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Be (c) Spectrum using barcode integrated for none control to generate, create none image in none barcode generator Figure 2.31 Optical transmitter. Microsoft Office Official Website Layered Architecture The optical eld E( none none t) of a laser tuned to an optical frequency s can be represented as the real part of a complex signal: E(t) = Re E(t)e j2 s t . The complex envelope E(t) can be written in the form E(t) = 2I (t)e j (t) , (2.15) (2.

14). where I (t) = . E(t). 2 /2 is the instant aneous optical signal intensity (proportional to power P) and (t) is the instantaneous phase. If the laser is intensity modulated by s(t), we have I (t) = I0 [1 + ms(t)] 0 < m 1, . s. 1, (2.16). where I0 is the int none for none ensity of the unmodulated signal and m is the modulation index. In analog or subcarrier modulation, s(t) is a continuously varying signal, and in digital modulation (e.g.

, OOK), s(t) is in the form of a sequence of pulses. The phase contains components due to the complexities of the modulation process (pure intensity modulation is not achievable in practice) as well as random phase uctuations due to the laser itself.20 If the modulating signal s(t) is restricted to a bandwidth Bs , then s and E have power spectral densities Ss ( f ) and Se ( ), respectively, as shown in Figure 2.

31(c). Note that only the positive frequencies of Se ( ) are shown. Recall that in amplitude modulation (as opposed to intensity modulation), the bandwidth of the modulated signal is just twice that of the baseband modulating signal.

However, in intensity modulation of a laser, the bandwidth Be of the optical eld is considerably more than this because of the square root in Equation (2.15) as well as the extraneous phase and frequency modulation represented by (t)..

Receiving Side There are several c none for none ommon OR structures. The simplest is the direct detection receiver shown in Figure 2.32.

In the tunable version shown in Figure 2.32(a), the optical signal is rst passed through an optical lter and is then detected by a photodetector (PD) to produce a photocurrent i(t) at point e. (Points d and e in Figure 2.

32 correspond to the same points in Figure 2.29.) The spectrum Se ( ) of the input optical eld E(t) and the spectrum Si ( f ) of the output photocurrent are shown in Figure 2.

32(b). Note that two -channels are shown on the access ber, at optical frequencies 1 and 2 .21 The receiver is tuned to select the former by appropriately positioning its optical lter transfer function HOF .

(Of course, this selection is possible only if the two optical signal spectra do not overlap.). 20 21. The randomness in t none for none he lasing process also produces uctuations in intensity, called relative intensity noise (RIN; see Section 4.6.4).

The effect of RIN is omitted here. The set of optical frequencies that reaches the input of a particular receiver depends on the selectivity (if any) of the splitting/WDMUX device in front of it, as well as the waveband selectivity and setting of the network node. Recall that a waveband-selective switching node directs all signal power in a selected waveband to each desired output port.

The waveband may contain a set of several closely spaced -channels, from which the desired channels must be selected by the receiver.. Multiwavelength Optical Networks PD Signal OF i(t). (a) Tunable Direct Detection Receiver Se( ) HOF 0 Si(f ). f (b) Spectra PD1 OF1 Signal d OFm PDm (c) Arrayed Receiver e e i(t) Switch e Figure 2.32 Optical receivers. Ideally the photode tector, acting as a photon counter, produces a photocurrent that is an exact replica of the instantaneous optical intensity impinging on it; in other words, it acts as a square-law detector of the optical eld. More precisely, i(t) = R I (t) = R . E(t). 2 , 2 (2.17). where R is the resp none for none onsivity of the photodetector. Thus, if intensity modulation is used to generate the optical signal E(t), as in Equation (2.16), this receiver will recover the transmission signal s(t).

It is important to note, however, that a direct detection receiver recovers no phase information, so it cannot be used to recover optical phase- or frequency-modulated signals.22. By appropriately sh aping the optical lter transfer function HOF , it is possible to introduce phase and frequency discrimination in front of the photodetector, thereby making it possible to detect optically phaseand frequency-modulated signals; e.g., optical frequency-shift keying and differential phase-shift keying (see Section 4.


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