Frontispiece: Dual‐wavelength optical parametric oscillator using antiresonant ring interferometer

The photograph shows an example of an antiresonant ring (ARR) interferometer, first proposed by Sagnac in 1913. The ARR consists of a beam‐splitter and two mirrors, which essentially form a ring. The input optical beams are split into clockwise and counterclockwise beams. After propagation inside the ring and recombination, the beams are totally reflected backwards, making the ARR function as a highly reflecting mirror. In their letter, A. Esteban‐Martin, V. Ramaiah‐Badarla, and M. Ebrahim‐Zadeh ( DOI 10.1002/lpor.201200018 pp. L7‐L11) report a novel technique for coupling of two resonant optical cavities using the ARR, without coupling between the gain media. While the technique has been demonstrated in a femtosecond synchronously‐pumped optical parametric oscillator (OPO), the method can be universally applied to any oscillator (OPOs, lasers, and their combination) and/or operating in any temporal regime (continuous‐wave, pulsed, ultrafast picosecond and femtosecond, or their combination), and in any spectral range of interest. The absence of gain coupling also enables unrestricted and uninterrupted tuning through wavelength degeneracy, together with the ability to tune the degenerate wavelength, making the technique highly desirable for the generation of widely tunable radiation in different regions across the THz spectrum.