A novel InP-based monolithically integrated multiwavelength detector is used as the central part of a fiber Bragg grating interrogator unit. We have designed the arrayed waveguide grating (AWG) in the device such that at least two detectors have a significant signal at any wavelength being measured. Consequently, the measurement resolution is increased, while keeping the area of device small. We demonstrate the resolution of the measurement to be 0.32 pm over a working range of 10 nm. The corresponding relative resolution of 0.003% is to our knowledge the best reported to date in an integrated interrogator.