System and method for optical heterodyne detection of an optical signal

A system for optical heterodyne detection comprising:

• an optical coupler having a first input for receiving an input signal and a second input for receiving a local oscillator signal, said optical coupler having first and second outputs, wherein a first beam that includes said input signal and said local oscillator signal is output from said first output and wherein a second beam that includes said input signal and said local oscillator signal is output from said second output;
• a polarizing beam splitter connected to receive said first beam from said optical coupler and to receive said second beam from said optical coupler, said polarizing beam splitter splitting said first beam into a first split beam having a first polarization state and into a second split beam having a second polarization state, said polarizing beam splitter splitting said second beam into a third split beam having said first polarization state and into a fourth split beam having said second polarization state;
• a first photodetector connected to receive said first split beam having said first polarization state and to generate a first electrical signal in response to said first split beam;
• a second photodetector connected to receive said second split beam having said second polarization state and to generate a second electrical signal in response to said second split beam;
• a third photodetector connected to receive said third split beam having said first polarization state and to generate a third electrical signal in response to said third split beam;
• a fourth photodetector connected to receive said fourth split beam having said second polarization state and to generate a fourth electrical signal in response to said fourth split beam;
• a processor connected to receive said first, second, third, and fourth electrical signals, and
• a switch located in an optical path before said first input of said optical coupler for selectively blocking transmission of said input signal to said optical coupler.

A method for monitoring an optical signal utilizing optical heterodyne detection comprising steps of:

• combining an input signal with a swept local oscillator signal and outputting a first beam and a second beam, said first beam including portions of said input signal and said swept local oscillator signal and said second beam including portions of said input signal and said swept local oscillator signal;
• splitting said first beam into a first split beam having a first polarization state and into a second split beam having a second polarization state;
• splitting said second beam into a third split beam having said first polarization state and into a fourth split beam having said second polarization state;
• generating a first electrical signal in response to said first split beam;
• generating a second electrical signal in response to said second split beam;
• generating a third electrical signal in response to said third split beam;
• generating a fourth electrical signal in response to said fourth split beam;
• and
• processing said first, second, third, and fourth electrical signals in order to suppress intensity noise components of said first, second, third, and fourth split beams and to achieve polarization independence.

A system for optical heterodyne detection comprising:

• an optical coupler for combining an input signal and a local oscillator signal into a combined optical signal, said optical coupler having two outputs for outputting a first beam and a second beam of said combined optical signal;
• polarizing beam splitter for splitting a beam based on its state of polarization, said polarizing beam splitter being optically connected to said two outputs of said optical coupler to receive said first beam and said second beam, said polarizing beam splitter having four outputs for outputting two polarized portions of said first beam and two polarized portions of said second beam;
• first, second, third, and fourth photodetectors, each of said photodetectors being optically connected to receive a different one of said two polarized portions of said first beam and said two polarized portions of said second beam, said photodetectors generating electrical signals in response to respective ones of said four beams, and
• a digital signal processor for providing an output signal in response to said electrical signals in which the intensity noise component of said input signal is suppressed.