RAPPORT TECHNIQUE CEI IEC TECHNICAL REPORT TR 61292 3 Première édition First edition 2003 06 Amplificateurs optiques – Partie 3 Classification, caractéristiques et applications Optical amplifiers – Pa[.]
Amplificateurs à fibre de silice dopée à l'erbium (EDFA ou EDSFA)
The concept of the erbium-doped fiber amplifier (EDFA) was first demonstrated in 1985 At a time when conventional systems without repeaters were nearing their peak performance, a research group at the University of Southampton revealed that optical fibers could exhibit optical gain at wavelengths close to.
Erbium-doped fiber amplifiers (EDFAs) have gained significant attention in the field of fiber optic communications due to their efficient operation in the preferred telecommunications window of approximately 1,550 nm, where loss is minimal These amplifiers are activated with low-power visible light and are currently the most widely used optical amplifiers in the industry.
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The previous text summarises optical amplifier wavelength bands from a historical perspective.
The rapid evolution of technology has led to the introduction of new bands and the expansion of existing band boundaries In 2001, the ITU-T established recommended band definitions to enhance discussions regarding single-mode optical transmission systems, resulting in the agreement on six contiguous spectral bands for these systems.
The U-band ultralong wavelength ranges from 1625 to 1675 nanometers Spectral bands are defined to aid in discussions and should not be considered as specifications The specific operating wavelength bands are detailed in the relevant system documentation.
The G.65x fibre Recommendations have not yet confirmed the applicability of all wavelength bands for system operation or maintenance The boundary at 1,460 nm between the E-band and S-band is still under investigation The U-band is designated for potential maintenance purposes only, with no current plans for traffic-bearing signal transmission; any non-transmission use must ensure minimal interference with other bands Future applications, both with and without optical amplifiers, are expected to utilize signal transmission across the full range of 1,260 nm to 1,625 nm.
3 Erbium doped fibre amplifiers (EDFAs)
3.1 Erbium doped silica fibre amplifiers (EDFAs or EDSFAs)
The erbium doped fibre amplifier (EDFA) was first demonstrated in 1985 by a research group at the University of Southampton, showcasing that optical fibres could achieve optical gain near the 1,550 nm wavelength By doping these fibres with the rare earth element erbium and activating them with low powers of visible light, EDFAs have gained significant attention in optical fibre communications for their ability to operate efficiently within the low-loss telecommunications spectral window.
1 550 nm EDFAs are the most widely used optical amplifiers today.
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Un EDFA peut être pompé de manière optique à certain nombre de longueurs d’onde avec des performances optimales atteintes aux longueurs d’onde de 980 nm et 1 480 nm.
Ils fournissent un gain aux longueurs d’onde comprises approximativement entre 1 520 nm et
An EDFA (Erbium-Doped Fiber Amplifier) typically consists of a single-mode erbium-doped fiber section, a pump laser, a WDM coupler to combine the pump power and signal within the erbium fiber, input and output isolators, tap couplers, and an electronic drive device Although the longer wavelength range of 1625 nm has not yet been finalized by various standardization bodies, this basic configuration is essential for effective signal amplification in optical communication systems.
The erbium atom has numerous energy levels, but only a select few are relevant for optical amplification in telecommunications systems These include the ground state and several of the lowest energy states Higher energy states correspond to transitions in the visible and ultraviolet parts of the spectrum and are largely unoccupied in EDFA applications Figure 1 below illustrates (a) the abbreviated energy levels for EDFA and (b) the primary energy levels utilized in EDFA.
~1 480 nm 1530 nm-1560 nm Non radiatif rapide
Figure 1 – Niveaux d’énergie abrégés et primaires pour EDFA
The EDFA demonstrates polarization-insensitive gain, immunity to crosstalk between channels, high saturation output power, and low noise close to the quantum limit EDFA can simultaneously amplify weak signals across its entire operating wavelength range, which varies based on the amplifier design This capability is essential for wavelength division multiplexing (WDM) Additionally, EDFAs provide all optical amplification in the 1,550 nm region, where silica transmission fiber experiences its minimum loss.
Erbium possesses excellent spectroscopic properties, including a limited radiative decay metastable lifetime and conveniently located auxiliary energy levels This has enabled the development of amplifiers that operate within fractions of a dB of the quantum limits for noise figure and power conversion efficiency Erbium-doped fiber amplifiers (EDFAs) have significantly increased the capacity of optical transmission systems while reducing system costs The high output powers provided by EDFAs support a greater number of channels, and their wide bandwidth and slow gain dynamics facilitate "transparent" multi-channel operation.
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An Erbium-Doped Fiber Amplifier (EDFA) is optimally pumped at wavelengths of 980 nm and 1,480 nm, providing gain for signals ranging from approximately 1,520 nm to 1,625 nm While the upper limit of this range is still under review by various standards bodies, a typical EDFA comprises a section of single-mode erbium-doped fiber, a pump laser, a WDM coupler to combine the signal and pump power, as well as input and output isolators, tap couplers, and drive electronics.
The erbium atom possesses numerous energy levels, but only a select few are relevant for optical amplification in telecommunication systems, specifically the ground state and several low-level states Higher energy states, which correspond to transitions in the visible and ultraviolet spectrum, remain largely unoccupied in Erbium-Doped Fiber Amplifier (EDFA) applications Figure 1 illustrates the abridged energy levels for EDFAs and highlights the primary energy levels utilized in these systems.
~1 480 nm 1530 nm-1560 nm Fast non-radiative
Figure 1 – Abridged and primary energy levels for EDFAs
Erbium-doped fiber amplifiers (EDFAs) offer polarization insensitive gain and immunity to inter-channel cross-talk, making them ideal for wavelength-division multiplexing (WDM) They can amplify weak signals across their entire operating range, which varies by design, and provide high saturation output power with low noise close to the quantum limit Operating primarily in the 1,550 nm region, where silica transmission fiber experiences minimal loss, EDFAs leverage erbium's excellent spectroscopic properties, including a radiative decay limited metastable lifetime and well-placed auxiliary energy levels This enables the creation of amplifiers that achieve noise figure and power conversion efficiency within fractions of a dB of quantum limits.
Erbium-doped fiber amplifiers (EDFAs) significantly enhance the capacity of optical transmission systems while lowering costs Their high output power enables the support of more channels, and their broad bandwidth combined with slow gain dynamics facilitates transparent multi-channel operation.
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A variety of host glasses, dopants, and fiber designs are under investigation to optimize amplifier characteristics, including pump efficiency and spectral bandwidth.
Amplificateurs à fibre de fluorure dopée à l'erbium (EDFA)
One of the major limitations in fully utilizing bandwidth in WDM systems is the spectral gain non-uniformity of conventional erbium-doped silica fiber amplifiers (EDFA or EDSFA) Due to the different spectroscopic behavior of erbium ions in fluoride materials, fluoride fiber amplifiers offer improved uniformity compared to traditional silica fiber amplifiers.
The first production of zirconofluoride glasses in 1975 led to the definition of the ZrF4-BaF2-LaF3-AlF3-NaF3 (ZBLAN) system in 1981 Significant research has since been conducted on the potential of this material to enable new optical amplification applications across various strategic wavelengths for fiber optic transmission, including the 1.5 µm window with erbium doping.
The primary difference between silica and fluoride amplifiers lies in the changes to the spectroscopic characteristics of lasers induced by the host environment and phonon energy In the case of fluoride-doped fiber amplifiers (EDFFA), there is a broader and smoother gain spectrum compared to erbium-doped fiber amplifiers (EDFA).
Another parameter influenced by the glass matrix is the phonon energy, which is lower in ZBLAN glasses compared to silica glasses The energy level during the 4 I 11/2 lifetime is too long in ZBLAN, hindering effective 980 nm pumping due to the excited state absorption (ESA) effect However, EDFFAs pumped at nearly 1,480 nm involve the same laser transitions as EDFAs, resulting in gain performance, output power, and noise levels comparable to those achieved by silica EDFAs.
The configuration of ZBLAN-based EDFFAs is quite similar to that of conventional silica EDFAs, with the exception of fiber splices Fusion splicing between ZBLAN and silica fibers is not feasible for two main reasons: first, the melting temperature of silica is around 2,300 K, while ZrF4 reaches its vapor phase at a much lower temperature of about 900 K Second, the thermal expansion coefficients of the two materials differ significantly, with ZBLAN's being more than ten times greater.
De ce fait, la technique utilisée consiste à épissurer mécaniquement la fibre dopée à une fibre de silice d’ouverture numérique (NA) élevée ayant un diamètre du champ de mode identique.
La fibre de silice à NA élevée est à son tour épissurée par fusion à une fibre de silice standard au moyen d’une technique du cœur à extension thermique.
EDFFAs can serve as power amplifiers, both in-line and as preamplifiers for single and multiple channel applications The characteristics for multi-channel operation are achieved with an optimized total input power to ensure the best gain uniformity across the output spectra.
Comparộ aux EDSFA conỗus sans aucun dispositif additionnel pour l’uniformitộ de gain, le
EDFFA provides improved gain uniformity essential for multi-channel operation The exceptional advantage of fluoride amplifiers is their ability to utilize a wide bandwidth in the C and L bands for the design of high-capacity WDM systems.
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[4] R.I Laming and D.N Payne, “Noise characterization of erbium-doped fiber amplifier pumped at 980 nm” IEEE Photonics Technology Letters, vol.2, pp418-421, 1990
[5] J Hecht, Understanding Fiber Optics – Third Edition, SAMS Publishing, 1999
3.2 Erbium doped fluoride fibre amplifiers (EDFFAs)
A significant limitation in utilizing bandwidth in WDM systems is the spectral non-uniformity of gain in conventional silica-based erbium doped fiber amplifiers (EDFAs) In contrast, fluoride-based fiber amplifiers demonstrate enhanced gain flatness due to the distinct spectroscopic behavior of erbium ions in fluoride materials, making them a superior alternative to traditional silica-based amplifiers.
The first realization of fluorozirconate glasses in late 1975 led to the definition of the ZrF 4-
The BaF\(_2\)-LaF\(_3\)-AlF\(_3\)-NaF\(_3\) (ZBLAN) system, developed in 1981, has been extensively researched for its potential in advancing optical amplification applications across various strategic wavelengths for optical fiber transmission, particularly in the 1.5 µm window with erbium doping.
Silica-based amplifiers differ from fluoride-based amplifiers primarily in how their laser spectroscopic characteristics are affected by the host environment and phonon energy Specifically, EDFFAs exhibit a broader and smoother gain spectrum compared to EDFAs.
The phonon energy in ZBLAN glasses is lower than that in silica glasses, which affects their performance The long lifetime of the energy level 4 I 11/2 in ZBLAN hinders efficient 980 nm pumping because of the influence of pump excited-state absorption (ESA).
EDFFAs pumped at near 1 480 nm involve the same laser transitions as EDSFAs resulting in gain, output power and noise performances close to that achieved by silica-based EDFAs.
The configuration of EDFFAs closely resembles that of traditional silica-based EDFAs, with the notable exception of fiber splices Fusion splicing between ZBLAN and silica fibers is unfeasible due to two main factors: the fusion temperature for silica is approximately 2,300 K, while ZrF4 transitions to its vapor phase at around 900 K, and the thermal expansion coefficients of the two materials differ significantly, with ZBLAN exhibiting more than ten times the dilatation coefficient of silica.
The technique involves mechanically splicing doped ZBLAN fiber to a high numerical-aperture (NA) silica fiber with the same mode-field diameter This high NA silica fiber is then fusion spliced to standard silica fiber using the thermally expanded core technique.
EDFFAs serve as power, in-line, and preamplifiers for both single and multi-channel applications, achieving optimal gain flatness in output spectra through total input power optimization Unlike EDSFAs that lack gain flattening devices, EDFFAs provide superior gain flatness essential for multi-channel operations A significant benefit of fluoride-based amplifiers is their ability to utilize a broad bandwidth in the C and L bands, making them ideal for designing high-capacity WDM systems.
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The transmission of sixteen 10 Gb/s channels over a distance of 531 km using conventional single-mode fiber was achieved with seven inline fluoride EDFAs Hybrid EDFAs (silica/fluoride) featuring dual-stage flat gain, which provide a wider amplification window and low noise figure, along with dispersion compensating fiber (DCF) sections, were utilized in the transmission process.
32 voies de 10 Gb/s à espacements égaux couvrant 25 nm sur 500 km de fibre monomodale conventionnelle [3].
Le matériel de transmission, y compris 20 amplificateurs de ligne à fluorure à gain plat avec gain de 22 dB à 27 dB, puissance de sortie type de +15 dBm, facteur de bruit type inférieur à
7 dB et 80 km de DCF, a été utilisé dans un essai sur site de mise en réseau WDM transfrontalier avec branchements [4] à transposition de longueurs d’onde tout optiques.
[1] “1.5 àm Fluoride-Based Amplifiers for Wideband Multichannel Transport Networks”,
B Clesca, D Bayard, and J.L Beylat Optical Fiber Technology 1, pp135-157, 1995
[2] “Transmission of 16x10Gbps channels spanning 24 nm over 531 km of conventional single-mode fiber using 7 in-line fluoride-based EDFAs”, S Artigaud, M Chbat,
P Nouchi, F Chiquet, D Bayart, L Hamon, A Pitel, F Goudeseune, P Bousselet,
[3] “320 Gbps WDM transmission over 500 km of conventional single-mode fiber with 125 km amplifier spacing”, S Bigo, Abertaina, M Chbat, S Gurib, J Da Loura, J.C Jacquinot,
J Hervo, P Bousselet, S Borne, D Bayart, L Gasca, J.L Beylat ECOC’97
[4] “A cross-border WDM networking field trial with all-optical wavelength-translating crossconnects”, L Berthelon, S Bjomstad, P Bonno, P Bousselet, M Chbat,
C Coeurjolly, P.J Godsvang, R Gronvold, P.M Kjeldsen, A Kleivstul, A Jourdan,
J.S Mapsen, A Noury, T Olsen, G Soulage OFC’98 Postdeadline paper
Amplificateurs à fibre de tellurure dopée à l'erbium (EDTFA)
Telluride glass is an oxide-based material with a high refractive index of approximately 2 A telluride EDFA (EDTFA) can achieve a broader amplification bandwidth compared to EDSFA and EDFFA Additionally, the EDTFA offers other fiber amplifier properties, including polarization insensitivity, low noise figure, and high saturation power.
The amplification mechanism of erbium-doped telluride fiber amplifiers (EDTFA) is similar to that of silica-based EDFA, relying on the stimulated emission from the 4 I 13/2 to the 4 I 15/2 energy levels of erbium ions Notably, erbium-doped telluride glass exhibits distinct optical properties, including a high refractive index of approximately 2.0, which results in a larger stimulated emission cross-section compared to conventional silica glass This characteristic is particularly significant in the wavelength region around 1,530 nm.
At 1580 nm, the refractive index is approximately 1.3 times greater than that of silica glass, and it exceeds a factor of 2 at around 1600 nm This analysis highlights the significant differences in optical properties at these wavelengths.
Judd-Ofelt, les paramètres Ω dans le verre au tellurure sont de Ω 2 = 4,116 × 10 –20 cm 2 ,
The cross-sections for the optical processes are Ω 4 = 1.805 × 10⁻²⁰ cm² and Ω 6 = 8.486 × 10⁻²¹ cm² The maximum wavelength limit, defined as the intersection of emission and absorption in the excited state, is 1,637 nm It is anticipated that the operational wavelength range of the EDTFA will extend by an additional 7 nm or 9 nm in the higher wavelength region compared to the silica or fluoride EDFA, respectively.
1 Les chiffres entre crochets se refèrent aux documents de référence à la fin du paragraphe.
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Transmission of sixteen 10 Gb/s channels spanning 24 nm between 1 536,61 nm and
A transmission distance of 531 km using conventional single-mode fiber was achieved at a wavelength of 1560.61 nm with the help of seven in-line fluoride-based EDFAs The system utilized dual-stage flat-gain hybrid EDFAs, which combined silica and fluoride to provide a broader amplification window and a low noise figure, along with sections of dispersion compensating fiber (DCF) This setup successfully transmitted 32 equally spaced channels at 10 Gb/s over a 25 nm range across 500 km of fiber.
The transmission equipment utilized in the cross-border WDM networking field trial comprised 20 flat-gain fluoride-based line amplifiers, offering a gain range of 22 dB to 27 dB, with a typical output power of +15 dBm and a noise figure of less than 7 dB Additionally, 80 km of dispersion compensating fiber (DCF) was employed alongside all-optical wavelength-translating crossconnects.
[1] “1.5 àm Fluoride-Based Amplifiers for Wideband Multichannel Transport Networks”,
B Clesca, D Bayard, and J.L Beylat Optical Fiber Technology 1, pp135-157, 1995
[2] “Transmission of 16x10Gbps channels spanning 24 nm over 531 km of conventional single-mode fiber using 7 in-line fluoride-based EDFAs”, S Artigaud, M Chbat, P Nouchi,
F Chiquet, D Bayart, L Hamon, A Pitel, F Goudeseune, P Bousselet, J.L Beylat.
[3] “320 Gbps WDM transmission over 500 km of conventional single-mode fiber with 125 km amplifier spacing”, S Bigo, Abertaina, M Chbat, S Gurib, J Da Loura, J.C Jacquinot,
J Hervo, P Bousselet, S Borne, D Bayart, L Gasca, J.L Beylat ECOC’97
[4] “A cross-border WDM networking field trial with all-optical wavelength-translating cross- connects”, L Berthelon, S Bjomstad, P Bonno, P Bousselet, M Chbat, C Coeurjolly,
P.J Godsvang, R Gronvold, P.M Kjeldsen, A Kleivstul, A Jourdan, J.S Mapsen, A.
Noury, T Olsen, G Soulage OFC’98 Postdeadline paper
3.3 Erbium doped tellurite fibre amplifiers (EDTFAs)
Tellurite glass is an oxide-based material with a high refractive index of about 2 A tellurite- based EDFA (EDTFA) can be used to achieve wider amplification bandwidth than EDSFA and
EDFFA In addition, EDTFA can also provide other optical fibre amplifier properties such as polarization insensitivity, low noise figure and high saturation power.
The amplification mechanism of erbium-doped tellurite-based fibre amplifiers (EDTFA) mirrors that of silica-based EDFA, relying on stimulated emission from the 4 I 13/2 to the 4 I 15/2 levels of erbium ions However, erbium-doped tellurite glass exhibits distinct optical properties, including a high refractive index of approximately 2.0, which leads to a larger stimulated emission cross-section compared to conventional silica-based glass Specifically, the cross-section in the 1,530 nm to 1,580 nm wavelength range is about 1.3 times larger than that of silica-based glass, and it exceeds that by a factor of more than 2 at around 1,600 nm.
Judd-Ofelt analysis, the Ω parameters in tellurite glass are Ω 2 = 4,116×10 –20 cm 2 ,
Ω 4 = 1,805×10 –20 cm 2 and Ω 6 = 8,486×10 –21 cm 2 The longer wavelength limit, which is defined as the intersection of the emission and the excited state absorption cross-section, is 1 637 nm.
The operational wavelength range of the EDTFA is expected to extend 7 or 9 nm further into the longer wavelength than that of silica or fluoride based EDFA respectively.
1 Figures in brackets refer to the reference documents at the end of the subclause.
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Erbium-doped telluride fiber (EDF) is produced using a drawing-coating method Preforms and coating tubes are prepared through casting-suction and rotational casting techniques, respectively The glass transition temperature is approximately 400 °C, and the background loss of telluride EDF is less than 30 dB/km at 1,200 nm.
The key distinction in EDF telluride modules lies in the technology employed for splicing telluride and silica fibers An inclined V-groove connection technique is utilized to achieve low-loss and low-reflection splicing between the fibers Typically, the reflection and insertion loss at the inclined V-groove connection are around 0.3 dB and less than -50 dB, respectively.
Les longueurs d’onde de pompe pour l’EDTFA sont de 980 nm et de 1 480 nm Le fonctionnement à faible bruit peut être aisément obtenu en utilisant un plan de pompage de
980 nm Les longueurs d’onde de fonctionnement pour les EDTFA pompés de 980 nm et
The gain profile is influenced by the average population inversion factor An EDTFA operating at a lower average population inversion can achieve a longer operating wavelength by utilizing a longer fiber, thereby shifting the gain window to higher wavelength regions.
For broadband operation between 1,530 nm and 1,610 nm, the EDFTA exhibits a high gain peak around 1,560 nm The gain profiles of telluride and silica amplifiers differ slightly at approximately 1,580 nm, with the telluride amplifier showing a plateau in its gain profile Additionally, the gain window is somewhat wider at the higher wavelength end Consequently, the EDFTA can be utilized to provide highly efficient broadband amplification with multi-stage gain equalizer configurations and intermediate stages.
Dans le fonctionnement de la bande de 1 580 nm, l’EDTFA possède une largeur de bande élevée de 50 nm comprise entre 1 560 nm et 1 610 nm avec une variation de gain d’environ
The EDSFA has a bandwidth of approximately 38 nm with consistent gain variation The conversion efficiency and noise figure of the EDTFA at a wavelength of 1,580 nm are nearly identical to those of the EDSFA.
EDTFA devices have applications similar to EDSFA devices They offer an intriguing method for obtaining optical, analog, or digital links, operating within a wavelength range of 1,530 nm to 1,620 nm.
De nos jours, les EDTFA avec une longueur d’onde de fonctionnement large peuvent être utilisés en tant que survolteurs, préamplificateurs et amplificateurs en ligne Une transmission
A 3 Tb/s WDM system (160 Gb/s channels × 19) has been demonstrated using an EDFA as a recovery amplifier An EDFA operating at a wavelength of 1580 nm was utilized in an error-free transmission experiment at 10 Gb/s.
L’EDTFA à haute concentration d’erbium a également été étudié en vue d’une utilisation dans un amplificateur de petite taille pour compenser la perte d’insertion de divers dispositifs optiques.
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Tellurite-based erbium-doped fiber (EDF) is produced through a jacketing-drawing technique, utilizing suction-casting for preforms and rotational casting for jacketing tubes This fiber exhibits a glass transition temperature of around 400 degrees Celsius and demonstrates a background loss of less than 30 dB/km at a wavelength of 1,200 nm.
Amplificateur à fibre dopée à l'ytterbium/erbium (EYDFA)
The relatively narrow absorption bands of Er$^{3+}$ in silica, combined with the lack of high-power lasers at these pump wavelengths during the early development of the EDFA, led to the creation of co-doped or sensitized fibers for 1.5 µm amplification This involves adding a second ion to the core of the fiber, which has a higher energy level that is close to or isodynamic with the energy level of the active ion.
Active ions, or amplifiers, are pumped through energy transfer between different ion species The most effective approach is co-doping Er:Yb in aluminum-phosphosilicate glass, which achieves efficient pumping and relatively high bandwidths Notably, the 2 F 5/2 level of Yb is nearly isodyname with the 4 I 11/2 level in Er 3+, facilitating direct energy transfer Yb ions possess a broad absorption band, enabling pumping across a range of wavelengths.
800 nm et 1 080 nm et permettant l’utilisation du pompage de 1 060 nm des lasers haute puissance Nd à solides ou fibres.
Satisfactory results using Er:Yb co-doped silica fibers rely on efficient energy transfer to Er$^{3+}$ ions Optically excited Yb ions transfer energy to the 4I$^{11/2}$ level of Erbium, which then non-radiatively relaxes to the upper laser level 4I$^{13/2}$ To prevent energy transfer back to Yb ions, a rapid decay from the 4I$^{11/2}$ level is essential, favoring glass hosts with high phonon energy Pure silica fibers have shown disappointing results due to their low phonon energy (~1,190 cm$^{-1}$) compared to phosphate glasses, which have a higher crystal lattice vibrational energy (~1,325 cm$^{-1}$) However, these soft glasses have several drawbacks and are not easily compatible with standard silica fibers Research has shown that adding Al$_2$O$_3$ to a phosphosilicate fiber can provide a good energy transfer rate to Erbium while remaining compatible with standard fibers Typical Yb:Er doping ratios range from 10:1 to 30:1, and this, combined with its high absorption cross-section, ensures that Yb dominates the pump absorption.
Yb ions enable pumping across a wide range of wavelengths and facilitate the use of high-power laser arrays through an intermediate laser pump A popular approach involves using 810 nm laser diode arrays to pump a solid-state Nd:YAG laser operating at 1,060 nm, which subsequently pumps the Er:Yb fiber amplifier The solid-state laser functions as a mode converter, transforming the multimode output from the diode array into a single-mode beam suitable for the fiber amplifier.
A pure phosphate glass host for the Er:Yb system exhibits a significantly narrower bandwidth compared to erbium in silica fiber This limitation poses clear disadvantages for WDM applications; however, research on phosphosilicate fibers has demonstrated that gain spectra similar to those of Er-doped silica fibers can be achieved.
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[1] Y Ohishi et al., Opt Lett., 23(4), pp274-276, 1998
[2] Mori et al., Electron Lett., 34(9), pp887-888, 1998
[3] T Nakai et al., Technical Digest OAA’98, PD5, 1998
[4] Mori et al., J Lightwave Technol., Vol 20, pp822-827, 2002.
3.4 Erbium/ytterbium doped fiber amplifiers (EYDFA)
The development of co-doped or sensitized fibers for 1.5 µm amplification was driven by the narrow absorption bands of Er³⁺ in silica and the absence of high-power lasers at these pump wavelengths during the early stages of EDFA development By adding a second ion with an upper energy level close to that of the active ion, energy transfer occurs between the ions, facilitating amplification The most effective approach is Er:Yb co-doping in alumino-phosphosilicate glass, which allows for efficient pumping and broad bandwidths The isoenergetic relationship between the Yb ²F₅/₂ level and the Er ⁴I₁₁/₂ level enables direct energy transfer, while the broad absorption band of Yb permits pumping across a wavelength range of 800 to 1,080 nm, including the use of 1,060 nm pumping from high-power Nd solid-state or fiber lasers.
Successful results using Er:Yb co-doped silica fibres rely on efficient energy transfer to the
Er 3+ ions [1] The optically excited Yb ions provide energy transfer to the 4 I 11/2 level of Erbium
The excitation level achieved with 980 nm pumping decays non-radiatively to the 4 I 13/2 upper laser level, necessitating a rapid decay from the 4 I 11/2 level to prevent energy transfer back to Yb ions This process is enhanced in glass hosts with high phonon energy, as seen in phosphate glasses (~1 325 cm –1), which outperform pure silica fibers (~1 190 cm –1) that exhibit lower phonon energy However, soft glasses present compatibility issues with standard silica fibers, particularly in splicing Research indicates that adding Al 2 O 3 to phosphosilicate fibers improves energy transfer rates to Er, maintaining compatibility with standard fibers Typical doping ratios of Yb:Er range from 10:1 to 30:1, ensuring that Yb effectively dominates pump absorption due to its high absorption cross-section.
Yb ions facilitate pumping across a broad wavelength spectrum and support the utilization of high-power laser arrays via an intermediate laser pump A widely adopted method involves employing 810 nm laser diode arrays to pump a Nd:YAG solid-state laser.
1 060 nm, which in turn pumps the Er:Yb fibre amplifier [2] The solid state laser acts as a
‘mode converter’, converting the multi-mode output of the laser diode array into a single-mode pump beam for the fibre amplifier.
A pure phosphate glass hosting the Er:Yb system exhibits a significantly narrower bandwidth compared to Er in silica fiber, which poses challenges for Wavelength Division Multiplexing (WDM) applications However, research on phosphosilicate fiber indicates that gain spectra comparable to those of Er in silica fiber can be achieved.
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Les premiers résultats utilisant le pompage de diode ont montré une sortie de +21 dBm à
A wavelength of 1542 nm was achieved using 390 mW of pump power in a bidirectional pumping scheme An optical small-signal gain of 40 dB and a gain exceeding 30 dB were recorded for an output of +20 dBm Additionally, optical conversion efficiencies of approximately 40% can be attained.
The use of solid-state lasers or fiber lasers as mode converters for laser diode arrays facilitates the relatively easy scaling of power A 900 mW pump from a Nd:YLF laser operating at 1,053 nm is employed in this process.
A 1,047 nm laser was utilized to achieve an output of +24.6 dBm Additionally, a high-power flash-lamp-pumped Nd:YAG laser was employed to demonstrate a power scaling of the linear amplifier up to +27 dBm with a 1.5 W pump.
Bien qu’il ne soit pas aussi critique que pour une application de préamplificateur, le facteur de bruit de tels amplificateurs de récupération représente également un paramètre important.
Les fibres co-dopées à l'Er:Yb se sont approchées du facteur de bruit à limite quantique similaire aux EDFA standard.
Recent advancements in cladding-pumped laser technology have demonstrated superior output powers Utilizing an intermediate pump laser as a mode converter reduces the overall efficiency and available power for fiber pumping Given the broad absorption of the Yb ion, directly pumping the fiber with a high-power diode laser is advantageous Fibers with two concentric cores have been developed to facilitate multimode pumping directly into the fiber The doped single-mode core is surrounded by a multimode core, which is ultimately encased in an outer cladding Pump light injected into the multimode core randomly intersects with the doped single-mode core, leading to absorption The pump absorption is reduced from the pure single-mode pumping plane in proportion to the area ratio of the multimode core to the single-mode core The high absorption coefficient of Yb in the 950 nm to 980 nm range, along with high Yb doping levels, enables efficient pump absorption and Er ion inversion in short segments of double-clad fibers, making this approach practical Final pumping at 620 mW at 962 nm yielded an output of +17 dBm in a non-optimized design More recently, a compact device utilizing side pumping has been reported, demonstrating an output of +25 dBm with a pump power of 1.25 W and a noise figure of 4.5 dB at 1,570 nm using a V-groove cut in the double-clad fiber to couple the pump light.
Les puissances de sortie élevées et les facteurs faibles de bruit que l’on peut obtenir avec le
EDFA technology has led to its application in CATV markets and optical broadcasting networks, which require high-output optical amplifiers Advances in single-mode laser diodes at 980 nm and 1480 nm now meet the power demands for telecommunications and data networks, allowing conventional EDFA systems to dominate this market Furthermore, the spectral bandwidth limitations of current Er:Yb fibers have hindered their use in network deployment.
WDM ó des largeurs de bande > 30 nm sont requises Cependant, de récents résultats ont montré des EYDFA donnant une ondulation du gain de 0,2 dB sur une largeur spectrale de
A hybrid amplifier featuring a standard erbium fiber first stage followed by an Er:Yb fiber second stage has achieved an output of +26 dBm over a wavelength range of 17 nm, with a noise figure of 5.2 dB This performance highlights the effectiveness of combining different fiber types to enhance output power.
0,5 dB et un facteur de bruit de 5 dB [6].
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Amplificateurs à fibre de fluorure dopée au praséodyme (PDFFA)
The praseodymium-doped fluoride fiber amplifier (PDFFA) is an optical fiber amplifier designed to operate at a wavelength of approximately 1,300 nm It offers several advantages, including high saturation output power, polarization-independent gain, low distortion, and a low noise figure compared to other amplifiers at this wavelength Consequently, the PDFFA is recognized as a highly promising candidate for 1,300 nm transmission systems.
The amplification mechanism of Praseodymium-doped fiber amplifiers (PDFFA) is classified as a four-level amplification system, relying on the stimulated emission from the 1 G 4 level to the 3 H 5 level of praseodymium ions Ground state absorption (GSA) occurs between the 3 H 4 and 1 G 4 levels, with a peak absorption wavelength of approximately 1,015 nm Excited praseodymium ions transition from the 1 G 4 level to the 3 F 4 level due to multiple phonon relaxation, as the energy difference is about 3,000 cm –1 To enhance quantum yield, selecting a low phonon energy glass as the fiber host is crucial, with fluoride glass being the most promising candidate for praseodymium ions The spontaneous lifetime of the 1 G 4 level is 110 µs for ZrF 4-doped fluoride fiber, while InF 4-doped fluoride fiber exhibits a longer lifetime of approximately 170 µs due to its lower multiple phonon relaxation rate The quantum yield for spontaneous emission from the 1 G 4 to 3 H 5 levels in indium fluoride glass is nearly double that of zirconium glass, and thermal analysis indicates sufficient thermal stability for fiber manufacturing Additionally, praseodymium-doped fiber (PDF) with a small core diameter structure can achieve highly efficient amplification.
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The high power capability has also been used in producing compact and efficient fibre lasers.
The Er:Yb fibre is utilized in high power continuous wave (CW), Q-switched, and mode-locked lasers, delivering high energy and peak power pulses for various applications, including laser marking, laser ranging, and inter-satellite communications Additionally, single frequency distributed feedback (DFB) fibre lasers have been successfully demonstrated, necessitating short gain media to achieve larger mode spacing and requiring strong pump absorption.
[1] S Grubb et al., Electron Lett 28 (13), pp1275-1276, 1992
[2] S Grubb et al., IEEE Photonics Tech Lett 4 (6), pp553-555, 1992
[3] J.D Minelly et al., IEEE Photonics Tech Lett 5 (3), pp301-303, 1993
[4] L Goldberg, J Koplow, Electron Lett 34 (21), pp2027-2028, 1998
[5] N Park et al., IEEE Photonics Tech Lett 8 (9), pp1148-1150, 1996
[6] P.F Wysocki et al., Opt Lett., 21 (21), pp1744-1746, 1996
[7] J.T Kringlebotn et al., Electron Lett 30 (12), pp972-973, 1994
4 Non-erbium doped fibre amplifiers
4.1 Praseodymium doped fluoride fibre amplifiers (PDFFAs)
Praseodymium doped fluoride-based fibre amplifiers (PDFFA) operate at approximately 1,300 nm and offer several advantages, including high saturation output power, polarization-independent gain, low distortion, and a low noise figure These characteristics make PDFFA a highly promising option for 1,300 nm transmission systems, outperforming other amplifiers in this wavelength range.
The amplification mechanism of PDFFAs is classified as a four-level amplification system It is based on the stimulated emission of the 1 G 4 level to the 3 H 5 level of praseodymium ions [1].
Pump photon ground state absorption (GSA) occurs between the 3 H 4 level and the 1 G 4 level.
The peak absorption wavelength for praseodymium ions is approximately 1,015 nm Due to multi-phonon relaxation, the transition from the excited 1 G 4 level to the 3 F 4 level occurs easily, as the energy difference is only about 3,000 cm⁻¹ To enhance quantum efficiency, selecting a low phonon energy glass as the fiber host is crucial Among the available options, fluoride glass stands out as the most promising host matrix for praseodymium ions Additionally, the spontaneous lifetime of the 1 G 4 level is measured at 110 µs.
Pr-doped ZrF4-based fluoride fibers exhibit unique properties, while Pr-doped InF4-based fluoride fibers have a spontaneous lifetime of approximately 170 µs due to the lower multi-phonon relaxation rate of InF4-based fluoride glass Additionally, the quantum efficiency of indium-based fluoride glass enhances the spontaneous emission characteristics.
The thermal stability of praseodymium-doped fibre (PDF) with a small-core diameter structure is sufficient for fibre fabrication, and its performance at the 3 H 5 level is nearly double that of zirconium-based glass, enabling highly efficient amplification.
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Fluoride PDF can be produced using a drawing-coating method, where the preform and coating tube are prepared through casting-suction and rotational casting, respectively The glass transition temperature for indium fluoride glass is approximately 250 °C Additionally, the background losses of fluoride PDF with a refractive index difference of 3.7% are below 50 dB/km at a wavelength of 1,200 nm.
The primary distinction in the fluorine PDF module set lies in the technology employed for splicing fluorine and silica fibers An inclined V-groove connection technique is utilized to achieve low-loss and low-reflection splicing between the fibers, with reflection and insertion loss at the inclined V-groove connection measuring 0.3 dB and