Huai-Chuan Lee

Diffusion-bonded composites of YAG single crystals

Diffusion bonding of single crystal Nd:YAG with another Nd:YAG or undoped YAG involves precision polishing, assembly by optical contacting and heat treatment. Preliminary data on bonding kinetics indicate an approximate (time)1/5 dependence which describes bulk sintering. Optical homogeneity across the interface did not exhibit a discontinuity for composites of Nd:YAG/Nd:YAG while a step at the Nd:YAG/undoped YAG interface is assumed to be due to a difference in refractive index. Parallel to the interface, stress birefringence values of up to about 5 nm/cm have been observed for Nd:YAG/Nd:YAG and about 20 nm/cm for Nd:YAG/undoped YAG. Corresponding values perpendicular to the interface are about 2 to 3 nm/cm for both types of composites. Reflectivity of about 3% and scattering of about 5% has been measured at 89 degrees incident angle while values below 0.03 and 0.05% respectively have been estimated for normal incidence.

Singly resonant optical parametric oscillator based on adhesive-free-bonded periodically inverted KTiOPO 4 plates: terahertz generation by mixing a pair of idler waves

We implemented a singly resonant optical parametric oscillator based on adhesive-free-bonded periodically inverted KTiOPO4 plates. It has major advantages such as walk-off compensation and oscillation at four wavelengths. The threshold of the oscillation was measured to be 8 MW/cm2, which is about a factor of 4 lower than that based in two separate KTiOPO4 crystals. By frequency-mixing the dual-wavelength output in GaP stacks, we generated the terahertz radiation at 2.54 THz. The tuning range of the terahertz output was demonstrated to be 2.19-2.77 THz.

Optical parametric oscillations of 2 μm in multiple-layer bonded walk-off compensated KTP stacks

Spatial walk-off can severely increase the threshold of critical phase-matched optical parametric oscillations (OPOs) and distort the output beam quality. With 16-layer KTP stacks for walk-off compensation, noncritical phase-match-like (NCPM-like) and quasi-phase-match-like OPOs with enhanced angular acceptance have been demonstrated based on two different KTP bonding orientations. In the NCPM-like composite, output pulse energy of 49 microJ has been achieved at pump energy of 523 microJ with pulse duration of 15 ns and repetition rate of 1 KHz. The OPO threshold as low as 254 microJ(44.6 MW/cm(2)) has been achieved.

Restoration of blurred images due to phase distortion based on polarization-insensitive phase conjugation in second-order nonlinear medium

Polarization-insensitive phase conjugation, achieved based on difference-frequency generation in a second-order nonlinear composite consisting of stacked KTP plates, was exploited to restore blurred images due to phase distortion as a novel scheme.

True crystalline fibers: double-clad LMA design concept of Tm:YAG-core fiber and its mode simulation

We propose a sub-100 μm double-clad single-crystalline Tm:YAG square core adhesive-free bond (AFB) waveguide structure as a prototype gain medium for an LMA ~2-μm fiber laser with ~790-nm diode laser pumping and present its mode simulation. The structure uses an undoped YAG and ceramic spinel for the inner and outer claddings, respectively. The mode simulation indicates that such double-clad structure can have over 1000-μm2 mode area in a 2% Tm:YAG core which will maintain a single transverse mode laser operation. The structure also has a large numerical aperture (~ 0.22) for the un-doped YAG inner cladding available for pumping.

High Efficiency, High Power 2.097-μm Ho:YAG Laser Pumped Mid-Infrared ZGP Optical Parametric Oscillation

A maximum output power of 18.6 W has been measured at a pump power of 23.7 W in a Tm:fiber laser pumped adhesive-free bonded (AFB®) YAG/Ho:YAG/YAG composite. The corresponding optical-to-optical efficiency and slope efficiency are 78.5% and 81.2%, respectively. When the laser is operated in Q-switched mode, the shortest laser pulse width of 11 ns has been measured in our experiment at repetition rate of 5 KHz. The laser has been used as pump source of a mid-infrared ZGP OPO. The wavelength tuning range from 3.06 to 6.6 μm has been achieved in a 15-mm long ZGP crystal. The maximum output power is 1.44 W at pump power of 10 W with repetition rate of 5 KHz.

Singly-resonant optical parametric oscillator in adhesive-free-bonded periodically inverted KTiOPO 4 plates: Achieving oscillations at dual wavelengths

Based on adhesive-free-bonded periodically inverted KTiOPO4 plates, we implemented a singly-resonant optical parametric oscillator, which has superior advantages of compensating walk-off and oscillating dual-signal wavelengths based on QPM.

Walk-off corrected KTP crystal for low pulse energy pumped optical parametric oscillation

A quasi-noncritical phase-matching (QNCPM) technology has been developed employing adhesive-free bonding (AFB) for high efficiency and high beam quality frequency conversion. A 16-layer KTP composite with total length of 32 mm was fabricated for a low pulse energy pumped 2-μm optical parametric oscillator (OPO). Our calculations indicate that the KTP composite has a 16 times lower walk-off effect and 16 times higher angle acceptance compared with the OPO in the same length single KTP crystal. Even only considering the walk-off correction, the threshold pump pulse energy in such a QNCPM OPO can be expected to be reduced by 256 times. In addition, the AFB technique was demonstrated to have uniform bonding quality and immeasurably small interface loss. Therefore, it can be expected to allow engineering of other critical phase-matched nonlinear optical devices into QNCPM devices.

Interferometric measurement of refractive index difference applied to composite waveguide lasers

A sensitive technique has been implemented to measure the differences in refractive indices between rare-earth doped YAG and un-doped YAG. Theoretically, a refractive index difference of less than 10−6 can be identified.

Ho:YAG laser pumped walk-off compensated mid-infrared ZGP optical parametric oscillation

We have demonstrated a novel folded linear resonator for walk-off compensated (WOC) optical parametric oscillation (OPO) in a single ZGP crystal. The OPO uses the doubled ZGP crystal length in a WOC configuration while simplifying requirements on crystal alignment. With a Q-switched Ho:YAG laser as a pump source, a maximum output power of 2 W at a wavelength of 4.72 μm has been measured for a pump power of 11.9 W in a 15-mm long type-I phase-matched ZGP crystal. The measured OPO quantum slope efficiency is above 50%.