C. Q. Xu

1.5 μm band efficient broadband wavelength conversion by difference frequency generation in a periodically domain‐inverted LiNbO3 channel waveguide

Efficient 1.5 μm band wavelength conversion by difference frequency generation (DFG) in a periodically domain‐inverted LiNbO3 channel waveguide is demonstrated, and DFG efficiency as high as 40% W−1 cm−2 is obtained. Nearly constant wavelength conversion efficiency has been confirmed over a broad wavelength range of 30 nm, which is the largest value ever reported.

Polarization insensitive semiconductor laser amplifiers with tensile strained InGaAsP/InGaAsP multiple quantum well structure

Data on semiconductor laser amplifiers with a small tensile strain in the wells of multiple quantum well structures are presented. Semiconductor amplifiers with a small strain of 0.2% exhibit polarization insensitive characteristics with a signal gain of 15 dB in the 1.5 μm wavelength range. The enhancement of TM mode gain due to tensile strain is studied by measuring the dependence of amplified spontaneous emission spectra on device length and tensile strain.

Wavelength conversions ∼1.5 μm by difference frequency generation in periodically domain‐inverted LiNbO3 channel waveguides

Wavelength conversions ∼1.5 μm are realized by difference frequency generation in periodically domain‐inverted LiNbO3 channel waveguides for the first time. A wide wavelength conversion bandwidth, which is very important for future wavelength division multiplexing optical communication system, is demonstrated in this kind of devices.

A comparison of optical second-harmonic generation efficiency using Bessel and Gaussian beams in bulk crystals

Abstract The conversion efficiencies of optical second-harmonic generation (SHG) of light by Bessel beams and Gaussian beams in nonlinear crystals are compared. A Bessel beam, which has a quasi-nondiffractive nature, is generated by illuminating an axicon lens with a Gaussian beam. The interaction path lengths are the same for both beams, and are set to be twice the confocal parameter of the Gaussian beams. The Bessel beam is found to have greater SHG efficiency than the Gaussian beam regardless of the interaction path length and the fundamental wavelength.

Photorefractive damage of LiNbO3 quasiphase matched wavelength converters

Photorefractive damage (PRD) of LiNbO3 quasiphase matched (QPM) wavelength converters is studied by a novel two-beam method, in which the second-harmonic generation (SHG) tuning curve of a broadband probe beam is monitored. The QPM condition and wavelength conversion efficiency, which are characterized by the peak wavelength and peak intensity of the SHG tuning curve respectively, are investigated in detail with respect to the irradiation power and irradiation time. It is found that the QPM condition and wavelength conversion efficiency change markedly even for low irradiation powers (<20 mW), indicating that the PRD effect is non-negligible in LiNbO3 QPM wavelength converters intended for practical applications.

Temperature and polarization dependence of LiNbO3 quasiphase-matched wavelength converters

Temperature and polarization characteristics of 1.55-μm-band LiNbO3 quasiphase-matched (QPM) wavelength converters have been studied by second-harmonic generation (SHG). It is found that the shift of the QPM wavelength is linearly proportional to temperature over the measured temperature range between 10 and 40 °C, with a temperature tolerance (corresponding to a 3 dB reduction in the QPM conversion efficiency) of greater than 10 °C for a 10-mm-long device. With respect to the angle θ of the injected fundamental light polarization direction, the SHG power changes as a function of sin4 θ, with a tolerance of over 60°. Theoretical explanations for the observed results are also presented.

THREE WAVE MIXING USING A FIBER RING RESONATOR

Three wave mixing using a fiber ring resonator scheme is discussed, and experimental results are presented. In this scheme, an optical nonlinear medium is placed into the fiber ring laser cavity, and a semiconductor optical amplifier and a tunable optical band pass filter (TOBPF) are used as the gain medium and lasing wavelength selector. The phase-matching condition, which is necessary for efficient wave mixing, can easily be satisfied by tuning the TOBPF. Second-harmonic generation (SHG) and sum-frequency generation (SFG) were demonstrated using a LiNbO3 waveguide with a periodically domain inverted structure. Enhancements of SHG and SFG were achieved.

Second‐harmonic generation using a fiber ring resonator with a LiNbO3 waveguide and a semiconductor optical amplifier

Optical second‐harmonic generation (SHG) using a fiber ring resonator has been demonstrated. A LiNbO3 waveguide with a periodically domain inverted structure and a semiconductor optical amplifier were used as the nonlinear and gain media, respectively. The quasiphase‐matched condition was satisfied simply by tuning an optical bandpass filter in the resonator. SHG was enhanced by a factor of 280.

Device length dependence of optical second-harmonic generation in AlGaAs quasiphase matched waveguides

Device length dependence of optical second-harmonic generation (SHG) in AlGaAs semiconductor quasiphase matched (QPM) waveguides with periodically crystal domain inverted gratings has been studied both experimentally and theoretically. It is found that the SHG power depends strongly on device length. An optimum device length for obtaining maximum SHG is shown for QPM-SHG devices with non-negligible waveguide propagation loss. It is also shown that this optimum device length can be predicted theoretically, making experimental optimization unnecessary.

A novel method to characterize photorefractive damage in quasiphase-matched wavelength converters

A novel two-beam method is proposed and applied for the first time, to characterize photorefractive damage (PRD) in a LiNbO3 quasiphase-matched (QPM) wavelength converter. In the proposed method, irradiation light from a Ti sapphire laser and a broadband probe beam from an erbium-doped fiber amplifier are coupled into a LiNbO3 QPM waveguide. The PRD effect caused by the irradiation is studied by monitoring the generated second-harmonic light spectrum of the probe light. It is shown that PRD in the LiNbO3 QPM waveguide can be qualitatively characterized by the proposed method, and relevant information relating to the QPM wavelength conversion can be extracted directly.