Transmission system for the polarisation-insensitive transmission of signals

Abstract: The invention relates to a transmission system for the polarisation-insensitive transmission of signals over a signal path between a transmitter (Z) and a receiver (O), provided with high-order retarder (R). The retarder is a combination of a polarisation-splitting coupling (1) and a loop-type delaying transmission route (5). The retarder (R) may be sited at the receiver end, with a suitably chosen angle between one of the principle axes of the retarder and the polarisation of a local oscillator (Lo) incorporated in the receiver. Advantages are reduction in size and number of components in the retarder, reduction in the number of retarders in such a transmission system and an increase of the capacity for integration.

Description: The invention relates to a transmission system for the polarisation-insensitive transmission of signals over a signal path between a transmitter and a receiver, provided with a high-order retarder, in which the receiver comprises a local oscillator and a coupling device for coupling it to the signal path. Such a transmission system is disclosed by the paper entitled "Polarization-Switching Techniques for Coherent Optical Communications" which appeared in Journal of Lightwave Technology, Vol.6, No.10, October 1988, pages 1537 to 1548. From this paper it is known to provide a high-order retarder (preferably a birefringent optical fibre) at the transmission end of a coherent communication system in order to render the latter insensitive to polarisation by means of the principle of "data-induced polarisation switching". If a receiver is shared by a number of transmitters, it will be advantageous to use the method mentioned at the receiving end. This method only works if the polarisation of the signal applied to the high-order retarder is at a fixed angle of virtually 45° with respect to the principle axis of the high-order retarder. In practice, however, the signal presented to the receiver has, in particular, a strongly fluctuating polarisation, and this will present problems in using the high-order retarder at the receiving end. Surprisingly, it has been found that the problems are avoided in that the high-order retarder is connected between the signal path and the coupling device and the angle between one of the principle axes of the retarder and the polarisation of the local oscillator is greater than 0° and less than, or equal to, 180° according to Poincaré's representation. Preferably, said angle is 90° according to Poincaré's representation. To summarise, application of the invention offers the advantage of a reduction in the number of such retarders in a transmission system for the polarisation-insensitive transmission of signals. The invention will be explained in greater detail below with reference to the drawings. In the drawings: Figure 1 shows a high-order retarder based on a known combination of a resonant ring and a polarisation splitting device; Figure 2 shows a diagram of the polarisation at the output of a retarder as a function of the frequency of the input signal; and Figure 3 shows a transmission system according to the invention. Polarised light can always be broken down into, or built up from, two orthogonally polarised components, with or without a phase difference. Light linearly polarised at 45° can, for example, be formed from a component having horizontal polarisation and a component having vertical polarisation, which components are precisely in phase. In the case of circularly polarised light, the components having horizontal and vertical polarisation, respectively, of the electric field are 90° out of phase. A high-order retarder may be based on the concept of retardation. This concept implies that one component is delayed with respect to the other. The retardation can be expressed in degrees. A retardation of 90° converts light linearly polarised at 45° into circularly polarised light. Retardation can also be expressed as a fraction of the wavelength. A retardation of 90° is equivalent to a quarter wavelength. A 90° retarder is therefore also referred to as a λ/4 retarder. As a third possibility, the retardation can also be defined as delay time. 1500 nm light has a cycle time of 5 fs (5.10⁻¹⁵ sec). A retardation of 90° is then equivalent to 1/4.5 fs = 1.25 fs. The high-order retarder is to be regarded as a retarder having a very large retardation. For example, a retardation of 15 cm (500 ps, 100,000λ, 36,000,000°). The order of a retarder is the number of times that 360° has to be subtracted from the retardation to arrive at an amount between -180° and +180°. A retarder with a retardation of 15 cm therefore has an order of 100,000. It is known to use a birefringent fibre of great length as a retarder. A typical value of the birefringence power of this fibre is 5.10⁻⁴, i.e. 0.5 mm retardation for 1 m of fibre. A retardation of 15 cm therefore requires a birefringent fibre 300 m long, as a result of which the size of the high-order retarder is large. In addition, birefringent fibres are expensive. Figure 1 shows a high-order retarder based on a combination of an optical fibre resonant ring and a polarisation splitting coupling device. A similar combination of a polarisation-splitting coupling device and a loop of polarisation-maintaining fibre is known from a paper by M.V. Andres et al. entitled "Optical-fiber resonant rings based on polarization-dependant couplers", published in Journal of Lightwave Technology, Vol. 8, No. 8, Aug. 1990, pages 1212-1220. In this high-order retarder, only one polarisation-splitting coupling device 1 having two inputs 6 and 7 and two outputs 8 and 9 is used. The input light signal applied to the input of the high-order retarder is applied to the input 6 of the polarisation-splitting coupling device 1, which splits the input light signal into a first component having a first polarisation and a second component having a second polarisation, which components are presented to the respective outputs 8 and 9 of the polarisation-splitting coupling device 1. For example, the component having horizontal polarisation direction appears at the output 8 of the polarisation-splitting coupling device 1 and therefore also at the output of the high-order retarder. A first transmission route therefore runs from "in", via 6, 1 and 8 to "out". The component having vertical polarisation appears, however, at the output 9 of the polarisation-splitting coupling device 1 and is applied, via a second transmission route, in this case formed by the delaying optical waveguide 5, to the second input 7 of the polarisation-splitting coupling device 1. The recombination of the two transmission routes takes place in the coupling device 1 itself, because the delayed component also appears at the output 8 of the polarisation-splitting coupling device 1 and is therefore combined with the component having horizontal polarisation direction, which combination is presented to the output of the high-order retarder. The component having horizontal polarisation direction therefore travels, as it were, straight through and the component having vertical polarisation direction always crisscrosses in the polarisation-splitting coupling device 1. As a result of using this property of the polarisation-splitting coupling device 1, one coupling device is therefore sufficient. Said high-order retarder has the property that the application thereto of an input signal having a predetermined frequency produces an output signal having a particular polarisation direction. If the frequency of the input signal is, for example, 1 GHz higher, a signal is produced at the output of the retarder with a polarisation whose direction is perpendicular to that of the first-mentioned output signal. Switching between the input frequencies therefore produces a switching between the two polarisation directions mentioned. The retarder then acts as a polarisation switch. The most important application of a retarder having high retardation is the conversion of optical frequency modulation into polarisation modulation, also referred to as "data-induced polarisation switching". For an optical frequency of 200,000 GHz (λ ≈ 1500 nm), the 15 cm retarder according to the invention behaves as a 36,000,000° retarder. Modulo 360°, that is a 0° retarder. For a somewhat higher frequency, the wavelength is somewhat smaller and somewhat more wavelengths therefore fit into the 15 cm, with the result that the retardation is rather greater. For an optical frequency of 200,000.5 GHz, the retardation becomes 36000090° (in this case, then it is a retardation of 90° modulo 360°). For 200,001 GHz, the retardation becomes 36000180°, etc. This behaviour is shown in Figure 2 for a number of frequencies. Because the frequency is swept over the correct interval, in this example over 1 GHz, the polarisation will be switched between two orthogonal positions. This is attractive for communication systems which are sensitive to polarisation fluctuations (coherent systems). If all the "zeros" are lost as a result of an accidental polarisation adjustment, all the "ones" can still be received and vice versa. The signal can never be completely lost. A second application of retarders having high retardation is "polarisation scrambling"/depolarisation of light. For this purpose, a light source is taken which has a coherence length which is much shorter than the retardation. In this example, a light source having a spectral width of many GHz is used. The effect of this is that all the polarisations occur simultaneously at the output of the high-order retarder during 1 bit and that the light is therefore in fact emitted in unpolarised form. According to the abovedescribed principle of "data-induced polarisation switching", the signal from the transmitter is coupled in a high-order retarder in such a way that the polarisation of the coupled-in signal is at 45° to the principle axis of the high-order retarder. The transmitter is FSK-modulated between two optical frequencies. The high-order retarder has the property that the output polarisation is a function of the optical frequency of the signal applied thereto, with the result that the polarisation is modulated with the optical frequencies as a result of using the FSK modulation. For the correct operation of the principle of "data-induced polarisation switching", the output polarisations for a "zero" state should be orthogonal with respect to a "one" state. For this purpose, inter alia, the polarisation of the signal from the transmitter should be polarised at 45° with respect to the principle axis of the high-order retarder. It is only in that case that the output polarisations associated with the "zero" and "one" states can be orthogonal, which is, after all, required. During the transmission of a light signal through an optical fibre, the polarisation changes randomly, but the orthogonality between the "zero" and "one" states is maintained. A coherent receiver is only sensitive to one component of the polarisation, with the result that the signal may be lost as a result of the change in the polarisation in the fibre. Because of the orthogonality mentioned between the polarisations of the "zero" and "one" states, for example, all the "zeros" may be lost, but, on the contrary, all the "ones" will be optimally detected. The converse and all the intermediate forms are also possible. In no case will the entire signal be lost. The principle of "data-induced polarisation switching" at the transmitting end is possible because the transmitter has a known fixed polarisation which can be coupled into the high-order retarder at the correct angle. The costs of the principle mentioned consist, in particular, of the price of the high-order retarder and the installation thereof. In addition, the space said retarder occupies is of great importance. If a receiver has to be shared by many transmitters, for example for the data traffic from the subscribers to a final exchange, it will be advantageous to apply the principle of "data-induced polarisation switching" to the receiver because the cost associated with the high-order retarder have to be incurred only once. The desired application appears, however, impossible. For the principle mentioned of "data-induced polarisation switching" it is, after all, required that the polarisation of the signal applied to the high-order retarder is precisely at 45° to the principle axis of the high-order retarder. It is only then that the output polarisations associated with the "zero" and "one" states can be orthogonal. As has been mentioned above, the polarisation of a light signal varies randomly during its transport through an optical fibre. At the receiving end, it is not possible therefore to satisfy the requirement that the polarisation is always at 45° with respect to the principle axes of the high-order retarder. It may even occur that the polarisation becomes precisely parallel to the principle axis of the retarder, as a result of which the polarisation is simply maintained in the high-order retarder and the "zero" and "one" states emerge from the high-order retarder with the same polarisation. There is then no longer any question of "data-induced polarisation switching". It therefore appears that the application of the "data-induced polarisation switching" principle at the receiver is not possible because after the signal has been transported through the optical fibre, the polarisation has been randomly altered and, in addition, varies with time, as a result of which it is impossible to couple the light into the high-order retarder at the correct angle. In spite of this, the solution according to the invention shown in Figure 3 has been found. The light signal originating from the transmitter Z is applied via a transmission path T and a high-order retarder R to one input of a coupling device K. Coupled to the output of said coupling device K is the receiver O. The signal from the local oscillator LO is applied to the other input of the coupling device K. The arrangement shown in Figure 3 is possible because a particular requirement is met. This requirement is that the polarisation of the light from the local oscillator LO is at 45° with respect to the principle axis of the high-order retarder R. If it is assumed that the polarisation of the received signal is accidentally at 45° with respect to the principle axis of the retarder R, the method of "data-induced polarisation switching" operates in the same way as if the retarder is provided at the transmitting end. The polarisations of the zeros and ones at the output of the retarder are in this case mutually orthogonal and if the "zeros" are lost, the "ones" will always be left over and vice versa, while all the possible intermediate forms do not present problems either. If the other extreme case occurs, in which the received polarisation runs precisely parallel to the principle axis of the high-order retarder, the polarisations of "zeros" and "ones" at the output of the high-order retarder have remained the same and they are still parallel to the principle axis. Because, however, the polarisation of the output signal of the local oscillator forms an angle of 45° with the principle axis of the high-order retarder, the coherent receiver will nevertheless receive all the "zeros" and "ones", albeit at half power. In terms of sensitivity, this is equivalent to losing all the "zeros" and completely receiving all the "ones", with the result that the sensitivity is in this case equal to the case where the high-order retarder is sited at the transmitter and the method of "data-induced polarisation switching" is used. In a corresponding manner, for polarisations at the input of the high-order retarder which are between the abovementioned extremes, that is to say 45° with respect to or parallel to the principle axis, the signal will never be lost. To summarise briefly, the "data-induced polarisation switching" principle is possible at the receiving end, provided the polarisation of the local oscillator is at 45° with respect to the principle axis of the high-order retarder. For the sake of the clarity of the explanation, it has been assumed above that the polarisation of the local oscillator or of the transmitter, respectively, is at an angle of 45° with respect to the principle axis of the high-order retarder. It is clear that the invention can be described much more generally by making use of Poincaré's representation or Poincaré's sphere. According to said representation, the polarisation of the signal from the local oscillator does not have to coincide with the principle axis of the high-order retarder, it being preferable that the principle axis of said retarder forms an angle of 90° with the polarisation of the output signal of the local oscillator according to Poincaré's representation. A special case of this is a local oscillator having a linearly polarised output signal which is at 45° to the principle axis of the high-order retarder. Moreover, mention has always been made of the principle axis of a high-order retarder. In fact, a high-order retarder has, however, two principle axes, namely the fast and the slow principle axis which are mutually perpendicular. It is therefore preferable to speak of one of the principle axes.

Claims: Transmission system for the polarisation-insensitive transmission of signals over a signal path (T) between a transmitter (Z) and a receiver (O), provided with a high-order retarder (R), in which the receiver comprises a local oscillator (LO) and a coupling device (K) for coupling it to the signal path, characterised in that the high-order retarder (R) is connected between the signal path (T) and the coupling device (K), and in that the angle between one of the principle axes of the retarder (R) and the polarisation of the local oscillator is greater than 0° and less than, or equal to, 180° according to Poincaré's representation. Transmission system according to Claim 1, characterised in that the angle between one of the principle axes of the retarder (R) and the polarisation of the local oscillator (LO) is equal to 90° according to Poincaré's representation. Transmission system according to Claim 1 or 2, characterised in that the receiver (O) is shared by at least two transmitters (Z) in a passive optical network. Transmission system according to Claim 1, in which the high-order retarder (R) comprises a polarisation-splitting device having at least one input (6) and two outputs (8, 9) for splitting the transmission between a first transmission route (8, out) and a second delaying transmission route (5) and a recombination device connected to the transmission routes having two inputs (6, 7) and one output (8) for recombining the transmitted signals, the signal path (T) being connected to the input (6) of the polarisation-splitting device and the output (8) of the recombination device being connected to the coupling device, the polarisation-splitting device, the first transmission route and the recombination device being formed by one polarisation-splitting coupling device (1) having a second input (7), of which the first input (6) and first output (8) are the input and the output, respectively, of the high-order retarder, the delaying transmission route (5) being incorporated between the second input (7) and the second output (9) of the polarisation-splitting coupling device (1).

Inventors: VAN DEVENTER MATTIJS OSKAR, VAN DEVENTER, MATTIJS OSKAR
Applicants: NEDERLAND PTT, KONINKLIJKE PTT NEDERLAND N.V.
Application year: 1991
Main classification: H04B10
IPC codes: H04B10, H04J14, G02B6