InP-based reversed-mesa ridge-waveguide structure for high-performance long-wavelength laser diodes

A novel structure for an InP-based ridge-waveguide (RWG) laser is demonstrated. It has a reverse-trapezoid-ridge shape which offers reduced threshold current and smaller electrical and thermal resistances over the conventional vertical-mesa (VM) RWG devices. We found this new RWG laser quite suitable for wide-temperature-range and high-power operations. 1.3-1.55-μm wavelength strained InGaAsP-InP multiple quantum-well (MQW) lasers were demonstrated with the new ridge structure, which showed sufficient lasing behavior such as high-temperature lasing up to 165°C and a high light output over 300 mW. This simple and high-performance laser structure was also applied for integrated lasers with other functional elements such as an absorption-type modulator or a beam-expander waveguide lens. To achieve low-threshold current operation in RWG lasers, suppression of the lateral-current spreading is important. We proposed a new method to effectively reduce this lateral diffusion current by using a doped active region with n-type dopant. Combined by the reduced transparency carrier density in the n-doped active region, this leads to a very low-threshold current of less than 2.2 mA for a coated 200-μm-long cavity device, which is comparable to those of recent advanced same-material buried-heterostructure (BH) lasers. These excellent lasing properties, along with the sufficient device reliability under strict environmental conditions, make this reversed-mesa (RM) RWG a promising candidate for the widespread use of low-cost long-wavelength light sources