Chapter 20: Optical Integrated Circuits

Source: Hunsperger, sixth edition (2009), Problems 20.1–20.8, printed page 447. Historical and technology-development questions are interpreted relative to that edition.

Problem 20.1 — How aggregate rates reached terabits per second

Working. Aggregate throughput is approximately \(R_{\rm total}=\sum_j R_j\). DWDM raises the number of parallel wavelength channels; narrow, stable lasers and selective filters reduce crosstalk between them. Optical amplifiers restore many channels without per-channel optical-electrical-optical conversion. Low-loss fiber and dispersion control preserve signals over longer spans, while faster modulators and receivers raise per-channel rate.

Answer. Channel parallelism, optical amplification, improved fiber transmission, and high-speed sources/modulators/detectors act together.

Check. A terabit aggregate link does not imply every laser was directly modulated at a terabit per second.

Problem 20.2 — Optical add/drop multiplexing

Working. An OADM separates selected wavelength channels from a multiplexed stream and directs them to a local drop port. It inserts local signals at designated wavelengths through an add port while forwarding the other channels.

Answer. It provides wavelength-selective access and routing without demultiplexing, detecting, and regenerating every through channel.

Check. An OADM must control insertion loss, isolation, and channel alignment. It is more selective than a power splitter that sends every wavelength to every output.

Problem 20.3 — Dispersion-shifted fiber

Working. Total chromatic dispersion contains material and waveguide contributions. Changing the refractive-index profile and core geometry changes the latter, moving the wavelength where their sum is zero.

Answer. Dispersion-shifted fiber places its low/zero-dispersion region nearer a chosen communication band, historically toward the low-loss 1.55 micrometre window.

Check. Low dispersion is not identical to low attenuation. In DWDM systems exactly zero dispersion can also enhance nonlinear mixing, motivating other dispersion profiles.

Problem 20.4 — Three integrated optical sensors

Working. A Mach–Zehnder interferometer senses changes in optical path, so pressure or strain can be converted into phase. An evanescent sensing guide changes response when analyte index or absorption changes. A resonator or grating shifts resonance with temperature or strain.

Answer. Examples are interferometric pressure/strain sensing, evanescent chemical sensing, and resonant temperature sensing.

Check. Each example includes a transduction mechanism; simply labelling an optical guide a “sensor” does not specify what measurable quantity changes.

Problem 20.6 — Blu-ray light source

Working. A shorter wavelength permits a smaller diffraction-limited focused spot at a given numerical aperture, increasing the disc’s achievable recording density.

Answer. A blue-violet semiconductor laser diode near 405 nm, using the GaN/InGaN material family.

Check. The short wavelength alone is not the complete storage-density design; objective NA and recording-layer properties also determine the spot and usable features.

Problem 20.7 — Dense wavelength-division multiplexing

Working. Give independent signals closely spaced optical carrier frequencies, combine them in one fiber, then separate them with wavelength-selective devices at the receiver. For \(M\) equal-rate channels, \(R_{\rm aggregate}\simeq M R_{\rm channel}\).

Answer. DWDM is wavelength multiplexing with closely spaced channels requiring precise spectral control.

Check. Frequency and wavelength spacings are not numerically interchangeable: \(|\Delta\nu|\simeq c|\Delta\lambda|/\lambda_0^2\). An amplifier’s usable band limits the total channel allocation.

Problem 20.8 — Benefits of optical sensing

Working. A dielectric sensing head can provide electrical isolation and little electromagnetic pickup. Light can interrogate a small remote sensor through a fiber, and wavelength/time multiplexing can combine several sensing positions. Optical phase and resonance can be highly sensitive to small geometrical or refractive changes.

Answer. Isolation, remote access, compactness, multiplexing, and sensitive phase/spectral readout are possible advantages.

Check. Source drift, temperature cross-sensitivity, coupling changes, and calibration still affect measurements. An optical sensor is not automatically more accurate for every measurand.