Digital-to-analog converters (DAC) are indispensable functional units in optical signal transmission and processing. The photonic DAC that converts electrical digital signals to an optical analog one will offer advantages in lowering system complexity, power, and cost. Especially with the required bandwidth increasing, it could mitigate the problems faced by its electrical counterparts in dealing with higher sampling rate. Achieving such a photonic DAC in silicon photonics is promising due to the integration capability of both electronics and photonics and large scale DAC-based photonic circuits can be further realized for on-chip optical signal processing. In this work, we demonstrate 2-bit D/A conversion for the simple proof of concept utilizing only one single silicon Mach-Zehnder modulator (MZM), which is much simpler than previously reported segmented MZM and microring resonator based DACs. One-single MZM capable of 2-bit DAC merits future higher bit resolution design and meanwhile guarantees wide spectral bandwidth. One arm of MZM is used for the MSB bit input, while the other for the LSB, both of them being accomplished by only one phase shifter. For each bit input, we utilize amplitude modulation, instead of phase modulation, by applying the carrier injection induced absorption in the phase shifters. For principle, by setting different bias points for two phase shifters, we can produce the condition at which the amplitude weighting ratio of LSB to MSB is 1/2 in order to obtain the linear amplitude DAC output. In other words, the output optical field has the analog linear amplitude levels (0,1,2,3) which corresponds to the power levels of (0,1,4,9) at the full extinction condition. For fabrication, this device was fabricated on a 220-nm SOI wafer with a 3-m buried-oxide layer at the AIST SCR 300-mm CMOS foundry. The 430-nm-wide fully etched channel waveguide was used for the components except for the pn phase shifter which adopted the shallow-etched rib waveguide structure with a slab thickness of about 110 nm and a width of about 600 nm. The doping density in the weak p/n regions was about 1.61018 cm-3. This MZM was arm-balanced with 2-mm-long phase shifters, adopting GSGSG configuration. Two 50- terminators were also integrated on-chip at the ends of two signal electrodes. For measurement, a two-channel pulse pattern generator produced bit sequences at various frequencies for both MSB and LSB which was applied to the signal electrodes through bias-tees and high-speed probes. The 1.55-m cw light at TE polarization was coupled into the chip via a tapered fiber and the optical output passed an EDFA and a bandpass filter and then was sent to a high-speed oscilloscope for examining DAC analog output. Using this device, we successfully achieved correct D/A conversions with the sampling rates up to 3 GS/s with <1 V peak-to-peak voltages. Note that this speed can be further enhanced to <10 GS/s by constructing the pn phase shifter into a MZM structure or replacing it with a SiGe electro-absorption modulator. In summary, this work verified the feasibility to realize high-sampling-rate 2-bit D/A conversion utilizing a single silicon MZM modulator.
We have developed a design method for a photonic crystal directional coupler switch (PCDCSw) with a short switching length and wide bandwidth. Usually, the relationship between the switching length and bandwidth has a trade-off. To overcome this trade-off relation, we invented the suitable dispersion curve of eigenmodes of the PCDCSw. To actually obtain this dispersion curve, we examine the electromagnetic field of an ordinary PCDC at each wavenumber, and modify the even-mode mainly by enlarging the radius of the airs-holes and by shifting the position of the air-holes closer to the waveguides in the photonic crystals. We confirm using numerical simulation that the switching length of the new designed structure is only 4% of that for the ordinary PCDC.
Various important scientific and engineering applications, such as control of spontaneous emission, zero-threshold lasing, sharp bending of light, and trapping of photons, are expected by using photonic bandgap (PBG) crystals with artificially introduced defect states and/ or light-emitters. Realizing the maximum potential of photonic crystals requires the following steps: (i) construct a three-dimensional (3D) crystal with a complete photonic bandgap in the optical wavelength region; (ii) introduce an arbitrary defect into the crystal at an arbitrary position; (iii) introduce an efficient light-emitter; and, (iv) use an electronically conductive crystal, as this is desirable for actual device application. Although various approaches to constructing 3D crystals have been proposed and investigated, none of these reports satisfies the above requirements simultaneously. To develop complete 3D crystals at infrared (5-10um) to near-infrared wavelengths (1-2um), we stacked III-V semiconductor gratings into a diamond structure by means of wafer bonding and a laser-beam-assisted very precise alignment technique. Since the crystal is constructed with III-V semiconductors, which are widely used for optoelectronic devices, requirement (iii) is satisfied. Moreover, as the wafer bonding enables us to construct an arbitrary structure and to form an electronically conductive interface, all the above requirements (i)-(iv) will be satisfied. In this paper, we review our approach for creating full 3D photonic bandgap crystals at near-infrared wavelengths.