Tunable optical frequency comb generation in a periodically poled lithium niobate waveguide based on stimulated Brillouin scattering

We propose and demonstrate a technique for the generation of an optical frequency comb (OFC) in a periodically poled lithium niobate (PPLN) waveguide based on stimulated Brillouin scattering. A single continuous wave laser is sent to a multi-wavelength Brillouin erbium fibre laser for generating a set of coherent and phase-locked multi-wavelength spectral lines. They are injected into a PPLN waveguide to obtain an OFC. We investigate the influence of the cavity structure on the OFC property in our two different schemes. The number of comb lines is affected by the 980 pump current and Brillouin pump power. The OFCs are tunable in a large range by changing their central wavelength.     

Effect of MgO doping of periodically poled lithium niobate on second-harmonic generation of femtosecond laser pulses

We theoretically analyze type-I broadband second-harmonic generation (SHG) of femtosecond laser pulses based on a quasi-phase-matching configuration in periodically poled congruent LiNbO3 (LN) andperiodically poled MgO:LiNbO3 (PPMgLN) (5% and 7%). Group-velocity matching (GVM) can be achieved at the fundamental waves of 1.59, 1.56, and 1.55 microm for SHG when the three types of crystals have grating periods of 22.31, 20.07, and 23.45 microm, respectively. It is found that the central wavelength of the fundamental wave for GVM will increase with the decrease of MgO doping in LN. It is concluded that the shift of the GVM central wavelength is due to the difference of MgO doping, which changes the dispersion of the crystal. Therefore, tunable and high efficiency broadband SHG of femtosecond laser pulses in a long crystal can be realized by selecting different doping rates of PPMgLN.     

Flat broadband wavelength conversion based on cascaded second-harmonic generation and difference frequency generation in segmented quasi-phase matched gratings

Bandwidth enhancement and response flattening of wavelength conversion based on single-pass and double-pass cascaded second harmonic generation and difference frequency generation were investigated in segmented quasi-phase matched (QPM) gratings. It is shown that the signal and pump bandwidths are both efficiently widened by increasing the segment number of the QPM grating and optimising the poling period of each segment. The ripple on the matching response is also very small. The conversion bandwidth in a 3-cm-long three-segment waveguide reaches 150–160?nm, which is over the whole conventional band and long-wavelength band. Larger signal bandwidth can be obtained with a little response flatness penalty and conversion efficiency penalty, which can be compensated by increasing the input pump power. Compared with a sinusoidally chirped optical superlattice device, a wavelength converter based on the segmented gratings has higher conversion efficiency, broader bandwidth and better pump-wavelength tolerance, and is easier to fabricate in practice.     

Femtosecond optical parametric oscillators based on periodically poled lithium niobate

Abstract

We describe two configurations of collinearly pumped femtosecond optical parametric oscillator based on periodically poled lithium niobate and tunable in the infrared from 975 nm to 4.98 μm. Maximum output powers of 240 mW for the signal and 106 mW for the idler were recorded with 25 mW of average power measured at 4.88 μm. An overall conversion efficiency of 35% and slope efficiencies for the signal of 46% at a wavelength of 1.04 μm and 70% at 1.1 μm were measured. Interferometric autocorrelations of the signal and idler pulses at various wavelengths within the tuning range have been obtained and imply nearly transform-limited pulse durations of about 140fs for the signal and about 190fs for the idler.     

Influence of magnetic field on terahertz wave generation in photorefractive periodically poled lithium niobate crystal

By employing femtosecond pump-probe configuration, we successfully realized narrowband terahertz wave generation and detection in both photorefractive periodically poled lithium niobate (PPLN) and periodically poled Mg:LiNb(3) (PP-Mg:LN) crystal. Using an applied magnetic field, we achieved modulation of the terahertz wave in a photorefractive PPLN crystal. The terahertz wave depends strongly on the magnitude of the applied magnetic field in the photorefractive PPLN crystal. Terahertz wave independence of the magnetic field in PP-Mg:LN crystal was also identified. The interaction of the magnetic field and photorefractive PPLN crystal is believed to occur due to the Lorentz force, which results in the buildup of a space-charge field in a photorefractive PPLN crystal.     

Fourth-harmonic generation in a single lithium niobate-crystal with cascaded second-harmonic generation

Tunable second- and fourth-harmonic radiation was generated in a single 1-cm-long lithium niobate (LiNbO(3)) crystal with the Mark III infrared free electron laser at Duke University. The fundamental laser radiation was tuned from 2 to 2.5 μm, yielding 1-1.25-μm radiation (second harmonic) and 0.5-0.625-μm radiation (fourth harmonic). A fundamental-second-harmonic energy conversion efficiency of 66% and a fundamental-fourth-harmonic energy conversion efficiency of 3.3 × 10(-6) were measured. The maximum energy in the fourth harmonic was 3.3 nJ.     

Enhanced second-harmonic generation in an electro-optic controlled periodically poled ferroelectric crystal

We present a novel design of enhanced second-harmonic generation in a periodically poled ferroelectric crystal. It profits from enhanced light intensity and slowed group velocity, which occur near photonic band edges through electro-optic modulation. The designed structure, with 108 periods and a length of 322 microm, can generate a second-harmonic beam whose conversion efficiency is 2 orders of magnitude larger than the one produced by an equivalent length of quasi-phase-matching bulk medium. This result might be beneficial to the development of optics integration and compact devices.     

Pulse shaping by the electro-optic effect in chirped periodically poled lithium niobate

We propose an ultrafast pulse shaping method by modulating the pulse phase and amplitude by the electro-optic effect and Bragg diffraction in the aperiodically optical superlattice. Linear-chirped periodically poled lithium niobate is used. The input pulse can be shaped, for example, by compressing it through the extraordinary refractive index change of the crystal by applying and changing the external electric field.     

The asymmetry between the domain walls of periodically poled lithium niobate crystals

The domain walls of periodically poled lithium niobate (PPLN) crystals were investigated using Raman spectrum mapping. The periodical changes of Raman shift in several Raman lines along the period of PPLN have been found. The experimental results show that the domain reversal has different degree of affection on its two neighbor regions, which is related to the asymmetry of crystal lattice. This phenomenon can be used to examine the periodical domain reversal of LN non-contacted, non-destructive and easily operated.     

Polarization control by use of the electro-optic effect in periodically poled lithium niobate

We propose a method to control the polarization of light by the electro-optic effect in periodically poled lithium niobate. A single integrated chip of Z-cut lithium niobate having two sections is used. The first section is not periodically poled, whereas the second section is. With an electric field applied along the Z axis of the first section and another electric field applied along the Y axis of the second section, light with an arbitrary elliptical polarization can be converted into a fixed linearly polarized state.     

Characterization of a periodically poled lithium niobate optical parametric oscillator pumped by a Bessel beam

Abstract

A Bessel beam pumped periodically poled lithium niobate optical parametric oscillator has been characterized. A slope efficiency of 5.4 ± 0.1% was obtained and a threshold pulse energy of 472 ± 12 μJ was measured, which corresponds to a threshold fluence of 0.207 ± 0.005 J cm^?2. The signal output was tuned over ～20 nm by changing the crystal temperature by 50°C. The FWHM bandwidth at each temperature was observed to be 0.3 ± 0.1 nm at each wavelength. The beam profiles of the Bessel pump and Gaussian signal beams were studied in both the near and far field. A beam divergence of 1.14 ± 0.02 mrad was measured.     

Stable high-power green light generation with a periodically poled stoichiometric lithium tantalate

Periodically poled stoichiometric lithium tantalate was used for an efficient high-power coherent green light generation based on second-harmonic generation (SHG) using a Q-switched Nd:YVO₄ laser. Achieved conversion efficiency was 69% that corresponds to SHG power density of 21.6 MW/cm² at the maximum input fundamental peak power density of 31.5 MW/cm². The SHG power fluctuation was less than 3.4% for 35 h, which was mainly caused by input power fluctuation of 2.6%. Obtained SHG powers of 4.4 and 3.4 W were sufficiently stable without spatial beam distortion at room temperature and a phase matching temperature adjustment was required precisely at high input power.     

Synchronously pumped optical parametric oscillation in periodically poled lithium niobate with 1-w average output power

We report on a rugged all-solid-state laser source of near-IR radiation in the range of 1461-1601 nm based on a high-power Nd:YVO(4) laser that is mode locked by a semiconductor saturable Bragg reflector as the pump source of a synchronously pumped optical parametric oscillator with a periodically poled lithium niobate crystal. The system produces 34-ps pulses with a high repetition rate of 235 MHz and an average output power of 1 W. The relatively long pulses lead to wide cavity detuning tolerances. The comparatively narrow spectral bandwidth of <15 GHz is suitable for applications such as pollutant detection.     

Effect of water vapor in a y-cut lithium niobate waveguide

In z-cut lithium niobate (LiNbO3) samples, surface damage has been observed after diffusion in a wet atmosphere, but recent reports show that with controlled flow of water vapor waveguides with good surface morphology and low loss can be obtained. Y-cut waveguides do not show any surface damage. Fabrication of y-cut waveguides diffused with controlled variation of water vapor in the ambient has not been reported to the best of our knowledge. We show that a minimum loss in y-cut waveguides is obtained at a particular water vapor content in the ambient, which is lower than the loss obtained for waveguides diffused in dry ambient. We have found a decrease in the waveguide loss to 0.3 dB/cm from 0.6 dB/ cm for 1 mL of water vapor passed per hour as compared with a dry atmosphere.     

First-order quasi-phase-matched second-harmonic generation in a LiTaO(3) waveguide

We achieved improvement in conversion efficiency in a first-order quasi-phase-matched second-harmonic generation device that uses a LiTaO(3) waveguide by experimentally characterizing the device process and the performances. Efficient overlaps among propagation light modes and the first-order periodically domain-inverted region are gained in a strongly confined waveguide fabricated by use of proton exchange annealed by a quick heat treatment. A blue-light power of 22 mW is obtained for a conversion efficiency of 18% by using a Ti:Al(2)O(3) laser. The observed FWHM temperature and wavelength acceptance bandwidths for second-harmonic generation power are 2.5 °C and 0.13 nm, respectively. Using this device with antireflection coating on the input and output facets of the waveguide, we generate 1.3 mW of blue light for a conversion efficiency of 4% by direct diode-laser doubling.     

Dispersion engineering of a silicon-nanocrystal-based slot waveguide for broadband wavelength conversion

A proposal for broadband wavelength conversion using four-wave mixing is presented based on a slot waveguide with silicon nanocrystals (Si-nc's) as the optical nonlinear material. The dispersion of the waveguide is engineered to realize a flat dispersion as well as a small effective mode area for better nonlinear interaction by optimizing the waveguide dimensions. The conversion performance is synthetically analyzed and numerical results show that a bandwidth of over 400?nm and an efficiency of -2.38?dB can be achieved using a pump power of 150?mW in a 4?mm long Si-nc slot waveguide with slot width of 50?nm, slab width of 310?nm, and height of 305?nm.     

Single-mode near-infrared optical parametric oscillator amplifier based on periodically poled KTiOPO4

A singly resonant, single-axial-mode, optical parametric oscillator (OPO) based on periodically poled KTiOPO(4) (PPKTP) is reported. Signal (1.68 mum) and idler (2.90 mum) optical bandwidths have been narrowed to <400 MHz by use of a diffraction grating at grazing incidence. The OPO generates 370 muJ of signal radiation when pumped by 3.1 mJ of 1.064-mum radiation. We implemented a single-pass PPKTP amplifier to yield 2.15-mJ signal and 1.17-mJ idler radiation without broadening the spectral bandwidths.     

Highly efficient coupling in lithium niobate photonic wires by the use of a segmented waveguide coupler

The purpose of this article is to show that efficient light coupling in lithium niobate waveguides presenting a strongly confined mode, such as photonic wires, is possible with the use of a periodically segmented waveguide coupler. The coupler consists in an input periodically segmented waveguide whose mode size is adapted to the mode of a standard single-mode fiber coupled to a photonic wire whose mode size is of the same order of the wavelength. The periodic segmentation of the input waveguide allows fulfilling the phase matching condition necessary to achieve an efficient light transfer between these waveguides. The coupling efficiency is typically 5 times higher than the butt-coupled configuration.     

Broadband wavelength conversion in hydrogenated amorphous silicon waveguide with silicon nitride layer

Broadband wavelength conversion based on degenerate four-wave mixing is theoretically investigated in a hydrogenated amorphous silicon (a-Si:H) waveguide with silicon nitride inter-cladding layer (a-Si:HN). We have found that enhancement of the non-linear effect of a-Si:H waveguide nitride intermediate layer facilitates broadband wavelength conversion. Conversion bandwidth of 490 nm and conversion efficiency of 11.4 dB were achieved in a numerical simulation of a 4 mm-long a-Si:HN waveguide under 1.55 μm continuous wave pumping. This broadband continuous-wave wavelength converter has potential applications in photonic networks, a type of readily manufactured low-cost highly integrated optical circuits.