External Cavity Diode Laser with VBG and PBS
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Solution Overview
Problem
Diode laser arrangements with optical feedback from volume Bragg gratings suffer from significant side mode energy outside the spectral band of interest, and optimizing the reflectivity of these gratings is challenging, leading to suboptimal spectral purity and increased costs due to the need for additional filtering and temperature control.
Innovation Solution
Incorporating a polarizing beam splitter and a polarization-modifying element, such as a wave-plate, between the laser element and the volume Bragg grating to alter the polarization state of light reflected from the VBG, allowing the PBS to divide the light into feedback and output beams, thereby eliminating side modes and optimizing optical feedback without external filtering.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If a volume Bragg grating is used to provide optical feedback to the laser element, then the lasing threshold for desired modes is lowered and spectral narrowing is achieved, but significant side mode energy remains outside the spectral band of interest
Solution Approach 1:
The optical feedback path is segmented into two separate paths: a main beam path that carries the majority of laser radiation power, and an additional feedback beam path that provides optical feedback to the laser element. This segmentation allows the frequency-selective element to be positioned externally where it can selectively feedback desired wavelengths while suppressing side modes, thereby improving spectral purity without being overwhelmed by the main beam energy
Solution Approach 2:
A frequency-selective element (volume Bragg grating) is introduced as an intermediary component in the additional feedback beam path. This intermediary selectively reflects desired wavelengths back to the laser element while allowing side modes to pass through or be suppressed, thereby mediating between the laser element's broadband emission and the desired narrowband output
2Productivity
If the VBG is positioned in the main beam path to provide optical feedback, then feedback efficiency is maximized, but thermal stress on the frequency-selective element increases
Solution Approach 1:
The beam path is divided into a main beam path carrying high power and an additional feedback beam path carrying lower power. The frequency-selective element is positioned in the additional feedback path, exposing it only to a fraction of the total laser power. This segmentation reduces thermal stress on the VBG while maintaining feedback efficiency through the separated paths
Solution Approach 2:
Instead of directing the full main beam through the frequency-selective element, only a partial amount of light (sufficient for feedback) is routed through the additional feedback path. This partial action provides adequate optical feedback to the laser element while avoiding excessive thermal loading on the VBG
3Manufacturing precision
If additional filtering components are added to suppress side modes, then spectral purity is improved, but device complexity and cost increase
Solution Approach 1:
The frequency-selective element performs multiple functions simultaneously: it provides optical feedback to the laser element to establish lasing modes, and it acts as a wavelength selector to suppress side modes in the output. This multi-functionality eliminates the need for separate filtering components, reducing device complexity while maintaining spectral purity
Solution Approach 2:
The feedback function and wavelength selection function are merged into a single frequency-selective element positioned in the additional feedback beam path. This consolidation achieves both spectral narrowing and side mode suppression without requiring multiple separate components, thereby reducing overall device complexity
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 configuration achieves excellent side mode suppression and spectral purity by ensuring the output beam is entirely determined by the VBG properties, eliminating sidebands and allowing for flexible optimization of optical feedback during assembly, reducing the need for additional components and temperature control.
Implementation Method 1
a volume Bragg grating (VBG) for providing optical feedback to the laser element... The VBG is designed to have partial reflectivity for the spectral band of interest, in order for part of the light in that spectral band impinging on the VBG to be reflected towards the laser element as feedback
Implementation Method 2
a polarizing beam splitter, PBS, arranged in the beam path between the laser element and the VBG
Implementation Method 3
a polarization-modifying element arranged in the beam path between the PBS and the VBG element, said polarization-modifying element being a wave-plate configured to modify linearly polarized light into generally elliptically polarized light
Data Source
Figure 1~3
Figure 4~5
Figure 6~7
AI summary
A laser arrangement comprising a laser element; a volume Bragg grating, VBG, for providing optical feedback to the laser element along a beam path; a polarizing beam splitter, PBS, arranged in the beam path between the laser element and the VBG; and a polarization-modifying element arranged in the beam path between the PBS and the VBG element; wherein said polarization- modifying element is structured and arranged to alter a polarization state of light reflected from said VBG such that said PBS is operative to divide said light reflected from said VBG into a first portion that provides the optical feedback to the laser element and a second portion that provides an output beam from the laser arrangement. Embodiments of the present invention are particularly useful for low wavenumber Raman spectroscopy.