A description of a novel Y-branch photonic crystal laser diode operating at 1.55 µm is presented. Optical gain calculations within an InGaAsP-InP multiple-quantum-well (MQW) gain medium predict a threshold current of 0.5 mA, which is lower than a typical ridge laser. The resonant structure of the beamsplitting laser is modeled using a scattering matrix approach incorporating the effects of multiple internal reflections on the cavity mode structure and mode spacing. Phase-matching conditions are derived, taking into account all the different resonant substructures. The results from the phase-matching analysis show an operating mode that reduces threshold current beyond that of a geometrically similar conventional ridge laser. This Y-branch photonic crystal laser, when cascaded as a multichannel resonant structure, produces numerous optical intensity levels that can be used in an optical analog-to-digital converter (ADC).