tokens: The signal source comes from a quasi-monochromatic Continuous Wave HeNe laser emitting at 3.39 μm. Figure 2. Description of the experimental setup. M: injection mirrors for the infrared signal at 3.39μm; L1: focusing lens for the MIR injection into the Periodically Poled Lithium Niobate (PPLN) waveguide; L2: focusing lens for the converted signal into a fibre single mode at 810 nm; L3: collimating lens for the pump signal; P: off-axis parabola; D: dichroic mirror used to separate converted signal from pump signal; BS: beam splitter used to separate the MIR signal in two separated ways. The 3.5 mW signal is shared equally between the two interferometric arms thanks to a pellicle beam splitter BS. Over the MIR stage, a set of mirrors and objectives allow the injection of the signal into frequency converters in each arm. These converters consist of periodically poled Ti-indiffused waveguides in LiNbO 3 . The waveguides are designed to provide low-loss single-mode guiding in the MIR range. Poling periods around 22 μm enable the phase-matching for the SFG process. The entire length of the non-linear conversion section is 92 mm. The non-linear stages are powered by a distributed feedback laser laser emitting at 1065 nm to generate a wave at 810 nm through the SFG process. On each interferometric arm, the maximum power conversion efficiency is measured in the range of 3·10 −5 when each non-linear crystal is powered by a 60 mW pump power. Notice that this overall throughput includes the non-linear efficiency and the whole losses resulting from the launching and collimating stages, including the stringent spatial filtering. This current low conversion efficiency is to be improved for future application, e.g. by using pigtailed techniques as demonstrated by Büchter (Büchter, Herrmann Sohler 2011 ) The pump signal is injected backward using a taper, and is then back-reflected towards the output of the Periodically Poled Lithium Niobate (PPLN) waveguide thanks to a dichroic mirror directly deposited at the waveguide endface [Anti-Reflection (AR) at 3.39 μm and High-Reflection (HR) at 1065 nm, see Fig. 3 ]. The waveguide taper allows us to ensure a better control of the pump beam geometry leading to the optimal spatial overlapping in the non-linear crystals between the pump at 1065 nm and the signal at 3.39 μm. Figure 3. PPLN injection scheme. The pump wave at 1065 nm propagates in the reverse direction, and is coupled into the waveguide thanks to a taper. After a back-reflection by a dichroic mirror, the pump field interacts with the signal at 3.39 μm to generate the converted wave at 810 nm. We define the normalized conversion efficiency curve of the SFG process as the ratio between the converted power and the signal one as a function of the signal wavelength. This curve depends on the crystal's optical and geometrical parameters (Fig. 4 ) and can be tuned by a fine control of its temperature. The full width at half-maximum of the conversion
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