Limits to Maximum Absorption Length in Waveguide Photodiodes

The maximum photocurrent, power dissipation, and linearity of waveguide photodiodes are limited by the length over which the input optical power is absorbed. This absorption length is determined by the absorption coefficient of the absorbing layer material (αo), the optical confinement factor (Γ), and the excess loss coefficient (αi). In this paper, we analyze the fundamental limits to maximizing the absorption length and demonstrate a new waveguide photodiode structure that approaches these limits. The new structure is referred to as a slab-coupled optical waveguide photodiode (SCOWPD) and is realized in the InGaAsP/InP material system. Assuming 100% coupling efficiency, the SCOWPD has an ultralow optical confinement factor and a low excess loss coefficient, both calculated from measurements, of Γ = 0.069% and αi = 1.65 cm^-1, respectively. This results in a 1/e absorption length of 2.1 mm. The SCOWPD exhibits an external responsivity of 0.8 A/W and a maximum photocurrent of 250 mA at a wavelength of 1.55 μm.