Yosuke Hinakura

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.

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.

Experimental simulation of ranging action using Si photonic crystal modulator and optical antenna

Time of flight LiDARs are used for auto-driving of vehicles, while FMCW LiDARs potentially achieve a higher sensitivity. In this study, we fabricated and tested each component of a FMCW LiDAR based on Si photonics and experimentally simulated the ranging action. Here, we drove a Si photonic crystal slow light modulator with linearly frequency-chirped signal in the frequency band of 500–1000 MHz and a repetition frequency of 100 kHz, to generate FM-signal light from a narrow-linewidth laser source. Next, we branched the signal light into two paths. One was inserted into a fiber delay line of 20–320 m and its output was irradiated to a photonic crystal slow beam steering device acting as an optical antenna via the free-space transmission. When the irradiation angle was optimized so that the antenna gain took maximum for a set laser wavelength, light was efficiently coupled into the antenna. We mixed the light output from the antenna with reference light of the other path with no delay, and detected it by balanced photodiodes. We observed a beat signal whose frequency well agreed with the theoretical value predicted from the length of the delay line. Thus, we succeeded in the experimental simulation of the FMCW LiDAR. We also observed a spectral sequence around the beat spectrum, in which the inter-frequency spacing equals the repetition frequency and corresponds to a range resolution of 30 cm which will be improved by expanding the modulation bandwidth.

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.

Wideband slow light effects in 25 Gbps Si photonic crystal Mach-Zehnder modulators

We evaluated wideband 25 Gbps error-free operation of Si slow light modulators. The fluctuation in extinction ratio was 1 dB over the bandwidth. Larger group indices produce larger extinction ratios and lower bit error rates.

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).