Optical Communications and Networking Oct. 9, 2017 1
Optical Amplifiers In optical communication systems, the optical signal from the transmitter are attenuated by the fiber and other passive components (couplers, filters, and etc.). After some distance, the signal can become too weak to be detected and its power needs to be restored. Optical amplifiers can restore optical signal power by generating photons that are at the same frequencies. Optical amplifiers Fiber-based: Erbium-doped fiber amplifier, Raman Amplifier Semiconductor-based amplifier 2
How to Generate a Photon? Well, we usually don t want to bother God for the trivial things like optical communications. Good News: photons can be easily generated with the emission of radiation by atoms in the presence of an electromagnetic field. Bad News: we need to review the principle of quantum mechanics to understand the emission by atoms. 3
Review of Quantum Mechanics An atom can be described as a quantum mechanical system. Energy-levels: a quantum mechanical system can only take on certain discrete values of energy. Transitions: Electrons in atoms can change their energylevels by emitting or absorbing a photon. Planck s constant: h, determines the energy of a photon as E = hv, where v is the frequency of the photon ( 波粒二象性 ). 4
Review of Quantum Mechanics Emitting a Photon Absorbing a Photon Two rules: the conservation of energy, and the discretization of energy. h: Planck s Constant. v: Frequency of the photon (color). 5
Stimulated Emission ( 受激辐射 ) The new and original photons have the Same phase, frequency, polarization and propagating direction. 6
Stimulated Emission ( 受激辐射 ) Stimulated emission: Perfect optical amplification Two reciprocal transitions: stimulated emission and stimulated absorption. In normal media at thermal equilibrium, absorption exceeds emission because most atoms are in the low energy-level. When a population inversion is present, emission can exceeds absorption optical amplification. Population inversion: electrons in the higher energy-level are more than those in the lower energy-level. 7
Optical Amplification Optical amplification: stimulated emission dominate over absorption. We need to figure out a way to create population inversion. Pumping: supplying additional energy to put more atoms to the higher energy level. Three-Level Case Four-Level Case 8
Population Inversion Optical Amplification 9
Spontaneous Emission ( 自激辐射 ) Spontaneous emission: independent of any external radiation that may be present, atoms in the higher energy level can transit to the lower one and emit a photon. The spontaneous emission process DOES NOT contribute to the gain of the optical amplifier. Although the emitted photons have the save frequency as those from the stimulated emission, they are emitted in random directions, polarization, and phase. Therefore, spontaneous emission is incoherent, but stimulated emission is coherent. 10
Spontaneous Emission ( 自激辐射 ) When there is population inversion, the optical amplifier treats spontaneous emission radiation as another input, and also amplifies it, causing amplified spontaneous emission (ASE). The ASE appears as noise at the output of optical amplifiers. 11
Erbium-Doped Fiber Amplifier Erbium-doped fiber amplifier (EDFA): a length of silica fiber whose core is doped ( 掺杂 ) with ionized erbium atoms ( 离子化的铒原子, 稀土元素 ). 掺铒光纤放大器 Population inversion is created with an optical pump signal. Optical signal in 1550 nm wavelength range can be amplified based on the stimulated emission effect. Why EDFA is the amplifier of choice in today s optical communication systems? The availability of compact and reliable high-power pump lasers. EDFA is an all-fiber device, and it is simple and easy to handle. EDFA introduces no crosstalk when amplifying WDM signals. 12
Erbium-Doped Fiber Amplifier Stark splitting: each energy-level that appears as a discrete line in an isolated ion of erbium is split into multiple energylevels when these ions are introduced into silica fiber. Spreading each discrete energy-level of an erbium ion into a continuous energy band can increase the frequency or wavelength range of the signals that can be amplified. Typically, the set of wavelengths that can be amplified by EDFA corresponds to the wavelength range 1525-1570 nm, with a bandwidth of 50 nm (5.64 THz). This range overlaps with the low-attenuation window of the optical fiber. 13
History of Erbium-Doped Fiber Amplifier Before the invention of EDFA, people have to use opticalto-electrical-to-optical (O/E/O) repeaters for amplifying optical signals. An O/E/O repeater can only recover the signal of a single wavelength channel and it is expensive. EDFA was invented in 1987 by researchers from Southampton University (UK) and AT&T Bell Labs (USA). E. Desurvide, et al., High-gain erbium-doped traveling-wave fiber amplifier, Optics Letters, vol. 12, no. 11, Nov. 1987. EDFA can amplify many wavelength channels simultaneously. A real technical break-through for modern telecommunication systems. 14
Optical Transmission System Evolution 15
History of Erbium-Doped Fiber Amplifier Prof. David Payne, one of the inventers of EDFA Burning down of the Optoelectronics Research Center in Southampton University (10/31/2005) 16
Working Principle of EDFA Pumping wavelengths: two-level case (1480 nm), three-level case (980 nm). 17
Frequency Response of EDFA 18
Optical Pumping in EDFA 980 nm pumping Higher degree of population inversion, and low noise insertion. More efficient, but high-power pump laser is not available. Pre-amplifier: amplifies the low-power input signal. 1480 nm pumping Not very efficient and high noise insertion. High-power pump laser is available, and can yield high signal output power. Post-amplifier: amplifies the signal for retransmission. 19
Structures of EDFA Wavelength Selective Coupler 1550 nm Signal Erbium-doped Fiber Single-Stage Configuration DCF Erbium-doped Fiber 980 nm Pump Two-Stage Configuration 1480 nm Pump 20
Gain Flattening for EDFA Make gain unique to all WDM channels Use fluoride glass fiber instead of silica fiber Worse noise performance Use a filter insider the amplifier Fiber Bragg grating, 21
Noise in EDFA: Amplified Spontaneous Emission Spontaneous emission in EDFA causes atoms in the higher energy-level to decay and generate photons with random phase, frequency, polarization and propagating direction. Amplified spontaneous emission (ASE): the light wave generated by the spontaneous emission gets amplified. The light wave of ASE becomes the noise in EDFA and degrades the signal-to-noise ratio (SNR) of the optical signal. Noise figure: the ratio of input SNR to output SNR, it is usually 6 ~ 8 db in EDFAs. 22
Noise in EDFA: Amplified Spontaneous Emission Without Input Input at 1550 nm Under the rule of the conservation of energy, the signal and ASE noise compete for power in EDFAs, i.e., EDFA should amplify an optical signal before its power is too weak. 23
What does an EDFA look like? 24
Raman Fiber Amplifier Review of stimulated Raman scattering Due to the interaction between the light wave and the molecules in fiber, energy gets transferred from shorter wavelengths to longer ones. The magic frequency spacing: 13 THz, by pumping an SMF using a high-power laser, we can provide gain to the signal whose frequency is 13 THz below the pump. Raman fiber amplifier: Nothing but a spool of SMF and high-power pump laser(s). Pumping at 1460-1480 nm is for amplification at 1550-1600 nm. 25
Raman Amplifier Based on stimulated Raman scattering Energy transfer from shorter wavelengths to longer wavelengths Magic frequency spacing: 10 THz Pumping at 1460-1480 nm for amplification at 1550-1600 nm Pump lasers can be distributed Better gain flattening when signal and pump are in the opposite directions Noise from Rayleigh scattering, lower noise figure. 26
Structure of Raman Fiber Amplifier Output Power at 30 dbm or more! 27
Raman Amplifier versus EDFA EDFA provides gain in 1528 1605 nm range, while a Raman amplifier can provide gain at any wavelength. Raman amplifiers can open up more WDM bands. EDFA works in a discrete manner, while Raman amplifier can work as a distributed amplifier with the pump attached to one end of the fiber span. ASE noise is not an issue for Raman amplifiers. In Raman amplifiers, fluctuations in pump power will cause the gain to vary and generate crosstalk. 28
Considerations for Raman Amplifiers For Raman amplifiers, it is important to keep the pump at a constant power to reduce crosstalk. Having the pump propagate in the opposite direction can help reduce noise insertion and crosstalk dramatically, since the negative effects are averaged over the propagation time over the fiber. 29