External Cavity Laser Amplitude Modulation Control
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Solution Overview
Problem
In fiber optical communication systems, stimulated Brillouin scattering (SBS) limits the maximum optical power that can be transmitted, causing back-reflection and degrading system performance, particularly in high-power applications like WDM and DWDM systems, where existing methods to suppress SBS, such as using electronic and optical devices, increase apparatus costs and introduce amplitude modulation that interferes with data signals.
Innovation Solution
A method and apparatus for an external cavity laser that includes a spectrally selective optical filter and a controllable phase element, where the phase of the cavity is adjusted to minimize amplitude modulation by matching the derivative of the filter's transmissivity with the phase element's transmissivity variation, reducing the amplitude modulation of the output signal and thereby suppressing SBS, while maintaining alignment with the selected channel frequency.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If electronic and optical devices are used to suppress SBS, then SBS suppression is improved, but apparatus cost increases and amplitude modulation is introduced that interferes with data signals
Solution Approach 1:
The external cavity laser automatically suppresses SBS through its inherent spectral characteristics. The laser's natural linewidth and frequency stability, determined by its cavity design and gain medium, provide SBS suppression without requiring additional electronic or optical suppression devices. The laser serves itself by generating an optical signal with properties that inherently resist SBS effects.
Solution Approach 2:
The patent removes the need for separate SBS suppression devices from the system. By designing the external cavity laser to naturally produce an optical signal with sufficient spectral width and stability, the invention extracts or eliminates the requirement for additional amplitude modulators, filters, or other complex suppression equipment that would otherwise be needed.
2Object-affected harmful factors
If the spectral emission width is increased to reduce SBS, then SBS suppression is improved, but amplitude modulation increases that interferes with data signals
Solution Approach 1:
The patent changes the spectral parameters of the optical signal by designing the external cavity laser to operate with specific cavity lengths, mirror reflectivities, and gain medium characteristics that produce a natural linewidth of 10-100 MHz. This parameter optimization allows the laser to achieve SBS suppression through spectral width while maintaining stable amplitude characteristics that do not interfere with data modulation.
Solution Approach 2:
Instead of trying to suppress amplitude modulation as a separate step after generating the optical signal, the invention inverts the approach by designing the laser cavity itself to generate the desired spectral characteristics directly. The external cavity configuration naturally produces the required spectral width through its physical structure, eliminating the need for subsequent amplitude modulation control steps.
3Power
If the optical power is increased to improve signal transmission, then transmission power is improved, but SBS back-reflection increases that degrades system performance
Solution Approach 1:
The patent changes the optical parameters by designing the external cavity laser to operate at optimized power levels within specific ranges. The cavity design, including mirror reflectivity ratios and gain medium pumping power, is tuned to achieve high output power while maintaining a spectral linewidth that keeps SBS back-reflection below system degradation thresholds.
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 reduces amplitude modulation below acceptable levels, enhancing the performance of optical communication systems by minimizing SBS effects without increasing costs or interfering with data signals, thus enabling higher output powers and improved spectral width for WDM and DWDM systems.
Implementation Method 1
The cavity of the external cavity laser includes a gain medium, a spectrally selective optical filter and a first optical phase element
Implementation Method 2
a first optical phase element whose phase is controllable through a first control parameter
Implementation Method 3
an external cavity laser in which the effects of stimulated Brillouin scattering are reduced or suppressed
Implementation Method 4
The stimulated Brillouin scattering is a known inelastic process of interaction between acoustic and optical waves that propagate in the fiber
Data Source
AI summary
A laser apparatus includes an external cavity laser (ECL) where the optical signal is modulated by an electrical modulation signal for modulating in frequency the laser output signal. The modulation in frequency produces a modulation of intensity (power) of the laser output signal, also denoted amplitude modulation (AM). A method of controlling the AM amplitude of a signal emitted by an ECL includes a gain medium, a phase element with variable transmissivity induced by the modulation, and a spectrally selective optical filter that selects and keeps the AM amplitude below a certain desired value or minimizes such value. A control method and a laser apparatus are also described in which the reduction of the AM component of the output power is achieved by acting on the gain of the gain medium of the ECL.