PPLN based optical parametric oscillators pumped by high power lasers around 1 µm are well established sources for generating light in the 3-5 µm spectral region, of interest for a wide range of scientific, commercial and military applications. We have been investigating optical parametric amplification (OPA), or difference-frequency generation (DFG), single-pass alternatives to conventional resonant OPOs. This avoids the need for a cavity and the corresponding design constraints that this can impose; such as, fixed repetition rates, sensitive alignment and/or poor output beam qualities at high average power levels. In this paper, we review recent results on high average power ( > 6 W) nanosecond pulse generation in the 3.3-3.5 μm region at MHz repetition rates, employing Yb:fibre and Er:fibre master oscillator power amplifiers (MOPA) systems pumping PPLN OPAs. We use focused Gaussian beam theory to validate the experimental results. We will also discuss spectral extension further into the mid-infrared, using different nonlinear crystal and alternative rare-earth doped fibre MOPA and Raman shifted fibre laser combinations. Ongoing work aimed at the power scaling of the mid-infrared light in both the nanosecond pulsed and continuous wave regimes will be presented.
Second harmonic generation (SHG) is a ubiquitous technique for extending the spectral coverage of laser sources into regions that would otherwise be technologically challenging to access. SHG schemes typically rely on the use of bulk optical components, resulting in systems with large footprints requiring precise optical alignment. Integration of the SHG components into a single unit facilitates the implementation of compact, robust and turn-key sources, suitable for applications in biophotonic imaging, amongst others. We report on the development of fiber-coupled frequency doubling modules and their application to novel fiberintegrated picosecond pulse sources in the visible and near-visible. The modules employ a simple, single-pass configuration using a periodically-poled lithium niobate (PPLN) crystal as the nonlinear conversion medium. They are readily adaptable for different fiber pump laser configurations and are configurable with either fiber-coupled or collimated free-space outputs. Two sources using the modules are presented, operating at 780 nm and 560 nm. The 780 nm source utilizes an erbium master oscillator power fiber amplifier (MOPFA) scheme. SHG was performed in a 35 mm long crystal, generating 3.5 W of 780 nm radiation with a pulse duration of 410 ps at 50 MHz and conversion efficiencies exceeding 20%. Results of this source being used for parametric wavelength conversion in photonic crystal fiber are discussed. The 560 nm source was based on SHG of a Raman amplified CW diode pumped by a pulsed ytterbium-fiber MOPFA. This source generated 450 mW of average power with conversion efficiencies greater than 20%.