Octave-spanning, continuously tunable injection-seeded backward terahertz-wave parametric oscillator

We present an all-optical, octave-spanning, continuously tunable injection-seeded backward terahertz-wave parametric oscillator (BW-TPO) pumped with a pulsed Nd:YAG laser along with a continuous-wave seed laser. As a BW-TPO gain medium, four crystals with specially designed slant-stripe-type periodically poled lithium niobate (PPLN) for different frequency ranges are proposed. By introducing a unique off-center rotation of the rectangular PPLN crystal to achieve maximum broadband tunability, a wide mode-hop-free tunable bandwidth of more than 400 GHz and a frequency span of up to 1.7 octaves from a single gain crystal was achieved. The total tunable coverage over all crystals spans 763 GHz, equivalent to over 2 octaves of tunability. It makes the BW-TPO an innovative source-emitter for nondestructive imaging, such as in swept-source optical coherence tomography, and for sub-THz frequency spectroscopy.