Infrared Receivers/Transmitters for Atmosphere Spectroscopy
Tunable IR Parametric Receivers/Transmitters with Heterodyne Detection for Remote Spectroscopy of the Atmosphere with High Spectral Resolution and Probing Distance
Tech Area / Field
- INF-SIG/Sensors and Signal Processing/Information and Communications
- ENV-APC/Air Pollution and Control/Environment
3 Approved without Funding
Russian Academy of Sciences / Institute of Applied Physics, Russia, N. Novgorod reg., N. Novgorod
- VNIIEF, Russia, N. Novgorod reg., Sarov
- Fast Metrix,Inc/Precision Measurement Systems, USA, AL, Huntsville\nUniversité des Sciences et Techniques de Lille / Laboratoire de Spectroscopie Hertzienne, France, Lille
Project summaryRemote spectroscopy of the atmosphere is used to solve a number of scientific and applied problems, including:
· Investigation of physicochemical processes in the atmosphere related to climate changes and transregional pollution transfer.
· Monitoring of the ecological state of the atmosphere and pollution sources.
The infrared (IR) region is the most informative one for the monitoring of the ecological state of the atmosphere, since in the visible and near-ultraviolet regions some molecular impurities in the atmosphere (for example, HCl, NH3, PH3 and CH4) don’t have strong absorption lines. The IR region is preferable also because here the probing distance depends weakly on the weather conditions, and the scattered radiation in this region is practically safe for human eyes.
However, today’s IR receivers do not allow benefiting from the remote spectroscopy opportunities in this most informative region where characteristic absorption lines of almost all atmospheric impurities are located.
Limited capabilities of the traditional mid IR lidars are mainly related to the high noise level, equivalent to approximately 103 – 104 photons per reception period of 10-8 – 10-6 s, of the usually applied direct photodetection receivers. This significantly limits the probing distance, which usually does not exceed 10-20 km even for lidars with rather powerful lasers (~ 1 J/pulse). Besides, IR lasers can, as a rule, generate only discrete lines, which also limits the opportunities of the remote spectroscopy.
The recent advances in the technology of growing nonlinear optical crystals and also in the development of powerful CW and pulse solid-state lasers allow the creation of efficient parametric transformers of their radiation into the tunable IR radiation (Optical parametric oscillators – OPOs). Using such oscillators, it is possible to design wide-range tunable receivers/transmitters of a new type, namely the parametric receivers/transmitters with heterodyne detection (PRTH). Lidars with PRTH will significantly broaden the opportunities of the atmosphere remote spectroscopy and advance in solving the problems related to its application. For example, the PRTH-based lidars will make it possible to monitor, from the Earth’s surface and space, such molecules as HCl, CH4, NH3, and N2O in the upper layers of the atmosphere (as high as 20-40 km). At present the monitoring of these molecules is possible only from aircrafts.
According to our analysis, the PRTH-based lidars can offer new opportunities, comparing to traditional IR lidars, due to the following characteristics:
- The possibility of smooth tuning over most informative parts of the IR region.
- The noise level of the heterodyne receiver is close to the quantum limit (approximately a photon per reception period of ~ 10-8 s). Consequently, it has high potential in the IR region (~ 1017 - 1019). This would allow, for example, to perform monitoring by scattering at aerosols up to distances of several tens of km with 300-m resolution in light mist at relatively low probing pulse energy (~ 10 - 100 mJ), small diameter of a receiving telescope (~ 10 cm), and high pulse repetition frequency (up to several tens of Hz).
- PRTH lidars would provide the possibility of making, from the Earth’s surface and space, spectroscopic tests of the highest atmospheric layers (up to 20 - 40 km) in the most promising for this purpose transparency windows of 2 - 2.5 m and 3 - 4 m. In these bands, scattering of the probing IR radiation from the highest layers is enough to be recorded by the PRTH lidars, in spite of a significant intensity decrease due to reduced aerosol concentration and large wavelength.
- High spectral resolution (as good as 0.003 cm-1). This, in particular, will provide a substantial increase in the accuracy and reliability of detection of impurities with low concentration (down to units and fractions of ppb).
- In addition, PRTHs are not sensitive to the background light radiation.
To create the PRTH-based lidars, it is necessary to solve a number of tasks. Most important of these tasks are as follows:
- Task 1. Finding the ways of creating an injection-seeded OPO (OPOIS) with frequency shift; development of such OPOIS and creation of an experimental PRTH model based on it. This OPOIS should generate single- or two-frequency pulses with frequencies shifted by intermediate frequencies relative to a heterodyne signal and with energy of tens of mJ.
- Task 2. Working out recommendations to reduce fluctuations of radiation parameters of CW OPOs. Development of a new type of CW OPOs – an optical parametric oscillator based on anisotropic waveguides (OPOAW) – as a source of heterodyne radiation of PRTH; an investigation and optimization of its characteristics.
The accomplishment of these tasks is the main objective of the Project.
The principal executor of the Project is the Institute of Applied Physics of the Russian Academy of Science (IAP RAS). The IAP RAS scientists have strong research experience in the field of theoretical and experimental studies of the processes of parametric conversion, amplification and generation of light in nonlinear crystals, and in the investigation of frequency fluctuations and frequency stabilization in gas and solid-state lasers.
The 1-st co-executor of the Project is the Russian Federal Nuclear Center – Russian Institute of Experimental Physics (RFNC – RIEP). This group participates in the numerical modeling of nonlinear wave processes, in design, engineering, and fabrication of separate units of experimental setups and models to be created; carries out experimental investigations. The members of this group have substantial expertise in numerical modeling of wave processes and are experienced in the design of opto-electrical and opto-mechanical units of high power laser systems.
The Project mainly relates to applied research. In addition to the investigations directed at solving the tasks stated above, it is intended to develop original OPO schemes and, based on these schemes, to create experimental models that are of independent interest and will be used at the further phase of development:
- a frequency-shifted LiNbO3-based OPOIS model pumped by YAG:Nd+3 laser pulses with energy of 0.2-0.5 J. Under injection of a monochromatic signal in the atmosphere transparency window of 3 - 4 m this OPO model will generate single- or two-frequency probing pulses with energy of tens of mJ and coherency necessary for PRTH.
- a PRTH model with a He-Ne laser as a heterodyne for remote spectroscopy of methane at a wavelength of 3.39 m with the potential close to the limit one (up to 1017- 1018) and the spectral resolution of about 0.003 cm-1.
- a model of a CW OPO based on planar waveguides made of AgGaS2 crystal.
On the basis of the investigations and experimental models developed under the Project, we plan to create a prototype of a PRTH lidar based on the differential absorption method (DIAL system) for high-resolution remote spectroscopic monitoring of the atmosphere in the 3 - 4 m spectral range with the probing distance up to several tens of km.
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