Lasers and Photonics

Physics of lasers, optical amplification, threshold, resonators, and photonic applications.

Conceptual context

A laser combines an active medium, pumping, and a cavity to amplify and select radiation. The central issue is when gain compensates loss. Below that point spontaneous emission dominates; above $P_u$, sustained oscillation develops and $P_out$ rises with $P_p$. Photonics applies this physics to light generation, guidance, modulation, and detection.

🟢 Essential level

The active medium stores energy supplied by $P_p$. With population inversion, a suitable photon can trigger stimulated emission and generate another photon with the same idealized $\nu$, phase, and direction.

The cavity returns part of the light through $R_1$ and $R_2$, so radiation crosses the medium repeatedly and is amplified. Losses represented by $\alpha_{i}$ remove energy. Laser oscillation begins when available gain reaches $g_u$, corresponding to $P_u$. Below it, useful laser $P_out$ is nearly zero; above it, output grows approximately linearly. The rate of this growth is $\eta_{s}$, which states how much useful optical power is obtained from each additional pump increase. It is therefore a practical measure of differential conversion after oscillation has already started, not a description of the entire device efficiency, and confusing the two can lead to overly optimistic estimates of how much optical power a given pump source will ultimately deliver to the working beam once real losses are taken into account.

🔵 Formal level

Basic formulas

The quantum description begins with the relation between $E_{\gamma}$ and $\nu$:

Photon energy

Vacuum $\lambda$ follows from $\nu$ and $c$:

Frequency and wavelength

Both relations are purely quantum in nature and do not yet depend on the cavity, the pump, or any other element of the specific laser device.

Real student questions

Is there light below threshold? Yes, from spontaneous emission. Does more pumping always raise output? Only in the valid range. Does larger $R_2$ always help? No, it improves feedback but reduces extraction. Does $\lambda$ determine power? No; it fixes photon energy, while power also depends on photon flux.

Final synthesis

Output appears when gain compensates loss. $P_u$ marks the transition, and $\eta_{s}$ controls subsequent $P_out$ growth.

Complete interpretation requires comparison of the ideal response with measured data, unit checks, and a decision about whether operation remains steady, single-mode, and free of significant saturation.