In this work, we have reported the design, MBE-growth and fabrication of strain-balanced n-type modulation-doped (MD) InGaAlAs/InGaAs multiple quantum wells laser/SOAs on InP. The quantum well contains a lattice-matched InGaAs core, a compressive-strained InGaAs padding, and a tensile-strained InGaAlAs spacer. Two kinds of samples having similar structure but different fundamental transition wavelength of 1.55 £gm and 1.48 £gm are separately prepared for investigating their characteristics in optical amplification under forward bias and electro-absorption under reversed bias. Also, the technique of growing high-quality InGaAlAs with solid-source molecular beam epitaxy has been established and the resulting InGaAlAs bulk and QWs samples are extensively characterized by double-crystal X-ray diffraction, transmission electron microscopy, electroluminescence, and photoluminescence measurements. For £f = 1.55 £gm samples, ridge-waveguide lasers of Fabry-Perot (FP) type and tilted-end-facet (TEF) type were fabricated by a new developed multi-step wet-etching process. When injection current density > 20A/cm^2, electroluminescence spectra show higher optical gain for the quantum well e1-hh2 transition at £f = 1460 nm than the e1-hh1 transition at £f = 1550 nm. The FP laser shows a lasing peak of £f = 1514 nm at threshold. Additional lasing wavelength at £f =1528 nm and 1545 nm were observed sequentially as the injection current increased. However, for the TEF laser, only the emission at £f = 1511 nm was observed. These TE-polarized lasing wavelengths are consistent with the £_-like absorption peaks in photocurrent spectra. The lasing performance is possible attributed to optical transitions within quantum dots/wires which are formed by the strain-field profile and alloy segregation/migration. For £f = 1.48 £gm samples, the differential absorption spectroscopy, which measures the change of transmission (£GT/T) in the presence of electric field, is used to study the electro-absorption modulation behavior of MD-SOA¡¦s. A sample with n-type modulation-doping amounting to a sheet density of 3.5 ¡Ñ 10^11 cm^-2 per QW and combining with a hole-stopping barrier represents the largest chirp parameter (£Gn/£Gk) under reversed bias, which offers an excellent platform to realize electro-refractive devices with larger refractive index changes (£Gn) but lower differential absorption (£G£\) near £f = 1.55 £gm, which is also our interested region of operation. In addition, we have succeeded in reducing the length of conventional constant-width multimode interference (MMI) coupler of K = 0.15 and 0.28 more than 32% by a novel stepped-width design concept. By extending the stepped-with idea, we show that it is possible to obtain 2x2 waveguide couplers with new power splitting ratios of 7%, 64%, 80% and 93% for cross coupling by cascading two short MMI sections. We further realize freely chosen power splitting ratio by interconnecting a pair of unequal-width waveguides as the phase-tuning section into the middle of two short MMI sections. These compact and low loss MMI-based devices use only rectangular geometry without any bent, curved, and tapered waveguides. They offer valuable new possibilities for designing waveguide-based photonic integrated circuits.