Frequency comb based absolute frequency reference design for future spaceborne multi species differential absorption lidar systems for green-house gases monitoring

Abstract

The Lidar Emitter and Multi-species greenhouse gases Observation iNstrument (LEMON) is a novel Differential Absorption Lidar (DIAL) sensor concept for greenhouse gases and water vapor measurements from space. 1,2 It is based on a versatile transmitter allowing for addressing various absorption lines of different molecules. This highly flexible emitter design requires a universal frequency referencing scheme. Here we present a concept employing a 1 GHz frequency comb, which allows the absolute referencing over a spectral range from 0.95 μm to 1.15 μm. By using an intermediate frequency doubling stage, this allows for DIAL measurements on CO2, H2O/HDO, and CH4 in the 2 μm range. Absolute referencing is obtained by using a GPS disciplined oscillator as the common time base for frequency measurements. The concept of the LEMON Frequency Reference UnIT (FRUIT) is designed to match the requirements of the vibration loads associated with airborne operation to allow implementation on the airborne demonstrator for LEMON. In addition, the requirements for a future space development are considered in the design. For example, radiation critical items have been identified and radiation tested within the project and a compact wavemeter design has been implemented.

Development and test of a broadband absolute frequency reference with sub-MHz precision based on a GHz mode locked laser source

“…The outline of the LEMON frequency reference unit is shown in the next figure and described in detail in a reference. 5 Figure 2. Overall outline of the LEMON frequency reference unit (FRUIT) with the main interfaces to the other instrument subunits.…”

“…Details about the electronic stabilization scheme are described in a reference. 5 A pulsed 'micro-laser' with about 15 ns pulses and a 4 kHz repetition rate has been used for testing the beating implementation during integration of the frequency reference unit (FRUIT). One of the single shot beating traces is presented in figure 4.…”

“…Focusing on the most critical issues with analysis and tests (radiation and vibration), the cornerstone for the space development of a multi species DIAL shall be set and lead to a defined roadmap. Apart from radiation testing of components, 5 the development of a space compatible design outline of the FRUIT is expected. The wavemeter, which is still part of the FRUIT as a coarse calibration tool to resolve the 1 GHz comb ambiguity, has a space compatible prototype that has been developed for the MERLIN FRU.…”

“…In order to stick closely to the design of MELRIN and keep the design compact, a two-dimensional approach was chosen, where the two Fizeaus deflect the laser in orthogonal directions. 5 Currently, we focus on the space development of the frequency comb. Within the space development work package, the technology of photonic integrated circuits (PIC) has been evaluated.…”

GHz Figure‐9 Er‐Doped Optical Frequency Comb Based on Nested Fiber Ring Resonators

“…Pulsed LIDAR systems often use nanosecond modulation, requiring combs with spacing larger than GHz to ensure that each LIDAR pulse can overlap with one of the OFC pulses in the time domain. [22,23] Nevertheless, excessively high repetition rates also reduce spectral sampling resolution. Therefore, OFCs with repetition rates near GHz offer an optimal choice for these applications.…”