Polarisation-independent coherent optical receiver

Patent metadata

Field	Value
Publication number	US-9755759-B2
Application number	US-201415039890-A
Country	US
Kind code	B2
Filing date	Nov 27, 2014
Priority date	Nov 29, 2013
Publication date	Sep 5, 2017
Grant date	Sep 5, 2017

Abstract

In a coherent optical receiver, a received signal and an oscillator-generated signal, having frequency difference such that the receiver operates under intradyne conditions, are made to beat in a 3×3 optical coupler. A polarizing beam-splitter splits one of the signals into components with orthogonal polarization which are applied to inputs of the coupler, which receives the other of the received or oscillator-generated signal. After photoelectric conversion, the signals are fed to analog processing devices generating an electrical signal representing the received signal that is fed to a low pass filter before being demodulated. The frequency difference between the signals and the passband of the filter are such that a component of the electrical signal, oscillating at a frequency depending on the frequency difference and having amplitude and phase depending on the instant state of polarization of the received signal, is suppressed. A method is also provided.

First claim

The invention claimed is:

1. A polarisation-independent coherent optical receiver, comprising:
an optical coupler ( 2 ) having three inputs and three outputs, which receives a modulated optical signal and an optical signal generated by a local oscillator ( 3 ) and creates the beat between such signals;
an optical signal splitter ( 5 ; 5 ′), which is located on the path of one out of the modulated optical signal and the signal generated by the local oscillator, splits the signal applied thereto into two components having orthogonal polarisations and sends such components to two inputs of the coupler ( 2 ), which receives at a third input the other out of the modulated optical signal and the signal generated by the local oscillator ( 3 );
three photodetectors ( 6 a , 6 b , 6 c ) respectively connected to the coupler outputs and converting the optical signals outgoing from the coupler ( 2 ) into electrical signals;
analogue processing circuitry ( 7 , 8 a , 8 b , 8 c , 9 a , 9 b , 9 c , 10 a , 10 b , 10 c , 11 ) receiving and processing the electrical signals outgoing from the photodetectors ( 6 a , 6 b , 6 c ) and generating a resultant electrical signal that represents the modulated optical signal and includes a baseband component; and
a low-pass filter ( 12 ) filtering the resultant electrical signal;

characterised in that:
the optical coupler ( 2 ) is of a kind providing at the three outputs thereof optical signals having the same mutual phase shift;
the analogue processing circuitry ( 7 - 11 ) includes at least:
three squaring circuits ( 10 a , 10 b , 10 c ) squaring said electrical signals or combinations thereof; and
an adder ( 11 ) summing up the signals outgoing from the squaring circuits ( 10 a , 10 b , 10 c ) and outputting said resultant electrical signal;

wherein the resultant electrical signal includes, besides the baseband component, a spurious component that is centred on a frequency depending on a frequency difference between the modulated optical signal and the signal generated by the local oscillator ( 3 ) and that has an amplitude and a phase depending on the instant states of polarisation of the modulated optical signal and the signal generated by the local oscillator;
said frequency difference is such that the receiver ( 100 ; 200 ) operates under intradyne conditions; and
a passband of said low pass filter ( 12 ) and said frequency difference are such that said spurious component falls outside said passband or has a minimum spectral overlap with the modulated optical signal.

2. The receiver as claimed in claim 1 , wherein the low pass filter ( 12 ) has a passband in a range from about 65% to about 100% of a bit rate of the modulated optical signal, preferably a passband of the order of 75% of said bit rate.

3. The receiver as claimed in claim 1 , wherein:
the local oscillator ( 3 ) is of a kind generating a polarised signal comprising two orthogonally polarised components having the same amplitude; and
the optical signal splitter ( 5 ) is located on the path of the signal generated by the local oscillator ( 3 ) and comprises either a polarising beam splitter, or a non-polarising beam splitter of a kind equally splitting the signal between its outputs and having controllers of the state of polarisation on each output.

4. The receiver as claimed in claim 3 , wherein said optical coupler ( 2 ), said optical signal splitter ( 5 , 5 ′) and said controllers of the state of polarisation said polarisation rotator ( 20 ) are implemented by means of a photonic integrated circuit technology.

5. The receiver as claimed in claim 1 , wherein:
the optical signal splitter ( 5 ′) is located on the path of the modulated optical signal;
an output of the optical signal splitter ( 5 ′) is associated with a polarisation rotator ( 20 ) such that both components of the modulated optical signal arrive at the coupler ( 2 ) with the same polarisation; and
the local oscillator ( 3 ) is configured so as to generate a signal having the same polarisation as the component outgoing from the rotator ( 20 ).

6. The receiver as claimed in claim 5 , wherein said optical coupler ( 2 ), said optical signal splitter ( 5 , 5 ′) and said polarisation rotator ( 20 ) are implemented by means of a photonic integrated circuit technology.

7. The receiver as claimed in claim 1 , wherein said low pass filter ( 12 ) is connected to means ( 13 ) for recovering a synchronism signal and information data from the filtered signal and/or for determining the average power of the modulated signal in a given frequency range.

8. The receiver as claimed in claim 1 , wherein said coupler ( 2 ) is a coupler in which the energy is coupled from each input to all outputs.

9. A method for polarisation-independent coherent reception of modulated optical signals, wherein:
the beat between a received modulated optical signal and an optical signal generated by a local oscillator ( 3 ) is created after having split one out of the modulated optical signal and the signal generated by the local oscillator into two components with orthogonal polarisations, the beat generating three optical signals;
said three optical signals are converted into respective electrical signals;
said electrical signals are subjected to an analogue processing such as to generate a resultant electrical signal representing the modulated optical signal and including a baseband component; and
the resultant electrical signal is subjected to low-pass filtering;
characterised in that:
said beat generates three optical signals having the same mutual phase shift;
the analogue processing includes squaring the signals resulting from the photoelectric conversion, or a combination thereof, and combining the signals resulting from the squaring into said resultant electrical signal, which comprises, besides the baseband component, a spurious component that is centred on a frequency depending on a frequency difference between the modulated optical signal and the signal generated by the local oscillator ( 3 ) and that has an amplitude and a phase depending on the instant states of polarisation of the modulated optical signal and the signal generated by the local oscillator ( 3 );
said frequency difference is such that reception takes place under intradyne conditions; and
a passband of the low pass filtering and said frequency difference are such that said spurious component falls outside said passband or has a minimum spectral overlap with the modulated optical signal.

10. The method as claimed in claim 9 , wherein said passband is in a range from about 65% to about 100% of a bit rate of the modulated optical signal, and preferably is of the order of 75% of said bit rate.

11. The method as claimed in claim 9 , wherein:
the signal generated by the local oscillator ( 3 ) is a signal comprising two components with orthogonal polarisations having the same amplitude; and
said signal generated by the local oscillator ( 3 ) is split into said components with orthogonal polarisations.

12. The method as claimed in claim 9 , wherein:
the modulated optical signal is split into said components with orthogonal polarisations;
the polarisation of one of the components resulting from the splitting is rotated so that devices ( 2 ) creating the beat receive signals having the same polarisation; and
a signal having the same polarisation as that obtained through said rotation is generated by means of the local oscillator ( 3 ).

Assignees

• ERICSSON TELEFON AB L M (publ)

Inventors

• Ciaramella Ernesto
• Presi Marco

Classifications

• H04B10/65 Primary

  Intradyne, i.e. coherent receivers with a free running local oscillator having a frequency close but not phase-locked to the carrier signal · CPC title

• H04B10/6166

  Polarisation demultiplexing, tracking or alignment of orthogonal polarisation components · CPC title

• H04B10/614 Primary

  comprising one or more polarization beam splitters, e.g. polarization multiplexed [PolMux] X-PSK coherent receivers, polarization diversity heterodyne coherent receivers (H04J14/06 takes precedence) · CPC title

• H04B10/611

  Electricity · mapped topic