Optical amplifiers:

In between transmitter and receiver; to boost the power of signal, different amplifiers are required.

A general representation of a fiber amplifier is shown in Figure 6.5.

Fig6.5: Schematic of a fiber amplifier

i. The gain medium normally comprises a length of single-mode fiber connected to a dichroic coupler which provides low insertion loss at both signal and pump wavelengths.

ii. Excitation occurs through optical pumping from a high-power solid-state or semiconductor laser which is combined with the optical input signal within the coupler. The amplified optical signal is therefore emitted from the other end of the gain medium.

iii. Optical amplifiers are placed at intervals along a fiber link to provide linear amplification of the transmitted optical signal.

iv. The optical amplifier, in principle, provides a much simpler solution in that it is a single in-line component which can be used for any kind of modulation at virtually any transmission rate.

v. The Erbium Dopped Fiber Amplifier (EDFA) is most commonly used optical amplifiers in majority application.

Low-impedance front-end:

i. Three basic amplifier configurations are frequently used in optical fiber communication receivers.

ii. The simplest, and perhaps the most common, is the voltage amplifier with an effective input resistance $R_a$ as shown in Figure 6.6.

Fig6.6: Low-impedance front-end optical fiber receiver with voltage amplifier

iii. In order to make suitable design choices, it is necessary to consider both bandwidth and noise.

iv. However, in most practical receivers the detector is loaded with a bias resistor $R_b $ and an amplifier (see Figure 6.6).

v. The bandwidth is determined by the passive impedance which appears across the detector terminals which is taken as $R_L $ in the bandwidth relationship.

vi. However, $R_L$ may be modified to incorporate the parallel resistance of the detector bias resistor $R_b$ and the amplifier input resistance $R_a$ . The modified total load resistance RTL is therefore given by:

$RTL = R_b R_a / R_b+ R_a$

To achieve an optimum bandwidth both $R_b$ and $R_a$ must be minimized.

High-impedance (integrating) front-end:

i. The second configuration consists of a high input impedance amplifier together with a large detector bias resistor in order to reduce the effect of thermal noise.

ii. However, this structure tends to give a degraded frequency response as the bandwidth relationship is not maintained for wideband operation.

iii. The detector output is effectively integrated over a large time constant and must be restored by differentiation.

iv. This may be performed by the correct equalization at a later stage as illustrated in Figure 6.7.

Fig6.7: High-impedance integrating front-end optical fiber receiver with equalized voltage amplifier

v. Therefore the high-impedance (integrating) front-end structure gives a significant improvement in sensitivity over the low-impedance front-end design, but it creates a heavy demand for equalization and has problems of limited dynamic range (the ratio of maximum to minimum input signals).

The Trans impedance front-end:

i. This configuration largely overcomes the drawbacks of the high-impedance front end by utilizing a low-noise, high-input-impedance amplifier with negative feedback.

ii. The device therefore operates as a current mode amplifier where the high input impedance is reduced by negative feedback.

iii. An equivalent circuit for an optical fiber receiver incorporating a transimpedance front-end structure is shown in Figure 6.8.

Fig6.8: An equivalent circuit for the optical fiber receiver incorporating a transimpedance preamplifier

iv. In this equivalent circuit the parallel resistances and capacitances are combined into $R_{TL}$ and $C_T$ respectively. The open loop current to voltage transfer function $H_{OL} (ω)$ for this transimpedance configuration corresponds to the transfer function for the two structures.