Helium–neon laser

A helium–neon laser, or He–Ne laser, is a specific type of gas laser. It uses a mixture of helium and neon gases to create a steady beam of light.

This device is highly valued for its high spectral purity. This means the light it produces is very concentrated in a narrow range of colors. Most people recognize these lasers by their bright red light. This light comes from a specific wavelength in the red part of the visible spectrum. Because they produce such high-quality light, these lasers are vital tools in scientific research and industrial manufacturing.

The mechanism of the laser begins with a gas mixture inside a glass envelope. The mixture consists of helium and neon in a ratio between 5:1 and 10:1.

The total pressure inside the tube is kept low, at approximately 1 Torr. To start the process, a high-voltage electrical discharge is passed through the gas between an anode and a cathode. This discharge provides the energy needed to excite the atoms. A DC current of 3 to 20 mA is typically used to maintain this continuous operation. This electrical energy is the "pump" that drives the entire system.

Once the electricity flows, the helium atoms begin to react. Energetic electrons from the discharge collide with ground-state helium atoms. These inelastic collisions push the helium atoms into higher, metastable excited states.

Because the mixture contains mostly helium, there are many excited helium atoms ready to interact. These excited helium atoms then collide with the neon atoms. Through this process, the energy is transferred from the helium to the neon atoms. This transfer increases the population of certain neon energy levels, such as the 4s2 and 5s2 levels. When more atoms exist in these upper levels than in the lower levels, a state called population inversion is achieved.

Population inversion is the key to light amplification. In a He–Ne laser, the neon atoms eventually drop from these excited states to a lower level called 3p4. As they drop, they release energy in the form of light. This is known as stimulated emission. To turn this light into a concentrated beam, the laser uses an optical cavity. This cavity usually consists of two mirrors at the ends of the tube. One mirror is highly reflective, often reflecting 99.9% of the light. The other mirror, called the output coupler, allows about 1% of the light to pass through.

As the light bounces back and forth between these mirrors, it passes through the gas repeatedly. Each pass amplifies the light until a stable, continuous beam exits through the output coupler.

The history of the He–Ne laser is tied to the early days of laser science. The first gas lasers were developed at Bell Telephone Laboratories. In December 1960, researchers demonstrated the first continuous-wave infrared He–Ne laser. This first version emitted light at a wavelength of 1150 nm, which is invisible to the human eye. Scientists soon realized that visible light was in much higher demand. They investigated many different neon transitions to find the best visible light source. They discovered that the 633 nm line provided the highest gain in the visible spectrum. This discovery led to the development of the red He–Ne lasers we use today.

Most standard red He–Ne lasers operate at a center wavelength of 632.8 nm in air. In a vacuum, this wavelength is precisely 632.991 nm.

The laser's output power is generally quite low, ranging from 0.5 to 50 mW. The length of the laser cavity typically stays between 15 and 50 cm, though some high-power versions reach 1 meter. These lasers are incredibly precise. For example, frequency-stabilized versions can specify a wavelength to within 1 part in 10^8. Using an iodine absorption cell, scientists can achieve absolute frequency stabilization as fine as 2.5 parts in 10^11. This level of precision makes them essential for measuring the physical world.

Because of this precision, He–Ne lasers have many important applications. They are used in holography and as wavelength references for spectroscopy. In the past, they were common in supermarket barcode scanners. They were also used in LaserDisc players from 1978 until the mid-1980s. Today, they are vital for nano-positioning in semiconductor device fabrication. They are also used in high-precision laser gyroscopes for navigation in commercial aircraft.

These gyroscopes use a ring laser configuration to help planes stay on course. From helping build computer chips to navigating the skies, the He–Ne laser remains a cornerstone of modern technology.