Lutong Cai

Photonic crystal slab fabricated on the platform of lithium niobate-on-insulator

We report on a photonic crystal slab patterned on a 690 nm thick LiNbO3 thin film bonded to SiO2 on lithium niobate substrate. The transmission spectrum is measured and a broad and clear photonic bandgap ranging from 1335 to 1535 nm with a maximum extinction ratio of more than 20 dB is observed. The bandgap is simulated by plane wave expansion and 3D finite-difference time-domain methods. Such a deep and broad bandgap structure can be used to form high-performance photonic devices and circuits on the platform of lithium niobate-on-insulator.

Low-loss waveguides in a single-crystal lithium niobate thin film.

We report low-loss channel waveguides in a single-crystal LiNbO(3) thin film achieved using the annealed proton exchange process. The simulation indicated that the mode size of the α phase channel waveguide could be as small as 1.2  μm(2). Waveguides with several different widths were fabricated, and the 4 μm-wide channel waveguide exhibited a mode size of 4.6  μm(2). Its propagation loss was accurately evaluated to be as low as 0.6 dB/cm at 1.55 μm. The single-crystal lattice structure in the LiNbO(3) thin film was preserved by a moderate annealed proton exchange process (5 min of proton exchange at 200°C, followed by 3 h annealing at 350°C), as revealed by measuring the extraordinary refractive index change and x ray rocking curve. A longer proton exchange time followed by stronger annealing would destroy the crystal structure and induce a high loss in the channel waveguides.

Waveguides consisting of single-crystal lithium niobate thin film and oxidized titanium stripe.

Strip-loaded waveguides were fabricated by the direct oxidation of a titanium film based on the single-crystal lithium niobate. The method avoided the surface roughness problems that are normally introduced during dry etching of waveguide sidewalls. Propagation modes of the composite strip waveguide were analyzed by a full-vectorial finite difference method. The minimum dimensions of the propagation modes were calculated to be 0.7 μm(2) and 1.1 μm(2) for quasi-TM mode and quasi-TE mode at 1550 nm when the thickness of the LN layer and TiO(2) strip was 660 nm and 95 nm, respectively. The optical intensity was as high as 93% and was well confined in the LN layer for quasi-TM polarization. In this experiment, the propagation losses for the composite strip waveguide with 6 μm wide TiO(2) were 14 dB/cm for quasi-TM mode and 5.8 dB/cm for quasi-TE mode, respectively. The compact hybrid structures have the potential to be utilized for compact photonic integrated devices.

Channel waveguides and y-junctions in x-cut single-crystal lithium niobate thin film.

Proton exchanged channel waveguides in x-cut single-crystal lithium niobate thin film could avoid optical leakage loss which existed in the z-cut case. Indicated by simulations, the mechanism and condition of the optical leakage loss were studied. The light energy in the exchanged layer and the mode sizes were calculated to optimize the parameters for fabrication. By a very short time (3 minutes) proton exchange process without anneal, the channel waveguide with 2 μm width and 0.16 μm exchanged depth in the x-cut lithium niobate thin film had a propagation loss as low as 0.2 dB/cm at 1.55 μm. Furthermore, the Y-junctions based on the low-loss waveguide were designed and fabricated. For a Y-junction based on the 3 μm wide channel waveguide with 8000 μm bending radius, the total transmission could reach 85% ~90% and the splitting ratio maintained at a stable level around 1:1. The total length was smaller than 1 mm, much shorter than the conventional Ti-diffused and proton exchanged Y-junctions in bulk lithium niobate.

Electric-optical property of the proton exchanged phase modulator in single-crystal lithium niobate thin film

The electric-optical property of the proton exchanged phase modulator in an x-cut single-crystal lithium niobate thin film was studied. Proton exchanged waveguides generally suffered from a deteriorated electric-optical coefficient. By introducing a shallow proton exchange layer (thickness = 0.165 μm), most energy of the optical mode was allowed to guide in the untouched single-crystal lithium niobate film, making contribution to the effective electric-optical coefficient as high as 29.5 pm/V, which was very close to that of the bulk lithium niobate (r33 = 31 pm/V). A 12 V voltage applied to the electrodes located on the two sides of the waveguide induced a 0.097 nm shift of the Fabry-Perot resonant peak. Considering the wavelength difference of the neighboring resonant peaks (0.228 nm) and the length of the electrodes (2.3 mm), the voltage-length product was as low as 6.5 V·cm, indicating the efficient electric-optical modulation.

Mean fringe contrast, optimum beam ratio and maximum diffraction efficiency for volume gratings written by coupled waves

Diffraction properties of both transmission and reflection volume gratings written in wave-coupling materials are analyzed based on the concept of mean fringe contrast of the interference pattern formed in the recording process. Optimum beam ratio and corresponding maximum diffraction efficiency are derived. This shows that the lossless transmission and reflection gratings have very similar diffraction behaviors. Experimental results for reflection gratings are also presented.

Wave design of the interference of three noncoplanar beams for microfiber fabrication

Abstract In our recent paper [Opt. Lett. 26 (2001) 1858] we have indicated that the interference of three noncoplanar beams (ITNB) can form a pipeline microstructure that may serve as a microfiber bundle. This communication further explains how to determine the wave vectors and polarizations of the three beams for a desired lattice. It shows that generally the recording geometry for a given pattern is not unique, and this fact gives us more freedom in choosing proper recording geometry to obtain the best uniform contrast of resultant pattern.

A compact photonic crystal micro-cavity on a single-mode lithium niobate photonic wire

The properties of the guided modes, including the single-mode conditions and the coupling of different polarized modes in the single-crystal lithium niobate photonic wires, were analyzed in detail. One-dimensional photonic crystal micro-cavities with several different patterns, which could be used as an ultra-compact optical filter, were designed and simulated in order to get high transmission at the resonant wavelength and the best preferment. The designed structure, with the whole size of 6.5 × 0.7 μm2, was fabricated on a single-mode photonic wire. A measured peak transmission of 0.34 at 1400 nm, an extinction ratio of 12.5 dB and a Q factor of 156 were obtained. The measured transmission spectrum was basically consistent with the simulation, although a slight shift of resonant wavelength occurred due to the fabrication errors.

Lithium-rich vapor transport equilibration in single-crystal lithium niobate thin film at low temperature

A lithium-rich vapor transport equilibration technique was used to increase the Li/Nb ratio in single crystal LiNbO3 thin film at low temperature (below 600 °C). The extraordinary refractive index ne of the Li-compensated thin film was measured and found to be 2.1983 at 632.8 nm using the prism coupling method, while the ne of the congruent LiNbO3 was 2.2024. The lattice parameter (cr) of Li-compensated LiNbO3 thin film was determined to be 13.8604 A using high-resolution x-ray (HRXRD) ω - 2θ scan, which was between the values of congruent LiNbO3 (13.8650 A) and stoichiometric LiNbO3 (13.8562 A). The Raman spectra showed significant differences in the relative intensity and the FWHM of Raman lines between the Li-compensated thin film and congruent thin film.

Observation of fractional Fourier transforms of continuously variable orders with a scale invariant input

Abstract Generally, in optical systems performing the fractional Fourier transform (FRT), the scale of input function must be changed to obtain transformations of different orders. To eliminate the inconvenience of using masks of different scales, the effects of employing an unchanged input with a certain fixed scale on the final FRTs are investigated. The feasibility of this approach to realize the FRTs is shown with three simple optical systems as examples. This analysis provides a useful method for observation of the FRT patterns of continuously varied orders with a scale invariant input pattern, and is helpful for the design and evaluation of optical systems implementing the FRT.