Optical Sources Questions and Answers

Answer: C

An InGaAsP/InP device emits at about 1.3 µm.

The passage says the second generation systems used 1.3 µm sources and names InGaAsP/InP LEDs and laser diodes as the typical ones. The other numbers in the passage are not emission wavelengths: 25 nm and 2 nm are the spectral widths of the LED and the laser diode, which is why options A and B are wrong. 0.8 µm is the first generation wavelength, with which the passage compares the 1.3 µm loss.

Answer: B

The keyed requirement is stimulated emission.

The word laser stands for Light Amplification by Stimulated Emission of Radiation. An incoming photon causes an excited atom to emit a second photon of the same frequency, phase, direction and polarisation; this is what makes laser light coherent and allows amplification.

Pumping (optical or electrical) and the optical feedback of the resonator are needed to create population inversion and sustain oscillation, but they serve the stimulated emission process. Optical pumping is only one way of pumping (diode lasers are pumped by current), and current modulation is a way of encoding data, not a requirement for lasing.

Answer: B

The source with the best directivity is the edge emitting LED.

A surface emitting LED, of which the Burrus LED is the classic type, emits from a broad area with a nearly Lambertian pattern, about 120° wide. An edge emitting LED confines the light in a thin active stripe sandwiched between heterojunction layers, like a waveguide, and emits from the edge in a much narrower beam, especially in the plane perpendicular to the junction (about 30°). Its higher radiance and directivity give better coupling into fibres.

The double heterojunction is a structure both types use for carrier and optical confinement, not a separate geometry with the best directivity.

Answer: C

In a four level laser the lower lasing level is empty.

In a four level system, atoms are pumped from the ground level to a high pump band, decay quickly to the upper laser level, lase down to the lower laser level, and then decay rapidly back to the ground state. Because the lower laser level lies above the ground state and empties quickly, it stays almost unpopulated, so population inversion between the laser levels is easy to reach, with far less pump power than in a three level laser such as ruby, where the lower level is the ground state. Nd:YAG and He-Ne lasers work this way.

The 'four levels' are energy levels, not blocks of the system, stages of pumping, or a sign of high power.

Answer: D

The keyed matching is a-ii, b-iv, c-i, d-iii.

• (a) A narrow spectral line width means a long coherence time, so line width is tied to coherence (ii).
• (b) Directionality, the spread of the beam, is degraded by scattering in the medium (iv).
• (c) Photon energy is fixed by the wavelength, \(E = hc/\lambda\) (i).
• (d) The quality factor of a laser cavity is the ratio of the energy stored to the energy lost per cycle (iii).

The other codes pair energy with coherence or stored energy, or line width with wavelength, which breaks these relationships.

Answer: A

Lithium niobate combines electro-optic, piezo-electric and optical properties.

LiNbO₃ is a ferroelectric crystal with a large electro-optic effect, so its refractive index changes with an applied voltage, which is used in high-speed optical modulators. It is strongly piezoelectric, used in surface acoustic wave filters, and it has good nonlinear optical properties for frequency doubling and waveguides.

The other options each name only part of this combination, or add properties such as optical activity or chemical behaviour that are not what makes lithium niobate useful.