Q19Engineering Physics

The Helium-Neon (He-Ne) laser, invented in 1960 by Ali Javan and his team at Bell Labs, was the very first gas laser ever developed. Unlike the Ruby laser which fires in intense, short pulses, the He-Ne laser is universally celebrated for its ability to produce a highly stable, continuous-wave (CW) beam of highly coherent light, most commonly at a wavelength of $632.8\text{ nm}$ in the visible red spectrum. It relies on a meticulously balanced mixture of two noble gases and the principle of resonant collisional energy transfer.

1. Fundamental Principle

The laser operates on the principle of stimulated emission in a four-level energy system. However, the unique aspect of the He-Ne laser is that the lasing action (the actual emission of photons) occurs entirely within the energy levels of the Neon atoms. The Helium atoms do not emit the laser light. Instead, the Helium atoms act solely as an intermediary 'pumping' mechanism. They absorb energy from an electrical discharge and efficiently transfer it to the Neon atoms to achieve the necessary population inversion.

2. Construction Details

• The Discharge Tube: The core of the laser is a long, narrow cylindrical tube made of fused quartz or borosilicate glass, typically about $10$ to $100\text{ cm}$ long and a few millimeters in diameter. A narrow bore restricts the discharge, increasing the current density and promoting collisions.
• The Gas Mixture: The tube is evacuated and then filled with a precise, high-purity mixture of Helium and Neon gases. The optimal ratio is crucial and is generally around 10 parts Helium to 1 part Neon (10:1 or sometimes 7:1). The total pressure inside the tube is very low, usually around $1\text{ Torr}(\text{mm of Hg})$. This specific low pressure is required to maintain a stable electrical glow discharge.
• The Excitation Source: To pump energy into the system, two heavy electrodes (an anode and a cathode) are sealed into the tube. A high-voltage DC power supply (typically a few kilovolts) is connected across them to generate a continuous electrical discharge (plasma) through the gas mixture.
• The Optical Resonator: To amplify the light, the ends of the tube are capped with two highly polished, optically flat mirrors that are aligned to be perfectly parallel to each other. One mirror is coated to be $100\%$ fully reflective, while the other is partially transmissive (about $99\%$ reflective and $1\%$ transmissive). The continuous laser beam escapes through this partially transmissive mirror. Often, the tube ends are sealed with optical windows set at Brewster's angle to ensure the output laser beam is perfectly linearly polarized.

3. The Working Mechanism: Energy Level Dynamics

The brilliance of the He-Ne laser lies in the fortuitous alignment of the energy levels of the two different atoms. The mechanism proceeds in several distinct, sequential steps:

Step 1: Excitation of Helium Atoms (Electrical Pumping) When the high voltage is applied, free electrons are massively accelerated down the tube. These high-speed energetic electrons violently collide with the abundant Helium atoms. These inelastic collisions transfer kinetic energy to the Helium atoms, kicking their electrons from the ground state ($F_1$) up to higher excited energy states, specifically the $2^{3}S$ (denoted as $F_2$) and $2^{1}S$ (denoted as $F_3$) levels.

Crucially, these $F_2$ and $F_3$ levels in Helium are metastable states. Quantum mechanical selection rules forbid these atoms from spontaneously emitting a photon and dropping back down to the ground state. Because they are 'stuck' there, excited Helium atoms begin to accumulate in massive numbers in these metastable states.

Step 2: Resonant Energy Transfer (Collisional Pumping) Neon is a much heavier and more complex atom with many closely spaced energy levels. By an incredible coincidence of nature, two specific excited energy levels of Neon (denoted as $4s$ and $5s$, or sometimes $E_4$ and $E_6$ in different notations) sit at almost the exact same energy values as the metastable $F_2$ and $F_3$ levels of Helium.

Because the gas mixture is $90\%$ Helium, a metastable Helium atom drifting through the tube will very quickly collide with a ground-state Neon atom. Because their energy levels match almost perfectly, a resonant energy transfer occurs upon collision. The Helium atom drops back down to its ground state, and the Neon atom absorbs the energy and is instantly excited up to the $4s$ or $5s$ level. Reaction: $H{e}^{\ast }(\text{excited})+Ne(\text{ground})\to He(\text{ground})+N{e}^{\ast }(\text{excited})+\Delta E$

Step 3: Achieving Population Inversion The $4s$ and $5s$ levels of Neon are also metastable. Meanwhile, the lower energy levels of Neon (like $3p$ and $4p$) are virtually empty because any atoms there instantly decay downwards. Thanks to the continuous, relentless pumping by the millions of colliding Helium atoms, the $4s$ and $5s$ levels of Neon quickly become heavily populated, while the $3p$ and $4p$ levels remain mostly empty. A massive population inversion is successfully established between these upper ($s$) and lower ($p$) states in the Neon gas.

Step 4: Stimulated Emission and Laser Action A stray Neon atom inevitably drops spontaneously from the $5s$ level to the $3p$ level, emitting a single photon of wavelength $632.8\text{ nm}$. This stray photon travels down the axis of the tube. As it passes other excited Neon atoms in the $5s$ state, it triggers stimulated emission. These atoms are forced to instantly drop to the $3p$ state, each emitting an identical, perfectly in-phase, $632.8\text{ nm}$ photon travelling in the exact same direction. This growing avalanche of photons hits the mirrors, reflects back through the gas, and triggers even more stimulated emissions on every pass. The light wave is rapidly and intensely amplified. A fraction of this highly intense, highly coherent, parallel red light leaks out of the partially transmissive mirror as the continuous laser beam.

Step 5: Rapid Depopulation (The Bottleneck Solution) For the laser to operate continuously without stopping, the Neon atoms that dropped into the $3p$ state must be quickly removed. If they pile up there, the population inversion will be destroyed. Fortunately, Neon atoms in the $3p$ state decay very rapidly via spontaneous emission down to the $3s$ state (emitting non-laser light at $\sim 600\text{ nm}$). Finally, the atoms in the $3s$ state drop back to the absolute ground state through physical collisions with the walls of the narrow glass tube. They are now ready to be hit by another Helium atom and repeat the cycle endlessly.