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.

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.

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.

In situ fabrication of far-detuned optical fiber wavelength converters.

We demonstrate an in situ approach for the fabrication of all-fiber wavelength converters with a wavelength offset that is both far-detuned and precisely engineered. Such wavelength converters are fabricated using the parametric gain of A2Se3 microwires and finely tuned from successive adjustments of microwire diameter along with real-time monitoring. Wavelength conversion is achieved from a pump at a wavelength of 1.938 µm to any far-detuned idler within the spectral range of 2.347-2.481 µm, resulting in a detuning of 27.0-33.9 THz with a wavelength offset precision within 3.1 THz.

Free-running mode-locked laser based dual-comb spectroscopy.

We present a real-time dual-comb spectrometer operated from a bidirectional mode-locked fiber laser in the wavelength range of 1.9 µm. Two pulsed signals emitted from a common cavity ensure mutual coherence and common mode noise rejection. The resulting spectrometer operates without any complex electronic feedback system.

Pulse characterization by cross-phase modulation in chalcogenide glass.

We report, to the best of our knowledge, the first all-fiber frequency-resolved optical gating (FROG) device based on cross-phase modulation in chalcogenide glass. The amplitude and phase of pulses as short as 390 fs at femtojoule energy levels are accurately characterized without direction-of-time ambiguity in the retrieved pulse. A measurement sensitivity of 18 mW2 is achieved from the strong nonlinearity of a 10 cm long chalcogenide microwire.

Bidirectional mode-locked thulium-doped fiber laser.

We report a bidirectional mode-locked thulium-doped fiber laser. Mode-locking is enabled by the combination of semiconductor saturable absorption and nonlinear polarization rotation. Two stable mode-locked picosecond pulse trains in opposite directions are generated with a fundamental repetition rate of ∼16.57 MHz. Output wavelengths are tunable over 35 nm.

All-fiber frequency-resolved optical gating pulse characterization from chalcogenide glass.

We report, to the best of our knowledge, the first all-fiber frequency-resolved optical gating device from nonlinear processing in chalcogenide glass. The strong four-wave mixing efficiency of an 11 cm long chalcogenide microwire enables a high sensitivity characterization of pulses in the 2 µm wavelength band. The amplitude and phase of chirped and unchirped picosecond pulses are accurately characterized with a high sensitivity of 0.16 mW2.

Mid-infrared wavelength conversion from As2Se3 microwires.

We demonstrate all-fiber far-detuned and widely tunable mid-infrared wavelength conversion using As2Se3 microwires. In a first experiment, an idler is generated and tuned from 2.351 to >2.500 µm from four-wave mixing in a 0.5 cm long microwire. In a second experiment, tunable parametric sidebands are generated via modulation instability in a 10 cm long microwire. The resulting parametric frequency conversion reaches up to 49.3 THz, the largest ever reported in soft glass materials.

Passively synchronized Q-switched and mode-locked dual-band Tm3+:ZBLAN fiber lasers using a common graphene saturable absorber.

Dual-band fiber lasers are emerging as a promising technology to penetrate new industrial and medical applications from their dual-band properties, in addition to providing compactness and environmental robustness from the waveguide structure. Here, we demonstrate the use of a common graphene saturable absorber and a single gain medium (Tm3+:ZBLAN fiber) to implement (1) a dual-band fiber ring laser with synchronized Q-switched pulses at wavelengths of 1480 nm and 1840 nm, and (2) a dual-band fiber linear laser with synchronized mode-locked pulses at wavelengths of 1480 nm and 1845 nm. Q-switched operation at 1480 nm and 1840 nm is achieved with a synchronized repetition rate from 20 kHz to 40.5 kHz. For synchronous mode-locked operation, pulses with full-width at half maximum durations of 610 fs and 1.68 ps at wavelengths of 1480 nm and 1845 nm, respectively, are obtained at a repetition rate of 12.3 MHz. These dual-band pulsed sources with an ultra-broadband wavelength separation of ~360 nm will add new capabilities in applications including optical sensing, spectroscopy, and communications.

Chalcogenide-based optical parametric oscillator at 2 µm.

We report the first chalcogenide-based optical parametric oscillator (OPO) relying on pure parametric gain. The all-fiber OPO operates in the wavelength range of 2 µm and is tunable over 290 nm from the combined Stokes and anti-Stokes contributions. The gain medium is a 10 cm long chalcogenide microwire made from a high modal confinement As2Se3 core with cyclo olefin polymer cladding, leading to optimized chromatic dispersion, high nonlinearity, and broadband transparency. With a power threshold of only a fraction of a milliwatt, this design is promising for the fabrication of tunable, compact, and low-power consumption mid-infrared sources.

Chalcogenide optical microwires cladded with fluorine-based CYTOP.

We demonstrate optical transmission results of highly nonlinear As2Se3 optical microwires cladded with fluorine-based CYTOP, and compare them with microwires cladded with typical hydrogen-based polymers. In the linear optics regime, the CYTOP-cladded microwire transmits light in the spectral range from 1.3 µm up to >2.5 µm without trace of absorption peaks such as those observed using hydrogen-based polymer claddings. The microwire is also pumped in the nonlinear optics regime, showing multiple-orders of four-wave mixing and supercontinuum generation spanning from 1.0 µm to >4.3 µm. We conclude that with such a broadband transparency and high nonlinearity, the As2Se3-CYTOP microwire is an appealing solution for nonlinear optical processing in the mid-infrared.