## 3 Membrane Photonic Devices on Si Substrate
### InGaAlAs Membrane Laser Array Grown on InP-on-Insulator Substrate  
Figure 9a-d show the fabrication procedure of a membrane distributed-reflector (DR) laser using InGaAlAs MQWs grown on an InP-on-insulator substrate [47]. The entire fabrication procedure (including the epitaxial growth of MQW, epitaxial regrowth of InP, and formation of the lateral _p-i-n_ junctions) was carried out using an InP-on-insulator substrate. To fabricate an electrically pumped membrane-laser array using thin BHs on Si, thermal diffusion of Zn (_p_-doping) and ion implantation of Si (_n_-doping) were carried out to form lateral _p-i-n_ junctions. The InP output waveguides were formed using RIE. To achieve single-mode lasing, gratings were formed by dry-etching the InP surface just above the BH and a rear output waveguide using RIE. Both InP waveguides have a laterally tapered structure at the edges. After metallization, the silicon oxynitride (SiON) layer was deposited by electron-cyclotron-resonance p-CVD. The SiON layer was functionalized as not only the core of the spot-size converter (SSC) but also the cladding of the III-V waveguide layer. After defining the SiON core structures by partial etching with RIE, a SiO\({}_{2}\) cladding layer was deposited by p-CVD. Finally, the contact-hall was formed to electrically drive the lasers. Figure 9e shows a schematic of cross-section of the fabricated laser, and Figure 9f show a bird's eye view of the fabricated membrane laser before integration of the SSC. The device has a buried active core that is 80 \(\upmu\)m long and 0.8 \(\upmu\)m wide. The DFB surface grating was formed just above the BH, and the distributed Bragg reflector (DBR) grating was also formed on the rear-side of the BJ InP waveguide. Since the Bragg wavelength of the DBR  
Figure 9: (**a–d**) Fabrication procedure of membrane distributed reflector (DR) laser using InGaAs MQWs grown on InP-on-insulator substrate; (**e**) schematic of cross-section of DR laser; (**f**) bird’s eye view of laser before integration of the spot-size converter (SSC). grating is slightly longer compared with that of the DFB grating, the longer side-mode of the uniform DFB grating was selected and the asymmetric optical output from the front InP waveguide was achieved. The coupling coefficient was set to ~800 cm\({}^{-1}\). The fabricated membrane laser is an eight-channel array and each laser has the same lasing wavelength. Figure 10a shows a photograph of the diced wafer that includes four eight-channel laser arrays (32 lasers). Figure 10a also shows a top-view photograph of an eight-channel laser array. The separation of each laser (laser pitch) was 250 \(\upmu\)m, the length of the tapered InP waveguide was 300 \(\upmu\)m, and total SSC length was 660 \(\upmu\)m. The overall chip size was 2.0 \(\times\) 1.0 mm\({}^{2}\). Figure 10b shows the measured optical output power and applied voltages of an eight-channel DR laser array as a function of injected current (L-I-V characteristics). The optical output power was first measured with a photodetector placed in front of the device facet. The fiber-coupled output power was then measured by butt-coupling the single-mode high numerical aperture fiber (HNAF) to the SSC facet. The reflection at the SSC facets originally caused kinks in the light-current (L-I) curves but were successfully suppressed when the HNAF was butt-coupled. The average fiber-coupling loss was 2.98 dB. The threshold current was ~1.35 mA for all eight channels and the I-V characteristics were almost the same. Figure 10c compares the lasing spectra of the eight lasers at a bias current of 4.0 mA. We achieved single-mode lasing at wavelengths from 1301.5 to 1301.8 nm with an average side-mode suppression ratio of 42.7 dB. The small wavelength deviations indicate the lasers' potential for use in wavelength division multiplexing (WDM) systems. We also demonstrated 25.8-Gbit/s direct modulation with a 2\({}^{31}-\)1 pseudo-random bit sequence (PRBS) non-return-to-zero (NRZ) signal.