Yosuke Terada

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

Abstract 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 ng. 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.

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.

Theoretical and experimental investigation of low-volgage and low-loss 25-Gbps Si photonic crystal slow light Mach–Zehnder modulators with interleaved p/n junction

In this study, we investigated the performance of Si lattice-shifted photonic crystal waveguide (LSPCW) Mach-Zehnder modulators theoretically and experimentally. The LSPCW increases the phase shift in modulator to 2.3 - 2.5 times higher, which allows for size reduction and high performance. On-chip passive loss was reduced to less than 5 dB by optimizing each component. We obtained 25 Gbps clear open eye and 3 dB extinction ratio at a drive voltage of 1.5 – 1.75 V for 200 μm phase shifter with linear p/n junction when we added a modulation loss of 7 dB. This modulation loss was reduced to 0.8 dB, maintaining other performance, by employing interleaved p/n junction and optimizing doping concentrations.

Slow-light effect in a silicon photonic crystal waveguide as a sub-bandgap photodiode

We demonstrate a Si sub-bandgap photodiode in a photonic crystal slow-light waveguide that operates at telecom wavelengths and can be fabricated using a Ge-free, standard Si-photonics CMOS process. In photodiodes based on absorption via mid-bandgap states, the slow-light enhancement enables performance that is well balanced among high responsivity, low dark current, high speed, wide working spectrum, and CMOS-process compatibility, all of which are otherwise difficult to achieve simultaneously. Owing to the slow-light effect and supplemental gain at a high reverse bias, the photodiode shows a responsivity of 0.15  A/W with a low dark current of 40 nA, which is attributed to no particular processes such as ion implantation and excess exposure of the Si surface. The maximum responsivity was 0.36  A/W. The modest gain allows for sufficient frequency bandwidth to observe an eye opening at up to 30  Gb/s.

Compact QPSK and PAM Modulators With Si Photonic Crystal Slow-Light Phase Shifters

We demonstrate quadrature phase-shift keying (QPSK) modulation and four-level pulse-amplitude modulation (4-PAM) in Si Mach-Zehnder modulators whose phase shifter length is reduced to 300 μm by the slow-light effect in photonic crystal lattice-shifted waveguides and increased modal overlap with interleaved p-n junctions. The QPSK constellation patterns were observed up to 15 Gbaud (30 Gb/s) with an error-vector magnitude of <;20% and 28 Gbaud (56 Gb/s) using an RF equalizer filter. In the 4-PAM, four separate eyes were observed up to 15 Gbaud (30 Gb/s). Improving the modulation efficiency and frequency bandwidth and suppressing the insertion loss may deliver practical modulator performance.

Low-voltage 25 Gbps modulators based on si photonic crystal slow light waveguides

25 Gbps operation was obtained with extinction ratios of 2 - 4 dB for Vpp = 1.00 - 1.75 V in MZI modulator consisting of 200-μm photonic crystal slow light waveguide phase shifters.

Si Photonic Crystal Slow-Light Modulators with Periodic p–n Junctions

We theoretically optimized and demonstrated the periodic p–n junction in silicon photonic crystal slow-light modulators to balance the efficiency and speed of phase shifters and reduce the power consumption compared with those of previous linear and interleaved p–n junctions. In particular, sawtooth and wavy junctions, whose profiles match with the distribution of the slow-light mode, theoretically prove effective in achieving these objectives. However, the sawtooth junction requires a high-resolution process. Therefore, we finally employed the wavy junction and obtained 25- and 32-Gb/s operations in a 200-μm device with extinction ratios of 4 and 3 dB, respectively, for an excess modulation loss of 1 dB.

WDM transmitter using Si photonic crystal optical modulators

We fabricated a WDM transmitter consisting of Si photonic crystal MZ modulators, triangular-shaped coupled-microring multiplexers and optical switches. 25 Gbps/ch operation and hitless switching of channel wavelength were successfully obtained.

Hitless tunable WDM transmitter using Si photonic crystal optical modulators

We fabricated a WDM transmitter consisting of Si photonic crystal MZ modulators, triangular-shaped coupled-microring multiplexers and optical switches. 25 Gbps/ch operation and hitless switching of channel wavelength were successfully obtained.

Thermally controlled Si photonic crystal waveguide slow light beam steering device

The double-periodic Si photonic crystal waveguide radiates guided slow light into free space as an optical beam. It also functions as a beam steering device, in which the steering angle is changed widely by the slight wavelength variant thanks to the large dispersion of slow light. A similar function is obtainable when the wavelength is fixed and the refractive index of the waveguide is changed. In this study, we integrated two kinds of heater structures in the waveguide and demonstrated the beam steering by the thermo-optic effect. For a p-i-p doped heater structure, we implanted a p-type dopant except around the waveguide core, and observed a beam steering angle Δθ = 26°, which is close to a theoretical value, with a relatively low heating power P = 1.6 W and high-speed response of 100 kHz order. However, the beam divergence increased up to δθ = 5°, which seemed to reflect the temperature nonuniformity in the Si slab. On the other hand, for the TiN heaters placed away from the waveguide core, we obtained a comparable steering angle with a narrower beam divergence of δθ < 0.3°. However, the required heating power was as large as P = 4.8 W, and the response speed was slow, reflecting its low heating efficiency and large heat capacity. We expect these problems to be solved by homogenizing the current and temperature distributions for the former and by optimizing the positioning of the heaters for the latter.