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Heterogeneously Integrated III-V/Si Distributed Bragg Reflector Laser with Adiabatic Coupling (1)
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A. Descos , C. Jany , D. Bordel , H. Duprez , G. Beninca de Farias , P. Brianceau , (1) (1) S. Menezo , and B. Ben Bakir (1)
CEA, LETI, Minatec Campus, 17 rue des Martyrs, F-38054 Grenoble cedex 9, France,
[email protected]. (2) III-V Lab, Joint lab of “Alcatel-Lucent Bell Labs”, “Thales Research and Technology” and “CEA Leti”, Minatec Campus, 17 rue des Martyrs, F-38054 Grenoble cedex 9, France. Abstract We report on a III-V on Silicon distributed Bragg reflector laser with adiabatic coupling operating continuous wave at 1547 nm. The lasing threshold at 20 °C and the maximum output power are 17 mA and 15 mW, respectively. The fiber-coupled power is higher than 4 mW. The device is directly modulated and generates open eye-diagram up to 12.5 Gb/s. Introduction Silicon photonics is a highly promising technological platform for large-scale photonic 1 integration . Hybrid laser integration on silicon via direct bonding of III-V epi-grown material on top of a patterned silicon-on-insulator (SOI) substrate is currently the most relevant solution 2 for low cost and high volume fabrication . Moreover, this approach takes advantage of both material properties: III-V for efficient light emission and silicon for its low-loss beyond 1.1 ȝm and high-index-contrast with its native oxide. In this communication, we report on experimental demonstration of a heterogeneously integrated III-V/Si distributed
Bragg reflector (DBR) laser with low threshold (< 20 mA) and high output power (> 15 mW). The laser performances are similar to those obtained by A. J. Zilkie et al. where the hybrid external-cavity DBR laser is built with a III-V reflective semiconductor optical amplifier (SOA) edge-coupled to a 3 μm-thick Si waveguide 3 containing a Bragg grating reflector . Intel, UCSB and Aurrion Inc. demonstrated Fabry-Pérot, distributed feedback (DFB) and 1, 4, 5 DBR lasers using an approach where the mode is mainly guided by the Si waveguide and evanescently coupled with the III-V waveguide, using a very thin bonding layer (< 5 nm, corresponding to the Si native oxide). In our 6, 7 approach , the mode in the hybrid section is
Fig. 1: a) Schematic representations of the hybrid III-V/Si laser, top and side views. b) A cross-section of the refractive index profile of the hybrid structure. c) Optical microscope image of the final fabricated device. d) Scanning electron micrograph of the Si-based distributed Bragg reflector. The grating period, Λ, is 237 nm.
Th.1.B.2.pdf mainly guided by the III-V waveguide, and an adiabatic Si-taper is used to couple the light from the III-V waveguide to the silicon waveguide. In this case, the bonding interface can be relatively thick (from 30 to 150 nm, typically). The advantage is that the optical mode experiences a high optical gain in the central region of the laser structure while maintaining a high coupling efficiency with the bottom silicon waveguide on the output part. This approach overcomes the trade-off between high modal gain and high coupling efficiency between the silicon and III-V waveguides that is inherent to the so-called Si evanescent lasers. This results in a larger gain available for amplification. Device structure The wafer-level fabrication and integration of the hybrid lasers have been described in our 6 earlier work . The laser structure, illustrated in Fig. 1, is formed by two vertically superimposed waveguides separated by a 100 nm-thick SiO2 layer. The top waveguide, fabricated in an InP/InGaAsP-based heterostructure, only serves 6 to provide optical gain . The bottom Siwaveguides system, which supports all optical functions, is constituted by two tapered ribwaveguides (mode transformers), two DBRs and a surface-grating coupler.
Fig. 2: L-I-V characteristics of the hybrid DBR laser for different temperatures.
The optical cavity is defined by the two DBRs spaced 600 ȝm apart. The grating reflectors have a width, an etch depth and a duty cycle of 10 μm, 10 nm and 50%, respectively, resulting -1 in a grating strength, κ, of 83 cm . Modal reflectivities of 97.3% and 46.4% were calculated for the back and front mirrors. The 3 dB bandwidths are 2.58 and 3.98 nm, respectively. The 400 ȝm-long III-V waveguide is placed inside the cavity. The mode transformers, positioned below the edges of the active waveguide, provide an adiabatic transition by varying the width of the Si rib-waveguide. Both 6, 8 the shape of the mode transformers and, in particular, the thickness of the oxide separation layer are optimized to be robust enough with respect to the variations induced by the fabrication processes. For specific configurations, coupling efficiencies higher than 97% were calculated. In addition, if we take into accounts fabrication process tolerances, the lower limit is found to be higher than 90%. Experimental results The continuous wave laser output power is collected through the surface-grating coupler by a multimode fiber and then characterized by using both a spectrum analyzer and an optical power-meter. To determine the output power at the low-reflectivity side of the hybrid DBR laser we measured, on the same wafer, the insertion loss of two surface grating couplers connected with a 500 ȝm-long and 10 ȝm-broad waveguide. Figure 2 shows the fiber-coupled and front mirror output power-current-voltage (L-I-V) characteristics for operating temperatures ranging from 15 to 65 °C. As can be seen from the L-I curves, the laser threshold is 17 mA with a maximum output power of 15 mW at 20 °C (> 4 mW in the fiber), leading to a differential efficiency of 13.3 %. The device has a lasing
Fig. 3: Lasing spectrum measured at a bias current of 118 mA.
Th.1.B.2.pdf turn-on voltage of 1.0 V and a series resistance of 7.5 Ω. The laser operates up to a stage temperature of 60 °C with a front mirror output power exceeding 2.4 mW. The kinks observed in the L-I curves are typical for DBR lasers. They are caused by a thermal detuning of the hybrid gain and reflector sections and are associated with mode hopping.
temperature increase with higher pumping current the lasing wavelength is red-shifted due to the thermooptic effect in the cavity. When the lasing mode is far enough from the reflectivity peak, a longitudinal mode hop to another mode occurs. The small signal modulation response of the laser is shown in Fig. 5. The 3 dB bandwidth of ~ 7 GHz is obtained for bias currents ranging from 125 mA to 150 mA.
Fig. 6: Eye diagrams of the directly modulated DBR laser at 5 Gb/s and 12.5 Gb/s.
Figure 6 shows eye diagrams, obtained for 5 and 12.5 Gb/s operations, in back-to-back 15 conditions for a 2 -1 pseudo-random bit stream (PRBS) with 17 mW of RF power. The bias current is 160 mA at room temperature. The extinction ratios are 6.2 dB at 5 Gb/s and 4.8 dB at 12.5 Gb/s.
Fig. 4: Fiber-coupled L-I characteristic and normalized contour plot of the lasing spectra at room-temperature.
Conclusions We demonstrated a hybrid III-V/Si DBR laser operating at 1547 nm with a sidemode suppression ration of 50 dB. The laser has a low threshold current of 17 mA and a maximum output power of 15 mW at 20°C. The device is directly modulated and generates open eyediagrams for data rates of 5 and 12.5 Gb/s with extinction ratios of 6.2 and 4.8 dB, respectively. Acknowledgements This work was funded thanks to the French national program “programme d’Investissements d’Avenir, IRT Nanoelec” ANR-10-AIRT-05.
Fig. 5: Room-temperature frequency response of the hybrid DBR laser for different bias currents.
The lasing spectrum is shown in Fig. 3 with a lasing peak at 1546.97 nm for a drive current of 118 mA. The sidemode suppression ratio is about 50 dB. Figure 4 shows the lasing spectrum as a function of the drive current along with the corresponding L-I curve. Measurements were performed at room temperature. We can clearly observed jumps in the lasing longitudinal mode due to mode hops. As the device
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