Optical Networks - _3_5 Transmitters_39

For this reason for several years after they were

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the device cannot be dissipated easily. For this reason, for several years after they were first demonstrated in 1979, VCSELs were not capable of operating at room temperature. However, significant research effort has been expended on new mate- rials and techniques, VCSELs operating at 1.3 μ m at room temperature have been demonstrated [Har00]. The advantages of VCSELs, compared to edge-emitting lasers, include simpler and more efficient fiber coupling, easier packaging and testing, and their ability
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180 Components Figure 3.48 A two-dimensional array of vertical cavity surface-emitting lasers. to be integrated into multiwavelength arrays. VCSELs operating at 0.85 μ m are commercially available and used for low-cost, short-distance multimode fiber inter- connections. In addition, 1.3 μ m VCSELs have been commercially available. In a WDM system, many wavelengths are transmitted simultaneously over each link. Usually, this requires a separate laser for each wavelength. The cost of the transmitters can be significantly reduced if all the lasers can be integrated on a single substrate. This is the main motivation for the development of arrayed lasers such as the DFB laser arrays that we discussed earlier. Moreover, an arrayed laser can be used as a tunable laser simply by turning on only the one required laser in the array. The use of surface-emitting lasers enables us to fabricate a two-dimensional array of lasers, as shown in Figure 3.48. Much higher array packing densities can be achieved using surface-emitting lasers than edge-emitting ones because of this added dimension. However, it is harder to couple light from the lasers in this array onto optical fiber since multiplexers that work conveniently with this two-dimensional geometry are not readily available. These arrayed lasers have the same yield problem as other arrayed laser structures; if one of the lasers does not meet specifications, the entire array will have to be discarded. Mode-Locked Lasers Mode-locked lasers are used to generate narrow optical pulses that are needed for the high-speed TDM systems that we will study in Chapter 12. Consider a Fabry-Perot laser that oscillates in N longitudinal modes, which are adjacent to each other. This means that if the wavelengths of the modes are λ 0 , λ 1 , . . . , λ N 1 , the cavity length l satisfies l = (k + i)λ i / 2 , i = 0 , 1 , . . . , N 1 , for some integer k . From this condition, it can be shown (see Problem 3.7) that the corresponding frequencies f 0 , f 1 , . . . , f N 1 of these modes must satisfy f i = f 0 + i f , i = 0 , 1 , . . . , N 1 . The oscillation at
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3.5 Transmitters 181 Figure 3.49 Output oscillation of a laser oscillating simultaneously in 10 longitudinal modes. (a) The phases of the modes are chosen at random. (b) All the phases are equal to each other; such a laser is said to be mode locked. frequency f i is of the form a i cos ( 2 πf i t + φ i ) , where a i is the amplitude and φ i the phase of mode i . (Strictly speaking, this is the distribution in time of the electric field associated with the longitudinal mode.) Thus the total laser output oscillation takes the form N 1 i = 0 a i cos ( 2 πf i t + φ i ).
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