Frequency-domain equalization of the fiber optic channel
View Patent ↗Systems and methods for frequency-domain compensation in optical communication systems. In pre-equalization embodiments, the transmitter transforms the data stream into a frequency domain signal and applies a compensation filter before transforming it back into a pre-distorted time domain signal. As the pre-distorted time domain signal propagates through the optical channel, optical dispersion effects counter the pre-distortion, producing an equalized signal at the channel output. In post-equalization embodiments, the receiver transforms the received signal into a frequency domain signal and applies a compensation filter before transforming it back into an equalized time domain signal. Pre-equalization may prove less expensive due to the square-law characteristic of photodetectors employed by most receivers.
1. A method for countering dispersion caused by a fiber optic medium, the method comprising:
converting an optical beam into an electrical receive signal;
converting the electrical receive signal into time-domain data blocks, wherein said converting includes:
digitizing the receive signal to obtain a receive data stream; and
forming time-domain data blocks from the receive data stream, wherein said forming includes discarding cyclic prefixes to each time-domain data block;
transforming the time-domain data blocks into frequency-domain data blocks;
applying a dispersion compensation filter to each frequency-domain data block to produce a corrected frequency-domain data block, wherein the dispersion compensation filter provides a phase correction that is a nonlinear function of frequency;
transforming each corrected frequency-domain data block into a corrected time-domain data block; and
reconstructing a transmitted data stream from the corrected time-domain data blocks.
2. The method of claim 1 , wherein the dispersion compensation filter is expressible with a frequency dependence in the form of exp(−j β 2 ω 2 L/2), where exp( ) is the exponential function, j is the square root of (−1), β 2 is a group velocity dispersion parameter, ω is frequency, and L is an effective length of a fiber channel that carries said optical beam.
3. An optical receiver that comprises:
a linear or linearized optical-to-electrical converter that converts a received optical signal into a received electrical signal;
an analog to digital converter that converts the received electrical signal into a received data stream;
a serial-to-parallel converter that forms time-domain data blocks from the received data stream, wherein the serial-to-parallel converter discards a cyclic prefix for each time-domain data block;
a frequency transformer that transforms the time-domain data blocks into frequency-domain data blocks;
a multiplier that applies a compensation filter to the frequency-domain data blocks to produce compensated frequency-domain data blocks, wherein the compensation filter provides a phase correction that is a nonlinear function of frequency;
an inverse frequency transformer that transforms the compensated frequency-domain data blocks into compensated time-domain data blocks; and
a decoder that reconstructs a transmit data stream from the compensated time-domain data blocks.
4. The optical receiver of claim 3 , wherein the compensation filter compensates for optical dispersion effects of an optical channel.
5. The optical receiver of claim 3 , wherein the compensation filter is expressible with a frequency dependence in the form of exp(−j β 2 ω 2 L/2), where exp( )is the exponential function, j is the square root of (−1), β 2 is a group velocity dispersion parameter, ω is frequency, and L is an effective length of a fiber channel that carries said optical beam.
6. The optical receiver of claim 3 , wherein the inverse frequency transformer further adds overlapped portions of the compensated time-domain data blocks.
7. The method of claim 1 , wherein said reconstructing includes adding overlapped portions of the compensated time-domain data blocks.