A spectrometer includes an input unit for receiving an optical signal, a diffraction grating disposed on the transmission path of the optical signal for dispersing the optical signal into a plurality of spectral rays, an image sensor disposed on the transmission path of at least a portion of the spectral rays, and a waveguide device. A waveguide space is formed between the first and second reflective surfaces of the waveguide device. The optical signal is transmitted from the input unit to the diffraction grating via the waveguide space. The portion of the spectral rays is transmitted to the image sensor via the waveguide space. At least one opening is formed on the waveguide device, and is substantially parallel to the first and/or second reflective surface. A portion of the spectral rays and/or the optical signal diffuses from the opening out of the waveguide space without reaching the image sensor.