Optical methods in earth sciences

Mankind progress was started by systematic observations of Earth and celestial rhythms. As soon as the technology of the first modern optical instruments became available, they were applied to astronomical and geodetical measurements. Modern science itself originates from the Copernican revolution based on new observations and interpretation of Earth and celestial bodies motions. The revolutionary telescope of Galileo Galilei represented the first 'scientific' instrument in the modern sense of the word, aimed to test a previous theory and eventually to put the foundations for a new one. Galileo's optical instruments put the basis of geography and cartography, solving problems like the determination of longitude and the detection of the cause of gravity variations with latitude. The advent of the laser started a new era in optical science, allowing the development of new methodologies and instruments based on coherent radiation. Despite the strong link between optics and Earth and planetary sciences of past times, laser applications in modern Earth Sciences have been rather limited. They have been mainly restricted to the field of geodesy, with laser telemetry for electronic distance measurements (EDM), some laser strain-meters, and interferometric absolute gravimeters. Some sporadic applications of optical techniques have also involved seismic data processing and filtering, now completely overcome by computer analysis. In very recent times, the use of InSAR interferometry for differential geodetic measurements has become widespread, and laser interferometry started to be commonly used. The increasing demand for telecommunication technologies, in the last few decades, has been sustained by the tremendous progress of optoelectronics, fibre optics and integrated optics. New perspectives of application in many different fields have been thus opened. The wide availability of low-cost, compact optical devices, allows to design new instruments for environmental applications. In this context, a crucial role has been played by diode lasers with output wavelengths from 760 to 2000 nm. These lasers have generally a distributed feedback (DFB) design enabling for single mode emissions with output power of milliwatts. They are currently used as sources in laser-based spectrometers to monitor trace-gas concentrations. Their characteristics of room-temperature operation and compatibility with telecommunication-grade optical components are being exploited to develop portable instrumentation for several applications, including environmental monitoring, factory-process control, bio-medical diagnostics. However, near-infrared diode lasers probe overtone or combination vibrational bands of simple molecules, which are orders of magnitude weaker than fundamental transitions occurring in the mid-infrared. Laser absorption spectroscopy would gain in sensitivity, precision and accuracy if the laser wavelength moves to the mid-infrared. This is one of the main reasons of interest towards the generation of coherent and tunable radiation with longer wavelength. In recent years, difference frequency generation in periodically poled crystals has emerged as a convenient technique to produce laser radiation between 3 and 5 ?m. At the same time, a great innovation in photonics dealt with a different way of producing coherent radiation in semiconductor heterostructures that led to the invention of quantum-cascade lasers. Despite their CW operation at cryogenic temperatures, these lasers might open interesting perspectives in sensing applications because of their excellent performances in the mid-infrared, in terms of spectral purity and emitted power. That may give rise to a new generation of portable optical sensors. Apart from these very recent developments