H. P. Chung

Adiabatic light transfer in titanium diffused lithium niobate waveguides.

Ultra-broadband adiabatic light transfer was demonstrated for the first time in lithium niobate. Coupling bandwidths and efficiencies of ~540 nm and >0.96, respectively, were obtained with 5-cm long Ti:LiNbO3 adiabatic couplers for both polarization modes.

Supercontinuum generation in quadratic nonlinear waveguides without quasi-phase matching.

Supercontinuum generation (SCG) is most efficient when the solitons can be excited directly at the pump laser wavelength. Quadratic nonlinear waveguides may induce an effective negative Kerr nonlinearity, so temporal solitons can be directly generated in the normal (positive) dispersion regime overlapping with common ultrafast laser wavelengths. There is no need for waveguide dispersion engineering. Here, we experimentally demonstrate SCG in standard lithium niobate (LN) waveguides without quasi-phase matching (QPM), pumped with femtosecond pulses in the normal dispersion regime. The observed large bandwidths (even octave spanning), together with other experimental data, indicate that negative nonlinearity solitons are indeed excited, which is backed up by numerical simulations. The QPM-free design reduces production complexity, extends the maximum waveguide length, and limits undesired spectral resonances. Finally, nonlinear crystals can be used where QPM is inefficient or impossible, which is important for mid-IR SCG. QPM-free waveguides in mid-IR nonlinear crystals can support negative nonlinearity solitons, as these waveguides have a normal dispersion at the emission wavelengths of mid-IR ultrafast lasers.

Characterization and analysis of finite-beam Bragg diffraction in a periodically poled lithium niobate electro-optic grating

We report the study, both theoretical and experimental, of the finite-beam Bragg diffraction behavior of an electro-optic (EO) volume grating made of a periodically poled lithium niobate (PPLN) crystal. When a Gaussian laser beam is used, the experimental observations show that the diffraction characteristics of the PPLN EO Bragg device, including the diffraction mode pattern and diffraction efficiency, are closely related to the interaction beam size and applied voltage, which cannot be modeled properly by a simplified theory using the plane-wave approximation. In this work, we have developed a theoretical model for describing the diffraction behavior of a PPLN EO Bragg device based on the coupled-wave theory with the aid of the plane-wave decomposition method. Specifically, we found that it is the angular distribution (or the dephasing bandwidth) of the plane wave elements decomposed from the incident Gaussian beam and grating strength that determine the Bragg coupling behavior of the device. We also identified some other electro-optically induced effects in the PPLN grating as an important mechanism in affecting the diffraction performance of the present device, especially at high working voltages.

Electro-optically tunable, multi-wavelength optical parametric generators in aperiodically poled lithium niobates

We report on the design and demonstration of electro-optically tunable, multi-wavelength optical parametric generators (OPGs) based on aperiodically poled lithium niobate (APPLN) crystals. Two methods have been proposed to significantly enhance the electro-optic (EO) tunability of an APPLN OPG constructed by the aperiodic optical superlattice (AOS) technique. This is done by engineering the APPLN domain structure either in the crystal fabrication or in the crystal design process to increase the length or block-number difference of the two opposite-polarity domains used in the structure. Several orders of magnitude enhancement on the EO tuning rate of the APPLN OPGs constructed by the proposed techniques for simultaneous multiple signal wavelength generation over a conventional one has been demonstrated in a near infrared band (1500-1600 nm).

Electro-optically spectrum tailorable intracavity optical parametric oscillator

We report a unique, pulsed intracavity optical parametric oscillator (IOPO) whose output spectrum is electro-optically (EO) tailorable based on an aperiodically poled lithium niobate (APPLN) working simultaneously as an optical parametric gain medium and an active gain spectrum filter in the system. We have successfully obtained from the IOPO the emission of single to multiple narrow-line signal spectral peaks in a near-infrared (1531 nm) band simply by electro-optic control. The power spectral density of the EO tailored signal can be enhanced by up to 10 times over the original (nontailored) signal.

Two-dimensional aperiodic nonlinear photonic crystal in a dual-wavelength Nd:YVO 4 laser for pulsed orange generation

We report the design and construction of a highly integrated two-dimensional (2D) aperiodic nonlinear photonic crystal (NPC) for working in a diode-pumped, dual-wavelength (1064 and 1342 nm) Nd:YVO₄ laser to demonstrate a compact, high-peak-power intracavity sum-frequency generator (ISFG) radiating at orange 593.5 nm. The 2D aperiodic NPC was built in quasi-phase-matched LiNbO₃whose crystal domain was structured based on the aperiodic optical superlattice technique to best achieve its simultaneous performance of a dual-wavelength electro-optic Bragg Q-switch and a SFG in the Nd:YVO₄ laser. When the NPC device was driven with a 350-V Q-switching voltage and a 1-kHz switching rate, we measured pulse energy of ~4.3 μJ (or peak power of ~531 W) at orange 593.5 nm from the constructed ISFG with 5.28-W diode power.

Tunable, pulsed multiline intracavity optical parametric oscillator using two-dimensional MgO: periodically poled lithium niobate–aperiodically poled lithium niobate

We report a tunable, pulsed multiline intracavity optical parametric oscillator (IOPO) realized in an Nd:YVO4 laser using a two-dimensionally domain engineered MgO:LiNbO3 as simultaneously an electro-optic Bragg Q switch and a multichannel optical parametric downconverter. The MgO:LiNbO3 was periodically and aperiodically poled along the crystallographic y and x axes, respectively, to simultaneously satisfy the phase-matching conditions required by the two quasi-phase-matching devices. When Q switched by 1 kHz, 300 V pulses, three signal lines at 1518, 1526, and 1534 nm were simultaneously generated, each with a peak power of ∼1  kW from the IOPO at 8.3 W diode power at 50°C. Spectral tuning of the three-line IOPO with temperature was demonstrated.

Double-prism domain PPLN for simultaneous laser Q-switching and optical parametric oscillation in a Nd:YVO 4 laser

We report a design and demonstration of an electro-optically Q-switched intracavity optical parametric oscillator (IOPO) based on a unique ramped duty-cycle periodically poled lithium niobate (PPLN) in a diode-pumped 1064-nm Nd:YVO(4) laser. The PPLN crystal, having a double-prism domain (DPD) structure with a domain period of 30 μm, can work simultaneously as an electro-optic (EO) beam deflector (and therefore an EO Q-switch in the laser cavity) and an optical parametric down converter. The characterized deflection sensitivity of the DPD PPLN device was 1.15°/kV-cm. At a 180-V Q-switching voltage and a 1-kHz switching rate, we measured a down-converted signal at 1550 nm with pulse energy of >8.1 μJ (or peak power of >2.3 kW) from the constructed IOPO at 7.5-W diode pump power. Continuous wavelength tuning of the IOPO signal was also demonstrated.

Parametrically Tunable Soliton-Induced Resonant Radiation by Three-Wave Mixing

A remarkable feature about the temporal optical soliton is that it can be phase-matched to new frequencies, emitting so-called resonant radiation (RR). This constitutes an efficient source of ultrafast pulses in emerging wavelength regimes, and plays a vital role in coherently extending the supercontinuum bandwidth [1]. RR waves are usually invoked by four-wave mixing (4WM) through the self-phase modulation (SPM) term A|A|2, or more recently through the conjugate SPM term A∗|A|2, or the third-harmonic generation (THG) term A3 [2, 3]. We here show with theoiy and experiments that three-wave mixing (3 WM) also supports soliton-induced RR waves [4].

Tunable soliton-induced resonant radiation by three-wave mixing

A remarkable feature about the temporal optical soliton is that it can be phase-matched to new frequencies, emitting so-called resonant radiation (RR). This constitutes an efficient source of ultrafast pulses in emerging wavelength regimes, and plays a vital role in coherently extending the supercontinuum bandwidth [1]. RR waves are usually invoked by four-wave mixing (4WM) through the self-phase modulation (SPM) term A|A|2, or more recently through the conjugate SPM term A∗|A|2, or the third-harmonic generation (THG) term A3 [2, 3]. We here show with theoiy and experiments that three-wave mixing (3 WM) also supports soliton-induced RR waves [4].