An optical fiber pH sensor based on a long-period fiber grating (LPFG) is reported. Two oppositely charged polymers, polyethylenimine (PEI) and polyacrylic acid (PAA), were alternately deposited on the sensing structure through a layer-by-layer (LbL) electrostatic self-assembly technique. Since the polymers are pH sensitive, their refractive index (RI) varies when the pH of the solution changes due to swelling/deswelling phenomena. The fabricated multilayer coating retained a similar property, enabling its use in pH-sensing applications. The pH of the PAA dipping solution was tuned so that a coated LPFG achieved a pH sensitivity of (6.3 ± 0.2) nm/pH in the 5.92–9.23 pH range. Only two bilayers of PEI/PAA were used as an overlay, which reduces the fabrication time and increases the reproducibility of the sensor, and its reversibility and repeatability were demonstrated by tracking the resonance band position throughout multiple cycles between different pH solutions. With simulation work and experimental results from a low-finesse Fabry–Perot (FP) cavity on a fiber tip, the coating properties were estimated. When saturated at low pH, it has a thickness of 200 nm and 1.53 ± 0.01 RI, expanding up to 310 nm with a 1.35 ± 0.01 RI at higher pH values, mostly due to the structural changes in the PAA.

We report, to the best of our knowledge, the first demonstration of dual-comb operation from a femtosecond optical parametric oscillator (OPO) at a 1-GHz pulse repetition rate. The singly-resonant OPO is fundamentally synchronously pumped by a high-power low-noise Yb:CaF₂ diode-pumped solid-state laser, enabling a compact system design, high parametric gain, and stable uniform pulse trains for both the signal and idler outputs. Dual-comb generation is realized in a spatially multiplexed single-cavity configuration for both the OPO and the pump laser. The center wavelength is tunable from 1415 nm to 1645 nm (signal) and 2960 nm to 4085 nm (idler) with average powers up to 650 mW and 200 mW per comb, respectively. The source delivers on average an instantaneous bandwidth of 2.5 THz, a power-per-combline up to 270 µW in the short-wave infrared and 75 µW in the mid-wave infrared is available. We demonstrate the potential of this source for fast dual-comb spectroscopy by detecting ambient methane (∼2 ppm) at 1645.5 nm over a 41-m path length, achieving a normalized spectral signal-to-noise ratio of 42.0 dB Hz^1/2. This measurement was performed without any active stabilization.

Gigahertz modelocked lasers operating in the crucial 2–3 µm spectral range are highly sought after for numerous applications. However, they currently rely on bulky and expensive fiber amplifiers for pumping. This paper presents the first, to our knowledge, diode-pumped gigahertz modelocked laser operating at 2.36 µm, utilizing Cr:ZnS as the gain medium and an InGaSb SESAM as the saturable absorber. The modelocked laser emits 199 fs pulses with an average power of 268 mW at a record high 1.06 GHz repetition rate for diode laser pumping, achieving a peak power exceeding 1 kW. The laser exhibits excellent long-term and short-term stability with an integrated relative intensity noise of 0.09% in the frequency interval of 10 Hz–10 MHz. The demonstrated cost-effective, compact yet sufficiently powerful, low-noise, high-repetition-rate femtosecond laser in the short-wave infrared range is a promising source for rapid molecular spectroscopy and efficient nonlinear conversion applications.