The effect of an optical pump on the absorption coefficient of magnesium-doped near-stoichiometric lithium niobate in terahertz range

The absorption coefficient of magnesium-doped near-stoichiometric lithium niobate crystal is measured by terahertz time-domain spectroscopy in a frequency range of 0.2 THz~0.9 THz at room temperature. The absorption coefficient is modulated by external optical pump fields. Experimental results show that the absorption coefficient of near-SLN：Mg crystal is approximately in a range of 22 cm- 1_35 cm- 1 in a frequency range of 0.2 THz-0.9 THz and tunable up to nearly 15%. Further theoretical analysis reveals that the variation of absorption coefficient is related to the number of light-induced carriers, domain reversal process, and OH- absorption in this crystal.     

Optical properties of highly photosensitive hybrid sol-gel films

<正>A new organic-inorganic hybrid material,which shows photo-induced reduction of refractive index as well as volume contraction,is prepared using a sol-gel method.This material is coated on a Si substrate by spin-coating to manufacture film.After irradiated by ultraviolet(UV) light with a deuterium lamp for 5 h, the thickness of film decreases largely by 55%,and the refractive index of film changes from 1.484 to 1.445 at 550 nm.The film's optical thickness exhibits an exponential change with the increasing irradiation time.Futhermore.the photo-patternning of the organic-inorganic hybrid film without any further process (wet etching and thermal curing) is performed utilizing the volume contraction on UV-light irradiation. This film has potential applications for fabrication of patterned filter array and apodizing filter by direct light writing,and also demonstrates good temperature stability and immunity to visible light exposure.     

Light-Induced Array of Three-Dimensional Waveguides in Lithium Niobate by Employing Two-Beam Interference Field

We propose a novel approach to fabricate an array of three-dimensional waveguides in lithium niobate by employing illuminations of a pair of mutually coherent or incoherent two-beam interference patterns. Adjusting periods of the patterns and the angle between the grating vectors of the two patterns, we find that arrays of square, rectangular or parallelogrammic waveguides can be created. An array of rectangular waveguides is demonstrated in an iron doped lithium niobate by the double exposures of two-beam interference patterns with different orientations. The measured index distribution and the guiding test results show that the waveguide array is successfully fabricated.     

Supercontinuum Generation in Lithium Niobate Ridge Waveguides Fabricated by Proton Exchange and Ion Beam Enhanced Etching

We report on the fabrication of the 10-mm-long lithium niobate ridge waveguide and its supercontinuum generation at near-visible wavelengths(around 800 nm). The waveguides are fabricated by a combination of MeV copper ion implantation followed by wet etching in a proton exchanged lithium niobate planar waveguide. Using a mode-locked Ti:sapphire laser with a central wavelength of 800 nm, the generated broadest supercontinuum through the ridge waveguides spans 302 nm(at-30 dB points), from 693 to 995 nm. Temporal coherence properties of the supercontinuum are experimentally studied by a Michelson interferometer and the coherence length of the broadest supercontinuum is measured to be 5.2μm. Our results offer potential for a compact and integrated supercontinuum source for applications including bio-imaging, spectroscopy and optical communication.     

Coherence preservation during light-surface plasmon polaritons-light transformation

We constructed a four-layer system composed of a prism,a silver film,an air layer and a lithium niobate crystal.Initially we used two coherent light beams to excite surface plasmons.The surface plasmons were then decoupled into light in the photorefractive crystal where a holographic grating was recorded.The two beams remained coherent through light to surface plasmons to light transformation.Studying the characteristics of the holographic grating we found out that the thickness of the grating was to the order of hundreds of microns.The thick holographic grating suggests that the holographic recording in the photorefractive materials was induced by the leaky waves rather than by surface plasmon polaritons directly.     

Amplification of fluorescence using collinear picosecond optical parametric amplification at degeneracy

We demonstrate the output characteristic of broadband parametric amplification of incoherent light pulses in a 355-nm pumped degenerate picosecond optical parametric amplification with either saturated or unsaturated amplification.The optical parametric amplifier is seeded by the fluorescence generated in a solution of pyridine-1 dye in ethanol.With the saturated amplification,we can obtain high energy incoherent light pulses,whose full width at half maximum bandwidth varies from 16 nm to 53 nm for the different phase matching angles near degeneracy.Moreover,the unsaturated bandwidth of the amplified pulses fits well to the calculated result at degeneracy.Selecting s-polarized fluorescence with a Glan-Taylor prism,the maximum bandwidth of the amplified fluorescence is found to be 59 nm for a purely s-polarized seed.The maximum output energy is 0.67 mJ for the optical parametric amplifier.By using an optical filter and compressor,the generated high energy incoherent light has great potential as the incoherent pump,signal or idler wave of a parametric down-conversion process,so that a wave with a high degree of coherence can be generated from an incoherent pump light.     

Tunable,continuous-wave single-resonant optical parametric oscillator with output coupling for resonant wave

We present a continuous-wave singly-resonant optical parametric oscillator with 1.5% output coupling of the resonant signal wave, based on an angle-polished Mg O-doped periodically poled lithium niobate(Mg O:PPLN), pumped by a commercial Nd:YVO4laser at 1064 nm. The output-coupled optical parametric oscillator delivers a maximum total output power of 4.19 W with 42.8% extraction efficiency, across a tuning range of 1717 nm in the near- and mid-infrared region.This indicates improvements of 1.87 W in output power, 19.1% in extraction efficiency and 213 nm in tuning range extension in comparison with the optical parametric oscillator with no output coupling, while at the expense of increasing the oscillation threshold by a factor of ～ 2. Moreover, it is confirmed that the finite output coupling also contributes to the reduction of the thermal effects in crystal.     

Experimental investigation on a software synchronous optical sampling technique based on periodically poled lithium niobate

A software synchronous optical sampling system is constructed based on sum frequency generation (SFG) in periodically poled lithium niobate (PPLN). Five gigahertz of a non-return-to-zero (NRZ) data signal is sampled by sampling pulse with the repetition frequency of 29.31 MHz. The power of the SFG light is set at 23 dBm, and an eye diagram is successfully recovered. A band-pass filter is added before the sampling pulse is subjected to erbium-doped fiber amplifier to reduce gain competition and ensure a high power level of SFG.     

Red Laser-Induced Domain Inversion in MgO-Doped Lithium Niobate Crystals

A laser beam at wavelength 647nm is focused on a sample of 5mol% MgO-doped lithium niobate crystal for domain inversion by a conventional external electric field. In this case, a reduction of 36% in the electric field required for domain nucleation （nucleation field） is observed. To the best of our knowledge, it is the longest wavelength reported for laser-induced domain inversion. This extends the spectrum of laser inducing, and the experimental results are helpful to understand the nucleation dynamics under laser illumination. The dependence of nucleation fields on intensities of laser beams is analysed in experiments.     

Lithium niobate planar and ridge waveguides fabricated by 3 MeV oxygen ion implantation and precise diamond dicing

We report on the fabrication and optimization of lithium niobate planar and ridge waveguides at the wavelength of 633 nm.To obtain a planar waveguide, oxygen ions with an energy of 3.0 Me V and a fluence of 1.5 × 10~(15) ions=cm~2 are implanted in the polished face of Li Nb O_3 crystals. For planar waveguides, a loss of 0.5 d B/cm is obtained after annealing at 300°C for30 min. The ridge waveguide is fabricated by the diamond blade dicing method on optimized planar waveguides. The guiding properties are investigated by prism coupling and end-face coupling methods.     

Intensity dependence and white-light gating of two-color photorefractive gratings in LiNbO(3)

Photorefractive gratings have been written in Pr:LiNbO(3) by use of a diode laser for writing and of filtered white light (390-520nm) as a gating source. The gating light increases the writing efficiency by more than 3 orders of magnitude, and the two-step writing process provides nondestructive readout. The material sensitivity for two-color writing rises strongly for gating wavelengths near the bandgap and approaches that of Fe-doped lithium niobate at power densities of a few watts per square centimeter. In addition, we show that the dynamic range of the recording process is strongly dependent on the writing intensity.     

掺杂对铌酸锂晶体非挥发全息存储性能的影响

通过研究掺镁、掺锌和掺铟同成分铌酸锂晶体的紫外-红光双色全息存储性能,发现双色记录响应时间均比单色记录时明显缩短,最多的可减小3个数量级;双色记录灵敏度大幅度提高,在掺镁5 mol.%的晶体中可达到11 cm/J.在掺杂浓度超过抗光损伤阈值的铌酸锂晶体中,均可实现非挥发全息存储.但是,在掺镁、锌样品中,深、浅能级中心上的光栅反相,而在掺铟样品中则表现为同相.这是由于掺杂离子的种类不同,在铌酸锂晶体中形成的缺陷中心也不同所引起的. 关键词： 掺杂 铌酸锂晶体 非挥发 全息存储     

连续光条件下对LiNbO3∶Fe∶Mn晶体全息存储性能的理论研究

以双中心模型为基础,在低光强连续光条件下研究了LiNbO3∶Fe∶Mn晶体在稳态情况下的非挥发双光双步全息存储性能。采用数值方法,通过比较双中心模型中深(Mn2+/Mn3+)、浅(Fe2+/Fe3+)能级之间所有可能的电子交换过程,发现由隧穿效应引起的深浅能级之间直接电子交换过程对LiNbO3∶Fe∶Mn晶体总的空间电荷场大小起着决定性的作用。同时,这一电子交换过程对晶体非挥发全息存储性能也起着至关重要的作用。此外,通过相同实验条件下LiNbO3∶Fe∶Mn晶体与近化学比LiNbO3∶Fe晶体总的空间电荷场的比较,显示LiNbO3∶Fe∶Mn晶体在低抽运光和高记录光光强条件下有着比近化学比LiNbO3∶Fe晶体更佳的全息存储性能。     

Comparison of two-color holography among different doped LiNbO3 crystals at low intensities

The effects of material and experimental parameters on the nonvolatile two-color holographic recording space charge field and sensitivity for different doped LiNbO₃:Fe crystals have been studied theoretically based on a two-center model. When the direct electron transfer between the deep-trap centers and the shallow-trap centers was considered, the near-stoichiometric LiNbO₃:Fe is confirmed theoretically to be of bigger space charge field and higher recording sensitivity than the LiNbO₃:Fe:Mn and LiNbO₃:Cu:Ce in the low intensity region. A further improvement of the recording sensitivity can be achieved by doping concentration, thermal reduction treatment of Fe, appropriate gating and recording wavelengths with large photo-excitation cross sections.     

Thermal tuning of a fixed Bragg grating for IR light fabricated in a LiNbO3:Ti channel waveguide

We investigate a thermally tunable integrated narrow-bandwidth interference filter for light in the infrared wavelength region around 5=1550 nm fabricated in a channel waveguide in lithium niobate. The filter is based on a holographically recorded and thermally fixed refractive-index Bragg grating. The device is connected to optical single-mode fibers and works polarization-independent. We measure a peak reflectivity of 95% and a linewidth (FWHM) of 0.09 nm. By temperature-controlling the peak wavelength of the filter is tuned. For the linear thermal expansion coefficient of lithium niobate along the c axis the value !₃₃=(4.5ǂ.5)᎒^-6 K^-1 is obtained. Permanent illumination of the filter with incoherent light allows us to adjust the peak reflectivity. Dedicated to Prof. K. Ringhofer on the occasion of his 60th birthday.     

Digital holographic storage system incorporating optical fixing

Digital holograms have been written in stoichiometric Pr:LiNbO(3) in a two-color recording scheme, demonstrating what is to our knowledge the first all-optical nondestructive readout of digital data. Using writing light at 800nm and gating light at 476nm, we stored and retrieved 256-kbit digital data pages with a raw bit-error rate BER of <10(-4) .     

Angle-multiplexed hologram storage in LiNbO(3):Tb, Fe

We have performed hologram multiplexing with a cw laser at lambda=532 nm in a near-stoichiometric LiNbO(3) :Tb, Fe crystal that exhibits a visible absorption band induced by UV light. When 50 plane-wave holograms were angle multiplexed from the UV preexposed state, the recording sensitivity gradually decreased, owing to bleaching of the induced absorption. However, a conventional recording schedule of multiplexing many holograms of equal diffraction efficiency can be used in this photochromic material. The gradual decrease in recording sensitivity throughout multiplexing can be compensated for by the increased erasure-time constants of later-written holograms. An M/# of 1.73 was obtained in a crystal of 3.3-mm thickness.     

近化学配比LiNbO3晶体双色光全息记录的实验研究

实验用红光记录紫光敏化,研究了几种不同掺杂浓度和氧化还原状态的近化学配比铌酸锂晶体的双色全息记录特性,如其饱和衍射效率和记录灵敏度的变化规律。实验结果表明,晶体的饱和衍射效率随透射光强与紫外光强比值的增加先增加后减小,而记录灵敏度在不断减小。Mn含量相同时,Tb含量越高的晶体饱和衍射效率越高,而记录灵敏度越低。Tb含量相同时,含Mn比不含Mn的晶体的饱和衍射效率低,但记录灵敏度更高。还原晶体的饱和衍射效率和记录灵敏度均比氧化晶体高。记录光与敏化光的比值、掺杂浓度和氧化还原状态对晶体的全息特性产生影响。通过适当调整上述参数之间的关系,可对材料的全息记录特性进行优化。     

Enhanced terahertz-wave parametric generation and oscillation in lithium niobate waveguides at terahertz frequencies

We observed parametric-generation efficiency of 1.61% from 1064 to 1071 nm and at 162 microm in a 0.5 mm thick, 45 mm long z-cut congruent lithium niobate waveguide with a pump energy of 2.2 mJ and a pump pulse width of 5.8 ns. We also measured an ultralow-threshold intensity of 70 MW/cm2 for a 1064 nm pumped parametric oscillator resonating at 1071 nm and emitting at 162 microm with a 1 mm thick, 45 mm long lithium niobate waveguide.     

Permanent narrow-band reflection holograms for infrared light recorded in LiNbO3:Ti:Cu channel waveguides

Permanent refractive-index gratings are generated by thermal fixing of holograms in photorefractive lithium niobate channel waveguides. The guides are fabricated by successive indiffusion of titanium stripes and thin layers of copper. After high-temperature recording with green light, refractive-index modulations exceeding Δn=8×10^-5 for light of the telecommunication wavelength 1550 nm appear without the need of any development process of the written holograms. The gratings are stable in the dark and no compensation mechanism via dark conductivity is observed. Thus this method may be well suited for long-time applications in holography and integrated optics. Received: 2 October 2000 / Revised version: 25 January 2001 / Published online: 22 March 2001