METHOD AND APPARATUS FOR ENHANCEMENT OF COMMON MODE REJECTION ON COHERENT OPTIC RECEIVERS

1 . An apparatus comprising:

optical circuitry configured to provide an optical signal source to be divided into a plurality of paths, a first path of the plurality of paths being input into an optical modulator, an output of the optical modulator being coupled to an input port of a first optical receiver, a second path of the plurality of paths being provided to a local oscillator port of the first optical receiver;

the first optical receiver comprising at least one coupler, an output of the coupler configured to provide input to at least one differentially configured pair of photodiodes, the photodiodes currents output being operably coupled to an analog to digital converter (ADC); and

feedback circuitry configured to provide outputs of the ADC to an algorithmic logic circuitry, the algorithmic logic circuitry being coupled to a variable voltage source.

2 . The apparatus of claim 1 , wherein at least one photodiode of the photodiodes is coupled to at least one variable voltage source to control the responsivity of the corresponding photodiode.

3 . The apparatus of claim 2 , wherein the algorithmic logic circuitry is configured to implement an adaptive filter algorithm.

4 . The apparatus of claim 3 , wherein the adaptive filter algorithm comprises one of a least means squares (LMS) algorithm, a Kalman algorithm, or a minimum mean-squared error algorithm.

5 . The apparatus of claim 3 , further comprising Fast-Fourier Transform (FFT) circuitry configured to provided transformed outputs of the ADC to the algorithmic logic circuitry.

6 . The apparatus of claim 2 , wherein the optical receiver comprises a dual-polarization receiver.

7 . The apparatus of claim 6 , wherein a hybrid coupler is configured to transform a local oscillator (LO) laser signal into an in-phase component and a quadrature component and wherein a hybrid coupler output is configured to generate an optical differential signal comprising at least the LO laser signal or a receiver input signal.

8 . The apparatus of claim 7 , wherein the photodiodes are configured to generate a difference product of a frequency of the LO laser frequency minus input signal frequency.

9 . The apparatus of claim 8 , further comprising bias circuitry to generate tuning voltages as inputs for the photodiodes.

10 . The apparatus of claim 1 , further comprising at least one optical switch, the at least one optical switch configured to switch signals from a transmission path of the apparatus to a receive path of the apparatus.

11 . The apparatus of claim 1 , wherein the apparatus is included in least one of a channel media, a fiber system, a terrestrial free-space optics (FSO) system, a satellite-to-satellite FSO system or a combination thereof.

12 . A method for calibrating a common mode rejection ratio of an optical transceiver, the method comprising:

configuring optical switches to enable routing at least one output signal of modulator circuitry operably coupled to a first receive path of a coherent optical transceiver;

configuring the input to at least one modulator to generate at least one first stimulus signal;

configuring a path from the first receiver analog-to-digital converter to an adaptive algorithm circuitry;

adapting at least one bias setting of a photodiode associated with the first receiver in response to at least one first stimulus detected at the first receiver analog-to-digital converter to an adaptive algorithm circuitry;

determining an optimum value of a photodiode associated with the first receiver through algorithm convergence; and

determining whether to terminate or repeat the configuring and adapting operations based on whether all receive paths have been configured.

13 . The method of claim 12 , wherein configuring the optical switches comprises configuring a subset of the optical switches to be calibrated and refraining from configuring optical switches that are not to be calibrated.

14 . The method of claim 12 , wherein providing the input stimulus comprises providing inputs from a signal generator.

15 . The method of claim 12 , where the algorithmic convergence solves for a minimization of the second harmonic in response to receiving a substantially one-hundred percent amplitude modulated tonally modulated signal.

16 . The method of claim 12 , wherein the method is applied to at least one of a channel media, a fiber system, a terrestrial free-space optics (FSO) system, a satellite-to-satellite FSO system or a combination thereof.

17 . An integrated circuit configured to perform operations comprising:

configuring optical switches to enable routing at least one output signal of modulator circuitry operably coupled to a first receive path of a coherent optical transceiver;

configuring the input to at least one modulator to generate at least one first stimulus signal;

configuring a path from the first receiver analog-to-digital converter to an adaptive algorithm circuitry;

adapting at least one bias setting of a photodiode associated with the first receiver in response to at least one first stimulus detected at the first receiver analog-to-digital converter to an adaptive algorithm circuitry;

determines an optimum value of a photodiode associated with the first receiver through algorithm convergence; and

determining whether to terminate or repeat the configuring and adapting operations based on whether all receive paths have been configured.

18 . The integrated circuit of claim 17 , wherein configuring the optical switches comprises configuring a subset of the optical switches to be calibrated and refraining from configuring optical switches that are not to be calibrated.

19 . The integrated circuit of claim 17 , wherein providing the input stimulus comprises providing inputs from one of a signal generator and a look up table.

20 . The integrated circuit of claim 17 , where the algorithmic convergence solves for a minimization of the second harmonic in response to receiving a substantially one-hundred percent amplitude modulated tonally modulated signal.