A Directly Modulated Laterally Coupled Distributed Feedback Laser Array Based on Sio2 Planarization Process

applied sciences

Article A Directly Modulated Laterally Coupled Distributed Feedback Laser Array Based on SiO2 Planarization Process

Qichao Wang 1, Jian Wang 1,2,*, Changzheng Sun 1,2 , Bing Xiong 1,2, Yi Luo 1,2,3,*, Zhibiao Hao 1,2, Yanjun Han 1,2,3, Lai Wang 1 , Hongtao Li 1 and Jiadong Yu 1,2,3

1 Beijing National Research Centre for Information Science and Technology, Department of Electronic Engineering, Tsinghua University, Beijing 100084, China; [email protected] (Q.W.); [email protected] (C.S.); [email protected] (B.X.); [email protected] (Z.H.); [email protected] (Y.H.); [email protected] (L.W.); [email protected] (H.L.); [email protected] (J.Y.) 2 Center for Flexible Electronics Technology, Tsinghua University, Beijing 100084, China 3 Flexible Intelligent Optoelectronic Device and Technology Center, Institute of Flexible Electronics Technology of THU, Jiaxing 314006, China * Correspondence: [email protected] (J.W.); [email protected] (Y.L.); Tel.: +86-10-6279-8300 (J.W.); +86-10-6279-4900 (Y.L.)

Abstract: Low-cost and high-speed single-mode semiconductor lasers are increasingly required as wide-band access ﬁber communication expands in recent years. Here, a high-speed laterally coupled

distributed feedback (LC-DFB) laser array is achieved based on a SiO2 planarization process. The device exhibits low threshold currents of about 12 mA and high slope efﬁciencies over 0.26 W/A. Stable single mode operation and high-speed performance are realized with side mode suppression ratios (SMSR) over 45 dB, and 3-dBe bandwidths exceed 14 GHz for all four channels. Such a high-speed and process simple LC-DFB laser array shows great potential to the low-cost ﬁber communication networks.

Keywords: laterally coupled gratings; high-speed modulation; SiO2 Planarization; 1.3 µm; Citation: Wang, Q.; Wang, J.; Sun, C.; distributed feedback laser Xiong, B.; Luo, Y.; Hao, Z.; Han, Y.; Wang, L.; Li, H.; Yu, J. A Directly Modulated Laterally Coupled Distributed Feedback Laser Array 1. Introduction Based on SiO2 Planarization Process. Appl. Sci. 2021, 11, 221. https:// High-speed optical communication develops rapidly with the rapid increase of the doi.org/10.3390/app11010221 amount of data. Low-cost and high-performance laser sources, such as distributed feedback (DFB) laser sources, are urgently needed. Various high-speed DFB lasers were demon- Received: 8 December 2020 strated [1,2], but most of them require at least one epitaxial regrowth step after grating Accepted: 25 December 2020 definition and thus have complicated process and high fabrication costs. Published: 29 December 2020 Laterally coupled (LC) DFB lasers use surface gratings beside the ridge waveguide to select longitudinal mode; thus, they circumvent the complex regrowth step and have the Publisher’s Note: MDPI stays neu- potential for low-cost fiber communication applications. LC-DFB lasers with good static tral with regard to jurisdictional claims performance, e.g., side mode suppression ratio (SMSR), were reported [3–6]. However, in published maps and institutional there are only a few reports about the high frequency performance, e.g., modulation affiliations. bandwidth [7–9]. It is actually a trade-off between the single mode performance and the modulated bandwidth, and it is especially challenging to achieve a high modulation bandwidth for LC-DFB lasers, which often suffer from a low coupling coefficient and thus

Copyright: © 2020 by the authors. Li- have a long cavity to select longitudinal mode. censee MDPI, Basel, Switzerland. This In this paper, a high-speed LC-DFB laser array with deep and planar lateral gratings is article is an open access article distributed demonstrated. We have developed a SiO2 planarization process to simplify the fabrication. under the terms and conditions of the The array exhibits low threshold currents of about 12 mA, high slope efﬁciencies over Creative Commons Attribution (CC BY) 0.26 W/A and high SMSRs over 45 dB. The 3-dBe small-signal modulation bandwidths license (https://creativecommons.org/ exceed 14 GHz for all four channels under injection currents of 100 mA. Such an LC- licenses/by/4.0/).

Appl. Sci. 2021, 11, 221. https://doi.org/10.3390/app11010221 https://www.mdpi.com/journal/applsci Appl. Sci. 2021, 11, 221 2 of 6

DFB laser array is very promising for applications of high-speed and low-cost optical communication.

2. Device Structure, Design and Fabrication Methods Figure1a depicts the schematic of the proposed LC-DFB laser array in which laterally coupled gratings are along both sides of the ridge waveguide to provide both lateral optical Appl. Sci. 2021, 11, x FOR PEER REVIEW 2 of 6 conﬁnement and longitudinal feedback. A 2-µm-thick SiO2 dielectric layer supports the planar electrodes and passivates the side wall of ridge waveguides. A p-type electrode pad with a diameter of 100LC-DFBµm introduces laser array is a very pad promising capacitance for applicationsC of 0.18 of pF. high-speed and low-cost optical The intrinsic modulationcommunication. speed of DMLs is limited by the relaxation oscillation frequency ( f ). The f is given by [10]. r r 2. Device Structure, Design and Fabrication Methods Figures 1a depicts thep schematic of the proposed LC-DFB laser array in which laterally coupled gratingsΓνga areηi along( bothI − Isidesth) of theq ridge waveguide to provide both lateral op- fr = = D (I − Ith) (1) tical confinementqV and longitudinal2π feedback. A 2-μm-thick SiO2 dielectric layer supports the planar electrodes and passivates the side wall of ridge waveguides. A p-type electrode pad with a diameter of 100 μm introduces a pad capacitance C of 0.18= pF. where Γ is the active regionThe optical intrinsic confinement modulation factor, speed ofνg DMLsis the is group limited velocity, by the relaxationa ∂g /oscillation∂N [11] fre- is the differential gain, quencyηi is the (𝑓 internal). The 𝑓 is quantum given by [10]. efficiency, and I and Ith are the bias current

and the threshold current, respectively. q is the elementary charge,( ) and V is the active volume. q 𝑓 = ( ) =𝐷(𝐼 𝐼 ) (1) The D factor D = 1/2π Γνgaηi /(qV) describes the slope of fr on the square root of the bias current above the thresholdwhere 𝛤 is [the12 ].active region optical confinement factor, 𝜈 is the group velocity, 𝑎= 𝜕𝑔⁄𝜕𝑁 𝜂 𝐼 𝐼 Strained multi-quantum [11] wells is the (MQWs)differential weregain, designed is the internal for quantum high speed efficiency, modulations and and are the bias current and the threshold current, respectively. 𝑞 is the elementary charge, which considered theand improving𝑉 is the active of the volume. differential The D factor gain 𝐷=1a [13⁄ ]. 2𝜋 The(𝛤𝜈𝑎𝜂 epitaxial)(𝑞𝑉)⁄ describes structure the isslope the same as Li, A.K., whichof 𝑓 on consists the square of root five of periods the bias current of 5-nm-thick above the threshold InGaAlAs [12]. quantum-well and 8.5-nm-thick InGaAlAsStrained barrier multi-quantum [14]. The optical wells (MQWs) confinement were designed factor for of high the activespeed modulations region which considered the improving of the differential gain 𝑎 [13]. The epitaxial structure is Γ is also an important parameter for fr. The relationship of the coupling coefficient κ the same as Li, A.K., which consists of five periods of 5-nm-thick InGaAlAs quantum-well and Γ versus the ridgeand width 8.5-nm-thick are calculated InGaAlAs bybarrier the [14]. finite The elementoptical confinement analysis factor (FEA) of methodthe active re- as shown in Figure1b.gion The 𝛤 optical is also an confinement important parameter factor for Γ𝑓.increases The relationship with of the the coupling width ofcoefficient the waveguide, yet a narrowκ and waveguide𝛤 versus the isridge needed width forare calculated an LC-DFB by the to finite increase element the analysis grating (FEA) coupling coefficient κ.method A narrow as shown waveguide in Figure 1b. would The optical result confinement in more power factor 𝛤 leakage increases out with of the the width of the waveguide, yet a narrow waveguide is needed for an LC-DFB to increase the grating waveguides, thus decreasingcouplingΓ coefficient. However, κ. A itnarrow enhances waveguide the optical would result field in in more the power grating leakage region out of and increases κ. The strengththe waveguides, of the thus grating’s decreasing feedback 𝛤. However, is it described enhances the by optical the field normalized in the grating 𝜅 coupling coefficient (κregionL). The and cavityincreases length . The strength is inverse of the to grating’sκ when feedbackκL is is a described constant. by the The normal-fr is proportional to theizedκ and couplingΓ by coefficient Equation (κL). (1). The cavity Theridge length is width inverse is to designedκ when κL is as a constant. 1.2 µm The 𝑓 is proportional to the 𝜅 and 𝛤 by Equation (1). The ridge width is designed as 1.2 μm considering a compromiseconsidering between a compromiseκ and Γ. between Figure1𝜅c and shows𝛤. Figure the variation 1c shows the of variationκ versus of dutyκ versus cycle γ assuming a ridgeduty widthcycle γ assuming of 1.2 µ ma ridge and width a grating of 1.2 μ orderm and ofa grating 3. A dutyorder of cycle 3. A ofduty 0.9 cycle is of designed to obtain a higher0.9 is designedκ taking to obtain the etching a higher broadening𝜅 taking the etching effect broadening into account. effect into account.

(a) (b) (c)

FigureFigure 1. 1.( a(a)) SchematicSchematic of the of thestructure structure of the laterally of the laterallycoupled distributed coupled feedback distributed (LC-DFB) feedback laser array (LC-DFB) with deep laserand planar lateral gratings. (b) κ (blue line) and 𝛤 (red line) versus the ridge width with the third order gratings and a duty arrayfactor with of 0.5. deep (c) κ andas a function planar of lateral the duty gratings. cycle γ on (thirdb) κ order(blue gratings line) andwith aΓ ridge(red width line) of versus 1.2 μm. the ridge width with the third order gratings and a duty factor of 0.5. (c) κ as a function of the duty cycle γ on third order gratings with a ridge width of 1.2 µm.

To fabricate the deep lateral gratings, the pattern was formed by electron beam lithography (EBL) on 400-nm-thick HSQ photo-resist. Deep lateral gratings were formed by Appl. Sci. 2021, 11, x FOR PEER REVIEW 3 of 6

by inductively coupled plasma (ICP) etching with CH4/H2/Ar gas mixture. Finally, diluted hydrochloric acid solution was adopted to smooth the side wall of lateral gratings, whose microscope image captured by scanning electron microscope (SEM) is shown in Figure 2a. Commented [M1]: All the figures must be cited in The lateral gratings along the 1.2-μm-wide strips have pitches of 599.7, 602, 604 and 606.7 numerical number. nm, a duty factor ~0.75 and a grating depth ~1.75 μm, which results in the coupling coefficient κ ~ 30 cm−1 according to the finite element analysis (FEA) simulation, as shown in We replaced the location of figure 2 and figure 3 Figure 1c. Appl. Sci. 2021, 11, 221 After the process of the deep gratings and waveguides, a planarization process3 of 6of silica was adopted instead of a benzocyclobutene (BCB) planarization process described in the article of Li, A.K. The process was significantly simplified as shown in Figure 3. A 2-μm-thick SiO2 supporting layer was deposited by PECVD, whose surface topography is Appl. Sci. 2021, 11, x FOR PEER REVIEW 3 of 6 shown in Figure 2b. SiO2 on top of the ridge waveguide was removed by CHF3 ICP after inductively coupled plasma (ICP) etching with CH4/H2/Ar gas mixture. Finally, diluted self-alignment exposure, as shown in Figure 2c. Then Ti/Pt/Au p-type electrodes and hydrochloric acidTi/Au solution n-type was electrodes adopted were to deposited, smooth therespectively. side wall Laser of lateralchips with gratings, a cavity whoselength of microscopeby inductively imagecoupled~350 captured plasma μm were(ICP) by etchingcleaved scanning without electronofCH the4/H 2wafer./Ar microscope gas High mixture. reflection Finally, (SEM) (HR) diluted is shownfilm with in reflectivityFigure2a. of Thehydrochloric lateral acid gratings solution98.66% along was and adopted anti-reflection the 1.2- to smoothµm-wide (AR) the film side strips with wall reflectivity haveof lateral pitches gratings,of 0.37% of werewhose 599.7, deposited 602, on 604 two and ends 606.7microscope nm, image a duty captured factorof lasers by ~0.75 respectively scanning and electron a by grating evaporation microscope depth after ~1.75(SEM) annealing isµ m,shown which to inincrease Figure results the2a. output in the Commentedpower coupling of the [M1]: All the figures must be cited in The lateral gratings alongfacet.− 1the 1.2-μm-wide strips have pitches of 599.7, 602, 604 and 606.7 coefﬁcient κ ~ 30 cm according to the ﬁnite element analysis (FEA) simulation,numerical as shown number. nm, a duty factor ~0.75 and a grating depth ~1.75 μm, which results in the coupling coef- in Figure1c. . ficient κ ~ 30 cm−1 according to the finite element analysis (FEA) simulation, as shown in We replaced the location of figure 2 and figure 3 Figure 1c. After the process of the deep gratings and waveguides, a planarization process of silica was adopted instead of a benzocyclobutene (BCB) planarization process described in the article of Li, A.K. The process was significantly simplified as shown in Figure 3. A 2-μm-thick SiO2 supporting layer was deposited by PECVD, whose surface topography is shown in Figure 2b. SiO2 on top of the ridge waveguide was removed by CHF3 ICP after self-alignment exposure, as shown in Figure 2c. Then Ti/Pt/Au p-type electrodes and Ti/Au n-type electrodes were deposited, respectively. Laser chips with a cavity length of ~350 μm were cleaved out of the wafer. High reflection (HR) film with reflectivity of

98.66% and anti-reflection (AR) film with reflectivity of 0.37% were deposited on two ends of lasers respectively(a) by evaporation after annealing(b) to increase the output power of(c )the facet. FigureFigure. 2. (a) 2.SEM(a )image SEM of image the as-etched of the as-etchedwaveguide and waveguide lateral gratings. and lateral (b) Surface gratings. topography (b) Surface of the SiO topography2 supporting of layer.the (c) SiO SEM supportingimage of the waveguide layer. (c) and SEM lateral image gratings of the after waveguide removing SiO and2 on lateral top of gratings the current after injection removing region. SiO3. Results and Discussion2 2 on top of the current injection region.

After the process of the deep gratings and waveguides, a planarization process of silica was adopted instead of a benzocyclobutene (BCB) planarization process described in the article of Li, A.K. The process was signiﬁcantly simpliﬁed(a) as shown in Figure3.A 2-µm-thick SiO2 supporting layer was deposited by PECVD, whose surface topography is shown in Figure2b. SiO 2 on top of the ridge waveguide was removed by CHF3 ICP after self-alignment exposure, as shown in Figure2c. Then Ti/Pt/Au p-type electrodes and

(a) Ti/Au n-type electrodes(b were) deposited, respectively.(c) Laser chips with a cavity length of ~350 µm were cleaved out of the wafer. High reflection (HR)(b) film with reflectivity of 98.66% Figure 2. (a) SEM imageFigure of andthe 3. as-etched The anti-reflection comparison waveguide diagram (AR) and lateral of film (a) BCBgratings. with planarization reflectivity (b) Surface process topography of0.37% and (b )of wereSiO the2 planarizationSiO deposited2 supporting process on two ends of lasers layer. (c) SEM image of therespectively waveguide and lateral by evaporation gratings after removing after annealing SiO2 on top to of increasethe current theinjection output region. power 3. of the facet. Results and Discussion The lasing characteristics were tested at 10 °C under continuous-wave conditions. Figure 4a shows the typical fiber coupled output power as a function of injection currents for a four-channel LC-DFB laser array. The threshold currents are about 12 mA. Excellent slope efficiencies of above 0.26 W/A are achieved for all channels. The output power is about 20 mW at an injection current of 100 mA. Such excellent output power performance for LC-DFB lasers is attributed to the combination of a good epitaxy of AlGaInAs wafer (a)

(b)

Figure 3. The comparison diagram of (a) BCB planarization process and (b) SiO2 planarization process Figure 3. The comparison diagram of (a) BCB planarization process and (b) SiO2 planarization process.The lasing characteristics were tested at 10 °C under continuous-wave conditions. Figure 4a shows the typical fiber coupled output power as a function of injection currents 3.for Resultsa four-channel and DiscussionLC-DFB laser array. The threshold currents are about 12 mA. Excellent slope Theefficiencies lasing of characteristicsabove 0.26 W/A are were achieved tested for at all 10 channels.◦C under The output continuous-wave power is conditions. Figureabout 204 mWa shows at an injection the typical current ﬁber of 100 coupled mA. Such output excellent power output as power a function performance of injection currents for LC-DFB lasers is attributed to the combination of a good epitaxy of AlGaInAs wafer for a four-channel LC-DFB laser array. The threshold currents are about 12 mA. Excellent slope efﬁciencies of above 0.26 W/A are achieved for all channels. The output power is about 20 mW at an injection current of 100 mA. Such excellent output power performance for LC-DFB lasers is attributed to the combination of a good epitaxy of AlGaInAs wafer and HR-AR facet coated. The four-channel wavelengths range from 1286 nm to 1303 nm with a channel spacing of about 1 THz, as shown in Figure4b. An average SMSR exceeding 50 dB is obtained, which is comparable to that of conventional DFB lasers, indicating the good longitudinal mode selectivity of deep and planar lateral gratings. Appl. Sci. 2021, 11, x FOR PEER REVIEW 4 of 6

about 20 mW at an injection current of 100 mA. Such excellent output power performance for LC-DFB lasers is attributed to the combination of a good epitaxy of AlGaInAs wafer and HR-AR facet coated. The four-channel wavelengths range from 1286 nm to 1303 nm with a channel spacing of about 1 THz, as shown in Figure 4b. An average SMSR exceeding 50 dB is obtained, which is comparable to that of conventional DFB lasers, indicating Appl. Sci. 2021, 11, 221 the good longitudinal mode selectivity of deep and planar lateral gratings. 4 of 6

10 Lane 1 Lane 2 5 Lane 3 Lane 4 0 20 40 60 80 100 Current (mA)

(a) (b)

FigureFigure 4.4. ((aa)) Fiber Fiber coupled coupled output output power power as a function as a function of injection of injection currents (L-I)currents of the (L-I) LC-DFB of the laser LC-DFB ◦ laserarray array with thewith operating the operating temperature temperature at 10 C. at (b )10 The °C. lasing (b) The spectra lasing under spectra injection under currents injection of currents ◦ of100 100 mA mA at 10 at C.10 °C. The DML array is p-side up soldered onto a copper block to improve the thermal characteristics.The DML Thearray p-type is p-side electrode up soldered is bonded onto to a AlNa copper microstrip block transmission to improve line the thermal characteristics.by gold wires through The p-type a matching electrode resistor is ofbonded 35 Ω. The to a 3-dBe AlN bandwidths microstrip oftransmission the small- line by goldsignal wires modulation through response a matching shown resistor in Figure of5 35 exceed Ω. The 14 GHz3-dBe for bandwidths all four channels. of the Asmall-signal modulationbandwidth of response about 18 GHz shown is obtained in Figure on lane5 exceed 3 under 14 anGHz injection for all current four channels. of 100 mA. A To bandwidth ofour about knowledge, 18 GHz this is is theobtained best small-signal on lane 3 modulation under an performanceinjection current for LC-DFB of 100 lasers. mA. To our The frequency response of the laser can be ﬁtted as follows [15], in which the junction knowledge, this is the best small-signal modulation performance for LC-DFB lasers. The capacitance is taken into account: frequency response of the laser can be fitted as follows [15], in which the junction capaci-

tance is taken into account: 2 fr 1 1 |H( f )| = q p q (2) ( 2 − 2)2 + γ 2 1 + f / fPN + ( )2 fr f 2π f 1 f / fRC |𝐻(𝑓)| = (2) ⁄ ()( ) (⁄ ) Here γ is the damping factor. The fPN and fRC are the cut-off frequencies caused by the junction capacitance, the contact capacitance and the series resistance. Figure6a showsHere the relationships𝛾 is the damping between factor.fr and The the 𝑓 square and root 𝑓 of are the the bias cut-off currents frequencies above the caused by thethreshold junction for capacitance, four channels. the The contactD factors capacitance of the four-channel and the series lasers resistance. are 1.56, 1.6, Figure 1.49 6a shows 1/2 theand relationships 1.23 GHz/mA between, respectively. 𝑓 and The the values square of theroot four-channel of the bias differentialcurrents above gain a theare threshold forobtained four fromchannels. the D factors,The D as factors shown inof Figurethe four-channel6b. The differential lasers gain are tends 1.56, to decrease1.6, 1.49 and 1.23 with the increase of the lasing wavelength. Generally, a lasing wavelength shorter than the GHz/mA1/2, respectively. The values of the four-channel differential gain 𝑎 are obtained material peak gain provides a larger differential gain. The maximum appears at the 5 nm Appl. Sci. 2021, 11, x FOR PEER REVIEWfrom the D factors, as shown in Figure 6b. The differential5 of 6 gain tends to decrease with the detuned grating wavelength to longer wavelengths with respect to the material peak gain, increasewhich is aboutof the 1285 lasing nm. wavelength. Generally, a lasing wavelength shorter than the material peak gain provides a larger differential gain. The maximum appears at the 5 nm detuned grating wavelength to longer wavelengths with respect to the material peak gain, which is about 1285 nm.

Figure 5. Small-signal frequency responses of the LC-DFB laser array with injection currents of 100 FiguremA at 10 5. °C.Small-signal frequency responses of the LC-DFB laser array with injection currents of 100 mA at 10 ◦C.

5 Lane 1 Lane 2 Lane 3 Lane 4 0 0510 (I-I )1/2 (mA)1/2 th

(a) (b)

Figure 6. (a) The relationship between the relaxation oscillation frequency 𝑓 and the square root of the bias current above the threshold, where the solid dots and lines are the experimental data and fitting results, respectively. (b) The relationship between the values of four-channel differential gain and lasing wavelengths.

4. Conclusions

A high-speed LC-DFB laser array based on a SiO2 planarization process is demonstrated. Each channel exhibits a low threshold current of about 12 mA and a high slope efficiency over 0.26 W/A. Stable single mode operation is demonstrated with SMSRs over 45 dB. The small-signal 3-dBe bandwidths exceed 14 GHz for all four channels under injection currents of 100 mA. The high modulation bandwidth benefits from high differential gain, and it can be further improved with a shorter cavity. Such results imply that the LC-DFB laser array can combine high efficiency slopes, high SMSRs, high modulation bandwidths and a simple fabrication process, and it shows great potential to low-cost optical communication.

Author Contributions: Conceptualization, J.W. and Y.L.; data curation, Q.W.; formal analysis, Q.W.; funding acquisition, Y.L.; investigation, Q.W. and J.W.; methodology, Q.W. and C.S.; project administration, Y.L.; resources, C.S. and Y.H.; supervision, Y.L.; validation, B.X., Z.H., L.W., H.L. and J.Y.; visualization, Q.W.; writing—original draft, Q.W.; writing—review and editing, Q.W. and J.W. All authors have read and agreed to the published version of the manuscript. Appl. Sci. 2021, 11, x FOR PEER REVIEW 5 of 6

Figure 5. Small-signal frequency responses of the LC-DFB laser array with injection currents of 100 Appl. Sci. 2021, 11, 221 5 of 6 mA at 10 °C.

5 Lane 1 Lane 2 Lane 3 Lane 4 0 0510 (I-I )1/2 (mA)1/2 th

(a) (b)

FigureFigure 6.6.( a(a)) The The relationship relationship between between the relaxation the relaxation oscillation oscillation frequency frequencyfr and the 𝑓 square and root the ofsquare root ofthe the bias bias current current above above the threshold, the threshold, where thewhere solid the dots solid and dots lines areand the lines experimental are the experimental data and data andfitting fitting results, results, respectively. respectively. (b) The relationship (b) The relationship between the between values of the four-channel values of differential four-channel gain differen- tialand gain lasing and wavelengths. lasing wavelengths. 4. Conclusions 4. Conclusions A high-speed LC-DFB laser array based on a SiO2 planarization process is demonstrated.A high-speed Each channel LC-DFB exhibits alaser low thresholdarray based current on ofa SiO about2 planarization 12 mA and a high process slope is demonstrated.efficiency Each over 0.26channel W/A. exhibits Stable single a low mode thresh operationold current is demonstrated of about with 12 mA SMSRs and over a high slope efficiency45 dB. The small-signalover 0.26 W/A. 3-dBe Stable bandwidths single exceed mode 14 operation GHz for all is four demonstrated channels under with injec- SMSRs over tion currents of 100 mA. The high modulation bandwidth benefits from high differential 45 dB. The small-signal 3-dBe bandwidths exceed 14 GHz for all four channels under in- gain, and it can be further improved with a shorter cavity. Such results imply that the jectionLC-DFB currents laser array of can100 combinemA. The high high efficiency modulation slopes, bandwidth high SMSRs, benefits high modulation from high differen- tialbandwidths gain, and and it can a simple be further fabrication improved process, with and a it shorter shows greatcavity. potential Such results to low-cost imply that the LC-DFBoptical communication. laser array can combine high efficiency slopes, high SMSRs, high modulation bandwidths and a simple fabrication process, and it shows great potential to low-cost op- ticalAuthor communication. Contributions: Conceptualization, J.W. and Y.L.; data curation, Q.W.; formal analysis, Q.W.; funding acquisition, Y.L.; investigation, Q.W. and J.W.; methodology, Q.W. and C.S.; project administration, Y.L.; resources, C.S. and Y.H.; supervision, Y.L.; validation, B.X., Z.H., L.W., H.L. and J.Y.; Authorvisualization, Contributions: Q.W.; writing—original Conceptualization, draft, Q.W.; J.W. writing—review and Y.L.; data and curation, editing, Q.W.Q.W.; and formal J.W. All analysis, Q.W.;authors funding have read acquisition, and agreed toY.L.; the investigation, published version Q.W. of the and manuscript. J.W.; methodology, Q.W. and C.S.; project administration, Y.L.; resources, C.S. and Y.H.; supervision, Y.L.; validation, B.X., Z.H., L.W., H.L. Funding: This research was funded by the National Key Research and Development Program and(Grant J.Y.; No. visualization, 2017YFA0205800); Q.W.; the writing—original Science Challenge draft, Project Q.W.; (Grant writing—review No. TZ2016003); the and National editing, Q.W. andNatural J.W. Science All authors Foundation have ofread China and (Grant agreed No. to 51561165012, the published 51561145005, version of 61574082, the manuscript. 61621064, 61822404, 61875104, 61904093, 61927811); Tsinghua University Initiative Scientific Research Pro- gram (Grant No. 20161080068, 20161080062); the China Postdoctoral Science Foundation (Grant No. 2018M640129, 2019T120090) and the Collaborative Innovation Centre of Solid-State Lighting and Energy-Saving Electronics. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: The data that support the plots within this paper and other findings of this study are available from the corresponding author upon reasonable request. Conflicts of Interest: The authors declare no conflict of interest.

References 1. Nakahara, K.; Wakayama, Y.; Kitatani, T.; Taniguchi, T.; Fukamachi, T.; Sakuma, Y.; Tanaka, S. Direct Modulation at 56 and 50 Gb/s of 1.3-µm InGaAlAs Ridge-Shaped-BH DFB Lasers. IEEE Photonics Technol. Lett. 2014, 27, 534–536. [CrossRef] 2. Liu, G.; Zhao, G.; Sun, J.; Gao, D.; Lu, Q.; Guo, W. Experimental demonstration of DFB lasers with active distributed reﬂector. Opt. Express 2018, 26, 29784–29795. [CrossRef][PubMed] 3. Kim, H.-C.; Wiedmann, J.; Matsui, K.; Tamura, S.; Arai, S. 1.5-m-wavelength distributed feedback lasers with deeply etched ﬁrst-order vertical grating. Jpn. J. Appl. Phys. 2001, 40, L1107. [CrossRef] Appl. Sci. 2021, 11, 221 6 of 6

4. Tolstikhin, V.I.; Watson, C.D.; Pimenov, K.; Moore, R.; Logvin, Y.; Wu, F. Laterally coupled DFB lasers for one-step growth photonic integrated circuits in InP. IEEE Photonics Technol. Lett. 2009, 21, 621–623. [CrossRef] 5. Telkkälä, J.; Viheriälä, J.; Bister, M.; Karinen, J.; Melanen, P.; Dumitrescu, M.; Guina, M. Narrow linewidth 1.55 µm laterallycoupled DFB lasers fabricated using nanoimprint lithography. In Proceedings of the IPRM 2011-23rd International Conference on Indium Phosphide and Related Materials, Institute of Electrical and Electronics Engineers, Berlin, Germany, 22 May 2011; pp. 1–4. 6. Tang, S.; Hou, L.; Chen, X.; Marsh, J.H. Multiple-wavelength distributed-feedback laser arrays with high coupling coefficients and precise channel spacing. Opt. Lett. 2017, 42, 1800–1803. [CrossRef][PubMed] 7. Schreiner, R.; Schweizer, H. High-frequency operation of 1.57-µm laterally coupled distributed feedback lasers with self-aligned Ohmic contacts and nickel surface gratings. IEEE J. Select Top Quantum Electron. 2003, 9, 1179–1182. [CrossRef] 8. Afzal, S.; Schnabel, F.; Scholz, W.; Reithmaier, J.P.; Gready, D.; Eisenstein, G.; Vallone, M. 1.3-µm Two-Section DBR Lasers Based on Surface Defined Gratings for High-Speed Telecommunication. IEEE Photonics Technol. Lett. 2011, 23, 411–413. [CrossRef] 9. Uusitalo, T.; Virtanen, H.; Bardella, P. Dumitrescu, M. Analysis of the photon–photon resonance influence on the direct modulation bandwidth of dual-longitudinal-mode distributed feedback lasers. Opt. Q. Electron. 2017, 49, 46. [CrossRef] 10. Coldren, L.A.; Corzine, S.W.; Masanovic, M.L. A Phenomenological Approach to Diode Lasers. In Diode Lasers and Photonic Integrated Circuits, 2nd ed.; John Wiley & Sons, Inc.: Hoboken, NJ, USA, 1995; p. 73. 11. Coldren, L.A.; Corzine, S.W.; Masanovic, M.L. Dynamic Effects. In Diode Lasers and Photonic Integrated Circuits, 2nd ed.; John Wiley & Sons, Inc.: Hoboken, NJ, USA, 1995; p. 258. 12. Matsuo, S.; Kakitsuka, T. Low-operating-energy directly modulated lasers for short-distance optical interconnects. Adv. Opt. Photonics 2018.[CrossRef] 13. Liu, G.; Chuang, S.L. High-speed modulation of long-wavelength In(1−x)GaxAsyP(1−y) and In(1−x−y)Gax AlyAs strained quantum- well lasers. IEEE J. Quantum Elect. 2001, 37, 1283–1291. [CrossRef] 14. Li, A.; Wang, J.; Sun, C.; Wang, Y.; Yang, S.; Xiong, B.; Luo, Y.; Hao, Z.B.; Han, Y.J.; Wang, L.; et al. 1.3 µm 10-Wavelength Laterally Coupled Distributed Feedback Laser Array with High-Duty-Ratio Gratings. Phys. Status Solid 2018, 215, 1800490. [CrossRef] 15. Olshansky, R.; Hill, P.; Lanzisera, V.; Powzainik, W. Frequency response of 1.3 µm InGaAsP high speed semiconductor lasers. IEEE J. Quantum Electron. 1987, 23, 1410–1418. [CrossRef]