Tunable light source cavity detection using a plurality of axial-plus-transverse modes

A system for detecting a substance, comprising:

• a tunable light source wherein the tunable light source generates a beam of electro-magnetic radiation, wherein a wavelength of the beam of electro-magnetic radiation is tuned to operate at a plurality of wavelengths;
• an optical cavity, wherein the optical cavity receives the beam of electro-magnetic radiation, wherein physical characteristics of the optical cavity define a plurality of allowed axial-plus-transverse electro-magnetic radiation modes, wherein only a subset of the plurality of allowed axial-plus-transverse electro-magnetic radiation modes are excited when the optical cavity receives the beam of electro-magnetic radiation;
• a normalization detector, wherein the normalization detector is operative to sense electro-magnetic radiation entering the optical cavity;
• a processor operating to:
• receive information relating to the sensed electro-magnetic radiation within or entering the optical cavity;
• detect the substance at least one of within or outside the optical cavity based on at least one of an intensity, an amplitude, a phase, or an amplitude and phase of the sensed electro-magnetic radiation emanating from at least one of within or entering the optical cavity at one or more of the plurality of wavelengths while the optical cavity receives the beam of electro-magnetic radiation;
• and
• account for effects of electro-magnetic radiation changes caused by changes in intensity, amplitude, phase, or amplitude and phase of the electro-magnetic radiation entering the cavity comprising measuring intensity, amplitude, phase, or amplitude and phase of the sensed electro-magnetic radiation entering the optical cavity.