A frequency stabilization unit was developed to stabilize high power laser diodes for differential absorption lidar. Three distributed-feedback (DFB) lasers were used in this approach. To achieve the required frequency stability, two opto-electrical feedback loops for on- and off-line channels were coupled to the third opto-electrical loop for CO2 locking. A master DFB laser is locked to the selected CO2 absorption line using a single pass reference cell and a custom feedback loop based on a commercially available equipment (LaseLock 4 Channel). The master DFB stabilization range is around 18 kHz at the interval of 23 s, measured with a frequency comb. The light emitted from the master laser is then fed into the on- and off-line frequency locking loops (phase-lock loops) and it is used to stabilize the beat note of the on-line and off-line channels with respect to the master laser frequency. The offsets read 350 MHz with tunable range of 150 MHz and fixed 10 GHz for ON- and OFF-line signals, respectively.
Sounding measurements with high spatial resolution (better than 10 km horizontal and 1 km vertical) and repeat cycles better than an hour over the full Earth disk will enhance the ability of National Meteorological Services to initialise models of observations of temperature, moisture and winds.
To meet those needs, trade-off’s were performed during the Meteosat Third Generation (MTG) mission study (2003-2005) where preliminary instrument concepts for the Infra-Red Sounding (IRS) mission were investigated allowing at the same time to consolidate the technical requirements for the overall system study. The trade-off’s demonstrated that two types of instrument could fulfill the requirements: a Fourier Transform Spectrometer and a Dispersive Spectrometer.
This paper aims at comparing these two MTG-IRS sensor concepts by highlighting the differences in the constraints imposed on the characteristics and required performance at hardware level. In addition, technology criticalities and some other aspects are discussed qualitatively.
The accurate determination of the atmospheric distribution of carbon dioxide (CO2) on planetary scale is a key requirement for setting up modeling tools able to make reliable predictions of Earth climate dynamics which are essential for the understanding of such important issues as climate change and global warming. Nowadays, the concentrations of CO2 are mainly measured in-situ at a number of surface stations that are unevenly distributed over the planet. Air-borne and space-borne missions have the potential to provide a denser and better distributed set of observations to complement those provided by the surface network.
For a prediction of the rate of climate change during the 21st century, there is an urgent need to better understand the global carbon cycle, in particular the processes that control the carbon flows between the various reservoirs, and their interactions with the climate system. Atmospheric carbon dioxide (CO2) represents the main atmospheric phase of this biogeochemical cycle. Due to human activities, the concentration of this most important of the Earth’s greenhouse gases has grown from a pre-industrial average atmospheric mole fraction of about 280 parts per million volume (ppm) to 390.5 ppm in 2011 which is an increase of 40%. CO2 contributes to ~63% to the overall global radiative forcing.
Carbon dioxide (CO2) is the major anthropogenic greenhouse gas contributing to global warming and climate change. Its concentration has recently reached the 400-ppm mark, representing a more than 40 % increase with respect to its level prior to the industrial revolution.
We propose an integrated path differential absorption lidar system based on all-semiconductor laser sources and single photon counting detection for column-averaged measurements of atmospheric CO2. The Random Modulated Continuous Wave (RM-CW) approach has been selected as the best suited to semiconductor lasers. In a RM-CW lidar, a pseudo random sequence is sent to the atmosphere and the received signal reflected from the target is correlated with the original sequence in order to retrieve the path length. The transmitter design is based on two monolithic Master Oscillator Power Amplifiers (MOPAs), providing the on-line and off-line wavelengths close to the selected absorption line around 1.57 µm. Each MOPA consists of a frequency stabilized distributed feedback master oscillator, a bent modulator section, and a tapered amplifier. This design allows the emitters to deliver high power and high quality laser beams with good spectral properties. An output power above 400 mW with a SMSR higher than 45 dB and modulation capability have been demonstrated. On the side of the receiver, our theoretical and experimental results indicate that the major noise contribution comes from the ambient light and detector noise. For this reason narrow band optical filters are required in the envisioned space-borne applications. In this contribution, we present the latest progresses regarding the design, modeling and characterization of the transmitter, the receiver, the frequency stabilization unit and the complete system.
The availability of suitable laser sources is one of the main challenges in future space missions for accurate measurement
of atmospheric CO2. The main objective of the European project BRITESPACE is to demonstrate the feasibility of an
all-semiconductor laser source to be used as a space-borne laser transmitter in an Integrated Path Differential Absorption
(IPDA) lidar system. We present here the proposed transmitter and system architectures, the initial device design and the
results of the simulations performed in order to estimate the source requirements in terms of power, beam quality, and
spectral properties to achieve the required measurement accuracy. The laser transmitter is based on two InGaAsP/InP
monolithic Master Oscillator Power Amplifiers (MOPAs), providing the ON and OFF wavelengths close to the selected
absorption line around 1.57 μm. Each MOPA consists of a frequency stabilized Distributed Feedback (DFB) master
oscillator, a modulator section, and a tapered semiconductor amplifier optimized to maximize the optical output power.
The design of the space-compliant laser module includes the beam forming optics and the thermoelectric coolers. The
proposed system replaces the conventional pulsed source with a modulated continuous wave source using the Random
Modulation-Continuous Wave (RM-CW) approach, allowing the designed semiconductor MOPA to be applicable in
such applications. The system requirements for obtaining a CO2 retrieval accuracy of 1 ppmv and a spatial resolution of less than 10 meters have been defined. Envelope estimated of the returns indicate that the average power needed is of a
few watts and that the main noise source is the ambient noise.
For the CO2 and CH4 IPDA lidar CHARM-F two single frequency Nd:YAG based MOPA systems were developed. Both lasers are used for OPO/OPA-pumping in order to generate laser radiation at 1645 nm for CH4 detection and 1572 nm for CO2 detection. By the use of a Q-switched, injection seeded and actively length-stabilized oscillator and a one-stage INNOSLAB amplifier about 85 mJ pulse energy could be generated for the CH4 system. For the CO2 system the energy was boosted in second INNOSLAB-stage to about 150 mJ. Both lasers emit laser pulses of about 30 ns pulse duration at a repetition rate of 100 Hz.
Carbon dioxide (CO2) and methane (CH4) are the most important of the greenhouse gases that are directly influenced by
human activities. The Integrated Path Differential Absorption (IPDA) lidar technique using hard target reflection in the
near IR (1.57μm and 1.64μm) to measure the column-averaged dry air mixing ratio of CO2 and CH4 with high precision
and low bias has the potential to deliver measurements from space and air that are needed to understand the sources and
sinks of these greenhouse gases. CO2 and CH4 IPDA require tunable laser sources at 1.57 μm and 1.64 μm that coincide
with appropriate absorption lines of these species having high pulse energy and average power as well as excellent
spectral and spatial properties.
Within this study we have realized more than 50mJ of pulse energy in the near IR coincident with appropriate absorption
lines using an injection-seeded optical parametric oscillator-amplifier system pumped at 100 Hz. At the same time this
device showed excellent spectral and spatial properties. Bandwidths of less than 100 MHz with a high degree of spectral
purity (> 99.9 %) have been achieved. The frequency stability was likewise excellent. The M2-factor was better than 2.3.
Owing to these outstanding properties optical parametric devices are currently under investigation for the CH4 lidar
instrument on the projected French-German climate satellite MERLIN. A similar device is under development at DLR
for the lidar demonstrator CHARM-F which will enable the simultaneous measurement of CO2 and CH4 from an
airborne platform.
Increasing user and service needs for Numerical Weather Prediction impose an Infrared Sounding (IRS) mission on Meteosat Third Generation platforms. A major objective of the IRS mission is to provide a vertical sounding capacity in the vicinity of clouds, above clouds and below semi-transparent clouds. Thus, cloud contamination has to be severely controlled at instrument or at system level. Pseudo-noise is defined as the measurement error generated by scene heterogeneity. Spectrally correlated, it affects the information content of the vertical sounding and thus the quality of high-level products. An IRS pseudo-noise estimation model has been developed in view of supporting the consolidation of observational requirements during the detailed instrument design and system specification process. Geophysical inputs to the model are a spatial atmosphere distribution and the corresponding high-resolution radiance spectra. Major instrument inputs are a PSF model and an ISRF model. The paper presents a mathematical specification of pseudo-noise components and their quantification upon current design assumptions for the two considered instrument concepts: dispersive and Fourier transform spectrometer. The pseudo-noise budget is established for meaningful geophysical scenarios, and the associated observational requirements are derived and discussed, providing useful orders of magnitude for further iteration of the trade-off between instrument specifications and system requirements.
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