Field Trial of Optical Fibre Cable-TV System Optical Fibre System for ...

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GRIN lenses \l, \o Fig. 6 The basic principle 01 the two-channel WDM couplers as used in the demonstration equipment. Cylindrical GRIN lenses and an interference filter are used to transmit or reflect the light, depending on the wavelength. In this way, an optical wavelength-dependent multiplexing and demultiplexing function is achieved Interferencefilter The three-channel multiplexing desired for the field trial is achieved by connecting two of these couplers in series and using filters for different wavelenghts. The most critical component in the coupler is the interference filter. This filter consists of a thin glass sheet onto which several layers of coating have been applied by means of vapour deposition in order to achieve the desired characteristics. The performance of the coupler is highly dependent upon the quality of the filter and its matching the wavelengths of the light sources. The LEDs used in the system have a relatively wide spectrum. The half-value width is 35-50 mm and the distance between the wavelength peaks is only 80 mm. Consideration must also be given to the fact that the wavelength peaks of the LEDs change by 0.3 nm/°C which gives a variation of 13.5 nm over the temperature range of 0-45°C. Because of this, the coupler filters are not sufficient to separate the two signals, and they must therefore be optically bandpass-filtered in order to reduce the spectral width in the channels. The transmission characteristics of the filters used in the couplers are shown in fig. 7. As can be seen in the figure, almost all signals at the higher wavelength are reflected, while at the lower wavelength, only 90% are transmitted. The remaining 10% will be reflected into the wrong channel and cause crosstalk. The reflected signal must therefore be bandpass-filtered at the detector. Fig. 6 also shows that the LEDs with the two highest wavelengths have very wide spectra, so that in fact their spectra overlap each other considerably. It is therefore necessary to bandpass-filter the signals directly after the LEDs, using bandpass filters of the same type as in the detector above, fig. 8. In both cases the bandpass filters are placed in the LED housing between the LED and the fibre connector, fig. 9. When manufacturing the couplers, the only operation requiring high mechanical precision is the attaching of the fibres to the end surfaces of the GRIN lenses. This is done using motor-driven high-precision positioners to optimize the positions of the fibres. The precision of this adjustment and the characteristics of the filter are the major factors determining the performance of the coupler. The remaining assembly operations may therefore be carried out with the aid of simple fixtures. The parts are assembled using carefully chosen glues which are transparent at the wavelengths concerned and the couplers are embedded in polyurethane for protection against mechanical stress. Fig. 10 shows the finished WDM coupler together with a coupler before the polyurethane is filled in. 100 - Fig. 7 Transmission characteristics of the two types of interference filter used in the WDM couples. Filter type 1 transmits only the 730-nm channel and reflects the 820- and 890-nm channels. Type 2 transmits the 730- and 820-nm channels and reflects the 890-nm channel
Fig. 8 Transmission characteristics of the bandpass filters used in the system. The filtering improves the crosstalk attenuation between the channels in the system, but also introduces extra losses which requires increased output power from the LEDs 100- % BP-820 50 BP-890 700 750 800 850 900 \(nm) When measuring the performance of the couplers in the laboratory, it was found that the attenuation values for the individual couplers were 0.5-1,0 dB for the reflected channel and 1.0-1.5 dB for the transmitted channel. The measurements were made under steady-state conditions using monochromatic light. The crosstalk attenuation is better than 25 dB for the transmitted channel, while the reflected channel only gives about 10 dB isolation. This is a consequence of the above-mentioned filter characteristics and is remedied by optical bandpass filtering at the detector. Fig. 9 Bandpass-filtered LED and APD in their housing. In the housing (drilled open) the optical bandpass filter can be seen as a thin disc between the active component and the sleeve of the optical fibre connector Fig. 10 The finished WDM coupler is shown at the top of the picture. The lower part of the picture shows the coupler before the polyurethane is injected. The interference filter in the middle is clearly seen surrounded by the two GRIN lenses. The red tubes protect the fibres, which are glued to the end surfaces of the GRIN lenses During temperature cycling within the temperature range 0-45°C, no noticeable changes in the attenuation values of the couplers have been measured. A complete three-channel WDM system is obtained by connecting the couplers together two by two as shown in fig. 4. In the field trial installation at Skarpnäck it has been demonstrated that WDM systems for subscriber connections over optical fibre is a promising technique from a technical as well as an economical point of view. This technique is also very suitable for mass production. Optical wavelength multiplexing should therefore be one of the major alternatives when designing the fibre-optic subscriber network of the future, and it will compete successfully with other forms of time and space multiplexing for equivalent grades of service. References 1. Bergsten, K. and Gobi, G: Field Trial of Optical Fibre Cable-TV Systern. Ericsson Rev. 62 (1985):4, pp. W 160. . , 2. Gobi, G. and Jacobson, S.: 0P" c JJ Fibre Systems for Digital Cable-1* Transmission, ZAV 280/4. Ericsson Rev. 62(1985):4, pp. 161-169.
Page 1 and 2: ERICSSON REVIEW 4 1985 Field Trial
Page 3 and 4: Trial of Optical Fibre Cable-TV Sys
Page 5 and 6: Head end (HC] 280 Mbit/s Farsta aut
Page 7 and 8: 158 Switching equipment Fig. 7 Futu
Page 9 and 10: 160 Fig. 10 The teleconference room
Page 11 and 12: GERHARD GOBL Public Telecommunicati
Page 13 and 14: Fig. 5, above left Two-channel A/D
Page 15 and 16: Digital input signals Digital outpu
Page 17 and 18: Power ZAV 280/4 has individual powe
Page 19 and 20: Wavelength Division Multiplexing fo
Page 21: 172 Subscriber side BP-890 Controll
Page 25 and 26: Most computer cabinets are 19" wide
Page 27 and 28: 178 Fig. 5 Modems in Series 7 Modem
Page 29 and 30: Computer Aided Production of Plasti
Page 31 and 32: Fig. 4 Product geometry displayed o
Page 33 and 34: Rectifier for Mobile Telephone Syst
Page 35 and 36: I OR | OFU OS I CMUP HlsHi W±±±h
Page 37 and 38: 188 Fig. 6 The rectifier with the o
Page 39 and 40: Fig. 9 Rectifier BMJ A01 001 with t
Page 41 and 42: The Renewal of the London Undergrou
Page 43 and 44: 194 Rickmansworth, and the Loughton
Page 45 and 46: 196 Fig. 5 Optical fibre cable rout
Page 47 and 48: 198 In planning the optical fibre n
Page 49 and 50: Fig. 9 The digital telephone DIAVOX
Page 51 and 52: 202 Fig. 11 Transmission equipment
Page 53 and 54: 204 Fig. 2 Automatic testing of mul
Page 55 and 56: 206 Relative occurrence for F Fig.
Page 57 and 58: Fig. 5 Field testing of a 140 Mbit/
Page 59 and 60: 210 Fig. 7 The system down time as
Page 61 and 62: MTBF (years) Equipment 2-5 Transmul

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