UC Santa Barbara

With the acceleration of global climate change caused by accumulation of greenhouse gases in the atmosphere, many adverse effects on the environment have been observed, including shrinking glaciers, early breaking up of ice in rivers and lakes, premature flowering of trees, and shifting plant and animal habitats. In recent years, more focus has been placed on accurate monitoring of atmospheric greenhouse gasses for informing climate modelling and policy. In 2009, NASA launched the Orbiting Carbon Observatory (OCO) satellite mission to remotely measure carbon dioxide (CO2) levels from low earth orbit using passive sensing technology. Around the same time, development of an active remote sensing system began under the Active Sensing of CO2 Emissions over Nights, Days, & Seasons (ASCENDS) mission. A complex instrument built from individually packaged components, the active sensing system is large, heavy and power hungry. To reduce the size, weight, and power (SWaP) of the sensor, an equivalent system using photonic integrated circuit (PIC) technology is a promising solution for the future of remote sensing. Indium phosphide (InP) PIC technology is especially interesting because it offers the capability to monolithically integrate high quality lasers along with modulators and passive waveguides.

In this work, InP PICs employing an integrated path differential absorption (IPDA) lidar topology were designed for CO2 active remote sensing. The fabricated PIC is about 1 × 10 mm in size and integrates two lasers, a phase modulator, a pulse generator, a photodiode, and several splitters and optical amplifiers. The overall sensing system consists of two parts: stabilization of a leader laser at a reference wavelength and offset locking of a follower laser to the leader laser. The leader laser stabilization is achieved using a frequency modulation technique, where the leader laser is locked to a CO2 reference cell by modulating the phase of the laser output signal at 125 MHz. The follower laser offset locking is accomplished using an optical phase lock loop, where an integrated photodiode detects the beat note between the leader and follower lasers. Compared to unlocked measurements, the leader laser stability improved by more than 20 dB for a two hour time period. The locked follower laser saw a stability improvement of more than 45 dB compared to unlocked. In addition, CO2 active sensing was successfully demonstrated in the lab using a continuous wave signal and a pulsed signal. For the continuous wave sampling, an 8.92 dBm fiber-coupled output power was measured. For the case of pulsed sampling, the output pulses had an extinction ratio greater than 45 dB. For both sampling conditions, the PIC successfully scanned over a 20 GHz range centered at 1572.335 nm and recovered the CO2 absorption spectrum.

In sum, we have demonstrated the feasibility of using InP PIC technology for CO2 active remote sensing. In future, efforts will be made towards the PIC packaging and photonic-electronic integration to further reduce the SWaP of the overall system.