PURPOSE: To quicken response by forming a diffraction lattice type lens, comprising a plurality of concentric grooves with the gap (d) between adjacent grooves being represented by specific conditions, on the surface of a substrate on the inside of a window.

CONSTITUTION: A diffraction lattice type leans 11 comprising a plurality of concentric grooves having the same depth is formed on the rear surface of an InP substrate 1. An antireflection film 10 is formed on the surface of the diffraction lattice lens 11 whereas an n-type ohmic electrode 9 is formed on the surface of the substrate on the outside of the diffraction lattice leans 11. An n-type InP buffer layer 2, an n-type InGaAs light absorption layer 3, and an n-type InP cap layer 4 are formed sequentially on the surface of the InP substrate 1. The diffraction lattice type lens 11 is obtained by making a plurality of grooves 11a, 11b, 11c, 11d concentrically at an interval d=λ/2n.sinθ (θ= tan-1(r/1)) and a depth λ/4.