Fu-Rong Feng

Type IIa Bragg grating based ultra-short DBR fiber laser with high temperature resistance.

We report on the fabrication of a thermally resistant ultra-short distributed Bragg reflector (DBR) fiber laser based on the photo inscription of two wavelength-matched type IIa gratings in a thin-core Er-doped fiber. With continuous UV exposure, each Bragg reflector initially grows as a type I grating, followed by decay in strength, and then re-grows as a type IIa grating with enhanced thermal resistance. The DBR laser, with an entire length of 13 mm, can stably operate at 600°C with single longitude mode, which provides potential applications in high temperature environments.

Thermally triggered fiber lasers based on secondary-type-In Bragg gratings

The secondary-type-In grating formed in a small-core photosensitivity active fiber is discovered and investigated. Due to the different grating types, the transmission dip of a secondary grating structure chases and integrates with the type-In grating structure as the temperature increases, which strengthens the reflectivity of the grating. By use of these secondary-type-In gratings as Bragg reflectors, a thermally activated distributed Bragg reflector (DBR) fiber laser is proposed, which can be potentially used in high-temperature alarms and sensors.

1-μm-wavelength ytterbium-doped fiber laser based on the third harmonic reflection in secondary-type-In Bragg gratings.

In this Letter, a 1-μm-wavelength fiber laser is proposed through the use of the ytterbium-doped active fiber and the third harmonic Bragg reflectors. Benefiting from the ratio below 0.5 between the core diameter of the fiber and the Talbot length of the UV fringes, enabling the formation of the secondary-type-In grating, the reflector with reflectivity higher than 95% at two-thirds of the designing Bragg wavelength can be fabricated within 2 min. Inherited by the lower temperature and strain sensitivity of the third harmonic grating, the laser has good wavelength stability and could be used as a reliable laser resource. In addition, this Letter can also be expected to provide great flexibility in fiber laser wavelength design.

Secondary Bragg grating based fiber sensors for the application in high temperature environment

As one of the most proven fiber optic sensors, novel fiber Bragg gratings are continually investigated to extend their roles in extreme environments. In this paper, a newly found “secondary Bragg grating” (SBG) is proposed. The presence of SBG occurs in the case of the type-IIa Bragg grating inscribed in small active fibers, where an additional resonance appears at the shorter wavelength. The SBG provides a variety of interesting characteristics, such as the dip integration, high temperature resistance and high reflectivity, showing promising potential in high temperature sensing.

Formation and Applications of the Secondary Fiber Bragg Grating

Being one of the most proven fiber optic devices, the fiber Bragg grating has developed continually to extend its applications, particularly in extreme environments. Accompanying the growth of Type-IIa Bragg gratings in some active fibers, a new resonance appears at the shorter wavelength. This new type of grating was named “secondary Bragg grating” (SBG). This paper describes the formation and applications of the SBGs. The formation of the SBG is attributed to the intracore Talbot-type-fringes as a result of multi-order diffractions of the inscribing beams. The SBG presents a variety of interesting characteristics, including dip merge, high-temperature resistance, distinct temperature response, and the strong higher-order harmonic reflection. These features enable its promising applications in fiber lasers and fiber sensing technology.

Secondary-type In Bragg grating formed in small core photosensitive fiber

An abnormal phenomenon of secondary dip existing in type In Bragg grating formation, which is highly dependent on the diameter and photosensitivity of the fiber core, is discovered and investigated.

Type In microfiber Bragg gratings

Type In Bragg gratings are inscribed in microfibers, which exhibit 800OC temperature resistance. The formation efficiency of those abnormal-type gratings can be improved by use of the microfibers with smaller diameters.

Type IIa Bragg gratings formed in active fibers

Type IIa gratings are formed in active fibers. The forming efficiency relies on the core diameter and the numerical aperture of the fiber. The proposed grating can be used as laser reflector with high temperature resistance.