A new coupled-cavity design for single-longitudinal-mode operation of an optical parametric oscillator (OPO) is presented. The OPO is based on a b-BaB 2 O 4 crystal and is pumped by the third harmonic of a Nd : YAG laser. With this design, we achieved single-longitudinal-mode operation of the OPO with a decrease in the threshold and an increase in external eff iciency compared with those of a conventional grazing-incidence OPO. A mathematical model that describes the mode spacings for this cavity is given. © 1995 Optical Society of America Optical parametric oscillators (OPO's) are attractive solid-state sources of coherent radiation with extensive tuning ranges and high efficiencies. However, free-running OPO's exhibit a broad spectral bandwidth that is not desirable for many applications. Several mechanisms have been explored to produce a narrow linewidth: injection seeding with a narrowband laser 2 and the use of frequency-selective intracavity elements such asétalons.
3Another technique to reduce the spectral bandwidth is to use a cavity in a grazing-incidence conf iguration. 4 For an OPO based on KTP in this conf iguration, single-longitudinal-mode (SLM) operation has been achieved. 4 This configuration has also been used to minimize the bandwidth of an OPO based on b-BaB 2 O 4 (BBO), as we reported recently.
5However, the external eff iciency of such a cavity was low, and we found stable SLM operation diff icult to achieve.In this Letter we present an OPO with a new cavity design based on the grazing-incidence conf iguration. The new design includes an additional mirror that couples back a small part of the zero-order output of the cavity. We have observed reliable SLM operation of this OPO at signif icantly higher external eff iciencies. It involves two coupled cavities: a grazing-incidence cavity and a cavity formed by the back mirror and a zero-order mirror placed in the zero-order beam ref lected from the grating. This second cavity is referred to as the linear cavity in the text. This composite resonator can be described as a Michelson-mirror cavity 6 with the conventional beam splitter replaced by a grating. Michelson-mirror cavities have been used to reduce the bandwidth of lasers, as reviewed by Smith.7 Using a mathematical model based on the description by Smith, 8 we are able to calculate the mode spacing of the coupled cavity accurately. Pinard and Young investigated a Fox -Smith-type coupled cavity for an OPO. 9 This coupled cavity, in which the gain medium is active in only one of the two cavities, differs from that in the Michelson case, in which the gain medium is active in both cavities.