Great question—this cuts to the key differences in gain media, operating conditions, and energy efficiency that set these two laser types apart. Let’s break it down clearly:

1. Gain Media: Atoms vs. Molecules #

HeNe lasers rely on atomic gas (helium + neon) as their gain medium. Atomic energy levels are simple—neon’s 3s₂ to 2p₄ transition (the iconic 632.8nm red line) is the primary one, but the number of atoms that can be excited to the upper energy level is inherently limited. There’s no efficient way to "recycle" atoms for repeated transitions without disrupting the delicate particle number inversion needed for laser action.

CO₂ lasers use molecular gas (CO₂, often mixed with nitrogen and helium). CO₂ molecules have a dense, complex set of vibrational-rotational energy levels. Nitrogen atoms act as a "middleman" to pump energy into CO₂’s upper vibrational levels, and molecular collisions actually help maintain a stable, large-scale particle inversion. Each CO₂ molecule can participate in multiple energy transitions before de-exciting, making the gain per unit volume far higher than HeNe’s.

2. Operating Pressure Tradeoffs #

HeNe lasers run at extremely low pressure (a few torr). This is non-negotiable: too many atomic collisions would cause the upper energy level to depopulate quickly, destroying the particle inversion. But low pressure means far fewer atoms per cubic centimeter, so the total available gain is inherently low.

CO₂ lasers operate at much higher pressures (tens to hundreds of torr). Higher pressure means more CO₂ molecules per unit volume, and counterintuitively, molecular collisions support the particle inversion (nitrogen transfers energy to CO₂ efficiently via collisions). This dense, collision-assisted gain medium is built for high-power output.

3. Why Longer Tubes Can’t Fix HeNe’s Power Limit #

You’re right that longer tubes boost HeNe power to a point, but there’s a hard ceiling for two critical reasons:

• Gain vs. Loss: HeNe’s gain is tiny (~0.1% per cm). Beyond a certain tube length, losses (wall scattering, diffraction, absorption) start to cancel out any additional gain from more atoms. Extending the tube further would result in more loss than gain, so no meaningful power increase.
• Discharge Stability: HeNe uses DC discharge to excite atoms. Longer tubes make it nearly impossible to maintain a uniform, stable discharge across the entire length. Localized breakdown or uneven excitation would reduce efficiency and could even damage the laser.

CO₂ lasers avoid this issue: their higher gain means longer tubes add meaningful power, and they use RF discharge systems that maintain uniform excitation even in very long cavities.

4. Energy Conversion Efficiency #

HeNe lasers have terrible electro-optical efficiency—only about 0.1%. Almost all input electricity is wasted as heat, which further degrades particle inversion. Pumping more power into a longer HeNe tube would just generate more heat, killing laser performance instead of boosting output.

CO₂ lasers, by contrast, have efficiencies of 10-20%. Most input energy goes directly into exciting CO₂ molecules, making it feasible to scale power up to kilowatt levels without overheating the gain medium.

内容的提问来源于stack exchange，提问作者jusaca