Comparison of Q-factors between TE and TM modes in 3-D microsquares by FDTD simulation

Mode characteristics of three-dimensional (3-D) microsquare resonators are investigated by finite-difference time-domain (FDTD) simulation for the transverse electric (TE)-like and the transverse magnetic (TM)-like modes. For a pillar microsquare with a side length of 2 pin in air, we have Q-factors about 5 X. 103 for TM-like modes at the wavelength of 1550 run, which are one order larger than those of TE-like modes, as vertical refractive index distribution is 3.17/3.4/3.17 and the cororresponding center layer thickness is 0.2 mu m. The mode field patterns show that TM-like modes have much weaker vertical radiation coupling loss than TE-like modes. TM-like modes can have high Q-factors in a microsquare with weak vertical field confinement.

Design of quantum cascade microcavity lasers based on Q factor versus etching depth

The choice of the etching depth for semiconductor microcavities is a compromise between a high Q factor and a difficult technique in a practical fabricating process. In this paper, the influences of the etching depth on mode Q factors for mid-infrared quantum cascade microcylinder and microsquare lasers around 4.8 and 7.8 mu m are simulated by three-dimensional (3D) finite-difference time-domain (FDTD) techniques. For the microcylinder and the microsquare resonators, the mode Q factors of the whispering-gallery modes (WGMs) increase exponentially and linearly with the increase in the etching depth, respectively Furthermore, the mode Q factors of some higher order transverse WGMs may be larger than that of the fundamental transverse WGM in 3D microsquares. Based on the field distribution of the vertical multilayer slab waveguide and the mode Q factors versus the etching depth, the necessary etching depth is chosen at the position where the field amplitude is 1% of the peak value of the slab waveguide. In addition, the influences of sidewall roughness on the mode Q factors are simulated for microsquare resonators by 2D FDTD simulation. (C) 2009 Optical Society of America

FREQUENCY RESPONSE OF FIBER RING RESONATOR AND ITS APPLICATION IN DFB LASER WAVELENGTH VARIATION MEASUREMENT

Frequency response of a fiber ring resonator (FRR) composed of an ordinary optical coupler and a segment of optical fiber is theoretically and experimentally investigated. The frequency response equation based oil small-signal modulation is derived and studied in detail. It is shown that the shape of the frequency response curve is very sensitive to the wavelength; as a result, the FRR can be applied to measure the wavelength of a lightwave source with high resolution. With this method, we demonstrate the measurement of tiny changes of wavelength of a DFB laser. (C) 2009 Wiley Periodicals. Inc. Microwave Opt Technol Lett 51 2444-2448, 2009 Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.24608

Equilateral-Triangle and Square Resonator Semiconductor Microlasers

The characteristics of equilateral-triangle resonator (ETR) and square resonator microlasers are reported, which are potential light sources in the photonic integrations. Based on the numerical simulations, we find that high-efficiency directional emission can be achieved for the triangle and square microlasers by directly connecting an output waveguide to the resonators. The electrically injected InP/InGaAsP ETR and square resonator microlasers with a 2-mu m-wide output waveguide were fabricated by standard photolithography and inductively coupled plasma etching techniques. Room-temperature continuous-wave (CW) operations were achieved for the ETR microlasers with the side length from 10 to 30 mu m and the square resonator microlasers with the side length of 20 mu m. The output power versus CW injection current and the laser spectra are presented for an ETR microlaser up to 310 K and a square resonator microlaser to 305 K. The lasing spectra with mode wavelength intervals as that of whispering-gallery-type modes and Fabry-Perot modes are observed for two square lasers, which can lase at low temperature and room temperature, respectively.

Four-channel reconfigurable optical add-drop multiplexer based on photonic wire waveguide

We designed and fabricated a four-channel reconfigurable optical add-drop multiplexer based on silicon photonic wire waveguide controlled through thermo-optic effect. The effective footprint of the device is about 1000 x 500 mu m(2). The minimum insertion loss is about 10.7 dB and the tuning bandwidth about 17 nm. The average tuning power efficiency is about 6.187 mW/nm and the tuning speed about 24.4 kHz. The thermo-optic polarization-rotation effect is firstly reported in this paper. (C) 2009 Optical Society of America

Calculation of light delay for coupled microrings by FDTD technique and Pade approximation

The Pade approximation with Baker's algorithm is compared with the least-squares Prony method and the generalized pencil-of-functions (GPOF) method for calculating mode frequencies and mode Q factors for coupled optical microdisks by FDTD technique. Comparisons of intensity spectra and the corresponding mode frequencies and Q factors show that the Pade approximation can yield more stable results than the Prony and the GPOF methods, especially the intensity spectrum. The results of the Prony method and the GPOF method are greatly influenced by the selected number of resonant modes, which need to be optimized during the data processing, in addition to the length of the time response signal. Furthermore, the Pade approximation is applied to calculate light delay for embedded microring resonators from complex transmission spectra obtained by the Pade approximation from a FDTD output. The Prony and the GPOF methods cannot be applied to calculate the transmission spectra, because the transmission signal obtained by the FDTD simulation cannot be expressed as a sum of damped complex exponentials. (C) 2009 Optical Society of America

Symmetry analysis and numerical simulation of mode characteristics for equilateral-polygonal optical microresonators

Mode characteristics for two-dimensional equilateral-polygonal microresonators are investigated based on symmetry analysis and finite-difference time-domain numerical simulation. The symmetries of the resonators can be described by the point group C-Nv, accordingly, the confined modes in these resonators can be classified into irreducible representations of the point group C-Nv. Compared with circular resonators, the modes in equilateral-polygonal resonators have different characteristics due to the break of symmetries, such as the split of double-degenerate modes, high field intensity in the center region, and anomalous traveling-wave modes, which should be considered in the designs of the polygonal resonator microlasers or optical add-drop filters.

Room-temperature continuous-wave electrically injected InP-GaInAsP equilateral-triangle-resonator lasers

Equilateral-triangle-resonator (ETR) microlasers with an output waveguide connected to one of the vertices of the ETR are suitable to be a light source for photonic integrated circuits. InP-GaInAsP ETR lasers with side length from 10 to 30 pm and the output-waveguide width of 1 or 2 pm are fabricated using standard photolithography and inductively coupled-plasma etching techniques. Continuous-wave electrically injected 1520-nm ETR laser with 20-mu m sides is realized with the maximum output power 0.17 and 0.067 mW and the threshold current 34 and 43 mA at 290 K and 295 K, respectively.

Prediction and suppression of strong dispersive coupling in microracetrack channel drop filters

For a four-port microracetrack channel drop filter, unexpected transmission characteristics due to strong dispersive coupling are demonstrated by the light tunneling between the input-output waveguides and the resonator, where a large dropping transmission at off-resonance wavelengths is observed by finite-difference time-domain simulation. It causes a severe decline of the extinction ratio and finesse. An appropriate decrease of the coupling strength is found to suppress the dispersive coupling and greately increase the extinction ratio and finesse, a decreased coupling strength can be realized by the application of an asymmetrical coupling waveguide structure. In addition, the profile of the coupling dispersion in the transmission spectra can be predicted based on a coupled mode theory analysis of an equivalent system consisting of two coupling straight waveguides. The effects of structure parameters on the transmission spectra obtained by this method agree well with the numerical results. It is useful to avoid the strong dispersive coupling region in the filter design. (c) 2007 Optical Society of America.

Raman scattering detection of stacking faults in free-standing cubic-SiC epilayer

We report on stacking fault (SF) detection in free-standing cubic-SiC epilayer by the Raman measurements. The epilayer with enhanced SFs is heteroepitaxially grown by low pressure chemical vapour deposition on a Si(100) substrate and is released in KOH solution by micromechanical manufacture, on which the Raman measurements are performed in a back scattering geometry. The TO line of the Raman spectra is considerably broadened and distorted. We discuss the influence of SFs on the intensity profiles of TO mode by comparing our experimental data with the simulated results based on the Raman bond polarizability (BP) model in the framework of linear-chain concept. Good agreement with respect to the linewidth and disorder-induced peak shift is found by assuming the mean distance of the SFs to be 11 angstrom in the BP model.

Distributed mode coupling in microring channel drop filters

The authors present the observation of wide transmission dips in a microring channel drop filter by two-dimensional finite-difference time-domain simulation. The authors show that distributed mode coupling between the input waveguide and the resonator results in the oscillations of the coupling efficiency and the envelope of transmission spectra with wavelength. The critical coupling as the light just passing through the coupling region is important for optimizing related devices. If the width of the input waveguide is different from that of the ring resonator, the phenomenon can be greatly reduced. (c) 2006 American Institute of Physics.

Investigation of mode characteristics for a square cavity with a pedestal by a three-dimensional finite-difference time-domain technique

Mode characteristics of a strongly confined square cavity suspended in air via a pedestal on the substrate are investigated by a three-dimensional finite-difference time-domain technique. The mode wavelengths and mode quality factors (Q factors) are calculated as the functions of the size of the pedestal and the slope angle 0 of the sidewalls of the square slab, respectively For the square slab with side length of 2 mu m, thickness of 0.2 mu m, and refractive index of 3.4, on a square pedestal with refractive index of 3.17, the Q factor of the whispering-gallery (WG)-like mode transverse-electric TE(3.5)o first increases with the side length b of the square pedestal and then quickly decreases as b > 0.4 mu m, but the Q factor of the WG-like mode TE(4.6)o drops down quickly as b > 0.2 mu m, owing to their different symmetries. The results indicate that the pedestal can also result in mode selection in the WG-like modes. In addition, the numerical results show that the Q factors decrease 50% as the slope angle of the sidewalls varies from 90 degrees to 80 degrees. The mode characteristics of WG-like modes in the square cavity with a rectangular pedestal are also discussed. The results show that the nonsquare pedestal largely degrades the WG-like modes. (c) 2006 Optical Society of America

WG-like modes selectivity in a square cavity with posts

We investigate the characteristics of Whispering-Gallery(WG)-like modes in a square cavity with posts by employing the two-dimentional (2D) finite-difference time-domain (FDTD) technique combined with the effective index method. The results indicate that the posts can result in mode selection in the WG-like modes. The WG-like modes with odd mode numbers are not much sensitive to the sizes of the posts. However, the quality factor (i.e. Q-factor) of the WG-like modes with even mode numbers decreases sharply with the increasing size of the posts. The decreasing Q-factor is attributed to mode leakage and scattering loss due to the presence of the post. The mode selection increases with the mode spacing of square cavity twice in an optimized strucure.

Analysis of mode quality factors and mode reflectivities for nanowire cavity by FDTD technique

The mode frequency and the quality factor of nanowire cavities are calculated from the intensity spectrum obtained by the finite-difference time-domain (FDTD) technique and the Pade approximation. In a free-standing nanowire cavity with dielectric constant epsilon = 6.0 and a length of 5 mu m, quality factors of 130, 159, and 151 are obtained for the HE11 modes with a wavelength around 375 nm, at cavity radius of 60, 75, and 90 nm, respectively. The corresponding quality factors reduce to 78, 94, and 86 for a nanowire cavity standing on a sapphire substrate with a refractive index of 1.8. The mode quality factors are also calculated for the TE01 and TM01 modes, and the mode reflectivities are calculated from the mode quality factors.

Analysis of output coupling efficiencies for directional emission triangle and square microlasers

Output coupling efficiencies are analyzed for triangular and square microlasers connected with an output waveguide by FDTD simulation. The results show that square resonator with an output waveguide connected to the midpoint of one side can have high output coupling efficiency and a good mode selection.