Masayuki Fujita

Ultrasmall and ultralow threshold GaInAsP-InP microdisk injection lasers: design, fabrication, lasing characteristics, and spontaneous emission factor

We have calculated lasing characteristics of current injection microdisk lasers of several microns in diameter, taking account of the scattering loss at center posts and the carrier diffusion effect. We found that the optimum width of the disk wing exposed to the air is 0.6-0.7 /spl mu/m and the minimum threshold current is nearly 10 /spl mu/A for the disk diameter of 2 /spl mu/m. The internal differential quantum efficiency can be 95% if the transparent carrier density is reduced to 7.5/spl times/10/sup 17/ cm/sup -3/ and the diffusion constant is increased to 8 cm/sup 2//s. In the experiment, we have obtained the room temperature continuous-wave operation of a GaInAsP-InP device of 3 /spl mu/m in diameter, for the first time, with a record low threshold of 150 /spl mu/A. This achievement was mainly owing to the reduction of the scattering loss at the disk edge, and hence the reduction of the threshold current density. The spontaneous emission factor was estimated to be 6/spl times/10/sup -3/. This value was much reduced by the large detuning of the lasing wavelength against the spontaneous emission peak. A larger value over 0.1, which is expected for such a small device, will be obtained by the wavelength tuning and the narrowing of the spontaneous emission spectrum.

Proposal and finite-difference time-domain simulation of whispering gallery mode microgear cavity

The existence of many high Q whispering gallery modes in microdisk and microcylinder lasers seriously affects the internal efficiency in lasing operation and disturbs the enhancement of the spontaneous emission factor. To suppress these modes except for one lasing mode, we propose the microgear cavity having a grating with the same period as that of the mode standing wave. A finite-difference time-domain simulation theoretically demonstrates that the microgear selects one resonant mode that satisfies the unique condition regarding the mode order and the phase. This paper describes the dependence of the mode behavior on some structural parameters, and concludes that a deep grating, which seems to be possible in an experiment, allows for sufficient suppression of any types of nonlasing modes and the enhancement of the Q factor of the lasing mode.

Low-Threshold Continuous-Wave Lasing in Photopumped GaInAsP Microdisk Lasers

We demonstrated continuous-wave lasing at room temperature in a photopumped GaInAsP microdisk laser fabricated by Cl2/Xe inductively coupled plasma etching. The minimum threshold pump power was as low as 30 µW. This value is 0.7 times the lowest threshold in the current injection device due to the uniform carrier distribution by photopumping. Higher thermal resistance and odd-order azimuthal mode lasing as a result of a narrower pedestal and no upper post structure were observed.

Strain relaxation effect in microdisk lasers with compressively strained quantum wells

From lasing wavelengths observed for many samples of microdisk lasers with GaInAsP compressively strained quantum wells (CS-QWs), we estimate the gain peak redshifted by ∼10 meV. We explain this phenomenon as the strain relaxation in the CS-QWs at the disk wing exposed to the air. Band and rate equation analyses show that the built-in potential by strain relaxation accelerates the carrier diffusion toward the disk edge and reduces the threshold to 30%–60% of that without strain relaxation. This result indicates the advantage of CS-QWs not only for the microdisk laser but also for various microlasers with a membrane structure, e.g., photonic crystal slab lasers.

A novel GaInAsP microcylinder laser with AlInAs(O/sub x/) claddings

We have proposed and demonstrated a novel 1.5-/spl mu/m-GaInAsP microcylinder laser with low refractive index InAlAs(O/sub x/) claddings, which allows the strong optical confinement as in a microdisk. The effective threshold current density was 25% lower than that for the previous 0.98-/spl mu/m-GaInAs device. The selective oxidation of the claddings is expected to further reduce the threshold and extend the feasibility of this type of device in large scale photonic circuits.

Raman spectroscopy measurements of CO2 dissolved in water and CO2 bubbles for laser remote sensing in water

We examined the applicability of Raman spectroscopy as a laser remote sensing tool for monitoring the gases dissolved in water and gas bubbles. A frequency doubled Q-switched Nd:YAG laser (532 nm) is irradiated to CO2 gas bubbles generated by an air pump. The Raman signals at 1383 cm-1 from CO2 and 1645 cm-1 from water were detected. It has been shown that the CO2/H2O Raman signal ratio is dependent on the CO2 bubble rate.

Dual beam laser grooving of CFRP by pulsed lasers

We have been investigating micromachining of CFRP by short laser pulses in order to minimize material damages and HAZ. The laser system used in this study was a Ti:Sapphire laser with pulsewidth of 100fs. For comparison, we also used 200ps stretched pulses from the Ti:Sapphire laser and output from a 200ns-pulsed fiber laser. In order to overcome slow processing speed by ultra-short pulses, we have proposed a novel dual beam grooving. In the first process, small-energy ultra-short pulses are used to cut the edges of the groove with multiple scanning. Secondly, high-energy long-pulses ablate the remained part with a single scanning. We used 1.3mm-thick uni-directional CFRP samples. Trenches with depth of 100µm and width of 50µm were machined by 200ps laser pulses. Then, 200ns fiber laser pulses were focused on one of the remained part in a single scanning. As a result, a groove with depth of 100u2005µm and width of 150u2005µm was formed without significant HAZ.We have been investigating micromachining of CFRP by short laser pulses in order to minimize material damages and HAZ. The laser system used in this study was a Ti:Sapphire laser with pulsewidth of 100fs. For comparison, we also used 200ps stretched pulses from the Ti:Sapphire laser and output from a 200ns-pulsed fiber laser. In order to overcome slow processing speed by ultra-short pulses, we have proposed a novel dual beam grooving. In the first process, small-energy ultra-short pulses are used to cut the edges of the groove with multiple scanning. Secondly, high-energy long-pulses ablate the remained part with a single scanning. We used 1.3mm-thick uni-directional CFRP samples. Trenches with depth of 100µm and width of 50µm were machined by 200ps laser pulses. Then, 200ns fiber laser pulses were focused on one of the remained part in a single scanning. As a result, a groove with depth of 100u2005µm and width of 150u2005µm was formed without significant HAZ.

Temperature dependence of laser-induced damage thresholds by short pulse laser

Temperature dependence of laser-induced damage thresholds were measured by Nd:YAG laser (1064-nm wavelength, 4- ns pulse width) and Ti:Sapphire laser (800-nm wavelength, 100-fs, 2-ps, and 200-ps pulse widths) to elucidate the effects of laser-induced damage mechanisms. As experimental samples, SiO2, MgF2, Al2O3, HfO2, ZrO2, and Ta2O5 were prepared by electron evaporation. With longer pulses than few picoseconds, laser-induced damage thresholds were increased with decreasing temperature. Temperature dependence was reversed for shorter pulses than a few picoseconds. The effects of temperature at different pulse width to laser-induced damage mechanisms were considered with separated processes. In the conclusions, a temperature effect to free-electron generations by photoionization and multi photon ionization is negligible. However, the temperature affects to electron multiple (electron avalanche) and critical density. Electron multiple decreased at low temperature and the laser-induced damage thresholds increased. On the other hand, critical density decreased at low temperature and the laser damage thresholds decreased. Influence of electron avalanche is much greater than the impact of critical density. Thus, the trend and the strength of the temperature dependence on laser-induced damage threshold will be decided by electron avalanche.

Laser micromachining of CFRP by ultra-short pulse lasers

CFRP (Carbon Fiber-Reinforced Plastic) has attractive features like high-durability, high-strength and light-weight. We have been investigating micromachining of CFRP by ultra-short laser pulses in order to minimize material damages and HAZ. The laser system used in this study was a commercially available Ti:Sapphire laser (Spectra Physics, Hurricane). Various samples of CFRP were supplied by Japanese leading manufacturing companies. Optimizations of the laser parameters were investigated.CFRP (Carbon Fiber-Reinforced Plastic) has attractive features like high-durability, high-strength and light-weight. We have been investigating micromachining of CFRP by ultra-short laser pulses in order to minimize material damages and HAZ. The laser system used in this study was a commercially available Ti:Sapphire laser (Spectra Physics, Hurricane). Various samples of CFRP were supplied by Japanese leading manufacturing companies. Optimizations of the laser parameters were investigated.

Large spontaneous emission factor confirmed in CW lasing microdisk injection lasers

Microdisk lasers are suitable for spontaneous emission control due to a high Q lasing mode with an ultra-small volume. However, the clear evaluation of spontaneous factor C is difficult for pulsed and/or photo-pumped devices due to the necessity of a wide range fitting of light-current characteristic from far below threshold. Recently, we have achieved CW lasing in GaInAsP devices with the record low threshold of 40 /spl mu/A. In this study, we fabricated 1.56 /spl mu/m GaInAsP compressively-strained QW microdisk injection lasers and observed some clear evidence of large C factors in the lasing characteristics.