Waveguide Mode Profiles Research Articles

Femtosecond laser micromachining of suspended silica-core liquid-cladding waveguides inside a microfluidic channel

This work addresses the fabrication of straight silica-core liquid-cladding suspended waveguides inside a microfluidic channel through fs-laser micromachining. These structures enable the reconfiguration of the waveguide's mode profile and enhance the evanescent interaction between light and analyte. Further, their geometry resembles a tapered optical fiber with the added advantage of being monolithically integrated within a microfluidic platform. The fabrication process includes an additional post-processing thermal treatment responsible for smoothening the waveguide surface and reshaping it into a circular cross-section. Suspended waveguides with a minimum core diameter of 3.8 µm were fabricated. Their insertion losses can be tuned and are mainly affected by mode mismatch between the coupling and suspended waveguides. The transmission spectrum was studied and it was numerically confirmed that it consists of interference between the guided LP01 mode and uncoupled light and of modal interference between the LP01 and LP02 modes.

Guided mode meta-optics: metasurface-dressed waveguides for arbitrary mode couplers and on-chip OAM emitters with a configurable topological charge.

Metasurface has achieved fruitful results in tailoring optical fields in free space. However, a systematic investigation on applying meta-optics to completely control waveguide modes is still elusive. Here we present a comprehensive catalog to selectively and exclusively couple free space light into arbitrary high-order waveguide modes of interest, leveraging silicon metasurface-patterned silicon nitride waveguides. By simultaneously engineering the matched phase gradient of the nanoantennas and the vectorial spatial modal overlap between the antenna near-field and target waveguide mode profile, either single or multiple high-order modes are successfully launched with high purity reaching 98%. Moreover, on-chip twisted light generators are theoretically proposed with configurable OAM topological charge ℓ from -3 to +2. This work may serve as a comprehensive framework for guided mode meta-optics and motivates further applications such as versatile integrated couplers, multiplexers, and mode-division multiplexing-based communication systems.

Utilizing polarization-selective mode shaping by chalcogenide thin film to enhance the performance of graphene-based integrated optical devices

High refractive index (RI) thin films are capable of pulling waveguide mode profiles towards themselves. In this study, it is shown that by applying high RI coatings with specific thicknesses on the side of optical waveguides, significantly different mode profiles for orthogonal polarizations can be achieved. This phenomenon, that we call it polarization-selective mode shaping, can be extensively used in the enhancement of polarization-dependent integrated optical devices. As an illustrating application, a tri-layer structure consisting of poly(methyl methacrylate)/graphene/chalcogenide on a side-polished fiber is designed to realize an extremely high extinction ratio polarizer. This structure changes the mode profiles in a way that the attenuation of TE mode is maximized, while the power carried by the TM mode remains relatively constant. Simulations and experimental characterizations confirm that polarization-selective mode shaping coordinates four loss mechanisms to maximize the extinction ratio and minimize the insertion loss of the polarizer. The fabricated polarizer is examined in the O, C, and L telecommunication frequency bands. This configuration achieves the high extinction ratio of 51.3 dB and its maximum insertion loss in the tested wavelengths is 1.79 dB. The proposed polarizer has been compared with other state-of-the-art polarizers in the conclusion section which shows its superiority.

Optical design of split-beam photonic crystal nanocavities

We design high-quality-factor photonic crystal nanobeam cavities formed by two mechanically isolated cantilevers. These "split-beam" cavities have a physical gap at the center, allowing mechanical excitations of one or both of the cavity halves. They are designed by analyzing the optical band structures and mode profiles of waveguides perforated by elliptical holes and rectangular gaps and are predicted to support optical resonances with quality factors exceeding 10(6) at wavelengths of ~1.6 μm.

Investigation of Waveguide Mode Profiles of AWGs Using a Low-Coherence Interferometric Method

In this paper, we propose and demonstrate a novel method to characterize waveguide mode profiles in arrayed waveguide grating (AWG) devices by utilizing a low-coherence interferometric measurement. Our method calculates the waveguide mode profile from its far-field diffraction pattern obtained by the low-coherence interferometric measurement utilizing AWG as a testing vehicle. The waveguide mode profile and its far-field diffraction pattern follow the Fourier transformation relationship. Detailed formulation and analysis procedures are presented in this paper. Experimental results of waveguide mode profiles for Gaussian waveguides and multimode interference waveguides are also shown. This method is very effective in characterizing complex waveguide designs in advanced AWG devices.

Analysis of a Synchronized Flattop AWG Using Low Coherence Interferometric Method

We have applied a low coherence interferometric method to investigate the thermal tuning effects of a synchronized arrayed waveguide grating (SAWG). We characterized the periodic waveguide mode profile of the input Mach-Zehnder interferometer (MZI) of an SAWG and measured phase delay changes of the two MZI arms introduced by thermal tuning. These phase delay changes, linear to applied heater power, provide simple and effective thermal control of this unique AWG.

Hollow-core waveguide characterization by optically induced particle transport

We introduce a method for optical characterization of hollow-core optical waveguides. Radiation pressure exerted by the waveguide modes on dielectric microspheres is used to analyze salient properties such as propagation loss and waveguide mode profiles. These quantities were measured for quasi-single-mode and multimode propagation in on-chip liquid-filled hollow-core antiresonant reflecting optical waveguides. Excellent agreement with analytical and numerical models is found, demonstrating that optically induced particle transport provides a simple, inexpensive, and nondestructive alternative to other characterization methods.

Ultrafast Response of Arrayed Waveguide Grating Pulse Shapers With Digital Amplitude and Phase Modulation

We present a general quantitative description of the temporal and spectral responses of arrayed waveguide gratings pulse shapers to a single ultrashort optical pulse. Operation of the pulse shaper was described in terms of superposition of pulse replicas that are overlapping in the slab waveguide region of the device. We demonstrate that the analytical formulation requires only a combination of fast Fourier transform algorithm with waveguide-mode profile simulation to accurately describe the spectral and temporal responses of the pulse shaper. The proposed model was applied to pulse shapers based on digital amplitude and phase modulations. A supporting evidence of the effectiveness of the model described in this paper was obtained from the good agreement between simulations and experiments.

Improvement of two-step K+–Na+ and Ag+–Na+ ion-exchanged glass waveguides by surface removal

We introduce a simple approach to improve the properties of waveguides fabricated by two-step K+–Na+ and Ag+–Na+ ion exchange in BK7 glass. By etching a 0.25-μm-thick layer off the waveguide surface, the propagation losses and polarization dependent losses caused by silver colloids formed during the ion exchange process can be significantly reduced. In addition, this method can improve the uniformity of the waveguides with varying waveguide patterns. After 24 h of annealing at 300 °C, the propagation losses of the etched waveguides have a value of 0.25–0.35dB∕cm⁠, which is about 0.1dB∕cm lower than the waveguides without etch. The etch has little impact on the waveguide mode profiles.

Optical properties of pseudomorphic Si/sub 1-x/Ge/sub x/ for Si-based waveguides at the λ=1300-nm and 1550-nm telecommunications wavelength bands

The index of refraction for pseudomorphic Si/sub 1-x/Ge/sub x/ layers grown on Si has been measured at wavelengths /spl lambda/=1310 mn and /spl lambda/=1550 nm. The refractive index values were obtained from waveguide mode profile measurements on a series of Si-Si/sub 1-x/Ge/sub x/-Si waveguides with Ge/sub x/ concentrations between x=0.01 and x=0.1. The index of refraction, n, is significantly larger for light polarized parallel to the growth direction than for light polarized in the plane of the epilayer. This birefringence is consistent with the anisotropic index change predicted using photoelastic theory, given the biaxial strain present in the pseudomorphic Si/sub 1-x/Ge/sub x/ layers. At all wavelengths and polarizations, n varies linearly with the Ge concentration. The pseudomorphic Si/sub 1-x/Ge/sub x/ waveguides layer are stable against lattice relaxation during short anneals at 950/spl deg/C, but exhibit partial relaxation after annealing at 1200/spl deg/C.

Frequency doubling in two-component self-assembled chromophoric waveguide structures

Self-assembled chromophoric multilayers having large second-order optical nonlinearities can be combined with linear guiding materials such as polymethylmethacrylate to produce two-component frequency doubling waveguides. This approach introduces considerable flexibility in optimizing the trade-off between the overlap of waveguide mode profiles and linear absorption. Extremely low waveguide propagation losses can be achieved because electric field poling and the accompanying poling-induced optical scattering are obviated.

Characterization of Er<sup>3+</sup>‐doped Ti:LiNbO<sub>3</sub> waveguides: losses, absorption spectra, and near field measurements

In this paper we present some experimental results about the characterization of Er^3+-doped Ti:LiNbO_3 waveguides using nondestructive methods. We report results concerning losses, near field and absorption measurements. Waveguide losses have been measured in the 1550 nm wavelength region through an interferometric method based on two different techniques: thermal cavity tuning and wavelength sweeping. Homogeneous absorption and emission cross sections have been calculated from transmitted spectra. The near field measurements have been used to obtain the waveguide mode profile through a deconvolution algorithm to eliminate the probe effect. The resulting field profile allowed to reconstruct the refractive index distribution.

Nondestructive waveguide loss-measurement method using self-pumped phase conjugation for optimum end-fire coupling

We present a new nondestructive method for measuring the propagation loss coefficient in waveguides based on optimum end-fire coupling by self-pumped phase conjugation in a photorefractive BaTiO3 crystal. In a first pass through the waveguide, the laser beam is adapted to the waveguide mode profile, and hence the phase-conjugated beam is automatically coupled back into the guide with the optimum coupling efficiency. The propagation loss coefficient is determined by measurement of the transmittance of the phase-conjugated beam in the second pass through the wave-guide. We apply this method to measure the propagation loss coefficient of ion-implanted KNbO3 channel waveguides with an accuracy of better than 5%.

Coupling of fibers to Ti-diffused LiNbO_3 waveguides by butt-joining

The optical losses due to butt-joining a single-mode optical fiber to a 32-microm wide Ti-diffused LiNbO(3) waveguide have been investigated. Measurements of the total insertion loss as a function of waveguide length show that the waveguide attenuation is 1.25 dB/cm and that coupling losses as low as 0.5 dB can be achieved. The coupling loss has been calculated by a numerical evaluation of the overlap integral of the fiber and waveguide mode profiles, and the results agree with the measured loss.