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Training Videos

Optical Transmitter Design

We have discussed the properties of optical sources. Although an optical source is a major component of optical transmitters, it is not the only component. Other components include a modulator for converting electrical data into optical form (if direct modulation is not used) and an electrical driving circuit for supplying current to the optical source. This tutorial covers the design of optical transmitters with emphasis on the packaging issues. 1. Source-Fiber Coupling The design object for any transmitter is to couple as much light as possible into the optical fiber. In practice, the coupling efficiency depends on the type of...

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Light-Emitting Diodes (LEDs)

In some local-area networks, a coherent source is not required, and one can employ a light-emitting diode (LED), a less expensive and longer-lasting optical source with a relatively wide optical spectrum. The basic structure of an LED is similar to that of semiconductor lasers in the sense that both employ an active layer sandwiched between two cladding layers and pumped using a forward-biased p-n junction. The main difference is that stimulations emission does not occur because a population inversion is not realized. Rather, radiative recombination of electron-hole pairs in the active layer generates light through spontaneous emission, some of which...

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Optical Signal Generation

The first step in the design of an optical communication system is to decide how the electrical data would be converted into an optical signal with the same information. The original electrical data can be in an analog form, but it is invariably converted into a digital bit stream (with the RZ or NRZ format) consisting of a pseudorandom sequence of 0 and 1 bits. Two techniques, know as (a) direct modulation and (b) external modulation, can be used to generate the corresponding optical bit stream. Both of them are discussed in this tutorial.

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Semiconductor Laser Characteristics

The operation of semiconductor lasers is well described by a set of rate equations that govern the interaction of photons and electrons inside the active region. In this tutorial, we use the rate equations to discuss both the continuous-wave (CW) and modulation characteristics.

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Single-Mode Semiconductor Lasers

Semiconductor lasers oscillate in several longitudinal modes simultaneously because of a relatively small gain difference (~0.1 cm-1) between two neighboring modes of the cavity. The resulting spectral width (2-4 nm) is acceptable for some applications but becomes a concern for many others. This tutorial is devoted to techniques that can be used to design semiconductor lasers such that they emit light predominantly in a single longitudinal mode.

The basic idea is to design the laser such that losses are different for different longitudinal modes of the cavity, in contrast with FP lasers whose losses are mode-independent. The following figure shows the gain and loss profiles schematically for such a laser.

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