We present a technique to improve signal strength and therefore sensitivity in distributed temperature and strain sensing (DTSS) using Frequency domain Rayleigh scatter. A simple UV exposure of a hydrogen loaded standard SMF-28 fibre core is shown to enhance the Rayleigh back-scattered light dramatically by ten-fold, independent of the presence of a Bragg grating and is therefore created by the UV exposure alone. This increase in Rayleigh back-scatter allows an order-of-magnitude increase in temperature and strain resolution for DTSS compared to un-exposed SMF-28 fibre used as a sensing element. This enhancement in sensitivity is effective for cm range or more sensor gauge length, below which is the theoretical cross-correlation limit. The detection of a 20 mK temperature rise with a spatial resolution of 2 cm is demonstrated. This gain in sensitivity for SMF-28 is compared with a high Ge doped photosensitive fibre with a characteristically high NA. For the latter, the UV enhancement is also present although of lower amplitude and enables an even lower noise level for sensing, due to the fibre’s intrinsically higher Rayleigh scatter signal.

We propose a novel device defined as Random Optical Grating by Ultraviolet or ultrafast laser Exposure (ROGUE), a new type of fiber Bragg grating (FBG), exhibiting a weak reflection over a large bandwidth, which is independent of the length of the grating. This FBG is fabricated simply by dithering the phase randomly during the writing process. This grating has an enhanced backscatter, several orders of magnitude above typical Rayleigh backscatter of standard SMF-28 optical fiber. The grating is used in distributed sensing using optical frequency domain reflectometry (OFDR), allowing a significant increase in signal to noise ratio for strain and temperature measurement. This enhancement results in significantly lower strain or temperature noise level and accuracy error, without sacrificing the spatial resolution. Using this method, we show a sensor with a backscatter level 50 dB higher than standard unexposed SMF-28, which can thus compensate for increased loss in the system.

Distributed feedback (DFB) fiber lasers have been demonstrated to be excellent narrow-linewidth (kilohertz range) single-frequency laser sources. However, this type of laser is normally limited to rare-earth-doped fibers. Raman gain offers an alternative, operating at any arbitrary wavelength. We demonstrate here linearly polarized, single-frequency, DFB fiber Bragg grating Raman lasers with, to the best of our knowledge, the lowest pump threshold of 350 mW, an output power of up to 50 mW at 1120 nm, an 8.5% slope efficiency, and 300 mW of output power at 1178 nm. In the high-power regime, stimulated Brillouin scattering plays an important role in the laser dynamics. We also report the characterization of the power profile inside the fiber along its axis through a novel side-scatter technique.

We propose a method to generate phase-locked pulses in the picosecond regime by using Stimulated Brillouin Scattering (SBS). The phase-locked comb is generated using only long length of fiber and a single frequency CW pump laser. We show that there is a phase relationship between multiple Stokes peaks in a cavity, which directly leads to pulsing without the need to add a mode-locking component. This generates highly coherent pulses in the order of ~10 ps. The repetition frequency, which is very stable is in the order of tens of GHz, is based on the SBS frequency shift and has a linear dependence with temperature (1 MHz/°C). Such a laser could therefore be used in high-speed optical clocks and optical communication system. This system allows the pulses to be generated at any wavelength by simply changing the pump wavelength.

The possibility to perform time-resolved spectroscopic studies in the molecular fingerprinting region or extending the cutoff wavelength of high-harmonic generation has recently boosted the development of efficient mid-infrared (mid-IR) ultrafast lasers. In particular, fiber lasers based on active media such as thulium or holmium are a very active area of research since they are robust, compact, and can operate at high repetition rates. These systems, however, are still complex, are unable to deliver pulses shorter than 100 fs, and are not yet as mature as their near-infrared counterparts. Here, we report the generation of sub-40 fs pulses at 1.8 µm, with quantum efficiencies of 50% and without the need for post-compression, in hydrogen-filled, hollow-core photonic crystal fiber pumped by a commercial high-repetition-rate 300 fs fiber laser at 1030 nm. This is achieved by pressure-tuning the dispersion and avoiding Raman gain suppression by adjusting the chirp of the pump pulses and the proportion of higher-order modes launched into the fiber. The system is optimized using a physical model that incorporates the main linear and nonlinear contributions to the optical response. The approach is average power-scalable, permits adjustment of the pulse shape, and can potentially allow access to much longer wavelengths.

Single-frequency laser sources have found a great number of applications, but are difficult to implement and suffer from poor robustness, poor quality (linewidth and stability), and are generally expensive to fabricate. One solution for a cheaper and simpler single-frequency source is a $\pi$-phase-shifted distributed feedback (DFB) fiber Bragg grating (FBG) based laser. Typically, such a laser usually uses a fiber with rare-earth dopants as an active medium for gain. However, its operating wavelength is limited to the emission bandwidth of the rare-earth dopant in the fiber. A proposed solution to overcome this limitation is to use Raman gain. Raman DFB fiber lasers have been successfully demonstrated, and a few simulations have been undertaken and reported. However, a thorough study of parameters and careful optimization has not been reported due to the long computation time and difficulty in the fabrication of long FBGs with known parameters. We demonstrate here, with the aid of a fast but exact method, a detailed optimization study on phase-shifted Raman DFB fiber lasers. These theoretical results are compared with the experimental operation of many fabricated FBGs thanks to a newly developed fabrication technique for the replication of FBGs. We show that fabricated lasers have poor performance compared to simulations of ideal lasers. We also show that the difference in performance is due to the high internal optical intensity induced nonlinear thermal gradient along the FBG.