We report on the fabrication of what we believe is the first example of a two-dimensional (2D) nonlinear photonic crystal [Berger, Phys. Rev. Lett. 81, 4136 (1998)], where the refractive index is constant but where the 2nd order nonlinear susceptibility is spatially periodic. Such crystals allow for efficient quasi-phase-matched 2nd harmonic generation using multiple reciprocal lattice vectors. External 2nd harmonic conversion efficiencies >60% were measured with picosecond pulses. The fabrication technique is extremely versatile and should allow for the fabrication of a broad range of 2D crystals including quasicrystals.

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We demonstrate what is to our knowledge the first synchronously pumped high-gain optical parametric oscillator (OPO) with feedback through a single-mode fiber. This device generates 2.3 -2.7 W of signal power in 700 -900-fs pulses tunable in a wavelength range from 1429 to 1473 nm. The necessary high gain was obtained from a periodically poled LiTaO 3 crystal pumped with as much as 8.2 W of power at 1030 nm from a passively mode-locked Yb:YAG laser with 600-fs pulse duration and a 35-MHz repetition rate. The fiberfeedback OPO setup is compact because most of the resonator feedback path consists of a standard telecom fiber. Because of the high parametric gain, the fiber-feedback OPO is highly insensitive to intracavity losses. For the same reason, the synchronization of the cavity with the pump laser is not critical, so active stabilization of the cavity length is not required. © 2001 Optical Society of America OCIS codes: 190.4970, 190.2620, 190.7110, 320.7080, 190.4360. Synchronously pumped optical parametric oscillators (OPO's) are interesting sources of broadly wavelength-tunable ultrashort pulses as required for many applications. Recently it was demonstrated 1 that power-scalable diode-pumped thin disk lasers 2 can be passively mode locked with a semiconductor saturable-absorber mirror. 3,4Such mirrors generate multiwatt average output powers, even in the subpicosecond domain. In this Letter we show that the combination of such a pump laser with a fiber-feedback OPO, a novel type of synchronously pumped OPO in which a single-mode fiber represents most of the cavity length, results in a system with a number of attractive features. Our concept has led to a stable and compact OPO setup that is unusually insensitive to intracavity losses and drifts of the OPO cavity length. Even with nonoptimal optical components, we obtained as much as 2.7 W of average power in 900-fs pulses tunable around 1.45 mm. In contrast to many other OPO's in this pulse duration regime, the fiber-feedback OPO does not need active stabilization of the cavity length.The incorporation of a f iber into a cavity that contains bulk components will, in general, introduce substantial losses, mainly at the fiber launch. Nevertheless, high power-conversion eff iciency can be achieved if a large parametric gain is available and most of the power of the resonant wave is coupled out directly after the nonlinear crystal. Other intracavity losses then affect only a small portion of the generated power. We achieved a small-signal gain of the order of 90 dB by applying a high average pump power of as much as 8.2 W to a periodically poled LiTaO 3 (PPLT) crystal, which has a relatively high nonlinearity [d eff ഠ 9 pm͞V (Ref. 5)]. The Yb:YAG pump laser is slightly modified from that described in Ref. 1, generating pulses with a duration of 0.6 ps at a repetition rate of 35 MHz and delivering as much as ഠ11 W of average power.After passing an isolator and an attenuator, the pump beam is focused with a curved mirror ͑M 1 ͒ to a waist with 90-mm radius in...

We report the generation of 450-mW average blue (473-nm) power by frequency doubling of a diode-pumped 946-nm Nd:YAG laser. We achieved pulsed operation at a high repetition rate (~160kHz) by driving the relaxation oscillations of the laser. A 40% conversion efficiency to the second harmonic was obtained in a single-pass, extracavity, first-order, quasi-phase-matched process in which periodically poled lithium niobate (period 4.5microm , thickness 0.5mm , and length 15mm) at 140 degrees C was used. The resulting high-power blue beam was circular in profile and nearly diffraction limited, indicating that photorefractive effects do not appear to limit device performance.