What are Amplifier Chains?

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- GoPhotonics

Feb 15, 2024

An Amplifier chain consists of several optical amplifiers in succession, often with intermediary optical elements between the stages, each capable of amplifying a signal sequentially. This arrangement is prevalent in both electronic and laser technologies. In laser systems, amplifier chaining is particularly common, wherein a series of optical amplifiers are employed to successively amplify a signal. This approach involves multiple stages, where the initial stage is commonly referred to as the preamplifier, while the final stage is designated as the power amplifier.

The fundamental motivation for employing amplifier chaining lies in the optimization of diverse performance parameters, necessitating trade-offs between factors such as saturated output power and gain. For instance, achieving a high saturated output power requires a large mode area in the final amplifier section, while the preamplifier benefits from a smaller effective mode area to enhance gain efficiency. Mode area refers to the spatial extent of the optical mode within the gain medium of the amplifier. The optical mode represents the distribution of the electromagnetic field associated with the propagating light wave. The mode area is a measure of the cross-sectional area over which the majority of the optical power is distributed. Mode area is a critical parameter associated with the optical mode. In the context of amplifier chaining, optimizing the mode area at each stage becomes crucial due to the inherent trade-offs between various performance parameters.

Gain Efficiency and Saturated Output Power 

The gain efficiency of an amplifier is influenced by the size of the mode area. A smaller mode area can enhance gain efficiency, as a larger portion of the input signal interacts with the gain medium, leading to a more effective amplification. On the other hand, achieving high saturated output power, which is the maximum output power a saturated amplifier can provide, often requires a larger mode area. This is because a larger mode area can accommodate higher optical power levels without reaching detrimental levels of nonlinear effects.

In amplifier chaining, where multiple stages are employed, optimizing the mode area at each stage provides a more refined strategy in balancing gain efficiency and saturated output power. The preamplifier stage may benefit from a smaller mode area to maximize gain efficiency, while the final power amplifier stage may require a larger mode area to achieve higher saturated output power.

By independently optimizing the mode area in different stages of the amplifier chain, the performance of each stage to specific requirements can be customized, which leads to an overall enhancement in the efficiency and effectiveness of the entire amplification process. In ultrashort pulse amplifiers, it may be essential to increase the mode area of the power amplifier for reducing nonlinear effects.

Combination of Amplifiers

Diverse types of amplifiers can be combined within a chain, such as a regenerative amplifier serving as a high-gain preamplifier coupled with a multipass amplifier functioning as a power amplifier with moderate gain. Alternatively, an optical parametric amplifier may be integrated with a laser amplifier.

In amplifier chaining, sometimes it is necessary to add certain optical elements between different stages to address specific requirements. Examples include optical isolators between stages in high-gain amplifiers to minimize sensitivity to amplified spontaneous emission (ASE) and back-reflections, the integration of bandpass filters to reduce total ASE power in both propagation directions, and the inclusion of optical modulators (e.g., acousto-optic modulators) to selectively transmit pulses during specific time intervals in ultrashort pulse amplification systems.

Utilizing amplifier chains enables a modular approach, wherein various performance metrics can be achieved by diverse combinations of existing amplifier stages (modules).

Example of Amplifier Chaining

Consider a scenario where there is a fiber optic communication link connecting two cities, City A and City B, which are 500 km apart. Due to the high attenuation of the optical signal over this distance, multiple optical amplifiers need to be deployed along the transmission path.

The transmitter, located in City A, converts electrical signals into optical signals and launches them into the optical fiber. Along the optical fiber path, optical amplifiers are strategically placed at certain distances, such as every 80 km. These amplifiers, starting with Optical Amplifier 1 (OA1), boost the power of the optical signal without converting it into an electrical signal. The amplified signal then travels further down the fiber. Additional optical amplifiers, such as Optical Amplifier 2 (OA2), Optical Amplifier 3 (OA3), and so forth, are installed at intervals along the fiber link. The number and placement of these amplifiers depend on factors such as the total length of the link and the required signal strength at the receiver end. Finally, at the receiver located in City B, the optical signal is received, converted back into electrical signals, and processed accordingly.

Applications of Amplifier Chains

Amplifier chains find diverse applications across various fields due to their ability to enhance signal strength and performance. Some notable applications include, long-distance fiber optic communication systems, laser technology, radar systems, medical imaging devices, radio frequency (RF) and microwave communication systems, military applications, broadcasting stations, and space communication systems.

Click here to know more about Amplified Spontaneous Emission (ASE).