Properties of Laser Light: Monochromaticity, Coherence, and More

Properties of Laser Light

1. Monochromaticity

Unlike discharge lamps that emit on all atomic transitions, laser emission typically corresponds to a single atomic transition of the gain medium. The spectral line width can be much smaller than the atomic transition's due to the influence of the optical cavity.

2. Coherence

Laser beams exhibit high spatial and temporal coherence.

Spatial coherence describes the regularity of the optical phase across a beam's cross-section.

Temporal coherence refers to the duration over which the beam's phase remains well-defined. The temporal coherence time (t_c) is generally the reciprocal of the spectral linewidth (ν). Consequently, the coherence length (l_c) is: l_c = ct_c = c/ν.

3. Directionality

A key characteristic of laser beams is their directionality. The light emerges as a focused beam, contrasting with light bulbs and discharge lamps, which emit light in all directions. This directionality stems from the cavity.

4. Brightness

The concentrated beam results in high power per unit area, even with low total power. Furthermore, all energy concentrates within the narrow spectrum of the active atomic transition, leading to even higher spectral brightness compared to white light sources. For instance, a 1mW laser beam can have millions of times greater spectral brightness than a 100W light bulb.

5. Ultrashort Pulse Generation

Lasers can operate continuously or in pulses. The pulse duration (t_p) relates to the spectral bandwidth (ν) through the uncertainty product: t_pν ∼ 1, meaning t_p ≥ 1/ν.

This arises from the Fourier transform of a pulse with duration t_p. Dye lasers, with bandwidths exceeding 10¹³ Hz, can generate sub-100 fs pulses (1 fs = 10^-15 s) using a technique called "mode-locking."