Abstract:
The arrangement includes a transmitter ( 1 - 6 ) with two optical sources ( 1, 2 ) generating two optical carrier signals (L 1 , L 2 ) having different frequencies. The optical carrier signals (L 1 , L 2 ) are combined and divided in a first coupler ( 2 ) and fed to carrier signal inputs of two modulators ( 4, 5 ). The mixed carrier signals (L 1 +jL 2 , jL 1 +L 2 ) are separately modulated by two modulation signals (a(t)) and (b(t)) and the modulated signals ((A 1 +jA 2 ), (jB 1 +B 2 )) are combined in a first combiner and emitted as transmission signal (X(t)). Only on demodulator is necessary to regain the modulation signals (a(t), b(t)).

Description:
FIELD OF THE INVENTION 
       [0001]    The invention refers to an optical transmission method and an optical transmission system with high spectral bandwidth efficiency. The method is applicable for ASK/OOK (amplitude shift/on off keying) and for different kinds of phase modulations like DPSK (differential phase-shift keying). 
       BACKGROUND OF THE INVENTION 
       [0002]    Nowadays optical data transmission systems are transmitting optical signals with high data rates. However, high data rates require not only high bandwidths and expensive components at the transmitter and at the receiver but also degrade for a given modulation schema the signal quality according to the system and fiber impairments, e.g. filter distortions, chromatic and polarisation mode dispersion. 
         [0003]    Different transmission methods like orthogonal frequency-diversity modulation OFDM or polarisation multiplex diversity are used to reduce the channel symbol rate and to overcome these impairments. But the realisation of these methods leads to complex systems. 
         [0004]    Wavelength division systems split high data rate signals into two or more signals with lower data rates to overcome the impairments scaling with the data rate. But filters and different demodulators are necessary to separate and regain the data signals. 
         [0005]    Caplan et al. OFC 2007, paper OThD3 describe a transmission method using only a single interferometer to demodulate several wavelength channels exploiting the periodic transfer function of an delay interferometer. Nevertheless separate filters and optical-electrical converters are still necessary for each channel. 
         [0006]    The transmission methods may make use of different kinds of modulation, e.g. as intensity modulation, phase or difference phase modulation. 
         [0007]    Especially difference phase shift keying DPSK is preferred for optical transmission systems. The performance of DPSK transmission is described by e.g. A. H. Gnauck and P. J. Winzer in IEEE Journal of Lightwave Technology, vol. 23, no. 1, pp. 115-130, January 2005. 
       OBJECTS AND SUMMARY OF THE INVENTION 
       [0008]    It is an object of the invention to disclose a method and an optical transmission system with high bandwidth efficiency and low complexity. 
         [0009]    This problem is solved by a method for transmitting and a receiving two modulated signals characterized by the features of claim  1  and  2  respectively, and a transmitter and a receiver characterized in claims  11  and  12  respectively. 
         [0010]    Additional advantageously features are described in dependent claims. 
         [0011]    The modulation of mixed carrier frequencies allows a simple separation of the modulation (data) signals. Only one optical demodulator and electrical-optical converter is necessary. The use of orthogonal signals allows the transmission in a narrow optical channel. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    Preferable embodiments of the invention will now be described, by way of an example, in more detail in conjunction with appended drawings, wherein: 
           [0013]      FIG. 1  shows a simplified diagram of a transmitter according to the invention, 
           [0014]      FIG. 2  shows an example of a spectrum of an optical transmission signal and a spectrum of a demodulated electrical signal, 
           [0015]      FIG. 3  shows a simplified diagram of a receiver according to the invention, 
           [0016]      FIG. 4  shows a simplified diagram of a DPSK receiver, and 
           [0017]      FIG. 5  shows a second example of a spectrum of an optical transmission signal and a spectrum of a demodulated electrical signal. 
       
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0018]    The simplified diagram of  FIG. 1  illustrates a transmitter for transmitting two modulation (data) signals a(t) and b(t). Optical amplifiers, polarisation controllers and control circuits well known for those skilled in the art are not shown for reasons of clarity. 
         [0019]    The transmitter comprises two laser sources  1  and  2 , each connected to an input of a first coupler  3  (3 dB optical coupler; combiner/splitter). The outputs of said first splitter  3  are connected to carrier inputs of a first modulator  4  and a second modulator  5  respectively. A first modulation signal a(t), corresponding e.g. to a digital data signal, is connected to a modulation signal input of the first modulator  4  and a second modulation signal b(t) is connected to a modulation signal input of the second modulator  5 . The outputs of both modulators  4  and  5  are connected to a first combiner (3 dB optical coupler)  6 . One output  7  is chosen as transmitter output. 
         [0020]    The laser sources  1  and  2  emit a first carrier signals L 1  with a first carrier frequency f 1  and second carrier signal L 2  with a second carrier frequency f 2  having a phase difference compared to f 1 . The carrier signals L 1  and L 2  are fed to the inputs of the first coupler  3  (3 dB coupler/splitter). 
         [0021]    The output signals of the first coupler  3  are the input signals of the modulators  4  and  5  neglecting constant factors can be derived as 
         [0000]    
       
         
           
             
               
                 
                   
                     
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         [0022]    The mixed carrier signals L 1 +jL 2 , jL 1 +L 2  can also be derived by modulation a single laser source, e.g. as described in an article by A Sano, Proceedings of ECEC 2007, incorporated by reference. 
         [0023]    The first mixed carrier signal L 1 +jL 2  is modulated by the first modulation signal a(t) and the second mixed carrier signal jL 1 +L 2  is modulated by the second modulation signal b(t): 
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         [0000]    with:
 
A 1 =Carrier L 1  with modulation signal a(t),
 
A 2 =Carrier L 2  modulated with modulation signal a(t),
 
B 1 =Carrier L 1  modulated with modulation signal b(t),
 
B 2 =Carrier L 2  modulated with modulation signal b(t),
 
j is the imaginary unit (sqrt(−1)),
 
the indices  1  and  2  are still indicating the carrier frequencies f 1  and f 2  respectively.
 
a(t) and b(t) are e.g. modulation signals representing a logic value of 1 or 0. Depending on the art of modulation the first modulated signal A 1 +jA 2  and the second modulated signal jB 1 +B 2  output from the modulators might be intensity or phase modulated.
 
         [0024]    The output signals of the modulators are combined by the first combiner  6  are 
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         [0025]    One of the output signals of the first combiner  6  is chosen as a transmission signal, e.g. the transmission signal according to the first line of the resulting matrix emitted at a first combiner output  7  (the signal emitted at a second combiner output  8  could also be used). 
         [0000]        X=A   1   +jA   2   −B   1   +jB   2   (4)
 
         [0000]      or 
         [0000]        X =( A   1   −B   1 )+ j ( A   2   +B   2 )  (5)
 
         [0026]    Written as time depending equation, the transmission signal is 
         [0000]        X ( t )=( A ( t )− B ( t )) e   j(2πf     1     t) +( A ( t )+ B ( t )) e   j(2πf     2     t+ΔΦ)   (6)
 
         [0000]    with
       A 1 =A (t)e j(2πf     1     t) , B 1 =B(t)e j(2πf     1     t) ;   jA 2 =A(t) e j(2πf     2     t+ΔΦ) , jB 2 =B(t) e j(2πf     2     t+ΔΦ)      and ΔΦ-phase difference.       
 
         [0030]    A(t) and B(t) correspond to baseband signals respectively modulation signals while the optical carrier signals are described in komplex form. 
         [0031]    Applying equations (1) and (4) an intensity modulated transmission signal comes out as 
         [0000]        X   A ( t )=( L   1   +jL   2 ) a ( t ))+(− L   1   +jL   2 ) b ( t ))  (7)
 
         [0032]    If the mixed carrier signals are intensity modulated, e.g. by a first binary or logical data signal a(t) and a second binary or logical data signal b(t), the standardised amplitudes of (A 1 +jA 2 )=(L 1 +L 2 )a(t) and (jB 1 +B 2 )=(−L 1 +jL 2 )b(t)) may vary between 0 and 1 as functions of the modulating signals a(t) and b(t) respectively. 
         [0033]    For a phase modulated signal, A(t) and B(t) correspond to baseband signals having a constant amplitude but different phases which might take the value of e jπ  or e −jπ  respectively and depend on the modulation signals a(t), b(t). 
         [0034]    If DPSK (Difference Phase Shift Keying) is used the corresponding DPSK transmission signal is designated as X D (t). 
         [0035]    An example of an optical spectrum S(X) (optical power P O  as a function of the frequency f) of an optical transmission signal X(t) is shown in  FIG. 2 . The spectra related to the carrier frequencies f 1  and f 2  are separated by Δf from each other, so that also the corresponding demodulated electrical spectra S(Y) (electrical power P E  as a function of the frequency f) do not interfere which each other. 
         [0036]      FIG. 3  shows a receiver for intensity modulated signals according to the invention. The receiver comprises an optical-electrical converter (photodiode)  10  receiving the transmission signal X AR (t) at its input  9 . An output of the optical-electrical converter  10  is connected to a first splitter  11  of a signal separation circuit  11 - 19 . An electrical signal Y A (t) output from the optical-electrical converter  10  is fed via said first splitter  11  directly to a first low pass filter  13 . Another identical part of the electrical signal Y(t) is fed from a second splitter output via a second splitter  12  and a modulator  15  to a second low pass filter  17 . The output signals of both filters  13 ,  17  are split and fed to a first adder  18  and a second adder  19 . 
         [0037]    Considering that the amplitude of the signals with different carrier frequencies f 1  and f 2  output from a modulator are always the same, we can simplify |A 1 |=|A 2 |=A and |B 1 |=|B 2 |=B. In general, the optical-electrical converter  10  squares the received transmission signal X(t) of equation (6). The different kinds of modulations need not be considered here. 
         [0038]    Making further use of the mathematical relations, the squared electrical signal output from the optical electrical converter  10  is: 
         [0000]        Y ( t )=( A−B )+( A+B ) 2 +2( A−B )( A+B )cos(2π( f   1 - f   2 ) t −ΔΦ)  (8)
 
         [0000]      which is equal to 
         [0000]        Y ( t )=2( A   2   +B   2 )+2( A   2   −B   2 )cos(2π( f   1 - f   2 ) t −ΔΦ)  (9)
 
         [0000]    After the first low pass filter  13  the signal 
         [0000]        S   13 =2( A   2   +B   2 )  (10)
 
         [0000]    remains. In  FIG. 3  Y(t) is denoted as Y A (t) because ASK is applied. 
         [0039]    In the lower signal path a synchronised oscillator  14 , which receives via the second splitter  12  the electrical signal Y(t) for synchronisation, generates a phase-locked signal with an angular frequency co according to the difference f 1 -f 2 . The squared optical signal is modulated by said synchronized signal cos(ωt−ΔΦ). 
         [0000]        Y   2 ( t )=2( A   2   +B   2 )+2( A   2   −B   2 )cos(ω t −ΔΦ)×cos(ω t −ΔΦ)  (11)
 
         [0000]        Y   2 ( t )=2( A   2   +B   2 )cos(ω t −ΔΦ)+2( A   2   −B   2 )cos 2 (ω t −ΔΦ)  (12)
 
         [0000]    Applying mathematical relations Y 2 (t) becomes 
         [0000]        Y   2 ( t )=2( A   2   +B   2 )cos(ω t −ΔΦ)+( A   2   −B   2 )+( A   2   −B   2 )cos 2 (2 ωt− 2ΔΦ)  (13)
 
         [0040]    This signal is amplified by factor 2 (or the signal S 13  is attenuated). After the amplifier  16  and the second low pass filter  17  a second filter output signal 
         [0000]        S   17 =2( A   2   −B   2 )  (14)
 
         [0000]    remains. This signal is added to the first filter output signal S 13  by the first adder  18  and subtracted from S 13  by the second adder  19 . Therefore a first output signal A 0  and a second output signal B 0  becomes 
         [0000]        A   0 =2( A   2   +B   2   +A   2   −B   2 )=4 A   2   (15)
 
         [0000]        B   0 =2( A   2   +B   2   −A   2   +B   2 )=4 B   2   (16)
 
         [0000]    which convey the logical values of the modulation signals a(t) and b(t). 
         [0041]    Neglecting the constant factors 4 (which are also neglected in the drawings) the signals A 2  and B 2  are output at a first and second receiver output  20  and  21  respectively. 
         [0042]    If for example DPSK (difference phase shift keying) is applied, phase modulators are used instead of intensity modulators ( FIG. 1 ) and the DPSK transmission signal X D (t) is emitted. 
         [0043]    The appropriate DPSK receiver comprises a common DPSK demodulator shown in  FIG. 4  using a delay interferometer  22 ,  23 ,  24  and a pair of optical-electrical converters  25 ,  26  (photodiodes). The signal separation circuit  11 - 19  remains the same as already described. 
         [0044]    A received DPSK modulated transmission signal X DR (t) is received at the input  9  and split into two parts by a further splitter  22 . A first signal part is led via a delay  23  to a first input to a second combiner (3 dB coupler)  24  and a sec- and signal part is directly fed to a second input of the combiner. Both outputs of the combiner are connected to a pair of electrical-optical converters  25  and  26 . The output signals of which are fed to a further adder  27  or the electrical-optical converters  25  and  26  are connected in series in a well kwon manner. Because the delay time of the time delay corresponds to a symbol length the phase difference of two adjacent symbols is directly converted into an amplitude modulated signal Y D (t). 
         [0045]    The unaltered separation circuit  11 - 19  regains both modulation signals. 
         [0046]    If multistage modulation is used, also multistage modulation signals representing symbols e.g. a(t)=f(a 0 (t), a 1 (t)) and b(t)=f(b 0 (t), b 1 ( t )) are employed. A corresponding receiver comprises appropriate decision circuits for signal separation. 
         [0047]    If e.g. a DQPSK (difference quadrature phase shift keying) transmission system is implemented, each modulated transmission signal has four different possible phases. The appropriate receiver comprises two of the receivers shown in  FIG. 4 , each with an interferometer and a separation circuits. 
         [0048]    To reduce the transmission bandwidth optical carrier signals frequencies with a low frequency difference Δf=f 1 -f 2  are chosen. To minimize the interaction between carriers and therefore the degradation due to intercarrier-interference, orthogonality is desired. Even an overlap of the spectra is feasible, when the carrier signals L 1  and L 2  are orthogonal: 
         [0000]    
       
         
           
             
               
                 
                   
                     
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         [0000]    or equivalent 
         [0000]        f 1- f 2 =Δf=n/T=n×symbol/s   (18),
 
         [0000]    wherein T is the symbol duration, n is an integer, and symbol/s is the symbol rate of the modulation signal. 
         [0049]    An appropriate example of an narrow optical spectrum S(X N ) of the optical transmission signal and an associated electrical spectra S(Y N ) is illustrated in  FIG. 5 . The optical spectra and the electrical spectra associated to the carrier frequencies are overlapping. To separate these signals low pass filters with correlation properties (integrate and dump filters) have to be used. 
         [0050]    General, to assure orthogonality, the duration of the pulse must be taken into account as well. But if modulation according to the invention is used, orthogonality is ensured regardless if RZ (return to zero) or NRZ (non return to zero) pulses are transmitted. 
         [0051]    DQPSK (difference quadrature phase shift keying) and OOK (on-off keying) are orthogonal when NRZ pulses are used and the frequency separation between carriers Δf=symbol rate. If the frequency separation between carriers Δf=n×symbol rate, n=2, 3, NRZ or RZ might be used. 
       REFERENCE SIGNS 
       [0000]    
       
           1  first optical source (laser) 
           2  second optical source (laser) 
           3  first coupler 
           1 , 2 , 3  carrier signal generator 
           4  first modulator 
           5  second modulator 
           6  first combiner 
           7  first combiner output 
           8  second combiner output 
           9  receiver input 
           10  optical-electrical converter 
           11  first splitter 
           11 - 19  signal separation unit 
           12  second splitter 
           13  first low pass filter 
           14  oscillator 
           15  electrical modulator 
           16  amplifier 
           17  second low pass filter 
           18  first adder 
           19  second adder 
           20  first receiver output 
           21  second receiver output 
           22  further splitter 
           23  time delay 
           24  second combiner 
           25  electrical-optical converter 
           26  electrical-optical converter 
           27  further adder 
         L 1  first carrier signal 
         L 2  second carrier signal 
         L 1 +L 2  first mixed carrier signal 
         jL 1 +L 2  second mixed carrier signal 
         X(t) transmission signal 
         X A (t) ASK modulated transmission signal 
         X D (t) DPSK modulated transmission signal 
         S(X) spectrum of the transmission signal 
         P E  (Y) spectrum of demodulated signal 
         X AR  (t) received ASK modulated transmission signal 
         X DR  (t) received DPSK modulated transmission signal 
         Y D  (t) received and converted transmission signal 
         Y A (t) received and converted ASK transmission signal