optical amplifiers

optical amplifiers PowerPoint PPT Presentation


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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: ReliabilityFlexibilityWavelength Division Multiplexing (WDM)Low CostVariety of optical amplifier types exists, including:Semiconductor Optical Amplifiers (SOAs)Erbium Doped Fibre Amplifiers (EDFAs) (most common).

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3: Traditional Optical Communication System

4: Optically Amplified Systems

5: Optical Amplification

6: Overview

7: 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 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. 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.

8: Erbium Doped Fibre Amplifier

9: Interior of an Erbium Doped Fibre Amplfier (EDFA)

10: Operation of an EDFA

11: Physics of an EDFA

12: 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 randomly (time constant ~1 ms) Stimulated emission By electromagnetic wave Emitted wavelength & phase are identical to incident one Erbium Properties Erbium is a rare earth element which can absorb and release light energy in the communications band around 1550 nm. When light at 980 or 1480 nm is applied to fiber doped with Er, the fiber absorbs this energy, i.e., electrons are excited to a higher energy level where they remain in a metastable state for some time. If left undisturbed, the Er doped fiber will eventually release this energy in the band of frequencies from about 1530 to 1565 nm. If stimulated by an input signal in this band, the Er doped fiber will emit the stored energy at the stimulated wavelength. The 980 nm absorption band is narrower than the 1480 nm band. In addition it is more difficult to make reliable 980 nm lasers. However, pumping the amplifier with 980 nm can result into a better noise figure of the amplifier. Erbium is a rare earth element which can absorb and release light energy in the communications band around 1550 nm. When light at 980 or 1480 nm is applied to fiber doped with Er, the fiber absorbs this energy, i.e., electrons are excited to a higher energy level where they remain in a metastable state for some time. If left undisturbed, the Er doped fiber will eventually release this energy in the band of frequencies from about 1530 to 1565 nm. If stimulated by an input signal in this band, the Er doped fiber will emit the stored energy at the stimulated wavelength. The 980 nm absorption band is narrower than the 1480 nm band. In addition it is more difficult to make reliable 980 nm lasers. However, pumping the amplifier with 980 nm can result into a better noise figure of the amplifier.

13: Erbium Doped Fibre Amplifiers

14: Er+3 Energy Levels

15: EDFA Operation

16: Technical Characteristics of EDFA

17: 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 In order to better understand noise generated by optical amplifiers we need to look at the spontaneous emission of the EDFA. As mentioned before, electrons will fall from the metastable state down to the ground stable either by stimulated emission due to an incoming photon (that is the amplification effect) or randomly with about a one millisecond time constant. Randomly emitted photons have a random phase, travel direction and wavelength within the amplifier’s wavelength range. This is called the spontaneous emission. Those photons travelling along the fiber will trigger stimulated emission that of course will have their wavelength, phase, etc. At the end almost all energy pumped into an amplifier without any input signal reappears as amplified spontaneous emission (ASE). However, if an input signal consumes electrons in the metastable state then fewer are left for spontaneous emission, therefore reducing the ASE.In order to better understand noise generated by optical amplifiers we need to look at the spontaneous emission of the EDFA. As mentioned before, electrons will fall from the metastable state down to the ground stable either by stimulated emission due to an incoming photon (that is the amplification effect) or randomly with about a one millisecond time constant. Randomly emitted photons have a random phase, travel direction and wavelength within the amplifier’s wavelength range. This is called the spontaneous emission. Those photons travelling along the fiber will trigger stimulated emission that of course will have their wavelength, phase, etc. At the end almost all energy pumped into an amplifier without any input signal reappears as amplified spontaneous emission (ASE). However, if an input signal consumes electrons in the metastable state then fewer are left for spontaneous emission, therefore reducing the ASE.

18: EDFA Behaviour at Gain Saturation

19: Saturation in EDFAs

20: 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 As discussed before, one benefit of the fact that EDFAs operate in saturation and lose about one dB of gain for one dBm increase in output power is that the output power of an EDFA will stay fairly constant over a variety of operating conditions. This amplifier has almost constant output power for a very wide input power range. At -30 dBm more than 50% of all metastable electrons are consumed for amplification. When the input power increases then this number approaches 100%. Because the pump power remains constant, the pool of excited electrons is limited. When used with a single carrier, if the input power of the EDFA were to drop by one dB, the gain would increase by one dB to reestablish the previous output power operating level. If the input power increased, the gain would drop, again, reestablishing the previous operating point. Again, we assume that the power fluctuations occur much slower than the time constant of the metastable state (~1 ms), and the modulation is much faster (20 kHz to many GHz).As discussed before, one benefit of the fact that EDFAs operate in saturation and lose about one dB of gain for one dBm increase in output power is that the output power of an EDFA will stay fairly constant over a variety of operating conditions. This amplifier has almost constant output power for a very wide input power range. At -30 dBm more than 50% of all metastable electrons are consumed for amplification. When the input power increases then this number approaches 100%. Because the pump power remains constant, the pool of excited electrons is limited. When used with a single carrier, if the input power of the EDFA were to drop by one dB, the gain would increase by one dB to reestablish the previous output power operating level. If the input power increased, the gain would drop, again, reestablishing the previous operating point. Again, we assume that the power fluctuations occur much slower than the time constant of the metastable state (~1 ms), and the modulation is much faster (20 kHz to many GHz).

21: Amplifier Length

22: 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 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. 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.

24: OFAs in the Network

25: Optical Amplifier Applications

26: Amplifier Applications

27: 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 Remotely pumped Electronic free extending links up to 200 km and more (often found in submarine applications) 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.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.

28: Example: Conventional EDFA

29: Gain Flattened EDFA for DWDM

30: Selecting Amplifiers

31: Pumping Directions

32: Multistage EDFAs

33: Commercial Designs Commercial amplifiers are optimized for performance needed in a particular application (booster/in-line/pre-amplifier) as well as to optimize cost and functionality. Input and output monitors are added for safety and reflection monitoring reasons. Power sensors monitor overall system health and provide aging information. Pumping at the input of the EDFA versus the output also improves the noise figure of the EDFA. If the EDFA is considered to be a system with an inherent noise figure and a gain block, placing the gain block early in the component cascade will reduce the overall noise figure of the cascade.Commercial amplifiers are optimized for performance needed in a particular application (booster/in-line/pre-amplifier) as well as to optimize cost and functionality. Input and output monitors are added for safety and reflection monitoring reasons. Power sensors monitor overall system health and provide aging information. Pumping at the input of the EDFA versus the output also improves the noise figure of the EDFA. If the EDFA is considered to be a system with an inherent noise figure and a gain block, placing the gain block early in the component cascade will reduce the overall noise figure of the cascade.

34: Security/Safety Features Input power monitor Turning on the input signal can cause high output power spikes that can damage the amplifier or following systems Control electronics turn the pump laser(s) down if the input signal stays below a given threshold for more than about 2 to 20 µs Backreflection monitor Open connector at the output can be a laser safety hazard Straight connectors typically reflect 4% of the light back Backreflection monitor shuts the amplifier down if backreflected light exceeds certain limits As discussed before, transient spikes can damage components in the amplifier and in the system. Input and reflection monitors help to significantly reduce or even eliminate this risk as well. For example, some amplifiers shutdown the pump laser if more than 0.1% of the output light is reflected back. A straight open connector has 4% backreflection (14 dB return loss) and therefore will cause such an amplifier to shut down. Someone may look at an open connector to check whether or not it is dirty or damaged. But few people manage to get dangerous power levels out of closed patchcords or cables.As discussed before, transient spikes can damage components in the amplifier and in the system. Input and reflection monitors help to significantly reduce or even eliminate this risk as well. For example, some amplifiers shutdown the pump laser if more than 0.1% of the output light is reflected back. A straight open connector has 4% backreflection (14 dB return loss) and therefore will cause such an amplifier to shut down. Someone may look at an open connector to check whether or not it is dirty or damaged. But few people manage to get dangerous power levels out of closed patchcords or cables.

36: Output Spectra This slide shows the typical ASE output spectra of an EDFA with no input signal and with a stimulating input signal. In addition to noting that most of the pump power appears at the stimulating wavelength, note also how the power distribution at the other wavelengths changes with a given input signal. The basic question for characterizing EDFAs is how to measure its noise figure. If the input signal is turned off then you measure a big ASE. If it is turned on then you measure a big signal. HP has developed several techniques to address this problem, including ASE interpolation, polarization nulling and time domain extinction.This slide shows the typical ASE output spectra of an EDFA with no input signal and with a stimulating input signal. In addition to noting that most of the pump power appears at the stimulating wavelength, note also how the power distribution at the other wavelengths changes with a given input signal. The basic question for characterizing EDFAs is how to measure its noise figure. If the input signal is turned off then you measure a big ASE. If it is turned on then you measure a big signal. HP has developed several techniques to address this problem, including ASE interpolation, polarization nulling and time domain extinction.

37: EDFA Gain Spectrum

38: Gain Characteristics of EDFA

39: Flattening of the Gain Curve Techniques

40: Gain Flattening Concept

41: Gain Flattening Filters (Equalizers)

42: Spectral Hole Burning (SHB) Gain depression around saturating signal Strong signals reduce average ion population Hole width 3 to 10 nm Hole depth 0.1 to 0.4 dB 1530 nm region more sensitive to SHB than 1550 nm region Implications Usually not an issue in transmission systems (single l or DWDM) Can affect accuracy of some lightwave measurements While spectral hole burning and PHB are small effects on a single EDFA, the effects may be more significant in long concatenated chains of similar EDFAs. Such systems are more common in long submarine links. While spectral hole burning and PHB are small effects on a single EDFA, the effects may be more significant in long concatenated chains of similar EDFAs. Such systems are more common in long submarine links.

43: Polarization Hole Burning (PHB) Polarization Dependent Gain (PDG) Gain of small signal polarized orthogonal to saturating signal 0.05 to 0.3 dB greater than the large signal gain Effect independent of the state of polarization of the large signal PDG recovery time constant relatively slow ASE power accumulation ASE power is minimally polarized ASE perpendicular to signal experiences higher gain PHB effects can be reduced effectively by quickly scrambling the state of polarization (SOP) of the input signal PDG: The polarization-dependent loss of components inside the amplifier depends on the state of polarization (SOP) of the input signal; PHB (see below) does not. ASE power accumulation: ASE power can be split into two parts, one half of the ASE is polarized parallel and the other half perpendicular to the saturating signal. Polarization hole burning (PHB) is an additional effect which can change the effective gain of an EDFA at a specific wavelength. Similar to spectral hole burning (see next slide), PHB is caused by a depletion of erbium ions in the polarization orientation of the stimulating signal in excess of the general rate of ion depletion across the EDFA emission band. In general, whenever you send one wavelength into an EDFA, the EDFA will tend to deplete all the energy stored in the EDFA at the wavelength input, at the polarization of the input signal. Typically PDG and PHB effects rarely exceed 0.1 to 0.5 dB.PDG: The polarization-dependent loss of components inside the amplifier depends on the state of polarization (SOP) of the input signal; PHB (see below) does not. ASE power accumulation: ASE power can be split into two parts, one half of the ASE is polarized parallel and the other half perpendicular to the saturating signal. Polarization hole burning (PHB) is an additional effect which can change the effective gain of an EDFA at a specific wavelength. Similar to spectral hole burning (see next slide), PHB is caused by a depletion of erbium ions in the polarization orientation of the stimulating signal in excess of the general rate of ion depletion across the EDFA emission band. In general, whenever you send one wavelength into an EDFA, the EDFA will tend to deplete all the energy stored in the EDFA at the wavelength input, at the polarization of the input signal. Typically PDG and PHB effects rarely exceed 0.1 to 0.5 dB.

45: Optical Amplifier Chains

46: Amplifiers Chains and Signal Level

47: Amplifiers Chains and Optical SNR

48: Noise Figure (NF) 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. 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.

50: Raman Amplifiers

51: Raman Effect Amplifiers

53: Distributed Raman Amplification (II)

54: Broadband Amplification using Raman Amplifiers

55: Advantages and Disadvantages of Raman Amplification

56: Semiconductor Optical/Laser Amplifiers (SOAs/SLAs)

57: Travelling Wave SLAs (TWSLAs)

58: Limitations/Advantages/Applications

59: Other Amplifier Types 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 Praseodymium-doped Fiber Amplifier (PDFA) Similar to EDFAs but 1310 nm optical window Deployed in CATV (limited situations) Not cost efficient for 1310 telecomm applications Fluoride based fiber needed (water soluble) Much less efficient (1 W pump @ 1017 nm for 50 mW output) 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. 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.

61: Miniature Optical Fibre Amp

62: A 1310 nm Band Raman Amplifier

63: 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 and other functions The recent explosion in WDM (see next section) has created a need to extend the band useful for amplifications. New developments show that you can extend the range of the optical amplifier possibly up to 1625 nm (or more), effectively allowing designers to send more optical channels through the system. Other WDM requirements include better gain flatness which can be achieved with doping modifications or filters.The recent explosion in WDM (see next section) has created a need to extend the band useful for amplifications. New developments show that you can extend the range of the optical amplifier possibly up to 1625 nm (or more), effectively allowing designers to send more optical channels through the system. Other WDM requirements include better gain flatness which can be achieved with doping modifications or filters.

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