O-band surface-grating-coupled Si wavelength demultiplexing filter using cascaded directional couplers with 30-nm passband linewidth, 6-dB insertion loss and 23.4-dB polarization extinction is demonstrated for 64-Gbit/s NRZ-OOK with WDM channel extinction enhanced from 8.9-dB@1330nm to 14.6-dB@1325 nm.
We successfully demonstrated the 4-channel Ge waveguide photodetectors with an MZI WDM as an O-band CWDM receiver to receive the 212-Gbps NRZ-OOK and 480-Gbps 16-QAM-DMT data streams without digital signal processing compensation in a 2-km SMF link for satisfying the IEEE 802.3bs and 802.3cu standards.
We report an optical–electrical (OE) oscillator using two new key components (a high-power integrated photoreceiver module and a low-driving-voltage lithium niobite (LN) Mach–Zehnder modulator (MZM)) to configure key parts of the entire OE oscillator system. A 60-GHz narrowband photoreceiver module integrated with a 60-GHz photodetector chip with a gain emphasis circuit and a 60-GHz high-power amplifier chip was designed and fabricated. By increasing the input photocurrent to 5 mA at 60 GHz, we successfully achieved a 1-dB power compression level of +16 dBm with good output linearity. The LN-based bulk MZM was specially designed to decrease the driving voltage to 1.5 V by optimizing both the waveguide and electrode length at high frequencies. Excellent performance with a -5 dB insertion loss at 60 GHz was achieved. The OE oscillator system was configured via a feedback loop using optical and electrical components, which primarily consisted of the newly developed photoreceiver and MZM through a 100 m–1 km long optical fiber. We successfully demonstrated -106 dBc/Hz at a 10-kHz offset near 60 GHz. The design and fabrication of the OE system are discussed in detail herein.
KEYWORDS: Digital signal processing, Radio optics, Forward error correction, Transmitters, Receivers, Standards development, Pulse signals, Power consumption, Optical transmission, Modulation
Future short-reach optical fiber communication links for datacenter and optical access applications would require not only high data rate but also low power/complexity. In this work, by leveraging a home-designed LiNbO3 intensity modulator with high slope-efficiency, we experimentally investigate a low-complexity/power optical pulse amplitude modulation (PAM) link with 300Gb/s data rate excluding FEC overhead and the following detailed merits: (1) at the transmitter side, no digital signal processing or a high-resolution wideband DAC is required, while only CMOS-class driving voltages less than 1 volt is needed; (2) at the receiver side, no optical or electrical amplifier is needed. It is found that for C-band 600m transmission, a symbol-spaced decision-feedback equalizer (DFE) with 91 feedforward taps and 1 feedback taps is sufficient to achieve a BER lower than the threshold of a low-power practical FEC. With these features, we expect that the overall system has low power consumption. A bitrate-distance product of >180Gb/s*km in C-band is achieved experimentally.
We developed a THz bandwidth quantum memory using a photon echo technique in InAs quantum dot ensemble with a large inhomogeneous broadning. To improve photon echo generation efficiency, we applied Adiabatic Rapid Passage (ARP) technique to quantum manipulation of quantum memory. In addition, we developed pulse pumped up conversion single photon detector (UCSPD) which enables us to achieve the femtosecond scale temporal resolution and we successfully demonstrated femtosecond time bin photon echo signals.
Efficient functionalization of the silicon nitride waveguide with bioreceptors, e.g. antibodies, is key to increase antigen binding activity in photonic sensors based on silicon nitride (SiN) waveguides. A bioreceptor coating technique using silica nanoparticles (NPs) to enhance the density of functionalized antibodies, by increasing the surface areas for biomolecule binding, is proposed. The detection of S100 Calcium-Binding Protein A6 (S100A6), a proposed cholangiocarcinoma marker, has been demonstrated using the SiN resonator sensor with 400nm thick waveguide, fabricated by low-cost 500 nm technology. The NPs were synthesized by silica condensation. Antibodies were attached to the NPs by 1-ethyl-3-(3- dimethyl aminopropyl)–carbodiimide (EDC)/ N-hydroxysuccinimide (NHS)-crosslinking. Then the NPs were coated on SiN sensor by N-terminal to N-terminal crosslinkers. It was found that the application of silica NPs coating showed increased sensor sensitivity at approximately 8.8 pm/(ng/ml) in optical resonant wavelength shift compared to 0.36 pm/(ng/ml) by our previous antibody coating technique using (3-Aminopropyl) triethoxysilane (APTES) silanization with EDC/NHS protein crosslink.
We have developed a quantum memory using quantum dots with a photon echo method for the storage of ultraweak femtosecond pulses. This quantum memory has a large bandwidth of 7.2 THz, which can be achieved due to the large inhomogeneous broadening of quantum dots. We successfully demonstrated femtosecond timebin pulse transfer to photon-echo-based quantum memory using quantum dots. We also succeeded in measuring the retrieved time-bin pulses as a photon echo from the quantum memory using a pulse-pumped frequency upconversion single-photon detector (UCSPD) with a temporal resolution of 429 fs. It is found that the retrieved time-bin photon echo pulse maintains the sub-picosecond time duration and the relative phase.
We investigated a growth technique for ultra-low-density self-assembled InAs QDs using Bi surfactant-assisted interdiffusion epitaxy (IDE). The samples were grown using a solid-source molecular beam epitaxy system. InP(311)B substrates were used to grow InAs QDs. After growing the InP buffer layer, a 100 nm-thick InGaAlAs barrier layer and a 1 nm-thick InP were used for the IDE process, and self-assembled InAs QD were formed. The density of QDs was very low, approximately 3.2×107/cm2, which is three orders of magnitude smaller than that of the conventional QD. Moreover, sharp photoluminescence was observed from a single QD at 1522 nm.
We present a 100-GHz narrowband optical-to-radio converter driven by power over fiber (PoF), and discuss about the fundamental radio over fiber communication performance and the settling time performance in radio launching driven by PoF. The 100-GHz optical-to-radio converter consists of a zero-bias operational 100-GHz UTC-PD and a 100-GHz GaAs-based enhancement type pHEMT amplifier. The generated electrical power through a power converter in PoF was applied to the drain bias, and the generated small power through the UTC-PD was applied to the gate bias. While turning ON/OFF optical power rapidly in PoF, the settling performance in 100-GHz radio launching was investigated.
KEYWORDS: Power supplies, Optical fibers, Radio over Fiber, Phase only filters, Hybrid fiber radio, Photodetectors, Space division multiplexing, Multiplexing, Receivers, Signal attenuation
We present a newly developed 90-GHz high O/E conversion efficient, high power photoreceiver for radio over fiber transmission, which is driven by photonic power supplies. The photoreceiver consisted of a hybrid integration with a 100 GHz photodetector and a 22-dB high gain RF amplifier. To save the fiber resources in the PoF system, bias-free operational design was employed for the photodetector. In the RF measurement result, we successfully achieved the high output power of +15 -dBm at 90-GHz, high linearity in RF output power against the input photo-current, and high 3dB bandwidth of 14 GHz (85-99 GHz).
In this study, we fabricated QD laser diodes using a digital embedding method (DEM) in which InAs QDs were embedded in an InGaAs/InAlAs superlattice whose miniband acts as an effective barrier for the QDs. We stacked 15 QD layers by using DEM with four monolayers in each InGaAs/InAlAs superlattice. The stripe laser structures were fabricated using conventional laser diode processes. The laser with a 600-µm cavity showed lasing at 1600 nm with a threshold current of 474 mA. The internal loss of this laser was 16.2 cm-1, which was similar that of the laser that uses a conventional quaternary InGaAlAs barrier material.
In this work we discuss technological aspects of creating a linear energy dispersion spectrum of charge carriers in semiconductor materials and report on the experimental realization of the topological Dirac semimetals (DSM) in nanostructurally engineered zero-gap InAs/GaInSb superlattices (SL) [1]. The SL samples are synthesized by molecular beam epitaxy, which provides monolayer accuracy for growing high-quality single-crystals on large area substrates. The prospects for designing the topological insulator (TI) SLs with the same approach and first results of experimental characterization of the TI candidates are also presented.
In quantum information technology, it is necessary to develop a light-matter quantum interface that transfers and stores quantum information. As a bandwidth of quantum entangled photon pairs used for quantum information increases, a quantum interface with broad bandwidth will be required. The combination of quantum dot (QD) ensemble and photon echo (PE) method is one of promising methods for broadband quantum interface. Since the bandwidth of the quantum interface using this method is limited only by the inhomogeneous width of the QDs, it is possible to implement a quantum interface with the bandwidth of 10 THz at telecommunication wavelength. However, in the PE method, the spatial inhomogeneousity of the laser intensity and the inhomogeneousity of the resonance frequency of the QDs result in the uniform quantum control of excitons in QDs. As a result, the regeneration efficiency of the PE light is significantly deteriorated.
To solve this problem, it is effective to introduce a quantum control method using chirped pulses (Adaptive Rapid Passages; ARPs) which is robust to inhomogeneousities. In this study, we demonstrate that the regeneration efficiency of PE in inhomogeneous QDs can be improved by ARPs using femtosecond pulses. By performing numerical simulation and optical experiments, it was found that the regeneration efficiency improves as the chirp amount and the pulse area increase, and saturates at a certain condition.
The efficient transfer of a quantum state from photons to matter qubits in order to momentarily store information has become a central problem in quantum information processing. A quantum memory turns out to be an essential tool to achieve advanced technologies such as quantum networks, quantum repeaters, deterministic single photon sources or linear optics quantum computers. The realization of a quantum interface has been investigated in various forms, among which one can find solid-state atomic ensembles, color centers in crystal lattices, cold atomic gases, optical phonons in diamond and many others. Here we focus on a broadband quantum interface for high repetition rate (76 MHz) ultrafast optical pulses (250 fs) at telecommunication wavelength (1530 nm) based on the photon echo process occurring in semiconductor quantum dots. We evaluated the quantum state of photonic qubits in order to characterize the impact of the storage on the transmitted signal. Homodyne traces corresponding to projections of the Wigner function of the signal on rotated quadrature components were obtained using broadband balanced homodyne detection, i.e. mixing the ultrafast optical pulses to analyze with a high repetition rate pulsed local oscillator. The reconstruction of the Wigner function from the homodyne traces was performed using three algorithms: the inverse Radon transform, the minimax adaptive reconstruction and the maximum likelihood estimation. The three methods lead to similar results, concluding that for an input pulse in a coherent state, the reemitted photon echo is also in a coherent state.
Most of space-based observing systems make water-vapor- and temperature-related measurements, while spacebased observing systems for wind measurement is limited. The current passive space-based observing systems for wind measurement has a large coverage area and high temporal and horizontal resolutions but has a low vertical resolution. The World Meteorological Organization (WMO) wants to develop space-based wind profiling systems. A Doppler Wind Lidar (DWL) is a useful and power technology for wind measurement and it can be designed as compact mobile, airborne, and space-based systems. DWL would provide us with a wind profile having high vertical resolution, low bias, and good precision, and it is necessary to fill the gap of current observations. The National Institute of Information and Communications Technology (NICT) is developing a single-frequency high-energy Tm,Ho:YLF laser, 2-μm key technology and instrument for a future space-based coherent DWL. We demonstrated the Tm,Ho:YLF laser producing a pulse energy of 125 mJ operating at 30 Hz meeting requirements for the future spacebased coherent DWL. In the paper, we will describe recent progress at NICT.
We designed and fabricated a low-bias operational uni-travelling carrier photodiode (UTC-PD) structure, which can be operated at over 100 GHz. The main structure of the device consisted of p-doped InGaAs for the photo-absorption layer and non-doped InP for the carrier collector layer, to obtain both a high electron drift velocity at a low bias and a small CR time constant based on the pn-junction capacitance. Through an on-wafer probing test, the frequency response was measured up to 210 GHz using a 1 mm coaxial connecter type (DC-110GHz), W-band (75–110 GHz) and G-band (140–220 GHz) waveguide probe with a spectrum analyzer. In the measurement results, a large bandwidth of 10 MHz-110 GHz could be obtained with good flatness within ±1 dB. When the W-band and G-band performance were characterized, the high-power characteristic of -3.8 dBm could be achieved at 106 GHz. and the output power level of - 19.8 dBm could be confirmed at 210 GHz as well.
We investigated the effect of Rapid Thermal Annealing (RTA) process on Quantum Dot Semiconductor Optical Amplifiers (QD SOAs). The devices are composed of 30-layer stacks of InAs quantum dot by using strain compensation method. The lateral size and height of QD are 30 nm and 4 nm, respectively. Our QD SOAs have emission wavelengths within 1.5 μm-band. We applied RTA process to improve the characteristics of internal quantum efficiency (ηi ) and optical loss (αi ) of ridge laser diode for QD SOAs. In this case, the operating temperatures of RTA process were set at 600°C, 620°C, 640°C and 660°C for 30 seconds each. In addition, the devices are cleaved to form a cavity length at 0.06 cm, 0.08 cm, 0.10 cm, 0.12 cm and 0.14 cm. According to the L-I characteristic result of ridge laser diode structure for QD SOAs at 640°C, the best minimum threshold current ( Ith ) is 47.93 mA. Moreover, according to the plot between 1 ηd−1 (external quantum efficiency) and cavity length, we can optimize the internal quantum efficiency and optical loss for a ridge laser diode structure to be 66.39% and 9.87 cm-1 respectively at 640°C RTA’s temperature. Finally, The RTA process helps to achieve 1.4 times higher in internal quantum efficiency as well as a minimal increase in internal optical loss comparing to without RTA.
High reflection dielectric distributed Bragg reflector (DBR) can be used with high Q optical cavity to narrow down the spectrum optical source, for experimental quantum electro-dynamics, etc. To construct a free space optical cavity for a local oscillator of the state-of-the-art optical frequency standards, concave mirrors are used as one of the end mirrors of the cavity. Usually, the high reflection DBR is fabricated by depositing dielectric materials on a glass substrate. However, the ultimate stability of the optical cavity is limited by the thermal noise of dielectric DBR. To overcome this problem, a crystalline DBR was proposed to stabilize the optical cavity, which can reduce thermal noise. In this study, we fabricated crystalline DBR by GaAs/AlAs compound semiconductor on a concave GaAs substrate. Although a traditional semiconductor substrate has atomically flat surface, we fabricated a concave surface with a curvature radius of 1000 mm on the GaAs substrate by optical quality polishing. Then, we carried out wet etching and introduced it in vacuum chamber for molecular beam epitaxy (MBE). In the MBE growth chamber, we carried out thermal cleaning with As4 at a substrate temperature of 600°C. Next, the GaAs/AlAs DBR structure was grown at 580°C. The evaluation of surface roughness was conducted by atomic force microscopy, which showed a roughness of 0.165 nm in 1 × 1 um measurement such that a very smooth surface can be obtained.
We investigated the temperature characteristics of a modularized semiconductor optical amplifier (SOA) utilizing InAs/AlGaAs quantum dot (QD) in the active layer operating at C-band (1.53μm–1.56μm). It has been reported by many literatures on physics that QDs are superior at energy efficiency and leads to less thermal energy generation. By changing the temperature of the Peltier element inside the module from 20℃ to 80℃, we measured the difference in the gain at each input power and injection current. The QD-SOA we measured was utilizing InAs QD in active layer and the laminated structure had 20 layers having 20nm of intermediate layers which refers to the width between QDs. When the input power was -50 dBm, we successfully confirmed more than 10 dB at the Peltier element temperature of 70℃ by injecting a current larger than 400 mA. In addition, we obtained a maximum gain of 20.68 dB at the center wavelength and a constant gain of approximately 15 dB at other Peltier element temperatures. It can be concluded from the output of the experiment that this QD-SOA can be put to use in optical communication in several situations where the temperature ranges between 20℃ to 80℃. This involves a new approach towards the application of QD amplifiers in the field of optical fiber communications.
Optical power-delivery systems are applied to distribute electrical power over optical fibers for systems such as remote wireless radio heads. Typically, the electrical voltage at the receiver side is low, around 1.0 V, owing to the use of a long-wavelength carrier. Consequently, we recently proposed a light-wave-modulation method for increasing the received electrical voltage. A 940-nm high-power laser was directly modulated to form a modulated light wave. We also used a small inductor to generate an induced electromotive force from the modulated light wave. We successfully obtained a peak voltage over 18 V using this simple technique.
We present a double cladding, high-mesa-type waveguide UTC photodetector with an improved the responsivity. In this device structure, an InGaAs thin core layer was sandwiched by p-InP/InGaAsP and n-InP/InGaAsP cladding layers, including a UTC structure, in order to obtain a good optical coupling between the waveguide and the fiber. By comparing the resulting mode field with that obtained with a single cladding layer structure, we confirmed that the vertical mode field was enlarged. Without a spot size converter, the measured responsivity was as high as 0.6 A/W at 1550 nm, which suggests a responsivity three times higher than that of back-illuminated structures, and higher responsivity than that of previous reports. A high frequency performance (f3dB = 100 GHz) was also measured. The device structure, including the layer, doping level conditions, and optical fiber coupling results are discussed, and its performance is characterized in detail.
A 100-GHz narrowband photoreceiver module integrated with a zero-bias operational uni-traveling-carrier photodiode (UTC-PD) and a GaAs-based pseudomorphic high-electron-mobility transistor (pHEMT) amplifier was fabricated and characterized. Both devices exhibited flat frequency response and outstanding overall performance. The UTC-PD showed a 3-dB bandwidth beyond 110 GHz while the pHEMT amplifier featured low power consumption and a gain of 24 dB over the 85-100 GHz range. A butterfly metal package equipped with a 1.0 mm (W) coaxial connector and a microstrip-coplanar waveguide conversion substrate was designed for low insertion loss and low return loss. The fabricated photoreceiver module demonstrated high conversion gain, a maximum output power of +9.5 dBm at 96 GHz, and DC-power consumption of 0.21 W.
Short-range interconnection and/or data center networks require high capacity and a large number of channels in order to support numerous connections. Solutions employed to meet these requirements involve the use of alternative wavebands to increase the usable optical frequency range. We recently proposed the use of the T- and O-bands (Thousand band: 1000–1260 nm, Original band: 1260–1360 nm) as alternative wavebands because large optical frequency resources (>60 THz) can be easily employed. In addition, a simple and compact Gb/s-order high-speed optical modulator is a critical photonic device for short-range communications. Therefore, to develop an optical modulator that acts as a highfunctional photonic device, we focused on the use of self-assembled quantum dots (QDs) as a three-dimensional (3D) confined structure because QD structures are highly suitable for realizing broadband optical gain media in the T+O bands. In this study, we use the high-quality broadband QD optical gain to develop a monolithically integrated QD optical gain modulator (QD-OGM) device that has a semiconductor optical amplifier (QD-SOA) for Gb/s-order highspeed optical data generation in the 1.3-μm waveband. The insertion loss of the device can be compensated through the SOA, and we obtained an optical gain change of up to ~7 dB in the OGM section. Further, we successfully demonstrate a 10-Gb/s clear eye opening using the QD-OGM/SOA device with a clock-data recovery sequence at the receiver end. These results suggest that the monolithic QD-EOM/SOA is suitable for increasing the number of wavelength channels for smart short-range communications.
High-performance photodetectors (PDs) for radio over fiber (RoF) applications over 60 GHz were designed and fabricated. The RF output was investigated while a high linearity was observed for two designs: a low carrier concentration InGaAs absorption layer in a PIN structure and a low carrier concentration collection layer in a unitravelling- carrier (UTC) structure. The RF output performances of both PIN and UTC structures were studied at 67 GHz and 100 GHz respectively. High photocurrent densities could be obtained from both structures (21.7 kA/cm2 in the PIN structure and 35.4 kA/cm2 in the UTC structure). The PIN structure exhibited a slightly higher current density of 1.6 times than the UTC structure. The frequency response of the UTC-PD exhibited excellent flatness up to 110 GHz, with a 3 dB bandwidth beyond 110 GHz. In addition, maximum RF output powers of +6.8 dBm at 67 GHz and -5 dBm at 100 GHz was successfully obtained. The space charge effect could be ruled out for the output linearity, but avoiding overheating in the p-contact metal had to be considered. By modifying impedance matching circuit designs, the maximum RF output power level of 3 dB can be improved.
We successfully developed the Quantum Dot (QD) light source in the T- and O-band (Thousand-band: 1.000–1.260 μm
and O-band: 1.260–1.360 μm).We used the optical gain chip properties of a single InAs/InGaAs QD to develop a 1.3
μm waveband QD optical frequency comb laser (QD-CML) as the novel functional wavelength divisio nmultiplexing
(WDM) light source. The QD-CML can be used for stable generation of multiple-wavelength peaks that the QD-CML
can be selected and controlled using the optical interference of the etalon filter. We used holey fiber (HF) transmission
line to successfully demonstrate an O-band photonic transport system. In this experiment, each single peak could be
clearly selected in the wavelength range the 1286- to 1302 nm. An error-free data transmission of the 10-Gb/s signal with
stable multiple-wavelength channels that were generated from a newly developed 1.3 μm waveband multiple-wavelength
quantum dot light source was achieved over the low-loss HF that was 8 -km long. We suggest that the low cross-talk for
a data transmission can be achieved using the multiple-wavelength peaks from the QD light source. Based on these
experimental results, it is clear that the generation of multiple-wavelength peaks in the 1.3 μm waveband can be
successfully achieved using from the multiple-wavelength QD light source, which acts as a single QD optical gain device.
Mid-infrared (mid-IR) light sources operating in the 3-μm waveband are useful photonic devices for the spectroscopic
detection of trace gases and biomaterials. In order to achieve efficient mid-IR light emissions with a small footprint, we
propose an attractive technique for developing a compact intracavity system that enables the reuse of the photons emitted
by the pump source. Periodically poled congruent LiNbO3 (PPCLN) and Nd-YVO4 crystals were both set in the
intracavity. An 805-nm waveband GaAs-based laser diode was used as the signal and pump light sources for differential
frequency generation (DFG), since GaAs-based device technology enables the construction of high-power, highefficiency
lasers operating in this waveband. We have successfully demonstrated that a 3-μm wavelength-tunable light
source using the photon reuse technique is the most effective and compact intracavity system; it possesses useful
broadband wavelength tunability characteristics up to approximately 90 nm and offers a small footprint (15 × 30 cm).
We obtained high output power of the order of a few milliwatts from the developed light source. In addition, since the
optical spectrum shape of the mid-IR light is also important, we propose a useful spectrum shaping technique using a
Fabry-Perot (FP) etalon filter included in the intracavity system; we obtained a fine single-peak spectrum in the 3-μm
waveband. The developed wavelength-tunable compact intracavity mid-IR light source using the photon reuse and
spectrum shaping techniques is attractive for optical communications and biomedical applications.
Semiconductor quantum dots (QDs) grown using self-assembly techniques in the Stranski-
Krastanov (S-K) mode are expected to be useful for high-performance optical devices such as QD
lasers. A significant amount of research has been carried out on the development of highperformance
QD lasers because they offer the advantages of a low threshold current, temperature
stability, high modulation bandwidth, and low chirp. To realize these high-performance devices, the
surface QD density should be increased by fabricating a stacked structure. We have developed a
growth method based on a strain-compensation technique that enables the fabrication of a high
number of stacked InAs QD layers on an InP(311)B substrate. In this study, we employed the
proposed method to fabricate QD laser diodes consisting of highly stacked QD layers and
investigated the dependence of the diode parameters on the stacking layer number. We fabricated
QD laser diodes with 5, 10, 15, and 20 QD layers in the active region. All of the laser diodes
operated at around 1.55 μm at room temperature, and their threshold currents showed clear
dependence on the stacking layer number. Laser diodes with more than 10 QD layers showed
sufficient gain, i.e., the threshold currents decreased with a decrease in the cavity length. On the
other hand, for laser diodes with less than 10 QD layers, the threshold currents increased with a
decrease in the cavity length.
Photonic transport systems in the C+L bands have been extensively employed in conventional networks. The
continuously expanding demand for greater photonic network capacities has created the need for the use of additional
wavebands to strengthen the transmission capacities. We recently focused on the use of a novel wavelength band such as
1.0-μm (thousand band: T band), together with the conventional C and L bands, to enhance the usable optical frequency
resources in future photonic networks employing waveband division multiplexing. Furthermore, we successfully
demonstrated an ultra-broadband T-band photonic transport system using a holey fiber (HF) transmission line to create a
wide range of usable optical frequency resources over 8.4 THz (wavelength range: 1037-1068 nm). In constructing an
ultra-broadband photonic transport system for the T, C, and L bands, HF is considered to be a great candidate for an
ultra-broadband and high-capacity data transmission line. In this study, we demonstrated a polarization division
multiplexing (PDM) photonic transport system for doubling the optical frequency resources in the T band. Error-free
PDM photonic transmissions in the T band with a clear eye opening at 10 Gb/s were successfully achieved over a long
distance using an 11.4-km HF transmission line for the first time. To upgrade the present photonic network system, we
believe the technologies of the demonstrated T-band PDM, together with WDM photonic transport systems using the
>10-km long HF transmission line, represent a pioneering breakthrough in the use of ultra-broadband optical frequency
resources.
We designed InAs/Ga0.6In0.4Sb superlattice (SL) material for terahertz-range photodetectors. Depending on the
thicknesses of the InAs and Ga0.6In0.4Sb layers, the SL energy gap Eg can be adjusted to be between 8-25 meV, which corresponds to a cut-off frequency from 2 to 6 THz. Different designs were numerically evaluated by using the eightband
k•p model. The calculations show that the SL energy gap is sensitive to monolayer (ML) scale variations in layer
thickness, and that realization of the design parameters requires better than 1ML accuracy of epitaxial growth.
A 40-period strained Ga0.6In0.4Sb SL with alternating InSb (1ML) and GaAs (1ML) interfaces was grown by a
molecular beam epitaxy on a GaSb substrate; the target energy gap Eg was 9 meV. The SL samples were characterized
by X-ray diffraction (XRD), atomic force microscopy (AFM), photoluminescence and absorption spectroscopy
measurements. Despite the large lattice mismatch between InAs and Ga0.6In0.4Sb, the XRD and AFM measurements
showed that the SL had good structural and surface quality and an accurate layer structure. The surface roughness was
0.22 nm.
To open up new optical frequency resources available for optical communications, the concept of all-band photonics
has been proposed, which is based on the utilization of broadband of optical frequencies from 1- to 2-μm waveband as a
novel photonic band for photonic transmission. In this study, an ultra-broadband photonic transport system was
developed by employing a long-distance holey-fiber transmission line to simultaneously use the new 1-μm waveband (Tband)
and a conventional waveband. We successfully demonstrate the use of a photonic transport system to achieve simultaneous 3x10-Gbps error-free optical data transmissions for waveband division multiplexing of the 1-μm waveband, C-band, and L-band.
Continually expanding demand for greater photonic network capacities has created a need for the use of alternative
wavebands and development of methods to strengthen transmission capacities. Photonic transport systems in the C band
(1530-1565 nm, 4.3-THz bandwidth) and L band (1565-1625 nm, 7.1-THz bandwidth) have been extensively employed
in conventional networks. We recently focused on use of a novel wavelength band such as 1.0-μm (thousand band: T
band) together with the conventional C and L bands for enhancing usable optical frequency resources in future photonic
networks employing wavelength division multiplexing (WDM). Therefore, here, we develop an ultra-broadband
photonic transport system in the T band to create novel optical frequency resources in that waveband. In the proposed
system, a holey fiber (HF) transmission line has been developed that is capable of ultra-broadband data transmission. In
this study, we demonstrated ultra-broadband, 10-Gbps, error-free operation in a T-band photonic transport system using
a wavelength tunable light source and an HF transmission line (typically >3.3 km). We successfully developed ultrabroadband
transmission capability beyond the 8.4-THz bandwidth (1037-1068 nm), from usable optical frequency
resources in the T band. In this band we also successfully demonstrate a polarization division multiplexing (PDM)
photonic transport system for achieving efficient use of optical frequency resources. To construct the photonic network
system of the future, we believe the technologies of the demonstrated T-band photonic transport systems using the HF
transmission line represent a pioneering breakthrough in the use of ultra-broadband optical frequency resources.
An all-optical switching device has been proposed by using self-assembled InAs/GaAs quantum dots (QDs) within a
vertical cavity structure for ultrafast optical communications. This device has several desirable properties, such as the
ultra-low power consumption, the micrometre size, and the polarization insensitive operation. Due to the threedimensional
confined carrier state and the broad size distribution of self-assembled InAs/GaAs QDs, it is crucial to
enhance the interaction between QDs and the cavity with appropriately designed 1D periodic structure. Significant
QD/cavity nonlinearity is theoretically observed by increasing the GaAs/AlAs pair number of the bottom mirror. By this
consideration, we have fabricated vertical-reflection type QD switches with 12 periods of GaAs/Al0.8Ga0.2As for the top mirror and 25 periods for the bottom mirror to give an asymmetric vertical cavity. Optical switching via the QD excited
state exhibits a fast switching process with a time constant down to 23 ps, confirming that the fast intersubband relaxation of carriers inside QDs is an effective means to speed up the switching process. A technique by changing the light incident angle realizes wavelength tunability over 30 nm for the QD/cavity switch.
To open up new optical frequency resources available for optical communications, the concept of all-band photonics
has been proposed, which is based on the utilization of broadband of optical frequencies in the 1-μm waveband as a
novel photonic band for photonic transmission. In this study, an ultra-broadband photonic transport system was
developed by employing a long-distance holey-fiber transmission line to simultaneously use the 1-μm waveband and a
conventional waveband such as the C-band. We successfully demonstrate the use of a photonic transport system to
achieve simultaneous 10-Gbps error-free optical data transmissions in the 1-μm waveband and C-band.
We propose a method for the fabrication of self-assembled semiconductor quantum dots (QDs) and the control of their
density and emission wavelength. This method would be fundamental toward the fabrication of nanophotonic devices.
We used molecular beam epitaxy and fabricated self-assembled QDs from various materials under various growth
conditions. We controlled the emission wavelength over a wide range (700-1700 nm) by changing the materials around
the QDs. We used antimonide-related materials or a highly stacked structure to control the density of the obtained QDs
within the range of 108-1013/cm2.
An injection-seeding bandwidth of 4 THz (1042-1057 nm) was successfully obtained for a quantum-dot (QD) Fabry-
Perot laser diode operating in a 1-μm optical waveband. The operation of a fabricated laser was investigated for
transmission through a 1-km hole-assisted fiber (HAF), and clear eye-openings and almost power-penalty-free
transmission were successfully demonstrated with respect to 2.5 Gbps for various wavelengths in the 1-μm band.
The population and coherent dynamics of excitons in InAs quantum dots were investigated using transient pump-probe and four-wave mixing spectroscopies in the telecommunications wavelength range. The sample
was fabricated on an InP(311)B substrate using strain compensation to control the emission wavelength. This technique also enabled us to stack over a hundred QD layers, which resulted in a significant enhancement of nonlinear signals. By controlling the polarization directions of incident pulses, we precisely estimated the radiative and non-radiative lifetimes, the transition dipole moment, and the dephasing time while taking into account their anisotropic properties. The measured radiative lifetime and dephasing time shows large anisotropies with respect to the crystal axes, which results from the anisotropic nature of the transition dipole moment. The
anisotropy is larger than that for InAs quantum dots on a GaAs(100) substrate, which seems to reflect a lack of symmetry on an (311)B substrate. A quantitative comparison of these anisotropies demonstrates that nonradiative population relaxation and pure dephasing are quite small in our QDs.
Silicon photonics technology is expected to be key to high-performance, low-power photonic networks. It is also
expected that using Si photonics technology will make it possible to integrate photonic key components, such as optical
waveguides, optical switches, polarization rotators, light emitters, and optical gain on Si chip C-MOS devices. We
propose approaches to fabricating these key components on Si wafers. That is, we introduce some key technologies; (a)
all-optical switching in a Si nanowire waveguide, (b) a Si waveguide polarization rotator, (c) material synthesis and
waveguide formation of rare earth-doped SiOx thin-films for Si-based light emitters, and (d) a technique for fabricating
nano-structured III-V semiconductors, such as Sb-based quantum dot structures, on Si wafers. We also propose a concept
for applying Si photonics, i.e., a way to use Si photonics in information and communications technology (ICT).
We present a fabrication technique for creating high-quality structures of antimonide-based quantum dots (Sb-based QDs), which show long-wavelength emissions for fiber-optic communications. By using the Sb-based QDs as the active medium, we successfully demonstrated optical-emissions in the 1.3- and 1.5-μm wavebands from a long-wavelength vertical-cavity surface emitting laser (VCSEL) structure fabricated on a GaAs substrate. Additionally, we describe a growth technique for Sb-based QDs on a silicon wafer, which may become novel-materials for silicon photonics technology.
We investigated the heteroepitaxial growth of GaSb on Si(001) substrates. High-quality GaSb films were grown on Si substrates by using an AlSb initiation layer. When small AlSb islands were formed on the Si substrate before the GaSb growth, two-dimensional GaSb film was grown. In contrast, without small AlSb islands, large GaSb islands formed on the substrate. Therefore, the AlSb islands played an important role in preventing excessive surface diffusion of Ga atoms on the Si surface and promoting two-dimensional growth of GaSb. A narrow X-ray diffraction rocking curve (around 200 arcsec) was obtained by optimizing the growth temperature and the thickness of the AlSb initiation layer. High-quality GaSb/AlGaSb and InGaSb/AlGaSb MQW samples were also grown on a Si substrate by using this method. At room temperature, these samples gave a strong emission at 1.55 μm, which is a wavelength used by fiber optic communications systems. Furthermore, we could control the emission wavelength by simply changing the well width. The emission energy was in good agreement with the theoretical curve. The temperature dependence of the PL intensity indicated a large activation energy (~77.6 meV) from the GaSb QWs. These results indicate that the fabricated QW structure had high crystalline quality and that GaSb quantum wells can be fabricated on Si for optical devices operating above room temperature.
Semiconductor quantum dots have unique characteristics which advantage especially for saturable
absorbers. We characterized nonlinear optical absorption of highly-stacked InAs quantum dot layers on an InP
(311)B substrate in 1.5-μm band. High-density (5 x 1012 cm-2) quantum dots by stacked more than 150 layers
were recently fabricated with a strain-controlled technique for 1.5-μm communication devices. The
transmission increase for the vertical incidence was observed as much as 1%, and transmittance decreased at
higher intensity of incident pulse. The temporal behavior of the transmission increase showed two decay
components of a few picoseconds and several hundred picoseconds decay. The nonlinear absorption was
explained by the saturable absorption of the quantum dots layer and the two photon absorption of the InP
substrate. The features of the saturable absorber are suitable for application to a mode-locker for generating
short optical pulses in the 1.5-μm band. Analysis showed that we can tune the saturable absorption
characteristics by adjusting the numbers of quantum dot layers, thickness of residual InP substrate and
antireflection coating on the surfaces. We obtained planar type saturable absorbers at a typical saturation
intensity of 35MW/cm2 with 3.5% transmittance change for a traveling type and 8% change for reflective type,
respectively.
We investigate the dephasing of excitons in InAs self-assembled
quantum dots by using a transient four-wave-mixing technique. A used sample is specially designed to compensate the strain. We observe long-lived coherence of excitons at 5 K which corresponds to the dephasing time longer than a nanosecond, where the photon energy of
the excitation pulse is 0.874~eV. We find that a pure dephasing due to exciton-phonon interactions dominates in the exciton dephasing
rather than the population decay and the exciton-exciton interaction
in the weak excitation region, by analyzing the population lifetime and the polarization-dependent dephasing time.
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