A smile-suppressed high-power InGaN laser array has been developed for a high beam quality material processing light source. The smile effect becomes apparent especially in InGaN laser array with large chip curvature due to lattice mismatch of epitaxial growth layers. To reduce the smile, periodic grooves are introduced to the epitaxial layers for removing the origin of strain. It also enables a two-dimensional strain management of remaining epitaxial layers. This technology improves the chip curvature within micron range, i.e. as small as 0.3 μm in a 9 mm-width InGaN laser array. We have successfully realized reducing the smile to 0.4 μm without degrading the laser light output characteristics.
We propose optical-loss suppressed thick-optical-waveguide (TOW) InGaN laser diodes (LDs) without operatingvoltage increase. A record high continuous-wave (CW) output of 7.2W for a single-emitting InGaN LD is achieved without thermal peak-out in the light-current curve. The TOW enables to confine major part of the propagating light into a transparent undoped region, and thus significantly reduces the optical-loss. An electron-overflow-suppression (EOS) layer placed between the waveguide layer and a p-cladding layer plays an important role to reduce the operating voltage after introduction of the undoped TOW layer. We executed a self-consisted calculation of voltage-current characteristics taking into account Schrödinger and Poisson equations in conjunction with a carrier continuity equation. The calculation result indicates possible presence of conductivity-modulation in the waveguide filled with electrons reflected backward by the EOS layer and holes injected from the p-type cladding layer. We successfully demonstrated the optical-loss suppressed operation resulting in the slope efficiency (SE) increase from 2.0W/A to 2.5W/A. It is noted that the operating voltage of the TOW LD is nearly identical to the conventional LD thanks to the above conductivitymodulation phenomenon. The presented result suggests that our TOW structure can overcome the optical-loss drawback of the InGaN LDs, and hence will lead them to the applications requiring high wattage light sources.
We report on design and lasing characteristics of GaN vertical-cavity surface-emitting lasers (VCSELs) with an
elongated cavity for use in uniform elements of a two-dimensional (2D) laser array. Calculations of VCSELs with the
elongated cavity taking into account the wavelength dispersion of the refractive index show that the transverse mode
spacing can be significantly narrower than the gain spectrum with a small tradeoff of the differential quantum efficiency.
The result clearly shows that the elongated cavity is robust against the thermally induced peakshift of the gain spectrum,
and thus preferable for use in elements of density packed laser array for which uniform operation of each element is
crucial. The VCSEL with the elongated cavity fabricated by the wafer thinning technique operates under current
injection by using highly reflective distributed Bragg reflectors (DBRs) made of transparent ZrO2 and SiO2 film stacks. Together with high reflectivity and wide stop band of the DBR, the elongated cavity of 6 μm (36λ) allows multimode lasing oscillation with a mode spacing of 2.9 nm, which is one order of magnitude narrower than the gain spectrum. In addition, we demonstrate a 5x5 GaN VCSEL array.
A novel GaN-based surface-emitting laser was realized by utilizing total internal reflection (TIR) by an
inclined mirror formed at one end of the horizontal cavity of an edge-emitting laser. The inclined mirror was fabricated
by focused ion beam (FIB) etching. The mirror was inclined by 45° with respect to the surface normal. The guided light
propagating along the horizontal-cavity is reflected at the mirror to the surface normal.
We analyzed optical losses in the laser. To increase the external quantum efficiency, removal of an
FIB-damaged layer and precise control of the mirror angle are important. Argon-milling was applied to the FIB-etched
surface to remove the FIB-damaged layer which causes an optical loss. The fabricated device with the stripe width of 8
µm and the cavity length of 600 μm lased at 390 nm with a threshold current of 260 mA. Surface-emission was obtained
with beam divergence angles of 24.0° and 6.2°, corresponding to perpendicular and parallel to the junction plane,
respectively. The presented surface-emitting laser is suitable to form high-power GaN-based 2D laser arrays.