Increased Modal Overlap for Improved Sensitivity in a Monolithic Intracavity Chemical Sensor

We discuss the design and fabrication of an InP-based single-chip chemical sensor with increased modal overlap with a chemical analyte. The fabricated devices use a sensor design with frequency tunable lasers and heterodyne spectrometers. By reducing the waveguide ridge width in one section of the laser, the transverse modal overlap with the analyte increases by 17 times, increasing the sensitivity by the same ratio. A frequency shift of 72 GHz/refractive index unit was measured with temperature effects removed. The frequency stability of this sensor is as low as 436 kHz leading to a minimum detectable index difference of 6 times 10^-6.