IEICE TRANS. ELECTRON., VOL.E101–C, NO.7 JULY 2018561

INVITED PAPER Special Section on Distinguished Papers in Photonics

Chirp Control of Semiconductor Laser by Using HybridModulation∗

Mitsunari KANNO†a), Student Member, Shigeru MIEDA†, Nobuhide YOKOTA†, Wataru KOBAYASHI††,and Hiroshi YASAKA†, Members

SUMMARY Frequency chirp of a semiconductor laser is controlledby using hybrid modulation, which simultaneously modulates intra-cavityloss and injection current to the laser. The positive adiabatic chirpof injection-current modulation is compensated with the negative adia-batic chirp created by intra-cavity-loss modulation, which enhances thechromatic-dispersion tolerance of the laser. A proof-of-concept transmis-sion experiment confirmed that the hybrid modulation laser has a largerdispersion tolerance than conventional directly modulated lasers due to thenegative frequency chirp originating from intra-cavity-loss modulation.key words: dispersion tolerance, frequency chirp, frequency response,semiconductor lasers

1. Introduction

A direct current modulation is the simplest modulationscheme to generate a high-speed optical signal by usingsemiconductor laser sources and is widely used for semi-conductor lasers applied in optical data communication sys-tems. Increasing the operation speed of directly currentmodulated lasers (DMLs) is one of the important targets toincrease the transmission capacity of data communicationsystems for data centers to keep up with explosively increas-ing data traffic. A recent bandwidth-enhancement schemeof semiconductor lasers, which uses the photon-photon res-onance (PPR) effect induced by the coupling between thelasing mode of the laser cavity and feedback light from anintegrated external cavity, has recently been attracting atten-tion for breaking through the limit of the modulation band-width of DMLs [1]–[4]. High-speed operation with a 40-Gbps non-return-to-zero (NRZ) signal has been achievedusing a PPR-introduced DML with an external cavity [4].To further enlarge the modulation bandwidth of the PPR-introduced DML, it is important to reduce the modulation-response degradation of the DML at a higher frequency re-gion than the relaxation oscillation frequency. We previ-ously proposed the hybrid modulation (HM) scheme, whichsimultaneously modulates the injection current and intra-cavity loss in a laser to modify the modulation-responsedegradation at a high-frequency region toward an ultra-high-

Manuscript received October 30, 2017.Manuscript revised February 8, 2018.∗This is an original article.†The authors are with Research Institute of Electrical Commu-

nication, Tohoku University, Sendai-shi, 980–8577 Japan.††The author is with NTT Device Technology Laboratories,

NTT Corporation, Atsugi-shi, 243–0198 Japan.a) E-mail: mi-kanno@riec.tohoku.ac.jp

DOI: 10.1587/transele.E101.C.561

speed semiconductor laser and demonstrated that our HMscheme can control the modulation response of a semicon-ductor laser [5]. Controlling an optical frequency chirp isalso important for optical-signal generation because it deter-mines the dispersion tolerance and transmission distance ofoptical signals for an optical fiber. The optical signal gen-erated using a conventional directly modulated distributedfeedback (DFB) laser has a positive frequency chirp, andits dispersion tolerance is limited to around 100 ps/nm for10-Gbps NRZ-signal operation. Therefore, the transmis-sion distance of the generated optical signal is limited toless than 10 km at a wavelength of 1550 nm [6]. To im-prove the dispersion tolerance and extend the transmissiondistance of DMLs, it is necessary to control the frequencychirp of the laser. There are several methods for control-ling the frequency chirp in semiconductor lasers. A chirp-managed semiconductor laser (CML) was proposed to tailorthe frequency chirp of a DML by applying a multi-cavityfilter to narrow the lasing spectrum under modulation, anda dispersion tolerance of 4200 ps/nm for a 10-Gbps signalwas demonstrated [7]. Another example is an FM-response-enhanced distributed Bragg reflector laser was used for 20-km standard single-mode fiber (SSMF) transmission under40-Gbps signal operation [8]. For an electroabsorption (EA)modulator integrated with a DFB laser, the transient chirpof the DFB laser was cancelled out by that of the EA mod-ulator by using a dual modulation scheme, in which theDFB laser is modulated with anti-phase signal comparedto the EA modulator, and 180-km SSMF transmission ofa 10-Gbps NRZ signal was confirmed [9]. We demonstratedthat our HM scheme can enhance the dispersion tolerancefor optical-fiber transmissions through numerical analysis ofdynamic operation of an HM laser using rate equations [10].Since our HM scheme can control the modulation responseand extend the modulation bandwidth of PPR-introducedlasers, simultaneous control of the frequency chirp by us-ing this scheme is attractive. In this study, we investigatedthe frequency chirp characteristics of the HM laser and con-firmed negative chirp operation through numerical calcula-tions and a proof-of-concept experiment.

2. Optical Frequency Chirp of Semiconductor Laser

The rate equations for a semiconductor laser are expressedas follows.

Copyright c© 2018 The Institute of Electronics, Information and Communication Engineers

562IEICE TRANS. ELECTRON., VOL.E101–C, NO.7 JULY 2018

dNdt=

IeV− Nτs− vgAg(N − Ntr)

1 + εSS (1)

dSdt=

[ΓvgAg(N − Ntr)

1 + εS− 1τp

]S (2)

dφdt=αΓvgAg(N − Nth)

1 + εS, (3)

where N, S , φ, I, e, V , τs, τp, Γ, vg, Ag, Ntr, ε, and αare carrier density, photon density, phase of light, injec-tion current, electron charge, volume of DFB active sec-tion (2.2 × 10−17 m3), carrier lifetime (0.8 ns), photon life-time (2.0 ps), optical confinement factor of light into ac-tive region (0.05), group velocity of light (8.8 × 107 m/s),differential gain coefficient (6.4 × 10−20 m2), transparentcarrier density (5.0 × 1024 m−3), nonlinear gain coefficient(5.5 × 10−23 m3), and linewidth enhancement factor (3.0),respectively. The typical values for InGaAsP multiple quan-tum well DFB lasers are used for parameters [11].

In our HM scheme, the injection current and intra-cavity loss 1/τp are simultaneously modulated. From therate equations, the optical frequency change Δω is approx-imately derived as follows when the term concerned withthe photon lifetime is expressed as 1/τp = 1/τp0 + Δ(1/τp),where 1/τp0 is a steady state value and Δ(1/τp) is a compo-nent arising from intra-cavity-loss modulation (ICLM).

Δω ≈ α2

{1S

dSdt+ε

τp0S + Δ

(1τp

)}(4)

The first term on the right side is known as a transient chirpcomponent proportional to the differential of the photondensity. The second term is known as an adiabatic chirpcomponent proportional to the photon density. These twocomponents determine the frequency chirp of conventionalDMLs [12]. The third term appears due to ICLM with ourHM scheme. When the third term has a negative value, theadiabatic chirp component can be compensated for. Thisindicates that the frequency chirp of the laser can be con-trolled by controlling the amount of ICLM. The optical fibertransmission characteristics can be improved when the adi-abatic chirp is dominant (a range in which (dS/dt)/S is notso large). Figure 1 shows a schematic of an HM laser. It hasa DFB active section and ICLM section in the laser cavity.The ICLM section is composed of an EA waveguide, andits optical loss increases when an applied negative bias isincreased. When in-phase electrical-modulation signals areapplied to both sections, they lead to output-power modula-tions of the HM laser with the same sign. For example, both

Fig. 1 Schematic of hybrid modulation laser.

sections act to increase the laser-output power when mod-ulation voltage increases (injection current increases andintra-cavity loss decreases). Under this condition, a negativevalue for Δ(1/τp) in Eq. (4) can be obtained and frequencychirp can be controlled. A semiconductor optical amplifier(SOA) section is also integrated in the laser cavity to com-pensate for the static loss of the ICLM section by apply-ing a small DC to avoid the extra chirp caused by the gain-saturation effect at SOA [13]. The SOA section is treated asa simple gain material, and its contribution is included in thephoton lifetime term. The facet of the SOA-section side iscoated with a high-reflectivity (∼80%) film; thus, the lasercavity of the HM laser is composed of the DFB, ICLM, andSOA sections.

3. Numerical Analyses

The time evolution of the photon density and lasing fre-quency change were calculated using the rate equations(Eqs. (1)–(3)). In the calculation, we used a relative modu-lation ratio η in dB defined as η = MICLM/MC where MICLM

and MC are, respectively, the intensity-modulation sensitiv-ities at 100 MHz for the ICLM and injection-current modu-lation [5], which represents the dominance of ICLM. Whenη is a large negative value, injection-current modulation isdominant. The η value does not affect the threshold currentin the following calculations since the DC loss in the ICLMsection and the DC current to the DFB section are kept con-stant.

Figures 2 (a) and 2 (b) show the time responses of pho-ton density in the HM laser for various η when a falling (a)or rising (b) step signal was applied at t = 0 ns. The opti-mum delay time Δt defined as a delay time of an injection-current modulation signal relative to that of an ICLM signalto minimize the relaxation-oscillation was kept at 20 ps [5].The photon density sharply responded to each step signalwith the reduced relaxation oscillation at η = −5 dB. Asη decreased, the response of photon density showed a de-lay and verges on conventional injection-current modulation

Fig. 2 Calculated time responses of photon density for (a) falling and(b) rising step signals and chirp for (c) falling and (d) rising step signals.

KANNO et al.: CHIRP CONTROL OF SEMICONDUCTOR LASER BY USING HYBRID MODULATION563

characteristics. As shown in Fig. 2 (c), the positive transientand positive adiabatic chirps arose at η = −20 dB for thefalling step signal. As η increased, the positive transientchirp weakened and the adiabatic chirp changed from posi-tive to negative due to ICLM. When the adiabatic chirp in-duced by ICLM was reasonably larger than that induced byinjection-current modulation, the total chirp correspondedto a quasi-negative transient chirp with a negative adiabaticchirp, as seen at η = −7 dB. A sign of these chirp character-istics invert for the rising step signal as shown in Fig. 2 (d).In this manner, our HM scheme controls the time responsesof photon density and chirp.

Next, we investigated eye patterns calculated with 10-Gbps NRZ signals. A Bessel Thomson filter with a cut-off frequency of 7.5 GHz was taken into account to rejectthe unnecessary high frequency components. The calcula-tion results of eye patterns at η = −5, −7, −9, and −20 dBunder back-to-back condition and after 20-km SSMF trans-mission are shown in Fig. 3. As shown in Fig. 3 (a), a clear

Fig. 3 Eye patterns calculated with 10-Gbps NRZ signals underback-to-back condition and after 20-km SSMF transmission.

eye opening was confirmed under the back-to-back condi-tion at η = −5 dB, where a relative contribution of ICLMwas strong and the ideal step response for the photon den-sity was obtained. However, as shown in Fig. 3 (b) a largeovershoot was observed at falling edge after the transmis-sion because of the large negative adiabatic chirp inducedby ICLM. Therefore, waveforms at η = −5 dB tendedto be distorted after optical fiber transmissions. Althoughthe waveform has slight overshoot, the distortion was sup-pressed at η = −7 dB (Fig. 3 (d)) compared to the conditionat η = −5 dB, where the negative adiabatic chirp gener-ated by ICLM was less dominant. As shown in Fig. 3 (f) atη = −9 dB, the overshoot of waveform after the transmis-sion was further smaller than that at η = −7 dB. However,leading edge of the eye pattern became gentle and extinctionratio degraded. At η = −20 dB, where the injection-currentmodulation was dominant, central part of the eye patternsshowed a dip after the transmission (Fig. 3 (h)) because ofthe positive adiabatic and positive transient chirps.

The relation between the extinction ratio of eye pat-terns and η under back-to-back condition and after 20-kmSSMF-transmission is shown in Fig. 4. The maximum ex-tinction ratio of 5.1 dB was obtained at η = −4 dB and an ex-tinction ratio of 4.8 dB was maintained even after the trans-mission. It is worth mentioning that the extinction ratios af-ter the transmission at the range from η = −8 to −5 dB werelarger than the extinction ratios under back to back condi-tion. This indicates that, under this condition, the HM laserhas a negative chirp-operation condition with a chromaticdispersion tolerance of 340 ps/nm (17 ps/nm/km × 20 km)due to the negative adiabatic chirp arising from the ICLMapplied with injection-current modulation. These calcula-tion results indicate that our HM scheme controls relativecontributions of transient and adiabatic chirps by tuning ηwhile maintaining the modified modulation response. Al-though simultaneous and arbitrary controls of both chirp andmodulation bandwidth are difficult, the extinction ratio ofeye pattern can be less degraded after 20-km SSMF trans-mission even when we use η from −8 to −5 dB where theoperation condition is optimum for obtaining the largest ex-tinction ratio and wider modulation bandwidth.

Fig. 4 Extinction-ratio dependence of HM laser under back-to-back con-dition (circles) and after 20-km SSMF transmission (triangles) as functionof η.

564IEICE TRANS. ELECTRON., VOL.E101–C, NO.7 JULY 2018

4. Transmission Experiment

To confirm the frequency-chirp characteristics of the HMlaser, we conducted a proof-of-concept experiment. We fab-ricated the HM laser having the same structure illustratedin Fig. 1. It consists of a 300-µm-long DFB active sec-tion, 150-µm-long ICLM section, and 50-µm-long SOA sec-tion. These sections have core regions of InGaAlAs quan-tum wells [14] and a ridge-waveguide structure buried inbenzocyclobutene (BCB) to reduce the parasitic capacitanceof the electrode pads. The waveguide width and thicknessof the InGaAlAs multiple quantum well layer are respec-tively 2 and 0.17 µm. We estimated the pad capacitancesof the DFB and ICLM sections to be 0.3 and 0.15 pF. A45-ohm resistor was inserted in series for the pad of theDFB section, and a 50-ohm resistor in parallel for that of theICLM section. The setup for the optical fiber transmissionexperiment is shown in Fig. 5. The electrical-modulationsignal generated by a 10-Gbps pseudo random pattern gen-erator was amplified then split using a power divider. TheΔt between the two electrical-modulation signals was con-trolled by the two phase shifters. Attenuators were insertedinto each path to control the ratio of the modulation depthsof injection-current modulation and ICLM. The electrical-modulation signals were superimposed on DC bias by usingbias-tees then applied to the DFB and ICLM sections. TheDC bias current to the DFB section and DC bias voltage tothe ICLM section were set to 90 mA (∼ 4.5Ith) and −1.82V, respectively. The bias current to the SOA section wasset to 5 mA to reduce its effect on frequency chirp. Theeye patterns of the optical signal from the HM laser weremonitored using a photo detector ( f3dB ∼ 15 GHz), BesselThomson filter ( f0 = 7.5 GHz), and digital oscilloscope.The Δt, AC modulation current to the DFB section, and ACmodulation voltage to the ICLM section were set to 43 ps,23.2 mA, and 0.56 V, respectively, which were determinedto generate the clearest eye pattern before and after the 20-km SSMF transmission. The difference in Δt between thecalculation and experiment may be due to the difference inthe distance between the RF connectors and two modula-tion pads due to the mounting process of the laser to a highspeed sub-assembly. Under the above conditions, dynamicsingle-mode operation was confirmed at a lasing wavelengthof 1555.7 nm.

Figures 6 (a) and 6 (b) show the measured eye patternsfor the DML and HM laser under the back-to-back condi-tion. Data for the DML were obtained by modulating onlythe DFB section of the HM laser. A clear eye opening witha dynamic extinction ratio of 1.3 dB was observed in bothcases. We note that the dynamic extinction ratio was lim-ited by a saturation power (23 dBm) of RF amplifier used inthe experiment. Figures 6 (c) and 6 (d) show the measuredeye patterns after 20-km SSMF transmission for optical sig-nals generated by the DML and HM laser. The eye patternsfor the DML severely degraded after 20-km SSMF trans-mission. The eye patterns for the HM laser, on the other

Fig. 5 Experimental setup for optical fiber transmission.

Fig. 6 Measured eye patterns for directly current modulated laser andHM laser under back-to-back condition and after 20-km SSMF transmis-sion.

hand, showed clear or clearer eye openings, even after 20-km SSMF transmission, which is similar to the case calcu-lated with η of −7 dB as shown in Figs. 3 (c) and (d). Al-though the extinction ratio of the eye patterns was limitedby the experimental setup, the results indicate that the HMlaser can control frequency chirp and enhance the dispersiontolerance for optical fiber transmissions.

5. Conclusion

Frequency chirps of a semiconductor laser were controlledwith our HM scheme, which simultaneously modulates theinjection current and intra-cavity loss of the semiconductorlaser. The positive transient chirp dominant in conventionalDMLs was found to be weakened due to ICLM, and negativechirp characteristics were obtained by tuning η. The proof-of-concept experiment confirmed that our HM scheme cancontrol the frequency chirp of a semiconductor laser, and20-km SSMF transmission was demonstrated at 10 Gbps,which cannot be explained by the characteristics of conven-tional DMLs.

KANNO et al.: CHIRP CONTROL OF SEMICONDUCTOR LASER BY USING HYBRID MODULATION565

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Mitsunari Kanno received his B.E. inelectrical and electoronic engineering from To-kyo University of Agriculture and Technologyin 2017. In the same year, he joined the AppliedQuantum Optics Laboratory, Research Instituteof Electrical Communication, Tohoku Univer-sity, Japan for pursuing his M.E. His researchinterest includes Laser Diode.

Shigeru Mieda was born in Hyogo Prefec-ture, Japan, on February 9, 1990. He receivedhis B.S., M.S. and Ph.D. in electronics engi-neering from Tohoku University, Miyagi, Japan,in 2012, 2014, and 2017. He has been a doc-toral student at Tohoku University since 2014.In 2017 he joined OCLARO Japan INC. He hasbeen engaged in research and development onhigh-speed semiconductor light sources. He is amember of the Institute of Electronics, Informa-tion and Communication Engineers (IEICE).

Nobuhide Yokota received his B.S. inelectrical engineering from Kochi National Col-lege of Technology, Kochi, Japan, in 2009 andhis M.S. and Ph.D. in materials science fromNara Institute of Science and Technology, Nara,Japan, in 2011 and 2014. He is currentlyan assistant professor at the Research Insti-tute of Electrical Communication, Tohoku Uni-versity, Miyagi, Japan. His research interestsinclude optoelectronics, semiconductor lasers,and semiconductor spintronics.

Wataru Kobayashi was born in Chiba,Japan, in 1980. He received his B.S. and M.E. inapplied physics and his Dr. Eng. in nano-scienceand nano-engineering from Waseda University,Tokyo, Japan, in 2003, 2005 and 2011. In 2005,he joined NTT Photonics Laboratories, Atsugi,Kanagawa, Japan. He has been engaged in theresearch and development of optical semicon-ductor devices. Dr. Kobayashi is a memberof the Institute of Electronics, Information andCommunication Engineers (IEICE) of Japan.

Hiroshi Yasaka received his B.S. and M.S.in physics from Kyushu University in 1983 and1985, and Ph.D. in electronics engineering fromHokkaido University in 1993. In 1985, he joinedAtsugi Electrical Communication Laboratories,Nippon Telegraph and Telephone Corporation(NTT). From then to 2008, he was engagedin research and development on semiconductorphotonic devices for optical fiber communica-tion systems. In 2008, he joined the ResearchInstitute of Electrical Communication (RIEC) at

Tohoku University as a professor and has been engaging in research onhighly functional semiconductor photonic devices and their monolithicallyintegrated devices. Professor Yasaka is a member of the Institute of Elec-tronics, Information and Communication Engineers (IEICE), the Japan So-ciety of Applied Physics (JSAP), the Physical Society of Japan (JPS), andIEEE/Photonics.