Tunable Laser Assembly Stabilizing Wavelength With Etalon
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Solution Overview
Problem
Tunable semiconductor lasers face challenges in maintaining stable wavelength and bandwidth over time due to environmental and aging effects, requiring accurate monitoring and control to ensure consistent performance in applications like optical coherence tomography, while existing methods like fiber Bragg gratings are not stable over temperature and introduce artifacts.
Innovation Solution
A tunable laser assembly with a VCSEL chip, semiconductor optical amplifier, wavelength monitoring optical elements, photodetectors, and an optical isolator mounted on a common baseplate, using a closed-loop controller to stabilize the center wavelength and bandwidth, and optical isolation to prevent reflections that could degrade performance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a fiber Bragg grating is used to monitor and control the center wavelength, then the wavelength reference is provided, but the FBG drifts with temperature at approximately 10 pm/°C rate which is not desirable for a stable wavelength reference
Solution Approach 1:
The patent uses an etalon with a fixed free spectral range that provides temperature-insensitive wavelength references. By changing from an FBG whose wavelength shifts with temperature to an etalon whose transmission peaks remain stable over temperature, the system achieves a stable wavelength reference that does not drift with environmental conditions.
Solution Approach 2:
The patent introduces an intermediary etalon between the VCSEL and the output optics to provide wavelength monitoring. This etalon acts as a mediator that converts the wavelength information into intensity modulation that can be detected by photodetectors, while itself remaining stable over temperature.
2Stability of the object's composition
If the VCSEL cavity size is kept small for single-mode operation, then mode-hop-free tuning is achieved, but the output power is limited to below 50 mW which is insufficient for high-speed applications
Solution Approach 1:
The patent segments the laser system into two functional parts: a small VCSEL cavity that maintains single-mode operation and provides wavelength tuning capability, and a separate semiconductor optical amplifier that boosts the power. This segmentation allows each component to optimize its function without compromise.
Solution Approach 2:
The patent merges the VCSEL and semiconductor optical amplifier into a single integrated package with shared mounting substrate and thermal management. This combining allows the small VCSEL to benefit from the power amplification of the SOA while maintaining the simplicity and single-mode operation characteristics of the VCSEL.
3Speed
If the laser sweep rate is increased to achieve high-speed imaging, then the imaging speed is improved, but the wavelength characterization becomes more difficult and requires more complex monitoring systems
Solution Approach 1:
The patent replaces complex mechanical wavelength monitoring systems with a simplified optical-intensity-based monitoring using etalons and photodetectors. This substitution of mechanical/wavelength-domain measurement with optical-intensity-domain measurement enables high-speed wavelength tracking that keeps pace with fast laser sweeps.
Solution Approach 2:
The patent implements feedback control where photodetectors monitor the etalon transmission peaks and provide error signals to a control system that adjusts the VCSEL tuning to maintain accurate wavelength tracking. This feedback loop enables real-time wavelength characterization even at high sweep rates.
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 provides a highly stable and cost-effective tunable laser assembly that maintains consistent spectral properties over the operating environment and lifetime, reducing system artifacts and enabling lower-cost, high-performance optical imaging and sensing applications.
Implementation Method 1
at least one optical isolator to prevent reflections from traveling backward through the semiconductor optical amplifier
Implementation Method 2
a beam splitter to provide two optical paths
Implementation Method 3
at least one wavelength monitoring optical element to generate a signal that is used to monitor and control at least one of an absolute wavelength and an optical bandwidth
Data Source
AI summary
A tunable laser assembly housed in a single enclosure and a method of control is described that provides high-speed monitoring and control of the spectral properties of widely tunable lasers, such as MEMS-tunable VCSELs, with an optical configuration that does not introduce perturbations into the swept-source laser output spectrum that would cause artifacts in imaging applications such as optical coherence tomography (OCT).