semiconductor optical amplifieris based on semiconductor gain mediaoptical amplifier. It is essentially like a fiber-coupled semiconductor laser tube, and the end mirror has been replaced by an antireflection film.; The inclined waveguide can be used to further reduce the end reflectivity. The signal light is typically transmitted through a semiconductor single-mode waveguide with lateral dimensions of 1-2 μm and a length of about 0.5-2mm. The waveguide mode has significant overlap with the active (amplified) region, which is pumped by the current. The injection current generates a certain carrier density in the conduction band, allowing an optical transition from the conduction band to the valence band. The gain maximum occurs at photon energies slightly above the bandgap energy.

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Working principle of semiconductor optical amplifier

The semiconductor optical amplifier amplifies the incident light signal through stimulated emission, and its mechanism is the same as that of the semiconductor laser.SOA Optical AmplifierIt is just a semiconductor laser without feedback. Its core is that when the semiconductor optical amplifier is optically or electrically pumped, the population inversion obtains optical gain, as shown in the figureAs shown in 1a. The optical amplifier gain distribution curve and the corresponding amplifier gain spectrum curve are shown in FIG. 1b.

However, the semiconductor laser has reflection at the cleavage plane (reflection coefficient R is about 32%), and has a considerable feedback. When the bias current is below the threshold, they are used as semiconductor optical amplifiers, but multiple reflections at the Fabry-Perot (F-P) cavity interface must be considered, as shown in Figure 2a. Such a semiconductor optical amplifier is called a F-P amplifier.

When R1 = R2 and ν = νm is taken into account, the amplification factor GFPA(ν) of the amplifier can be obtained using F-P interference theory.

Figure1 Semiconductor optical amplifier principle and gain distribution curve.

a) traveling wave semiconductor optical amplifier B) optical amplifier gain distribution curve g(ν) and corresponding amplifier gain spectrum curve G(ν)

When the frequency νs of the incident light signal is equal to the cavity resonance frequency νm, the gain GFPA(ν) reaches a peak, and when νs deviates from νm, the GFPA(ν) decreases rapidly, as shown in FIG. 2b. As can be seen from the figure, when the reflectivity of the semiconductor cleavage plane and air is R = 0.32, the peak value of the F-P amplifier at the resonant frequency is the largest. The smaller the reflectivity, the smaller the gain. When R = 0, the semiconductor laser becomes a semiconductor traveling wave optical amplifier, and its gain spectrum characteristic is a Gaussian curve.

Figure2 Fabry-Perot (F-P) Semiconductor Optical Amplifier (SOA)

a) The structure and schematic diagram of SOA B) The gain spectrum curve of different reflectivity of SOA.

     From the above discussion, it can be seen that increasing the reflectivity R of the F-P resonator that provides optical feedback significantly increases the gain of the SOA, the greater the reflectivity R, the greater the gain at the resonant frequency. However, when R exceeds a certain value, the semiconductor optical amplifier becomes a semiconductor laser. When GR = 1, equation (1) will become infinite, at which time the SOA produces laser emission.

SOA Amplification Features

Inphenix semiconductor optical amplifiers have proven to be versatile and multifunctional devices and are key building blocks of optical networks. There are five main parameters used to characterize a semiconductor optical amplifier (SOA):

Small signal gain (gs); up to 25 dB at-22dBm input power;

Gain bandwidth; up to 3 nm at 80 dB

· Saturated output power （psat) up to 17 dBm;

· Noise coefficient （NF） 7.0-8.0 dB;

• Polarization dependent gain （PDG)<1 dB or up to 10 dB

SOA Optical AmplifierIt shall have the highest gain suitable for the application. wide optical bandwidth is also required in orderThe SOA can amplify a wide range of signal wavelengths. Gain saturation effects can introduce undesirable distortion to the output, so an ideal SOA should have a very high saturated output power to achieve good linearity and maximize its dynamic range with minimal distortion. The ideal SOA should also have a very low noise figure (physical limit of 3dB) to minimize the amplified spontaneous emission (ASE) power at the output. Finally, an ideal SOA should have very low polarization sensitivity to minimize the gain difference between transverse electric (TE) and transverse magnetic (TM) polarization states. However, an ideal SOA is impossible to achieve because of the physical limitations of the various processes that occur within it.

Figure3- SOA interrelated parameters

SOA parameters are closely related, and in order to achieve an optimal value of one parameter, other specifications and/or control spectrum operating areas should be compromised, as shown in Figure 3.

Types of SOA

AccordingThe role of SOA in the customer system, they can be divided into four categories: tandem, booster; switching SOA and preamplifier;

· In-line: higher gain, medium Psat; lower NF and lower PDG, usually polarization-independent SOA;

· Enhancer: higher Psat, lower gain, usually depending on polarization;

· Switch: higher extinction ratio and faster rise/fall time;

· Preamplifier: Suitable for longer transmission distance, lower NF and higher gain.

in addition,The PDG may determine the polarity of the SOA. For example, if the PDG is less than 1.5dB, the SOA is polarity independent (P-I), and if the PDG is up to 10 dB, the SOA is polarity dependent (P-D).

Application of SOA

1 Traditional applications

Amplification isApplication of basic principles of SOA in optical communication systems. SOAsarea highly versatile components for various amplification and routing functions in telecommunications. Commercialized SOA is now widely used in the market and is rapidly becoming a cost-effective solution for optical amplification in advanced optical systems for core, metro, and final access applications. The SOA can be used in any optical communication network to regenerate the signal at various points in the link by acting as a boost amplifier (post amplifier), an in-line amplifier.

(2) Telecommunications

SOA is widely used in many industries. One of the most important industries is telecommunications, which are valued in routing and switching. In addition, SOA is also used to enhance or amplify the signal output of long-distance optical fiber communication. In this application, telecommunications companies use fiber optic lines from the headquarters to the data center. These transmission lines may exceed 10 kilometers or more and require the use of SOAs to enhance/amplify the signal from the usual light sources.

The diagram in the photon carrier4-SOA (top) can be used in photonic integrated circuits (PIC)(bottom left)

3. Functional application

SOAs may also be used to perform functions useful in future optically transparent networks. These all-optical capabilities can help overcome the so-called "electronic bottleneck", which is currently the main limiting factor for the deployment of high-speed optical communication networks (such as optical wavelength converters). SOA function applications are always based on SOA non-linearity. These non-linearities are mainly caused by the carrier density changes caused by the amplifier input signal. The four main types of nonlinearities commonly used in SOAs are cross-gain modulation (XGM), cross-phase modulation (XPM), self-phase modulation (SPM), and four-wave mixing (FWM).

4. Sensing

Sensing is used in many applicationsAnother important industry for SOA. An important use of SOA in sensors is fiber Bragg demodulators. In this setup,,SLDorDFB Used as an input light source.SOALift the optical signal to the fiber Bragg grating (FBG), usually through the circulator to control the direction of the optical signal. Changes in temperature or strain can change the wavelength or timing of the optical signal to the PD/sensor. This may alert the user to a possible malfunction.

Figure5-Fiber Bragg Grating with SOA In-line Amplifier

Another important use of SOA in sensing is light detection and ranging (LiDAR). LiDAR devices can be small and used only for Doppler ranging, or they can be displayed as an array capable of mapping. One example of a LiDAR application uses frequency modulated continuous wave (FMCW) to detect the Doppler effect of motion, such as autonomous vehicles and drones. In addition, FMCW can also be used for mapping and inspection. The SOA used in narrowband (typically used with DFB) can have a high output power> 20mW for longer ranges.

Figure6-SOA-integrated LiDAR chip. An array of these chips provides wide area scanning.

Figure7-Frequency Modulated Continuous Wave (FMCW) Lidar. Green SOA.

5、CWDM

The small size, good integration capabilities and great potential for cost reduction through scaled manufacturing processes highlighted previously will continue to ensureSOA will play an increasingly important role in the future of advanced optical networks.coarse wavelength division multiplexing （CWDM） is an economical route that provides connectivity flexibility and increased throughput for the metro and enterprise network layers. ExtensionThe capacity and distance of CWDM systems (>100 kilometers) require low cost to operate over the entire optical bandwidth (from 1260 nm to 1620 nm)optical amplifier.SOA is currently the only viable technology that can be deployed to meet these expanding applications.

6. Bodom-PON

An extended role of SOAs in telecommunications is their use in wavelength division multiplexing-passive optical networks (WDM-PON). Cable companies utilize fiber optic lines from home offices to customers receiving data, often with nodes or distribution centers to assist in switching and routing data. This setup allows for the efficient distribution of data to a large customer base. SOA was an early application of WDM-PON, but it is likely to grow in the future.
There are other attractive applications of SOA, suchintensity modulatorandphase modulatorfor optical signal processingSOA logic, SOA add/drop multiplexer for optical time division multiplexing networks, SOA pulse generator that can easily generate pulses at high frequencies (> 10 GHz,optical receiverandThe SOA clock recovery required by the 3R generator, the SOA dispersion compensator overcomes the limitation of dispersion on the transmission distance, and the SOA detector detects the optical signal. The SOA may also be used to gate an optical signal, I .e. the signal may be amplified or absorbed by the SOA. The blocking characteristics of SOAs at low bias currents are very useful because they support channel routing functions, such as reconfigurable add/drop multiplexers (ROADM), which can be generated with channel isolation better than 50 dB.

Advantages of SOA

1. The optical gain provided by the SOA within the bandwidth is relatively independent of the wavelength of the incident optical signal.

2. The injected current is used as an amplified pump signal rather than an optical pump.

3. Due to its compact size, the SOA can be integrated with multiple waveguide photonic devices on a single planar substrate.

4. They use the same technology as diode lasers.

5. SOA can operate in 1300 nm and 1550 nm communication spectral bands with wider bandwidth (up to 100 nm).

6. They can be configured and integrated to be used as preamplifiers at the optical receiver end.

7. SOA can be usedWDMSimple logic gates in optical networks.

Limitations of SOA

1. SOAs can provide up to tens of milliwatts (mW) of output optical power, which is usually sufficient for single-channel operation in optical fiber communication links. However, WDM systems may require up to a few mW of power per channel.

Since the coupling of the input fiber into and out of the SOA integrated chip tends to cause signal loss, the SOA must provide additional optical gain to minimize the impact of this loss on the input/output side of the active area.

3. SOA is highly sensitive to the polarization of the input optical signal.

4. They generate higher noise levels in the active mediumfiber amplifier.

5. If multiple optical channels are enlarged as needed in a WDM application, the SOA can cause severe crosstalk.

How to turn a semiconductor laser into a semiconductor optical amplifier

Reduce the LD cleavage end reflection feedback, so thatYou can make travel wave semiconductors.optical amplifier. Tilt the strip-shaped active area with the normal cleavage plane or insert a transparent window area between the end face of the active layer and the cleavage plane, as shown in the figure.8 is shown. In the output transparent window region of Figure 8b, where the beam has diverged before reaching the semiconductor and air interface, the beam reflected from the interface diverges further and only a very small portion of the light is coupled into the thin active layer. When used with an anti-reflection film, the reflectivity can be as small as 10 − 4, making the LD into an SOA.

Figure8 Reduce reflection to make LD approximate to traveling wave (TW) semiconductor optical amplifier (SOA)

a) The strip-shaped active region and the cleavage plane form an inclined structure B) The window cleavage plane structure

Source of this article: Koneng Convergence Communications