In this work we report on a specially optimized type-I InAsSb/InAsSbP double heterostructure (DH) ridge laser grown by liquid phase epitaxy (LPE). To remove residual impurities and reduce Shockley-Read recombination, the active region was purified using a Gd gettering technique. In addition free carrier absorption loss was minimized by the introduction of two undoped quaternary layers with the same composition of the cladding layers either side of the active region. The inserted layers also helped alleviate inter-diffusion of unwanted dopants towards the active region during or after growth and reduced current leakage of the device. The diode lasers operate readily in pulsed mode at elevated temperatures and emit near 3.45 mu m at 170 K with a threshold current density as low as 118 A/cm(2) at 85 K. Compared to the conventional 3-layer DH laser, the optimized 5-layer structure with reduced optical loss can raise the maximum lasing temperature by 95K to similar to 210K.