A frequency modulation coherent optical communications system with continuous polarisation scrambling

Abstract: A coherent optical communications system with continuous polarisation scrambling, wherein such scrambling is obtained by combining orthogonally polarised radiations at different frequencies emitted by two independent sources (1, 2; 201, 202). The two sources may be employed to build the transmitter or the local oscillator of the system.

Description: The present invention relates to optical communications systems, and more particularly it concerns a frequency modulation, optical-fibre coherent communications system with continuous polarisation scrambling. It is known that conventional monomode optical fibres used in optical communications systems exhibit birefringence characteristics varying both in time and along the fibre axis, so that the state of polarisation of the propagating signals varies in continuous and unpredictable manner. In coherent communications systems, and especially systems with heterodyne reception which are the most widely used, it is essential to have a definite and constant state of polarisation at the fibre output. Indeed, the receivers can correctly operate only if the state of polarisation of the received signal coincides with that of the signal generated by the local oscillator, otherwise only part of the electromagnetic field undergoes heterodyning, with consequent signal fading which may be even total. The problem could be solved by using polarisation maintaining fibres, yet that solution is scarcely adopted since such fibres are rather expensive, have a high attenuation and may give rise to a noise increase if the alignment of the fibre axes at the joints, if any, is not correct. Thus, in coherent communications systems it is preferred to use conventional, low-birefringence optical fibres and to take measures for making the receiver polarisation-insensitive. This is generally achieved by scrambling the state of polarisation of either the transmitted signal or the signal generated by the local oscillator in non-adaptive continuous or discrete manner within each bit period. Generally such scrambling is performed by means of modulators external to the source. Transmission rates of the order of some hundred Mbit/s or some Gbit/s are used in the coherent systems and the modulators allowing the state of polarisation of the signals to be scrambled at a rate compatible with such bit rates are integrated-optics guide elements. Yet such elements are generally expensive and moreover give rise to high losses, due to both the attenuation of the guide itself and the coupling between the guide and the possible fibre piece allowing connection to an optical fibre. In accordance with the invention, we provide a coherent optical communications system wherein polarisation scrambling is obtained without using devices external to the source but by exploiting two orthogonally-polarised radiations at different frequencies. The combination of two orthogonally-polarised radiations at different frequencies to obtain insensitivity to polarisation fluctuations in detection systems or in optical fibre communications systems has been disclosed in the literature, yet the proposed systems exploiting such combination cannot be used to solve the problem of obtaining polarisation scrambling without use of external modulators. More particularly, the paper "Depolarised source for fiber-optic applications" presented by W.K. Burns et al. at the OFC'91 Conference (San Diego, California, 18-22 February 1991) and published at page 205 of the conference proceedings, describes a system in which the two radiations, generated by respective lasers, are combined to obtain in line a depolarised radiation. The paper does not disclose how such depolarised radiation can be combined with information transmission in a coherent system, whereas the invention just aims to provide a coherent system which does not require external devices to obtain polarisation scrambling. The two lasers emit always at a constant frequency, and the difference between the two frequencies is large if compared to the frequencies encountered in the application of interest: was such a system employed to obtain polarisation scrambling in a coherent communications system, the risk would exist that the beats with the signal generated by a local oscillator are outside the detector band, so that demodulation of the information would be impossible. Moreover, it is to be taken into account that polarisation scrambling causes a broadening of the information signal band which increases with the polarisation scrambling rate: should the system disclosed in the paper be used, receivers with very wide band should be employed, what causes a great increase in the noise present in the received signal. The article "New polarisation-insensitive detection technique for coherent optical fibre heterodyne communications" by A.D. Kersey et al., Electronics Letters, Vol. 23, No. 18, 27 August 1987, discloses the use of the two orthogonally-polarised radiations at different frequencies to obtain the output signal of the local oscillator in a polarisation diversity heterodyne receiver for a coherent communications system. The two radiations are obtained from a single source, by means of a polarising beam splitter generating the two orthogonal polarisations and of an acousto-optic modulator placed downstream the beam splitter, on the path of only one of the two orthogonally-polarised radiations. Also in this case, the two radiations always have a constant frequency and hence the considerations already made for OFC'91 still hold. Besides, even assuming that both signals outgoing from the acousto-optic modulator are used, and not only the frequency shifted signal, the invention cannot be obtained, since only one of the two radiations would be frequency-modulated. EP-A-0 428 119 discloses a coherent optical-fibre communication system using polarisation scrambling of the transmitted signal, the scrambling being performed by a modulator external to the light source. The features of the invention are disclosed in the appended claims. The invention will now be described in greater details with reference to the annexed drawings, which show two possible applications of the invention. Fig. 1 refers to the case in which the information to be transmitted is frequency modulated and the polarisation scrambling is applied to the transmitted signal. In the drawing, two semiconductor lasers 1, 2, which are as far as possible similar to each other, are connected to a source 3 of the binary data flow, which frequency modulates the two lasers. To this aim, the signals coming from source 3 are fed to control devices 4, 5 of lasers 1,2, where they are superimposed to the d.c. bias voltages VB1, VB2 and cause the lasers to oscillate at a respective frequency f₀ or f'₀ in correspondence with logic value 0 of the binary signal, and at a frequency f₁ or f'₁ in correspondence with logic value 1. The radiation emitted by laser 1, which is a linearly polarised radiation, is sent to a polarising beam splitter 7 and is received e. g. on the transmitting face thereof; splitter 7 receives also, on the reflecting face, the radiation emitted by laser 2, which has passed through a half-wave plate 8 and hence is polarised in an orthogonal plane to that of the radiation emitted by laser 1. Isolators 9, 10 prevent the rays reflected by the mirror or the plate from re-entering lasers 1, 2. It is worth noting that two individual sources, even though nominally equal, always present a certain frequency difference Δf due to the random fluctuations of the respective oscillation frequency, and that difference is practically impossible to be compensated. The invention just exploits that difference which, being the radiation linearly polarised in orthogonal planes, produces a resulting linear polarisation the plane of which rotates at an angular velocity Δω = 2πΔf . More particularly, voltages VB1, VB2 will be chosen so that, neglecting the fluctuations, difference Δf between frequencies f₀, f'₀ and f₁, f'₁, respectively, is at least equal to the inverse of the bit period of the information signal. Preferably such difference is exactly equal to the inverse of the bit period, for the reasons which will be explained later on. Splitter 7 combines the two beams and sends the resulting radiation, through a suitable optical system schematised by lens 11, into a low-birefringence monomode fibre 12, forming the system transmission line. Two orthogonally polarised components, with slightly different frequencies, will then be present in line for each bit of the information signal. At the fibre output, the transmitted signal is combined in a coupler 15 with the signal emitted by a local oscillator, namely a semiconductor laser 16 having a line width as far as possible equal to that of lasers 1, 2 and operating at a frequency fL different from f₀, f₁ and f'₀, f'₁. Isolator 18, having the same functions as isolators 9, 10, is placed between oscillator 16 and coupler 15 . The beam resulting from the combination is sent to a photodetector 20 the output signal of which, through amplifier 21, is fed to two passband filters 24, 25 having such a passband as to comprise F0, F'0 and F1, F'1, respectively (where Fi, F'i indicate frequencies f L - f i and f L - f' i , respectively, with i = 0, 1). The filtered signals are then demodulated in demodulators 26, 27, e.g. of the square law type. The signal outgoing from demodulator 27 is subtracted in a subtractor 28 from the signal outgoing from demodulator 26, so as to double the dynamics, and the signal resulting from the subtraction is supplied to a low pass filter 29 delimiting the signal base band. A threshold circuit 30, e. g. a zero crossing detector, supplies the recovered data signal. The operation of the described embodiment is as follows. Whatever the logic value of the bit to be transmitted, two orthogonally-polarised radiations at slightly different frequencies f₀, f'₀ or respectively f₁, f'₁ are transmitted over the line. Combination of such radiations results, as said, in a radiation the state of polarisation of which rotates at a frequency equal to the frequency difference between the two component radiations. Since such a difference is at least equal to the bit rate, during a bit period the polarisation of the signal sent over the line by the transmitter will pass through all possible states where the relative phase of the resulting field (leaving the amplitudes unchanged) continuously varies. Thus, the continuous polarisation scrambling necessary to eliminate the effects of random polarisation variations caused by the fibre is achieved. Indeed, during the bit period, the line signal will have at least a component in the same polarisation plane as the signal generated by the local oscillator, and the heterodyne beat necessary for the coherent demodulation can be obtained. As said, the frequency difference between lasers 1, 2 in both emission conditions must be at least equal to the inverse of the bit period, in order that the state of polarisation effects at least one complete rotation in the bit period. Such a difference could also be greater, yet the polarisation scrambling causes a widening of the received signal band which increases with the scrambling rate: the drawbacks inherent in a band widening are well known to the skilled in the art, and therefore preferably the state of polarisation performs a single rotation in the bit period. It is to be appreciated that the receiver scheme is identical to that of a conventional FSK receiver; only, the band requirements of filters 24, 25 must be compatible with the presence of two components at different frequencies, i.e. the filters must be transparent to the polarisation scrambling frequency. In practice, this entails the use of filters whose passband is almost twice that necessary for a conventional receiver without polarisation scrambling. The principle adopted can also be used to build a polarisation diversity receiver, as shown in Fig. 2. Here, the transmitter comprises a single laser, for instance laser 1, frequency modulated by the data signal. The signal emitted by said laser is the line signal. At the receiving side, the local oscillator comprises two lasers 201, 202 emitting radiations at frequencies f L , f' L whose difference is at least equal to the inverse of the bit period of the information signal, polarising beam splitter 207, half-wave plate 208 and isolators 209, 210. Devices 207 - 210 have exactly the same tasks as devices 7 - 10 in Fig. 1. In the electrical part of the receiver, demodulation is carried out by using a single channel: amplifier 21 is followed by a passband filter 31 with a passband tuned on frequency F0 = f 0 - f L , and by a demodulator 32, directly connected to low-pass filter 29. This solution (single-channel FSK), which may be employed also in the diagram of Fig. 1, is constructionally simpler than the one employing two-channel demodulation, yet the greater constructional simplicity entails the loss of the power associated with the second frequency present in the beam sent to detector 20. The operation principle is similar to that of Fig. 1, the only difference being that the state of polarisation continuously varying in the bit period is that of the signal generated by the local oscillator instead of that of the line signal. In the drawing, thin lines represent the optical signal paths and thick lines the electrical signal paths.

Claims: A method of polarisation scrambling in an optical signal transmission system, wherein optical signals are modulated by a binary digital information signal and undergo heterodyne coherent demodulation, characterised in that: two sources (1, 2) of linearly polarised radiation are frequency modulated by the information signal, so that the first source (1) emits radiations at a first (f₀) or a second (f₁) frequency ant the second source (2) emits radiations at a third (f₀') or a fourth (f₁') frequency, in correspondence with the first and the second logic level of the information signal, the third and fourth frequencies differing respectively from the first and the second frequencies by an amount at least equal to the inverse of the bit period in the binary signal; the states of polarisation of the radiations emitted by the two sources (1, 2) are rendered orthogonal in correspondence with each bit of the information signal; the two orthogonally polarised radiations are combined in order to send on a transmission line (12), for each bit, a signal comprising two orthogonally-polarised components at the first and the third frequency or at the second and the fourth frequency, whereby a linear polarisation is obtained the plane of which rotates by at least 360° in the bit period. A method according to claim 1, characterised in that the frequency difference is equal to the inverse of the bit period. A coherent optical fibre transmission system using polarisation scrambling, comprising a transmitter with means for modulating an optical signal with a binary information signal and means for continuously scrambling, in each bit period, the state of polarisation of the modulated signals, and a receiver comprising a local oscillator generating a signal which is combined with the line signal, characterised in that the means for sending the modulated signals on the line and for scrambling the state of polarisation of such signals are made by a same group of devices and comprise: a first source (1) of light radiations, which emits radiations linearly polarised in a first plane and is biased by the information signal so as to emit radiations at a first or a second frequency according to whether said information signal presents the first or the second of the two logic levels; a second source (2, 8) of light radiations, which emits radiations linearly polarised in a second plane orthogonal to the first plane and is biased by the information signal so as to emit radiations at a third or a fourth frequency, according to whether said information signal presents the first or the second of the two logic levels, the third and fourth frequencies being very close to the first and second frequencies respectively but differing therefrom by an amount at least equal to the inverse of the bit period; means (7) for combining the radiations emitted by the two sources into a single radiation which is sent onto an optical fibre transmission line and comprises, for each of the two logic levels of the information signal, two orthogonally-polarised components having the first and third frequencies or the second and fourth frequencies, respectively, so that, for both logic levels, a resulting linear polarisation is obtained the plane of which rotates by at least 360° in the bit period.

Inventors: CALVANI RICCARDO, CAPONI RENATO, MARONE GIUSEPPE, CALVANI, RICCARDO, CAPONI, RENATO, MARONE, GIUSEPPE
Applicants: CSELT CENTRO STUDI LAB TELECOM, CSELT CENTRO STUDI E LABORATORI TELECOMUNICAZIONI S.P.A.
Application year: 1992
Main classification: H04B10
IPC codes: H04B10, G02F2