Apparatus and method based on cavity ring-down spectroscopy

A cavity ring-down sensor comprising:

• a ring-down optical cavity for sensing a substance modifying a ring-down characteristic of the cavity;
• a continuous wave light source for exciting said cavity;
• and
• a detector for monitoring said ring-down characteristic;
• and
• wherein said light source has a power and bandwidth sufficient to couple energy into at least two modes of oscillation of said cavity to overcome losses within the cavity and excite said two modes of oscillation.

An evanescent wave cavity-based optical sensor, the sensor comprising:

• an optical cavity formed by a pair of highly reflective surfaces such that light within said cavity makes a plurality of passes between said surfaces, an optical path between said surfaces including a reflection from a totally internally reflecting (TIR) surface, said reflection from said TIR surface generating an evanescent wave to provide a sensing function;
• a light source to inject light into said cavity;
• and
• a detector to detect a light level within said cavity;
• whereby absorption of said evanescent wave is detectable using said detector to provide said sensing function;
• wherein said light source comprises a continuous wave light source;
• and
• wherein said light source has a power and bandwidth sufficient to couple energy into at least two modes of oscillation of said cavity to overcome losses within the cavity and excite said two modes of oscillation.

An optical cavity-based sensing device comprising:

• an optical cavity absorption sensor comprising an optical cavity formed by a pair of reflecting surfaces;
• a light source for providing light to couple into said cavity;
• and
• a light detector for detecting a level of light escaping from said cavity;
• said cavity being configured such that light within said cavity makes at least ten absorption sensing passes through said cavity before decaying to a half intensity value;
• and wherein
• said light source is operable as a substantially continuous source and has a bandwidth sufficient to provide at least a half maximum power output across a range of frequencies equal to a free spectral range of said cavity.

A cavity ring-down sensor comprising:

• a ring-down optical cavity for sensing a substance modifying a ring-down characteristic of the cavity;
• a light source for exciting said cavity;
• and
• a detector for monitoring said ring-down characteristic;
• and
• wherein said cavity comprises a fibre optic sensor including a fibre optic cable configured to provide access to an evanescent field of light guided within the cable for said sensing.