Slow-light Photonic Crystal Waveguides Research Articles

Compact and Highly Spectral Selective Asymmetric Co-Directional Coupler Using Slow Light Photonic Crystal Waveguide

This letter addresses the design of an ultra-compact narrowband transmission co-directional coupler suitable for 100-GHz wavelength-division-multiplexing applications. The originality of the considered approach stems from the use of laterally coupled strip and photonic crystal waveguides, the latter one operating near the high group index flat region of the dispersion diagram. The performed finite-difference time-domain modeling demonstrates the improvement of the spectral selectivity by a factor of 70 with respect to the state-of-the-art conventional symmetric co-directional couplers. A 1-nm half-width transmission band is achieved for a 55-μm coupler length. The corresponding length bandwidth merit factor is 0.055 nm×mm. The spectral selectivity improvement is achieved without significant extra-loss penalty. The design performed in view of implementation in silicon photonic technology is compatible with the existing micro-nano-fabrication facilities.

Improved design of photonic crystal waveguides with elliptical holes for enhanced slow light performance

We propose a new design for slow-light photonic crystal W1 waveguide, which uses a combination of circular and elliptical air holes arranged in a hexagonal lattice with background material of refractive index n = 2.84. Large value of normalized delay bandwidth product (NDBP = 0.3708) is obtained. We have also analyzed dispersion property for the structure and achieved nearly zero-dispersion for a very large bandwidth. Our proposed photonic crystal waveguide has slow light applications such as reduction in length and power consumption of all-optical and electro-optic switches at optical frequency.

Low loss propagation in slow light photonic crystal waveguides at group indices up to 60

We have designed slow light photonic crystal waveguides operating in a low loss and constant dispersion window of Δλ=2nm around λ=1565nm with a group index of ng=60. We experimentally demonstrate a relatively low propagation loss, of 130dB/cm, for waveguides up to 800μm in length. This result is particularly remarkable given that the waveguides were written on an electron-beam lithography tool with a writefield of 100μm that exhibits stitching errors of typically 10–50nm. We reduced the impact of these stitching errors by introducing “slow–fast–slow” mode conversion interfaces and show that these interfaces reduce the loss from 320dB/cm to 130dB/cm at ng=60. This significant improvement highlights the importance of the slow–fast–slow method and shows that high performance slow light waveguides can be realised with lengths much longer than the writing field of a given e-beam lithography tool.

Photonic crystal tunable slow light device integrated with multi-heaters

We fabricated photonic crystal slow light waveguides integrated with multi-heaters, using CMOS-compatible process. By optimizing heating powers and adjusting the index distribution, a clear delay peak was observed, which suggests that the fabrication errors were compensated for completely. When a linear index chirp was added to this condition, the delay was tuned by 54 ps. When a quadratic chirp was added, arbitrary group delay dispersion was generated at wavelengths around 1550 nm within a 3 nm bandwidth. The continuously tunable range was from −32 to 54 ps/nm/mm. Using this as a dispersion compensator, we compressed pre-chirped pico-second pulses.

Ultrafast Tunable Optical Delay Line Based on Indirect Photonic Transitions

We introduce the concept of an indirect photonic transition and demonstrate its use in a dynamic delay line to alter the group velocity of an optical pulse. Operating on an ultrafast time scale, we show continuously tunable delays of up to 20 ps, using a slow light photonic crystal waveguide only 300 μm in length. Our approach is flexible, in that individual pulses in a pulse stream can be controlled independently, which we demonstrate by operating on pulses separated by just 30 ps. The two-step indirect transition is demonstrated here with a 30% conversion efficiency.

High sensitivity gas sensing method based on slow light in photonic crystal waveguide

Abstract This paper proposed a high sensitivity gas sensing system based on slow light in photonic crystal waveguide (PCW). The PCW was used as the gas chamber, and the harmonic detection method was adopted for signal processing. Combined the Maxwell equation with perturbation theory, the effective gas absorption factor was deduced which was greatly enhanced due to slow light in PCW. For the measurement of CO gas, a PCW was designed as the gas chamber and the detection sensitivity could be increased by 14.45 times compared with the traditional system that without PCW as the chamber. Thus the measuring detection limit of 21.5 ppm is achieved with active region is only 1 mm long. The proposed method provided a potential for expending the applications of slow light in the sensor fields, having powerful practicability and favorable application prospects.

High-performance slow light photonic crystal waveguides with topology optimized or circular-hole based material layouts

Photonic crystal waveguides are optimized for modal confinement and loss related to slow light with high group index. A detailed comparison between optimized circular-hole based waveguides and optimized waveguides with free topology is performed. Design robustness with respect to manufacturing imperfections is enforced by considering different design realizations generated from under-, standard- and over-etching processes in the optimization procedure. A constraint ensures a certain modal confinement, and loss related to slow light with high group index is indirectly treated by penalizing field energy located in air regions. It is demonstrated that slow light with a group index up to ng=278 can be achieved by topology optimized waveguides with promising modal confinement and restricted group-velocity-dispersion. All the topology optimized waveguides achieve a normalized group-index bandwidth of 0.48 or above. The comparisons between circular-hole based designs and topology optimized designs illustrate that the former can be efficient for dispersion engineering but that larger improvements are possible if irregular geometries are allowed.

Semi-analytic method for slow light photonic crystal waveguide design

We present a semi-analytic method to calculate the dispersion curves and the group velocity of photonic crystal waveguide modes in two-dimensional geometries. We model the waveguide as a homogenous strip, surrounded by photonic crystal acting as diffracting mirrors. Following conventional guided-wave optics, the properties of the photonic crystal waveguide may be calculated from the phase upon propagation over the strip and the phase upon reflection. The cases of interest require a theory including the specular order and one other diffracted reflected order. The computational advantages let us scan a large parameter space, allowing us to find novel types of solutions.

Thermo-optic characteristics and switching power limit of slow-light photonic crystal structures on a silicon-on-insulator platform

Employing a semi-analytic approach, we study the influence of key structural and optical parameters on the thermo-optic characteristics of photonic crystal waveguide (PCW) structures on a silicon-on-insulator (SOI) platform. The power consumption and spatial temperature profile of such structures are given as explicit functions of various structural, thermal and optical parameters, offering physical insight not available in finite-element simulations. Agreement with finite-element simulations and experiments is demonstrated. Thermal enhancement of the air-bridge structure is analyzed. The practical limit of thermo-optic switching power in slow light PCWs is discussed, and the scaling with key parameters is analyzed. Optical switching with sub-milliwatt power is shown viable.

Coupling loss minimization of slow light slotted photonic crystal waveguides using mode matching with continuous group index perturbation

We experimentally demonstrate highly efficient coupling into a slow light slotted photonic crystal waveguide. With optical mode converters and group index tapers that provide good optical mode matching and impedance matching, a nearly flat transmission over the entire guided mode spectrum of 68.8 nm range with 2.4 dB minimum insertion loss is demonstrated. Measurements also show up to 20 dB baseline enhancement and 30 dB enhancement in the slow light region, indicating that it is possible to design highly efficient and compact devices that benefit from the slow light enhancement without increasing the coupling loss.

Compact coupling of light from conventional photonic wire to slow light waveguides

In this study, efficient input and output power coupling schemes for transition regions at the interface of conventional and slow light waveguides are investigated. By optimizing the tapered nano-tip of the input and output slab waveguides that support a group index of 3.58, we achieved 97% coupling efficiency to a square-lattice based slow light photonic crystal waveguide with a group index of 1200. The complementary slow waveguide structure based on triangular-lattice is also designed to support same order of magnitude slow light mode and targeted to alleviate the severity of the coupling loss. An acceptable efficiency value is recorded for the second type of slow waveguide mode. For the sake of targeting only input and output coupling losses, we made an assumption that other loss mechanisms are absent in the structure. The successful demonstration of effective and compact slow light couplers will assist the deployment of slow light devices in important applications, such as nonlinear optics, optical buffers, and optical delay lines.

Characteristics of Correlated Photon Pairs Generated in Ultracompact Silicon Slow-Light Photonic Crystal Waveguides

We report the characterization of correlated photon pairs generated in dispersion-engineered silicon slow-light photonic crystal waveguides pumped by picosecond pulses. We found that taking advantage of the 15 nm flat-band slow-light window (vg ~ c/30) the bandwidth for correlated photon-pair generation in 96 and 196 \mum long waveguides was at least 11.2 nm; while a 396 \mum long waveguide reduced the bandwidth to 8 nm (only half of the slow-light bandwidth due to the increased impact of phase matching in a longer waveguide). The key metrics for a photon-pair source: coincidence to accidental ratio (CAR) and pair brightness were measured to be a maximum 33 at a pair generation rate of 0.004 pair per pulse in a 196 \mum long waveguide. Within the measurement errors the maximum CAR achieved in 96, 196 and 396 \mum long waveguides is constant. The noise analysis shows that detector dark counts, leaked pump light, linear and nonlinear losses, multiple pair generation and detector jitter are the limiting factors to the CAR performance of the sources.

Third-harmonic generation in slow-light chalcogenide glass photonic crystal waveguides

We demonstrate third-harmonic generation (THG) in a dispersion-engineered slow-light photonic crystal waveguide fabricated in AMTIR-1 chalcogenide glass. Owing to the relatively low loss and low dispersion in the slow-light (c/30) regime, combined with the high nonlinear figure of merit of the material (∼2), we obtain a relatively large conversion efficiency (1.4×10(-8)/W(2)), which is 30× higher than in comparable silicon waveguides, and observe a uniform visible light pattern along the waveguide. These results widen the number of applications underpinned by THG in slow-light platforms, such as the direct observation of the spatial evolution of the propagating mode.

Scaling of Raman amplification in realistic slow-light photonic crystal waveguides

The prospect for low pump-power Raman amplification in silicon waveguides has recently been boosted by theoretical studies discussing the enhancement of nonlinear phenomena in slow-light structures. In principle, the slowing down of either the pump or the signal beam is equivalent in terms of Raman gain, but in the presence of losses, we show that they play different roles in determining the net signal gain. We also investigate the impact of the mode profile in realistic slow-light waveguides on the total gain, an effect that is usually neglected in the context of stimulated Raman scattering. By taking representative losses and mode shapes into account, we provide a realistic estimation of the achievable performance of slow-light photonic crystal waveguides.

Lasing in localized modes of a slow light photonic crystal waveguide

We demonstrate lasing in GaAs photonic crystal waveguides with InAs quantum dots as gain medium. Structural disorder is present due to fabrication imperfection and causes multiple scattering of light and localization of light. Lasing modes with varying spatial extend are observed at random locations along the guide. Lasing frequencies are determined by the local structure and occur within a narrow frequency band which coincides with the slow light regime of the waveguide mode. The three-dimensional numerical simulation reveals that the main loss channel for lasing modes located away from the waveguide end is out-of-plane scattering by structural disorder.

Effect of multiphoton absorption and free carriers in slow-light photonic crystal waveguides

We examine the effects of multiphoton absorption, free carriers, and disorder-induced linear scattering in slow-light photonic crystal waveguides. We derive an analytic formulation for self-phase modulation including the group velocity scaling of the nonlinear phase shift in materials limited by three-photon absorption as a representative nonlinear process. We investigate the role of free carriers and derive an approximate critical intensity at which these effects begin to strongly modify the optical field. This critical intensity is employed to determine an optimal group index for the self-phase modulation in the slow-light devices. These observations are confirmed with numerical modeling.

Experimental observation of evanescent modes at the interface to slow-light photonic crystal waveguides

We experimentally study the fields close to an interface between two photonic crystal waveguides that have different dispersion properties. After the transition from a waveguide in which the group velocity of light is v(g) ~ c/10 to a waveguide in which it is v(g) ~ c/100, we observe a gradual increase in the field intensity and the lateral spreading of the mode. We attribute this evolution to the existence of a weakly evanescent mode that exponentially decays away from the interface. We compare this to the situation where the transition between the waveguides only leads to a minor change in group velocity and show that, in that case, the evolution is absent. Furthermore, we apply novel numerical mode extraction techniques to confirm experimental results.

On the role of evanescent modes and group index tapering in slow light photonic crystal waveguide coupling efficiency

We investigate effects of different mechanisms on coupling efficiency between strip waveguides and the slow light mode in photonic crystal waveguides (PCWs). Both numerical simulations and experimental results show that group index (ng) tapering improves strip-PCW butt-coupling efficiency when compared to a direct coupling between a strip waveguide and a high-ng PCW. However, coupling efficiency is even higher when an intermediate low-ng PCW is used to couple from a strip waveguide to a high-ng PCW without an ng tapering. Our results suggest that the role of evanescent mode is more dominant in efficient coupling between two PCWs with large ng mismatch.

Study of different spectral regions and delay bandwidth relation in slow light photonic crystal waveguides

We investigate slow light propagation in monomode photonic crystal waveguides with different spectral features such as constant group index, high bandwidth and low group velocity dispersion. The form of the waveguide mode alters dramatically and spans three different spectral intervals by tuning the size of the boundary holes. Namely, slope of the band gap guided mode changes sign from negative to positive toward the Brillouin zone edge. In between there is a transition region where modes have nearly zero slopes. Maximum group index occurs at these turning points at the expense of high dispersion and narrow bandwidth. The apparent trade-off relationship between group index and bandwidth is revealed systematically. We show that as the radius of the innermost hole is increased above a certain value, the former one decreases and the latter one increases both exponentially but with a different ratio. The product of average group index and bandwidth is defined as a figure of merit which reaches up to a value of approximately 0.30 after a detailed parametric search. The findings of the frequency domain analysis obtained by plane wave expansion method are confirmed via finite-difference time-domain study.

Wideband group velocity independent coupling into slow light silicon photonic crystal waveguide

We experimentally demonstrate efficient optical coupling into a slow light photonic crystal waveguide (PCW) that is independent of the group velocity of the guided mode. With a group index taper to match the group velocity between a strip waveguide and a PCW, the optical coupling efficiency is nearly constant throughout the spectrum of the defect-mode, including the slow light region near the band edge. Compared to strip-PCW butt-coupling without a group index taper, our measurement results show a 20 dB enhancement in coupling efficiency with 5 dB less Fabry–Perot fluctuations. The measurements show excellent agreement with two-dimensional finite-difference time domain simulations.