Fiber Bragg grating
This is achieved by creating a periodic variation in the refractive index of the fiber core, which generates a wavelength-specific dielectric mirror.
Hence a fiber Bragg grating can be used as an inline optical filter to block certain wavelengths, can be used for sensing applications,[1] or it can be used as wavelength-specific reflector.
In 1989, Gerald Meltz and colleagues demonstrated the much more flexible transverse holographic inscription technique where the laser illumination came from the side of the fiber.
This technique uses the interference pattern of ultraviolet laser light[3] to create the periodic structure of the fiber Bragg grating.
), is given by, where, The term type in this context refers to the underlying photosensitivity mechanism by which grating fringes are produced in the fiber.
Later work showed that the increase in Bragg wavelength began once an initial type I grating had reached peak reflectivity and begun to weaken.
The gratings were formed in germanosilicate fibers with pulses from a frequency doubled XeCl pumped dye laser.
It was shown that initial exposure formed a standard (type I) grating within the fiber which underwent a small red shift before being erased.
Further exposure showed that a grating reformed which underwent a steady blue shift whilst growing in strength.
Archambault et al. showed that it was possible to inscribe gratings of ~100% (>99.8%) reflectance with a single UV pulse in fibers on the draw tower.
The resulting gratings were shown to be stable at temperatures as high as 800 °C (up to 1,000 °C in some cases, and higher with femtosecond laser inscription).
Apodized gratings offer significant improvement in side-lobe suppression while maintaining reflectivity and a narrow bandwidth.
[21] Addressed fiber Bragg structures (AFBS) is an emerging class of FBGs developed in order to simplify interrogation and enhance performance of FBG-based sensors.
The central wavelength of AFBS can be defined without scanning its spectral response, unlike conventional FBGs that are probed by optoelectronic interrogators.
[24] The germanium-doped fiber is photosensitive, which means that the refractive index of the core changes with exposure to UV light.
This method allows for quick and easy changes to the Bragg wavelength, which is directly related to the interference period and a function of the incident angle of the laser light.
The shadow of the photomask then determines the grating structure based on the transmitted intensity of light striking the fiber.
The main difference of this method lies in the interaction mechanisms between infrared laser radiation and dielectric material - multiphoton absorption and tunnel ionization.
As well as reducing associated costs and time, this also enables the mass production of fiber Bragg gratings.
In a tunable demultiplexer or OADM, the Bragg wavelength of the FBG can be tuned by strain applied by a piezoelectric transducer.
They can also be used as transduction elements, converting the output of another sensor, which generates a strain or temperature change from the measurand, for example fiber Bragg grating gas sensors use an absorbent coating, which in the presence of a gas expands generating a strain, which is measurable by the grating.
Specifically, fiber Bragg gratings are finding uses in instrumentation applications such as seismology,[29] pressure sensors for extremely harsh environments, and as downhole sensors in oil and gas wells for measurement of the effects of external pressure, temperature, seismic vibrations and inline flow measurement.
As such they offer a significant advantage over traditional electronic gauges used for these applications in that they are less sensitive to vibration or heat and consequently are far more reliable.
The challenge is to operate these monolithic cavities at the kW CW power level in large mode area (LMA) fibers such as 20/400 (20 μm diameter core and 400 μm diameter inner cladding) without premature failures at the intra-cavity splice points and the gratings.
Once optimized, these monolithic cavities do not need realignment during the life of the device, removing any cleaning and degradation of fiber surface from the maintenance schedule of the laser.
Although the power handling capability of the fiber itself far exceeds this level, and is possibly as high as >30 kW CW, the practical limit is much lower due to component reliability and splice losses.
The matching of active and passive fibers for improved signal integrity requires optimization of the core/clad concentricity, and the MFD through the core diameter and NA, which reduces splice loss.
The easiest method for matching the signal carrying light is to have identical NA and core diameters for each fiber.
It has been shown that matching all of these components provides the best set of fibers to build high power amplifiers and lasers.
This approach accounts for details of the refractive index profile which can be measured easily and with high accuracy on the preform, before it is drawn into fiber.
