Monolithically Integrated Active-Cavity “Laser-as-a-Sensor”: Active Intracavity Enhancement for Oxygen Detection with a Subnanoliter Sampling Volume

Tunable diode laser absorption spectroscopy (TDLAS) is widely used in O2 gas sensing; however, conventional schemes rely on meter-scale optical path lengths and hundred-milliliter absorption cells, making it difficult to meet the demands of chip-scale integration and distributed sensing. This paper proposes and demonstrates a 760 nm microelectromechanical (MEMS) vertical-cavity surface-emitting laser (VCSEL) gas sensor with a monolithically integrated active cavity. The active cavity and the sensing function are monolithically integrated; the active cavity simultaneously serves as the gas absorption cell, integrating wavelength tuning, gain compensation, and gas absorption into the same active microcavity. Continuous single-mode tuning is achieved in the 760–764 nm range (side-mode suppression ratio >30 dB). The intracavity amplified absorption (IAA) effect enables a sampling volume of only subnanoliter, which is 7–8 orders of magnitude smaller than conventional hundred-milliliter gas cells. Based on the nonlinear modulation response of threshold output power to intracavity absorption, an equivalent optical path enhancement factor of 1.3 × 105 is achieved, with a limit of detection reaching 37 ppm. This “laser-as-a-sensor” architecture offers a compact route for chip-scale distributed gas sensing networks.