Redouane Katouf

Quantum Dot Optical Frequency Comb Laser with Mode-Selection Technique for 1-µm Waveband Photonic Transport System

An optical frequency comb was generated from a single quantum dot laser diode (QD-LD) in the 1-µm waveband using an Sb-irradiated InGaAs/GaAs QD active medium. A single-mode-selection technique and interference injection-seeding technique are proposed for selecting the optical mode of a QD optical frequency comb laser (QD-CML). In the 1-µm waveband, a wavelength-tunable single-mode light source and a multiple-wavelength generator of a comb with 100-GHz spacing and ultrafine teeth are successfully demonstrated by applying the optical-mode-selection techniques to the QD-CML. Additionally, by applying the single-mode-selection technique to the QD-CML, a 10-Gbps clear eye opening for multiple-wavelengths in 1-µm waveband photonic transport over a 1.5-km-long holey fiber is obtained.

Ultrafast Response Induced by Interference Effects between Weakly Confined Exciton States

We report an ultrafast response of weakly confined excitons in GaAs thin films observed by a degenerate four-wave-mixing (DFWM) technique. The time-domain signals excited by broadband pulses show an oscillatory structure with a period that varies according to the pulse spectral width and an ultrafast response comparable to the pulse width; on the other hand, the exciton dephasing time is hardly changed by the excitation spectral width. This ultrafast response is much faster than the radiative lifetime expected on the basis of the nonlocal response theory. The DFWM spectra measured at various time delays clearly indicate the contribution of weakly confined excitons to the ultrafast response. Moreover, in the measurement of excitation-energy dependence, the time-domain signals excited by a controlled spectral width exhibit the variation of the oscillation period and response time. These characteristics are similar to those of the excitation spectral width dependence. From the analysis of oscillatory structu...

Ultrahigh Relative Refractive Index Contrast GaAs Nanowire Waveguides

An effective and practical fabrication procedure toward compound semiconductor nanowire waveguide on insulator is developed. GaAs nanowire waveguides with a cross-section of 400×400 nm2, bend radius of 10 µm, and a high relative refractive index contrast, Δ, of 40% were successfully fabricated for the first time. The propagation loss at 1.55 µm was measured to be approximately 4.5 dB/mm, and the bend loss was less than 0.8 dB per 90° bend. The losses were considered to be dominated by light scattering at the rough sidewalls.

1-µm waveband, 10Gbps transmission with a wavelength tunable single-mode selected quantum-dot optical frequency comb laser

1-µm waveband, 10Gbps transmission over 1.5-km single-mode holey fiber is demonstrated with clear eye-openings. A wavelength tunable single-mode selected quantum-dot optical frequency comb laser is utilized as the optical source potentially capable of WDM source.

Low-voltage quantum well microring-enhanced Mach-Zehnder modulator

Modulation characteristics of a novel InGaAs/InAlAs multiple quantum well (MQW) microring-enhanced Mach-Zehnder modulator (MRE-MZM) is investigated in detail and its low-voltage operation with high extinction ratio is demonstrated. The MZM has a single microring resonator in one arm and is driven by the change in electrorefractive index induced by the quantum-confined Stark effect in the MQW core layer. As the MQW, a multiple five-layer asymmetric coupled quantum well (FACQW) is used to obtain a large electrorefractive index change. The driving voltage of the proposed MZM is significantly reduced owing to the enhanced phase shift in the microring resonator. High-mesa waveguide structures are grown by solid-source molecular beam epitaxy and fabricated by inductively coupled plasma etching. A directional coupler with an asymmetric branching ratio is used as an input coupler to prevent the degradation of the extinction ratio of the MZM. The extinction ratio of the fabricated MRE-MZM is approximately 27 dB. The product of the half-wave voltage and phase shifter length, V(π) · L, is 1.7 Vmm in static modulation. This value is one-quarter that of a conventional MZM with the same waveguide structure.

1-μm- band transmission by use of a wavelength tunable quantum-dot laser over a hole-assisted fiber

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.

Thermo-optically driven silicon microring-resonator-loaded Mach-Zehnder modulator for low-power consumption and multiple-wavelength modulation

Low-power-consumption thermo-optically controlled silicon-microring-resonator loaded Mach–Zehnder modulators (MRR-loaded MZMs) are demonstrated. We experimentally characterized a single microring and cascaded-multiple-microring resonators coupled to one arm of a Mach–Zehnder interferometer (MZI). The driving power consumption of the proposed MZM is significantly reduced owing to the enhanced phase shift in the MRR. The device was fabricated on a silicon-on-insulator (SOI) waveguide structure, and each microring is equipped with TiN microheater for thermo-optic tuning. The coupling efficiency between the microring and a busline waveguide was regulated by varying the gap between two waveguides at a directional coupler. The power consumption of single microring and cascaded MRR-loaded MZMs was approximately 0.4 and 1 mW, respectively. The phase-shift enhancement factor of up to 19 with a maximum extinction ratio of 18 dB was obtained experimentally. Multiple-wavelength operation was also demonstrated in the cascaded MRR-loaded MZM.

Hitless wavelength-selective switch with quadruple series-coupled microring resonators using multiple-quantum-well waveguides

We demonstrate a hitless wavelength-selective switch (WSS) based on InGaAs/InAlAs five-layer asymmetric coupled quantum well (FACQW) quadruple series-coupled microring resonators. The WSS is driven by the electric-field-induced change in refractive index in the FACQW core layer caused by the quantum-confined Stark effect (QCSE) for high-speed operation. The WSS with high-mesa waveguides is fabricated on a molecular beam epitaxy-grown wafer by dry etching. The fabricated WSS consists of four microrings, each with a round-trip length of 350 μm and five directional couplers with shallow grooves. A boxlike spectral response and hitless switching with higher extinction ratios than a double series-coupled microring resonator are successfully demonstrated. In addition, we propose the improvement of switching characteristics by controlling the coupling efficiencies at the directional couplers.

All-optical flip-flop and inverter using adjacent lasing wavelengths by semiconductor microring laser

We demonstrated an all-optical flip-flop and inverter using two adjacent lasing wavelengths (FSR = 1.7 nm) from an InGaAs/InGaAsP multiple quantum well (MQW) microring laser. The main lasing wavelength (1567.54 nm) and both side adjacent lasing wavelengths (1565.84 and 1569.24 nm) were used as the injection light wavelength. These operations were realized in the same device with different injected currents. The injected light power and detuning dependence of the inverter operation were clarified.

1-µm waveband, 12.5-Gbps transmission with a wavelength tunable single-mode selected quantum-dot optical frequency comb laser

1-µm waveband, 12.5-Gbps transmission over a 1.5-km single-mode holey-fiber is demonstrated with clear eye-openings. A wavelength tunable single-mode selected quantum-dot optical frequency-comb laser is used as the optical-source potentially capable of wavelength division multiplexing (WDM).