Optical Amplification

Optical Amplification
Source: Master 7_5

Optical Amplifiers An optical amplifier is a device which amplifies the optical signal directly without ever changing it to electricity. The light itself is amplified. Reasons to use the optical amplifiers: Reliability Flexibility Wavelength Division Multiplexing (WDM) Low Cost Variety of optical amplifier types exists, including: Semiconductor Optical Amplifiers (SOAs) Erbium Doped Fibre Amplifiers (EDFAs) (most common)

Traditional Optical Communication System
Loss compensation: Repeaters at every km

Optically Amplified Systems
EDFA = Erbium Doped Fibre Amplifier

Optical Amplification
Variety of optical amplifier types exist, including: Semiconductor optical amplifiers Optical fibre amplifiers (Erbium Doped Fibre Amplifiers) Distributed fibre amplifiers (Raman Amplifiers) Optical fibre amplifiers are now the most common type One of the most successful optical processing functions Also used as a building block in DWDM systems Source: Master 7_5

Erbium doped fibre amplifiers Amplifier applications
Overview Erbium doped fibre amplifiers Amplifier applications Issues: Gain flattening and Noise Raman amplification

Basic EDF Amplifier Design
Erbium-doped fiber amplifier (EDFA) most common Commercially available since the early 1990’s Works best in the range 1530 to 1565 nm Gain up to 30 dB (1000 photons out per photon in!) Optically transparent “Unlimited” RF bandwidth Wavelength transparent Input 1480 or 980 nm Pump Laser Erbium Doped Fiber Output Isolator Coupler This diagram shows a very basic amplifier that possibly has 5 to 15 dB gain and less than 10 dB noise figure. High performance commercial designs provide output powers from 10 to 23 dBm (10 mW to 200 mW) and noise figures between 3.5 and 5 dB (the physical limit is 3.01 dB). EDFAs have been deployed in terrestrial and submarine links and now are considered as standard components using a well understood technology. EDFAs are self-regulating amplifiers. When all metastable electrons are consumed then no further amplification occurs. Therefore a system stabilizes itself because the output power of the amplifier remains more or less constant even if the input power fluctuates significantly.

Erbium Doped Fibre Amplifier
A pump optical signal is added to an input signal by a WDM coupler Within a length of doped fibre part of the pump energy is transferred to the input signal by stimulated emission For operation circa 1550 nm the fibre dopant is Erbium Pump wavelength is 980 nm or 1480 nm, pump power circa 50 mW Gains of dB possible Isolator Isolator Input WDM Output Erbium Doped Fibre Pump Source = Fusion Splice Source: Master 7_5

Interior of an Erbium Doped Fibre Amplfier (EDFA)
Pump laser WDM Fibre coupler Erbium doped fibre loop Fibre input/output Source: Master 7_5

between pump and data signals
Operation of an EDFA Power interchange between pump and data signals Power level Power level 980 nm signal 1550 nm data signal 980 nm signal 1550 nm data signal Isolator Isolator Input Output WDM Erbium Doped Fibre Pump Source = Fusion Splice

Physics of an EDFA

Erbium Properties Erbium: rare element with phosphorescent properties
Photons at 1480 or 980 nm activate electrons into a metastable state Electrons falling back emit light in the 1550 nm range Spontaneous emission Occurs randomly (time constant ~1 ms) Stimulated emission By electromagnetic wave Emitted wavelength & phase are identical to incident one 1480 980 820 540 670 Ground state Metastable state Erbium is a rare earth element which can absorb and release light energy in the communications band around 1550 nm. When light at 980 or 1480 nm is applied to fiber doped with Er, the fiber absorbs this energy, i.e., electrons are excited to a higher energy level where they remain in a metastable state for some time. If left undisturbed, the Er doped fiber will eventually release this energy in the band of frequencies from about 1530 to 1565 nm. If stimulated by an input signal in this band, the Er doped fiber will emit the stored energy at the stimulated wavelength. The 980 nm absorption band is narrower than the 1480 nm band. In addition it is more difficult to make reliable 980 nm lasers. However, pumping the amplifier with 980 nm can result into a better noise figure of the amplifier.

Erbium Doped Fibre Amplifiers
Consists of a short (typically ten metres or so) section of fibre which has a small controlled amount of the rare earth element erbium added to the glass in the form of an ion (Er3+). The principle involved is the principle of a laser. When an erbium ion is in a high-energy state, a photon of light will stimulate it to give up some of its energy (also in the form of light) and return to a lower-energy (more stable) state (“stimulated emission”). The laser diode in the diagram generates a high-powered (between 10 and 200mW) beam of light at a wavelength such that the erbium ions will absorb it and jump to their excited state. (Light at either 980 or 1,480 nm wavelengths.)

Er+3 Energy Levels Pump: 980 or 1480 nm Pump power >5 mW Emission:
Long living upper state (10 ms) Gain  30 dB

EDFA Operation A (relatively) high-powered beam of light is mixed with the input signal using a wavelength selective coupler. The mixed light is guided into a section of fibre with erbium ions included in the core. This high-powered light beam excites the erbium ions to their higher-energy state. When the photons belonging to the signal (at a different wavelength from the pump light) meet the excited erbium atoms, the erbium atoms give up some of their energy to the signal and return to their lower-energy state. A significant point is that the erbium gives up its energy in the form of additional photons which are exactly in the same phase and direction as the signal being amplified. There is usually an isolator placed at the output to prevent reflections returning from the attached fibre. Such reflections disrupt amplifier operation and in the extreme case can cause the amplifier to become a laser!

Technical Characteristics of EDFA
EDFAs have a number of attractive technical characteristics: Efficient pumping Minimal polarisation sensitivity Low insertion loss High output power (this is not gain but raw amount of possible output power) Low noise Very high sensitivity Low distortion and minimal interchannel crosstalk

Amplified Spontaneous Emission
Erbium randomly emits photons between 1520 and 1570 nm Spontaneous emission (SE) is not polarized or coherent Like any photon, SE stimulates emission of other photons With no input signal, eventually all optical energy is consumed into amplified spontaneous emission Input signal(s) consume metastable electrons  much less ASE Random spontaneous emission (SE) In order to better understand noise generated by optical amplifiers we need to look at the spontaneous emission of the EDFA. As mentioned before, electrons will fall from the metastable state down to the ground stable either by stimulated emission due to an incoming photon (that is the amplification effect) or randomly with about a one millisecond time constant. Randomly emitted photons have a random phase, travel direction and wavelength within the amplifier’s wavelength range. This is called the spontaneous emission. Those photons travelling along the fiber will trigger stimulated emission that of course will have their wavelength, phase, etc. At the end almost all energy pumped into an amplifier without any input signal reappears as amplified spontaneous emission (ASE). However, if an input signal consumes electrons in the metastable state then fewer are left for spontaneous emission, therefore reducing the ASE. Amplified spontaneous emission (ASE) Amplification along fiber

EDFA Behaviour at Gain Saturation
There are two main differences between the behaviour of electronic amplifiers and of EDFAs in gain saturation: 1) As input power is increased on the EDFA the total gain of the amplifier increases slowly. An electronic amplifier operates relatively linearly until its gain saturates and then it just produces all it can. This means that an electronic amplifier operated near saturation introduces significant distortions into the signal (it just clips the peaks off). 2) An erbium amplifier at saturation simply applies less gain to all of its input regardless of the instantaneous signal level. Thus it does not distort the signal. There is little or no crosstalk between WDM channels even in saturation.

Saturation in EDFAs Total output power:
Amplified signal + Noise (Amplified Spontaneous Emission ASE) Total P Max out -3 dB Gain - 30 - 20 - 10 P in (dBm) EDFA is in saturation if almost all Erbium ions are consumed for amplification Total output power remains almost constant, regardless of input power changes

Gain Compression Total output power: Amplified signal + ASE
EDFA is in saturation if almost all Erbium ions are consumed for amplification Total output power remains almost constant Lowest noise figure Preferred operating point Power levels in link stabilize automatically Total P out Max -3 dB Gain As discussed before, one benefit of the fact that EDFAs operate in saturation and lose about one dB of gain for one dBm increase in output power is that the output power of an EDFA will stay fairly constant over a variety of operating conditions. This amplifier has almost constant output power for a very wide input power range. At -30 dBm more than 50% of all metastable electrons are consumed for amplification. When the input power increases then this number approaches 100%. Because the pump power remains constant, the pool of excited electrons is limited. When used with a single carrier, if the input power of the EDFA were to drop by one dB, the gain would increase by one dB to reestablish the previous output power operating level. If the input power increased, the gain would drop, again, reestablishing the previous operating point. Again, we assume that the power fluctuations occur much slower than the time constant of the metastable state (~1 ms), and the modulation is much faster (20 kHz to many GHz). -30 -20 -10 P in (dBm)

Amplifier Length As the signal travels along the length of the amplifier it becomes stronger due to amplification. As the pump power travels through the amplifier its level decreases due to absorption. Thus, both the signal power level and the pump power level vary along the length of the amplifier. At any point we can have only a finite number of erbium ions and therefore we can only achieve a finite gain (and a finite maximum power) per unit length of the amplifier. In an amplifier designed for single wavelength operation the optimal amplifier length is a function of the signal power, the pump power, the erbium concentration and the amount of gain required. In an amplifier designed for multiwavelength operation there is another consideration - the flatness of the gain curve over the range of amplified wavelengths. With a careful design and optimisation of the amplifier's length we can produce a nearly flat amplifier gain curve.

Optical Gain (G) G = S Output / S Input S Output: output signal (without noise from amplifier) S Input: input signal Input signal dependent Operating point (saturation) of EDFA strongly depends on power and wavelength of incoming signal Gain (dB) 1540 1560 1580 10 1520 20 40 30 -5 dBm -20 dBm -10 dBm P Input: -30 dBm The gain versus wavelength curve of the EDFA (as well as the ASE versus wavelength plot) can vary with input signal wavelength and power. Carefully watch how the gain decreases with increasing input power. If the input is -20 dBm then the gain is about 30 dB at 1550 nm, resulting in +10 dBm output. If the input is -10 dBm then the gain is about 25 dB and the output about +15 dBm. In other words, when the input changes by a factor of ten then the output changes only by a factor of three in this power range. Above -10 dBm input the amplifier is in full compression: -5 dBm input power has 20 dB gain, therefore the 5 dB increase in input power has no effect on the output power (but it may have improved the noise figure). You can also recognize the saturation by the fact that the traces become more flat when the input power increases. Saturation is a preferred point of operation because it stabilizes the system and reduces noise without causing nonlinear effects (like clipping) inside the amplifier for high speed modulation. Wavelength (nm)

Selection/Applications
EDFA Applications & Selection/Applications Source: Master 7_5

OFAs in the Network Several attractive features for network use:
Relatively simple construction Reliable, due to the number of passive components Allows easy connection to external fibres Broadband operation > 20 nm Bit rate transparent Ideally suited to long span systems Integral part of DWDM systems Undersea applications for OFAs are now common Source: Master 7_5

Optical Amplifier Applications
Fibre Link In-line Amplifier Optical Receiver Transmitter Optical Amplifiers Fibre Link Power Amplifier Optical Receiver Transmitter Optical Amplifier Optical Receiver Preamplifier Transmitter Fibre Link Optical Amplifier Source: Master 7_5

Amplifier Applications
Preamplifiers An optical preamplifier is placed immediately before a receiver to improve its sensitivity. Since the input signal level is usually very low a low noise characteristic is essential. However, only a moderate gain figure is needed since the signal is being fed directly into a receiver. Typically a preamplifier will not have feedback control as it can be run well below saturation. Power amplifiers Most DFB lasers have an output of only around 2 mW but a fibre can aggregate power levels of up to 100 to 200 mW before nonlinear effects start to occur. A power amplifier may be employed to boost the signal immediately following the transmitter. Typical EDFA power amplifiers have an output of around 100 mW. Line amplifiers In this application the amplifier replaces a repeater within a long communication line. In many situations there will be multiple amplifiers sited at way-points along a long link. Both high gain at the input and high power output are needed while maintaining a very low noise figure. This is really a preamplifier cascaded with a power amplifier. Sophisticated line amplifiers today tend to be made just this way - as a multi-section amplifier separated by an isolator.

EDFA Categories In-line amplifiers Power boosters Pre-amplifiers
Installed every 30 to 70 km along a link Good noise figure, medium output power Power boosters Up to +17 dBm power, amplifies transmitter output Also used in cable TV systems before a star coupler Pre-amplifiers Low noise amplifier in front of receiver Remotely pumped Electronic free extending links up to 200 km and more (often found in submarine applications) The output gain, output power and noise figure of EDFAs can be tweaked by various design modifications. Using 980 nm pumps usually produce EDFAs with lower noise figures. EDFAs of this type make better preamps. It is thought that the reason for this NF improvement is that the pump wavelength is farther out of the emission band and, for this reason, reduces ASE. Remotely pumped EDFAs allow system designers to extend medium range submarine links, such as those between islands. Their main advantage is that there are no electronics and therefore no power needs along the link, a fact that improves reliability and reduces cost. TX Pump RX Pump

Example: Conventional EDFA
Best used for single channel systems in the 1550 nm region, Systems are designed for use as boosters, in-line amplifiers or preamplifiers. Bandwidth is not wide enough for DWDM, special EDFA needed Source: Master 7_5

Gain Flattened EDFA for DWDM
Gain flatness is now within 1 dB from nm ITU-T DWDM C band is 1530 to 1567 nm Source: Master 7_5

Low value < 5 dB essential
Selecting Amplifiers Type Gain Maximum Output power Noise figure Power Amplifier High gain High output power Not very important In-line Medium gain Medium output power Good noise figure Preamplifier Low gain Low output power Low value < 5 dB essential

Additional pumping options
Pumping Directions Additional pumping options

Multistage EDFAs Two-Stage EDFA Line Amplifier with Shared Pump. Pump power would typically be split in a ratio different from 50:50. Some new EDFA designs concatenate two or even three amplifier stages. An amplifier “stage” is considered to consist of any unbroken section of erbium doped fibre. Multistage amplifiers are built for a number of reasons: 1. To increase the power output whilst retaining low noise 2. To flatten the total amplifier gain response 3. To reduce amplified stimulated emission noise

Commercial Designs EDF EDF Input Output Isolator Isolator Pump Lasers
Telemetry & Remote Control Input Monitor Output Monitor Commercial amplifiers are optimized for performance needed in a particular application (booster/in-line/pre-amplifier) as well as to optimize cost and functionality. Input and output monitors are added for safety and reflection monitoring reasons. Power sensors monitor overall system health and provide aging information. Pumping at the input of the EDFA versus the output also improves the noise figure of the EDFA. If the EDFA is considered to be a system with an inherent noise figure and a gain block, placing the gain block early in the component cascade will reduce the overall noise figure of the cascade.

Security/Safety Features
Input power monitor Turning on the input signal can cause high output power spikes that can damage the amplifier or following systems Control electronics turn the pump laser(s) down if the input signal stays below a given threshold for more than about 2 to 20 µs Backreflection monitor Open connector at the output can be a laser safety hazard Straight connectors typically reflect 4% of the light back Backreflection monitor shuts the amplifier down if backreflected light exceeds certain limits As discussed before, transient spikes can damage components in the amplifier and in the system. Input and reflection monitors help to significantly reduce or even eliminate this risk as well. For example, some amplifiers shutdown the pump laser if more than 0.1% of the output light is reflected back. A straight open connector has 4% backreflection (14 dB return loss) and therefore will cause such an amplifier to shut down. Someone may look at an open connector to check whether or not it is dirty or damaged. But few people manage to get dangerous power levels out of closed patchcords or cables.

Spectral Response of EDFAs
& Gain Flattening Source: Master 7_5

Output Spectra Amplified signal spectrum
+10 dBm Amplified signal spectrum (input signal saturates the optical amplifier) ASE spectrum when no input signal is present This slide shows the typical ASE output spectra of an EDFA with no input signal and with a stimulating input signal. In addition to noting that most of the pump power appears at the stimulating wavelength, note also how the power distribution at the other wavelengths changes with a given input signal. The basic question for characterizing EDFAs is how to measure its noise figure. If the input signal is turned off then you measure a big ASE. If it is turned on then you measure a big signal. HP has developed several techniques to address this problem, including ASE interpolation, polarization nulling and time domain extinction. -40 dBm 1575 nm 1525 nm

EDFA Gain Spectrum Erbium can provide about nm of bandwidth, from 1520 to 1570 nm Gain spectrum depends on the glass used, eg. silica or zblan glass Gain spectrum is not flat, significant gain variations 30 20 10 EDFA gain spectrum Gain (dB) Wavelength (nm) Source: Master 7_5

Gain Characteristics of EDFA
Gain (amplifier) - is the ratio in decibels of input power to output power. Gain at 1560 nm is some 3 dB higher than gain at 1540 nm (this is twice as much). In most applications (if there is only a single channel or if there are only a few amplifiers in the circuit) this is not too much of a limitation. WDM systems use many wavelengths within the amplified band. If we have a very long WDM link with many amplifiers the difference in response in various channels adds up.

Flattening of the Gain Curve Techniques
Operating the device at 77° K. This produces a much better (flatter) gain curve but it's not all that practical. Introducing other dopant materials (such as aluminium or ytterbium) along with the erbium into the fibre core. Amplifier length is another factor influencing the flatness of the gain curve. Controlling the pump power (through a feedback loop) is routine to reduce amplified spontaneous emission. Adding an extra WDM channel locally at the amplifier (“gain clamping”). Manipulating the shape of the fibre waveguide within the amplifier. At the systems level there are other things that can be done to compensate: Using “blazed” fibre Bragg gratings as filters to reduce the peaks in the response curve. To transmit different WDM channels at different power levels to compensate for later amplifier gain characteristics.

Gain Flattening Concept

Gain Flattening Filters (Equalizers)
Used to reduce variation in amplifier gain with wavelength, used in DWDM systems The gain equalisation is realised by inserting tapered long period gratings within the erbium doped fibre. Designed to have approximately the opposite spectral response to that of an EDFA Inline Dicon gain flattening filter spectral response Inline Dicon gain flattening filter Source: Master 7_5

Spectral Hole Burning (SHB)
Gain depression around saturating signal Strong signals reduce average ion population Hole width 3 to 10 nm Hole depth 0.1 to 0.4 dB 1530 nm region more sensitive to SHB than 1550 nm region Implications Usually not an issue in transmission systems (single l or DWDM) Can affect accuracy of some lightwave measurements 1545 1550 1560 1540 Wavelength (nm) 7 nm 0.36 dB While spectral hole burning and PHB are small effects on a single EDFA, the effects may be more significant in long concatenated chains of similar EDFAs. Such systems are more common in long submarine links.

Polarization Hole Burning (PHB)
Polarization Dependent Gain (PDG) Gain of small signal polarized orthogonal to saturating signal 0.05 to 0.3 dB greater than the large signal gain Effect independent of the state of polarization of the large signal PDG recovery time constant relatively slow ASE power accumulation ASE power is minimally polarized ASE perpendicular to signal experiences higher gain PHB effects can be reduced effectively by quickly scrambling the state of polarization (SOP) of the input signal PDG: The polarization-dependent loss of components inside the amplifier depends on the state of polarization (SOP) of the input signal; PHB (see below) does not. ASE power accumulation: ASE power can be split into two parts, one half of the ASE is polarized parallel and the other half perpendicular to the saturating signal. Polarization hole burning (PHB) is an additional effect which can change the effective gain of an EDFA at a specific wavelength. Similar to spectral hole burning (see next slide), PHB is caused by a depletion of erbium ions in the polarization orientation of the stimulating signal in excess of the general rate of ion depletion across the EDFA emission band. In general, whenever you send one wavelength into an EDFA, the EDFA will tend to deplete all the energy stored in the EDFA at the wavelength input, at the polarization of the input signal. Typically PDG and PHB effects rarely exceed 0.1 to 0.5 dB.

Noise in EDFAs Source: Master 7_5

Optical Amplifier Chains
Optical amplifiers allow one to extend link distance between a transmitter and receiver Amplifier can compensate for attenuation Cannot compensate for dispersion (and crosstalk in DWDM systems) Amplifiers also introduce noise, as each amplifier reduces the Optical SNR by a small amount (noise figure) Fibre Link Optical Receiver Transmitter 1 2 N Optical Amplifiers Fibre Section Source: Master 7_5

Amplifiers Chains and Signal Level
Sample system uses 0.25 atten fibre, 80 km fibre sections, 19 dB amplifiers with a noise figure of 5 dB Fibre Link Each amplifier restores the signal level to a value almost equivalent to the level at the start of the section - in principle reach is extended to 700 km + Source: Master 7_5

Amplifiers Chains and Optical SNR
Fibre Link Same sample system: Transmitter SNR is 50 dB, amplifier noise figure of 5 dB, Optical SNR drops with distance, so that if we take 30 dB as a reasonable limit, the max distance between T/X and R/X is only 300 km Source: Master 7_5

Noise Figure (NF) NF = P ASE / (h• • G • B OSA) P ASE: ASE power measured by OSA h: Plank’s constant : Optical frequency G: Gain of EDFA B OSA: Optical bandwidth [Hz] of OSA Input signal dependent In a saturated EDFA, the NF depends mostly on the wavelength of the signal Physical limit: 3.0 dB Noise Figure (dB) 10 7.5 Noise figure describes how close an amplifier comes to an ideal amplifier that amplifies the input spectrum including noise but does not add any noise. According to quantum physics, it is impossible to build an optical amplifier with better than 3.0 dB noise figure. The noise figure formula compares noise density (PASE / BOSA) measured at the output but normalized to the input (1/G term) with the “quantum noise” of the incoming photons (h*). Note that this formula is based on some assumptions usually present in fiberoptic communication systems (for a detailed discussion, see “Fiber Optic Test And Measurement” by Dennis Derickson, ISBN ). As we can see from the measurements shown here, the noise figure becomes better with increasing wavelength. The traces overlap significantly because even at -30 dBm input power the amplifier is already saturated sufficiently. You have to drop input power considerably before the noise figure becomes noticeably worse. 5.0 1520 1540 1560 1580 Wavelength (nm)

Raman Amplification Source: Master 7_5

Raman Amplifiers Raman Fibre Amplifiers (RFAs) rely on an intrinsic nonlinearity in silica fibre Variable wavelength amplification: Depends on pump wavelength For example pumping at 1500 nm produces gain at about nm RFAs can be used as a standalone amplifier or as a distributed amplifier in conjunction with an EDFA Source: Master 7_5

Raman Effect Amplifiers
Stimulated Raman Scattering (SRS) causes a new signal (a Stokes wave) to be generated in the same direction as the pump wave down-shifted in frequency by 13.2 THz (due to molecular vibrations) provided that the pump signal is of sufficient strength. In addition SRS causes the amplification of a signal if it's lower in frequency than the pump. Optimal amplification occurs when the difference in wavelengths is around 13.2 THz. The signal to be amplified must be lower in frequency (longer in wavelength) than the pump. It is easy to build a Raman amplifier, but there is a big problem: we just can't build very high power (around half a watt or more) pump lasers at any wavelength we desire! Laser wavelengths are very specific and high power lasers are quite hard to build.

Distributed Raman Amplification (I)
Raman pumping takes place backwards over the fibre Gain is a maximum close to the receiver and decreases in the transmitter direction Long Fibre Span Optical Receiver Transmitter EDFA Raman Pump Laser Source: Master 7_5

Distributed Raman Amplification (II)
With only an EDFA at the transmit end the optical power level decreases over the fibre length With an EDFA and Raman the minimum optical power level occurs toward the middle, not the end, of the fibre. EDFA + Raman Optical Power EDFA only Distance Source: Master 7_5 Animation

Broadband Amplification using Raman Amplifiers
Raman amplification can provides very broadband amplification Multiple high-power "pump" lasers are used to produce very high gain over a range of wavelengths. 93 nm bandwidth has been demonstrated with just two pumps sources 400 nm bandwidth possible? Source: Master 7_5

Advantages and Disadvantages of Raman Amplification
Variable wavelength amplification possible Compatible with installed SM fibre Can be used to "extend" EDFAs Can result in a lower average power over a span, good for lower crosstalk Very broadband operation may be possible Disadvantages High pump power requirements, high pump power lasers have only recently arrived Sophisticated gain control needed Noise is also an issue Source: Master 7_5

Semiconductor Optical/Laser Amplifiers (SOAs/SLAs)
There are two varieties: Simple SOA are almost the same as regular index-guided FP lasers. The back facet is pigtailed to allow the input of signal light. The main problem is that it has been difficult to make SOAs longer than about 450 m. In this short distance there is not sufficient gain available on a single pass through the device for useful amplification to be obtained. One solution to this is to retain the reflective facets (mirrors) characteristic of laser operation. Typical SOAs have a mirror reflectivity of around 30%. Thus the signal has a chance to reflect a few times within the cavity and obtain useful amplification. Travelling wave SLA (TWSLA)

Travelling Wave SLAs (TWSLAs)
The TWSLA is different from the SOA in a number of ways: 1. The cavity is lengthened (doubled or tripled) to allow enough room for sufficient gain (since the amplifier uses a single pass through the device and doesn't resonate like a laser). Devices with cavities as long as 2 mm are available. 2. The back facet is anti-reflection coated and pigtailed to provide entry for the input light. 3. The exit facet of the amplifier is just the same as for a laser except that it is also anti-reflection coated. 4. Because of the absence of feedback the TWSLA can be operated above the lasing threshold giving higher gain per unit of length than the simple SOA (Gains of up to 25 dB over a bandwidth range of 40 nm).

Limitations/Advantages/Applications
SOAs have severe limitations: Insufficient power (only a few mW). This is usually sufficient for single channel operation but in a WDM system you usually want up to a few mW per channel. Coupling the input fibre into the chip tends to be very lossy. The amplifier must have additional gain to overcome the loss on the input facet. SOAs tend to be noisy. They are highly polarisation sensitive. They can produce severe crosstalk when multiple optical channels are amplified. This latter characteristic makes them unusable as amplifiers in WDM systems but gives them the ability to act as wavelength changers and as simple logic gates in optical network systems. A major advantage of SOAs is that they can be integrated with other components on a single planar substrate. For example, a WDM transmitter device may be constructed including perhaps 10 lasers and a coupler all on the same substrate. In this case an SOA could be integrated into the output to overcome some of the coupling losses.

Other Amplifier Types Semiconductor Optical Amplifier (SOA)
Basically a laser chip without any mirrors Metastable state has nanoseconds lifetime (-> nonlinearity and crosstalk problems) Potential for switches and wavelength converters Praseodymium-doped Fiber Amplifier (PDFA) Similar to EDFAs but 1310 nm optical window Deployed in CATV (limited situations) Not cost efficient for 1310 telecomm applications Fluoride based fiber needed (water soluble) Much less efficient (1 W 1017 nm for 50 mW output) Research labs around the world have been working on other optical amplifier designs but very few of them have been deployed (besides field trials). SOAs possibly will be used to compensate losses in integrated optics or for special switch applications. PDFAs are very difficult to manufacture and yet have not been very cost effective for large-scale use in networks.

Small Footprint Amplifiers and 1300 nm Amplifiers
Source: Master 7_5

Miniature Optical Fibre Amp
Erbium doped aluminium oxide spiral waveguide 1 mm square waveguide Pumped at 1480 nm Low pump power of 10 mW Gain only 2.3 dB at present 20 dB gain possible With the permission of the FOM Institute Amsterdam and the University of Holland at Delft Source: Master 7_5

A 1310 nm Band Raman Amplifier
Operation is as follows: 1. Signal light and pump light enter the device together through a wavelength selective coupler. 2. The pump light at 1064 nm is shifted to 1117 nm and then in stages to 1240 nm. 3. The 1240 nm light then pumps the 1310 band signal by the SRS and amplification is obtained. To gain efficiency a narrow core size is used to increase the intensity of the light. Also, a high level of Ge dopant is used (around 20%) to increase the SRS effect. This is a very effective, low noise process with good gain at small signal levels.

Future Developments Broadened gain spectrum Gain flattening
2 EDFs with different co-dopants (phosphor, aluminum) Can cover 1525 to 1610 nm Gain flattening Erbium Fluoride designs (flatter gain profile) Incorporation of Fiber Bragg Gratings (passive compensation) Increased complexity Active add/drop, monitoring and other functions The recent explosion in WDM (see next section) has created a need to extend the band useful for amplifications. New developments show that you can extend the range of the optical amplifier possibly up to 1625 nm (or more), effectively allowing designers to send more optical channels through the system. Other WDM requirements include better gain flatness which can be achieved with doping modifications or filters.

Optical Amplification

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