Optical Amplification EDFA Applications & Selection/Applications Spectral Response of EDFAs

Optical Amplification EDFA Applications & Selection/Applications Spectral Response of EDFAs www.phwiki.com

Optical Amplification EDFA Applications & Selection/Applications Spectral Response of EDFAs

Matlosz, Felicia, Features Reporter has reference to this Academic Journal, PHwiki organized this Journal 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 20-50 km

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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 Overview Erbium doped fibre amplifiers Amplifier applications Issues: Gain flattening in addition to 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 b in addition to width Wavelength transparent Erbium Doped Fibre Amplifier Pump Source WDM Erbium Doped Fibre Isolator Isolator = Fusion Splice Input Output 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 30-40 dB possible Source: Master 7-5 Interior of an Erbium Doped Fibre Amplfier (EDFA) Fibre input/output Erbium doped fibre loop Pump laser WDM Fibre coupler Source: Master 7-5

Operation of an EDFA Pump Source WDM Erbium Doped Fibre Isolator Isolator = Fusion Splice Input Output 980 nm signal 1550 nm data signal Power level 980 nm signal 1550 nm data signal Power level Power interchange between pump in addition to data signals 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 r in addition to omly (time constant ~1 ms) Stimulated emission By electromagnetic wave Emitted wavelength & phase are identical to incident one

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 as long as m 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 as long as m of light) in addition to return to a lower-energy (more stable) state (“stimulated emission”). The laser diode in the diagram generates a high-powered (between 10 in addition to 200mW) beam of light at a wavelength such that the erbium ions will absorb it in addition to 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: 1.52-1.57 m 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 in addition to return to their lower-energy state. A significant point is that the erbium gives up its energy in the as long as m of additional photons which are exactly in the same phase in addition to 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 in addition to 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 in addition to minimal interchannel crosstalk Amplified Spontaneous Emission Erbium r in addition to omly emits photons between 1520 in addition to 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 Amplified spontaneous emission (ASE) R in addition to om spontaneous emission (SE) Amplification along fiber EDFA Behaviour at Gain Saturation There are two main differences between the behaviour of electronic amplifiers in addition to 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 in addition to 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) EDFA is in saturation if almost all Erbium ions are consumed as long as amplification Total output power remains almost constant, regardless of input power changes P in (dBm) Total P out -3 dB Max – 20 – 30 – 10 Gain Gain Compression Total output power: Amplified signal + ASE EDFA is in saturation if almost all Erbium ions are consumed as long as amplification Total output power remains almost constant Lowest noise figure Preferred operating point Power levels in link stabilize automatically P in (dBm) Total P out -3 dB Max -20 -30 -10 Gain 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 in addition to the pump power level vary along the length of the amplifier. At any point we can have only a finite number of erbium ions in addition to there as long as e we can only achieve a finite gain ( in addition to a finite maximum power) per unit length of the amplifier. In an amplifier designed as long as single wavelength operation the optimal amplifier length is a function of the signal power, the pump power, the erbium concentration in addition to the amount of gain required. In an amplifier designed as long as multiwavelength operation there is another consideration – the flatness of the gain curve over the range of amplified wavelengths. With a careful design in addition to 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 in addition to wavelength of incoming signal Wavelength (nm) EDFA Applications & Selection/Applications Source: Master 7-5 OFAs in the Network Several attractive features as long as network use: Relatively simple construction Reliable, due to the number of passive components Allows easy connection to external fibres Broadb in addition to operation > 20 nm Bit rate transparent Ideally suited to long span systems Integral part of DWDM systems Undersea applications as long as OFAs are now common Source: Master 7-5

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Optical Amplifier Applications In-line Amplifier Power Amplifier Preamplifier Fibre Link Transmitter Transmitter Transmitter Optical Amplifiers Optical Amplifier Fibre Link Optical Amplifier Fibre Link Optical Receiver Optical Receiver Optical Receiver Source: Master 7-5 Amplifier Applications Preamplifiers An optical preamplifier is placed immediately be as long as e 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 be as long as e 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 in addition to 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 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 be as long as e a star coupler Pre-amplifiers Low noise amplifier in front of receiver Remotely pumped Electronic free extending links up to 200 km in addition to more (often found in submarine applications) RX Pump

Example: Conventional EDFA Best used as long as single channel systems in the 1550 nm region, Systems are designed as long as use as boosters, in-line amplifiers or preamplifiers. B in addition to width is not wide enough as long as DWDM, special EDFA needed Source: Master 7-5 Gain Flattened EDFA as long as DWDM Source: Master 7-5 Gain flatness is now within 1 dB from 1530-1560 nm ITU-T DWDM C b in addition to is 1530 to 1567 nm Selecting Amplifiers

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 in addition to the University of Holl in addition to at Delft Source: Master 7-5 A 1310 nm B in addition to Raman Amplifier Operation is as follows: 1. Signal light in addition to pump light enter the device together through a wavelength selective coupler. 2. The pump light at 1064 nm is shifted to 1117 nm in addition to then in stages to 1240 nm. 3. The 1240 nm light then pumps the 1310 b in addition to signal by the SRS in addition to 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 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 in addition to other functions

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