Silica-embedded silicon photonic crystal waveguides

BER evaluation in a multi-channel graphene-silicon photonic crystal hybrid interconnect: a study of fast- and slow-light effects

A comprehensive theoretical investigation on the bit-error ratio (BER) performance of multi-channel photonic interconnects operating in pulsed regimes is presented. Specifically, the optical link contains either a silicon photonic crystal (SiPhC) or a SiPhC-graphene (SiPhC-GRA) waveguide, possessing slow-light (SL) and fast-light (FL) regimes. A series of Gaussian pulses plus complex white noise are placed at input of each channel, with output signals demultiplexed and analyzed by a direct-detection receiver. Moreover, a rigorous theoretical model is proposed to measure signal propagation in SiPhC and SiPhC-GRA, which incorporates all crucial linear and nonlinear optical effects, as well as influences of free-carriers and SL effects. BER results of multi-channel systems are evaluated by utilizing the Fourier series Karhunen-Loeve expansion method. Our findings reveal that good BER performance is acquired at SiPhCs and SiPhC-GRAs in SL regimes but with their footprint about 2.5-fold smaller than FL waveguides. Moreover, the enhanced nonlinearity in SiPhC-GRAs induced by strong graphene-SiPhC coupling causes extra signal degradation than SiPhCs at the same length. This work provides additional insights into the coupling effect between SiPhCs operating in SL regimes and graphene, and their influence on WDM signal transmission, highlighting the potential applications of SiPhC-GRA interconnects in next-generation super-computing systems.

Hybrid External Cavity Laser with an Amorphous Silicon-Based Photonic Crystal Cavity Mirror

The authors present results on the performance of a hybrid external cavity photonic crystal laser-comprising semiconductor optical amplifier, and a 2D photonic crystal cavity fabricated in low-temperature amorphous silicon. The authors demonstrate that lithographic control over amorphous silicon photonic crystal cavity-resonant wavelengths is possible, and that single-mode lasing at optical telecommunications wavelengths is possible on an amorphous silicon platform.

M-line spectroscopy on mid-infrared Si photonic crystals for fluid sensing and chemical imaging

The presented work demonstrates the design and characterization of Si-based photonic crystal waveguides operating as an evanescent wave absorption sensor in the mid-IR range λ = 5-6 µm. The photonic crystal structure is fabricated in a Si slab upon a thin Si(3)N(4)/TEOS/Si(3)N(4) membrane. M-line spectroscopy is used to verify the presence of guided waves. Different fillings of the photonic crystal holes have been realized to avoid sample residuals in the holes and, at the same time, to obtain spectral tuning of the structures by modification of the refractive index contrast with the photonic background. The chip displays sensitivity to fluid droplets in two-prism experiments. The output signal is quantitatively related to the fluid's absorption coefficient thereby validating the experimental method.

Zero-coupling-gap degenerate band edge resonators in silicon photonics

Resonances near regular photonic band edges are limited by quality factors that scale only to the third power of the number of periods. In contrast, resonances near degenerate photonic band edges can scale to the fifth power of the number periods, yielding a route to significant device miniaturization. For applications in silicon integrated photonics, we present the design and analysis of zero-coupling-gap degenerate band edge resonators. Complex band diagrams are computed for the unit cell with periodic boundary conditions that convey characteristics of propagating and evanescent modes. Dispersion features of the band diagram are used to describe changes in resonance scaling in finite length resonators. Resonators with non-zero and zero coupling gap are compared. Analysis of quality factor and resonance frequency indicates significant reduction in the number of periods required to observe fifth power scaling when degenerate band edge resonators are realized with zero-coupling-gap. High transmission is achieved by optimizing the waveguide feed to the resonator. Compact band edge cavities with large optical field distribution are envisioned for light emitters, switches, and sensors.

CMOS compatible high-Q photonic crystal nanocavity fabricated with photolithography on silicon photonic platform

Progress on the fabrication of ultrahigh-Q photonic-crystal nanocavities (PhC-NCs) has revealed the prospect for new applications including silicon Raman lasers that require a strong confinement of light. Among various PhC-NCs, the highest Q has been recorded with silicon. On the other hand, microcavity is one of the basic building blocks in silicon photonics. However, the fusion between PhC-NCs and silicon photonics has yet to be exploited, since PhC-NCs are usually fabricated with electron-beam lithography and require an air-bridge structure. Here we show that a 2D-PhC-NC fabricated with deep-UV photolithography on a silica-clad silicon-on-insulator (SOI) structure will exhibit a high-Q of 2.2 × 10(5) with a mode-volume of ~ 1.7(λ/n)(3). This is the highest Q demonstrated with photolithography. We also show that this device exhibits an efficient thermal diffusion and enables high-speed switching. The demonstration of the photolithographic fabrication of high-Q silica-clad PhC-NCs will open possibility for mass-manufacturing and boost the fusion between silicon photonics and CMOS devices.

Maximizing slow-light enhancement in one-dimensional photonic crystal ring resonators

Photonic crystal ring resonators (PhCRR) combine the features of ring resonators with the slow-light effects present in photonic crystal waveguides, resulting in better mode confinement and increased light-matter interaction. When the resonator modes are near the photonic band edge, this enhancement is maximized. However, for this to be useful it is necessary to design the resonator so that these modes are at a desired wavelength. We introduce a design prescription, based on a theoretical analysis of the mode spectrum of PhCRRs, that maximizes these effects at a given wavelength. We test the procedure using numerical simulations, finding a good agreement between the design objectives and the simulated mode structures. We also consider the effects of disorder on the device.

Slow-light Mach-Zehnder modulators based on Si photonic crystals

Mach-Zehnder optical modulators are the key devices for high-speed electrical-to-optical conversion in Si photonics. Si rib waveguides with a p-n diode structure operated in the carrier depletion mode have mainly been developed as their phase shifters. Their length is usually longer than millimeters due to the limited change in the refractive index due to the carrier depletion in a Si p-n diode. This length is shorter than commercial LiNbO₃ modulators, but still much shorter devices are desired for large-scale integration and for simplifying the high-speed RF modulation. A promising solution is to use slow light in photonic crystal waveguides, which enhances the modulation efficiency in proportion to the group-velocity refractive index n_g. In particular, dispersion-engineered slow light allows more than five-fold enhancement, maintaining a wide working spectrum as well as large temperature tolerance. The devices with a phase shifter length of around 100 μm are fabricated by a standard process compatible with complementary metal-oxide semiconductors. The operation at 10 Gbps and higher speeds are obtained in the wavelength range of 16.9 nm and temperature range of 105 K.

All-optical tunable slow light achievement in photonic crystal coupled-cavity waveguides

In this paper, a tunable low power slow light photonic crystal device with a silicon-on-insulator platform is proposed based on the combination of an asymmetric defects coupled-cavity waveguide and the electromagnetically induced transparency (EIT) phenomenon. Modulating the refractive index of special regions in the suggested structure by the EIT phenomenon leads to a relatively wideband slow light device with adjustable group index in the same structure. Using this feature, a small and compact delay line is introduced that has many applications in optical telecommunications, especially in buffers. The numerical calculations show that the group index of 80-98 over the slow light bandwidth from 3.2 to 2.6 nm is achievable for the central wavelength of 1546-1555 nm, respectively. The device malfunction, due to fabrication errors, is modeled, and the tunable characteristics of the proposed structure are verified.

Cascaded modulator architecture for WDM applications

Integration density, channel scalability, low switching energy and low insertion loss are the major prerequisites for on-chip WDM systems. A number of device geometries have already been demonstrated that fulfill these criteria, at least in part, but combining all of the requirements is still a difficult challenge. Here, we propose and demonstrate a novel architecture consisting of an array of photonic crystal modulators connected by a dielectric bus waveguide. The device architecture features very high scalability and the modulators operate with an AC energy consumption of less than 1fJ/bit. Furthermore, we demonstrate cascadeability and multichannel operation by using a comb laser as the source that simultaneously drives 5 channels.

Four-wave mixing in slow light photonic crystal waveguides with very high group index

We report efficient four-wave mixing in dispersion engineered slow light silicon photonic crystal waveguides with a flat band group index of n(g) = 60. Using only 15 mW continuous wave coupled input power, we observe a conversion efficiency of -28 dB. This efficiency represents a 30 dB enhancement compared to a silicon nanowire of the same length. At higher powers, thermal redshifting due to linear absorption was found to detune the slow light regime preventing the expected improvement in efficiency. We then overcome this thermal limitation by using oxide-clad waveguides, which we demonstrate for group indices of n_g = 30. Higher group indices may be achieved with oxide clad-waveguides, and we predict conversion efficiencies approaching -10 dB, which is equivalent to that already achieved in silicon nanowires but for a 50x shorter length.

Efficient adiabatic wavelength conversion in Gires-Tournois resonators

We compare theoretically the performance capabilities of Fabry-Perot and Gires-Tournois resonators when used for adiabatic wavelength conversion. It is shown that the Gires-Tournois device will exhibit superior performance and is able to convert the wavelength of optical pulses with >74% efficiency while nearly preserving their temporal duration.

Nonlinear enhancement in photonic crystal slow light waveguides fabricated using CMOS-compatible process

We have studied low-dispersion slow light and its nonlinear enhancement in photonic crystal waveguides. In this work, we fabricated the waveguides using Si CMOS-compatible process. It enables us to integrate spotsize converters, which greatly simplifies the optical coupling from fibers as well as demonstration of the nonlinear enhancement. Two-photon absorption, self-phase modulation and four-wave mixing were observed clearly for picosecond pulses in a 200-μm-long device. In comparison with Si wire waveguides, a 60-120 fold higher nonlinearity was evaluated for a group index of 51. Unique intensity response also occurred due to the specific transmission spectrum and enhanced nonlinearities. Such slow light may add various functionalities in Si photonics, while loss reduction is desired for ensuring the advantage of slow light.

10 Gb/s operation of photonic crystal silicon optical modulators

We report the first experimental demonstration of 10 Gb/s modulation in a photonic crystal silicon optical modulator. The device consists of a 200 μm-long SiO2-clad photonic crystal waveguide, with an embedded p-n junction, incorporated into an asymmetric Mach-Zehnder interferometer. The device is integrated on a SOI chip and fabricated by CMOS-compatible processes. With the bias voltage set at 0 V, we measure a V(π)L < 0.056 V∙cm. Optical modulation is demonstrated by electrically driving the device with a 2(31) - 1 bit non-return-to-zero pseudo-random bit sequence signal. An open eye pattern is observed at bitrates of 10 Gb/s and 2 Gb/s, with and without pre-emphasis of the drive signal, respectively.

Fabrication of low loss dispersion engineered chalcogenide photonic crystals

We demonstrate low loss photonic crystal waveguides in chalcogenide (Ge(33)As(12)Se(55)) glasses. The measured losses are as low as 21 dB/cm. We experimentally determine the refractive index of the thin film chalcogenide glass to be n = 2.6 and demonstrate that dispersion engineering can be performed up to a group index of ng = 40 in this relatively low refractive index contrast system.

Loss engineered slow light waveguides

Slow light devices such as photonic crystal waveguides (PhCW) and coupled resonator optical waveguides (CROW) have much promise for optical signal processing applications and a number of successful demonstrations underpinning this promise have already been made. Most of these applications are limited by propagation losses, especially for higher group indices. These losses are caused by technological imperfections ("extrinsic loss") that cause scattering of light from the waveguide mode. The relationship between this loss and the group velocity is complex and until now has not been fully understood. Here, we present a comprehensive explanation of the extrinsic loss mechanisms in PhC waveguides and address some misconceptions surrounding loss and slow light that have arisen in recent years. We develop a theoretical model that accurately describes the loss spectra of PhC waveguides. One of the key insights of the model is that the entire hole contributes coherently to the scattering process, in contrast to previous models that added up the scattering from short sections incoherently. As a result, we have already realised waveguides with significantly lower losses than comparable photonic crystal waveguides as well as achieving propagation losses, in units of loss per unit time (dB/ns) that are even lower than those of state-of-the-art coupled resonator optical waveguides based on silicon photonic wires. The model will enable more advanced designs with further loss reduction within existing technological constraints.

Glass-embedded two-dimensional silicon photonic crystal devices with a broad bandwidth waveguide and a high quality nanocavity

To enhance the mechanical stability of a two-dimensional photonic crystal slab structure and maintain its excellent performance, we designed a glass-embedded silicon photonic crystal device consisting of a broad bandwidth waveguide and a nanocavity with a high quality (Q) factor, and then fabricated the structure using spin-on glass (SOG). Furthermore, we showed that the refractive index of the SOG could be tuned from 1.37 to 1.57 by varying the curing temperature of the SOG. Finally, we demonstrated a glass-embedded heterostructured cavity with an ultrahigh Q factor of 160,000 by adjusting the refractive index of the SOG.

Wideband slow light in chirped slot photonic-crystal coupled waveguides

Wideband dispersion-free slow light in chirped-slot photonic-crystal coupled waveguides is proposed and theoretically investigated in detail. By systematically analyzing the dependence of band shape on various structure parameters, unique inflection points in the key photonic band with approximate zero group velocity can be obtained in an optimized slot photonic-crystal coupled waveguide. By simply chirping the widths of the photonic-crystal waveguides in the optimized structure, wideband (up to 20 nm) slow-light with optical confinement in the low dielectric slot is demonstrated numerically with relative temporal pulse-width spreading well below 8% as obtained from two-dimensional finite-difference time-domain simulations. The wideband slow-light operation of the proposed structures would offer significant potential for novel compact high-speed optical-signal-processing devices in silicon-based systems.

Losses and group index dispersion in insulator-on-silicon-on-insulator ridge waveguides

We present polarization-dependent optical transmission properties of a completely symmetric silicon-on-insulator (SOI) microphotonic material system. In contrast to typical SOI based photonic materials, here an insulator-on-silicon-on-insulator (IOSOI) material system has been fabricated. This symmetric structure exhibits average losses between 1510 and 1630 nm of around 0.5 dB/mm for TE and 0.3 dB/mm for TM-polarization. The good transmission for TM-polarization can be explained by the thick insulting cladding layer of 3 microm thickness. Moreover, group index dispersion diagrams are presented and discussed for both polarizations.

Optical filter with very large stopband (approximately 300 nm) based on a photonic-crystal vertical-directional coupler

We have designed, fabricated, and demonstrated a vertical directional coupler based on the coupling between a polymer waveguide and a W1 photonic crystal waveguide. The filters have a bandwidth of approximately 2 nm within a stopband of Delta lambda approximately 300 nm and an on-chip insertion loss of 1 dB. This is the first (to our knowledge) demonstration of a filter with such a large stopband that overcomes the bandwidth limitation of existing filters.