Si Photonic Crystal Waveguide Research Articles

Si photonic crystal slow-light waveguides optimized through informatics technology.

We modeled the photonic bands of SiO2-cladded Si lattice-shifted photonic crystal waveguides via machine learning and found a structure that generates low-dispersion slow light with a group index of approximately 20 in the full C-band at telecom wavelengths. The normalized delay-bandwidth product is as large as 0.45, which is close to the theoretical upper limit. The transition structure between this waveguide and a Si-channel waveguide was designed using an evolutional optimization, and a C-band average loss of 0.116 dB/transition was calculated. These results prove the possibility of further enhancing the versatility of slow light.

Observation of Ultrashort Laser Pulse Evolution in a Silicon Photonic Crystal Waveguide

Using the sum frequency generation cross-correlation frequency-resolved optical gating (SFG-XFROG) measurement setup, we observed the soliton evolution of low energy pulse in an Si photonic crystal waveguide, and it exhibited the pulse broadening, blue shift, and evident pulse acceleration. The soliton evolution was also investigated by nonlinear Schrödinger equation (NLSE) modelling simulation, and the simulated results agreed well with the experimental measurements. The effects of waveguide length on the pulse evolution were analyzed; the results showed that the pulse width changed periodically with increasing waveguide length. The results further the understanding of the ultra-fast nonlinear dynamics of solitons in silicon waveguides, and are helpful to soliton-based functional elements on CMOS-compatible platforms.

Space-time-domain observation of high-speed optical beam scanning in a thermo-optic Si photonic crystal slow-light beam scanner.

We developed a thermo-optically controlled nonmechanical optical beam scanner using a Si photonic crystal slow-light waveguide with a diffraction grating to achieve on-chip light detection and ranging (LIDAR). This Letter applies pre-emphasis signals to the thermo-optic control, and the cutoff frequency increases to 500 kHz. Observing the beam scanning in the space-time domain showed that the turn-on and turn-off times of the scanner for a rectangular drive voltage were 10 µs and reduced to 2.7 µs when the pre-emphasis signals were optimized. This new, to the best of our knowledge, result enables a frame rate of 29 fps for 12,800 resolution points in LIDAR.

Particle swarm optimization of silicon photonic crystal waveguide transition.

Slow light generated through silicon (Si) photonic crystal waveguides (PCWs) is useful for improving the performance of Si photonic devices. However, the accumulation of coupling loss between a PCW and Si optical wiring waveguides is a problem when slow-light devices are connected in a series in a photonic integrated circuit. Previously, we reported a tapered transition structure between these waveguides and observed a coupling loss of 0.46 dB per transition. This Letter employed particle swarm optimization to engineer the arrangement of photonic crystal holes to reduce loss and succeeded in demonstrating theoretical loss value of 0.12 dB on average in the wavelength range of 1540-1560 nm and an experimental one of 0.21 dB. Crucially, this structure enhances the versatility of slow light.

Reconfigurable nanocavity formation in graphene-loaded Si photonic crystal structures.

We propose and numerically demonstrate that a reconfigurable nanocavity can be created in a graphene-loaded Si photonic crystal waveguide. The cavity formation is caused by the local mode-gap modulation induced by electrostatic gate-tuning of graphene. Although most recent graphene photonic devices are based on a change in the imaginary part of the refractive index, here we make use of a change in the real part of the refractive index for gated graphene. We clarify that nanocavities can be formed in two different cases, red-shifted and blue-shifted tunings. These novel formation mechanisms enable us to create and annihilate a nanocavity in a reconfigurable way by varying the gate voltage, which is promising for novel control in photonic processing.

Prism lens for beam collimation in a silicon photonic crystal beam-steering device.

The doubly periodic Si photonic crystal waveguide operates as a nonmechanical beam-steering device that can be applied in light detection and ranging. In this Letter, we develop and describe a prism lens that collimates a fan-shaped beam, emitted from the waveguide independent of the steering angle. Its fundamental profile is investigated using a theoretical analysis for thick lenses, and then, its detailed aspherical design is obtained. In ray tracing, this prism lens suppresses the beam divergence to less than the diffraction limit in most of the targeted beam-steering range. The prism lens is fabricated by acrylic cutting, and its expected characteristics are observed.

Thermally controlled Si photonic crystal slow light waveguide beam steering device.

The doubly periodic Si photonic crystal waveguide radiates the guided slow light into free space as an optical beam. The waveguide also functions as a beam steering device, in which the steering angle is changed substantially by a slight variation in the wavelength generated due to the large angular dispersion of the slow light. A similar function is obtained when the wavelength is fixed and the refractive index of the waveguide is changed. In this study, we tested two kinds of integrated heater structures and observed the beam steering using the thermo-optic effect. For a p-i-p doped waveguide, the heating current was made to flow directly across the waveguide and a beam steering range of 21° was obtained with a relatively low heating power and high-speed response of the order of 100 kHz, maintaining a narrow beam divergence of 0.1-0.3° and a 120 resolution points. We also performed a preliminary life test of the device but did not observe any severe degradation in the temperature variation of 80-430 K for the duration up to 20‒40 h. For a TiN heater device, we obtained the comparable beam steering characteristics, but the required heating power increased, and the response speed decreased drastically.

Electro-optic phase matching in a Si photonic crystal slow light modulator using meander-line electrodes.

We demonstrate a Si photonic crystal waveguide Mach-Zehnder modulator that incorporates meander-line electrodes to compensate for the phase mismatch between slow light and RF signals. We first employed commonized ground electrodes in the modulator to suppress undesired fluctuations in the electro-optic (EO) response due to coupled slot-line modes of RF signals. Then, we theoretically and experimentally investigated the effect of the phase mismatch on the EO response. We confirmed that meander-line electrodes improve the EO response, particularly in the absence of internal reflection of the RF signals. The cut-off frequency of this device can reach 27 GHz, which allows high-speed modulation up to 50 Gbps.

Adiabatic wavelength redshift by dynamic carrier depletion using p−i−n -diode–loaded photonic crystal waveguides

We demonstrate an adiabatic wavelength redshift using dynamic carrier depletion. Free carriers are first induced through two-photon absorption of a control pulse and then extracted by a reverse-biased p-i-n diode formed on a Si photonic crystal waveguide, resulting in rapid carrier depletion. A copropagating signal pulse is redshifted by the consequent increase in refractive index. We experimentally evaluated the dynamics of the carrier depletion by the pump-probe method and explored suitable conditions for adiabatic redshift. The signal's redshift was observed, and was confirmed to originate in the dynamic carrier depletion. The redshift was experimentally determined as 0.21 nm.

Fan-beam steering device using a photonic crystal slow-light waveguide with surface diffraction grating.

Compact non-mechanical beam steering devices are desired not only for current common applications, but also for advanced applications such as light detection and ranging. We use a Si photonic crystal slow-light waveguide with a diffraction grating, which radiates the guided mode to free space and steers a fan beam by sweeping the wavelength. Due to its large angular dispersion, slow light enhances the steering range without degrading the beam quality, resulting in more resolution points. We fabricated 600μm devices and observed a 23° steering range and a beam divergence of 0.23°, which resulted in 100 resolution points.

Theoretical Comparative Analysis of BER in Multi-Channel Systems With Strip and Photonic Crystal Silicon Waveguides

We present a comparative analysis of the performance of multi-channel photonic systems containing strip or photonic crystal (PhC) silicon (Si) waveguides in the presence of white Gaussian noise. Specifically, we consider a multi-wavelength optical signal propagating either in a strip single-mode Si photonic waveguide (Si-PhW) or in a Si PhC waveguide (Si-PhCW), each channel consisting of a CW nonreturn-to-zero ON–OFF keying modulated signal. The output signal is demultiplexed and the signal in each channel is analyzed using direct-detection optical receivers. We describe the propagation of the optical signal in the Si-PhW and Si-PhCW using a linearized model and validate our main results by employing a rigorous theoretical model that incorporates all relevant linear and nonlinear optical effects and the mutual interaction between free-carriers and the optical field. The bit error rate (BER) of the transmitted signal is calculated by using both a time- and frequency-domain Karhunen–Loeve expansion method. Our analysis suggests that due to enhanced nonlinear effects, for comparable optical power, a similar BER is achieved in a PhC waveguide about $100\times$ shorter than a strip one, with a particularly strong signal degradation being observed in the slow-light regime of the Si-PhCW.

Group index oscillations in photonic crystal waveguides

This work studies the phase contributions of a photonic crystal waveguide resonator. When studying the transmission spectrum of such resonators oscillations of group index are observed that can not be described by typical models conventionally used for analysis. We develop a simple mathematical tool capable of describing these oscillations. Furthermore, we carry out experiments on a Si photonic crystal waveguide resonator and apply that tool extracting phase contributions of the photonic crystal waveguide terminations.

Simulation of Optical Properties of Si Photonic Crystals

To investigate optical properties of Si photonic crystal waveguides, a mathematical model was set up. Finite difference time domain method was used to calculate the Maxwell’s equations numerically. For the evolution of the electromagnetic fields in the photonic crystals, simulations were done for a small lattices using Yee lattice approach. The properties of a waveguide and a power divider were investigated for 3λx3λ photonic crystal formed from Si circular rods in air for telecommunication wavelength 1.55 µm. The model developed was satisfactory in predicting the behaviour of light in linear photonic crystals

Simulation of Optical Properties of Si Photonic Crystals

To investigate optical properties of Si photonic crystal waveguides, a mathematical model was set up. Finite difference time domain method was used to calculate the Maxwell’s equations numerically. For the evolution of the electromagnetic fields in the photonic crystals, simulations were done for a small lattices using Yee lattice approach. The properties of a waveguide and a power divider were investigated for 3λx3λ photonic crystal formed from Si circular rods in air for telecommunication wavelength 1.55 µm. The model developed was satisfactory in predicting the behaviour of light in linear photonic crystals

Dynamic Wavelength Conversion in Copropagating Slow-Light Pulses

Dynamic wavelength conversion (DWC) is obtained by controlling copropagating slow-light signal and control pulse trajectories. Our method is based on the understanding that conventional resonator-based DWC can be generalized, and is linked to cross-phase modulation. Dispersion-engineered Si photonic crystal waveguides produce such slow-light pulses. Free carriers generated by two-photon absorption of the control pulse dynamically shift the signal wavelength. Matching the group velocities of the two pulses enhances the shift, elongating the interaction length. We demonstrate an extremely large wavelength shift in DWC (4.9nm blueshift) for the signal wavelength. Although DWC is similar to the Doppler effect, we highlight their essential differences.

Air-band optical resonators in one-dimensional Si photonic crystal waveguides for biosensing applications

Optical cavities using air-band states in Si photonic crystals (PhCs) are proposed as biosensors. Since the light in the air band of PhCs is preferably located at the lower-index cladding rather than the inside of higher-index Si, interactions are enhanced between the light and the target molecules in the cladding, being effective for the sensitive detections. Such an air-band PhC is used as an optical cavity, which is surrounded with another PhC as the photonic-bandgap mirrors. Simulations are performed for air-band resonators in one-dimensional (1D) PhC of arrayed holes in a Si channel waveguide, in order to analyze a figure of sensing, i.e., red shifts in the resonance wavelengths induced by the presence of target molecules. The S value, the amount of red shift caused by the unit change in the cladding index, can be as large as 200 nm/RIU, which is 2 times larger than those for ordinary 1D-PhC resonators. Fabricated devices successfully show the resonance peaks, whose wavelengths and quality factors agree well with theoretical ones. These results suggest that the air-band resonators are promising for high-performance biosensors.

High-speed delay tuning of slow light in pin-diode-incorporated photonic crystal waveguide

We demonstrate the high-speed electrical delay tuning of slow light pulses using Si photonic crystal waveguides. The device has an i-region-chirped pin diode, within which thermo-optic and carrier plasma effects are generated by forward bias. The former changes the delay up to 62 ps for the DC bias. The latter changes the delay for 1 Gbps pseudo random bit sequence tuning signals, which will be applicable to advanced time-domain optical signal processing.

Two-photon-absorption photodiodes in Si photonic-crystal slow-light waveguides

We demonstrate two-photon-absorption photodiodes in Si photonic-crystal waveguides, which shows wideband low-dispersion slow light. The device was fabricated on SOI substrate by CMOS-compatible process. The responsivity was improved by higher group indexes of slow light up to 0.052 A/W for pulses at wavelengths around 1550 nm with a 2.7 ps width and sub-watt peak powers. We applied this device to an optical correlator and dispersion detector. In the former, the correlation waveforms of 0.7−10 ps pulses were observed with small errors. In the latter, photocurrents inversely proportional to the pulse width were detected.