Ultrashort Pulse Fiber Bragg Grating for High Temperature Stability
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Solution Overview
Problem
Existing Fiber Bragg grating sensors face limitations in thermal stability, reflectivity, and scattering loss, particularly at high temperatures, which hinders their ability to be multiplexed along a single optical waveguide effectively.
Innovation Solution
A method involving the use of ultrashort pulse duration lasers with intensities above the threshold for Type II grating formation, combined with thermal post-processing, to create a Type II grating structure that is stable up to the glass transition temperature of the fiber, with low scattering loss and high reflectivity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If UV photo-inscribed Type I gratings are used, then the fiber Bragg grating sensors can be created with standard UV laser beams, but the gratings are removed or annealed at elevated temperatures approaching the glass transition temperature
Solution Approach 1:
The patent changes the fundamental parameter of laser pulse duration from conventional long pulses to ultrashort pulses (femtosecond to picosecond range). This parameter change enables Type II grating formation through multiphoton ionization and plasma-mediated index modification, which are thermally stable unlike Type I gratings. The ultrashort pulse duration prevents thermal diffusion while enabling sufficient energy deposition for permanent index changes that survive high-temperature annealing.
Solution Approach 2:
The patent employs periodic action through the use of pulsed laser irradiation with specific pulse durations and repetition rates. The periodic pulsing allows controlled energy deposition that accumulates to form the grating structure through multiphoton processes, while the intervals between pulses allow thermal relaxation. This periodic action enables formation of thermally stable Type II gratings that maintain their index modulation at elevated temperatures.
2Reliability
If intense UV beams with high fluence are used to create permanent photoretractive index changes, then the induced index change is more robust and temperature-stable, but the Bragg gratings have relatively low refractive index modulations and are mechanically weak
Solution Approach 1:
The patent optimizes the laser pulse duration parameter to the ultrashort range (femtosecond to picosecond), which fundamentally changes the interaction mechanism from conventional linear absorption to multiphoton ionization and plasma formation. This parameter change enables the creation of Type II gratings with both high thermal stability and adequate mechanical strength, as the index modification arises from permanent structural changes in the glass matrix rather than temporary color center formation.
3Use of energy by moving object
If long laser pulse durations are used, then energy can be transferred to the surrounding lattice, but thermal diffusion removes the energy resulting in damage propagation and high scattering loss
Solution Approach 1:
The patent applies the skipping principle by using ultrashort laser pulses that deliver energy to the glass matrix faster than thermal diffusion can remove it. The pulse duration is so short (femtosecond to picosecond) that the energy deposition occurs before heat can diffuse away, creating localized plasma and multiphoton ionization zones. This rushing through of the energy transfer process prevents thermal diffusion from causing damage propagation and minimizes scattering loss.
Solution Approach 2:
The patent uses periodic pulsed laser irradiation where each pulse is ultrashort in duration. The periodic nature allows the material to cool between pulses while the cumulative effect of multiple pulses builds up the permanent index modulation. This periodic action with ultrashort pulses prevents thermal accumulation that would lead to damage, while still achieving sufficient energy deposition for grating formation.
4Ease of manufacture
If conventional UV laser beams are used to inscribe gratings, then the process is simple and straightforward, but the gratings exhibit high scattering loss and cannot be effectively multiplexed along a single optical waveguide
Solution Approach 1:
The patent fundamentally changes the laser pulse duration parameter from conventional continuous or long-pulse operation to ultrashort pulse operation (femtosecond to picosecond). This parameter change modifies the energy deposition mechanism to multiphoton ionization and plasma formation, which creates thermally stable Type II gratings with significantly reduced scattering loss. The simplified process requires only standard ultrashort pulse lasers and phase masks, making it as easy as conventional methods but with superior performance.
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 approach results in high temperature stable Fiber Bragg grating sensors with low insertion loss and high reflectivity, enabling effective multiplexing along a single optical waveguide without the need for hydrogen loading or specialty fibers.
Implementation Method 1
the formation of thermally stable Bragg gratings with very high index modulations... formed as a result of traversing an intensity threshold... of multiple pulse infrared (fs-IR) laser... The threshold nature of the process also makes it more difficult to tailor the induced index profile
Implementation Method 2
multiphoton absorption and multiphoton ionization that results in plasma formation and the possible creation of microvoids
Implementation Method 3
heating the optical waveguide to a temperature and for a duration sufficient to substantially remove a non-permanent grating formed in the optical waveguide by the irradiating step
Implementation Method 4
providing a diffractive optical element which, when exposed to the ultrashort laser pulse, creates an interference pattern on the optical waveguide
Implementation Method 5
creates an interference pattern on the optical waveguide, wherein the irradiation step comprises irradiating a surface of the diffractive optical element
Implementation Method 6
Each FBG sensor has a characteristic retro-reflective Bragg resonance or Bragg resonance wavelength, which is dependent upon the periodicity of the grating photo-inscribed within the fiber
Implementation Method 7
The techniques taught by Glenn and Hill result in gratings that are typically referred to as Type I gratings... spatially modulated UV beam can be created by using a two-beam interference technique
Data Source
AI summary
Provided is an optical waveguide with an inscribed Bragg grating, where the Bragg grating is stable at high temperature, has low scattering loss and high reflectivity. Also provided is a method for inscribing a Bragg grating in an optical waveguide, the method comprising irradiating the optical waveguide with electromagnetic radiation from an ultrashort pulse duration laser of sufficient intensity to cause a permanent change in an index of refraction within a core of the optical waveguide, where the irradiating step is terminated prior to erasure of a Bragg resonance, and heating the optical waveguide to a temperature and for a duration sufficient to substantially remove a non-permanent grating formed in the optical waveguide by the irradiating step.