Generation of tunable high-purity microwave and terahertz signals by two-frequency solid-state lasers

We show that diode-pumped solid-state lasers can generate tunable high-purity microwave signals. In the case of a single-axis cavity containing an adjustable linear phase anisotropy, orthogonal linear eigenstates oscillate with a continuously tunable frequency difference. The maximum beat frequency is fixed by the laser cavity length and can reach a few tens of GHz. In order to reach the THz range, insertion of a double refraction crystal inside the laser cavity creates a two-axis laser that allows one to choose independently the frequencies of the two eigenstates. In this case the maximum beat frequency is fixed by the active medium gain bandwidth which is of a few THz for an Er: Yb:glass active medium. We show that doubling the two frequencies emitted by such a two-axis laser at 1.55 μm yields a source of tunable cw THz beat notes suitable for photomixing in GaAs-based THz emitters. Moreover, the beat notes generated by diode-pumped solid-state lasers can be phase-locked to microwave local oscillators. In particular, we show that a single-axis Er:Yb:glass laser provides a beat note continuously tunable from 0 to 20 GHz with a 170 μHz line width. The phase noise of such a source is measured to be lower than -130 dBc/Hz at 100 kHz offset from the carrier.