CHAPTER 6 OPTICAL AMPLIFIERS

Why Optical Amplifiers? Increase transmission distance by increasing optical power coupled to transmission fiber(power booster) by compensating optical fiber losses(in-line amplifier, remote pump amplifier) by improving receiver sensitivity (optical preamplifier) Function : Amplification of optical signal without conversion to electrical signal Ingredients : Pump energy, amplification medium Optical output Pumping of energy l l l l l l l l l 1 2 3 4 5 6 7 8 16 ... ... Optical input amplification medium l l l l l l l l l

The Need of Optical Amplification Why? – Extend distance light signal can travel without regeneration Erbium-Doped Fiber Amplifiers (EDFAs) – application in long haul. Today’s amplifier of choice. Erbium-Doped Waveguide Amplifiers (EDWAs) – application in metro and access networks Raman Amplifiers – application in DWDM Semiconductor Optical Amplifiers (SOA) – not fiber based type, application in metro and access networks Standard Fiber Amplifier Pump Lasers

General Application of Optical Amplification In-line amplifier: Able to amplify an attenuated signal so that it can travel an additional length of fiber, is called an optical line amplifier. Preamplifier: is placed directly before the detector and may be integrated with it. Power (booster) amplifier: it is placed right after the source, and thus may also be integrated with it. LAN booster amplifier Standard Fiber Amplifier Pump Lasers

Amplifier Operation Points 10 15 20 25 30 35 40 Booster amplifier 60 mW preamplifier 45 mW in-line amplifier 30 mW -10 -5 5 10 15 Output power[dBm]

Improvement of System Gain Booster amplifier Preamplifier In-line Remote pump Improvement in gain(dB) Key technology in length(km) 10 - 15 5 - 10 (APD) 10 - 15 (PIN) 15 - 30 5 - 15 40 - 60 20 - 40 40- 60 60 - 120 30 - 60 High efficiency Low noise Supervisory High pumping power

Erbium Doped Fiber Amplifiers (EDFA) Any discussion of lightwave transmission systems must include, at a minimum, a brief description of EDFAs, erbium doped fiber amplifiers. When coupled with DWDM filters, EDFAs are the enabling technology behind the huge growth in lightwave transmission capacity.

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 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. Input 1480 or 980 nm Pump Laser Erbium Doped Fiber Output Isolator WDM

Erbium-doped Fibre Amplifier When you actually look at an EDFA block diagram you see the input on the left the output on the right, photo diodes to measure input and output power levels, pump lasers which are used to excite this piece of erbium doped fiber to a higher energy state, and 1550 light is allowed to pass through. The isolators are used to keep the 980 and 1480 pump wavelengths contained in the fiber but allow 1550nm light to pass through.

Issues of Amplifier Design Optimization maximum efficiency minimum noise figure maximum gain maximum gain flatness/gain peak wavelength Dynamic range, operation wavelength Gain equalization Control circuit Monitoring of amplifier performance

Erbium-doped Fibre Amplifier Gain, G (dB) 10log[(PSignal_Out - Pase) / PSignal_In] Noise Figure, F = SNRout / SNRin S-band : 1440 - 1530nm C-band : 1530 - 1565nm L-band : 1565 - 1625nm Spectrum of a typical EDFA

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)

Noise Figure (NF) NF = P ASE / (h• • G • B OSA) P ASE: Amplified Spontaneous Emission (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 0-13-534330-5). 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)

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 before a star coupler Pre-amplifiers Low noise amplifier in front of receiver Bi-directional amplifiers An amplifier which work in both ways 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.

EDFA Categories

Typical Design Figure in the slide shows an EDFA with all the options design Coupler #1 is for monitoring input light Coupler #2 is optional and is used for monitoring backreflections. The microcontroller can be set to disable the pump lasers in case the connector on the output has been disconnected. This provides a measure of safety for technicians working with EDFA’s.

Typical Design

Erbium-Doped Fiber Amplifiers Figure in the next slide shows two single-stage EDFA’s packaged together Mid stage access is important for high performance fiber optic systems requiring a section of dispersion-compensating fiber (DCF) The high loss of DCF (10dB+) is compensated by the EDFA\ The main advantage of SOA’s and EDFA’s is that amplification is done with photons, as opposed to electronic amplification (requires O/E and E/O conversion)

Erbium-Doped Fiber Amplifiers

Typical Packaged EDFA

EDFAs In DWDM Systems Gain flatness requirements Gain competition Optical amplifiers in DWDM systems require special considerations because of: Gain flatness requirements Gain competition Nonlinear effects in fibers DWDM systems can cause additional complications for designers. Let us look at the key issues one at a time. Note that most of these aspects become irrelevant on DWDM systems over short distances. For example, many municipal links do not need any amplifier at all (the average cable length between network terminals in big cities can be as short as seven miles).

Gain Flatness Gain versus wavelength Compensation techniques The gain of optical amplifiers depends on wavelength Signal-to-noise ratios can degrade below acceptable levels (long links with cascaded amplifiers) Compensation techniques Signal pre-emphasis Gain flattening filters Additional doping of amplifier with Fluorides G  The gain and noise figure of the first generations of erbium-doped fiber amplifiers (EDFAs) were not flat over wavelength. The amplitude or signal-to- noise ratio (SNR) can degrade quicker than desired if amplifiers are cascaded. Therefore they may not be well suited for DWDM applications. More recent designs compensate this effect with different fiber doping, compensation filters or simply by pre-emphasizing the channels at the transmitter side in order to achieve a good signal-to-noise ratio for all channels.

Gain Competition Total output power of a standard EDFA remains almost constant even if input power fluctuates significantly If one channel fails (or is added) then the remaining ones increase (or decrease) their output power Output power after channel one failed In a simple model, an amplifier has a pool of optical energy available for amplification. If a channel drops in a DWDM system (or is added) then fewer (more) signals compete for this energy. As a result, the output power for the other channels increases (decreases) accordingly. These unwanted fluctuations must be compensated by either a high tolerance of the receiver for power fluctuations or by adjusting the power of the pump lasers inside the amplifier. Equal power of all four channels

Output Power Limitations High power densities in SM fiber can cause Stimulated Brillouin scattering (SBS) Stimulated Raman scattering (SRS) Four wave mixing (FWM) Self-phase and cross-phase modulation (SPM, CPM) Most designs limit total output power to +17 dBm Available channel power: 50/N mW (N = number of channels) The total power (sum of all channels) coming out of an amplifier can cause nonlinear effects in the fiber behind the amplifier. Because the effects can cause significant distortions, system designers must carefully balance EDFA power levels, amplifier spacing and signal-to-noise requirements. Standards for long haul systems typically propose up to eight channels at 100 or 200 GHz spacing with up to twelve EDFAs to cover a span up to 600 km. Companies offering systems with more channels often target them for regional links with less or no amplifiers.

Erbium Doped Waveguide Amplifier (EDWA) An erbium-doped waveguide amplifier (EDWA) consists of waveguides embedded in an amorphous erbium-doped glass substrate. The erbium atoms provide the glass with gain in the 1,550-nm fibre-optic window. The waveguide itself is a localised increase in the glass refractive index. Today's manufacturers have several methods available to produce erbium-doped glass waveguides: PECVD and flame hydrolysis deposition, sputtering, ion-exchange, or ion implantation. For the manufacture of waveguide amplifiers, the two most advanced methods are ion-exchange and sputtering.

Erbium Doped Waveguide Amplifier (EDWA) EDWA advantages EDWAs are inherently compact. One of the smallest gain block amplifiers to date, featuring 15-dB gain at 1,535 nm, fits in a 130x11x6-mm package. EDWAs also offer a better price/performance ratio than comparable EDFAs for access and metro network applications.

Raman Amplifier Raman Amplifier was demonstrated in the 1980s Unavailability of high-power diode laser pump source Why do you need it : amplify signals from 1270 to 1670 nm any optical fiber can serve as the amplifying medium Raman process itself provides high-power laser Disadvantage: Cross-talk

Raman Amplifier Amplifier Gain

Raman Amplifier Wide bandwidth Raman amplifier can be realized using multiple pumps

Raman Amplifier Multistage Amplifier Counter propagating pump

EDFA+Raman Combined Raman and EDFA transmission Experiment: DWDM (Lucent)

Raman Amplifier High-power laser sources are available by Raman process itself. 200 to 400 nm Bandwidth Amplifiers are possible Raman, EDFA combination Flat gain for WDM New fiber lasers and gratings make it practical System with Terabits of capacity are possible

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 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.

Semiconductor optical amplifiers, like their semiconductor-laser, consist of gain and passive regions. Layers of antireflective coatings prevent light from reflecting back into the circuit while the incoming signal stimulates electrons in the gain region

Semiconductor Optical Amplifier (SOA) Semiconductor optical amplifiers are similar in construction to semiconductor lasers. They consist of a gain (active) section and a passive section constructed of a semiconductor material such as indium phosphide. The main difference is that SOAs are made with layers of antireflection coatings to prevent light from reflecting back into the circuit. Optical gain occurs as excited electrons in the semiconductor material are stimulated by incoming light signals; when current is applied across the p-n junction the process causes the photons to replicate, producing signal gain. The gain medium can be either a bulk or a multiple-quantum-well active layer.

Semiconductor Optical Amplifiers Semiconductor Optical Amplifiers (SOA’s) Laser diodes without end mirrors Fiber attached to both ends Amplified version of the optical signal is produced Advantage: bidirectional Drawbacks High-coupling losses High noise figure

Semiconductor By adjusting the chemical composition of III-V semiconductors (typically GaInAsP) the band gap can be adjusted to give optical gain in the telecommunications windows of interest. The devices are typically 250mm long although devices of up to 1mm have been made. In general the longer devices can achieve higher gain and wider bandwidths. The optical bandwidth of a semiconductor optical amplifier (SOA) can be as large as 100 nm and the fact that the energy source is a DC electrical current makes these devices look promising as optical amplifiers.

Semiconductor There are two aspects of their properties which has prevented their widescale deployment. The SOA is fundamentally nonlinear as the device refractive index and the device gain depend on the amount of population inversion. Since this inversion changes as the signal is amplified this leads to amplitude and phase changes being applied to the signal. The short lifetimes of the device (100ps-1ns) also make these effects apparent at the single pulse level and as a pattern dependent effect. The second problem is that the noise performance of these devices is inferior to the Erbium fibre amplifier. One final drawback is that the usual geometry of the semiconductor waveguide is rectangular and the silica waveguide is circular. This leads to a mismatch of the fundamental mode patterns and the typical loss in going from one to the other is 3dB. Even so fibre to fibre gains of over 20dB can be achieved.

SOA operation E Conduction band Optical transition k Valence band

SOA operation The DC current applied to the device results in electrons being pumped into the (normally empty) conduction band and removed from the (normally full) valence band. This creates the population inversion which is a pre-cursor to optical gain. When signal photons travel through the device they cause stimulated emission to occur when an electron and hole recombine. The phase effects in these devices can be quite strong with a gain change of 3dB corresponding to roughly a phase change of p.