A Guide to Laser History

Lasers, an essential invention of the 20th century, are used in many modern technologies, from CD players to fibre-optic networks. The term LASER stands for Light Amplification by Stimulated Emission of Radiation, a concept first predicted by Albert Einstein. The first working laser was built in 1960 by Theodore Maiman using a ruby. It followed the earlier development of the maser (Microwave Amplification by Stimulated Emission of Radiation), which operates similarly but uses longer-wavelength photons outside the visible or infrared spectrum.

How a Laser Works

A laser produces visible or infrared light through stimulated emission, which builds on two key concepts: absorption, where an atom absorbs energy, and emission, where an excited atom releases energy as a photon. Spontaneous emission happens naturally, while stimulated emission occurs when an excited atom is triggered by an incoming photon to emit two identical photons.

In a basic laser, energy is added to a material (solid, liquid, or gas) inside a mirrored cavity, causing a population inversion- more excited atoms than grounded ones. This leads to stimulated emission. The photons bounce between mirrors, building energy, and some escape through a partially transparent mirror as a coherent laser beam.

Laser Communications History

Laser communications began with early experiments by NASA and the US Air Force, including one where Morse code was transmitted by manually chopping a laser beam. The first laser communication patents appeared in the 1960s, and from then through the 1980s, various defence organisations developed and refined the technology. Over the past 40 years, most of the core engineering work behind modern laser communications was driven by aerospace and defence needs. Applications tested included ground-to-aircraft, ground-to-satellite, satellite-to-satellite, and even satellite-to-submarine links. This military foundation paved the way for today’s commercial optical wireless systems.

Laser Safety and Classifications

Laser communication systems can be designed to be eye-safe, following international safety standards set by the IEC (adopted by most countries) and regulated in the US by the FDA’s CDRH. A laser deemed completely safe to view with the naked eye is classified as IEC Class 1M.

Eye safety depends heavily on wavelength. Most commercial systems operate around 800 nm or 1550 nm:

• 800 nm lasers (near-infrared) are invisible but focus directly on the retina, posing a serious eye hazard due to concentrated light intensity- potentially causing permanent damage without warning.

• 1550 nm lasers are absorbed by the cornea and lens, not reaching the retina, making them much safer. These can legally transmit at 50x higher power, enabling longer range and higher data rates.

Historically, 780–850 nm lasers were used due to lower cost and availability (e.g. from CD technology). However, for high-performance, eye-safe, and scalable systems, 1550 nm is preferred- especially since it’s aligned with commercial fibre-optic network wavelengths and supports better operation under conditions like fog.

Terrestrial Laser Communications Challenges

Fog

Fog significantly attenuates both visible and near-infrared light, affecting laser communications similarly to how rain affects RF wireless. However, fog is not a deal-breaker for optical wireless systems, as links can be engineered to maintain sufficient power even during heavy fog most of the time. Hybrid systems that combine laser (FSO) and RF technologies can be used to further increase availability and reliability.

Physical Obstructions

Laser communications systems that employ multiple, spatially diverse transmitters and large receive optics will eliminate interference concerns from objects such as birds.

Pointing Stability

Pointing stability in commercial laser communications systems is achieved by one of two methods. The simpler, less costly method is to widen the beam divergence so that if either end of the link moves the receiver will still be within the beam. The second method is to employ a beam tracking system. While more costly, such systems allow for a tighter beam to be transmitted allowing for higher security and longer distance transmissions.

Scintillation

Performance of many laser communications systems is adversely affected by scintillation on bright sunny days. Through a large aperture receiver, widely spaced transmitters, finely tuned receive filtering, and automatic gain control, downtime due to scintillation can be avoided.

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