Laser Technology: Principles, Properties, and Applications

Posted on Jan 21, 2025 in Physics

LASER = Light Amplification by Stimulated Emission of Radiation

A laser is actually an oscillator rather than a simple amplifier. The difference is that an oscillator has positive feedback in addition to the amplifier.

Light is understood in a general sense: electromagnetic radiation with a wavelength around 1 μm. Thus, one can have infrared, visible, or ultraviolet lasers.

The atomic medium with population inversion used in the laser is called the active medium. The positive optical feedback is obtained by placing the active medium between two mirrors. One of them (M1) totally reflects back the light (The mirror reflectivity is R1 = 1) to the active medium, while the other one (M2, called the output coupler) has a reflectivity less than unity (R2 < 1) and allows some of the light to be transmitted as the output of the laser. The two mirrors form a resonant cavity for the optical radiation.

Lasers come in many shapes and sizes. They are classified by various criteria:

Gain medium is solid, liquid, or gas
Wavelength is in the infrared, visible, or ultraviolet spectral region
Mode of operation is continuous or pulsed
Wavelength is fixed or tunable

Key Laser Properties

1. Monochromaticity

The emission of the laser generally corresponds to just one of the atomic transitions of the gain medium, in contrast to discharge lamps, which emit on all the transitions. The spectral line width can be much smaller than that of the atomic transition. This is because the emission is affected by the optical cavity. In certain cases, the laser can be made to operate on just one of the modes of the cavity. Since the quality factor of the cavity is generally rather large, the mode is usually much narrower than the atomic transition, and the spectral line width is orders of magnitude smaller than the atomic transition.

This is particularly useful for high-resolution spectroscopy and applications such as interferometry and holography that require high coherence.

2. Coherence

In discussing the coherence of an optical beam, we must distinguish between spatial and temporal coherence. Laser beams have a high degree of both.

Spatial coherence refers to whether there are irregularities in the optical phase in a cross-sectional slice of the beam.

Temporal coherence refers to the time duration over which the phase of the beam is well defined. In general, the temporal coherence time tc is given by the reciprocal of the spectral linewidth ν. Thus, the coherence length lc is given by: lc = ctc = c/ν.

3. Directionality

This is perhaps the most obvious aspect of a laser beam: the light comes out as a highly directional beam. This contrasts with light bulbs and discharge lamps, in which the light is emitted in all directions. The directionality is a consequence of the cavity.

4. Brightness

The brightness of lasers arises from two factors. First of all, the fact that the light is emitted in a well-defined beam means that the power per unit area is very high, even though the total amount of power can be rather low. Then we must consider that all the energy is concentrated within the narrow spectrum of the active atomic transition. This means that the spectral brightness is even higher in comparison with a white light source like a light bulb. For example, the spectral brightness of a 1mW laser beam could easily be millions of times greater than that of a 100W light bulb.

5. Ultrashort Pulse Generation

Lasers can be made to operate continuously or in pulses. The time duration of the pulses tp is linked to the spectral bandwidth of the laser light ν by the “uncertainty” product tν ∼ 1 : tp ≥ 1/ν.