A twin-crystal device consisting of a pair of two identically cut KTP crystals of length l = 6 mm mounted with their optical axes symmetrically crossed is used to circumvent conversion-efficiency limitations that are due to the aperture effect in type-II (oeo) critically phase-matched second-harmonic generation (SHG) at λ = 1.3 and λ = 2.532 μm. A single-pass increase of as great as 3.2–3.5 times the conversion efficiency of a single crystal is obtained with this angle-tuned device at these wavelengths. This enhancement results from an increased effective coherence length of the interaction compared with that of a single bulk crystal of length 2l. We show that cavity-enhanced critically phase-matched type-II SHG can be easily performed with the twin device, allowing for walk-off as well as phase compensation for the fundamental-resonating ordinary and extraordinary waves at λ = 1.30 μm. Potential implementations of this device in other single-cavity-enhanced parametric interactions, such as doubly resonant optical parametric oscillation and degenerate sum-or difference-frequency mixing, are discussed.
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Zondy et al. (1994) studied this question.
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