External Cavity Laser Mode Hopping Reduction
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Solution Overview
Problem
Lasers with broad bandwidth gain chips in external cavities experience unpredictable mode hopping due to insufficient grating resolution and temperature variations, leading to instability in laser beam pointing and wavelength tuning, which is critical for precise measurements.
Innovation Solution
An external cavity laser system with a grating and actuator driver that dithers the grating angle about an equilibrium position to excite multiple adjacent laser modes, using a servo system to maintain peak diffraction alignment with the desired laser cavity mode, and employing a long focal length lens with a large numerical aperture to improve selectivity and stability.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If the grating resolution is increased to prevent mode hopping, then wavelength selectivity is improved, but device complexity and cost increase
Solution Approach 1:
The patent applies dynamic dithering of the grating angle at frequencies above the laser pulse repetition rate. This dynamic modulation allows the system to maintain lower grating resolution while achieving effective wavelength selectivity through temporal averaging, resolving the contradiction between selectivity and device complexity
Solution Approach 2:
The patent implements periodic dithering of the grating angle at specific frequencies (above the pulse repetition rate). This periodic action creates a time-averaged effect that enhances wavelength selectivity without requiring higher static grating resolution, thus reducing device complexity while maintaining measurement precision
2Stability of the object's composition
If the grating angle is fixed to maintain stable beam pointing, then beam pointing stability is improved, but wavelength tuning repeatability deteriorates due to mode hopping
Solution Approach 1:
The patent dynamically modulates the grating angle through dithering at frequencies above the pulse repetition rate. This dynamic approach prevents mode hopping by continuously modulating the cavity conditions, ensuring reliable wavelength tuning repeatability while maintaining beam pointing stability through controlled modulation rather than fixed positioning
Solution Approach 2:
The patent employs a feedback mechanism where the dithered grating angle modulation is synchronized with the pulse repetition rate. This feedback control ensures that the laser operates at the desired wavelength by actively compensating for mode hopping tendencies, thereby improving wavelength tuning repeatability while maintaining beam pointing stability
3Reliability
If dithering amplitude is increased to excite multiple modes for averaging, then mode hopping is reduced, but beam pointing stability deteriorates
Solution Approach 1:
The patent optimizes the dithering parameters (amplitude and frequency) to achieve effective mode averaging while minimizing beam pointing instability. By carefully selecting the dithering amplitude to excite multiple modes and setting the frequency above the pulse repetition rate, the system achieves mode hopping reduction through parameter optimization rather than excessive modulation
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 provides reproducible and stable laser output by averaging over multiple modes, reducing mode hopping and improving wavelength tuning repeatability and beam pointing stability, essential for precise measurements.
Implementation Method 1
a grating diffracts light leaving the gain chip in a band of wavelengths back to the gain chip, the alignment of the grating is characterized by an angle relative to light leaving the gain chip
Implementation Method 2
The actuator driver causes the angle to be dithered about an equilibrium angle in a motion characterized by an amplitude and average frequency. The amplitude is sufficient to excite a first plurality of adjacent laser modes of the gain chip.
Implementation Method 3
Part of the diffracted light is amplified by the gain chip
Data Source
AI summary
An external cavity laser and method for operating same are disclosed. The external cavity laser includes a gain chip, a grating, an actuator and actuator driver. The grating diffracts light leaving the gain chip in a band of wavelengths back to the gain chip, the grating is characterized by an angle relative to light leaving the gain chip and distance from the gain chip, the angle and distance is controlled by the actuator. Part of the diffracted light is amplified by the gain chip. The actuator driver causes the angle to be dithered about an equilibrium angle in a motion characterized by an amplitude and average frequency. The amplitude of the dithering motion is chosen to either excite a plurality of adjacent laser modes of the gain chip or provide a servo signal for maintaining the grating angle at a target value.