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We present a wavelength tunable vertical-cavity surface-emitting laser (VCSEL) constructed by die-bonding a half-cavity InGaAs laser diode (LD) chip onto a silicon-on-insulator chip with a microelectromechanical system electrostatic diaphragm mirror that functions as a Fabry–Perot interferometer. As a result of the short cavity length, the integrated tunable LD has single-mode lasing characteristics with an extremely large coherence length of 150 m or more. The developed wavelength tunable LD is used to perform swept-source optical coherence tomography with a large scan depth, which is applicable to ophthalmic observation for the diagnosis of pathologic nearsightedness based on the measurement of the axial length of an eye.

Heterogeneous material integration technology gives us freedom of material choices in both electronic and photonic devices. In this presentation, status, technology and characteristics of photonic devices in photonic integrated circuits (PICs) on Si (SOI) will be reviewed. Membrane (thin III-V film) PICs can realize low power consumption data transmission on Si substrate. This PICs can be applicable to on-chip interconnection to reduce power dissipation under higher speed transmission. 93 fJ/bit transmission with 20 Gbps has been demonstrated. Hybrid PICs were also demonstrated to realize 10-Tbps-class transceiver with low energy cost for distributed computing. This structure can integrate multiple function and many array devices in one chip. Also, by dense integration, some function of electronics can be moved to photonics part. This enables power consumption reduction.

Infrared refractive index is an indispensable parameter for various fields including infrared photonics. To date, critical-angle refractometers, V-block refractometers, and spectroscopic ellipsometry have been commonly used to measure the refractive index. Although every method has an accuracy of four decimal places for the refractive index, a measurable wavelength region is limited up to about 2 µm. In this study, we demonstrated a metamaterial infrared refractometer for determining broadband complex refractive index. Using the device, a broadband (40–120 THz; wavelength 2.5–7.5 µm) and high-precision(< 5 ×10−3) complex refractive index of polymethyl methacrylate was measured for the first time.

Novel magneto-optical isolator and self-holding optical switch with an a-Si:H microring resonator are demonstrated. The devices are driven by the remanence of integrated thin-film magnet and, therefore, maintain their state without any power supply.

Recently a novel photonic integration technology, based on a thin InP-based membrane, is emerging. This technology offers monolithic integration of active and passive functions in a sub-micron thick membrane. The enhanced optical confinement in the membrane results in ultracompact active and passive devices. The membrane also enables approaches to converge with electronics. It has shown high potential in breaking the speed, energy and density bottlenecks in conventional photonic integration technologies. This paper explains the concept of the InP membrane, discusses the versatility of various technology approaches and reviews the recent advancement in this field.

We propose an Si-based optical vortex beam splitter consisting of topological edge state waveguides with C6 symmetry. The device can continuously change the branching ratio of the propagate light in the range from 16% to 84%.

We propose and demonstrate a near-infrared invisibility cloaking by using a double-layered metamaterial film. 1020-nm-band light is guided so as not to hit a tungsten wire around which the film is wound.

We demonstrate a novel self-holding function of a magneto-optical waveguide switch. The switching state is flipped by a pulsed current and maintained without any power supply by virtue of the nonvolatility of the thin-film magnet. Extinction ratios up to 15.4 dB were demonstrated. The switch state was controlled by a 1- $\mu \text{s}$ pulsed electrical current.

The energy consumption of light sources must be reduced to realize on-chip optical interconnects. This study proposes a buried-ridge-waveguide (BRW) structure to enhance the lateral optical confinement. An increase in the lateral optical confinement induces the reduction in the threshold current, and it also provides a differential resistance due to a reduction in the distance between the electrode and active region. The stripe width and ridge height are designed considering the internal loss and differential resistance. A comparison between the BRW and conventional flat structures demonstrates a 20% and 35% reduction in the threshold current and differential resistance, respectively.

To advance on-chip optical interconnections, membrane distributed-reflector (DR) lasers with low threshold current and high-efficiency operation at one side output were realized. First, a membrane distributed Bragg reflector (DBR) laser with 80- μ m-long active section and 50- μ m-long DBR section was fabricated to clarify the DBR reflectivity. An external differential quantum efficiency of 35% for the output from the front facet was obtained, and the DBR reflectivity was estimated to be 75%. Next, a membrane DR laser with 61- μ m-long distributed feedback section and 50- μ m-long DBR section was fabricated. A threshold current of 0.48 mA, external differential quantum efficiency from the front side waveguide of 26%, and light output ratio from the front to the rear sides of 13 were obtained. The lasing spectrum showed a single-mode operation with a side-mode suppression-ratio (SMSR) of 40 dB. Finally, small-signal direct modulation was carried out and a modulation current efficiency factor of 7.9 GHz/mA1/2 and 7 GHz/mA1/2 were, respectively, obtained for the 30- μ m-long and 61- μ m-long devices.

On-chip optical interconnection is a promising technology for wiring future large-scale integrated circuits, as a means to mitigate the considerable power dissipation of traditional wiring layers. Here, we fabricate an integrated optical link using a membrane distributed-feedback (DFB) laser and a p-i-n photodiode (PD) in a butt-jointed built-in coupling geometry. The optical link is formed on a Si substrate by benzocyclobutene bonding. The integrated DFB laser shows a low-threshold current of 0.48 mA. Light transmission between the DFB laser and the p-i-n PD is confirmed with static measurements of the optical link. The optical link has a 3-dB bandwidth of 11.3 GHz at a 2.73 mA DFB laser bias current and a –3 V p-i-n PD bias voltage. A data transmission experiment of the optical link is performed, using a nonreturn to zero, pseudorandom-bit-sequence with a word length of 231-1 signals. With a DFB laser bias current of 2.5 mA, 10 Gbit/s data transmission with a bit-error-rate of 6 × 10–7 is successfully achieved.

Record high power conversion efficiency operation of a GaInAsP/InP membrane distributed-reflector laser on silicon substrate was realized. In order to enhance the efficiency, both an improvement of external differential quantum efficiency and a reduction of differential resistance were achieved. As a result, an external differential quantum efficiency from the front facet of 36%, and a maximum power conversion efficiency of 14.6% were obtained for a device with a 40- $\mu \text{m}$ -long active region.

We demonstrate a silicon Mach-Zehnder modulator (MZM) based on hydrogenated amorphous silicon (a-Si:H) strip-loaded waveguides on a silicon on insulator (SOI) platform, which can be fabricated by using a complementary metal-oxide semiconductor (CMOS) compatible process without half etching of the SOI layer. Constructing a vertical p-n junction in a flat etchless SOI layer provides superior controllability and uniformity of carrier profiles. Moreover, the waveguide structure based on a thin a-Si:H strip line can be fabricated easily and precisely. Thanks to a large overlap between the depletion region and optical field in the SOI layer with a vertical p-n junction, the MZM provides 0.80- to 1.86-Vcm modulation efficiency and a 12.1- to 16.9-dBV loss-efficiency product, besides guaranteeing a 3-dB bandwidth of about 17 GHz and 28-Gbps high-speed operation. The αVπL is considerably lower than that of conventional high-speed modulators.

We fabricated a magneto-optical (MO) isolator with a TE mode nonreciprocal phase shift. The isolator is based on a Mach-Zehnder interferometer composed of 3-dB directional couplers, a reciprocal phase shifter, and a nonreciprocal phase shifter. To realize TE mode operation in the optical isolator, we designed a novel waveguide structure composed of a hydrogenated amorphous silicon waveguide with an asymmetric MO garnet lateral clad on a garnet substrate. The isolator operation is successfully demonstrated in a fabricated device showing the different transmittances between forward and backward directions. The maximum isolation of the fabricated isolator is 17.9 dB at a wavelength of 1561 nm for the TE mode.

We proposed a wavelength-grid free WDM system, which allows wavelength drift so that use of uncooled laser and high-density WDM is enabled. A multi-wavelength QD-LD for the system was also demonstrated.

A broadband Si waveguide loop mirror utilizing a curved directional coupler (CDC) was realized for application to an external cavity type III-V/SOI hybrid laser. Reflectance with broad wavelength characteristics was obtained while controlling reflectance value by varying the CDC coupling angle.

We experimentally demonstrate a microring-type magneto-optical (MO) switch with a-Si.H waveguides on a garnet substrate. By controlling the direction of the external current induced magnetic field, an extinction ratio of 9 dB was achieved.

We demonstrated a novel slow-light Si-wire waveguide combined with metamaterials, which can be easily integrated with other Si photonics devices. The slow-light effect can be produced simply by placing metamaterials at an appropriate position on a Si-wire waveguide. It was confirmed that the large group index of more than 40 could be obtained because of a steep and discontinuous change of dispersion relation near the resonance frequency of metamaterials.

Toward the realization of photonic integrated circuits (PICs) on silicon platform, wafer bonding is desirable approach comparing with direct epitaxial growth and flip chip bonding in recent years. However, conventional wafer bonding technologies, such as hydrophilic bonding and plasma activated bonding (PAB), introduce thermal stress caused by difference of thermal expansion coefficient between bonded wafers and long cooling time during the wafer bonding process. In order to deal with the previous problems, surface activated direct wafer bonding (SAB) technology based on fast atom beam (FAB) is considered one of attractive technologies. In this paper, the influence of various FAB sources irradiation on GaInAsP/InP wafers was investigated by comparing their photoluminescence (PL) properties for figuring out the suitable FAB source under the same FAB irradiation conditions, including the irradiation time and irradiation current. The results indicated Xe-FAB has the lowest damage to PL intensity of sample wafer. Also, InP/Si bonding experiment was conducted by Xe-FAB. Results of mentioned investigations proved Xe-FAB is the choice for achieving enough bonding strength of hybrid laser process as well as maintaining low damage to PL intensity.

The bonding strength dependence on stress by the bowing of chips was investigated for the InP/Si chip-on-wafer plasma activated bonding. The high bonding strength of 20 MPa was achieved with tensile stress occurred by the strain-controlled interlayer between InP chips and Si substrates.

Investigation of thin film characteristics of InP/Si hybrid wafers by novel chip-on-wafer direct transfer bonding technology (CoW-DTB) was carried out. 36 InP chips were successfully bonded on Si wafers by CoW-DTB and photoluminescence properties were measured and compared by $1.3-\boldsymbol{\mu} \mathbf{m}$ AlGaInAs quantum wells samples with different bonding interface after removing InP substrate.

In III-V/SOI hybrid photonic devices, the mode size between III-V/SOI hybrid section and Si waveguide are very different. To obtain high coupling efficiency between them, we investigated taper length dependence of a double taper-type coupler which enables efficient mode conversion even with a short taper length.

An optical-vortex multiplexer/demultiplexer using vertically curved Si waveguides was developed. Multiplexing/demultiplexing with the lowest crosstalk of 23 dB was demonstrated for five multiple optical vortices.

Low-power consumption directly-modulated lasers are a key device for on-chip optical interconnection. We fabricated a GaInAsP/InP membrane DFB laser that exhibited a low-threshold current of 0.21 mA and single-mode operation with a sub-mode suppression ratio of 47 dB at a bias current of 2 mA. A high modulation efficiency of 11 GHz/mAsup1/2/supwas obtained. A 10 Gbit/s direct modulation using a non-return-to-zero 2sup31/sup-1 pseudo-random bit sequence signal was performed with a bias current of 1 mA, which is the lowest bias current ever reported for direct modulation of a DFB laser. A bit-error rate of 10sup-9/supwas successfully achieved.

In this work, electron beam lithography proximity effect correction (PEC) was experimentally studied for patterning of Si photonic waveguides with a relatively thick resist mask. Beam’s energy density distribution (EDD) was experimentally extracted by the line exposure method; however, exposure lines in this work were developed after cleavage with a high-contrast process to reduce developer-related effects. The measured line spread function was fitted to a 4-Gaussian function to model mid-range energy densities accurately. The extracted EDD showed less proximity effects compared to conventional Monte-Carlo simulation performed by a commercial software. PEC processes with both techniques were experimentally compared for a Si photonic waveguide pattern with different side-cladding trench widths. Microscopic images confirmed that the presented calibration method could achieve better development conditions near the required clearance dosage. Single-mode propagation loss for a 500 × 220 nm Si wire waveguide was reduced from 3.2 to 2.4 dB cm−1 using the presented process.

Replacing part of a conventional optical circuit with a topological photonic system allows for various controls of optical vortices in the optical circuit. As an underlying technology for this, in this study, we have realized a topological converter that provides high coupling efficiency between a normal silicon wire waveguide and a topological edge waveguide. After expanding the waveguide width while maintaining single-mode transmission from the Si wire waveguide, the waveguides are gradually narrowed from both sides by using a structure in which nanoholes with C6 symmetry are arranged in a honeycomb lattice. On the basis of the analysis using the three-dimensional finite-difference time-domain method, we actually fabricated a device in which a Si wire waveguide and a topological edge waveguide were connected via the proposed topological converter and evaluated its transmission characteristics. The resulting coupling efficiency between the Si wire waveguide and the topological edge waveguide through the converter was –4.49 dB/taper, and the coupling efficiency was improved by 5.12 dB/taper compared to the case where the Si wire waveguide and the topological edge waveguide were connected directly.

A metamaterial is an artificial material designed to control the electric permittivity and magnetic permeability freely beyond naturally existing values. A promising application is a slow-light device realized using a combination of optical waveguides and metamaterials. This paper proposes a method to dynamically control the slow-light effect in a metamaterial-loaded Si waveguide. In this method, the slow-light effect (i.e., group index) is controlled by changing the phase of the control light incident on the device from a direction opposite to that of the signal light. The group index of the device could be continuously controlled from 63.6 to 4.2 at a wavelength of 1.55 µm.

Heterogeneous wafer bonding of InP/Si at room temperature is studied using surface-activated bonding (SAB) technology. To minimize the degradation of optical property while maintaining enough bonding strength, various bonding conditions including gas species of fast atom beam (FAB) were examined. The results show that a bonding strength of over 0.5 MPa can be obtained with less degradation of photoluminescence (PL) property of InP/Si hybrid wafer by combining Xe and Ar gases for FAB. Using this condition, hybrid wafer including quantum wells were fabricated, which showed sufficient PL property for the fabrication of hybrid photonic devices. The bonding conditions described in this study enabled realization of continuous wave (CW) operation of InP-based layers/SOI hybrid laser.

A transistor laser (TL) is a device that operates at a high-speed with multiple functions such as output control with low wavelength shift and signal mixing. By adopting a high heat dissipation structure with a high-speed compatible wide electrode pad and thick Au plating in TLs, improvement of temperature performance in 1.3 μm wavelength npn AlGaInAs/InP TL was demonstrated. As a result, continuous-wave operation of a 1.3 μm TL up to 90 °C was achieved. The thermal resistance was estimated to be 25 K W−1, based on the spectrum behavior, which is at least four times lower than the previously observed value.

Transistor Laser (TL) has multiple modulation methods by a combination of terminal connection because it has three electrical terminals unlike the laser diodes. In this report, for the voltage modulation of the TL, the light output response under a large signal voltage modulation was simulated. By considering the transient change of the current in the active layer, we succeeded in reproducing the light output having the peak in the opposite direction to the steady state observed with the actual device so far. Based on the simulation, the optimum collector-base capacitance in the actual device and the minimum required end-facet reflectance for securing a high extinction ratio were obtained.

Membrane distributed reflector (DR) laser exhibited a low threshold current (0.21 mA) and high modulation current efficiency factor of 12 GHz/mA1/2. A 20 Gbit/s direct modulation was obtained with the bias current of 1 mA, which corresponded to the energy cost of 93 fJ/bit.

In order to improve lasing characteristics of 1.3-μm wavelength transistor lasers, device structure was changed by focusing on optical confinement and heat dissipation. As the results, the highest continuous-wave operation temperature up to 90 °C was obtained for the first time.

We demonstrate high-speed direct-modulation of InAs/InGaAs quantum dot lasers grown on on-axis (001) silicon. The devices exhibited a 12.5 Gbit/s NRZ operation and a 25 Gbit/s PAM-4 operation confirmed from eye diagrams and BER measurements.

An energy cost of ridge-waveguide type membrane Distributed-Reflector laser was investigated for 10 Gbit/s and 20 Gbit/s operations. Thanks to its strong optical confinement and reduced resistance, the energy cost could be reduced to approximately 1/2 of that for conventional flat-top-waveguide type membrane DR laser.

We investigated high-power pulse semiconductor lasers based on epitaxially stacked double active layer through tunnel junction composed of MOCVD grown highly carbon-doped GaInAs on InP substrate. The peak pulse optical power was 50 W at a current of 65 A.

To realize next-generation photonic integrated circuits based on the III–V/Si hybrid integration platform using chip-on-wafer (CoW) direct bonding technologies, high-yield collective bonding of InP chips on Si substrates with a high bonding strength is required. This study demonstrates high-yield InP/Si CoW plasma-activated bonding using a chip holder with pockets, the depth of which is precisely controlled. Additionally, finite element simulations are used to determine that the stress-controlled interlayer consisting of InP-based epitaxial layers with tensile strain effectively suppresses stress at the InP/Si bonding interface, which affects the bonding strength. Thus, a high bonding strength of 20 MPa in 2 mm × 2 mm InP chips on the Si substrate was achieved by introducing a superlattice structure consisting of GaInAsP and InP (with tensile strain) as the stress-controlled interlayer.

Direct modulation lasers are attractive for short-to-mid range fiber communication system. Transistor lasers (TLs), which are hetero-bipolar transistor with an active layer in the base, have a potential to overcome modulation bandwidth limitation of conventional laser diodes (LDs). One of the reasons of the modulation bandwidth limitation of the conventional LDs is delay of carrier diffusion to quantum wells (QWs). With proper design of TLs with some current gain, this delay can be reduced and TLs can have wider modulation bandwidth under the emitter current modulation configuration. In the first part of the presentation, these theoretical analyses will be discussed. By theoretical analysis of TLs with rate equations taking account of carrier diffusion lifetime, 50 GHz modulation bandwidth can be expected with a current gain of ~1.7. Also, by applying voltage bias between the base and collector under the common-base configuration, which causes Frantz-Keldysh effect at the interface layer (GaInAsP in our case), internal loss can be controlled if the bandgap of layers at the interface is appropriately wider than the energy corresponding to the lasing wavelength. This is a kind of “built-in” electro absorption modulator. Therefore, voltage modulation, which corresponds to “loss” modulation, can be configured and has wider modulation bandwidth. The reason why loss modulation has wider modulation bandwidth is that the slope of decay curve of small signal modulation over a relaxation oscillation frequency is slow compared with that of conventional current modulation. In the second part, buried-hetero (BH) regrowth techniques of AlGaInAs QWs will be discussed. AlGaInAs TLs with lasing wavelength of 1.3 µm is very important for fiber communication system. By adopting proper annealing conditions in an organo-metallic vapor-phase-epitaxy (OMVPE) reactor, non-radiative recombination at the AlGaInAs/InP regrowth interface can be significantly reduced even after exposing in the air. Annealing gas, temperature, and the time are important parameters to find optimum annealing conditions. We found using PH3, instead of AsH3, as the annealing gas is effective to remove oxidation at the interface under 650°C, 45-min. annealing. Using this annealing condition, 1.3-µm AlGaInAs BH-LD was realized with a low threshold current density of 140A/cm2/well. Finally, the fabrication process and characteristics of 1.3-µm AlGaInAs TLs will be discussed. Several regrowth steps by MOVPE were required including above mentioned BH regrowth. CW operation up to the temperature of 40°C was demonstrated with both 2 and 3-terminal configurations. For 2-terminal configuration (this means the collector was floating), the threshold current of 33 mA was achieved with the cavity length of 1000 µm and stripe width of 1.7 µm. For 3-terminal configuration, the device shows optical characteristics (I-L) and transistor (collector current) characteristics, simultaneously, under common-emitter and common-base configuration. For the common-emitter configuration, threshold “base” current was reduced by applying voltage between the emitter and collector, On the other side, for the common-base configuration, we observed threshold “emitter” current increase by applying voltage between the base and collector due to Frantz-Keldysh effect. From transistor characteristics, we found Early effect also had an important role for TL characteristics.

We propose and demonstrate a method of making an infrared (∼60 THz) invisibility cloaking device by simply rolling a metamaterial film around an object that we want to hide.

We report the first demonstration of direct modulation of InAs/GaAs quantum dot (QD) lasers grown on on-axis (001) Si substrate. A low threading dislocation density GaAs buffer layer enables us to grow a high quality 5-layered QD active region on on-axis Si substrate. The active layer has p-modulation doped GaAs barrier layers with a hole concentration of 5 × 1017 cm−3to suppress gain saturation. Small-signal measurement on a 3 × 580 μm2 Fabry-Perot laser showed a 3dB bandwidth of 6.5 GHz at a bias current of 116 mA. A 12.5 Gbit/s non-return-to-zero signal modulation was achieved by directly probing the chip. Open eyes with an extinction ration of 3.3dB was observed at room temperature. The bit-error-rate (BER) curve showed no error-floor up to BER of 1 × 10−13. 12 km single-mode fiber transmission experiments using the QD laser on Si showed a low power penalty of 1 dB at 5 Gbit/s. These results demonstrate the potential for QD lasers epitaxially grown on Si to be used as a low-cost light source for optical communication systems.

We propose a novel slow-light Si-wire waveguide using metamaterials, which can be easily integrated with other Si photonics devices, and the group index of more than 40 was obtained in both theoretical and experimental investigations.

A novel self-holding switch is demonstrated by a magneto-optical waveguide switch. The switching state is flipped by a pulsed current and maintained without any power supply by virtue of the non-volatility of thin-film magnet.

A GalnAs/InP membrane p-i-n photodiode (PD) integrated with a back end DBR was theoretically investigated under a condition of the back reflection of −30dB. As the result, it was found that the 3dB bandwidth of 17 GHz can be obtained with the absorption section length of 12 μm. These are approximately 3 times faster and 1/3 times smaller than those of a similar PD without the back end DBR.

We demonstrate a monolithic integration of lateral-current-injection (LCI)-type membrane-based distributed-feedback (DFB) lasers and p-i-n-photodiodes (PDs) using a butt-jointed built-in (BJB) structure bonded on a Si substrate using benzocyclobutene. The BJB structure to be integrated to the membrane optical devices was prepared by organometallic vapor-phase epitaxy. A threshold current of 280 μA was obtained under a room-temperature continuous-wave condition by adopting a strongly index-coupled DFB laser with a surface grating structure and a λ/4-shift region. A low dark current of 0.8 nA was obtained with the p-i-n-PD at a bias voltage of –1 V, and its photocurrent property coincided with the light output property of the membrane DFB laser.

The direct modulation characteristics of a membrane distributed feedback (DFB) laser on a silicon substrate were investigated. Enhancement of the optical confinement factor in the membrane structure facilitates the fabrication of a strongly index-coupled (κ(I) = 1500 cm(-1)) DFB laser with the cavity length of 80 µm and a threshold current of 270 µA. Small-signal modulation measurements yielded a -3dB bandwidth of 9.5 GHz at 1.03-mA bias current, with modulation efficiency of 9.9 GHz/mA(1/2), which is, to the best of our knowledge, the highest value among those reported for DFB lasers.