An In-Line optical amplifier is a device placed within the optical fiber network, at specific intervals, to amplify optical signals without the need for converting them into electrical signals. These amplifiers are essential for overcoming signal losses that occur as light travels through long stretches of optical fiber. 

The in-line optical amplifier needs only a small amount of signal to work and makes it stronger for sending it further through the fiber optic cable. If the weak signal is controlled and noise is reduced, the system will work better. However, if too many amplifiers are added in a row, it can limit the distance that the signal can travel in the system.

In-line optical amplifiers are very simple and easy. Sometimes, the initial amplification provided by the pre-amplifier and booster amplifier may not be sufficient due to the signal loss during long-distance transmission through optical fibers. So, in-line optical amplifiers are placed in the optical fiber network every 80-100 kilometers. These amplifiers offer a moderate boost in signal strength and have a similar output power to booster amplifiers.

In optical communication systems, besides providing substantial optical amplification, in-line amplifiers also offer a broad optical bandwidth and maintain consistent amplification across that bandwidth to support complex optical systems like wavelength-division multiplexed (WDM) systems. Also, their amplification should be linear to prevent unwanted interference between different wavelength channels.

In-Line Optical Amplifier in Fiber Optic Connection

An in-line optical amplifier functions at the midpoint of a fiber-optic connection. An in-line optical amplifier within the fiber link connecting an optical transmitter to an optical receiver is illustrated below. 

The primary purpose of an in-line optical amplifier is to counteract signal attenuation resulting from factors such as fiber loss, interconnection losses, and signal dispersion in WDM networks. In a typical case, an optoelectronic repeater, comprised of a photodiode, timing, and shaping circuits, and a laser, can be substituted with a suitable in-line optical amplifier in an extended-distance optical fiber communication link, as shown in the figure below. 

Multiple in-line optical amplifiers can be daisy-chained along the fiber link in a long-distance fiber-optic communication setup, as depicted below. 

In this configuration, it is essential to ensure that the system's performance remains relatively unaffected by cumulative factors like amplified spontaneous emission (ASE) noise, dispersion, non-linearity, and stability over the entire WDM spectrum. To offset the cumulative ASE noise level, there should be a nearly linear rise in optical signal power with increasing fiber link length. 

This is crucial for sustaining a constant Signal-to-Noise Ratio (SNR). Also, achieving a low noise figure increases power output and optical gain. An essential parameter associated with cascaded in-line optical amplifiers is the noise penalty factor (N_pf). This factor measures the increase in path-average signal energy required to maintain a fixed SNR as optical gain grows. It is expressed as 

where G denotes the optical gain (as a ratio) of an in-line amplifier. 

To get the best overall optical gain and output SNR, it is necessary to evenly distribute cascaded in-line optical amplifiers throughout the fiber optic communication link. Typical input optical power levels for these in-line optical amplifiers typically range from -26 dBm to -9 dBm, corresponding to 2.5 μW to 125 μW, with optical gains exceeding 15 dB.

Benefits of In-Line Optical Amplifiers

• Long-Distance Transmission: In-line optical amplifiers enable data to be transmitted over extremely long distances without significant signal degradation. This is important for undersea cables and cross-country fiber-optic networks.
• High Data Rates: By maintaining signal quality, in-line optical amplifiers allow for the transmission of high data rates, making them ideal for applications like internet backbone networks and data center interconnects.
• Reduced Latency: In-line optical amplifiers eliminate the need for signal conversion to electrical form and back to optical, reducing latency in the communication process.

Applications of In-Line Optical Amplifiers

In-line optical amplifiers find a wide range of applications in the field of optical communications and beyond. One important application is in long-haul optical fiber networks, where in-line optical amplifiers are strategically placed at intervals to combat signal loss over extensive distances. This is crucial for transmitting data across continents, enabling international telecommunications and high-speed internet connections. In addition to telecommunications, in-line optical amplifiers are also utilized in optical sensing systems, such as distributed temperature or strain sensors, where they enhance the sensitivity and reach of these sensors by amplifying weak optical signals. Also, in scientific research, in-line optical amplifiers play a vital role in experiments involving precise optical measurements, where signal integrity and strength are paramount for accurate data collection.