Lattice-shifted Photonic Crystal Waveguides 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.

Wide Spectral Characteristics of Si Photonic Crystal Mach-Zehnder Modulator Fabricated by Complementary Metal-Oxide-Semiconductor Process

Optical modulators for optical interconnects require a small size, small voltage, high speed and wide working spectrum. For this purpose, we developed Si slow-light Mach-Zehnder modulators via a 180 nm complementary metal-oxide-semiconductor process. We employed 200 μm lattice-shifted photonic crystal waveguides with interleaved p-n junctions as phase shifters. The group index spectrum of slow light was almost flat at ng ≈ 20 but exhibited ±10% fluctuation over a wavelength bandwidth of 20 nm. The cutoff frequency measured in this bandwidth ranged from 15 to 20 GHz; thus, clear open eyes were observed in the 25 Gbps modulation. However, the fluctuation in ng was reflected in the extinction ratio and bit-error rate. For a stable error-free operation, a 1 dB margin is necessary in the extinction ratio. In addition, we constructed a device with varied values of ng and confirmed that the extinction ratio at this speed was enhanced by larger ng up to 60. However, this larger ng reduced the cutoff frequency because of increased phase mismatch between slow light and radio frequency signals. Therefore, ng available for 25 Gbps modulation is limited to up to 40 for the current device design.

High Performance Optical Modulator Based on Electro-Optic Polymer Filled Silicon Slot Photonic Crystal Waveguide

Silicon-organic hybrid integrated devices have emerging applications ranging from high-speed optical interconnects to photonic electromagnetic-field sensors. Silicon slot photonic crystal waveguides (PCWs) filled with electro-optic (EO) polymers combine the slow-light effect in PCWs with the high polarizability of EO polymers, which promises the realization of high-performance optical modulators. In this paper, a high-speed, power-efficient, low-dispersion, and compact optical modulator based on an EO polymer filled silicon slot PCW is presented. Lattice-shifted PCWs are utilized to engineer the photonic band diagram and thus enable an 8 nm-wide low-dispersion spectrum range, which is over an order of magnitude wider than that in modulators based on non-band-engineered PCWs and ring-resonators. A small voltage-length product of Vπ × L = 0.282 V × mm measured at 100 KHz is achieved by slow-light enhancement, corresponding to an unprecedented record-high effective in-device EO coefficient (r33) of 1230 pm/V among silicon-organic hybrid modulators. Excluding the slow-light effect, the actual in-device r33 is estimated to be 98 pm/V. By engineering the RC time constant via silicon doping and also utilizing a backside gate technique, the 3-dB modulation bandwidth of the device is measured to be 15 GHz. In addition, the RF power consumption of the modulator is estimated to be 24 mW at 10 GHz, and the estimated energy consumption for potential digital modulations is approximately 94.4 fJ/bit at 10 Gb/s.

Hitless tunable WDM transmitter using Si photonic crystal optical modulators.

We have developed compact Si Mach-Zehnder modulators that are assisted by wideband low-dispersion slow light in lattice-shifted photonic crystal waveguides. We have also developed Si triangular-shaped coupled-microring multiplexers that allow a box-like spectrum, a wide free spectral range, and an efficient thermal tuning. In this study, we integrated three sets of these devices in a small footprint of 2.0 × 0.7 mm(2) and achieved their 25 Gbps/ch operation as a wavelength division multiplexing transmitter. Moreover, we demonstrated hitless wavelength tuning using thermo-optic switches loaded in the bus waveguide.

Silica-Clad Silicon Photonic Crystal Waveguides for Wideband Dispersion–Free Slow Light

We comprehensively calculated the photonic bands of the waveguide modes in practical lattice-shifted photonic crystal waveguides, which are completely cladded by silica. We assumed various lattice shifts and found that the shift of the second rows and the mixed shift of the first and third rows along the waveguide generate low-dispersion slow light with group indices of 34-36, which is higher than those with a conventional shift of the third rows, maintaining a wide bandwidth over 10 nm at telecom wavelengths. We fabricated the waveguides using a CMOS-compatible process and confirmed correspondence with the calculation results. We also compared 25-Gb/s photonic crystal slow light Mach-Zehnder modulators and confirmed the improvement of the modulation efficiency by second-row shifts.

Wideband and Low Dispersion Slow Light in Lattice-Shifted Photonic Crystal Waveguides

We propose a new type of photonic crystal waveguide structure to achieve wideband slow light with large group index and low dispersion. The waveguide is based on triangular lattice photonic crystal imposed by simply a selective altering the locations of the holes adjacent to the line defect. Keeping the group index at 31, 44, 61, 71, 94, and 132, respectively, while restricting its variation within a ±10% range, we accordingly attain an available bandwidth of 19.1, 13.0, 9.0, 7.5, 5.5, and 3.6 nm around 1550 nm. The normalized delay-bandwidth product keeps around 0.35 for all cases. Low dispersion slow light propagation is confirmed by studying the relative temporal pulsewidth spreading with the 2-D finite-difference time-domain method.

Ultrafast Slow-Light Tuning Beyond the Carrier Lifetime Using Photonic Crystal Waveguides

We demonstrate ultrafast delay tuning of a slow-light pulse with a response time <10 ps. This is achieved using two types of slow light: dispersion-compensated slow light for the signal pulse, and low-dispersion slow light to enhance nonlinear effects of the control pulse. These two types of slow light are generated simultaneously in Si lattice-shifted photonic crystal waveguides, arising from flat and straight photonic bands, respectively. The control pulse blueshifts the signal pulse spectrum, through dynamic tuning caused by the plasma effect of two-photon-absorption-induced carriers. This changes the delay by up to 10 ps only when the two pulses overlap within the waveguide and enables ultrafast tuning that is not limited by the carrier lifetime. Using this, we succeeded in tuning the delay of one target pulse within a pulse train with 12 ps intervals.

Compact and fast photonic crystal silicon optical modulators

We demonstrate the first sub-100 μm silicon Mach-Zehnder modulators (MZMs) that operate at >10 Gb/s, by exploiting low-dispersion slow-light in lattice-shifted photonic crystal waveguides (LSPCWs). We use two LSPCW-MZM structures, one with LSPCWs in both arms of the MZM, and the other with an LSPCW in only one of the arms. Using the first structure we demonstrate 10 Gb/s operation with a operating bandwidth of 12.5 nm, in a device with a phase-shifter length of only 50 μm. Using the second structure, owing to a larger group index as well as lower spectral noise, we demonstrate 40 Gb/s operation with a phase-shifter length of only 90 μm, which is more than an order-of-magnitude shorter than most 40 Gb/s MZMs.