Stephen James Mihailov

1100°C-resistant Bragg gratings with a 181 nm period were fabricated in non-photosensitized silica-based optical fibers using tightly focused deep-ultraviolet (267 nm) femtosecond laser pulses and the phase-mask technique. Changes in the transmission spectra of the fiber Bragg gratings were monitored as the surrounding temperature was increased in steps from 20°C to 1100°C and then held at 1100°C for, depending on the fiber, 60 or 100 minutes. The results showed that, both before and after the annealing, the gratings strongly and directionally outcoupled light and did not undergo a regeneration cycle during the annealing. Therefore, the gratings almost certainly consist of truly periodic mesoscopic structural disruptions in the host material.

Volume Bragg gratings with a 1 μm grating pitch and high diffraction efficiency of up to 94% are fabricated using line-by-line femtosecond laser writing of filaments of modified material in fused silica plates. Uniform filaments‒up to 150 μm in length‒are produced by tightly focusing the femtosecond pulses deep inside the plates and exploiting spherical aberration, induced by the planar air-sample interface, to stretch the focal volume of the focused pulses. The resultant VBGs exhibit high diffraction efficiency, high angular selectivity, and low polarization dependence. The VBGs also preserve their optical characteristics after isothermal annealing for 2 hours at 1000 °C.

Fiber Bragg gratings (FBGs) were fabricated in tapered fibers using both the plane-by-plane and phase mask approaches with a femtosecond infrared laser. A thin layer of boron nitride nanotubes (BNNTs) was deposited onto the FBGs through a dip-coating process from a BNNT water solution. The reflection spectra of the FBGs were then recorded when they were placed in ammonia (NH3), hydrogen chloride (HCl) and bromine (Br2) gases. It was demonstrated experimentally that, due to the BNNT coating, the return losses of the FBGs increased when they were surrounded with these hydrophilic gas vapors. However, when the FBGs were in other atmospheres such as air, methanol and acetone vapors, no change in the FBG reflection spectra was observed. The results of this work indicate that BNNT coatings on FBGs can play an important role for hydrophilic gas detection with large measurement range and reusability due to their quick release of adsorbed gases. Moreover, the BNNT coating induced losses with different ammonia gas concentrations were quantitatively measured. It was observed that the Bragg wavelengths of the FBGs remained the same when the FBGs were tested in hydrophilic gases suggesting the proposed device can be used for temperature or strain sensing at the same time. The response of the Bragg wavelength of BNNT coated FBGs to temperature was also studied and it was found that the thin BNNT coating did not affect the FBG's temperature sensitivity. Due to the properties of BNNTs and optical fibers, the proposed fiber sensor can be used in harsh environments.

Boron nitride nanotubes (BNNTs) are predicted to be promising one-dimensional nonlinear optical materials, but to date, only one experimental observation has been made using individual nanotubes. In this work, second harmonic generation (SHG) was achieved from free-standing bulk BNNT sheets and BNNT coatings on silica substrates. Focusing femtosecond infrared (fs-IR) laser pulses with a wavelength of 800 nm onto the BNNT assemblies resulted in strong SHG at a wavelength of 400 nm. It was observed that due to the thickness variation of the BNNT assemblies and orientational alignment of BNNTs in the assemblies, the intensity of the second-harmonic (SH) radiation changed dramatically when different locations on the samples were investigated. Among all the BNNT assemblies tested, the localized SH response and its dependence on the polarization of the excitation fs-IR pulses were the strongest in BNNT coatings produced by a dip-coating process. By measuring the SH response, the uniformity, reproducibility, and efficiency of BNNT deposition processes could be assessed. For applications requiring a high SH response from BNNT assemblies, the process of dip coating is preferred.

Fiber Bragg gratings (FBGs) are useful components in fiber optic sensing systems, which can be highly multiplexed and distributed. In recent years, fabrication using ultrafast lasers has made these devices much more versatile and robust, but questions concerning their high-temperature performance remain. The wavelength resonance of an FBG is naturally sensitive to various parameters of its environment; in particular, changes in the temperature or strain of a fiber tend to induce observable shifts in the Bragg wavelength. Thus, FBGs can offer reliable sensing solutions, provided they are isolated from other influences and their wavelength responses remain well characterized. Nonetheless, it is important to be aware that the isothermal wavelength drift of unstrained FBGs has been previously observed. When this occurs, it can lead to measurement errors and a requirement for sensor recalibration. This study presents a comparison of long-term isothermal wavelength drifts observed at 600 °C, 800 °C, 900 °C and 1000 °C for large numbers of Type II FBGs in different kinds of single-mode fibers. The results provide guidance for the design of high-temperature sensing systems, both in terms of fiber selection and for estimating the maximum time before recalibration becomes necessary to maintain a specified accuracy.

All-fiber visible spectrometers with a sub-nanometer resolution and record-high light outcoupling (70%) are fabricated using violet (400 nm) and near-infrared (800 nm) femtosecond laser pulses and the phase mask technique. The spectrometers are based on highly localized uniform Bragg gratings produced by tightly focusing the femtosecond pulses into the core of visible single-mode fibers. The unique nanoscale morphology of the resultant Bragg gratings ensures very strong outcoupling of light from the fiber, while bending of the fiber provides the focusing capability to define an efficient all-fiber spectrometer.

1000°C-resistant Bragg gratings (i.e., Type-II Bragg gratings) with a 300 nm period are fabricated in non-photosensitized silica-based optical fibers using tightly focused ultraviolet/visible (400 nm) femtosecond pulses and a phase mask. Five millimeter-long Bragg gratings with a -10 dB transmission dip and -0.1 dB off-resonance insertion loss are demonstrated. Inscription of such gratings is also possible through protective acrylate coatings on the fiber. The Bragg wavelength drift and changes in the transmission of the fiber Bragg gratings are monitored in the course of isothermal annealing performed at 1000°C for 120 hours.

The optical and structural properties of boron nitride nanotube (BNNT) assembly were characterized using optical fiber Bragg grating (FBG) sensors. FBGs were fabricated in tapered fibers with a diameter of less than 30 μm. After tapered fibers were etched in hydrofluoric acid solution for a few minutes, BNNTs were then deposited on the fibers through a dip-coating process. The Bragg wavelengths of the FBGs were measured before and after BNNT coating such that the effective refractive index (RI) of the guided fundamental HE11 mode in the fiber was obtained. By numerical modeling of the dependence of HE11 mode effective RI on the RI of the surrounding material, both the RI and the air void content of BNNT coating were obtained. To verify the accuracy of the modeling results, a FBG sensor fabricated in a tapered fiber was immerged in various RI standard solutions and the corresponding Bragg wavelengths were measured and compared to numerical simulations. It was shown that the experimental data agreed well with simulation results.

Ultrafast laser micro-/nanofabrication and processing technology using femtosecond laser beams has emerged as a pivotal tool in the arena of photonic device fab Ultrafast laser micro-/nanofabrication and processing technology using femtosecond laser beams has emerged as a pivotal tool in the arena of photonic device fab

Durable fiber Bragg grating (FBG) relative humidity (RH) sensors are fabricated in commercially available polyimide (PI) coated 50-µm diameter Ge-doped silica optical fibers using an infrared femtosecond pulse duration laser and a phase mask. FBGs are inscribed directly through the 10-µm thick PI coating, which is subsequently thickened with PI using a customized fiber recoating system. Devices with varying PI thicknesses are evaluated for their response to RH. For telecom C-band wavelengths, FBG sensor sensitivities vary from 2.4 to 19 pm/%RH for 10 to 60-µm thick PI coatings, respectively. The 19 pm/%RH response represents the highest sensitivity reported for a PI coated silica FBG RH sensor. RH sensors with thinner coatings respond more rapidly to small changes in humidity and show smaller hysteresis when compared with devices with thicker coatings.