Optical Spectrum Reshaper Spatial Filtering for Long-Haul Transmission
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Solution Overview
Problem
Directly modulated laser (DML) transmitters in optical fiber systems face severe signal distortion due to inherent time-dependent chirp, limiting their reach to less than 5 km, whereas chirp-managed laser transmitters can only extend to approximately 80 km, and there is a need to further enhance the transmission distance in standard single mode fiber at 10 Gb/s and 1550 nm without incurring significant dispersion penalty.
Innovation Solution
The optical system incorporates a frequency modulated laser source, an optical spectrum reshaper (OSR) with spatial filtering, where the optical fiber's central axis is laterally offset from the OSR and focusing lens's central axis, to modify the spectral response and extend the transmission distance by converting frequency modulation to amplitude modulation, thereby optimizing the spectral response for longer reach.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If directly modulated laser (DML) transmitters are used, then the device complexity is reduced, but the transmission distance is severely limited to less than 5 km due to time-dependent chirp distortion
Solution Approach 1:
An optical spectrum reshaper (OSR) is introduced as an intermediary component between the DML transmitter and the optical fiber. The OSR converts the frequency-modulated signal with time-dependent chirp into an amplitude-modulated signal, eliminating the harmful chirp effects while preserving the simple DML transmitter structure.
Solution Approach 2:
The invention changes the modulation parameter from frequency modulation (FM) to amplitude modulation (AM) through the OSR. This parameter transformation converts the chirp-distorted FM signal into a clean AM signal that can propagate over long distances without severe distortion, extending transmission distance from <5 km to >250 km.
2Length of moving object
If chirp-managed laser transmitters with optical spectrum reshaper are used, then the transmission distance is extended to approximately 80 km, but the reach is still insufficient for long-haul applications
Solution Approach 1:
The invention introduces spatial filtering as an additional dimension of signal processing. By laterally offsetting the optical fiber's central axis from the OSR and focusing lens axes, spatial filtering is applied to further purify the optical signal, removing residual distortions and extending transmission distance beyond 250 km while maintaining signal quality.
Solution Approach 2:
The optical spectrum reshaper performs preliminary conversion of the FM signal to AM signal before transmission, and the spatial filtering performs preliminary cleaning of the signal before it enters the optical fiber. These preliminary actions prevent distortion accumulation during propagation, enabling extended reach while maintaining reliability.
3Length of moving object
If spatial filtering with lateral offset of optical fiber axis is implemented, then the transmission distance is extended beyond 250 km, but the alignment complexity increases
Solution Approach 1:
The invention applies local quality by creating a controlled asymmetry in the optical path. The optical fiber is deliberately positioned with its central axis laterally offset from the OSR and focusing lens axes, creating a specific spatial filtering condition at the fiber input. This localized geometric adjustment enables extended transmission distance through spatial filtering while maintaining overall system simplicity.
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 configuration effectively extends the reach of chirp-managed laser transmitters beyond 250 km in standard single mode fiber at 10 Gb/s and 1550 nm, improving the bit error rate and optical power sensitivity while maintaining acceptable dispersion penalty.
Implementation Method 1
an optical spectrum reshaper (OSR) which uses the frequency modulation to increase the amplitude modulated signal and partially compensate for dispersion in the transmission fiber
Implementation Method 2
a focusing lens adapted to receive the amplitude modulated optical beam from the OSR and focus the same
Implementation Method 3
the OSR has a central axis, the focusing lens has a central axis, and the optical fiber has a central axis, with the central axis of the optical fiber being laterally offset from at least one of the central axis of the OSR and the central axis of the focusing lens so as to effect spatial filtering
Data Source
AI summary
An optical system comprising a frequency modulated laser source adapted to produce a frequency modulated optical beam, an optical spectrum reshaper (OSR) adapted to receive the frequency modulated optical beam from the laser source and convert it into an amplitude modulated optical beam, a focusing lens adapted to receive the amplitude modulated optical beam from the OSR and focus the same, and an optical fiber adapted to receive the amplitude modulated optical beam from the focusing lens and transmit an optical signal;characterized in that the OSR has a central axis, the focusing lens has a central axis, and the optical fiber has a central axis, with the central axis of the optical fiber being laterally offset from at least one of the central axis of the OSR and the central axis of the focusing lens so as to effect spatial filtering and thereby generate the desired optical transmission characteristics for the resulting optical signal in the optical fiber.