Nobuhide Yokota

1.55-μm VCSEL with polarization-independent HCG mirror on SOI

We fabricated a VCSEL incorporating a polarization-independent HCG mirror on SOI for a novel polarization-bistable functional device. The VCSEL oscillated under an optical short pulse excitation at 300 K.

Harmonic superposition for tailored optical frequency comb generation by a Mach–Zehnder modulator

This Letter demonstrates tailored optical frequency comb (OFC) generation using a LiNbO3 Mach-Zehnder modulator driven by a combination of first- and second-order harmonics of the RF signal. A quasi-rectangular-shaped OFC with less than 1 dB flatness among 11 lines was experimentally obtained when a slight second-order harmonic of the RF signal (0.1 times the half-wavelength voltage) was introduced. Good agreement was obtained between the measured and calculation results for OFCs. We discuss conditions to obtain flat OFCs using this method along with details concerning OFC conversion efficiency and bandwidth.

Optical Negative Feedback for Linewidth Reduction of Semiconductor Lasers

We propose a coherent optical negative feedback method for reducing linewidth of single-mode semiconductor lasers, which requires only an optical filter. Results of numerical analysis indicate that the power spectral density of FM noise can be reduced by more than 40 dB and the subkilohertz spectral linewidth can be obtained with our method when the spectral linewidth is 10 MHz under a free running condition. From a proof-of-concept experiment, we confirmed that the spectral linewidth of a single-mode semiconductor laser is reduced when using our method from 6.4 MHz to 6.5 kHz. This result confirms that the spectral linewidth can be reduced by at least 30 dB, which proves the validity of our method.

Experimental demonstration of linewidth reduction of laser diode by compact coherent optical negative feedback system

We demonstrate the linewidth reduction of a laser diode from 6.4 MHz to 6.5 kHz using a compact coherent optical negative feedback system, which consists of only a lens and an optical filter. The FM noise power spectral density (PSD) and the relative intensity noise (RIN) of the laser output are measured to discuss the performance of the system. The FM noise PSD is reduced by ∼30 dB at <100 MHz. The RIN of ∼130 dB/Hz is not found to be increased by the feedback at <7 GHz, where the system is driven under negative feedback conditions.

Nonlinearity of Semiconductor Mach-Zehnder Modulator for Flat Optical Frequency Comb

Optical frequency combs (OFCs) generated by an InP-based semiconductor Mach-Zehnder modulator (MZM) are numerically investigated. Nonlinear phase modulation induced by drive voltages applied to the MZM prove to be useful in obtaining a flat OFC. The experimentally measured intensity deviations of OFCs reported previously are quantitatively reproduced using a numerical model considering nonlinear change of refractive index and optical absorption induced by the drive voltages applied to the MZM. It is found that such nonlinearities generate a flat nine-channel (9-ch) OFC with an intensity deviation of <;1 dB when a moderate RF drive voltage of <;5 V is used. This flat 9-ch OFC is sustained regardless of the ratio of RF drive voltages of two arms in the MZM. Conversion efficiency is reasonably high (27%), but it is decreased by optical absorption and extinction losses in the MZM.

Carrier Lifetime and Electron Spin Relaxation Time in (110)-Oriented GaAs–AlGaAs Quantum-Well Micro-Posts

Carrier lifetime and electron spin relaxation time in (110)-oriented GaAs-AlGaAs quantum-well (QW) micro-posts are investigated by polarization- and time-resolved photoluminescence measurements. The long electron spin relaxation time in QWs on GaAs (110) is found to be preserved even when the sidewall boundaries with fast surface recombination are introduced and the carrier lifetime is drastically shortened. Rate equation analysis shows that spin-controlled vertical-cavity surface-emitting lasers with such (110)-QW micro-posts will exhibit faster switching of lasing circular polarizations than that we previously demonstrated. In particular, 20-GHz switching is expected with 0.5-μm posts.

Ultrahigh-speed operation of laser diode by cross-gain modulation using external cavity

An optically controlled laser diode with an intensity- and phase-controllable external cavity was studied in order to achieve an increased bandwidth. The external cavity functions as the feedback section for the laser diode, using time-delayed laser light. A 3-dB bandwidth of over 70 GHz was numerically predicted, using rate equations that include cross-gain modulation and multiple optical feedback from the external cavity. A 3-dB bandwidth of more than 40 GHz was experimentally achieved with a fabricated device.

Room temperature spin injection into (110) GaAs quantum wells using Fe/x-AlOx contacts in the regime of current density comparable to laser oscillation

We investigate the electrical spin injection into (110) GaAs single quantum wells (SQWs) and multiple quantum wells (MQWs) using light-emitting diodes (LEDs) having Fe/crystalline-AlOx (x-AlOx) tunnel barrier contacts. A degree of circular polarization (Pc) of 5.0% is obtained for the SQW LED at 4 K with the current density of 1 kA/cm2 which is comparable to that for the laser oscillation in vertical-cavity surface-emitting lasers (VCSELs). On the basis of electron spin relaxation time and carrier lifetime in the (110) GaAs SQW measured by time-dependent photoluminescence and the value of Pc = 5.0%, the degree of spin polarization of initially injected electrons (P0) in the SQW is estimated to be 6.6% at 4 K. By using the MQW LED having a much stronger electroluminescence, a Pc value of 2.6% is obtained at room temperature (RT) with the current density of 1.5 kA/cm2. The temperature and current density dependences of Pc are found to be weak in both the SQW and MQW LEDs. The estimated P0 of 9.3% at RT sugg...

Intra-cavity loss modulation for ultrahigh-speed direct modulation lasers based on photon–photon resonance

The characteristics of the intra-cavity loss modulation laser diode (ICLM-LD) proposed in this paper were numerically and experimentally evaluated. A lower modulation sensitivity degradation rate (MSDR) of −0.30 dB/GHz and a wider 3 dB bandwidth of 49 GHz compared to those of direct modulation lasers (−0.49 dB/Hz, 16 GHz) were numerically confirmed. In addition, an MSDR of −0.29 dB/GHz and a 3 dB bandwidth wider than 30 GHz were experimentally confirmed using the fabricated ICLM-LD. The ICLM-LD will thus become a key part of ultrahigh-speed direct modulation lasers with external cavities whose modulation sensitivity is enhanced by the photon–photon resonance effect.

Room temperature spin transport in undoped (110) GaAs/AlGaAs quantum wells

We are reporting on our first observation of a micrometer-order electron spin transport in a (110) GaAs/AlGaAs multiple quantum well (QW) at room temperature using a space- and time-resolved Kerr rotation technique. A 37-μm transport was observed within an electron spin lifetime of 1.2 ns at room temperature when using an in-plane electric field of 1.75 kV/cm. The spatio-temporal profiles of electron spins were well reproduced by the spin drift-diffusion equations coupled with the Poisson equation, supporting the validity of the measurement. The results suggest that (110) QWs are useful as a spin transport layer for semiconductor spintronic devices operating at room temperature.