AU726368B2

AU726368B2 - Optical transmission system

Info

Publication number: AU726368B2
Application number: AU12533/97A
Authority: AU
Inventors: Rofl Heidemann; Rolf Hofstetter; Berhard Junginger; Harald Schmuck
Original assignee: Alcatel NV
Current assignee: Alcatel Lucent NV
Priority date: 1997-02-07
Filing date: 1997-02-07
Publication date: 2000-11-02
Anticipated expiration: 2017-02-07
Also published as: AU1253397A

Description

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ORIGINAL

COMPLETE SPECIFICATION STANDARD PATENT Invention Title: "OPTICAL TRANSMISSION SYSTEM" The following statement is a full description of this ivention, including the best methiod of perfon-nig it known to us:- This invention relates to an optical transmission system wherein a centre is connected to at least one terminal through a fibre-optic network, wherein the centre comprises a light source, an optical-to-electrical transducer, and coupling means for coupling light emitted by the light source into the fibre-optic network and for coupling light to be received out of the fibre-optic network and feeding it to the optical-toelectrical transducer, and wherein the at least one terminal comprises a facility for deriving from light received by it an upstream signal destined for the optical-toelectrical transducer at the centre.

Such an optical transmission system is known, e.g. from the American patent 10 US-A-5,361,157. The optical transmission system known therefrom comprises a transmitter and a receiver which are connected by means of an optical waveguide.

This optical transmission system enables a bidirectional point-to-point signal transmission between the transmitter and the receiver. The transmitter comprises a light source, a coupler and a light receiving portion. The coupler feeds the light 15 emitted by the light source into the optical waveguide; furthermore, the coupler extracts light from the optical waveguide, which is then fed into the light receiving portion contained in the transmitter. The receiver comprises an optical phase modulator, a light reflector, a coupler and a light receiving portion. The light emitted by the transmitter is reflected by the light reflector and fed to the optical phase modulator, which modulates the intensity of the reflected light and returns it to the transmitter. Several practical examples are specified for the optical phase modulator designed on LiNbO 3 substrate.

Besides the stated optical transmission system for a bidirectional point-to-point signal transmission, another optical transmission system is known, e.g. from W.

Schmid et al, "FITL CATV: ein Ubertragungssystem mit optischen Verstarkern foir analoge TV-Signale (transmission system with optical amplifiers for analog TV signals), Elektrisches Nachrichtenwesen (Alcatel), 3rd quarter 1993, pages 248 to 259, said system enabling a point-to-multipoint signal transmission via a fibre-optic path. In this optical transmission system, TV and audio signals are distributed from a head-end station to multiple subscribers by means of two network hierarchies the transport network between the head-end station and a distribution centre and the connection network between the distribution centre and the optical broadband- network termination. Network topologies for such an optical transmission system are shown in the corresponding Figure 2.

To be able to supply interactive services to subscribers, e.g. video-on-demand, the subscribers must have the facility of sending upstream signals to the head-end station. This is made possible by, e.g. having a laser, which emits the upstream signal, available at each optical termination. The lasers must be selected according to emitted wave length and beam width, which requires a great deal of effort.

Noise interference of great bandwidth occurs in an optical transmission system where the centre comprises, a transmitter and a receiver and where each terminal 10 comprises a reflective optical facility.

This occurs through scattering (Rayleigh-scattering) of the light emitted by the transmitter to different places of the fibre-optic path. Due to the resultant reflections, an "interference signal" occurs in the electrical signal generated by the receiver present in the centre. The generation of this "interference signal" can be regarded 15 as homodyne reception of the optical signal reflected by scattering; a "close" (or "remote") optical signal reflected by scattering acts as a local oscillator. The Homodyne -reception method is known from the overview article "Optischer O berlagerungsempfang eine Obersicht "(optical overlay reception an overview) by O. Strobel et al, Frequenz, 41 (1987) 8, pages 210 to 208.

S 20 It is an object of the present invention to provide an optical transmission system suitable for interactive services, where a high signal-noise power ratio is achieved in the centre.

According to the invention there is provided an optical transmission system wherein a centre is connected to at least one terminal through a fibre-optic network, wherein the centre comprises a light source, an optical-to-electrical transducer, and coupling means for coupling light emitted by the light source into the fibre-optic network and for coupling light to be received out of the fibre-optic network and feeding it to the optical-to-electrical transducer, and wherein the at least one terminal comprises a facility for deriving from light received by it an upstream signal destined for the optical-to-electrical transducer at the centre, wherein the centre further comprises an optical filter which is connected between the light source and the coupling means and to which the light emitted by the light source is feedable, the optical filter having the property of producing minima and maxima in the spectrum of the emitted light, and the upstream signal derived in the facility of the at least one terminal has a selected carrier frequency.

An advantage of the invention is that the signal-noise power ratio can be improved significantly, e.g. by 50 dB, in the optical transmission system. A further advantage of the invention is that lasers are not required in the optical network terminations (terminals), making costly selections redundant; a single, high-quality laser suffices for the invention.

In order that the invention may be readily carried into effect, embodiments 10 thereof will now be described with reference to the accompanying drawings, in which: Figure 1 shows schematic optical transmission system with a centre and :two terminals, and Figure 2 shows schematic display of three signal spectra occurring in the centre, light source being a multimode laser, 15 Figure 3 shows further schematic display of three signal spectra occurring in the centre, light source being a monomode laser.

Figure 1 schematically shows an optical transmission system comprising a centre 1, two terminals 9, 13 and a fibre-optic network 8. In the simplest case, the fibre-optic network 8 is an optical waveguide which connects centre 1 with a terminal. Usually however, the fibre-optic network 8 consists of a multiple of individual optical waveguide paths which are connected by means of optical distributors and sometimes by optical amplifiers to distribute the light transmitted from centre 1 to multiple terminals. The transmitted light can be modulated by intelligence signals, which generates an optical signal to be distributed. In the optical transmission system shown in Figure 1, a connection 15 of centre 1 is connected with the fibre-optic network 8. Terminal 9 is connected to the fibre-optic network 8 by means of an optical waveguide 14 and terminal 13 by means of optical waveguide 17. Optical waveguides 14, 17 are of course part of the fibre-optic network 8 and only highlighted here for a better understanding.

Centre 1 comprises a light source 2, an optical filter 3, a coupler 7 and an optical-to-electrical transducer 4, in the following designated as a photodiode. Light source 2 is arranged so that light emitted by it can be directed to optical filter 3; this can be done by means ofian optical waveguide 6 which connects light source 2 with the optical filter 3. Coupler 7 is connected with the optical filter 3 and the photodiode 4 and couples light coming from the optical filter 3 into the fibre-optic network 8 via connection 15. The coupler 7 furthermore decouples light arriving at connection and directs it to photodiode 4 via an optical waveguide Light source 2 is usually a junction laser which emits light with a spectrum characteristic for the junction laser. The junction laser can be, e.g. a DFB laser (monomode laser) or a Fabry-Perot MQW laser (multimode laser). When a multimode laser is used, attention must be paid, that its spectrum only changes very 10 slowly over time. For the further description, light source 2 is designated as a laser.

The optical filter 3 has the property of producing minima (zero places) and i°:'°maxima in the laser light spectrum, so that the minima and maxima occur strictly periodically over a wide wave length range. An optical filter is characterised, amongst other things, by its transmission function HF Preferentially, the optical 15 filter 3 is a Mach-Zehnder filter designed with the help of optical waveguides. Mach- Zehnder structures based on optical interference are known, e.g. from Robert G.

Walker, "High-Speed Ill-V Semiconductor Intensity Modulators", IEEE Journal of Quantum Electronics, Vol 27, No. 3, March 1991, pages 654 to 667. However, any other optical filter with the stated property of producing periodical minima and 20 maxima in the spectrum is suitable.

Further to the terminals 9, 13 shown in Fig 1, which transmit upstream signals to the centre 1 and are thereby able to utilise interactive services, terminals which do not want to utilise interactive services can be connected to the fibre-optic network 8 in an optical transmission system. In the following, we are only looking at terminals 9, 13 which want to utilise interactive services and which therefore comprise a modulation facility 10, which is shown schematically in Figure 1 in terminal 9. This modulation facility 10 comprises a reflective optical modulator 11, a modulator 12 and a carrier-frequency generator 16. The carrier-frequency generator 16 generates a sine-wave signal with a selected carrier frequency f g characteristic for terminal 9.

To be able to transmit the upstream signals by frequency -division multiplexing, terminal 13 is allocated its own characteristically selected carrier frequency f 13. The significance for the totaloptical transmission system is that each terminal desiring to utilise interactive services is allocated a selected, carrier frequency individual to the terminal.

As an alternative to frequency-division multiplexing, time-division multiplexing can be used in the optical transmission system. The significance for the total optical transmission system is that each terminal desiring to utilise interactive services is allocated the same selected carrier frequency.

It is however also possible to use a combination of frequency-division and time-division multiplexing in the optical transmission system.

Modulator 12 connected to the carrier frequency generator 16 modulates the 10 sine-wave signal with an intelligence signal N directed to the modulator 12, thus "'generating a control signal C. This control signal C controls the reflective optical modulator 11. The signal light reflected by reflective optical modulator 11 is thereby g* modulated according to the control signal. The modulation facility 10 therefore has the task of generating an upstream signal for photo diode 4 contained in centre 1, 15 from the received signal light.

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There are three arrows drawn in Figure 1 which point to various positions within centre 1; this is to indicate that one of three power density spectra,

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2 in the following designated simply as spectra, occur there. Reference is made to Figure 2 and Figure 3 by means of an index x (x 2, The arrow for spectrum points to optical waveguide 6, which connects laser 2 with the optical filter 3. The arrow for spectrum S2 points to an output of optical filter 3, which is connected to coupler 7. The arrow for spectrum Sx3 points to an output of the photodiode, where an electrical signal exits.

10 Figure 2 shows the already mentioned spectra, S 21

S

22

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23 (x 2) in logarithmical display and the transmission function HF of optical filter 3 in linear display; for spectra S21 S 22 each optical power density is shown as a function of the optical frequency v; for spectrum S 23 the electrical power density is shown as a function of the electric frequency f. The multiplexing shown in Figure 2 and Figure 15 3 is the frequency-division multiplexing.

The transmission function HF of optical filter 3 has a sinusoidal progression with regular zero places in a range of, e.g. 50 MHz For example, a multimode laser emits light which consists of several single modes. The spectrum S 21 which consists of two single modes as shown in Figure 2, 20 is only a part of the whole spectrum which the light emitted by a multimode laser has.

Each single mode has, e.g. a half-width value of 10 GHz. The optical filter 3 with transmission function HF(v) changes the optical signal with the spectrum S 21 fed to it, so that the optical signal exiting optical filter 3 has the spectrum S 22 It is characteristic for this spectrum S 22 that minima (zero places) and maxima occur within each individual mode, which have a regular interval of e.g. 50 MHz, as given by optical filter 3.

In the following it is explained how an electrical signal is generated in the centre 1 shown in Fig 1, which has the spectrum S23 shown in Figure 2. Within the fibre-optic network 8, the optical signal transmitted from centre 1 is subject to different scattering mechanisms, particularly the Rayleigh-scattering occurring in optical systems, which are explained e.g. in the book "Systemgrundlagen und Messtechnik in der optischen Obertragungstechnik" (System principals and measuring techniques in optical transmission technology) by W. Bludau et al, Teubner Studienskripten, 105: Angewandte Physik (Applied Physics) Elektrotechnik, Stuttgart 1985, chapter 2.2.3, pages 82 to 89. This Rayleigh-scattering causes scattered light in the fibre-optic network 8, which diffuses against the actual direction of diffusion of the optical signal. This scattered light is decoupled by coupler 7 and directed to photodiode 4. Therefore, the scattered light from the optical signal, which is generated at close and remote locations of the fibre-optic network and sent to terminals 9, 13, overlays the upstream signals coming from terminals 9, 13 on the photodiode 4. The scattered light from the optical signal and the upstream signals 10 have the same wavelength, as the optical transmission system only comprises laser 2.

Three sub-spectra K21 K2 and K 3 are drawn in spectrum S 23 Subspectrum K 21 is the electrical spectrum of the "interference signal", which results from the folding of the spectra of the "near" and "remote" optical signal reflected by scattering. If the spectrum S 22 has a regular peak structure as in the here 15 described case, the sub-spectrum K 2 1 resulting from it also has a comparable, regular peak structure. This means, that "gaps", i.e. frequency ranges with very low noise power density occur in the electrical signal. In Figure 2 these gaps occur at intervals of 25 MHz, 75 MHz and 125 MHz. When the carrier frequency f 9 allocated to terminal 9 equals 25 MHz, the spectrum of the upstream signal of terminal 9 S 20 occurs in the frequency range of around 25 Mhz in spectrum S 3 this sub-spectrum K, is indicated by a hatched triangle. If the carrier frequency f, 3 allocated to terminal 13 is 75 Mhz, the spectrum of the upstream signal of terminal 13 occurs in the frequency range of around 75 Mhz, in spectrum S33 and this sub-spectrum K,(f) is also shown by a hatched triangle.

As the optical properties of the optical filter 3 are known, the carrier frequencies fg, f, 3 can be purposely selected ahead so that the upstream signals occupy a frequency range in which the noise-power density is very low.

In Figure 3 the spectra S31 S 32

S

33 (x 3) are shown in logarithmic display and the transmission function HF of the optical filter 3 in linear display: the optical power density is shown as function of the optical frequency v for spectra S31

S

32 for spectrum S 33 the electrical power density is shown as a function of electrical frequency f.

Transmission function HF has, as is also shown in Figure 2, a sinusoidal progression with regular zero places at intervals of, e.g. 50 Mhz.

In variation from Figure 2, a monomode laser spectrum is shown. Spectrum S3, consists of only one single mode, which has a half-width value of 2 MHz, which, for example, a DFB laser can have. The optical filter 3 with the transmission function HF changes the optical signal with the spectrum S31 fed to it, so that the optical signal exiting the optical filter 3 has the spectrum S32 Minima (zero places) and maxima occur within spectrum S,3 which have a regular interval of e.g. 50 MHz, as given by the optical filter 3. Spectrum S 33 also consists of three sub-spectra K 3 1 10 K 2 and K 3 These sub-spectra K 3 1 K, and K 3 are generated in the same way as the sub-spectra K 2 1

K

2 and K 3 described in connection with Figure 2.

With the help of Figure 2 and Figure 3 it becomes clear, that the signal-noise Spower ratio is definitely improved by, e.g. 50 dB in the so called "gaps". This is achieved by impressing minima and maxima on the complete emission spectrum of 15 laser 2 by the optical filter 3.

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Claims

1. An optical transmission system wherein a centre is connected to at least one terminal through a fibre-optic network, wherein the centre comprises a light source, an optical-to-electrical transducer, and coupling means for coupling light emitted by the light source into the fibre-optic network and for coupling light to be received out of the fibre-optic network and feeding it to the optical-to-electrical transducer, and wherein the at least one terminal comprises a facility for deriving from light received by it an upstream signal destined for the optical-to-electrical transducer at the centre, wherein the centre further comprises an optical filter which is connected between the 10 light source and the coupling means and to which the light emitted by the light source is feedable, the optical filter having the property of producing minima and maxima in the spectrum of the emitted light, and the upstream signal derived in the facility of the at least one terminal has a selected carrier frequency.

2. An optical transmission system as claimed in claim 1, wherein the optical filter is an interference filter.

3. An optical transmission system as claimed in claim 2, wherein the optical filter is a Mach-Zehnder filter.

4. An optical transmission system as claimed in claim 1 or 3, wherein the facility in the at least one terminal comprises a reflective optical modulator, a carrier-frequency S 20 generator, and a modulator, wherein the modulator is feedable with a sine-wave signal generated by the carrier frequency generator and with an intelligence signal, wherein the modulator derives a control signal for the reflective optical modulator from the sine-wave signal and the intelligence signal, and wherein the reflective optical modulator modulates the light emitted by the centre with the control signal to produce the upstream signal.

An optical transmission system as claimed in claim 4, comprising two or more terminals, wherein each of the facilities in the terminals transmits the upstream signal at a different selected carrier frequency, so that the upstream signals are transmitted to the centre using frequency-division multiplexing.

6. An optical transmission system as claimed in claim 4, comprising two or more terminals, wherein eachof the facilities in the terminals transmits the upstream signal at the same selected carrier frequency, and wherein the upstream signals are 11 transmitted to the centre using time-division multiplexing.

7. An optical transmission system as claimed in claim 4, comprising two or more terminals, wherein each of the facilities in the terminals transmits the upstream signal at a selected carrier frequency, and wherein the upstream signals are transmitted to the centre using a combination of time-division and frequency-division multiplexing.

8. An optical transmission system substantially as herein described with reference to Figures 1 3 of the accompanying drawings. DATED THIS FOURTH DAY OF FEBRUARY 1997 10 ALCATEL ALSTI r~M MPA& r ONlrALE d'ElE TDIIT r o e• 4 o 4e f