Chapter 12: Optical Amplifiers: Erbium Doped Fiber Amplifiers (EDFAs) Prof. Dr. Yaocheng SHI ( 时尧成 ) yaocheng@zju.edu.cn http://mypage.zju.edu.cn/yaocheng 1
Traditional Optical Communication System Loss compensation: Repeaters at every 20-50 km 2
Repeaters 3R (regeneration with retiming and reshaping) produces a fresh copy of received signal 2R (regeneration with reshaping) 1R (regeneration) Repeaters and amplifiers Repeaters do not work for fiber-optic networks, where many transmitters send signals to many receivers at different bit rates and in different formats Optical amplifier Support any bit rate and signal format (transparent) Support the entire region of wavelengths Increase the capacity of fiberoptic links by using WDM Provide the capability of alloptical networks, not just point-to-point links 3
Optically Amplified Systems EDFA = Erbium Doped fiber Amplifier 4
Amplifier types and applications The main types of optical amplifiers: Doped fiber amplifier (eg. EDFA) Semiconductor optical amplifier (SOA) Fiber Raman amplifier Fiber Brillouin amplifer Fiber optical parametric amplifier (FOPA) For overcoming or compensating for the optical loss Important properties to consider: Gain and gain bandwidth Gain flatness and saturation Noise 5
Common characteristics of all amplifiers 6
Comparison of real and ideal amplifiers 7
Operating band of rare-earth doped fiber amplifiers 8
What is EDFA? Why EDFA? 9
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 10
Erbium Doped Fiber Amplifier A pump optical signal is added to an input signal by a WDM coupler Within a length of doped fiber part of the pump energy is transferred to the input signal by stimulated emission For operation circa 1550 nm the fiber dopant is Erbium Pump wavelength is 980 nm or 1480 nm, pump power circa 50 mw Gains of 30-40 db possible 11
Interior of an Erbium Doped fiber Amplifier (EDFA) WDM fiber coupler Pump laser Erbium doped fiber loop fiber input/output 12
Operation of an EDFA 13
Erbium energy levels and pump sources 14
Er +3 Energy Levels Pump: (Optical) 980 or 1480 nm Pump power >5 mw Emission: 1.52-1.57 m Long living upper state (10 ms) Gain 30 db 15
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 fiber 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. Why? 16
Structures of EDFAs Components: Er-doped fiber Pump laser WDM coupler Isolator Filter 17
Output Spectra If the input signal is turned off then you measure a big ASE. If it is turned on then you measure a big signal. Why? 18
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) Amplification along fiber Amplified spontaneous emission (ASE) 19
Gain in EDFAs The amplification achieved in a EDFA is due to the Er-ions returning to the ground state and then emitting the excessive energy as coherent light when stimulated by the incoming light. Definition: Gain is the ratio of output to input light power Gain P / P out in 10 Gain( db) 10log ( P / P ) Gain ( P P ) / P Gain( db) 10log 10[( P P ) / P ] out ASE in Gain coefficient: db/m Gain bandwidth: From about 1500 nm to 1600 nm C-band (1530-1560 nm): most EDFA L-band (1560-1610 nm): depress four-wave mixing out in out ASE in 20
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 fiber 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 fiber waveguide within the amplifier. At the systems level there are other things that can be done to compensate: Using blazed fiber 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. 21
Gain in EDFAs Gain flatness (for multiple channel systems) Equalization filter (eg. long-period fiber grating) 22
Gain saturation Gain in EDFAs Gain is decreased because of the depletion of the population of intermediate level with a high-power input signal Gain saturation determines the maximum output power (saturated output power) The higher the pump power, the more excited Er-ions and the higher saturation power. Gain vs. fiber length The typical length of an active fiber is from 20 to 50 meters. An active fiber has an optimal length (doping concentration, gain bandwidth and shape). 23
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. 24
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 25
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 40 30 20 10 1520 Gain (db) P Input: -30 dbm -20 dbm -10 dbm -5 dbm 1540 1560 1580 Wavelength (nm) 26
Pumping power The saturation power depends on the pumping power 980nm, 165mW and 1480nm, 140mW, enough to 8 channels and 16 channels, but not enough to 32 even 40 channels Double-pumping configurations 27
Pumping Length The optimal length of the active fiber depends on the pumping power Why there are drops? 28
Noise in EDFAs An optical amplifier generates its own noise. EDFA noise is caused by amplified spontaneous emission (ASE). Population inversion factor (spontaneous emission factor): n N /( N N ) sp 2 2 1 n sp 1, ideal amplifier (1.4 ~ 4) Noise Figure: F ( SNR) /( SNR) n in out ASE average total power: P ASE 2 n hf G BW sp In EDFA, noise figure based on signal-noise beating: F P ( ) /[ hf G BW ( )] 2n 2 n ASE s s sp In an ideal EDFA, F n = 3 db, and F n = 4 ~ 7 db in a normal EDFA. 29
Gain and noise vs. EDFA parameters Length of active fiber Input power Signal wavelength 30
Gain and noise vs. Pumping Direction (1) Co-propagating Pumping Low Noise (2) Counter-propagation Pumping High Gain (3) Bi-directional Pumping 31
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) 32
Amplifiers Chains and Signal Level Sample system uses 0.25 atten fiber, 80 km fiber sections, 19 db amplifiers with a noise figure of 5 db 10 Signal level (dbm) 0-10 -20-30 0 100 200 300 400 500 600 700 800 Location (km) 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 + 33
Amplifiers Chains and Optical SNR Same sample system: Transmitter SNR is 50 db, amplifier noise figure of 5 db 60 50 Optical SNR (db) 40 30 20 10 0 0 100 200 300 400 500 600 700 800 Location (km) 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 34
Components of an EDFA module Components of an EDFA: Er-doped fiber Pump laser diodes WDM Coupler Isolator Filter Er-doped fiber Small core diameter: 2.8~5.2 m (SMF: 8.3 m, MMF: 50, 62.5 m) Small mode-field diameter (MED): 3~6 m (regular fiber: 9~11 m) Concentrate most erbium ions in the center region of the small core 35
Coupling loss and polarization fiber modes. Er-doped fiber modes and the transmission 36
Splicing an Erbium doped fiber 37
Several passive components for EDFAs 38
Advantages and disadvantages of EDFAs 39
Optical Amplifier Applications 40
EDFA Categories Power boosters Up to +17 dbm power, amplifies transmitter output Also used in cable TV systems before a star coupler In-line amplifiers Installed every 30 to 70 km along a link Good noise figure, medium output power Pre-amplifiers Low noise amplifier in front of receiver 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 41
Requirement of EDFA applications Booster amplifier High saturation power: to provide maximum output power 16 dbm (39.8 mw) ~ 19 dbm (79.4 mw) In-line amplifier Moderate gain, noise, and saturation output characteristics (14 ~ 18 dbm) High gain flatness Gain tilt: Tilt( db / db) G( ) / G( ) 0 Pre-amplifier Low noise: Fn = 4.0 db High gain High sensitivity: -40 dbm (0.1 W) 42
Raman Amplifiers Raman Fiber Amplifiers (RFAs) rely on an intrinsic non-linearity in silica fiber Variable wavelength amplification: Depends on pump wavelength For example pumping at 1500 nm produces gain at about 1560-1570 nm RFAs can be used as a standalone amplifier or as a distributed amplifier in conjunction with an EDFA
Fiber Raman Amplifier Principle: Stimulated Raman Scattering (SRS) An incident photon excites an electron to the virtual state and the stimulated emission occurs when the electron de-excites down to the vibrational state of glass molecule. The Stokes shift corresponding to the eigen-energy of a phonon is approximately 13.2 THz for all optical fibers.
Stimulated Raman Scattering (SRS) Gain Spectrum 13.2THZ 1. The Raman gain spectrum of optical fiber exhibits a broad continuum shape due to amorphous nature of the material. 2. The peak value of Raman gain coefficient is inversely proportional to pump wavelength. In other word, Raman gain shape is wavelength/frequency dependent. 3. Exists in all fiber. 4. Ultra-fast response.
power(db) Power(dB) Fiber Raman Amplifier 1550nm Fiber λ (a) Without Pump 1550nm The input signal will be attenuated in the fiber. 1450nm 1550nm λ Fiber 1450nm 1550nm When the signal and a high-power pump are injected into a fiber together, and the signal is within the Raman gain region of the pump, the signal will be amplified. (b) With Pump
Backward and Forward Pump
Gain Flatting At 1550 nm window, the peak wavelength of Raman gain profile is 100 nm larger than the pump wavelength. Using several pump lights of different wavelengths (15 nm difference) and powers, a 100 nm-broadband of gain is achieved. SMF
Fiber Raman Amplifier
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 With only an EDFA at the transmit end the optical power level decreases over the fiber length With an EDFA and Raman the minimum optical power level occurs toward the middle, not the end, of the fiber.
Advantages and Disadvantages of Raman Amplification Advantages Variable wavelength amplification possible Compatible with installed SM fiber 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
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Semiconductor optical amplifier (SOA) Principle Stimulated emission to amplify an optical signal. Active region of the semiconductor. Injection current to pump electrons at the conduction band. The input signal stimulates the transition of electrons down to the valence band to acquire an amplification.
Fabry-Perot and Traveling-Wave Amplifiers Gain of FPA Gain ripple: An FPA exhibits peaks of gain at resonant wavelengths. The gain can be increased by optical feedback. Increasing the reflectance beyond a certain point can turn the amplifier to a laser. The smaller the reflectance, the less pronounced the gain peaks. R=0 TWA Gain of TWA The input signal is amplified by a single passage through the active region.
Traveling-Wave Amplifiers Covering the facets with an antireflection coating Tilting the active region with respect to the facets Using buffer material between the active region and the facets
Gain Saturation and Polarization Dependence The gain coefficient depends on the frequency and power of the signal being amplified. max P sat G s Gs 1 ln Ps, in Gs The saturation output power for an SOA range from 10 to 15 mw. The gain of SOA depends on the state of polarization of the signal.
Bandwidth Bandwidth: The frequency range at which the gain drops 3 db from its maximum value. Bandwidth difference between FPA and TWA FPA: large gain, small bandwidth TWA: small gain, large bandwidth
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 fiber 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.
Miniature Optical Amp Erbium doped aluminum 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
Homework 12.70, 12.77 Think: 12.72 61