-called at-tractor ruins of the destabilized mode with a direction of mo-tion toward the stable high gain mode. The power dropoutsare caused by the merging of attractor ruins of external-cavitymodes and antimodes. Noise has much effect on the statisticsof the power dropouts [16] and the noise of lasers is affected bythe optical feedback. The noise slowly increases with increasingfeedback level in the coherence collapse regime when lasers biasnear their threshold current [17], [18]. When the external cavitylength is set near the coherence length of the VCSEL, the rela-tive noise of the VCSEL is reduced due to the fact that only apart of the feedback light interacts with the VCSEL lasing field.As such the laser is less likely to be pushed off the stable stateby the noise when the trajectory reaches the stable high gainmode. In consequence the VCSEL remains in the stable state fora longer time and the coexistence of LFF and stable regime per-sists [11]. When the external cavity length is further increased,the feedback light has less interaction with the lasing field ofthe VCSEL, the stable high gain mode cannot be reached andthe VCSEL remains in the LFF regime. Fig. 4 also shows thatthe normalized standard deviation of the average period of LFFdecreases with the increasing external cavity length when theexternal cavity length was longer than the coherence length ofthe VCSEL. This means that the power dropouts occur moreregularly with longer external cavity lengths.

The optical feedback strength is one of the most importantparameters in an optical feedback system. Within the LFF op-erating regime, the average period of LFF as a function of theoptical feedback ratio is shown in Fig. 6. The VCSEL bias cur-rent and the external cavity length were fixed at 0.96 mA and134 cm, respectively. Fig. 6 shows that time intervals betweenthe intensity dropouts increase with increasing feedback power,which is in agreement with theoretical predictions [2]. In the op-erating range, there is no evidence of the coexistence of stableoperation and LFF. The results of Figs. 3 and 6 indicate that theLFF period is not simply determined by the feedback strength,but is also dependent on the interplay of the dynamics inducedby the round trip time with the dynamics of the solitary laser.

We also find that LFF exist in some range of bias current forfixed external cavity length and optical feedback level. Fig. 7shows the average period of LFF versus the bias current for a

Fig. 7. Average period of LFF as a function of the bias current.

Fig. 8. Normalized standard deviation (NSD) as a function of the laser biascurrent.

39-cm external cavity length and 9-dB feedback ratio. Withthis feedback level, the threshold current of the VCSEL reducedfrom 0.95 to 0.91 mA. Fig. 7 shows that the average period ofLFF decreases with increasing bias current until the bias cur-rent reaches 0.96 mA. With further increased bias current, theaverage period was stable at around 58 ns, which is similar tothe case in edge-emitting semiconductor lasers [16], [19], [20]or with polarized optical feedback in VCSELs [5]. Howeverthis is different from the case of isotropic optical feedback inVCSELs with low value of the dichroism [8], [9], where theaverage periods of LFF decrease approximately linearly withthe injection current within LFF existence regime. This differ-ence in behavior is seen to arise from the fact that only one po-larization component is excited in VCSELs with a high valueof the dichroism. Thus for VCSELs with a large value of thedichroism it could be possible to make a quantitative compar-ison between experiment and theory using the Lang–Kobayashimodel. It would seem that such a comparison cannot be made inedge-emitting semiconductor lasers due to multimode operationin LFF [20].

The normalized standard deviation of the average period ofLFF in Fig. 7 is shown in Fig. 8. The figure shows that the powerdrops out more regularly at the bias current near the thresholdcurrent of the stand-alone VCSEL, which has been observed inedge-emitting semiconductor lasers [20]. The time traces corre-sponding to Fig. 8 show that the VCSEL operated in a regimeof coexistence of LFF and stable emission for low bias currentand exhibits irregular fluctuations in the output power for higherbias current.

IV. CONCLUSION

We have experimentally investigated the influence of delaytime on LFF in VCSELs subject to isotropic optical feedback.The optical feedback time delay is shown to have a significanteffect on LFF power dropouts. For short external cavities, LFF

HONG AND SHORE: INFLUENCE OF OPTICAL FEEDBACK TIME-DELAY ON POWER-DROPS 1057

operation is lost and the VCSEL enters the coherence collapseregime. When the external cavity length is set near the coherencelength of the VCSEL, coexistence of LFF and stable emissionis observed. The power drops out more regularly with externalcavities longer than the VCSEL coherence length. We have alsoconfirmed that the average periods of LFF increase with the in-creasing feedback level and decreasing laser bias current, whichis similar to the behavior of edge-emitting semiconductor lasers.

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Yanhua Hong received the B.Sc. degree in physics from Fujian Normal Univer-sity, Fuzhou, China, in 1987, the M.S. degree from Beijing Normal University,Beijing, China, in 1990, and the Ph.D. degree from the Institute of Physics, Chi-nese Academy of Science, Beijing, China, in 1993.

She was a Lecturer at the Beijing University of Aeronautics and Astronautics,Beijing, China, during 1993–1997. Since 1997, she has been a PostdoctorateResearcher in the University of Wales, Bangor, U.K., where her research mainlyincludes multiwave mixing in semiconductor lasers and semiconductor opticalamplifiers and polarized dynamics in VCSELs.

K. Alan Shore (M’88–SM’95) received the degree in mathematics from theUniversity of Oxford, Oxford, U.K., and the Ph.D. degree at University Col-lege, Cardiff, Wales, U.K. His thesis work was concerned with the electricaland optical properties of double-heterostructure semiconductor lasers.

He was a Lecturer at the University of Liverpool (1979–1983), Liverpool,U.K., and then at the University of Bath, Bath, U.K., where he becameSenior Lecturer (1986), Reader (1990), and Professor (1995). In 1995, hewas appointed to the Chair of Electronic Engineering, University of Wales,Bangor, U.K., where he is currently the Head of the School of Informatics. Heis the Director of Industrial and Commercial Optoelectronics (ICON) a WelshDevelopment Agency Centre of Excellence which has a mission to “makelight work” through the utilization of Bangor optoelectronics expertise andfacilities. He is also the Deputy Chair of the Welsh Optoelectronics Forum. Hisresearch work has been principally in the area of semiconductor optoelectronicdevice design and experimental characterization with particular emphasison nonlinearities in laser diodes and semiconductor optical waveguides. Hehas authored or co-authored almost 700 contributions to archival journals,books and technical conferences. He was a Visiting Researcher at the Centerfor High Technology Materials, University of New Mexico, Albuquerque,in 1987. He received a Royal Society Travel Grant to visit universities andlaboratories in Japan in July 1988. In 1989, he was a Visiting Researcherat the Huygens Laboratory, Leiden University, The Netherlands. Duringthe summers of 1990 and 1991 he worked at the Teledanmark ResearchLaboratory and the MIDIT Center of the Technical University of Denmark,Lyngby. He was a Guest Researcher at the Electrotechnical Laboratory(ETL), Tsukuba, Japan in 1991. In 1992, he was a Visiting Professor atthe Department of Physics, University de les Illes Balears, Palma-Mallorca,Spain. He was a Visiting Lecturer in the Instituto de Fisica de Cantabria,Santander, Spain, in June 1996 and 1998, and a Visiting Researcher in theCentre for Laser Applications, Department of Physics, Macquarie University,Sydney, Australia, in July–August 1996, 1998, 2000 and April 2002. InJuly–August 2001, he was a Visiting Researcher at the ATR AdaptiveCommunications Laboratories, Kyoto, Japan. His current research interestsinclude multiwave mixing and optical switching in semiconductor lasers,design and fabrication of intersubband semiconductor lasers and organicsemiconductor lasers, dynamics of vertical-cavity semiconductor lasers andapplications of nonlinear dynamics in semiconductor lasers to optical dataencryption.

Dr. Shore co-founded and acts as Organizer and Programme CommitteeChair for the International Conference on Semiconductor and IntegratedOptoelectronics (SIOE) which, since 1987, has been held annually in Cardiff,Wales, U.K. He was Programme Chair for the U.K. National Quantum Elec-tronic Conference (QE’13) and was European Liaison Committee chair forthe OSA Integrated Photonics Research (IPR’98) conference in Victoria BC,Canada, 1998. He was a member of the programme committee for IPR’99Santa Barbara, California, July, 1999, and also the European Conference onIntegrated Optics (ECIO) Torino, Italy, in April 1999. He was a co-organizerof a Rank Prize Symposium on Nonlinear Dynamics in Lasers held in the LakeDistrict, U.K. in August 2002. He was a technical committee member for theConference on Lasers and Electro-Optics (CLEO), Europe, 2003.