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Residual light amplifiers and lasers

The graphic shows a thermal imaging device

This third article from the series of the Cluster Optronics in the specialist area Command and Control Systems aims to introduce readers to the world of residual light amplifiers and laser systems.

Two military personnel are equipped with a residual light amplifier and a laser light module in a stairwell.
Residual light amplifier (RLV 19) and laser light module (LLM 19) in house-to-house fighting

Residual light amplifiers

Using residual light amplifiers, reconnaissance, intelligence, surveillance and watch orders are possible at night, without white light (such as utility lights or headlights) being used and thus revealing one’s own location. The Armed Forces assigns the new residual light amplifiers to the infantry, intelligence and mechanised infantry platoons. The new residual light amplifiers are attached to the helmet or can be placed on the head using a support device.

Laser light module

Laser light modules enable target acquisition at night, in conjunction with a residual light amplifier. They are mounted on an assault rifle or a light machine gun. In addition to an invisible infra-red laser and an infra-red battlefield illuminator, they also contain a visible target laser for use during the day as well as a powerful white flashlight. The infra-red laser is only used in combination with a residual light amplifier, while the visible laser, on the other hand, can also be used during the day and without residual light amplifier. 


Theory

Residual light amplifiers

There is no complete darkness in the wild. The light from the moon and stars, scattered light from distant residential areas or vehicles is always present, even if the sky is overcast. An illuminance of around 0.25 lux can be measured on a starry night with a full moon.  A cloudy night sky without a moon and extraneous light has around 0.00013 lux. The few available photons can now be multiplied so many times that a person can perceive a clear image.

: A moon is shown to the left, and the light from the moon shines on a deer in grey. The residual light from the deer, consisting of photons, is focused in a lens on a photocathode. The released photoelectrons pass through the micro-channel plate. The multiplied photoelectrons emerge as secondary electrons from the micro-channel plate and gather on a phosphor screen. On the phosphor screen, an electron emits one photon at a time. The photons are transferred via a fibre optic. The compact fibre optic consisting of light guides corrects the upside down image and projects it onto the outlet aperture or a flat image sensor (such as CCD). The deer can now be clearly recognised.
Functional principle of a (passive) night vision device

The residual light is focused in a lens on a photocathode. The incident photons release electrons from the cathode material through the photoelectric effect. A micro-channel plate under high voltage attracts the electrons, accelerates them and multiplies their number. The electron shower hits a phosphor screen located at the output electrode of the micro-channel plate, whereupon each electron now releases one photon at a time: An upside down and reversed bright image of the recorded scene becomes visible on the phosphor screen. A compact fibre optic consisting of light guides corrects the upside down image and projects it onto the outlet aperture or a flat image sensor (such as CCD).

Laser

A LASER, an acronym of the English Light Amplification by Stimulated Emission of Radiation, is a light source characterised by four main aspects. The first aspect is its monochromatic character – a laser emits in a narrow wavelength range. The second is its coherence – the phases of its photons are correlated in space and time. The third is its direction – the beam travels in one direction. And finally, the fourth is its high brightness level. The radiance – or power emitted for each unit of surface area – is extremely important.

All these characteristics mean that this type of light is not found in nature and that a laser will maintain a very concentrated beam over long distances, thanks to which all of its applications are possible.

The beam is generated in an optical cavity (or resonator) in which the atoms are excited in order to store energy. This input of energy is called pumping. The external source of energy may be a flash of light, an electrical discharge or even another laser. The stimulated emission – which is what we are trying to achieve – is obtained when an excited atom (in the state E2) returns to the level of E1 as a result of the shock with a photon.  A photon is emitted which is exactly the same as the first (same wavelength, same phase).

To create this particular beam, the active medium of the laser, in other words, where the generation and amplification of the light takes place, can be a gas, a liquid, a solid or a semiconductor. It is the nature of the medium which will define the wavelength of the beam and therefore its colour. The active medium is composed of a very pure material, origin of the monochromatic character of the emitted laser.

An optical resonator can be seen in the image. The resonator consists of a mirror with 100% reflectivity on the left and a semi-reflecting mirror on the right. The semi-reflecting mirror ensures that a part of the radiation can leave the device as a laser beam.
Principle of operation of a laser.

Below, a flash of light strikes the resonator. The flash is an application of energy and excites the atoms in the resonator. This input of energy is called pumping.

The energy states E1 and E2 from one atom can be seen above.

The stimulated emission by the flash excites the atom and changes it from the state E1 to E2. After the stimulated emission, the atom returns from the state E2 to E1 and emits a photon which is exactly the same as the first (same wavelength, same phase).