1、3404583 01109055 ElSI TECHNICAL REPORT ETR I26 March 1994 Source: ETSI TC-TM Reference: DTRTTM-O1 O1 9 UDC: 621.395 Key words: Amplification, fibre, optical, network Transmission and Multiplexing (TM); Applications of optical fibre amplifiers in long distance and optical access networks ETSI Europea
2、n Telecommunications Standards Institute ETSI Secretariat Postal address: 06921 Sophia Antipolis Cedex - FRANCE Office address: Route des Lucioles - Sophia Antipolis - Valbonne - FRANCE Tel.: +33 92 94 42 O0 - Fax: +33 93 65 47 16 European Telecommunications Standards Institute 1 994. All rights res
3、erved. No part may be reproduced except as authorised by written permission. The copyright and the foregoing restriction on reproduction extend to all media in which the information may be embodied. 3404583 0309056 7T3 Page 2 ETR 126:1994 Whilst every care has been taken in the preparation and publi
4、cation of this document, errors in content, typographical or otherwise, may occur. If you have comments concerning its accuracy, please write to “ETSI Editing and Standards Approval Dept.“ at the address shown on the title page. m 3404583 0109057 b3T m Page 3 ETR 126: 1994 Contents Foreword . 5 Scop
5、e . 7 References . 7 Abbreviations . 7 OFA classification . 8 4.1 Types of OFAs 8 4.2 Power amplifier 9 4.3 Line amplifier . 9 4.4 Pre-amplifier 9 System aspects 10 Applications of OFAs in public telecommunication networks . 10 6.1 General . 10 6.2 Application of OFAs in long distance networks . 11
6、6.3 Application of OFAs in OANs . 12 Laser safety considerations 12 History 13 m 3404583 0107058 576 m Page 5 ETR 126:1994 . Foreword This ETSI Technical Report (ETR) has been produced by the Transmission and Multiplexing (TMI Technical Committee of the European Telecommunications Standards Institut
7、e (ETSI). This ETR describes possible applications sf optical fibre amplifiers in long distance and optical access networks. Previous page is blank 3404583 0309059 402 Page 7 ETR 126:1994 1 Scope This ETSI Technical Report (ETR) describes possible applications of Optical Fibre Amplifiers (OFAs) in b
8、oth long distance and Optical Access Networks (OANs). The impact of applying OFAs in both installed systems and systems under development are also discussed. Descriptions given in this ETR are as general as possibe, in order to allow the deployment of OFAs in both the 1 310 nm and the 1 550 nm wavel
9、ength regions. Because of current commercially available technology the emphasis of this ETR is on OFAs using rare-earth doped fibres as the active medium operating in the 1 550 nm wavelength region. 2 References For the purposes of this ETR, the following references apply: 11 I 41 51 ITU-T Recommen
10、dation G.661 (1 993): “Definition and test methods for the relevant generic parameters of optical fibre amplifiers“. ITU-T Recommendation G.652 (1 993): “Characteristics of a single-mode optical fibre cable“. ITU-T Recommendation (2.653 (1 993): “Characteristics of a dispersion- shifted single-mode
11、optical fibre cable“. EN 60825: “Radiation safety of laser products, equipment classification, requirements and users guide“. I EC-825: “Radiation safety of laser products, equipment classification, requirements and users guide“, 3 Abbreviations For the purposes of this ETR, the following abbreviati
12、ons apply: AM ASE CATV cw LOS OAM OAN OAR OAT O FA ONU PDH SBS SDH STM WDM Amplitude Modulated Amplified Spontaneous Emission Cable Television Continuous Wave Loss of Optical input Signal Operation And Maintenance Optical Access Network Optically Amplified Receiver Optically Amplified Transmitter Op
13、tical Fibre Amplifier Optical Network Unit (customer side) Plesiochronous Digital Hierarchy Stimulated Brillouin Scattering Synchronous Digital Hierarchy Synchronous Transport Module Wavelength Division Multiplexing Previous page is blank = 3404583 OLOOb0 124 TX . Page 8 ETR 126:1994 oooo RX 4 OFA c
14、lassification 4.1 Types of OFAc The following categories of OFAs are considered in this ETR: a) power, post, or booster amplifier; b) line amplifier; C) pre-amplifier. An overview of the three categories under consideration is given in figure 1. This ETR only addresses discrete OFAs, leaving the so-
15、called distributed amplifiers (lightly doped transmission fibres using “remote“ pumping at terminal ends) outside the scope of this ETR. Furthermore, only uni-directional operation of OFAs is assumed. As well as separate deployment of the above three options, various combinations are possible, e.g.
16、power and pre-amplifier or power, pre-amplifier and line amplifier. I 1 - Standard Booster amplifier P re- amplifier Line amplifier RX I oooo ITX I U u Figure 1 : Overview of OFA categories m 3404583 OLOObL Ob0 m Page 9 ETR 126: 1994 4.2 Power amplifier A power, post-amplifier or booster amplifier i
17、s an OFA integrated with, or inserted in, the optical line just after a laser transmitter, where the first option results in an Optically Amplified Transmitter (OAT), and the second option results in a non-integrated optical power amplifier, as described in ITU-T Recommendation G.661 111 (see figure
18、 2). In the latter case, the OAM functions may or may not be shared with the optical transmitter. This OFA, operating most likely in a saturated mode, considerably increases the optical output power of commercially available laser sources. Furthermore, this OFA category does not need stringent requi
19、rements for noise and optical filtering. Because of the relatively high level of the OFA output power, the undesirable Amplified Spontaneous Emission (ASE) noise, inherently present due to the statistical process of photon generation inside the OFA, is usually negligible. I i S RI- S Figure 2: Schem
20、e of the optical power amplifier as a) OAT; and b) non-integrated OFA 4.3 Line amplifier A line amplifier is a low noise OFA, inserted between two passive fibre sections, in order to compensate for attenuation losses in long-haul fibre sections or to compensate for branching losses in point-to-multi
21、point OANs. If designed properly, a line amplifier may replace a part of or all conventional regenerators in long-haul fibre sections. Depending on its capabilities, it can be envisioned that even more than one conventional regenerator can be replaced by a single line amplifier, with the evident adv
22、antage of reduced equipment in transmission links. Furthermore, a situation can be envisaged, where both line amplifiers, for compensation of signal attenuation, and conventional regenerators, for compensation of signal distortion, appear in long-distance networks. A line amplifier should have a suf
23、ficient dynamic range to ensure practical deployment, related to existing ITU-T Recommendations. 4.4 Pre-amplifier A pre-amplifier is a very low noise OFA, which improves the receiver sensitivity considerably, when being integrated with, or inserted in, the optical path just before the optical recei
24、ver. The first option will create an Optically Amplified Receiver (OAR), and the second option results in a non-integrated optical pre-amplifier as described in ITU-T Recommendation G.661 Ill (see figure 3). In the latter case, the OAM functions may or may not be shared with the optical receiver. Th
25、e required low level of the ASE can be achieved by using narrow band optical filters. An automatic tuning capability of the centre wavelength of the pre-amplifier filter to the transmitter wavelength is very much preferable in order to relax requirements on initial transmitter wavelength tolerance a
26、nd its long-term stability. The pre-amplifier, in combination with the optical receiver, should have a sufficient dynamic range to ensure practical deployment. W 3404583 OL090b2 TT7 Page 10 ETR 126: 1994 Figure 3: Scheme of the optical pre-amplifier as a) OAR; and 6) non-integrated OFA 5 System aspe
27、cts Regarding maintenance and supervision, a distinction should be made between power amplifiers and pre-amplifiers on one side and line amplifiers on the other side, In case of pre-amplifiers and power amplifiers, it is preferred that the OFA maintenance channels will be connected to the existing m
28、aintenance circuitry of the ordinary terminal equipment. This is trivial in the case of OATS and OARS. Therefore, dedicated service channels will not be necessary. It is, however, mandatory to have a separate communication channel in case of line amplifiers, allowing alarming, supervision and contro
29、l of installed remote line amplifiers. Such a supervision channel should be implemented in such a way that no restrictions of pump laser wavelength choices or operating wavelength window exist. One possibility is to superimpose a low-frequency Amplitude Modulated (AM) channel to the regular signal,
30、but this will certainly have an impact on (existing) transmitter designs. Another option is to use a specific wavelength channel within or outside of the OFA operating wavelength window. The number of fault information signals which could be taken into consideration should be limited and only relate
31、d to the OFA as a “black-box“, especially in the non-integrated cases e.g. Loss of Optical input Signal (LOS), OFA-degrade (increase of pump laser current above a fixed threshold) and OFA-fail (fatal failures, which can seriously degrade system performance, e.g. reduction of output power below a cer
32、tain threshold). Although some OFAs could be designed for a specific application, privileging the corresponding relevant characteristics, in general maintenance schemes (management and supervision) independent of the application are preferable. This allows the re-use of the OFA in case of system upg
33、rading (when, for example, passing from PDH to SDH networks, or increasing the bit-rate, e.g. from STM4 to STM16, inserting line amplifiers into the installed equipment, or adding additional channels via WDMJ. However, some other approaches using embedded channels could be implemented. 6 Application
34、s of OFAs in public telecommunication networks 6.1 General OFAs can be generally applied in both long distance networks and in OANs with the enormous advantage of increasing the available power budget, allowing longer section lengths and higher splitting ratios respectively. Furthermore, OFAs are in
35、 principle transparent devices, independent of bit-rate, signal format and, therefore, ideally suitable for network upgrades. = 3404583 0309063 933 Page 11 ETR 126:1994 6.2 Application of OFAs in long distance networks In long-haul systems, the available power budget can be increased considerably by
36、 adding OFAs (power amplifiers, pre-amplifiers or line amplifiers) to the conventional terminal equipment. Especially in case of long-haul transmission the advantages of OFA appfication are evident, allowing not only upgrading from PDH to SDH, but also to higher bit-ratesor even adding future wavele
37、ngth channels within the same operating window. However, this upgrading is only possible if sufficient margins are available for the parameters involved (e.g. saturation output power, total dispersion of the line). In general OFAs will compensate for attenuation losses in telecommunication networks.
38、 Therefore, systems which were attenuation limited at first, may now become dispersion limited. Nevertheless, OFAs can still off er longer section lengths on both standard, non-dispersion shifted fibres (ITU-T Recommendation G.652 121) as well as dispersion shifted fibres (ITU-T Recommendation G.653
39、 131). Dispersion limits can occur sooner with dispersion-shifted fibres operated in the 1 310 nm wavelength region or non-dispersion-shifted fibres operated in the 1 550 nm wavelength region. However, several methods exist to minimize dispersion problems, e.g. using dispersion compensating fibres o
40、r Continuous Wave (CW) operated narrow-linewidth lasers with external modulators. The application of power and pre-amplifiers proves to be very efficient in those cases, where intermediate locations with active equipment are either undesirable or inaccessible (.e. like in submarine systems). Further
41、more, less intermediate locations implies easier maintenance for the network operator. Therefore, the first, and by far the easiest, step to increase the available power budget is to use either an OAT (instead of a regular transmitter) or a power amplifier directly after the regular transmitter. An
42、equivalent increase in power budget can be realized by using an OAR or a pre-amplifier just before the conventional receiver. However, because pre-amplifier configurations are generally more complex than power amplifiers, due to the necessity of additional narrow band optical filters to reduce ASE l
43、evels, pre-amplifiers are likely to be used in combination with, rather than instead of, power amplifiers. In general, system enhancement will be achieved by power amplifiers only, pre-amplifiers only, or a combination of both. Also, from a system point of view, these are the preferred configuration
44、s (see Clause 5). The use of line amplifiers should, therefore, be considered only in case of longitudinal compatibility or joint engineering, thus avoiding the difficult requirements related to transverse compatibility of communication channels between multi-source line amplifiers. Theoretically, u
45、ltra-long (thousands of kilometres) transmission distances can be realized by periodically inserting line amplifiers in the optical path. However, when many OFAs are cascaded, problems can occur due to noise accumulation, spectral dependency of total gain, effects of polarization, chromatic dispersi
46、on and non-linear effects, causing deteriorated system performance. Laboratory tests have indicated that the overall system behaviour in the case of many cascaded line amplifiers is much more complex than in the case of a few cascaded line amplifiers. In particular, the total gain of a chain of line
47、 amplifiers in series is generally peaked around a specific wavelength, depending on the specific amplifier configuration, giving a considerable reduction of the usable OFA operating wavelength range. Therefore, design for this type of system will be very much different from the situation with a few
48、 cascaded line amplifiers. With respect to finding solutions to the above mentioned problems of cascading OFAs, a distinction should be made between single channel transmission and multiple wavelength channel transmission. In case of single channel transmission, noise accumulation can be reduced by
49、using low noise OFAs in combination with adequate band pass optical filtering. The dispersion limitations can usually be minimized by operating close to the fibre zero dispersion wavelength. Furthermore, care needs to be taken to keep non-linear effects under control, e.g. Stimulated Brillouin Scattering (SBS), (see subclause 6.3) and self phase modulation (changes of fibre index of refraction caused by high signal intensities). - 3404583 0107064 87T Page 12 ETR 126:1994 When introducing multi-wavelength channels, new problems can occur due to insufficient gain unifor
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