Andrew M. Schober

Measurement of the nonlinear coefficient of orientation-patterned GaAs and demonstration of highly efficient second-harmonic generation

Quasi-phase-matched (QPM) GaAs structures, 0.5 mm thick, 10 mm long, and with 61-mum grating periods, were grown by a combination of molecular-beam epitaxy and hydride vapor phase epitaxy. These were characterized by use of mid-IR second-harmonic generation (SHG) with a ZnGeP(2) (ZGP) optical parametric oscillator as a pump source. The SHG efficiencies of QPM GaAs and QPM LiNbO(3) were directly compared, and a ratio of nonlinear coefficients d(14)(GaAs)/d(33) (LiNbO(3))=5.01+/-0.3 was found at 4.1-mum fundamental wavelength. For input pulse energies as low as 50muJ and approximately 60-ns pulse duration, an internal SHG conversion efficiency of 33% was measured in QPM GaAs.

Broadband quasi-phase-matched second-harmonic generation of ultrashort optical pulses with spectral angular dispersion

Noncollinear quasi-phase matching, in combination with spectral angular dispersion, can be used to broaden the bandwidth of second-harmonic generation (SHG) beyond the bandwidth for collinear, nondispersed interactions. A general theoretical treatment is presented, in addition to a solution that predicts the generated field for the case of a Gaussian input field; a comparison is made between this technique and others available for broadband SHG. An experiment in periodically poled lithium niobate demonstrates SHG of a 138 fs pulse at 1550 nm in a 1 cm length crystal (with a collinear acceptance bandwidth 13 times narrower than the first-harmonic bandwidth) with minimal spectral narrowing.

Picojoule threshold, picosecond optical parametric generation in reverse proton-exchanged lithium niobate waveguides

We have demonstrated picosecond optical parametric generation in reverse proton-exchanged waveguides in periodically poled lithium niobate with thresholds as low as 200 pJ. Near-transform-limited near-infrared pulses were obtained from cascaded optical parametric generation. For a 1.8-ps (FWHM) pump pulse at 769.6 nm, a saturated internal photon-conversion efficiency of 33% was obtained with 1 nJ of pump energy. The signal-wavelength tuning range was from 1.15 μm to 2.3 μm with a pump wavelength between 770 nm and 789.5 nm. Numerical simulations well predicted the mechanism for the transform-limited pulse generation. These results enable optical parametric generation devices with low threshold and good temporal properties with a simple single-pass setup.

Quasi-group-velocity matching using integrated-optic structures

We propose a device to compensate for group-velocity mismatch (GVM) effects that limit the efficiency-bandwidth product in nonlinear frequency-mixing devices. Integrated wavelength-dependent delay lines are introduced periodically in a waveguide containing a series of quasi-phase-matching (QPM) gratings. Appropriate choice of the time delays can compensate for GVM. We have demonstrated a two-stage device in a periodically poled lithium niobate waveguide. Two approximately 150-fs-long pulses generated 6 ps apart by second-harmonic generation in two QPM gratings were resynchronized by a fixed delay line, and their relative phase was fine controlled by temperature tuning. This technique, which can be iterated to more than two segments, permits optical frequency mixers with a higher efficiency-bandwidth product than would be possible in a single grating short enough to avoid GVM effects.

Broadband optical parametric amplifier using chirped quasi-phase-matched gratings

We demonstrate operation of an optical parametric amplifier using a chirped quasi-phase-matching grating. Such devices enable engineering of gain spectra over wide bandwidths, applicable to femtosecond pulse amplification and other wide-band devices.

Multi-color spatial solitons through seeded optical parametric amplification tailored with chirped-period quasi-phase-matching gratings

We demonstrate the tailoring of quadratic spatial solitons using chirped gratings of periodically-poled lithium niobate, providing a method for creating solitons at large wavevector mismatch that are difficult to launch in uniform crystals.

Ultra-broadband second-harmonic generation using spectral angular dispersion in quasi-phase-matched nonlinear materials

We demonstrate the use of spectral angular dispersion in quasi-phase-matched second-harmonic generation (SHG) of 138 fs pulse at 1550 nm in a 1-cm-long crystal of periodically-poled lithium niobate (collinear acceptance bandwidth 13 times narrower than the first-harmonic bandwidth) with minimal spectral narrowing. In addition, we discuss the unique potential of quasi-phase-matched nonlinear materials in noncollinear SHG, and compare this technique to other methods for broadband frequency conversion.

Engineering of multicolor spatial solitons with chirped-period quasi-phase-matching gratings in optical parametric amplification.

We demonstrate the excitation of solitons in a parametric amplifier with enhanced signal content through the use of a chirped-period quasi-phase-matching grating. This technique affords a low soliton threshold at the input end of a parametric amplifier, and the subsequent transformation to a desired soliton that exists at nonzero wave-vector mismatch through the use of a linearly chirped quasi-phase-matching grating. This approach has an advantage over direct excitation of solitons at nonzero wave-vector mismatch in uniform nonlinear materials and holds potential for improving the efficiency and mode quality of high-gain parametric amplifiers.