Laser Physics 3.6 Photon Lifetime and Cavity Q-factor

Laser Physics 3.6 Photon Lifetime and Cavity Q-factor

by Fysiikkaa kotisohvalle

Formal & Physical Sciences Physics PHJOptical physicsPHJLLaser physics
🎙 Fysiikkaa kotisohvalle 👥 316 📅 June 23, 2026 ⏱ 18 min 👁 29 📄 tutorial 🧭 2026-08-16
Available in: English (current) Français

Keywords

photon lifetimecavity Q-factorlaser resonatormirror reflectivitylinewidth

Summary

This lecture, part of a laser physics course, focuses on the temporal dynamics of a laser resonator, specifically the photon lifetime and the cavity Q-factor. The instructor begins by motivating the topic, explaining that these parameters dictate the linewidth and coherence properties of a laser. He then derives the photon decay rate in a cavity, considering mirror reflectivities and internal losses, leading to an expression for the photon lifetime tau_c. The physical meaning of tau_c is discussed as the average time a photon spends in the cavity. Next, the Q-factor is introduced as a measure of energy storage relative to energy loss, and it is shown to be proportional to the product of the oscillation frequency and the photon lifetime. The lecture concludes with a design rule: cavities should have the highest possible Q-factor for efficient lasing, except for broadband or mode-locked lasers. A discussion question is posed about the effect of Q-factor on laser output power.

157 words

Critical Evaluation

Value of the Information & Strength of the Argument

The lecture provides a solid, step-by-step derivation of the photon lifetime and Q-factor, making the physics accessible. The argumentation is logical and builds on previous lectures, connecting concepts like finesse and linewidth. The instructor emphasizes the physical meaning of each parameter, which aids understanding. However, the presentation is somewhat informal and includes asides that may distract. The value lies in its clear pedagogical approach, though it lacks depth in discussing practical implications or alternative derivations.

84 words

Title / Content Match

The title accurately reflects the content, which focuses on photon lifetime and cavity Q-factor.

Quality & Reliability

7/10

The lecture provides a clear derivation of photon lifetime and Q-factor from first principles, with correct physics. However, it lacks citations to external sources and contains minor imprecisions in terminology (e.g., 'intensity' vs 'irradiance').

Key Moments

Markers derived by PSI from the transcript: the creator did not define chapters.

Contribution & Novelties

The lecture provides a clear and systematic derivation of photon lifetime and Q-factor, which is standard but well-explained. It connects these concepts to practical design considerations, such as the trade-off between Q-factor and output power. The pedagogical approach is effective for students.

Pour aller plus loin :

  • Laser resonator — Overview of optical cavities and their properties.
  • Quality factor — General definition and applications.
  • Photon lifetime — Specific concept in laser physics.

72 words

Radar Profile

The radar profile shows balanced scores across information quantity, quality, technical level, and reliability, indicating a well-rounded educational content. The high technical level is matched by good reliability, making it suitable for advanced students.

Reliability 7/10