Bidirectional Optical Link Pre-Compensation for Chromatic Dispersion
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Solution Overview
Problem
Current optical transmission systems using intensity modulation and direct detection face bandwidth limitations and chromatic dispersion issues, particularly in standard C-band DWDM systems, which restrict data rate and transmission reach, especially for mobile fronthaul applications.
Innovation Solution
A method for automatically determining pre-compensation parameters, such as filter coefficients and chirp parameters, at the transmitting side of a bidirectional optical transmission link to reduce inter-symbol interference and bit error rate, using an initialization process involving training signals and algorithms like zero-forcing or least mean square error to optimize digital filtering and chirp control.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If the data rate is increased using non-return-to-zero coded optical signals, then the bandwidth utilization is improved, but the transmission reach is reduced due to chromatic dispersion
Solution Approach 1:
The patent applies preliminary action by implementing pre-compensation of chromatic dispersion at the transmitting side before the signal enters the optical fiber. Digital filters and chirp modulation are used to pre-distort the signal in a way that counteracts the expected dispersion effects, allowing high data rates to be maintained over extended transmission distances without requiring complex receiver-side equalization
Solution Approach 2:
The patent utilizes parameter changes by dynamically adjusting the chirp parameter of the electro-optical converter and the coefficients of digital filters based on the calculated chromatic dispersion characteristics of the optical path. These parameter adjustments enable the system to adapt to different transmission distances and fiber conditions, optimizing both data rate and reach
2Productivity
If standard C-band DWDM systems are used with improved component bandwidth, then the data transmission rate is increased, but the transmission reach is significantly reduced due to dispersion penalty
Solution Approach 1:
The patent implements preliminary anti-action by pre-compensating for chromatic dispersion effects at the transmitter side. The system calculates the dispersion characteristics of the optical path and applies opposite-phase pre-distortion through digital filtering and chirp modulation, effectively canceling out the harmful dispersion effects before they can degrade the signal over long distances
3Reliability
If laser wavelengths within the O-band are used to avoid chromatic dispersion, then the dispersion penalty is reduced, but the propagation loss is larger and amplification is less mature
Solution Approach 1:
The patent applies parameter changes by utilizing chirp modulation in the electro-optical converter, where the instantaneous frequency of the optical signal is varied in correlation with the data signal. By optimizing the chirp parameter, the system can achieve dispersion compensation at C-band wavelengths, avoiding the need to switch to O-band wavelengths and their associated higher propagation losses
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach enhances the performance and reach of optical transmission links while reducing transceiver costs, allowing for increased data rates while maintaining low bit error rates.
Implementation Method 1
an electro-optical converter configured to convert the pre-emphasized electrical PAM-n transmit signal into an optical PAM-n transmit signal with a positive or negative chirp
Implementation Method 2
optical transceivers based on intensity modulation (IM) and direct detection (DD)
Implementation Method 3
optical transceivers based on intensity modulation (IM) and direct detection (DD)
Implementation Method 4
the chromatic dispersion (CD) of the optical path
Data Source
AI summary
The invention relates to a method and apparatus of operating a bidirectional optical transmission link. The optical transmission link includes a first and a second optical transceiver at a dedicated end of the optical transmission link and an optical path connecting the first and second optical transceiver. The optical transceivers apply the methods of converting an electrical digital transmit signal into an electrical PAM-n transmit signal, pre-emphasizing the electrical PAM-n transmit signal) by digital filtering and using the pre-emphasized electrical PAM-n signal2) as modulating signal for optically modulating an optical carrier signal. The optical modulation method deployed is configured to create an optical PAM-n transmit signal with a positive or negative chirp. For initializing the optical transmission link (100), an initialization process is performed in which at least one loop including the following steps is run through creating, in the first optical transceiver, an optical PAM-n training transmit signal and transmitting it to the second optical transceiver, the optical PAM-n training transmit signal being created using an electrical PAM-n training transmit signal including a binary training sequence. Initial values for filter parameters are used for pre-emphasizing the electrical PAM-n training transmit signal and an initial value is used for a chirp parameter that defines the positive or negative chirp of the optical PAM-n training transmit signal receiving, in the second optical transceiver, the optical PAM-n training transmit signal as an optical PAM-n training receive signal using direct detection. The optical PAM-n training receive signal is converted into an electrical PAM-n training receive signal. The method includes obtaining sampled values of the electrical PAM-n training receive signal (RPel,1) by sampling this signal at predetermined points in time; and using the sampled values obtained and corresponding sampled values of an ideal electrical PAM-n transmit signal to determine operating values for the filter parameters and an operating value for the chirp parameter.