Y. Sugimoto

Room‐temperature lasing operation of a quantum‐dot vertical‐cavity surface‐emitting laser

Self‐assembled growth of quantum dots by molecular‐beam epitaxy is used to form the active region of a vertical‐cavity surface‐emitting laser (VCSEL). Ten layers of InGaAs quantum dots are stacked in order to increase the gain. This quantum‐dot VCSEL has a continuous‐wave operating current of 32 mA at room temperature. Emission spectra at various current injections demonstrate that the lasing action is associated with a higher‐order transition in the quantum dots.

EXPERIMENTAL INVESTIGATION ON HIGH-SPEED SWITCHING CHARACTERISTICS OF A NOVEL SYMMETRIC MACH-ZEHNDER ALL-OPTICAL SWITCH

We report the experimental results of a novel symmetric Mach–Zehnder (SMZ) all‐optical switch. Although the switching is based on the band‐filling effect in the semiconductor waveguide, the switching speed is not restricted by the slow relaxation time. We obtain fully switched, nearly square modulation, which is characteristic of the SMZ all‐optical switch. We also achieve high‐speed switching; the full width at half‐maximum of the obtained wave form is 8 ps, which is only limited by the time resolution of the detection system.

Ultrafast polarization‐discriminating Mach‐Zehnder all‐optical switch

We propose a polarization‐discriminating Mach‐Zehnder (PDMZ) all‐optical switch. The switching speed of this PDMZ all‐optical switch is not limited by the slow relaxation time of highly efficient incoherent nonlinearities. We demonstrate a squarelike modulation characteristic, which is necessary in most switching applications, at a switching speed (on‐off time) of 40 ps. We also demonstrate ultrafast switching at a detector limited speed of ∼8 ps.

Complete polarization control of 8×8 vertical‐cavity surface‐emitting laser matrix arrays

Polarization of 8×8 vertical cavity surface emitting laser (VCSEL) arrays is completely controlled. These index‐guided VCSELs have a 6×5 μm rectangular poststructure consisting of DBR mirrors. All 64 VCSELs emit fundamental single‐mode and linearly polarized light with a polarization angle deviation of only 2.9°. Their light output characteristics are almost the same as those of conventional 6×6 μm polarization‐uncontrolled VCSELs.

CONTROLLING POLARIZATION OF QUANTUM-DOT SURFACE-EMITTING LASERS BY USING STRUCTURALLY ANISOTROPIC SELF-ASSEMBLED QUANTUM DOTS

Polarization control is achieved in vertical-cavity surface-emitting lasers (VCSELs) by using structurally anisotropic self-assembled quantum dots (QDs) as active layers. The dot shape is long in the [011] direction on the (100) surface. The photoluminescence from the dots has a polarization dependence whose intensity along the [011] direction is 1.37 times stronger than that along the orthogonal [011] direction. The QD VCSEL operates at the wavelength of the QD ground state transition. Lasing emission shows polarization along the [011] direction and an orthogonal polarization suppression ratio of 18 dB.

Temperature dependence of impact ionization coefficients in InP

Impact ionization coefficients for electrons and holes in InP were measured experimentally at 25–175 °C in the 400–600 kV/cm electric field range with planar avalanche photodiodes, in which the n‐InP avalanche region was separated from the light absorbing InGaAs and/or InGaAsP layers. α and β monotonically decreased with elevated temperatures; β/α slightly decreased with increasing temperature. Comparison of the experimental results with Okuto–Crowell formula on the impact ionization coefficient gave the phonon energy ERO=46 meV and the phonon scattering mean free path λ0=41.7 A for electron impact ionization and ERO=36 meV and λ0=41.3 A for hole impact ionization, respectively. Curves calculated by using these parameters agree with the experimental results quite satisfactorily at each temperature.

Design, fabrication, and characterization of coupling-strength-controlled directional coupler based on two-dimensional photonic-crystal slab waveguides

A directional coupler (DC) with a coupling-strength control defect (CCD) based on a two-dimensional photonic-crystal slab waveguide is numerically and experimentally studied. The structure consists of two parallel-coupled single-defect waveguides separated by a triple-line air-hole array, with some missing air holes in the middle of the triple line. Using a two-dimensional finite-difference time-domain method, a 50% coupler was designed and almost 50% coupling performance was experimentally confirmed at a wavelength of ∼1.3 μm with a coupling length of about 7 μm, about 1/3 in length as compared to the DC without the CCD.

A two-dimensional optical parallel transmission using a vertical-cavity surface-emitting laser array module and an image fiber

The possibility of high-density optical parallel transmission using an arrayed vertical-cavity surface-emitting laser (VCSEL) and an image fiber was demonstrated. Thirty six (6/spl times/6) laser lights from a VCSEL-array module were transmitted through an image fiber with a 460-/spl mu/m diameter and consisting of 10,000 super-thin micro cores. The optical crosstalk was measured spatially and dynamically, and was less than -27 dB from dc to 3 GHz when a 1-m-long image fiber was used. We have actually transmitted bit data at a rate of 1 Gb/s, and the bit-error rate became 1/spl times/10/sup -8/ at the average received power of -27.7 dBm.

Polarization-controlled single-mode VCSEL

Relative intensity noise (RIN) in a vertical-cavity surface-emitting laser (VCSEL) was greatly reduced through the use of polarization control to eliminate competition between two orthogonal polarization states by ensuring there was only one polarization state. Polarization was stable with optical feedback of up to 10%. Polarization control was achieved by inducing a small loss anisotropy in fundamental transversal mode VCSELs. Anisotropic post structures, such as a rectangular post, an oblique post, or a zigzag-sidewall post, were found to be effective in creating loss anisotropy without serious degradation of other VCSEL characteristics such as light-output power or beam profile.

Gigabit-rate optical-signal transmission using vertical-cavity surface-emitting lasers with large-core plastic-cladding fibers

We have demonstrated multi-gigabit-rate large-signal modulation of vertical-cavity bottom-surface-emitting lasers (VCSELs) mounted on a high-speed carrier package that can individually operate 64 VCSELs. Current-modulation efficiency of 5.0 GHz//spl radic/(mA) and 3-dB modulation bandwidth of up to 7.0 GHz have been obtained for VCSELs that include flip-chip bonding pads and packaging. We have also demonstrated easily aligned optical signal transmission using our VCSELs with large-core graded-index plastic cladding fibers (GI-PCFs) at bit-rates of 3 Gb/s for 100 m, 2 Gb/s for 500 m, and 1.5 Gb/s for 1 km.<<ETX>>