Wavelength diversity turbulence mitigation method and apparatus for free-space optical communications
Aspects of the subject disclosure may include, for example, receiving a plurality of modulated optical wavelengths generated from digital input data and unmodulated optical wavelengths. The one or more of the plurality of modulated optical wavelengths undergo an optical operation and are converted into one or more electrical signals. The optical operation increases a signal to noise ratio (SNR) of the one or more electrical signals to exceed an SNR threshold. Additionally, the one or more electrical signals are processed to produce digital output data. Other embodiments are disclosed.
1 . A device configured for a wavelength-diverse optical communications link, comprising:
circuitry configured to perform operations, the operations comprising
receiving a plurality of modulated optical wavelengths, wherein the plurality of modulated optical wavelengths are generated by a transmitter modem from digital input data and transmitter-side unmodulated optical wavelengths, wherein generating of the plurality of modulated optical wavelengths involves subcarrier multiplexing, and wherein, after the transmitter modem, one or more of the plurality of modulated optical wavelengths undergo at least one optical operation that involves subcarrier selection prior to being transmitted across a turbulent channel,
converting of the plurality of modulated optical wavelengths into one or more electrical signals, wherein the at least one optical operation ensures a signal to noise ratio (SNR) of the one or more electrical signals exceeds an SNR threshold, and
processing the one or more electrical signals in a receiver modem to produce digital output data.
2 . The device of claim 1 , wherein the at least one optical operation comprises an optical delay, a polarization rotation, a nonlinear multiplication, an amplitude or phase modulation, an optical filter, or another linear or nonlinear process.
3 . The device of claim 2 , wherein the optical delay is an integer unit interval of time defined by an inverse of a symbol rate.
4 . The device of claim 1 , wherein the transmitter modem and the receiver modem are configured for a single wavelength.
5 . The device of claim 1 , wherein the transmitter-side unmodulated optical wavelengths are generated by one or more optical sources where a phase difference between the transmitter-side unmodulated optical wavelengths is unknown or varying.
6 . The device of claim 1 , wherein the turbulent channel introduces partially correlated or uncorrelated phases and amplitudes of the plurality of modulated optical wavelengths.
7 . The device of claim 1 , wherein the converting is performed with a coherent optical hybrid and photodetectors, and wherein a local oscillator is mixed with the plurality of modulated optical wavelengths.
8 . The device of claim 7 , wherein the local oscillator is composed of one or more receiver-side unmodulated wavelengths where phases of the receiver-side unmodulated wavelengths or the plurality of modulated optical wavelengths or both are unknown or time-varying.
9 . The device of claim 1 , wherein the wavelength-diverse optical communications link is characterized by a channel response, and wherein the at least one optical operation changes the channel response.
10 . The device of claim 9 , wherein the processing utilizes an adaptive equalizer of the receiver modem that compensates for the channel response, and wherein the adaptive equalizer is configured to utilize a least mean squares algorithm, a recursive least squares algorithm, a stochastic gradient descent algorithm, a constant modulus algorithm, or any combination thereof.
11 . The device of claim 9 , wherein the plurality of modulated optical wavelengths and the one or more electrical signals include one or more subcarriers.
12 . The device of claim 11 , wherein the one or more subcarriers are subjected to intersymbol interference, power loss, or noise enhancement based on the channel response.
13 . The device of claim 11 , wherein the one or more of the subcarriers comprises at least two subcarriers that are similar or identical to each other.
14 . The device of claim 13 , wherein the at least two subcarriers are combined in the receiver modem based on maximal ratio combining (MRC), equal gain combining (EGC), selective combining (SC), or any combinations thereof.
15 . The device of claim 11 , wherein the one or more subcarriers or information derived from the one or more subcarriers is further processed according to MRC, EGC, SC, or combinations thereof.
16 . The device of claim 1 , wherein the wavelength-diverse optical communications link utilizes spatial or temporal diversity techniques.
17 . A method for a wavelength-diverse optical communications link, comprising:
receiving a plurality of modulated optical wavelengths, wherein the plurality of modulated optical wavelengths are generated by a transmitter modem from digital input data and unmodulated optical wavelengths, wherein generating of the plurality of modulated optical wavelengths involves subcarrier multiplexing, and wherein, after the transmitter modem, one or more of the plurality of modulated optical wavelengths undergo at least one optical operation that involves subcarrier selection prior to being transmitted across a turbulent channel;
converting of the plurality of modulated optical wavelengths into one or more electrical signals, wherein the at least one optical operation results in a signal to noise ratio (SNR) of the one or more electrical signals exceeding an SNR threshold; and
processing the one or more electrical signals to produce digital output data.
18 . The method of claim 17 , wherein the at least one optical operation comprises an optical delay, a polarization rotation, a nonlinear multiplication, an amplitude or phase modulation, an optical filter, or another linear or nonlinear process.
19 . A system comprising:
first circuitry configured to generate a plurality of modulated optical wavelengths from digital input data and unmodulated optical wavelengths, wherein generating of the plurality of modulated optical wavelengths involves subcarrier multiplexing, and wherein one or more of the plurality of modulated optical wavelengths undergo at least one optical operation that involves subcarrier selection prior to being transmitted across a turbulent channel; and
second circuitry configured to perform operations including
receiving a plurality of optical signals from the turbulent channel that correspond the plurality of modulated optical wavelengths transmitted across the turbulent channel,
converting of the plurality of modulated optical wavelengths into one or more electrical signals, wherein the at least one optical operation increases a signal to noise ratio (SNR) of the one or more electrical signals to exceed an SNR threshold, and
processing the one or more electrical signals to produce digital output data.
20 . The system of claim 19 , wherein the at least one optical operation comprises an optical delay, a polarization rotation, a nonlinear multiplication, an amplitude or phase modulation, an optical filter, or another linear or nonlinear process.