Optical Device Narrow Linewidth Laser Feedback
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Solution Overview
Problem
Conventional semiconductor-based narrow linewidth lasers, such as DFB and DBR lasers, face limitations in achieving very narrow linewidths due to grating constraints, and electro-optical modulation techniques require large cavities and additional electronics, which are not always feasible.
Innovation Solution
An optical device employing an all-optical feedback mechanism using a passive optical cavity and an unbalanced optical coupler to filter and inject a seed signal back into the laser source, effectively narrowing the linewidth through preferential stimulated emission, with the seed signal being 20 dB or more less powerful than the source signal.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If conventional DFB or DBR laser diodes are used, then the laser can operate at specific wavelengths, but the linewidth is limited by the grating structure
Solution Approach 1:
An external optical cavity is introduced as an intermediary resonant structure to filter and define the laser linewidth. The cavity acts as a mediator between the laser diode and the output, providing precise frequency selection through its resonant modes while the laser diode continues to operate with its standard grating structure for wavelength generation
Solution Approach 2:
A feedback mechanism is implemented where a portion of the laser output is coupled back into the laser diode through an unbalanced optical coupler. This feedback signal, filtered by the external cavity, reinforces the desired frequency and suppresses unwanted modes, thereby narrowing the linewidth without requiring complex internal grating designs
2Manufacturing precision
If electro-optical modulation techniques are used to achieve narrower linewidth, then the linewidth can be reduced, but large cavities and additional high frequency modulation electronics are required
Solution Approach 1:
The patent replaces the electro-optical modulation approach with an all-optical feedback mechanism. Instead of using high-frequency electrical modulation signals and large modulator cavities, the invention uses optical field feedback through an unbalanced optical coupler and external cavity to achieve linewidth narrowing, eliminating the need for complex modulation electronics
Solution Approach 2:
The linewidth narrowing function is extracted from the laser diode itself and implemented in a separate external optical cavity. This allows the laser diode to maintain its simple structure while the external cavity provides the precise frequency filtering, separating the wavelength generation function from the linewidth control function
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 device achieves a significantly narrower linewidth, typically less than 600 kHz, allowing for precise tuning and improved performance in applications like sensors and communications without the need for complex electronics.
Implementation Method 1
The optical cavity is a resonant structure that acts to filter the source signal to yield the seed signal
Implementation Method 2
the unbalanced optical coupler is arranged to couple a source signal from the laser source into the optical cavity, and to couple a seed signal received back from the optical cavity into the laser source
Implementation Method 3
The seed signal is used as a feedback mechanism, and is injected directly into the lasing cavity. Providing a narrower linewidth seed signal within the cavity acts to promote preferential stimulated emission within the narrower linewidth
Data Source
Figure 1~2
AI summary
A narrow linewidth laser in which an all-optical feedback line-up is used to improve the linewidth from a conventional laser source, such as a laser diode. The feedback line-up comprises an optical device having a controllable unbalanced optical coupler which may be an optical isolator (110) arranged on a cavity input path to couple a source signal from the laser source into the optical cavity, and to couple a seed signal received back from the optical cavity (124) into the laser source which may be a DFB laser diode (104). The seed signal has a lower power than the source signal. The unbalanced optical coupler may be an optical isolator arranged to couple the seed signal into the laser source at a power level selected to promote preferential stimulated emission within a narrower linewidth. By controlling the power of seed signal such that only a small portion thereof influences the lasing cavity, the narrowing effect of the preferential stimulated emission can be enhanced and ASE may be suppressed ,ore efficiently. The beam path between the LD (104) and the optical cavity (124) may further comprise two lenses (108,112) and two beam steering mirrors with one fixed mirror (116) and an adjustable mirror (118). The optical cavity may be a folded Fabry-Perot cavity with a folding mirror (128) and two end mirrors (120,122).