Multiplexed Volume Bragg Grating Mode Area Expansion
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Existing optical systems employing multiplexed volume Bragg gratings face limitations in increasing mode area without requiring multiple gratings, leading to constraints on thickness and heating issues in high-power systems, and previous methods for mode conversion lack angular selectivity and are difficult to manufacture.
Innovation Solution
A method using a multiplexed volume Bragg grating system with two volume Bragg gratings, where the angular selectivity of each grating matches the divergence of higher order modes, allowing for reversible conversion between fundamental and higher order spatial modes by diffracting higher order mode beams into a fundamental mode, and vice versa, while maintaining low divergence and angular selectivity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Area of moving object
If multiple volume Bragg gratings are recorded in a single optical element to increase mode area, then the mode area increases by a factor equal to the number of gratings, but the thickness of the optical element increases and heating issues occur in high-power systems
Solution Approach 1:
The patent combines multiple volume Bragg gratings into a single optical element (PTR glass) where multiple holographic elements are recorded within the same photosensitive material. This merging approach allows N gratings to be integrated in one element, increasing mode area by N times while avoiding the need for multiple separate elements that would require greater total thickness and generate more heat.
Solution Approach 2:
The single optical element serves multiple functions by containing multiple VBGs that can selectively diffract different incident beams at different angles along a shared path. This multi-functional design allows the same element to perform mode area expansion, spectral filtering, and angular filtering simultaneously, eliminating the need for multiple separate components.
2Ease of operation
If binary phase plates are used for mode conversion, then phase discontinuities can be corrected to convert higher order modes to fundamental modes, but the elements lack angular selectivity and are difficult to manufacture due to strict slope requirements
Solution Approach 1:
The patent replaces mechanical binary phase plates with a volumetric optical element (PTR glass) where phase modulation is achieved through holographic refractive index modulation. This substitution eliminates the need for precise mechanical fabrication of phase discontinuities with strict slope requirements, as the phase conversion is achieved through the volumetric interference pattern recorded in the photosensitive material.
Solution Approach 2:
The invention changes the physical parameter of phase modulation from surface-level binary phase discontinuities to volumetric refractive index modulation. By recording holographic patterns throughout the volume of the PTR glass, the phase conversion function is achieved through distributed refractive index changes rather than sharp surface discontinuities, making the element easier to manufacture while maintaining mode conversion capability.
3Speed
If transverse mode selection is performed inside the gain medium using angularly selective elements, then beam divergence is improved and mode area is reduced, but the fundamental mode is forced to oscillate within the gain medium
Solution Approach 1:
The patent moves the mode selection function from inside the gain medium to outside the gain medium by placing the VBG in the resonator cavity. This spatial relocation allows the gain medium to support larger mode areas without the constraining effect of intra-gain-medium angular filtering, while the VBG performs angular selectivity in a different spatial location, decoupling the mode area from the gain medium dimensions.
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
Enables increased mode area without the need for multiple gratings, reducing heating and maintaining low beam divergence, and provides both mode selection and conversion within a resonator or passive system, improving beam quality and brightness.
Implementation Method 1
An incident wave that satisfies the Bragg condition will be diffracted at an angle determined by a grating vector and a wave vector of an incident beam
Implementation Method 2
Volume Bragg gratings (VBG) are diffractive optical elements. They are typically holographically recorded by exposing a photo-sensitive material to a two-beam interference pattern, creating a spatial sinusoidal refractive index modulation
Implementation Method 3
They are typically holographically recorded by exposing a photo-sensitive material to a two-beam interference pattern, creating a spatial sinusoidal refractive index modulation
Implementation Method 4
This glass is photosensitive in the near UV spectral region and it is transparent from 350 to 2700 nm
Implementation Method 5
By designing such a holographic element to diffract two incident beams of a specific wavelength, which are incident at different angles, along a shared path, each grating will interact with one another, allowing multiple incident wavefronts to coherently interact
Data Source
AI summary
A method of reversible spatial mode selection and conversion between waveguides and free space is presented using a multiplexed volume Bragg grating (MVBG). The MVBG has an inherent angular selectivity, providing different losses for different transverse modes and converting a higher order mode in waveguide to a single fundamental mode in free space. Using the device in a resonator allows for a pure higher order mode to be guided and amplified in the gain medium, to increase the mode area, to extract accumulated excitation more efficiently, and, therefore, to increase gain of the amplifier. In the same resonator, the device is able to convert the higher order mode to a high brightness Gaussian beam in free space or to a fundamental mode in a waveguide.