Optical All-Pass Filter in Silicon-on-Insulator

In this work, an optical beamforming network (OBFN) is proposed and simulated to verify its effectiveness on a 4 × 4 2D antenna array, enabling efficient beam steering for millimeter-wave systems. The network employs an optical all-pass filter (APF) based on a single micro-ring resonator (MRR) coupled with a straight waveguide for delay control. The APF is designed to achieve a maximum delay of approximately 100 ps in the 28 GHz communication band while maintaining a constant amplitude response during phase adjustment. This design significantly reduces amplitude variations associated with delay control, eliminating the amplitude effects typically observed in conventional MRR-based delay methods. Simulation results demonstrate that the design supports simultaneous lateral and longitudinal beam sweeps from -60° to 60° on a 4 × 4 2D antenna array. This approach offers a high-performance solution for OBFN architectures.

As the bitrate per channel increases in optical systems, the allowable tolerance for dispersive effects, such as chromatic and polarization mode dispersion, decreases substantially. At 40 Gb/s and beyond, the tolerance window has shrunk to an extent where adjustable dispersion compensation is required to accommodate beginning-of-life distributions, wavelength dependence, measurement uncertainties, and temperature fluctuations in the fiber dispersion. We focus on optical allpass filters. Allpass filters are unique in their ability to provide a flexible phase response that can approximate any desired dispersion over a bandwidth which is large relative to the filter period and, in the limit of low feedback path losses, which is decoupled from the magnitude response.

Filters are key devices in optical systems and are usually applied to signal suppression or selection based on their amplitude frequency responses. Different from amplitude filtering, an all-pass filter (APF) is a unique type of filter that exhibits a uniform amplitude and variable phase in the frequency response. Traditionally, an APF is designed to be ideal and waveguide loss as well as coupling loss are omitted, which is impossible in practice. Here we demonstrate a method of realizing an APF even when waveguide loss and coupling loss exist, and a feasible structure for realizing the APF with waveguide loss and coupling loss is proposed. The method indicates that an APF can be realized by introducing self-compensation of loss to the structure of an ideal single-stage APF, and further, an APF based on silicon-on-insulator foundry is fabricated and demonstrated. The method of self-compensation of loss can also be used to realize APFs in the electrical domain. We anticipate that our work will inspire realizations of more APF-related devices that once seemed impossible in practice, as well as loss related devices or structures.

All-pass filters (APFs) are devices that allow phase correction or equalization without introducing any amplitude distortion. An optical implementation of such devices is very attractive since they can be used for dispersion compensation. In contrast to other dispersion control devices, optical APFs can correct any order of dispersion. This can be achieved by careful design of multistage APFs to approximate a target phase profile. However, large dispersion is usually narrow band or requires many filter stages. These performance tradeoffs and the general phase properties of optical APFs are reviewed and clarified in the first part of this paper. In the second part, a general design methodology of optical APFs is introduced. We show that any all-pass structure may be constructed from simple N-port devices (such as directional couplers or Mach-Zehnder interferometers) with N-1 outputs fed back to any of the N-1 inputs. The feedback paths may contain delays or further APFs (recursive design). This set of design rules allows for constructing complex all-pass filters of any number of stages starting with very simple elements. We use this technique to demonstrate a number of optical all-pass structures that may be implemented in planar waveguide or using thin-film filter technology.

Purpose: To compare amplitude sorting and phase sorting techniques using external respiratory measurements for 4D‐CT for patients undergoing quiet breathing. Method and Materials: We have developed a 4D CT technique for mapping respiratory motion in radiotherapy treatment planning. A 16‐slice CT scanner was operated in ciné mode to acquire 25 scans consecutively at each couch position. The scans were sorted into 12 respiratory‐windows based on the amplitude and direction (inhalation or exhalation), and on the phase (0–360°) of a synchronized external respiratory measurement. An air content measure (the amount of air in a 16‐slice CT segment, used as a surrogate for internal motion) was correlated to the respiratory amplitude and phase throughout the lung. Images reconstructed based on the two sorting techniques were displayed for a qualitative comparison. Also, the variations in the amplitude of the respiratory measurement during the entire scan session were compared using 8, 12, 24, and 48 respiratory windows. Results: The air content showed a higher correlation with the respiratory amplitude than with the respiratory phase for most cases. Images reconstructed based on the amplitude sorting technique displayed fewer artifacts, especially at the lung‐diaphragm boundaries, than images reconstructed based on the phase sorting technique. The variations in the respiratory amplitude were much smaller with amplitude sorting than those with phase sorting. These variations decreased significantly with finer amplitude respiratory windows while showed insignificant changes with finer phase respiratory windows. Conclusion: The amplitude sorting was generally better than phase sorting, especially for patients whose breathing was less reproducible. The use of finer respiratory windows did not improve the consistency for phase sorting. Keywords: 4‐D CT, respiratory sorting, motion, radiotherapy.

A microring resonator with a narrow gap between a waveguide and a microring resonator has been fabricated by using nanoimprint lithography. An organic spin-on-glass has been used as the material for the microring resonator. The outer diameter of the microring resonator is 20 µm and the coupling gap between the straight waveguide and the microring resonator is 40 nm. The filtering characteristics of the microring resonator have been evaluated using a metal halide lamp. The free spectral range is 3.9 nm and the Q-factor is 310. The structure of the microring resonator with a narrow gap can be used as a fundamental component of optical devices.

As a kind of optical comb filter, double-ring resonator formed by a 3 x 3 fiber directional coupler can be used in optical communication system. By changing the parameters such as the coupling angle of the directional coupler, physical lengths ofthe double rings, loss, and gain factors. It behaves like a device with multi-function involving multichannel band-pass and band-stop filter, laser resonator or amplifier, optical all-pass filter and so on. In this paper, characteristics of double ring resonator formed by an equal triangle 3 x 3 fiber directional coupler is numerically analyzed in detail, for the double-ring resonator with equal lengths, a minor peak appears near every main resonant peak, they change in reverse dynamically with the increase of the gain coefficient for fixed loss. For unequal lengths case, active cavity brings about clear Vemier effect, and free spectral range (FSR) is increased. At last, applications of this component in the construction of flattop interleaver and dispersion compensator in FDM system are discussed.

The nature of the observed amplitude variability of several modes in the δ Scuti star FG Vir is examined. This is made possible by the extensive photometry obtained during 2002–04, as well as the long photometric time-base starting in 1992. In this star, three frequencies show strong amplitude and phase variations. In the power spectrum, these frequencies also show up as frequency doublets. However, since true amplitude variability of a single frequency can also lead to (false) frequency doublets in the power spectrum, a specific test examining in detail the observed amplitude and the phase variations of an assumed single frequency is applied. For the frequencies at 12.15 and 23.40 cycle d−1, it is shown that the amplitude variability of a single mode can be ruled out. In particular, an important property of beating between two modes is fulfilled: the amplitude and the phase vary synchronously with a phase shift close to 90°. The origin of the amplitude variability of a third mode, namely, near 19.86 cycle d−1, is not clear due to the long beat period of 20+ yr, for which the amplitude–phase test suffers from gaps in the coverage. However, even for this frequency, the amplitude variations can be expressed well by a mathematical two-mode model. If we examine these three close frequency pairs together with other (usually more widely separated) close frequencies in FG Vir, 18 pairs of frequencies with separations closer than 0.10 cycle d−1 have been detected. It is shown that the majority of the pairs occur near the theoretically expected frequencies of radial modes. Mode identifications are available for only a few modes: the only detected radial mode at 12.15 cycle d−1 is part of a close pair. It is shown that accidental agreements between the frequencies of excited modes can be ruled out because of the large number of detected close frequency doublets.

Investigating the effects of structural parameters on the optical characteristics of add-drop filters

Optical fiber is capable of providing the solution to the need of high bandwidth communication. The main degradation to the optical signal integrity in single mode fibers (SMFs) is chromatic dispersion (CD). The effect of CD is pulse spreading which in turn leads to inter-symbol interference (ISI), thus increasing the bit error rate (BER) of the system. This paper presents a CD compensation technique that utilizes a parallel optical all pass filter (p-OAPF), where the design is based on the inverse phase response of the SMF. The p-OAPF is based on a class of all-pass filter. Simulation results of the proposed technique show an increase in the repeater-less length of a point-to-point optical communication system by 3 times with Gaussian pulses at 10 Gb/s and BER of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-9</sup> . A passive infinite impulse response filter structure is proposed for the implementation of the p-OAPF.

Micro-resonators (MR) have become a key element for integrated optical sensors due to their integration capability and their easy fabrication with low cost polymer materials. Nowadays, there is a growing need on MRs as highly sensitive and selective functions especially in the areas of food and health. The context of this work is to implement and study integrated micro-ring resonators devoted to sensing applications. They are fabricated by processing SU8 polymer as core layer and PMATRIFE polymer as lower cladding layer. The refractive index of the polymers and of the waveguide structure as a function of the wavelength is presented. Using these results, a theoretical study of the coupling between ring and straight waveguides has been undertaken in order to define the MR design. Sub-micronic gaps of 0.5 μm to 1 μm between the ring and the straight waveguides have been successfully achieved with UV (i-lines) photolithography. Different superstrates such as air, water, and aqueous solutions with glucose at different concentrations have been studied. First results show a good normalized transmission contrast of 0.98, a resonator quality factor around 1.5 × 104 corresponding to a coupling ratio of 14.7%, and ring propagation losses around 5 dB/cm. Preliminary sensing experiments have been performed for different concentrations of glucose; a sensitivity of 115 ± 8 nm/RIU at 1550 nm has been obtained with this couple of polymers.

A refractive index (RI) sensor based on straight silicon strip waveguide and a hybrid plasmonic microring resonator is presented. The hybrid waveguide is made up of a Metal-Air-Silicon structure, where the electric field is significantly enhanced in the narrow air gap. The transmission spectrum and electric field distribution of this sensor structure are simulated using Finite Element Method (FEM). Due to a large overlap of the optical field in the hybrid microring resonator and the sensing medium, this sensor can be used for both gas and bio sensing applications. The best sensitivity of 690 nm/RIU and 401 nm/RIU is obtained for gas and bio sensing, respectively. The obtained values are relatively high as compared to conventional dielectric microring resonator sensors. Our proposed sensor design has a potential to find further applications in biomedical science and nanophotonic circuits.

This letter presents a new circuit topology for a vector sum phase shifter where the amplitudes and phases of I and Q are simultaneously varied using a single analog control signal. The simultaneous adjustment of amplitudes and phases of I and Q allows for the vector sum's large phase tuning while maintaining its amplitude. Moreover, various functions of vector summation are all performed in a single stage, minimizing the chip real estate and power consumption. The maximum range of analog phase tuning with a constant amplitude is theoretically 180°, but in practice, it is limited by the tuning range of the utilized varactors. To achieve a large phase tuning range given varactors with limited tunability, digital control has been added to allow for coarse tuning of the phase. The utilized approach for coarse phase tuning preserves the chip area and power consumption but causes amplitude variation. The phase shifter performance is verified by simulations and measurements.

Optical phased arrays are promising candidates for both RF signal processing and optical beam forming and steering. These platforms not only enable accurate electrically controlled beam steering at high frequencies but also have the potential to significantly improve the performance of future free-space optical communications systems. In this work we exploit recent advancements in both nano-scale hybrid silicon-slot waveguides and electro-optic (EO) polymers to demonstrate an integrated optical phased-array antenna. Specifically, we create a hybrid integrated "photonic circuit" that connects an array of optical phase modulators, fed by a common optical signal and a 1x4 splitter, to a compact optical waveguide diffraction array for optical beam steering applications. The fundamental characteristics of the resulting integrated optical beam former, including the optical insertion loss, driving voltage, and phase control from the waveguide aperture are summarized in this letter.

BackgroundThermal stimulation has been proposed as a modality to facilitate motor recovery in neurological populations, such as stroke. Recently (Ansari, Remaud & Tremblay, 2018), we showed that application of cold or warm stimuli distally to a single digit produced a variable and short lasting modulation in corticomotor excitability. Here, our goal was to extend these observations to determine whether an increase in stimulation area could elicit more consistent modulation.MethodsParticipants (n = 22) consisted of a subset who participated in our initial study. Participants were asked to come for a second testing session where the thermal protocol was repeated but with extending the stimulation area from single-digit (SD) to multi-digits (MD, four fingers, no thumb). As in the first session, skin temperature and motor evoked potentials (MEPs) elicited with transcranial magnetic stimulation were measured at baseline (BL, neutral gel pack at 22 °C), at 1 min during the cooling application (pre-cooled 10 °C gel pack) and 5 and 10 min post-cooling (PC5 and PC10). The analysis combined the data obtained previously with single-SD cooling (Ansari, Remaud & Tremblay, 2018) with those obtained here for MD cooling.ResultsAt BL, participants exhibited comparable measures of resting corticomotor excitability between testing sessions. MD cooling induced similar reductions in skin temperature as those recorded with SD cooling with a peak decline at C1 of respectively, −11.0 and −10.3 °C. For MEPs, the primary analysis revealed no main effect attributable to the stimulation area. A secondary analysis of individual responses to MD cooling revealed that half of the participants exhibited delayed MEP facilitation (11/22), while the other half showed delayed inhibition (10/22); which was sustained in the post-cooling phase. More importantly, a correlation between variations in MEP amplitude recorded during the SD cooling session with those recorded in the second session with MD cooling, revealed a very good degree of correspondence between the two at the individual level.ConclusionThese results indicate that increasing the cooling area in the distal hand, while still eliciting variable responses, did produce more sustained modulation in MEP amplitude in the post-cooling phase. Our results also highlight that responses to cooling in terms of either depression or facilitation of corticomotor excitability tend to be fairly consistent in a given individual with repeated applications.