A spectrometer comprising: a resonator comprising a waveguide ; and a Bragg grating disposed at one of two different ends of the resonator; and wherein the Bragg grating comprises: a core member extending from the waveguide of the resonator in a lengthwise direction of the waveguide and having a width (W2) that is less than a width (D) of the waveguide of the resonator; a plurality of first refractive members protruding from the core member from opposite sides of the core member in a width direction of the core member and spaced apart from each other along the lengthwise direction, wherein pitches (P) between the first refractive members vary along the lengthwise direction; and a second refractive member filling spaces between the first refractive members and having a refractive index different from a refractive index of the first refractive members; and wherein the width (W2) of the core member gradually reduces toward a center portion of the core member along the lengthwise direction; and the protruding lengths (L) of the first refractive members gradually increase toward the center portion of the core member along the lengthwise direction. a resonator comprising a waveguide ; and a Bragg grating disposed at one of two different ends of the resonator; and wherein the Bragg grating comprises: a core member extending from the waveguide of the resonator in a lengthwise direction of the waveguide and having a width (W2) that is less than a width (D) of the waveguide of the resonator; a plurality of first refractive members protruding from the core member from opposite sides of the core member in a width direction of the core member and spaced apart from each other along the lengthwise direction, wherein pitches (P) between the first refractive members vary along the lengthwise direction; and a second refractive member filling spaces between the first refractive members and having a refractive index different from a refractive index of the first refractive members; and wherein the width (W2) of the core member gradually reduces toward a center portion of the core member along the lengthwise direction; and the protruding lengths (L) of the first refractive members gradually increase toward the center portion of the core member along the lengthwise direction. a core member extending from the waveguide of the resonator in a lengthwise direction of the waveguide and having a width (W2) that is less than a width (D) of the waveguide of the resonator; a plurality of first refractive members protruding from the core member from opposite sides of the core member in a width direction of the core member and spaced apart from each other along the lengthwise direction, wherein pitches (P) between the first refractive members vary along the lengthwise direction; and a second refractive member filling spaces between the first refractive members and having a refractive index different from a refractive index of the first refractive members; and wherein the width (W2) of the core member gradually reduces toward a center portion of the core member along the lengthwise direction; and the protruding lengths (L) of the first refractive members gradually increase toward the center portion of the core member along the lengthwise direction. <EOS>