An optical instrument for splitting optical spectrum, comprising a series of hollow core optical wave guides (12, 22, and so on) connected by optical couplers (16, 26, and so on) transporting broad band incident optical wave from one stage to another, a narrow band optical wave guide (14, 24, and so on) made of photonic crystal materials mounted inside each of said hollow core optical wave guide and along it, and specified for confining certain portion of the incident optical spectrum. Said inner narrow band optical wave guides bend out at the ends of said outer hollow core optical wave guides to extract and guide out the selected component of the incident optical spectrum. Said optical couplers couple the output optical waves of said outer hollow core wave guides of the previous stages into said inner optical wave guides of the next stages for further splitting.