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Continuously tunable cw emission near 4.5 μm was generated by difference frequency generation (DFG) within a Nd:YAG pumped optical parametric oscillator (OPO). The periodically poled lithium niobate crystal used for DFG contained eight grating bands that enabled wavelength tuning between 4.25 and 4.65 μm. As much as 90 mW of power at 4.48 μm was achieved for 16.7 W of pump.
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Da-Wun Chen (2003) studied this question.
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