Abstract
A review of recent progress in the use of infrared femtosecond lasers to fabricate optical fiber sensors that incorporate fiber Bragg gratings (FBG) and random fiber gratings (RFG) is presented. The important advancements in femtosecond laser writing based on the phase mask technique now allow through-the-coating (TTC) fabrication of Bragg gratings in ultra-thin fiber filaments, tilted fiber Bragg gratings, and 1000 °C-resistant fiber Bragg gratings with very strong cladding modes. As an example, through-the-coating femtosecond laser writing is used to manufacture distributed fiber Bragg grating sensor arrays for oil pipeline leak detection. The plane-by-plane femtosecond laser writing technique used for the inscription of random fiber gratings is also reviewed and novel applications of the resultant devices in distributed temperature sensing, fiber lasers and fiber laser sensors are discussed.
Highlights
High-power near infrared (IR) ultrafast laser systems, such as femtosecond regeneratively amplified Ti:sapphire lasers, proved themselves to be effective tools for machining of optical fibers for sensing applications [1]
The PbP approach involves the focusing of single fs laser pulses into the core region of the fiber using a high numerical aperture microscope objective
The second approach to fabricate random fiber gratings (RFG) requires the introduction of random changes to the laser repetition rate while the air-bearing stage moves at a constant speed and the piezo actuator is stationary
Summary
High-power near infrared (IR) ultrafast laser systems, such as femtosecond (fs) regeneratively amplified Ti:sapphire lasers, proved themselves to be effective tools for machining of optical fibers for sensing applications [1]. Focused beam intensities of over 1013 W/cm are realized and are sufficient to initiate nonlinear light absorption in the optical fiber glass during the laser pulse [2,3] This highly localized energy deposition produces an electron plasma within the focal volume which after several picoseconds transfers its energy to the bulk material. FBGs were manufactured by exposing the germaniumdoped (Ge-doped) core of silica (SiO2) telecommunications fiber to periodically modulated high intensity ultraviolet (UV) laser radiation. In such cases, the induced index change and modulation pattern in the core of the fiber results from color center formation [13] and localized compaction of the glass matrix [14]. All of the exotic Bragg grating structures that have been fabricated with UV laser systems in photosensitive telecommunications fibers, such as chirped gratings, phase shifted gratings, apodized gratings, gratings with suppressed cladding modes, and tilted or blazed gratings, can be fabricated with fs lasers using similar processes
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