Injection-seeded optical parametric oscillator at 1645 nm for space-borne remote sensing of CH4

Abstract

Space-borne integrated path differential absorption (IPDA) lidar for global observation of methane (CH4) requires a tunable single-longitudinal mode (SLM) pulsed laser source at 1645 nm, which coincides with appropriate absorption line of CH4 molecules. To meet this application, a pulsed injection-seeded optical parametric oscillator (OPO) using potassium titanyle arsenate (KTA) as the nonlinear crystal is developed. The OPO set-up is a four-mirror stable ring cavity with two pieces of 15-mm-long KTA crystal in critical phase-matching cut for wavelengths around 1645 nm. A single frequency Nd:YAG master oscillator power amplifier (MOPA) laser at 1064 nm serves as the pump. A distributed feedback (DFB) fiber laser with a linewidth of 3 MHz is used for injection of the OPO. To insure successful injection seeding process and enough frequency stability, a cavity-length control method based on the optical heterodyne technique is applied on the OPO cavity. Root-mean-square (RMS) of the frequency variation of the signal pulse compared to the seed laser is measured to be 9.9 MHz, and the Allan deviation is less than 0.25 MHz for averaging time of more than 10 s. With 11 mJ pump pulse input at 50 Hz repetition rate, a signal pulse energy of 1.8 mJ is obtained. The pulse width of this OPO is 15 ns and corresponding linewidth is 45 MHz.