Negative-index gratings formed by a 193-nm excimer laser

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.

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.

Negative-index gratings formed by femtosecond laser overexposure and thermal regeneration

We demonstrate a method for the preparation of negative-index fibre Bragg gratings (FBGs) using 800 nm femtosecond laser overexposure and thermal regeneration. A positive-index type I-IR FBG was first inscribed in H₂-free single-mode fibre using a femtosecond laser directed through a phase mask, and then a highly polarization dependant phase-shifted FBG (P-PSFBG) was fabricated from the type I-IR FBG by overexposure to the femtosecond laser. Subsequently, the P-PSFBG was thermally annealed at 800 °C for 12 hours. Grating regeneration was observed during thermal annealing, and a negative-index FBG was finally obtained with a high reflectivity of 99.22%, an ultra-low insertion loss of 0.08 dB, a blueshift of 0.83 nm in the Bragg wavelength, and an operating temperature of up to 1000 °C for more than 10 hours. Further annealing tests showed that the thermal stability of the negative-index FBG was lower than that of a type II-IR FBG, but much higher than that of a type I-IR FBG. Moreover, the formation of such a negative-index grating may result from thermally regenerated type IIA photosensitivity.

Single-mode deep-UV light source at 191.7 nm by seventh-harmonic generation of a high-power, Q-switched, injection-locked 1342 nm Nd:YVO₄ laser

A single-mode deep-UV laser at 191.7 nm is demonstrated by seventh-harmonic generation of a single-mode 1342 nm Nd:YVO₄ laser. The fundamental laser is an injection-locked, Q-switched ring laser at 10 kHz pulse repetition frequency. By cascaded second-harmonic and sum-frequency generation, an average power of 230 mW at 191.7 nm is achieved. At 185 mW, the setup features a Gaussian beam with a beam quality factor of M²<1.5. A pulse duration of 9 ns with pulse energy fluctuations of σ<3% is obtained. The long-term spectral width is approximately 240 MHz full width at half-maximum, measured with a homemade scanning confocal Fabry-Perot interferometer. This work opens possibilities in fiber Bragg grating production, allowing the inscription of long gratings.

Influence of pre-annealing on the thermal regeneration of fiber Bragg gratings in standard optical fibers

A detailed study of the dynamics during thermal regeneration of fiber Bragg gratings, written in hydrogen-loaded standard single-mode fibers using a ns pulsed 213 nm UV laser, is reported. Isothermal pre-annealing performed in the range 85 °C to 1100 °C, with subsequent grating regeneration at 1100 °C, resulted in a maximum refractive index modulation, Δn(m) ~1.4⋅10(-4), for gratings pre-annealed near 900 °C while a minimum value of Δn(m) ~2⋅10(-5) was achieved irrespective of pre-annealing temperature. This optimum denote an inflection point between opposing thermally triggered processes, which we ascribe to the reaction-diffusion mechanism of molecular water and hydroxyl species in silica. The results shed new light on the mechanisms underlying thermal grating regeneration in optical fibers.

Type IIa Bragg gratings formed in microfibers

In this Letter, Type IIa Bragg gratings are inscribed into microfibers. The large germanium-doped core region of the multimode fiber provides the necessary photosensitivity to form a Type IIa grating when it is drawn down to the microscale. Reducing the diameter of the microfiber due to lower saturate modulation and the amplified tension-strain transformation effect can accelerate the formation of a Type IIa grating. This provides an efficient method for the fabrication of fiber gratings with 800°C temperature resistance.

Post-hydrogen-loaded draw tower fiber Bragg gratings and their thermal regeneration

The idea of Bragg gratings generated during the drawing process of a fiber dates back almost 20 years. The technical improvement of the draw tower grating (DTG) process today results in highly reliable and cost-effective Bragg gratings for versatile application in the optical fiber sensor market. Because of the single-pulse exposure of the fiber, the gratings behave typically like type I gratings with respect to their temperature stability. This means that such gratings only work up to temperatures of about 300 °C. To increase temperature stability, we combined DTG arrays with hydrogen postloading and a thermal regeneration process that enables their use in high-temperature environments. The regenerated draw tower gratings are demonstrated to be suitable for temperatures of more than 800 °C.

Germania-glass-core silica-glass-cladding modified chemical-vapor deposition optical fibers: optical losses, photorefractivity, and Raman amplification

Germania-glass-core silica-glass-cladding single-mode fibers (deltan as great as 0.143) with a minimum loss of 20 dB/km at 1.85 microm were fabricated by modified chemical-vapor deposition. The fibers exhibit strong photorefractivity, with type IIa index modulation of 2 x 10(-3). A Raman gain of 300 dB/(kmW) was determined at 1.12 microm. Only 3 m of such fibers is sufficient for constructing the 10-W Raman laser at 1.12 microm with a 13-W pump at 1.07 microm.

Enhanced type IIA gratings for high-temperature operation

The inscription of type IIA fiber Bragg gratings in standard boron-codoped germanosilicate fiber has been demonstrated to show marked differences from that reported in the literature. These gratings were subjected to high temperatures, and their decay behavior was evaluated. Gratings resistant to heat up to 800 degrees C for a moderate length of time are demonstrated.

Use of dual-grating sensors formed by different types of fiber Bragg gratings for simultaneous temperature and strain measurements

We report on a systematic investigation of the dependence of both temperature and strain sensitivities on the fiber Bragg grating type, including the well-known Type I, Type IIA, and a new type that we have designated Type IA, using both hydrogen-free and hydrogenated B/Ge codoped fibres. We have identified distinct sensitivity characteristics for each grating type, and we have used them to implement a novel dual-grating, dual-parameter sensor device. Three dual-grating sensing schemes with different combinations of grating type have been constructed and compared, and that of a Type IA-Type IIA combination exhibits the best performance, which is also superior to that of previously reported grating-based structures. The characteristics of the measurement errors in such dual-grating sensor systems is also presented in detail.

Dependence of temperature and strain coefficients on fiber grating type and its application to simultaneous temperature and strain measurement

We report an investigation of the dependence of the temperature and strain coefficients on the grating type for fiber Bragg gratings that are UV inscribed in B/Ge-codoped fiber with and without hydrogenation. The results reveal that all types of grating exhibit similar strain sensitivities but markedly different temperature sensitivities, greater for gratings inscribed in hydrogen-free rather than hydrogenated fiber and substantially less in type IA gratings than all others. The sensitivity characteristics of these gratings have been used to implement a new type of dual-grating sensor for simultaneous measurement of temperature and strain that has properties superior to those of previously reported structures.

Effect of periodic background loss on grating spectra

The effect of periodic loss on the performance of refractive-index gratings has been studied in detail. It is shown that the loss periodicity and relative phase strongly affects the symmetry of the reflection, transmission, and loss spectra. This asymmetry is explained successfully through consideration of the overlap between the standing-wave intensity distribution and the periodic loss pattern.

Abnormal spectral evolution of fiber Bragg gratings in hydrogenated fibers

We report the observation of abnormal spectral evolution in regenerated fiber Bragg gratings in hydrogenated B-Ge-codoped and standard telecom fiber with UV overexposure. The behavior of this new type of regenerated grating, which we have designated type IA, contrasts with that of the previously reported type IIA grating in nonhydrogenated fiber by exhibiting a large redshift in Bragg wavelength with increasing exposure of as much as 18 nm from a strong (16-dB) regenerated grating in B-Ge fiber.

High-temperature stable gratings in germanosilicate planar waveguides

Negative index gratings in planar germanosilicate waveguides were found to be stable up to 500 degrees C. The annealing properties are similar to those of negative index fiber gratings.

Complex photosensitivity observed in germanosilica planar waveguides

Photosensitive effects distinguished as type I and type IIA photosensitivity within optical fibers were observed in a much more pronounced form within germanosilica waveguides deposited by hollow-cathode plasma-enhanced chemical-vapor deposition. With increasing exposure to 193-nm UV light, positive index changes greater than 2 x 10(-3) were observed, followed by negative index changes greater than -5 x 10(-3) . These behaviors are attributed to an increase in macroscopic polarizability and a reduction in material density, respectively. The negative index change is more temperature resistant and is fully annealed only at 900 degrees C, whereas the positive one is annealed at 500 degrees C.

Thermal decay of fiber Bragg gratings of positive and negative index changes formed at 193 nm in a boron-codoped germanosilicate fiber

A complex grating decay process is observed at elevated temperatures as predicted by a recently proposed three-energy-level model. We have also measured thermal stability of fiber gratings of both positive and negative index changes in a boron-codoped germanosilicate fiber in order to characterize the energy levels of the system and to predict grating lifetimes. The negative index gratings are found to be able to operate at 300 degrees C for more than 25 years without significant degradation.