An environmental sensor
The laser is not only a valuable tool for exploring the unknown at very small scales. In environmental research and monitoring, the laser is an important tool when it comes to understanding relationships in nature, or revealing environmental contamination, on a larger scale. Remote sensing instruments that analyze their surroundings using a laser are known as LIDAR instruments. LIDAR stands for: Light Detection and Ranging. Geoscientists use LIDAR to explore the composition of the atmosphere or the ocean. It may be that the laser is positioned on an aircraft or a satellite. The principle is simple: a laser transmits light pulses. These pulses partially bounce back, either off a body of water or the atmosphere. This light is reflected by molecules or particles. From that, researchers can draw conclusions about the composition of the examined body.
Using LIDAR, the fluorescent properties of many materials can be exploited. Fluorescent material absorbs the laser light, then sends out light itself, which usually consists of a different wavelength than the original radiation. The dye Chlorophyll a, for instance, is found in algae. When irradiated with a laser, it emits light in the red spectral range. Similarly, using fluorescence, one can detect humic substances – water-soluble organic substances in soil. These arise from plant matter rotting on land, which is then transported into the sea by rivers. Humic substances absorb light in the ultraviolet and blue range, and emit light that extends over the entire visible range.
Stars on earth
The most powerful laser in the world is located near San Francisco, at the National Ignition Facility (NIF). The laser is housed in a building about the size of three football pitches. With this laser system, researchers generate higher temperatures than those found at the center of stars. The researchers use these to studying the energy of the sun, in order to work towards one day using this energy to produce electricity.
In stars, like our sun, nuclear fusion is occurring. Hydrogen is fused into helium, releasing energy. This energy causes the star to burn and emit light. Mankind wants to use this system to produce energy on earth. The objective of this fusion research is to fuse the hydrogen isotopes deuterium and tritium. Here, a helium nucleus, a neutron, and also large amounts of usable energy are created: one gram of this fuel in a power plant could generate 90,000 kilowatt hours of energy – equivalent to that produced from the combustion of eleven tons of coal.
But under terrestrial conditions, there is a problem with this nuclear fusion: so that the reaction gets underway and keeps running, it needs extreme conditions, enormous pressure and hot temperatures. These exist only in the sun. But we can recreate these technical conditions on earth. The NIF laser helps ignite the fire. NIF physicists position a wheat grain-sized bead of frozen hydrogen into a small gold cylinder. This is simultaneously irradiated by 192 laser cannons, with a total capacity of about 500 terawatt – corresponding to five trillion 100-watt light bulbs. The average level of the electricity consumption of earth is approximately 30 times lower than this. However, the flash of light lasts only a billionth of a second. It heats the hydrogen atoms to about 100 million degrees Celsius. That's enough to ignite nuclear fusion, because at these temperatures, hydrogen atoms overcome their repelling forces, fuse to form helium, and the excess mass is transformed into energy. But to keep this nuclear fusion going – and to gain energy from it – has not been possible to date.