Sensor for measuring a flow of a fluid

A sensor for measuring a flow of a fluid in a physiological environment, the sensor comprising:

• an interrogation light guide extending from a proximal end to a distal end of the sensor, the interrogation light guide being configured to transmit interrogation light to, and receive reflected interrogation light from, the distal end of the sensor;
• an excitation light guide configured to transmit excitation light to a first position of the sensor, wherein the excitation light is provided for heating the fluid;
• and
• a sensing element located at the distal end of the sensor, the sensing element comprising at least two etalons for reflecting interrogation light back along the interrogation light guide towards the proximal end of the sensor, each etalon having a respective optical path length and at least one of the etalons having at least one reflective surface external to the interrogation light guide;
• wherein the sensing element is configured to be in thermal contact with the fluid such that the optical path length of at least one of the etalons is dependent on a temperature of the fluid, and wherein the reflected interrogation light forms an interferogram which is dependent on the optical path lengths of the respective etalons;
• wherein at least one of the etalons includes a gas cavity, where the sensor further comprises a capillary tube with a membrane closing the distal end of the sensor to form the gas cavity within the capillary tube between the membrane and the distal end of the interrogation light guide.

A sensor for measuring a flow of a fluid in a physiological environment, the sensor comprising:

• an interrogation light guide extending from a proximal end to a distal end of the sensor, the interrogation light guide being configured to transmit interrogation light to, and receive reflected interrogation light from, the distal end of the sensor;
• an excitation light guide configured to transmit excitation light to a first position of the sensor, wherein the excitation light is provided for heating the fluid;
• and
• a sensing element located at the distal end of the sensor, the sensing element comprising at least two etalons for reflecting interrogation light back along the interrogation light guide towards the proximal end of the sensor, each etalon having a respective optical path length and at least one of the etalons having at least one reflective surface external to the interrogation light guide;
• wherein the sensing element is configured to be in thermal contact with the fluid such that the optical path length of at least one of the etalons is dependent on a temperature of the fluid, and wherein the reflected interrogation light forms an interferogram which is dependent on the optical path lengths of the respective etalons;
• wherein one etalon comprises a dichroic dielectric coating that provides a low finesse in a first wavelength range corresponding to the interrogation light and provides a high finesse in a second wavelength range for acting as a hydrophone to detect ultrasound waves incident from the fluid onto the sensor.