Yao Jianquan

Fe-Doped Polycrystalline CeO2 as Terahertz Optical Material

Fe-doped CeO2 is synthesized by ceramic method and the effects of Fe doping on the structure and properties are characterized by ordinary methods and terahertz-time domain spectrometer (THz-TDS) technique. Our results show that pure CeO2 only has a small dielectric constant of 4, while a small amount of Fe (0.9 at. %) doping into CeO2 promotes densification and induces a large e of 23. From the THz spectroscopy, it is found that for undoped CeO2 both the power absorption and the index of refraction increase with frequency, while for Fe-doped CeO2 we measure a remarkable transparency together with a flat index curve. The absorption coefficient of Fe-doped CeO2 at frequency ranging from 0.2 to 1.8 THz is less than 0.35 cm−1, implying that Fe-doped CeO2 is a potential THz optical material.

LARGE-SIGNAL RESULTS FOR DEGENERATE FOUR-WAVE MIXING AND PHASE CONJUGATE RESONATORS

A numerical calculation procedure and various large-signal numerical solutions are presented for degenerate four-wave mixing in optical Kerr media, and for phase conjugate resonators using degenerate four-wave mixing. The solutions presented take full account of nonlinear refractive index changes, pump depletion, signal saturation, distributed losses, and possible external mirrors with laser gain. We find that including the nonlinear index change generally causes little change in the reflectivity or power output of degenerate four-wave mixing devices, at least with symmetric pumping. The optimum power output from a phase conjugate resonator with and without a laser gain medium is calculated. The results provide some theoretical guidance for designing phase conjugate resonators.

Multiband Metamaterial Absorber at Terahertz Frequencies

We propose a multi-band metamaterial absorber operating at terahertz frequencies. The design, characterization, and theoretical calculation of the high performance metamaterial absorber are reported. The multi-band metamaterial absorber consists of two metallic layers separated by a dielectric spacer. Theoretical and simulated results show that the metamaterial absorber has four distinct absorption points at frequencies 0.57 THz, 1.03 THz, 1.44 THz and 1.89 THz, with the absorption rates of 99.9%, 90.3%, 83.0%, 96.1%, respectively. Two single band metamaterial absorbers and a dual band metamaterial absorber on the top layer are designed. Some multi-band absorbers can be designed by virtue of combining some single band absorbers. The multiple-reflection theory is used to explain the absorption mechanism of our investigated structures.

Spectral Optical Coherence Tomography Using Two-Phase Shifting Method

A two-phase shifting method is introduced to eliminate the strong autocorrelation noise inherent in spectral optical coherence tomography and to mitigate the unwanted auto- and cross-coherent terms introduced by the reflections from various optical interfaces present in the system. Furthermore, this method is also able to amplify the desired signal by a factor of 2. The feasibility of such a method is demonstrated using a mirror-like object. An intact porcine cornea tissue in vitro is also used to show the potential of this method for biological imaging.

Low Threshold and High Conversion Efficiency Nanosecond Mid-Infrared KTA OPO

Based on a Type II non-critically phase-matched KTA crystal, a low-threshold and high conversion efficiency mid-infrared optical parametric oscillator (OPO) pumped by a diode-end-pumped Nd:YVO4 laser is demonstrated. The OPO threshold is only 0.825 W. The maximum output power of 435mW at 3.47 μm is achieved with the repetition rate of 30 kHz, corresponding to an optical-to-optical conversion efficiency of 4.4%. The photon conversion efficiency is as high as about 64%. The pulse width is 3.5 ns with a peak power of 4kW for the maximum output power.

Thermal Effect in KTP Crystals During High Power Laser Operation

We report on the theoretical and experimental studies of the thermal effect of the KTP crystal during high power operation. From the dependence of the refractive index temperature coefficients on wavelength, the dependence of the optimum phase-matching angles on temperature is derived. In the experiment, the angle of the frequency-doubled KTP crystal is tilted to compensate for the thermal effect and to obtain Δ = 0.7° when the green laser output power is 30 W and the KTP crystal temperature is about 80°C. We obtained the highest stable output power greater than 40 W with an L-shaped flat-flat intracavity frequency-doubled Nd:YAG laser. The experimental results are very consistent with the theoretical analysis.

Single Mode Condition and Power Fraction of Air-Cladding Total Refractive Guided Porous Polymer Terahertz Fibers

We investigate the single-mode condition and power fraction of an air cladding total internal reflection (TIR) guided porous polymer THz fiber. It is shown that the single mode condition and a high fraction of power in air holes cannot be simultaneously met in porous fibers. Employing the V parameter and the Lorentz—Lorenz formula, we analyze this problem theoretically.

Nearly-noncritical phase matching in MgO : LiNbO3 optical parametric oscillators

We have proposed and demonstrated a nearly-noncritical phase-matched (NCPM) optical parametric oscillation (OPO) in an MgO:LiNbO3 crystal with 5 mol% MgO by temperature tuning. By giving up perfect NCPM, the practical tuning range for the OPO is increased by two times. For the crystal, the operated temperature decreases with the phase-matching angle at degeneracy. With a cutting angle of 82° instead of the noncritical case of 90°, the tuning range was increased. In order to obtain a sufficiently high output pulse energy for both signal and idler throughout the entire tuning range, five sets of mirrors were used for the resonator. The tuning range of the OPO was 800-1700 nm with temperatures tuning from 83°C to 224.2°C. The output energy was about 6.45 mJ with a conversion efficiency of nearly 13%. The bandwidth of the output was 1.0-1.1 nm.

Efficient Compound-Cavity Eye-Safe KTP OPO at 1.57 μm Pumped by an Electro-Optic Q-Switched Nd:YAG Laser

An efficient high-energy eye-safe optical parametric oscillator (OPO) based on a type-II non-critically phase-matched KTP crystal is demonstrated. The KTP OPO is pumped by a quasi-cw diode side-pumped electro-optic Q-switched Nd:YAG laser in a compound-cavity configuration. The maximum output energy of the signal wavelength at 1.57 μm is 66.5 mJ, corresponding to an electrical-to-optical conversion efficiency of 4.47% and an optical-to-optical conversion efficiency of 12.1%. The pulse width (FWHM) is about 3.6ns with a peak power of 18.5 MW. The output energy is insensitive to repetition rate and demonstrates good stability.

Numerical Modelling of QCW-Pumped Passively Q-Switched Nd:YAG Lasers with Cr4+:YAG as Saturable Absorber

Passively Q-switched quasi-continuous-wave (QCW) diode-pumped Nd:YAG laser with Cr4+:YAG as saturable absorber is numerically investigated by solving the coupled rate equations. The threshold pump rate for passively Q-switched QCW-pumped laser is derived. The effects of the pump rate and pump-pulse duration on the laser operation characteristics are studied theoretically. The pump power range can be estimated according to the number of output pulses. The numerical simulation results are in good agreement with the experimental results.