1 . 55 μ m high speed low chirp electroabsorption modulated laser arrays based on SAG scheme

We demonstrate a cost-effective 1.55 μm low chirp 4 × 25 Gbit/s electroabsorption modulated laser (EML) array with 0.8 nm channel spacing by varying ridge width of the lasers and using selective area growth (SAG) integration scheme. The devices for all the 4 channels within the EML array show uniform threshold currents around 18 mA and high SMSRs over 45 dB. The output optical power of each channel is about 9 mW at an injection current of 100 mA. The typical chirp value of single EML measured by a fiber resonance method varied from 2.2 to −4 as the bias voltage was increased from 0 V to 2.5 V. These results show that the EML array is a suitable light source for 100 Gbit/s optical transmissions. ©2014 Optical Society of America OCIS codes: (250.0250) Optoelectronics; (250.5300) Photonic integrated circuits; (250.5960) Semiconductor lasers; (140.3290) Laser arrays References and links 1. IEEE 802.3ba 40 Gb/s and 100 Gb/s Ethernet Task Force Public Area; http://www.ieee802.org/3/ba/index.html. 2. Y. B. Cheng, J. Q. Pan, Y. Wang, F. Zhou, B. J. Wang, L. J. Zhao, H. L. Zhu, and W. Wang, “40-Gbit/s low chirp electroabsorption modulator integrated with DFB laser,” IEEE Photon. Technol. Lett. 21(6), 356–358 (2009). 3. Y. B. Cheng, J. Q. Pan, S. Liang, W. Feng, Z. Y. Liao, F. Zhou, B. J. Wang, L. J. Zhao, H. L. Zhu, and W. Wang, “Butt-coupled MOVPE growth for high-performance electro-absorption modulator integrated with a DFB laser,” J. Cryst. Growth 308(2), 297–301 (2007). 4. W. Kobayashi, T. Fujisawa, T. Ito, S. Kanazawa, Y. Ueda, and H. Sanjoh, “Advantages of EADFB laser for 25 Gbaud/s 4-PAM (50 Gbit/s) modulation and 10 km single-mode fibre transmission,” Electron. Lett. 50(9), 683– 685 (2014). 5. M. N. Ngo, H. T. Nguyen, C. Gosset, D. Erasme, Q. Deniel, N. Genay, R. Guillamet, N. Lagay, J. Decobert, F. Poingt, and R. Brenot, “Electroabsorption Modulated Laser Integrated with a Semiconductor Optical Amplifier for 100km 10.3 Gb/s Dispersion-Penalty-Free Transmission,” IEEE J. Lightw. Technol. 31(2), 232–238 (2013). 6. J. Kreissl, C. Bornholdt, T. Gaertner, L. Moerl, G. Przyrembel, and W. Rehbein, “Flip-Chip Compatible Electroabsorption Modulator for up to 40 Gb/s, Integrated with 1.55 mu m DFB Laser and Spot-Size Expander,” IEEE J. Quantum Electron. 47(7), 1036–1042 (2011). 7. S. Makino, K. Shinoda, T. Kitatani, H. Hayashi, T. Shiota, S. Tanaka, et al., “High-Speed EA-DFB Laser for 40G and 100-Gbps,” IEICE.T.Electron. E92c(7), 937–941 (2009). 8. D. Ton, G. W. Yoffe, J. F. Heanue, M. A. Emanuel, S. Y. Zou, J. Kubicky, B. Pezeshki, and E. C. Vail, “2.5Gb/s modulated widely-tunable laser using an electroabsorption modulated DFB array and MEMS selection,” IEEE Photon. Technol. Lett. 16(6), 1573–1575 (2004). 9. T. Fujisawa, S. Kanazawa, K. Takahata, W. Kobayashi, T. Tadokoro, H. Ishii, and F. Kano, “1.3-μm, 4 × 25Gbit/s, EADFB laser array module with large-output-power and low-driving-voltage for energy-efficient 100GbE transmitter,” Opt. Express 20(1), 614–620 (2012). 10. S. Kanazawa, T. Fujisawa, A. Ohki, H. Ishii, N. Nunoya, Y. Kawaguchi, N. Fujiwara, K. Takahata, R. Iga, F. K ano, and H. Oohashi, “A compact EADFB laser array module for a future 100-Gbit/s Ethernet transceiver,” IEEE J. Sel. Top. Quantum Electron. 17(5), 1191–1197 (2011). 11. T. Fujisawa, T. Itoh, S. Kanazawa, K. Takahata, Y. Ueda, R. Iga, H. Sanjo, T. Yamanaka, M. Kotoku, and H. Ishii, “Ultracompact, 160-Gbit/s transmitter optical subassembly based on 40-Gbit/s × 4 monolithically integrated light source,” Opt. Express 21(1), 182–189 (2013). 12. J. S. Sim, S. B. Kim, Y. H. Kwon, Y. S. Baek, and S. W. Ryu, “A Four-Channel Laser Array with Four 10 Gbps Monolithic EAMs Each Integrated with a DBR Laser,” Etri. J. 28(4), 533–536 (2006). 13. S. W. Ryu, J. S. Sim, Y. H. Kwan, S. B. Kim, and Y. S. Baek, “10 Gbps transmission of electroabsorption modulators integrated with a 4-channel distributed Bragg reflector laser array,” Semicond. Sci. Technol. 23(5), 055012 (2008). 14. Y. Nakano and K. Tada, “Analysis, design, and fabrication of GaAlAs/GaAs DFB lasers with modulated stripe width structure for complete single longitudinal mode oscillation,” IEEE J. Quantum Electron. 24(10), 2017– 2033 (1988). 15. L. Tao, L. J. Yuan, Y. P. Li, H. Y. Yu, B. J. Wang, Q. Kan, W. X. Chen, J. Q. Pan, G. Z. Ran, and W. Wang, “4λ InGaAsP-Si distributed feedback evanescent lasers with varying silicon waveguide width,” Opt. Express 22(5), 5448–5454 (2014). 16. J. Hong, W. P. Huang, T. Makino, and G. Pakulski, “Static and Dynamic Characteristics of MQW DFB Lasers with Varying Ridge Width,” IEE Proc., Optoelectron. 141(5), 303–310 (1994). 17. S. R. Jain, M. N. Sysak, G. Kurczveil, and J. E. Bowers, “Integrated hybrid silicon DFB laser-EAM array using quantum well intermixing,” Opt. Express 19(14), 13692–13699 (2011). 18. B. K. Saravanan, T. Wenger, C. Hanke, P. Gerlach, M. Peschke, and R. Macaluso, “Wide temperature operation of 40-Gb/s 1550-nm electroabsorption modulated lasers,” IEEE Photon. Technol. Lett. 18(7), 862–864 (2006). 19. F. Devaux, Y. Sorel, and J. F. Kerdiles, “Simple measurement of fiber dispersion and of chirp parameter of intensity modulated light emitter,” J. Lightwave Technol. 11(12), 1937–1940 (1993).


Introduction
To cope with the demand for huge data capabilities in Internet and associated data-driven applications, 100-Gb/s transmission technology has been widely considered as a strong candidate for next-generation terabit per second communication networks [1].The Electroabsorption Modulated Laser (EML) has attracted much attention due to its compactness, low packaging cost, low driving voltage and high stability [2][3][4][5][6][7].Monolithically integrated EML arrays are promising light sources for modern reconfigurable dense wavelength division multiplexing (DWDM) systems.The devices can potentially reduce optical network costs with simplified optical alignment and packaging processes.Compared with the monolithically integrated distributed feedback (DFB) laser arrays and an electroabsorption modulator (EAM) which operates as the wavelength selectable light sources [8], the EML arrays can handle much higher data capabilities and the device performance of different channels in EML arrays can be optimized separately.There have been several reports on EML arrays that operate more than 40 Gbit/s over the past years.Compact monolithically integrated light sources based on 1.310 µm 4 × 25 Gbit/s and 4 × 40 Gbit/s electroabsorption modulated DFB laser array were demonstrated for future long distance data communication system, respectively [9][10][11].A 4 × 1 optical mulitiplexer was used to reduce the chip size.A 1.55 µm 4 × 10 Gbit/s electroabsorption modulated distributed Bragg reflector (DBR) laser array was also proposed for better control of the output wavelength due to the inherent wavelength tunability of DBR laser [12,13].
Although much progress is made for integrated light sources as mentioned above, it is expected that integrated light sources will be cost-effective and they will demonstrate improved performance of both the laser and the modulator simultaneously with relative simple fabrication processes for practical applications.To fabricate EML arrays, one of the key issues is that different wavelength channel have to be defined side by side on one chip and the lasing wavelength between neighboring channels must have a uniform wavelength space.Up to now, electron-beam lithography (EBL) is the most widely used techniques to fabricate gratings with different periods [9][10][11].However, drawbacks such as high costs, time consumption, low yields, and complex manufacture processes make this method not the best choice for mass production.As shown in earlier reports [14], the variation in the ridge width would lead to a slight difference in the effective index and then in the lasing wavelength.And this method was used to fabricate multi-wavelength laser arrays with the same grating period [15,16].Such width-varying DFB waveguides can be made by standard photolithography and holographic lithography.
Another key issue is the wavelength compatibility between the DFB and the EAM.The DFB lasing wavelength is longer than the absorption-peak wavelength of the EAM.The wavelength detuning between the DFB lasing wavelength and the EAM absorption-peak wavelength can greatly affect the devices performances such as extinction ratio, output power and 3dB bandwidth.Among the many ways of laser-modulator monolithic integration explored [2-13, 17,18], selective area growth has large degree of freedom in bandgap energy control and almost 100% optical coupling achievable between the components.This method allows definition of different regions of MQW bandgaps on a single masked substrate with a one-step growth process.Unlike the butt-joint integration scheme, no partial removal of the active region by selective etching and no different active material by regrowth are necessary for the SAG method.Thus, the fabrication complexity of the EMLs is significantly reduced by using SAG and no postgrowth annealing such as quantum-well intermixing is required.In this work, selective area growth (SAG) is employed to grow an active layer in the regions of the laser and the modulator.The ridge width varying techniques are used to monolithically integrate a 4 × 25 Gbit/s EML array.As a result, the fabrication process of the integrated device is significantly simplified.Our method has great potential for the fabrication of low-cost, high quality integrated multi-wavelength laser arrays.

Device design and fabrication
Each EML of the array is composed of a 250 µm long DFB laser and a 170 µm long EAM.Different active regions of laser and EAM at 1552 nm and 1495 nm respectively, have been obtained by the SAG method.The laser array is designed to have a frequency spacing of 100 GHz (0.8 nm) which is obtained by slightly changing the ridge waveguide width according to: Where the is the grating period, B is the Bragg wavelength determined by the hologram lithography, n eff is the effective refractive index in waveguide structure and m is grating order which is 1 here for the first order grating.The EML arrays have a typically 2.0 to 3.5 µmwide ridge waveguide structures to achieve transverse fundamental mode operation.The transverse fundamental mode operation is necessary for EML with single longitudinal mode output because high order transverse modes lead to power kinks, side-mode-suppression reduction or multimode lasing.The effective refractive index of the ridge waveguide as a function of the waveguide width was calculated by using beam propagation method (BPM).For all of the four-channel EML in the array, waveguide width are designed as 2 µm, 2.35 µm, 2.6 µm and 3.1 µm, which corresponds to the calculated effective index of 3.21, 3.2116, 3.2133 and 3.2149, respectively.The designed EML array was fabricated using the similar process in [2].Device fabrication started with deposition of 200 nm thick SiO 2 dielectric films on the S-doped (100) InP substrates by plasma-enhanced chemical vapor deposition (PECVD).Masks were patterned along the [011] direction by conventional photolithography.Then, an n-InP buffer layer and an eight-pair InGaAsP/InGaAsP MQWs structure were selectively grown on the patterned substrates by ultra-low-pressure MOCVD (30 mbar, 655°C).The MQWs consist of un-doped 8-nm-thick 0.7% compressive strain InGaAsP wells separated by 9-nm-thick 0.3% tensile strain In 0.85 Ga 0.15 As 0.42 P 0.58 barriers, sandwiched between 100-nm-thick latticematched In 0.78 Ga 0.22 As 0.47 P 0.53 optical confinement layers.It is well known that the principal mechanism of selectively grown MOCVD lies in the lateral gas phase diffusion of group-III precursors.At ultra-low pressures, the reagent particles can easily diffuse out of the stagnation gas phase layer.The measured growth enhancement rate between the stripes is 20% which fits well with the lateral gas phase diffusion model.A first-order grating with period of 241.7 nm is partially formed on the laser section by conventional holographic exposure followed by chemical etching.The exposure and dry-etching processes were controlled to achieve a grating duty factor of approximately 50% for an optimum coupling coefficient.To complete the epitaxial structure, a p-type InP cladding layer and p + -InGaAs contact layer were successively grown.
To ensure a small series resistance of the DFB laser and a low capacitance of the EAM, a single reverse-mesa-ridge structure was formed by conventional photolithographic technology and etching along the [011] direction.RIE was first employed to remove the p + -InGaAs contact layer during the ridge etching.After that, the p-type InP cladding layer was etched by using selective wet chemical etching in HCl:H 2 O solution.The etching processes were to achieve the low loss reverse-mesa-ridge.The EA modulator section was further processed into deep mesa ridge waveguide structure by the second RIE step to reduce the junction capacitance of the device.Electrical isolation between the laser and the modulator were realized by forming 50--wide trench between them and adopting He + implantation in the trench .Three steps of He + implantation were used for a flat ion distribution with doses of 1 × 10 14 , 8 × 10 13 , 5 × 10 13 cm 2 and at energies of 180kev, 100kev,80kev respectively.An to reduce the parasitic electrode pad capacitance of EAM.The ridge waveguide was well protected and passivated.Thus low capacitance of the modulator and low leakage currents were obtained.Then, patterned Ti/Au p-electrode was formed on top of the planarized wafer.Au/Ge/Ni n-electrode was evaporated onto the backside of the device after thinning the wafer y, the wafer was cleaved into device chips, and the facet of the EA modulator and DFB laser were antireflection (AR) and high reflection (HR) coated with dielectric layers deposited by PECVD, respectively.A 40-GHz vector network analyzer and a calibrated receiver were applied for a high-speed measurement.

Characteristics of the device
The device was mounted on a copper submount using an optimized Au 80 Sn 20 solder.The copper sub-mount was placed on a thermoelectric cooler (TEC) to control the chip temperature under continuous-wave (CW) operation.During the measurement, the EMLs Figure 1 shows the measured light output power versus injection current curve of a typical chip of EML array at the temperature of 25°C.A threshold current as low as 18 mA and an output power of about 9mW at 100mA was achieved for each EML channel.The output light from the EML array was collected by a lensed single mode fibre and coupled to an optical spectrum analyzer (OSA) for spectrum analyse.Figure 2 shows the recorded spectra for all 4 channel lasers at the injection current of around 65 mA.The lasing wavelengths were around 1551.80 nm, 1552.56 nm, 1553.28 nm, and 1554.04 nm, which had a slight shift of around ± 0.1 nm from the design due to lithography resolutions.The measured single-mode suppression ratios (SMSRs) are larger than 40 dB and the average value is 45 dB, which exhibits good single longitudinal mode property in the DFB laser array.The measured Fig. 2. Typical lasing spectrum of the EML at a driving current of 65mA.
lasing wavelengths appear to have good linearity.After linear fitting, the residual wavelength changes only from 0.004 to 0.012 nm, this is relatively quite small.Supposing that the gratings are fabricated by other advanced fabrication methods such as EBL, the deviation of period must be controlled within a very small value of 0.124 nm, corresponding to the tolerance of 0.8 nm.As we know, it is quite challenge to obtain space of grating pitches less than 1nm with typical EBL.We also measure the output optical spectrum of the EML under different operation injection current.The wavelength shift is about 0.012 nm/mA.Thus, the EML array can be finely tuned by changing the injection current of each channels and the required injection current range is about 10 mA to make the lasing wavelengths exactly linear.The divergence angles from the EAM output are 40.2° × 34.6° in the vertical and horizontal, respectively; the coupling efficiency to single mode fiber reached 42% in the experiment.In Fig. 3, the dc extinction ratio of all the four EML channels measured using an integrating sphere is plotted as a function of the reverse bias applied to the EAM.The extinction ratio at 5V reverse bias is estimated to be approximately 15 dB.When coupled to single mode fiber the dc extinction ratio at 5V reverse bias is more than 35dB.By adopting a deep ridge waveguide and planar electrode structures combined with buried BCB, the capacitance of the EAM is reduced to 0.22 pF.The measured relative E/O response of a typical EML channel  (channel 2) is shown in Fig. 4, which has been calibrated to account for the frequency response of the photo detector and high frequency probe.The measured 3dB bandwidth is more than 18.5 GHz (~24 GHz by fitting the experimental result) for all the four channels, which are sufficiently large for 25 Gbit/s NRZ signal operation.The intrinsic bandwidth of the EML will be larger if the resonance is removed by optimizing the design of the submount.
Wavelength chirping which is defined as the ratio of the increments of the real and imaginary parts of the EAM complex refractive index is important to the transmission characteristics of an EAM.The wavelength chirping is estimated from the small signal parameter measurements using a fiber resonance method proposed by Devaux et al. [19].Figure 5 shows the reverse bias voltage dependence of the parameter for EML channel 2. The parameter varies from 2.2 to 4, as the reverse bias voltage increases from 0V to 2.5V.A zero chirp parameter was achieved at a bias voltage of 1.58 V.The transfer characteristic of the device offers potential for getting a low dispersion penalty.

Discussions and conclusions
A low chirp 4 × Gbit/s EML array with 0.8 nm channel spacing has been fabricated by varying the laser ridge width and SAG.The grating was fabricated only by common holographic exposure and an additional micrometer-level standard photolithography, which results in a low cost.The device shows uniform threshold current around 18 mA and high SMSRs over 45 dB.The optical output power of each channel is about 9 mW at an injection current of 100 mA.The chirp values of the EML chip was measured using a fiber resonance method.The chirp varied from 2.2 to 4 as the bias voltage was increased from 0 V to 2.5 V respectively.The transfer characteristic of the device indicates potential for transmission with a low dispersion penalty.The experiment results show that our technique is a simple and powerful tool for the fabrication of low-cost, high quality multi-wavelength laser arrays.

Fig. 1 .
Fig. 1.Typical light output power versus current curve of a typical chip of EML array at the temperature of 25°C under CW condition.

Fig. 3 .
Fig.3.Measured extinction behavior of the integrated light source with 170µm EA modulator section using an integrating sphere.