Optical Networks - _3_5 Transmitters_39

Both semiconductor and erbium fiber lasers are

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Both semiconductor and erbium fiber lasers are capable of achieving high output powers, typically between 0 and 20 dBm, although semiconductor lasers used as WDM sources typically have output powers between 0 and 10 dBm. Fiber lasers are used mostly to generate periodic trains of very short pulses (by using a technique called mode locking, discussed later in this section). Principle of Operation Consider any of the optical amplifiers described, and assume that a part of the optical energy is reflected at the ends of the amplifying or gain medium, or cavity, as shown in Figure 3.42. Further assume that the two ends of the cavity are plane and parallel to each other. Thus the gain medium is placed in a Fabry-Perot cavity (see Section 3.3.5). Such an optical amplifier is called a Fabry-Perot amplifier. The two end faces of the cavity (which play the role of the mirrors) are called facets. The result of placing the gain medium in a Fabry-Perot cavity is that the gain is high only for the resonant wavelengths of the cavity. The argument is the same as that used in the case of the Fabry-Perot filter (Section 3.3.5). After one pass through the cavity, as shown in Figure 3.42, part of the light leaves the cavity through the right facet, and part is reflected. Part of the reflected wave is again reflected by the left facet to the right facet. For the resonant wavelengths of the cavity, all the light waves transmitted through the right facet add in phase. As a result of in-phase addition, the amplitude of the transmitted wave is greatly increased for these resonant wavelengths
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174 Components compared to other wavelengths. Thus, when the facets are at least partially reflecting, the gain of the optical amplifier becomes a function of the wavelength. If the combination of the amplifier gain and the facet reflectivity is sufficiently large, the amplifier will start to “oscillate,” or produce light output, even in the absence of an input signal. For a given device, the point at which this happens is called its lasing threshold. Beyond the threshold, the device is no longer an ampli- fier but an oscillator or laser. This occurs because the stray spontaneous emission, which is always present at all wavelengths within the bandwidth of the amplifier, gets amplified even without an input signal and appears as the light output. This process is quite similar to what happens in an electronic oscillator, which can be viewed as an (electronic) amplifier with positive feedback. (In electronic oscillators, the thermal noise current due to the random motion of electrons serves the same purpose as spontaneous emission.) Since the amplification process is due to stimu- lated emission, the light output of a laser is coherent. The term laser is an acronym for light amplification by stimulated emission of radiation.
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