# Optical Amplifier | Power Amplifier, In-line, Pre-amplifier

## Optical Amplifier

Optical Amplifier: An optical amplifier is a widely used device in optical fiber communications for the purpose of amplification the optical signal generated by an optical transmitter. The optical signal is directly amplified to yield optical signal without any conversion to electrical signal first and then restoring it to the amplified optical signal. This implies that optical amplifiers operate on photons in the all-optical domains. In essence, optical amplifiers do require source and photodetector as optoelectronic devices and additional electronic circuits for other operations such as the shaping of optical pulses including retiming.

why do we need optical amplifiers? We know that typical fiber loss around 1500-nm wavelength is approximately 0.2 dB/km. Let us examine what happens after the optical signals travel about 100 km down the fiber. The signals are attenuated by 20 dB. So they need to be amplified, otherwise, the signal-to-noise ratio of detected signals is too low and the bit-error-rate (BER) becomes too high (typically desired value of BER is <10-9). One way to resolve this issue is to detect the weak signals, followed by modulating a new laser (optical-to-electrical-to-optical conversions) which requires high-speed (>10 GHz) electronic circuitry. Therefore, the best way to amplify the signal is only optically and the preferred method is fiber amplifier which is the most efficient, the most stable, and the one with the lowest loss.

## Functional Types of Optical Amplifiers

An optical amplifier is nothing but a laser diode with anti-reflection coatings in place of end mirrors. The signal light is passed through a semiconductor which is essentially a single-mode waveguide having ~ 0.5–2 mm length and 1–2 μm transverse dimensions. The figure is shown below a simplified functional block schematic of a generic optical amplifier.

The optical input signal flowing through the optical fiber is applied to an active medium (an amplifying region) through a fiber-to-amplifier coupler. The active medium is pumped with an electric current from a pump source. This results in optical transitions from the conduction band to the valence band because the injection current from the pump source creates a certain carrier density in the conduction band. Since the photon energies are slightly above the bandgap energy, the maximum optical gain occurs.

The distance between a transmitter and receiver link is extended by the use of optical amplifiers.

They can compensate for signal attenuation mainly due to fiber loss. But this may reduce the optical SNR (and hence noise figure) by a small amount. Moreover, dispersion (and crosstalk in DWDM systems) cannot be compensated by optical amplifiers. In terms of the functions performed in a typical optical fiber communications link, optical amplifiers can be broadly classified in three basic types as given below:

1. Power Amplifier or Booster (at Optical Transmitter)
2. In-line Optical Amplifier (along with the fiber)

### Power Amplifier or Booster

It is a power amplifier that raises the power of an optical signal available at the output of an optical transmitter to the highest level before sending it down the optical fiber. The figure is shown below an arrangement of deploying optical amplifier as a power amplifier.

Depending on the optical gain of the power amplifier and fiber losses, it is possible to increase the transmission distance by 100 km or more. For example, most of the DFB lasers used as optical sources deliver very small output power of the order of 2 mW only. A power amplifier used immediately after the optical transmitter can boost the optical signal.

### In-line Optical Amplifier

An in-line optical amplifier operates with a signal in the middle of a fiber–optic link. The figure is shown below the use of optical amplifiers as in-line amplifiers along with the fiber link between the optical transmitter and the optical receiver.

The main function of an in-line optical amplifier is to compensate for signal losses caused by fiber attenuation, losses due to interconnections, and signal distribution in WDM networks. In a typical application, an optoelectronic repeater (comprising of a photodiode, timing and shaping circuits, and laser) can be replaced with an appropriate in-line optical amplifier within a long-haul optical fiber communication link, as shown in the figure below.

Many in-line optical amplifiers can be cascaded along the fiber link in a long-haul fiber–optic communication link, as shown in Fig. 5.5.

In this arrangement, we have to ensure that the system performance is not degraded significantly by cumulative effects of amplified spontaneous emission (ASE) noise, dispersion, non-linearity, and stability over entire WDM bandwidth. To compensate for the accumulated ASE noise level, there should be almost linear increase of optical signal power with the length of the optical fiber link.

This helps to maintain a constant SNR. Moreover, for a very low noise figure, high power output, as well as high optical gain, is needed.

There is an important parameter, noise penalty factor (Npf) associated with cascaded in-line optical amplifiers. It is a measure of the path-average signal energy that must be increased (as optical gain increases) in order to maintain a fixed SNR. It is expressed as

$N_{pf}=\frac{1}{G}\left (\frac{G-1}{\ln G} \right )^{2}$

where, G represents the optical gain (in ratio) of an in-line amplifier.

It is imperative to say here that in order to yield the best combination of overall optical gain and output SNR, the location of cascaded in-line optical amplifiers should be uniform along with the fiber optic communication link. Typical values of input optical power level for these in-line optical amplifiers range from -26 dBm to -9 dBm (i.e., 2.5 μW to 125 μW), with optical gains of more than 15 dB.

### Pre-amplifier in Optical Amplifier

When an optical amplifier is used as a pre-amplifier, it amplifies an optical signal just before it reaches the optical receiver, as depicted in figure below

By using an optical amplifier as a pre-amplifier just before an optical receiver, what exactly do we achieve? Obviously, the sensitivity of direct-detection optical receivers is significantly improved which has been limited by thermal noise. Another advantage is to compensate for distribution losses in LANs. The next question arises: what is the most essential requirement of an optical amplifier to be used as a pre-amplifier? It is, of course, a low noise characteristic because the input signal level is usually very low. The optical gain requirement may not be very high because the received optical signal is applied directly into an optical receiver. Moreover, a pre-amplifier can operate well below saturation; it will not have any feedback control. So, a pre-amplifier should have good sensitivity, high gain, and low noise.

The detector sensitivity or the improvement of minimum detectable signal of an optical amplifier employed as a pre-amplifier at the receiver end can be defined as the ratio of minimum value of the electrical signal power, Smin, that is required for it to perform with an acceptable bit-error rate (BER) to the new minimum detectable electrical signal level S¢min that is needed to maintain the same signal-to-noise ratio. It is given as

$\frac{S_{min}}{S_{min}^{'}}=G^{2}\frac{N}{N+N^{'}}> 1$

where, G represents the gain of an optical pre-amplifier, N represents the receiver’s noise power level, N’ is the spontaneous emission from the optical pre-amplifier that gets converted by the photodiode in the receiver to an additional background noise.

## Comparison between optical amplifiers for a type of usage

Now the question arises of how to select an optical amplifier to be used either as a power amplifier at the optical transmitter end, or in-line amplifier along with a fiber link, or a pre-amplifier at the optical receiver end. There are three major performance parameters: optical gain, maximum output power, and noise figure which decides its usage. The table is shown below the requirements for these three types of usage of optical amplifiers in optical fiber communication link applications.

Table: Selecting optical amplifiers for type of usage

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