Semiconductor Optical Amplifier

semiconductor optical amplifier

A Semiconductor Optical Amplifier (SOA) is essentially a laser diode (LD) with no feedback from its input and output ports and hence is also referred to as a Traveling-Wave Amplifier (TWA). Semiconductor optical amplifiers (SOAs) have proven to be versatile and multifunctional devices that are key building blocks for optical networks.

There are five parameters used to characterize a Semiconductor Optical Amplifier (SOA): (1) Gain (Gs); (2) Gain Bandwidth; (3) Saturation Output Power (PSAT); (4) Noise Figure (NF); (5) Polarization Dependent Gain (PDG).

A Semiconductor Optical Amplifier (SOA) should have the highest gain appropriate to the application. A wide optical bandwidth is also desirable so that the Semiconductor Optical Amplifier (SOA) can amplify a wide range of signal wavelengths. Gain saturation effects introduce undesirable distortion to the output so an ideal SOA should have very high saturation output power to achieve good linearity and to maximize its dynamic range with minimum distortion. An ideal Semiconductor Optical Amplifier (SOA) should also have a very low noise figure (the physical limit is 3dB) to minimize the amplified spontaneous emission (ASE) power at the output. Finally, an ideal Semiconductor Optical Amplifier (SOA) should have very low polarization sensitivity to minimize the gain difference between the transverse-electric (TE) and transverse-magnetic (TM) polarization states. However, an ideal Semiconductor Optical Amplifier (SOA) is impossible to realize because of the physical limitations of the various processes taking place within it.

InPhenix’s 1310nm Semiconductor Optical Amplifiers (SOAs) are ideal O-Band Optical Amplifiers. Inphenix also offers SOAs for E-band, S-band, C-band, L-band, and U-band wavelengths.

O-Band Optical Amplifier

Benefits

· Saturated power > 10dB
· Small signal gain > 10dB over the entire O-band (1270 to 1340nm)
· Ripple < 1 dB
· Compact + Low Cost

Applications

· 100GBASE-LR4 Transceiver test and measurement
· Passive component testing
· Splitting/tap loss compensation
· Reach extension (increase link budget): booster amplifier and per-amplifier

Peak Wavelength(nm)
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Category Typical Gain(dB)
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Typcial Psat(dBm)
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Typical Spectral Width(nm)
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Max. Noise Figure
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Typical Current(mA)
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Polarization Dependent or Independent Package Part Number
Peak Wavelength(nm) Category Typical Gain(dB) Typcial Psat(dBm) Typical Spectral Width(nm) Max. Noise Figure(dB) Typical Current(mA) Polarization Dependent
or Independent
Package Part Number
880 In-Line 20 10 40 8 150 Dependent Butterfly IPSAD0801
1015 In-Line 20 11 80 8 250 Dependent Butterfly IPSAD1002
1040 In-Line 25 10 55 8 250 Dependent Butterfly IPSAD1003
1050 In-Line 25 10 45 10 300 Dependent Butterfly IPSAD1001
1285 In-Line 30 17 60 7.5 700 Dependent Butterfly IPSAD1201
1300 In-Line 30 15 60 7 700 Dependent Butterfly IPSAD1305
1300 In-Line 19 7 80 7.5 300 Dependent Butterfly IPSAD1306
1310 Booster 10 11 40 7.5 300 Independent Butterfly IPSAD1307
1310 Booster 22 10 45 7.5 250 Independent Butterfly IPSAD1301
1310 In-Line 19 7 50 7.5 250 Independent Butterfly IPSAD1303
1310 In-Line 29 9 40 7.5 200 Independent Butterfly IPSAD1308
1310 Pre-amplifier 16 10 55 7 250 Independent Butterfly IPSAD1304
1310 Pre-amplifier 15 7 45 7.5 250 Independent 8-Pin IPSAD1309
1310 Switch 18 6 40 9 100 Dependent Butterfly IPRAD1301
1310 Switch 10 4 50 7.5 100 Independent Butterfly IPSAD1302
1490 Pre-amplifier 16 11 55 8 300 Independent Butterfly IPSAD1402
1490 Pre-amplifier 26 9 40 8 400 Independent Butterfly IPSAD1401
1525 In-Line 19 7 50 8 300 Dependent Butterfly IPSAD1506
1550 Booster 13 14 55 9 500 Independent Butterfly IPSAD1505
1550 Booster 20 10 45 9 350 Independent Butterfly IPSAD1501
1550 Booster 15 10 55 9 350 Independent Butterfly IPSAD1504
1550 Booster 14 16 60 9 700 Dependent Butterfly IPSAD1507
1550 Booster 25 12 60 9 500 Dependent Butterfly IPSAD1509
1550 Booster 10 12 40 9 300 Independent Butterfly IPSAD1510
1550 In-Line 16 5 50 9 350 Independent Butterfly IPSAD1503
1550 In-Line 28 8 45 9 350 Dependent Butterfly IPSAD1508
1550 Switch 10 3 50 10 120 Independent Butterfly IPSAD1502
1550 Switch 18 6 40 9 100 Independent Butterfly IPRAD1501
1550 Switch 22 8 60 9.5 600 Dependent Butterfly IPSAD1511
1550 Switch 13 12 60 9.5 400 Independent Butterfly IPSAD1512
1550 Switch 22 13 60 9.5 600 Independent Butterfly IPSAD1513
1550 Switch 25 13 40 9.5 600 Dependent Butterfly IPSAD1514
1550 Booster 20 20 40 8.0 1000 Dependent Butterfly IPSAD1515
1600 Booster 20 9 50 10.0 400 Independent Butterfly IPSAD1601
1650 Booster 25 13 40 9.5 600 Dependent Butterfly IPSAD1602

A Semiconductor Optical Amplifier (SOA) is an optoelectronic device that can amplify an input optical signal under appropriate operating conditions. A schematic diagram of a basic SOA is presented in Fig.1 (below).

Fig.1-SOA simplified diagram.

An electric current externally applied to the Semiconductor Optical Amplifier excites electrons in the active region of forward biased pn-junction. When photons travel through the active area they can cause these electrons to lose some of their extra energy and generate more photons that match the wavelength of the initial ones, so called stimulated photon emission.The embedded waveguide is used to confine the propagating signal wave to the active area. Therefore, an optical signal passing through the active region is amplified and is said to have experienced optical gain.

Inphenix’s SOAs come in different form factors starting as small as a 6-pin mini butterfly that can be mounted in a CFP/CFP2 transceiver to a desktop that can be integrated with drivers and custom ordered passive components.

Inphenix SOA’s have been tested to meet Telcordia GR-468-CORE for extremely high reliability, as well as the RoHS directive.

SOA Amplification Characteristics

Inphenix’s semiconductor optical amplifiers have proven to be versatile and multifunctional devices that are key building blocks for optical networks.There are five main parameters used to characterize a Semiconductor Optical Amplifier (SOA):

A Semiconductor Optical Amplifier should have the highest gain appropriate to the application. A wide optical bandwidth is also desirable so that the SOA can amplify a wide range of signal wavelengths. Gain saturation effects introduce undesirable distortion to the output so an ideal SOA should have very high saturation output power to achieve good linearity and to maximize its dynamic range with minimum distortion. An ideal SOA should also have a very low noise figure (the physical limit is 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 the transverse-electric (TE) and transverse-magnetic (TM) polarization states. However, an ideal SOA is impossible to realize because of the physical limitations of the various processes taking place within it.

Fig. 2- SOA interrelated parameters.

SOA parameters are strongly interrelated, and in order to achieve the best value of one parameter, the other specification(s) should be compromised and/or spectral operation area should be controlled, as it is schematically shown on Fig.2.

Types of SOAs

Depending on the role SOAs play in the customer’s system, they can be classified into four categories: in-line,booster; switch SOA and preamplifier;

In addition, PDG can determine polarity of an SOA.  For instance, if the PDG is less than 1.5dB, the SOA is Polar Independent (P-I) and if the PDG is up to 10 dB, the SOA is Polar Dependent (P-D).

Applications of SOA

Traditional applications

Amplification is a basic principle application of SOAs in optical communication system. SOAsarea highly versatile component that can be used for various amplifications and routing functions in telecommunications. Commercialized SOAs are now widely available in the market and are fast-becoming a cost-effective solution to optical amplification in advanced optical systems for core, metro, and ultimately access applications. SOA can be employed in any optical communication network to regenerate signals at various points in the link by operating either as a booster amplifier (post-amplifier), in-line amplifier.

Telecoms

SOAs are in use by a wide variety of industries.  One of the most important industries is telecoms, where they are valued for routing and switching.  In addition, SOAs are used to boost or amplify signal output for long-distance fiber-optic communications.  In this application, telecom firms employ fiber-optic lines from headquarters to the data centers.  These transmission lines can exceed 10km or more, requiring the use of SOAs to boost/amplify the signal from the usual light sources.

Fig. 3-SOAs in photonic carriers (top) can be used in Photonic Integrated Circuit (PIC) (bottom-left).

Functional applications

SOAs can also be used to perform functions that are, and will be, useful in future optically transparent networks. These all-optical functions can help to overcome the so called ‘electronic bottleneck’ which is presently a major limiting factor in the deployment of high-speed optical communication networks, like, for example, optical wavelength converter. Invariably, SOA functional applications are based on SOA nonlinearities. These nonlinearities are principally caused by carrier density changes induced by the amplifier input signals. The four main types of nonlinearity commonly exploited in SOAs arecross-gain modulation (XGM), cross-phase modulation (XPM), self-phase modulation (SPM) and four-wave mixing (FWM).

Sensing

Sensing is another important industry utilizing SOAs in many applications.  One important use of SOAs in sensors is the Fiber-Bragg Interrogators.  In this setup, either an SLD or DFB is used as the input light-source.  An SOA boosts the optical signal to a fiber-Bragg grating (FBG), often through a circulator to control direction of the optical signals.  Changes in temperature or strain change the wavelength or timing of the optical signals to a PD/sensor.  This can alert the user to possible malfunctions.

Fig. 4- Fiber-Bragg Grating with SOA in-line amplifier

Another important use of SOAs in sensing is with Light Detection and Ranging (LiDAR).   LiDAR devices can be small, for only Doppler-ranging or appear as an array capable for mapping.An example of LiDAR application uses Frequency Modulated Continuous Wave (FMCW) to detect the Doppler effect of movement, such as with autonomous cars and drones.  In addition, FMCW can be used for cartography and inspection.  SOAs used in narrowband, usually with DFBs, can have high output power >20mW, for longer range.

Fig. 5- LiDAR chip with integrated SOAs. An array of these chips gives a wide-area scan.

Fig.6- Frequency Modulated Continuous Wave (FMCW) LiDAR. SOAs in green.

CWDM

The previously highlighted features of small size, good integration capability and high potential for cost reduction through scaled manufacturing processes will continue to ensure that the SOA plays an increasingly important role in future advanced optical networks. Coarse wavelength division multiplexing (CWDM) is an economic route for giving connection flexibility and increased throughput for metro and enterprise network layers. Extending the capacity and distance of CWDM systems (>100 km) requires a low cost optical amplifier operating across the entire optical bandwidth (from 1260 nm to 1620 nm). The SOA is the only viable technology available today that can be deployed to meet these expanding applications.

WDM-PON

An expanded role for SOAs in telecoms is in their use in Wavelength Division Multiplexing-Passive Optical Networks (WDM-PON).  Cable companies utilizing fiber-optic lines from the home office to the customer receiving the data, often have nodes, or distribution centers, to assist in switching and routing data.  This setup allows the efficient distribution of data to a large customer base.  SOAs are an early application of WDM-PON but may see growth in the future.

There are several other attractive applications of SOA, such as intensity and phase modulation, SOA logic for the use of optical signal processing, SOA add/drop multiplexer for optical time division multiplexed network, SOA pulse generator to generate pulse easily at high frequencies (> 10 GHz), SOA clock recovery required in optical receivers and 3R generators, SOA dispersion compensator to overcome the chromatic dispersion which limits the transmission distance, and SOA detector to detect optical signal. SOAs can also be used for gating optical signals, i.e. signals can be either amplified or absorbed by SOAs. The blocking properties of SOAs at low bias currents are extremely useful because they enable channel routing functions, such as reconfigurable add/drop multiplexers (ROADM), to be produced with off channel isolation better than 50 dB.

Advantages of SOAs

  1. The optical gain provided by SOAs within a bandwidth is relatively independent of the wavelength of the incident optical signal.
  2. The injection electric current serves as the pump signal for amplification instead of optical pumping.
  3. Due to their compact size, SOAs can be integrated with several waveguide photonic devices on a single planar substrate.
  4. They use the same technology as diode lasers.
  5. SOAs have the ability to operate at communication spectral bands of 1300 nm and 1550 nm with wider bandwidth (up to 100 nm).
  6. They can be configured and integrated to function as pre-amplifiers at the optical receiver end.
  7. SOAs can function as simple logic gates in WDM optical networks.

Limitations of SOAs

  1. SOAs can deliver output optical power up to a few tens of milliwatts (mW) only which is usually sufficient for single channel operation in a fiber–optic communication link. However, a WDM system may require up to a few mW of power per channel.
  2. Since the coupling of the input optical fiber into and out of the SOA integrated chip tends to induce signal loss, SOA must provide additional optical gain in order to minimize the impact of this loss on the input/output facets of the active region.
  3. SOAs are highly sensitive to the polarization of the input optical signal.
  4. They generate higher noise levels in the active medium than optical fiber amplifiers.
  5. In case multiple optical channels are amplified as required in WDM applications, SOAs can produce severe crosstalk.

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