Ever wondered what a semiconductor optical amplifier (SOA) was and what sort of applications it is used for? Do you wish to know how semiconductor optical amplifiers perform in comparison to other optical amplification methods like optical fiber amplifiers? Then you’re at the right place. Learn all about optical amplifiers, what makes semiconductor optical amplifiers great, what applications they have in digital communications, and a few tips on what to consider when choosing a semiconductor optical amplifier for yourself. Let’s begin!
What Are Optical Amplifiers And Why Are They Important For Transmitting An Optical Signal?
An optical amplifier is a device that can directly boost (amplify) an optical signal without converting it into an electrical signal. Optical amplifiers of all kinds are used primarily in fiber optical communication applications where data is transmitted in the form of light through a fiber optic cable.
When an optical signal is sent through a fiber cable, the quality of the signal gets degraded as it travels due to various reasons. The transmission loss when light is transmitted through an optical fiber is minuscule with less than 0.2 dB per km for optical wavelengths in the 1,550 nm band. This is negligible for small distances like within a room or a building. However, when data has to be transmitted over large distances like hundreds of kilometers, this transmission loss cannot be ignored.
By the time the optical signal is received at the output end, it might be nearly impossible to retrieve all the original information it had near the input. An optical amplifier can ensure that the integrity of the optical signal is maintained and degradation is minimal to receive the data without it being heavily distorted.
Due to dramatic advancements in communications, processing, and even consumer electronics, more and more data needs to be transmitted. The latest smartphone apps, voice assistants, and other Internet of Things (IoT) applications require data communication between these devices and data centers where all the sophisticated analytics are done.
Communication within data centers is handled by the Ethernet standard IEEE802.3ba which utilizes the 100GBASE-LR4/ER4 technology. To communicate with other data centers or to send and receive data to and from mobile phone base stations that are 10 to 40 kilometers away, optical amplifiers are required to compensate for the transmission loss. For the 1.3 μm band light sources used in these optical signals, a semiconductor optical amplifier (SOA) is used on the receiver end.
How Optical Amplifiers Work
Any sort of optical network generally has transmitters and receivers connected by fiber cable to carry information between these ends. Optical amplifiers are placed somewhere in between for signal amplification. The following method of amplification used by optical amplifiers is called ‘stimulated emission’.
The optical amplifier has an active medium that receives energy from a ‘pump source’ which is an electrical current. The electrons in the active medium get excited to a higher energy level. When an optical input signal is provided, this signal travels through the active medium. This triggers the excited electrons to lose energy in the form of photons and return to their ground state. The emitted photons will have the same wavelength range as the input signal. These photons when added to the input signal strengthen it, and the resulting output signal is amplified.
Optical Amplifiers Vs Electrical Amplifiers
An electrical amplifier can also work although it requires the optical signal to be converted to an electrical signal and back again. This conversion requires additional pieces of equipment which can slow down the data processing and transmission rate. Optical amplifiers don’t require those components. Faster data rates over longer distances without signal attenuation were made possible through the development of optical amplifiers.
The Semiconductor Optical Amplifier (SOA)
Semiconductor optical amplifiers work on the same stimulated emission method. The active medium is made from semiconducting material from Group III and V such as GaAs/AlGaAs, and InP/InGaAs. Newer models of semiconductor optical amplifiers have introduced anti-reflective coatings and tilted waveguides to reduce unwanted reflective properties at the end faces.
Semiconductor optical amplifiers are used by the telecommunication industry to obtain a gain of up to 30 dB. SOAs usually operate in the 850 nm and 1600 nm signal wavelengths. SOAs are small devices that connect to fiber pigtail components. Semiconductor optical amplifiers are known for their conveniently small size and affordability.
Tapered Semiconductor Optical Amplifiers
Regular semiconductor optical amplifiers are limited when it comes to output power. Tapered amplifiers are optimized to deliver a high output power that can be from hundreds of mW up to 03 Watts. It can deliver a higher output power and a broader wavelength range than the regular SOAs while maintaining high beam quality. The input power is 10 to 50 mW and the typical wavelengths are from 633 to 1480 nm.
This is possible through the tapered section which starts with a single-mode waveguide of limited transverse dimensions. The cross-sectional area of the amplified beam is gradually increased along the direction of light propagation.
Vertical Cavity Semiconductor Optical Amplifiers (VCSOA)
This is a fairly recent addition to the semiconductor optical amplifiers that work similarly to the vertical cavity surface emitting lasers (VCSELs). The difference is that mirror reflectivity in the amplifier cavity is reduced to prevent reaching the laser threshold.
Compared to standard semiconductor optical amplifiers, the VCSOAs are smaller, more affordable, and can be fabricated in high volumes in arrays. They can even be used by a lower drive current with even a 10mA current sufficient to obtain a gain of 20dB.
Other Optical Amplifiers
Semiconductor amplifiers are only one type of amplifier and there have been many other technologies developed over the years that can serve the same function.
Erbium-Doped Fiber Amplifiers
Before the optical amplifier came into existence, optical repeaters were used. This involved temporarily converting the signal light into an electrical signal, amplifying and regenerating the waveform before converting it back into light. This all changed after the invention of the doped fiber amplifier (DFA) in 1987. Since the early 1990s, the DFA has remained the most widely used form of optical amplifier in the telecommunications industry.
It bears this name because the active medium is made by doping a silica core with rare earth elements. Erbium is the element usually used for this purpose. The amplification through an Erbium-doped fiber amplifier (EDFA) promises high gain and a low noise figure in the 1.55 μm band or 1.58 μm band. It is also not dependent on the polarization of the signal.
Fiber Raman Amplifiers
In a Raman amplifier method, it is the nonlinear interaction between the signal and a pump laser that allows the amplification. The amount of power required for the pump exceeds that of EDFA-style amplifiers, requiring up to 500mW or more. An optical power of 1W may be necessary for the lumped-type Raman amplifiers.
Semiconductor Optical Amplifier Vs Optical Fiber Amplifier
With competing technologies being used in the telecommunications industry, let’s compare the semiconductor optical amplifier against the doped fiber amplifier to see what each one is good at. An SOA is very lightweight and compact, often comprising a single semiconductor chip that has electrical and optical ports attached to it. The cost is naturally much lower as well.
An EDFA is a larger device that may also include other components such as Faraday isolators that block light in one direction. The models used for very long distances up to 500 kilometers and more are usually large rack-mounted devices with multiple channels. The output power of an EDFA is significantly higher and so is the required drive current to make it work.
The gain bandwidth is actually similar in both cases since different devices can be built to the specifications required. An SOA has a higher noise figure and exhibits nonlinear distortions at a moderate power level. Normally these distortions are undesirable although there are cases where they can be useful in optical signal processing. Finally, the amplification in SOA is polarization dependent.
Things To Consider When Choosing Your Semiconductor Optical Amplifier
If you’re looking for a semiconductor optical amplifier, here are a few simple parameters you need to consider and check on the product data sheet. First on the list is the amplifier gain which indicates the factor by which the input is amplified. It is measured in dB which is the ratio of output power to input power. A high-gain amplifier will deliver a high-output signal.
Next is the gain bandwidth which is the range of signal wavelengths that will get amplified. The wider the gain bandwidth, the more wavelengths can be amplified. The saturation output power is the maximum output power that the amplifier can deliver. A higher saturation output power is desirable to have a high dynamic range and for the amplifier to remain in the linear working region.
The final point to consider when choosing a semiconductor optical amplifier is its noise figure. It is the measure of the signal-to-noise ratio (SNR) degradation that is caused by the amplifier itself during normal operation. The amplifier noise figure is normally around 5 dB.
Other Applications Of A Semiconductor Optical Amplifier
Apart from simply amplifying optical signals, SOAs are useful in other applications in telecommunications. Nonlinear qualities in gain saturation and changes in the refractive index due to the carrier density are both properties that can be useful. Channel translation in wavelength division multiplexing systems, clock recovery, pattern recognition, modulation format conversion, and signal regeneration are some of these applications.
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A semiconductor optical amplifier contains a component called an ‘active medium’ that amplifies the input light signal through a process called stimulated emission.
An optical amplifier is used in long-distance communication networks to maintain the integrity of the data being transmitted without getting degraded.
The gain, gain bandwidth, saturation output power, and noise figure are the parameters that need to be considered when choosing an optical amplifier for a specific application.