Chapter 8 Wavelength-Division Multiplexing (WDM) Part II: Amplifiers
Introduction Traditionally, when setting up an optical link, one formulates a power budget and adds repeaters when the path loss exceeds the available power margin. To amplify an optical signal with a conventional repeater, one performs photon-to-electron conversion, electrical amplification, retiming, pulse shaping, and then electron-to-photon conversion. Although this process works well for moderate-speed single-wavelength operation, it can be fairly complex and expensive for high-speed multiwavelength systems. Thus, a great deal of effort has been expended to develop all-optical amplifiers. These devices operate completely in the optical domain to boost the power levels of lightwave signals for the two long-wavelength transmission windows of optical fibers.
Optical amplifiers A fiber-optic transmission could be made much longer before introducing a regenerator. It also allowed the practical application of wavelength division multiplexing by operating completely in the optical domain to boost the power levels of lightwave signals for the two long-wavelength transmission windows of optical fibers. All the signals (e.g. WDM signal with many different wavelengths) could be amplified simultaneously, and the number of regenerators is greatly reduced to make the system less complex and much cost effective. Two fundamental types of optical amplifier: Semiconductor optical amplifier (SOA), doped fiber amplifier (DFA) All optical amplifiers increase the power level of incident light through a stimulated emission process.
Basic operation The device absorbs energy supplied from an external source called the pump. The pump supplies energy in an active medium, which raises them to higher energy levels to produce a population inversion. An incoming signal photon will trigger these excited electrons to drop to lower levels through a stimulated emission process, thereby producing an amplified signal. Optical Fiber communications, 3 rd ed.,g.keiser,mcgrawhill, 2000
Gain One of important parameters of an optical amplifier is the signal gain or amplifier gain G, which is defined as G P sout, = = P sin, exp[ g( zl ) ] P P s, out where s, in and are the input and output powers, respectively, of the optical signal being amplified. g( z) is the overall gain per unit length. Gain saturation gz ( ) = g0 P ( z) + P 1 s amp, sat g 0 is the unsaturated medium gain per unit length in the absence of signal input, Ps ( z) is the internal signal power at point z, and P amp is the amplifier, sat saturation power, which is defined as the internal power level at which the gain per unit length has been halved.
Dependence of the gain on the input power The curve shows that as the input signal power is increased, the gain first stays near the small-signal level and then starts to decrease. After decreasing linearly in the gain saturation region, it finally approaches 0 db for high input powers.
SOAs SOAs are based on conventional laser principles; an active waveguide region is sandwiched between a p-region and an n-region. A bias voltage is applied to excite ions in the region and create electron-hole pairs. Then, as light of a specific wavelength is coupled in the active waveguide, stimulation takes place and causes electron-hole pair to recombine and generate more photons (of the same wavelength as the optical signal), and hence optical amplification is achieved. Alloys of semiconductor elements from groups III and V (e.g., phosphorus, gallium, indium, and arsenic) make up the active medium in SOAs. They work in both the 1300 nm and 1550 nm low attenuation windows, and they can be easily integrated on the same substrate as other optical devices and circuits. Two types of SOAs 1) Fabry-Perot amplifier (the reflectivity of the end faces is about 32 percent) 2) Traveling wave amplifier (the internal reflection does not take place)
DFAs The active medium in an optical fiber amplifier consists of a nominally 10- to 30-m length of optical fiber that has been lightly doped with a rare-earth element, such as erbium (Er), Ytterbium (Yb), neodymium (Nd), etc. The host fiber material can be either standard silica, a fluoride-based glass, or a multicomponent glass. The doped fiber is active medium itself. Very compact and compatible with optical fiber devices Whereas semiconductor optical amplifiers use external current injection to excite electrons to higher energy levels, optical fiber amplifiers use optical pumping. In this process, one uses photons to directly raise electrons into excited states.
EDFA The most popular material for long-haul telecommunication application is a silica fiber doped with erbium, which is known as an erbium-doped fiber amplifier or EDFA.
Amplifier noise The dominant noise generated in an optical amplifier is amplified spontaneous emission (ASE). The origin of this is the spontaneous recombination of electrons and holes in the amplifier medium. This recombination gives rise to a broad spectral background of photons that get amplified along with optical signal. Optical Fiber communications, 3 rd ed.,g.keiser,mcgrawhill, 2000
Amplifier noise (Cont.)
System applications In designing an optical fiber link that requires optical amplifiers, there are three possible locations where the amplifiers can be placed. Although the physical amplification process is the same in all three configurations, the various uses require operation of the device over different input power ranges. In-line amplifier Preamplifier Power amplifier
System performance (Cont.) In-line amplifiers In a long transmission system, optical amplifiers are needed to periodically restore the power level after it has decreased due to attenuation in the fiber. Normally, the gain of each EDFA in this amplifier chain is chosen to compensate exactly for the signal loss incurred in the preceding fiber section of length. The accumulated ASE noise is the dominant degradation factor in such cascaded chain of amplifiers. Power amplifiers For the power amplifier, the input power is high, since the device immediately follows an optical transmitter. High pump powers are normally required for this application. Preamplifers An optical amplifier can be used as a preamplifier to improve the sensitivity of directdetection receivers that are limited by thermal noise.
Multichannel operation Another characteristic of an EDFA is that its gain is wavelength dependent in its normal operating window of 1530-1560 nm. If it is not equalized over the spectral range of operation in a multichannel system, this gain variation will create a large signal-to-noise ratio differential among the channels after passing through a cascaded EDFAs. Numerous techniques have been tried for this equalization.
In-line amplifier gain control In a long-distance fiber transmission system using optical amplifiers, it is desirable to keep the output power of the in-line amplifiers constant when there are fluctuations in the input power level. A practical way to keep the output power constant is to operate the optical amplifier in the gain-saturation region.
Erbium-doped fiber lasers (EDFLs) Resonators Ring configurations Linear configurations Advantages Compact, small size, stable, does not need complex cooling system, compatible to fiber systems Applications Medical, military, communication, micromachining, etc.
Multiwavelength EDFLs
Raman amplifier Raman scattering Raman scattering results from the interaction of light with the vibrational modes of the molecules constituting the scattering medium. Raman scattering can be equivalently be described as the scattering of light from optical phonons. Stimulated Raman scattering (SRS) SRS is an important nonlinear process that can turn optical fibers into broadband Raman amplifiers and tunable Raman lasers. It can also severely limit the performance of multichannel lightwave systems by transferring energy from one channel to the neighboring channels.
Light scattering Spontaneous light scattering By spontaneous light scattering, we mean light scattering under condition such that the optical properties of the material system are unmodified by the presence of the incident light beam. A light scattering process is said to be spontaneous if the fluctuations (typically in the dielectric constant) that cause the light scattering are excited by thermal or by quantum-mechanical zero-point effects.
Stimulated light scattering Stimulated light scattering The character of the light-scattering process is profoundly modified whenever the intensity of incident light is sufficiently large to modify the optical properties of the material system. A light scattering process is said to be stimulated if the fluctuations (typically in the dielectric constant) are induced by the presence of the light field. Stimulated Brillouin scattering (SBS) Brilloiun scattering is the scattering of light from sound waves, that is, from propagating pressure waves. Brillouin scattering can also be considered to be the scattering of light from acoustic phonons. SBS is that the interference of the incident light and scattered light contains a frequency component at the frequency of sound wave. The response of the material system can act as a source that tends to increase the amplitude of the sound wave. Thus the beating of the incident light wave with the sound wave tends to reinforce the scatted light wave. Stimulated Raman scattering (SRS)
Spontaneous Raman effect In general, the scattered light contains frequencies different from those of the excitation source. Those new components shifted to lower frequencies are called Stokes components, and those shifted to higher frequencies are called anti-stokes. Raman Stokes scattering consists of a transition from the ground state g to the final state n by means of an intermediate transition to a virtual level associated with excited state n. Raman anti-stokes scattering entails a transition from level n to level g with n serving as the intermediate level. The anti-stokes lines are typically much weaker than the Stokes lines because, in thermal equilibrium, the population of level n is smaller than the population in level g.
Stimulated Raman scattering The spontaneous Raman scattering process described in the previous section is typically a rather weak process. SRS is typically a very strong scattering process: 10% or more of the energy of the incident laser beam is often converted into the Stokes frequency.
Raman gain
Raman fiber amplifiers