A delay line interferometer comprising an optical waveguide having a distributed Bragg reflector, e.g. Bragg grating, fabricated therein. The distributed Bragg reflector has a refractive index modulation with a period variation Λ(z) along its length z that is arranged to output in transmission an output optical signal fout(t) in response to a input optical signal fin(t), wherein the output optical signal fout(t) is the result of temporal interference between one or more time-delayed replicas of the input optical signal fin(t). In other words, the distributed Bragg reflector is operable to generate and permit temporal interference between two or more time-delayed replicas of the input optical signal fin(t). The invention may thus mimic the behaviour of one or more MZIs.