Fiber Optics

Sources, Modulators, and Detectors For Fiber-Optic Communication Systems Part 1

INTRODUCTION Optical communication systems utilize fiber optics to transmit the light that carries the signals. Such systems require optoelectronic devices as sources and detectors of such light, and they need modulators to impress the telecommunication signals onto the light. This topic outlines the basics of these devices. Characteristics of devices designed for both high-performance, high-speed […]

Sources, Modulators, and Detectors For Fiber-Optic Communication Systems Part 2

OPERATING CHARACTERISTICS OF LASER DIODES A forward-biased pn junction injects carriers into the active region. As the drive current increases, the carrier density in the active region increases. This reduces the absorption from an initially high value (at thermal equilibrium the absorption coefficient a = 500 cm-1) to zero, at which point the active layer […]

Sources, Modulators, and Detectors For Fiber-Optic Communication Systems Part 3

Relaxation Oscillations An important characteristic of the output of any rapidly switched laser (not just semiconductor lasers) is the relaxation oscillations that can be observed in Fig. 6. These overshoots occur as the photon dynamics and carrier dynamics are coming into equilibrium. Such oscillations are characteristic of the nonlinear coupled laser rate equations and can […]

Sources, Modulators, and Detectors For Fiber-Optic Communication Systems Part 4

NOISE CHARACTERISTICS OF LASER DIODES Noise in LDs results from fluctuations in spontaneous emission and from the carrier generation-recombination process (shot noise). To analyze the response of LDs to noise, one starts with rate equations, introduces Langevin noise sources as small perturbations, and linearizes (performs a small-signal analysis). Finally, one solves in the frequency domain […]

Sources, Modulators, and Detectors For Fiber-Optic Communication Systems Part 5

QUANTUM WELL AND STRAINED LASERS Quantum Well Lasers We have seen that the optimum design for low-threshold LDs uses the thinnest possible active region to confine free carriers, as long as the laser light is wave guided. When the active layer has a thickness less than a few tens of nanometers (hundreds of angstroms), it […]

Sources, Modulators, and Detectors For Fiber-Optic Communication Systems Part 6

Distributed Feedback (DFB) Lasers When the corrugation is put directly on the active region or its cladding, this is called distributed feedback (DFB). One typical example is shown in Fig. 19. As before, the grating spacing is chosen such that, for a desired wavelength nearwhere now ng is the effective group refractive index of the […]

Sources, Modulators, and Detectors For Fiber-Optic Communication Systems Part 7

Operating Characteristics of LEDs In an LED, the output optical power Popt is linearly proportional to the drive current; the relation defines the output efficiency n: This efficiency is strongly affected by the geometry of the LED. The power coupled into a fiber is further reduced by the coupling efficiency between the LED emitter and […]

Sources, Modulators, and Detectors For Fiber-Optic Communication Systems Part 8

LITHIUM NIOBATE MODULATORS The most direct way to create a modulated optical signal for communications applications is to directly modulate the current driving the laser diode. However, as discussed in the sections on lasers, this may cause turn-on delay, relaxation oscillation, mode-hopping, and/or chirping of the optical wavelength. Therefore, an alternative often used is to […]

Sources, Modulators, and Detectors For Fiber-Optic Communication Systems Part 9

ELECTROABSORPTION MODULATORS FOR FIBER-OPTIC SYSTEMS When modulators are composed of III-V semiconductors, they can be integrated directly on the same chip as the laser, or placed external to the laser chip. External modulators may be butt-coupled to the laser, coupled by means of a microlens, or coupled by means of a fiber pigtail. Electroabsorption. Semiconductor […]

Sources, Modulators, and Detectors For Fiber-Optic Communication Systems Part 10

Electrorefraction in Semiconductors Near the band edge in semiconductors, the change in refractive index with applied field can be particularly large, especially in quantum wells, and is termed electrorefraction, or the electrore-fractive effect. Electrorefraction is calculated from the spectrum of electroabsorption using the Kramers-Kronig relations. Enhanced electroabsorption means enhanced electrorefraction at wavelengths below the band […]

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