Compact chip-scale comb sources are of significant interest for many practical applications. Here, we experimentally study the generation of supercontinuum (SC) in an axially varying integrated waveguide. We show that the local tuning of the dispersion enables the continuous blue shift of dispersive waves by more than 200 nm due to their trapping by the strongly compressed pump pulse. This mechanism provides an insight into supercontinuum generation in varying-dispersion integrated waveguides. Pumped close to 2.2 μm in the femtosecond regime and at a pulse energy of approximately 4 pJ, the output spectrum extends from 1.1 μm up to 2.76 μm and shows good coherence properties. Octave-spanning SC is also observed at an input energy as low as approximately 0.9 pJ. We show that the supercontinuum is more robust against variations of the input-pulse parameters and is also spectrally flatter in waveguides with an optimized dispersion profile than in dispersion-engineered ones. This research demonstrates the potential of varying-dispersion waveguides for coherent SC generation and paves the way for integrated low-power applications, such as chip-scale frequency-comb generation, precision spectroscopy, optical-frequency metrology, and wide-band wavelength division multiplexing in the near infrared.

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We report the first optical parametric generator (OPG) based on the new nonlinear material, orientationpatterned gallium phosphide (OP-GaP). Pumped by a Qswitched nanosecond Nd:YAG laser at 1064 nm with 25 kHz pulse repetition rate, the OPG can be tuned across 1721-1850 nm in the signal and 2504-2787 nm in the idler. Using a 40-mm-long crystal in single pass, we have generated a total average output power of up to ~18 mW, with ~5 mW of idler power at 2670 nm, for 2 W of input pump power. The OPG exhibits a passive stability in total output power better than 0.87% rms over 1 hour, at a crystal temperature of 120 ºC, compared to 0.14% rms for the input pump. The output signal pulses, recorded at 1769 nm, have duration of 5.9 ns for input pump pulses of 9 ns. Temperature-dependent loss measurements for the pump polarization along the [100] axis in the OP-GaP crystal have also been performed, for the first time, indicating a drop in transmission from 28.8% at 50ºC to 19.4% at 160ºC. © 2015 Optical Society of America OCIS codes : (190.4360) Nonlinear optics, devices; (190.4400) Nonlinear optics, materials; (190.4410 Mid-infrared (mid-IR) coherent sources based on optical parametric down-conversion are of great significance for a variety of applications such as spectroscopy [1], up-conversion imaging [2] and as pump source for other nonlinear processes [3]. Oxide-based nonlinear crystals, and especially their quasi-phase matched (QPM) counterparts such as MgO-doped periodically-poled LiNbO3 (MgO:PPLN), stoichiometric LiTaO3 (MgO:sPPLT) and KTiOPO4 (PPKTP), have enabled broadband mid-IR coverage up to ~4 μm, in all time-scales from continuous-wave (CW) to femtosecond domain [4][5][6][7]. However, the onset of multiphonon absorption in these nonlinear crystals has been a fundamental barrier to reach spectral regions into the deep mid-IR. Non-oxide-based nonlinear materials with extended transparency in the mid-IR, such as ZnGeP2 (ZGP) and orientation-patterned GaAs (OP-GaAs), offer potential for the development of parametric sources beyond 4 μm. However, their low bandgap requires pumping beyond 2 μm to avoid two-photon absorption. The more recently developed nonlinear crystal, CdSiP2 (CSP), is an important new addition to the family of birefringent nonlinear materials for parametric downconversion into the deep mid-IR. The high optical nonlinearity, noncritical phase-matching (NCPM) capability, and a wide transparency range extending down to below ~1 μm, enable the deployment of the widely available Nd-based solid-state or Ybbased fiber lasers as pump source for deep mid-IR wavelength generation up to ~7 μm [8,9]. On the other hand, the new semiconductor nonlinear material, orientation-patterned GaP (OPGaP) can be considered as the QPM analog of CSP, and a promising alternative for deep mid-IR generation. As a QPM material, OP-GaP offers unique flexibility for tunable wavelength generation into the deep mid-IR using grating engineering under NCPM with no spatial walk-off. Moreover, unlike ZGP and OP-GaAs, it has sig...

We report a tunable, single-pass, pulsed nanosecond difference-frequency-generation (DFG) source based on the new semiconductor nonlinear material, orientationpatterned gallium phosphide (OP-GaP). The DFG source is realized by mixing the output signal of a nanosecond OPO tunable over 1723-1827 nm with the input pump pulses of the same OPO at 1064 nm in an OP-GaP crystal, resulting in tunable generation over 233 nm in the midinfrared from 2548 to 2781 nm. Using a 40-mm-long crystal, we have produced ~14 mW of average DFG output power at 2719 nm for a pump power of 5 W and signal power of 1 W at 80 kHz repetition rate. To the best of our knowledge, this is the first single-pass nanosecond DFG source based on OP-GaP. The DFG output beam has a TEM00 spatial mode profile and exhibits passive power stability better than 1.7% rms over 1.4 hours at 2774 nm, compared to 1.6% and 0.1% rms for the signal and pump, respectively. The OP-GaP crystal is recorded to have a temperature acceptance bandwidth of 17.7 ºC. Mid-infrared (Mid-IR) pulsed sources in the 2-3 µm wavelength range are highly desirable for industrial, medical and securityrelated applications [1]. Access to this spectral region can be achieved by using conventional bulk/fiber lasers as well as optically pumped semiconductor lasers [2][3][4]. However, stable, high output power together with spectral agility can be best achieved by nonlinear frequency conversion techniques, such as difference-frequency-generation (DFG) and optical parametric oscillators (OPOs). While OPOs can provide tunability together with high efficiency, DFG sources can be realized in simple singlepass architecture. The ability to tune one of the input beams in the DFG process results in tunable output at longer wavelengths. Such DFG sources based on near-IR nonlinear materials such as -BaB2O4 (BBO) [5], KTiOPO4 (KTP) [6] and MgO-doped periodically poled LiNbO3 (MgO:PPLN) [7], and mid-IR nonlinear materials such as ZnGeP2 (ZGP) [8] and orientation-patterned GaAs (OPGaAs) [9] have been previously demonstrated. Although ZGP and OP-GaAs can provide spectral coverage deep into the mid-IR, they require pumping beyond 2 μm due to two-photon absorption at short-wavelength transparency cut-off [10]. On the other hand, BBO, PPKTP and MgO:PPLN can be pumped using the readily available near-IR laser sources in the 1-2 μm range, but their longwavelength transparency cut-off limits efficient DFG beyond ~4 μm. As such, it is important to investigate alternative nonlinear materials, which can be pumped in the near-IR to generate deep mid-IR radiation. One such material is the recently developed birefringent nonlinear crystal, CdSiP2 (CSP), which enables DFG at wavelengths as long as 6 µm when pumped in the near-IR [11]. Orientation-patterned gallium phosphide (OP-GaP) is a new quasiphase-matched (QPM) nonlinear material with transparency across 0.8-12 µm and a high nonlinear coefficient of ~70 pm/V [12]. It also possesses a high thermal conductivity of 110 W/m-K, leading to a high damage threshold of 0.8 ...