Isao Morohashi

20-Gb/s QPSK W-band (75-110GHz) wireless link in free space using radio-over-fiber technique

We demonstrate 20-Gb/s W-band wireless transmission in free space with a distance of 30mm using optical signal generation. Optically synthesized QPSK signal and direct optical upconversion technique ease generation of W-band RoF signals for dual purpose of wireline and wireless transmission link. A W-band radio receiver with W-band frequency downconversion and digital signal processing will be applicable for any W-band radio detection.

Broadband wavelength-tunable ultrashort pulse source using a Mach-Zehnder modulator and dispersion-flattened dispersion-decreasing fiber

The broadband wavelength tunability of femtosecond pulse generation using a Mach-Zehnder-modulator-based flat-comb generator (MZ-FCG) and a dispersion-flattened dispersion-decreasing fiber (DF-DDF) was demonstrated. Near-Fourier-transform-limit picosecond pulses generated from the MZ-FCG were compressed into femtosecond pulses by adiabatic soliton compression. By tuning the wavelength of the input cw light, 200 fs, 10 GHz pulses were generated in the wavelength range of 1,535 to 1,570 nm. Such wide-range wavelength tunability was realized by both the independence of a comb-flattening condition from the inputted wavelength and the dispersion flatness of the DF-DDF.

Widely repetition-tunable 200 fs pulse source using a Mach–Zehnder-modulator-based flat comb generator and dispersion-flattened dispersion-decreasing fiber

By combining a Mach-Zehnder-modulator-based flat comb generator (MZ-FCG) with a dispersion-flattened dispersion-decreasing fiber, femtosecond pulses have been generated from a cw light. Near-Fourier-transform-limit picosecond pulses generated from the MZ-FCG were compressed into femtosecond order by pulse compression. Our system enables flexible tuning of the repetition rate and pulse width, because those depend on the driving signal of the MZ-FCG. Pulse trains of 200 fs width were continuously and stably generated without mode hopping, with a repetition rate range from 5 to 17 GHz. Our system consists of a modulator and compression fiber; thus, the configuration is simpler and more stable.

16-Gbaud QPSK radio transmission using optical frequency comb with recirculating frequency shifter for 300-GHz RoF signal

We demonstrate 300-GHz-band radio transmission with a digital coherent detection technique. Quadrature-phaseshift-keying modulation can provide a data rate of 29.9 Gb/s, with 7% forward error correction overhead. A frequency-locked optical frequency comb for a 300-GHz radio-over-fiber signal can be generated by a recirculating frequency shifter with a single sideband suppressed-carrier modulator in an amplified optical fiber loop.

Efficient generation of twin photons at telecom wavelengths with 2.5 GHz repetition-rate-tunable comb laser

We demonstrate a down-conversion-based twin photon source pumped by a 10 GHz repetition-rate-tunable comb laser at 1553 nm wavelength. We show high Hong-Ou-Mandel interference visibilities, which are free from the pump-power induced degradation.

Observation of interdot correlation in single pair of electromagnetically coupled quantum dots

The interdot correlation in a single pair of InAs∕GaAs barrier-coupled quantum dots (QDs) is investigated by microphotoluminescence spectroscopy, in which each QD is individually excited at unique energy levels. Surprisingly, we observe an anomalous increase in the luminescence intensity when the two QDs are excited simultaneously. This remarkable finding can be interpreted in terms of the electromagnetic coupling between QDs with thick barrier layers.

Precise Evaluation of a Phase-Locked THz Quantum Cascade Laser

We have demonstrated the phase-locking of a THz quantum cascade laser (THz-QCL) toward the realization of an accurate and stable local oscillator for a high-resolution receiver. The beat note between the THz-QCL and a THz reference was obtained by heterodyne mixing in a superconducting hot electron bolometer mixer (HEBM) and used for stabilizing the phase of 3.1 THz radiation from the THz-QCL. The phase-locked 3.1 THz radiation was fully evaluated with a superlattice harmonic mixer operating in the THz band, and this revealed that the THz-QCL synchronized with the microwave reference with a fractional frequency instability of 3×10-15 at an averaging time of 100 s, corresponding to a center frequency deviation within 1 mHz, and the imposed phase noises during the heterodyne mixing were negligibly small.

Electronic structures in single pair of InAs∕GaAs coupled quantum dots with various interdot spacings

The electronic structures in a single pair of InAs∕GaAs coupled quantum dots (CQDs) with various interdot spacings are investigated by performing photoluminescence (PL) and photoluminescence excitation (PLE) measurements. Luminescence from the bonding (X+) and antibonding (X−) states caused by electron-wave-function coupling was observed in the micro-PL spectra of the CQDs. We indicate the contribution of the hole excited states to the PL spectra in QDs based on the results for the spectral dependence on circularly polarized light and the structures of PLE spectra. PLE spectra reveal the electronic structures of the CQD system at higher energy states where both the common excited levels due to the level sharing between the electron excited states and the individual excited levels related to the hole excited states coexist. In addition, we mention that the energy-level mixing due to the strong-wave-function coupling between two QDs influences the decoherence of the carrier relaxation processes.

Observation of exciton molecule consisting of two different excitons in coupled quantum dots

We propose and experimentally investigate an exciton molecule consisting of two different excitons in coupled quantum dots (QDs). Quantum mechanical coupling between double QDs leads to the creation of bonding and antibonding states and should yield an exciton molecule consisting of two excitons that originate from these two states. We prepared a quantum mechanically coupled QD system and succeeded in observing a single exciton molecule in a single pair of coupled QDs by means of a two-color excitation photoluminescence measurement.

Coherent MMW/terahertz signal transmission with frequency-reconfigurable RoF transmitter based on an optical frequency comb

A frequency-reconfigurable radio-over-fiber transmitter is designed and demonstrated with an optical frequency comb source for millimeter-wave and terahertz signal transmission over a fiber-wireless hybrid link. Optical-to-radio and radio-to-optical converters with a high-speed photomixer and frequency-heterodyne technique successfully perform coherent quadrature-phase-shift-keying signal generation and reception in the optical domain at 90 GHz and 300 GHz. A digital radio receiver also receives and demodulates the transmitted QPSK signal at 300 GHz with the same bit error rate behavior as in an optical link.