LASERS AND FIBRE OPTICS - Flashcards (Question and Answer)

Question and answer flashcards covering key concepts from lasers and optical fibres, based on the provided notes.

What are the four main characteristics of laser light?

Monochromatic, highly coherent, highly directional, and can be sharply focused.

Name the three basic interaction mechanisms of radiation with matter discussed in the notes.

Absorption, spontaneous emission, and stimulated emission.

Under what condition does absorption occur between two atomic levels E1 and E2?

Absorption occurs when E2 − E1 = hf (the photon energy matches the energy difference).

Define population inversion.

A non‑equilibrium condition where the upper energy state is more populated than the lower one: N2 > N1.

State the Boltzmann relation for the ratio N2/N1 at temperature T.

N2/N1 = exp[−(E2−E1)/(kT)] = exp[−hf/(kT)].

What are Einstein coefficients in radiation interactions?

A21: spontaneous emission; B12: induced absorption; B21: stimulated emission.

Give the rate expressions for absorption, spontaneous emission, and stimulated emission.

Rate of absorption ∝ B12 N1 If; rate of spontaneous emission ∝ A21 N2; rate of stimulated emission ∝ B21 N2 If.

At thermal equilibrium, what is the relationship between absorption and emission rates?

B12 N1 If = A21 N2 + B21 N2 If.

What is the common relation between B12 and B21?

B12 = B21.

How are A21 and B21 related to the spectral energy density at equilibrium?

A21 B21 = 8πh f^3 / c^3, and If = (8πh f^3 / c^3) / (e^{hf/(kT)} − 1). Also B12 = B21.

Why is population inversion essential for laser action?

It makes the rate of emission exceed the rate of absorption, enabling net amplification via stimulated emission.

What are the three essential components of a laser system?

Pumping system, lasing medium (active medium), and a resonant/optical cavity.

In the He–Ne laser, which energy level pair in Neon forms the lasing transition, and how is inversion achieved?

Lasing transition is Ne E2 → E1. Inversion is achieved by energy transfer from excited Helium (E3) to Ne (E2), with E3 metastable and E1→E0 decaying quickly.

Describe the lasing transition and inversion mechanism in a ruby laser.

Lasing occurs from Cr3+ energy level E2 to E1 (Cr-doped Al2O3). Population inversion is achieved at E2, pumped to higher levels (E3) and relaxations populate E2.

What is the active medium in a semiconductor laser and how is it driven to lase?

A heavily doped p–n junction diode; forward bias injects carriers, recombination yields stimulated emission, end faces act as mirrors.

What are the key features of a semiconductor (injection) laser?

P–N junction as gain medium, high current density to achieve population inversion, highly polished end facets forming a resonator, one facet partially reflecting.

List three common applications of lasers mentioned in the notes.

Barcode scanners, laser printers, and laser cooling (Doppler and Sisyphus cooling).

What is the principle behind laser cooling and its two main types?

Dissipative light forces reduce kinetic energy of atoms; two types are Doppler cooling and Sisyphus cooling.

What is an optical fibre made of?

A core (n1) with higher refractive index surrounded by cladding (n2 < n1) and a protective jacket.

What is total internal reflection and why is it crucial for optical fibres?

Reflection that occurs when light in a denser medium hits a boundary at an angle above the critical angle, enabling guided light with many reflections along the fibre.

Define numerical aperture (NA) and its relation to the acceptance angle.

NA = n0 sin θ0; it measures light‑gathering ability; for air (n0 ≈ 1), NA ≈ sin θ0 and typical acceptance angles are ~5° (single‑mode) and 10–15° (multimode).

Define Δ (fractional refractive index change) and its relation to NA.

Δ = (n1 − n2)/n1; for small Δ, NA ≈ n1√(2Δ) (approximate relation between NA and core–cladding contrast).

What is skip distance in optical fibres?

The distance between successive internal reflections of a ray inside the fibre.

Name the three main types of optical fibres described.

Single‑mode step‑index, multimode step‑index, and multimode graded‑index (GRIN).

What is the normalized frequency V and its significance?

V = π d/λ · sqrt(n1^2 − n2^2); number of supported modes scales ~2V^2 for V >> 1; single‑mode when V is small (≈ below 2.405).

State the material dispersion expression given (D_m).

D_m = [λ (Δλ) / (c L)] · (d^2 n / dλ^2).

Identify the three distortion mechanisms in optical fibres.

Material dispersion, waveguide dispersion, and intermodal dispersion.

What are the main attenuation mechanisms in optical fibres?

Absorption by impurities and intrinsic material, scattering from inhomogeneities, and other losses such as bends (micro and macro).

What is fibre optic networking?

A telecommunication network using optical fibre as the transmission medium with optical nodes and light paths.

What is optical communication?

Transmission of information using light to carry the signal through a channel (fibre) from transmitter to receiver.