Femtosecond laser writing of fiber Bragg gratings using the phase mask technique: a geometrical optics analysis based on the Bravais refractive index.

Material modification is produced inside silica-based optical fibers of different diameters using tightly focused near-infrared (central wavelength at 800 nm) femtosecond laser pulses and the phase mask technique which is often employed for laser inscription of fiber Bragg gratings. 1st-, 2nd-, and 3rd-order phase masks designed for the operation at 800 nm are used in the experiments. The inscription is performed at different distances from the fiber's front surface by translating the focusing cylindrical lens along the laser beam propagation direction. The results show that the material modification produced by means of the 2nd- and 3rd-order phase mask can be positioned at any predetermined distance from the fiber's front surface. In contrast, when the 1st-order mask is used for laser writing, the maximum distance from the fiber's front surface at which material modification can be produced is limited and determined by three main parameters: the diffraction angle of the phase mask, the refractive index of the fiber and the diameter of the fiber.

Nanoscale morphology and thermal properties of low insertion loss fiber Bragg gratings produced using the phase mask technique and a single femtosecond laser pulse.

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.

Toward a Quantum Memory in a Fiber Cavity Controlled by Intracavity Frequency Translation.

We propose a quantum memory protocol based on trapping photons in a fiber-integrated cavity, comprised of a birefringent fiber with dichroic reflective end facets. Photons are switched into resonance with the fiber cavity by intracavity Bragg-scattering frequency translation, driven by ancillary control pulses. After the storage delay, photons are switched out of resonance with the cavity, again by intracavity frequency translation. We demonstrate storage of quantum-level THz-bandwidth coherent states for a lifetime up to 16 cavity round trips, or 200 ns, and a maximum overall efficiency of 73%.

Ultrafast Laser Processing of Optical Fibers for Sensing Applications.

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.

Fiber Bragg Grating Wavelength Drift in Long-Term High Temperature Annealing.

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.

High-temperature stable fiber Bragg gratings with ultra strong cladding modes written using the phase mask technique and an infrared femtosecond laser: erratum.

In this erratum, we correct the mistakes in Eqs. (2) and (2a) in Opt. Lett.45, 443 (2020).OPLEDP0146-959210.1364/OL.381111.

Through-the-coating writing of tilted fiber Bragg gratings with the phase mask technique.

Tilted fiber Bragg gratings are inscribed in non-photosensitized single mode fibers through the polyimide coating using a femtosecond infrared laser and a phase mask. The inscription technique used is based on simultaneously translating the fiber along its axis and the focusing cylindrical lens in the orthogonal direction by means of piezoelectric actuators. The grating plane tilt angles up to 10.3° are achieved with a 1.07 µm-pitch phase mask. The cladding modes reach ∼5 dB in strength in transmission despite the presence of the polyimide coating. The effectiveness of the fabricated tilted fiber Bragg gratings for refractive index sensing through the polyimide coating is also demonstrated. Additionally, we show that the classical approach for the inscription of tilted Bragg gratings, which is based on simply tilting the fiber with respect to the interference fringes, cannot be used in tight focusing geometries that are necessary for through-the-coating inscription.

Complex diffraction and dispersion effects in femtosecond laser writing of fiber Bragg gratings using the phase mask technique.

The combined effect of chromatic dispersion and conical diffraction (i.e., off-plane diffraction) in femtosecond laser inscription of fiber Bragg gratings using the phase mask technique is characterized by measuring the light intensity distribution after the phase mask. As the distance from the mask and the observation point grows, chromatic dispersion and conical diffraction introduced by the mask gradually decrease the peak intensity inside the line-shaped focal volume of the cylindrical lens that is used to focus the femtosecond pulses inside the fiber. We also show that at a certain distance from the mask spherical aberration introduced by the plane-parallel mask substrate is cancelled out by conical diffraction and, at a different distance, chromatic aberration of the cylindrical lens is cancelled out by chromatic dispersion of the mask. These two independent cancellation effects lead to sharpening of the line-shaped focus and the consequent growth of peak light intensity inside it. The above phenomena become especially pronounced for tightly focused femtosecond laser beams and small-pitch phase masks, which, in turn, allows one to choose experimental conditions to inscribe Bragg gratings in polymer-coated non-sensitized 50 µm fibers.

Real-time and high-precision interrogation of a linearly chirped fiber Bragg grating sensor array based on dispersive time delay and optical pulse compression.

A novel technique to achieve real-time and high-precision interrogation of a linearly chirped fiber Bragg (LCFBG) grating sensor array based on dispersive time delay and optical pulse compression is proposed and experimentally demonstrated. In the proposed system, an ultra-short optical pulse is launched into an LCFBG sensor array consisting of N identical LCFBGs, to produce N reflected pulses with their time delay changes corresponding to the strain or temperature changes. The reflected pulses are sent to another LCFBG having an opposite dispersion coefficient to achieve optical pulse compression to improve the measurement accuracy. A proof-of-concept experiment is demonstrated. A 2×3 LCFBG sensor array using six identical LCFBGs is implemented, and the performance is evaluated experimentally. The sensing speed can be as high as 48.6 MHz, and the sensing accuracy can be as high as 0.26 µÎµ for strain and 0.045°C for temperature.

Spectral degradation of Type II π-phase-shifted fiber Bragg gratings due to femtosecond laser induced loss.

Narrowband high-temperature stable fiber Bragg gratings (FBGs) can be made by introducing a π-phase shift in the middle of a Type II periodic grating structure. This creates a passband inside the wavelength rejection band. During the inscription of Type II Bragg gratings broadband, optical loss is induced in the fiber core as a result of interaction between the inscription beam and the silica host. The amount of broadband loss will determine the passband's spectral characteristics (bandwidth and loss).

Real-time random grating sensor array for quasi-distributed sensing based on wavelength-to-time mapping and time-division multiplexing.

A real-time random grating sensor array for quasi-distributed sensing based on spectral-shaping and wavelength-to-time (SS-WTT) mapping and time-division multiplexing is proposed and experimentally demonstrated. The sensor array consists of multiple random gratings written in a single-mode fiber (SMF) at different physical locations. When the temperature or strain applied to a particular random grating is changed, the central wavelength of the reflection spectrum of the random grating will change, which is converted to the time domain as a time shift based on SS-WTT using a linearly chirped fiber Bragg grating. After detection at a photodetector, an electrical waveform with the time shift information encoded in the random waveform is obtained, which is further compressed by correlation to increase the time resolution. As a demonstration, a real-time quasi-distributed sensing system based on a two-random-grating array is implemented. The results show that the sensing resolutions for temperature and strain are 0.23°C and 2.5 µÏµ, respectively, and the accuracies for temperature and strain are 0.11°C and 1.2 µÏµ, respectively. Compared with a conventional quasi-distributed sensor, our proposed sensing system has key advantages, including real-time sensing, high-resolution interrogation, and large scalability.

High-temperature stable π-phase-shifted fiber Bragg gratings inscribed using infrared femtosecond pulses and a phase mask.

Type II π-phase-shifted Bragg gratings stable up to ~1000°C are written inside a standard single mode silica optical fiber (SMF-28) with infrared femtosecond pulses and a special phase mask. Inscription through the protective polyimide fiber coating is also demonstrated. The birefringence of the Bragg gratings and, as a result, the polarization dependence of their spectra are strongly affected by the femtosecond laser polarization. Using optimized writing conditions, the full width at half maximum of the π-phase-shifted passband feature can be ~30 pm in transmission, while the polarization-dependent shift of its central wavelength can be less than 8 pm, for a 7 mm long grating structure. This makes such gratings a unique tool for high-resolution measurements of temperature, load and vibration in extreme temperature environments.

Extreme Environment Sensing Using Femtosecond Laser-Inscribed Fiber Bragg Gratings.

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.

Through-the-coating femtosecond laser inscription of very short fiber Bragg gratings for acoustic and high temperature sensing applications.

Very short Type I and Type II Bragg gratings, on the order of 100 µm in length, are written through the protective polyimide coating of high NA and standard single mode silica optical fibers with infrared femtosecond pulses and a phase mask. By exploiting the transverse walk-off of apertured diffracted beams produced by the phase mask and a slit placed proximate the mask, complex grating structures are fabricated and characterized. These gratings are suitable for structural health monitoring based on acoustic measurements or localized high-temperature measurements.

Nonlinear photoluminescence imaging applied to femtosecond laser manufacturing of fiber Bragg gratings.

Nonlinear photoluminescence imaging is used to visualize the intensity distribution of femtosecond laser pulses inside the optical fiber during Bragg grating inscription based on side illumination through a phase mask. This technique, which results in direct imaging of the inscription laser field inside the optical fiber, facilitates i) the characterization of the laser focus in the vicinity of the fiber core and ii) the optimization of the fiber alignment with respect to the laser focus while using pulses with energies several times lower than those used during the actual inscription process. The applicability of this imaging technique is demonstrated for Bragg grating inscription in different optical fibers, including direct inscription through the fiber coating.

Self-organized nanostructure formation during femtosecond-laser inscription of fiber Bragg gratings.

Periodic planar nanostructures are found in Type II-IR Bragg gratings produced in SMF-28 fiber by side-illuminating it with infrared femtosecond-laser pulses through a phase mask. The planar nanostructures are aligned perpendicular to the laser polarization, as demonstrated using scanning electron microscopy analysis of cleaved fiber samples. Dark field optical microscopy is employed for real-time monitoring of structural changes occurring inside the fiber during the inscription process.

Low loss Type II regenerative Bragg gratings made with ultrafast radiation.

A novel type of fiber Bragg grating is produced by annealing a type I-like grating that is written with multiple infrared femtosecond laser pulses through a phase mask under conditions that are typically used to fabricate thermally stable type II gratings. This new grating is created through a process similar to a regenerative one and displays low loss and high resilience in a 1000 °C ambient environment. Such gratings are ideally suited for quasi-distributed sensing at high temperatures.

Fiber Bragg grating sensors for harsh environments.

Because of their small size, passive nature, immunity to electromagnetic interference, and capability to directly measure physical parameters such as temperature and strain, fiber Bragg grating sensors have developed beyond a laboratory curiosity and are becoming a mainstream sensing technology. Recently, high temperature stable gratings based on regeneration techniques and femtosecond infrared laser processing have shown promise for use in extreme environments such as high temperature, pressure or ionizing radiation. Such gratings are ideally suited for energy production applications where there is a requirement for advanced energy system instrumentation and controls that are operable in harsh environments. This paper will present a review of some of the more recent developments.

High-temperature multiparameter sensor based on sapphire fiber Bragg gratings.

We present, for the first time to our knowledge, a dual strain/temperature sapphire fiber Bragg grating sensor. Temperature and strain coefficients of the grating are evaluated. By recording the blackbody radiation level above 650 degrees C, wavelength shifts due to temperature can be decoupled from those due to strain.

Ultrafast femtosecond-laser-induced fiber Bragg gratings in air-hole microstructured fibers for high-temperature pressure sensing.

We present fiber Bragg grating pressure sensors in air-hole microstructured fibers for high-temperature operation above 800 degrees C. An ultrafast laser was used to inscribe Type II grating in two-hole optical fibers. The fiber Bragg grating resonance wavelength shift and peak splits were studied as a function of external hydrostatic pressure from 15 psi to 2000 psi. The grating pressure sensor shows stable and reproducible operation above 800 degrees C. We demonstrate a multiplexible pressure sensor technology for a high-temperature environment using a single fiber and a single-fiber feedthrough.