Optical Transmitter SBS Suppression via Laser Dithering
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Solution Overview
Problem
Current optical communication systems face limitations in launch power due to stimulated Brillouin scattering (SBS), which reduces signal quality and performance, especially at higher power levels required for long-distance transmissions, as the SBS threshold is typically around 6-7 dBm, restricting the maximum power that can be launched into optical fibers.
Innovation Solution
The technique involves direct dithering of the laser with a high frequency signal at least twice the highest signal frequency, combined with external modulation and phase/gain adjustments to spread the optical power across a wider spectral range, suppressing SBS and increasing the SBS threshold, thereby allowing higher launch powers without introducing significant distortions or noise.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Power
If high fiber launch power is used to achieve better signal to noise ratio for long distance transmissions, then transmission performance is improved, but stimulated Brillouin scattering occurs which reduces transmitted power below intended level
Solution Approach 1:
The patent applies periodic dithering modulation to the optical carrier signal, varying the frequency periodically to spread the spectral power distribution. This periodic frequency variation prevents the buildup of coherent acoustic phonons that cause SBS, allowing higher launch powers without triggering the scattering effect.
Solution Approach 2:
The patent dynamically modulates the optical carrier frequency using dithering signals, transforming the static narrow-linewidth spectrum into a dynamic broadened spectrum. This dynamic frequency variation keeps the instantaneous power spectral density below the SBS threshold while maintaining high average launch power.
2Power
If optical power input to fiber is increased to improve transmission performance, then signal to noise ratio improves, but SBS reflects portion of light and reduces transmitted power level
Solution Approach 1:
Periodic dithering modulation is applied to the optical carrier, creating time-varying frequency shifts that spread the optical power spectrally. This periodic action ensures that at any instant, the power within the narrow SBS gain bandwidth remains below the threshold, preventing energy reflection while maintaining high input power.
Solution Approach 2:
The patent transitions from a single-frequency narrow bandwidth approach to a multi-frequency broadened spectrum approach by applying dithering. This dimensional expansion in the frequency domain distributes the optical power across a wider range, reducing the power density within the SBS gain bandwidth and minimizing reflected energy.
3Power
If SBS threshold power level is maintained at conventional levels, then system simplicity is preserved, but maximum launch power is limited to 6-7 dBm for standard single mode fiber
Solution Approach 1:
The patent implements periodic dithering modulation as a relatively simple technique compared to other SBS suppression methods. By applying a periodic frequency modulation signal to the laser driver, the system achieves SBS suppression and increased launch power capability without requiring complex optical components or system architecture changes.
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 effectively suppresses SBS, enhances signal quality, and increases the maximum launch power into optical fibers, improving transmission performance and distance without degrading the system further, thus overcoming the limitations imposed by the SBS threshold.
Implementation Method 1
stimulated Brillouin scattering (SBS) suppression in an optical transmission system... a phenomenon known as Stimulated Brillouin Scattering (SBS) may occur... If the power within the SBS gain line width is kept below a SBS threshold power level, the SBS should remain adequately suppressed
Data Source
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AI summary
Techniques for transmitting an optical signal through optical fiber with an improved stimulated Brillouin scattering (SBS) suppression and an improved transmitters signal to noise ratio (SNR) include externally modulating a light beam emitted from a light source with a high frequency signal. The light beam is also modulated externally with an RF information-carrying signal. The high frequency signal is at least twice a highest frequency of the RF signal. The high frequency signal modulating the light source can be split, providing a portion of the split signal to a phase and gain control circuit for adjusting a phase/gain. The output of phase and gain control circuit can be applied to the external modulator to eliminate intensity modulation for SBS suppression improvement. The optical transmitters SNR is further improved by cancelling a beat between SBS suppression modulation tone and out of band distortion spectrum of information bearing RF signal.