Tomofumi Furuta

120-GHz-band millimeter-wave photonic wireless link for 10-Gb/s data transmission

A 120-GHz-band wireless link that uses millimeter-wave (MMW) photonic techniques was developed. The output power and noise characteristics of 120-GHz-band MMWs generated by converting a 125-GHz optical subcarrier signal were evaluated. It was then shown that the noise characteristics of the 125-GHz signal generated with these photonic technologies is sufficient for 10-Gb/s data transmission. We constructed a compact 120-GHz-band wireless link system, and evaluated its data transmission characteristics. This system achieved error-free transmission of OC-192 and 10-GbE signals over a distance of more than 200 m with a received power of below -30 dBm.

High-Speed Response of Uni-Traveling-Carrier Photodiodes.

The photoresponse of a uni-traveling-carrier photodiode (UTC-PD), which is configured with a neutral narrow-gap light absorption layer and a depleted wide-gap carrier collecting layer, is investigated by small-signal analysis. Drift-diffusion model was used for analyzing carrier dynamics in the absorption layer. For accurately predicting the frequency response, a boundary condition at the edge of the absorption layer was carefully treated by taking into account the electron thermionic emission velocity. High electron mobility in the absorption layer and high drift velocity in the carrier collecting layer associated with the velocity overshoot effect are both essential for short response times. Calculations performed on InP/InGaAsP UTC-PDs with the same absorption and carrier collecting layer thicknesses show that the response can be dominated by the electron transport in the absorption layer provided that the significant velocity overshoot occurs in the carrier collecting layer. Furthermore, a UTC-PD with a quasi-field in the absorption layer can generate a several times broader bandwidth than conventional pin PDs, while maintaining a similar internal quantum efficiency.

Continuous THz-wave generation using antenna-integrated uni-travelling-carrier photodiodes

Photonic generation of continuous millimetre- and sub-millimetre waves up to the THz range using antenna-integrated uni-travelling-carrier photodiodes is described. A device integrating a wideband log-periodic antenna exhibits a maximum output power of 2.6 ?W at 1.04 THz with good linearity. A module with a quasi-optical output port fabricated for practical use generates almost the same output power as the chip at around 1 THz and operates at frequencies of up to 1.5 THz. The output power level and the operation frequency are records for wideband photodiodes operating at 1.55 ?m. Devices integrating resonant narrowband dipole antennae have also been fabricated and the output power increases at resonant peak frequencies confirmed. The device having a peak at 1.04 THz exhibits a maximum (detected) output power of 10.9??W at 1.04 THz with good linearity. This output power is the highest value ever directly generated from a photodiode in the THz range, and several times higher than the maximum value reported by the low-temperature-grown GaAs photoconductive switch at around 1 THz.

InP-InGaAs uni-traveling-carrier photodiode with improved 3-dB bandwidth of over 150 GHz

Uni-traveling-carrier photodiodes (UTC-PDs) with ultrafast response and high-saturation output are reported. It is experimentally demonstrated that the photoresponse of UTC-PDs is improved by incorporating a step-like potential profile in the photoabsorption layer. The fabricated device shows a peak electrical 3-dB bandwidth of 152 GHz at a low reverse bias voltage of -1.5 V. The output voltage can be increased to as high as 1.9 V at higher reverse bias voltages with the 3-dB bandwidth staying at over three-quarters of the maximum value. To our knowledge, the obtained response is the fastest among those reported for 1.55-/spl mu/m wavelength photodiodes.

Broadband-Frequency-Tunable Sub-Terahertz Wave Generation Using an Optical Comb, AWGs, Optical Switches, and a Uni-Traveling Carrier Photodiode for Spectroscopic Applications

We present a monochromatic sub-terahertz signal generation technique using an optical comb signal, arrayed waveguide gratings (AWGs), and a uni-traveling carrier photodiode (UTC-PD) for spectroscopic applications. This scheme offers random or continuous frequency tuning in the range between 100 GHz and up to 1 THz. In addition, since a RF synthesizer is employed as a reference signal source of the photonic frequency multiplier, frequency locking with external instruments and reliable operation are offered. Highly coherent optical comb signal for the photonic frequency multiplication provides a narrow linewidth and very low phase noise in the generated sub-terahertz signal. For 125 GHz, the phase noise is approximately -92 dBc/Hz at the offset frequency of 10 kHz. This is larger than that of the 25-GHz RF source by about 13 dB and agrees well with the theory regarding phase noise multiplications due to frequency multiplication. For generating monochromatic signals, unwanted spurious signals are suppressed in the optical domain over a wide range with two AWGs, and the suppression ratio is expected to be better than 46 dBc. Utilizing the implemented sub-terahertz signal generator with a J-band UTC-PD module, absorption lines of N2O were measured in the frequency range between 240 and 360 GHz and compared with theoretical calculations.

Strong photoluminescence emission at room temperature of strained InGaAs quantum disks (200–30 nm diameter) self‐organized on GaAs (311)B substrates

We have recently found that quantum‐box‐like structures are formed during spontaneous reorganization of a sequence of AlGaAs and strained InGaAs epitaxial films grown on GaAs (311)B substrates by metalorganic vapor‐phase epitaxy into InGaAs islands (disks) buried beneath AlGaAs. The size of the disks is directly controlled by the In content in the range 200–30 nm. Strong photoluminescence (PL) efficiency at room temperature is observed in these strained quantum disks. Even for the 30 nm disk the radiative efficiency is not reduced compared to the reference (100) quantum well. The PL spectra are characterized by narrow linewidth and well resolved exciton resonances in excitation spectroscopy.

Improved Response of Uni-Traveling-Carrier Photodiodes by Carrier Injection

We have studied the ultrafast response of uni-traveling-carrier photodiodes with photo-absorption layer doping levels from 2.5 ×1017 to 2.5 ×1018 cm-3. It is found that 3-dB band width increases with the output voltage in the low output region. This enhancement is more prominent for a lower doping level in the photo-absorption layer. From the analysis of the carrier transport in the photo-absorption layer, we attribute the observed enhanced bandwidth as a result of the self-induced electric field associated with carrier injection.

High-power and broadband sub-terahertz wave generation using a J-band photomixer module with rectangular-waveguide output port

A high-output-power photomixer module operating at 200-500 GHz has been developed. The module consists of a uni-traveling-carrier photodiode, a PD-waveguide mode-conversion coupler, and a compact metal package with a rectangular-waveguide that outputs sub-terahertz wave. The fabricated module exhibits a 10- dB down bandwidth of about 300 GHz and an output power as high as -2.7 dBm at 350 GHz. This is the highest value ever reported for a photomixer module operating at 350-GHz band.

InP-Based Planar-Antenna-Integrated Schottky-Barrier Diode for Millimeter- and Sub-Millimeter-Wave Detection

An InP-based Schottky-barrier diode (SBD) is monolithically integrated with a wideband log-periodic toothed antenna for detecting millimeter- and sub-millimeter-waves at frequencies of up to the terahertz (THz) range. A module with a quasi-optical collimation lens fabricated for practical use exhibits sensitivities of 1000 V/W at 300 GHz and 125 V/W at 1.2 THz with good linearity. Near-distance wireless transmission of 5 Gbit/s data with a 240 GHz carrier is also examined using the fabricated SBD module.

Photoresponse characteristics of uni-traveling-carrier photodiodes

Uni-traveling-carrier photodiode (UTC-PD) is a newly developed high-speed photodiode for generating high output current. The high saturation current is realized by its operation mode in which only electrons are used as active carriers. The bandwidth of a UTC-PD increases with increasing photocurrent level, which results from the fact that electron transport changes from diffusive to drift/diffusive motion due to the self-induced field in the absorption layer. In this report, photoresponse characteristics of InP/InGaAs UTC-PDs, which depend on device parameters and operation conditions, are discussed. Theoretical analysis for calculating 3-dB-down bandwidth as a function of operation current is also presented. The charge-control model for photodiodes, used in this study, allows us to predict small-signal bandwidth by numerical calculation.