We propose a scheme based on two-ring resonator system that can realize flat delay and transmission response with
delay-bandwidth product (DBP) higher that those achieved in previously proposed schemes. The spectrum flatness and
DBP are two key parameters that characterize the maximum number of bits that can be buffered without distortion for
certain signal operating bandwidth. Simple time domain simulation shows that our scheme can achieve the same
buffering time with 2 to 4 times smaller number of modules, which indicates DBP of 2 to 4 times larger than those of
side-coupled ring structure and coupled resonator optical.
In this proceeding, we present for the first time, a nested-ring Mach-Zehnder interferometer (NRMZI) on SOI (Silicon-on-
insulator), realized using a CMOS based process. We show that the device operates in two propagating resonance
modes: (1) The inner-loop resonant mode due to strong build-up inside the inner-ring and (2) the double Fano-resonance
mode due to strong light interaction with the outer loop. The experimental data shows that the inner-loop resonance is
highly sensitive to the MZI arm imbalance as compared to the double-Fano resonance mode. With such considerations, a
good fit is acquired between theory and experiment.
We propose a finesse enhancement scheme by a simple two-ring system, in which the resonance finesse is dependent on
the relative intensity buildup of the second ring with respect to the first. In lossless case, it is possible to obtain finesse
two orders of magnitude higher than that of the single ring system. The two-ring system is fabricated in silicon-on-insulator
using deep UV (DUV) lithography and shown to exhibit the finesse of 100 to 300. The associated finesse
enhancement of 20 is in a good agreement with the theory.
Electroabsorption modulator has been widely used in modern optical fiber communication system and analog RF link system. In this paper, the design of a high-performance EAM with low coupling loss, high saturation power and high speed was demonstrated, which include the waveguide, active core and electrodes. A novel EAM with large optical cavity (LOC) waveguide structure, intrastep quantum well (IQW) active core and traveling wave electrodes was presented and fabricated successfully. Our results show that the LOC waveguide effectively improved the optical profile of EAM and reduced the coupling loss. The obtained traveling wave EAM achieved 21dBm saturation power and 23GHz 3-dB bandwidth.
1×2 and 1×3 planar optical splitters based on Benzocyclobutene (BCB 4024-40) polymeric material are demonstrated for the first time. The devices are designed based on symmetric interference and fabricated on BK7 glass substrates with a thin layer of SiO2 as cover. A cost-effective chemical etching technique is used in the fabrication process to take advantage of the photosensitivity of the polymer. The waveguide loss was measured using the cutback method to be 3.5 dB/cm. The splitting uniformity is better than 0.02 and 1.5 dB at 1550-nm optical wavelength for the 1×2 and 1×3 splitters, respectively.
A new design of vertical coupler (VC) based on only a single mesa is presented. The newly proposed vertical coupler
has the potential of improving the fabrication yield, as it requires much simpler fabrication process. The vertical coupler
comprises a large underlying rib waveguide to which light is coupled. The lower waveguide is formed by the loading
effect provided by the smaller upper ridge waveguide. The upper ridge waveguide contains the actual device (e.g.
modulator or photodetector) and is tapered in a way to maximize the vertical transfer of light from the lower waveguide,
and thus the vertical coupler can be used as a spot-size converter (SSC) between a single-mode fiber and the actual
device. We investigate the transfer efficiency of the single-mesa vertical coupler with different configurations. The
single-mesa design is found to give a lower total loss (i.e., fiber coupling loss plus transfer loss) compared with direct
coupling to the actual device. The transfer efficiencies obtained are more than 90% for both TE and TM polarizations,
and are polarization independent.
We present a transfer matrix analysis of a 2-D filter to study its frequency response functions. The (M × N) array consists of N independent columns of micro-ring resonators side-coupled to two channel bus waveguides, with equal spacing between columns and each column consisting of M coupled resonators. We show that the bandgap of the 2-D structure is a superposition of the non-overlapping bandgap of the two 1-D arrays. This non-overlapping property can be
used to realize the "near-ideal" filter with flat and sharp passband, negligible sidelobes in the stop bands, and linear
phase response over 80% of the passband. The existence of defect mode in linear and lossless ring resonator arrays is also demonstrated. The defect can be introduced by removing one ring or by making one ring bigger or smaller. Defect states within the photonic bandgaps behave like either donor or acceptor modes similar to other photonic crystals. The results based on transfer matrix model shows reasonable agreement with finite difference time domain (FDTD) simulations.
Vertical coupling between waveguides is a critical component for three-dimensional (3-D) integrated optics. Vertical integration adds flexibility in integrating different devices that require different materials, and facilitates coupling of the miniature devices with optical fibers. We propose a systematic design of a relatively simple and versatile vertical coupler that provides not only vertical interconnection, but also mode-size transformation and polarization mode selection all in one. As a vertical polarization splitter, it separates the TE and TM polarizations onto different vertical levels of a 3-D photonics structure, and is thus uniquely different from conventional splitters based on directional couplers or other planar devices. The vertical coupler consists of a larger bottom waveguide that serves to improve the fiber coupling, and a smaller top waveguide that contains the actual photonic device. As a polarization-independent coupler the vertical coupler is shown to transfer light with more than 90% efficiency for all polarizations over a transfer length of only 150 µm. As a polarization mode splitter, the vertical coupler preferentially couples TE or TM polarization with a contrast ratio up to 20dB. This versatility renders the vertical coupler a compact and useful input-stage device that improves the fiber coupling to small active devices and also provides a mechanism of polarization control.
In this paper, we present a comparison analysis between directional couplers (DC) and multi-mode interferometers (MMI) based on high-index contrast ridge waveguides. It is found that the two devices are intimately related as the MMI is structurally derived from the DC. For the first time, the continuous evolution from the two-mode coupling characteristic of DC to the two-mode interference and multi-mode interference of MMI is demonstrated. The resulting MMIs are compared with the DC in terms of coupling length, polarization dependence, excess loss, and fabrication tolerances. We show that practical directional couplers with reasonable gap size can also be quite compact and have the same coupling length for both TE and TM polarizations. Consequently, the DC can be just as polarization insensitive as the MMI. These features, however, require careful design control involving a large set of design parameters. By comparison, the MMI design is more robust and involves fewer design variables.
In photonic integration, high index contrast waveguides (e.g. ridge waveguides) have been widely used for interconnection and device construction because of their compactness. However, the strong confinement also means that the ridge waveguide is generally more multi-moded laterally than other waveguides of similar dimensions. A laterally single mode ridge waveguide is typically about 0.5 micrometer or less in width, which could incur high propagation and fiber coupling losses. Hence, a wider waveguide is often used to minimize loss at the expense of the single-mode characteristic, especially for long devices.
However, many devices, such as Mach-Zehnder interferometers and directional couplers, require single-mode waveguides for proper operation. In these cases, the multimode waveguides can still be used if appropriate mode-filters are strategically located at the input and within the device to remove potential higher-order modes.
We propose a higher-order mode filter using two back-to-back lateral waveguide tapers, which could be as short as 150 micrometers. Mode discrimination occurs in the down-taper where the higher-order modes become leaky when the taper width reaches their cut-off points (i.e., points where the modes are no longer guided by total internal reflection). The taper imposes only about 0.2dB loss to the fundamental mode itself. The up-taper at the other end restores the fundamental mode back to the original size. Simulations using Beam Propagation Method (BPM) shows that this mode filter is insensitive to the taper lengths and has reasonable fabrication tolerances.
Micro-ring resonators, by virtue of the geometrical birefringence of the strongly-confined waveguides used to construct them, are polarization dependent, and thus of limited application in fiber networks unless integrated with a laser or polarization controller. This paper addresses the polarization sensitivity issue by proposing a novel design of an MMI-coupled resonator with substantially reduced polarization sensitivity, while maintaining single-mode and low-loss conditions. The polarization-independent, single-mode waveguide is obtained by using the proper combination of ridge width and etching depth. The design is applicable to relatively large resonators, and may provide an intrinsic solution to small-FSR (free spectral range) polarization-independent applications.
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