Tunable filter

A method of matching a known waveband spectrum of a selected substance, the method comprising the steps of:

• providing an optical waveguide having a core material, a nominal core refractive index, an input end and an output end;
• forming periodic variations in the core material to change the nominal core refractive index of the optical waveguide at the periodic variations between the input and the output ends so that the periodic variations reflect selected wavelengths of light back toward the input end to produce a reference spectrum that matches the known spectrum of the selected substance;
• coupling a light source to the input end of the optical waveguide;
• transmitting light energy from the light source into the input end and through the core material of the optical waveguide to the output end;
• and
• modulating the periodic variations in the core material to selectively and intermittently shift the reflected wavelengths of the optical waveguide.

A method of detecting the presence of a selected substance in a test environment where the selected substance has a known waveband spectrum, the method comprising the steps of:

• passing light energy into the test environment;
• detecting the light energy after passing through the test environment representing a sample spectrum of the environment;
• providing an optical waveguide having a core material with a nominal core refractive index and an input end and an output end;
• forming periodic variations in the core material to alter the core refractive index of the optical waveguide at the periodic variations between the input and the output ends so that the optical waveguide produces a reference spectrum that matches the known spectrum of the selected substance;
• passing light energy into the input end of the optical waveguide;
• detecting the light energy emitted by the optical waveguide representing the reference spectrum;
• and
• comparing the reference spectrum to the sample spectrum.

An apparatus for matching a known spectrum of a selected substance, the apparatus comprising:

• an optical waveguide having a core material, a nominal core refractive index, an input end and an output end wherein the optical waveguide transmits light energy from the input to the output end through the core material;
• a plurality of periodic variations in the refractive index carried in the optical waveguide core between the input and the output ends changing the nominal core refractive index to reflect selected wavelengths of light back toward the input end to produce a reference spectrum at the output end of the waveguide having characteristic wavelengths matching the known spectrum of the selected substance;
• and
• a modulator coupled to the optical waveguide to modulate the periodic variations in the core refractive index to selectively and intermittently shift the reflected wavelengths in the reference spectrum of the optical waveguide.

An apparatus for matching a known spectrum of a selected substance, the apparatus comprising:

• an optical waveguide having a core material, a nominal core refractive index, an input end and an output end wherein the optical waveguide transmits light energy from the input to the output end through the core material;
• a plurality of periodic variations in the refractive index carried in the optical waveguide core between the input and the output ends changing the nominal core refractive index to reflect selected wavelengths of light back toward the input end to produce a reference spectrum at the input end of the waveguide having characteristic wavelengths matching the known spectrum of the selected substance;
• and
• a modulator coupled to the optical waveguide to modulate the periodic variations in the core refractive index to selectively and intermittently shift the reflected wavelengths in the reference spectrum of the optical waveguide.