Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
1998-08-14
2001-01-23
Pascal, Leslie (Department: 2733)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C359S199200, C359S199200, C359S199200
Reexamination Certificate
active
06178023
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to an optical telecommunication system. More particularly it refers to a telecommuncation system having an optical-fibre transmitting line in which an independent channel for service communications comprising a data-transmitting unit and a data-receiving unit for transmission/reception over said service channel is provided.
Optical-fibre telecommunication systems adapted to enable signal transmission for communication over long distances usually provide, in addition to channels intended for communication signals and put at the users' disposal, also an independent channel adapted to enable transmission of service signals.
BACKGROUND OF THE INVENTION
Such service signals can be of various types, for example command or control signals for apparatus disposed along the line, such as repeaters or amplifiers, or communication signals between the maintenance staff acting at a line point and an intermediate station or terminal of the line itself.
For service signals a restricted transmission band as compared with the band of the communication channels is usually sufficient. An overall transmission speed of 300 Kbit/s, in the case in which service signals are coded on a single digital channel is deemed sufficient, for example.
In an optical-fibre telecommunication system a remedy for signal attenuation along the fibres is necessarily provided by periodically amplifying the signals. The use of optical amplifiers disposed at regular intervals along the transmission line has proved to be convenient.
Such amplifiers, that can be made of optical fibres doped with a fluorescent substance and submitted to optical pumping, are capable of amplifying the signals without converting them to an electric form.
In these lines it is impossible to insert and extract signals into and from the fibre along which they are transmitted by means of known electronic apparatus, because signals are available in the optical form even close to the amplifiers.
U.S. Pat. No. 5,113,459 in the name of the assingee of this application describes an optical telecommunication system optionally provided with optical amplifiers along the line, in which insertion and extraction of the service channel takes place by dichroic couplers.
Also provided in this system are receiving and emitting units connected to the dichroic couplers, adapted to receive optical service signals from the line, convert them to electric signals and electronically amplify them, and to receive the amplified electric signals, convert them to optical signals at the wavelength of the service channel and send them to the line, respectively.
In order to carry out separation between the signals by means of dichroic couplers, the wavelength of the service channel has been selected considerably different from that of the communication channels. In addition, for minimizing attenuation for the service channel, this wavelength has been selected substantially coincident with or to a small distance from a minimum of the spectral attenuation curve of light in the optical fibre.
In the case in which the telecommunication signal wavelength is substantially included between 1500 and 1600 nm (the so-called “third window” for silica-based optical fibres) and the service channel wavelength is included in the so-called “second window”, for silica-based optical fibres located in the vicinity of the relative attenuation minimum at about 1300 nm, the attenuation to which the service signals are submitted is much greater than that relating to the communication channels.
In fact, at the second window wavelengths, the attenuation coefficient for silica-glass optical fibres usually in use has a value typically included between 0.37 and 0.41 dB/km, against a typical value of about 0.2 dB/km for wavelengths within the third window.
The length of the line portion included between two amplifiers or between one of the end stations and one of the amplifiers is given by the maximum acceptable attenuation at the wavelengths of the communication channels, in turn linked to the maximum available gain at those wavelengths.
With the optical amplifiers presently in use this maximum gain is about 25-30 dB.
The overall attenuation value at the wavelength of the service channel along the portion between two amplifiers may therefore reach values higher than 50 dB, for example.
In order to generate the radiation to be used for transmission of the service channel, the use of semiconductor lasers is provided. Semiconductor lasers with emission at the wavelength of the second window have a typical output power of about 1 mW (0 dBm). Lasers having a greater output power are undesirable due to their high costs.
Taking into account the reduction of the laser output power in time and aging of the passive optical components along the transmission line, a further power reduction at the receiver of about 8 dB can be expected.
The problem exists therefore of transmitting digital signals, in particular service signals, along an optical communication line and receiving them with a sufficiently low error rate, in the presence of a limited power at the receiver.
SUMMARY OF THE INVENTION
The present invention in one aspect consists of a digital optical telecommunication method comprising the steps of:
receiving a first electric signal carrying a piece of information at an optical transmission station and generating a digital modulated optical signal at a predetermined wavelength, corresponding to said electric signal;
feeding said modulated optical signal to an optical-fibre line having a predetermined unitary-attenuation value at said wavelength;
receiving said modulated optical signal transmitted from said optical-fibre line to a given optical-power level, at an optical receiving station, converting it to an electric form and thereat generating a second digital electric signal;
characterized in that:
said step of generating a modulated optical signal comprises coding said piece of information of said first electric signal in a sequence of elementary information units univocally associated with said piece of information, said units following one after the other according to a first predetermined cyclic time rate, and
said step of generating a second digital electric signal comprising detecting in said converted signal, an electric signal having a second cyclic time rate higher than said first cyclic time rate and recognizing in the detected signal, the phase of an electric signal at said first time rate by comparing a received sequence of elementary information units with at least one reference sequence and verifying the correspondence of result in said comparison at a given condition.
Preferably said second time rate is a multiple of said first time rate.
In a preferential version, said step of generating a modulated optical signal comprises generating a third digital electric signal having said second time rate, starting from said sequence of elementary information units following one after the other according to a predetermined first time rate. Said modulated optical signal can be generated to advantage by modulating the emission of a coherent-radiation source, by means of said third digital electric signal. Advantageously, said third digital electric signal can be generated by phase-modulating a carrier having said second time rate.
Preferably said step of converting said received optical signal to an electric form comprises detecting said received optical signal, converting it to an electric signal, filtering said electric signal and amplifying said filtered signal.
Preferably said reference sequence has said first time rate.
In a particular embodiment, said step of recognizing in the detected signal, the phase of an electric signal at said first time rate comprises:
generating a timing signal at said first time rate and with a random phase;
determining, within each period of said timing signal at said first time rate, the leading edges of said electric signal with said second time rate that are not simultaneous with the leading edge of the timing signal;
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Nava Adriano
Tamburello Mario
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Pascal Leslie
Phan Hanh
Pirelli Cavi S.p.A.
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