Development of dual-wavelength pumped mid-infrared fibre laser

Abstract

There is an urgent need for efficient and compact sources of coherent mid-infrared wavelength radiation (3-5 μm). Cost-effective and bright mid-infrared sources will lead to exciting new sensing applications, ranging from the remote sensing of greenhouse gases, such as CO₂ and methane, to analysis of trace gases in exhaled breath for disease marker identification. The last decade has seen a continuing increase in the output power of various types of mid-infrared sources, such as optical parametric oscillators and amplifiers, quantum cascade lasers and fibre lasers. However, advances in brightness, efficiency, peak power and tunability are still necessary for many applications. In this thesis, we describe a new concept for a fibre laser based on Er³⁺ doped ZBLAN glass that operates in the mid-infrared with lasing centred around 3.5 μm. We used a novel dual-wavelength pumping (DWP) scheme to achieve world-leading efficiency for this material and an output power of 260 mW. The DWP technique uses long-lived excited states in our Er³⁺ ion doped ZBLAN glass gain medium to improve the Stokes efficiency. A low power, 985 nm pump source excites ions from the ⁴I₁₅ ̷₂ ground state to the long lived ⁴I₁₁ ̷₂ state. A large fraction of the ion population can be stored in this level because of its long lifetime, creating a \virtual ground state." A concurrent 1973 nm pump source is then used as the main pump source. This pump excites the ions further to the upper laser level ⁴F₉ ̷₂. Post lasing, the 1973 nm pump cycles the ions between the \virtual ground state" at ⁴I₁₁ ̷₂ and the ⁴F₉ ̷₂ level. The first pump at 985 nm maintains the population in the “virtual ground" as this population is diminished by spontaneous emission and energy-transfer processes, which eject ions from the lasing cycle. In this thesis, we review the literature and the current state-of-the-art in mid-infrared fibre lasers. An overview of the spectroscopic properties of Er³⁺:ZBLAN relevant to mid-infrared operation is presented. The difficulties and issues associated with the creation of mid-infrared radiation are discussed and our spectroscopic investigations of ZBLAN glass and glass fibres are summarised. Multiple wavelengths were used as pump sources for our DWP laser. Our investigation of the optimal wavelength for the DWP technique and the development of suitable sources is described as well. The 3.5 μm laser system is discussed, including the full characterisation of the laser. The thesis is concluded with a summary of the results and an outlook for the future.