Tunable Single-, Dual- and Multi-wavelength Fibre Lasers by Using Twin Core Fibre-based Filters

Tunable fibre lasers draw intensive attention because their emission wavelength can be systematically tuned within a certain spectral range, which allows using a single laser source instead of several sources. This is convenient and cost-effective for many applications in a range of fields, such as telecom, material processing, microscopy, medicine and imaging and so on. The laser wavelength can be tuned in a certain range of wavelength by inserting wavelength-selective elements into the laser’s optical cavity. This chapter describes the twin core fibre (TCF)-based filters, which work as the wavelength-selective element. They are introduced into the ring cavity to implement tunable single-, dual- and multi-wavelength fibre lasers. First, we deduced the coupled-mode theory of TCF-based filter. Second, we experimentally and numerically characterized the optical properties of TCF-based filters including free spectral range, polarization dependence, strain effect and bending effect. Finally, we investigated three tunable fibre lasers which operate at single-, dual- and multi-wavelengths, respectively. The operation mechanism of the fibre lasers mainly involved the elastic-optic effect, polarization hole burning effect and non-linear optical loop mirror. We emphasized the tuning mechanism and the tuning characteristics of the tunable fibre lasers

Chemically functionalised suspended-core fibre for ammonia gas detection

An optical fibre ammonia gas sensor utilising a functionalised four-leaf-clover-shaped suspended-core fibre (SCF) is demonstrated. The fibre is functionalised by depositing a thin layer of an ammonia sensitive dye (tetraphenylporphyrin tetrasulfonic acid hydrate) on the wall of the inner cavities of the SCF through capillary action. An in-line fibre sensing structure is designed for light transmission through the SCF and also allows gas exchange with the atmosphere through two porous polyethylene housings. The sensing structure exhibits a temperature-dependent transmission due to the thermal expansion of the housings. A ratiometric method is applied for signal processing which is demonstrated to reduce significantly the effect of temperature change in the tested range (20-30 C). The sensor is tested in ammonia concentration ranges from 0-10 ppm and demonstrates capability of detecting ammonia at ppb levels. The minimum tested concentration is 150 ppb in the experiment with a calculated limit of detection of 20 ppb. The sensor response time (T10-90%) is 160 s and it is demonstrated to be reusable after treatment with hydrogen chloride vapour. Numerical simulation of evanescent absorption features of such an SCF indicates that approximately 0.02% of the optical power exists in the air holes for the fundamental propagation mode. Distinctive absorption bands observed in the transmission spectrum of the fabricated sensor can also be observed in the simulated model after adding the TPPS absorption layer in the holes

Tunable fiber laser by cascading twin core fiber-based directional couplers

A C-band tunable erbium-doped fiber laser is proposed and experimentally demonstrated by cascading two twin-core fiber (TCF)-based directional couplers. Two TCF-based couplers have different TCF lengths and gave rise to a dense fringe pattern modulated by a sparse one. The lasing wavelength was coarsely tuned by bending one TCF-based coupler with TCF length of 0.14 m, and finely tuned by stretching another TCF-based coupler with TCF length of 1.37 m. Experimental results have shown that the lasing wavelength was linearly and monotonously tuned over 23.2 nm (1541.8-1560 nm) with a uniform tuning step of 0.22 nm, side mode suppression ratio of up to 58 dB, power uniformity of ±0.1 dB. In addition, the output power fluctuation and the wavelength drift have been monitored and found to be <±0.05 dB and 0.004 nm, respectively

Bend resistant large mode area fiber with multi-trench in the core

A bend resistant large mode area fiber with multitrench in the core is proposed. Four layers of trenches with high refractive index are introduced to modulate the mode field distribution. Low refractive index trenches in traditional designs are replaced with pure silica trenches to reduce the difficulty of manufacture. Meanwhile, the core region with a refractive index higher than pure silica cladding conforms to the practical requirement for active fibers. Numerical investigations show that single mode operation with a mode field area of 1100 µm2 is achieved at a bend radius of 15 cm. This design shows the potential of mode field scaling for multitrench fibers and makes a contribution to compact high power fiber lasers