Optical Receiver
A receiver for a differentially phase shift keying formatted optical signal, such as an RZ-DPSK formatted optical signal. Dither control loops are provided for controlling path length in a demodulator and/or for controlling the center wavelength of an optical band pass filter. A feedback loop is provided for controlling the gain of a pre-amplifier, and a method of protecting against optical transients by disabling a pre-amplifier is also provided. A preset delay may be provided to compensate for the differential delay in paths associated with the demodulator arms. When the signal is an RZ-DPSK modulated signal, a clock for retiming data from the optical signal may be derived from a signal on the data path.
1 . An apparatus comprising:
a data path comprising:
a demodulator for demodulating an optical input signal having data modulated thereon according to a RZ-DPSK modulation format, said demodulator being configured to split said optical input signal onto first and second arms and provide first and second optical outputs, and
first and second photo detectors, said first photodetector being configured for providing a first electrical output in response to said first optical output being imparted thereon, said second photodetector being configured for providing a second electrical output in response to said second optical output being imparted thereon;
a circuit coupled to said first and second electrical outputs, said circuit being configured to provide an electrical output data stream representative of said data in response to a clock input representative of a clock associated with said data; and
a clock path coupling a portion of a signal on said data path to said circuit as said clock input.
2 . An apparatus according to claim 1 , wherein said clock path is coupled between an input to said demodulator and said clock input.
3 . An apparatus according to claim 1 , wherein clock path is coupled to said data path for receiving a portion of said first electrical output and a portion of said second electrical output.
4 . An apparatus according to claim 1 , wherein first photodetector is configured for providing a third electrical output in response to said first optical output being imparted thereon, and said second photodetector is configured for providing a fourth electrical output in response to said second optical output being imparted thereon, and wherein said clock path is coupled to said data path for receiving said third and fourth electrical outputs.
5 . An apparatus according to claim 1 , wherein said clock path comprises an electrical delay circuit for imparting an adjustable delay to said portion of said signal.
6 . An apparatus according to claim 1 , wherein said clock path comprises a band pass filter.
7 . An apparatus according to claim 1 , wherein said clock path comprises a limiting amplifier.
8 . An apparatus according to claim 1 , wherein said circuit comprises a D-flip-flop.
9 . A method of recovering data in an optical receiver having a data path including a demodulator for demodulating an optical input signal having said data modulated thereon according to a RZ-DPSK modulation format, the demodulator being configured to split said optical input signal onto first and second arms and provide first and second optical outputs, and first and second photo detectors, the first photodetector being configured for providing a first electrical output in response to said first optical output being imparted thereon, the second photodetector being configured for providing a second electrical output in response to said second optical output being imparted thereon, said method comprising:
providing a circuit coupled to the first and second electrical outputs, said circuit being configured to provide an electrical output data stream representative of said data in response to a clock input representative of a clock associated with said data; and
coupling a portion of a signal on said data path to said circuit as said clock input.
10 . A method according to claim 9 , wherein said coupling comprises coupling a portion of an input to said demodulator to said circuit as said clock input.
11 . A method according to claim 9 , wherein said coupling comprises coupling a portion of said first electrical output and a portion of said second electrical to said circuit as said clock input.
12 . A method according to claim 9 , wherein said first photodetector is configured for providing a third electrical output in response to said first optical output being imparted thereon, and said second photodetector is configured for providing a fourth electrical output in response to said second optical output being imparted thereon, and wherein said coupling comprises coupling at least a portion of said third and fourth electrical outputs to said circuit as said clock input.
13 . A method according to claim 9 , said method further comprising imparting an adjustable delay to said signal.
14 . A method according to claim 9 , wherein said circuit comprises a D-flip-flop.
15 . An apparatus comprising:
a demodulator for demodulating an optical input signal having data modulated thereon according to a DPSK modulation format, said demodulator being configured to split said optical input signal onto first and second arms and provide first and second optical outputs;
first and second photo detectors, said first photodetector being configured for providing a first electrical output in response to said first optical output being imparted thereon, said second photodetector being configured for providing a second electrical output in response to said second optical output being imparted thereon;
a clock and data recovery circuit coupled to first and second electrical outputs, said clock and data recovery circuit being configured to provide an output representative of said data; and
a electrical delay circuit coupled between said first electrical output and said clock and data recovery circuit, said electrical delay circuit being configured to provide an electrical delay of said first electrical output relative to said second electrical output.
16 . A method of compensating for a differential delay between first and second electrical outputs of associated first and second photodetectors in a receiver for an optical signal having data modulated thereon according to a DPSK modulation format, said method comprising:
tuning a value of an electrical delay line to compensate for said differential delay; and
coupling said electrical delay line to one of said first and second outputs.
17 . A method according to claim 16 , wherein said tuning said value of said electrical delay line is performed before said coupling said electrical delay line to said one of said first and second outputs.
18 . A method according to claim 16 , wherein said tuning said value of said electrical delay line is performed after said coupling said electrical delay line to said one of said first and second outputs.