Light In LiNbO3 Research Articles

Terahertz spectral nondestructive detection of amino acid molecules in aqueous solution

Only in liquid environment can most biomolecules maintain their biological activity, the realization of terahertz (THz) spectrum detection of water-bearing biomolecules has an important application value for the research of the function and structure of active biomolecules and non-invasive biomedical detection. In this paper, by using the THz radiation source generated by LiNbO3 light rectification, and combing with the local electric field enhancement effect of the flared gradient parallel flat waveguide, we have built a THz time domain spectral measurement system that can detect the trace amounts of living cells and aqueous biomolecules, which can implement THz spectral detection of 4-aminobenzoic acid and glutamic acid in solution. The results show that 4-aminobenzoic acid and glutamic acid in aqueous solution have obvious and unique characteristic absorption peaks in the frequency band of 0.1–2.0 THz when the effect of water on THz wave absorption is removed. The acquisition of characteristic absorption spectrum of amino acid aqueous solution in THz frequency band lays a foundation for the detection of active biological macromolecules or tissues.

Special scattering of extraordinary light in Photorefractive LiNbO3: Fe crystal

In this paper, we reported our experimental observations of the special scattered light inLiNbO3:Fe crystal. We studied the far-field scattering light of five kinds of configurations, respectively, when the angle between the direction of the c axis of the crystal and the direction of the incident long linear beam is 0°, 30°, 45°, 60°, 90°. We observed an interesting phenomenon firstly that strong light scattering appeared along the direction of the incident long linear beam on the crystal, and the scattering light is constituted of the filaments that perpendicular to the direction of the incident beam on the crystal in each situation. We also observed a new phenomenon that the scattering light appear perpendicular to the direction of the incident beam when the angle is 90°.

Electro-optic modulation analysis of a Y-cut Z-propagation LiNbO3 light modulator: Comparison with an X-cut Z-propagation LiNbO3 light modulator and a dual LiNbO3 crystal type modulator

We analyzed dependence of modulation characteristics on manufacturing errors for a Y-cut Z-propagation (YZ) and an X-cut Z-propagation (XZ) LiNbO3 (LN) crystal light modulators. We investigated that the modulation of the YZ modulator is hardly affected by small rotation error of Z-axis, while the XZ modulator suffers significant influence. We also analyzed temperature characteristics of modulation of conventional temperature-compensation LN light modulator. These numerical results show that the modulated signal change of the YZ modulator due to temperature variance is not more than 2.5% of that of the temperature-compensation modulator, in the presence of applied electric field of integral times of the half-wave voltage, modulated signal of the YZ modulator is almost independent on the temperature.

Temperature characteristics of a Y-cut Z-propagation LiNbO3 light modulator for application to polarimeters

The temperature characteristics of a Y-cut Z-propagation LiNbO3 crystal light modulator, with manufacturing errors, in the absence and presence of an electric field have been investigated by analyses and experiments. According to our analyses, when the Z-axis of the LiNbO3 crystal is at an angle of 0.22° with respect to the normal of the input surface of the crystal, we found the theoretical fluctuation of the normalized output-light intensity with temperature to be less than 7:75 × 10−6/°C. This magnitude is less than 1% of the theoretical intensity fluctuation of a conventional temperature-compensation LiNbO3 light modulator. The measured temperature characteristics of a prototype of this modulator were 2 × 10−4/°C in the absence of an electric field (OFF state) and 2:8 × 10−4/°C in the presence of an external field (ON state). During a running test of longer than 8 hours at room temperature, the intensity fluctuation of this prototype was 0.01% in the OFF state, and 0.07% in the ON state.

Role of charge-transfer states of Cr3+ in creation and annihilation of light-induced polarons in LiNbO3:Cr3+, MgO crystals

It is shown that Cr impurity takes a crucial role in the process of creation of hole O− and electron NbNb4+ polarons by the UV light in LiNbO3:Cr crystal codoped with MgO above threshold concentration. Thermal stability of both types of polarons shows that they are created and they annihilate simultaneously with each other in such crystals.

Broadband absorption changes and sensitization of near-infrared photorefractivity induced by ultraviolet light in LiNbO3:Mg

We observe ultraviolet-light-induced absorption changes in spectral regions from the near-ultraviolet to the near-infrared in as-grown LiNbO3:Mg. Such a broad light-induced absorption band is unstable at room temperature and decays nonmonoexponentially in the dark with a time constant on the order of tens of seconds. We attribute the appearance of the broadband light-induced absorption to the creation of the intermediate state O− in which holes excited from deep defect centers are trapped at O2− sites near cation vacancies. It is also found that near-infrared photorefractive sensitivities increase by several orders of magnitude by pre-exposing the crystal to incoherent ultraviolet light. These properties may be useful for an application of LiNbO3:Mg to nonvolatile two-color holography.

Copper Diffusion into Lithium Niobate

Holographically recorded refractive index gratings in lithium niobate crystals are of potential use for applications in integrated optics. In this field commercially available wafers serve as the substrate material for waveguide fabrication. To increase the number of photorefractive centers [1] the wafers can be surface-doped by additionally indiffusing [2] a thin layer of a transition metal, e.g, iron. Iron-doped substrates have been used to fabricate reflection filters for infrared light in LiNbO3:Ti channel waveguides [3] for dense WDM applications or laser diode wavelength stabilisation. Also DBR waveguide lasers [4] have been realized in LiNbO3:Fe utilizing holographically recorded laser mirrors. A main disadvantage of the indiffusion of iron is its small diffusion constant even at high temperatures. As an alternative, in this contribution we investigate copper diffusion at T ˆ 1000 C into lithium niobate. The sample was prepared in the following way: An undoped polished lithium niobate wafer of congruently melting composition was cut into pieces of x0 z0 ˆ 8 10 mm2, the c-axis of the crystal pointing along the larger side. The direction of the y-axis was parallel to the top face normal. The thickness y0 of the sample was 1 mm. A 500 nm-thin layer of copper was deposited by thermal evaporation on the top face of the crystal. We successively indiffused the layer into the substrate at T ˆ 1000 C in air. The annealing steps were Dt ˆ 1 h, 1 h, 1 h, 2 h and 2 h, resulting in a total diffusion time of t ˆ 1 h, 2 h, 3 h, 5 h and 7 h. After each annealing step the c-face of the sample (orientated rectangular to the top face) was precisely polished. Then it was scanned in the y-direction by the focused electron beam of a microprobe (focus diameter < 1 mm, acceleration voltage 25 keV) to determine the distribution of copper atoms in the sample. As a reference the signals of oxygen and niobium atoms were also checked. One scan step of the beam was dy ˆ 1 mm. After 1000 steps the whole sample was scanned and the copper indiffusion was carried on with the next annealing step Dt. For a total annealing time below t ˆ 7 h the evaporated copper layer had not yet indiffused completely into the sample. An additional sharp peak on the top appeared in the copper signal during the microprobe scan. This indicates that up to this point indiffusion from an infinite source had taken place. The solution of Fick's laws results in

Four-wave polariton scattering of light in LiNbO3

Four-wave Stokes k-spectra for light scattering on polaritons in lithium niobate crystals with an Mg impurity are studied experimentally. The mechanisms for direct, cascade, coherent, and incoherent four-wave mixing of light are discussed in the course of interpreting the angular dependences of the scattered light intensity. It is shown that the dispersion of the real part of the polariton wave vector and the refractive index of the crystals at the polariton frequencies can be measured with an order of magnitude greater accuracy than by spontaneous three-wave polariton light scattering. A significant discrepancy is found between determinations of the polariton absorption coefficient from the angular spectra of three-wave scattering and four-wave scattering in terms of the model employed here.

5 Gbit/s, 250 km error-free soliton transmission with Er 3+ -doped fibre amplifiers and repeaters

Error-free solition transmission at a bit rate of 5 Gbit/s over 250 km has been accomplished. The transform-limited soliton pulse source is a gain-switched DFB laser diode with a narrow-band spectral filter and Er amplifiers. A LiNbO3 light intensity modulator is used for pulse switching. A new ‘pre-emphasis technique’ for sending solitons over a longdistance is used, in which an Er3+-doped fibre amplifier is installed every 25 km as a lumped amplifier.

Advanced Ti-Implanted Optical Waveguides in LiNbO3

ABSTRACTLiNbO3 is the best substrate for modulators and switches for integrated optics. Efficient low loss waveguides for light in LiNbO3 are formed by introducing Ti-ions into its lattice, thus increasing locally the ordinary and the extraordinary indices of refraction. We are the first to use the very versatile technique of ion-implantation to administer Ti into LiNbO3. This implantation process offers the possibility to introduce significantly more Ti into a well-defined volume than conventional diffusion techniques. During this process first an amorphous non-equilibrium phase is generated, which has to be kept at low temperatures in order to prevent segregation. Subsequent thermal treatment leads to solid phase epitaxy and restores the desired stable crystalline state. We have used this technique to fabricate excellent planar waveguides, channel waveguides and Mach-Zehnder modulators.

Anomalous effect of drag of photovoltaic electrons by light in linbo3:Fe crystals

The anomalous effect of drag of photovoltaic electrons, which have a small effective mass (∼0.5 m0) and large photohall mobility (∼102-103 cm2/V ˙ sec) by light in LiNbO3:Fe crystals has been discovered. The mechanisms of the drag current are discussed. The dependence of the photovoltaic current's change on the magnetic field has been obtained.

Suppression of Li2O out-diffusion from Ti-diffused LiNbO3 optical waveguides

Light guides fabricated by Ti diffusion into y-cut LiNbO3 are attractive because of the possibility of using the larest electro-optic coefficient (r33). However, the accompanying Li2O out-diffusion gives rise to unwanted guided modes which are a problem in electro-optic modulators. To overcome this problem it is necessary to reintroduce Li back into the crystal by some means. Here we report on a simplebut effective process (using Li2O) to suppress Li2O out-diffusion and realize single-mode Ti-diffused LiNbO3 light guides in a y-cut orientation. A natural extension of the above process to Ti-diffused LiTaO3 light-guiding structure should be possible.

Lithium niobate crystal as a light modulator

A new LiNbO3 light modulator (designated as ML-4) was developed for modulation of λ = 0.45–2 μ radiation in the 0–100 MHz range. The modulator is based on the transverse linear electrooptic effect in LiNbO3. It is in the form of an electrooptic element placed in a casing which can be used for adjustment. The modulator can operate continuously for 300 h and it is resistant to mechanical and climatic perturbations. These properties make it useful in a wide range of practical applications.

LiNbO3 Light Modulator

Up to this time, LiNbO3 has not been considered to be a desirable material for a light modulator because of its susceptibility to ``optical damage.'' We ascertained that the LiNbO3 light modulator could be constructed practically without optical damage when the crystal was heated to ∼170–180 °C. A pair of polished LiNbO3 single crystals were soldered on a piezoelectric resonance damper of copper with pure tin, and were encapsulated in a glass vacuum vessel which prevented turbulence by heated air. It was demonstrated that the performance of the modulator was fine, especially with regard to extinction ratio, temperature characteristics, frequency response, and so on. In this report, we describe the optical-damage-free LiNbO3 modulator generally, from the growth condition of a high optical quality crystal to the construction and the performances of the modulator.