High-power operation of AlGaAs SQW-SCH broad-area laser diodes for Nd:YAG solid-state laser pumping

AlGaAs single-quantum-well separate-confinement-heterostructure (SQW-SCH) single-stripe broad-area laser diodes (LDs) for Nd:YAG solid-state laser pumping were developed. The high-power operation of the SQW-SCH LD was demonstrated. The maximum output power under continuous wave operation at room temperature was 2.6 W in the range of the Nd:YAG absorption band. Stable operation was also confirmed for over 500 h under the condition of 25 degrees C at 1 W under continuous-wave operation.<>     

Characteristics of laser diodes with a partially intermixedGaAs-AlGaAs quantum well

Laser diodes (LD's) with a partially intermixed quantum-well (QW) active layer are fabricated by Zn out-diffusion from a p-cladding layer to the QW region. The dependencies of the degree of intermixing, measured by the photoluminescence (PL) shift, on Zn concentration of the p-cladding layer (P_clad) and the Al content of the guiding layer (X_g) in a separate-confinement-heterostructure (SCH) are investigated. P_clad changes in the range from 1×10 ¹⁸ cm^-3 to 4×10¹⁸ cm^-3 and X_g changes in the range from 0.21-0.37. When P_clad is 2×10¹⁸ cm^-3 and X_g is 0.37, large bandgap energy shift of 96.1 meV is observed. The lasing wavelengths of the LD's, with the partially intermixed QW, are blue-shifted linearly with increasing P_clad and X_g. For the bandgap energy shift of 66.8 meV by PL, the threshold current density is increased by 33% from that of the nonintermixed LD. Reliability of LD's with the partially intermixed QW is investigated for the first time. In spite of a large degree of intermixing the reliability of the LD with the partially intermixed QW of 66.8 meV energy shift by PL is the same as the nonintermixed one, which is confirmed by the aging test of 2500 hours at 45°C with the output power of 1 W under CW operation     

High-power and highly reliable 1020-nm ridge waveguide laser diodes with small aspect ratio as a pumping source for praseodymium-doped fiber amplifiers

The structure of an asymmetrically expanded optical field normal to the active layer was applied to 1020-nm ridge waveguide laser diodes in order to increase their maximum kink-free output power and reduce the aspect ratio of the output beams. As a result, a kink-free output power of more than 400 mW and an aspect ratio of less than 2.5 were achieved by this structure. Good reliability was also obtained at an output power of 300 mW at 50/spl deg/C.     

Achieving broad-area laser diodes with high output power andsingle-lobed far-field patterns in the lateral direction by loading amodal reflector

A modal reflector, which consists of a high-reflectivity region surrounded by low-reflectivity regions, is added to the front facet of two types of broad-area laser diodes (LD's) to control the lateral modes. One type of LD is the self-aligned laser with a bent active layer (SBA LD) that has a real index-guiding mechanism. The other is a planar-stripe LD that consists of a Zn-diffused region to confine the current flow and has a gain-guiding mechanism. For the SBA LD's with a modal reflector, stable single-lobed far-field patterns (FFP's) are obtained at up to 0.3 and 0.4 W output powers in continuous wave (CW) operation and pulsed operation, respectively. In addition, for planar-stripe LD's with a modal reflector, stable single-lobed FFP's are obtained at up to 0.4 W in CW operation. The lateral modes inside the cavity are analyzed by utilizing a slit model and FFP's are calculated. Good agreement is found between experimental and calculated FFP's for a large Fresnel number     

Leaching of cell wall components caused by acid deposition on fir needles and trees

Virgin fir forests have been declining since the 1960s at Mt. Oyama, which is located at the eastern edge of the Tanzawa Mountains and adjacent to the Kanto plain in Japan. An acid fog frequently occurs in the mountains. We collected throughfall and stemflow under fir trees and rainfall every week during January-December 2004 at Mt. Oyama to clarify the influence of acid fog on the decline of fir (Abies firma) needles. In relation to throughfall and stemflow, D-mannose, D-galactose, and D-glucose are the major neutral sugar components; only D-glucose is a major component of rainfall. The correlation coefficient between the total neutral sugars and uronic acid (as D-galacturonic acid), which is a key component of the cross-linking between pectic polysaccharides, was high except for rainfall. The leached amount of calcium ion, neutral sugars, uronic acid, and boron is related to the nitrate ion concentration in throughfall. Results of a laboratory exposure experiment using artificial fog water simulating the average composition of fog water observed at Mt. Oyama (simulated acid fog: SAF) on the fir seedling needles also shows a large leaching of these components from the cell walls of fir needles. The leaching amount increased concomitantly with decreasing pH of the SAF solution. We also observed that a dimeric rhamnogalacturonan II-borate complex (dRG-II-B) that exists in the cell wall as pectic polysaccharide was converted to monomeric RG-II (mRG-II) by the leaching of calcium ion and boron. Results not only of field observations but also those of laboratory experiments indicate a large effect of acid depositions on fir needles.     

Laser diodes in photon number squeezed state

A laser diode with an intrinsic layer as the space charge limited current region is expected to emit a low noise (less than the shot noise level) light. However, when one applies the intrinsic layer to the laser diode, severe difficulty is faced. Because the intrinsic layer has a very high resistivity, the applied voltage to operate the laser diode is too large and causes catastrophic damage to the laser diode. Here we propose novel laser diodes which emit a low noise light. The first is an AlGaAs laser diode having an undoped layer between the active layer and the cladding layer which acts as the space charge limited current region. Fano factor, F_m, of this laser diode is 28% smaller than the shot noise level (standard quantum limit, F_m=1) at 21 mA (output power, P₀=20 mW). The second one is an InGaAsP laser diode having two tunnel barrier layers whose bandgap energy is larger than that of the cladding layer. The region between the barriers acts as the space charge limited current region, Fano factor, F_m of this laser diode is 47% smaller than the shot noise level at 21 mA (P₀=10 mW). On the other hand, an AlGaAs laser diode with the two tunnel barrier layers has Fano factor, F_m which is 43% smaller than the shot noise level at 21 mA (P₀=20 mW). The calculated amplitude noise spectral densities of the latter two laser diodes are in good agreement with the calculated values from Langevin method. However, the calculated amplitude noise spectral density of the former laser diode does not agree with the calculated value from Langevin method. This disagreement is also discussed     

Anomalous wavelength changes of 980-nm pump laser diodes with antireflection coatings

We have observed abrupt wavelength jumps form 980-nm pump laser diodes with an antireflection (AR) coating when the injection currents were changed. The width of the anomalous jumps becomes wider as the cavity length is shortened. It was clarified from theoretical calculations that the phenomenon is caused by narrow mirror loss distributions. It was also confirmed that wide AR coatings or a long cavity can avoid these abrupt wavelength jumps.     

0.78- and 0.98-μm ridge-waveguide lasers buried with AlGaAsconfinement layer selectively grown by chloride-assisted MOCVD

The 0.78- and 0.98-μm buried-ridge AlGaAs laser diodes (LD's) with a high Al-content AlGaAs confinement layer selectively grown by using a Cl-assisted MOCVD are demonstrated. By employing the AlGaAs confinement layer, the threshold current and the slope efficiency of the 0.78-μm LD are improved by ~40%, compared to those of the conventional loss-guided LD with the GaAs confinement layer. In addition, the stable fundamental mode up to 150 mW and the small astigmatic distance less than 1 μm are obtained. The 0.78-μm LD also shows the excellent high-power and high-temperature characteristic such as 100 mW CW operation at 100°C and the reliable 2,000-hour operation under the condition of 60°C and 55 mW. In the 0.98-μm LD, the narrow beam with the low aspect ratio of 1.86 and the stable fundamental transverse mode over 200 mW are exhibited. As a result, the 0.98-μm LD realizes the high fiber-coupled-power of 148 mW. Moreover, the high-power and high-temperature operation of 150 mW at 90°C is obtained. In the preliminary aging test, the LD's have been stably operating for over 900 hours under the condition of 50°C and 100 mW     

High-power 980-nm ridge waveguide laser diodes including an asymmetrically expanded optical field normal to the active layer

We propose a new ridge waveguide laser diode (LD) which supports an asymmetrically expanded optical field normal to the active layer in order to increase the maximum kink-free output power and reduce the aspect ratio of output beams. The dependence of maximum kink-free output power on facet reflectivity was analyzed from the viewpoint of the total optical power in the cavity. It was clarified that the maximum kink-free output power is influenced by the facet reflectivity which affects the refractive index changes of the ridge region via the total optical power in the cavity. More than 600 mW of maximum kink-free output power and an aspect ratio of less than 2.5 were achieved in experiments with 980-nm ridge waveguide LDs by means of this proposed new structure.     

Fabrication of broad-area laser diodes on a three-inch wafer by asolid-phase diffusion method

We have developed extremely uniform Zn diffusion on a three-inch GaAs-AlGaAs QW laser wafer by an open tube solid-phase diffusion technique and have applied it to fabrication of broad-area laser diodes (LD's) for the first time using 3-inch full-wafer processing. Excellent uniformity of device characteristics have been obtained on a 3-inch wafer; threshold current distribution along the radial direction of a wafer is 356.2±8.3 mA (1σ), emission wavelength is 879.7±0.6 nm (1σ). Such high uniformity is caused by not only uniform Zn diffusion but also uniform epitaxial layer thickness over a three-inch wafer