Broadly wavelength versatile mid-infra red sources based upon difference frequency generation and parametric conversion.

The mid-infrared region of the electromagnetic spectrum, defined (ISO 20473) as radiation with wavelengths from 3-50 microns, is vitally important for applications in healthcare, manufacturing and defence. Key organic molecules exhibit strong unique absorption of mid-infrared light, including many atmospheric gases, pollutants and the complex proteins characterised by the term "molecular fingerprint region". Laser sources in the mid-infrared thus act as important enabling tools, allowing one to probe the inner workings of an enormous variety of chemical and biological processes, from monitoring pollution levels in cities and at high powers processing polymer materials to accurately differentiating cancerous and healthy tissue in early-stage disease diagnosis.
In comparison to the near-infrared region of the spectrum (0.7-3 microns), it is extremely challenging to build compact, economically viable and user-friendly lasers in the mid infra-red, that can be power scaled, exhibit broad wavelength tunability and versatility of pulse duration and repetition rate.
In this project various experimental routes will be taken to demonstrate the proposed versatility. Fibre based infrared light sources will be upconverted to the mid-infrared, employing nonlinear parametric conversion techniques where a crystalline host will be used to split an individual near-infrared photon into two photons of different wavelengths, one of which lies in the mid-infrared, with seeding enhancing the conversion process.
Using periodically poled lithium niobate (PPLN), consequently restricting wavelength operation to below 5 m, two initial areas will be investigated :- Power scaling nanosecond generation at 3.3-3.5 m to generate wavelength and pulse duration tunable radiation at the > 20 W level. In addition, CW generation at 3.4 m will be undertaken with probable scaling to the > 30 W level, using a 300 W ytterbium (Yb) and an 80 W erbium (Er) fiber laser.
To extend the wavelength range, new nonlinear materials will be investigated and characterised - BaGa4Se7 [BGSe], targeting 7-15 microns and cadmium silicon phosphide (CdSiP2) [CSP], targeting 6-7 microns. Initially 7-10 microns in BGSe will be examined through mixing Er:fibre (1.55 microns) and Tm:fibre (1.91 microns) systems and 6-7 microns in CSP through mixing of Yb:fibre (1.06 microns) with tuneable continuous wave semiconductor diodes (1.26-1.29 microns). Alternatively, all required pump wavelengths throughout the complete near infra red can be provided by our unique fibre Raman MOPFA sources (1-1.8 m), permitting extreme versatility in the parametric wavelengths generated.
In the femtosecond regime, initial studies will concentrate on the 3-5 m region, employing an in-house constructed high average power (~30W), 100 fs Yb oscillator-amplifier scheme co-seeded with radiation from either fibre Raman lasers or high power semiconductor lasers. Extension beyond 5 m will then be investigated in the alternative crystalline materials noted above.