Roadmap on optical sensors.

Optical sensors and sensing technologies are playing a more and more important role in our modern world. From micro-probes to large devices used in such diverse areas like medical diagnosis, defence, monitoring of industrial and environmental conditions, optics can be used in a variety of ways to achieve compact, low cost, stand-off sensing with extreme sensitivity and selectivity. Actually, the challenges to the design and functioning of an optical sensor for a particular application requires intimate knowledge of the optical, material, and environmental properties that can affect its performance. This roadmap on optical sensors addresses different technologies and application areas. It is constituted by twelve contributions authored by world-leading experts, providing insight into the current state-of-the-art and the challenges their respective fields face. Two articles address the area of optical fibre sensors, encompassing both conventional and specialty optical fibres. Several other articles are dedicated to laser-based sensors, micro- and nano-engineered sensors, whispering-gallery mode and plasmonic sensors. The use of optical sensors in chemical, biological and biomedical areas is discussed in some other papers. Different approaches required to satisfy applications at visible, infrared and THz spectral regions are also discussed.

Mitigating stimulated Brillouin scattering in multimode fibers with focused output via wavefront shaping.

The key challenge for high-power delivery through optical fibers is overcoming nonlinear optical effects. To keep a smooth output beam, most techniques for mitigating optical nonlinearities are restricted to single-mode fibers. Moving out of the single-mode paradigm, we show experimentally that wavefront-shaping of coherent input light to a highly multimode fiber can increase the power threshold for stimulated Brillouin scattering (SBS) by an order of magnitude, whilst simultaneously controlling the output beam profile. The SBS suppression results from an effective broadening of the Brillouin spectrum under multimode excitation, without broadening of transmitted light. Strongest suppression is achieved with selective mode excitation that gives the broadest Brillouin spectrum. Our method is efficient, robust, and applicable to continuous waves and pulses. This work points toward a promising route for mitigating detrimental nonlinear effects in optical fibers, enabling further power scaling of high-power fiber systems for applications to directed energy, remote sensing, and gravitational-wave detection.

Ultrafast 3.5 µm fiber laser.

We report, to the best of our knowledge, the first mode-locked fiber laser to operate in the femtosecond regime well beyond 3 µm. The laser uses dual-wavelength pumping and nonlinear polarization rotation to produce 3.5 µm wavelength pulses with minimum duration of 580 fs at a repetition rate of 68 MHz. The pulse energy is 3.2 nJ, corresponding to a peak power of 5.5 kW.

Actively Q-switched dual-wavelength pumped Er3+ :ZBLAN fiber laser at 3.47 µm.

We demonstrate the first actively Q-switched fiber laser operating in the 3.5 µm regime. The dual-wavelength pumped system makes use of an Er3+ doped ZBLAN fiber and a germanium acousto-optic modulator. Robust Q-switching saw a pulse energy of 7.8 µJ achieved at a repetition rate of 15 kHz, corresponding to a peak power of 14.5 W.

New energy-transfer upconversion process in Er3+:ZBLAN mid-infrared fiber lasers.

We report a new energy-transfer process in erbium doped ZBLAN glass, which is critical for optimizing the operation of lasers that use the 3.5 µm band 4F9/2 to 4I9/2 transition. The magnitude of this energy-transfer process is measured for two different doping levels in Er3+:ZBLAN and the requirement for low doping in these lasers established.

Versatile and widely tunable mid-infrared erbium doped ZBLAN fiber laser.

We report on a long wavelength emitting rare earth doped fiber laser with the emission centered at 3.5 µm and tunable across 450 nm. The longest wavelength emission was 3.78 µm which is the longest emission from a fiber laser operating at room temperature. In a simple optical arrangement employing dielectric mirrors for feedback, the laser was capable of emitting 1.45 W of near diffraction limited output power at 3.47 µm. These emission characteristics complement the emissions from quantum cascade lasers and demonstrate how all infrared dual wavelength pumping can be used to access high lying rare earth ion transitions that have previously relied on visible wavelength pumping.

Mid-infrared fiber lasers at and beyond 3.5 µm using dual-wavelength pumping.

We report the first, to the best of our knowledge, erbium-doped zirconium-fluoride-based glass fiber laser operating well beyond 3 µm with significant power. This fiber laser achieved 260 mW in CW at room temperature. The use of two different wavelength pump sources allows us to take advantage of the long-lived excited states that would normally cause a bottleneck, and this enables maximum incident optical-to-optical efficiency of 16% with respect to the total incident pump power. Both output power and efficiency are an order of magnitude improvement over similar lasers demonstrated previously. The fiber laser operating at 3.604 µm also exhibited the longest wavelength of operation obtained to date for a room temperature, nonsupercontinuum fiber laser.