Photo-acoustic gas sensor module having light emitter and detector units

A photo-acoustic gas sensor, comprising:

• a light emitter unit comprising a stationary light emitter configured to emit a beam of light pulses with a predetermined repetition frequency and a wavelength corresponding to an absorption band of a gas to be sensed;
• and
• a single detector unit comprising a microphone,
• wherein the light emitter unit is arranged so that the beam of light pulses traverses an area configured to accommodate the gas and the single detector unit is arranged so that the microphone can receive a signal oscillating with the repetition frequency,
• wherein the detector unit is disposed on a first substrate within a hermetically sealed chamber, wherein the hermetically sealed chamber comprises a light entrance window opposite from the first substrate and a reference gas within the hermetically sealed chamber, and
• wherein the light emitter unit is disposed on a second substrate that is outside of the hermetically sealed chamber and is arranged face-to-face with the first substrate.