Low-power-consumption modulation of short-cavity DBR laser on SiO2/Si substrate.

We developed short-active-length distributed Bragg reflector (DBR) lasers to reduce the power consumption of chip-to-chip optical interconnects. These lasers have buried bulk InGaAsP waveguides to increase the coupling efficiency between the active region and DBR to 99.79% from the 98.14% of our previous DBR lasers that had InP channel waveguides. We achieved continuous wave operation of 5- to 80-µm active-length DBR lasers and the 5-µm-long laser consumed 24 fJ/bit with a 10-Gbps NRZ signal. The threshold current of the 5-µm laser was 51 µA, which compares favorably to our previous 10-µm DBR lasers with a threshold current of 170 µA.

Cross-gain modulation-based photonic reservoir computing using low-power-consumption membrane SOA on Si.

We demonstrate photonic reservoir computing (RC) utilizing cross-gain modulation (XGM) in a membrane semiconductor optical amplifier (SOA) on a Si platform. The membrane SOA's features of small active volume and strong optical confinement enable low-power nonlinear operation of the reservoir, with 101-mW-scale power consumption and 102-µW-scale optical input power. The power consumption is about an order of magnitude lower than that of conventional SOAs that exhibit saturable nonlinearity. The XGM-based reservoir is configured by injecting a delayed feedback signal into the SOA from a direction opposite to the input signal. This configuration provides robust operation of the feedback circuit because of the phase insensitivity and the elimination of loop oscillation risk. The RC performance is evaluated via the information processing capacity (IPC) and a nonlinear benchmark task. It is revealed that the XGM-based reservoir performs strong nonlinear transformation of input time-series signals. The series of results consistently show that the membrane SOA performs RC-applicable nonlinear operations through XGM at a low power scale.

Heterogeneously integrated widely tunable laser using lattice filter and ring resonator on Si photonics platform.

We fabricated a tunable laser consisting of a Si lattice filter, a Si ring resonator, and a III-V gain region. The lattice filter, a cascade of interferometers with the same delay length, has periodic transmission peaks with a wide free spectral range (FSR). By connecting the lattice filter to a ring resonator with a narrow FSR, the lasing mode is selected from one of the resonance modes of the ring resonator. The lasing wavelength can be tuned by changing the transmission peak wavelength of the lattice filter, in which an integrated micro heater controls the refractive index of the longer or shorter arm. Since the length of the refractive index control region on both arms of the lattice filter can be extended while maintaining a wide FSR, a wide tuning range can be obtained. This laser facilitates the control of the lasing wavelength because of the simple configuration. The Si lattice filter and the Si ring resonator were fabricated on a Si photonics platform by a Si photonics foundry, and III-V gain region was heterogeneously integrated. The lasing wavelength is shifted to a longer (shorter) one by heating the longer (shorter) arm of the lattice filter, in which the tuning wavelength is 1529 to 1561 nm and side-mode suppression ratio is more than 40 dB. A Lorentzian linewidth for lasing wavelengths narrower than 40 kHz is also demonstrated.

Integration of a high-efficiency Mach-Zehnder modulator with a DFB laser using membrane InP-based devices on a Si photonics platform.

We demonstrate a wafer-level integration of a distributed feedback laser diode (DFB LD) and high-efficiency Mach-Zehnder modulator (MZM) using InGaAsP phase shifters on Si waveguide circuits. The key to integrating materials with different bandgaps is to combine direct wafer bonding of a multiple quantum well layer for the DFB LD and regrowth of a bulk layer for the phase shifter. Buried regrowth of an InP layer is also employed to define the waveguide cores for the LD and phase shifters on a Si substrate. Both the LD and phase shifters have 230-nm-thick lateral diodes, whose thickness is less than the critical thickness of the III-V compound semiconductor layers on the Si substrate. The fabricated device has a 500-µm-long DFB LD and 500-µm-long carrier-depletion InGaAsP-bulk phase shifters, which provide a total footprint of only 1.9 × 0.31 mm2. Thanks to the low losses of the silica-based fiber couplers, InP/Si narrow tapers, and the phase shifters, the fiber-coupled output power of 3.2 mW is achieved with the LD current of 80 mA. The MZM has a VπL of around 0.4 Vcm, which overcomes the VπL limit of typical carrier-depletion Si MZMs. Thanks to the high modulation efficiency, the device shows an extinction ratio of 5 dB for 50-Gbit/s NRZ signal with a low peak-to-peak voltage of 2.5 V, despite the short phase shifters and single-arm driving.

Optical links on silicon photonic chips using ultralow-power consumption photonic-crystal lasers.

Ultrashort-distance optical interconnects are becoming increasingly important due to continuous improvements in servers and high-performance computers. As light sources in such interconnects, directly modulated semiconductor lasers with an ultrasmall active region are promising. In addition, using Si waveguides is important to provide low loss optical links with functions such as wavelength filtering and switching. In this paper, we demonstrate a wafer-scale heterogeneous integration of lambda-scale embedded active-region photonic-crystal (LEAP) lasers and Si waveguides, achieved through precise alignment. We numerically and experimentally demonstrated the coupling design between the LEAP lasers and Si waveguides; it is important to match propagation constants of Si waveguides and wavenumber of the optical cavity modes. The LEAP lasers exhibit an ultralow threshold current of 13.2-µA and 10-Gbit/s direct modulation. We also achieved the first data transmission using an optical link consisting of a LEAP laser, Si waveguide, and photodetector and obtained an averaged eye diagram at a bit rate of 10 Gbit/s with a bias current of 150 µA.

Membrane buried-heterostructure DFB laser with an optically coupled III-V/Si waveguide.

We have developed a membrane buried-heterostructure (BH) distributed feedback (DFB) laser consisting of an optically coupled III-V/Si waveguide and SiN surface grating. A 230-nm-thick membrane III-V layer enables us to construct an optical supermode in a 220-nm-thick Si waveguide and control the optical confinement factor in both the III-V and Si layers by changing Si waveguide width. This makes it possible to use a conventional Si photonics platform because the Si waveguides widely used on it are around 220-nm thick. To fabricate the BH-the key component for constructing a membrane laser with a lateral current-injection structure-we used direct wafer bonding and regrowth by metalorganic vapor phase epitaxy. Light output from the DFB laser is transferred to the Si waveguide through a short inverse-taper InP waveguide. A fiber-chip interface constructed by using inverse-taper Si waveguides and SiOx waveguides provides 2-dB fiber coupling loss. Fiber coupling power of 7.9 mW is obtained with a λ/4-shifted DFB laser with a 500-µm-long cavity. Single-mode lasing with a side-mode suppression ratio of 50 dB and lasing up to 120°C are also demonstrated.

Membrane InGaAsP Mach-Zehnder modulator with SiN:D waveguides on Si platform.

A high-efficiency InGaAsP Mach-Zehnder modulator is integrated with hydrogen-free deuterated silicon nitride (SiN:D) waveguide circuits on a Si substrate. A thin InP-based layer provides a high optical confinement factor of around 50% and enables easy optical coupling to the SiN:D waveguides, which are fabricated by a low-temperature backend process. The fabricated Mach-Zehnder modulator with a 300-µm-long phase shifter shows a VπL of 0.4 Vcm, insertion loss of ~4.5 dB, and an error-free operation for 40-Gbit/s non-return-to-zero signal.

Amplifierless PAM-4/PAM-8 transmissions in O-band using a directly modulated laser for optical data-center interconnects.

Toward the realization of low-cost, long-, and extended-reach 400GbE data-center applications, the performance of pulse amplitude modulated (PAM) signals is studied using a state-of-the-art, high-performance 1.3-µm distributed feedback directly modulated laser, without any optical amplification or complex digital processing. Amplifierless PAM-4 transmissions of up to 64-Gb/s are achieved over 40 km of standard single-mode fiber (SSMF) for standard KP4-FEC, while 84-Gb/s PAM-8 signals are evaluated over 10 km SSMF.

Twin-mirror membrane distributed-reflector lasers using 20-µm-long active region on Si substrates.

We demonstrate 20-µm-long twin-mirror membrane distributed-reflector (DR) lasers for chip-to-chip optical interconnects. The lasers employ distributed Bragg reflectors (DBRs) at both ends of a 20-µm-long λ/4-phase shifted distributed feedback (DFB) section. We achieve single-mode lasing in a λ/4-phase shifted DFB mode at room temperature with a threshold current of 0.39 mA. The lasing wavelength remains stable while the injected current is varied, and it is determined by the λ/4 phase-shifted DFB. The modulation current efficiency is 11.4 GHz/mA1/2, which is measured by using relative intensity noise spectra. We also demonstrate the direct modulation of the DR lasers at a bit rate of 25.8 Gbit/s with an energy cost of 163 fJ/bit.

Room temperature continuous-wave nanolaser diode utilized by ultrahigh-Q few-cell photonic crystal nanocavities.

Few-cell point-defect photonic crystal (PhC) nanocavities (such as LX and H1 type cavities), have several unique characteristics including an ultra-small mode volume (Vm), a small device footprint advantageous for dense integration, and a large mode spacing advantageous for high spontaneous-emission coupling coefficient (ß), which are promising for energy-efficient densely-integratable on-chip laser light sources enhanced by the cavity QED effect. To achieve this goal, a high quality factor (Q) is essential, but conventional few-cell point-defect cavities do not have a sufficiently high Q. Here we adopt a series of modified designs of LX cavities with a buried heterostructure (BH) multi-quantum-well (MQW) active region that can achieve a high Q while maintaining their original advantages and fabricate current-injection laser devices. We have successfully observed continuous-wave (CW) lasing in InP-based L1, L2, L3 and L5 PhC nanocavities at 23°C with a DC current injection lower than 10 µA and a bias voltage lower than 0.9 V. The active volume is ultra-small while maintaining a sufficiently high confinement factor, which is as low as ~10-15 cm3 for a single-cell (L1) nanocavity. This is the first room-temperature current-injection CW lasing from any types of few-cell point-defect PhC nanocavities (LX or H1 types). Our report marks an important step towards realizing a nanolaser diode with a high cavity-QED effect, which is promising for use with on-chip densely integrated laser sources in photonic networks-on-chip combined with CMOS processors.

Membrane distributed-reflector laser integrated with SiOx-based spot-size converter on Si substrate.

We demonstrate monolithic integration of a 50-µm-long-cavity membrane distributed-reflector laser with a spot-size converter, consisting of a tapered InP wire waveguide and an SiOx waveguide, on SiO2/Si substrate. The device exhibits 9.4-GHz/mA0.5 modulation efficiency with a 2.2-dB fiber coupling loss. We demonstrate 25.8-Gbit/s direct modulation with a bias current of 2.5 mA, resulting in a low energy cost of 132 fJ/bit.

Systematic study of thresholdless oscillation in high-ß buried multiple-quantum-well photonic crystal nanocavity lasers.

Buried multiple-quantum-well (MQW) 2D photonic crystal cavities (PhC) achieve low non-radiative recombination and high carrier confinement thus making them highly efficient emitters. In this study, we have investigated the lasing characteristics of high-ß(spontaneous emission coupling factor) buried MQW photonic crystal nanocavity lasers to clarify the theoretically-predicted thresholdless operation in high-ß nanolasers. The strong light and carrier confinement and low non-radiative recombination in our nanolasers have enabled us to clearly demonstrate very smooth lasing transition in terms of the light-in vs light-out curve and cavity linewidth. To clarify the thresholdless lasing behavior, we carried out a lifetime measurement and a photon correlation measurement, which also confirmed the predicted behavior. In addition, we systematically investigated the dependence of ß on the detuning frequency, which was in good agreement with a numerical simulation based on the finite-difference time-domain method. This is the first convincing systematic study of nanolasers based on an MQW close to the thresholdless regime.

All-optical switching for 10-Gb/s packet data by using an ultralow-power optical bistability of photonic-crystal nanocavities.

An all-optical packet switching using bistable photonic crystal nanocavity memories was demonstrated for the first time. Nanocavity-waveguide coupling systems were configured for 1 × 1, 1 × 2, and 1 × 3 switches for 10-Gb/s optical packet, and they were all operated with an optical bias power of only a few µW. The power is several magnitudes lower than that of previously reported all-optical packet switches incorporating all-optical memories. A theoretical investigation indicated the optimum design for reducing the power consumption even further, and for realizing a higher data-rate capability and higher extinction. A small footprint and integrability are also features of our switches, which make them attractive for constructing an all-optical packet switching subsystem with a view to realizing optical routing on a chip.

Heterogeneously integrated photonic-crystal lasers on silicon for on/off chip optical interconnects.

We demonstrate the continuous-wave operation of lambda-scale embedded active-region photonic-crystal (LEAP) lasers at room temperature, which we fabricated on a Si wafer. The on-Si LEAP lasers exhibit a threshold current of 31 µA, which is the lowest reported value for any type of semiconductor laser on Si. This reveals the great potential of LEAP lasers as light sources for on- or off-chip optical interconnects with ultra-low power consumption in future information communication technology devices including CMOS processors.

Directly modulated buried heterostructure DFB laser on SiO2/Si substrate fabricated by regrowth of InP using bonded active layer.

We describe the growth of InP layer using an ultrathin III-V active layer that is directly bonded to SiO2/Si substrate to fabricate a buried heterostructure (BH) laser. Using a 250-nm-thick bonded active layer, we succeeded in fabricating a BH distributed feedback (DFB) laser on SiO2/Si substrate. The use of a lateral current injection structure is important for forming a p-i-n junction using bonded thin film. The fabricated DFB laser is directly modulated by a 25.8-Gbit/s NRZ signal at 50°C. These results indicate that our fabrication method is a promising way to fabricate high-efficiency lasers at a low cost.

Two-stage surgery for hidradenitis suppurativa: staged artificial dermis and skin grafting.

BACKGROUND: Hidradenitis suppurativa is a chronic and often refractory skin disease that can require radical excision of the full layer of fatty tissue under the lesion. Closure using a split-thickness skin graft often results in depression deformity and lack of tissue flexibility. We have developed a two-stage procedure to preserve fatty tissue during radical excision and apply an artificial dermis graft, and we have performed this procedure in 18 patients (33 lesions). OBJECTIVE: To describe our two-stage procedure and report results of the procedure in our patient series. METHODS: In the first step, all diseased skin including the superficial subcutaneous fatty tissue is excised; normal deep subcutaneous fatty tissue is preserved. Artificial dermis is then grafted to the preserved fatty tissue. Two weeks later, split-thickness skin grafts are applied to the skin defects. We evaluated graft success, any recurrence, and postoperative appearance in our patients, who were followed up for 8 to 36 months. RESULTS: All 32 skin grafts were successful. There was only one recurrence, which was treated using reoperation, and postoperative appearances were good. CONCLUSIONS: Our new procedure incorporating artificial dermis appears to be a good treatment option for advanced hidradenitis suppurativa.

Effect of applying Lactiplantibacillus plantarum subsp. plantarum N793 to the scalps of men and women with thinning hair: a randomized, double-blind, placebo-controlled, parallel-group study.

Lactiplantibacillus plantarum subsp. plantarum N793 (N793) is a lactic acid bacterium (LAB) isolated from corn. We previously showed that N793 increases the level of keratinocyte growth factor, which is required for hair growth, in the culture supernatant of human follicle dermal papilla cells. Additionally, an open-label, single-arm study reported that applying a lotion containing N793 to the scalp for 24 weeks improved hair density in men and women with thinning hair. The present study was a double-blind, placebo-controlled, parallel-group study aimed at verifying the efficacy of N793 for thinning hair. A lotion containing N793, and a control lotion (placebo) were applied once daily for 24 weeks to 104 healthy Japanese men and women. Analysis of all participants revealed no difference in hair density between the N793 and placebo groups. However, an additional analysis limited to participants with relatively mild progression of thinning hair showed a significantly better hair density in the N793 group than in the placebo group. These findings suggest that topical application of N793 improves thinning hair in men and women when the condition's progression is relatively mild.

InGaAs nano-photodetectors based on photonic crystal waveguide including ultracompact buried heterostructure.

Ultrasmall InGaAs photodetectors based on a photonic crystal waveguide with a buried heterostructure (BH) were demonstrated for the first time. A sufficiently high DC responsivity of ~1 A/W was achieved for the 3.4-µm-long detector. The dynamic response revealed a 3-dB bandwidth of 6 GHz and a 10-Gb/s eye pattern. These results were thanks to the strong confinement of both photons and carriers in a small BH and will pave the way for unprecedented nano-photodetectors with a high quantum efficiency and small capacitance. Our device potentially has an ultrasmall junction capacitance of much less than 1 fF and may enable us to eliminate electrical amplifiers for future optical receivers and subsequent ultralow-power optical links on a chip.

Ultralow-energy and high-contrast all-optical switch involving Fano resonance based on coupled photonic crystal nanocavities.

We experimentally and theoretically clarified that a Fano resonant system based on a coupled optical cavity has better performance when used as an all-optical switch than a single cavity in terms of switching energy, contrast, and operation bandwidth. We successfully fabricated a Fano system consisting of doubly coupled photonic-crystal (PhC) nanocavities, and demonstrated all-optical switching for the first time. A steep asymmetric transmission spectrum was clearly observed, thereby enabling a low-energy and high-contrast switching operation. We achieved the switching with a pump energy of a few fJ, a contrast of more than 10 dB, and an 18 ps switching time window. These levels of performance are actually better than those for Lorentzian resonance in a single cavity. We also theoretically investigated the achievable performance in a well-designed Fano system, which suggested a high contrast for the switching of more than 20 dB in a fJ energy regime.

A flat-output widely tunable laser based on parallel-ring resonator integrated with electroabsorption modulator.

We present a novel parallel-ring-resonator tunable laser monolithically integrated with an InGaAlAs electroabsorption modulator. The fabricated tunable laser exhibits stable wavelength tuning with a step of 200 GHz over a wide tuning range of 35 nm, achieved with a single electrode control. The variation in the laser output power with wavelength tuning is less than 1 dB even at an injected tuning current of 20 mA. Clear eye openings at 25 Gbit/s with a dynamic extinction ratio of more than 10 dB are demonstrated over a wavelength range of 25.7 nm with a constant voltage swing of 2 V at 45 °C. Error-free operation is confirmed under the same operating conditions.