Naoya Yazawa

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.

Sub-100 μm Photonic Crystal Si Optical Modulators: Spectral, Athermal, and High-Speed Performance

We report on our recent progress on Si Mach-Zehnder modulators (MZMs) incorporating sub-100 μm slow-light photonic crystal waveguide (PCW) phase-shifters. In the standard MZM with a PCW in each of the arms, we study the power-dependent bit-error-rate (BER) characteristics at 10 Gb/s, and measure BER = 1 × 10^-9 and 1 × 10^-8 even with phase-shifter lengths of 90 and 50 μm, respectively. Furthermore, by exploiting the low-dispersion slow-light in the 90 μm device, we measure a spectral operating bandwidth of 16.9 nm and temperature tolerance between 19-124°C, where the eye pattern amplitude is consistent to within ±25%. In the device with a 90 μm PCW in only one of the MZM arms, designed for large-n_g operation, we achieve BER = 1 × 10^-9 at 10 Gb/s and also observed barely but open eye patterns at 25 and 40 Gb/s.

One-chip integration of optical correlator based on slow-light devices.

We propose and demonstrate an on-chip optical correlator, in which two types of photonic crystal slow-light waveguides are integrated and operated as an optical delay scanner and a two-photon-absorption photodetector. The footprint of the device, which was fabricated using a CMOS-compatible process, was 1.0 × 0.3 mm(2), which is substantially smaller than that of conventional optical correlators with free-space optics. We observed optical pulses using this device and confirmed the correspondence of pulse waveforms with those observed using a commercial correlator when the pulse width was 5-7 ps. This device will achieve one-chipping of an optical correlator and related measurement instruments.

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.

On-chip optical correlator

Using all technologies of photonic crystal, slow light and Si photonics, we first fabricated an on-chip optical correlator whose footprint is only 2.08 × 0.3 mm2. Picosecond pulse waveform was successfully measured with this device.

Si photonic crystal compact multilevel modulators

We fabricated Si QPSK and PAM modulators with photonic crystal slow light waveguides and interleaved p/n junction, both of which have footprints less than 1 mm2. 30-35 Gbps operation were observed in 300-450 μm devices.

OOK and QPSK operation with wide working spectrum in 200–300 μm Si photonic crystal slow light modulators

25 Gbps error-free operation and 16-nm working spectrum were obtained in 200-μm photonic crystal slow light MZ OOK modulators. QPSK modulation was also obtained in a 300-μm device (1.0 × 0.5 mm² footprint).

High-extinction Si photonic-crystal optical modulators at 10 Gb/s

We optimized the 10 Gb/s operation in Si photonic crystal optical modulators with 50-200 μm long phase shifters. By incorporating additional phase tuners, we obtained over 10 dB extinction ratio and error-free operation.

Athermal Sub-100 μm Si photonic crystal optical modulator

We demonstrate athermal operation at 10 Gb/s, across 19-124°C, in a 90 μm Si photonic crystal optical modulator. This is enabled by low-dispersion slow-light and the resulting 17.9 nm spectral operating bandwidth.

Electrically-controlled photonic crystal slow light device and its application to optical correlator

Lattice-shifted photonic crystal waveguides (LSPCWs) are effective for generating wide-band on-chip slow light at room temperature. In this study, we integrated the LSPCW with multi-heaters using CMOS-compatible process, and demonstrated electrically-tunable slow light. Seven heaters were placed on each side of the LSPCW with air slots for thermal isolation. On-demand temperature distributions were formed by controlling each heating power independently. When the heating powers were optimized, a clear delay peak of slow light corresponding to the flat photonic band of the LSPCW was observed, which suggests that fabrication errors in the air-hole diameters were less than 5 nm and related band fluctuation was well compensated by the heating. When a linear temperature distribution was added to this condition, the delay was reduced up to 54 ps. When a quadratic distribution was added, the group delay dispersion was generated in the range from -10.2 to 17.5 ps/nm. We applied the tunable delay to the delay scanning in optical correlator. Here, output pulse were compressed to 0.6 ps through self-phase modulation and dispersion compensation in external fibers, and its delay was tuned in range of 17 ps. At a scanning frequency of 100 Hz, which is <10 times faster than that of conventional mechanical delay scanners, pulse lengths of 0.3 - 6 ps were measured with a <95% accuracy. We are also fabricating all on-chip optical correlator consisting of this slow light delay scanner and two-photon-absorption photo-diodes.