--- title: "He-Ne laser working with energy levels" subject: "Engineering Physics" chapter: 6 key: "semester_1/engineering_physics:he-ne-laser-working-energy-levels" source: https://www.bctengineer.com/q/semester_1/engineering_physics/he-ne-laser-working-energy-levels site: BCT Engineering --- # He-Ne laser working with energy levels Engineering Physics · Chapter 6: Laser and Fiber Optics - Asked in: 2073 Shrawan, 5 marks; 2073 Chaitra, 5 marks; 2071 Chaitra, 5 marks; 2070 Asar, 5 marks; 2068 Chaitra, 5 marks; 2067 Asoj, 5 marks (asked 6 times) - Video: [Helium Neon Laser](https://www.youtube.com/watch?v=ppwM8SYJx-4) — Physics Jessy - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_1/engineering_physics/he-ne-laser-working-energy-levels - Whole chapter: https://www.bctengineer.com/md/semester_1/engineering_physics/chapter-6 ## Question What is population inversion (and optical pumping)? Explain the construction and working of a He-Ne laser with a suitable energy level diagram. ## Short answer Population inversion is a non-equilibrium condition where a higher energy level has more atoms than a lower one, essential for laser action. In a He-Ne laser, a discharge excites He atoms to metastable states; they transfer energy to Ne atoms, creating inversion between Ne 3s and 2p levels. Stimulated emission produces red 632.8 nm light; mirrors provide feedback. ## Answer #### Population inversion and optical pumping In thermal equilibrium, lower levels hold more atoms than higher levels (N₂ < N₁). **Population inversion** is the non-equilibrium condition in which a higher energy level holds more atoms than a lower level (N₂ > N₁). It is necessary for light amplification by stimulated emission. **Pumping** is the process of raising atoms to the excited state to achieve inversion. In **optical pumping** an intense lamp (for example, a flash lamp for a ruby laser) provides the exciting light. In the He-Ne laser, pumping is by electric discharge (electron collisions) and energy transfer between atoms. #### Construction ```text Brewster window Brewster window M1 (100%) | He:Ne = 10:1 , ~1 torr | M2 (~99%) | |====== discharge tube =====| | --> laser | | anode (+) cathode(-) | | 632.8 nm +------[ high-voltage DC ]--+ ``` - A glass tube about 30 cm long and a few mm in diameter, filled with helium and neon in a ratio of about 10 : 1 at a total pressure of about 1 torr. - Two electrodes carry a DC discharge of about 1 to 2 kV (or an RF source is used). - Two parallel mirrors form the optical cavity: one is totally reflecting, the other about 98 to 99 % reflecting so that some light leaves as the beam. - Windows are set at the Brewster angle to give a linearly polarized output. #### Working ```text eV He Ne 20.61 2¹S ====collision====> 3s (20.66) 3s -> 3p (20.30): 3.39 µm 3s -> 2p : 632.8 nm (red) 19.82 2³S ====collision====> 2s (19.78) 2s -> 2p : 1.15 µm ^ electron impact 2p (18.70) | | fast decay (~10⁻⁸ s) | 1s (16.6) -> ground by wall collisions 0 ground ground ``` 1. The discharge accelerates electrons, which collide with He atoms and excite them to the metastable states 2³S (19.82 eV) and 2¹S (20.61 eV). These states cannot quickly decay by emitting a photon, so many He atoms accumulate there. 2. The Ne levels 3s (20.66 eV) and 2s (19.78 eV) have almost the same energies. In collisions the excited He atoms transfer their energy to ground-state Ne atoms by resonant energy transfer, exciting them to these levels, while the He atoms return to the ground state. 3. Ne atoms are then much more numerous in 3s and 2s than in the lower levels 3p (about 20.3 eV) and 2p (about 18.7 eV), so **population inversion** occurs between 3s and 3p/2p, and between 2s and 2p. 4. A spontaneously emitted photon stimulates other Ne atoms; the photons move back and forth between the mirrors and the beam grows by stimulated emission. 5. The main transitions are 3s → 2p at **632.8 nm** (red, the usual output), 3s → 3p at 3.39 µm and 2s → 2p at 1.15 µm (infrared). 6. Atoms in the lower level 2p decay quickly (about 10⁻⁸ s) to the 1s level, and then to the ground state by collisions with the tube wall. This keeps the lower level empty, which maintains the inversion. A narrow bore helps this wall relaxation. #### Properties Continuous output of a few mW, highly monochromatic, very directional (divergence about 1 mrad), efficiency about 0.1 %. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is population inversion? A. A higher level has more atoms than a lower level (N2 > N1) B. Fewer atoms in all levels C. Equal populations D. All atoms in ground state Answer: A. It is a non-equilibrium condition required for amplification. (Key point: Population inversion) 2. What is helium's role? A. Acts as the mirror B. Excited by electron impact into metastable states, then transfers energy to Ne by collision C. Emits the laser light D. Cools the tube Answer: B. Resonant energy transfer to Ne's 3s and 2s levels. (Key point: Role of helium) 3. Why is a narrow tube bore helpful? A. It cools electrodes B. It increases pressure C. Lower-level atoms relax at the wall, keeping the lower level empty D. It reflects light Answer: C. Fast wall relaxation depopulates the lower level. (Key point: Maintaining inversion) 4. The usual red output wavelength is from: A. 3s → 2p at 632.8 nm B. 3s → 3p at 3.39 µm C. 1s → ground D. 2s → 2p at 1.15 µm Answer: A. The usual output is red 632.8 nm. (Key point: Output) ## Related in Engineering Physics - [Why laser needs population inversion](https://www.bctengineer.com/question.md?key=semester_1%2Fengineering_physics%3Apopulation-inversion-laser-necessity) — Engineering Physics, chapter 6 ## Related in other subjects - [Types of optical fibre: construction and propagation](https://www.bctengineer.com/question.md?key=semester_6%2Fpropagation_and_antenna%3Aoptical-fiber-types-construction) — Propagation and Antenna, chapter 6, asked 2× - [Construction and types of optical fibre](https://www.bctengineer.com/question.md?key=semester_6%2Fpropagation_and_antenna%3Afiber-construction-types) — Propagation and Antenna, chapter 6 - [n-channel E-MOSFET construction and working](https://www.bctengineer.com/question.md?key=semester_3%2Felectronics_devices_and_circuit%3An-emosfet-construction-working) — Electronics Devices and Circuit, chapter 3, asked 9× - [Rhombic antenna construction and working](https://www.bctengineer.com/question.md?key=semester_6%2Fpropagation_and_antenna%3Arhombic-antenna) — Propagation and Antenna, chapter 3, asked 5×