Toshimasa Umezawa

Highly Sensitive Photodetector Using Ultra-High-Density 1.5-μm Quantum Dots for Advanced Optical Fiber Communications

We have fabricated a high-density 1.5-μm quantum dot photodetector for advanced optical fiber communications and have found unique optical properties, including avalanche multiplication. The structure of the absorption layer had stacked InAs/InGaAlAs layers with a high density of 1 × 1012 cm-2, which consisted of strained 1.5-μm InAs quantum dots and a strain compensation layer of InGaAlAs. A three times larger absorption coefficient than the InGaAs layer, an avalanche multiplication effect, and a low dark current are reported with InAs quantum dot conditions.

Investigation of a 1.5-µm-wavelength InAs-quantum-dot absorption layer for high-speed photodetector

We investigated a 1.5-µm-wavelength InAs-based quantum-dot (QD) absorption layer for high-speed and high-sensitivity photodetectors in advanced optical-fiber communications. The photodetector, which contained 20 stacked absorption layers of InAs/InGaAlAs QDs using the strain-compensation technique, exhibited a high absorption coefficient, avalanche multiplication effect, and a low dark current. It could operate at a low bias voltage for a p-type–intrinsic–n-type (PIN) diode structure. We expect that the 3-dB bandwidth at a low bias voltage would be approximately 50 GHz, and that the gain and bandwidth product in the avalanche multiplication region would be 150 GHz at a high bias voltage.

Bias-Free Operational UTC-PD above 110 GHz and Its Application to High Baud Rate Fixed-Fiber Communication and W-Band Photonic Wireless Communication

We present the design of a bias-free uni-travelling-carrier photodiode (UTC-PD) that is operational above 110 GHz, and its application to 100-GBd fixed-fiber communication and 109-GHz carrier-photonic wireless communication for advanced optical networks. The design and analysis for bias-free operation were explained in detail using the carrier concentration in the photo-absorption layer and carrier collector layer. In the experimental results, the developed UTC-PD with back-illuminated structure exhibited a wide 3-dB bandwidth of over 110 GHz at 0 V. For fixed-fiber communications, a high-baud-rate photoreceiver based on the UTC-PD was fabricated. Eye diagrams showed clear openings at up to 107 GBd and high photo-currents of 3-7 mA at 100 GHz signal light were observed at zero-bias. Assuming photonic wireless communication using photonic power supply through optical fiber, the developed UTC-PD contributed to reducing bias feeds to the photoreceiver module. Additionally, low power consumption was achieved when using the newly developed 110 GHz InP-PHEMT amplifier with the UTC-PD.

Zero-bias operational ultra-broadband UTC-PD above 110 GHz for high symbol rate PD-array in high-density photonic integration

We have successfully developed a zero-bias operational ultra-broadband uni-traveling-carrier photodiode (UTC-PD) with a frequency bandwidth above 110 GHz using a low carrier concentration of 3 × 10¹⁴ cm^-3 in the carrier collection layer.

A semiconductor optical amplifier comprising highly stacked InAs quantum dots fabricated using the strain-compensation technique

A semiconductor optical amplifier was fabricated by incorporating highly stacked InAs quantum dots (QDs) as a gain media. Twenty InAs QD layers were successfully stacked through the strain-compensation technique, without any deterioration of crystal quality. A wide-range 1.55 µm band gain was observed, and maximum gain reached 25 dB at 1530 nm when the input power was below −40 dBm.

Stable Two-Mode Emission from Semiconductor Quantum Dot Laser

A two-wavelength emission laser (two-mode laser) has been fabricated using semiconductor quantum dots as the gain medium in the external cavity configuration. Two-mode laser emission with a separation of 98.8 GHz has been achieved by controlling the etalon and narrow-band filter setting. A 98.4 GHz beat signal was observed using the Michelson interferometer configuration, indicating that these emissions occurred simultaneously.

100-GHz Fiber-Fed Optical-To-Radio Converter for Radio-And Power-Over-Fiber Transmission

We have developed a 100-GHz fiber-fed optical-to-radio converter for radio- and power-over-fiber transmission. A zero-bias operational high-speed photodetector (3-dB bandwidth >100 GHz) could be applied for the optical-to-radio converter, taking significant advantage of no-bias feed 100-GHz operation. We have successfully demonstrated simultaneous radio and power generation from one device through a bias-T circuit. A 100-GHz pseudomorphic high electron mobility transistor amplifier allowed a hybrid integrated photoreceiver to enhance the O/E conversion gain. Using the generated electric power, a gate bias in the 100-GHz amplifier was driven. The performance of each device and the integrated photoreceiver is described. High data rate photonic wireless transmission based on photonic power supply without external electric power supply is demonstrated.

Bias free operational 107-Gbaud ultra-fast photodetector

We designed a high-baud-rate UTC photodetector without a bias circuit for the first time. The photodetectors wideband frequency response over 100 GHz and clear eye diagrams up to 107 Gbaud were confirmed at 0 V. Bit error rate is discussed for 56 Gbaud and 107 Gbaud.

Data rate penalty for high-density photonic integrated circuits in advanced modulation formats

We report on a simulation investigation of the effects of signal lines and core pitch on RF and optical crosstalk in integrated photonic circuits, and we discuss the data rate with photonic circuit density in advanced modulation formats such as 64QAM.

Crosstalk reduction for large scale photonic integrated circuits

The fundamental crosstalk characteristics related to design of highly compact optical and RF signal lines has been studied in this work. The systematic investigation of the crosstalk of parallel waveguides are carried out by modal analysis and the beam propagation analysis. The new idea of crosstalk mitigation in PIC by using uniform air gap along the longitudinal structure has been proposed. The crosstalk mitigation up to - 40dB by using air gap can successfully achieve, which results in reduction of spacing between signal lines.