choice in next-generation optical systems. Continuous innovations in efficiency, miniaturization, integration,

cost-effectiveness and manufacturing techniques concerning this device are rapidly overcoming historical

limitations, ensuring its ascendancy in both academic research and industrial applications. This paper explores

the evolution of the SOA, highlighting key advancements in its development and future prospects.

Collectively, all the evidence points to its inevitable dominance in future practical optical communications,

mainly by virtue of its adaptivity, either as a standalone competitive device or with hybrid-like integration with

other types of amplifiers.

The main optical amplifier types currently in use today commercially are the erbium-doped fibre amplifier

wavelengths (1565–1625 nm) [4]. However, as network demands evolve, researchers are exploring alternatives

that could complement or even replace EDFAs in certain scenarios [5]. These C- and L-bandwidth limitations

are incompatible with future networks requiring broader or different spectral ranges (e.g., the O-band for

shorter-reach optics) [6]. Future amplifiers will also need to offer more compatibility and better power

efficiency, crucial for data centres and space-constrained applications [7]. Apart from the SOA, all the other

amplifiers are fibre-based and bulky compared to on-chip amplifiers like the SOA, and emerging technologies

such as coherent optics and free-space optical communications may need different amplification approaches

[8]. The key demands for the amplifier of the future will be driven by future bandwidth requirements,

integration needs and power-efficiency targets utilizing ultra-efficient on-chip amplification [9].

One of the most likely candidates to replace the EDFA in the future is the SOA. At present, the SOA occupies

modulators, and detectors on a single chip—an essential feature for co-packaged optics and future photonic

interconnects [13]. Being already compact and chip-scale, SOAs are also viewed as promising candidates for

nanophotonics [14] and quantum communication systems [15], where ultra-compact and low-power

amplification is critical. Also, their ultrafast gain dynamics enable many applications in optical switching and

high-speed signal processing [16]. Nevertheless, SOAs currently face challenges such as relatively higher

noise figures (5 to 8 dB, compared to ~4 dB for EDFAs) and nonlinear effects like cross-gain modulation,

which limit their effectiveness in long-haul transmission [17]. With continuous advances in fabrication

techniques and the growing demand for miniaturization, SOAs are expected to dominate short-reach

applications, and potentially challenge EDFAs in future long-haul networks [18].

Moreover, SOAs are not only compact amplifiers but also versatile functional devices due to their nonlinear

switching and routing in next-generation network architectures [26]. Their ability to amplify and process

optical packets is instrumental in ensuring efficient data transmission and routing in these advanced systems

[27]. The most fundamental function of an SOA is to boost signals all-optically—such as serving as an in-line

all-optical regenerator (2R/3R) or as a pre-amplifier [28]. SOAs are currently essential for maintaining signal

integrity in short-range optical communication systems like MANs and LANs, where they amplify over

relatively short distances [29], [30]. Due to ever-increasing data internet traffic, and hence the need to avoid

bottlenecks, the SOA is increasingly becoming integrated in wavelength conversion functions [31]. This is

internally facilitated through non-linear optical effects; such as cross-gain modulation (XGM) [32], cross-

phase modulation (CPM) [33] and FWM [34]. SOAs can also perform optical logic operations such as AND,

OR, and NOT, using nonlinear interactions and carrier dynamics [35] and perform pulse reshaping [16, 24,

consequence of higher input power, the saturation effects of the SOA considerably reduced the measured

relative-intensity noise (RIN) in the output FWM signal. Work done since with ASE FWM has improved the

technique and practically eliminated polarization effects from the SOA when converting wavelength in such

systems [38].

This paper reviews how the SOA device has changed over the years and its strong potential to dominate in

future optical systems. Section 2 introduces the scope of the device and application ideas. Section 3 reviews

the theory behind the practical workings of the SOA. Section 4 looks at changes in manufacturing trends over

the years. Section 5 reviews former (historical), current (modern), and future (emerging) roles in photonic and

telecommunication systems. Finally, conclusions are made in Section 6.

TABLE 1 Timeline of Developments in SOAs

1990s

commercializatio

n begins

introduction of

traveling-wave SOAs

(TWSOAs),

introduction of

quantum-well

structures

fibre optic communication systems -

signal regeneration and wavelength

conversion

Future

ultra-fast,

quantum & green

photonics

ultrafast, low-energy

SOAs,

spintronic and quantum

SOAs

quantum networks, sustainable

photonic systems

From table 1, it is worth highlighting that before the advent of QD-SOAs, conventional semiconductor optical

amplifiers (using bulk materials or quantum wells) suffered from slow gain recovery (limiting data speeds),

temperature sensitivity (requiring coolers) and high signal distortion. The development of Quantum Dot SOAs

compatibility with semiconductor processes, SOAs can more easily be integrated with lasers, modulators and

detectors on-chip [25, 55]. This allows for the development of more compact, power-efficient and higher-

SOAs have now also entered the realms of optical sensing and biomedical imaging systems. For example, in

optical coherence tomography (OCT) [56], it has been shown that the SOAs broad bandwidth, compactness,

ruggedness, electrically pumped advantages, tunable gain capability and cost effectiveness were all factors

instrumental in providing the necessary broadband light for high-resolution OCT imaging. Of course, SOAs

themselves are not classical sensors like photodiodes or strain gauges - they do not directly measure

temperature, pressure, chemicals, etc. However, because they are very sensitive to changes in their

environment (like temperature, input optical power, bias current, and even surrounding refractive index), these

sensitivities can be used to detect changes [57]. In this work, the SOA acted as the active element enabling a

fibre ring laser to function effectively as a dynamic strain sensor, where strain-induced changes modulated the

laser output detected through the arrayed waveguide grating demodulator.

saturation, and input-output power relationships—critical factors in optical communication and signal

processing applications.

The theoretical gain of a SOA is typically given by G = exp(Γ · g · L), where Γ is the optical confinement

factor, g is the material gain coefficient (in cm⁻¹), and L is the length of the active region (in cm) [42]. Under

small-signal conditions, where input power is low, the gain simplifies to G₀ = exp(g₀ · L), with g₀ representing

the unsaturated material gain. At higher input powers, gain saturation occurs and the gain becomes power-

dependent, following G(P) = G₀ / (1 + P_in / P_sat), where G(P) represents the saturated gain of the

semiconductor optical amplifier (SOA) as a function of input power, P_in is the input optical power and P_sat

100 to 2000 cm⁻¹ and device lengths between 200 µm and 2 mm. This SOA gain mathematics is shown in