Optically amplified transmission systems

Abstract

In an optically amplified transmission system including a concatenation of optical amplifiers in the transmission path between an optical transmitter and a non-coherent receiver, the transmitter includes, in addition to a modulator for impressing data modulation, a polarisation state modulator for improving the signal-to-noise ratio at the detector.

I claim: 
     
       1. An optical data transmission system employing non-coherent detection, the system including an optical transmitter optically coupled with a non-coherent optical receiver by an optical transmission path that includes a concatenation of optical amplifiers, wherein the transmitter includes a data modulator adapted to impress data modulation upon an optical signal transmitted by the transmitter to the receiver, and wherein the transmitter additionally includes a second modulator, which second modulator is optically in series with the first modulator and is adapted to modulate between a pair of substantially orthogonally polarised states the state of polarisation of the optical signal transmitted by the transmitter. 
     
     
       2. A data transmission system as claimed in claim 1, wherein the data modulator is adapted to impress data modulation in the form of intensity modulation. 
     
     
       3. A data transmission system as claimed in claim 1, wherein the second modulator is adapted to modulate at a frequency greater than that of said data modulation. 
     
     
       4. A data transmission system as claimed in claim 3, wherein the data modulator is adapted to impress data modulation in the form of intensity modulation. 
     
     
       5. A data transmission system as claimed in claim 3, wherein the second modulator is adapted to modulate at a frequency which is twice, or greater integral multiple of, the frequency of said data modulation. 
     
     
       6. A data transmission system as claimed in claim 5, wherein the data modulator is adapted to impress data modulation in the form of intensity modulation. 
     
     
       7. A data transmission system as claimed in claim 1, wherein the system includes a feedback control loop adapted to regulate the operation of a polarisation state adjuster located physically in series with the second modulator so as to minimise the component of the signal detected by the receiver that is at the same frequency as that of the modulation that is provided by the second modulator. 
     
     
       8. A data transmission system as claimed in claim 7, wherein the data modulator is adapted to impress data modulation in the form of intensity modulation. .Iadd. 
     
     
       9.  An optical data transmission system employing non-coherent detection, the system including an optical transmitter having a first modulator adapted to impress data modulation upon an optical signal transmitted by the transmitter wherein the transmitter additionally includes a second modulator optically in series with the first modulator, the second modulator adapted to modulate the state of polarization of the optical signal transmitted by the transmitter between a pair of substantially orthogonally polarized states. .Iaddend..Iadd.10. The system of claim 9 wherein the first modulator is adapted to impress data modulation in the form of intensity modulation. .Iaddend..Iadd.11. The system of claim 9 wherein the second modulator is adapted to modulate at a frequency greater than that of said data modulation. .Iaddend..Iadd.12. The system of claim 9 wherein the first modulator is adapted to impress data modulation in the form of intensity modulation. .Iaddend..Iadd.13. The system of claim 11 wherein the second modulator is adapted to modulate at a frequency which is an integral multiple of the frequency impressed by the first modulator. .Iaddend..Iadd.14. The system of claim 13 wherein the first modulator is adapted to impress data modulation in the form of intensity modulation. 
     
     
        .Iaddend..Iadd.15.  The system of claim 9 wherein the optical signal is amplified by at least one optical amplifier and wherein the system includes feedback circuitry to regulate the operation of the transmitter so as to minimize a component of the amplified signal that is at the same frequency as the modulation provided by the second modulator. .Iaddend..Iadd.16. The system of claim 7 wherein the first modulator is adapted to impress data modulation in the form of intensity modulation. .Iaddend..Iadd.17. An optical transmitter in an optical data transmission system employing non-coherent detection comprising means for transmitting an optical signal, a first modulator adapted to impress data modulation upon the optical signal, a second modulator optically in series with the first modulator and adapted to sweep the state of polarization of the optical signal between substantially orthogonally polarized states, and at least one optical amplifier for amplifying the transmitted optical signal. .Iaddend..Iadd.18. The system of claim 17 wherein the first modulator is adapted to impress data modulation in the form of intensity modulation. .Iaddend..Iadd.19. The system of claim 17 wherein the second modulator is adapted to modulate at a frequency greater than that of said data 
     
     
        modulation. .Iaddend..Iadd.20.  The system of claim 17 wherein the first modulator is adapted to impress data modulation in the form of intensity modulation. .Iaddend..Iadd.21. The system of claim 19 wherein the second modulator is adapted to modulate at a frequency which is an integral multiple of the frequency impressed by the first modulator. .Iaddend..Iadd.22. The system of claim 21 wherein the first modulator is adapted to impress data modulation in the form of intensity modulation. .Iaddend..Iadd.23. The data transmission system of claim 17 wherein the system includes feedback circuitry to regulate the operation of the transmitter to minimize a component of the amplified signal that is at the same frequency as that of the modulation provided by the second modulator. .Iaddend..Iadd.24. The system of claim 23 wherein the first modulator is adapted to impress data modulation in the form of intensity modulation. 
     
     
        .Iaddend..Iadd.25.  An optical transmitter in an optical data transmission system employing non-coherent detection comprising: means for generating an optical signal;   a first modulator adapted to impress data modulation upon the optical signal;   a second modulator, optically in series with the first modulator, adapted to modulate the state of polarization of the optical signal transmitted by the transmitter; and   a polarization adjuster, operative to regulate the operation of the transmitter based on a feedback signal to improve the quality of the amplified optical signal. .Iaddend..Iadd.26. The transmitter of claim 25 wherein the polarization adjuster minimizes a component of the amplified signal that is at the same frequency as that of the modulation provided by the second modulator. .Iaddend..Iadd.27. A method for optically amplifying an optical signal transmitted by an optical transmitter comprising the steps of:   generating an optical signal;   modulating the optical signal by impressing data upon the signal;   modulating the optical signal by sweeping the polarization state of the optical signal between substantially orthogonally polarized states; and   
     
     
       amplifying the modulated optical signal. .Iaddend..Iadd.28.  The method of claim 27 further comprising the step of providing a feedback signal to the transmitter for regulating operation of the transmitter to minimize a component of the amplified signal that is at the same frequency as that of the modulation provided by the second modulator. .Iaddend..Iadd.29. A method for optically amplifying an optical signal transmitted by an optical transmitter comprising the steps of: generating an optical signal;   impressing data upon the optical signal;   modulating the state of polarization of the optical signal transmitted by the transmitter; and   regulating the operation the transmitter with a feedback optical signal to improve the quality of the amplified optical signal. .Iaddend.