Periodically Poled LiNbO3 Research Articles

Thermal inhibition of high-power second-harmonic generation in periodically poled LiNbO3 and LiTaO3 crystals

A coupled thermo-optical model shows strong thermal inhibition of second-harmonic generation (SHG) in periodically poled (PP) LiNbO3 (PPLN) and stoichiometric LiTaO3 (PPSLT) crystals. Three-dimensional simulations performed for a 1.064μm fundamental wavelength pulsed nanosecond laser beam 150μm in radius show the onset of significant temperature nonuniformities along and across the irradiated zone, and strong thermal dephasing and inhibition of SHG in these crystals for input powers >10W. PPSLT is found to have a significant advantage over PPLN due to the higher heat conductance, decreasing these temperature nonuniformities and allowing one to maintain the irradiated zone within the corresponding SHG temperature tolerance range (ΔT≈3K).

Study of Noncollinear Quasi-Phase Matching Optical Parametric Amplification with Group-Velocity Matching Based on Periodically Poled Crystals

The properties of noncollinear optical parametric amplification (NOPA) based on quasi-phase matching of periodically poled crystals are investigated, under the condition that the group velocity matching (GVM) of the signal and idler pulses is satisfied. Our study focuses on the dependence of the gain spectrum upon the noncollinear angle, crystal temperature, and crystal angle with periodically poled KTiOPO4 (PPKTP), periodically poled LiNbO3 (PPLN), and periodically poled LiTaO3 (PPLT), and the NOPA gain properties of the three crystals are compared. Broad gain bandwidth exists above 85 nm at a signal wavelength of 800 nm with a 532 nm pump pulse, with proper noncollinear angle and grating period at a fixed temperature for GVM. Deviation from the group-velocity-matched noncollinear angle can be compensated by accurately tuning the crystal angle or temperature with a fixed grating period for phase matching. Moreover, there is a large capability of crystal angle tuning.

Three-Dimensional Measurement for Absolute Value of Polarization Angle by Scanning Nonlinear Dielectric Microscopy

A technique for measuring the absolute value of ferroelectric polarization angles by scanning nonlinear dielectric microscopy (SNDM) is proposed and demonstrated. In this experiment, a periodically poled LiNbO3 (PPLN), which has a three-dimensional domain structure, is observed. The measured polarization angles agreed well with the actual polarization orientations, and the obtained values of polarization angles allowed an accurate visualization of the microdomain structure in PPLN. Through this experiment, we confirmed that 3D SNDM, which can measure in both the vertical and lateral polarization directions, is a useful technique for the absolute evaluation of the three-dimensional polarization direction.

Quasi-phase-matched second-harmonic-generation in bulk periodically poled LiNbO3 and optimal design

Quasi-phase-matched second harmonic generation(SHG) in bulk periodically poled L iNbO3(PPLN) was studied , the coupled-wave equations for quasi-phase- matched S HG was accurately resolved, and the formula for calculating SHG conversion effic iency under focusing Gauss beams was given. The relations between focusing beams  waist and crystal length were analyzed. Furthermore, the SHG cavity was opti mized to obtain the maximum conversion efficiency.

Experimental demonstration of wavelength conversion between ps-pulses based on cascaded sum- and difference frequency generation (SFG+DFG) in LiNbO3 waveguides

All-optical wavelength conversion between ps-pulses based on cascaded sum- and difference frequency generation (SFG+DFG) is proposed and experimentally demonstrated in periodically poled LiNbO3 (PPLN) waveguides. The signal pulse with 40-GHz repetition rate and 1.57- ps pulse width is adopted. The converted idler wavelength can be tuned from 1527.4 to 1540.5nm as the signal wavelength is varied from 1561.9 to 1548.4nm. No obvious changes of the pulse shape and width, also no chirp are observed in the converted idler pulse. The results imply that single-to-multiple channel wavelength conversions can be achieved by appropriately tuning the two pump wavelengths.

Investigation of one of the most fundamental limits to parametric processes: two-photon absorption

Among almost all the work done on parametric processes in the past, two-photon absorption (TPA) has been intentionally avoided by properly choosing the pump wavelength. On the contrary, we have purposely explored how TPA sets the fundamental limit on the optical parametric generation (OPG) and oscillation (OPO) by using periodically poled LiNbO3 (PPLN) as a testing nonlinear medium. Indeed, we have demonstrated that the TPA dramatically affects the characteristics of these two parametric processes. To our surprise, the OPO is actually not a preferred parametric process anymore compared with the OPG due to the presence of the strong TPA. In addition, the TPA can further be enhanced by self-focusing effect caused by accumulated thermal effect (ATE).

Mid-infrared generation based on a periodically poled LiNbO 3 optical parametric oscillator

We report a nanosecond Nd:YVO4-pumped optical parametric oscillator (OPO) based on periodically poled LiNbO3 (PPLN). Tuning is achieved in this experiment by varying the temperature and period of the PPLN. The design of double-pass singly resonant oscillator (DSRO) and confocal cavity enables the OPO threshold to be lowered considerably, resulting in a simple, compact, all-solid-state configuration with the mid-infrared idler powers of up to 466mW at 3.41μm.

Inspection of periodically poled waveguide devices by confocal luminescence microscopy

Using confocal defect-luminescence microscopy, we investigated Er3+-doped periodically-poled LiNbO3 waveguide devices and found that the Er3+ ion and vibrational Raman modes can be used effectively as probes to inspect and characterize the devices with high spatial resolution, non-destructively, and free of topographical artifacts. The technique is based on scanning over the sample, while simultaneously measuring and evaluating the emission or Raman spectra in terms of their moments (i.e., area, average emission wavelength, and spectral width). The contrast is observed even in tempered samples because of differences in intrinsic electric (strain) fields and defect arrangements that occur in regions inside and outside the waveguide as well as in regions in which domains have been inverted, or which are as grown. Evaluating different parts of the Er3+ spectra and/or using different moments, different aspects of the device can be highlighted and regions inside and outside the waveguide, in which domains have been inverted or not inverted, as well as domain walls, can be identified. Raman spectroscopy is less sensitive, but it still can identify the domain wall regions.

Dual-wavelength optical-pulse source based on diode lasers for high-repetition-rate, narrow-bandwidth terahertz-wave generation

Synchronized dual-wavelength 1-MHz repetition-rate optical pulses were successfully generated by the combination of a gain-switched diode laser and a wavelength-tunable continuous-wave diode laser at 1.5 mum. The timing synchronization of dual-wavelength optical pulses was achieved with four-wave mixing by use of a highly nonlinear optical fiber. This optical pulse source was utilized for terahertz (THz)-wave differencefrequency generation by use of slanted periodically poled LiNbO3 (PPLN). We generated between 1.05 and 2.1 THz by use of the proper grating period of slanted PPLN with a 10-GHz bandwidth.

Comparison of continuous-wave optical parametric oscillators based on periodically poled LiNbO_3 and periodically poled RbTiOAsO_4 pumped internal to a high-power Nd:YVO_4 laser

We present and compare two continuous-wave optical parametric oscillators based on the nonlinear crystals periodically poled LiNbO3 (PPLN) and periodically poled RbTiOAsO4 (PPRTA) pumped internal to a high-power, all-solid-state Nd:YVO4 laser. At a diode pump power of 12 W, similar total extracted idler powers of over 440 mW were obtained from each system even though the nonlinearity of PPRTA is significantly less than that of PPLN. The spatial, power, and frequency stability of the PPRTA-based system outperformed those of the PPLN system. We attribute the poor performance of PPLN in these parameters to its susceptibility to thermal lensing.

Temperature Tunable Infrared Optical Parametric Oscillatorwith Periodically Poled LiNbO3

We demonstrate a temperature tunable infrared optical parametricoscillator (OPO) based on periodically poled LiNbO3 (PPLN)pumped by an acousto-optically Q-switched cw-diode-end-pumpedNd:YVO4 laser with the signal output from 1.48 µm to1.54 µm by tuning the work temperature from 60°C to250°C and the maximum average signal power (1500 nm) of 137 mW.We report the demonstration of the sum frequency of the pump and idlerin a single-grating PPLN crystal.In addition, we present the theoretical analysis of the signalwavelength change rate with the changing temperature for a quasi-phasematching OPO, which is in good agreement with our experimental results.

Mid-Infrared Optical Parametric Oscillator Based onAll-Solid-State-Pumped Periodically Poled LiNbO

We report the tunable mid-infrared generation with a periodically poledLiNbO3 (PPLN). Using an all-solid-state-pumped Nd:YVO4laser as the pump source and a PPLN nonlinear crystal with gratingperiods of 28.2-30.8 µm, we have achieved wavelength conversion in the2.90-4.05 µm spectral range by period tuning. The use of confocalcavity design has brought a compact, all-solid-state configuration withan average output powers of idler up to ~200 mW. The maximumpower of 277 mW was obtained at the wavelength of 3.35 µm.

Structural Characterization Using X-ray Diffraction of Ti Indiffused Periodically Poled LiNbO3 Fabricated by Direct Electron Beam Bombardment

Periodic poling in titanium indiffused lithium niobate by direct electron beam irradiation is reported in this paper and the benefits the technique of X-ray diffraction (XRD) as a method to examine the structure of ferroelectric domains is presented. It is shown that in the case of periodically poled crystals with a grating period lower or equal than a few microns, XRD enables to straightforwardly access the domain inverted grating period. The use of analytical functions to model the XRD profiles allows this method to be easily implemented for on line non-destructive characterization of the devices.

Period and temperature tuning of cascaded optical parametric oscillator based on periodically poled LiNbO 3

We report the broadly tunable source by a cascaded optical parametric oscillator in the periodically poled LiNbO3 (PPLN) with domain grating period and temperature tuning. The optical parametric oscillator was pumped by a passive Q-switched Nd:YVO4 laser. Multi-wavelength outputs from visible to infrared were obtained. The temperature of the PPLN crystal changed within the range of 70-150°C with different periods of PPLN. The tunable range covered from 433 to 1657nm.

Femtosecond near-IR optical parametric oscillator with efficient intracavity generation of visible light

We present an experimental and a theoretical analysis of the operation characteristics of a femtosecond optical parametric oscillator (OPO) based on periodically poled LiNbO3 (PPLN). It provides bright visible pulses through second-harmonic generation (SHG) of the signal and sum-frequency generation (SFG) between the signal and the pump, simultaneously with the parametric oscillation in the near IR. Using a duty cycle of the poling period of approximately 56%, we achieved strong enhancement of even-order quasi-phase-matched (QPM) SHG and SFG, whereas the reduction in the efficiencies of odd-order QPM processes is negligible. Femtosecond pulses with output powers of up to 14 mW in the blue, 12 mW in the green, and 18 mW in the red were obtained for a pump power of only 480 mW. The tuning ranges extended from 460 nm to 500 nm and from 520 nm to 660 nm for SFG between the pump and signal and SHG of the signal, respectively, with at least 2-mW output power. Our work demonstrates that proper choice of the duty cycle of the poling period allows exploitation of higher-order QPM for intracavity SHG and SFG processes in PPLN OPOs.

Squeezed singly resonant second-harmonic generation in periodically poled lithium niobate

We determine the theoretical singly resonant second-harmonic generation squeezing performance of several cavity designs and nonlinear materials. We show that, for the doubling of the 1064-nm output of Nd:YAG lasers, monolithic cavities made from periodically poled LiNbO3 (PPLN) will produce the largest amount of squeezing. We also present experimental results for a free-space cavity with a PPLN sample that yielded slightly less than 0.6 dB of measured squeezing. The cavity performance as a function of input power agrees with our theoretical predictions and shows the superior performance of periodically poled materials for the production of bright squeezed light.

Polarization dependence of quasi-phase-matched second-harmonic generation in bulk periodically poled LiNbO3

Numerical comparisons of the acceptance bandwidths for quasi-phase-matched E Zω E Zω–E Z2ω (ee–e) and EYω EYω–EZ 2ω (oo–e) second-harmonic generation processes in bulk periodically poled LiNbO3 (PPLN) are given. The larger acceptance bandwidths and grating periods for the latter process lower the fabrication constraints, especially in the short-wavelength region, and also enhance the frequency conversion efficiency. The corresponding (oo–e) quasi-phase-matching conditions in bulk PPLN are discussed for different fundamental wavelength regions.

COHERENT AND TUNABLE TERAHERTZ OSCILLATORS, GENERATORS, AND AMPLIFIERS

We have considered feasibilities of using optical parametric oscillation and amplification, and difference-frequency generation for efficiently generating and amplifying terahertz waves in several second-order nonlinear optical materials. They include GaAs, GaP, GaSe, CdSe, LiNbO3 and LiTaO3. The advantage of using birefringence in CdSe and GaSe is tunability of the output terahertz frequency. Furthermore, both CdSe and GaSe can be used to achieve the backward parametric oscillation without any cavity. On the other hand, in periodically-poled LiNbO3 and LiTaO3, one can take advantage of large diagonal elements of second-order nonlinear susceptibility tensor. In the diffusion-bonded-stacked GaAs and GaP plates, quasi-phase matching can be achieved by alternatively rotating the plates. It is also feasible to achieve phase matching in a single GaAs or GaP plate due to the presence of the absorption tail of polariton modes. In this case, tuning can be achieved by changing the pump wavelength. The advantages of using coherent parametric processes are possibilities of efficiently generating and amplifying temporally-coherent and narrow-linewidth tunable terahertz waves at room temperature. However, all other schemes do not possess these advantages. Compared with a noncollinear configuration, by using the parallel wave propagation configurations, the conversion efficiency can be higher because of longer effective interaction length among all the waves.

Scanning-nonlinear-dielectric-microscopy study on periodically poled LiNbO3 for a high-performance quasi-phase matching device

Studies of the domain distribution of periodically poled LiNbO3 (PPLN) using scanning nonlinear dielectric microscopy (SNDM) were performed for high-quality quasi-phase matching (QPM) devices and some very important results were obtained. First, a single domain surface layer formed on PPLN was found and second, the SNDM studies also revealed that a very strong residual stress or an internal electric field remained in PPLN fabricated using the application of a high voltage due to the pinning effect in single crystal LiNbO3. This stress or internal field substantially reduced the nonlinear dielectric constant of LiNbO3. By annealing to release this stress, it was confirmed that the LiNbO3 recovered from the reduced nonlinear dielectric constant. In the optical region, this constant is equivalent to the second harmonic generation constant, therefore, it is proposed that annealing is an effective method for obtaining a highly efficient high-performance QPM device.

Ambient formaldehyde detection with a laser spectrometer based on difference-frequency generation in PPLN.

A laser spectrometer based on difference-frequency generation in periodically poled LiNbO3 (PPLN) has been used to quantify atmospheric formaldehyde with a detection limit of 0.32 parts per billion in a given volume (ppbV) using specifically developed data-processing techniques. With state-of-the-art fiber-coupled diode-laser pump sources at 1083 nm and 1561 nm, difference-frequency radiation has been generated in the 3.53-micrometers (2832-cm-1) spectral region. Formaldehyde in ambient air in the 1- to 10-ppb V range has been detected continuously for nine and five days at two separate field sites in the Greater Houston area operated by the Texas Natural Resource Conservation Commission (TNRCC) and the Houston Regional Monitoring Corporation (HRM). The acquired spectroscopic data are compared with results obtained by a well-established wet-chemical o-(2,3,4,5,6-pentafluorobenzyl) hydroxylamine (PFBHA) technique.