Optical Transmitter Amplitude Control for RIN Suppression
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Solution Overview
Problem
In optical transmitters, particularly in Radio over Fiber systems, relative intensity noise (RIN) deteriorates the signal-to-noise ratio (SNR) and spurious free dynamic range (SFDR) due to varying amplitudes of drive signals in modulation schemes like OFDM, M-QAM, and W-CDMA, which are not effectively addressed by increasing laser injection current alone.
Innovation Solution
An optical transmitter with an amplitude control circuit to generate a constant amplitude electric signal, an E/O circuit for direct modulation, and an optical power control circuit based on detected amplitude modulation components to suppress RIN and maintain signal quality.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the injection current of the laser is increased to suppress relative intensity noise, then the signal-to-noise ratio and spurious free dynamic range are improved, but the device complexity and power consumption increase
Solution Approach 1:
The patent segments the amplitude control function into two parts: an amplitude control circuit that generates a constant amplitude electric signal, and an optical power control circuit that adjusts optical signal power based on detected amplitude modulation components. This segmentation allows RIN suppression without requiring uniformly high laser injection current, thereby reducing device complexity while maintaining SNR performance.
Solution Approach 2:
The patent changes the parameter control strategy from uniformly increasing laser injection current to dynamically adjusting optical signal power based on detected amplitude modulation components. This parameter change allows suppression of relative intensity noise while avoiding the need for continuously high current operation, thus improving SNR without proportionally increasing device complexity and power consumption.
2Reliability
If the injection current of the laser is increased to suppress relative intensity noise, then the signal-to-noise ratio and spurious free dynamic range are improved, but the power consumption increases
Solution Approach 1:
The patent segments the power control approach into an amplitude control circuit for constant amplitude signal generation and an optical power control circuit for dynamic power adjustment based on detected amplitude modulation. This allows SFDR improvement through controlled power management without requiring continuously high laser injection current, thereby reducing overall power consumption while maintaining spurious free dynamic range performance.
Solution Approach 2:
The patent changes from uniformly high power operation to dynamic power adjustment based on detected amplitude modulation components. This allows suppression of relative intensity noise and improvement of SFDR while avoiding continuous high power consumption, as the optical power is adjusted only when needed based on signal conditions.
3Device complexity
If direct modulation is used for simplicity, then the device complexity is reduced, but relative intensity noise is generated due to varying amplitude of drive signals
Solution Approach 1:
The patent applies preliminary action by using an amplitude control circuit to generate a constant amplitude electric signal before the E/O conversion stage. This preliminary amplitude stabilization prevents the generation of relative intensity noise during direct modulation, while maintaining the simplicity of the direct modulation approach itself. The harmful amplitude variations are eliminated before they can cause RIN.
Solution Approach 2:
The patent introduces an amplitude control circuit as an intermediary between the signal source and the E/O conversion stage. This intermediary generates a constant amplitude electric signal that drives the laser, thereby eliminating amplitude variations that would cause relative intensity noise during direct modulation, while preserving the overall simplicity of the direct modulation architecture.
4Reliability
If the amplitude of the drive signal is increased to suppress relative intensity noise, then the signal-to-noise ratio is improved, but the distortion of the modulated optical signal increases
Solution Approach 1:
The patent segments the signal processing into amplitude control (generating constant amplitude signal) and optical power control (adjusting power based on detected amplitude modulation). This segmentation allows SNR improvement through controlled power adjustment without requiring high drive signal amplitudes that would cause distortion, as the power control operates on the optical signal rather than electrically amplifying the drive signal.
Solution Approach 2:
The patent changes the approach from electrically amplifying the drive signal (which causes distortion at high amplitudes) to optically adjusting the signal power based on detected amplitude modulation components. This parameter change allows SNR improvement through controlled optical power adjustment without introducing the distortion that would result from high-amplitude electrical drive signals.
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
The solution effectively suppresses relative intensity noise, improving the SNR and SFDR of optical signals by controlling the amplitude and power of the modulated optical signals, even under varying drive signal conditions.
Implementation Method 1
an E/O (Electrical-to-Optical) circuit configured to generate a modulated optical signal from the constant amplitude electric signal by a direct modulation
Implementation Method 2
a detector configured to detect an amplitude modulation component of the input electric signal
Data Source
AI summary
An optical transmitter includes: an amplitude control circuit, an E/O (Electrical-to-Optical) circuit, a detector and an optical power control circuit. The amplitude control circuit controls an amplitude of an input electric signal to generate a constant amplitude electric signal. The E/O circuit generates a modulated optical signal from the constant amplitude electric signal by a direct modulation. The detector detects an amplitude modulation component of the input electric signal. The optical power control circuit controls a power of the modulated optical signal based on the amplitude modulation component detected by the detector.