Laser-based Mid-infrared Sources and Applications

5Mid‐IR by Nonlinear Optical Frequency Conversion

This chapter reviews devices based on nonlinear optical (NLO) frequency downconversion that operate in the 2–20 μm mid‐IR region. It discusses the principles of nonlinear frequency conversion, gives a brief assessment of existing and emerging NLO materials suitable for mid‐IR, and reviews current approaches for obtaining tunable mid‐IR output. We start first with the techniques that involve three‐wave processes, namely those based on the second‐order nonlinearity χ⁽²⁾.

5.1 Two Approaches to Frequency Downconversion Using Second‐order Nonlinearity

There are two basic techniques (Figure 5.1) for achieving broadly tunable mid‐IR output via frequency downconversion in a three‐wave process that relies upon second‐order nonlinearity χ⁽²⁾ of optical materials, namely (i) using difference frequency generation (DFG) and (ii) using optical parametric oscillators (OPOs). The OPO variations include traveling‐wave optical parametric generators (OPGs) and optical parametric amplifiers (OPAs).

While both DFG and OPO are based on three‐photon processes, the main distinction between them is that in the former case one needs two pump sources (and at least one of them has to be tunable to achieve mid‐IR tunability) while in the latter case one uses just one pump laser source (and a “seed” wave in the case of an OPA).

In both DFG and OPO processes, the photon energy conservation dictates that

(5.1)

while the photon momentum conservation (also ...