K. Nakahara

GaInNAs: a novel material for long-wavelength semiconductor lasers

GaInNAs was proposed and created in 1995 by the authors. It can be grown pseudomorphically on a GaAs substrate and is a light-emitting material having a bandgap energy suitable for long-wavelength laser diodes (1.3-1.55 /spl mu/m and longer wavelengths). By combining GaInNAs with GaAs or other wide-gap materials that can be grown on a GaAs substrate, a type-I band lineup is achieved and, thus, very deep quantum wells can be fabricated, especially in the conduction band. Since the electron overflow from the wells to the barrier layers at high temperatures can he suppressed, the novel material of GaInNAs is very attractive to overcome the poor temperature characteristics of conventional long-wavelength laser diodes used for optical fiber communication systems. GaInNAs with excellent crystallinity was grown by gas-source molecular beam epitaxy in which a nitrogen radical was used as the nitrogen source. GaInNAs was applied in both edge-emitting and vertical-cavity surface-emitting lasers (VCSELs) in the long-wavelength range. In edge-emitting laser diodes, operation under room temperature continuous-wave (CW) conditions with record high temperature performance (T/sub 0/=126 K) was achieved. The optical and physical parameters, such as quantum efficiency and gain constant, are also systematically investigated to confirm the applicability of GaInNAs to laser diodes for optical fiber communications. In a VCSEL, successful lasing action was obtained under room-temperature (RT) CW conditions by photopumping with a low threshold pump intensity and a lasing wavelength of 1.22 /spl mu/m.

1.3-μm continuous-wave lasing operation in GaInNAs quantum-well lasers

A 1.3-/spl mu/m continuous wave lasing operation is demonstrated, for the first time, in a GaInNAs quantum-well laser at room temperature. This lasing performance is achieved by increasing the nitrogen content (up to 1%) in GaInNAs quantum layer. It is thus confirmed that this type of laser is suitable for use as a light source for optical fiber communications.

GaInNAs-GaAs long-wavelength vertical-cavity surface-emitting laser diodes

Vertical-cavity surface-emitting laser diodes with GaInNAs-GaAs quantum-well (QW) active layers are demonstrated for the first time. GaInNAs permits the realization of a long-wavelength vertical-cavity laser grown directly on a GaAs substrate. Room-temperature (RT) pulsed operation is achieved, with an active wavelength near 1.18 /spl mu/m, threshold current density of 3.1 kA/cm/sup 2/, slope efficiency of /spl sim/0.04 W/A, and output power above 5 mW for 45-/spl mu/m-diameter devices. Laser oscillation is observed for temperatures at high as 95/spl deg/C.

Mechanism analysis of improved GaInNAs optical properties through thermal annealing

We investigated the mechanisms of improved GaInNAs optical properties by thermal annealing. The absorption spectra measured for the bulk layer indicated that the large shift in the PL wavelength was probably caused by a bandgap shift in the GaInNAs itself. The cathodeluminescence measurements revealed that the enhancement of the PL intensity was generated by uniform emission from the entire region; in comparison, nonuniform dot-like regions exist in an as-grown GaInNAs layer. These analyses, which is peculiar to this type of material system, should be helpful for further improving the crystal quality, thus helping to enable semiconductor lasers with excellent high-temperature performance.

Gas-source MBE of GaInNAs for long-wavelength laser diodes

We have grown GaInNAs using gas source molecular beam epitaxy (GS-MBE) in which a nitrogen radical was used as the nitrogen source. This growth method produces more reactive-N and fewer reactions between the sources. Excellent crystalline quality and luminescence properties of the GaInNAs/GaAs quantum well were confirmed by transmission electron microscopic observation and photoluminescence measurement. We have also succeeded in applying GaInNAs to long-wavelength laser diodes that lased under room temperature continuous wave operation in the 1.3 μm wavelength range suitable for optical fiber communications. Thus, we have experimentally demonstrated that GS-MBE is a suitable method to grow GaInNAs with sufficient quality for device applications.

40-Gb/s Direct Modulation With High Extinction Ratio Operation of 1.3-

Direct modulation at 40 Gb/s of a 1.3-mum InGaAlAs distributed feedback ridge waveguide laser is experimentally demonstrated. By combination of the high differential gain of an InGaAlAs multiquantum well active layer, a short cavity length of 100 mum, and a low-resistance notch-free grating, it achieves high bandwidth of 29 GHz and high-extinction ratio of 5 dB at 40-Gb/s modulation. Moreover, the laser operates at a record maximum ambient temperature of 60degC under 40-Gb/s directly modulation. It also achieves 40-Gb/s modulated transmission over 2 km with a low power penalty of 0.25 dB at 25degC .

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We have obtained a high characteristic temperature (T0) of 215 K from a 1.3 µm GaInNAs/GaAs single-quantum-well laser under pulsed operation at 20°C to 80°C. To our knowledge, this T0 is the highest yet reported for 1.3 µm band edge emitters suitable for optical-fiber communication systems. The use of GaInNAs as an active layer is, therefore, very promising for the fabrication of long-wavelength laser diodes with excellent high-temperature performance.

m InGaAlAs Multiquantum Well Ridge Waveguide Distributed Feedback Lasers

A 1.3-µm-range GaInNAs/GaAs single-quantum-well laser was tested for 1000 hours at 24°C under an auto-current-control condition. No degradation was observed. The threshold current fell slightly (14%). The lasing wavelength was stable. The lack of rapid degradation under an auto-power-control condition at a high temperature (50°C) suggests that a reliable GaInNAs laser diode with a practical lifetime may soon be realized.

A 1.3-µm GaInNAs/GaAs Single-Quantum-Well Laser Diode with a High Characteristic Temperature over 200 K

The temperature dependence of lasing wavelength in 1.2-/spl mu/m or 1.3-/spl mu/m-range GaInNAs edge-emitting laser diodes (LD) was found to be small. It is almost independent of the characteristic temperature (T/sub 0/) and is equivalent to the temperature shift of the bandgap wavelength of GaInNAs (0.42 nm//spl deg/C). Since the dependence is smaller than that of 1.3-/spl mu/m-range conventional InGaAsP LDs and also smaller than the required value (<0.48 nm//spl deg/C), it is concluded that the GaInNAs LDs are promising for use as 1.3-/spl mu/m-range light sources because of their lasing-wavelength stability against temperature shift and a high T/sub 0/. The small dependence is due to the small effect of band filling on lasing wavelength from the deep quantum well in GaInNAs LDs.

A 1.3-µm GaInNAs Laser Diode with a Lifetime of over 1000 Hours

A novel structure for an InP-based ridge-waveguide (RWG) laser is demonstrated. It has a reverse-trapezoid-ridge shape which offers reduced threshold current and smaller electrical and thermal resistances over the conventional vertical-mesa (VM) RWG devices. We found this new RWG laser quite suitable for wide-temperature-range and high-power operations. 1.3-1.55-/spl mu/m wavelength strained InGaAsP-InP multiple quantum-well (MQW) lasers were demonstrated with the new ridge structure, which showed sufficient lasing behavior such as high-temperature lasing up to 165/spl deg/C and a high light output over 300 mW. This simple and high-performance laser structure was also applied for integrated lasers with other functional elements such as an absorption-type modulator or a beam-expander waveguide lens. To achieve low-threshold current operation in RWG lasers, suppression of the lateral-current spreading is important. We proposed a new method to effectively reduce this lateral diffusion current by using a doped active region with n-type dopant. Combined by the reduced transparency carrier density in the n-doped active region, this leads to a very low-threshold current of less than 2.2 mA for a coated 200-/spl mu/m-long cavity device, which is comparable to those of recent advanced same-material buried-heterostructure (BH) lasers. These excellent lasing properties, along with the sufficient device reliability under strict environmental conditions, make this reversed-mesa (RM) RWG a promising candidate for the widespread use of low-cost long-wavelength light sources.