Method and system for receiving an optical-duo-binary signal

1. Method of receiving an optical-duo-binary, ODB, signal (S), which has a predefined ODB-transmission bit-rate (B), using a photoreceiver, said method comprising the step of filtering the ODB signal,

wherein said ODB signal is filtered using a filter which provides a frequency peak in the photoreceiver's frequency response, said frequency peak being located in the spectral range between 30% and 70% of the predefined ODB-transmission bit-rate, and,

wherein before said step of filtering the ODB signal using said filter, the ODB signal (S) is pre-filtered using an optical line pre-filter having a full-width-half-maximum bandwidth of 75% or less of the ODB-transmission bit-rate.

2. Method of claim 1 , wherein said frequency peak exceeds the frequency response for an input signal having a frequency of 0 Hz.

3. Method of claim 1 ,

wherein the frequency peak level of said frequency peak is between 5 dB and 7 dB over the DC-value, and wherein the DC-value refers to the frequency response for an input signal having a frequency of 0 Hz.

4. Method according to claim 1 ,

wherein the ODB signal is filtered in the optical domain with a delay line interferometer filter having a free spectral range corresponding to between 60% and 140% of the ODB-transmission bit-rate.

5. Method according to claim 1 ,

wherein the optical ODB signal (S) is split into a first signal portion (SP 1 ) and a second signal portion (SP 2 ),

wherein the first signal portion is filtered by a Mach-Zehnder-Interferometer filter ( 20 ) having a free spectral range (FSR) corresponding to between 60% and 14% of the ODB-transmission bit-rate,

wherein the filtered first signal portion and the second signal portion are added by coupler ( 40 ) having two exit ports, each of which being connected to a photodiode (D 1 , D 2 ), and

wherein the added signal portions (SA 1 , SA 2 ) at each exit port are detected with the respective photodiode.

6. Photoreceiver

adapted for receiving an optical-duo-binary, ODB, signal (S) having a predefined ODB-transmission bit-rate comprising:

at least one photodetector (D 1 , D 2 ), and

a filter, said filter provides a frequency peak in the photoreceiver's frequency response, said frequency peak being located in the spectral range corresponding to between 30% and 70% of the predefined ODB-transmission bit-rate (B),

wherein the filter ( 10 ) comprises a Mach-Zehnder-Interferometer ( 20 ) having a free spectral range (FSR), and

wherein the free spectral range (FSR) of the Mach-Zehnder-Interferometer corresponds to the ODB-transmission bit-rate (B),

a first coupler ( 30 ) having a first output ( 31 ) and a second output ( 32 ), said first output being connected to an input ( 21 ) of the Mach-Zehnder-Interferometer ( 20 ), and,

a second coupler ( 40 ) having a first input ( 41 ) and a second input ( 42 ), said first input being connected to the output ( 22 ) of said Mach-Zehnder-Interferometer ( 20 ), and said second input ( 42 ) being connected to said second output ( 32 ) of the first coupler ( 30 ).

7. Photoreceiver of claim 6 ,

wherein said frequency peak level is between 5 dB and 7 dB, preferably 6 dB, over the DC-value, wherein the DC-value refers to the frequency response for an input signal having a frequency of 0 Hz and/or

wherein the frequency peak is located at a frequency corresponding to half of the predefined ODB-transmission bit-rate.

8. Photoreceiver according to claim 6 ,

wherein the filter ( 10 ) comprises a Delay Line Interferometer structure having a free spectral range (FSR) corresponding to between 60% and 140% of the ODB-transmission bit-rate.

9. Photoreceiver according to claim 6 ,

wherein the first and second couplers have coupling ratios between 20% and 80%.

10. Optical system ( 100 ) comprising a photoreceiver ( 5 ) according to claim 6 , and an optical line pre-filter ( 120 ), wherein the filter of said photoreceiver provides a frequency peak in the photoreceiver's frequency response, said frequency peak being located in the spectral range between 50% and 80% of the pre-filters ( 120 ) bandwidth and having a peak level between 5 dB and 7 dB over the DC-value.

11. Photoreceiver ( 5 ) adapted for receiving an optical-duo-binary, ODB, signal (S) having a predefined ODB-transmission bit-rate (B), said photoreceiver ( 5 ) comprising at least one photodetector (D 1 , D 2 ), and a at least one filter ( 20 ), wherein said filter ( 20 ) comprises

a first coupler ( 30 ) having a first output ( 31 ) and a second output ( 32 ), said first output ( 31 ) being connected to an input ( 21 ) of a Mach-Zehnder-Interferometer ( 20 ), said Mach-Zehnder-Interferometer having a free spectral range (FSR),

a second coupler ( 40 ) having a first input ( 41 ) and a second input ( 42 ), said first input ( 41 ) being connected to the output ( 22 ) of said Mach-Zehnder-Interferometer, and said second input ( 42 ) being connected to said second output ( 32 ) of the first coupler ( 30 ).

12. Photoreceiver according to claim 11 ,

wherein the free spectral range (FSR) of the Mach-Zehnder-Interferometer ( 20 ) corresponds to the ODB-transmission bit-rate (B).