Optical Networks - _3_5 Transmitters_39

These modulators have very good extinction ratios

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These modulators have very good extinction ratios ranging from 15 to 20 dB, and we can control the chirp very precisely. Due to the high polarization dependence of the device, a polarization maintaining fiber is used between the laser and the modulator. The EA modulator is an attractive alternative to lithium niobate modulators because it can be fabricated using the same material and techniques used to fabricate semiconductor lasers. This allows an EA modulator to be integrated along with a DFB laser in the same package and results in a very compact, lower-cost solution, compared to using an external lithium niobate modulator. In simple terms, the EA
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3.5 Transmitters 195 Modulator Laser Periodic pulses Modulated pulses 1 0 0 1 1 1 0 0 1 1 Electrical NRZ data Stage 1 Stage 2 DFB laser CW signal Periodic pulses Modulated pulses 1 0 0 1 1 1 0 0 1 1 Electrical clock input Electrical NRZ data Time Time (a) (b) Two-stage modulator Figure 3.57 Using external modulators to realize transmitters for systems using RZ or soliton pulses. (a) A laser emitting a periodic pulse train, with the external modulator used to block the 0 bits and pass through the 1 bits. (b) A more common approach using a continuous-wave (CW) DFB laser followed by a two-stage modulator. Input Output V Unmodulated light Modulated light Figure 3.58 A lithium niobate external modulator using a directional coupler configu- ration.
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196 Components Input Output V Unmodulated light Modulated light Applied voltage ( ) V/V p 0 1 2 3 - 1 - 2 - 3 Output signal (a) (b) Figure 3.59 A lithium niobate external modulator using a Mach-Zehnder interferom- eter (MZI) configuration. (a) Device configuration. (b) Theoretical switching response as a function of applied voltage, V . V π denotes the voltage required to achieve a π phase shift between the two arms. Note that the MZI has a periodic response. modulator uses a material such that under normal conditions, its bandgap is higher than the photon energy of the incident light signal. This allows the light signal to propagate through. Applying an electric field to the modulator results in shrinking the bandgap of the material, causing the incident photons to be absorbed by the material. This effect is called the Franz-Keldysh effect or the Stark effect. The response time of this effect is sufficiently fast to enable us to realize 2.5 Gb/s and 10 Gb/s modulators. The chirp performance of EA modulators, though much better than directly modulated lasers, is not as good as that of lithium niobate MZI modulators. (While ideally there is no chirp in an external modulator, in practice, some chirp is induced in EA modulators because of residual phase modulation effects. This chirp can be controlled precisely in lithium niobate modulators.) 3.5.5 Pump Sources for Raman Amplifiers One of the biggest challenges in realizing the Raman amplifiers that we discussed in Section 3.4.4 is a practical high-power pump source at the right wavelength. Since
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3.5 Transmitters 197 1155 1155 1218 1218 1288 1288 1366 1366 1455 1455 Input pump 1100 nm Output pump 1455 nm High-reflectivity fiber Bragg gratings Low-reflectivity fiber Bragg grating Figure 3.60 A high-power pump laser obtained by cascading resonators (after [Gru95]).
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  • Spring '09
  • Boussert
  • cavity, DFB, DBR, DBR Lasers

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