Fiber Bragg Gratings
How Fiber Bragg Gratings Work
Fiber Bragg Gratings (FBGs) are optical components that utilize the properties of light reflection and refractive index modulation to serve as effective sensors and filters. This technology relies on a recurring change in the refractive index within an optical fiber, enabling it to reflect certain light wavelengths while permitting others to pass along the fiber.
Underlying technology
The core manufacturing operation of FBGs involves inscribing a pattern of refractive index changes along an optical fiber segment, using techniques such as ultraviolet (UV) light exposure. When light travels through a fiber, only those wavelengths that match the Bragg condition are reflected. The reflected wavelength known as Bragg wavelength changes in response to external factors such as temperature and strain, making FBGs excellent for sensing applications.
Key components and principles
An optical fiber acts as a light conduit, while a grating selectively reflects particular wavelengths due to its alternating refractive index. As light travels through the fiber and encounters these variations, certain wavelengths experience structural interferences causing them to be reflected back along the fiber.
This reflective process turns FBGs into precision optical filters, isolating and reflecting specific wavelengths while permitting others to continue their path. Moreover, the sensitivity of Bragg wavelengths to external factors allows FBGs to serve as precise detectors of temperature, strain, or pressure. Changes in these conditions alter the refractive index which, in turn, alters the reflected wavelength.
FBG advantages
Fiber Bragg Gratings (FBGs) provide several benefits making them a preferable option for different sensing applications.
- High Sensitivity and Accuracy
FBGs are well-known for their outstanding sensitivity to temperature and strain variations, allowing for highly reliable measurements. The light wavelength reflected by an FBG alters according to these external changes, providing vital data to observe changes in various environmental factors. - Resistance to Electromagnetic Interference
FBGs can withstand electromagnetic interference. In contrast to conventional electrical sensors, FBGs transmit optical signals through fibers protecting them from electromagnetic noise that may distort measurements. Thus, FBGs are particularly suitable for environments with high electromagnetic radiation. - Corrosion Resistance and Durability
FBG components are highly resistant to corrosion, so they are of great use in tough surroundings where traditional sensors may fail. This exceptional durability ensures that they can maintain stability and reliability even in chemically aggressive settings, which ultimately decreases maintenance costs. - Lightweight and Compact Design
FBGs are designed to be both light and compact, so they are easily incorporated into a range of applications. Their reduced dimensions are particularly advantageous for structures and locations of limited space. - Multi-Point Measurement Capability
FBGs can be densely integrated within one optical fiber, enabling measurements at various points along its length. This capability guarantees the thorough collection of data from different sites without multiple cables, simplifying the installation process and cutting down on expenses. - Wide Range of Measurement
FBGs can monitor a diverse set of variables, including temperature, pressure, strain, and more. This diversity of monitoring options suggests applying them in many sectors such as aerospace, civil engineering, and biomedical industry.
Specifications Overview
Optromix manufactures various types of Fiber Bragg Gratings. Our specialists are able to produce a special fiber product considering all your needs and requirements
| Central Wavelength (CW), nm | 600 - 2300 |
| Wavelength Tolerance, nm | < 0.1 |
| Reflectivity, % | 0.1 … 99.9 |
| Bandwidth (FWHM), nm | 0.03 - 3 |
| Sidelobe Suppression Ratio (SLSR), dB | > 20 |
| FBG Length, mm | 0.1 - 50 |
| Coating | Acrylate; Polyimide; No coating |
| Temperature range (acrylate), °C | -20 … +85 |
| Temperature range (polyimide), °C | -300 .. +350 |
| Connectors | FC/PC; FC/APC; any other |
Types Of Fiber Bragg Gratings
- FBG writing into multicore fibers means inscribing Fiber Bragg Gratings within fibers with multiple closely spaced cores enhancing their sensing capabilities. Each core can host an independent FBG sensor, allowing for simultaneous measurements of various parameters through a single optical fiber.
- Chirped Fiber Bragg Gratings have a refractive index pattern that gradually changes along the fiber and produces a wide reflection spectrum capable of covering various wavelengths. They effectively control chromatic dispersion in optical communication systems and minimize signal distortions.
- Apodized Fiber Bragg Gratings are designed to produce a single, sharp reflection peak without side lobes. Therefore, they can be ideally utilized in lasers and filters, where precise, single-peak reflections are required for optimal performance.
- Pi-phase-shifted Fiber Bragg Gratings (π-phase-shifted FBGs) are a specific variation designed with phase shifts at designated points within their structure. Their key benefit lies in boosting sensitivity and narrowing reflection bandwidth.
- DFB (Distributed Feedback) lasers Fiber Bragg Gratings (FBGs) inscription into non-photosensitive fibers involves creating FBGs using a DFB laser. This process is achieved through high-intensity laser techniques such as focused ion beams or pulsed laser writing to induce refractive index changes in the non-photosensitive material.
- Long-period Fiber Gratings (LPFGs) are advanced optical fibers with the periodic modulation of the refractive index along their length to couple light from the core with the cladding. Unlike regular ones, LPFGs have a more extended distance between the refractive index changes. They are designed to selectively block or diminish specific wavelengths, producing a transmission spectrum marked by prominent resonance dips.
- Tilted Fiber Bragg Gratings (TFBGs) feature grating planes set at the angle of the optical fiber axis. This unique alignment allows transferring light between the core and cladding modes, significantly enhancing their sensing abilities.
- Customized Fiber Bragg Gratings. FBGs writing into special optical fibers (Panda, active, PCF, etc.)
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Three Methods of FBGs Inscription
Currently, there are three known methods of FBGs inscription. Optromix is able to write fiber Bragg gratings by all 3 techniques, using 4 available laser stands.
Interferometric method. When fiber core is illuminated with a fringe pattern created by a phase mask (in this case FBG period is defined by a phase mask period)
Direct point-by-point method. When each FBG “pitch” is formed as a result of the nonlinear fs laser pulse absorption (in this case FBG period is defined by laser pulse repetition rate and by velocity of a fiber translation along its axis)
Continuous core-scanning method. When fiber position is modulated in transverse plane (in this case FBG period is defined by modulation frequency and by velocity of a fiber translation along its axis)