Spectrally multiplexed, high-repetition-rate lidar instrument

We propose a novel, spectrally multiplexed, multi-channel lidar receiver technology with unprecedented spectral resolution and efficiency that will enable multiple laser-based Earth science measurements.  The receiver concept is based on combining two novel ideas – (1) to improve wavelength resolution with an Advanced Coupled-cavity Etalon (ACE) and (2) to use a Recirculating Etalon Receiver (RER) to improve the instrument throughput. This research will mitigate one of the fundamental limitations for lidar architectures – pulse repetition frequency (PRF).

Lidar technology is trending toward photon counting detectors, increased PRF and higher measurement (photon) efficiency.  These trends work well together however, when doing lidar measurements from a satellite, there has been an upper limit on the repetition rate of the laser because of the round-trip time of a laser pulse through the atmosphere.  Due to cloud reflections and atmospheric scatter, only one laser pulse can be in the atmosphere at any given time or there is ambiguity between multiple targets and pulses.  (For the Earth, that limits the repetition rate to about 10 KHz corresponding to a 100 µs round-trip through the atmosphere.  For planetary atmospheres, the round-trip time will change depending on the thickness of the atmosphere but there is still an upper limit on how closely spaced consecutive laser pulses can be.)  Essentially the returned signal from a single pulse through the Earth can spread in extent across 100 µs.  If the pulses are closer together than 100 µs, then they overlap and the pulse returns become indistinguishable.  This limitation on the laser repetition rate then requires high laser pulse energies which can be challenging to achieve – particularly with certain laser architectures like fiber lasers and optical parametric amplifiers (OPAs).  In general higher PRF laser architectures are more efficient and more reliable and therefore more compatible for space-flight requirements.  Higher pulse energies and commensurate higher peak optical powers are often limiting because of nonlinear optical parasitic effects that reduce laser efficiency and performance.  Rather than attack the laser problem directly (which many people are already doing), we propose a system architecture approach that fundamentally simplifies the laser requirements.  So we will develop an instrument architecture that relaxes the laser requirements, makes it simpler to build and offers more freedom in laser architectures.

The higher PRF also specifically addresses a major criticism of trace-gas lidars that are limited to 10 KHz.  In an ideal scenario, the different wavelengths would sample the same column of atmosphere and the same footprint so the only changes in return would be wavelength dependent.  In a 10 KHz lidar in low-earth orbit, the spacing between consecutive laser pulses is ~10 meters.  In a higher PRF instrument the spacing of these pulses is much closer together and it removes a potentially large error source from the measurement by getting much closer to the ideal case of overlapped sampling.