Data transmission system and method

1. A data transmission system comprising:

a transmitter ( 20 ) for transmitting data using polarized electromagnetic waves, comprising:

a first signal source ( 22 ) being controllable to emit a first electromagnetic signal (S 1 ) having a first wavelength (λ 1 ), said first electromagnetic signal (S 1 )comprising a first polarization component (S 1x ) having a first polarization direction and a second polarization component (S 1y ) having a second polarization direction orthogonal to the first polarization direction;

a second signal source ( 23 ) being controllable to emit a second electromagnetic signal (S 2 ) having a second wavelength (λ 2 ), said second electromagnetic signal (S 2 ) comprising a first polarization component (S 2x ) having said first polarization direction and a second polarization component (S 2y ) having said second polarization direction orthogonal to the first polarization direction;

a first modulator ( 26 ) arranged and configured to modulate the first polarization component (S 1x ) of said first electromagnetic signal (S 1 ) to encode a first data stream (DS 1 ); and

a second modulator ( 27 ) arranged and configured to modulate the second polarization component (S 2y ) of said second electromagnetic signal (S 2 ) to encode a second data stream (DS 2 ):and

a receiver ( 40 ) for retrieving a first data stream (DS 1 ) and a second data stream (DS 2 ) from a combined electromagnetic signal (S comb ) comprising:

a first polarization component (S x ), and a second polarization component (S y ) orthogonal to said first polarization component,

said first polarization component (S x ) comprising a first modulated signal (S 1x,mod ) having a first carrier wavelength (λ 1 ) and a second unmodulated signal (S 2x ) having a second carrier wavelength (λ 2 ) different from the first carrier wavelength, and said second polarization component (S y ) comprising a first unmodulated signal (S 1y ) having said first carrier wavelength (λ 1 ) and a second modulated signal (S 2y,mod ) having said second carrier wavelength (λ 2 ),

said first data stream (DS 1 ) being carried by said first modulated signal (S 1x,mod ) and said second data stream (DS 2 ) being carried by said second modulated signal (S 2y,mod ),

the receiver ( 40 ) comprising:

a first receiver branch ( 41 ) comprising:

a first band pass filter ( 43 ) configured to allow passage of said first modulated signal (S 1x,mod ) and prevent passage of said second unmodulated signal (S 2x );

a polarization splitter ( 45 ) arranged and configured to divide an input electromagnetic signal into said first polarization component (S x ) and said second polarization component (S y );

a second band pass filter ( 46 ) arranged to receive said second polarization component (S y ), and configured to output a filtered portion of said second polarization component comprising said first unmodulated signal (S 1y ); and

a demodulator ( 47 ) arranged to receive said first polarization component (S x ) comprising said first modulated signal (S 1x,mod ), and said filtered portion of the second polarization component comprising said first unmodulated signal (S 1y ) and configured to demodulate said first modulated signal using said first unmodulated signal, to thereby retrieve said first data stream (DS 1 ):

and a second receiver branch ( 42 ) comprising:

a first band pass filter ( 50 ) configured to allow passage of said second modulated signal (S 2y,mod ) and prevent passage of said first unmodulated signal (S 1y );

a polarization splitter ( 52 ) arranged and configured to divide an input electromagnetic signal into said first polarization component (S x ) and said second polarization component (S y );

a second band pass filter ( 53 ) arranged to receive said first polarization component (S x ), and configured to output a filtered portion of said first polarization component comprising said second unmodulated signal (S 2x ); and

a demodulator ( 54 ) arranged to receive said second polarization component comprising said second modulated signal (S 2y,mod ) and said filtered portion of the first polarization component comprising said second unmodulated signal (S 2x ) and configured to demodulate said second modulated signal using said second unmodulated signal, to thereby retrieve said second data stream (DS 2 );

the receiver ( 40 ) configured to receive an electromagnetic signal transmitted by said transmitter.

2. The data system according to claim 1 , wherein the transmitter ( 20 ) further comprises a polarization combiner ( 28 ) arranged and configured to combine the first polarization components (S 1x ; S 2x ) of the first electromagnetic signal (S 1 ) and the second electromagnetic signal (S 2 ) with the second polarization components (S 1y ; S 2y ) of the first electromagnetic signal (S 1 ) and the second electromagnetic signal (S 2 ) to form a combined electromagnetic signal (S comb ).

3. The data system according to claim 1 , wherein said first modulator ( 26 ) is configured to modulate at least a phase of the first polarization component of said first electromagnetic signal, and said second modulator ( 27 ) is configured to modulate at least a phase of the second polarization component of said second electromagnetic signal.

4. The data system according to claim 3 , wherein said first modulator ( 26 ) is configured to modulate a phase and an amplitude of the first polarization component of said first electromagnetic signal, and said second modulator ( 27 ) is configured to modulate a phase and an amplitude of the second polarization component of said second electromagnetic signal.

5. The data system according to claim 1 , wherein each of said first signal source ( 22 ) and said second signal source ( 23 ) is a laser.

6. A receiver ( 40 ) for retrieving a first data stream (DS 1 ) and a second data stream (DS 2 ) from a combined electromagnetic signal (S comb ) comprising:

a first polarization component (S x ), and a second polarization component (S y ) orthogonal to said first polarization component,

said first polarization component (S x ) comprising a first modulated signal (S 1x,mod ) having a first carrier wavelength (λ 1 ) and a second unmodulated signal (S 2x ) having a second carrier wavelength (λ 2 ) different from the first carrier wavelength, and said second polarization component (S y ) comprising a first unmodulated signal (S 1y ) having said first carrier wavelength (λ 1 ) and a second modulated signal (S 2y,mod ) having said second carrier wavelength (λ 2 ),

said first data stream (DS 1 ) being carried by said first modulated signal (S 1x,mod ) and said second data stream (DS 2 ) being carried by said second modulated signal (S 2y,mod ),

the receiver ( 40 ) comprising:

a first receiver branch ( 41 ) comprising:

a first band pass filter ( 43 ) configured to allow passage of said first modulated signal (S 1x,mod ) and prevent passage of said second unmodulated signal (S 2x );

a polarization splitter ( 45 ) arranged and configured to divide an input electromagnetic signal into said first polarization component (S x ) and said second polarization component (S y );

a second band pass filter ( 46 ) arranged to receive said second polarization component (S y ), and configured to output a filtered portion of said second polarization component comprising said first unmodulated signal (S 1y ); and

a demodulator ( 47 ) arranged to receive said first polarization component (S x ) comprising said first modulated signal (S 1x,mod )and said filtered portion of the second polarization component comprising said first unmodulated signal (S 1y ) and configured to demodulate said first modulated signal using said first unmodulated signal, to thereby retrieve said first data stream (DS 1 );

and a second receiver branch ( 42 ) comprising:

a first band pass filter ( 50 ) configured to allow passage of said second modulated signal (S 2y,mod ) and prevent passage of said first unmodulated signal (S 1y );

a polarization splitter ( 52 ) arranged and configured to divide an input electromagnetic signal into said first polarization component (S x ) and said second polarization component (S y );

a second band pass filter ( 53 ) arranged to receive said first polarization component (S x ), and configured to output a filtered portion of said first polarization component comprising said second unmodulated signal (S 2x ); and

a demodulator ( 54 ) arranged to receive said second polarization component comprising said second modulated signal (S 2y,mod ) and said filtered portion of the first polarization component comprising said second unmodulated signal (S 2x ) and configured to demodulate said second modulated signal using said second unmodulated signal, to thereby retrieve said second data stream (DS 2 ).

7. The receiver ( 40 ) according to claim 6 , wherein:

said first receiver branch ( 41 ) further comprises a polarization control device for controlling a polarization state of said electromagnetic signal input to said polarization splitter ( 45 ); and

said second receiver branch ( 42 ) further comprises a polarization control device for controlling a polarization state of said electromagnetic signal input to said polarization splitter ( 52 ).

8. The receiver ( 40 ) according to claim 7 ,

wherein said polarization control device of the first receiver branch ( 41 ) comprises:

a polarization controller ( 44 )polarization controller arranged to output a polarization-controlled electromagnetic signal to said polarization splitter ( 45 ) and

a control unit ( 49 ) arranged in a feedback configuration to control said polarization controller ( 44 ) based on at least one measured property of at least one of said first polarization component and said filtered portion of the second polarization component; and

wherein said polarization control device of the second receiver branch ( 42 ) comprises:

a polarization controller ( 51 ) arranged to output a polarization-controlled electromagnetic signal to said polarization splitter ( 52 ); and

a control unit ( 56 ) arranged in a feedback configuration to control said polarization controller ( 51 ) based on at least one measured property of at least one of said second polarization component and said filtered portion of the first polarization component.

9. The receiver ( 40 ) according to claim 8 ,

wherein the control unit ( 49 ) of said first receiver branch ( 41 ) is arranged to receive an input signal indicative of a power of said filtered portion of the second polarization component and to control said polarization controller ( 44 ) towards a state where the power of said filtered portion of the second polarization component is substantially constant; and

wherein the control unit ( 56 ) of said second receiver branch ( 42 ) is arranged to receive an input signal indicative of a power of said filtered portion of the first polarization component and to control said polarization controller ( 51 ) towards a state where the power of said filtered portion of the first polarization component is substantially constant.