LASER DIODE FUNDAMENTALS: WHAT ARE LONGITUDINAL MODES?

Laser diodes must meet two critical requirements to facilitate the lasing process. The first requires that, at a minimum, the laser cavity's gain increases to the point that it reaches unity with the level of losses. This unity state is known as the gain threshold. If you missed it, read our previous blog, titled "Laser Diode Fundamentals: Diode Gain Threshold," to get an in-depth understanding of this phenomenon. The second requirement is that there must be a longitudinal mode present inside the optical cavity, coinciding with the laser's gain curve. This article explores precisely what longitudinal modes are and how they affect the laser diode's performance.

All lasers must contain three key components: an active gain medium, a resonator cavity, and an excitation/pump source. To understand what longitudinal mode is and how it's derived, we will focus solely on the resonator cavity, ignoring the gain medium and excitation/pump source. To further clarify our initial analysis, we will also assume that both mirrors in the resonator cavity are 100% reflective, effectively