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Compact lasers capable of producing kilowatt class peak power are highly desirable for applications in various fields, including laser remote sensing, laser micromachining, and biomedical photonics. In this paper, we propose a high-peak-power chip-scale semiconductor/solid-state vertically integrated laser in which two cavities are optically coupled at the solid-state laser gain medium. The first cavity is for the intra-pumping of ytterbium-doped yttrium aluminum garnet (Yb:YAG) with an electrically driven indium gallium arsenide (InGaAs) quantum well, and the second cavity consists of Yb:YAG and chromium-doped yttrium aluminum garnet (Cr:YAG) for passive Q-switching. The proposed laser produces pulses as short as 450 ps, and an estimated peak power of 57.0 kW with a laser chip dimension of 1 mm3. To the best of our knowledge, this is the first monolithic integration of semiconductor and solid-state laser gain mediums to realize a compact high-peak-power laser.

In post-stroke patients, muscle synergy (the coordination of motor modules during walking) is impaired. In some patients, the muscle synergy termed module 1 (hip/knee extensors) is merged with module 2 (ankle plantar flexors), and in other cases, module 1 is merged with module 4 (knee flexors). However, post-stroke individuals with a merging pattern of module 3 (hip flexor and ankle dorsiflexor) and module 4, which is the swing-muscle synergy, have not been reported. This study aimed to determine the muscle-synergy merging subtypes of post-stroke during comfortable walking speed (cws). We also examined the effect of experimental lower-limb angle modulation on the muscle synergy patterns of walking in each subtype. Forty-one participants were assessed under three conditions: cws, long stepping on the paretic side (p-long), and long stepping on the non-paretic side (np-long). Lower-limb flexion and extension angles and the electromyogram were measured during walking. Subtype classification was based on the merging pattern of the muscle synergies, and we examined the effect of different lower-limb angles on the muscle synergies. We identified three merging subtypes: module 1 with module 2 (subtype 1), module 1 with module 4 (subtype 2), and module 3 with module 4 (subtype 3). In the cws condition, the lower-limb flexion angle was reduced in subtype 3, and the lower-limb extension angle was decreased in subtype 1. A more complex muscle synergy was observed only in subtype 3 in the p-long condition versus cws (p = 0.036). This subtype classification of walking impairments based on the merging pattern of the muscle synergies could be useful for the selection of a rehabilitation strategy according to the individual’s particular neurological condition. Rehabilitation with increased lower-limb flexion may be effective for the training of patients with merging of modules 3 and 4 in comfortable walking.

A chip-sized passively Q-switched laser is demonstrated, in which a solid-state gain medium is intra-cavity pumped and vertically integrated with a semiconductor laser. A laser chip volume of 1 mm3 and peak power of 50 kW are achieved simultaneously.

To examine the relationship between temporal asymmetry and complexity of muscle synergy during walking using rhythmic auditory cueing (RAC) and the factors related to changes in muscle synergy during walking with RAC in survivors of stroke.Cross-sectional study.Wards at 2 medical corporation hospitals.Forty survivors of stroke (N=40; mean age, 70.4±10.3 years; time since stroke, 72.2±32.3 days) who could walk without physical assistance.Not applicable.The participants were assessed in a random block design under 2 conditions: comfortable walking speed (CWS) and walking with RAC. Single-leg support time, kinematics, and electromyograms were measured. Factors related to the complexity of muscle synergy (variance accounted for by 1 synergy [VAF1]) between the walking conditions were examined using hierarchical multiple regression analysis.In the RAC condition, lower limb flexion and knee flexion angles, single-leg support time on the paretic side, and the symmetry index of single-leg support time were increased compared with those in the CWS condition. VAF1 was decreased in the RAC condition (73.9±0.15) compared with that in the CWS condition (76.9±0.13,The RAC condition demonstrated a more complex representation of muscle synergy than the CWS condition; the change in single-leg support time on the paretic side related to the changes in muscle synergy more than changes in lower limb angle. These findings can help in the walking-training concept to improve muscle synergy deficits in survivors of stroke.

A compact tunable laser module integrating a newly developed DBR/Ring laser and an extremely-high- Δ PLC wavelength locker is demonstrated with narrow spectral linewidth of

We investigated high-power pulse semiconductor lasers based on epitaxially stacked double active layer through tunnel junction composed of MOCVD grown highly carbon-doped GaInAs on InP substrate. The peak pulse optical power was 50 W at a current of 65 A.

Carrier injection rate in tunnel injection quantum well structures used in semiconductor lasers was investigated through theoretical analysis. Carrier capture time from 3D-state to 2D-state in a conventional quantum well was several ten picoseconds and it is almost uncontrollable characteristics. The tunnel injection structure can be designed to control the carrier injection rate. The carrier transition time from carrier reservoir-region of the tunnel injection structure to the active well was a few picoseconds when the structure was designed for high speed transition. As results, carrier injection to ground state of the active well takes less than 10 picoseconds. The change of the radiative recombination rate in the active well also defines the optimal design of the tunnel injection structure. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

A quantum well design for controlling of the carrier dynamics was investigated for high speed direct modulation semiconductor lasers. The proposed Well-in-Well structure decreases the carrier relaxation time into an active well by designing the wave function which relates to the LO phonon scattering rate. By analyzing the scattering rate of each energy state, the electron relaxation time was estimated to be a few picoseconds which is 2–10 times smaller than that of a conventional quantum well structure. The result is desirable for improving the modulation damping effect which limits the modulation band width. f-3dB is expected above 50GHz. The effect of an asymmetrical Well-in-Well structure was also discussed.

We provide numerically simulated dynamic properties of a SOA with a tunnel-injection structure considering tunneling current and biased potential. As result, the tunneling injection SOA has potential of 40 Gbit/s or more high-speed operation.

Carrier relaxation in tunnel-injection quantum structures are analyzed using thousands-sets of rate equations of each quantum state. The results suggest that modulation-bandwidth of TI-QW lasers is broaden by suppression of lack of carriers.

We propose a SOA with a tunnel injection structure and analyze the operation properties. High-speed carrier recovery was suggested in the tunnel-injection SOA. The results indicate tunnel-injection SOAs have a potential of high-speed operation.

A quantum well design for controlling of the carrier relaxation process and reducing of the gain damping effect was investigated for improvement of high speed direct modulation semiconductor lasers. The proposed Well-in-Well structure decreases the carrier relaxation time into an active well by carefully designed wave function which relates to the LO phonon scattering rate. The decrease of relaxation time and the increase of modulation bandwidth was analyzed numerically. The reduction of damping factor γ was also demonstrated by experimental measurement of the relative intensity noise spectrum.

A quantum-well design for controlling carrier dynamics was investigated for high-speed direct-modulation semiconductor lasers. The proposed well-in-well (WWell) structure decreases the carrier relaxation time into an active well by designing the wave function that is related to longitudinal optical phonon scattering rate. The electron relaxation time was analyzed numerically and was estimated to become 1/2–1/10 times smaller than that of a conventional quantum-well structure. This reduction of the relaxation time is effective for reducing the optical gain damping. The advantage of an asymmetrical WWell structure is also shown.

A large kink in the light output versus current characteristics was observed in semiconductor lasers with a tunnel injection quantum well structure. The output power was changed from 13 to 1.9 mW under a small current increase of 0.06I th (30 mA). The peak-to-valley (P/V) ratio of the output power was as high as 6.8. The kink current and the P/V ratio changed greatly within a monolayer thickness variation of the tunnel injection well. The kink mechanism that can be assumed was presented on the basis of the threshold current and the threshold voltage characteristics of the experimental results.