Satoshi Ashihara

Soliton compression of femtosecond pulses in quadratic media

We describe efficient soliton compression of femtosecond pulses by use of cascade quadratic nonlinearity and normal dispersion in quadratic media. Pulse compression by a factor of ∼3 is achieved in ∼30-mm-long beta-barium borate at a wavelength of 800 nm. We investigate the dependence of compression performance on phase mismatch, input intensity, and propagation length. The compressed pulses are fully characterized by use of the frequency-resolved optical gating method.

Group-velocity matched second-harmonic generation in tilted quasi-phase-matched gratings

We propose a novel scheme of achromatic phase matching for second-harmonic generation (SHG) in tilted quasi-phase matching gratings. The spectral angular dispersion is introduced in fundamental waves such that each frequency component satisfies the two-dimensional quasi-phase matched condition. This is equivalent to simultaneous quasi-phase- and group-velocity-matched SHG for ultrashort pulses. Equations to describe achromatic conditions are derived and applied specifically to periodically poled lithium niobate (PPLN). The phase-matching bandwidth for 10 mm-long PPLN increases by factors of 110 and 32 at wavelengths of 0.8 and 1.55 μm, respectively, compared with those for the conventional quasi-phase matching.

Optical pulse compression using cascaded quadratic nonlinearities in periodically poled lithium niobate

Efficient pulse compression was achieved by the use of cascaded quadratic nonlinearities in 10-mm-long periodically poled MgO-doped lithium niobate. An off-diagonal component of the nonlinear coefficient was utilized for group-velocity matched cascaded interactions at the fundamental wavelength of 1560 nm. Simultaneously compressed fundamental and second-harmonic pulses of ∼35 fs duration were obtained from the 110-fs-pump pulses.

Efficient frequency doubling of a femtosecond pulse with simultaneous group-velocity matching and quasi phase matching in periodically poled, MgO-doped lithium niobate

We demonstrate efficient frequency doubling of 95-fs pulses with a small temporal broadening in a 10-mm-long, periodically poled, 5-mol % MgO-doped lithium niobate crystal. Simultaneous group-velocity matching and quasi phase matching were achieved by the off-diagonal nonlinear optical coefficient d32 at telecommunications range.

Polaron dynamics in lithium niobate upon femtosecond pulse irradiation: Influence of magnesium doping and stoichiometry control

The formation and relaxation dynamics of electron polarons in lithium niobate crystals were investigated by measuring transient absorption induced by blue femtosecond pulses. Anisotropy in the absorption change distinguished between small free polarons and small bound polarons, revealing that the dynamics were influenced by MgO doping and stoichiometry control. In crystals doped with MgO at concentrations above threshold, small free polarons were generated within 100 fs and decayed after tens of nanoseconds. In the presence of antisite defects, sequential formation of polaronic states occurred: electrons initially trapped as small free polarons became trapped as small bound polarons on picosecond time scale. The results are relevant for nonlinear optical applications of pulsed or high-power lasers.

Group-velocity-matched noncollinear second-harmonic generation in quasi-phase matching

We demonstrate group-velocity-matched second-harmonic generation of 80-fs pulses under noncollinear quasi-phase-matching (QPM) geometry. Second-harmonic pulses of 100-fs duration, 11-nm bandwidth, and0.78-µm center wavelength were generated with 50% efficiency with 3-mm-long periodically poled lithium niobate. We have also numerically studied the dependence of conversion efficiency and pulse durations on pump intensity, propagation length, and noncollinear angle. It is confirmed that our group-velocity-matching scheme requires a much lower pump intensity than the conventional collinear QPM scheme.

Linear sensing of speckle-pattern displacements using a photorefractive GaP crystal

Linear transformation of lateral speckle-pattern displacements into the transmitted intensity modulation is achieved in a photorefractive GaP crystal using the polarization self-modulation effect. The optical system is very simple, self-referenced, and as sensitive as an interferometer.

Adiabatic compression of quadratic temporal solitons in aperiodic quasi-phase-matching gratings

We numerically show that it is possible to achieve adiabatic compression of femtosecond quadratic solitons in aperiodically poled lithium niobate device. Two-colored solitons of the fundamental wavelength of 1560 nm can be adiabatically shaped by using group-velocity matching schemes available in quasi-phase-matching (QPM) devices. We investigate the performance of the adiabatic compression based on two different group-velocity matching schemes: type-I (e: o + o) collinear QPM geometry and type-0 (e: e + e) non-collinear QPM geometry. Two-colored temporal solitons with pulse duration of 35 fs are generated without visible pedestals from 100-fs fundamental pulse. We also show that walking solitons with shorter pulse durations are adiabatically excited under small group-velocity mismatch condition. The walking solitons experience deceleration or acceleration during compression, depending on the sign of the groupvelocity- mismatch. The demonstrated adiabatic pulse shaping is useful for generation of shorter pulses with clean temporal profiles, efficient femtosecond second harmonic generation and group-velocity control.

Cascaded third-harmonic generation of ultrashort optical pulses in two-dimensional quasi-phase-matching gratings

Cascaded third-harmonic generation (THG) of femtosecond pulses in a two-dimensional periodically poled lithium niobate is demonstrated experimentally. The poling pattern satisfies quasi-phase-matching conditions for both second-harmonic generation (SHG) and the following sum-frequency generation. The pulse-front tilt in noncollinear interactions compensates for group-velocity mismatch among fundamental, second-harmonic (SH), and third-harmonic (TH) pulses, thereby achieving broad acceptance bandwidths in SHG and THG. A 2 mm thick device generates SH and TH pulses of 106 and 110 fs, respectively, from fundamental pulses with temporal duration of 118fs and a 1568 nm center wavelength. The frequency conversion properties are studied theoretically, and the optimal focusing condition for maximum conversion efficiency is derived.

Spectral broadening of mid-infrared femtosecond pulses in GaAs

We achieved efficient spectral broadening for mid-IR pulses of few-microjoule energy. The spectral bandwidth of the femtosecond pulses at the center wavelength of 5000 nm increased from 540 nm to 2060 nm (from 220 to 910 cm(-1) in frequency) by nonlinear propagation in a gallium arsenide single crystal. The spectral broadening was accompanied by nonlinear absorption loss of 25%. The demonstrated scheme should be available at any operation wavelength within the material transparency range and provides a useful tool in nonlinear vibrational spectroscopy.