We propose a very low loss multiple-light-source red-green-blue (RGB) power combiner by optical waveguide mode coupling technique. The combiner consists of a two-step circuit that performs both power coupling and wavelength multiplexing for an RBG multiple-light source. The first step of the circuit combines first R, G, and B as the 0th-order mode. The second step combines second R and G by mode conversion from the 0th-order mode to second-order modes using waveguide mode couplers. We used an even mode configuration to avoid asymmetric deformation of the beam due to interference between the modes. By using all of these coupler functions in the two steps, the circuit provides multiplelight-source (RRGGB) power combining. The combiner was fabricated by silica planar lightwave circuit (PLC) technology. The coupler length is about 4.5 mm, including 2.3 mm for the 0th-order coupler and 2 mm for the secondorder coupler. We estimated the coupling loss of both the 0th-order RGB coupler and second-order RG power coupler to be about 1 dB by evaluating the combined power for the 0th-order RGB couplers and the complementary output powers for mode couplers. To the best of our knowledge, this is the first demonstration of a multiple-light-source RRGGB power combiner using multimode coupling. This method enables us to combine a much larger number of light sources using multi-stage coupling for different modes as well. Moreover, the beam shape can be controlled by mode selection.
A novel approach is demonstrated for overcoming the trade-off relationship between the power consumption and transmission distance of an electro-absorption modulator integrated with a DFB laser (EADFB laser). We demonstrate that the monolithic integration of a short semiconductor optical amplifier (SOA) with an EADFB laser is effective in overcoming the limitation imposed by the Kramers-Kronig (K-K) relation of the EA modulator, which cannot be overcome with the conventional method of optimizing the MQW structure of the EA modulator. Our approach provides an EADFB laser with two advantages. One is that we can realize a higher optical output power with smaller power consumption than with a conventional EADFB laser by reducing the DFB laser injection current and allocating it to the SOA section. We design the SOA length based on this concept. The other advantage is the chirp compensation of the EA modulator with the SOA. To confirm the validity of this approach, we investigate the SOA length dependence on the basic characteristics. By using an EADFB laser integrated with a 50-μm-long SOA, we achieve a 2 dB increase in the modulated output power compared with a stand-alone EADFB laser with the same power consumption. We realize an extended transmission distance of 5 km at 40 Gbit/s, and a 1.55-μm-wavelength window, which is conventionally achieved for a 2-km SMF transmission with an EADFB laser. These results indicate that this approach is a promising way to realize a high-speed light source with low power consumption for future large capacity optical network systems.
We review our recent progress on the heterogeneous integration of high-speed InP photodiodes (PDs) on a silica-based planar lightwave circuit (PLC) with the goal of realizing a small photonic integrated circuit (PIC) device with excellent performance. Heterogeneous integration technology is a combination of monolithic InP fabrication and angled micro-mirror etching processes, and provides low-loss optical coupling between them without any complicated optical alignment. Also, by employing 2.5%-Δ waveguides with a bending radius of 1 mm, we can reduce the chip size required for compact PIC devices. We demonstrated a dual-polarization quadrature-phase-shift-keying (DP-QPSK) coherent detector composed of a variable optical attenuator (VOA), a polarization beam splitter (PBS), eight high-speed PDs, two 90-degree optical hybrids (OHs) and micro-mirrors. Heterogeneous technology on silica-based PLCs will be an attractive way to provide compact high performance PICs for future photonic networks.
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