Volume Bragg gratings with a 1 mu m 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 mu m 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 degrees C.

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.

Femtosecond laser pulses at two different wavelengths (400 and 800 nm) are used to fabricate highly efficient FBG-based spectrometers featuring a micropore-based internal structure. The unique cylindrical geometry of the micropores allows for strong outcoupling of light from the fiber, with relatively low polarization dependency. Bending of the fiber enables focusing of the spectrally dispersed outcoupled light on an external sensor at different locations, thereby defining an all-fiber spectrometer.

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.

A comparison of Type II Fiber Bragg Grating (FBG) wavelength drift is reported for 600°C, 800°C, 900°C and 1000°C. Significant prolonged redshift was observed at 800°C, transitioning to blueshift more quickly at higher temperatures.

Practical quasi-distributed sensing with Fiber Bragg Gratings (FBGs) depends on the repeatable fabrication of numerous, mechanically robust, low loss FBGs. This work reports an array of 1008 FBGs written with a phase mask through polyimide.

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 pm/%RH to 19 pm/%RH for 10 μm 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 to devices with thicker coatings.

A super-efficient all-fiber visible spectrometer with sub-nanometer resolution is fabricated using one infrared femtosecond laser pulse and a phase mask. The strong light outcoupling from the fiber is due to micropores formed in its core.

Fiber Bragg grating (FBG) relative humidity (RH) sensors are fabricated in commercially available polyimide (PI)-coated optical fibers with diameters of 50 and 125 μm. Infrared (800 nm) femtosecond pulse duration laser pulses and a phase mask are used to inscribe Type-I and Type-II FBGs directly through the protective polyimide coatings of both 50 and 125 μm diameter fibers without typical fiber processing such as hydrogen loading, cryogenic storage, stripping, recoating or annealing. The devices are then evaluated for their performance as humidity sensors. At telecom wavelengths, the 50 μm diameter fiber devices with a 10 μm thick PI coating had a wavelength shift of the Bragg resonance at a constant temperature of 2.7 pm/%RH, whereas the 125 μm diameter fiber devices with a 17 μm thick PI coating had a wavelength shift of 1.8 pm/%RH. The humidity sensors in the 50 µm diameter fiber demonstrated a more rapid response time to small changes in humidity and a weaker hysteresis when compared to the 125 µm diameter fiber devices. No modification to the PI coatings was observed during fabrication. No difference in RH sensitivity was observed for Type-I devices when compared with Type-II devices with the same fiber. The applicability of this approach for fabricating distributed RH sensing arrays with hundreds of sensing elements on a single fiber is discussed.

Fiber Bragg gratings with a very low insertion loss are inscribed using the phase mask technique and a single infrared (800 nm) femtosecond laser pulse. The morphology of the resultant light-induced structural changes in the Ge-doped silica fiber (SMF-28) is analyzed using scanning electron microscopy. The electron microscopy images reveal that each Bragg grating period incorporates an elongated micropore embedded in a region of homogeneous material modification. The Bragg wavelength drift and reflectivity of fiber Bragg gratings produced with single pulses having the same energy but different duration (80 fs and 350 fs) are monitored for 1000 hours in the course of isothermal annealing at 1000°C. The annealing data demonstrate that both the isothermal Bragg wavelength drift and the decrease in the reflectivity of the fiber Bragg gratings under test are statistically slower for the 350 fs inscription pulses.

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.

Pipeline leak detection sensors are created by using fiber Bragg gratings (FBGs) that are inscribed through polyimide coatings of optical fiber with an infrared femtosecond laser and then specially packaged in materials susceptible to hydrocarbons. Depending on the sensor geometry, strain is either applied or released upon exposure to toluene or crude oil.

High-temperature-resistant fiber Bragg gratings (FBGs) are the main competitors to thermocouples as sensors in applications for high temperature environments defined as being in the 600–1200°C temperature range. Due to their small size, capacity to be multiplexed into high density distributed sensor arrays and survivability in extreme ambient temperatures, they could provide the essential sensing support that is needed in high temperature processes. While capable of providing reliable sensing information in the short term, their long-term functionality is affected by the drift of the characteristic Bragg wavelength or resonance that is used to derive the temperature. A number of physical processes have been proposed as the cause of the high temperature wavelength drift but there is yet no credible description of this process. In this paper we review the literature related to the long-term wavelength drift of FBGs at high temperature and provide our recent results of more than 4000 h of high temperature testing in the 900 –1000°C range. We identify the major components of the high temperature wavelength drift and we propose mechanisms that could be causing them.

The graded-index fiber lens was used to couple probe light into the multimode sapphire fiber containing a Bragg grating (SFBG). It improved the spectral resolution of the SFBG and facilitated the packaging of the SFBG as a fiber sensor for applications in high temperature environments.

Femtosecond laser written fiber Bragg gratings are useful for extreme sensing, including combustor applications for energy and aerospace. This paper reviews our fabrication and deployment of such sensors, for monitoring temperature gradients within such environments.

Femtosecond Infrared (fs-IR) laser written fiber Bragg gratings (FBGs), have shown great potential for sensing in extreme environments. This paper presents the fabrication and deployment of two fs-IR laser written FBG temperature probes, for monitoring temperature gradients on the flame tube of a low emission burner, during a high pressure combustor test of an optically accessible combustor rig (OACR). Results of this work include: contour plots of measured internal and exhaust temperature gradients, contrast of FBG measurements with thermocouple data, discussion of deployment strategies, as well as comments on reliability and other important considerations.

Femtosecond Infrared (fs-IR) laser written fiber Bragg gratings (FBGs), have demonstrated great potential for extreme sensing. Such conditions are inherent to advanced power plant technologies and gas turbine engines, under development to reduce greenhouse gas emissions; and the ability to measure temperature gradients in these harsh environments is currently limited by the lack of sensors and controls capable of withstanding the high temperature, pressure and corrosive conditions present. This paper reviews our fabrication and deployment of hundreds of fs-IR written FBGs, for monitoring temperature gradients of an oxy-fuel fluidized bed combustor and an aerospace gas turbine combustor simulator.

The femtosecond laser-induced fiber Bragg grating is an effective sensor technology that can be deployed in harsh environments. Depending on the optical fiber chosen and the inscription parameters that are used, devices suitable for high temperature, pressure, ionizing radiation and strain sensor applications are possible. Such devices are appropriate for aerospace or energy production applications where there is a need for components, instrumentation and controls that can function in harsh environments. This paper will present a review of some of the more recent developments in this field.

Femtosecond infrared (fs-IR) laser written fiber Bragg gratings (FBGs), have demonstrated great potential for extreme sensing. Such conditions are inherent in advanced gas turbine engines under development to reduce greenhouse gas emissions; and the ability to measure temperature gradients in these harsh environments is currently limited by the lack of sensors and controls capable of withstanding the high temperature, pressure and corrosive conditions present. This paper discusses fabrication and deployment of several fs-IR written FBG arrays, for monitoring exhaust temperature gradients of a gas turbine combustor simulator. Results include: contour plots of measured temperature gradients, contrast with thermocouple data.